SYNTHESIS AND MODIFICATION OF INTERFACE LAYER IN BIPOLAR MEMBRANE FOR ELECTRODIALYSIS APPLICATIONS

Abstract

Global water and energy demand coupled with sustainable developmental goals emerged the concept of water-energy nexus, where ion exchange membranes (IEMs) play an important role addressing challenging issues on waste water treatment which are otherwise extremely difficult, energy intensive and time consuming. Electrodialysis, electro-electrodialysis, etc., are some of the well established techniques, where monopolar membrane(s): cation exchange membrane (CEM), or anion exchange membrane (AEM) are being applied for selective (counterion-specific) transport/recovery of electrolyte(s) from its solution. Temperature resistant monopolar membranes are now being explored in fuel cells as solid electrolyte for direct conversion of electrical energy from chemical reaction. Bipolar membrane (BPM) is another category of IEM, where an interface catalyst layer is sandwiched between two monopolar layers (CEL and AEL). Under application of reverse bias, the water molecules present in the interface layer of BPM undergo heterolytic cleavage (H2O → H+ and OH−) forming H+ and OH− ions, which get transported respectively through CEL and AEL, and released in the adjacent chambers forming acid and base of the corresponding electrolyte once BPM is placed in a electrodialysis set up (BMED, bipolar membrane electrodialysis). Since 1980, besides desalination and waste water treatment of process industries, this BMED technology is being used as sustainable technique for its ability to generate acid and base from the waste electrolyte. Around 2010, the BPM was first reported as solid electrolyte in fuel cell for green energy production using hydrogen and oxygen gases as fuels. Subsequently, successful application of BPMs in photo electrochemical cells, water electrolyzer, flow batteries, and CO2 reduction process have motivated several researchers in development of commercial application. Development of any new application using BPM is solely dependent on its long-term stability (mechanical, chemical, thermal, etc.), and performance consistency, e.g., during heterolytic water cleavage using BPM demands rapid water dissociation (kinetics) with minimum applied potential, i.e., low energy requirement. Adequate mechanical stability, long durability, high water permeability towards the interface and increased selectivity for H+/OH− ions are some of the crucial properties expected in this BPM. Synthesis of such BPMs incorporating all these properties is truly challenging as it requires optimization of several parameters including: (i) Properties and thickness of monopolar CEL and AEL (i.e., type of functional group, degree of functionalization) (ii) Reactivity and thickness of interfacial catalyst layer (i.e., type of functional group, extent of hydrophilicity, and its catalytic activity towards water dissociation) (iii) Extent of compatibility of two monolayers (most crucial for mechanical stability imparting resistance towards layer delamination) (iv) Synthesis route and method of casting during preparation of BPM (Presence of multiple layers in BPM complicates its synthesis often leading to non-reproducibility) Optimization of all these parameters will lead to BPM with superior properties, but it’s very challenging to achieve all desired properties in a single BPM. Some of these properties will get compromised with other. High water dissociation kinetics at lower BPM potential is one of the most desirable property with BPM, provided the production cost remains affordable. Complexity in synthesis of BPM have made these membranes highly expensive (600.00 €∙m−2, fumasep®FBM, Fumatech GmbH, Germany), which is the greatest rudiment towards its large-scale commercialization applications. Therefore, this thesis investigates synthesis of low cost BPMs with performance comparable with commercial BPM, where following approaches were made: (i) selection of novel interface catalyst, (ii) improvement of fabrication technique, and (iii) identification of low cost functionalized CEL/AEL. This thesis reports synthesis, characterization, and performance evaluation of both homogenous and heterogenous BPMs where optimization of physical properties of each monolayer and interface catalyst layer were investigated. Physicochemical properties of synthesized BPMs were compared with commercial BPM (fumasep®FBM) and optimized variation among synthesized BPM was then applied for acid-base production from NaCl solution through BMED technology. A brief literature review on bipolar membrane materials, fabrication techniques, application in various sectors, fundamental studies on water dissociation phenomena and its mechanism, and techniques used to evaluate properties of BPM is discussed in Chapter 1. Additionally, challenges associated with development of superior quality BPMs and problems experienced during large scale commercialization of BPM based technologies were discussed. Finally, the chapter concluded with specific research gaps, objectives and organization of the thesis. The fabrication difficulties experienced during synthesis of BPM through layer-by-layer solvent casting method and role of reinforcement fabric to overcome major challenges were taken up in Chapter 2. Where, sulfonated poly-ether-ether-ketone (SPEEK), quaternized polysulfone, and nylon based woven fabric were used as CEL, AEL and reinforcement fabric, respectively. Dipicolinic acid was chosen to be an interface catalyst for electro-splitting of water molecules by ii BPM, which could dissociate water molecules at 0.70 V. Other properties of synthesized BPM such as mechanical stability, chemical stability, membrane durability and acid-alkali production via BMED setup remained comparable with those recorded with commercial BPM (fumasep®FBM). Potential of montmorillonite (MMT) nanoclay as water dissociating catalyst was evaluated in Chapter 3 by incorporating this at the interface of synthesized BPM and observed performances were compared with commercial BPM. Optimization of MMT loading (1.0 mg∙cm−2) at the interface of SPEEK based CEL, and quaternized polysulfone based AEL resulted water dissociation occurring at 0.73 V. Presence of charged sites in MMT platelets due to isomorphic substitution in alumino silicate layers and availability of interlayer galleries in MMT nanoclay facilitated protonation deprotonation phenomena of water molecules causing dissociation of water molecules. Thus, MMT nanoclay was found to be an inexpensive catalyst material suitable for interface layer of BPM. In spite of inferior performance commonly reported with heterogeneous IEMs and BPM, its ease of synthesis often attracted many researchers to explore it at greater depth. Similarly, attempts were made to synthesize heterogenous BPM (HBM) using cation exchange resin (sulfonic acid group as CEL), anion exchange resin (quaternary ammonium group as AEL) and PVC as binder using common solvent for all layers and details are presented in Chapter 4. Variations were made in terms of (i) thickness of individual monopolar layers: CEL/AEL, without interface catalyst layer, (ii) with SPEEK as catalyst and its thickness variation, and (iii) GO (Graphene oxide) and SPEEK based nanocomposite as interface catalyst layer. Optimum GO loading in nanocomposite interface showed considerable lowering in water dissociation potential from 3.27 V (without interface catalyst) to 1.80 V (with GO nanocomposite catalyst). Further, HBMs were also synthesized using cation exchange resin (amino methyl phosphonic acid group as CEL), anion exchange resin (quaternary amine group as AEL), MMT nanoclay as the interface catalyst and PVC as common binder for both AEL and CEL. Details of synthesis are presented in Chapter 5 and optimum thickness ratio between AEL:CEL, MMT loading (1.0 mg∙cm−2) could reduce water dissociation potential from 2.86 V (without interface catalyst) to 2.15 V (with MMT interface catalyst). Morphology analysis of synthesized HBM, mechanical/chemical stability, and acid-base production ability showed that these HBMs might have commercial potential in BMED application, obviously recurring energy cost will be substantially higher compared to homogeneous BPM. iii Finally, Chapter 6 is dedicated to the application of BPM addressed to overcome lime based industrial causticization of green liquor to white liquor in paper mills. Lime based causticization of green liquor (GL, rich in Na2CO3) to white liquor (rich in NaOH, and recycled for digestion of wood chips) generates lime mud (rich is CaCO3), which is recycled as CaO after high temperature (energy intensive) calcination in rotary kiln. As an alternative to conventional causticization technique, feasibility of using BMED (bipolar membrane electrodialysis) technique to recover caustic from green liquor is investigated under different process conditions. With 50 mA⋅cm−2, 1.6 mol⋅L−1 of NaOH (500 mL) could be produced from 500 mL of synthetic GL in 5 h of three-cell pair BMED setup (32 cm2 effective area per cell pair). Current efficiency and energy consumption estimates were ~88.0% and ~4.7 kWh⋅kg−1, respectively. Thus, application of BMED technology might be an alternative to conventional energy intensive and environmentally hazardous multistep causticization of lime based green liquor process. An overall summary of the research work done and knowledge developed are highlighted in Chapter 7. Additionally, based on this experience future recommendations are also made for further improvement in performance of BPM.