Operando µ-Raman study of the membrane water content in the polymer electrolyte membrane fuel cell: Effects of gas flow-field geometry and temperature

Abstract

The energy conversion efficiency of the polymer electrolyte membrane fuel cell needs improved thermal and water managements. Here, the actual water concentration of the NafionⓇ membrane in the fuel cell working at constant stoichiometry and relative humidity is measured by operando Raman microspectroscopy. Through-plane water content profiles with μm resolution are obtained across the membrane at locations corresponding to the gas distribution channel and under the lands for the current collection, at the center of the active surface. The influence of the cell working temperature, of the current density and of the geometry of the gas feed channel (serpentine vs. parallel) are investigated. The combined measurement of the cell resistance and mass balance allow to establish relationships between the local water distribution throughout the cell and electrochemical performances. The membrane water content lowers with the increase of both current density and temperature regardless to the flow-field channel geometry. The membrane dehydration with current is ascribed to the concomitant raise of pressure losses and the spontaneous increase of the fuel cell inner temperature, with the last prevailing when the cell is managed at low temperatures. In that case, the distribution of water at the channel/lands scale has a distinct effect on the electrochemical behavior and performance of the cell. The parallel geometry exhibits easier accumulation of water at the under-lands location, which induces detrimental local water condensation but limits the membrane dehydration with current. The larger pressure losses generated by the serpentine geometry allow less inhomogeneous water distribution at the channel/lands scale, lower mass-transport over-voltage and, thus, a more efficient use of the active surface. The cell electrochemical behavior results from the interplay between the hydration of the membrane and the repartition of water, in such a way that the parallel geometry exhibits better performances at high current density when the cell is managed at low temperature. The serpentine design shows better performances for a large number of temperature and current conditions, namely when the cell is operated at the standard t = 80 °C.