Abstract
We address the low selectivity problem faced by the electrochemical nitrogen (N2) reduction reaction (NRR) to ammonia (NH3) by exploiting the Mars-van Krevelen (MvK) mechanism on two-dimensional (2D) Ti2N nitride MXene. NRR technology is a viable alternative to reducing the energy and greenhouse gas emission footprint from NH3 production. Most NRR catalysts operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. The MvK mechanism reduces this competition by eliminating the adsorption and dissociation processes at the sites for NH3 synthesis. We show that the new class of 2D materials, nitride MXenes, evoke the MvK mechanism to achieve the highest Faradaic efficiency (FE) towards NH3 reported for any pristine transition metal-based catalyst—19.85% with a yield of 11.33 μg/cm2/hr at an applied potential of − 250 mV versus RHE. These results can be expanded to a broad class of systems evoking the MvK mechanism and constitute the foundation of NRR technology based on MXenes.
Highlights
We address the low selectivity problem faced by the electrochemical nitrogen (N2) reduction reaction (NRR) to ammonia (NH3) by exploiting the Mars-van Krevelen (MvK) mechanism on two-dimensional (2D) Ti2N nitride MXene
It’s imperative to understand the morphological changes after the synthesis prior to the nitrogen reduction reaction (NRR) characterization, as these can affect the MvK mechanism
Successful removal of the fluoride salts via sulfuric acid washing is evidenced by the thinning of the individual layers (Fig. 1e purple outline), consistent with the X-ray diffraction (XRD) results (Fig. 1f), and the low amount of sodium and potassium present in the electron dispersive spectroscopy (EDS) analysis (Fig. S1, supplementary information)
Summary
The two main drawbacks to the HB process are (1) the high temperature and pressure necessary for the reaction to take place, which causes a large energy footprint for the process; (2) the majority of the hydrogen feedstock comes from non-renewable sources such as steam reforming of natural g as[4] This results in an environmentally damaging process where approximately 2 equivalents of carbon dioxide ( CO2) are released into the atmosphere for every 1 equivalent of NH3 that gets produced[5]. The vacancy that is formed from this dissociation is replenished by gaseous N 2 allowing for the cycle to continue without the need for regeneration time or large energy input to break the N≡N bond[14] For this reason, it is believed that nitride catalysts provide the most favorable pathway for the realization of the NRR technology[20]. There exists no experimental work on nitride MXene NRR or their ability to evoke the MvK mechanism
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