Ni–Pd Nanocomposites on Reduced Graphene Oxide Support as Electrocatalysts for Hydrogen Evolution Reactions
Anna Prytkova, Maria A. Kirsanova, Airat Kiiamov, D. A. Tayurskiı̆, Ayrat M. Dimiev
Abstract
Synthesis of bimetallic nanoparticles is a popular approach in developing novel electrocatalytic materials. In this work, by wet chemical protocols, we synthesized the mixed nickel–palladium nanocomposite on reduced graphene oxide support (Ni–Pd/rGO) alone with its monometallic analogues Ni/rGO and Pd/rGO as reference samples. The structure of the three nanocomposites was revealed by a set of advanced instrumental methods. In Ni/rGO, nickel evenly covers the rGO support in the form of single ions, chemically bound to the surface. In Pd/rGO, palladium is in form of nanoparticles with the size of 3–8 nm. In Ni–Pd/rGO, nickel uniformly covers the rGO surface, and Pd forms nanoparticles, similar to that in the monometallic analogues. At the same time, a thin surface layer of the Pd nanoparticles is enriched by Ni atoms. The nickel-enriched layer is not continuous, with a gradient of Ni content from the particle surface toward its center; its thickness does not exceed dimensions of two to three atomic layers. Only Pd/rGO and Ni–Pd/rGO demonstrated catalytic activity toward the hydrogen evolution reaction (HER), suggesting that catalytic properties stem from Pd, not Ni. Ni–Pd/rGO exhibits a significantly higher electrocatalytic surface area of 2.421 m 2 /g, compared to 0.278 m 2 /g for Pd/rGO, which could be explained by agglomeration of Pd nanoparticles in the latter and their lower availability to reagents. Both nanocomposites demonstrated good stability after 1000 cycles. Despite reduced palladium content, Ni–Pd/rGO demonstrated higher efficiency toward HER with overpotential of 63 mV compared to 116 mV for Pd/rGO: the catalytic efficiency is increased simultaneously with reducing the content of precious Pd by half. These observations can be explained by the alteration of the surface energy of the particles due to the difference in electronegativity and the lattice mismatch between the two metals.