Effects of metal alkoxides on electro-assisted water dissociation across bipolar membranes

A bipolar membrane (BPM) was prepared by the layer-by-layer solution casting method (in the same solvent to achieve good adhesion between the layers). Thin films of different metal alkoxides (M(OR)n, where M is Si, Ti or Zr, and R is (CH2)nCH3) were used as interfacial layers (ILs), which were sandwiched between heterogeneous anion-exchange layers (AELs) and homogeneous cation-exchange layers (CELs) of sulfonated poly (ether sulfone) (SPS). The CELs and the AELs were separately evaluated for their physicochemical and electrochemical properties. BPMs prepared with ILs (containing different metal alkoxides) showed good stability and low resistance. Current–voltage (i–V) curves were explained in terms of the first limiting current density (ilim1), the second limiting current density (ilim2), the water dissociation potential (Vdiss), the water dissociation resistance (Rdiss) and the operational area resistance (Rop). The catalytic activity of the metal alkoxides improved the water dissociation performance of the BPMs. The BPM–SiOH performed better than the commercially available BPMs. Furthermore, electrodialysis with a bipolar membrane (EDBPM) experiments confirmed that water dissociation occurred at the BPM interface and thus acid and base formation occurred.