Friday, May 27, 2005

Little to Big?
The development of fuel cells built around thin polymer membranes has generated interest in a wide range of applications for cars, homes, and portable devices. One of the unresolved questions, however, is whether experience in one area of fuel cell application, such as small electronics, benefits work in another, such as automotive fuel cells. This story from Technology Review highlights a case in point: a small fuel cell extracting its hydrogen from a novel fuel, sodium borohydride. If this technology proves out, will it resolve the hydrogen production, storage and distribution challenges that stand in the way of practical hydrogen fuel cell cars?

Fuel cells generate electricity through an electrochemical reaction of hydrogen and oxygen to make water. Supplying that hydrogen has been a problem, particularly where size is a factor and pressurized storage of hydrogen gas is inconvenient or unsafe. As the article indicates, many developers have focused on methanol, a simple alcohol that can be broken down relatively easily to give up its hydrogen content . Unfortunately, methanol--also known as wood alcohol--is both flammable and poisonous. In fact, it is more toxic to humans than gasoline, and there are unresolved questions about its biodegradability outside ideal laboratory conditions.

Millennium Cell, the company featured in the article, has proposed an alternative to methanol for small fuel cells, in the form of a relatively common inorganic chemical, sodium borohydride, which is made from sodium and borax. The borohydride reacts with water to produce sodium borate and hydrogen. The spent borate can then be stored and recycled to make more borohydride. I can see this working for small devices where energy density is more important than cost and efficiency, but there are two basic problems in trying to scale this idea up to cars.

As the company's own materials admit, current processes for making borohydride are expensive and inefficient. Advanced processes would have to be evaluated on the same kind of "well-to-wheels" basis as other ways to fuel a car. The more basic problem, though, seems to be weight. The great paradox of hydrogen for transportation is that the lightest element in the universe always seems to require heavy storage systems, either mechanical or chemical, to contain it. The amount of hydrogen by weight in this sodium borohydride system is around 4%, compared to about 7% for metal hydride storage. Imagine a gasoline tank for your car weighing 500 lb. empty, and you start to get the idea. With a higher proportion of vehicle weight devoted to hydrogen storage, your overall efficiency and payload will be lower.

This combination of high cost--or low energy efficiency--and high total weight compared to the amount of hydrogen stored look like compelling reasons why this approach might not be suitable at larger scales. As a result, I'm skeptical that liquid chemical solutions like borohydrate will turn out to be the answer for transportation. But for those of us worried about longer laptop and cellphone life, sodium borohydride fuel cells might well offer practical alternatives to methanol fuel cells and batteries. The market will sort that out.

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