Mixed-Functionalized Sc2CTx (T=O, OH, F) MXene for Electrocatalytic CO2 Reduction: Insight from First-Principles Calculations

A microscopic understanding of the mixed-functionalized MXenes (${M}_{2}{X\ensuremath{-}T}_{x}$; ${T}_{x}=\text{O, OH,}$ and $\mathrm{F}$) are extremely important to the design of efficient ${\mathrm{CO}}_{2}$ catalytic activity. Here, we report a first-principles study of the ${\mathrm{CO}}_{2}$ activation on pure $\mathrm{O}$, $\mathrm{OH}$, and $\mathrm{F}$ and mixed-functionalized ${\mathrm{Sc}}_{2}\mathrm{C}$ MXene surfaces. We find that ${\mathrm{CO}}_{2}$ adsorption energy can be tuned by changing the coverage of $\mathrm{O}$, $\mathrm{OH}$, and $\mathrm{F}$ functional groups on the surface. Fully terminated $\mathrm{O}$ and $\mathrm{F}$ ${\mathrm{Sc}}_{2}\mathrm{C}$ forms weak interactions with ${\mathrm{CO}}_{2}$ molecules (binding energy $\ensuremath{-}0.136$ and $\ensuremath{-}0.168\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$) whereas mixed-functionalized ${\mathrm{Sc}}_{2}\mathrm{C}$ surface exhibits higher binding energy($\ensuremath{-}0.364\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$). In the mixed-functionalized ${\mathrm{Sc}}_{2}\mathrm{C}$ case, only $\mathrm{O}$ sites allow ${\mathrm{CO}}_{2}$ reduction ($\mathrm{F}$ and $\mathrm{OH}$ are inactive) and finally converts into methane (${\mathrm{CH}}_{4}$). Ab-initio-based Bader charge analysis and projected density of state calculations reveal strong bonding between the C atom of ${\mathrm{CO}}_{2}$ and $\mathrm{O}$ functional group. The Gibbs free-energy calculation confirms the conversion of $\mathrm{HCO}$ into ${\mathrm{H}}_{2}\mathrm{CO}$ to be a rate-limiting step with the limiting potential 1.387 eV. In the mixed-functionalized surface, as we increase the number of $\mathrm{OH}$ groups in the vicinity of $\mathrm{O}$ sites, the binding energy increases (transiting from a physisorption to a chemisorption regime). However, increasing the amount of O coverage turns out to be detrimental to the catalytic activity. Our study highlights the role of different functional groups in achieving efficient ${\mathrm{CO}}_{2}$ catalytic activity on ${\mathrm{Sc}}_{2}\mathrm{C}$ MXene, which can further help us to design experiments accordingly.