Interpretation and Significance of Heats of Activation for Electrochemical Reactions Exhibiting Anomalous Tafel Slopes

Abstract

Heats of activation for electrochemical reactions studied over a wide range of temperatures are considered in relation to the temperature dependence of the Tafel slope factor b. Experimentally, b is found to have anomalous temperature dependence, i.e., other than the classical form RT/αF , for hydrogen evolution at Hg, Cd, Pb, Ni, and Pt and the consequences of this behavior in calculation of heats of activation are evaluated. Curvature of the electrochemical Arrhenius plots is shown to arise for certain of the cases treated and is discussed in relation to the problem of detection of proton tunneling at low temperatures. Other difficulties associated with temperature dependence of coverage by atomic H and temperature‐dependent structure changes in the solvent are considered.

Significance of Effects of High Pressure on Kinetics of Electrode Reactions: I . Applications to Transition States in Hydrogen Evolution Reaction Mechanisms

“…At 104 bars in aqueous solution it gives a 20% effect on rates but appears to have been neglected in previous work. It is seen that evaluation of AVt~ for an electrode process involves problems similar to those arising (13,14) in evaluation of the activation energy of electrode processes from determinations of the temperature dependence of currents at a given ~1 as pointed out in Ref. (7).…”

“…Then from Eq. [8] AVt,24= = AVa,2+ + AVe(1 + ~ --0H) AV1 [14] Under (experimentally inaccessible) conditions of constant r the true volume of activation for the overall processes [I] and [II], with [II] rate controlling, would therefore be 1 hVt,24= : ~V24= -~ (1 --OH)aV1 [15] or with Eq. ] 14]…”

“…In the present paper, pressure effects on the Fe(CN)6 ~--~ e ~ Fe(CN)64-reaction at a gold electrode are reported. This type of reaction is of special interest with regard to current theories of electron transfer (7)(8)(9)(10)(11)(12)(13)(14)where the reorganization (10) of the initial state is involved in the activation process for electron transfer. One of the questions involved is the extent to which the activation process involves longrange solvent polarization fluctuations (10,11) around the initial state ion, or more specific short-range changes of hydration shell configuration or inner coordination shell reorganizatiOn (7,8,12).…”

Effect of temperature on cyclic voltammograms during the electrodeposition of lead

“…10). The slopes of these curves (d log zld T 1 ) give the corresponding AHS values according to the equation (29). [3] …”

Temperature dependence of kinetic parameters related to oxygen electroreduction in acid solutions on platinum electrodes

“…Conversely, Damjanovic and co-workers [11][12][13] have reported values of (bT)ncn increasing with temperature in diluted aqueous HCIO 4 and H2SO 4 below 70°C, as predicted by classical theories of electron transfer reactions at electrified interfaces. Reports by Conway and McKinnon on the hydrogen evolution reaction (HER) in ethanolic and methanolic acid solutions [14], and by Conway et al on the bromine evolution reaction (BER) on graphite [15], have shown that the temperature dependence of (bT)nC D is related to the nature of the electrode and the operating temperature range [14,16,17]. It is clear that, despite the number of works about OERR on PC Pt and well-defined Pt electrodes [17][18][19][20][21], there are some fundamental kinetic aspects which are still far from being well established.…”