We have developed a pyroelectric sensor for electrochemical microcalorimetry, based on LiTaO3, which provides unprecedented sensitivity for the detection of electrochemically induced heat effects. Deterioration of the heat signal by electrostriction effects on the electrode surface is suppressed by a multilayered construction, where an intermediate sapphire sheet dampens mechanical deformations. Thus, well textured thin metal films become viable candidates as electrodes. We demonstrate the sensor performance for Cu underpotential deposition on (111)-textured Au films on sapphire. The sensor signal compares well with a purely thermal signal induced by heating with laser pulses. The high sensitivity of the sensor is demonstrated by measuring heat effects upon double layer charging in perchloric acid, i.e., in the absence of electrochemical charge- or ion-transfer reactions.

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We measured the heat which is reversibly exchanged during the course of an electrochemical surface reaction, i.e., the deposition/dissolution of the first two monolayers of Ag on a Au(111) surface in (bi)sulfate and perchlorate containing electrolytes. The reversibly exchanged heat corresponds to the Peltier heat of the reaction and is linearly related to its entropy change, including also non-Faradaic side processes. Hence, the measurement of the Peltier heat provides thermodynamic information on the electrochemical processes which is complementary to the current-potential relations usually obtained by conventional electrochemical methods. From the variation of the molar Peltier heat during the various stages of the deposition reaction we inferred that co-adsorption processes of anions and Ag do not play a prominent role, while we find strong indications for a charge neutral substitution reaction of adsorbed anions by hydroxide, which would not show up in cyclic voltammetry.

We measured the molar Peltier heat during the course of Cu underpotential deposition (UPD) in sulfate-containing solutions by electrochemical microcalorimetry. The molar Peltier heat during Cu UPD deviates considerably from that of Cu bulk deposition in the corresponding solution. Since the molar Peltier heat directly reflects the reaction entropy of the involved electrochemical reaction, including also charge-neutral side processes, this finding signals significant contributions of non-Faradaic side reactions. For the first stage of the Cu UPD this process is known to be coadsorption of Cu2+ and sulfate species. We retrieved the potential dependent surface coverages of Cu and sulfate during the first Cu UPD stage with the single additional assumption that the Cu coverage in the first stage of the Cu UPD reaches 2/3 ML. In addition, we found that both HSO4(ad) and SO4(ad) are adsorbed on the surface with a noticeable surplus of bisulfate. For the second stage of Cu UPD, i.e., the completion of the first (1 × 1) Cu monolayer, our data indicates a charge-neutral side process, which cannot be inferred from the current potential relationship as measured, e.g., by cyclic voltammetry. This side process has considerably high positive reaction entropy and is present also for Cu UPD in perchlorate solutions. We interpret this side process as substitution of adsorbed sulfate or perchlorate anions by oxygen species upon completion of the first Cu monolayer, whereby the potential of zero charge shifts negatively.

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.