Nano-engineering platinum surfaces for improved fuel cells

The development of hydrogen fuel cells for vehicles, the ultimate green dream in transportation energy, is another step closer thanks to recent work in the US.  Researchers with the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and Argonne National Laboratory (ANL) have identified a new variation of a familiar platinum-nickel alloy that they say is far and away the most active oxygen-reducing catalyst ever reported.

The slow rate of oxygen-reduction catalysis on the cathode – a fuel cell’s positively charged electrode – has been a primary factor hindering development of the polymer electrolyte membrane (PEM) fuel cells favoured for use in vehicles powered by hydrogen.

“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,” said Vojislav Stamenkovic, a scientist with dual appointments in the Materials Sciences Division of both Berkeley Lab and Argonne. Stamenkovic and Argonne senior scientist Nenad Markovic are the corresponding authors of a study whose results are now available online from the journal Science, with highlights on the Eureka website. The paper, entitled Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability, reports a platinum-nickel alloy that increased the catalytic activity of a fuel cell cathode by an astonishing 90-fold over the platinum-carbon cathode catalysts used today..

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. The hydrogen-powered fuel cells most talked about for use in vehicles are PEM fuel cells – also known as proton exchange membrane fuel cells – because they can deliver high power in a relative small, light-weight device. Unlike batteries, PEM fuel cells do not require recharging, but rely on a supply of hydrogen and access to oxygen from the atmosphere.

PEM fuel cells have admirably served NASA’s space programme, but they remain far too expensive for use in cars or most other Earth-bound applications. The biggest cost factor is their dependency on platinum, which is used as the cathode catalyst. A PEM fuel cell consists of a cathode 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 types of 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 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.

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 (Fig.1). 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 – 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.