High Rate Performance of Flexible Pseudocapacitors fabricated using Ionic-Liquid-Based Proton Conducting Polymer Electrolyte with Poly(3, 4‑ethylenedioxythiophene):Poly(styrene sulfonate) and Its Hydrous Ruthenium Oxide Composite Electrodes

We report the studies on all-solid-state flexible pseudocapacitors based on poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) (PEDOT-PSS) and PEDOT-PSS/hydrous ruthenium oxide composite electrodes separated by nonaqueous proton conducting polymer electrolyte. Structural, thermal and electrochemical properties including high ionic conductivity (6.2 × 10–2 S cm–1 at 20 °C) of the polymer electrolyte, comprising ionic liquid 1-ethyl 3-methyl imidazolium hydrogen sulfate (EMIHSO4) immobilized in the blend of poly (vinyl alcohol) (PVA) and poly (vinyl pyrrolidone) (PVP), demonstrate its excellent suitability in supercapacitor fabrication. A substantial improvement in the specific capacitance (hence the specific energy) has been obtained when the PEDOT-PSS electrodes in the symmetrical pseudocapacitor are replaced by the composite electrodes PEDOT-PSS/RuO2·xH2O. High rate capability of the capacitor cell (with PEDOT-PSS electrodes) has been observed, as evidenced from the high knee frequency (∼966 Hz), low response time (∼70 ms) and high pulse power (∼10.2 kW kg–1), observed by impedance analysis. Almost rectangular (capacitive) cyclic voltammetric patterns for high scan rates (up to 15 V s–1) confirm the high rate performance of the pseudocapacitor. The PEDOT-PSS/RuO2·xH2O composite electrodes show the lower rate capability (knee frequency ∼312 Hz, response time ∼1 s, pulse power ∼3.2 kW kg–1 and capacitive CV response up to 500 mV s–1) because of slow exchange of charges at the interfaces via RuO2·xH2O. However, the pseudocapacitor with composite electrodes shows higher rate performance relative to many reported RuO2·xH2O systems. About 15% improvement is noticed in the capacitance value when the capacitor with composite electrodes is initially charged and discharged up to ∼200 cycles. Thereafter, the cell shows almost constant value of specific capacitance (∼70 F g–1) for 1000 cycles.