Types of Fuel Cell - An Overview and Applications of Fuel Cells Currently under Development

Polymer Electrolyte Membrane (PEM) Fuel Cells
Direct Methanol Fuel Cells
Alkaline Fuel Cells
Phosphoric Acid Fuel Cells
Molten Carbonate Fuel Cells
Solid Oxide Fuel Cells
Regenerative Fuel Cells

Fuel Cells are mainly categorized by the type of electrolyte use. This in turn governs the type of catalysts needed and the type of chemical reactions that occurs inside the fuel cell. The type of electrolyte used also determine factors such as the type of fuel required and the temperature range the cell operates.

Most importantly, these properties ultimately decide the applications for which these fuel cells are most suitable.

Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells provide high power density and the advantages of low weight and volume compared to other types of fuel cells. Also referred to as proton exchange membrane fuel cells, polymer electrolyte membrane fuel cells employ porous carbon electrodes containing a platinum catalyst and a solid polymer as the electrolyte. In order for the polymer electrolyte membrane fuel cells to operate, hydrogen, water and oxygen from air are required. Polymer electrolyte membrane fuel cells are typically fueled with pure hydrogen supplied from onboard reformers or storage tanks.

Figure 1. Polymer Electrolyte Membrane Fuel Cell. (Source: Dept. of Energy)

Polymer electrolyte membrane fuel cells are mainly used for transportation applications such as cars and buses and they operate at relatively low temperatures.

Direct Methanol Fuel Cells

Direct methanol fuel cells are powered by pure methanol. Methanol has a lower energy density than gasoline or diesel fuel, but has a higher energy density than hydrogen. Since it is a liquid similar to gasoline, methanol is also easier to transport and supply to the public using existing infrastructure.

The methanol used in direct methanol fuel cell is mixed with steam which is then fed directly to the fuel cell anode. Direct methanol fuel cell technology is relatively new compared to hydrogen fuel cells.

Alkaline Fuel Cells

Alkaline fuel cells can employ a number of non-precious metals as a catalyst at the cathode and anode and a solution of potassium hydroxide in water as the electrolyte.

Figure 2. Alkaline Fuel Cell. (Source: Dept. of Energy)

The high performance of alkaline fuel cells is due to the rate at which chemical reactions take place in the cell. However, alkaline fuel cells are easily poisoned by carbon dioxide; a small amount of carbon dioxide in the air can affect the operation of the fuel cell.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells employ porous carbon electrodes containing a platinum catalyst and liquid phosphoric acid, which is stored in a Teflon-bonded silicon carbide matrix as an electrolyte.

Figure 3. Phosphoric Acid Fuel Cell. (Source: Dept. of Energy)

Phosphoric acid fuel cells are around 85 percent efficient when they are used for the generation of heat and electricity. However, phosphoric acid fuel cell is less efficient when they are used to generate only electricity. Phosphoric acid fuel cells are generally large and heavy as a result of them being less powerful when compared to other types of fuel cells. Phosphoric acid fuel cells are also more expensive since the fuel cells require an expensive platinum catalyst.

Molten Carbonate Fuel Cells

Molten carbonate fuel cells employ a molten carbonate salt mixture electrolyte which is suspended in a porous chemically inert lithium aluminum oxide ceramic matrix. Molten carbonate fuel cells are high temperature fuel cells that offer significant cost reduction over phosphoric acid fuel cells. Molten carbonate fuel cells also offer significant increase in efficiency of around 60 percent when it is used to generate electricity than compared to phosphoric acid fuel cells. The major drawback of current molten carbonate fuel cell technology is its durability.

Figure 4. Molten Carbonate Fuel Cell. (Source: Dept. of Energy)

Solid Oxide Fuel Cells

Solid oxide fuel cells use a hard, non porous ceramic compound is used as the electrolyte. An efficiency of around 50 to 60 percent can be expected when solid oxide fuel cells are used to convert fuel to electricity. Since solid oxide fuel cells operate at very high temperatures, this removes the need for catalysts made from precious metals. As a result, the cost of solid oxide fuel cells is reduced.

Figure 5. Solid Oxide Fuel Cell. (Source: Dept. of Energy)

Regenerative Fuel Cells

Just like other fuel cells, regenerative fuel cells produce electricity from oxygen and hydrogen and heat and water as by-products. Regenerative fuel cells can also used solar power or other sources of electricity to split water produced as by-product into hydrogen and oxygen in a process referred to as electrolysis. Regenerative fuel cells are comparatively new type of fuel cell technology which is currently under development by organization such as NASA.

Source: AzoCleantech

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