Improving the capacitance of hydrogel-based supercapacitors by pre-depositing a zwitterionic 3,4-ethylenedioxythiophene

Abstract

Polymer hydrogel electrolytes show great advantages in the flexible electronic applications of supercapacitors, but the poor interfaces between electrode and electrolyte are limiting their energy and power densities. In this work, by taking the well-developed pair of poly(vinyl alcohol)/H2SO4 gel electrolyte and graphene film electrode as a prototype, after electrochemically depositing a functional monomer of zwitterionic 3,4-ethylenedioxythiophene (EDOTSB) onto the graphene film electrodes, the specific capacitance of supercapacitors receives a 2.5-fold increase up to 248 mF cm−2 (346 F g−1). By comparing with other pendent groups on EDOT, the decisive role of the zwitterionic group is demonstrated. Attributed to the strong hydration ability of the zwitterion group, the deposition of EDOTSB effectively improves the hydrophilicity of graphene film electrodes, so that the interfacial affinity between the electrode and hydrogel electrolyte is greatly promoted. On the one hand, the EIS measurements certify that the charge-transfer resistance is largely reduced. On the other hand, tests with electrochemical quartz crystal microbalance demonstrate that the interior of graphene film is highly accessible for EDOTSB, thereby enlarging the accessible surface area on the electrode and accelerating the electrolyte infilling process. Furthermore, the general comparison with the liquid electrolyte counterpart is also discussed. This developed method is highly universalizable and can find applications in different electrode/electrolyte supercapacitors and other high-performance flexible energy storage devices.