A four-fluid bipolar plate for a fuel cell includes a nonporous sub-plate and a porous sub-plate. The nonporous sub-plate includes a water management side, an opposing reactant side, and an internal coolant passage therebetween. The water management side includes a recessed region over an area approximately equal to the active area, and the porous sub-plate is nested and sealed in the recessed region. The porous sub-plate includes a reactant side and an opposing water management side. The water management side is in fluid communication with the water management side of the nonporous sub-plate.

Proposed are a fuel cell membrane humidifier and a fuel cell system having the same in which humidification by moisture exchange and cooling by heat exchange are performed in one membrane humidifier such that the fuel cell system can be simplified and be miniaturized. The fuel cell membrane humidifier includes a housing part having a space divided by a partition, a humidification module formed in a first portion of the divided space and having a plurality of hollow fiber membranes allowing a first fluid flowing thereinside to perform moisture exchange with a second fluid flowing thereoutside, a heat exchange module formed in a second portion of the divided space and configured to cool a first fluid flowing inside the heat exchange module, and a flow control part configured to actively control a flow direction of the first fluid.

A four-fluid bipolar plate for a fuel cell includes a nonporous sub-plate comprising a first reactant half-plate joined to a second reactant half-plate. The nonporous sub-plate includes an internal coolant passage network having coolant flow field passages extending across an active area of the fuel cell. The nonporous sub-plate defines fuel supply and fuel return internal manifolds, oxidant supply and oxidant return internal manifolds, water management supply and water management return internal manifolds, and coolant supply and coolant return internal manifolds. In one embodiment, a method of preventing corrosion at a carbon/metal interface in a fuel cell is disclosed. In other embodiments, a method of operating a four-fluid fuel cell in thermal boost mode is disclosed, and a method of accumulating and retaining product water in a four-fluid fuel cell is disclosed.