Carbon-Based Electrodes for Supercapacitors, with a Focus on Carbon Nanotubes—A Brief Overview

Increased needs arising from efficient utilization of renewable energy sources and the emerging use of portable electronic devices have introduced new requirements and challenges, such as fast charging and discharging, high-speed energy delivery, longer lifetime, and recyclability. To meet these demands, the innovative use of supercapacitors is essential, as they can complement the batteries currently in use. One of the major disadvantages of supercapacitors is that their energy storage capacity (5–20 Wh/kg) is currently insufficient, compared to the capacity of batteries (~1000 Wh/kg). Supercapacitors have higher specific power (10 kW/kg) but lower specific energy density, which is another significant disadvantage compared to batteries. This has prompted researchers around the world to find innovative solutions to enhance the energy density of these materials. Carbon-based nanomaterials are one of the most widely used electrode materials for supercapacitors; therefore, the development of carbon-based nanomaterials plays crucial role in evolution of supercapacitors, due to their high electrical conductivity, large specific surface area, and excellent mechanical strength compared to conventional electrode materials graphite, copper, platinum, etc. Significant results have been reported in the scientific literature on novel carbon-based nanostructured materials such as carbon nanotubes, vertically aligned carbon nanotubes, graphene, activated carbon, or carbon nanoballs, which have a hierarchical pore structure, as well as hybrid systems combining these materials and the introduction of alternative electrolytes. This manuscript reviews briefly the background and fundamental characteristics of supercapacitors, classifying them. It also mentions the general electrochemical measurement methods used to evaluate the energy storage properties of supercapacitors, with emphasis on their specific characteristics and limitations. The integral components of supercapacitors, especially electrode materials, are considered to have considerable impact on the performance of supercapacitor devices (e.g., long life cycle, storage capacity, and high power density).