Using Layer-by-layer Assembled Clay Composite Junctions to Enhance the Water Dissociation in Bipolar Membranes

Bipolar membranes (BPMs) with a layer-by-layer (LbL) assembled montmorillonite (K30 MMT) clay-polyelectrolyte (PE) composite junction coated onto a sulfonated poly(ether ether ketone (SPEEK)) electrospun support are prepared, characterized and their water dissociation performance is analyzed. In particular, the focus is on the effect of the presence of the K30 MMT clay as a catalyst for water dissociation, the bilayer number (three, six, and nine), and the PE strength (poly(ethylenimine) (PEI) as a weak PE and poly(diallyl dimethylammonium chloride) (PDADMAC) as a strong PE) on the BPM performance. The BPMs are prepared by electrospinning and hot pressing SPEEK and the Fumion FAA-3 polymer. Adding the composite multilayers in the BPM junction decreases the membrane area resistance in reverse bias from 560 to 21 Ohms cm2 for the best-performing modified BPM. The bilayer number has limited influence on the overall membrane resistance, while the PDADMAC BPMs outperform the PEI BPMs due to the higher and more stable PE and clay adsorptions. Electrochemical impedance spectroscopy shows that the depletion layer thickness decreases exponentially with the number of bilayers as the water dissociation reaction becomes less dependent on the junction electric field. Furthermore, the higher Donnan exclusion at the modified junctions improves the BPM permselectivity 3-fold compared to the BPM containing no catalyst. Altogether, these improvements lead to 6.7 times less energy being used in BPM electrodialysis for the production of acid and base when a BPM with composite LBL junction is used compared to a BPM without catalyst. Thus, adding MMT clay composite LbL catalyst to BPM junctions is a promising method to improve the efficiency and reduce the energy consumption of electrochemical processes that rely on BPMs.