Two metals better than one for fuel cell catalysts

US scientists have reported a dramatic improvement in the activity of catalytic nanoparticles destined to replace platinum in fuel cells. They say their bimetallic ’nanodendrites’ are more than twice as effective than current state-of-the-art platinum catalysts and could help make future fuel cells more economically viable.

Proton-exchange membrane (PEM) systems are promising fuel cell technologies, but they require rare metal catalysts. The new catalyst, developed by a team at Washington University in St Louis and Brookhaven National Laboratory in New York, consists of platinum ’arms’ anchored to a palladium core. These arms increase the surface area of the precious metal available for the oxygen reduction reaction (ORR) in a fuel cell and thus could substantially reduce the amount of catalyst needed.

’One of the main barriers for commercialisation of fuel cells is the expense of the catalyst - we have to reduce the price by at least three or four times,’ says Younan Xia, who led the team. ’As we have shown, if you can increase the activity, then you can reduce the amount you need on the electrodes and significantly reduce the price.’

Xia uses 9nm octahedral palladium nanocrystals as ’seeds’ from which to grow his dendrite structures. The platinum arms, which bear facets that are particularly active for the ORR, are formed via the reduction of K₂PtCl?₄ by L-ascorbic acid (vitamin C). Although the team haven’t yet pinned down the exact mechanism of growth, Xia says the palladium is crucial.

’If you just use platinum, the particle you form is pretty close packed, so somehow this palladium is very important.,’ he explains. ’It defines where you’re going to have nucleations for platinum and forms this open structure. The arms stay separate from each other so the reagents can still diffuse to this area.’

As a result, their catalyst is far more effective by mass at reducing oxygen than current commercial catalysts - two and half times that of platinum-carbon catalysts and five times that of platinum black.

’I really like their synthetic approach, and praise them in their accomplishment in terms of faster rates for the ORR,’ says Francisco Zaera, a surface chemist at the University of California Riverside. ’If there is a weakness, it is in their limited characterisation of the stability of the catalyst. This is critical if the technology is to be used in practical applications.’

Adding gold, says Xia, could make the catalyst more durable - currently it loses around half its active surface area after 10,000 cycles. Zaera thinks more research is needed to determine what causes the decrease in activity, which he suggests could be related to loss of the original morphology or mixing of the platinum and palladium phases.

Hayley Birch