As a type of energy storage device between traditional capacitors and batteries, the supercapacitor has the advantages of energy saving and environmental protection, high power density, fast charging and discharging speed, long cycle life, and so forth. One of the key factors affecting the performance of supercapacitor is the electrode material. Carbon materials, such as carbon nanotube, graphene, activated carbon, and carbon nanocage, are most widely concerned in the application of supercapacitors. The synergistic effect of composites can often obtain excellent results, which is one of the common strategies to increase the electrochemical performance of supercapacitors. To further improve the performance of binary composites, it is a relatively simple method to increase the components as the “bridge” between the two materials to form the ternary composites. The review mainly introduces the current research progress of supercapacitors with pure carbon nanomaterials and multistage carbon nanostructures (composites) as electrodes. The characteristics and application directions of different pure carbon nanomaterials are introduced in detail. Different ways of multilevel structure (material) composite have their own effects on the development of high‐performance supercapacitors. We also highlight the recent advances related to these fields and provide our insight into high‐energy supercapacitors.

Flexible electrodes with superior mechanical and electrochemical properties are essential for flexible supercapacitors. A convenient and scalable colloidal film‐assisted chemical vapor deposition (CF‐CVD) method is developed for the one‐step fabrication of the carbon microspheres films composed of carbon nano‐onions (CMS‐CNO films). The influence of growth conditions (such as growth temperature, time, and gas ratio) during CF‐CVD process on the carbon structures and the growth mechanism of the CMS‐CNO films have been investigated. By controlling the growth conditions, the controllable preparation of CMS‐CNO films is realized. Such binder‐free films can be used for the assembly of flexible supercapacitors, and unique architecture can achieve excellent performance. Benefitting from the composite of nano‐micro zero dimensional structures, the performance of the film in supercapacitors is remarkably improved. At the current density of 5 mA cm⁻², the area‐specific capacity can be 903 mF cm⁻². When the current density is increased to 500 mA cm⁻², the area‐specific capacity can be increased to 729 mF cm⁻². This simple and low‐cost preparation process and the superb electrochemical performance suggest great potential applications of CMS‐CNO films in flexible supercapacitors.

Supercapacitors have become attractive energy storage devices due to their high power density, good cycling stability, and fast charging and discharging speeds. Porous carbon has great specific surface area, high energy density and good conversion performance, so porous carbon as supercapacitor electrode material has been widely concerned. Carbon materials with different dimensions and sizes, such as porous carbon spheres, porous carbon nanotubes, porous carbon nanofibers porous graphene and activated carbon can provide different performance advantages. At the same time, the composite of porous carbon with metal compounds, conductive polymers and particles containing N/P/O/S can further optimize electrode materials, as well as the significant effects on the increase of specific surface area and energy density are obtained. This article introduces the porous carbon materials used as electrode materials in recent years, as well as their multi-level structural materials and related composite materials. We first introduced porous carbon electrode materials with different dimensions and compared their electrochemical performance. Then, based on various research results, the factors affecting its electrochemical performance were discussed in detail. As well as, the preparation methods of porous carbon electrode materials were introduced, and the specific requirements, advantages and disadvantages of different preparation methods were briefly analyzed. The application of porous carbon electrode materials combined with other materials in supercapacitors is listed. Finally, a summary and outlook of the current research status were supplied, providing reference for the rational design of porous carbon supercapacitors in the future.

A convenient and scalable hydrothermal method was developed for the fabrication of the core-branch Fe₂O₃@NiO nanorods arrays directly grown on flexible carbon cloth (denoted as Fe₂O₃@NiO/CC). Such a unique architecture was applied as an electrode of the supercapacitors. As a result, the Fe₂O₃@NiO/CC exhibited a high areal capacitance ~800 mF cm⁻² at 10 mA cm⁻², which was about 10 times increase with respect to Fe₂O₃ nanorods array grown on carbon cloth (Fe₂O₃/CC). The Fe₂O₃@NiO/CC also had the long life cycle (96.8 % capacitance retention after 16,000 cycles) and remarkable rate capability (44.0 % capacitance loss at a very large current density of 100 mA cm⁻²). The superior performance of the Fe₂O₃@NiO/CC should be ascribed to the reduction of the contact resistance and the free-standing structure of the flexible electrode. This study provides a novel strategy to construct high-performance flexible electrode materials with unique core-branch structure by incorporating two different pseudocapacitive materials.

In order to fulfil the requirements of various equipment in different fields for energy storage components, there is an increasing number of studies being conducted on the development of electrode materials for supercapacitors. At present, carbon materials used in electrode components in supercapacitors are mostly graphene, porous carbon, activated carbon, and carbon nanotubes. Due to the fast advancements in modern technology and science, which have stimulated the demand for sustainable and eco-friendly energy storage materials, biomass-derived carbon materials have gradually emerged in the public eye. The carbon source of biomass-derived carbon is usually a natural substance, which has advantages such as low cost, easy availability, and environmental friendliness. Due to the diversity of material structures, the required electrode materials can be designed and prepared according to performance requirements. At the same time, biomass-derived carbon is also capable of being combined alongside various electrode materials to create asymmetric capacitors, combining the advantages of the two to enhance capacitor electrochemical performance. This article first introduces different sources of biomass-derived carbon-based electrode materials and compares their electrochemical performance. Then, based on various research results, the factors that affect their electrochemical performance are discussed in detail. Then, the preparation methods of biomass-derived carbon electrode materials are introduced, and the specific requirements, advantages and disadvantages of different preparation methods are briefly analyzed. The application of biomass-derived carbon electrode materials in supercapacitors in combination with other materials is listed. Finally, a summary and outlook of the current research status are provided to provide a reference for the rational design of biomass carbon supercapacitors in the future.