Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers: Mass Transport Resistance and Liquid Water Accumulation at Limiting Current Density with in operando Synchrotron X-ray Radiography

Abstract

The polymer electrolyte membrane fuel cell (PEMFC) continues to develop as a viable alternative to the combustion engine in automotive applications. As this technology advances, it is critical that the PEMFC is capable of maintaining performance for long term operation. In an effort to understand the degradation of fuel cell performance over time, a study into the development of an accelerated gas diffusion layer degradation protocol was completed. As part of an effort to quantify the effects of this accelerated degradation, in situ synchrotron radiography imaging techniques were coupled with performance testing in order to quantify the effects of this degradation on water saturation profiles, transport resistance, and limiting current in an operating fuel cell. Gas diffusion layer samples of SGL 25 BC and SGL 29 BC were artificially aged in a concentrated solution of hydrogen peroxide (30% wt.) for a period of 12 hours at an elevated temperature of 90 degrees Celsius. Hydrogen peroxide facilitates chemical corrosion of the carbon material in the gas diffusion layer and was found to significantly affect the wettability of the degraded samples. Hydrogen peroxide was selected to facilitate the degradation mechanism due to the fact that it is a recognized chemical species found in operating fuel cells and produced at the catalyst layer (1, 2). The accelerated degradation procedure that was used in this study was found to primarily affect the wettability of the tested gas diffusion layers. Consequently, it was expected that the most significant impact of this degradation mechanism would be in the mass transport losses at high current densities. For this reason, a limiting current investigation was performed. Unlike other studies of limiting current in which the cathode oxygen concentration is varied (3), a limiting current study based on varying the relative humidity of reactant gases was performed. In this study, limiting current was measured for relative humidities of 0%, 50%, 80%, 90%, and 100% for several fuel cells with fresh and degraded GDLs. Simultaneous synchrotron imaging was used to quantify the distribution of liquid water in the anode and cathode of the operating cell during these limiting current studies. Trends with respect to reactant transport resistance, water saturation profiles, and limiting current were quantified as a function of relative humidity and degree of degradation. For all tested samples, as the relative humidity was reduced, the corresponding limiting current increased. It was also found that the limiting current liquid water saturation profile was independent of the reactant gas relative humidity. Additionally, artificial degradation led to increased liquid water saturation levels in the operating fuel cell. This work illustrates the potential impacts of long term fuel cell operation on the water management of PEMFC gas diffusion layers.