Improved Activity and Durability of Carbon-Capped Pd and PdNi Catalysts─From Model Active Layers to Anion-Exchange Membrane Fuel Cell Electrodes

Carbon-supported and carbon-capped Pd (Pd@C/C) and PdNi (PdNi@C/C) nanoparticles were synthesized and characterized physicochemically. The presence of the carbon cap minimizes surface metal oxides after synthesis, while the metal loading, size, and composition of the nanoparticles on the carbon support can be tuned. After mild electrochemical activation, these catalysts outperform commercial (non-carbon-capped) Pd/C and PdNi/C catalysts for the alkaline hydrogen oxidation reaction (HOR): the beneficial effect of Ni alloying and of the carbon cap makes PdNi@C/C the best catalyst because it exhibits a metal–metal oxide surface compatible with fast HOR, both initially and during accelerated stress tests performed using rotating disk electrodes. The carbon cap prevents significant oxidation/passivation and leaching of the metals, thereby enhancing the materials’ stability and minimizing their passivation. The Pd@C/C and PdNi@C/C catalysts were upscaled and integrated in low-loaded anodes in Pt-free anion-exchange membrane fuel cells designed to be anode-limiting. The Pd@C/C and PdNi@C/C anodes surpassed Pd/C and PdNi/C anodes, in terms of both activity and stability, thereby confirming the results obtained in liquid alkaline electrolytes, with PdNi@C/C being the winner HOR catalyst. Carbon capping is, therefore, a successful strategy to obtain active and stable HOR catalysts.