Effect of Phosphomolybdate Anionic Redox Species on the Performance of Redox Flow Polymer Electrolyte Fuel Cells
Authors: Naruse Shinji , Muroyama Hiroki , Matsui Toshiaki , Eguchi Koichi 
Published: December 2024

Redox flow polymer electrolyte fuel cells (RFFCs) have attracted attention to establish precious metal catalysts–free polymer electrolyte fuel cells (PEFCs) [1]. In RFFCs, an aqueous solution containing redox mediators is supplied to the electrodes for power generation. The aged redox mediators are oxidized or reduced to be the initial state outside the cell, and then the regenerated mediators are fed to the electrodes again. The development of mediators with high redox ability is essential to achieve high performance in RFFCs. Phosphomolybdate anion [PMo12O40]3– (PMA) is known as one of the candidates for an anode redox mediator for RFFCs [2]. Studies adapting PMA aqueous solutions as mediators for RFFCs 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 [3]. When PMA is reduced, it receives N electrons to form reduced–PMA (hereafter abbreviated as PMA–[N], N = I, II, IV) [4]. Thus, it is beneficial to clarify the compositional properties in order to design high–performance anolytes suitable for RFFCs.
In this study, the constituent amounts of PMA redox species were semi–quantitatively determined by 31P NMR measurements [5]. Power generation tests have revealed the effect of redox species of heteropolyanion on the performance of RFFCs using the reduced PMA as an anolyte. H3PMo12O40·19.6H2O and H2SO4 were added to ultrapure water to prepare an oxidized 0.1 M PMA–0.5 M H2SO4 aqueous solution. This solution was subjected to a constant current electrolysis (5.00 mA cm–2) to obtain the reduced PMA. A membrane–electrode assembly was prepared using Nafion® membrane as an electrolyte, carbon black as an anode, and Pt/C as a cathode. The pre–reduced PMA aqueous solution bubbled with argon was supplied to the anode by a liquid feed pump. Humidified oxygen (relative humidity; 100%) was supplied to the cathode. Current–voltage (I–V) measurements and galvanostatic operation tests (5.00 mA cm–2) were performed using this single cell at 80 ºC. The 31P NMR spectra of PMA aqueous solutions were measured using a 0.1 M phosphonoacetic acid (C2H5O5P)–D2O as an internal standard. As a result, 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.

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