Dear Md Nazmul Islam

The Electrochemical Active Surface Area (ECSA) always differs from the real surface area. ECSA again depends on the application of interest. The ECSA measured using standard ferri-ferro cyanide reaction helps in the measuring the ECSA of different modified electrodes. (i.e., Bare vs nanoparticle modified electrodes). More over the ECSA varies from one electrolyte to other.

For Noble metals such as platinum, palladium and their alloys, either oxide stripping or hydrogen adsorption and desorption was followed and the values were compared with theoretical values.

For gold surfaces it is better to measure the change in the double layer capacitance during the formation of the self assembled mono layer (Ex. thiols).

Hi Md Nazmul Islam

I use the Randles-Sevick equation with [Fe(CN)6] 3- regularly to find the area of Pt electrodes. Its simple, quick, and non-destructive, but it isn't a direct measurement of area. Any error included in the equation (poorly determined concentration, inaccurate diffusion coefficient, and so on), will cause inaccuracies in the area you determine for the electrode. If it's to be used, therefore, care must be taken when setting up your experiment. I usually see poor results for two reasons now (1) My test solution is poor and (2) My electrode is not clean.

It depends which area is relevant. The length scale varies with technique. You will always find a much larger area on atomic scale measurements such as gold oxide reduction peak areas or capacitance than you will get by diffusion methods which have a length scale of sqrt(2Dt). The IUPAC review by Trassati and Petrii in Pure and Applied Chemistry is the definitive source of information on methods and meanings.

Dear Md Nazmul Islam

Firstly, there is an important distinction to be made between the “surface area” and “geometric area” of an electrode. Surface specific methods, such as gold oxide stripping, will tell you about the former, as pointed out by Danny O'Hare , in his response. Redox mediator methods, such as the oxidation of Fc or reduction of [Fe(CN)6]3- will tell you about the latter. This is because even an atomically rough surface will look “flat” on the scale of the diffusion layer, hence the diffusion limited current is sensitive to the geometric area, rather than the true surface area (note that the ratio between the two, surface area/geometric area, is the “roughness factor” of your electrode).

As for your original question, while it is true that Prof Bond often uses the Randles-Sevcik Method to calibrate the geometric area of electrodes used in electroanalysis, two important assumptions need to be made in this case:

1) The reaction is electrochemically reversible (Nernstian) on the timescale of the experiment –this is always true for Fc on metal electrodes at scan rates less than 1 V/s, but may not be true for the case of [Fe(CN)6]3-, which can exhibit some deviation from reversibility on some electrodes.

2) The contribution from uncompensated resistance, that is iRu, is negligible.

The voltammetric peak current, ip, is influence by both the electron-transfer kinetics (k0) and iRu, making it a less than ideal form of analysis.

Another method that you could try is semiintegration, which converts your peak-shaped transient voltammogram into a sigmoidal-shaped semiintegral voltammogram. Unlike the peak current, the diffusion-limited plateau of a semiintegral voltammogram is insensitive to both k0 and iRu.

Details can be found in: Bentley, C. L.; Bond, A. M.; Hollenkamp, A. F.; Mahon, P. J.; Zhang, J., Advantages available in the application of the semi-integral electroanalysis technique for the determination of diffusion coefficients in the highly viscous ionic liquid 1-methyl-3-octylimidazolium hexafluorophosphate. Anal Chem 2013, 85 (4), 2239-45.

Hope this helps,

Cameron Bentley

Hi, Md Nazmul Islam

I do not think that the Randles-Sevick equation would be such a good tool for it. The sepecies are normally not adsorbed, so the roughness factor (rf) should be very small for this. During my Ph.D. I was T.A. for a electrochemical lab course and one of the experiments was like this, the students should measure the area of a Pt electrode using a caliper ruler, by RS equation in the Ferri-Ferro system and them by Hupd. Some groups also used Cotrell's equation for chronoamperometry instead of RS, and the conclusions were always the same, the rf was almost 1 by the redox couple and higher with the Hupd (we used the same paper from Trassati and Petrii as basis for Hupd measurements).

So, for Au the idea should not be different, so the idea that Rajendra Kumar Reddy Gajjala gave to use thiols should be a better option. In my works using Au (you can check 10.1021/acs.jpcc.7b11460), we used UPD of Cu on Au, which worked very well. The idea is very similar to the one proposed by Trassati and Petrii for Hupd (for Pt and similar behavior metals) or oxide layers, or the thiol idea, but in my opinion it is more well-behaved then thiol, especially if the carbon chain is big, you should observe some steric blocking.

As was said in Trassati and Petrii's work, the oxide layer couls also be used, but this should be very controled, for Pt, a friend published a work in which you can see the PtOH and PtO generation and the multi-layer oxide after the oxide inversion by electrochemical techniques ( 10.1016/j.elecom.2015.02.002 ), so the potential range to do apply this approach is limited and for Au, I think that it is not so well behaved, since the error is huge for the standard surface charge, you can check in Trassati and Petrii.

In a recent editorial in ACS Nano (10.1021/acsnano.8b07700) they also suggest the use of capacitive current for materials that do not present an Hupd region or it is not possible to use metallic UPD on it, it should also be a good idea, but I would suggest it for oxide materials, metallic ones you normally have an alternative...

Sorry for the long post, I hope this can help the discussion.

Best,

André Dourado

Dear Md Nazmul Islam

Please try to see the two paper , I hope this helps.

Kafia OULMI

I would also recommend UPD deposition as André H. B. Dourado proposed. We used this as well. Very well defined system.

Dear Md Nazmul Islam

Please try to see the two paper , I hope this helps.

Kafia OULMI