Silicon and Silicon Carbide Nanowires: Synthesis, Characterization, Modification, and Application as Micro-Supercapacitor Electrodes

For applicationin mobile and remote sensing platforms, microsupercapacitors are attractive energy storage devices due to their robust lifetimes and high specific power capacity. Utilization of green electrolytes in these devices reduces environmental impact and simplifies packaging by avoiding the stringent oxygen and moisture free conditions required for organic and ionic liquid based electrolytes. Porous silicon nanowire based microsupercapacitor electrode materials are promising for onchip applications using an environmentally benign aqueous electrolyte, 1M KCl, however they are prone to oxidationA silicon carbide coating was found to mitigate this issue. The fabrication techniques, involving lowtemperature electroless etching of silicon, are compatible with current integrated circuit processing methods and may be readily integrated at the microdevice level. The electrode materials are in good electrical contact with the underlying substrate and require no additional current collector. The base porous silicon nanowires are coated with a thin silicon carbide passivation layer by low pressure chemical vapor deposition. The demonstrated capacitance of the electrode materials, ~1700 μF/cm2 projected area, is comparable to other carbon based microsupercapacitor electrodes, remains stable over many charge/discharge cycles, and maintains capacitive behavior over a wide range of charge/discharge rates.
An improved passivation method for the porous silicon nanowires has also been developedThe selective coating procedure deposits an ultrathin (~ 13 nm) carbon sheath over the nanowires and passivates them. The ultrathin nature of the coating enables solvent access to the pore area and hence a large improvement of active specific surface over the SiC coated PSiNWs discussed above. The electrochemical performance of these coated nanowires is characterized in both an aqueous electrolyte and an ionic liquid electrolyte. Specific capacitance values reaching 325 mF cm^-2are achieved in ionic liquid, and calculations indicate that the theoretical maximum capacitance of the pristine wires is reached. TEM studies confirm the coating thickness and its conformality. Raman spectroscopy indicates that the carbon in the coating is mainly sp2 hybridized, with corresponding high conductivity.  At the time of writing, these materials represent the largest specific energy microsupercapacitor electrode published. A test device is prepared and demonstrated powering an LED.
The testing resultsof silicon carbide (SiC) nanowires (NW) as an electrode material for micro-supercapacitorsis described.SiC NWs are grown on a SiC thin film coated with a thin Ni catalyst layer via chemical vapor deposition. A specific capacitance of ~240 μF cm-2is demonstrated. Charge-discharge studies demonstratethe SiC nanowires exhibit exceptional stability, with 95% capacitance retention after 2×105charge/discharge cycles in an environmentally benign, aqueous electrolyte.Doping of the nanowires with nitrogen through the addition of 5 at% ammonia to the precursor gas flow rate improves the conductivity of the nanowire films by over an order of magnitude leading to increased power capabilities.
A method to transfer silicon and silicon carbide nanowire arrays to arbitrary substrates while maintaining electricalcontact through the entire array is elucidated. The nanowires are grown on graphene sheets on SiO2 coupons. The graphene acts as both the flexible material for maintaining structural continuity and electrical contact through the array during transfer. The SiO2 acts as the sacrificial growth substrate which is etched after growth in order to release the nanowire/graphene hybrid. The nanowire/graphene hybrids are structurally characterized by XRD and electron microscopy. Good electrical contact is confirmed through testing of the SiCNW/graphene hybrids as supercapacitor electrode materials in an aqueous electrolyte. The specific capacitance, ~340 F cm-2, is similar to SiCNWarrays grown on oxide while the electrical conductivity is improved and cycling stability tests show less than a 1% decrease in capacitance after 10,000 cycles.
John Paul Alper
David B. Graves
Publication date: 
May 31, 2014
Publication type: 
Ph.D. Dissertation
Alper, J. P. (2014). Silicon and Silicon Carbide Nanowires: Synthesis, Characterization, Modification, and Application as Micro-Supercapacitor Electrodes. United States: University of California, Berkeley.

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