Symposium S.EN09—Flow-Based Open Electrochemical Systems

Two major limitations of Vanadium Redox Flow Batteries (VRFBs) are (1) the transport losses of getting the electrolyte into the electrode and to the reaction sites with minimum resistance, and (2) Lack of access to all reaction sites due to relatively low saturation levels of the electrolyte [1,2]. Although the porous carbon electrodes have a high porosity, a large pressure may be required to pump the electrolyte through the electrode during operation. The flow-through porous carbon electrode can be further hampered due to the presence of hydrogen bubbles, which are formed as a parasitic side product of the V³⁺ reduction reaction at the anode in VRFBs [3]. This affects the efficiency and stability of the VRFB cells. Besides mixed potentials, the Hydrogen bubbles could also damage the pore structure of the electrode and create inaccessible surface area where otherwise the reaction would occur. To investigate the formation of gas bubbles inside the small pores of the carbon electrode, we carried out an experiment based on a novel beamline half-cell measurement setup.
During the experiment the electrolyte was injected into the cell using a peristaltic pump. Radiography was conducted during the injection process to track the flow of the electrolyte through the electrode. Simultaneously, pressure drop was measured to characterize the transport process and identify the breakthrough of the electrolyte on the opposite side of the electrode.
The setup also allows us to evaluate various types of carbon electrodes such as felts, woven or paper style materials. Flow-by and flow-through geometries and manifolds can be exchanged to study their impact on the flow dynamics. These measurements help us develop theoretical models for a better understanding of the multiphase and interfacial flow phenomena within the porous electrode. These experiments are essential for the evaluation and optimization of electrode materials and manifolds currently being used in VRFBs.

References
[1] N. Bevilacqua, L. Eifert, R. Banerjee, K. Köble, T. Faragó, M. Zuber, A. Bazylak, R. Zeis, Journal of Power Sources, Volume 439, 227071, (2019)
[2] R. Banerjee, N. Bevilacqua, L. Eifert, R. Zeis, Journal of Energy Storage 21, 163-171, (2019)
[3] L. Eifert, Z. Jusys, R. Banerjee, R.J. Behm, R. Zeis, ACS Applied Energy Materials 1 (12), 6714-6718, (2018)

Membranes need to balance a low resistance and a low crossover of redox-active species, and they have to be chemically stable in the electrolyte. For vanadium redox flow batteries, this implies stability against sulphuric acid and stability against VO₂⁺ ions. For many years, the latter criteria strongly limited the choice of membrane materials, and perfluorinated Nafion membranes became the state of the art.[1]

A very recent development is the use of polybenzimidazole membranes, which do not conduct ions in the pure form, but become proton conductive in contact with the sulfuric acid containing electrolyte. The positive charge on the protonated polymer backbone and the narrow size of the electrolyte filled voids between the polymer chains successfully repels vanadium ions in ex-situ permeability measurements. Most interestingly, we observed that both voltage efficiency and coulomb efficiency increase when the membrane thickness decreases. This probably is related to the average charging potential, which increases with the membrane thickness and thus a) increases migration in the electric field, providing some vanadium ions with the energy to pass the energy barrier which hinders them from entering the membrane, and b) possibly enhances side reactions. This result suggests that cells with thin PBI films would be very efficient in terms of both voltage and coulomb efficiency; the thinner, the better.[2]

Building up on this finding, we prepared porous Nafion membranes coated with a thin PBI layer[3] and porous PVDF membranes coated with a thin PBI layer.[4]

Another way to improve the voltage efficiency of PBI membranes is to blend them with anion exchange ionomers.[5, 6]

The very low permeability for metal ions of PBI membranes also enables the operation of iron-vanadium flow batteries (Fe-VRFB), reducing the costs of the electrolyte and increasing the upper temperature limit.[7]

By alkylating PBI membranes, a permanent charge can be fixed on the PBI backbone, turning the membranes into anion exchange membranes with a high IEC.[8] While the alkyl groups may be a starting point for degradation by VO₂⁺ , this type of anion exchange membranes can be used well in aqueous organic flow batteries.[9]

References
[1] J. Electrochem. En. Conv. Stor., 2018; 15, article 010801. DOI: 10.1115/1.4037248
[2] ACS Appl. Mater. Interfaces, 2017, 9, 36799-36809. DOI: 10.1021/acsami.7b10598
[3] Appl. Surface Sci., 2018, 450, 301–311. DOI: 10.1016/j.apsusc.2018.04.198
[4] J. Membr. Sci. 2019, 591, 117333. DOI: 10.1016/j.memsci.2019.117333
[5] Europ. Polym. J., 2017, 96, 383-392. DOI: 10.1016/j.eurpolymj.2017.09.031
[6] J. Membr. Sci, 2019, 580, 110-116. DOI: 10.1016/j.memsci.2019.03.014
[7] J. Power Sources, 2019, 439, 227079. DOI: 10.1016/j.jpowsour.2019.227079
[8] Macromol. Mater. Eng., 2011, 296, 899 - 908. DOI: 10.1002/mame.201100100
[9] ChemSusChem, 2017, 10, 3193-3197. DOI : 10.1002/cssc.201701060

Redox flow batteries are quickly developing as a potential solution for current grid-level electrochemical energy storage concerns where high cycle number durability, long calendar life, high efficiency, low cost and fast response times are required.^1,2 However, currently low energy density^3–5 and high capital costs^3,4 limit the industrial implementation of such systems.
It should come as no surprise therefore that electrolytes are a topic of active research, with a view to increasing the energy density while reducing the cost. Increasing energy density can be achieved in various ways: expanding the voltage window or by minimising the mass and/or volume per electron transferred.⁶ These strategies correspondingly open up various avenues through which novel electrolytes can be explored.
Previous examples of electrolytes for flow batteries have included a variety of metal-based systems and a range of organic molecules.^7,8 Of these, quinones are a common electroactive class of organic molecules that have been investigated due to their fast kinetics, high tunability and low cost.⁸ In the current work, benzoquinone derivatives were synthesised and investigated as potential anolytes for redox flow batteries. Benzoquinone has a higher aqueous solubility and a lower molecular weight than anthraquinone, but it is more prone to electrochemical degradation.⁸
The derivatised quinones explored in this work are anticipated to benefit, in comparison to previous anthraquinones, from a higher energy density through greater solubility, and a lower cost. Electrochemical screening was carried out using cyclic voltammetry as an initial test of redox properties and stability. Selected molecules that exhibited favourable behaviour were then run in a lab-scale full cell and through the aid of in situ NMR and EPR spectroscopy, the behaviour of the species under cycling conditions was investigated. Density functional theory modelling was used to complement the analysis. The current investigation has shown promising results towards the possibility for new routes towards functionalisation and stabilisation of such organic systems.

1. Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).
2. Gyuk, I. et al. Grid Energy Storage. (2013).
3. Potash, R. A., McKone, J. R., Conte, S. & Abruña, H. D. On the Benefits of a Symmetric Redox Flow Battery. J. Electrochem. Soc. 163, A338–A344 (2016).
4. Wang, W. et al. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986 (2013).
5. Alotto, P., Guarnieri, M. & Moro, F. Redox flow batteries for the storage of renewable energy: A review. Renew. Sustain. Energy Rev. 29, 325–335 (2014).
6. Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652–657 (2008).
7. Noack, J., Roznyatovskaya, N., Herr, T. & Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chemie - Int. Ed. 54, 9776–9809 (2015).
8. Ding, Y., Zhang, C., Zhang, L., Zhou, Y. & Yu, G. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev. 47, 69–103 (2017).

We report two in situ NMR methods to study flow batteries. While flow batteries are low-cost and environmentally benign large-scale energy storage devices, an increased understanding of the battery chemistry is required to improve their energy densities, lifetimes and to reduce cost. Demonstrating the approach on two separate anthraquinones, 2,6-dihydroxyanthraquinone, DHAQ and 4,4’-((9,10-anthraquinone-2,6-diyl) dioxy) dibutyrate, DBEAQ as redox-active electrolytes, we confirm that the reduction reactions occur via a two-step, one-electron redox process coupled with a comproportionation reaction. Using the bulk magnetisation changes, observed via the ¹H NMR shift of the water resonance, we determine the potential differences of the two one-electron couples. Electron transfer between radical and the diamagnetic anions is observed and is quantified via the linebroadening of the resonances of the diamagnetic anions. This linebroadening and shifting of the resonance – coupled with EPR measurements - provide direct evidence for the degree of electron delocalization of unpaired spin over the radical anions. Electrolyte decomposition and battery self-discharge is explored in real time, the results indicating that DHAQ decomposition is an electrochemical reaction which can be minimized by limiting the voltage used on charging. Applications of the new NMR metrologies to understand a wide range of redox processes in flow and other battery systems is readily foreseen.

Redox flow batteries (RFBs) are a highly promising large-scale energy storage technology for mitigating the problematic intermittency of renewable energy sources such as solar and wind. Organic redox species in aqueous RFBs have attracted substantial attention due to their facile synthesis, structural diversity, and low cost. Here, our group present the design and implementation of a micellization strategy in a pH-neutral, nontoxic, metal-free, aqueous RFB. The PEGylated micelle is constructed in an aqueous KCl solution as the anolyte. The micellization strategy (1) improves stability by protecting the redox-active molecules with the hydrophilic PEG tail to suppress parasitic reactions; and (2) increases the overall size to a mean hydrodynamic radius of 500 nm to mitigate the crossover issue via the physical blocking mechanism. Paired with a well-established potassium ferrocyanide catholyte, the micelle-based RFB in a pH-neutral aqueous electrolyte solution displayed an excellent capacity retention of 90.7% after 3600 charge/discharge cycles (28.3 days), corresponding to a capacity retention of 99.67% per day and 99.998% per cycle. Mechanistic studies of redox-active materials are also conducted during and after the cycling by proton nuclear resonance spectroscopy and cyclic voltammetry and indicate the absence of side reactions that are commonly observed in other RFBs. This work represents the longest consecutive running time and the highest charge/discharge cycles among the reported organic material-based aqueous RFBs. The outstanding performance of our RFB demonstrates the effectiveness of the micellization strategy for enhancing the performance of organic material-based aqueous RFBs.

The ability to store large amounts of electrical energy is of increasing importance with the growing fraction of electricity generation from intermittent renewable sources such as wind and solar. Wide-scale utilization of flow batteries is limited by the cost of redox-active metals such as vanadium or precious metal electrocatalysts. We have developed high performance flow batteries based on the aqueous redox behavior of small organic and organometallic molecules, e.g. [1-7]. These redox active materials can be very inexpensive and exhibit rapid redox kinetics and high solubilities, potentially enabling massive electrical energy storage at greatly reduced cost. We have developed new protocols for measuring capacity fade rates, which are particularly important for establishing very low capacity fade rates, and have discovered that the capacity fade rate is determined by the molecular calendar life, which can depend on state of charge, but is independent of the number of charge-discharge cycles imposed [6]. We will report the performance of the very few chemistries with long enough calendar life for practical application in stationary storage, and on the prospects for reversing capacity fade by recomposing decomposed molecules [7].
[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909

[2] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz and M.P. Marshak, "Alkaline Quinone Flow Battery", Science 349, 1529 (2015), http://dx.doi.org/10.1126/science.aab3033

[3] K. Lin, R. Gómez-Bombarelli, E.S. Beh, L. Tong, Q. Chen, A.W. Valle, A. Aspuru-Guzik, M.J. Aziz, and R.G. Gordon, "A redox flow battery with an alloxazine-based organic electrolyte", Nature Energy 1, 16102 (2016). http://dx.doi.org/10.1038/nenergy.2016.102

[4] E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon and M.J. Aziz, "A Neutral pH Aqueous Organic/Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). http://dx.doi.org/10.1021/acsenergylett.7b00019

[5] D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. DePorcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, “Alkaline Quinone Flow Battery with Long Lifetime at pH 12” Joule 2, 1907 (2018). https://doi.org/10.1016/j.joule.2018.07.005

[6] M.-A. Goulet & M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, J. Electrochem. Soc. 165, A1466 (2018). http://dx.doi.org/10.1149/2.0891807jes

[7] M.-A. Goulet, L. Tong, D.A. Pollack, D.P. Tabor, S.A. Odom, A. Aspuru-Guzik, E.E. Kwan, R.G. Gordon, and M.J. Aziz, “Extending the lifetime of organic flow batteries via redox state management” J. Am. Chem. Soc. 141, 8014 (2019); https://doi.org/10.1021/jacs.8b13295

Electrification of modern society including portable devices, electrical vehicles, and grid-scale storage has driven intensive research development of inexpensive, safe, long-cycling life, high performance energy storage technologies. The presentation will primarily discuss our recent research efforts in developing viologen and quinone anolyte materials for both anion and cation exchange Aqueous Organic Redox Flow Batteries (AORFBs) with a variety of catholyte materials including ferrocene, TEMPO, ferrocyanide and halides. Particularly, the presentation emphasizes that fundamental understandings of redox active electrolytes at molecular level are crucial to develop new generations of redox flow and Mg batteries for sustainable energy storage.

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We report novel hybrid solar-charged storage devices that integrate redox flow batteries (RFBs) and regenerative semiconductor solar cells that share the same pair of redox couples. In these integrated solar flow batteries (SFBs), photoexcited carriers are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to charge up the battery without external electric bias; which can be discharged to generate the electricity when electricity is needed. Carefully matching high performance III-V solar cells with various high voltage organic couples and optimizing several generations of SFB device designs enabled integrated SFB device with an overall direct solar-to-output electricity efficiency (SOEE) of 14%. We have further improved the cycling performance of the SFBs by integrating robust organic redox couples and photoelectrodes. I will also describe our recent effort in achieving a record 20% SOEE SFB with at least hundreds of hours of lifetime using novel tandem devices designed with the quantitative voltage matching model. Such highly efficient and robust SFBs that can be fabricated at low cost would enable practical distributed and standalone solar energy conversion and storage systems in remote locations.

This paper will detail the use of quantitative structure-property relationships for the design of next-generation redoxmers. This will include redoxmers design for fuel cells and redox flow batteries. The first half of the talk will discuss predictive modeling of nitroxyl radical-based redoxmers for the anode of fuel cells. The second half of the talk will discuss predictive modeling of electrolytes for non-aqueous redox flow batteries. This will discuss parameterization for improving stability and cyclability. The talk will finish with a discussion of future needs in redoxmer design and engineering.

Redoxmer is a term coined for redox active materials with tunable architectures. In nonaqueous redox flow batteries, the properties of redoxmers often dominate the cycling performance, including energy density, power, and cycle life. However, the choice of supporting electrolytes can also dramatically impact the properties of redoxmer via tuning solvating environment; therefore it is of crucial importance to build understanding of such phenomenon. In this talk, a systemically approach was adapted to probe solvating impact of 2,1,3-benzothiadiazole (BzNSN) in various supporting electrolytes. BzNSN is an energy dense anolyte molecule with a low redox potential. When cycled in flow cells, the choice of salts significantly impact the measured redox potentials and cycling life. As shown in Figure 1, when measured in electrolytes containing Li salts, BzNSN delivers increased redox potentials, implying Li⁺ involved interactions. Moreover, by changing cations of different sizes, such as Na⁺, K⁺, and NEt₄⁺, the redox potential of BzNSN shift nearly 200 mV from -1.65 V for Li⁺ to -1.85 V for NEt₄⁺. For an anolyte material, the lower redox potential usually leads to worse stability as it becomes thermal dynamically unstable. Surprisingly, this is not the case for BzNSN in electrolytes with different cations. Despites the lower redox potential, BzNSN radical cation delivers a much longer life time (t1/2 = 90 h) in 0.5 M KTFSI acetonitrile electrolyte than in LiTFSI acetonitrile electrolyte (t1/2 = 42 h). This is the first time that we observe an anolyte redoxmer can deliver a longer life time at a lower potential, which might be a practical approach for designing high energy density flow cells. Molecular dynamic simulation was used to explain the observed results.

Redox reaction involving charge transfer at the electrode-electrolyte interface represents an essential process for various electrochemical energy conversion and storage applications, such as fuel cell, electrolyzer and battery, etc. As a result, the operation (i.e. cell voltage, current density, number of charges, etc.) of the above devices is inherently dictated and constrained by the redox reactions. The redox-mediated process, a chemical reaction between an electrolyte-borne redox species electrochemically generated on electrode and a material (generally insoluble in electrolyte) off the electrode, provides additional flexibility in circumventing the constraints intrinsically confronted by the conventional electrochemical devices. One example is the redox targeting of energy storage materials for flow batteries. The redox-mediated reactions of high capacity solid material stored in the tank with redox electrolyte flowing through it considerably boost the energy density of redox flow battery without compromising its operation flexibility. Another example is redox-mediated oxygen evolution reaction (OER) for water electrolysis. The concurrent electrochemical-chemical cycle enables continuous reaction between an electrolyte-borne redox mediator and an OER catalyst loaded in a fixed-bed reactor spatially separated from the cell, which is believed to be advantageous to enhanced safety.

In this talk, I will report our latest advancement in the above area. In addition, I will briefly introduce some other studies on redox-mediated reactions, such as low-grade waste heat harnessing based on a thermal-electrochemical cycle and battery material recycling based on a one-way redox targeting reaction.

References:
M. Zhou, Q. Huang. T. N. P. Truong, J. Ghilane, Y. G. Zhu, C. Jia, R. Yan, L. Fan, H. Randriamahazaka, Q. Wang, Nernstian Potential-driven Redox Targeting Reactions of Battery Materials. Chem, 3 (6), 1036-1049 (2017).
R. Yan and Q. Wang, Redox-Targeting-Based Flow Batteries for Large-Scale Energy Storage. Adv. Mater., 30, 1802406 (2018).
Y. Chen, M. Zhou, Y. Xia, X. Wang, Y. Liu, Y. Yao, H. Zhang, Y. Li, S. Lu, W. Qin, X. Wu, and Q. Wang, A Stable and High Capacity Redox Targeting-based Electrolyte for Aqueous Flow Batteries. Joule, 3 (9), 2255-2267 (2019).
J. Yu, X. Wang, M. Zhou, and Q. Wang, A Redox Targeting-based Materials Recycling Strategy for Spent Lithium Ion Batteries. Energy & Environ. Sci., 12, 2672-2677 (2019).

Given the advantages of low cost, high concentration and potential biodegradability, the concept of deep eutectic solvents (DESs) is beneficial to developing cost-effective and sustainable batteries with high-energy density. In this talk, we will present our recent study on the Al- and Fe-based DESs as potential anolytes and catholytes, respectively, for hybrid redox flow batteries. Benefitted from special interactions between metal chlorides and urea, both Al and Fe DESs have high concentrations of active species, showing great potential to boost the energy density. Moreover, through investigating the redox chemistry and coordination chemistry of metal centers with possible additives, the stable deposition and stripping of Al and long cycling of Fe DES with high utilization are achieved. An all-DES-based hybrid flow battery taking advantages of the Al DES and Fe DES has been demonstrated with high performance. As the chemical and physical properties of DESs can be tuned by appropriate additives, it provides a new platform for developing promising redox flow batteries based on new chemistry.

Non-aqueous redox flow batteries (NRFBs) are attractive technologies that offer the promise of providing higher cell voltages than their aqueous counterparts. In NRFBs, redoxmers are charged and discharged through electrodes such as carbon. Interactions between the electrode structure and concentrated redoxmer solutions cause adsorption of these species, which may lead to subsequent degradation reactions and state-of-charge dependent phenomena. Likewise, the interaction of the redoxmers with highly polarized electrodes may lead to a modified chemical reactivity at the electrochemical interface. In this work, we will describe our experiments probing the chemical and electrochemical reversibility of catholyte redoxmers such as alkoxybenzene derivatives on model carbon electrodes. Using scanning electrochemical microscopy (SECM), we probe the real-time evolution of the reactivity of the redoxmer-carbon interface by measuring the local redox kinetics, film formation processes, and the generation of reactive intermediates. Chemical interactions at the interface were further probed via in-situ Raman electrochemical measurements, and the newly developed Raman-SECM approach. We will describe the reactivity of different carbon features, as well as the interactions with model redoxmers. SECM revealed a wealth of reversible and irreversible changes in the kinetics of electron transfer, including the observation of transient enhancements in electrochemical rates and slow passivation processes. Simultaneous operation of SECM and Raman also created new opportunities to probe the impact of reactive radical cations on the chemistry of the carbon host and on film-formation processes through electropolymerization. These studies help direct the design of redoxmers with increased stability at the electrode-electrolyte interface and reveal the complex interplay of redoxmer and electrode chemistries at NRFB structures

The discovery of electrolytes that have low vapor pressures, wide electrochemical window and high solubility towards redox active species with an ability to undergo multiple electron transfer reactions is a challenge to realize large-scale energy storage. Deep eutectic solvents (DESs), which are eutectic mixtures of hydrogen bond donors and acceptors, are attractive electrolytes as they have good solvent strength that is also tunable along with other physical, thermal, electrochemical and transport properties. We have examined the electrochemical behavior of various organic redox active solutes, specifically methyl viologen (MV), quinones and nitroxyl radicals, in a common DES known as ethaline; a 1 to 2 molar mixture of choline chloride and ethylene glycol. While the reactivity of most quinones investigated are found to fall outside of the electrochemical window of ethaline, nitroxy radicals and MV provide reversible redox couples that are practical. In particular, TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) demonstrates two single electron transfer reactions separated by 1.3 V in ethaline. However, the solubility of TEMPO in ethaline is found to be limited to 0.17 M at room temperature. The hydrogen bonding achieved by –OH substitution on TEMPO and electrostatic interactions achieved by the functionalization of halide salts with TEMPO are shown to improve solubility in ethaline close to 2 M. This study illustrates that chemical modification of redox active organic molecules impacts both their solubility in a DES electrolyte, as well as their redox performance and physical properties of the solutions.

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Here, we show that enhanced electrical potential differences (i.e., streaming potential) may be obtained through the flow of salt water on liquid-filled surfaces that are infiltrated with a lower dielectric constant liquid, such as oil, to harness electrolyte slip and associated surface charge. A record-high figure of merit, in terms of the voltage generated per unit applied pressure, of 0.04 mV/Pa is obtained through the use of the liquid-filled surfaces. In comparison with air-filled surfaces (AFS), the figure of merit associated with the liquid-filled surface (LFS) increases by a factor of 1.4. These results lay the basis for innovative surface charge engineering methodology for the study of electrokinetic phenomena at the microscale, with possible application in new electrical power sources.

Our work has experimentally demonstrated the largest figure of merit thus far, to the best of our knowledge, with primary focus on methodologies related to enhance the streaming potential (V_s) per unit pressure difference (ΔP) through the use of LFS. The use of the LFS yields a figure of merit increase in comparison to that obtained using an AFS. It has been shown that larger voltages, through a measured streaming potential, may be achieved through careful engineering of the coupled electric field and fluid flow. The application of the related increase in the electrokinetic energy conversion efficiency would need further optimization of the fluidic and electrical impedances, in concert with the streaming conductance, as matched to an appropriate load. Concomitantly, unipolar transport (where for example, either Na⁺ or Cl⁻ ions are transported) through EDL (electrical double layer) overlap in nanoscale channels may be coupled with LFS to yield much larger voltages, comparable to that of batteries.

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. A significant enhancement in the streaming potential (V_s) was obtained in experiments considering the flow of electrolyte over liquid-filled surfaces (LFSs), where the grooves in patterned substrates are filled with electrolyte immiscible oils. Such LFSs yield larger V_s (by a factor of 1.5) compared to superhydrophobic surfaces, with air-filled grooves, and offer tunability of electrokinetic flow. It is shown that the density, viscosity, conductivity, as well as the dielectric constant of the filling oil, in the LFS, determine V_s. Relating a hydrodynamic slip length to the obtained V_s offers insight into flow characteristics, as modulated by the liquid interfaces in the LFS.

A significant enhancement of solar irradiation induced evaporation of water, and ethanol–water mixtures, through the use of carbon foam based porous media, is demonstrated. A relationship between the consequent rate of mass loss, with respect to the equilibrium vapor pressure, dynamic viscosity, surface tension, and density, was developed to explain experimental observations. The evaporative heat loss was parametrized through two convective heat transfer coefficients—one related to the surface and another related to the vapor external to the surface. The work promotes a better understanding of thermal processes in binary liquid mixtures with applications ranging from phase separation to distillation and desalination.

Desalination of seawater has emerged as an area of critical need, but remains challenging, to solve the fresh water supply problem caused by the steady growth of human population and industry. Although the previous research efforts led to development of a number of desalination methods, for instance distillation, reverse osmosis, electrodialysis and capacitive deionization, their applications were largely restricted by the inherent energy efficiency and effectiveness issues. Thus, there is a demand of new methods that allow efficient and effective separation of salt ions from water.
In this talk we review recent progress of water desalination research based on battery mechanism, and report our efforts in developing flow and redox-based electrochemical systems for water desalination application. Specifically, we have prepared FePO₄-based electrode material for sodium ion removal and copper-based electrode material for chloride ion removal. An electrochemical cell composing of the two electrode materials has been demonstrated with promising desalination capacity and efficiency.

Electrochemical conversion of CO₂ into useful products with good efficiency and high selectivity presents challenges due to the distinctive bonding strengths of the various intermediates and products. Reducing CO₂ into C-C coupled products, such as ethylene, ethanol and other hydrocarbons or oxygenates with 2 or more carbon atoms, is particularly interesting technically and scientifically. However, because of the so-called “scaling relationship”, tuning the selectivity on a single catalyst surface is rather difficult as the intermediates leading to specific products have similar binding energies. Cascade catalysis breaks this scaling relationship by separating the catalysis processes into two or more steps on different catalyst surfaces with transport of the intermediates between them.

Here, we report a flow cell with a cascade configuration of two closely spaced working electrodes (Ag and Cu) for electrochemical reduction of CO₂ into C-C coupled products. The Ag electrode is located upstream in the reactor to convert CO₂ into CO, and CO is convectively transported to the downstream Cu electrode for further C-C coupling catalytic reactions. The porous Ag and Cu working electrodes are separately controlled by two different potentiolstat channels allowing for precise control of the conversion rates in the cascade. The porosity of the metal electrodes was controlled by precision laser drilling. A COMSOL model was developed using preliminary experimental data to tune the cell design and operating conditions, such as the flow rate of electrolyte and optimal pore size of the working electrodes. When both Ag and Cu electrodes are activated, CO is generated at Ag and is transported to Cu, leading to a significant increase in generated C2-C3 oxygenates compared to control experiments with only the Cu activated. The flow cell configuration can also be extended to other working electrodes by using carbon cloth as a supporting substrate and depositing powder catalysts.

Lime scaling, the precipitation of calcium salts, is a major hindrance encountered in desalination processes. Recently¹, nanopore precipitation has been shown to cause oscillation of ion-current through nanopore membranes, a similar model system. Unfortunately, due to the geometry of the pore, direct observation of the precipitates is not possible, so the exact structure is unknown. We report on progress towards direct observation measurements, with transmission electron microscopy (TEM), of the nanoprecipitation that causes ion current oscillations. We have engineered a novel, monolithic in-situ TEM liquid cell that overcomes common shortcomings of other two-piece TEM liquid cells such as liquid thickness and alignment. The customizable geometry and reproducibility of this TEM liquid cell enables investigation of various chemistries and confinement physics.
1. Powell, M. R. et al. Nanoprecipitation-assisted ion current oscillations. Nat. Nanotechnol. 3, 51–57 (2008).

Layered double hydroxide (LDH) has been proven to have excellent performance in water splitting reactions, especially toward the oxygen evolution reaction (OER).The morphology of NiFe LDH is one of the key factors in improving its electrochemical performance. However, the controlling factors and mechanisms of morphological evolution of the nanoflower-shaped NiFe LDH remain poorly understood. In this study, NiFe LDH nanoflowers were synthesized by hydrothermal method, and the morphology and structure of NiFe LDH crystals are carefully controlled during the synthetic stage. The results indicate that NiFe LDH primary nano flakes rapidly agglomerate to form a nucleus matrix at the initial stage. Subsequently, these nano flakes are laterally aggregated and connected orderly on the substrate to form nanoflowers, and the nano-petals are generated by a rotating and edge-to-edge orientation attachment (OA) mechanism. The morphological evolution occurred by staged three-dimensional OA process plays an essential role in the self-assembly of flower-like NiFe LDH crystals. The well-formed NiFe LHD nanoflowers, grown by 10 hours of hydrothermal treatment, exhibits excellent OER performance in alkaline electrolyte, which displays very low overpotential of 190 mV at the current density of 10 mA cm^-2 and small Tafel slope of 35 mV per decade (much better than RuO₂ catalyst). Furthermore, the prepared NiFe LDH still showed high stability after 10,000 cycles of cyclic voltammetry tests. This work provides new insights into the relationship between catalyst morphology and OER performance, and provides new fundamental understanding of hydrothermal synthetic process of NiFe LDH.

Some emerging alloys, such as Mg alloy, are susceptible to galvanic corrosion, consequently accelerating corrosion process and resulting in severe financial loss. The galvanic corrosion potential and current obtained by experimental polarization curves could differ a lot between different literature. Herein, we proposed a semi-empirical model based on the mixed potential theory and first principles calculation to analyze the galvanic corrosion of the metal alloys. Our model is further validated in the case of Mg-Ge alloys, which is composed of anode Mg matrix and cathode Mg₂Ge second phase. The combination of the large anode equilibrium potential difference between Mg and Mg₂Ge, and the Schottky barrier across the interface indicates that the Mg₂Ge second phase can prevent the Mg grain from serving as the cathode and impede the electron transfer between the Mg grains. First principles calculations on the kinetics of hydrogen evolution reaction upon Mg₂Ge reveal that the rate-determining step is the hydrogen adsorption, which is extremely energetically unfavored but an inevitable intermediate state. The estimated exchange current of the hydrogen evolution upon Mg₂Ge is about 3 orders of magnitude smaller than that on pure Mg, depressing the hydrogen exchange current upon Mg₂Ge and hence the galvanic corrosion of the Mg-Ge alloys. Moreover, some other Mg alloys, such as Mg-Zn and Mg-Sc, were also investigated, which is in close agreement with the experimental observations. Our model is capable of predicting the galvanic corrosion behavior and provide a promising perspective for designing better corrosion-resistant metal alloys.

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We report novel hybrid solar-charged storage devices that integrate redox flow batteries (RFBs) and regenerative semiconductor solar cells that share the same pair of redox couples. In these integrated solar flow batteries (SFBs), photoexcited carriers are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to charge up the battery without external electric bias; which can be discharged to generate the electricity when electricity is needed. Carefully matching high performance III-V solar cells with various high voltage organic couples and optimizing several generations of SFB device designs enabled integrated SFB device with an overall direct solar-to-output electricity efficiency (SOEE) of 14%. We have further improved the cycling performance of the SFBs by integrating robust organic redox couples and photoelectrodes. I will also describe our recent effort in achieving a record 20% SOEE SFB with at least hundreds of hours of lifetime using novel tandem devices designed with the quantitative voltage matching model. Such highly efficient and robust SFBs that can be fabricated at low cost would enable practical distributed and standalone solar energy conversion and storage systems in remote locations.

Non-aqueous redox flow batteries (NRFBs) are attractive technologies that offer the promise of providing higher cell voltages than their aqueous counterparts. In NRFBs, redoxmers are charged and discharged through electrodes such as carbon. Interactions between the electrode structure and concentrated redoxmer solutions cause adsorption of these species, which may lead to subsequent degradation reactions and state-of-charge dependent phenomena. Likewise, the interaction of the redoxmers with highly polarized electrodes may lead to a modified chemical reactivity at the electrochemical interface. In this work, we will describe our experiments probing the chemical and electrochemical reversibility of catholyte redoxmers such as alkoxybenzene derivatives on model carbon electrodes. Using scanning electrochemical microscopy (SECM), we probe the real-time evolution of the reactivity of the redoxmer-carbon interface by measuring the local redox kinetics, film formation processes, and the generation of reactive intermediates. Chemical interactions at the interface were further probed via in-situ Raman electrochemical measurements, and the newly developed Raman-SECM approach. We will describe the reactivity of different carbon features, as well as the interactions with model redoxmers. SECM revealed a wealth of reversible and irreversible changes in the kinetics of electron transfer, including the observation of transient enhancements in electrochemical rates and slow passivation processes. Simultaneous operation of SECM and Raman also created new opportunities to probe the impact of reactive radical cations on the chemistry of the carbon host and on film-formation processes through electropolymerization. These studies help direct the design of redoxmers with increased stability at the electrode-electrolyte interface and reveal the complex interplay of redoxmer and electrode chemistries at NRFB structures

Desalination of seawater has emerged as an area of critical need, but remains challenging, to solve the fresh water supply problem caused by the steady growth of human population and industry. Although the previous research efforts led to development of a number of desalination methods, for instance distillation, reverse osmosis, electrodialysis and capacitive deionization, their applications were largely restricted by the inherent energy efficiency and effectiveness issues. Thus, there is a demand of new methods that allow efficient and effective separation of salt ions from water.
In this talk we review recent progress of water desalination research based on battery mechanism, and report our efforts in developing flow and redox-based electrochemical systems for water desalination application. Specifically, we have prepared FePO₄-based electrode material for sodium ion removal and copper-based electrode material for chloride ion removal. An electrochemical cell composing of the two electrode materials has been demonstrated with promising desalination capacity and efficiency.

The ability to store large amounts of electrical energy is of increasing importance with the growing fraction of electricity generation from intermittent renewable sources such as wind and solar. Wide-scale utilization of flow batteries is limited by the cost of redox-active metals such as vanadium or precious metal electrocatalysts. We have developed high performance flow batteries based on the aqueous redox behavior of small organic and organometallic molecules, e.g. [1-7]. These redox active materials can be very inexpensive and exhibit rapid redox kinetics and high solubilities, potentially enabling massive electrical energy storage at greatly reduced cost. We have developed new protocols for measuring capacity fade rates, which are particularly important for establishing very low capacity fade rates, and have discovered that the capacity fade rate is determined by the molecular calendar life, which can depend on state of charge, but is independent of the number of charge-discharge cycles imposed [6]. We will report the performance of the very few chemistries with long enough calendar life for practical application in stationary storage, and on the prospects for reversing capacity fade by recomposing decomposed molecules [7].
[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909

[2] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz and M.P. Marshak, "Alkaline Quinone Flow Battery", Science 349, 1529 (2015), http://dx.doi.org/10.1126/science.aab3033

[3] K. Lin, R. Gómez-Bombarelli, E.S. Beh, L. Tong, Q. Chen, A.W. Valle, A. Aspuru-Guzik, M.J. Aziz, and R.G. Gordon, "A redox flow battery with an alloxazine-based organic electrolyte", Nature Energy 1, 16102 (2016). http://dx.doi.org/10.1038/nenergy.2016.102

[4] E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon and M.J. Aziz, "A Neutral pH Aqueous Organic/Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). http://dx.doi.org/10.1021/acsenergylett.7b00019

[5] D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. DePorcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, “Alkaline Quinone Flow Battery with Long Lifetime at pH 12” Joule 2, 1907 (2018). https://doi.org/10.1016/j.joule.2018.07.005

[6] M.-A. Goulet & M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, J. Electrochem. Soc. 165, A1466 (2018). http://dx.doi.org/10.1149/2.0891807jes

[7] M.-A. Goulet, L. Tong, D.A. Pollack, D.P. Tabor, S.A. Odom, A. Aspuru-Guzik, E.E. Kwan, R.G. Gordon, and M.J. Aziz, “Extending the lifetime of organic flow batteries via redox state management” J. Am. Chem. Soc. 141, 8014 (2019); https://doi.org/10.1021/jacs.8b13295

Electrification of modern society including portable devices, electrical vehicles, and grid-scale storage has driven intensive research development of inexpensive, safe, long-cycling life, high performance energy storage technologies. The presentation will primarily discuss our recent research efforts in developing viologen and quinone anolyte materials for both anion and cation exchange Aqueous Organic Redox Flow Batteries (AORFBs) with a variety of catholyte materials including ferrocene, TEMPO, ferrocyanide and halides. Particularly, the presentation emphasizes that fundamental understandings of redox active electrolytes at molecular level are crucial to develop new generations of redox flow and Mg batteries for sustainable energy storage.

Redoxmer is a term coined for redox active materials with tunable architectures. In nonaqueous redox flow batteries, the properties of redoxmers often dominate the cycling performance, including energy density, power, and cycle life. However, the choice of supporting electrolytes can also dramatically impact the properties of redoxmer via tuning solvating environment; therefore it is of crucial importance to build understanding of such phenomenon. In this talk, a systemically approach was adapted to probe solvating impact of 2,1,3-benzothiadiazole (BzNSN) in various supporting electrolytes. BzNSN is an energy dense anolyte molecule with a low redox potential. When cycled in flow cells, the choice of salts significantly impact the measured redox potentials and cycling life. As shown in Figure 1, when measured in electrolytes containing Li salts, BzNSN delivers increased redox potentials, implying Li⁺ involved interactions. Moreover, by changing cations of different sizes, such as Na⁺, K⁺, and NEt₄⁺, the redox potential of BzNSN shift nearly 200 mV from -1.65 V for Li⁺ to -1.85 V for NEt₄⁺. For an anolyte material, the lower redox potential usually leads to worse stability as it becomes thermal dynamically unstable. Surprisingly, this is not the case for BzNSN in electrolytes with different cations. Despites the lower redox potential, BzNSN radical cation delivers a much longer life time (t1/2 = 90 h) in 0.5 M KTFSI acetonitrile electrolyte than in LiTFSI acetonitrile electrolyte (t1/2 = 42 h). This is the first time that we observe an anolyte redoxmer can deliver a longer life time at a lower potential, which might be a practical approach for designing high energy density flow cells. Molecular dynamic simulation was used to explain the observed results.

The discovery of electrolytes that have low vapor pressures, wide electrochemical window and high solubility towards redox active species with an ability to undergo multiple electron transfer reactions is a challenge to realize large-scale energy storage. Deep eutectic solvents (DESs), which are eutectic mixtures of hydrogen bond donors and acceptors, are attractive electrolytes as they have good solvent strength that is also tunable along with other physical, thermal, electrochemical and transport properties. We have examined the electrochemical behavior of various organic redox active solutes, specifically methyl viologen (MV), quinones and nitroxyl radicals, in a common DES known as ethaline; a 1 to 2 molar mixture of choline chloride and ethylene glycol. While the reactivity of most quinones investigated are found to fall outside of the electrochemical window of ethaline, nitroxy radicals and MV provide reversible redox couples that are practical. In particular, TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) demonstrates two single electron transfer reactions separated by 1.3 V in ethaline. However, the solubility of TEMPO in ethaline is found to be limited to 0.17 M at room temperature. The hydrogen bonding achieved by –OH substitution on TEMPO and electrostatic interactions achieved by the functionalization of halide salts with TEMPO are shown to improve solubility in ethaline close to 2 M. This study illustrates that chemical modification of redox active organic molecules impacts both their solubility in a DES electrolyte, as well as their redox performance and physical properties of the solutions.