(528a) Layer-by-Layer Electrospun Mat Composite Membranes for Fuel Cell Applications

The development of thin solid polymer electrolytes with improved performance is critical for the advancement of electrochemical energy devices including hydrogen and direct methanol fuel cells (DMFCs) and polymer-ion batteries^[1-2].  An essential property for polymer electrolytes in these applications is high ionic conductivity with low fuel/charge cross-over and mechanical stability.  Often, when no single polymer or material has all these desired attributes, composite membranes, ones where the support and physical separator material is different than the conducting, functional, material, are researched to provide better overall performance by allowing for the tuning of individual components and performance characteristics.  In the field of Li-ion batteries, water filtration, and fuel cells, there has been increasing work done on composite membranes, particularly on the nano and micro scale. 

In the case of methanol fuel cells, where the proton exchange membrane (PEM) has to be mechanically strong, ionically conducting, and relatively impermeable to methanol, a composite membrane becomes desirable, as it provides a means of combining different material properties such as permeability while still maintaining desired water and proton flux and chemical and mechanical stability during fuel cell operation^[3].  For the PEM the electrostatic layer-by-layer (LbL) assembly will be used to generate ultrathin (submicron) films through the alternation of polycationic and anionic polymer systems, which enables the nanoscale manipulation of thin film composition, and the generation of molecular level blends that would be difficult to achieve using traditional means of creating materials composites^[4-6].  These nanolayered thin film systems will act as methanol regulating, ionically conducting media for fuel cell membranes.  LbL assembly of sulfonated poly phenylene oxide and poly diallyl dimethyl ammonium chloride (sPPO/PDAC) been shown to provide ionic conductivity comparable to that of Nafion®, the industry standard, but with two orders of magnitude reduction in methanol crossover ^[7]; however, multilayer assembled PEMs suffer from poor mechanical performance. 

Consequently this research has sought to improve the mechanical performance of multilayer assembled PEMs by integrating the PEM within a highly tunable mechanically stable porous support membrane, Nylon electrospun fiber mats, while maintaining high proton conductivity and low methanol permeability to provide a cheaper and more methanol impermeable alternative to Nafion.  At the same time, this research seeks to exploit the unique properties of LBL assembly, namely nanometer growth rate and substrate flexibility, to create controlled nanometer and micrometer sized morphologies.   Electrospun fiber mats (EFM) are nonwoven polymer mats whose fiber size, thickness, and mechanical properties could be controlled.  Three different LBL assembly methods were explored: each resulting in unique morphology. 

Coating an EFM with the LBL dipping process produces composite membranes with interesting “webbed” morphologies, joining the fibers while growing on each one. Coating an EFM with the vacuum assisted spray LBL produces composite membranes with conformally coated individual fibers throughout the bulk of the EFM and gives nanometer control of the thickness of the coating.  Since the LBL treats each fiber as its substrate, entire EFMs (thickness from 25 – 80μm) can be filled with less than 1μm thick coating.  Currently we have been able to fill up to 70% of the EFM and recover up to 50% of the conductivity.  Without vacuum, spray LBL can form a film covering all the pores of the EFM without penetrating into the fibers.  These no-vacuum coated EFM have methanol permeability similar to free-standing PDAC/sPPO.  Thus the overall methanol permeance of the composite membrane may be tuned by adjusting how many bilayers have been sprayed.  Additionally, the mechanical properties of the composite mats are drastically improved, displaying composite-like properties, having the strength of PDAC/sPPO films when dry and the properties of the underlying EFM when wet.  Likewise, when the properties of the underlying EFM are improved, by heat treatment, the properties of the overall composite membrane are improved.   Furthermore, thinner membranes (~25μm) have been explored to capture the advantages of low methanol permeability of PDAC/sPPO.  These composite membranes have been tested in a DMFC and preliminary results have shown higher OCV as expected from reduced methanol permeability.

References:

1.      Winter, M., Brodd, R.J. What are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245-4269

2.      Frano Barbir. PEM Fuel Cells: Theory and Practice. Published 2005, Elsevier.

3.      Li, X., Roberts, E.P.L., Holmes, S.M., Evaluation of Composite Membrane for Direct Methanol Fuel Cells.  Journal of Power Sources. 154 (2006) 115-123

4.      Dercher, G. Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science 277, 1232-1237 (1997)

5.      Hammond, P. T. Form and function in multilayer assembly: New applications at the nanoscale. Adv. Mater. 16, 1271-1293 (2004)

6.      Lutkenhaus, J. L.; Hammond, P. T., "Electrochemically Enabled Polyelectrolyte Multilayer Devices: From Fuel Cells to Sensors" Soft Matter 2007, 3, 804-816

7.      A.A. Argun, J.N. Ashcraft, and P.T. Hammond, Highly Conductive, Methanol Resistant Polyelectrolyte Multilayers. Advanced Materials, 2008, 20, 1539-1543.