Electrons, flow across interfaces

Electrons, flow across interfaces, 8 Electron transfer processes. 427 Electrostatic potential, function of distance, from central ion, 242 Electrostatics, and work done, 366 Electrostriction, 185 calculated, 188 and other systems, 190 and volume changes, in water, 187, 189 Eley and Evans... 

This sounds like a grand definition. It is, and one can see what it means by looking at Figs. 7.1 and 7.2. In Fig. 7.1, one sees the first part of the definition (substances from electricity). Copper ions, invisible and dissolved in solution, are converted into visible metallic copper by means of the electrons flowing across the interfaces to the copper ions in solution. A new substance is produced by means of the flow of electricity. In Fig. 7.2, the reverse occurs One puts in a substance at one electrode and another substance at the other, and gets electricity So, electrochemistry has (as its name suggests), a chemical and an electrical side. 

If all interfaces remained at equilibrium, electrochemical devices would be limited in their possibilities. Substances could not be produced electrochemically neither would power production in fuel cells be possible. Net currents must flow across interfaces for devices to work. There must be net electronation or net dcclectronation. Interfaces need to move away from equilibrium and the corresponding Gal van i potential difference,... 

In all cases the electrode reaction secures continuity of current flow across the interface, a relay type of transfer of charges (current) from the carriers in one phase to the carriers in the other phase. In the reaction, the interface as a rule is crossed by species of one type electrons [e.g., in reaction (1.22)] or ions [e.g., in reaction (1.21)]. 

In electrochemistry, the electrode at which no transfer of electrons and ions occurs is called the polarizable electrode, and the electrode at which the transfer of electrons and/or ions takes place is called the nonpolarizable electrode as shown in Fig. 4-4. The term of polarization in electrochemistry, different from dipole polarization in physics, indicates the deviation in the electrode potential from a specific potential this specific potential is usually the potential at which no electric current flows across the electrode interface. To polarize" means to shift the electrode potential from a specific potential in the anodic (anodic polarization) or in the cathodic (cathodic polarization) direction. 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

Electroanalyte movement through solution. If electron conduction through the electrode and electron transfer across the interface are both fast, then the rate that limits the overall rate of charge flow will be that at which the electroactive material moves from the solution to approach close enough to the electrode for electron transfer to occur. 

From equation (3.4.31), if the ratio n/nso is unity there will be no net current flow across the interface this condition is depicted in Fig. 3.13(a) for an n-type semiconductor. Under this equilibrium state surface electrons can undergo isoenergetic electron transfers to the redox species due to a built-in potential, equal to the difference of potential between Ecb and Eredox- Equilibrium can be perturbed, with a resulting observable transient current flow, by varying the concentrations of the redox species. The surface electron concentration ng is related to the bulk concentration no by the potential difference of the space charge layer as follows ... 

As shown in Fig. 3.13(b) and 3.13(c) when ratio n/nsfl is less than or greater than 1 the system is in non-equilibrium resulting in a net current, with the electron transfer kinetics at the semiconductor-electrolyte interface largely determined by changes in the electron surface concentration and the application of a bias potential. Under reverse bias voltage, Vei > 0 and ns,o > ns as illustrated in Fig. 3.13(b), anodic current will flow across the interface enabling oxidized species to convert to reduced species (reduction process). Similarly, under forward bias, Ve2 < 0 and ns > ns,o as illustrated in Fig. 3.13(c), a net cathodic current will flow. 

The rectifier, or diode, is an electronic device that allows current to flow in only one direction. There is low resistance to current flow in one direction, called the forward bias, and a high resistance to current flow in the opposite direction, known as the reverse bias. The operation of a pn rectifying junction is shown in Figure 6.17. If initially there is no electric field across the junction, no net current flows across the junction under thermal equilibrium conditions (Figure 6.17a). Holes are the dominant carriers on the / -side, and electrons predominate on the n-side. This is a dynamic equilibrium Holes and conduction electrons are being formed due to thermal agitation. When a hole and an electron meet at the interface, they recombine with the simultaneous emission of radiation photons. This causes a small flow of holes from the jp-region... 

Thus, the electronation and deelectronation reactions modify the electric field across the interface, and the field, in feedback style, alters the rates until the rates of M+ + e — M and M — M+ + e become equal. This is equilibrium. Underlying the condition of zero net current, an equilibrium exchange-current density Iq, flows across the interface in both directions. The potential difference across the interface at equilibrium depends upon the activity ratio of electron acceptor to electron donor in the solution. Alter the ratio, and the equilibrium potential changes.14... 

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

Fig. 8.2. An early transient. Current density is constant. Potential builds up first through charging of the double layer, but at a higher potential, electrons pass across the interface, i.e., current flows and the double layer behaves as a leaky capacitor. The very early sections of the transient (double-layer condenser not leaking) can be used to obtain the capacity of the double layer because, there, there is a negligible Faradaic current through the interfacial region and the current goes overwhelmingly to charging the double layer. C = (dq/dV) = (idt/dV).

When an ion transfers at the interface between water, W, and an organic solution, O, a current flows across the interface, and the potential difference at the interface varies depending on the amount of the ion transferred, since the ion carries a charge similarly to an electron. Therefore, the ion transfer reaction can be regarded as an electrochemical process. Another electrochemical process at the W/0 interface is the electron transfer, which proceeds when a reductant or an oxidant in W comes into contact with an oxidant or a reductant, respectively, in O at the interface, resulting in an interfacial redox reaction. 

The simplest model of electron transfer across a semiconductor/metal interface assumes that the current depends linearly on the concentration of electrons at the semiconductor surface, s. It also assumes that the concentrations of acceptor and donor states in a metal are extremely large thus, these variables can be assumed to be constant and can be incorporated as time-independent quantities into the appropriate rate equations. This assumption is extremely reasonable for the moderate current densities that flow through typical semiconductor/metal interfaces. ... 

The first goal of this work is to develop a sound theoretical foundation for the description of ion transport along a channel. Once this description is established, it is possible to consider refinements interactions with channel wall vibrations and ion transfer across interfaces that control the flow of ions from solution, for example, into the channel. In this paper, I examine a model for ion transport in screened, but otherwise electrically neutral channels. Band states may exist for ions in such systems. There is evidence [10] that ion conduction channels do not need to have incorporated water to solvate mobile ions effectively aromatic pi-electrons are sufficiently polarizable to interact strongly with a simple cation to create an association that is as effective as water solvation. Thus, the models constructed assume only that the sources (molecules) that make up the channel walls... 

A model system for DSC was developed by Flynn (16) in which the electronic response of the instrument is coupled with the heat flow across an interface. Equations are derived that relate the time constants for this two-step process with the thermal properties of the sample and the amplitudes, areas, slopes, and dwell time of the DSC curves. Flynn (17) has also developed a simple theory to utilize DSC for the determination of heat capacities, glass transition, and enthalpies of transition. 

The faradaic current ip arises from any residual electroactive species that can be electrolyzed in this potential range, such as traces of metals or organic contaminants. Such impurities may be introduced by the supporting electrolyte, which will have to be purihed in some trace analyses. The charging current is nonfaradaic in other words, no electron flow occurs across the metal-solution interface, and neither redox nor permanent chemical changes result from its presence. The mercury-solution interface acts, to a... 

Fundamentals. A microelectrode with a small diameter (e.g. 10-20 pm, such an electrode is sometimes also called ultramicroelectrode (UME) [112-116]) is exposed to an electrolyte solution containing an electrochemically active substance. The electrode potential is adjusted to a value sufficiently negative to drive the electrochemical reaction O -h riQ R under diffusion control. Diffusion of reactive species to the electrode surface is hemispherical instead of planar, as in the case of large electrodes. The current I flowing across the solid/electrolyte solution interface of the microelectrode tip quickly reaches a steady state value /xa = nFDcr with n as the number of electrons transferred in the electrochemical reaction step, F the Faraday constant, D the diffusion coefficient of the reacting species, c its concentration and r the tip radius. The experimental setup is pictured schematically in Fig. 7.10. 

Figure 15 shows the situation when the Fe and Pt electrodes are shorted so that the metal are equal for the two electrodes. The potential drops across the interfaces change and electrons flow through the wire connecting them and an ionic current flows through the electrolyte. As a result of the ionic current, there is a potential drop in the solution. The couple potential of the Fe and Pt measured by a voltmeter using a reference electrode will now depend on... 

Further increase in forward bias allows electrons to flow across the Schottky barrier. These excess electrons bind with H+, form atomic hydrogen and gradually destroy the dipole layer at the interface, thereby losing the hydrogen detection sensitivity. 

When a metal, M, is immersed in a solution containing its ions, M, several reactions may occur. The metal atoms may lose electrons (oxidation reaction) to become metaUic ions, or the metal ions in solution may gain electrons (reduction reaction) to become soHd metal atoms. The equihbrium conditions across the metal-solution interface controls which reaction, if any, will take place. When the metal is immersed in the electrolyte, electrons wiU be transferred across the interface until the electrochemical potentials or chemical potentials (Gibbs ffee-energies) on both sides of the interface are balanced, that is, Absolution electrode Until thermodynamic equihbrium is reached. The charge transfer rate at the electrode-electrolyte interface depends on the electric field across the interface and on the chemical potential gradient. At equihbrium, the net current is zero and the rates of the oxidation and reduction reactions become equal. The potential when the electrode is at equilibrium is known as the reversible half-ceU potential or equihbrium potential, Ceq. The net equivalent current that flows across the interface per unit surface area when there is no external current source is known as the exchange current density, f. 

If the corrosion product on the metal surface is also an electronic conductor, the corrosion product does not hinder the flow of electrons, and the electrochemical reaction, instead of taking place at the metal/solution interface, occurs at the corrosion product/solution interface. In this case, if the process is under cathodic control, its total current may be also greatly increased by the presence of the corrosion product both for the possible greater catalytic activity of the corrosion product compared to that of the metal on the cathodic process (e.g., in the case of some iron sulfides with respect to hydrogen evolution) and for the much higher surface area of the corrosion product compared to that of the metal. On the other hand, if the corrosion product does not have the characteristics of an electronic conductor, the electron cannot flow across the corrosion product/solution interface, and the cathodic process occurs only on the limited free metal surface through the porosity of the corrosion product and with hindered diffusion. In the latter situation, the current density of the cathodic process has an upper limit, and it is drastically reduced. 

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