Optimizing the pair carbon xerogels-electrolyte for high performance supercapacitors

Supercapacitors have received a lot of research attention and are promising energy storage devices due to their high power and long cycle life. In order to developed an advanced device with significant capacity for storing charge and cheap carbon materials, efforts must focus not only on improving synthesis by controlling the morphology and pore size, but also on improving electrode-electrolyte compatibility of the resulting systems. The present study examines the relationship between the surface chemistry of different kind activated carbon xerogels, the electrolyte type and the electrochemical properties of supercapacitors. Activated carbon xerogels were prepared by varying the initial pH of the resorcinol-formaldehyde aqueous solution (AX6.5, AX5.97, pH=6.5 and 5.97, respectively). An activated carbon xerogel doped with graphene (AX6.5-G) was also evaluated. This latter material has a very good combination of high porosity and a superior electrical conductivity. The materials produced are physicochemical characterized by DTA/TGA, porous characterization and SEM analysis. The carbon xerogel based electrodes were prepared by spreading over glass plate a slurry containing the carbon gel, graphite and poly vinylidene difluoride (PVDF) binder. The layer formed was thermally treated to improve its mechanical stability. The developed electrode materials and the sodium form (for soaking in 1M Na2SO4) of Aquivion® membrane (code membrane E87-05S, Solvay Specialty Polymers), were used to assembly symmetric solid- state supercapacitor. The supercapacitor cells composed by same electrodes and 1 M KOH electrolytes are also assembled and tested for comparison. The supercapacitor performances are verified by different electrochemical methods - cyclic voltammetry, galvanostatic charge/discharge measurements and long-term durability tests in neutral and alkaline environments. Specific capacitance, energy and power density, energy efficiency and lifetime were compared in the studied supercapacitors. The results are discussed on the basis of electrode-electrolyte interactions and a correlation is found that could be important to design sustainable solid-state supercapacitors with high power density and faster charge and discharge rates.