No abstract

We present a microcalorimeter for measuring heat effects during electrochemical reactions with conversions down to a few percent of a monolayer, referenced to the electrode's surface atoms. The design uses a thin pyroelectric polymer foil for temperature measurement at the backside of a thin electrode, similar to the concepts pioneered by the groups of D. A. King and Ch. T. Campbell for UHV adsorption microcalorimetry. To establish intimate thermal contact between electrode and sensor and utmost sensitivity, the free standing sensor and electrode foils are pressed together by air pressure, acting on the electrochemical cell. Pyroelectric temperature sensing is combined with pulsed electrochemistry, where the electrochemical heat is released on a time scale of about 10 ms, which is long enough for thermal equalization of the electrode-sensor assembly but short enough to avoid significant heat loss into electrolyte and cell compartment. As examples heat effects upon Ag deposition and dissolution as well as the electron transfer reaction of [Fe(CN)(6)](4-)/[Fe(CN)(6)](3-) are presented. The latter reaction was also employed for the calibration of the calorimeter.

Heat effects upon Cu deposition on polycrystalline Au surfaces from sulphuric acid electrolytes were calorimetrically measured. By combination of pyroelectric temperature detection at the backside of a thin electrode foil with pulsed electrochemistry, sensitivities to electrochemical conversions of a few percent of a Cu monolayer (ML), corresponding to about 1 microJ cm(-2) were achieved. We compared the heat evolution upon Cu under potential deposition (UPD), Cu deposition onto a fully developed Cu UPD layer and bulk Cu deposition onto a 300 ML thick Cu layer on Au. The heat effects were measured dependent on the amplitudes of the applied potential steps, that is, the driving forces of the respective reactions. From the differences of the heat effects among the Cu deposition processes, we deduced implications on the reaction mechanisms. For Cu UPD, the heat effects were explicable by the deposition of 1.3 Cu atoms per two electrons flowing to the electrode accompanied by sulphate coadsorption, similar to Cu UPD from sulphuric acid solutions on Au(111). Upon Cu deposition on a Cu UPD layer the heat effects signal considerable anion coadsorption up to the deposition of about 0.5 ML of Cu. At higher conversions the deposition mechanism gradually changes towards bulk Cu deposition, accompanied by reduction of the sulphate coverage.