Mechanistic Insights of V2+/V3+ Reaction on Glassy Carbon for Vanadium Redox Flow Batteries

Abstract

Energy demand is increasing due to the world’s growing population and rising quality of life, accompanied by increasing CO2 emissions. A shift towards renewable energy sources like solar and wind is needed to sustainably meet our growing energy requirements. However, the intermittent nature of renewable resources creates the need for energy storage technologies, which would store excess energy and then supply this stored energy when it is needed. Pumped hydroelectric is the most used electrical energy storage method, but is limited to specific geographical regions, leading to a push towards development of redox flow batteries (RFBs). Despite the unique ability of RFBs to decouple energy and power density while having high current densities, their commercial viability is limited by high costs. In particular, All Vanadium Redox Flow Batteries (VRFBs, i.e., (VO2+/VO2 +//V2+/V3+)) have received much interest due to the use of the common element vanadium that removes the issue of cross-contamination, leading to higher Coulombic efficiencies. The sluggish kinetics of V2+/V3+ redox couple on the negative electrode (Reaction 1), however, is found to limit VRFBs efficiency.1 Thus, improving V2+/V3+ kinetics can lead to higher current densities, reducing the overall cost of energy storage. Studies indicate the V2+/V3+ reaction is an inner sphere electron transfer based on differences in activity on certain surfaces,2 but there is a lack of the mechanistic understanding needed to further improve rates. V2+→ V3+ + e‾ (Reaction 1) Eº = −0.255 V vs SHE Traditionally, VRFBs use porous graphite felts with sulfuric acid (H2SO4) as an electrolyte, often without accurate area estimates needed for obtaining normalized current densities. Recent studies have shown increased stability of V ions and higher energy capacities for mixed H2SO4 and HCl electrolytes,3 but the effect of electrolyte on kinetics is also not well understood. Here, we study the V2+/V3+ reaction kinetics on a controlled electrode surface in H2SO4 and HCl, and show that the kinetics increase with State of Charge (SoC) and in the presence of Cl-, where Cl- also changes the V3+ coordination shell as detected by UV-Vis. We use a polished glassy carbon disk as the reaction surface, with high conductivity and controlled electrochemical surface area as compared to graphite felts. We find the V2+/V3+ reaction rates are independent of the total vanadium ion concentration, but instead depend on SoC. SoC was estimated by deconvoluting the individual V2+ and V3+ concentrations from UV-Vis. Exchange current densities for oxidation (iO,oxidation ) decrease with decreasing SoC, whereas Tafel slopes for oxidation (boxidation ) remain constant with SoC. Rate measurements at varied temperature (23.3 – 40.0 °C) were conducted to evaluate effective activation energies (Ea ) for the V2+/V3+ reaction at different SoC by two independent measurements, a) iO,oxidation and b) Charge Transfer Resistance (Rct ). The two independent methods for determining Ea matched closely, and show that Ea increases with decreasing SoC (Figure 1). This dependence of Ea on SoC along with high magnitudes (42 – 44 kJ mol-1 for 50% SoC, as compared to ∼13 kJ mol-1 for the outer sphere ferri-ferrocyanide redox couple4) confirms V2+/V3+ is an inner sphere reaction. This variation of iO,oxidation and Ea with SoC is indicative of a mechanism involving an adsorbed V intermediate affecting reaction kinetics. We measure higher iO,oxidation in HCl compared to H2SO4, which we hypothesize is due to a change in the coordination shell of the reacting specie, leading to formation of a different transition state with lower Ea . V2+ and V3+ ions in pure water typically exist as [V(H2O)6]2+ and [V(H2O)6]3+ (Oh symmetry) respectively with six labile water molecules constituting their first coordination sphere.5 We use UV-Vis to determine the structure of the coordination shell with anions present, by a shift in peak locations (based on ligand field strength) and absolute absorbance (depicting changes in molecular symmetry). The V2+ UV-Vis spectra remained unchanged in H2SO4, HCl, and mixed acid electrolytes of varied concentrations, indicating [V(H2O)6]2+ was the dominant species, with no anions in the first coordination sphere. However, the UV-Vis spectra of V3+ had a shift in peak locations with reduced absolute absorbance in HCl relative to H2SO4, indicating formation of a different V3+ complex, which we hypothesize is related to the change in redox kinetics.