Metal Phosphide-Based Novel Anodes for Intermediate Temperature Fuel Cells

Abstract

Metal phosphides are world-widely investigated as highly-active and selective catalysts for hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation. These reactions require catalytic ability for dissociation of H-H bond as well as C-S, C-N, or C-O bond. In addition to the catalytic activity, metal phosphides are known to be thermally and chemically stable and to possess electric conductivity comparable to metals and hardness similar to ceramics [1]. Because of these features, metal phosphides are expected to function as electrodes in electrochemical cells. Recently, metal phosphide based electrodes have been investigated as an electrode for hydrogen evolution reaction in H2O electrolysis. McEnaney et al. carried out cyclic voltammetry of phosphide anodes in sulfuric acid and obtained stable current-voltage curves [2]. This indicates that metal phosphides have potentials to be used as anodes of fuel cells employing acidic electrolytes. So far, application of metal phosphides to a fuel cell anode is quite limited. Chang et al. developed anodes of Pd supported on Ni2P/C for direct formic acid fuel cells [3].The Pd anode with nickel phosphide exhibited improved power generation characteristics when they were compared with the cell without the nickel phosphide. The role of the nickel phosphide seems, however, still unclear in the anode, since the power generation temperature was ca. 30ºC, which is too low for the phosphide-based catalysts to exert catalytic activity.In this study, metal phosphides such as Ni2P, CoP, FeP, WP, and MoP have been investigated as anode catalysts for intermediate temperature fuel cells. Composites composed of CsH2PO4 and SiP2O7, the metal phosphates, and a commercial Pt/C electrode were used as the electrolyte, anode, and cathode, respectively, and power generation characteristics were evaluated at 220ºC as H2-O2 fuel cells. Cesium dihydrogen phosphate, CsH2PO4, was prepared by dissolving stoichiometric quantities of Cs2CO3 and H3PO4 in distilled water and drying overnight at 120 °C. SiP2O7 was prepared from mesoporous SiO2 and H3PO4 as reported previously [4]. As a cathode catalyst Pt/C-loaded carbon paper (Pt loading 1 mg cm-2, ElectroChem Inc.) was used. As an anode catalyst, the phosphide-loaded carbon paper was prepared by filtration of the phosphide dispersed in an ethanol solution. The membrane electrode assembly (MEA) was prepared by uniaxial pressing at 250 MPa for 10 min to form the MEA. In power generation experiments, humidified H2 and O2 were supplied to anode and cathode, respectively, at 50 cm3 min-1. A water vapor concentration of 30 vol% was obtained by bubbling the gas flow through water at 70 °C. Current-voltage characteristics were measured at 220 °C using a potentiostat (Solartron 1287), and AC impedance measurements were carried out at open circuit condition with a frequency-response analyzer (Solartron 1260).Figure 1 summarizes current-voltage characteristics of H2-O2 fuel cells with phosphide anodes at 220°C. The phosphide electrocatalysts were found to function as anodes of intermediate temperature fuel cells. The power generation characteristics are in the following order: MoP > WP > FeP > CoP > Ni2P. Impedance analysis showed that ohmic resistance of the cells with the phosphides except FeP was small and comparable to that of the Pt/C anode. This result indicates that these phosphide anodes possess good conductivity applicable to fuel cell anodes. On the other hand, the non-ohmic resistance of FeP, CoP, and Ni2P was considerably large, which corresponds to the order of the power generation performance. [1] S.T. Oyama, T. Gott, H. Zhao, Y.-K. Lee, Catal. Today 143, 94-107 (2009).[2] J.M. McEnaney, J.C. Crompton, J.F. Callejas, E.J. Popczun, A.J. Biacchi, N.S. Lewis, R.E. Schaak, Chem. Mater. 26, 4826-4831(2014).[3] J. Chang, L. Feng, C.P. Liu, W. Xing, X. Huet, Angew. Chem. Int. Ed. 53, 122-126 (2014).[4] T. Matsui, T. Kukino, R. Kikuchi, K. Eguchi, Electrochem. Solid-State Lett. 8, A256-A258 (2005). Figure 1