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 H 3 PO 3 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 H 3 PO 3 via H 2 O yielding H 3 PO 4 and H 2 . A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation-induced effects, enabling a more accurate determination of the aqueous H 3 PO 3 oxidation mechanism. In situ XANES studies of aqueous 5 mol dm -3 H 3 PO 3 on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H 3 PO 3 to H 3 PO 4 . 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 H 2 O, as further supported by XANES measurements of aqueous H 3 PO 3 of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H 2 O content. The significant role of water in the oxidation of H 3 PO 3 to H 3 PO 4 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 H 3 PO 3 during HT-PEMFC operation.