This contribution describes how Raman spectroscopy can be applied to investigate the distribution of molecular species inside an ABPBI membrane of a running PEM fuel cell. For the in-situ measurements an experimental setup was first developed and then used to identify phosphoric acid species and to record how they change their concentrations when the current consumption of the fuel cell was changed. The observation port to look inside the membrane was placed next to the cathode. At that location among orthophosphoric acid, H3PO4·H2O, a lot of oligophosphoric acid species are present where pyrophosphoric acid could unambiguously be identified. In comparison to all identified species pyrophoshoric acid and polyphosphoric acid undergo largest concentration changes when the current was changed.

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