(Digital Presentation) Achieving High Selectivity for the Nitrogen Reduction Reaction through the Mars-Van Krevelen Mechanism

Abstract

Electrochemical nitrogen reduction reaction (NRR) technology is a viable alternative to reducing the energy and greenhouse gas footprint from the current ammonia (NH3) production technology. Most NRR catalysts suffer from low selectivity towards NH3 because they operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER). In this presentation, we report on a new catalyst and untapped mechanism for NRR to increase the selectivity towards NH3. This untapped Mars-van Krevelen (MvK) mechanism reduces the competition between NRR and HER by eliminating the sluggish hydrogenation reactions of the dissolved N2 molecule. The new catalyst, two-dimensional (2D) Ti2N nitride MXene, was synthesized via an oxygen-assisted molten salt fluoride etching technique. We confirmed its phase purity and stability in aqueous electrolytes using various characterization techniques, including x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and cyclic voltammetry (CV). Through an MvK mechanism, the Ti2N nitride MXene catalyst achieved a high Faradaic efficiency (FE) of 19.85% towards NH3 at an applied potential of –250 mV vs. RHE with a yield of 11.33 μg/cm2/hr in a 0.1M hydrochloric acid (HCl) N2-saturated electrolyte. These results constitute the foundation of NRR technology based on MXenes and can be expanded to a broad class of systems evoking the MvK mechanism.