Power Generation Characteristics of PEFCs Using Heteropolyacids As Anode Redox Mediators

Abstract

Polymer electrolyte fuel cells (PEFCs) have attracted much attention as clean power generation systems. PEFCs have been employed for residential cogeneration systems, fuel cell vehicles and spaceships due to short startup/shutdown times, small size, and low weight, compared to high– temperature FCs. Because precious metals such as Pt are utilized for electrocatalysts in order to obtain high power generation efficiency, high cost of the system is one of the issues in the spread period. In addition, hydrogen fuel has problems in safety and transportation.Therefore, in recent years, PEFC using heteropolyacid (HPA) as a mediator has been reported for the solution of these problems [1]. In this system, reduced HPA is supplied to the anode, and is converted to oxidized HPA over the electrode via the following reaction. [H nXM V nM VI 12–n O40] z – → [XM VI 12O40] z – + ne– + nH+ (1) where X = Si, P and M = W, Mo, V On the other hand, oxygen is reduced to produce water on the cathode, as in the case of general PEFCs. This PEFC system can be continuously operated by reducing the oxidized HPA and supplying it to the anode again. Electrochemical methods and addition of reducing agents can be considered for reducing oxidized HPA [2]. However, more effective reduction methods have been needed to achieve high performance and long-term operation.In this study, various metal pieces were utilized for the reduction of oxidized HPA in order to establish an effective method. The effect of metal pieces on the power generation performance and long-term operation of PEFC was evaluated. H3PW12O40·28H2O (PW12) and H2SO4 were added to ultrapure water to prepare an oxidized PW12 solution (0.01 M PW12–0.5 M H2SO4). This solution was subjected to a constant current electrolysis (1.00 mA cm–2) or an addition of metal pieces (Al, Ni, Fe, Co) to obtain the reduced PW12. A membrane–electrode assembly was prepared using Nafion® membrane as an electrolyte, carbon black as an anode, and Pt/C as a cathode. The operating temperature was 80 ºC, and O2 (relative humidity: 100%) was supplied to the cathode. The reduced PW12 solution bubbled with Ar gas was supplied to the anode by a liquid feed pump. Current–voltage (I–V) measurements and galvanostatic operation tests (5.00 mA cm–2) were performed using this single cell. As a result, the addition of Co pieces achieved the high reduction degree of HPA and the superior cell characteristics. The suppression of the side reactions such as hydrogen generation and the increase of the output power were necessary for the improvement of PEFC system using HPA as the anode redox mediator.[1] R. Singh, A. A. Shah, A. Potter, B. Clarkson, A. Creeth, C. Downs, F. C. Walsh, J. Power Sources, 201 (2012) 159–163.[2] S. Nakada, E. Morikawa, T. Okanishi, H. Muroyama, T. Matsui, Y. Hirao, K. Eguchi, of 82th Meeting of Electrochem. Soc. Jpn., 3B09 (2015).