Development of electrocatalysts for efficient and selective electrochemical CO2 reduction has been challenging as protons are usually easier to be reduced than CO2 in an aqueous electrolyte. Recently, single-atom catalysts have been studied for various kinds of chemical reactions due to their unique catalytic properties and high specific activities. To prepare single atom electrocatalysts, it is very important to stabilize the atomic metal uniformly over the conducting supports such as graphene sheets with prevention of metal aggregation. In this research, to stabilize nickel ions over the graphene oxide (GO) sheets homogeneously, tris(2-benzimidazolyl-methyl)amine (NTB) ligand was introduced as a binder for nickel ions on the graphene oxide sheets. As NTB ligands have strong interactions with GO sheets by pi-pi interactions and with nickel ions by ligation, they can effectively stabilize nickel ions on GO sheets forming Ni(NTB)-GO complexes. After high-temperature annealing of Ni(NTB)-GO under an inert atmosphere, nickel and nitrogen-doped reduced graphene oxide sheets (Ni-N-RGO) was successfully prepared. Presence of single-atom nickel species on the graphene lattice were directly observed by HAADF-STEM imaging suggesting that NTB can effectively stabilize nickel ions on graphene oxide sheets. Moreover, from the XANES and EXAFS analysis, we confirmed that the nickel centers have square planar geometries with four nitrogen coordination. Ni-N-RGO shows high CO2 reduction selectivity in the reduction of CO2 to CO with 97% faradaic efficiency at -0.8 V vs RHE. Furthermore, iron and cobalt doped N-RGO were prepared with the same approach to see whether they can also selectively reduce CO2 to CO. However, compared to Ni-N-RGO, Fe and Co-N-RGO shows less selectivity toward CO2 reduction producing hydrogen as main products. And, from the DFT calculation, Ni-N-RGO shows a highest activation energy for proton reduction compared to its cobalt and iron counterparts leading to highly selective CO2 reduction activity in the aqueous electrolyte.

Ligand-assisted formation of single-atom nickel sites over graphene sheets for selective electrochemical reduction of CO2 to CO

2019 Spring National Meeting

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