Dear All
I have been working on electrooxidation of formic acid. I have two catalysts. One of them
exhibits much higher activity (Catalyst A) than the other one (Catalyst B). Right now, I am trying to make tafel plot and determine their exchange current density (i0). However, I am not sure which the equilibrium potential of formic acid oxidation (FAO) is the correct one for me to calculate overpotential.
I found two "standard potentials" for FAO:
-0.25 V vs NHE
0 V vs RHE (from CO2 reduction papers)
When I use these two potentials to calculate overpotential for creating Tafel plots, the i0 of the Catalyst A is lower than Catalyst B, which does not make any sense. The equilibrium potential is way lower than the onset potential which I measured from exps. Since the Tafel slope of Catalyst A is lower than Catalyst B, the two straight lines will intersect eventually at the potential which is higher than the equilibrium potential.
I checked some papers and they all show the similar trend I get. The i0 of their good catalyst is lower if I use the potentials above to calculate overpotential.
My questions are:
1. What is the equilibrium potential of FAO?
2. Does anybody have the same problem? (None of the papers use "overpotential" as the y-axis when they made their Tafel plots. So, it is also very weird.)
Thank you for all your help!
Couple quick comments. "Tafel plot" has been derived for a reversible reaction. The oxidation of formic acid to CO2 is a multistep process and Tafel analysis might not be applicable for this reaction. The standard oxidation potential of formic acid is pH dependent
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