Achieving High Capacitance of Paper-Like Graphene Films by Adsorbing Molecules from Hydrolyzed Polyimide

To date, graphene-based electric double layer supercapacitors have not shown the remarkable specific capacitance as theoretically predicted. An efficient strategy toward boosting the overall capacitance is to endow graphene with pseudocapacitance. Herein, molecules of hydrolyzed polyimide (HPI) are used to functionalize N-doped graphene (NG) via π–π interaction and the resulting enhanced electrochemical energy storage is reported. These aromatic molecules in monolayer form on graphene contribute strong pseudocapacitance. Paper-like NG films with different areal mass loadings ranging from 0.5 to 4.8 mg cm⁻² are prepared for supercapacitor electrodes. It is shown that the gravimetric capacitance can be increased by 50–60% after the surface functionalization by HPI molecules. A high specific capacitance of 553 F g⁻¹ at 5 mV s⁻¹ is achieved by the HPI-NG film with a graphene mass loading of 0.5 mg cm⁻² in H₂SO₄ aqueous electrolyte. For the HPI-NG film with highest mass loading, the gravimetric specific capacitance drops to 340 F g⁻¹ while the areal specific capacitance reaches a high value of 1.7 F cm⁻². HPI-NG films are also tested in Li₂SO₄ aqueous electrolyte, over an extended voltage window of 1.6 V. High specific energy densities up to 40 Wh kg⁻¹ are achieved with the Li₂SO₄ electrolyte.