A Bipolar Membrane Containing Core-Shell Structured Fe₃O₄-Chitosan Nanoparticles for Direct Seawater Electrolysis

Seawater has attracted increasing attention as a promising resource for hydrogen production via electrolysis. However, multivalent ions present in seawater can reduce the efficiency of direct seawater electrolysis (DSWE) by forming inorganic precipitates at the cathode. Bipolar membranes (BPMs) can mitigate precipitate formation by regulating local pH, thereby enhancing DSWE efficiency. Accordingly, this study focuses on the fabrication of a high-performance BPM for DSWE applications. The water-splitting performance of BPMs is strongly dependent on the properties of the catalyst at the bipolar junction. Herein, iron oxide (Fe₃O₄) nanoparticles were coated with cross-linked chitosan to improve solvent dispersibility and catalytic activity. The resulting core-shell catalyst exhibited excellent dispersibility, facilitating uniform incorporation into the BPM. Water-splitting flux measurements identified an optimal catalyst loading of approximately 3 μg cm^-2. The BPM containing Fe₃O₄-chitosan nanoparticles achieved a water-splitting flux of 26.2 μmol cm^-2 min^-1, which is 18.6% higher than that of a commercial BPM (BP-1E, Astom Corp., Tokyo, Japan). DSWE tests using artificial seawater as the catholyte and NaOH as the anolyte demonstrated lower cell voltage and stable catholyte acidification over 100 h compared to the commercial membrane.