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 Mo 2 CT x :Co phase (where T x are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (β-Mo 2 C:Co) through a two-step synthesis: first an intercalation of gallium yielding Mo 2 Ga 2 C: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 Mo 2 CT x 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 Mo 2 CT x :Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo 2 CT x catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo 2 CT x :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 Mo 2 CT x lattice. In addition to the remarkable HER activity, Mo 2 CT x :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 Mo 2 CT x . 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.

Microenvironment modulation of single-atom catalysts and their roles in electrochemical energy conversion

“…MXenes have been recently applied as promising hosts for anchoring single metal atoms with controlled microenvironment for boosting HER (118,119). In the work done by Zhang et al (47), electrochemical, exfoliated, double transition metal MXene nanosheets (Mo 2 TiC 2 T x ) were used to immobilize single Pt atoms for enhancing HER performance.…”

On a Two-Dimensional MoS2/Mo2CTx Hydrogen Evolution Catalyst Obtained by the Topotactic Sulfurization of Mo2CTx MXene

“…28 During etching, the A layers -Ga in this case -are replaced by terminal groups such as hydroxyl (-OH), oxygen (-O) and/or fluorine (-F) leading to the general formula Mo 2 CT x , where T denotes the MXene terminal groups and x is the number of such groups. 18,29 Among the different MXenes synthesized to date, Mo 2 CT x is one of the most promising for HER. 30,31 The choice of this MXene as HER catalyst support was also made, instead of the more classical carbon, for three reasons: (i) it is more conductive, (ii) it is more electroactive, allowing to use it as co-catalyst and (iii) the MoS 2 /Mo 2 CT x composite can be produced, as shown herein, in a one-step process.…”

Recent Progress in MXene‐Based Materials: Potential High‐Performance Electrocatalysts

“…It has been found that MXene stoichiometry and atomic surface structure are critical for catalytic activity while having less impact on operational durability. Kuznetsov et al [51] synthesized Mo 2 CT x :Co MXene phase solid solution with cobalt substituent on molybdenum site and showed excellent HER activity. Through DFT calculation, the improvement of HER activity after cobalt substitution was attributed to the thermodynamics of improved hydrogen bonding interaction on the surface of MXene, thus promoting the release reaction of H 2 .…”

“…Reproduced with permission. [51] Copyright 2019, American Chemical Society. c) Schematic illustration of synthetic strategy of the Co x Mo 2−x C/MXene/NC catalyst.…”