No abstract

Beginning‐of‐life H 2 ‐fed fuel cell performance of membrane electrode assemblies (MEA) made using state‐of‐the‐art subcomponents (25–50 µm membranes, 47 wt% Pt on carbon, 0.15–0.4 mg cm −2 Pt cathode loading, ≤1100 equivalent weight (EW) ionomer/binder) was studied. Performance data were taken using small‐scale (50 cm 2 ) cells and a large‐scale (500 cm 2 , 22 cell) stack, and voltages were corrected for membrane resistance in order to isolate cathode polarization losses. Based on catalyst and MEA cyclic voltammetry and in‐situ testing with pure O 2 feed, we observed loading independent O 2 kinetic control up to 1.8 A cm −2 at 80 °C under fully humidified conditions; this indicates negligible effects of cathode layer proton transport limitations with pure O 2 . With air as the oxidant, we observed O 2 kinetic control below 0.2 A cm −2 and the onset of O 2 transport resistances at higher current densities. Results with helox (21% O 2 in helium) as the cathode feed revealed that the O 2 transport resistances were roughly evenly distributed between gas and solid phases. With incomplete reactant humidification, additional membrane resistance was observed and proton conductivity losses in the cathode layer became significant. Moreover, thin (25 µm) low‐equivalent weight (EW) MEAs outperformed thicker (50 µm) high EW MEAs even after correcting for membrane ohmic losses. This is due to faster water transport of the thinner low EW MEAs resulting in improved retention of cathode layer proton conductivity under dry conditions.

Membrane failures at catalyst layer edges in proton exchange membrane fuel cell ͑PEMFC͒ membrane electrode assemblies ͑MEAs͒ were investigated using MEAs with segmented electrodes. A mathematical model was developed to predict the potential distribution at the edge of the MEA with misaligned electrodes. Control experiments were performed using an accelerated membrane durability test protocol and significant membrane degradation was observed in the region where the cathode overlaps the anode. The model-experiment comparisons suggest that a high cathode potential contributes to the membrane failure. A dependence of membrane degradation on relative humidity ͑RH͒ was observed in the experiments, regardless of the electrode overlap. The observed membrane degradation in the overlap region of MEAs with an anode catalyst overlap, run at low RH, is not explained by the model and needs further investigation.

A tandem surface-transmission Fourier transform infrared (FTIR) method permits the simultaneous investigation of adsorbed and desorbed species formed at the electrode surface of direct methanol fuel cell (DMFC) membrane electrode assemblies (MEAs). Potential dependent, in situ specular reflectance spectra and on-line transmission FTIR spectroscopy of adsorbed and desorbed species on working fuel cell electrode surfaces, confirm that linear bound CO, the primary intermediate on unsupported Pt and PtRu surfaces of fuel cell MEAs, have lower Stark tuning rates than the corresponding on arc-melted alloys. Two peaks corresponding to CO stretching modes were observed on a PtRu fuel cell anode surface prepared with Johnson Matthey catalysts. The higher frequency peak is ascribed to a Pt-rich alloy and the low frequency peak is ascribed to either pure Ru or an Ru-rich phase. The tandem technique confirms that a peak easily misassigned as a linear bound CO stretching mode false(2083 cm−1false) is a methanol overtone peak. In addition to CO2, methylformate is a major product of a gas fed DMFC, which is detected at the anode exhaust by on-line FTIR. © 2002 The Electrochemical Society. All rights reserved.

Adsorbed CO Stark tuning rates have been studied for the first time in direct methanol fuel cells on Pt black catalysts supported only on the polymer electrolyte (Nafion) in membrane electrode assemblies. The bipolar peaks resulting from the Stark shift of CO absorbance peaks are inverted, indicating an anomalous increase in the reflectivity where CO infrared absorption occurs. The vibrational Stark tuning data suggests that CO oxidation occurs on the perimeter of COads islands, which is consistent with the formation of CO within and above Pt double layer potentials as reported by Kunimatsu. This is expected since methanol is continuously delivered to the anode at all potentials in direct methanol fuel cells.