project of redox ultracapacitors using stainless ... - ulisboa

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Abstract—It was in 1950s when the first ultracapacitor was developed. However, only at the beginning of the XXI century this technology started to be explored, and, since then, it started to impose as a solution in the market of energy storage systems, just between the batteries and the conventional capacitors.

This work aims at studying a redox ultracapacitor prototype, its characteristics and, in particular, the development and upgrade of the prototypes that were produced using new metallic oxide-based materials and that needed to be electrically tested. Therefore, these work contributed to develop a stable prototype, to study its voltage, current and capacity, and use it as part of some particular systems, like the DC/DC converter. A computer simulation was also performed. The results evidenced the great potential for the proposed technology and revealed that the characteristics of these prototypes are similar to the ones showed by carbon-based ultracapacitors. Keywords—Ultracapacitors, Capacity, DC/DC Converter

I. INTRODUCTION The growth in the development of electric and electronic devices has increased exponentially in the past few decades [1]. Therefore, a parallel growth in the energy sector is required to fulfill the needs of those devices and to improve their autonomy.

Among the most common energy storage devices, there are not only the well-known batteries but also the ultracapacitors. Although their use is still growing, already being useful in several applications such as electronic, biomedical, and aerospace components, there are still improvements that can be made to the ultracapacitors. For this to be achieved, there is the need to create new electrodes based on different materials. The batteries, in comparison with the ultracapacitors, display much higher energy densities. On the other hand, the ultracapacitors are able to charge and supply energy much faster, because they have a much higher power density. In addition, ultracapacitors can also perform at a good efficiency level during longer periods of time compared to the batteries. Due to the fact the ultracapacitors support a higher amount charge-discharge cycles, they are able to operate inside a wider range of temperatures and they have more attractive dimensions (are smaller and lighter).

Figure 1. Ragone Chart [2]

A. Proposed Approach The present work focused on the development and upgrade

of redox ultracapacitors prototypes that were produced using new metallic oxide-based materials. In this context, it is required to characterize the prototypes, to quantify their properties and to determine their efficiency. This means to estimate their current, voltage and capacity values as well as predict their lifespan.

The prototypes were also tested as part of a DC/DC converter and it was also performed a computer simulation with the same implementation to verify the results obtained.

B. State of the Art In a large growing process within the energy market,

ultracapacitors have been progressively incorporated into new applications.

One of the first major uses for the ultracapacitors has been in the automotive industry. The regenerative braking system is one of the most frequent uses of this technology [3]. This system, which uses an internal combustion engine, takes advantage of the alternator for, in situations of deceleration, charge the ultracapacitor. Also in the field of the renewable energies there can be found several applications for ultracapacitors. They are often used to temporarily store energy obtained from the sea waves, wind and sun.

Project of redox ultracapacitors using stainless steel electrodes with electrodeposited transition

metals oxides João Saraiva Rafael

Department of Electrical and Computer Engineering Instituto Superior Técnico

Av.Rovisco Pais, 1049-001 Lisboa, Portugal

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For instance, in the wind energy production the ultracapacitors are widely used. The wind turbines can turn the wind into electrical energy. However, in emergency situations such as strong winds or, especially, power failures, it is necessary a safety control of the pitch of each blade, and that can be provided by the ultracapacitors. Being able to support a high range of temperatures combined with the ability of having a fast response time, the ultracapacitors have characteristics that makes them very attractive to this purpose [4]. For example, when compared to batteries, they are very light weighted.

In the field of solar energy, ultracapacitors are also starting to have an important role. In cloudy days, the amount of solar radiation incident on the panels can vary a lot. Therefore, ultracapacitors can be used to minimize those oscillations helping the panel to supply a constant amount of energy [5].

Another innovative application that leverages the capabilities of the ultracapacitor is the “Bluetooth” portable speaker [6]. This device, made with “Maxwell” ultracapacitors take only five minutes to charge for almost ten hours of usage.

More recently, other applications of ultracapacitors begin to emerge. In China, there is now into service a new concept of public transportation. There is an electrical bus that uses only the energy stored in ultracapacitors to move. As has been said, the greatest fault of ultracapacitors is their relatively low level values of energy density, so, in this case, this problem is cleverly bypassed by quickly charging the ultracapacitors at each bus stop. [7]

There is a wide variety of equipment that employs this technology. In some airplanes, like the Airbus 380, ultracapacitors are used as a source of energy supply in emergency situations. Some manufacturers, for example, are employing them to provide lift to electric forklifts [8]. Thus, any application that requires to charge and discharge quickly, can find in the ultracapacitors an extremely useful device for supplying energy.

C. Ultracapacitors The redox ultracapacitor is, as its name suggests, a type of

capacitor that was improved. A capacitor is an electrical component that stores energy and typically consists of two electrodes (two plates) separated by an insulating medium called dielectric or/and by a conductive medium called electrolyte. There are three types of ultracapacitors: i) Electrostatic ii) Electrolytic iii) Electrochemical. Those used in this work, the redox ultracapacitors, are within the electrochemical category of capacitors. This type of ultracapacitors can store energy due to the faradic reactions that are made between the electrodes and the electrolyte, oxidation-reduction reactions responsible for the charge storage.

Figure 2. Redox Ultracapacitor representation [9]

Rechargeable and single use batteries mainly share the

storage devices market. Ultracapacitors, for now, only have less than 1% of this share. However, by the year of 2020, it is predicted that ultracapacitors will represent around 5% of the storage units market [10]. For now, there are already a few companies that manufacture ultracapacitors. “Maxwell”, “Ioxus”, “Shanghai Green Tech Co” and “VinaTech” are between the biggest ones. Their ultracapacitors have, in their great majority, a voltage that rounds the 2,7V, a cycle life between 500000 and 1000000 cycles and capacity values that can reach the 5000F according to their size. [11][12][13][14]

Figure 3. Predicted growth of ultracapacitors market [15]

II. METHODS The study on these prototypes can be described in the

following steps:

A) Charge and Discharge Tests B) Prototype Improvement C) Capacity Calculation D) Voltage Improvement E) DC/DC Converter Application F) Computer Simulation

A. Charge and Discharge tests

The first phase of the experimental trials in this work was the characterization of this prototype using different

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electrodes, in different conditions, and repeating the tests under the same conditions to ensure their consistency. Therefore, the first test carried was to obtain discharge curves of the device. For this purpose, the ultracapacitor was charged with the aid of a DC voltage source and then it was observed being discharged through an oscilloscope. At first the ultracapacitors were fully charged and then, it was also conducted the same experiment but charging the ultracapacitor only until reaching 50% of its maximum voltage value.

B. Prototype Improvement

As the results of the first prototypes were not as good as expected, it came the need to improve them. Therefore, it was built a new optimized prototype. This prototype has electrodes much bigger than the previous ones. For the new ultracapacitor it were then repeated the charge and discharge experiments and it were also calculated its voltage and current values.

C. Capacity Calculation

The capacity, 𝐶, in a capacitor can be obtained based on the voltage and current discharge curves and can be calculated by the following formula:

𝐶 = 2∆𝑊!

(𝑈!! − 𝑈!!) (1)

In (1), 𝑈! and 𝑈! are the maximum and minimum voltage values respectively for a given period of time and ∆𝑊! is the variation of electrical energy stored in a capacitor, that can be given by the following formula:

𝑊! = 𝑢 𝑡!

𝑖 𝑡 𝑑𝑡 (2)

Basically, 𝑊! is the area below the voltage and current curves multiplied.

With the help of the curves obtained in B and using the given formulas, it was estimated the capacity for several optimized prototypes and during different periods of time.

D. Voltage Improvement To accomplish further testing, is necessary to raise the

maximum voltage values that the ultracapacitors can withstand. For this purpose, there were connected in series several cells. Thus, the potential difference in the system terminals will be equal to the sum of the potential differences in each ultracapacitor terminals. After the system assembly, it was made a characterization of its proprieties. Likewise, it was obtained the discharged curves of the new prototype, and it were calculated its voltage, current and capacity values. It was also carried out a theoretical proof for the systems capacity to show if the value previously obtained matched the one that should be obtained (and that was calculated with the individual values from each ultracapacitor).

E. DC/DC Converter Application

Having already accomplished the raising of the voltage values, the ultracapacitor (now constituted by seven cells) was used in a DC/DC converter. The DC/DC converter pretends to supply a direct current in the output given a direct voltage at the input. Therefore, a voltage source, a semiconductor, a diode and a load can be found as part of the circuit. Firstly, it was performed the montage with the ultracapacitor as part of the load as shown in Fig. 4.

Figure 4. Converter with the ultracapacitor as part of the load

It was measured the current that flows by the diode and the voltage at the ultracapacitor terminals. Then, it was performed the same test but with the ultracapacitor replacing the voltage source as is showed in Fig. 5.

Figure 5. Converter with the ultracapacitor playing the role of voltage source

It was measured once more the current that flows by the diode and the voltage at the ultracapacitors terminal. Those experiments were performed at first with only a resistance as the load and afterwards was also added an inductor.

The resistance, 𝑅, will take values along the trials between 𝑅 = 6 Ω and 𝑅 = 30Ω. The inductor, 𝐿, has a value of 𝐿 = 15 mH.

The voltage at the load (RL) terminals was also measured.

F. Computer simulation

In order to compare with the results obtained in the laboratory experiments, it was performed a simulation in

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Matlab/Simulink of each of the previously tested assemblies. In the simulated ultracapacitor, was used a capacity value of 𝐶 = 20𝐹, this value was the one estimated for the system when it was developed. For the initial ultracapacitor voltage was set a value of 6𝑉 as the cell was only at half of its load when the experience was made. In Figure 6 it is shown the simulated diagram of the DC/DC converter assembled like the experiment.

Figure 6. Simulation diagram of the DC/DC converter assembled like the first experiment

Not only in this simulation but also in the following one, it was given values to resistance just like in the experimental tests (𝑅 = 6 Ω and 𝑅 = 30Ω).

The next simulation was performed with the ultracapacitor replacing the voltage source, as is shown in Figure 7.

Figure 7. Simulation diagram of the DC/DC converter assembled like the second experiment

The load used in this experiment begun as just a resistance, 𝑅, and then was introduced an inductor, 𝐿, with a value like as in the laboratory experiments, 𝐿 = 15 mH. Therefore, in the final trials, there was a RL load.

In these simulations, it was obtained the curves of the current that flows by the diode, the voltage in the ultracapacitor terminals and the voltage at the load terminals. These parameters were obtained in the experimental tests and were compared to those. It was also simulated the curves of current and voltage that flows by the IGBT, the voltage in the

diode terminals, and the current and voltage at the output of the circuit.

III. RESULTS The explained procedures provided the results that will be

shown in this section. From the first experiment, the discharge tests that were made to the first prototype resulted the following potential curves shown in Figures 8, and 9.

Figure 8. First prototype discharge curve starting at full potential

Figure 9. First prototype discharge curve starting at half of its full potential

Each color represents the average curve of each ultracapacitor. The graphs obtained show that the voltage decreases significantly and very quickly. It is normal for a storage device to have, over time, its characteristics deteriorated. The batteries have usually a self-discharge rate of 5%-30%, but for the capacitors tested, this rate is much higher. Therefore, the sudden decrease of voltage that occurs is too high, even more counting that it happens with semi-new cells that are still far from reaching its lasts life cycles. The consequence is, that after just a very short time gap, the voltage values are already very small compared to the default ones. Still, despite the poor performance of the test results, one can notice a consistency between the results obtained for different electrodes, which indicate that the tested cells have a similar aging process and there is no cell with results significantly worse than the others. This indicator should be considered promising since the repeatability of results helps to identify the phenomena that led to the observed behavior. Given these results, an attempt was made to optimize the electrodes and, thus, to develop a new prototype.

The new prototype was built with larger electrodes, which means that, at least theoretically, the ultracapacitor should have higher values of capacity.

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Figure 10. Discharge curves for the optimized ultracapacitor

Figure 10, depicts the voltage curves of the optimized ultracapacitor. Once again, the different colors represent the average curve from different prototypes. Compared to the results that were obtained in the tests made to the first prototype, there was a significant improvement of the discharge curves that were obtained. The ultracapacitor looses its charge at a much lower rate and, therefore, the capacity value for this new prototype is much higher than those obtained before. Thus, given the presented results, it is possible to estimate the capacity value of this ultracapacitor with the assurance that this will present a value close to those that ultracapacitors currently on the market have. On the other hand, the maximum voltage value that the ultracapacitor reaches remains the same as before, which was expected since this parameter does not vary with increasing the electrodes area.

Figure 11. Voltage and current discharge curves of the ultracapacitor

The graphics in Figure 11 show the voltage and the current curves during a small period of time for an optimized prototype. According to the formulas (1) and (2) and based on this curves, it was estimated the capacity for this ultracapacitor as 𝐶 = 216𝐹. However, this estimation is only based in the first few minutes since the discharge process started. Thus, the value that can be obtained for a larger period of time should be a lot bigger since the voltage curve tends to stabilize.

In the Figure 12, is represented a discharge curve of an ultracapacitor during a longer period of time.

Figure 12. Discharge curve for an ultracapacitor during a larger period of time

Based on this curve and the current values, the capacity value of this prototype was estimated. The result, 𝐶 = 564𝐹, was, as expected, bigger than the previously obtained. So, it can be concluded that the longer the ultracapacitor remains discharging, the higher the capacity values that can be obtained. Furthermore, if the capacity is estimated for intervals such as between the 4000-6000 seconds, the value obtained rounds 𝐶 = 1000𝐹, where the curve tends to stabilize. Therefore, if waited long enough, its not wrong to say that the capacity of this prototype takes values around 𝐶 = 1000𝐹.

So far, there was a problem with the capacity levels of the ultracapacitors, which were too low, but now that problem is solved. On the other hand, the voltage values of the prototypes (that are perfectly reasonably and in the range of those presented by commercialized ultracapacitors) are still low for performing some experiments. Therefore, the maximum voltage value reached by the prototypes was increased by connecting seven ultracapacitors in series.

Figure 13. Discharge current and voltage curves of the ultracapacitors assembled in series

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As it can been observed, the maximum potential reached is

10.5V which is exactly seven times more than the potential of each prototype and that makes total sense because the voltage of the equivalent of a sum of capacitors should be equal, when connected in series, to the sum of each capacitor voltage value. By a simple observation of the voltage curve, it is evident that the curve decreases at a much faster rate than before. That is also a consequence of the series connection as the capacity of a series of capacitors, 𝐶! , varies with the capacity of each one, 𝐶!, according to the following formula:

1𝐶!=1𝐶!+1𝐶!+1𝐶!+ . . .+

1𝐶! (3)

Then, the capacity value of the system was calculated by

the usual method (through the voltage and current curves) and the result was 𝐶 = 30𝐹. This value was calculated for a period of time of 4000 seconds, and the discharge voltage curve for the given parameters is shown in Figure 14. In order to compare both values, it was also calculated the capacity using the individual capacity values of all the prototypes and replacing them in (3). It was obtained a value of 𝐶! = 67𝐹 which is in the same range as the previous one.

Figure 14. Discharge voltage curve of the series system

The ultracapacitors system assembled in series was then ready to be tested in a DC/DC converter. The Figures 15 and 16 were obtained as a result of assembling the circuit like in Figure 4.

Figure 15. Ultracapacitor Voltage (in yellow) and Diode Current (in blue)

Figure 16. Ultracapacitor Voltage (in yellow) and Diode Current (in blue)

The Figure 15 was obtained with a resistance, 𝑅 = 6 Ω, and Figure 16 was obtained with a resistance, 𝑅 = 30Ω. As it can easily been observed, the diode current in the second image is significantly smaller compared with the previously image, which is due to the resistance increased value. The switching frequency chosen was 𝑓 = 0.1𝐻𝑧 and a duty cycle selected was 𝛿 = 0.5,

The ultracapacitor was then assembled to replace the voltage source instead of being in parallel with the resistance in the load like how it is showed in Figure 5. The results are showed in the Figures 17-18, also with a 𝑅 = 30Ω.

Figure 17. Ultracapacitor Voltage (in yellow), Diode Current (in blue)

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With a new value for the switching frequency, 𝑓 =100𝐻𝑧, and the same duty cycle, 𝛿 = 0.5, the current that flows by the diode and the voltage at the ultracapacitor terminals were then measured (Figure 17), the waveforms can be observed also with more detail in Figure 18, which is a zoom of the previous figure. With the same resistance value, an inductor, 𝐿 = 15mH, was added to the load.



The Figures 19 and 20 show the results of this experiment. The current, as a result of the addition of the inductor in the load, increased to 200𝑚𝐴. Still with a RL load, the resistance value was decreased to 𝑅 = 6Ω. The results can be seen in the Figures 21-22.



Decreasing the resistance value, in a RL load, resulted in another increase of the current in the diode that reached now 600𝑚𝐴. The switching frequency value was also increased and it was observed that, with this increase, the inductor current ripple decreased as a result of the ultracapacitor discharge time also decreasing, and, therefore, the current flowing in the inductor will be approximately constant. Finally, in the experimental work, it was measured the voltage at the RL terminals together with the current in the diode as shown in Figure 23.

Figure 23. Voltage at RL terminals (in yellow), Diode Current (in blue)

The value of the voltage at the RL terminals is lower than

the voltage at the source, which is mainly due to voltage drop at the IGBT.

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It was performed a computer simulation from the laboratorial experiments. Figure 24 and Figure 25 show the results obtained with the ultracapacitor connected in parallel with a resistance with a value of 𝑅 = 30Ω.

Figure 24. Voltage measure at the ultracapacitors terminals

Figure 25. Voltage and Current measured by the diode

Both figures show waveforms similar as the experimental ones that were also obtained, the voltage at ultracapacitor terminals and the current by the diode. Then, the ultracapacitor replaced the voltage source and it was performed the simulation with just a resistance as load with a value of 𝑅 = 30Ω

Figure 26. Voltage and current at the output

Figure 27. Current and voltage by the IGBT

Figures 27 and 28 show the current and voltage at the

output of the circuit and by the IGBT respectively. These waveforms were not obtained in the experimental work, however, they match perfectly to what was expected after theoretically analyzing the system. In the Figure 28 there it is shown the voltage waveform at the ultracapacitors terminals.

Figure 28. Voltage at the ultracapacitor terminals

At last, it was simulated the converter DC/DC with an

inductor with 𝐿 = 15𝑚𝐻. So, for the remaining of the work, the circuit will have a RL load.

Figure 29. Current and Voltage by the IGBT

Figure 29 shows the waveforms at the IGBT. In the following simulations it was obtained the current that flows by the diode for different resistance values.

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Figure 30. Current and voltage by the diode for 𝑹 = 𝟔𝛀

Figure 31. Current by the diode for 𝑹 = 𝟑𝟎𝛀

Figures 30 and 31 show the different curves for 𝑅 = 6Ω and 𝑅 = 30Ω. As expected, the current decreases when the resistance values increase. The values obtained were absolutely identical to the ones achieved at the experiments, 200𝑚𝐴 and 600𝑚𝐴.

IV. CONCLUSIONS & 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 analyzed the properties of this device and its advantages and disadvantages compared to the batteries. A study of the ultracapacitors market, including the main companies involved in the business, the products sold to public, and their characteristics, was also made. It was concluded that this market is clearly growing, and, because the technology is relatively new, new applications for its use are appearing at a quick rate.

Then, the first prototype was analyzed. The conclusion drawn was that the capacity levels obtained were under the expectations and it would be necessary an improvement at that level. For that purpose, it was developed a new prototype and its capacity levels were estimated and proven to be a lot better, within the range of those that can be find in the commercialized ultracapacitors. It was then given a great step in the development of the redox ultracapacitors working based on aqueous electrolytes. Several prototypes were analyzed, with the same basic principle (electrodes with the same characteristics), but slightly differently constructed to evaluate the consistency of the results. It was concluded, successfully, that there is no significant variation between the results of the different prototypes, that means this system is quite stable and is not very sensitive to assembly variations. Later in this work, it was revealed the need to increase the voltage of the

ultracapacitor. Therefore, it was developed an assembly of multiple prototypes that were subject of performance experiments, which revealed results in accordance with the foreseen ones. The objective of increasing the voltage at the terminals of the ultracapacitor was completed with success.

In the last section of this work, it was used the ultracapacitors as part of the load and replacing a voltage source in a DC/DC converter. The results, observed in the graphics obtained through the experiments, show results similar to the ones predicted theoretically. Then, a computer simulation was carried out, and the results were very close to the experimental ones. Therefore, in this section of the work, the results obtained were accordingly to the expected.

In conclusion, this study achieved the main objectives that have been proposed, contributing in the development of the technology. It is noted that the redox ultracapacitors capable of operating in aqueous electrolytes are a priority of the current “SET plan” as published by the European Union.

Upon the completion of this work, the great development that can be done in the future is the improvement in the construction of the prototypes. It would be interesting to develop a compact device, capable of producing, at least, similar results but that could have a much smaller size to be able to, perhaps, be used in applications that require a device with smaller dimensions. The decrease in weight and volume is essential to optimize the energy and power densities. This improvement can be achieved because in each ultracapacitor there is much excess space between the electrodes, which not only contributed to the increase in volume of the prototype but also certainly contributed to many of the observed results may not be as satisfactory as it was expected. For these reasons, improvements in the construction of the prototype would be an important step for this technology to become capable of competing with the existing on the market, which uses carbon and organic solvents that have many environmental problems.

Moreover, it would also be interesting to carry out further studies on the ultracapacitor, particularly, test the system powering a generator using a proportional integral current controller and to analyze the results in with the ultracapacitor charging and discharging. To do so, it would be helpful to use the ultracapacitors system assembled in series developed in this work as the minimum voltage value required to perform this tests would be around 6V. Finally, it could also be held tests like the ones already made but for cells in the beginning and end of life and record the differences that exist between each of the cases.

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REFERENCES

[1] Zhao, X., Sanchez, B. M., Dobson, P. J., Grant, P. S. (2011). The Role

of Nanomaterials in Redox-Based Supercapacitors for Next Generation Energy Storage Devices. Nanoscale, 3(3), 839-855.

[2] Ragone chart “http://www.mpoweruk.com/performance.htm” [3] Ultracapacitors in breaking systems

“http://www.mazda.com/technology/env/i-eloop/”” [4] Ultracapacitors in wind energy

“http://www.altenergymag.com/emagazine.php?issue_number=04.10.01&article=maxwell”

[5] Ultracapacitors in Solar energy

”http://www.greentechmedia.com/articles/read/Ultracapacitors-Provide-Solar-to-Grid-Smoothing-in-San-Diego”

[6] Portable Speakers “http://www.gizmag.com/helium-capacitor-powered-speakers/29938/”

[7] Capa Vehicle “http://www.technologyreview.com/news/415773/next-stop-ultracapacitor-buses/”

[8] Ultracapacitors in Forklifts ”http://www.maxwell.com/ultracapacitors/applications”

[9] Redox Ultracapacitors

“http://www.alrc.doe.gov/newsroom/labnotes/2011/06-2011.html”

[10] Storage Market “http://www.marketresearchmedia.com/?p=912”

[11] “Maxwell” ultracapacitors ”http://www.maxwell.com/ultracapacitors/”

[12] “Ioxus” ultracapacitors “http://www.ioxus.com/ultracapacitors/”

[13] “Shanghai Greentech” ultracapacitors “http://www.greentechee.com/product/&pmcId=22.html”

[14] “VinaTech” Ultracapacitors “http://www.supercapacitorvina.com/product/edlc.html”

[15] Predicted growth of ultracapacitors “http://www.idtechex.com/research/articles/change-of-leadership-of-the-global-market-value-of-supercapacitors-00005344.asp”

project of redox ultracapacitors using stainless ... - ulisboa

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