Fluorine-free strategy for green synthesis of 2D layered Co-boride (CoB) nanosheets with ultra-high energy density in supercapacitors and exceptional stability

Abstract

Two-dimensional (2D) metal boride (MBene) exhibit energy storage capabilities owing to their high specific surface area, tunable interlayer spacing, and electrical conductivity. Traditionally, the interlayer spacing in 2D metal borides is achieved by selectively etching the aluminum (Al) layers from layered transition metal borides (3D MAB phases) using hydrothermal chemical etching. Here, we report the first-ever synthesis of 2D Cobalt boride (CoB) utilizing a fluoride-free etching method with sodium hydroxide (NaOH) as the etchant. The successful synthesis is achieved through annealing followed by pro-longed ultrasonication and hydrothermal technique, which results in 2D cabbage-like layered porous nanosheet morphology as confirmed by scanning electron microscopy (SEM). Structural and compositional analyses using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) validate the removal of aluminum layers and formation of 2D CoB. Electrochemical measurements reveal that the 2D CoB has an areal capacitance of 692 mF·cm⁻² at a current density of 1 mA·cm⁻² within an optimized potential window of −0.3 to 0.45 V in 1 M H₂SO₄ supporting electrolyte. Further, an asymmetric device (CoB//Gr) operates within a wide voltage window of 1.5 V demonstrating an energy density of 84 μWh·cm⁻² at 1405 μW·cm⁻². Moreover, it demonstrates cycle life, retaining nearly 93.3 % of its initial capacity after 20,000 long cycles. These findings establish layered CoB as a promising material for high-performance energy storage, addressing the demand for efficient and sustainable energy solutions.