Electrocatalytic Reduction Mechanisms of CO₂ on MoS₂ Edges Using Grand-Canonical DFT: From CO₂ Adsorption to HCOOH or CO

Efficiently converting carbon dioxide (CO2) into valuable fuels or chemicals represents one of the great challenges at the core of current scientific research. Here, we report a DFT-based theoretical study of the reactivity of two main MoS2 edges for electrocatalytic, or eventually photocatalytic, reduction of CO2. By explicitly accounting for the electrode potential via a grand-canonical ensemble that controls the number of electrons, we show that the two edges exhibit different H coverage, which, in turn, directly influences the CO2-reduction energy profile under relevant reducing potentials. Specifically, on the S-edge, 0.375 ML H coverage enables CO2 activation through its adsorption in a bidentate mode with a limiting potential of 0.07 V vs SHE. By contrast, H coverage on the Mo-edge at reducing potentials relevant for CO2 reduction was determined to be 0 ML. On this bare Mo-edge, CO2 activation occurs at an additional energy cost of 0.44 eV for −0.80 V. Consequently, the activated CO2 on the S-edge exhibits comparable and thermodynamically favorable reactivity for the two possible two-electron products, formic acid and carbon monoxide (CO), with limiting potentials of −0.54 V and −0.34 V, respectively. The two products, however, show different desorption behavior, with CO desorption being endergonic and independent of electrode potential. Conversely, the reactivity of the Mo-edge is less favored, and it is anticipated to favor formic acid over CO due to a highly endergonic C–O bond-breaking step in the adsorbed COOH intermediate. This step was determined to be exergonic on the S-edge. This study paves the way for a better understanding of CO2 reduction mechanisms on MoS2 edges and highlights the key role of the CO2 adsorption step which was generally neglected in prior investigations.