Systematic evaluation of factors influencing electrochemical and morphological characteristics of free-standing 3D graphene hydrogels as electrode material for supercapacitors

Here, we demonstrate how the oxidation level of graphene oxide, pH, temperature and duration of hydrothermal process affects the physicochemical and electrochemical characteristics of resulting 3D graphene hydrogels as electrode material in supercapacitors. Two graphene oxide samples are prepared using Hummers and Tour methods to follow the influences of oxidation level and the type of oxygenated functional groups on the electrochemical and morphological characteristics of resulting 3D graphene hydrogels. The optimized conditions for the hydrothermal process are determined with respect to achieving samples with the highest capacitive characteristics. Hydrogels obtained using Tour's graphene oxide has a hierarchical and interconnected porous framework and provides the best electrochemical characteristics. It also exhibits specific surface area of 921 m(2) g(-1), ionic conductivity of 0.44 S cm(-1) and double layer capacitance of 200 F g(-1) at 5 mV s(-1) in 6 M KOH, which are about 12%, 5% and 7% higher than that achieved for the best sample prepared based on Hummers' graphene oxide, respectively. The assembled symmetric supercapacitor prepared based on this hydrogel exhibits a remarkable gravimetric capacitance of 245 F g(-1) at 0.5 A g(-1) and an excellent cyclic stability with 78% capacitance retention at high current density of 50 A g(-1). (C) 2019 Elsevier Ltd. All rights reserved.