Enhanced energy storage in supercapacitors using R-TiO₂ nanotube and graphene-based electrodes

Conventional thin-film supercapacitors are limited by low energy density and poor charge balance between electrodes, restricting their integration into miniaturized electronic devices. In this study, reduced TiO₂ nanotubes (R-TiO₂ NTs) were fabricated via a straightforward anodization process followed by electrochemical reduction (self-doping) and further decorated with Ni(OH)₂ nanospheres. These R-TiO₂ NTs/Ni(OH)₂ NSs electrodes were employed as both positive and negative electrodes for symmetric supercapacitors, and as positive electrodes in asymmetric configurations. To develop a suitable negative electrode, few-layer graphene (FLG) and graphene nanoplatelets (GNP) were combined, and the optimal FLG/GNP weight ratio was identified to balance charge storage. This electrode design enabled the fabrication of an asymmetric supercapacitor (ASC) with significantly enhanced energy storage performance. The superior performance of the ASC is attributed to a synergistic charge storage mechanism, where surface-controlled pseudocapacitive reactions of Ni(OH)₂ nanosheets complement the double-layer capacitance of the FLG-GNP electrode, ensuring rapid charge-discharge kinetics, high rate capability, and excellent cycling stability. The ASC achieved an areal capacitance of 118.26 mF cm^-2 and an energy density of 42.05 µWh cm^-2 at 0.25 mA cm^-2, compared to 19.38 mF cm^-2 and 6.89 µWh cm^-2 for the symmetric device. This work demonstrates a promising strategy for high-performance, scalable micro-supercapacitors with potential applications in flexible and miniaturized electronics.