In situ quantification of the in-plane water content in the Nafion~? membrane of an operating polymer-electrolyte membrane fuel cell using ~1H micro-magnetic resonance imaging experiments

摘要

Spatial, quantitative, and temporal information regarding the water content distribution in the transverse-plane between the catalyst layers of an operating polymer-electrolyte membrane fuel cell (PEMFC) is essential to develop a fundamental understanding of water dynamics in these systems. We report ~1H micro-magnetic resonance imaging (MRI) experiments that measure the number of water molecules per SO_3H group, λ, within a Nafion~?-117 membrane between the catalyst stamps of a membrane-electrode assembly, MEA. The measurements were made both ex situ, and inside a PEMFC operating on hydrogen and oxygen. The observed ~1H MRI T_2 relaxation time of water in the PEM was measured for several known values of λ. The signal intensity of the images was then corrected for T_2 weighting to yield proton density-weighted images, thereby establishing a calibration curve that correlates the ~1H MRI density-weighted signal with λ. Subsequently, the calibration curve was used with proton density weighted (i.e., T_2-corrected) signal intensities of transverse-plane 'H MRI images of water in the PEM between the catalyst stamps of an operating PEMFC to determine λ under various operational conditions. For example, the steady state, transverse-plane λ was 9±1 for a PEMFC operating at ～26.4mWcm~(-2) (～20.0 mA, ～0.661 V, 20 ℃, flow rates of the dry H_2(g) and O_2(g) were 5.0 and 2.5 mL min~(-1), respectively).