Single-atom-sized Ni-N(4)sites anchored in three-dimensional hierarchical carbon nanostructures for the oxygen reduction reaction

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-N(4)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 similar to 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-N(4)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 similar to 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.