Phase formation in electrode reactions

Such electrode reactions frequently have a unique feature namely, the I-E characteristics before and after the formation of the new phase are quite different. For example, the I-E curves for a solution of copper ions at an inert electrode (e.g. carbon) and the same electrode covered with a thin layer of copper will be totally different, the latter being similar to a bulk copper cathode. Similarly the I E curve for a recharging lead/acid positive electrode will change dramatically 

While nucleation phenomena are important in electrochemical technology, it must be recognized that for most of a process the nuclei will exist and the main reaction is expansion of the new lattice as the deposit thickens. Hence we need to consider two distinct situations (i) the initial growth where a very thin layer or small centres of the deposit are created by nucleation and start to grow, and (ii) the subsequent growth to a macrophase. 

The total number of nuclei growing will depend upon the time dependence of the nucleation process. This usually follows a first-order law 

We shall again consider only the growth of hemispherical centres although similar equations can be derived for any shape of nuclei. Growth may be controlled either by kinetics or by mass transport. In the former case, the current at a fixed overpotential will depend only on the surface area iS of a nucleus 

This equation, however, gives only the current at a single nucleus and to obtain the observed I-t response we must combine equation (1.112) with the time dependence of nucleation, i.e. equation (1.109) or (1.110). Hence for instantaneous nucleation 

These features are due to the difficulties of nucieation, i.e. of forming small centres of a new phase. Throughout nature, nudeation is an improbable event small particles have a high area volume ratio and, hence, the positive surface-free energy exceeds the negative lattice energy and the nuclei should redissolve. 

Typical consequences of the difficulty of nucieation are the stability of supercooled water and the need to seed when recrystallizing many organic compounds. 

The formation of a thick eiectrodeposit requires several stages (1) nucieation  

If the deposit is an oxide or an anodic him, the mechanism of the growth process is quite different and dependent on (1) whether the film is an electronic or an ionic conductor and (2) whether both components of Ihe film come from the solution or, as is much more common, one lattice component is supplied by the solution and the other by the electrode. 

The formation of a thick electrodeposit requires several stages (1) nucleation (2) growth of the isolated centres (3) overlap of the centres into a continuous layer and (4) thickening of the layer. Each step will influence the properties of the final layer. 

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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. 

Intermediates are commonly formed in chemical reactions, as well as in electrode reactions. The preferred mechanism is that which involves the most stable intermediates, since this is the path of lowest energy of activation. For reactions taking place in the gas phase or in the bulk of the solution, the stability of different species can be calculated, or at least estimated, from existing thermodynamic data. This is not the case for electrode reactions. For the h.e.r. discussed earlier, a hydrogen atom was assumed to be an intermediate. The standard reversible potential for the formation of this species in solution, that is, for the reaction... 

On solid metals the situation for ion deposition or dissolution in electrode reactions is much more complicated. The models for crystal growth from the vapor phase or atomic evaporation have to be applied, being modified by ion discharge or ion formation in passing the electrical double layer at the interface. Figure 2.31 represents the main positions of atoms on the surface of a low index face of a metal with one monoatomic step. It is assumed that the edge of the step is not smooth and contains several kink sites. 

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent electrodes, we see that the incorporation of electroactive species such as lithium into a crystalline electrode can occur in two basic ways. In the examples discussed above, and in which complete equilibrium is assumed, the introduction of the guest species can either involve a simple change in the composition of an existing phase by solid solution, or it can result in the formation of new phases with different crystal structures from that of the initial host material. When the identity and/or amounts of phases present in the electrode change, the process is described as a reconstitution reaction. That is, the microstructure is reconstituted. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

The electrode reaction can involve the formation of a new phase ( e.g. electro-deposition processes used in galvanizing metals). The formation of a new phase is a multi-stage process since it requires a first nucleation step followed by crystal growth (in which atoms must diffuse through the solid phase to then become located in the appropriate site of the crystal lattice). 

In Sections 4.1 and 4.2, the electron transfer and the mass transport involved in a simple electrode reaction [simple = not complicated by preceding or following reactions, by absorption, or by formation of phases (see Section 2.2)] have been treated separately. However, it is to be expected that in reality both phenomena act in a concerted manner during a faradaic process. Thus, as seen previously, even the simple electrode process ... 

Therefore, this chapter provides a summarized data on the preparation of organic ion-radicals as independent particles that can be free or bound with counterions in ion pairs. The chapter considers liquid-phase equilibria in electron transfer reactions and compares electrode and liquid-phase processes for the same organic compounds. Isotope-containing molecules have specific features as ion-radical precursors, therefore, the generation of the corresponding ion-radicals is considered in Section 2.6 of this chapter. This chapter also pays some attention to the peculiarities of ion-radical formation in living organisms. 

The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of crown ether, the eight-membered complex depicted in Scheme 2.16 is destroyed. The unprotected anion-radicals of azoxybenzene are further reduced by cyclooctatet-raene dianion, losing oxygen and transforming into azodianion. The same particle is formed in the electrode reaction shown in Scheme 2.13. In the chemical reduction, stabilization of azodianion is reached by protonation. Namely, addition of sulfuric acid to the reaction results in the formation of hydrazobenzene, which instantly rearranges into benzidine (4,4 -diamino-l,l"-diphenyl). The latter was isolated from the reaction, which proceeded in the presence of crown ether. 

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

Adsorption. In the examples discussed above it was assumed that the reactants and products are soluble in the solution and that surface processes (adsorption of the reactants or products, and phase formation and removal) can be neglected. However, if the shape of the peak is unusual (sharp), the electrochemical reaction probably is complicated by a surface process. Cyclic voltammetry is especially sensitive to such phenomena and is a useful characterization tool.13 14 Adsorption of an electroactive species usually favors the electrode reaction that takes place at a lower potential. 

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