MXenes is a family of 2D nanomaterials constituted by the stacking of alternate one-atom thick sheets of an early transition metal with other of carbide or carbonitride. These materials were reported for the first time in 2011 by Barsoum, Naguib, Gogotsi and coworkers¹ and very soon they become favourite materials for supercapacitors and Li-ion battery cathodes. Due to the unique optoelectronic properties and electrical and thermal conductivity, MXenes have also found application as electro- and photocatalysts.² This presentation will describe some of the results that have been recently obtained in our group on the use of MXenes in these two fields.

Regarding electrocatalysis, it will be shown the preparation from commercially available Ti₃AlC₂ in a single step, a Ni-Fe alloy strongly anchored on Ti₃C₂ support using the Lewis acid molten salt (Fig. 1).³ By changing the NiCl₂ to FeCl₂ proportion in the NaCl/KCl eutectic mixture, it is possible to obtain various Ni/Fe atomic ratios. These Ni_xFe_y/Ti₃C₂ samples were tested as anodes for the electrocatalytic oxygen evolution reaction (OER) at neutral pH that is a process occurring in water electrolyzers for hydrogen production. It was found that the Ni₁Fe₁/Ti₃C₂ sample having Ni/Fe atomic ratio of 1 was the best performing OER material exhibiting an overpotential of 310 mV at 10 mA cm^-2, and a Tafel slope of 48 mV dec⁻¹ (Fig. 1).³ DFT calculations suggest that the adsorbate evolution mechanism is more likely than the lattice oxygen mechanism when either Ni or Fe act as the active centers, with higher activity on the Ni sites.

Ni supported on Ti₃C₂ was also used as photothermal catalyst to perform the CO₂ hydrogenation to CH₄ and CH₃OH.⁴ CH₄ and CH₃OH  can be considered as hydrogen carriers and specifically CH₃OH is most wanted liquid organic hydrogen carrier given the high H₂ content and its liquid state at ambient temperature. In this case, it was found that the thin oxide layer of NiO over the core Ni nanopatches establishes a S-heterojunction with the Ti₃C₂ MXene as evidenced by the in situ irradiated XPS (Fig. 2). Fs-TAS reveals an ultrafast charge transfer dynamics in the heterojunction. The combination of the photocatalytic and photothermal effects is demonstrated by determining the influence of the temperature on CO₂ conversion, while the influence of the irradiation wavelength is compatible with the occurrence of photoinduced charge separation (Fig. 2). DFT calculations reveal the important role of Ni metal. Ni oxide and MXene on the adsorption of H₂, CO₂ and CH₃OH formation. Overall, the data that will be materials to establish heterojunctions and Schottky barriers with other semiconductors.

The photocatalytic activity of MXene dots for hydrogen generation and overall water splitting will also be presented.