Improving Stability and Kinetics of Alkaline HOR Catalysts – Towards Reduced System Cost

Abstract

Anion exchange membrane (AEM) fuel cells (FC) show great promise in overcoming monetary drawbacks of currently state of the art proton exchange membrane (PEM) FC, that rely on expensive Pt catalysts for both the hydrogen oxidation reaction (HOR) as well as the oxygen reduction reaction (ORR). With recent advancements in demonstrating high ORR activity and stability of Fe-N-C catalysts in alkaline environments,1 the HOR suddenly became the bottleneck reaction towards high power density platinum group metal (PGM) -low or -free AEM-FCs. While application of PGM-free ORR catalysts is an eventual goal, current state of the art NiM nanoparticles show relatively poor performance in terms of both activity and irreversible oxidation and dissolution.2 Hence, at least for some time, one will need to rely on PGM catalysts for alkaline HOR, where the biggest challenge will be lowering the loading and meanwhile increasing the stability in an alkaline environment. To this end, fundamental understanding of surface processes is required.In this work we investigate model CeOx covered Pt and Pd electrocatalysts as benchmark materials for the HOR reaction.3,4 To prepare such model electrodes, we use physical vapor deposition and atomic layer deposition techniques for formation of PGM and a variable thicknesses CeOx surface layers, subsequently. Activity and stability of the CeOx@Pt and CeOx@Pd electrodes are studied in common rotating disk electrode (RDE) and on-line inductively coupled plasma mass spectrometry setups, respectively. External analytics, e.g. X-ray photoelectron spectroscopy (XPS), are used to further characterized the electrodes before and after degradation studies. We show that stability of the electrodes at the higher anodic potentials can be improved greatly by tuning the thickness of the CeOx layer. Interestingly, this can be achieved without sacrificing and even with a significant enhancement in the HOR activity. This unique performance is explained by assuming that the deposited CeOx films form an ion selective semipermeable passivation layer on the surface of Pd and Pt. We anticipate that as a next logical step such layers can be used to stabilize PGM free Ni alloys and reach the goal of having completely PGM-free FCs.