Rapid Microwave-Assisted Synthesis of a 2D Borophene-Graphene Composite Embedded in a 3D Porous Hydrogel for Flexible Solid-State Supercapacitors with High Energy Density

Flexible solid-state supercapacitors play a crucial role in meeting the energy storage demands of modern electronics, particularly, portable and wearable devices. This work reports a 2D borophene and graphene composite (BG)-based hydrogel as a high-performance solid-state symmetric supercapacitor. The BG is synthesized via a cost-effective microwave synthesis technique resulting in a distinctive 3D nanosponge-like surface morphology confirmed by scanning electron microscopy. Subsequently, through lyophilization, it yields an interconnected, large surface area, porous structure of the borophene-graphene hydrogel (BGH). The BGH demonstrates a specific capacitance of 455.1 F/g at 1 A/g within a potential window of −1 to 0 V with an aqueous KOH electrolyte. Furthermore, a flexible solid-state symmetric supercapacitor device is constructed using BGH electrodes sandwiched between gel-coated electrolytes comprising poly(vinyl alcohol) (PVA) and potassium hydroxide (KOH). This BGH/PVA-KOH/BGH device delivers a high energy density of 36.77 Wh/kg and a power density of 585.5 W/kg at a current density of 0.5 A/g within a 1.25 V voltage window. Impressively, the device remains highly effective even when bent at a 45° angle, demonstrating excellent mechanical resilience. It also retains 80.8% of its capacitance over 20,000 cycles, highlighting its durability for flexible applications. The excellent performance of the BGH can be accredited to borophene’s outstanding electronic properties in conjunction with graphene’s conductivity and mechanical strength. This synergy results in superior conductivity and mechanical resilience with a reduced risk of degradation, rendering it ideal for flexible energy storage devices. This study marks significant progress in energy storage technology with potential applications in wearable electronics, smart packaging, and low-power sensor systems.