As wearable electronic devices are becoming an integral part of modern life, there is a vast demand for safe and efficient energy storage devices to power them. While the research and development of microbatteries and supercapacitors (SCs) have significantly progressed, the latter has attracted much attention due to their excellent power density, longevity, and safety. Furthermore, SCs with a 1D fiber shape are preferred because of their ease of integration into today’s smart garments and other wearable devices. Fiber supercapacitors based on carbon nanotubes (CNT) are promising candidates with a unique 1D structure, high electrical and thermal conductivity, outstanding flexibility, excellent mechanical strength, and low gravimetric density. This review aims to serve as a comprehensive publication presenting the fundamentals and recent developments on CNT-fiber-based SCs. The first section gives a general overview of the supercapacitor types based on the charge storage mechanisms and electrode configuration, followed by the various fiber fabrication methods. The next section explores the different strategies used to enhance the electrochemical performance of these SCs, followed by a broad study on their stretchability and multifunctionality. Finally, the review presents the current performance and scalability challenges affecting the CNT-based SCs, highlighting their prospects.

Nitrogen-doped, 3-dimensional graphene (N3DG), synthesized as a one-step thermal CVD process, was further functionalized with atmospheric pressure oxygen plasma. Electrodes were fabricated and tested based on the functionalized N3DG. Their characterization included scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer–Emmet–Teller (BET), and electrochemical measurements. The tested electrodes revealed a 208% increase in the specific capacitance compared to pristine 3D graphene electrodes in a three-electrode configuration. The performed doping and plasma treatment enabled an increase in the electrode‘s surface area by 4 times compared to pristine samples. Furthermore, the XPS results revealed the presence of nitrogen and oxygen functional groups in the doped and functionalized material. Symmetric supercapacitors assembled from the functionalized 3D graphene using aqueous and organic electrolytes were compared for electrochemical performance. The device with ionic electrolyte EMIMB₄ electrolyte exhibited a superior energy density of 54 Wh/kg and power density of 1224 W/kg. It also demonstrated a high-cyclic stability of 15,000 cycles with a capacitance retention of 107%.

Carbon nanotube (CNT) hybrid composites were formed by combining a CNT and silicone elastomer solution with Kevlar yarn, Kevlar fabric, and Kevlar veil materials. The integration of a CNT-silicone matrix with Kevlar yarn and fabric materials produced a composite with moderate electrical and thermal conductivity due to CNT fabric combined with the strength of Kevlar fabric or yarn. In the material synthesis, a notable difficulty was that the CNT-silicone did not bond strongly to the Kevlar. The composites passed the Vertical Flame Test ASTM D6413 and the Forced Air Oven Test NFPA 1971. These hybrid composites can have multiple applications in areas requiring favorable conductivity, strength, and flame and heat resistance. The application areas include firefighter apparel, military equipment, conductive/smart structures, and flexible electronics. The synthesis process used to manufacture CNT-silicone/Kevlar composites yielded composite sheets with an area of 2250 cm². The process is scalable and customizable for the synthesis of CNT composites with tailored properties. Improvements in the bonding of CNT-silicone to Kevlar are being investigated.

The substitution of traditional copper power transmission cables with lightweight copper–carbon nanotube (Cu–CNT) composite fibers is critical for reducing the weight, fuel consumption, and CO₂ emissions of automobiles and aircrafts. Such a replacement will also allow for lowering the transmission power loss in copper cables resulting in a decrease in coal and gas consumption, and ultimately diminishing the carbon footprint. In this work, we created a lightweight Cu–CNT composite fiber through a multistep scalable process, including spinning, densification, functionalization, and double-layer copper deposition. The characterization and testing of the fabricated fiber included surface morphology, electrical conductivity, mechanical strength, crystallinity, and ampacity (current density). The electrical conductivity of the resultant composite fiber was measured to be 0.5 × 10⁶ S/m with an ampacity of 0.18 × 10⁵ A/cm². The copper-coated CNT fibers were 16 times lighter and 2.7 times stronger than copper wire, as they revealed a gravimetric density of 0.4 g/cm³ and a mechanical strength of 0.68 GPa, suggesting a great potential in future applications as lightweight power transmission cables.

Less defective, nitrogen-doped 3-dimensional graphene (N3DG) and defect-rich, nitrogen-doped 3-dimensional graphene (N3DG-D) were made by the thermal CVD (Chemical Vapor Deposition) process via varying the carbon precursors and synthesis temperature. These modified 3D graphene materials were compared with pristine 3-dimensional graphene (P3DG), which has fewer defects and no nitrogen in its structure. The different types of graphene obtained were characterized for morphological, structural, and compositional assessment through Scanning Electron Microscopy (SEM), Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS) techniques. Electrodes were fabricated, and electrochemical characterizations were conducted to evaluate the suitability of the three types of graphene for heavy metal sensing (lead) and Electric Double-Layer Capacitor (EDLC) applications. Initially, the various electrodes were treated with a mixture of 2.5 mM Ruhex (Ru (NH₃)₆Cl₃ and 25 mM KCl to confirm that all the electrodes underwent a reversible and diffusion-controlled electrochemical process. Defect-rich graphene (N3DG-D) revealed the highest current density, followed by pristine (P3DG) and less-defect graphene (N3DG). Further, the three types of graphene were subjected to a sensing test by square wave anodic stripping voltammetry (SWASV) for lead detection. The obtained preliminary results showed that the N3DG material provided a great lead-sensing capability, detecting as little as 1 µM of lead in a water solution. The suitability of the electrodes to be employed in an Electric Double-Layer Capacitor (EDLC) was also comparatively assessed. Electrochemical characterization using 1 M sodium sulfate electrolyte was conducted through cyclic voltammetry and galvanostatic charge-discharge studies. The voltammogram and the galvanostatic charge-discharge (GCD) curves of the three types of graphene confirmed their suitability to be used as EDLC. The N3DG electrode proved superior with a gravimetric capacitance of 6.1 mF/g, followed by P3DG and N3DG, exhibiting 1.74 mF/g and 0.32 mF/g, respectively, at a current density of 2 A/g.