Polymer exchange membrane (PEM) fuel cell, also known as the proton exchange membrane fuel cell, uses a water-based, acidic polymer membrane as its electrolyte. It uses pure hydrogen, which is processed at the anode. On the cathode side, electrode combines the proton and electron with oxygen to produce water and heat as a byproduct. Oxygen can be supplied in a purified form or extracted from directly from the air. As a result, proton pass through the membrane to the cathode side and electron travels in an external circuit, generating the electrical output of the cell. For instance, in 2017, trucks and other heavy vehicles have become the most important sector for polymer electrolytes membrane fuel cells.