Chemical state changes of Nafion in model polymer electrolyte fuel cell under oxygen/hydrogen gas atmosphere observed by S-K XANES spectroscopy

Changes in the chemical states of the sulfonic groups of Nafion in a model polymer electrolyte fuel cell under an oxygen/hydrogen gas atmosphere were studied using sulfur K-edge XANES spectroscopy. First, the chemical state changes in the sulfonic acid groups of both cathode and anode electrodes due to humidity under oxygen/hydrogen gas flow were observed. Reversible spectral changes ascribed to the hydration and dehydration of the sulfonic acid group were observed at both electrodes. This result is similar to the experimental results obtained without introducing oxygen (helium/hydrogen). On the anode, some of the sulfonic acid groups were decomposed to atomic sulfur adsorbed on platinum (S_ad) and the amount increased with time. On the cathode, the formation of S_ad was suppressed under the oxygen atmosphere. Next, the effects of oxygen gas introduction onto S_ad were examined. S_ad was at once formed on both electrodes under dry conditions without an oxygen supply. By supplying oxygen gas, S_ad on the cathode disappears. Therefore, the catalyst of the cathode has the ability to recover against the poisoning S_ad, while that on the anode accumulates.