Carbon corrosion is an important degradation mechanism that can impair PEMFC performance through the destruction of catalyst connectivity, collapse of the electrode pore structure, loss of hydrophobic character, and an increase of the catalyst particle size. In this study, carbon corrosion was quantified in situ by measurement of carbon dioxide in the fuel cell exhaust gases through non-dispersive infrared spectroscopy during simulated drive cycle operations consisting of potential cycling with varying upper and lower potential limits. These studies were conducted for three different types of carbon supports. A reduction in the catalyst layer thickness was observed during a simulated drive cycle operation with a concomitant decrease in catalyst layer porosity, which led to performance losses due to increased mass transport limitations. The observed thickness reduction was primarily due to compaction of the catalyst layer, with the actual mass of carbon oxidation (loss) contributing only a small fraction (< 20%). The dynamics of carbon corrosion are presented along with a model that simulates the transient and dynamic corrosion rates observed in our experiments. Accelerated carbon corrosion stress tests are presented and their effects are compared to those observed for the drive cycle test.

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The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeder's paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. At open circuit and during fuel cell operation, the measured water content indicates that the membrane is operating between the vapor- and liquid-equilibrated states.

Cerium is a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability. During operation, however, cerium rapidly migrates in the PEM and into the catalyst layers (CLs). In this work, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) under different humidity conditions. Cerium migration was characterized in the MEAs after ASTs using X-ray fluorescence. During fully humidified operation, water flux from cell inlet to outlet generated in-plane cerium gradients. Conversely, cerium profiles were flat during low humidity operation, where in-plane water flux was negligible, however, migration from the PEM into the CLs was enhanced. Humidity cycling resulted in both in-plane cerium gradients due to water flux during the hydration component of the cycle, and significant migration into the CLs. Fluoride and cerium emissions into effluent cell waters were measured during ASTs and correlated, which signifies that ionomer degradation products serve as possible counter-ions for cerium emissions. Fluoride emission rates were also correlated to final PEM cerium contents, which indicates that PEM degradation and cerium migration are coupled. It is proposed that cerium migrates from the PEM due to humidification conditions and degradation, and is subsequently stabilized in the CLs by carbon catalyst supports. Widespread adoption of polymer electrolyte membrane (PEM) fuel cell technology is currently hindered by insufficient component durability and high cost.1 During operation, reactive radical species generated by electrochemical fuel cell processes attack vulnerable functional groups in the ion-conducting, or ionomer, molecules which constitute the PEM and are present in the catalyst layers (CLs).2 These attacks reduce PEM thickness and generate local pinholes, which release hydrofluoric acid (HF), sulfuric acid (H 2 SO 4 ), and fluorinated polymer fragments into effluent cell waters; increase crossover of reactant gases through the PEM; and lead to cell failure.1,2 Since the PEM is constrained by cell hardware, hygrothermal cycling generates mechanical stresses which cause physical damage to the PEM in the form of cracks, tears, and pinholes.1,2 Furthermore, during typical operation, cells experience both chemical and mechanical stresses simultaneously, which results in synergy between the degradation modes. Localized mechanical stresses increase PEM susceptibility to radical attack by reducing the activation energy necessary for such attacks to proceed 3,4 and chemical degradation of the ionomer diminishes the bulk mechanical properties of the PEM, such as ultimate tensile strength, strain-to-failure, and fracture toughness, which further increases its susceptibility to physical failure. 5-11Owing to its rapid and regenerative redox with radical species and stability in acidic media, 7 cerium dramatically improves PEM durability by neutralizing radicals before they attack the ionomer. Cerium ions may be directly exchanged with protons in the ionomer 12,13 or ...

Component durability of polymer-electrolyte membrane (PEM) fuel cells can be improved by adding cerium cations, which serve to scavenge harmful free radicals and selectively decompose hydrogen peroxide, which are formed during the oxidation reduction reaction (ORR). We have investigated the change in distribution of cerium cations in a hydrogen fuel cell as a function of operating time, considering both cerium containing membranes (commercial XL by DuPont) as well as fuel cells with CeO 2 in the cathode catalyst layer. Our results show cerium cations are very mobile in Nafion, and migrate into both the anode and cathode catalyst layers.