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issues remain whether to design and operate PAFC units at atmospheric vs. pressurized conditions. Primarily, small, multi-kW PAFC power units that were the focus of initial commercial applications led to atmospheric pressure operation. Although pressurization increased efficiency (lower fuel cost), it complicated the power unit - resulting in higher capital cost. The economic trade-off favored simpler, atmospheric operation for early commercial units. Another important issue, independent of power unit size, is that pressure promotes corrosion. Phosphoric acid electrolyte (H3PO4) produces a vapor. This vapor, which forms over the electrolyte, is corrosive to cell locations other than the active cell area. These cell locations are at a mixed voltage (open circuit and cell voltage), that can be over ~0.8V/cell. That is the limit above which corrosion occurs (active area limited to operation under ~0.8 V/cell). An increase in cell total pressure causes the partial pressure of the H3PO4 vapor to increase, causing increased corrosion in the cell. Cell temperature must also be increased with pressurized conditions to produce steam for the steam reformer (3). A major breakthrough in PAFC technology that occurred in the late 1960s was the development of carbon black and graphite for cell construction materials; this and other developments are reviewed by Appleby (4) and Kordesch (5). Carbon black and graphite were sufficiently stable to replace the more expensive gold-plated tantalum cell hardware used at the time. The use of high-surface- area graphite to support Pt permitted a dramatic reduction in Pt loading without sacrificing electrode performance. It was reported (4) that "without graphite, a reasonably inexpensive acid fuel cell would be impossible, since no other material combines the necessary properties of electronic conductivity, good corrosion resistance, low density, surface properties (especially in high area form) and, above all, low cost." However, carbon corrosion and Pt dissolution become an issue at cell voltages above ~0.8 V. Consequently, low current densities at cell voltage above 0.8 V and hot idle at open circuit potential should be avoided. The porous electrodes used in PAFCs have been described extensively in patent literature (6); see also the review by Kordesch (5). These electrodes contain a mixture of electro-catalyst supported on carbon black and a polymeric binder, usually PTFE (30 to 50 wt percent). The PTFE binds the carbon black particles together to form an integral, but porous, structure that is supported on a porous graphite substrate. The graphite structure serves as a support for the electro-catalyst layer, as well as the current collector. A typical graphite structure used in PAFCs has an initial porosity of about 90 percent, which is reduced to about 60 percent by impregnation with 40 wt percent PTFE. This wet-proof graphite structure contains macropores of 3 to 50 μm diameter (median pore diameter of about 12.5 μm) and micropores with a median pore diameter of about 34 Å for gas permeability. The composite structure, consisting of a carbon black/PTFE layer on the graphite substrate, forms a stable, three-phase interface in the fuel cell, with H3PO4 electrolyte on one side (electro-catalyst side) and the reactant gas environment on the other. 5-3PDF Image | Fuel Cell Handbook (Seventh Edition)
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