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 V_o with capacitance C_o and series resistance R_o yields a capacitor with a voltage rating M·V_o, capacitance C_o/M, and series resistance M·R_o.   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).

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