Hydrogen Evolution and Oxygen Reduction on OH/F-Terminated Titanium Nitride MXene Reveal the Role of the Surface Termination Group in Electrocatalysis

Abstract

MXenes, a class of two-dimensional (2D) carbides and/or nitrides, are increasingly utilized in various electrochemical reduction reactions owing to their electronic conductivity, specific surface area, and tunable surface chemistry. Previous studies have indicated that the performance of MXenes in catalyzing the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is influenced by their surface termination groups. However, our understanding of how these groups affect electrocatalytic performance remains limited, especially for nitride MXenes. This work investigates the effects of termination group modification on the HER and ORR activity of Ti₄N₃ nitride MXene in alkaline media. Ti₄N₃ MXene was synthesized via oxygen-assisted molten salt fluoride etching and delaminated using different solvents, including tetramethylammonium hydroxide (TMAOH), dimethyl sulfoxide (DMSO), water (H₂O), and tetrabutylammonium hydroxide (TBAOH). Characterization through FTIR, EDS, and XPS revealed that all delaminated MXenes have hydroxyl and fluoro terminations, with the former being the predominant group. Among the samples, Ti₄N₃ delaminated with TBAOH (referred to as Ti₄N₃-TBAOH) had the highest −OH surface coverage. While the initial HER activity was comparable for all the nitride samples, we observed different onset and overpotentials after activation through chronopotentiometry, likely due to the removal of the passivation layer and the consequent increase in the −OH surface coverage. Ti₄N₃-TMAOH demonstrated the highest improvement, with a nearly 300-mV decrease in the overpotential at −10 mA/cm². For the ORR activity, all OH-terminated Ti₄N₃ MXenes exhibited very similar onset and half-wave potentials despite having different surface coverages. Overall, our results show that while varying the delamination agent alters the coverage of OH/F functional groups, it does not significantly affect the overall catalytic performance, which offers flexibility when preparing nitride MXenes for these applications. These insights provide an experimental basis to further exploration of surface modification of nitride MXenes for fuel cell and water splitting applications.