Defect chemistry in perovskite titanate : from materials to interfaces

Abstract

The increasing demand for energy consumption and environmental protection accelerates renewable energy applications. Due to the intermittence supply of renewables such as wind and wave, the development of energy conversion and storage techniques is in urgent need. H₂, with high energy density, huge reserves and zero emissions, becomes one of the most promising energy carriers in the future. High temperature steam electrolysis and photoelectrolysis

are two promising methods for H₂ production that can use renewable energies like wind and solar. However, the catalysts degradation is still the main drawback for their wide application. This thesis explores a promising candidate perovskite titanate that can be used as catalyst substrates. The influences of A-site cation deficiency on materials microstructure, electronic structure and redox stability are the main issues studied in this project. This work illustrates the metal-oxide interactions enhancements when cation deficiency exists in perovskite titanate oxides. Promoted cation migration results in metal-oxide interface reconstruction, which in turn increases contact area and adhesion force between catalyst and substrate. Electron microscopy and thermogravimetry analysis showed enhanced particle stability on A-site deficient perovskite at 700 °C in redox atmosphere. The influence of cation deficiency on perovskites electronic structure was also discussed based on La and Cr co-doped SrTiO₃. Although A-site deficiency doesn’t contribute to band structure, it introduces electrons and increases carrier mobility. Thus, a dramatical enhancement in H₂ production rate was achieved in materials containing a small amount of cation deficiency. The highest H₂ production rate is 7.5 μM·h⁻¹ under visible light (>420 nm, 250 W).

The reduction of perovskite in strong reducing atmosphere not only creates oxygen vacancies, but also pushes B-site cations out the lattice when A-site cation deficiency exists. Here, a series of materials doped with Fe, Co, Ni and Cu were prepared and analysed to compare the different exsolution ability. Cation size, oxygen vacancies and doping level all affect the exsolution process of transition metals. N-type conductivity due to Ti reduction suggests the possibility usage for anode material in SOFC. The initial trial with such material achieved 0.7 W·cm⁻² for single fuel cell with wet H₂ (3%H₂O) at 900 °C. Also, it can produce H₂ efficiently when working in SOEC mode. The fundamental properties of A-site deficient perovskite titanate explored in this thesis gives insight for further material design related to various functionality.