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May 09, 2010

Inexpensive Metal Catalyst for Generating Hydrogen from Water and Cheaper and More Efficient Fuel Cell Catalysts

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1. A new proton reduction catalyst based on a molybdenum-oxo metal complex is about 70 times cheaper than platinum, today’s most widely used metal catalyst for splitting the water molecule. This work that appears in the April 29, 2010 issue of the journal Nature, titled “A molecular molybdenum-oxo catalyst for generating hydrogen from water.” [5 page pdf]

(H/T reader Goatguy)

Hydrogen gas, whether combusted or used in fuel cells to generate electricity, emits only water vapor as an exhaust product, which is why this nation would already be rolling towards a hydrogen economy if only there were hydrogen wells to tap.

Nature has developed extremely efficient water-splitting enzymes – called hydrogenases – for use by plants during photosynthesis, however, these enzymes are highly unstable and easily deactivated when removed from their native environment. Human activities demand a stable metal catalyst that can operate under non-biological settings. Metal catalysts are commercially available, but they are low valence precious metals whose high costs make their widespread use prohibitive. For example, platinum, the best of them, costs some $2,000 an ounce

2. MIT Technology Review : New material could cut the use of expensive platinum by 80 percent in fuel cells.

The new material already meets the U.S. Department of Energy's 2015 target for platinum catalysts: producing at least 0.44 amperes of electric current per milligram of platinum. It produces up to 0.49 amps per milligram of platinum, and the researchers believe it should be possible to increase the material's catalytic activity even more. "If we could get another factor of two [improvement in catalytic activity], we think that the cost of platinum in these fuel cells would make the technology more practical," says SLAC physicist Anders Nilsson.

At the anode of a conventional proton exchange membrane (PEM) fuel cell, the catalyst splits hydrogen into hydrogen ions and electrons, with the latter flowing out of the cell to create current. At the cathode, oxygen molecules combine with electrons and hydrogen ions to form water. This reaction is sluggish and speeding it up requires 10 times as much platinum as is used at the anode. "If you're trying to replace platinum, it is more important to replace the platinum at the cathode," says Dodelet.

In a recent Nature Chemistry paper, the researchers reveal the mechanism that makes this catalyst more active than regular platinum. By studying how x-ray beams are scattered by the new catalyst, they discovered that the distance between the platinum atoms that are left on the surface of the nanoparticles is less than the distance in pure platinum nanoparticles. A good catalyst should be able to split up oxygen molecules into atoms but should not bind too strongly with the free atoms; the shorter distance between platinum atoms in the new material makes it a more effective catalyst because it binds even more weakly with the oxygen atoms.

There are alternatives to using platinum as a catalyst. Dodelet and his group have worked with General Motors to develop a promising iron-based catalyst that they are now working to commercialize. Meanwhile, low-cost carbon nanotube catalysts and nickel catalysts are in the works for alkaline fuel-cell chemistries.

Platinum-free catalysts have advantages other than their low cost, says Liming Dai, a materials engineering professor at the University of Dayton, in Ohio, who is working on carbon nanotube catalysts. Platinum nanoparticles tend to lose their catalytic efficiency by aggregating into larger particles over time or when carbon monoxide sticks to their surface. Carbon nanotubes are more robust in the long-term, Dai says.

"This is interesting work and an important advance because the mechanism could be applied to other catalysts," Dai says of the new platinum catalyst. "It would be interesting to check out the long-term stability and carbon monoxide [surface] poisoning effect for this kind of core-shell catalyst."

Nature Chemistry - Lattice-strain control of the activity in dealloyed core–shell fuel cell catalysts

Electrocatalysis will play a key role in future energy conversion and storage technologies, such as water electrolysers, fuel cells and metal–air batteries. Molecular interactions between chemical reactants and the catalytic surface control the activity and efficiency, and hence need to be optimized; however, generalized experimental strategies to do so are scarce. Here we show how lattice strain can be used experimentally to tune the catalytic activity of dealloyed bimetallic nanoparticles for the oxygen-reduction reaction, a key barrier to the application of fuel cells and metal–air batteries. We demonstrate the core–shell structure of the catalyst and clarify the mechanistic origin of its activity. The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and weakening chemisorption of oxygenated species. We combine synthesis, measurements and an understanding of strain from theory to generate a reactivity–strain relationship that provides guidelines for tuning electrocatalytic activity.


23 pages of supplemental material

3. Perla Balbuena, professor in Texas A&M University’s Artie McFerrin Department of Chemical Engineering, has found a class of composite materials that show early indications of being just as effective — and even more durable — than the costly platinum catalysts typically used in fuel cells.

* demonstrated the potential durability and activity properties of a new “core-shell” composite material that can serve as a catalyst within a fuel cell. The material, she explains, still uses platinum but less of it, meaning it’s cheaper. What’s more, in its core, the material uses other key elements in a way that ensures the core particles will not segregate to the surface and dissolve in the polymeric membrane.

* The DOE’s Solid State Energy Conversion Alliance estimates fuel cells will need to cost $700 per kilowatt to serve as a viable energy alternative. Current technology, however, costs nearly 10 times that amount per kilowatt.

* A more affordable, durable catalyst could help lower the cost of fuel cell production, says Balbuena, who notes the composite material she has found meets a set of standard properties that DOE has set for the durability and makeup of such catalysts. Having successfully met those criteria, the next step for the composite material, Balbuena says, is actual production and laboratory testing



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