Molecular Level Study of Graphene Networks Functionalized with Phenylenediamine Monomers for Supercapacitor Electrodes

Three phenylenediamine (PD) monomers, o-phenylenediamine (OPD), m-phenylenediamine (MPD), and p-phenylenediamine (PPD), were used to prepare the functionalized graphene (PD/rGO) networks. The results obtained from a series of chemical, thermal, and rheological analyses elucidated the mechanism of the covalent bonding and the existence of cross-linked graphene networks. The measured XRD patterns and molecular dynamic calculations discovered that those PPD and MPD molecules could enlarge graphene interlayer spacing to 1.41 and 1.30 nm, respectively, while OPD molecules were disorderly bonded or nonbonded to the basal planes of graphene layers, resulting in small and variable interlayer distances. The loadings of PD monomers were optimized to achieve superior supercapacitor performance. Electrochemical study showed that PPD/rGO exhibited the largest specific capacitance of 422 F/g with excellent cycling stability and low charge transfer resistance. The large variations in the capacitance values among PD/rGO networks with different PD monomers were explained by the difference in the graphene nanostructures, reversible redox transitions, and charge transfer characteristics. Particularly, density function theory calculations were adopted to compare electronic properties of the PD/rGO composites, including formation energy, electron density distribution, HOMO energy levels, and electron density of states near the Fermi level.