High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes

Abstract

We report a new class of electrochemical capacitors by utilizing vertically aligned carbon nanotubes as the electrodes and environmentally friendly ionic liquids (ILs) as the electrolytes. With their vertically aligned structures and well spacing, aligned carbon nanotubes showed a strong capacitive behavior in the ionic liquid electrolyte. Plasma etching played an important role in opening the end tips of nanotubes and in introducing defects and oxygenated functionalization to the nanotubes, further enhancing the capacitive behavior of carbon nanotubes. With the combined contribution from double-layer capacitance and redox pseudocapacitance, carbon nanotubes showed a remarkable capacitance in ionic liquid electrolyte. Combining the highly capacitive behavior of carbon nanotube electrodes with the large electrochemical window of ionic liquid electrolytes, the resultant capacitors showed a high cell voltage, high energy density, and high power density, potentially outperforming the current electrochemical capacitor technology. The device configuration incorporating vertically aligned nanostructured electrodes and inherently safe electrolytes would be useful for improving performances for new energy storage technologies.

Introduction

Electrochemical capacitors (aka, supercapacitors or ultracapacitors) are energy storage devices that combine the high energy storage capability of batteries with the high power delivery capability of capacitors [1], [2]. Electrochemical capacitors have been developed to provide power pulses for a wide range of applications including transportation, consumer electronics, medical electronics, and military devices. However, performance (energy and power densities, safety, and cycle life) of the state-of-the-art electrochemical capacitors is needed to improve to satisfy the rapidly increasing performance demands for these applications. The maximum energy (E_max) and power (P_max) of an electrochemical capacitor are given by: $E_{\text{max}}=(C{U}^2)/2$ (Eq. (1)) and $P_{\max }=U^2/(4R)$ (Eq. (2)), respectively (where C is capacitance, U is cell voltage, and R is total equivalent series resistance (ESR) of the capacitor) [2]. Limited properties of the presently available electrodes (low electrolyte accessibility and low capacitance) and electrolytes (narrow electrochemical window, flammability, toxicity, volatility, and thermal instability) are needed to overcome to develop high performance electrochemical capacitors with high energy and power densities, safe operation, and long lifetimes.

High-surface-area activated carbons (ACs) are currently used electrode materials in commercial electrochemical capacitors [3]. While these ACs possess a high specific surface area (1000–2000 m² g⁻¹) they have a limited capacitance. This lack of capacitance of ACs is largely associated with their low mesoporosity and low electrolyte accessibility [4]. Development of electrode materials having an appropriate balance between the surface area and the mesoporosity has been a bottleneck to the development of advanced electrochemical capacitors. Since their discovery in 1991 [5], carbon nanotubes (CNTs) have become an important class of electrode material for various electrochemical devices, including electrochemical capacitors [4], [6], [7], [8], [9], [10], [11]. In spite of their moderate surface area compared to ACs, CNTs show reasonably high capacitances (e.g., 102 F g⁻¹ for multi-walled nanotubes [6] and 180 F g⁻¹ for single-walled nanotubes [9]) due to their large mesoporosity and high electrolyte accessibility. Excellent electrical conductivity, high mesoporosity, and high electrolyte accessibility of CNTs ensure a high charge transport capability and hence a high power density for the electrochemical capacitor. Conventional aqueous [6], [9], [12] and organic [13], [14], [15] electrolytes have been employed for the development of CNT electrochemical capacitors. The aqueous electrolyte-based electrochemical capacitors showed reasonably high power densities (>7 kW kg⁻¹) but their energy densities (∼4 Wh kg⁻¹) are still limited. Narrow electrochemical window of the aqueous electrolytes used, and hence small cell voltages and low energy (as described in Eq. (1)) of the capacitors, is a major reason for this drawback. The advantage of the use of organic electrolytes is mainly associated with their relatively large electrochemical windows. To this end, in order to further enhance the capacitor performance, especially the energy density required for advanced CNT electrochemical capacitors [7], new electrolytes having even larger electrochemical windows are needed. Further, the ideal electrolytes will also offer superior safety properties over the current technology.

Currently employed electrolytes in commercial electrochemical capacitors include aqueous and organic [16]. The narrow electrochemical windows of these electrolytes (aqueous: ∼1.2 V, organic: 2–3 V) lead to small cell voltages and hence limited energy and power of the capacitor (Eqs. (1) and (2)). Upon charge, ions of the electrolyte are transported into the double-layers at the electrode–electrolyte interfaces, resulting in the decrease of salt concentration in the electrolyte (the so-called electrolyte depletion) and hence the limit of energy density of the capacitor [17]. Also, this electrolyte depletion increases the cell resistance and thus lowers the maximum power density achievable for the capacitor. As a result, fabricated from these electrolytes and AC electrodes, the commercially available electrochemical capacitors possess a low energy density (4–5 Wh kg⁻¹) and a low power density (1–2 kW kg⁻¹) [3]. Furthermore, some organic electrolytes suffer from serious health and safety problems as they are inherently volatile, flammable, and toxic [18], resulting in a narrow operational temperature range and potential for explosion during outlying circumstances (e.g., during car accidents). Since their first description in 1914 [19], ionic liquids (ILs) have been used in a wide range of applications [20], [21], [22]. Certain unique properties of these environmentally friendly ILs, including high ionic conductivity (up to 10⁻² S cm⁻¹), large liquid phase range (−100–400 °C), wide electrochemical window (4–6 V), non-volatility, non-flammability, and non-toxicity, have made them an excellent electrolyte for various electrochemical systems [23].

Given that the performance of an electrochemical capacitor is directly proportional to the square of its cell voltage, Eqs. (1) and (2), the use of large electrochemical window ILs would significantly boost the performance for CNT electrochemical capacitors. Compared to conventional electrolytes, ILs have a unique property that they are both 100% solvents and also 100% salts. It is unnecessary to add other salts into an IL to achieve ionic conductivity. The very high ionic concentration of ILs would be able to eliminate the electrolyte depletion problem as encountered with conventional electrolytes and therefore enhance the capacitor performances. Further, the superior chemical and environmental stability of ILs ensures safe operation and long lifetimes for capacitors. Previously, electrochemical behavior of randomly entangled CNT electrodes has been studied in IL electrolytes, showing a large potential window but with a limited capacitance for the CNTs [24], [25]. It is likely that a facilitated access of the IL ions could not take place within the CNTs due to the mismatch between the irregular pore structures of the randomly entangled CNTs and the relatively high viscosity of the IL electrolytes (comparing to conventional aqueous and organic electrolytes). Recent research demonstrated that vertically aligned CNTs are advantageous over their randomly entangled counterparts for electrochemical capacitor applications, especially, in improving rate capability of the capacitors [14], [15], [26]. Specifically, in IL electrolytes, improved rate capability has been achieved for CNTs when vertically aligned structures were employed [27]. Nevertheless, for the development of high performance electrochemical capacitors, capacitance (24 F g⁻¹) of these CNTs (obtained in IL electrolytes) is still limited. This, therefore, indicates a need to improve the properties of CNTs in order to match with an IL to achieve a high capacitance for the CNTs and thus a high performance for the CNT-IL electrochemical capacitors.

In this work, we investigated the electrochemical behavior of plasma-etched, vertically aligned CNTs (abbreviated as ACNTs) in IL electrolytes. Unique properties of the ACNTs made them highly capacitive in the ILs. Moreover, combining the novel properties of ACNT electrodes with the large electrochemical window of environmentally friendly IL electrolytes, we developed a new class of electrochemical capacitors, showing high cell voltages (4 V) and superior energy and power densities (148 Wh kg⁻¹and 315 kW kg⁻¹, based on the mass of the active electrode materials), potentially outperforming the currently available electrochemical capacitor technology.