Electrochemical Processes Involving Porous Materials

In this chapter, theoretical approaches for describing electrochemical processes in porous solids containing immobile redox-active centers will be discussed, whereas cases in which the entire porous material is electroactive will be further treated. In 

FIGURE 2.1 Schematics of possible combinations of conducting properties with redox-active centers distribution for porous materials. 

FIGURE 2.2 Schematic representation of possible configurations for studying the electrochemistry of microporous materials, (a) Discontinuous deposit of microparticles, (b) continuous layer, (c) material sandwiched between two electrodes. 

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Electrochemical data recorded under no steady-state conditions can also be used for studying electrocatalytic processes involving porous materials. In cases where the catalytic system can be approached by homogeneous electrocatalysis in solution phase, variation of cyclic voltammetric profiles with potential scan rate (Nicholson and Shain, 1964) and/or, for instance, square-wave voltammetric responses with square-wave frequency (O Dea et al., 1981 O Dea and Osteryoung, 1993 Lovric, 2002) can be used. This situation can, in principle, be taken for highly porous materials where substrate transport, as well as charge-balancing ion transport, is allowed. On first examination, the catalytic process can be approached in the same manner... 

An interesting but relatively unexplored aspect is the existence of site-characteristic electrochemical responses in porous materials containing electroactive centers. As far as the electrochemical processes involve an interfacial electron transfer between the electroactive species and the electrode surface, it is reasonable to expect that the kinetics of that electron transfer should be conditioned by the... 

A metal CMP process involves an electrochemical alteration of the metal surface and a mechanical removal of the modified film. More specifically, an oxidizer reacts with the metal surface to raise the oxidation state of the material, which may result in either the dissolution of the metal or the formation of a surface film that is more porous and can be removed more easily by the mechanical component of the process. The oxidizer, therefore, is one of the most important components of the CMP slurry. Electrochemical properties of the oxidizer and the metal involved can offer insights in terms of reaction tendency and products. For example, relative redox potentials and chemical composition of the modified surface film under thermodynamically equilibrium condition can be illustrated by a relevant Pourbaix diagram [1]. Because a CMP process rarely reaches a thermodynamically equilibrium state, many kinetic factors control the relative rates of the surface film formation and its removal. It is important to find the right balance between the formation of a modified film with the right property and the removal of such a film at the appropriate rate. 

In contact with aqueous alkaline media, metal oxide electrochemistry is dominated by hydroxylation processes. However, in contact with acidic media, proton and cation insertion processes occur, eventually leading to complicated responses where reductive or oxidative dissolution processes frequently take place (Scholz and Meyer, 1998 Grygar et al., 2002 Scholz et al., 2005). As far as such processes involve disintegration of the porous structure of the material, electrochemically assisted dissolution processes will be taken only tangentially here. 

Bsiesy et al. (1991) believe that electrochemical oxidation of PSi has the following advantages (1) electrochemical oxidation of porous silicon can be achieved easily and (2) it is possible to oxidize either the lower part of the porous layer, or the whole depth, at a level which depends on the exchanged charge. This method therefore appears to be more attractive than thermal oxidation when incomplete oxidation is required. In particular, such a requirement appears during silicon (or other material) epitaxy on porous silicon. These processes generally involve temperatures above 400 °C and porous silicon must be stabilized by a preoxidation step in order to conserve its very thin microstructure. If this preoxidation is achieved by thermal oxidation, there is also oxide growth on top of the sample, which must be eliminated before subsequent epitaxy. Electrochemical oxidation, with an appropriate choice of experimental conditions, can lead to oxidation limited to the inner part of the porous layer. 

In the industrial applications of electrochemistiy, the use of smooth surfaces is impractical and the electrodes must possess a large real surface area in order to increase the total current per unit of geometric surface area. For that reason porous electrodes are usually used, for example, in industrial electrolysis, fuel cells, batteries, and supercapacitors [400]. Porous siufaces are different from rough surfaces in the depth, /, and diameter, r, of pores for porous electrodes the ratio Hr is very important. Characterization of porous electrodes can supply information about their real surface area and electrochemical utilization. These factors are important in their design, and it makes no sense to design pores that are too long and that are impenetrable by a current. Impedance studies provide simple tools to characterize such materials. Initially, an electrode model was developed by several authors for dc response of porous electrodes [401-406]. Such solutions must be known first to be able to develop the ac response. In what follows, porous electrode response for ideally polarizable electrodes will be presented, followed by a response in the presence of redox processes. Finally, more elaborate models involving pore size distribution and continuous porous models will be presented. 

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