Abstract text (incl. figure legends and references)

Proton exchange membranes (PEM) are crucial to the functionality of fuel cells (FC). In those cells H₂ and O₂ is reacting to H₂O, generating electric energy and heat. Material types, composition and particle distribution within the catalyst coated membrane determine the efficiency of the hydrogen/oxygen to water chemical conversion process. The generated microscopic water distribution cannot be determined with sufficient resolution during the cell operation. Furthermore, ex-situ determination of the product water distribution within the meso-porous cell components and the membrane is neither easy nor unambiguous. The establishment of a reliable workflow for the microscopic determination of the material distribution, including water, would be very beneficial. Therefore, we elaborated a correlative workflow, using TESCANs cryo-SEM/FIB approach, for the determination of water and material distributions in PEMs. The correlative cryo-EDS approach enables the determination of the PEM layer- and material-compositions in different membrane types, whether fresh or used after operation in a FC stack. Fig.1 shows i.e., the uneven water distribution of a PEM FIB cross-section by cryo-EDS imaging. PEM-materials of different ageing states can thus reliably be tested. Generated water agglomerations can be located and imaged down to the nano-meter scale within the PEM. The relative amount can be correlated to the microporous structure. This enables to test different PEM materials which contributes to the understanding of the FC stack efficiency. (Authors 1-4 contributed equally)

(Fig.1: Water distribution (blue) of a fresh cryo-FIB cross-sectioned PEM)