by Chris Woodford. Last updated: September 20, 2016.
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
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
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.
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
Types of fuel cell
PEM fuel cells (sometimes called PEMFCs)
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
(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 (see box below). 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.)
Where will all the hydrogen come from?
For the last 150 years or so, virtually every car has
run on a liquid we rather confusingly call gas. But in the next 150
years, many people think cars will run on a real gas:
hydrogen. In theory, running cars off hydrogen is a great idea: it's the simplest
and most common chemical element and it makes up the vast majority
(something like three quarters) of the entire matter in the Universe.
Plenty for everyone, then! But there's a snag: poke about in the air
around you and you won't find much hydrogen at all—only about one
liter of hydrogen in every million liters of air. (In volume terms,
that's the same as hunting down about two liters of water randomly
mixed up in every Olympic swimming pool full). So where will all the vast clouds of hydrogen come from to run our global car fleet?
We'll need to make it ourselves from water, the magic substance that covers 70 percent of Earth's surface, is made partly from hydrogen.
Split good old H2O into its parts and you get H2 (hydrogen) and O2 (oxygen). How do you do it? With an electrolyzer!
Electrolyzers and electrolysis
An electrolyzer is a piece of electrochemical apparatus (something
that uses electricity and chemistry at the same time) designed to
perform electrolysis: splitting a solution into the atoms from which it's made by passing electricity through it. Electrolysis was
pioneered in the 18th century by British chemist Sir Humphry Davy
(1778–1829), who used a primitive battery called a
to discover a number of chemical elements including sodium and potassium.
An electrolyzer is a bit like a battery working in reverse:
- In a battery, you have chemicals packed into a sealed container with two
electrical terminals dipping into them. When you connect the
terminals into a circuit, the chemicals undergo reactions inside the
container and produce electricity that flows through the circuit.
(Read more about this in our main article on batteries.)
- In an electrolyzer, you place a solution in a container and dip two
terminals into it. You connect the terminals up to a battery or other
power supply and pass electricity through the solution. Chemical
reactions take place and the solution splits up into its atoms. If
the solution you use is pure water (H2O), you find it quickly splitting up
into hydrogen gas (at the negative electrode) and oxygen gas (at the
positive electrode). It's relatively easy to collect and store these
gases for use in future.
Photo: Demonstrating hydrogen power. Light (from the Sun) hits a solar cell (the blue rectangle on the left),
making electricity. An electrolyzer uses this electrical energy to split water into oxygen and hydrogen
(collected in the test tubes in the middle of the picture). The hydrogen is then fed into a fuel cell (metal
box on the right), which produces electricity
and lights a lamp (right). Photos by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
How does an electrolyzer work?
Here's how a very simple electrolyzer makes hydrogen gas from water:
- A battery connects the positive terminal (sometimes called the anode) to the negative terminal (or cathode) through an
electrolyte. In a simple laboratory experiment, the electrolyte could be pure water. In a real electrolyzer, performance
is improved considerably by using a solid polymer membrane as the electrolyte, which allows ions to move through it.
- When the power is switched on, water (H2O—shown here as two red blobs joined to one green one) splits into positively charged hydrogen ions (hydrogen atoms missing electrons, shown in red) and negatively charged oxygen ions (oxygen atoms with extra electrons, shown in green).
- The positive hydrogen ions are attracted to the negative terminal and recombine in pairs to form hydrogen gas
- Likewise, the negative oxygen ions are drawn to the positive terminal and recombine in pairs there to form oxygen gas
Why are fuel cells taking so long to catch on?
Photo: It could be a while before hydrogen
filling pumps like this become commonplace.
Photo by courtesy of US Department of Energy.
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, until recently, a fuel-cell car would have set you back hundreds of
thousands. (Toyota's "relatively affordable" Mirai finally became widely available in 2016
at a cost of just under US$60,000, twice the price of its hybrid Prius.)
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. The "hydrogen economy" is a far-off dream.
It's easy to see how a world full of hydrogen cars might work.
We'd have lots of electrolyzer factories all over the place making
hydrogen gas from water. Now gases take up a vast amount more
space than liquids or solids, so we'd need to turn the hydrogen
gas into liquid hydrogen, making it easier to transport and store,
by compressing it to a high pressure. Then we'd transport the hydrogen to gas stations ("hydrogen stations"?)
where people could pump it into their cars, which would be powered by fuel cells instead of conventional
The trouble with hydrogen
But do you see the problem? Producing hydrogen by electrolysis uses energy—and quite a lot of it: we have to use electricity to split up water.
If we use typical solar cells to provide that electricity, they might be about 10 percent efficient,
while an electrolyzer might be 75 percent efficient, giving a miserable overall efficiency of just
7.5 percent. That's quite a poor start—and it's only the start!
We also use energy transporting hydrogen and compressing it (turning hydrogen gas into a liquid) so cars can carry enough of it in their tanks to go anywhere. That's a real problem because the energy density of
hydrogen (the amount of energy it carries per unit of its volume or mass) is
only about a fifth that of gasoline. In other words, you need five times more to go as far
(assuming your hydrogen car is heavy as your gasoline one, which may not be the case—because gasoline cars need heavy engines and transmissions). Another problem is that hydrogen
is difficult to store for long periods because its extremely
tiny molecules easily leak out of most containers—and since
hydrogen is flammable, leaks can cause horrific explosions.
And then, of course, there are all the inefficiencies at the opposite end of the process, when a fuel-cell
car turns hydrogen back into electricity to power the electric motors that drive its wheels.
Hydrogen isn't a fuel
"...hydrogen is a hyped-up bandwagon... Hydrogen is not a miraculous source of energy; it's just an
energy carrier, like a rechargeable battery. And it is a rather inefficient energy
carrier, with a whole bunch of practical defects."
Professor David MacKay
Sustainable Energy Without the Hot Air
Hydrogen is not, itself a fuel, but simply a way of transporting fuel made by some other process.
So it's better to compare it to batteries (another way of packaging and transporting energy)
than to gasoline (a genuine fuel). All told, today's hydrogen cars are considerably less efficient than the best electric cars running off batteries and often less efficient than ordinary gasoline or diesel engine vehicles!
We could use solar cells to do the electrolysis of water "for
free," but we could just as easily store the same energy in batteries and use those to power our cars instead.
Fuel-cell cars sound promising, but if battery cars really are better, hydrogen may turn out to be an expensive
distraction from the important business of switching the world from fossil fuels
to renewable energy.
Anything but oil?
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—maybe even
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.