Study On The Synthesis And Supercapacitor Properties Of Vertically Aligned Graphene

Electric double-layer capacitors(EDLC), also known as supercapacitors or ultracapacitors, are promising for energy storage because of advantages including high power capability, high cycle efficiency, long cycle life and being maintenance free. Although tremendous progress has been made, the relatively low specific capacitance of EDLCs still limits their practical application. Vertically aligned few-layer graphene(VFG), which are synthesized by plasma enhanced chemical vapor deposition(PECVD), can be used as electrode materials to fabricate supercapacitors with large specific surface area, good conductivity and ultra-high capacitance. In this paper, VFG were in-situ synthesized on the Pt/Si substrate by PECVD and used directly as the supercapacitor collector, then the EDLC electrochemical properties were investigated.The preparation technology of VFG synthesized on the Pt/Si substrate by PECVD were studied. We focused on the influence of parameters, such as deposition temperature, deposition time, radio-frequency power, total gas pressure and gas proportion on the VFG synthesis process. Optimized parameters were achieved and the as-synthesized VFG owns good quality, appropriate quantity, low-degree defects with transparent and thin sheets, the sheet sizes are around 100 nm. The synthesis mechanism of VFG on the Pt/Si substrate by PECVD are also be explained by analyzing the SEM and HRTEM results of the interface between VFG and Pt substrate.A way to improve the performance of VFG-based supercapacitors by introducing self-defects is proposed to simultaneously realize 3D structure and good wettability in VFG, aiming to overcome the VFG’s poor wettability. The degree of surface defects in the VFG samples is found to be sensitive to slight changes of Ar:H2 gas ratio. The VFG with a high density of defects owns a better wettability with liquid electrolyte because of its relatively high surface energy, the ratio of ID/IG rises from 1.3 to 2.0, the contact angle drops from 131° to 73°, the specific capacitance can peach up to 700 μF/cm2@2m V/s, it also exhibited good stability, retaining about 91.2% of the original capacitance after 3000 cycles. The defect-stimulated wettability way to improve capacitance reveals that a correlation exists between the degree of defects, wettability and supercapacitor properties of VFG.Based on the thought of defect-stimulated wettability to promote capacitance, we also proposed a surface treatment method for VFG electrode material by Ar ion post-etching. Ar ion vertically post-etching method could be “one step” operated and cost low energy, the capacitance can be elevated 110%. The improved Ar ion post-etching ultilizes an angular surface, after two-side etching, the degree of defects were significantly increased, and the contact angle decreased from 131° to 33°, the wettability gets improved. The capacitance lifts significantly from 263 μF/cm2 to 1014 μF/cm2 C@ 2 m V/s, the capacitance elevates 285%. Eventually we achieve ultra-high VFG-based electric double-layer capacitor performance.