Abstract
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.