Reframing water as a tunable design parameter in polymer electrolyte fuel cells by exploring multiscale perspectives

Water plays a central role in governing the performance and durability of polymer electrolyte fuel cells (PEFCs). However, its functions across length scales remain difficult to correlate and control. This perspective integrates recent advances in in situ and operando imaging, scattering, reflectometry, and spectroscopy using synchrotron X-rays and neutrons to elucidate the multiscale behavior of water in PEFCs. We highlight how liquid water governs microscale mass transport, how confined water-cluster networks regulate proton conductivity at the nanoscale, and how interfacial water structure modulates catalytic activity and stability at the atomic scale. By bridging these hierarchical insights, this perspective moves beyond descriptive water management and reframes water as a tunable design parameter. Elevating water chemistry from observation to predictive design provides guiding principles for simultaneously optimizing performance and durability in next-generation PEFC catalysts and electrode architectures.