Silk screen printing is a familiar process used to print T-shirts, stickers, textiles and circuit boards. But can you use this technique to print capacitors? A company called Cellergy thinks you can. It has adapted this method to print bipolar electrochemical double layer capacitors (ELDCs). But before we get into this printing technique, let's take a closer look at an ELDC.
An electrochemical double layer capacitor is essentially a battery that has low energy content but a very high power capability. Its large capacitance results from the physical interface between solid and liquid. Upon applying voltage to this interface, charge separation occurs. On the solid side, electrons accumulate, and on the liquid side, counter ions accumulate thus forming an EDLC. Enormous capacitances can be achieved. They are much larger than the capacitances of standard capacitors due to the use of activated carbon with large surface areas (~2000 m2/gr) as the solid electrode material.
EDLCs, often called super capacitors, are found in some power related applications in the electronics industry such as GSM burst transmission, PCMCIA cards, and medical devices. An EDLC coupled with a battery can supply large power pulses backed by the energy pool of the battery. Most EDLC manufacturers commonly use comparatively slow manufacturing processes to produce EDLCs, such as winding of films or "pick and place" of capacitor components, thus leading to a costly product. The production of some products even necessitate the use of extremely dry electrolytes requiring dry rooms and complex electrolyte filling processes.
Cellergy recognized that the high price of EDLCs is the main issue slowing the entrance of these devices into the electronics market. A new core technology implementing screen printing and the use of cheap raw materials is expected to lead to low price EDLCs. In principle, screen printing can allow printing of many capacitors simultaneously in a prefixed array using a simple and well known technique. This process can be repeated multiple times, forming a wafer consisting of many separate EDLCs. The wafer can then be cut to produce hundreds of individual multi-layer capacitors.
This method can be used to print many capacitor shapes and sizes easily, rapidly and cost-effectively. Many barriers had to be overcome to adapt this printing technology to multi-layer electrochemical printing. Unique electrode pastes had to be developed to achieve screen or stencil printing capability. Because the maximum voltage of a single aqueous capacitor is one volt and thus much less than that used in electronics, Cellergy had to achieve higher EDLC voltages. This could have been done by connecting single one-volt capacitors in series to achieve higher voltages, but this method is inefficient. Instead, the company developed an accurate printing process enabling consecutive printing of electrode paste in a matrix array layer while insuring an accurate method to encapsulate the paste. This process of printing wet material is repeated multiple times to achieve the required voltage and also to achieve a lock-tight and bipolar structure.
This innovative technology is protected by several PCTs and international patents describing the different aspects involved in Cellergy’s manufacturing process and the material composition of the EDLCs. The screen printing technology permits printing of varying thickness of electrodes resulting in Cellergy capacitors with similar sized footprints (see Table 1), but with a wide range of capacitances as seen in Figure 1. Ion movement in aqueous electrolytes is much faster than in organic electrolytes giving the EDLC a fast response time. Due to the choice of aqueous electrolyte, Cellergy achieves a wide working temperature window (see Figure 2) difficult to achieve in aqueous EDLCs. Some examples of Cellergy ELDCs are shown in Figure 3.
Contact Information: Cellergy Ltd., +972-3-9415751; fax: +972-3-9415753; E-mail: [email protected]; Web site: www.cellergy.co.il
Company: CELLERGY LTD.
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