Fuel cells start to look real
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Fuel-cell technology
Unlike electrochemical batteries, which use chemical reactions to store and discharge electricity, fuel cells generate electricity from hydrogen fuel. Haul around enough fuel, and the fuel cell will power an electric vehicle as far as the motorist wants to drive.
The fuel cell was first demonstrated in principle by British scientist Sir William Robert Grove in 1839. Grove's invention was based on the idea that it should be possible to reverse the already well-known electrolysis process to produce electricity. In electrolysis, an electric current is introduced into a conducting liquid known as an electrolyte, where it flows between two electrodes causing the splitting of water or other chemical compounds into their ionic (charged) components, which then react chemically.
A fuel cell likewise consists of two electrodes: a positive electrode called an anode and a negative electrode called a cathode. Pure hydrogen gas - or hydrogen extracted (or reformed) from a hydrocarbon fuel such as methanol or gasoline - together with oxygen is fed into the cell. A catalyst at the anode (usually based on a platinum-family element) causes hydrogen atoms to give up their negatively charged electrons, leaving positively charged protons. Negatively charged oxygen ions (from ionized oxygen gas) at the cathode side attract the hydrogen protons. As the protons pass selectively through a semipermeable solid electrolyte membrane (in the most common fuel-cell type), the remaining electrons are redirected to the cathode by way of an external circuit, thus producing current that powers an electric motor. The electrons combine with the hydrogen protons and oxygen ions at the cathode forming the fuel cell's major byproduct, water. The other principal end-product is heat, which can be captured and reused, or released. Because a single cell generally produces only a few volts, fuel cells are typically piled into "stacks" to generate more useful voltage. The exhaust emissions of a pure hydrogen fuel cell are clean, but the extraction of hydrogen from hydrocarbon fuels in reformer systems does release some atmospheric pollutants.
Though there are several types of fuel cells that use different fuels and materials, one version - the proton exchange membrane (PEM) variety - has emerged as the clear favorite for automotive use. Another type, the solid oxide fuel cell (SOFC), is seen as the dark-horse alternate. Another fuel-cell technology called an alkaline/air cell is being developed, though its prospects are considered more speculative. The biggest difference between the SOFC and PEM technologies is their operating temperatures. While PEM cells run at 80°C (176°F), SOFC units function at anywhere from 700 to 1000°C (1290 to 1830°F). Another difference is the membrane, which is a polymer in the PEM and a ceramic in the SOFC.
Many engineers believe that SOFCs, together with an onboard gasoline fuel processor or reformer, would be highly suited as auxiliary power units (APUs) for cars and trucks in the relatively near term. Engineers have long desired to rid automobiles of the alternator and its notoriously low efficiency. And as vehicles are crammed with more and more electronic equipment and move toward higher electrical loads, a larger burden will be placed on the alternator. An auxiliary power unit based on SOFC technology could provide an ideal alternative. A research alliance including BMW, Renault, and Delphi Automotive Systems is pursuing this fuel-cell application.
PEM fuel cells
The proton exchange membrane cell, which was developed by General Electric for NASA's Gemini space program nearly four decades ago, is the favored technology for auto applications because it is compact, runs at a low operating temperature, permits an adjustable power output, and can be started relatively rapidly. Innovations made in the 1980s at Los Alamos National Laboratories made the PEM cell more practical by substantially cutting the amount of precious metal catalyst needed to coat the cell's ultra-thin polymer membrane.
Ballard's prototype fuel-cell units, which are used by DaimlerChrysler, Ford, Honda, and Nissan (and others yet unacknowledged), comprise a series of carbon plate/PEM electrode assemblies. "Each assembly includes five main components," explained Paul Lancaster, Vice President, Finance at Ballard. "At either end is an electrode made of a carbon material with a coating of platinum-family catalyst that ionizes hydrogen on one side of the unit and oxygen on the other. In the middle is a thin proton exchange membrane, which is a rubbery hydrophilic polymer electrolyte with solid sulfonic acid sites bonded onto it. These sites allow protons to be transported selectively through the membrane."
Although the latest enhancements to fuel cells are rather recent, researchers at Ballard and presumably GM have worked out most of the technical problems, boosting the stack's power density by determining how to keep the membranes moist but not flooded, and by optimizing the flow lines that transport hydrogen, oxygen, and water through the stacks. Ballard, which has obtained nearly 400 patents in PEM technology, intends to have a car-sized power unit ready to go within four years at prices comparable to IC engines, according to Lancaster.
Clearly, fuel cells provide some advantages over IC engines: they are more efficient in extracting energy from fuel; they are quieter; and they could form the basis of a zero- or very-low-emissions engine that runs on a renewable fuel. It should be noted that some engineers expect practical fuel-cell vehicles to be hybridized with batteries, ultracapacitors, or other energy storage devices to allow them to be run at lower power output levels and, thus, at higher efficiencies. "Fuel-cell stacks operate at 50 to 70% efficiency in the current power load of interest - about 60% in around-town driving," explained Byron McCormick, Co-director of GM's Global Alternative Propulsion Center.
Ballard researchers say that they surpassed the minimum power density for automobile applications - about 1 kW/L - when they brought out the Mark 700 fuel cell a couple of years ago. The newer Mark 900 unit puts out as much as 1.35 kW/L, making it "sufficiently powerful for today's vehicles," Lancaster said. "Though that output translates into less power than today's IC engines provide, the torque characteristics of electric motors allow electric cars to offer superior performance."
But much more work remains to make the fuel-cell vehicle truly practical. "Though it's a solid-state device with no moving parts, which tends to keep things simple, it's a catalytic device - like a catalytic converter - so it has different wear modes," said Staley. "This means that after a certain amount of time it'll run out of gas, so to speak. Though we see no obvious roadblocks to being able to cycle a fuel cell through the lifetime of the car, we're not there yet."
