Flow Batteries: Current Status and Trends - ACS Publications

Review pubs.acs.org/CR

Flow Batteries: Current Status and Trends Grigorii L. Soloveichik*,† GE Global Research, 1 Research Circle, Niskayuna, New York 12309, United States 12. Conclusions Author Information Corresponding Author Present Address Notes Biography Acknowledgments Abbreviations References

CONTENTS 1. Introduction 1.1. Energy-Storage Needs and Application Space 1.2. Electrochemical Energy-Storage Technologies 2. Types of Redox Flow Batteries 3. All-Liquid Aqueous Flow Batteries 3.1. All-Vanadium RFB 3.2. Iron−Chromium RFB 3.3. Polysulfide−Bromine RFB 3.4. Soluble Metal−Bromine RFB 3.5. Other Chemistries 4. Flow Batteries with Metal Anode 4.1. Zinc-Based Flow Batteries 4.1.1. Zinc−Halogen RFB 4.1.2. Zinc−Cerium RFB 4.1.3. Other Zinc-Based RFBs 4.2. Single-Metal Flow Batteries 4.2.1. All-Iron RFB 4.2.2. All-Lead RFB 4.2.3. All-Copper RFB 5. Flow Batteries with Hydrogen Anode 5.1. Hydrogen−Halogen RFB 5.1.1. Hydrogen−Chlorine RFB 5.1.2. Hydrogen−Bromine RFB 5.2. Other RFBs with Hydrogen Anode 6. Flow Batteries with Oxygen Cathode 7. Flow Batteries with Nonaqueous Electrolyte 8. Organic Flow Batteries 8.1. Organic RFB with Aqueous Electrolytes 8.2. Organic Regenerative Fuel Cells 8.3. Organic RFB with Nonaqueous Electrolytes 9. Semisolid Flow Batteries 10. Technical and Economic Issues of Flow Batteries 11. Current Trends and Perspectives 11.1. Increasing Energy Density 11.2. Increasing Power Density 11.3. Maximizing Cell Efficiency 11.4. Maximizing System Efficiency 11.5. Modular Design © XXXX American Chemical Society

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1. INTRODUCTION

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1.1. Energy-Storage Needs and Application Space A

Electricity is a major form of energy usable by mankind (e.g., 40% of all energy consumption in the United States in 2012),1 and the demand for it is growing worldwide, to be doubled by 2050.2 About 12% of electricity is generated from renewable sources including hydroelectric power, geothermal, solar, wind, and biomass. The most environmentally friendly intermittent energy sources (solar and wind) constitute yet a small fraction compared to hydropower. Globally, solar and wind installed capacities are growing at rates of 60% and 20% per year, respectively, driven by the necessity to reduce carbon emissions. Recently, renewables-based energy-generation installations surpassed fossil-fuel-based ones as a share of new electricity-generation capacity. The goal, in many states legislatively adopted, is to reach at least 12% of total electricity production from renewables by 2020 in the United States; an even higher target (20%) was set in Europe. The time- and weather-dependent nature of the fastest growing energygeneration sources, wind and solar, demands the mitigation of their power output with either power generation on demand (peaking power plants that usually use gas turbines) or energy storage (e.g., mechanical, electrochemical, or thermal). Electricity energy storage (EES) is also vital for the smart grid and distributed power generation development. The U.S. Department of Energy has identified four major challenges to the widespread implementation of EES: cost, reliability and safety, equitable regulatory environments, and industry acceptance.3 The development of novel EES technologies capable of resolving these challenges is critical. Another very important application of EES is electrification of on-ground transportation, mostly light-duty vehicles. Replacement of traditional internal combustion engines with hybrid, plug-in hybrid, and pure electric vehicles (EVs) allows for reduction of carbon emissions and fuel savings.4 Different technologies serve multiple grid and off-grid energy-storage applications. These applications may be grouped

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Received: December 30, 2014

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Table 1. Electrochemical Energy-Storage Technologies for Stationary Applications technology supercapacitors regenerative fuel cells with hydrogen storage lead-acid batteries

typical power (MW)

discharge time

storage capacity cost ($/kWh)

0.25 10a

5 h

500−3000

0.5−20

3−5 h

Li-ion batteries NAS battery flow battery (VRB) a

0.25−1 0.5−12

efficiency (%)

drawbacks

500000/20 13

>90 40−50

explosion hazard, low energy density, cost low-density storage, high cost, safety

65−120

1000−1200/3−4

70−80

1−5 h

400−600

750−3000/6−8

80−90

6−8 h 10 h

360−500 150−2500

2500−4500/6−12 500−2000/10

87 70

low energy density, short lifetime, temperature sensitive cost, safety, short lifetime, self-discharge, temperature sensitive cost, high-temperature operation, safety low energy density

life time (cycle/years)

Projected.

by scale, response time, and discharge duration into five service categories: bulk energy (arbitrage and supply capacity), ancillary (regulation, spinning reserve, voltage control, etc.), transmission infrastructure (transmission upgrade deferral and congestion relief), distribution infrastructure (distribution upgrade deferral and voltage support), and customer energy management (power quality and reliability, retail energy time shift, and demand charge management).5 The EES widely varies in capacity (from 1 kWh to hundreds of MWh), response time (from milliseconds to minutes), and discharge duration (from minutes to hours). Economics analysis showed that the most impact can be obtained in energy management and renewables that require discharge duration from 2 to 6 h and scaled from 1 kW to 500 MW.6,7 The variety of applications dictates a diversity of EES technologies (mechanical, thermal, electrical, and electrochemical). Flywheel EES, (ultra)capacitors, superconducting magnetic energy storage (SMES), and batteries (e.g., lead-acid and lithium-ion) are used for high-power applications such as power quality and frequency regulation. In the energy-management applications space, pumped hydroelectric energy storage (PHES), compressed air energy storage (CAES), thermal EES, and batteries compete with flow batteries.8 PHES technology takes the lion’s share of current grid-scale storage capacity (>99%) followed by underground CAES with 0.35%. Both technologies are the least expensive but are economical only at the large scale and have geographic restrictions such as the presence of an underground cavern for CAES. Thermal EES, which is comparable with CAES in installed capacity, is usually associated with solar power plants and is used for the peak shaving.3 This technology has low energy density (10−50 Wh/kg) and requires high capital costs. All these technologies require big capital investments, have a large footprint, and lack flexibility. In contrast, electrochemical EES technologies have better flexibility due to a modular design and higher energy density. Increased demand for vehicle electrification including a power train also requires on-board energy storage with high energy density and the capacity within the 40−120 kWh range.

carbon material and the aqueous or nonaqueous electrolyte. Additionally, electrodes may contain electrochemically active components, such as metal oxides, that undergo fast, reversible redox transformations.18 Supercapacitors have an excellent power capability, efficiency, and cycle life, but their energy density is very low and their cost is high. Secondary batteries, in which electroactive materials do not move (stationary electrodes), represent the biggest and most developed group of electrochemical EES. The most widely used secondary batteries are lead-acid, Ni−Cd, Ni−metal hydride, sodium−sulfur, lithium-ion, and Li-polymer.19 The main disadvantages of such batteries are high cost, short lifetime, and safety issues. In addition, power and energy in secondary batteries are coupled as illustrated by a Ragone plot (Figure 1), which leads to an excess of either power or energy and therefore higher costs for many applications.

Figure 1. Ragone plot for traditional batteries with stationary electrodes. Reprinted with permission from ref 20. Copyright 2011 Royal Society of Chemistry.

In traditional fuel cells, e.g., hydrogen−air or direct methanol fuel cell, a fuel and a gaseous oxidant are consumed, respectively, at the anode and cathode separated by an ionexchange membrane to generate power. If the fuel cell can function in the reverse, e.g., electrolytic mode performing water electrolysis (regenerative fuel cell), it can be used for energystorage applications (eq 1).21 RFCs are usually combined with hydrogen storage and in some cases, e.g., space applications, with oxygen storage that adds cost but provides more power.22 Compared to batteries, RFCs have higher energy density but lower round-trip efficiencies and higher cost. Nevertheless, RFCs become more cost-effective than stationary batteries when the storage time exceeds 3 days.23

1.2. Electrochemical Energy-Storage Technologies

There are several electrochemical technologies suitable for EES including supercapacitors, stationary batteries, regenerative fuel cells (RFCs), rechargeable metal−air batteries, and flow batteries. Several reviews comparing different electrochemical EES options for stationary applications have been recently published.9−17 Comparative analysis of these options is given in Table 1. Supercapacitors or electric double-layer capacitors store charge by electrostatic interactions within the double layer at the interface between the electrode made of highly porous B

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although they have some advantages compared to conventional batteries, namely, separation of location, sizing of the power and energy components, and possibility of rapid mechanical charging.29 Several excellent reviews on RFBs have been published.30−33 Therefore, this review mainly focuses on recent works and trends in the development of RFBs as well as the challenges of their practical realization.

(1)

Rechargeable metal−air batteries where metal is lithium24 or zinc25 resemble RFCs but with the anode comprising solid active materials and a liquid aqueous (eqs 2a and 3) or nonaqueous (eq 2b) electrolyte, respectively. Although the demonstrated practical energy density of lithium−air batteries (1700 Wh/kg)26 and zinc−air (700 Wh/kg)27 is much less than the theoretical energy density (e.g., the density of the Li−O2 couple is comparable with that of gasoline, 12.2 kWh/kg), it is still much higher than for known Li-ion batteries. However, all electrochemical cells with oxygen cathode suffer from sluggish kinetics, which results in high cell overpotentials and low energy efficiency. In addition, metal−air batteries suffer from low lifetime and poisoning of a basic electrolyte with CO2 from air. 4Li + 6H 2O + O2 ⇌ 4(LiOH ·H 2O) (E 0 = 3.0 V in LiOH)

(2a)

2Li + O2 ⇌ Li 2O2 (E 0 = 2.96 V)

(2b)

2Zn + O2 ⇌ 2ZnO (E 0 = 1.59 V)

(3)

2. TYPES OF REDOX FLOW BATTERIES The first flow cell concept based on the SnII/SnIV couple in combination with Br−/Br2 or FeII/FeIII cathodes was proposed by Posner almost 60 years ago.34 The tin−bromine cell had an OCV of 0.8 V and achieved 30 mA/cm2 at 0.4 V. The main polarization occurred at the tin anode.34 Three flow cell chemistries with the same VO+2 /VO2+-cathode (eq 4) and different anodes (SnII/SnIV, FeII/FeIII, and Cu0/CuII redox couples) were reported by Oei.35 The cell demonstrated a power density of 49 mW/cm2 (for tin and iron anodes) and 93 mW/cm2 (for the metal copper anode). It was assumed that regeneration of the discharged anode and cathode would be done chemically.35 VO2+ + 2H+ + e− ⇌ VO2 + + H 2O (E 0 = +0.991)

Cost is a major driver for EES, but all current electrochemical EES technologies are still too expensive to be implemented on a large scale for both stationary and automotive applications. In redox flow batteries (RFBs) at least one electrode comprises a solution (or sometimes movable slurry) of an electroactive material, and energy is generated/stored when the redox species flow through the electrochemical cell anode and cathode chambers separated by an ion-exchange (cation or anion) or microporous membrane and undergo electrontransfer reactions at inert electrodes. One of the RFB electroactive materials may be a flowing gas. A number of electrochemical cells are connected in series (usually using a bipolar electrode design) or parallel, forming a cell stack. Electrolytes are stored in separate tanks and flow into the stack with the help of pumps (Figure 2) and sometimes gravity. RFBs

(4)

A regenerative redox fuel cell based on the cathodic reaction (eq 4) and the anodic reaction (eq 5) with a cell potential of ∼1 V was described.36 Regeneration of the catholyte was performed by direct oxidation with O2 using the soluble heteropoly acid catalysts, H3PMo12O40 or H5PMo10V2O40, whereas the anolyte was regenerated by direct Pt-catalyzed reduction with H2.36 SiW12O40 5 − → SiW12O40 4 − + e−

(5)

In another flow cell using the FeII/FeIII and VIV/VV couples, the vanadium catholyte was regenerated by oxygen in the presence of nitric acid and the iron anolyte was reduced by oxidation of methane at 120 °C catalyzed by Pt.37 The opencircuit voltage of the cell was 0.48 V, and a maximum power output was 8.1 mW/cm3 on a graphite felt electrode.37 As one can see, the first flow cell concepts did not assume electrochemical charging. The chemically regenerative redox fuel cell is the most attractive for mediating the sluggish oxygen reduction reaction (ORR) kinetics at a gas diffusion air or oxygen electrode. The FeII/FeIII aqueous cathode is also being developed by ACAL Energy Ltd. for the fuel cell application.38 In this approach reduced FeII species are oxidized by oxygen in the presence of a heteropolyoxometalate catalyst in a separate reactor. Energy density is higher than that of comparable flow batteries but lower than that of fuel cells. Added system complexity and cost may be compensated by eliminating the need for expensive noble metal catalysts and higher ORR rate. The first practical flow battery using CrII/CrIII and FeII/FeIII redox couples was invented by Thaller.39,40 It represents the most common all-liquid flow battery type where both negative and positive electroactive materials in the charged and discharged form are dissolved to create anolyte (negolyte) and catholyte (posolyte), respectively (Figure 2). The most common solvent is water, although a number of nonaqueous solvents have been proposed. To increase conductivity, adjust pH, and in some cases increase solubility, additional components could be added to anolyte and catholyte.

Figure 2. Schematic of a redox flow cell.

are similar to fuel cells because in both cases fuel and oxidant are stored externally and supplied to the electrochemical cell on demand. Essentially, RFBs are regenerative fuel cells28 except both discharged reductant and oxidant are stored for electrochemical regeneration (charging). RFBs are relatively new technology compared with well-developed lead-acid, Ni− Cd, and Li-ion secondary batteries.11 Because of the low energy density, RFBs are not considered for mobile applications, C

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Figure 3. Schematics of a hybrid redox flow cell with a solid anode (a), a gas cathode (b), and a semiflow cell (c). Reprinted with permission from ref 41. Copyright 2011 John Wiley and Sons.

If RFB’s half-cell reactions include the deposition of solid species (usually in the charged form) on an electrode or one of them involves a gaseous material (e.g., hydrogen), such RFBs are called “hybrid” (Figure 3a and b, respectively). The solid species are usually a metal (e.g., Zn, Fe) depositing on an anode or metal oxide (e.g., PbO2) depositing on a cathode. Basic operations of such RFBs are very similar to all-liquid RFBs, although the design of electrodes should be tuned to the deposition process. The metal deposition electrodes have limitations of the capacity and power density due to formation of metal dendrites at high current densities. Therefore, in contrast to all-liquid RFBs, power and energy of such systems are not fully decoupled. To return this very important advantage of RFBs and increase energy density, a semiflow design has been proposed where an electroactive material deposited on conductive particles or formed a slurry that can be moved through an electrochemical cell and stored outside the stack (Figure 3c).41−43 As membranes present a significant part of the stack material and manufacturing cost, attempts are made to develop membraneless RFBs. This concept is based on the phenomenon that at low Reynolds numbers (Re < 10) two laminar streams flow side by side with minimal mixing that occurs only by cross-stream diffusion.44 Known RFB chemistries have been used in the membraneless setup, e.g., all-vanadium,45,46 hydrogen−bromine,47 and zinc−bromine.48 This type of RFB is good for low-power (e.g., “on-chip”) applications, but the issues of their scale-up to medium-power applications using layered stack structure (sealing, high manufacturing cost, etc.) have to be solved. Some RFB chemistries where both electroactive materials in the charged state are solids do not require a membrane (Figure 4). Examples are an all-lead RFB proposed by Pletcher and coworkers49 and a single-flow Zn/NiOOH battery proposed by Cheng et al.50 Theoretically, a plethora of redox couples could be used in RFBs. Practically, chemistries that are applicable to large-scale RFB’s are limited by many factors including redox potential, electrochemical reversibility, solubility, cost, availability, and toxicity. Possible redox couples for negative (anolyte) and positive (catholyte) active materials are listed in Table 2. Charge-transfer and ion-carrier-migration modes for exemplary redox couples used in RFBs are shown in Figure 5.

Figure 4. Schematic of a single-flow redox flow cell.

Table 2. Potentials of Redox Couples Used in RFBs vs normal hydrogen electrode (NHE)51 reductant −

E0 (V)

2H2O + 2e → H2 + 2OH− MHx−1 + H2O + e− → MHx + OH− Zn2+ + 2e− → Zn S42+ + 2e− → 2 S22−

−0.828

−0.763 −0.45

Fe2+ + 2e− → Fe Cr3+ + e− → Cr2+

−0.440 −0.408

V3+ + e− → V2+

−0.255

Pb2+ + 2e− → Pb 2H+ + 2e− → H2 TiO2+ + 2H+ + e− → Ti3+ + H2O Fe(EDTA)2− + e− → Fe(EDTA)− Sn4+ + 2e− → Sn2+ Cu2+ + 2e− → Cu

−0.126 0.000 0.1

−0.80

+0.140 +0.150 +0.337

E0 (V)

oxidant Fe(CN)63− + Fe(CN)64−

−

e → −

O2 + 2H2O + 4e → 4OH− − I + 2e− → I3− C6H4O2 + 2e− → C6H4(OH)2 Fe3+ + e− → Fe2+ ClBr2− + 2e− → 2Br− + Cl VO2+ + 2H+ + e− → VO2+ + H2O Br2 + 2e− → 2Br− O2 + 4H+ + 4e− → 2H2O Cl2 + 2e− → 2Cl− PbO2 + 4H+ + 2e− → Pb2+ + 2H2O Mn3+ + e− → Mn2+ Ce4+ + e− → Ce3+

+0.360 +0.401 +0.536 +0.699 +0.771 +0.80 +0.991 +1.065 +1.229 +1.360 +1.455 +1.510 +1.610

Wales (Australia) by Skyllas-Kazacos et al. in 1986.1,52−54 The battery chemistry is based on VII/ VIII redox reaction in anolyte, which is highly reversible and fast (eq 6), and VIV/ VV redox reaction in catholyte (eq 4), which is considerably more complicated and therefore slower (Figure 5a). Sulfuric acid is commonly used as a supporting electrolyte, which ensures low pH and thus increases the stability of electroactive species in solution. The acid concentration is high (up to 4 M) to provide higher conductivity.55 The solubility of vanadium species determined the theoretical energy density of 60.5 Wh/kg. It was assumed that the catholyte reaction is mix-controlled by

3. ALL-LIQUID AQUEOUS FLOW BATTERIES 3.1. All-Vanadium RFB

The all-vanadium redox battery is the most studied and advanced RFB. It was invented at the University of New South D

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Figure 5. Schematic of charge transport in various redox-flow systems: all-vanadium (a), vanadium/bromine (b), iron/chromium (c), Fe-EDTA (ethylenediaminetetraacetate)/bromine (d), zinc/cerium (e), bromine/polysulfide (f), nonaqueous ruthenium/bipyridine (g), nonaqueous vanadium/acetylacetonate (h), and nonaqueous chromium/acetylacetonate (i) (the values give the potential of the cell). Adapted from ref 33 with permission. Copyright 2011 Springer.

concentration above 3.5 M is outside of the hydraulic flow region. It was concluded that V concentration in catholyte may be maintained at the level of 3 M in 6 M sulfate/bisulfate at temperatures below 40 °C. It was shown that poly(acrylic acid) and its mixture with methanesulfonic acid can effectively stabilize all four vanadium ions (V2+, V3+, VO2+, and VO2+) in electrolyte solutions up to 1.8 M.62 To stabilize the catholyte, several additives such as ammonium oxalate have been proposed.63 The use of mixed acids (H2SO4 and HCl) improves vanadium solubility and thermal stability of the catholyte. Using density functional theory (DFT) modeling and 51V and 35Cl NMR, it was shown that the vanadium(V) cation at higher vanadium concentrations (≥1.75 M) exists as a dinuclear, chlorine-bonded [V2O3Cl2· 6H2O]2+ complex. It is assumed that this complex is resistant to the deprotonation reaction, which is the initial step in the V2O5

charge transfer and diffusion steps. A carbon-based electrode treatment56 and several electrocatalysts including PGM metal catalysts such as CuPt357 and non-PGM oxides, e.g., Mn3O458 and WO3,59 have been proposed to accelerate this reaction. V3 + + e− ⇌ V2 + (E 0 = −0.255)

(6)

The solubility of the vanadium species in the sulfuric acid electrolyte limits the system energy density. VOSO4, the species which exists in the discharged positive half-cell, is soluble in water (3.3 M), but addition of sulfuric acid effectively decreases the solubility (to 0.26 M in 9 M H2SO4).60 All-vanadium RFB is prone to precipitation of V2O5 at elevated temperatures, which limits its energy density and operation range. It was found that the concentration of VV may reach 5 M in concentrated sulfuric acid at ambient temperature but drops to 2 M above 50 °C.61 The viscosity of VV solutions with a E

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precipitation.64 The use of sulfate−chloride mixed electrolytes allows for dissolving 2.5 M vanadium, which results in about a 70% increase in energy capacity over the pure sulfate system and widens the operational temperature range to −5 to +50 °C.65 The use of a mixture of VBr3 and VBr4 (1:1) in HBr as a starting electrolyte for both anode and cathode allowed for increasing the vanadium concentration to 4 M and transferring 1.5 electrons per V atom at the anode as well as getting an additional capacity at the cathode due to bromide to bromine oxidation.66 This approach substantially increases the RFB energy density, but a Coulombic efficiency is rather low (88%) and toxic bromine is generated in the catholyte. Addition of polyethylene glycol reduces Br2 vapor pressure but decreases a voltage efficiency.66 Performance of all-vanadium RFB can be significantly improved using advanced electrode materials and electrode design. Graphite felt treated by microwave at 400 °C shows enhanced electrocatalytic activity and improved efficiency, probably due to the increase of concentration of hydrophilic OH-groups.56 The surface area of a cathode made of flexible carbon nanofibers was doubled (up to 350 cm2/cm3) compared to commonly used carbon felt.67 Addition of flow fields to VRFB electrodes increases discharge voltage and energy efficiency by 5% but increases a pressure drop.68 An improved cell design allowed for an increase of peak power density up to 500 mW/cm2.69 These improvements in electrode/cell design can be used for different flow battery chemistries. As crossover of electrochemically active species reduces the cell efficiency and may require a periodic electrolyte reconstitution, the development of selective ion-conductive membranes is of great importance. Sol−gel-derived Nafion/ SiO2 hybrid membrane lowers vanadium ions permeability and increases Coulombic and energy efficiencies compared to pure Nafion membrane.70,71 The pore size and pore-size distribution of poly(ether sulfone) (PES)/silica composite porous membranes can be controlled by the amount of silica gels inside the pores to reach a Coulombic efficiency of 97%.72 Sulfonated poly(ether ether ketone) (SPEEK) membranes embedded with graphene73 or the short-carboxylic multiwalled carbon nanotubes74 demonstrate decreased permeability of vanadium ions, resulting in 4−7% higher Coulombic efficiency and high mechanical strength. A similar effect is seen by attaching pendent carboxylic acid groups to the SPEEK backbone.75 A porous polytetrafluoroethylene (PTFE) substrate with pores filled with a SPEEK−graphene oxide composite demonstrated stable performance for 1200 cycles.76 A double-layer membrane made of ultrathin Nafion film on the PES/SPEEK porous membranes showed very high ion selectivity and energy efficiency (86.5% at 80 mA/cm2).77 Anion-exchange membranes improve water transport.78 The use of an anion-exchange membrane (AEM) made of quaternary ammonium-functionalized poly(fluorenyl ether) resulted in 100% Coulombic efficiency due to extremely low VO2+ permeation.79 At low current densities (10−5 m/s.112

Open circuit voltage is changing with the state of charge due to the dominance of different hydrated complexes of Cr(III) at different stages.103 Slow chromium redox kinetics called for development of redox catalysts, like bismuth or bismuth−lead on carbon.104 Addition of EDTA to the chromium electrolyte allows for stabilization of Cr(V), which makes an all-chromium battery possible, but the kinetics of the anodic reaction is slow.105 Addition of bismuth to the single electrolyte containing iron and chromium chlorides in HCl solution, which is used as both anolyte and catholyte, reduces the cell polarization by almost 2 times.106 EnerVault recently installed a demonstration 250 kW/1MWh battery in California (http://enervault.com). Imergy was developing an iron−chromium 5 kW/30 kWh RFB but recently switched to vanadium RFB technology.

3.4. Soluble Metal−Bromine RFB

A vanadium polyhalide RFB that employs the VCl2/VCl3 couple in the anolyte (1 M VCl3 in 1.5 M HCl) and the Br−/ClBr2− couple in the catholyte (1 M NaBr in 1.5 M HCl) with a Nafion 112 membrane demonstrated Coulombic and voltage efficiencies of 83 and 80%, respectively (Figure 5b).113,114 A similar titanium polyhalide cell that uses the same solution of TiCl4 in HBr/HCl in both anolyte and catholyte (iodide forms insoluble deposits) has an OCV of 0.9 V and high Coulombic efficiencies.115 The use of inexpensive titanium instead of vanadium offers a significant cost benefit. However, the cell voltage efficiency is low due to high ohmic losses and requires improved cell design to be practical.115 A chromium−bromine RFB using 1 M chromium chloride or bromide solution in 0.1 M HCl and Br2 to generate 30 mA/cm2 has been patented.116 Addition of phosphoric acid, citric acid, and EDTA to the anolyte to form chromium chelate complexes with more negative redox potentials (up to −1.0 V vs NHE) increases the cell voltage and energy density up to 24 Wh/L.117

3.3. Polysulfide−Bromine RFB

Polysulfide−bromine RFB (PSBFB) described by eq 7 was invented in 1984.107 2NaBr + (x − 1)Na 2Sx ⇌ Br2 + x Na 2Sx − 1 where x = 2−4

(7)

In flow cells, bromine is present in catholyte in the form of polybromide anions Br(2n+1)− where n = 1−6.108 Because anolyte and catholyte contain different species, the PSBFB cells suffer from the crossover of active species through a membrane. Replacement of the carbon anode for an open pore nickel foam allowed for increasing energy efficiency (77% at 40 mA/ cm2).109 With cycling of PSBFB with a carbon felt cathode and the nickel anode, the Coulombic efficiency was actually growing, which was explained by reducing self-discharge due to sulfur precipitate build up within micropores of the Nafion membrane. This effect results also in the decrease of the voltage efficiency, so the energy efficiency remains the same.109 WS2 was proposed as an electrocatalyst for polysulfide redox processes in aqueous alkaline solutions to improve the efficiency of PSBFB.110 The catalyst prepared by ball-milling was the most active. Practical PSBFB was under development by Regenesys Technologies Ltd., and a demonstration plant with an energystorage capacity of 120 MWh and a discharge time of 8 h was to be built in Little Barford (U.K.). However, the project was stopped in 2003 before completion by then owner RWE, the German utilities group. Numeric modeling of PSBFB was developed and validated using the data acquired from a pilotscale system.111,112 The model showed that the drift in performance was caused mostly by self-discharge and electro-

3.5. Other Chemistries

A catholyte based on the Mn2+/Mn3+ couple in sulfuric acid (1 M in 4 M H2SO4) was combined with various anolytes using Ti3+/Ti4+, Cr2+/Cr3+, V2+/V3+, Sn2+/Sn4+, and Zn/Zn2+ redox couples to produce electrochemical cells with theoretical OCVs of 1.61, 1.92, 1.76, 1.66, and 2.27 V, respectively.118 Manganese concentration can be increased up to 4 M with decreasing the concentration of sulfuric acid to 1 M. The major problem of RFBs with this catholyte is stability of Mn3+ ions in solution, which easily disproportionate according to eq 8. The problem was solved by addition of titanium salts, which suppress the disproportionation, to the catholyte, and the cell can be safely cycled up to 90% SOC.118 A Ti−Mn flow cell was cycled with both cation- and anion-exchange membranes showing similar performance during at least the first cycles.118 2Mn 2 + + 2H 2O ⇌ MnO2 ↓ + 4H+ + 4e−

(8)

The redox potential of the FeII/FeIII couple shifts in the negative direction by complexation with EDTA, oxalate, and citrate, which allows their use as anolyte materials (Table 3).119 Fe complexes with citrate demonstrate better solubility than G

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exchange membrane is extremely low. The measured fluxes for titanium tris(catecholate) and ferri/ferro-cyanide anion were 6.5 × 10−8 and 6.5 × 10−8 mol/cm2 per day, respectively, which are ∼6 orders of magnitude lower than the flux of chargecompensating cations (Na+ and K+). A single-metal undivided redox flow battery using single aqueous 0.2 M CrIII−EDTA electrolyte demonstrated 100% current efficiency in the anolyte reaction (CrII/CrIII) but very high overpotential of the cathode reaction (CrIII/CrV), which resulted in low energy efficiency, 70% at the current density of 20 mA/cm2.182 The use of corrugated graphite and reticulated vitreous carbon (RVC) as positive and negative current collectors increased the cell capacity to 114 mAh/g of total active materials with 95% Coulombic efficiency at 20 mA/cm2.183 It was shown that morphology and phase composition (α-PbO2 vs β-PbO2) of lead dioxide deposit at the positive electrode plays a major role in cyclability, and optimizing the deposition conditions increased the battery cycle life to >2000 cycles at 79% energy efficiency.184 Power of 160 mW/cm2 has been demonstrated for the all-lead RFB, and major failure modes have been identified (Pb dendrites formation and PbO2 creeping and sludging).185 The soluble lead RFB has a better charge efficiency but lower discharge efficiency than the conventional lead-acid battery (overall energy efficiency is about the same), but it could recover its performance after a single charge− discharge cycle.186 4.2.3. All-Copper RFB. An all-copper RFB based on reaction 15 using an aqueous solution of CuCl in both catholyte and anolyte has been proposed.187 The high concentration of copper (3 M) that was achieved due to addition of HCl and CaCl2 (4 M) partially compensates the relatively low cell open-circuit potential (OCP) (0.6 V) and allows for reaching an energy density of 20 Wh/L. Copper can be electroplated on different materials with Coulombic efficiency increasing in the order graphite < stainless steel < titanium. A nonoptimized cell with a nanoporous composite poly(vinyl chloride) (PVC)/silica separator demonstrated energy efficiency of 90% but at very low current densities (5 mA/cm2). The current density increase above 20 mA/cm2

Hydrogen−halogen RFBs are similar to hydrogen−oxygen regenerative fuel cells, but the kinetics of the cathodic reaction (eq 16) is much faster and typically does not require an electrocatalyst.190 However, RFBs with fluorine catholyte are too hazardous, and the ones with iodine catholyte seem to be impractical due to the formation of an insoluble iodine precipitate. Hydrogen−chloride and hydrogen−bromide cells have high OCV (Table 1) and theoretical energy density (993 and 353 Wh/kg, respectively) and may be appealing for energy storage. 5.1.1. Hydrogen−Chlorine RFB. The use of a hydrogen− chlorine regenerative fuel cell for energy storage promises higher round-trip energy efficiency (∼70%) compared with the hydrogen−oxygen cell (30−40%) due to faster cathode reaction (eq 16).191 Initially, the hydrogen−chlorine regenerative fuel cells were based on both reagents in a gas form with aqueous solutions of NaCl acidified with HCl192 or other acids193 as electrolytes (eq 17). The study of H2−Cl2 fuel cells with Pt and Rh anode catalysts and Nafion and PBI/H3PO4 proton-conducting membranes showed low polarization and fast electrode kinetics resulting in a power density of 500 mW/ cm2.194,195 Pressurization of the system above 10 bar allowed conversion of chlorine gas to chlorine hydrate, which can be stored in liquid form, and provided higher current density.196 A cell with a Pt/C anode catalyst (0.5 mg Pt/cm2), a (Ru0.1Co0.9)3O4 cathode catalyst, and Nafion 112 PEM demonstrated a peak power of 1 W/cm2 (at 56% energy efficiency) and 0.4 W/cm2 at 90% energy efficiency at 70 psig.197 Cl 2 + 2e− ⇌ 2Cl− (E 0 = + 1.36 V)

(16)

H 2(g) + Cl 2(g) ⇌ 2HCl(aq)

(17)

5.1.2. Hydrogen−Bromine RFB. A hydrogen−bromine fuel cell with a proton-conducting membrane has been known for more than 50 years198 and later was considered as a regenerative one with high round-trip efficiency for energy K

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storage.199−201 A bromine/bromide cathode, where Br− is in equilibrium with Br3− and Br5−,202 is essentially the same as that used in the zinc−bromine and polysulfide−bromine RFBs. It was recognized early that a Coulombic efficiency increased if bromine was separated from the catholyte.203 To reduce the vapor pressure of extremely toxic bromine, several complexing agents for both H2−Br2 and Zn−Br RFBs have been proposed, e.g., polyethylene glycol140 and quaternary ammonium bromides.204,205 A nanoporous membrane can be used as a separator in a nanoporous proton-conducting membrane H2−Br2 RFB. In the fuel cell mode, such a cell design using a diaphragm made of a ceramic nanopowder and polyvinylidene fluoride (PVDF) demonstrated maximum power densities of >1.5 W/cm2 and an energy efficiency close to 90% with a dry hydrogen feed at 80 °C.206 Similar results (1.4 W/cm2) were achieved with a PEM cell using 0.9 M Br2 in 1 M HBr catholyte and Nafion NR212 membrane with thicker multilayer electrodes in a flowthrough mode.207 Several papers were dedicated to numeric modeling of the H2−Br2 RFB and showed the high power capabilities of this RFB.202,207,208 With good cathode kinetics, a major problem of the H2−Br2 RFB is poisoning of Pt catalysts with bromine and bromide due to the crossover through a membrane.209−211 This effect results in high Pt loading required for the cell functioning and, therefore, high RFB cost. Attempts to develop non-Pt electrocatalysts for this RFB were made.212 Less expensive ruthenium−cobalt and ruthenium−manganese alloy oxides on a titanium metal electrode demonstrated good catalytic activity and chemical stability in a cell,213 and MoS2 and IrO2 showed catalytic activity on the rotating disk electrode.214 Another cell degradation mode is carbon electrode oxidation to CO2, which limits the operating cell voltage to 1.4 V.215 Little is known about commercialization efforts on the H2−Br2 RFB except recent announcement by EnStorage (www.enstorageinc.com) on installation of a grid-connected 50 kW/100 kWh battery.

cycles. Irreversible degradation of Coulombic and voltage efficiencies was attributed to the loss of active material due to the crossover and corrosion.220 Another type of vanadium−air battery proposed by Wen et al. separates charge and discharge processes that occur in two different cells with a common VSO4/V2(SO4)3 sulfuric acid electrolyte.221,222 In this bifunctional RFB, electrochemical oxidation of VII with air is used only for the discharge process, and for the charging the VIII reduction is paired with electrochemical oxidation of organic compounds, e.g., glyoxal to glyoxylic acid221 or L-cystine to L-cystinic acid.222 Generation of value-added organic products may potentially improve the RFB economics, but added system complexity, low Coulombic efficiency, and sheer volume of chemical to be produced make this approach impractical for large-scale applications.

7. FLOW BATTERIES WITH NONAQUEOUS ELECTROLYTE Nonaqueous electrolytes for RFBs are attractive because of possible cell voltage increase beyond the hydrogen and oxygen evolution potential limits in aqueous electrolytes. There is also a hope that solubility of active species could be increased by the tuning of ligands and solvent selection. Both trends could increase the RFB power density. In contrast to aqueous RFBs that employ metal deposition/dissolution, zerovalent metal complexes stabilized by donor ligands such as bipyridyl are stable in organic solvents that allow for an all-liquid RFB operation. However, lower dielectric constant and dissociation constants of metal complexes in organic solvents lead to lower electrolyte conductivity compared to aqueous systems and require the presence of supporting electrolytes such as tetraalkylammonium salts. The first demonstrated nonaqueous RFB contained [Ru(bpy)3]2+ complex in acetonitrile as an active material for both redox reactions and exhibited an OCV of 2.6 V and a discharge current of 5 mA/cm2 at a smooth carbon electrode (Figure 5g).223 Testing of an Ru(acac)3-based system with an H-type cell with a Neosepta anion-exchange membrane showed an energy efficiency of 74% but could be charged only to 9.5% because of a sharp increase of the cell potential.125 An undivided RFB based on 0.1 M solution of ruthenium acetylacetonate in acetonitrile demonstrated 55% voltage efficiency, though at very low current densities.224 All-uranium124,225,226 and all-neptunium123,227,228 RFBs based on eqs 18 and 19 in nonaqueous solvents (dimethylformamide (DMF) and propylene carbonate) were proposed by Yamamura et al. Uranium and neptunium cations are stabilized by β-diketonate and polyketonate ligands and have a solubility up to 0.8 M for MVI and 0.4 M for MIV. The cell OCV is estimated as ∼1 V, whereas in aqueous solutions the OCV is much lower (0.68 V for U).124

5.2. Other RFBs with Hydrogen Anode

Several other batteries with hydrogen anode and an inorganic aqueous catholyte have been proposed. The major advantage of such a system is the absence of cross-contamination of electrolyte that is offset by the necessity to compress and store hydrogen. Hydrogen−vanadium RFB uses the same VIV/ VV redox couple as in all-vanadium RFB. It showed good reversibility and reached a peak power density of 114 mW/ cm2.216 A cell using 1.1 mg/cm2 WO3-modified graphite felt obtained via a hydrothermal method as its catalytic cathode had higher voltage efficiency (88.7% vs 78.9% for untreated graphite at 70 mA/cm2).59 The increase of the electrocatalyst coverage had a small effect on the efficiency. Hydrogen−iron RFB is based on the FeII/FeIII redox couple in sulfuric acid electrolyte and has an OCV of 0.77 V.217 It demonstrated good Coulombic (96%) and energy (85%) efficiencies at 200 mA/cm2. In galvanic mode it reached a power density of 71 mW/cm2.218

6. FLOW BATTERIES WITH OXYGEN CATHODE To minimize the size and weight of RFB, it was proposed to apply an air (oxygen) cathode similar to regenerative fuel cells. A vanadium−air RFB was patented in 1994.219 Testing of a flow cell with the 1.2 M V2(SO4)3 in 2 M H2SO4 anolyte and a bidirectional cathode with IrO2-based catalyst showed an initial energy efficiency of 41.6%, which decreased with the number of

MO2 2 + + e− ⇌ MO2+ (M = U, Np)

(18)

M 4 + + e− ⇌ M3 + (M = U, Np)

(19)

An identical structure of uranyl cations allows for fast electron transfer in the catholyte.124 The redox reaction of neptunium on a plastic-formed carbon electrode is ∼2 orders of magnitude faster than the analogous reactions of vanadium ions.228 This results in the energy efficiency of the uranium and neptunium RFB at 70 mA/cm2 being 98%226 and 99%,227 respectively, which was much higher than that for the allL

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catholyte in RFB with a cell potential of 2.0 V.237 Coulombic efficiency of this RFB was ∼90% at very low current density due to a lower crossover through an anion-exchange membrane, but ohmic losses were very high.237 A nonaqueous analogue of polyoxometalate RFB with (NBu4)4H3SiV3W9O40 as a single active material and propylene carbonate/0.5 M NBu4OTf as an electrolyte showed higher operational voltage (∼0.3 V more than the aqueous analogue) but suffered from faster capacity fade and lower Coulombic efficiency.134 It was found that electrochemical reactions of copper chloride anions in a mixture of choline chloride and ethylene glycol (1:2) are fast (eqs 21 and 22) and can be used to build an all-copper RFB.129 However, small cell OCV (0.75 V) and low energy efficiency (52% at 10 mA/cm2) make it impractical.

vanadium RFB. Although these RFBs are impractical due to radioactivity and high cost of active components, their energy efficiency is remarkable. A single-metal RFB using vanadium(III) acetylacetonate in acetonitrile with NEt4BF4 as a supporting electrolyte (Figure 5h) has been proposed.126,229 V(acac)3 is reduced in the anolyte and oxidized in the catholyte during charge. The quasireversible VII/VIII and VIII/VIV couples generated a potential of 2.2 V. The maximum V(acac)3 concentration was only 0.4 M (in 0.5 M), and the Coulombic efficiency was low (50%).126 Degradation of nonaqueous vanadium acetylacetonate RFB was associated with a side reaction involving environmental oxygen and water that caused the electrode passivation and the formation of oxovanadium species.230 Analysis of 20 electrolyte/solvent pairs for a vanadium acetylacetonate RFB showed that the solvent should have a low molar volume for better solubility of active materials and a high Hansen polarity for conductivity.231 Acetonitrile demonstrated the best combination of parameters, but its high volatility and moderate toxicity would prevent wide usage. An RFB with propylene carbonate solutions of tris(2,2′bipyridine)nickel(II) tetrafluoroborate in anolyte and tris(2,2′bipyridine)iron(II) tetrafluoroborate in catholyte showed a cell voltage of 2.2 V and initial Coulombic and energy efficiencies of 90.4 and 81.8%, respectively. This cell demonstrated a stable performance during 100 cycles.232 The replacement of bipyridyl ligand in these complexes with phenanthroline and of the solvent with gamma-butyrolactone increased the cell OCV to 2.4 V with average energy efficiency of 81.6%.233 A Ni complex with multidentate ligand, (nickel(II)-1,4,8,11-tetracyclotetradecane, (Ni(cyclam)2+), demonstrates reversible reduction and oxidation behavior and can be used as a single electrolyte for nonaqueous RFBs with a theoretical cell voltage of 2.52 V and a reasonably high solubility of ∼0.4 M.234 Addition of a phosphoric ester polymer to solutions of chelated Ni and Fe complexes increases their solubility in propylene carbonate (PC) by 14−24%.235 A nonaqueous RFB utilizing [Co(bpy)3]+/2+ (anolyte) and [Fe(bpy)3]2+/3+ (catholyte) redox couples in 0.5 M solution of NEt4BF4 in propylene carbonate and carbon-coated Ni-FeCrAl and Cu metal foam electrodes exhibited a practical operating voltage of >2.1 V and an energy efficiency of 85% with no loss of energy efficiency over 300 cycles.128 At the same time an alliron RFB using bipyridyl complexes with ClO4− anion (eq 20) demonstrated very low energy efficiency (6%),125 which possibly indicates the occurrence of side reactions on the anode. Fe(bpy)3

3+

−

+ e ⇌ Fe(bpy)3

2+

+

⇌ Fe(bpy)3 − e

Anode: Cathode:

CuCl32 − + e− ⇌ Cu(s) + 3Cl− CuCl4 2 − + e− ⇌ CuCl32 − + Cl−

(21) (22)

An attempt to combine an active material and a solvent in the form of am electroactive ionic liquid was made by Anderson et al. at Sandia National Laboratory.238,239 Metal complexes containing cations of copper, iron, and other metal stabilized with ethanolamine or diethanolamine and ethylhexanoate or triflate anions are liquids at ambient temperature and show in some cases reversible electrochemical behavior, but all of them are very viscous, which results in poor reversibility.238,239 The high molecular weight of these ionic liquids, especially for more stable diethanolamine-based ones, leads to a single-digit Wh/kg energy density. A hybrid Li-ion−polysulfide RFB with a solid anode comprising a lithium intercalation material, e.g., lithiated graphitic carbon, and a nonaqueous polysulfide catholyte was claimed.240 A hybrid RFB constructed from a Li anode and catholyte containing 0.2 M Li2S8 and 0.2 M Li2S6 in 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/0.1 M LiNO3 in a mixture of dimethoxyethane/dioxolane (1:1) demonstrated a power density of 1.823 mW/cm2 at 4 mA/ cm2.241

8. ORGANIC FLOW BATTERIES Organic electroactive materials for RFBs have potentially low cost, good solubility in various electrolytes, and an opportunity to tune redox properties, solubility, and crossover transport via introducing different substituents. 8.1. Organic RFB with Aqueous Electrolytes

Reversible quinone/hydroquinone couples were previously proposed as an anode material for fuel cells242 and a cathode material for rechargeable lithium batteries.243,244 The discharge capacity of 2,5-dimethoxy-1,4-benzoquinone (312 mAh/g) is more than twice that of LiCoO2, the conventional positiveelectrode material. In a contrast to organic aprotic solvents, where quinones demonstrate two sequential one-electron reductions, in aqueous solutions they undergo fast two-electron reduction with or without proton transfer depending on pH.245 Currently these redox couples are being considered for RFBs as promising active materials for both catholyte and anolyte (Figure 7). A single-flow RFB using water-insoluble tetrachloro-pbenzoquinone (chloranil) as a cathode material and a Cd/ CdSO4 anode with a H2SO4−(NH4)2SO4−CdSO4 electrolyte was described.166 A small cell demonstrated an average Coulombic efficiency of 99% and an energy efficiency of 82%

−

(20)

The use of phosphate ligands instead of bpy allowed for increasing the active materials solubility up to 1.5−1.9 M for trimethyl phosphate (TMPO) metal complexes in propylene carbonate (PC). 2 3 6 Anolyte/catholyte couples Co(bpy)3(BF4)2/Fe(TMPO)5(BF4)2 and Ni(TMPO)3BF4/Fe(bpy)3(BF4)2 in PC produced OCVs of 1.95 and 1.12 V and have theoretical energy densities of 29 and 25 Wh/kg, respectively. For the former cell, an average energy efficiency was 89.5%.236 A Schiff base cobalt complex with bis(acetylacetone)ethylenediamine (acacen), Co(acacen), is reversibly oxidized and reduced; therefore, combined with NBu4PF6 in acetonitrile, it was used as a single-source electrolyte for anolyte and M

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hydrides such as hydrocarbons and alcohols serve as virtual hydrogen carriers generating stable organic molecules, protons, and electrons upon electrooxidation (eq 23, direct). Aromatic or unsaturated dehydrogenation products can be electrochemically hydrogenated (eq 23, reverse). The total cell reaction is described by eq 24. LHn ⇌ L + nH+ + ne−

(23)

LHn + n/4O2 ⇌ L + n/2H 2O

(24)

The theoretical OCV of electrochemical cells utilizing eq 11 is in the range 1.06−1.11 V if the dehydrogenation product is an aromatic or carbonyl compound but only ∼0.9 V if the product is an olefin.252 The energy density of these systems is lower than traditional fuel cells based on the full oxidation. Nevertheless, the theoretical energy densities for practical fuels are in the range 1600−2200 Wh/L, which is comparable with that of liquid hydrogen (2540 Wh/L). In addition, the theoretical efficiency of organic fuel cells is higher than that of hydrogen (93−95% vs 83%),252 which could result in higher round-trip efficiency. In a fuel cell mode the possibility of partial electrooxidation of organic hydrides was demonstrated for vaporized organic fuel couples such as cyclohexane/benzene (OCV = 920 mV)253 and isopropanol/acetone (OCV = 790 mV)254 using Pt/C electrocatalyst (eq 25). The power density was low (15 and 78 mW/cm2, respectively) but was higher for Pt alloys, e.g., with Ni.255 Electrochemical hydrogenation of acetone to isopropyl alcohol (IPA) and reverse IPA dehydrogenation in aqueous acidic solutions for use in a thermally regenerative fuel cell was studied by Ando et al. (eq 26).256,257 The PEM cell using eq 26 has a small OCV and a current density too low for practical application. Replacing the PtRu/C electrocatalyst with PdRu/C or PdFe/C258 or addition of sulfuric acid to the catholyte259 increased the cell OCV by a factor of 2−4. The cell efficiency peaked at different IPA and acetone concentrations.260 Acetone hydrogenation at the cathode was the rate-determining reaction, and its Coulombic efficiency was low due to the competing reaction of hydrogen evolution.261 Raney Ni was also used as an electrocatalyst for hydrogenation of ketones.262,263 Electrochemical hydrogenation of other types of substrates such as furfural,264 levulinic acid,265 and oxylene266 in a PEM reactor using a hydrogen anode as a proton source was recently reported. In the latter case low current density and higher temperature favors electrohydrogenation over hydrogen evolution.266

Figure 7. Structures of some quinones used as electroactive materials in RFBs.

over 100 cycles at the current density of 10 mA/cm2. A combination of soluble sulfonated quinones such as tiron (5,6dioxocyclohexa-1,3-diene-1,3-disulfonic acid disodium salt) in H2SO4 as catholyte with a Pb/PbSO4 anode resulted in an RFB with an OCV in the range from 0.7 to 1.0 V (for the latter, which showed 70% energy efficiency and only small capacity loss during 100 cycles at 10 mA/cm2).246 An aqueous RFB using the well-known bromine/bromide catholyte and an organic anolyte based on sulfonated anthraquinones demonstrated very high power densities, up to 0.6 W/cm2 at 1.3 A/cm2 in the galvanic mode.247 The cell uses fast redox reactions on both anode and cathode that do not require an electrocatalyst. The kinetic rate constant k0 of 9,10-anthraquinone-2,7-disulfonic acid (AQDS) reduction in acidic media was 7.2 × 10−3 cm/s,247 which was higher than those for most of the other active species used in RFBs.33 A flow cell was cycled for 50 cycles with ∼99% capacity retention per cycle. Although a crossover of bulky and negatively charged sulfonate species through a cation-exchange membrane should be minimal, the bromine crossover is still an issue. Addition of two hydroxyl groups to the AQDS molecule increases solubility and negatively shifts the redox potential by 95 mV; both effects potentially increase energy density.247 The rate constant was also higher, which was attributed to the formation of intramolecular hydrogen bonds. A similar anolyte was used in an all-organic RFB that utilizes quinone/hydroquinone redox couples in both anolyte and catholyte.248 A monosubstituted anthraquinone-2-sulfonic acid (AQS) with E0 = +0.09 V served as anolyte, and 1,2benzoquinone-3,5,-disulfonic acid (BQDS) with E0 = +0.85 V served as catholyte. The reduction rate constant of BQDS was comparable with that of AQDS and was even higher for AQS, so no electrocatalyst was needed.248 A flow cell was cycled with 0.2 M solutions of active species in 1 M H2SO4 at 10 mA/cm2 to exhibit 90% capacity release. It was noted that power density decreased substantially below 50% state-of-charge.248 The use of organic redox materials opens a wide space for increasing their solubilities above 1 M and tuning electrochemical properties via attaching various functional groups; however, these modifications will lead to higher molecular weight and, therefore, to lower specific energy.

An attempt to use a neat liquid fuel (N-ethyldodecahydrocarbazole, dodecahydrofluorene) using PtRu catalyst in a fuel cell resulted in a high OCV close to theoretical but very low current density.242 Therefore, the development of effective and selective electrocatalysts for liquid organic fuels as well as development of compatible PEMs is a prerequisite for their practical use.

8.2. Organic Regenerative Fuel Cells

8.3. Organic RFB with Nonaqueous Electrolytes

The direct use of organic hydrides as an anode active material in RFBs was proposed249−251 and then studied by DOE-funded GE-led Energy Frontier Research Center for Electrocatalysis, Transport Phenomena, and Materials. In this case organic

The use of a catholyte based on a substituted anthraquinone in nonaqueous solvent (propylene carbonate) with a Li metal anode results in two discharge/charge plateaus (2.40 and 2.20 V), which correlates with observation of two redox events in N

CH3CHOHCH3 + 1/2O2 ⇌ CH3COCH3 + H 2O

(25)

CH3CHOHCH3 ⇌ CH3COCH3 + H 2

(26)

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CV. However, even at very low current density (0.1 mA/cm2), the energy efficiency drops in 10 cycles in a static cell.267 A lithium-based RFB using organic solutions of polyromatic hydrocarbon such as naphtalene or anthracene in an organic solvent as an anolyte and of a quinone with high redox potential such as dichlorodicyanoquinodimethane as a catholyte was claimed.268 A solid Li-ion conductor like LISICON was used as a separator. Concentration of active species in the anolyte may be as high as 10 M, which may lead to a theoretical energy density of 1800 Wh/kg.268 A nonaqueous organic RFB using carbonate solutions of quinoxalines, dipyridylketones, and viologens as anolytes and solutions of 1,4-dialkylbenzenes, phenothiazines, and catechol ethers as catholytes with charge-balancing Li+ ions was claimed.269 Modification of redox-active species such as ferrocenes and pyrazines with an ionic moiety, e.g., quaternary ammonium, results in increased solubility and higher energy density of propylene carbonate-based catholytes, with a potential to reach 80−140 Wh/L.270

and Li2S4, which is described by the two-electron reaction (eq 27). Addition of 2.5 wt % LiNO3 to a dioxolane/dimethoxyethane to the catholyte effectively suppressed the internal shuttle effect. Average discharge and charge cell voltages were 2.30 and 2.45 V, respectively, at a current density of 1.5 mA/ cm2.275 The cell was cycled with little capacity fade more than 200 cycles at this current density, but the capacity fade rate was much faster for higher polysulfide concentrations.

2Li + 4S ⇌ Li 2S4

(27)

10. TECHNICAL AND ECONOMIC ISSUES OF FLOW BATTERIES The attractiveness of RFBs for energy-storage applications is tarnished by several technical and economic issues that should be resolved to ensure wide industry acceptance. Some issues are independent of chemistry, e.g., how to improve the flow-field design to maximize transport of active species. Performance of RFBs is strongly dependent on the rate of active species crossover, which leads to capacity fade, and on the rate of membrane and electrolyte degradation. RFBs are similar to regenerative fuel cells and can use the advances in fuel cell development as was recently demonstrated by UTRC and LBNL to increase the power density.276 The major difference is the dependence of RFB performance (change of the cell voltage) on the state of charge. Performance independence of SOC and, therefore, better efficiency can be achieved using a four-tank design. but it practically doubles the cost of storage/pumping components and can be uneconomical.277 From the other side, the battery power can be adjusted by changing of the electrolytes flow rate. Aqueous RFB electrolytes are nonflammable, but a largevolume catastrophic spill (e.g., from zinc−bromine RFB) can present health and safety risks that should be mitigated. In addition to performance issues, economic issues have to be resolved as well. The potential low cost of RFBs can be realized only if the cost of the active materials and other electrolyte components is low. The active materials cost is defined by reaction stoichiometry and does not depend on energy density (concentration) of anolyte and catholyte. Major contributors to the material cost are cost of metals (in the case of rare and expensive metals, e.g., vanadium), manufacturing cost of ligands for metal complex active materials, and, for nonaqueous RFBs, cost of a solvent and an electrolyte salt. The cost of vanadium only for the all-vanadium RFB calculated from its current market price is about $84/kWh, which makes the current DOE cost target outside the reach. Cost for most commodity chemicals is unlikely to change much, but for design chemicals like ligands and organic and ionic liquid active materials, the initial high cost will go down with increasing production volume. Major issues related to the system cost are low energy density leading to a large footprint and increased capital (storage tanks and pumps) and operational (pumping losses) costs, as well as the high cost of the stack (bipolar and end plates, membrane, flow fields, etc.). The electrolyte storage and delivery system cost scales with energy while the stack cost scales with power. The use of nonaqueous solvents in RFBs may add a substantial cost. For example, propylene carbonate, one of the least expensive solvents, would add $45−50/kWh to the material cost if used as a basis for 1 M electrolyte. Electrolyte salts that are used to balance the charge usually contain fluoride

9. SEMISOLID FLOW BATTERIES The concept of a semisolid RFB using a flowable conducting suspension of active materials as electrolyte was proposed by Chiang and co-workers.41 This design allows for using active materials, which are insoluble in both charged and discharged forms or only in the charged form (e.g., metal), or overcoming the solubility limitation for soluble materials. A semisolid RFB LiFePO4/LiPF6 in ethylene carbonate−dimethyl carbonate (EC-DMC)/Li with 12.6 vol % of cathode solids demonstrated a power density of 328 mW/cm2, larger than the ones observed in Li-ion batteries with stationary electrodes, and energy densities up to 50 Wh/kg (67 Wh/L).271 A nonaqueous lithium polysulfide RFB with the 1.5 vol % Ketjen black carbon suspension in the 2.5 M lithium polysulfides/0.5 M LiTFSI catholyte was claimed.272 The catholyte discharge energy density (vs Li+/Li°) was 34 Wh/L if all active species are in solution and 234 Wh/L assuming precipitation of Li2S.272 An aqueous Li-ion semisolid flow cell using slurry electrodes based on the LiTi2(PO4)3−LiFePO4 couple demonstrated an average cell voltage of 0.9 V with a Coulombic efficiency of 98− 99% at 10 mA/cm2.273 Conductive suspensions of active materials were prepared by mixing of 10−18 vol % of active materials with 1−2 vol % Ketjen Black carbon in 1 M LiNO3 aqueous electrolyte (pH = 11−12). The cell showed an energy density of 63 Wh/kg (202 Wh/L), but capacity fade was pronounced (40% in 100 cycles).273 Another aqueous Li metal RFB uses high solubility of lithium polysulfides in water and a flowable current collector (1.5 vol % Ketjen Black) to reach an energy density of 20.9 Wh/kg (26.7 Wh/L).43 Discharge to Li2S increases energy density to 36.6 Wh/kg (46.7 Wh/L). Disadvantages of the semisolid RFBs include high viscosity of electrode slurries that leads to high pumping losses and higher requirements to the electrolyte delivery system. It was emphasized that plug flow is necessary to achieve high energy efficiency with highly non-Newtonian flow electrodes.273 An intermittent (stop−go pulses) flow cell operation improves cell efficiencies.43,273,274 A membraneless hybrid RFB with Li metal anode and a nonaqueous Li polysulfide catholyte demonstrated a high energy density of 97 Wh/kg and 108 Wh/L using 5 M catholyte, which constitutes 57% of theoretical values.275 To keep all species in solution, the cathode is cycled between sulfur O

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anions such as PF6− and N(SO2CF3)2−, which increases the cost even more. Several cost evaluations of practical flow batteries cost as well as predictions of emerging ones have been published.69,90,278−281 On the basis of the developed flow sheet of a grid-size PSBFB (3 MW/18 MWh), an economic analysis showed that, for 5 M NaBr and 1 M polysulfide solutions, even assuming zero crossover of active species through a PEM, the battery is currently not cost-competitive with other batteries at a current density of 50 mA/cm2 and an energy efficiency of 77%.280 The two largest contributors to the capital cost are the PEM and the pumps, the size and cost of which is defined by the pressure drop across the bipolar plates. At an assumed membrane cost of $1000/m2, the capital cost was estimated at $833/kWh, but reducing the membrane cost to $100/m2 would reduce the cost to $204/kWh.280 An assessment of technical and economic feasibility of the Zn−Br RFB showed that this system becomes economical only at a scale above 100 kWh and a discharge time of >5 h.282 More effective flow plate designs such as serpentine have higher pressure drop,283 so a trade-off between the performance (operational cost) and capital cost should be made. The major problem for bipolar plates made from graphite is carbon corrosion at high potentials (eq 28), which is kinetically slow. C + 2H 2O → CO2 + 4H+ + 4e−

E 0= 1.24 V

the Faraday constant (26.8 Ah/mol); and V is the voltage of the cell. When catholyte and anolyte concentrations and number of electrons in each half-reaction are equal, the energy density E = nCFV/2. E=

ncCcFV 1+

ncCc naCa

=

naCaFV 1+

naCa ncCc

(29)

Because RFB capacity is the product of n and concentration of active species and is not sensitive to the cell chemistry for the same n values, the theoretical energy density can be visualized in Figure 8. One can see that, to reach an energy density

(28)

Figure 8. RFB theoretical energy density as a function of number of electrons, concentration of active species, and cell voltage.

11. CURRENT TRENDS AND PERSPECTIVES To be competitive in the energy-storage space, RFBs should have lower capital and operational costs, longer life/cycling time, high round-trip efficiency, and safety. Aqueous RFBs are intrinsically not flammable, although component toxicity (e.g., bromine and heavy metals) must be taken into account due to possible spills. Nonaqueous RFBs based on organic solvents are less safe but are still safer than Li-ion batteries due to physical separation of major amounts of catholyte and anolyte. All RFBs have similar basic architecture including electrolytes, storage and pumping systems, an electrochemical cell stack, an inverter, and a control system. The cost of active materials is defined by cell chemistry and is independent of power and energy densities. The energy density determines the size and cost of storage and pumping systems and also affects the cost of solvent and supporting electrolyte. The cost of the stack including felt electrodes, frames, bipolar and end plates, flow channels, ion-exchange membrane, or microporous separator scales with the system power density. Therefore, there are several pathways to reduce overall cost of RFBs, namely, to increase energy density, power density, energy efficiency, and lifetime as well as to simplify the balance of the plant.

competitive with traditional Li-ion batteries (>250 Wh/L), it is necessary to achieve an active species concentration above 4 M for aqueous systems and above 2.5 M for nonaqueous highvoltage systems for two-electron reactions (and twice as much for one-electron reactions). The increase of RFB energy density could be achieved via increasing one or more n, C, and V parameters. In many cases it requires the development of new chemistries that define n and V. Due to the cell-voltage limitations in aqueous media (H2 and O2 evolution on the anode and cathode, respectively), organic solvents with a much wider electrochemical window seem to be a natural choice. Deep eutectic solvents and ionic liquids, which exhibit the widest electrochemical window, were used as RFB solvents.129,173,284 In addition, ionic liquid cations and anions could potentially provide higher current densities285 or be electroactive, thus increasing the concentration of active species.108,238,239 However, high cost and extremely high viscosity (thousands of cP) severely limit their practical application in RFBs. Nonaqueous RFBs indeed exhibit higher cell voltages (2.2− 2.5 V), although not much higher than some aqueous RFBs, e.g., Zn−Ce (2.4 V).157,158 Only semisolid RFBs with lithium− graphite or anode and LiCoO241 or modified ferrocene286 cathode demonstrated that the cell voltage is >3.4 V. In most cases the solubility of redox-active metal complexes in nonaqueous solvents is in the range 1−1.5 M, which is not higher than the solubility of vanadium species in the allvanadium RFB and much lower than the solubility of some inorganic salts such as ZnBr2 (>6 M). A comparison of aqueous and nonaqueous RFBs implies that, at the same equivalent weight and material cost and very inexpensive solvents, a similar performance and cost may be achieved at concentration 2 M for nonaqueous and 5 M for aqueous electrolytes.281 On the other side, the use of flammable and more expensive organic solvents takes away major advantages of RFBssafety and cost.

11.1. Increasing Energy Density

Energy density of RFBs is notoriously low, and its increase would allow for reducing the system footprint and storage size, thus expanding the application space. The energy density is determined by the number of transferred electrons, the concentration of active species in the electrolyte, and the cell voltage. Taking into account both catholyte and anolyte, which may have different number of electrons and concentrations, the energy density E can be described by eq 29, where nc and na are the number of electrons involved in the redox reactions on the cathode and anode, respectively; Cc and Ca are maximum concentrations of the less soluble of charged and discharged active redox species in catholyte and anolyte, respectively; F is P

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chromium (or titanium) and manganese, respectively, improved the utilization rate of the vanadium ion and, therefore, the energy density due to suppression of oxygen evolution.118,292 A similar approach targeting the increase of energy density was used in a RFB with a VII/VIII anolyte and a catholyte containing two active species (1.5 M VIV/VV and 1.M FeII/FeIII in HCl−H2SO4 solution) that effectively doubled its capacity.293 The anolyte volume should be doubled, and the overall reaction is described by eq 30.

However, two major issues of nonaqueous RFBs are low faradaic efficiency that rarely exceeds 90%126,127,229,230,287,288 and high resistance of electrolyte and membrane (45−50 times higher than that for water based electrolytes289 and ionexchange membranes290,291). Tuning of ligands for metal complex or organic compounds to increase their solubility and/or improve electrochemical properties will inevitably decrease their diffusivity and thus further limit the RFB power density. Therefore, nonaqueous RFBs can hardly win over aqueous ones in terms of performance and cost. Another approach to increase the cell voltage in aqueous RFBs and to avoid side reactions of hydrogen and oxygen evolution is maintaining different pH’s in anolyte and catholyte. In a vanadium−metal hydride semiflow battery, a basic anolyte and an acidic vanadium catholyte are separated by a bipolar membrane to deliver a practical cell voltage of 1.7 V (Figure 9).168 In another example, a three-compartment cell consisting

VO2+ + Fe 2 + + 2V3 + + H 2O ⇌ VO2 + + Fe3 + + 2V2 + + 2H+

(30)

11.2. Increasing Power Density

A stack cost of RFBs as well as of fuel cells nonlinearly decreases with the power density.69,294 A cost analysis of RFB stack repeatable components (separator, electrodes, bipolar plates, etc.) showed that the cost levels out above 500 mW/cm2 (Figure 10).187 Although even higher power densities have

Figure 9. Scheme of metal hydride−vanadium hybrid flow battery. Adapted with permission from ref 168. Copyright 2013 The Electrochemical Society. Figure 10. Normalized cost of RFB repeatable stack components per power capability and per energy capacity for a C/5 rated system Adapted with permission from ref 69. Copyright 2013 The Electrochemical Society.

of a basic zincate anolyte, an acidic cerium catholyte, and an intermediate chamber with a neutral redox-inactive salt separated by cation-exchange and anion-exchange membranes, respectively (Figure 6), demonstrated a cell voltage of 3.08 V.163 This concept can work with all anion−anion (AN2−/AN−) vs cation−cation (CP2+/CP+) redox pairs. During charging the charge-balancing M+ cations cross the CEM and X− anions cross the AEM from the middle electrolyte to the anolyte and catholyte, respectively. In this setup the overall ion crossover rate between electrolytes is significantly lower (2−3 orders of magnitude) than for a single-membrane design, resulting in good energy efficiency.163 However, higher current density will substantially reduce voltage efficiency due to the large internal resistance caused by additional membrane and electrolyte. In addition, a third storage tank required for the middle electrolyte will increase the capital cost and reduce the system energy density that probably will not be compensated by higher cell voltage and lower crossover. If different pH values in anolyte (high) and catholyte (low) are not maintained, the higher RFB capacities are achieved as a result of the extra energy stored/ released in the acid−base neutralization reaction (ΔGo = −79.85 kJ/mol). Improved utilization of active materials effectively increases the energy density without an increase in concentration of active species and cell voltage. Addition of a second redox couple to the all-vanadium RFB anolyte (with lower redox potential) and catholyte (with higher redox potential), e.g.,

been demonstrated by UTRC for the all-vanadium RFB187 and by LBNL for the hydrogen−bromine RFB207,276 using a fuel cell-like design, the most known RFBs exhibit power densities in the range 50−100 mW/cm2, which makes them not competitive with traditional batteries at short discharge times. Besides an improved flow cell design, the power density may be increased by choosing fast electrochemical redox couples, e.g., quinones/hydroquinones in acidic media,247 or by using electrocatalysts to accelerate slow electrode reactions, e.g., Pt/ C,295 CuPt3,57 and Mn3O458 for the all-vanadium RFB cathode, or by cathode surface modification, e.g., using fiber-based materials67 or carbon nanotubes.148 The use of a serpentine flow field in the single-flow zinc− nickel RFB enhances mass transport, thus reducing the cathode polarization, improving the energy efficiency to 75.2%, and increasing the charge−discharge current density 4 times to 80 mA/cm2.296 An improved flow-field design with interdigitated channels was claimed.297 11.3. Maximizing Cell Efficiency

RFB energy efficiency is defined by a product of Coulombic and voltaic efficiencies. Low Coulombic efficiency in aqueous electrolytes is usually caused by the side reaction of water Q

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12. CONCLUSIONS The projected life of RFB systems is longer than that of conventional batteries, although it should be still verified in field tests. Combined with higher usable storage capacity of RFBs, it results in their lower levelized cost of electricity even when the capital cost is twice as much.305 RFBs are the most economical when the discharge time is >3 h (C-rate < 1/3) and the system cost decreases with the duration of an application.306 They have less manufacturability issues due to the uniformity of electrolytes and cells. Independence of power and energy in RFBs allows for using the same chemistry in different applications. However, both fundamental and applied research is needed for industry acceptance and wide implementation of this technology in energy-storage systems. Specific needs in chemistry, materials science, and engineering are as follows: (1) The search for novel electrochemical couples based on inexpensive components, with high solubility and redox potential, and exhibiting fast electrode kinetics. Recently the design space for redox couples was substantially expanded due to introduction of organic active materials and redox-active ligands for metal complexes. Employing multielectron transfer may increase an energy density without an increase in solubility, which in many cases is close to the practical limit due to high viscosity. (2) The development of electrocatalysts with high activity, which could be incorporated into an electrode structure with high surface area. In many cases electrode kinetics for promising energy-dense materials is too slow for practical use but can be improved using electrocatalysts or electron-transfer mediators. (3) A fundamental understanding of complex electrochemical processes in electrolytes, especially for multielectron active materials. Many processes in electrolytes initiated by an electron transfer are coupled with chemical reactions in solution or on the electrode surface. Understanding of these processes will help in selection of electrode materials, electrocatalysts, and additives for dendrite-free electrodeposition. (4) The development of ion-selective, cost-effective membranes with high conductivity and long lifetime. This is a critical component of RFBs that defines efficiency and lifetime of the whole system. Although 100% ion selectivity may be not achievable, the use of charged ionic materials may radically reduce the crossover. (5) Advanced cell/stack design and electrochemical engineering. Bipolar electrodes represent a big part of the RFB cost structure and may be an important resource for the cost reduction. In addition, an advanced design of flow fields could make in impact on RFB power density and, therefore, on the cost. Issues of materials degradation, water management, cell impedance, and balance-of-plant losses should be also addressed. (6) The development of detailed computational models for steady-state and transient operations. Computational models could save resources and time in scaling up and accurately predict the state of health and lifetime of an RFB for different applications. There are a few results on thorough testing data of RFB stacks published in the literature. (7) The evaluation of RFB environmental safety in long-term operations. Although intrinsically safer than conventional batteries, an industrial-scale RFB is essentially a miniature

electrolysis. These reactions could be regulated by the electrolyte composition and pH. It was found that in the presence of PbII ions the O2 evolution in the all-lead RFB is substantially suppressed.177 Reducing crossover of active species through an ionexchange membrane or a microporous separator increases the cell efficiency while minimizing capacity fade and electrolyte cross-contamination. To reduce the crossover of active material through a membrane, different strategies are applied such as using hybrid membranes consisting of an ionomer and an inorganic proton conductor70,71 and layered bipolar168 and multijunction298 AEM/PEM membranes. A cylindrical battery with carbon fibers electrodes and a porous silica glass with high chemical stability as a membrane was tested with all-vanadium chemistry and showed a power density of 24 Wh/L at a current density of 55 mA/cm2 and negligible self-discharge.299 Another approach is to use an indirect flow battery design with an intermediate electrolyte163 or hydrogen electrode.300 Allvanadium RFB with flow fields has a higher discharge voltage at higher flow rates and exhibits 5% higher energy efficiency than the battery without flow fields but at the expense of a larger pressure drop.68 11.4. Maximizing System Efficiency

Maximizing energy efficiency at the system level can be achieved by reducing pumping losses by reducing volume and viscosity of electrolytes. It was proposed to use gravity forces to move electrolytes through a RFB using a tilted stack design.301 It means that there is a trade-off between concentration of active species and viscosity of the solutions. In the case of slurry electrolytes in semiflow batteries, use of intermittent flow pulses of controlled volume and duration reduces the pumping losses by about half compared to the continuous mode.274 Minimizing a shunt current is also very important for improving the system efficiency because of high conductivity of liquid RFB electrolytes. A model containing an electrochemical performance, a shunt current, and a pumping loss module was developed to estimate actual system efficiency as a function of state of charge and operating conditions.294 11.5. Modular Design

Initially it was assumed that for large-scale applications electrolytes would be stored in a single tank to get the benefits of economy of scale. For example, a 120 MWh/15 MW energystorage plant projected by Regenesys consisted of 150 100-kW stacks and just two storage tanks that need ∼40 rail tank cars to fill. Such a size requires long maintenance time for filling/ draining. The most contemporary designs are based on full system transportable modules that fit into a shipping container and may have ∼250 kWh of storage. Flow batteries can be used for energy storage in the form of chemical storage, e.g., hydrogen. A vanadium−cerium RFB was proposed to generate hydrogen via water reduction with VII on a molybdenum carbide catalyst and oxygen via water oxidation with CeIV on a RuO2 catalyst in two separate reactors.302 A solar-rechargeable redox flow battery based on photoregeneration of discharged I3−/I− and FeCp2+/ FeCp2 in a separate dye-sensitized solar cell has been proposed.303 A similar approach was realized in an RFB using an aqueous solution of Li2WO4 as anolyte and LiI solution in organic electrolyte as catholyte, with LISICON film serving as Li-ion conducting.304 R

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Biography

chemical plant containing toxic and/or corrosive high energy substances. Therefore, all hazards and environmental and safety issues should be adequately addressed during a scale-up including development of preventative and protective measures. Research in these directions requires new breakthroughs and inventions but if successful it will enable an “ideal” RFB, which can cover the wide spectrum of stationary energy-storage applications using the same chemistry. The lower energy density of RFBs compared to that for Li-ion batteries makes their use in mobile applications unlikely. Worth noting, however, is the recent announcement by NanoFlowcell AG (http://www.nanoflowcell.com) of the Quant car powered by an RFB with a claimed energy density of 200 Wh/L. Is it possible to predict the “ideal” RFB or, at least, point to chemistries having the greatest potential? It is a difficult question. Sheer volume of needed energy storage measured in GWh demands millions of tons of active materials. A few redoxactive metals (Fe, Cu, Zn, and Pb) and basic inorganic (e.g., sulfuric and hydrochloric acids, sodium hydroxide, and chlorine) and organic (e.g., methanol, acetic acid, and phenol) chemicals are produced worldwide at such a scale. Therefore, based on scale and availability, the “ideal” flow battery for largescale deployment should be aqueous and use one (or more) of the most abundant metals (Fe, Cu, Zn, or Pb) or organic active materials and basic inorganic salts as electrolytes. On the basis of the cost criteria, in addition to inexpensive electrolyte materials, RFB electrode reactions should be fast and electrolyte conductivity should be high to provide high power and thus reduce the stack size, which again points to aqueous electrolytes. The importance of RFB efficiency criteria will grow with time, so minimization of the active species crossover will be achieved via development of high selectivity membranes or adjusting the size and charge of active materials. To minimize the system size and footprint, it is necessary to use highly soluble, multielectron active materials, inorganic or possibly organic. Summarizing, the “ideal” RFB most probably will employ a single, high energy density electrolyte based on inexpensive materials and an advanced electrode design and materials to maximize power density.

Grigorii L. Soloveichik is a Senior Chemist at GE Global Research, where he is leading the efforts to develop novel electrochemical storage technologies. For the past five years he served as a Director of DOE-funded Energy Frontier Research Center on Electrocatalysis, Transport Phenomena and Materials. His research interests include understanding and development of electrochemical cells (batteries, flow batteries, electrolyzers, and fuel cells), electrosynthesis, homogeneous and heterogeneous catalysts, hydrogen generation and storage, and CO2 capture. Previously, he worked as a Leading Researcher at the Institute of New Problems of Chemical Physics of Russian Academy of Sciences and a Senior Scientist at Moltech (now Sion) Corporation. Dr. Soloveichik has authored/coauthored more than 130 research papers and is an inventor/coinventor of more than 75 patents. He received his M.S., Ph.D. and Dr. Sci. (habilitation) degrees in inorganic and organometallic chemistry from the M.V. Lomonosov Moscow State University, Russia. Recently he assumed a Program Director position at Advanced Research Projects Agency− Energy (ARPA-E), U.S. Department of Energy.

ACKNOWLEDGMENTS The work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award no. DE-AR0000381, and funding was provided by a grant from Empire State Development. I thank Drs. Gary Yeager and Thomas Miebach of GE Global Research for help with the manuscript preparation. ABBREVIATIONS CAES compressed air energy storage EES electrical energy storage HER hydrogen evolution reaction ORR oxygen-reduction reaction OCV open-circuit voltage PC propylene carbonate PHES pumped hydroelectric energy storage RFC regenerative fuel cell RFB redox flow battery SMES superconducting magnetic energy storage

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (202) 2876415. Fax: (202) 287-5450. Present Address

REFERENCES

†

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Advanced Research Projects Agency−Energy, Washington, DC 20585, United States. Notes

The authors declare no competing financial interest. S

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