Spray Layer-by-Layer Electrospun Composite Proton Exchange Membranes

Polymer electrolyte films are deposited onto highly porous electrospun mats using layer-by-layer (LbL) processing to fabricate composite proton conducting membranes. By simply changing the assembly conditions for generation of the LbL film on the nanofiber mat substrate, three different and unique composite film morphologies can be achieved in which the electrospun mats provide mechanical support; the LbL assembly produces highly conductive films that coat the mats in a controlled fashion, separately providing the ionic conductivity and fuel blocking characteristics of the composite membrane. Coating an electrospun mat with the LbL dipping process produces composite membranes with webbed morphologies that link the fibers in-plane and give the composite membrane in-plane proton conductivities similar to that of the pristine LbL system. In contrast, coating an electrospun mat using the spray-LbL process without vacuum produces a uniform film that bridges across all of the pores of the mat. These membranes have methanol permeability similar to free-standing poly(diallyl dimethyl ammonium chloride)/sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (PDAC/sPPO) thin films. Coating an electrospun mat with the vacuum-assisted spray-LbL process produces composite membranes with conformally coated fibers throughout the bulk of the mat with nanometer control of the coating thickness on each fiber. The mechanical properties of the LbL-coated mats display composite properties, exhibiting the strength of the glassy PDAC/sPPO films when dry and the properties of the underlying electrospun polyamide mat when hydrated. By combining the different spray-LbL fabrication techniques with electrospun fiber supports and tuning the parameters, mechanically stable membranes with high selectivity can be produced, potentially for use in fuel cell applications.