Electrochemical reduction of CO2 to fuels and chemicals carries extraordinary significance for industry era, and is highly competitive to water electrolysis and downstream gas-phase CO2 reduction for addressing the energy problem. Single atom materials endowed with maximum atom efficiency, tunable coordination environments and electronic structures have emerged as highly active catalysts for converting CO2 to CO. However, practical application of single atom catalysts still seems to be too far due to their complicated and high-cost materials synthesis as well as low performance metrics. In this work, Ni single atoms on a low-cost carbon nanoparticle support are developed via a simple and scalable method, with record-high selectivity and activity toward CO production. Moreover, scaling up the electrodes into a modular cell achieves a high overall current while maintaining an exclusive CO evolution.