title: Earth-abundant Materials as High-Performance Catalysts for Renewable Energy Applications
Creators: name: Liu, Xu
Publisher: name: Griffith University
Year: 2018
Subjects: subject: Renewable energy
subject: Earth-abundant materials
subject: Electrochemical catalysts
subject: Graphene
subject: Theoretical calculation

description: With increasing demand of renewable energy sources, electrochemical catalysts have attracted great attention as they are critically important for energy transformation devices such as fuel cell and metal-air batteries. Traditional high-performance electrocatalytic materials such as noble metal and noble metal-based oxides suffer from high cost and poor stability. To address this problem, other materials such as transition-metal-based compound-like oxides, chalcogenides, carbides, complexes and carbon-based materials have been considered and proven as promising candidates for alternatives to current noble metal-based electrocatalysts. This thesis attempts to develop high performance electrocatalysts based on earth abundant materials for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) process. To tackle the issues that limit the performance of such catalysts like poor electron conductivity, insufficient active sites and low mass transfer rate, various optimization methods were applied, including heteroatom doping, fabrication of porous structure and combining with a conductive substrate. Graphene, known as a star material, has been widely used in electrocatalysis because of its outstanding conductivity and large specific surface area. Extensive studies have been conducted to improve the catalytic performance of graphene-based materials. Generally, catalytic activity can be optimised in two methods: enhancing the intrinsic activity of the active sites and/or increasing the number of active sites. Both can be achieved at the same time. By chemically crafting diamino-benzene derivatives to ortho-quinone sites on holey graphene oxide (GO) edges via a simple condensation reaction, pyridinic nitrogen-decorated active sites for ORR can be obtained. The graphene-based electrocatalyst produced outperform Pt/C electrocatalyst in Zn-air batteries. This will be elaborated in Chapter 2. Transition metal-based compounds are also popular choices for electrocatalysts. A controllable synthesis method, developed to fabricate atomically thin CoSe2 nanobelt structures for water oxidation catalysis, will be discussed in Chapter 3. The as-synthesized material shows low overpotential, high current density, small Tafel slope and excellent catalytic stability, outperforming other assembled nano-structures. Furthermore, the electrode constructed from these nanobelts possesses a porous structure with highly accessible channels that allows facile electrolyte diffusion and efficient mass transfer. Spinel oxides are another class of material which also exhibits remarkable electrocatalytic activity. In Chapter 4, we investigated a series of Cobalt-doped spinel manganese oxide nanoparticles hybrid with GO nanosheets. The doping process alters the surface atomic arrangement and electronic property, which contribute to optimum adsorption behaviour of oxygen molecular and the ORR activities. When the dopant was 20 wt% (CMG-3), the doped material showed the best performance. The Zn-air battery assembled using CMG-3 as the air-cathode catalyst achieved good cycling performance at various current densities. Theoretical calculation is an effective tool for the design of catalyst and the exploration of the reaction mechanism. A study for low-dimensional metal-organic frameworks (MOFs) as HER catalyst was conducted. Using the Gibbs free energy of the adsorption of hydrogen atoms as a key descriptor, S atoms within one-dimensional MOFs are identified to be the preferred catalytic active sites for HER. The calculation results further revealed that the activities of part S atoms can be improved by interacting with alkali metal cations from the electrolytes; specifically, the influence of cations on the performance is dependent on the electron affinity of cations. This thesis highlight several achievements in developing earth abundant material-based electrocatalysts: (i) designing novel electrocatalysts based on the understanding of relevant mechanistic chemistry at the atomic level; (ii) developing facial synthetic routes for the preparation of earth-abundant carbon material, metal oxide and metal chalcogenide electrocatalysts; (iii) developing a surface doping approach to manipulate the surface electronic structure of metal catalysts; and (iv) evaluating the application potential of obtained material in Zn-air battery.