Electrode Treatments for Redox Flow Batteries: Translating Our Understanding from Vanadium to Aqueous‐Organic

Abstract Redox flow batteries (RFBs) are a promising technology for long‐duration energy storage; but they suffer from inefficiencies in part due to the overvoltages at the electrode surface. In this work, more than 70 electrode treatments are reviewed that are previously shown to reduce the overvoltages and improve performance for vanadium RFBs (VRFBs), the most commercialized RFB technology. However, identifying treatments that improve performance the most and whether they are industrially implementable is challenging. This study attempts to address this challenge by comparing treatments under similar operating conditions and accounting for the treatment process complexity. The different treatments are compared at laboratory and industrial scale based on criteria for VRFB performance, treatment stability, economic feasibility, and ease of industrial implementation. Thermal, plasma, electrochemical oxidation, CO2 treatments, as well as Bi, Ag, and Cu catalysts loaded on electrodes are identified as the most promising for adoption in large scale VRFBs. The similarity in electrode treatments for aqueous‐organic RFBs (AORFBs) and VRFBs is also identified. The need of standardization in RFBs testing along with fundamental studies to understand charge transfer reactions in redox active species used in RFBs moving forward is emphasized.


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Gamma ray
Steps: 1) 150 kGy optimized dose for carbon felt at a rate of 10 kGy h −1 in a gamma irradiator with 60 Co source PU: Gamma irradiator 1/1/2 [3] Acid ID Chemical Treatment Details

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Echem Oxidation-1 H2SO4
Steps: 1) Graphite felt was used as anode, and graphite plate was used as cathode.The felt was electro-oxidized at 100 mA cm −2 in 1 M H2SO4 passing charge of 560 mAh g −1 , 2) Graphite felt was then washed with deionized water, and 3) Dried at 70 °C for 48 h PU: Potentiostat, Washing vessel, Oven 3/3/6 [12] Echem Oxidation-2 H2SO4 Steps: 1) Graphite Felt was used as anode, and Ti plate as cathode, carried out in 1 M H2SO4.Potential between 5-15 V is applied to galvanically oxidize passing 3000 C g −1 mass of felt, 2) Felt was washed with deionized water, and 3) Dried in a vacuum oven at 120 °C for 5 h PU: Potentiostat, Washing vessel, Oven 3/3/6 [13] Echem Pulse NaOH Steps: 1) Graphite Felt was used as working, Saturated Calomel as reference, and Pt as counter electrode.Square wave potential pulse between −1.3 V to 0.6 V at a frequency of 5 Hz was applied for 1600 seconds (optimized) in 2 M NaOH PU: Potentiostat, Washing vessel 1/2/3

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Steps:
Preparation of RSF solution: 1) Cocoons from silkworms obtained from Uljin farm were boiled for 25 mins in 0.02 M Na2CO3, 2) Washed with deionized water, 3) Dried at room temperature for 3 days, 4) Dissolved in an aqueous solution of 9.3 M LiBr at 60 °C for 6 h, 5) Dialyzed in water using dialysis cassette for 2 days to get 8 wt% RSF solution Coating on carbon felt: 1) Carbon felt was dip-coated in an aqueous 0.5 wt% solution of RSF (or glucose O source, or melamine N source), 2) Dried in a vacuum oven for 30 °C for 3 h, 3) Heated in tube furnace for 2 h at 400 °C (at 5 °C min −1 ) in Ar (200 mL min −1 ) PU: Boiler, Washer, Oven (2), Mixer, Dialysis cassette, Dip-coater, Furnace 8/8/16 [19] N, S doped Ammonium persulfate Steps: 1) 0.25 M Ammonium persulfate is dissolved in 100 mL deionized water and 1 g of carbon felt was immersed in it, 2) The solution with felt was then placed in a 200 mL Teflon lined hydrothermal reactor heated in an air furnace at 180 °C for 12 h, 3) Rinsed with deionized water to remove residual salt from the surface PU: Mixer, Autoclave, Furnace, Washer 3/4/7 [20] B, O doped Boric acid Steps: 1) Thermally treated at 400 °C for 5 h in air (ramp rate 15 °C min −1 ), 2) Oxygen-treated GF was immersed in ethanol solution (100 mL) of boric acid (0.1 and 1 M) at 298 K for 1 h with stirring, 3) Dried in oven at 80 °C for 4 h, 4) Thermally treated in N2 for 1.5 h at 700 °C, 5) Washed with ethanol at 50 °C under sonication for 1 h and dried at ambient temperature PU: Furnace (2), Mixer, Oven, Washer with sonication 5/5/10 [6] P doped

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Ozone
Steps: 1) Felt placed in a cylindrical quartz column (55 mm outer diameter, 50 mm inner diameter) of an electric furnace and was heat treated.Exposed to ozone at 1 L min −1 (during heat treatment) generated from gaseous oxygen via a commercial ozone generator (2OG-030, Ozone Engineering), 2) Column purged with Ar to remove ozone molecules after treatment PU: Ozone generator, Furnace 2/2/4 [5] CO2 Steps: Carbon felt treated in a tube furnace at 1000 °C for 30 mins at 20 °C min −1 and flow rate of 50 sccm PU: Furnace 1/1/2 [24] Porous Felts by Chemical Reaction ID Chemical Treatment Details

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Defects via NiO/Ni NiO/Ni
Steps: 1) Cleaned with acetone for 10 mins, 2) Precursor solution was prepared by dissolving Nickel nitrate hexahydrate (Ni(NO3)2.6H2O) in acetone followed by ultrasonication for 5 mins, 3) Felt immersed in precursor solution (5 wt%) for 2 h, 4) Dried at 60 °C for 1.5 h in a vacuum oven to obtain a uniform layer of precursor on carbon felt, 5) Thermal treatment at 500 °C in Ar to form NiO nanoparticles, 6) Temperature increased to 600 °C for 30 mins to etch the surface of felt by NiO reduction in Ar gas, 7) Oxidation at 500 °C for 30 mins in air atmosphere to recover NiO etching source (these oxidation at 500 and 600 °C repeated multiple times to control pore size), 8) Felt dipped in 2 M HNO3 followed by 3 M HCl to remove the metals from the surface, and 9) Thermally treated at 500 °C for 3 h in air PU: Vessel for cleaning, Mixer, Ultrasonicator, Oven (min 2), Furnace (min 3), Acid-resistant containers (2) 9/10/19 [25] Porous felt using K2FeO4 K2FeO4 Steps: 1) Scapium scaphigerum was soaked in deionized water for 3 h to make it fully expand, 2) Core and meridian of scapium scaphigerum was removed, 3) Pup was moved to a Teflon lined autoclave which was sealed and heated at 180 °C for 15 h, 4) Immersed in ethanol to remove organic matter, 5) Dried in oven at 80 °C for 8 h and 6) ground uniformly, 7) K2FeO4 and this grounded mixture were mixed at as mass ratio of 1:1, 8) Mixture was carbonized at 800 °C for 2 h in Ar atmosphere, 9) Sample washed with diluted HCl to remove K, Fe, and residual inorganic impurities, 10) Washed with deionized water, 11) Dried at 80 °C, 12) Mixture of 10 mg of prepared catalyst and 30 mL of DMG were mixed to create a suspension, 13) Felt was immersed in this suspension and dried at 80 °C for 4 h PU: Soaking container, Tools for core and meridian removal, Autoclave, Furnace (2), Mixer (3), Oven (2), Washer (3) 13/13/26 [26] Porous felt using Fe Fe Steps: 1) Thermally treated in air at 420 °C for 10 h, 2) FeOOH nanorods grown on thermally treated felt by hydrothermal method, 3) Annealed in N2 gas at 900 °C at 5 °C min −1 to form Fe3O4 from FeOOH on thermally treated felt, 4) Fe3O4 nanoparticles were then dissolved by treating with concentrated HCl, and 5) Thermally activated in air at 420 °C for 10 h PU: Furnace (4), Autoclave, Mixer 5/6/11 [8] Carbon catalysts

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Sulfonated CNTs Chlorosulfon ic acid
Steps: 1) MWCNTs (Shenzhen Nanotech) were added to chlorosulfonic acid and uniformly mixed, 2) Mixture was transferred to three Teflon autoclaves and then hydrothermally treated for 10 h at 200 °C, 3) Washed with deionized water until neutralized, 4) Dried at 80 °C for 8 h, 5) 3 mg of this catalyst was mixed with 10 mL of DMF to form an ink which is sonicated for 3 h, 6) Felt was then immersed in the ink and dried at 100 °C for 4 h PU: Mixer, Autoclaves, Furnace, Washer, pH meter, Oven (2), Sonicator 6/8/14 [32] N doped CNTs Ferrocene Steps: 1) N-CNT/GF was synthesized using a single injection CVD system, using ferrocene (2.5 wt%) as a growth catalyst dissolved in ethylenediamine, 2) Synthesis carried out at 800 °C under inert gas flow and steady source solution injection in a horizontal quartz tube reactor (GF placed at center of tube), 3) Reactor chamber cooled to 400 °C and air flow for 1 h to burn off amorphous carbon species PU: Mixer, CVD Reactor and associated tools, Furnace (2) 3/4/7 [33] P doped CNTs Phosphorylet hanolamine Steps: 1) 20 mg of carboxylic MWCNTs were mixed with 10 mL of phosphorylethanolamine solution which was prepared by mixing 500 mg pf phosphorylethanolamine with 10 mL deionized water and stirred for 24 h at room temperature, 2) Mixture centrifuged to remove unreacted solution, 3) Mixture dried at 60 °C for 12 h, 4) 20 mg of catalyst powder mixed with 7 mL of IPA followed by ultrasonication for 30 seconds, 5) Graphite felt submerged in the solution and shaken for 10 mins, 7) Each felt was mixed with 19.9 mL of IPA and 0.1 mL of Nafion and shook for 5 mins, 8) Catalyst coated carbon felt, and 9) Felt was then dried at 60 °C for 12 h PU: Mixer (3), Centrifuge, Oven (2), Ultrasonicator, Coater 9/8/17 [34] N, S doped MWCNTs Thiourea Steps: 1) MWCNTs and thiourea (4:1 molar ratio) were added to 30 mL of ethanol and stirred for 30 mins, 2) Heated under agitation at 50 °C to evaporate ethanol, 3) Sintered at 800 °C for 2 h under Ar atmosphere, 4) Treated graphite felts were washed with water, 5) Felts dried at 100 °C for 10 h, 6) Felts immersed in MWCNTs dispersed in dimethylformide with concentration of 0.4 mg mL −1 , and 7) Dried at 100 °C for 24 h PU: Mixer, Heater, Furnace, Washer, Oven (2), Sonicator 7/7/14 [35] Carbon Nanofiber/ CNT composite Steps: 1) Felts washed by ultrasonication in acetone for 20 mins, 2) Dried at 100 °C for 4 h, 3) Felts immersed in nickel nitrate solution (1wt% Ni(NO3)2) dissolved in acetone (60 mL) and dried for 3 h at 100 °C, 4) Calcined in an inert atmosphere, 5) Reduced in a mixture of 10% H2/rest Ar gas at 600 °C for 3 h and cooled to room temperature, 6) Heated in inert gas at 700 °C at 0.5 L min −1 flow rate, 7) Acetylene flowed (9.94% C2H2/balance Ar) for 10 mins and then cooled to room temperature, 8) Resultant CNF/CNT was refluxed in concentrated HCl for 2 h to remove metal impurities, 9) Washed with deionize water, and 10) Dried at 100 °C in air for 12 h PU: Ultrasonicator, Oven (3), Mixer (2), Furnace (2), Washer, Mixer 10/10/20 [36] Tris(hydroxymet hyl) Aminomethane CNTs Steps: 1) 0.61 g of tris (hydroxymethyl)aminomethane was mixed with 10 mL of deionized water at 60 °C, and 40 mg of carboxylic MWCNTs were added to this solution, 2) Stirred at 60 °C for 15 mins, 3) Centrifuged to remove the unreacted species (conducted 5 times), 4) Dried in an oven for 60 °C for 12 h, 5) 20 mg of catalyst was mixed in 7 mL of isopropyl alcohol and sonicated for 30 seconds for proper dispersion, 6) Graphite felt was dipped and stirred for 10 mins in the solution, 7) Graphite felt with ink was soaked into the mixed solution of 19.9 mL of IPA and 0.1 mL of Nafion 117 solution and stirred for 5 mins, 8) Catalyst coated graphite felt was dried at 60 °C for 12 h PU: Mixer (3), Centrifuge, Oven (2), Sonicator, Catalyst coater 8/8/16 [37] Graphene Nanoplatelets Steps: 1) A 5% by weight suspension of hydrolyzed carbon nanoparticles was prepared using deionized water and alcohol-free media was employed to prepare the graphene nanoplatelets suspension, 2) The pH of the colloidal suspension was varied with a 0.5 M sulfuric acid solution to give the nanoplatelets sufficient negative surface charge (zeta potential∼−38 mV at a pH of 7), 3) The suspension was then sonicated for 30 min prior to multilayer assembly, 4) An automated dipping tool (Beckmann Biomek 2000) was used in the preparation of the larger number of layered assemblies.The polymer solution and the GNP suspension were continuously stirred during this operation and dipped felt was rinsed repeatedly.This sequence of steps yielded a single bilayer of nanoplatelets-polyelectrolyte deposition.PU: Mixer, pH meter, Sonicator, Dipping tool, Washer 4/5/9 [38] Br doped graphene nanoplatelets Steps: 1) Pristine graphite (5 g) was placed in diluted halogen (Br2) in a stainless steel container with stainless steel balls (500 g, 5 mm diameter), 2) The steel container was sealed and degassed after reducing pressure (0.05 mm Hg) to remove air and then fixed in a plenary ball-mill machine and agitated at 500 rpm for 48 h, 3) Prepared samples were extracted with acetone to remove unreactive materials, 4) 8/7/15 [39] Washed with 1 M HCl solution to remove metallic impurities, 5) Dark black powders of Br-Graphene nanoplatelets were obtained by freeze drying them at −120 °C for 48 h, 6) Catalysts ink was prepared by dissolving 20 mg of catalyst particles in a mixture of 100 μL of 5wt% Nafion and 900 μL of ethanol by ultrasonically blending for 20 mins, 7) 5 mg cm -2 of ink was coated onto a carbon felt, and 8) Carbon felt was then dried at 60 °C for 12 h PU: Ball mill, Extractor, Washer, Freeze Dryer, Sonicator, Ink coater, Oven

Carbon dots
Steps: Carbon dots/Graphite Felt were synthesized by solvothermal process: 1) 0.6 g of p-phenyldiamine was dispersed in 90 mL ethanol under sonication, 2) Felt was immersed in solution and the solution is then transferred to a 200 mL Teflon lined autoclaves, and the reaction was conducted at 180 °C for 9 h, followed by cooling down to room temperature, 3) Felt was taken out and washed with water, and 4) Dried at 60 °C for 12 h PU: Sonicator, Mixer, Autoclave, Furnace, Washer, Oven 4/6/10

N, P doped carbon microspheres
Steps: Synthesis of Carbon microspheres: 1) 0.5 mol L −1 glucose solution was filled in a Teflon tank where hydrothermal reaction occurred, 2) This system was placed in an oven at 180 °C for 24 h, 3) Carbon microspheres obtained by this hydrothermal treatment was washed with anhydrous alcohol, and 4) Dried at 80 °C Doping of Carbon microspheres: 1) Carbonized at 800 °C for 2 h in Ar, 2) Mixed with NH4Cl as N source and (NH4)2HPO4 as N and P source by grinding, 3) Heated at 800 °C for 2 h in Ar to obtain N, P doped carbon microspheres Deposition on Carbon Felt: 1) 10 mg of prepared microspheres was dispersed in 5 mL of N,N-dimethyl formamide by ultrasonication, and then 2) Coated on felt PU: Mixer (2), Autoclave, Furnace (2), Oven (2), Washer, Ultrasonicator, Ink coater 9/10/19 [41] N doped carbon nanospheres Steps: 1) 200 mg of dopamine was dissolved in 100 mL of distilled water with stirring for 30 mins to form a solution at room temperature, 2) Felt was immersed in solution and hold in vacuum chamber for 3 h (for allowing it to penetrate), 3) 75 μL of tris buffer was added to initiate self-polymerization of dopamine, 4) Reaction was carried out for 8 h (optimized) at room temperature with continuous stirring, 5) Felt was washed with deionized water, 6) Dried with 50 °C for 12 h, 7) Carbonized in a tube furnace for 2 h at 900 °C (optimized).PU: Mixer (2), Vacuum chamber, Reaction vessel, Washer, Oven, Furnace 7/7/14 [42] N doped carbon black Steps: 1) 0.1 g of zein powder was stirred in a solvent containing 3 mL ethanol and 3 mL deionized water for 10 mins to obtain a yellow solution, 2) This solution was blended with 0.3 g of carbon black particles (optimum ratio 1:3) to form a coating of zein particles on carbon black, 3) Evaporated at 60 °C to allow particles self-assemble on carbon black, 4) Powder was placed on a furnace for 3 h in Ar at 800 °C (optimized), 5) 20 mg of N doped carbon black particles were dissolved in 100 μL of 5wt% Nafion and 900 μL of ethanol followed by sonication for 20 mins, 6) 5 mg cm −2 ink was coated on carbon felt, and 7) Felt was dried at 60 °C for 12 h.PU: Mixer (2), Blender, Oven (2), Furnace, Sonicator, Ink coated 7/8/15 [43] Graphite oxidebased graphene Steps: Prepared using modified Hummer's method requiring NaNO3 and KMnO4. 1) Concentrated H2SO4 (360 mL) was added to a mixture of synthetic graphite (7.5 g) and NaNO3 (7.5 g), and the resulting mixture was cooled down using ice bath, 2) KMnO4 (45 g) was added slowly in small doses to keep temperature below 20 °C, 3) Mixture was heated to 35 °C and stirred for 3 h, 4) 3% H2O2 (1.5 L) slowly added raising the temperature to 98 °C, 5) Stirred for 30 mins, 6) Centrifuged (3700 rpm for 30 min), 7) Decantation of mixture, 8) Washed with water and 9) Centrifuged again until pH was neutral, 10) Thermally treated at 1000 °C for 1 h under N2 to obtain thermally prepared graphene oxide PU: Mixer (3), Ice bath, Centrifuge (2), Decanter, Washer, pH meter, Furnace 10/10/20 [44] GO-rGO on Graphene Foam Steps: 1) Graphene was first grown on Ni foam by CVD to obtain NiF/graphene foam (GF), 2) Graphene Oxide (GO) was prepared by a modified chemical exfoliation method with natural graphite as starting material, 3) The NiF/GF was dispersed in acid GO aqueous dispersion (6 mg mL −1 , pH =3) for 24 h at 60 °C, 4) The obtained NiF/GF was filled with wet GO gels, 5) Reduced on a Zn foil surface for 3 mins, 6) Sample was then frozen in liquid nitrogen and 7) Frozen sample was freeze-dried for 24 h to obtain a gradient of oxygen functional groups, 8) Immersed in 2 M HCl for 24 h to remove NiF, 9) Repeated washing with purified water, and 10) Wet GO-RGO/GF material was further subjected to freeze drying for 24 h PU: CVD Reactor, Sonicator, Potentiostat, Liquid nitrogen container, Freeze dryer (2), Mixer, Washer (2) 10/9/19 [45] Table S4.Conditions used for preparation and testing of metal and metal-oxide electrocatalysts for VRFBs from literature and corresponding unique IDs that are used to identify them in this work.Number of steps and process units (PU) needed to implement the process industrially are identified to evaluate complexity of treatment.

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Prussian Blue Electrodeposit ion
Steps: 1) Felts were washed with 5 wt% HCl to remove impurities, 2) Washed with water and ethanol three times, 3) This was followed by drying at 50 °C for 12 h, 4) 2.5 mM K3Fe(CN)6 and 2.5 mM FeCl3 was added to 0.5 M KCl solution and several drops of 5 wt% HCl was added to adjust the pH to 2, and 5) Graphite plate was used as counter electrode and 0.1 V (vs Hg/Hg2SO4) was applied for 500 seconds to electrodeposit on carbon felt (optimized) PU: Washer (3), Oven, Mixer, pH meter, Reaction vessel, Potentiostat 9/8/17 [69] Nanofibers ID Deposition Method Treatment Details

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S2. Techno-economic Model for Evaluating Affordable Capital Cost
An in-house technoeconomic model is developed to evaluate the total overall capital expenditure (CAPEX) of the flow battery.The current density vs energy efficiency relationships developed for various treatments are used to obtain the number of stacks required to deliver a fixed capacity of 12 MWh (Table S5).Since the number of stacks required to deliver a fixed capacity changes with operating current density, as shown in Table S5, the CAPEX for VRFBs with treated and untreated carbon felts are different.The cost associated with balance of plant hardware, electrolyte, and utilities are unchanged with the change in number of stacks for an RFB with fixed energy capacity.Thus, the total CAPEX for the flow battery system with fixed energy capacity is the sum of the cost of battery stack and balance of plant hardware component evaluated in Table S6.The CAPEX for VRFBs with treated and untreated CFs (UT and UT/S) are evaluated to obtain the affordable capital cost (ACCUT and ACCUT/S), using the methodology discussed in Figure 6 of main text.

S3. Cost Estimate of Carbon Felts and Treatments
The costs of carbon felts (CFs) are evaluated using the cost of polyacrylonitrile (PAN) precursor and adding the costs associated with textile and pressing.All calculations are done assuming total production of carbon fiber is 1500 ton year −1 .

ID Thermal-2:
The CF is treated at 750 °C for 5 mins in ID Thermal-2 treatment (Table S3).We scale the cost based on the change in energy required, estimated by the change in duty.

ID Thermal-5:
The CF is treated at 500 °C for 5 h in ID Thermal-5 treatment (Table S3).Using the process same as for ID Thermal-2, we get: Cost of Thermal-5 treatment = The difference between the cost of untreated and thermally treated CF of same thickness from SGL Carbon is ~ 30-40 € m −2 (Figure S10).Using € to $ conversion ratio as 1.18, this cost difference lies between ~ 35-47 $ m −2 , which is close to the cost of Thermal-5 treatment evaluated above.b) Plasma: Carbon fibers are treated in plasma ~20-24 mins (  , assuming an average of 22 mins for calculations). [85] Estimated costs of plasma treatment of carbon fibers by researchers at Oak Ridge National Lab, [79,83-85]

ID Plasma-3:
The CFs is treated with plasma for 10 mins ( −3 ) in ID Plasma-3 treatment (Table S3).We scale the cost based on the change in time of plasma treatment.The surface treatment for carbon fibers involves passing 500 C g −1 (= 139 mAh g −1 ) of charge. [86] Estimated costs of surface treatment of carbon fibers by researchers at Oak Ridge National Lab, [79,83-85] as of 2013 = 0.

Figure S1 .
Figure S1.Relative change in Coulombic Efficiency for VRFBs with carbon felts with and without treatments at fixed current density of 50 mA cm −2 .Only the treatments for which Coulombic Efficiency at 50 mA cm −2 is reported are considered.The change in Coulombic Efficiency after treatments is < 5 % for VRFBs.

Figure S2 .
Figure S2.Energy Efficiency of laboratory scale VRFB with various carbon felt treatments at three different current densities.(a) 50, (b) 75, and (c) 100 mA cm −2 .The Energy Efficiency for a scaled up (200 kW/ 400 kWh) VRFB with untreated carbon felt (UT/S) as electrodes is shown by dotted line for each current density.If the Energy Efficiency of laboratory scale VRFB for at least one of the three current densities exceed Energy Efficiency of VRFB with UT/S,

Figure S3 .
Figure S3.Relative increase in Energy Efficiency of laboratory scale VRFB with various carbon felt treatments at three different current densities.(a) 50, (b) 75, and (c) 100 mA cm −2 .If the relative increase in Energy Efficiency of

Figure S4 .
Figure S4.Energy Efficiency of laboratory scale VRFB with different metal and metal oxide based electrocatalysts at three different current densities.(a) 75, (b) 100, and (c) 125 mA cm −2 .The Energy Efficiency for a scaled up (200 kW/ 400 kWh) VRFB with untreated carbon felt (UT/S) as electrodes is shown by dotted line for each current density.If the Energy Efficiency of laboratory scale VRFB for at least one of the three current densities exceed Energy Efficiency of VRFB with UT/S, first part of the performance criteria as discussed in the main text is satisfied.The green arrow highlights the region where energy efficiency should lie for each current density to satisfy the criteria.

Figure S5 .
Figure S5.Relative increase in Energy Efficiency of laboratory scale VRFB with different metal and metal oxide based electrocatalysts at three different current densities.(a) 75, (b) 100, and (c) 120 mA cm −2 .If the relative increase in Energy Efficiency of laboratory scale VRFB for at least one of the three current densities is greater than 5 %, second part of the performance criteria as discussed in the main text is satisfied.The green arrow highlights the region where relative increase in energy efficiency should lie for each current density to satisfy the criteria.

Figure
Figure S6.Energy Efficiency DegradationFactor for laboratory scale VRFB with various carbon felt treatments.If the Energy Efficiency Degradation Factor of laboratory scale VRFB is < 0.0012 % cycle −1 , stability criteria as discussed in the main text is satisfied.The red arrow highlights the region where Energy Efficiency Degradation Factor should not lie for to satisfy the criteria.

[ 76 ] 15 Figure S8 .
Figure S8.Affordable Capital Cost for laboratory scale VRFB with various carbon felt treatments at energy efficiencies of (a) 67.3,(b) 73, and (c) 78 %.The green arrow highlights the region where affordable capital cost should lie for each energy efficiency to satisfy the criteria.

Figure S9 .
Figure S9.Affordable Capital Cost for laboratory scale VRFB with various metal and metal-oxide electrocatalysts at energy efficiencies of (a) 67.3,(b) 73, and (c) 78 %.The green arrow highlights the region where affordable capital cost should lie for each energy efficiency to satisfy the criteria.

Figure S10 .
Figure S10.Screenshot of the quote from SGL Carbon for thermally treated carbon felts, obtained in July 2021.2.6 and 4.6 mm (2.6 EA and 4.6 EA) are two thicknesses in which thermally treated carbon felts are sold by SGL Carbon.