Acta Chimica 126. (1989)

1. szám - Szabó Kálmán–Mika József: Anomalous temperature dependence of the slope of the tafel equation in the case of H2 evolution on gallium electrodes

7* Acta Chimica Hungarica 126 (1), pp. 99—109 (1989) ANOMALOUS TEMPERATURE DEPENDENCE OF THE SLOPE OF THE TAFEL EQUATION IN THE CASE OF EL EVOLUTION ON GALLIUM - ELECTRODES Kálmán Szabó* and József Mika (Department of Physical Chemistry and Radiology, L. Eötvös University, H-1088 Budapest, Puskin и. 11—13.) Received July 2, 1987 Accepted for publication November 16, 1987 The temperature dependence of H2 evolution has been studied on solid and liquid gallium electrodes in perchloric acid — water — methanol mixtures. We have established that the slope of the Tafel straight line (6) shows an anomalous temperature dependence in both states, which can be explained for the liquid gallium electrode by the temperature dependence of the transfer coefficint, ß. In the case of the solid gallium electrode, the temperature dependence of b in the perchloric acid — water — methanol mixture differs from that on a liquid gallium electrode .This is probably due to the con­siderable temperature dependence of adsorption on solid Ga, and in addition to the tempe­rature dependence of ß, this also influences the course of the b—T curve. Upon comparing the above results with our earlier studies, we have established that on a gallium eectrode ß is a function of temperature also in aqueous solution, and this function may be written an ß = ßfj -f- ßgT. ßfj and ßs change with the composition of the solution, and there is a correlation between the corresponding value pairs. Correlation has also been found between activa­tion energy, E*, and ß^, and, on the other hand, between the logarithm of the preexpo­nential factor, lg A and ß$, as well as between E* and lg A. This latter correlation refers to a compensation effect. The trends of the above correlations are similar, in certain cases they are identical with those observed on a mercury electrode [4]. The study of the temperature dependence of the rate of electrode pro­cesses shows that the temperature dependence of the slopes of Tafel equations describing correlations between the overvoltage of H2(r?) and current density (j) usually cannot be represented by the relationship b = RT/ßF with the condition that the transfer coefficient ß is independent of temperature [1—5a, 7—9]. The deviation of function b —T at constant ß 0.5 was demonstrated first by Conway. According to his hypotheses, the anomalous temperature dependence of b can be derived from the temperature dependence of ß, if the adsorption or its temperature dependence for the components in the system are negligible [2, 3], which can be described as ß = ßH + ßsT * To whom correspondence should be addressed. Acta Chim. llung. 126, 1989 Akadémiai Kiadó, Budapest

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