Electrode Preparation Considerations

The application of steady-state methods to anodic organic oxidation reactions has been criticized because of the time variation observed which can lead to considerable irreproducibility. While the time variation remains a problem, if standardized procedures and electrode preparations are employed, good reproduction of data (within 10%) can be obtained. Steady-state methods are limited to the study of reactions involving a high energy of activation where diffusion control is not important. All of the anodic oxidations of organic species reported up to now have satisfied this requirement. The experimental details for steady-state methods have been well established in the literature and require no further elaboration. 

Unfortunately, while catalyst components and structures in low-temperature MEAs have attracted considerable attention, optimization of high-temperature catalyst layer structures and components seems little studied. Lobato et al. [83] investigated the effect of the catalytic ink preparation method on the performance of HT-PEMFCs. They employed two methods for catalyst layer preparation the solution method and the colloid method. In the solution method, catalyst ink was prepared by mixing the catalyst (20% Pt/C) and PBl solution (5% PBl in dimethylacetamide). In the colloid method, acetone was added to the mixture of catalyst and PBI solution, which made the PBI form a colloid suspended in the solvent. They found that electrodes prepared by the solution method showed better performances at 150 °C and 175 °C, and that the electrodes prepared by die colloid method gave a better performance at 125 °C. This is probably due to differences in catalyst layer structure (see Section 18.2.7). 

The experimental study of the initial stages of M02C electrocrystallization from tungstate-molybdate-carbonate melts with electrodes prepared from various materials over a wide temperature range allows us to put forward the following concepts of nucleation. Thus, using inert substrates at r< 1073-1173 K, we observed considerable crystallization hindrances associated with the formation of three-dimensional nuclei. An increase in the electrolysis temperature facilitates the diffusion of atoms of the components into the substrate, which results in a decrease of crystallization overvoltage. Simultaneously, a transition from three- to two-dimensional nucleation is observed and, in some instances, to depolarization phenomena due to solid-phase saturation of the boundary layers of the electrode with the components (molybdenum and carbon) and the formation of an alloy with the material of the electrode. 

Table 26 shows some steps in the chronological sequence of compilations, which are evidently related to improvements in the preparation and control of electrode surfaces. In second order, the control of the cleanliness of the electrolyte solution has to be taken into consideration since its effect becomes more and more remarkable with solid surfaces. A transfer of emphasis can in fact be recognized from Hg (late 1800s) to sp-metals, to sd-metals, to single-crystal faces, to d-metals, although a sharp chronological separation cannot be made. 

Nowadays, such hydride electrodes are used widely to make alkaline storage batteries which in their design are similar to Ni-Cd batteries but exhibit a considerably higher capacity than these. These two types of storage battery are interchangeable, since the potential of the hydride electrode is similar to that of the cadmium electrode. The metal alloys used to prepare the hydride electrodes are multicomponent alloys, usually with a high content of rare-earth elements. These cadmium-free batteries are regarded as environmentally preferable. 

Of considerable interest was the demonstration that metalloporphyrins and the like can be used as nonmetallic catalysts in electrochemical reactions, nourishing hopes that in the future, expensive platinum catalysts could be replaced. Starting in 1968, dimensionally stable electrodes with a catalyst prepared from the mixed oxides of titanium and ruthenium found widespread use in the chlorine industry. 

The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. 

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