New two-dimensional transition metal borides for Li ion batteries and electrocatalysis

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.