Competing effects of potassium hydroxide activation of graphene on gravimetric and volumetric capacitances

Potassium hydroxide (KOH)-activated graphene is actively investigated as an electrode material with high gravimetric capacitance for electrical double-layer capacitors. However, extremely low density (≪ 0.1 g cm⁻³) of activated graphene and the resulting low volumetric capacitance severely limit its practical applications. Despite its critical importance to volumetric capacitance, however, material density of activated graphene has barely been previously discussed. In this study, we comprehensively investigate material density, compressive strength, gravimetric capacitance, and volumetric capacitance of KOH-activated graphene microspheres (A-GMs). Competing effects of KOH activation are clearly observed for the material density and gravimetric capacitance. For the A-GMs treated using GMs/KOH mixtures in ratios of 1:3–1:12, the gravimetric capacitance increases from 209.6 F g⁻¹ for GMs to 320.5 F g⁻¹ in organic electrolyte. However, the material density diminishes from 0.91 g cm⁻³ for GMs to 0.47 g cm⁻³ with a concomitant decrease in volumetric capacitance from 190.7 F cm⁻³ to 150.6 F cm⁻³. Notably, volumetric capacitance reaches the highest value of 201.3 F cm⁻³ for the A-GMs activated with a GMs/KOH mixture (1:6), demonstrating that a balance of material density and gravimetric capacitance is of critical importance for high volumetric capacitance of A-GMs.