Using redox chemistry as a source of mobile electrical energy is inspired! I mean..without this you'd NEVER be able to say "I'm on the train...!".
Two half equations with different E^o values is all that it takes. Thes can be [++] or [+-] or [--].Whether this cell turns out to be "practical" or not depends on many things...not least the states of the reagents used!
Inspect the two E^o vlaues: one will be more negative, even if they are both positive.
The couple which is more negative becomes the ....negative [!] pole of the cell. Well it would do , wouldn't it.
The process which makes this electrode negative is the one in which [negative] electrons are lost...oxidation.
The other couple must therefore be the POSITIVE pole, where Reduction happens.
The REDUCTION couple is placed on the RIGHT and the Oxidation [LOSS] of electrons on the LEFT
Ecell = E red - E ox
Please remember the SIGN and UNITS [V] for your answer.
The cell emf will be positive. This means that the cell reaction is spontaneous in the forward direction, and when the circuit is made current will flow until equilibrium is reached, at which point the cell emf becomes zero.
There is no connection between the magnitude of Ecell and the RATE of the reaction...so even a cell with a very hign voltage may not be practical in allowing current to be drawn from it.