New PEM Membrane Material Retains Conductivity During Low Humidity
11 September 2006
Researchers at Virginia Tech have developed a new proton exchange membrane (PEM) polymer material for fuel cells that retains conductivity during low humidity.
James McGrath, University Distinguished Professor of Chemistry at Virginia Tech, presented his group’s work during a plenary lecture at the Challenges for the Hydrogen Economy symposium during the 232nd National Meeting of the American Chemical Society (ACS) in San Francisco.
Fuel cells convert chemical energy to electrical energy. In a PEM fuel cell, the critical exchange takes place through a thin water-swollen co-polymer film that contains sulfonic acid (SO3H) groups. Electrons are peeled off by oxidation of the hydrogen atoms and hydrated 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. But, in the desert, that is pretty inefficient.—James McGrath
Instead of stirring two kinds of reactive monomers, or small molecules, together to form a new random copolymer, the new material links blocks of two different short polymers in sequences.
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.
Subsequent presentations on the material during the course of the ACS conference will present more details on the alternative PEM materials, including atomic force microscope images of the new polymer; a report on the synthesis of the new material; and measurements of cell conductivity.
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 the Department of Energy.
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