Identifying the physical properties related to the primary functions of the porous transport layer (PTL) (thermal, electrical, and mass transport) is necessary for understanding the impact of PTL design choices on electrolyzer performance. In this thesis, two-dimensional (2D) representations of three common PTLs were fabricated on microfluidic chips in order to study the impact of PTL microstructure on multiphase transport via visualization of air bubble transport in liquid-saturated porous networks. Air bubble patterns and breakthrough gas saturation were used to characterize the impact of porosity and average throat size on displacement patterns and oxygen saturation values. Next, a multiphase computational fluid dynamics (CFD) simulation tool was developed and validated with experimental results to simulate the dynamic displacement process of oxygen gas injection into liquid water saturated porous media. Using this CFD tool, a comprehensive phase diagram was developed through a parametric characterization of the porous media in terms of material hydrophobicity and gas flow rates. In addition to calculating the saturation of the invading gas, gas pressure variations were calculated and used to identify the locations of the phase diagram boundaries. The proposed phase diagram serves to inform the design of efficient PTL for improved polymer electrolyte membrane (PEM) electrolyzer performance.