Electrochemical activation parameters

In a new twist on this subject, electrochemical activation parameters have been obtained for two series of redox couples that undergo coupled spin-state change and electron transfer (28). One series is [M(tacn)2]3+/2+ where M = Fe, Co, Ni, and Ru, and tacn = 1,4,7-triazacyclononane. The other is [Fe(pzb)2]+/0, where pzb-= hydrotris(pyrazol-l-yl)borate... 

Additional insight can be obtained by comparing the electrochemical and homogeneous activation parameters. Table II... 

Once again, it is important to recognize that these current-voltage curves pertain only to the situation in which the activation parameters of the electron transfer itself are controlling. This circumstance is unusual for most analytical electrochemical techniques, where it is not desirable to be rate-limited by the actual exchange of electrons at the surface. 

In addition to rate constants measured as a function of potential at a given temperature, electrochemical activation parameters obtained from temperature-dependent rate data also yield useful information. Unfortunately, such measurements have seldom been made by electrochemists, probably due largely to confusion on the most appropriate way of controlling the electrical variable as the temperature is varied and the widespread (al-... 

In comparison with the perturbations to rate constants induced by the electrode potential, relatively little attention has been directed to experimental examinations of temperature effects in electrochemical kinetics. This is probably due, in part, to uncertainties in how to control the electrical variable while the temperature is altered. However, as noted in Sect. 3.4, in actuality there are no more ambiguities in interpreting electrochemical activation parameters than for the commonly encountered Arrhenius par-... 

Electrochemical rate constants and activation parameters for inorganic electrochemical reactions span wide ranges e.g., standard rate constants vary from ca. 10 to > 10 cm s. For transition-metal redox couples, values of AH vary - from 0 to 80 kJ mol". As for homogeneous redox processes, it is highly desirable to elucidate the underlying reasons that are responsible for the observed rate parameters. In the following sections, the theory of electrochemical electron-transfer kinetics is outlined, followed by a discussion of some pertinent experimental results. 

Thus, as expected, temperature responses of linear voltammograms in case of reversible electrochemical reactions do not allow obtaining data on kinetic characteristics of the charge transfer proeess only activation parameters (i.e. pre-exponential factor and activation energy) of the diffusion process may be calculated from experimental dependences of peak current and peak potential on the temperature. 

---