Transition-metal based oxides for oxygen storage and energy-related applications
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The development of energy storage and conversion techniques with high efficiency and power density is of great importance for the sustainable development of our green world. Li-O₂ batteries with high theoretical energy density has attracted extensive attention. However there are still many issues waiting to be solved, such as low stability of cathode catalyst, as well as the deactivation of cathode by H₂O and CO₂ from air. Reversible solid oxide fuel cells can be used for electricity production by SOFCs and fuel production (H₂ and O₂) by SOECs. Thus, oxygen storage materials can bridge Li-O₂ batteries and reversible SOFCs with the purpose of increasing the whole efficiency of the system. The discovery of oxygen storage materials with reversible oxygen release/storage behaviours and high oxygen storage capacities dependent on temperature or oxygen partial pressures (e.g., inert and oxidation gases) still needs further research. The work in this thesis mainly focuses on the preparation of transition-metal based oxides (such as perovskite oxides, brownmillerite-type oxides, layered-perovskite oxides, coated β-MnO₂ nanorods, transition-metal doped CeO₂ nanocrystals) as oxygen storage materials and their energy-related applications, seeking to discover the principles for oxygen storage/release properties and their performance in energy conversion and storage applications. The prepared materials included nanostructured and bulk materials via various synthesis methods, including citrate-modified evaporation-induced self-assembly method, hydrothermal method, pechini method, as well as solid state method. This work investigated the oxygen storage capacities of several crystal structure types oxides based on transition-metals. Nanostructured La₀.₆Ca₀.₄Fe₁₋ₓCoₓO[sub](3-δ) and La₀.₆Ca₀.₄Mn₁₋ₓFeₓO[sub](3-δ) exhibit high oxygen storage capacities and stability under reductive 5%H₂/Ar, but the oxygen-storage content under inert argon is low, just about 0.2 wt%. Brownmillerite-type Ca₂AlMnO₅ is demonstrated to possess a large amount of oxygen release/storage capacities depending on temperature even under flowing oxygen, as well as high oxygen storage/release properties and reversibility under alternating inert and oxygen gases at 500 °C. Substituting Ga on the Al-site would reduce the oxygen storage capacities, even though these substituted samples still posses good reversibility. The effect of A-site species (Mg, Ca, Sr) have been also investigated and demonstrated. It can't obtain pure brownmillerite-type crystal structure when Ca is partially or totally substituted by Mg or Sr, resulting in poor reversibility and low oxygen storage capacities. Nanostructured layered-perovskite La₁.₇Ca₀.₃M₁₋ₓCuₓO[sub](4-δ) (M = Fe, Co, Ni, Cu) have also been investigated for oxygen storage and as potential cathodes for IT-SOFCs. Even though the as-prepared layered-perovskite oxides have been demonstrated to be good candidates as cathode materials for IT-SOFCs with high performance, they do not possess high amount of oxygen storage/release ability under inert atmospheres because of the robust phase stability. β-MnO₂ nanorods can release large amount of oxygen (ca. 9.2 wt%) with increasing temperature at about 560 °C under various gases (air, N₂). Coating β-MnO₂ nanorods with CeO₂ nanocrystals could result in lower temperatures for oxygen mobility and removal under N₂ because of the enhanced oxygen mobility between CeO₂₋ₓ and β-MnO₂, while coating β-MnO₂ nanorods with SnO₂ nanocrystals have no enhanced oxygen mobility behaviours. The results demonstrate the positive and negative synergetic effect between other metal oxides and β-MnO₂ on the oxygen migration. Cr- and Cu-doped CeO₂ nanocrystals (i.e. nanorods, nanocubes and nanoparticles) were chosen to investigate the effect of transition-metal doping on CeO₂ and their valence changes with temperature and various atmospheres, as well as their oxygen storage capacities. The effect of Cr- or Cu- doping on CeO₂ nanocrystal morphology and oxygen storage capacities have been investigated and demonstrated. This provides some basic information for transition-metals doped CeO₂ nanocrystal evolution and stability, as well as further applications in energy-related fields, such as three-way catalysts, electrode materials in solid oxide fuel cells and Li-air batteries.
Thesis, PhD Doctor of Philosophy
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Embargo Date: 2019-02-27
Embargo Reason: Thesis restricted in accordance with University regulations. Electronic copy restricted until 27th February 2019
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