Facile Design of Multiscale Cellulose-Enhanced Hydrogel Electrolytes for Flexible Zn-Ion Capacitors in Wearable Electronics

Abstract

Flexible solid-state supercapacitors show significant potential for wearable electronics; however, achieving simultaneous mechanical robustness and high ionic conductivity remains challenging. In this work, a polyacrylamide (PAM)/cellulose nanocrystal (CNC)-based hydrogel electrolyte loading with carboxymethyl cellulose (CMC) is engineered to address this limitation (PAM/CNC-CMC-Zn2+). Incorporating CNC improved the mechanical properties of hydrogels, while subsequently adding CMC-Na enriched with hydrophilic groups (─OH and ─COO−) into PAM/CNC hydrogels disrupted hydrogen-bond networks within the ZnSO4 electrolyte, thereby optimizing Zn2+ solvation sheath structure. This modification suppressed corrosion currents and minimized side reactions. The hydrogel demonstrated outstanding mechanical properties, including a tensile strength of 0.22 MPa, high stretchability (1452.1%), and remarkable fracture toughness (0.98 MJ m−3). The zinc-ion capacitors (Zn // PAM/CNC-CMC-Zn2+ // AC) demonstrate exceptional electrochemical performance, achieving a significant specific capacitance of 151.4 F g⁻¹ at 0.5 A g⁻¹, coupled with a remarkable power density of 1150 W kg⁻¹ (at 10.9 Wh kg⁻¹). Notably, the device exhibits outstanding performance stability, maintaining its functionality under mechanical folding and retaining its efficiency after 10 000 long charge–discharge cycles. These multiscale cellulose-based design highlights the hydrogel electrolyte's dual functionality in balancing mechanical adaptability and electrochemical efficiency, offering a potential solution for next-generation wearable energy storage systems.