Hierarchically ordered mesoporous carbon/graphene composites as supercapacitor electrode materials

Abstract

Hierarchically ordered mesoporous carbon/graphene (OMC/G) composites have been fabricated by means of a solvent-evaporation-induced self-assembly (EISA) method. The structures of these composites are characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and nitrogen adsorption-desorption at 77 K. These results indicate that OMC/G composites possess the hierarchically ordered hexagonal p6mm mesostructure with the lattice unit parameter and pore diameter close to 10 nm and 3 nm, respectively. The specific surface area of OMC/G composites after KOH activation is high up to 2109.2 m(2) g(-1), which is significantly greater than OMC after activation (1474.6 m(2) g(-1)). Subsequently, the resulting OMC/G composites as supercapacitor electrode materials exhibit an outstanding capacitance as high as 329.5 F g(-1) in 6 M KOH electrolyte at a current density of 0.5 A g(-1), which is much higher than both OMC (234.2 F g(-1)) and a sample made by mechanical mixing of OMC with graphene (217.7 F g(-1)). In addition, the obtained OMC/G composites display good cyclic stability, and the final capacitance retention is approximately 96% after 5000 cycles. These ordered mesopores in the OMC/G composites are beneficial to the accessibility and rapid diffusion of the electrolyte, while graphene in OMC/G composites can also facilitate the transport of electrons during the processes of charging and discharging owing to its high conductivity, thereby leading to an excellent energy storage performance. The method demonstrated in this work would open up a new route to design and develop graphene-based architectures for supercapacitor applications.

Ordered Mesoporous Carbons Loading on Graphene after Different Molten Salt Activations for Supercapacitor Applications

“…However, it is noted that the H 4 hysteresis loop of OMC/G‐K 2 CO 3 has closed up at the relatively higher pressure P/P 0 , indicating OMC/G‐K 2 CO 3 contains much larger mesopores, as also confirmed in Table . In addition, the H 4 hysteresis loop becomes unclear after KOH activation as reported previously ,. It is therefore inferred that OMC/G‐ x after oxysalts activations could partially keep the original mesostructures of OMC/G before activation.…”

“…The capacitance of all samples are found to decrease along with the increase of the current density, but when the current density increases up to 20 A g −1 , the capacitance of OMC/G‐ x changes less along with the further increase of the current density. The specific capacitance of OMC/G‐K 2 CO 3 is high up to 359.04±27.71 F g −1 at a current density of 0.5 A g −1 , which is significantly larger than those of other three OMC/G‐ x , OMC/G (133.8±4.42 F g −1 , see SI) before activation and after KOH activation . When the current density increases from 0.5 to 100 A g −1 , the retention rate of OMC/G‐K 2 CO 3 is close to 55 %, and that of other OMC/G‐ x is approximately close to 50 %.…”

“…Therefore, we can infer from Table that the mesoporous volume of OMC/G is evidently increased after four oxysalts activations, but the microporous volume of OMC/G is predominately increased only after K 2 CO 3 activation. In particular, OMC/G‐KOH after KOH activation occupies high microporous volume (V micro /V total is 0.49), but OMC/G‐K 2 CO 3 exhibit relatively high mesoporous volume (V micro /V total is 0.36), indicating that KOH activation provides much more micropores while K 2 CO 3 molten salt activation provides much more mesopores. In addition, after oxysalts activations, the mesoporous sizes of OMC/G‐ x become much more inhomogeneous, which is attributed to the distortions and collapses of the mesostructure during the activation process.…”

“…A controllable one‐pot method is reported to synthesize N‐doped OMC, which has a high surface area of 586 m 2 g −1 and a specific capacitance of 227 F g −1 in 6 M KOH at a current density of 0.2 A g −1 . OMC and graphene composites fabricated by soft template also exhibit low specific surface area (less than 700 m 2 g −1 ) and low specific capacitance (less than 200 F g −1 ) ,…”

“…The relatively low specific surface area of ordered mesoporous carbons would dramatically limit their applications of supercapacitors. The further generation of porosity in the mesoporous wall or OMC surface is therefore required to increase the specific surface area and also promote the diffusion rate of electrolyte ions within the electrode, which is often realized by either physical or chemical post‐synthesis activation processes ,,. The physical activation includes that precursors are first carbonation at high temperature (600‐1000 °C) in N 2 or Ar atmosphere followed by activation with CO 2 steam, air, or their mixtures .…”