Rational selection of small aromatic molecules to functionalize graphene for enhancing capacitive energy storage

Surface functionalization of graphene sheets with redox-active small molecules can help store more charges due to the added pseudocapacitance. However, a clue on selecting appropriate molecules to enhance the energy storage of graphene-based supercapacitors is unclear. Herein, four different types of aromatic molecules containing amino or hydroxyl groups, or both, are selected to functionalize N-doped graphene (NG) films for a comparison study, aiming to explore factors that can enhance the capacitive performance, such as adsorption affinity to graphene, type of redox group, number of groups in a single molecule, electrochemical redox potential, capability of sustaining higher voltage in Li-salt electrolyte, and cycling performance. Among the selected organic compounds, 4,4′-oxydianiline molecules are found to be a good match for graphene. In a symmetric cell with Li₂SO₄ electrolyte, the functionalized NG film exhibits a high specific capacitance of 612 F g^-1 as well as maintaining high coulombic efficiency within a voltage window of 1.6 V.