Fuel cells convert chemical energy, usually from hydrogen, to electrical energy. In a proton exchange membrane in fuel cells, the critical exchange takes place through a thin water swollen copolymer film that contains sulfonic acid (SO3H) groups. Electrons are peeled off by oxidation of the hydrogen atoms and protons pass through the film to combine with oxygen on the other side to form water as a byproduct.
The efficiency of the exchange process depends upon water, so efficiency – measured as proton conductivity – goes down as humidity goes down. “Up to now, a lot of water has been needed to assist the proton transfer process,” says chemistry Professor James McGrath. “But, in the desert, that is pretty inefficient.”
Last year (2005), McGrath, chemical engineering Professor Don Baird, and their students demonstrated a method for creating a material with improved conductivity even at lower humidity. In March 2006, the U.S. Department of Energy awarded McGrath and Bard’s groups $1.5 million over five years to advance the research.
The basic concept goes back to a book McGrath wrote on “block” copolymers in 1977, he says. Instead of stirring two kinds of reactive monomers, or small molecules, together to form a new random copolymer, McGrath links blocks of two different short polymers in sequences. For example, he would link polymer W (loves water) and polymer d (dry but strong) into a chain this way: WWWWWdddddddWWWWWdddddddd.
For the new materials, the researchers can link a 10- to 50-unit block of a polymer containing acidic groups (SO3H) that like water (hydrophilic) to an equally long block of a polymer that has mechanical strength, thermal stability, and endurance, but hates water (hydrophobic). The chains self-assemble into flexible thin films. Under an atomic force microscope, the film’s swirling surface looks like a fingerprint, with light ridges and dark channels. It turns out that the soft hydrophilic polymer forms the dark channels where water is easily absorbed so that the entire film – or proton exchange membrane (PEM) – has an affinity for water transport that is two to three times higher than the present commercially available PEM.
In addition to making PEM materials with better qualities, another goal of the research is to make PEM materials that can be easily manufactured. The self-assembling nature of the block copolymer material into a nanocomposite film is an important attribute. In addition, Baird is working on processing the film from powders using a reverse roll coater, equipment commonly available in the materials industry but not yet being used to produce PEM.
McGrath’s group has been developing fuel cell materials since the late 1990s with funding from the National Science Foundation, Department of Defense, NASA, and DOE. In 2004, McGrath and his Ph.D. student, Michael Hickner (now at Sandia National Labs), received an R&D 100 award with Battelle for a high-temperature PEM that is now being developed for automotive, stationary, and portable power fuel cells. The Virginia Tech-Battelle material is projected to be less expensive to produce than previous materials because it applies new processes to largely commercially available materials.
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