Low-Valence Zn^δ+ (0<δ2 Reduction

A nitrogen-stabilized single-atom catalyst containing low-valence zinc atoms (Zn^δ+ -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. Zn^δ+ -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 Zn^δ+ -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.