A nitrogen-stabilized single-atom catalyst containing low-valence zinc atoms (Znδ+-NC) is reported. It contains saturated four-coordinate (Zn-N4) and unsaturated three-coordinate (Zn-N3)
sites. The latter makes Zn a low-valence state, as deduced from X-ray
photoelectron spectroscopy, X-ray absorption spectroscopy, electron
paramagnetic resonance, and density functional theory. Znδ+-NC catalyzes electrochemical reduction of CO2 to CO with near-unity selectivity in water at an overpotential as low as 310 mV. A current density up to 1 A cm−2 can be achieved together with high CO selectivity of >95 % using Znδ+-NC in a flow cell. Calculations suggest that the unsaturated Zn-N3
could dramatically reduce the energy barrier by stabilizing the COOH*
intermediate owing to the electron-rich environment of Zn. This work
sheds light on the relationship among coordination number, valence
state, and catalytic performance and achieves high current densities
relevant for industrial applications.
