Fuel cell types are generally characterized by electrolyte material. The electrolyte is the substance between the positive and negative terminals, serving as the bridge for the ion exchange that generates electrical current.
While there are dozens of types of fuel cells, there are six principle kinds in various stages of commercial availability, or undergoing research, development and demonstration. These six fuel cell types are significantly different from each other in many respects; however, the key distinguishing feature is the electrolyte material.
1. Alkaline Fuel Cell (AFC)
2. Molten Carbonate Fuel Cell (MCFC)
3. Phosphoric Acid Fuel Cell (PAFC)
4. Proton Exchange Membrane Fuel Cell (PEMFC)
5. Solid Oxide Fuel Cell (SOFC)
6. Direct Methanol Fuel Cell
Figure 2. The Five Principal Types of Fuel Cells and their Electrochemical Reactions.
The following sections describe each of these fuel cell types.
Alkaline Fuel Cells (AFCs) were the first type of fuel cell to be widely used for manned space applications. AFCs contain a potassium hydroxide (KOH) solution as the electrolyte. AFCs operate at temperatures between 100°C and 250°C (211°F and 482°F). Higher temperature AFCs use a concentrated (85wt%) KOH solution while lower temperature AFCs use a more dilute
KOH solution (35-50wt%). The electrolyte is contained in and/or supported by a matrix (usually asbestos) which wicks the electrolyte over the entire surface of the electrodes. A wide range of electro-catalysts can be used in the electrodes (e.g., Ni, Ag, spinels, metal oxides, and noble metals). The fuel supplied to an AFC must be pure hydrogen. Carbon monoxide (CO) poisons an AFC and carbon dioxide (CO2) reacts with the electrolyte to form potassium carbonate (K2CO3). Even the small amount of CO2 in the atmosphere (about 370 ppm) must be accounted for operation of an AFC (Hirschenhofer et al., 1998).
Full-scale demonstration plants are now testing molten carbonate fuel cells (MCFCs). The electrolyte in an MCFC is an alkali carbonate (sodium, potassium, or lithium salts, i.e., Na2CO3, K2CO2, or Li2CO3) or a combination of alkali carbonates that is retained in a ceramic matrix of lithium aluminum oxide (LiAlO2). An MCFC operates at 600 to 700°C where the alkali carbonates form a highly conductive molten salt with carbonate ions (CO3=) providing ionic conduction through the electrolyte matrix. Relatively inexpensive nickel (Ni) and nickel oxide (NiO) are adequate to promote reaction on the anode and cathode respectively at the high operating temperatures of an MCFC (Baker, 1997).
MCFCs offer greater fuel flexibility and higher fuel-to-electricity efficiencies than lower temperature fuel cells, approaching 60 percent. The higher operating temperatures of MCFCs make them candidates for combined-cycle applications, in which the exhaust heat is used to generate additional electricity. When the waste heat is used for co-generation, total thermal efficiencies can approach 85 percent.
Phosphoric Acid Fuel Cell (PAFC) technology is the most mature of the types in use today. PAFCs use a concentrated 100% phosphoric acid (H3PO4) electrolyte retained on a silicon carbide matrix and operate at temperatures between 150 and 220°C. Concentrated H3PO4 is a relatively stable acid, which allows operation at these temperatures. At lower temperatures, problems with CO poisoning of the anode electro-catalyst (usually platinum) and poor ionic conduction in the electrolyte become problems (Hirschenhofer et al., 1998). The electrodes typically consist of TeflonTM-bonded platinum and carbon (PTFE-bonded Pt/C).
PAFC fuel cells produced by UTC Fuel Cells (previously named ONSI and International Fuel Cells) were the world's first commercially available fuel cell product (King and Ishikawa, 1996). Turnkey 200-kilowatt plants are now available and have been installed at more than 200 sites in the United States, Europe, and Asia (principally Japan). Operating at about 200°C, the PAFC plant also produces heat for domestic hot water and space heating, and its electrical efficiency is 36-40 percent. The development and implementation of this commercial fuel cell product is a result of several years of research development and demonstration by the U.S. Department of Energy, U.S. Department of Defense, Gas Research Institute.
Proton Exchange Membrane
The proton exchange membrane fuel cell (PEMFC) is also known as the solid polymer or polymer electrolyte fuel cell. A PEMFC contains an electrolyte that is a layer of solid polymer (usually a sulfonic acid polymer, whose commercial name is NafionTM) that allows protons to be transmitted from one face to the other (Gottesfeld and Zawadinski, 1998). PEMFCs require hydrogen and oxygen as inputs, though the oxidant may also be ambient air, and these gases must be humidified. PEMFCs operate at a temperature much lower than other fuel cells, because of the limitations imposed by the thermal properties of the membrane itself (Appleby and Yeager, 1986). The operating temperatures are around 90°C. The PEMFC can be contaminated by CO, reducing the performance and damaging catalytic materials within the cell. A PEMFC requires cooling and management of the exhaust water to function properly (Gottesfeld and Zawadinski, 1998).
Solid Oxide Fuel Cells (SOFCs) are currently being demonstrated
in sizes from 1kW up to 250-kW plants, with plans to reach
the multi-MW range. SOFCs utilize a non-porous metal oxide
(usually yttria-stabilized zirconia, Y2O3-stabilized ZrO2)
electrolyte material. SOFCs operate between 650 and 1000°C,
where ionic conduction is accomplished by oxygen ions
(O=). Typically the anode of an SOFC is cobalt or nickel
zirconia (Co-ZrO2 or Ni-ZrO2) and the cathode is strontium-doped
lanthanum manganite (Sr-doped LaMnO3) (Singhal, 1997;
SOFCs offer the stability and reliability of all-solid-state ceramic construction. High-temperature operation, up to 1,000°C, allows more flexibility in the choice of fuels and can produce very good performance in combined-cycle applications. SOFCs approach 60 percent electrical efficiency in the
simple cycle system, and 85 percent total thermal efficiency in co-generation applications (Singhal, 1997).
The flat plate and monolithic designs are at a much earlier stage of development typified by sub-scale, single cell and short stack development (kW scale). At this juncture, tubular SOFC designs are closer to commercialization.
The direct-methanol fuel cell (DMFC) is similar to the PEM cell in that it uses a polymer membrane as an electrolyte. However, a catalyst on the DMFC anode draws hydrogen from liquid methanol, eliminating the need for a fuel reformer. While potentially a very attractive solution to the issues of hydrogen storage and transportation (particularly for portable applications), the principal problem facing the commercial application of the DMFC today stems from its relatively low performance in comparison to hydrogen fueled PEMFCs.