Synergistic Effects of Nitrogen Doping on MXene for Enhancement of Hydrogen Evolution Reaction

Earth-abundant, nonprecious, and efficient electrocatalysts for effective hydrogen evolution reaction (HER) are crucial for future large-scale green energy production. Low-cost two-dimensional MXenes have been widely studied in energy-storage devices owing to their unique chemical and physical properties and have recently attracted scientists in the electrocatalysis field. Nevertheless, their electrocatalytic activity still remains unsatisfactory. Herein, we present a facile and general strategy using ammonia heat treatment to enhance the hydrogen evolution catalysis of Ti3C2Tx MXenes by modification with a nitrogen heteroatom. Importantly, our approach is focused on revealing: (1) the contribution of all possible incorporated N species including Ti–N, N–H, and N in O–Ti–N, rather than considering only that of Ti–Nx motifs as previously reported for N-doped MXene electrocatalysts, and their role in inducing a change in the electronic configuration of the as-prepared catalysts, which then leads to increased electrical conductivity and improved intrinsic catalytic reactivity; and (2) the importance of controlling the proper amount of N obtained at a suitable calcined temperature to assist the shift of the Gibbs free energy for hydrogen adsorption (ΔHad*) approaching 0 eV (ideal value), as proved by the density functional theory. Moreover, experimental findings indicate that nitrogen-doped Ti3C2Tx annealed at 600 °C shows superior improved HER electrocatalytic performance compared to pristine Ti3C2Tx, with an onset potential of −30 mV and an overpotential as low as 198 at 10 mA cm–2, as well as a much smaller Tafel slope of 92 mV dec–1.