Performance and Stability of Pt-Based Alloy Catalysts for High-Temperature PEM Fuel Cell Application

Ever increasing energy demands and push for low-carbon imprint technologies require at least partial replacement of internal combustion engines. For many stationary and heavy-duty applications, high-temperature fuel cells with proton-exchange membrane (HTPEMFC) represent a feasible solution for electric energy and heat co-generation. HTPEMFC operates with hydrogen as a fuel and atmospheric oxygen as an oxidant. The core of the technology is membrane (PEM), based on polymer doped with phosphoric acid, enabling operating temperature between 120 and 200 °C. Elevated temperature is necessary for heat cogeneration and enables use of hydrogen produced from fossil fuels and other sources without additional demanding purification. On the other hand, in synergy with phosphoric acid, it also accelerates degradation processes in HTPEMFC, making relatively high loadings of Pt on the electrodes necessary. The basic catalyst used in HTPEMFC comprises Pt nanoparticles on carbon black graphitized support, exhibiting reasonable activity and stability. Phosphoric acid is an adsorbing electrolyte, decreasing Pt catalyst activity by blocking the active Pt surface with phosphate anions and products of phosphoric acid reduction on the anode. Additionally, dissolution of Pt in phosphoric acid at elevated temperature is relatively high, leading to pronounced Ostwald ripening process on the cathode resulting in the progressive loss of Pt surface area. Both these effects can be, to a certain degree, mitigated by the introduction of highly active, Pt-based alloy catalysts on stable, carbon-based supports. Nanoparticles with Pt-rich shell and intermetallic Pt-X core can have activity superior to Pt, lower susceptibility to phosphate adsorption and, possibly, different kinetics of degradation. Introduction of novel, oxidation-resistant supports can further improve catalyst durability by nanoparticle-support interaction and minimize nanoparticle sintering due to support corrosion. Accordingly, this study investigates possibility of introduction of Pt-Co and Pt-Ni nanoparticular catalysts on supports including Ketjenblack and reduced graphene oxide to HTPEMFC. The study is divided to stages, including the investigation of alloying metal dissolution in phosphoric acid at elevated temperatures, determination of catalyst activity at conditions relevant to HTPEMFC operation using thin film-modified rotating rod disc electrode and finally, single cell testing. Leaching of Pt alloy catalysts proved that during relatively short time, majority of alloying metal is lost in case of Ketjenblack support. However, supporting of Pt alloys on reduced graphene oxide significantly improved alloy stability. The activity towards oxygen reduction reaction of Pt alloy catalysts was in majority of cases superior to commercial Pt catalyst, underlining the positive effect of alloying metal presence. Single cell experiments with Pt-Co and commercial Pt catalyst proved feasibility of alloy catalyst application. When used on the cathode and commercial Pt catalyst on the anode, Pt-Co catalysts led to superior cell performance in comparison with cell utilizing commercial Pt catalyst on both electrodes. Moreover, the performance of cells with Pt-Co on the cathode progressively improved during 300 hours of operation, pointing out to the combination of dealloying and restructuring of nanoparticle. In line with these results, it is evident that Pt alloy catalysts are an interesting alternative to Pt catalysts and, after optimization of alloy nanoparticle-support combination, can lead to improved performance and stability of HTPEMFC, making the technology even more attractive for market. The study was funded by the Czech Science Foundation (GAČR), project No. 22-23668K. This work was supported by the project "The Energy Conversion and Storage", funded as project No. CZ. 02. 01. 01/00/22₀08/0004617 by Programme Johannes Amos Commenius, call Excellent Research.