Abstract
We propose that graphene quantum dots (GQDs) with a sufficient number of acidic oxygen-bearing functional groups such as -COOH and -OH can serve as solution- and solid- type electrolytes for supercapacitors. Moreover, we found that the ionic conductivity and ion-donating ability of the GQDs could be markedly improved by simply neutralizing their acidic functional groups by using KOH. These neutralized GQDs as the solution- or solid-type electrolytes greatly enhanced the capacitive performance and rate capability of the supercapacitors. The reason for the enhancement can be ascribed to the fully ionization of the weak acidic oxygen-bearing functional groups after neutralization.
Highlights
Because of their unique two-dimensional feature, large specific surface area, and favorable electronic properties, graphene-based materials are promising candidates for the electrode materials of supercapacitors[1]
Three dimensional graphene-graphene quantum dots (GQDs) hydrogels prepared by Qu et al demonstrated high capacitive and cyclic performance for supercapacitors[17].GQDs derived from chemical oxidation of carbonaceous precursors can be considered extremely small Graphene oxide (GO) debris with a sufficient number of acidic oxygen-bearing functional groups[18,19]
The Fourier-transform infrared (FT-IR) and 13C SSNMR spectra of the GQDs displayed in Fig. 2a,b indicated that a sufficient number of oxygen-bearing functional groups such as C-O-C, -C-OH, -C = O, and -COOH were decorated on the GQDs22–24
Summary
The CV curves of the supercapacitors using the GQD solution (Fig. 3c) and solid-state films (Fig. 3d) were approximately rectangular without obvious redox peaks This indicated the electrochemical double-layer capacitive performance of the GNS electrodes in which the energy storage mechanism is based on reversible ion adsorption-desorption on its surface[26,27,28]. The weak acidic oxygen-bearing functional groups on GQDs must be ionized further The kinetics of this process results in large electrochemical polarization of the electrode at high scan rate, and is considered the main reason for the distortion of the CV curves. We propose that KOH neutralized GQDs can be considered a type of salt (Fig. 4b) These NGQDs almost fully ionize in a humid environment, resulting in considerable improvement of the ionic conductivity and ion-donating ability of the GQDs under humid conditions. We believed that our findings can broaden the applications of GQDs and provide new insights into designing solid-state electrolytes
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