Fuel cells operating on hydrocarbon fuels require water vapor injection into the fuel stream for fuel reforming and the prevention of carbon fouling. Compared to active water recovery systems, a passive approach would eliminate the need for a separate water source, pumps, and actuators, and thus reduce parasitic thermal losses. The passive approach developed in this paper employs a capillary pump that recovers the water vapor from the exhaust, while providing a diffusion barrier that prevents exhaust gases from entering the fuel stream. Benchtop proof tests have proven the feasibility of the passive fuel humidifier concept. and have provided a calibration factor for a computational design tool that can be used for industrial applications.