The miniature fuel cell (BU52A)

For a long time, manufacturers have been exploring ways to replace the electrochemical battery. Higher energy densities, smaller size, lower cost per watt and faster charging are on the wish list. Electrical energy from rechargeable batteries is expensive when considering the high purchase price, the limited life span and the small amount of power they can deliver. (See The cost of portable power.)

Will the fuel cell replace the battery?

During the last years, fuel cell technology has gained much hype and many see this power source as the gateway to the future. Since its invention in 1839 by Sir William Grove, the fuel cell remained a scientific oddity until the 1950s when it was used for US space and military programs for the first time. In the 1980s, the fuel cell had another rebirth when scientists and stock promoters envisioned a world powered by a clean power source fed by an inexhaustible fuel, hydrogen. They forecasted that cars would be run by fuel cells and households be powered by electricity generated from back-yard fuel cell units. High manufacturing costs and short service life have been in the way of making this a reality.

The fuel cell uses hydrogen and oxygen as fuel. Combining the two gases generates electricity and water. There is not combustion; no pollution. The byproduct is pure water. The system runs so clean that Ballard, a developer of fuel cell stacks, offered the guests tea from the hot water produced by the fuel cell. This makes it possible to run a fuel cell in an enclosed room, such as an office of living room. The theoretical energy output of the fuel cell is high, but over half is lost in heat.

During the past years, portable versions of the fuel cells have emerged. The most promising miniature fuel cell is the direct methanol fuel cell (DMCF). DMCF is inexpensive, convenient, does not require pressurized hydrogen gas and provides a reasonably good electrochemical performance. Current systems produce 900 Wh of power and offer an energy density of 102 Wh/l. This is still large in size compared to an electrochemical battery and further reductions will be needed. Charging consists of replacing the cartridge on the fly. This provides a continued source of energy, similar to fueling a car.

Toshiba unveiled a prototype fuel cell for a laptop but described the technology as being in its 'infancy.' The company gave no indication as to when the product would be commercially available. A direct battery replacement that offers high power, small size and competitive price is still several years away. Figure 1 shows a DMFC by Toshiba. The micro fuel cell on the left is capable of providing 300mW of continuous power. The fuel is 99.5% pure methanol stored in a 10 mL tank. The refueling process is shown on the right.


Figure 1: Toshiba fuel cell with refueling cartridge. Only a limited number of these devices are being made. Courtesy of Toshiba

Angstrom Power is developing a portable fuel cell that runs on stored hydrogen and oxygen from the air. The system operates at ambient conditions and has no pump and fan. The advantage of pure hydrogen over methanol is increased efficiency and smaller size. The aim is to offer a power source that is clean, quiet and can be refueled on the fly. Figure 2 illustrates an application as a bicycle lamp.

Figure 2: Fuel cell powered bicycle lamp. The 21cc cartridge provides the equivalent energy of about 10 AA disposable alkaline batteries. The only by-product is water vapor. The runtime between refueling is 20 hours. Courtesy of Angstrom Power

According to Angstrom Power, the micro hydrogen™ bike lights have delivered good performance in winter and spring conditions and the user feedback is positive. The hydrogen fuel is stored in a 21cc cartridge, providing the equivalent energy of about 10 AA disposable alkaline batteries. The only by-product is water vapor. Refueling takes a few minutes and provides a continuous runtime of about 20 hours.
As good as the fuel cell may look from the outside, 15-years of experiments has not solved a number of persistent problems. One is the slow start-up; another is the low electrochemical activity at the anode. This is especially apparent with the DMCF. Each cell produces about one volt and when loaded, the relatively high internal resistance causes the voltage drops quickly. Figure 3 illustrates the voltage drop as a function of load current. As can be seen, the power band is quite narrow.

Figure 3: Power band of a portable fuel cell. High resistance causes the cell voltage to drop rapidly with load. The power band is limited to about 300-800mA.

Current loading is not critical with a small bicycle light, especially when low-drain LED technology is used. A laptop, on the other hand, requires about 40 watts of power and a small fuel cell cannot provide enough output to sustain the demand. The system needs a battery as back up. In essence, the fuel cell becomes a slave to the battery and serves more like a charger. The same applies to a fuel cell-powered cell phones and cameras.

The fuel cell has not seen the same earth-shattering breakthroughs that microelectronics has enjoyed. The Moore's laws don't apply here. The continued struggle is low power, large size, premature aging and high cost. There are also transportation issues that inhibit passengers from bringing fuel on an aircraft. These rules will likely change in the next two years. The ICAO dangerous goods panel (DGP) has already established an exclusion to allow the transport and operation of methanol fuel cells on commercial flights. This same standard will not yet apply to storage of hydrogen gas, however.

When examining alternative power sources, the traditional battery starts to look awfully good. It is small, clean, quiet and provides an instant source of high power when needed. Similar to the combustion engine in a car, the battery is hard to replace with something that offers equivalent energy density and is affordable. An inexhaustible fuel cell would be nice, but for now we are beholden to the old-fashioned electrochemical concept, called a battery. There are no major developments on the horizon that will change the way we use portable equipment and atomic fusion as a potential portable power source hasn't entered the race yet. It is our hope, however, that the fuel cell will succeed as a clean energy source for our future generations.

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Created: November 2006