Electrochemical Properties of Proton-Conducting Solid Oxide Fuel Cells with Y-doped BaZrO3 Electrolytes Fabricated by Low-Temperature Processes

Abstract

The demand for low operating temperatures in solid oxide fuel cells (SOFCs) is increasing due to the numerous problems induced by high operating temperatures including the high cost of peripherals, degradation of anodes due to particle coarsening, and Cr evaporation and corrosion of metal interconnects. The primary obstacles in reducing the operating temperature include the high Ohmic resistance of the electrolytes. Among the high temperature proton conductors (HTPCs), especially yttrium-doped barium zirconate (BZY) that has high bulk conductivity at low temperatures and excellent chemical stability has been widely investigated in order to replace the conventional electrolyte materials, such as yttria-stabilized zirconia (YSZ). However, it is a big challenge to obtain a good BZY electrolyte for SOFCs because of its refractory properties and high grain boundary resistance. For the last decade, there have been numerous attempts to achieve high total conductivity in BZY, which can be categorized into three approaches: i) use of sintering aids to lower the sintering temperature, ii) process controls for sintering or powder preparation, and iii) thin film without resistant grain boundaries. However, there is a significant reduction in the total conductivity or the sintering temperatures of these attempts are still too high to be utilized in the fabrication of SOFC. In addition, it is very difficult to obtain highly-textured film with large scale on polycrystalline substrate despite of its promising high total conductivity. The objectives of this dissertation are the fabrication and assessment of BZY proton conducting oxide films that have high electrical (protonic) conductivity comparable to the bulk conductivity of pellet BZY, and that can be suitable for the electrolyte of proton-conducting SOFCs with large scale. As the alternatives of conventional approaches, aerosol deposition (AD) and phase transformation methods are used for obtaining dense BZY at low temperature. Phase transformation method is also effectively utilized in the modification of anode microstructure. The feasibility study of AD technique by using Gd-doped ceria films (Chapter III) shows that the microstructure and conductivities of films deposited by AD were dependent on the choice of substrate, and high conductivity was obtained in the film of which microstructure was dense in cross-section and smooth in surface. The study on the highly-oriented BZY film by pulsed laser deposition (PLD) (Chapter IV) shows that not only the deposition rate exhibits decisive effects on the microstructure but also Ba deficiency of BZY films is dependent on the substrate regardless of the deposition rate. Thus, the film quality and Ba deficiency are two critical factors that determine the magnitude of the electrical conductivity for BZY films. As an alternative of conventional methods, AD and/or phase transformation techniques are employed for the fabrication of BZY electrolyte proton conducting SOFC (Chapter V, VI, VII) The single cell with BZY electrolyte coated by AD was embodied and the electrochemical properties was assessed (Chapter V). In the preparation of powder for AD, it was found that Y-doping in BaZrO3 is very difficult during calcination process. Highly Y-doped BaZrO3 powders without second phase were synthesized successfully without use of sintering additives through a series of careful preparation processes. High content of yttrium in prepared powder resulted in high conductivity of BZY electrolyte. The power density of this cell is also considerably high even though the sintering temperature of cell, 1200 °C, is much lower than those shown in other popular studies (≥1400 °C). Phase transformation technique is introduced for the modification of anode functional layer (AFL) from Ni-YSZ into Ni-BZY by heat treatment with atmospheric powder at temperature (1200 °C) substantially lower than the normal sintering temperature of cell fabrication (Chapter VI). Very fine BZY particles are newly formed and homogeneously distributed on top of large NiO particles in the AFL of the cell with atmospheric powder. This results in a dramatic decrease of area-specific polarization resistance (0.21 Ω∙cm2 at 600 °C) and increase of peak power density (246 mW∙cm-2 at 600 °C) of cell compared to those values (1.19 Ω∙cm2 and 84 mW∙cm-2) of cell fabricated without the atmospheric powder. The formed BZY particles, transformed from YSZ particles, contribute a significant enlargement of the triple-phase-boundaries (TPB). Finally, the new approach using the phase transformation from YSZ into BZY is applied for the formation of BZY electrolyte which can be suitable for the electrolyte of proton-conducting SOFCs with large scale at low temperature. (Chapter 7) Simple heat treatment changed the YSZ-electrolyte cell embedded in an atmospheric powder into BZY-electrolyte cell. The original YSZ electrolyte layer heat-treated at 1350 °C fully transformed into BZY layer with increased grain size and increase of film thickness. Electrochemical cell tests show very high peak power density, 250 mW∙cm-2 at 600 °C, which is one of the highest among the reported values. The high power density is attributed to the lowest RΩ, 0.42 Ω∙cm2 at 600 °C, as well as considerably low RP, 0.35 Ω∙cm2 at 600 °C. The origin of the low RΩ is the increase of grain size with reduction of grain boundary resistance, and that for the low RP is the modified microstructure of AFL with fine BZY particles. Although the conductivity of transformed BZY electrolyte is still lower than the reported values of highly-textured thin films due to the insufficient doping of Y and/or Ni diffusion from AFL, it is valuable that this process is very useful for the fabrication of proton conducting fuel cell with very low RΩ compared to the pellets sintered at very high temperature or the films made by PLD. In conclusion, BZY electrolyte proton conducting SOFC has been successfully fabricated by low temperature processes, AD or phase transformation methods, as the alternatives of conventional approaches. In AD process, obtaining dense BZY film with high content of Y doping is crucial for the conductivity of electrolyte and power density of cell. In phase transformation method, electrolyte and AFL with BZY proton conductor are easily formed from YSZ oxygen ion conductor reacting with BaO vapor at low temperature without sintering aids, which provides a very simple and useful way for the fabrication of proton conducting SOFC having highest power density with lowest Ohmic or polarization resistance.