Unraveling V(V)-V(IV)-V(III)-V(II) Redox Electrochemistry in Highly Concentrated Mixed Acidic Media for a Vanadium Redox Flow Battery: Origin of the Parasitic Hydrogen Evolution Reaction.

We present a mechanistic understanding of the full redox electrochemistry of V(V)-V(IV)-V(III)-V(II) and the origin of the parasitic hydrogen evolution reaction (HER) during electroreduction of either V3+ or VO2+ in a highly concentrated mixed acidic solution based on both electroanalytical and computational approaches. First, we found that the VO2+/VO2+ redox reaction is well explained by the EC/EC square scheme. We also found that V3+ is electrochemically oxidized to V4+ and subsequently undergoes a transition to stable VO2+ via hydrolysis. In the V3+/V2+ redox reaction via voltammetric analysis at scan rates greater than 0.05 V/s, the voltammograms are well explained based on a simple 1e- transfer reaction scheme. However, at the longer time scale observed in the chronoamperograms with constantly applied potentials where V3+ is electrochemically reduced to V2+, we found that a significant HER occurs because of possible formation of an electrocatalyst related to the V(II)O species, V(II)catalyst. We suggest that V(II)O is kinetically formed from V2+ via hydrolysis only when a local concentration of V2+ is high in the vicinity of a GC electrode surface, and V(II)O is adsorbed on a GC surface to form V(II)catalyst. To extend our mechanistic pathway, electroreduction of VO2+ to V(II) was also analyzed, revealing that VO2+ is electroreduced to VO+ and further reduced to VO in addition to disproportionation of VO+. Eventually, V(II)catalyst forms on a GC electrode, resulting in a significant HER. The computational calculation strongly supports the possible formation of V(II)catalyst. The calculation shows that neither V3+ nor V2+ can form stable intermediates during the HER, while V(II)O has the highest proton affinity compared with V(III)O+ and V(IV)O2+, indicating a plausible electrocatalytic property of V(II)O for the HER.