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

Abstract

This study explores the electrocatalytic nitrate reduction reaction (NO₃^–RR) using nitride-based two-dimensional Ti₂NT_x MXene (also known as MNene) synthesized via O₂-assisted molten salt fluoride etching and its parent Ti₂AlN MAX phase. Ti₂NT_x 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.