With the rapid development of portable and wearable smart electronic devices, the demand for energy storage devices has become increasingly stringent. Traditional supercapacitors have struggled to meet these evolving requirements. In contrast, flexible supercapacitors—owing to their lightweight, bendability, and excellent cycling stability—have emerged as a new generation of energy storage devices with immense potential. This article reviews recent advancements in the application of carbon fibers within flexible supercapacitors, summarizes the existing challenges faced by this technology, and provides a forward-looking perspective on its future development.

Electrode materials are critical in determining the performance of supercapacitors. Achieving both the flexibility of active electrodes and high energy density simultaneously remains a significant challenge. Among various materials, carbon exhibits superior properties such as hardness, optical characteristics, thermal stability, and excellent conductivity, making it an ideal candidate for use as a foundational electrode material in flexible supercapacitors. In particular, carbon fibers stand out due to their remarkable electrical and thermal conductivity, along with outstanding chemical stability, attracting considerable attention from researchers. By employing diverse structural designs, it becomes possible to better integrate electrodes, electrolytes, and flexible substrates. This approach not only ensures the flexibility of the supercapacitor but also enhances its energy storage capabilities.

Carbon fiber, a new type of fiber material with a carbon content exceeding 95%, exhibits superior corrosion resistance compared to traditional glass fibers. It is lighter in weight than metallic aluminum yet boasts higher strength than steel, combining the characteristics of flexibility on the outside and rigidity on the inside. Fiber-shaped carbon materials, such as carbon cloth and nanofiber paper, simultaneously possess excellent conductivity and flexibility, making them ideal for fabricating electrodes in flexible supercapacitors. However, fiber-based supercapacitors suffer from relatively low energy density, which often limits their practical applications—this limitation also applies to carbon fiber when used as an electrode material.

To enhance energy density, two approaches can be employed: optimizing the working potential window and maximizing the specific capacitance. Utilizing organic or ionic electrolytes can significantly improve conductivity, while assembling asymmetric supercapacitors effectively expands the potential window. MnO₃ boasts a work function as high as 6.9 eV, making it suitable for use as the positive electrode, whereas MnO₂ has a work function of 4.4 eV, ideal for the negative electrode. Given the substantial difference in work functions between the positive and negative electrodes, this disparity can effectively increase the overall voltage of the potential window. To boost specific capacitance, one strategy involves integrating pseudocapacitive materials to induce pseudocapacitive effects, thereby enhancing the electrode material's ability to store charge. Carbon fibers are frequently used as flexible substrates, onto which metal oxides or conductive polymers are deposited and grown, serving as the active electrode materials. Researchers have successfully developed modified nitrogen-doped carbon fiber cloth electrodes that exhibit outstanding electrochemical performance, offering a simple yet effective pathway toward large-scale production of high-performance carbon fiber-based electrode materials. Additionally, researchers have employed an electrodeposition method to grow nickel-cobalt layered double hydroxides directly onto carbon fiber cloth, resulting in an electrode that delivers an impressive specific capacitance of 1540 F/g at a current density of 1 A/g.

The key to flexible supercapacitors lies in electrodes that combine high performance with excellent flexibility. Carbon fibers, with their outstanding physicochemical properties and inherent weaveability, are ideally suited as electrodes for flexible supercapacitors. However, a limitation of carbon fiber electrodes is their relatively low energy density, which can be improved by expanding the electrochemical potential window and enhancing the specific capacitance. Furthermore, the integration of carbon fibers with various metal oxides significantly boosts their energy storage capacity. Additionally, employing organic or ionic electrolytes not only enhances electrical conductivity but also allows for tailored performance based on the unique work functions of different metal oxides. By assembling asymmetric supercapacitors, it becomes possible to further widen the operating voltage range, thereby maximizing energy storage efficiency.

In summary, nano-carbon materials such as carbon fiber hold great potential for application in flexible supercapacitors; however, several issues still require careful consideration and resolution, including:

(1) There is no unified evaluation standard established for the performance of flexible supercapacitors.

(2) Nanocarbon materials are generally expensive, making it currently challenging to apply them in industrial-scale production.

(3) New materials with outstanding electrochemical performance should be continuously explored. The energy storage mechanisms should be analyzed in detail to improve existing materials. Additionally, the structure of supercapacitors can be optimized according to different application scenarios.