Thin film solid oxide fuel cells (SOFCs) are one of the promising power sources for portable gadget devices in terms of a fast on/off, a compact system volume, and a high power density, and thus this thesis discuss fabrications of thin films, characterizations and system integrations between a cell and a afterburner, which operate at the temperature range of 300-500 ˚C. The fabricating processes of both the electrolyte and the electrode via sputtering, pulsed laser deposition and atomic layer deposition are discussed.
First, this study investigates the microstructural structure of Y-doped Ba0.8Zr0.2O3-δ (BZY) thin films for porous/dense morphologies via pulsed laser deposition (PLD) and the crystallinity of BZY thin films on different substrate temperatures; BZY thin films deposited under 10 mTorr oxygen pressure are dense regardless of the substrate temperature (25 ˚C ≤ T ≤ 600 ˚C) while oxygen partial pressure of 100 mTorr, BZY thin films deposited at a low temperature (T ≤ 200 ˚C) have porous structures. Also, BZY thin films deposited at 200 mTorr in low/intermediate temperature ranges (25 ˚C ≤ T ≤ 400 ˚C) have porous and columnar structures. Regardless of chamber pressure, BZY thin films with dense structures can be obtained when the substrate temperature exceed 500˚C.
Second, this thesis reports the effect of platinum catalyst thickness and morphology on the electrochemical surface area using electron microscopy and cyclic voltammetry. The roughness and porosity of Platinum catalyst are varied by using different Argon pressure during Platinum sputter-deposition process. Lower Argon pressure results in denser and smoother Platinum films, which are confirmed by scanning electron microscopy and atomic force microscopy, respectively. The cyclic voltammetry is then used to quantify the catalytic activity of the films with various thickness and morphology. The electrochemical surface area extracted from the cyclic voltammetry shows that the current density is enhanced by 4 ? 5 orders of magnitude through the formation of finite amount of porosity using a higher Argon pressure (> 60 mTorr) compared to a highly dense film.
Third, we demonstrate a simple yet effective approach of stabilizing the nanostructure of porous metal-based electrodes and thus extending the life of micro solid oxide fuel cells. In an effort to avoid thermal agglomeration of metal electrodes, an ultrathin yttria stabilized zirconia (YSZ) was coated on the porous metal (Pt) cathode by the atomic layer deposition, a scalable and potentially high-throughput deposition technique. A very thin YSZ coating was indeed found to maintain the morphology of its underlying nanoporous Pt during high temperature operations (500°C). More interestingly, the YSZ coating was also found to improve oxygen reduction reaction activity by ~2.5 times. This improvement is attributed to an enhanced triple phase area especially in the vicinity of the Pt|electrolyte interface; cross-sectional electron microscopy images indicate that the initially uniform ultrathin YSZ layer becomes a partially agglomerated coating, a favorable structure for a maximized reaction area and fluent oxygen access to the Pt|electrolyte interface.
Finally, a thermally stable and self-sustainable portable energy conversion system is designed using low-temperature thin film SOFCs. Hydrogen and air gases are fed into this system, which is successfully operated at 319 °C. Each cell, comprised of Pt (anode)/YSZ-gadolinia doped ceria(GDC)-YSZ/Pt (cathode), is manufactured using sputtering and atomic layer deposition, and the area of a single cell is 2.56 cm2. The maximum absolute power and power densities at 500 °C are measured to be 44 mW and 17 mW/cm2, respectively. To the best of our knowledge, this represents the highest absolute power reported at such a low operating temperature regime. To increase the system’s temperature, a catalytic burner using Al2O3 and a Pt catalyst is manufactured by dip coating. After hydrogen gas and air pass through the anode and cathode sides, respectively, the mixed fuel gases are consequently supplied to the catalytic burner, which undergoes an exothermic reaction. We successfully demonstrate that this system is heated up to 319 °C (from room temperature) without any other initial heat source; we also measure the electrical power simultaneously.