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Thermodynamics and kinetics of electrochemical reactions

Foto Aufbau microcal
sketch Aufbau microcal
Experimental setup: We use a three electrode set-up consisting of reference- (RE), counter- (CE) and working electrode (WE). The pyroelelectric foil is in tight contact with the thin WE.
Transient
Potential- (E), current (I), and temperature (ΔT) transients for copper bulk dissolution. The reaction was conducted by a 10 ms potential pulse of 20 mV amplitude.
heatVSconversion
Absolute heat versus conversion for 3 different copper deposition processes. For details see Ref. [2]

 

The measurement of heat changes upon electrochemical reactions provides valuable information on the entropy of reaction as well as on possible irreversible reactions accompanying the charge transfer. However, in electrochemical environment calorimetric measurements usually require large conversions, due to the high heat capacity of electrode and electrolyte. We recently developed an experimental approach, which allows the investigation of electrochemical reactions with submonolayer conversions. We achieved the high sensitivity by adapting the calorimetric method, introduced by David King's and Charles Campbell's groups for the measurement of heats of adsorption in UHV [1], to electrochemical systems. Combining the use of a thin electrode-sensor assembly with pulsed electrochemical reactions resulted in sensitivities high enough for measuring heat changes upon conversions of a few percent of a monolayer (see figures 1 and 2). Calibration of the calorimeter, e.g., with the Fe2+/Fe3+ redox reaction allows quantitative determination of the evolved heat and therefore of thermodynamic quantities of electrochemical reactions with submonolayer conversion.

 

We employed electrochemical microcalorimetry for studying the electrodeposition of Cu on Au from sulphuric acid electrolytes under different potential and surface conditions. Differences in the entropies of reaction among the different deposition processes were attributed to coadsorption of anions as an important side reaction upon the Cu deposition process. Furthermore, entropic contributions to the free enthalpy were shown to play an important role for the stabilization of electrochemically formed adlayers.  E.g., in a Cu UPD layer on Au the entropy gain due to coadsorption of  anions compensates for lower binding enthalpy of Cu in the Cu UPD layer, if compared to Cu bulk (see figures 3 and 4).

 

A second project deals with the heat evolution upon electrochemically induced phase transitions in a dodecyl sulphate adlayer on Au, where the adlayer structure changes from a hemimicellar structure towards a densely packed bilayer, coinciding with additional adsorption of dodeclyl sulphate. Upon adsorption of dodecyl sulphate the displacement of water by the hydrophobic tail of dodecyl sulphate ions in solution is diminished, which results in strong entropy production and therefore measurable cooling of the electrode.

 

[1] J. T. Stuckless, N. A. Frei and C. T. Campbell, Rev. Sci. Instrum. 69 (1998) 2427.

[2] R. Schuster, R. Rösch and A. E. Timm, Z. für Phys. Chem., 221 (2007) 1479.

[3] K. D. Etzel, K. R. Bickel and Rolf Schuster, Rev. Sci. Instr. 81 (2010) 034101.

[4] K. D. Etzel, K. R. Bickel and Rolf Schuster, Chem. Phys. Chem. 11 (2010) 1416.