Water splitting promoted by electronically conducting interlayer material in bipolar membranes

Abstract

Bipolar membranes are used in a variety of industrial applications to split water into hydronium and hydroxide ions. This research investigated the hypothesis that an electronically conducting material between the anion and cation exchange membranes can increase the rate of water splitting by increasing the electric field intensity in the mobile ion depleted region. Bipolar membranes were constructed with electronically conducting (graphene and carbon nanotubes) and electronically insulating (graphene oxide) interlayer materials of varying thickness. All three interlayer materials decreased the voltage required for water splitting compared to a bipolar membrane with no interlayer material. Quantum chemistry simulations were used to determine the catalytic effect of proton accepting and proton releasing sites on the three interlayer materials. Neither graphene nor carbon nanotubes had catalytic sites for water splitting. Thicker layers of graphene oxide resulted in decreased rates of water splitting at each applied potential. This effect can be attributed to a diminished electric field in the mobile ion depleted region with increasing catalyst layer thickness. In contrast, membrane performance with the electronically conducting graphene and carbon nanotube interlayers was independent of the interlayer thickness. An electrostatic model was used to show that interlayer electronic conductance can increase the electric field intensity in the mobile ion depleted region as compared to an electronically insulating material. Thus, including electronically conducting material in addition to a traditional catalyst may be a viable strategy for improving the performance of bipolar membranes.