The unintended accumulation of oxygen gas in polymer electrolyte membrane (PEM) electrolyzers has been recently identified as one of the main hurdles to achieving high cell efficiencies. Oxygen is a by-product of the electrochemical reaction used to produce hydrogen, and this oxygen must be removed in order to reduce mass transport losses. The porous transport layer (PTL) is a key component of the PEM electrolyzer which facilitates mass transport and electrical conductance. However, oxygen bubble accumulation potentially dominates the total mass transport losses during operation. Many experimental and computational studies have been performed in an attempt to understand the relationship between the morphology of the PTL and the voltage loss of the electrolyzer, but this relationship has yet to be fully defined. In this work, efforts towards identifying and understanding mass transport losses are discussed. PTL structural parameters that were shown to affect performance, such as bulk porosity, particle size, pore size, thickness, and permeability are reviewed. Visualization techniques that have been employed to investigate the behavior of oxygen bubbles are also discussed. This work presents a summary of the studies which have been performed to investigate the key parameters of the PTL that should be tailored for improved PEM electrolyzer performance.

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