Operando Observation of Sulfur Species Poisoning Polymer Electrolyte Fuel Cell Studied by Near Ambient Pressure Hard X-ray Photoelectron Spectroscopy

Operando S 1s hard X-ray photoelectron spectroscopic (HAXPES) measurements were performed to observe sulfur species poisoning polymer electrolyte fuel cell electrodes. Without artificial introduction of S-containing species, we observed several S species other than the sulfonic acid group of Nafion: anionic atomic S species adsorbed on the Pt nanoparticles, SO32 in the electrolyte, and S in the C support. As the voltage applied between the cathode and the anode increases, the adsorbed S species on the Pt nanoparticles at the cathode electrode are gradually diminished and completely disappear at >= 0.8 V due to oxidation reactions from adsorbed S to SO32 and possibly SO42 but is reproduced with a decrease in the voltage. With oxygen introduction to the cathode electrode, no sulfur species except the sulfonic acid group of Nafion were detected even at the lowest voltage. Schematic electric potential diagrams of the cathode, electrolyte, and anode are depicted. Upon the voltage change, the electric double layer varies on the cathode while it is kept constant (similar to 1.0 V) on the anode. It is emphasized that the electric potentials of the chemical species observed by HAXPES can distinguish the location of the specimen; the SO32 species is found not to be adsorbed on the electrode but to be solved in the electrolyte.