Utilization of sulfonated cellulose membrane for Zn ion hybrid capacitors

Zinc ion hybrid capacitors (ZHCs) are regarded as sustainable energy storage devices, largely due to the abundance of zinc and its compatibility with aqueous electrolytes. Thick glass microfiber separators are commonly employed in ZHCs because they resist penetration by Zn dendrites, a prevalent issue in these devices. However, glass fiber separators not only reduce the volumetric energy but also raise environmental concerns due to their production processes, which generate significant amounts of greenhouse gases. In this study, we propose using a sulfonated cellulose membrane (SCM) derived from softwood cellulose nanofibrils as an eco-friendly and sustainable separator for ZHCs. Utilizing this sulfonated cellulose membrane, we achieved 2000 h of continuous plating/stripping of Zn and more than 95% coulombic efficiency. Additionally, the efficacy of SCM as a separator was validated through the successful deployment of a Zn ion hybrid capacitor, which exhibited specific energies of 42 Wh/kg. The ZHC demonstrated remarkable cyclic stability, enduring over 10 000 cycles with minimal self-discharge behavior. This study highlights the use of a cost-effective, thin, mechanically robust, and highly cross-linked cellulose nanofibrils membrane for ZHCs, showcasing its potential for broader utilization in various energy storage devices.