Construct Flexible, Durable Supercapacitors via Antifreezing Polyampholyte Hydrogels

Deformability and resistance to freezing are critical attributes to flexible supercapacitors to be operational in extreme environments, besides stable and sustainable power output. Much slowed rates of ion diffusion in a frozen electrolyte usually lead to device malfunction, posing significant limits to the application of such devices. Polyampholyte (PA) hydrogels are inherently antifreezing due to the inherent anion–cation electrostatic interaction in the polymer network. Here, we report the fabrication of poly(sodium p-styrenesulfonate-co-dimethylaminoethyl acrylate quaternized ammonium) [P(NaSS-co-DMAEA-Q)] conductive PA hydrogels cross-linked by ethylene glycol diacrylate urethane (BAGU) and doped with phosphoric acid (H3PO4) for achieving favorable antifreezing attributes. We further assemble sandwich-structured all-PA hydrogel supercapacitors PA@BAGU-SC (PA@BAGUACP/H3PO4/PA@BAGUH3PO4/PA@BAGUACP/H3PO4) with PA@BAGUH3PO4 as electrolyte and PA@BAGUACP/H3PO4, PA@BAGUH3PO4, further doped with activated carbon particles (ACPs) as electrodes, and assess their electrochemical performances at low temperature environments. The PA@BAGUH3PO4 hydrogel electrolytes exhibited sustained high conductivity, with 8.23 and 6.88 mS cm–1 in a broad −30–20 °C temperature range and after 5 freeze–thaw cycles, respectively, for typical PA@BAGUH3PO4(0.9). The optimal all-PA hydrogel supercapacitor PA@BAGUACP(1.0)/H3PO4(0.9)/PA@BAGUH3PO4(0.9)/PA@BAGUACP(1.0)/H3PO4(0.9) exhibited 104.9 mF cm–2 initial areal capacitance at 1 mV s–1 scan rate and 97.01 and 45.77% capacitance retentions after 500 charge–discharge cycles at 20 and −30 °C, respectively, both superior to its MBAA counterpart of 96.3 and 25.92% retentions, respectively. The ∼20% antifreezing performance improvement in PA@BAGU-SC realized at −30 °C was most likely due to the additional hydrogen-bonding capability of BAGU deliberately introduced into the hydrogel network, allowing more free water molecules to be immobilized. The strategy we presented in this work of fabricating BAGU cross-linked, antifreezing P(NaSS-co-DMAEA-Q) PA hydrogel electrolytes, PA@BAGUH3PO4 and assembling their-based all-PA supercapacitors may be readily applied to the development of a broad range of hydrogel-based materials for biomedical and bioelectronic applications, especially as portable and wearable energy storage devices operating at low-temperature harsh environments.