| Author: | Li, Zhen |
| Title: | Development of transition-metal-based electrocatalysts for efficient seawater oxidation |
| Advisors: | Lee, Yoon Suk Lawrence (ABCT) Yao, Zhongping (ABCT) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Subject: | Oxidation-reduction reaction Transition metal catalysts Seawater Electrolysis Hydrogen as fuel Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Biology and Chemical Technology |
| Pages: | xi, 228 pages : color illustrations |
| Language: | English |
| Abstract: | Green hydrogen produced via water electrolysis is pivotal for replacing fossil fuels and achieving carbon neutrality. However, the large-scale deployment of water electrolyzers, which heavily rely on freshwater, raises concerns about water resource sustainability. Given that seawater constitutes 96.5 % of the Earth's water resources, producing green hydrogen through seawater electrolysis is an alternative and viable strategy for achieving dual-carbon goals. Compared to indirect seawater electrolysis, which requires a desalination pre-treatment, direct seawater electrolysis offers a simplified system with easier scalability and economic advantages. However, the efficiency of direct seawater electrolysis is hindered by the high thermodynamic energy barrier of water oxidation (1.23 V vs. reversible hydrogen electrode, RHE) and serious corrosion due to chlorine evolution reaction (CER) in the Cl⁻-rich seawater environment. Although noble-metal-based oxides, such as IrO₂ and RuO₂, exhibit excellent water oxidation performance, their scarcity limits their wide applications. Earth-abundant transition metals, with their d-orbital valence electronic structures, can interact with oxygen-containing intermediates, making them promising candidates for catalyzing the oxygen evolution reaction (OER). Despite intensive efforts in recent years, the electrocatalytic seawater OER performance of transition-metal-based catalysts remains unsatisfactory, largely due to the competition between OER and CER and severe electrode corrosion. Therefore, designing efficient OER electrocatalysts based on transition metals with strong anti-corrosion properties is essential to advance seawater electrolysis techniques. In this thesis, several important strategies, including element doping, heterojunction construction, and microenvironmental modulation, are adopted to regulate the electronic structure of active sites, optimize OH⁻ adsorption, and increase the overpotential gap between OER and CER. Additionally, the Lewis-acid adsorption principle and electrostatic-repelling effect are applied to reduce the Cl⁻ adsorption, thereby mitigating CER-induced corrosion. Four types of OER electrocatalysts, g-C3N4/Li-NiFe layered double hydroxides (LDH), Ni(OH)2/LiFePO4, Ni(OH)2/NiMoO4, and MoO3/Fe2O3/MoS2 have been successfully designed, exhibiting excellent OER activity and durability in seawater electrolyte. Chapter I summarizes the mechanisms of OER and CER, along with a brief review of recent progress in transition-metal-based electrocatalysts. Strategies widely used for improving OER activity and anti-corrosion properties are also introduced. Brief descriptions of characterization techniques and electrochemical methods are summarized in Chapter II. In Chapter III, Li doping and g-C3N4 hybridization were employed together to modify the structure of NiFe-LDH and study their effect on electrocatalytic seawater oxidation performance. Li-ion doping increases the Ni3+ population, while NiFe-LDH/g-C3N4 heterointerface redistributes interfacial charge and constructs a built-in electric field, thereby improving selectivity towards OH⁻. These strategies further decrease the OER Gibbs free energy from 0.72 to 0.53 eV, enabling the g-C3N4/Li-NiFe-LDH catalyst to stably operate seawater oxidation at 200 mA cm⁻² for 100 h. Chapter IV forges a bridge between the recycling of spent Li-ion batteries (LIBs) and seawater electrolysis. By employing pulsed laser ablation and electrodeposition techniques, Ni(OH)2 interfaced with laser-ablated LiFePO4 (Ni(OH)2/L-LFP) was fabricated. The NiOOH/Fe3(PO4)2 active species formed after surface reconstruction are particularly advantageous for promoting OH⁻ while concurrently suppressing Cl⁻ adsorption. Additionally, PO4³⁻ ions, leached during the reconstruction process, contribute to Cl⁻ ion repelling in seawater, mitigating catalyst corrosion. The Ni(OH)2/L-LFP demonstrates exceptional OER performance, achieving a current density of 10 mA cm⁻² at a low overpotential of 237 mV in alkaline seawater. It also maintains excellent stability at 100 mA cm⁻² for 600 h. Chapters V and VI focus on energy-saving seawater electrolysis. In Chapter V, the anion-adsorption strategy was used to modulate the local microenvironment on the catalyst surface, improving methanol-assisted seawater electrolysis performance. In situ leached MoO4²⁻ during the reconstruction process of Ni(OH)2/NiMoO4 (Ni(OH)2/NMO) pre-catalyst optimizes the coordination environment on NiOOH surface, simultaneously decreasing the adsorption energy for Cl⁻ and accelerating the proton-coupled electron transfer in the methanol oxidation reaction (MOR). Consequently, the Ni(OH)2/NMO-based full cell achieves current densities of 0.1 and 0.5 A cm⁻² at considerably lower cell voltages (1.840 and 2.324 V, respectively) in methanol-hybrid seawater compared to seawater electrolyte (1.904 and 2.392 V, respectively). Chapter VI discusses ternary heterojunctions of MoO3/Fe2O3/MoS2 and their effect on light-assisted seawater oxidation. The three-phase heterointerface favors OH⁻ adsorption and increases the overpotential gap between OER and CER, ensuring high OER selectivity. In situ leached MoO4²⁻ and SO4²⁻ species further reduce Cl⁻ adsorption, enhancing anti-corrosion properties. The catalyst demonstrates excellent stability at 300 mA cm⁻² for 500 h. Built-in electric fields at interfaces lower interfacial resistance and extend the lifetime of photo-generated carriers by 1.47-fold, achieving a 20.4 % increase in seawater OER current density under light irradiation. Finally, Chapter VII presents the conclusions of all research work and perspectives for future research directions. |
| Rights: | All rights reserved |
| Access: | open access |
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