In many cases the design of the pseudocapacitor device includes two electrodes, one of which is real pseudocapacitor and another is battery-type. In this case the voltammetry of the device will show the peaks coming from the battery-type electrode. Even worse, some authors use the term 'pseudocapacitor' when describe the battery-type electrode of such devices separately. Please, refer to http://jes.ecsdl.org/content/162/5/A5185.full for detailed discussion.

Dear Rui Zhou 

Have a nice time and good day

I concur with Prof. Alexander B. Missyul’ and Prof. Artur Braun.

Pseudocapacitors store electrical energy faradaically by electron charge transfer between electrode and electrolyte. This is accomplished through electrosorption, reduction-oxidation reactions (redox reactions), and intercalation processes, termed pseudocapacitance.

A pseudocapacitor is part of an electrochemical capacitor, and forms together with an electric double-layer capacitor (EDLC) to create a supercapacitor.

For more details please go to these links:

Pseudocapacitors and batteries;

https://ecee.colorado.edu/~ecen4555/SourceMaterial/ElectricalEnerStor1208.pdf

https://en.wikipedia.org/wiki/Pseudocapacitor

The attached files may also help you.

Good luck

Dear Alexander B. Missyul’

Thanks for your answer. The article you suggested was really helpful. 

As the article states, the asymmetric electrode pairing can increase the energy density of the device efficiently. How about the power density? In my view, the power density is mainly limited by the diffusion of electrons and ions as well as the chemical reaction. No matter the typically EDLC electrode or the called “real pseudocapacitor electrode” can ensure the fast transfer of ions. The main limits come from the chemical reaction in the “battery-type electrode”. In this way, does the power density of the hybrid (asymmetric) device can be compared to supercapacitor’s? Or this kind of devices just can be called hybrid (asymmetric) devices or electrochemical energy storage devices which could combine the advantages of supercapacitor (high power density but not sure) and batteries (high energy density)?

Dear Artur Braun

Thanks for your helpful explanations.

As my own understanding for your answer, the mainly differences between the “battery” and “pseudocapacitor” are the performances (powder density and energy density). The electrode of the “battery” suffers from low power density which is limited by the chemical reaction rate. While the supercapacitor owns high power density but low energy density. The application of “pseudocapacitor electrode” can efficiently increase the energy density of supercapacitor. The materials owned similar characters (redox peaks and charge/ discharge plateaus) can be distinguished by the different applications (used in supercapacitors and batteries).

If my understanding is correct, my confusion is that is this classification suitable and can the “pseudocapacitor electrodes” ensure the characters of supercapacitor (such as high power density)?

Dear Ahmed Saeed Hassanien

Thanks for your response and suggestions. I would learn the information you suggested for further discussion.

Have a nice day.

Dear Rui Zhou

You are welcome. With my best wishes and good luck

Yours,

Ahmed Saeed Hassanien

I am afraid pseudocapacitance must only be used in a specific case where the electrochemical signature of an oxide or nitride electrode (MnO2 or RuO2 for example) "looks like" capacitive (but is not due to specific electrochemical reactions). This is the meaning of the prefix "pseudo" and of the definition of capacitance some centuries ago (see for example: https://en.wikipedia.org/wiki/Capacitance). This has nothing to do with energy or power densities.

I am afraid too many authors are presenting battery type materials that have nothing to do with pseudocapacitance neither "supercapacitive" properties. In such case, it makes no sense to talk about capacitance (in F/g or F/cm2 or F/cm3) but capacity must be used instead (in C/g or mAh/g, /cm2 or /cm3).

I suggest these authors take a look to Conway's handbook on Electrochemical Capacitors: "Pseudocapacitance is Faradaic in origin, involving the passage of charge across the double layer, as in battery charging or discharging, BUT capacitance arises on account of the special relation that can originate for thermodynamic reasons between the extent of charge acceptance (deltaQ) and the change of potential (deltaV), so that a derivative d(deltaQ)/d(deltaV), which is equivalent to a capacitance, can be formulated and experimentally measured by dc, ac, or transient techniques."

Obviously, the electrode materials presented in such papers do not exhibit any relationship of that type (which may result in a quasi rectangular CV) and instead demonstrates a strong Faradaic behavior which has nothing to do with capacitance.

Unfortunately many confusing papers are reporting so called "pseudocapacitive behavior" of nickel or cobalt based oxides or hydroxides in the literature which is wrong and misleading. A recent paper describes the difference between this different kind of behavior ("To Be or Not To Be Pseudocapacitive?", D. Bélanger et al., J. Electrochem. Soc. 2015 volume 162, issue 5, A5185-A5189).

 Thierry Brousse

Plateaus are only observed in battery type electrodes and the charge storage must be given in C/g (/cm2, /cm3, ...) or mAh/g.

Pseudocapacitive materials only show triangular shape charge/discharge curves or rectangular shape CVs so that a CONSTANT capacitance value can be defined in order to fulfil the Q = C deltaU relationship over a given potential window.

If such electrochemical signature is not shown, thus this cannot be called a pseudocapacitive material. This is simply a fast rate battery type electrode.

following Artur Braun, we know that a (lead acid) car battery may extent it's cost effective application: from a racing car to a (long) bus, using the "same" materials properties. In conventional car producers' internal plans there is a trend to apply for (at least) two units of car batteries: one optimized for high current (as "supercapacitor") and the second optimized for high charge. So, one will be used for starting the engine (as "supercapacitor", optimized for high current car battery). The  second will be used as storage car battery powering the rest of the (today and future) ever growing internal electrical systems as well as the additional passengers' needs like computers, tablets, cell-phones etc.

Dear Thierry Brousse

Thanks，appreciate for your answer． 

Dear Ioannis Samaras

Thanks for your answer. If the two systems be used in EVs commercially, the issue of energy shortage may be largely solved. Looking forward to that day.

Rui Zhou : Another difference perhaps is that in pseudocapacitors, there is electrostatic capacitance plus a electrochemical capacitance which arises due to a REDOX reaction at the electrode surface but electrode itself does not undergo any change in this reaction. On the other hand, in a Battery, the electrode does undergo a change and quite often a change in phase too.