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air at 200 mA/cm2 on the advanced electrode were 10 mV higher than those of the conventional electrode. There is a trade-off between high power density and cell life performance. One of the major causes of declining cell performance over its life is that electrode flooding and drying, caused by migration of phosphoric acid between the matrix and the electrodes, occurs during cell load cycling. Researchers at Fuji Electric addressed two approaches to improve cell life performance while keeping power density high (31). In one, the wettability of the cathode and anode were optimized, and in the other a heat treatment was applied to the carbon support for the cathode catalyst. During tests, it was observed that a cell with low cathode wettability and high anode wettability was more than 50 mV higher than a cell with the reverse wetting conditions after 40 start/stop cycles. The use of carbon black with large surface area to improve platinum dispersion on supports was investigated as a method to increase the power density of a cell (32). However, some large surface area carbon blacks are fairly corrosive in hot potassium acid, resulting in a loss of catalytic activity. The corrosivity of the carbon support affects both the rate of catalyst loss and electrode flooding and, in turn, the life performance of a cell. Furnace black has been heat treated at high temperature by Fuji Electric to increase its resistance to corrosion. It was found that corrosion could be reduced and cell life performance improved by heat treating carbon supports at high temperature, at least to around 3,000 °C (5,432 °F). More recently, UTC Fuel Cells cites improvements to achieve 40,000 hour cell life through better cell temperature control, increasing H3PO4 inventory, and incorporating electrolyte reservoir plates in the cell stack (3). 5.2 Performance There have been only minor changes in documented cell performance since the mid-1980s - mostly due to the operating conditions of the cell. The changes are reported in performance trends shown in this section that were primarily gained from contracts that UTC Fuel Cells had with the Department of Energy or outside institutions. New, proprietary PAFC performance data may likely have been observed by the manufacturer (3). Cell performance for any fuel cell is a function of pressure, temperature, reactant gas composition, and fuel utilization. In addition, performance can be adversely affected by impurities in both the fuel and oxidant gases. The sources of polarization in PAFCs (with cathode and anode Pt loadings of 0.5 mg Pt/cm2, 180 °C, 1 atm, 100 percent H3PO4) were discussed in Section 2 and were illustrated as half cell performance in Figure 2-4. From Figure 2-4 it is clear that the major polarization occurs at the cathode, and furthermore, the polarization is greater with air (560 mV at 300 mA/cm2) than with pure oxygen (480 mV at 300 mA/cm2) because of dilution of the reactant. The anode exhibits very low polarization (-4 mV/100 mA/cm2) on pure H2, and increases when CO is present in the fuel gas. The ohmic (iR) loss in PAFCs is also relatively small, amounting to about 12 mV at 100 mA/cm2. 5-11PDF Image | Fuel Cell Handbook (Seventh Edition)
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