Graphene Foam Cantilever Produced via Simultaneous Foaming and Doping Effect of an Organic Coagulant

Sub-Second Joule-Heated RuO2-Decorated Nitrogen- and Sulfur-Doped Graphene Fibers for Flexible Fiber-type Supercapacitors

Graphene-based fiber-shaped supercapacitors (FSSCs) have received considerable attention as potential wearable energy storage devices owing to their simple operating mechanism, flexibility, and long-term stability. To date, energy storage capacities of supercapacitors have been significantly improved via strategies such as heteroatom doping and the incorporation of pseudocapacitive metal oxides. Herein, we develop a novel and scalable direct-hybridization method that combines heteroatom doping and metal oxide hybridization for the fabrication of high-performance FSSCs. Using porous and highly conductive nitrogen and sulfur co-doped graphene fibers (NS-GFs) as self-heating units, we successfully convert ruthenium hydroxide anchored to the surface into ruthenium oxide nanoparticles after programmed sub-second electrothermal annealing without structural damage of the fibers. The resulting fibers show an increased gravimetric capacitance of 68.88 F g^-1 compared to that of the pristine NS-GF (8.32 F g^-1), excellent cyclic stability maintaining 96.67% of the initial capacitance after 20 000 continuous charging/discharging cycles, and good mechanical flexibility. The findings of this work advocate a successful Joule heating strategy for preparing high-performance graphene-based metal oxide hybrid FSSCs for use in energy storage applications.

Cold-Resistant Nitrogen/Sulfur Dual-Doped Graphene Fiber Supercapacitors with Solar-Thermal Energy Conversion Effect

Although graphene fiber-based supercapacitors are promising for wearable electronic devices, the low energy density of electrodes and poor cold resistance of aqueous electrolytes limit their wide application in cold environments. Herein, porous nitrogen/sulfur dual-doped graphene fibers (NS-GFs) are synthesized by hydrothermal self-assembly followed by thermal annealing, exhibiting an excellent capacitive performance of 401 F cm^-3 at 400 mA cm^-3 because of the synergistic effect of heteroatom dual-doping. The assembled symmetric all-solid-state supercapacitor with polyvinyl alcohol/H₂ SO₄ /graphene oxide gel electrolyte exhibits a high capacitance of 221 F cm^-3 and a high energy density of 7.7 mWh cm^-3 at 80 mA cm^-3 . Interestingly, solar-thermal energy conversion of the electrolyte with 0.1 wt % graphene oxide extends the operating temperature range of the supercapacitor to 0 °C. Furthermore, the photocatalysis effect of the dual-doped heteroatoms increases the capacitance of NS-GFs. At an ambient temperature of 0 °C, the capacitance increases from 0 to 182 F cm^-3 under 1 sun irradiation because of the excellent solar light absorption and efficient solar-thermal energy conversion of graphene oxide, preventing the aqueous electrolyte from freezing. The flexible supercapacitor exhibits a long cycle life, good bending resistance, reliable scalability, and ability to power visual electronics, showing great potential for outdoor electronics in cold environments.