A systematic study was conducted on small Pd nanocrystals (5−6 nm) to understand the effects of catalyst structure and electrolyte on the oxygen reduction reaction (ORR) and formic acid oxidation (FAO). The ORR activities of Pd catalysts strongly depended on their structure and the electrolyte used. It was found that Pd cubes were 10 times more active than Pd octahedra for ORR in an aqueous HClO 4 solution due to higher onset potential of OH ad formation on the cubic surface. In the case of a H 2 SO 4 solution, the ORR activity of Pd cubes was 17 times higher than that of Pd octahedra due to the stronger adsorption of (bi)sulfate on the surface of octahedral nanocrystals in addition to OH ad . In alkaline solutions, however, no structure dependence was observed for ORR due to the outer-sphere electron-transfer mechanism in the potential region for Pd oxide formation. For FAO, no advantage was observed on shape-controlled Pd nanocrystals in comparison to conventional Pd catalysts. The FAO current densities, both at peak current and at 0.4 V, followed the order of conventional Pd > octahedral Pd > cubic Pd. It was hypothesized that steps and defects were more active for FAO than terraces, which could be used to explain why the shape-selective materials were less active than conventional Pd because they contained fewer defects and edge sites.

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A comprehensive experimental study was conducted on the dealloying of PdNi6 nanoparticles under various conditions. A two-stage dealloying protocol was developed to leach >95% of Ni while minimizing the dissolution of Pd. The final structure of the dealloyed particle was strongly dependent on the acid used and temperature. When H2SO4 and HNO3 solutions were used in the first stage of dealloying, solid and porous particles were generated, respectively. The porous particles have a 3-fold higher electrochemical surface area per Pd mass than the solid ones. The dealloyed PdNi6 nanoparticles were then used as a core material for the synthesis of core-shell catalysts. These catalysts were synthesized in gram-size batches and involved Pt displacement of an underpotentially deposited (UPD) Cu monolayer. The resulting materials were characterized by scanning transmission electron microscopy (STEM) and in situ X-ray diffraction (XRD). The oxygen reduction reaction (ORR) activity of the core-shell catalysts is 7-fold higher than the state-of-the-art Pt/C. The high activity was confirmed by a more than 40 mV improvement in fuel cell performance with a Pt loading of 0.1 mg cm(-2) by using the core-shell catalysts.

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