Effect of phosphomolybdate anionic redox species on the redox flow polymer electrolyte fuel cell performance

Abstract

Phosphomolybdate anion [PMo12O40]3– (PMA) is known as one of the candidates for an anode redox mediator for redox flow polymer electrolyte fuel cells (PEFCs). Studies adapting PMA aqueous solutions as mediators for redox flow PEFCs have reported only properties related to the preparation composition and comprehensive systematic studies on the redox component of heteropolyanions are lacking. For example, PMA undergoes hydrolysis and dimerization in an aqueous solution to form heteropolyanions with different molybdenum coordination numbers spontaneously. When PMA is reduced, it receives N electrons to form reduced–PMA (hereafter abbreviated as PMA–[N], NI, II, IV). Thus, it is beneficial to clarify the compositional properties in order to design high–performance anolytes suitable for redox flow PEFCs. In this study, the constituent amounts of PMA redox species were semi–quantitatively determined by 31P NMR measurements. Power generation tests have revealed the effect of redox species of heteropolyanion on the performance of redox flow PEFCs using the reduced PMA as an anolyte. The molar percentages of PMA–[N] species changed during the galvanostatic electrolysis reduction and the current passage test. The power density improved from 0.43 mW cm−2 to 3.52 mW cm−2 by increasing the molar percentages of the PMA–[N] in the aqueous solution.