高效能電催化與光電催化劑之開發暨催化反應之臨場與非臨場分析

Development of High-efficient Electrocatalysts and Photoelectrocatalysts and Their In-/Ex-situ Analysis

洪崧富(Sung-Fu Hung)

指導教授 ： 陳浩銘

國立臺灣大學/理學院/化學研究所/博士(2018年)

https://doi.org/10.6342/NTU201802945

摘要

光電催化反應為一個能直接將太陽能有效轉化成化學能的方式。在實際應用上，不同的催化劑在其本質上大幅地影響催化效能。吸光能力優異的光敏材料往往僅擁有較低的表面催化活性；反之，具有優異表面催化活性的電催化劑卻通常沒有光電轉換能力。因此，光敏劑與電催化劑的複合材料可有效結合兩者的長處，大幅增進光電催化活性。而電催化劑可在材料體與電解液間扮演重要的橋樑，其複雜的界面效應也會大幅影響催化活性。故除了新催化材料的開發外，也需要透過學理的探討以及臨場或非臨場量測的分析，才能夠藉由了解其催化反應的關鍵步驟而改良催化特性。
本研究論文著重於新催化材料的開發以及臨場分析技術的開發與反應機制的確立。本研究論文分為四個部分：第一部分為合成具有表面電漿共振特性吸收可見光的奈米金作為光敏劑，研究中發現奈米金的產氧催化能力不佳且光激發熱電洞的生命週期太短，故利用3-硫醇丙酸 (3-Mercaptopropionic acid) 複合產氧催化能力優異的氧化銥於奈米金表面來增強光激發熱電洞的動力學性質及催化能力，以改善熱電洞運用於水分解之效能，並以照光之X光吸收光譜觀測到奈米金-氧化銥複合材料中的熱電子注入半導體數目高於純奈米金系統，亦是使得光催化效果大幅提升的原因之一。由於修飾電催化劑可顯著改善光電催化能力，故第二部分著重於開發低成本、高效能的電催化劑，相對於氧化銥，氧化鈷為一個低成本的電催化劑，在掺雜鐵離子後可大幅降低產氧反應所需過電壓 (ηj=10 mA/cm2 = 229 mV)，並藉由X光吸收譜及電化學分析發現鈷離子在產氧反應中為主要催化中心，而鐵離子的主要作用為侷限鈷離子於四面體位點，使其在活性位點上能被持續催化，抑制鈷離子在催化反應的動態結構轉相時的多重路徑轉移，而大幅提升鈷離子的催化性能。由於傳統臨場X光吸收譜及X光繞射圖譜僅能觀測在催化環境中的材料結構變化，故第三部分著重在開發催化反應中的新臨場量測，藉由開發臨場高解析螢光偵測之X光吸收光譜 (High-energy-resolution Fluorence Detaction X-ray Absorption Spectroscopy, HERFD)，成功觀測到在產氧反應時鈷離子與電解液間的軌域交互作用，而將鐵離子摻雜至系統中時，鐵離子本身並不參與和電解液間的交互作用，但使得更多鈷離子的軌域與電解液產生交互作用，使得催化劑的本質活性大幅提升。為進一步降低設備複雜度及材料成本來實現具體應用的可能性，第四部分開發高效能、低成本之雙功能電催化劑，相較於磷化鈷之雙功能效能(554 mV)，鐵摻雜磷化鈷(355 mV)可大幅降低其過電位並大幅增加交換電流密度，藉由臨場穿透式X光繞射圖譜及臨場X光吸收光譜得知鐵離子可穩定磷化物之晶相並抑制氫氧化物之生成，使其在產氧反應時能快速轉換成具活性之非晶羥基氧化物，在產氫反應時則維持磷化物，而得到高效雙功能之表現。本研究論文成功開發新穎複合光電催化劑、高效能產氧電催化劑、高效能雙功能電催化劑，並開發新穎臨場觀測軌域交互作用之技術，期望這些研究成果能為現在面臨的能源及環境危機挹注些許幫助，渡過所面臨的危機並讓未來的生活更好。

關鍵字

光電催化劑 ； 電催化劑 ； 雙功能電催化劑 ； 電化學 ； 臨場分析 ； X光吸收光譜 ； X光繞射圖譜

並列摘要

Photoelectrolysis has been regarded as an effective approach to converting solar energy into chemical fuels. In empirical application, various catalysts can remarkably affect the overall intrinsic catalytic performance. Photocatalysts with superior light absorbing ability frequently exhibit weak surface catalytic ability, while electrocatalyst with the outstanding surface catalytic capability commonly cannot proceed the photoelectric conversion. Hence, the integration of photocatalysts and electrocatalysts can present their superb natures to considerably enhance the entire catalytic activities. Besides, the synergy in the composite system would also influence the activities. By conducting in-situ techniques, the reaction mechanism can be figured out and the catalytic properties can be manipulated and enhanced.
This dissertation is focused on the development of new catalysts, the synergy of composite material on photoelectrolysis, and the development of new operando X-ray technique. In this work, the results can be separated into four chapters, including: 1) The development of composite photocatalyst TiO2 NR-Au-IrOx. Plasmonic gold exhibits the superior visible light absorption via localized surface plasmon resonance effect, but its poor surface catalytic ability and the short lifetime of hot holes limit its catalytic performance. Hence, modification of iridium oxide on Au surface would greatly improve the kinetics of hot holes and the catalytic capability by forming tiered energy levels. The results of Ti L-edge of plasmonic electrodes under illumination indicates that the number of injected hot electrons in Au-IrOx is higher than that in pure Au, which is also the reason of high catalytic activity. 2) The development of electrocatalyst with low cost and high activity. As doping iron ions into cobalt oxides, the composite catalyst exhibits superb catalytic ability for oxygen evolution reaction (ηj=10 mA/cm2 = 229 mV). By means of X-ray absorption spectroscopy and electrochemical analysis, it is found that cobalt ions are in charge of the catalytic center, while iron ions primarily confine cobalt ions to the tetrahedral site to restrain the multi-path-transfer of cobalt ions during the dynamic structural transformation between spinel and oxyhydroxide, continuously activating the catalytic behavior of Co2+(Td) ions. 3) The development of operando high-energy resolution fluorescence-detected X-ray absorption spectroscopy. It is found that 3d orbitals of cobalt ions directly interacted with the electrolyte. Instead of involving themselves to interact with the electrolyte, iron ions in the system lead more 3d orbitals of cobalt ions to hybridize with the electrolyte, resulting in the enhancement of intrinsic catalytic activity. 4) The development of high-efficient and low-cost bi-functional electrocatalyst. The synthesized iron-doped cobalt phosphides present high activity in bi-functional performance in alkaline solution with nearly 100% Faradaic efficiency. In-situ transmitted X-ray diffraction identifies that iron ions would stabilize the phosphide phase, restraining the formation of hydroxides that impedes the active substance, and quickly convert into active oxyhydroxide for OER as well as preserve the active phosphide phase for HER to achieve high bi-functional activity.

並列關鍵字

photoelectrocatalyst ； electrocatalyst ； bifunctional electrocatalyst ； electrochemistry ； in-situ analysis ； X-ray absorption spectroscopy ； X-ray diffraction

參考文獻

Chapter 1

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(3) Listorti, A.; Durrant, J.; Barber, J. Solar to Fuel. Nat. Mater. 2009, 8, 929-930.

(4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38.