Symposium ES2 : Materials Challenges for Flow-Based Energy Conversion and Storage

Flow batteries are one of the most promising storage technologies for electricity generated by renewable sources. State-of-the-art all-vanadium redox-flow batteries (VRFBs) and zinc/bromine hybrid-flow batteries (HFBs) utilize dangerous and highly corrosive acidic electrolytes, and, therefore, cannot be taken into consideration as “green” energy storage systems. Organic charge storage materials improve the socio-economic footprint in reference to conventional flow batteries, since they rely on inexpensive, save and sustainable electrolytes. In terms of system capital costs VRFBs suffer from expensive raw material prices for pure vanadium pentoxide and perfluorinated proton exchange membranes. In contrast, organic charge storage materials can be obtained via petrochemistry or even from renewable resources at lower costs. In addition, polymeric charge storage materials allow the application of low cost dialysis membranes. These polymer-based redox-flow batteries (pRFBs) offer an economically reasonable and environmentally friendly battery operation.
Several polymers were developed in our group for the utilization in pRFBs. They were optimized in terms of electrochemical reversibility, solubility and viscosity. Therefore, different solubility promoting co-monomers and polymerization procedures were investigated, yielding tailored polymers for the application as cathodic or anodic charge storage materials. Also the combination of polymeric active materials with zinc to semi-organic polymer HFBs offers several opportunities like an increased cell voltage of 1.7 V, an extended electrochemical operation window of up to 2.1 V in water, low acquisition costs and insensibility against atmospheric oxygen. First investigations were performed on special polymer architectures like micelles in order to reduce the viscosity of the electrolyte and on polymers containing bipolar redox active pendant groups that can be used both as cathode and anode active material.


References:
[1] T. Janoschka et al., Nature 2015, 527, 78.
[2] T. Janoschka et al., Polym. Chem. 2015, 6, 7801.
[2] J. Winsberg et al., Adv. Mater. 2016, 28, 2238.
[3] J. Winsberg et al., Polym. Chem. 2016, 7, 1711.
[4] J. Winsberg et al., Chem. Mater. 2016, 28, 3401.

Energy storage concentration cells are based on the concentration gradient of redox-active reactants, where the increased entropy is transformed into electric energy as the concentration gradient reaches equilibrium between two half cells. However, the diffusion-limited low concentration gradient in electrolytes confines their energy density, while low voltage output and short lifetime have precluded many applications of concentration cells towards the utilization of chemical energy. Here we report a recyclable and flow-controlled magnetic electrolyte concentration cell. The hybrid inorganic-organic nanocrystal based electrolyte, consisting of molecular redox-active ligands adsorbed on the surface of magnetic nanocrystals, leads to a magnetic field driven concentration gradient of redox molecules. The energy storage performance of concentration cells is dictated by magnetic characteristics of cobalt ferrite nanocrystal carriers. The enhanced conductivity and kinetics of redox-active electrolytes could further induce a sharp concentration gradient to improve the energy density and voltage switching of magnetic electrolyte concentration cells. This study provides a new and viable strategy to magnetically control electrical generation and storage by concentrating non-magnetic redox-active molecules through particle-surface ligand interactions.

Among the grid-scale energy storage options such as pumped hydroelectric, compressed air and lithium ion batteries, redox flow batteries (RFBs) offer a number of attractive features including long cycle lives, and improved energy management as a consequence of the decoupling of power and energy. Commercially available RFBs are based on aqueous electrolytes, consequently the cell potential is limited by the stability window of water (1.23V). Efforts to increase energy density and reduce cost have focused on non-aqueous chemistries with cell potentials that can approach 5V. Despite their promise, there are significant materials limitations associated with non-aqueous RFBs including the lack of active species that are sufficiently robust to achieve cycling and efficiency targets, and permselective membranes that minimize parasitic losses due to cross-over and irreversible reactions. Metal coordination complexes (MCCs) offer the possibility of multiple electron transfers, high solubilities in non-aqueous solvents and low cost. This presentation will review our work to develop MCCs and novel nanoporous membranes to meet targets for the commercialization of non-aqueous RFBs. A particular focus for work has been the development of structure-composition-function relationships for complexes including acetylacetonate, terperidine, and bipyridylimino isoindoline ligands based on experimental and computational results. This information informs our design of new, better performing MCCs. The nanoporous membranes are based on nanofiber based mats produced using layer-by-layer and spin coating methods. We are also exploring a common ion design that should reduce the cost of RFBs.

Aqueous organic redox-active electrolytes are steadily becoming viable alternatives to metal- and halogen-based chemistries for aqueous redox flow batteries. However, the cell hardware, particularly the membrane-separator, remain cost- and performance-limiting components. Large, non-toxic, non-corrosive redox-active molecules present a new opportunity for exploring alternative membrane-separators with potentially lower cost and improved performance. Inexpensive membranes including hydrocarbon-based chemistries are evaluated in comparison to perflourinated membranes suggesting the opportunity for cost reductions without sacrificing cell performance. We systematically study the permeation rates of several redox-active organic molecules through various membranes after different pretreatments. We find many orders of magnitude variation in permeability among molecules, and interpret the results in terms of size exclusion and charge exclusion effects. Full alkaline cells with alternative membranes have been evaluated showing, at the time of submission of this abstract, >98% current efficiency and >65% energy efficiency at 0.25A/cm² with <0.04% capacity fade/cycle. These results highlight paths toward low-cost, high-performance aqueous organic redox flow batteries.

We have recently proposed a disruptively different redox flow battery system on the basis of a novel concept — “redox targeting reaction” of battery materials, which holds great promise for enhancing the energy density and bringing down the cost of flow batteries. This new concept can be applied to different battery chemistries, so that a versatile “Redox Flow X-Battery” (RFXB, X represents different battery chemistries) can be developed. With lithium-ion battery chemistry as an example, high energy density redox flow lithium battery (RFLB) has been successfully demonstrated. The active materials in RFLB are stored statically in two separate tanks while power is generated in the cell stack by the redox reactions of redox mediators. The transport of electrons between the active materials and the current collector is mediated by the circulation of redox shuttle molecules in the electrolyte fluids. Compared with vanadium flow battery, the energy of RFLB is not stored in the electrolyte fluids but in the solid lithium-ion battery materials kept in the tanks. So the energy density of RFLB is much higher than the conventional liquid redox flow battery. Besides high energy density, other advantages of RFLB include: (1) Compared with the commercial lithium ion batteries, no binder and bulky conducting additives are required, which makes the loading of energy storage materials very easy. (2) RFLB has greater tolerance to the volume variations of electrodes during repeated lithiation/delithiation cycles — one of the most challenging technology barriers for achieving long cycle life. (3) RFLB has greater tolerance to overcharging/ overdischarging and hence safety, since the active materials are not directly charged or discharged. Here the latest progress on the development of RFLB half-cell, single full cell and cell stack will be reported. This involves the extensive investigations of various cathodic and anodic Li⁺ storage materials and the matched redox mediators, as well as Li⁺-conducting membranes. In addition, the application of redox targeting concept to other battery chemistries, such as Li-O₂ batteries will also be discussed.

Demand for large-scale energy storage is increasing, especially as renewable energy resources, such as solar and wind, become more prevalent. These variable energy generators require that large-scale energy storage is used to arbitrage and minimize fluctuations onto the grid. As a promising candidate for energy storage and load leveling, redox flow batteries (RFBs) have been considered. However, due to the challenging issues such as low cell performance, durability, and high electrolyte cost, their wide-spread adoption has not occurred. Aqueous RFBs are promising for large-scale energy storage, due to their high power and energy density, potential for very large (MWh) systems, suitability for long discharge times, and favorable cost and safety. RFB research at LBNL focuses on cell-level and component-level analysis, diagnostics, and optimization. Substantial improvements to power density and energy efficiency have been achieved, and principles for optimizing cell metrics and improving durability have been elucidated. An overview of our work on the specific cell chemistries listed will be provided.
The main research focus is on the hydrogen-bromine system. It has highly reversible and kinetically favored electrochemical reactions, soluble chemical species, high current-density operation, and a relatively inexpensive electrolyte. We have shown that the optimization and understanding of the primary losses can result in power densities of 1.5 W/cm² and limiting current densities over 4 A/cm² for discharge at ambient conditions. In addition, other hydrogen chemistries will be discussed including inexpensive (H₂-Fe-ion) and high-voltage (H₂-Ce) systems. The various performances and tradeoffs of these systems will be detailed. Finally, the various results will be put into context using experiments and simulations to elucidate the most critical issues in the field and outline areas of needed research, especially from the materials perspective.

Electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required components to enhance performance of energy conversion and storage technologies. Noble metals, e.g. Pt, and their oxides such as Ir- and Ru-based materials show high activity for either the ORR or OER. Along with this, nanocrystalline structure of noble metal-based electrocatalysts (e.g. Pt, Pt₃Ni, Pd-Pt) have been studied their catalytic activities depending on various facets. However, the ORR and OER activities with diverse facets of bi-functional electrocatalysts have rarely reported owing to the difficulty in controlling morphology.
Herein, we firstly demonstrate the relationship between bi-functional catalytic activities and various facets of polyhedral-shaped pyrochlore oxides (Bi₂Ru₂O₇) electrocatalysts. Notably, the pyrochlore oxides show microcrystalline structure consisting of (100), (110), (111) and (522) planes. According to enclosed type of facets, they are classified into low- and high-index planes dominant groups. Among them, low-index planes dominant polyhedral-shaped Bi₂Ru₂O₇ shows outstanding ORR and OER activities in half- and full-cells for primary and rechargeable Zn–air flow batteries.

Al-air batteries have been considered as a promising candidate for efficient power delivery in the field of transportation and uninterrupted power supply (UPS) owing to its high specific energy (theoretically 8,100 Wh kg⁻¹) with abundant reserves, low cost and lightweight of aluminum. However, there are intrinsic problems in Al-air batteries such as a sluggish rate of the oxygen reduction reaction (ORR) and precipitation of byproducts (e.g. Al₂O₃, AlOH₃) for the broad application and further commercialization.
Herein we suggest the relationship between electronegativity (x) and catalytic activity of silver-based electrocatalysts by scanning transition metals from group 6 to 9 (Cr, Mn, Fe, and Co). Further, we fabricated the high energy aluminum air flow batteries with alloy-based silver manganese oxides electrocatalysts. Insufficient electrical conductivities of manganese oxide have been overcome by combining Ag that has lower resistivity than Pt. Notably, the intrinsic precipitation problems have been solved by introducing new aluminum air flow batteries system, which showed discharge time of 7.3 hours and specific energy of 2552 Wh kg⁻¹ at a current density of 100 mA cm⁻². In consequence, the aluminum air flow batteries are promising technology for next generation energy conversion devices.

The escalation of power system promotes the development of battery technologies with its huge application market. Redox flow batteries (RFBs) are very attractive to customers in the energy grid system, and their noticeable technological innovations in the past decades have driven them to gradually replace the conventional energy storage methods under certain circumstance. Portable batteries used in electronics and fully electric vehicles ought to be designed to fulfill the growing desires on supplying energy quicker and longer. Therefore, many significant improvements have been achieved on battery manufacture and mechanism optimization.

Here, the first fully-flow-able zinc nickel flow battery (ZNFB) is proposed, whose performance is supposed to be suitable for various application scales. Based on the semi-solid fuel cell (SSFC) technology concept, we incorporated the beneficial features of conventional aqueous Zn/Ni chemistry (essentially sustainable, eco-friendly and deposit-abundant) into the fuel cell structure to effectuate a “hybrid” flow battery system. With the nano-sized carbon network, the balance between electron transfer and ion transfer were obtained in three-dimensions space. Basic electrochemical tests have demonstrated the feasibility of the ZNFB technology. Fundamental characteristics of suspension electrodes were studied and analyzed to improve the electrochemical properties of the ZNFB system. More researches are undergoing on the electrode optimization and new system layout design to overcome the interaction effects between the electronic conductivity and the ionic conductivity, which is caused by the unique “semi-solid” electrode design of SSFC technology.

Redox flow batteries (RFBs) offer several advantages over enclosed batteries in grid-scale applications such as decoupled power rating (reactor size) and energy capacity (tank size), high active-to-inactive materials ratio (especially at long durations), and improved safety characteristics.¹ Recent literature has focused on emerging aqueous organic RFBs (AORFBs) that employ abundant redox active organic species dissolved in inexpensive acidic electrolytes. Anthraquinone disulfonic acid (AQDS) is considered a benchmark compound for AORFBs^2–5 due to its good charge retention (days), low crossover rates (through Nafion), high solubility (>1 M), and low redox potential (~0.2 V vs RHE). Some studies report idealized electrochemical behavior of AQDS at low concentrations (1 mM),⁶ however, at higher concentrations AQDS deviates from ideal behavior and undergoes intramolecular reactions, which contribute to changes in storage capacity, redox potential, and solubility.

Here, we examine the complexities of AQDS in aqueous electrolytes utilizing cyclic voltammetry (CV), bulk electrolysis (BE), chemical titration, and nuclear magnetic resonance (NMR) spectroscopy. Depending on the AQDS concentration and electrolyte choice, the observed electrochemical behavior can vary indicating that different mechanistic pathways are accessed. To elucidate these reaction mechanisms, we synthesize various derivatives of AQDS and examine their electrochemistry, via CV and BE, as well as their chemical structures via NMR. Furthermore, we correlate the chemical structure of AQDS in solution with the observed electrochemistry to understand variations in reversible capacity and to provide guidelines for maximizing storage performance. The complex, solution-phase interactions experienced by AQDS suggest that other novel organic compounds may display similar, or more convoluted, phenomena. To this end, the suite of chemical and electrochemical methods presented here are portable and can be applied to other candidate materials for RFBs to better understand inconsistencies in storage capacity, solubility, and stability.

Acknowledgments
We thank the Joint Center for Energy Storage Research for financial support. This research was conducted with Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a.

References
1. Su, L., et al. Rechargeable Batteries (eds. Zhang, Z. & Zhang, S. S.) 673–712 (Springer International Publishing, 2015).
2. Huskinson, B. et al. Nature 505, 195–198 (2014).
3. Yang, B., et al. J. Electrochem. Soc. 161, A1371–A1380 (2014).
4. Lin, K. et al. Science 349, 1529–1532 (2015).
5. Yang, B. et al. J. Electrochem. Soc. 163, A1442–A1449 (2016).
6. Batchelor-McAuley, C. et al. J. Phys. Chem. B 114, 4094–4100 (2010).

Redox flow batteries have shown great prospects in grid-scale stationary energy storage applications. The decoupling of the energy and power allows independent scaling of tank volume and stack size, which offers tremendous design flexibility to meet different energy/power ratio requirements. Traditional flow batteries are based on inorganic redox materials but generally suffer limited solubility, low electrochemical reactivity and/or high cost. On the contrary, electrochemically active organic compounds are promising alternative material candidates because of their molecular diversity, structural tailorability, environmental benignity, and potentially low cost. There has merged a gradual shift to organics-based redox flow chemistries to push the boundaries of grid storage technology development.^1,2
Here we report a series of organic redox flow chemistries in nonaqueous electrolytes.³ Promising redox material candidates have been identified with desirable redox potentials, high solubilities, and good electrochemical properties. Flow cells of these systems resulted in remarkable cell efficiencies, high rate performance, and great cycling stability. Rational molecular engineering has been demonstrated as a general strategy to improve the battery performance and durability. Moreover, we have developed FTIR-based diagnostics of the state of charge (SOC) of a nonaqueous symmetric organic flow battery.⁴
References
(1) K. Gong, Q. Fang, S. Gu, S. Li, and Y. Yan, Energ. Environ. Sci. 2015, DOI: 10.1039/C5EE02341F.
(2) W. Wang and V. Sprenkle, Nat. Chem. 2016, 8 (3), 204.
(3) X. Wei, W. Xu, J. Huang, L. Zhang, E. Walter, C. Lawrence, M. Vijayakumar, W. A. Henderson, T. Liu, L. Cosimbescu, B. Li, V. Sprenkle, and W. Wang, Angew. Chem. Int. Ed. 2015, 54, 8684.
(4) W. Duan, R. S. Vemuri, J. D. Milshtein, S. Laramie, R. D. Dmello, J. Huang, L. Zhang, D. Hu, M. Vijayakumar, W. Wang, J. Liu, R. M. Darling, L. Thompson, K. Smith, J. S. Moore, F. R. Brushett, and X. Wei J. Mater. Chem. A, 2016, 4, 5448 – 5456.

Nonaqueous redox flow batteries (NAqRFBs) are an emerging class of grid-scale, electrochemical energy storage devices that promise high cell potentials and a broad materials space. Although NAqRFBs could achieve future prices approaching $100 kWh^-1, current prices are substantially higher, and chemical cost contributions must decrease significantly to reach commercial viability [1]. Several studies (e.g., [2]) sought to increase the charge stored per unit mass of active species as a pathway to decrease cost. To date, however, no studies have considered designing a supporting electrolyte to reduce chemical costs. Typical fluorinated salts for NAqRFBs will likely have higher costs than the active species [1], and increased salt concentrations can suppress active species solubility, limiting energy density [3]. Reducing salt content, while maintaining electroneutrality and conductivity, is an overlooked challenge. Here, we analyze the importance of common-ion cell configurations, which enable lean electrolytes, for NAqRFBs.
The majority of reported NAqRFBs employ salt-splitting cell configurations, where, upon charging, the positive active species oxidizes to a cation and the negative active species reduces to an anion. This charge distribution necessitates that salt cations and anions migrate to the negative and positive electrolytes, respectively. Salt-splitting cells require a minimum 1:1 molar ratio of salt-to-active species to maintain electroneutrality, and ion-exchange membranes are typically not designed to transfer both cations and anions. Contrastingly, common-ion RFBs use at least one ionic active species at 0% state-of-charge (SOC), and the active species charges do not change sign during cycling. The active species counter ion affords charge balance, and the supporting salt merely imparts ionic conductivity on the electrolyte, lowering salt requirements. This study employs iron tris(2,2’-bipyridine) tetrafluoroborate (Fe(bpy)₃(BF₄)) and ferrocenylmethyl dimethyl ethyl ammonium tetrafluoroborate (Fc1N112-BF₄) as ionic active species to show the benefits of common-ion exchange. We extend this concept further by studying active species that remain as ions across all SOCs, and show that ionic active species, such as Fe(bpy)₃(BF₄) and Fc1N112-BF₄, can exhibit higher conductivities than typical supporting salts (e.g., LiBF₄). Combining these ionic active species in a common-ion configuration yields a NAqRFB devoid of any supporting salt, operating at moderate current densities (20 mA cm^-2) and greater than 85% current and voltaic efficiencies.
Acknowledgments
We acknowledge the Joint Center for Energy Storage Research and National Science Foundation Graduate Research Fellowship Program for financial support.
References
[1] R.M. Darling et al., Energy Environ. Sci. 7 (2014) 3459–3477.
[2] C.S. Sevov et al., J. Am. Chem. Soc. 137 (2015) 14465–14472.
[3] A.A. Shinkle et al., J. Power Sources. 248 (2014) 1299–1305.

Alternative energy sources such as solar and wind provide clean energy into the electrical grid, yet their inherent intermittency requires the development of energy storage systems to provide continuous supply. Non-aqueous redox flow batteries (RFBs) promise efficient grid-scale energy storage owing to their straightforward scalability and ample choice for active materials¹. However, currently used ion-exchange membranes (IEMs) in RFBs show low ionic conductivity in organic media.² An alternative to the use of IEMs is a size-exclusion approach that incorporates redox active polymers (RAPs) as storage media with porous separators.³ RAPs exhibit fast charge transfer, chemical reversibility, and high solubility in nonaqueous systems, making them attractive for flow applications. Here, we evaluate the performance of this approach under flow conditions using a model non-aqueous redox system.
Size-exclusion principles based on RAP size and membrane pore radius were used to suppress crossover while allowing high ionic conductivity. We will present on the development and characterization of RAPs that can be paired and cycled reversibly. These RAPs individually show promising electrochemical properties in voltammetric and electrolysis experiments, with Coulombic efficiency values consistently higher than 95%, high voltage efficiency and rate capabilities above 1C, as well as minimal redox active material crossover. Flow cell testing demonstrates the performance of these polymers under varying rate capabilities and material loadings for enabling a new generation of flow battery research based on non-aqueous redox systems.
[1] Energy Environ. Sci. 2014, 7(11), 3459. [2] J. Electrochem. Soc. 2016, 163(1), A5253. [3] J. Am. Chem. Soc. 2014, 136(46), 16309.

In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density could capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules, among which, stability is the crucial factor for determining the real cycling performance. ^1,2
The derivatives of 1,4-dimethoxybenzene are thus far the best performing catholyte molecules and have been intensively studied for non-aqueous redox flow batteries (NRFBs). However, most of these molecules have been found by trial and error and the structural factors that determine durability of their radical cations and even the nature of reactions controlling the life times of these radical cations are insufficiently understood. In this report, an exploratory study focusing on the correlation between molecular symmetry and electrochemical stability was conducted. As shown in Fig 1, we examined quite a few previous reported stable redox molecules^3-5 for the root causes for radical cation stability. By comparing their cycling performance, we observed that symmetric molecules are more likely stable than the relative asymmetric ones. Evidenced by DFT calculations, we rationalize this connection through the effect of this symmetry on fragmentation in the stability of a conformationally frozen radical cation; in particular, on the rates of radical deprotonation to fragmentation reactions. In asymmetrical radical cations, the extra positive charge residing on some functional groups becomes greater than on other such groups, causing their faster fragmentation. Following this observation, we design and synthesize several new structures that are expected to be even more stable redox molecules.


(1) Wei, X.; Xu, W.; Huang, J.; Zhang, L.; Walter, E.; Lawrence, D.; Vijayakumar, M.; Henderson, W. A.; Liu, T.; Cosimbescu, L.; Li, B.; Sprenkle, V.; Wang, W. Angewandte Chemie International Edition 2015, 54, 8684.
(2) Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J. Z.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; Sprenkle, V.; Wang, W. Adv. Energy Mater. 2015, 5, 10.1002/aenm.201570002.
(3) Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L. Adv. Energy Mater. 2015, 5, 1401782.
(4) Zhang, L.; Zhang, Z.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Energy & Environ. Sci. 2012, 5, 8204.
(5) Zhang, Z.; Zhang, L.; Schlueter, J. A.; Redfern, P. C.; Curtiss, L.; Amine, K. J. Power Sources 2010, 195, 4957.

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. Solid-electrode batteries are drained far too soon, when discharged at their rated power, to economically fill the gaps in photovoltaic or wind temporal power profiles. Flow batteries show promise because the designer can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all electro-active species in fluids. Wide-scale utilization of flow batteries is limited by the abundance and cost of these materials, particularly those utilizing redox-active metals such as vanadium or precious metal electrocatalysts. We have developed [1-5] high performance flow batteries based on the aqueous redox behavior of small organic molecules such as quinones. The redox active materials can be very inexpensive and sometimes exhibit rapid redox kinetics and long lifetimes. This new approach should enable massive electrical energy storage at greatly reduced cost. Relevant materials and molecular issues will be discussed.

[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] Q. Chen, M.R. Gerhardt, L. Hartle, and M.J. Aziz, "A quinone-bromide battery with 1 W/cm² power density", J. Electrochem. Soc. 163, A5010 (2016), doi: 10.1149/2.0021601jes
[3] Q. Chen. L. Eisenach, and M.J. Aziz, "Cycling analysis of a quinone-bromide redox flow battery", J. Electrochem. Soc. 163, A5057 (2016), doi: 10.1149/2.0081601jes
[4] 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
[5] K. Lin, R. Gómez-Bombarelli, E.S. Beh, L. Tong, Q. Chen, A.W. Valle, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, Nature Energy, in press (2016).

Aqueous quinone/hydroquinone couples exhibit rapid redox kinetics, require no electrocatalyst, and are potentially very inexpensive, making them attractive candidates for large-scale energy storage devices such as flow batteries [1-3]. We evaluate four anthraquinone derivatives for use as the negolyte in aqueous flow batteries. These anthraquinones are shown to undergo reversible two-electron reduction and oxidation in acidic aqueous solution, with tunable reduction potentials. When paired with a posolyte containing bromine and hydrobromic acid, three of these quinone derivatives exhibit cell voltages above 1 V. These derivatives are evaluated for chemical stability against bromination and stability upon electrochemical reduction and oxidation. DFT calculations paired with bromine exposure experiments demonstrate each derivative’s susceptibility, or lack thereof, to bromination in the oxidized and reduced forms. Electrochemical reduction and oxidation mechanisms of these quinones are explored via cyclic voltammetric studies. Two anthraquinone derivatives apparently stable to bromination and electrochemical reduction are cycled in a flow cell, showing greater than 98% current efficiency at 0.25 A/cm² cycling current. Mechanisms for capacity loss and energy inefficiency are discussed to guide further research.

[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] B. Huskinson, M.P. Marshak, M.R. Gerhardt and M.J. Aziz, “Cycling of a quinone-bromide flow battery for large-scale electrochemical energy storage”, ECS Trans. 61, 27 (2014)
[3] B. Yang, L. Hoober-Burkhardt, F. Wang, G.K. Surya Prakash, and S.R. Narayanan, J. Electrochem. Soc. 161, A1371 (2014)

Quinone and its hydroquinone counterpart can form quinhydrone dimer through dispersion interactions, with the best-known case being the quinhydrone reference electrode based on p-benzoquinone. Similar phenomena may occur in quinone-based flow batteries, as the reduced and oxidized forms of quinone coexist in all states of charge except 0 and 100%. We use UV-Vis spectroscopy to confirm the quinhydrone formation of the 9,10-anthraquinone-2,7-disulfonic acid (AQDS) molecule. An in-line UV-Vis spectrometer is also constructed to conduct such an observation in a flow cell during charge and discharge processes. These measurements help us quantify the equilibrium constant, and provide insights into the formation mechanisms corroborated by density functional theory calculation. We will also discuss how the quinhydrone formation, as well as the tribromide formation in the bromine/bromide electrolyte, impacts the cell voltage of the AQDS-bromide flow battery.

The state of the art of our understanding of electrolyte functions in all-vanadium redox flow batteries will be presented and then used as a platform for discussion of the use of polymer-based electrolytes in other contexts. Descriptions of processes occurring at scales from molecular to macro-scale will be discussed. Of particular interest in the flow battery context, and of general interest for electrosynthesis, is a description of the complex interplay between component interactions with each other and with membranes. The trade-off between membrane conductance and species transport across the membrane is key and is largely driven by the tendency of the membrane to imbibe different solution components as well as their intrinsic mobility. For the former, we will describe a thermodynamic approach to describing the uptake of various components. For the latter we discuss our recent work to probe the motions of key species. Based on this, we will outline a few implications of the fundamental aspects.

Vanadium flow batteries are one of the most promising candidates for large scale energy storage application, due to their attractive features like flexible design, high efficiency, long cycle life and high safety. Exploring new membranes for VFB application is one of most important targets in this field. To address the problem of hydrocarbon ion exchange membranes (poor chemical stability), brought by the introduction of ion exchange groups in polymer, porous membranes, based on the idea of separating vanadium ions from protons via pore size exclusion, have been successfully introduced in VFB recently by our group.
The first reported porous membrane for VFBs is made from hydrolyzed polyacronitrile (PAN).[1] The membranes show good prospects in VFBs, exhibiting columbic efficiency of 95 % and energy efficiency of 76 % at the current density of 80 mA cm^-2. To further optimize the membranes selectively and battery performance, the nanofiltration (NF) membranes with in situ assembled silica on the surface and in the pores are prepared.[2] The modified membranes show much higher V/H ion selectively by achieving a CE up to 98 % (EE: 79 %) at the current density of 80 mA cm^-2. Later, to further achieve impressive VFB performance, more effort was devoted to optimize the membrane morphology and materials. Quite recently, very impressive results were achieved on the design and fabrication of high performance porous membranes and further upscale was carried out [3].
In this presentation, the recent progress and challenge of porous membranes in the application of VFB will be demonstrated.

[1] H. Zhang, H. Zhang, X. Li, Z. Mai, J. Zhang, Nanofiltration (NF) membranes: the next generation separators for all vanadium redox flow batteries (VRBs)?, Energy & Environmental Science, 4 (2011) 1676.
[2] H. Zhang, H. Zhang, X. Li, Z. Mai, W. Wei, Silica modified nanofiltration membranes with improved selectivity for redox flow battery application, Energy Environ. Sci., 5 (2012) 6299-6303.
[3] H. Zhang, H. Zhang, F. Zhang, X. Li, Y. Li, I. Vankelecom, Advanced charged membranes with highly symmetric spongy structures for vanadium flow battery application, Energy Environ. Sci., 6 (2013) 776-781.

Redox flow batteries (RFBs), as one of the most promising electrical energy storage systems, provide an alternative solution to the problems of balancing power generation and consumption. RFBs are designed to convert and store electrical energy into chemical energy and release it in a controlled fashion when required. In particular, aqueous RFBs are attracting more attentions because of their good safety, low cost and high power density comparing with non-aqueous RFBs. Therefore, aqueous RFB chemistries with appropriate redox potential, high soluble and low-cost active species are critically required.^[1]
During the past few years, PNNL invented several aqueous inorganic RFB systems, including mixed-acid vanadium flow battery and zinc-polyiodide flow battery. In 2011, we introduced chlorine acid into the traditional VRB^[2], thereby improving the solubility of the active element of vanadium. The improved energy density reached 25 Wh/L. It is well known that high charge/discharge rate (charge/discharge current density) is prone to generate high power density, but the energy efficiency (EE) was significantly declined. In 2015, we reported a new environmentally friendly flow battery system:^[3] zinc-polyiodide redox flow battery (ZIB) system. A high energy density of 167 Wh/L approaching that of Li-ion batteries is achievable based on the high solubility of zinc iodide electrolytes (> 7M). However, the energy efficiency is still very low, exhibiting with the values of 63% at a charge/discharge rate of 30 mA/cm² at 2.5 M ZnI₂. Thus, the low energy efficiency means that more active species are required for identical energy output, which eventually results in increased cost. In order to increase EE especially at higher current density, much attention has been paid on the improvement of electrodes. Commonly, carbon-based materials are used as electrodes in VRBs and ZIBs because they are readily available, highly stable, corrosion resistant, economical and conductive. However, they were proved to show poor kinetic reversibility. Therefore, we developed low-cost and highly active catalysts, such as metal (Bi) ^[4], metal oxide (Nb₂O₅) ^[5], carbon oxygen functional groups ^[6] as well as metal-organic frameworks ^[7], decorating graphite felts greatly enhanced the electrode performances.

References:

[1] Soloveichik, Grigorii L. Chemical reviews 115.20 (2015): 11533-11558.

[2] Li, L et al. Advanced Energy Materials 1.3 (2011): 394-400.

[3] Li, B et al., Nature communications, 2015

[4] Li, B et al. Nano letters 13.3 (2013): 1330-1335.

[5] Li, B et al. Nano letters 14.1 (2013): 158-165.

[6] Li, B et al. ChemSusChem (2016).

[7] Li, B et al., Nano Letters, 2016

With the increasing penetration of variable renewable generation, energy storage is now becoming one of the hottest topics in the utility industry. The commercial success of energy storage applications will depend on aligning the cost of a project with the benefits of the technology. Research on materials and devices has increased cost effectiveness, cycle life and safety of these systems. Besides Li-ion batteries, flywheels, flow batteries, and advanced lead-carbon batteries are being deployed. The presentation will discuss research on flow batteries as an example of consistent, device driven reduction in cost. Markets are now gradually taking shape as changes in the regulatory framework result in more equitable valuation of storage benefits. The presentation will discuss diverse monetized and unmonetized benefit streams, using multi-megawatt applications of a variety of energy storage technologies and highlight outcomes and objectives of DOE supported energy storage deployments in California, Massachusetts, Washington, Oregon, and Vermont. As major players begin deploying more storage projects, operators are recognizing their value for ancillary services. In particular, smoothing and ramping of wind and solar PV are being addressed. Emergency preparedness through storage microgrids is another important development. There are now over 1500 storage projects listed in the Global Energy Storage Data Base, but with the continuation of the California mandate for 1.3MW of storage and new emphasis in other states, we can expect an exciting upsurge in storage research and many new projects to be realized.

Chloride-containing all vanadium redox flow battery system was developed at US DOE Pacific Northwest National Laboratories in 2010. By complexing VO₂⁺ with Cl^- in the catholyte solution, the poor stability of VO₂⁺ in the conventional sulfuric acid-based supporting solution at high vanadium concentration and high temperature conditions can be largely improved. This electrolyte stability improvement significantly eases battery system design and operation requirements, and thus, advances the system reliability and durability. It also enhances the vanadium flow battery safety through containerization and onsite chemical volume reduction. From MW-level product point of view, compared to the traditional vanadium sulfate system, this improvement practically double the electrolyte energy density, allowing compact product design with 5X footprint reduction. In this presentation, two unique advantages of this redox flow battery system, i.e. No Capacity Fading feature and Over-Charge Safe feature, will be discussed. Some materials issues associating with this new system in our MW-scale product will also be discussed.

Electrolytes store energy through electrochemical process by serving as solvent and transport medium for redox active ions of redox flow battery (RFB). Functional properties of electrolytes such as ionic conductivity, viscosity, and stability evolve from molecular interactions between ionic solutes and solvent molecules (commonly referred to as solvation phenomena). It is critical to understand the role of competing counter ions and solvent mixtures to predict and design the optimal electrolyte for a target application. Solvation structure and dynamics were analyzed for various electrolytes through a comprehensive study of density functional theory and nuclear magnetic resonance spectroscopy. The chemical and thermal stability of redox species are correlated to the role of co-solvent and counter anion ligand exchange process as part of solvation structure, which helped in building design formulations for aqueous redox flow batteries. Comprehensive view of solvation phenomenon of transition metal based aqua-cation species and their impact on RFB electrolyte chemical stability will be discussed.

Redox flow batteries are emerging candidates for grid-scale energy storage solutions. In redox flow batteries, electrolytes are stored externally, which allows customizable battery designs to meet various demands of a wide range of energy/power ratios. Chemical cost holds a significant in the overall capital cost of redox flow batteries,¹ and therefore it is highly desirable to improve utilization ratios of active materials in current redox flow batteries.
Herein we report such efforts on several redox flow batteries, such as all vanadium and zinc iodide systems. Utilization ratios of active materials, as well as specific capacities, are significantly improved by revisiting the flow chemistry, optimizing electrolyte components, and developing new electrodes.

1. V. Viswanathan, A. Crawford, D. Stephenson, S. Kim, W. Wang, B. Li, G. Coffey, E. Thomsen, G. Graff, P. Balducci, M. Kintner-Meyer and V. Sprenkle, J. Power Sources, 2014, 247, 1040-1051.

Vanadium flow batteries (VFBs) are an attractive technology for a variety of energy storage applications^1-4. They offer the advantage that, because the species on both sides of the membrane are just different forms of vanadium in H₂SO₄, cross-contamination problems are effectively eliminated. In addition, because the reactants and products of the reactions at the positive and negative electrodes are stored in solution, these batteries have relatively long lifetimes. Since aqueous vanadium species are highly colored, the vanadium concentrations and state-of-charge of both sides of a VFB may be precisely monitored using UV-visible spectroscopy³.
The energy density of VFBs is limited by the solubility of V^II, V^III, V^IV and V^V in the electrolyte. On the negative side, the solubility of V³⁺ and V²⁺ generally increases with temperature and decreases with increasing concentration of H₂SO₄ and this is also true of the solubility of the V^IV species, vanadyl ion (VO²⁺), on the positive side. However, the V^V species in the catholyte, pervanadyl ion (VO₂⁺), can precipitate as V₂O₅. This reaction is usually found to be very slow and, in practice, supersaturated solutions of V^V in sulphuric acid can persist for very long periods of time¹. The stability of such metastable solutions decreases, as expected, as the concentration of V^V increases. The stability increases with increasing concentration of sulphate, and the presence of certain additives such as H₃PO₄ further increases the stability. There have been several studies^1,4 of the stability of V^V in the catholyte of VFBs, but there is an absence of detailed kinetic studies of the precipitation process. In this paper we report a quantitative study of the kinetics of precipitation of V^V from H₂SO₄ solutions in the absence and in the presence of additives and as a function of composition and temperature.
In agreement with earlier studies, when a solution of V^V in H₂SO₄ was held above room temperature an induction period was observed after which precipitation occurred. The induction time depended on the solution composition and the temperature. We investigated this induction time in detail for a range of catholyte compositions and temperatures. Both light scattering measurements and direct visual observations were used to investigate the precipitation process in detail.
Detailed quantitative measurements of the effects of the concentrations of vanadium, sulphate and various additives on the kinetics of precipitation at various temperatures will be presented and discussed. A simulation method based on these results will be described which can predict the stability of catholytes with reasonable accuracy.
1. S. Roe et al., J. Electrochem. Soc., 163, A5023 (2016)
2. A. Bourke et al., J. Electrochem. Soc., 163, A5097 (2016)
3. C. Petchsingh et al., J. Electrochem. Soc. 163, A5068 (2016)
4. M. Skyllas-Kazacos et al., J. Electrochem. Soc., 158, R55 (2011)

Typical flow batteries^1-3 have carbon-based electrodes separated by a proton exchange membrane. For example, vanadium flow batteries (VFB) typically have carbon felt flow-through electrodes. In order to better understand and improve the performance of flow battery electrodes, it is important to investigate the electrode kinetics.
The kinetics of both the V^II/V^III and V^IV/V^V redox couples depends strongly both on the type of carbon used and on the preparation of the electrode surface. Generally, the kinetics is found to be faster for V^IV/V^V than for V^II/V^III but there are also reports that the kinetics is slower for V^IV/V^V than for V^II/V^III. Thus the literature on the electrode kinetics of vanadium on carbon has a number of inconsistencies and contradictions despite the importance of the topic for flow batteries.
This paper discusses our recent work which explains the origins of these inconsistincies in the previous literature. It also demonstrates how a proper understanding of the effect of electrode treatment on kinetics can be exploited to significantly improve the performance of flow batteries.
Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of V^II/V^III and V^IV/V^V in H₂SO₄ on several different carbon materials: glassy carbon, carbon paper, carbon xerogel, and, importantly, carbon fibers extracted from a carbon felt typically used in flow-battery electrodes. For all carbon materials investigated, the kinetics of V^II/V^III was enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of V^IV/V^V was inhibited by anodic, and enhanced by cathodic, treatment. The potential region for each of these effects varied only slightly with carbon material. Rate constants were always greater for V^IV/V^V than for V^II/V^III except when anodized electrodes were compared.
Based on these results, the range of potential experienced by a negative electrode in a flow cell suggests that the kinetics will become slower with time. This prediction is supported by a series of flow-cell experiments which showed that the overpotential at the negative electrode increased significantly with charge-discharge cycling. Furthermore, the redox potential of the positive electrolyte falls in a region where the electrode is activated for the V^II/V^III reaction, which suggests that the overpotential at the negative electrode would be reduced by interchanging the electrodes or, equivalently, by a short application of a suitable positive potential. This prediction was also supported by the flow-cell experiments.
Effects of electrode treatment on the kinetics of other relevant redox couples at carbon will also be briefly discussed.
1. Bourke et al., J. Electrochem. Soc., 163, A5097 (2016)
2. Petchsingh et al., J. Electrochem. Soc. 163, A5068 (2016)
3. Bourke et al., J. Electrochem. Soc. 162, A1547 (2015)

Large-scale energy storage has attracted increasing interests due to its urgent need in grid management (load leveling and peak shaving), the grid reliability and utilization and the integration of renewable energy sources. Among a wide range of energy storage technologies, vanadium flow battery (VFB) has a unique combination of high efficiency, high reliability, flexible design and long cycle life, which makes it an ideal candidate for large scale energy storage. Numerous application demonstrations illustrated that VFB can meet the demands of large-scale energy storage and is suitable for the applications like renewable energy generation and the distributed energy supply etc. However, the relatively low power density of VFB leads to its too high cost and further hinders its further commercialization. Therefore the current efforts were mostly focus on lowering the VFB cost by exploring key materials with improved performance. The key materials of a VFB mainly composed of the electrolyte, the carbon felt electrode and membranes.
Dalian Institute of Chemical and Physics (DICP) has devoted to VFB research for more than 10 years from materials to system integration. The key materials including electrolytes, electrodes and membranes were succesfully explored and realized mass production via investigating the structure-performance relation of the materials.
For the electrolytes, the transfer behavior of electrolytes were clarified via investigating the transfer behavior in membranes with different morphologies, the results showed the capacity decay rate of VFB could be lowered via changing membranes materials.
For the bipolar plates, the concept of carbon composite bipolar plate was proposed and the batched preparation techniques and continuous modeling equipments for carbon composite bipolar plate were estabilished, where the production capacity has reached 20,000 m²/year. Meanwhile, porous membrane separator was first introduced into VFB based on the idea of separating vanadium ions from proton via pore size exclusion. This new concept successfully overcomes the restriction caused by ion exchange groups from traditional ion exchange membranes, which broadens the materials option of VFB membranes.
In this presentation, the research on VFB key materials will be introduced and the progress of key materials in DICP will be presented.
References:
Cong Ding, Huamin Zhang, Xianfeng Li, Tao Liu, and Feng Xing, Vanadium Flow Battery for Energy Storage: Prospects and Challenges, J. Phys. Chem. Lett., 2013, 4 (8), pp 1281–1294
Hongzhang Zhang, Huamin Zhang, Xianfeng Li, Zhensheng Mai, Jianlu Zhang, Nanofiltration (NF) membranes: the next generation separators for all vanadium redox flow batteries (VRBs) Energy & Environmental Science 2011, 4 (5), 1676 – 1679

Development of efficient and cost-effective energy storage systems is nowadays crucial considering the huge amount of electrical energy supplied by the intermittent renewable energy sources and its related issues, such as grid instabilities and volatility in power prices in liberalized electricity markets [1]. Among the energy storage technology, one of the most promising is Vanadium Redox Flow Battery (VRFB) due to the peculiarities to separate power and energy, high efficiency and extremely long charge/discharge cycle life. However the commercialization of VRFB is still hindered by some technological issues, among which low power density is one of the most important. Recently Aaron et al. [2] increased the peak power density of a VRFB system utilizing a serpentine flow field, coupled with a commercial non-wetproofed carbon paper electrodes, Sigracet® SGL 10 AA.
This work presents the results of a novel method for treating commercial carbon electrodes with carbon nanostructures, in order to increase the specific active area. Treated samples are firstly analyzed with ex-situ techniques in order to evaluate and understand material nanostructure. Raman spectroscopy, X-Ray diffraction, Scanning Electron Microscope (SEM) and BET surface area measurement are performed.
Afterwards, polarization curves, cyclic voltammetries and electrochemical impedance spectroscopies are performed in a 25 cm² modified cell architecture [3], where the negative electrode is a commercial GDE fed with hydrogen, acting as a reference electrode. This hardware permits to characterize the treated materials as VRFB positive electrode, fed with VO²⁺/VO₂⁺ liquid electrolyte, eliminating V²⁺/V³⁺ cross contamination effects. The experimental setup is provided with two tanks, keeping constant the state of charge during the measurements. Stability of carbon nanoparticles to charge/discharge cycles is also verified.
Preliminary results from cyclic voltammetries on a carbon electrode show that this method permit to increase electrode performance, as also confirmed by polarization curves.


References:
[1] B. Zakeri, S. Syri, Electrical energy storage systems: a comparative life cycle cost analysis, Renewable and Sustainable Energy Reviews 42 (2015) 569-596.
[2] D.S. Aaron, Q. Liu, Z. Tang, G.M. Grim, A.B. Papandrew, A. Turhan, T.A. Zawodzinski, M.M. Mench, Dramatic performance gains in vanadium redox flow batteries through modified cell architecture, Journal of Power Sources 206 (2012) 450-453.
[3] V. Yufit, B. Hale, M. Matian, P. Mazur, N. P. Brandon, Development of a regenerative hydrogen-vanadium fuel cell for energy storage applications, Journal of The Electrochemical Society 160 (2013) A856-A861.

Flow batteries and vanadium flow batteries (VFB) in particular have attracted keen attention for electrochemical energy storage. Following the design strategies of flow batteries, VFBs have an ion exchange membrane that is responsible for transferring non-reaction ions to complete the internal circuit as well as for separating the positive and negative electrolytes, which are stored in separate storage tanks. The mostly currently used membranes for VFBs are perfluorosulfonic polymers such as Dupont Nafion®. They show excellent chemical stability and very high proton conductivity in VFBs, but the high cost and low selectivity on vanadium ions have hampered their further application in VFBs. The hydrocarbon ion exchange membranes such as sulfonated poly(ether ether ketone) (SPEEK), polysulfone-based anion exchange membranes etc are the lately most studied systems due to their tunable ions conductivity and low cost. However, these membranes are suffered from low chemical stability under VFB medium. The degradation mechanism of these hydrocarbon ion exchange membranes is not very clear, since the medium of VFB is very complicated. As a consequence, the studies of degradation mechanism are rarely reported and the detailed degradation reactions of these membranes under VFB medium are not clear, which leads to very few relevant strategies to synthesize of new materials with excellent chemical stability.
In this study, aimed at understanding the degradation of hydrocarbon ions exchange membranes, SPEEK membranes with different DS were selected as the model compounds. The influence of ion exchange groups on oxidation stability during ex-situ test was investigated, indicating that the oxidation stability of membranes decreases with the increasing DS or the sulfonic acid groups accelerated the degradation of the membrane. To clarify the degradation mechanism of SPEEK membranes during ex situ tests, SPEEK-3 was selected as example to investigate the chemical structure of the degradation products. The chemical structure of the degradation products was then analyzed by FTIR, NMR and LC/MS spectrogram etc. Based on the these characterization and analyzation, the degradation mechanism of SPEEK membrane was then proposed. Under the strong acidic conditions, the ethereal oxygen atoms in SPEEK could be easily pronated and become strong electro-withdrawing groups. The protonated ether functionality together with the strong electron-withdrawing sulfonic acid group induce a strong electrophilic carbon center on the benzene ring, which can be easily attacked by the lone pair electron on the vanadium(V) oxygen species while leaving the sulfonic acid functional groups stable. Based on this degradation mechanism, strategies such as protecting the ether bond or synthetic new compounds without ether bond or introducing electron-donating groups to aromatic backbone could be very effective to improve the oxidation stability of the membrane for VFB application.

Redox flow batteries (RFBs) have increasingly being recognized as a prominent candidate for large-scale energy storage due to their unique advantages of high safety, decoupling of power and energy, long lifespan, quick response, and potentially low cost. This presentation describes the recent progress of the advanced redox flow battery technologies developed at Pacific Northwest National Laboratory. On the component level, new developments on the high energy density electrolyte, high selective membrane, and catalytic electrode will be reported. New redox flow battery chemistry based on soluble low-cost redox active materials will be discussed, which eliminate the need for expensive metal-based redox couples. Compared with other reports in this area, the new redox flow battery technology demonstrated higher voltage, energy density, rate capability, and improved cycling stability.

The storage of electrical energy will become a key issue with increasing amounts of fluctuating renewable energies in a grid. Different technologies are established or in development to solve the need for storage at low cost. One of the technologies are redox flow batteries which can be separately scaled in terms of power and energy. Thus leads to potentially low storage cost if the storage medium has low cost and the storage time is in the range of some hours. Currently over 58 different types of flow batteries are in development which claim to allow low cost for energy storage but only a few have been commercialized [1]. Beside the cost of the active materials, the cost of the stack components, their electrochemical performance and the reliability of the overall system are important to the levelized cost of energy. Key components are the electrochemical components membrane, electrodes and electrolytes, which behavior strongly influences the system cost through a complex network of dependencies.

During this talk we will present an overview about the recent results of our onging research on active materials, membranes, electrodes and a material based techno-economical model for flow batteries and we will discuss advantages and disadvantages of different concepts. Our focus is mainly contributed with the studies of vanadium systems, but also includes organic, hydrogen/bromine, zinc/bromine, vandium/air and other chemistries. We studied electrochemical reaction mechanisms with different electrochemical and spectroelectrochemical methods, electrolyte properties, aging of materials, cell, stack and system behavior. Our developments focused on cost reduction of flow batteries by improved production methods and materials like carbon nanotube based and mouldable thermoplastic electrode materials, injection moulded components and system optimization. Additionally we studied failure mechanisms on materials of different types of flow batteries with RAMAN-Spectroscopy, REM-EDX and XPS to improve the reliability of the materials.

1 J. Noack, N. Roznyatovskaya, T. Herr, P. Fischer, Angew. Chem. Int. Ed. 54 (2015) 9776–9809.

A large fraction of the cost of a flow battery installation comes from the high cost of separators and bipolar plates¹. Optimizing the use of active area in flow batteries by maximizing power density allows cell stack engineers to reduce the amount of Nafion or other separator materials required. The design of flow fields has a significant impact on cell performance, including peak galvanic power density^2,3. In this work, we present computational fluid dynamics models of several potential flow field geometries. We present a model of an interdigitated flow field, coupling fluid dynamics and electrochemistry. The model is used to optimize the channel, land, and electrode dimensions in the flow field, demonstrating energy efficiency and power density increases by reducing resistive and pumping losses, and improving reactant utilization in the porous electrode. Model results are compared to experimental results from anthraquinone-based flow batteries to guide future flow battery design.

1. V. Viswanathan et al., J. Power Sources, 247, 1040–1051 (2014) http://linkinghub.elsevier.com/retrieve/pii/S0378775312018691.
2. M. L. Perry, R. M. Darling, and R. Zaffou, ECS Trans., 53, 7–16 (2013) http://ecst.ecsdl.org/cgi/doi/10.1149/05307.0007ecst.
3. R. M. Darling and M. L. Perry, J. Electrochem. Soc., 161, A1381–A1387 (2014) http://jes.ecsdl.org/cgi/doi/10.1149/2.0941409jes.

Redox flow batteries (RFBs) are an attractive technology for grid-level energy storage due to scalable power and energy, long lifetimes, and facile manufacturing [1], but the battery price must be sufficiently low to encourage widespread implementation. The United States Department of Energy (DOE) established that RFB system price, including the battery, inverter, and installation costs, must drop under $150 kWh^-1 to enable market penetration for 2–4 h discharge applications [2]; RFB prices in 2014 exceeded $500 kWh^-1 [3]. Despite existing high prices, a techno-economic (TE) study indicated that RFBs could achieve the DOE target by reducing materials and reactor costs [3]. Recently, RFB literature has focused on new electrolyte materials, discovering low-cost active species, such as tailored organic molecules [4] and widely abundant inorganics (e.g. [5]), or implementing nonaqueous (NAq) solvents [4]. Given the wide array of newly proposed RFB materials, this work develops a TE model to guide cost-effective electrolyte materials selection (i.e., active species, salt, solvent) for designing RFBs aimed at the DOE target price.
TE modeling is a technique that evaluates the price performance of an energy storage device by relating system price to cell performance, component costs, and materials properties. First, we quantify cost-constraining variables for both aqueous (Aq) and NAqRFBs, spanning a broad range of materials options. AqRFBs benefit from small area-specific resistance (ASR) and low supporting electrolyte costs, but suffer from narrow electrochemical windows, typically ≤ 1.5 V, limiting electrolyte energy density. Contrastingly, NAqRFBs offer larger electrochemical windows (3–4 V), but suffer from expensive supporting electrolytes and high ASR. Second, this work illustrates that AqRFBs are sensitive to changes only in cell potential and active species molecular weight / cost, but NAqRFBs are sensitive to nearly every model parameter due to similar cost contributions from the reactor and all electrolyte components. Third, we explore the financially feasible space of RFB materials by generating design maps from the TE model, illustrating tradeoffs in adjusting multiple design parameters simultaneously. The analysis culminates in a set of suggested electrolyte design pathways to realistically bring RFB prices to the DOE target of $150 kWh^-1 or below.
Acknowledgments
We thank the Joint Center for Energy Storage Research and National Science Foundation Graduate Research Fellowship Program for financial support.
References
[1] A.Z. Weber et al., J. Appl. Electrochem. 41 (2011) 1137–1164.
[2] Energy Storage: Program Planning Document, U.S. Department of Energy: Office of Electricity Delivery and Energy Reliability, 2011.
[4] R.M. Darling et al., Energy Environ. Sci. 7 (2014) 3459–3477.
[5] W. Wang, V. Sprenkle, Nat. Chem. 8 (2016) 204–206.
[6] X. Wei et al., J. Electrochem. Soc. 163 (2016) A5150–A5153.

The practical utilization of solar energy demands not only efficient energy conversion but also inexpensive large scale energy storage. Building on mature regenerative photoelectrochemical solar cells and emerging electrochemical redox flow batteries (RFBs), more efficient, scalable, compact and cost-effective hybrid energy conversion and storage devices could be realized. Here we present an integrated PEC solar energy conversion and electrochemical storage device by integrating regenerative photoelectrochemical solar cells in aqueous electrolytes with RFBs using the same pair of organic redox couples. In such an integrated PEC-RFB device, solar energy is absorbed by semiconductor electrodes and photoexcited caries are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to fully charge up the battery (i.e. store the solar energy into the redox couples). When electricity is needed, the charged up redox couples will be discharged on the surface of carbon felt electrodes as one would do in the discharge of a RFB to generate the electricity. We demonstrated that such an integrated PEC-RFB device can be charged under solar illumination without external electric bias and deliver a high discharge capacity comparable with state of the art RFBs over many cycles. This integrated device can utilize solar energy efficiently -- an overall direct solar-to-output electricity efficiency (SOEE) of 1.7% have been achieved without significant performance optimization.

All-vanadium redox flow batteries (VRFBs) are considered as one of promising candidates for large-scale energy storage systems. The performance of a VRFB depends on stack design, component materials, and operating conditions. A VRFB stack mainly consists of bipolar plates, porous electrodes, and separators. Electrochemical reaction takes place on the fiber surface of the porous electrode and the reaction kinetics depends on properties of the porous electrode. Many studies have focused on the surface modification of the porous electrode to improve the kinetics of electrochemical reaction; however, the effect of compression ratio and porosity of porous electrodes on the performance of VRFBs are rarely reported.
The compression ratio of a graphite felt has a significant influence on the porosity of the electrode and contact resistance between components. When the porous electrode is compressed, the contact resistance and porosity are reduced. The electrolyte velocity within the electrode increases with decreasing porosity of the electrode, resulting in the decrease of concentration overpotential. Ohmic overpotential is also decreased due to decreased contact resistance. As a result, the cell performance can be improved by increasing the compression ratio of the porous electrode. However, the power consumption of pumping electrolytes through porous electrodes is increased due to increased pressure drop through the felt. As a result, it is required to understand the effect of compression ratio of the porous electrode on the VRFB performance and efficiency.
In this study, we specially designed a VRFB single cell to improve the distribution of electrolytes. Frames with three thicknesses of 3, 4, and 5 mm were fabricated to control the thickness of compressed porous electrodes. Graphite felts with two thicknesses of 6 and 10 mm were employed as porous electrodes. The assembled single cell was operated under different current density levels and electrolyte flow rates. The efficiencies of the single cell will be calculated and analyzed. The effective porosity of the graphite felts at different compression ratios will be determined and the results will be discussed.

The random and intermittent nature of renewable energies, such as solar and wind, hinder their widespread application. This issue can be solved by employing energy storage systems, which play a role of power balancing and peak shaving for power generation systems. Among energy storage systems, all-vanadium redox flow batteries (VRFB) have drawn much attention due to their long cycle life, deep discharge, and independence of power and energy ratings. However, the design of battery structure and optimization of operating parameters are remained issues to be solved.
In practical applications the active area of a VRFB is usually over one thousand square centimeters due to inherently low operating current density. Therefore, electrolyte distribution within the active area is a key factor that influences the performance of a VRFB. The uniformity of electrolyte distribution can be improved by flow field designs on electrically insulated frames. When the electrolyte flows within the porous electrode of a VRFB, the concentration of electroactive species in the electrolytes gradually decreases along the flow direction due to electrochemical reaction, resulting in the increase of mass transfer overpotential near the outlet and the decrease of uniformity of local current density. The battery performance can be improved by increasing the uniformity of local current density. Accordingly, to reduce the cost of developing a VRFB, a mathematical model that can describe the distribution of local current density at various operating conditions is required.
In this study, a three-dimensional mathematical model was developed to describe local current density distribution within a VRFB. The electrolytes were assumed to uniformly flow into the reaction area. As a result, we considered the variation of local current density along the flow direction and across the electrode direction. The porous electrode was described by a tertiary current distribution module using a commercial software COMSOL. This model was calibrated using experimental data. The effect of operating conditions and cell configuration on local current density distribution will be investigated and discussed in this research.

For previous decades, the polymer electrolyte membrane fuel cell (PEMFC) has been researched extensively due to its high energy conversion efficiency without generating pollutant emissions such as CO2, NOx, SOx and hydrocarbon chemicals. However, there have been many challenging issues such as sluggish electrochemical reaction of cathode catalyst with oxygen, so called oxygen reduction reaction (ORR), catalyst poisoning by carbon monoxide and water transport problem in the membrane electrode assembly (MEA). Among these issues, water management at the cathode is one of the most important issues for enhancing PEMFC performance, because if water produced at the cathode is not removed properly, it slow down the oxygen transport to triple-phase boundary (TPB) in cathode catalyst layer and substantial cell voltage drop takes place in sequence.
In this work, we introduce a simple and effective strategy to enhance the performance of the PEMFC by imprinting prism-patterned arrays onto the Nafion membrane, which provides three combined effects directly related to the device performance. First, a locally thinned membrane via imprinted micro prism-structures lead to reduced membrane resistance, which is confirmed by electrochemical impedance spectroscopy. Second, increments of the geometrical surface area of the prism-patterned Nafion membrane compared to a flat membrane result in the increase in the electrochemical active surface area. Third, the vertically asymmetric geometry of prism structures in the cathode catalyst layer lead to enhanced water transport. To explain the enhanced water transport, we propose a simple theoretical model on removal of water droplets existing in the asymmetric catalyst layer. These three combined effects achieved via incorporating prism patterned arrays into the Nafion membrane effectively enhance the performance of the PEMFC.

To prevent Cr-poisoning in solid oxide fuel cells (SOFCs) and to decrease area specific resistance by limiting the rate of chromia growth on Crofer 22 APU interconnects, a protective coating layer on the interconnects is necessary. In this study, glycine nitrate combustion process (GNP) was used to synthesize nano-sized powders of two spinels, Mn_1.5Co_1.5O₄, and CuMn_1.8O₄, which then were deposited on grooved Crofer 22 APU by electrophoretic deposition (EPD) method, followed by post-processing thermo-mechanical treatments. Uniformity, thickness and porosity of the coating layers were evaluated by scanning electron microscopy (SEM). The effectiveness of the coating layer to prevent oxygen and chromium diffusion was studied by thermogravimetric analysis and energy dispersive spectroscopy (EDS), respectively. The area specific resistance (ASR) of the coated samples was measured as a function of time and temperature. Comparison of the Mn_1.5Co_1.5O₄ and CuMn_1.8O₄ coatings will be discussed in this presentation.

A cost-effective modified combustion route using citric acid and glycine has recently been developed to synthesize high quality, and fully dense holmium-doped barium zirconate (BZH) ceramic pellets. The modified combustion route products obtained were characterized by X-ray diffraction (XRD), differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, high-resolution transmission electron microscopy. All XRD patterns were studied by using rietveld analysis have shown that the powder calcined at1100 °C for 4 h and sinter pellet at 1600 °C for 8 h are phase pure BaZr_0.9Ho_0.1O₃ and have a cubic (Pm-3m space group symmetry) perovskite (ABO₃) structure with lattice constant 4.1994 and 4.2035 Å respectively. The transmission electron microscopic investigation has shown that the particle size of the powder calcined at1100 °C for was in the range 10–40 nm. The FESEM image of sintered pellet at 1600 °C for 8 h reveals dense nature of BaZr_0.9Ho_0.1O₃ with 96.3 theoretical density. The observed raman spectrum of BaZr_0.9Ho_0.1O₃ is assigned to have its source in the nanodomains with the symmetry different than cubic one. AC electrical properties reveal three type relaxations in the 300 °C to 800 °C temperature region as studied at different frequencies over 100 Hz to 1 MHz in 3% humidified O₂ atmosphere. The total conductivity of BaZr_0.9Ho_0.1O₃ dense ceramic is found 2.76 x 10^-4 S-cm^-1 at 800 °C in 3% humidified O₂ atmosphere. The giant dielectric constant (ε’) of BaZr_0.9Ho_0.1O₃ is attributed to the Maxwell–Wagner polarization mechanism as well as to the thermally activated mechanism of charge carriers. The dielectric measurements were studied by fitting the electrical modulus with the Kohlrausch–Williams–Watts (KWW) function.

Solid oxide fuel cells (SOFC) are an alternative to traditional power sources. In order to reduce material cost and to improve SOFC lifetime, operating temperatures are sought to be decreased to 500-700C, i.e. intermediate temperature SOFC (IT-SOFC). Any fuel cell consists of three parts; anode, electrolyte, and cathode. The cathode should have a small thermal expansion coefficient compatible with its coupled electrolyte, high surface area to increase the active site for the oxygen reduction reaction (ORR), and high ORR catalytic activity at the operating temperature. It is possible to modify the properties of the material by doping, and thus enhance ionic and electronic conductivity. This has led to the development of cathode materials that are Mixed Ionic and Electronic Conductors (MIEC), from which doped SmCoO₃ is a good example. By doping the Sm-site with Ba²⁺, Ca²⁺ or Sr²⁺, oxygen vacancies (Vo) are generated, enhancing oxygen diffusion, whereas Co-site-doping with Fe³⁺, Cu³⁺, Ni³⁺, or Mn³⁺, enhance electronic properties. To fully understand its properties, atomic level computational studies are necessary. Despite theoretical publications regarding similar materials, SmCoO₃ has not been widely assessed.

We here present a theoretical study, using density functional theory (DFT) and molecular dynamics (MD), of the catalytic properties of SmCoO₃. ORR is influenced by bulk and surface Vo. Having investigated bulk [Vo], Vo surface formation is studied, as this is vital to ORR. The subsequent oxygen transport through cathode to electrolyte through different catalytic pathways are then explored. The oxygen bulk diffusion and electronic conductivity are investigated for different dopant systems, dopant concentrations, and SOFC operating conditions.

The capability of fabricating flexible power sources with light-weight and volume is a key to cope with the increasing demand for smart and ubiquitous energy sources for flexible electronics such as bio-integrated sensors and wearable devices. While, so far, most of the effort has focused on fabricating light-weight lithium-ion batteries as a portable power sources, polymer electrolyte membrane fuel cells (PEMFCs) are a respectable alternative thanks to their high energy and power density. That is why there have been several reports about flexible PEMFCs in literature, though, they were too thick to be highly flexible and their size was too small to be applied as portable power sources.
We report herein a portable, lightweight, and rollable PEMFC based on thin polycarbonate film as a flow-field plate and metal-grid as a current collector. We fabricated thin, flexible flow-field plates (~ 200 μm) by thermal imprint lithography using SUS master with flow channel shape. Current collectors were fabricated by laser-cutting thin metal sheets (~ 50 μm). In addition, the membrane-electrode assembly (MEA) was prepared by spraying Pt/C ink slurry onto the anode and cathode sides of the Nafion membrane and combined with carbon paper. All components were assembled into a single cell using hot-pressing method.
The as-fabricated rollable PEMFC showed the entire thickness of ~ 0.9 mm, weight of ~2.2 g, and reactive area of 9 cm2 with the peak power density of 56 mW/cm2. Furthermore, we realized a portable flexible planar stack of ten fuel cells. The stack showed open circuit voltage of ~ 9.6 V, maximum power of ~2.4 W. It was successfully demonstrated by charging a smartphone and operating an electric LED fan simultaneously, showing the technological level of this flexible fuel cell near commercialization.

All-vanadium redox flow battery (VRFB) is an attractive option for electrochemical energy storage for medium-to-large scale (upto MW) grid applications, owing to its salient features of independent energy and power ratings, high energy efficiency, high capacity, high safety, long life, environmental friendliness and flexibility in design. Electrodes are important component of VRFB technology as they provide platform for redox reactions to take place. Among the several carbon materials used as electrodes for VRFB, PAN-based carbon felt (CF) and graphite felt (GF) are the most widely used because of their stability and compatibility.
In the current work, firstly, performance evaluation of CF and GF at high current densities (up to 300 mA cm^-2) is carried along with the effects of electrode compression and thermal treatment. CF exhibits superior performance to GF especially at high current densities. At 200 mA cm^-2, CF gives a specific discharge capacity of 14.2 Ah L^-1 with energy efficiency (EE) of 68.4 % where as GF shows no performance at all. At a current density of 150 mA cm^-2, EE with CF is 4.8 % higher than GF while the corresponding increase in specific discharge capacity and discharge energy density is ~ 36 and 45 %, respectively. Compression of the electrodes also shows positive impact on performance. Thermal treatment at 500 °C marginally improves the performance with both CF and GF. Overpotentials for charging and discharging are higher for GF than CF at all current densities.
Secondly, effect of Sn, as an advanced electrocatalyst, on performance of CF and GF is investigated. VRFB shows remarkably enhanced performance with Sn. Sn increases the EE of CF by 2.8 % with the corresponding increase in specific discharge capacity and discharge energy density by ~23 and 27 % at 150 mA cm^-2, respectively. The catalytic effects of Sn on GF are even higher. At 150 mA cm^-2, Sn increases the EE on GF by 5.1% with corresponding increase in specific discharge capacity and discharge energy density of ~76 and 87 %, respectively. Sn helps to reduce the charging overpotential by 49 mV and 29 mV on GF and CF, respectively. Overall, useful potential window is increased by 149 mV in case of GF and 93 mV in case of CF.
Significant difference in performance of CF and GF as well as different catalytic activity of Sn towards them are further probed by electrochemical techniques, XRD, XPS and NEXAFS. Therefore, appropriate electrode material and catalyst can increase the market penetration of VRFB technology by increasing fuel utilization and enabling faster charge / discharge.

Nitrogen-doped (N-doped) nanoporous carbonaceous materials have recently shown a significant impact on the research areas of energy storage and conversion including electrodes for fuel cell and CO₂ adsorbents and so on. Herein, we report a highly N-doped nanoporous carbon spheres synthesized from melanin-like polymer (MP) and sepia from squid in the seawater, and utilization of such carbon samples as CO₂ adsorbents and metal free Oxygen Reduction Reaction (ORR) catalysts. First, monodisperse MP with size of 100-500 nm was synthesized using a dopamine precursor. As prepared MP was carefully characterized by FT-IR, UV-vis spectrometer, SEM, EA (elemental analysis), ¹³C solid state NMR, and it turned out that the chemical and structural features resembled sepia from squid in the seawater: The sepia is a natural melanin and it shows spherical shape with 100-200 nm in size. Nitrogen-enriched MP and sepia polymer (SP) from squid were then carbonized to provide highly N-doped and monodisperse carbons of which nitrogen content was as high as 12 wt%. Highly N-doped melanin-like carbon (MC) and sepia carbon (SC) were then activated to increase specific surface area and pore volume as well as control the pore size distribution (PSD) by hot CO₂ treatment: for example, surface area of 2677 m²/g and pore volume of 1.72 cm³/g for MC, and surface area of 2506 m²/g and pore volume of 1.47 cm³/g for SC. Highly N-doped and largely developed ultramicropore (< 1 nm) for activated MC and SC samples, which have been known for key factors for enhancing CO₂ adsorption, were then utilized as CO₂ adsorbents, and one of which shows outstanding CO₂ adsorption capacities of 7.8 mmolg^-1 at 273K and 4.8 mmolg^-1 at 298K (1bar). In addition, as a metal free ORR catalyst, highly N-doped MC and SC with large surface areas of 2677 m²/g and 2506 m²/g, respectively, show excellent activity for ORR, which is comparable ORR performance to commercial Pt-based catalyst. It is noted that largely developed surface areas with mesoporosity and numerously created active catalytic nitrogen sites of MC and SC samples favored four-electron reduction pathway, which is important factor for a promising Pt-free ORR catalyst.

The transition from fossil fuels has increased the need for cost-effective grid storage due to the intermittent nature of many renewable energy sources. Non-aqueous redox-flow batteries (NRFB) are suited to grid storage for many reasons, including the ability to decouple the power out put from capacity. Current NRFB technology has been limited by poor stability of the charge-carrying metal complexes. Among several degradation mechanisms that have been identified, ligand substitution of transition-metal-containing active materials has been shown to have a deleterious effect on long-term cycling and overall battery performance, especially in the presence of trace oxygen and water. While characteristics of putative active materials such as open-circuit voltage and solubility are critical to successful implementation of this technology, our focus is on elucidating an electrolyte that is stable to deep redox cycling, providing a scaffold for optimization.
Amavadin is a naturally occurring molecule that is synthesized by mushrooms in the Amanita genus. It consists of a vanadium ion tightly bound by two tetradentate N-hydroxyiminodipropionate ligands. This redox-active molecule is among the most stable vanadium(IV) complexes known to date. The electrochemical properties of Amavadin analogues as NRFB electrolytes will be presented, including their stability to deep redox-cycling under various conditions and in the presence of water and oxygen. We will also present our progress toward optimization of these electrolytes by altering counter ions and by covalent modification of the ligands.

Ethanol electro-oxidation (EOR) has been widely studied due to high theoretical specific energy (8.0 kWh/kg) and less toxicity of ethanol when compared with methanol. Owing to a complex multiple-electron process of EOR involving many intermediates and products, the EOR mechanism is rather complex and the design of electro-catalysts for EOR is still challenging. Palladium has been considered as a promising anode catalyst for EOR due to its high electro-catalytic activity for C-C bond breaking and tolerance stability. Metal oxide-loaded Pd catalysts (i.e. CeO₂, NiO, Mn₃O₄, and etc.) have been recently investigated and found that they have high electro-catalytic activity toward EOR. In this work, we electrodeposited Pd@PdO core-shell nanodendrites on flexible graphene foam produced by a CVD technique. The as-synthesized Pd@PdO provides ultrahigh electro-catalytic activity toward EOR in alkaline solution with a current density of 386.86 mA/cm². To further investigate the reaction mechanism of EOR at the Pd@PdO electrocatalyst, in situ electrochemical quartz crystal microbalance (EQCM), ex situ X-ray photo-electron spectroscopy (XPS), ex situ Fourier transform infrared (FTIR) and ex situ X-ray adsorption near-edge structure (XANES) were carried out. The significant changes in the masses of adsorbed intermediate at the surface of the Pd@PdO were observed during testing by the in situ EQCM. Other ex situ techniques confirm that acetaldehyde and acetic acid are the major intermediates of EOR in alkaline media.

Incorporating catalytic nanoparticles (NPs) into nanoporous supports is of particular interest for the excellent surface area to volume ratios of the NPs and these supports, which could aid in reducing the amount of precious metals required in these materials. Precious metal NPs that exhibit catalytic activity are especially attractive for their application in power generating systems, such as proton exchange membrane fuel cells and methanol oxidation fuel cells. Nanoparticle supports can further enhance the chemical and electrochemical stability of these materials, as well as positively influence their catalytic efficiency. It would be ideal to have a versatile process that could be easily tuned to adjust the composition of both the catalyst NPs and the porous support. For ease of materials processing and tuning composition, many of the sought after catalyst layers are prepared as two-dimensional substrates. There are elegant techniques, such as atomic layer deposition and chemical vapour deposition, which can be used to create 3D structures. In this study, a relatively simple method is demonstrated for preparing a variety of combinations of NPs within a 3D porous matrix of tunable compositions. This process was demonstrated using different industrially relevant and scalable processes that included physical vapor deposition, electrodeposition, and sol-gel techniques. Confirmation of the composition, structure of the porous support, and loading of the NPs were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Thin cross-sections of these materials were prepared by focused ion beam milling (FIB) with further analysis by TEM based EDS mapping. A range of catalytic supports were prepared, including a range of compositions, which could be further utilized for increased catalytic efficiency and improved chemical and electrochemical stability.

We have recently demonstrated the use of binder-free carbon nanotube membranes as electrode in two-electrode 1M H₂SO₄ aqueous double layer supercapacitor that shows very high power density ~1040 kW/kg based on the mass of both electrodes and time constant of ~ 15 ms with no degradation in performance even after 10,000 cycles. We take advantage of the unique properties of the carbon nanotube membrane to develop a high power/energy density ultra lithium ion battery using the carbon nanotubes as a scaffold for MoO₃ (theoretical capacity ~1117 mAh g^-1) anode. We present the designing and results on the energy/power densities, charge/discharge rate, voltage, and cyclability.

We have demonstrated a new approach to increase the capacitance of super-capacitors. The capacitance of these were increased by more than 40%. This approach is general and may be adopted for any supercapacitor layout.

A typical supercapacitor is composed of two electrodes: an anode and a cathode. The electrodes are immersed in an electrolyte. The capacitor takes advantage of the thin double-layer (Helmholtz layer) interface at each electrode. A separator layer, made of a polymeric porous film (a membrane) is added between the electrodes in order to minimize unintentional discharge. We modified the otherwise passive separator layer into an active diode-like structure. The diode-like structure was made of two electronic layers, p-type and n-type, each made of functionalized carbon nano-tubes. It was permeable to the flowing ions. While dry, the diode-like structure exhibited a typical current-voltage curve of an electronic diode. We postulate that when wet, a local ionic charge separation occurs, which results in an overall capacitance increase.

The electrolyte was 1 M NaCl. Various characterization methods were used: three and two-electrode cyclic voltammetry (CV), chrono-potentiometry and electrochemical impedance spectroscopy; all demonstrated the increase in the overall cell’s capacitance by the structured diode-like separator layer.


[1] Y. Zhang, H. Grebel, “Controlling Ionic Currents with Transistor-like Structure”, ECS Transactions 2 (18). 2007
[2] S. Sreevatsa, H. Grebel, “Carbon Nanotube Structures as Ionic Barriers: A New Corrosion Prevention Concept”, ECS Transactions 19 (29) 91-100. 2009
[3] T. Chowdhury and H. Grebel, "Effect of Gate Electrode in Electrochemical Cells", Session B01: Energy Storage: Batteries and Supercapacitors, Abstract MA2015-01 721, 227 ECS Annual Meeting, Chicago, IL, (May 2015).

There is a specific need for flexible devices that use lightweight and high efficient energy storage units. Super-capacitors can be used for this application since they have good flexibility and are lightweight. MnO₂ has a high theoretical specific capacitance but can rarely be achieved. In this case, a conductive polymer can be coupled with MnO₂ to improve capacitance and conductivity of a hybrid structure based on MnO₂. The need for flexibility in the super-capacitor is based on the current collector, so PET/ITO was utilized as a substrate for the device. The objective of this research was to fabricate and characterize a solid-state electrolyte for super capacitor devices. In this work a flexible super-capacitor is fabricated in order to characterize the solid-state electrolyte. In order to obtain a solid electrolyte, in preliminary work a gel electrolyte was fabricated first and utilized in the flexible super-capacitor for testing. The MnO₂-Polypyrrole composite layer was deposited using Electrodepositing method. Topography studies have been conducted by the use of Atomic Force Microscopy, Scanning Electron Microscope, and Profilometer. Specific Capacitance and current density are also measure for the Super Capacitor device.

Bacteria are ubiquitous organisms found across a bewildering range of habitats. Amongst gram-negative bacterium, Sporosarcina pasteurii is a particularly intriguing bacterial species due to its ability to precipitate calcite under favorable environmental conditions. Given the right chemical environment, this bacterium can catalyze a set of complex reactions via an enzyme urease which ultimately leads to the formation of negatively charged carbonate ions resulting from hydrolysis of naturally occurring urea molecules. These negative ions in turn react with positive metal ions like calcium, which incidentally is present in almost all natural environments, to form carbonates of calcium. This phenomenon is usually classified under the broad domain known as Microbiologically Induced Calcite Precipitation (MICP). An important application of the chemical precipitation lies in its ability to clog pores in an extended porous structure. This has made MICP relevant to several novel engineering applications. For example, this can be applied to heal ageing heritage structures which are too vulnerable to be treated via conventional structural engineering approaches. A suitably enriched bacterial culture, if injected with proper additives, can completely repair the developed cracks from deep within a masonry structure. This has been experimentally studied by monitoring the dynamic evolution of permeability and porosity and the associated improvement in structural strength.

We utilized polymer-based commercial sponge blocks to mimic natural porous media. The blocks were treated with an enriched culture medium full of bacteria cells and compared with untreated control samples. The samples dipped in bacteria solution exhibited superior mechanical strength and reduced porosity. This was quantified using standard mechanical characterization through compressive tests. Pore-scale visualization was also performed using micro-CT imaging. Image analysis on sections of the samples revealed long-range pore-blockage. The present research addresses key issues in the physical understanding of the mechanism of pore-clogging and its relationship to fluid dynamic aspects of the system.

In this study carbon nanofibers with high values of thermal conductivity have been embedded into paraffin wax phase change thermal energy storage material (PCM) in order to enhance its bulk thermal conductivity. The effects of implanting carbon nanofibers (average diameter = 50 nm, average length = 25 µm) on the bulk thermal properties of an organic paraffin PCM (IGI 1245A) is experimentally investigated. The bulk thermal conductivity, volumetric heat capacity and thermal diffusivity of the nanocomposites are obtained as a function of temperature and nanofiber mass fraction. The effect of nanofiber concentration on the PCM’s latent heat of fusion and melting temperature is measured in order to determine the applicability of this nanocomposite in electrical vehicle thermal management systems. Maximum thermal conductivity enhancement in solid and liquid phases at 2 vol% is found to be 31% and 19%, respectively. The nanocomposites’ thermal conductivity enhancement is compared with calculations of effective medium theory considering the role of interfacial thermal transport. Such a nanocomposite with enhanced thermal transport is a promising candidate for passive thermal management applications.

Porous carbon papers are essential components for polymer-electrolyte fuel cells (PEFCs) and redox flow batteries (RFBs). Understanding morphology and associated liquid and gas transport in these porous papers is critical in optimizing RFB and PEFC performance. These components are selected based on their structural stability, high electrical and thermal conductivity, large surface area, and high fluid transport properties owing to their relatively large pores (O(10 mm)) and porosity. To understand and improve these fibrous media, three-dimensional studies are required due to their intrinsic complex and anisotropic structure. For high current-density performance of PEFCs during start-up and at low temperatures, water transport is critical. Without proper management, liquid water may block pores in the electrode porous media and thus restrict reactants' access to catalyst sites. During operation, the oxygen reduction reaction produces liquid water and a significant thermal gradient in the through-plane direction. In turn, the thermal gradient facilitates evaporation and thus increases the relevance of phase-change-induced (PCI) flow. In order to improve water management, it is vital to understand the interplay between evaporation/condensation, PCI flow, and capillary-driven flow. Factors such as heat redistribution due to evaporation/condensation and the dependence of gas-diffusion-layer (GDL) thermal conductivity on liquid-water content increase the complexity of heat and mass transport.

Despite previous investigations (theoretical, model-based, and experimental), evaporation and PCI flow within GDLs is still not well understood. A key reason for this is the challenge of in-situ measurements and visualization of the evaporating water front. This presentation discusses water distribution and quantifies evaporation/condensation under thermal gradients within GDLs. In-situ synchrotron-based X-ray micro computed tomography (CT) was used to visualize water front within the GDLs under various thermal gradients, liquid water saturations, and GDL materials. In addition, water distribution within microporous layers (MPLs), which are carbon materials with nano-porosity, was quantified using nano-X-ray CT.

Acknowledgement:
The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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.

The need for inexpensive energy storage and water treatment technologies have emerged as key technological challenges facing our society. Electrochemical systems such as flow batteries, electrochemical flow capacitors and capacitive deionization cells are among the most promising solutions to these challenges [1]. The recent application of flowable suspension electrodes to the latter devices has led to new functionalities and performance regimes. One major limitation of such electrodes is the poor electronic conductivity of the slurry-based suspension electrodes, which are typically of order 1-10 mS/cm. This low conductivity is the result of transporting electric charge through a discontinuous network of carbon particles, and is many orders of magnitude lower than that achieved by traditional solid electrodes. It has been demonstrated previously that the slurry's electronic conductivity is a strong function of the electrode's carbon weight percentage (CWP, expressed in wt%), as increasing this percentage enables more effective electron transport. However, until now, the practical CWP of slurry electrodes has been limited to about 20 wt% as more concentrated electrodes are no longer flowable. Our recent work has demonstrated a breakthrough in suspension electrodes, achieving a flowable electrode with up to 35 wt% by utilizing upflow fluidized bed electrodes rather than typical slurry electrodes [2]. The high CWP is achieved via leveraging gravitational forces to allow for a highly packed electrode structure. While a highly dense bed forms in the electrode compartment, simultaneously the CWP in the surrounding flow system remains very low (here ~2.5 wt%), thus preventing clogging of the flow system and minimizing pumping requirements. We further will present the first fluidized bed electrode capacitive deionization system, in which we leveraged bead sedimentation to enable closed-loop and continuous water desalination for several days.[2]

References:
[1] Suss, M.E., et al. Energy Environ. Sci. 8.8, 2296-2319 (2015).
[2] Doornbusch, G., Dykstra, J., Biesheuvel, P.M., & Suss, M.E. J. Mat. Chem. A (2016)

Capacitive deionization (CDI) technique has been of great interests in desalination process due to its high-energy efficiency compared to conventionally utilized techniques including reverse osmosis and multistage flash distillation. So far, most CDI systems are fabricated from fixed carbon based electrodes. However, since the ion adsorption capacity of such fixed carbon electrodes is limited, flushing process under the reverse applied potential is absolutely required for desorption of ions from the electrodes, greatly lowering desalination efficiency and generating issue on separation of deionized water from saline water. In order to overcome such limitation of current CDI system, we designed flow-electrode capacitive deionization (FCDI) system utilizing flow-electrode instead of fixed electrode. The flow-electrode, consisting of suspension of activated carbon and NaCl, continuously flows through a flow-path carved on the current collector while the carbon electrode of conventional CDI remains fixed. FCDI unit cell exhibited continuous desalting performance higher than 95 % of removal efficiency at the target saline water with various NaCl concentrations ranging from 2.0 g/L to 32.1 g/L. Our result demonstrates that the flow-electrodes, fed in the FCDI cell, have infinite ion adsorption capacity allowing continuous seawater desalination without the need of flushing process. Also, we figured out that various parameters including salt and electrode concentration of electrode suspension and the surface property of electrode play a key role in determining desalination phenomena. For scaling up the desalination capacity, FCDI stack with five unit cells was realized as well. Furthermore, it was confirmed that the ion storage and extraction (or the ion charge and discharge) of the flow-electrode, charged by constant voltage, generated around 20% of the supplied energy in an FCDI cell during constant current discharge of flow-electrode at the NaCl concentration of 35.0 g/L, suggesting their potential application to energy storage system.

This work demonstrates that redox active, inexpensive, and environmentally friendly manganese oxides show great promise in creating low cost and high performance flowable desalination systems. Capacitive deionization (CDI) utilizing one faradaic electrode and one capacitive electrode, or hybrid capacitive deionization (HCDI), has the potential to achieve significantly enhanced ion removal capacities over standard CDI systems. This was demonstrated in a recent report by the high ion removal capacity achieved with a tunnel structured manganese oxide (Na₄Mn₉O₁₈) in a HCDI configuration [1]. Motivated by this study, we investigate for the first time the ion absorption capacities of other manganese oxides with tunnel crystal structures. These manganese oxide phases, which are composed of MnO₆ octahedra arranged in various tunnel configurations around different stabilizing cations, are attractive for HCDI applications due to the fact that their well defined, open tunnel structures provide crystallographically defined positions for ions intercalation. Further, by changing the stabilizing cation, the size and shape of the tunnels and thus crystallographic positions for ions insertion can be tuned, thereby opening up a unique opportunity for selective ions extraction from water.

Three different tunnel structured manganese oxides are synthesized in this work with a nanowire morphology, an advantageous nanostructure for flow-based systems since it allows for excellent access of water to and utilization of active material surfaces. The three new materials studied for HCDI are α-MnO₂ (K_0.11MnO₂) consisting of 2x2 octahedra square structural tunnels, todorokite MnO₂ (Mg_0.20MnO₂) with 3x3 octahedra square tunnels, and a hybrid sodium stabilized tunnel structure (Na_0.20MnO₂) possessing tunnels with varying dimensions, including 2x2, 2x3, 2x4, and 3x3 octahedra tunnels. The ion removal capacity of each phase is investigated in KCl, MgCl₂, and NaCl solutions, and it is found that each material achieved high ion absorption capacities of over 30 mg of salt per gram of electrode, which is among the highest values ever reported for HCDI. The different materials exhibited selectivity in terms of different ions absorption, and interestingly, the highest absorption capacity for α-MnO₂ and todorokite MnO₂ was exhibited in KCl and MgCl₂ solutions, respectively, which contain the same cation as each structure’s respective stabilizing cations. Further, the hybrid tunnel structure displayed the highest overall ion removal capacities, resulting from the varying large tunnel dimensions allowing for effective accommodation of the most ions. This work demonstrates the promising potential of tunnel manganese oxides for use as efficient and high capacity HCDI electrodes, as well as the opportunity to use these materials for selective ions absorption from aqueous solutions.

[1] J. Lee et al., Energy Environ. Sci., 2014, 7, 3683

Flowable (suspension) electrodes have recently gained a great attention for enabling scalability in various electrochemical applications including grid-scale energy storage and capacitive deionization.¹ A flowable electrode is a multi-phase mixture consisting of electroactive particles suspended in an ionic solution (electrolyte) for charge storage. Upon applied potential, the suspended particles rely on readily formed percolation networks to conduct electrons to/from the current collectors for charge transfer. Upon complete charge, the charged suspensions can be pumped out of the cell and stored for the future use. This principle enables scalability for many energy storage electrode materials that were initially limited to small-scale applications.² The electrochemical flow capacitor utilizes the same principle with suspensions of high surface area carbons, which store charges electrostatically.³ Here, the use of various carbon nanostructures, optimizations and use of carbon particles as a conductive support to incorporate pseudocapacitive organic materials will be presented. Moreover, the viability of the recently discovered 2D nanomaterial, MXenes,⁴ as flowable electrodes will be discussed.

References

1 K. B. Hatzell, M. Boota and Y. Gogotsi, Chem. Soc. Rev., 2015, 44, 8664–8687.
2 M. Duduta, B. Ho, V. C. Wood, P. Limthongkul, V. E. Brunini, W. C. Carter and Y.-M. Chiang, Adv. Energy Mater., 2011, 1, 511–516.
3 V. Presser, C. R. Dennison, J. Campos, K. W. Knehr, E. C. Kumbur and Y. Gogotsi, Adv. Energy Mater., 2012, 2, 895–902.
4 M. Naguib, V. N. Mochalin, M. W. Barsoum and Y. Gogotsi, Adv. Mater., 2014, 26, 992–1005.

Large-scale energy storage is required to meet a multitude of current energy challenges. These challenges include modernizing the grid, incorporating intermittent renewable energy sources (so as to dispatch continuous electrical energy), improving the efficiency of electricity transmission and distribution, and providing flexibility of storage independent of geographical and geological location. In addition, such storage should be scalable for centralized or distributed use.
Through efforts supported by ARPA-E and the Department of Energy Office of Electricity, the technology approach we are developing is based on using very low cost iron electrolytes in a flow battery that will be economically feasible. Additional advantages of the IFB include abundant, non-toxic, and non-corrosive materials that are used to provide an energy storage solution that has inherently safe operation and is environmentally friendly.
In this presentation I will address some of the challenges of this approach. Specifically, I will discuss properties and characteristics of a carbon slurry electrode as they relate to controlling the performance and cost of a slurry iron flow battery that decouples power and energy sizing.

Recently, electrochemical flow capacitor (EFC) has been proposed as the grid-scale electrical energy storage [1]. EFC has the advantages of both supercapacitors and flow batteries, so exhibited high power density and long cycle lifetime like supercapacitors and the scalable energy capacity like flow batteries. However, compared to the flow battery system, the slurry electrolyte has limited capacitance only derived from EDLC.
Our previous study on aqueous electrochemical capacitor revealed that holding quinone derivatives inside the nanopores of activated carbon multiples the energy density of the activated carbon with maintaining high power capability comparable to EDL capacitor, and long lifetime above 1000 cycles [2]. This is derived from the fast kinetics and high reversibility for proton-insertion/extraction reaction with quinone derivatives.
Herein, to address the limitation of the energy density of EFC slurry, in this study, we enhanced the capacity of the carbon by complexing with redox-active organic compounds, quinone derivatives. The quinone derivatives were conjugated with activated carbon beads by simple adsorption, and then the quinone/carbon complex was dispersed in aqueous sulfuric acid solution to form flowable slurry electrolyte. Tetrachlorohydroquinone (TCHQ) and Dichloroanthraquinone (DCAQ) were employed as redox active materials for positive and negative electrodes, respectively. The ideal capacities of TCHQ and DCAQ were 216 and 193 mAh g^-1, and the redox potentials of TCHQ and DCAQ with protons are 0.50 and -0.05 V vs. Ag/AgCl, respectively. This difference in the redox potentials between anodic and cathodic slurry electrolytes facilitates proton-shuttle charge-discharge mechanism to enhance the capacitance. This couple of slurry electrolytes exhibited high energy density (19.4 Wh/kg) with maintaining high power capability of electrochemical capacitor. This energy density is about two times larger than that of the EFC using the unmodified carbon slurry. Moreover, charge-discharge measurement in intermittent flow system confirmed the prepared slurry electrolytes were applicable to electrochemical flow capacitor system.
[1] V. Presser, et al., Adv. Energy Mater, 2, 895 (2012).
[2] T. Tomai, et al., Sci. Rep., 4, 3591, (2014).

Despite extensive research on Zinc Bromine redox flow batteries (RFB), there has been limited commercial implementation of bromine bearing systems in general, and the Zn-Br₂ RFB systems in particular. The root causes are the following: i) Br₂(l) generated during charging can diffuse and react with the Zn deposited at the negative electrode (crossover), leading to self-discharge; ii) repeated zinc plating leads to dendrite formation, which can form a conductive bridge between the electrodes (shorting), and iii) Br₂(l) has low miscibility in aqueous solutions (~2.8 vol%) and tends to stratify, resulting in non-uniform concentration distributions. In the membrane-free, non-flowing, single-chamber zinc-bromine (SC-Zn-Br₂) battery design zinc dendrites are allowed to form freely and Br₂(l) is allowed to stratify; expensive membranes, complexing agents, pumps, and flow controllers are eliminated; and yet overall energy efficiency is improved. This is achieved by using a highly-porous, hydrophobic carbon foam electrode (CFE) for local containment of Br₂ generated during cycling, thus preventing crossover. Zinc dendrites are allowed to grow, but react with the highly corrosive Br₂(l) on the surface of the CFE and dissolve back into the electrolyte as Zn²⁺ and Br^- ions. This prevents direct electrical shorting, and keeps the cell running without generation of irreversible chemical products in the system.

Here, we discuss the design, composition, and fabrication of the CFE, and its effects on the performance of SC-Zn-Br₂ battery. In particular, we demonstrate the 3D design strategies of the CFE to maximize the ‘loose’ storage of Br₂(l) within it. We utilize color-concentration tracking and feedback monitoring scheme to actively control and quantify the deep red Br₂(l), yellow Br₂(aq), and colorless ZnBr₂ electrolyte species generation and transport. Finally, we demonstrate the effect of prolonged exposure to Br₂(l) on the CFE using XPS analysis, and the potential degradation mechanisms. We report coulombic and energy efficiencies of up to 95% and 75%, respectively, for over 1000 cycles of SC-Zn-Br₂ cells at moderate currents using carbon-black and graphite CFEs. The cost, lifetime, and performance potential of this battery design are attractive for grid-scale energy storage applications.

Non-aqueous redox flow batteries have attracted great attentions in electrical energy storage (EES) fields. Organic electrode materials as redox species are considered as a promising candidate due to their low cost, tunable property, and non-toxicity. The researchers in Pacific Northwest National Laboratory (PNNL) reported a novel non-aqueous lithium hybrid redox flow battery using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and ferrocene compound as catholyte in order to achieve high energy density and cell voltage. Recently, we studied the isomeric effect of two quinone-based materials using 5,12-naph-thacenequinone (NAQ) and 1,2-benzanthraquinone (BAQ), and found that BAQ showed higher redox potentials that NAQ due to a lower value of lowest unoccupied molecular orbital (LUMO) energy levels. Particularly, we firstly developed tube-type flow battery system under intermittent flow mode. However, solubility of those materials still limited to obtain high energy density.
In this presentation, we will briefly touch on the research progress of various redox flow battery systems. Then, the effect of the position of methoxy group on 2-phenyl-1,4-naphthoquinone (NQ) will be discussed. We synthesized five NQ derivatives with methoxy group on ortho, para, meta, 2,3,4 position, and 3,4,5 position. As for the redox potential, methoxy functional group could decrease LUMO energy level because of an electron donating property. The prepared materials can reversibly uptake two lithium ions in liquid phase after dissolving in tetraethylene glycol dimethyl ether (TEGDME) containing 1.3 M lithium bis(trifluoromethane sulfonyl) imide salt (LiTFSI). In half-cell test, the different voltage profiles of NQ derivatives were observed, and their cycle stability for 100 cycles and rate capability were compared. Notably, the rate capability of 2-(2-Methoxyphenyl)-1,4-naphthoquinone catholyte was highly improved even at 10C rate. In terms of battery application, we will demonstrate the tube-type Li-organic batteries using the organic catholyte.

Electrochemical energy storage systems such as secondary batteries are the principle power source for portable electronics, electric vehicles and stationary energy storage. Meanwhile the increasing deployment of renewable energy technologies requires efficient energy storage systems to mitigate the intermittency of renewable energy from solar and wind, particularly important for the power grid. As an emerging battery technology, Lithium-based redox flow batteries (LRFBs) inherit the advantageous features of modular design of conventional redox flow batteries and high voltage and energy efficiency of Li−ion batteries, and show great promise as efficient electrochemical energy storage system in potential transportation and residential applications. The unique chemistry of LRFBs with aqueous or non-aqueous electrolytes potentially provides widened electrochemical potential windows, thus may provide greater energy density and efficiency than those of conventional redox flow batteries based on proton chemistry.
In this talk, we will discuss the design rationale, fundamentals and characterization of LRFBs from a chemistry and material perspective, with particular emphasis on the emerging chemistries and materials. We will focus on recent advances in the field, especially Li-based organic redox flow batteries and some critical challenges and opportunities with this new promising battery system.

Flow electrodes based on semi-solid suspensions have the potential to lower the cost of traditional flow batteries, but their yield-stress increases with carbon loading and is a major challenge. Enabling higher carbon loading in the flow electrode has the potential to enable higher current densities and lower polarization. In previous works, the carbon loading has been limited to <0.5vol% even when using slippery surfaces such as Teflon. This work introduces lubricant-impregnated surfaces (LIS) to promote the flow of electrodes with >0.5vol% carbon black (KB) and outlines the design of LIS for flow cells based on interfacial thermodynamics and electrochemical stability. A 0.75vol% KB flow electrode flows on LIS whereas the same flow electrode cannot flow on Teflon. Rheological measurements quantify the amount of slip LIS provide over the state-of-the-art Teflon surface. LIS are then applied to a new flow battery architecture, the gravity-induced flow cell (GIFcell), to demonstrate the flow of an otherwise non-flowing Lithium polysulfide flow electrode in a half-flow cell configuration.

Renewable energies conservation and energy storage have become significant issues to control and regulate demand of on-grid energy supply. Redox flow batteries are promising technologies in the advancement of energy storage which use flowing electrolytes with dissolved redox species. These electrolytes (catholyte and anolyte) are pumped to electrochemical cells and stored in big tanks. This strategy enables long-term energy storage since they exhibit very long-life cyclability. Moreover, their design allows easy uncoupling between energy (size of the storage electrolyte tanks) and power (size of the electrode) providing a very flexible technology to improve both characteristics. Soluble redox species (vanadium, cerium…) have been widely studied [1]. However, one limiting aspect of these technologies is solubility (1-2M) of the compounds which implies a reduced capacity around 50Wh/L. Research has been undertaken to use non-soluble compounds as lithium intercalation particles (LiTi₂(PO₄)₃, LiFePO₄ [2]…), in liquid dispersions, to improve the mass capacity. In this present study, symmetrical flow battery LiMn₂O₄/LiMn₂O₄ are investigated and in particular water electrolyte. This battery demonstrates a theoretical voltage of 1V, very close to vanadium flow battery (1.26V). However, the major advantage, here, is the harmlessness of the electrolytes (water versus sulfuric acid in the case of vanadium) and the symmetry of the battery which allows the design of a unique electrolyte. In particular, electrolyte formulation in one of the crucial barrier to overcome for particle-based flow batteries. The electrolyte must demonstrate high electronical conductivity without losing its flowing properties. The paper focuses specifically on optimized flowable carbon-based dispersions, used to incorporate LiMn₂O₄ particles. They exhibit a conductivity around 3mS/cm, at least twenty times more important than in litterature [3]. The characteristics of several carbons are studied (surface charge, interaction with matrix...) and related to rheology as well as percolation properties in aqueous dispersions. This paper demonstrates the feasibility of using these optimized "liquid electrodes" for flow batteries.



[1] P. Leung, P. De Leon, The Royal Society Of Chemistry, 2012
[2] Z. Li, Y.M Chiang, Physical Chemistry Chemical Physics, 2013
[3] H.Parant, A.Colin, G.Muller, T. Le Mercier, Patent deposit No. 15307101.4 : Flowable aqueous conductive dispersions of carbon particles with and without active additives for flow systems.

Lithium-sulfur (Li-S) batteries have the potential to outcompete their Li-ion battery counterparts and become the next generation of rechargeable batteries for portable electronics and larger-scale power-demanding utilities.¹ However, the insoluble sulfur and various sulfide intermediates (e.g., Li₂S, Li₂S₂)^{2, 3} formed during the cell’s operation continue to impede battery performance through the occurrence of heterogeneous solid-liquid interfaces. Recently, the concept of “semi-liquid battery” was described as one possible approach to avoid the formation of solid Li₂S₂ and Li₂S products by cycling the cell in a voltage range where only S₈ and Li₂S₄ co-exist.⁴ Despite the encouraging performance of the “semi-liquid battery”, the cell’s energy density remains significantly lower than that of conventional Li-S batteries because of the incomplete lithiation of sulfur.

In contrast, our recent studies geared to the use of S-rich functional polymers in Li-S batteries show that intermediate lithium sulfides and fully lithiated products tend to remain well soluble in 1,3-dioxolane/1,2-dimethoxyethane, the electrolyte typically used for Li-S batteries. The presence of functional polymers is also found to help approach the cell’s theoretical energy density maximum by further improving the extent of sulfur utilization across the Li-S battery device. This finding also enables the application of Li-S flow battery for large-scale energy storage.

1. Rosenman, A.; Markevich, E.; Salitra, G.; Aurbach, D.; Garsuch, A.; Chesneau, F. F. Advanced Energy Materials 2015, 5, (16), 1500212.
2. Zheng, J.; Lv, D.; Gu, M.; Wang, C.; Zhang, J.-G.; Liu, J.; Xiao, J. Journal of The Electrochemical Society 2013, 160, (11), A2288-A2292.
3. Nazar, L. F.; Cuisinier, M.; Pang, Q. MRS Bulletin 2014, 39, (05), 436-442.
4. Yang, Y.; Zheng, G.; Cui, Y. Energy & Environmental Science 2013, 6, (5), 1552-1558.

Fuel cell systems are in an increased demand with recent advances in alternative renewable energy sources for reducing fossil fuel consumption and combating climate change. The electrochemical generation of gaseous fuels remains a relatively inefficient component of the fuel cell infrastructure due to mechanisms for passivation of the electrode surfaces. Therefore this study sought to improve the performance of electrode materials for the production of gaseous fuels through correlating this function with the structure of the electrode surfaces. Ordered micro-structured nickel surfaces were prepared by a series of lithographic techniques for their potential influence on enhancing transport efficiencies at the electrode surfaces for electrochemical oxygen evolution reactions (OER) in alkaline media. The electrochemical stability, surface wettability, and potential (and current) dependant efficiencies of OER were investigated for these structured electrodes. Nickel structures with micro-sized features improved the efficiency of the OER processes, exhibiting approximately twice as much current at 100 mV overpotential in contrast to flat surfaces and other control experiments. These structures can enhance efficiency of fuel production, such as through water electrolysis and metal oxide electroxidation.

For polymer electrolyte fuel cells (PEFCs) to achieve broad commercialization the high cost associated with the use of Pt as electrocatalyst in the electrodes, and durability concerns due to carbon corrosion should be resolved. PEFCs’ conventional carbon-supported platinum (Pt/C) electrodes currently use 0.25 mg/cm² of Pt loading, whereas the set target for 2020 by the U.S. Department of Energy (DOE) is 0.125 mg/cm². Non-conventional thin-film electrodes prepared by atomic layer deposition (ALD) are a promising alternative to conventional designs.
In this work, we propose an ALD thermal exposure mode to deposit Pt nanostructures onto a sacrificial anodized aluminum oxide (AAO) substrate using trimethylcyclopentadienylmethylplatinum (IV) and oxygen gas as precursors. Thermal exposure mode provides additional time for the reactions in the ALD chamber and allows higher penetration of the precursor into the substrate. After the deposition the ALD electrodes were hot-pressed onto Nafion XL membrane and sacrificial AAO template was etched. We have demonstrated the feasibility of long free-standing structures of about 6-8 μm.
The ALD cathode electrodes were electrochemically characterized within a fuel cell, where an anode was house-made anode gas diffusion electrodes (GDE). Polarization curves, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were performed at dry and wet conditions to assess activity and water management of these electrodes. In this presentation we will compare various ALD electrode fabrication conditions, ionomer content and provide detailed electrochemical analysis of these nanoelectrode arrays.
Acknowledgements
This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrustructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. CNS is part of Harvard University. We thank Mr. Jake Berliner for helping in ALD electrodes characterization and Mr. Berney Peng for SEM sample preparation. Fabrication of the structures was carried out in part at the Tufts Micro and Nano Fabrication Facility.

The production of complex multiscale architectures is of significant interest in materials science, and the integration of those structures into engineering applications provides methodological breakthroughs in practical issues by exploiting a multiscale approach that integrates rational advantages from both the microscale and nanoscale. Complex hierarchical microarchitectures have rationally emerged, nearly invariably, from the multiscale needs for overcoming engineering issues in a single scale; however, the production of complex three-dimensional architectures with soft materials remains challenging, mainly because soft lithography basically employs sufficient thermal or ultraviolet (UV) treatments to fully solidify the raw materials for high pattern fidelity. UV-curable resins that guarantee simple yet rapid replication within minutes at both the microscale and nanoscale are typically manipulated into patterns under a solid cross-linking with polymer chains, preventing reliable and sustainable secondary deformations into predefined structures because of the restorations from elasticity and mechanical failures after the fracture point. To address those challenges, we demonstrate a multiplex lithography process that is a simple strategy for the stepwise, multistep treatment of UV-curable resin. A LEGO®-like multiplex stacking that employs the deformation of a viscoelastic coating on a multiple contrast brick (MCB) is delineated in our method, together with the concept of the scavenging effect of infiltrated oxygen, to form multiscale hierarchical architectures via a replica moulding (REM) process. To demonstrate outstanding multiscale engineering in energy applications, the architecture from multiplex lithography was adapted to polymer electrolyte membrane fuel cells (PEMFCs). The PEMFC with multiscale architectured Nafion membrane exhibited largely enhanced performances at various operating conditions due to significantly lowered membrane resistance from reduced effective thickness and increased electrochemical active surface area (ECSA), resulting from augmenting the triple-phase boundaries (TPBs) which are areas related to electrochemical reactions with the aid of multiscale architectures.

Anion exchange membrane (AEM) fuel cells are a possible route to overcoming fundamental issues with acid-based fuel cells. The issues include the high cost of platinum catalysts, complex water management, and sluggish electrochemical reactions. However, the performance of AEM fuel cells is not as good as that of proton exchange membrane (PEM) fuel cells partially because of the limitations of current anion exchange membranes and ionomers, such as low ionic conductivity, poor stability at high pH, and high water uptake.
A series of partially fluorinated multiblock copoly(arylene ether)s with long head-group tether were synthesized for use in AEM fuel cells and electrolyzers. The nanophase-separation resulting from the multiblock structure facilitates the formation of efficient conductive channels for ion transportation. Multiblock copolymers with different block lengths and ion exchange capacity (IEC) were synthesized to explore the relationship between chemical structures and membrane properties and identify the optimized channel size to maximize ionic conductivity and lower water uptake. Channel sizes in the range of 5 to 25 nm were observed from atomic force microscope (AFM) phase image. Large channel size led to high ionic conductivity but relatively high water uptake as well. A non-linear relationship was found between the number of head-groups on a hydrophilic block and the conductivity. Doubling the number of head-groups more than doubled the hydroxide conductivity. Hydroxide conductivity as high as 119 mS/cm at 80°C have been observed with a specific size block size: X_5.4Y₇, where X is the hydrophobic block and Y is the hydrophilic block. The hydrophobicity of the backbone has allowed synthesis of polymers with minimal water uptake. The ratio of conductivity-to-water uptake shows that less water is absorbed compared to conventional materials. The number of waters per ion within the polymer is as low as 4. This is reflected in the measurement of the amount of free-water and bound-water. No conductivity loss was observed after soaking the membrane in 1M NaOH solution at 60°C for over 1000 hours.
Financial support from US Office of the Deputy Assistant Secretary of the Army for Defense Exports and Cooperation (DASA-DE&C) is gratefully acknowledged.

Polymer electrolyte membrane fuel cells (PEMFCs) are a critical technology towards a sustainable energy future as they represent a highly efficient and zero-emission power alternative for automotive transportation. The ionomer electrolyte dispersion and pore structures within PEMFC catalyst layers (CLs) are crucial in providing pathways for rapid proton exchange, gas/mass transport, and water management. Understanding the 3D dispersion of the ionomer network within CLs at multiple length scales can greatly facilitate the optimization of ink processing variables. Ionomer visualization and analysis is challenging via standard scanning transmission electron microscopy (STEM)-based tomography, primarily due to the low contrast exhibited by the ionomer, insufficient X-ray counts generated, and rapid ionomer damage during tilt series data acquisition. To reliably image/map the 3D distribution of ionomer within the CL structure, we combined ultramicrotomy serial-sectioning across CLs, with analysis conducted using a high brightness, high X-ray collection efficiency STEM/energy dispersive X-ray spectroscopy (EDS) system. The EDS-acquired fluorine maps acquired from each of the thin sections of the CL are aligned and stacked using gold fiducial markers; the reconstructed 3D CL volumes demonstrate the extent of ionomer thickness variations and ionomer aggregation association with secondary pores, which calls into question the use of conventional spherical-pore models that include uniformly thick ionomer thin films ranging in thickness from 3-10 nm. Rather, isolated ionomer aggregates, which range in size from 10 to >>100nm, are directly associated with irregularly shaped secondary pores having a branched morphology that indicate a disrupted thin ionomer film network. Future work is aimed at the correlation of 3D datasets from CLs prepared via different processing routes and incorporating different catalyst and/or support materials, from the micro- to the nano-scale.