Measuring the heat during electrochemical reactions provides direct information on the reaction entropy of the electrochemical processes. These include also charge neutral side processes, e.g., coadsorption of anions with metal ions or ordering processes of the solvent at the interface, which will not show up in the electrochemical charge balance. Electrochemical microcalorimetry hence provides complementary information to the current voltage relationship as measured by conventional electrochemical techniques and may allow for the identification of side processes often obscured in electrochemical systems (1, 2). In this presentation we will present two examples, i) we will investigate anionic side processes during the Cu UPD formation on Au(111) in the presence of sulfate (3) and ii) we will address the reaction steps and the role of Cu⁺during the Cu-bulk deposition.

For Cu UPD it is well accepted that Cu deposition is accompanied by strong coadsorption of anions. From the course of the reaction entropy and the charge balance during the UPD process we determined the potential dependent anion coverages. From experiments with pH variation it followed that sulfate and bisulfate become adsorbed with roughly equal amounts. In addition we found highly positive reaction entropy upon the completion of the first metal monolayer, which signals a charge neutral substitution process of adsorbed anions by oxygenated species.

During pulsed Cu-bulk deposition we found that the heat evolution continues after the current pulse, i.e., without external current flow (see figure). This directly signals Cu deposition from Cu⁺ followed by a slow reestablishment of the Cu⁺/Cu²⁺ equilibrium, in agreement with the reaction mechanism of Mattsson and Bockris, proposed on the basis of the Tafel plot (4).

References:

1. S. Frittmann, V. Halka, C. Jaramillo and R. Schuster, Rev. Sci. Instrum., 86, 064102 (2015).

2. S. Frittmann, V. Halka and R. Schuster, Angew. Chem. Int. Ed., 55, 4688 (2016).

3. S. Frittmann and R. Schuster, The Journal of Physical Chemistry C, 120, 21522 (2016).

4. E. Mattsson and J. O. M. Bockris, Transactions of the Faraday Society, 55, 1586 (1959).