The physical properties of polymer membranes pose severe limits on performance when used in applications such as fuel cells and air separation modules. In the case of proton-exchange membranes (PEMs) used in fuel cells such as Nafion®, temperatures must be kept below 80°C in order to keep polyfluorocarbon membranes sufficiently hydrated for proton conduction, but the performance of fuel cell electrodes will be improved if the operational temperatures is increased to above 100°C. In fuel cells that derive protons from liquid hydrocarbon such as direct methanol fuel cells (DMFCs), there is an additional problem of performance being further degraded by PEM permeability to methanol (“methanol crossover”).
In the case of air separation modules, polymer membranes suffer from low O2/N2 selectivity (~6). Modestly higher selectivities can be achieved by adding silica particles to the polymer (up to ~9), but selectivities need to be at least 30 for membranes tightly-packed into compact modules to provide efficient oxygen enrichment. The highly selective materials that have been tried so far, such as carbon molecular sieves and zeolite membranes, are too fragile and expensive for practical use as substitutes for fluorocarbon polymer membranes in air separation modules.
A University of California researcher has invented a nanocomposite membrane that uses an alternative to silica particles—nanocrystallite zeolite particles. Both high temperature dehydration and methanol crossover can be countered by incorporating nanocrystalline zeolites into the fluorocarbon polymers.
For air separation, unlike silica particles the nanocrystals used in this invention have excellent proton conductivity and are more efficient in increasing the O2/N2 selectivity of polymer membranes.
UC nanocomposite membranes might become standard components of PEM fuel cells, which have great commercial significance as the leading candidate to replace internal combustion engines in automobiles. The UC membranes are also likely to be the preferred membrane for use in hollow-fiber and multi-leaf spiral-wound air separation modules.
The UC nanocomposite membranes:
|United States Of America||Issued Patent||7,829,620||11/09/2010||2001-195|