AC Line Filtering Research Articles

Vertically-Oriented Graphene Electric Double Layer Capacitor Designs

High-voltage electric double layer capacitors (EDLCs) capable of efficient AC line-filtering have been developed. They were fabricated with vertically-oriented graphene nanosheet (VOGN) electrodes using a planar design. Two approaches were examined to series connect EDLC cells and thus achieve high-voltage operation. Electrical performance of VOGN electric double layer capacitors fabricated with an ionic liquid electrolyte was measured at temperatures up to 125°C. Volume comparisons are made between VOGN electric double layer capacitors and aluminum electrolytic capacitors. A practical design is presented that provides the VOGN electric double layer capacitor with more than an order-of-magnitude higher ripple-current filtering performance (120-Hz) than available from present capacitor technology.

High-Voltage, High-Frequency Electrochemical Capacitor

Electric double layer capacitors (EDLCs) with electrical performance like aluminum electrolytic capacitors have been developed.(1) They can be smaller in size and offer higher reliability. Operation of such devices relies on charge stored on vertically oriented graphene nanosheet (VOGN) carbon. Vertically oriented graphene nanosheets are prepared using RF plasma enhanced chemical deposition of a hydrocarbon feed gas.(2) Under proper growth conditions, VOGN are ohmically connected to the metal current collector on which they grow, offer high electronic conductivity, and have an “open” structure totally free of porous electrode behavior. Each of these characteristics is necessary for efficient operation at frequencies of 1 kHz and higher.EDLC cells, like battery cells, operate at low voltage. Thus cells must be series connected to operate at high-voltage. Figure 1 shows an approach devised for interconnecting the EDLC cells to create a high-voltage, high-frequency EDLC. It involves interconnecting planar EDLC cells using the substrate metallization. As shown, interdigitated gaps are created in the VOGN that has been grown on a thin metal layer covering an insulating substrate, for instance, nickel on alumina. The gap is created using a laser to cut through the VOGN and metal down to the insulating substrate. This then creates multiple electrically-isolated regions (six are shown in the figure). Two adjacent regions become a single EDLC after electrolyte is applied over the gap and the surface of that region. To make a series connection of cells, each gap must be individually covered with an electrolyte, making sure that the electrolyte band does not touch the neighboring bands. This is a “bipolar” design in two dimensions, the substrate metallization serving as the bipolar plate. Connecting M cells in series each rated at voltage Vo with capacitance Co and series resistance Ro yields a capacitor with a voltage rating M·Vo, capacitance Co/M, and series resistance M·Ro. The frequency response of this high-voltage capacitor is nearly identical to that of one of its cells because the interconnects add minimal resistance. Voltage-balance resistors can be added using, for instance, thick-film printing processes. Details of the fabrication process are described and electrical performance data on these high-voltage, high-frequency electric double layer capacitors is presented. References 1J.R. Miller el al, “Graphene Double-Layer Capacitor with ac Line-Filtering Performance”, Science 329 , 1637 (2010). 2Cai M, Outlaw RA, Butler SM, and Miller JR, “A High Density of Vertically-Oriented Graphenes for Use in Electric Double Layer Capacitors”, Carbon 50, 5481–5488 (2012)..

Ultrahigh-rate supercapacitors based on eletrochemically reduced graphene oxide for ac line-filtering

The recent boom in multifunction portable electronic equipments requires the development of compact and miniaturized electronic circuits with high efficiencies, low costs and long lasting time. For the operation of most line-powered electronics, alternating current (ac) line-filters are used to attenuate the leftover ac ripples on direct current (dc) voltage busses. Today, aluminum electrolytic capacitors (AECs) are widely applied for this purpose. However, they are usually the largest components in electronic circuits. Replacing AECs by more compact capacitors will have an immense impact on future electronic devices. Here, we report a double-layer capacitor based on three-dimensional (3D) interpenetrating graphene electrodes fabricated by electrochemical reduction of graphene oxide (ErGO-DLC). At 120-hertz, the ErGO-DLC exhibited a phase angle of −84 degrees, a specific capacitance of 283 microfaradays per centimeter square and a resistor-capacitor (RC) time constant of 1.35 milliseconds, making it capable of replacing AECs for the application of 120-hertz filtering.

Graphene Double-Layer Capacitor with ac Line-Filtering Performance

Electric double-layer capacitors (DLCs) can have high storage capacity, but their porous electrodes cause them to perform like resistors in filter circuits that remove ripple from rectified direct current. We have demonstrated efficient filtering of 120-hertz current with DLCs with electrodes made from vertically oriented graphene nanosheets grown directly on metal current collectors. This design minimized electronic and ionic resistances and produced capacitors with RC time constants of less than 200 microseconds, in contrast with ~1 second for typical DLCs. Graphene nanosheets have a preponderance of exposed edge planes that greatly increases charge storage as compared with that of designs that rely on basal plane surfaces. Capacitors constructed with these electrodes could be smaller than the low-voltage aluminum electrolyte capacitors that are typically used in electronic devices.