Interphase electrochemistry

This review has been restricted mainly to clarification ofthe fundamentals and to presenting a coherent view ofthe actual state of research on voltaic cells, as well as their applications. Voltaic cells are, or may be, used in various branches of electrochemistry and surface chemistry, both in basic and applied research. They particularly enable interpretations of the potentials of various interphase and electrode boundaries, including those that are employed in galvanic and electroanalytical cells. 

This definition requires some explanation. (1) By interface we denote those regions of the two adjoining phases whose properties differ significantly from those of the bulk. These interfacial regions can be quite extended, particularly in those cases where a metal or semiconducting electrode is covered by a thin film. Sometimes the term interphase is used to indicate the spatial extention. (2) It would have been more natural to restrict the definition to the interface between an electronic and an ionic conductor only, and, indeed, this is generally what we mean by the term electrochemical interface. However, the study of the interface between two immiscible electrolyte solutions is so similar that it is natural to include it under the scope of electrochemistry. 

So, in general when two conducting phases are brought into contact, an interphase electric potential vill develop. The exploitation of this phenomenon is one of the subjects of electrochemistry and we can define electrochemical reactions as ones in which... 

When charges are separated, a potential difference develops across the interface. The electrical forces that operate between the metal and the solution constitute the electrical field across the electrode/electrolyte phase boundary. It will be seen that although the potential differences across the interface are not large ( 1 V), the dimensions of the interphase region are very small (—0.1) and thus the field strength (gradient of potential) is enormous—it is on the order of 10 V cm. The effect of this enormous field at the electrode/electrolyte interface is, in a sense, the essence of electrochemistry. 

It is clear that the adsorption of species in the metal-solution interphase region needs a subtle analysis. The unraveling of the complex situation and the building up of a basic picture of the accumulation and depletion of species at an electrified interface is one of the principal achievements of the new electrochemistry and is largely due to the American electrochemist, Grahame. 

R. Parsons, Equilibrium Properties of Electrified Interphases, in Modem Aspects cf Electrochemistry, J. O M. Bockris and B. Conway, eds., Vol. 1, Butterworths London (1954). 

Interfacial electrochemistry is about electric charges at interfaces between phases, one of which is an electron conductor and the other an ion conductor. The kinetic part of the subject is about the rate at which these charges transfer across the interphase. However, this definition clearly embraces two limiting cases. 

Figure 2.13 Model of the electrode-solution interphase as described by Bockris, Devanathan, and Muller [J. O M. Bockris and A. K. N. Reddy, Modem Electrochemistry, Vol. 2, Chap. 7, Plenum, New York, 1970.]...

Usually, in a given electrolyte solution, there is a similarity in the mechanism of SEI formation on carbon and metallic lithium.285 353 354 The mechanisms of SEI formation on lithium in numerous electrolytes are investigated since about three decades. In about the last 15 years, the focus continuously shifted from metallic lithium to carbon. There are a huge number of publications covering manifold aspects of the carbon s reactivity with the electrolytes and/or the SEI formation. The reader of this chapter is referred to the books published in this field recently and especially to the primary literature listed therein. Examples include Nonaqueous Electrochemistry from 1999 edited by Aurbach,355 Advances in Lithium-Ion Batteries from 2002 edited by van Schalkwijk and Scrosati,356 and Lithium-Ion Batteries Solid-Electrolyte Interphase from 2004 edited by Balbuena and Wang.281... 

In the mechanisms to be described in this section, one of the idealizations of electrochemistry is being portrayed. Thus, in perfectly polarizable metal electrodes, it is accepted that no charge passes when the potential is changed. However, in reality, a small current does pass across a perfectly polarizable electrode/solution interphase. In the same way, here the statement free from surface states (which has been assumed in the account given above) means in reality that the concentration of surface states in certain semiconductors is relatively small, say, less than 10 states cm. So when one refers to the low surface state case, as here, one means that the surface of the semiconductor, particularly in respect to sites energetically in the energy gap, is covered with less than the stated number per unit area. A surface absolutely free of electronic states in the surface is an idealization. (If 1012 sounds like a large number, it is in fact only about one surface site in a thousand.) A consequence of this is the location of the potential difference at the interphase of a semiconductor with a solution. As shown in Fig. 10.1(a), the potential difference is inside the semiconductor, and outside in the solution there is almost no potential difference at all. 

Since the advantage of using nonaqueous systems in electrochemistry lies in their wide electrochemical windows and low reactivity toward active electrodes, it is crucial to minimize atmospheric contaminants such as 02, H20, N2, C02, as well as possible protic contaminants such as alcoholic and acidic precursors of these solvents. In aprotic media, these contaminants may be electrochemically active on electrode surfaces, even at the ppm level. In particular, when the electrolytes comprise metallic cations (e.g., Li, Mg, Na), the reduction of all the above-mentioned atmospheric contaminants at low potentials may form surface films as the insoluble products precipitate on the electrode surfaces. In such cases, the metal-solution interface becomes much more complicated than their original design. Electron transfer, for instance, takes place through electrode-solution rate limiting interphase. Hence, the commonly distributed solvents and salts for usual R D in chemistry, even in an analytical grade, may not be sufficient for use as received in electrochemical systems. 

Ref. [i] Parsons R (1954) Equilibrium properties of electrified interphases. In Bockris JO M, Conway BE (eds) Modern aspects of electrochemistry. Academic Press, New York... 

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]. 

Refs. [i] Frumkin AN (1979) Potentsialy nulevogo zaryada (in Russian) (Potentials of zero charge). Nauka, Moscow [ii] Parsons R (1954) Equilibrium properties of electrified interphases. In Bockris JO M (ed) Modern aspects of electrochemistry, vol. 1. Buttersworth, London, p 103 [in] Trasatti S (1975) JElectroanal Chem 66 155 [iv] Hillier AC, Kim S, Bard AJ (1996) J Phys Chem 100 18808 [v] Thomas RC, TangyunyongP, Houston JE, Michalske TA, Crooks RM (1994) J Phys Chem 98 4493... 

These early observations serve to introduce a subject—the formation of mobile ions in solution—that is as basic to electrochemistry as is the process often considered its fundamental act the transfer of an electron across the double layer to or from an ion in solution. Thus, in an electrochemical system (Fig. 2.1), the electrons that leave an electronically conducting phase and cross the region of a solvent in contact with it (the interphase) must have an ion as the bearer of empty electronic states in which the exiting electron can be received (electrochemical reduction). Convo sely, the filled electronic states of these ions are the origin of the electrons that ente the metal in the... 

The surface excess is an integral quantity. This has the advantage of relieving us of the need to define the boundary of the interphase. On the other hand, it cannot yield any information on the variation of the concentration inside the inlerphase. Another point to remember is that the surface excess, as defined here, can have both positive and negative values. This statement is generally correct, but its validity can most easily be seen in electrochemistry. Thus, a negative charge on the metal (q < 0) causes a positive surface excess of cations and a... 

The differential relationships just derived represent the equivalent of the Maxwell equations in thermodynamics. Seldom used in electrochemistry, these equations have been employed in relation to the study of adsorption, particularly at the mercury-solution interphase. 

To be more exact, we should be talking about the interfacial tension, which is the surface tension between two specified phases. In electrochemistry it is customary to use the term surface tension to refer to the interfacial tension at the metal-solution interphase. 

The state of any electrified interphase depends on the potential. It is well known from the electrochemistry that the adsorption, electrosorption processes of ions and neutral molecules are strongly influenced by the potential of the electrode. 

At any given interface between two phases the properties of both phases close to the interface and, in particular, those of the topmost layers are different from those in the bulk. In order to separate this special portion of a system from both bulk phases the term interphase has been coined for this quasi-phase in between the bulk phases. This term considerably expands the two-dimensional view of the phase boundary as a simple interface between two completely homogenous phases. The particular properties of these interphases are of pivotal importance for their behavior in many areas of science and technology. In applied sciences an improvement of these properties is possible only with knowledge of these properties that is as broad and deep as possible. In electrochemistry the interphase properties are further complicated by the involvement of charged particles and extremely high electric fields. A broader overview of the electrochemical interface will identify further adjacent domains ... 

The clearest introduction to the electrochemical potential is given by its creator in Chap. 8 of E. A. Guggenheim, Thermodynamics (North-Holland, Amsterdam, 1%7). Applications to soil solutions are reviewed in Chap. 4 of G. Sposito, op. cit. The issue of electric potentials near interfaces is discussed in detail in R. Parsons, Equilibrium properties of electrified interphases. Modern Aspects of Electrochemistry 1 103 (1954). 

Posdorfer J, Olbrich-Stock M, Schindler RN (1994) Electrochim Acta 39 2005 Posdorfer J, Olbrich-Stock M, Schindler RN (1994) J Electroanal Chem 368 173 Hansen WN (1973) Internal reflection spectroscopy in electrochemistry. In Muller RH (ed) Advances in electrochemistry tmd electrochemical engineering, vol 9. John Wiley, New York McIntyre IDE (1973) Specular reflection spectroscopy of the electrode-solution interphase. In Muller RH (ed) Advances in electrochemistry and electrochemical engineering, vol 9. John WUey, New York... 

We have seen that the idea of an electrode film system is useful for electrochemistry of molten salts including low-temperature ionic liquids. It is not restricted, however, to this field only. As an example, the protective layer on lithium metal in aprotic organic electrolytes could be mentioned. This layer, so-called solid electrolyte interphase (SEl), exhibits properties of a polyfunctional conductor with high ionic conductivity (Li ions are the carriers) and low electronic conductivity of semiconductive nature. Some peculiarities of film systems with semiconductive character of electronic conductivity are considered below. 

Infrared spectroelectrochemistry has been established as a versatile tool to obtain detailed information about processes occurring in the electrochemical interphase and found application in many subfields of electrochemistry. Following shortly, important applications are summarized for more details, the reader is referred to the above cited reviews. 

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