How Fuel Cells Work

If you want to be technical about it, a fuel cell is an electrochemical energy conversion device. Discovered in the 1800s, 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 more 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.

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.

A fuel cell provides a DC (direct current) voltage that can be used to power motors, lights or any number of electrical appliances.

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 Electrolyte Membrane Fuel Cell (PEMFC)

The PEMFC, also known as the proton exchange membrane fuel cells, 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.

Solid Oxide Fuel Cell (SOFC)

Factories or towns could receive their electricity from solid oxide fuel cells, which work best with large-scale stationary power generators. Fuel cells of this kind operate 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 that the fuel cells produce 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; this type was the first the United States space program used to produce electricity and drinkable water on-board rockets and shuttles. NASA favored this method because the alkaline cell has high efficiency and a low operating temperature. However, because it’s highly susceptible to contamination, it requires pure hydrogen and oxygen. And it is unlikely to be commercialized because of the high cost.

Molten Carbonate Fuel Cell (MCFC)

Like the SOFC, these fuel cells are also best suited for large, stationary power generators. They operate at over 600 degrees Celsius, so they can generate steam that can be used to generate more power. The cell uses a molten salt solution as a catalyst that separates hydrogen particles from a traditional fuel like natural gas.

This process means that the fuel cell doesn’t require external refining equipment, but the high operating temperature also makes it susceptible to corrosion. 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 been reliably used for the longest amount of time when it comes to hydrogen energy technology. This type of cell sees use in stationary power generation as well as industrial vehicles and buses. It operates at a higher temperature than proton exchange membrane fuel cells, so it has a longer warm-up time. This makes it unsuitable for use in cars. Phosphoric-acid systems are likely to fall out of favor because they are less efficient than other fuel cells and require a toxic gas as their catalyst.

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.

Reversible Fuel Cells

A reversible fuel cell combines two methods of energy production, uniting one of the previous fuel cell types with a solar or a wind generator. Like all other fuel, it produces energy and water vapor, but the water is then stored for later. During times of high wind or solar activity, the other side of the system produces power.

Some of that power can then be run back through the fuel cell to convert the stored water back to oxygen and hydrogen fuel through electrolysis. This system boasts very high efficiency, but is also massively complex and expensive to produce.

The polymer electrolyte 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:

Picture 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.

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.

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 researchers have decided to concentrate on pure hydrogen fuel-cell vehicles, despite challenges associated with hydrogen production and storage.

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 77 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 77 percent for the car, 40 percent for the power plant and 90 percent for charging the car. That gives an overall efficiency of 28 percent. The overall efficiency varies considerably depending on what sort of power plant is used. If a hydroelectric plant, for instance, generates the electricity for the car, then it is basically free (we didn't burn any fuel to generate it), and the efficiency of the electric car is about 69 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. Most modern hydrogen cars, such as the Honda Clarity, use a lithium battery and electric motors 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.

Fuel cells might be the answer to our power problems, but first scientists will have to sort out a few major issues.

Cost

A major issue that hydrogen energy production faces is the cost of fuel. The Department of Energy estimated that the 2020 cost of switching over to hydrogen electrolysis was a whopping $279 per kilowatt-hour of energy produced, and they expect this to drop to $182 by 2030. By comparison, an existing natural gas plant can produce one kilowatt-hour for only 55 cents.

These costs can be brought down using traditional fossil fuels instead of water to derive the hydrogen molecules, but that also defeats the purpose since more fossil fuels will create more pollution. When it comes to long-term efficiency and money-saving, reversible fuel cells as mentioned above may be the saving grace for hydrogen power.

Durability

Researchers must develop PEMFC membranes that are durable and can operate at temperatures greater than 100 degrees Celsius and still function at subzero ambient temperatures. A 100 degrees Celsius temperature target makes it possible 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.

Hydration

Because PEMFC membranes must by hydrated in order to transfer hydrogen protons, researchers must find a way to develop fuel cell systems that can continue to operate in subzero 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 SOFCs 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.

Infrastructure

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.

Most of the hydrogen supply in the U.S. is created as a byproduct of natural gas and petroleum refinement. The other reliable method of collecting hydrogen fuel is through electrolysis, which entails energizing fresh water and separating it into base components: hydrogen and oxygen. The downside to electrolysis is that it requires a secondary power source, reducing net energy produced.

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.

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.

The U.S. has already pumped billions of dollars into hydrogen fuel cell production, and will likely add billions more by the time they reach the end of that 2050 roadmap. The United States, and nations across the globe are investing in hydrogen for the same reason they’re investing in other renewable energy sources like wind and solar: to cut back on oil dependence. Although the U.S. is able to produce most of its own oil, the country still feels some of the effects through the globalized economy.

Burning fossil fuels also adds carbon gasses to our atmosphere at a much greater rate than can be filtered out naturally. We’ve seen the effects through our lifetimes, with ambient temperatures rising, extreme weather events becoming more common and respiratory problems arising in high-population urban centers. Sharply reducing fossil fuel consumption is the only way to maintain the Earth’s ideal environment.

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 through electrolysis, the United States could increasingly rely on domestic sources for energy production. This could also do away with some of the inefficiencies and environmental hazards of shipping fuel across international waters.

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.