Facile Synthesis of Polyhedral Pd Nanocrystals as a Highly Active and Methanol-Tolerant Electrocatalyst for Oxygen Reduction

Abstract

The polyhedral Pd nanocrystals are synthesized by a facile conventional hydrothermal method, in which the cetyltrimethylammonium bromide, Fe3+ and formaldehyde are designed as structural orientation, etching and reduction agents, respectively. The crystal and morphological characteristics show that the crystal facets of (111) and (220) planes are exposed on the surface of face-centered cubic crystal structure of Pd polyhedron. Electrochemical analyses indicate that the polyhedral Pd/XC-72 catalyst shows the highest electrochemically active surface area (57.6 m2 g−1), similar half-wave potential and diffusion-limiting current density (-4.79 mA cm−2) with state-of-the-art commercial Pd/C in alkaline electrolytes. The excellent electrocatalytic activity supports a four-electron-transfer pathway and smaller Tafel slope (67.2 mV dec−1) reveals a higher intrinsic activity for ORR. Additionally, the polyhedral Pd/XC-72 catalyst also exhibits much better methanol tolerance crossover effects and long-term stability than commercial Pd/C. All these excellent electrocatalytic performances are considered to be the coaction of the exposed high surface energy facets and more of active sites on the polyhedral crystals surface, which could significantly improve the adsorption and activation energies with oxygenated intermediates in alkaline solution. This work provides a new prospect in designing high active, non-easy tolerance and more stable Pd-based catalyst in alkaline fuel cells.