Additionally, X-ray imaging of water transport can be compromised due to the poor contrast between water and fuel cell materials such as carbon fibre. Among the various microscopy options, neutron imaging is a powerful technique that is sensitive to low density materials, which allows direct mapping of the water content in operating fuel cells and provides quantitative data. Notably, neutron imaging demonstrates a superior ability to penetrate dense structural materials without the need for significant modifications, making it versatile for applications ranging from small-scale lto full-size single cells to stacks containing multiple cells. Neutron imaging has been extensively used to investigate water management-related problems, exploring the impact of cell structure design and compression , operation conditions, materials and degradation . Lee et al. conducted neutron imaging on an operating PEFC under various humidity levels, in parallel with impedance measurements at high frequencies, to determine the membrane resistance. The results confirmed that the Ohmic resistance increases with decreasing RH levels, and that there is an inverse relationship between the Ohmic resistance and the membrane water content, in line with earlier discussions. Despite its advantages, neutron imaging is not a realistic option for online implementation of water content characterisation in vehicular or stationary fuel cell systems due to the requirement for nuclear reactors or spallation sources to produce neutrons, and it is likely to remain confined to specialist laboratories and beamlines for the foreseeable future.