Symposium EN09 : Materials and Systems for Grid Energy Storage---Redox Flow Batteries

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. Compared with separated solar conversion and storage devices, combining both functions into a single integrated device represents a more efficient, compact and cost-effective approach to harvest, store, and utilize solar energy. We have developed novel hybrid solar-charged storage devices that integrate regenerative semiconductor solar cells and redox flow batteries (RFBs) that share the same pair of redox couples. In these integrated solar flow batteries (SFBs), 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. When electricity is needed, the charged up redox couples will be discharged on carbon electrodes to generate the electricity as in a RFB. We have demonstrated that such SFB devices can be charged under solar light without external electric bias and deliver a high discharge capacity comparable with state-of-the-art RFBs over many cycles. After developing silicon solar cells and high performance solar cells that are carefully matched with various organic or inorganic redox couples and optimizing several generations of SFB device designs, we have recently achieved integrated SFB device with an overall direct solar-to-output electricity efficiency (SOEE) of 14%. These high performance SFBs can serve as distributed and standalone solar energy conversion and storage systems in remote locations and enable practical off-gird electrification.

Challenges posed by the intermittency of solar energy source necessitate its integration with proper energy storage systems. Bridged by carefully chosen reversible redox reactions, the monolithic integration of photoelectrochemical solar energy conversion and electrochemical energy storage offers an efficient and compact approach toward practical solar energy utilization. Here we present an integrated solar flow battery (SFB) device by integrating regenerative solar cells in aqueous electrolytes with RFBs using the same pair of organic redox couples. The SFB can be configured to perform all the requisite functions, including solar energy harvest, conversion, storage and redelivery, without external bias. Exploiting high efficiency photoelectrode, properly chosen redox couples and optimized flow filed design, we demonstrate a record solar-to-output electricity efficiency (SOEE) of 14.1%. In addition, we describe the major advantages of individual components used to assemble the record-breaking device, and discuss the possible pathways for future developments. This work paved the way for a promising new approach to harvesting, storing and utilizing the intermittent solar energy with high energy conversion efficiency and energy storage density. These integrated SFBs will be especially suitable as distributed and standalone solar energy conversion and storage systems in remote locations and enable practical off-gird electrification.

The increasing penetration of solar generated electricity in the United States will eventually lead to a glut of grid power during times of peak solar intensities. We discuss and present preliminary studies of an entirely new approach for converting and storing intermittent solar energy by directly storing the sunlight as electrochemical energy. This solar charged redox flow battery has the additional advantage of being able to capture and store the heat energy, that is 80% of the incoming solar flux, that is not used in the electrochemical reactions to then be used for space and water heating. This hybrid system could replace both a rooftop solar water heating systems and some photovoltaic panels resulting in substantial cost savings and wider implementation of solar energy and load leveling of the electrical grid. Compared to photoelectrochemical water splitting this approach has several other advantages including not having to deal with gases and bubbles that block light, many possible semiconductor redox couple combinatons rather than being restricted to semiconductors with band edges that match only water oxidation and water reduction potentials and use fast one-electron couples require no multielectron electrocatalysts reducing overpotential losses. We will present results using p-type InP and n-type GaP photoelectrodes with oxide protection layers to drive the well studied V(III) reduction and V(IV) oxidation reactions to directly store solar energy.

The growing deployment of variable renewables has created both need and opportunity for affordable energy storage over multi-day and longer durations, which may then enable renewable power plants that are is cost-competitive with fossil fuel generation. To reach this goal, stationary storage is desired that adds a cost of ~$0.03/kWh-cycle or less to the cost of renewable generation. However, at long durations, the decreasing number of operating cycles over lifetime dictates that the capital cost of a storage system must proportionally decrease in order to meet such metrics. Storage technologies are desired that have exceptionally low cost of stored energy, while operating at much lower C-rates than for most battery applications. Flow batteries have the flexibility of design to meet these requirements if the underlying chemical cost of storage is low enough. This talk will give examples of use-case and techno-economic analyses that help to define requirements, and report on progress towards these goals using aqueous sulfur based flow chemistries.

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. Electrolyte is de facto the most critical component in a RFB system. This presentation describes development of new electrolyte chemistries at Pacific Northwest National Laboratory. Solvation chemistry of the different electrolyte systems will be discussed, which provide a greater understanding of dynamic interactions between solvent-solvent, ion-solvent, and ion-ion at the molecular level. Such understanding is pivotal in developing new redox flow system with higher energy density and temperature stability. A new RFB based hydrogen generation technology will also be presented.

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. 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 high performance flow batteries based on the aqueous redox behavior of small organic and organometallic molecules, e.g. [1-5]. These redox active materials can be very inexpensive and exhibit rapid redox kinetics and long lifetimes. This new approach could enable massive electrical energy storage at greatly reduced cost. I will discuss the latest developments from our laboratories.

[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909

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

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

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

[5] http://aziz.seas.harvard.edu/electrochemistry

Large energy storage are achieving a high attention due to the wide application of the renewable energy such as solar, wind and hydroelectric power. Among all the technologies, a growing interest in flow battery has been observed due to their features of high efficiency, high safety, long cycle life as well as separation of the energy and power density. Although much work has focused on the development of FB technologies, low energy densities, high cost, and poor reliability have hindered their further commercialization. Therefore, it is vitally important to find a new flow battery system with low cost, high energy density, and excellent electrochemical kinetics. Zinc and iron are two common elements that have the abundant reserve in the earth crust with the relative high electrochemical activity. Furthermore, due to the high solubility of zinc and iron salts, the battery has the potential to achieve a high energy density. Most importantly, compared with some redox couples such as Br/Br^- and Pb/Pb²⁺, zinc and iron salts are environment-friendly.
In this study, we use the FeCl₂ and ZnBr₂ as the redox reactants to establish neutral zinc-iron flow battery system. Using the glycine as the complex agent to overcome the intrinsic problem of the hydrolysis of Fe²⁺/Fe³⁺, the electrolyte stability can be greatly improved. Moreover, the complex effect of glycine enlarges the volume of the redox moiety which will alleviate the crossover problem. Furthermore, the application of the low-cost PBI porous membrane to replace the expensive ion exchange membrane ensure the battery with high ion conductivity. Besides, the ion exchange membrane suffers from the membrane fouling and may result in a high omhic resistance which can be confirmed by the area resistance test. The battery test indicate neutral zinc iron flow battery can be operated at 40 mA/cm² with a high energy efficiency of 86%, even at 80 mA/cm², an efficiency of nearly 78% can be obtained for more than 100 cycles. Above all, an energy density about 50 Wh/L can be achieved with the electrolyte of 2 M with the cost of less than 150 $/kW h. Based this study, we offer a good strategy to applies the FeCl₂ electrolyte and provide a reference to develop low-cost battery system.

In this presentation we will show that, by emphasizing physical organic chemistry, molecular engineering is a powerful strategy to develop high performance redox active molecules for flow battery application. We will discuss the development of pH neutral aqueous organic redox flow batteries (AORFBs) employing water-soluble redox active molecules including viologen, ferrocene, TEMPO, and Ferrocyanide.^[1-5] Particularly, the chemistry and battery performance of two electron storage viologen molecules will be highlighted.^[6] In addition, our effort in developing non-aqueous organic redox flow batteries (NAORFBs) will be also covered.

References: (1) Hu, B.; DeBruler C., Rhodes, Z.; Liu, T. L., J. Amer. Chem. Soc., 2017, 1207-1214. (2) Liu, T.;* Wei, X.; Nie, Z.; Vince, S.; Wang, W. Adv. Energy Mat. 2016. (3) B. Hu, C. Seefeldt, C. DeBruler, T. L. Liu, J. Mater. Chem. A 2017, DOI: 10.1039/C1037TA06573F. (4) Filed US Patents covering applications of water-soluble viologen, ferrocene derivatives, and TEMPO for RFBs. (5) Luo, J.; Sam, A.; Hu, B.; DeBruler, C.; Wei, X.; Wang, W.; Liu, T. L. Nano Energy 2017, ASAP. (6) DeBruler, C.; Hu, B.; Moss, J.; Liu, X.; Luo, J.; Sun, Y.; Liu, T. L. Chem 2017, ASAP.

Biography: Dr. Tianbiao Leo Liu received his Ph.D from Texas A&M University in 2009, served as staff scientist at Pacific Northwest National Laboratory from 2013 to 2015, and is currently an assistant professor at Utah State University. His research is broadly spread on energy and green chemistry including electrocatalysis, electrochemical energy storage, and environmentally benign chemical transformations.

Redox flow batteries (RFBs) have become indispensable in the debate on the storage of renewable energies. To date, over 60 different combinations of active materials have been released, but only a small number are in commercialization. The reasons for this are complex but superficially determined by the specific chemical and physical properties of the active materials, as well as the production technologies used. The supposedly simple electrochemical reactions ultimately result in a complex system for reducing undesirable effects. Currently the most important commercial systems are mainly V/V-, Zn/Br-, Zn/Fe- and H/Br-RFBs, as well as the significantly increased number of water-soluble organic RFBs. VRFBs have today reached a stage of development where costs can be reduced, notably through alternative materials such as microporous separators and low-cost electrode materials, as well as optimized production while increasing life time. However, it should also be noted that side reactions such as hydrogen evolution and oxidation by atmospheric oxygen possibly require more complex balancing measures through the use of recombination or regeneration cells and thus makes the system more expensive, especially if service lives of 20 years are to be achieved. In addition, the question of the electrode material must be clarified, i.e. how an inexpensive electrode material must be designed to achieve a favorable power density/cost ratio and, in this context, the influence of the structure and composition of the electrode surface. Thermal stability of the electrolyte solution plays more than higher energy density an important role to reduce costs. With the use of additives or alternative solvents, the interplay of physical and electrochemical properties and side reactions is a challenge that is still in need of improvement. Organic compounds as active material represent an interesting alternative to potentially more expensive inorganic redox pairs. However, some fundamental questions have to be clarified. Pure organic electrolytes have low conductivities, low solubilities and thus low power densities which leads to high storage costs. Water-soluble organic compounds circumvent this problem, but it is necessary to add conductive salts that alter the properties of the electrolytes. However, one of the biggest challenges is the possibility of side reactions in the potential range of the other active material and generally the stability of the radicals over a longer period of time at higher states of charge and in particular at higher concentrations of active material. Electro-migration effects, the impact of real concentrations and real storage times must not be disregarded when evaluating a new electrolyte in order to be able to assess the application potential. In this talk, we give an overview of today's general challenges to RFBs, as well as in detail to the main commercial representatives VRFB and Zn / Br-RFB, as well as organic based RFBs.

In this work, we introduce new electrode catalyst for vanadium redox flow batteries (VRFBs) and new activated organic species and electrode catalyst including improved redox reactivity by adoption of new catalyst for aqueous organic redox flow batteries (AORFBs). First, in terms of the catalysts for VRFB, new hydroxamic acid functionalized carbon nanotube (HAA-CNT) catalyst is suggested to evaluate its impact on redox reactions of vanadium ions and performance of VRFB with comparison with them using pure CNT and carboxylic acid functionalized CNT catalysts [1]. With the associated measurements, it is proved that HAA-CNT indicates excellent catalytic activity and reaction reversibility because of chelation ability of hydroxamic acid included in the HAA-CNT. Second, regarding the activated species for AORFB, modified form of alloxazine (M-Alloxazine) that is initially suggested by Aziz and Gordon group [2] is used as the species and its optimized synthetic conditions are determined with improved redox reactivity and cell potential. According to the related measurements, performance and durability of the AORFB using the species are compatible with previous result [2]. In addition, when the acidified CNT (A-CNT) catalyst is further involved, the redox reactivity and cell potential of the M-Alloxazine is more improved due to electron transfer promoted by proper difference in electronegativity between the M-Alloxazine and A-CNT. The effects of the new electrode catalysts and activated organic species on performance and durability of AORFB and VRFB are more introduced in this MRD meeting presentation.

References
[1] C. Noh, S. Moon, Y. Chung and Y. Kwon, Journal of Materials Chemistry A, 5, 21334, 2017.
[2] K. Lin, R. Gomez-Bombarelli, E.S. Beh, L. Tong, Q.Chen, A. Valle, A. AspurGuzik, M.J. Aziz, and R.G. Gordon, Nature Energy, 1, 1, 2016.

Redox flow batteries (RFBs) are promising candidates for grid storage, with a few large-scale systems currently in operation. However, current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials costs, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable an increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. A limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small-molecule two-electron donors and/or two-electron acceptors. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase solubility as well as molecular stability in all relevant states of charge. In particular, we highlight the design, synthesis, and electrochemical analysis of posolyte materials with high oxidation potentials. Molecular solubility and stability in various electrolytes will be considered. Spectroscopic, electrochemical, and flow battery results will be presented.

Redox flow batteries (RFBs) are attractive for large scale energy storage applications for the electric grid of the future due to their scalable energy capacity and sustained discharge at peak power. RFBs store charge in energized fluids circulating between the cell and the external tanks. The performance of these devices critically depends on the energy density and chemical stability of the redox active molecules (ROMs) in these fluids.
In this talk, two novel bicyclical substituted dialkoxybenzene molecules, BODMA and BODEA, will be discussed for use as catholyte materials in NRFBs. These molecules have been engineered to provide greater solubility (in their neutral state) and improved chemical stability (in their charged state) as compared to other tetra-substituted dialkoxybenzenes. The structural differences between these two bicyclo-alkyl substituents have considerable effect on their electrochemical behaviour and physical properties. A hybrid flow cell using BODMA as the catholyte material demonstrated stable efficiencies and capacity over 150 cycles. Our study indicates that (when designed properly) full substitution in a small-molecule catholyte material can significantly improve electrochemical cycling performance, widening dramatically the structural space for optimization of such materials for NRFBs.

Organic redox flow batteries (RFB) are possible alternatives to aqueous redox flow systems for large scale energy storage. The advantages of organic based RFB’s over aqueous systems are increased solubility of redox couple and a larger voltage operating window. A sodium based system is of interest due to sodium’s low cost and natural abundance. One major obstacle for organic RFB’s is the lack of available membranes that can withstand the harsh conditions of the system. A membrane for RFB needs to possess sufficient mechanical strength, high conductivity, ion selectivity and excellent chemical resistance. In this project, mechanically robust, chemically resistant, crosslinked membranes have been fabricated via thermal curing or UV curing. In one system, crosslinked poly(ethylene oxide)-based membranes containing 10 wt% to 40 wt% sodium triflate have been characterized as dry and plasticized samples. Membranes are free standing, flexible and robust with and without plasticizer. Specific ionic conductivity is governed by the polymer’s segmental dynamics and coordination of the anion and cation. Specific ionic conductivity for dry membranes ranges from 1.5 x 10^-8 S/cm to 3.0 x 10 ^-6 S/cm at 20°C. Infrared and Ramen spectroscopy indicates formation of ion-aggregates and ion pairs when the sodium triflate concentration exceeds 24 wt%, elucidating the trend of the ionic conductivity. Glass transition temperature (T_g) increases from -40 °C to -6 °C as sodium triflate content increases from 10 wt% to 40 wt%, while T_g of plasticized membranes were almost constant around -55 °C. Furthermore, our efforts on the synthesis and characterization of sodium sulfonate functionalized crosslinked membranes will also be discussed.

Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy.

Redox flow batteries continue to attract much attention as a scalable solution for the ever increasing demand of storing energy from naturally intermittent energy sources such as solar and wind energy farms. Organometallic complexes are attractive candidates for serving as electrolytes, as they provide a rich and versatile electronic structure that can be controlled conveniently by varying the composition of the ligands, which in principle allows for engineering effective solutions. Here we examined strategies for rationally modifying the redox properties of a common and representative Co-complex, namely Co(II) bearing bidentate or tridentate pyridyl and pyrazolyl ligands to optimize their redox behavior. We profiled galvanostatic performances of Co-complexes as catholytes and anolytes in prototype battery cells over 600 cycles and demonstrated enhanced cell voltage and stability through tuning of Co metal-ligand bonding strength and denticity. Unlike organic or single-atom systems, these complexes allow for dissipating the electronic stress associated with the excess electron by spin-crossover. In addition, the chelation effect can be used to enhance the stability of the complex: By increasing the denticity of the ligands, the number of ligands on the metal can be reduced to two from three while maintaining the core structure of the metal complex. As a consequence, the entropic penalty associated with the complexation is lowered by nearly 10 kcal/mol, contributing to the stability of the catholyte during the charge-discharge cycles. This approach allowed for identifying specific guiding principles for designing efficient and robust electrochemical materials that display promising cycling stabilities.

Nonaqueous redox flow batteries are often limited by the solubility of the redox active species, constraining the maximum energy density. One route to increasing the energy density is through chemical mediation of solid energy-storing materials, such as common Li-ion battery anodes and cathodes. These solid energy-storing materials can be contained in canisters through which an electrolyte with redox active species (mediators) is flowed in typical flow battery fashion. The redox potentials of the flowing species are chosen to mediate the chemical reduction and oxidation of the solid energy-storing materials. Afterwards, the mediators are recharged in the electrochemical cell. This strategy is advantageous in that it allows for independent optimization of the flowing electrolyte (e.g. for low viscosity, high discharging rates) and the solid energy-storing media (e.g. for high energy density). We demonstrate application of this strategy to some common Li-ion battery materials and show how choice of membrane and mediator influences overall cell performance in terms of efficiencies and capacity fade. Optimization of this mediated system’s parameters promises flexible, high density energy storage for nonaqueous redox flow battery systems.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

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. They also hold promise for significantly reducing cost. Commercially available RFBs are based on aqueous electrolytes, consequently the cell potential is limited by the stability window of water. 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. Metal coordination complexes (MCCs) offer the possibility of multiple electron transfers, high solubilities in non-aqueous solvents and low cost. This presentation will describe our efforts to correlate experimentally measured standard potentials, solubilities, and cycle lifes, with selected chemical, structural and electronic properties determined using density functional theory (DFT) calculations. A particular focus for our 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. These relationships and associated predictive models have been used to design next generation MCCs. In addition, this presentation will explore a common ion design that holds promise for significantly reducing the cost of RFBs.

In conventional batteries with enclosed configuration, active materials are coated on current collector with binder and carbon additives in order to form conducting electrode sheets. In 2006, “redox targeting” of poorly conductive materials such as LiFePO₄ was proposed by us to eliminate the need for carbon additives. In the presence of redox shuttle molecules, an active electrode material can be reversibly delithiated/lithiated via redox targeting reactions without being attached to the current collector, for which the transport of electrons between the material and the current collector is mediated by the diffusion of redox molecules dissolved in the electrolyte. The application of redox targeting reactions to both the anode and cathode intuitively leads to a novel energy storage device — redox targeting-based flow battery. In this novel device, the active materials are stored statically in two separate tanks and power is produced in the cell stack by the redox reactions of redox mediators, disruptively changing the operation mode of the conventional batteries. The transport of electrons between the active materials and the current collector is mediated by the circulation of redox shuttle molecules in the electrolyte. Such flow battery devices present significant advantages over other types of electrochemical energy storage devices in terms of energy density, safety, and operation flexibility for large-scale stationary energy storage. In this talk, the latest progress on the development of various redox targeting-based flow batteries, with the application of redox targeting concept to various battery chemistries, will be reported. The present status, challenges and future development will be addressed in the talk.

Non-aqueous redox-flow batteries have emerged as promising systems for large-capacity, reversible energy storage capable of meeting the variable demands of the electrical grid. Here, we investigate the potential for a Lindqvist polyoxovanadate-alkoxide (POV-alkoxide) clusters to serve as the electroactive species for a symmetric, non-aqueous redox-flow battery. POV-alkoxides display four quasi-reversible redox events, and demonstrate significant solubility and stability in acetonitrile across all charge-states. Using the POV-alkoxide cluster in acetonitrile as the electrolyte solution at both the anode and cathode of a static H-cell, we obtain coulombic efficiencies of nearly 100% for over 500 charge-discharge cycles. This application of hexavanadate clusters as electrolytes in organic media demonstrates that the remarkable redox properties of multi-metallic metal-oxide assemblies can be harnessed for non-aqueous energy storage applications, and represents an important new direction for the generation of high performance non-aqueous redox-flow batteries.

Redox flow batteries (RFBs) are promising energy storage devices for grid-level applications due to their extraordinarily long cycle life and the ability to independently scale their energy and power densities. The energy density of conventional RFBs is dictated by their capacity (which is directly related to the solubility of the redox species in the electrolyte) and operating potential. Numerous RFB chemistries utilizing both aqueous and non-aqueous electrolytes have been explored.^[1-3] In general, aqueous RFBs have low operating potentials ca. 1.5 V, resulting in poor energy densities (25 – 30 Wh/kg for an all vanadium RFB), whereas systems containing organic electrolytes with wider electrochemical windows have moderately higher energy densities.

The present study describes a revolutionary new approach^[4-7] which uses mediated electrochemical reactions involving anion radical species to utilize a high capacity anode in a redox flow configuration. Rather than using the solvated anion radicals as the primary energy storage medium, herein the anion radicals are transferred to a plug flow reactor containing an active material powder which is charged/discharged through mediated electrochemical reactions. In this configuration, the anion radical species can be recycled several times throughout the cell stack during a single charge/discharge cycle, effectively decoupling the RFB’s energy density from the redox species’ solubility in the electrolyte. This approach can conceivably be used to achieve energy densities exceeding 200 Wh/kg which is ~10 times greater than that of conventional RFBs.

This presentation will describe the mechanism of a mediated RFB and discuss important design considerations (e.g., selection of appropriate electrolyte, mediators, active materials, etc.). Results showing electrochemically mediated sodiation and desodiation of an anode in a redox flow configuration will be presented. The structural evolution of the active material during charge/discharge as determined from Raman spectroscopy and X-ray diffraction (XRD) will also be discussed.

Acknowledgements
Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.

References
[1] X. Wei et al., ACS Energy Lett. 2017, 2, 2187-2204.
[2] D. S. Aaron et al., J. Power Sources 2012, 206, 450-453.
[3] C. N. Sun et al., J. Electrochem. Soc. 2014, 161, A981-A988.
[4] Q. Huang, H. Li, M. Grätzel, Q. Wang, Phys. Chem. Chem. Phys. 2013, 15, 1793-1797.
[5] Q. Wang, Q. Huang, US Patent Application, Pub. No. US 2014/0178735 A1 2014.
[6] J. Yu, et al., Nat. Commun. 2017, 8, 14629.
[7] T. M. Anderson, et al., US Patent No. 9,548,509 B2 2017.

Rotating Disk Electrode (RDE) measurements on model glassy carbon (GC) substrates and Cyclic Voltammetry on more practical commercial carbon supports are used to demonstrate that the kinetics of the positive VO²⁺/VO₂⁺ and negative V³⁺/V²⁺ redox reactions can be substantially enhanced by using electrostatic layer-by-layer assembly (LbL) to decorate their surface with graphene and bismuth nanoplatelets (NPs). Substantial increases in exchange current densities, i₀, are observed relative to standard carbon electrodes. Tafel slope analysis is compared to electron microscopy imaging to conclude that while faster redox kinetics is associated with an increase in the available active area, the prevalence of smaller NPs and associated edge sites the can attenuate activity gains with increasing number of layers. Practical implementation to existing VRFB configurations was demonstrated through the application of NP coatings on carbon felt (CF). The NP coatings yielded a significant increase relative in voltage and overall efficiency of charge discharge curves obtained under typical VRFB cell operating conditions. Furthermore, a substantial increase in the discharge time is observed.

Renewable energy sources, like solar and wind power, are among the central topics of our times with the issues of energy crisis and environment pollution. However, the random nature of these intermittent renewable energy sources (change with synoptic conditions, diurnal variation, etc.) makes it quite challenging for its utilization and dispatch through the grid. One of effective solutions is to connect the power station and the grid using electrochemical energy storage techniques.
Vanadium flow battery (VFB), one kind of electrochemical energy storage techniques, is rather suitable for this application due to its features like long life time, active thermal management as well as the independence of energy and power ratings. Dalian Institute of Chemical and Physics (DICP) has devoted to VFB research for more than 10 years from materials to system integration, where the VFB stacks with power rating from 5 W to 32 kW were successfully developed. To further accelerate the commercialization of VFBs, a spin-off company (Rongke power Co.Ltd) was established in 2008, after which different demonstrations of VFBs in different application field were carried out. In 2012, a world largest 5 MW/10 MWh VFB system was successfully implemented by Ronke Power.
In this presentation, the detailed research and development of VFBs including key materials, stacks and application will be introduced. The challenge and prospect will be summarized as well.

Vanadium flow battery (VFB) is deemed to be one of the most promising technologies for large-scale industrial energy storage, due to its attractive features like flexible capacity, active thermal management, long cycle life and high safety. Dalian Rongke Power Co., Ltd. (RKP) has been devoted to the fundamental study and advanced research of Vanadium flow battery applications. It has become one of the world-leading enterprises in VFB development, manufacturing and delivery industry. The products have evolved from earlier indoor structure to the latest containerized system designs. More than a dozen VRFB systems have been deployed and end products have been launched into operations across the world over the past decades.

The 500kW/2MWh VFB product was developed for the large scale energy application, which is consisted of 16 cell stacks. The high integration level, low shunt current loss, low auxiliary power supply consumption and high reliability were achieved. The novel cell stack is designed by optimizing the key materials and cell stack structure, which can achieve a better performance and higher reliability at a relatively low cost. The energy efficiency (EE) could reach 80% at the rated power of 33kW, and the current density could be improved to 140~150 mA/cm2.The cycle life test indicated that no obvious performance decay was found even after 10000 cycles. The advanced two-stage power conversion system (PCS) was selected in order to realize the independent control of each sub-systems, which would improve the overall reliability ultimately. The AC/AC energy efficiency could reach 70% at the rated power of 500kW. The 500kW/2MWh VFB product has been installed in power plants for wind and solar energy storage and has obtained better application effect.

The world largest battery system, a 200MW/800MWh VFB peaking power plant conducted by RKP, is currently in progress in Dalian, China. The purpose is to improve the Northeast grid resiliency and facilitate smooth integration of renewables. A 500kW/2MWh VFB product is the minimum storage unit of the project. Each unit product could be controlled independently. The foreseeable success of the project would exert great influence on the development of the flow battery in the future.

In this presentation, new technology and product developments of vanadium flow battery at RKP will be introduced and further challenges will be addressed as well.

Vanadium flow batteries (VFBs) are an attractive technology for a variety of energy storage applications^1-5. They offer the advantage that cross-contamination due to transport through the membrane is effectively eliminated because the anolyte and catholyte differ only in the oxidation state of the vanadium. 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. 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⁺). 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 time⁴. There have been several studies^1,2,4,5 of the stability of V^V in the catholyte of VFBs. In this paper we discuss our recent work in measuring and modelling the kinetics of precipitation of V^V from H₂SO₄ solutions in the absence and in the presence of additives.

We investigated the stability of typical vanadium flow battery (VFB) catholytes at temperatures in the range 30–60°C for V^V concentrations of 1.4–2.2 mol dm^-3 and sulfate concentrations of 3.6–5.4 mol dm^-3. In all cases, V₂O₅ precipitates after an induction time, which decreases with increasing temperature. The logarithm of induction time for precipitation increases linearly with inverse temperature and with sulfate concentration and decreases linearly with V^V concentration. The slopes of these plots give values of activation energy and concentration coefficients which we used to generate a quantitative model of catholyte stability.

The addition of H₃PO₄ has a strong stabilizing effect on VFB catholytes at higher temperatures. For example, at 50°C the induction time for precipitation for a typical catholyte is enhanced ~12.5-fold by 0.1 M added H₃PO₄. However, the behavior is rather complex and at higher concentrations induction time begins to decrease with increasing concentration of H₃PO₄. Arrhenius plots for low concentrations of added H₃PO₄ show reasonable fits to straight lines. Experiments at 70°C using other phosphate additives (sodium triphosphate, Na₅P₃O₁₀, and sodium hexametaphosphate, (NaPO₃)₆) showed similar results to H₃PO₄. Other additives were similarly investigated: the results will be presented and discussed.

1. D. Oboroceanu et al., J. Electrochem. Soc., 164, A2101 (2017).
2. D. Oboroceanu et al., J. Electrochem. Soc., 163, A2919 (2016).
3. C. Petchsingh et al., J. Electrochem. Soc. 163, A5068 (2016)
4. S. Roe et al., J. Electrochem. Soc., 163, A5023 (2016)
5. M. Skyllas-Kazacos et al., J. Electrochem. Soc., 158, R55 (2011).

Transport of ions and reagents through electrodes and membranes determine the performance limits of redo flow batteries. In this talk, we will discuss our present understanding of the interplay amongst structure, mechanical properties and chemical interactions of materials in determining these properties. We will describe the state of understanding of how those factors control ion and solvent uptake in membranes and how the latter relate to conduction and permeation processes. We will begin to probe similar aspects of porous electrodes use in flow batteries. The molecular scale underpinnings of these various features will also be discussed.

Acknowledgements
We would like to gratefully acknowledge the current support of this work by the Office of Naval Research and the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk) and the DoD Office of Naval Research.

Redox flow batteries (RFBs) are open batteries that can be scaled for output power or duration of service (energy storage capacity). Among various chemistries developed for RFBs, all-vanadium redox flow batteries (VRFBs) are unique due to utilization of the same element (vanadium) in four oxidation states for the negative and positive electrolytes. Many strategies have been explored for improving VRFB performance including improved cell design, superior material selection, and electrolyte engineering [1,2]. Although significant advances have yielded improved performance, reversible capacity decline during cycling is still a major issue yet to be addressed. Several parameters influence rapid capacity decay during cycling; the major contributor is unwanted transport of vanadium ions and water (crossover) through the ion-exchange membrane [3]. Some previous works have focused on measuring and mathematical modeling of crossover for various types of ion-exchange membranes [4,5,6]. Therefore, the major thrust within the literature has been focused on designing and fabricating high-performance ion-exchange membrane capable of mitigating crossover [7]. However, few efforts have considered other methods of mitigating crossover based on cell architecture. In this talk, we will introduce novel cell designs for crossover mitigation during VRFB cycling. We have designed, engineered and prototyped cell architectures with asymmetric configurations for the negative and positive sides that are capable of retaining discharge capacity over long-term cycling. We have utilized a unique setup equipped with UV/Vis spectroscopy for real-time measurement of crossover and consequently validated the asymmetric cell approach for retaining discharge capacity. The novel cell configurations we have engineered are under invention disclosures at University of Tennessee and provide an inexpensive solution for adopting VRFBs in industry as reliable and robust technology for grid-scale storage.


References:
1. D. S. Aaron, S. Yeom, K. Kihm, Y. Ashraf Gandomi, T. Ertugrul, M. M. Mench, Journal of Power Sources, 366, 241-248 (2017). https://doi.org/10.1016/j.jpowsour.2017.08.108.
2. Y. A. Gandomi, D. Aaron, T. Zawodzinski, and M. Mench, Journal of The Electrochemical Society, 163 (1), A5188-A5201 (2016); doi: 10.1149/2.0211601jes.
3. Y. A. Gandomi, T. A. Zawodzinski, and M. M. Mench, ECS Transactions, 61 (13); 23-32 (2014). doi:10.1149/06113.0023ecst.
4. Y. A. Gandomi, M. Edmundson, F. Busby, and M. M. Mench, Journal of The Electrochemical Society, 163 (8), F933-F944 (2016); doi: 10.1149/2.1331608jes.
5. Y. A. Gandomi, D. Aaron, and M. Mench, Electrochimica Acta, 218, 174-190 (2016); http://dx.doi.org/10.1016/j.electacta.2016.09.087.
6. Y. A. Gandomi and M. M. Mench, ECS Transactions, 58 (1), 1375-1382 (2013); doi: 10.1149/05801.1375ecst.
7. Y. Ashraf Gandomi, D. S. Aaron, M. M. Mench, Membranes, 7(2), 29 (2017); doi:10.3390/membranes7020029.

Redox flow batteries (RFBs) are open batteries, thus scalability is a major advantage of these devices; engineering cell reactors for desired output power can be performed independently, without altering the available capacity and vice versa [1]. All-vanadium redox flow batteries (VRFBs) are a type of RFB with the unique benefit of not suffering from cross contamination and irreversible capacity decay as a function of crossover (both water and vanadium) [2,3].

Crossover of vanadium ions and water through the ion-exchange membrane occurs due to several drivers; of them, concentration gradient and electric field are considered dominating contributors [4,5,6]. Among various components being used within the VRFB architecture, ion-exchange membranes play a significant role, influencing not only the rate of crossover but also affecting the performance of the cell directly [7]. It is well-known that the ohmic overpotential is dominated by the membrane in VRFBs. Therefore, engineering ion-exchange membranes must improve ionic conductivity while reducing crossover; this optimization effort is the subject of much research in the literature.

One of the major issues to be addressed is the contribution from interfacial phenomena (contact resistance between electrodes and membrane) to the ionic conductivity along with ionic and water transport through the ion-exchange membrane. In this work, we have utilized a novel experimental set-up (capable of measuring the ionic crossover in real-time) to quantify the permeability of ionic species. In addition, we have designed, engineered and prototyped several conductivity cells for measuring the ionic conductivity of the ion-exchange membranes and electrolytes ex-situ. Such a comprehensive experimental diagnostic has enabled us to provide further details regarding the impacts of interfacial phenomena on ionic conductivity and crossover. The results of this study provide deeper insight into the optimization of VRFBs for high-performance and robust applications.


References:
1. D. S. Aaron, S. Yeom, K. Kihm, Y. Ashraf Gandomi, T. Ertugrul, M. M. Mench, Journal of Power Sources, 366, 241-248 (2017). https://doi.org/10.1016/j.jpowsour.2017.08.108.
2. Y. A. Gandomi, D. Aaron, T. Zawodzinski, and M. Mench, Journal of The Electrochemical Society, 163 (1), A5188-A5201 (2016); doi: 10.1149/2.0211601jes.
3. Y. A. Gandomi, T. A. Zawodzinski, and M. M. Mench, ECS Transactions, 61 (13); 23-32 (2014). doi:10.1149/06113.0023ecst.
4. Y. A. Gandomi, M. Edmundson, F. Busby, and M. M. Mench, Journal of The Electrochemical Society, 163 (8), F933-F944 (2016); doi: 10.1149/2.1331608jes.
5. Y. A. Gandomi, D. Aaron, and M. Mench, Electrochimica Acta, 218, 174-190 (2016); http://dx.doi.org/10.1016/j.electacta.2016.09.087.
6. Y. A. Gandomi and M. M. Mench, ECS Transactions, 58 (1), 1375-1382 (2013); doi: 10.1149/05801.1375ecst.
7. Y. Ashraf Gandomi, D. S. Aaron, M. M. Mench, Membranes, 7(2), 29 (2017); doi:10.3390/membranes7020029.

We introduce an aqueous flow battery based on low-cost, non-flammable, non-corrosive and Earth-abundant elements. During charging, electrons are stored in a concentrated water solution of 2,5-dihydroxy-1,4-benzoquinone (DHBQ), which rapidly receives electrons with inexpensive carbon electrodes without the assistance of any metal electro-catalyst. Electrons are withdrawn from a second water solution of a food additive, potassium ferrocyanide (K₄Fe(CN)₆). When these two solutions flow along opposite sides of a cation-conducting membrane, this flow battery delivers a cell potential of 1.21 V, a peak galvanic power density of 300 mW/cm² and a coulombic efficiency exceeding 99%. Continuous cell cycling at 100 mA/cm² shows a capacity retention rate of 99.76%/cycle over 150 cycles. Various molecular modifications involving substitution for hydrogens on the aryl ring were implemented to block decomposition by nucleophilic attack of hydroxide ions in solution. These modifications resulted in increased capacity retention rates of up to 99.962%/cycle over 400 consecutive cycles, accompanied by changes in voltage, solubility, kinetics and cell resistance. Quantum chemistry calculations of a large number of organic compounds predicted a number of related structures that should have even higher performance and stability. Flow batteries based on alkaline-soluble dihydroxybenzoquinones and derivatives are promising candidates for large-scale, stationary-storage of electrical energy.

Aluminum-air battery is the promising candidate for the next-generation high-energy density batteries, but inherent limitations make it difficult in practical use. Here we show that silver nanoparticle-mediated silver manganate nanoplate as an oxygen reduction catalyst greatly improves the catalytic activity and chemical stability in alkaline solution. By means of atomic resolved transmission electron microscopy, we find that the formation of stripe patterns on the surface of silver manganate nanoplate originates from the zigzag atomic arrangement of silver and manganese, creating high concentration of dislocations in crystal lattice. This structure can provide abundant active sites for ion adsorption and high electrical conductivity for fast electron transfer kinetics. We also confirm the outstanding performance of our catalyst in flow-based aluminum-air batteries, demonstrating high gravimetric and volumetric energy densities of ~2,552 Wh kg_Al⁻¹ and ~6,890 Wh l_Al⁻¹ at 100 mA cm⁻² and high stability during mechanical recharging process.

Zn-Br battery is a promising technology for large-scale energy storage for the electric grid. Fundamental understanding of Br2/Br- redox, which is key to Zn-Br battery, has only been provided by electrochemical measurements. My new research group at Georgia Tech (started Jan 2017) has successfully developed a methodology based on optical microscopy to visualize Br electrochemical reactions in real time and in native liquid electrolyte. In this presentation, I will show our results including electrochemically-correlated time-lapse microscopic videos supporting these discoveries: (1) bromine complexing agent such as N-methyl-N-ethyl pyrrolidinium bromide is necessary for the formation of phase-separated bromine; (2) individual bromine microdroplets (1~20 um) tend to adhere to electrode surface at low current density, while leave electrode surface and enter electrolyte solution at high current density; (3) bromine forms with different morphology on metal and carbon electrode; (4) bromine microdroplets can be reversibly consumed by applying reductive current; (5) the morphology of bromine microdroplets that form on cycled electrode surface is different from that on fresh surface, indicating the change of surface chemistry after only one cycle; (6) bromine microdroplets slowly dissolve without electrical current, which explains their self-discharge; (7) bromine microdroplets are stable at temperature much lower their melting point, when they are super-cooled. Our findings will be interesting to a broad audience in the field of flow battery, by revealing mechanistic insight, and provide guidance on the optimization of materials and system. This platform we have developed can be adapted to study other electrochemical systems as well.

Obtaining pure nanocarbons with a high yield is a great challenge in the field of material science. Research into nanocarbons for bi-functional oxygen reduction reaction (ORR) and evolution reaction (OER) electrocatalysts are classified according to their dimensionality from 0D to 3D nanocarbons. Among them, 1D epitaxial heterostructures with modulated composition enable the generation of devices with diverse catalytic activities. Despite significant progress, these nanocarbons still suffer from insufficient activity and stability. To further enhance catalytic activities and chemical stability, metals with nitrogen are used as a dopant (M-N-C). In current stage, conventional synthesis (for example, chemical vapour deposition (CVD), arc discharge or laser ablation methods) for M-N-C catalysts leads to mixtures of metals and carbon where exposed metals on the carbon composites are easily degraded into highly concentrated alkaline electrolyte for practical applications. Therefore, the controlled growth in nanoscale building blocks with chemically incorporated M-N-C is crucial for the development of high performance ORR and OER electrocatalyst.
Herein we suggest novel bottom-up synthetic process for core-shell Fe/Cu@Carbon Nitride Nanotubes (FeCu@CNNT) via supercritical reaction where Fe and Cu atoms are encapsulated within carbon nitride nanotubes. The morphology of the FeCu@CNNT was directly visualized by using scanning transmission electron microscopy (STEM). Electron energy-loss spectroscopy (EELS) mapping image and spectra was used to clearly tract the Fe and Cu arrangements within CNNT. ORR and OER activities of catalysts were measured using a rotating ring disk electrode (RRDE), showing higher half-wave potential of 0.890 V for ORR than that of Pt/C (0.850 V) and current densities of 2.924 mA cm⁻² at 1.60 V for OER (2.276 mA cm⁻² for IrO₂). Further, we fabricated the rechargeable Zn-air batteries and flow-assisted Al-air batteries with FeCu@CNNT electrocatalyst. For the Zn-air batteries, the FeCu@CNNT showed better bi-functional properties than the mixture of Pt/C and IrO₂ under high depth of discharge (DOD) of ~ 29% (12 h per cycle). The intrinsic precipitation problems of the Al-air batteries have been solved by introducing new Al-air flow batteries system, which showed the specific energy of 2287 Wh kg_Al⁻¹ at a current density of 25 mA cm⁻². We believe that the bottom-up growth of the FeCu@CNNT with a high yield of ~40% is promising strategy for the synthesis of high quality M-N-C catalysts and the practical mass production.

Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost, have a low toxicity and high chemical stability associated with them. To make Zn-air batteries viable for large scale use, issues at the Zn electrode need to be addressed that lead to a loss of capacity during cycling. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging. During the oxidation of Zn in the battery, a passive layer of ZnO is formed on the electrode. Additionally, electrode morphology changes between each cycle, producing inconsistent performance. Here we describe studies of electrode structures and processes targeting improvement of these issues. Porous carbon electrodes similar that used in air electrodes provide a conductive and permanent structure for Zn deposition and removal during battery operation. We show that flowing electrolyte through such a porous electrode during electrochemical cycling allows for significant mitigation the mass transport issues at the Zn electrode. Carbon felts were studied for their use as the negative electrode in rechargeable Zinc-based batteries. Polarization studies showed that flow improved the performance of the both deposition and dissolution reactions. A current density of 100mA/cm² was observed with only 20mV of overpotential during deposition and dissolution at a flow rate of 15ml/min of electrolyte through the electrode. Cycling shows a long stable charging step with the overpotential decreasing as charge is passed. Due to the Zn area in the electrode increasing, which provides more sites for Zinc deposition to occur. During discharge overpotential remains stable over a long period of time, eventually increasing as most of the Zn has been removed indicating that the electrode is fully discharged. Cycling remains stable over long periods of time: a symmetric cell was cycled for 250 hours at 50mA/cm² without any problems with cell shorting or major deviation of the cell performance. Behavior of the electrode is specific to the conditions of the electrolyte used in the cell and will be discussed further. Zinc deposition in the carbon felt varies based on the flow and potential applied. Generally, Zinc deposits appear to form a needle like coating around the carbon fibers in the felt. Zinc then fills in the gaps between fibers as more metal is deposited. Constant potential deposition at higher applied overpotential leads to more deposition away from the current collector, while lower overpotentials during deposition produce deposits throughout the electrode volume. Reasoning for why this occurs and implications for the electrode performance will be presented.
We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk) and by the ARPA-e IDEAS program.

Redox flow batteries (RFBs) are a promising large-scale energy storage technology forintegration of intermittent renewable sources, such as wind and solar, into the electrical grid.Among several types of RFBs under development, non-aqueous redox flow batteries (NRFBs) have recently gained significant interest due to their wide electrochemical potential windows and improved range of operating temperature, offering high performance operation compared to their aqueous counterparts [1]. Despite the promise of NRFBs, state-of-the-art systems are limited by decomposition of active materials, often exhibiting nearly quantitative capacity-fade after only modest cycling [2-3]. This underscores a critical challenge currently limiting the advancement of this technology – stability. To address the instability issues of these systems, we demonstrate a fundamentally new design strategy for NRFB active materials. This approach leverages millions of generations of biological evolution as a toolkit to elucidate molecules that provide a stable scaffold for further development. The compounds investigated in this study are based on chelators that have evolved as part of biological metal-transport systems [4]. As a result of natural-selection they exhibit extremely strong metal-binding properties, shutting down decomposition pathways. In this presentation, we will demonstrate the performance characteristics of the proposed NRFB system using charge/discharge cycling, capacity fade and efficiency analyses. Using in-situ spectroscopic analysis, we demonstrate chemical stability during cycling and tight coupling between current and electrochemical formation of the oxidized and reduced form of the active material. Additionally, recent progress to reduce area specific resistance and improve current
density applied during operation will be discussed.
References:
1. K. Gong, Q. Fang, S. Gu, S. F. Y. Li, Y. Yan, Energy Environ. Sci., 2015, 8 (12), 3515-3530.
2. 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 Engl, 2015, 54, 8684-8687.
3. I. L. Escalante-García, J. S. Wainright, L. T. Thompson and R. F. Savinell, J. Electrochem. Soc., 2015,
162, A363-A372.
4. H. Huang, R. Howland, E. Agar, M. Nourani, J. A. Golden, P. J. Cappillino, J. Mater. Chem. A., 2017,
5, 11586-11591.

Non-aqueous redox flow batteries (NRFBs) promise efficient grid-scale energy storage due to the straightforward scalability and ample choice for active materials.¹ They also promise a diverse library of highly soluble species and high energy densities due to wide potential windows for efficient grid-scale energy storage.¹ Yet, small molecules still pose the challenge of preventing crossover while maintaining high conductivity when using ion exchange membranes.² Macromolecular redox designs for NRFBs have been of recent interest in mitigating this parasitic crossover to prevent capacity fades. These size-exclusion materials, typically polymers,^3-4 are designed for use with porous membranes to maintain high conductivity. With this strategy, our laboratories have synthesized and electrochemically evaluated redox active polymers (RAPs)³ and colloids (RACs)⁵ as storage materials with porous membranes. Our results demonstrated that this strategy effectively blocks active material crossover through nanoporous separators while allowing high ionic conductivity.^3,5-6 Moreover, these RAPs and RACs undergo effective charge transfer, chemical reversibility and have high solubility in non-aqueous media, which are required for flow applications. Here we report on ongoing work designing and characterizing RAPs that together cycle reversibly as catholyte/anolyte pairs. For these studies, we have used a combination of methods including computational methods such as DFT along with experimental results to evaluate polymeric designs for a catholyte polymer containing cyclopropenium redox centers as the active unit. Experimentally we have used electroanalytical techniques such as cyclic voltammetry, potential-controlled electrolysis, spectro-electrochemistry, and Galvanostatic cycling. Initial results show that these catholyte RAPs exhibit electrochemical reversibility and cycle at Coulombic efficiencies near unity in a half-cell configurations. Thus, we will present on the performance of these RAPs for their potential use in enabling next-generation NRFBs.
[1] Energy Environ. Sci. 2014 [2] RSC Adv. 2013, 3, 9095. [3] J. Am. Chem. Soc. 2014, 136, 16309. [4] Chem. Mater. 2016, 28, 3401. [5] J. Am. Chem. Soc. 2016, 138(40), 13230. [6] J. Electrochem. Soc. 2017, 164(7), A1688-A1694.

Zero-emission proton exchange membrane fuel cell (PEMFC) is promisingly considering as a great candidate promoting the alternative energy to overcome the pollution issues emitted by fossil fuel. PEMFC is still expensive, especially commercial platinum (Pt)/Carbon in cathode-side catalyst. This research has then been developed to prepare economical PEMFC catalyst by synthesizing non-precious metal, iron (Fe), nanoparticles embedded in carbon particles using waste oil from local community as carbon source. Considering the methodology, solution plasma process (SPP), a single-step discharge in solution, has been utilized to produce metallic nanoparticles embedded carbon nanoparticle. Fe nanoparticles-embedded carbon (FeNPs/Carbon) particles are simply obtained after applying high voltage from bipolar-pulsed power generator to Fe electrodes submerged in waste oil for 90 min under ambient condition and heat treatment process are further accomplished to improve its electrical conductivity. The FeNPs with particle size about 20 nm dispersed in carbon matrix are observed by transmission electron microscope (TEM). The morphology and chemical composition of FeNPs/Carbon are confirmed by scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS), respectively. The BCC nanostructure of FeNPs is observed using x-ray diffraction measurement (XRD). The catalytic activity of FeNPs/Carbon is measured by cyclic voltammetry (CV), it shows that FeNPs/Carbon provides a good response in oxygen reduction reaction (ORR). Hence, the synthesized material is capable to promote a great impact to the field of energy in near future.

Dye-sensitized solar cells (DSCs) have been often used as photo-energy converter in hybrid rechargeable system because of their several advantages, especially sensitivity to light intensity. Nevertheless, the influential factors for the performance of DSC-combined energy storage devices are still veiled. In this work, we designed 3-electrode photo-energy conversion/storage system including dye-sensitized TiO₂ photo-electrode, functionalized membrane electrode, and LiMn₂O₄ storage-electrode, as named dye-sensitized solar battery (DSSB). Three kinds of redox mediators, such as I⁻/I₃⁻, Co^+2/+3(bpy)₃, and Cu^+/+2(dmp)₂ were introduced as charge regenerator for oxidized dye in order to investigate their impacts on the photo-energy conversion/storage performance of DSSB. As a result, I⁻/I₃⁻ showing the highest photo-injection current generally provided the highest photo-charging current (J_Ch) and thus total stored energy (E_stored) under one sun condition (1000 W m⁻²), followed by Co^+2/+3(bpy)₃ and Cu^+/+2(dmp)₂. However, under dim lighting (2.4 W m⁻²), Cu^+/+2(dmp)₂ showed the highest E_stored corresponding to 6.2% of photo-energy conversion/storage efficiency (η_overall). The trend of η_overall under dim lighting indicates that output voltage gradually becomes influential for E_stored as the incident light intensity decreases, while the impact of J_Ch becomes negligible. These findings present which factors we should consider to develop the DSSBs suitable for indoor application.

Flow batteries are receiving wide attention for electrochemical energy storage that can be combined with renewable energies due to their perfect combination of high efficiency, high reliability and long cycle life.¹ Alkaline zinc-iron flow battery owns the characteristics of low cost and easy upscale, together with two-electron-redox properties resulting in high capacity.² However, further work on this type of flow battery has been broken off, because of the technological reasons, e.g. poor coulombic efficiency (76%) and electrochemical efficiency (61.5%) at low working current density (35 mA cm^-2) because of zinc electrode etc.³ And since then, the alkaline zinc-iron flow battery has been rarely reported. Here we present an alkaline flow battery system that uses earth-abundant zinc and iron redox pairs as redox couples in combination with self-made, low cost membranes and 3D carbon felts electrode. The alkaline zinc-iron single cell with a coulombic efficiency of 99.49%, an energy efficiency of 82.78% is demonstrated at a current density of 160 mA cm^-2, along with a stable long term cycling capacity (more than 150 cycles) and a well-defined open cell voltage of 1.83 V at 50% SOC. The performance is up to now the highest value for reported flow battery system. Most importantly, the practical application of this battery system is well confirmed by assembling a kW stack, affording an average CE of 98.84%, an average EE of 84.17% and an average output power of 1.127 kW at the current density of 80 mA cm^-2. These fabulous results indicated that the alkaline zinc iron flow battery shows very promising prospect for stationary energy storage applications.
Reference:
1. W. Lu, Z. Yuan, Y. Zhao, H. Zhang, H. Zhang and X. Li, Chem. Soc. Rev., 2017, 46, 2199-2236.
2. K. Gong, X. Ma, K. M. Conforti, K. J. Kuttler, J. B. Grunewald, K. L. Yeager, M. Z. Bazant, S. Gu and Y. Yan, Energy Environ. Sci., 2015, 8, 2941-2945.
3. J. McBreen, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1984, 168, 415-432.