Interpolymer ion exchange membranes for CapMix process

Desalination 482 (2020) 114384

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Interpolymer ion exchange membranes for CapMix process ⁎

T

Katarzyna Smolinska-Kempisty , Anna Siekierka, Marek Bryjak Wroclaw University of Science and Technology, Faculty of Chemistry, Department of Polymer and Carbon Materials, Wyb. St. Wyspianskiego 27, 50-370 Wrocław, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Interpolymers Ion exchange membrane CapMix process

In the presented paper interpolymeric ions exchange membranes were used in a capacitive mixing process. This was reach to increase the amount of energy generated during mixing waters with two different salinities. Membranes were prepared by extrusion of ethylene/styrene-co-divinylbenzene interpolymer followed by chemical modification of obtained foils. The extrudate contained 30 wt% of polystyrene crosslinked with different amount of divinylbenzene. Four pairs of membranes of different thickness were tested in CapMix process. It was found that the best membranes for energy harvesting were obtained from interpolymer with 2 wt% of DVB.

1. Introduction Renewable energy sources has gained constantly rising popularity for the last years. The enormous demand for energy results in the development of technology due to increased requests to the standard of life, population growth, etc. The Division for Sustainable Development Goals (DSDG) in the United Nations Department of Economic and Social Affairs (UNDESA) has reported that the total energy demand doubled between 1971 and 2012 and its consumption will grow dramatically in the next 30–40 years [1]. As the ‘traditional’ sources of energy are getting exhausted a need for searching some other energy origins has been launched. It seems that Blue Energy can be such new and dynamically developing approach [2]. Blue energy officially called Salinity Difference Energy (SGE) is a method of generation of energy by mixture two solutions with different salt concentrations. [3] The operation mechanism of this method is based on controlled mixing of high saline water, e.g. seawater, with less saline water, e.g. treated wastewater or river water [1]. Today, harvesting energy from the salt gradient can be done by three methods: Pressure Retarded Osmosis (PRO), Reversed Electrodialysis (RED) including Capacitive Reversed Electrodialysis (CRED) and Capacitive Mixing (CapMix) [1–5]. According to data collected by Mora nad Rijck energy production in SGE system generates over 100 times less carbon dioxide than energy obtained from coal combustion, however the price

is still more than twice as large [1,4,5]. When PRO and RED systems are well-recognized now, the Capacitive Mixing method is a relatively new one and is still in the research phase [6,7]. In the case of PRO, osmotic active membrane is placed between two solutions and osmotic flow propels a turbine, where the electricity is produced. In the case of RED, the pairs of ion-exchange membranes are permeable to cations and anions, and ion transport constitutes a current that is extracted as electricity. The CapMix process is based on mixing two solutions in a controlled way and allows generation of electrical current by periodically switching between high saline and low-saline solutions [6]. The key point of production energy by CapMix is related to accumulation of ions in electric double layer (EDL) of activated carbon as well as increase transportation of ions by wrapping electrodes with ion exchange membranes. It merges technologies of energy storage devices like supercapacitors or batteries and produces electricity using salt induced electrode potentials originated by faradaic and non-faradaic reactions. It is worth to note here that in opposition to PRO and RED methods, supported by mechanical or redox intermediates, the CapMix process produces electrical energy directly from the controlled mixing of solutions. No intermediates are employed [1–8]. The European Commission Report states that SGE technology costs are still high. What is more, reliability and survivability of most existing technologies in the saline environment are not fully recognized yet. In effect, the generated electricity is expensive so most of the current

Abbreviations: AC, activated carbon; AEM, anion exchange membrane; CapMix, capacitive mixing; CEM, cation exchange membrane; CH, high salt concentration solution; CL, low salt concentration solution; CRED, capacitive reversed electrodialysis; d, membrane thickness; IEM, ion exchange membrane; P, power production; PE//St-co-DVB, polyethylene//styrene-co-divinylbenzene; Pefficiency, efficiency of power during CapMix; Poperation, real power of CapMix; Ppump, power of pump; PRO, pressure retarded osmosis; Ptheoretical, theoretical power of CapMix; Q, charge in mAh/g; RED, reverse electrodialysis; SGE, salinity difference energy; Wd, water absorption calculated in relation to the dry membrane; Ww, water absorption calculated in relation to the membrane swelling; Zc, ion-exchange capacity; ΔGmix, Gibbs free energy of mixing; ΔV, potential rise ⁎ Corresponding author. E-mail address: [email protected] (K. Smolinska-Kempisty). https://doi.org/10.1016/j.desal.2020.114384 Received 18 November 2019; Received in revised form 13 February 2020; Accepted 15 February 2020 0011-9164/ © 2020 Elsevier B.V. All rights reserved.

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Fig. 1. The CapMix cell construction.

type Bralen FB 2-30 was from SLOVNAFT. Purified water (Milipore, Progard TS2, 18 MΩ/cm2) was used for solutions preparation as well as for cleaning procedure.

Table 1 IEM composition. No

St content %

PE content %

DVB content %

1 2 3 4

30 30 30 30

69.5 69 68 66

0.5 1 2 4

2.2. IEM preparation Ion-exchange membranes (IEM) were obtained by modification of PE//St-co-DVB interpolymer films. The solid interpolymer films were extruded on LabTech ultra-micro extruder to get film of 30–70 μm thickness. Four kinds of interpolymer with 30 wt% content of styrene and different amount of divinylbenzene were processed. The composition of PE//St-co-DVB interpolymers are shown in Table 1. Ion exchange membranes were prepared as follows:

recipients would not be able to use it. However, the same European Commission's Report forecasts that these costs may be reduced together with the improvement and development of SGE technologies [9]. To increase the efficiency of ion accumulation at the electrode, solid ion exchange membranes (IEM) were used in the performed research. The membranes were made on polyethylene/styrene-co-divinylbenzene (PE//St-co-DVB) interpolymer where polyethylene served as a matrix. Styrene polymerizes and penetrates the matrix forming the interpenetrating polymer network. Aromatic rings of styrene were chlorosulfonated and finally hydrolysis or aminated to get ion exchange membranes; cation exchange for hydrolyzed and anion exchange for aminated routes [10–13]. The detailed procedures of membrane preparation were described previously [10–13]. For purpose of this work, the thin membranes (around 30 μm) were prepared by extrusion of interpolymer matrix followed by chemical modification. Interpolymer ion exchange membranes were successfully used for electrochemical separation processes so far [14–16]. Below can be found the scheme for of CapMix setup assembled with interpolymer membranes (Fig. 1).

1. Swell polyethylene pellets with mixture of St and DVB with dissolved benzoyl peroxide BP (Brabender mixer, ambient temperature, 30 wt% of St and DVB, 1% BP, 3 h.) 2. Polymerize St and DVB into polyethylene pellets (Brabender mixer, 90 °C, 3 h.) 3. Homogenize interpolymer in two-screw extruder (120 °C) followed by granulation 4. Extrude thin films (LabTech ultra-micro extruder, 30–70 μm films) 5. Chlorosulfonate the interpolymer films 6. Aminate films of chlorosulfonated interpolymer with ethylenediamine. Resultant AESD-2 membrane (see Fig. 2) 7. Hydrolyse films of chlorosulfonated interpolymer in 0.5 M NaOH. Resultant KESD-2 membrane (see Fig. 2).

2. Materials and method

2.2.1. Chlorosulfonation of the interpolymer The membranes were placed in a mixture of chlorosulfonic acid (15 mL) and dichloroethane (35 mL) for 1 h at ambient temperature. After this time, they were transferred to dioxane (25 mL) for 30 min. Finally, they were toughly washed by methanol.

2.1. Materials Ethylenediamine (EDA), styrene (St), divinylbenzene (DVB) and chlorosulfonic acid were from Sigma-Aldrich. Dioxane was from Honeywell. Methanol form PPH Stanlab. Sodium chloride, dichloroethane, sodium hydroxide, benzoyl peroxide and hydrochloric acid were from POCH, Poland. Dimethylformamide (DMF) and acetone, ethanol were from Krakchemia SA, Poland. Poly(vinyl chloride) (PVC) Ongrovil® S-5167 was supplied by BorsodChem. Polyethylene LDPE

2.2.2. Hydrolysis The chlorosulfonated membranes were placed in 20% w/w aqueous solution of sodium hydroxide for 24 h at ambient temperature. Finally, they were washed and storage at purified water. 2

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Fig. 2. Preparation of ion-exchange membranes.

2.2.3. Amination The chlorosulfonated membranes were kept for 24 h in 50% solution of ethylenediamine in mixture of 1:1 of water and methanol at ambient temperature. After this time, the membranes were transferred to 1 M hydrochloric acid and kept for 24 h. Finally, they were washed and stored in water.

were measured by goniometer PG-X (Fibro Systems) for all membranes. Total surface energy of samples as well as its acid–base components were calculated according to the harmonic averaging protocol and the van Oss' approach, respectively.

2.3. Analytical section

To study the energy production phenomenon by the CapMix process the laboratory electrodialyzer FT-ED-100-4 was employed. The stack was comprised of two electrodes covered by investigated ion exchange membranes, which had been divided by polymeric spacer (thickness 200 μm). The electrolyser was biased by Multi-Range Programmable DC Power Supplies BK Precision 9201 and controlled by DC Electronic Load BK Precision 8601.

2.4. CapMix setup

2.3.1. Water uptake Water uptake was determined in relation to both, the swollen (Ww) and dry membrane (Wd), according to Eqs. (1) and (2). After modification, the membranes were kept in water, then weighed and dried at 110 °C for 24 h.

m w − md × 100% md

(1)

m − md Ww = w × 100% mw

(2)

Wd =

2.4.1. Electrodes The electrodes were made of activated carbon (AC) YP-50F delivered by Kuraray Chemical Co. LTD. Osaka, Japan. Its characteristic can be found in Table 2. Electrodes were prepared via casting knife. A 25 wt % carbon suspension in 3.5 wt% solution of PVC in DMF was applied to a 0.2 mm thick graphite substrate and solvent was allowed to evaporate at room temperature. After that electrodes were immersed and kept in water. The detailed procedure for the preparation of electrodes has been described previously [16,17].

where: Wd: water absorption calculated in relation to the dry membrane (%),Ww: water absorption calculated in relation to the membrane swelling (%), md: dry membrane mass (g), mm: mass of the swollen membrane (g), 2.3.2. Ion exchange capacity The ion-exchange capacity was determined by the OH-/Cl- cycle according to the Eq. (3)

Zc =

(CHCI X VHCI − VNaOH X CNaOH ) X A m

2.4.2. Solutions For the CapMix process NaCl solutions were used as feed. As a higher concentrated solution (CH) 20, 40, 60, 80 and 100 g/L were investigated while as lower concentrated solution (CL) 4 g/L were employed. To monitor the conductivity, pH and temperature values the CX-601 multimeter was applied.

(3)

where: Zc – ion-exchange capacity (mol/g) CHCl - concentration of HCl used for titration (mol/g) CNaOH - NaOH concentration (mol/g) VHCl volume of HCl (cm3) VNaOH - volume of NaOH (cm3), m - mass of the samples (membrane), A - dilution factor (2.5).

2.4.3. CapMix procedure The CapMix procedure was performed according to the protocol provided in [18]. In study of energy production by the CapMix cell,

2.3.3. IR spectroscopy To monitor the progress of surface modification ATR-IR spectra were collected. The studies were carried out on Perkin-System 2000 where 64 scans with resolution of 4 cm−1 were gathered.

Table 2 Material parameters of activated carbon [1,2].

2.3.4. Contact angle and surface energy Dynamic contact angles of water, diiodomethane, and formamide 3

Sample

Specific Surface area (m2g−1)

Pore volume (cm3g−1)

Pore diameter (nm)

AC

1670

0.757

1.65

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Fig. 3. The CapMix cell processing cycle.

To determine the power efficiency of the CapMix process, power of pump and electrical power were related to particular operation power (Eq. (7)):

mixing of two solution with various salt concentration was used. The main idea of CapMix performances is shown in Fig. 3. The cell was formed by composite electrodes assembled of ion exchange membranes and activated carbon layer. The energy harvesting process followed four steps:

PEffciency =

1. High-saline solution filled the cell when circuit was open (phase I), 2. The cell was charged by means of an external device (phase II), 3. High-saline solution was replaced by low-saline one. The circuit open (phase III), 4. The cell was discharged. In this step the energy was extracted (phase IV).

Poperation (Ppump + Ptheoretical )

∙100%

(7)

where Poperation is an integral of current, I, and voltage, V, during the charging step. 3. Results and discussion 3.1. Interpolymer membrane characteristics

2.4.4. CapMix calculations The power production can be evaluated taking into account that charge is stored into the cell during II phase (at time Tcharge). The storage is assisted with current I and voltage V. During phase IV the charge is extracted at a higher voltage and is shown by the voltage rise ΔV. It appears when time of discharge is equivalent to time of charge Tdischarge = Tcharge (phase IV). Hence, the power production (mW/m2) can be expressed by [19]:

Tdischarge ⎞ P = ⎛∆VI /A Ttotal ⎠ ⎝ ⎜

The solid interpolymer films have been extruded from the pelleted PE/St-co-DVB interpolymer. The thinnest foils, 30 μm, were obtained from the interpolymer with 2% DVB content. Other foils had a thickness of about 40–50 μm (see Table 3). After extrusion, the foils were subjected to chemical modification. The effectiveness of the modification process was monitored by infrared spectrophotometry. Fig. 4 shows the IR spectra for membranes before (NM) and after modification (CEM, AEM) for representative samples (with 2% DVB). In the spectrum of cation exchange membranes a peak characteristic for SeO bond can be observed. This band usually appears at wavelength between 1560 and 1730 cm−1. Additionally, the strong peak is observed between 3550 and 3200 cm−1 which is related to appearance OeH stretching. The presence of OeH group confirmed the second step of modification and incorporation cation exchange group into polymeric chains. In the spectrum of anion exchange membranes several peaks confirm the

⎟

(4)

where A is a total area of electrode. Critical parameter in electro-membrane processes is power (or energy) efficiency. Due to leak of other forces producing energy in the CapMix process, the power of pump is necessary to evaluate power efficiency of the operation. The power of pump is given by Eq. (5) according to Rica et al. [21]:

Ppump = ∆P∙ν∙ncell

Table 3 CEM characteristic.

(5)

where ΔP is pressure drop (according to peristaltic pump specification, 1 bar of pressure drop was used), ν is volumetric flow rate (m3/s) and ncell is a number of cell pairs. The electrical power of process is calculated by means of Eq. (6):

ptheoretical = I∙V

(6) 4

No

d μm

1 2 3 4

54.00 43.02 33.01 46.00

± ± ± ±

20.61 17.70 13.15 17.08

Ww %

Wd %

Zc mmol/g

50 48 52 45

99 93 111 81

0.91 1.10 1.03 0.97

± ± ± ±

0,253 0,192 0,010 0,191

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Fig. 4. IR spectra, unmodified membrane (NM, green), cation exchange membrane (CEM, blue), anion exchange membrane (AEM, orange).

effectiveness of the amination. At the wavelength between 1007 and 1170 cm−1 it is a peak attributed to CeN bond while between 1600 and 1790 cm−1 - peak for NeH bond. Additionally, medium peak is observed between 3500 and 3300 cm−1 that, in this case, comes from NeH stretching. It directly confirms presence of aliphatic primary amines after amination reaction. To confirm modification extent the ion exchange capacity was checked. The affinity of membranes to ions was evaluated by their ion exchange capacity (Zc). As is presented in Table 3, the CEM ion exchange capacity was in the range 0.91 mmol/g to 1.1 mmol/g. Slightly lower values were achieved for anion exchange membranes: 0.63 mmol/g to 0.8 mmol/g (Table 4). The smaller Zc of AEM resulted of lower ion exchange capability of amino groups and two-stage modification process. Similarly, water uptake by both membranes was higher for CEM than for AEM. Water uptake was from 45 to 50% for CEM, and 17–27% for AEM (see Tables 3, 4). In order to check the character of the membrane surface, the contact angle was measured and the surface energetics was calculated. The contact angle of membranes was determined by three liquids: water, formamide (FA) and diiodomethane (DIM). This allowed to determine the surface energy and its acidic and basic components. The water contact angle of unmodified membrane was about 72.9° for water, 61.3° for FA and 38° for DIM. For both CEM and AEM modified membranes, the values of all angles were slightly smaller (Fig. 5). This additional confirmed the effectiveness of the modification process. In the case of cation exchange membranes, sulfonic groups were located on the membrane surface thus an increase of acid component of the surface energy was observed. In the case of AEM, the presence of eNH2 component of ethylenediamine made the surface more basic (Table 5).

Fig. 5. Contact angels measured for IEM.

Table 5 Surface Energy of IEM.

Energy extraction in the CapMix process should be evaluated for the following operation parameters: time of charging and discharging, time of switching the solutions, current densities and concentrations of highsaline and low-saline solutions. The pair of IEM with the same Table 4 AEM characteristic. d μm

1 2 3 4

54,00 43,02 33,01 46,00

± ± ± ±

20,61 17,70 13,15 17,08

Ww %

Wd %

Zc mmol/g

24 20 27 17

32 25 38 21

0,69 0,72 0,80 0,63

± ± ± ±

Surface energy mJ/m

Basic component mJ/ m

Acidic component mJ/ m

NM CEM AEM

44.2 41.3 46.4

15.1 13.4 25.3

0.2 1.9 0.2

percentage of DVB were tested for energy production. Hence the CapMix cell was equipped with AEM or CEM prepared from interpolymer with 0.5% of DVB, or 1% of DVB, or 2% DVB, or 4% DVB. To check if IEMs were able to affect energy production, the effect of their presence was compared to the system with non-covered electrodes. It was observed the presence of ion exchange membranes increased energy production (Fig. 6A), Taking into account the principle of energy production, the critical parameter seemed to be voltage rise (ΔV), shown in Fig. 3 for III phase of CapMix cycle [21,22]. By increasing value of voltage rise, the amount of power production was boosted. The phenomenon of high energy production for systems with IEM is shown in Fig. 6A and B, where the relationship of voltage rise to ion exchange capacity of membranes is presented. By increasing the value of IEC the voltage rise and power production were raised. The most effective pairs of membranes for energy production have been marked by the grey field in Fig. 6A. However, for more complex inside into the power production the other process parameters had to be checked. As it is presented in Fig. 7, the voltage rise dropped with an increase membrane thickness. This phenomenon was related to obstacles in ions transfer to electrical

3.2. CapMix process with interpolymer membranes

No

Membrane

0,003 0,11 0,06 0,02

5

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Fig. 6. Power production depends on the type of applied membranes (A) (CL = 4 g/L, CH = 80 g/L, tch = tdisch = 3 min, ttot = 8 min, j = 1.38 A/m2), different concentration of saline solutions (B) (IEMs:2 wt% of DVB, CL = 4 g/L, tch = tdisch = 3 min, ttot = 8 min, j = 1.38A/m2), current densities (C) (IEMs:2 wt% of DVB, CL = 4 g/L, CH = 80 g/L, tch = tdisch = 3 min, ttot = 8 min), and time of charge and discharge (D) (IEMs:2 wt% of DVB, CL = 4 g/L, CH = 80 g/L, j = 1.38 A/m2).

Fig. 7. The relations of voltage rise and IEC for CEM (A) and AEM (B) and for thickness of CEM (C) and AEM (D). 6

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factor of 1.5. In the meantime, power consumption for charging step increased from 33 to 184 mW/m2 for 3 and 10 min, respectively. Hence, the power requested for prolonged charging step exceeded power production. In the case of 10 min charging, the system lost the energy instead to produce it. The conducted studies allowed to select the best cation and anion exchange membranes. They were prepared from PE//St-co-DVB interpolymer that contained 2 wt% of divinylbenzene. The best ion exchange membrane of each group was used as the standard in assembling the CapMix system composed of CEM and AEM. Fig. 8A shows power production for systems build of the best CEM and AEM. The higher power production was observed for two pairs: for CEM and AEM with 2 wt% of DVB and for CEM with 2 wt% of DVB and AEM with 4 wt % of DVB. When the system is reversed, it means AEM had 2 wt% of crosslinker and CEM was changed, the CapMix system produced the largest portion of energy for cation exchange membrane with 2 wt% of DVB (Fig. 8B). Hence, the best system for energy production should be composed of cation and anion exchange membranes obtained from interpolymer that contains 2 wt% of divinylbenzene. As it was mentioned before higher voltage rise increases production of power. The relationship of voltage rise over charge for charging and discharging steps was monitored for selected membrane systems. Fig. 9C, D and F presented the relationships for CEM(2 wt%):AEM(2 wt%), CEM(2 wt%):AEM(4 wt%) and CEM (1 wt%):AEM(2 wt%), respectively. Unlike to the other configurations, the systems listed above exhibited the higher voltage rise that was estimated to 164 mV, 193 mV and 169 mV, respectively. Hence, the highest power production for these systems was consequence of the highest voltage rise . The last analysed parameter was the power efficiency in the charging step. According to Eqs. (5), (6) and (7) it was possible to estimate the theoretical and operational power. Moreover to estimated real power efficiency for the CapMix process the power of pump was calculated according to Eq. (5). This operation was conducted for all selected configurations and results are listed in Table 7. Taking into account the energy efficiency, the lowest ratio was found for the systems with higher power production. It was noted that charging curve for the systems CEM(2 wt%):AEM (2 wt%), CEM(2 wt%):AEM(4 wt%) and CEM(1 wt%):AEM (2 wt%), has slightly different shape that in other systems. The linear shape corresponds to “accumulator mixing” (AccMix) that could be obtained when the electrode surface is assembled with ion exchange membranes [19]. The operation with IEMs allowed to rise potential during mixing of two solutions. For this reason, the properties of IEMs play a key role in power production of the CapMix process. The CapMix process is one of three methods that allows to generate power from salinity difference. The others methods are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Taking into account the principles of these methods, CapMix is characterized by the lowers energy requirements without application high pressure pump (PRO) and high current density (RED). In Table 8 the comparison of these methods is presented. Brogioli et al. tested chemically inert activated carbons at the CapMix process without application IEMs [22]. They found that the main drawbacks of the CapMix process is the loss of stored charge due to undesired chemical reactions on activated carbon [22]. Power production was estimated around 50 mW/m2 [22]. The Zhan's group [29] applied positively charged quaternized poly(4-vinylpyridine) coated activated carbon and negatively charged oxidized activated carbon as a asymmetric CapMix (Asy-CapMix) [29]. They found that this configuration allowed to reach voltage rise of 150.0 mV and power production of 65.0 mW/m2 [29]. Hence, the modification of activated carbon electrodes raised power extraction in the CapMix process. Finally, Fernández et al. compared the “soft” IEMs with commercial Fumasep FAS and Fumasep FKS ion exchange membranes. The “soft electrodes” were combinations of the thin layered polyelectrolytes on activated carbon electrodes. They applied cationic polymers as

double layers of carbon electrode. Resistance of membranes and voltage drop in charging step were related to the membrane thickness. These relationships can be withdrawn form GSC theory of electrical double layers [21]. Moreover, water uptake in particular membranes could affect their intrinsic resistance [23]. In the case of evaluated KESD2 and AESD2 membranes, water uptake was the highest for IEMs with 2 wt% of DVB (Tables 2, 3). That directly corresponds to voltage rise and finally to power production by the CapMix system. In the CapMix process the compositions of the high-saline and low-saline water could increase the free Gibbs energy and, in the same way, affect voltage rise and result in enlarging production of energy. After selection of the most promising pair of IEMs, the process parameters of CapMix were evaluated. The effective of such factors were determined: current densities, time of charging/discharging and concentration of high-saline and low-saline solutions. Fig. 6B presents the relationship of power production due to the concentration of high-saline solutions. The right selection of Clower-saline and Chigher-saline solution allowed increasing the energy upper limitation [20]. Any increase of salt concentration causes the energy rise according to (Eq. (8)):

− ∆Gmix ≅ 2RT [cmixing ln cmixing − ϕcL ln cL − (1 − ϕ) cH ln cH ]

(8)

where: R: gas constant, T: temperature, Cmixing: concentration occur after mixing and ϕ is fraction of low saline water per total volume of applied solutions. The calculated data of Gibbs free energy of mixing per unit volume are presented in Table 6. Based on these calculations it can be expected that any increase of high-saline concentration increases the energy to be extracted by means of the CapMix system. Current density from external power supply was the next verified parameter. The charge density had the direct dependency on the surface charge density and affected the rise of potential difference [6]. The results are shown in Fig. 6C. It was observed the linear relationship between the power production and applied current density. Hence, the power production increased linearly with increasing external charge. However, the increase of current densities over 3 times caused raise of power production of 30% only. According to [24], the dissipative lost during the process could be negligible when the power energy over the current density approaches asymptotically the straight line. This behaviour described that during electrochemical process the oxidation or reduction reactions were not observed. In the case of nonlinear correlation the power lost should be effected by degradation of electrode material. In our case the CapMix process was conducted at voltage lover than 1.23 V to prevent water split. Hence, for the use of interpolymer membranes, it can be stated that the loss of extracted energy was not observed. The last evaluated parameter was the contact time for charging/ discharging steps. The results are present in Fig. 6D. By making the charging time longer, the EDLs of II phase took higher capacitance what increased the potentially extracted power. Hence, with increasing time of charging, the charge accumulated in EDLs was higher, the potential raised, the capacitance expanded and in consequence more energy was extracted by mixing two solutions [25]. When one triplicates charging and discharging times one gets an increase of power production by Table 6 Gibbs free energy of mixing solution depends on their salt concentrations. CL [g/dm3]

CH [g/dm3]

ΔGmix [J/dm3]

4

20 40 60 80 100

−246 −724 −1246 −1787 −2337

7

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140

A

140

CEM with 2% of DVB

120

B

AEM with 2% of DVB

P (mW/m2)

P (mW/m2)

120

100

100

80

80 0.5

1

2

4

0.5

1

2

4

Type of cation exchange membrane

Type of anion exchange membrane

Fig. 8. Powers production with changing type of (A) canion exchange membranes and (B) ation exchange membranes. Conditions: (CL = 4 g/L, CH = 80 g/L, tch = tdisch = 3 min, ttot = 12 min, j = 1.38A/m2).

electric double layers [EDLs] [30]. Taking into account the above outcomes, application of stable IEMs seems to be a key point to increase power production by CapMix. All of three possible methods of power production using salinity gradient phenomenon were compered. In Table 8 are summarized results of power extraction for described above different CapMix conception as well as for RED and PRO processes. The different processes were compared using the net value of power production which means

PDADMAC (poly(diallyldimethyl ammonium chloride)) or PEI (polyethyleneimine) and anionic polymers PSS (poly(sodium 4-styrenesulfonate)) or PAA (poly(acrylic acid)). They found that application of PSS and PDADMAC allowed to extracted 50 mW/m2 [30]. The low power production in comparison to commercial IEMs (105 mW/m2) can be explained by inhomogeneous distribution of polyelectrolytes on the electrodes surface. It could be an effect of penetration of polyelectrolytes into electrodes, blocking porous structure or decreasing of

Fig. 9. The voltage rise over the charge for mixed IEM with different content of DVB: CEM with 2% of DVB and AEM with 0.5% of DVB (A), CEM with 2% of DVB and AEM with 1% of DVB (B), CEM with 2% of DVB and AEM with 2% of DVB (C), CEM with 2% of DVB and AEM with 4% of DVB (D), CEM with 0.5% of DVB and AEM with 2% of DVB (E), CEM with 1% of DVB and AEM with 2% of DVB (F), CEM with 4% of DVB and AEM with 2% of DVB (G), without IEM (H). Conditions: (CL = 4 g/ L, CH = 80 g/L, tch = tdisch = 3 min, ttot = 12 min, j = 1.38 A/m2). 8

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200

Type of CEM [wt % of DVB]

Type of AEM [wt% of DVB]

Energy efficiency [%]

Power production [mW/m2]

2 2 2 2 0.5 1 4

0.5 1 2 4 2 2 2

86.3 85.1 36.0 45.3 69.6 32.4 61.7

64.4 116.0 165.7 152.2 125.7 167.7 143.9

Power production (mW/m2)

Table 7 Power efficiency and power production for investigated configurations.

that in the CapMix, RED and PRO processes the energy consumption of peristaltic or high-pressure pump, were subtracted. Vermaas et al. presented that in small scale of RED stack it was possible to generate the net power at 0.7 W/m2 level [31]. The same team provided that manipulating of intermembrane distance in the RED stack, the power generation was risen and could be estimated to 1.2 W/m2 by mixing river and sea water [26,27]. Kim and Elimelech reported that for NaCl solutions with 12.5 bar of hydraulic pressure difference it was possible to generate 4.72 W/m2 (theoretic power production was 6.76 W/m2) [28]. Due to different requirements for compared systems it is difficult to judge now about the best method for large scale application. However, such comparison can be helpful for further development of all blue energy methods. From the chemical engineering point of view, the crucial element is stability of power production in time. Fig. 10 presents stability studies for two configurations of IEMs. The red colour is related to CEM with 2% of DVB and AEM with 2% of DVB, while black one to CEM with 1% of DVB and AEM with 2% of DVB. Based on these date, it can be concluded that investigated IEMs did not change the system properties and offered stable power production of 168.5 mW/m2 for the first membrane system and 162.5 mW/m2 for the second system. In summary, evaluated PE//St-co-DVB membranes guarantee the power production in the CapMix process in a stable way.

CEM:2, AEM:2 CEM:1, AEM:2

180

160

140

120

100 0

2

4

6

8

10

Cycles Fig. 10. Power production for two systems over 10 cycles.

the systems with the largest voltage rise and the highest power production were built from CEM(2 wt%):AEM(2 wt%), CEM(2 wt%):AEM (4 wt%) and CEM(1 wt%):AEM(2 wt%) membranes. Above configurations showed the lowest energy efficiency during charging step. This fact can be seen by different shape of curves during the charging step that is typical to Accumulator Capacitance (AccCap). Acknowledgment The authors wish to acknowledge the financial support of the bilateral collaboration program (TUBITAK-NCBR-2549) between Turkey and Poland and express their thanks to Dr. K. Szustakiewicz for extrusion of IPN membranes.

4. Conclusions

Author's contributions

The ion exchange membranes application have an vital effect on rising power production by the CapMix process. Application of IEMs increased power production over 60 times compare to configuration without membranes. The best configuration is comprised with membranes build from 2 wt% of DVB. This fact is caused by increased voltage rise at switching point of salt solutions. To control the voltage rise the following IEMs parameters should be monitored: ion exchange capacity, thickness and water uptake. In mixed membrane configuration,

KSK obtained and characterized IEM and participated in the preparation of the manuscript, AS performed and characterized the CapMix process and participated in manuscript preparation, MB corrected the manuscript. Authors statement Katarzyna Smolinska-Kempisty obtained and characterized IEM and

Table 8 Comparison of different methods of power production from salinity gradient. Type of process

Current density [A/m2]

Hydraulic pressure [bar]

Membrane

Salinity of feed

Power production [W/m2]

Reverse Electrodialysis

15

–

Commercial

1.2

[28]

Reverse Electrodialysis

n/a

–

Commercial

0.7

[32]

Pressure Retarded Osmosis

–

12.5

Commercial

4.72

[29]

CapMix

n/a

–

Without

0.05

[23]

CapMix

6.1

–

Without

0.065

[30]

CapMix

n/a

–

“Soft electrodes”

0.05

[31]

CapMix

n/a

–

Commercial

0.105

[31]

CapMix

1.38

–

PE/St-co-DVB

CH = 30 g NaCl CL = 1 g NaCl CH = 30 g NaCl CL = 1 g NaCl CH = 117 g NaCl CL = 29 g NaCl CH = 29 g NaCl CL = 1.7 g NaCl CH = 29 g NaCl CL = 1.7 g NaCl CH = 29 g NaCl CL = 1.7 g NaCl CH = 29 g NaCl CL = 1.7 g NaCl CH = 80 g NaCl CL = 4 g NaCl

0.16

This study

9

Ref.

Desalination 482 (2020) 114384

K. Smolinska-Kempisty, et al.

participated in the preparation of the manuscript, Anna Siekierka performed and characterized the CapMix process and participated in manuscript preparation, Marek Bryjak corrected the manuscript.

127 (1) (1984) 171–186. [14] V. Bhadja, S. Chakraborty, S. Pal, R. Mondal, B.S. Makwana, U. Chatterjee, A sustainable and efficient process for the preparation of polyethylene–polystyrene interpolymer based anion exchange membranes by in situ chloromethylation for electrodialytic applications, Sustainable Energy Fuels 1 (3) (2017) 583–592. [15] V. Bhadja, J.S. Trivedi, U. Chatterjee, Efficacy of polyethylene Interpolymer membranes for fluoride and arsenic ion removal during desalination of water via electrodialysis, RSC Adv. 6 (71) (2016) 67118–67126. [16] M. Dinda, U. Chatterjee, V. Kulshrestha, S. Sharma, S. Ghosh, G.R. Desale, et al., Sustainable synthesis of a high performance inter-polymer anion exchange membrane employing concentrated solar radiation in a crucial functionalization step, J. Membr. Sci. 493 (2015) 373–381. [17] A. Siekierka, J. Kujawa, W. Kujawski, M. Bryjak, Lithium dedicated adsorbent for the preparation of electrodes useful in the ion pumping method, Sep. Purif. Technol. 194 (2018) 231–238. [18] A. Siekierka, Preparation of electrodes for hybrid capacitive deionization and its influence on the adsorption behavior, Sep. Sci. Technol. (2019) 1–12. [19] D. Brogioli, R. Ziano, R.A. Rica, D. Salerno, F. Mantegazza, Capacitive mixing for the extraction of energy from salinity differences: survey of experimental results and electrochemical models, J. Colloid Interface Sci. 407 (2013) 457–466. [20] M. Marino, L. Misuri, R. Ruffo, D. Brogioli, Electrode kinetics in the ‘capacitive mixing’ and ‘battery mixing’ techniques for energy production from salinity differences, Electrochim. Acta 176 (2015) 1065–1073. [21] R.A. Rica, R. Ziano, D. Salerno, F. Mantegazza, R. van Roij, D. Brogioli, Capacitive mixing for harvesting the free energy of solutions at different concentrations, Entropy 15 (4) (2014) 1388–1407. [22] D. Brogioli, et al., Exploiting the spontaneous potential of the electrodes used in the capacitive mixing technique for the extraction of energy from salinity difference, Energy Environ. Sci. 5 (12) (2012) 9870–9880. [23] N. Boon, R. Van Roij, Blue energy’ from ion adsorption and electrode charging in sea and river water, Mol. Phys. 109 (7–10) (2011) 1229–1241. [24] G.M. Geise, M.A. Hickner, B.E. Logan, Ionic resistance and permselectivity tradeoffs in anion exchange membranes, ACS Appl. Mater. Interfaces 5 (20) (2013) 10294–10301. [25] M. Marino, et al., Modification of the surface of activated carbon electrodes for capacitive mixing energy extraction from salinity differences, J. Colloid Interface Sci. 436 (2014) 146–153. [26] G.R. Iglesias, M.M. Fernández, S. Ahualli, M.L. Jiménez, O.P. Kozynchenko, A.V. Delgado, Materials selection for optimum energy production by double layer expansion methods, J. Power Sources 261 (2014) 371–377. [27] D.A. Vaermaas, M. Saakes, K. Nijmeijer, Doubled power density from salinity gradients at reduced intermembrane distance, Environ. Sci. Technol. 45 (2011) 7089–7095. [28] Y.C. Kim, M. Elimelech, Potential of osmotic power generation by pressure retarded osmosis using seawater as feed solution: analysis and experiments, J. Mem. Sci. 429 (2013) 330–337. [29] F. Zhan, G. Wang, T. Wu, Q. Dong, Y. Meng, J. Wang, J. Yang, S. Li, J. Qiu, High performance asymmetric capacitive mixing with oppositely charged carbon electrodes for energy production from salinity differences, J. Mater. Chem. A 5 (2017) 20374–20380. [30] M.M. Fernandez, R.M. Wagterveld, S. Ahualli, F. Liu, A.V. Delgado, H.V.M. Hamelers, Polyelectrolyte-versus membrane-coated electrodes for energy production by capmix salinity exchange methods, J. Power Sources 302 (2016) 387–393. [31] J. Veerman, M. Saakes, S.J. Metz, G.J. Harmsen, Electrical power from sea and river water by reverse electrodialysis: a first step from laboratory to a real power plant, Environ. Sci. Technol. 44 (2010) 9207–9212.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D.A. Mora, A. de Rijck, Blue Energy: Salinity Gradient Power in Practice, GSDR, 2015 (Brief). [2] R.A. Tufa, E. Curcio, E. Fontananova, G.D. Profio, Membrane-based processes for sustainable power generation using water: pressure-retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CAPMIX), Compr. Membr. Sci. Eng. 2 (2017) 206–248. [3] M.M. Fernández, O.O. Flores, G.R. Iglesias, G.R. Castellanos, A.V. Delgado, L.A. Martinez, New energy sources: blue energy study in Central America, J Renew. Sustain. Ener. 9 (2017) pp.014101-8. [4] R.A. Tufa, S. Pawlowski, J. Veerman, K. Bouzek, E. Fontananova, G. di Profio, Progress and prospects in reverse electrodialysis for salinity gradient energy conversion and storage, Appl. Ener. 225 (2018) 290–331. [5] M.A. Abdelkareem, M.E.H. Assad, E.T. Sayed, B. Soudan, Recent progress in the use of renewable energy sources to power water desalination plants, Desalination 435 (2018) 97–113. [6] D. Brogioli, R. Ziano, R.A. Rica, D. Salerno, et al., Exploiting the spontaneous potential of the electrodes used in the capacitive mixing technique for the extraction of energy from salinity difference, Energy Environ. Sci. 5 (2012) 9870–9880. [7] A. Cipollina, G. Micale, Capacitive mixing and mixing entropy battery, Sustainable Energy from Salinity Gradients (2016) 181–218. [8] M. Marino, O. Kozynchenko, S. Tennison, D. Brogioli, Capacitive mixing with electrodes of the same kind for energy production from salinity differences, J Phys.: Condensed Matter 28 (11) (2016) 114004 1–10. [9] Blue Energy Action Needed to Deliver on the Potential of Ocean Energy in European Seas and Oceans by 2020 and beyond Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of Regionsand Impact Assessment Synopsis. Directorate-General for Maritime Affairs and Fisheries, European Union, 2014, https://doi.org/10. 2771/32703. [10] M. Bryjak, G. Poźniak, W. Trochimczuk, Synthesis and properties of porous ionexchange membranes, Angew. Makromol. Chem.: Appl. Macromol. Chem. Phys. 200 (1) (1992) 93–108. [11] M. Bryjak, G. Poźniak, N. Kabay, Donnan dialysis of borate anions through anion exchange membranes: a new method for regeneration of boron selective resins, React. Funct. Polym. 67 (12) (2007) 1635–1642. [12] M. Bryjak, G. Poźniak, W. Trochimczuk, Porous ion-exchange membranes, 2. Effect of adhesion promoters, Angew. Makromol. Chem.: Appl. Macromol. Chem. Phys. 205 (1) (1993) 131–139. [13] G. Poźniak, W. Trochimczuk, Tubular interpolymer ion exchange membranes. Cation exchange membranes based on polyethylene modified with styrene-divinylbenzene copolymers, Angew. Makromol. Chem.: Appl. Macromol. Chem. Phys.

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