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The cathode CL of a polymer electrolyte membrane fuel cell (PEMFC) was exposed to high potentials, 1.0 to 1.4 V versus a reversible hydrogen electrode (RHE), that are typically encountered during start up/shut down operation. While both platinum dissolution and carbon corrosion occurred, the carbon corrosion effects were isolated and modeled. The presented model separates the carbon corrosion process into two reaction steps; (1) oxidation of the carbon surface to carbon-oxygen groups, and (2) further corrosion of the oxidized surface to carbon dioxide/monoxide. To oxidize and corrode the cathode catalyst carbon support, the CL was subjected to an accelerated stress test cycled the potential from 0.6 V RHE to an upper potential limit (UPL) ranging from 0.9 to 1.4 V RHE at varying dwell times. The reaction rate constants and specific capacitances of carbon and platinum were fitted by evaluating the double layer capacitance (Cdl) trends. Carbon surface oxidation increased the Cdl due to increased specific capacitance for carbon surfaces with carbon-oxygen groups, while the second corrosion reaction decreased the Cdl due to loss of the overall carbon surface area. The first oxidation step differed between carbon types, while both reaction rate constants were found to have a dependency on UPL, temperature, and gas relative humidity. Early polymer electrolyte membrane fuel cell (PEMFC) research used platinum black as the catalyst for both the cathode and anode electrodes. These electrodes were costly due to their very high Pt loadings (>>1.0 mg/cm 2 ). One of the ways of reducing the platinum loading was to replace platinum black with dispersed platinum nanoparticles on a carbon support. The smaller platinum particles on carbon enabled a reduced Pt loading of the cathode to less than 0.5 mg/cm 2 , while maintaining the required platinum surface area required for high fuel cell performance. Although cost was reduced, the durability of the fuel cell was negatively impacted. At potentials greater than 0.2 V RHE (reversible hydrogen electrode), the carbon support is thermodynamically unstable and able to oxidize to carbon dioxide (CO 2 ) and/or carbon monoxide (CO) [Eqs. 1-3], 1 leaving the platinum unsupported and inactive. Furthermore, due to the loss of support the platinum particles have been shown to agglomerate into larger particles, dissolve into the ionomer, or get washed out of the system.Even in the presence of platinum, the kinetics of carbon oxidation/corrosion is relatively slow; therefore, carbon is quite stable under normal PEMFC operating conditions. In practice, elevated cathode potentials of greater than 1.2 V RHE are required to oxidize carbon at reaction rates high enough to cause significant structural degradation. Normal PEMFC operation occurs between 0-1.0 V RHE ; however, upon fuel starvation or gas switching conditions (start-up and shutdown operation due to infiltration of oxygen into the normally hydrogen filled anode compartment during fuel cell off conditions) the cathode potential can exceed ...

Durability of the catalyst layer (CL) is of vital importance in the large-scale deployment of PEMFCs. It is necessary to determine parameters that represent properties of catalysts layer and other cathode components for optimization of fuel cell performance and durability. The structure, morphology and surface chemistry of the catalyst powder affects the ionomer and catalyst interaction, ionomer dispersion in the catalyst layer and, for this reason, its morphology and chemistry. These, in turn, affect the catalyst layer effective properties such as thickness, porosity, tortuosity, diffusivity, conductivity and others, directly influencing electrode performance and durability. In this study, X-ray Photoelectron Spectroscopy and SEM are used to quantify surface species and morphology of membrane electrode assemblies (MEAs) tested under different accelerated stress test (AST) conditions. Correlations between composition, structure and morphological properties of cathode components and the catalyst layer have been developed and linked to catalyst layer performance losses. The key relationships between the catalyst layer effective properties and performance and durability provide design and optimization levers for making MEAs for different operating regimes.

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