Theoretical investigations of electrochemical CO2 reduction by transition metals anchored on CNTs

Abstract

Transition metals supported on nitrogen doped carbon materials are a class of promising electrochemical catalysts toward electrochemical CO₂ reduction reactions (CO₂RR) that have exhibited excellent catalytic performance. Herein, M–N₄ (M = Fe, Co and Ni) coordination structures embedded in carbon nanotubes (CNTs) were constructed to explore detailed mechanisms as electrocatalysts for CO₂RR via density functional theory (DFT) calculations. Nitrogen atoms of coordination structures rather than only single transition metal atoms were demonstrated to be effective active sites toward CO₂RR. For two possible pathways of the first step, forming *COOH or *OCHO, according to the catalytic activity of nitrogen atoms to the H atom, *COOH generation was facilitated in terms of kinetics over *OCHO in three systems. Meantime, it is suggested that the products of electrochemical CO₂RR on the three catalysts are heavily dependent on the interaction of CO and the catalysts. Ni–N₄/CNT exhibits a considerable selectivity to CO due to the weak interaction of CO and the substrates with a limiting potential of 1.79 V. On the contrary, CO prefers to remain on Fe–N₄/CNT due to the strong binding energy of CO adsorbed on Fe–N₄/CNT. Then the CO of the Fe–N₄/CNT system undergoes further hydrogenation to produce CH₃OH and CH₄ at the same limiting potential of 0.68 V, indicating that there is no distinct selectivity in forming CH₃OH and CH₄. Nevertheless, the limiting potentials of CO, CH₃OH and CH₄ on Co–N₄/CNT are totally different, 0.43 V, 0.56 V and 0.87 V, respectively. In particular, CO₂RR to CO on Co–N₄/CNT has the lowest potential limit. Thus, Co–N₄/CNT has a considerable potential as the catalyst for electrochemical CO₂RR. In addition, the catalysts of Ni single atoms, unsaturated coordination with nitrogen atoms, edge-anchored on CNTs were investigated for their activity for the CO₂RR. Our comprehensive understanding of M–N₄/CNT materials may be instructive and meaningful to design advanced catalysts from nonprecious metals for electrochemical CO₂RR.