DEVELOPMENT OF BILAYER ELECTROLYTE LT-SOFCs USING ALTERNATIVE BISMUTH OXIDES
Wachsman, Eric D
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This work is primarily focused on the fabrication and performance of anode supported cells based on gadolinium doped ceria (GDC) and alternative stabilized bismuth oxide bilayer electrolyte that can operate at low temperatures (500-650 °C).The well-known bismuth-based electrolyte (ESB) undergoes ordering phenomena at temperatures ≤600°C, causing rapid decay in conductivity, and in turn the power output. As alternatives, two bismuth oxide compositions: (a) Neodymium-gadolinium stabilized bismuth oxide (NGSB) in rhombohedral phase, and (b)heavily doped composition, (Bi0.75Y0.25)1.86Ce0.14O3±δ (YCSB), with Y and Ce as co-dopants in cubic phase were evaluated in this work. For the GDC/NGSB bilayer SOFCs, thickness of GDC and NGSB layers was varied between 23-70 μm and 0-25 μm, respectively. The results showed the addition of NGSB layer was effective in blocking electronic conduction which increased the OCV compared to the baseline single layer GDC cell. Further, the relative and total thickness of the two layers showed a significant impact on the OCV of the cell at different temperatures with best performance obtained with cells with lower GDC thickness and higher NGSB thickness. For the GDC/YCSB bilayer electrolyte SOFCs, a cell with a ~ 20 μm GDC layer and a ~12-13 μm YCSB layer, OCV and MPD of the cell at 650 ℃ reached 0.833 V and 760 mW/cm2 respectively. OCV stability of this bilayer was measured for 50 hours at 625 and 600 ℃ (100 hours in total) and exceptional stability of OCV and ohmic ASR was observed. In comparison, the cell with 10GDC/ESB bilayer electrolyte showed a very rapid degradation of OCV at 600 ℃ (average hourly degradation rate of 0f -0.55%/h). and the ohmic ASR of the cell with GDC/ESB bilayer electrolyte at 600 ℃ increased by 5 times over the first 50 hours of operation mainly due to the conductivity decay of ESB. Following the stable performance of GDC/YCSB bilayer electrolyte SOFCs, effect of GDC/YCSB thickness ratio on the performance of the cell was studied. It was shown that MPD of the bilayer electrolyte cells is higher than pristine GDC based cells with reduced ohmic ASR values. Specifically, a high MPD of ~1 W/cm2 at 650℃ was achieved on a GDC(20μm)/ YCSB(12μm) bilayer electrolyte based SOFC, which is 62% higher than pristine GDC based SOFC (0.64 W/cm2). Such enhancement is due to the 9.3% improvement in OCV (from 0.791 to 0.865 V) and a considerable 36% reduction in ohmic ASR values (from 0.094 to 0.069 Ω.cm2). Finally, to achieve high power density at low temperatures (≤ 600 ℃), a thin GDC (7-8 μm)/ YCSB (2 μm) bilayer electrolyte was used, and non-ohmic ASR of the cell was drastically lowered via infiltration of Ni/GDC and LSM on anode and cathode, respectively. At 600 and 550 ℃, maximum power density (MPD) of the cell reached 1.73 and 1.25 W/cm2, respectively, significantly higher than all previously reported values using non-cobalt cathode materials. The effect of infiltrating LSM on LSM-YCSB cathode was studied by varying temperatures and partial pressure of oxygen. It was revealed that by infiltrating the LSM/YCSB cathode with LSM nanoparticles, non-ohmic ASR of cathode reduced remarkably by one order of magnitude. Stability of the infiltrated symmetrical cell at 550 ℃ was measured over a period of 500 hours with no sign of decay.