What do batteries and fuel cells have in common? They are both electrochemical energy conversion devices that produce electricity. Also, they both have anodes, cathodes, and an electrolyte. There are also big differences. Batteries produce electricity until completely discharged, then they have to be replaced or recharged. A fuel cell continues to produce electricity as long as it is supplied with fuel and oxygen. In the typical fuel cell used in transportation, that’s hydrogen and air. A battery produces essentially no emissions and little heat, while a hydrogen fuel cell emits water and more heat.
While there are several different types of fuel cells, they all work on the same basic principle. The proton exchange membrane (PEM) fuel cell will be discussed here. With rare exception, this is the technology being developed for use in cars, trucks, and buses. PEM fuel cells appear to be the most promising for vehicles because the reactions are about the simplest of any fuel cell design. They also have a high kilowatts-per-cubic-inch power density. Their relatively low operating temperature of 140 to 176 degrees F means they start to produce electricity quickly and don’t require expensive cooling systems.
In a PEM fuel cell, pressurized hydrogen gas enters on the anode side and is forced through the catalyst. Here, H2 molecules come in contact with catalyst, splitting it into two H+ ions (protons).and two electrons. The proton exchange membrane and electrolyte let positively charged proton through and block negatively charged electrons.
Electrons are conducted through the anode and travel through the external circuit as DC (direct current) electric power, which can useful for purposes such as powering an electric motor, and then they reach the cathode. Here they combine on the cathode’s catalyst with the proton coming through the membrane and with oxygen gas, or air, forced through the catalyst, where they form two oxygen atoms with a strong negative charge. This negative charge attracts the two H+ ions, which combine with an oxygen atom and two of the electrons to form a water molecule.
The proton exchange membrane is a specially treated material that looks somewhat like ordinary kitchen plastic wrap. The membrane must be hydrated to transfer protons and remain stable. Thus, fuel cell systems must be designed to operate in sub-zero temperatures, low humidity environments, and high operating temperatures. At about 70 degrees F, hydration is lost without a high-pressure hydration system.
Catalysts play the crucial role of separating hydrogen into ions and protons at the anode and combining them, plus water, at the cathode. Typically these use a platinum group metal or alloy with platinum nanoparticles very thinly coated onto carbon paper or cloth. The catalyst is rough and porous to expose maximum surface area to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the membrane.
Precious metal catalysts plus proton exchange membranes, gas diffusion layers, and bipolar plates make up about 70 percent of a current fuel cell’s cost. Because of this, plus the rarity of precious metals and competition from other uses such as catalytic converters, some critics say platinum is the PEM fuel cell’s Achilles heel. Research is under way to solve this potential impediment. For example, researchers are looking at ways to use less of the precious metals and to find alternatives. Recycling platinum, especially from catalytic converters, is already common practice. More abundant gold, reduced to nanometer size, could be used as a catalyst as well. Enhancing a catalyst with carbon silk can also reduce the amount of precious metals required.
Another problem with PEM fuel cells is that impurities can poison the catalysts, resulting in reduced efficiency and activity so more dense catalysts are required and more platinum is used. Again, research is underway to solve the problem with various promising techniques being explored, like using a gold-palladium coating that may be less susceptible to poisoning.
Since a single fuel cell produces only about 0.7 volts, many separate fuel cells are combined to form a fuel cell stack. They can be connected in a parallel circuit for higher current and in series for higher voltage.
Fuel cells are very efficient. If supplied with pure hydrogen they can convert 80 percent of the hydrogen’s energy content to electric power. If the electricity is used by an electric motor and inverter in a fuel cell vehicle – which are about 80 percent efficient – the overall efficiency is 64 percent. This compares to the approximate 20 percent energy conversion efficiency of the typical gasoline-fueled vehicle, providing yet another reason why fuel cell vehicles hold such promise for the future.
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