71Supercapacitors as Energy Storage Devices

SUPERCAPACITORS AS ENERGY STORAGE DEVICES

Anna Lisowska-Oleksiak / Gdańsk University of Technology Andrzej P. Nowak / Gdańsk University of Technology

Monika Wilamowska / Gdańsk University of Technology

1. INTRODUCTION

During recent years the fuel crisis combined with the issue of climate change has forced administrations of highly developed countries to introduce legislative solutions aimed at decreasing CO2 emissions and diversi-fication of energy sources. Novel systems using renewable energy sources, i.e. wind turbines or solar batteries are now being rapidly developed, on research, implementation and operation levels. Incorporation of multiple sources of electrical power into one system requires appropriate tools for energy storage and conversion. Countries experienced in wind and solar power applications propose various solutions. These include systems of galvanic cells, so-called redox flow cells (RFCs) and electrochemical capacitors. Technologies based on electro-chemical capacitors have already been practically implemented in the world [1]. Cars with hybrid systems are also equipped with supercapacitors, which act as components of high power density [2].

Electrochemical capacitors (ECs) have been known for many years. In 1957 Becker (General Electric) patented a capacitor design in which carbon material with a well-developed surface area acted as an electrode and sulphuric acid was used as electrolyte [3]. In 1970, which is seen as the beginning of EC commercial ap-plications, the company SOHIO attempted to introduce these devices into the market [4]. The 1990s saw a huge intensification of scientific and technical research on electrochemical capacitors. This is related to application of ECs in vehicles with electric or hybrid propulsion systems. Electrochemical capacitors have been comprehen-sively described in Conway’s monograph [5].

2. PRINCIPLES OF ENERGY STORAGE IN ELECTROCHEMICAL CAPACITORS

Generally electric energy can be stored in electrochemical devices in two main ways:1) by using chemical reactions and/or2) directly by the concentration of electrostatic charge in the interface between electrode and electrolyte.In the first case the energy of chemical reaction is transformed into electricity according to the equation

W = -z × F × E (W – work which can be performed, z – number of transferred electrons, F – Faraday constant 96485 C/mol, E – potential change. This type of conversion process is known as faradaic reaction. Devices which utilize faradaic reactions are galvanic cells (batteries1, accumulators, fuel cells) and redox supercapacitors.

Abstract

Electrochemical capacitors, also known as superca-pacitors or ultracapacitors, store energy in an electric field within an electrochemical double layer. Using electrodes with a developed surface allows obtaining high capacitance values. Small electrochemical capacitors have been avail-able on the market for many years and applied in small electronic devices. Rapid progress in materials engineer-ing evolving towards nanotechnologies has resulted

in increasing reliability of supercapacitors which work both with wind turbines and systems of photovoltaic cells. Fur-ther development of supercapacitor technology is achieved by improving their operational parameters, particularly voltage range and power rating. This paper presents basic principles of supercapacitors, their characteristics and ex-amples of their application.

1. Theoretically the word “battery” should only be used to describe a set of cells connected in parallel or series, but it has become a standard of common language to call such a commercial popular product [6].

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The other mechanism for charge storage, the so-called nonfaradaic mechanism, is a principle for electro-chemical capacitor operation. At the electrode/electrolyte interface a capacitor with an electrical double layer dl is formed. It is composed of charges on a metal surface and ions of opposite charge in the solution directly adjacent to the electrode surface.

ELECTRICAL DOUBLE LAYER MODELSThe concept of the electrical double layer has a long history. In modern times it can be traced back to

studies on dispersed phases carried out by Helmholtz (1857). Scientific development in this field is presented in fig. 1. First models take into account the charge ordering (Helmholtz 1857) (fig. 1a) and diffuse layer effect caused by thermal movements (Gouy-Chapman model) (fig. 1b). The Stern model (1927) (fig. 1c) combines these two approaches. The resulting theory says that there are actually two capacitors connected in series. One of them is a Helmholtz capacitor with a capacitance CH and the other – a capacitor with diffused layer with a capacitance Cdif. Total capacitance of an electrical double layer is Cdl

-1 = CH-1 + Cdyf

-1. . Therefore charge order-ing at the interface of two conductive phases results in the formation of a capacitor. Capacitance C of a double layer capacitor results from a charge accumulated at a proper range of potentials C = dq/dV and depends on the geometry (area A and plate separation d). An interfacial capacitor has a thickness of d depending on the size of solvent molecules, and in this case d denotes the diameter of those molecules of their clusters. Studies by Graham (1947) and the model created by Parsons (1978) take into account the presence of solvent dipoles in an inner layer capacitor, see fig. 1d.

a) b) c)

d)

Fig. 1. Electrode/electrolyte interface according to Helmholtz model (a), Gouy-Chapman diffuse layer (b), Stern diffuse layer (c) and Graham diffuse layer (d), where φM denotes Galvani potential and ψ is Volta potential, IHP and OHP are inner and outer Helmholtz planes respectively [5]

diffuse layer

solvent particle

anion

cation

Gouy-Chapman diffuse layer

IPH OHP

OHP

Anna Lisowska-Oleksiak; Andrzej P. Nowak; Monika Wilamowska / Gdańsk University of Technology

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2.1. ELECTROCHEMICAL DOUBLE-LAYER CAPACITORSThe literature refers to the group of electrochemical capacitors using electrical double layer charge as

Electrochemical Double-Layer Capacitors or ECDLs.Capacitance of a capacitor is proportional to the surface of its plates and dielectric constant of the sub-

stance contained between those plates, and inversely proportional to the plate separation.

(1)

where C is capacitance [Farad], A – electrode surface, d – plate separation, ε0 – permittivity of free space, and εr – relative permittivity of the medium.

Capacitance of a capacitor with an electrical double layer metal (e.g. Pt, Au)/electrolyte Cdl varies be-tween 16 and 50 μF/cm². This value is not attractive for practical applications.

According to the equation (1) significant increase of capacitance Cdl can be achieved by using electrode materials with a so-called developed surface area. Those may be conductive carbons with porous structure, oxides of transition metals and electroactive polymers. Activated carbon with a surface of 1000 m²/g and double layer capacitance Cdl 15 μF/cm² allows achieving a specific capacitance of 150 F/g (1000 m²/g × 10,000 cm²/m² × 15 μF/cm² = 150 F/g). Hence the name “supercapacitor” or “ultracapacitor” used for devices which use capaci-tance with a dual layer of electrodes made of materials with highly developed surfaces. In practical applications electrode layers with a thickness of several micrometers are used.

2.2. ELECTROCHEMICAL CAPACITORS USING SO-CALLED REDOX PSEUDOCAPACITANCEElectrochemical capacitors using so-called redox pseudocapacitance are another type of chemical capaci-

tor widely used in practical applications. They are known as redox electrochemical capacitors or redox super-capacitors. These devices, apart from utilizing charge of electrical double layer, are a source of current result-ing from a charge transfer process occurring at substances adsorbed on the surface and undergoing faradaic reaction. The difference between a normal redox reaction and the described process is substrate availability. In capacitors charge transfer is limited to the electrode surface only. The process occurs in multi-electron mode and in a wide range of potentials. This type of device use materials able to undergo surface redox reactions (e.g. ruthenium oxides, electroactive polymers). An example reaction where redox pseudocapacitance is used may be a reaction which occurs on the surface of ruthenium oxides, where surface hydroxyl groups in an acidic environment undergo reaction according to the following equation [7]:

(2)

It is worth noting that the stoichiometry of this reaction is very complex and the surface layer of –OH groups is treated as a whole. Stoichiometric coefficients of the reaction refer to the monolayer of active surfacegroups.

3. DIFFERENCES BETWEEN ELECTROCHEMICAL CAPACITORS AND GALVANIC CELLS

Both capacitors and galvanic cells consist of two electrodes separated with electrolyte (fig. 2). The differ-ence lies in the character of used electrode material and charge accumulation mechanisms. In electrochemical capacitors both electrodes are in most cases made of the same material. In galvanic cells the electrodes have different chemical properties (the anode material is different from that of the cathode).

�d

AC r×× 0ε ε

� δ yδzyz (OH)RuO(OH)RuO eH δ δ

Supercapacitors as Energy Storage Devices

74

Fig. 2. Schematic diagram of an electrochemical capacitor

In the process of capacitor charging or discharging an internal change of electrode potential V is continu-ously observed. It follows the equation:

C = q/V or = C × V (3)

Curves for discharge of an electrochemical capacitor and a battery are presented in fig. 3.

Fig. 3. Discharge curves of an electrochemical capacitor and a battery

In contrast to that characteristic, charging/discharging of galvanic cells is carried out at constant poten-tial, except for values near 0 and 100% (fig. 2).

This difference results in the fact that energy E accumulated by a capacitor is:

E = 1/2 C × U2 or E = 1/2 q × U (4)

while for a battery the energy level is q × U, i.e. it is twice higher than the energy level in a capacitor of the same voltage U = ΔV. Power of a supercapacitor is expressed by the equation:

Separator with electrolyte

Conductive casing wall

Electrode material in contact with electrolyte

Battery discharge curve

Electrochemical capacitor discharge curve

Time

U [V

]

Anna Lisowska-Oleksiak; Andrzej P. Nowak; Monika Wilamowska / Gdańsk University of Technology

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where R stands for the device’s resistance.

(5)

Equation (4) indicates that increase of energy accumulated in a capacitor can be achieved by:1) capacitance increase, which may be realized by:

a. increasing electrode active surface,b. decreasing plate separation,c. increasing relative dielectric permittivity of the medium

2) voltage increase.

According to the equation (5) power can be increased by:1) voltage increase2) resistance decrease.When designing a device we can influence the useful power level by appropriate selection of materials,

electrode geometry and electrochemical stability of the electrolyte.Electrochemical capacitors as energy storage and conversion devices can be placed between electrolytic

capacitors2 and batteries. This is illustrated by the Ragone diagram (fig. 4).

� 2

4UPR

Fig. 4. Ragone diagram for various electrochemical devices [8, 9]

Time constants (RC) (dashed lines in the diagram) indicate that the charging/discharging time for revers-ible galvanic cells is considerably longer than corresponding times for electrochemical capacitors.

Batteries, just like low-temperature fuel cells, display low power density when compared to electrolytic capacitors. At the same time batteries have higher energy density than capacitors. Using both batteries and electrochemical capacitors in the same device can improve its operational parameters. The number of charg-ing/discharging cycles for electrochemical capacitors is much higher than for batteries. This results from the fact that in the cells new phases are created during the charge transfer process and difficulties resulting fromside effects occur. An electrochemical capacitor uses mainly electrostatic charge, so from a theoretical point of view there is no limit for the number of charging/discharging cycles.

2 In contrast to electrochemical capacitors, electrolytic capacitors (with dimensions of several centimetres) have very small capacitances, measu-red in micro- or nanofarads. Electrolytic capacitors owe their name to the method of separator creation between the plates. Plates of electrolytic capacitors are made of metals like aluminium, tantalum, titanium, niobium, etc. Plates are separated with a thin (10…100 nm) film of an oxide of therespective metal. This film occurs as a result of anodic polarization of both plates. Electrolytic capacitors should not be confused with electrochemicalcapacitors.

Spe

cific

pow

er[W

/kg]

Electrolyte capacitor

Electrochemical double-layer

capacitor

Cadmium nickel and nickel-metal hydride cells

UltracapacitorLithium-ion batteries

Acid-head cells

Fuel cells

Specific energy [Wh/kg]

Supercapacitors as Energy Storage Devices

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4. ELECTRODE MATERIALS

4.1. CONDUCTIVE CARBONSCarbon materials are often used for electrodes of electrochemical capacitors [5, 7, 10]. As one knows,

they owe their ability to conduct electric current to the presence of graphene layers where carbon atoms C have a hybridization sp2 (as opposed to non-conductive carbon types sp3). Resistance of graphite or so-called highly oriented pyrolitic graphites (HOPG) is high and depends on their structure and porosity.

Activated carbons are materials of very well developed surface. In a technical scale they are obtained from natural materials i.e. fossil fuels and organic substances (e.g. wood, fruit stones, nutshells). In laboratory conditions also sucrose and synthetic resins are used.

Available carbon electrodes can have specific surfaces area reaching even 2500 m²/g. Carbon materialis used in the form of powder, fabric, felt or fibres. Electricity storage on carbon electrodes is capacitive in anelectrochemical double layer. These are so-called electrochemical double-layer capacitors (ECDLs).

Progress in the field of nanotechnology allows one to expect that in the near future application of carbonnanomaterials in the form of single-walled and multi-walled nanotubes or nanoparticles will enable obtaining a higher specific capacitance of electrode materials.

4.2. METAL OXIDESOxides of transition metals are commonly used in redox electrochemical capacitors. The most popular

types are ruthenium oxides (RuOx) [5,7] for which x value varies from 1.9 to 2.0. Specific capacitance for ruthe-nium oxide capacitors can even reach 720 F/g. This is the highest value of specific capacitance achieved for anyknown electrode material; however, RuOx applications are limited due to the high cost of this material. Promis-ing alternatives include oxides of manganese, iron, indium, tin, vanadium and their combinations. For these, specific capacitance is around 150 F/g. A supercapacitor composed of Fe3O4 as a negative electrode and MnO2 as a positive electrode is characterized by an operating voltage up to 1.8 V in aqueous electrolyte. Specific ca-pacitance of such a device is 21.5 F/g, actual specific power – 405 W/kg and specific energy – 8.1 Wh/kg [11].

Oxide materials, just like carbons, are stable during thousands of charging/discharging cycles.

4.3. CONDUCTIVE POLYMERSConductive polymers, also known as synthetic metals, represent a very attractive group of electrode ma-

terials, which have found application in supercapacitors [5, 12]. These are mixed electron-ion conductors. The most popularly used polymers include polypyrrole and derivatives of tiophene. Advantages of those materials include fast oxidation and reduction processes during charging and discharging, high charge density (~500 C/g) and easy synthesis of electrode material.

Thanks to their energy accumulation properties, conductive polymers have found application as electrode materials in supercapacitors. These can be both p and n type polymers. Rudge et al have divided polymer super-capacitors into three categories.

Type I, where both electrodes are made of identical p-conducting polymer. When the capacitor is fully charged one electrode is oxidized (positively charged) and the other remains neutral (without charge). Potential difference between electrodes is ca 0.8-1.0 V.

Type II, where electrodes are made of different p-conducting polymers which have different oxidation-reduction potentials. Using different polymers allows enhancing the potential range.

Type III – contains both p-conducting polymer (e.g. polythiophene, poly(3-methylthiophene)) and an n-conducting polymer (derivative of bithienyl). Type III supercapacitors offer a wide range of operational poten-tials (up to 3 V for non-aqueous electrolytes) and appropriately higher energy density.

Conductive polymers used as electrodes in electrochemical capacitors may be modified in order to im-prove their operational parameters. Most often modification is made with oxides of transition metals – man-ganese and vanadium. Another option is modification of conductive polymers by attaching a redox group to apolymer chain. Yet another method is the introduction of inorganic substance into a polymer matrix. This addi-tive fulfils the role of a multicentric redox system [13-15]. There are also methods for modifying electroactivepolymers with nanomaterials [16]. In all cases activity of the composite material is created by both electroactive material and the modifying agent.

Anna Lisowska-Oleksiak; Andrzej P. Nowak; Monika Wilamowska / Gdańsk University of Technology

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5. ELECTROLYTES

Electrolyte type is another classification criterion for electrochemical capacitors. Both aqueous and non-aqueous electrolytes are used (with aprotic solvents and ionic liquids).

5.1. AQUEOUS ELECTROLYTESAqueous electrolytes restrict operational voltage to 1 V, as above this value during the charging process

molecules decompose on the positively polarized electrode and oxygen is generated, while on the negative electrode water is decomposed and hydrogen is generated. An advantage of aqueous electrolytes is the high conductivity value (e.g. 0.8 S/cm for sulphuric acid), and simple cleaning and drying of electrode material dur-ing the manufacturing process. Moreover, the price of aqueous electrolytes is considerably lower than that of non-aqueous ones.

In order to avoid problems related to decreasing effectiveness of supercapacitor charging, high-concen-tration electrolytes are used. They guarantee sufficiently low resistance values.

5.2. NON-AQUEOUS ELECTROLYTES WITH APROTIC SOLVENTSUsing organic liquids which do not contain chemically active hydrogen atoms in their molecules in the role

of solvent enhances the stability window of the system (decomposition of the solvent’s molecules does not oc-cur). This allows achieving higher operating voltage values. The higher the voltage level, the higher the amount of energy that can be accumulated (see equation (4)). Non-aqueous electrolytes allow achieving voltages up to 3 V. Higher values are prevented by traces of water present in solvents.

An adverse effect of using non-aqueous electrolytes is their high specific resistance value, which affectsthe capacitor’s power. Nonetheless, the loss of power is usually compensated by a possibility to achieve higher voltage.

5.3. IONIC LIQUIDSIonic liquids are salts which are liquid in ambient temperatures. A low melting point results from the

structure of those salts, which consist of a large and asymmetric cation (e.g. 1-alkyl-3-methylimidazolium, 1-alkyl-pyridinium) and a small anion. The range of their electrolytic stability depends only on the type of ions which the ionic liquid is composed of. Appropriate ion selection allows constructing supercapacitors operating in a wide potential spectrum. There are known designs where the operational voltage is 3 V. Usage of ionic liq-uids is limited by low conductivity value, in the range of mS/cm. Because of this feature ionic liquids are used in supercapacitors which are operated at higher temperatures.

6. SUPERCAPACITOR APPLICATION EXAMPLESElectrochemical capacitors are increasingly reliable devices which can work with wind turbines or pho-

tovoltaic cell systems [17]. Very fast charging/discharging rates offered by supercapacitors allow them to promptly adapt to load changes. Supercapacitors have found applications in household appliances, electronic tools, mobile telephones, cameras etc. They are also used in the power supply systems of electrically driven cars. In the automotive industry the main purpose of supercapacitors is to provide support for classic batter-ies – they act as an additional buffer during acceleration and braking. Such an arrangement lowers operational costs of the vehicle, as it extends battery lifetime. Supercapacitors protect the battery from harmful effects of peak loads. Recovery of braking energy by supercapacitors also allows reducing operational costs by decreasing energy consumption.

Supercapacitors as Energy Storage Devices

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REFERENCES

7. CONCLUSIONSupercapacitors are dynamically entering the power engineering market. Legal regulations concerning

environmental protection and sustainable development foster installation of renewable energy sources, and this in turn generates the demand for reliable energy storage and conversion systems.

Electrochemical capacitors are able to quickly charge and discharge. They also have long lifetimes, though they are not able to store volumes of electricity as high as classic batteries or fuel cells. Comparison of elec-trochemical capacitors and batteries reveals that they are in fact complementary systems. For that reason it is a very good solution to combine supercapacitors with chemical sources of electricity.

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Anna Lisowska-Oleksiak; Andrzej P. Nowak; Monika Wilamowska / Gdańsk University of Technology