Molecularly Thin Nanosheet Films as Water Dissociation Reaction Catalysts Enhanced by Strong Electric Fields in Bipolar Membranes

Bipolar membranes (BPMs) are interesting materials for the development of next-generation electrochemical energy conversion and separations processes. One of the key challenges in optimizing BPM performance is enhancing the rate of the water dissociation (WD) reaction. While electric field effects, specifically the second Wien effect, have been demonstrated to enhance the rate of WD reaction, making BPMs with low overpotentials for WD using primary electric field effects has been difficult to achieve. In this study, we constructed an abrupt interfacial structure between the anion exchange membrane (AEM) and cation exchange membrane (CEM) of BPMs to maximize the intensity of local electric field. A film of densely tiled, molecularly thin titanium oxide nanosheets was deposited as the interfacial layer to create an abrupt interface for studying extreme electric field effects. Although BPMs with titanium oxide nanosheet films exhibited higher WD reaction resistance compared to thicker catalyst layers composed of nanoparticles at low current density, they showed superior performance at higher current densities, where strong electric fields were present, and an apparent WD overpotential of 0.25 V at 300 mA cm^-2 was extracted from electrochemical impedance measurements. These results highlight the potential of optimizing BPM performance by maximizing the second Wien effect through the utilization of two-dimensionally assembled nanosheet films.