Analysis of Mass Transport Phenomena in PEMFC Cathode Electrode: Effects of Operating Conditions

Abstract

Electrochemical conversion systems that use air as the oxidant, like proton exchange membrane fuel cells (PEMFCs), direct liquid fuel cells and metal/air batteries, strongly depend on mass transport of reagents for power production. To meet the power density, reliability and cost requirements that promote further commercialization of fuel cell vehicles, a sufficient understanding of properties-performance correlations is strictly desired. In particular, the dependence of fuel cell performance and mass transport parameters on operating conditions and structural properties of the membrane electrode assembly (MEA) should be better understood. Previously we developed a method for determining the oxygen mass transfer coefficient which is based on measurements of limiting current distributions using O2 mixtures with different diluents (from He to C3H8) [1]. The method allows us to separate gas phase molecular diffusion (km, N2 ) and a combination of Knudsen diffusion and ionomer/water films (kK+film ). In this work we studied effects of operating conditions and gas diffusion layer (GDL) properties on oxygen mass transport and PEMFC performance to gain a better understanding of the transport phenomena in fuel cell cathodes and to further develop the aforementioned method. A segmented cell and data acquisition system were operated with commercially available 100 cm2 MEAs with anode and cathode loadings of 0.1 and 0.2 mgPt cm-2 and thicknesses of 3-4 and 6-7 μm, respectively. Cathode GDLs were also varied to elucidate the impacts of the microporous layer (MPL) and catalyst layer. 25BA (no MPL) and 25BC (with MPL) diffusion medias from Sigracet SGL were used for the cathode electrode. Thus, operation of MEA-25BA provided us with results on mass transport predominantly in the catalyst layer, since the GDL does not include MPL. The experiments were performed at two operating temperatures: 60 and 80°C, varied relative gas humidities and back pressures. Details of experimental procedures and the determination of the mass transfer coefficient are presented in [1]. Fig. 1 presents results illustrating the impact of reagent humidification on the mass transfer coefficients and PEMFC performance. The oxygen mass transfer coefficients (kK+film and km, N2 ) determined for MEA-25BA are higher than for MEA-25BC at all studied humidification values and temperatures, most likely due to the effect of MPL. kK+film increases with RH and temperature, however, temperature does not have a significant influence. The oxygen mass transfer coefficient in the gas phase (N2) was found to grow when RH increased from 32 to 50% at 60°C, but further increases in gas humidification did not affect its value (Fig. 1 a). At the same time, operation at 80°C demonstrated an increase in km, N2 with humidification (Fig. 1 b). Behavior of mass transfer coefficient arising from the MPL (kK, MPL ) showed a similar development at both temperatures: a decrease in oxygen transport with RH. The observed trends in mass transfer coefficients with RH and temperature can be attributed to the gas permeability of Nafion and its dependence on humidification, which is more critical at low RH operation [2]. Effects of high gas humidification are associated with liquid water formation and its removal through the evaporation mechanism as well as though liquid-phase motion due to capillary effects of the MPL [3]. Apparently, the effects of water seem to be more pronounced at 60°C, where mass transfer coefficients in the gas phase and in the MPL decrease with RH. Higher operating temperature appears to be beneficial for oxygen transport in GDE and results in greater values of the mass transfer coefficients in the gas phase. At the same time variation of back pressure allowed us to determine the pressure-independent component of oxygen transport and further separate contributions to the oxygen transport in the cathode electrode structure [4]. Detailed analysis of the PEMFC performance, voltage losses and oxygen mass transfer coefficients at varied operating conditions will be presented and discussed. ACKNOWLEDGEMENTS We gratefully acknowledge funding from ONR (N00014-18-1-2127) and ARO (W911NF-15-1-0188). The authors are thankful to Hawaiian Electric Company, G. Randolf and J. Huizingh for support of the operation as well as T. Carvalho for assisting with SEM. REFERENCES T. V. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161, F1089-F1100 (2014).K. Broka, P. Ekdunge, J. Appl. Electrochem., 27, 117-123 (1997).J.H. Nam, M. Kaviany, Int. J. Heat Mass Transfer, 46, 4595-4611 (2003).D.R. Baker, D.A. Caulk, K.C. Neyerlin, M.W. Murphy, J. Electrochem. Soc., 156, B991-B1003 (2009). Figure 1