Hydrogen fuel cells convert chemical energy from hydrogen into electrical energy through separate reactions of hydrogen fuels and oxidizing agents (oxygen). Among hydrogen fuel cells, high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are attractive for micro-stationary electricity sources. One disadvantage of these HT-PEMFCs is that the phosphoric acid (H3PO4) proton conductor leaches out of the H3PO4-doped polybenzimidazole membrane and poisons the platinum catalyst. Recent studies show further complications during the operation of the HT-PEMFC, where some of H3PO4 might be reduced to H3PO3, which may further poison the platinum catalysts, leading to a significant loss of performance.
An earlier study by Prof. Dr Marcus Bär’s team showed that opposite processes also take place at the interface between Pt and aqueous H3PO3 and that the interactions between the platinum catalyst and the H3PO3/H3PO4 are very complex: while H3PO3 can lead to poisoning of the platinum catalyst, at the same time platinum might catalyzes the oxidation of H3PO3 back to H3PO4.
In order to investigate the oxidation behaviour of aqueous H3PO3 under conditions close to HT-PEMFCs working conditions, Bär’s team has now analysed the chemical processes using an in-housed designed heatable electrochemical cell compatible for in situ tender X-ray studies at the OÆSE end-station recently set up in the Energy Materials In-situ Laboratory Berlin (EMIL). They used intense X-ray light in the tender X-ray energy range (2 keV – 5 keV), which is provided by the EMIL beamline at the X-ray source BESSY II. In this energy range, X-ray absorption near-edge structure spectroscopy (XANES) at the P K-edge is used to monitor oxidation processes from H3PO3 to H3PO4.
“We have thus uncovered different processes for this oxidation reaction, including platinum-catalysed chemical oxidation, electrochemical oxidation under positive potential bias at the platinum electrode, and heat-promoted oxidation. These in situ spectroscopic results are also confirmed by ion-exchange chromatography and in situ electrochemical characterisations,” explains Enggar Wibowo, first author of the study and a PhD candidate in Bär’s team. “Remarkably, all of these oxidation pathways involve reactions with water, which shows that the humidity inside the fuel cell has a significant influence on these processes.”
Read more on HZB website
Image: The illustration shows four different oxidation pathways (1-4) of aqueous phosphoric acid (H3PO3), which could be elucidated by XANES at BESSY II. All these reactions depend on the humidity present.
Credit: HZB