This review highlights recent advances of MXenes and their composites in the environment-related applications including catalysis, water purification and sensors.

Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr₂CO₂ and Hf₂CO₂ as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr₂CO₂ and Hf₂CO₂ is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr₂CO₂ and Hf₂CO₂ in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr₂CO₂ surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.

Exploring new two-dimensional (2D) crystals attracts great interest in the materials community due to their potential intriguing properties. Here, we report a new family of 2D transition metal borides (labeled as MBenes) that can be produced by selectively etching the A layer from a family of layered transition metal borides (MAB phases). The emerged MBenes are demonstrated to possess great stability with isotropic and ultrahigh Young's modulus. Meanwhile, our results show that 2D Mo₂B₂ and Fe₂B₂ MBenes are metallic with excellent electronic conductivity, which are highly desirable for applications in Li-ion batteries (LIB) and electrocatalysis. Furthermore, 2D Mo₂B₂ and Fe₂B₂ are confirmed to have an omnidirectional small diffusion energy barrier and high storage capacity for Li atoms, which highlight MBenes as appealing electrode materials for LIBs. Moreover, 2D Fe₂B₂ MBene also exhibits superior catalytic activity for the hydrogen evolution reaction (HER) with hydrogen adsorption Gibbs free energy close to the optimal value (0 eV), indicating its promising application as an electrocatalyst for hydrogen evolution. Considering the large number of possible MAB phases, more MBenes with attractive applications are anticipated theoretically and/or experimentally in the near future.

Ni@Mo ₂ B ₂ can be used as a HER/OER bifunctional electrocatalyst while Cu@Mo ₂ B ₂ can be used as an OER/ORR bifunctional electrocatalyst.

The prevalence of MXenes, two-dimensional (2D) transition metal carbides/nitrides separated from layered MAX phases, has generated much interest in exploring 2D transition metal borides during the past decade. In 2017, we first reported a new family of 2D transition metal borides as analogues to MXenes and coined a catchy name for them, MBenes. Over the last five years, MBenes have gained more and more attentions in the fields of nanomaterials, physics and chemistry. The promising future of MBenes is foreseeable since they have exhibited many intriguing properties and been widely studied for energy storage and electrocatalysis applications. However, research on MBenes is still in its infancy, with many expected properties and applications awaiting to be explored. In this review, first we made a scientific classification of 2D transition metal borides and proposed an explicit definition of MBenes: a family of 2D transition metal borides with sandwich-like structures which are derived from layered MAB phases. Then we reviewed the progress of synthesis methods, properties and applications of MBenes for energy storage and electrocatalysis. Finally, the discussions on the existing problems in experimental synthesis and theoretical calculations along with the perspectives and prospects of MBenes were presented.