Some Electrochemical Devices

This chapter has presented the current status of electrochemical, impedi-metric and field-effect solid-state gas sensors for CO2 detection. As can be inferred from the works reviewed, although there are already some electrochemical devices for CO2 detection in the market, there is significant room for improvement and new detection principles to investigate in order to realize cheaper and more reliable low-cost CO2 solid-state sensors. Researchers try to improve sensitivity, selectivity and long-term stability of the devices by exploring new fabrication methods that are cheap and reliable. As presented in Section 16.5.2, nanostructured materials are becoming more popular because of their improved sensing properties. Other approaches, such as the UV stimulation of metal oxide materials (also of nanostructured oxides), open new possibilities for room-temperature detection. 

Generally, the systems used for gas detection are heterogenous, that is to say of solid-liquid or gas-liquid type. However, since this book in only interested in devices using solid materials, we will not deal with the gas-liquid aspects particular to some electrochemical devices. 

At present, in electrochemistry, the most commonly studied ionic liquids belong to the family of the imidazolium-based ones. This is due to their low melting points and high electrochemical stability. The potential application of these ionic liquids in electrodeposition processes [26] and development of some electrochemical devices, such as biosensors [28] and lithium batteries [29], have been largely considered. Evidently, when accounting for toxicity and environmental persistence, these solvents do not fully address these important principles of green chemistry. 

The manufacture of ionic liquids on an industrial scale is also to be considered. Some ionic liquids have already been commercialized for electrochemical devices (such as capacitors) applications [45]. 

In this section we treat some electrochemical reactions at interfaces with solid electrolytes that have been chosen for both their technological relevance and their scientific relevance. The understanding of the pecularities of these reactions is needed for the technological development of fuel cells and other devices. Investigation of hydrogen or oxygen evolution reactions in some systems is very important to understand deeply complex electrocatalytic reactions, on the one hand, and to develop promising electrocatalysts, on the other. 

Despite the fact that in many cases, metal electrodes with adatoms are catalyti-cally highly active, they have found rather limited practical nse in electrochemical devices. This is dne to the low stability of these electrodes The adatoms readily nndergo oxidation and desorption from the surface, whereupon the catalytic activity is no longer boosted. In some cases, attempts have been made to extend the existence of the active condition by adding the corresponding ions to the working electrolyte of the electrochemical device so as to secure permanent renewal of the adatom layer. 

The development of solid state conducting solids that are on a par with liquid electrolytes has revolutionized the design of batteries and other solid state ionic devices (SSIs) in recent years, and this section explains the operating principles behind some of these devices. Figure 5.15 is a simple schematic diagram which we can use to explain the operation of several different types of electrochemical device. 

In this chapter, some characteristics and applications of gel electrolyte systems are reviewed, with emphasis on their application to electrochemical devices. We also include a report on our own work in this field. 

Capacitors can be polarized or non-polarized, depending on the - dielectric. Non-polarized devices have dielectrics consisting of ceramics or polymers (such as polystyrene, polyester, or polypropylene). They are normally box-shaped and their capacity is usually in the range from pF to pF, the maximum voltage up to 1000 V. Polarized capacitors are electrochemical devices the dielectric is an anodic oxide of A1 (pF to 100 mF, potentials up to 1000 V), Ta (capacities pF to 100 pF, potentials up to 20 V), or Nb (- electrolytic capacitor) or a double layer (- supercapacitor, capacities up to some 10 F and potentials up to 2.5 V or 5 V). Aluminum electrolytic capacitors are normally of cylindrical shape with radial or axial leads. Tantalum capacitors are of spherical shape and super capacitors form flat cylinders. 

For non-ohmic resistors, R is a function of current and the definition R = dV/dl is far more useful. This is sometimes called the dynamic resistance. Solid state devices such as thermistors are non-ohmic, and non-linear. A thermistor s resistance decreases as it warms up, so its dynamic resistance is negative. Tunnel diodes and some electrochemical processes have a complicated /-Vcurve with a negative resistance region of operation. 

In addition to commercial systems, there are quite a number of oscillator circuits that can be built from relatively inexpensive components to perform tiie essential measurements without the functions and convenience of a packaged instrument [22-28]. Both the commercial systems and most of these home-built oscillator circuits yield just one piece of information the resonant frequency of the TSM device. While this is sufficient for mass-loading-only applications like vacuum deposition of metal films, for some electrochemical processes, and even for appropriately selected chemically sensitive films, it can fall short when changes in the mechanical properties of a surface layer or contacting medium are significant [29]. 

Some examples of sohd electrolytes are presented in Table 1. In the hmited scope of this article, only a few examples of some of the most important (i.e. for potential commercial apphcations) monovalent cation (Li+, Na+, and H+) and anion (oxide and fluoride) conductors will be discussed. Amorphous materials, glasses, and polymers are treated in Section 4. However, it should be noted that relatively good ionic conductors are known with many other monovalent ions including K+, Rb+, Cu+, T1+, and Ag+, divalent ions, for example, Pb +, Ca +, Ba +, Zn +, Sn + in jS -alumina, and even trivalent cations,and tetravalent cations. In Section 5, the application of some of these materials in electrochemical devices including batteries, sensors, smart windows and fuels cells are discussed. 

Lithium is the most electropositive metal and the lightest consequently there has been a lot of interest in electrochemical devices using components that include Li metal electrodes and Li+ conducting solid electrolytes. Hundreds of materials have been studied for applications as potential hfhium conducting solid electrolytes. Tables 1 and 2 give representative examples of the more important Li+ sohd electrolytes, some of which are discussed in this section. 

Perovskite (ABO3 in which A is divalent and B is tetravalent) and pyrochlore (AaBaOi in which A is trivalent and B is tetravalent) oxide compounds have been proposed as oxygen ion conducting electrolytes for electrochemical devices. Some of the perovskite structures (e.g., BaCeO and SrCeOs) are generating interest because of... 

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