Preparation of Bipolar Membranes for Water Splitting and Water Generation

Abstract

BPMs are laminates of cation-exchange polymer (CEP) and anion-exchange polymer (AEP) layers. At the interfacial junction where the two layers meet, water is split into H+ and OH- during current flow at a voltage less than the normal thermodynamic electrode potential of 1.23 V (so-called reverse bias mode). In forward bias mode, H+ and OH- are driven to the junction and combine to form water. BPMs are used in electrodialysis separations for water reclamation and for energy and sustainability applications, such as CO2 capture. Conventional 2D BPMs, with a planar junction interface have intrinsic flaws, such as a low contact area of the AEP/CEP layers, poor adhesion between the layers, and limitations on the ion-exchange capacity of the AEP and CEP polymers. This Ph.D. dissertation project sought to address these shortcomings by fabricating trilayer BPMs with a high surface area 3D junction composed of interpenetrating and interlocking AEP and CEP nanofibers, with outer layers of neat and dense AEP and CEP films. The BPMs were made by hot-pressing pre-formed outer films to a dual fiber junction layer, with the incorporation of catalyst particles in the junction. Different types of AEP and CEP ionomers were assessed to balance the swelling of the two ionomers. In addition, the thickness/composition of each layer was changed to minimize the overall membrane voltage drop while ensuring no co-ion leakage (high water-splitting selectivity). BPMs were investigated in forward (water generation) and reverse bias (water splitting) modes, and in two important applications: as a water-splitting membrane for electrodialysis water reuse and recovery and as a self-hydrating membrane in a hydrogen/oxygen fuel cell. Trilayer 3D junction BPMs were contrasted with a commercial 2D junction bipolar membrane and a preferred 3D junction membrane composition/morphology for both forward and reverse bias modes was identified, in terms of transmembrane voltage drop and durability (maintaining constant transmembrane voltage drop over time at a fixed current density). Membrane-electrode-assemblies with a hybrid bipolar junction produced high power in a H2/O2 fuel cell at low feed gas humidities, thus demonstrating the self-humidification capability of a BPM in water generation mode.