Nanocomposite and nanostructured ion-exchange membrane in salinity gradient power generation using reverse electrodialysis

CHAPTER

Nanocomposite and nanostructured ionexchange membrane in salinity gradient power generation using reverse electrodialysis

13

Jin Gi Hong*, Haiping Gao†, Lan Gan†, Xin Tong†, Chengchao Xiao†, Su Liu†, Bopeng Zhang†, Yongsheng Chen† Department of Civil Engineering and Construction Engineering Management, California State University, Long Beach, CA, United States* School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, United States†

13.1 INTRODUCTION The development of renewable and sustainable energy-conversion technology is widely recognized as an important strategy for global energy security. The need for new, clean energy sources is becoming extremely important due to the escalating environmental concerns of continued reliance on current energy sources. Water salinity is one such renewable energy source that has yet to be fully explored and its utilization through the ion exchange process is widely recognized as more feasible and applicable considering the abundance of seawater and various other water sources of different salinities. The technology of harvesting electrical energy from water salinities is named blue energy or salinity gradient energy (SGE). SGE has gained a considerable amount of attention as a zero-emitting and nonpolluting energy technology over the last decade. Through the mixing of fresh water with salt water, free energy is created, and thus the chemical potential of low salinity (LS) water and high-salinity (HS) water converts into electrical energy. The theoretical amount of worldwide available energy due to the salinity gradient of seawater mixed with all river water is estimated at nearly 2 TW of power production (Hong et al., 2013; Logan and Elimelech, 2012). One of the most promising techniques to harvest SGE is reverse electrodialysis (RED). In RED, the mixing of two aqueous solutions with different salinities leads to Advanced Nanomaterials for Membrane Synthesis and Its Applications. https://doi.org/10.1016/B978-0-12-814503-6.00013-6 Copyright # 2019 Elsevier Inc. All rights reserved.

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a change in Gibbs free energy that can be liberated as electrical energy through ion transport in selective membranes (Pattle, 1954). Specifically, RED utilizes the transport of cations and anions in the water through selective ion exchange membranes to produce electricity. In a RED stack, cation exchange membranes (CEMs) and anion exchange membranes (AEMs) are placed alternately between electrodes. The concentrated salt water and diluted fresh water are fed through narrow compartments, which are formed by spacers between each membrane. Ionic diffusion in water allows the migration of ions that are selectively determined by corresponding ion exchange membranes (IEMs): the passage of cations through CEMs and anions through AEMs. This ion discrimination by IEMs results in an electrochemical potential difference and further drives cations toward the cathode and anions in the other direction toward the anode. At the surface of electrodes, these ionic species convert to electron current via oxidation-reduction reactions facilitated by the electrode rinse solution used in the system. As a result, an electron can travel from the anode to the cathode through an external electrical circuit, which generates an electrical current (Fig. 13.1). A full stack in an RED system can be composed of numerous membrane cell pairs used to generate electrical power for an external load or energy consumer. RED power generation using salinity gradient has great potential to play a vital role in the sustainable development of energy sources, due to its technical and economical superiority when compared to fossil-fuel systems or other forms of renewable sources such as solar and wind. The successful application of RED for salinity gradient power mainly depends on membrane performance, like many other membrane-based systems, so the role of membranes is of considerable importance in maximizing the RED power performance. However, the application is currently limited by high-membrane cost, the absence of RED-optimized IEMs, and thus a low gross power density. Today, IEMs have attracted wide interest with growing research

FIG. 13.1 A schematic of RED. Only one pair of ion exchange membranes is shown as a representative example.

13.1 Introduction

and practical applications across various fields, ranging from water treatment to industrial separation, to power generation, particularly using electrodialysis (ED), electrodialysis reversal (EDR), and fuel cells. Normally, each application emphasizes its own physical and electrochemical requirements as the properties of the membranes. Since the membranes are key components in this electrochemical system, their properties have been studied by many researchers to determine preferred characteristics and to investigate their significance to the power performance. Considering the nature of the electrochemistry cell system, it is feasible that the membrane properties, such as electrical area resistance, permselectivity, and ion exchange capacity, would have significant effects on RED power performance. Specifically, low area resistance, high selectivity, high-ion exchange capacity, and high-charge density are reported to be desirable for ion exchanging processes such as RED. However, it is often challenging to tailor membranes for such features and the membranes with different chemistry formation and materials result in inconsistent characteristic performance. Among many different physical formations of IEMs, nanocomposite structural combinations of the membrane are discussed in this chapter. Nanocomposite structure of IEMs specifically for RED applications was first proposed by Hong and Chen in 2014 (Hong and Chen, 2014). Composite membranes created by introducing inorganic nanomaterials into an organic polymer matrix are viable candidates for RED, as they allow carrying extra ion-exchangeable functional groups by modulating the membrane structure. Nanomaterials have long been incorporated in various forms and applications and are often considered as good filler material due to their versatile characteristics: (1) large specific surface area, (2) wide use, (3) cost effectiveness (easy to produce), (4) ability to adsorb various inorganic and organic materials, and (5) ease of being coated on various surfaces. Moreover, embedding functionalized inorganic nanomaterials in polymeric structures can be considered as homogeneous up to certain level of loadings. It is important to have a homogeneous ion exchange body of structure, because the fixed charge groups are more evenly distributed over the entire membrane matrix in homogeneous regions, while heterogeneous membranes have distinct regions of uncharged polymer of ion exchange groups in the membrane matrix. Therefore, charge density in heterogeneous IEMs is relatively low compared to homogeneous membranes, and electrical resistance is higher owing to its membrane structure where an uncharged domain exists in the same polymer matrix. Moreover, organic-inorganic composites combine the unique features of inorganic materials with those of organic materials and have received increasing attention for more than a decade, owing to their remarkable synergized properties. Therefore, nanocomposite structure of the IEMs can be a promising approach to enhance the physical and electrochemical properties of membranes, which directly affects the power performance in RED systems. In this chapter, key IEM properties, methods of membrane synthesis, and advanced nanomaterials and nanostructures (e.g., nanopore) will be discussed in conjunction with RED for salinity gradient power generation.

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13.2 KEY PROPERTIES OF ION EXCHANGE MEMBRANES IEMs can be classified by their different charged functional groups: cation exchange membranes (CEMs) contain negatively charged functional groups (such as sulfonic acid (―SO3
), carboxylic acid (―COO
), etc.), and permit the passage of cations but exclude all anions; anion exchange membranes (AEMs) contain positively charged functional groups (ammonium (―NH3+), secondary amine (―NRH2+), tertiary amine (―NR2H+), quaternary amine (―NR3+)), and can thus transport anions but reject cations (Mei and Tang, 2018). For both type of IEMs, membrane properties concerned in RED systems are generally the same: high-ionic conductivity, high selectivity over counter-ion and coions, high-ion exchange capacity, and moderate mechanical strength to maintain integrity in applications. Membrane thermal and chemical stabilities are important indicators of the durability of the membranes; in addition, membrane mechanical strength is also important to ensure the durability of the membranes, because hydraulic pressure and osmotic pressure difference exist inside the RED stack. The thermal stability of the IEMs is related to the crosslinking degree of the polymeric material, thermal stability of the inert polymeric material, and reinforcing fabric (Hong et al., 2015b). The common temperature for the RED system is around room temperature so the requirement for thermal stability of the membranes in RED is not high. Membrane chemical stability measures the durability of the membranes in different acidic and/or basic solutions. Chemical stability is vital in membrane systems such as ED. In ED systems, the electrical current applied can inevitably dissociate water and generate proton and hydroxyl ions. It is important for the membranes to have the ability to withstand harsh pH conditions in such systems. However, in RED systems, the dissociation of water molecules is limited to a negligible degree, so the pH of the whole system is expected to be stable. In addition, since the feed solutions (river water, seawater, concentrated brines, etc.) are close to neutral, the membranes used in the RED system do not need to have high-chemical stability. The membranes need enough mechanical strength to withstand the osmotic pressure difference between solutions with different salinities, as well as the hydraulic pressure from water flow. Membrane permselectivity and ionic resistance are the most important properties determining the power generation performance of the RED system. Other important membrane properties include ion exchange capacity (IEC), membrane swelling degree (SD), and fixed charge density (CD) (G€uler et al., 2013; Yip et al., 2014). Membrane permselectivity characterizes the ability of a membrane to select counter-ions (cations for CEM and anions for AEM) and repulse coions (anions for CEM and cations for AEM). A perfect IEM has a permselectivity of unity since it can eliminate the passage of coions. In real systems, a small portion of coions could also contribute to the ionic current, so the permselectivity would be smaller than one. The apparent permselectivity is applied in practice in RED to characterize the actual membrane potential Em under given solution concentrations. The relationship is given by the following equation:

13.2 Key properties of ion exchange membranes

  RT ac Em ¼ αm ln ad F

(13.1)

where αm is the apparent membrane permselectivity, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and αc and αd are the activities of the concentrated solution and the diluted solution, respectively. Membrane ionic resistance measures the ability of the membrane to oppose the passage of ionic current. Thus, if the level of ionic resistance increases, the ability of conducting ions in the system (i.e., ionic conductivity) would decrease. The resistance in RED systems is determined by experiment. Membrane resistance is usually measured indirectly, since the membrane is in solution, and the working conditions such as temperature, solution concentrations, and concentration gradient across the membrane can significantly affect the result. A common set-up for the resistance measurement is shown in Fig. 13.2. A four-electrode system has the advantage to account for only the potential drop across the membrane under a given current density. Therefore, the membrane resistance can be measured either in direct current mode from the current-to-voltage curve or in alternating current mode using EIS (Dlugolecki et al., 2010; Zhang et al., 2017). If the two compartments contain solutions of different salinity, direct measurement of membrane potential can be used to deduce the permselectivity of the IEM of interest. With membrane permselectivity and resistance measured, the performance of the RED system can be estimated by calculating maximum power output (Eq. 13.2) if the resistance from other parts of the stack is known: Pmax ¼

E2stack 4ARstack

(13.2)

where Pmax is the maximum of power output, Estack is the accumulative potential over the whole stack, A is the effective area of membranes, and Rstack is the overall electrical resistance of the whole stack including membrane resistance.

FIG. 13.2 Experimental set-up for membrane resistance and permselectivity measurement.

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Membrane IEC indicates the number of fixed charged groups in the membrane matrix in a unit weight of dry membrane. The IEC of the ion exchange membrane is often determined experimentally by a titration method using strong acid (HCl) for CEMs and strong base (NaOH) for AEMs, respectively. Swelling degree represents the water content or uptake of the membrane under a given solution condition. The SD of a membrane relates to the nature of the membrane materials (structure, hydrophilicity, etc.) as well as the solution conditions. The SD can decrease the ionic resistance of the membrane, especially for AEMs (Długołęcki et al., 2008); it can also decrease the permselectivity of the membrane. For application in RED systems, sacrificing membrane permselectivity to reduce membrane ionic resistance may be beneficial if a much lower membrane ionic resistance is achieved. However, excessively high SD may also lead to the increase of ionic resistance by more than three orders of magnitude (Geise et al., 2013). In addition, membrane mechanical strength also decreases by membrane swelling. Thus, membrane swelling degree needs to be confined within a certain range to achieve optimal performance in RED systems. Higher membrane IEC indicates larger density of charged functional groups present in the membrane matrix, which is beneficial for an ion transport; however, membrane swelling can also decrease the density of functional groups. The membrane fixed CD, the ratio of membrane IEC and SD, describes the overall relationship between the two membrane properties. By directly comparing CD of different membranes, an overall understanding of membrane properties can be easily obtained, since in many cases other membrane properties (IEC, permselectivity, SD) do not change simultaneously (Hong et al., 2015b). The morphology of homogeneous IEMs tells important structural information, but morphological study is even more critical in the case of nanocomposite membranes. Characteristics such as the distribution of nanoparticles within the matrix, phase interaction, and change of surface appearance can all be revealed under microscopy. Typical techniques include scanning electron microscopy (SEM) and coupled energy-dispersive X-ray spectrometry (EDX), which provide geographical analysis of elements on the sample surface (Fig. 13.3). Polymer-filler interfacial

FIG. 13.3 SEM image of nanocomposite cation exchange membrane (SiO2-sPPO) surface (left) and EDX analysis of iron on the surface (right) (Hong et al., 2015a).

13.3 Nanocomposite IEMs for RED

morphology studied through SEM builds the fundamentals for surface property tuning and nanofiller agglomeration control (Li et al., 2013; Tong et al., 2017). Further investigation of surface hydrophilicity can be conducted using atomic force microscopy (AFM).

13.3 NANOCOMPOSITE IEMs FOR RED 13.3.1 SYNTHESIS OF CEMs The selective function of CEMs is realized by the negatively charged functional groups. Nanocomposite CEMs serve the same purpose. These membranes mainly consist of two parts, the polymeric matrix and the nanofiller. Both parts can be modified to improve membrane properties. Several methods have been developed to synthesize nanocomposite CEMs. Generally, these methods can be divided into three types: physical blending method, sol-gel method, and infiltration method (Tripathi and Shahi, 2011; Li et al., 2013). For different composite materials, different methods may apply. Physical blending methods disperse the prepared nanoparticles into the polymer matrix by solution blending or melt blending, followed by the solidification of the membrane (Fig. 13.4). This method is simple and makes it easy to combine multicomponents for hybrid formation. The polymeric solution and the nanofiller are independently prepared and then mixed. In this way, physical blending has good flexibility. Multiple types of polymers and nanofillers can be employed to make nanocomposite CEMs through this method. There are no fussy restrictions on the use of certain polymers and nanofiller materials caused by physical and intrinsic properties such as chemical structure, composition, size, shape, etc. However, it is often challenging to have uniform distribution of nanoparticles in the polymeric matrix. In situations

FIG. 13.4 Schematics of a typical procedure of direct blending method in nanocomposite membrane synthesis.

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like this, the nanofillers tend to aggregate, leading to the uneven distribution of nanoparticles in the membrane matrix, change of membrane morphology and properties, and defects. The dosage of nanofillers needs to be carefully optimized to mitigate the aggregation problem and to ensure the best membrane performance. Sol-gel methods are classical in situ processes to attach nanoparticles onto the polymeric membrane. The precursors of the desired nanoparticle are suspended in a solution that is deposited on the polymer substrate by coating, dipping, or spinning. Then the precursors condense into the nanoparticles through chemical reactions initiated by heating, addition of initiators, etc. Gel is formed in the condensation process. The main disadvantage of the sol-gel method is the small range of available types of membrane materials, concentrating on silicon and metal materials. Also, the modification of the nanofillers is relatively complicated. However, this method provides good dispersion of nanofillers in the membrane matrix, which brings better contact of nanoparticles and polymer than the physical blending method does. This method has been studied for many years, especially for membrane applications in fuel cells (Brandon et al., 2003; Merle et al., 2011). In the infiltration method, pores or void spaces within the polymer membrane are first enlarged by swelling them in the solution, and then the nanoparticle precursors are infiltrated. Nanoparticles can grow in the swollen space of the polymer membrane (Tripathi and Shahi, 2011). This method allows wider options of materials than the sol-gel method, while keeping the nanofillers well-dispersed. The main drawback of this method is the effect of diffusion resistance of the precursor suspension. It causes a concentration gradient from the membrane surface to the inside, so a certain extent of inhomogeneity is inevitable (Klein et al., 2005). The first work on nanocomposite CEMs for RED applications was reported by Hong and
Chen (2014). In this study, functionalized iron (III) oxide Fe2 O3
SO4 2
acted as the inorganic filler and was combined with a sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (sPPO) polymer matrix. This study compared the electrochemical properties and the power generation performance of the tailor-made nanocomposite membrane and the commercial membrane, presenting the potential of nanocomposite CEMs for RED power generation systems. Tong et al. reported on nanocomposite CEMs made from oxidized multiwalled carbon nanotubes (O-MWCNTs) blended with sPPO, showing the possibility of using O-MWCNTs to improve the properties of CEMs (Tong et al., 2016), and studied the mechanism of ion transport in nanocomposite CEMs by comparing the performance of sPPO-based CEMs blended with sulfonated silica nanoparticles and unsulfonated silica nanoparticles (Tong et al., 2017).

13.3.2 SYNTHESIS OF AEMs A functioning AEM relies on cationic moieties, positively charged ligands on a polymer matrix that attracts anions. Quaternary ammoniums are usually the ionic sites present on an AEM. To get cationic moieties, dissolved polymers can be modified to contain positively charged functional groups, or a preformed membrane can be modified chemically to carry the required ligands. The most common modification

13.3 Nanocomposite IEMs for RED

process involves a chloromethylation process, to get the backbone ready for the quaternary ammonium function group. Then, the following quaternization reaction inserts the functional group. During the chloromethylation process, however, a highly toxic chemical, chloromethyl methyl ether, is commonly used. Recent developments in green chemical usage have found multiple substitutions that have less toxicity, such as N-bromosuccinimide (NBS) as a halomethylation agent for bromomethylation (Zhao et al., 2014) or para-formaldehyde for chloromethylation (Hibbs et al., 2009). Direct chemical modification performs well with homogeneous membrane preparation. However, nanocomposite membranes contain organic and inorganic segments. Technically, these membranes qualify as heterogenous membranes. For nanocomposite membranes, nanoparticles as inorganic materials are introduced during the casting process by distributing nanoparticles into the casting solution. As a result, this kind of membrane is slightly different from heterogeneous membranes with continuously separable two or more phases, but with intertwined organic and inorganic segments. A hybrid membrane is therefore a more specific name (Merle et al., 2011). In practice, the membrane can be fabricated by direct mixing of ion exchange resin with backbone polymer. For example, a mixer of polyvinyl alcohol (PVA) and Indoin FFIP (chloromethylated polystyrene), plasticized by polyethylene glycol, can render the required exchange capacity to the matrix (Merle et al., 2011). PVA, mixed with Amberlyst IRA-402 as ion exchange resin, and additional polyaniline and MWCNTs have been used to fabricate nanocomposite AEMs used in electrodialysis systems (Hosseini et al., 2014). Similar to the synthesis of nanocomposite CEMs, the sol-gel process plays an important role in nanocomposite membrane synthesis, because covalent bonds are formed between inorganic and organic molecules. In this case, the inorganic part is usually silane or siloxane for mechanical support. Crosslinkers are important to form chemical bonds between polymer chains. Organic compounds such as formaldehyde blended with acids, or polymers such as poly(GMA-co-γ-MPS) (copolymer of glycidylmethacrylate(GMA) and γ-methacryloxypropyl trimethoxy Silane) have been used in nanocomposite AEM synthesis as crosslinking agents (Wu et al., 2010; Nagarale et al., 2005). Nonetheless, specific nanocomposite AEMs for RED applications have not been reported in the field. Nanocomposite membranes from related fields may provide the experience and insight to develop RED-specific AEMs. Due to relatively low requirements of mechanical strength, membranes synthesized for RED applications usually do not require backing fabric for support. Moreover, ionic resistance and permselectivity overweigh all other membrane characters in RED applications, and thus they guide the focus for future development.

13.3.3 POTENTIAL ADVANCED NANOMATERIALS Inorganic nanomaterials have been used to fabricate ion-selective membranes for application in RED systems (Hong et al., 2015a; Hong and Chen, 2014; Feng et al., 2016; Tong et al., 2016), even though the type of nanomaterials and polymer materials that have been applied in membrane synthesis still need further investigation.

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13.3.3.1 Inorganic nanomaterials in IEM synthesis Several inorganic nanomaterials have been used to synthesize inorganic-organic nanocomposite IEMs for RED applications, including iron oxide and silica oxide (Hong and Chen, 2014; Hong et al., 2015a). Those nanocomposite materials can combine the good features of both inorganic nanomaterials and organic polymeric materials. By controlling the membrane synthesis process, the obtained nanocomposite IEMs can have reduced ionic resistance and increased permselectivity, which is desired for RED applications (Hong et al., 2015b; Table 13.1). Hong and Chen (2014) successfully developed RED-specific CEMs using sPPO and iron oxide nanoparticles. The membrane resistance has been decreased to as low as 0.87 Ω cm2 with moderate permselectivity (87.65%). Iron oxide nanoparticles are incorporated into the polymer matrix by direct blending before casting. An optimal dosage of nanoparticles as related to membrane electrochemical properties has also been reported. Hong et al. (2015a) further expanded the search for nanoparticle and polymer combinations related to different sizes of sulfonated silica oxide. Membrane resistance was decreased to 0.85 Ω cm2. Membrane morphology changes have been linked to nanoparticle filler sizes and potentially affected the resistance and permselectivity. Existence of an optimal dosage of nanoparticles of different types of nanomaterials used in nanocomposite membrane synthesis implies a complex correlation between membrane properties, morphological changes, and phase separation and interactions. However, there is limited literature discussing the mechanism of nanocomposite structure and its effect on membrane characteristics. A previous study indicates that the membrane nanoscale structure change upon the addition of inorganic nanomaterials can increase the efficiency of ion transport, which could be the origin of membrane property enhancement (Tong et al., 2017). The dispersion of nanomaterials in an organic solvent and polymeric materials is also closely related to the transport property of the resulted nanocomposite membranes. Many studies have concluded that nanoparticle aggregation has a negative influence on membrane properties and transport (Tong et al., 2017; Hong et al., 2015a; Klaysom et al., 2011; Zuo et al., 2009b). Moreover, the surface characteristics of nanomaterials are important since they are related to the dispersion properties of the nanomaterials. Nanomaterials with more functional groups on the surface generally have better dispersion properties, since their surface is more hydrophilic. Also, different fabrication methods, such as direct blending and the sol-gel process, have been applied to increase the affinity between the nanomaterials and polymeric materials.

13.3.3.2 Carbon-based nanomaterials in IEM synthesis Carbon-based nanofillers, such as carbon black, carbon nanotubes (CNTs), carbon nanofibers, graphene, and its derivatives such as graphene oxide (GO), have been extensively investigated to develop polymer/organic-inorganic nanocomposite membranes. Among them, CNTs and GO are some of the most promising materials used for advanced polymeric nanocomposites. Due to their outstanding properties, including low mass density, high-aspect ratio, high flexibility, excellent mechanical

Table 13.1 Existing Nanocomposite Ion Exchange Membranes and Nanostructures Nano Material

Organic Material

Swelling Degree (%)

IEC (mEq/g)

Conductivity/ Resistance

Al2O3

PVA

–

–

0.2–0.4 Ω cm

CeO2

Nafion

17–22

–

0.018 S/cm

Fe2O3

Nafion

22.9–40.7

–

–

Fe2O3

PPO

20–26

0.87–1.4

0.87–2.05 Ω cm2

Fe2NiO4

PVC

17–23

1.5–1.6

9.1–12.8 Ω cm2

MWCNT

PVA

38.2–284.0

0.7–2.25

–

SiO2

PVDF

10–26.2

1.25–2.0

0.0026–0.0041 S/cm

SiO2

PES

9.7–14.3

0.74–1.1

0.00007–0.00024 S/cm

SiO2

PPEK

28–70

–

–

SiO2 SiH4

PAES PEO

26–37.5 127–203

– 0.4–0.99

0.08–0.13 S/cm –

TiO2

Nafion

30–36.5

–

0.0705–0.0947 S/cm

TiO2

Nafion

–

–

–

ZrO2

Nafion

21–27

0.9–1.13

–

2

Applications

References

Quaternized composite membrane for alkaline direct methanol fuel cell (DMFC) Chemically durable proton exchange membrane for fuel cells High-proton conductivity composite membrane for DMFC Salinity gradient power generation using RED Performance evaluation of heterogeneous CEM Crosslinked nanocomposite membrane for DMFC Electrochemical characterization of CEM Electrodialysis IEM for desalination Proton exchange membrane for DMFC Fuel cell application Thermally stable negatively charged NF membrane Solid superacid composite membrane for DMFC Electrochemical performance for DMFC Conductive composite membrane for PEMFC

Yang et al. (2010)

Wang et al. (2012) Sun et al. (2010)

Hong and Chen (2014) Hosseini et al. (2012) Yun et al. (2011) Zuo et al. (2009a) Klaysom et al. (2010) Su et al. (2007) Lee et al. (2007) Wu et al. (2005) Wu et al. (2008) Baglio et al. (2005) Zhai et al. (2006) Continued

Table 13.1 Existing Nanocomposite Ion Exchange Membranes and Nanostructures—cont’d Nano Material

Organic Material

Swelling Degree (%)

IEC (mEq/g)

Conductivity/ Resistance

ZrO2

Nafion

–

–

0.13–0.15 Ω cm

ZrO2

Nafion

20–30

0.84–0.92

–

GO Sulfonated GO

SPEEK SPEEK

24–38 10–37

0.75–1.11 0.75–1.65

1.08–2.87 mS/cm 1.08–8.42 mS/cm

Zwitterionic GO

PBI

10.2–35.2

0.27–1.12

7.1–19.5 mS/cm

Multifunctional GO

Nafion

20.9–29.9

0.92–1.02

–

GO

SPES

12.12–15.19

1.27–1.40

23.4–64 mS/cm

GO

PVC

8.5–13

1.15–1.40

–

ZrO2

Nafion

–

–

–

ZnO

PVC

10–50

–

15–18 Ω cm2

SiO2

Commercial Fumasep FAP

20–22

1.07–1.16

0.7–1.088 Ω cm2

2

Applications

References

Solid polymer electrolyte electrolyzer application Performance at high temperature/low humidity for PEMFC DMFC Sulfonated GO composite membrane for DMFC Zwitterionic GO composite membrane for DMFC High-proton conductivity composite membrane at low humidity High stability GO composite IEM for DMFC and ED GO composite CEM for water deionization Proton conductivity for hightemperature DMFC Electrodialysis for water treatment Vanadium redox flow batteries

Siracusano et al. (2012) Sacca et al. (2006) Heo et al. (2013) Heo et al. (2013) Chu et al. (2015) He et al. (2017)

Gahlot et al. (2014) Hosseini et al. (2017) Navarra et al. (2009) Parvizian et al. (2014) Leung et al. (2013)

GO, graphene oxide; MWCNT, multiwalled carbon nanotube; PAES, poly(arylene ether sulfone); PBI, polybenzimidazole; PEO, polyethylene oxide; PES, polyethersulfone; PPEK, poly(phthalazinone ether ketone); PPO, poly(2,6-dimethyl-1,4-phenyleneoxide); PVA, polyvinyl alcohol; PVC, polyvinyl chloride; PVDF, polyvinylidene fluoride; SPEEK, sulfonated polyetheretherketone; SPES, sulfonated polyethersulfone.

13.3 Nanocomposite IEMs for RED

strength, and electronic conductivity, CNT nanocomposite membranes have been extensively explored to enhance the membrane stability, proton/ionic conductivity, and other membrane properties in electrochemical energy conversion and storage systems, especially in fuel cells applications (Spitalsky et al., 2010). It has been reported that the CNT/Nafion nanocomposite proton exchange membrane (PEM) exhibited improved dimensional stability and mechanical properties, while no apparent enhancement was achieved in proton conductivity (Liu et al., 2006). This result suggests that the absence of surface functional groups and the resultant poor dispersion in the polymeric matrix might be critical hurdles for CNTs being employed to effectively modify polymer membranes (Ou et al., 2018). Surface functionalization is the commonly used route to obtain uniform dispersion of CNTs within polymer matrixes. Surface functionalization of CNTs can be accomplished by coating or grafting simple acid groups (e.g., carboxylic, phosphoric, and sulfonic acids), inorganic proton conducting materials (e.g., boron phosphate, BPO4), and polyelectrolytes (e.g., polystyrene sulfonic acid). CNTs can also be modified to carry positive charge with ammonium salt functionalization (Gao et al., 2013). Enhanced proton conductivity is achieved with incorporating sulfonic acid or phosphoric acid group functionalized CNTs into PEMs (Kannan et al., 2008, 2009, 2011; Jha et al., 2013). The acid groups in CNTs interconnect some of the proton conductive domains present in the polymer, thus providing a proton conducting network (Kannan et al., 2011). Apart from functionalizing CNTs with simple acid groups, inorganic proton conductors such as BPO4 have also been utilized to modify CNTs (Gong et al., 2016). BPO4 can be effectively coated on CNTs through a facile sol-gel method. The nanocomposite membrane composed of polymer electrolyte and BPO4 functionalized CNTs exhibits improved proton conductivity and stability simultaneously. In comparison to the previously mentioned functionalized CNTs, polyelectrolyte-functionalized CNTs are proven to be more compatible with polymer matrixes, and to possess more active sites for further modification depending on specific application (Liu et al., 2009). Ou et al. (2018) fabricated chitosan-based PEMs by blending chitosan matrix with chitosan-coated CNTs. Chitosan is introduced onto the CNT surface by a facile noncovalent deposition assisted with a crosslinker. The chitosan coating layer can significantly improve the dispersity of CNTs in a chitosan matrix, which can largely increase the opportunity for transferring the excellent properties of CNTs to nanocomposite membranes and obtain the synergistic effect of polymer and nanofiller (Datsyuk et al., 2008). Based on the PEM studies, it is suggested that functionalized CNTs could be used as nanofiller to fabricate nanocomposite membranes for RED (Ou et al., 2018). The incorporation of functionalized CNTs into nanocomposite IEMs might provide a continuous network or additional ionic pathways to facilitate ion transport, which is a desired property for IEMs in RED applications. Tong et al. (2016) fabricated carboxylic acid-functionalized CNT nanocomposite CEMs for RED. The physiochemical and electrochemical properties of the prepared CNT nanocomposite membranes are reported as improved. With optimal oxidized CNT loading, much lower membrane resistance, that is, better ion conductivity,

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higher permselectivity and IEC, can be obtained, which finally leads to elevated power generation. Meanwhile, it is found that the antiscaling potential can be improved with an oxidized CNT nanocomposite membrane. The hydrophilic property and surface charge introduced by the functionalized groups on CNTs results in enhanced hydrophilicity, surface charge density, and surface morphology change, which leads to better antifouling potential of nanocomposite membranes (Tong et al., 2016; Vatanpour et al., 2011). However, the relatively high cost, complicated process of fabricating high-density vertically aligned CNTs, and difficulties in obtaining large-scale production are the primary obstacles for the practical application of CNTs (Liu et al., 2015). The development of graphene-based materials has given rise to a new class in materials science, owing to its exciting properties and low price in recent years. As one of the most attractive graphene derivatives, GO can be easily synthesized from graphite through various chemical oxidation methods (e.g., Hummers method). GO is a two-dimensional material with epoxy and hydroxyl groups on the basal plane and carbonyl and carboxyl groups on the edges. The abundant oxygen-containing functional groups offer many reactive sites for interacting with organic and inorganic materials to prepare a functional graphene-based nanocomposite membrane with high hydrophilicity, outstanding mechanical stability, and good electrochemical properties. Additionally, owing to the presence of these oxygen-containing groups, GO exhibits good solubility both in water and commonly used solvents (e.g., dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP)) and is easily modified to possess other functional groups such as sulfonic acid, amino, and zwitterionic groups (Heo et al., 2013; Chu et al., 2015; Enotiadis et al., 2012). The uniform distribution of GO in polymer matrixes has been shown via characterization with transmission electron microscope (TEM) and SEM (Gahlot et al., 2014; Hosseini et al., 2017). In recent years, GO has been investigated increasingly as nanofiller for the development of innovative graphene-based nanocomposite polymer electrolyte membranes for different applications (Hou et al., 2011). The amphiphilic properties attributed to the hydrophilic functional groups and hydrophobic conjugation give GO the ability to modify the microstructure of the hydrophobic backbone and hydrophilic ionic cluster of the polymer. Then, the transport properties of nanocomposite membranes are favorably manipulated by incorporation of GO or functionalized GO (f-GO) (Choi et al., 2012). It has been reported that the ionic conductivity can be remarkable enhanced owing to the manipulated ion channels in a GO nanocomposite IEM with optimal GO content (Gahlot et al., 2014). Recently, GO-embedded CEMs utilized in water deionization have been reported (Hosseini et al., 2017). The ion permeability, transport number, IEC, and membrane surface hydrophilicity of GO nanocomposite membranes have been increased to different extents. These results indicate that it is possible to fabricate desired GO nanocomposite IEMs for RED applications. On the other hand, due to the very highhydrophilic property, GO has been intensively studied for its membrane antifouling potential in water treatment (Lee et al., 2013; Zinadini et al., 2014). With the

13.4 Emerging nanostructures in SGE harvesting

increased hydrophilicity of GO nanocomposite membranes, the antifouling potential is expected to be enhanced significantly, especially for AEMs sensitive to organic fouling (Vermaas et al., 2013).

13.4 EMERGING NANOSTRUCTURES IN SGE HARVESTING Due to transport limitation of regular IEMs, some researchers have switched gear to investigate the possibility of taking advantage of two-dimensional materials under nanoscale to fabricate high-electron density, highly selective, and highly conductive nanostructures to enhance ionic transportation. Nanopore membrane and nanofluidic channels are good examples in this field. Even though some of these studies targeted other applications such as electrodialysis, the principles and working conditions of these nanostructures make them applicable to RED as well. Zhao et al. (2013) proposed charged nanopores drilled in a graphene sheet as IEMs for the electrodialysis application. Because of the negatively charged nanopore edge, the pore exhibited permselectivity of K+ ion over Cl
ion. The scale of the pore makes the permselectivity highly dependent on charge density in space affected by the nanopore diameter and charge numbers. Ionic conductivity was also enhanced remarkably by two orders of magnitude compared to regular IEMs with permselectivity of nearly unity. Rollings et al. (2016) found similar selectivity of single graphene nanopores as a CEM in a KCl solution. Additionally, monovalent selectivity has also been shown in fabricated nanopores. They also observed K+/Cl
selectivity holds when the pore diameter is about 20 nm, which significantly reduces difficulty of fabrication on a large scale. To explain this phenomenon, they proposed that the graphene surface may carry a pH-dependent surface charge due to deprotonatable oxygen-containing chemical groups on the graphene surface, rendering the pore with permselectivity due to electrostatic repulsion. Feng et al. (2016) demonstrated the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators that can produce a large, osmotically induced current from a salt gradient with an estimated power density of up to 106 W/m2. MoS2 has electron redistribution between Mo and S atoms, giving rise to charged edge atoms once nanopores are created. According to the charge polarity, for S2
terminated pores, the S2
would highly cover the edge of the pore, making the nanopore very selective. Mo-terminated nanopores are positively charged and can create a high Coulombic barrier for cationic species, implying a very highenergy barrier for Na+ ion transport but not for water molecules or Cl
ions (Li et al., 2016). Therefore, electrostatic repulsion is an important rejection mechanism of the nanoporous MoS2 membrane (Wang and Mi, 2017). In addition, ultrahigh-power density was obtained recently by using a single transmembrane boron nitride nanotube and a single-layered MoS2 nanopore (Siria et al., 2013; Feng et al., 2016), although large-scale fabrication of those materials remains a challenge. Larger scale membrane fabrication based on nanopores has also been reported. Kim et al. (2013) has successfully fabricated an anodic alumina nanopore array

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and silica-coated alumina array as anion selective and cation selective membranes, respectively. Applying these nanopore arrays in an RED system resulted in a 542 nW power output on the NaCl salinity difference of 100 and 10 mM. Three major methods are frequently used to drill the hole on the membrane: TEM drilling, ion beam sculpting, and electrical pulse fabrication. The most popular method is TEM drilling, in which the individual nanopore is drilled with a wellfocused electron beam, using a JEOL 2010F EM operating at 200 kV (Garaj et al., 2013). However, the TEM drilling process is too expensive to be widely used. Also, it is highly susceptible to hydrocarbon contamination. Ion beam sculpting is implemented in a feedback-controlled sputtering system that provides fine control over ion beam exposure and sample temperature to make a hole on the membrane (Li et al., 2001). For the electrical pulse fabrication method, it just requires a simple fluidic cell and modest electronics, and can dramatically increase the accuracy and reliability of nanopore production, allowing consistent production of single nanopores down to subnanometer sizes (Kuan et al., 2015). In nature, some organisms can capture the energy from seawater by using their highly selective ion channel on a cell membrane (Feng et al., 2017). For this reason, Xue and coworkers drilled a bioinspired single pore on a polyimide membrane and found out the maximum power output with an individual nanopore approaches 26 pW, and they expect the power density to be enhanced by one to three orders of magnitude over precious IEMs (Guo et al., 2010). Nanofluidic channels fabricated by using various inorganic nanomaterials have also received great attention as promising substitutes for polymeric IEMs for use in RED (Ouyang et al., 2013). The devices built up by using those ion selective nanofluidic channels can be applied to not only harvest energy from various sources in nature but also to act as a power supply for self-powered micro/nanoscale systems. For example, silica nanochannels with heights of a few nanometers were introduced on a silicon wafer (Kim et al., 2010). Those silica nanochannels acted as selective ion channels, which can facilitate the transport of counter-ions but reject coions, i.e., serve as an IEM. Salinity gradient power was successfully converted to electricity by using potassium chloride solutions of different concentrations. To sum up, nanopores and nanochannels serving as membranes are also considered to be promising techniques that can significantly increase the efficiency of IEMs, but their wide application is still limited by the drilling method and materials. Therefore, improving the production efficiency of nanostructured membranes is still a challenge and needs substantial improvement to advance applications.

13.5 CONCLUSIONS AND PERSPECTIVES Nanocomposite membranes specifically designed for RED applications are still in an early phase of development. Only limited types of nanomaterials and polymeric materials have been tested to synthesize RED-specific IEMs. Due to similar functionality of membranes, many more potentially viable membranes in related fields,

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such as fuel cells and desalination, may have already provided insights for RED applications. A significant amount of work is expected in this field to develop nanocomposite membranes with optimal characteristics that enable more efficient energy harvesting. Other than conventional IEM, nanomaterials such as GO and nanostructures have shown orders of magnitude improvement in ion transport efficiency. Any successful scale-up of these novel approaches will be revolutionary, if a feasible balance of fabrication cost and consistent structure can be maintained at larger scales. Nanostructures working on a smaller scale of salinity gradient energy harvest have also provided a novel perspective in the combination of nanotechnology and RED.

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