Dynamic Measurement and Modeling of the Water Vapor Concentration during ac Impedance Measurements in Polymer Electrolyte Fuel Cells (PEFCs)

摘要

The build up of water concentration oscillations along the flow channels of a low humidity polymer electrolyte fuel cell (PEFC) during ac impedance measurements is investigated. A pseudo 2D PEFC transient model is presented to describe the effect of ac current on the local water concentration in the flow channels. Furthermore, the frequency response of the water concentration in the anode and cathode exhaust gas is measured in situ via tunable diode laser absorption spectroscopy. The results show a strong dependence of the local phase and amplitude on modulation frequency. At higher modulation frequencies the water concentration oscillation in the membrane and in the gas channels is very small (f ≥ 10Hz). However, at frequencies below f=1Hz the water concentration oscillations start to penetrate into the membrane, and water vapor concentration oscillations build up along the flow channels. This process governs the impedance response of a low humidity PEFC at low frequencies.