| Author: | Yao, Yunduo |
| Title: | Understanding water-oxidation reaction on nanocatalyst surface using in-situ and ex-situ microscopy and spectroscopy |
| Advisors: | Zhu, Ye (AP) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Hydrogen Water -- Electrolysis Electrocatalysis Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxvi, 166 pages : color illustrations |
| Language: | English |
| Abstract: | Hydrogen produced via water electrolysis is a promising approach to ease the current energy crisis and address the associated environmental concern. Traditional water electrolysis technology relies on precious-metal-based electrocatalysts (e.g., Pt-based electrocatalysts and Ir- or Ru-based oxides) to catalyze the sluggish two semi-reactions, anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), respectively. Hence, intensive efforts have been devoted to developing non-precious metal-based electrocatalysts as cost-effective alternatives, among which nickel-based electrocatalysts showed inherent activity in alkaline electrolytes, especially for OER. The rational design and development of the electrocatalysts highly relied on advanced characterization techniques to uncover the active state for electrocatalysis, including the morphology, structure, composition, and valance states, ect. Especially, the development of the operando/in-situ characterization techniques could capture the local evolution of the electrocatalyst during the synthesis and catalysis process in real-time, overcovering the limitation of ex-situ techniques. In this thesis, by combining various in-situ and ex-situ microscopy and spectroscopy techniques, the dynamic synthesis process, facet-dependent electrocatalytic properties, and catalytic mechanism of nickel-based electrocatalysts for OER and HER in alkaline electrolyte were investigated to reveal the dynamic structure-performance relationship. Guided by the comprehensive investigation of in-situ and ex-situ characterization, the modulation strategies on nickel hydroxides and oxides by facet engineering and reconstruction control were proposed to enhance the electrocatalytic performance. Initially, single-crystal β-Ni(OH)2 with a well-defined hexagonal platelets structure was synthesized, which was investigated by ex-situ and in-situ microscopy and spectroscopy for alkaline OER. Through investigation of the pristine and after OER state of the β-Ni(OH)2 via collaborated ex-situ studies, facet-dependent restructuring on the edge {101̅0} facets to form Ni3+ rich Ni1-xO was revealed, along with performance enhancement of ~16 fold. Further operando Raman spectroscopy study revealed the vital role of the intermediate NiOOH phase in OER catalysis and the restructuring pathway to Ni1-xO. The activity enhancement was attributed to an increase in the exposed active sites and optimized *OH affinity after surface-dependent surface restructuring. Hence, this study proposed an effective OER performance modulation strategy for nickel hydroxides, which is increasing the active edge facets area and facilitating the reconstructing. Furthermore, guided by the above study, assembled β-Ni(OH)2 with an around 3.8-fold edge thickness of the previous hexagonal platelets were prepared, which consisted of single-crystal platelets tightly assembled in the form of hollow spheres. The ex-situ TEM and operando Raman spectroscopy studies on the assembled β-Ni(OH)2 revealed the same facet-dependent reconstruction and the active role of the intermediate NiOOH, indicating the internal properties of β-Ni(OH)2. Furthermore, after the CV activation, the assembled β-Ni(OH)2@Ni1-xO performed excellent OER activity, which only 300 mV was demanded to reach the current density of 200 mA/cm2 and a 470-fold current enhancement than that of the benchmark IrO2 at the 1.5 V anodic potential was achieved. This study demonstrates a promising strategy to optimize metal-(oxy)hydroxide-based electrocatalysts for large-scale alkaline OER applications. Another research object is focused on the investigation of nickel oxides (NixO) by in-situ TEM and ex-situ characterization technologies comprehensively. Firstly, the facet-dependent OER activity of NixO was demonstrated by studying a series of single-crystal NixO with different exposed facets, the low-index (100), (111), and the high-index (311) facets, respectively. Attributed to the higher activity of NixO with {311} high-index facets exposed, further heterostructure Ni/NixO fabrication was on {311}-NixO via reduction reaction in the reduced environment (high vacuum and H2/N2, respectively) to further improve the activity. Attributed to the surface-sensitive in-situ STEM, the direct observation of the dynamic Ni exsolution and growth process during heating in vacuo were captured. Combined with the ex-situ and in-situ SAED, EELS, and TEM investigation, it demonstrated the epitaxial growth of metallic Ni particles on the NixO surface during heating in vacuum, resulting in the Ni/NixO heterostructure with the orientational relationship of [100]NiO||[100]Ni and [110]NiO||[110]Ni. Meanwhile, it was found that the optimized Ni/NixO heterostructure presented enhanced activity for alkaline OER and HER compared to the original NixO and metallic Ni, which was attributed to the improved electronic conductivity, reduced charge transfer resistance and synergistic effect of the constructed heterostructure. This work provides a fundamental understanding of the reduction process of nickel oxides vacuum annealing. It also gives further insight into the design and synthesis of excellent electrocatalysts by crystal-facet and heterostructure engineering. |
| Rights: | All rights reserved |
| Access: | open access |
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