Charge across metal-oxide interface

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

When a zinc strip is dipped into the solution, the initial rates of these two processes are different. The different rates of reaction lead to a charge imbalance across the metal-solution interface. If the concentration of zinc ions in solution is low enough, the initial rate of oxidation is more rapid than the initial rate of reduction. Under these conditions, excess electrons accumulate in the metal, and excess cationic charges accumulate in the solution. As excess charge builds, however, the rates of reaction change until the rate of reduction is balanced by the rate of oxidation. When this balance is reached, the system is at dynamic equilibrium. Oxidation and reduction continue, but the net rate of exchange is zero Zn (.S ) Zn (aq) + 2 e (me t a i)... 

Among electrode processes with at least one charge transfer step, several different types of reaction can be found. The simplest interfacial electrochemical reactions are the exchange of electrons across the electrochemical interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface. The electrode in this case is merely the source or sink of electrons, uptaking electrons from the reduced species and releasing them to the oxidized redox species in solution. Examples of simple electron transfer reactions are... 

An electrode at which no charge transfer can occur across the metal-solution interface, regardless of the potential imposed by an outside source of voltage, is called an ideal polarized (or ideal polarizable) electrode (IPE). While no real electrode can behave as an IPE over the whole potential range available in a solution, some electrode-solution systems can approach ideal polarizability over limited potential ranges,. For example, a mercury electrode in contact with a deaerated potassium chloride solution approaches the behavior of an IPE over a potential range about 2 V wide. At sufficiently positive potentials, the mercury can oxidize in a charge-transfer reaction ... 

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. 

Any chemical transformation that implies the transfer of charge across the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) is referred to as an electrochemical reaction. An electrochemical reaction can include one or several electrode reactions. For example the reaction (1.3) is an electrochemical reaction each atom of iron that passes into solution implies the exchange of two electrons between the metal and the protons. Two electrode reactions are involved the oxidation of the iron and the reduction of the proton. According to the definition given above, all corrosion reactions that involve metal oxidation are electrochemical reactions. In order to understand and control corrosion phenomena it is essential to study the thermodynamics and kinetics of electrochemical reactions. 

As shown in Eqn. 26, the degree of oxidation or reduction of the electroactive coating is determined by the potential drop across the metal/polymer interface. Thus to describe the degree of charging of the system as a function of the electrode polarization E (relative to a reference electrode in the electrolyte), we must take into account the Donnan potential Disregarding short-range interactions of ionic... 

Two types of impedances are measured in electrochemical impedance spectroscopy (EIS) faradic and nonfaradic. Faradic impedance is associated with the process which involves transfer of charge across an interface. In faradic impedance measurement, a redox probe is used which is alternately oxidized and reduced due to transfer of electrons to and from the metal electrode resulting from the biological events occurring near the electrode surface. Nonfaradic impedance (mostly capacitive measurements) on the other hand is associated with transient flow of current or displacement current without actual transfer of any electron. In this case, no redox probes are required. 

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