testing a hybrid supercapacitor using stainless ... - ulisboa

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Abstract—The development of energy storing devices with high density power has become an area of major interest to wide range of applications. Therefore, ultracapacitors have been in the spotlight because, beyond its high power density, it also has a lot of advantages when compared to other devices of energy storage

available in nowadays. Its characteristics have been explored since the beginning of the 21st century, which made ultracapacitors conquer a significative market share (and still increasing) in the market of energy storage devices.

This work has been done in the framework of an investigation project in the electrochemical area, and it pretends to study a redox ultracapacitor prototype with an aqueous electrolyte. This work aims to study its electrical characteristics, in order to

develop and improve even more the prototypes of ultracapacitors using this kind of materials.

The final objective of this work is to characterize a prototype electrically, estimating its capacity, internal resistance and self-discharge characteristic. It was also simulated its behaviour when applied in a DC/DC converter. The results obtained showed a lot of potential in the redox ultracapacitors technology, and that the prototype has similar

characteristics when compared to carbon based ultracapacitors, which are usually available in the market. In spite of this results, were identified aspects that could be improved, which is in line with the objectives of this work. Keywords – Ultracapacitor, Capacity, Internal resistance, Self-discharge, DC/DC converter

I. INTRODUCTION

LETRICAL and electronic devices have an increasingly

important role in the population quotidian and, as a result

of that, its growth has increased over the last years.

Among the most common energy storage devices are

ultracapacitors. This type of devices have, simultaneously,

comparable energy and power densities when compared to batteries.

Ultracapacitors main advantages when compared to

batteries are: high rates of charge/discharge, low degradation

over hundred thousand cycles, high efficiency and good

reversibility. Among the main disadvantages, the quantity of

stored energy per weight unit is relatively low, the voltage

varies with the stored energy and the dielectric has a high

absorption rate. [1]

Figura I.1 - Ragone chart [2]

A. Proposed work

As referred in the previous chapter, ultracapacitors have

been growing in the electrical energy storage market.

However, there are still a lot of limitations and a long way to

go. Therefore, ultracapacitors are subject to an improvement

phase of its technology.

The present work comes under a project on the

electrochemical area, whose main objective is to develop an ultracapacitor prototype using stainless steel electrodes with

electrodeposited transition metal oxides, manufactured in

IST’s laboratory of electrochemical technology. This

prototype should have its electrical properties characterized

and quantified.

It was performed a computer simulation of the prototype, to

test this device as a part of a DC-DC converter.

B. Applications

Ultracapacitors are still not being used in a lot of its

potential applications. Initially, were limited by its low energy

and power densities, being used only in low power and energy

applications. Due to the technological advancements, this

characteristics have been improving, and the application for

this type of devices has been increasing rapidly.

The main ultracapacitors applications are:

Military (for engines starting and substitution of

missiles batteries);

Memory Backup (ultracapacitors provide energy

right after the power cut, until the generator isn’t

working);

Testing a hybrid supercapacitor using stainless steel

electrodes with electrodeposited transition metal

oxides

João André Gama Correia, Instituto Superior Técnico, November 2015, Lisbon, Portugal,

[email protected]

E

2

Electrical vehicles ( due to its high efficiency and

the possibility of regenerative braking);

Energy quality (ultracapacitors can provide a

small discharge necessary to smooth the

interferences);

Portable energy sources;

Renewable energy sources.

II. FRAMEWORK

A. Capacitors

Conventional capacitors consist in two conductive

electrodes, separated by a dielectric medium. When a voltage

is applied at its terminals, opposite charges accumulate in the surface of each electrode, producing an electric field and

making possible energy storage. The capacity can be

calculated by:

(1)

The stored energy in a conventional capacitor is given by

the following expression:

(2)

To analyse a capacitors performance there’s an essential

concept, ESR (Equivalent Series Resistance). It represents the resistance of the dielectric, electrodes, electrolytic solution and

terminals under a certain frequency. It is, basically, a

representation of all the resistive components of the capacitor.

It’s responsible for the heating of the device, and can harm the

capacitors performance[3]. It’s also determinant in the power

of the capacitor, which is given by the following formula[2]:

(3)

B. Ultracapacitors

Ultracapacitors working principles are similar to capacitors,

with the major difference being the electrode area (which is

much larger) and distance (which is much smaller). Analysing

(1) and this characteristics, it’s possible to tell that

ultracapacitors can achieve much higher values of capacitance, which results in a larger amount of stored energy. [2]

The internal resistance can’t only be represented by ESR, and

EDR (Equivalent Distributed Resistance) must also be taken

in account. It corresponds to the additional contribution of the

charge redistribution in electrodes pores, which takes place in

every voltage “jump” and is due to the inhomogeneous

structure of the electrode. It can increase significantly the

heating of the device trough Joule’s effect.[4]

There are three types of ultrapacitors:

Eletric dual layer capacitors – Store charge

electrostatically;

Pseudocapacitors – Store charge electrochemically;

Hybrid Ultracapactors – Store charge

electrostatically and electrochemically.

The discharge characteristics of ultracapacitors, capacitors and

ideal batteries are represented in the following figure.

Figure II.1 - Discharge characteristics of energy storage devices

1) Electric dual layer capacitor

The electrodes of this type of ultracapacitors consist in a

microporous structure (usually made of active carbon) which

allows to increase the surface area. These are immersed in an

organic or aqueous electrolyte and separated by a porous

membrane, which is called the separator.[5]

The major function of the electrolyte is to provide the ions

responsible for the transportation of electric charges between

electrodes.[6]

Figur II.2 - Charge distribution in an electric dual layer capacitor (charge and discharged)[7]

Due to the fact that there are no charge transfer between the

electrolyte and the electrodes, there aren’t also chemical

reactions or composition changes associated to chemical

phenomenon’s (these are purely electrostatic). This results in a

high cyclic stability and, consequently, in a large number of

life cycles.

2) Pseudocapacitors In this type of ultracapacitors the energy storage resorts to

electrochemical phenomenon’s (there are charge transfer

between electrodes and electrolyte). This is done due to redox

reactions, electroabsorption processes and chemical

intercalation. This results in capacity values much superior to

the ones found in electric dual layer capacitors.[7]

The electrodes are usually made of conducting polymers or

metal oxides, which insure high reversibility in redox

reactions.

3

Table II.1 - Comparison between EDLC and pseudocapacitors[8]

Electric Dual

Layer capacitor

(EDLC)

Pseudocapacitor

Life cycles >500000 >100000

Energy

density

Low Medium

Power

density

Very high High

Cost per

energy unit

High Medium (half of

EDLC)

3) Hybrid ultracapacitors

Hybrid ultracapacitors aggregate pseudocapacitors and EDLC,

trying to utilize the advantages of each one of them.

Uses electrostatic and electrochemical processes to store

energy, so are able to achieve even higher energy and power

densities than EDLC, without sacrificing cyclic stability.

4) Ultracapacitor electric circuit Models

The electric circuit represented by the Figure II.3 represents a first order model of an ultracapacitor. However, this circuit

doesn’t take into account the non-linear behaviour caused by

the porosity of the material that composes the electrodes.

A more suitable electric circuit is represented by Figure II.4.

Figure II.3 - First order model of ultracapacitor electric circuit[9]

Figure II.4 - Non-linear model of the ultracapacitor [9]

5) Ultracapacitors market

Nowadays, the energy storage device market is essentially

divided in rechargeable batteries, ultracapacitors and single

use batteries. Ultracapacitors only have 0.57% of this market

share, but this share is expected to grow.[10] Ultracapacitors evolution resulted in a lot of new applications.

The initial growth was due to users electronics, but recently,

the growth has been more accentuated, driven by the growth

of electric vehicles industry, as it can be seen in the figure

below.[11]

Figure II.5 - Ultracapacitor market shares in 2014 and 2020 [12]

III. USED METHODS IN ULTRACAPACITOR CHARACTERIZATION

The objective of this chapter is to test a certain methodology

that allows the determination of the ultracapacitors electrical

characteristics. As it is know the properties of an ELNA (2,5V 200F), this methodology was applied and the results were

compared to the ones in its datasheet. The electrical properties

tested were:

Capacity;

Internal Resistance.

It was also observed the self-discharge characteristic.

A. Capacity

Recording the voltage and current characteristics during the

discharge of the ultracapacitor, it is possible to calculate the

energy spent using the following formula:

(4)

Basically, (4) results in the area below the voltage and current multiplied.

Knowing the energy spent during the discharge, it is possible

to calculate the capacity by the formula below:

(5)

In (5), U1 and U2 are, respectively, the maximum and

minimum voltage values obtained for a given period of time,

while is the variation of electrical energy store in the

capacitor, which is given by (4).

1) Experimental results

Using two ELNA ultracapacitors connected in series (to allow

a greater voltage value and, consequently, more precision in

the measurement), were realized three different tests with

three different values of discharge resistances. This way, it is

possible to test different current discharge values.

All the discharge figures are similar (only varying the current

values) so, below, there’s only a picture of a discharge to a 6Ω

resistance, and the capacity values obtained for each discharge

resistance.

4

Figure III.1 - ELNA discharge to a 6 ohm resistance

R=6Ω ⇒ C=222,6F

R=12Ω ⇒ C=208,2F

R=33Ω ⇒ C=222,9F As the capacity value given by the manufacturer is 200F,

analysing this results it’s possible to conclude that this method

is reliable.

B. ESR

To calculate ESR it’s necessary to calculate de transient

regime between the charge/discharge, as it can be seen in the

figure below.

Figure III.2 - Method to calculate ESR and EDR[4]

ESR value it’s given by the following formula:

(6)

To test different current discharges, were used three different

values of discharge resistances: 5,6 Ω, 8,2 Ω and 10 Ω. Higher

resistance values would lead to imperceptible transient

regimes.

The figure below represents the transient of the 5,6 Ω

discharge resistance, where this phenomenon is perceptible.

Figure III.3 - Transient regime for a 5,6 ohm discharge resistance

The results obtained for all the discharge resistances were:

R=5,6Ω ⇒ ESR=0,049 Ω

R=8,2Ω ⇒ ESR=0,045 Ω

R=10Ω ⇒ ESR=0,044 Ω It was not possible to find the ESR value for this specific

ultracapacitor, however, analysing the “normal” values for this

type of devices, the results are acceptable.[12][13][14] Also,

the ESR value is supposed to maintain its value even to

different discharge conditions, which can be seen in the figure

below.

Figure III.4 - ESR and EDR variation with the current per Farad unit [4]

C. EDR

To calculate this value, a lot of different voltage values were

registered when the ultracapacitor was discharging. This way,

it is possible to characterize the discharge behaviour trough a straight line, which can be given by the equation y=mx+b.

EDR can be calculated by the following formula:

(7)

Knowing the value of b, and subtracting it to initial voltage

value, its possible to calculate , as it can be seen in Figure III.2.

The results obtained, using three different discharge

resistances were the following:

R=5,6Ω ⇒ EDR=0,501 Ω

R=8,2Ω ⇒ ESR=0,478 Ω

-2

-1

0

1

2

3

4

5

6

0 100 200 300

Corrente[A]

Tensão[V]

Current [A]

Voltage[V]

-2

-1

0

1

2

3

4

5

6

-0,003 -0,002 -0,001 0 0,001 0,002 0,003

Tempo[s]

Corrente[A]

Tensão[V]

Current [A]

Voltage[V]

5

R=10Ω ⇒ ESR=0,477 Ω EDR values are supposed to vary with the current, as it can be

seen in Figure III.4. However, the discharge current values

must be much more different to see this difference and due to

the device characteristics it was not possible to obtain more

results. However, as the prototype values will be similar to

this, it’s interesting to do this comparison.

D. Self-Discharge characteristic

The self-discharge characteristic shows the quantity of charge

lost by the ultracapacitor when there are no components

connected to its terminals. The best way to find out how much

charge was lost, in %, it through the following formula:

[%] (8)

This calculation was made for different time values after the ultracapatior was left in an open circuit, and the results can be

seen below:

After 1h: %self-discharge=1,2%



Analysing this results, it’s possible to conclude that this device

has good charge retention.

E. Electrical circuit representation

In this chapter, ultracapacitors equivalent electric circuits

show in II.B.4) were simulated in order to see if this circuits

were able to give a response similar to the ones obtained

during the discharge tests.

As it was expected, the non-linear model gave a better

response when compared to the one obtained during the

testing, although it stills as an error associated.

Figure III.5 - Discharge simulation of the non-linear electric circuit

IV. EXPERIMENTAL RESULTS

The prototype consists in two cells connected in series,

constituted by two straight parallel electrodes

(electrodeposited NI-Co), separated by and aqueous

electrolyte (KOH) and a separator (paper).

The procedures used to calculate its characteristics were equal

to the ones used in III.

A. Capacity

This time, to calculate the prototype capacity, it were used

four different resistances to make the discharge. Each one was

tested, at least, 4 times, in order to produce the best results.

The picture below only shows one of the discharges, just to

have an idea of the prototype behaviour when discharging.

Figure IV.1 - Prototype discharge to a 6ohm resistance

The results obtained to all the discharges are in the tables

below.

R=6Ω Table IV.1 - Capacity values of the prototype when discharged to a 6 ohm resistance

Test1 Test2 Test3 Test4

[J] 13,60 14,77 14,23 14,45

[V] 2,04 2,38 2,42 2,36

[V] 0,02 0,04 0,02 0,02

Ctot [F] 4,72 5,22 4,86 5,19



[J] 17,09 18,42 17,11 20,50

[V] 2,58 2,68 2,56 2,50

[V] 0,04 0,04 0,04 0,04

Ctot [F] 5,14 5,13 5,22 6,56



[J] 25,34 27,15 26,52 26,09

[V] 2,66 2,72 2,74 2,72

[V] 0,58 0,64 0,58 0,58

Ctot [F] 7,52 7,77 7,40 7,39

-2

-1

0

1

2

3

4 Corrente Tensão

Current [A]

Voltage[V]

U[V]

I [A]

t[s]

6



[J] 29,88 29,32 29,14 28,13

[V] 2,72 2,82 2,82 2,86

[V] 1,08 1,04 1,06 1,0

Ctot [F] 9,59 8,54 8,54 7,84

Analysing all the results, it’s possible to verify a variation of

the prototypes capacity from 4,72F to 9,59F. It was also noticed a high voltage drop in the moment the discharge

began.

Making a mean value of all the results obtained, it’s possible

to say that this device has a capacity of 8,89F.

As it can be seen in Figure IV.12, there are clearly three

different discharge characteristics. This are discretized in the

figure below.

Figure IV.2 - Prototypes different discharge zones

Each zone represents a different discharge behaviour, and it’s

interesting to know if this characteristic is maintained no

matter what are the conditions. So all the zones from all the

curves obtained had their slope tested, and the results are in

the table below. Table IV.5 - Slope of the different discharge zones for every discharge resistance

6Ω 12Ω 33Ω 56Ω

Zone 1 0,35 0,153 0,066 0,057 Zone 2 0,044 0,02 0,0106 0,0073 Zone 3 0,0029 0,0032 0,0014 0,0011

As it can be seen in the table above, there is always a decrease

of the time constant (corresponding to the slope) on the same

order of magnitude for every discharge zone. This behaviour

is constant and independent of the charge of power values

involved.

B. ESR

To test this parameter, the resistance values were equal to the

ones used in III.B, 5,6Ω, 8,2Ω and 10Ω.

In the figure below there’s a representation of the transient

when using the 5,6Ω.

Figure IV.3 - Transient of the prototype using a 5,6 ohm discharge resistance

R=5,6Ω ⇒ ESR=0,5689 Ω

R=8,2Ω ⇒ ESR=0,5256 Ω

R=10Ω ⇒ ESR=0,5750 Ω

Analysing the results, they are all similar independently of the discharge current, which is supposed. Making a mean value,

it’s possible to say that this prototype as an ESR value of:

Comparing this results with the ones obtained for the ELNA

ultracapacitor, it’s possible to conclude that the prototype ESR

is, approximately, ten times superior! As refered in Erro! A

origem da referência não foi encontrada., this can lead to

device heating, interfere in the self-discharge characteristic

and even decrease the power value.

C. EDR

To calculate EDR, this time were used five different resistance

values, due to the prototype characteristics. Overall, all the

resistances have a bigger value, which results in a lower

discharge current. If it were used the same resistances, the

initial voltage drop would make much more difficult to obtain the results.

The EDR values calculated were the following:

R=33Ω ⇒ EDR=0,472 Ω

R=39Ω ⇒ EDR=0,482 Ω

R=47Ω ⇒ EDR=0,413 Ω

R=56Ω ⇒ EDR=0,372 Ω

R=62Ω ⇒ EDR=0,482 Ω

The results obtained are similar to each other, presenting some

variations due to prototype behaviour. The EDR value is

supposed to change with the current applied, as it is shown in

Figure III.4, but the difference between currents is not enough

to see this phenomenon.

D. Self-discharge characteristic

The self-discharge test were made with different initial

voltages: 3V (maximum voltage) and 2V. The tests resulted in

the following potential curves:

-1

-0,5

0

0,5

1

1,5

2

2,5

3

-0,004 -0,002 0 0,002 0,004

Tempo[s]

Corrente[A]

Tensão[V]

Current [A]

Voltage [V]

7

Figure IV.4 - Self-discharge with initial voltage of 3V

Figure IV.5 - Self-discharge with initial voltage of 2V

It must be noticed that the voltage drop is much more

significant when the initial voltage is higher, which is

expected due to capacitors characteristics.

The curves obtained show that the voltage decreases

significantly and very quickly. It’s normal for an energy

storage device to have, over time, its characteristics

deteriorated, however, in this case, the devices were new and

far from reaching its lasts life cycles. In spite of the poor

performance, the tests are consistent and prove the self-

discharge characteristic.

Similarly to what has been done to ELNA ultracapacitors,

some values were taken to see, in %, how much charge the prototype can retain after different periods of time. The results

were:

After 1min: %self-discharge=7,2%

After 10min: %self-discharge=16,5%




E. Efficiency

The prototype efficiency can be calculated by:

(9)

Edischarge is the energy spent during the device discharge. Its

maximum value can be obtained by the data present in Table

IV.4, and is equal to 29,89J.

Emax can be calculated by equation (2), and results in 59,13J.

Taken all this into account, it’s possible to say that the

prototype efficiency is, in the best case:

F. Energy and power densities, Maximum power

The values of energy and power density can be calculated by

the following formulas:[4]

(10)

(11)

Taking into account that the mass of the device is 4,3g, and all

the values obtain trough this chapter, the energy and power

densities calculated are:

The maximum power can by calculated by (3), the result

being:

G. Prototype electrical circuit representation

Using the non-linear electrical circuit, the result obtained was:

Figura IV.6 - Discharge simulation of the non-linear electric circuit (prototype)

The result goes accordingly to the behaviour observed in

laboratory. However, the specific behaviour of the prototype

mentioned before prevent this simulation to achieve better

results.

V. DC-DC CONVERTER APPLICATION

In this chapter some simulations have been done to predict

how this device would behave when subject to certain

conditions.

A. Voltage control in with internal current control

Using the first order model of the ultracapacitor, it’s possible

to obtain the following circuit to calculate the current in the

inductor:

U[V]

I [A]

t[s]

I [A]

8

Figure V.4 - Discharge of the ultracapacitor using voltage

control with internal current control

Figura V.1 - Electrical circuit considered

To control the current of the inductor, using the Laplace

transform:

(11)

The controller used was a PI, because normally is used in

systems with frequent charge changes, being efficient and with

simple implementation.

The block diagram the make this controller is the following:

Figura V.2 - Voltage control with internal current control

After calculating the converter gains, this system was

simulated to obtain the results in the charge and discharge

situations. The results can be seen in figure V.3 and V.4.

Regarding Figure V.3, it is possible to see that the

ultracapacitor achieves 10V in less than 40s, with a maximum

current value of approximately 11A.

Figura V.3 - Charge of the ultracapacitor using voltage control with internal current control

In Figure V.4, the first 30s correspond to charging the

ultracapacito to 10V. The discharge occurs after the 30th

second.

B. Simulation with a DC-DC converter model

1) Charge

To simulate the charge of the prototype, it was used the

following schematic:

Figura V.5 - Schematic used to simulate the charge of the prototype

Simulating the charge process from 0V to 10V, while

maintaining a current of 7A on the device, the following

results were obtained:

U[V]

t[s]

U[V]

t[s]

I [A]

I [A]

9

Figura V.6 - Voltage and current at the ultracapacitor during the

charge using DC-DC converter schematic

Analysing the results, the device charged to the 10V as

intended, and the current controller kept the current around 7A

as desired.

2) Discharge

Due to the purely resistive characteristic of the charge, the

control of voltage and current must be done separately. It was

used only a hysteretic controller for each situation.

3) Discharge with current control

The schematic used to simulate the discharge with current

control was:

Figura V.7 - Schematic for ultracapacitor discharge with current control

In this simulation, it is intended to maintain 7A in the resistance. The results obtained were:

Figura V.8 - Voltage and current in the resistance, using the ultracapacitor as source, when controlling the charge current

The current controller was able to keep the current in the

resistance at 7A, as desired, while the ultracapacitor had

enough power to support it.

4) Discharge with voltage control

The schematic used to control the voltage during the discharge

is similar to the one in FigureV.7, but with one of the inputs of

the hysteretic cycle being the voltage across the resistance terminals.

In this simulation, it was pretended to have 2V across the

resistance. The result can be seen in the figure below.

Figura V.9 - Voltage and current in the resistance, using the ultracapacitor as source, when controlling the charge voltage

VI. CONCLUSIONS AND FUTURE WORK

This work achieved interesting and promising results and

contributed to improve the experimental procedures in the

characterization of redox ultracapacitors. Firstly, it was made

a parallelism between capacitors and ultracapacitors, as well

as its fundamental concepts, operating mechanisms and

different types. It was also made a brief overview on the

market for this type of devices.

U[V]

t[s] t[s]

I [A]

U[V]

I [A]

I [A]

U[V]

t[s]

10

Then, it was necessary to define a methodology to test the

prototype but, in order to do it correctly, the methodology had

to be tested, to assure that the results are reliable. The

characteristics measured and tested were capacity, internal

resistance (ESR and EDR) and self-discharge. This

methodology was applied to a commercial ultracapacitor (ELNA 2,5V 200F), and the results compared with its

datasheet or usual values for the characteristics. As all the

values obtained were similar, this methodology was proved to

produce trustworthy results.

Applying the methodology to the prototype given by the

electrochemical department, when comparing the results

obtained with the ones of commercial ultracapacitors, the

prototype shows similar characteristics. However, it presents a

high self-discharge characteristic and also a high internal

resistance. It was also observed the electrolyte’s evaporation

and bubble. It was also observed different discharge

behaviours through the discharge, showing different time constants that maintain even with charge change.

In the last section of this work, some simulations were made

to predict the prototype’s behaviour when applied in a DC-DC

converter. It was simulated the mathematical model of the

circuit (block diagram) when charging and discharging, and it

was also tested the circuit of a DC-DC converter, with the

ultracapacitor functioning as source and as charge. The results

were as expected, and show that this device can be used to this

purpose.

In conclusion, in this work the main objectives were achieved,

and are important to contribute to the evolution of this technology, that shows to be very promising.

Another good point is use of ecological components

(electrolyte) in this type of devices.

As negative points, the main characteristic to improve is self-

discharge. This results in lower capacity values and higher

values of internal resistance. The ESR values are also very

high, which makes this point a priority to improve. It was

noticed the electrolyte evaporation in short periods of time and

the release of gases, when in the charge/discharge process

were used slightly greater current values.

Analysing all the work, the major improvement is the

construction of a more robust prototype, which can fix the structure that allows the immobilization of the electrodes and

avoids the electrolyte evaporation.

It would be interesting to develop a more compact device, in

order to increase the number of possible applications. This

would also improve the energy and power density values that

could be much higher with the decrease of weight and volume.

It’s necessary to increase the voltage of the device by, for

example, associating more cells. This would allow to test the

system in many different applications, using the controllers

simulated in this work.

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http://electronics.wesrch.com/paper-details/press-

paper-EL11TZ000HAWC-ultra-capacitor-recent-

technology-and-market-forecast-2020

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[13] T. Yuden, “Wire-wound chip power inductors (BR

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[14] NEC Tokin, “Vol.14” 2013.

testing a hybrid supercapacitor using stainless ... - ulisboa

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