How Fuel Cells Work

While fuel cells can be composed of different materials, they all generally work the same way. To be technical, fuel cells are electrochemical energy conversion devices. This means they convert chemical energy into electrical energy. As pointed out elsewhere, this is similar to how batteries work and very different from how internal combustion engines work.

Internal combustion engines simply burn fuel to create an explosion. The explosion is used to perform mechanical work by moving a piston. Energy from the momentum of the piston is then used to turn the transmission and operate other components of the car. Thus, internal combustion engines (ICEs) convert chemical energy into mechanical energy, which is rather inefficient due to the fact that most energy from burning is lost in the form of heat. The theoretical efficiency of an ICE is 58%, but they seldom reach over 25% in most applications. The same argument applies to any other process that relies on burning a fuel to produce energy. This would include the generation of electricity and heat for homes.

Batteries are strictly storage devices. They store energy in the form of chemical energy, which can be converted into electrical energy when needed. Batteries can be used over and over again, because electrical energy can be applied to create chemical energy in the charging process and that chemical energy can then be reconverted into electrical energy during the discharge process. This cycle can often be repeated several hundred times before the chemicals become exhausted and the battery is no longer useful.

Basics of Fuel Cell Operation

Fuel cells can be envisioned as a hybrid between ICEs and batteries. Fuel cells do not burn fuel, but rather disassemble it to create electricity. Thus, they are like batteries in that they convert chemical energy to electrical energy. However, unlike batteries, fuel cells are usually not recharged (though it is possible to run them in reverse). Rather, they depend on a constant supply of fuel to run, which is how they are similar to ICEs. Overall, fuel cells have a theoretical efficiency of 90%, reaching 40 to 60% in common usage.

To convert chemical energy to electrical, fuel cells are built in a manner very similar to that of a battery. They contain an anode, cathode, and electrolyte. The anode in a fuel cell is always negatively charged and the cathode is always positively charged when energy is being created (this is reversed if the fuel is being run in reverse). The electrolyte, which can be solid, liquid, or specialized polymer, is the heart of the fuel cell and what makes it work. The nature of the electrolyte determines the type of fuel cell.

The electrolyte separates the anode from the cathode. Because it is semi-permeable, it allows certain things to pass through it and not others. In the case of fuel cells, the electrolyte allows certain charged ions (like hydroxyl ions or protons) to past through, but not negatively charged electrons. This essentially traps electrons on the anode side of the fuel cell, which creates an electrical gradient we commonly refer to as voltage. Creating a supply of electrons is the first step in the operation of a fuel can takes place at the anode.

At the anode, a molecule is split to create positively and negatively charged particles. The positive particles are free to enter the electrolyte but the electrons are not. The result is a build-up of negative charge around the anode that is attracted to the cathode, but which cannot reach it. This creates a voltage gradient as indicated above. Voltage is the force that propels electrons through metal wires, which is what we commonly call electricity. For the electrons in a fuel cell to move, they must be provided with a path to the cathode.

To get to the cathode, an extra circuit is added that allows the electrons to bypass the electrolyte. As they flow through this external circuit, pushed by the voltage gradient, the electrons can be used to do work. This is how a fuel harnesses electricity. Once the electrons reach the cathode, they are reunited with the positively charged particles and combined with oxygen to produce water.

Fuel Cell

Solid oxide fuel cells actually work in a slight different way. While their products are ultimately the same and the electrons flow in the same direction, it is the movement of particles through the electrolyte that is different. Rather than positive particles moving from anode to cathode, negative particles (usually oxygen ions) move from cathode to anode. Once at the anode, the oxygen ions are able to oxidize a fuel to produce water and electrons. The electrons then flow through an external circuit to the cathode where they participate in the reaction again.
Solid Oxide Fuel Cell

Types of Fuel Cells

While the general operation of all fuel cells is the same, the exact components used can differ. Most commonly, hydrogen is used as a fuel because, when combined with oxygen at the cathode, only water is produced. In fuel cells that use fossil fuels rather than hydrogen, water is still produced at the cathode, but the breakdown of the fossil fuel at the anode also produces carbon dioxide. This is generally deemed undesirable because carbon dioxide is a greenhouse gas. However, because hydrogen is difficult to produce, transport, and store, hydrogen only fuel cells are currently impractical.

The other functional impact that the choice of components has on fuel cells is operating temperature. Some fuel cells, generally those that burn hydrogen, can operate at standard temperatures. Other fuel cells, however, have to operate at temperatures as high as 1000 C (1800 F). This impacts start-up time, reliability, and efficiency.

There are roughly 20 different types of fuels cells, each with slightly different operating characteristics. The specifics of each type of fuel cell and how it operates are discussed on separate pages for each.

Name

Fuel

Temperature (Degrees Celsius)

Polymer Electrolyte Membrane Fuel Cells

Proton Exchange Membrane

Hydrogen

50 to 220

Direct Formic Acid

Formic Acid

< 40

Direct Methanol

Methanol

< 40

Reformed Methanol

Methanol

250 - 300

Direct Ethanol

Ethanol

< 40

Reformed Ethanol

Ethanol

250 - 300

Microbial

Organic Matter

< 40

Solid Fuel Cells

Tubular Solid Oxide

Oxygen

850 – 1100

Protonic Ceramic

Hydrogen

700

Direct Carbon

Multiple (Coal most commonly)

700 – 850

Planar Solid Oxide

Oxygen

500 – 1100

Alkaline Fuel Cells

Metal Hydride

Hydrogen

0 C

Zinc-air

Zinc

< 40

Direct borohydride

Potassium borohydride/Sodium borohydride

70

Molten Carbonate Fuel Cell

Molten Carbonate

Hydrogen/Natural Gas

600 – 650

Phosphoric Acid Fuel Cell

Phosphoric Acid

Hydrocarbons (various)

150 - 200

Other

Magnesium-Air

Hydrocarbon

-20 - 50