Review on supercapacitors: Technologies and materials
Introduction
The increasing cost of fuels, pollution, global warming and geopolitical concerns are among the problems connected with the dependence of modern societies on fossil fuels. Reducing these issues is an increasingly important goal that can be achieved through developing other energy sources and storage technologies. As a result, recently there has been a growing interest in high power and high energy density storage systems. A more widespread use of renewable sources and a better efficiency of transportation systems are two important goals to be pursued to overcome this problem.
Energy storage systems (ESSs) are the key to deal with the intermittent nature of renewable energy sources and increase the power transmitted into the grid from systems such as wind and solar power. In addition, an increase in the efficiency of a vehicle requires kinetic energy to be stored somewhere whenever the vehicle slows down or stops. Although these operations have been successfully performed with batteries on a low-power scale, new methods for efficiency enhancement will require large amounts of power that can only be provided by other energy storage technologies such as supercapacitors. These have attracted significant attention due to their high power capabilities and long cycle-life, giving a very good chance to build more advanced hybrid ESSs, for both on-board and stationary applications.
Section snippets
Supercapacitors within energy storage systems
Supercapacitors are devices capable of managing high power rates compared to batteries. Although supercapacitors provide hundred to many thousand times higher power in the same volume [1], they are not able to store the same amount of charge as batteries do, which is usually 3–30 times lower [1]. This makes supercapacitors suitable for those applications in which power bursts are needed, but high energy storage capacity is not required. Supercapacitors can also be included within a
Electric double layer
An electric double layer is a structure appearing when a charged object is placed into a liquid. The balancing counter charge for this charged surface will form on the liquid, concentrating near the surface. There are several theories or models for this interface between a solid and a liquid. In Fig. 6 the Helmholtz model, the Gouy–Chapman model and the Stern model are illustrated, where Ψ is the potential, Ψ0 is the electrode potential, IHP refers to the inner Helmholtz plane, and OHP refers
Pseudocapacitance
Pseudocapacitance is a Faradaic charge storage mechanism based on fast and highly reversible surface or near-surface redox reactions. Importantly, the electrical response of a pseudocapacitive material is ideally the same as the one of a double-layer capacitor, i.e. the state of charge changes continuously with the potential, leading to proportionality constant that can be formally considered as capacitance. Some materials can also store a significant charge in a double layer such as
Electrode materials
The most important electrode materials are gathered here, giving a brief explanation of their characteristics. This section is divided in three subsections comprising carbon based materials, metal oxides, and conducting polymers.
Electrolytes
The electrolyte also plays an important role in the supercapacitor performance. The electrolyte concentration has to be high so as to avoid depletion problems during the charge of the supercapacitor, especially for organic electrolyte (“the electrolyte starvation effect”) [176]. If the electrolyte reservoir is too small as compared to the large electrode surface the performance of a supercapacitor cell will be reduced. Concentrations higher than 0.2 molar are normally sufficient [3].
Most
Electrochemical configuration of supercapacitor cells
The electrochemical configuration of supercapacitor cells is detailed in Fig. 3, which shows that supercapacitors can be symmetric or asymmetric. Typically, the symmetric supercapacitors are made up of two identical carbon electrodes, schematically shown in Fig. 4.
The asymmetric supercapacitors are fabricated with different electrodes. This can be two electrodes made of the same carbons but having different thicknesses (masses) or two different carbons, or a pseudocapacitive material in at
Manufacturers
The number of supercapacitor developer and manufacturers is growing rapidly. Most of the market is taken by organic-based supercapacitors using acetonitrile or propylene carbonate-based electrolytes, but nowadays near the 50% of the available manufacturers offer devices based on non-flammable and relatively non-toxic electrolytes, which is an advantage.
There are several applications in which a supercapacitor can be the best solution, and these applications differ from each other in terms of
Conclusions
Supercapacitors are a very interesting technology for different applications requiring high power ratings, long cycle and calendar life, and reliability. Those requirements are stipulated by renewable energy systems such as wind power conversion and solar systems. The first requires high power burst for blade-pitch adjusting or enhancing low voltage ride-through capability. The second requires output power smoothing, which is classically done with the batteries that do not last more than few
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