Recent progress in carbon fiber electrodes for structural supercapacitors composites

The energy crisis and the continued deterioration of ecological environment pose a huge challenge to the sustainable development of the mankind. Energy conservation and emission reduction have become challenging goals in fields of automobiles, railway transportation and aerospace. The development of advanced lightweight composite structures and high-performance energy storage systems have become a research focus in various countries. Moreover, modern equipments have also put forward technological requirements for lightweight, integrated, and multifunctional energy storage systems. A new concept of "stealth" of energy storage systems, namely, the integration of energy storage systems with structural components, attracts extensive attentions from academia and industry.Inspired by the energy storage principle of double layer supercapacitors and the lightweight and the laminated structure of carbon fiber reinforced resin composite, a kind of structural supercapacitor composites (SSC) with both energy storage and structural bearing capacity are developed by using carbon fiber (with excellent mechanical and electrical conductivity properties) as multifunctional structural electrode reinforcements. It is expected to simultaneously meet the dual needs of equipment systems for structural lightweight and highly efficient energy storage, improve the integration and system efficiency of equipment systems, and thus have broad application prospects in high-tech fields such as new energy vehicles and aerospace. It is currently a research hotspot in the area of advanced composite materials and energy storage materials, and therefore also has received widespread attention from domestic and foreign researchers.Carbon fiber structural electrode is an important component of SSC, which is responsible for the charge accumulation and mechanical loading, and is also one of the key factors affecting the energy storage performance and structural loading performance of SSCs. Therefore, carbon fiber structural electrode should have high specific surface area, porous structures, excellent mechanical properties, and good wettability with polymer electrolyte. However, the conventional commercial carbon fibers have the disadvantage of smooth surface, low specific surface area, high surface chemical inertness and insufficient charge storage space, which is not conducive to ion storage and resin electrolyte infiltration in carbon fiber structural electrodes, and thus limits the preparation and application of high-performance SSC. Therefore, how to construct the porous structures with high specific surface area on the carbon fiber surfaces and improve the specific capacitance of carbon fiber electrodes without deteriorating the structural integrity and mechanical properties of carbon fiber, is one of the key challenges that is urgently need to be overcome in the current research of SSC.In recent years, domestic and foreign researchers have developed many methods such as etching activation and surface deposition of carbon based active materials to address the above challenges. The specific capacities of carbon fiber electrodes have been improved by increasing the specific surface area of carbon fiber structural electrodes. Moreover, nano metal compounds, polyaniline and other pseudo capacitive electrode materials are also assembled on the surfaces of carbon fiber electrodes, and a series of high-performance carbon fiber structural electrodes with both pseudo capacitive behavior have been developed. This review paper introduces the current research status of carbon fiber structural electrodes for SSC, mainly focuses on the impact of different modification methods on the electrochemical and mechanical properties of carbon fiber structural electrodes and the resultant SSC, as well as the corresponding mechanisms. It systematically compares the advantages and disadvantages of various modification methods and the differences in the development level of SSC at home and abroad.Finally, the challenges and development trends of carbon fiber structural electrodes for SSC are further prospected, and several key areas that still need to be broken through are proposed as follows: (1) Improving the capacitance performance of carbon fiber electrodes while avoiding the damage to its structural integrity. (2) Constructing a hierarchical structure on the surfaces of carbon fiber electrodes to better match with viscous polymer electrolytes; (3) Exploring new methods for the preparation of heteroatom doping carbon materials to improve the energy storage performance of carbon fiber electrodes; (4) Developing new and efficient nano metal compound active materials to improve the energy storage performance and cycling stability of carbon fiber electrodes.