A membrane exchange humidifier employs a water permeable membrane
comprising a microporous polymer and a hydrophilic additive. In
operation, the membrane preferably has favorable water transmission
properties and resists transmission of reactant gas or other components.
The membrane is suitable for use even when permeable in its dry condition
to the wet or dry gases in the humidifier, and/or when the wet and dry
gases are of different composition. By wetting the membrane, the presence
of an amount of liquid water in the wet gas can reduce gas transmission
through the membrane to an acceptable level. The humidifier is useful in
fuel cell systems in which a reactant gas supply stream, such as the
oxidant supply stream, is humidified primarily using water vapor from a
fuel cell reactant exhaust stream. The humidifier is particularly
suitable for use in conjunction with solid polymer fuel cell systems. The
improved mechanical and welding properties of the membrane allow for a
simpler humidifier configuration.