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In order to characterize high temperature polymer electrolyte fuel cells (HT-PEFCs) in operando, neutron radiography imaging, in combination with the deuterium contrast method, was used to analyze the hydrogen distribution and proton exchange processes in operando. These measurements were then combined with the electrochemical impedance spectroscopy measurements. The cell was operated under different current densities and stoichiometries. Neutron images of the active area of the cell were captured in order to study the changeover times when the fuel supply was switched between hydrogen and deuterium, as well as to analyze the cell during steady state conditions. This work demonstrates that the changeover from proton to deuteron (and vice versa) leads to local varying media distributions in the electrolyte, independent of the overall exchange dynamics. A faster proton-to-deuteron exchange was re-discovered when switching the gas supply from H 2 to D 2 than that from D 2 to H 2 . Furthermore, the D 2 uptake and discharge were faster at a higher current density. Specifically, the changeover from H to D takes 5–6 min at 200 mA cm −2 , 2–3 min at 400 mA cm −2 and 1–2 min at 600 mA cm −2 . An effect on the transmittance changes is apparent when the stoichiometry changes.

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In Operando Neutron Radiography Analysis of a High-Temperature Polymer Electrolyte Fuel Cell Based on a Phosphoric Acid-Doped Polybenzimidazole Membrane Using the Hydrogen-Deuterium Contrast Method