Layer-By-Layer Graphene Structures As Supercapacitor Electrode Materials

Supercapacitors, also called electrochemical capacitors, are one kind of energy storing devices with faster and higher-power energy storage. Compared to the traditional energy storage devices, supercapacitors are more suitable for rapid storing and releasing energy. During the past several years, a variety of graphene-based materials have been applied in supercapacitors, such as reduced graphene oxides, active carbon, microporous carbon and so on. However, very less attention has been paid recently to the electrochemical properties of graphene films with intrinsic flat structure prepared by chemical vapor deposition (CVD).

    In this study, in order to evaluate the supercapacitor performance of pristine CVD-grown graphene films, button supercapacitors were fabricated using layer-by-layer (LBL) graphene structures as electrode materials. Graphene was prepared on copper foils by CVD using methane as carbon source, and transferred to the current collectors layer by layer. MnO2 particles were deposited on graphene films using KMnO4 and alcohol as the chemicals. Button supercapacitors were assembled in a glove box with 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-PF4) as the non-aqueous electrolyte. As a result, the CV curves deviate from the rectangular shape due to the high internal resistance, the surface functional group and the properties of electrolyte. The fair symmetry of CV curves confirms the presence of the electrochemical double layer capacitance without pseudocapacitive effects. The CV curves are gradually closing to the standard rectangular shape along with the increase of the layer number. The specific capacitance increases from 0.10 mF/cm2 (1G, 50 mV/s) to 0.29 mF/cm2 (10G, 50 mV/s). At higher scan rates, the increase of specific capacitances is apparent, from 0.05 mF/cm2 (1G) to 0.20 mF/cm2 (10G) (200 mV/s). The increase of each layer of graphene film results in a ~20% increase (50 mV/s) and a ~30% increase (200 mV/s) in capacitance, indicating that electrolyte ions could be absorbed by each graphene layer, and stacked graphene layers on the current collector could improve the ability of storing ions. MnO₂ particles could increase the specific capacitance from 0.10 to 0.12 mF/cm² with the scan rate of 50 mV/s and from 0.05 to 0.06 mF/cm² at the scan rate of 200 mV/s, which can be attributed to the pseudocapacitive behavior of MnO₂ particles to introduce a high redox capacitance. The maximum energy density is 1.5 mWh/cm² and the highest power density is 0.09 mW/cm².

    In summary, button supercapacitors were assembled with LBL graphene films. The number of graphene layers influenced the performance of device. The sandwiched multi-layer structures with oxide deposition further improved the device electrochemical properties. However, the polycrystalline nature of CVD-grown graphene films introduced structural instability during charge-discharge process, resulting in degraded capacitive performance and cycling stability. Therefore, the flat graphene films with intrinsic “in-plane” structure might not be ideal candidates for electrode materials.