PEM fuel cells use a proton-conducting polymer membrane as the electrolyte and pure hydrogen as the fuel. PEM fuel cells were invented in the 1960s and have since been used in applications like submarines, portable power devices, and vehicles. A typical PEM fuel cell consists of an anode and cathode separated by a polymer electrolyte membrane. At the anode, hydrogen gas is oxidized to produce protons and electrons. The protons pass through the membrane to the cathode while the electrons are routed through an external circuit, producing electricity. At the cathode, oxygen and protons react to form water. PEM fuel cells are efficient, have high power density, and are well-suited for transportation and small stationary power applications.