o-Benzenediol-Functionalized Carbon Nanosheets as Low Self-Discharge Aqueous Supercapacitors.

Widening the voltage window is often proposed as a way to increase the energy density of aqueous supercapacitors. However, attempting to operate beyond the aqueous supercapacitor stability region can undermine the supercapacitor reliability due to pronounced electrolyte decomposition, which can lead to a significant self-discharge process. To minimize this challenge, charge injection by grafting o-benzenediol onto the carbon electrode is proposed through a simple electrochemical cycling technique. Due to charge injection from o-benzenediol into the carbon electrode, the equilibrium potential of the individual electrode can be reduced. In addition, due to its small molecular size, charge distribution, which is commonly faced by bulk pseudocapacitive materials, is also avoided. The assembled supercapacitor based on the o-benzenediol-grafted carbon demonstrated a maximum energy density of 24 Wh kg-1 and a maximum power density of 69 kW kg-1 , with a retention of 89 % after 10 000 cycles at 10 A g-1 . A low self-discharge of about 4 h was recorded; this could be attributed to the low driving force arising from the lower equilibrium potential. Thus, the proposed technique may provide insight towards the tuning of the equilibrium potential to attain reliable, high-performing supercapacitors with a low self-discharge process.