Impact of pH and Composition of Phosphovanadomolybdate Anionic Catholytes on the Performance of Redox Flow Polymer Electrolyte Fuel Cells (Supporting Information)

A H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution containing phosphovanadomolybdate anions such as [PV₃Mo₉O₄₀]⁶⁻ and [PV₂Mo₁₀O₄₀]⁵⁻ were applied to a catholyte of redox flow polymer electrolyte fuel cells (PEFCs). This system enables continuous power generation by the electrochemical reduction of the phosphovanadomolybdate anions over the carbon cathode and subsequent reoxidation of the reduced phosphovanadomolybdate anions in the cathode tank. Currently, investigations on H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution have been limited to compositional analysis only, and there are no reports of its application to the catholyte. In this study, the performance of H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution using as the catholyte in redox flow PEFCs was investigated. In addition, the pH of catholytes was increased, and effects on the performance were compared. Current–voltage measurements showed that increasing the pH (0.3 → 2.5) decreased the power density (42 mW cm⁻² → 17 mW cm⁻²). On the other hand, the reduction–reoxidation operation showed that increasing the pH increased the reoxidation rate of the phosphovanadomolybdate anions (0.5 mmol min⁻¹ → 2.2 mmol min⁻¹). This study is the first step toward the use of H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution as a catholyte in redox flow PEFCs.