METHOD OF MAKING AN ELECTRODE HAVING MULTI-WALLED CARBON NANOTUBES

Abstract

A method of making a multi-walled carbon nanotubes (MWCNTs) electrode is a deposition-based method for growing MWCNTs on copper (Cu) foils to make binder-free electrodes for energy storage devices, such as those used in batteries and supercapacitors. A chromium layer is sputter coated on a copper foil substrate, and a nickel catalyst layer is sputter coated on the chromium layer, such that the chromium layer forms an electrically conductive barrier layer between the nickel catalyst layer and the copper foil substrate. The multi-walled carbon nanotubes are then formed on the copper foil substrate using plasma enhanced chemical vapor deposition.

Claims

1. A method of making a MWCNTs electrode, consisting of the steps of: sputter coating a chromium layer on a copper foil substrate; sputter coating a nickel catalyst layer on the chromium layer, such that the chromium layer forms an electrically conductive barrier layer between the nickel catalyst layer and the copper foil substrate; and using plasma enhanced chemical vapor deposition to form the MWCNTs electrode; wherein the MWCNTs have diameters between 10-15 nm and lengths between 10-12 μm.

2. The method of making a MWCNTs electrode as recited in claim 1, wherein the chromium layer sputter coated on the copper foil substrate has a thickness between 5 nm and 10 nm.

3. The method of making a MWCNTs electrode as recited in claim 2, wherein the nickel catalyst layer sputter coated on the chromium layer has a thickness between 20 nm and 25 nm.

4. The method of making a MWCNTs electrode as recited in claim 1, further comprising the step of cooling the MWCNT electrode in a gaseous hydrogen atmosphere.

5. The method of making a MWCNTs electrode as recited in claim 1, wherein, prior to the step of sputter coating the chromium layer on the copper foil substrate, the copper foil substrate is cleaned with deionized water and hydrochloric acid.

6. A supercapacitor comprising the electrode of claim 1 as a working electrode.

7. The supercapacitor of claim 6, further comprising three electrode cells.

8. The supercapacitor of claim 6, further comprising a counter electrode comprising a Pt wire.

9. The supercapacitor of claim 6, further comprising a reference electrode comprising Ag/AgCl.

10. The supercapacitor of claim 6, further comprising an electrolyte comprising KCl and at least one other neutral electrolyte.

11. The supercapacitor of claim 6, wherein the MWCNTs electrode is configured to sustain at least one thousand charge/discharge cycles.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] A method of making an electrode having multi-walled carbon nanotubes (alternatively referred to herein as an MWCNTs electrode) is a deposition-based method for growing multi-walled carbon nanotubes (MWCNTs) on copper (Cu) foils to make binder-free electrodes for energy storage devices, such as those used in batteries and supercapacitors. The method includes sputter coating a copper foil substrate with a layer of chromium, and sputter coating a nickel catalyst layer on the chromium layer, such that the chromium layer forms an electrically conductive barrier layer between the nickel catalyst layer and the copper foil substrate. After this step, Plasma Enhanced Chemical Vapor Deposition (PECVD) can be performed to facilitate growth of MWCNTs on the copper foil substrate. At the end of the growth period, the MWCNTs can be cooled, preferably under a H.sub.2 gas environment. The incorporation of the chromium in the catalyst support prevents diffusion of the nickel catalyst into the underlying copper, and further promotes the formation of a high-density population of catalyst particles, resulting in high density MWCNT growth.

[0011] In experiments, commercial grade copper (Cu) foil with a thickness of ˜0.1 mm (purity: 99.9% Cu) was used. Prior to growth, the Cu foil was cleaned by washing with deionized (DI) water, immersing in 10% HCl solution for 2-3 minutes, rinsing in DI water and drying with flowing air. To grow the multi-walled carbon nanotubes (MWCNTs) on the Cu foil, the copper substrate was placed in a sputter coater system and ˜20-25 nm of nickel (Ni) catalyst layer was deposited on its surface. Additionally, a chromium (Cr) layer of ˜5-10 nm was deposited, forming an electrically conductive, thin barrier layer between the copper substrate and the Ni catalyst layer. Following the sputter coating, plasma enhanced chemical vapor deposition (PECVD) was performed for the growth of the MWCNTs. At the end of the growth period, the samples were slowly cooled within the furnace, under a hydrogen gas (H.sub.2) environment. The MWCNTs grown on the Cu foil were characterized and used as binder-free electrodes for the fabrication of supercapacitors.

[0012] For material characterization, the structural properties of the samples were obtained using a Philips® X'Pert MPD X-ray diffraction system equipped with Cu Kα radiation in the 20 range of 10°-70°. The grown product was studied using field emission scanning electron microscopy (FESEM) with a JEOL® JSM-7600F Schottky field emission scanning electron microscope, and also transmission electron microscopy (TEM) with a JEOL® JEM-2100F field emission electron microscope operated at 200 kV. Room temperature Raman spectroscopy was carried out using a LabRAM® HR800 confocal Raman microscope in an ambient atmosphere with a He—Ne wavelength laser of 633 nm and power of 20 mW. The characterization revealed that the grown MWCNTs had a crystalline structure with diameters ranging from 10-15 nm and lengths of ˜10-12 μm.

[0013] Electrochemical measurements of the MWCNTs were performed using three electrode cells in an electrochemical analyzer system. For the working electrode, binder-free MWCNTs on a Cu substrate were used, prepared as described above. All electrochemical studies were carried out in electrolytes including KCl and other neutral electrolytes with a counter electrode of a Pt wire, and Ag/AgCl served for the reference electrode. Cyclic voltammetry (CV) studies and charge-discharge (CD) studies were conducted. A frequency ranging from 1 Hz to 100 kHz was used for an electrochemical impedance spectroscopy (EIS) analysis. The electrochemical studies showed that the MWCNTs electrodes demonstrated good electrochemical performance and cyclic stability for thousands of charge/discharge cycles. Electrochemical studies were also performed to study the effect of thickness of the Ni catalyst layer, with or without the Cr barrier layer.

[0014] It is to be understood that the method of making an electrode having MWCNTs electrode is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.