A high-rate graphene-based supercapacitor is very attractive for the practical application of graphene. Here, we first synthesized the films of the hybrids of biomass cellulose and large literal sheet sizes and weakly defective graphene flakes reaching high thermal conductivity and then converted them into hierarchical porous graphene carbon materials reaching superior supercapacity. The interconnected porous carbon framework, with macroporous walls sandwiched by micro/mesoporous activated carbon covering graphene flakes, was synthesized by template-free low-temperature activation of the cellulose/graphene hybrids at 650°C. The graphene flakes could probably assist both the decrease in the temperature of the chemical activation of cellulose and the formation of the hierarchical carbon pores without destroying their sp 2 bonds. The porous graphene carbon-based supercapacitors exhibit a reversible specific capacitance of ∼300 F/g and ultrahigh energy storage performance of 67 Wh/kg, 54 Wh/L, and 60 kW/kg over a 45 s discharge time. ■ INTRODUCTIONHigh-energy density storage and fast response supercapacitors are needed to serve the people to keep up with the high pace of modern life. 1−6 A hierarchical porous carbon framework with micro-, meso-, and macropores can be made into desirable electrodes for high-performance supercapacitors. 7−10 The macropores can work as a fast buffering reservoir for electrolytes, minimizing the diffusion distance of the ions and electrolytes from each pore, while the meso-and micropores provide a large accessible surface area for ion transport and charge accommodation. 11,12 Recently, porous graphene-based composites have received intense attention because the flat open atomic structure of graphene allows ions and electrolytes fast access to its surface with the result being a fast charging or discharging rate for energy storage. 13−17 On the other hand, although the theoretical specific surface area of a single graphene sheet is 2630 m 2 /g, experimentally accessible surface areas of graphene materials are far below this value because of the strong self-aggregation/stacking tendency of graphene flakes (GFs). To prevent the aggregation, many scientists and engineers are trying to design a three-dimensional (3D) framework, including converting flat flexible two-dimensional (2D) into 3D structure or making activated carbon and graphene hybrids. 18−22 Recently, Zhu and his co-workers 23 reported that reduced graphene oxide activated KOH at 800°C to yield a special 3D activated carbon analogue with a large surface area of >3000 m 2 /g as the electrode in a two-electrode symmetrical supercapacitor with excellent electrochemical performance. More recently, graphene oxide and polymer were also activated to produce 3D porous carbon with a large surface area and high specific capacity. 24,25 However, in most cases, (reduced) graphene oxides were used as a starting material, which remains costly and is not competitive with commercial activated carbon. In addition, (reduced) graphene oxides were c...

The rapid development of many emerging technologies (e.g., electric vehicles and smart grids) requires advanced energy storage and conversion systems that have higher energy and power density, longer operational life, and better safety. A low‐cost, green, and sustainable process for fabrication of all‐solid‐state asymmetric supercapacitors (ASC) composed of a hierarchically porous carbonized wood (CW) anode, a cellulose paper separator, and a Co(OH)2@CW cathode is reported here. The hierarchically porous wood‐derived electrode exhibits a high areal capacitance of 3.723 F cm−2 (with an areal loading Co(OH)2 of 5.7 mg cm−2) at a current density 1.0 mA cm−2, and 1.568 F cm−2 at a current density of 30 mA cm−2. Moreover, the all‐solid‐state ASC exhibits outstanding energy density of 0.69 mWh cm−2 (10.87 Wh kg−1) at power density of 1.126 W cm−2 (17.75 W kg−1) while maintaining a capacitance retention of 85% after 10 000 continuous charge–discharge cycles. The high energy/power‐densities are attributed to the unique architecture of the electrodes derived from natural wood, which allow full exposure of active electrode materials, efficient current collection, and fast ion transport. Further, the materials are renewable, environmentally friendly, and biodegradable.

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Over the past decade, wood‐derived materials have attracted enormous interest for both fundamental research and practical applications in various functional devices. In addition to being renewable, environmentally benign, naturally abundant, and biodegradable, wood‐derived materials have several unique advantages, including hierarchically porous structures, excellent mechanical flexibility and integrity, and tunable multifunctionality, making them ideally suited for efficient energy storage and conversion. In this article, the latest advances in the development of wood‐derived materials are discussed for electrochemical energy storage systems and devices (e.g., supercapacitors and rechargeable batteries), highlighting their micro/nanostructures, strategies for tailoring the structures and morphologies, as well as their impact on electrochemical performance (energy and power density and long‐term durability). Furthermore, the scientific and technical challenges, together with new directions of future research in this exciting field, are also outlined for electrochemical energy storage applications.