Researchers at the University of Houston have developed a platinum-based catalyst for the cathode side of polymer electrolyte membrane fuel cells (PEMFCs) that is at least four-times and up to six-times more efficient than existing catalysts.
The catalyst material consists of nanoparticles with a platinum-rich shell and a core made of an alloy of copper, cobalt, and platinum. This catalyst demonstrates the highest activity yet observed for the reduction of oxygen.
Peter Strasser, an assistant professor of chemical and biomolecular engineering, leads the group that conducted this research. Their findings are published in the journal Angewandte Chemie.
Platinum-based compounds typically serve as catalyst materials in PEM fuel cells. Platinum, however, is extremely expensive, and the application of the material in fuel cells drives up costs.
The automobile companies have been asking for a platinum-based catalyst that is four-times more efficient, and therefore four-times cheaper, than what is currently available. That’s the magic number.—Peter Strasser
A PEM fuel cell powerful enough to operate a small two- or four-door automobile would require around 3.33 ounces of platinum costing approximately $3,600, according to Strasser. The use of a catalyst such as the group developed would allow a fuel cell-powered car to operate on about .66 of an ounce of platinum, cutting the cost by almost $3,000.
Strasser and his team deposited an alloy of platinum, copper, and cobalt onto carbon supports in the form of nanoparticles. The active catalytic phase is formed in situ: when a cyclic alternating current is applied to the electrode, the less precious metals, especially the copper, on the surface of the nanoparticles separate from the alloy. This process results in nanoparticles with a core made of the original copper-rich alloy and a shell containing almost exclusively platinum.
The observed increase in surface area of the nanoparticles is not enough to explain the increased activity. Strasser suspects that special altered structural characteristics of the surface play a role. Although the surface consists mostly of platinum, the distances between the platinum atoms on the particle surface seem to be shorter than those in pure platinum. This compression can be stabilized by the alloy core, which shows even shorter Pt-Pt distances because of the presence of copper and cobalt. In addition, the copper-rich core seems to influence the electronic properties of the platinum shell. Theoretical calculations have suggested that the oxygen can thus bind optimally to the particle surface, allowing it to be more easily reduced.
The oxygen-reducing activity of our new electrocatalytic material is unsurpassed—it is four to five times higher than that of pure platinum. In addition, we have demonstrated how to incorporate and activate this material in situ in a fuel cell.—Peter Strasser
Ratndeep Srivastava, Prasanna Mani, Nathan Hahn, Peter Strasser. “Efficient Oxygen Reduction Fuel Cell Electrocatalysis on Voltammetrically Dealloyed Pt-Cu-Co Nanoparticles” Angew Chem Int Ed Engl 2007 Sep 24