Why Is C–C Coupling in CO2 Reduction Still Difficult on Dual-Atom Electrocatalysts?

The emerging metal–nitrogen–carbon (M–N–C) dual–atom catalysts (DACs) have been expected to generate multicarbon products in the CO2 reduction reaction (CO2RR) due to the presence of multimetal sites of DACs. Unfortunately, numerous recent experiments suggested that almost no DAC could effectively produce a high quantity of multicarbon products. To uncover the reason for this phenomenon, we probed the surface states of typical homonuclear and heteronuclear DACs and explored the reaction mechanisms in the CO2RR by spin-polarized density functional theory calculations with van der Waals interactions. Contrary to the conventional hypothesis that C–C coupling can occur through the metal-top sites, surface Pourbaix analyses indicate that CO preferentially occupies the bridge sites between two metals, which would hinder the subsequent C–C coupling. Moreover, according to the energy variation, the C–C coupling occurring on the surface of a DAC is not feasible in both thermodynamics and kinetics. Based on the derived microkinetic models of DACs in the CO2RR, CO formation is more favorable than other reduction products, which is consistent with current experimental results. Furthermore, we found that double-side occupancy is also favorable if the molecules can penetrate the carbon layer through a large defect, which would lead to a more favorable HCOOH formation in the CO2RR. By developing an analytical framework combining surface state analysis, activity modeling, and electronic structure analysis, this work reveals why C–C coupling in the CO2RR remains difficult on DACs and provides insights into regulating the adsorption strength of *CO on the bridge site to enhance the selectivity and activity of the CO2RR at DACs.