Binder-free reduced graphene oxide 3D structures based on ultra large graphene oxide sheets: High performance green micro-supercapacitor using NaCl electrolyte

We here report a breakthrough in the fabrication of green high performance symmetric supercapacitors. Our system is based on binder-free reduced graphene oxide foam electrodes using NaCl aqueous electrolyte (1 M). A liquid crystal ultra large graphene oxide sheets dispersion in water was used for the fabrication of foams through freeze drying. Proper initial self-assemblage/alignment of graphene oxide (GO) sheets of the aqueous mixture on the substrate and exposure to vacuum sublimation conditions resulted in a unique 3D open structure. A novel technique based on nitrogen adsorption has been applied for the first time to estimate the average wall thickness. The combination of the use of ultra large GO, freeze drying, and employment of NaCl electrolyte resulted in an exceptionally high capacitance of 618.15 F g(-1) at a current density of 3.0 A g(-1). This is the highest value ever reported for a pure carbonaceous electrode in a two electrode system. Although electrical double layer storage is dominant, our device exhibits a high capacitance typical of pseudo capacitive systems and does not suffer from a limited cycle life. Another excellent feature of the novel supercapacitor is the persistence of high capacitive behavior (132.13 F g(-1)) even at a high charge/discharge rate of 51.5 A g(-1). This is mainly attributed to the amphiphilic character of the reduced graphene oxide sheets resulting from the high percentage of the carbonyl/carboxyl surface residual oxygenate groups which impart an enhanced wetting of the surface by the electrolyte in addition to inducing a higher concentration of Na+ and Cl- ions in the electrical double layers. Our optimum combination of energy and power densities is 42.92 W kg(-1) and 1.35 kWh kg(-1), respectively.