Construction of PAFCs

Another featnre of concentrated phosphoric acid solutions that is very important for fnel cells is the water vapor pressnre, which decreases drastically with increasing acid concentration. This featnre allows the phosphoric acid solution to be immobilized in a porons solid matrix, greatly simplifying the elimination of water as a reaction prodnct from the fnel cell s cathode space by gas (oxygen or air) circulation. It is safe to adjnst this circulation to the maximum current load (maximum rate of water production) without the need to readjust it at lower loads, as because of the immobilization, there is no risk of excessive drying of the matrix. In this way, water elimination has a peculiar self-regulation. No such feature exists in snlfnric acid solntion, where for water elimination, the acid itself would have to be circulated, which would cause problems of sealing and of corrosion. 

Basically, the construction of PAFCs differs little from what was said in Section 1.4 about fuel cells with liquid acidic electrolyte. In the development of PAFCs and two decades later in the development of PEMFCs (described in Chapter 3), many similar steps can be distinguished, such as the change from pure platinum catalysts to catalysts consisting of highly disperse platinum deposited on a carbon support with a gradual reduction of platinum content in the catalyst from 4 to 0.4 and then to 0.25 mg/cm, and the change from pure platinum to Pt-Ru catalysts. The bipolar graphite plates that have special channels for reactant snpply and distribution over the entire electrode surface now used widely in PEMFC stacks were first used in PAFCs. 

The concentrated phosphoric acid solution in a PAFC is absorbed into the pores of a porous matrix with fine pores and a total thickness of about 50 xm. From the outside, this matrix electrolyte behaves like a solid electrolyte (like the membrane in PEMFCs), preventing the reactant gases hydrogen and oxygen from getting to the foreign electrode and mixing. 

In early work, Kynar poly(vinylidene fluoride), a thermoplastic material, was used to make the matrix. It was soon discovered that in concentrated phosphoric acid at high temperatures, it is not sufficiently stable chemically and produces fluorine-containing impurities tending to adsorb on the catalyst surface, lowering the catalyst s activity and with it the fuel cell s performance. 

Among new materials suggested for the porous electrolyte matrix in PAFCs, we mention a mixture of silicon carbide (SiC) and PTFE (Mori et ah, 1998). A suspension of the components is mixed in a ball mill for a long time, then spread onto the surfaces of the cathode and anode. This assures good contact between the electrodes and the electrolyte immobilized in the matrix. 

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