A Bottom-Up Synthesis of Multi-component Metal Oxides for Energy-storage and Catalytic Applications

Abstract

The investigation into high-entropy alloys (HEAs) was inspired by the ‘cocktail effect’ of alloy manufacture. However, unlike conventional alloys that usually compose of a principal element and multiple additional elements, these HEAs are extreme examples of alloys as they usually involve multifold principal elements in the same ratio. The study of high-entropy materials was then extended to high-entropy oxides (HEOs), or more precisely, entropy-stabilized oxides (ESOs). Intriguingly, these multi-component solid solutions can be crystallized into a variety of crystal structures and stabilized as single-phase materials, indicating that all metal species are randomly incorporated in the crystal structures of HEOs. These structural and compositional features of HEOs enable them to be different from any of their component binary oxides. Therefore, studying the impact of structure and composition of HEOs on their physical and chemical properties is of great interest for materials scientists and inorganic chemists. In this doctoral project, we successfully deposit five different cations (Mg, Co, Ni, Cu, Zn) into one-dimensional metal-organic chain precursor particles by employing oxalate ions as the bridging ligands. Followed by annealing of the precursor, the metal-oxalate complex precursor can be transformed into an entropy-stabilised oxide (ESO). The obtained ESO has a single rock salt phase. The as-obtained ESO particles are employed as the anode material for lithium-ion batteries and show outstanding cycle performance. One step further, we extend our synthesis strategy to the fabrication of a brand-new HEO material, i.e., (MgMnFeCoNi)O. In this compound, all involved metal ions are presented as divalent cations and the final oxide product has a single-phase rock salt structure. Subsequently, we define this HEO material as multi-component solid solution (MSS) oxide because the cation sites in this oxide are equally occupied by different cations. The microstructure and local environment of this MSS oxide are then investigated in detail. This MMS oxide is further treated to be a potential catalyst for dry reforming of methane.