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 types of 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 below shows
the main parts of a fuel cell
The diagram above shows an outline of a typical fuel cell. This one is a
hydrogen-oxygen
fuel cell.
In this cell hydrogen gas, the fuel, enters the porous
carbon anode. Here the hydrogen is
oxidised by the
catalysts on the anode surface 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 produced at the anode then
flow through the wire connected to the bulb, but it can be any electrical item.
The hydrogen ions (H+) diffuse across the semi-permeable 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 → 2H2O(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
half-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.
Starting from the left hand-side of the fuel cell in the diagram we can say that:
At the anode:
- The hydrogen fuel is fed into the fuel cell where it is
oxidised on the surface of the anode.
The catalyst which is
on surface of the anode will oxidise the
hydrogen gas. As we saw above once the hydrogen gas has been
oxidised it forms hydrogen ions (H+)
and releases 2 electrons.
- The hydrogen ions flow through the semi-permeable membrane
and head towards the cathode. The electrons
flow through the
wire (the external circuit) and light up the bulb; as shown in the diagram.
At the cathode:
- Oxygen gas is fed into the fuel cell
at the cathode. On the surface of the
cathode the oxygen gas reacts with
the hydrogen ions from the electrolyte. The oxygen atoms also gain electrons from the external circuit flowing through the bulb.
- The oxygen atoms on the cathode are
reduced to form water as the only product of this reaction.
- The overall reaction that happens inside the fuel cell is simply
hydrogen reacting with oxygen to form water
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 the main 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 also 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 friendly 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. Fuel cells have longer working life than batteries and so could be considered more environmentally
friendly and sustainable. Hydrogen-oxygen fuel cells also produce water as the only waste product and so are
much more environmentally friendly than fossil fuels, no carbon dioxide, sulfur dioxide
or nitrogen oxides are produced, or batteries which contain some corrosive and toxic chemicals.
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