Recent progress on synthesis, structure and electrocatalytic applications of MXenes

Two-dimensional transition metal carbides and nitrides, i.e MXenes, have been at the center of extensive research since the first MXene material, Ti3C2Tx, was experimentally realized via selective etching of Al layers from its MAX phase, Ti3AlC2, in 2011. Recent years have seen the booming growth of MXenes as promising candidates in wide applications due to their metallic conductivity and surface hydrophilicity. Generally, pristine MXenes are terminated with such functional groups as –F, –OH, and –O. Surface terminations of MXenes have a huge influence on its electronic properties, metallic or semiconductoring, and dynamical stability. Functionalized MXenes are shown to be promising electrocatalysts either alone or as part of MXene-based hybrid materials. The MXene family has been enlarged by extending its parent MAX phases to quaternary phases with multiple metal sites, offering a rich variation of chemistries, compositions, structures, and catalytic performances. In this review, recent advances are provided regarding synthetic strategies, surface structures, electronic structures, and electrocatalytic applications.