Solid solutions, oxide cathodes

Fukui, T. Ohara, S. Hotta, T. Okawa, H. Naito, M. Properties of NiO cathode coated with lithiated Co and Ni solid solution oxide for MCFCs. J. Power Sources 2000, 86 (1-2), 340-346. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

For a number of reasons the TiS2 battery was not widely exploited commercially, but similar batteries based upon intercalation into transition-metal oxides LiJCTO2, where T is a 3d transition metal such as Ni, Co, or Mn (or a solid solution of these metals, LiJt.T1 yTj,02) are widely available. The first of these, the Sony cell, introduced in 1991, employs LixCo02 as the cathode and the intercalation of Li into graphite as the anode, to form LivC6. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

For oxide electrocrystallization, the last condition is the most strenuous, sine many oxides are insulating non-stoichiometric compounds, however, are sufficient conductive. When two (or more) substances are codeposited, certain specific feature of the crystallization can be expressed for both cathodic and anodic processes on th basis of the thermodynamics of binary (or more complex) systems. If stabl multicomponent phases exist, then it is their deposition (not the deposition of mixture of simpler products) that preferentially proceeds in a certain potential regioi In such cases, intermetallic compounds are deposited in cathodic processes, and th deposition of mixed oxides takes place in anodic processes. These products ca represent both chemical compounds and solid solutions. 

Lanthanum nickel-iron oxide cathodes, La(Ni,Fe)03, have attracted much attention because they show essentially no effects of chromium poisoning [59]. This is mainly because there is no chance of SrCr04 formation. From the thermodynamic point of view, there is a possibility that the La(Ni,Fe)03 perovskites react with chromium-containing vapors to form the La(Ni,Fe,Cr)03 solid solution [42]. Even so, such perovskites must still be electro-conductive, although some surface kinetic activity will be lowered due to the fact that lanthanum chromite has lower catalytic activity among the transition metal perovskite oxides. 

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