ReviewMetal oxide thin film based supercapacitors
Research highlights
► Supercapacitor performance of metal oxide thin film. ► Supercapacitors exhibited the specific capacitance between 50 to 1100 F g-1. ► Metal oxide films play a major role in supercapacitor technology.
Introduction
Supercapacitors, also known as, electrochemical capacitors, have been known for over fifty years and are considered as one of the potential energy storage systems in addition to batteries [1]. There are two types of electrochemical capacitors based on the type of electrochemical response. The most commonly known capacitors are those exhibiting high surface reactivity resulting in the formation of a double layer, also originally called electrical double layer (EDLC) capacitors. The second category of capacitor materials are the ones showing Faradaic electrochemical reactions occurring on the surface. Supercapacitors can store substantially more energy (per unit mass or volume) than a conventional capacitor because: (i) charge separation takes place across a very small distance in the electrical double layer that constitutes the inter phase between an electrode and the adjacent electrolyte; and (ii) an increased amount of charge can be stored on the highly extended electrode surface area created by a large number of pores within a high surface area electrode material. The mechanism of energy storage is inherently rapid because it simply involves movement of ions to and from electrode surfaces. Supercapacitors exhibit a very high degree of reversibility in repetitive charge–discharge cycling and demonstrated cycle life in excess of 500,000 cycles. High cycle life and good stability make supercapacitors useful in applications such as lightweight electronic fuses, backup power sources for calculators and digital calipers, surge-power delivery devices for electric vehicles etc. Supercapacitors are maintenance-free substitutes for batteries in these applications [1], [2].
A large number of review articles including a special issue by The Electrochemical society [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The capacitance of a device is largely dependent on the characteristics of the electrode material; particularly, the surface area and the pore-size distribution [14]. The operating voltage of supercapacitors is usually dependant on electrolyte stability. Aqueous electrolytes, such as acids (e.g., H2SO4) and alkalis (e.g., KOH) have the advantage of high ionic conductivity (up to 1 S cm−1), low cost and wide acceptance however; they have the inherent disadvantage of a relatively low decomposition voltage of 1.23 V [15]. Non-aqueous electrolytes allow the use of cell operating voltages above 2.5 V. Non-aqueous electrolyte mixtures such as propylene carbonate or acetonitrile, containing dissolved quaternary alkyl ammonium salts have been employed in many commercial supercapacitors. The electrical resistivity of non-aqueous electrolytes is, however, at least an order of magnitude higher than that of aqueous electrolytes and therefore the resulting capacitors generally have a high internal resistance [14]. In supercapacitors, a number of sources contribute to the internal resistance and are collectively measured and referred to as the equivalent series resistance, or ESR. Contributors to the ESR of supercapacitors include: (i) electronic resistance of the electrode material, (ii) the interfacial resistance between the electrode and the current collector, (iii) the ionic (diffusion) resistance of ions moving in small pores, (iv) the ionic resistance of ions moving through the separator, and (v) the electrolyte resistance [16].
The required electrode properties of supercapacitor electrode material arise from a combination of chemical and physical properties, namely: high conductivity, high surface area range (1 to >2000 m2 g−1), good corrosion resistance, high temperature stability, controlled pore structure, processability and compatibility and relatively low cost. It has been possible to generate high surface area materials exhibiting Faradaic electrochemical as well as electrical double layer type responses. Among the several transition metal oxides and carbon xerogels studied for capacitor response, the only oxide that has been widely researched and known for its superior electrochemical capacitor response to date is the various crystallographic and morphological forms of ruthenium oxide [17]. Unfortunately, the expensive nature of ruthenium has limited the technological viability of this material. Consequently, the field of supercapacitor has not witnessed many advances.
Many chemically deposited metal oxide thin films including ruthenium oxide, iridium oxide, manganese oxide, cobalt oxide, nickel oxide, tin oxide, iron oxide, pervoskites, ferrites, etc. have been applied in supercapacitors. The thin film deposition methods involving the growth from solution are called as chemical methods. Here, a fluid surface precursor undergoes a chemical change at a solid surface, leaving a solid layer. In chemical deposition the solutions contain precursor molecules for a variety of elements in the thin film of interest. These methods are inexpensive and enable the synthesis of film materials with complex chemical compositions [18]. Depending on applications, one would prefer thin films which have a special texture, low grain boundary density, or smooth surfaces. The methods usually have low operating temperature. Apart from the obvious advantages in terms of energy saving, the low deposition temperature avoids high temperature effects such as interdiffusion, contamination and dopant redistribution. They offer mysterious morphologies of the thin films which can be easily controlled by preparative parameters. Unlike physical deposition methods, they do not require high quality target and/or substrates nor do they require vacuum at any stage, which is a great advantage if the methods are used for industrial applications. Chemical methods include electrodeposition, chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), electroless deposition, anodization, spray pyrolysis, liquid phase epitaxy, spin coating, dip drying, etc.
This review presents the survey of supercapacitive performance of chemically deposited metal oxide thin film electrode materials. The supercapacitors have exhibited the specific capacitance (Sc) values between 50 and 1100 F g−1, which are quite comparable with bulk electrode values. Therefore, it is likely that these metal oxide thin films will continue to play a major role in supercapacitor technology.
Section snippets
Supercapacitor parameters
The specific energy stored and the specific power that can be delivered to the load are the crucial characteristics of a supercapacitor device along with others, such as its cycling life, self-discharge current and efficiency.
The double layer capacitance, Cdl, at each electrode interface is given by,where ɛ is the dielectric constant of the electrical double layer region, A the surface area of the electrode and t is the thickness of the electrical double layer. The specific energy (E)
Types of supercapacitors
Supercapacitors may be distinguished by several criteria such as the electrode material utilized, the electrolyte, or the cell design [4]. With respect to electrode materials there are three main types: carbon based, metal oxides and polymeric materials. Carbon in various modifications is the electrode material used most frequently for electrodes of electrochemical capacitors [14]. Reasons for using carbon are low cost, high surface area, availability, and established electrode production
Metal oxide thin films based supercapacitors
Literature survey of supercapacitors shows that a variety of metal oxide thin films have been employed. These include thin films of RuO2, MnO2, NiO, In2O3, Co3O4, V2O5, Fe3O4, Bi2O3, IrO2, NiFe2O4, BiFeO3 etc.
Conclusions
Fig. 5 shows the Sc values obtained for different metal oxide thin film based supercapacitors. The highest value (2104 F g−1) is reported for Co–Ni hydroxide based supercapacitor. However, all other values range between 50 and 1100 F g−1. From the published data, following conclusions can be drawn:
- (i)
Simple chemical methods such as electrodeposition, spray pyrolysis, chemical deposition, spin coating, dip dry etc. have been employed to deposit metal oxide thin films.
- (ii)
For most of the systems, only
Acknowledgement
One of the authors, (CDL), wishes to thank Korean Federation of Science and Technology Societies (KOFST), Korea for the award of Brain Pool Fellowship. This work was supported by the Hydrogen Energy R and D Centre, one of the 21st Century Frontier R and D programs, funded by the Ministry of Science and Technology of Korea. Authors are also grateful to All India Council for Technical Education (AICTE), New Delhi for financial support through the scheme F. No. 8023/BOR/RID/RPS-165/2009-10 and
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