Batteries are the bane of consumer electronics users. They provide only a limited amount of power, take hours to recharge, and over time become less long-lasting. For years, engineers have been eyeing fuel cells–devices that produce electricity by mixing a fuel with oxygen molecules–as a longer-lasting alternative power supply. But the technology has always encountered hurdles that keep it from being as practical and cost effective as batteries.
Now researchers at Arizona State University’s Biodesign Institute in Tempe have developed a technique that could help make better fuel cells for laptops, military-grade communication devices, and, potentially, cell phones. In research presented yesterday at the American Chemical Society meeting in San Francisco, Dominic Gervasio, associate professor in the Center for Applied Nanobioscience at Arizona State, and his team showed that by adding a chemical found in antifreeze to sodium borohydride–a liquid used to store hydrogen, the molecule that powers fuel cells–they can make a longer-lasting fuel cell. The resulting fuel could power a laptop twice as long as any battery on the market, while allowing room temperature operation, unlike many other fuel cells.
Fuel cells for portable devices have been gaining traction in the past few years as the technology behind them has steadily improved. Indeed, they’ve now reached beyond research labs, and made their way into various forms of production. Millennium Cell, an Eatontown, NJ-based company, supplies fuel cells for military applications. And by year-end, New York-based Medis Technologies plans to offer a consumer fuel cell device designed to instantly recharge standard batteries in cell phones, MP3 players, and laptops.
Sodium borohydride, the solution used by Millennium Cell, Medis, and the Arizona State team is becoming a popular choice to store hydrogen for portable fuel cells, says Gervasio. One reason is that it’s used with the most-established fuel cell design. This type of fuel cell works by combining hydrogen with oxygen from the air to produce electric current. In addition, systems that use sodium borohydride can be made as small as conventional batteries because the solution stores a large amount of hydrogen in a small volume. Moreover, it’s a relatively safe liquid that isn’t flammable. “You could take a match and put it out in it,” Gervasio says.
But to succeed as a replacement for batteries, these fuel cells have to prove themselves significantly better than the batteries they aim to replace, says Gervasio. Currently, most sodium borohydride fuel cells produce only slightly more electrical energy per volume of fuel than conventional batteries, he says. To increase the performance of their fuel cell system, Gervasio and his team knew they needed to increase the amount of hydrogen available to the fuel cell from the sodium borohydride solution.
Micro-fuel-cell systems generally contain three parts: a cartridge of fuel, a hydrolysis chamber where hydrogen is extracted from the liquid fuel, and a fuel cell where hydrogen mixes with oxygen, creating electricity. In the Arizona State researchers’ system, a mixture of water and sodium borohydride is pumped from the cartridge into the hydrolysis chamber, which contains a catalyst. The catalyst sets off a chemical reaction that liberates hydrogen from the sodium borohydride solution, and also creates byproducts that are pumped out of the chamber and back into the fuel cartridge to be disposed later.
It would seem logical that increasing the concentration of sodium borohydride would increase the production of hydrogen, and make a better-performing fuel cell system. However, Gervasio says, “there’s a hidden problem” with this scenario. One of the byproducts of the hydrolysis reaction is boron oxide, a compound that, doesn’t dissolve readily in water. So when the concentration of sodium borohydride increases, so does the concentration of the solid boron oxide, which “gums up” the pumping system, limiting how much sodium borohydride can be used, says Gervasio.
To address this problem, Gervasio and his team tested solvents that dissolve boron oxide. They found that by adding ethylene glycol to the boron hydride solution, they could use a concentration of sodium borohydride 50 percent stronger than one without ethylene glycol in the solution–increasing the amount of hydrogen that can be stored and liberated–without producing the unwanted clumps of boron oxide. The difference, says Gervasio, is a fuel cell system that can power a device roughly twice as long as a battery of the same size and weight.
Ethylene glycol is useful not only for its ability to dissolve boron oxide, but also for managing the temperature of water in the fuel cell, Gervasio adds. Ethylene glycol reduces the freezing point and increases the boiling point of water in fuel cells, just as it does in the antifreeze of a car’s cooling system. A reduced freezing point keeps the water in the sodium borohydride solution from turning to ice on a cold day, while an increased boiling point could keep the system running smoother at higher temperatures.
Indeed, managing heat is one aspect of fuel cell technology that engineers must consider when designing efficient fuel cells, says Jack Brouwer, associate director of the National Fuel Cell Research Center at the University of California, Irvine. And he feels that the Arizona State research is “really interesting work in that regard.”
John Battaglini, vice president of sales, marketing, and product management for Millennium Cell, says his company has taken similar approaches in its fuel cell development; and adds that he’s “happy to see more people looking at sodium borohydride,” and expects it to lead to other advances down the line.
Right now, Gervasio’s team is looking at different types of alcohol additives that dissolve boron oxide as well as or better than ethylene glycol. He estimates that it could be about five years before his system is incorporated into a consumer laptop. But the ball is rolling: he’s filed for a number of patents on the technology and is in talks with device makers about his recent advances. “I have a lot of hope for this,” he says.
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