Single-atom-sized Ni–N4 sites anchored in three-dimensional hierarchical carbon nanostructures for the oxygen reduction reaction

Abstract

The four-electron (4e⁻) oxygen reduction reaction (ORR) is a basic reaction in fuel cells and metal–air batteries, but its wide use requires the development of efficient and inexpensive catalysts. This work demonstrates that single-atom-sized Ni–N₄ sites embedded in three-dimensional and hierarchically structured carbon (NiN₄–C) exhibit a high catalytic activity for the ORR under alkaline conditions, in which their activity is better than, or at least as high as, that of commercial Pt/C catalysts. The catalyst is synthesized by simply pyrolyzing a mixture of nickel salt and EMIM-dca (1-ethyl-3-methylimidazolium dicyanamide). The product comprises many single Ni atom active sites (the content of Ni atoms in the catalyst is ∼4.2 ± 0.4 wt% as estimated by ICP-OES), in which each Ni atom is coordinated with four N atoms through the formation of a Ni–N₄ planar configuration. Both theoretical simulations and electrochemical measurements demonstrate that the catalyst has a high 4e⁻ selectivity, i.e., it facilitates an effective 4e⁻ ORR with a limiting 2e⁻ reaction. Moreover, when it is integrated into a Zn–air battery, the catalyst shows a maximum power density of ∼95 mW cm⁻², similar to the power density of Pt/C catalysts. The results are helpful for understanding and analyzing the catalytically active sites of carbon-supported single metal atom catalysts.