Nanoalloy catalyst may cut fuel cell platinum

FUEL cell technology in cars could become more competitive thanks to a new catalyst that delivers improved activity from lower quantities of platinum.

Fuel cells convert chemically-stored energy to electricity, and are foreseen to play an important role in future sustainable energy systems such as the automotive industry. One specific technology, the polymer electrolyte membrane fuel cell, has a particularly high energy density, but presents kinetic challenges regarding oxygen reduction at the cathode. Currently, a large amount of platinum is needed to achieve reasonable efficiency, but this precious metal can account for almost half of the cost.

Now, researchers at Sweden’s Chalmers University of Technology and Technical University of Denmark may have found a solution. Their work has demonstrated a new type of catalyst that requires less platinum, but can deliver specific activity up to an order of magnitude higher than alternatives such as polycrystalline platinum or platinum nanoparticles.

Their technique built upon previous research that demonstrated it is possible to mix platinum with other metals, such as yttrium, to reduce the amount of the expensive metal in a fuel cell. The main challenge faced when scaling this research up was that yttrium oxidised, instead of forming an alloy with the platinum. The researchers found, however, that a deposition technique called sputtering, applied in a vacuum chamber, could result in a nanometre-thin film supported on an underlying alloy.

Lead researcher, Björn Wickman, said: “A nano solution is needed to mass-produce resource-efficient catalysts for fuel cells. With our method, only one tenth as much platinum is needed for the most demanding reactions. This can reduce the amount of platinum required for a fuel cell by about 70%.'

The authors of the work, published in Advanced Materials Interfaces, believe that the high activity is likely related to compressive strain in the thin overlayer. They reported that thin alloy films of single-target cosputtered platinum-yttrium exhibit up to seven times higher specific activity for the oxygen reduction reaction than polycrystalline platinum, and up to one order of magnitude higher mass activity than platinum nanoparticles.

If this level of efficiency could be achieved in a fuel cell, the authors hope that this could prove a real breakthrough in the automotive industry – as the amount of platinum needed would be comparable to the common catalytic converter. Furthermore, the technique of sputtering is well-suited for mass production.

The paper’s lead author, Niklas Lindahl, said: 'When we can use our resources better, we save both the environment and lower costs. Fuel cells convert chemical energy into electrical energy using hydrogen and oxygen, with water as the only product. They have huge potential for sustainable energy solutions in transport, portable electronics and energy.'

The researchers anticipate that their next steps towards commercialisation are to combine the sputter deposition of the platinum-yttrium alloy with a promising alternative electrode design called nanostructure thin film (NSTF), then perform tests in real fuel cells.

Wickman said: “The electrodes in today’s fuel cells have been developed for catalysts prepared by traditional chemical methods, and a new electrode design is needed in order to incorporate sputtering. This might not be as difficult as one might think, as there has been a lot of research on alternative electrode designs.

“For example, NSTF electrodes have been identified by the US department of energy to be promising for future fuel cells and a large amount of research is devoted to develop this type of electrode. Most NSTF designs would be well suited to combine with sputtering to deposit the catalyst material.

'Hopefully, this will allow fuel cells to replace fossil fuels and also be a complement to battery-powered cars.' 

Advanced Materials Interfaces: http://doi.org/b7k8