In a proton exchange membrane (PEM) fuel cell, one of several types under development, hydrogen reacts with oxygen in the presence of a platinum catalyst to create voltage between two electrodes. A thin membrane serves as a solid electrolyte, instead of the liquid electrolyte used in a lead-acid car battery. Each fuel-cell membrane is sandwiched between two bipolar plates (graphite sheets machined to create flow channels for hydrogen, air, and water). Each individual fuel cell “sandwich” produces just less than one volt of electricity. To produce a higher voltage, many cells are stacked together.

Hydrogen is fed through channels in one bipolar plate. A platinum catalyst starts a reaction at the anode (negative electrode) that strips the electron from each hydrogen atom. These electrons flow through a circuit, providing power. The protons (hydrogen ions) that are left behind flow through the polymer membrane to the platinum-coated cathode. At the cathode, these protons combine with oxygen drawn from the air through the other bipolar plate to form water.

Figure 10 Fuel Cell Stack