Download III. Characterization Platform

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
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[1]
Figure 3. Variation of the leakage current