This paper proposes a cyanide-containing polymer as the framework of gel electrolytes for electric double layer capacitors (EDLCs). Dielectric studies in a frequency range of 10¹-10⁵ Hz demonstrate the fast polarization and low dielectric loss features of the gel electrolyte that entraps 1-M TEABF₄/PC. The swift polarization of the gel electrolyte can be attributed to the high dipole moment of the contained cyanide functionality on the polymer chains. When supported with separator membranes, the ionic conductivity of the gel electrolyte is one order of magnitude higher than that of the 1-M TEMABF₄/PC liquid electrolyte (10^-2 S cm^-2 vs. 10^-3 S cm^-2 at room temperature) owning to the presence of polarizable cyanide groups, which facilitate ion motion under the action of an applied electric field. In addition, the polymer framework improves the dissociation of ion-complexes to promote ion transport. Symmetric EDLCs assembled using the gel electrolyte exhibits specific capacitances of 199 and 130 F g ^-1 (within 0-2.7 V) at galvanostatic discharge rates of 0.5 and 100 A g^-1, respectively, based on the carbon mass. The corresponding capacitances for liquid-electrolyte EDLCs are 172 and 109 F g^-1. The gel EDLCs exhibit high stability, retaining ~95% of their initial capacitance after 20,000 cycles. An important feature of this gel electrolyte is its controllable gelling time. The electrolyte can be used in a manner similar to that of an ordinary liquid electrolyte during assembly and auto-gelation of the electrolyte subsequently takes place in the assembled EDLC. The implemented auto-gelation greatly enhances the durability of the resulting devices and is applicable in a wide range of energy storage systems.

Acknowledgments: This research was supported by the Ministry of Science and Technology, Taiwan (104-2221-E-006-231-MY3, 104-2221-E-006-234-MY3, and 106-3113-E-006-009) and by the Ministry of Education’s Aim for the Top University Project at National Cheng Kung University, Taiwan.