O. Gomez-Zavala, L. A. Garcés-Patiño, Y. Velazquez-Galvan, M. Perez-Chavez, P. Salas, J. Oliva
The sustained increase in global demand for portable electronic devices, electric vehicles, and power backup systems has intensified the search for storage technologies with better performance than conventional batteries. Although lithium-ion batteries offer high energy densities (100–265 Wh/kg), their power density is between 50 and 200 W/kg, with long charging times (1–5 h) and a lifespan limited to 1000–2000 cycles with charge–discharge efficiencies of 70–85%. In contrast, supercapacitors (SCs) exhibit lower energy densities (1–20 Wh/kg), but stand out for their very high power density (up to 10,000 W/kg), with charging times ranging from seconds to minutes, and an extraordinary operational lifetime (50,000–1,000,000 cycles), with efficiencies of over 95% [ 1 , 2 ]. In recent years, the development of flexible SCs based on carbonaceous materials has gained great interest due to their potential to be integrated into wearable electronic devices, smart textiles, and self-powered sensors. However, conventional carbonaceous materials have limitations: activated carbon, although reaching surface areas > 3000 m 2 /g, rarely exceeds 100 F/g in specific capacitance [ 2 ]. This restricts its energy density to below 10 Wh/kg. Graphene, despite its theoretical surface area of 2630 m 2 /g and excellent conductivity, tends to reaggregate (restack), reducing its actual active surface area and effective capacitance [ 3 ]. On the other hand, carbon nanotubes (CNTs), although they allow higher capacitances (200–400 F/g) when combined with metal oxides, have complex and expensive synthesis processes [ 4 , 5 ]. In addition, recent literature emphasizes that the combination of materials (composites/hybrids) is the most effective strategy to obtain high surface area, conductivity, and redox sites, that is, combining high-conductivity carbons with redox phases to increase energy density without sacrificing power [ 6 ].