Nano-scale Electromechanics for Supercapacitors
Supercapacitors are known by several names including ultracapacitors, double-layer capacitors, and electrochemical supercapacitors, which is a misnomer because unlike the case for a battery, there are no chemical reactions between the ions and the electrode. In the uncharged state the positive and negative ions are uniformly distributed in the electrolyte. When a potential difference is applied between two electrodes, the ions diffuse to form electrostatic double layers at the electrode/electrolyte interface. The thickness of the double layer is typically 1 nm.
Because a supercapacitor does not undergo a chemical reaction, the charge and discharge is due solely to ionic diffusion. Thus, a supercapacitor can charge and discharge at a rate that is an order of magnitude faster than the rate for a battery. Supercapacitors also have ten times greater specific power and fifty times more cycles (~1 million).[i]
Current supercapacitor designs are engineered on the micro-scale and therefore suffer from two shortcomings: (1) Most supercapacitors use carbon aerogels for electrodes. Although carbon aerogels are nano-porous, each aerogel electrode is ten to hundreds of microns thick. Thus, the ionic diffusion paths (microns) are long relative to the porosity (nanometers), so the maximum power density is not realized; and (2) A porous polymer “separator” is inserted between the two electrodes to prevent them from contacting and short-circuiting. These separators are typically 30 microns thick and they contribute nothing to the energy or power of the device, and they therefore lower the energy and power densities.
Both of these shortcomings may be eliminated by using nanofabrication to make supercapacitors in which the electrodes are separated by only tens of nanometers and the polymer separator is eliminated. Such nano-engineered supercapacitors will have a power density that is at least ten times higher than current designs.
Freely-suspended nano-scale electrodes will tend to move due to osmotic forces, electrostatic forces, and van der Waals forces.[ii] With no separator, it is imperative to understand this motion (or kinematics) to prevent the electrodes from contacting.
The objective is two-fold: (1) Derive theoretical relationships for the fully coupled energy, power, and mechanics (both forces and kinematics) of nano-scale supercapacitor electrodes; (2) Experimentally verify this theory by using nanolithography to make supercapacitors with nano-scale-features.
We will use nanolithography to make nano-scale arrays of interdigitated electrodes. The electrodes will have the form of fibers fixed only at their ends. These fibers will be fabricated by first evaporating metal (Cr/Au) on a sacrificial insulator such as silicon nitride. The electron beam will then be used to pattern e-beam resist on the metal. The exposed metal will be etched away using the patterned resist as a mask, the resist will be stripped, and the sacrificial substrate under the fiber electrodes will be etched away. What remains is a fiber array with mechanical supports and electrical contacts at the two ends of each fiber.
The electromechanical properties will be characterized by augmenting the current/voltage measurements with a nanoindentor (with a non-conducting tip) to measure the force and displacement of the fiber electrodes. The PIs recently purchased (installed August 2001) this nanoindenter (Hysitron, Inc., approximately $220,000) using funds provided by Texas A&M University. The nanoindenter includes an integral scanning probe microscope, a type of AFM. The load resolution is 1 nN and the displacement resolution is less than one angstrom.
Electric and hybrid electric vehicles can gain widespread use only if their power-to-weight ratios are dramatically increased. The industry/government collaborative effort “Partnership for a New Generation of Vehicles” calls for weight reductions in cars of about 50%, from 3,000 pounds to 1,500 pounds. Equivalently, supercapacitors could be used to increase the power density. Thus, supercapacitors will have significant environmental impact because they will facilitate the introduction of electric and hybrid electric vehicles, which have greatly reduced toxic emissions and greenhouse gases with respect to conventionally-powered vehicles.
Urban traffic results in stop-and-go driving. In heavy urban traffic more than half of the total energy consumed is dissipated in the brakes.[iii] Therefore regenerative braking, which recovers and stores the kinetic energy that is lost during braking, will be included on electric and hybrid electric vehicles. Because the braking typically occurs in less than 20 seconds, the energy storage device must be able to accept the energy at a high rate. Supercapacitors can be 50% charged in 20 seconds, whereas batteries require at least 5 minutes for a 50% charge, and hours for a full charge. Also, in urban driving an acceleration typically follows a breaking, and the supercapacitor will be ideal for providing the high power needed for acceleration. The nano-scale supercapacitors proposed herein will have even faster charging times.
[i] 1999 Conway, B.E., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic / Plenum Publishers, New York.
[ii] 1992 Israelachvili, J., Intermolecular and Surface Forces, Academic Press, N.Y.
[iii] 1999 Gao, Y., Chen, L., and Ehsani, M., “Investigation of the Effectiveness of Regenerative Braking for EV and HEV,” in Electric and Hybrid Electric Vehicles and Fuel Cell Technology, SAE SP-1466, Society of Automotive Engineers, Inc.,Warrendale, PA, pp. 25-31.