Highly Durable Pt-Based Core–Shell Catalysts with Metallic and Oxidized Co Species for Boosting the Oxygen Reduction Reaction

Over the last 15 years, core–shell nanoparticles with a Pt-enriched shell have emerged as efficient electrocatalysts for the oxygen reduction reaction (ORR). However, to what extent the chemical state of the cobalt species inside the particle core has influence on the ORR performance and particularly on the long-term durability has not yet fully understood to date. In this study, we investigate the chemical state of the internal cobalt atoms and their stability within the PtxCo1–x core–shell catalysts during the ORR as well as after applying different accelerated stress test (AST) protocols. Remarkably, at the begin-of-life, the activated PtxCo1–x core–shell catalysts exhibit over 3.7–5.3 increase in Pt mass-based activity and over 5.8–10.6 increase in Pt surface area-specific activity at 0.95 VRHE compared to Pt/C. The superior ORR activity originates from the chemical composition of the particle core, where cobalt not only exists in the metallic state but also as 40–60% of Co oxide species detected by X-ray photoelectron spectroscopy. The Co oxide species are very likely relics of the precursor catalyst from the activation process via electrochemical dealloying. Moreover, the PtxCo1–x core–shell catalysts show improved durability and high cobalt retention against electrochemical dissolution during the AST protocols (e.g., >70% of Co after 2000 cycles between 0.5 and 1.5 VRHE). Although the potential cycle-dependent changes in the electrochemically active Pt surface area and particle size are negligible or moderate, the ORR activities of PtxCo1–x core–shell catalysts decrease but still surpass that of Pt/C by a factor of 2–3. The observed loss of ORR performance for PtxCo1–x core–shell catalysts is very likely related to Ostwald ripening as the main degradation process, which leads to increasing thickness of the Pt-enriched particle shell. Remarkably, the stability of the internal Co oxide species is barely affected under the aggressive AST conditions. Thus, we suggest that the Co oxide species might have a positive effect and could even be a yet undiscovered alternative to metallic cobalt to boost the ORR activity and the long-term durability of PtxCo1–x core–shell catalysts beyond their expected useful life.