Abstract
Atomically dispersed and nitrogen-coordinated single Ni sites (i.e., NiNx moieties) embedded in partially graphitized carbon have emerged as effective catalysts for CO2 electroreduction to CO. However, much mystery remains behind the extrinsic and intrinsic factors that govern the overall catalytic CO2 electrolysis performance. Here, we designed a high-performance single Ni site catalyst through elucidating the structural evolution of NiNx sites during thermal activation and other critical external factors (e.g., carbon particle sizes and Ni content) by using Ni–N–C model catalysts derived from nitrogen-doped carbon carbonized from a zeolitic imidazolate framework (ZIF)-8. The N coordination, metal–N bond length, and thermal wrinkling of carbon planes in Ni–N–C catalysts significantly depend on thermal temperatures. Density functional theory (DFT) calculations reveal that the shortening Ni–N bonds in compressively strained NiN4 sites could intrinsically enhance the CO2RR activity and selectivity of the Ni–N–C catalyst. Notably, the NiN3 active sites with optimal local structures formed at higher temperatures (e.g., 1200 °C) are intrinsically more active and CO selective than NiN4, providing a new opportunity to design a highly active catalyst via populating NiN3 sites with increased density. We also studied how morphological factors such as the carbon host particle size and Ni loading alter the final catalyst structure and performance. The implementation of this catalyst in an industrial flow-cell electrolyzer demonstrated an impressive performance for CO generation, achieving a current density of CO up to 726 mA cm−2 with faradaic efficiency of CO above 90%, representing one of the best catalysts for CO2 reduction to CO.
