|The green dots in this Low Energy Electron Diffraction pattern for a single crystal of Pt3Ni(111) reveal a tightly packed arrangement of surface atoms that wards off platinum-grabbing hydroxide ions and boosts catalytic performance. Click to enlarge.|
Researchers with the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Argonne National Laboratory (ANL) have identified a new variation of a familiar platinum-nickel alloy that is the most active oxygen-reducing catalyst ever reported.
The team found a Pt3Ni(111) material that is 10-fold more active for the oxygen-reduction reaction (ORR) than the corresponding Pt(111) surface and 90-fold more active than the current state-of-the-art platinum-carbon cathode catalysts used today. The work is reported online in the journal Science.
The slow rate of oxygen-reduction catalysis on the cathode has been one of the main limitations for the automotive application of the polymer electrolyte membrane fuel cell (PEMFC). The new discovery could result in fuel cells with higher power densities without a loss in cell voltage.
The existing limitations facing PEM fuel cell technology applications in the transportation sector could be eliminated with the development of stable cathode catalysts with several orders of magnitude increase in activity over today’s state-of-the-art catalysts, and that is what our discovery has the potential to provide.—Vojislav Stamenkovic, a scientist with dual appointments in the Materials Sciences Division of both Berkeley Lab and Argonne
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 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 to 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.
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 of the layers underneath those 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. 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 characterizing nanoscale surfaces in order to fine-tune their properties in a desired direction.—Vojislav Stamenkovic
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).
This research was funded by the US Department of Energy’s Hydrogen Program. It was also supported through funding by General Motors.
PEM cell background. A PEM fuel cell consists of a cathode (the positively charged electrode) and an anode (the negatively charged electrode) that are positioned on either side of a polymer electrolyte membrane, which is a specially treated substance that conducts positively charged protons and blocks negatively charged electrons.
Like other fuel cells, PEM fuel 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 (O) comes from molecules in the air (O2) that are split into pairs of O atoms by the cathode catalyst.
A challenge for PEM cells has been the platinum. While pure platinum is an exceptionally active catalyst, it is quite expensive and its performance can quickly degrade through the creation of unwanted by-products, such as hydroxide ions.
Hydroxides have an affinity for binding 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 erodes. 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.
“Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability”; Vojislav R. Stamenkovic, Ben Fowler, Bongjin Simon Mun, Guofeng Wang, Philip N. Ross, Christopher A. Lucas, Nenad M. Markovic; Science Online, 11 January 2007 Science DOI: 10.1126/science.1135941