What they all said.

You can have a 6 Volt cell if you try hard enough.
Practicality may be low.

See table on this page

  http://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)

Difference between the values for each half cell here gives overall
cellpotential.
Some may not be especially mutually realisable in a practical battery

From:  http://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page=
)
--=20
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