title: Elucidating the Complex Oxidation Behaviour of Phosphorous Impurities at Pt|Aqueous H3PO3 Interface in HT-PEMFC by a Combination of X-ray Spectroscopies
Creators: name: Wibowo, Enggar
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: spectroelectrochemistry
subject: X-ray spectroscopy
subject: solid-electrode|liquid-electrolyte interface

description: 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 lower-temperature 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-and-pull" 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 K-edge 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. The nine P-containing solids references included GaP, InP, red-P, H3PO3, Na2H2P2O6, H3PO4, KH2PO4, Na2HPO4, and InPO4. Shifts in the absorption-edge positions between different compounds are found, likely correlated with the electronegativity of atoms surrounding the P atom, which causes different core–hole screening effects for different compounds. Complementing ab initio many-body calculations of XANES spectra further provided insights into the electronic structure of the observed spectral features and their impact on the electronic structure of the materials. Secondly, P K-edge XANES measurements on aqueous H3PO3, H3PO4 and their mixtures without the presence of Pt catalysts were conducted, to determine the effect of water solvation on the XANES spectral fingerprint, and to determine the possibility of speciation between H3PO3 and H3PO4 within aqueous solutions. The XANES spectra of the aqueous H3PO3 and H3PO4 solutions show analogous spectral features to their solid counterparts. However, upon close examination, the spectra of aqueous H3PO3 displayed fewer spectral features compared to its solid counterpart. All of the aqueous acids XANES spectra also display slight shifts in their absorption-edge position to lower energy values compared to the solid counterparts. Additionally, mixtures of aqueous acids display the major spectral features corresponding to the individual components, indicating the ability for speciation through fingerprinting. Lastly, to ensure the collection of meaningful data, in situ P K-edge XANES experiments coupled with simultaneous electrochemical characterization by recording of the OCP values were conducted on the Pt-electrode|aqueous H3PO3-electrolyte systems with different synchrotron irradiation doses (i.e., photon fluxes, exposure time), to determine if there are any radiation-induced effects occurring (e.g., radiolysis and/or beam damage) that may corrupt any related interpretations and conclusions from the spectra. High irradiation doses induced oxidation of aqueous H3PO3, leading to the generation of H3PO4 and H2. This was observed both spectroscopically by changes in the major spectral features associated with H3PO3 and H3PO4 and electrochemically by means of changing OCP values. To minimize these undesirable irradiation-induced effects, a broadly applicable experimental procedure was developed to suppress these undesirable radiation-induced effects, thereby enabling a more accurate determination of the aqueous H3PO3 oxidation mechanism. Finally, to unravel the complex oxidation behavior of aqueous H3PO3 at conditions relevant to HT-PEMFCs operation, in situ P K-edge XANES measurements of aqueous H3PO3 were conducted at several different experimental conditions of interest implementing this developed experimental procedure to minimize radiation-induced effects, namely: (i) electrodes with different Pt availability, thickness, and surface roughness ('Pt free' Kapton substrate, (flat) planar Pt, and (rough) Pt black; (ii) electrode potentials (OCP, and more positive potentials: +0.8 V, +0.9 V, and +1.0 V); (iii) temperatures ( 25°C and 75°C); and (iv) different electrolyte concentrations (0.1, 1, and 5 mol dm-3) thereby tuning the quantity of H2O. For in situ P K-edge XANES measurements at elevated temperatures, a custom heatable three electrode flow cell that enables precise temperature control and monitoring was developed. The experiments with different electrodes confirmed that Pt is catalyzing the chemical oxidation of H3PO3 to H3PO4. This oxidation is enhanced upon applying positive potentials to the Pt electrode or raising the electrolyte temperature; the latter being corroborated by complementary IEC measurements. Notably, all these oxidation processes involve reactions with H2O, as further supported by XANES measurements of aqueous H3PO3 of with different H2O concentrations, showing more pronounced oxidation of H3PO3 to H3PO4 in electrolytes with higher H2O content. These results highlight the significant role of H2O in the oxidation of H3PO3 to H3PO4. The findings revealed in this dissertation provide valuable insights into potential strategies to mitigate Pt catalyst poisoning by H3PO3 during the HT-PEMFCs operation.