Insights into the Complex Oxidation Behaviour of Aqueous H3PO3 through in Situ P K- Edge XANES
Authors: Wibowo Enggar , Garcia-Diez Raul , Bystron Tomas , van der Merwe Marianne , Prokop Martin , Efimenko Anna , Arce Mauricio , Steigert Alexander , Wilks Regan George , Bouzek Karel , Bär Marcus 
Published: February 2024

High-temperature electrolyte membrane fuel cells (HT-PEMFCs) using H3PO4-doped membranes are an enticing choice for electrochemical energy sources at elevated temperatures [1]. Yet, one of the challenges concerning these systems is the reduction of H3PO4 to lower valency phosphorus compounds, such as H3PO3, during operation [2,3,4]. H3PO3 can strongly adsorb on the Pt catalysts, hence poisoning it and thereby lowering the fuel cell performance [5,6]. Interestingly, the presence of Pt may, under certain conditions, catalyze the chemical (re-)oxidation of aqueous H3PO3 back to H3PO4, illustrating the complexity of the H3POx – Pt interaction. Therefore, to optimize the performance of HT-PEMFCs by rational design of the catalyst/electrolyte interface, further insight into H3PO3 oxidation behaviour and Pt/H3PO3 interaction under conditions relevant to HT-PEMFCs operation is required.
In this study, in situ P K- edge XANES (x-ray absorption near edge structure) spectroscopy was conducted to shed light on the oxidation behaviour of aqueous H3PO3 by investigating the impact of (i) different temperatures (25°C vs 75°C), (ii) varying electrode roughness (flat planar Pt vs rough Pt black), (iii) different electrode potentials (open circuit potentials vs more positive potentials [+0.8, +0.9, and +1.0 VRHE]), and (iv) varying molar concentrations of H3PO3 (0.1 , 1 , and 5 mol dm-3). Initially, XANES coupled with electrochemical characterization (e.g. OCP, cyclic voltammetry) of the system was performed under different radiation doses (i.e.: photon fluxes, exposure time) to determine the effect of radiolysis and/or radiation damage during experiments that may lead to misinterpretations of XANES results. Under high radiation dose, we find evidence of H2 presence in the vicinity of the Pt electrode, likely generated by water radiolysis. We have developed experimental procedures to suppress these undesirable effects during the collection of the XANES data, enabling an accurate determination of H3PO3 oxidation behaviour and minimizing the radiolysis contribution to the process. It was found that higher temperature facilitates the oxidation of aqueous H3PO3 to H3PO4, presumably because H3PO3 exists in the thermodynamically preferred “active” pyramidal form at elevated temperatures, which is more prone to react with H2O, forming H3PO4 and H2. In aqueous H3PO3 solutions with higher H2O contents, also more pronounced oxidation of H3PO3 is observed, indicating that oxidation of H3PO3 to H3PO4 in aqueous solutions proceeds via the presence of H2O. The experiments using rough Pt black electrodes additionally hint at the observation of electrochemical oxidation of H3PO3 to H3PO4 during the application of positive potentials. This work provides insights into the underlying chemical processes that occur at conditions relevant for HT-PEMFCs operation and thus it paves the way for possible strategies to mitigate H3PO3 poisoning of the Pt electrode during operation.

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