Phase boundaries, electron transfer across

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

When considering the energy changes associated with electron transfer across a phase boundary it is necessary to refer the energy levels on either side of the boundary to the same reference state. The potentials of redox couples in solution are generally expressed relative to the normal hydrogen electrode (NHE) ... 

The observed Volta potential does not change at a sufficiently large buffer capacity of the solution (from 5 to 100 mM Tris-HCI) [47,48]. This indicates that the potential shift at the octane/water interface results from electron transfer across the interface and not from the pH change in the boundary layer. During a redox reaction on BLM containing chlorophyll, a layer adjacent to the membrane is formed with a proton concentration different from that in the bulk phase [7, 38]. Boguslavsky et al. [47,48]... 

The presupposition is that parallel electrochemical reactions (i.e., ion or electron transfer) occur across the phase boundary, if the measured ions and interfering ions are both present in the solution. A redox process in which electrons pass the phase boundary is also considered an interfering electrochemical reaction. 

It will be shown further on that the phases on either side of the boundary become charged to an equal and opposite extent and this gives rise to a potential difference across the boundary. There are several ways in which this potential difference can arise. If one of the phases is an electronic conductor and the other is an ionic conductor, electron-transfer reactions can occur at the boundary and lead to the development of a potential difference. A discussion of this type of mechanism will be reserved for Section 7.5. Or, the electronic conductor can be deliberately charged by a flow of electrons from an external source of electricity. The electrolyte side of the boundary then responds with an equal and opposite charge, and a potential difference develops across the boundary. However, even without an external connection or the occurrence... 

Charge will spontaneously develop at the interface between two phases when there is a difference in the ease with which particles with charge of opposite sign can be transferred across the phase boundary. One example of this is at the interface between a metal and a solution, where metallic ions, but not electrons, can dissolve in the solution.10 Another example is at the interface between two metals, where electrons, but not ions, undergo rapid transfer. In the latter case, the electron transfer depends on temperature and forms the basis for measuring temperature differences by means of thermocouples. 

Charge-transfer overpotential — The essential step of an - electrode reaction is the charge (- electron or - ion) transfer across the phase boundary (- interface). In order to overcome the activation barrier related to this process and thus enhance the desirable reaction, an - overpotential is needed. It is called charge-transfer (or transfer or electron transfer) overpotential (f/ct). This overpotential is identical with the - activation overpotential. Both expressions are used in the literature [i-iv]. Refs. [i] Bard A], Faulkner LR (2001) Electrochemical methods. Wiley, New York, pp 87-124 [ii] Erdey-Gruz T (1972) Kinetics of electrode processes. Akademiai Kiadd, Budapest, pp 19-56 [Hi] Inzelt G (2002) Kinetics of electrochemical reactions. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 29-33 [iv] Hamann CH, Hamnett A, Viel-stich W (1998) Electrochemistry. Wiley VCH, Weinheim, p 145... 

Many different types of interfacial boundaries can be probed by SECM. The use of the SECM for studies of surface reactions and phase transfer processes is based on its abilities to perturb the local equilibrium and measure the resulting flux of species across the phase boundary. This may be a flux of electrons or ions across the liquid/liquid interface, a flux of species desorbing from the substrate surface, etc. Furthermore, as long as the mediator is regenerated by a first-order irreversible heterogeneous reaction at the substrate, the current-distance curves are described by the same Eqs. (34) regardless of the nature of the interfacial process. When the regeneration kinetics are more complicated, the theory has to be modified. A rather complete discussion of the theory of adsorption/desorption reactions, crystal dissolution by SECM, and a description of the liquid/liquid interface under SECM conditions can be found in other chapters of this book. In this section we consider only some basic ideas and list the key references. 

In evaporation-intercalation devices solar energy conversion would, at least in the more efficient case of a thermal system, not be converted by exciting electrons and rapidly separating them from holes, but by transferring atoms or molecules across a phase boundary by evaporation which is usually a very efficient process. It is, consequently, neither necessary to use materials which are well crystallized like those developed for photovoltaic cells nor is it necessary to prepare sophisticated junctions. A compacted polycrystalline sheet of a two-dimensional material which is on one side placed in contact with an electrolyte, sandwiched between the layer-type electrode and a porous counter electrode, as it is used in fuel cells, would constitute the central energy conversion unit. Some care would have to be taken to choose an electrolyte which is suitable for intercalation reactions and which is not easily evaporated through leaks in the electrodes. Thin layers of polymeric or solid electrolytes would seem to be promising. 

Photocatalytic processes in two-phase systems involve either a homogeneous photoreaction followed by the transfer of intermediate species across the interface, or a heterogeneous electron transfer between the photoactive species and the substrate. At the polarizable ITIES, both processes would manifest themselves by an increase in the current on illumination at constant potential, i.e., a photocurrent response. Indeed, photocurrent measurements have been recorded for the transfer of photogenerated ions at a liquid liquid boundary, as well as for heterogeneous redox quenching. We shall review some of these studies in this section. 

Periodic perturbations of the potential across the Hquid/liquid boundary induce a modulation of the concentration of species located in the interfacial region. By collecting the spectroscopic signals at the same frequency as that of the potential perturbation, employing phase-sensitive detection, the interfacial sensitivity of the measurements is tremendously enhanced, as the contribution from species in the bulk of the electrolyte solutions can be effectively neglected. Based on this principle, Fermfn and co-workers introduced potential-modulated reflectance (PMR) and potential-modulated fluorescence (PMF) to study a variety of processes including ion transfer [22], electron transfer [20], and the specific adsorption of ionic species [15]. 

The electrochemical reactions at the electrodes are causing charge transfer across the phase boundary between the electron conducting phase of the electrodes and the ion conducting phase of the electrolyte. 

In this chapter, the transfer of electrons across the phase boundary is examined, and the kinetics of interfacial charge transfer are considered from empirical and fundamental points of view. The treatment is restricted largely to simple charge-transfer reactions since mechanistic complications are dealt with elsewhere in this book. 

In lithium-ion batteries substances should be used as cathode material which can intercalate and discharge lithium ions at a highly positive potential - compared to the intercalation into the carbon anode - and with only low kinetic hindrance, i.e. at low over-voltage or nearly reversible. The first requirement is fulfilled especially by transition metal oxides and halides and also, to a lesser extent, by sulfides. The second requirement of low kinetic hindrance for insertion and release of lithium ions is meant as a requirement of high mobility of lithium ions and electrons within the cathodic lattice and of unhindered mass transfer across phase boundaries as far as phase transitions happen in the host lattice during in- and excorporation of lithium. As the transition metal halides are poorer electronic conductors than oxides, only the latter are used in practice. 

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