Surface-mediated electrocatalysts upon laser irradiation technique for water electrolysis

• Main Author: Lu, Yu
• Other Authors: Huang Yizhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Chemistry::Physical chemistry::Electrochemistry; Engineering::Materials::Nanostructured materials
• Online Access: https://hdl.handle.net/10356/163870
• Institution: Nanyang Technological University

In the modern world, the energy crisis and air pollution problem trigger the research to find out efficient, sustainable, and eco-friendly energy resources to replace fossil fuels. One of the promising substitutes is hydrogen energy, which is considered a clean, abundantly available fuel with high energy density. Besides, hydrogen can be a superior medium that stores and transports the energy in a chemical form, able to improve the flexibility and efficiency of energy system. Few sectors such as aerospace has started to use hydrogen energy. The utilization of hydrogen energy extending to other industrial areas and social communities still relies on the advancement of hydrogen energy conversion system. One of the most significant hydrogen conversion systems is water electrolysis, which has been widely implemented to generate hydrogen from earth-abundant water. The electrochemical conversion from electrical energy to chemical energy in hydrogen production requires overcoming a high energy barrier. Therefore, water electrolysis always accompanies catalysis to facilitate the hydrogen generation efficiency. In this case, developing novel electrocatalysts with low cost and excellent performance becomes a key issue in research, which can promote the progress of next-generation electrochemical energy conversion techniques. This project focuses to improve the catalytic performance of electrocatalysts through the surface process using the laser irradiation technique. Two kinds of surface process approaches, i.e. vacancy engineering, and heteroatom doping are implemented by laser irradiation to manipulate the active sites of the electrocatalysts and transform them from ordinary to extraordinary ones. For the vacancy engineering strategy, Co3O4, one of the promising OER electrocatalysts, is chosen for this thesis, which is grown in the form of nanosheet arrays. Massive vacancy defects are generated within the nanosheets upon the laser irradiation The laser irradiation ablates the Co3O4 surface forming nanosized holes, which expose more active sites in contact with electrolyte. Different atomic morphologies of the Co3O4 nanosheet surface were manipulated in response to the various laser parameters. The vacancy defects in the Co3O4 lattice can be characterized using scanning transmission electron microscopy (STEM) in high angle annular dark-field (HAADF) mode. As to be expected, the laser irradiated Co3O4 nanosheet shows significant improvement of the OER performance, as proved by the electrochemical surface area (ECSA) and linear sweep voltammetry (LSV). It is the result of the superior kinetic activity of vacancy sites that brings about the substantial increase of exposed active sites over the surface of Co3O4 nanosheet. DFT calculations confirm that both Co and O vacancies could optimize the electronic structure of Co3O4, and the adsorption energy of the H2O molecule is enhanced at the vacancy sites. For the heteroatom doping strategy, the laser-induced carbothermal shock is firstly used to fabricate nanosized multi-element alloy nanoparticles on the multi-wall carbon nanotube (MWCNT). The morphology and lattice structure of synthesized alloys are manipulated when subject to the laser irradiation parameters. Various hollow active sites are formed on the catalyst surface by tuning the composition of alloys. HAADF-STEM is performed to identify the hollow active sites formed by heteroatoms. Furthermore, the various kinds of hollow active sites play bifunctional roles for oxygen evolution and hydrogen evolution reaction (OER and HER), which simplify the complex electrocatalysis system. DFT calculations prove that the hollow sites optimize desorption energies for the products, facilitating the rate of hydrogen and oxygen generation. This research provides a powerful laser technology to engineer vacancies and heteroatom doping to created surface-mediated electrocatalysts. The strategies based on laser irradiation are rapid, facile, cost-effective, and controllable, which could place an insight into the design and improvement of a wide range of metal-based electrocatalysts for the large scaling in industry.