PEMFC Reactant Mass Transfer Coefficient Measurement and Separation – Method Extension to the Mixed Kinetic and Mass Transfer Control Regime

Abstract

Additional cost reductions are needed for proton exchange membrane fuel cells (PEMFC) to effectively compete with internal combustion engine powered vehicles. Recent trends have focused on the use of a high current density and a lower Pt catalyst loading to reduce the quantity of expensive materials. However, such strategies have led to higher mass transfer overpotentials (1). These performance losses are advantageously characterized by the mass transfer coefficient. Its separation into fundamental contributions will focus research efforts to improve cell performance. A separation method previously proposed (2) was extended, which provides information for a wider cell voltage range of relevance to duty cycling operation. The extended model yields a closed form and implicit current distribution expression (Eq. 1) where X represents the dimensionless Cartesian coordinate along the flow field channel, ie the inlet oxygen flow rate equivalent current density, ik the kinetic current density, iL ,0 the inlet limiting current density and i the current density. Experimental data obtained with a segmented single cell of 100 cm2 active area (Gore catalyst coated membrane, 18 μm thick membrane, 0.2 and 0.1 mg Pt cm- 2 catalyst loading, segmented and unsegmented Sigracet 25BC gas diffusion layer for the cathode and anode, respectively) validated Eq. 1 (Fig. 1a) and led to overall oxygen mass transfer coefficients k (derived from iL ,0 and operating conditions). A plot of 1/k versus M, the diluent molecular weight, yields a linear relation (Eq. 2) enabling the separation of the overall mass transfer coefficient into 2 contributions where ke +K represents the oxygen mass transfer coefficient in the electrolyte and the gas phase (Knudsen diffusion), km the oxygen mass transfer coefficient in the gas phase (molecular diffusion) for the specified diluent (subscript He for helium, N for N2, CF for C3F8) and b a parameter. The molecular diffusion mass transfer coefficient km is larger with a lighter diluent (Fig. 1b). Each mass transfer coefficient increases as the cell voltage is decreased. However, the increase in the mass transfer coefficient in the electrolyte and the gas phase (Knudsen diffusion) ke +K is more substantial than for the other contribution. As a result, the k/ke +K ratio decreases for lower cell voltages. Specifically for N2, the mass transfer rate limiting step is associated with the electrolyte and the gas phase (Knudsen diffusion), which suggests a focus on the micro-porous and catalyst layers to increase performance. Two reasons are proposed to explain the cell voltage dependency observed in Fig. 1b. First, the local change in gas phase composition promotes convection. This effect is particularly important before reaching the mass transfer control regime, in the mixed kinetic and mass transfer control region, as the interfacial oxygen concentration changes from its bulk flow field channel value to near 0 in the 0.75 to 0.6 V cell potential range creating a diluent enriched layer. Second, the local temperature in the gas diffusion electrode (through plane direction) is also higher at a lower cell voltage owing to additional heat production. The larger local temperatures in turn promote larger diffusion coefficients in the gas phase (molecular and Knudsen) and permeabilities in the ionomer phase. A comparison with other mass transfer coefficient values derived from impedance data (3) and the model limits of validity will also be discussed. Acknowledgments We gratefully acknowledge funding from the Army Research Office (W911NF-15-1-0188). The authors are grateful to the Hawaiian Electric Company for their ongoing support of the operations of the Hawaii Sustainable Energy Research Facility.