Dopant-induced enhanced ion diffusion in flower-shaped hollandite-MnO2: a wearable supercapacitor with improved energy storage

Abstract

The rapid technological advancement in portable devices has increased the demand for novel materials and fabrication strategies for transforming electronic devices into wearable gadgets. This also applies to supercapacitors, which are widely used energy storage devices. Herein, the transition metal ion Ni²⁺ was doped in α-MnO₂ to fabricate a 180° bendable supercapacitor, and subsequently, a wristband-type device based on a prototype device was designed for high-power density charge storage application. Ni²⁺–doped α-MnO₂ was characterized via SEM, TEM, and XPS and was found to possess a flower-like morphology with flake-like petals forming groves that act as active sites for the diffusion of ions during electrochemical cycling. The doping of Ni²⁺ ions in the hollandite 2 × 2 tunnel structure of α-MnO₂ led to structural distortion, generation of oxygen vacancy defects, and an increase in the surface area, thus promoting reaction kinetics and specific capacitance. The Ni²⁺–MnO₂ electrode showed a high specific capacitance of ∼757.5 mF cm⁻² (378.75 F g⁻¹), which is impressive as it could be achieved with a very low Ni doping content of ∼0.77%. Furthermore, the wearable wristband supercapacitor displayed a high power density of 6056 mW cm⁻² at a minimum energy density of 28 mW h cm⁻², revealing the practical importance of the device in flexible electronics. To showcase the industrial importance of this work in flexible electronics, the wristband-type device was used to light LEDs under bending conditions, making it suitable for on-site application.