Fast Response, Vertically Oriented Graphene Nanosheet Electric Double Layer Capacitors Synthesized from C₂H₂

Abstract

The growth and electrical characteristics of vertically oriented graphene nanosheets grown by radio frequency plasma-enhanced chemical vapor deposition from C2H2 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 CH4 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.

Ultrahigh-rate supercapacitors with large capacitance based on edge oriented graphene coated carbonized cellulous paper as flexible freestanding electrodes

“…At 120 Hz, these two numbers are 0.6 mF cm À2 and 0.6 F cm À3 for our EOG/GCF electrodes. The corresponding phase angle at 120 Hz is À83 , comparable with the state of the art such as VOG on Ni substrate (À82 to À85 ) [8,11,12], POG in Ni foam (À82 ) [13], electrochemically reduced graphene oxide (ErGO) on Au (À84 ) [8], and patterned graphene CNT on metal coated silicon (À81.5 ) [23], but with a much larger volumetric capacitance when the whole electrode volume is considered. The frequency-dependent capacitance can also be estimated based on the method of Taberna et al [4] to define complex capacitance C c ðuÞ as C c ðuÞ ¼ C 0 ðuÞ À jC 00 ðuÞ ¼ 1=ðjuZÞ (2) where C 0 ðuÞ is the accessible capacitance at the corresponding frequency, while C 00 ðuÞ corresponds to energy dissipation due to irreversible process (resistive loss).…”

“…A comparison of the EOG/CCP electrode with others in the literature is helpful to reveal their performance difference. In the work by Miller et al [8], 0.6 mm thick VOG was grown on 75 mm thick Ni substrate to obtain a capacitance of 0.175 mF cm 2 at 120 Hz, which were further improved by Cai et al [12] to 0.53 mF cm 2 by using 2 mm thick VOG film on Ni substrate. Liu et al [9] reported electrodes of 20 mm thick ErGO foam deposited on Au substrate with a 120 Hz capacitance of 0.566 mF cm 2 .…”

Vertically Aligned Graphene Nanosheet Arrays: Synthesis, Properties and Applications in Electrochemical Energy Conversion and Storage

“…In contrast, the VAGNA electrodes grown at the reduced temperature facilitated the access of electrolyte ions and thus showed better frequency response, which was favorable for their high-efficiency filtering applications. [85] Nitrogen doping in graphene has been shown to be able to induce n-type conductivity and enhance its electron transfer capability. By an in situ NH 3 plasma treatment, N-doped VAGNAs were obtained on flexible carbon cloth substrates, and N atoms were predominantly bonded to carbon in pyridinic structure (Figure 6a).…”

“…Ascribing to the abundant open pores formed by surrounding vertical graphene nanosheets, VAGNA/Ni electrodes enabled much improved electrochemical reaction kinetics and demonstrated as a near-ideal structure for EDLC electrodes being capable to operate at high frequencies, as shown in Figure 6e. [27,85] The design of VAGNAs minimized the electronic and ionic resistances and delivered a resistorcapacitor time constant less than 200 microseconds (about 1/5 of that of conventional EDLCs).…”

Vertically-Oriented Graphene Electric Double Layer Capacitor Designs

“…5 Generally 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.…”