Wood inspired anisotropic polyvinyl alcohol hydrogel with hierarchical channels for low temperature tolerant solid-state supercapacitors via directional freezing

Abstract

Hierarchical microstructures are unique characteristic in various natural organisms with optimized functions. The anisotropic structure offers low-tortuosity with hierarchical porosity, providing sufficient multifunctional channels for mass/ion transport, which is desirable for the design of low temperature tolerant energy storage device. Conductive hydrogels are elastic crosslinked polymeric networks that adaptable in various environmental conditions, exhibiting promising potentials for energy storage and conversion devices. However, conventional hydrogel based energy storage devices would inevitably freeze at low temperatures (< 0 °C) due to the high content of solvent water, which fundamentally lead to the fatal deterioration of the ionic conductivity and limit their applications in extreme conditions. In this paper, a flexible solid-state supercapacitor was prepared using wood inspired anisotropic polyvinyl alcohol (PVA) hydrogel electrolyte via directional freezing strategy. The hierarchical channels are tuneable by varying the concentrations of PVA solutions. The ionic conductivity of hydrogel electrolyte was significantly enhanced in the full temperature range attributing to the hierarchical microstructures, specifically, the microscale vessel-like directional ion transport channels coupled with nanopores on the inner walls. With co-mixing of dimethyl sulfoxide/ethylene glycol as the organic solvent medium, the assembled solid-state supercapacitors exhibited excellent electrochemical performance of 65% capacity retention at -30 °C, remarkably flexibility and decent cycling stability of 75% capacity retention after 5000 cycles at -20 °C. This work demonstrates that the hierarchical anisotropic channels and organic anti-frozen additive could significantly enhance the low temperature performance of solid-state supercapacitors, opening up new strategy for advancing energy storage devices at extreme conditions via the bio-mimicked optimization of microstructure and electrolyte composition.