全球暖化以及石油資源的減少，促使著科學家尋找乾淨的替代能源載體。由於氫有著高能量密度以及對環境友善的特質，普遍被認為是理想的選擇。經由太陽能或電力來分解水分子以生產氫的作法，與直接使用光催化來裂解水相比之下較為理想，因而在近年來獲得了極大的關注。但由於相關技術尚未成熟，並且必須配合地區與當地氣候。產氫反應（HER）以電化學方式將水分解為氫可以生產大量高純度的氫，在現階段仍是最實用的方式。在HER的使用上，Pt貴金屬是最有效的低過電位電催化劑，但是Pt的高成本加上Pt在地球上很稀少，使之無法大規模的應用。因此，科學家正積極的研究，期望開發一個高效率，高穩定性，低成本並兼顧環保的HER電催化劑。

本研究的主要目的是開發原創的Co-Nx電催化劑，並通過調整d波段中心來促進析氫反應。我們結合實驗和密度泛函理論（DFT），將金屬配位的變化與催化劑活性的變化聯繫起來。在這個實驗中，我們研發出兩個可用於HER的CO-Nx電催化劑。第一種是N-Co-C的HER電催化劑。 N-Co-C催化劑是使用維生素B12與還原的氧化石墨烯（RGO）作為金屬前體，在高溫下透過一步熱裂解方法製作而成。第二項實驗則是使用衍生自維生素B12和硫脲以及RGO作為催化劑載體的N-Co-S電催化劑。我們觀察到N-Co-X（X = C和S）催化劑的電催化活性與熱解溫度，酸處理和金屬負載有著密切的關連性。合成的催化劑可透過不同的特徵分類為XRD，XPS，XAS，TEM和DFT。實驗結果顯示，當Co-corrin合成物（維生素B12）會與RGO一起在高溫下分解形成N-Co-X結構。這個結合物引發鈷的d-帶中心的下降，進而導致氫的結合能量降低。這反過來有利於Hads的電化學脫附並導致相對適中的Co-H結合強度，進而增強產氫反應。

Running out of petroleum resources and rising the global temperature have urged the search for clean alternatives as energy carriers. Hydrogen is considered as an ideal candidate; thanks to its high energy density and environmental friendliness. Hydrogen production via water splitting by solar energy or electricity has attracted great attention in recent years. In comparison with photocatalytic water-splitting directly using solar light, which is ideal however the relevant technologies are not yet mature and mainly depend on weather and regions. Electrochemical splitting of water into hydrogen gas via the hydrogen evolution reaction (HER) offers a promising solution for high purity and large quantities of hydrogen production, which is more practical at the current stage. For HER, Pt-group noble metals are the most effective electrocatalysts with low overpotential; however, Pt is a high-cost metal and rare on earth, which significantly hindered the large scale application. Therefore, tremendous efforts have been actively made in developing electrocatalysts with high efficiency, high stability, low cost, and environmental friendliness for HER.
The main goal of this study is to develop and understand the novel Co-Nx electrocatalyst to promote the hydrogen evolution reaction by tailoring the d-band center. Here, we combine the experimental and density functional theory (DFT) to correlate changes in the metal coordination to changes in catalyst activity. In this work, two Co-Nx electrocatalyst systems are developed for their potential effect on HER. The first is HER electrocatalyst based on N-Co-C system. The N-Co-C catalysts are prepared by a one-step pyrolysis process at the high temperature in which vitamin-B12 was used as a metal precursor together with reduced graphene oxide (RGO). The second is N-Co-S electrocatalyst, which is derived from vitamin-B12 and thiourea together with RGO as catalyst support. We observed that the electrocatalytic activities of N-Co-X (X = C and S) catalysts are strongly related to the pyrolysis temperature, acid treatment, and metal loading. The as-synthesized catalysts were characterized by XRD, XPS, XAS, TEM, and DFT. The results indicate that Co-corrin complexes (Vitamin B12) together with RGO have been decomposed at high temperatures to form N-Co-X structure. This conjugation induces a downshift of the d-band center of cobalt, and thereby decreases its hydrogen binding energy. This, in turn, favors the electrochemical desorption of Hads and leads to a relatively moderate Co−H binding strength, resulting in the enhancement of the hydrogen evolution reaction.

摘要...........................................................i
ABSTRACT.......................................................iii
ACKNOWLADGEMENTS...............................................v
TABLE OF CONTENTS..............................................vi
LIST OF ABBREVIATIONS..........................................ix
LIST OF FIGURES................................................xi
LIST OF TABLES.................................................xx
Chapter 1 Introduction.........................................1
1.1 General background.....................................1
1.2 Hydrogen energy...............................2
1.2.1 Hydrogen energy carrier................................2
1.2.2 Hydrogen production....................................3
1.3 Hydrogen evolution catalysts...................6
1.3.1 Electrochemistry of the hydrogen evolution reaction....6
1.3.2 Platinum group metal (PGM) electrocatalysts............9
1.3.3 Non-PGM electrocatalysts...............................9
1.4 Motivation....................................10
1.5 Thesis layout..................................13
Chapter 2 Experimental methods and characterization techniques.....................................................14
2.1 X-ray analysis.........................................14
2.1.1 X-ray photoelectron spectroscopy (XPS).........14
2.1.2 X-ray diffraction (XRD)........................15
2.1.3 X-ray absorption spectroscopy (XAS)............15
2.2 Electron microscopy analysis...........................16
2.2.1 Scanning electron microscopy (SEM).............16
2.2.2 Transmission electron microscopy (TEM).........17
2.3 Other characterizations................................18
2.4 Electrocatalyst preparation............................18
2.5 Electrochemical measurements...........................20
Chapter 3 N-Co-C electrocatalyst for hydrogen evolution reaction.......................................................23
3.1 Introduction...........................................23
3.2 XRD and XPS characterizations..........................24
3.3 Microscopy analysis (SEM and TEM) .....................27
3.4 Effect of acid treatment...............................28
3.5 XAS and other characterizations........................31
3.6 Hydrogen evolution reaction activities of as-prepared N-Co-C electrocatalyst................................................36
3.7 d-band calculation from DFT............................46
3.8 Summary................................................47
3.9 Supplementary notes and figures........................49
Chapter 4 N-Co-S electrocatalyst for hydrogen evolution reaction.......................................................58
4.1 Introduction...........................................58
4.2 Microscopy analysis (SEM, TEM, and STEM) ..............60
4.3 XRD and XPS characterizations..........................62
4.4 XAS and other characterizations.….....................66
4.5 Hydrogen evolution reaction activities of as-prepared N-Co-S electrocatalyst................................................71
4.6 Summary................................................84
4.7 Supplementary figures..................................85
Chapter 5......................................................92
5.1 Conclusions............................................92
5.2 Future direction.......................................93
References.....................................................95
Publications...................................................106
Appendix A.....................................................107

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