Beneficial Use of a Coordination Complex As the Junction Catalyst in a Bipolar Membrane

Bipolar membrane (BPM) has been used commercially in electrodialysis separation processes for the generation of acid and base from aqueous salt solutions and electrochemical water splitting processes for hydrogen generation. Research advances have demonstrated that, upon a sufficient applied reverse bias, water molecules at the junction zone of the BPM can dissociate into protons (H⁺) and hydroxide anions (OH^–). Therefore, a stable and catalytic active junction for rapid water dissociation is highly desired, but it still remains a challenge for current bipolar membrane designs. Here, we demonstrate a versatile strategy for fabricating a thin metal–polymer coordination complex junction-based bipolar membrane. The complex used consists of polyethylenimine (PEI) that coordinate to Fe­(III) centers through the amine–iron interaction (Fe­(III)@PEI). The unique coordination interaction enables to promote water dissociation and suppress catalyst leakage issue. In addition, to prevent junction layer dehydration from highly efficient water dissociation, the cation-exchange membrane containing porous water channels is utilized to sufficiently replenish the water molecules consumed at the junction zone. Notably, under a current density of 320 mA cm^–2, Fe­(III)@PEI-based BPM exhibits a voltage of 1.88 V, which is 56 and 36% lower than PEI-based BPM and FeCl₃-based BPM, respectively. Moreover, during a constant current density operation at 60 mA cm^–2, Fe­(III)@PEI-based BPM exhibits a much lower voltage increasing speed (5.93 mV h^–1) than the FeCl₃-based BPM (32.64 mV h^–1), indicating the improved durability of the metal–polymer coordination complex junction.