Unraveling Factors Leading to High Pseudocapacitance of Redox-Active Small Aromatics on Graphene

Graphene sheets functionalized by redox-active small aromatics can exhibit enhanced capacitance because of the introduced faradaic process. However, the immense number of possible molecules for energy storage makes the selection of the appropriate ones difficult. This study combines experiment and theory to unveil factors behind the different pseudocapacitance contributions of some aromatic isomers adsorbed onto graphene, aiming to provide a guideline for computationally screening out optimal molecules for supercapacitor electrodes. Eight kinds of molecules containing amino groups are intentionally selected to functionalize N-doped graphene (NG) and their electrochemical properties are compared. The highest occupied molecular orbital level of a molecule is found to play an important role in rendering a high pseudocapacitance. Also, remarkable efficacies from two kinds of molecules, 4-aminophenol and 1,5-naphthalenediamine (1,5-NAPD), are unveiled, and the role of the amino group in charge storage is discussed. As a result, the graphene film absorbed with 1,5-NAPD molecules shows a high specific gravimetric capacitance of 877 F g ^-1 within the voltage window of 1 V, corresponding to a high areal specific capacitance of 1.14 F cm ^-2 from the thin film with a mass loading of 1.3 mg cm ^-2 . Also, the 1,5-NAPD/NG film shows good cycling stability, achieving 105% capacitance retention after 5000 charge-discharge cycles.