Bacterial cellulose (BC), an eco-friendly bio-product obtained from fermentation of various microorganism, consisting of the interconnected networks structure attracted widespread interest due to its unique physical properties, including the large specific surface area, remarkable mechanical strength, high water-holding ability, good chemical stability, and environmental benign material. These advantages enable BC to be applied to fabricate the highly versatile three-dimensional (3D) carbon nanomaterials, and tunable flexible scaffold to support other multifunctional materials. In this review, the production process of various carbon nanofibrous composites based on BC, such as carbon nanofiber (CNF), doped CNF, CNF/metal oxide and CNF/conducting polymer, is presented. Their emerging applications in supercapacitors are illustrated, in particularly, the design of hybrid bendable electrodes based on BC substrate for flexible supercapacitor is highlighted. The challenges and opportunities in this fascinating area of designing functional nanomaterials and flexible electrode from BC for various energy storage are addressed. Moreover, the perspectives are given for the future development, including several significant kinds of study for applications in the rechargeable battery.

A new inorganic/organic composite film PEDOT:Ce@TiO₂ was prepared through drop casting-secondary polymerization method using the mixture solution of PEDOT oligomer and cerium-containing polyoxotitanate cage [Ti₈O₇(HOEt)(OEt)₂₁Ce]. PEDOT:Ce@TiO₂ exhibits a special nanostructure with gully-like rough surface, which shows strong hydrophobicity while good wettability to acetonitrile solution. PEDOT:Ce@TiO₂ can be used as cathodically electrochromic film and supercapacitor electrode material. The mass specific capacitance of PEDOT:Ce@TiO₂ is 71.2 F/g at a current density of 1 A/g, which is 1.7 times higher than that of the PEDOT film. An all-solid-state electrochromic supercapacitor prototype device was further assembled using PEDOT:Ce@TiO₂. With charging completed the electrochromic area of this device shows dark green, and it turns bright yellow with discharging completed.

Recently, a new type of 2D transition metal carbides or nitrides (MXene) has attracted wide attention due to its large specific surface area, good hydrophilicity, metallic conductivity and other physical and chemical properties. 2D Ti₃C₂T_x MXene was obtained by etching Al layer of Ti₃AlC₂ with LiF and HCl and then mechanically delaminated. And the monolayer Ti₃C₂T_x nanosheets with lateral dimension of 625 and 2562 nm can be prepared by changing the intensity and way of mechanically delamination, as well as the centrifugation rate and time. Then their morphology, structure, composition, and electrochemical performance of Ti₃C₂T_x were studied. The results showed that the specific capacitance of Ti₃C₂T_x with smaller lateral size (<1 μm) can reach 561.9 F/g, higher than that of reported graphene, carbon tube and MnO₂ in the repotted literatures. And the Ti₃C₂T_x electrode still remained 96% of the initial specific capacitance after 10 ⁴ testing cycles.

As a new class of two-dimensional transition metal carbon/nitride, MXenes have been proven to be a kind of pseudocapacitive supercapacitor electrode materials with excellent electrochemical property, and hold promise in practical use in the near future. In practical applications, it is required to make the electrode materials into planar porous electrodes for capacitor assembly. Herein, a simultaneous ammonization/carbonization method is proposed for the preparation of MXene planar porous electrode. Filter paper was used as a planar porous template, and MXene was coated on the fibers of the filter paper by means of dipping-drying, and then heat-treated in an ammonia atmosphere to obtain MXene/carbon planar porous composite electrodes. Analysis results show that the MXene nanosheets are uniformly coated on the carbonization-derived carbon fibers of the filter paper. When immersed 5 times, the areal capacitance reaches 403 mF/cm ² at a scan rate of 2 mV/s. After the composite electrode was tested for 2500 times in a galvanostatic charge-discharge cycle at a current density of 10 mA/cm ², the capacitance was almost the same as the initial capacitance, showing good rate performance and cycle stability. The MXene/carbon planar porous composite electrodes prepared by simultaneous ammonia/carbonization exhibit excellent electrochemical performance without using either polymer binder or metal current collector.

Ti₃C₂T_x@MnO₂ composites with different morphologies were prepared by liquid-phase coprecipitation and hydrothermal method using Ti₃C₂T_x@PDA as matrix, KMnO₄ as manganese source, CTAB or PEG as surfactant. Effect of MnO₂ morphology (δ-MnO₂ nanofragments, α-MnO₂ nanorods, α-MnO₂ nanoflowers and α-MnO₂ nanowires) on phase structure and electrochemical performance of Ti₃C₂T_x was analyzed by FE-SEM, XRD, Raman, FT-IR, BET, and electrochemical measurements. The results show that Ti₃C₂T_x@α-MnO₂ nanowires possesses better comprehensive electrochemical properties (340.9 F?g ^-1 at 2 mV?s ^-1), nearly 2.5 times higher than using CTAB, and smaller charge transfer resistance, as well as excellent cycle stability.

MXenes—two-dimensional (2D) compounds generated from layered bulk materials, have attracted significant attention in energy storage fields. However, low mass loading of MXenes results in low areal capacity and impedes the practical use of MXenes electrodes. Inspired by natural basswood, an ideal architecture with natural, three-dimensionally (3D) aligned open microchannels was developed for high Ti₃C₂T_x mass loading. Compared with reported Ti₃C₂T_x electrode structure, the 3D porous carbon matrix has several advantages including low tortuosity, high conductivity and good structure stability. The Ti₃C₂T_x assembled with the wood carbon can deliver a high areal capacity of 1983 mF/cm ² at 2 mV/s with a high Ti₃C₂T_x mass loading of 17.9 mg/cm ² when used as electrode for supercapacitors. This work provides a new strategy to develop 3D porous electrodes for MXenes, which can achieve high areal capacity.

Bio-source materials have attracted wide attention due to their rich sources, recyclability, pollution-free and multi-functional properties. Herein, a lithium-sulfur battery interlayer was obtained by slicing the birch at different angles, following with the delignification and carbonization processes. Biochar containing lots of micropores and mesopores shows a specific surface area up to 267.7 m ²/g. The biochar interlayer sawing along 45° achieved the highest electrochemical performances. Based on this biochar interlayer, lithium-sulfur battery exhibits a high initial capacity of 979.4 mAh/g at 0.2C (1C=1650 mA/g) and retains a discharge specific capacity of 625.4 mAh/g after 200 cycles, which degrades only 0.18% per cycle. Furthermore, the hierarchical microchannel and transpiration of birch effectively adsorb polysulfide and reduce the shuttle effect, suggesting that it can be widely employed in commercial lithium sulfur battery.