Fuel cells
by Chris Woodford. Last updated: August 30, 2011.
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!
Photo: Ford Motor Company's hydrogen fuel cell demonstration car (a modified Ford Focus).
Photo by courtesy of NASA Kennedy Space Center (NASA-KSC).
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!
Photo: Under the hood of Ford's hydrogen fuel cell car.
Photo by courtesy of Ford Motor Company and
US Department of Energy/National Renewable Energy Laboratory.
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:
- 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.
- Oxygen from the air (big turquoise blobs) comes down a second pipe to the negative terminal.
- 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.
- 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.
- The electrons, meanwhile, flow through the outer circuit.
- 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.
- At the negative terminal, the protons and electrons recombine with oxygen from the air in a chemical reaction that produces water.
- 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. Here's what a PEM fuel cell actually looks like:
Photo: A typical proton exchange membrane (PEM) hydrogen fuel-cell. This one 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.)
Why are fuel cells taking so long to catch on?
People have been heralding fuel cells as the next big thing in power
supplies since the 1960s, when the Apollo space
rockets first
demonstrated that the technology was practical. Four decades later,
there are hardly any fuel-cell cars on our streets—for a variety of
reasons. First, the world is geared up to producing gasoline engines by
the million, so they're naturally much cheaper, better tested, and more
reliable. You can buy an ordinary car for a few thousand
dollars/pounds, but a fuel-cell car will set you back hundreds of
thousands! Cost isn't the only problem. There's also a massive
oil-based economy to support gasoline engines: there are garages
everywhere that can service gasoline-powered cars and filling stations
all over the place to supply them with fuel. By contrast, hardly anyone
knows anything about fuel-cell cars and there are virtually no filling
stations supplying pressurized hydrogen.
Photo: It could be a while before hydrogen
filling pumps like this become commonplace.
Photo by courtesy of US Department of Energy.
Until oil becomes much more expensive, motorists will have little or
no incentive to switch to fuel-cell cars. Even then, there are rival
technologies that may stop fuel-cell cars from ever catching on. We
might stick with internal combustion engines, but power them with biofuels. Or it might turn out more efficient
to build electric cars with onboard batteries that you charge up at
home. Or perhaps a mass switch to hybrid cars, running gasoline engines
and electric motors, will extend world oil supplies long enough for us
to come up with an entirely new technology—like
nuclear-powered cars.
No-one knows what the future holds, but one thing is certain: petroleum
will be playing a much smaller part in it. The sooner we embrace
alternatives—electric cars, biofuels, fuel cells, or whatever—the better.
Further Reading
On this website
On other websites
- Hydrogen fuel cell: A Californian government website explaining the basics of fuel cells and their drawbacks.
- US Department of Energy: Fuel cell vehicles: A US government guide to the pros and cons of fuel-cell cars, including how they work, what's good and bad about them, and what kind of fuel economy you can expect. There are also test-drive videos of some current hydrogen fuel-cell cars.
- California Fuel Cell Partnership: Industry lobby promoting the use of fuel-cell technology. Lots of good background information on the site.
Articles
Books
- Hydrogen cars—blimp your ride: Cambridge physics professor David MacKay doesn't believe the future lies with hydrogen cars. His view is that hydrogen is "an inefficient energy carrier" (because it takes so much energy to make hydrogen fuel) with "a whole bunch of practical defects" (because hydrogen is very hard to store easily and transport safely).
- The Hydrogen Economy: Opportunities and Challenges by Michael Ball and Martin Wietschel. Cambridge University Press, 2009. Are hydrogen cars the way forward? What obstacles do we need to overcome first?.
- The Hydrogen Economy by Jeremy Rifkin. J.P. Tarcher/Penguin, 2003. A more general guide to a world filled with hydrogen-powered, fuel-cell cars.
