November 2, 2012 | Leave a Comment
Fuel cells are for some the nirvana of portable or mobile energy production. But the problem of a catalyst that tears the hydrogen atom into the parts needed to generate electricity still bedevils the progress to widespread commercial marketability.
The best catalyst for efficiency is platinum, the rare, expensive, and beautiful silvery metal. Other ideas are in research and some early claims are looking positive, but for now platinum is king. If platinum is going to stay on top, how will costs be cut and longevity increased?
A research team at the Cornell Energy Materials Center has taken an important step forward with a chemical process that creates platinum-cobalt nanoparticles with a platinum enriched shell that show improved catalytic activity.
The new work also addresses another catalyst problem. A fuel cell is pretty much a steady state energy production device. No throttle, instant powerup, power on demand or other customary variances that would be needed for particularly electric vehicle use. The variance is going to need storage batteries or more likely capacitors for power bursts and energy recovery surges. The sluggish sensation of fuel cells working alone isn’t going to make drivers happy at all.
Héctor Abruña, the E.M. Chamot Professor of Chemistry and Chemical Biology believes the Cornell work. “ . . . could be a real significant improvement. It enhances the catalysis and cuts down the cost by a factor of five.”
In a hydrogen fuel cell, a catalyst at one electrode cracks hydrogen atoms into their component protons and electrons. The electrons travel through an external circuit to create an electric current to the other electrode, where a second catalyst combines the incoming electrons, free protons and oxygen to form water. In current commercial fuel cells, that catalyst is pure platinum.
The Cornell research team has previously created nanoparticles of a palladium-cobalt alloy coated with a thin layer of platinum that worked like pure platinum at lower cost. Forming the catalyst as nanoparticles – typically about 5 nanometers in diameter and distributed on a carbon support – provides more surface area to react with the fuel.
Abruña explains computer simulations of the catalytic reaction predicted that there should be an increase in catalytic activity if the platinum atoms are pushed a bit together or “strained”.
Deli Wang, a post-doctoral researcher in Abruña’s group, devised a new chemical process to manufacture nanoparticles of a platinum-cobalt alloy that included an annealing (heating) step, where the randomly distributed atoms in the alloy form an orderly crystal structure. Rather than just being jumbled together, the metal atoms arrange themselves in an orderly lattice.
This innovation is absolutely key – the platinum atoms layered onto these particles line up with the lattice and are pushed closer together than they would be in pure platinum, with the resulting “strain” enhancing the catalytic activity.
Huolin Xin, a graduate student in Muller’s group, used a scanning tunneling electron microscope to confirm the structure.
In preliminary tests the new nanoparticles supported in the lattice showed about three and a half times higher catalytic activity (measured by current flow) than similar particles with a disordered core, and more than 12 times more than pure platinum.
The new catalysts also are more durable.
Fuel cell catalysts lose their effectiveness as platinum atoms are oxidized away or as nanoparticles clump together, deceasing the surface area they can offer to react with fuel.
After 5,000 on-off cycles of a test cell, catalytic activity of the Cornell ordered lattice nanoparticles remained steady, while that of similar cobalt-platinum nanoparticles with a disordered core rapidly fell off.
The ordered structure is more stable, Abruña said. The platinum skin may be bonded more strongly to the ordered core than to the disordered alloy, so it would be less likely to fuse with the platinum on other nanoparticles to cause clumping. “We have not gone beyond 5,000 cycles but the results up to that point look very, very good,” he said.
Along with lead author Abruña, Wang and Xin co authors include Francis DiSalvo, the John Newman Professor of Chemistry and Chemical Biology, and David Muller, professor of applied and engineering physics and co-director of the Kavli Institute at Cornell for Nanoscale Science. Their paper, “Structurally Ordered Intermetallic Platinum–Cobalt Core–Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts” has been published in Nature Materials.
Lots of claims are being made on solving the fuel cell catalyst problem. So far no commercial or mass production scale is taking place from the new ideas. But the platinum fuel cell is known technology in manufacturing and if Abruña is right on a commercial application needing 80% less platinum, without a huge processing cost, fuel cells could find a much larger market.