I am unable to understand whether i should draw 2 slopes or 1 for the attached tafel plot. Please help.
Hi there,
You need to plot it versus the overpotential of the reaction you are doing on the y-axis, not just against the reference electrode. From there, then you can identify the linear region more clearly, and you can get the useful data out of the equation for the line.
If you still have two major linear regions as it goes to higher overpotential, you may have a change in mechanism or another reaction occurring after that potential.
I hope that helps. You can also check out a standard electrochemistry text like Bard and Faulkner or Joe Wang's books for more info.
-Kyle Grice
Ok Sir then like for ORR the standard potential is 1.23V vs SHE ..So i have to deduct the potential from the standard and then plot it on y axis..
Helllo Disha,
Plot it such that the larger the overpotential goals, the larger the log of current gets. Then look to see how many linear areas you have. Often time the area near overpotential = 0 is excluded from the fitting.
Dr. Grice
Sir can you please explain me how tafel slope values are interpreted . I know that we can determine the mechanism out of this but i don't understand the significnace of tne no.(magnitude)??
see the book by Conway and S.Srinivasan--- fuel cells books too
the subject is well studied one
sometimes 2 electron transfer and 4 electrons too
see the role of H2O2 and OH radical
Hello, Disha!
What exactly you want? to determine the value of the slope? to interprete the value? to understand how to determine Tafel slope correctly?
As to the methodology of presenting the data : if you are sure, that the current values are purely kinetic and not influenced by diffusion, than you just determine the lgi (or ln i) and plot its dependence of the potential or overpotential. But: as ORR is a coplicated reaction, you cannot determine the value of overpotential just extracting 1.23 from the the potential (vs normal hydrogen electrode) that you mesured . So just feel free to plot E = f (ldi).
The next difficulty is how to choose the part of the plot which is relatively linear. On the figure you presented you managed to do this? Can you please tell me, how?
You know, there are papers when they try to inteprete the values 30 and 45 mV / decade and to see the difference of the mechanisms, but if you choose arbitrarily you can have 35 for one experiment, 42 for the other and so on. And what to do in this case.
For simple reactions when you carry out very precise experiments, you can freely try to determine and interprete Tafel slopes. But in case of ORR it's mostly just a convenient instrument to test yourself, the correctness and precision of you experiments.
And, by the way, what is the material used to catalyse ORR on your figure and the medium?
ma'am
actually I don't understand how to identify linear regions..as such I can find many ..i mean if there is any proper method to determine so..like for Pt there r two.
I am also unable to interpret what does slope value signifies.I know for good ORR SLOPE should haave a lower value
don't worry, nobody knows what the values signify. Till now nobody could prove if the first or the second or the third or finally the last electron transfer limitates the reaction rate.
Use Tafel slope for ORR just as the semi-quantitative and mostly qualitative parameter.
just curious, how did you make them stable ? both are not stable even if they are decorated as a core shell structure
You need to select few potential values starting from the onset potential from your LSV curve for X-axis or you can use Overpotential having at different scan scan rate
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