The accurate evaluation of the activation overpotential of a membrane electrode assembly (MEA) is essential for fuel cell design. We have developed a new method of precisely determining the activity of cathode catalyst layer in MEA by maintaining the effect of platinum oxides at a constant level and have subsequently studied activity changes resulting from various operational histories. Cyclic voltammetry was used to quantitatively correlate activity changes in the cathode catalyst layer with various operational conditions and demonstrated that activity decreased after low humidity operation and recovered after high humidity operation in a N2 atmosphere at potentials below 0.2 V. The activity changes were found to be very slow and a span of more than 8 hours was required for the activity to reach the steady state. This phenomenon is presumably caused by the adsorption/desorption of ionomer sulfonate groups on the Pt surface, based on observations that the activity changes are consistent with sulfonate group adsorption and dependent on the ionomer structure. Oxygen transport resistance in the catalyst layer also varied in conjunction with the activity changes. Two possible Pt/ionomer interface models are proposed in an attempt to explain the above observations.

No abstract

Changes in the chemical states of sulfonic groups of Nafion in polymer electrolyte fuel cells (PEFCs) under gas-flowing conditions were studied using in situ S-K XANES spectroscopy. The applied potential to the electrodes and the humidity of the cell were changed under flowing H gas in the anode and He gas in the cathode. While the potential shows no significant effect on the S-K XANES spectra, the humidity is found to induce reversible changes in the spectra. Comparison of the spectral changes with simulations based on the density functional theory calculations indicates that the humidity influences the chemical state of the sulfonic group; under wet conditions the sulfonic group is in the form of a sulfonate ion. By drying treatment the sulfonate ion binds to hydrogen and becomes sulfonic acid. Furthermore, a small fraction of the sulfonic acid irreversibly decomposes to atomic sulfur. The peak energy of the atomic sulfur suggests that the generated atomic sulfur is adsorbed on the Pt catalyst surfaces.

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