(381f) Incorporating Phosphoric Acid Into Layer-By-Layer Assembled Proton Exchange Membranes for Higher Conductivity At Lower Relative Humidities

Authors: 
Liu, D. S., Massachusetts Institute of Technology



Fuel cells and Li-ion batteries are under intense research for their possible role in a more carbon neutral future.  To have an economical hydrogen fuel cell car with minimal additional parts, the development of a thin proton exchange membrane with sustained performance up to 120⁰ C and down to 50% RH is necessary.  Nafion, the industrial standard for proton exchange membranes, begins to deform near this temperature and, like other acid based membranes, loses its conductivity as the relative humidity decreases.  It is therefore necessary to find an alternative proton exchange membrane that is stable at higher temperatures and can better conduct protons in lower relative humidity conditions.  Phosphoric acid doped poly(benzimidazole) (PBI) based membranes have been used for anhydrous hydrogen fuel cells at higher temperatures (up to 200 ⁰C), but is not quite conductive enough at lower temperatures and has issues with retaining the phosphoric acid.

LbL assembly allows for the controlled deposition of alternating polyelectrolytes at the nanometer scale.  In addition, it allows the use of water soluble polymers and electrostatically binds them to an opposite charge.  By use of a phosphoric acid doped polymer, poly(2-vinyl pyridine) (P2VP), as the cation and sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO) as the anion we were able to make films that when doped mildly with phosphoric acid (at or less than 1:1 phosphoric acid: P2VP amine ratio) has drastically improved conductivity at 50% relative humidty compared with films made from sPPO alone (poly(diallyl dimethyl ammonium chloride) (PDAC) / sPPO).  The phosphoric acid concentration in the film, and thus its proton transport properties, could be controlled post film fabrication by changing the concentration of the phosphoric acid the film is doped in. Higher doping conditions results in increased conductivity, but the film may be less able to retain the phosphoric acid beyond a certain concentration. In addition, the films have higher conductivities than either component of the film alone at that relative humidity, indicating a proton transport mechanism that unilizes both phosphoric and sulfonic acid groups.