Water is produced inside a PEMFC's cathode catalyst layer (O2+4H++4e- =2H2O), a nano-scale porous composite thin-film, and the rate of water-production is proportional to electrical current density. A further increase of the electrical current density requires a better understanding of how and why water phase-change, liquid water morphology, water content inside electrolyte materials, proton conductivity and active surface area, and water transport mechanisms are related to operating conditions, catalyst layers properties, gas diffusion layer properties and flow-channel surface properties. This presentation talks about our efforts to model complex mass water transport phenomena and their interplay with the microstructures of catalyst layers, and it also discusses the multidisciplinary challenges and research opportunities.