Designing of zwitterionic proline hydrogel electrolytes for anti-freezing supercapacitors

Hydrogel electrolytes containing a large amount of freezable water tend to freeze at subzero temperatures , which catastrophically reduces their ionic conductivity and thus limits their practical applications. In this work, we propose a new type anti-freezing hydrogel electrolyte based on an additive of zwitterionic proline , which can maintain high ionic conductivities of hydrogel electrolytes at subzero temperatures. The unique zwitterionic structure leads to several interesting characters like strong hydration, strong ionic interactions and low self-associations, which is proved to be the keys for the high performance of hydrogel electrolytes under low temperatures. As a result, the proline hydrogel electrolytes show a high ionic conductivity of 4.2 mS cm −1 even at −40 °C. The activated carbon electrode of supercapacitors based on proline hydrogel electrolytes delivers high specific capacitances of 145.8 (at 0.5 A g −1 ) and 116.1 F g −1 (at 0.5 A g −1 ) at 25 and −30 °C, respectively. Furthermore, the specific capacitance still shows a high retention of 71% after 12,000 charge/discharge cycles at −30 °C, confirming the good low-temperature adaptability. Such anti-freezing electrolytes with high ionic conductivity will open up a new avenue for anti-freezing energy storage devices, not limited to supercapacitors.