Anodes ionic conductivity requirements

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

In the systems illustrated in Figure 53.1, the anodic reaction has to be electrically balanced by the cathodic reaction, since electrical charge cannot build up at any location. A continuous electrical circuit is required through the metal (for electron conduction) and the environment (for ionic conduction). 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

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... 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

The anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

The electrolyte in this fuel cell is generally a combination of alkali carbonates, which are retained in a ceramic matrix of LiA102 [8], This fuel cell type works at 600°C-700°C, where the alkali carbonates form a highly conductive molten salt with carbonate ions providing ionic conduction. At the high operating temperatures in the molten carbonate fuel cell, a metallic nickel anode and a nickel oxide cathode are adequate to promote the reaction [9], Noble metals are not required. 

The vast majority of engineering materials dissolve via electrochemical reactions. Chemical processes are often important, but the dissolution of metallic materials requires an oxidation of the metallic element in order to render it soluble in a liquid phase. In fact, there are four requirements for corrosion an anode (where oxidation of the metal occurs), a cathode (where reduction of a different species occurs), an electrolytic path for ionic conduction between the two reaction sites, and an electrical path for electron conduction between the reaction sites. These requirements are illustrated schematically in Fig. 1. 

The complete circuit in local-cell eonosioa involves ion conduction in solution between llte anodic and cathodic sites (Figure 7.391. When llte solvent is very polar, e.g.. water, ionic conduction is excellent and the anodic and cathodic sites can be maeroscopically separated. The conductivities of solutions of RMgX and MgX.i in DI-.F. however, are very low. It is douhtful that the requirement for a complete electrical circuit will allow anodic and cathodic sites to he separated by much more than A-scale distances, if any, in DKH. In principle, anodic and cathodic sites, if they exist, might be detected by scanning electrochemical devices or STM. 

Anode system. The anode system, which consists of the anode material plus its overlay, must supply the required current for the anticipated service life and distribute it to the reinforcement that needs to be protected. Anode materials and current density aspects have been dealt with in a previous section. The general requirements of an anode system are it has to adhere to the concrete surface it should be suitable for appHcation to the surface needing protection (top, bottom, horizontal, vertical, flat, curved), it should be durable and have low installation cost it should produce acceptable weight addition and change of the appearance and dimensions of the structure. If an overlay is used, it should have durable bond to the substrate concrete, sufficient mechanical strength and electrical characteristics equal to those of base concrete (ionic conductivity). 

A gas that is normally considered aggressive, such as chlorine, if anhydrous, it may be contained in iron cylinders because the ionic conduction factor is absent. Gold is found in nature in its native state and is classified as a noble metal because in nature, there are no cathodic processes that satisfy the requirements of energy balance in relation to the anodic processes of gold dissolution The cathodic process is lacking. Lead dioxide is an electronic conductor that can be used as insoluble anode as the process of anodic dissolution is absent. 

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