• Alkaline Fuel Cell (AFC)
• Phosphoric Acid Fuel Cell (PAFC)
• Molten Carbonate Fuel Cell (MCFC)
• Solid Oxide Fuel Cells
• Proton Exchange Membranes
• Direct Methanol
• Direct Carbon Fuel Cells
• Endnotes

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Phosphoric Acid Fuel Cell (PAFC)
The Phosphoric Acid Fuel Cell is the most mature fuel cell technology in terms of system development and commercialization activities. It has been under development for more than 20 years. The phosphoric acid fuel cell uses liquid phosphoric acid as the electrolyte. The phosphoric acid is contained in a Teflon bonded silicone carbide matrix. The small pore structure of this matrix preferentially keeps the acid in place through capillary action. Some acid may be entrained in the fuel or oxidant streams and addition of acid may be required after many hours of operation. Platinum catalyzed, porous carbon electrodes are used on both the fuel (anode) and oxidant (cathode) sides of the electrolyte. phosphoric acid fuel cell power plant designs show electrical efficiencies in the range from 36% to 42% (HHV² ). The higher efficiency designs operate with pressurized reactants. The higher efficiency pressurized design requires more components and likely higher cost. A portion of the thermal energy can be supplied at temperatures of ~ 250°F to ~ 300°F. However, the majority of the thermal energy is supplied at ~150°F. The phosphoric acid fuel cell has a power density of 160 to 175 watts/ft2 of active cell area.³

One issue for phosphoric acid fuel cells is that if the source of its hydrogen fuel is reformed gasoline, sulfur must be removed from the fuel entering the cell or it will damage the electrode catalyst.

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Molten Carbonate Fuel Cell (MCFC)
The Molten Carbonate Fuel Cell uses a molten carbonate salt mixture, usually lithium carbonate and potassium carbonate, as its electrolyte. The electrolyte is suspended in ceramic matrix. The anode is a nickel-chromium alloy, and the cathode is a lithium-doped nickel oxide.⁴ Cell operating temperatures range from 600-800°C.

High-temperature molten carbonate fuel cells can extract hydrogen from a variety of fuels using either an internal or external reformer. They are also less prone to carbon monoxide "poisoning" than lower temperature fuel cells, which makes coal-based fuels more attractive for this type of fuel cell. Molten carbonate fuel cells work well with catalysts made of nickel, which is much less expensive than platinum. Molten carbonate fuel cells exhibit up to 60 percent efficiency, and this can rise to 80 percent if the waste heat is utilized for cogeneration. Currently, demonstration units have produced up to 2 megawatts (MW), but designs exist for units of 50 to 100 MW capacity. Two major difficulties with molten carbonate technology put it at a disadvantage compared to solid oxide cells. One is the complexity of working with a liquid electrolyte rather than a solid. The other stems from the chemical reaction inside a molten carbonate cell. Carbonate ions from the electrolyte are used up in the reactions at the anode, making it necessary to compensate by injecting carbon dioxide at the cathode.⁵

In addition, the electrolyte used in molten carbonate fuel cells is highly corrosive, limiting some of it potential applications.

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A. Solid Oxide Fuel Cell (SOFC)
The Solid Oxide Fuel Cell uses a ceramic, solid-phase electrolyte which reduces corrosion considerations and eliminates the electrolyte management problems associated with the liquid electrolyte fuel cells. The solid oxide fuel cell is based upon the use of a solid ceramic as the electrolyte. The preferred electrolyte material is dense yttria-stabilized zirconia. The solid oxide fuel cell is a solid state device and shares certain properties and fabrication techniques with semi-conductor devices. The anode is a porous nickel/zirconia cermet while the cathode is magnesium-doped lanthanum manganate. In development cells and small stacks, the solid oxide fuel cell has demonstrated 0.6V/cell at about 232 A/ft2. Lifetimes in excess of 30,000 hours for single cells have been demonstrated as have a number of heat/cool cycles. Presently available, unpressurized solid oxide fuel cells deliver fuel to electric efficiencies in the range of 45% (HHV). Argonne National Laboratories suggests that pressurized systems could yield fuel efficiencies of 60% (HHV). The high operating temperature of the solid oxide fuel cell offers the possibility of internal reforming. As in the molten carbonate fuel cell, CO does not act as a poison and can be used directly as a fuel. The solid oxide fuel cell is also the most tolerant of any fuel cell type to sulfur. It can tolerant several orders of magnitude more sulfur than other fuel cells. With a 1830°F (1000°C) operating temperature, the solid oxide fuel cell requires a significant start-up time.

It is estimated that the fuel to electricity efficiency of solid oxide fuel cells to range from 50-70%, depending on the size of the power plant. In addition, these efficiencies hold from about 15%-100% power, making the cells ideal for applications in which a wide range of loads is found. Because most solid oxide fuel cells utilize both hydrogen and carbon monoxide fuel inside the cell, they can readily operate on hydrocarbon fuels such as coal gas, gasoline, diesel fuel, jet fuel, alcohol, and natural gas.⁶

The efficiency of the solid oxide fuel cell used in CHP applications will be higher than the polymer electrolyte fuel cells for two major reasons. The first reason is that the hydrocarbon fuel is reformed into hydrogen and carbon monoxide fuel largely inside the cell. This results in some of the high temperature waste heat being recycled back into the fuel. The second reason is that air compression is not required. Especially on smaller systems, this results in a higher amount of net electricity being produced and quieter operation. Because of the high temperatures that the solid oxide fuel cell must run, they may not be practical for sizes much below 1,000 watts or when portable applications are involved.⁷

B. Solid Oxide-Hybrid Fuel Cell Power Systems
A recent development in high temperature stationary fuel cell power plants is the coupling of a microturbine generator with a high-pressure, natural gas-fueled solid oxide fuel cell. High pressure waste heat from solid oxide fuel cell is routed into a microturbine, generating 10% or more additional power than if the exhaust gas energy had not been recaptured. In a recent test by Siemens-Westinghouse, the output of a 200 kW solid oxide fuel cell was boosted to 220 kW through use of a microturbine hybrid configuration.⁸ A new configuration using higher gas pressures and a 50 kW gas turbine is expected to boost output to 250 kW.⁹ These systems are to 55-60% efficient in converting the energy in natural gas into power, better than the current 50% efficiency of natural gas turbines. According to Siemens-Westinghouse, hybrid solid oxide fuel cells may have the potential to reach 70% efficiency as hybrid technology improves. A 1MW hybrid solid oxide fuel cell demonstration plant is planned for installation in 2002.

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A. Proton Exchange Membrane (PEM)
The Proton Exchange Membrane Fuel Cell¹⁰ offers an order of magnitude higher power density than any other fuel cell system, with the exception of the advanced aerospace alkaline fuel cell, which has comparable performance. The proton exchange membrane can operate on reformed hydrocarbon fuels, with pretreatment, and on air. The use of a solid polymer electrolyte eliminates the corrosion and safety concerns associated with liquid electrolyte fuel cells. The anode and cathode are prepared by applying a small amount of platinum black to one surface of a thin sheet of porous, graphitized paper which has previously been wet-proofed with Teflon. Platinum requirements are currently 0.60 oz/kW. Improvements in proton exchange membrane performance can reasonably be expected to reduce platinum requirements to 0.035 oz/kW or about $2/kW.

The proton exchange membrane typically operates at 70-85°C. Its low operating temperature provides instant start-up and requires no thermal shielding to protect personnel. About 50% of maximum power is available immediately at room temperature. Full operating power is available within about 3 minutes under normal conditions. Recent advances in performance and design offer the possibility of lower cost than any other fuel cell system.

In 5 kW production fuel cell stacks, Ballard is achieving a stack-only power density of over 5.4 kW/ft3. Power densities approaching 14.2 kW/ft3 are certainly feasible. A near-term system, including fuel and oxidant controls, cooling, and product water removal which operates on hydrogen and air at 45 psia will provide 1.25 kW/ft3 and 40 W/lb. As is true with all fuel cells, performance is improved by pressurizing the air. In any application, there will be a trade-off between the energy and financial cost associated with compressing air to higher pressures and the improved performance. Pressures above 45 psia are not likely to be advantageous for most applications. Cell operating lifetimes in excess of 50,000 hours were demonstrated for the proton exchange membrane during the NASA program.¹¹

B. Regenerative Proton Exchange Membrane-based fuel cells
Properly designed, a proton exchange membrane fuel cell can be run in reverse, acting as an electrolyzer. This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). A regenerative fuel cell uses water and electrical energy as inputs, electrolyzes the water, and emits hydrogen and oxygen as outputs. These units are currently in the prototype stage, with novel applications such as creating hydrogen during the day with solar electric power, then using the hydrogen fuel at night to power a hybrid solar/hydrogen fuel cell high-altitude unmanned reconnaissance airplane.

The URFC is an excellent energy source in situations where weight is a concern because it is lighter than a separate electrolyzer and generator system. In 1995, the regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density of any chemical battery--about 450 watt-hours per kilogram.¹²

Energy Conversion Devices, Inc. (ECDI) is currently developing a regenerative fuel cell. According to ECDI, their product is less costly than proton exchange membrane cells because it does not require platinum catalysts, and has a broader operating temperature range. A proton exchange membrane fuel cell's operating temperature is 60°C to 80°C, while the newly designed regenerative fuel cell is able to operate within a range of -20°C to 120°C.¹³

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Direct Methanol (DMFC)
Direct methanol fuel cells are similar to the proton exchange membrane cells in that they both use a polymer membrane as the electrolyte. However, in the direct methanol fuel cell, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120-190 degrees F. Higher efficiencies are achieved at higher temperatures.¹⁴

Direct methanol fuel cells are being considered for a number of applications, including transport, portable power including cellular phones and laptop computers, auxiliary power for instrumentation and vehicles, and as a battery replacement for combat personnel and for battlefield applications. The U.S. Office of Transportation Technologies states that onboard reforming of liquid fuels, such as gasoline, eliminates concerns about hydrogen storage and a refueling infrastructure. This could increase customer acceptance of the technology and accelerate near-term introduction of fuel cell vehicles. In addition, when compared with conventional internal combustion engines, the fuel cell system’s increased efficiency will lower fuel consumption and reduce criteria pollutants and carbon monoxide emissions which contribute to global warming.¹⁵ Motorola, Mechanical Technology, Inc., Los Alamos National Laboratory, the Defense Advanced Research Projects Agency (DARPA), and the U.S. Department of Energy are among the major players in direct methanol fuel cell research, development, and commercialization.

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Direct Carbon Fuel Cells
This very new type of fuel cell is based on a process called direct carbon conversion, in which carbon particles are joined in an electrochemical process with oxygen molecules to produce CO2 and electricity. The direct carbon fuel cell technology was developed at Lawrence Livermore National Laboratory (LLNL). The source of carbon fuel can be any type of hydrocarbon, including coal, lignite, natural gas, petroleum, petroleum, coke, and biomass. Because it is carbon, and not hydrogen, that fuels this cell, hydrogen is released as a byproduct of the cell reaction and could potentially be captured for use in a separate hydrogen-powered fuel cell.

According to a recent LLNL newsletter, the technology uses aggregates of extremely fine carbon particles, from 10 to 1,000 nanometers in diameter, distributed in a mixture of molten lithium, sodium, or potassium carbonate at 750-850°C.¹⁶ Total cell efficiencies are projected to be 70-80%, with power generation in the 1 kW/m2 range, sufficient for practical applications. The carbon fuel particles can be produced through pyrolysis of hydrocarbons, a thermal decomposition method well-known as the source of carbon black for tires, ink, and other applications in manufacturing industries. While the concept has been successfully demonstrated with a 3 W cell, this technology is still in the experimental phase of development. Because this is a high-temperature cell, it would be best suited for stationary applications, particularly in combination with CHP utilizing the waste heat energy.

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