Fuel Cell FAQs

What are the Different Types of Fuel Cells?

Direct Methanol Fuel Cell (DMFC)

Direct Methanol Fuel Cells (DMFC), a subcategory of proton-exchange fuel cells, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode. Their main advantage is the ease of transport of methanol, an energy-dense yet reasonably stable liquid at all environmental conditions. Efficiency is low for these cells, so they are targeted especially to portable applications, where energy and power density are more important than efficiency.

Polymer Electrolyte Membrane (PEM) Fuel Cell

Polymer Electrolyte Membrane (PEM) Fuel Cells (PEMFC), also known as proton exchange membrane fuel cells, deliver high-power density and offer the advantages of low weight and volume, compared with other fuel cells. Their distinguishing features include lower temperature/pressure ranges (50 to 100 °C), a solid polymer electrolyte membrane and porous carbon electrodes containing a platinum catalyst. PEM fuel cells need hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen that is supplied from storage tanks or on-board reformers.

Alkaline Fuel Cell (AFC)

Alkaline fuel cells (AFC), also known as the Bacon fuel cell after its British inventor, were one of the first fuel cell technologies developed, and they were the first type widely used by NASA to produce electrical energy and water on-board spacecrafts. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F)

AFCs’ high performance is due to the rate at which chemical reactions take place in the cell. They have also demonstrated efficiencies near 60% in space applications.

The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can affect this cell’s operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell’s lifetime (the amount of time before it must be replaced), further adding to cost.

Cost is less of a factor for remote locations, such as space or under the sea. However, to effectively compete in most mainstream commercial markets, these fuel cells will have to become more cost-effective. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours, something that has not yet been achieved due to material durability issues. This obstacle is possibly the most significant in commercializing this fuel cell technology.

Phosphoric Acid Fuel Cell (PAFC)

Phosphoric Acid Fuel Cells (PAFC) use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst.

The PAFC is considered the “first generation” of modern fuel cells. It is one of the most mature cell types and the first to be used commercially. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily “poisoned” by carbon monoxide because carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell’s efficiency. They are 85% efficient when used for the co-generation of electricity and heat but less efficient at generating electricity alone (37%–42%). This is only slightly more efficient than combustion-based power plants, which typically operate at 33%–35% efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell.

Molten Carbonate Fuel Cell (MCFC)

Molten carbonate fuel cells (MCFC) are currently being developed for natural gas, biogas (produced as a result of anaerobic digestion or biomass gasification), and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells, operating at temperatures of 600 °C and above, that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix of beta-alumina solid electrolyte (BASE). Because they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85%.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs do not require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.

The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.

Solid Oxide Fuel Cell (SOFC)

Solid Oxide Fuel Cells (SOFC) use a hard, non-porous ceramic compound as its electrolyte. Because the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50%–60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system’s waste heat (co-generation), overall fuel use efficiencies could top 80%–85%.

Solid oxide fuel cells operate at very high temperatures—around 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more of sulfur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This property allows SOFCs to use gases made from coal.

Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues.

Microbial Fuel Cell (MFC)

A Microbial Fuel Cell (MFC), or biological fuel cell, is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature. MFCs can be grouped into two general categories, those that use a mediator and those that are mediator-less. The first MFCs, demonstrated in the early 20th century, used a mediator, a chemical that transfers electrons from the bacteria in the cell to the anode. Mediator-less MFCs are a more recent development dating to the 1970s; in this type of MFC bacteria in mediator-less MFCs typically have electrochemically active redox proteins such as cytochromes on their outer membrane that can transfer electrons directly to the anode. Since the turn of the 21st century MFCs have started to find a commercial use in the treatment of waste water.