Stack Performance

Two 2 kW stacks have been tested since this February. First stack(ST-14) was operated more than 1,000 hours but failed in the continuous operation under the load of 150A because of separator defaults. Second stack(ST-17) has been operating for 800 hours (as of writing this paper) without any degradation of voltage under the 150A electric load. Fig. 1 shows the I-V characteristics of the 2kW stacks ST-14 and ST-17. The fuel and oxidant utilization at 150mA/ctif were kept 40% in the test and 70% of air and 30% of CC>2 were used as an oxidant gas. It is seen that the OCVs were 21.0 V(1.050V/cell) in stack ST-14 and 21.6 V (1.805V/cell) in stack ST-17 while the voltages at current density of 150mA/crf were 14.74V (0.737V/ceU) in the stack ST-14 and 16.52V (0.826V/ceU) in ST-17, respectively. The corresponding analysis of the voltage loss reveals that the loss composed of Nemst loss, the internal resistance loss, and the electrode polarization. Most of voltage loss came from the electrode polarization and the internal resistance.

Fig. 2 showd the voltage distribution of 20 cells in the stack ST-17. The cell by cell voltage variation during the nomal operation under the load of 150A showed that the voltages ranged from 800 to 860 mV except No.ll cell which had malfunction in the cathode channel. The standard deviation of the cell voltages in the stack ST-17 showed 8.8mV which was less than the previous stack. Thus, uniformity of the cell to cell performance seems to be very good in the stack ST-17.

Fig.3 showes the endurance test results of stack ST-17. The stack is now being operated for 800 hours with the performance of no decay rate, which also turned out a huge progress compared to previous the small stack because of A1 coating in the separator's wet-seal area.

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