The design of a porous transport layer (PTL) that exhibits effective two-phase transport characteristics and rigid contact at the catalyst layer (CL)-PTL interface is a prerequisite for improving the efficiency of polymer electrolyte membrane (PEM) water electrolyzers at high current densities (i >​ 4 A/cm²). In this thesis, key design considerations were presented based on extensive investigations performed via comprehensive numerical models and in operando imaging.

A numerical investigation was performed to determine the impact of PTL structures on reactant liquid water delivery to reaction sites. A comprehensive model for designing PTLs was presented, where a stochastic model was used to numerically generate PTLs with realistic morphologies and tunable PTL structures, and a pore network model was used to perform two-phase flow calculations and determine their transport properties. A trade-off relationship was observed between achieving an effective CL-PTL interfacial contact and favourable reactant transport behaviour. Finer powder diameters and lower porosities improved the CL-PTL interfacial contact but led to the reduced permeability of liquid water. Conversely, increasing either powder diameter or porosity improved permeability but deteriorated the CL-PTL interfacial contact. Therefore, it is crucial to consider the operating conditions of an electrolyzer when designing PTLs.

Mass transport behaviour in an electrolyzer operating at high current densities was studied using in operando imaging techniques. A sufficient reactant flow rate was essential for mitigating electrolyzer cell failures at high current densities. Specifically, the critical current density was observed when inadequate reactant water was supplied at high current densities. Both gas content in the PTL and mass transport overpotential significantly increased at the critical current density, and the electrolyzer failed to operate beyond the critical current density. Moreover, mass transport in the PTL surprisingly improved when patterned through-pores (PTPs) were implemented with a commercial PTL. The usage of a novel PTP PTL led to the reduced accumulation of product gas at the CL-PTL interface by 43.5%. Furthermore, PTPs led to more frequent gas removal, which subsequently improved water intake to the reaction sites. The findings in this thesis provide valuable insights for designing novel PTLs in PEM electrolyzers operating at high current densities.