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Wisconsin engineers clear bottleneck in production of hydrogen

Dumesic lab

Left to right: Professor James Dumesic, graduate students Steven T. Evans, Tobias Voitl, Gabriel R. Rodriguez-Rivera, and post-doctoral researcher Won Bae Kim. (large image)

Discovery could lead to new strategies for operating fuel cells

Hydrocarbons such as gasoline, natural gas or ethanol must be reformed into a hydrogen-rich gas to be useful in a power-generating fuel cell. A large, costly and critical step to this process requires generating steam and reacting it with CO in a process called water-gas shift (WGS) to produce hydrogen and carbon dioxide (CO2). Additional steps reduce the CO levels further before the hydrogen enters a fuel cell.

Diagram of CO oxidation reactor with gold catalyst and fuel cell
                        powered with reduced polyoxometalate as a shuttle for energy storage
                        from CO.

Diagram of CO oxidation reactor with gold catalyst and fuel cell powered with reduced polyoxometalate as a shuttle for energy storage from CO. (large image)

Reporting in the August 27 issue of Science, Chemical and Biological Engineering Professor James Dumesic, post-doctoral researcher Won Bae Kim, and graduate students Tobias Voitl and Gabriel Rodriguez-Rivera eliminated the water-gas shift reaction from the process, removing the need to transport and vaporize liquid water in the production of energy for portable applications. The team uses an environmentally benign polyoxometalate (POM) compound to oxidize CO in liquid water at room temperature. The compound not only removes CO from gas streams for fuel cells, but also converts the energy content of CO into a liquid that subsequently can be used to power a fuel cell.

"CO has essentially as much energy as hydrogen," Dumesic says. “It has a lot of energy in it. If you take a hydrocarbon and partially oxidize it at high temperature, it primarily makes CO and hydrogen. Conventional systems follow that with a series of these 'water-gas shift' steps. Our discovery has the potential of eliminating those steps. Instead, you can send the CO through our process which works efficiently at room temperature, and takes the CO out of the gas to make energy.”

Demonstration of CO oxidation by polyoxometalate in the membrane
                        reactor with gold nanotube membrane catalyst

Demonstration of CO oxidation by polyoxometalate in the membrane reactor with gold nanotube membrane catalyst. The oxidized POM (left bottom line, yellow color) is introduced while CO is fed in the back side (gas chamber), resulting in the reduction of POM (right upper line, dark green color). The color of outlet POM varies with the degree of reduction from yellow to blue. (large image)

The research team says the process is especially promising for producing electrical energy from renewable biomass-derived oxygenated hydrocarbons, such as ethylene glycol derived from corn, because these fuels generate H2 and CO in nearly equal amounts during catalytic decomposition. The hydrogen could be used directly in a proton-exchange-membrane fuel cell operating at 50-percent efficiency and the remaining CO could be converted to electricity via the researchers' new process. The overall efficiency of such a system is equal to 40-percent and does not require water as would be needed in traditional ethylene glycol reforming. The overall efficiency is equivalent to 60 percent of the energy content of octane.

Dumesic's team believes the advance will make possible a new generation of inexpensive fuel cells operating with solutions of reduced POM compounds. While higher current densities can be achieved in fuel cells using electrodes containing precious metals, the researchers found that good current densities can be generated using a simple carbon anode.


 

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8/26/2004