Going back to the analogy given by Professor Lewin where he briefly describes the action of a Van de Graaff generator.
An uncharged conducting dome is separated from the ground using an insulator.
Near the ground negative charges (electrons) are sprayed onto a conveyor belt made of an insulator.
The electrons travel towards the conducting dome.
The electrons leave the belt and move onto the dome which now has a net negative charge.
The potential of the dome is now less than that of the ground and an electric field exists in the region between the dome and the ground.
The direction of the electric field is from the ground towards the dome.
As more and more electrons are collected by the dome the electric field gets stronger and stronger and the potential difference between the dome increases.
To get the electrons to move from the ground to the dome (negative potential relative to the ground) requires work to be done and that work is done by whatever is making belt move.
It could be you moving the belt using the chemical energy stored in the food that you have eaten, a motor using electrical energy, a water wheel using the gravitational potential energy, etc, and as a result of the work done moving the belt is an increase in the electrical potential energy stored in the Van der Graaff generator.
Now imagine a situation where the force between the electrons and the belt and the electrons on the dome is so large that the belts stops moving because whatever has been moving the belt cannot evert enough force to move the belt.
(If you have ever seen a demonstration which uses a Van de Graaff generator you may have noticed that the belt slows down as the charge on the dome increases due to the increased difficulty in moving charges onto the dome as the charge on the dome increases).
You now have the dome at a constant negative potential relative to the ground and no electrons are moving.
Hopefully you can understand the analogy of this process with that of a pump moving electrons from one place (the ground) to another (the dome) and the pump having a finite capability in terms of setting up a maximum (constant) potential difference (pressure difference) between the ground and the dome.
So overall there is a conversion of some form of energy into electric potential energy using a "pump" which moves the electrons.
Note that this pump moves the electrons in the direction of the electric field set up between the ground (positive) and the dome (negative) which is opposite to the direction you would normally expect them to move.
An equivalent thing happens in a cell with the “pump” powered by chemical energy and that pump moving electrons from one terminal (which we call the positive terminal = deficit of electrons) to the other terminal (negative terminal = surplus of electrons) until such a time that the electric field between the positive plate and the negative field is so strong that the electro-chemical reaction cannot move any more electrons in the same direction as the electric field (from the positive terminal to the negative terminal).
There is then a steady potential difference between the terminals of the cell which is called the emf of the cell.
There is also an electric field set up outside the cell between its terminals (in a direction from the positive terminal to the negative terminal) but no current flows because air is an insulator (it contains no mobile charge carriers).
If a conductor, which contains mobile charge carriers, is placed between the terminals of the call then electrons move from the negative terminal towards the positive terminal (in the opposite direction to that of the electric field which has been set up in the conductor).
The electron pump in the battery moves electrons from the positive terminal to the negative terminal to make up for the electrons which have left the negative terminal and travelled outside the cell to the positive terminal and in doing so keeps the potential difference between the terminals of the cell constant.