To improve the atomic utilization of metals and reduce the cost of industrialization, the one-step total monoatomization of macroscopic bulk metals, as opposed to nanoscale metals, is effective. In this study, we used a thermal diffusion method to directly convert commercial centimeter-scale Ni foam to porous Ni single-atom-loaded carbon nanotubes (CNTs). As expected, owing to the coating of single-atom on porous, highly conductive CNT carriers, Ni single-atom electrocatalysts (Ni-SACs) exhibit extremely high activity and selectivity in CO2 electroreduction (CO2RR), yielding a current density of > 350 mA/cm2, a selectivity for CO of > 91% under a flow cell configuration using a 1 M potassium chloride (KCl) electrolyte. Based on the superior activity of the Ni-SACs electrocatalyst, an integrated gas-phase electrochemical zero-gap reactor was introduced to generate a significant amount of CO current for potential practical applications. The overall current can be increased to 800 mA, while maintaining CO Faradaic efficiencies (FEs) at above 90% per unit cell. Our findings and insights on the active site transformation mechanism for macroscopic bulk Ni foam conversion into single atoms can inform the design of highly active single-atom catalysts used in industrial CO2RR systems.