Although internal humidification was not a requirement of the program, in the interest of obtaining the greatest possible power density, it was incorpora ted in the stack while greatly reducing the required volume. As shown in Figure 4, current density performance with internal humidification can equal or slightly better external humidification. The selected stack design used conventional filter press construction with all components designed and fabricated by MTI from raw materials.

Stack'Testing - Mil designed and built test stations were used for all testing. The larger stacks were tested using an 800 SLM scroll type air compressor, pressurized hydrogen gas, a closed loop heating/cooling system, and a custom designed load bank all controlled through LabVIEW. Four stacks of various power levels were built and tested using MTI internal funds including lkW, 1.7kW, 5kW and lOkW versions each of which provided valuable learning experiences.

Figure 3 - Single Cell Performance 50 cm2 vs. 250 cm2
Figure 4 - Internal vs. External Humidification 50 cm2

The first stack to be tested was the 1 kW version which verified the scale up from laboratory size to full size cells. The performance test results shown in Figure 5 compare single cell with stack results and indicate their good agreement The tests also demonstrated that there was inadequate air supply capacity in the test station above the 1 kW level which was corrected in the design for a 10 kW test station.

Test results for the 10 kW stack are shown in Figure 6 which, indicate that the target performance was reached at 0.6 volts per cell. While" this performance was expected to have been reached with MEAs having 0.25 mg Pt/cm2, independent assay of the MEAs demonstrated that it had been reached with a loading of only 0.17 mg Pt/cm2. This result was unexpected and led to the decision to build another stack with the design Platinum loading.

A 1.7kW stack was then built using MEAs with Platinum loading of 0.25 mg/cm2. This power level was selected because it would utilize available hardware and raw materials and was consistent with the modular configuration of the stack structure. Performance results for this stack when scaled to the number of modules in the 10 kW stack design indicated that a power level of llkW would be reached. Thus, the additional 0.08 mg Pt/cm2 provided an increase of approximately 10% in stack power level. The power density at 11 kW. would be 3.54 kg/kW (7.8 Ibm/kW) with internal humidification and 3.22 kg/kW (7.1 Ibm/kW) without internal humidification. These results are consistent with the overall program requirements

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