Electrochemical Surface Processing

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

Not much has been done in discussing ways to improve medical device surfaces using electrochemical coating methods in particular, with the aim of accomplishing the medically relevant goal, aside from discussion in the relevant literature, of the processing of nitinol (an alloy of nickel and titanium see Section 21.4). 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Diagnostic plots for heterogeneous catalytic electrode reactions at the RRDE have many features in common with those for simple parallel reactions [178]. This type of analysis is important in the investigation of the oxygen electrode reaction where non-electrochemical surface processes can occur. 

Radicals are generated at a tubular electrode and are then transported by laminar flow into the ESR cavity which, as a downstream detector, is analogous to a second electrode. The theoretical response for the cases where the radicals are stable or decompose by first- or second-order kinetics has been derived and experimentally confirmed [126, 301, 302]. The flow-rate dependence is different for each of the three situations which provides a diagnostic for the type of kinetics. Further information may be obtained from galvanostatic transients which allow the elucidation of electrode and radical surface processes [303]. Very recently, an in situ channel tube electrode has been described for electrochemical ESR which also allows shorter-lived species to be observed and smaller surface coverages to be analysed [304—306]. 

Cyclic voltammetry is also very useful for the study of adsorbed species1415,28-30. In the examples discussed above, it was assumed that the electroactive species and its reaction products are soluble in the solution and that surface processes can be neglected. However, if the shape of the peak is unusual (e.g. very sharp), the electrochemical reaction is probably complicated by surface processes, such as adsorption. Usually, adsorption of species favours the electrode reaction taking place at lower potentials in the case of a deposition, one speaks about under potential deposition. Different ways of adsorption can be obtained ... 

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). 

The importance of the defect electron in surface processes on ZnO is also demonstrated in electrochemical investigations 81). 

Modeling Electrochemical Phenomena via Markov Chains and Processes gives an introduction to Markov Theory, then discusses applications to electrochemistry, including modeling electrode surface processes, electrolyzers, the repair of failed cells, analysis of switching-circuit operations, and other electrochemical systems... 

Since the early work of Kanazawa [1] and Bruckenstein in 1985 [2], quartz crystal resonators have been used for more than 12 years in contact with liquids to assess changes in mass during electrochemical surface processes. Extensive use of the electrochemical quartz crystal microbalance (EQCM) has been done in the study of electrode processes with change of mass simultaneous to charge transfer. 

The kinetic expressions shown before explain the direct electrochemical processes. However, many of the processes with interest in electrochemical oxidation or coagulation treatments are not direct processes, but simply chemical processes caused by the products generated at the electrode surface (mediated electrochemical processes). In addition, several chemical processes not related to the electrochemical process can occur in the electrochemical cell. Thus, in electrooxidation, the most common case is the mediated oxidation carried out by oxidants electrochemically generated on the electrode surface, such as hydroxyl radicals, hypochlorite, peroxo-sulphates, or peroxophosphates. In electrochemical coagulation, aluminum species formed during the electrochemical dissolution of the anodes are responsible for the later coagulation reactions. 

Radiochemical methods are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows - adsorption and -> electrosorption occurring on the surface of electrodes the role of electrosorption in -> electrocatalysis -> deposition and dissolution of metals - corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes in these films study of the dynamics of - electrosorption and - electrode processes under steady-state and equilibrium conditions (exchange and mobility of surface species) electroanalytical methods (e.g., radiopolarog-raphy). 

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