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Accurate determination of the temperature dynamics at the electrode surface is crucial for advancing electrocatalysis, particularly in the development of stable materials that aid energy conversion and storage technologies. Here, lanthanide-based in situ luminescence thermometry was used to probe local heat effects at the platinum electrode surface during alkaline water electrolysis. It is demonstrated that the oxygen evolution reaction (OER) induces a more significant temperature increase compared to the hydrogen evolution reaction (HER) under the same electrochemical conditions. This difference is attributed to variations in overpotential heating and local effects on Joule heating. Furthermore, local heat effects are not observed at increased electrolyte concentrations during the HER, whereas substantial temperature variations (up to 2 K) are detected for the OER at higher electrolyte concentrations. Our observations highlight the potential of in situ luminescence thermometry to measure interfacial temperature effects during electrocatalytic reactions., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

We measure the entropy of formation of the interface upon anion adsorption (Cl - , Br - I - and SO 4 2- ) on Au(111) as an important indicator for the structure, order and composition of the interface. The entropy of formation of the interface exhibits a rather universal behaviour for all anions with a steep decrease upon initial adsorption followed by a shallow minimum at intermediate anion coverages and a strong increase close to the completion of the adsorbate adlayer. The strong variation of the entropy signals significant entropic contributions to the free enthalpy of the adsorption process and thus the stability of the adsorbed phase. At low anion coverages, close to the potential of zero charge, we attribute the entropy variations to the rearrangement of the interfacial water structure. At intermediate and high anion coverages, a comparison with the results of a lattice-gas model, considering pairwise repulsive interactions within the quasi-chemical approximation, shows that the entropy changes upon anion adsorption can be explained by the configurational entropy of the adsorbed phase. Thus, entropic contributions from both the solvent and the adsorbate are important for the stability of surface phases, particularly for disordered systems.

The front cover artwork is provided by Dr. Marco Schönig from Prof. Rolf Schuster's group at the Karlsruhe Institute of Technology. The image shows a gold surface under electrochemical control covered by two different, unspecified adsorbed species. In the inset, we present the result of our heat of reaction measurements, which showed that for Au(111) in sulfuric acid solutions the adsorbing anionic species is sulfate. Read the full text of the Research Article at 10.1002/cphc.202200227., (© 2022 Wiley-VCH GmbH.)

We investigate compositional changes of an electrochemical interface upon polarization with electrochemical microcalorimetry. From the heat exchanged at a Au(111) electrode upon sulfate adsorption, we determine the reaction entropy of the adsorption process for both neutral and acidic solutions, where the dominant species in solution changes from SO 4 2- to HSO 4 - . In neutral solution, the reaction entropy is about 40 J mol -1 K -1 more positive than that in acidic solution over the complete sulfate adsorption region. This entropy offset is explicable by a deprotonation step of HSO 4 - preceding sulfate adsorption in acidic solution, which shows that the adsorbing species is SO 4 * in both solutions. The observed overall variation of the reaction entropy in the sulfate adsorption region of ca. 80 J mol -1  K -1 indicates significant sulfate-coverage dependent entropic contributions to the Free Enthalpy of the surface system., (© 2022 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)