The growing demand for energy and rising climate concerns call for an urgent need to decarbonize the highly polluting energy sector by adopting hydrogen economy. Polymer electrolyte membrane water electrolyzers (PEMWEs) are a promising technology to produce green hydrogen by water electrolysis at high efficiencies and low temperatures. The hydrogen evolution reaction (HER) on cathode has fast kinetics hence it can happen with low precious metal loadings. However, the Oxygen Evolution Reaction (OER) occurring on iridium-based (IrOx) electrocatalyst on the anode has extremely sluggish kinetics and imparts significant overpotential to the system. Hence, the widespread deployment of this technology is stifled today by the use of expensive and rare iridium and high catalyst loadings (~2 mg/cm2). Additionally, the absence of a stable and durable catalyst support leads to underutilization of the available catalyst. The interfacial contact between the catalyst layer, the proton conducting media, and the porous transport layer (PTL) morphology plays a major role in influencing PEMWE performance. The nature of these microscopic interfaces and their effect on different cell overpotentials and system durability are not well understood. Hence, understanding the factors affecting catalyst utilization by interfacial analysis and engineering better interfaces is imperative to realize GW scale deployment of these systems.This dissertation aims to characterize the nature of interfaces and its effect of bulk transport phenomenon primarily with the use of X-ray computed tomography. We developed the method for operando X-ray characterization of PEMWEs by designing and optimizing operando cell designs. With image analysis we developed formulation for calculating the Triple Phase Contact Area (TPCA) which quantifies the electrochemically active area that otherwise cannot be calculated with electrochemical techniques in full cells. We used the developed TPCA method along with electrochemical and modeling techniques to characterize the interface for two different porous transport layers (PTLs) and catalyst layers at various loadings. We show that low porosity sintered PTLs exhibit higher TPCA that results in improved kinetics. Radiography and modeling results indicate that oxygen taking multiple transport pathways through the PTL results in slug flow through the channels that reduces mass transport overpotential. We further this study using microporous layers (MPLs) on sintered PTLs. Along with interface characterization, for the first time, we showed time averaged oxygen pathways through PTLs using X-ray tomography. We showed how MPLs shift oxygen invasion patterns through PTLs and increase catalyst utilization by suppressing bubble masking. Based on the results, we suggest optimal MPL design strategies to enable low catalyst loadings. We developed an accelerated stress test protocol by understanding OER kinetics on IrOx catalysts and its effect on dissolution. Following the developed protocol, we concluded that most degradation is kinetically dominated resulting from loss of Ir from the cell. From XPS analysis, we saw Ir(III)-Ir(IV) redox shift during OER onset and potential cycling. Finally, we conducted 1000 hr durability tests and found that optimal MPL design can increase system lifetime and cause least voltage decay thus achieving a step forward in enabling large-scale green hydrogen production.