Chemical vapor-deposited carbon nanofibers on carbon fabric for supercapacitor electrode applications
© Gao et al.; licensee Springer. 2012
Received: 19 October 2012
Accepted: 16 November 2012
Published: 27 November 2012
Entangled carbon nanofibers (CNFs) were synthesized on a flexible carbon fabric (CF) via water-assisted chemical vapor deposition at 800°C at atmospheric pressure utilizing iron (Fe) nanoparticles as catalysts, ethylene (C2H4) as the precursor gas, and argon (Ar) and hydrogen (H2) as the carrier gases. Scanning electron microscopy, transmission electron microscopy, and electron dispersive spectroscopy were employed to characterize the morphology and structure of the CNFs. It has been found that the catalyst (Fe) thickness affected the morphology of the CNFs on the CF, resulting in different capacitive behaviors of the CNF/CF electrodes. Two different Fe thicknesses (5 and 10 nm) were studied. The capacitance behaviors of the CNF/CF electrodes were evaluated by cyclic voltammetry measurements. The highest specific capacitance, approximately 140 F g−1, has been obtained in the electrode grown with the 5-nm thickness of Fe. Samples with both Fe thicknesses showed good cycling performance over 2,000 cycles.
Electrochemical capacitors, also known as supercapacitors or ultracapacitors, are energy storage systems that differ from regular capacitors in that they have ultrahigh capacitance, long cycle life, and high power density[1–3]. Supercapacitors have many applications ranging from hybrid automobiles and large industrial equipment to storage devices for solar cells and portable consumer electronics[3, 4]. Supercapacitors can be divided into two categories: electrical double-layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, different forms of carbon are commonly used as active electrode materials, and the capacitance results from electrostatic charge accumulations at the electrode/electrolyte interfaces[5–7]. In contrast, in redox or pseudocapacitors, in which transition metal oxides such as RuO2·x H2O and MnO2 and electronically conducting polymers such as polyaniline and polypyrrole are used as active electrode materials[8–11], charge storage results from fast and reversible faradic reactions at the surface of the electroactive materials. Among the many candidates for supercapacitor electrode materials, mesoporous carbon spheres, carbon nanotubes (CNTs) and/or carbon nanofibers (CNFs)[13–16], CNT/polypyrrole composites, and MnO2/CNT composites have attracted much attention due to their excellent electrical conductivity, large surface area, chemical inertness, and high operating temperature range. Several methods have been developed to synthesize CNTs and CNFs including arc discharge, laser ablation, and chemical vapor deposition (CVD)[19–21]. In the CVD process, transition metals such as nickel (Ni), cobalt (Co), iron (Fe), or their combination are used as the catalyst and are often deposited onto the substrates before the CNTs and CNFs are grown. Then, carbon-containing precursor gases such as methane (CH4), acetylene (C2H2), ethylene (C2H4), or ethane (C2H6) with the carrier gases (argon and/or hydrogen) are introduced into the CVD system and decompose at the catalyst sites to form CNTs or CNFs at the corresponding gas decomposition temperature.
In the water-assisted chemical vapor deposition (WA-CVD), water vapor is introduced during the CVD process to enhance CNT/CNF growth. Two main contributions of the water vapor are as follows: (1) it inhibits catalyst nanoparticles formed at CVD temperature from diffusing into the substrates by oxidizing metal nanoparticles such as Fe; (2) it removes amorphous carbon that is formed on the active catalyst surface, thereby increasing the catalyst lifetime.
Compared to commonly used silicon substrates, weaved carbon fabric (CF) has several advantages such as flexibility, scalability, light weight, and low cost. In addition, due to its weave structure, it has more surface area than other conventional substrates and is more advantageous for supercapacitor applications. In recent studies, active carbon, multi-walled carbon nanotubes (MWCNTs)[30–33], single-walled carbon nanotubes, CNT and polypyrrole composites, TiO2/MWCNTs, and graphene have been successfully incorporated into the CF via various growth methods for supercapacitor applications.
In this work, CNFs are grown on CF substrates using the aforementioned WA-CVD method with Fe as the catalyst and C2H4 as the precursor gas. Furthermore, the effect of the CNF morphology on the capacitive performance is discussed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) are utilized to characterize the structure and morphology of the CNFs. The capacitive behaviors of the CNF/CF electrodes are investigated by cyclic voltammetry (CV) via a three-electrode system in a neutral aqueous Na2SO4 electrolyte solution.
Panex 30 carbon fabrics made from spun yarn (plain weaved; density, 1.75 g cm−3; thickness, 406 μm) were purchased from Zoltek (St. Louis, MO, USA). The fabrics were PAN-based materials that are >99% carbonized.
Synthesis of the CNFs on CF
First, a thin film of Fe was deposited onto the CF substrate via DC sputtering at a base pressure of 10−5 Torr. The deposition rate of Fe was about 1.25 Å/s (RF power, 50W).The thickness of the Fe catalyst can significantly affect CNF morphology and distribution. Two thicknesses (5 and 10 nm) of the Fe catalyst layer were deposited, and their influence on CNF morphology was compared.
Characterization of the CNFs/CF
The Fe catalyst nanoparticle formation on the CFs as well as the morphology of the CNFs was investigated using a Zeiss Supra 55 field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). EDS provides information for elemental analysis. The microstructure of the CNFs was studied by TEM using JEOL JEM 2100 F (JEOL Ltd., Akishima, Tokyo, Japan). The crystallinity of the CNFs was observed by electron diffraction (ED). A Sartorius CPA225D microbalance (Sartorius AG, Göttingen, Germany) with a resolution of 0.01 mg was used to measure the weight of the CNFs for the specific capacitance calculation. Before electrochemical measurement, the grown CNFs were treated with nanostrip (commercial mixture of concentrated H2SO4 and H2O2) to remove the remaining Fe catalyst particles to accurately measure the intrinsic capacitance of the CNFs on CF. Electrochemical measurements were carried out using Solartron SI 1287 electrochemical interface system (Solartron Analytical, Farnborough, UK) via a three-electrode configuration using the CNFs/CF as the working electrodes, a platinum plate as the counter electrode, and standard saturated calomel electrode as the reference electrode. A 0.5 M Na2SO4 aqueous neutral solution was used as the electrolyte. Cyclic voltammetry was performed over the potential range from −0.2 to 0.5 V at scan rates ranging from 5 to 100 mV s−1. Cycling tests were also conducted using the same configuration in order to investigate the specific capacitance behavior over 2,000 cycles.
Results and discussion
Morphology and structure of CNFs/CF
CNF/CF properties as supercapacitor electrodes
The as-grown CNFs were treated with nanostrip for 2 h to remove the Fe catalyst particles so that the calculated specific capacitances are exclusively from CNFs/CF. It is also noticed that the CNFs/CF changed from hydrophobic to hydrophilic as a result of the nanostrip treatment. This is because the acid attacks the defects in the CNFs, forming carboxylic groups on the sidewalls as well as at the tip[47, 48].
where Cp is the specific capacitance, m is the mass of the CNFs, ΔE is the potential range, qa and qc are the anodic and cathodic charges during the positive and negative going scan, ia and ic are the anodic and cathodic currents, and E1 and E2 are the switching potentials of the CV.
Specific capacitance can be affected by many factors such as specific surface area, pore size, and conductivity[52, 53]. However, these factors are interrelated, and a trade-off is usually needed when optimizing the specific capacitance. For instance, a small pore size may provide a large specific surface area, but it may also slow the diffusion of the electrolyte ions at interface; CNTs have less defects which leads to higher conductivity than CNFs, but the specific area of CNTs is much less than that of CNFs. In this case, it is desirable for the carbon supercapacitor materials to have relatively high conductivity and also mesopores that are large enough for the electrolyte ions to diffuse and small enough to provide a large surface area.
CNFs were directly grown on flexible CF substrates via the WA-CVD method using Fe as the catalysts and C2H4 as the precursor gas. Different thicknesses of the catalyst (5 and 10 nm) led to different morphologies and densities of the CNFs on the CF, thus resulting in different capacitive performances of the CNF/CF electrode as a supercapacitor. CNFs grown with 5 nm of Fe demonstrated better capacitive behaviors with a specific capacitance of approximately 140 F g−1 at the scan rate of 5 mV s−1, compared to 99 F g−1 for its counterpart. The electrode shows good cycling stability for more than 2,000 cycles. The CNF/CF electrodes are flexible, stretchable, and scalable, and hence, they could be a good candidate for flexible supercapacitor applications.
We are grateful to the Center for Autonomous Solar Power (CASP) of the State University of New York at Binghamton for funding this work. We are also grateful to Daniel VanHart from CASP for the TEM sample preparation and Dr. In-Tae Bae from the Analytical and Diagnostics Laboratory (ADL) of Binghamton University for the TEM and ED data analysis. We would like to thank Mr. Siva P. Adusumilli for helping in the CVD experiment.
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