What chemical action within a battery generates the potential difference ?
The chemical reaction taking place within a battery is responsible for the electricity. When the battery is not connected to an external circuit the reaction within the battery doesn’t continue since the electrons at the negative terminal cannot flow anywhere. When the battery is connected to an external circuit the reaction takes place continuously and we get a constant current from the battery.
Thus, we can say the chemical reactions taking place within the battery constitutes the chemical action that generates potential difference in a battery.