Impact of size on energy storage performance of graphene based supercapacitor electrode

Manipulating structure of graphene and its aggregates has been found to be important for graphene based supercapacitor performance enhancement. However, seldom work has investigated impact of graphene size on its energy storage performance. In this work we have tailored size of graphene oxide from micrometers to tens of nanometers via sonochemical approach. The converted graphene by thermal annealing graphene oxides of different size have been used as electrode materials for constructing supercapacitor with electrolyte of ionic liquid. Electrochemical analysis indicates that there is no obviously positively correlation between energy storage performance and structure characters (specific surface area and electrochemical active edge sites) of graphene, which is contrary to our common knowledge. Based on structure and electrochemical analysis of graphene electrodes, we proposed that reducing the size of graphene results in low edge activity of it for the disrupted aromatic structure on the edges with enriched sp3-carbon defects, where electrical charge is hard to transfer to and through. The practical available area for double layer capacitance setup does not include these electrical charge inaccessible edge regions if the size of graphene is very small. And thus increasing edges of small size graphene has no positive impact on its capacitance enhancement. Furthermore, structure of graphene has shown key influence on equivalent series resistance of bulk electrode. Increased charge transfer frequency between graphene layers of size reduced graphene and higher charge transfer resistance between layers of graphene with increased defective edge were attributed to reduced power density of electrode of graphene with reduced size.