Practically every article about fuel cells over the past 10 years has predicted that fuel cell-powered vehicles are ‘just around the corner’. If they are then they must have stalled, because fuel-cell powered vehicles have still not put in an appearance.
The main stumbling block would appear to involve catalysis. This is especially true for a type of fuel cell known as a polymer electrolyte membrane fuel cells (PEMFCs), which in most other ways are the most suitable fuel cells for the majority of applications, including transport, as they are fairly small, can generate quite a bit of electrical power and operate at fairly low temperatures.
Like all other fuel cells, PEMFCs generate electrical power from reactions between hydrogen and oxygen, and produce water as the only waste product. But these oxidation and reduction reactions don’t happen spontaneously: they need some form of catalyst. And the most efficient catalyst so far discovered is platinum, which is coated onto the electrodes in the current generation of PEMFCs.
The clear disadvantage with platinum is its cost. At $1500 an ounce, the platinum catalyst alone contributes around $3000 to the cost of a small fuel cell-powered car, equal to the cost of an entire petrol-powered car engine. To compound the problem, the cost of platinum has recently soared, driven by supply problems in South Africa, which produces around 80% of the world’s supply. Furthermore, the catalytic activity of the platinum tends to decline with use, as a result of the platinum molecules both becoming oxidised and binding with unwanted byproducts, such as hydroxide ions.
So scientists are on the hunt for cheaper and more durable catalysts, or at least catalysts that don’t use as much platinum. Making things a bit easier is the fact that it is novel cathode catalysts that are most urgently required. This is because the oxygen reduction reaction (ORR) that occurs here is slower than the hydrogen oxidation reaction at the anode, which means that cathodes need to contain more platinum than anodes.
One of the simplest ways to create cheaper catalysts is to combine platinum with one or more less expensive metals, such as copper or nickel. Some of these alloys have proved to be more efficient catalysts than platinum, but unfortunately they’re still not efficient enough to make up for the high cost of platinum.
‘The automobile companies have been asking for a platinum-based catalyst that is four times more efficient, and, therefore, four times cheaper, than what is currently available,’ says Peter Strasser, an assistant professor of chemical and biomolecular engineering at the University of Houston. ‘That’s the magic number.’
Last year, Strasser led a team that managed to hit that magic number with a catalyst made of platinum, copper and cobalt. The catalyst is in the form of nanoparticles that Strasser and his team synthesised directly onto a carbon cathode. They did this by impregnating the cathode with liquid solutions of the three metals in turn, with each solution left to dry and then reduced by annealing (controlled heating). The end result was nanoparticles made of platinum, copper and cobalt, although they were not yet catalytic.
‘The active phase of the catalyst is created by applying several hundred potential cycles in order to remove the copper and cobalt atoms near the surface of the particles,’ explains Strasser. ‘Platinum is more noble and does not dissolve during the potential cycling.’ This produces nanoparticles with a defined structure, in which a platinum-rich outer shell covers a copper-rich core.
Strasser thinks that this shell-core structure is the secret behind the impressive catalytic ability of the nanoparticles, which are four to five times more efficient than pure platinum. By analysing the nanoparticles, Strasser has uncovered evidence that the platinum molecules on the surface are closer together than in pure platinum, probably as a result of the copper-rich core, which also affects the electronic properties of the platinum shell. This all seems to help oxygen molecules bind to the surface of the nanoparticle, thereby making it easier for the ORR to occur. Strasser has since shown that nanoparticles consisting of just platinum and copper have similar catalytic abilities.
Confirming the benefits of platinum-based nanoparticles with defined structures is work being conducted by Vojislav Stamenkovic at Argonne National Laboratory, Illinois. Together with colleagues from the Lawrence Berkeley National Laboratory, the University of South Carolina and the University of Liverpool in the UK, he has developed nanoparticles with a similar core-shell structure that are an impressive 90 times more efficient than the platinum–based catalysts used in current fuel cells.
To develop these nanoparticles, Stamenkovic and his team carried out some of the first detailed studies of the relationship between the surface properties of platinum-based nanoparticles, including surface composition and electronic properties, and their catalytic abilities. Using analytical techniques such as electron spectroscopy and ultraviolet photoemission spectroscopy, they discovered that these catalytic abilities seem to be dictated by the different rates at which molecules involved in the ORR process, including oxygen and reactive intermediates, and unwanted by-products bind to the platinum molecules.
Based on this finding, they created a platinum–nickel nanoparticle with the chemical formula Pt3Ni(111), which showed the highest ORR activity ever detected. This nanoparticle has a similar shell–core structure to Strasser’s nanoparticle, consisting of a layer of tightly-packed platinum atoms over a layer with an equal number of platinum and nickel atoms surrounding a platinum-rich core. Like Strasser, Stamenkovic thinks that it is the specific structure of the nanoparticle that is responsible for its impressive catalytic abilities, with the close configuration of the surface platinum atoms preventing unwanted by-products from binding and thereby allowing more ORR activity.
‘At this phase of our research we have several goals,’ he says. ‘To make a catalyst which can mimic the activity of the Pt3Ni(111)-skin surface, to make a nanoscale version of this surface, to find an appropriate substrate which would not lead to a drop in specific activity, and to improve the durability of the cathode catalyst.’
One way in which Stamenkovic could improve the durability of his cathode catalyst is to take advantage of recent work conducted by a team from Brookhaven National Laboratory, New York, led by Radoslav Adzic. They have discovered that simply adding gold clusters to platinum catalysts can make them more durable. The gold clusters appear to do this by preventing the platinum atoms from being oxidised to platinum oxide and hydroxide, which are more prone to dissolving than pure platinum.
‘Gold is nobler than platinum [and] changes its electronic properties to some extent,’ explains Adzic. ‘XANES [X-ray absorption near edge spectroscopy] data clearly show in the presence of gold a shift of platinum oxidation to more positive potentials.’ Unlike conventional cathodes, which can lose 45% of their platinum over five days under certain operating conditions, Adzic found that adding gold clusters prevented the loss of practically any platinum from the cathode. He and his team are now testing the clusters in real fuel cells.
Others researchers are trying to ditch the platinum completely, although this has so far proved far from easy. After more than 30 years of searching for non-precious metal catalysts, scientists had only really managed to come up with cobalt and iron porphyrins modified by pyrolysis (heating in the absence of air). This pyrolysis process creates metal–nitrogen sites on the pyrolysis products where ORR is thought to take place, but unfortunately these pyrolised metal porphyrins are even less stable than platinum.
Then, in 2006, Rajesh Bashyam and Piotr Zelenay from Los Alamos National Laboratory, New Mexico, reported a new non-precious metal cathode catalyst based on a cobalt–polypyrrole–carbon composite. Bashyam and Zelenay specifically designed this composite to have a similar structure to the pyrolised metal porphyrins, with the polypyrrole acting as a framework for hosting cobalt-nitrogen sites. ‘The idea was to entrap catalytic centres by co-ordinating the metal centre with heteroatoms present in such polymers,’ explains Zelenay.
They found that not only could this composite catalyse the ORR, helping to generate electrical power when incorporated into a fuel cell, but it also showed no sign of degrading over more than 100 hours of continual use. Zelenay and Bashyam have since gone on to develop similar catalytic polymers comprising iron and nickel, and are now attempting to increase the catalytic activity of these materials.
As is so often the way, however, if you want to discover really effective non-precious metal catalysts then you need to turn to nature. For there are enzymes that are able to conduct the ORR, which are known as laccases and have active sites based on copper, and enzymes that are able to oxidise hydrogen atoms, which are known as hydrogenases and have active sites based on iron and nickel. And Oxford University’s Fraser Armstrong has managed to take advantage of this catalytic ability to develop enzymatic fuel cells.
‘Our inspiration came about from the realisation that the active sites of enzymes were in many cases as good – and we now believe probably even better – than that most expensive resource platinum,’ Armstrong explains.
The other advantage of enzymes is that they are very specific catalysts. So, unlike platinum, hydrogenases will only catalyse the oxidation of hydrogen atoms and laccases will only catalyse the ORR. This means that if you incorporate hydrogenases into the anode and laccases into the cathode of a fuel cell, then you can pass a hydrogen-air mix over both electrodes rather than needing separate streams of hydrogen and air. Unfortunately, enzymatic fuel cells can’t produce much power, but Armstrong believes that they could find use in certain niche applications, such as powering implantable devices.
Armstrong hopes that this work will also inspire the development of the next generation of non-precious metal catalysts. ‘We’re looking at these enzymes as a benchmark for what is definitely possible if we are able to synthesise small analogues that mimic this activity,’ he says.
If such analogues can be found, then perhaps fuel cell-powered vehicles will finally appear from around that corner.
How it works
A typical PEMFC fuel cell consists of an anode and a cathode, both usually made of carbon, separated by an electrolyte, which in this case is a solid ionic polymer. A stream of hydrogen is passed over the anode (blue arrows), while oxygen in the form of air is passed over the cathode (white arrows).
At the anode, the hydrogen molecules undergo oxidation reactions, splitting into electrons (yellow, inset) and hydrogen ions (blue, inset), which are able to pass through the solid ionic polymer (yellow) to the cathode. But the electrons are unable to follow because the polymer is non-conducting and so they are forced to travel along an external circuit between the two electrodes, generating electrical power. At the cathode, the electrons and hydrogen ions combine with oxygen molecules in the air to produce water, in what is known as the oxygen reduction reaction (ORR).