Three dimensional MEMS supercapacitors
Appears in the following Collection
- Fysisk institutt 
AbstractThe overall objective of this research is to achieve compact supercapacitors with high capacitance, large power density, and long cycle life for using as micropower sources to drive low power devices and sensors. The main shortcoming of supercapacitors as a power source is that its energy density typically is about 1/10 of that of batteries. To achieve compact supercapacitors of large energy density, supercapacitors must be developed with high capacitance and power density which are mainly depended on the effective surface area of the electrodes of the supercapacitors. Many studies have been done to increase the effective surface area by modifying the electrode materials, however, much less investigations are focus on machining the electrodes. In my thesis work, micro- and nano-technologies are applied as technology approaches for machining the electrodes with three dimensional (3D) microstructures. More specific, Micro-electro-mechanical system (MEMS) fabrication process flow, which integrates the key process such as LIGA-like (German acronym for Lithographie, Galvanoformung, Abformung, which mean Lithography, Electroplating and Molding) technology or DRIE (deep reactive ion etching), has been developed to enable innovative designs of 3D MEMS supercapacitors which own the electrodes of significantly increased geometric area. Two types of 3D MEMS supercapcitors, based on LIGA-like and DRIE technology respectively, were designed and successfully created. The LIGA-like based 3D MEMS supercapacitor is with an interdigital 3D structure, and consists of silicon substrate, two electroplated nickel current collectors, two PPy (polypyrrole) electrodes, and solid state electrolyte. The fabrication process flow developed includes the flowing key processes, SU-8 lithography, nickel electroplating, PPy polymerization and solid state electrolyte coating. Electrochemical tests showed that the single electrode of the supercapacitor has the specific capacitance of 0.058 Fcm-2 and the specific power of 0.58 mWcm-2 at 20mVs-1 scan rate. The 3D MEMS supercapacitor fabricated in this project has the specific capacitance and the specific power of 0.029 Fcm-2 and 2.2 mWcm-2 respectively at a relative large discharge rate of 5 mAcm-2. It is also found vi that the supercapacitors have the performance of broad frequency range up to 300Hz. For DRIE based 3D MEMS supercapacitor, the innovative designs were developed based on silicon micromachining process flow which includes the key processes such as thermal oxidation, RF sputtering, wet etching, DRIE, electroless plating and PPy polymerization. The optimized PPy electrode doping with TOS- performed ideal supercapacitor properties in NaCl electrolyte. The single PPy electrode of the 3D MEMS supercapacitors can provide 0.128 Fcm-2 specific capacitance and 1.28 mWcm-2 specific power at 20 mVs-1 scan rate. The specific capacitance of the 3D MEMS supercapacitors equals 0.056 Fcm-2, and the specific power at 20 mVs-1 scan rate equals 0.56 mWcm-2. In addition, novel supercapacitors based on wafer level process are designed for flexible integration in applications such as high temperature electronics and hybrid power system for electric vehicles. Experimental work on TiO2 anodic oxidation, which enables the fabrication of the one of these designs, has been carried out. Dense TiO2 nanoholes with diameters ranged from about 90 to 270 nm were obtained in 0.05 wt% HF aqueous solutions with two-step anodic oxidation method. Comparing to the published specific capacitance (about 2 mFcm-2) for microsupercapacitors , I have achieved much larger specific capacitance (typically 0.029 to 0.056 Fcm-2) for 3D MEMS supercapacitors. The above results have been presented in 3 international conferences. Total 4 journal articles have been published, and one has been submitted. The article in the Journal of Power Source (appendix 3) has been cited 9 times after published in April 2009, and the article in Microelectronic Engineering (appendix 1) has been cited 4 times after published in December 2008, which shows the originality and the importance of the work. Keywords: MEMS, supercapacitors, three dimensional, specific capacitance, specific power.
List of papers. Papers I-IV are removed from the thesis due to copyright restrictions.
Paper I Wei Sun, Xuyuan Chen: Fabrication and tests of a novel three dimensional micro supercapacitor. Microelectronic Engineering 86 (2009) 1307–1310 doi:10.1016/10.1016/j.mee.2008.12.010
Paper II Wei Sun, Hai Sheng San, Ying Xian Duo, Xu Yuan Chen: Preparation and Characterization of Micro Polypyrrole Electrodes for Supercapacitor. Advanced Materials Research 60-61 (2009) 375-379 doi:10.4028/www.scientific.net/AMR.60-61.375
Paper III Wei Sun, Xuyuan Chen: Preparation and characterization of polypyrrole films for three-dimensional micro supercapacitor. Journal of Power Sources 193 (2009) 924–929 doi:10.1016/j.jpowsour.2009.04.063
Paper IV Wei Sun, Ruilin Zheng and Xuyuan Chen: Symmetric redox supercapacitor based on micro-fabrication with three-dimensional polypyrrole electrodes Journal of Power Sources 195 (2010) 7120–7125 doi:10.1016/j.jpowsour.2010.05.012