Why is it important that the polymer membrane in a PEM fuel cell allows only protons to move through it?
It forces the electrons to move through an external circuit, where they can do work.
It prevents the protons from reacting with the compounds in the rest of the fuel cell.
It allows the protons to build up potential energy and release more energy when they react with oxygen.
It keeps the electrons from corroding the interior of the PEM fuel cell.