Single-Ni-atoms on nitrogenated humic acid based porous carbon for CO₂ electroreduction

We proposed a facile synthesis of single-Ni-atom catalysts on low-cost porous carbon using a calcination method at the temperatures of 850–1000 °C, which were used for CO₂ electrochemical reduction to CO. The porous carbon was prepared by carbonizing cheap and abundant humic acid. The structural characterizations of the as-synthesized catalysts and their electrocatalytic performances were analyzed. The results showed that the single-Ni-atom catalyst activated at 950 °C showed an optimum catalytic performance, and it reached a CO Faradaic efficiency of 91.9% with a CO partial current density of 6.9 mA·cm⁻² at −0.9 V vs. reversible hydrogen electrode (RHE). Additionally, the CO Faradaic efficiency and current density of the optimum catalyst changed slightly after 8 h of continuous operation, suggesting that it possessed an excellent stability. The structure-activity relations indicate that the variation in the CO₂ electrochemical reduction performance for the as-synthesized catalysts is ascribed to the combined effects of the increase in the content of pyrrolic N, the evaporation of Ni and N, the decrease in pore volume, and the change in graphitization degree.