Electrodes electronic conductor

The processes analyzed in this section refer to catalytically active molecules attached to the electrode (electron conductor), a case which has been named molecular electrocatalysis by Saveant [40]. 

When an electrode (electronic conductor) is contacted with an electrolyte (ionic conductor), it shows some potential and attracts ions with opposite sign, forming electrical double-layer at the electrode/electrolyte interface, as shown in Figure 17. lu. Increasing its electrode potential causes further adsorption of ions... 

Although powder routes have been successfully developed for synthesis of catalysts used in low temperature fuel cells, electrodeposition offers the highest noble metal utilization and is preferred over the alternative chemical methods. Electrodeposition enables the formation of catalyst particles on specific sites where they can be essentially utilized, i.e., the triple phase boundary where the membrane (ionic conductor), electrode (electronic conductor), and reactants meet. Powder methods do not guarantee that all catalyst particles are in contact with both electrode and membrane materials, and therefore, a portion of catalyst particles may remain inactive. 

Fuel cells, like batteries, are a variety of galvanic cells, that is, devices in which two or more electrodes (electronic conductor) are in contact with an electrolyte (ionie conductors). Another variety of galvanic cells are electrolyzers in which electric current is used to generate chemicals in a process that is the opposite of those oceurring in fuel cells and involving the conversion of electrical to chemical energy. 

The majority of solid electrolyte sensors are based on proton conductors (Miura et al. 1989, Alberti and Casciola 2(X)1). Metal oxides that can potentially meet the requirements for application in solid electrolyte sensors are listed in Table 2.7. These proton condnctors typically do not have high porosity but rather can reach 96-99% of the theoretical density (Jacobs et al. 1993). Similar to oxygen sensors, solid-state electrochemical cells for hydrogen sensing are typically constructed by combining a membrane of solid electrolyte (proton conductor) with a pair of electrodes (electronic conductors) Most of the sensors that use solid electrolytes are operated potentiometrically. The voltage produced is from the concentration dependence of the chenucal potential, which at eqnihbrium is represented by the Nemst equation (Eq. 2.3). 

Similar to liquid systems, solid-state electrochemical cells for gas sensing are typically constructed by combining a membrane of solid electrolyte (ion conductor) with a pair of electrodes (electronic conductors). As typical of all electrochemical systems, the interface of solid electrolyte and electrode... 

By injection of electrical energy and heat into the component, the water is decomposed into hydrogen and oxygen. More specifically, this is a redox reaction which takes place within the electrochemical component. Like all electrochemical components, an electrolyzer is made up of two electrodes (electron conductors) connected through an electrolyte (ionic conductor electron insulator). Depending on the nature of the electrolyte, the reaction will be different at each electrode. 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

When two different metals are immersed in the same electrolyte solution they will usually exhibit different electrode potentials. If they are then connected by an electronic conductor there will be a tendency for the potentials of the two metals to move towards one another they are said to mutually polarise. The polarisation will be accompanied by a flow of ionic current through the solution from the more negative metal (the anode) to the more positive metal (the cathode), and electrons will be transferred through the conductor from the anode to the cathode. Thus the cathode will benefit from the supply of electrons, in that it will dissolve at a reduced rate. It is said to be cathodically protected . Conversely, in supplying electrons to the cathode the anode will be consumed more rapidly, and thus will act as a sacrificial anode. 

Electrode an electron conductor by means of which electrons are provided for, or removed from, an electrode reaction. 

Galvanic Cell an electrochemical cell having two electronic conductors (commonly dissimilar metals) as electrodes. 

Before examining the processes in a cell, we should name the parts of a cell and clear away some language matters. The electrons enter and leave the cell through electrical conductors—the copper rod and the silver rod in Figure 12-5— called electrodes. At one electrode, the copper electrode, electrons are released and oxidation occurs. The electrode where oxidation occurs is called the anode. At the other electrode, the silver electrode, electrons are gained and reduction occurs. The electrode where reduction occurs is called the cathode. 

If these two electrodes are connected by an electronic conductor, the electron flow starts from the negative electrode (with higher electron density) to the positive electrode. The electrode A/electrolyte system tries to keep the electron density constant. As a consequence additional metal A dissolves at the negative electrode, forming A+ in solution and electrons e, which are located on the surface of metal A ... 

For increased power requirements, electrode constructions have been developed which bring the electronic conductors in closer contact with the active material particles first, around 1930, the sinter electrode [110], recently in sealed cells largely replaced by the nichel-foam electrode, and then, around 1980, the fiber structure electrode [111]. In order to take full advantage of their increased perform-... 

A quite different approach was introduced in the early 1980s [44-46], in which a dense solid electrode is fabricated which has a composite microstructure in which particles of the reactant phase are finely dispersed within a solid, electronically conducting matrix in which the electroactive species is also mobile. There is thus a large internal reactant/mixed-conductor matrix interfacial area. The electroactive species is transported through the solid matrix to this interfacial region, where it undergoes the chemical part of the electrode reaction. Since the matrix material is also an electronic conductor, it can also act as the electrode s current collector. The electrochemical part of the reaction takes place on the outer surface of the composite electrode. 

From Eq. (18) the concentration of electrons, and according to Eq. (11) the concentration of holes also, depend on the lithium activity of the electrode phases with which the electrolyte is in contact. Since anode and cathode have quite different lithium activities, the electronic concentration may vary to a large extent and an ionically conducting material may readily turn into an electronic conductor. 

At that time it was first reported that the catalytic activity and selectivity of conductive catalysts deposited on solid electrolytes can be altered in a very pronounced, reversible and, to some extent, predictable manner by applying electrical currents or potentials (typically up to 2 V) between the catalyst and a second electronic conductor (counter electrode) also deposited... 

At junctions between electronic conductors and electrolytes, the exchange is associated with continuing anodic and cathodic partial reactions. It therefore follows that equilibrium can be established for an electrode reaction only when this reaction is invertible (i.e., can be made to occur in the opposite direction). 

Two directions of current flow in galvanic cells are possible a spontaneous direction and an imposed direction. When the cell circuit is closed with the aid of electronic conductors, current will flow from the cell s positive electrode to its negative electrode in the external part of the circuit, and from the negative to the positive electrode within the cell (Fig. 2.2a). In this case the current arises from the cell s own voltage, and the cell acts as a chemical source of electric current or battery. But when a power source of higher voltage, connected so as to oppose the cell, is present in the external circuit, it will cause current to flow in the opposite direction (Fig. 2.2b), and the cell works as an electrolyzer. 

Experimental studies in electrochemistry deal with the bulk properties of electrolytes (conductivity, etc.) equilibrium and nonequilibrium electrode potentials the structure, properties, and condition of interfaces between different phases (electrolytes and electronic conductors, other electrolytes, or insulators) and the namre, kinetics, and mechanism of electrochemical reactions. 

These ideas can be applied to electrochemical reactions, treating the electrode as one of the reacting partners. There is, however, an important difference electrodes are electronic conductors and do not posses discrete electronic levels but electronic bands. In particular, metal electrodes, to which we restrict our subsequent treatment, have a wide band of states near the Fermi level. Thus, a model Hamiltonian for electron transfer must contains terms for an electronic level on the reactant, a band of states on the metal, and interaction terms. It can be conveniently written in second quantized form, as was first proposed by one of the authors [Schmickler, 1986] ... 

An electrode is an electronic conductor in contact with an ionic conductor, the electrolyte. The electrode reaction is an electrochemical process in which charge transfer at the interface between the electrode and the electrolyte takes place, and two types of reaction can occur, viz. ... 

The electrode is considered to be a part of the galvanic cell that consists of an electronic conductor and an electrolyte solution (or fused or solid electrolyte), or of an electronic conductor in contact with a solid electrolyte which is in turn in contact with an electrolyte solution. This definition differs from Faraday s original concept (who introduced the term electrode) where the electrode was simply the boundary between a metal and an electrolyte solution. 

Electrochemistry is the study of structures and processes at the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) or at the interface between two electrolytes. 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

A redox electrode acts as a reagent as well as an electron conductor, as the metal of an electrode can also be one component part of a redox couple. 

In general, an electronic conductor which is used to introduce an electric field or electric current is called an electrode. In electrochemistry, an electronic conductor immersed in an electrolyte of ionic conductor is conventionally called the electrode. Since the function of an electrode to provide electric current does not work in isolation but requires the presence of an electrolyte in contact with the electrode, the term of "electrode" is defined as a combination of an electronic conductor and an ionic electrolyte. Usually, an electrode is used in the form of its partial inunersion in an electrolyte as shown in Fig. 4-1 (a). It is, however, more common to define the electrode in the form of complete immersion in the electrolyte as shown in Fig. 4—1 (b). 

The electronic conductor of an electrode may be either a metal or a semiconductor, and the electrol3de may be either an aqueous solution, fused salt, solid electrolyte, or gaseous electrol34e (gaseous plasma). In this textbook, we deal mainly with metal and semiconductor electrodes in aqueous electrolytes. 

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