The type of electrolyte that fuel cells use is primarily what differentiates them. The variety of electrochemical reactions that take place within the cell, the kind of catalysts that are required, the temperature range over which the cell operates, the amount of fuel that is required, and other aspects are all determined by this classification. The applications for which these cells are best suited are affected by these characteristics in turn. There are a number of different kinds of fuel cells that are being developed at the moment, each of which has its own benefits, drawbacks, and potential applications. Find out more about the fuel cells listed below.
POLYMER ELECTROLYTE MEMBRANE FUEL CELLS
When compared to other types of fuel cells, polymer electrolyte membrane (PEM) fuel cells—also known as proton exchange membrane fuel cells—offer advantages such as low weight and volume and high power density. The electrolyte in PEM fuel cells is a solid polymer, and the electrodes are porous carbon with a platinum or platinum alloy catalyst. To function, they require only water, hydrogen, and air oxygen. They typically run on pure hydrogen that comes from reformers or storage tanks.
PEM fuel cells operate at temperatures around 80 degrees Celsius (176 degrees Fahrenheit). They can start up quickly, requiring less warm-up time, and low-temperature operation reduces wear on system components, enhancing their durability. However, in order to separate the hydrogen’s electrons and protons, a noble metal catalyst—typically platinum—is required, which raises system costs. Additionally, the platinum catalyst is extremely sensitive to carbon monoxide poisoning, necessitating the use of an additional reactor to reduce the amount of carbon monoxide in the fuel gas in the event that the hydrogen comes from a fuel containing hydrocarbons. Additionally, this reactor adds cost.
PEM fuel cells are primarily used in stationary and transportation applications. PEM fuel cells are particularly well-suited for use in heavy-duty trucks, buses, and automobiles.
DIRECT METHANOL FUEL CELLS
Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is usually mixed with water and fed directly to the fuel cell anode.
Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cell systems because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline. DMFCs are often used to provide power for portable fuel cell applications such as cell phones or laptop computers.
ALKALINE FUEL CELLS
Alkaline fuel cells (AFCs) were one of the first fuel cell technologies to be developed. They were the first type of fuel cell that was widely used in the U.S. space program to produce water and electrical energy aboard spacecraft. These fuel cells can use a variety of non-precious metals as catalysts at the anode and cathode, and their electrolyte is a solution of potassium hydroxide in water. New AFCs with a polymer membrane as the electrolyte have been developed in recent years. The only difference between these fuel cells and conventional PEM fuel cells is that they employ an alkaline membrane rather than an acid one. The rapidity of the cell’s electrochemical reactions is what gives AFCs their high performance. In space applications, they have also demonstrated efficiencies exceeding 60%.
Due to its susceptibility to carbon dioxide (CO2) poisoning, this type of fuel cell faces significant difficulties. Due to carbonate formation, even a small amount of CO2 in the air can have a significant impact on cell performance and durability. Recirculating alkaline cells with liquid electrolytes can help reduce the effects of carbonate formation in the electrolyte by allowing for electrolyte regeneration. However, the recirculating mode introduces problems with shunt currents. Additional issues with the liquid electrolyte systems include wettability, increased corrosion, and difficulties coping with differential pressures. These issues are addressed by alkaline membrane fuel cells (AMFCs), which are less susceptible to CO2 poisoning than liquid-electrolyte AFCs. In any case, CO2 actually influences execution, execution sturdiness of the AMFCs actually slack that of PEMFCs. AMFCs are being considered for W-to-kW-scale applications. Tolerance to carbon dioxide, membrane conductivity and durability, operation at higher temperatures, water management, power density, and anode electrocatalysis are obstacles for AMFCs.
PHOSPHORIC ACID FUEL CELLS
Phosphoric acid fuel cells (PAFCs) make use of porous carbon electrodes with a platinum catalyst and liquid phosphoric acid as an electrolyte. The acid is contained in a silicon carbide matrix that has been bonded with Teflon. The diagram to the right depicts the electrochemical reactions that take place within the cell.
The PAFC is thought of as the “original” of present day energy components. It is the first commercially available cell type and one of the most mature types. Although PAFCs have been used to power large vehicles like city buses, this kind of fuel cell is typically used for power generation at a stationary location.
PEM cells, on the other hand, are easily “poisoned” by carbon monoxide because carbon monoxide binds to the platinum catalyst at the anode, reducing the fuel cell’s efficiency. PAFCs, on the other hand, are more tolerant of impurities in fossil fuels that have been transformed into hydrogen. When used to co-generate heat and electricity, PAFCs are more than 85% efficient, but they are only 37%–42% efficient at generating electricity. The efficiency of PAFCs is only marginally higher than that of combustion-based power plants, which typically achieve an efficiency of around 33%. PAFCs are likewise less strong than other power devices, given a similar weight and volume. As a result, fuel cells tend to be bulky and heavy. PAFCs also cost a lot. They cost more to produce because they require more expensive platinum catalyst to be loaded into them than do other kinds of fuel cells.
MOLTEN CARBONATE FUEL CELLS
Molten carbonate fuel cells, or MCFCs, are being developed for use in natural gas and coal-based power plants for industrial, military, and electrical utility purposes. An electrolyte of molten carbonate salts suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix is used in MCFCs, which are fuel cells that operate at high temperatures. Non-precious metals can be utilized as catalysts at the anode and cathode, which results in cost savings because they operate at temperatures of approximately 1,200°F (650°C).
Further developed productivity is one more explanation MCFCs offer huge expense decreases over phosphoric corrosive energy components. When combined with a turbine, molten carbonate fuel cells can achieve efficiencies approaching 65 percent, which is significantly higher than the 37–42 percent efficiencies of a phosphoric acid fuel cell plant. Fuel efficiencies can reach over 85% when waste heat is captured and used.
In contrast to soluble, phosphoric corrosive, and PEM energy components, MCFCs don’t need an outside reformer to change over energizes like flammable gas and biogas to hydrogen. Internal reforming, which also reduces costs, is the process by which methane and other light hydrocarbons in these fuels are converted to hydrogen within the fuel cell itself at the high temperatures at which MCFCs operate.
Durability is the primary drawback of the current MCFC technology. The corrosive electrolyte and high operating temperatures of these cells reduce cell life by speeding up component breakdown and corrosion. Researchers are currently looking into fuel cell designs that double the cell life from the current 40,000 hours (5 years) without sacrificing performance, as well as corrosion-resistant materials for components.
SOLID OXIDE FUEL CELLS
Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. SOFCs are around 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 85%.
SOFCs operate at very high temperatures—as high as 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 sulfur than other cell types can. In addition, they are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.
Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 700°C that have fewer durability problems and cost less. Lower-temperature SOFCs have not yet matched the performance of the higher temperature systems, however, and stack materials that will function in this lower temperature range are still under development.
REVERSIBLE FUEL CELLS
Similar to other types of fuel cells, reversible fuel cells generate heat and water as byproducts as well as electricity from hydrogen and oxygen. Reversible fuel cell systems, on the other hand, can use electricity from solar, wind, or other sources to electrolyze water to produce hydrogen fuel and oxygen. Reversible fuel cells can provide power when it is needed, but they can also store the excess energy in the form of hydrogen when other technologies produce a lot of power (like when high winds make too much wind power available). It’s possible that this ability to store energy will be a major enabler for intermittent renewable energy technologies.
