Wibowo, Enggar Pramanto: Elucidating the Complex Oxidation Behavior of Phosphorus Impurities at the Pt|Aqueous H3PO3 Interface in HT-PEMFCs by a Combination of X-ray Spectroscopies. , Dissertation, FAU Erlangen-Nürnberg, 2024
10.25593/open-fau-915
Open Access Version
Abstract:
High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), employing polybenzimidazole (PBI) membranes doped with concentrated phosphoric acid (H3PO4), are an attractive choice for a micro stationary clean electric energy source. Due to the high operation temperature (120°-180°C), HT-PEMFCs offer distinct advantages over their lowertemperature counterparts, including the possibility of operation with reformers, enhanced resistance against CO poisoning, and potential for combined heat and electricity generation. Among the challenges associated with HT-PEMFCs, the H3PO4 electrolyte may undergo reduction during the fuel cell operation, leading to the formation of phosphorus impurities with a lower oxidation state, such as phosphorus oxo-acids (H3PO3). Recent studies indicate that the H3PO3 may strongly adsorb on the state-of-the-art Pt catalysts on both electrodes in the HT-PEMFCs, and thus, adversely influence the O2 reduction reaction (ORR). During HT-PEMFCs operation, the H3PO3 that is formed on the anode might be transported to the cathode and poison Pt catalysts on the cathode. Such catalyst poisoning will significantly limit the ORR, thereby decreasing the HT-PEMFCs performance. Therefore, comprehensive investigations of the Pt-H3PO3 interaction and its oxidation behavior under relevant HT-PEMFCs conditions (e.g., at elevated temperatures and under positive potentials similar to the cathode) are necessary for an insight-driven optimization of HT-PEMFCs. Yet, literature on the interaction of H3PO3 with Pt and the oxidation behavior of H3PO3 are currently extremely scarce. This dissertation aims to unravel the interaction between Pt catalysts and H3PO3 and to elucidate its complex oxidation behavior through the combination of in situ X-ray spectroscopies, electrochemical methods, and other complementing characterizations such as chromatography. To investigate the interaction at the Pt|aqueous H3PO3 interface, in situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) combined with the ”dip-andpull” configuration was performed at the electrode|aqueous electrolyte interface at room temperature and open circuit potential (OCP) conditions. For further insights into the Pt- H3PO3 interactions, the stability of aqueous H3PO3 with and without the presence of Pt catalysts was assessed by using ion exchange chromatography (IEC), gas chromatography (GC), and X-ray photoelectron spectroscopy (XPS). Results from these experiments show that even though theoretically aqueous H3PO3 is considered thermodynamically unstable, in the absence of O2, on its own aqueous H3PO3 is stable for at least a week. This can be attributed to the fact that on its own H3PO3 exists in a highly stable and less reactive tetrahedral tautomeric form. However, upon contact with Pt, Pt immediately catalyzes the oxidation of aqueous H3PO3 to H3PO4. Likely, H3PO3 adsorbs on the Pt surface in the highly reactive pyramidal tautomeric form, thus the H3PO3 is more susceptible to react with H2O leading to the formation of H3PO4 and H2. To unravel the complex oxidation behavior of aqueous H3PO3 at conditions relevant to HT-PEMFCs operation, in situ P K-edge X-ray absorption near edge structure spectroscopy (XANES) measurements were conducted. Several preliminary experiments are conducted to ensure a comprehensive understanding and accurate interpretation of the in situ P Kedge XANES dataset. First, the acquisition of the P K-edge XANES and complementary P L2,3-edge XANES of a well-understood set of reference phosphorus (P) containing compounds with a wide range of oxidation states and chemical environments was performed. This enabled the collection of reference XANES spectral fingerprints associated with different chemical environments. ...