Redox supercapacitors based on sulfonated PEEK and iodide species in the electrolyte

Recently, solid polymer electrolytes are largely considered as possible advanced materials for the development of flexible and safer solid-state supercapacitors. Here, we report the study of supercapacitor cells constructed with carbon xerogel electrodes and with solid polymer electrolyte based on sulfonated polyether ether ketone (sPEEK). The carbon xerogel is a pre-commercial carbon kindly furnished from CSIC-INCAR of Oviedo (Spain); whereas the sulfonated PEEK was synthesized in our lab by a well-experienced process based on heterogeneous sulfonation of the bare PEEK polymer with sulfuric acid. Various types of solid-state supercapacitor cells were realized by contacting face-to-face the SPEEK electrolyte membrane and the two 2 cm2 electrodes. The electrochemical characteristics of supercapacitors were investigated in a specific designed titanium cell provided of a reference electrode. A KI solution was used as an additive of electrolyte and impregnate in the positive electrode while the Na2SO4 was used to realize a cation exchange SPEEK membrane and to impregnate the negative electrode. The SPEEK membrane in the supercapacitors acted as an ion conductor and electronic insulator between the two carbon composite electrodes. Whereas, the KI salt added to the positive side of supercapacitor had the task of providing additional capacity through the I-/I3-redox reaction, to which possible hydrogen electro-sorption reaction occurred to the negative electrode of the carbon-carbon capacitor. These supercapacitors were electrochemical investigated by cyclic voltammetry (CV), DC galvanostatic charge/discharge and AC electrochemical impedance spectroscopy (EIS). As results, it were found that the solid-state supercapacitor based on Na-SPEEK membrane and with iodide species in the positive electrode exhibited higher specific capacitance (180 Fg-1) compared to these with the porous separator impregnated with 1M Na2SO4 (93 Fg-1), while the specific capacitance obtained with Na-SPEEK electrolyte membrane (83 Fg-1). The EIS analysis highlighted that the solid-state supercapacitor based on Na-SPEEK and KI showed very stable low resistance and full capacitance retention during a long stability test with 20000 cycles at 2 Ag-1 and more than 200 h in voltage holding condition at 1.6 Volt. The results of the remarkable electrochemical performance ( 200 Fg-1, 20 Wh kg-1) and excellent lifetime stability displayed from this configuration of supercapacitors is promising for the development of next-generation low cost, high-performance, solid-state and flexible energy storage devices.