以支撐式類囊體膜片形成具耐受性和穩定性之生物陽極及其在光電解產氫之應用

Abstract

光合作用在類囊體膜上透過電子傳遞的方式以高效率進行光能轉換。人們為 了能直接利用這個高效率的系統作為光能轉換媒介，因此嘗試研發以類囊體為基 的生物陽極作為多樣化的應用。以往關於類囊體陽極的研究著重於電極材料開發 的研究，至今無人針對類囊體本身的型態對於類囊體陽極可能造成的影響作出探 討。在本論文中，我們通過調控滲透壓使原本堆疊的類囊體膨脹。相較於其他研 究中所使用的未膨脹類囊體，膨脹的類囊體囊泡進行鋪膜可使類囊體膜與電極基 材間擁有較有效的接觸。此提升的接觸可使我們的類囊體陽極收穫更多的光電流 且對於空氣水界面張力更具耐受性。我們也發現添加較多類囊體的陽極與白金電 極製成的光燃料電池，其光電流在較高操作電壓以及長時間連續的照光下會有更 顯著的衰退。我們認為多層類囊體膜使電子傳遞效率降低因而產生活性氧化物致 使類囊體膜蛋白失活，導致光電流衰退。這部分的結果也支持了我們以膨脹類囊 體鋪膜的方式製備與電極基材良好接觸使電子有效傳遞的生物陽極之重要性。 接著我們將支撐式類囊體膜片應用於光電解池產氫上，並找尋方法降低產氫 所需的額外電壓。透過循環伏安法分析金與白金的氧化還原，並觀察了不同電壓 下的電位變化情形，提出了金在氧化水的反應產生前為速率限制電極，而初始的 電壓改變的幾乎都是金的電位;水解反應開始後白金轉為速率限制電極，使陰極 電位下降。因此，我們提出需將陽極材料換成可在較低電位行水解反應的碳紙及 調整了陰陽極間的相對大小，發現皆可使白金在較小偏壓時即轉變為速率限制電 極，降低系統產氫所需的電壓。微生物電解池因為有生物物質的參與，無法像一 般的電解池在強酸環境下操作，以及需要緩衝液來維持恆定的環境。因此我們使 用了弱酸緩衝鹽-檸檬酸鈉，來維持酸鹼恆定並同時進行弱酸催化效應。我們發現 在酸鹼值一旦低於檸檬酸緩衝鹽的解離常數時，所需產氫電壓皆有顯著降低;也 發現類囊體陽極光電流在扣除掉陽極基材本身造成的影響後，會隨著氫離子濃度 下降而下降，但過酸的環境也會造成其光電流下降，推測是超出其原生酸鹼環境 所致。綜合以上的結果，我們認為類囊體膜以直接電子傳遞的方式在光電解產氫 的反應中扮演的角色是提供額外的電子來源，使光電解系統有更佳的產氫效率。

Photosynthesis is a high-efficiency energy conversion process through electron transfer on thylakoid membranes. People sought to utilize this high-efficiency system directly and therefore develop thylakoid membrane-based bioanodes for various applications. Most research focused on electrode material developments. To the best of our knowledge, no one ever reported the influence of the physical structures of thylakoids on the performance of TM-based bioanode. In this study, we manipulated the osmotic pressure and expanded the naturally stacked thylakoids into expanded thylakoid liposomes. The thus deposited thylakoid membranes (TMs) have a larger and more effective contact area than the naturally stacked thylakoid membranes used in other research. The effective contact elevates the photocurrent yield and durability to air-water interfacial peeling force for the expanded TM/Au bioanode. Furthermore, we found that PFCs made of high-load TM bioanodes had fast photocurrent decay under continuous operation at high cell voltages. We reasoned that this observation with the poor communication of large numbers of TMs at the high-load TM bioanodes could cause more ROS accumulation and therefore decrease the operational stability. This result supports the importance of effective contact between TMs and the electrodes. Second, we applied the TM/Au bioanode to the photo-electrolysis cell for hydrogen production and sought ways to lower the required bias. We analyzed the cyclic voltammograms of Au and Pt electrodes and the characterization results of the TM/Au-Pt PEC. We hypothesized that the Au was the rate-limiting electrode before reaching the potential where water oxidation occurred. Therefore, the applied bias allocated more increased potential to the Au anode. After the water oxidation occurred at the anode, the cathodic potential decreased, and the Pt cathode became the rate-limiting electrode. Hence, we devised two strategies, including altering the anode substrate to carbon paper with a lower water oxidation potential and enlarging the relative electrode size ratio between the anode and the cathode. Results show that both strategies can make the Pt cathode become the rate-limiting electrode at a lower bias and therefore lower the required bias for hydrogen generation. Since MEC systems cannot work under strong acid or strong base environments like the commercial electrolyzers. The near-neutral environment has low aqueous free protons and has difficulty in hydrogen production. Therefore, the use of buffer solution as an electrolyte can not only maintain the pH but also serve as another proton donating source for HER, which is called the weak acid catalytic effect. We applied the concept of weak acid catalysis to our PEC system. We found that the required biases decreased significantly from pH 6.8 to pH 3.0 when the pH was lower than the pKas of the citric acid but remained unchanged when the pH was further lowered to pH 2.0. The photocurrents exclusively from TM could be elevated with the pH lowering to the philosophical relevant pH of the TM but decreasing when it was too acidic. Our results suggest that the TM can play a role in supplying more electrons to elevate the energy conversion efficiency.