Oxygen-terminated Ti3C2Tx MXene via electrophoretic functionalization for electrochemical nitrate reduction to ammonia

Abstract

The electrochemical nitrate reduction reaction (eNO3RR) is a promising method for efficiently producing ammonia (NH3), which is a carbon-free fuel and hydrogen carrier. However, eNO3RR suffers from low NH3 selectivity due to the sluggish protonation steps (*NH2 → *NH3), widely regarded as the rate-determining step (RDS), competing with N2 formation. Ti3C2Tx MXene (MX) has emerged as a potential electrocatalyst for eNO3RR due to its excellent NO3− adsorption capability, high conductivity, and large surface area. Despite these advantages, MX exhibits poor catalytic activity in practice, primarily due to the non-uniform distribution of various terminal groups (H, O, OH, F, and Cl), which significantly affect the initial NO3− activation and subsequent protonation steps. To overcome this limitation, we introduce oxygen-terminated Ti3C2Tx MXene (o-MX) by replacing inactive fluorine groups with oxygen groups via a controlled electrophoresis process with optimized duration. The successful substitution of terminal groups was confirmed through XPS, Raman, and FT-IR analyses, revealing a significant increase in oxygen terminations without structural degradation of MX. As a result, the o-MX exhibited significantly enhanced catalytic activity, achieving a maximum faradaic efficiency of 92.8% and an NH3 production rate of 3.278 mg h−1 cm−2 at −0.85 VRHE. This study demonstrates the potential of functionalized MX as an active electrocatalyst without any addition of metal and highlights that surface termination engineering can be broadly applied to other electrocatalytic reactions beyond eNO3RR.