觸媒/Nafion/溶液之黏彈性研究及觸媒漿料製備

Viscoelasticity of catalyst/Nafion solutions and catalyst ink preparation

陳衣伶(Yi-Ling Chen)

指導教授 ： 林秀麗

共同指導教授 ： 余子隆(Tyzz-Lung Yu)

元智大學/工程學院/化學工程與材料科學學系/碩士(2008年)

摘要

全氟磺酸化樹酯( Nafion )是一個具有氟碳主鏈及帶-SO3-的支鏈，且具有二個溶解參數之高分子。Nafion分子在溶劑中之溶解性及分子構形( conformation )受溶劑之溶解參數及介電常數影響很大，進一步會影響到觸媒粒子在Nafion溶液中分散性。本論文擬根據溶劑之介電常數及溶解參數，選擇使用適當之溶劑或混合溶劑配製觸媒漿料溶液。利用動態黏彈儀 ( viscoelasticity analyzer )，穿透式電子顯微鏡( Transmission electron microscope，TEM )及場發射掃瞄式電子顯微鏡 ( Field-emission scanning electron microscope，FE-SEM )觀察觸媒粒子在Nafion溶液中分散性，再利用電化學分析方法 ( CV curve )探討觸媒之使用效率，以期尋求適當的溶劑，Nafion濃度、觸媒濃度、及塗佈方法與程序製作分散均勻之觸媒層。本論文所使用之溶劑如：甲基甲醯胺( NMF)；二甲基甲醯胺( DMF )；二甲基乙醯胺( DMAc )甲醇/水( MeOH/H2O ( 4:1 g/g) )；乙醇/水( EtOH/H2O ( 4:1 g/g) )；異丙醇/水( IPA/H2O ( 4:1 g/g) )。
根據TEM實驗可以瞭解Nafion在稀釋溶液裡的構型與溶劑的溶解參數和介電常數有密不可分的關係。Nafion在NMF中分子呈現的是細長桿狀(rod-like)結構；而Nafion在MeOH/H2O溶液裡，分子呈現的是長方形與正方形的聚集結構。主要是因為NMF和MeOH/H2O的溶解參數接近Nafion離子支鏈的溶解參數，導致帶電荷的支鏈在聚集粒子的外側表面與溶劑接觸，而主鏈則形成一個疏水性的聚集。而DMAc與DMF則因為溶解參數接近Nafion主鏈的溶解參數，所以導致Nafion分子呈現一個團狀(coil-like)結構。所以Nafion形成團狀結構主鏈在結構外圍，因此很少有聚集的發生。
動態黏彈儀、FE-SEM及CV curve實驗顯示，Pt-C/Nafion/DMAc及Pt-C/Nafion/DMF溶液之黏度較低，所以觸媒/Nafion分散性佳，可提高其觸媒活性面積。由單電池測試結果所示，自製之MEA的性能均與商購的MEA差異不大，尤其以MEA-3 (125-Nafion-Pt-C-DMAc)具有最大功率為294.54 mW/cm2。

關鍵字

Nafion ； solution viscoelasticity ； catalyst solutions

並列摘要

Nafion, an ionomer composed of perfluorocarbon backbone and vinyl ether side chains terminated with –SO3- groups, has dual solubility parameters, i.e. δ1= 9.7 (cal/cm3)1/2 relating to perfluorocarbon backbones and δ2= 17.3 (cal/cm3)1/2 relating to sulfonic acid side chains. In this study, the conformations of Nafion molecules in dilute solutions with solvents of various dielectric constants (ε) and solubility parameters (δ) , were investigated using viscoelasticity analyzer and transmission electron microscope (TEM). These observations showed conformations of Nafion molecules in dilute solutions strongly depended on δ and ε of solvents. Nafion molecules behave rod-like structures in NMF and methanol/water. Nafion molecules behave cubic and globular structures in Ethanol/water and 2-propanol/water, respectively. The NMF and methanol/water solvents haveδvalues close toδ2 of Nafion vinylether side chains, leading to some hydrophobic backbones aggregations with the highly ionized side chains surrounding on the surface of aggregate particles and in contact with solvents. The very high ε =182.4 of NMF causes Nafion side chains to be highly ionized and high electrostatic charge repulsion of the aggregated particles, and thus rod-like structures of Nafion molecules in NMF. The DMAc and DMF have δ values close to δ1 of Nafion perfluorocarbon backbones and lower ?? values, leading Nafion molecules to behave coil-like structures in these solvents with perfluorocarbon backbones in contact with solvent. The Pt/Nafion and Pt-C/Nafion solution in DMAc, DMF and methanol/water solvents haves lower viscosity than other solutions.
According to of viscoelasticity measurements, FE-SEM and CV curve observation, we found that the viscosities of Pt-C/Nafion/DMAc and Pt-C/Nafion/DMF solutions are lower than viscosity of Pt-C/Nafion/solutions prepared from other solvents. The electrodes prepared by coating Pt-C/Nafion/DMAc and Pt-C/Nafion/DMF solutions on carbon paper GDL have a better Pt-C catalysts dispersion than those prepared from other solvents. The MEAs prepared from Pt-C/Nafion/ DMAc catalyst solutions with Nafion membranes obtained by casting from 5 wt % Du Pont Nafion solutions and annealing at 125 ℃ showed better PEMFC performance than those prepared from commercial Du Pont Nafion-212 membrane with E-Tek LT 120 electrodes.

並列關鍵字

Nafion ； solution viscoelasticity ； catalyst solutions

參考文獻

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