By: David DeLeonardo, Analog Specialist AE, Future Electronics
In this Tech Brief, we will take a look at the class of components loosely referred to as “Super Capacitors”. A range of distinctly different components are often lumped together under this nomenclature, but we will differentiate them by type and characteristics. Finally, a summary chart is provided that characterizes each type of component across a set of parameters.
The class of components called “Super Capacitors” is best distinguished into sub-classes of components based on the dominant mechanism of electron storage. These sub-classes are Electric Double-Layer Capacitors which use an electrostatic means of charge storage and Pseudo Capacitors which are primarily electrochemical devices. The mechanisms of charge storage used in each of two types can be combined into another separate class of component called Hybrid Capacitors. This classification is shown in Figure 1:
Now, before we discuss the characteristics of each type of Super Capacitor, we need to examine the parameters that are most commonly used to characterize them.
Because all Super Capacitors are electrochemical in nature (even the electrostatic ELDC devices), there is a chemical aspect to their construction, if only in the electrolyte. However, while the chemical nature of the electrolyte is certainly a competitive differentiator, it is not a means of distinguishing among various devices as is often the case with batteries. As to the electrodes, the case is somewhat different. While ELD devices have no chemical reactions at their electrodes, Pseudo and Hybrid Capacitors do, and the composition of their electrodes is a means for characterization. The newest sub-class of Super Capacitors is the Hybrid Capacitor which, as seen in Figure 1, has both an electrostatic as well as electrochemical nature. These have been commercialized as “Lithium Ion Capacitors”.
Current flow into/out of a Super Capacitor involves the flow of ions in the electrolyte across the separator to the electrode’s surfaces and porous interior. This results in losses during the charge carrier movement that are measured as DC resistance. For a given discharge current and test condition, there will be a drop in the terminal voltage as a result of this internal resistance given by: R(int)=V(drop)/I(test). R(int) is central in determining many critical Super Capacitor
parameters such as peak charge and discharge current and their respective times. This ISR is a DC characteristic and must not be confused with the AC parameter “Equivalent Series Resistance” (ESR) which is normally given for capacitors. ESR is much smaller than ISR and is not related to peak performance currents for Super Capacitors.
Time constant is the key figure of merit for Super Capacitors. Ideally, it is the product of the capacitance of the device times the internal resistance: T = R(int) * C. For example, a 1000F device with an internal resistance of 25mΩ has a time constant of 25 seconds. As with any capacitor, when charged or discharge via a constant current, the terminal voltage will linearly increase or decrease respectively. So, discharging a fully charged 1000F device with a current limited ONLY by internal resistance, the voltage will decline by approximately 63% after 25 seconds. Again, the specifics will depend on the particular construction of the device under test. This is due to the fact that, because R(int) changes during charge and discharge, this is only an idealized approximation. Each specific device will have a particular charge/discharge curve determined by the particulars of its internal construction.
Capacitance and Frequency Response
Super Capacitors do have DC capacitance values three to seven orders of magnitude greater than “traditional” Electrolytic Capacitors. However, this is only under DC conditions. Due to the nature of their internal construction, they have a much longer time constant. In the frequency domain, this translates to a very rapid fall off in effective capacitance with rising frequency as shown in Figure 2.
Despite their rapid “roll-off” in effective capacitance with increasing frequency, Super Capacitors have so much capacitance to start with that they are actually far superior to batteries in terms of being able to deliver or accept extremely high currents. This allows them to be used in applications that require very high pulse currents like motor drives, regenerative braking, battery powered radios, etc.
Specific Energy Density
This parameter is a measure of the availableto-use energy stored in the capacitor divided by its mass. An ideal capacitor would then contain E(sp) = ½ * (C *(Vmax-Vmin) ^ 2) / mass. Here Vmax is the highest allowable terminal voltage and Vmin is the lowest. For ELD and most pseudo capacitors, Vmin is zero, but for Lithium Ion Capacitors, Vmin is approximately 2.6V. It will be shown in the summary chart at the end of this article, Super Capacitors have much lower specific energy densities than batteries.
Specific Power Density
This parameter is a measure of the instantaneous power that is available at the terminals of the device. It is calculated as: 1/8 * ((V^2/ESR) / mass). Here, the Effective Series Resistance is a critical limiting factor. It is also critical to note that, given the strong frequency dependency present in Super Capacitors, the nature of the test pulse also affects the result. It is in this parameter that Super Capacitors far exceed batteries of comparable masses.
Cycle Endurance/Service Lifetime
In this parameter, Super Capacitors again far exceed batteries. Since they do not rely upon chemical exchanges in the electrodes, the life of Super Capacitors is limited by evaporation and integrity of the liquid electrolyte. This, in turn, is a function of service temperature and operating conditions such as peak load current and terminal voltage.
Self Discharge Time
Due to the very small distances separating the charge carriers, Super Capacitors exhibit a leakage current that results in slowly diminishing terminal voltage with time. Thus, a fully charged device will decline from the maximum charge voltage to some specified lower value in a given time, usually measured in weeks. In this parameter, they are inferior to most types of batteries.
Now let’s look at the main types of Super Capacitors with the above parameters in mind.
EDLCs: The First Super Capacitors
The earliest Super Capacitors date from the 1950s and were constructed with activated carbon electrodes immersed in an electrolyte and separated by a thin porous insulator. This yielded devices of approximately 1F of capacitance and a nearly 5 second time constant to a terminal voltage of less than 2V. The EDL (Electric Double Layer) mechanism of charge storage would not be well understood for another 30 years, but by then commercial products were already in use for computer memory backup. Today, Super Capacitors that rely primarily on the ELD effect are primarily applied to memory backup applications due to their relatively lower energy density, cost and capacitance.
ELDC devices use only the double-layer capacitance effect at each electrode to create electrostatic charge storage. This does NOT involve any chemical reactions with the electrodes themselves. Although the specifics of the EDL charge storage mechanism are beyond the scope of this paper, suffice it to say here that, while it DOES involve Ion/Cation grouping at the molecular level, it is an electro STATIC effect that does NOT involve ionic chemical reactions. Thus, the electrodes are NOT degraded during use and exhibit symmetrical charge/charge behaviors that are largely free of temperature effects. Still, due the need for ionic motion, this mechanism does result in relatively long time constants compared to other types of capacitors.
Electrochemical Pseudo Capacitors
The second sub-class of Super Capacitors is called Pseudo Capacitors. These devices primarily use chemical redox reactions to achieve electron charge-transfer. In addition to the chemical redox reactions, there are also secondary effects used to increase the amount of charge transfer. These include Elecrosorption, which is the absorption of electrons onto the surface of an electrode and intercalation, which is the reversible insertion or inclusion of electrons into a material, especially one with a layered molecular structure.
On the other hand, Pseudo Capacitors do use electrochemical redox reaction resulting in charge transfer between the molecules of the electrode and the electrolyte. This results in gradual degradation of the electrodes, asymmetrical charge/discharge profiles and a marked temperature dependency with regard to performance. However, these drawbacks are also accompanied by a significant increase in energy and power density.
Hybrid Super Capacitors/Capattery
Finally, the newest sub-class of Super Capacitors is Hybrid Capacitors. As the name suggests, these devices use BOTH of the above effects to create a product that has the strengths of each, albeit at a substantial cost premium. Thus, they exhibit both the high pulse currents of ELDC devices while also having the very high energy density of Electrochemical Pseudo Capacitors.
Super Capacitors Applications and Trends
Despite their relatively long lineage, Super Capacitors have undergone a surge of development over the past 15 years that is resulting in rapidly increased capabilities, falling prices (in Wh/Kg) and wider applications. While they are still used in their original niche of memory backup, they are rapidly expanding into application markets that are as diverse as 5kF units used in electric vehicles down to 0.5F devices used in small sensor array networks.
Today, rising performance and falling costs have resulted in this product class entering a time of a new “virtuous cycle” whereby increasing capabilities allow wider applications which results in higher production volumes which drives down prices. For example, 10 years ago, an 80F/1.7V device was the largest commercially available Super Capacitor. Today, multi-kF devices are available from numerous vendors with terminal voltages from 2.5V to 2.7V.
This fall in price then supports the continuation of the cycle. Of course, eventually, margins will decrease to the point where further product development is no longer profitable and the cycle will slow and halt until the next disruptive development.
To review a wide selection of super capacitor devices, go to bit.ly/1QtQaRm.
|Parameter||EDLC||High Power Super Capacitors||Pseudo and Hybrid Capacitor||Li-Ion Battery (comparison)||Aluminum Electrolyte (comparison)|
|Specific Cost (C*V/$ )||18||54||25||n/a||0.03 - 0.07|
|Cell Voltage (V)||1.5 - 5.5||2.3 - 3.5||2.2 - 3.8||2.6 -4.2||5 to 550|
|Cycle Endurance (80%/20% cycle range )||100K - 1M||100K - 1M||50K - 80K||1K - 8K||Infinite within specifications|
|Capacitance Range and Typical Value, Cost and Size||<150F 120F at 2.7V for $17 35D x 42H (mm)||150 - 15KF 5kF at 2.7V for $250 70 x 70 x 60mm||10 - 300F 270F at 3.8V for $42 25D x 42H (mm)||n/a||0.22μF - 2.3F 2.3F at 10V for $300 100D x 254H (mm)|
|Volumetric Energy Density (Wh/L)||7.2K||72.3K||297K||n/a||0.9K|
|Self-Discharge Time (HOURS to decline 50% from full charge)||Hundreds||Hundreds||Thousands||Thousands||Tens|
|Life Time at Standard Temperature (YEARS)||6 to 12||6 to 12||6 to 12||4 to 15||>>15|
|Key Applications||Memory backup, battery pulse assist, energy harvesting||EV power train, high power conversion and storage||Wind and PV systems, UPS, EV power train||Power tools, portable electronics, EVs, UPS||Universal|