The Structure of Hydrogen-Oxygen MCFC

The working temperature of molten carbonate fuel cells is around 600-650°C. Mixed carbonate melts containing 62-70 mol% of lithium carbonate and 30-38 mol% of potassium carbonate, with compositions close to the eutectic point, are used in molten carbonate fuel cells as an electrolyte. Sometimes, sodium carbonate and other salts are added to the melts. This liquid melt is immobilized in the pores of a ceramic fine-pore matrix, made of sintered magnesium oxide or lithium aluminate powders. 

Porous metallic gas diffusion electrodes are used in these fuel cells. The anode consists of a nickel alloy with 2% of chromium. Chromium that is added prevents recrystallization and sintering of the porous nickel though it works as an electrode. This action is based on chromium forming a thin layer of chromium oxide at the nickel grain boundaries, which interferes with the surface diffusion of the nickel atoms. 

The cathode consists of lithiated nickel oxide. Nickel oxide is a p-type semiconductor, having a rather low conductivity. When doped with lithium oxide, its conductivity increases tens of times, owing to a partial change of Ni + to Ni + ions. The lithiation is accomplished by treating the porous nickel electrode with a lithium hydroxide solution in the presence of air oxygen. The compound produced has a composition given as Lij +Nii j( Nijj +0. This lithiation of nickel oxide was first applied in 1960 by Bacon in his alkaline fuel cell. 

The matrix with its fine pores filled by the carbonate melt is a reliable protection against gases bubbling through and getting into the wrong electrode compartment. Therefore, there is no need to provide the gas diffusion electrodes with a special gas barrier layer. 

The fuel cell components have thicknesses as follows the anode is 0.8-1.5mm thick the cathode, 0.4-1.5 mm, the matrix, 0.5-1 mm. In a fuel cell of the filter-press type, the individual cells are separated by bipolar plates made of nickel-plated stainless steel, contacting the anode with their nickel side, and the cathode with their steel side. All structural parts are made of nickel or nickel-plated steel. In a working fuel cell, the temperature of the outer part of the matrix electrolyte is lower than that of the inner part, so that in the outer part the electrolyte is solidified. This provides for tight sealing around the periphery of the individual fuel cells. 

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