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Performance tests on the two initially candidate third generation short-stacks showed the excellent performance due to the LSCF cathodes: at standard Real-SOFC conditions for the current-density/voltage measurements the ASR of all cells was about 0.26 Ù•cm² at 700 °C! This is the same value as obtained for both first and second generation short-stacks at 800 °C. It was therefore decided to perform the durability tests on these candidate third generation short-stacks under standard baseline conditions at 700 °C, thus operating the short-stacks at a similar voltage and power-density. The durability tests on both candidate third generation short-stacks successfully passed 3000 h, at which time the stack F1002-97 with the ITM interconnect plates was selected as third generation (G3). The durability test on this stack was continued with the aim to reach 10,000 h of operation. Because of its excellent behavior it was decided to continue the durability test on the remaining candidate third generation short-stack F1002‑95 with the CroFer22APU interconnect plates as well (G3-TK).

Figure 1 shows the current status of the durability tests of the two third generation stacks, being operated on hydrogen at 0.5 A/cm² at a fuel utilization of 40%. The first one just passed 6200 h of operation time, the latter trailing 500 h behind. The drop in performance at about 700 h for the stack F1002‑95 was caused by a failure of the electronic load.

First noteworthy observation is the absence of the large drop in performance in the initial 300 to 500 h of operation. This must be caused either by the lower operating temperature or by the changed composition or increased thickness of the cathode contact layer. Second noteworthy observation is the absence – so far – of the progressive degradation. Over the full period of operation still a quasi-linear degradation is observed. Over the first 3000 h of operation a degradation rate of 22 mÙ•cm²/kh was determined for both cells in the stack F1002‑97. After the drop in performance at 700 h the cells in stack F1002-95 show similar degradation rates.

Four third generation short-stacks were meanwhile assembled and after passing their acceptance tests delivered to the partners for testing. These are being tested by the partners CEA, VTT and EBZ at benchmark conditions set for the G3 stacks: a current density of 0.7 A/cm² with hydrogen and methane, respectively, at a fuel utilisation of 75% and at 700 and at 800 °C, respectively.

Figure: Durability tests on third generation stacks F1002‑97 (G3), and ‑95 (G3-TK)
operated at 700 °C on hydrogen (u_F = 40%) at 0.5 A/cm²

From an I-V at constant u_F (60%) at 800°C with syngas, an ASR of 0.44 Wcm² was derived. From 1300 to 3600 h, the total ohmic resistance for the SRU increased from 0.125 to 0.175 Wcm², as determined from impedance spectroscopy, which indicates that the contribution of ohmic resistance to the overall degradation is predominant (equivalent to ca. -11 mV/kh). This trend continued until the end (impedance at 5000 h).

At the same time, the insitu monitored contact resistances (anode and cathode sides) show virtually stable behaviour. The observed overall ohmic increase is thus due to other sources such as delamination or interphase formation.

At 4700h, a brief fuel supply interruption occurred (30 sec), after which degradation greatly increased. The test was stopped at 5200h and dismantled. OCV remained stable at 1050 mV to within 3 mV throughout the test, except after the interruption, where it dropped to 1030 mV. The cell was found intact and fully reduced at the anode side, with the exception of incipient microcracks at the anode inlet, that could explain the change in degradation slope.

Figure: Voltage stability of G3 SRU over 5000h.