title: Development of perovskite materials for electrocatalysis
Creators: name: Pan, Yangli
Publisher: name: University of Southern Queensland
Year: 2021
Subjects: classificationCode: 401699
subject: Materials engineering not elsewhere classified
valueUri: https://www.abs.gov.au/statistics/classifications/australian-and-new-zealand-standard-research-classification-anzsrc/latest-release
classificationCode: 400499
subject: Chemical engineering not elsewhere classified
valueUri: https://www.abs.gov.au/statistics/classifications/australian-and-new-zealand-standard-research-classification-anzsrc/latest-release
classificationCode: 340604
subject: Electrochemistry
valueUri: https://www.abs.gov.au/statistics/classifications/australian-and-new-zealand-standard-research-classification-anzsrc/latest-release

description: There is growing interest worldwide in developing energy storage and conversion technologies for a sustainable energy future. In this pursuit, electro-catalysis plays a central role in enabling key electrochemical transformations involved in these energy technologies, for example, the oxygen evolution reaction (OER), an anodic reaction that is of paramount importance to the realization of water electrolysis, carbon dioxide reduction, and nitrogen fixation. However, because of a complex four-electron transfer process, the OER intrinsically suffers from slow reaction kinetics, which can cause considerable energy losses. Noble metal iridium- and ruthenium-based materials are recognized as the state-of-the-art OER catalysts, but their widespread use is limited due to the high cost and low abundance of these noble metals. Recently, perovskite oxides have demonstrated great promise as low-cost alternatives showing excellent catalytic activity toward the OER in alkaline solutions. The currently available perovskite candidates, however, still suffer from relatively poor activity, which is intrinsically associated with the scaling relations-induced limitations where the conventional adsorbate evolution mechanism (AEM) is at play during the OER. To address this concern, a new reaction mechanism that involves the participation of lattice oxygen, named lattice oxygen-mediated mechanism (LOM), has been proposed for perovskite oxide catalysts and demonstrated to be capable of bypassing the scaling relations and contributing to enhanced OER catalysis. Till date, a systematic understanding of the structure–activity relationship is still lacking on perovskite catalysts which operate via the LOM mechanism. This thesis focuses on the materials engineering of novel perovskite oxides in favor of the LOM mechanism to achieve catalytic enhancements toward the alkaline OER. By taking advantage of the perovskite's versatility in chemical composition, defect, and structure, diverse material design strategies are explored, including doping of foreign elements, introduction of cation deficiencies, and formation of composites, which are found to induce the lattice oxygen participation to lead to improved OER catalysis. To provide direct evidence about the LOM participation and its contribution to OER catalysis, a series of silicon (Si)-doped strontium cobaltite perovskites, SrCo1−ySiyO3−δ, are designed in the first experimental chapter. Si is selected as the dopant because its incorporation can modify the oxygen vacancy content without contributing to additional OER catalysis due to its inertness. The SrCo1−ySiyO3−δ series are found with similar surface transition metal properties but different levels of oxygen vacancy concentrations and oxygen diffusion rates, providing an excellent platform for studying the role and degree of lattice oxygen participation in the OER. Notably, the doped sample with optimum Sr inclusion exhibits an order of magnitude higher OER intrinsic activity, matching closely with a 12.8-fold enhancement in the oxygen mobility. These results strongly support the important role of LOM in substantially contributing to the OER activity. To further understand the LOM participation and its relationship with the oxygen vacancy defect, a group of A-site Sr-deficient perovskite oxides, La1/3Sr(2−3z)/3Co0.5Fe0.5O3−δ, are designed in the second experimental chapter. Unlike the doped perovskite systems where the incorporation of dopant can also modify the perovskite crystal structure or electronic structure, the introduction of Sr deficiency is found to have little impact on these structural parameters, as verified by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy, but only to modify the oxygen vacancy defect. Based on defect chemistry, introducing A-site Sr-deficiency increases the amount of oxygen vacancies and concurrently improves the oxygen mobility, both of which strongly correlates with the enhanced OER activity. These results provide solid evidence that enhanced lattice oxygen participation is crucial to boosting the OER on LOM-based perovskite catalysts. While the LOM participation is beneficial to the OER catalysis, the activity of the single-phase perovskites remains to be insufficient to meet the requirements for practical use. To achieve higher activity, a number of Ruddlesden–Popper perovskite/simple perovskite (RP/SP) dual-phase composites, LaSr3−yCo1.5Fe1.5O10, are designed in the third experimental chapter. By utilizing a cation deficiency-induced phase separation strategy, RP/SP composites are self-assembled in a one-pot synthesis and feature a strongly interacted interface, as supported by transmission electron microscopy imaging. Markedly, all the RP/SP composites share the same phase structure and phase composition and only differ in the phase concentration, thereby serving as an ideal platform to distinguish the contribution of interfacial interaction from other interfering factors to the enhanced OER activity of composite catalysts. The strong coupling between the RP and SP phases contributes to enhanced oxygen ionic transport, which favors the OER catalysis utilizing the LOM mechanism. By optimizing the phase concentration, exceptional OER activity is achieved on the RP/SP composite catalyst, outperforming most of the reported single-phase or dual-phase perovskite-based catalysts. Overall, this thesis highlights the advantages and prospects of tuning perovskite properties to develop better OER catalysts through leveraging the LOM mechanism. The results obtained from this thesis aim to further our understanding of the structure–activity relationship on perovskite oxide catalysts. This thesis is expected to have implications for the design of perovskite electro-catalysts and other types of catalyst materials for a wide variety of energy applications.