Hello,

When you use ohm, it refers to a specific object. So if people do not know anything about your object it has no meaning. This is why we usually normalise to something meaningfull and directly comparable.

For instance, for electrolytes, we are interested in ion conduction. In this case, the meaningful is conductivity: resistance= 1/conductivity * (thickness/surface). The unit of conductivity is ohm-1.m-1 or S.m-1 or S.cm-1.

For electrodes, the property is not intrinsical to a material i.e. it is related to material properties because it is due to ion/electron conductivity but also to electrochemical reaction, diffusion of gases, etc... and what you measure is not linear with the thickness of the electrode. 

In this case, the meaningfull value is the one that characterises how good is your electrode for a given surface. So the unit will be ohm.cm2.

The thickness of the electrode has no meaning because part of it simply serves as electrical contact. The active zone is most often smaller than the thickness, which simply means that the rest of the electrode does affect electrode properties.

Hoping it helps

Guilhem

It is not Ohm / cm2 but Ohm cm2, for impedance. It could also be mm2, dm2, m2.....

Typically it is referred to the projected surface area of the working electrode or of the rate-limiting component of the system (e.g. anode, cathode, membrane...).

You need to multiply the resistance obtained in the experiment on the area of the sample studied.  This will allow other researchers to compare the results. As in the case of the current and current density, the resistance depends on the geometry of the sample.

Hello,

When you use ohm, it refers to a specific object. So if people do not know anything about your object it has no meaning. This is why we usually normalise to something meaningfull and directly comparable.

For instance, for electrolytes, we are interested in ion conduction. In this case, the meaningful is conductivity: resistance= 1/conductivity * (thickness/surface). The unit of conductivity is ohm-1.m-1 or S.m-1 or S.cm-1.

For electrodes, the property is not intrinsical to a material i.e. it is related to material properties because it is due to ion/electron conductivity but also to electrochemical reaction, diffusion of gases, etc... and what you measure is not linear with the thickness of the electrode. 

In this case, the meaningfull value is the one that characterises how good is your electrode for a given surface. So the unit will be ohm.cm2.

The thickness of the electrode has no meaning because part of it simply serves as electrical contact. The active zone is most often smaller than the thickness, which simply means that the rest of the electrode does affect electrode properties.

Hoping it helps

Guilhem

Asstated by Stanislav Melnikov You need to multiply the resistance obtained in the experiment on the area of the sample studied. If you are mentioning simply by Ohm, it is the total resistance of the system, if you are normalizing to the area area under the experimrnt it is Ohm cm2.

Generally supporting others replies if you want to compare your data with data collected for other samples they all need to be normalized but if you want to show only raw data use Ohms (e.g. to show how your fitting looks like)

It seems we are missing something. If the unit is in Ohm/cm2, it means that area (cm2) is the actual area of the object which is incontact with electrolyte. For instance, total area of object might be larger than the actual aperture/opening/pores in the object through which ions flow to and fro. Total impedance is based on this flow area in the object. This is my understanding. 

Consider this: the resistance is a derived quantity; in the EIS measurement, the voltage and current (and phase of current with respect to voltage) are what are actually measured. 

Current is an extensive property - the magnitude of the current depends on the size of the working electrode.  In this case, the size of interest is area, since we are discussing an interfacial phenomenon - electron transfer occurs at the solution - electrode interface.  Current density, on the other hand, is an intensive property - it does not depend on the area of the electrode.  This is why current density is most often reported in literature (for DC techniques like potentiodynamic polarization).

According to Ohm's Law, resistance is R = V/I.  If you replace current I with current density i, the units on R change from V/A (Ohms) to V/A/sq. cm (if area is in cm. sq.), which become Ohms*cm^2.  This makes explicit why the units are Ohms*cm^2 instead of Ohms/cm^2.

So, to get your impedance in Ohms*cm^2, you could use current density instead of current to calculate the impedance.

When it comes to electro-chemistry and performing electrochemical impedance spectroscopy (EIS) it can be a little confusing.

If we perform an experiment with say a 10cm^2 electrode and a 1cm^2 electrode, in terms of pure resistance the 10cm^2 electrode will give a lower resistance - let say 20 ohms. In the same system the 1cm^2 electrode will produce a resistance ten times this i.e. 200 ohm. As mentioned above, to compare them we use ohm cm^2 and would multiply the 20 ohms x 10 cm^2 = 200 ohm cm^2. The 200 ohms from the smaller 1 cm^2 electrode would by multiplied by one i.e. 200 ohm x 1 cm^2 = 200 ohm cm^2 to obtain the same or comparable values.

Hello Brian. I run EIS and obtained half nyquist plot (quarter of circle) for 1Hz - 100kHz. Same is repeated with 0.01Hz - 100KHz. Why it is happening?  

Hello Brian. I think the only half nyquist plot means your system has a large resistance. And there is no Warburg impedence which means your system is not controlled by diffusion. 

Hi Degang Li, can you please explain this more. What do you mean by this statement "system is not controlled by diffusion".

Hi, Muhammad Shuja Khan. The Warburg impedence will appear in your EIS when the diffusion is the rate controlling step. Please see one of my paper: The corrosion inhibition of the self-assembled Au, and Ag nanoparticles films on the surface of copper, Colloids and Surfaces A: Physicochemical and Engineering Aspects,  2006, Volume 273, Issues 1–3, Pages 16–23.

Hi, I would like to inform that impedance in ohm cm2 is as intesive quantity and due to Z=E/I the current density is in A/cm2 therefore the unite of Z will be: volt/Amper.cm-2 or volt/Amper*cm2=ohm*cm2. It means the Impedance for using 1cm2  electrode surface interface.

ohm * Area= ohm.cm2

If we consider that we are measuring the ionic conductivity by the impedance method, but expressed by the ionic impedance or resistivity, it can be easily understand that the unit Ohm*cm2 is reciprocal of specific conductivity S/cm2.

G. Dezanneau

Brian Kinsella

Does that mean to multpliy Z' and Z'' by S(cm²)?

Another question, do I need to normalize Z' and Z'' values for the Nyquist plot?

Please see this paper for more details:

Article Electrophysiology of Epithelial Sodium Channel (ENaC) Embedd...

Dear Dario,

Assume that we have two electrodes immersed in electrolyte where the immersed cross sectional area of the twp electrodes are equal of area A. assume that the spacing between the two electrodes is L and the resitivity of the electrolyte is roh, then accordingly one can expels the resistance between the two electrodes as

R= roh L/A,

Rearranging

R XA= roh/L

From this relation it is clear that

When A =1cm^2, then it will give the properties of the electrolyte roh and its path length. The unit of R is in Ohm which depends on the electrode area. Ohm x cm^2 is resistance for for an electrode area of one cm^2. It is an electrode area specific resistance,

Best wishes