Chemistry only
Fuel cells
One of the problems with cells and batteries is that the chemicals that react to produce
useful electrical energy eventually run out and the
cell/battery has to be disposed of, that
is unless the cell is of a type which can be recharged.
However even rechargeable cells
can only be recharged so many times before they need replacing. A potential solution to this
problem is to use a fuel cell. A fuel cell is similar to an ordinary
cell/battery but the
chemicals which react to produce electrical energy are not contained inside the
battery but are
supplied continually from an external source.
There are several different types of fuel cell but they have many similarities in common. A fuel,
commonly hydrogen, alcohol or methane is supplied to the fuel cell. All
fuel cells contain an anode,
a cathode and an electrolyte. In
cells the anode has a negative charge and the
cathode is positively charged, this is the
other round round in electrolysis, so be careful not to get them mixed up! The
electrolyte varies in different fuel cells but common
electrolytes are phosphoric acid or potassium hydroxide solution. The
anodes and cathodes
are made from porous carbon and they have a coating of small catalyst particles on their
surface. The anode catalyst is normally platinum and the cathode
catalyst is commonly nickel.
The anode, cathode and electrolyte are kept apart by a semi-permeable membrane. This membrane
allows ions to move across it but not electrons.
The diagram above shows an outline of a typical fuel cell. This one is a hydrogen-oxygen
fuel cell.
In this cell hydrogen gas enters the porous carbon anode. Here the hydrogen is oxidised by the
catalysts on the anode to form hydrogen ions. This can be shown as:
Anode half-equation: H2(g) → 2H+(aq) + 2e
This is oxidation- loss of electrons
The electrons flow through the wire, in this case to the bulb, but it can be any electrical item.
The hydrogen ions (H+) diffuse across a membrane and enter the electrolyte solution.
At the cathode oxygen gas (from the air) is fed in and here it undergoes a catalysed reaction on
the cathode to form water. The oxygen gas reacts with the hydrogen ions from the
electrolyte and the electrons which flow from the anode to the cathode via the external circuit.
Cathode half-equation: O2(g) + 4H+(aq) + 4e → 2H2(g) This is reduction- gain of electrons
The overall equation is simply obtained by combining the anode and cathode half-equations, although we need to multiply the anode
equation by two to balance off the electrons:
Overall cell equation: oxidation at the anode and reduction at cathode- redox reaction
cathode half-equation O2(g) + 4H+(aq) + 4e → 2H20(l)
anode half-equation 2H2(g) → 4H+(aq) + 4e
Overall equation: 2H2(g) + O2(g) + → 2H20(g)
The cell operates at temperatures between 150-2000C, this means that the water
which is produced can be converted into steam which can be used for other purposes e.g.
generating electricity or supplying hot water to homes, this can increases the efficiency
of the fuel cell from around 50% to as much as 90% efficient.
How fuel cells work
The image below shows how a fuel cell works
Uses of fuel cells
Fuel cells can be made in a large range of sizes and power outputs, from units
which can be used to
generate electricity for factories, homes and offices to small scale devices which can charge your
phone and other electronic devices. Car manufacturers including Honda, Toyota, and Hyundai have
models of cars running on hydrogen fuel cells.
Advantages and disadvantages of fuel cells
The main drawback with fuel cells is their fuel, hydrogen.
Though hydrogen is an excellent fuel,
which burns to release large amounts of clean energy, hydrogen is not widely available. Large
amounts of energy are needed to split water
using electrolysis as a source of hydrogen. Unless
the energy used for this electrolysis comes from renewable sources then the carbon footprint is
likely to be large. Hydrogen can also be obtained from methane but as this is a fossil fuel the
long term sustainability of this is questionable. The extraction of hydrogen from methane also
requires large amounts of energy This energy would need to be obtained from renewable sources.
It is unlikely that there is sufficient renewable energy resources available to supply enough
hydrogen to convert all cars, bikes, trains etc away from traditional fossil fuels.
The costs to the country of converting all garages from selling petrol and diesel to hydrogen
would be massive. There would also be considerable costs in storing and transporting hydrogen
safely around the country in sufficiently large quantities. The other main problem with hydrogen
is that it is a gas and so would need to be cooled and stored under pressure as a liquid. This
again costs money and also you have the additional problem that hydrogen is a very explosive gas.
Would there be a high risk of death and serious injury in fitting hydrogen tanks to all motor vehicles?
However provided that the hydrogen needed for the fuel cells is produced in an environmentally way then
fuel cells, with their high efficiencies, high energy outputs and low emissions are viable alternative
to providing clean energy for the future.
Fuel cells have no moving parts so are reliable and useful for providing power for remote and isolated
communities and businesses. Fuel cells can provide more energy and are cheaper to produce than
batteries, they are also not as polluting or harmful to the environment to produce. They will
also will have longer working life than batteries and so could be considered more environmentally
friendly and sustainable. Fuel cells also produce water as the only waste product and so are
much more environmentally friendly than either fossil fuels, no carbon dioxide, sulfur dioxide
or nitrogen oxides are produced, or batteries which contain some corrosive and toxic chemicals.
Practice questions
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