| Author: | Kim, Daekyu |
| Title: | A study on electronic structure modulation of 3d transition metal-based heterostructures for efficient electrochemical water oxidation |
| Advisors: | Lee, Yoon Suk Lawrence (ABCT) Wong, K. Y. (ABCT) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Electrocatalysis Catalysts Metallic oxides Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Biology and Chemical Technology |
| Pages: | xix, 215 pages : color illustrations |
| Language: | English |
| Abstract: | Amid the growing concerns regarding the energy crisis resulting from the depletion of fossil fuels and related environmental deterioration, electrochemical water splitting has emerged as a promising technology for producing clean and high-purity hydrogen. However, the overall water-splitting efficiency is hindered primarily by the sluggish kinetics of oxygen evolution reaction (OER). Although noble metals such as Ru-and Ir-based materials have shown excellent OER activities, their scarcity and high-cost limit practical applications. As an alternative, transition metal-based nanomaterials have been explored due to their tunable catalytic properties, good stabilities, and cost-effectiveness. Nevertheless, achieving the same performance level of noble metal-based catalysts remains a challenge, and it requires the rational design and proper tuning of transition metal-based electrocatalysts. Recent studies have revealed that the intrinsic catalytic properties are closely linked to the electronic structure of catalysts. Tuning their electronic structures through interface engineering and heteroatom doping can optimize catalytic properties for OER. This thesis presents three unique NiFe-based oxides constructed as heterostructure OER catalysts using interface/doping strategies. 1) A novel nanoarchitecture of oxygen vacancy-rich NiFe2O4−x nanoparticles (NPs) anchored on NiMoO4 (NMO) nanowire arrays was prepared. By adjusting the synthetic conditions of the precursor, NiFe Prussian blue analogue (NiFe PBA), the physicochemical properties of resulting NiFe2O4−x NPs were successfully tailored. The intimate interface between NiFe2O4−x and NMO nanowire enables excellent OER activity with a low overpotential of 326 mV at a high current density of 600 mA cm−2 and good stability. In situ Raman spectroelectrochemical investigations reveal that the interface is crucial for facilitating a rapid phase transition to the active γ-NiOOH phase, which lowers the overpotential of water oxidation. 2) A defect-rich samarium orthoferrite interfaced with samarium (Sm)-doped nickel ferrite (SFO/Sm-NFO) nanoarchitecture was synthesized. The incorporation of Sm forms the SFO interfacing with Sm-NFO, inducing charge re-distribution and generating high-valent Ni species. In addition, Sm occupying the tetrahedral sites in NFO yields Sm-NFO with a reinforced inverse degree. These synergistic effects improve OER performance, as manifested by a low overpotential of 370 mV at 1,000 mA cm−2 and excellent stability for 100 h. In situ Raman spectroelectrochemistry and density functional theory (DFT) calculations elucidate the active species and the optimal binding energies of the reaction intermediate, providing insights into the optimized OER activity of SFO/Sm-NFO. 3) Dynamic surface reconstruction processes play a crucial role in the electrocatalytic activity of most OER electrocatalysts. This study investigates the surface reconstruction process of NiFe2O4 interfaced with NiMoO4 (NFO/NMO) and the impact of Ru doping on this process and catalytic activity. Ru doping was found to tune the electronic configuration of NFO/NMO and induce the high-valence state of Ni3.6+δ. This effect promotes the surface reconstruction to highly active Ru-doped NiFeOOH/NiOOH, resulting in excellent OER performance with a low overpotential of 350 mV at 1,000 mA cm−2 and good stability at varying current densities for 80 h. The remarkable improvement in OER activity is attributed to the increased charge density and optimized intermediate adsorption energy through Ru doping. |
| Rights: | All rights reserved |
| Access: | open access |
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