A comparison of single-wall carbon nanotube electrochemical capacitor electrode fabrication methods

Abstract

Carbon nanotubes (CNTs) are being widely investigated as a replacement for activated carbon in supercapacitors. A wide range of CNT specific capacitances have been reported in the literature based on experiments using different CNT materials, fabrication methods, and characterization routines; making it difficult to draw conclusions about the relative merits of the different fabrication methods. This work systematically compares four solution-based electrode fabrication methods (drop casting, air brushing, filtration, and electrospraying) and, to a lesser extent, some solution preparation techniques to determine if there is an optimum method for fabricating electrochemical capacitor electrodes out of single-wall CNTs (SWCNTs). We have found that it is best to use CNT solutions free from additives that may be difficult to remove from the fabricated electrode. In addition, the CNT solution preparation (e.g., dilution and sonication) had little effect on the resulting specific capacitance. Large differences in performance due to the fabrication methods were not seen, and the differences that were seen could be ascribed to material loss or contamination during the deposition. A single-layer graphene electrode was also fabricated and tested to obtain an estimate of the specific capacitance potentially achievable with SWCNTs, with 550 F/g demonstrated using 1 molar (M) sulfuric acid.

Introduction

Electrochemical capacitors, also referred to as supercapacitors, have several advantages over conventional batteries, including higher specific power (∼2 orders of magnitude higher), higher cycle life (millions of charge/discharge cycles), rapid charge/discharge times (seconds to minutes), high efficiencies (up to 98%), and unaltered performance in extreme heat and cold (1). However, electrochemical capacitors have low energy density compared to batteries which is a significant disadvantage for energy storage. Increasing electrochemical capacitor energy and power densities will make them more useful for portable power applications.

An electrochemical capacitor consists of two solid dielectric-free electrodes, in contact with an electrolyte, which store charge by adsorption of ions onto the electrodes. The capacitance due to the adsorption of ions onto the electrodes is referred to as electrochemical double-layer capacitance, since there is a layer of ions on the electrode with a second layer of counter-ions (oppositely charged ions) next to the adsorbed ions. Energy can also be stored through reduction and oxidation (redox) reactions at the electrodes, whose rates are potential dependent. This type of energy storage is referred to as pseudocapacitance since it behaves electrically as a capacitance, though the charge transfer reactions are more like that of a battery. Since there is no dielectric on the electrochemical capacitor electrodes, the applied biases must remain low enough that electrochemical breakdown of the electrolyte does not occur. This limits the voltage rating on individual electrochemical capacitor cells to ∼1 V when using aqueous electrolytes, and ∼2.7 V when using organic electrolytes.

Electrochemical capacitors achieve large capacitances by using electrodes with very large surface areas. Carbon electrodes are desirable because they are conductive and have high surface area, good corrosion resistance, and good thermal stability [1]. Carbon materials with improved surface area may increase the capacitance of electrochemical capacitors. Two materials being studied for this are CNTs and graphene. Graphene is a single atomic layer of graphite. Similarly, a SWCNT is a single atomic layer of graphite that curves back on itself to form a tube. Multi-wall carbon nanotubes (MWCNTs) are carbon nanotubes that are more than one atomic layer thick and they were not used in this study. This study focused on SWCNTs since they have the largest surface area to mass ratio given that any interior walls in a MWCNT contribute mass but not surface area. Extremely large capacitances may be obtainable if these materials can be assembled in a manner that optimizes the electrode surface area that is accessible to the electrolyte.

This study details investigations of various solution-based electrode fabrication methods to determine if there is an optimum method for fabricating SWCNT electrodes for electrochemical capacitors. Solution-based processing was chosen, as it is manufacturable and compatible with roll-to-roll processing. It also does not impose significant thermal and chemical constraints on the underlying current collector as direct growth methods at 900 °C in a reducing environment would. When SWCNTs are deposited from solution, they typically do so in bundles. It is not yet clear if this bundling is detrimental to the resulting accessible surface area and, therefore, the resulting capacitance. In addition, the deposition method may also affect the porosity of the electrode, which will affect how easily the electrolyte ions can move into and out of the electrode (the Warburg impedance) during the charge/discharge process.

Many CNT solution-based processing approaches have been demonstrated with measured specific capacitances of 23–200 F/g reported [2], [3], [4], [5], [6], [7], [8], [9]. It is difficult to draw direct comparisons from these studies because they use different: CNT sources including single- and multi-walled tubes, solution compositions, fabrication methods, and characterization protocols. According to Stoller and Ruoff: “Methodology to reliably measure a material's performance for use as an ultracapacitor electrode is not well standardized with various techniques yielding widely varying results” [10]. In addition, many of the higher specific capacitances reported likely include pseudocapacitive contributions in the measured specific capacitance. Such pseudocapacitance contributions may overwhelm the double-layer capacitance, which is the focus of this work, as double-layer capacitance is a measure of the accessible CNT surface area being produced by the fabrication methods tested. We have systematically investigated the individual contributions of the solution preparation and the deposition methods to the achieved double-layer capacitance.

In order to determine how high a capacitance might be achieved with SWCNTs, a model system of a single-layer graphene electrode on sapphire was tested. A dielectric substrate is used to eliminate any contribution to capacitance from the substrate. This should be a reasonable model for SWCNTs since the electrolyte can only access one side of the graphene (corresponding to the outside of a SWCNT).