Comprehensive Studies of Spatial PEMFC Performance Under CH3 Br Poisoning of Cathode

Abstract

Air is the most practical and economic oxidant for fuel cell operation, so any impurities in the air will be a significant concern for performance and durability of proton exchange membrane fuel cells (PEMFCs). Exposure of the cathode to airborne contaminants was found to cause serious performance loss and degradation in many cases (1). Seven compounds were chosen for a detailed investigation from the inventory of 260 possible air pollutants suggested by Environmental Protection Agency (2, 3). CH3Br was selected for further studies as a potential air pollutant due to its natural and anthropogenic origin. The application of CH3Br as an agricultural pesticide has been reduced from 2005 since it is an ozone-depleting compound, however, CH3Br has a major natural emission source from oceans (4). The work focuses on comprehensive analysis of localized long-term PEMFC performance exposed to 5 ppm CH3Br for improving adaptability and durability and understanding the poisoning mechanism. A segmented cell and data acquisition system were used (5) with a commercially available 100 cm2 membrane/electrode assembly (MEA). Each electrode contained a Pt/C catalyst with a loading of 0.4 mgPt cm-2. A segmented SGL 25BC gas diffusion layer (GDL, 10 segments of 7.6 cm2) and a Teflon gasket were employed at the cathode whereas a single GDL piece was applied at the anode. The MEA was operated under galvanostatic control of the whole cell current. Other operating conditions were: 80°C, 48.3 kPagback pressure, 100/50% relative humidity and 2/2 stoichiometry for the anode and cathode respectively. The dry contaminant was injected into the humidified cathode air stream. The poisoning proceeded until the cell voltage reached a steady value. MEAs were analyzed by SEM, TEM, XPS and electrochemical methods. Fig. 1 a) shows the voltage response and normalized current density for each segment and 1.0 A cm-2. For the first 18 hours, the cell was operated with air resulting in a cell voltage of 0.650 V. The injection of 5 ppm CH3Br decreased the voltage over a long transition period (~50 h) and eventually resulted in a steady stateof 0.335 V. The voltage decrease was accompanied by a redistribution of local current densities. Operation with pure air for 70 h after the poisoning phase did not recover the original cell voltage. XPS analysis of the MEA exposed to CH3Br for 147 h showed the presence of Br- that suggests bromomethane hydrolysis yielding CH3OH, Br- and H+ (6). CH3OH appears to be oxidized at the conditions of the cathode, whereas Pt can adsorb Br-. Chemisorption of Br- results in a decrease of the electrochemical area (ECA), suppression of O2 adsorption and shift of the oxygen reduction from a 4-electron to a 2-electron mechanism with H2O2intermediate formation, which negatively impact PEMFC performance (7). Cyclic voltammetry (CV) measurements demonstrated that the ECA of both anode and cathode after CH3Br exposure for 147 h decreased by 50%. The cathode ECA loss was only 30% for an MEA aged under the same conditions without CH3Br. The observed ECA drop is explained by Pt particles size growth to 6-8 nm under contamination (Fig. 1 b) and 4-5 nm without CH3Br (Fig. 1 c) compared to the initial particle size of 2-2.5 nm. The absence of a self-recovery indicates that Br- can be strongly adsorbed on Pt which reduces performance. However, polarization curves measured after CV scans revealed a cell recovery and performance loss was only 25-50 mV mainly due to increased activation overpotential. Thus, CV accompanied by operation with fully humidified gases could remove Br-. A detailed discussion of results, CH3Br poisoning mechanism and possible mitigation procedures will be presented. ACKNOWLEDGMENTS We gratefully acknowledge ONR (N00014-13-1-0463), DOE EERE (DE-EE0000467) and Hawaiian Electric Company. REFERENCES O.A. Baturina, Y. Garsany, B.D. Gould, K.E. Swider-Lyons, in: H. Wang, H. Li, X.-Z. Yuan (Eds.), PEM fuel cell failure mode analysis, CRC Press, 2011, p. 199.T.V. Reshetenko, J. St-Pierre, J. Power Sources, 293, 929 (2015).T.V. Reshetenko, J. St-Pierre, J. Power Sources, 287, 401 (2015).S.A. Yvon-Lewis, E.S. Saltzman, S.A. Montzka, Atmos. Chem. Phys., 9, 5963 (2009).T.V. Reshetenko, G. Bender, K. Bethune, R. Rocheleau, Electrochim. Acta, 56, 8700 (2011).W. Mabey, T. Mill, J. Phys. Chem. Ref. Data, 7, 383 (1978).N.M. Marković, H.A. Gasteiger, B.N. Grgur, P.N. Ross, J. Electroanal. Chem., 467, 157 (1999). Figure 1