The value of 1.229 V is based on the thermodynamic value for the Gibbs energy change (Delta G) for the reaction H2O = H2 + 0.5 O2. In reality, you will not measure any reaction of water until a voltage of at least 1.5 V or higher is present. The experimental value depends on the overvoltage (overpotential), and this differs greatly from one metal to another. The lead acid battery operates at 2 V per cell in H2O + H2SO4 because lead has a high overvoltage for water electrolysis. Different electrolytes, different electrodes and different solvents will give greatly differing potential windows. The limit for any combination is about 6 V. The lithium - ion battery will give about 4 V per cell.

The value of 1.23 V origins from the reaction thermodynamics. The reaction always requires to apply a so called overpotential. The real problem is not to INCREASE overpotential but to DECREASE it. The overpotential depends on the materials used for cathode/anode. You an also use non-aqueous electrolyte.

Dr. Yurii V Geletii is fully true. The decomposition potential of water is 1.23 volts. This can be found in any tutorial. Therefore, if you want to work with same electrodes under higher potentials, it is necessary to use electrolyte on a different basis (for example, non-aqueous). An alternative is also a "water-in-salt" electrolyte (I read the article somehow, I think the authors are from Maryland).

Could you, please, show (or message?) this

eccentric report[1] or ref. ?

1. Aranganathan Viswanathan's question; quotation: ... there is report that claims potential window of 2.6 V with 1 M Na2SO4,...

Aranganathan Viswanathan

Did you change the question after the first answers?

Sumptuously...

Dear Aranganathan Viswanathan,

The possibility of having high cell voltage aqueous SC is realized by constructing asymmetric supercapacitors because positive and negative electrodes are working in separate potential windows. But individually they are limited by 1.23 V. The reaction overpotential adds a little towards that theoretical 1.23 V limit, so some of the two can be 2.6 V.

Usman Zubair : "positive and negative electrodes are working in separate potential windows. But individually they are limited by 1.23 V. "

This is wrong. The thermodynamic potential limit for water oxidation is E1 = 1.23 -0.06*pH , for water reduction E2 = 0.06* pH. Then, instead of "so some of the two can be 2.6 V," should be sum of the two is 1.23 V.

If you use different pHs in anodic and cathodic compartments, then you have a very high junction potential resulting in a significant increase of voltage between anode and cathode

I think that I am a bit sloppy in my previous answer. Yurii V Geletii You are perfectly right thermodynamic potential limit for water electrolysis is 1.23 V (standard). It is all about reference point we are considering. In case of asymmetric supercapacitor (ASC), negative electrode work in -ve potential window and positive electrode work in +ve potential window. On balancing, the two electrodes the potential difference of the two extreme is 2.6 V. may be following image can communicate it better.

Dear Usman Zubair,

could you, please, share (or message?) a full (public or even an initial)

report[1,2], about/for "ASC.jpg",

or any related ref(s) ?

1. What is the (aq.) electrolyte, the V-rate(s), SOH, SOC, etc. ?

2. Is there, any, EIS, having the polarization Vdc (parameter), near the V-limits ?

The value of 1.229 V is based on the thermodynamic value for the Gibbs energy change (Delta G) for the reaction H2O = H2 + 0.5 O2. In reality, you will not measure any reaction of water until a voltage of at least 1.5 V or higher is present. The experimental value depends on the overvoltage (overpotential), and this differs greatly from one metal to another. The lead acid battery operates at 2 V per cell in H2O + H2SO4 because lead has a high overvoltage for water electrolysis. Different electrolytes, different electrodes and different solvents will give greatly differing potential windows. The limit for any combination is about 6 V. The lithium - ion battery will give about 4 V per cell.

In reality, EIS[1] is very sensitive, illuminating details of most WEs' interfacial properties, at nanoscale regime. EIS is able to "predict" a near-DC, kinetically hinted, ("silent") electrolysis in (water electrolyte//) electrode's interfaces near (Vdc~) 1.23V // SHE, or higher (Vdc>1.23V vs. SHE, at 25°C, pH=7), due to it's wide (multi-freq.) Z-spectra "effective visibility".

1. Basics of Electrochemical Impedance Spectroscopy https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/

Dear Ioannis Samaras please find the article in attachment...Article High-Performance 2.6 V Aqueous Asymmetric Supercapacitors ba...

Melvin Henry Miles has highlighted the same rightly.

Dear Usman Zubair, I thank you for this article link, it is very good, and we are waiting for the next (supplementary) article. Maybe, we are not too far from a "green" replacement, for a LA (car) battery, with a "safe" water based electrolyte.

This, so high, 2.6 V nominal voltage (source scanning window), is far from the (near) thermodynamic voltage window value for this SC. One could, simply, approach it by measuring the OCV[1](time) value(s), after a full charge (or a full discharge).

Also, there are methods to avoid most of the kinetic[2] effects, during these (article) measurements.

1. With a two electrode configuration, or even better, the two OCPs in a three electrode configuration.

2. like the iR drop(s) in the Carbon cloth, the substrate for the active (electrode) materials.