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Capacitors are electronic devices which store electrical charge. They consist of a pair of conductors, with a dielectric in between. When a potential difference is applied across the conductors, an electric field forms in the dielectric, and energy is stored between them. A capacitor is described based on its capacitance value, which is the ratio of the electric charge on each conductor to the voltage between the two. Most practical capacitors have values in the range of ÂµF (1x10-6) or mF (1x10-3). Capacitors store charge physically, hence there are no chemical or phase changes, and therefore capacitors may be charged or discharged countless number of times. Thus, capacitors are used to smooth numerous circuits, including power supply and timer circuits. Capacitors also have the ability to block DC hence they are used in filtering circuits.
Electrolytic capacitors are the second generation type of capacitors. They differ from traditional capacitors because at least one of their plates is a non-metallic conductor, also known as an electrolyte. Electrolyte capacitors are very cost effective and provide a greater capacitance per volume than traditional capacitors. Hence, they are normally found in circuits which utilise high currents or low frequencies, such as audio amplifiers. Their construction is similar to that of a cell, but the anode and cathode materials are the same. Aluminium, tantalum and ceramic are all electrolyte capacitors which use solid or liquid electrolytes.
The third generation of capacitors are the supercapacitors, also known as ultracapacitors or electrochemical capacitors. These latest type of capacitors make use of high surface area electrode materials together with thin electrolytic dielectrics to allow the supercapacitor to reach capacitances of much larger values than traditional capacitors. Thus they achieve higher energy densities whilst keeping the high power density which make capacitors so popular.
Supercapacitors can be split into three types, based on their mechanism for storing charge: electrochemical double layer capacitors, pseudocapacitors and hybrid capacitors. Pseudocapacitors use a Faradiac mechanism which involves the transfer of charge between electrode and electrolyte, through the chemical processes such as oxidation-reduction reactions. Electrochemical double layer capacitors use a non-Faradaic process, whereby charges are dispersed across surfaces by physical process. These physical processes are non-chemical processes; hence no chemical bonds are made or broken. Hybrid capacitors use a combination of both non-Faradaic and Faradaic mechanisms. Error: Reference source not found shows the difference in design between the electrostatic, electrolytic and the electrical double layer capacitor. Figure illustrates the categorization of supercapacitors.
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Figure -Three Types of Capacitors illustrating the Differences in Design
Figure -Supercapacitors Classification
Supercapacitors have many advantages over standard capacitors, although they share the same principles. These standard capacitors have comparatively high power densities, but relatively low energy densities when compared to electrochemical batteries and fuel cells. This means that a battery is able to store larger amounts of energy than a capacitor, but is unable to release its energy as rapidly as a capacitor. Capacitors cannot store high amounts of energy, and their energy per unit mass/volume is low, however they are able to release their energy quickly, so they have high power densities.
A Ragone plot is used to display typical energy storage and conversion devices based on their specific energy and specific power. Supercapacitors are situated between batteries and standard capacitors. Supercapacitors may be combined with either batteries or capacitors to improve their performance; the power density in the case of batteries and fuel cells, and energy density when combined with conventional capacitors. Moreover, supercapacitors have a much longer cycle life than batteries since negligible chemical charge reactions take place. Figure 3 is the Ragone plot for capacitors, supercapacitors, batteries and fuel cells.
Figure -Ragone Plot sketch of Storage and Energy Devices
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Supercapacitors have additional electrodes with greater surface areas and much thinner dielectrics that decrease the distance between the two electrodes. Hence, taking A as the surface area and D as the distance between the electrodes, and analysing the capacitance equation of the standard capacitor (Equation ), it is clear that supercapacitors can achieve a much larger capacitance. Also, from Error: Reference source not found, this higher capacitance leads to higher energy levels compared to conventional capacitors. Supercapacitors also keep the low Equivalent Series Resistance (ESR) property of conventional capacitors to achieve similar power densities. Figure illustrates these properties through the schematic diagrams of the conventional capacitor and the supercapacitor.
A < A
D > D
Figure -Schematic of Conventional Capacitor and Supercapacitor
Electrical Double Layer Capacitors (EDLC)
EDLC use an electrochemical double layer formed at an electrolyte interface to store electric energy. They are constructed from two carbon-based electrodes, an electrolyte and a separator in between the two electrodes. EDLCs, as has been mentioned earlier, store charge non-Faradaically and thus there is no charge transfers between electrode and electrolyte. When a voltage is applied, electric charges are amassed on the electrode surfaces. Unlike charges are attracted to each other, hence the ions found in the electrolyte solution move through the separator, into the pores of the electrode of the opposite side. The electrodes are designed specifically to prevent recombination of ions since they are separated by a membrane, which permits the mobility of charged ions whilst preventing electronic contact. Hence a double layer of charge is created at each electrode. It is the combination of these double layers, the increased surface area of the electrode and the smaller distance between the electrodes that allow EDLCs to achieve high energy densities.
Storing charge is highly reversible in EDLCs, since there are no chemical or composition changes as there are no transfer of charge between electrolyte and electrode. Therefore, EDLCs can achieve very high cycling stabilities. EDLCs have a cycling life of millions of cycles, all at a stable level of performance. Electrochemical batteries can only be charged and discharged around 103 cycles.
Classification of Electrochemical Double Layer Capacitors
The performance level of an EDLC, most specifically its operating voltage, is determined by its electrolyte. An EDLC have either an aqueous or organic electrolyte.
Aqueous electrolytes such as acids and alkalis (H2SO4 and KOH respectively) have high ionic conductivity levels, low cost and are among the most popular types of electrolytes. However, their operating voltage is limited to around 1.23V. Their Farads per gram ratio is higher compared to non-aqueous (organic) electrolytes, due to the higher dielectric constant.
Non-aqueous electrolytes allow the operating voltage of the cell to reach around 2.5V-2.7V. Non-aqueous electrolytes include propylene carbonate or acetonitrile. These organic electrolytes are used in many commercial supercapacitors, specifically in higher energy applications. This is due to the high specific energy values these supercapacitors can reach, since the specific energy is proportional to the square of the operational voltage. A major disadvantage of non-aqueous electrolytes is that their electrical resistivity is at least 20 times greater than aqueous electrolytes. Thus, supercapacitors with organic electrolytes have a larger internal resistance. This reduces the maximum usable power, and hence limits their use. However, part of the reduction in power is compensated by the larger operational voltage attainable. **PRINCIPLES & APPLICATION â€¦ ***
Ionic liquid electrolytes (RTILs) increase the maximum operation voltage possible to 3.5V. Using RTILs, the energy capabilities of the supercapacitor is increased. **IONIC LIQUID ..**
Carbon Material used for Electrolyte Construction
EDLCs are also split into different categories based on the type of carbon used as the electrode material. Carbon has many advantages when used as the electrode substance, including:
High surface area range
Good corrosion resistance
High temperature stability
Measured pore structure
Process ability and compatibility in composite materials
Other electrode materials include conducting polymers and metal oxides. The first two properties are very important in the construction of supercapacitor electrodes. These properties of carbon permit both the conductivity and surface-area to be modified as required. There are three types of carbon materials that are employed to store charge in EDLC electrodes: activated carbons, carbon aerogels and carbon nanotubes. To understand the electrical properties of supercapacitors, the structure and behaviour of carbon is explained briefly below.
Properties of Carbon
Carbon has four allotropes: natural graphite and diamond, which are found as minerals on earth; and carbine and fullerenes, which are synthetic. Carbon is unique due to its high number of allotropes and its large range of structural forms and physical properties. Most carbons in use today are manufactured or engineered carbons. They have an amorphous structure with a disordered microstructure similar to the one found in graphite. The electrical properties of carbon have a large effect on the electrical properties of supercapacitors. The electrical properties of carbon materials depend mainly on their structure. The resistivity and conductivity levels are the vital specifications of an electrode. Conductivity of solid carbons is increased significantly at temperatures up ~700 Â°C, and at a slower rate up this temperature. During this heat treatment, electrons are delocalized from their bonds and become charge carriers. Also, electrical conductivity increases as separate conjugated systems connect together to form a conducting network. Therefore, heat treatment increases the conductivity of carbons by changing the scale of structural disorder.
The electronic resistivity of carbons is dependent on their degree of crystal positioning. The intrinsic resistivity of a carbon material is determined by its chemical and structural morphology. The electric resistance of a stream of combined carbon particles is dependent on the intra-particle resistance and the inter-particle resistance. There is also a resistance in a current-carrying path, through the carbon particle, past the collector interfaces and through the metallic conductor.
It has also been studied that the greater the compaction pressure, the lower is the resistivity of the carbon powder, and at high compaction pressures this approaches the intrinsic resistivity of the carbon material itself. Packing pressure reduces the path length across the bed by compressing the particle bed. The use of carbon powders means that the thickness of the carbon coating can easily be modified, allowing for the construction of electrodes in various shapes and capacities. Thin coating films reduce the ESR of electrodes, which is vital for high-charge applications.
Other factors which affect the electrical properties of carbon include:
Surface oxygen: Increasing the surface oxygen of carbon increases their electrical resistivity, whereas reducing the surface oxygen content decreases their electrical resitivity.
Organic binding agent: The presence of an organic binding agent maintains the structure of the carbon electrode. Excessive amounts of binder will increase the electrode resistance and hence the capacitor ESR, hence the binder must not be used regularly.
As explained earlier, there are three types of carbon materials that are used as electrodes in an EDLC. These are activated carbons, carbon aerogels and carbon nanotubes.
Most commercial EDLCs use electrode material made of activated carbon, usually blended with a conductive carbon black or graphite. Activated carbons produced from carbonized phenolic resins or petroleum cokes are suitable for use in supercapacitors, especially organic-based capacitors. (Hollenkamp, 2006) Activated carbon materials store charge in EDLCs through their double layer capacitances. Large surface areas (around 1000's of m2/g) can be formed using activated carbon materials. Activated carbons use a complex porous structure made up of various sizes of structures: macro (>50nm), meso (2-50nm) and micro (<2nm) structures. These structures allow activated carbons to have high surface areas. Theoretically, specific capacitance of an activate carbon is directly proportional to the surface area, however this theory does not hold in practice. There have been cases where activated carbons with a lower surface area gave a larger specific capacitance, when compared to those with a larger surface area. This is due to ionic transport necessary in the electrolyte fluid. Electrolyte ions that are too big to be diffused into smaller micro pores prevent some pores from having an effect on charge storage. Large pore sizes are associated with high power densities, and smaller pore sizes are associated with higher energy densities. Thus, recent EDLC design research has focussed on determining the optimal pore size for a given ion size, as well as finding improvements to techniques to control pore size distribution during fabrication.
Carbon aerogels are highly porous materials formed by the pyrolysis of organic aerogels. They are normally synthesized by the poly-condensation of resorcinol and formaldehyde, via a sol-gel process and subsequent pyrolysis. Varying the conditions of the sol-gel process allows the macroscopic properties of aerogels, such as density, pore size and form to be controlled. (Hollenkamp, 2006)The carbon aerogel material has a high usable surface area and high electrical conductivity. Carbon aerogels are formed from a continuous network of conductive carbon nanoparticles with interspersed mesopores. This continuous structure, coupled with their ability to form chemical bonds to the current collector, means that carbon aerogels do not need an adhesive binding agent. Hence, carbon aerogels have a lower ESR than activated carbons, which results in a higher power being delivered. Aerogel supercapacitors find uses as energy storage in DSL modems (Emily Ng, 2006) as well as in high power applications (Cooper Bussman, PowerStor)
Carbon Nanotubes (CNTs) are nanoscale devices are formed when a grapheme sheet of carbon atoms is rolled into a cylindrical structure. These structures have a wall thickness of one atom, a diameter on the order of ten atoms and are typically several micrometers in length. (Johnson) Electrodes made from carbon nanotubes are usually grown as an entangled mat of carbon nanotubes, with an open network of mesopores. The mesopores in carbon nanotube electrodes are interconnected, unlike in other carbon-based electrodes. Therefore, this permits a continuous charge distribution that consumes almost all the available surface area. This means that the surface area is used up more efficiently, allowing for capacitance levels similar to those found in activated carbon supercapacitors, despite the fact that carbon nanotube electrodes have a smaller surface area.
CNTs can act either as a metallic or semiconductor material, based on their structure. CNT electrodes have a lower ESR than activated carbon as electrolyte ions can diffuse into a mesoporous network much more easily. Fabrication techniques have also been developed to decrease the ESR of CNT supercapacitors further. CNTs can be grown directly on current collectors, exposed to heat-treatment or formed into colloidal suspension thin films. The efficiency of the mat structure means that CNT capacitors reach energy densities comparable to activated carbon and carbon aerogel materials. They also achieve higher power densities due to their low ESR levels. Carbon nanotubes are used in microchips as well as hybrid cards. (Nano World: Carbon nanotube capacitors, 2006) They are also used in spaceflight applications, superconductors, hydrogen storage, field emission, logic circuits and other emerging areas. (S. Arepalli, 2005)