Atomic Ordering Effect\nof Intermetallic PdCoNi/rGO\nCatalysts on Formic Acid Electro-oxidation

Abstract

Pd-based alloys, especially for alloys containing early\ntransition\nmetals, have been extensively designed and applied to electro-oxidize\nformic acid for direct formic acid fuel cells owing to their capability\nto directly oxidize formic acid molecules via the dehydrogenation\npathway. Adjusting the strain and electronic effects of Pd-based alloys\nis an effective method to regulate the adsorbing phenomena of intermediates\non catalysts during formic acid oxidation and improve the catalytic\nactivity and stability. The ordering degree of alloys plays an important\nrole in modulating the adsorbing ability. In this work, we report\na strategy varying the atomic ordering degree of PdCoNi/rGO trimetallic\nalloys to manipulate the strain and electronic effects and electrocatalytic\nperformance toward formic acid oxidation. A series of PdCoNi/rGO-<i>T</i> trimetallic catalysts are synthesized, in which the atomic\norder of the trimetallic catalysts is regulated by annealing the wet-chemistry-synthesized\nPdCoNi/rGO alloy. As the annealing temperature increases, the atomic\narrangement among Pd, Co, and Ni is ordered. Eventually, ordered intermetallic\nPdCoNi/rGO-<i>T</i> catalysts are generated. As the atomic\nordering degree increases, the lattice constant decreases, and more\ncharge transfers from Pd to Ni and Co, leading to enhanced strain\nand electronic effects. Moreover, the ordered intermetallic structure\nstabilizes Co and Ni atoms to prevent the dissolution of the transition\nmetals in acidic electrolyte, and the strain and electronic effects\nin ordered PdCoNi/rGO catalysts are maintained. With an annealing\ntemperature of 700 °C, the intermetallic PdCoNi/rGO-700 exhibits\nthe highest specific activity of 8.33 mA/cm², which is\n1.95-fold improved compared to pristine PdCoNi/rGO alloy (3.23 mA/cm²) and 2.48 times compared to Pd/rGO-700 (2.54 mA/cm²). Moreover, PdCoNi/rGO-700 also shows outstanding catalytic durability\ndue to its excellent structural stability. We believe that this research\nhelps in developing Pd-based alloy catalysts with high activity and\nstability for formic acid electro-oxidation.