Chemguide: Support for CIE A level Chemistry


Learning outcome 6.4(a)

This statement is about batteries and fuel cells.

Before you go on, you should find and read the statement in your copy of the syllabus.


It is very difficult to know exactly what CIE want for this statement, because although it is based on a similar statement in earlier syllabuses, nothing had ever been directly asked about it up to November 2013.


A simple fuel cell

There is no suggestion in the syllabus that you should know how a fuel cell works, but you can't really talk about its advantages unless you have some idea. Read the next bit, but don't waste time learning it.

One of these types is called a Proton Exchange Membrane Fuel Cell (PEMFC), or a Polymer Electrolyte Membrane Fuel Cell (also PEMFC). There is a simplified diagram below:

Essentially, this has two metal electrodes covered with a fine layer of metal catalyst, for example platinum. This catalyses the reactions at the two electrodes.

Sandwiched between these is a polymer-based membrane through which protons (hydrogen ions) can diffuse.

One electrode is exposed to hydrogen gas; the other to oxygen or air.

The catalyst on the electrode exposed to the hydrogen catalyses the reaction:

You may notice that I have doubled that equation on the diagram above. I will explain why in a moment.

On the electrode exposed to the oxygen, this happens:

So, overall, at the left-hand electrode, hydrogen molecules ionise to give hydrogen ions and electrons.

Those hydrogen ions diffuse through the membrane to the right-hand electrode, where they react with oxygen to make water.

The overall reaction in the fuel cell can be found by adding the two equations together, but because of the need for four electrons at the right-hand electrode, the hydrogen equation has to happen twice.

Giving:

Notice that the only waste product from this cell is water. This is a major advantage. It is also much more efficient in energy terms to get electricity directly from the reaction between hydrogen and oxygen than to burn the hydrogen to produce heat, and then use that to generate electricity.


Rechargeable batteries

The bit about rechargeable batteries is new.

I'm not sure what they want for this other than the obvious. Using rechargeable batteries saves long-term expense in buying new batteries, and saves wasting expensive finite resources when non-rechargeable batteries are thrown away.


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© Jim Clark 2011 (last modified April 2014)