Two-dimensional nitride MXenes have a promising potential in electrochemical reactions due to their high electronic conductivity, high surface area, and tunable surfaces. Among these properties, investigating the surface chemistry presents the most challenge, hindering experimental exploration in the applications of hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Herein, we systematically varied the surface chemistry of Ti₄N₃T_x nitride MXenes by utilizing different delaminating agents such as tetramethylammonium hydroxide (TMAOH), dimethyl sulfoxide (DMSO), water (H₂O), and tetrabutylammonium hydroxide (TBAOH) and measure the HER and ORR electrocatalytic activity and stability in aqueous alkaline media. Findings from FTIR show the presence of hydroxyl and fluoro groups on the Ti₄N₃T_x surface while XPS deconvolution show hydroxyl group being the major component and fluoro group with the least coverage (< 7%), regardless of the delaminating agent. Despite the difference in surface composition, all Ti₄N₃T_x samples initially showed comparable HER activity. However, after activation through chronopotentiometry, the HER performance of each Ti₄N₃T_x material improved differently. Notably, Ti₄N₃T_x-TMAOH demonstrated a 295-mV improvement in the overpotential at -10 mA/cm² and we attribute this to the low -F coverage on the basal plane and the dissolution of surface TiO₂ during electrolysis at cathodic potentials. Moreover, the increased -OH coverage post-HER corroborates previously reported computational findings suggesting -OH as an active site for MXenes during HER. Similarly, while the Ti₄N₃T_x MXene samples also exhibited very close initial ORR activity and selectivity, long-term chronoamperometry revealed a drastic decrease in the ORR performance. This implies that further material investigation is a requisite for Ti₄N₃T_x MXene to be a promising ORR electrocatalyst. Post-characterization XPS revealed a high increase in the -OH coverage which may have negatively affected the ORR performance. These insights offer a foundation for optimizing surface modifications of nitride MXenes for fuel cell and water splitting applications.

Eugenie Marie Pranada

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