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reforming the fuel cell

24 MARCH 2005

Fool for fuel cells
Two-sided green cube encloses bending pipes.Still waiting to buy a car powered by pollution-free fuel cells? So are we. 2004 drove off without a single fuel-cell car reaching the market, mainly due to high cost of the cells, and a shortage of hydrogen fuel. But if fuel cells haven't hit the road, they haven't cruised off to the scrapyard, either. Instead of driving wheels, some fuel cell designs may become a quiet, affordable and (we hope) efficient electric generator that makes heat as a useful byproduct.

A proposed five-kilowatt solid oxide fuel cell module Graphic: Pacific Northwest National Laboratory

All fuel cells make electricity by oxidizing -- but not burning -- fuel. In a planet striving to trim global warming, the ideal fuel cell would oxidize hydrogen, producing the harmless byproduct water, rather than the greenhouse gas carbon dioxide that's made whenever you burn fossil-fuel.

But where to get the hydrogen?

That problem has stumped fuel-cell designers. Pure hydrogen is costly and a lot scarcer than gasoline, so many designers opt to "reform" hydrocarbon fuel -- to separate the hydrogen from the carbon so both elements can be oxidized. But reforming cuts efficiency and dumps carbon dioxide into the atmosphere.

And the separated carbon can clog the fuel cell and shut it down.

Adding a catalyst allows a fuel cell to make its own fuel without getting clogged.Breaking up is hard to do
To overcome the last hurdle, Scott Barnett, a professor of material science at Northwestern University, added a catalyst of the rare-earth metal ruthenium to the input side of a solid-oxide fuel cell. The catalyst -- his key innovation -- separates the incoming fuel into hydrogen and carbon monoxide.

The fuel cell then oxidizes the hydrogen into water, and the carbon monoxide into carbon dioxide.

The result was a solid-oxide fuel cell that didn't clog, but still produced 0.3 to 0.6 watts of electricity per square centimeter, about as much as similar cells, says Barnett. One experimental cell refused to clog during a 100-hour run. Among many possible designs of fuel cells, solid oxide cells are probably the second-most popular approach right now, he adds.

By breaking down the hydrocarbon fuel as it enters the cell, the catalyst raises the efficiency of reforming, Barnett wrote with colleague Zhongliang Zhan. "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." (Endothermic reactions require heat from outside.)

Animation shows that energy is given off by movement of oxygen and hydrogen.
Solid oxide fuel cells use a hard, non-porous ceramic compound as the electrolyte. These cells can use waste heat to extract the hydrogen from hydrocarbon fuel, but they tend to clog. Animation: Science @ NASA

Using waste heat, Barnett says, raises overall system efficiency from about 29 percent to at least 50 percent. Higher efficiency should cut both costs and environmental pollution.

Keep in mind, however, that the researchers tested one fuel cell on a pure, not mixed, fuel, and, to achieve the necessary 600° to 800°C temperatures, they had to roast the cell in a furnace. In other words, the discovery is nowhere near the market.

The next steps, Barnett says, are to see if the fuel cell will run on ordinary fuels like jet fuel, diesel fuel and gasoline, and to build stacks of cells to create larger amounts of power.

Diagram shows creation of oxygen and carbon monoxide with arrows
Iso-octane fuel enters the fuel cell, is "reformed" by the catalyst, creating oxygen and carbon monoxide, which are oxidized, creating heat, free electrons, and releasing carbon dioxide and water. Courtesy Scott Barnett.

Hydrogen: An energy free lunch?
So far, we've been discussing stationary fuel cells -- gizmos that need to run hot, and aren't likely to power your Mazda. Unfortunately, heat is not the only barrier to achieving the supposed nirvana of the hydrogen economy, where we fuel up our cars with hydrogen, and they make water as a waste product rather than carbon dioxide.

Don't expect environmental miracles from fuel cells.Even if fuel cells become affordable, they will not solve our energy and environmental woes unless the hydrogen is made sustainably. Hydrogen loves to combine, so you don't find it in nature, and both reasonable sources of hydrogen raise problems: You can separate hydrogen from a hydrocarbon, as Barnett and Zhan did, you make carbon dioxide. And if you burn hydrocarbons to drive a generator to separate hydrogen from water through electrolysis, you still make carbon dioxide.

To Barnett, these facts are cause for caution: "A lot of people think about hydrogen fuel-cell vehicles as being completely CO2 free, but that's not true at all. In the production of the hydrogen ... you produce quite a bit of CO2."

And neither method of making hydrogen is dead-sure to solve our environmental problems, Barnett adds. "People have done life-cycle analyses, from taking the fuel from the ground, to making electricity in the fuel cell, and it doesn't help." In a recent study, hybrid cars were at least as good as hydrogen fuel cells in terms of energy efficiency and the amount of carbon dioxide, he adds.

If you're moving people by car, efficiency is not a matter of whether your car burns gasoline or oxidizes it in a fuel cell: To reduce carbon-dioxide pollution, the car must drive further on each gallon of fuel. And while fuel-cell cars are years down the road, hybrid cars are on sale today.

Line of many brightly colored truck cabs sit idle.When trucks idle overnight, a fuel cell, not the diesel engine, could power the air-conditioning. Photo: EPA

Still, fuel cells have some advantages, since in principle, they needn't make carbon dioxide. Like early photovoltaic panels, the first fuel cells may be sold to solve niche problems, Barnett says. One possibility is powering a parked semi-truck's heating, cooling and lighting. Big diesels that run all night long at truck stops generate a bit of electricity but make a basket of carbon dioxide and particle pollution.

Eventually, a stationary fuel cell in your basement could make electricity, heat and hot water more efficiently than today's technology.

-- David Tenenbaum

Bibliography
An Octane-Fueled Solid Oxide Fuel Cell, Zhongliang Zhan and Scott A. Barnett, ScienceExpress, 31 March 2005.


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