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Oxide conduction in Solid Oxide Fuel Cells (SOFC)

Dear Reader,

Recently I was asked about the mechanism by which thorium dioxide is reduced by lithium metal, this made me think more about metal oxides. I also happened to attend a series of talks recently which included some work on fuel cells (SOFC). This made me think more and I have drawn some diagrams to explain things.

Now lets start with the 111 face of zirconium dioxide. Below is a diagram of the oxide with orange for oxygen and blue for zirconium. You can see that the solid has a series of oxygen and zirconium atoms. We have got this by slicing a cube of zirconium dioxide to cut off a corner.

ZrO2 111 face

Now we take a different view of the solid, this is now a 110 face. We get a slightly different arrangement of the atoms on the surface. What we have done is to slice the cube along a diagonal which goes between two opposite edges.

Zirconium dioxide 110 slice

Now to improve the conductivity some of the 4+ zirconium ions have been replaced with 3+ ions such as yttrium ions. These ones are in brown, every time a pair of 3+ ions are used to replace zirconium ions an oxygen is lost from the solid. This is done to keep the solid electrically neutral. Below is a picture of a doped zirconium oxide.

Doped zirconium dioxide with one oxygen now missing, this is a "hole"

By moving the oxygens around we can move the hole through the solid, by this the solid can be made to conduct electricity. Please look at the following pictures.


What you should see is that the hole is moved from one side to the other.

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