Selective Nitrate Electroreduction to Ammonia on CNT Electrodes with Controllable Interfacial Wettability

The development of electrocatalysts that can efficiently reduce nitrate (NO₃^-) to ammonia (NH₃) has garnered increasing attention due to their potential to reduce carbon emissions and promote environmental protection. Intensive efforts have focused on catalyst development, but a thorough understanding of the effect of the microenvironment around the reactive sites of the catalyst is also crucial to maximize the performance of the electrocatalysts. This study explored an electrocatalytic system that utilized quaternary ammonium surfactants with a range of alkyl chain lengths to modify an electrode made of carbon nanotubes (CNT), with the goal of regulating interfacial wettability toward NO₃^- reduction. Trimethyltetradecylammonium bromide with a moderate alkyl chain length created a very hydrophobic interface, which led to a high selectivity in the production of NH₃ (∼87%). Detailed mechanistic investigations that used operando Fourier-transform infrared (FTIR) spectroscopy and online differential electrochemical mass spectrometry (DEMS) revealed that the construction of a hydrophobic modified CNT played a synergistic role in suppressing a side reaction involving the generation of hydrogen, which would compete with the reduction of NO₃^-. This electrocatalytic system led to a favorable process for the reduction of NO₃^- to NH₃ through a direct electron transfer pathway. Our findings underscore the significance of controlling the hydrophobic surface of electrocatalysts as an effective means to enhance electrochemical performance in aqueous media.