奈米碳管於直接甲醇燃料電池電極上之應用與特性分析

Abstract

燃料電池由於具有無污染、高效率、低噪音等優點，被視為下一個世紀最具淺力的替代能源，其中以可微小化的直接甲醇燃料電池最受青睞。本研究主要乃是將具有高表面積、高導電性之奈米碳管作為直接甲醇燃料電池之電極材料，藉以改善直接甲醇燃料電池中觸媒用量過大、利用率不佳及反應速率過低等缺點。 本研究分兩個階段實驗，第一是將奈米碳管直接成長於碳布上作為直接甲醇燃料電池電極之觸媒載體，嘗試分別以濺鍍法及濕式沉浸法(wet impregnation)來披覆鉑(Pt)金屬觸媒，針對所製作之膜電極組(membrane electrode assembly)進行電化學分析及單電池效能測試。實驗結果發現，以濺鍍法雖可合成較小粒徑之鉑觸媒，但礙於載體的遮蔽效應(shield effect)，鉑觸媒的質量效能(mass efficiency)，濺鍍時間超過15分鐘以上時有漸減的趨勢，造成燃料電池的效能提升幅度有限。然而利用濕式沉浸法雖無遮蔽效應的影響，但由於碳管表面為化學惰性，導致觸媒的群聚現象，造成直接甲醇燃料電池的能量密度(power density)，在鉑的披覆量為0.4 mg/cm2時，70oC的操作溫度之下，甲醇燃料電池的能量密度只有4.6 mW/cm2。而在2.0 mg/cm2的高鉑披覆量之下，亦只達到27.4mW/cm2。 因此，為了解決觸媒群聚的現象及有效提升電池效能，第二階段我們嘗試對奈米碳管表面進行改質處理，首先我們利用硝酸溶液來進行奈米碳管的表面改質，結果顯示改質後的奈米碳管有效解決了觸媒的群聚效應，且電池效能在0.4 mg/cm2鉑批覆量，70oC的操作溫度下的能量密度可達到15.8 mW/cm2；於30oC的室溫下，亦可有8.6 mW/cm2的效能表現。接著我們利用微波消化 (microwave digestion) 法對奈米碳管做快速及有效率的表面改質；我們從FTIR, Raman光譜和XPS的結果發現，藉由微波消化法進行奈米碳管的表面改質，可以在較短時間內(1小時) 比硝酸處理18小時後的碳管，能接上較多的官能基及有較高的觸媒質量效能，有效率且大幅度地縮短了奈米碳管表面改質的時間。 除此之外，我們更嘗試比較使用硫酸、氫氧化鉀與2-硫醇乙醇作為蝕刻溶劑，針對不同溶液的改質後所形成的官能基與觸媒間的影響進行分析及探討，實驗結果發現以2-硫醇乙醇硫化後的奈米碳管，有較小的觸媒粒徑及其質量效能(mass efficiency)可達到304.2 mA/mg，電池效能在0.5 mg/cm2鉑批覆量，70oC的操作溫度下的能量密度可達到20.6 mW/cm2,高於一般以硝酸溶液酸化後的奈米碳管。

Fuel cells are regarded as the most potential power source to replace fossil oil due to their low pollution, high efficiency and low noise. Among all, direct methanol fuel cell (DMFC) which can be minified catches everyone eyes. However, high catalysts loading, low utility of catalysts and low reaction rates are advantages of DMFC. In this thesis, high surface area and high conductivity carbon nanotubes (CNTs) were used as the electrodes to improve the utility of catalysts of the direct methanol fuel cell. This research was divided into two parts. The first part includes directly synthesizes CNTs on carbon cloth as the catalyst supports and disperses metallic catalysts on CNTs by sputtering and wet impregnation. After that, we fabricate and characterize the membrane electrode assembly of a DMFC. We found that although dispersing catalysts by sputtering method can form Pt catalysts with small size (about 3nm), the mass efficiency will decrease with long sputtering time due to the shield effect of CNTs. This will result in the degradation of cell performance. On the other hand, the shield effect can be solved by wet impregnation method, but the agglomeration of catalysts occurred and strongly decreases the performance of DMFC. The second part of this thesis focuses on the surface modification/ functionalization of CNTs for solving the agglomeration of catalysts and improving the cell performance. The surface modifications of CNTs were carried out by refluxing pristine CNTs in HNO3 for attaching the functional groups. From the results of cyclic voltammograms (CVs) and single cell test of Pt/MWCNTs electrodes, we can found the significant improvement of cell performance of DMFC after modifying MWCNTs. The power density of Pt dispersed on HNO3 treated MWCNTs is 15.8, 12.6 and 8.6 mW/cm2 at 70oC, 50oC and 30oC with 0.4 mg/cm2 Pt loading ,respectively. This is much higher than the DMFCs with Pt/pristine-MWCNTs and sputter deposited Pt/MWCNTs based cells. Furthermore, we introduce a fast and effective method for modifying CNTs by microwave digestion method. The functionalization of CNTs for increasing more anchoring sites can be achieved in a short time by this approach. The CVs results show that the microwave digestion modified Pt/MWCNTs electrode exhibits the larger electrochemical Pt surface area and higher current density of methanol oxidation than pristine and HNO3 treated Pt/MWCNTs electrodes. This technique can be widely used for effective modifying CNTs and shorting the process time. Finally, the effects of functionalization of CNTs with different functional groups, such H2SO4, KOH and 2-mercaptoethanol were discussed in this part. The Pt/sulfonated-MWCNTs has the maximum methanol oxidation current density about 152.1 mA/cm2 and mass efficiency about 304mA/g. The power density of Pt dispersed on thiolated MWCNTs is 20.6 mW/cm2 at 70oC, with 0.5 mg/cm2 Pt loading, which is much higher than the Pt/HNO3 treated-MWCNTs.