Micro-structure change and crystal-structure modulated of oxygen carriers for chemical looping: Controlling local chemical environment of lattice oxygen

Chemical looping conversion shows great potential in the renewable energy sector. The successful execution of chemical looping conversions relies heavily on oxygen carrier, which is also referred to as the redox catalyst or metal oxide materials. The oxygen carrier supplies the necessary quantity of oxygen ions (specifically lattice oxygen) for the conversion of fuels during the process of reduction. The oxygen-deficient oxygen carrier is refilled with molecular oxygen from air in an oxidizer. The redox-driven ionic diffusion and the formation of surface oxygen vacancies leads to a modification in the microstructure and crystal-Structure of oxygen carrier particles, resulting in the development of a new local chemical environment and the creation of oxygen vacancies. These are considered as active sites in chemical looping. Significant research has been conducted in recent years to enhance the performance of oxygen carrier through the modulation of their component design, structural construction, and modification for different chemical looping conversions. Nevertheless, the oxygen carriers' micro-structure and crystal-structure has been given inadequate focus, encompassing aspects such as lattice deformation, site asymmetry, strength of Me-O bonds, and tilting of the crystal lattice, among others. The focus of this review is on the progress made in controlling the diffusion of ions, phase segregation, and manipulating the local chemical environment of O in oxygen carrier. These advancements will aid in the investigation of oxygen carrier for effective chemical looping conversion.