Interfacial H 2 O reactivity in bipolar membrane junctions Carlos Gomez Rodellar, Beatriz Roldan Cuenya, Sebastian Z. Öner Interface Science Department of the Fritz Haber Institute of the Max Planck Society, Germany H 2 O is omnipresent in electrochemistry, yet, its interfacial reactivity at metal oxide surfaces that exhibit a rich acid- base site chemistry is poorly understood. Here, we study interfacial water dissociation and formation (H 2 O <-> H + + OH - ) over metal(oxide) nanoparticle surfaces inside bipolar membrane (BPM) junctions. BPM junctions are comprised of a polymeric hydroxide ion conducting anion exchange layer (AEL), a metal (oxide) catalyst layer and a proton conducting cation exchange layer (CEL). Nafion is the prototypical example for a CEL. When applying a bias across the system, water can be dissociated (formed) inside the catalyst layer and hydroxide ions and protons are moving outward (inward). Despite the use of BPMs in industrial electrodialysis, the BPM catalyst activity is poorly understood. We are performing Tafel and Arrhenius analyses on pristine and TiO 2 -catalzed BPM junctions, and show that not only water dissociation can be catalyzed, as has been shown previously, 1,2 but also the reverse process of interfacial water formation. Studying the TiO 2 -loading dependence, we observe a transition of varying Ea(V) for pristine BPM junctions to a region of constant E a for TiO 2 -catalzed junctions. These results are paramount to understand the activity of pristine AEL and CEL sites and further show that additional, active metal oxide catalysts are maintained in their reversible state up to 100’s mA cm -2 . Conversely, for active catalysts with constant Ea, positive differential resistances are likely caused by a potential-dependent pre-exponential factor, A app (V), as we observe from Arrhenius analysis, and which might be indicative of electric field effects on interfacial water molecules. Further, we combine our experimental findings with Multiphysics simulations to develop an experimentally verifiable model about the BPM junction avoiding commonly made speculations. Last, we use our insights to demonstrate BPM fuel cells, that operate the hydrogen oxidation reaction in acidic and oxygen reduction reaction in alkaline conditions. References 1. S.Z. Oener, M. Foster, S.W. Boettcher, Accelerating Water Dissociation in Bipolar Membranes and Electrocatalysis, Science 369, 6507, 1099-1103 (2020), https://doi.org/10.1126/science.aaz1487 2. S. S. Mel’nikov, O. V. Shapovalova, N. V. Shel’deshov, V. I. Zabolotskii, Effect of d-metal hydroxides on water dissociation in bipolar membranes. Petrol. Chem. 51, 577–584 (2011). https://doi.org/10.1134/S0965544111070097
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