Types of Fuel Cells
Fuel cells are a family of technologies that generate electricity through electrochemical processes, rather than combustion. There are many fuel cell types, but the principal ones include the alkaline fuel cell (AFC), proton exchange membrane (PEM) fuel cell, direct methanol fuel cell (DMFC), molten carbonate fuel cell (MCFC), phosphoric acid fuel cell (PAFC), and solid oxide fuel cell (SOFC). A number of these fuel cell types are commercially available today.
Each fuel cell type has its own unique chemistry, such as different operating temperatures, catalysts, and electrolytes. A fuel cell’s operating characteristics help define its application – for example, lower temperature PEM and DMFC fuel cells are used to power passenger vehicles and forklifts, while larger, higher temperature MCFC and PAFC fuel cells are used for stationary power generation.
Researchers continue to improve fuel cell technologies, examining different catalysts and electrolytes in order to improve performance and reduce costs. New fuel cell technologies, such as microbial fuel cells, are also being examined in the lab.
Proton Exchange Membrane Fuel Cell (PEM)
Electrolyte: Solid polymer membrane
Catalyst: Platinum is the most active catalyst for low-temperature fuel cells
Operating Temperature: Around 175-200⁰F
Electrical Efficiency: 40-60 percent
PEM fuel cells operate at relatively low temperatures, have high power density, and can vary output quickly to meet shifts in power demand. PEMs are well-suited to power applications where quick startup is required, such as automobiles or forklifts. Single PEM units range from several watts to several kilowatts, and can be scaled into larger systems – the largest to date is a 1 megawatt PEM stationary power plant. PEM systems are available today for a variety of applications, with sales focused in the telecommunications, data center and residential markets (primary or backup power), and to power forklifts and other material handling vehicles. PEM fuel cells are also used in buses and demonstration passenger vehicles – major auto manufacturers anticipate the start of commercial fuel cell vehicle sales around 2014-2016. PEMs are fueled with hydrogen gas, methanol, or reformed fuels.
High-temperature PEM (HT-PEM) fuel cells are similar to PEM fuel cells, but operate at higher temperatures, between 250⁰F and 390⁰F. HT-PEMs are often integrated with fuel reformers, permitting operation using wider variety of input fuels. HT-PEMs can be used to power vehicles as range extenders for batteries, and small scale commercial buildings and homes.
Direct Methanol Fuel Cell (DMFC)
Electrolyte: Solid polymer membrane
Catalyst: Platinum is the most common
Operating Temperature: Around 125-250⁰F
Electrical Efficiency: Up to 40 percent
DMFCs are similar to PEM fuel cells in that they both use a polymer membrane as the electrolyte. However, in DMFC systems the anode catalyst itself draws the hydrogen from liquid methanol, eliminating the need for a fuel reformer. The low operating temperature makes DMFCs attractive for miniature applications such as cell phones, laptops, and battery chargers for consumer electronics, to mid-size applications powering electronics on RVs, boats, or camping cabins.
Alkaline Fuel Cell (AFC)
Electrolyte: Potassium hydroxide solution in water
Catalyst: Can use a variety of non-precious metal catalysts
Operating Temperature: Around 225-475⁰F
Electrical Efficiency: 60-70 percent
NASA has used hydrogen-fueled AFCs on space missions since the 1960s to provide both electricity and drinking water. AFCs are poisoned easily by small quantities of CO2, and are thus deployed primarily in controlled aerospace and underwater environments.
Phosphoric Acid Fuel Cell (PAFC)
Electrolyte: Liquid phosphoric acid ceramic in a lithium aluminum oxide matrix
Catalyst: Carbon-supported platinum catalyst
Operating Temperature: 350-400⁰F
Electrical Efficiency: 36-42 percent
PAFCs can operate using reformed hydrocarbon fuels or biogas. Anode and cathode reactions are similar to PEMs, but since operating temperatures are higher, PAFCs are more tolerant of fuel impurities. PAFCs are frequently used in a cogeneration mode, in which byproduct heat is captured for onsite heating, cooling, and hot water (also called combined heat and power, or CHP). PAFCs are commercially available today with systems operating around the world at high-energy demand sites such as hospitals, schools, office buildings, grocery stores, manufacturing or processing centers, and wastewater treatment plants.
Molten Carbonate Fuel Cell (MCFC)
Electrolyte: Typically consists of alkali (Na & K) carbonates retained in a ceramic matrix of LiHO2
Catalyst: High MCFC operating temperature permits the use of lower-cost, non-platinum group catalysts
Operating Temperature: Around 1,200 ⁰F
Electrical Efficiency: 50-60 percent
The high operating temperatures of MCFCs means that hydrocarbon fuels can be converted to hydrogen within the fuel cell itself (internal reforming). MCFCs are not prone to CO or CO2 “poisoning” – they can even use carbon oxides as fuel – making them more attractive for fueling with gases made from coal. MCFCs are ideal for large stationary power and CHP applications, and are available as commercial products, with dozens of power plants deployed at food and beverage processing facilities, manufacturing plants, hospitals, prisons, hotels, colleges and universities, utilities, and wastewater treatment plants worldwide.
Solid Oxide Fuel Cells (SOFC)
Electrolyte: A solid ceramic, typically yttria-stabilized zirconia (YSZ)
Catalyst: High SOFC operating temperature permits the use of lower-cost, non-platinum group catalysts
Operating Temperature: About 1,800⁰F
Electrical Efficiency: 50-60 percent
High-temperature SOFCs are capable of internal reforming of “light” hydrocarbons such as natural gas, but heavier hydrocarbons (gasoline, jet fuel) can be used, though they require an external reformer. There are two configurations of SOFC fuel cell systems: one type uses an array of meter-long tubes, and another uses compressed discs. Tubular SOFC designs are closer to commercialization and are being produced by companies around the world. SOFCs are suitable for large stationary applications, and are being deployed across the country at data centers, office buildings and retail stores. SOFCs are also being demonstrated for use as vehicle auxiliary power units and tested for small stationary applications, such as homes and apartments in the U.S., Japan, and Germany.
Other Fuel Cell Types
Regenerative Fuel Cells (RFCs) are attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyzer, and then is directed to the fuel cell, where electricity, heat and water are generated. The byproduct water is re-circulated back to the electrolyzer where the process begins again. PEM and SOFC regenerative fuel cell system systems are currently in development.
Zinc Air Fuel Cells (ZAFCs) combine zinc pellets and air with an electrolyte to create electricity, generating significantly more power than lead-acid batteries of the same weight. ZAFC systems have potential use in transport applications.
Microbial Fuel Cells (MFCs) use the catalytic reaction of microorganisms to convert virtually any organic matter (e.g. glucose, acetate, wastewater) into fuel. Enclosed in oxygen-free anodes, organic compounds are consumed by bacteria or other microbes. As part of the digestive process, electrons are pulled from the fuel and conducted into a circuit with the help of inorganic mediator chemicals. MFCs operate in mild conditions between 68-104⁰F. These systems are capable of efficiencies up to 50 percent, and will be suitable for small to miniature applications such as medical devices.