Cathode Materials General Requirements

A further argument for the choice of the cathode material may be the catalytic activity for hydrogenation reactions. Vice versa, this is also important if the cathode is the counter electrode - usually evolving hydrogen - where hydrogenation reactions are undesired. 

Anode Investigations using cyclovoltammetry confirm an important effect of surface oxides (see Vols. 3, 4). A known example of the different anodic activity is the poisoning of platinum by adsorbed carbon monoxide species, for example, in the direct methanol fuel cell (DMFC), 

Anodic corrosion in case of platinum metals mostly is insignificant or at least small for most anolyte compositions and conditions. But it may be an economic problem for industrial applications. Furthermore, as aforementioned, it can be the reason of cathode poisoning. The corrosion rate of gold, and especially of the less noble metals, is very dependent on the pH value of the anolyte. 

Cathode Platinum metals, especially platinum and palladium, achieve the lowest known overvoltages for hydrogen. Moreover, they are effective catalysts for hydrogenation reactions [15]. 

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In general, only atoms in the flame that are the same as in the hollow cathode material can absorb the specific lines emitted by this material. The only requirement of the monochromator, then, is to isolate the desired line from other lines of the cathode material and the lines of the filler gas. One line of the element is usually absorbed more strongly than others (it has a higher oscillator strength ). This often, but not necessarily, corresponds to the electronic transition from the ground state to the lowest excited state. This line is selected for maximum sensitivity measurements. For high concentrations, a line with a lower oscillator strength may be selected. 

Unlike the constraints on anode material, the constraints on cathode materials are usually lower because typically they do not need to constitute the transparent electrode material. In certain instances, where a completely transparent OLED is needed (windshield and heads-up displays), ITO may also be used as the cathode with suitable modification [12]. In general, cathode materials are pure metals or metal alloys. The requirements for cathode materials are as follows ... 

A number of "multi-element" lamps are also available commercially. Various metals, in powdered form, are mixed in predetermined ratios, pressed and sintered to produce the cathode material. However, only certain combinations are practicable. The obvious advantages of this format is that fewer lamps are required and the time required to switch from one element to another is minimised, however, the intensity of the emissions are generally lower than from the corresponding single element lamps. 

The porous cathode is generally made of Sr- and/or Co-doped LaMnOa perovskite mixed conductors. The LaCoOa (Sr-doped) material exhibits a high oxygen diffusivity and is therefore very attractive from an electrochemical point of view, but unfortunately suffers from an unacceptably high reactivity with zirconia and high coefficient of thermal expansion. The LaMnOa electrode (doped with Sr) on the other hand, is predominantly an electronic conductor. Various compositions are being considered to optimise the required properties. 

James Hunter of Eveready Battery Co. was the first to patent spinel cathode material. The application of material to Li-ion system has been developed by J. M. Tarascon [59] and extensively studied by M. Thackeray [60]. Generally, lithium spinel oxides suitable for the cathode are limited to those with a normal spinel in which the lithium ions occupy the tetrahedral (8a) sites and the transition-metal ions reside at the octahedral (16d) sites. Currently, spinel is the center of much interest as the cathode material for large format lithium-ion cell for hybrid electric vehicle applications where high power, safety, and low cost are the strongly required features. 

Primary batteries are generally available in two basic form factors, cylindrical and coin. Within these form factors, the arrangement of the working electrodes can vary considerably depending on the volumetric differences in anode and cathode materials, changes in the volume of these materials during electrochemical discharge, the application current, and the necessary interfacial surface area need to support the current Additionally, factors such as material electronic conductivity, electrolyte ionic conductivity, separator requirements, and safety features of the battery need to be considered. 

We remain, in practice, far from meeting these apparently trivial requirements so-called inert electrodes have a finite lifetime due to corrosion and physical wear while it is common, even normal, to accept an ovcrpotential of several hundred millivolts. Only in the chlor-alkali process and, to a lesser extent, in water electrolysis and electrowinning has significant progress towards improved electrode materials been made. Generalizations concerning electrode materials are probably unwise and the choice of electrodes for particular industrial processes will be discussed in subsequent chapters Table 2 2, however, lists some common anode and cathode materials. 

In general, a cathode material for lithium ion batteries needs to meet the following requirements f] ... 

In this chapter, general features of perovskite-type oxides are summarized from the point of view of the required properties as a cathode material. Then,... 

Over fhe pasf decade, a variety of electrode materials and configurations have been explored. Generally, both the anode and cathode materials should have high electric conductivity, chemical stability, mechanical strength and low cost. Fulfilling fhese requirements, carbon-based materials and non-corrosive metals are currently most-widely used as the base electrode materials. 

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