Tailoring surface termination and intrinsic metal phase of MXene via Lewis acidic molten salt etching for electrocatalysis

Two-dimensional transition metal carbides/nitrides/carbonitrides (MXene) have been widely applied in various electrocatalytic fields due to the unique layered structure, excellent intrinsic conductivity, tunable surface chemistry and high specific surface area. However, the traditional synthesis strategies of MXene involve dangerous fluorine-containing etching reagents, which have drawbacks such as strong corrosiveness, high safety risks and deficient active sites, thus severely limiting the regulation of active sites and the optimization of electrocatalytic performance. The Lewis acidic molten salt (LAMS) strategy utilizes the redox reaction between metal cations in the LAMS and the A-site atoms in the MAX phase to achieve selective etching and obtain MXene, featuring high safety, universality and structural controllability. More importantly, the LAMS strategy can simultaneously achieve surface termination modification and in-situ metal phase derivation during the etching process, ingeniously constructing dual functional site for electrocatalysis. This review comprehensively summarizes the structure and properties of MXene and the improvement methods and etching principles of the LAMS strategy. Additionally, the catalytic mechanism of surface termination and metal phase, and the latest progress in electrocatalysis are systematically sorted out. Furthermore, the currently challenges and the future prospects for the LAMS strategy are proposed. This review aims to break the conventional mindset of harmless treatment of etching by-products, as well as strengthen the resource utilization of LAMS derived by-products as active sites in electrocatalysis, providing new idea and theoretical reference for the development of green, low-cost and high-performance MXene-based catalysts.