Partially Nitrided Ni Nanoclusters Achieve Energy-Efficient Electrocatalytic CO2 Reduction to CO at Ultralow Overpotential

Electrocatalytic CO 2 reduction reaction (CO 2 RR) offers a promising strategy to lower CO 2 emission while producing value-added chemicals. A great challenge facing CO 2 RR is how to improve energy efficiency by reducing overpotentials. Herein, partially nitrided Ni nanoclusters (NiN x ) immobilized on N-doped carbon nanotubes (NCNT) for CO 2 RR are reported, which achieves the lowest onset overpotential of 16 mV for CO 2 -to-CO and the highest cathode energy efficiency of 86.9% with CO Faraday efficiency >99.0% to date. Interestingly, NiN x /NCNT affords a CO generation rate of 43.0 mol g −1 h −1 at a low potential of −0.572 V (vs RHE). DFT calculations reveal that the NiN x nanoclusters favor *COOH formation with lower Gibbs free energy than isolated Ni single-atom, hence lowering CO 2 RR overpotential. As NiN x /NCNT is applied to a membrane electrode assembly system coupled with oxygen evolution reaction, a cell voltage of only 2.13 V is required to reach 100 mA cm −2 , with total energy efficiency of 62.2%.