(Digital Presentation) In-Situ Spectroelectrochemistry to Unveil the Mechanism of Green Ammonia Production on the Ti2n Nitride Mxene

Abstract

Electrochemical nitrogen reduction reaction (NRR) is used to convert nitrogen (N2) to ammonia (NH3) at ambient temperature and pressure. In general, acidic electrolytes are used to provide protons (H+) for the reduction process. However, this leads to a low NH3 selectivity because of H2 by-product formation via the hydrogen evolution reaction (HER). Recently, we showed that a Ti2N MXene is active and selective for NRR, and provided preliminary evidence that this catalyst works through a Mars-van Krevelen (MvK) mechanism rather than conventional associative/dissociative mechanisms. To improve the selectivity of the NRR process, the electrolyte was shifted from acidic to neutral pH values. From this, it was seen that NRR in 0.1M Na2SO4 was able to achieve a selectivity significantly greater than that of 0.1M HCl. We also observed that at more cathodic potentials, the material begins to irreversibly decompose into TiO2 as evidenced by cyclic voltammetry analysis before and after chronoamperometry experiments. To further understand the performance of this material, we use in-situ/operando techniques, including X-ray absorption spectroscopy (XAS) and Fourier-transform infrared (FTIR) spectroscopy, to deconvolute the mechanism of NH3 production. We use XAS to track the Ti oxidation state shift and degree of shift as a function of reaction time and applied potential. We also use XAS to track the change in the atomic bond lengths at the local coordination environment of the Ti atoms throughout time and voltage. Furthermore, we use FTIR to elucidate in real-time the adsorption of NRR reactive species and intermediates on the MXene surface. All findings are further corroborated with density functional theory (DFT) calculations to track the reaction energy pathway over each intermediate. We plan to expand these findings to other materials and systems, and use the knowledge obtained from these studies to design optimal electrolytic conditions to produce NH3 through NRR.