What is a Pseudocapacitor?

The word ‘Pseudo’ is derived from Greek ‘pseuds’, meaning ‘false’ or ‘lying’. Pseudocapacitor is not strictly a capacitor, but looks and behaves like one. It does not work on pure electrostatic process like that in EDLC, but also additionally involves fast and reversible electrochemical process (Faradaic Reaction) using near-surface charge transfer for energy storage. This charge is in addition to electrostatic charge storage, and is the result of charge transfer at electrode / electrolyte contact surface. Faradaic reactions occur at or near the surface of electrode materials. Involving small number of electrons between valence states of electrode materials.

Faradaic current in electrochemistry is electric current (or charge transfer) generated by reduction or oxidation of some chemical substance at electrodes. Electron transfer causes a change in state of material by oxidation or reduction. It is governed by Faraday’s Law which states that sum of a chemical reaction caused by electric current is proportional to the charge transferred, and such process is called Faradaic reaction. Ion insertion and surface redox reaction process characterize the term ‘pseudocapacitor’, and charge transfer and energy storage can be defined in terms of capacitor absorption / desorption model.

Equivalent circuit

Equivalent circuit of pseudocapacitor is represented as below:

C_dl is EDLC capacitor, representing electrostatic charge storage as in an EDLC capacitor. R_s is equivalent series resistance of the whole capacitor. R_f is the electrode/ electrolyte resistance, and R_d represents losses during charge transfer by faradaic process. C_f is the faradaic component of capacitance, representing charge transfer by faradaic process.

TMO Pseudocapacitors

TMO pseudocapacitors have charge storage much higher than EDLC, offering higher energy. They use materials like transition metal oxides (TMO) or conducting polymers which offer high capacitance and power density.  TMO pseudocapacitors use materials like Ruthenium dioxide RuO₂, Manganese dioxide MnO₂, Nickel oxide NiO, Copper oxide CuO, and cobalt oxide Co₃O₄. High capacitance and power density is due to their ability to undergo redox reactions, which involve transfer of small numbers of electrons between valence states of metal ions. TMOs have two or more oxidation states, and during charging or discharging, materials change from one state to another.

In redox reaction, while charging / discharging, charge transfer is brought about by gaining or losing valence electrons at the surface layer. This process, along with electrostatic charge accumulation, enables much higher energy storage compared with EDLC. Ruthenium oxide exhibits effective area as high as 1400-2000 sq. m. /gm, and is therefore most widely used.’ Energy storage densities are in between EDLC and Li-ion hybrid capacitors.

Recent years has seen a great progress in these capacitors because of their far greater amount of energy storage capacity than EDLC, and higher voltage ratings. A number of redox-active nanotechnology materials have made the surge possible. Electrodes in pseudocapacitor are usually made from transition metal oxides like manganese, ruthenium, or conducting polymers, while a wide variety of electrolytes may be used. Demand for high energy storage and smaller sizes have seen development of new materials with ever increasing energy capacities. Existence of electrochemical reactions (energy storage not fully in electrode materials, but partly in chemical bonds, reduces life of these capacitors compared with EDLC, but the benefits drawn make these acceptable.

Polymer pseudocapacitors

Polymer pseudocapacitors use conducting polymers like polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTh) as electrode materials. Advantages are high energy density, cost-effectiveness, and versatile properties. They offer properties similar to TMO, but are inexpensive and have tunable electronic conductivity, as also high cyclic stability. Charge/ discharge rates can be higher. Conducting polymer capacitors can be designed to have tailored properties, which makes them interesting for researchers.

Polymer pseudocapacitors are environ-friendly, being made from organic materials. This makes them a preferred choice in several applications. They are also light weight, which makes them suitable for wearable and portable electronic devices. Conducting polymers can be combined with other materials such as carbonaceous materials, transition metals to create composite or hybrid materials within capacity to store higher energies.

Limitations of polymer pseudocapacitors

• Lower energy density than TMO type
• Cyclic stability is poorer than EDLC
• Conductivity of polymers is lower than TMOs
• Though costs are generally lower, high quality polymers can be costly

Advantages of conductive polymers have led to continued research as a promising alternative to other types of pseudocapacitors. Life of pseudocapacitors may go to several hundred thousand cycles. The cycle life is around 20,000 cycles, much lower than EDLC, but is acceptable in most cases. Current capacity of pseudocapacitors is also lower than EDLC because of slower electrode reactions. Those based on vanadium dioxide (V₂O₃) and manganese dioxide (MnO₂) have high stability with capacitance retention of nearly 86% after 15,000 cycles. By comparison, hybrid capacitors, combining the characteristics of pseudocapacitors and battery-like electrodes, offer higher energy density, but have inferior cycle life and current capacity. While pseudocapacitors are symmetric, hybrid capacitors are essentially asymmetric in construction- a balance between pseudocapacitors and battery. Several new materials are under intense study for betterment in properties and life cycles.

Applications of pseudocapacitors

Pseudocapacitors are extensively used in electric vehicles, power backup and memory retention, as also for grid storage. In EVs they provide high acceleration and store energy fast during regenerative braking.