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May 19, 2016
Heat engines
May 19, 2016

Fuel cells

Fuel cells

Acentury or so ago, the number of cars on Earth numbered in the thousands. Today, there are something like a billion cars—roughly one for every seven people on the planet. Think of Earth as a giant gas station with only a limited supply of fuel and you’ll realize quite quickly that we have a problem. Many geologists think we’re reaching a point they call “peak oil” and, in the next few decades, supplies of gasoline (and everything else made from petroleum) will start to dwindle. If that happens, where will all our cars get their fuel from? The short-term fix is to get better fuel efficiency from existing cars. In the longer term, the solution may be to switch vehicles over from gasoline engines and diesel to electric fuel cells, which are a bit like batteries powered by hydrogen gas that never run flat. Silent and pollution free, they’re among the cleanest and greenest power sources yet developed. Are they all they’re promised to be? Let’s take a closer look at how they work!

What are fuel cells?

There are really just two ways to power a modern car. Most cars on the road today use an internal-combustion engine to burn petroleum-based fuel, generate heat, and push pistons up and down to drive the transmission and the wheels. Electric cars work an entirely different way. Instead of an engine, they rely on batteries that feed electric power to electric motors that drive the wheels directly. Hybrid cars have both internal-combustion engines and electric motors and switch between the two to suit the driving conditions.

Fuel cells are a bit like a cross between an internal-combustion engine and battery power. Like an internal-combustion engine, they make power by using fuel from a tank (though the fuel is pressurized hydrogen gas rather than gasoline or diesel). But, unlike an engine, a fuel cell doesn’t burn the hydrogen. Instead, it’s fused chemically with oxygen from the air to make water. In the process, which resembles what happens in a battery, electricity is released and this is used to power an electric motor (or motors) that can drive a vehicle. The only waste product is the water—and that’s so pure you can drink it!

Think of fuel cells as batteries that never run flat. Instead of slowly depleting the chemicals inside them (as normal batteries do), fuel cells run on a steady supply of hydrogen and keep making electricity for as long as there’s fuel in the tank.

How does a fuel cell make electricity from hydrogen?

What happens in a fuel cell is called an electrochemical reaction. It’s a chemical reaction, because it involves two chemicals joining together, but it’s an electrical reaction too because electricity is produced as the reaction runs its course.

A fuel cell has three key parts similar to those in a battery. It has a positively charged terminal (shown here in red), a negatively charged terminal (blue), and a separating chemical called an electrolyte in between the two (yellow) keeping them apart. (Think of the whole thing as a ham sandwich. The two terminals are the pieces of bread and the electrolyte is the ham in between.)

Here’s how a fuel cell produces electricity:

  1. Hydrogen gas from the tank (shown here as big brown blobs) feeds down a pipe to the positive terminal. Hydrogen is flammable and explosive, so the tank has to be extremely strong.
  2. Oxygen from the air (big turquoise blobs) comes down a second pipe to the negative terminal.
  3. The positive terminal (red) is made of platinum, a precious metal catalyst designed to speed up the chemistry that happens in the fuel cell. When atoms of hydrogen gas reach the catalyst, they split up into hydrogen ions (protons) and electrons (small black blobs). In case you’re confused: hydrogen ions are simply hydrogen atoms with their electrons removed. Since they have only one proton and one electron to start with, a hydrogen ion is the same thing as a proton.
  4. The protons, being positively charged, are attracted to the negative terminal (blue) and travel through the electrolyte (yellow) towards it. The electrolyte is a thin membrane made of a special polymer (plastic) film and only the protons can pass through it.

  5. The electrons, meanwhile, flow through the outer circuit.
  6. As they do so, they power the electric motor (orange and black) that drives the car’s wheels. Eventually, they arrive at the negative terminal (blue) too.
  7. At the negative terminal, the protons and electrons recombine with oxygen from the air in a chemical reaction that produces water.
  8. The water is given off from the exhaust pipe as water vapor or steam.

This type of fuel cell is called a PEM (different people say this stands for polymer exchange membrane or proton exchange membrane because it involves an exchange of protons across a polymer membrane). It’ll keep running for as long as there are supplies of hydrogen and oxygen. Since there’s always plenty of oxygen in the air, the only limiting factor is how much hydrogen there is in the tank.

Photo: Here’s what a fuel cell actually looks like. This is a typical proton exchange membrane (PEM) hydrogen fuel-cell that can produce 5 kilowatts (5000 watts) of power. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Fuel cell stacks

A single fuel cell produces only about as much electricity as a single dry-cell battery—nowhere near enough to power a laptop computer, let alone a car. That’s why fuel cells designed for vehicles use stacks of fuel cells linked together in a series. The total electricity they produce is equal to the number of cells multiplied by the power each cell produces.

Types of fuel cell

PEM fuel cells (sometimes called PEMFCs) are currently favored by engineers for powering vehicles, but they’re by no means the only design possible. Just as there are many kinds of batteries, each using different chemical reactions, so there are many kinds of fuel cell too. Spacecraft use a more primitive design called an alkaline fuel cell (AFC), while much greater amounts of power could be generated by an alternative design known as a solid-oxide fuel cell (SOFC). Microbial fuel cells have an extra feature: they use a tank of bacteria to digest sugar, organic matter, or some other fuel and produce either an electric current (which can be used to power a motor) or hydrogen (which can power a fuel cell in the usual way). Another possibility is to have a vehicle with a solar panel on the roof that uses the Sun’s electricity to split water into hydrogen and oxygen gases with an electrolyzer. These gases are then recombined in the fuel cell to produce electricity. (The advantage of doing things that way, rather than using the Sun’s energy directly, is that you can store up hydrogen in the daytime when the Sun’s shining and then use it to drive the fuel cell at night.)

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