In this work, as-received pristine GDLs were degraded (aged) through an accelerated ageing process by immersion into a 35% wt. solution of H 2 O 2 at 90 • C for 12 hours. Pristine and degraded GDLs were assembled in custom-built fuel cells for in operando synchrotron X-ray visualization. Electrochemical impedance spectroscopy (EIS) measurements were obtained, while synchrotron X-ray radiography was performed to investigate the effect of ageing on liquid water transport behavior. In both pristine and aged GDLs, the liquid water profile converged to an upper (terminal) threshold in the saturation pattern as the current density increased. The terminal steady state liquid water profile was established at a lower current density for the aged GDLs compared to that of the pristine GDLs. Higher water quantities of liquid water accumulated at the GDL/flow field channel and MPL/carbon fiber substrate interfaces of the fuel cell with an aged GDL compared to the fuel cell with a pristine GDL. The aged GDLs exhibited more liquid water accumulation at lower current densities. Increases in liquid water accumulation and reduced performance were attributed to the reduction in hydrophobicity of the GDL.

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In this thesis, the relative humidity (RH) of the cathode reactant gas was investigated as a factor which influences gas diffusion layer (GDL) liquid water accumulation and mass transport-related efficiency losses over a range of operating current densities in a polymer electrolyte membrane (PEM) fuel cell. Limiting current measurements were used to characterize fuel cell oxygen transport resistance while simultaneous measurements of liquid water accumulation were conducted using synchrotron X-ray radiography. GDL porosity distributions were characterized with micro-computed tomography (μCT). The work presented here can be used by researchers to develop improved numerical models to predict GDL liquid water accumulation and to inform the design of next-generation GDL materials to mitigate mass transport-related efficiency losses. This work also contributes an extensive set of concurrent performance and liquid water visualization data to the PEM fuel cell field that can be used for validating multiphase transport models.iii

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We have conducted a series of experiments designed to measure the flashover strength of various azimuthally symmetric 45 vacuum-insulator configurations. The principal objective of the experiments was to identify a configuration with a flashover strength greater than that of the standard design, which consists of a 45 polymethyl-methacrylate (PMMA) insulator between flat electrodes. The thickness d and circumference C of the insulators tested were held constant at 4.318 and 95.74 cm, respectively. The peak voltage applied to the insulators ranged from 0.8 to 2.2 MV. The rise time of the voltage pulse was 40 -60 ns; the effective pulse width [as defined in Phys. Rev. ST Accel. Beams 7, 070401 (2004)] was on the order of 10 ns. Experiments conducted with flat aluminum electrodes demonstrate that the flashover strength of a crosslinked polystyrene (Rexolite) insulator is 18 7% higher than that of PMMA. Experiments conducted with a Rexolite insulator and an anode plug, i.e., an extension of the anode into the insulator, demonstrate that a plug can increase the flashover strength by an additional 44 11%. The results are consistent with the Anderson model of anode-initiated flashover, and confirm previous measurements. It appears that a Rexolite insulator with an anode plug can, in principle, increase the peak electromagnetic power that can be transmitted across a vacuum interface by a factor of 1:18 1:44 2 2:9 over that which can be achieved with the standard design.

In this work, in-operando synchrotron X-ray radiography was used to capture the changes in the liquid water saturation of gas diffusion layers (GDLs) during changes in operating current density. Through in-operando visualizations at high temporal and spatial resolutions, we observed that the liquid water saturation increased with increasing current density. Eventually, a threshold water content in the GDL was reached despite further increases in current density. A time lag between the change in current density and the onset of increasing GDL water content was also observed. Current density consistently reached a steady state value before the GDL water content reached steady state, and the trends in liquid water distributions in the MPL were distinct from, yet influential to the accumulation in the substrate. We present a logarithmic growth function that describes the dynamic changes in GDL water content. The formulation of the GDL liquid water content transient response to changes in operating conditions provides a new metric for designing next generation fuel cell powertrains with fast dynamic responses. Through plane water thickness profiles were also used to show that the water accumulation patterns continued to evolve for up to 15 minutes at the low current density.

The presence of multiwall carbon nanotubes (MWCNTs) corresponded to a dispersion of carbon black particles in the microporous layer (MPL) of the polymer electrolyte membrane (PEM) fuel cell. The gas diffusion layer (GDL) with a MWCNT-based MPL exhibited larger pores and a higher porosity compared to a conventional GDL, and less MPL intrusion into the GDL substrate was observed with the MWCNTs-based MPL. The GDLs were evaluated in operando in a fuel cell that was customized for concurrent liquid water visualization (synchrotron X-ray radiography) and electrochemical characterization. The MWCNT-based fuel cell exhibited higher power densities and lower mass transport resistances compared to the fuel cell with the conventional GDL; however, a higher liquid water saturation was observed for the MWCNT-based GDL. Although the liquid water saturation in the MWCNTbased GDL was higher, its higher effective porosity led to superior performance compared to the conventional fuel cell. The use of the MWCNTs-based MPL resulted in improved oxygen transport in the fuel cell, particularly at high current densities. The achievement of effective water management in the polymer electrolyte membrane (PEM) fuel cell remains a challenge. Excess liquid water accumulation in the gas diffusion layer (GDL) hinders the transport of oxygen in the fuel cell. Reduced oxygen diffusion in the liquid water saturated GDL leads to efficiency losses and unstable performance.1,2 However, the commonly used bi-layered GDL (containing a carbon fiber substrate and a microporous layer (MPL)) has been shown to enhance liquid water management and the overall performance of the fuel cell. [3][4][5][6][7][8] In recent years, significant effort has been devoted to developing novel carbon fiber substrates and MPLs.9-18 Thomas et al. 9 developed a novel method for preparing hydrophobic GDLs via the electrochemical reduction of diazonium salts. GDLs were prepared with homogenous hydrophobicity distributions in the absence of pore structure modifications. Nguyen et al. 10 introduced the direct fluorination of the GDL for providing homogenous hydrophobicity. Less liquid water content was observed in these novel GDLs compared to the conventional GDL (SGL 24 BC) when visualized with soft X-rays. In the works of Forner-Cuenca et al.,11,12 a patterning of wettability was applied to a Toray GDL through radiation grafting. Hydrophilic regions facilitated preferential pathways for liquid water transport, while hydrophobic regions facilitated gas transport. Chevalier et al.13 created a novel GDL via electrospinning and radiation grafting. Ko et al.14 proposed a novel GDL through the dry deposition of hydrophobic silicone nanolayer on carbon fibers. The longterm operational stability and dynamic response of the GDL were improved.On the other hand, a number of works have focused on MPL additives for improving water management in fuel cells.19-29 While conventional MPLs consist of carbon black particles and a hydrophobic agent, such as polytetrafluoroethylene (PTFE), recent ...