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.