What is a PEM fuel cell?

Polymer (or proton) electrolyte membrane (PEM) fuel cells are used in vehicles to create electricity by combining a hydrogen fuel source and oxygen in the air. These fuel cells' only waste product is water.

A membrane in the PEM cell permits only protons from the hydrogen to pass into the cell, as electrons would short the circuit. PEM fuel cells have traditionally been much more expensive than other options and initially were only used in space missions, but research aimed at bringing down production costs has made them potentially viable for a much wider range of uses, including mobile phones.