Consumer fuel cells
In search of forever
Jun 12th 2008
From The Economist print edition
As a source of power for cars, fuel cells have been a disappointment. For laptops and mobile phones, they are just about to take off
Illustration by Stephen Jeffrey
METHANOL is nasty stuff. Careless distillation in many a backwoods still has caused it to blind the imbibers of “alternative” alcoholic drinks. Yet it has its uses, and one of them may be to restore fuel cells to their oft-vaunted role as the power packs of the future—but with a twist. The main role that has been discussed for fuel cells over the past few decades is as replacements for the internal-combustion engine. Their actual use may turn out to be to provide power for portable electronic devices.
A fuel cell is a device that combines hydrogen with oxygen to generate electricity. The traditional approach has been to use the gas itself in the cell—and that is the approach taken by the world's carmakers in their so-far not very successful attempts to make a commercial fuel-cell-driven car. Since gaseous hydrogen is hard to store and handle, an alternative that some people have considered is to lock the hydrogen up in methanol, a liquid whose molecules are made of a carbon atom, an oxygen atom and four hydrogen atoms. Methanol will react with water in the form of steam to make hydrogen and carbon dioxide—a process known as steam reformation. Put a steam reformer in a car along with the fuel cell and you can fill the tank with methanol instead of hydrogen.
That idea has not gone very far, either. But it has provoked another thought. What if it were possible to decompose the methanol without steam, and within the fuel cell itself? And that has, indeed, turned out to be possible. The resulting cells are nowhere near powerful enough to run cars, but they are plenty powerful enough to stand in for small batteries. What is more, they last far longer than batteries and when they do need recharging, it is the work of a moment.
Proton power
In a direct-methanol fuel cell (DMFC) the methanol is oxidised at the anode in the presence of liquid water. The reaction, which requires a catalyst, turns the methanol and water into protons and electrons (in other words, dissociated hydrogen atoms) and carbon dioxide. While the electrons pass along an external circuit as an electric current, the protons diffuse through a membrane to the cathode, where they recombine with the incoming electrons to form hydrogen atoms that react instantly with oxygen to make water. With pleasing symmetry the water is then channelled back to mix with the incoming methanol. Even though DMFCs produce carbon dioxide, the amount is small enough for the cells to count as a much greener technology than batteries. Some companies also think the new cells could be safer than batteries, which can burst into flame if short-circuited.
The efficiency of a DMFC is determined by its membrane. One of the most commonly used sorts is made of Nafion, a polymer developed by DuPont from a variation of Teflon. Nafion, however, can be expensive and it allows some methanol to seep through, which wastes fuel. Researchers are therefore trying to come up with more efficient membranes—and one group, led by Paula Hammond of the Massachusetts Institute of Technology (MIT), appears to have done so.
Dr Hammond and her colleagues used a newish thin-film fabrication technique known as “layer-by-layer”. This repeatedly dips a material into a solution, to build it up one layer at a time, while the properties of the liquid are gradually changed. That enables the structure of the resulting film to be fine tuned. When Dr Hammond coated a Nafion membrane in this way it became less permeable to methanol but kept its ability to transport protons. The effect, which the group reported in a recent issue of Advanced Materials, was to boost the cell's electrical output by more than 50%. The next stage, which the team has now embarked on, is to build complete membranes rather than mere coatings. The researchers think these may be able to work as proton-exchange membranes in their own right.
Squeeze me, please me
Toshiba, a large Japanese electronics firm, reckons that DMFCs can be used to produce mobile devices that have no need for batteries at all. In its latest investment plan, it says it will begin making such cells within a year for mobile phones and laptops. Sharp, one of its rivals, recently said that it had developed new microfabrication techniques to build DMFCs with the highest power densities yet achieved. Sharp reckons this will enable it to produce cells that are the same size as the lithium-ion batteries used in mobile devices, but which can run those devices for much longer. Some in the industry talk of mobile phones capable of operating continuously for several weeks before their fuel cells need topping up.
The most likely way that topping up will be done is with a cartridge of methanol that is inserted into the device and replaced when it is running low. As portable devices become more sophisticated, with added functions and large colour screens, they are draining batteries faster. MTI Micro, an American company, has put its version of a DMFC into satellite-navigation devices, which are often used for long periods. The company says it can run even a power-hungry model for up to 60 hours before the gadget needs refuelling.
Longer life is a big appeal; some people would like to run their laptops continuously on a 12-hour flight. Hence, new rules are being drawn up for aircraft. America's Department of Transportation is planning a rule change from October 1st to allow passengers and crew to bring fuel-cell-powered electronic devices and one or two fuel cartridges on board in their carry-on baggage. To qualify, the devices will have to meet certain safety standards. It is proposed that each passenger would be limited to about 200ml of fuel.
Successful work like that at MIT will help to make DMFCs cheaper and more efficient, which will, in turn, make them even more attractive as power sources for portable devices. Already, some companies are predicting that sales of refuelling cartridges could run into the billions within a few years of them coming into the market. Forget, then, the familiar cry: “Has anyone got a charger I can borrow?” It will be replaced by: “Can you spare me a squirt of methanol?”—and that won't mean in your hooch.
Source
www.economist.com/science/displaystory.cfm?story_id=11529364