Does an external field act on the pre-exponential factor?
How do activation energy and pre-exponential factor of electrocatalytic reactions depend on the structure of the interfacial water, and how can the two quantities be separated experimentally?
In an Arrhenius analysis, activation energy and pre-exponential factor are determined together from the temperature dependence. Temperature, however, changes the water structure and the reference potential at the same time.
EFCE provides a second variable that acts on the interface at constant temperature and constant potential. We suspect that an external field, via the orientation of the interfacial water, mainly affects the pre-exponential factor.
Hydrogen evolution at Pt with and without an additional field
- Arrhenius analysis of hydrogen evolution at Pt in 0.05 M H₂SO₄ at several temperatures in the established set-up.
- EFCE at the 5 µm Pt microdisc: the same temperature series with and without a constant additional field.
- Field sweep at fixed potential: current change per field change (dI/dF) as a function of temperature.
The field changes the pre-exponential factor at unchanged activation energy. The water structure thus becomes accessible isothermally.
The field acts on the interfacial state, not on the reaction rate. The control curve is then to be read exclusively as a state variable.
- Whether the field has a measurable influence on the faradaic current is unresolved; the field sweep at fixed potential is planned as the first experiment.
- The temperature dependence of the Hg/Hg₂SO₄ reference has to be taken into account in the evaluation.
- Tafel slopes alone do not allow the mechanism to be assigned; the evaluation is therefore based on the comparison with and without field.