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Defect & Nanostructure Engineering of Copper-based Materials for (Photo) Electrochemical H₂ and NH₃ Productions

  • QU LI (아주대학교 일반대학원)

초록/요약

Copper-based oxides (e.g., CuO, Cu₂O, CuBi₂O₄, and CuFe₂O₄) are widely used materials for (photo)electrochemical (PEC) water splitting and nitrate reduction to produce hydrogen and ammonia. Due to its wide bandgap value (1.2 to 3 eV), it provides many promising light absorber candidates for applications of solar energy conversion. It is worth noting that the conduction band edges in most copper-based oxides surpass the hydrogen evolution potential, thereby thermodynamically promoting the water reduction reaction. Additionally, the d-orbital electron of Cu can inject charge into the high-energy π- orbital of nitrate to active N=O, thus promoting the electrochemical nitrate reduction reaction (e-NO₃RR) to synthesize ammonia. However, the stability of the copper-based catalysts needs to be improved. For PEC water splitting, we fabricated nanoporous CuBi₂O₄ (np-CBO) photocathodes using a facile solution method and studied their rapid thermal processing in controlled atmospheres (O₂, N₂, and vacuum) to control the surface point defects. We illuminated that controlling the RTP atmospheres and sequence strongly influenced the formation of point defects (copper/oxygen vacancy and Cu¹⁺), which is evident from the scanning transmission electron microscopy, X-ray photoelectron spectroscopy and electron paramagnetic resonance analyses. Significantly, the O₂-RTP treated CBO photocathode exhibited a greatly enhanced photocurrent density and stability than the pristine CBO. Also, we showed the reversibility of the formation of point defects and photocurrent responses via sequential RTP treatments. Conclusively, surface point defect engineering via RTP treatment in a controlled atmosphere is a rapid and facile strategy to improve charge transport and transfer properties of photoelectrodes for efficient solar water splitting. For the e-NO₃RR study, we synthesized nanoporous defect-rich CuO nanowires (nd-CuO NWs) electrocatalysts using a facile solution-flame (sol-flame) reduction strategy, and successfully controlled both surface defects and morphology to construct a highly active surface for converting NO₃- to NH₃. Obviously, the surface defects of oxygen vacancies were analyzed by X-ray photoelectron spectroscopy and electron paramagnetic resonance spectroscopy. Significantly, the nd-CuO NWs exhibited a greatly enhanced NH₃ yield rate, Faradaic efficiency, and selectivity of 0.48 mmol h-1 cm-2, 97.3%, and 86.2% at a lower reduction potential of -0.2 V vs. RHE in 1 M KOH with 2000 ppm NO₃- electrolyte, which are higher than other controlled samples. Noticeably, the defect-rich, manoporous structure provided a high electrochemically active surface area and fast electron transfer properties, leading to a high e-NO₃RR performance and stability. Hence, this work provided a rational design strategy for the rapid fabrication of defect-rich nanoporous catalysts for efficient electrochemical nitrate-to-ammonia conversion. Our study provides new insights into nanoporous structure design and point defects engineering to develop efficient PEC water splitting and electrochemical e- NO₃RR. Furthermore, it is expected that more efficient (photo)electrodes can be developed through additional research and development in the future. Also, the optimized synthesis method on the defects control applies to various oxide synthesis methods and is expected to be expanded to other energy fields, such as (photo)electrochemical, energy conversion, and storage devices.

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목차

Chapter 1. Introduction 1
1.1 Global energy crisis and climate change 1
1.2 Renewable and carbon-free energy sources 4
1.2.1 Hydrogen as a potential clean energy solution 4
1.2.2 Ammonia as a renewable energy source 4
Chapter 2. Literature Survey 6
2.1 Hydrogen energy evaluation 6
2.1.1 Hydrogen production via photoelectrochemical cell 6
2.1.2 Basic principle of photoelectrochemical water splitting 7
2.1.3 Copper-based oxides photocathodes 10
2.2 Ammonia synthesis methods 13
2.3 Electrochemical conversion of nitrate-to-ammonia 17
2.3.1 Mechanism and pathway of e-NO3RR 17
2.3.2 Application of copper-based materials in e-NO₃RR 21
2.3.3 Difficulties and challenges of e-NO₃RR conversion 24
Chapter 3. Point-defect engineering of nanoporous CuBi₂O₄ photocathode via rapid thermal processing for enhanced photoelectrochemical activity 26
3.1 Introduction 26
3.2 Experimental 29
3.2.1 Materials 29
3.2.2 Synthesis of nanoporous CBO (np-CBO) photocathode 29
3.2.3 RTP treatments 29
3.2.4 Material characterizations 30
3.2.5 PEC measurements 30
3.3 Result and Discussion 32
3.3.1 Synthesis and RTP treatment of np-CBO 32
3.3.2 Microstructure analysis of np-CBO 43
3.3.3 Point defect analyses 45
3.3.4 Optical properties 52
3.3.5 Comparison of PEC performance 54
3.4 Conclusion 82
Chapter 4. Defect-rich nanoporous CuOx nanowires for improved electrochemical nitrate-to-ammonia conversion 83
4.1 Introduction 83
4.2 Experimental 86
4.2.1 Preparation of Cu(OH)₂ NWs 86
4.2.2 Solution-flame (Sol-flame) synthesis of nanoporous and defect-rich CuO nanowires (nd-CuO NWs) 86
4.2.3 Synthesis of defective CuO NWs (d-CuO NWs) and control CuO NWs (c-CuO NWs) 87
4.2.4 Materials characterization 87
4.2.5 Electrochemical measurements 88
4.2.6 Detection method 88
4.2.7 N Isotope labeling experiments 89
4.2.8 Calculation of NH₃ yield rate, Faradaic efficiency, conversion, and selectivity 90
4.3 Result and Discussion 90
4.3.1 Synthesis, chemical and structural analyses of nanoporous and defect-rich CuOx nanowires (nd-CuO NWs) 90
4.3.2 Microstructural analysis control CuO nanowires (c-CuO NWs), defective CuO nanowires (d-CuO NWs), and nd-CuO NWs 99
4.3.3 Defects (oxygen vacancy and Cu¹⁺) analyses 106
4.3.4 E-NO₃RR performance of the electrodes 112
4.4 Conclusion 134
References 135
List of Publications 149
Curriculum vitae 150

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