- 學位論文
高分子靜電紡絲纖維之製備及作為 燃料電池/鋰離子電池元件之應用
Preparation of Polymeric Electrospun Fibers and Their Applications for Fuel Cells and Lithium-Ion Batteries
摘要
本研究以靜電紡絲高分子纖維作為研究主軸，並探討其應用於燃料電池之質子交換膜和鋰離子電池的隔離膜的性質和潛力。燃料電池聚電解質膜材開發係製備靜電紡絲高分子纖維，將之與高分子電解質成形複合薄膜分為二方向，其一為聚苯并咪唑與聚苯并環己烷摻混聚電解質纖維應用於高溫燃料電池中（磷酸燃料電池），其二係以聚偏二氟乙烯( Poly(vinylidene fluoride), PVDF)電紡纖維以全氟磺酸樹脂(Nafion)高分子鏈或是聚磺酸苯乙烯高分子鏈(PSSA)反應接枝形成仿聚電解質纖維應用於低溫燃料電池中。新穎鋰離子電池隔離膜開發方面係以聚苯并環己烷( Polybenzoxazine, PBz)靜電紡絲纖維技術製備與膜材表面性質並應用於鋰離子電池效能探討。 第一部分的研究包括兩個方向，其一為高溫燃料電池中的聚電解質薄膜，以聚苯并環己烷(PBz)與聚苯并咪唑( Polybenzimidazole, PBI)兩者摻混共紡製備電紡高分子纖維，其中聚苯并環己烷作為高分子交聯劑而交聯固定纖維結構，再將之含浸聚苯并咪唑形成複合纖維膜。聚苯并咪唑與聚苯并環己烷摻混電紡纖維(PBI-PBz NF)能有效存在於聚苯并咪唑膜材中，提升複合膜材楊氏係數3倍與最大應力值1.35倍，提高複合膜材的質子導通率( 0.17 S cm-1於160 oC)與尺寸變化量可降至20 %以下，並提升對應的電池性能以及操作持久性。 其二，由全氟磺酸樹脂作為聚電解質膜基材部分，與聚電解質改質的電紡絲纖維形成複合膜。利用不同化學反應步驟，分別將全氟磺酸樹脂高分子鏈或是聚磺酸苯乙烯(PSSA)高分子鏈反應接枝於聚偏二氟乙烯的電紡絲纖維表面，形成類似聚電解質電紡絲纖維纖維。以全氟磺酸樹脂纖維表面改質研究方面( Nafion-functionalized poly(vinylidene fluoride) electrospun nanofibers, PVDFNF-Nafion)，全氟磺酸樹脂與聚偏二氟乙烯電紡纖維以臭氧改質方式與官能基置換交替使用方式，形成聚偏二氟乙烯電紡纖維表面修飾全氟磺酸樹脂，再將此改質電紡纖維與全氟磺酸樹脂形成複合膜(Nafion-CM1)。其氫氧燃料電池(PEMFC)性能顯示Nafion-CM1膜材最高功率密度可達700 mW cm-2，並且具有優異的甲醇阻抗能力與尺寸安定性，因此於直接甲醇燃料電池(DMFC)性能上，於高甲醇濃度時(5M)，電池最高功率密度可達122 mW cm-2。 以聚磺酸苯乙烯纖維表面改質研究方面( Poly(styrenesulfonic acid)-graft-poly(vinylidene fluoride) electrospun nanofibers, PSSA-g-PVDFNF)，透過苯乙烯磺酸鈉(4-Styrenesulfonic acid sodium, Nass)以自由基轉移聚合(ATRP)的聚合方式修聚偏二氟乙烯電紡纖維表面，使聚偏二氟乙烯電紡纖維表面帶有具柔軟可移動式磺酸根基團的聚磺酸苯乙烯高分子，並與全氟磺酸樹脂形成複合膜(Nafion-CM)，其質子導通率可達106 mS cm-1，氫氧燃料電池性能最高功率密度高達770 mW cm-2。比較同樣以全氟磺酸樹脂為基材的聚電解質纖維膜，Nafion-CM1與Nafion-CM的質子導通率皆高於全氟磺酸重鑄膜(Nafion-RC)及商業膜全氟磺酸膜Nafion 212，且有效降低質子導通活化能至3.0 kJ mol-1。 本研究第二部份係以可交聯的聚苯并環己烷高分子製備靜電紡絲纖維(CR-PBz-FbM)，探討其基本性質以及於鋰離子電池隔離膜之應用。使用二氨基二苯醚(4,4’-diaminodiphenyl ether)、雙酚A(Bisphenol-A)與聚甲醛(Paraformaldehyde)所製備而成的聚苯并環己烷高分子(PBz-oda)作為電紡纖維材料的選擇，在電紡過程以不同參數調控纖維型態及熱處理對材料極性影響，且疏水特性因聚苯并環己烷電紡纖維膜形成纖維結構及熱處理方式不同而有所差異(於107o - 147o之間)。在生物沾黏部分，由於聚苯并環己烷材料低表面能特性，能有效阻止血液以及抗蛋白質吸附現象。 另一方面，經由交聯過後聚苯并環己烷高分子表面帶有許多羥基(-OH)，使聚苯并環己烷電紡纖維膜(CR-PBz-FbM)具備水釘扎(water-pinning)能力與水分傳遞運輸特性。且聚苯并環己烷高分子為熱固性材料，具備良好熱穩定性及化學穩定性。經由物理與化學損害測試，呈現耐磨、對不同溶劑與酸鹼忍受度高、高撓曲性及高機械強度等特性。 鋰離子電池隔離膜應用上，證實聚苯并環己烷電紡纖維膜確實有良好的離子導通率(2.92 mS cm-1)、高形狀維持能力及含浸電解液過後具備自熄行為及阻燃能力，能夠有效避免熱失控現象。在電池充放電性能上。電紡纖維結構具相當高的孔隙率(77 %)以及較大的孔洞尺寸(孔洞大小約為4.0 μm)，高速充放電(2C)數據顯示，聚苯并環己烷電紡纖維膜在鋰離子電池性能上具有優異的高速充放電能力(118 mAh g-1)外，同時也具備充放電穩定性。
並列摘要
This research focuses on the preparation and surface modification of electrospun nanofibers and their application in the proton exchange membranes for fuel cells and the porous separators for lithium-ion batteries. In the first part, polybenzimidazole (PBI) is electrospun into nanofiber mats with a polybenzoxazine as a crosslinking agent. The thermally crosslinked PBI electrospun nanofiber mats (CR-PBI-NF) are impregnated with PBI solutions to result in CR-PBI-NF reinforced PBI composite membranes. Based on the crosslinked structures, the nanofiber morphology could be maintained in the PBI composite membranes, so as to enhance their mechanical strength with a Young’s modulus of about 2200 MPa and stress strength of 85 MPa, which is 3.0-fold and 1.35-fold of the values recorded with the neat PBI membrane. The composite membranes also exhibit good dimensional stability upon acid-doping with dimensional changes less than 20%. The nanofibers also provide as proton-conducting pathways in the composite membranes so as to increase their proton conductivity from 0.85 to 0.17 S cm-1 at160 oC. As a result, the single cell employing the composite membranes shows better cell performance than the results observed with the pristine PBI membrane. The nanofiber-reinforcement approach is further applied to Nafion-based membranes with surface-modified poly(vinylidene fluoride) electrospun nanofibers (PVDFNF) as the reinforcements. Both Nafion and poly(styrene sulfonic acid) chains are chemically incorporated to the PVDFNF surfaces to improve the interfacial compatibility between the nanofiobers and Nafion matrix and to induce proton-conducting channels along the nanofiber surfaces in the composite membranes. With the formation of proton-conducting pathways, the Nafion composite membranes exhibit low activation energy of proton conduction (about 2.4-3.0 kJ mol-1), high proton conductivity (about 106 mS cm-1), and depressed methanol permeability compared to the neat Nafion membrane. Consequently, the Nafion composite membrane based H2/O2 single cells show a maximum power density of 770 mW cm-2, which is 1.5-fold of the value recorded with the commercial Nafion 212 membrane. Meanwhile, the low methanol permeability of Nafion-based composite membranes makes it be suitable for direct methanol fuel cells (DMFCs). With a 5 M methanol solution as a feeding fuel, the single cell shows a maximum power density of 122 mW cm-2 and a current density at 0.2 V of 610 mA cm-2. The other part of this work involves the preparation of electrospun nanofiber mats of a mainchain polybenzoxazine (PBz, number averaged molecular weight: about 6,700 g mol-1) prepared with 4,4’-diaminodophenyl ether, bisphenol-A, and paraformaldehyde. The thermally crosslinked electrospun PBz nanofiber mats (CR-PBz-FbM) show some attractive properties, including hydrophobic surface with a water contact angle of about 147o, water-pinning durability, blood and protein repellency, shape-reforming ability, and robust mechanical and chemical resistance. The CR-PBz-FbM (thickness of about 80 μm, porosity of 76 %, and mean pore size of 4.0 μm) has been evaluated as a separator for lithium-ion batteries. They exhibit an very high electrolyte uptake (about 825 %), high ionic conductivity (2.92 mS cm-1), and a near-zero thermal shrinkage at 150 oC for 0.5 h. As a result, the performance of the half-cell tests on the CR-PBz-FbM-based lithium-ion battery demonstrate a high energy density of 118 mAh g-1 at 2.0 C is and good cycling stability after 50 charge-discharge cycles at 0.2 C.