First-principles investigation of surface functionalization and CO₂ adsorption in MXenes

Ever-increasing amounts of carbon dioxide (CO₂) are being expelled into the atmosphere, and without some type of countermeasure the environmental consequences will be dire. Motivated by the fact that CO₂ generators are a fixed part of our technological society which may be reduced, but not fully removed; research efforts have been directed towards utilizing these high concentrations of atmospheric CO₂ as a sustainable resource. Carbon capture, specifically post-combustion, captures CO₂ from a single source, removing it from the air and allowing further operations to be performed later. 2D early transition metal carbides and nitrides known as MXenes are investigated as an effective sorbent material thanks to their diverse composition, chemically active surface stemming from surface functionalizations and overall versatile and tunable nature. The computational characterization of MXene surface functionalizations, both the native surface inherited from synthesis and non-native surface groups introduced afterwards, are being systematically explored through calculated adsorption energies from density functional theory (DFT) as well as material optimization via machine learning (ML) methods. Molecules and surfaces being modeled are: pristine surfaces (Ti₃C₂, Mo₃C₂), attaching molecules (CO₂, H₂O, O₂, N₂, SO₂), native surface functional groups (F, O₂, O, OH, Cl and H), non-native surface functional group (NH₂) and the interactions between them. The characterizations so far have shown that when adsorbing to a pristine Ti₃C₂surface, single ions of F, O, OH, Cl and attaching molecules CO₂, H₂O and O₂ all preferred binding to hollow sites, while ions of H and NH₂ favored the bridge site before relaxing into a hollow site. Single fluorine ions bound to the surface were more thermodynamically stable when H₂O adsorbed, when compared to CO₂ and O₂, but saturation of the surface with fluorine resulted in CO₂ and H₂O having similar adsorption energies. The unknowns revolving around MXenes and their surface functionalizations is a knowledge gap that is preventing MXenes from being fully utilized in the field of sustainable chemistry. The characterizations and MXene designs generated by this research will not only fill in the divide between MXenes and carbon capture, but also unite industries and our environment through more accessible carbon capture methods.