January 27, 2016

An alternative to platinum: iron-nitrogen compounds as catalysts in graphene

Nano-island of graphene in which iron-nitrogen complexes are embedded.
The FeN4 complexes (shown in orange) are catalytically active. Image: S. Fiechter/HZB

(January 27, 2016)  Teams at HZB and TU Darmstadt have produced a cost-effective catalyst material for fuel cells using a new preparation process which they analysed in detail. It consists of iron-nitrogen complexes embedded in tiny islands of graphene only a few nanometres in diameter. It is only the FeN4 centres that provide the excellent catalytic efficiency – approaching that of platinum. The results are interesting for solar fuels research as well and have been published in the Journal of the American Chemical Society.

Fuel cells convert the chemical energy stored in hydrogen (H2) into electrical energy by electrochemically “combusting" hydrogen gas with oxygen (O2) from the air into water (H2O), thereby generating electricity. As a result, future electric automobiles might be operated quite well with fuel cells instead of with heavy batteries. But for “cold” combustion of hydrogen and oxygen to function well, the anode and cathode of the fuel cell must be coated with extremely active catalysts. The problem is that the platinum-based catalysts employed for this contribute about 25 per cent of the total fuel-cell costs.

However, iron-nitrogen complexes in graphene (known as Fe-N-C catalysts) have been achieving levels of activity comparable to Pt/C catalysts for several years already. “Systematic investigation of Fe-N-C catalysts was difficult though, since most approaches for preparing the materials lead to heterogeneous compounds. These contain various species of iron compounds such as iron carbides or nitrides besides the intended FeN4 centres”, explains Sebastian Fiechter of HZB.

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