Diamond and graphite are both allotropes of carbon. However only graphite can be used as an electrode explain why this is so even though they are both forms of carbon?

Answer 1

Diamond is a tightly bound molecule of carbon atoms. Highly covalent bonds. Graphite, however, occurs in layers and electrons can be found in the areas between layers. These de-localized electrons are responsible for the electrical conductivity of graphite. Diamond is not a layered material.
Here is a site about graphite.
http://en.wikipedia.org/wiki/Graphite