Elucidating the Complex Oxidation Behavior of Aqueous H 3 PO 3 on Pt Electrodes via In Situ Tender X-ray Absorption Near-Edge Structure Spectroscopy at the P K -Edge.

In situ tender X-ray absorption near-edge structure (XANES) spectroscopy at the P K -edge was utilized to investigate the oxidation mechanism of aqueous H3 PO3 on Pt electrodes under various conditions relevant to high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) applications. XANES and electrochemical analysis were conducted under different tender X-ray irradiation doses, revealing that intense radiation induces the oxidation of aqueous H3 PO3 via H2 O yielding H3 PO4 and H2 . A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation-induced effects, enabling a more accurate determination of the aqueous H3 PO3 oxidation mechanism. In situ XANES studies of aqueous 5 mol dm-3 H3 PO3 on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H3 PO3 to H3 PO4 . This oxidation is enhanced upon applying a positive potential to the Pt electrode or raising the electrolyte temperature, the latter being corroborated by complementary ion-exchange chromatography measurements. Notably, all of these oxidation processes involve reactions with H2 O, as further supported by XANES measurements of aqueous H3 PO3 of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H2 O content. The significant role of water in the oxidation of H3 PO3 to H3 PO4 supports the reaction mechanisms proposed for various chemical processes observed in this work and provides valuable insights into potential strategies to mitigate Pt catalyst poisoning by H3 PO3 during HT-PEMFC operation.