Silica Nanoparticles With an Outer Oligomeric Ionomer Improve Fuel Cell Proton Exchange Membranes Say A*Star Scientists
May 4, 20100
The Agency for Science, Technology and Research ((A*STAR) Singapore, SG) researchers have developed an improved proton exchange membrane containing nanoparticles with an outer oligomeric ionomer for fuel cells. The membrane has improved properties such as proton-conductivity, thermal stability, and reduced permeability of water and methanol which are detailed in U.S. Patent 7,709,542.
According to inventors Liang Hong, Zhaolin Liu, Xinhui Zhang and Bing Guo the polymer has side chains with ionic groups. The particles have an average particle size of less than 20 nm and include an oligomeric ionomer that interacts with the polymer and attracts the ionic groups on its side chains.
The composite may be formed by a method in which an initiator is bonded to silica particulates. The initiator is used to initiate polymerization of a precursor monomer to form a salt form of the oligomeric ionomer bonded to the silica particulates, which is then reacted with an acid to produce the oligomeric ionomer, thus forming the ionomer particles. The ionomer particles are dispersed in a solution containing a solvent and the polymer dissolved therein. The solvent is removed. The residue is cured to form the composite.
Proton-exchange membranes (PEM) are useful in many applications, including fuel cells such as hydrogen fuel cells (H2–FC) and direct methanol fuel cells (DMFC). A PEM is typically a semipermeable membrane generally made from ionomers that conducts protons but is impermeable to gasses such as oxygen or hydrogen. In a PEM fuel cell (PEMFC), the PEM severs two functions: separating the reactants, and transporting protons across the membrane. A PEM functions as a polymer electrolyte membrane.
The composite comprises a polymer matrix formed from a proton-exchange polymer and ionomer particles. The proton-exchange polymer has side chains which have ionic groups. The ionomer particles have an average particle size of less than 20 nm and comprise an oligomeric ionomer that interacts with the proton-exchange polymer and attracts the ionic groups on the side chains of the proton-exchange polymer.
The ionomer particles are distributed in the matrix. The oligomeric ionomer may have a pendant sulfopropyl group. The oligomeric ionomer may comprise a sulfopropyl acrylate (SPA) repeating unit. The oligomeric ionomer may also comprise an N,N’-methyl-(6-hexylcarbamatoethyl-methacrylate) imidazolonium bromide (EMACI) repeating unit. The oligomeric ionomer may comprise less than 11 repeating units. The ionomer particles may comprise a silica core and an oligomeric ionomer bonded to the silica core. The silica core may have a core size of about 7 nm. The ionomer particles may have an average particle size less than 10 nm. The proton-exchange polymer may be a sulfonated perfluoro-polymer (SPFP). The SPFP may comprise fluorinated polyethylene-polypropylene. The composite may comprise from 2 to 6 wt %, such as about 4 wt %, of the ionomer particles. The ionomer particles may be uniformly distributed throughout the polymer matrix. The composite may be in the form of a membrane or resin.
FIGS. 10A to 10C are transmission electron microscopic images of different types of silica nanoparticles used to form improved fuel cell membranes. FIGS. 10A to 10C show, respectively, the TEM images of nanoparticles: a) PSPA-silica nanoparticles, b) PSPA-K-silica particles , and c) poly(SPA-co-EMACI)-silica particles in their respective dry forms.
It has also been found that the PSPA ionomer grafted onto the silica cores facilitated uniform dispersion of the particles in the sample SPFP membrane matrix. In comparison, pristine nano-silica particles were observed to aggregate spontaneously in a SPFP matrix. The difference can be seen in FIGS. 12A and 12B.
FIG. 12A shows a field emission scanning electron microscopic (FESEM) image of a cross-section of a SPFP Membrane Doped with PSPA-Silica where aggregation of the silica particles can be clearly seen (shown as white dots). The sizes of the silica particles were up to about 0.1 micron. FIG. 12B shows an FESEM image of a cross-section of the SPFP Membrane Doped with PSPA-Silica, where no significant agglomerates were observable. Further, it was observed that the oligomeric ionomer grafted silica particles were uniformly distributed in the membrane matrix, as can be seen in FIG. 12B.