What is a Pseudocapacitor : Working & Its Applications

A supercapacitor is a special type of energy storage device which has an extremely large capacitance by combining the capacitors & batteries properties into one device. These capacitors can store more energy as compared to other regular types of capacitors and provides high output power than batteries. Supercapacitors are also called an ultracapacitor which is easy to operate & very safe. Supercapacitors are available in three types based on requirements like EDLC or Electrostatic double-layer capacitors, Pseudo-capacitors, and Hybrid capacitors. This article discusses one of the types of supercapacitors like Pseudocapacitor – working and its applications.

What is a Pseudocapacitor?

Pseudocapacitor Definition: Pseudocapacitors or faradaic supercapacitors are devices that are different from EDLCs. The electrodes of this capacitor include redox-active materials to store electrical energy using a different mechanism as compared to EDLCs.

In fact, only a part of the charge is achieved because of the EDLC whereas a large amount of energy transfer & storage can be achieved through faradaic mechanisms like redox reactions electrosorption & intercalation.

When an exterior potential is applied to this capacitor then quick and reversible redox reactions can occur on the electrode which includes the passage of charges between electrode & electrolyte. The charge & discharge mechanism of this capacitor is related to the one of the electric batteries.

Pseudocapacitor Diagram

A pseudocapacitor is a hybrid in between a battery & an EDLC (electric double layer capacitor). This capacitor includes two electrodes which are separated through an electrolyte. The storage of charge mainly occurs through chemical & electrostatic processes.

The chemical process mainly involves transferring charge through Redox or reduction-oxidation reactions. When the charge transfer is comparable to that within a battery, then transfer rates are superior due to thinner redox material over the electrode otherwise fewer ions diffusion from the electrolyte into the structure. The values of capacitance are higher within pseudocapacitors due to several processes performing to store charge.

Working Principle

The working principle of Pseufdocapacitor is to store electrical energy by transferring electron charge between electrode & electrolyte through reduction-oxidation reactions, electrosorption & intercalation processes called pseudocapacitance. In an electrochemical capacitor, a pseudocapacitor is an essential part that forms a supercapacitor together with an EDLC or electric double-layer capacitor.

Pseudocapacitive are generally made up of metal sulfides, metal oxides, metal hydroxides, metal nitrides & conducting polymers. Examples of Pseudocapacitor materials are Metal oxides like RuO2, NiO, MnO2, Co3O430, and conducting polymers like polyaniline & polypyrrole.

The energy storage in Pseudocapacitors can be done throughout the faradaic reactions. So they store charge electrostatically where the transfer of charge can be done between electrode & electrolyte. Once the voltage is applied to a pseudocapacitor, then both reduction & oxidation occurs on the material of the electrode. The faradic process used in these capacitors will enhance the electrochemical reactions which provide greater specific capacitance & energy densities as compared to EDLCs.

The materials of pseudocapacitor enhance the energy density to permit the energy storage density in the volume of electrode materials at their surface. The main characteristics of these capacitor materials are electrically conductive otherwise including two oxidation conditions within a particular potential window.

These capacitors have the highest capacitance density as compared to other capacitors due to their special storage charge principles. So the sum of electric charge stored within a pseudocapacitance is linearly proportional to the voltage applied.

Faradaic Reaction

Electron transfer can reason to occur oxidation or reduction because this reaction is mainly governed by Faraday’s law which states that the sum of a chemical reaction caused through the current flow is proportional to the sum of electricity passed is known as a faradaic reaction.

Redox Reaction

A redox reaction is a chemical reaction where electrons are moved in between two reactants participating in it. These electrons transfer can be identified simply by monitoring the changes within the oxidation conditions of the reacting species.

How to Connect a Pseudo Capacitor?

The circuit of the pseudocapacitor is shown below. As shown in the following diagram, ‘Rs’ is the series resistance and the pseudocapacitance ‘Cᵩ’ mainly depends on the potential, ‘R_D’ is the Faradaic resistance that may perform throughout discharge until the ions disappear & ‘R_F’ is the electrode-electrolyte resistance. So at particular potentials, the pseudocapacitance exceeds the dual-layer capacitance ‘Cdl’. So the parallel combination of capacitors will assist greatly to increase the pseudocapacitor’s capacitance due to the additive law of capacitance.

Types of Pseudocapacitor

Pseudocapacitors are classified into two types based on electrode materials used to store charge within pseudocapacitors like the following.

• Metal oxide
• Conducting Polymers

Metal Oxide

The metal oxide is one kind of pseudocapacitive material that exhibit reversible as well as fast redox reactions at the outside of the electrode materials. This material exhibits low resistance & high specific capacitance so constructing supercapacitors is very easy with high power.

The most frequently used metal oxides are MnO2, RuO2, NiO, SnO2, IrO2, Fe3O4, V2O5, Co2O3 & MoO as supercapacitor electrodes. From these metal oxide electrode materials RuO2 is considered the most capable electrode material in supercapacitor applications due to its high electrical conductivity & specific capacitance. But practically, the non-existence of ‘Ru’ on the earth has restricted its application. So by considering this main issue, the recognition of the less cost pseudocapacitance materials is necessary.

Metal oxides exhibit different oxidation conditions at various potentials & possess crystalline arrangements that allow maximum conductivity to enable charges to broadcast in their network. Metal oxides are capable of changing their oxidation conditions & protons can add to and remove from the oxide lattice throughout oxidation & reduction surface reactions.

Conducting Polymers

Conducting polymers are materials used for redox pseudocapacitors because of their rapid & reversible oxidation or reduction processes, comparatively low cost & good electrical conductivity.

The most frequently used conducting polymers are PPy (Polypyrrole), PANI (Polyaniline), PTh (Polythiophene), and PPV (p-Poly p-phenylene vinylene) & PEDOIT (p-poly e-ethylene dioxythiophene). These materials are normally produced either through electrochemical oxidation or chemical of the monomer and rendered conductive using a conjugated bond system & the polymer backbone.

As compared to carbon-based electrode materials, conducting polymer materials provide the capacitance, enhanced conductivity & decreased equal series resistance. Once oxidation & reduction occurs then the ions are simply observed or drifted from the electrolyte to the conducting polymers & released into the electrolyte.

These polymers exhibit extremely reversible effects that lead to better cycling stability due to no phase transition occurring. They are charged positively or negatively through the redox reactions that encourage enhanced conductivity.

Pseudo Capacitor Vs Supercapacitor

The difference between a pseudo capacitor and a supercapacitor includes the following.

Advantages

The advantages of pseudocapacitor include the following.

• These capacitors have a higher power density.
• They have significantly longer lifetimes.
• As compared to lithium-ion batteries, they charge & discharge very quickly.
• The materials of pseudocapacitor materials will enhance the density of energy & allows the energy density storage within the bulk of electrode materials & at their surface.

Disadvantages

The disadvantages of pseudocapacitor include the following.

• These capacitors have less energy density, so they cannot be used in place of batteries in energy storage applications.
• They are not suitable for long-term energy storage devices.
• The output voltage of these capacitors refuses with their charge linearly.

Applications

The applications of pseudocapacitor include the following.

• Pseudocapacitors store electrical energy through a faradaic reaction.
• It is part of an electrochemical capacitor that uses an electric double-layer capacitor to form a supercapacitor.
• These are used in consumer electronics.
• In wearable or flexible electronics
• Regenerative braking within automobile applications
• Kinetic energy (K.E) recovery systems like cranes, elevators, wind turbines, etc.

Thus, this is brief information about Pseudocapacitor and its working with applications. The EDLC materials cannot increase energy density sufficiently but pseudocapacitor materials enhance the energy density & allow the energy density storage within the volume of electrode materials & at their outside. The main benefits of Pseudocapacitor mainly include cost-effective, lightweight, eco-friendly, comfortable, flexible, improved safety, tunable electrochemical properties, etc. Here is a question for you, what is EDLC?