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EDLC Characterization Platform Alexandru Vasile, Paul Svasta, Andreea Brodeală, Cristina Marghescu, Ciprian Ionescu University “POLITEHNICA” of Bucharest, Faculty of Electronics, Telecommunications and Information Technology Center for Technological Electronics and Interconnection Techniques Bucharest, Romania [email protected]; [email protected] Abstract: Technological improvements have led to the development of low resistivity materials with higher surface which are capable of storing more energy in the form of electrical charge. These developments as well as a better theoretical understanding of the process of charge transfer, a process that appears in double layer type materials, led to the development of capacitors with high capacitance, known as EDLC (Electrochemical Double-Layer Capacitor). It is important for the user to be able to predict the behaviour of the EDLC in different conditions. This paper introduces a platform which can be used to obtain the capacitance as a function of voltage, time or temperature. Keywords-component: Electrochemical Capacitor, characterization platform I. Double-Layer INTRODUCTION (HEADING 1) An electrochemical double-layer capacitor (EDLC) (other terms: supercapacitor, supercondenser, pseudocapacitor or ultracapacitor) is an electrochemical capacitor that has an unusually high energy density compared to common capacitors (typically on the order of thousands of times greater than a high capacity electrolytic capacitor). If we take as an example a Dcell sized electrolytic capacitor, this will have a capacitance in the range of tens of milifarads. An EDLC with the same size would have a capacitance of several farads. This represents an improvement of about two or three orders of magnitude in capacitance; the main disadvantage is that this happens usually at a lower working voltage. Emilian Ceuca University „1 Decembrie 1918” Alba Iulia, Romania model the double layer is seen as being composed by two electrostatic charged simple layers. A layer is on the electrode and the other is composed of ions from the electrolyte. The specific capacitance of this double layer is defined by the following equation: C A 4 Where: C- the capacitance, A - plate surface, - dielectric constant of the medium between the two layers (electrolyte), - Distance between the two layers (distance from the surface of the plate to the center of the ionic layer). The basic architecture of EDLCs is that of two electrodes; this has as result the building-up of two double layers, one for each interface electrode/electrolyte. A membrane separates the two layers and does not allow electric contact between them. The membrane allows ions to pass through, from one side to the other (figure 1). The basic operating principle of EDLC is similar to that of conventional electrostatic capacitors. Electrochemical capacitors store energy in an analogous way, but the charge is not accumulated on the two plates. The charge gathers at the interface between the surface of the conductor and an electrolyte solution (fig.1). The charge accumulated thus, forms a double layer that is electrically charged, the separation between each layer being of a few Anstrong. An estimation of the capacity can be obtained using the double layer model developed by Helmholz in 1853. In this (1) Figure 1. Storage mechanism in EDLC The plates are realized from materials with an effective surface processed to increase the surface of the double layer: porous carbon, carbon aerogels. High energy density’s can be obtained due to the high specific capacitance. This capacitance values are obtained through: The existence of the plate/electrolyte interface on the plates; The existence of the thin layer (of atomic dimensions) that separates the charge. The highest energy density in production is 30 Wh/kg. This is below rapid-charging Lithium-titanate batteries. EDLCs have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. They have applications as energy-storage devices used in vehicles and for smaller applications like home solar systems where extremely fast charging is a valuable feature. In many applications where batteries are used, along with a battery EDLCs lead to improved performance in operation. Starting the internal combustion engine at low temperatures is facilitated by the use EDLCs. To use energy efficient hybrid electric vehicles using EDLCs increase that recovers energy during braking. The Electric Power systems renewable energy such as photovoltaic or wind energy use is beneficial EDLCs. EDLCs can meet peaks of power when needed that the batteries can not generate. In a conventional capacitor, energy is stored by the removal of charge carriers, typically electrons, from one metal plate and depositing them on another. This charge separation creates a potential between the two plates, which can be harnessed in an external circuit. The total energy stored in this fashion is proportional to both the amount of charge stored and the potential between the plates. The amount of charge stored per unit voltage is essentially a function of size and the material properties of the plates, while the potential between the plates is limited by dielectric breakdown of the substance separating the plates. Different materials sandwiched between the plates to separate them result in different voltages to be stored. Optimizing the material leads to higher energy densities for any given size of capacitor. We cannot speak of a conventional dielectric in the case of EDLCs. In the case of an electrostatic capacitor the basic architecture is that of two plates separated by an intervening substance. In contrast to conventional capacitors EDLCs use "plates" that are in fact two layers of the same substrate. The electrical properties of these plates, the so-called "electrical double layer", result in the effective separation of charge despite the extremely thin (nanometers) physical separation of the layers. Since a bulky dielectric layer is unnecessary, the packing of "plates" with much larger surface area into a given size is possible. We obtain thus very high capacitances in practical-sized packages. The plates are realized from materials with an effective surface processed to increase the surface of the double layer: porous carbon, carbon aerogels. High energy density’s can be obtained due to the high specific capacitance. Storing charge in double layer is a surface process, so it can be concluded that the characteristics of the plate surface bear a direct influence on the capacitance value of the EDLC. The material that is most widely used for the plates is carbon, but research in the field point to metallic oxides and conducting polymers as interesting alternatives. Metallic oxides represent an alternative, mainly due to their high specific capacitance and low electrical resistance, thus allowing EDLCs with high power and energy density to be built. The maximum allowable voltage is dependent on the breakdown voltage of the electrolyte. The energy density (which is dependent on the voltage) is also restricted by the electrolyte. The power density is dependent of the ESR (Equivalent Series Resistance), which in turn depends on the electrolyte type: organic or aqueous. Due to their high decomposition voltage, organic electrolytes are usually used in commercial EDLCs. EDLC that use organic electrolytes can reach voltages of 2-2,5 V. The electrical resistance of organic electrolytes is relatively high, thus limiting the power density. A simple model describes a double-layer capacitor as a capacity (C) connected in parallel with an equivalent series resistance (ESR) and an equivalent parallel resistance (EPR). ESR is used to model power loss due to internal heating; internal heating is an important parameter for the charging/recharging processes. EPR models the current flow which influences the energy storage on the long term. One method to evaluate the energy density and the power density for super capacitors is charging them with a constant current. By charging and discharging super capacitors repeatedly we can raise the voltage characteristic. Main advantages of EDLCs over batteries are: Virtually unlimited operating life; Lower series resistance allowing higher power extracted; Fast cargo handling; Loading simple methods, not necessarily full load detection circuit; Efficient electrical energy storage, energy density is offset by lower long operating; Some of their main disadvantages are: It is not possible to use all available energy spectrum; The energy density is lower compared with batteries (one fifth to one tenth of the specific energy of batteries; Self is more powerful as a battery; II. PARAMETERS OF EDLCS The main parameters of EDLC are: nominal voltage, short circuit current, current losses. Other parameters are presented in the table below. TABLE I. EDLC PARAMETERS Parameter Values Operating temperature Rated voltage -40 to +70℃ Surge voltage 3.0VDC Capacitance tolerance Temperature Characteristics -20% to +80% High Temperature load High Temperature without load Humidity Resistance III. The platform also includes a load system, for the controlled discharge of the EDLC. Using this system the value and the variation of the discharge current can be determined. 2.5VDC +70℃│ΔC/C│≤30%, ESR≤100% of specified value at 25℃ -40 ℃ │ΔC/C│≤50%, ESR≤400% time of specified value at 25℃ After 1000 hours, +70 ℃ ± 2, nominal voltage, │ΔC/C│≤30% of the initial measured value, ESR≤400% of the initial specified value After 1000 hours, +70 ℃ ± 2, nominal voltage, │ΔC/C│≤30% of the initial measured value, ESR≤200% of the initial specified value After +40℃±2,90~95%RH,240 hour│ΔC/C│≤30% of the initial measured value, IL≤200% of the initial specified value,ESR≤4 time of the initial specified value CHARACTERIZATION PLATFORM The characterization of an EDLC – the value of the capacitance as a function of time, voltage or temperature - was performed using a platform developed for this type of measurements. The block diagram of the developed platform is presented in fig.1. A. Block diagramm components When developing the platform the following problems were considered: Separating the width band signal generator in direct current through the capacitor C1; The generator’s load has to be 50 ohm, so that the EDLC will act as a short circuit at frequencies that are higher than 100 Hz. This load is provided by R1, with a value of 50 . The current through the EDLC is read using R3. The voltage at R3s terminals is amplified through an instrumental amplifier U2. U2s terminals are connected at the input of a port of the NI acquisition system, installed on the computer. Through another port the voltage at the EDLCs terminals can be read using a very high impedance amplifier U1. Separating the alternative signal generator from the direct voltage source’s zero impedance through L1. Figure 1 Block diagram of the characterization platform B. Workingoutline of the block diagramm The frequency is fixed at the signal generator. At he time t0 the SW1 switch closes and the EDLC is charged with electrical charge in accordance with the parameters programmed at the energy source. The system starts the acquisition program and stores the data. C. Measuring limitations The maximal parameters for the platforms are imposed by the constructive elements and by the technical parameters of the auxiliary devices. The maximum parameters are as follows: Maximum EDLC charge current 5A; Maximum voltage 12V; Generator frequency 0 …60 MHz; Maximum generator signal 5 VVV; Maximum discharge current 20 A. IV. MEASUREMENT AND RESULT USING THE PLATFORM A. EDLCs characteristics of suite used in measurements: For the measurements presented in this paper, ten EDLCs were used. Processing and results of measurements made with the platform: V. CONCLUSIONS Constructive elements of a EDLCs having a direct impact on its performance are the electrolyte, separator and fixtures; Surface properties of a material used to manufacture have a significant impact on the specific capacity of the capacitor; chemical properties also impact the end product where pseudo-capacitance appears. Currently, the most common materials for the plates are activated carbon and conductive polymers, and metal oxides represent a future alternative; Separator properties also directly impact the performance of the EDLC; The electrolyte influences directly the specific capacitance and has a direct impact on energy density. Aqueous electrolyte solutions have better conductivity than organic solutions, but have a much lower breakdown voltage. The leakage current increases dramatically when the temperature rises. EDLC is recommended to be used for storage of large loads. The applications from the auto field use a special circuit which allows it to be coupled with a Pb battery. Figure 2. Voltage variation at the capacitor terminals for a charging current of 2A B. Drain currents maximum / minimum as function of temperature Drain current shown, 0.241 mA, is calculated as the average of 10 EDLCs, measured with the platform at the same initial conditions 1A and 2.5 V 0.241 mA. ACKNOWLEDGMENT The work has been funded by the Sectoral Operational Programmer Human Resources Development 2007-2013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreement POSDRU/6/1.5/S/16 and through the Financial Agreement POSDRU/88/1.5/S/60203. Stabilization time varies between 94 min - 150min REFERENCES P. Svasta, V. Golumbeanu, C. Ionescu, A. Vasile, “Rezistoare”, Cavallioti, Bucharest, 2010. [2] P. Svasta, A. Vasile, C. Ionescu, V. Golumbeanu, “Condensatoare”, Cavallioti, Bucharest 2010. [3] ***, Raychem Corporation, PolySwitch Resettable Fuses, Circuit Protection Databook, 1997. [4] www.passives.tycoelectronics.com [5] www.welwyn-tt.com [6] www.bccomponents.com [7] www.lcrcapacitors.co.uk [8] Do Yang Jung, „Shield Ultracapacitor String From Overvoltage Yet Maintain Efficiency”, Electronic Design, 27 May 2002, http://www.nesscap.com, Accesat: noiembrie 2009. [9] B.E. Cnway Electrochemical, EDLCs, Scientific Fundamentals an technological Application, 1999 Kluer Academic, New York. [10] Anca Duta, Tailoring Energy Storage Capacity in Lithium Ion Bateries ECOST 2009 Brasov. [11] Ronald K. Jurgen , Automotive Electronics Handbook, second editions, McGraw-Hill, Inc, New York 1999. [1] Figure 3. Variation of the leakage current