Recent advancements in carbon fiber-based sustainable electrodes for flexible and wearable supercapacitors

Electrochemical energy storage devices such as rechargeable batteries and supercapacitors have replaced conventional batteries and dielectric capacitors owing to their excellent charge storage capabilities. Among them, supercapacitors (SCs) are excellent owing to their high-power density and ability to deliver high-power on demand within a fraction of a second. Furthermore, SCs utilize water-based electrolytes, and hence they are safe and reliable energy storage devices for application in portable and wearable electronic devices. However, a major challenge in the fabrication of flexible and wearable SCs is the rigidity of their electrodes due to the use of rigid metallic current collectors, hindering the successful implementation of SCs to power commercial wearable electronic devices. Thus, the flexibility of SCs is mainly attributed to their electrodes, and hence their preparation is crucial. In this review, we present the facile fabrication of SCs using carbon fibers (CFs) including carbon microfibers and carbon nanofibers. CFs are a sustainable environment-friendly material that can be employed for the fabrication of electrochemical energy storage devices. CFs function as both the electrode-active material and current collector during the fabrication of SCs. However, a major bottleneck in the use of CFs as electrode-active materials in SCs is their low specific capacitance. In this case, the specific capacitance of CF-based SCs can be enhanced via the preparation of hybrid or nanocomposite electrodes by combining CFs with other high-performing electrode-active materials such as electronically conducting polymers, nanocarbons, MXenes, and transition metal oxides. We provide a detailed discussion on various strategies adopted for the synthesis of CF-based hybrid/nanocomposite flexible electrodes for application in SCs. Furthermore, the evaluation of the electrochemical performance of CF-based SC electrodes is reviewed, with emphasis on their flexible and wearable features. This review will give readers an in-depth insight into the preparation of sustainable CF-based flexible electrodes for application in next-generation wearable SCs.