Vertically-Oriented Graphene Electric Double Layer Capacitor Designs

Abstract

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 Efficient AC line-filtering (120 Hz) by an electric double layer capacitor (EDLC) was first demonstrated in 2010 using electrodes of vertically-oriented graphene (VOGN) grown directly on nickel.1 This electrode material and its structure ( Figure 1) reduce series resistance to an absolute minimum value and effectively eliminate distributed charge storage, i.e. porous electrode behavior with its corresponding transmission-line-like electrical response. In brief, EDLC electrodes are made by growing the VOGN on bare nickel current collectors using radio frequency plasma enhanced chemical vapor deposition of CH 4 /H 2 or C 2 H 2 /H 2 feed gases.2,3 Growth temperature is typically in the 550 to 850• C range. No catalytic seed material is required or added to the nickel surface. Growth proceeds by Volmer-Weber island impingement, 4 which yields an upturned structure at carbon island boundaries. There is significant carbon dissolution into the nickel during growth that helps ensure low-resistance ohmic connection between the high-conductivity graphene sheets and the metal current collector. Typical characteristic VOGN spacing is ∼200 nm and heights ∼1 μm, a void length-to-width ratio that thwarts porous electrode behavior as found with activated-carbon. 5Generally the Raman D band to G band intensity ratio decreases with growth time indicating that crystalline order of the VOGN increases with nanosheet height, which increases approximately linearly with time at growth rates of 70 nm/minute using CH 4 and 190 nm/minute using C 2 H 2 .6 Electrode capacitance at 120 Hz typically is in the 200 to 300 μF/cm 2 range, depending on VOGN growth conditions and growth time. The frequency response reported for EDLCs made using other external-surface-area nanomaterials is generally below that demonstrated by VOGN grown on nickel because of higher series resistance and/or distributed charge storage behavior. 7-12State of the art commercial EDLCs, which use internal-surface-area activated carbon electrode materials, typically have a -45• phase angle at ∼0.2 Hz and show inductive behavior (positive phase angle) at 120 Hz, rendering them totally useless for AC line filtering. 13 EDLC DesignsA conventional EDLC "sandwich" design, as depicted in Figure 2, is poorly suited for capacitors made using V...

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