以電化學方式於碳布上成長金奈米結構及其在直接甘油燃料電池和直接乙二醇燃料電池 之應用

Abstract

近年來有愈來愈多在直接醇類燃料電池上的研究，包含了乙醇，IPA，乙二醇及甘油。比起氣態燃料，液態燃料擁有高能量密度及較佳的能源轉換效率，並且容易儲存和運送，因此被視為在未來有希望的替代能源。鹼性燃料電池可以使用較不昂貴的非鉑金屬作為催化劑，如金，銀，鎳等。而金在鹼性環境下有高度電催化活性並且十分穩定，因此我們選用金作電池的觸媒。 在本篇論文中，我們利用簡單的電化學成長法在碳布上製備出具有奈米結構的金，作為鹼性醇類燃料電池陽極觸媒。以金氯酸作為金的來源，並加入不同濃度的介面活性劑CTAC(cetyltrimethylammonium chloride)和硝酸鈉，固定還原電位成長出金奈米珊瑚和金奈米顆粒。 經實驗證實，金奈米珊瑚具有高表面積及對甘油和乙二醇有較高的催化能力和叫好的毒性耐受力。進一步我們將製備的奈米結構金電極作為陽極與自製的鉑電極和陰離子交換膜組成膜電極組，並應用於直接甘油燃料電池(DGFC)及直接乙二醇燃料電池。結果顯示以金奈米珊瑚為電極的情況下，有不錯的表現。DGFC可得開路電壓0.68 V以及最高輸出功率36.4 mW/cm2，DEGFC可得開路電壓0.61 V 及最高輸出功率30.1 mW/cm2。另外我們將電池連續作極化測試，實驗顯示經過五十次極化作用之後，DGFC的OCV 及最高輸出功率降幅分別為4.4 % 及 20.6%，而DEGFC的OCV 及最高輸出功率降幅分別為3.3 % 及 15.6%，在長時間作用之下降幅皆不大。因此，我們所製備的奈米珊瑚金電極不僅在電池上有相對良好的表現，且可經長時間使用。經過比較可以發現DGFC擁有較好的性能因金奈米電極催化甘油的能力較好，而DEGFC則有較好的耐久能力因乙二醇的氧化中間產物對金電極的毒化能力較弱。 最後，我們將燃料固定在五毫升，作用於電阻五歐姆的馬達運轉。實驗顯示兩者的工作時間皆相當長(超過2個小時)，但是由於副產物毒化了電極表面，產出電子與反應物的莫爾數比值仍不到1，仍有未來改善的空間。

Recently, more and more studies about direct alcohol fuel cell (DAFC), such as ethanol, iso-propanol, ethylene glycol and glycerol. Liquid fuels have a higher energy density and better energy efficiency, are more easily stored and transported than gaseous fuels, thus have been regarded as a very promising fuel cell technique in the future. It is well known that the alkaline fuel cell allows the use of inexpensive non-Pt metal catalyst such as gold, silver and nickel. Especially, Au is considered one of the promising catalysts as its good electrocatalytic ability and stable in alkaline media. In this work, we demonstrated nanostructured Au material deposited on carbon cloths can be used for direct glycerol fuel cell and direct ethylene glycol fuel cell. A simple two-electrode electrochemical deposition process was developed to grow gold nanostructures, including nanocorals and nanoparticles, on carbon cloths by reducing HAuCl4 under constant potential in mixtures containing cetyltrimethylammonium chloride (CTAC) and/or NaNO3. Electrochemical characterization showed that Au nanocorals exhibited high surface area and high electrocatalytic activity for glycerol and ethylene glycol oxidation in alkaline media, both the alcohols showed better toxic tolerances. Further, Au nanostructures as anode were assembled with anion exchange membrane (AEM) and Pt cathode for fuel cell. The open circuit voltages (OCV) and maximum power density for DGFC were 0.68 V and 36.4 mW/cm2, for DEGFC were 0.61 V and 30.1 mW/cm2, respectively. After 50 cycle polarization, the decay rates of OCV and power density for DEGFC are 3.3% and 15.6%, which for DGFC are 4.4% and 20.6%, respectively. DGFC showed higher OCV and power density, but DEGFC showed more stable output than DGFC in long-term performance, the results were corresponded to the half-cell test. However, although the operation time were quite long (over two hours) by using only 5 mL fuel, the electron/reactant molar ratio of both of them were lower than 1 because of the byproduct poisoning the Au nanocoral surface.