Nafion Composite Membrane Reinforced By Phosphonated Polypentafluorostyrene Nanofibers

Abstract

The membrane is one of the crucial components of fuel cells. Applying composite membranes for fuel cells is a promising option due to better mechanical properties compared to membranes without reinforcement. Composite membranes can be prepared by combining ionomer with a filler which can be selected from many types of materials, such as polymers, ceramics, carbons, and metals. Filler materials exist in different nanostructures which provide flexible designs for composite membranes. However, the main issue in composite membranes is a trade-off among properties when adjusting the ratio between ionomer and filler, especially between ionic conductivity and mechanical modulus. On the one hand, maintaining high protonic conductivity is possible when small concentrations of reinforcing fillers are incorporated. On the other hand, a high amount of reinforcement can improve the mechanical properties significantly but could result in low protonic conductivity as well. Our strategy to overcome this issue is by employing protonic conductive nanofibers as reinforcement. Electrospinning is a versatile method to transform polymer solutions into long and solid nanofibers. Electrospun fibermats possess a high porosity and contain voids which can be filled with an ionomer like Nafion by spraycoating to form a dense composite membrane.We were successful in producing electrospun nanofibers from phosphonated polypentafluorostyrene (PWN70) and unmodified polypentafluorostyrene (PPFSt). PWN70/Nafion and PPFSt/Nafion composite membranes were prepared separately by spraycoating of a Nafion solution into PWN70 and PPFSt fibermats that have comparable thickness and fiber loading. From tensile tests, we found that composite membranes made from PWN70/Nafion and PPFSt/Nafion have much higher Youngs’ modulus (E) than pure Nafion (Figure 1A). Although PWN70/Nafion is a relatively brittle membrane, it has the best Youngs’ modulus and yield stress. Protonic conductivity is also a crucial membrane property which can be determined by electrochemical impedance spectroscopy. In Figure 1B, Nafion reinforced by PPFSt fibers has a reduced conductivity due to non-ion-conductive PPFSt. Surprisingly, the protonic conductivity of a PWN70/Nafion composite membrane is similar to spraycoated Nafion. Without reducing much the protonic conductivity, the PWN70/Nafion composite membrane shows comparable ohmic resistance to the spraycoated D2020 in fuel cell operation which has also been done in this work. Since the PWN70 nanofibers are ion-conductive and electro-spinnable, the nanofibers offer benefits when designing fiber-reinforced composite membrane possessing both good mechanical stability and protonic conductivity. Figure 1