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 V 3+ or VO 2+ in a highly concentrated mixed acidic solution based on both electroanalytical and computational approaches. First, we found that the VO 2+ /VO 2 + redox reaction is well explained by the EC/EC square scheme. We also found that V 3+ is electrochemically oxidized to V 4+ and subsequently undergoes a transition to stable VO 2+ via hydrolysis. In the V 3+ /V 2+ 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 V 3+ is electrochemically reduced to V 2+, 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 V 2+ via hydrolysis only when a local concentration of V 2+ 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 VO 2+ to V(II) was also analyzed, revealing that VO 2+ 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 V 3+ nor V 2+ can form stable intermediates during the HER, while V(II)O has the highest proton affinity compared with V(III)O + and V(IV)O 2+, indicating a plausible electrocatalytic property of V(II)O for the HER.