HYDROGEN - A clean, green fuel for the
future?
Unless you live in your own little isolated bubble, you are
probably all too aware from reports in the media that our
continued and increased reliance on fossils fuels could be
accelerating global warming. Though there is still a great
deal of debate about exactly how much of an influence our
use of these fuels, in particular the amount of greenhouse
gases like carbon dioxide that we are pumping into the
atmosphere, is having on climate change, one this is certain:
fossil fuels are finite fuels and one day they will run out, or
become uneconomical to use! For many years now we have
been looking at developing sustainable and renewable
energy sources, using natural resources like the wind, sun,
water and the heat of the earth to generate power.
One area that has been of particular interest is the possible
use of hydrogen as a fuel. Hydrogen, the first element to be
formed in the early Universe, is the simplest element
consisting of just one proton and one electron. So how
exactly could this substance change the way we produce and
use energy?
What is the Hydrogen Economy?
The Hydrogen Economy is a term that has been around now
for many years and basically describes an economy in which
hydrogen replaces traditional fossil fuels as the main source
of energy. Energy from hydrogen could be used to generate
electricity for use around the home or office, or for more
mobile applications ranging from transport to portable
electronic devices.
Obviously making such a drastic change in the type of
primary energy source that we use would not be easy. A
whole new infrastructure for production, storage and
distribution would be required and existing systems would
need to be replaced. However, research teams around the
world are working right now on ways of overcoming these
obstacles one at a time.
The chief advantage of using hydrogen as a fuel is that
carbon dioxide is not produced at the site of end use.
However, this greenhouse gas may be generated during the
This article appeared in
Issue 28 of the SAS
newsletter in Spring 2008.
processes that are used to produce the hydrogen in the first
place. But, it is possible to produce hydrogen cleanly and
eliminate the generation of green house gases altogether.
How can we produce hydrogen?
Hydrogen is the most abundant element in the Universe;
however, its low density means that it is only found in the
Earth’s atmosphere, in its gaseous form, at a concentration
of about 1ppm. There are many abundant and easily
obtainable hydrogen-containing compounds, the most
important if which is water. Hydrocarbons such as methane
and indeed oil are also rich in hydrogen.
Hydrogen has been produced commercially for many years
by mixing methane with steam at a temperature of around
1000°C, where the following chemical reaction occurs:
CH4 + H20  3H2 +CO
Although this does produce lots of hydrogen gas, the byproduct, carbon monoxide is an important greenhouse gas
and this is exactly what the hydrogen economy aims to
eliminate.
This is the key issue with producing hydrogen as a fuel - how
can hydrogen be produced without generating greenhouse
gases?
There are really three main options for producing hydrogen.
Firstly, it can be extracted from methane in the reaction
described above and this technique has been used
commercially for many years for producing hydrogen for use
in the Haber Process to make ammonia. As mentioned
already though, this slightly defeats the object of using
hydrogen as a fuel, as greenhouse gases are produced as a
by-product of hydrogen generation. Secondly, hydrogen gas
can be filtered from natural gas where it occurs in its diatomic
form. Doing this requires the natural gas to be passed
through a filter with sufficiently small pores to only allow the
hydrogen through. Research is being carried out to develop
a material which is capable of doing this on a commercial
level. The downside to both of these options is that they still
rely on fossil fuels as the raw material from which hydrogen
is extracted. The third option could completely remove our
reliance on fossil fuels and uses water as the hydrogen
source. Electrolysis of water provides a sustainable source
of ‘clean’ hydrogen, but it does require the input of electrical
energy to drive the process. The ultimate solution to this
would be to use energy from a sustainable source (e.g.
geothermal energy, solar energy etc.). Producing hydrogen
in this way could be done centrally on a large scale for
distribution to end users or it could be produced locally and
used directly by the end user. In either case the hydrogen
produced would need to be stored, perhaps only for a short
amount of time and this brings with it a whole host of other
problems which need to be solved.
How can we store hydrogen?
One of the biggest hurdles to overcome if we are to use
hydrogen on a large scale is how to store it prior to use.
Hydrogen gas has a low density and storing it in its ambient
form would require tanks which would simply be impractical
in size. It is possible to store compressed hydrogen gas or
even liquefied hydrogen in tanks, but these methods also
have problems. The tanks needed to store the hydrogen
under pressure would need to be pretty substantial and very
heavy and the tanks needed to store liquid hydrogen would
need to be very well insulated to prevent it from evaporating.
All three of these techniques have one serious safety
consideration. Hydrogen is a very combustible element and
storage in either its liquid or gaseous form would be
dangerous (liquid hydrogen and liquid oxygen are mixed
together in rocket fuel!).
There are two other methods for storing hydrogen which are
less risky. In the first method hydrogen is stored chemically
as a hydride or other hydrogen-containing compound. In the
form of a stable metal hydride, for example, the hydrogen
can be stored and transported safely to the point of use,
where it can be liberated. Again this technique has problems
associated with it. At the moment high temperatures and
pressures are required to get hydrides to form in the first
place and to then decompose and liberate hydrogen for use.
The final method uses a solid material to contain hydrogen in
its molecular form. Storage materials include carbon
nanotubes and metal-organic frameworks. These materials
contain pores in which hydrogen gas can be stored. Since
the hydrogen is stored in its molecular form this method does
not have the same problems associated with it as using a
hydride and similar storage densities to liquefied hydrogen
can be achieved.
Once a safe and effective method of storing and transporting
hydrogen is found it may then be used as a fuel.
How can we use hydrogen?
The chief way of using hydrogen as a fuel is in a fuel cell.
Fuel cells have been around for many years and are
basically a way of converting chemical energy into electrical
energy. Unlike a battery, which is also an electrochemical
conversion device, fuel cells do not store energy, they
generate a DC voltage and must be connected to the
equipment they will power or to batteries which will store the
energy.
The hydrogen-oxygen fuel cell was invented in 1839 by Sir
William Grove. He knew that he could separate water into its
constituent elements using electrolysis and suggested that
electricity and water could be made by recombining
hydrogen and oxygen. Using his very primitive ‘gas voltaic
battery’ he proved his theory to be true.
A number of different types of fuel cells exist. Some of these
are better for large stationary power generation plants and
others are better for more portable devices.
Alkaline fuel cells (AFC) have been used by the United States
Space program since the 1960s and are the oldest design of
fuel cell. However they are very susceptible to contamination
and require the hydrogen and oxygen to be very pure.
Solid oxide fuel cells (SOFC) again use the combination of
hydrogen and oxygen to generate electricity but they do this
at very high temperatures (700-1000°C). When in
continuous use this type of system is very stable and could
be used to generate electricity on a large scale. Electricity
can be generated from a SOFC in two ways increasing their
overall efficiency. Firstly, electricity is generated by the
recombination of hydrogen and oxygen in the fuel cell itself,
and secondly the high temperature steam produced by the
fuel cell is used to drive a more conventional generator.
Proton exchange membrane fuel cells (PEMFC) offer the
most promise as they operate at room temperature and can
be made in a variety of sizes. It is also one of the simplest
types of fuel cell consisting of just four parts.
 The anode has channels etched in to it, to ensure that
the hydrogen gas is dispersed equally over its surface,
where the following reaction occurs:
H2
2H+ + 2e-
 The cathode is also etched to ensure that the oxygen is
distributed evenly. At the cathode the electrons from the
 external circuit recombine with hydrogen ions that have
diffused through the membrane and oxygen to produce
water.
 The catalyst facilitates the reaction between hydrogen
and oxygen and is usually made from platinum in the
form of nanoparticles coated on to carbon paper or cloth.
 The proton exchange membrane is the electrolyte
between the anode and cathode. It only allows the
passage of positively charged particles and forces
electrons to flow around the external circuit.
A schematic diagram of a PEMFC is shown below.
Where can hydrogen be used?
The main reason for using fuel cells is to reduce pollution,
whether this is from a power station or exhaust fumes from a
car. However, it is important the make sure that the fuel cells
used are as energy efficient as possible. In theory, using
pure hydrogen a fuel cell can achieve an efficiency of 80%.
In reality, when connected to systems which will convert the
electrical energy into mechanical work an overall efficiency of
about 64% is more realistic.
Fuel cell powered vehicles are not very common at the
moment and indeed it could be another 5 or 10 years before
they are a practical solution. In these vehicles the electricity
generated by the fuel cell would be used to power electric
motors, making the vehicles very quiet.
As well as being used to power cars one of the main
suggested uses of fuel cells is to power buses. Many cities
across the world are now looking at investing in this
technology for clean, pollution-free public transport.
It has also been suggested that one day we will all have a
bank of fuel cells in our home to generate electricity when we
need it to power our appliances.
Can we learn from the stars?
Using hydrogen in a fuel cell is just one way of producing
clean energy, but we could look to the starts to find an
alternative way of generating energy on a large scale.
At the heart of our Sun hydrogen is combining to produce
helium which generates huge amounts of energy. Nuclear
fusion has been the subject of scientific research for many
years and we may be getting closer to finding a way of using
hydrogen fusion on a practical scale.
The JET (Joint European Torus) project, located at the UK
AEA Culham Science Centre in Oxfordshire is home to the
largest and most powerful magnetic confinement fusion
device in the world and is carrying out fusion experiments on
a scale close to that of a possible commercial fusion reactor.
Where can I find out more?
Both Wikipedia and ‘How Stuff Works’ have good articles
looking at all aspects of using hydrogen as a fuel, however,
the data that is quoted by both of these sites is very focussed
on the USA.
The Hydrogen Materials Group in the Materials Department
at the University of Birmingham is carrying out research on
all aspects of hydrogen production, storage and use. They
even have a working fuel cell-powered narrow boat! You can
find out more at www.hydrogen.bham.ac.uk.
For more information about fusion you can visit
www.fusion.org.uk. The AEAs laboratories at Culham in
Oxfordshire offer a range of resources, activities and visits
for schools.