Atomic Ordering Effect of Intermetallic PdCoNi/rGO Catalysts on Formic Acid Electro-oxidation

Abstract

Pd-based alloys, especially for alloys containing early transition metals, have been extensively designed and applied to electro-oxidize formic acid for direct formic acid fuel cells owing to their capability to directly oxidize formic acid molecules via the dehydrogenation pathway. Adjusting the strain and electronic effects of Pd-based alloys is an effective method to regulate the adsorbing phenomena of intermediates on catalysts during formic acid oxidation and improve the catalytic activity and stability. The ordering degree of alloys plays an important role in modulating the adsorbing ability. In this work, we report a strategy varying the atomic ordering degree of PdCoNi/rGO trimetallic alloys to manipulate the strain and electronic effects and electrocatalytic performance toward formic acid oxidation. A series of PdCoNi/rGO-T trimetallic catalysts are synthesized, in which the atomic order of the trimetallic catalysts is regulated by annealing the wet-chemistry-synthesized PdCoNi/rGO alloy. As the annealing temperature increases, the atomic arrangement among Pd, Co, and Ni is ordered. Eventually, ordered intermetallic PdCoNi/rGO-T catalysts are generated. As the atomic ordering degree increases, the lattice constant decreases, and more charge transfers from Pd to Ni and Co, leading to enhanced strain and electronic effects. Moreover, the ordered intermetallic structure stabilizes Co and Ni atoms to prevent the dissolution of the transition metals in acidic electrolyte, and the strain and electronic effects in ordered PdCoNi/rGO catalysts are maintained. With an annealing temperature of 700 °C, the intermetallic PdCoNi/rGO-700 exhibits the highest specific activity of 8.33 mA/cm2, which is 1.95-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 due to its excellent structural stability. We believe that this research helps in developing Pd-based alloy catalysts with high activity and stability for formic acid electro-oxidation.