Flexible 2D borophene-stacked MXene heterostructure for high-performance supercapacitors

Two-dimensional (2D) materials provide slit-shaped ion diffusion channels that enable the rapid movement of ions. Building high-performance energy storage devices requires electrode materials with enhanced electronic conductivity, quantity of intercalation sites, and stability during prolonged cycling to maximize the energy density, power density, and cycle life of the devices. In this work, we develop the flexible ultrafast supercapacitor derived from conductive 2D MXene/Borophene (MxB) electrodes via an electrophoretic deposition method. The electrostatically self-assembled positively charged Borophene and negatively charged MXene nanosheets effectively avoid MXene nanosheet self-restacking, thus resulting in a significantly larger interlayer gap, as well as reaching more exposed electroactive sites owing to accelerated electrolyte ion diffusion. The freestanding MxB (50:50) electrode exhibits a remarkable rate capability with 85.14% capacitance retention at 20 A·g−1, long-term cycle life, and high gravimetric capacitance of 626.7F·g−1 at 1 A·g−1. The developed supercapacitor displays an exceptional energy density of 75.6 Wh·kg−1, the highest values recorded for carbon- or MXene-based materials in aqueous electrolytes. This work illuminates the design of next-generation flexible, portable, and highly integrated supercapacitor with high volumetric and rate capabilities, while offering basic insight into the impact of interlayer spacing on the electrochemical performance of 2D hybrid materials.