Ti2N nitride MXene evokes the Mars-van Krevelen mechanism to achieve high selectivity for nitrogen reduction reaction

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.

Metal Oxynitrides for the Electrocatalytic Reduction of Nitrogen to Ammonia

“…Metal-based catalysts reported in the literature have generally shown poor selectivity and slow ammonia production rates that are below DOE and commercial targets, with few exceptions. Figure shows the NH 3 production rates and Faradaic efficiencies toward ammonia for a variety of metal, metal oxide, metal chalcogenide, metal nitride, and metal oxynitride catalysts. ,− ,,,,− Commercialization and DOE targets are overlaid on Figure a,b. Metal nitride and N-doped porous carbon (CN x ) electrocatalysts tend to have NH 3 production rates that are 1 or 2 orders of magnitude lower than those of metal oxynitride electrocatalysts (i.e., VNiON and VN 0.7 O 0.45 ).…”

“…However, Figure b shows that any advantages in activity from using metal oxynitrides are generally offset by lower Faradaic efficiencies, with metals, metal oxides, metal nitrides, and metal oxynitrides having Faradaic efficiencies under 20%. Two catalysts, Au/CoO x and Ti 2 N MXene, have both relatively high rates and Faradaic efficiencies, but the Faradaic efficiencies are still less than 20%. , Other metal-based catalysts reported to exhibit unusually high Faradaic efficiencies include metals on N-doped carbons such as Mo on holey N-doped graphene (Mo/HNG) (50%), single-atom Fe on N-doped carbon (Fe SA -N-C) (56%), Co on N-doped mesoporous hollow carbon nanofibers (Co-SA/N-SCF) (57%), and Bi nanocrystals (66%) . While the reported Faradaic efficiencies in these works are impressive, these studies lack key NO x controls that would set them apart from other works that have been shown to be false positive results.…”

“…The question of whether the dominant e-NRR reaction mechanism on metal nitrides is facilitated by surface vacancies has been studied for a variety of transition metal nitrides. Experiments and computations predict that e-NRR likely proceeds via an MvK mechanism on metal nitrides, where a surface N vacancy acts as an active site. ,, For example, e-NRR has been shown to proceed via an MvK mechanism on (100) facets of VN, ZrN, NbN, and CrN . The degree to which a surface can use an MvK mechanism and achieve higher activity compared to the corresponding Langmuir–Hinshelwood mechanism depends on the ease with which the surface forms N vacancies.…”

Cu/CuOx In-Plane Heterostructured Nanosheet Arrays with Rich Oxygen Vacancies Enhance Nitrate Electroreduction to Ammonia

“…There is little doubt that nitrogen-containing compounds serve as an indispensable role in the biologic life for living things. , However, nitrogen compounds have been overproduced because of their extensive application, which may cause a series of ecological and environmental problems. There is, in addition, one further point to make that nitrate (NO 3 – ) pollution in natural resources is aggravated by anthropogenic nitrogen inputs, such as overapplication of agricultural fertilizers and discharge of industrial wastewater and municipal sewage. − Therefore, efforts have been made to convert nitrate pollutants into other nitrogen species through biological nitrogen removal, chemical reduction, or electrocatalysis. − An emerging research topic in the treatment of nitrate pollutants is to transform them into value-added chemicals [e.g., ammonia (NH 3 )]. It is universally acknowledged that ammonia is a momentous energy storage species and a clean CO x -free energetic vehicle. , At present, the proverbial Haber–Bosch process is principally applied to produce ammonia for industrial applications, and with that comes massive amounts of energy consumption. − Electrochemical nitrate-to-ammonia conversion has been recognized as an emerging and promising technology. − The electrochemical nitrate reduction to ammonia (NRA) involves eight-electron and nine-proton transfer, which not only significantly reduces the total kinetic rate but also generates other unnecessary byproducts in the reaction process.…”

Ni-Doped Ti3CNTx-Coated Nanoporous Covalent Organic Frameworks to Accelerate Hydrogen Diffusion for Enhanced Hydrogen Evolution

“…The high specific surface area of MXenes means that they have more active sites and the potential for strong adhesion of active catalytic materials . High conductivity is beneficial to charge transfer, and they can be combined with active catalytic materials to build an electronic structure suitable for catalytic reactions . Therefore, MXenes are considered to be a kind of electrocatalytic substrate with low cost and great potential in hydrogen evolution reaction (HER) .…”