CAI ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Fast Response, Vertically Oriented Graphene Nanosheet Electric Double Layer Capacitors Synthesized from C 2 H 2 Minzhen Cai, † Ronald A. Outlaw, †, * Ronald A. Quinlan, †,‡ Dilshan Premathilake, † Sue M. Butler, § and John R. Miller §,^ † Department of Applied Science, The College of William and Mary, Williamsburg, Virginia 23187, United States, ‡ Materials and Power Systems Branch, Naval Surface Warfare Center, Carderock Division, West Bethesda, Maryland 20817, United States, § JME, Inc., 23500 Mercantile Road Suite L, Beachwood, Ohio 44122, United States, and ^ Case Western Reserve University, Great Lakes Energy Institute, Cleveland, Ohio 44106, United States C apacitors capable of working at high frequency are in a growing demand. 1 Many applications, such as high-power load leveling, ripple-current filtering, power conversion, and fast pulsed power generation, require capacitors to have quick response in absorbing and releasing energy. 13 To date, aluminum electrolytic capacitors are still the primary capacitors used in these applica- tions; however, they are large, bulky and typically fail abruptly, impacting their reli- ability. The traditional electric double layer capacitors (EDLCs) that are typically based on porous carbon electrodes are considered slow response devices when compared to electrolytic capacitors and cannot meet the filtering requirements. For example, conventional carbon-based EDLCs have an impedance phase angle that is near 0° at 120 Hz, which is far from the 90° value required for ripple-current filtering. 46 Sub- stantial efforts have been devoted to devel- oping fast response EDLCs. Carbon nano- tube EDLCs achieved an impedance phase angle at 120 Hz of 10° to 65° but were unable to efficiently filter ripples. 710 Chem- ically modified graphene EDLCs show very high capacitance; however, the inher- ent small pore diameters and long path lengths create unwanted porous electrode behavior that severely limit effective filtering. 1114 Recently, electrochemically reduced graphite oxide electrodes were reported and exhibited an impedance phase angle of 84° at 120 Hz, however, with limited capacitance. 15 EDLCs made from vertically oriented graphene nano- sheet (VOGN) using radio frequency plasma * Address correspondence to raoutl@wm.edu. Received for review February 16, 2014 and accepted May 5, 2014. Published online 10.1021/nn5009319 ABSTRACT The growth and electrical characteristics of vertically oriented graphene nanosheets grown by radio frequency plasma-enhanced chemical vapor deposition from C 2 H 2 feedstock on nickel substrates and used as electrodes in symmetric electric double layer capacitors (EDLC) are presented. The nanosheets exhibited 2.7 times faster growth rate and much greater specific capacitance for a given growth time than CH 4 synthesized films. Raman spectra showed that the intensity ratio of the D band to G band versus temperature initially decreased to a minimum value of 0.45 at a growth temperature of 750 °C, but increased rapidly with further temperature increase (1.15 at 850 °C). The AC specific capacitance at 120 Hz of these EDLC devices increased in a linear fashion with growth temperature, up to 265 μF/cm 2 (2 μm high film, 850 °C with 10 min growth). These devices exhibited ultrafast frequency response: the frequency response at 45° phase angle reached over 20 kHz. Consistent with the increase in D band to G band ratio, the morphology of the films became less vertical, less crystalline, and disordered at substrate temperatures of 800 °C and above. This deterioration in morphology resulted in an increase in graphene surface area and defect density, which, in turn, contributed to the increased capacitance, as well as a slight decrease in frequency response. The low equivalent series resistance varied from 0.07 to 0.08 Ω and was attributed to the significant carbon incorporation into the Ni substrate. KEYWORDS: graphene . EDLC . electrodes . RF-PECVD . AC filtering . energy storage ARTICLE