Single Site Cobalt Substitution in 2D Molybdenum Carbide (MXene) Enhances Catalytic Activity in the Hydrogen Evolution Reaction

Abstract

Two-dimensional (2D) carbides, nitrides, and carbonitrides known as MXenes are emerging materials with a wealth of useful applications. However, the range of metals capable of forming stable MXenes is limited mostly to early transition metals of groups 3-6, making the exploration of properties inherent to mid or late transition metal MXenes very challenging. To circumvent the inaccessibility of MXene phases derived from mid-to-late transition metals, we have developed a synthetic strategy that allows the incorporation of such transition metal sites into a host MXene matrix. Here, we report the structural characterization of a Mo₂C<i>T</i>_<i>x</i>:Co phase (where <i>T</i>_<i>x</i> are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (β-Mo₂C:Co) through a two-step synthesis: first an intercalation of gallium yielding Mo₂Ga₂C:Co followed by removal of Ga via HF treatment. Extended X-ray absorption fine structure (EXAFS) analysis confirms that Co atoms occupy Mo positions in the Mo₂C<i>T</i>_<i>x</i> lattice, providing isolated Co centers without any detectable formation of other cobalt-containing phases. The beneficial effect of cobalt substitution on the redox properties of Mo₂C<i>T</i>_<i>x</i>:Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo₂C<i>T</i>_<i>x</i> catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo₂C<i>T</i>_<i>x</i>:Co to the favorable binding of hydrogen on the oxygen terminated MXene surface that is strongly influenced by the substitution of Mo by Co in the Mo₂C<i>T</i>_<i>x</i> lattice. In addition to the remarkable HER activity, Mo₂C<i>T</i>_<i>x</i>:Co features excellent operational and structural stability, on par with the best performing non-noble metal-based HER catalysts. Overall, our work expands the compositional space of the MXene family by introducing a material with site-isolated cobalt centers embedded in the stable matrix of Mo₂C<i>T</i>_<i>x</i>. The synthetic approach presented here illustrates that tailoring the properties of MXenes for a specific application can be achieved via substitution of the host metal sites by mid or late transition metals.