Review and Perspectives of Electrochemical Energy Conversion and Storage in Metal Diborides and XBenes

AlB2-type metal diborides are receiving renewed interest owing to their layered crystal structure. These materials have extraordinary diversity and topology owing to multicentered boron bonds, which impart unique physicochemical properties. Recently, several bottom-up and top-down approaches have been developed to experimentally realize their quasi-two-dimensional (2D) forms, named XBenes. This has added a new dimension to the spectrum of metal diboride research. The electrochemical properties of metal diborides are promising for a variety of energy applications, including lithium-ion batteries and supercapacitors. Their quasi-2D structures further enhance their electrochemical properties, such as high conductivity and large surface area. This makes them promising candidates for next-generation energy storage and conversion devices. In this review, we highlight the electrochemical properties and related energy applications of metal diborides and their quasi-2D structures (XBenes) where boron planes are stitched together by metal atoms, constituting a mixture of covalent and ionic bonding. Diverse examples of bulk metal diborides and their 2D nanostructures are surveyed and discussed, depending on their versatile electrochemical kinetics and reaction pathways. We elucidate the unique advantages offered by quasi-2D metal diborides over bulk metal diborides for the design of electrocatalysts and electrodes due to their high surface area, high density of active sites, and distinctive electrical and structural properties. The future research perspectives for the exploration of quasi-2D forms of electrocatalysts and electrodes are presented. These perspectives provide valuable insights into recent developments in sustainable energy technologies.