Green car one step closer
Monday, 14 May, 2007
A new variation of a familiar platinum-nickel alloy has been identified and claimed to be the most active oxygen-reducing catalyst ever reported.
This discovery by researchers with the US Department of Energy's Lawrence Berkeley National Laboratory and Argonne National Laboratory could mean that the development of hydrogen fuel cells for vehicles, the ultimate green dream in transport energy, is another step closer.
Polymer electrolyte membrane (PEM) fuel cells are suitable for vehicles powered by hydrogen but the slow rate of oxygen-reduction catalysis on the cathode has been hindering the development of these fuel cells.
PEM fuel cells consist of electrodes containing a platinum catalyst and a solid polymer electrolyte. By splitting hydrogen molecules at the anode and oxygen molecules at the cathode, PEM fuel cells generate a current with only heat and water as a by-product.
Vojislav Stamenkovic in the Materials Sciences Division of Berkeley Lab and senior scientist Nenad Markovic have found a platinum-nickel alloy that increased the catalytic activity of a fuel cell cathode 90-fold over the currently used platinum-carbon catalysts.
"This new catalyst is 90 times more active than currently used state-of-the-art Pt/Carbon catalyst. The improved activity is coming from modified electronic properties of topmost Pt layer over Ni rich subsurface layer," Stamenkovic said.
By converting chemical energy into electrical energy without combustion, fuel cells represent perhaps the most efficient and clean technology for generating electricity. This is especially true for fuel cells designed to directly run off hydrogen, which produce only water as a by-product.
Also, unlike batteries, PEM fuel cells do not require recharging, but rely on a supply of hydrogen and access to oxygen from the atmosphere to work effectively.
However, while PEM fuel cells have been used in NASA's space program, they remain far too expensive for use in cars.
The biggest cost factor is the cells' dependency on platinum, which is used as the cathode catalyst.
While pure platinum is an exceptionally active catalyst, it is expensive and its performance can quickly degrade through the creation of unwanted by-products, such as hydroxide ions.
Hydroxides can bind with platinum atoms and when they do this they take those platinum atoms out of the catalytic game. As this platinum-binding continues, the catalytic ability of the cathode becomes weaker. Consequently, researchers have been investigating the use of platinum alloys in combination with a surface enrichment technique.
Under this scenario, the surface of the cathode is covered with a 'skin' of platinum atoms, and beneath are layers of atoms made from a combination of platinum and a non-precious metal, such as nickel or cobalt. The subsurface alloy interacts with the skin in a way that enhances the overall performance of the cathode.
Like other types of fuel cells, PEM cells carry out two reactions, an oxidation reaction at the anode and an oxygen reduction reaction (ORR) at the cathode. For PEMs, this means that hydrogen molecules are split into pairs of protons and electrons at the anode. While the protons pass through the membrane, the blocked electrons are conducted via a wire (the electrical current), through a load and eventually onto the cathode.
At the cathode, the electrons combine with the protons that passed through the membrane plus atoms of oxygen to produce water. The oxygen comes from molecules in the air that are split into pairs of O atoms by the catalyst.
"Massive application of PEM fuel cells as the basis for a renewable hydrogen-based energy economy is a leading concept for meeting global energy needs," said Stamenkovic.
"Since the only by-product of PEM fuel cell exploitation is water vapour, their widespread use should have a tremendously beneficial impact on greenhouse gas emissions and global warming."
For this latest study, Stamenkovic and Markovic and their colleagues created pure single crystals of platinum-nickel alloys across a range of atomic lattice structures in an ultra-high vacuum (UHV) chamber. They are then used a combination of surface-sensitive probes and electrochemical techniques to measure the respective abilities of these crystals to perform ORR catalysis. The ORR activity of each sample was then compared with that of platinum single crystals and platinum-carbon catalysts.
The researchers identified the platinum-nickel alloy configuration Pt3Ni(111) as displaying the highest ORR activity that has ever been detected on a cathode catalyst - 10 times better than a single crystal surface of pure platinum(111) and 90 times better than platinum-carbon. In this (111) configuration, the surface skin is a layer of tightly packed platinum atoms that sits on top of a layer made up of equal numbers of platinum and nickel atoms. All the layers underneath these top two layers consist of three atoms of platinum for every atom of nickel.
According to Stamenkovic, the Pt3Ni(111) configuration acts as a buffer against hydroxide and other platinum-binding molecules, blunting their interactions with the cathode surface and allowing for far more ORR activity. The reduced platinum-binding also cuts down on the degradation of the cathode surface.
"We have identified a cathode surface that is capable of achieving and even exceeding the target for catalytic activity, with improved stability for the cathodic reaction in fuel cells," said Stamenkovic.
"Although the platinum-nickel alloy itself is well known, we were able to control and tune key parameters which enabled us to make this discovery. Our study demonstrates the potential of new analytical tools for characterising nanoscale surfaces to fine-tune their properties in a desired direction."
"This material can be used for any application where fuel cells can be applied: direct methanol fuel cells (laptop computers, mobile phones etc), stationary fuel cells (power units for buildings and hospitals), etc," Stamenkovic said.
So what does the future hold for the platinum/nickel alloy? According to Stamenkovic it will take several years of extensive fundamental research to fully engineer and develop the catalyst at the nanoscale level.
The next step, Stamenkovic said, will be to engineer nanoparticle catalysts with electronic and morphological properties that mimic the surfaces of pure single crystals of Pt3Ni(111).
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