What additions must you make to the cell for it to generate a standard emf?

Addition of salt bridge

To generate a standard electromotive force, an addition of salt bridge is essential to help maintain electrical neutrality within the internal circuit and prevent the cell from rapidly running its reaction to equilibrium.
In addition, salt bridge contains potassium sulfate, an inert electrolyte where its ions will diffuse into the separate half-cells to stabilize the building charges at the electrodes and produce electricity.