Symposium F.EN09—Developing In Situ and Operando Methodology for Observation of Energy Conversion, Storage and Transport Processes in Materials and Devices

Electron microscopes have become very power tools to study the structure of materials at unprecedented resolution. From their invention, these tools have provided essential information on the structure and properties of the broadest range of materials used today in structural and functional applications. In-situ electron microscopy has evolved over the last few decades from dynamic experiments (heating, straining) to reaction in gases, and, more recently, even electrochemical experiments either in vacuum or in liquids. While the spatial resolution remains the first motivation for such in situ experiments in electron microscopy (so that even nucleation and early stages of growth of nanostructures formed during electrodeposition can be investigated), the flexibility and the more realistic environments remain the realm of synchrotron beam lines. Higher pressures, higher (or lower) temperatures, experiments in liquids and gases, can be carried out with exquisite spectroscopic resolution, penetrating power with diffraction techniques, in realistic and macroscopic samples and experimental conditions more suitable to represent operating conditions.
In this presentation, a comparison of the types of experiments that can be carried out with electron and photon techniques are discussed, in particular in the context of the nature of the samples, the time resolved sensitivity and the potential artefacts.

Rechargeable non-aqueous lithium-oxygen batteries with a large theoretical capacity are emerging as a high-energy electrochemical device for sustainable energy strategy. Although the fundamental electrochemical principle of lithium-oxygen batteries has been established for decades, the cathodic reaction kinetics, associated with the formation and decomposition of a solid Li₂O₂ phase during charging and discharging, remains debating. Here we report direct visualization of the charge/discharge reactions on Au and RuO₂ cathodes in non-aqueous electrolytes with and without redox mediators using liquid-cell aberration-corrected scanning transmission electron microscopy (STEM) combining with synchronized electrochemical measurements. The real-time and real-space characterization by time-resolved quantitative STEM with ~1 nm spatial resolution reveals the electrochemical correspondence of discharge/charge overpotentials with the nucleation, growth and decomposition of Li₂O₂ which are mediated by solid and liquid catalysts at constant current densities. The nano-scale operando observations unveil the underlying reaction pathways of lithium-oxygen batteries under different electrochemical conditions during round-trip discharging and charging and shed lights on the strategies in improving the performances of lithium-oxygen batteries by tailoring the cathodic reactions through cahode and electrolyte design.

The solid electrolyte interphase (SEI) is regarded as the most complex but the least understood constituent in secondary batteries using liquid and solid electrolytes. The nanostructures of SEIs were recently reported to be equally important to the chemistry of SEIs for stabilizing Li metal in liquid electrolyte. However, the dearth of such knowledge in all-solid-state battery (ASSB) has hindered a complete understanding of how certain solid-state electrolytes, such as LiPON, manifest exemplary stability against Li metal. Characterizing such solid-solid interfaces is difficult due to the buried, highly reactive, and beam-sensitive nature of the constituents within. By employing cryogenic electron microscopy (cryo-EM), the interphase between Li metal and LiPON is successfully preserved and probed, revealing a multilayer mosaic SEI structure with concentration gradients of nitrogen and phosphorous, materializing as crystallites within an amorphous matrix. This unique SEI nanostructure is less than 80 nm and is shown stable and free of any organic lithium containing species or lithium fluoride components, in contrast to SEIs often found in state-of-the-art organic liquid electrolytes. Our findings reveal insights on the nanostructures and chemistry of such SEIs as a key component in lithium metal batteries to stabilize Li metal anode.

As current memory technologies approach their fundamental limits, increasing demand for novel non-volatile memories is driving the development of new architectures, which are already showing their potential in high performance computing [1, 2]. As a result of their non-volatile nature, byte-addressability and fast response times, which are comparable to those of dynamic random-access memory, resistive-switching-based phase change materials (PCMs) are strong candidates for memory applications with reduced hardware cost and power consumption [3, 4].

The use of advanced transmission electron microscopy combined with in situ electrical biasing provides a powerful method for studying the local relationship between atomic structure and electrical properties in individual nanoscale memory elements, in order to identify the factors that affect their switching reproducibility and dynamics.

We have developed a multi-step lithographic procedure, which allows dedicated nanostructured line cells that containPCM switching layers, electrical contacts and protective layers to be fabricated directly on electron-transparent silicon nitride membranes and used for switching experiments both outside and inside the transmission electron microscope. We used a special electrical setup to provide short (50 ns) electrical current pulses to individual line cells with sharp (approximately 3 ns) rising and falling edges. By using these pulses, PCM devices could be switched between crystalline low resistance and amorphous high resistance states inside a transmission electron microscope.

Even though each device consisted of several layers of different materials, resulting in a total thickness of between 200 and 300 nm, a fast pixelated direct electron detector could be used to record scanning electron diffraction patterns from amorphous/crystalline regions before and after successive switching cycles, while measuring the electric properties of the same devices. Newly-developed algorithms based on LiberTEM software [5] were used to follow the evolution of individual crystallites and to show that the switching mechanisms involved slow crystal growth or rapid recrystallization, depending on the details of the current pulses.

[1] K. Wu, F. Ober, S. Hamlin and D. Li, ArXiv:1708.02199 [Cs] (2017).
[2] Y. Zhang and S. Swanson, 31^st Symposium MSST (IEEE, Santa Clara, CA, USA, 2015), pp. 1-10.
[3] T. Coughlin, IEEE Consumer Electronics Magazine 5, 133 (2016).
[4] S. Chhabra and Y. Solihin, 38^th Annual ISCA (2011), pp. 177-188.
[5] A. Clausen et al. LiberTEM/LiberTEM: 0.5.0. (Zenodo, 2020). doi:10.5281/zenodo.3763313.

Electrically charged liquid-solid interfaces play a vital role in many electrochemical phenomena encountered in biology, energy, and the environment. At electrically charged interfaces, ions and solvent molecules rearrange to screen the surface charge, forming the electrical double layers. The ability to probe such systems with single-ion resolution is important to a basic understanding of their behaviors and the development of new ionic technologies. Although many techniques have been used to probe liquid-solid interfaces, it remains a challenge to acquire spatial information down to the single ion level. Here, using cryo-electron microscopy, we directly visualize individual ions and their distributions at electrically charged liquid-solid interfaces. We observe discrete layering of multivalent counterions, which arises from the finite ion size and the strong electrostatic interactions. In addition, correlated ion structures under extreme confinement are revealed, with the channel widths approaching the ion diameter (~1 nm). Our experiment opens up a new arena to study liquid-solid interfaces at the single ion level.

Synchrotron X-ray absorption spectroscopy and associated techniques are useful tools for the element-specific analysis of the structure and properties of materials. In this talk, the application of such techniques in the study of some noble metal nanostructures toward catalytic applications will be presented. Materials of interest include nanoclusters, nanoalloys, and single-atomic-site catalysts. It will be demonstrated that the X-ray spectroscopy techniques, in association with other experimental and computational methods, can sensitively probe the structural and property changes of the catalysts induced by effects such as sample size, protecting ligands, and alloy bonding. The structural information revealed by the X-ray techniques is found useful to more thoroughly understand the structure-property relationships of the nanocatalysts.

MnO₂ nanosheets demonstrate an interesting response to reduction of Mn⁴⁺ to Mn³⁺, where the Jahn-Teller distorted Mn³⁺ is displaced out of the plane of the nanosheet forming a “surface Frenkel” defect. Atomic scale point defects enhance capacitance beyond 300F/g while lowering charge transfer resistance of δ-MnO₂ nanosheets by a factor of 10 and increasing cycle stability.

In-situ and in-operando X-ray total scattering and XRD was teamed with X-ray absorption spectroscopy, Raman spectroscopy, and related tools to probe the local atomic structure and defect chemistry during electrochemical cycling. The result is a description of the charging mechanism in high surface area MnO₂ nanosheet electrodes: reversible in-plane strains of up to 0.7% occured during charging of nanosheets with little restacking, accompanied by Mn reduction. Bond distances obtained from Pair Distribution Functions correlated with the XRD where Mn-O and Mn-Mn bond lengths reversibly increased during charging. Raman spectra also demonstrate an increased number of Jahn-Teller distorted MnO₆ octahedra in the charged state.

Changes to the interlayer during electrochemical cycling were morphology dependent. Random assemblies of nanosheets with little sheet-to-sheet restacking showed only in-plane response, without changes in sheet-to-sheet distances. However, highly restacked nanosheets contract in the c direction on charging due to electrostatic changes to the interlayer. The results suggest that improved cycling stability is feasible using high-porosity, random assemblies of nanosheets and that the interlayer hydration is a key parameter in diffusion of the charging ions.

For the more than half a century the small polaron hopping model has been used to describe the electronic charge transport in semiconducting oxides. This model was originally developed for binary oxides, and despite its vast usage, the model’s accuracy has not been rigorously tested for complex oxides (ternarys and higher order oxides). Ternary spinel oxides are an important class of materials actively researched for applications in energy storage, catalysis, and data storage due to their exciting electrochemical, catalytic, and magnetic properties; and better understanding of charge transport in such systems can lead to higher performance devices. Previous experimental investigations of charge transport in ternary spinels are sparse and limited owing to the lack of accessible characterization techniques to correlate site occupation to properties, and also due to difficulties in achieving phase-pure ternarys over a wide stoichiometric range. To overcome these challenges, we choose the Mn_xFe_3-xO₄ system, which has exciting electrochemical and catalytic properties, and mixed oxidation states that enables us to examine the mechanisms of small polaron transport. We fabricate epitaxial and phase pure Mn_xFe_3-xO₄ thin films over the complete ‘x’ range using molecular beam epitaxy (MBE); and use our recently developed method using x-ray emission spectroscopy (XES) to determine the site occupation of cations. Previously we have shown that XES is an accessible and a more accurate method than traditional methods such as x-ray absorption spectroscopy (XAS) in extracting cation site occupation. We find that in Mn_xFe_3-xO₄, polarons preferably localize at Mn octahedral sites and polaron hopping occurs only between like-elements (Fe to Fe and Mn to Mn), and as a result the conduction occurs along decoupled pathways formed by Mn and Fe cations. Since the role of cross-hopping and decoupled pathways have not been considered in the conventional polaron hopping model, we introduce new parameters in the conventional transport model. We test the validity of the new model on the Mn_xFe_3-xO₄ system by comparing the concentration of charge carriers calculated using the conventional and the new model to the experimental conductivity trends. An excellent overlap of the new model with experimental trends confirms the precision of our augmented model and the limitation of the conventional model for ternary spinels. With this work, we use new synchrotron methods to provide a pragmatic tool for designing non-equilibrium oxides with enhanced performance with the aim to incorporate spinels in devices such as batteries, supercapacitors, and fuel cells.

Perovskite nanoparticles, composed of LaCo_xFe_1-xO₃, are a promising heterogenous catalyst for oxidation reactions. As-synthesized nanocatalysts often exhibit a complex structure with a variety of defects. During operando experiments, the complexity of the structure increases by the addition of a boundary layer. Furthermore, the catalytic activity itself can result in structural changes. However, the understanding of the structure-activity relationship is critical to ultimately improve catalytic performance.

Due to their large surface to volume ration, the bulk as well as the surface of small nanoparticles can be probed by Extended X-Ray Absorption Fine Structure (EXAFS). Measurement at the different elemental absorption edges resolves the local structure around each atom type of the complex oxide.

The presentation is focused on the EXAFS measurements of LaCo_xFe_1-xO₃ nanoparticles at the Co-K, Fe-K and La-K edge to refine a mutual particle model by Reverse Monte Carlo (RMC) analysis.

Acknowledgements:
Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 388390466 – TRR 247.

A central theme in renewable energy technologies today is to design nanostructured catalysts towards desired reactions. Engineering the functional defects in materials is a key step in this direction. For this, we have to advance the understanding of the role of defect structures in the relevant properties of materials. Here we develop a generalizable experimental and analysis framework using ambient pressure X-ray spectroscopy (APXPS), density functional theory (DFT), and Monte Carlo (MC) calculations, to understand the role of point defects in catalyst generation under real working conditions.
A recent advance in materials for electrochemical energy and fuels conversion is to synthesize metal nanoparticles in a process termed “exsolution”. Unlike traditional nanoparticle infiltration techniques, the nanoparticle catalysts from exsolution are anchored in the parent oxide. This strong metal-oxide interaction makes the exsolved nanoparticles more resistant against particle agglomeration as compared to the infiltrated ones. In addition, the exsolved particles also open up the possibility of regeneration of catalysts. While being an exciting and promising pathway for generating stable oxide supported nanoparticles, rational control over the exsolved particles has yet to be achieved. In particular, it remains a big challenge to control the size and density of the exsolved nanoparticles.
In this work, we propose that point defect formation in the oxide lattice to be the fundamental knob to control exsolution, and demonstrate this approach in epitaxial La_0.6Sr_0.4FeO₃ (LSF) thin films. By quantifying surface defect states with APXPS, we identify the initial oxygen vacancy (V_O) formation and the following Schottky defect formation as the primary defect reactions in exsolution. Then, by tuning the formation energy of these two defects in LSF with biaxial lattice strain, we examined their impacts on metallic iron (Fe⁰) exsolution. As a result, the LSF surface with that has more of these point defects, also has the highest Fe⁰ concentration, the largest particle density, as well as the finest particle size. Finally, with the aid of DFT calculations and MC simulations, we suggest surface oxygen vacancy clusters to be the nucleation sites for the exsolved particles. These observations highlight the critical role of surface point defects in controlling the size and density of the nanoparticles.

For the inspection of processes involving condensed matter, in situ neutron powder diffraction proves to be a versatile tool, giving insight into processes of technological pertinence. Only a few high-intensity powder diffractometers at intense neutron sources allow for this. D20 at Institut Laue-Langevin provides the highest available intensity in constant wavelength neutron powder diffraction. A stationary, curved linear position sensitive detector allows for in-situ diffraction studies down to a second and encourages the use of complex sample environments with inherently small sample sizes. D20 adapts to various levels of crystallographic complexity and rapidity of an observed phenomenon.
Rapid growth of the portable electronics market, as well as non-polluting ground transportation, needs portable energy storage solutions with improved characteristics. One way to improve Li-ion batteries are solid electrolytes to overcome issues of liquid electrolytes in battery safety and high-voltage operation. Neutron diffraction determines the Li diffusion pathway in solid-state Li-ion conductors. Development of better electrode materials in terms of gravimetric and volumetric energy density, temperature operation range and cycling stability needs understanding of lithium (de)intercalation phenomena. Operando diffraction techniques are well suited here. Electrochemical cells based on a neutron-transparent (Ti,Zr) alloy combine good electrochemical properties and the ability to collect neutron diffraction patterns with reasonable statistics and no other Bragg peaks than those of the electrode material. This allows detailed structural determination of electrode materials by Rietveld refinement during operation.
Solid-oxide fuel cells convert chemical energy into electricity at higher efficiency than conventional methods, with less pollution. The anode (fuel electrode) must not alter at high temperature (thermal stability), not form nonconductive phases at interfaces (chemical stability) and not degrade upon reduction and oxidation cycles (redox stability). The state-of-the-art “cermet” of Ni and yttria-stabilized zirconia ceramic loses performance upon usage as its porosity is reduced by Ni agglomeration and as oxidation of Ni causes redox instability. Cermet deactivates through carbon coking and sulfur poisoning, making it unsuited for hydrocarbon fuels. Single-phase mixed ionic and electronic conductors provide microstructural stability and increase the electrode fraction accessible to oxide ions. Many of those oxides have been investigated successfully in operando at high temperature under oxidizing and reducing gas flow by neutron diffraction, following the crystal chemistry of oxide ions during the process.
Classical in situ work (thermo-diffractometry) has been done on the new photovoltaic materials, MaPbI₃ and derivates, and neutron diffraction turned out to be a perfect tool to screen the crystal chemistry of light organic atoms beside the heavy metal atoms over a wide range of temperatures.
Hydrogen is an attractive energy carrier for renewable energy sources due to its high energy density. Solid-state hydrogen storage provides higher storage capacities than compressed or liquefied hydrogen. Complex metal hydrides have high hydrogen storage capacities but suffer from poor kinetic and thermodynamic properties. Tuning the thermodynamics for dehydrogenation, in order to reduce the temperature at which hydrogen is evolved is achieved through the addition of a second phase, which will lead to the formation of a more stable product upon decomposition and thereby reducing the enthalpy for dehydrogenation. Neutron powder diffraction comes screens the crystal chemistry of the different phases in situ and localizes hydrogen in the structures.

The performance of functional materials central to energy devices, including rechargeable batteries, fuel cells, as well as gas separation and storage technologies, is determined largely by atomic-scale materials structure and dynamic-function relations. Many energy materials function by undergoing change, such as compositional change in rechargeable battery electrodes that reversibly host charge-carrying ions and in gas storage and separation materials. Robust characterization methods that quantitatively and accurately capture these changes are essential to the strategic design of superior materials function, and consequently to improving energy device performance. Historically, a conflict existed between fast and detailed measurements of materials structure, particularly for measurements of materials within devices. This conflict was sometimes alleviated by using small model systems in which the material function was not representative of that in the corresponding commercially used device. Today, advanced characterization methods that capture material changes in detail while they are occurring is possible in whole devices under real-life operating conditions, as facilitated by advances in instrumentation.

This talk will give examples of in operando and in situ neutron measurements of energy materials used for rechargeable batteries capturing compositional change and/or non-equilibrium processes, with a focus on neutron powder diffraction using the high intensity neutron powder diffractometer Wombat at the Australian Centre for Neutron Scattering

The field of all-solid-state-batteries (ASSBs) is expected to potentially overcome associated issues of conventional LIBs and has recently attracted both industrial and academic attention ^{(1) (2)}. Indeed, by removing flammable liquid solvents, this technology may enable improved intrinsic safety at cell level and increased temperature stability, resulting in system level benefits compared with conventional LIBs based on liquid electrolytes. SEs should also allow the possibility to use lithium metal anode leading to an enhancement of the energy density. To develop the ASSB technology, it is mandatory to first understand the (de)lithiation mechanisms into/from active materials and investigate the stability of the solid electrolyte itself. Among in-situ techniques used to understand electrode materials, neutron diffraction (ND) is one of the most powerful due to its high penetration depth allowing simultaneous data collection from both electrodes.

Operando ND in Li-ion batteries has been widely used to determine lattice parameters, phase transitions, and detailed structural information such as Li position and content in anode and cathode materials ^{(3) (4) (5) (6)}, but also useful for commercial cell analysis ^{(7) (8) (9)}. In the long term, this tool could help the ASSB community with trending problems such as SE decomposition ⁽¹⁰⁾ or stability of crystalline coating on active materials ⁽¹¹⁾. A new operando electrochemical cell made of a Ti-Zr alloy completely transparent to neutrons was designed by the Institut Laue-Langevin, allowing operando neutron diffraction on the whole solid state battery, in a wide temperature range from -100°C to + 400 °C. The cell is completely airtight and offers the possibility of applying pressure. A first demonstration of the capabilities of the cell and of its limitations will be presented, using a solid-state battery based on a LiCoO₂ cathode, a Li₄Ti₅O₁₂ anode and Argyrodite Li₆PS₅Cl as the solid electrolyte. Temperature-controlled Li⁺ extraction from LiCoO₂, up to 150°C, made possible by the use of an ASSB, reveal new interesting phenomena. Over one electrochemical cycle, the structural integrity of the Argyrodite solid electrolyte appears to be maintained.

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Surface engineering is the most effective method in regulating the chemo-mechanical interaction behavior of electrodes and further enhancing their electrochemical performance in lithium ion batteries (LIBs). Although extensive studies have been carried out to investigate the effect of coatings on the electrodes surface degradation, limited understanding have been obtained regarding how these coatings regulate the electrodes mechanical behavior. Here, for the first time, we combine in-operando synchrotron X-ray scattering technique and high-resolution transmission X-ray microscopy (TXM) technique to investigate the effect of a poly(3,4-ethylenedioxythiophene) (PEDOT) artificial coating on the electro-chemo-mechanical behavior of LiCoO₂. TXM provides straightforward time-dependent images of LiCoO₂ particles, while X-ray scattering offers volume-averaged quantitative information of the LiCoO₂ electrodes during cycling. The complimentary of the two techniques is examined by an over-lithiation experiment of LiCoO₂ between 3.0 V – 0.5 V, during which LiCoO₂ particles/agglomerates crack and eventually pulverize. The two in-operando techniques are then applied to uncover the largely improved 4.5 V high-voltage cycling stability of LiCoO₂ from the PEDOT coating. The results suggest that the PEDOT coating not only alleviates LiCoO₂ surface deterioration and electrolyte decomposition, but also inhibits LiCoO₂ particles/agglomerates expansion during the cycling test. The understanding of the electro-chemo-mechanical behavior of battery electrodes will stimulate the development of advanced LIBs with fast charging ability and long cycle life.

Photoelectrochemical water splitting is a promising route for efficient conversion of solar energy into chemical fuels. The chemical transformation of water into oxygen and hydrogen takes place at the photoelectrode surface. Consequently, the activity, efficiency, and reaction pathway are critically controlled by the material surface properties. Under operating conditions, surface properties depend on the surrounding environment, and may be altered in the course of the reaction. Thereby, absorption of molecules can modify the chemistry at the surface, for example by influencing the kinetics of reactants, products, or reaction intermediates, but they can also directly impact the electronic transport properties by acting as surface trap states. In this context, improved understanding of these complex surface interactions will aid the development of highly efficient light absorbers as well as the integration of effective passivation and catalyst layers for these materials.

Among different photoelectrode materials, bismuth vanadate (BiVO₄) is one of the most actively investigated oxide semiconductors. Here, we employ photoconductive AFM under controlled in-situ conditions to gain insight into the relationship between surface interactions and interfacial charge transport characteristics in polycrystalline BiVO₄ thin films. We demonstrate that the low intrinsic bulk conductivity of BiVO₄ limits charge transport through the film, and that the transport mechanism can be attributed to space charge limited current in the presence of trap states. [1] By analyzing the space charge limited current in selective gas environments, we are able to quantify the impact of surface adsorbates on bulk transport properties. We find that surface adsorbed oxygen acts as a shallow trap state and accounts for 40% of the effective trap density in BiVO₄ thin films. [2] For humid environments, our results are consistent with the adsorption of water as an oriented dipole layer, which does not induce a surface charge transfer but partially inhibits the adsorption of oxygen at the surface. Disentangling the individual effects of oxygen and water on charge carrier trapping underpins the importance of trap state passivation for efficient transport of photogenerated charge carriers in BiVO₄.

[1] Eichhorn et al., Nat. Commun. 9, 2597 (2018).
[2] Eichhorn et al., ACS Appl. Mater. Interfaces 10, 35129 (2018).

Super-resolved fluorescence microscopy techniques have enabled significant advances in chemical and biological sciences. However, they can only interrogate entities that fluoresce — and most chemical or biological processes do not involve fluorescent species. Here we report ‘COMPEITS’ — a ‘COMPetition-Enabled Imaging Technique with Super-resolution’, that enables quantitative super-resolution imaging of nonfluorescent processes. It is based on the incorporation of competition into a single-molecule fluorescence detection scheme. We demonstrate COMPEITS by investigating a photoelectrocatalytic reaction; we map with nanometer precision a nonfluorescent surface reaction on single photocatalyst particles that is important for water decontamination. The sub-particle-level quantitative information of reactant adsorption affinities unambiguously decouples size and shape scaling laws on specific particle facets and uncovers surprising biphasic shape dependence, leading to catalyst design principles for optimal reactant adsorption efficacy. With its ability to provide spatially-resolved information on the behaviors of unlabeled, nonfluorescent entities under operando conditions, COMPEITS can interrogate a variety of surface processes in fields ranging from heterogeneous catalysis and materials engineering to nanotechnology and energy sciences. (Reference: Mao et al. Nature Chem. 2019, 11, 687-694)

The next generation of solar energy conversion systems requires design and integration of new semiconductor materials. Detailed understanding of the opto-electronic properties of these materials, their driving forces and the loss mechanisms that limit device performance is essential to the development of high efficiency systems. However, these materials and systems are difficult to model and only few experimental methods are available for direct characterization of dominant loss processes under relevant operating conditions. To this end, empirical extraction of the spatial collection efficiency (SCE) and the spatial photon recycling efficiency (SPRE) are operando, analytical tools that provide functional depth profiles of the active regions in the device.
By coupling external quantum efficiency (EQE) measurements and optical modeling, SCE extraction allows quantifying charge transport properties and loss mechanisms across the device depth profile under real operating conditions with very few assumptions. Similar to SCE, combining optical modeling with wavelength dependent photoluminescence quantum yield (PLQY) measurements enables extracting the SPRE- the probability that an electron hole pair photogenerated at a specific point will contribute to photoluminescence from the device. In this contribution we will introduce the SPRE concept and will show a first demonstration of the SPRE extraction method applied to InP samples. Extracting the SPRE enables simple distinction between different losses such as surface recombination and self-absorption. The quantification of surface recombination losses makes this an excellent tool for characterizing the effect of surface passivation layers. Furthermore, since the PLQY is directly related to the obtainable photovoltage from the device, the SPRE also maps the contribution of different regions in the device to the photovoltage. As a result, combining the SPRE and SCE profiles at specific operating points provides detailed spatial information on charge extraction, contribution to the photovoltage, and discrimination between radiative and non-radiative recombination processes at the surface and in the bulk of the device.

New semiconductor metal oxides capable of driving water-splitting reactions by solar irradiation alone are required for sustainable hydrogen production. Whereas most metal oxides only marginally deliver the photochemical energy to split water molecules, uranium oxides are efficient photoelectrocatalysts due to their absorption properties (E_g ~ 2.0 - 2.6 eV) and easy valence switching among uranium centers that additionally augment the photocatalytic efficiency. Although considered a scarce resource, the abundance of uranium compounds in the environment is manifested in the huge quantities of stored UF₆ gas, produced as waste streams in the nuclear fuel enrichment process. Here we demonstrate that thin films of depleted uranium oxide (U₃O₈) and their bilayers with hematite (a-Fe₂O₃) are high activity water oxidation catalysts due to electronically coupled phase boundaries. The electronic structure of uranium oxides showed an optimal band edge alignment in U₃O₈//Fe₂O₃ bilayers (DFT calculations) resulting in improved charge-transfer at the heterojunction as supported by TAS and XAS measurements. The enhanced photocurrent density of the heterostructures with respect to well-known hematite offers unexplored potential of uranium oxide in artificial photosynthesis.

BiFeO₃ (BFO) is a multiferriocs that combines antiferromagnetic and ferroelectric order well above room temperature. It is narrow band gap semiconductor (~ 2.2-2.7 eV) hence, it can harvest significant amount of visible light. Combining such desired properties on simple ternary compound makes it easier to utilize in many different folds. However, the performance of BFO in the field of photocatalysis is still poor due to the fast recombination of the photogenerated charges which will negatively affect their performance. Here we report on the photocatalytic oxygen evolution reaction, OER, of BiFeO₃ nanoparticles synthesized by sol gel. Oxygen evolution reaction is challenging because production of one molecule of gaseous oxygen, O₂, requires four holes (oxidize two molecules of water), and occurs on a time scale approximately 5 orders of magnitude slower than that required for H₂ evolution. There are two important issues to address when photocatalysis intended for water redox reaction to takes place; sacrificial agents to be used and the co-catalyst to be loaded on surface. For the former and in case of OER a sacrificial agent acts as an electron acceptor. Although many choices of sacrificial electron donner are available for water reduction to produce H₂, the choices are narrowing down to only few for sacrificial electron acceptors used for water oxidation to produce O₂. Ag⁺ and Fe⁺³ ions are commonly used for this purpose and to a lower extend sodium persulfate, Na₂S₂O₈. The redox potential of Ag⁺, Fe³⁺and S₂O₈ at pH zero are 0.8 V, 0.77 V and 2.05 V, respectively. It is reported that both the quantum efficiency and the stability of the colloidal nanocrystals in solution improve with increasing redox potential of the scavenger.The higher redox potential leads to faster scavenging, which in turn increases quantum efficiency and stability of the catalyst since electron hole recombination and oxidation or reduction of the catalyst become less important.This finding is important for choosing hole/electron scavengers and for comparing efficiencies and stabilities for different photocatalytic nano systems. Hence the resolve to Na₂S₂O₈ as electron scavenger. Mott Schottky measurements and the Uv-Vis spectroscopy showed that the band positions of BFO, the conduction band and valence band lie at 0.46V and 2.69 V with respect to NHE, respectively.To enhance BFO photocatalytic OER, IrO₂ nanoparticles as a co-catalyst were loaded on the surface using impregnation method. OER showed two folds enhancement upon loading with 2wt% IrO₂. IrO₂ is one of the best catalysts for OER, unfortunately it is also one of the most expensive rare elements, so their applicability is limited by the high cost. Reducing IrO₂ content onto the system should be an option to make its application feasible and cost effective and loading on IrO₂ nanoparticles on the surface could be feasible solution. Scanning the loaded IrO₂ content on BFO from 0.5 wt% up to 4 wt% showed a maximum of the evolved oxygen at 2wt % followed by a decrease in activity. XPS showed the 4f peaks of Ir, it shows symmetric two peaks at binding energies 64.9 and 61.87 eV corresponding to the 4f 5/2 and 4f 7/2 of Ir (IV) respectively. TEM indicated non uniform distribution of it on the surface. Charge carrier lifetime and dynamics for pure BFO and IrO₂ loaded BFO have been studied by means of laser transient absorption spectroscopy, TAS, laser pulses of 20 ns and of λ = 540 nm were used. 2wt.% IrO₂ loading showed the fastest decay of holes compared to other loading percentage indicating a significant role of the IrO2 nanoparticles mediating the hole transfer process of the solar irradiated BFO system. This loading percentage corresponds to the observed highest OER. Band positions between BFO and IrO₂ favors the formation of heterojunction at the interface between IrO₂ and BiFeO₃ that enhances the separation of the photogenerated charges and the photocatalytic OER performance.

On Cu electrodes, the electrochemical conversion of CO₂ to hydrocarbons occurs via surface-adsorbed CO. Understanding the molecular factors that determine the adsorption of CO on Cu under electrochemical conditions is therefore essential to improving the product selectivity of this electrocatalytic process. Previous studies probing the adsorption of CO on Cu electrodes have suggested that, at low overpotentials, CO likely competes with anions from the electrolyte for surface sites. However, prior work has not systematically explored the effects of anion concentration and identity on the coverage of CO on the electrode. In this work, we examine the impact of adsorbing and non-adsorbing anions (Cl^-, SO₄^2-, ClO₄^-) at varying concentrations (10 mM and 1 M) on the adsorption and desorption of CO from polycrystalline Cu electrodes. Using surface-enhanced infrared absorption spectroscopy (SEIRAS), we observed the interplay between adsorbed CO and interfacial anions through the lens of the anion hydration shells, CO coverage, and CO stretch frequency under electrochemical conditions. In the presence of 10 mM anion, the potential dependent CO coverage and stretch frequency during a cyclic voltammogram (CV) is virtually independent of the anion identity. By contrast, at 1 M anion, the CO coverage and stretch frequency are distinctly dependent on the anion identity of the electrolyte. With weakly adsorbing and non-adsorbing anions (SO₄^2- and ClO₄^-), the CO saturation coverage is lowered by ≈35% compared to the corresponding 10 mM experiments. We attribute the lower coverage to quasi-specifically adsorbed anions that block adsorption sites. With 1 M Cl^-, the CO saturation coverage is only lowered by ≈12% relative to the coverage in the presence of 10 mM Cl^-. This moderate decrease is attributed to a decrease in the binding strength of CO to Cu in the presence of co-adsorbed Cl^-, which is evidenced by the ~19 cm^-1 blue shift in the CO stretch frequency. In contrast to all other electrolytes, there is no hysteresis in the adsorption/desorption profiles of CO in the presence of 1 M Cl^-. This hysteresis between the forward and reverse scans in the CV is due to a CO-induced reversible reconstruction of the Cu electrode surface. In the presence of Cl^-, this reconstruction is suppressed. We discuss our findings in the context of the effects of halides on the product selectivity of the CO₂ reduction reaction on Cu electrodes. Our study provides critical insight into the structure of the electric double layer and its evolution during the CO adsorption process.

The surface morphology of rough Cu electrodes can profoundly affect the product selectivity and activity for the reduction of CO and CO₂ to hydrocarbons. Because of the scarcity of in situ methods for probing the atomic-level surface structure under electrochemical conditions, it is still debated how the atomic-level surface morphology gives rise to desirable catalytic activity and selectivity. In this work, we introduce a novel method for the determination of the prevalent surface facets on rough metal electrodes under electrochemical conditions. As model systems, we employed two types of rough Cu thin-film electrodes that are widely applied in surface-enhanced infrared absorption spectroscopy (SEIRAS). With differential electrochemical mass spectrometry (DEMS), we show that Cu films that are electrochemically deposited on Si-supported Au films (CuAu–Si) exhibit an onset potential for ethylene that is ∼200 ± 65 mV more cathodic than the one of copper films (Cu–Si) that are electrolessly deposited onto Si crystals. With SEIRAS, we find that lineshapes of the CO stretch mode of surface-adsorbed CO exhibit greatly different potential-dependences for the two types of electrodes. Lineshape analysis reveals the presence of different prevalent terraces on these two types of electrodes. On the basis of this finding, we rationalize the higher overpotential for the formation of ethylene on the CuAu-Si film. We expect this novel methodology to be broadly applicable for the in situ surface characterization of rough metal electrodes. This structural information is critical for the development of rough metal electrodes with desirable catalytic characteristics.

Mitigating activation polarization and maintaining long-term stability in solid oxide fuel cell (SOFC) electrodes represent major challenges in improving their intermediate-temperature operation and applications. It has been shown that the infiltration of nanoparticle electrocatalysts increases the number of electrochemically active sites to decrease charge transfer polarization losses and improve performance. In this study, nickel and mixed ionic and electronic conducting (MIEC) phases were infiltrated into Ni-YSZ and Ni-doped YSZ anodes of symmetric cells, and their performances characterized in-operando using electrochemical impedance spectroscopy (EIS). The kinetics of various charge transfer processes are modeled by distribution of relaxation times (DRT) analysis of the EIS spectra, allowing for an understanding of the role of the nickel nanoparticles and the consequences of nickel coarsening on the catalytic activity of the anode.

Prussian blue (Fe₄[Fe(CN₆)]₃) and its analogues are of interest as electrode materials for electrochemical energy storage. The open cubic framework of Prussian blue allows for long cycle life and reversible intercalation kinetics during cation insertion/extraction with capacities of up to 125 mAh/g in aqueous electrolytes. However, this performance is highly dependent on the electrolyte cation, and prior results indicate that a smaller hydrated cation radius leads to the best cyclability. The hypothesis guiding this study is that the cycling stability with smaller hydrated cations is tied to lower structural deformation of Prussian blue during electrochemical intercalation. In this study, we investigate the electrochemomechanics of Prussian blue due to the intercalation of three different aqueous cations, K⁺, Na⁺, and Li⁺. Operando atomic force microscopy (AFM) is used to measure the mechanical deformation of the electrode during electrochemical cycling as a function of electrolyte and cyclic voltammetry sweep rate. This technique confirms that the electrochemical stability as well as the local deformation of the Prussian blue is dependent on the hydrated cation radius, with smaller cations giving rise to larger deformation. The superior cycling stability seen during K⁺ insertion is commensurate with the high mechanical stability measured via operando AFM. This presentation will provide an understanding of the coupling between electrolyte cation size and electrochemomechanical response of a framework-type energy storage material.

Transition metal dichalcogenides are gaining much interest in the energy storage sector due to the 2D nature and conductivity of the materials. However, single transition metal dichalcogenides are not stable, preventing their practical use in real devices. Herein, we demonstrate the synthesis of binary metal dichalcogenides (Co-W-S) via carbonization of (ZIF-67) metal organic framework encapsulated with phosphotungstic acid (PTA@ZIF-67). The morphology and surface functional groups of the as-synthesized Co-W-S composite are characterized via (FESEM), (HRTEM), and (FTIR). Furthermore, the crystal structure and the elemental composition of the fabricated Co-W-S composite are elucidated using (XRD) and (XPS) analyses. Upon testing its electrochemical performance as a supercapacitor electrode, the fabricated Co-W-S@N,S co-doped porous carbon (N,S-PC) shows exceptional specific capacitance (1929 Fg^-1 at 5 mVs^-1). Furthermore, the constructed asymmetric supercapacitor device using Co-W-S@N,S-PC and activated carbon as positive and negative poles, respectively displays superior energy density and power density of 32.9 Wh Kg^-1 and 700.2 W kg^-1, respectively with a high Columbic efficiency over 10000 charge/discharge cycles at 10 A g^-1.

Iron hydroxide is a promising photovoltaic material because of its abundance on earth, chemical stability in aqueous media, and outstanding optical absorbance. Here, iron hydroxide nanostructures are synthesized by the atmospheric micro-plasma and characterized. The nanostructures are fabricated into dye-sensitized solar cells and their photovoltaic behaviors are measured. The in-situ Raman scattering of the integrated nanostructures is investigated under various experimental conditions. The in-situ investigation would enhance our understanding of the dynamic behaviors of the photovoltaic nanomaterial in solar cells.

Many chemical transitions of organic semiconductors occur at the interface during device operations. Charges are transfered or accumulated at the interface between organic semiconductors and intimate functional materials as function of operating conditions. However, the kinetic materials properties at the interface of organic semiconductor are hardly observed with real-time operational transition. Operando metrology is used to analyze charge transfers at the interface of organic semiconductors by photoelectron spectroscopy during operating bottom gate top electrode p-type organic field-effect transistors (OFETs). The interfacial chemical potential shift of organic semiconductor is simultaneously compared according to electrical behaviors. The electronic activation in the nearest effective monolayers of organic semiconductors at the interface is explained to correlate with gating, doping, or transporting behaviors of OFETs. This operando metrology will shed a light on many veiled intermediate transitions during the operatation of organic semiconductors.

We report the growth of nanoscale multilayered thermoelectric thin films and the fabrication of integrated thermoelectric devices for high-efficiency energy conversion and solid-state cooling. Nano multilayered Bi₂Te₃/Sb₂Te₃, Sb/Sb₂Te₃, and Te/Bi₂Te₃ thermoelectric thin-film materials were grown using the e-beam evaporation. The multilayered thin films were prepared with 100 to 300 layers, where each layer is about 3 to 5 nm thick. The effects of DC substrate bias on the growth of films such as the crystalline structures in the films were studied. Integrated thermoelectric devices with a high density of thermoelectric elements were fabricated with the nanoscale multilayered thin films using the clean room-based microfabrication techniques such as UV lithography. Plasma-enhanced atomic layer deposition (PE-ALD) was used to grow zirconium dioxide (ZrO₂) as the insulation layer in the device fabrication. X-ray diffraction and high-resolution tunneling electron micrograph (HR-TEM) were used to analyze the nanoscale multilayered thin films. The fabricated device was cooled in liquid nitrogen or annealed at 150 ^oC for 30 min, respectively. HR-TEM was used to analyze the cross-section of the nanoscale multilayered thin films for the three cases: as-grown, after being cooled in the liquid nitrogen, and after being annealed, showing that the cooling made the multilayer structures to be sharper while the annealing made the multilayer structures to become weaker. The thermoelectric characteristics of the devices were measured and analyzed for the three cases of as fabricated, after being cooled, and after being annealed, respectively, showing that the integrated thermoelectric device had the highest efficiency of thermal-to-electrical energy conversion after being cooled in the liquid nitrogen.

Identifying the catalyst properties that determine catalyst activity and selectivity is of significant importance, as an understating of the catalytic mechanism can provide a rational catalyst design principle for efficient energy devices. However, understanding the relationships between catalyst properties and performance remain challenges in general because of the co-existence of multiple factors that are irrelevant to the catalytic cycle, and the dynamic nature of the catalytic chemistry which is responsive to reaction rates. To investigate the complex dynamic catalyst structure, an operando characterization under realistic operation, particularly at electrochemical conditions approaching those used to evaluate and benchmark electrocatalysts, is needed. Here, we report a comprehensive study of catalyst properties that can determine the electrochemical CO reduction reaction (CORR) in a gas-fed cell. We first employed operando X-ray absorption spectroscopy (XAS) synchronized with simultaneous measurements of catalyst performance by gas chromatography (GC) to investigate the relationship between valence states of Cu-based catalysts and activity for ethylene (C₂H₄) production. By introducing an anodic oxidation process, which enhanced CORR performance, we demonstrated that the oxidation states do not correlate with the activity or selectivity of Cu-based catalysts. We also found that an increase in the number of active sites estimated by electrochemical surface area (ECSA) was not proportional to the C₂H₄ yield, whereas hydrogen evolution reaction (HER) activity is positively correlated to the ECSA changes. Based on the post-mortem microscopic investigations of Cu-based catalysts, we suggest that the increased C₂H₄ activity and selectivity may arise from a morphological reconstruction that originates from the electrochemical oxidation-reduction process. The rapid reduction of the initial oxide structure into metallic Cu at the early stage of the CORR could generate a more active structure. This electrochemical oxidation-reduction process was used as a regeneration method that recovered the original catalyst activity and selectivity in the gas-fed cell without requiring cell disassembly.

Heterogeneous catalysis plays a crucial role in the production of chemicals and requires the design of active and selective nanostructures. In situ transmission electron microscopy (TEM) can determine morphology or chemistry changes in novel catalysts during a chemical reaction. In this work, bimetallic Au1-xPdx particles supported on a raspberry colloid templated (RCT) silica support were studied under oxidative and reactive conditions with a temperature of up to 500 °C and a pressure of 1 bar. Pd is the minority component, representing no more than 25 weight percent of the total metallic loading, and the particles have an average size of 5.5 nm. The remarkable stability of this system, with limited particle migration and coalescence phenomena, was demonstrated. Sharp facets perpendicular to [100] and [111] directions are present when Pd's concentration is below four atomic percent. When the concentration of Pd exceeds four atomic percent, the particles have a spherical shape, and high-resolution spectroscopy revealed the presence of small Pd clusters on the surface of the as-prepared particles with an average size of 1-2 nm. In situ results combined with ex situ TEM and infrared spectroscopy show the growth of small Pd clusters on the nanoparticles' surface that explain changes in activity and selectivity observed after treating the sample with oxygen or hydrogen. These observations demonstrate how a combination of operando methods, aberration-corrected microscopy, and direct electron detection can be used to guide the design of future catalytic structures.

Due to the rising demand for sustainable energy, there has been a global drive for developing clean energy from renewable sources (solar, wind, etc.) which will reduce our dependence on fossil fuels. Fuel cells represent an important vector in the development of clean and renewable energy, but major challenges for the mass commercialization of fuel cells are the high cost and poor durability of the catalysts at the cathode, which requires the prohibitive Pt as a catalyst to overcome the sluggish oxygen reduction reaction kinetics. This presentation will discuss recent results of our investigations of low-Pt binary and ternary nanoalloy electrocatalysts using different characterization techniques, including in operando synchrotron X-ray diffraction technique in standard electrochemical cells and proton exchange membrane fuel cells (PEMFC). The focus is to develop the fundamental understanding of the nanophase structures of the catalysts, which provide insights into the correlation between the durability and the composition of the nanoalloy catalysts in the PEMFCs.

The discovery of electrocatalysts for efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is pivotal to the growth of a new generation of energy technologies. In this study, for the first time, we present an effective strategy for a nano-dispersed low amount of Co₃O₄ materials (i.e., 2.67 wt %) on La_0.5Sr_0.5MnO_3-δ catalyst substrate for synthesizing an efficient bifunctional ORR/OER catalyst by utilizing an advanced ALD technique. Interestingly, the cation diffusion between LSM and Co₃O₄ introduces a self-intercalated MnCo₂O₄ spinel structure at the interface. The existence of MnCo₂O₄ was assisted by TEM-EDS, XANES. Benefiting from the MnCo₂O₄ interlayer, the LSM-20-Co catalyst exhibits excellent maximum power density (146.5 mW cm^-2), specific capacity (755.0 mA gZn^-1) and outstanding stability over 450mn without any increase of charge-discharge voltage gap. Consequently, the LSM-20-Co catalyst is an attractive bifunctional catalyst as an efficient air cathode catalyst for the alkaline metal-air system and provides insight into the rational design of a thin metal oxide with the ALD process.

Nanomaterials are important for a wide range of applications because of their unique properties, which are strongly connected to their three-dimensional (3D) structure. Electron tomography has therefore been used in an increasing number of studies. Most of these investigations resulted in 3D reconstructions with a resolution at the nanometer scale, but also atomic resolution was achieved in 3D.

In order to preserve the carefully designed morphologies and functionalities, understanding the stability of nanomaterials during application is crucial. It is hereby important to note that most electron tomography investigations have been performed at the conventional conditions of an electron microscope. An emerging challenge is therefore to fully understand the connection between the 3D structure and properties under more relevant conditions, including high temperatures as well as in the presence of liquids and gases. Therefore, innovative methodologies are required to track the fast 3D changes of nanomaterials that occur under such conditions.

Recently, we proposed an acquisition approach where a tilt series of 2D HAADF-STEM projection images is acquired within a few minutes. By continuously tilting the holder and simultaneously acquiring projection images while focusing and tracking the particle, we were able to reduce the total acquisition time for a tilt series by a factor of ten [1]. Moreover, a new approach was developed to compute high quality 2D virtual slices through nanoparticles in approximately 60 ms time. This technique drastically improves the efficiency of 3D characterization of nanomaterials by TEM. It enables explorative imaging and provides valuable information to dynamically adjust the acquisition parameters during an electron tomography experiment [2].

In this manner, we were able to study the 3D morphological evolution of anisotropic Au and AuPd nanoparticles as a function of both heating time and temperature [1,3]. Moreover, we measured the elemental diffusion dynamics of individual anisotropic Au-Ag nanoparticles in 3D [4]. We conclude that for a given composition, the shape of the nanoparticle does not influence the alloying process significantly. Based on our analysis, it is clear that interdiffusion of metals at the nanoscale is more complex than predicted by simple Fickian diffusion and that other factors such as surface diffusion need to be taken into account.

To quantify nanoparticle shape dynamics in a gaseous environment in 3D, HAADF-STEM images served as an input for atom counting procedures followed by 3D relaxation of the structure. In this manner, we characterized shape changes of a Pt nanoparticle in a gaseous environment. The conditions have been varied from vacuum to a 1 bar H2 flow, followed by a 1 bar O2 environment. To investigate the behaviour during cycling, we repeated the experiment several times using the same particle. We clearly observe morphology changes and we were even able to quantify the occurrence of the different surface facets [5].

References
[1] H. Vanrompay et al., Nanoscale 10 (2018), p. 22792
[2] H. Vanrompay et al., Particle and Particle System Characterization (2020) in press
[3] W. Albrecht et al., ACS Nano 13 (2019), p.6522
[4] A. Skorikov et al., ACS Nano 13 (2019), p.13421
[5] T. Altantzis et al., Nano Letters 19 (2019), p.477
[6] This work was supported by the European Research Council (Grant 815128 REALNANO to SB, Grant 770887 PICOMETRICS to SVA and Grant 797153 SOPMEN to WA, Grant 823717 ESTEEM3). The authors acknowledge financial support from the Research Foundation Flanders (FWO, Belgium) through project fundings, a postdoctoral grant to EB and a doctoral grant to HV.

Lithium-ion batteries (LIBs) have profoundly advanced the development of electric vehicles (EVs). However, one of their remaining bottlenecks in the widespread deployment is the long charging time required for commercial LIBs. There is a global push towards extreme fast charging (XFC) to reduce charging times to 10-15 minutes.¹ But existing LIBs cannot achieve this goal without significantly reducing battery performance, mainly due to the loss of active lithium (Li). The lost Li either becomes “dead” from being electronically disconnected after plating on the anode or “inactive” because of the irreversible reaction of Li with the electrolyte to form a solid electrolyte interphase. Since Li plating is identified as one of the major degradation mechanisms from XFC in LIBs,² understanding its origin and characteristics is essential to developing designs to enable fast-charged batteries. Several studies have characterized Li plating using techniques such as cryo-electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray micro-computed tomography (CT) among others.³ While valuable, these characterization techniques have limitations in terms of their sample preparation, spatial resolution, and imaging contrast required to distinguish between materials of similar atomic number (Z). In addition, due to the complex electrochemical reactions of Li plating, multiple methods for detecting Li metal and understanding the mechanistic details of Li plating are needed.

Here, we are using simultaneous neutron and X-ray-based dual-mode micro-computed tomography, a non-destructive imaging modality, to investigate the characteristics of Li plating during XFC in LIBs. Since X-rays and neutrons are sensitive to the electron and nuclear density of the material, respectively, dual-mode X-ray and neutron CT allows an easy segmentation of the different anode components such as Li, graphite, and pores. Higher energy X-rays are needed to get through the metallic components in batteries such as copper and aluminum. However, they do not have sufficient contrast to separate Li from graphite. Thus, neutrons are used due to their high sensitivity to lighter elements such as Li, especially ⁶Li, and carbon.

We performed multi-modal imaging experiments at the Neutron and X-ray Tomography (NeXT)⁴ system located on the BT-2 beamline at the National Institute of Standards and Technology Center for Neutron Research. We characterized pristine and cycled graphite anode strips containing plated lithium at different regions. For cycled anode strips, we disassembled the battery pouch cells and imaged the graphite anodes cycled under XFC conditions for 450 cycles at fully discharged condition. The spatial resolution achieved was ~10-15 μm, thus providing sufficient resolution to pinpoint the location of Li plating within the anode thickness. With the dual-imaging datasets, we are now investigating a link between the spatial heterogeneity of Li plating and the local microstructure of the battery anode including porosity, tortuosity, and thickness. We want to investigate why and where Li plating occurs on the battery anode. Addressing these fundamental questions will inform improved battery designs, graphite anode architectures, and charging protocols that will reduce Li plating during XFC in LIBs.

References:

1. Liu, Y., Zhu, Y., & Cui, Y. (2019). Challenges and opportunities towards fast-charging battery materials. Nature Energy, 1.
2. Tomaszewska, A., Chu, Z., Feng, X., O'Kane, S., Liu, X., Chen, J., ... & Li, Y. (2019). Lithium-ion battery fast charging: A review. eTransportation, 1, 100011.
3. Liu, Qianqian, et al. "Understanding undesirable anode lithium plating issues in lithium-ion batteries." RSC advances 6.91 (2016): 88683-88700.
4. LaManna, J. M., Hussey, D. S., Baltic, E., & Jacobson, D. L. (2017). Neutron and X-ray Tomography (NeXT) system for simultaneous, dual modality tomography. Review of Scientific Instruments, 88(11), 113702.

Recent advances in laboratory-based X-ray microscopy (XRM) have led to the development of new opportunities for the visualization and characterization of a variety of energy materials. Through X-ray tomography, the process of reconstructing a volumetric data set from a series of 2D projection images, we can gain vital insight to the internal 3D structure of these types of materials over a range of relevant length scales spanning the millimeter to nanometer range. By combining 3D visualization through repeated identical location tomography scans at various temporal stages, powerful in-situ investigations of dynamic material properties can be obtained. This methodology is often termed as 4DCT (4-dimensional computed tomography) and is particularly well suited to study various dynamic and evolutionary processes in energy materials. The capability to incorporate a variety of in situ rigs within the flexible XRM architecture provides a multitude of possibilities for the direct observation of change in a sample’s microstructure in response to a stimulus. This data can lead to improved understanding of material performance, degradation, or failure processes in a quantitative fashion. Recent advances and applications of XRM for energy materials will be presented, including in situ visualization of polymer electrolyte fuel cells and quantification of water distribution changes while monitoring cathode catalyst layer morphological changes, solid oxide fuel cells and temperature induced structural changes. Additional examples of imaging lithium ion batteries undergoing structural changes following charge cycles, combined with correlative post-mortem analysis are also discussed.

Heterogeneous catalysis is a stone edge of chemical industry and “green chemistry”. The sparse understanding of surface mechanisms at molecular scale impedes the development of new high-performance catalysts and requires the development of new tools and methodological approaches. Cobalt is capable to dissociate CO₂, allowing further conversion to higher value compounds. Such mechanism will conduct to Co surface oxidation, which formation mechanism remains uncertain. In this work, we propose to investigate this process with Atom Probe Tomography and expose the Co to O₂(g) by two different methods: one in-situ using an annex reactor chamber connected on APT; and one in operando by introducing O₂(g) directly in APT analytic chamber during the ongoing analysis. After a cleaning process of the surface by field evaporation and surface imaging by FIM, the Co, shaped as a sharp needle, is transferred under UHV into a reactor chamber in which it is exposed to pure O₂(g) at different temperatures and times. Oxidation is quenched on the APT cryogenic stage. Oxidation is revealed to be inhomogeneous and strongly influenced by surface structures. We detect local distribution of Co₂Oⁿ⁺ and CoOⁿ⁺ where Co₂Oⁿ⁺ is preferentially found deep in the Co volume, drawing the progression of the oxygen in the metal. In operando mode allows a real time direct observation of the surface oxidation. Surface composition is probed by applying periodic electric pulses, resulting into a virtual “reconstruction” representing a stack of the different surface composition states over time.
As oxidation rate progressively increases while P(O₂) increases, starting from pure Co surface to successively an early and an advanced oxidation stage with Co₂O and CoO species. Inspired by the principles of the 1-D Atom Probe developed by Kruse et al.¹, the influence of the electric potential, the pulses frequencies and how to study surface kinetics with an in-operando Atom Probe will be discussed.
¹Kruse N., Abend G. and Block J.H., The kinetics of adsorption and thermal desorption of NO on stepped Pt single crystal surfaces, Journal of chemical Physics 88 (1987) 1307-1312

A fundamental understanding of transport properties (e.g., ion transport, thermal transport, etc.) in electrolytes during battery operation is important for optimal operation and design of batteries especially for high-power applications such as vehicles. It is known that internal resistance of the lithium-ion batteries (LIBs) temporarily rises by repeating high-rate charge/discharge cycles and that it decreases by stopping the battery operation and rest. Because such a reversible resistance rise occurs only during the battery operation, this phenomenon is speculated to be related to the ion concentration distributions in the electrolyte. Such ion concentration distributions in the electrolyte have more serious effect on the performance of lead-acid batteries (LABs). The development of a vertical ion concentration gradient of sulfuric acid is called electrolyte stratification, which causes inhomogeneous current distribution in the vertical distribution of the electrodes by frequent charging-discharging, reducing the life of LABs. However, there are few techniques that can be used to quantitatively characterize ion concentration distribution that arise in electrolytes during the battery operation.
Phase-contrast X-ray imaging is a technique that can be used to visualize density differences by detecting the X-ray phase shift caused by an X-ray passing through a sample. The sensitivity of the phase-contrast X-ray imaging is about 1000 times higher than that of conventional X-ray absorption imaging in a hard X-ray region for light elements. Since the electrolytes of batteries are also composed of light elements, the phase-contrast X-ray imaging is expected to be effective in visualizing small density differences in an electrolyte during battery operation. We demonstrated that in situ phase-contrast X-ray imaging technique can quantitatively visualize ion concentration distributions that arise in electrolytes during battery operation (cf. LIBs [1], LABs [2]).
Another crucial concern about batteries used in high-power applications is the thermal management. It is known that temperature of the batteries temporarily rises by repeating high-rate charge/discharge cycles and such a temperature distribution causes inhomogeneous reaction distribution of batteries. Although there are several conventional temperature measurement methods, such as thermistors, resistive temperature detectors, thermocouples, infrared thermography, etc., these methods detect the temperature directory through physical contact or using infrared and visible light from the object, and therefore, the detectable depth is limited to a few mm from its surface. So, its inner temperature distribution cannot be measured in principle. We have successfully demonstrated that nondestructive observations of inner temperature and thermal flow in heated water by using phase-contrast X-ray imaging [3]. Recently, thermo-electrochemical cells (TECs), which generate electricity directly from low-grade heat, have attracted attention for the need to power sources of Internet-of-things (IoT) sensors. We have also performed operando visualization of ion and thermal transport behavior in electrolytes of TECs by phase-contrast X-ray imaging and succeeded in visualizing the convection generation behavior caused by the temperature gradient and ion gradient occurring in the electrolyte in real time.
In this presentation, we will show the quantitative evaluation of ion concentration distribution and thermal distribution (inner temperature gradient) in the electrolyte simultaneously during electrochemical device operation with high temporal (a few seconds) and spatial (a few microns) resolutions.

References: [1] J. Am. Chem. Soc., 140, 1608 (2018). [2] Chem. Commun., 56, 9553 (2020). [3] Sci. Rep., 8, 12674 (2018).

There is a growing consensus that the theoretical limit of one electron intercalation reactions will soon be reached. Recent discoveries on anion redox electrochemistry offer new opportunities and a possible paradigm shift in energy storage. By activating anion (e.g. oxygen, sulfur) for reversible electrochemical reactions, increased energy density has been demonstrated. In the past decade, the research community has made great progress on understanding key features of the anionic electrochemical activities and dynamic phenomena that govern the performance limitations including cycling voltage hysteresis and decay. However, the exact mechanism at molecular/atomic scales is still largely under-debate. The lack of conscience on the anionic redox mechanism which can be fueled by possible misinterpretation of experimental results. Of particular importance is determining what techniques provide accurate and reliable evidence for reversible bulk oxygen redox.
Compared with transition metal cations, oxygen anions are more challenging to be probed and quantified, which requires a multi-modal experimental approach with exceptional energy and spatial resolution combining computational modelling. Here, we advance: 1) from X-ray absorption spectroscopy to resonant inelastic X-ray scattering which shows a sharp feature with the corresponding excitation energy at 531 eV corresponding to the extra unoccupied electron orbitals of oxidized oxygen; 2) from neutron diffraction to neutron pair distribution function which is enormously valuable as a local probe that can reveal local distortions, when present along the oxygen redox reactions; 3) from X-ray diffraction to Bragg coherent diffraction imaging which will enable the direct observation of the strain dynamics with the exceptional sensitivity as small as 0.05% and phase evolution induced by the oxygen redox activity at nanoscale. It is through the in-depth understanding of electrochemical reactions mechanism at microscopic and macroscopic level during the operation of batteries; we can successfully formulate strategies to achieve reversible and stable anionic redox for next generation lithium-ion batteries.

In the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase [1]. Why is that? And why, on the other hand, compounds predicted as thermodynamically stable often cannot be synthesized experimentally? Understanding the competition between thermodynamics and kinetics is fundamental for answering these question and towards the rational synthesis of target materials [2]. In this presentation, we will highlight the use of in situ synchrotron x-ray diffraction to yield insights on the real time behavior of powder precursors during synthesis. We investigate the multistage crystallization pathways of the important two-layer (P2) sodium oxides Na_0.67MO₂ (M = Co, Mn) [3]. We find a series of fast non-equilibrium phase transformations through metastable three-layer O3, O3’ or P3 phases, before formation of the equilibrium two-layer P2 polymorph. A theoretical framework to rationalize the observed phase progression will be presented, demonstrating that even though P2 is the equilibrium phase that we target, the first phase to form will be the one with the most negative compositionally-unconstrained reaction energy from the powder precursors. In other words, the first phase to form is guided by thermodynamics and it does not necessarily have the composition corresponding to the overall precursor composition in the reaction vessel. Afterwards, the first phase may develop according to kinetic or thermodynamic arguments, depending on the system of choice. Further examples will be presented for industrially-relevant LiNiO₂ and related compounds [4], where lithiation pathways can be instead more straightforward than sodiation ones; nonetheless, the high resolution of synchrotron XRD allows to observe detailed structural information revealing a complex behavior where metal vacancies play an important role. The insights we present constitutes a step forward towards unraveling the complex interplay between thermodynamics and kinetics during synthesis. Moreover, they can guide the choice of precursors and parameters towards the optimization of the solid-state synthesis of ceramic materials.

References:
[1] M. Aykol et al., Science Advances, 2018, 4(4), 148.
[2] Z. Jiang et al., Journal of Materials Chemistry C, 2017, 5(23), 5709-5717.
[3] M. Bianchini, J. Wang et al., Nature Materials. 2020, https://doi.org/10.1038/s41563-020-0688-6.
[4] M. Bianchini et al., Journal of Materials Chemistry A, 2020, 8(4), 1808-1820.

The properties of emerging non-precious metal oxide electrocatalysts for the alkaline oxygen evolution reaction (OER, 4OH^- → O₂ + 2H₂O + 4e^-) evolve dynamically with voltage during operation. While it is appreciated that the voltage-dependent behavior of the electrode may convert an inactive material into an active electrocatalyst or vice versa through bulk and surface ion-coupled electron transfer, the chemical and structural identity of the electrode in its active state cannot be directly extracted from the electrochemical data alone. Importantly, the active state of the electrocatalyst is generally a non-equilibrium state that can only be obtained at the voltages necessary to drive the OER. Thus, the accurate understanding of how material properties govern the kinetics of electron and ion transfer during the OER requires integrating electrochemical cells into advanced characterization methods to study reactivity during operation. Further, the sub-particle spatial heterogeneity of electrocatalysts during operation requires that these operando methods resolve the local chemical, physical, and electronic structure at the nanoscale and link these properties to the local electrochemical activity.

This talk will describe a suite of correlative operando microscopy techniques developed to study the OER on faceted Co (oxy)hydroxide nanoplatelets. Using operando Scanning X-Ray Transmission Microscopy (STXM) and Electrochemical Atomic Force Microscopy (EC-AFM), the local Co oxidation state and particle morphology are mapped across voltages spanning the bulk ion insertion reactions and OER regimes. These results are correlated to local electrochemical activity maps obtained through Scanning Electrochemical Cell Microscopy (SECCM) providing insight into where and why oxygen is evolved on the Co (oxy)hydroxide platelets. Combined with ensemble averaged bulk techniques including operando X-ray diffraction (XRD), UV-Vis spectroelectrochemistry, and Electrochemical Quartz Crystal Microbalance (EQCM), we provide a comprehensive description of the chemistry of an operating OER electrocatalyst. These experimental material properties and local reactivity measurements are integrated into a computational model that combines first principles thermodynamic adsorption energetics of OER surface intermediates and microkinetic modeling to derive the experimental electrochemical Tafel behavior. The results from our multi-modal correlative approach alter long standing assumptions about the OER-active properties of the Co (oxy)hydroxide system and provide an improved methodology towards developing predictive electrocatalytic theories based on material properties that govern reactivity away from open circuit conditions.

Layered transition metal oxides that incorporate water or other solvents provide fundamental understanding of the effects of confinement on electrochemical ion transport and reactivity. They also tend to exhibit fast electrochemical ion insertion kinetics making them of interest for high power electrochemical energy storage. These materials are hybrids that consist of transition metal oxide layers separated by fluids that can be either ordered or disordered, and strongly or weakly bound to the oxide. Our group has investigated ion insertion into a range of hydrated and solvated metal oxides with operando characterization to connect structural changes to the electrochemical response. Since many of these materials exhibit fast kinetics (full charge/discharge in minutes), operando characterization techniques must have suitable temporal resolution to capture structural or chemical changes at these rates. This presentation will describe our use of three types of operando techniques to probe insertion kinetics at minute-to-second timescales: X-ray diffraction, atomic force microscopy, and electrochemical quartz crystal microbalance. It will also discuss how knowledge of the dynamics of ion insertion in these materials help us understand ion transport under confinement.

Catalytic oxidation of air pollutants plays a major role in in sustainable energy and environment fronts, e.g., fuel reforming and elimination of atmosphere emission exhaust. Recent interests in low temperature catalytic conversion of air pollutants are largely driven by increasingly-stringent environmental regulations. Understanding how surface atomic-scale active sites are created and stabilized on supported nanoalloy catalysts under reaction conditions is crucial for the understanding of catalytic activity and stability which requires probing the dynamic structures in real time. However, little has been understood about the role of metal components in surface oxygenated highly active sites creation and stabilization in hydrocarbon and carbon monoxide oxidation. In this work, we will focus on recent findings in the investigation of oxygenated Pt (and/or Pd, Au) alloyed bi-/trimetallic catalysts for low temperature carbon monoxide and propane oxidation. The analysis of operando results on catalytic synergy from in-situ/operando characterization technique combining Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTs), Synchrotron High-Energy X-ray Diffraction coupled with Pair Distribution Function analysis (HE-XRD/PDFs) and online mass spectrometer will be discussed. The results reveal largely-irregular oscillatory kinetics associated with the dynamic lattice expansion/shrinking, ordering/disordering, and formation of surface-oxygenated sites and intermediates. This work reveals new insights on catalyst design strategies into the catalytic reaction pathways and the structure-composition-activity correlation.

Global demand for high-performance energy storage systems has grown tremendously in recent years. The all-solid-state lithium-ion batteries (ASSLIBs) are promising for such storage solutions on account of the lithium metal anodes with high areal capacity and the solid-state electrolytes (SSEs) with critical safety.¹ However, the interfacial failure of the ASSLIBs poses a major hurdle towards the implementation of practical high-performance energy storage devices.^2,3,4 Time-resolved in operando techniques (e.g., in situ microscopy, synchrotron X-ray tomography) are ideal for the direct and quantitative characterization of the interfacial deteriorations. However, the in situ analysis of solid-state batteries is still in its primary stage due to the difficulties in probing the buried interfacial layer. The specific causes of the high interfacial impedance and the lithium dendrite growth that deteriorates the battery in a few cycles remain nebulous.

To this end, we developed a multi-dimensional diagnostic platform that allows the direct probe of the chemical and spatial distribution on the interface while the cell operates. Parameters such as pressure and temperature, which have a strong influence on cell behavior, are also enabled by the platform. Moreover, the platform can be air-free transferred between analytical instruments like SEM, ToF-SIMS, and Raman, for a synergistic diagnose of the structural and chemical evolutions in the interfaces. A deeper understanding of interfaces and dendrites evolutions can be acquired by the testing platform, including (1) correlation between void formation and electrolyte decomposition, and (2) creeping mechanical properties of the lithium metal during the cell operations. These in-depth understandings will allow us to effectively optimize and enable further enhancements of solid-state Li batteries.

References
[1] T. Famprikis and C Masquelier et al, Nat. Mater. 18, (2019) 1278-1291
[2] K Park and J Goodenough et al, Chem. Mater. 28, (2016) 8051-8059
[3] S Wang and A Manthiram et al, J. Am. Chem. Soc. 140 (2018) 250-257
[4] Y Tian and G Ceder et al, Energy Environ. Sci. 10 (2017) 1150-1166

Chemical substitution of transition metal species is often used to tune the performance and cost of electrode materials. However, substitutions that improve one performance metric can be coupled to unintended consequences. This presentation will show how we have used operando X-ray scattering to understand kinetic limitations and O-redox in Li-layered transition metal oxide electrodes.
Substituted Li layered transition metal oxide (LTMO) electrodes such as Li_xNi_yMn_zCo_1-y-zO₂ (NMC) and Li_xNi_yCo_1-y-zAl_zO₂ (NCA) show reduced first cycle Coulombic efficiency (90-87 % in standard cycling conditions) compared with archetypal Li_xCoO₂ (LCO – ~98 % efficiency). We use operando x-ray diffraction (XRD) to demonstrate that the apparent first cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.85, with near full capacity recovered and slow Li reinsertion during a potentiostatic hold following the galvanostatic charge–discharge cycle. The kinetic limitation manifests not only as the kinetic capacity loss during discharge, but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge–discharge voltage hysteresis at x > 0.85.

Li-ion insertion electrodes store and release charge through an assortment of reversible interactions, including surface capacitance, single-phase Li insertion and two-phase transformations. Several in situ and in operando characterization tools have been developed to deconvolute and quantify these processes to engineer better batteries. However, it is very difficult to resolve how batteries charge in operando at the time and length scales relevant to these processes simultaneously. Color is a simple, yet often overlooked, indicator of the convoluted processes that occur in electrodes. Indeed, electrochromism is commonly observed in many ion-insertion electrodes (e.g. graphite, Li_xTi₅O₁₂, Li_xFePO₄ and Li_xCoO₂). Transitions due to the band-gap (UV-visible), local defects (visible), electronic oscillations (near-IR) cause distinct features, which can be used to deconvolute charging processes. Coupled with electrochemistry, these measurements reveal rate-limiting processes and track the state-of-charge of independent charging transformations.

In this talk, we demonstrate how optical characterization of nanostructured TiO₂ thin-film electrodes deconvolutes dynamic charging processes and reveals rate-limiting steps during (de)lithiation. Anatase TiO₂ nanocrystal films, synthesized with nm-scale control of size and shape, are studied as a reliable model Li-ion anode that undergoes a distinct phase transformation between low- and high-Li content states. Using optical measurements during charge titration experiments, coupled with phase-field models of individual particles, we find that particle size systematically tunes the two-phase transformation potentials of nanocrystalline TiO₂. We demonstrate how the distribution of critical transformation potentials in an ensemble of particles changes non-equilibrium charging rates. These measurements yield an unexpected relationship between the initial state-of-charge and charging rates across a range of different nanocrystal particle morphologies. We conclude that neither Li diffusion nor phase propagation kinetics limit charging rates in this system. Rather, kinetics are limited by slow accumulation of Li in the particle ensemble prior to distinct crystalline transformations. More broadly, these results indicate a path forward to deconvolute non-equilibrium charging behavior in nanostructured systems through a combination of precise colloidal synthesis, first-principles modeling, and in operando resolution of dynamic processes with optical spectroscopy.

The development of next-generation electrodes for metal-ion batteries requires an understanding of intercalation dynamics in nanomaterials. Lithium intercalation into graphite is the foundation for the lithium-ion battery, and the thermodynamics of the lithiation of graphitic electrodes have been heavily investigated. Intercalated lithium undergoes structural ordering known as staging. Initially, lithium atoms are randomly distributed throughout the gaps between carbon sheets (Dilute Stage I), but increasing lithium concentration triggers a phase transition to ordered structures comprised of intercalated regions vertically separated by unintercalated regions. Lithium typically initiates staging by forming a Stage IV structure in graphite at a concentration around LiC₂₄, where the staging index indicates the number of graphene layers between the intercalant layers. This process is well-understood for bulk graphite; however, confinement effects become important at the nanoscale, which can significantly impact the electrochemistry of nanostructured electrodes.
Here, we show that small mechanical strain in graphene microflakes can significantly modulate the thermodynamics of lithium staging. We performed electrochemical lithium intercalation on nanodevices with mechanically exfoliated graphene microflakes and monitored the staging of intercalated lithium in situ. In situ Raman spectroscopy of graphene microflakes mechanically constrained at the edge during lithium intercalation reveals a thickness-dependent increase of up to 1.26 V in the electrochemical potential that induces lithium staging, marking a clear departure from the bulk behavior. Thus, we show that microscale mechanical strain significantly affects the formation of ordered lithium phases in graphene. While the induced mechanical strain energy increases with graphene thickness to the fourth power, its magnitude is small compared to the observed increase in electrochemical energy. We hypothesize that the mechanical strain increases a nucleation barrier for lithium staging, greatly delaying the formation of ordered lithium phases.
Our results suggest that electrode assembly can critically impact lithium staging dynamics important for cycling rates and power generation for batteries. The observed effects of strain reach beyond energy storage. Strain engineering has been used to modify the local electronic band structure of two-dimensional (2D) nanomaterials for optoelectronics and catalysis, and our results indicate that strain can also be used to modulate the intercalation-induced phase transitions of 2D materials.

In situ studies of model systems have always been central to fundamental electrochemistry as they provide precise potential-dependent quantities that can be used to assess theoretical models. In both fields of energy sciences and electrochemistry, structural evolution of electrocatalysts exposed to harsh oxidizing environment and the associated (electro)chemical pathways have remained rather mysterious. To gain the necessary insight, one has to employ not only a combination of advanced in situ characterization techniques, but also well-defined materials that can be directly modelled.
For example, some complex oxides demonstrated a high activity toward electrochemical water oxidation, and yet they are rarely used in precise in situ surface studies due to the difficulty in their controlled synthesis. The progress in the growth of epitaxial oxide films and superlattices (the area primarily being the realms of condensed matter physicists) has enabled us with a perfect tool to address some of the most pressing questions in the field of oxide electrocatalysis. A perfect control over the quality and composition of epitaxial films provides specimens that are ideally suited for precise in situ experiments. In this talk, I will discuss how model systems such as epitaxial films can be used for real-time observations of materials’ evolution during electrocatalytic water splitting.

With the increasing demand for electric vehicles, there is an urgent need for Li-ion batteries (LIBs) with extreme fast charging (XFC) capabilities [1]. The need arises from making recharging times for commercial electric vehicles comparable to traditional refueling times. However, such XFC rates are associated with a drastic decrease in performance, as well as safety hazards. Current literature suggests that parasitic Li plating on the anode is the major factor contributing to decreasing battery capacity and therefore shortened battery life in LIBs [2]. Techniques used for understanding Li plating under in situ conditions have primarily involved two approaches. The first involves taking global measurements, averaged over the entire cell, to quantify the losses associated with lithium plating [3]. While such studies provide a good staring point, the lithium plating process is also known to be spatially heterogeneous across the cell. Therefore, other approaches probe the spatially heterogeneous nature of lithium plating across the cell, often using imaging based methods [1, 4]. There is a need to tie the two length scales together, in order to obtain a comprehensive picture of the origins and properties of lithium plating across the cell. In this work, high-energy X-ray diffraction (XRD) is presented as such a method, to bridge the two length scales. Firstly, XRD provides a way to quantify the amount of Li plating, as well as other loss mechanisms, and tie them to the global cell performance after XFC cycling. Second, XRD is an in-situ technique, allowing to characterize the entire battery in the fully assembled condition. Thus, local heterogeneities in the cathode and anode can be studied and correlated to heterogeneities in Li plating.

In this woork, single layer pouch cells (3.5cm x 4cm, 3 mAh/cm²) with a porous graphite anode, NMC (LiNi_xMn_yCo_zO₂) cathode and EC:MC (with LiPF₆ additive) electrolyte were charged at different rates within the XFC regime (4C-9C, 10-15 minutes charge) and characterized using in-situ XRD. The cells are all characterized after hundreds of cycles, in the fully discharged condition. At the local level, the characteristics of plated lithium crystallites such as the preferred crystallographic orientations and size of plated Li on graphite are revealed. Additionally, the regions with local lithium plating are correlated with the local SOC (lithium occupancy) and loss of active surface area in the cathode and anode, in the discharged state. Finally, the capacity fade of the cycled cells is correlated to the amount of dead Li, with separated contributions from irreversibly plated Li, Li trapped in graphite as Li\textsubscript{x}C (which cannot be reversibly extracted from the anode) and reaction of plated Li with the electrolyte. Based on this knowledge of the properties of lithium plating and the conditions that favor it, as well as it's effect on overall battery performance, new approaches towards designing batteries can be realized, such that irreversible Li plating is minimized. This step will in turn help to guide the rational design of the next generation of XFC capable LIBs with a consistent and safe performance.

References:
[1] T. R. Tanim, et al. Extreme fast charge challenges for lithium-ion battery: Variability and positiveelectrode issues.Journal of The Electrochemical Society, 166(10):A1926–A1938, 2019.
[2] A. Tomaszewska, et al. Lithium-ionbattery fast charging: A review.eTransportation, 1:100011, 2019.
[3] L. Gold, et al. "Probing lithium-ion batteries' state-of-charge using ultrasonic transmission – Concept and laboratory testing," Journal of Power Sources, vol. 343, pp. 536-544, 2017.
[4] F. Shi, et al. Strong texturing of lithium metal inbatteries.Proceedings of the National Academy of Sciences, 114(46):12138–12143, 2017.

(Electro)chemical reaction at the electrode-electrolyte interface dramatically affects the performance of lithium secondary batteries, including cycle life and capacity¹. Much effort has been made to reveal the (electro)chemical reaction at the electrode-electrolyte interface by using operando measurements, e.g., operando attenuated total reflection infrared (ATR-IR) spectroscopy, which successfully probes the surface reaction in liquid electrolyte². Although solid polymer electrolyte (SPE) is considered as a next-generation electrolyte with flame retardancy and high mechanical stability³, its (electro)chemical reaction at the electrode-electrolyte interface is not well understood.
Here, we employed operando ATR-IR and various electrochemical measurements, including electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV), to understand the surface reaction on model electrode surface in the SPE system. We selected the most widely used polymer electrolyte, polyethylene oxide (PEO)-based electrolyte with LiTFSA⁴, as a target material. Cu nanofilm was used as a model electrode to achieve good S/N and avoid the undesirable reaction.
Linear sweep voltammetry (LSV) showed a small reduction current at ca. 2.0 V_Li and ca. 1.0 V_Li, which suggested the growth of the solid-electrolyte interphase (SEI) layer on the Cu electrode. The assignment was further supported by the EIS spectra, where we observed the increase in low-frequency semicircle during LSV, indicating the change in the charge transfer and SEI resistance. Operando ATR-IR spectra, obtained simultaneously with LSV, showed the voltage-dependent peaks at ca. 1000 cm^-1 and ca. 1200 cm^-1 along with the small reduction current, which corresponds to the vibration signals from SEI components. In this talk, we will discuss the formation mechanisms of the SEI and surface (electro)chemical reaction in the SPE system, based on the detailed analysis of operando ATR-IR spectra. Our findings will highlight the importance of the electrode-electrolyte interface in the SPE system, which enables the design of polymer electrolytes with superior cyclability and ion conductivity.

[Reference ]
1. An, S. J. et al. Carbon N. Y. 105, 52–76 (2016).
2. Zhang, Y. et al. Energy Environ. Sci. 13, 183–199 (2020).
3. Murata, K., Izuchi, S. & Yoshihisa, Y. Electrochim. Acta 45, 1501–1508 (2000).
4. Mindemark, J., Lacey, M. J., Bowden, T. & Brandell, D. Prog. Polym. Sci. 81, 114–143 (2018).

A battery is comprised of a large number of interfaces, each of which are subject to reactions. The most important of these reactions being that occurring between the electrode and electrolyte – the so called solid-electrolyte interphase (SEI). Understanding the underlying mechanism that promotes the formation and dynamics of the SEI on battery electrode surfaces is central to the development of better performing lithium-ion batteries. Neutron reflectometry (NR) is ideal for studying interfacial processes in battery materials, in situ. We have used NR to study SEI formation and growth on amorphous silicon (aSi) anodes. Silicon electrodes are of interest to battery researchers due to their large theoretical capacity (Li₁₅Si₄ – 3579 mAh g^-1) and low potential versus Li (< 0. 4 V). In our work we have focused on investigating the amorphous silicon electrode surface with a standard battery electrolyte (1.2M LiPF₆-3:7 wt% ethylene carbonate (EC):dimethyl carbonate (DMC)). Combining NR with various ex situ complementary techniques we have determined the chemical reactivity of an aSi anode with electrolyte, the formation and growth of the SEI as a function of charge state, the formation and growth of the SEI with electrolyte additive as a function charge state and the role of binder material on SEI formation.

It is often said that the highly developed surface area of electrode materials based on activated carbons (AC) ensures high capacitance, power and energy especially in electrochemical capacitors (EC). However, the real ‘efficiency’ of the surface exploited in charge storage remains often unknown. The unexploited potential of a highly developed AC surface comes often from a limited packing of electrolyte ions in carbon micropores, namely - relatively poor matching between pore and ion size. Only several techniques for effective specific surface area involvement in charge accumulation have been proposed recently. It turned out that the Step Potential Electrochemical Spectroscopy (SPECS) technique can be a useful tool for determining the real electrode/electrolyte interface surface during the operation of the device.

The SPECS technique allows for the comprehensive analysis of the total current output and evaluation of the capacitive, redox and other components. Together with the physicochemical data from nitrogen and CO₂ adsorption techniques, we aim at the determination of the real surface area of AC used for charge accumulation in EC. SPECS measurements have been done in Swagelok^® system equipped with reference electrode. Organic electrolyte 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM⁺TFSI^-) was used as an electrolytic medium. The basis for choosing such an electrolyte was very small difference between cation and anion dimensions. The results will be supported by the recent findings from electrochemical dilatometry experiments. The tested materials include popular Kuraray YP-50F and Cabot BP-2000 with significant differences in pore distribution and structure.

Based on the knowledge gained to date, we assume that the efficiency of the interface ‘exploitation’ depends on many factors: pore size distribution, wettability of the electrode material by electrolyte, as well as the size of the ions. Thus, inappropriate selection of one of these parameters can aggravate the device performance, even if the excellent carbon material is applied as electrode component. The paper will provide the results from SPECS correlated with the physicochemical properties of carbon materials. It appears that the electrochemically accessible surface does not exceed 30% of the values estimated by N₂ or CO₂ adsorption.

The interest in all solid-state batteries (ASSB) is widely known for two main things, one cited for its safety and the other its’ ability to use the metallic anodes for enhancing the gravimetric energy density. In order to make solid state batteries a reality it’s important ¹ to understand the process responsible for failure in ASSB. 1. During the cell assembly of all solid-state batteries, parameters such as pressure, temperature, preparation of the cathode composite², etc. will influence the interface quality and in turn affect the battery cycling. 2. Also, during battery operation, parameters such as the operating current density³ and stack pressure influence battery cycling. They both (1&2) impact cycling performance leading to formation of new phases or interphases, the grains volumetric expansion inducing contact loss and crack formation, the possible dendrites formation leading to the battery short circuit. Among different techniques being used to understand the fundamentals of battery degradation, the scanning electron microscopy (SEM) offers a good compromise in terms of size and resolution of observation with the possibility to perform studies on the different types of interface with a good spatial resolution. By combining the imaging with chemical analyses by X-ray energy dispersive spectroscopy (EDX), a complete survey of morphological and chemical modification induced at the interfaces is possible.
A home-made ASSB cell has been designed to perform in-situ and operando studies under real time in ESEM.To begin with, we studied the impact of current density(J) for NMC-LPS-Li ASSB system, keeping other parameters of ASSB constant and relating cycling at different J to the battery degradation. This study has been carried out in two parts coupled with other techniques, ex-situ (SEM, XRD, Raman) and in-situ (SEM). The investigated ASSB system shows degradation of LPS at both interfaces, formation of voids, changes in LPS morphology depending on the cycling conditions. The plating and stripping of Li-metal was also investigated. These observations, along with other researches³ aimed at improving the primary metal anode⁴ or interface between metal anode and electrolyte⁵ and understanding solid electrolyte⁶ will help us to deal and design our future ASSB with improved performances. This kind of study will be further extended to understand other interesting ASSB system.

Understanding the structural changes at the atomic scale of nano-oxide formation via metal oxidation is critical for the rational design and control of oxides for a variety of technology-critical applications including heterogeneous catalysts, sensors, and corrosion protection in extreme environments. Though classical oxidation theories, such as Cabrera-Mott, exist, little is known concerning the initial oxidation stages at the atomic scale. With the developments of in situ characterization tools, such as Environmental Transmission Electron Microscope (ETEM), we are now able to directly observe the structural changes during oxidation. In this work, we combine in situ ETEM with density functional theory (DFT) to fundamentally understand oxide island formation and growth at the atomic scale; Cu(001) was selected as the model metal system. As an example of this correlative approach, we recently observed a layer-by-layer growth of Cu₂O island along Cu₂O(110) plane, where our analysis of these growth dynamics suggests a diffusion limited monolayer growth process. DFT results supported these experimental observations of preferential growth along Cu₂O(110) based on surface energies, diffusion barriers and adsorption energies simulations. We are currently investigating these critical surface reactions for binary alloys. Support for this research program is from NSF-DMR 1410055, 1508417, and 1410335.

The processes related to the formation and dynamics of the electrical double layers (EDLs) at the solid-liquid electrolyte interfaces are crucial to numerous applications. However, EDLs formed in realistic interfaces are nanometer(s) thin. The shortage, therefore, exists in the experimental analytical tools capable of probing the concentration, composition, and potential profiles within nanometers thin EDLs with sufficient depth and spatial resolution. The escape depth of low energy electrons in liquids, on the other hand, is in the order of nanometer making electrons, and SEM in particular, to be a suitable probe for EDL. There is, however, an apparent experimental challenge to probe EDL with SEM due to ca. nine orders of magnitude pressure gap between the liquid sample and microscope chamber environments.
In our current communication, we report on a novel experimental approach to probe and image the electrical double layers (EDLs) at the electrified solid-liquid electrolyte interfaces using SEM. Our work resolves the “pressure gap” challenge using graphene as an electron-transparent vacuum separating membrane and demonstrates the sensitivity of the secondary electron yield from the electrified graphene-electrolyte interface to the interfacial processes such as electrolyte polarization and Faradaic processes. We developed finite element analysis models of the potential distribution near the interface and relay them with Monte Carlo simulations to explain the experimentally observed trends of bias-dependent SEM contrast [1]. The proposed method is most useful for studying electrochemical systems where laterally heterogeneous interfaces are present and high-resolution polarization mapping is required
References

H. Guo et al. Probing Electrified Liquid-Solid Interfaces with Scanning Electron Microscopy arXiv:2006.04283

Selectively reducible oxides form the foundation of “regenerable” oxide-supported metal catalysts, where transition metals may be reduced from the oxide, then resorbed via oxidation to regenerate spent catalysts. We present in-situ studies of a model defective spinel catalyst under redox cycling conditions that mirror operation conditions for methane reforming and related reactions. Experiments show that these catalysts have excellent sulfur tolerance and high turnover frequencies for methane dry reforming. In-situ studies show that reduction causes crystallographically oriented surface cracking and complex surface structures decorated by metal nanoparticles assembled on crystal facets. In-situ XRD shows the spinel is completely regenerable during redox cycling, allowing for oxidative sulfur removal and metal oxide reincorporation. Microstructural development upon redox cycling is observed by in-situ HT-FESEM and is linked to improved catalytic performance upon redox cycling. Subsequent redox cycles create more surface roughness, as demonstrated by surface investigation after each redox cycle. The in-situ FESEM study even at high vacuum provides sufficient oxygen for reoxidiation of the active metal particles, and agrees with calculated thermodynamic stability. Metal particle coarsening and migration is shown to be a function of temperature and the extent of faceting of the oxide, and re-oxidation is found to be a function of the ambient pO2. Under high oxygen activity, the transition metal first oxidizes and then reacts with the defect spinel to regenerate the spinel, while at pO2<200 ppm full oxidation does not occur but instead the oxidation and resorption occur at the triple phase boundaries. Enhanced mass transport on the surfaces of the partially-reduced and defective spinel is likely a major contributor to the high catalytic activity and unusually high oxidative sulfur removal reaction.

Development of high efficiency and stable solar fuels production systems is one of the most promising routes to address the increasing energy demands and finite source of fossil fuels available around the world. Long-term stability of such systems has always been an obstacle for widespread implementation of solar fuels; hence, investigation of the mechanism of self-decomposition and corrosion of photoelectrodes under photoelectrochemical operation is of significant importance. In Striving to develop an in-situ photo-corrosion detection technique which is widely available in most labs we studied the photo-corrosion of GaP photoanodes, a promising III-V material for tandem top subcells ,in strongly acidic condition via in-situ UV-Vis spectroscopy technique along with ex-situ SEM and XPS characterization to monitor the dissolved Ga and P species during photoelectrochemical reaction. The corrosion faradaic efficiency of n-GaP and p⁺-GaP was calculated as a function of applied bias and time and found to be very different in corrosion process. p⁺-GaP was uniformly dissolved across the active area, while n-GaP was etched in an anisotropic way with micropores formed on the surface of photoanode. Ultimately, a thin layer of TiO₂ protective layer was coated on n-GaP photoanodes to explore its consequences on corrosion mechanism which slowed the rate of GaP dissolution but also introduced a large charge-transfer resistance to the system.

The development of materials for sustainable energy conversion and storage requires new approaches on both the materials and process side. These approaches rely on the use of scattering techniques offering access to the molecular and nanoscale and exploiting both the virtues of X-rays and neutrons [1]. One crucial point is to observe the materials structural transformations in situ from the nano- to the microscale during materials’ synthesis and subsequent device fabrication. These changes must be related to their (macroscopic) functionality. Preferentially the materials itself are sustainable, water-based, or biomaterials. To start with, we focus on wood-based biomaterials, such as cellulose and lignin. Introducing spray deposition as versatile process, ultrasmooth cellulose nanofibril films with controlled thickness are investigated [1,2]. Depending on their charge and roughness, the wettability can be controlled. This is crucial for optimizing the fabrication of hybrid materials, typical for energy conversion and storage. The tailoring of wettability is further pursued by surface functionalization of water-based latex colloids. These materials offer the possibility to tailor and fine-tune the wettability by tuning the core-shell colloid morphology in thin films, thus providing a facile template methodology for repellent surfaces of devices [3]. In materials synthesis, chemical modification of lignin affects the molecular network, aggregate stacking, and size of lignin thermosets. This in turn changes the macroscopic properties of the resulting thermoset [4]. Electronic properties manifest themselves for example in electrical conductivity changes and optical response. The electronic properties are governed by the nanoscale morphology. Imaging via scattering methods allows to extract the granular morphology up to and beyond the percolation threshold in situ during fabrication. At the same time, this analysis allows to predict the electrical behavior of hybrid composites and thus to optimized for example electrodes in such devices [5]. Silver nanowires offers electrical conductivity, mechanical flexibility as well as well high optical transparency when applied as meshes in top electrodes. Here, printing applications offer great potential for photovoltaics and flexible electronics. Two examples are presented: First, a tough composite for a 2D-printed film as top contact on a solar cell, and secondly a flexible composite applied for a 3D-printed flexible capacitor [6]. These examples highlight the advanced capabilities of scattering methods for in situ imaging of the nanoscale structure and for establishing the nanostructure-function relationship.
[1] C.J. Brett et al., Macromolecules 52, 4721 (2019)
[2] S. V. Roth, J. Phys.: Condens. Matter 28, 403003 (2016)
[3] J. Engström et al.: Adv. Funct. Mater., 30, 1907720 (2020)
[4] M. E. Jawerth et al., ACS ACS Appl. Polym. Mater. 2, 668 (2020)
[5] M. Gensch et al., ACS Appl. Mater. Interfaces 11, 29416 (2019)
[6] T. E. Glier et al., Scientific Reports 9, 6465 (2019)

Large scale production of sustainable devices experiences a massive increase of attention over the last years. The global economy is initiating projects to minimize the use of fossil resources. However, it is yet unclear how sustainable bio-based devices perform and degrade under environmental influences and how we could improve possible drawbacks. We believe that wood-based cellulose is a key enabler for hierarchical bio-nanocomposites with unprecedented properties as lightweight, flexibility and durability [1]. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is nowadays widely used as high performance conductive polymer in the field of bioelectronics, solar cells, supercapacitors. Due to its water-solubility, it fulfils the sustainable approach of using less solvents to achieve environmentally friendly devices. The aim of this in situ spray deposition study was to correlate directly the inner morphology of the deposited thin films with its macro-scale physical properties. Herein we used spray deposition as industrial deposition technique on a roll-to-roll coater to fabricate large-scale films with unprecedented low surface roughness down to 2 nm [2-3]. The sprayed flexible and conductive bionanocomposite with thicknesses up to 30 µm are fabricated to provide high energy densities for battery and supercapacitor technologies. We investigated the effect of different layer formation parameters such as drying times, temperature and additives, in particular organization of PEDOT:PSS on CNF and film formation mechanism with spray coating method by means of surface sensitive X-ray scattering [2-4].We thus achieved a better understanding of the morphology of such films and their electrochemical performance in order to create better energy storage and healthcare devices in the future.

References
[1] Mittal, N. et al. ACS Nano 12, 6378-6388 (2018)
[2] C. J. Brett et al., Macromolecules 52, 12, 4721-4728 (2019)
[3] W. Ohm et al., J. Coat. Technol. Res. 15, 759 (2018)
[4] J. Engström*, C. J. Brett* et al., Adv. Funct. Mater. 30, 1907720 (2020)

Nanoparticles are promising building blocks for materials in various energy applications i. e. fuel cells, batteries, photocatalysts. These materials are often produced by solution-based processing of colloidal nanoparticles. Detrimental in this process is the tendency of nanoparticles to agglomerate due to their high surface energy. Thus, agglomeration also influences for example catalytic properties and adsorption behavior of nanoparticles, as the effective specific surface area available is reduced. Nevertheless, it can be prevented by colloidal stabilization. However, before stabilization the agglomerates must be broken apart at best into individual (primary) nanoparticles. This can be achieved by energy input into the colloidal system. Ultrasonic deagglomeration is promising because, through ultrasonic cavitation, it generates high mechanical shear forces, shock waves and microjets that break down nanoparticle agglomerates.

In this context, it will be shown how synchrotron-based in situ small-angle X-ray scattering (SAXS) can be used to study the deagglomeration process in order to possibly produce tailor-made agglomerates on the nanoscale with respect to morphology and size. Therefore, the fractal structure and radii of gyration of differently sized SnO₂ nanocrystals synthesized in the gas phase are continuously monitored during sonication in a specifically designed X-ray transparent in situ cell as a function of time and energy input.

Acknowledgement
This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Chemical vapor synthesis (CVS) is a technique enabling reproducible synthesis of narrow sized nanocrystals of high purity from the gas phase. By variation of essential synthesis parameters, such as the time-temperature (t-T) -profile, synthesis pressure and partial pressure of the reactants, different materials and morphologies can be created in the form of nanoparticles with the same synthesis assembly [1, 2]. One of the commonly used materials is iron. In combination with oxygen it can form a variety of compounds (e.g. FeO, Fe₂O₃) and phases (e.g. α-Fe₂O₃, γ-Fe₂O₃) containing iron in different oxidation states and crystalline systems.

In order to design efficient and sustainable processes for the synthesis of tailored materials with desired properties and access to novel materials or morphologies, the synthesis process must be studied and understood in detail. This is enabled by providing an in-situ view of the synthesis process.

X-ray absorption spectroscopy (XAS) reveals the electronic properties of the material, its local structure and composition. We will present our ongoing study on the in-situ investigation of the CVS process using compounds containing iron and oxygen as model system. So far we studied the precursor and its vapor [3]. Furthermore, we describe the design of the mobile CVS reactor for in-situ investigation of the process with control over the t-T-profile. The parameter window for the reactor is defined based on investigations of iron oxide nanoparticles and evaluation of the reaction chamber with computational fluid dynamic simulations.

[1] Winterer, M., Discovering paths to optimized nanoparticle characteristics, Chem. Eng. Sci. 186, (2018).
[2] Levish, A. and Winterer, M., Nanoparticles generated by combining hot wall and microwave plasma chemical vapor synthesis, MRS Adv. 1-6, (2018).
[3] Levish, A. and Winterer, M., In situ cell for x-ray absorption spectroscopy of low volatility compound vapors, Rev. Sci. Instr. 91, (2020).
Acknowledgment:
This research was partially carried out at PETRA III at DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, under Proposal No. I-20170647 and at the Diamond Light Source facilities at beamline I20 under Proposal No. 19229. We acknowledge the German Research Foundation (DFG) for funding through research unit FOR 2284 ‘Model-based scalable gas-phase synthesis of complex nanoparticles’ (Project No. WI-981/14).

Electron microscopes have become very power tools to study the structure of materials at unprecedented resolution. From their invention, these tools have provided essential information on the structure and properties of the broadest range of materials used today in structural and functional applications. In-situ electron microscopy has evolved over the last few decades from dynamic experiments (heating, straining) to reaction in gases, and, more recently, even electrochemical experiments either in vacuum or in liquids. While the spatial resolution remains the first motivation for such in situ experiments in electron microscopy (so that even nucleation and early stages of growth of nanostructures formed during electrodeposition can be investigated), the flexibility and the more realistic environments remain the realm of synchrotron beam lines. Higher pressures, higher (or lower) temperatures, experiments in liquids and gases, can be carried out with exquisite spectroscopic resolution, penetrating power with diffraction techniques, in realistic and macroscopic samples and experimental conditions more suitable to represent operating conditions.
In this presentation, a comparison of the types of experiments that can be carried out with electron and photon techniques are discussed, in particular in the context of the nature of the samples, the time resolved sensitivity and the potential artefacts.

As current memory technologies approach their fundamental limits, increasing demand for novel non-volatile memories is driving the development of new architectures, which are already showing their potential in high performance computing [1, 2]. As a result of their non-volatile nature, byte-addressability and fast response times, which are comparable to those of dynamic random-access memory, resistive-switching-based phase change materials (PCMs) are strong candidates for memory applications with reduced hardware cost and power consumption [3, 4].

The use of advanced transmission electron microscopy combined with in situ electrical biasing provides a powerful method for studying the local relationship between atomic structure and electrical properties in individual nanoscale memory elements, in order to identify the factors that affect their switching reproducibility and dynamics.

We have developed a multi-step lithographic procedure, which allows dedicated nanostructured line cells that containPCM switching layers, electrical contacts and protective layers to be fabricated directly on electron-transparent silicon nitride membranes and used for switching experiments both outside and inside the transmission electron microscope. We used a special electrical setup to provide short (50 ns) electrical current pulses to individual line cells with sharp (approximately 3 ns) rising and falling edges. By using these pulses, PCM devices could be switched between crystalline low resistance and amorphous high resistance states inside a transmission electron microscope.

Even though each device consisted of several layers of different materials, resulting in a total thickness of between 200 and 300 nm, a fast pixelated direct electron detector could be used to record scanning electron diffraction patterns from amorphous/crystalline regions before and after successive switching cycles, while measuring the electric properties of the same devices. Newly-developed algorithms based on LiberTEM software [5] were used to follow the evolution of individual crystallites and to show that the switching mechanisms involved slow crystal growth or rapid recrystallization, depending on the details of the current pulses.

[1] K. Wu, F. Ober, S. Hamlin and D. Li, ArXiv:1708.02199 [Cs] (2017).
[2] Y. Zhang and S. Swanson, 31^st Symposium MSST (IEEE, Santa Clara, CA, USA, 2015), pp. 1-10.
[3] T. Coughlin, IEEE Consumer Electronics Magazine 5, 133 (2016).
[4] S. Chhabra and Y. Solihin, 38^th Annual ISCA (2011), pp. 177-188.
[5] A. Clausen et al. LiberTEM/LiberTEM: 0.5.0. (Zenodo, 2020). doi:10.5281/zenodo.3763313.

Synchrotron X-ray absorption spectroscopy and associated techniques are useful tools for the element-specific analysis of the structure and properties of materials. In this talk, the application of such techniques in the study of some noble metal nanostructures toward catalytic applications will be presented. Materials of interest include nanoclusters, nanoalloys, and single-atomic-site catalysts. It will be demonstrated that the X-ray spectroscopy techniques, in association with other experimental and computational methods, can sensitively probe the structural and property changes of the catalysts induced by effects such as sample size, protecting ligands, and alloy bonding. The structural information revealed by the X-ray techniques is found useful to more thoroughly understand the structure-property relationships of the nanocatalysts.

For the inspection of processes involving condensed matter, in situ neutron powder diffraction proves to be a versatile tool, giving insight into processes of technological pertinence. Only a few high-intensity powder diffractometers at intense neutron sources allow for this. D20 at Institut Laue-Langevin provides the highest available intensity in constant wavelength neutron powder diffraction. A stationary, curved linear position sensitive detector allows for in-situ diffraction studies down to a second and encourages the use of complex sample environments with inherently small sample sizes. D20 adapts to various levels of crystallographic complexity and rapidity of an observed phenomenon.
Rapid growth of the portable electronics market, as well as non-polluting ground transportation, needs portable energy storage solutions with improved characteristics. One way to improve Li-ion batteries are solid electrolytes to overcome issues of liquid electrolytes in battery safety and high-voltage operation. Neutron diffraction determines the Li diffusion pathway in solid-state Li-ion conductors. Development of better electrode materials in terms of gravimetric and volumetric energy density, temperature operation range and cycling stability needs understanding of lithium (de)intercalation phenomena. Operando diffraction techniques are well suited here. Electrochemical cells based on a neutron-transparent (Ti,Zr) alloy combine good electrochemical properties and the ability to collect neutron diffraction patterns with reasonable statistics and no other Bragg peaks than those of the electrode material. This allows detailed structural determination of electrode materials by Rietveld refinement during operation.
Solid-oxide fuel cells convert chemical energy into electricity at higher efficiency than conventional methods, with less pollution. The anode (fuel electrode) must not alter at high temperature (thermal stability), not form nonconductive phases at interfaces (chemical stability) and not degrade upon reduction and oxidation cycles (redox stability). The state-of-the-art “cermet” of Ni and yttria-stabilized zirconia ceramic loses performance upon usage as its porosity is reduced by Ni agglomeration and as oxidation of Ni causes redox instability. Cermet deactivates through carbon coking and sulfur poisoning, making it unsuited for hydrocarbon fuels. Single-phase mixed ionic and electronic conductors provide microstructural stability and increase the electrode fraction accessible to oxide ions. Many of those oxides have been investigated successfully in operando at high temperature under oxidizing and reducing gas flow by neutron diffraction, following the crystal chemistry of oxide ions during the process.
Classical in situ work (thermo-diffractometry) has been done on the new photovoltaic materials, MaPbI₃ and derivates, and neutron diffraction turned out to be a perfect tool to screen the crystal chemistry of light organic atoms beside the heavy metal atoms over a wide range of temperatures.
Hydrogen is an attractive energy carrier for renewable energy sources due to its high energy density. Solid-state hydrogen storage provides higher storage capacities than compressed or liquefied hydrogen. Complex metal hydrides have high hydrogen storage capacities but suffer from poor kinetic and thermodynamic properties. Tuning the thermodynamics for dehydrogenation, in order to reduce the temperature at which hydrogen is evolved is achieved through the addition of a second phase, which will lead to the formation of a more stable product upon decomposition and thereby reducing the enthalpy for dehydrogenation. Neutron powder diffraction comes screens the crystal chemistry of the different phases in situ and localizes hydrogen in the structures.

There is a growing consensus that the theoretical limit of one electron intercalation reactions will soon be reached. Recent discoveries on anion redox electrochemistry offer new opportunities and a possible paradigm shift in energy storage. By activating anion (e.g. oxygen, sulfur) for reversible electrochemical reactions, increased energy density has been demonstrated. In the past decade, the research community has made great progress on understanding key features of the anionic electrochemical activities and dynamic phenomena that govern the performance limitations including cycling voltage hysteresis and decay. However, the exact mechanism at molecular/atomic scales is still largely under-debate. The lack of conscience on the anionic redox mechanism which can be fueled by possible misinterpretation of experimental results. Of particular importance is determining what techniques provide accurate and reliable evidence for reversible bulk oxygen redox.
Compared with transition metal cations, oxygen anions are more challenging to be probed and quantified, which requires a multi-modal experimental approach with exceptional energy and spatial resolution combining computational modelling. Here, we advance: 1) from X-ray absorption spectroscopy to resonant inelastic X-ray scattering which shows a sharp feature with the corresponding excitation energy at 531 eV corresponding to the extra unoccupied electron orbitals of oxidized oxygen; 2) from neutron diffraction to neutron pair distribution function which is enormously valuable as a local probe that can reveal local distortions, when present along the oxygen redox reactions; 3) from X-ray diffraction to Bragg coherent diffraction imaging which will enable the direct observation of the strain dynamics with the exceptional sensitivity as small as 0.05% and phase evolution induced by the oxygen redox activity at nanoscale. It is through the in-depth understanding of electrochemical reactions mechanism at microscopic and macroscopic level during the operation of batteries; we can successfully formulate strategies to achieve reversible and stable anionic redox for next generation lithium-ion batteries.

The performance of functional materials central to energy devices, including rechargeable batteries, fuel cells, as well as gas separation and storage technologies, is determined largely by atomic-scale materials structure and dynamic-function relations. Many energy materials function by undergoing change, such as compositional change in rechargeable battery electrodes that reversibly host charge-carrying ions and in gas storage and separation materials. Robust characterization methods that quantitatively and accurately capture these changes are essential to the strategic design of superior materials function, and consequently to improving energy device performance. Historically, a conflict existed between fast and detailed measurements of materials structure, particularly for measurements of materials within devices. This conflict was sometimes alleviated by using small model systems in which the material function was not representative of that in the corresponding commercially used device. Today, advanced characterization methods that capture material changes in detail while they are occurring is possible in whole devices under real-life operating conditions, as facilitated by advances in instrumentation.

This talk will give examples of in operando and in situ neutron measurements of energy materials used for rechargeable batteries capturing compositional change and/or non-equilibrium processes, with a focus on neutron powder diffraction using the high intensity neutron powder diffractometer Wombat at the Australian Centre for Neutron Scattering

Nanomaterials are important for a wide range of applications because of their unique properties, which are strongly connected to their three-dimensional (3D) structure. Electron tomography has therefore been used in an increasing number of studies. Most of these investigations resulted in 3D reconstructions with a resolution at the nanometer scale, but also atomic resolution was achieved in 3D.

In order to preserve the carefully designed morphologies and functionalities, understanding the stability of nanomaterials during application is crucial. It is hereby important to note that most electron tomography investigations have been performed at the conventional conditions of an electron microscope. An emerging challenge is therefore to fully understand the connection between the 3D structure and properties under more relevant conditions, including high temperatures as well as in the presence of liquids and gases. Therefore, innovative methodologies are required to track the fast 3D changes of nanomaterials that occur under such conditions.

Recently, we proposed an acquisition approach where a tilt series of 2D HAADF-STEM projection images is acquired within a few minutes. By continuously tilting the holder and simultaneously acquiring projection images while focusing and tracking the particle, we were able to reduce the total acquisition time for a tilt series by a factor of ten [1]. Moreover, a new approach was developed to compute high quality 2D virtual slices through nanoparticles in approximately 60 ms time. This technique drastically improves the efficiency of 3D characterization of nanomaterials by TEM. It enables explorative imaging and provides valuable information to dynamically adjust the acquisition parameters during an electron tomography experiment [2].

In this manner, we were able to study the 3D morphological evolution of anisotropic Au and AuPd nanoparticles as a function of both heating time and temperature [1,3]. Moreover, we measured the elemental diffusion dynamics of individual anisotropic Au-Ag nanoparticles in 3D [4]. We conclude that for a given composition, the shape of the nanoparticle does not influence the alloying process significantly. Based on our analysis, it is clear that interdiffusion of metals at the nanoscale is more complex than predicted by simple Fickian diffusion and that other factors such as surface diffusion need to be taken into account.

To quantify nanoparticle shape dynamics in a gaseous environment in 3D, HAADF-STEM images served as an input for atom counting procedures followed by 3D relaxation of the structure. In this manner, we characterized shape changes of a Pt nanoparticle in a gaseous environment. The conditions have been varied from vacuum to a 1 bar H2 flow, followed by a 1 bar O2 environment. To investigate the behaviour during cycling, we repeated the experiment several times using the same particle. We clearly observe morphology changes and we were even able to quantify the occurrence of the different surface facets [5].

References
[1] H. Vanrompay et al., Nanoscale 10 (2018), p. 22792
[2] H. Vanrompay et al., Particle and Particle System Characterization (2020) in press
[3] W. Albrecht et al., ACS Nano 13 (2019), p.6522
[4] A. Skorikov et al., ACS Nano 13 (2019), p.13421
[5] T. Altantzis et al., Nano Letters 19 (2019), p.477
[6] This work was supported by the European Research Council (Grant 815128 REALNANO to SB, Grant 770887 PICOMETRICS to SVA and Grant 797153 SOPMEN to WA, Grant 823717 ESTEEM3). The authors acknowledge financial support from the Research Foundation Flanders (FWO, Belgium) through project fundings, a postdoctoral grant to EB and a doctoral grant to HV.

Layered transition metal oxides that incorporate water or other solvents provide fundamental understanding of the effects of confinement on electrochemical ion transport and reactivity. They also tend to exhibit fast electrochemical ion insertion kinetics making them of interest for high power electrochemical energy storage. These materials are hybrids that consist of transition metal oxide layers separated by fluids that can be either ordered or disordered, and strongly or weakly bound to the oxide. Our group has investigated ion insertion into a range of hydrated and solvated metal oxides with operando characterization to connect structural changes to the electrochemical response. Since many of these materials exhibit fast kinetics (full charge/discharge in minutes), operando characterization techniques must have suitable temporal resolution to capture structural or chemical changes at these rates. This presentation will describe our use of three types of operando techniques to probe insertion kinetics at minute-to-second timescales: X-ray diffraction, atomic force microscopy, and electrochemical quartz crystal microbalance. It will also discuss how knowledge of the dynamics of ion insertion in these materials help us understand ion transport under confinement.

Chemical substitution of transition metal species is often used to tune the performance and cost of electrode materials. However, substitutions that improve one performance metric can be coupled to unintended consequences. This presentation will show how we have used operando X-ray scattering to understand kinetic limitations and O-redox in Li-layered transition metal oxide electrodes.
Substituted Li layered transition metal oxide (LTMO) electrodes such as Li_xNi_yMn_zCo_1-y-zO₂ (NMC) and Li_xNi_yCo_1-y-zAl_zO₂ (NCA) show reduced first cycle Coulombic efficiency (90-87 % in standard cycling conditions) compared with archetypal Li_xCoO₂ (LCO – ~98 % efficiency). We use operando x-ray diffraction (XRD) to demonstrate that the apparent first cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.85, with near full capacity recovered and slow Li reinsertion during a potentiostatic hold following the galvanostatic charge–discharge cycle. The kinetic limitation manifests not only as the kinetic capacity loss during discharge, but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge–discharge voltage hysteresis at x > 0.85.

Understanding the structural changes at the atomic scale of nano-oxide formation via metal oxidation is critical for the rational design and control of oxides for a variety of technology-critical applications including heterogeneous catalysts, sensors, and corrosion protection in extreme environments. Though classical oxidation theories, such as Cabrera-Mott, exist, little is known concerning the initial oxidation stages at the atomic scale. With the developments of in situ characterization tools, such as Environmental Transmission Electron Microscope (ETEM), we are now able to directly observe the structural changes during oxidation. In this work, we combine in situ ETEM with density functional theory (DFT) to fundamentally understand oxide island formation and growth at the atomic scale; Cu(001) was selected as the model metal system. As an example of this correlative approach, we recently observed a layer-by-layer growth of Cu₂O island along Cu₂O(110) plane, where our analysis of these growth dynamics suggests a diffusion limited monolayer growth process. DFT results supported these experimental observations of preferential growth along Cu₂O(110) based on surface energies, diffusion barriers and adsorption energies simulations. We are currently investigating these critical surface reactions for binary alloys. Support for this research program is from NSF-DMR 1410055, 1508417, and 1410335.

The development of materials for sustainable energy conversion and storage requires new approaches on both the materials and process side. These approaches rely on the use of scattering techniques offering access to the molecular and nanoscale and exploiting both the virtues of X-rays and neutrons [1]. One crucial point is to observe the materials structural transformations in situ from the nano- to the microscale during materials’ synthesis and subsequent device fabrication. These changes must be related to their (macroscopic) functionality. Preferentially the materials itself are sustainable, water-based, or biomaterials. To start with, we focus on wood-based biomaterials, such as cellulose and lignin. Introducing spray deposition as versatile process, ultrasmooth cellulose nanofibril films with controlled thickness are investigated [1,2]. Depending on their charge and roughness, the wettability can be controlled. This is crucial for optimizing the fabrication of hybrid materials, typical for energy conversion and storage. The tailoring of wettability is further pursued by surface functionalization of water-based latex colloids. These materials offer the possibility to tailor and fine-tune the wettability by tuning the core-shell colloid morphology in thin films, thus providing a facile template methodology for repellent surfaces of devices [3]. In materials synthesis, chemical modification of lignin affects the molecular network, aggregate stacking, and size of lignin thermosets. This in turn changes the macroscopic properties of the resulting thermoset [4]. Electronic properties manifest themselves for example in electrical conductivity changes and optical response. The electronic properties are governed by the nanoscale morphology. Imaging via scattering methods allows to extract the granular morphology up to and beyond the percolation threshold in situ during fabrication. At the same time, this analysis allows to predict the electrical behavior of hybrid composites and thus to optimized for example electrodes in such devices [5]. Silver nanowires offers electrical conductivity, mechanical flexibility as well as well high optical transparency when applied as meshes in top electrodes. Here, printing applications offer great potential for photovoltaics and flexible electronics. Two examples are presented: First, a tough composite for a 2D-printed film as top contact on a solar cell, and secondly a flexible composite applied for a 3D-printed flexible capacitor [6]. These examples highlight the advanced capabilities of scattering methods for in situ imaging of the nanoscale structure and for establishing the nanostructure-function relationship.
[1] C.J. Brett et al., Macromolecules 52, 4721 (2019)
[2] S. V. Roth, J. Phys.: Condens. Matter 28, 403003 (2016)
[3] J. Engström et al.: Adv. Funct. Mater., 30, 1907720 (2020)
[4] M. E. Jawerth et al., ACS ACS Appl. Polym. Mater. 2, 668 (2020)
[5] M. Gensch et al., ACS Appl. Mater. Interfaces 11, 29416 (2019)
[6] T. E. Glier et al., Scientific Reports 9, 6465 (2019)