A simple cadmium sulfide nanomaterial is found to be an efficient and stable electrocatalyst for CO2 reduction in aqueous medium for more than 40 h with a steady CO faradaic efficiency of approximately 95 %. Moreover, it can realize a current density of −10 mA cm−2 at an overpotential of −0.55 V on a porous substrate with similar selectivity. Theoretical and experimental results confirm that the high selectivity for CO2 reduction is due to its (0 0 0 2) face with sulfur vacancies that prefers CO2 molecule reduction in aqueous medium.