Electrode perovskites

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

A signihcant problem in tire combination of solid electrolytes with oxide electrodes arises from the difference in thermal expansion coefficients of the materials, leading to rupture of tire electrode/electrolyte interface when the fuel cell is, inevitably, subject to temperature cycles. Insufficient experimental data are available for most of tire elecuolytes and the perovskites as a function of temperature and oxygen partial pressure, which determines the stoichiometty of the perovskites, to make a quantitative assessment at the present time, and mostly decisions must be made from direct experiment. However, Steele (loc. cit.) observes that tire electrode Lao.eSro.rCoo.aFeo.sOs-j functions well in combination widr a ceria-gadolinia electrolyte since botlr have closely similar thermal expansion coefficients. 

Although several metals, such as Pt and Ag, can also act as electrocatalysts for reaction (3.7) the most efficient electrocatalysts known so far are perovskites such as Lai-xSrxMn03. These materials are mixed conductors, i.e., they exhibit both anionic (O2 ) and electronic conductivity. This, in principle, can extend the electrocatalytically active zone to include not only the three-phase-boundaries but also the entire gas-exposed electrode surface. 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

Fig. 43. Full-cell performance with hot-pressed membrane, perovskite electrodes. Cathode removal and anode generation as a function of applied current. Lines calculated from stoichiometry, 1 mol/2 F.

Our recent our works show that even higher activity and stability can be demonstrated by the three-layer electrodes with nickel layer, active in the oxygen evolution, middle layer with catalyst, active in the oxygen reduction (Mn02, pyropolymer or a perovskite), and a diffusion (waterproof) layer,... 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

Imanishi N, Matsumura T, Sumiya Y, Yoshimura K, Hirano A, Takeda Y et al. Impedance spectroscopy of perovskite air electrodes for SOFC preparated by laser ablation method. Solid State Ionics 2004 174 245-252. 

Oxide ratio, 18 815 Oxides, 16 598 acidic, 22 190-191 bond strengths and coordination numbers of, 22 570t diorganotin, 24 819 glass electrodes and, 14 28 gold, 22 707 iron, 14 541-542 lead, 14 786-788 manganese, 15 581-592 nickel, 27 106-108 niobium, 27 151 plutonium, 29 688-689 in perovskite-type electronic ceramics, 14 102... 

O2 and H2 dissociation kinetics are better at higher temperatures (>400 °C), low-cost electrode structures of high surface area Ni and oxides such as spinels or perovskites to replace the very effective, but costly, Pt catalysts have been sought. 

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