Amalgam-solution interface

An estimated profile of the diffusion layers across both sides of the amalgam-solution interface is shown in Fig. 13. Thus, the anodic current density (/) is given by... 

The potentials of half-cells that are constructed using amalgams are important whenever one is interested in performing reactions in a media in which hydrogen evolution at the metal-solution interface might interfere with the reaction of interest. Consequently, half-cell potentials for alkali metal amalgams are important to synthetic chemists and electrochemists alike. The standard potential for a metal amalgam in contact with a solution of its monovalent cation ( °(M" /M(Hg))) can be related to that of the pure metal in contact with its monovalent cation (EJ ) as shown in Eq. (14)... 

As discussed in Ref. 84, Li/Hg amalgam cannot be a model system for solid Li surfaces, because reduction of solution species on the liquid Li/Hg interface does not produce stable surface films. Thus, a massive solvent reduction may occur on Li/Hg in which each solvent molecule reacts directly with the bare active surface. In such a situation, PC and EC are indeed reduced directly to Li2C03 [84,131], However, R0C02Li species are major reduction products of PC and EC on Li/Hg as well. It should be noted that when the Li is initially covered by native surface films (Li20, Li2C03), the situation is more complicated. Only part of the native surface films may be replaced upon storage in the solutions thus, in such a case the nature of the surface films remains more inorganic than in the case of fresh Li surfaces [101-105], In any event, upon Li deposition or dissolution, the replacement of the native surface films by solution reduction products is fast and pronounced, and the above-described surface chemistry is very relevant to practical Li anodes in batteries. 

Multiphase system — An inhomogeneous system consists of two or more phases of one or more substances. In electrochemistry, where all processes occur at the interface thus all measurement systems must contain at least two - phases. In common understanding so-called multi-phase systems contain more than two phases. Good examples of such systems are -> electrode contacting a solid phase (immobilized at the electrode electroactive material or heterogeneous -> amalgams) and electrolyte solution, and an electrode that remains in contact with two immiscible liquids [i]. All phenomena appearing in such multi-phase systems are usually more complicated and additional effects as - interphase formation and -> mass transport often combined with - ion transfer must be taken into account [ii]. 

The generic process for electrochemical synthesis of sp-carbon chains was electrochemical reductive carbonization (corrosion) of poly(tetrafluoro-ethylene) (PTFE) by alkali metal amalgams, pioneered by Jansta and dousek [6 9] (for review see Reference 3). The reaction occurs at the interface of a dry contact between PTFE and alkali metal amalgams, hence, it does not seem to recall an electrochemical synthesis in its classical sense. The purely electrochemical carbonization of PTFE on a Pt electrode in aprotic electrolyte solution is also possible [3], but the amalgam-driven process is superior, presenting a clean and well-defined alternative to classical (wet) electrochemistry. 

The theoretical treatment presented (Eqs 4.1-4.5) is applicable also for direct wet electrochemistry on Pt cathode in aprotic electrolyte solution [12,13] (Table 4.1) and for some other chemical reductants, Rj, viz. benzoin dianion [14] and sodium dihydronaphthylide [15] (Table 4.1). Apparently, the decision between chemical and electrochemical carbonization may not be straightforward. The latter scenario requires a compact solid electrolyte with mixed electron/ion conductivity to be present at the interface. This occurs almost ideally in the reactions of solid fluoropolymers with diluted alkali metal amalgams [3]. If the interfacial layer is mechanically cracked, both electrochemical and chemical carbonization may take place, and the actual kinetics deviates from that predicted by Eq. 4.4 [10]. There is, however, another mechanism, leading to the perturbations of the Jansta and Dousek s electrochemical model (Eq. 4.4). This situation typically occurs if gaseous perfluorinated precursors react with Li-amalgam [4,5], and it will be theoretically treated in the next section. 

FIGURE 4.2.10. Concentration profile of Na inci Na" " near the interface between sodium amalgam and NaCI solution. 

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