In situ measurements of water vapor partial pressure and temperature dynamics in a PEM fuel cell

A fast in situ sensor was developed for detection of water vapor partial pressure and temperature simultaneously in the flow channels of a proton exchange membrane (PEM) fuel cell. Utilizing tunable diode laser absorption spectroscopy with wavelength modulation, this technique determined the ratio of harmonic signals of spectral absorption using a software lock-in amplifier. A curve-fitting analysis ensured errors less than ±2.5% in water partial pressure and ±3°C in gas temperature. Measurements were taken at steady and dynamic operating conditions in a sE_rpentine channel prototypical PEM fuel cell while simultaneously sampling the anode and cathode gas channels near the inlet and outlet ports. External load and inlet humidity conditions were varied. With increasing current density, water vapor concentration increased toward both outlets, but the increase was more prominent on the cathode side. The temporal variation in water vapor during dynamic operation of the fuel cell was examined with a time resolution of 0.2 s. For sudden external load changes, transients in water concentration were observed near the anode outlet due to the increase in electro-osmotic drag through the membrane. This dynamic water transport was observed and can be used to characterize the step response of the cell to external changes in current.