Applications of Magnetic Resonance Imaging for the Investigation of Water Refilling of Embolized Xylem Vessels in Plant Stem Segments and of Water Management in Operating Polymer Electrolyte Membrane Fuel Cells

Novel applications of magnetic resonance imaging (MRI) for the investigation of water refilling of embolized xylem vessels in plant stem segments, and for the investigation of water management in an operating polymer electrolyte membrane fuel cell (PEMFC) are presented.;The second project described in this thesis involved the application of 1H MRI for the investigation of water management in an operating PEMFC. Spatial, quantitative, and temporal information regarding the water distribution in the transverse plane between the catalyst layers of an operating PEMFC is essential to develop a fundamental understanding of water dynamics in these systems. 1H micro-MRI experiments that measure the number of water molecules per SO3H group, λ, within a Nafion ® 117 membrane between the catalyst stamps of a membrane electrode assembly are reported. The measurements were made both ex situ and inside a PEMFC operating on hydrogen and oxygen. The observed 1H MRI T2 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 T2 relaxation 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., T2-corrected) signal intensities of transverse plane 1H MR images of water located in the PEM between the catalyst stamps of an operating PEMFC to determine λ under a steady-state operational condition. The proposed method allows the determination of in situ in-plane λ values under various operational conditions; such values are important information for understanding the role of water in operating PEMFCs and for water management. In addition, water in the gas diffusion layers of the operating PEMFC was investigated using an ultra-short echo-time MRI method.;This thesis research demonstrates that MRI is a powerful and versatile technique for investigating many important problems such as those encountered in the plant sciences and in fuel cell research.;In the first project, water refilling of embolized xylem vessels in grapevine stem segments was investigated using high-resolution 1H MRI experiments. Determining temporal and spatial patterns of water refilling of embolized xylem vessels, and the rate of water ascent in these vessels, is critical to the understanding of embolism repair in living vascular plants. High-resolution 1H MRI experiments were performed on embolized grapevine stem segments while they were subjected to water refilling at two different applied xylem pressures. MRI difference images show that, with exceptions, vessels located near the bark tend to refill faster than do inner ones, hinting that vessel position within the cross section of the stem may affect the refilling process within the vessel. An MRI method for determining the water ascent velocity in each individual embolized xylem vessel is presented. The relationship between the experimentally determined ascent velocity and the vessel diameter is examined; no clear correlation is observed indicating that other parameters must also influence the refilling process. The experimental results suggest that theoretical modeling, in which homogenous vessel diameter distribution and a regular vessel arrangement can be assumed, is necessary. A theoretical model that assumes steady-state bubble dissolution is presented; model calculations show that vessel diameter and position within the cross section of the stem influence the refilling process within the vessels.