藉由官能化導電高分子調控電極-電解質界面以探討析氫反應中的界面現象

Abstract

析氫反應在永續能源的應用中扮演關鍵角色。提升析氫反應的效率是釋放氫氣作為乾淨、可再生能源潛力的關鍵所在。除了催化劑本身的性能外，電極的表面性質以及局部電解質環境也對析氫反應效率有重大的影響。深入研究這些界面現象，對於提升反應性能至關重要。本文中通過設計多種類型的官能化聚(3,4-乙烯二氧噻吩)(poly(3,4-ethylenedioxythiophene), PEDOT)，系統性的探討電極表面性質、局部電解質環境與析氫反應效能之間的關係。 導電高分子在電化學析氫反應中被廣泛應用。其中，PEDOT及其衍生物具有出色的電化學性質與穩定性，使其成為各種電化學應用中極具潛力的材料。透過在EDOT分子中引入不同的官能基團，可以靈活的調控其表面與電化學性質，突顯了其在電極改質方面的優勢。 在本論文的第一部分，我們結合了親水性官能化PEDOT （poly(EDOT-SuNa)）與膠體微影技術，製備超疏氣電極表面。我們使用原子力顯微鏡以及潤濕性測試，分析電極表面性質，並測試其在1 M KOH中的析氫反應效率。研究結果顯示，電極之反應過電位顯著降低，這是由於電極表面的設計提升了氣泡脫附。之後，我們進一步探討了導電高分子薄膜之表面電荷對鹼性環境下析氫反應效率的影響，結果顯示不同分子之間並無顯著差異。我們推測這是因為電解質溶液中高濃度的離子主導了反應過程，從而削弱了高分子薄膜之表面電荷對析氫反應性能的影響。 延續上一部分的研究，在本文的第二部分，我們將重點轉向研究電極表面與電解質溶液中陽離子之間的相互作用。我們探討了官能化EDOT之分子結構與官能基如何影響鹼性析氫反應的效率。由於乙二醇(ethylene glycol, EG)結構單元對金屬陽離子具有親和力，因此我們合成了三種具有EG官能基的EDOT分子，將其以電化學聚合的方式塗佈在鎳網基材上，研究結果顯示，所有塗佈EG官能化PEDOT後的電極之析氫反應效能均有所提升。為了深入了解其背後的原理，我們使用了電化學耗散型石英晶體微天平、電化學阻抗分析和X射線光電子能譜。結果顯示在施加電位時，EG結構能在電極表面形成局部濃縮的陽離子環境，並通過陽離子與水分子間的非共價相互作用促進水分子的解離。此外，我們合成了具有二乙二醇、四乙二醇和六乙二醇官能基團的分子，用以研究EG鏈長度對析氫反應效率的影響。 在本文的第三部分，我們著重研究電極/電解質界面中的鹼金屬陽離子（Li+、Na+、K+ 和 Cs+）對鹼性析氫反應效率的影響。我們合成了側鏈上帶有冠醚的EDOT分子，並將其電聚合在金電極上。冠醚官能化的EDOT與金屬陽離子具有特異性的吸附作用，從而建構具有高陽離子濃度的電極表面，我們使用耗散型石英晶體微天平對此現象進行了探討。我們進一步在不同鹼性溶液中進行析氫反應的實驗，以探討陽離子種類對反應效率的影響。我們觀察到在不同陽離子環境下，經冠醚官能化EDOT修飾的電極與未修飾的金電極相比，析氫反應活性呈現不同的趨勢，這表明表面陽離子濃度的變化確實會影響析氫反應的效能。為了對此陽離子效應進行分子層級的解析，我們結合電化學與和頻光譜術，研究析氫反應過程中的界面水分子結構變化，雖未取得明確的光譜結果，但此系列實驗提供了寶貴經驗，包含儀器架設、樣品製備，以及電化學系統與非線性光譜整合時所面臨的挑戰，為後續深入探討電極/電解質界面中陽離子與水分子間的相互作用奠定基礎。

The hydrogen evolution reaction (HER) is pivotal for numerous sustainable energy applications. Improving HER efficiency is critical to unlocking the full potential of hydrogen as a clean and renewable energy source. In addition to the intrinsic catalytic ability of the catalysts, factors such as electrode surface properties and the local electrolyte environment can significantly impact the HER efficiency. Exploring these interfacial phenomena is crucial for optimizing the reaction performance. In this dissertation, we investigated the relationship between electrode surface properties, local electrolyte environments, and HER performance by introducing various types of functionalized poly(3,4-ethylenedioxythiophene) (PEDOT). Conducting polymers have been widely used in HER electrocatalysts. PEDOT and its derivatives exhibit outstanding electronic properties and stability, making them highly promising materials for various electrochemical applications. The ability to modify surface and electrochemical properties by incorporating different functional groups into the EDOT molecule highlights its advantages in electrode modifications. In the first section of this dissertation, we combined a hydrophilic functionalized PEDOT (poly(EDOT-SuNa)) with colloidal lithography (CL) to create superaerophobic electrode surfaces. Atomic force microscopy (AFM) and wettability measurements were conducted to investigate the surface properties of the nanostructured electrodes. HER efficiencies were tested in 1 M KOH, showing significant reductions in overpotentials, which were attributed to the enhanced bubble-releasing ability of the electrode design. We also investigated the impact of surface charge on HER efficiency by introducing polymers with different surface charges. However, no significant difference was observed, possibly due to the highly concentrated ions in the electrolyte solution, which minimized the influence of surface charge on HER performance. In the second section, we further investigated the interaction between the electrode surface and the cations in the electrolyte solution. Three types of ethylene glycol (EG)-functionalized EDOTs were synthesized. The EG group was selected due to its strong affinity to metal cations. Improved HER performance was observed on nickel foams (NF) coated with EG-functionalized PEDOTs. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) were employed to understand the origin of the enhanced HER performance. The results indicated that the EG groups create locally concentrated cations near the electrode surface under applied potential, promoting water dissociation through noncovalent interactions. Additionally, the impact of EG chain length was studied by synthesizing di-EG, tetra-EG, and hexa-EG functionalized molecules. In the third section, we focused on elucidating the role of alkali metal cations (Li+, Na+, K+, and Cs+) at the electrode/electrolyte interface during HER. Crown-ether-functionalized EDOT molecules were synthesized and electropolymerized onto Au substrates. The specific adsorption of metal cations on crown-ether-functionalized EDOTs enables the construction of electrode surfaces with elevated cation concentrations, which was probed by QCM-D. The HER efficiencies in different alkaline solutions were investigated to unravel the influence of cation identity on HER performance. The observed variation in HER performance indicated that surface cation concentration could indeed alter HER performance. To gain molecular-level insights, electrochemical sum frequency generation (SFG) spectroscopy was employed to probe cation-induced changes in interfacial water structure. While conclusive spectra were not obtained, the experiments offered valuable insights into the instrumental setup, sample preparation, and the technical challenges of integrating electrochemical measurements with nonlinear spectroscopy. These experiences provide a solid foundation for future studies of cation-water interactions at electrified interfaces.