A general synthetic approach for hexagonal phase tungsten nitride composites and their application in the hydrogen evolution reaction

Abstract

Tungsten nitride (WN) materials have shown unique properties in many fields such as energy conversion and storage. Although many studies have been devoted to the synthesis of WN-based materials, the range of materials that can be made is mainly limited to the common cubic β-WN. Herein, we developed a β-to-δ phase transformation approach by introducing a second transition-metal as a general synthetic method to prepare hexagonal δ-WN and its composites. δ-WN was successfully achieved by introducing Co, Ni or Fe from metal–organic frameworks into tungsten precursors. EXAFS and XRD results confirmed the phase transformation from β-WN to δ-WN promoted by metallic Co. The δ-WN exhibited superior performance for the hydrogen evolution reaction (HER) compared to β-WN, i.e. 23-fold increase in TOF. The incorporation of Co into δ-WN further increased its activity; for example, δ-WN/Co_2.45 only required 76 mV overpotential to achieve 10 mA cm⁻². XPS analysis and density functional theory calculations revealed that the enhanced activity was due to the synergistic effect between Co and δ-WN which optimized the free energy of hydrogen adsorption and therefore was beneficial to the HER process. Ni and Fe could also induce the formation of δ-NW, and the as-prepared δ-WN/Ni and δ-WN/Fe both demonstrated satisfactory catalytic activity towards the HER. More importantly, the preparation method introduced in this work may be extended to other metal nitrides or carbides which couldn’t be synthesized previously for a variety of potential applications.