Worldwide government spending for fuel cell and hydrogen infrastructure surpassed $1.5B in 2004, and it is estimated that world markets for fuel cells systems will grow tenfold by 2007 and reach $12.6 billion per year by 2012. Despite the gains that have been made in fuel cell technology over the last decade, major barriers to implementation of commercial fuel cell generation remain. One of the key shortcomings of contemporary fuel cells is the proton exchange membrane (PEM), which serves as the electrolyte for proton transfer and as the separator to prevent direct physical mixing of the fuel (e.g., hydrogen, methanol) and the oxygen at the anode and cathode.
The objective of this research effort is to investigate the use of solution and melt processing to control the PEM microstructure of multicomponent, multiphase polymer systems and to understand the effect of the controlled microstructure on the transport and mechanical properties of the PEMs. The polymer systems that will be investigated include polymer blends, block copolymers and polymer nanocomposites. Methods will be developed for manufacturing membranes and simultaneously orienting the conductive phase in the direction of proton transport using external electrical and magnetic fields. Use of a multicomponent/multiphase system will allow one to decouple the transport and mechanical properties, which is a major problem in current PEM technology. Performance of the membranes in an actual fuel cell environment will be evaluated at an on-campus fuel cell center, equipped with facilities for manufacturing membrane electrode assemblies (MEAs) and fuel cell test stations.
The program is fully expected to produce novel results that will be published in scientific journals and presented at national scientific meetings. As well, this program will train students in polymer membrane manufacturing and fuel cell technology. Undergraduate students will participate either through independent studies during the academic year or as REU students. Under-represented minority students and women will be especially targeted for involvement in the project. A lecture and demonstration teaching module about fuel cell technology will be developed and offered to students and faculty at high schools and primarily-undergraduate colleges. Research collaborations will also be established with science and engineering faculty at primarily-undergraduate institutions. Industry will be exposed to the results of the research through the IMS Associates Program, an industrial outreach program in the Institute of Materials Science at the University of Connecticut.
Intellectual Merit. The specific focuses of the research were as follows: 1) the manufacture of proton exchange membranes by alignment of micro- and nanoparticles dispersed in a poorly conducting matrix polymer using electric fields, 2) the synthesis of highly charged crosslinked polyelectrolyte nanoparticles, and 3) manufacture of proton exchange membranes with nanofibers aligned in the permeation direction. The research demonstrated the use of field alignment of the proton-conducting domains in a blend of a highly charged ionomer dispersed within a non-charged, relatively hydrophobic polymer matrix to increase the conductivity of the manufactured polymer blend PEM. A blend of sulfonated poly(ether ketone ketone), SPEKK, and a polyether imide, PEI, was used to demonstrate this approach. Blends of SPEKK/PEI with a 3:7 mass ratio were aligned in solution and in the melt using electric field strengths varying from 0 to 30 V/mm and frequencies varying from 0 to 10 kHz, see Fig. 2. In general, the degree of alignment agreed with theoretical predictions for the alignment of drops or particles suspended in a fluid with a different dielectric constant. Alignment resulted in up to a three orders of magnitude increase in conductivity at low humidity. High ion-exchange capacity (IEC) sulfonated polystyrene nanoparticles were synthesized by an emulsion copolymerization of styrene, divinyl benzene and sulfonated styrene (SS), see Fig. 1. The effects of varying the counterion of the sulfonated styrene monomer, the SS concentration, the surfactant and the addition of a crosslinking agent on the ability to stabilize the emulsion nanoparticles to high IEC were studied. Water-insoluble nanoparticles, 20-160 nm in diameter, with IEC as high as 5 meq/g were achieved using sulfonated styrene with a quaternary alkyl ammonium cation, a non-ionic surfactant and a crosslinking agent in the emulsion formulation. That IEC corresponds to nearly fully sulfonated crosslinked polystyrene (IEC ~ 5.4 meq/g). Proton exchange membranes were prepared by dispersing these nanoparticles in a relatively hydrophobic matrix polymer. An alternative approach for incorporating a highly conductive phase with large specific surface area in a composite PEM is to use ionomeric nanofibers., which has the advantage of high aspect ratio that can provide alignment of the conducting phase. High sulfonation level (IEC ~ 4.8 meq/g), sulfonated PS fibers were electrospun, and composite membranes of SPS and poly(dimethyl siloxane) were prepared by embedding an electrospun nanofiber mat in siloxane monomer and curing the siloxane. A method was developed to chemically crosslink the polymer to improve stability of the electrospun mats in water. The addition of PEO to the spinning dope not only helped to improve spinnability of the polyelectrolyte but also, with subsequent heat-treatment, improved the stability of fiber mats in water. As-spun SPS-PEO mats readily dissolved in water, but adding PEO concentration and heat-treating the fiber mats decreased swelling due to crosslinking of the nanoparticles. The crosslinking was due to the formation of sulfonic acid esters at high temperature. 70/30 w/w SPS-PEO electrospun fiber mats appeared to be optimal in terms of low shrinkage on heating and the fiber mats also retained a fibrous morphology on exposure to water. Broader Impacts. Three graduate students participated in the research. Two have graduated with PhD degrees in Polymer Science from the University of Connecticut and currently work for U.S. companies. The other graduate student will graduate with a PhD in Polymer Engineering from the University of Akron in January, 2012. Two of the students were members of underrepresented groups for STEM. Seven undergraduate chemical engineering students worked on the project, and three were members of underrepresented groups for STEM. Several new courses were developed as a result of the grant, including a freshman honors seminar at the University of Connecticut and a graduate course on fuel cell technology at the University of Akron in the Spring semester of 2010. The PIs participated in local and regional science fairs and Olympiads held in CT and OH. Two high school students from an urban school in Akron, OH, conducted inquiry-based science projects at the University of Akron and the CT PI participated in an NSF-RET program (Joule Fellows in Sustainable Energies) at the University of Connecticut.
