I have searched on Research Gate and others have asked the same question of why the estimated electron transfer number calculated from Koutecky–Levich (K-L) plots is sometimes over 4 but this has yet to be answered.
I have also experienced this from my own samples and I am not sure what to conclude or what to do about this. I have been recommended to try a lower mass loading (currently using 0.4 mA/cm2) but this is already relatively low and I would not think this should solve the issue as the obtained value should be the same if normalized.
Any thoughts or advice are appreciated.
My suggestion for you is to: (1) do iR correction on the recorded polarization curves, and (2) subtract the ORR polariztion curves by pure CV recored in N2-saturated electrolyte.
You did not specify what electrode and catalyst(s) were used in your experiments.
The slope of the plot is more important, I also experienced this kind of problem. I think in my case the error from instrument which means the rotation they have mentioned is no longer correct. Hence, i just manually adjust the rpm by the calculation and got exact 4 electron transfer. However, we can't do like that, so make sure your instruments.
I also faced same problem. I used Cu-Ag bimetallic catalysts. I realized that in N2 saturated solution, there are reduction peaks that belong to surface oxide ( this result was confirmed by XPS). Therefore, because of this contribution, calculated electron number is higher than 4.
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