ELECTROCHEMICAL-BEHAVIOR OF MOLTEN V2O5-K2S2O7-KHSO4 SYSTEMS

The electrochemical behavior of K2S2O7-KHSO4-V2O5, K2S2O7-V2O4 and K2S 2O7-KHSO4-V2O4 melts was studied in argon and SO2/air atmospheres usin g a gold electrode. In order to identify the voltammetric waves due to KHSO4, molten KHSO4 and mixtures of K2S2O7-KHSO4 were investigated by voltammetry performed with Au and Pt electrodes in an argon atmospher e. It was shown that H+ reduction took place at 0.26 V vs. an Ag+/Ag r eference electrode, i.e., at a potential in between the V(V) --> V(IV) and V(IV) --> V(III) reduction stages. The presence of KHSO4 caused a n increased concentration of V(III) species in the V2O5 containing mol ten electrolytes. This effect may be caused either by protonic promoti on of the V(IV) --> V(III) reduction (VO2+ + 2H(+) + e(-) --> V3+ + H2 O) or by chemical reduction of V(IV) complexes with hydrogen, formed f rom H+ as the product of the electrochemical reduction. Both the V(V) --> V(IV) reduction and the V(IV) --> V(V) oxidation remained one-elec tron electrochemical reactions after the addition of KHSO4 (or water) to the H2S2O7-V2O5 melt. Water had no noticeable effect on the V(V) -- > V(IV) reduction but the V(IV) --> V(V) oxidation proceeded at higher polarizations in the water-containing melts in both argon and SO2/air atmospheres. This effect may be explained by participation of the wat er molecules in the V(IV) active complexes.