To transition to clean, renewable energy technologies, reliable energy storage is crucial for filling gaps when sources like solar and wind are intermittent. Hydrogen (H2) is a key contender for energy storage, with its ability to produce electricity through fuel cells or for higher power applications through combustion, producing only water as a byproduct. The focus is now on producing H2 efficiently with minimal CO2 emissions, where water electrolyzers play a vital role in splitting water into hydrogen and oxygen. Recent industry developments have showcased the potential of nano-structured catalysts to achieve ultra-high current densities in PEMWEs, enhancing performance significantly. However, challenges arise at ultra-high current densities, particularly with mass transport limitations due to oxygen gas blocking liquid water flow, impacting the efficiency of the system. Vapor-fed PEMWEs emerge as a promising alternative to liquid-fed systems, potentially reducing degradation risks and enabling operation in water-scarce regions. While some studies have explored vapor-fed PEM electrolyzers, the mass transport losses and operational limits associated with this method remain largely unexplored. The assessment of limiting current density in vapor-reliant PEMWE operation is crucial to understanding and optimizing performance. This study delves into the mass transport limitations of vapor-fed PEMWEs by assessing polarization curves to identify the cell's limiting current density. By varying the relative humidity (RH), the research aims to provide insights into the resistance mechanisms within the system and enhance the efficiency of hydrogen production in PEM electrolyzers.