Hierarchical Aerogel-Confined Deep Eutectic Electrolytes for Complete Water Immobilization and High-Performance, Freeze-Tolerant Supercapacitors

Abstract

Deep eutectic solvents (DESs) have emerged as sustainable electrolyte candidates for advanced electrochemical energy storage owing to their wide electrochemical stability windows, low volatility, and environmental compatibility. However, their practical application in supercapacitors remains hindered by high viscosity, limited electrode wettability, and severe interfacial resistance, which collectively compromise rate capability and long-term stability. Herein, we report an environmentally benign, dual-function electrolyte based on a CaCl₂-ethylene glycol DES immobilized within a hierarchical carboxymethyl cellulose (CMC) aerogel framework (CaCl₂-EG@Aerogel) via a facile immersion strategy. The aerogel-confined electrolyte architecture establishes continuous, low-tortuosity ion transport pathways while simultaneously serving as a leakage-free separator, ensuring mechanical robustness and operational safety. Nanoscale confinement within the CMC aerogel effectively immobilizes water in a strongly bound, non-freezable state, suppressing crystallization down to −65 °C, reducing parasitic side reactions, and lowering interfacial resistance without sacrificing electrochemical stability. As a result, the CaCl₂-EG@Aerogel exhibits enhanced ionic conductivity and a widened stable voltage window. Supercapacitors assembled with activated carbon electrodes and the proposed electrolyte deliver a high specific capacitance of 139 F g⁻¹ over a 2.6 V operating window, corresponding to an exceptional energy density of 130.5 Wh kg⁻¹ and a power density of 23,140 W kg⁻¹. The devices demonstrate remarkable durability, retaining 82% of their initial capacitance after 50,000 charge-discharge cycles with nearly 100% coulombic efficiency, and preserve 71.28% capacitance after repeated freezing–thawing cycles at −35 °C. Molecular dynamics simulations and density functional theory calculations reveal that strong Ca²⁺–CMC coordination, extensive hydrogen bonding with ethylene glycol, and hierarchical confinement synergistically regulate ion transport and suppress water crystallization. This work establishes a rational, sustainable electrolyte design strategy for high-voltage, freeze-tolerant supercapacitors operating under extreme conditions.