Optimizing Cu doping on carbon nitrogen holly shell for enhanced selectivity towards formate in CO₂ reduction

Electrochemical carbon dioxide reduction reaction (ECO₂RR) to produce high value-added products is a promising and effective strategy for closing the artificial carbon cycle and achieving sustainable development of resource. However, catalyst structural reorganization and agglomeration caused by the reduction process will reduce the catalytic performance. In this study, a carbon nitrogen shell with cupper-doped (CNCu shell) catalyst was prepared using silicon dioxide (SiO₂) as a template. The selectivity of the catalyst was controlled by precisely adjusting the form of Cu doping in the catalyst. When doped as single-atoms (CNCu_2.5), the catalyst exhibited a Faraday efficiency of up to 85 % for formate at -0.9 V versus reversible hydrogen electrode (vs. RHE). In contrast, when both Cu clusters and single-atoms coexisted (CNCu₂₅), the catalyst favored multi-carbon products, with a Faraday efficiency of 45 % for ethanol and 23 % for acetic acid. Density functional theory (DFT) calculations revealed the key mechanism for the difference in catalyst selectivity between the two doping forms. Cu single-atoms provided suitable binding energy to HCOO*, which increased the rate of CO₂ conversion to formate, while the combination of Cu clusters and single-atoms increased the adsorption of HCOO*, raising the rate-determining step energy of the formate pathway, which favored multi-carbon products. This study fundamentally revealed how different doping forms of metals affect catalyst selectivity, providing new insights and strategies for developing superior metal-carbon-nitrogen (MCN) catalysts.