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 ...

Catalyst coated membranes (CCMs) in polymer electrolyte fuel cells are subjected to mechanical stresses in the form of fatigue and creep that deteriorate the durability and lifetime of the cells. The present article aims to determine the effect of in-situ hygrothermal fatigue on the microstructure and mechanical properties of the CCM. The fatigue process is systematically explored by the application of two custom-developed accelerated mechanical stress test (AMST) experiments with periodic extraction of partially degraded CCMs. Cross sectional and top surface scanning electron microscope (SEM) images of the end-of-test CCMs reveal the formation of mechanically induced cracks and delamination due to cyclic tensile and compressive fatigue stress. Tensile and expansion tests are conducted at different stages of degradation to evaluate the evolution in the mechanical and hygrothermal properties of the CCM. The tensile test results indicate gradual reductions in final strain, ultimate tensile strength, and fracture toughness with increasing number of fatigue cycles. The decay in tensile properties is attributed to the microstructural damage and micro-cracks formed during the AMST. Moreover, it is shown that the hygrothermal expansion of the CCM is more sensitive to conditioning than mechanical degradation. Polymer electrolyte fuel cells (PEFCs) are a prime candidate to replace gasoline and diesel internal combustion engines for transportation applications due to their environmental benefits combined with rapid start-up, high efficiency, and high power density at relatively low operating temperature.1 The commercialization of PEFCs is dependent on the development of membrane electrode assemblies (MEA) capable of meeting the automotive industry durability targets.2 However, the current PEFC technology is facing insufficient longevity, mainly because of the deterioration of the proton exchange membrane (PEM) component.1 Hence, an essential step to accomplish the commercialization requirements for PEFCs is to enhance the membrane durability and lifetime. Among various types of membranes utilized in PEFCs, perfluorosulfonic acid (PFSA) ionomer membranes (e.g., Nafion from DuPont) are the most widely used materials due to the superior chemical stability attributed to the chemically inert C-F bonds of the polytetrafluoroethylene (PTFE) base structure. 3Chemical and mechanical degradation mechanisms are recognized as the principal root causes for lifetime limiting failures of PFSA ionomer membranes in fuel cells. Understanding of the degradation mechanisms, their interactions, and the corresponding failure modes could provide valuable insight toward decelerating the rate of the membrane degradation and thereby extend the lifetime.2 Chemical degradation is caused by the attack of radical species in the form of hydroxyl (•OH) and hydroperoxyl (•OOH) radicals generated through decomposition of hydrogen peroxide (H 2 O 2 ) by metal contaminants.2,4,5 Hydroxyl radicals also form as a by-product of the electrochemical reaction bet...

Catalyst coated perfluorosulfonic acid ionomer membranes (CCMs) were subjected to a combined chemical/mechanical accelerated stress test (AST) designed for rapid benchmarking of in situ membrane stability in polymer electrolyte fuel cells. In order to understand the evolution of the ionomer water sorption characteristics during combined chemical/mechanical degradation, CCM samples were periodically extracted from the AST and analyzed for ionomer mass fraction and water sorption properties. In spite of severe fluoride release and membrane thinning, the water uptake per unit mass of the partially degraded CCMs was found to be essentially constant. The mass fraction of ionomer in the CCM samples determined from thermogravimetric analysis (TGA) showed significant material loss throughout the AST process due to ionomer degradation and fluoride release, up to roughly 50% at end-of-life. The effects proceeding at different stages of degradation were therefore more accurately revealed by ionomer mass-normalized data. The water uptake per unit gram of ionomer was shown to increase significantly with degradation, in contrast to the previous results normalized by CCM dry mass. Although increased water sorption may indicate enlarged solvated hydrophilic domains in the membrane, which would be beneficial for enhanced proton mobility, the proton conductivity was found to decrease. This finding suggests that the additional water sorbed in the membrane was not contributing to proton conduction and was therefore likely situated in non-ionic cavities formed through degradation rather than in the ionic clusters.

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