Tuning The Surface Roughness And Wettability Of Vertical Graphene Channels For Supercapacitor Energy Storage

Graphene has been regarded as the most promising active material in supercapacitor. But restacking of graphene sheets has limited its energy storage ability. A novel vertically-oriented graphene (VG), with non-agglomerated morphology and exposed edges, was proposed for supercapacitor energy storage. And the wettability of VG has strong effects on the actual electrochemically accessible electrode surface area. In this thesis, the wettability of VG was controlled by tuning the surface roughness, which will promote the development of VG supercapacitor.VG with high density of graphene branches was fabricated with MW-PECVD method by changing the synthesis condition. Further, the morphology was modified with the capillarity-driven modification method. With the interaction of liquid and solid, the density of graphene branches on the VG was controlled by employing several solvent with different surface tensions. VG with low density of branches was gained. However, the unique orientation and network were kept and the chemical compositions of VG has not changed. The VG was reconstructed with a facile method for the topographical surface structuring.Since the structure of VG was essential to its wetting behavior. Further investigations on the wettability on VG with different morphology were conducted. The water contact angles of VG were measured. Water droplet spread on the surface of VG with plenty of branches (VGs-O), forming a contact angle of 3°, which was regarded as the superhydrophilic surface. The case of VG with relatively low density branches (VGs-W) was extremely different from the case of VGs-O. The contact angle of VGs-W was measured as 108.8°, a typical hydrophobic state. Also, VGs-A, with the branches density between two above types of VG, formed a contact angle of 40.6°. Thus the wettability of VG, ranging from superhydrophilicity to hydrophobicity, was controlled by changing the structure of VG. The capillary units formed by tiny branches played the significant role in the wetting behavior of VG.As an application of morphology-controlled VG, the tunable supercapacitors were fabricated. The electrochemical behaviors including cyclic voltammetry and electrochemical impedance spectroscopy were measured. The high density graphene branches on VGs-O generated plenty of open edges for capturing ions in the electrolyte. Hence the volumetric capacitance of VGs-O was 4.08 F/cm3, far beyond the capacitance of VGs-W. Though, the relatively smooth channels in the VGs-W made it quite easier for ions the pass through. And the -45° response frequency in Bode plots was 1258 Hz. The frequency of VGs-O is relatively low 50 Hz. The tunable electrochemical behaviors supercapacitors were fabricated with structure modified VG electrodes.In this research, a facile macro-approach was employed into modifying microstructure of nanomaterials, achieving the control of wetting and electrochemical behaviors.