High performance all-carbon thin film supercapacitors

Highlights

• Advance in supercapacitor technology by materials engineering all based on carbon.

• Our devices achieve volume energy density comparable to Li-ion batteries.

• Our devices achieve power density better than laser-scribed-graphene supercapacitors.

Abstract

We fabricated high performance supercapacitors by using all carbon electrodes, with volume energy in the order of 10⁻³ Whcm⁻³, comparable to Li-ion batteries, and power densities in the range of 10 Wcm⁻³, better than laser-scribed-graphene supercapacitors. All-carbon supercapacitor electrodes are made by solution processing and filtering electrochemically-exfoliated graphene sheets mixed with clusters of spontaneously entangled multiwall carbon nanotubes. We maximize the capacitance by using a 1:1 weight ratio of graphene to multi-wall carbon nanotubes and by controlling their packing in the electrode film so as to maximize accessible surface and further enhance the charge collection. This electrode is transferred onto a plastic-paper-supported double-wall carbon nanotube film used as current collector. These all-carbon thin films are combined with plastic paper and gelled electrolyte to produce solid-state bendable thin film supercapacitors. We assembled supercapacitor cells in series in a planar configuration to increase the operating voltage and find that the shape of our supercapacitor film strongly affects its capacitance. An in-line superposition of rectangular sheets is superior to a cross superposition in maintaining high capacitance when subject to fast charge/discharge cycles. The effect is explained by addressing the mechanism of ion diffusion into stacked graphene sheets.

Introduction

Carbon nanomaterials have a unique place in nanoscience, due to their excellent physical and chemical properties. Energy storage is one of the important applications of carbon nanomaterials, because of their ability to trap charges coupled to a good conductivity. Electrodes made of carbon nanostructures have been applied in fuel cells [1], [2], lithium batteries [3], and supercapacitors [4]. Active-carbon-based electrochemical supercapacitors are commercially available and have wide applications in both energy storage and power supply. Supercapacitors have been proposed to store extra energy from solar panels and wind turbines [5]. Supercapacitors are superior to batteries in power density, but inferior in energy density. The supercapacitor–battery combination has been used in electrical vehicles, in which the supercapacitor provided a peak power during acceleration and hill-climbing, and it could be recharged by the battery during regeneration braking [6]. Commercial supercapacitors are bulky, due to the strong encapsulation required for the liquid electrolyte. On the other hand, energy storage components have to be as small as possible for lightweight and portable gadgets. In this regard, the development of solid-state thin film supercapacitors that use carbon nanomaterials is highly desired. Research advancement in this area can also lead to novel design and manufacture of electrical vehicles: large-area thin film supercapacitors, for example, could be stacked together and combined with strong composites to make vehicle's body panel with energy storage function.

In recent years, the use of carbon nanotubes (CNTs) and graphene to make supercapacitors has attracted the attention of researchers since they have a higher specific capacitance than active carbon. Research on CNT-based supercapacitors started before the isolation of graphene in 2004. So far, either forest-like arrays [7], [8] or networks [9] of CNTs have been tested as supercapacitor electrodes. However, the capacitance of these CNT-based supercapacitors is not satisfactory. A second approach is to use CNTs to support metal oxides [10], [11] or conductive polymers [12], [13] to create high-capacitance pseudocapacitors, which exploit the oxidation-reduction process to charge and discharge. Yet, these structures are inherently three-dimensional; thin-film supercapacitors have remained elusive because they employ a liquid electrolyte for the redox reactions, which is not easily supported in a two-dimensional geometry. Compared to CNTs (either single-wall or multi-wall), graphene sheets have higher surface-to-volume ratio and result in higher capacitance when used as supercapacitor electrodes. Composites of graphene or graphene oxide (GO) and other materials such as polyaniline [14], TiN [15], MnO₂ [16], RuO₂ [17], and NiO [18] have been used as 3D-structured electrodes of liquid-phase electrochemical supercapacitors; however, electrodes made out of sole-nanocarbon 2D structures are more interesting as they allow the use of a gelled electrolyte and the fabrication of metal-free and solid-state thin film supercapacitors.

Graphene used in supercapacitors is commonly produced by high-temperature catalytic growth or wet-chemical exfoliation of graphite. Supercapacitors have been made [19] using the first method by growing graphene sheets onto a nickel foam. The nickel foam acts as both the catalyst to grow graphene sheets in a 3D structure and as current collector for the supercapacitor. Though high-temperature-grown graphene provides a clean surface and high conductivity, the technique is costly and the yield of graphene is low. Many researchers used reduced graphene oxide (rGO), mostly prepared by wet-chemical methods, to make supercapacitors. GO sheets can be mass-produced at low cost by using graphite powder and chemicals including oxidizing agents and acids, according to the Hummer's method [20]. Various wet-chemical methods have been developed to reduce GO, but the resistance of rGO is considerably higher than that of pure graphene [21]. To make a supercapacitor electrode, the rGO sheets were processed into 3D porous structure to improve their capacitance [22]. Such an electrode is usually tens of microns in thickness and is more suitable for liquid-electrolyte-based capacitors. Sheng et al. electrochemically reduced and deposited GO sheets from an aqueous solution onto Au foils and made supercapacitors with capacitance of 240–350 μFcm⁻² [23]. Kaner et al. developed a novel technique to thermally reduce GO film by using the laser of a standard DVD optical drive, and combined the laser-scribed graphene film and gelled electrolyte to make solid-state supercapacitors with a capacitance of ∼5 mFcm⁻² [24], [25]. The primary step in the fabrication of these supercapacitors is the preparation of GO, which uses many chemicals and is time-consuming. In this paper we explore a greener way to produce graphene and a novel method to make solid-state thin film supercapacitors with high performance.

The use of aqueous electrolyte containing ${\text{SO}}_4^{2-}$ ions to electrochemically exfoliate graphite is simple and time-saving, and the produced graphene is of high quality, comparable or even better than rGO [26], [27], [28]. In comparison, the standard wet chemical preparation of GO and rGO includes several steps for chemical reaction and purification. Here we produce graphene flakes by a fast electrochemical exfoliation method with mild chemicals, typically NaSO₄ in water, and blend them to multi-wall CNTs (MWCNTs) to make thin electrode films with high surface area for high-capacitance supercapacitors. We use highly-conductive double-wall CNT (DWCNT) films coated onto plastic papers as current collectors. Metal foils commonly used in electrochemical supercapacitors account for 20–30% of the total weight and have to undergo an anti-corrosion treatment. In our work, the DWCNT film is flexible and lightweight, free from any corrosion problem. A gelled electrolyte is used to bind the two carbon-nanomaterial layers to make a supercapacitor in the form of flexible film. We show that an in-plane assembly of supercapacitor cells increases the voltage output.