Carbon Nanotube-based MEMS Energy Storage Devices

Carbon nanotube (CNT) forests have been utilized as electrodes in supercapacitors in this work for energy storage applications. High surface area to volume ratio, good electrical conductivity, and low contact resistance to a bottom metal electrode make CNT forests attractive as electrodes in supercapacitors. Several approaches have been investigated to improve the performances such as configurations, power and energy density of CNT-based supercapacitors, including the single layer architecture by utilizing interdigitated finger electrodes, pseudo capacitors based on electroplated nickel nanoparticles, ultra-long and densified CNT forests electrodes.
Vertically aligned CNT forests have been synthesized using the thermal CVD process and their sheet and contact resistances have been characterized with four distinct methods: (1) the transfer length method (TLM), (2) the contact chain method, (3) the Kelvin method, and (4) the four pointprobe method. Experimental results show that CNT forests of 100μm in height and 100μm in width have a sheet resistance of about 100Ω/□. The specific contact resistance to a current collector is 5×10^4Ω·μm^2. Consistent results from these methods have been observed and less than 0.9% resistance deviations were measured after two months of open-air storage.
In the first development stage, planar supercapacitors based on CNT forests electrodes with interdigitated finger shapes have been fabricated using a combination of Mo/Al/Fe metal stack layers to achieve dense growth of CNT with low resistance. The specific capacitances of the prototype electrodes were measured to be about 1000 times higher than those made of flat metal electrodes. Furthermore, charging/discharging experiments show over 92% energy storage efficiency and robust cycling stability.
In the second development stage, CNT forests with embedded nickel nanoparticles have been used as electrodes for pseudo supercapacitors. A vacuum infiltration process is used in the electroplating process for the uniform deposition of 30-200nm in diameter nickel nanoparticles within the 80μm-high CNT forests. The measured specific capacitances are up to one order of magnitude higher than those CNT forests electrodes without nickel nanoparticles. No visual morphologic change was observed on nanoparticles after 1000 cycles of cyclic voltammetry tests.
In the third development stage, ultra long CNT forests were synthesized using a water-assisted CVD process. Experimental results confirmed the capacitance increments were linearly proportion to the height increase of CNT forests with good long term stability.
In the fourth development stage, a two-stage, self-aligned liquid densification process has applied on CNT forest to shrink the volume of CNT forests electrodes. By combining both mechanical bending and liquid densification, the height of CNT forest shrunk from 320μm to 21μm. Experimental results show self-aligned and continuous CNT films with preserved bottom contacts to the conductive metal layer. These densified CNT forests electrodes had similar total capacitances before and after the densification process while the volumetric specific capacitance increased from 1.07F/cm3 to 10.7F/cm3 because of the volume reductions. 
Publication date: 
May 31, 2011
Publication type: 
Ph.D. Dissertation
Jiang, Y. (2011). Carbon Nanotube-based MEMS Energy Storage Devices. United States: University of California, Berkeley.

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