Molecular engineering of aromatic amine spacers for high-performance graphene-based supercapacitors

In this work, chemical compounds of graphene oxide (GO) and amine molecules as spacers were synthesized by one-step hydrothermal reactions. The products of the reactions, referred to as rGO-Spacers samples, were fabricated into electrodes for supercapacitors. By engineering the molecular structures of the spacers, the specific capacitance (C-s) of the rGO-Spacers can reach up to 612 F/g at 2 mV/s in cyclic voltammetry (CV) measurement, 597 F/g at 0.5 A/g and 512 F/g at 10 A/g in galvanostatic charge-discharge measurement using two-electrode setup with 1 M sulfuric acid aqueous solution at the electrolyte. 97% of the high C-s was retained after 10 000 charge-discharge cycles. In comparison, the control sample without spacers presents a C-s of 194 F/g measured in CV measurement under the same conditions. The increased interlayer distance up to 1.9 nm between graphene sheets caused by addition of the amine spacers was confirmed by X-ray diffraction. The morphology of rGO-Spacers samples was characterized by scanning electron microscope, transmission electron microscope and atomic force microscope, where crumpled thin sheets were observed. The spacer molecules were found to disperse uniformly on the sheets by energy dispersive spectroscope mapping. Chemical reactions and covalent bonding between GO and the spacers are evidenced by thermogravimetric analysis, Xray photoelectron spectroscopy (XPS), UV-visible spectroscopy, Fourier-transform infrared spectroscope and Raman spectroscopy.. Based on the characterization results, the amide formation reactions and epoxide ring opening reactions between amine spacers and GO are proposed. The weight measurement and the atomic ratio information from XPS further reveal different reactivity of the spacers to GO, which could account for the different C-s of the samples with spacers of different molecular structures. From the results of these comparative studies, the engineering of the molecular structures was found to play a pivotal role in determining the value of C-s. The findings also provide guidance to achieve even higher C-s by optimization of the molecular structures of the spacers, as well as other experimental parameters which can facilitate the reaction between the spacers and GO. (C) 2016 Elsevier Ltd. All rights reserved.