A proton exchange membrane fuel cell (PEMFC) uses a water-based acidic polymer membrane as its electrolyte and has a platinum-based electrode. PEMFC batteries operate at relatively low temperatures (below 100 degrees Celsius) and can customize electrical output to meet dynamic power requirements. Due to the relatively low temperatures and the use of precious metal-based electrodes, these cells must be operated under pure hydrogen. PEMFC batteries are currently the leading technology for light vehicles and materials handling vehicles, and to a lesser extent for stationary and other applications. PEMFC fuel cells are also sometimes referred to as polymer electrolyte membrane fuel cells (also known as PEMFCs).
The hydrogen fuel is treated at the anode where electrons are separated from the protons on the surface of the platinum-based catalyst. Protons pass through the membrane to the cathode side of the cell while electrons travel in an external circuit, producing an electrical output of the cell. On the cathode side, another precious metal electrode combines protons and electrons with oxygen to produce water, which is discharged as the only waste; oxygen can be provided in purified form or directly at the electrode from the air.
Variants of PEMFCs operating at elevated temperatures are referred to as high temperature PEMFCs (HTPEMFCs). The HTPEMFC can operate up to 200 degrees Celsius by changing the electrolyte from a water base to a mineral acid based system. This overcomes some of the current limitations with respect to fuel purity, where the HTPEMFC is capable of treating reformate containing small amounts of carbon monoxide (CO). It is also possible to simplify the balance of the equipment by eliminating the humidifier.
Direct methanol fuel cells (DMFCs) are a relatively new set of fuel cell technologies; they were invented and developed by researchers in several U.S. agencies in the 1990s, including NASA and the Jet Propulsion Laboratory. It is similar to a PEM battery because it uses a polymer membrane as an electrolyte. However, the platinum-ruthenium catalyst on the anode of the DMFC can absorb hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Therefore pure methanol can be used as fuel, hence the name.
Methanol provides several advantages as a fuel. It is cheap but has a relatively high energy density and can be easily transported and stored. It can be supplied to the fuel cell unit from a reservoir that can remain full or in a cartridge that can be quickly replaced when in use.
DMFCs operate over a temperature range of 60°C to 130°C and tend to be used for applications with modest power requirements, such as mobile electronic devices or chargers and portable power packs. One particular application of the DMFC seeing commercial traction in various countries is the use of DMFC power units for material handling vehicles. Many of these units have been sold to commercial warehouses, where forklifts are usually powered by batteries. By switching to the fuel cell, the warehouse can refuel its truck within minutes, compared to the time required to charge the battery. Fuel cells also eliminate the need for a battery charging infrastructure in the warehouse, allowing more floor space for other uses.
Solid oxide fuel cells operate at very high temperatures, with the highest of all fuel cell types being around 800°C to 1000°C. When converting fuel into electricity, their efficiencies can exceed 60%; if the heat they generate is also used; their total efficiency of converting fuel into energy can exceed 80%.
SOFC uses solid ceramic electrolytes, such as yttria-stabilized zirconia, rather than liquids or membranes. Their high operating temperature means that the fuel can be reformed within the fuel cell itself, eliminating the need for external reforming and allowing the unit to be used with a variety of hydrocarbon fuels. They are also relatively resistant to small amounts of sulfur in the fuel compared to other types of fuel cells and can therefore be used with gas.
Another advantage of high operating temperatures is improved reaction kinetics, eliminating the need for metal catalysts. However, there are some drawbacks to high temperatures: these batteries require longer starting and operating temperatures, they must be constructed of strong, heat-resistant materials, and they must be shielded to prevent heat loss.
SOFC has three different SOFC geometries: Planar, Coplanar, and Microtube. In a planar design, components are assembled into a flat stack where air and hydrogen are traditionally flowed through the cells through channels built into the anode and cathode. In a tubular design, air is supplied to the inside of an elongated solid oxide tube (which is sealed at one end) while the fuel flows around the outside of the tube. The tube itself forms the cathode, and the battery components are constructed in layers around the tube.
SOFC is widely used for large and small stationary power generation: flat-type power generation is applied to 100kW off-grid generators such as BloomEnergy and SOFCs with several kilowatts of output, and is being tested for smaller cogeneration applications such as home combinatorial heat and Power (CHP). Microtube SOFCs in the output power range are also being developed for small portable chargers.
The alkaline fuel cell (AFC) is one of the first fuel cell technologies to be developed and was originally used by NASA for space programs to generate electricity and water on spacecraft. Throughout the planning period, NASA continues to use NASA space shuttles, as well as a few commercial applications.
AFCs use alkaline electrolytes such as potassium hydroxide in water, and usually use pure hydrogen fuel. The first AFC operates between 100°C and 250°C, but the typical operating temperature is now around 70°C. As a result of the low operating temperatures, there is no need to use platinum catalysts in the system, but various non-noble metals can be used as catalysts to accelerate the reactions occurring at the anode and cathode. Nickel is the most commonly used catalyst in AFC units.
Due to the rate at which chemical reactions occur, these batteries provide relatively high fuel-to-electric conversion efficiency, up to 60% in some applications.
Molten carbonate fuel cells (MCFCs) use molten carbonate suspended in a porous ceramic matrix as an electrolyte. The commonly used salts include lithium carbonate, potassium carbonate and sodium carbonate.
They operate at high temperatures, about 650°C, and have several advantages associated with this. First, high operating temperatures significantly increase the reaction kinetics, so there is no need to increase these with noble metal catalysts. Higher temperatures also make the system less susceptible to carbon monoxide poisoning in batteries with lower temperatures. As a result, MCFC systems can operate on a variety of different fuels, including coal-derived fuel gases, methane or natural gas, eliminating the need for an external reformer.
The disadvantages associated with MCFC units stem from the use of liquid electrolytes rather than solids and the need to inject carbon dioxide at the cathode because carbonate ions are consumed in the reactions that take place at the anode. There are also some problems with high temperature corrosion and the corrosive nature of the electrolyte, but these problems can now be controlled to achieve an actual lifetime.
MCFC is used for large stationary power generation. Most of the megawatt capacity of the fuel cell power plant uses MCFC, Large Scale Cogeneration (CHP) and Combined Cooling and Power (CCP) plants. These fuel cells can operate at fuel electrotransformation efficiencies of up to 60%, and overall efficiency can exceed 80% in CHP or CCP applications that also utilize process heat.
The phosphoric acid fuel cell (PAFC) consists of an anode and a cathode, the anode and cathode being made of a finely dispersed platinum catalyst on carbon and a silicon carbide structure that holds a phosphoric acid electrolyte. They are quite resistant to carbon monoxide poisoning, but they are often less efficient than other fuel cell types in producing electricity. However, these cells operate at a moderately high temperature of about 180°C, and if the process heat is used for cogeneration, the overall efficiency can exceed 80%.
This type of fuel cell is used in stationary generators with outputs ranging from 100 kW to 400 kW to power many commercial locations around the world, and they are also used in large vehicles such as buses. Most of the fuel cell units sold before 2001 use PAFC technology