Development of carbon xerogel and sulfonated PEEK materials for redox solid-state supercapacitors

Recently, solid polymer electrolytes are intensely investigated as suitable materials to be used in the development of flexible, compact and safer solid-state supercapacitors. Here, we report a study supercapacitors composed of electrodes based on a carbon xerogel of high surface area (e.g. 3000 m2g-1) and a 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 and the membrane electrolyte were prepared in our lab by a well-experienced process using H2SO4 as the sulfonating agent of the bare PEEK polymer. A series of solid-state flexible supercapacitors were realized by contacting face-to-face the SPEEK membrane/electrolyte or a porous paper separator and the two 2 cm2 electrodes. The electrochemical characteristics of supercapacitors were investigated in a specific designed titanium cell in which a neutral aqueous electrolyte solution of Na2SO4/kI is used. The KI solution was here used to impregnate the positive electrode while the Na2SO4 was employed to make the Na-SPEEK electrolyte membrane. The latter in these supercapacitors acted as ionic conductor and electronic insulator between two composite electrodes. The KI salt added to aqueous electrolyte (1M Na2SO4) is confined to the positive side of supercapacitor supplying additional capacitance through the redox reaction I-/I3-, while a hydrogen electro-sorption occurs in the negative electrode of the carbon-carbon capacitor. These developed redox 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 with Na-SPEEK membrane and Na2SO4-KI solution in positive electrode exhibited higher specific capacitance (123 Fg-1) compared to that with the reference porous separator in 1M Na2SO4 (103 Fg-1), while the specific capacitance obtained with Na-SPEEK electrolyte reached 83 Fg-1. The EIS analysis highlighted that the solid-state supercapacitors based on Na-SPEEK and Na2SO4-KI showed very stable resistance and full capacitance retention during the cycling life test (20000 cycles at 2 Ag-1), while the reference aqueous capacitors exhibited a slight increases of ionic resistance with cycling life. Because the remarkable electrochemical performance and excellent cycle life displayed from the investigated supercapacitor in solid-state configuration, these materials may be considered promising for the development of next-generation low cost, high-performance, solid-state and flexible energy storage devices.