Polymer electrolyte membrane or proton exchange membrane fuel cell where air and pure hydrogen are used to produce elelectrical energy.

PEM is short for Polymer Electrolyte Membrane Fuel Cell. Hydrogen gas is used in the anode as a fuel which is oxidized into protons and electrons. H2 ----  protons + e-

Protons migrate through the membrane and reach the cathodic regions where oxygen gas from the air is reduced through the oxygen reduction reaction  O2  + 4 protons + 4 electrons +2 H2O . The protons react with oxygen