Microwave-assisted synthesis of metal oxide/hydroxide composite electrodes for high power supercapacitors – A review

Highlights

• Supercapacitor of metal oxide/hydroxide thin film by microwave method studied.

• Supercapacitors exhibit high specific capacitance as potential energy storage.

• Metal oxide/hydroxide films play a major role in supercapacitor technology.

Abstract

Electrochemical capacitors (ECs), also known as pseudocapacitors or supercapacitors (SCs), is receiving great attention for its potential applications in electric and hybrid electric vehicles because of their ability to store energy, alongside with the advantage of delivering the stored energy much more rapidly than batteries, namely power density. To become primary devices for power supply, supercapacitors must be developed further to improve their ability to deliver high energy and power simultaneously. In this concern, a lot of effort is devoted to the investigation of pseudocapacitive transition-metal-based oxides/hydroxides such as ruthenium oxide, manganese oxide, cobalt oxide, nickel oxide, cobalt hydroxide, nickel hydroxide, and mixed metal oxides/hydroxides such as nickel cobaltite and nickel–cobalt oxy-hydroxides. This is mainly due to the fact that they can produce much higher specific capacitances than typical carbon-based electric double-layer capacitors and electronically conducting polymers. This review presents supercapacitor performance data of metal oxide thin film electrodes by microwave-assisted as an inexpensive, quick and versatile technique. Supercapacitors have established the specific capacitance (Cs) principles, therefore, it is likely that metal oxide films will continue to play a major role in supercapacitor technology and are expected to considerably increase the capabilities of these devices in near future.

Introduction

The electrochemical capacitor (EC) is often known and called supercapacitor [1]. As an energy storage/delivery device, supercapacitor to be of great technical potential in power systems owing to its high-power characteristics with acceptable capacity and long cycle life. Supercapacitors, sometimes called pseudocapacitors or electric double-layer capacitor (EDLC) or ultracapacitors, do not have a conventional solid dielectric. The capacitance value of an electrochemical capacitor is determined by two storage principles, both of which contribute to total capacitance of capacitor [2], [3], [4]:

• Double-layer capacitance (DLC) – electrostatic storage of electrical energy achieved by separation of charge in a Helmholtz double layer at the interface between surface of a conductive electrode and an electrolyte. The distance of static separation of charge in a double-layer is of the order of a few angstroms (0.3–0.8 nm), which is enormously small [3], [4].

• Peseudocapacitance – Electrochemical storage of electrical energy with electron transfer, achieved by redox reactions with specially adsorbed ions from electrolyte, intercalation of atoms in the layer lattice or electrosorption, under potential deposition of hydrogen or metal adatoms in surface lattice sites which results in a reversible faradaic charge transfer [3], [4].

The ratio of storage resulting from each opinion can vary significantly, depending on electrode design and electrolyte composition. Supercapacitance can enhance the capacitance value by as much as an order of magnitude over that of the double-layer by itself [3]. Supercapacitors are divided into three families, based on design of the electrodes [3], [4], [5], [6], [7]:

• Double-layer capacitors – with carbon electrodes or derivates with much higher static double-layer capacitance than faradaic pseudocapacitance.

• Supercapacitors – with electrodes out of metal oxides or conducting polymers with a high amount of faradaic pseudocapacitance.

• Hybrid capacitors – capacitors with special and asymmetric electrodes that exhibit both major double-layer capacitance and pseudocapacitance, such as lithium-ion capacitors.

In recent years, supercapacitors have attracted significant attention, mainly due to their high power density and long lifecycle [1], [8]. They have the maximum available capacitance values per unit volume and the greatest energy density of all capacitors. They support up to 12,000 F/1.2 V, with capacitance values up to 10,000 times that of electrolytic capacitors. While existing supercapacitors have energy densities that are approximately 10% of a conventional battery, their power density is generally 10 to 100 times greater. Power density is defined as the product of energy density, multiplied by the speed at which the energy is delivered to the load. Greater power density results in much shorter charge/discharge cycles than a battery is capable of, and a greater tolerance for numerous charge/discharge cycles [3], [9].

In order to understand the inherent differences between these two electrochemical storage systems better, supercapacitors and batteries, as well as electrochemical energy conversion systems, a Ragone plot can be drawn to demonstrate their respective performances. Ragone plots are often used to graph the characteristic power density in relation to energy density of such systems as shown in Fig. 1 [8]. The unique role that each energy storage or conversion system plays is evident by their region of dominance. While batteries are the popular choice for high portable energy storage, with Li-ion batteries achieving energy densities of 180 Wh kg⁻¹ [10], the electrode materials suffer strenuous volume and irreversible phase changes during charge discharge cycling that limits their cycle-life. This disadvantage further impedes their application for high power performance applications which often require rapid charging and discharging in short intervals. These shortcomings draw attention to the characteristic strengths of ECs. Evident by Ragone plot (Fig. 1), commercial ECs do not currently possess the large energy densities of batteries, with commercial devices ranging between 5 and 10 Wh kg⁻¹. Power density of ECs far exceeds that of batteries with the ability to charge and discharge stored energy within seconds. ECs compliment this characteristic very well with a cycle life in excess of 10⁶ cycles of deep discharge within a wide operational temperature range and require no further maintenance upon integration. Conscientious of environmental standards, these devices are also recyclable [11].

The electrolytes within an EC play an equally vital function in development of electrostatic and reversible redox processes necessary for charge storage and overall energy density. Choices among electrolytes range between aqueous to organic and ionic liquid, where consideration is given to electrolyte conductivity, ion size and electrochemical stability (voltage limitations). Within electrochemical capacitors, the electrolyte is the conductive connection between the two electrodes, distinguishing them from electrolytic capacitors, in which the electrolyte only forms the cathode, the second electrode. Supercapacitors are polarized and should operate with correct polarity. Polarity is controlled by design with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during the manufacturing process. Supercapacitors support a broad spectrum of applications for energy and power requirements. EC was supposed to enhance the fuel cell or the battery in the hybrid electric vehicle to supply the necessary power for acceleration, and additionally allow for recuperation of brake energy. Today, several companies such as Siemens Matsushita (now EPCOS), Maxwell Technologies, Panasonic, TOKIN, ELNA, NEC and several others invest in electrochemical capacitor development [3], [6], [7], [9], [12], [13].

Microwaves are being in many areas of chemistry and microwave techniques have become suitable for industrial applications such as food processing [14], [15] and industrial materials [16], [17], [18], [19]. In particular, the microwave assisted technique is regarded as a novel method in synthesis of inorganic solids and is a rapidly developing area of research. This method is facile, fast, quite, secure, controllable and energy-saving. Besides decreasing synthesis time, it was duly demonstrated that microwave technique provides an effective way to control particle size distribution and macroscopic morphology in the synthesis [20]. Microwave-assisted methods have been tried to synthesize metal oxide composite electrodes as supercapacitors [21], [22], [23], [24]. Nanostructured metal oxides, which exhibit pseudocapacitance behaviour, are considered to be excellent materials for achieving high specific capacitance.

This review presents the investigation of supercapacitive performance of metal oxide/hydroxide thin film electrode materials by microwave-assisted (MV) technique. Supercapacitors have exposed the specific capacitance (Cs) values, which are moderately comparable with bulk electrode values. Therefore, it is expected that these metal oxide thin films will continue to play a significant role in supercapacitor technology.