(486d) Controlling Mechanical and Transport Properties of Layer-By-Layer Electrospun Mat Composite Membranes for Fuel Cell Applications

Liu, D. S., Massachusetts Institute of Technology
Rutledge, G. C., Massachusetts Institute of Technology

The continued development of thin-film solid polymer electrolytes with improved mechanical and ion transport properties is critical for the further advancement of electrochemical energy devices.  For hydrogen and methanol fuel cells, the proton exchange membrane (PEM) is the electrolyte and has to have high protonic conductivity, low fuel crossover, and be mechanically and chemically stable.  More recently, there has been considerable interest in improving the mechanical properties and durability of the proton exchange membranes (PEMs) without sacrificing its transport properties.  For methanol fuel cells in particular, there is a need for a cheap, mechanically robust PEM that also has high protonic conductivity and low methanol permeability.

LbL assembly allows for the controlled deposition of alternating polyelectrolytes at the nanometer scale.  An LbL system composted of poly(diallyl dimethyl ammonium chloride) (PDAC) and sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO) has shown to have proton conductivity comparable to that of Nafion®, the industry standard, but with methanol permeability two orders of magnitude less than Nafion’s.  While this system by itself is mechanically deficient when hydrated, the mechanical properties can be greatly improved if the film is spray-coated on an electrospun fiber mat to form a composite membrane.  With vacuum assisted spray-LbL assembly, the fibers can be individually coated, and the mat filled; mats as thin as 15 μm can be coated. Without vacuum, the spray-LbL assembly can form a continuous fuel-blocking layer with properties similar to the free-standing LbL film by itself.  Combined, composite membranes with methanol permeability twenty times lower than Nafion and through-plane proton selectivity five times greater than Nafion could be fabricated.  

As anticipated, the mechanical properties of the underlying electrospun mat were passed on to the composite membrane.  The composite membranes were further strengthened by annealing the electrospun mat prior to coating.  In addition, the swelling of the composite membranes in water was significantly lower than the LbL-only system and over a magnitude lower than Nafion.  The mechanical hysteresis was also lower than Nafion under hydrated conditions.  The composite membranes were tested in an operational direct methanol fuel cell with results showing a higher open circuit voltage and comparable cell resistances when compared to Nafion.