Is it difference between the HCOOH (aq) (0 V) and maximum activation barrier height or the difference between the energy of HCOOH adsorption (-0.90 eV) and maximum activation barrier height (please see the attached Fig.)?
Hej Radhey,
you are putting forward a very important question. In the absence of computed activation barriers it is common practice to simple use the step with the highest redox potential (for oxidations) or the lowest redox potential (for reductions). This is the so called potential determining step. We have used this method for example to explore the oxidation of Glycerol over Au:
http://dx.doi.org/10.1021/acs.jpcc.8b02685
In your case you are presenting a more complete picture which also includes the barrier of electrochemical steps. These steps do of course also contribute to the experimentally measured current (see Butler-Volmer equation). So in principle this contribution would be have to be added when predicting the potential required to obtain a given current. But even if you would follow this path you would very likely still not predict the true onset potential since this value also contains many other contributions such as the conductivity of the material. In addition your error bars are typically also much larger than the accuracy of electrochemical measurements.
It is therefore more practical to simply split the discussion into 2 parts. In the first part you use the step with the highest/lowest potential to compute the thermodynamic onset potential and then complement this by a evaluation of the kinetics to determine the true path your reaction will follow. In your case this approach is also more honest since in principle also a non-electrochemical step with a too high barrier can block the overall reaction. In this case the reaction can not proceed through the mechanism even if one would increase the potential. So the true overpotential is determined by the remaining paths with lower activation barriers
We have used this approach for example in some of our more recent works:
https://doi.org/10.1016/j.electacta.2022.141784
doi.org/10.1002/cctc.202201671
So in a nutshell:
Use the thermodyamics to determine the approximate overpotential of the individual routes and complement this by a discussion of the kinetics to select the most likely path for which you then can suggest the thermodynamic onset potential of the reaction from the step with the highest redox potential.
Best wishes
Michael
Dear Michael Busch ,
I wanted to express my sincere appreciation for taking the time to answer my question. Your response and recommended articles were extremely helpful and provided valuable insights into my research. I am grateful for your expertise and willingness to share your knowledge.
Once again, thank you so much for your prompt and comprehensive answer.
Best regards,
Radhey
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