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estimate of the relationship between current density and degradation rate. The two initial cell tests presented within this paper were operated in long term SOEC mode for approximately 95 and 114 hours respectively. Prior to the shutdown of cells held under long-term test, final DC-potential sweeps were performed in both SOFC and SOEC modes of operation. PRELIMINARY RESULTS After initial cell characterization was complete at the beginning of each test, the cells were placed into a long-term SOEC mode of operation. Figure 8 below illustrates the Area Specific resistances and cell operating voltages as a function of time for each test. The cell in Test 1 was operated at a current density of 0.5 A/cm2 for the first 86 hours and lowered to 0.2 A/cm2. The cell in Test 2 was operated at 0.2 A/cm2 throughout the test. Mid-term DC-potential sweeps were performed at approximately half way through each test. These appear as vertical lines in data. The DC-potential sweep data acquired during initial cell characterization, mid-term and at the end of each test are plotted on figures 9 and 10 for tests 1 and 2 respectively. The open cell voltages (OCVs) of the cells at each measurement stage can be determined through the examination of figures 9 and 10 on the respective y-axis intercepts (I = 0 A). Both cells behaved consistently during initial characterization. The Dry OCV values at the peak operating temperature of 850 °C in both cases was approximately 1.03 Volts. Similarly, when steam was introduced to the cells at 33 %Mol, both cells had an OCV of 0.911 Volts. The OCV values at the test termination points differed as anticipated due to the cell degradation process, the rate of which was driven by the test conditions. The initial cell characterizations illustrated through the DC- potential sweeps presented in figures 9 and 10 demonstrate the cells’ expected responses to the changes in both operating temperature and steam content. Cell degradation is clearly exhibited in the DC-potential plots and the change in the gradients of each time dependant sweep is directly proportional to the increase in area specific resistance [23]. Examination of the cell voltages in Figure 8 can yield estimates for the cell degradation rates. For the cell in test number 1 while operated at a current density of 0.5 A/cm2, the apparent cell degradation rate was approximately 323 % per thousand hours of operation. When its current density was reduced to 0.2 A/cm2 during the final 9 hours of test number 1, the cell exhibited an apparent degradation rate of 109 % per thousand hours of operation. The cell operated at 0.2 A/cm2 throughout test number 2 exhibited an apparent degradation rate of approximately 100 % per thousand hours. Thus, for the single cells tested, there is a clear relationship between current density and cell degradation rates. Figure 8: Long term cell voltages and ASR values for Test 1 and Test 2. The cell in Test 1 was operated at a current density of 0.5 A/cm2 for the first 86 hours and lowered to 0.2 A/cm2. The cell in Test 2 was operated at 0.2 A/cm2 throughout the test. Mid-term DC-potential sweeps appear as vertical lines in data. 7 Figure 9: DC-potential sweeps for initial, mid-term and final stages of long term SOEC operation during test number 1. 9 hours separate the mid-term and final sweeps.PDF Image | Electrolysis Cells Operated Fuel Cell Steam Electrolysis
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