Time-Resolved Fourier Transform Infrared Spectroelectrochemical Investigation of Nitrate Reduction to Ammonia

This study explores the electrocatalytic nitrate reduction reaction (NO3–RR) using nitride-based two-dimensional Ti2NTx MXene (also known as MNene) synthesized via O2-assisted molten salt fluoride etching and its parent Ti2AlN MAX phase. Ti2NTx MNene achieved an ammonia (NH3) yield rate of ∼550 μmol h–1 g–1 with a Faradaic efficiency (FE) of ∼80%. Unexpectedly, the Ti2AlN MAX phase exhibited an even higher NH3 yield rate of ∼800 μmol h–1 g–1 at a comparable FE, despite its lower surface area and being traditionally considered a poor electrocatalyst. The enhanced performance of the MAX phase is likely due to −OH functionalization under alkaline conditions, leading to enhanced reaction kinetics. Postelectrolysis analyses, including Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), confirmed no significant changes in crystallinity but indicated surface chemical changes. Control experiments with blank electrolytes and isotopically labelled 15NO3– substantiate that NH3 originates exclusively from nitrate reduction on the surface terminations. Time-resolved in situ spectroelectrochemical studies identified nitrite (NO2–) reduction to further intermediates as the rate-determining step. These findings not only challenge the conventional perception of MAX phases as poor electrocatalysts but also underscore the potential of nitride-based MAX and MXene materials as robust and efficient electrocatalysts for the NO3–RR.