Conventional Redox Flow Batteries Research Articles

Beyond conventional batteries: a review on semi-solid and redox targeting flow batteries-LiFePO4 as a case study

Semi-solid and redox targeting flow batteries present high energy densities compared to conventional redox flow batteries. LiFePO4 active material is a promising catholyte for semi-solid and redox targeting flow batteries.

Towards Low Cost and Long Duration Iron-Air Flow Batteries

The electrification of the energy economy necessitates the development of low cost and large scale energy storage technologies that can integrate intermittent renewables (e.g. solar and wind). Among the various technological options (e.g. lithium ion batteries, vanadium redox flow batteries), metal-air batteries are promising alternatives owing to their high energy density, safety, and use of abundant raw materials. Moreover, when combined with the principles of redox flow batteries, mass transfer is enhanced, which results in a more uniform metal deposition and a reduction in the metal electrode passivation [2]. Furthermore, metal-air flow batteries could enable decoupling of power and energy, while retaining the scalability advantages of redox flow batteries. Additionally, they are more compact than conventional redox flow batteries since only one electrolyte tank is needed.Several metal-air flow battery systems have been investigated such as vanadium-air [3], lithium-air [4] and zinc-air batteries [5]. However, they suffer from limitations which challenge their scalability. Specifically, vanadium is costly and only available in certain regions, lithium requires the use of organic solvents which compromises the battery safety, and zinc forms dendrites leading to a lower performance and safety issues. In this context, iron-based batteries are a promising alternative due to their high specific capacity, low cost, large availability, safety, non-toxicity, and recyclability. Compared to zinc, iron is also less prone to dendrite formation during cycling in aqueous electrolytes since iron deposition kinetics is slower[6]. Despite these advantages, the performance of the iron anode is limited by electrode degradation during cycling due to phase transformations, hydrogen evolution, and low utilization of the activate material [6], which motivates fundamental research to advance the performance and durability of the system.In this poster presentation, we will discuss the main goals of our new project FAIR-RFB [7]. Our overall aim is to develop a low cost and durable iron-air flow battery system and, to this goal, we will investigate, (i) the role of porous electrode microstructure in defining the performance of iron-air flow batteries, (ii) the influence of electrode surface properties in determining transport phenomena, kinetics, selectivity and durability, and (iii) new electrochemical reactor architectures for high power iron-air redox flow batteries.

Assessing Mediated Redox Flow Battery Reaction Progression

One potential next generation battery system is mediated redox flow batteries (RFBs). With mediated RFBs, soluble electroactive species (redox shuttles) deliver power via electrochemical reactions in a flow-through stack reactor much like a conventional RFB. However, the redox shuttles then undergo chemical redox in a coupled chemical reactor system with solid electroactive materials. The use of solid electroactive material for chemical energy storage results in substantial increases in volumetric energy density for the system.Mediated RFBs with different configurations for the chemical redox reactor have previously been reported. In this work, a packed bed reactor system will be described, where the impact of varying different experimental parameters on the chemical redox progression will be highlighted. Generally, the progression of the chemical redox can be tracked in the lab by taking advantage of electrochemical analytical techniques. However, additional assessment of the reaction progression using neutron imaging will also be discussed. Neutron imaging offered a unique opportunity to access the spatial progression of the chemical redox in the packed bed reactor. The combination of techniques for assessing the chemical redox progression provides insights into the limiting processes in the reactor systems.

New Developments in the High-Energy-Density Solid-Liquid Storage Technology for Redox Flow Batteries

The environmental impact of the use of fossil fuels for energy can be reduced if electricity, which represents one-third of all energy uses, can be generated totally from renewable/sustainable sources such as wind and solar. However, this is only possible if cost-effective long-duration storage technologies are available to allow the highly variable and unpredictable wind and solar energy sources to become reliable baseline energy sources like coal, nuclear or natural gases. Redox flow battery (RFB) energy storage systems are highly suitable for this large-scale, long-duration storage application because while their power output scales with the size of the battery, their energy content resides in the amount of active materials that are stored in external tanks and can be easily scaled up for longer duration.1 The conventional redox flow batteries store electrical energy in the form of some aqueous or non-aqueous soluble ions or compounds in the electrolyte solution. Because of the low solubility (< 2M) of most ions and compounds in aqueous and non-aqueous solvents, these redox flow battery systems have low energy density.2–4 For example, the commercialized all-vanadium RFB system has an average energy density of 20 Wh/kg while that of the lithium-ion battery system is 100-265 Wh/kg.5 To store enough energy for 3-5 days in these RFBs requires a very large volume of solution in a large number of tanks, making these RFB systems expensive due to the cost of tanks and the fluid distribution system and floor space. Our group recently developed a new storage approach that can greatly increase the energy storage density while still enabling the flow battery concept.6 In this approach, the reactants are stored as both soluble ions and their undissolved solid forms and only the liquid containing the soluble ions is circulated through the batteries. This approach potentially enables >4X increase in the storage energy density. This technology was recently demonstrated in a hydrogen-vanadium (VI/V) system, and new test results and findings in this area will be presented in this talk.References H. Zhang, W. Lu, and X. Li, Electrochemical Energy Reviews, 1–15 (2019).D. G. Kwabi et al., Joule, 2, 1894–1906 (2018).M. Wu, T. Zhao, H. Jiang, Y. Zeng, and Y. Ren, Journal of Power Sources, 355, 62–68 (2017).C. Ding, H. Zhang, X. Li, T. Liu, and F. Xing, The Journal of Physical Chemistry Letters, 4, 1281–1294 (2013).A. Manthiram, ACS Central Science, 3, 1063–1069 (2017).Y. Li and T.V. Nguyen, “A Solid-Liquid High-Energy-Density Storage Concept for Redox Flow Batteries and Its Demonstration in an H2-V System,” Paper ID APEN-MIT-2021_023, Applied Energy Symposium: MIT A+B, Aug. 11-13, 2021, Cambridge, MA, USA.

Close Contact without Mixing: All-Aqueous Membrane-Free Flow Battery

The accumulation of energy produced by renewable sources to be used when demand is high is a key aspect of a truly decarbonized grid. Among all energy accumulation technologies, Redox Flow Batteries (RFBs) are specially promising due to its safety, flexibility of design and the ability to decouple capacity and power of the system. In a standard configuration, both redox electrolytes that are stored in external tanks are pumped in the electrochemical reactor separated only by an ion-selective membrane. This membrane is a critical part of the battery as it allows the selective pass of certain ions that compensate the charge imbalances while maintaining separated the active species in their respective compartments. Membranes used in RFB are expensive (up to 25% of the total cost in vanadium RFB), contribute greatly to the ohmic resistance of the battery, and the undesired crossover through it is never fully avoided which produces a performance decay of the battery over time. These issues are addressed by membrane-free RFB designs developed in our group.The idea behind MFreeB ERC project is to take advantage of the immiscibility of certain liquids to naturally separate redox active species in a membrane-free battery design [1]. Aqueous Biphasic Systems (ABSs) are a special example of all-aqueous natural occurring immiscible phases made by mixing water with two compounds that can be either polymers, inorganic salts or both. The two redox active species added to the biphasic system can be rationally selected and optimized so that they partition specifically to one of the two phases used as anolyte and catholyte [2]. These types of batteries have been proposed by several groups, including us, mostly as “static batteries” because filter-press reactors, typically used in conventional RFB, cannot be easily adapted to this membrane-free concept. In order to develop truly flowing systems, a totally different electrochemical reactor should be designed.Here, we report on the development of an electrochemical reactor that allows the formation of the two immiscible phases making possible the testing of immiscible batteries under flowing conditions for the first time. A counterflow flow-through configuration was used since it reduces flow instabilities while at the same time it allows high conversion rates in the carbon felt electrodes avoiding mass transport issues [3]. Several tests were performed in the battery; e.g. polarization curves, charge and discharge cycling and efficiency over time were measured. Moreover, the self-discharge phenomena, occurring near the interface when charged species enter in contact and provoking a loss of efficiency, was evaluated as a function of operating conditions. Special attention was given to the study of the different processes occurring in the region near the interface in order to mitigate losses and further optimize the design. Acknowledgments This work has been partially funded by project MFreeB which have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 726217).

(Invited) Modeling Electrochemical and Rheological Characteristics of Suspension-Based Electrolytes for Redox Flow Batteries

Electrochemical energy storage technologies are poised to play a key role in enabling society-wide decarbonization by facilitating the deployment of variable electricity generators and enhancing existing grid infrastructure. Redox flow batteries (RFBs) possess several technology features which are favorable for cost-effective, long-duration energy storage, including independent scaling of power and energy, long service life, and simplified manufacturing.1 In particular, their configuration enables a panoply of electrolyte compositions and cell materials to be considered, either at the beginning of life or during the installation lifetime (e.g., chemistry swaps). While conventional RFBs are based on redox couples dissolved in electrolyte solutions, more recently, suspension-based electrolytes containing solid energy storage materials have emerged as an alternative formulation. Flowable suspensions enable higher charge storage capacities (and thus cell energy densities) and unlock new operating modes (e.g., dissolution and precipitation on suspended particles).2,3 However, such electrolytes also frustrate traditional approaches to cell design and operation, as their complex electrochemical and rheological characteristics present multifaceted operational tradeoffs.4,5 By exploring essential features of these tradeoffs, we aim to better understand fluid dynamic and electrochemical engineering criteria for high-performance flow cells that operate with suspension-based electrolytes.In this presentation, we develop and apply a one-dimensional model to derive scaling relationships for suspension-based electrolytes in RFBs. Specifically, we investigate connections between rheological (i.e., non-Newtonian behavior, shear stress) and electrochemical (i.e., species and charge transport) phenomena for pressure-driven flow through a planar channel. identify key dimensionless groups which describe the relative magnitudes of relevant processes within the flowing suspension-based electrolyte. Through scaling analyses, we assess the importance of each parameter under different dynamic and geometric constraints to enable the identification of favorable materials sets and operating conditions. Ultimately, these dimensionless quantities offer a compact representation of the design considerations for suspension-based electrolytes, allowing more informed materials selection, cell engineering, and system formats. Finally, we hypothesize that these results can be generalized to describe flowable and stationary solid suspensions of utility in multiple electrochemical systems. Acknowledgments This work was funded by the Skoltech – MIT Next Generation Program. B.J.N, N.J.M, and K.R.L gratefully acknowledge the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. References M. L. Perry and A. Z. Weber, J. Electrochem. Soc., 163, A5064–A5067 (2016).H. Parant et al., Carbon, 119, 10–20 (2017).X. Wang, J. Chai, and J. “Jimmy” Jiang, Nano Materials Science, 3, 17–24 (2021).V. E. Brunini, Y.-M. Chiang, and W. C. Carter, Electrochimica Acta, 69, 301–307 (2012).N. C. Hoyt, R. F. Savinell, and J. S. Wainright, Chemical Engineering Science, 144, 288–297 (2016).

A Novel High-Energy-Density Storage and Operation Concept for Redox Flow Batteries

The environmental impact of the use of fossil fuels for energy can be greatly reduced if electricity, which represents one-third of all energy uses, can be generated totally from renewable/sustainable sources such as wind and solar. However, this is only possible if cost-effective long-duration (>5 days) storage technologies are available to allow the highly variable and unpredictable wind and solar energy sources to become reliable baseline energy sources like coal or natural gases. Redox flow battery (RFB) energy storage systems are highly suitable for this large-scale, long-duration storage application because while their power output scales with the size of the battery, an expensive component of the storage system, their energy content resides in the amount of active materials that are stored in external tanks and can be easily scaled up for longer duration.1 A stationary battery system like the lithium-ion battery does not have this separation of power and energy characteristic. Therefore, an approach that can significantly increase the operational duration of the RFB systems at a minimal cost is of great interest. The conventional redox flow batteries store electrical energy in the form of some aqueous or non-aqueous soluble ions or compounds in the electrolyte solution. Because of the low solubility (~1M-2M) of most ions and compounds in aqueous and non-aqueous solvents, these redox flow battery systems have low energy density (as compared to solid reactants like lithium in lithium batteries).2–4 For example, the commercialized all-vanadium RFB system has an average energy density of 20 Wh/kg while that of the lithium-ion battery system is 100-265 Wh/kg.5 To store enough energy for 3-5 days in these RFBs requires a very large volume of solution in a large number of tanks, making these RFB systems expensive due to the cost of tanks and the fluid distribution system and floor space. This presentation will discuss a concept that increases the amount of active materials stored in a given storage volume while still enabling the flow battery concept. The novel storage design is to i) store these reactants in a solution of soluble ions and an additional amount of these ions in their undissolved solid form and ii) to circulate only the liquids through the batteries. In their solid form, their energy densities will be much higher than that of the aqueous solution. For example, the concentration of vanadyl sulfate (VOSO4), an active compound used in the all-vanadium RFB system, is around 1.5M. At 1.5M, one has 1.5 moles of vanadium ions per liter or 40.2 Ah/L. In its 6-ligand-water solid form (VOSO4·6H2O) which has a density of 1920 g/L,6 a liter of this solid contains 7.1 moles of vanadium ions or 189 Ah energy. This is a tremendous increase and will put the energy storage density of the RFB systems within those of solid systems like lithium-ion batteries. An existing 6-hr RFB system can become a >24-hr system with minimal modifications. This concept and the processes used to demonstrate it will be discussed in this presentation. A demonstration of this aqueous/solid storage concept in a hydrogen-vanadium flow battery will be presented in another presentation from our group.Acknowledgments This work was also supported by the National Science Foundation under grant number CBET-2024378.

Polyoxometalate/Hydroquinone Hybrid Redox Electrolyte for Electrochemical Energy Storage

There has been progress and growing interest in the development of novel and efficient energy storage systems meeting high power and energy demands of modern devices and advanced technologies. Among important requirements for such charge storage devices are high volumetric and gravimetric energy and power, large number of charging/discharging cycles, improved safety and reasonable cost. At the current stage of technology, common charge storage systems, such as batteries and electrochemical capacitors, rarely meet those expectations. Indeed the present systems offer either high energy density (batteries) or high peak power (electrochemical capacitors) but barely both characteristics. Lately, there have been attempts to consider aqueous electrolytes containing dissolved redox species capable of undergoing fast electrode reactions at the interfaces formed with highly porous carbon electrodes. Consequently, the energy density significantly increases while keeping the power performance of a typical double-layer capacitor. Since most of redox-electrolyte-based systems utilize aqueous solutions, such advantages as enhanced safety, environment-friendliness and reasonable cost could be retained. Such redox-active species as iodide, quinone, phenelenediamine, ferrocyanide, methylene blue, thiocyanide and cerium sulphate have been successfully applied as positive or negative electrode-supporting electrolytes. The redox processes (battery-type characteristics) are typically observable in the potential range of a single electrode only and, therefore, the opposite electrode operating according to the electrical double-layer mechanism (capacitive behavior) limits the overall cell performance in terms of capacity and energy. To overcome this drawback, application of dual redox-type electrolytes comprising two independent redox couples separated in the positive and negative electrode compartments has been recently proposed. From the mechanistic point of view, such devices resemble conventional redox-flow batteries but operating in static conditions, i.e. more favourable from the practical and economical point of view.By taking into account the above deliberations, a dual redox-active (proton-conductive) electrolyte composed of the mixture of two different soluble redox species is considered here for application in energy storage devices with an enhanced specific energy [1]. The Keggin-type polyoxometalate (phosphotungstic acid), undergoing fast (electrochemically reversible) one-electron redox processes has been selected to support the negative electrode. In order to develop the potential difference in the cell, the hydroquinone, capable of oxidation/reduction according to the fast two-electron mechanism has been adopted and its electroactivity was predominantly confined to the positive electrode. It enabled us to design a hybrid charge storage cell with the operating voltage of 0.8 V and the specific energy of 20 Wh kg-1 (per total mass of both electrodes). In the constant power discharge mode, 13 Wh kg-1 of the energy has been preserved at the power of 1 kW kg-1 during discharging down to ½ of the maximum voltage which is consistent with the fast charging/discharging dynamics of the proposed hybrid system. A mixed hierarchical micro-porous (predominantly mesoporous) porous carbon of high (>2 cm3g-1) total pore volume and the BET surface area approaching 1000 m2 g-1 has been selected as the electrode material permitting electrical double layer charging and supporting electroactivity of redox species and unimpeded mass transport. Also, the normalization of electrical parameters against the total mass of electrodes and electrolyte resulted in a ca. five-fold increase of discharge capacity and specific energy in comparison to the performance of a simple electrical double-layer capacitor. Acknowledgement Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018 as well as by the NCBR in the framework of European Union POWER 3.2 Project no. POWR.03.02.00-00-I007/16-00. 1. M. Skunik-Nuckowska, K. Węgrzyn, S. Dyjak, N. H. Wisińska, P. J. Kulesza, Energy Storage Mater., 21 (2019) 427-438

A Study on Flow Characteristics and Flow Uniformity for the Efficient Design of a Flow Frame in a Redox Flow Battery

As global environmental problems are worsening, the efficiency of storage systems for renewable energy are gaining importance. The redox flow battery (RFB), a promising energy storage system (ESS), is a device that generates or stores electricity through reduction–oxidation reactions between active materials constituting electrolytes. Herein, we proposed a flow frame design that reduces flow resistance in the flow path and causes uniform flow distribution in the electrode to develop an efficient redox flow battery. Through computational fluid dynamics (CFD) and experimental verification, we investigated the flow characteristics and flow uniformity inside the conventional redox flow battery cell. An analysis of the flow characteristics of the conventional flow frame revealed a non-uniform distribution of the flow discharged to the electrodes, owing to the complex (branched) flow path geometry of the inlet channel. To address this problem, we proposed a new flow frame design that removed and integrated bifurcations in the flow path. This new design significantly improved flow uniformity parameters, such as the symmetry coefficient ( C s y m ), variability range coefficient ( R i ), and maximum flow rate deviation ( D m ). Ultimately, we decreased the pressure drop by 15.3% by reducing the number of flow path bifurcations and chevron repositioning.

Aqueous organic and redox-mediated redox flow batteries: a review

Abstract Redox flow batteries (RFBs) are among the most investigated technologies for large-scale energy storage applications. Since the first commercialization of all-vanadium RFB (in the early 90s), the technology has evolved towards the development of new systems. This review focuses on three innovative concepts including aqueous organic RFB (AO-RFB), dual-circuit RFB and redox solid booster–based RFB. We will highlight the recent advances in the last five years and discuss the main challenges encountered. Particularly, we focused on the use of redox-mediated process to reach higher energy density than conventional RFB.

Development of High-Performance Ion Selective Membranes for Lithium Polysulfide Redox Flow Batteries (Li-PS RFBs)

There has been a surge of research interest in the lithium polysulfide (Li-PS) redox flow batteries (RFBs) for large scale energy storage applications because of their high theoretical energy density (~2600 Wh/kg and 2199 Wh/L for elemental Li and S) and low material cost. They also can bring additional advantages, such as design flexibility, easy scalability, and safe operation condition. However, the Li-PS battery systems suffer from polysulfide crossover (polysulfide shuttling) between their two working electrodes which decreases the batteries’ columbic efficiency and shortens their cycle lives. This shuttling effect is more detrimental in Li-PS RFBs which doesn’t have carbon matrix to immobilize the solid-state sulfur. In conventional redox flow batteries (e.g. vanadium redox flow battery), the membrane separators has an important role by suppressing the active species crossover between catholyte and anolyte. However, commercial porous battery separators (e.g. Celgard) cannot be adopted due to their low rejection for polysulfide (PSn-) active species and fast capacity decay. Recent few research works revealed the feasibility of using ion exchange membranes (IEMs) as a barrier layer to selectively transport Li+ ions and block PSn- ions. Unfortunately, most commercial IEMs are unstable in the organic polysulfide electrolyte because of its high swelling ratio which can cause the electrolytes crossover. Hence, there is a critical need to develop high performance membrane materials for the Li-PS RFB applications. In this work, we will present our recent progress on developing novel ion exchange membrane materials for the Li-PS RFBs which can greatly suppress the polysulfide shuttling while maintaining high Li+ conductivity and mechanical stability. Our biphenyl polymer membrane (BP-SA) presents excellent stability the DOL/DME organic electrolyte solution, exhibiting potential for Li-PS RFBs application. We studied its selective ion transport properties through diffusion and electrochemical measurement. Diffusion test shows that there is close to 100 % rejection of Li-PS ions by the BP-SA membrane, while considerable amount of Li-PS crossover was found for Celgard and Nafion. Moreover, the BP-SA membrane possess comparable Li conductivity (0.13 mS/cm2) to Nafion (0.29 mS/cm2) in organic solvent. As a result, the Li-PS single cell with the IEM/Celgard composite membrane shows better efficiencies in comparison to the cell with Celgard 2325. Our results unambiguously demonstrate that the BP-SA membrane has greater Li+/PS anion selectivity than Nafion and can be a promising base material to make separators for Li-PS RFBs.

Flow investigation of complex suspension electrodes for battery applications using ultrasound imaging velocimetry

Flow batteries using suspension electrodes are currently investigated due to their potentially high energy densities and individual scalability of capacity and power density. In contrast to batteries with statically contacted electrodes or conventional redox flow batteries, their performance depend significantly on the local flow conditions of the electrode suspension in the electrochemical cell. Hence, it is crucial to understand and model the complex flow characteristics of such solid-liquid suspensions.Detailed ultrasound flow measurements are combined with multidimensional modeling of the bulk rheology and wall interactions for opaque, thixotropic multiphase fluids in a real-world flow setup. The present flow investigations are conducted for zinc-air type of batteries using a gelled alkaline electrolyte with suspended microscopic zinc particles, however the utilized methods could be adapted to other complex suspension flows of interest. Due to the opacity of the suspension, an ultrasound flow mapping technique, the ultrasound imaging velocimetry (UIV), was applied. In a reference measurement using the model fluid glycerol, the UIV is validated by comparing the measured flow field to optical particle image velocimetry (PIV) measurements and a numerical simulation.In contrast to a Newtonian fluid, the gelled suspension with 8 vol.% particle loading exhibits significant time-dependent non-Newtonian flow characteristics with slip at the wall. While initially strongly sheared zones retain a comparatively low viscosity and allow for high velocity gradients, in areas with low initial shearing (e.g. in corners) the fluid structure is able to rebuild, resulting in a solid-like behavior. An in-depth understanding of the development and the extent of such zero-flow zones can be critical for reliable and efficient designs of the relevant fluidic structures. The measurement data of the zinc-electrolyte suspension flow were used as reference to estimate parameters for a computational fluid dynamics model considering the time-dependent, non-Newtonian rheology of the bulk fluid and the apparent slip at the walls. The simulation model is able to predict the main characteristics of the measured two-dimensional flow fields. Despite some uncertainties, the methods prove to be effective for investigating complex suspension flows and they can be applied to improve and optimize the corresponding fluidic structures, either in flow batteries with flowing suspension electrodes or other industrial applications, including the processing of polymers, minerals, cosmetics, food products, pharmaceuticals and ceramics.

Ionic Liquids-Promoted Utilization of Redox-Active Organic Materials for Flow Batteries

Redox flow batteries (RFBs) are regarded as technique-of-choice for load levelling and peak shaving for the utilization of renewable energy sources, and are key component in smart grid network.[1,2] Electrolyte chemistry is a key consideration for the performance enhancement.[3-9] The conventional RFBs using non-organic redox species in aqueous H2SO4, KOH and NaCl, with limited operating voltage and volumetric energy density. Development of new high-performance electrolytes is highly important for speeding up the market penetration of RFBs. Herein, we explore the feasibility of using “water-in-ionic liquid” supporting electrolytes for tailoring the properties of electrolytes, including the electrochemical stability window, temperature adaptability, solubility of redox-active organics and chemical/electrochemical stability and reversibility of active materials.

Enhanced cyclability of organic redox flow batteries enabled by an artificial bipolar molecule in neutral aqueous electrolyte

Organic redox flow batteries receive increasing interest for large-scale electrical energy storage owing to the structural diversity and elemental abundance of organic compounds. However, their implementation is challenged by the cyclability, which is an order of magnitude lower than traditional redox flow batteries using inorganic redox couples. The unsatisfactory cyclability mainly arises from electrolyte contamination and irreversible losses of active material in the electrolyte. To address such issues, a highly water-soluble artificial bipolar molecule containing cathode-active ferrocene moiety and anode-active bipyridinium moiety is realized and used as the sole active material in a symmetric organic redox flow cell. Owing to the good reversibility, minimum electrolyte contamination, and stability of the synthesized molecule, the demonstrated cell boosts the cyclability to 4000 cycles with capacity retention of ca. 75% at 100% depth of discharge, which fills in the gap of cyclability between organic redox flow batteries and conventional redox flow batteries.

Metal–Air Batteries: From Static to Flow System

AbstractAs an emerging battery technology, metal–air flow batteries inherit the advantageous features of the unique structural design of conventional redox flow batteries and the high energy density of metal–air batteries, thus showing great potential as efficient electrochemical systems for large‐scale electrical energy storage. This review summarizes the operating principles and recent progress of metal–air flow batteries from a materials and chemistry perspective, with particular emphasis on the latest advanced materials design and cell configuration engineering, which the authors divide into three categories based on the anode species: vanadium–air, zinc–air, and lithium–air flow batteries. Since some of the capabilities developed for metal–air static batteries can be leveraged for next‐generation flow systems, classical works on conventional metal–air batteries are selected and compared with the metal–air flow systems, highlighting the prominent advantages of the latter in achieving high energy capacity and long cycle performance. At the end, a general perspective on current challenges/opportunities and future research directions to promote the commercial application of the metal–air flow battery technology is provided. The aim is to provide a comprehensive overview and to set up a road map for guiding development from conventional static to advanced flow technologies of metal–air batteries.

Anion Radical Mediated High Capacity Redox Flow Battery

The energy density of conventional redox flow batteries (RFBs) is dictated by their capacity (which is directly related to the solubility of the redox species in the electrolyte) and operating potential. 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 can potentially have moderately higher energy densities. A new approach to improve the performance of nonaqueous redox flow batteries (RFBs) through electrochemically-mediated reactions is demonstrated. Soluble anion radical species (biphenyl and pyrene) mediate reversible sodium storage in a red phosphorus anode located in an external plug flow reactor in the absence of binders or conductive additives. Because the anion radical species can be recycled several times throughout the cell stack during a single charge/discharge cycle, a mediated RFB effectively decouples the battery’s energy density from the redox species’ solubility in the electrolyte. Furthermore, this technology offers inherently safe operation and is unaffected by the large volume changes of red P (>300%) due to the use of an external plug flow reactor. 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 a phosphorus anode in a redox flow configuration will be presented This work is supported by Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy and The Office of Electricity Delivery and Reliability, U. S. Department of Energy.

A Re-Examination of Acid-Base Flow Batteries

The need for large scale energy storage is becoming more and more apparent as renewable energy sources such as wind and solar power gain a larger fraction of the electricity generation market. Redox flow batteries (RFBs) offer a promising chance to address that need at an acceptable cost, and the market for RFBs is expected to grow dramatically in the near future. However, none of the current flow battery technologies under development are ideal; generally, one or more deficiencies are found in areas of safety, cost, durability or efficiency. One concept for large scale energy storage that we are considering is an acid-base flow battery. In this device, the cell potential is developed from the proton concentration difference between a strongly acidic electrolyte (positive) and a strongly basic electrolyte (negative). Both electrodes employ the hydrogen oxidation/hydrogen evolution reactions. As hydrogen is evolved at one electrode, it is consumed at the other, thus only a small volume of hydrogen is required; the energy is stored in the changing concentrations of the acidic and basic electrolytes. Thus, an acid-base flow battery combines aspects of both a fuel cell/electrolyzer and a conventional redox flow battery. The advantages of this approach are 1) that no metal salts or redox active organic species are required, only the acidic and basic electrolytes, thus providing an advantage in terms of both cost and safety, 2) while the reactions require a catalyst, the kinetics are fast with relatively small amounts of catalyst required, and 3) unlike fuel cells and electrolyzers, strongly oxidizing conditions and the formation of peroxides are avoided, allowing for non perfluorinated membranes to be considered. The disadvantages include a relatively low cell voltage (≈0.8V) and the possibility of low coulombic efficiency due to reactant crossover, depending on the diffusion and migration of the active species (H+, OH-) through the membrane. The general concept of an acid-base flow battery is known (1,2), but relatively little work has been done in this area, and has not taken full advantage of the current state of the art in membranes, catalysis, and electrode/flow field designs. Significant advances have been made in these areas with regards to PEM hydrogen-air fuel cells, electrolyzers, and hydrogen/bromine flow batteries, which suggests that acceptable voltaic efficiency (over 80% at current densities greater than 100 mA/cm2) is possible, even with the relatively low cell voltage of this chemistry. As a result, a new investigation of the acid-base flow battery concept is warranted. In this presentation, we will focus on the coulombic efficiency of an acid-base flow battery using either cationic or anionic ion-exchange membranes and with various electrolyte compositions. An example is shown below in Fig. 1 for a cell with a Nafion 212 membrane. Initially both electrolytes were 1.5M Na2SO4. The cell was then charged at different current densities to a maximum of either 0.1M H+ / OH- or 1.0M H+ / OH-. The electrolyte volume was varied so that the same total time was required to reach full charge. The coulombic efficiency increases with increasing current density, suggesting that both migration and diffusion are significant factors in determining the coulombic efficiency. As expected, the efficiency is higher when lower concentrations of H+ and OH- are present at the end of charge. These conclusions suggest that an optimal concentration, that allows for high coulombic efficiency and an acceptable capital cost associated with the volume of electrolyte required may exist.

Improvement of Performance By Using Catalytically Etched Carbon Paper As Electrode for Vanadium Redox Flow Batteries

The environmental aspects of fossil fuels and depletion of their reservoirs are main reasons for the growing use of renewable energy sources and environmentally sustainable storage technologies in recent years. Solar, wind, biomass and tidal are among climate-friendly energy sources. But the integration of these sources into large scale grid application is limited by their intermittent nature. Electrical energy storage (EES) technology solves this issue by storing energy on large scales. Redox flow batteries (RFBs) due to unique advantages of design flexibility, long cycle life, decoupled scaling of power and energy, are amongst suitable candidates of EES. All Vanadium Redox Flow Battery (VRFB) introduced in 1980s by Maria Skyllas-Kzacos et al has advantage over conventional RFBs as it employs four different oxidation states of vanadium; V4+/V5+ to cathode side and V2+/V3+ to anode side hence minimizes crossover effect through membrane. A typical VRFB contains two electrolyte reservoirs connected to cell sandwiching an ion exchange membrane between electrodes. V4+ is oxidized to V5+ on cathode while V3+ is reduced to V2+ on anode during charging and these charged species are reversed back during discharging. Carbon paper or felt due to its large reactive surface area, chemical stability in highly acidic solution has been widely used as electrode in VRFB, however poor electrochemical activity, low kinetics toward oxidation and reduction of vanadium ions, and less wettability arising from its hydrophobic nature limit its wide-spread application. Several modification techniques have been used to improve the catalytic activity of carbon electrode toward vanadium redox couples mainly including thermal, chemical and electrochemical treatments. Most of the modification techniques result increased activity of carbon electrode attributed to its enhanced available surface area and surface functional groups. In this work carbon paper is etched by using cobalt oxide (Co3O4) as catalyst and is used as electrode for vanadium redox flow battery cell. The etched carbon paper is found better than pristine one in terms of charging overpotential, IR drop, charge/discharge capacities and energy efficiency at all working current densities. Etched carbon paper showed almost 31.0 Ah/L discharge capacity at initial working current density of 50 mA/cm2 compare to 16.0 Ah/L of pristine one, While 16 Ah/L at 150 mA/cm2 where pristine and thermally treated (TT) carbon paper have no performance. Similar effect was noticed in terms of energy efficiency, the etched carbon paper showed 70% energy efficiency at 150 mA/cm2 which is dramatically higher than pristine and thermally treated carbon paper electrodes. The improved performance is attributed to increased surface area, wettability and presence of microscopic pores on the surface of etched carbon paper.

Modeling Slurry Electrodes for Redox Flow Batteries Using Kinetic Monte Carlo

A new technological avenue, that allows utilization of conventional rechargeable battery material at grid scale, has opened up with the development of slurry redox flow batteries (SRFBs) [1]. These can offer volumetric capacity of up to 5-20 times greater than conventional redox flow batteries based on aqueous solutions [1]. However optimization and commercialization of SRFBs requires better understanding of suspension stability, non-Newtonian fluid flow behavior, electrochemical cycling phenomena, and consequent particle dynamics of the solid material in suspension. In this work we present a multiphysics mesoscopic modeling tool resolved using an kinetic Monte Carlo algorithm [2,3] in three dimensions. This model simulates the phenomena which take place during the discharge of a SRFB in static mode with a silicon-carbon based slurry in organic electrolyte [4]. It captures the dynamics of electronic percolation networks of carbon particles created during Brownian motion. These percolation networks form electron conduction pathways for silicon particles to discharge and eventually undergo volume expansion [Fig.1]. Results are discussed in contrast to in-house experimental data [4]. Our model offers a first look into the complex interplay of intrinsic parameters that need to be understood to achieve optimization based on factors like carbon volume fractions and C-rates [5]. Fig. 1. Snapshot of the channel during galvanostatic discharge slurry containing carbon (in red) and silicon particles (in blue). When silicon is in contact with carbon percolation networks (in yellow), it can discharge. The channel thickness is 1.5 μm and the area of the current collector is 6.25 μm2. References : [1] M. Duduta, B. Ho, V. C. Wood, P. Limthongkul, V. E. Brunini, W. C. Carter, and Y. Chiang, Adv. Energy Mater., 1(2011) 511 [2] M. A. Quiroga, and A. A. Franco, J. Electrochem Soc. 161(7) (2015) E73. [3] G. Blanquer, Y. Yin, M. A. Quiroga, and A. A. Franco, J. Electrochem Soc., 163(3) (2016) A329 [4] S. Hamelet, D. Larcher, L. Duport, and J. M. Tarascon, J. Electrochem Soc., 160(3) (2013) A516. [5] G.Shukla, D. del Olmo Diaz, V. Thangavel, and A. A. Franco, in preparation (2016) Figure 1

Membrane-less hybrid flow battery based on low-cost elements

The capital cost of conventional redox flow batteries is relatively high (>USD$ 200/kWh) due to the use of expensive active materials and ion-exchange membranes. This paper presents a membrane-less hybrid organic-inorganic flow battery based on the low-cost elements zinc (<USD$ 3 Kg−1) and para-benzoquinone (<USD$ 8 Kg−1). Redox potential and voltammetric studies show that the open-circuit voltage of the battery is 1.17–1.59 V over a wide range of pH. Half-cell charge-discharge and dissolution experiments indicate that the negative electrode reaction is limiting due to the presence of chemical side reactions on the electrode surface. The positive electrode redox reactions are not affected and exhibit (half-cell) coulombic efficiencies of >92.7% with the use of carbon felt electrodes. In the presence of a fully oxidized active species close to its solubility limit, dissolution of the deposited anode is relatively slow (<2.37 g h−1 cm−2) with an equivalent corrosion current density of <1.9 mA cm−2. In a parallel plate flow configuration, the resulting battery was charge-discharge cycled at 30 mA cm−2 with average coulombic and energy efficiencies of c.a. 71.8 and c.a. 42.0% over 20 cycles, respectively.