Machine Design

Fuel cells' long and winding road

Researchers have brought down the cost and improved the performance of fuel cells. But they still have a way to go before they become a practical source of automotive power.

DaimlerChrysler's first three fuel-cell vehicles used fuel cells built by Ballard Power Systems Inc., a Vancouver, Canada-based company that has been working with fuel-cell technology for almost 20 years. DaimlerChrysler's fifth-generation fuel-cell car, the NECAR 5, reforms hydrogen from methanol and can generate 75 kW of power. That's enough to power the A-Class Mercedes to over 90 mph while carrying five passengers and their luggage.

A PEM cell uses a semipermeable membrane that lets positive hydrogen ions pass through. Negative electrons are conducted through an outside circuit in the form of usable electricity. In a fuel-cell system for a vehicle, the fuel cell is just one component. The system also requires a reformer and air and thermal management subsystems.

William Grove, a British lawyer and amateur inventor, designed and built the first fuel cell in 1839.

Ballard Power Systems' Mark 900 fuel cell, designed for mass production, could be used to power cars and trucks. It generates 80 kW of power.

Fuel cells could find use in the consumer electronic market. This portable Ballard fuel cell provides 110 W of electricity, enough to power a TV/VCR.

Fuel-cell technology sat on the shelf for over a hundred years before NASA called on it in the 1960s to power its Gemini, Apollo, and even the current shuttle spacecraft. Theoretically, fuel cells were an almost perfect power source for NASA — clean, quiet, and reliable — even if they would require expensive new systems for handling liquid and gaseous hydrogen and oxygen. And it was considered safer than nuclear power and more practical than solar-powered photovoltaics. Still, the price for the custom-made cells was probably over $50,000/kW, making them impractical for commercial applications.

Today, issues such as pollution and an overreliance on foreign oil, together with technological advances made over the last two decades, have focused much of the automakers' and world governments' R&D on fuel-cell-powered vehicles. The California Fuel Cell Partnership, for example, will, over the next three years, demonstrate 50 cars and 30 buses that use fuel-cell technology. They will come from a variety of companies and countries. Policy makers are also realizing the public just won't buy short-range, low-power battery-powered vehicles. And improvements in battery technology are too far off. All this makes fuel cells look like the best path to emission-free "green" automobiles. But even fuel cells have problems and it could be a decade or more before cars powered by fuel cells become a common sight on America's highways.

Consumer-friendly electric car
The goal for most auto companies and fuel-cell researchers is to come up with a small, lightweight 70-hp powerplant for under $2,500. That translates into about $50/kW. (Stationary fuel cells, those that provide back-up power for computers, hospitals, or building sites far from the power grid, can be marketed at $750 to $1,000/kW). Automotive fuel cells should also operate 5,000 hr over a seven-year period. It's difficult to calculate what a complete fuel-cell system would cost today since none are being commercially produced, but some analysts peg the price at about $300/kW, or $15,000 per car.

Researchers also want fuel-cell vehicles to deliver about a 350-mile range, be simple to refuel, carry four passengers and luggage, and start-up in no less than 30 sec. One nonautomotive advantage fuel-cell cars will have is that they are, in fact, portable generators. In case of a blackout, owners start up the fuel cell, and have up to 50 kW to keep a refrigerator and other electric devices up and running.

Fueling controversy
Polymer electrolyte membrane (PEM) fuel cells, with their relatively low operating temperatures and high efficiencies at turning fuel into electricity (between 30 and 60%), are the most likely type of fuel cells to power electric vehicles. However, PEM cells rely on platinum catalysts. Currently it takes about 2 gm of platinum per kilowatt, a figure the DoE and the Big Three want to cut to 0.2 gm by 2008. Fortunately, 95% of the platinum used in a fuel cell can be recovered and reused once the cell is scrapped.

Fuel can come from a variety of hydrocarbons as well as hydrogen itself. But if pure hydrogen is used, zoning laws and federal regulations on handling and storing liquid and gaseous hydrogen will have to be changed, along with the public's fear of it. And hydrogen, pressurized to 3,500 psi, contains only one-tenth the energy of a similar volume of gasoline, making it hard to pack 350 miles worth of hydrogen in a car. Hydrogen is more expensive than other fuels such as coal, oil, and natural gas. There also has to be an infrastructure for delivering hydrogen to the public, much like the network of gas stations that covers the U.S. After clearing these hurdles, hydrogen may become practical for fuel cells in 2020.

Most researchers are looking at reforming hydrocarbons as the most likely source of hydrogen. (Reforming hydrocarbons extracts a hydrogen-rich gas using high temperatures). The Partnership for a New Generation of Vehicles (PNGV), a consortium of the Big Three car companies and the federal government, claim that only a "flexible" reformer that works on almost any hydrocarbon is the route to fuel cell success.

The reformer, which has yet to be designed, would work on gasoline, so there would be no problem with infrastructure or ramping up supply capacity. Then consumers could be weaned from gasoline to methanol and eventually to hydrogen.

"Unfortunately, gas is a very complex hydrocarbon with hundreds of different chemical components," says Gregory Dolan, a vice president with the Methanol Institute, a trade association for the methanol industry. "Many of those components are bad for fuel cells. Sulfur, in particular, destroys fuel cells.

The gasoline that would be needed is not today's pump grade. It would have to be a super-low or no-sulfur blend, and it's a fuel that is not being commercially refined. So there's still the question of how much would such a blend of gasoline cost?"

Dolan and his colleagues at the Methanol Institute believe a faster route to fuel cell acceptance would be to use much simpler reformers optimized for methanol, a liquid refined from natural gas. Reforming methanol takes temperatures of between 250 and 400°C, much lower than the 800°C thought to be necessary to reform gasoline. "Methanol is a simple molecule with only one carbon bond so it is much easier to reform than most other hydrocarbons. And it comes from refineries with purities of 99.99%."

Dolan also points out some of methanol's other advantages. "Worldwide capacity is about 12.5 billion gallons annually, and there are several new plants coming on-line with 1 billion gallon capacities. Oil companies have larger reserves of natural gas than oil. The price is about 35 cents/gallon, says Dolan. And since it is a liquid at ambient temperatures and pressures, it can be shipped in any carbon-steel tank, he adds. That means it can be produced where natural gas is cheap, such as Trinidad, Venezuela, Chile, or off the coast of Africa, and shipped anywhere.

Methanol is also biodegradable. "If the Exxon Valdeez had been filled with methanol, you wouldn't have found a trace of environmental damage two or three days after the spill," notes Dolan.

Gas stations could convert to methanol for less than $50/pump, according to Dolan. "With hydrogen, that figure jumps to a minimum of $500,000, if you can get around the restrictions."

The only potential drawback for methanol is that, by volume, it contains less than half the energy of gasoline. Therefore, at current efficiencies, a methanol-powered fuel-cell vehicle would travel only 60% as far as a same-sized gas-powered car on the same volume of fuel. Researchers at the Methanol Institute believe technologic advances will boost fuel-cell efficiencies to where consumers will get roughly the same mileage from a tank of methanol. In other words, a methanol-powered car, from reformer to electric motor, will be twice as efficient as today's gas-powered cars, thereby offsetting methanol's lower energy content per gallon.

Manufacturing headaches
Fuel cells have to be collected into a fuel stack, like batteries in a series, to generate enough voltage. They will need thermal-management systems to ensure cells don't overheat and reformers are kept at optimum temperatures. They will also need water-management systems to store water, a fuel-cell by-product, or vent it overboard. Naturally these systems will have to work in California deserts as well as Minnesota winters.

Fuel cells tend to be more efficient when hydrogen and outside air are slight pressurized. This means systems have to be designed that will clean and filter the air and then pump it and the hydrogen up by 5 psi.

Fuel-cell efficiencies also rely on completely separating the hydrogen, oxygen, and any coolants used, and uniform contact between anode, electrolyte and cathode. Therefore close tolerances will have to be kept on fuel-cell components.

Handling the thin membrane in PEM cells is another manufacturing issue. Researchers have largely solved the dimensional issues with modern manufacturing practices, and they are turning to experienced sheet-good makers for help in handling the membranes.

Despite the technological hurdles to fielding fuel-cell-powered cars and trucks, their success seems to hinge on public acceptance, the price of gasoline, and how fast an alternative fueling infrastructure can be developed. A likely timeline for fuel-cell vehicles is that the Big Three will introduce small volumes (in the hundreds) of them to the market in about 2004-3. By 2008-10, auto companies will each have scaled up so that each builds about 200,000 such vehicles per year. At that point, costs should drop and they should be competitive with internal combustion cars.

Fuel cells
Fuel cells combine hydrogen and oxygen to produce electricity, heat, and water through chemical reactions. There is no combustion, making the cells efficient at converting fuel into electricity. Fuel cells also have no moving parts, so they are quiet and reliable. With virtually no wear on parts (the platinum catalyst, for example, does not degrade or get dissolved), fuel cells generate electricity whenever hydrogen and oxygen are supplied. The hydrogen can be pure hydrogen gas or a hydrogen-rich gas reformed from a variety of hydrocarbons such as methanol, gasoline, or even coal gas. The oxygen usually comes from the air.

Fuel cells consist of an anode, electrolyte, and cathode. Hydrogen molecules flowing into the anode interact with platinum catalysts and are converted into two positively charged hydrogen ions and two negatively charged electrons. The electrolyte permits only the ions to pass through.

Negatively charged electrons are collected and travel through an external circuit as electricity, which can power an electric load. Oxygen flowing into the cathode, like the hydrogen, reacts with a platinum catalyst to combine with hydrogen ions and returning electrons to form water and heat. Depending on the electrolyte and fuels, the electrochemical reactions usually are more efficient when temperature is controlled.

The surface areas of the anode/electrolyte/cathode interface determines the current of an individual fuel cell, while its voltage is a function of the specific chemical reactions occurring. To increase the voltage, fuel cells are assembled into fuel stacks, with the number of fuel cells determining the total voltage. A fuel stack's output can be regulated by controlling the amount of hydrogen flowing into the stack.

Proton exchange membrane fuel cells
PEM cells use a semipermeable membrane as the electrolyte. It is coated on both sides with platinum catalysts. The membrane is less than a millimeter thick and consists of a solid-organic polymer — polyperfluorosulfonic acid —, sandwiched between two platinum-coated sheets of paper. A widely used polymer is DuPont's Nafion which is based on perfluorosulfonic acid and PTFE copolymer chemistry.

PEM cells are 40 to 45% efficient and operate at 60 to 100°C. PEM cells provide peak power quickly and at relatively low temperatures. And like alkaline cells in aerospace applications, PEM cells offer power densities an order of magnitude greater than other types of fuel cells. Because it does not use a liquid electrolyte, there are no corrosion problems or hazards due to leakage. All this makes them the fuel cell of choice for electric vehicles. One drawback is the cell's sensitivity to impurities in the hydrogen.

Solid-oxide fuel cells
SOFCs use ceramics, usually zirconia with some yttria added to stabilize the compound, as the electrolyte. To make the ceramic ionically conductive, however, the cell must operate at 800 to 1,000°C. This makes it easy to reform hydrocarbons. It also makes it easier to use the heat the cell generates. Cell temperature is controlled by regulating the amount of air entering the cell. From an efficiency standpoint, it's critical to keep the cell near its optimal operating point, 1,000°C. A 10% drop in temperature leads to a 12% drop in output. Due to the heat, SOFCs require significant thermal shielding and insulation, making them impractical for portable applications. Unpressurized SOFC have efficiencies of about 45%, but researchers at Argonne National Laboratory believe increasing the hydrogen and air pressures could lift that to 60%.

Phosphoric-acid fuel cells
PAF cells have been in development for 20 years and are being used to power at least 200 public buildings around the world. The technology received early attention because it was the only one that could use reformed hydrocarbons as a source of hydrogen. PAF use a phosphoric acid electrolyte contained in a Teflon-bonded silicone-carbide matrix and operate at 175 to 200°C. Water usually leaves the cell as steam, which is often used for heating. PAF cells are about 36% efficient at producing electricity, but include usable thermal output and efficiencies jump to 85%. On the downside, the cells rely on platinum catalysts and are large and heavy with relatively low current and power levels.

Molten-carbonate fuel cells
In the 60's, researchers tried developing a fuel cell that would use coal as fuel. The result was the MCFC, which has been shown to work using simulated coal-gassification fuels, as well as hydrogen, carbon monoxide, natural gas, propane, methane-rich gas from landfills, and marine diesel fuel. One reason it can use such a wide range of fuels is that it operates at between 600 and 1,000°C, hot enough to reform many hydrocarbons. (Heat makes it easier to break carbon-carbon bonds and lets designers use less-expensive catalysts.) Italian and Japanese researchers have built carbonate cells that produce 10 kW to 2 MW.

The electrolyte in carbonate cells is a mixture of lithium carbonate and potassium carbonate which is a liquid and good ionic conductor at the cell's operating temperature. The electrolyte is contained in a porous and chemically inert lithium-based matrix.

TAGS: Technologies
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