Iron-Tungsten Redox Flow Battery
Authors: Anwar M Shariq , Sarkar Arindam 
Published: July 2021

Redox flow batteries are hailed as a panacea for grid level storage. Since the successful commercialization of vanadium redox batteries, a lot of efforts are being made to develop alternative chemistries. Most of them are based on metal ions which can exist in multiple oxidation states. One of them being, Fe based batteries developed first by R. F. Savinell.[1] Iron based batteries offer significantly cheaper and abundant alternatives to vanadium chemistry and is crucial from a strategic viewpoint. We present here initial development of an iron-tungsten based redox flow battery. One of the key advantages of the battery system presented here is the determination of state of charge by colorimetry. The iron salt as ferrous sulphate was used as an electrolyte for the positive electrode and the tungsten salt as phosphotungstic acid was used as the negative electrode. Although the Fe reaction involves 1 electron only, the reaction of phosphotungstic acid 4 electrons, thus the battery is expected to provide higher capacity compared to a normal Fe battery. Moreover, the maximum potential that can be achieved by using the highest reduced form of phosphotungstic acid is comparable to a vanadium or an all-iron battery.
A flow cell was constructed and assembled in house using normal single cell fuel cell setup complete with gold-plated copper current collectors, graphite plates engraved with a serpentine flow field, porous graphite felt as electrode or reaction surface and nafion 117 as the selective proton conducting membrane. The schematic of the flow cell is presented in Figure 1(a). The electrolyte used for the positive electrode was iron(II) sulphate heptahydrate and for the negative electrode was phosphotungstic acid. Prior to the flow cell tests, CV experiments were conducted in a traditional 3 electrode setup. A usual CV (see Figure 1(b)) for Fe2+ and Fe3+ was obtained while the CV of phosphotungstic acid showed 3 oxidation and reduction waves corresponding to the 3 reactions. At very low negative potential, the hydrogen evolution reaction severely interfered with the 3rd reduction wave. An analysis of the peak potential with scan rate revealed that the rate constant for the Fe reaction to be 3x10-4 cm/s close to the reported value of 3.3x10-5 cm/s.[2] Meanwhile, an analysis of the peak separation of the 3 peaks for the phosphotungstic acid revealed that the reactions are reversible with a peak separation of ~ 59 mV and invariant with respect to scan rate.
Meanwhile, for the flow cell experiments, the solution concentrations were similar to that for the CV and initially the battery was charged by applying a potential of 1.3 V for 30 minutes. It may be mentioned here that the volume of each of the electrolytes was same and equal to 100 mL. The flow rate of the electrolyte through the cell was kept constant at 120 mL/min. The discharge was performed by drawing current from 10 mA to 1000 mA for 10 s and the potential was recorded to ensure pseudo steady state. Finally the current and the potential values were rearranged to obtain the IV (current-potential) plot along with the power density curve (see Figure 1(c)). The maximum power obtained was 50 mW/cm2 and the maximum current that could be drawn was 160 mA/cm2, which is slightly low compared to the all-iron battery reported by R. F. Savinell.[1] The results obtained here suggest that the Iron-PTA battery demonstrated here could indeed prove to be a viable alternative to the vanadium flow battery. The electrochemical reactions for the iron-phosphotungstic acid redox flow battery are as follows:
Negative:
[PW12O40]3- + e

-
↔ [PW12O40]4- Eo
' = +0.222V vs. RHE
[PW12O40]4- + e
- ↔ [PW12O40]5- Eo
' = -0.033V vs. RHE
[PW12O40]5- + 2e
- + H+ ↔ H[PW12O40]6- Eo
' = -0.348V vs. RHE
Positive:
Fe2+ ↔ Fe3+ + e

-
Eo
' = +0.705V vs. RHE
Overall:
4Fe2+ + [PW12O40]3- ↔ 4Fe3+ + H[PW12O40]6- Eo
' = +1.053V vs. RHE
References
[1] Hruska, L. W., & Savinell, R. F. (1981). Investigation of Factors Affecting Performance of the Iron‐Redox Battery. Journal of The Electrochemical Society, 128(1), 18–25.
[2] Tanimoto, S., & Ichimura, A. (2013). Discrimination of Inner- and Outer-Sphere Electrode Reactions by Cyclic Voltammetry Experiments. Journal of Chemical Education, 90(6), 778–781.

Figure 1