Fabrication and Application of Stable Bipolar Membranes: Effect of Layer Thickness and Interfacial Layer

In this research, efficient and stable BPMs are developed by simple and cost-effective layer-by-layer solution casting method. Selection of polymers/materials of cation exchange layer (CEL), anion exchange layer (AEL) and interfacial layer (IL) have been carried out that no delamination and ballooning is observed even prolonged duration. Initially, IONSEP-HC-A commercial anion exchange membrane was used as AEL and sulfonated poly (ether sulfone) (SPES) was taken as CEL with different content of nano MoS2 as IL. The IL of nano MoS2 acts as water splitting (WS) catalyst and enhances the water dissociation efficiency. For the dissociation of 0.4 M NaCl, a power consumption of 2.16 kWh kg−1 and a current efficiency of 84.46 % were found for BPM-5 at 8 V/pair. Moreover, to reduce the cost of BPMs, quaternized poly phenylene oxide (QPPO) and sulfonated poly (ether ether ketone) (SPEEK) were used as AEL and CEL respectively for tailoring BPMs using l-b-l assembly. Different functionalized MOF (metal organic framework-MIL-101-Cr) were synthesized for IL of BPM. Organic-inorganic catalytic sites with large surface area and porosity boost up the WS rate. The bifunctional groups of B-MOF serve as buffer for H+ and OH- ions and enhance the WS rate. BPM-BMOF was able to hydrolysed different sodium carboxylates salts and 1.0 M of NaCl. BPM-BMOF showed ~94 % acid recovery and 0.496 g h-1 WS rate during the hydrolysis of 0.2 M NaCl. Also, various inorganic materials such as sulfonated graphene oxide (S-GO), sulfonated molybdenum sulphide (S-MoS2) and iron-nickel doped graphene oxide with MXene (GMX) were synthesized as IL of BPM and their comparative study was performed. In the traditional lithium extraction process lithium sulfate (Li2SO4) is generated as an intermediate which is converted into lithium hydroxide (LiOH) used for various lithium ion battery applications. This work also represents the more convenient technique for the production of LiOH (and H2SO4) from purified lithium sulfate using bipolar membrane electrodialysis (BMED) process at different applied potentials and different feed concentrations using best optimized BM-SMoS2. To produce high purity of LiOH the lower concentration of (0.0075 M and 0.015 M) H2SO4 was circulated without recirculation (Acid mode) which reduced the migration of sulfate ions from acid to base solution. This reduces the average sulfate flux from 0.0305 x 10-4 mol m-2 s-1 to 0.0172 x 10-4 mol m-2 s-1 for the dissociation of 0.2 M Li¬2SO4 at 6 V/pair. All acid-mode experiments showed ≥ 90 % lithium recovery and ≥ 90 % product purity with ≤ 1.77 kWh kg-1 of consumption of energy and ≥ 80 % of current efficiency. As proton migrates faster than hydroxyl ion, an efficient transparent asymmetric BPM has been fabricated using SPES as CEL and QPPO as AEL. BPM with 40 % less AEL thickness (BPM-60 membrane) exhibited the better performance. BPM-60 showed the highest water splitting rate and alkali recovery of 0.491 g h-1 and 95.5 %, respectively with the lowest energy consumption of 1.11 kWh kg-1 and high current efficiency of 90.32 % for the dissociation of 0.2 M NaCl. An asymmetric BPM-60 membrane is efficient for enhancing product recovery with high current efficiency. We believe that the negligible co-ion leakage, high ionic conductivity and stability of BPM-BMOF and BM-SMoS2 make it industrially applicable for water splitting.