Fig. 1: Proton exchange membrane fuel cells convert hydrogen and oxygen into electrical energy by a catalytic process that conventionally requires the use of platinum-based catalysts. Nabae and co-workers have developed a durable platinum-free catalyst that could serve as a viable replacement for expensive platinum-based catalysts.

Proton exchange membrane fuel cells are able to generate electricity from hydrogen while producing only water as an emission. Fuel cells operate by reacting oxygen from the air with electrons and hydrogen ions using catalysts, which typically contain rare and expensive materials such as platinum (Fig. 1). The use of such expensive materials, however, drives up fuel cell costs and makes them harder to scale to high-volume production.

Precious-metal-free catalysts can be produced by heating together materials containing nitrogen, carbon and either iron or cobalt. Unfortunately, however, the resultant catalysts have relatively poor durability, hindering their practical use. Yuta Nabae and colleagues at the Tokyo Institute of Technology and Gunma University in Japan1 have now developed a technique that allows for the production of durable platinum-free catalysts.

Their method is based on the observation that iron contributes to the formation of active catalytic sites during heating up to 600 °C, but at temperatures higher than 700 °C, the presence of iron nanoparticles actually degrades catalytic activity. By removing iron at a certain stage in preparation, the catalysts can be heated to higher temperature to increase durability without degrading catalytic activity.

The researchers first heated iron phthalocyanine — a macrocyclic organic compound with an iron atom at its center — and phenolic resin at 600 °C in the presence of nitrogen. After contributing to the formation of active sites, the iron forms nanoparticles encapsulated in a carbon shell. Simple washing with acid, however, was unable to remove the nanoparticles for subsequent heating due to the presence of the carbon shell, so the researchers heated the material under ammonia, which broke down the carbon shell and allowd the iron nanoparticles to be removed by acid washing. Heating one more time at 1000 °C then gave a more durable catalyst with high activity.

In working fuel cells, the catalyst was found to have superior performance to other precious-metal-free catalysts, and the fuel cells were able to produce a steady voltage for hundreds of hours. The results imply that the conventionally accepted trade-off in precious-metal-free catalysts between catalytic activity and durability can be avoided. “Our technology will contribute to bringing down the cost of fuel cells,” says Nabae, “and making them practical for applications ranging from vehicles to the home and industrial manufacturing.”