I determine the surface area of my electrode using Randles Sevcik equation, however this surface area is less than the geometrical/physical surface area of the electrode. What is this means? Does geometrical surface area might greater than electrochemiocal surface area, which obtained using Randles Sevcik equation?
Hello Mulatu
In general I would say that the real area is always grater than the geometrical area. If you look at a d=1mm disk electrode the surface area is 3.14*1^2=3.14mm^2 now this is assuming that the surface is completely flat. HOWEVER this is never the case since you have scratches, bumps and curves in a microscopic scale so your real surface area will be grater.
So now you are asking how come you got a different result? well assuming your calculation was perfect, using a reversible couple and all the right constants and the temperature was accurate my best guess is that your surface was fouled. I mean you had some contamination or something blocking the electrode surface so the calculated surface was bigger that the real ACTIVE surface.
I guess that your electrode is covered in part by electrochemically inactive film
Hello Mulatu
In general I would say that the real area is always grater than the geometrical area. If you look at a d=1mm disk electrode the surface area is 3.14*1^2=3.14mm^2 now this is assuming that the surface is completely flat. HOWEVER this is never the case since you have scratches, bumps and curves in a microscopic scale so your real surface area will be grater.
So now you are asking how come you got a different result? well assuming your calculation was perfect, using a reversible couple and all the right constants and the temperature was accurate my best guess is that your surface was fouled. I mean you had some contamination or something blocking the electrode surface so the calculated surface was bigger that the real ACTIVE surface.
Dear Mulatu: There are not doubt, you have found out, without looking for it intentionally, an important aspect of electrochemica, ie, the active surface of the working electrode, it is less than the área of the electrode. Why?. the área it is not identical because the methods of clean are not very effective, and part of the area it is not active.
Regards
Dear Mulatu. If your electrode is a film model electrode, the different might be not too big (The surface is nearly flat). In the case your electrode is porous, the electrochemical surface area is much bigger. If your electrode was fabricated from powder, I recommend you measure BET of your active material.
Regards
It would help if you could tell us which kind of electrode are u dealing with?
Classical Randles-Sevcik equation is for the reversible processes only and you have to know the diffusion coefficient and the concentration of the electroactive species precisely. CV is not very good for such measurements. Much better for the determination of the surface area is the chronoamperometry because you can work at the limiting current plateau and plot current vs. 1/sqrt(t). In both techniques, at small diameter disk or spherical or cylindrical electrodes diffusion is not linear and equations must be modified. Of course your surface area must not be too large because of the IR drop. These techniques are based on diffusion so they are not sensitive to small surface roughness (see Bard & Faulkner, Electrochemical methods).
Another method is the impedance, where you can determine the real surface area from the capacitance measurements.
Thank you very much all!
Mr. Carlos José La Rosa and Aleksandar Karajic
My electrode(working electrode) is prepared in the laboratory from powder substance and changed in to rectangular shape.
Hi. Mulatu. An electrode surface is electroactive when is suitable for electrons transfer, ie, the rol of the of the active surface implies redox process. This means, that all the electrode surface is no homegeneous, and of course, the active surface it is always minor than the geometric. There are cases where the use of some chemical methods, laser or Chemical Vapor deposition (CVD) improves, significantly, the electrode active area. Some of these methods are very expensive with the exception of chemical methods.
Regards
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