New On-Board Solid Oxide Fuel Cell Delivers 50% Efficiency
31 March 2005
Northwestern University researchers have developed a new small solid oxide fuel cell (SOFC) that converts iso-octane (C8H18), a highly-pure hydrocarbon compound that is a component of gasoline, to hydrogen, The hydrogen is then used by the fuel cell to produce electrical energy with an overall fuel efficiency of up to 50%.
Their paper, published online by the journal Science, describes the combination of a special thin-film catalyst layer, through which the iso-octane flows, with a conventional anode. That porous layer, which contains stabilized zirconia and small amounts of the metals ruthenium and cerium, chemically and cleanly converts the fuel to hydrogen.
This approach is potentially the basis of a simple low-cost system that can provide significantly higher fuel efficiency by using excess fuel cell heat for the endothermic reforming reaction.—An Octane-Fueled Solid Oxide Fuel Cell [DOI: 10.1126/science.1109213]
Current internal combustion engines have a “well-to-wheels“ efficiency of only 10%–15%. Current PEM fuel cells using hydrogen from the steam reforming of natural gas offer 29% overall efficiency, while current gas/electric hybrids have achieved up to 32%.
(The different efficiencies are estimated here, in the supporting online material for the paper.)
The advent of hybrid vehicles has shaken up the fuel cell community and made researchers rethink hydrogen as a fuel. We need to look at the solid oxide fuel cell—the one kind of fuel cell that can work with other fuels beside hydrogen—as an option.—Scott A. Barnett, professor of materials science and engineering, NU
Although conceptually similar (hydrogen in, electricity out) the solid oxide fuel cell is different than the PEM (Proton Exchange Membrane) fuel cells most often discussed as power plants for transportation. PEM fuel cells tend to be smaller, run at lower temperatures, produce less power and require an external supply of hydrogen.
Solid oxide fuel cells use a hard, ceramic compound of metal (like calcium or zirconium) oxides as an electrolyte, rather than the thin, permeable polymer electrolyte sheet in a PEM. SOFCs tend to be more efficient, and run at a higher temperature. It is that higher temperature that the Northwestern team is using for the chemical reforming of the iso-octane.
A major drawback of using solid oxide fuel cells is that carbon from the fuel is deposited all over the anode because of the high temperatures. But our thin film catalyst, plus the addition of a small amount of oxygen, eliminates those deposits, making it a viable technology to pursue with further research. We have shown that the fuel cell is much more stable with the catalyst and air than without.—Scott Barnett
With its higher efficiency, such an SOFC approach would reduce our distillate usage compared to straight ICE vehicles or current hybrids, and would reduce the need for the supporting hydrogen infrastructure required by PEM cells. Of course, you still need a ready supply of iso-octane.
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