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

Abstract

Efficiently converting carbon dioxide (CO 2 ) 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 MoS 2 edges for electrocatalytic, or eventually photocatalytic, reduction of CO 2 . 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 CO 2 -reduction energy profile under relevant reducing potentials. Specifically, on the S-edge, 0.375 ML H coverage enables CO 2 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 CO 2 reduction was determined to be 0 ML. On this bare Mo-edge, CO 2 activation occurs at an additional energy cost of 0.44 eV for −0.80 V. Consequently, the activated CO 2 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 CO 2 reduction mechanisms on MoS 2 edges and highlights the key role of the CO 2 adsorption step which was generally neglected in prior investigations.

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