Study on activity and durability of bismuth-based oxide ion conductors for intermediate temperature solid oxide fuel cells

Abstract

The importance of the future renewable energy-based society is emerging worldwide by providing a new basis for economic growth and environmental benefits based on highly sustainable and reliable energy conversion and storage systems. In particular, the solid oxide fuel cell (SOFC) is an eco-friendly energy conversion device that generates electricity from chemical energy of injected fuels, and is spotlighted for its important role of distributed power generation and energy conversion in a sustainable energy system. However, due to durability and cost issues arising from high-temperature operation above 700 oC, recent approaches are heavily aimed to lower the operation temperature of SOFC and following two major polarization loss should be surely achieved in order to reach the goal: (1) ohmic loss across the electrolyte, (2) activity loss in electrodes reaction kinetics. In this respect, bismuth oxide with a fluorite-based cubic structure is very promising for the development of intermediate-temperature solid oxide fuel cells (IT-SOFCs), due to its high oxygen ion conductivity and surface oxygen exchange properties. However, the analysis of the intrinsic electrochemical catalytic properties of bismuth oxide is very lacking, and furthermore, the phase transition into less-conductive rhombohedral phase is one of the critical issues which limit the versatility of the material. Therefore, in this dissertation, I re-investigated the bismuth oxide by in-depth study of its surface activity and improvement strategy of its structural durability. Firstly, I embarked on a quantitative approach to enable identification of reaction pathway and facilitate measurement of the site-specific electro-catalytic activity. The dense oxide thin film and metal microelectrodes are fabricated by pulsed laser deposition (PLD) and photolithography process, hence the surface and interface of metal|oxide system is precisely controlled. Briefly, we observed that exceptionally high site-specific electro-catalytic activity of stabilized Bi2O3 compare to Y-stabilized ZrO2 (YSZ) case. Furthermore, its active feature was resulted independent on a choice of metals (Au, Pt), which suggests the significant role of bismuth oxide phase for oxygen electro-catalysis. Second, by introducing two brand-new strategies

grain boundary microstructure control and anion doping

I tried to solve the phase instability issue encountered in the long-term operation of bismuth oxide at intermediate-temperature, and further to explore the dynamics of phase transition and stabilization in depth. The former topic includes the well-defined study on how the grain boundary affects on phase transition kinetics of bismuth oxide. The epitaxial and nano-crystalline thin films, which possess completely different crystal growth kinetics, were rigorously manufactured by means of PLD method. For brief results obtained from electrochemical analysis, diffusion behavior of cations, and DFT calculations suggested that the phase transition associated with the rearrangement of cation sublattice can be prevented through removal of grain boundaries. Next, development of highly durable bismuth oxide based on a new composition through a fluorine (F-) anion doping strategy was proposed. The universal solid-state method was used to create new lattice environment with multiple anions rather than single oxygen ions, therefore aiming both kinetic effects based on application of local lattice strain and thermodynamic effect by increasing the configurational entropy of system. Briefly, by introducing fluorine elements up to relatively 10 mol% of the anion sites, roughly 14 times or more degree of improvement in long-term degradation characteristics was achieved. In detail, the fundamental cause of cubic phase instability was eliminated by inducing a decrease in the concentration of intrinsic oxygen vacant sites, and a decrease in the phase transition energy through introduction of fluorine was predicted through DFT calculation. The two strategies of durability improvement were carried out in this dissertation provided new methodologies of cubic phase stabilization of bismuth oxide, which had been limited within the introduction of single/multi-cation heterogeneous elements.