What are Different types of Ultracapacitors?

The two electrodes of an ultracapacitor may be made from identical materials (symmetric capacitors), or they may use different combinations (or two different technologies (asymmetric capacitors). These two broad constructions are in turn made from different types of material combinations.

Symmetric capacitors

Symmetric capacitors may have following compositions.

1- Both electrodes made from carbon (activated carbon, graphene, aerogel, nanotubes) [EDLC]. These capacitors, called Electric Double Layer Capacitors (EDLC), store energy by simple electrostatic attraction of charges to opposite polarity, as in conventional capacitors, but with very high surface area and extremely thin effective dielectric thinness. Hence they get charged or discharged extremely fast, and have highest power delivery.

Energy density goes up with type of carbon used, but is much smaller than other types of ultracapacitors. These were the first ultracapacitors developed, and still continue to be common usage in several applications because of their extremely high power density (specific power). Voltage of EDLC varies from 1.3V to 1.8 V for aqueous electrolytes, and from 2.3 V to 3 V for organic electrolytes. Energy density varies from 5- 8 Wh/Kg.

2- Both electrodes made from pseudocapacitor materials. Charge transfer and storage takes place via reversible reduction-oxidation (Redox) chemical reaction at Helmholtz layer formed at electrode / electrolyte surface (Faradaic reaction), in addition to electrostatic transfer as in EDLC. Energy density, and hence storage capacity is nearly double that of EDLC. Electrodes of pseudocapacitor can be made of metal oxides like Ruthenium oxide (RuO₂), or conducting polymers. This construction allows them to surpass the capacity limitations of EDLC, and the mass transfer limitations of batteries.

• Since a Faradaic chemical reaction is involved, response time for power delivery, and power density is smaller than EDLC. Capacitance value is several times that of EDLC in same volume, depending on electrode composition. Voltage of pseudocapacitor is higher than EDLC, varying from 3.2 V to 4V.  Energy density of pseudocapacitors is about double that of EDLC, and varies from 10 to 15 Wh/Kg, going to 20 Wh/kg in some cases.

Asymmetric capacitors

Asymmetric capacitor, as its name suggests, is a combination of two different technology materials at the two electrodes. The combinations possible are from EDLC and pseudocapacitor, or from either EDLC / Pseudocapacitor at one electrode, and a battery type electrode at the other end.

The figure below shows different combinations of EDLC and Pseudocapacitor based asymmetric capacitors.  Both electrodes could be carbon based EDLC materials of different types like CNT and Graphene, both of same material but with different thickness (graphene/graphene), or one EDLC electrode and the other a pseudocapacitor electrode. Voltages at the two electrodes are different, and depend on the types of electrode and electrolyte.

Some capacitors may use one electrode drawn from battery technology, which uses transfer of ions physically into structure of electrode material under electric field, through a process called intercalation. In Lithium ion capacitors, one electrode is similar to Li-ion battery technology. This is basically an asymmetric capacitor, but since a battery type material is used, it is more specifically called Hybrid capacitor.

Charge movement in battery electrode is through intercalation process, which is different from pseudocapacitance. Free Li-ions from electrolyte enter and occupy free spaces in electrode when charged, and return to electrolyte as capacitor discharges. This action is similar to that in a Li-ion battery. Lithium ion capacitors usually have maximum energy density among hybrid capacitors, in between that of a pseudocapacitor and a Li-ion battery.

• Energy density of Hybrid capacitors is quite high compared to other types, and can vary from 30 Wh/ Kg to 80 Wh/Kg. Recent advances have pushed the densities over 100 Wh/kg. Voltage per cell varies from 3.2 V to 4.2 V. Materials of hybrid capacitors have been a subject of intense research in recent years, and good amount of progress has been made in their capabilities.

The figure below summarized the classification of ultracapacitors.

General Review

Choice of a particular type of ultracapacitor depends on application, its temperature, power backup required and the function to be served by capacitor. For example, battery backup applications do not need high energy density, but high power delivery is critical. This directs one to choose an EDLC, which have power density (specific power) as high as 10 kW/Kg or even higher, though storage capacity is just around 4-8 Wh/Kg.

A good amount of research in electrolytes has enabled voltage ratings of EDLC to go higher from 2.5 V to 3.0 V in past few years, with corresponding increase in energy density. Hybrid capacitor voltages have gone up from 3.6 V to 4.2 V in some constructions. Graphene has attracted particular attraction in past decade due availability of large area and excellent electrolyte penetration, thereby enabling higher energy density.

A comparison of EDLC, Lithium ion capacitors and batteries gives the following figures.

Entity	EDLC	Li-ion Capacitor	Li-ion Battery
Anode	Activated carbon	Activated carbon	LiCoO2 / LIMnO4
Cathode	Activated carbon	Graphite / Doped Li-ion	Graphite / Doped Li-ion
Energy Storage Principle	Ion adsorption	Anode– Ion adsorption Cathode– ion adsorption + charge transfer	Reversible redox reaction
Temp. Range	-40℃ to 85℃	-25℃ to 70℃	-25℃ to  45℃
Max. rated Voltage	2.3-3.0 V	3.6-4.2 V	3.6-4.2 V
Charge Discharge cycle life	>1,000,000	50,000- 500,000	1000
Energy Density	5-10 Wh/Kg	40-80 Wh/Kg	75-250 Wh/Kg
Self-discharge	>30% after 2000 h	<5% after 2500 h	<5% after 2500 h