Electrospun Nanofiber Composite Proton Exchange Membranes
Ballengee, Jason Bryan
Proton exchange membrane fuel cells (PEMFCs) are an attractive power plant for applications ranging from automobiles to personal electronic devices. However, PEMFCs still need improvement, particularly regarding the proton-exchange membrane at the heart of the fuel cell’s membrane-electrode-assembly (MEA). This dissertation describes novel composite membranes that address current limitations in proton exchange membrane (PEM) performance. A new dual-fiber electrospinning technique is introduced for PEM fabrication. Two polymers, an ionomer with fixed sulfonic acid sites and an uncharged/inert material, are electrospun simultaneously onto a common collecting surface from separate syringes. Follow-on processing can create two distinct structures: i) an ionomer film reinforced by inert fibers or ii) ionomer fibers encapsulated in inert polymer. This “forced-assembly” approach provides: i) ease of fabrication, in particular the elimination of a polymer solution impregnation step often required for other composite membrane systems, ii) versatility of membrane composition, i.e. normally immiscible/incompatible polymers can be blended together on a sub-micron scale, and iii) a high degree of morphological control, specifically two distinct structures are fabricated from the same dual-fiber mat where the nanofiber diameter and volume fractions of the two electrospun fiber types can be independently controlled. Furthermore, the method can be extended to create layered composite structures. The nanofiber composite membranes described herein have outstanding material properties, namely, low water swelling, good mechanical strength, and attractive proton conductivity. Such material properties result in excellent hydrogen/air fuel cell performance when these membranes are incorporated into a MEA. For example, a nanofiber membrane of Nafion and polyphenylsulfone lasts over 50% longer than a commercial Nafion film in an accelerated fuel cell durability test. Furthermore, a MEA containing a membrane with 3M Company’s 660EW perfluorosulfonic acid polymer reinforced by polyphenylsulfone nanofibers has higher power output than a commercial Nafion MEA. Layered nanofiber membranes with Nafion and polyphenylsulfone are shown to have acceptable sheet resistance and low methanol permeability, two desirable properties for direct methanol fuel cell (DMFC) applications. Additionally, a trilayer nanofiber membrane’s DMFC power output is ~3 times higher than commercial Nafion 117’s when operating at 0.4V with a 10M methanol feed.