Transition Metal Nitrides: Activity Origin, Synthesis and Electrocatalytic Applications

Abstract

Currently, because of the worldwide over-exploitation and consumption of fossil fuels, energy crisis and environmental pollution are becoming more prominent. Hence, the production and utilization of clean energy such as hydrogen are crucial. As significant electrochemical reactions in energy conversion devices, the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR) have garnered considerable attention. However, the sluggish kinetics of these reactions, especially of the OER and ORR because of the multiple electron transfer steps, and the inevitable usage of noble metal catalysts (such as those based on Pt for HER/ORR and Ru/Ir for HER/OER) are the bottlenecks to realizing energy conversion devices, including overall water-splitting electrolyzers, fuel cells, and metal-air batteries. Therefore, the (Figure presented.)development of efficient non-precious metal catalysts is imperative. Transition metal nitrides (TMNs) have been recently studied and shown to exhibit high catalytic activity because of their ability to alter the electronic structure of host metals, specifically the downshift of the d-band center, the contraction of the filled state, and the broadening of the unfilled state. This high activity is attributed to the optimization of the adsorption energy between metals and adsorbates. In addition, metallic bonding in TMNs increases the conductivity of the catalysts. Thus, in this review, we focus on the latest developments in TMNs and their application as high-activity and high-stability electrocatalysts for water splitting and in fuel cells and zinc-air batteries. First, the origin of the high activity of TMNs is explained with the help of the d-band theory. The effect of nitrogen on TMNs, such as in terms of the location in the crystal structure, is briefly discussed. The preparation strategies for TMNs, including physical and chemical methods as well as the modification techniques such as doping, changing carrier properties, and defect construction, are outlined. Next, we summarize the applications of TMNs as an electrocatalyst for the HER, OER, and ORR. At the same time, to explain the bifunctional catalytic activity of TMNs, we discuss the modification strategies for single-metal-based nitrides, such as doping with other highly active atoms to adjust the electronic structure and increase the catalytic activities as well as using coupling materials with different catalytic selectivities to construct heterostructures. Finally, we discuss the challenges and development approaches for realizing the electrocatalytic applications of TMNs, such as through further improvement in catalytic activity, and for facilitating in-depth understanding of electrocatalytic processes through in situ characterization to reveal the electrocatalytic mechanism of TMNs. Undoubtedly, this review will promote the application of TMNs in the field of electrocatalysis.