I am beginning to study oxygen reduction (ORR) at carbon surfaces in KOH. I have read a few literature papers and reviews and I realized that, almost every study deduces the mechanism (whether 2 or 4 electron transfer) from RDE and RRDE experiments. The unfortunate thing is, I don't have access to these;
1. Could the Randles-Sevcik relation be applied to obtain the number of electrons transferred? Will it be accurate? Otherwise, how can I know how many electrons are involved without RDE and/or RRDE?
2. It also appears there is a current density limit that can be used to judge whether the process is a 2 or 4-electron transfer one. Am I right with this ? Please enlighten me.
I will appreciate some papers
Thanks
At first you have do ORR voltammograms recorded at different rotation speed (like 400 rpm to 2400 rpm). From all the date you will get current density and correspondig rpm value. if you plot J‒1 vs ω‒1/2, you will get Koutecky–Levich (K-L) plots.from the slope of the line you will get the number of electron transfer in oxygen reduction reaction, was determinate by the K–Lequation.
1/J = 1/JL + 1/JK =1/Bω1/2 + 1/JK
where B = 0.62nFC0 (D0)2/3ʋ−1/6
where J is the current density, JK and JLare the kinetic and diffusion-limiting current densities, ω is the angular velocity of the disk, n is the number of electrons transferred in ORR, F is the Faraday constant (F = 96,485 C mol−1), C0 is the bulk concentration of O2 (C0= 1.2 × 10−6mol cm−1),ʋ is the kinematic viscosity of the electrolyte (ʋ = 0.01 cm2s−1), and D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10−5cm2s−1).
then put all the value in equation you will get the n value.
many papers are there regarding this.
You have here the answer, page 6:
http://www.doiserbia.nb.rs/img/doi/0352-5139/2007/0352-51390707699E.pdf
dear Bapi Bera, i can't find out the parameters used for calculating electron transfer numbers ( the bulk concentration of O2, the kinematic viscosity of the electrolyte, diffusion coefficient of O2 in 1 M KOH), can you tell me, thank you.
Dear Shifeng,
Most of the people did ORR in 0.1 M KOH electrolyte, because ORR activity in 0.1 M KOH very good compare to other concentration. So, the value of all the parameters are given in the literature in 0.1 M KOH electrolyte. But you can find those value in the attached document in below...{Sorry for late reply, i didn't get notified for this}.
Even we can find the No. of electron using ring current with following equation
Id = 4 [Id / Id+Ir/N]
where Id = Disk current , Ir = Ring current and N= current collection = 0.41
But the best method is deriving from K-L plot
you see following reference.
Article Bifunctional Electrocatalytic Behavior of Sodium Cobalt Phos...
Dear Isaiah Gyan,
I have read a recent articles on best way to find out number of electron transfer for ORR reactions.
Since you don't have access to Either RDE/RRDE it is very difficult to get things done. The method that you have suggested is no longer helpful to you ( Randles-Sevcik relation ).
The best way to find no of electron transfer is using RRDE method utilizing both ring and disk current current.
Please go through the attached reference where K-L method is proven to be wrong also you will get clear idea what needs to be done.
Article Determination of Electron Transfer Number for Oxygen Reducti...
1- Record the ORR voltammograms at a different rotation speed
2-Estimate the current density with rpm from the date.
3- plot the current with the angular velocity of the disk ω using Koutecky–Levich
4- Find the number of electron transfers in oxygen reduction
from the slope of the plot
4- Use relation 1/J = 1/JL + 1/JK
where JL=Bω1/2
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