Van Marum Colloquium - Microcalorimetric investigation of the effect of ions on surface processes - From double layer charging to catalytic reactions
- Dr. Marco Schönig (Karlsruhe Institute of Technology)
- Monday 19 September 2022
2333 CC Leiden
Composition and structure of the electrochemical interface are key factors in electrochemical processes, spanning from electrosorption to catalytic reactions. To gather information on the involved ions typically the potential-charge relationship is investigated in electrochemistry. However, species that are not involved directly in the reaction and only alter the mechanism or speed of the reaction or charge-neutral side-processes cannot be detected by charge measurements alone. A complementary observable to the electrochemical variables is the reaction entropy, which contains information on all involved species and is accessible by the electrochemical microcalorimetry.
With the electrochemical microcalorimetry the exchanged heat during electrochemically driven processes is measured, whereby the reversibly exchanged heat represents the reaction entropy and the irreversible contribution to the exchanged heat reflects the overpotential. This contribution focusses on the influence of ions on different electrochemical systems.
First the entropy variation during double-layer charging and specific anion adsorption on Au(111) will be discussed. To explain the measured variation a lattice-gas model, based on the quasi-chemical approximation is proposed. Additionally, the question whether SO42- or HSO4- is adsorbed from sulphuric acid solutions is addressed, which is a commonly discussed topic in the electrochemical community.
Afterwards the focus will shift to platinum surfaces, where in contrast to gold hydrogen underpotential deposition can be observed. In this regard, the effect of different ions and the pH on the reaction entropy of the hydrogen adsorption will be shown and from this the entropy of the adsorbed hydrogen species will be determined.
Finally, the hydrogen evolution reaction (HER) on platinum is addressed, where in addition to the reaction entropy also kinetic aspects will be discussed. While the variation of different ions does not alter the reaction entropy at a given pH, the kinetics of the HER is strongly influenced. To illustrate this point the time-resolved heat flux during the reaction will be analysed and the exchange current density will be determined.