There seems to be no one clean, renewable energy technology that suits every location in the world, at least not one that is currently available. But what about the future? Is there a method of producing power that is cleaner and does not rely as heavily on coal, oil, and natural gas?

One likely candidate is the fuel cell. Fuel cells continuously convert chemicals into electricity through a series of chemical reactions. The advantages are many. Fuel cells are quiet and more energy efficient than burning fossil fuels for energy. Many are up to 60% efficient currently and with some adaptations of technology could reach close to 100%. For comparison, turbines run about 40-50% efficient, while internal combustion engines are 10-20% efficient.

Most fuel cells use hydrogen for fuel, rather than petroleum. Therefore, they produce little or no polluting emissions, depending on the type of fuel cell. Water and, for some types of cells, heat are the main by-products of fuel cell operations. If the cell can use fuel that is not purified, some carbon dioxide (CO2) is produced. However, the amount of these emissions is much lower than that produced by burning coal or gasoline.

This does not mean that fossil fuels are completely eliminated, though. The hydrogen needed to power fuel cells must come from somewhere. But hydrogen does not exist independently in the environment. One readily available source of hydrogen is fossil fuels. This means that some emissions will be produced in the process of making hydrogen. Thus, fuel cells are not a completely clean source of power, not yet.

Fuel cells currently provide electricity and water for manned space vehicles from the United States. Hydrogen-fueled cars are being developed worldwide, especially in China, Japan, and Europe. But fuel cells must still cross a few hurdles before they make it into common use. A hydrogen fuel stock needs to be developed. That means we will still rely on oil, natural gas, and coal, not for energy but to create the hydrogen needed to power fuel cells. Other issues for fuel cells include size, weight, durability, and cost. A closer look at different types of fuel cells will help us see if fuel cells will power the future.

Key Parts

A fuel cell is a relatively simple device. It contains two electrodes: the anode (the negative electrode) and the cathode (the positive electrode). All the chemical reactions occur at the electrodes. To speed up the chemical reaction, a catalyst is used to coat the two electrodes. The fuel cell also contains an electrolyte to carry the charged particles from one electrode to the other. The fuel cell needs two more things to make the reaction occur: oxygen, and of course fuel. Most of the fuel cells under development use hydrogen for fuel.

The hydrogen fuel cell uses chemical reactions to create electricity. Hydrogen fuel enters the cell, and through a series of chemical reactions combines with oxygen to produce water and electricity. Click for animation.

In the proton exchange membrane, or polymer electrolyte membrane, (PEM) fuel cell, hydrogen fuel enters at the anode, where it divides into hydrogen ions and electrons. The electrons carry the charge outside the cell, while the ions move through the cell. At the cathode the ions react with oxygen and the electrons, to form water.

How It Works

This is a simplified diagram of a fuel cell. These are used in space flights. Illustration courtesy of NASA.

All hydrogen fuel cells work on the same principles, with variations depending on the type of cell. Hydrogen fuel enters the cell at the anode. Oxidation occurs at the anode, when the positive ions (protons) are removed from hydrogen atoms by a chemical reaction that is assisted by the catalyst. The anode is porous, so the hydrogen can pass through it. The cathode is also porous, so the oxygen can pass through it.

The electrolyte conducts the charged ions from the anode to the cathode. However, the electrons are forced to go to an outside circuit. There, they create an electrical current that can be used for power.

Reduction occurs at the cathode, when the electrons combine with the positive hydrogen ions and oxygen to form water. The water drains out of the fuel cell. If pure hydrogen is used as the fuel, no other emissions are created. If the hydrogen is not pure, then small amounts of other gases are produced as well. Some fuel cells operate at very high temperatures, and therefore produce a lot of heat.

Fuel Cell Systems

A single fuel cell does not produce enough electricity for most uses. To produce adequate power, fuel cells are arranged in fuel cell stacks. The size of the stack depends on the amount of electricity needed for the intended use, the type of fuel cell, the size of the fuel cell, the operating temperature, and the pressure of the gases involved.

Some fuel cell systems use pure hydrogen as fuel. However, many use hydrogen that has not been purified, or hydrocarbons such as methanol, gasoline, or diesel. The problem with these fuels is that they contain molecules such as hydrogen sulfide and carbonyl sulfide. These molecules can “poison” the fuel cell, significantly reducing its effectiveness. Fuel cells that use these unpurified hydrogen or hydrocarbon fuels require a fuel processor. The fuel processor converts hydrogen-rich fuel into a form that the fuel cell can use. The type of processing depends on the fuel. Less-than-pure hydrogen only needs to be filtered. Hydrocarbons must be converted to hydrogen gas and carbon compounds, a mixture called reformate. In some cases the reformate also needs to be processed to remove impurities before it can be used in the fuel cell. Fuel cells that run at very high temperatures can do the reforming inside the cell. Reforming creates some CO2 but less than that from a typical combustion engine.

The electricity created by fuel cells is direct current, which flows in one direction. However, most electricity supplied to homes and businesses is alternating current, which flows in both directions in cycles that alternate. The flow, voltage, and frequency of the electric current must also be controlled. So a fuel cell needs current inverters and conditioners to adapt the electricity produced.

The final piece of a fuel cell system is the heat recovery system. Depending on the type of cell, fuel cells generate some to a lot of heat during operation. This heat can be used to create steam to operate a turbine and generator, and so make more electricity. This increases the energy efficiency of a fuel cell system.

Types of Fuel Cells

There are a number of fuel cells being developed in laboratories around the world. Each uses different electrolytes and catalysts, and runs at different operating temperatures. Other variations include energy efficiency and material durability. Some fuel cells must run on pure hydrogen, whereas others can take the hydrogen from fossil fuel. These variations make some fuel cells better suited for use in cars or buses, while other cell types work better for nonmobile uses such as power generation.

One of the first fuel cells to be developed was the alkaline fuel cell. This type of cell has produced electricity and water in space for the U.S. space program since the Apollo missions of the 1960s. Another early fuel cell still in use is the phosphoric acid fuel cell. More than 200 units of this first-generation fuel cell currently operate. Most make electricity for buildings, but some power vehicles, such as city buses.

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types of fuel cells…

Originally developed in the mid-1960s, molten carbonate fuel cells are now being developed for industrial, utility, and military use. The main reason: they operate at high temperatures, about 650°C (roughly 1,200°F). Researchers have worked with solid oxide cells for many years, starting in the 1950s. Solid oxide cells are also high-temperature cells. Because of the high operating temperatures, this type of fuel cell will likely be used to generate power for buildings and public utilities.

The polymer electrolyte membrane, also called proton exchange membrane (PEM), fuel cell is the one being developed to replace gasoline and diesel engines. Like some of the others, these cells originally provided electricity for spaceflights. PEM cells went up with America’s Gemini spaceflights in the 1960s. These cells operate at a relatively low temperature of 100°C (212°F). Thus they reach full operational capacity relatively quickly, a plus for use in transportation.

Direct methanol fuel cells are in the early stages of development, about three to four years behind the other fuel cell technologies. Small appliances, including laptop computers and mobile phones, may one day be powered by direct methanol cells. Another type under development is the zinc-air fuel cell. The zinc-air fuel cell is a kind of hybrid of a traditional battery and a fuel cell. The anode, made of zinc, gets used up over time and needs to be replaced. This cell is already in use in electric vehicles.

Most fuel cells use hydrogen that has been derived from fossil fuels. But what if we could replace fossil fuels with a renewable hydrogen fuel? That’s the idea behind regenerative fuel cells. The hydrogen fuel would be made by splitting water. Water produced by the fuel cell would be turned back into hydrogen through electrolysis. The electricity for these processes would come from renewable sources, such as solar power or wind energy. This approach could be used with any type of hydrogen fuel cell. While this eliminates fossil fuels and pollutants, right now there is no infrastructure for this type of hydrogen-based power. However, the National Aeronautics and Space Administration (NASA), the U.S. space agency, is developing a small-scale regenerative system powered by solar panels, to be used in space.

History

This is a diagram of William Robert Grove’s fuel cell. Illustration courtesy of Royal Society, National Museum of Natural History.

Fuel cells have been around a long time. The earliest experiments date back to 1838. William Robert Grove, a lawyer/scientist from Wales, created the “Grove cell.” This device had a platinum electrode immersed in nitric acid and a zinc electrode in zinc sulfate. It generated 12 amps of current at 8 volts. Grove called this a wet cell battery. He then created a fuel cell using two platinum electrodes. One end of each electrode sat in sulfuric acid, while the other ends were sealed in containers holding hydrogen and oxygen. The device produced a steady current between the two electrodes, and an increasing amount of water in the sealed containers. Grove was able to decompose and reform water with this device. He combined a number of these electrode sets into a “gas battery.”

In 1889, British chemists Ludwig Mond and Charles Langer used air and industrial coal gas in an attempt to build a practical version of Grove’s gas battery. They called this device a “fuel cell.”

British engineer Francis Thomas Bacon modified the design of Mond and Langer’s fuel cell in 1932. He substituted nickel gauze for the platinum in the electrodes and replaced the sulfuric acid electrolyte with alkali potassium, which is less corrosive. He called this the “Bacon cell”; this device was an early version of the alkaline fuel cell. It was not until 1959 that Bacon finally built a machine that could produce a significant amount of power. His machine made 5 kilowatts, to power a welding machine.

That same year, Harry Karl Ihrig, an engineer working for the U.S. farm equipment manufacturer Allis-Chalmers, built the first fuel cell-powered tractor. Ihrig created a fuel cell stack of 1,008 cells, which propelled a 20-horsepower tractor.

The next champion of fuel cell energy was the U.S. space agency NASA. In the early 1960s, NASA needed a way to provide electricity for manned spaceflights. Fuel cells provided the answer. They were safer than nuclear power, cheaper than solar power, and lighter than batteries. Interest in using fuel cells on the Earth grew in the United States with the oil import problems of the 1970s. Research continues to find the best combination of fuel, electrodes, and electrolyte for all kinds of applications.

Issues

Hydrogen fuel cells are still a long way from supplying power for every need. The number-one issue is cost: right now fuels cells are vastly more expensive than current internal combustion engines that run automobiles. They are also more expensive than any of the power sources that produce electricity. The most commonly used fuel cells cost about  US $4,500 per kilowatt; a diesel-powered generator costs between US $800 and US $1,500 per kilowatt, and a natural-gas-powered turbine costs US $400 per kilowatt or less. Automobile engines cost even less, between US $25 and US $30 per kilowatt. So, the operating costs of fuel cells need to drop significantly for them to even be considered for use in routine, day-to-day power production.

Cost is not the only issue. Presently, fuel cells still rely on fossil fuels for hydrogen. If another source of hydrogen develops, fuel cells could become a truly clean source of energy.

Furthermore, there is little knowledge of how durable and reliable the equipment is over the long run, particularly for the range of temperature conditions a vehicle faces. The size and weight of the whole system poses another problem for transportation applications. Besides the fuel cell stack, the lower-temperature cells require a processor, fuel tanks, air compressors and expanders, and sensors. Current internal combustion engines are more compact.  

The heat produced by even lower-temperature cells must be either recovered for reuse or cooled and released. Finally, there is no hydrogen supply system in place.

All this will eventually be resolved, but for now, fuel cells remain useful in limited applications.

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