Electrode Materials and Reactions

Tsipis EV, Kharton VV (2008) Electrode materials and reaction mechanisms in solid oxide fuel cells A brief review I. Performance-determining factors. J Solid State Electrochem 12 1039-1060 II. Electrochemical behavior vs. materials science aspects, ibid 1367-1391... 

The Combined Role of Electrode Material and Reaction Medium... 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

The success of an electrolysis process depends on the choice of a suitable electrochemical cell and optimal operation conditions because there is a widespread variety of electrolyte composition, cell constructions, electrode materials, and electrochemical reaction parameters. 

Because of the low-cost construction and simple operation, an undivided cell is always desired but it cannot be realized in all cases. A precondition for electrolysis in an undivided cell is that disadvantageous reactions and reaction products at the counter electrode can be avoided, for example, by selection of the electrode material and/or of the electrolyte composition. 

The decisive influence of the current density on the electrode reactions - including their selectivity - has been discussed in Sect. 2.3.2.1. Therefore, a uniform current density on the entire electrode area usually is indispensable. Precondition is a sufficiently constant overall cell resistance - of the electrodes, the electrolytes, and the cell separator - for every point of the electrode area. This needs satisfactory conductivities of the electrode materials and adequately dimensioned, symmetrical current feeders. For a constant electrolyte resistance, the electrodes have to be mounted parallel. This becomes more and more important... 

One of the most heavily studied factors thought to influence cathode performance has been the issue of reactivity between the electrode material and the electrolyte (usually YSZ) to form insulating secondary phases. This subject is sufficiently broad and complex to warrant its own review, and readers having a detailed interest in this topic are encouraged to read previous literature reviews in papers by Kawada and Mitterdorfer. Our main focus here is on how these secondary phases (or other impurities) appear to retard the reaction, particularly electrochemical kinetic processes occurring at the interface. 

Table 4.3. They vary with the electrode material and with the tetraalkylammonium cation used. Early workers used mercury electrodes but mercury may be involved in the overall reaction. Glassy carbon is generally favoured as the electrode material. Reproducibility of data depends critically on methods used for cleaning the glassy carbon surface [33].

The redox properties of an electrode are determined by its potential measured relative to some reference electrode. Many different reference electrodes are used in the literature. In order to make cross comparisons easily, most of the electrode potential quoted for reactions have been converted to the scale based on the saturated calomel electrode as reference. Electrode materials and electrolyte solutions used by the original workers are quoted. In many cases, the electrodes could be fabricated from more modem materials without affecting the outcome of the reactions. In the not too distant past perchlorate salts were frequently used as electrolytes. This practise must be discouraged for preparative scale reactions because of the danger of an explosion when perchlorates and organic compounds are mixed. Alternative electrolytes are now readily available. 

As in solution phase electrochemistry, selection of solvent and supporting electrolytes, electrode material, and method of electrode modification, electrochemical technique, parameters and data treatment, is required. In general, long-time voltam-metric experiments will be preferred because solid state electrochemical processes involve diffusion and surface reactions whose typical rates are lower than those involved in solution phase electrochemistry. 

The oxygen/water half-cell reaction has been one of the most challenging electrode systems for decades. Despite enormous research, the detailed reaction mechanism of this complex multi-step process has remained elusive. Also elusive has been an electrode material and surface that significantly reduces the rate-determining kinetic activation barriers, and hence shows improvements in the catalytic activity compared to that of the single-noble-metal electrodes such as Pt or Au. 

The point is that measurements of the dependence of the rate of the electrode reaction on important variables—potential, concentration, temperature, pressure, etc.—must be made under conditions in which the influences of contamination and diffusion control are eliminated. If, for example, when one varies the pressure, some other influence is at work (say, H diffusing into the electrode material and changing the properties of the surface), then of course an innocent analysis of the result of pressure variation (which may take many minutes to accomplish) as though the changes observed were due only to that variation and not contamination of the electrode surface, may yield puzzling conclusions. 

Moreover, each of the chemical and electrochemical reactions can have different reaction rates and reversibilities. All of them are reflected in cyclic voltammograms. If we measure cyclic voltammograms of an electrode reaction, changing parameters such as potential range, voltage scan rate, temperature, electrode material and solution composition, and analyze the voltammograms appropriately, we can obtain information about the electrode reaction. However, except for cases where the electrode process is very simple, it is not easy to analyze the cyclic voltammograms appropriately. 

Tab. 12.2 Influence of electrode materials and solvents on the reaction path of the reduction of C02...

In certain cases and together with the electrode reaction, in particular that of oxygen reduction at metal surfaces, a non-electrochemical regeneration mechanism operates which is heterogeneous in nature and involves the adsorbed product [165] obviously, it is very dependent on the electrode material and the available surface states. The reaction scheme is thus of the type... 

The electrode material and the electrolyte composition can be used as parameters for controlling the selectivity and reaction rate. 

Amperometry is the most widely reported EC detection mode for CE microchips, which primarily relies on oxidation or reduction of elect-rochemically active species by applying a constant potential to a working electrode. The current is then monitored as a function of time. Since it is based on the redox reaction that occurs at the electrode surface, electrodes can be miniaturised without loss in sensitivity. The relevance of this simple technique is reported in several reviews [48,74], In this section, a general overview of the combination of this detection technique to CE microchips together with special sections for different amperometric techniques and electrode materials and types are considered. 

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