Fig 5 - uploaded by Hyung-Tae Lim
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Voltage vs. current density and power density plot of the planar type cell corresponding to : (a) initial test, (b) after operated under ~ 0.5V for 4 h, (c) after operated under ~-0.2V for 4 h, and (d) after operated under ~-0.5V for 4 h.
Source publication
The stability of a solid oxide fuel cell (SOFC) stack is strongly dependent on the magnitude and profile of the internal chemical potential of the solid electrolyte. If the internal partial pressure is too high, the electrolyte can be delaminated from the electrodes. The formation of high internal pressure is attributed to a negative cell voltage,...
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Micro-sized anode materials demonstrate greater potential for practical applications than nanomaterials in the aspects of volumetric energy density, coulombic efficiency, fabrication process, and cost. However, the huge volume changes of alloy anodes (up to ∼500%) during repeated charge/discharge has led to a series of challenging issues including...
Citations
Anode supported BCY (anode side) - BZY (cathode side) cells were fabricated and their durability was investigated under constant current tests. The cells remained stable under the severe operating conditions of negative cell voltage and high air utilization. This is because the BZY layer at the cathode side provided local electronic conduction, which prevented electrolyte/electrode delamination under negative voltage, as well as chemical stability against water vapor attack under high air utilization conditions. No significant changes in composition or phase were observed in the cells following the tests. These results are contrary to those reported for a single layer BCY cell. The present work suggests that a protonic ceramic fuel cell fabricated with BZY+LSCF cathode, BZY/BCY electrolyte, and Ni + BCY anode provides superior durability under severe operating conditions.
Solid oxide fuel cells (SOFCs) are promising power generation systems that electrochemically convert chemical energy into electrical energy with little or no emission of pollutants [1–3]. Moreover, a high-temperature fuel cell has many advantages such as a high efficiency and fuel flexibility in comparison with a low-temperature fuel cell. For these reasons, a considerable amount of attention has been paid to SOFCs in recent years for application to medium- to large-scale power generation and combined heat and power (CHP).
Anode supported BCY (at the anode side) – BZY (at the cathode side) cells were fabricated, and local electronic conduction and internal partial pressures in the electrolyte were investigated using embedded Pt probes. It was observed that BCY-BZY electrolyte close to electrode interfaces exhibits n- and p-type electronic conduction in the electrolyte region close to the anode and the cathode, respectively. Based on these results, internal and in the electrolyte were estimated, and accordingly the change in internal and were relatively small in the electrolyte regions close to the anode and the cathode interfaces, respectively. Constant current tests were also conducted on the BCY-BZY electrolyte cells, and they were very stable under negative voltage operation, contrary to BCY single layer cells. Thus, the present study indicates that the utilization of a BZY layer at the cathode side is effective for establishing electronic conduction as well as chemical stability, which should improve the durability of BCY based PCFCs under severe operating conditions such as negative voltage.
The correlation between internal chemical potential, and durability of anode-supported BCY (BaCe0.85Y0.15O3-δ) electrolyte cells, was investigated as a function of electrolyte thickness. Internal hydrogen and oxygen chemical potentials were measured on thin (∼ 10 μm) and thick (∼ 50 μm) BCY electrolyte using an embedded Pt probe. As the electrolyte thickness decreased, the internal chemical potential became dominated by the gases surrounding the cathode, indicating that the thin BCY cell may be vulnerable to water vapor produced at the cathode during operation. Constant-current tests were conducted on a thin (∼ 10 μm) BCY electrolyte cell with a BCY + LSCF ((La,Sr)(Co,Fe)O3-δ) cathode, and a thick (∼ 35 μm) BCY electrolyte cell with a BZY (BaZr0.85Y0.15O3-δ) + LSCF cathode, under positive and negative voltage conditions. Consistent with the results of the internal chemical-potential measurements, the former showed a significant rate of degradation due to changes in BCY composition in the electrolyte and cathode, while the latter showed much better durability, regardless of the sign of cell voltage. Thus, the present work shows the effect of BCY electrolyte thickness and selection of cathode material, on the durability of BCY based cells, in terms of their internal chemical potential.
Yttria doped barium cerate (BCY) electrolyte, Ni+BCY anode supported cells were fabricated, and their stability and the mechanism of their degradation were investigated through constant current tests under various operating conditions, especially negative cell voltage operation (with respect to degradation phenomenon due to cell imbalance in a series connected stack). The results of electrochemical tests (I-V characteristics and impedance spectra) indicate that the degradation rate was significant when the cell was operated under higher current densities (regardless of the sign of cell voltage) and only the ambient air was used for the cathode. Post-material analyses revealed that microstructural and compositional changes were obvious in the BCY electrolyte and the BCY of the cathode functional layer because of BCY decomposition in the wet atmosphere at the cathode. Thus, the present work concludes that the degradation rate of BCY electrolyte-based cell depends on operating conditions, i.e., the amount of current density (the water vapor production rate at the cathode) and the air flow rate (flushing water vapor at the cathode).
Yttria stabilized zirconia (YSZ) thin films are deposited on thin Si3N4 layer coated Si substrates by RF-sputtering with different substrate temperatures, room temperature (YSZ-RT) and 550°C (YSZ-550). Fine polycrystalline structure is observed for YSZ-RT (thickness = 620 nm), while columnar crystal structure (111) oriented for YSZ-550 (thickness = 660 nm), respectively. The conductivity measurement shows that the activation energies of YSZ-RT and YSZ-550 are 1.34 and 1.58 eV, respectively, indicating that grain boundary conduction is dominant. Shear strengths of the YSZ films are determined by using a micro-blade cutting system to be 150 MPa for YSZ-RT and 200 MPa for TSZ-550, respectively. YSZ-550 is found to be better for solid oxide fuel cell application from mechanical and electrically properties point of view.
