Electrochemical versus Chemical Reactions

Consideration of Chemical versus Electrochemical Reaction. Based on a study of the effect of pH on the relative contribution of the chemical versus electrochemical reactions in HF-NH4F solutions, Allongue proposed a reaction scheme 

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Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

It is worth mentioning that the use of microelectrodes (dimensions of micrometers or less) [248] allows the investigation of the electrochemical stability range of solvents without addition of a salt [249]. These studies allow a distinction between chemical and electrochemical reactions at the electrodes. In contrast to the results of Ue et al. [247], THF is not reduced at potentials down to —2 V vs Li/Li+, but oxidizes already at +4 V vs Li/Li+, whereas PC is stable up to 5 V, but already reduces at potentials of less than 1 V vs Li/Li+. Nonetheless, electrochemical stability ranges from CV experiments may be used for screening useful electrolytes. Table 17.9 shows the electrochemical stability limits of several nonaqueous lithium salt electrolytes versus different REFs. Furthermore the used WEs and the experimental conditions, like scan rate v and the onset current density io, with their references are listed. 

According to the literature [21], all reported electrochemical oscillations can be classified into four classes depending on the roles of the true electrode potential (or Helmholtz-layer potential, E). Electrochemical oscillations in which E plays no essential role and remains essentially constant are known as strictly potentiostatic (Class I) oscillations, which can be regarded as chemical oscillations containing electrochemical reactions. Electrochemical oscillations in which E is involved as an essential variable but not as the autocatalytic variable are known as S-NDR (Class II) oscillations, which arise from an S-shaped negative differential resistance (S-NDR) in the current density (/) versus E curve. Oscillations in which E is the autocatalytic variable are knovm as N-NDR (Class III) oscillations, which have an N-shaped NDR. Oscillations in which the N-NDR is obscured by a current increase from another process are knovm as hidden N-NDR (HN-NDR Class IV) oscillations. It is known that N-NDR oscillations are purely current oscillations, whereas HN-NDR oscillations occur in both current and potential. The HN-NDR oscillations can be further divided into three or four subcategories, depending on how the NDR is hidden. 

Changes in the scaling of chemical potential with /d [Nagaev, 1991, 1992 Parmon, 2007 Savinova, 2006]. This could account for the similar slopes of the SA versus 1 /d plots for different electrochemical reactions. 

Scheme 20. Redox chemistry of [Os,oX(CO)24] (X = C, H4). Unless specified, chemical reactions are as in CHjClj. Electrochemical potentials versus Ag/Ag. ...

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