Fabrication of a symmetric supercapacitor device using MnO₂/cellulose nanocrystals/graphite electrodes <i>via</i> sputtering for energy storage

The growing commercialization of flexible electronic goods has led to increased interest in flexible wearable energy storage devices, particularly supercapacitors. The development of supercapacitive electrodes from low-cost, sustainable, and renewable materials is essential for promoting a green and eco-friendly approach. Cellulose nanocrystals (CNCs) with unique properties and structures hold the potential to produce a 3-D network-based electrode, which is necessary to utilize high-quality carbon materials. Integration of metal oxides on the CNCs/graphite surface exhibits excellent structural stability due to CNCs and electrical characteristics of the graphite substrate. In this work, we demonstrate a self-standing MnO₂/CNCs/graphite-based hybrid electrode with excellent supercapacitance for energy storage. An MnO₂ thin film was produced using the radio frequency (RF) magnetron sputtering technique, while CNCs were extracted from sugarcane bagasse. The MnO₂/CNCs/graphite hybrid electrode and device demonstrated superior electrochemical performance in 1 M Na₂SO₄ electrolyte. It offered a 1.2 V potential window with an areal capacitance of 149 mF cm^-2, energy density of 75 mW h cm^-2 at 2 mA cm^-2, and a power density of 2977 μW cm^-2 with a low solution resistance of 5.67 Ω, comparable to the very high value of CNCs, <i>i.e.</i>, <i>R</i>_s 6.13 KΩ. Moreover, the MnO₂/CNCs/graphite device demonstrated outstanding cyclic retention, <i>i.e.</i>, 85.27% after 15 000 cycles, owing to the structural stability imparted by CNCs, making it a great contender as supercapacitors.