Figure 1: Schematic diagram of synthesis and activation process of the Ni–C catalysts.

The Ni-MOF used as a precursor consists of an orthorhombic crystal. Atoms are shown as follows: C, black; H, white; O, red; N, blue; Ni, royal blue. Carbonization of the synthesized Ni-MOF was at 700 °C in N2 atmosphere to obtain Ni@C. HCl leaching treatment was repeated three times to sufficiently dissolve exposed Ni metal. Constant potential and CV treatments were performed to activate the catalysts until they reached the optimal performance and remained stable. During the activation process, Ni single atoms formed in situ anchored on the graphitized carbon.