Enhancing the energy density and discharge times of flexible graphene supercapacitors by introducing porous oxides on their anodes

Abstract

This work reports the electrochemical performance of flexible graphene-based supercapacitors (SCs) fabricated with and without porous oxides of: Al2O3, Sr2CeO4 and ZnO. According to the scanning electron microscopy images, the Al2O3, Sr2CeO4, and ZnO have particle sizes of 20−40 μm, 0.4−5 μm, and 100−200 nm, respectively. In addition, the X-ray diffraction analysis revealed that Al2O3, Sr2CeO4 and ZnO have rhombohedral, orthorhombic, and wurtzite crystalline structures, respectively. The electrochemical performance of the SCs devices was also evaluated and we found that the SC devices made with Al2O3 (G/Al-SC device), Sr2CeO4 (G/Sr-SC device) and ZnO (G/Zn-Sc device) had 40.5, 67 and 78 times more capacitance than the SC device made without porous oxides. Additionally, the energy density of the devices increased in the order: G/Al-SC < G/Sr-SC < G/Zn-SC, in agreement with the order of increase of the surface area for the porous oxides according to the BET measurements: Al2O3 (65.06 m2 g−1)<Sr2CeO4 (214.7 m2 g−1)<ZnO (335.6 m2 g−1). Ce3+/Ce4+ species and carboxylic groups were detected by X-ray photoelectron spectroscopy and Fourier transform infrared techniques, respectively. The presence of such species and carboxylic groups was important because they worked as redox centers for the charge storage. Thus, the results presented here could be useful for the development of graphene-based batteries with high energy density and prolonged discharge times.