Low-cost Supercapacitors for Household Electrical Energy Storage and Harvesting J. Chmiola, P. Gogotsi, R. Weerasooriya,* Y. Gogotsi Drexel University, Department of Materials Science and Engineering 3141 Chestnut Street, Philadelphia, PA 19104, USA *Permanent address: J.R. Masterman High School, Philadelphia, PA 19130, USA Large-scale adoption of alternative energy sources, and more efficient use and recovery of energy will require the development of new, better and less expensive devices for electrical energy storage [1]. These devices must store energy generated by multiple sources (e.g., a wind turbine and a solar panel), and deliver it to various systems such as lights, emergency sensors, or automatic door openers in every household. Supercapacitors, as devices that store electrical energy electrostatically, can be used in applications where batteries cannot provide sufficient power or charge- discharge rates [2]. They can also harvest/recover energy from fast repetitive motion that is wasted nowadays, decreasing the consumption of energy from the grid. However, until now, their high cost, compared to batteries with similar performance, has been limiting the use of supercapacitors in many household and other cost- sensitive applications [3]. The objective of this work was to build supercapacitors for electrical energy storage and harvesting by using inexpensive activated carbon and other commonly available materials. In this project, a high-school student and a high- school teacher, guided by a graduate student, built a supercapacitor that had performance comparable to expensive commercial supercapacitors, showing the feasibility of designing and manufacturing inexpensive supercapacitors for every home. (a) (b) (c) Figure 1. 15cm x 15cm electrodes with average thickness of 656µm constructed by Method 1 (a) and 15cm x 7.5cm electrodes with average thickness of 583µm constructed by Method 2 (b). Both are shown after testing, which produced some rusting of stainless steel mesh. Electrodes assembled on stainless steel foil are shown in (c). The basic design of the prototype was a two electrode prismatic cell. This design (a pouch cell) is used by JEOL, Japan (http://www.jeol.com/), for solar battery supercapacitors [4]. The prototype contains two electrodes, on current collectors, pressed together but separated by a porous membrane (Figure 2). Multiple prismatic cells can be assembled to reach higher voltage if necessary (110 V is required in many household devices and for grid power stabilization). Two methods were used for electrode preparation. In Method 1, we mixed 5wt% PTFE with 95wt% carbon in ethanol (solvent). After that, it was necessary to heat and stir the mixture until ethanol evaporated and PTFE (binder) evenly distributed in carbon. We worked the electrode until pliable and rolled it into an electrolyte sheet. Finally, the electrode was cut to size (Fig. 1a). In Method 2, paint was prepared by mixing 5.5g of activated carbon with 0.55g of PVDF (binder) and 50mL of DMF (solvent). These chemicals can easily be ordered via online catalogues and were used for the paint, because PTFE-based paint does not produce a smooth and adhesive coating. The solution was heated, stirred and decanted until it had become thick in consistency. The cut electrodes (Fig. 1b) were painted onto current collectors and dried in an oven at 80ºC. A simple supercapacitor was constructed by inserting two electrodes in a beaker or an empty milk container containing a 1M NaCl solution in water as an electrolyte, and connecting them to the potentiostat. Capacitance values were calculated from the slope of the discharge curve at constant current for the designed and commercial capacitors at currents ranging from 10mA to 2A. Capacitance of up to 80F was achieved at currents below 0.5A, but it decreased at 1A and above. On the other hand, the capacitor electrodes produced by Method 2 showed a lower total capacitance (10-20 F), probably due to a lower surface area available, but its capacitance was as stable as that of commercial 22F and 50F capacitors up to 2A. Other methods of process simplification and optimization, such as electrode preparation by mixing ground activated carbon with a saturated NaCl solution and selection of the best household tissue for separators, are being explored. A capacitor of that kind can be constructed using activated carbon, rock salt, metal foil and old plastic containers at home and can provide back- up power being charged from renewable energy sources or harvesting energy lost nowadays. This will decrease our dependence on oil and the electrical grid, helping in the construction of smart and green housing. Acknowledgements: We acknowledge funding from the NSF Research Experience for Teachers program. J.C. was supported by a NSF graduate fellowship. References: 1. B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. 1st ed. Springer, 2004. 2. Basic Research Needs for Electrical Energy Storage. Report of the Basic Energy Science Workshop in Electrical Energy Storage. U.S. Department of Energy, 2007. 3. J. Chmiola, Y. Gogotsi, Supercapacitors as Advanced Energy Storage Devices, Nanotechnology Business and Law, 4, 577-584 (2007). 214th ECS Meeting, Abstract #46, © The Electrochemical Society ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 54.166.214.45 Downloaded on 2016-07-31 to IP