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Fuel cells: Will a nanowire net cut catalyst costs?
Platinum nanowire net with (left) and without problematic "beads." Credit: Univ. of Rochester
One of the big divides the world of proton exchange fuel cell research is between those who are looking for an alternative to platinum (such as the University of Dayton’s Liming Dai) and those who are sticking with a platinum catalyst.
The pro-platinum group, populated by realists, are quick to acknowledge that ordinary catalyst systems are prohibitively expensive because the cost of the precious metal makes fuel cells containing them unaffordable except for military uses, space applications and specialized research centers. For them, the trick now is to find a way to use the least amount of platinum possible without reducing a fuel cell’s power output. Not surprisingly, they think the platinum Holy Grail can be found in nanotechnology.
Along these lines, one research team from the University of Rochester thinks they may have found the solution: long platinum nanowires.
According to a paper in Nano Letters, the concept is to use wires only 10 nanometers wide but several centimeters long to create a catalytic web of platinum. Lead author James C. M. Li, a professor of mechanical engineering at the university, and graduate student Jianglan Shui says they learned how to produce the long wires using electrospinning techniques.
The platinum nanowires produced by Li are roughly ten nanometers in diameter and also centimeters in length-long enough to create the first self-supporting “web” of pure platinum that can serve as an electrode in a fuel cell.
Much shorter nanowires have already been used in a variety of technologies, such as nanocomputers and nanoscale sensors. But the duo turned to a process known as electrospinning, a relatively old, noninvasive technique that uses an electrical charge to draw very fine (typically on the micro or nano scale) fibers from a liquid or molten material.
It’s easy to understand the attractiveness of electrospinning in this application. The technique is known for producing high surface-to-volume ratios, strong (approaching theoretical maximum strength) and defect-free structures. Li and Shui apparently are able to use this method to create platinum nanowires that are thousands of times longer than any previous such wires.
The electrospinning wasn’t without problems. Initial attempts at it left Li and Shui with platinum beads projecting the platinum nanowires. The beads block the surface of the wires, and if enough are present, large amounts of catalytic surface are effectively inaccessible. “With platinum being so costly, it’s quite important that none of it goes to waste when making a fuel cell,” says Li. “We studied five variables that affect bead formation and we finally got it – nanowires that are almost bead free.
Li and Shui say their approach avoids some of the pitfalls that other researchers have run into when using nanoscale amounts of platinum, such as the tendency of nanoparticles of the metal to merge through surface diffusion, and to become dislodged by oxidation of the support material.
Li says he understands why few have used his long-wire method. “The reason people have not come to nanowires before is that it’s very hard to make them. The parameters affecting the morphology of the wires are complex. And when they are not sufficiently long, they behave the same as nanoparticles,” says Li.
Li and Shui are now working on methods to make the wires longer, more uniform and with even fewer beads. “After that, we’re going to make a fuel cell and demonstrate this technology,” says Li.
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Making cheaper fuel cells with carbon nanotubes
Liming Dai
A University of Dayton research team - led by Liming Dai, UD’s Wright Brothers Institute endowed chair in nanomaterials - says it has developed a technique that makes carbon nanotubes a cheaper and better fuel cell catalyst than platinum.
The Feb. 6th online edition of Science magazine reports on the team’s findings. Since that announcement, interviews with Dai - a professor of materials engineering in UD’s Department of Chemical and Materials Engineering - also have appeared in online science-community websites and in print trade magazines.
As reported, Dai’s team has shown that vertically-grown arrays of carbon nanotubes act as effectively as platinum in alkaline fuel cells if - and this is key - the carbon nanotubes are doped with nitrogen.
Reporter Stephen Battersby explains the need for nitrogen in NewScientist:
Unaided, this reaction would happen only very slowly, so the cathode has to be formed of a chemical catalyst to speed up the reaction. Traditionally, the only substance that has worked well enough is platinum.
Battersby notes, when carbon nanotubes were used without nitrogen doping in earlier experiments, they catalyzed fuel cell action but “were much less effective than platinum nanoparticles.”
Dai reveals his methodology in a Feb. 5 article in Technology Review. The first step, he says, is starting with a compound comprised of carbon, nitrogen and iron. The next step is placing the compound on a quartz substrate and “heating it in the presence of ammonia, resulting in nitrogen-doped carbon nanotubes growing straight up from the surface.”
Then any latent iron needs to be eliminated by oxidizing the array and moving it to a polymer film. The electrode is then emerged in a potassium hydroxide electrolyte.
It was at this point, Dai says, when his team noticed the technology’s ability to speed up the cathode reaction of oxygen and electrons.
In Dai’s estimation, carbon nanotubes doped with nitrogen “are even better than platinum.” He says they produce four times as much electric current as platinum and “where platinum nanoparticles can lose their effectiveness when they cluster together or become tainted by carbon monoxide, the nanotubes are immune to these degradations.”
Dai believes his team may be able to produce the same results using other forms of nitrogen-doped carbon. “Now we have discovered how this chemistry works,” he points out, “It may not be necessary to use nanotubes.”
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Learning from lizards how to improve dry adhesives
Change in vertically-aligned multi-walled carbon nanotubes during adhesion measurements.
What can a team of highly trained researchers from four different U.S. universities learn from the feet of a gecko lizard? According to the researchers, how to improve carbon nanotube-based material so that, for the first time, it demonstrates “directionally varied (anisotropic) adhesive force” and gripping power nearly three times the level of existing nanotube dry adhesives.
The team - comprised of researchers from the University of Dayton, the Georgia Institute of Technology, the Air Force Research Lab and the University of Akron - describes its achievement in a paper published in the Oct. 10 edition of Science magazine.
As Science reports, the team believes their advance could lead to solder-free connections between electronic devices, longer-lasting adhesives for use in outer space and a broad range of other important applications.
In the article, team members explain that a gecko’s ability to scale vertical walls is due to atomic-scale van der Waals interactions that occur naturally in the microscopic hairs on the lizard’s toes.
These hairs - actually minute setae - give the gecko resistance to perpendicular shear force, enabling it to grab vertical surfaces with surprising strength. The setae also allow the gecko to easily release its strong hold.
By manipulating carbon nanotubes to simulate and intensify the anisotropic adhesive forces at work in gecko hairs, the team has created a carbon nanotube dry adhesive that is “ten times better than a real gecko at resisting perpendicular shear forces.”
According to team member Zhong Lin Wang, a Regents Professor in Georgia Tech’s School of Material Science and Engineering, the newly developed adhesive’s performance depends on the use of “rationally designed multi-walled carbon nanotubes formed into arrays with curly entangled tops.”
Wang likens the tangled tops to a “jungle of vines” that replicates the structure of a gecko’s foot, down to its “branching hairs of different diameters.”
These tangled tops become aligned with a surface when pressed against it, significantly increasing the contact area between the tops and the surface, Wang says.
Wang says, “When lifted off the surface in a direction parallel to the main body of nanotubes, only the [nanotube] tips remain in contact [with the surface], minimizing the forces of attraction.” He claims this “allows us to truly mimic what the gecko does naturally.”
Wang explains that, “When you have line contact along [a surface], you have van der Waals forces acting along the entire length of the nanotubes but, when you have a point contact, the van der Waals forces act only at the tip of the nanotubes.”
As the Science article reports, the researchers have tested their new adhesive’s grip on a number of surfaces, including glass, polymer, Teflon and rough sand paper. Wang says they found it measured up to 100 newtons per square centimeter in the shear direction and only 10 newtons per square centimeter in the normal direction. The team’s conclusion, he says, is that resistance to shear increases with nanotube length, while resistance to normal force is independent of tube length.
Funded by NSF, the project is led by the University of Dayton’s Liming Dai, the Wright Brothers Institute Endowed Chair in the UD’s School of Engineering. The research team also includes the UD’s Liangti Qu, Morley Stone from the Air Force Research Lab and Zhenhai Xia from the University of Akron.
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