Charge transfer dynamics and tuning of reorganization energy in graphene-encapsulated co-based alloy catalysts for fuel cells†

Graphene-encapsulated metal alloy nanocatalysts with a metal@graphene type architecture are unique catalysts with multifunctional properties. However, effectively integrating these physicochemical properties to enhance electrocatalytic performance still presents a significant challenge. Here, using density functional theory (DFT) calculations, we first reveal that tuning the composition of Co_xPd_y (x ≫ y) alloys optimizes structural reorganization energy and enhances charge transfer from the metal core to the graphene shell, thereby improving oxygen reduction reaction (ORR) activity. Guided by these computational insights, we synthesize graphitic carbon shell-encapsulated and carbon-supported Co_xPd_y alloy catalysts (Co_xPd_y@Gr/C) with varying Co : Pd ratios via a solvothermal decomposition followed by heat-treatment. Interestingly, electrochemical screening of various synthesized catalysts demonstrates that Co₉Pd₁@Gr/C catalyst with an ultra-low Pd loading exhibits the highest ORR activity in KOH, which is comparable to that of the commercial Pt/C catalyst. This enhanced activity originates from the synergistic effects where the Co-rich composition provides strong electron transfer while the presence of Pd creates beneficial lattice strain through atomic size differences. Moreover, electrochemical poisoning tests and accelerated durability tests provide strong evidence supporting the dual role of the graphene shell in improving ORR activity and stability. The graphene shell not only acted as the sole active site for the catalytic activity but also served as protective layers, preventing the corrosion of the alloy particles. Thus, this work will pave a new way for designing and developing cost-effective and high-performance cathode catalysts for anion exchange membrane fuel cells.