Other solid electrode materials

Rotating optically semii-transparent electrodes for spectroelectro-chemical or photoelectrochemical studies can be fabricated by vapour deposition techniques on a quartz substrate. In this way, tin oxide, platinum and gold electrodes, amongst others, can be made. Electrical contact is with silver paint. 

In the study of the electrochemistry of single crystals, e.g. semiconductors, at rotating electrodes, an RDE can be fabricated with a shallow hole and the crystal, after appropriate machining, cemented in place with silver epoxy resin. 

Mercury is a very widely used electrode material for studying cathodic processes owing to its very high hydrogen over-potential however, its anodic range is small. For use in dropping electrodes, mercury purity is most important. Its purification has been described extensively and is in four parts. 

There are two useful tests for mercury purity which are easy to carry out. If base metals are present, the mercury will leave a thin film on a glass vessel. Secondly, if a small quantity of mercury is shaken in a stoppered flask with about three times its volume of pure distilled water, foaming will occur lasting for 5—15 s if it is pure. 

Mercury used for electrochemical purposes should be recycled distillation, once performed, should not be necessary again. 

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Other solid electrode materials used are semiconductors, for example metal oxides12,13, and conducting organic salts14. These last are of much interest at present for the immobilization of organic compounds such as enzymes, given their compatibility with these macromolecules (Chapter 17). 

When the source of material is a solid electrode, it is consumed under current. Therefore the lifetime of the electrode is limited. In S S E, care must be taken to press the electrode onto the SE continuously. At the other solid electrode, space must be left for the electrode to grow. In contrast, in LSE the LE stays always in good contact with the electrode. Furthermore, when the source of material is the LE, then material can be supplied indefinitely since the LE can be replenished. The geometry of LSE cells closest to all solid cells is when the LE is soaked in a porous solid. 

Although the HMDE is the most commonly used working electrode for CSV, the applications of other types of electrode materials have been reported. For example, silver electrodes have been used for the CSV determination of halides and sulfides. In addition, the use of carbon and platinum electrodes has been reported for metal cation determinations, including iron(II), cerium(in), manganese(II), thalliu-m(I), and lead(II). Such CSV measurements involve the precipitation of insoluble metal hydroxides on the electrode surface during the precipitation step. The drawback of the use of solid electrode materials is the poor reproducibility of the analysis and the low sensitivity, as a result of irregularities on the electrode surface and irreversible reduction of hydroxides (in the case of metals). 

Carbon is a commonly used solid electrode material as a substrate for biosensors, particularly in the form of glassy carbon, due to its wide positive potential window, mechanical stability, and low porosity. Carbon film, carbon composite and graphite are other substrates which have been investigated. Carbon film electrodes made from carbon film electrical resistors have been extensively employed by us as electrode substrate [43], obtained by pyrolytically coating a ceramic cylindrical substrate with a thin carbon layer. 

As described above, LiNi02 is an attractive material for lithium-ion batteries. However, it is difficult to operate high-volume lithium-ion batteries consisting of LiNiOj and (natural) graphite (or other negative electrode materials) safely for thousands of cycles. The difficulty is associated with the formation of nickel dioxide, so that it is hopeless to cope with this problem in a usual manner. However, it may be possible using a characteristic feature of the solid-state redox reaction of... 

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

In 1968 DairOlio et al. published the first report of analogous electrosyntheses in other systems. They had observed the formation of brittle, filmlike pyrrole black on a Pt-electrode during the anodic oxidation of pyrrole in dilute sulphuric acid. Conductivity measurements carried out on the isolated solid state materials gave a value of 8 Scm . In addition, a strong ESR signal was evidence of a high number of unpaired spins. Earlier, in 1961, H. Lund had reported — in a virtually unobtainable publication — that PPy can be produced by electrochemical polymerization. 

The type of electrode reaction that will occur depends on the electrode and electrolyte and also on external conditions the temperature, impurities that may be present, and so on. Possible reactants and products in these reactions are (1) the electrode material, (2) components of the electrolyte, and (3) other substances (gases, liquids, or solids) which are not themselves component parts of an electrode or the electrolyte but can reach or leave the electrode surface. Therefore, when discussing the properties or behavior of any electrode, we must indicate not merely the electrode material but the full electrode system comprising electrode and electrolyte as well as additional substances that may be involved in the reaction for example, ZnCl2, ag I (Clj), graphite [the right-hand electrode in (1.19)]. 

In addition to metals, other substances that are solids and have at least some electronic conductivity can be used as reacting electrodes. During reaction, such a solid is converted to the solid phase of another substance (this is called a solid-state reaction), or soluble reaction products are formed. Reactions involving nomnetaUic solids occur primarily in batteries, where various oxides (MnOj, PbOj, NiOOH, Ag20, and others) and insoluble salts (PbS04, AgCl, and others) are widely used as electrode materials. These compounds are converted in an electrochemical reaction to the metal or to compounds of the metal in a different oxidation state. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

With metals, semiconductors, and insulators as possible electrode materials, and solutions, molten salts, and solid electrolytes as ionic conductors, there is a fair number of different classes of electrochemical interfaces. However, not all of these are equally important The majority of contemporary electrochemical investigations is carried out at metal-solution or at semiconductor-solution interfaces. We shall focus on these two cases, and consider some of the others briefly. 

Solid polymer and gel polymer electrolytes could be viewed as the special variation of the solution-type electrolyte. In the former, the solvents are polar macromolecules that dissolve salts, while, in the latter, only a small portion of high polymer is employed as the mechanical matrix, which is either soaked with or swollen by essentially the same liquid electrolytes. One exception exists molten salt (ionic liquid) electrolytes where no solvent is present and the dissociation of opposite ions is solely achieved by the thermal disintegration of the salt lattice (melting). Polymer electrolyte will be reviewed in section 8 ( Novel Electrolyte Systems ), although lithium ion technology based on gel polymer electrolytes has in fact entered the market and accounted for 4% of lithium ion cells manufactured in 2000. On the other hand, ionic liquid electrolytes will be omitted, due to both the limited literature concerning this topic and the fact that the application of ionic liquid electrolytes in lithium ion devices remains dubious. Since most of the ionic liquid systems are still in a supercooled state at ambient temperature, it is unlikely that the metastable liquid state could be maintained in an actual electrochemical device, wherein electrode materials would serve as effective nucleation sites for crystallization. 

The literature reviewed in sections 2—3.6 has shown that oxygen reduction on Pt is quite complex, involving several possible rate-limiting (or co-limit-ing) steps. As we will see in sections 4 and 5, this complexity is a universal feature of all SOFC cathodes, with many of the same themes and issues reappearing for other materials. We therefore highlight below several general observations about the mechanism of Pt that frame the discussion for other solid-state gas-diffusion electrodes involving O2. These observations are as follows. 

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

By far the most important practical use of this sensor is for automotive applications, namely for the control of the air to fuel ratio. It compares favorably with the surface conductivity or high temperature potentiometric sensor (Logothetis, 1987). Other gases could be detected on the same principle provided that the right materials for the electrochemical pump were used. The electrode materials/solid electrolytes used for the construction of potentiometric high temperature sensors (see Table 6.7) could serve as guidance. 

In the case of solid electrodes, knowledge of the real surface area is a prerequisite for the proper evaluation of the activity with respect to other samples from the same laboratory, or for the comparison of results from different laboratories. It is very difficult to determine the real surface area because there are no unique techniques applicable to all materials. When the surface area determination is lacking, an evaluation in terms of synergetic effects can only be ambiguous. 

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