Uncovering multielectron transfer in phosphomolybdate cluster electrolytes for redox flow battery application

Abstract

This work reports, for the first time, the use of phosphomolybdic acid (PMA) solutions as high-storage capacity electrolytes for aqueous redox flow cells. The electrochemical performance of PMA, paired with a vanadium posolyte, was investigated to optimize discharge capacity, current output, and power density. Cyclic voltammetry revealed multiple charge-transfer processes in the PMA-based electrolyte. In flow cell configurations, these processes enabled deep reduction of the PMA negolyte - confirmed by permanganate titration - resulting in 8 to 12 electrons exchanged per cluster at moderate to high PMA concentrations, as determined by galvanostatic discharge tests. This multicharge storage capability yielded an impressive capacity of 126 Ah center dot L- 1 for the PMA negolyte, significantly outperforming conventional vanadium-based systems. FTIR and NMR analyses confirmed the structural stability of PMA after galvanostatic cycling. Additionally, electrochemical impedance spectroscopy (EIS) was conducted to further understand the battery performance. Key factors influencing the assembly of highcapacity and high-power-density flow cells are discussed, positioning PMA as a highly promising electrolyte for next-generation redox flow batteries (RFB).