Polymer electrolyte membrane (PEM) fuel cells show voltage degradation when subjected to long-term drive cycle operation. While the kinetic losses in the fuel cells can be assigned to catalyst layer degradation, there is a significant mass transport loss of up to 100mV at 2 A/cm2 in 400 hours of operation. This mass transport loss can be attributed to the Gas Diffusion Layer (GDL) degradation especially when using low surface area carbon based catalyst materials. Ex situ ageing studies where the GDL is oxidized in a peroxide solution seem to simulate this degradation mechanism in terms of both the increase in graphitic carbon and surface-oxide species. The ex situ aged GDLs result in similar voltage losses when operated with fresh membrane-electrode assemblies (MEAs). AC impedance results confirm that these losses are due to mass transport limitations, probably arising from loss of hydrophobicity of the GDL material. To alleviate these mass transport losses, various combinations of materials have been examined; improved performance under drive cycle operation was obtained with a GDL material that had carbon nano-tubes in the MPL.

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The feasibility of using fiber-optic Raman probes to identify and quantify gases in enclosures is investigated by measuring and comparing detection thresholds using several probe and enclosure designs. Unfiltered, non-imaging, fiber-optic probes are shown to achieve lower detection thresholds than a filtered, imaging, fiberoptic probe, provided that light scattering within the sample enclosure is minimized and provided that a window is not used between the probe and the analyte gas. Achievable thresholds for hydrogen, oxygen, nitrogen, carbon monoxide, and methane in gas mixtures are demonstrated to be below 1 kPa with ten seconds signal acquisition and 0.1 kPa with twenty minutes signal acquisition with the use of 0.4 W of 532-nm excitation. Ambient carbon dioxide in air (.03 kPa) is shown to be detectable in a twenty minute acquisition, and ambient water vapor is well above the detection threshold. Background signals generated within the optical fibers remain the principal factors limiting detection thresholds. Factors affecting the magnitudes of these signals reaching the detector are investigated and discussed. A flat piece of light-absorbing colored glass tilted to direct reflected light away from the fiber-optic probe performs well as a beam stop to reduce background signal in a simple, cylindrical sample enclosure.

Numerous interlinked processes are responsible for limited durability of polymer electrolyte membrane fuel cells. One of the major phenomena that contribute to accelerated degradation of PEMFCs is loss of hydrophobicity of diffusion media caused by oxidation of carbon in the gas diffusion layer.1 The process is accompanied by formation of oxygen containing surface functional groups. While the vast majority of these groups produce characteristic signals in XPS, DRIFTS, FTIR ATR, etc., frequently an unfavorable signal to noise ratio in such measurements prevents accurate determination of the surface concentration of those functionalities. In this paper, we demonstrate successful application of Boehm type acid base titrations to determine concentrations of acidic functionalities in gas diffusion layer samples that underwent in situ and ex situ oxidation under different conditions and demonstrate correlations between those conditions and carbon oxidation.

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