Discussion Forum

There are different kinds of overpotentials. The most important and usual are
1. activation overpotential
2. concentration overpotential

The first one is the deviation from the reversible potential at a certain current. It is of the form

RT/(alpha*n*F)ln(i/i0)

where i is the current density, i0 the exchange current density and alpha is the charge transfer coefficient. Concentration overpotential comes from the concentration polarization due to electric current. It is of the form

RT/(nF)ln(cs/cb)

where cs is the surface and cb the bulk concentration, respectively. Thus can also be written as

RT/(nF)ln(1-i/ilim)

where ilim is the limiting current density. These issues are explained in any electrochemistry textbook.

BR
Lasse

Thank you for your reply. However, what I want is the exact origin/reference for this particular expression
eta = phi_s - phi_l - E_eq
Thank you anyway.

Are there any comsol models that looks at strictly distributing components of these Overpotential as you change the adjustable parameter?

Could you please clarify your question?

Lasse

Dear Lasse Murtomäki,

Would you please guide me what is the exact meaning of “the electrode potential (phi L)” and “electrolyte potential (Phi S)”?

Let me make my question clear:

I am trying to model an accelerated corrosion test on steel rebar embedded in concrete cylinder. In this test I am applying a potential of 10V by a DC power supply to the anode (steel rebar) and cathode (stainless steel plate which is placed outside the cylinder in a 5% salt solution (electrolyte)) - I don't exactly know what the potential is in the salt solution, I can just measure the applied voltage and current (A) through the circuit - please see the attached picture.

1- My first question is; what modulus is proper to model an Accelerated Corrosion Test: AC/DC Modulus or Secondary Corrosion Modulus or ... ? (Please note that I am not going to model a natural corrosion)

2- I am going to calculate the expansion due to the rust formation around the rebar and then calculate the concrete cracking due to this phenomenon, is COMSOL able to model rust formation and expansion around the anode?

3- Is there any practical example that I can use it as an initial guideline (please note that I have already seen the "Cathodic Protection of Steel in Reinforced Concrete" example, this is not really relevant to my test since it is just considering the concentration and diffusion of concrete – not any rust formation or mass loss)

Sincerely Yours,
Hamid

I am using the "Tertiary Current Distribution, Nernst-Planck", there I do use an "Electrolyte-Electrode Boundary Interface". For the kinetics you do need eta, which is used for example in the Butler-Volmer equation.
I guess, I do have the same problem like Kunna Wu had:

"Thank you for your reply. However, what I want is the exact origin/reference for this particular expression
eta = phi_s - phi_l - E_eq"

So my assumptions are:
eta is the over-potential
E_eq is the concentration depending electrolyte potential, where you do have the equilibrium potential (Nernst-Equation).
phi_s is the potential at the solid electrode.

My questions are:
Are my assumptions correct so far?
What stands phi_l for?
How to get sensible potential depending boundary conditions for phi_l

Thank you and best regards,

Martin

Dear all

phi_l is the Galvani potential of the liquid phase and phi_s is that of the solid phase, viz, the electrode. The theoretical basis of phi_s is in the Fermi levels where electrons reside, but we do not have to consider that. The difference phi_s - phi_l is the electrode potential that follows the Nernst equation at equilibrium. The deviation from the equilibrium potential is the definition of the overpotential:

eta = (phi_s - phi_l ) - E_eq

phi_l is needed because also in the solution phase potential drops occur due to ohmic loss and concentration polarization; they are calculated with the Nernst-Planck equation if needed.

Potential is quite versatile a concept in electrochemistry and confuses students all the time. If a reference to a textbook is required, I would suggest Hubert Girault's book "Analytical and physical electrochemistry", EPFL Press (Marcel Dekker) 2004.

Wish this helps

cheers
Lasse

hello,

I am as well facing the same confusion,

I am trying to model half cell reaction (Anode) and get the activation overpotential or anodic polarization. I .

Can anybody guide me on
1. if I am using current density as the boundary condition , which potential , Phi_s or Phi_l should I consider for activation over potential

2. to generate the polarization curve, which over potential should I plot. ( I am getting Nan values when I use reaction electrode potential or Phis)

thank you so much

Regards
Nive

Hi

Use the difference phis - phil and subfract for Eeq where no current is flowing.


In the current-voltage simulation, choose Electric potential.

Lasse

Hello Lass,

Thank you so much... :)

Nive