High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) employing a phosphoric acid H₃PO₄-doped membrane are considered to be promising sustainable electrochemical energy storage. The high-temperature operation has several advantages, such as a higher tolerance to CO poisoning, allowing coupling of HT-PEMFCs with reformers [1-3], as well the possibility for heat and electric energy co-generation [1,2]. However, during operation, phosphorus oxo-acids (e.g.: H₃PO₃) are generated on the anode. These impurities adsorb on the Pt catalyst [4-6], thus possibly negatively affecting the HT-PEMFCs performance. A detailed understanding of the H₃PO₃-catalyst (Pt) interaction is hence necessary for further HT-PEMFC optimization. However, besides an investigation of the H₃PO₃ adsorption behavior on Pt [6,7], literature on the behavior of the H₃PO₃ in contact with Pt (with/without polarization) is scarce.

In this work, the oxidation mechanism of H₃PO₃ was investigated using a combination of in situ x-ray spectroscopy techniques that directly probe the H₃PO₃/Pt interface interaction, complemented by ex situ x-ray photoelectron spectroscopy (XPS) and ion-exchange chromatography (IEC). IEC gave insights into the effect of Pt on the stability of deaerated aqueous H₃PO₃ solutions. XPS was conducted on H₃PO₃/support structures (including Au and Pt supports) to determine to what extent the support affects the H₃PO₃ oxidation. Furthermore, in-situ dip and pull near-ambient pressure (NAP-)XPS was conducted to investigate the state of H₃PO₃ at the H₃PO₃/Pt interface and in solution bulk. It was observed that at the H₃PO₃/Pt interface, H₃PO₃ was chemically oxidized to H₃PO₄, while in the bulk solution it remains stable, as shown in Figure 1. Moreover, in situ x-ray absorption spectroscopy at the P K-edge was conducted at different concentrations of H₃PO₃ in aqueous solutions (i.e., different amounts of H₂O) in contact with Pt, to determine the role of H₂O in the oxidation of H₃PO₃. A higher degree of oxidation was observed for the less concentrated H₃PO₃, implying that H₂O participates in the oxidation mechanism of H₃PO₃ to H₃PO₄.

References:

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