- 學位論文
利用溶劑揮發法與旋轉塗佈法合成Nafion®/MCM-22複合膜應用於直接甲醇燃料電池
Nafion®/MCM-22 Composite Membranes for DMFC Prepared Using Solvent Recasting and Spin-coating Methods
摘要
直接甲醇燃料電池是一種使用甲醇作為發電的燃料，利用質子交換膜做為電解質的燃料電池，其優點在於甲醇方便攜帶與其高能源密度可以致使較低的環境汙染；缺點在於反應中間產物一氧化碳CO會有毒化觸媒電極的問題，因此需要增加單位活性面積的觸媒含量，使得DMFC價格較昂貴；而且甲醇會經由質子交換膜從陽極穿透至陰極，降低電壓及電池功率。 本研究嘗試合成無機沸石材料MCM-22，其屬於MWW層狀堆疊結構，經由不同的修飾與處理將其以不同方法添加在杜邦公司所生產的Nafion®質子交換膜中，希望能藉此阻擋住甲醇穿透與提升質子的傳導率；研究的第一部分會製備初合成的MCM-22(P)，再經由鍛燒將層之間連結起來形成MCM-22，並利用反微胞方法使層與層之間結構撐開後再分散(disperse)可形成dis-MCM-22，再經由萃取和嫁接磺酸根等步驟合成ex-dis-MCM-22和SO3H-dis-MCM-22等材料，另外dis-MCM-22經由鍛燒可形成堆疊方式較不同的ITQ-2。第二部分則是將MCM-22(P)等衍生材料與Nafion膜形成複合膜，會利用溶劑揮發法(Solvent recasting)和旋轉塗佈法(Spin coating)兩種方式製備並比較其在甲醇燃料電池的電性效果與複合膜的性質。 合成出的MCM-22(P)等衍生材料會經由X光粉末繞射儀(XRD)、掃描式電子顯微鏡(SEM)、熱重分析儀(TGA)、元素分析儀(EA)和氮氣吸脫附恆溫曲線儀(BET)等儀器做性質的鑑定，而製備出的複合膜則會經由掃描式電子顯微鏡(SEM)、動態熱機械分析儀(DMA)、小角度X光散射(SAXS)鑑定複合膜的性質，並用甲醇擴散實驗量測甲醇穿透率、利用交流阻抗儀量測阻抗來換算出質子傳導率，並量測含水量和離子交換容積(IEC)，最後組成單電池的電極組量測直接甲醇燃料電池的功率密度；而實驗結果看到以溶劑揮發法製備的MCM-22(P)和dis-MCM-22複合膜其甲醇穿透和質子傳導率都隨著材料添加有下降的趨勢；而ex-MCM-22(P)、SO3H-dis-MCM-22和ITQ-2複合膜則有相反的趨勢，其餘複合膜則較無規律；單電池的功率密度在MCM-22(P)、MCM-22的複合膜有最高數值，可達約120 mW cm-2，相較於添加材料前的reacsting膜只有72 mW cm-2，而商業用Nafion®117則是90 mW cm-2。另外以旋轉塗佈法製備的數組複合膜，其功率密度大約落在60-80 mW cm-2左右，以複合膜的性質鑑定結果，推測單電池的發電效率和複合膜的機械強度性質有關，膜必須要和觸媒層表面有良好的銜接，才能發揮最大的效率，因此越軟越有彈性的複合膜，其功率密度皆較高。
並列摘要
Direct methanol fuel cells (DMFCs) are a subcategory of proton-exchange fuel cells which methanol is used as the fuel. The advantage of DMFCs is the east of transport methanol, a higher energy-density and caused lower environmental pollution. The disadvantages of DMFCs are that the intermediate product carbon monoxide is a poison to catalyst which lead to the increase in Pt and Ru contents of the catalyst so that DMFCs is more expensive. Moreover, methanol permeation from anode to cathode decreases the cell voltage and power efficiency MCM-22 is a zeolite with stacked MWW layers. In the present study, MCM-22 materials after different treatments were used as inorganic fillers in Nafion® membranes in order to block methanol permeability for the application in direct methanol fuel cell. Treatments of MCM-22 included calcination of as-synthesized MCM-22(P) at 560 ℃ to obtain MCM-22, dispersion of MWW layers by an inverse-micelle method to obtain dis-MCM-22, and calcination of dis-MCM-22 to obtain ITQ-2. The dis-MCM-22 was also modified by grafting with propylsulfonic acid groups. Two different ways were applied to prepare Nafion®/MCM-22 composite membranes. Solvent recasting method would disperse inorganic fillers homogeneously in Nafion® matrix, while spin-coating method would coat the inorganic species on one side surface of Nafion® membrane. The materials were characterized by XRD, TGA, EA, BET and SEM. The properties of resulting composite membranes were examined with DMA, SAXS, water uptake, ion exchange capacity, methanol permeability, proton conductivity, and single cell test. Methanol permeability and proton conductivity of the composite membranes filled with MCM-22(P) and dis-MCM-22 prepared by solvent recasting method decreased with the increasing amounts of fillers, while those filled with ex-MCM-22(P), SO3H-dis-MCM-22 and ITQ-2 had the opposite trend. The values had no significant difference for the membrane filled with MCM-22, ex-dis-MCM-22 and SO3H-ex-dis-MCM-22. The membranes filled with dis-MCM-22 gave the lowest methanol permeabilities among all. SC-MCM-22(P) and SC-dis-MCM-22 composite membranes prepared by spin coating method had lower methanol permeabilities and higher proton conductivity than membranes prepared by solvent recasting method. As to the single cell test, the membranes filled with MCM-22(P) and MCM-22 gave the highest power densities around 120 mW cm-2, which was higher than 72 mW cm-2 of the cell assembled with recasting Nafion® membrane. On the other hand, the single cell with SC-MCM-22(P), SC-dis-MCM-22, ITQ-2 and MCM-22 membranes gave relatively low power densities around 60-80 mW cm-2. It is attributed to that the latter membranes were harder than those prepared by recasting. The key factor affecting the power density of single cell turns out to be the mechanical strength of composite membranes. Soft membrane facilitates single cell assembly and good contact between the electrodes and electrolyte membrane.