• Electrical India
  • Dec 5, 2016

Supercapacitors 
Myths vs Facts

Supercapacitors and ultracapacitors are one and the same. The difference in the nomenclature can be attributed to the Europeans and the Americans. Europeans call the same device supercapacitor while the Americans know it as ultracapacitors...

- Dr P B Karandikar, 

Dr N R Kulkarni, 

  Seema Mathew


 Supercapacitors or ultracapacitors, have garnered a lot of interest as an emerging environment friendly device with its unique characteristics, like high power density and cyclability, that is currently unmatched by other storage devices. Similar to the conventional capacitor, the supercapacitor stores energy by charge separation.These characteristics are already being exploited in variousapplications ranging frommobile charging, toys, sensors,large scale transport systems like subway trains and buses, energy storage at intermittent generators and smart grid applications. The enthusiasts of this technology believe that it can garner a large part of the global $85billion battery market. But due to various factors this seems to be far from the reality.Here are some common myths about this technology, along with the facts to set the record straight.

Myth

Supercapacitors, ultracapacitors and electrochemical double layer capacitors are different devices.

Fact

  Supercapacitors and ultracapacitors are one and the same. The difference in the nomenclature can be attributed to the Europeans and the Americans. Europeans call the same device supercapacitor while the Americans know it as ultracapacitors. Electrochemical Double Layer Capacitors (EDLCs) are the most common type of supercapacitors which store charge at the positive and negative electrodes by the separation of charges at the electrode-electrolyte interface. Each electrode–electrolyte interface represents a capacitor and the complete cellcan be considered as two capacitors connected in series thereby making the double layer capacitor. Contrary to what most people believe, there are other types of supercapacitors that use surface chemistry like pseudocapacitance for charge storage. They are technically known as pseudocapacitors. Another variant is the hybrid capacitor which stores charges similar to the pseudocapacitor on one electrode and like the EDLC on the other electrode. The figure below shows the classification of supercapacitors depending on the mechanism of charge storage.

 

Figure 1: Types of Supercapacitors...

Myth

It can replace batteries.

Fact

  Replacing batteries with supercapacitors entirely is just not possible except in very few applications where for short bursts of current is the main requirement. This is due to the fact that supercapacitors do not have the high energy density which is the hallmark of batteries. A typical supercapacitor has an energy density in the range of 1-5Wh/kg while the same of the battery is range of 8-400Wh/kg. This means that when energy is required for extended periods of time, supercapacitors will not help. Thus in the near future the chances of the supercapacitor replacing the battery are remote. However, battery-supercapacitor with electronic controller combine power pack is likely to find use in many low voltage applications.

Myth

It can be used like a plug and play energy storage device.

Fact

  Due to their fast charging-discharging times and high currents, the supercapacitors requires very specialized power electronic interfaces for proper functioning in an application. Unfortunately, there is a scarcity of such modules and these interfaces have to be application specific. In other words, it is not possible to use the same power electronic interface for two applications which require the same energy storage ratings with different loads.Another issue is that power electronic devices work at high frequencies but manufacturers do not provide the necessary data like bode plots and frequency response in their datasheets. Thus, it becomes difficult for any application engineer to incorporate a supercapacitor in their circuit.

Myth

It has infinite life.

Fact

  Having infinite life is an ideal condition. In general supercapacitor lifetime is dependent on three things: electrolyte life, voltage rerating and temperature dissipation. The fluid in an electrolyte can evaporate and cause the supercapacitor to fail under extreme operating conditions. This device is also highly vulnerable to excess heat. Moreover, using a supercapacitor at close to its maximum voltage will cause it to fail more quickly than using it at a lower voltage. Typically, a manufacturer provides data regarding the change in capacitance and internal resistance to be expected after 1000 cycles.

  If this value is less than what is required after the 1000 cycles, then the conditions. This device is also highly vulnerable to excess heat. Moreover, using a supercapacitor at close to its maximum voltage will cause it to fail more quickly than using it at a lower voltage. Typically, a manufacturer provides data regarding the change in capacitance and internal resistance to be expected after 1000 cycles.

  If this value is less than what is required after the 1000 cycles, then the supercapacitor might need to be replaced. If proper attention is given to the rating and characteristics of the supercapacitor while choosing it for an application, the supercapacitor will easily outlive the system in which it is being used. The figure 2, illustrates the life in hours as a function of temperature and voltage...

Figure 2: Supercapacitor life as a function of temperature and voltage...

Myth
Electrode area and weight is used in calculating energy density and power density.
Fact
  Energy density is a measure of how much energy the supercapacitor can store, in a given mass. So a supercapacitor with a higher energy density can power a load longer than one with a low energy density and the same physical size or mass. Its SI unit isWh/kg. Power density measures how quickly the supercapacitor can deliver energy and its unit is W/kg. While calculating these densities, the total mass of the active material of the electrode is used. Many times researchers consider the mass of activated carbon or that of metal oxide used in the electrode for calculations which leads to confusion about values of energy and power densities in comparative analysis. In calculation of area based specific capacitance, some researchers use total measurable area of both electrodes and of both sides of electrodes. However, correct practice is to use only area of one electrode and that too of one side only. This often leads to confusion in comparative analysis of specific capacitance.
Myth
It does not require voltage balancing like in a battery stack.
Fact
  It is practically not possible for two supercapacitors to have thesame value of equivalent series resistance. The equivalent series resistancemainly depends on the electrolyte, specifically its ion concentration, ionmobility, solvent or solvent mixtures, and temperature.Even if two supercapacitors are of the same rating and brand all of these parameters like temperature can never be equal. Another issue is the leakage current which is caused due to unwanted oxidation-reduction reactions, ionic charge diffusion or/and electronic partial discharge through the separator and impurities in electrode-electrolyte materials. Non uniformity of cell voltage in a stack of supercapacitors connected in series is mainly due to the variation in these two parameters between cells. Those cells that are under higher voltage stress tend to have lower life compared to those under less stress. To combat these issues a stack of supercapacitors requires active or passive cell balancing. This is the main hurdle in its use in high voltage applications. The figure shows a passive and active cell balancing circuits that are typically employed.
Figure 3: Cell balancing circuits for supercapacitors in series (a) Passive and (b) Active... 
Myth
It can be used in AC circuits.
Fact
  A typical supercapacitor is a polar capacitor with fixed polarity i.e. positive and negative terminal similar to a battery, so they cannot be used in AC circuits. Moreover, since the time constant is high they are not suitable for high frequency applications. However, for pulsating DC supply they will not pose any problems. 
Myth
It can withstand higher operating voltages.
Fact
  The operating voltage of the supercapacitor is completely dependent on the voltage window or the electrochemical stability of the electrolyte. When a supercapacitor is subjected to more than its rated voltage, the electrolyte within the cell begins to decompose, producing a gaseous byproduct. If the overvoltage condition persists long enough, the pressure may build up until the safety vent on the supercapacitor's package opens. Consequently, more of the electrolyte will decompose and vaporize until the supercapacitor's effective internal resistance increases and becomes an open circuit. Presently, the various electrolytes used in its construction have low voltage windows such as 1.6V for aqueous electrolytes, 2.7V for organic electrolytes and 3V for ionic liquids. Thus for a typical application, supercapacitor cells need to be connected in series for higher operating voltages with proper voltage balancing circuits and power electronic interface.
Myth
A short circuit can reduce the life of a supercapacitor.
Fact
  Even if a charged supercapacitor is externally short-circuited by any possibility, there is no leakage of electrolyte, no smoke, no ignition or no rupture. This is due tothe fact that it has low energy inside unlike in a battery.Heating depends on handling of energy during charge or discharge of supercapacitor.As the discharge time is very small the temperature increase is momentary and it will decrease quickly by heat dissipation. 
Myth
Due to low voltage and low energy, supercapacitors are safe compared to other energy storage devices.
Fact
  In a supercapacitor the discharge time is very low and all of the stored energy is dumped into the load in a matter of milli-seconds. This causes very high currents to flow during discharge which can reach dangerous levels and induce sparking if proper precaution is not taken. This type of arc discharge can result in damage to both human life and property.
Myth
Non aqueous supercapacitors are better. 
Fact
  Most of the commercially available supercapacitors use organic electrolytes since their voltage window is larger. But on the flipside, these non-aqueous supercapacitors use salts dissolved in organic solvents which are poisonous. This causes issues during disposal. Aqueous electrolytes do not have any such environmental blueprint. A few of them do have a larger voltage (upto 2.2V) making them a better option. Another electrolyte which looks promising in the laboratory is the ionic liquid. 
Myth
Capacitor or battery manufacturing companies can easily manufacture supercapacitors.
Fact
  Supercapacitor manufacturing requirements are completely different from that of capacitors or battery. Of the two, capacitor manufacturing companies are better equipped as some of their existing facilities can be used for the manufacture of supercapacitors. 
  In this niche market, the top vendors are largely either aluminium electrolytic capacitor manufacturers or power film capacitor manufacturers. Some of the vendors (only a few) were involved in the battery mbeforehand, but an increasing number of manufacturers are standalone vendors who produce just supercapacitors. As of now, the most successful companiesin the manufacture of supercapacitors are manufacturers of capacitors for which they have existing channels of distribution and contacts with customers who have an interest in expanding their consumption to include capacitors in the Farad range.
Myth
It is easy to do research in this field.
Fact
  Even though there is a lot of interest in supercapacitors, it is still not amenable to research. This is because any research in this field requires different types of costly instruments for the preparation of prototypes and for their testing such as cyclic voltammetry, charge discharge machine, glove box, calendaring machine and packaging machine. A few of these instruments are shown in figure 4. 
  Furthermore, the different materials required for the preparation of prototypes are difficult to procure and are costly. Funding agencies are hesitant to provide the required capital as the technology is still in the nascent phase and the risk is high. Researchers are also discouraged from taking up this field of research as it requires a thorough knowledge of various diverse fields like chemistry, material science and electrical engineering thereby making it an interdisciplinary subject.
Figure 4: Some of the instruments required for supercapacitor research. Anticlockwise from the top (a) Calandering machine (b) Cyclic Voltammeter (c) Glove box (d) High Precision Electronic balance (e) Muffle Furnace...

Myth

Commercially available materials can be used in manufacturing of supercapacitors.

Fact

  The activated carbon that is used as supercapacitor electrodes have to be of very high purity with less than 1% ash content. Attaining this level of purity is costly and there are very few carbon manufacturers who have the requisite technology.

  Other materials which are used as the active electrode material like carbon nano tubes, carbon aerogel and graphene have still not managed to crossover successfully from the laboratory to the industry.

  Proper optimizing of the electrode-electrolyte pairing, suitable method for coating the current collector with active material so that the penetration of electrolyte is maximum and reducing the equivalent series resistance are some of the issues that need to be addressed for the proper scaling up of these technologies to make them suitable for mass production. Debate is still ongoing about the pros and cons on stacked type configuration vis-a-vis rolled configuration for the cell design. In the former, the electrodes and separatorsare stacked like in a battery and the cell has a cubical shape. In the latter case, the electrodes and separator are wound and the cell has a cylindrical shape similar to the conventional capacitor. Another major deterrent is the non-availability of the necessary grade of separators, electrolytes and current collector. The materials for electrode, current collector and separator materials needs more research. As these components need to be of very high quality, an OEM finds it very difficult and costly to procure.

Myth

This technology has potential to generate its own market.

Fact

  Though partly true, this is not really the case on the ground. There is still a lot of reluctance in industry to embrace this technology. Even though the automobile industry is the largest perceived market and a few automobile manufacturers are claiming to use the supercapacitor in some of their models, we haven’t yet seen them accept this technology in their entire range. Moreover, as most of the supercapacitor manufacturers in possession of path breaking technology are small players in the energy storage market, they are not able to use existing channels of distribution and contact with customers.

  This keeps them from completely exploiting the energy needs of the consumer. In many countries, apart from manufacturing of this device, developing market for this is also a challenge.

Figure 5: Supercapacitor Market Shares by Application...

  In conclusion, the myths surrounding the supercapacitor are many and it is high time that they are dispelled so that the supercapacitor can be given its proper place among the various energy storage devices. At present, the low energy density, the requirement of voltage balancing circuit and power electronic interface are still a major problem for the widespread use of supercapacitors.

  Another area which is a major hindrance to its acceptance is the cost. This is in spite of the fact that in the 1980s, a 470F 2.3V supercapacitor cost $2.00/F while in 2012 the same had come down to $0.03/F.

  It is expected that supercapacitor demand will grow in value by about 30% overall by 2020 and remain in the hundreds of millions of dollars and not billions, as was earlier stated by various experts.

  At present, the United States is the market leader for supercapacitors with more than 40% share in the market.
Europe and countries like China and Japan are also active players in this market. India is still taking baby steps and there is an urgent need to adopt this technology into the mainstream.

  The present push by the government towards electric mobility and the commitment to bring in 5 to 7 million electric vehicles on the Indian roads by 2020, will certainly boost this sector.


Dr P B Karandikar
Associate professor
Army Institute of Technology
Pune

Dr N R Kulkarni
Vice Principal and Head of Electrical Department
P.E.S’s Modern College of Engineering
Pune

Seema Mathew
Assistant Professor
P.E.S’s Modern College of Engineering
Pune

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