Besides fuel, a fuel cell also requires an oxidant (usually air). Depending on the application and design, air provided to the fuel cell cathode can be at a low pressure or a high pressure. High pressure of the air improves the reaction kinetics and increases the power density and efficiency of the stack. But increasing the air pressure reduces the water-holding capacity of the air and therefore reduces the humidification require­ments of the membrane (PEMFC). It also increases the power required to compress the air to a high pressure and thereby reduces the net power available. At present, most fuel cell stacks for stationary power applications are designed for operating pressures in the range of 1—8 atm, while automotive fuel cell systems based on the PEMFC technol­ogy are designed to operate at lower pressures of 2—3 atm to increase power density and improve water management.