Polarization contrast neutron imaging enables operando visualization of iron plating in redox flow batteries.

Visualizing multiphase (solid-liquid-gas) electrochemical transformations during operation is essential to advancing sustainable energy storage technologies. All-iron redox flow batteries represent a promising candidate due to their low-cost and earth-abundant materials; however, they suffer from non-uniform plating rates, cycling instability, and parasitic hydrogen evolution. Here, we introduce an operando neutron imaging methodology - combining polarized and transmission modes - to simultaneously track iron plating/stripping and hydrogen evolution. We uncover preferential iron deposition near the membrane indicating a non-uniform current density distribution. We find that pH plays a major role in reaction selectivity and cycling performance, where less acidic pH results in inaccessible iron deposits. Our experiments with different flow field geometries show that both reaction selectivity and spatial distribution are strongly influenced by electrolyte flow patterns. This multimodal imaging approach provides critical insights into reactive transport and multiphase behavior in flow batteries.