A direct-growth approach for fabricating aligned edge-oriented graphene array in an on-chip manner towards integrated electrochemical (EC) energy devices will be presented. Two types of substrates as current collectors, specifically, Si wafer and carbon cloth (CC) will be demonstrated. The use of Si wafer for direct graphene growth makes ease a potential integration with the matured Si-based device technology; whereas CC offers the opportunity to make flexible energy devices. However, for all energy devices, high-performance power/energy output requires efficient carrier generation and subsequent transfer across heterogeneous interfaces (e.g., electrode-electrolyte, catalyst-electrolyte, catalyst-support and electrode-collector). Both the architecture design of graphene-based electrode (or catalyst support) and the associated interface control are crucial. The edge-oriented graphene arrays were first synthesized by microwave plasma-enhanced chemical vapor deposition (CVD). By tuning the CVD process parameters, Raman spectroscopic evidence of strain, both in tensile and compressive, in the SiC-supported graphene has been observed. Chemical doping can be done in the same plasma reactor via either in-situ or ex-situ manners. Interestingly, for nitrogen doping in graphene, plasma process resulted in selective bond configuration to be pure pyridinic N-doping, as verified by the core-level X-ray photoemission spectroscopy studies. It was found that N-doping improves the wettability of graphene to electrolyte solution, and in turn increases its electrochemical activity. When applied as an EC electrode, the arrayed graphene exhibits nearly reversible redox characteristics. Cyclic voltammetry (CV) and EC impedance measurements reveal a fast electron transport and a low resistance of charge transfer of graphene. Remarkably, the arrayed N-doped graphene exhibits the maximum specific capacitance of 991.6 F/g with a measured energy density of 275.4 Wh/kg, at power density of 74.1 kW/kg in an aqueous electrolyte; all these are record high values reported till date. A simple route to further attach Pt and Ru nanoparticles onto graphene with more efficient utilization of precious metals, while achieving enhanced catalytic activity and electron kinetics has also been developed. Graphene samples with controllable compressive or tensile strains show strong effects on the resultant catalytic and EC properties. CV studies for electro-oxidation of methanol show a high ratio of forward anodic peak current to reverse peak current, suggesting an efficient oxidation of methanol to CO2 on the Pt/graphene electrode. Our approach offers an alternative route toward wafer-scale fabrication of graphene not only for fundamental research but also opens up various potential applications in electronics, sensing, catalysis, and energy.

Plasma-assisted CVD growth and chemical doping of arrayed graphene for electrochemical energy applications

244th National Meeting (2012)

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