You've probably heard about fuel cells. In 2003, President Bush announced a program called the Hydrogen Fuel Initiative (HFI) during his State of the Union Address. This initiative, supported by legislation in the Energy Policy Act of 2005 (EPACT 2005) and the Advanced Energy Initiative of 2006, aims to develop hydrogen, fuel cell and infrastructure technologies to make fuel-cell vehicles practical and cost-effective by 2020. The United States has dedicated more than one billion dollars to fuel cell research and development so far.
So what exactly is a fuel cell, anyway? Why are governments, private businesses and academic institutions collaborating to develop and produce them? Fuel cells generate electrical power quietly and efficiently, without pollution. Unlike power sources that use fossil fuels, the by-products from an operating fuel cell are heat and water. But how does it do this?
In this article, we'll take a quick look at each of the existing or emerging fuel-cell technologies. We'll detail how polymer electrolyte membrane fuel cells (PEMFC) work and examine how fuel cells compare against other forms of power generation. We'll also explore some of the obstacles researchers face to make fuel cells practical and affordable for our use, and we'll discuss the potential applications of fuel cells.
If you want to be technical about it, a fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity.
The other electrochemical device that we are all familiar with is the battery. A battery has all of its chemicals stored inside, and it converts those chemicals into electricity too. This means that a battery eventually "goes dead" and you either throw it away or recharge it.
With a fuel cell, chemicals constantly flow into the cell so it never goes dead -- as long as there is a flow of chemicals into the cell, the electricity flows out of the cell. Most fuel cells in use today use hydrogen and oxygen as the chemicals.
In the next section, we will look at the different types of fuel cells.
Types of Fuel Cells
The fuel cell will compete with many other energy conversion devices, including the gas turbine in your city's power plant, the gasoline engine in your car and the battery in your laptop. Combustion engines like the turbine and the gasoline engine burn fuels and use the pressure created by the expansion of the gases to do mechanical work. Batteries convert chemical energy back into electrical energy when needed. Fuel cells should do both tasks more efficiently.
There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by their operating temperature and the type of electrolyte they use. Some types of fuel cells work well for use in stationary power generation plants. Others may be useful for small portable applications or for powering cars. The main types of fuel cells include:
Polymer exchange membrane fuel cell (PEMFC)
The Department of Energy (DOE) is focusing on the PEMFC as the most likely candidate for transportation applications. The PEMFC has a high power density and a relatively low operating temperature (ranging from 60 to 80 degrees Celsius, or 140 to 176 degrees Fahrenheit). The low operating temperature means that it doesn't take very long for the fuel cell to warm up and begin generating electricity. We?ll take a closer look at the PEMFC in the next section.
Solid oxide fuel cell (SOFC)
These fuel cells are best suited for large-scale stationary power generators that could provide electricity for factories or towns. This type of fuel cell operates at very high temperatures (between 700 and 1,000 degrees Celsius). This high temperature makes reliability a problem, because parts of the fuel cell can break down after cycling on and off repeatedly. However, solid oxide fuel cells are very stable when in continuous use. In fact, the SOFC has demonstrated the longest operating life of any fuel cell under certain operating conditions. The high temperature also has an advantage: the steam produced by the fuel cell can be channeled into turbines to generate more electricity. This process is called co-generation of heat and power (CHP) and it improves the overall efficiency of the system.
Alkaline fuel cell (AFC)
This is one of the oldest designs for fuel cells; the United States space program has used them since the 1960s. The AFC is very susceptible to contamination, so it requires pure hydrogen and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be commercialized.
Molten-carbonate fuel cell (MCFC)
Like the SOFC, these fuel cells are also best suited for large stationary power generators. They operate at 600 degrees Celsius, so they can generate steam that can be used to generate more power. They have a lower operating temperature than solid oxide fuel cells, which means they don't need such exotic materials. This makes the design a little less expensive.
Phosphoric-acid fuel cell (PAFC)
The phosphoric-acid fuel cell has potential for use in small stationary power-generation systems. It operates at a higher temperature than polymer exchange membrane fuel cells, so it has a longer warm-up time. This makes it unsuitable for use in cars.
Direct-methanol fuel cell (DMFC)
Methanol fuel cells are comparable to a PEMFC in regards to operating temperature, but are not as efficient. Also, the DMFC requires a relatively large amount of platinum to act as a catalyst, which makes these fuel cells expensive.
In the following section, we will take a closer look at the kind of fuel cell the DOE plans to use to power future vehicles -- the PEMFC.
Polymer Exchange Membrane Fuel Cells
The polymer exchange membrane fuel cell (PEMFC) is one of the most promising fuel cell technologies. This type of fuel cell will probably end up powering cars, buses and maybe even your house. The PEMFC uses one of the simplest reactions of any fuel cell. First, let's take a look at what's in a PEM fuel cell:
In Figure 1 you can see there are four basic elements of a PEMFC:
- The anode, the negative post of the fuel cell, has several jobs. It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. It has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst.
- The cathode, the positive post of the fuel cell, has channels etched into it that distribute the oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.
- The electrolyte is the proton exchange membrane. This specially treated material, which looks something like ordinary kitchen plastic wrap, only conducts positively charged ions. The membrane blocks electrons. For a PEMFC, the membrane must be hydrated in order to function and remain stable.
- The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. It is usually made of platinum nanoparticles very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM.
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Figure 2. Animation of a working fuel cell
Figure 2 shows the pressurized hydrogen gas (H2) entering the fuel cell on the anode side. This gas is forced through the catalyst by the pressure. When an H2 molecule comes in contact with the platinum on the catalyst, it splits into two H+ ions and two electrons (e-). The electrons are conducted through the anode, where they make their way through the external circuit (doing useful work such as turning a motor) and return to the cathode side of the fuel cell.
Meanwhile, on the cathode side of the fuel cell, oxygen gas (O2) is being forced through the catalyst, where it forms two oxygen atoms. Each of these atoms has a strong negative charge. This negative charge attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two of the electrons from the external circuit to form a water molecule (H2O).
This reaction in a single fuel cell produces only about 0.7 volts. To get this voltage up to a reasonable level, many separate fuel cells must be combined to form a fuel-cell stack. Bipolar plates are used to connect one fuel cell to another and are subjected to both oxidizing and reducing conditions and potentials. A big issue with bipolar plates is stability. Metallic bipolar plates can corrode, and the byproducts of corrosion (iron and chromium ions) can decrease the effectiveness of fuel cell membranes and electrodes. Low-temperature fuel cells use lightweight metals, graphite and carbon/thermoset composites (thermoset is a kind of plastic that remains rigid even when subjected to high temperatures) as bipolar plate material.
In the next section, we'll see how efficient fuel-cell vehicles can be.
Fuel Cell Efficiency
Pollution reduction is one of the primary goals of the fuel cell. By comparing a fuel-cell-powered car to a gasoline-engine-powered car and a battery-powered car, you can see how fuel cells might improve the efficiency of cars today.
Since all three types of cars have many of the same components (tires, transmissions, et cetera), we'll ignore that part of the car and compare efficiencies up to the point where mechanical power is generated. Let's start with the fuel-cell car. (All of these efficiencies are approximations, but they should be close enough to make a rough comparison.)
If the fuel cell is powered with pure hydrogen, it has the potential to be up to 80-percent efficient. That is, it converts 80 percent of the energy content of the hydrogen into electrical energy. However, we still need to convert the electrical energy into mechanical work. This is accomplished by the electric motor and inverter. A reasonable number for the efficiency of the motor/inverter is about 80 percent. So we have 80-percent efficiency in generating electricity, and 80-percent efficiency converting it to mechanical power. That gives an overall efficiency of about 64 percent. Honda's FCX concept vehicle reportedly has 60-percent energy efficiency.
If the fuel source isn't pure hydrogen, then the vehicle will also need a reformer. A reformer turns hydrocarbon or alcohol fuels into hydrogen. They generate heat and produce other gases besides hydrogen. They use various devices to try to clean up the hydrogen, but even so, the hydrogen that comes out of them is not pure, and this lowers the efficiency of the fuel cell. Because reformers impact fuel cell efficiency, DOE researches have decided to concentrate on pure hydrogen fuel-cell vehicles, despite challenges associated with hydrogen production and storage.
Next, we'll learn about the efficiency of gasoline- and battery-powered cars.
Gasoline and Battery Power Efficiency
The efficiency of a gasoline-powered car is surprisingly low. All of the heat that comes out as exhaust or goes into the radiator is wasted energy. The engine also uses a lot of energy turning the various pumps, fans and generators that keep it going. So the overall efficiency of an automotive gas engine is about 20 percent. That is, only about 20 percent of the thermal-energy content of the gasoline is converted into mechanical work.
A battery-powered electric car has a fairly high efficiency. The battery is about 90-percent efficient (most batteries generate some heat, or require heating), and the electric motor/inverter is about 80-percent efficient. This gives an overall efficiency of about 72 percent.
But that is not the whole story. The electricity used to power the car had to be generated somewhere. If it was generated at a power plant that used a combustion process (rather than nuclear, hydroelectric, solar or wind), then only about 40 percent of the fuel required by the power plant was converted into electricity. The process of charging the car requires the conversion of alternating current (AC) power to direct current (DC) power. This process has an efficiency of about 90 percent.
So, if we look at the whole cycle, the efficiency of an electric car is 72 percent for the car, 40 percent for the power plant and 90 percent for charging the car. That gives an overall efficiency of 26 percent. The overall efficiency varies considerably depending on what sort of power plant is used. If the electricity for the car is generated by a hydroelectric plant for instance, then it is basically free (we didn't burn any fuel to generate it), and the efficiency of the electric car is about 65 percent.
Scientists are researching and refining designs to continue to boost fuel cell efficiency. One approach is to combine fuel cell and battery-powered vehicles. Ford Motors and Airstream are developing a concept vehicle powered by a hybrid fuel cell drivetrain named the HySeries Drive. Ford claims the vehicle has a fuel economy comparable to 41 miles per gallon. The vehicle uses a lithium battery to power the car, while the fuel cell recharges the battery.
Fuel-cell vehicles are potentially as efficient as a battery-powered car that relies on a non-fuel-burning power plant. But reaching that potential in a practical and affordable way might be difficult. In the next section, we will examine some of the challenges of making a fuel-cell energy system a reality.
Fuel Cell Problems
Fuel cells might be the answer to our power problems, but first scientists will have to sort out a few major issues:
Chief among the problems associated with fuel cells is how expensive they are. Many of the component pieces of a fuel cell are costly. For PEMFC systems, proton exchange membranes, precious metal catalysts (usually platinum), gas diffusion layers, and bipolar plates make up 70 percent of a system's cost [Source: Basic Research Needs for a Hydrogen Economy]. In order to be competitively priced (compared to gasoline-powered vehicles), fuel cell systems must cost $35 per kilowatt. Currently, the projected high-volume production price is $73 per kilowatt [Source: Garland]. In particular, researchers must either decrease the amount of platinum needed to act as a catalyst or find an alternative.
Researchers must develop PEMFC membranes that are durable and can operate at temperatures greater than 100 degrees Celsius and still function at sub-zero ambient temperatures. A 100 degrees Celsius temperature target is required in order for a fuel cell to have a higher tolerance to impurities in fuel. Because you start and stop a car relatively frequently, it is important for the membrane to remain stable under cycling conditions. Currently membranes tend to degrade while fuel cells cycle on and off, particularly as operating temperatures rise.
Because PEMFC membranes must by hydrated in order to transfer hydrogen protons, researches must find a way to develop fuel cell systems that can continue to operate in sub-zero temperatures, low humidity environments and high operating temperatures. At around 80 degrees Celsius, hydration is lost without a high-pressure hydration system.
The SOFC has a related problem with durability. Solid oxide systems have issues with material corrosion. Seal integrity is also a major concern. The cost goal for SOFC?s is less restrictive than for PEMFC systems at $400 per kilowatt, but there are no obvious means of achieving that goal due to high material costs. SOFC durability suffers after the cell repeatedly heats up to operating temperature and then cools down to room temperature.
The Department of Energy?s Technical Plan for Fuel Cells states that the air compressor technologies currently available are not suitable for vehicle use, which makes designing a hydrogen fuel delivery system problematic.
In order for PEMFC vehicles to become a viable alternative for consumers, there must be a hydrogen generation and delivery infrastructure. This infrastructure might include pipelines, truck transport, fueling stations and hydrogen generation plants. The DOE hopes that development of a marketable vehicle model will drive the development of an infrastructure to support it.
Storage and Other Considerations
Three hundred miles is a conventional driving range (the distance you can drive in a car with a full tank of gas). In order to create a comparable result with a fuel cell vehicle, researchers must overcome hydrogen storage considerations, vehicle weight and volume, cost, and safety.
While PEMFC systems have become lighter and smaller as improvements are made, they still are too large and heavy for use in standard vehicles.
There are also safety concerns related to fuel cell use. Legislators will have to create new processes for first responders to follow when they must handle an incident involving a fuel cell vehicle or generator. Engineers will have to design safe, reliable hydrogen delivery systems.
Researchers face considerable challenges. In the next section, we will explore why the United States and other nations are investing in research to overcome these obstacles.
Why Use Fuel Cells?
Why is the U.S. government working with universities, public organizations and private companies to overcome all the challenges of making fuel cells a practical source for energy? More than a billion dollars has been spent on research and development on fuel cells. A hydrogen infrastructure will cost considerably more to construct and maintain (some estimates top 500 billion dollars). Why does the president think fuel cells are worth the investment?
The main reasons have everything to do with oil. America must import 55 percent of its oil. By 2025 this is expected to grow to 68 percent. Two thirds of the oil Americans use every day is for transportation. Even if every vehicle on the street were a hybrid car, by 2025 we would still need to use the same amount of oil then as we do right now [Source: Fuel Cells 2000]. In fact, America consumes one quarter of all the oil produced in the world, though only 4.6 percent of the world population lives here [Source: National Security Consequences of U.S. Oil Dependency].
Experts expect oil prices to continue to rise over the next few decades as more low-cost sources are depleted. Oil companies will have to look in increasingly challenging environments for oil deposits, which will drive oil prices higher.
Concerns extend far beyond economic security. The Council on Foreign Relations released a report in 2006 titled "National Security Consequences of U.S. Oil Dependency." A task force detailed numerous concerns about how America's growing reliance on oil compromises the safety of the nation. Much of the report focused on the political relationships between nations that demand oil and the nations that supply it. Many of these oil rich nations are in areas filled with political instability or hostility. Other nations violate human rights or even support policies like genocide. It is in the best interests of the United States and the world to look into alternatives to oil in order to avoid funding such policies.
Using oil and other fossil fuels for energy produces pollution. Pollution issues have been in the news a lot recently -- from the film "An Inconvenient Truth" to the announcement that climate change and global warming would factor into future adjustments of the Doomsday Clock. It is in the best interest for everyone find an alternative to burning fossil fuels for energy.
Fuel cell technologies are an attractive alternative to oil dependency. Fuel cells give off no pollution, and in fact produce pure water as a byproduct. Though engineers are concentrating on producing hydrogen from sources such as natural gas for the short-term, the Hydrogen Initiative has plans to look into renewable, environmentally-friendly ways of producing hydrogen in the future. Because you can produce hydrogen from water, the United States could increasingly rely on domestic sources for energy production.
Other countries are also exploring fuel-cell applications. Oil dependency and global warming are international problems. Several countries are partnering to advance research and development efforts in fuel cell technologies. One partnership is The International Partnership for the Hydrogen Economy.
Clearly scientists and manufacturers have a lot of work to do before fuel cells become a practical alternative to current energy production methods. Still, with worldwide support and cooperation, the goal to have a viable fuel cell-based energy system may be a reality in a couple of decades.
Related HowStuffWorks Articles
More Great Links
- "Basic Research Needs for the Hydrogen Economy." Office of Science, Department of Energy.http://www.sc.doe.gov/bes/hydrogen.pdf
- Deutch, John, et al. "National Security Consequences of U.S. Oil Dependency." Independent Task Force Report No. 58.http://www.cfr.org/content/publications/attachments/EnergyTFR.pdf
- Garland, Nancy. "Fuel Cells Sub-Program Overview." U.S. Department of Energy. Dec. 19, 2008. (March 19, 2009)http://www.hydrogen.energy.gov/pdfs/progress08/v_0_fuel_cells_overview.pdf
- Goho, Alexandra. "Micropower Heats Up: Propane fuel cell packs a lot of punch." McGraw-Hill Encyclopedia of Science and Technology.
- Goho, Alexandra. "Special Treatment: Fuel cell draws energy from waste." McGraw-Hill Encyclopedia of Science and Technology.
- "Hydrogen Posture Plan." United States Department of Energy. http://www1.eere.energy.gov/hydrogenandfuelcells /pdfs/hydrogen_posture_plan.pdf
- Rose, Robert. "Questions and Answers about Hydrogen and Fuel Cells." Breakthrough Technologies Institute.http://www.fuelcells.org
- Testimony of David Garman, Under-Secretary of Energy. Committee on Energy and National Resources, United States Senate. http://www1.eere.energy.gov/office_eere/ congressional_test_071706_senate.html
- U.S. Department of Energy Hydrogen Programhttp://www.hydrogen.energy.gov