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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Efficient Bipolar Membrane with Functionalized Graphene Oxide Interfacial Layer for Water Splitting and Converting Salt into Acid/ Base by Electrodialysis Murli Manohar,†,‡ Arindam K. Das,†,‡ and Vinod K. Shahi*,†,‡ †

Electro-Membrane Processes Division, and ‡Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Gijubhai Badheka Marg, Bhavnagar 364 002, Gujarat, India S Supporting Information *

ABSTRACT: The layer-by-layer casting technique was adopted to fabricate bipolar membranes (BPMs) comprising a cation-exchange layer (CEL), an interfacial layer (IL, phosphorylated graphene oxide (PGO) and/or quaternized graphene oxide (QGO)), and an anion-exchange layer (AEL). Under applied dc potential gradient, water molecules dissociate at the IL and generate H+ and OH−. The effect of the IL nature (PGO, QGO, and mixed PGO/QGO) on the water splitting performance of the BPM was thoroughly studied. Well optimized BPM-PGO/QGO exhibited high current efficiency (CE) and low energy consumption and was assessed to be superior in comparison with different commercial BPMs. The reported BPM showed improved water splitting performance, significant reduction in resistance, and high permselectivity (low co-ion leakage) for over 10 experimental cycles. Relatively stable performance of BPM-PGO/QGO made it industrially viable for water splitting. activation energy of water dissociation.1,14 The water splitting performance of the BPM may be tuned by the contact region (IL), and different types of materials such as phosphoric acid,15 carboxylic acids,1 poly(acrylic acid),13 pyridines,16 amino acids,13 proteins,17 different inorganic materials,13,16,18 polyethylene glycol,19 poly vinyl alcohol,20 and graphene oxide (GO)6,21,22 were explored as water dissociation catalysts at the IL. GO is stable and a potential vehicle for ion transport, composed of thick sheets of macromolecular single carbon atom and containing oxygenated functional groups such as  O, −OH, and −COOH.23−25 GO based materials showed diversified applications due to their outstanding adaptability and stable functional properties.26 McDonald et al. reported GO as a water dissociation catalyst in the bipolar membrane IL.6,21 Sulfonic acid functionalized GO showed high conductivity.27 Thus, it was conceptualized that inserting functionalized GO (acidic or alkaline) rather than pristine GO offers a unique method for architecting a polar IL for a high performance BPM. We reported preparation of PGO and QGO as potential candidates for IL fabrication. To achieve an efficient BPM, a CEL and AEL were prepared by sulfonation or quaternization

1. INTRODUCTION Electrodialysis (ED) using cation- and anion-exchange membranes is a mature technology for separation/isolation of targeted molecules, purification, including desalination of brackish and seawater, and effluent treatment.1−5 Bipolar membranes (BPMs) significantly extended the diversified applications of ED for maintaining the pH gradient by water splitting and recovering of salts as their corresponding acid/ base.1,6−9 BPMs are laminated/sandwiched polymeric films of the cation-exchange layer (CEL) and anion-exchange layer (AEL) via a hydrophilic interfacial layer (IL). In the trilayer sandwiched structure, the IL acts as analogous to p−n junction (contact layer) and is responsible for water splitting under the influence of electric potential gradient.10 The products, H+ and OH−, are the major current carriers and are traversing toward respective electrodes across the CEL and AEL, in the bipolar membrane electrodialysis (BMED). The proton transfer mechanism is governed by formation of H+ and OH− due to protonation and deprotonation reaction between H2O and fixed charge groups (−SO3H and −N+H3).11,12 Formation of acids and bases from the corresponding salt solution is a popular scenario of BMED. The water splitting across to the BPM is reported to be 7 times higher than that of the free solution.1,13 The water splitting capability of BPMs has been generally impelled by incorporation of catalyst in the IL, which reduced the overpotential responsible for an efficient BPM. Generally, catalyst provides an alternative water splitting site (by formation of reactive activated complexes) by reducing the © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

September 19, 2017 November 30, 2017 December 25, 2017 December 25, 2017 DOI: 10.1021/acs.iecr.7b03885 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research of polysulfone (PS), respectively. Also, to explore the effect of charge density in the IL, PGO, QGO, or amphoteric PGO/ QGO (mixed PGO and QGO) separately were used to cast the IL. The reported protocol for BPM preparation is based on layer-by-layer casting of different layers (CEL, AEL, and IL) in the same solvent onto the dried surface of the former one. The same type of main polymer chain for the CEL, and the AEL, dissolved in the same solvent was used for the casting of different layers and was responsible for good adhesion between them. The BPM properties and efficiency were monitored for different ILs (PGO, QGO, and PGO/QGO) along with their stabilities and water splitting efficiency.

Figure 1. Functionalization of GO by grafting of phosphonic (PGO) or quaternary ammonium (QGO) groups.

2. EXPERIMENTAL SECTION 2.1. Materials. Polysulfone (PS) (poly[oxy-1,4-phenylenesulfonyl-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenylene (1-methylethylidene)-1,4-phenylene]) (Mw = 85 157, Mn = 38721; Mw/Mn = 2.199) was obtained from SigmaAldrich Chemicals. Para-formaldehyde (HCHO)n, stannic chloride anhydrous (SnCl4), trimethylsilyl chloride (TMSCl), chloroform (CHCl3), 2-propanol (CH3CHOHCH3), methanol (CH3OH), water, graphite powder (GtO), sulfuric acid (H 2 SO 4 ), phosphorous acid (H 3 PO 3 ), (aminopropyl)trimethoxysilane (APTMS), tetrahydrofuran (THF), N-methyl 2-pyrrolidone (NMP), trimethylamine (TMA), formaldehyde (HCHO), dichloromethane (DCM), potassium hydroxide (KOH), sodium chloride (NaCl), etc. of AR grade were obtained from SD fine Chemicals and used without any purification. 2.2. Preparation of the AEL, CEL, and IL. Quaternized PS was synthesized according to the method reported earlier and used to fabricate the AEL.28 For the fabricating CEL, PS was sulfonated by dissolving in H2SO4 under constant stirring 60 °C for 6 h, and precipitated in an excess of water, followed by washing several times for the complete removal of H2SO4 residue. Graphite oxide was synthesized by Hummer’s method from purified natural flake graphite powder according to the earlier reported procedure.29 Graphene oxide (GO) was exfoliated from graphite oxide by ultrasonication.30 The IL was prepared by unmodified GO, PGO, QGO, and an equal mixture of PGO and QGO, separately, to explore the nature of IL-forming material (acidic or alkaline) on the BPM performance. To synthesize PGO or QGO, first silica-modified GO was synthesized via condensation reaction between GO and (aminopropyl)trimethoxysilane (APTMS). In a typical procedure, GO (150 mg) was dispersed in anhydrous THF (500 mL) and sonicated to obtain a homogeneous dispersion. Further, APTMS (1.5 mL) was added, and reaction mixture was refluxed for 15 h at 80 °C. Cooled (30 °C) reaction mass was filtered and rinsed several times with THF, and dried under vacuum overnight. 31 Obtained silica-modified GO was phosphorylated using 1:1 (w/w) dispersion of formaldehyde and phosphorous acid solution in a closed round-bottom flask (RBF) under continuous stirring for 3 h at 70 °C. After cooling, PGO was filtrated, washed several times with water, and dried under vacuum oven overnight. To synthesize QGO, the desired amount of silica-modified GO was dispersed in methanol (100 mL); after addition of methyl iodide (10 mL) the mixture was stirred for 24 h at 30 °C, filtrated, washed several times with methanol, and dried under vacuum overnight, to obtain QGO. A schematic reaction for the preparation of PGO and QGO has been depicted in Figure 1.

2.3. Fabrication of BPMs. BPMs were prepared by layerby-layer casting of the CEL, IL, and AEL, respectively. To prepare the CEL, the desired amount of sulfonated PS (SPS) was dissolved in NMP (20 wt %) under constant stirring for 6 h at 30 °C. Resultant viscous solution was transformed as a thin film of known thickness (∼100 μm) on the cleaned glass plate and dried under the IR lamp. To cast the IL (∼40 μm) on the top of the CEL, pristine GO, PGO, QGO, and PGO-QGO (equal weight ratio) were dispersed homogeneously in NMP (6 mg/mL) under stirring followed by sonication, and dried under an IR lamp, whereas the AEL (∼100 μm) was cast on the top of the IL, by dissolving CMPS in NMP (20 wt %) under stirring for 20 h (50−60 °C). Prepared composite film was dried at 80 °C in a vacuum oven. Good adhesion and firm contact among all the three layers (CEL, IL, and AEL) was obtained because the same solvent (NMP) was used for the casting of different layers.32,33 Prepared BPMs were successively equilibrated in acid/base (1.0 M HCl/NaOH), separately, and finally washed with deionized water. Prepared BPMs were named BPM-GO, BPM-PGO, BPM-QGO, and BPM-PGO/QGO, depending on the IL. 2.4. Instrumental Analysis, Stabilities of the BPM, and Characterizations. Prepared BPMs were characterized by different instrumental analysis (scanning electron microscope (SEM), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), stress−strain curve, and burst strength) (section S1 of the Supporting Information). Detailed methods used for assessing oxidative and hydrolytic stabilities of BPMs, and measuring their properties (water uptake, cation- and anion-exchange capacity, and resistance (Rm)) are included in sections S2 and S3 of the Supporting Information, respectively. 2.5. i−V Curves and Water Splitting Experiments. A two-compartment cell (50 cm3) fitted with Pt electrodes and separated by the BPM was used to record i−V curves under a known electrolytic environment (0.10 M NaCl), whereas membrane potentials were measured using salt bridges (placed near membrane surfaces) and saturated calomel electrodes (SCE) (Figure S1, Supporting Information). The desired current density in steps was applied using dc power supply (Aplab India, model L1285), and the resultant potential was measured by a digital multimeter connected through SCEs. Different characteristics parameters such as overlimiting currents (Ilim1 and Ilim2), dissociation voltage (Vdiss), and open voltage (Vop) were obtained from i−V curves.5,17 Water splitting (BMED) experiments were performed in an electrodialysis unit consisting of five compartments separated by one piece of the BPM and two pieces of the CEL and AEL (effective area: 73.70 cm2) (Figure 2). The unit was fitted with a precious metal oxide coated titanium sheet electrodes (a TiO2 sheet was coated with a triple precious metal oxide (titanium− B

DOI: 10.1021/acs.iecr.7b03885 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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tration, pH, and conductivity for different compartments output were regularly monitored.

3. RESULTS AND DISCUSSION 3.1. Fabrication of the CEL, AEL, and BPM. The CEL and AEL were separately prepared by sulfonation and quaternization of PS according to the method reported earlier.17,28 The characteristics of the individual CEL and AEL using cation- and anion-exchange membrane in the BMED unit are summarized in Table S1 (Supporting Information). The BPMs reported in this manuscript were fabricated by layer-by-layer casting procedure of the CEL, IL, and AEL, respectively. Pristine GO, PGO, QGO, and an equiweight mixture of both PGO and QGO were used as the IL for the preparation of different BPMs. Graphite oxide (GtO) was exfoliated (oxidized) to GO, and later on silica (APTMS) was introduced for quarterization with CH3I or phosphorylation with H3PO4 (Figure 1). The BPM was fabricated by layer-bylayer casting of individual layers (CEL, IL, and AEL) on top of the semidried layer, and the dried thickness of the sandwiched structure was ∼220 μm (casting thickness: CEL, 500 μm; AEL, 500 μm). The thickness of the dried IL was kept at ∼22 μm (10% of BPM), and fixed charges in the CEL, AEL, and the IL avoid the dehydration.18 This also improved the mobility of H+ and OH− (water splitting product under influence of electrical gradient) due to the presence of −OH, −COOH, −H3PO4, and −N+(CH3)3 hydrophilic groups.

Figure 2. BMED unit for water splitting experiments.

ruthenium−platinum) of 6.0 μm thickness, obtained from Titanium Tantalum Products (TITAN, Chennai, India)). Parallel-cum-series flow arrangement was used by employing peristaltic pumps for different feeds (500 cm3) in the recirculation mode of operation with a constant flow rate (0.006 m3/h). A constant voltage was imposed by a dc power supply (Aplab India, model L1285), and the corresponding current was recorded. Both EW compartments were fed with Na2SO4 solution (0.10 M), whereas NaCl solutions of known concentration and volume were fed into a treated compartment (TC), and distilled water was fed into acid compartment (AC) and base compartment (BC). Changes in electrolyte concen-

Figure 3. Cross-sectional SEM images of (A) BPM-PGO/QGO, (B) BPM-GO, (C) BPM-QGO, and (D) BPM-PGO membranes. C

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Table 1. Thickness, WU, IEC, and Rm Values for the Different Commercial and Prepared CEL, AEL, and BPMs

The cross-sectional SEM images for BPM-PGO/QGO, BPM-GO, BPM-QGO, and BPM-PGO showed homogeneous trilayered structure with good adhesion without any phase separation (Figure 3A−D). Use of the same solvent/polymer matrix and layer-by-layer casting technique results in a robust BPM. 3.2. Thermal, Mechanical, Oxidative, Hydrolytic, and Acid−Base Stabilities of BPMs. Thermal stability of BPMs was analyzed by TGA curves, and all BPMs showed almost a similar weight loss pattern (Figure S2, Supporting Information). Below 100 °C absorbed water contributed to weight loss in the first step, whereas a small weight loss between 100 and 200 °C confirmed the thermally stable nature of BPMs up to 200 °C. Beyond 200 °C, steep weight loss was observed due to oxidation of functional groups. Pristine BPM-GO showed a relatively low storage modulus in DMA curves, whereas the storage modulus of the BPM with functionalized GO as the IL was increased slightly (Figure S3, Supporting Information). The enhanced storage modulus may be attributed to the strong bonding among the three layers (CEL, IL, and AEL) due to the presence of sulfonic acid and quaternary ammonium groups, which helped properly align the different layers in the composite BPM structure. Proper alignment of the IL between the CEL and AEL restricted the movement of the polymer chains and thus improved the polymer stiffness and storage modulus. Storage modulus data for the different BPMs also support the above and BPM-PGO/QGO the highest storage modulus (13.07 kg cm−2; Figure S4, Supporting Information). Oxidative stability was assessed by estimating the weight loss after treatment in Fenton’s reagent for 3 h at 60 °C and different BPMs showed 7.0−8.0% weight loss under a harassed oxidative environment (Figure 4). Under strong hydrolytic

membrane CEL (in this case) Neosepta CMX (CEL) AEL (in this case) Neosepta AMX (AEL) BPM-GO BPM-PGO BPM-QGO BPM-PGO/QGO ASTOM Neosepta BP-1E Fumasep FBM

thickness (dry) (μm)

WUa (%)

IEC (mequiv/g)

Rm b (Ω cm2)

100

18.6 17.0

1.67 1.62

2.67 2.35

100

19.3 16.0

1.34 1.25

3.74 2.91

220 220 220 220 220

15.98 18.45 16.62 21.65

10.14 8.22 8.99 6.48 820

200

4600

a

b

Measured in deionized water; measuring error: 0.01%. Measured in equilibration with 0.1 M NaCl solution; measuring error: 0.01 Ω cm2.

different thicknesses of these membranes. The individual CEL and AEL were prepared by layer-by-layer solution casting technique in same solvent, and the thickness of contact or adhesion layer (IL) was kept about 10% of the total BPM thickness (220 μm) (Table 1). Among the different BPMs, BPM-GO showed low WU and high resistance (10.14 Ω cm2) in comparison with other BPMs containing PGO, QGO, or PGO/QGO IL (Table 2). GO is hydrophilic in nature, but Table 2. Ilim and Vdiss Values Estimated from i−V Curves BPMs

Ilim1 (mA cm−2)

Ilim2 (mA cm−2)

Vdiss (V)

BPM-GO BPM-QGO BPM-PGO BPM-PGO/QGO

2.50 3.75 6.00 8.75

33.75 39.90 46.25 57.50

3.42 3.11 2.83 2.39

functionalized GO grafted with acidic or alkaline groups seems to be appropriate for the IL formation as it forms uniform layers containing plenty of fixed charge groups and mobile counterions, responsible for high WU value.34 The GO does not make good adhesion, and neat interfacial contact with the CEL and AEL may be due to hydrophilic−hydrophobic balance among the CEL, AEL, and IL. In the case of the BPM with mixed cationic and anionic GO (PGO and QGO) IL, extremely low resistance (6.48 Ω cm2) revealed the suitability of BPMPGO/QGO for efficient water splitting. Introduction of an IL, which contains organic acid and/or base groups is very advantageous because cationic and anionic charged materials act as ionically conductive buffer suitable for charge transfer under applied electrical potential and responsible for water splitting.22,32,35 A modern BPM also contains a hydrophilic IL for the facilitation of water dissociation and significant reduction in the resistance of the BPM. The most optimized reported BPM (BPM-PGO/QGO) showed low ionic resistance in comparison with other commercialized BPMs (ASTOM Neosepta BP-1E, and Fumasep FBM).36 3.4. i−V Curves. The i−V curve of the BPM comprised three characteristic regions and revealed limiting current density (Ilim1) due to the transport of ions, which depends on the solution concentration, and counterion diffusion coefficient values (Figure 5).37,38 Vdiss, the water dissociation potential, is responsible for the formation of OH−/H+, and current density

Figure 4. Weight loss (%) for different BPMs after treatment under acidic, oxidative, hydrolytic, and basic environments.

conditions (boil water test at 95 °C for 24 h) about 9.0−10.0% weight loss for different BPMs was estimated. In highly acidic/ basic conditions (treatment with 5.0 M HCl or 5.0 M NaOH for 24 h) about 5.0−10.0% weight loss confirmed the stable nature of the reported BPMs. 3.3. Properties of the CEL, AEL, and BPMs. The physicochemical and electrochemical properties such as thickness, water content (WU), ion-exchange capacity (IEC), and areal resistance (Rm) for the CEL and AEL are compared with commercialized Neosepta CMX and Neosepta AMX membranes in Table 1.36 The prepared CEL (2.67 Ω cm2) and AEL (3.74 Ω cm2) exhibited marginally high areal resistance in comparison with commercialized Neosepta CMX (2.35 Ω cm2), and AMX (2.91 Ω cm2), in spite of comparable IEC for the prepared CEL and AEL, which may be attributed to D

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CEL and AEL allowed facilitated transport of counterions for the formation of acid and base (maintain the pH gradient).41 Quaternary ammonium groups were chosen to functionalize both the AEL and QGO due to their highly dissociable nature,1 while the presence of strong acidic −SO3H (CEL) and −PO3H2 (PGO) maintained high cation selectivity. These factors are very important for fabricating a high performance BPM. A modern BPM often incorporated with the IL for the facilitation of water dissociation due to orientation of water molecules under the influence of applied electric field.42 Generally, macromolecules/polyelectrolytes with functional groups are used as the IL and act as water dissociation catalysts. The presence of functional groups increases the hydrophilic nature and thus improve the polarization of water molecules and water retention capacity to avoid the dehydration in the IL region. The BMED cell containing five cell pairs (CP) with 73.70 cm2 effective area was used to assess the water splitting efficiency of different BPMs at 9.6−17.0 mA cm−2 applied current density using 0.10 M NaCl solution for converting into it corresponding acid and base. The detailed methodology for estimating theoretical rate of water splitting has been included in section S4 of the Supporting Information. Theoretical rates of water splitting for any efficient BPM at 9.6, 11.1, and 17.0 mA cm−2 applied current density also have been

Figure 5. i−V curves for different BPMs in equilibration with 0.10 M NaCl solution.

(Ilim2) (the current carried by co-ions due to leakage) is responsible for water diffusion to the IL.31,39 The high value of Ilim2 allowed the sufficient water refilling to the IL necessary for splitting and to avoid the BPM dry-out. Thus, Ilim2 is a measure of co-ion leakage across the BPM, whereas the magnitude of Vdiss depends on the catalytic activity groups present in the IL necessary for water splitting. The Ilim1, Ilim2, and Vdiss values were obtained from Figure 5 and are included in Table 2.40 Relatively high Ilim1 values for BPM-PGO/QGO in comparison with other prepared BPMs, confirmed a broad window for applying current/voltage across the BPM to achieve the water splitting without any electrode reaction (Table 2). Thus, fast and efficient water splitting across the reported BPM is an attractive feature. The BPM resistance leads to required voltage for water dissociation during electrodialysis (Vdiss), and BPM-PGO/QGO showed comparatively low Vdiss in comparison to BPM-PGO, BPM-QGO, or BPM-GO (Table 2). Thus, introduction of equi-weight mixed PGO/QGO as the IL showed a good catalytic effect on water splitting. It seems the water content of the CEL and AEL, and the dual charged nature of the IL controls the water splitting efficiency of the BPM, and BPM-PGO/QGO is highly efficient for electrochemical water dissociation. Further, the acidic/basic charged nature of the IL promotes the formation of H+/OH−. Further, co-ion leakage across the BPM occurs due to unequal permselectivities of the CEL and AEL. The formation of a resistive region (limiting region) beyond the water dissociation potential and relatively low Ilim2 may be the indicator of a maximum rate of co-ion leakage. BPM-PGO/QGO showed the highest Ilim2 values (50.50 mA cm−2) responsible for the minimum co-ion leakage. Thus, i−V curves revealed efficient water splitting, low resistance, and minimum co-ion leakage across the BPM-PGO/QGO membrane in comparison with other BPMs with PGO, QGO, or GO as the IL. 3.5. Bipolar Membrane Electrodialysis (BMED) for Water Splitting. The performance of the prepared BPM was also assessed by sustaining the pH gradient by water splitting at the IL using BMED. In BMED, electrode wash chambers were separate and water splitting species (H+/OH−) were reduced or oxidized accordingly in cathode or anode chambers, respectively. The electroneutrality in the two chambers adjacent to the BPM was maintained by the formation of acid and base product by salt (NaCl) splitting and electromigration of ions from desalinated chambers. The high permselectivity of the

Figure 6. Theoretical and experimental rate of water splitting (g/h−1) for different BPMs.

included in Figure 6. The following equation was used for the estimation of experimental rate of water splitting (g h−1). Rate of water splitting = [concentration of acid/base (mol/L) × volulme of H 2O(l) × 18.02 g/mol] ÷ time allowed for electrodialysis (h)

(1)

To assess the BPM performance, the BMED experiment was conducted for 8 h at different applied current densities (9.6, 11.1, and 17.0 mA cm−2). The experimental rates of water splitting for different BPMs at different applied current densities are also included in Figure 6. It was observed that at each applied current density, the theoretical rate of water splitting was comparatively higher than the experimental value. Further, the experimental rate of water splitting across BPMGO was the lowest and BPM-PGO/QGO exhibited the highest rate of water and different BPMs followed the sequence BPMGO < BPM-QGO < BPM-PGO < BPM-PGO/QGO. E

DOI: 10.1021/acs.iecr.7b03885 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research The current efficiency (CE) value for the BMED process was evaluated by formation of acid via water splitting using the following equation. η=

z(Ct − C0)VtF × 100% NIt

Current efficiency and energy consumption values for different BPMs reported in the literature and in this manuscript for water splitting under different electrolytic environment are also compared in Table 3.32,43−46 Different commercial BPMs

(2)

Table 3. Current Efficiency and Energy Consumption Values for Different BPMs Reported in the Literature and in This Manuscript for Water Splitting under Different Electrolytic Environment

C0 and Ct are the concentration of acid (mol/L) in the acid compartment at time 0 and t (s), respectively. z is the ion valence (1 for H+ ions). Vt is the solution volume of acid compartment at time t, F is Faraday’s constant (96 500 C/mol), I is the current (A), and N is the number of repeating units. The energy consumption E (kWh kg−1) was calculated by extrapolating the results for the production of 1 kg of acid in the following equation: E (kWh kg −1) =

∫0

t

UI dt CtVt

membrane Neosepta BP-1a BPMb FKB-PEEKc sBPMc FT-BP (FuMA-Tech GmbH)d BPM-PGO/QGOe

(3)

where U is the voltage across the BMED stack. Current efficiency (CE) values were estimated from the ratio of theoretical and experimental rate of water splitting. CE values for different BPMs at 17.0 mA cm2 applied current density have been included in Figure 7. Water splitting CE for

current efficiency (%)

energy consumption (kWh kg−1)

77−90 68.8 46.9 44.7 92

4.3 3.10 4.48 3.93 12−16

43, 44 32 45 45 46

87.2

2.73

in this manuscript

ref

a

Measured for the splitting of 0.39−0.50 M NaCl solution. bMeasured for the splitting of 0.50 M NaCl solution. cCommercial BPM used for producing alanine (amino acid) from sodium salt. dCommercial BPM used for producing tartaric acid from sodium salt. eMeasured for the splitting of 0.10 M NaCl solution.

or those reported in the literature showed either low CE or high energy consumption values in comparison with BPMPGO/QGO (reported in this manuscript) for converting electrolytes into their corresponding acid and base. These informations also revealed the suitability of reported BPMPGO/QGO for BMED or other elecro-membrane processes. Stability and performance of the BPM as a function of operating time determine the suitability for prolonged applications and confirm successful industrial operation of BMED technology. The permselectivity of the BPM implies its reluctance for co-ion leakage and governs the product (acid/ base) purity. Different BPMs (BPM-GO, BPM-QGO, BPMPGO, and BPM-PGO/QGO) were used for at least 10 experimental cycles of BMED at 17.0 mA cm−2 constant applied current density for 5 h, and co-ion leakage across the BPM was assessed by measuring the Cl−/OH− concentration in the acid/base compartments. These experimental conditions are relevant to pilot scale industrial operations. In the case of BPM-GO, BPM-QGO, and BPM-PGO, co-ion leakage was relatively high and further increased with the number of experimental cycles (Figure 8). BPM-PGO/QGO exhibited comparatively low co-ion leakage, and after six cycles marginal

Figure 7. Current efficiency (CE) of different BPMs for water splitting in 0.1 M NaCl solution at 17.0 mA cm−2 current density.

different BPMs increased with applied current density and BPM-PGO/QGO showed the highest CE among different reported BPMs and was assessed to be most suitable for the water splitting. Correspondingly, under 17.0 mA cm2 applied current density during BMED using the BPM-PGO/QGO membrane, for producing the corresponding acid and base from 0.10 M NaCl, 2.73 kWhr/kg of NaCl energy was consumed against 87.2 CE value. These observations may be attributed to alteration in the IL and thus BPM performance. Similarly, the CEL and AEL were used to prepare the different BPMs by altering the IL. Thus, the IL has a significant impact on the performance of the BPM, and resistance (Rm) of BPMs changed significantly with the alteration of the IL. BPM-GO showed 10.14 Ω cm2 resistance, and the Rm for BPM-PGO and BPM-QGO was measured as 8.22 and 8.99 Ω cm2, respectively. But, in the case of BPM-PGO/QGO, extremely low resistance (6.48 Ω cm2) indicates its high performance. It seems that charge polarization in the IL is more dominant in the case of a dual charged IL (as in the case of BPM-PGO/QGO) under the influence of an electric field, which is responsible for high water splitting and thus CE. Chemical defects caused by molecular polarization catalyzed by the IL bound with ion-exchange groups are also responsible for water splitting across the BPM.14

Figure 8. Co-ion leakage for different BPMs, after different experimental cycles. F

DOI: 10.1021/acs.iecr.7b03885 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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consumption values in comparison with different commercial BPMs, for converting electrolytes into their corresponding acid and base. Thus, a fundamental understanding of dual charged GO as an efficient water splitting catalyst may be a critical step for application of BPM in fuel cells and membrane electrolyzers in the broader context.

co-ion leakage was recorded. Although co-ion leakage across BPM-GO was relatively high, permselectivity (co-ion leakage) was stabilized after six experimental cycles. The permselectivity of BPM-PGO/QGO was initially almost constant, and after six experimental cycles a marginal increase in co-ion leakage was observed. These observations can be attributed to rearrangement of GO and mixed PGO/QGO interfaces under the influence of an electric field.6 Polar oxygenic and nonoxygenic groups present in GO or functionalized GO were strongly influenced by the polarity of the electric field and functional charged nature of the adjacent CEL and AEL. It seems that electrostatic forces among the CEL, IL, and AEL exclude coions due to structural alignment and BPM exhibited robust nature without any mass or co-ion crossover. This study confirmed a significant increase in the BPM performance when equi-weight mixed acidic and alkaline functionalized GO was used as the IL in comparison with pristine GO, PGO, or QGO. In this case, only the nature of the IL altered the BPM performance rather than the CEL or AEL, and the dual charged nature of GO improved the stability of the IL as well as BPM due to electrostatic interactions between opposite charged groups and their orientation in the applied electric field. Further, good adhesion among the CEL, IL, and AEL (same solvent) was responsible for intact trilayer structure with a charged region for high performance water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03885. Details about instrumental analysis, oxidative and hydrolytic stabilities, water uptake, ion-exchange, and membrane resistance measurements, estimation of theoretical rate of water splitting, two-compartment i− V measurement cell, and TGA and DMA curves (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*V. K. Shahi. E-mail: [email protected]; vinodshahi1@ yahoo.com. Fax: +91-0278-2566970. Tel.: +91-2782569445. ORCID

Vinod K. Shahi: 0000-0002-1622-053X Notes

The authors declare no competing financial interest.

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4. CONCLUSION CEL- and AEL-forming material was prepared by controlled sulfonation or chloromethylation and quaternization of PS. The layer-by-layer assembly of polymer electrolytes (CEL, IL, and AEL) was successfully prepared in the same solvent and the resultant trilayer robust BPM showed good adhesion among the three layers to avoid delamination. BPMs were prepared using GO, PGO, QGO, or an equi-weight mixture of PGO/ QGO in the IL, separately, and their stabilities were assessed under thermal, oxidative, and hydrolytic conditions. Prepared BPMs were characterized by WU and membrane resistance values, whereas the water splitting performance of the BPM was studied by i−V curves in terms of Ilim1, Ilim2, and Vdiss values. It was observed that not only ion-exchange layers (CEL and AEL) but also the IL played a crucial role in achieving the high performance BPM. Further, a highly charged (acidic and alkaline) thin IL acts as a catalytic layer to improve the water splitting performance of BPM-PGO/QGO. Mixed dual charged PGO/QGO was assessed to be an excellent water splitting catalyst due to significant reduction in BPM resistance and maintenance of the BPM permselectivity and stability over about 10 experimental cycles. The investigated conditions for the IL coating of dual functionalized GO with controlled thickness between the CEL and AEL also control the BPM permselectivity and thus co-ion leakage due to strong electrostatic interactions between the acidic and alkaline charged groups of the CEL, IL, and AEL. The presence of acidic and alkaline groups in the IL provides a hydrophilic bipolar environment, which acts as a proton/ hydroxide buffer and facilitates the transport of water splitting product (H+/OH−) across the CEL and AEL toward the respective electrode. The presence of dual fixed charged groups in the IL also promoted polarization of water molecules under the influence of electrical gradient. Good adhesion and interactions between the IL and ion-exchange layers are the controlling factors for BPM quality and stability. The reported BPM-PGO/QGO exhibited either high CE or low energy

ACKNOWLEDGMENTS Manuscript registration number: CSIR-CSMCRI-169/2017. The authors are grateful for financial support from ONGC Energy Trust (ONGC, India). Central instrumental facilities of CSIR-CSMCRI (Analytical Science Division) are also acknowledged.

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DOI: 10.1021/acs.iecr.7b03885 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX