New method for isolating iron catalysts

US RESEARCHERS have developed a carbon-coated iron catalyst which could lead to more detailed research for developing efficient catalysts for fuel cells.

A team lead by the University of Illinois has identified the active form of an iron-containing catalyst, the most difficult part of developing an oxygen-reducing catalyst for a hydrogen fuel cell. These catalysts break the bonds between oxygen atoms, allowing ionised hydrogen to combine with the oxygen to form water. The findings can help researchers refine better catalysts while developing cheaper and more efficient cells.

The process for producing iron-based catalysts for oxygen reduction often yields a mixture of different compounds containing iron, nitrogen and carbon. Since the various compounds are difficult to separate, the form or forms that behave as the active catalyst have eluded researchers, making it difficult to refine or improve the catalyst. Iron-based catalysts are more desirable than precious metal catalysts as they are cheaper and less prone to degradation.

The team used high-temperature gas-phase chlorine and hydrogen treatments to selectively remove from the mixture particles that were not active for oxygen reduction, refining the mixture until only the active carbon-encapsulated iron nanoparticle remained.

Andrew Gewirth, professor of chemistry at the university, explained to The Chemical Engineer that the chlorine treatment has two functions. It first disrupts the carbon coating on the nanoparticles, allowing the chlorine to react with the iron, forming FeCl₃, and it also removes all the other iron-containing species from the electrode. The hydrogen treatment then reduces FeCl₃, leading to the synthesis of only the carbon-coated iron nanoparticles.

Graduate student Jason Varnell said: “Previously, we didn't know what these catalysts were made of because they had a lot of different things inside them. Now we've narrowed it down to one component. Since we know what it looks like, we can change it and work to make it better.”

The team hopes that narrowing down the active form of the catalyst can open new possibilities for making purer forms of the active catalyst, or for tweaking the composition to make it more active.

The next stage will be further research on the specific properties of the carbon-coated iron structure that was not possible to address before, such as the optimal size for the catalyst, the optimal density and the optimal coating material.

Gewirth said: “We're trying alternative methods for synthesising the active catalyst and making multicomponent nanoparticles with certain amounts of different metals. Previously, people would add some metal salt into the tube furnace, like cooking – a little of this, a little of that. But now we know we also need to do things at different temperatures to put other metals in it. It gives us the ability to make it a more active catalyst.”

Ultimately, the team hopes that improved catalyst function and manufacturability will lead to more-efficient commercial fuel cells, which could be used for vehicles or other power-intensive applications.

Nature Communications, DOI: http://doi.org/bqjm