A Ni single-atom catalyst with N-N4-xCx active sites is prepared in a single pyrolysis step in which the Ni single atom is incorporated in the carbon framework through nitrogen and carbon coordination utilizing the ionothermal synthesis method. In comparison to the complicated synthesis procedures of single-atom catalysts, this method provides a general and facile method to obtain single-atom catalysts with an opportunity to synthesize catalysts at a large scale. The precursors used in this method such as adenine, fructose, and glucose are derived from the biomass which is the essential requirement for a green process. The synthetic procedure developed here enables tunable properties of the catalysts, such as the density of active sites and characteristics of the carbon framework. In this study, the catalytic properties of our materials are investigated for an electrochemical CO2 reduction reaction. The overall catalytic activity of the catalyst depends on the density of active sites, but the properties of the carbon framework also affect the intrinsic activity of the catalyst. From the commercial prospect, a Ni single-atom catalyst with a high density of N -N4-xCx active sites can achieve a current density of -300 mA cm(-2) with a CO faradaic efficiency of 99.4% at an overpotential of 235 mV in a gas diffusion electrode cell system.