Activated graphene as a material for supercapacitor electrodes: effects of surface area, pore size distribution and hydrophilicity

Activated reduced graphene oxide (a-rGO) is a material with rigid 3D porous structure and high Specific Surface Area (SSA). Using variation of activation parameters and post-synthesis mechanical treatment we prepared two sets of materials with a broad range of BET (N2) SSA ~1000- 3000 m2/g, significant difference in pore size distribution and oxygen content. The performance of activated graphene as an electrode in supercapacitor with KOH electrolyte was correlated with structural parameters of the materials and water sorption properties. The a-rGO is hydrophobic material as evidenced by negligibly small BET (H2O) SSA determined using analysis of water vapor sorption isotherms. However, total pore volume determined using water vapor sorption and sorption of liquid water is almost the same as the one found by analysis of nitrogen sorption isotherms. Ball milling is found to provide improved bulk density of activated graphene and collapse of all pores except the smallest ones (<2 nm). Decrease in activation temperature from 850°C to 550°C is found to result in materials with narrow micropore size distribution and increased oxygen content. Elimination of mesopores using ball milling or lower activation temperature provided materials with better specific capacitance despite significant decrease (by ~¨30%) of BET(N2) SSA. The best gravimetric and volumetric capacitance in KOH electrolyte was achieved not for samples with the highest value of BET(N2) SSA but for materials with 80-90% of total pore volume in micropores and increased BET (H2O) SSA. Comparing performance of electrodes prepared using rGO and a-rGO shows that more hydrophilic surface is favorable for charge storage in supercapacitors with KOH electrolyte