No, a supercapacitor is not a capacitor

Inside a supercapacitor
Inside a supercapacitor

Many people confuse supercapacitor and capacitor by thinking that a supercapacitor is just an improved capacitor.

In fact, it is a little bit true from an electronic point of view, but it is wrong from a technological point of view.

Indeed, a supercapacitor has the same characteristics of charge and discharge as a capacitor. Except we do not manage necessarily in the same way a 200V 100μF capacitor and a 2.7V 3000F supercapacitor. On the other hand, a supercapacitor is not designed in the same way as a ceramic capacitor or an electrolytic capacitor.

From this point of view, the confusion between both systems systems complicates discussions on technological breakthroughs concerning supercapacitors. Indeed, people familiar with capacitors are trying to imagine what a very good capacitor can do and they do not see always a great interest on it. This is unfortunate because the supercapacitor is an electricity storage system that is technologically different, which has its own characteristics and important margins of progress.

When there is confusion between capacitor and supercapacitor, it is difficult to talk about the interest of some materials because some materials can be very inefficient to create high-performance capacitors, but very effective to improve supercapacitors. Hence discussions among people who do not speak about the same component.

It is therefore useful to explain clearly and forcefully the difference between a conventional capacitor and a supercapacitor.

We are going to show that if we wish to designate the supercapacitor as a capacitor, then we may as well designate it as an electric accumulator (often called "battery"), because the supercapacitor is technically between the capacitor and the electrochemical accumulator.

Examples of supercapacitors

Supercapacitors can be marketed in single-cell or be an assembly of several supercapacitors (like car batteries consist of multiple cells).

Some supercapacitors:

Maxwell Technologies supercapacitors
Maxwell Technologies supercapacitors

Voltage and capacitance

Voltage and capacitance of different capacitor technologies
Voltage and capacitance of different capacitor technologies

Supercapacitors have a voltage between 2.5V and 3V and until 3.8V for hybrid supercapacitors. The 5.5V supercapacitors sold in the market are in reality composed of two supercapacitors connected in series.

In terms of energy storage, ultracapacitors have a capacity between 0,1 farads and 9500 farads.

The energy density does not still reach that of batteries : lithium-ion batteries can store 10 times more energy in the same volume, but many supercapacitors should improve through the development of nanotechnology. See our article Graphene Supercapacitors 2.8V and 3V 30000F 12000F made in China.

Capacitor, supercapacitor and battery diagrams

Here are diagrams showing a ceramic capacitor, an electrolytic capacitor, a supercapacitor and a pseudo-supercapacitor (click to zoom):

capacitor diagrams

Another diagram showing a supercapacitor:

supercapacitor diagram

And here's a diagram showing a lithium-ion battery:

Li-ion battery diagram

Capacitors

A conventional capacitor can store electricity through two conducting plates (called electrodes) separated by an insulating material called a dielectric. During charging, electrons are transferred from an electrode to the other one through an external circuit. There is then in the capacitor an electric field that polarizes the material within it : the charged atoms that constitute the material rearrange themself. This polarization is very fast, which allows almost instantaneous charge or discharge.

The electric capacity of a capacitor is essentially determined according to the geometry of the electrodes and to the nature of the insulator ; the following simplified formula is often used to estimate its value: C = ε x S / e

  • S : surface area of the plates
  • e : distance between the plates
  • ε : dielectric permittivity

To make large-capacity capacitors in the smallest possible volume, it is therefore sought to minimize the thickness of the insulator between the two plates. But beware, each type of insulator allows to reach a voltage limit according to its nature and its thickness. Beyond this voltage limit, it may appear a violent breakdown current that results in destruction of the component.

Electrolytic capacitor

Let us return to our diagram of the capacitors :

Ceramic and electrolytic capacitors

Note that in this diagram, the electrolytic capacitor contains an electrolyte which acts as a cathode (negative electrode). The distance between the electrodes (d) is much smaller than for the ceramic capacitor. Furthermore, the anode surface is bigger because it is not flat but rough.

The electrolytic capacitor is different from a conventional capacitor because of an insulating film is created on the anode that will act as dielectric. Only specific materials allow, by oxidation treatment, forming an insulating film on their surface. This is the case of aluminum or tantalum for example. An aluminum electrochemical capacitor therefore uses aluminum foil as the anode. This sheet is first electrochemically etched to increase its surface by developing some roughness (to increase the S term in the formula of the capacity). Next, an anodizing treatment allow to form on the surface an insulating layer of Alumina (Al2O3) which will act as dielectric. This layer will have a thickness (d) which is controlled by the anodizing process, and varies from a few hundred nanometers to more than one micron. The anode is thus formed : aluminum foil, which will act as anode current collector coated with an Al2O3 film witch is the dielectric. To obtain a capacitor, it is necessary to add the cathode. It also uses an aluminum foil but to expect reporter the potential over the entire surface of the anode (and thus preserve S) is placed between the cathode foil and the dielectric, a liquid electrolyte which will form a continuous interface.

Principle of producing an electrolytic capacitor [1]
Principle of producing an electrolytic capacitor [1]

To form the anode of an aluminum electrolytic capacitor, we use the process of anodisation. The previously etched aluminum foil is placed as an anode in an electrolyzer ; the cathode is generally some lead. During the reaction, we oxidize the aluminum (Al) forming the surface the oxide (Al2O3) in a controlled manner. Since aluminum is a passivatable metal, the oxide layer which grows is electrically insulating and forms the dielectric.

Rolled up electrodes separated by a paper sheet with collectors of current that stand out [1]
Rolled up electrodes separated by a paper sheet with collectors of current that stand out [1]

To make electrolytic capacitors of big capacity, we can make a very long strip of Anode to which we add a sheet of paper of the same size, then a cathode strip. We roll up the whole and we inject an electrolyte which is going to soak the paper sheet and enter the roughness of the electrodes. The paper sheet serves only as a separator between the anode and the cathode, it is not used as dielectric.

Focus on rough anode, dielectric, electrolyte (cathode) and metal cathode [1]
Focus on rough anode, dielectric, electrolyte (cathode) and metal cathode [1]

On charging, the electrolyte itself becomes a part of the cathode. We see here that the roughness of the anode increases its surface. The electrolyte which acts as cathode conforms to this surface (with a dielectric between them) and therefore also present a rough "surface". The surface thus created may actually be 200 times greater than for a fully planar electrode.

Simplified view of anode, dielectric and cathode of an electrolytic capacitor
Simplified view of anode, dielectric and cathode of an electrolytic capacitor
Construction of an aluminum electrolytic capacitor [1]
Construction of an aluminum electrolytic capacitor [1]

Finally, the use of an electrolyte in an electrolytic capacitor is only a way used to design a capacitor with a very thin dielectric and to use a larger electrode surface. But it is still the same architecture as a conventional capacitor with an Anode, a Dielectric, a Cathode and fast charging by creation of an electrostatic field.

Explanatory video on capacitors:

Batteries

Diagram of a Lithium-ion battery
Diagram of a Lithium-ion battery

In a battery, the energy storage is achieved through its ability to transfer and store the charged particles called ions. A lithium-ion battery is constitued by a metal oxide Cathode (Co02 cobalt oxide or manganese MnO2 or MnO4) and a graphite anode (carbon structure) which are bathed in a liquid electrolyte. To prevent short circuits, we place a microporous separator (property that allows lithium ions to pass through this layer) in the electrolyte. An aluminum collector is placed on the Cathode in order to channel the electron flow generated or absorbed by this electrode witch is the positive pole of the battery, while a copper collector is placed at the anode for the same reason.

During the charging cycle, electrochemical reactions release lithium ions positively charged (Li+) from the cathode, which then move towards the anode, passing through the separator. During the discharge cycle, these ions move from the anode to the cathode by passing through the separator, while the electrons move from the anode to the cathode through the external circuit, which creates the current with useful output power. Lithium ions move and pass through the separator within the liquid electrolyte.

Here is another figure also represents a Lithium-ion battery :

Lithium-ion battery with oxide Cobalt cathode [2]
Lithium-ion battery with oxide Cobalt cathode [2]

During discharge of the accumulator, there is the following chemical equations:

At the cathode (+):

LiCoO2 <--> Li1-xCoO2 + xLi+ + xe-

At the anode (-):

xLi+ + xe- + xC6 <--> xLiC6

When charging, the equations are to be considered in the other direction. The charging process is limited by the supersaturation of the cobalt oxide and the production of lithium oxide Li2O witch is no longer capable of reproducing the Li+ ion. A 5.2 V overload leads to the synthesis of cobalt oxide CoO2. In the lithium-ion battery, the Li+ ions thus commute between the two electrodes at each cycle of charge / discharge but the reversibility is possible only for x < 0.5. [3]

Explanatory video on the Lithium-ion battery:

Supercapacitors

Here is a diagram of a supercapacitor:

Diagram of a supercapacitor with cathode, anode, separator and electrolyte ions [4]
Diagram of a supercapacitor with cathode, anode, separator and electrolyte ions [4]

The diagram of a supercapacitor reminder a little that of the Lithium-ion battery. Indeed, the mode of operation of a supercapacitor is fairly close to that of a battery. Both devices have an electrolyte : mixture of positive and negative ions. In a battery, chemical reactions move the ions from the electrolyte to the inside or outside of the atomic structure of the material composing the electrode, causing a change in oxidation state of the material, depending on whether the battery is charged or discharged. In contrast, in a supercapacitor, an electric field causes the ions to move to or from the electrode surface.

Unlike the electrolytic capacitor, the electrolyte is not used to create a chemical reaction and to create a dielectric. A supercapacitor shows no apparent dielectric layer, it uses the properties of the electrode-electrolyte interface forming an electric double layer (model described by Helmholtz Stern ...). Thus, in a supercapacitor, the electrolyte ions participate directly in the storage of electricity by going towards the one or the other one of the electrodes and by being adsorbed (fixed by attraction) on their surface.

When the supercapacitor is charged, the negative ions contained in the electrolyte migrate towards the positive electrode and positive ions migrate to the negative electrode. The ions are then "adsorbed" on the surface of the electrodes to compensate for the charge thereof.

In an accumulator, the ions are somehow "absorbed" in the electrodes, while in a supercapacitor, ions are "adsorbed" on the electrodes. We imagine very well that it is easier to store many ions inside the material by chemical reaction than by contenting with hanging them on the surface of the electrodes. But for storing still a lot of energy, supercapacitors have a trick : they use porous electrodes allowing the ions to enter deeply into the electrodes while contenting with remaining adsorbed on their surface.

The key to a good energy storage by supercapacitor is simple : provide more electrode surface for a large amount of ions can hang on it. During more than two centuries, chemists have synthesized electrode materials with a surface area increasingly high. Today in commercial supercapacitors, the electrode surface is mostly composed of activated carbon, a full of pores material witch providing a high surface by volume unit. Pores are small areas that are larger than a nanometer (a nanometer is a million times smaller than a millimeter). The electrode full of pores acts like an electrical sponge.

Activated carbon used in supercapacitors is often made from coconut bark (From a lecture by Patrice Simon [5])
Activated carbon used in supercapacitors is often made from coconut bark (Lecture by Patrice Simon [5])

The electrode surface, the pore size and the electrolyte used are crucial for a high-capacity storage in supercapacitors. To improve the capacitance of supercaps, researchers test other materials which can have qualities even more interesting than the activated carbon : graphene, carbon nanotubes, carbons derived of carbides... Materials with a large surface area and that can be arranged and structured to create pores of interesting size.

Activated Carbon (50µm)
Activated Carbon (50µm)
Carbons derived of carbides (5µm)
Carbons derived of carbides
(5µm)
3D Graphene
3D Graphene

Another diagram of a supercapacitor:

Schematic of discharged and charged supercapacitor [6]
Schematic of discharged and charged supercapacitor [6]

A supercapacitor is constituted by four main elements : current collectors, active material, electrolyte and separator. These elements are assembled to form a complete electrochemical cell and the current collectors are connected to an external electrical circuit.

The current collectors are the link between the active material and the external electrical circuit. Their function is to efficiently collect the charges developed at the level of the active material.

The electrolyte is a conductive substance containing movable ions. It may be a solvent containing dissolved ions (aqueous or organic electrolyte), or a pure ionic liquid without solvent. Ionic liquids are very expensive, so still very little used.

The separator is used to prevent short circuits in the system by isolating electrically both electrodes, but this separator must still let pass ions of the electrolyte.

The active material is the porous material of the electrode on which the ions are adsorbed. This is the electrode / electrolyte interface which is the basis of energy storage in the electrochemical double layer capacitors that are supercapacitors.

In the diagram above: when no potential difference is applied across the supercapacitor, ions are adsorbed (by attraction) on the electrodes without special segregation between anions and cations.

In the lower diagram: when a non-zero potential difference is applied, ions are selectively adsorbed onto the positive electrode and the negative electrode by compensating for the charge of the electrode.

We are used to say that in a supercapacitor, there is an electrostatic energy storage whereas in batteries there is an electrochemical energy storage. This is true, except for the pseudo-supercapacitors...

Pseudo-supercapacitors

Here's our first diagram:

Supercapacitor and pseudo-supercapacitor
Supercapacitor and pseudo-supercapacitor

The first diagram shows a standard supercapacitor with the ions that are adsorbed (fixed by attraction) to the porous electrodes surface: there is electrostatic energy storage. In the second diagram, there are redox reactions as in batteries. But unlike batteries, chemical reactions take place on the surface of the electrodes. Result: a little more energy storage density than with standard supercapacitors, but a speed charge / discharge still very fast, because the ions do not migrate to or from the inside of the material of electrodes...

Hybrid supercapacitors

There are also hybrid supercapacitors that are composed of a supercapacitor or pseudo supercapacitor electrode and a battery electrode. We so try to gather the best of both storage while trying to erase the most the weak points of these devices...

Here is a video not exempt from defects but still interesting to represent the operation of a supercapacitor:

Another video in which an electrolytic capacitor and a supercapacitor are opened and unwound to see what it looks like inside these components:

By detailing the operation of capacitors, batteries and supercapacitors, we demonstrated that supercapacitors are at least as technically close as batteries to capacitors and therefore it is better to avoid confusing capacitor and supercapacitor if we wish to speak seriously about these electricity storage systems.

To conclude, we can say that the supercapacitor is a kind of high-capacity capacitor (there are 9,500 farads supercapacitors) and/or a kind of ultra-fast battery with very long life (more than 1 million recharges).



Sources :

1 comment

#1  - custom best dissertations said :

Really , I never thought that there is no such thing as a super capicitor, this is really eye opening! I went through a couple of best dissertations after learning about it from here and seems like it is true. You have really done a good job with it, thanks!

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