Unraveling Low Overpotential Pathways for Electrochemical CO<sub>2</sub> Reduction to CH<sub>4</sub> on Pure and Doped MoS<sub>2</sub> Edges
Dhruv Lal, Tanmay Konnur, Anand Mohan Verma, M. Shaneeth, Ananth Govind Rajan
Abstract
The electrochemical reduction of carbon dioxide (CO 2 ) has received considerable attention from the scientific community for its promising applications in the selective production of useful hydrocarbons, such as synthetic natural gas, i.e., methane (CH 4 ). In the field of extraterrestrial exploration, it can enable the conversion of the metabolite CO 2 as well as that present in the Martian atmosphere into CH 4, which can be used as fuel. In this work, we investigate vertically aligned 2H molybdenum disulfide (MoS 2 ) and its edge-doped alternatives as heterogeneous electrocatalysts for the reduction of CO 2 using density functional theory (DFT) calculations. Via a comprehensive reaction pathway analysis, we show that the edges of MoS 2 offer a significantly low overpotential of 0.62 V for CO 2 reduction to CH 4 as compared to a value of 0.86 V on copper, a prominent electrocatalyst. Furthermore, by screening 8 dopants (Al, Co, Cr, Cu, Fe, Mn, Ni, and Rh), we find that Al-doped MoS 2 yields CH 4 at a remarkably low overpotential of 0.41 V, owing to a different potential-determining step (PDS) (*COOH → *CO) as compared to the PDS on pure MoS 2 (*CO → *CHO). Other promising dopants include Ni and Rh, offering overpotentials of 0.58 and 0.62 V, respectively, for CH 4 production. Investigation of the competing hydrogen evolution reaction (HER) reveals that, while the CO 2 RR is significantly more favorable on Al-doped MoS 2, the HER outcompetes the CO 2 RR on pure, Ni-doped, and Rh-doped MoS 2 . Mechanistic insights obtained by comparing various reaction pathways (via *COOH/*HCOO and *CH 2 /*CH 3 OH) are complemented by density of states and charge density difference calculations, which rationalize the favored mechanism for each catalyst considered. Overall, our thorough, DFT-based mechanistic investigation of CO 2 reduction on pure and doped MoS 2 presents Al-doped MoS 2 edges as a promising material for the thermodynamically facile electroreduction of CO 2 to CH 4 .