(Invited) Performance and Durability of Automotive Fuel Cell Stacks and Systems with Low-Loaded d-PtCo/C Cathode Catalyst in Membrane Electrode Assemblies

Abstract

Previous studies have shown that meeting the 30 $/kWe cost and 8,000-h durability targets for automotive fuel cell systems (FCS) requires high-activity oxygen reduction reaction (ORR) cathode catalysts that are stable under cyclic potentials at low Pt loadings (<0.1 mg-Pt/cm2). We have evaluated the performance and durability of a state-of-the-art (SOA) fuel cell stack and system under automotive relevant conditions using data obtained for a membrane electrode assembly (MEA) operated under differential cell conditions. The MEA has de-alloyed Pt3Co catalyst supported on a high surface area carbon (HSAC) in cathode, Pt catalyst supported on Vulcan carbon in anode, thin 12-μm reinforced PFSA membrane, and 200-μm thick diffusion media with microporous layers. The Pt loadings are 0.1 mg/cm2 in the cathode catalyst and 0.025 mg/cm2 in the anode catalyst. We formulated a model for ORR kinetics on d-PtCo/C catalyst by using the measured polarization data at low current densities together with the ionic conductivity of the cathode catalyst layer derived from the Galvanostatic impedance data obtained in H2/N2, and a transient solid solution model for oxide coverage as a function of potential, relative humidity and temperature. The kinetic model indicates that d-PtCo/C catalyst has 650 A/gPt mass activity, which is double the mass activity of annealed Pt catalyst that has nearly the same particle size, and easily exceeds the target of 440 A/gPt. In accelerated stress tests (AST) with 0.6-0.95 V square wave potentials in cyclic voltammetry, there are negligible changes in kinetic parameters denoting the reaction order for O2 partial pressure, activation energy and relative humidity (RH) dependence. As indicated in Fig. 1, the specific activity (µA/cmPt 2) degraded by ~33% in AST during which time the electrochemically active surface area (ECSA) decreases by 7.5% and nearly stayed constant with further decrease in ECSA. We concluded that the mass activity, which is a product of specific activity and ECSA, degrades in AST initially because of decrease in specific activity due to cobalt dissolution and subsequently because of decrease in ECSA due to coarsening of catalyst particles. We determined the oxygen transport resistance of the d-PtCo/C electrode (Rcf) by (a) using the measured polarization data at high current densities; (b) estimating mass transfer overpotentials (ηm) from the ORR kinetic model; (c) determining the limiting current densities (iL) at which ηm equals 400 mV; (d) estimating the oxygen transport resistance (ηm) from iL; and (e) extracting the pressure-dependent and pressure-independent (Rcf) parts of Rm. We concluded that after 30,000 AST cycles, the pressure-dependent part of Rm representing O2 transport resistance in gas channel and gas diffusion layer (GDL) showed small changes; however Rcf has increased by >50% at 40% RH exposure and >125% at 100% RH exposure. Lower roughness (SPt, cmPt 2/cm2) because of catalyst coarsening accounts for 8% of the increase in Rcf at 40% exposure and for 45% increase at 100% exposure. Also, 48-60% of the increase in Rcf may be associated with the reduction in O2 permeability through the ionomer film on the catalyst particles supported on HSAC. We developed an integral cell model using the differential cell data to evaluate the performance and cost of an automotive FCS with the d-PtCo/C catalyst relative to the targets of 65% peak efficiency, 8.0 kWe/g stack Pt utilization, 1000 mW/cm2 stack power density and 40 $/kW cost, all subject to 1.45 kW/K (Q/ΔT) heat rejection constraint. The analysis indicates that the SOA catalyst can achieve 9.5 ± 0.5 kWe/g stack Pt utilization and 1180 ± 55 mW/cm2 gross stack power density at 656 mV cell voltage, 2.5 atm stack inlet pressure, 95oC stack coolant outlet temperature, and 1.5 cathode stoichiometry. The projected system cost is 46.0 ± 0.7 $/kWe at high manufacturing volume (500,000 units/year). At lower manufacturing volumes, the estimated costs are 51 $/kWe at 100,000 units/year and 88 $/kWe at 10,000 units/year. We projected FCS performance degradation in terms of ECSA loss by conducting simulations at constant air flow rate (variable cathode stoichiometry) and heat rejection (Q). The simulations indicate that meeting the target of 10% derating in FCS power over lifetime requires controlling the stack operating conditions to limit the ECSA loss to 35%. At end of life, we estimate that mass activity has degraded by ~50%, specific activity by 25%, and the cell voltage at rated power by 30 mV. Increase in kinetic overpotential due to Co dissolution and coarsening of catalyst particles accounts for the majority of the projected cell voltage loss at rated power. Figure 1