Electrode lattice sites

Electrodes responding to other halides, sulphide, cyanide, silver, lead, copper and cadmium are made using membranes fabricated from pure or doped silver sulphide (Ag2S). The membrane potential is affected by the movement of Ag+ ions between cationic lattice sites which in turn is determined by the activities of the Ag+ ion in the internal and sample solutions. As the activity of the former is fixed, that of the latter alone influences the membrane potential. The electrode will also respond to the presence of S2- ions because of their effect on the Ag+ ion activity via the solubility product expression ... 

The presence of iron in nickel oxyhydroxide electrodes has been found to reduce considerably the overpotential for oxygen evolution in alkaline media associated with the otherwise iron free material.(10) An in situ Mossbauer study of a composite Ni/Fe oxyhydroxide was undertaken in order to gain insight into the nature of the species responsible for the electrocatalytic activity.(IT) This specific system appeared particularly interesting as it offered a unique opportunity for determining whether redox reactions involving the host lattice sites can alter the structural and/or electronic characteristics of other species present in the material. 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

Let us now consider a transfer reaction of iron ions Fc from the lattice site in a metallic iron electrode to the hydrated state of iron ion Fe in an aqueous solution at the standard temperature 298 K and pressure 101.3 kPa as shown in Eq. 9.34 ... 

A fluoride electrode, in which the membrane is a single crystal of lanthanum fluoride doped with europium to increase the conductivity, is one of the best ion-selective electrodes available. Conduction through the membrane is facilitated by the movement of F" ions between anionic lattice sites which in turn is influenced by the F ion activities on each side of the membrane. If the electrode is filled with a standard solution of sodium fluoride, the membrane potential is a function of the fluoride activity in the sample solution only. Thus,... 

See Bockris, loc, cit.f for detailed discussion. The deposition of metals from solvated ions is particularly difficult to study because of the speed and sensitivity to electrode surface of the electrode process. For deposition the chemical process on the surface may involve migration of a chemisorbed metal atom (or ion) to lattice sites. 

The adsorption of polymers, poly(vinyl pyridine) or poly(acrylonitrile) either to coordinate metal atoms or to adsorb biopolymers has been used to prepare chemically modified electrodes for immobihzation of enzymes either by physical or by chemical adsorption (carrier binding), cross-linking, and entrapping at lattice sites or in microcapsules [43]. A wide application of these types of electrodes has been made for electrochemical reactions of biological interest [44]. 

However, there is evidence that this reduction involves at least a charge-transfer step, creating an adsorbed silver atom adatom), and a crystallization step, in which the adatom migrates across the surface until it finds a vacant lattice site. Electrode processes may also involve adsorption and desorption kinetics of primary reactants, intermediates, and products. 

Materials for cathodes and anodes (insertion electrodes) for rechargeable lithium batteries are called intercalation compounds and constimte a special class of electroactive material [122]. The intercalation refers to the reversible insertion of mobile guest species into a crystalline host lattice, which contains an interconnected system of empty lattice sites of appropriate size, while the structural integrity of the host lattice is formally conserved [122]. 

Phase-formation phenomena at electrode-electrolyte interfaces can be conveniently treated with lattice gas concepts [38, 60, 67]. Such models consider that the entities, atoms, ions, or molecules, are fixed to particular cells /, j. (M. Fisher explicitly pointed out ... that instead of imagining the particles confined to lattice sites, one may suppose that they move continuously in space divided into cells, but that their interactions are determined solely by which particular cells are occupied [52].) The configurational energy of the adlayer on a (L x L) square lattice is given, as an example, by the following Grand Canonical Hamiltonian [38, 56, 57, 63]... 

It is possible, however, that an ion can interact chemically with the electrode material. If this happens the ion may break through the solvent layers or, as in the case of the solid, become displaced from a normal lattice site. This possibility is known as specific adsorption. In aqueous electrochemistry the locus of the centers of the specifically adsorbed ions is known as the inner Helmholtz plane. Neutral molecules may also adsorb and hence affect the faradic current, for example by blockage of the reaction sites. Neutral molecule effects have not been studied in the case of solid systems and will therefore not be considered further. 

DICKINSON I do not understand why you use the term chemisorption. In a solid electrolyte, chemisorption must surely involve either some degree of charge transfer between the ion and the electrode or some movement of the ion from its normal lattice site. Your model does not involve either of these phenomena, but you still describe the ions as being specifically adsorbed. 

Ionic transport in solid electrolytes and electrodes may also be treated by the statistical process of successive jumps between the various accessible sites of the lattice. For random motion in a three-dimensional isotropic crystal, the diffusivity is related to the jump distance r and the jump frequency v by [3] ... 

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