Influence of Aromatic Hydrocarbons in Air on Spatial PEMFC Performance

Abstract

The successful commercial deployment of proton exchange membrane fuel cells (PEMFCs) depends on the achievement of stringent performance and durability requirements. Currently, air is the most convenient oxidant for fuel cell applications, and its quality is an important consideration for operation because airborne contaminants can negatively affect fuel cell performance, cause premature degradation and decrease durability (1). Aromatic compounds are hazardous pollutants produced or used in many industrial processes. Benzene and naphthalene are the main representatives of aromatic hydrocarbons and are widely used as precursors for chemical syntheses. More than half of the entire benzene production is processed to styrene to manufacture polymers and plastics. Benzene is originated in the air from emissions of chemical plants, burning coal and oil, gasoline stations and vehicle exhausts. Naphthalene is used in the production of phthalic anhydride and as a pest control agent. Determination of the impact of C6H6 and C10H8 on PEMFC performance is critical to establish environmental requirements for fuel cell usage, define specifications for air filtration systems and support understanding of fundamental aspects of PEMFC operation and maintenance. A segmented cell and data acquisition system were used (2) 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. Subsequently, the contaminant injection was stopped to evaluate the cell self-recovery. Fig. 1 a) shows the voltage response and normalized current density for each segment at 1.0 A cm-2 under benzene contamination. For the first 18 hours, the cell was operated with pure air resulting in a cell voltage of 0.670 V. The injection of 2 ppm C6H6 decreased the voltage to a steady state of 0.560 V and caused a redistribution of local current densities. Operation with pure air fully recovered the initial cell performance. Effects of naphthalene are shown in Fig. 1 b). The introduction of 2.3 ppm C10H8to air stream led to a significant voltage drop within 10 h and a different current redistribution pattern. Voltage oscillations from 0.100 to 0.170 V were observed as soon as the cell reached 0.12 V. Recovery took 2 h and was accompanied by further redistribution of localized currents. Benzene and naphthalene have similar electrochemical properties (3, 4). Cathodic desorption of the adsorbed species on Pt occurs at hydrogen adsorption potentials (< 0.1 V) and is accompanied by partial hydrogenation, while electrooxidation takes place at 1.35 V with formation of CO2 as the main product. The strong adsorption of C6H6 and C10H8 occurs at 0.1-0.6 V without electrochemical reactions, and results in an in-plane adsorbate configuration due to the interaction between the aromatic ring and the Pt surface (5). Contaminant adsorption results in a decrease of the electrochemical area, suppression of O2 adsorption and shift the oxygen reduction from a 4-electron to a 2-electron mechanism, which negatively impact PEMFC performance. The data demonstrated that C10H8 has a severer effect on PEMFC than C6H6 which is most likely due to a higher adsorption energy and abilty to form multilayer adsorption (6, 7). A detailed discussion of the results and a poisoning mechanism 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, G. Bender, K. Bethune, R. Rocheleau, Electrochim. Acta, 56, 8700 (2011).F. Montilla, F. Huerta, E. Morallon, J.L. Vazquez, Electrochim. Acta, 45, 4271 (2000).T. Löffler, E. Drbalkova, P. Janderka, P. Königshoven, H. Baltruschat, J. Electroanal. Chem., 550-551, 81 (2003).M.P. Soriaga, A.T. Hubbard, J. Am. Chem. Soc., 104, 2735 (1982).C. Morin, D. Simon, P. Sautet, J. Phys. Chem. B, 108, 12084 (2004).J.M. Gottfried, E.K. Vestergaard, P. Bera, C.T. Campbell, J. Phys. Chem. B, 110, 17539 (2006). Figure 1