Water in polymer electrolyte fuel cells: Friend or foe?

The complex role of water as reaction product, proton shuttle, and asphyxiant in polymer electrolyte fuel cells is discussed. Water is transported into the fuel cell as part of the reactants and it is also considered to be a product of the overall reaction. Water exists as a liquid in the polymer electrolyte that separates the anode and cathode, and equilibrates with gaseous reactants that flow through the electrodes and supporting structures in the fuel cell. Insufficient water in the polymer electrolyte and catalyst layers causes poor reactivity and transport of proton, and the excess heat produced by insufficient operation hastens the fuel cells degradation. Hydrogen in a polymer electrolyte fuel cell, or methanol in a directed methanol fuel cell is oxidized in the catalyst layer on the anode side of the fuel cell. Water in polymer electrolyte serves as the proton shuttle and improves the mobility of hydrogen ions.