Electrochemical Kinetics

Motivation:

It is well-known and intuitive that the kinetics of redox reaction that involve protons must depend on pH. However, despite decades of research, the details of such dependence are still unclear, even for seemingly simple redox reactions. The dimensionless electron transfer parameter is a fundamental kinetic parameter which is a measure of the sensitivity of the transition state to the thermodynamic drive for the reaction . For an electrochemical reaction, within certain experimental constraints, an empirical manifestation of this is the rate of change of the logarithm of current with respect to the potential . When the kinetics of electron transfer is coupled to proton transfer, the above quantity becomes pH-dependent. Given the complexities of the interfacial protonation equilibria in the presence of an applied bias, it is expected that unraveling the dependence of on pH would be a non-trivial task. In our electrochemical work, we have taken steps towards this fundamental problem.

The pH-Dependence of Electron Transfer Coefficient:

We have used a relatively common 2-D free energy surface representation for the electron-proton transfer to understand this problem. We have used the approach by Koper and Schmickler to identify the influence of the experimental parameters, potential and pH, on this surface. We have extended Koper and Schmickler’s work and have recognized that the change in pH can shift the transition state position and consequently its sensitivity to potential. For proton reduction at high pH values, the transition state lies in the middle of the surface, favoring a concerted electron-proton transfer path. In contrast, at low pH values protonation of the electrode is readily possible and the transition state is that of the electron transfer. We have shown that in the concerted path the transition state is practically insensitive to pH, while in the step-wise path it shows linear dependence on pH. We have shown preliminary evidence for this in our experimental work on measuring for hydrogen evolution. Although, several complications exist in the experiment and in the in the interpretation, we have observed behavior that is qualitatively similar to the predictions of the model. (Link to paper)