Tungstosilicic Acid: A Promising Electrolyte for Redox Flow Battery

Abstract

Large-scale energy-storage systems are vital for the integration of renewable energy technologies. Vanadium redox flow battery (VRFB) is one of the most developed and mature energy storage technologies available today. However, VRFB suffers from low energy density and non-availability of active materials. [1]. Recently, polyoxometalates are emerging as promising electrolyte materials with high energy density owing to their capability of multi-electron-transfer per molecule and faster redox reactions [2-3]. From the number of polyoxometalates which are available, tungstosilicic acid (TSA) is a good candidate as a negative electrolyte and is investigated in RFB. The equilibrium redox potential and the corresponding electron-transfer number of all the three redox peaks of TSA are determined. One electron-transfer in the first two redox peaks (0.008 V vs. RHE and –0.248 V vs. RHE) and approximately 14 electrons in the third redox reaction (-0.374 V vs. RHE) are determined using chronoamperometry. The equilibrium redox potential of the third redox peak is comparable to that of the V2+/3+ redox couple (−0.295 V vs. RHE), which is commonly used as a negative electrolyte in VRFB. There is a significant potential difference between the third redox peak of TSA (−0.374 V vs. RHE) and VO2 +/ VO2+ redox couple (1.118 V vs. RHE). The current loss due to parasitic hydrogen evolution at the third redox peak potential is very less in comparison to TSA reduction current and the same is confirmed by chronoamperometry and linear sweep voltammetry. Thus, multi-electron-transfer, optimum redox potential and negligible HER current is the motivation to fabricate a TSA-VO2+ RFB with vanadyl sulfate (VOSO4) as positive electrolyte. The high-electron transfer per TSA molecule leads to higher power density at relatively low concentrations of TSA. Power density of 100 mM TSA – 1 M VO2+ flow battery is higher than that of 1 M all vanadium RFB and the impact of concentration ratio between the electrolytes on the power density is studied. Higher concentration of vanadium electrolyte for a given TSA concentration gives better power density and charge capacity of the flow battery. On over-charging of the battery, TSA changes to irreversible state and further may precipitate [4]; thus, charging potential of RFB is optimized. The improvement in performance is also validated by the two-electrode cell electrochemical impedance spectroscopy results. Figure: Cyclic Voltammograms of 100 mM TSA in 0.5 M H2SO4 (black) over bare glassy carbon electrode, equimolar (1 M each) VO2 +/VO2+ (blue) and equimolar V3+/V2+ (red) in 3M H2SO4 over Vulcan-C coated GC electrode recorded at scan rate of 20 mV s-1.