Power Generation from Concentration Gradient by Reverse Electrodialysis in Anisotropic Nanoporous Anodic Aluminum Oxide Membranes

energies

Article Power Generation from Concentration Gradient by Reverse Electrodialysis in Anisotropic Nanoporous Anodic Aluminum Oxide Membranes

Yunhyun Lee, Hyun Jung Kim and Dong-Kwon Kim *

Department of Mechanical Engineering, Ajou University, Suwon 443-749, Korea; [email protected] (Y.L.); [email protected] (H.J.K.) * Correspondence: [email protected]

Received: 20 January 2020; Accepted: 13 February 2020; Published: 18 February 2020

Abstract: In this study, reverse electrodialysis power generation using an anisotropic anodic aluminum oxide membrane with nanopores of two different pore diameters is proposed and experimentally investigated for the first time. A number of experiments were carried out for various combinations of concentrations to show that the anisotropic anodic aluminum oxide membrane is superior to the conventional isotropic membrane. As a result, the highest power density that was measured from the anisotropic membrane was 15.0 mW/m2, and it was 7.2 times higher than that from the isotropic membrane. The reasons why the anisotropic membrane is superior to the isotropic membrane are explained in detail. The experiments on the anisotropic membranes with various active layer lengths and pore diameters were also conducted for exploring the effects of these engineering parameters on the power generation performance. As a result, it was shown that the length of the active layer is a more important engineering parameter than the pore diameter of the active layer. Additionally, it was also shown that a low concentration solution should be brought into contact with the active layer side of the membrane whenever an anisotropic membrane is used for reverse electrodialysis.

Keywords: power generation; reverse electrodialysis; anisotropic nanoporous membrane

1. Introduction Recently, nanoporous membranes have received significant attention for their electrical power generation by salinity gradient energy, which is available from the difference in salt concentrations between two aqueous solutions. Nanopores in various nanoporous membranes have ion selectivity when they are filled with an aqueous solution, so that one kind of ions (i.e., cations or anions) can pass better than the other kind. For example, silica nanopores preferably pass cations and not anions, whereas alumina nanopores pass anions better than cations [1,2]. As shown in Figure1, when the concentration of the aqueous solution on one side of a nanopore is higher than that on the other, a natural flow of cations and anions occurs. Ions move from the side with high concentration to the low concentration side by diffusion. Owing to the ion selectivity of nanopores, flow rates of cations and anions are not equal, and as a result, an electric current flows in a nanopore. Therefore, electrical power can be obtained from this electrical current by placing electrodes in solutions. This electrical power harvesting from the difference in ion concentrations by using ion-selective nanopores in nanoporous membranes is termed reverse electrodialysis. Power generation from reverse electrodialysis is simple and does not require any moving structures. Moreover, it is environmentally friendly and allows obtaining instant power with only two solutions with different ion concentrations. Thus, reverse electrodialysis has a potential to be used as a power source for various small and disposable systems [3,4]. Many researchers have investigated power

Energies 2020, 13, 904; doi:10.3390/en13040904 www.mdpi.com/journal/energies EnergiesEnergies2020 2020, 13, 13, 904, x FOR PEER REVIEW 2 ofof 1515

investigated power generation from reverse electrodialysis using a number of ion selective generationmembranes. from These reverse studies electrodialysis are well summarized using a number in the review of ion paper selective by membranes.Mei and Tang These [1]. Guo studies et al. arefabricated well summarized an ion selective in the polyimide review paper membrane by Mei andwith Tang a single [1]. nanopore Guo et al. by fabricated using ion-track-etch an ion selective and polyimideconducted membrane experimental with investigation a single nanopore on the power by using generation ion-track-etch of this and membrane conducted [5]. experimentalThey showed investigationthat about 26 on pW the powerof power generation could be of produced this membrane from [5the]. They membrane. showed Kim that aboutet al. 26fabricated pW of power silica couldnanochannels be produced of various from the heights membrane. by a fabrication Kim et al. process fabricated compatible silica nanochannels with the standard of various CMOS heights process by aand fabrication used those process nanochannels compatible for with power the standardgeneration CMOS experiments process [6]. and As used a result, those nanochannelsthey obtained forthe 2 powerhighest generation power density experiments of 7.7 W/m [6].2 As from a result, the silica they nanochannels. obtained the highest Ji et al. powerfabricated density and ofinvestigated 7.7 W/m fromcharged the silicalamellar nanochannels. nanochannels Ji cascaded et al. fabricated in graphene and investigated oxide membranes charged for lamellar reverse nanochannelselectrodialysis cascaded[7]. As a inresult, graphene they oxideobtained membranes the highest for reversepower electrodialysisdensity of 0.77 [7W/m]. As2. aSiria result, et theyal. fabricated obtained theand highestinvestigated power boron density nitride of 0.77 nanotubes W/m2. Siria for et reverse al. fabricated electrodialysis and investigated [8]. They boron showed nitride their nanotubes nanotubes for reversehave a electrodialysis potential to [achieve8]. They showedthe power their density nanotubes of have4 kW/m a potential2. Feng toet achieve al. investigated the power densityreverse 2 ofelectrodialysis 4 kW/m . Feng in single-layer et al. investigated molybdenum reverse disulfide electrodialysis (MoS2) in nanopores single-layer [9]. molybdenum They found their disulﬁde MoS2 (MoSnanopores2) nanopores have a [potential9]. They foundto reach their the MoSpower2 nanopores density of have 106 W/m a potential2. to reach the power density of 106 W/m2.

FigureFigure 1.1. ReverseReverse electrodialysiselectrodialysis inin anan anisotropicanisotropic nanoporousnanoporous anodic anodic aluminum aluminum oxide oxide membrane. membrane.

Moreover,Moreover, since since the the amount amount of energy of energy generated genera by reverseted by electrodialysisreverse electrodialysis increases inincreases proportion in toproportion the number to the of nanopores,number of nanopores, many studies many have studies been have conducted been conducted on how membraneson how membranes with dense with nanoporesdense nanopores increase increase power production. power production. Ouyang etOuyang al. fabricated et al. fabricated a nanofluidic a nanofluidic crystal packed crystal with packed silica nanoparticleswith silica nanoparticles in a micropore in anda micropore measured and power measur generationed power from generation it [10]. It was from shown it [10]. that It nanofluidicwas shown crystalsthat nanofluidic can achieve crystals power generationcan achieve of 1.17power nW. Choigeneration et al. suggested of 1.17 nW. and experimentallyChoi et al. suggested investigated and aexperimentally nanofluidic reverse investigated electrodialysis a nanofluidic platform reverse in which electrodialysis crystallized nanoparticlesplatform in which are geometrically crystallized controllednanoparticles [11]. are It was geometrically shown that controlled their nanofluidic [11]. It platform was shown can achievethat their power nanofluidic generation platform of 320 pW.can Kimachieve et al. power used a nanoporousgeneration of anodic 320 aluminumpW. Kim oxideet al. membraneused a nanoporous with a membrane anodic diameter aluminum of 47 oxide mm, membranemembrane thickness with a membrane of 60 µm, anddiameter nominal of 47 pore mm, radius membrane of 10 nm. thickness Such a membraneof 60 μm, and produced nominal 542 pore nW ofradius power of [102]. nm. Such a membrane produced 542 nW of power [2]. NanoporousNanoporous anodicanodic aluminumaluminum oxideoxide membranemembrane cancan bebe fabricatedfabricated byby electrochemicalelectrochemical oxidation oxidation ofof anan aluminumaluminum membranemembrane [12[12,13].,13]. ItIt hashas attractedattracted closeclose attentionattention becausebecause itit isis suitablesuitable forfor powerpower generationgeneration byby reversereverse electrodialysis. electrodialysis. NanoporousNanoporous anodicanodic aluminumaluminum oxideoxide membranesmembranes areare suitablesuitable forfor reversereverse electrodialysiselectrodialysis beingbeing characterizedcharacterized byby aa highlyhighly homogeneoushomogeneous andand densedense structurestructure withwith straight and parallel nanopores. Today, researchers have focused mainly on isotropic anodic aluminum oxide membranes with nanopores of a uniform diameter (Figure 2a). However, as

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Energies 2020, 13, x FOR PEER REVIEW 3 of 15 straight and parallel nanopores. Today, researchers have focused mainly on isotropic anodic aluminum oxide membranes with nanopores of a uniform diameter (Figure2a). However, as previous studies previous studies demonstrate [14,15], isotropic membranes with a small nanopore diameter exhibit demonstrate [14,15], isotropic membranes with a small nanopore diameter exhibit excessively high excessively high electric resistance and generate a negligible amount of electric current. On the electric resistance and generate a negligible amount of electric current. On the contrary, nanopores contrary, nanopores with a large diameter cause excessively low ion selectivity, equal cation and with a large diameter cause excessively low ion selectivity, equal cation and anion ﬂow rates, and as anion flow rates, and as a result, no electric current is produced. Evidently, isotropic nanoporous a result, no electric current is produced. Evidently, isotropic nanoporous anodic aluminum oxide anodic aluminum oxide membranes have inherent limitations for power maximization. membranes have inherent limitations for power maximization.

(a) (b)

FigureFigure 2. 2. SchematicSchematic diagramsdiagrams of of nanoporous nanoporous anodic anodic aluminum aluminum oxide membranes oxide membranes for reverse for electrodialysis: reverse electrodialysis:(a) isotropic membrane; (a) isotropic (b membrane;) anisotropic (b membrane.) anisotropic membrane.

InIn this this study, study, we we propose propose reverse reverse electrodialysis electrodialysis power power generation generation using using an an anisotropic anisotropic anodic anodic aluminumaluminum oxide oxide membrane membrane with with the the nanopores nanopores of oftw twoo different different pore pore diameter diameterss (Figures (Figures 1 1and and 2b).2b). InIn an an anisotropic anisotropic membrane, membrane, the the active active layer layer consists consists of ofnanopores nanopores with with a small a small diameter, diameter, causing causing sufficientsufficient ion ion selectivity, selectivity, and and a veryvery shortshort length, length, causing causing lower lower electrical electrical resistance. resistance. The The support support layer layerconsists consists ofnanopores of nanopores with with alarge a large diameter, diameter, causing causing very very low low electrical electrical resistance,resistance, andand provides a a structural support support for for the the thin thin and and fragile fragile active active la layer.yer. In In short, short, it itis isexpected expected that that the the anisotropic anisotropic anodicanodic aluminum aluminum oxide oxide membrane membrane can can provide high powerpower generationgeneration due due to to high high ion ion selectivity selectivity and andlow low resistance. resistance. Systematic Systematic studies studies on on the the anisotropic anisotropic aluminum aluminum oxide oxide membrane membrane have have not not been been yet yetconducted. conducted. Hence, Hence, a proof-of-concepta proof-of-concept experiment experiment is required is required to showcase to showcase that thethat anisotropic the anisotropic anodic anodicaluminum aluminum oxide oxide membrane membrane is superior is superior to the to isotropic the isotropic membrane. membrane. InIn this this study, study, power power generation generation from from a concentration a concentration gradient gradient by reverse by reverse electrodialysis electrodialysis in anisotropicin anisotropic nanoporous nanoporous anodic anodic aluminum aluminum oxide oxidemembranes membranes was experimentally was experimentally investigated. investigated. The experimentsThe experiments were werecarried carried out for out various for various combinatio combinationsns of concentrations. of concentrations. The The experimental experimental results results werewere also also obtained obtained for for the the isotropic isotropic membranes membranes wi withth the the same same combinations combinations of of concentrations concentrations and and werewere compared compared to to those those for for the the anisotropic anisotropic memb membranes.ranes. Experiments Experiments on on the the anisotropic anisotropic membranes membranes withwith various various active active layer layer lengths lengths and and pore pore diameter diameterss were were conducted conducted to toreveal reveal the the effects effects of ofthese these engineeringengineering parameters parameters onon the the power power generation generation performance. performance. In addition, In addition, the anisotropic the anisotropic membranes membraneshave directionality have directionality in installation, in installation, and the effects and of the the effects installation of the direction installation on power direction generation on power were generationalso scrutinized. were also scrutinized.

2. 2.Materials Materials and and Methods Methods FourFour kinds kinds of ofanisotropic anisotropic aluminum aluminum oxide oxide me membranesmbranes (AAO|AM-013-002-B20, (AAO|AM-013-002-B20, InRedox InRedox Inc., Inc., Longmont,Longmont, CO, CO, USA) USA) were were used used in in this study. OneOne typetype ofof an an isotropic isotropic aluminum aluminum oxide oxide membrane membrane was was also tested in order to compare the performances of the anisotropic and isotropic membranes. Table 1 presents the membrane dimensions. All membranes had a support layer of the same length and pore diameter. The anisotropic aluminum oxide membrane had a very thin active layer added to

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also tested in order to compare the performances of the anisotropic and isotropic membranes. Table1 presents the membrane dimensions. All membranes had a support layer of the same length and pore diameter. The anisotropic aluminum oxide membrane had a very thin active layer added to the support layer. The active layer length and pore diameter were examined using several membranes with different active layer lengths and pore diameters. Figure3 presents cross section SEM (Scanning Electron Microscope) images of the anisotropic aluminum oxide membrane used in this study. As demonstrated, the actual anisotropic aluminum oxide membrane is not as simple as it is shown in Figures1 and2b. In Figure3a, it is visible that the support layer nanopores with a large pore diameter are connected to the active layer nanopores with a small diameter. In the case of the isotropic membrane, there is no active layer, and the cross-section SEM image is similar to Figure3b. A schematic diagram of Energiesthe experimental 2020, 13, x FOR PEER setup REVIEW is shown in Figure4. The membrane was sandwiched between two4 silicone of 15 rubber plates with round holes of a 2.5-mm radius. Two Ag/AgCl mesh electrodes were constructed theby support applying layer. Ag/AgCl The inkactive on silverlayer meshes.length and Then, pore each diameter electrode were was examined placed on using the plate. several Next, membranestwo acrylic with structures different were active attached layer lengths to the silicone and pore rubber diameters. plates toFigure create 3 reservoirspresents cross on both section sides SEMof the(Scanning membrane. Electron The Microscope) reservoirs were images filled of withthe anisotropic a NaCl solution aluminum with oxide ten di membranefferent combinations used in thisof study. concentrations As demonstrated, (Table2). the The actual reservoirs anisotropic on the aluminum active layer oxide side membrane and the support is not layeras simple side as of theit is shownanisotropic in Figures membrane 1 and were2b. In filled Figure with 3a, lowit is andvisible high that concentration the support layer solutions, nanopores respectively with a large (except porefor diameter special cases). are connected During the to experiments,the active layer both nanopores solutions with were a stirredsmall diameter. thoroughly In bythe stirrers case of so the that isotropicboth solutions membrane, were there well is mixed. no active Finally, layer, the and output the cross-section currents from SEM the image membrane is similar were to measured Figure 3b. for A schematicvarious input diagram voltages of th usinge experimental an Electrometer setup is (Sourceshown in Meter, Figure Keithley 4. The membrane Instruments was Inc., sandwiched Solon, OH, betweenUSA). Thetwo current silicone was rubber measured plates each with time roun afterd aholes<1% changeof a 2.5-mm in the currentradius. occurredTwo Ag/AgCl for 2 min. mesh Each electrodesmeasurement were wasconstructed repeated by four applying times to Ag/AgCl ensure the ink reliability on silver of meshes. data. The Then, surrounding each electrode temperature was placedwas 25on the1 ◦ plate.C. Next, two acrylic structures were attached to the silicone rubber plates to create reservoirs± on both sides of the membrane. The reservoirs were filled with a NaCl solution with ten different combinationsTable 1. Dimensions of concentrations of the nanoporous (Table 2) aluminum. The reservoirs oxide membranes on the active used in layer the study. side and the support layer side of the anisotropic Activemembrane Layer were filled with low and Support high Layer concentration solutions, respectivelyType (except for special cases). During the experiments, both solutions were stirred Pore Diameter (Da) Pore Length (La) Pore Diameter (Ds) Pore Length (Ls) thoroughlyAnisotropic by stirrers membrane so that both 2 solutions nm were well 1 µm mixed. Finally, 100 the nm output currents 50 µfromm the membraneAnisotropic were membranemeasured for various 2 nm input voltages 2 µusingm an Electrometer 100 nm (Source Meter, 50 µ Keithleym InstrumentsAnisotropic Inc., membrane Solon, OH, USA). 5nm The current was 1 measuredµm each time 100 nm after a <1% change 50 µm in the currentAnisotropic occurred membrane for 2 min. Each 10measurement nm was 1reµmpeated four times 100 nmto ensure the reliability 50 µm of data. TheIsotropic surrounding membrane temperature was 25 ± 1 °C. 0 µm 100 nm 50 µm −

(a) (b)

FigureFigure 3. 3.ScanningScanning Electron Electron Microscope Microscope (SEM) (SEM) cross-sectional cross-sectional view view of ofthe the anisotropic anisotropic nanoporous nanoporous anodicanodic aluminum aluminum oxide oxide membrane: membrane: (a) (activea) active layer layer side; side; (b)(b) supportsupport layer layer side. side.

Table 1. Dimensions of the nanoporous aluminum oxide membranes used in the study.

Active Layer Support Layer Type Pore Diameter (Da) Pore Length (La) Pore Diameter (Ds) Pore Length (Ls) Anisotropic membrane 2 nm 1 μm 100 nm 50 μm Anisotropic membrane 2 nm 2 μm 100 nm 50 μm Anisotropic membrane 5 nm 1 μm 100 nm 50 μm Anisotropic membrane 10 nm 1 μm 100 nm 50 μm Isotropic membrane − 0 μm 100 nm 50 μm

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Figure 4. SchematicSchematic diagram diagram of the experimental apparatus.

TableTable 2. CombinationsCombinations of ion concentrations used in the study.

ConcentrationsConcentrations of Br ofine Brine NaCl NaCl Solution Solution (cH) (cH)

0.001 M0.001 M 0.01 M 0.01 M 0.1 0.1 M M 0.5 M 0.0001 M O O O O Concentrations of 0.0001 M O O O O 0.001 M O O O dilute NaCl solution 0.001 M O O O Concentrations of dilute NaCl0.01 M O O (cL) solution (cL) 0.1 M 0.01 M OO 0.1 M O 3. Results and Discussion 3. ResultsIt is well and known Discussion that electricity generation by reverse electrodialysis from the experimental setup can be represented by an electric circuit (Figure 5a), and the following equation should be satisfied. It is well known that electricity generation by reverse electrodialysis from the experimental setup can be represented by an electric circuit (Figure5a), and the following equation should be satisﬁed.

V = Vapp + E = E IR (1) redox di f f − where V is the voltage generated in the membrane, Vapp is the applied voltage, Ediff is the diffusion potential of the membrane, Eredox is the voltage generated by the reactions at the electrodes due to the difference in solution concentrations between two reservoirs, R is the electrical resistance of the membrane, and I is the generated current. As Figure5 shows, the membrane, where reverse electrodialysis occurs, is equivalent to a series of connections of the electrical voltage source Ediff and the electrical resistance R. The(a) value of Eredox can be obtained from the following(b) equation, which is well known in the field of electrochemistry: Figure 5. Two electrical circuits represent the experimental setup. ! RT γcH cH =+Eredox = ln = − (2) VVapp EzF redoxγ EcL diffcL IR (1) where RV, isT, thez, F ,voltage and γ are generated the gas constant,in the membrane, temperature, Vapp charge is the number,applied voltage, Faraday E constant,diff is the anddiffusion mean potentialactivity coe offfi thecient, membrane, respectively. Eredox is Thus, the voltage by using generated Equations by (1) the and reac (2),tions the at voltage the electrodesV generated due to in the differencemembrane in can solution be calculated concentrations from the voltagebetweenV apptwoapplied reservoirs, to the R electrodes is the electrical by the electrometer.resistance of Thethe membrane,diffusion potential and I Eisdi ffthealso generated satisfiesthe current. following As equation:Figure 5 shows, the membrane, where reverse electrodialysis occurs, is equivalent to a series of connections of the! electrical voltage source Ediff and RT γc cH redox H the electrical resistance R. The valueEdi of f f E= (2 tcan be1) obtainedln from the following equation, which(3) is − zF c well known in the field of electrochemistry: − γcL L

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Figure 4. Schematic diagram of the experimental apparatus.

Table 2. Combinations of ion concentrations used in the study.

Concentrations of Brine NaCl Solution (cH)

0.001 M 0.01 M 0.1 M 0.5 M 0.0001 M O O O O EnergiesConcentrations2020, 13, 904 of 6 of 15 0.001 M O O O dilute NaCl solution 0.01 M O O (cL) where t is the transference0.1 number M for the anions and indicates the ion selectivity of the membrane.O If − the t value of the membrane is 1, the membrane passes only the anions and not the cations. When t − − 3. equalsResults 0.5, and the Discussion membrane passes equally the cations and anions, so that no voltage is generated at the membrane, as shown in Equation (3). Therefore, the higher the t number, the greater the diffusion It is well known that electricity generation by reverse electrodialysis− from the experimental setup canpotential be representedEdiff, the by voltage an electricV, and circuit the generated (Figure 5a), power. and Equation the following (3) is usefulequation for should understanding be satisfied. various trends of the diffusion potential.

(a) (b)

FigureFigure 5. Two 5. Two electrical electrical circuits circuits repres representent the the experimental experimental setup. setup.

Figure6 shows a typical voltage–current curve of the anisotropic membrane as measured in the VV=+ E = E − IR experimental setup. As the voltage increases,app redoxthe current diff decreases linearly. In this voltage-current(1) curve, the diffusion potential E can be calculated by using the voltage at the point where the where V is the voltage generated indiff the membrane, Vapp is the applied voltage, Ediff is the diffusion current equals zero. The slope of the voltage–current curve is related to the electrical resistance R potential of the membrane, Eredox is the voltage generated by the reactions at the electrodes due to the differenceof the membrane, in solution and concentrations the electrical between resistance two can reservoirs, be obtained R fromis the the electrical absolute resistance value of theof slope.the membrane,As depicted and in FigureI is the6, electricalgenerated resistance current. is As constant Figure regardless 5 shows, of the amountmembrane, of the where current reverse flowing. This arises because the concentration profile inside the nanopores is almost independent of the electrical electrodialysis occurs, is equivalent to a series of connections of the electrical voltage source Ediff and potential under small electric fields [16–18]; electrical resistance was also constant regardless of the the electrical resistance R. The value of Eredox can be obtained from the following equation, which is wellamount known of in current the field flowing. of electrochemistry: Therefore, the characteristics of voltage–current curve shown in Figure6 Energiesare similar 2020, 13 to, x thoseFOR PEER presented REVIEW in the previous studies. 7 of 15

FigureFigure 6. 6. TypicalTypical voltage–current voltage–current curve curve of ofthe the anisot anisotropicropic nanoporous nanoporous anodic anodic aluminum aluminum oxide oxide

membranemembrane (D (Da =a 2= nm,2 nm, La L=a 2= μ2m,µ m,cH =c H0.5= M,0.5 c M,L = c0.01L = 0.01M). M).

To understand better the power generation characteristics of the membranes, the applied voltage Vapp and the redox potential Eredox (Figure5a) can be simpliﬁed by considering them equivalent to

Figure 7. Typical power generation–electrical load curve of the anisotropic nanoporous anodic

aluminum oxide membrane (Da = 2 nm, La = 2 μm, cH = 0.5 M, cL = 0.01 M).

Especially, the area specific resistance is the resistance per unit membrane area. It can be obtained by multiplying the resistance and the membrane area. Contrary to the resistance, which depends on the size of the membrane, the area specific resistance is independent to the size of the membrane. Therefore, it is considered as one of the membrane properties, and it is widely used for characterizing the electrical resistance of the membrane, as shown [19,20]. The main purpose of this study is to show the anisotropic membrane is superior to the isotropic membrane. Therefore, it is important to compare the power densities of the anisotropic membrane and the isotropic membrane, because the power density is the most important performance index. In addition, the power density strongly depends on the dilute solution concentration and the brine solution concentration. Therefore, it is needed to compare the power densities for various combinations of concentrations. In addition, as shown in Equation (5), the power density is the function of the diffusion potential and the area specific resistance. Therefore, for understanding various results on the power density, it is also needed to compare the diffusion potentials and the

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the load resistance Rload (Figure5b). Then, the load resistance Rload and power generation P in the membrane can be determined as follows:

Vapp + Eredox V R = = , P = VI (4) load I I Figure7 shows a typical power–resistance curve of the anisotropic membrane measured in the experimental setup. The power reaches its maximum when the load resistance is equal to membrane resistance (Rload = R). Maximum power generation Pmax is calculated from Equations (1) and (4) as follows: 2 Edi f f P = (5) max 4R

Therefore, Pmax increases with an increase in the diffusion potential Ediff and a decrease in the electric resistance R. Reverse electrodialysis power generation aims to maximize power generation using the least amount of a membrane material. Therefore, the power density Pmax/A, where A is the membrane area, hasFigure to be maximized. 6. Typical voltage–current As shown in Equation curve of (5), the the anisot maximumropic nanoporous power density anodicPmax aluminum/A increases oxide with an increasemembrane in the (D dia ff=usion 2 nm, potentialLa = 2 μm, EcHdi =ff 0.5and M, a c decreaseL = 0.01 M). in the area specific membrane resistance RA.

FigureFigure 7. 7. TypicalTypical power power generation–electrical generation–electrical load load cu curverve of of the the anisotropic anisotropic nanoporous nanoporous anodic anodic µ aluminumaluminum oxide oxide membrane membrane (D (Da a= =2 2nm, nm, LaL =a 2= μ2m,m, cH c=H 0.5= 0.5M, M,cL =c L0.01= 0.01 M). M). Especially, the area specific resistance is the resistance per unit membrane area. It can be obtained Especially, the area specific resistance is the resistance per unit membrane area. It can be by multiplying the resistance and the membrane area. Contrary to the resistance, which depends obtained by multiplying the resistance and the membrane area. Contrary to the resistance, which on the size of the membrane, the area specific resistance is independent to the size of the membrane. depends on the size of the membrane, the area specific resistance is independent to the size of the Therefore, it is considered as one of the membrane properties, and it is widely used for characterizing membrane. Therefore, it is considered as one of the membrane properties, and it is widely used for the electrical resistance of the membrane, as shown [19,20]. characterizing the electrical resistance of the membrane, as shown [19,20]. The main purpose of this study is to show the anisotropic membrane is superior to the isotropic The main purpose of this study is to show the anisotropic membrane is superior to the isotropic membrane. Therefore, it is important to compare the power densities of the anisotropic membrane membrane. Therefore, it is important to compare the power densities of the anisotropic membrane and the isotropic membrane, because the power density is the most important performance index. and the isotropic membrane, because the power density is the most important performance index. In In addition, the power density strongly depends on the dilute solution concentration and the brine addition, the power density strongly depends on the dilute solution concentration and the brine solution concentration. Therefore, it is needed to compare the power densities for various combinations solution concentration. Therefore, it is needed to compare the power densities for various of concentrations. In addition, as shown in Equation (5), the power density is the function of the combinations of concentrations. In addition, as shown in Equation (5), the power density is the diffusion potential and the area specific resistance. Therefore, for understanding various results on the function of the diffusion potential and the area specific resistance. Therefore, for understanding power density, it is also needed to compare the diffusion potentials and the area specific resistances of various results on the power density, it is also needed to compare the diffusion potentials and the the anisotropic membrane and the isotropic membrane for various combinations of concentration.

Energies 2020, 13, x FOR PEER REVIEW 8 of 15 Energies 2020, 13, 904 8 of 15 area specific resistances of the anisotropic membrane and the isotropic membrane for various combinations of concentration. Therefore,Therefore, in in Figure Figure8 ,8, comparisons comparisons of power power densities densities PmaxP/Amax, diffusion/A, diffusion potentials potentials Ediff, andEdi ffarea, and areaspecific specific resistances resistances RARA of ofthe the anisotropic anisotropic membrane membrane and and the the isotropic isotropic membrane membrane for for various various combinationscombinations of of brine brine solution solution concentrations concentrationsc cHH and dilute solution solution concentrations concentrations cLc areL are presented. presented. AsAs mentioned mentioned in the in Sectionthe Section2, four 2, kinds four of kinds anisotropic of anisotropic membranes membranes were experimentally were experimentally investigated. However,investigated. in Figure However,8, only in the Figure results 8, only for the the anisotropic results for the membrane anisotropic with membrane active layer with pore active diameter layer Da poreof 2 nmdiameter and active Da of layer 2 nm pore and length activeL alayerof 2 µporem are length presented, La of 2 because μm are this presented, anisotropic because membrane this showsanisotropic highest membrane power density. shows highest power density. AsAs shown shown in in Figure Figure8a, 8a, the the highest highest power power density density thatthat waswas measuredmeasured from the the anisotropic anisotropic membranemembrane is is 15.0 15.0 mW mW/m/m2.2. This This value value was was measured measured when when the the brine brine solution solution concentration concentration was 0.5 was 0.5M, M, and and the the dilute dilute solution solution concentration concentration was was 0.01 0.01 M. M. On On the the other other hand, hand, th thee highest highest power power density density 2 thatthat was was measured measured from from the the isotropic isotropic membrane membrane isis 2.12.1 mWmW/m/m2 atat the the brine brine solution solution concentration concentration of of 0.50.5 M M and and the the dilute dilute solution solution concentration concentration ofof 0.010.01 M. As a result, the the highest highest power power density density of ofthe the anisotropic membrane is 7.2 times higher than that of the isotropic membrane. This power density anisotropic membrane is 7.2 times higher than that of the isotropic membrane. This power density from the anisotropic membrane is also much greater than the power density of 7 mW/m2 from another from the anisotropic membrane is also much greater than the power density of 7 mW/m2 from another anodic aluminum oxide isotropic membrane presented in the previous study [2]. Therefore, for anodic aluminum oxide isotropic membrane presented in the previous study [2]. Therefore, for reverse reverse electrodialysis, the anisotropic membrane is much superior to the isotropic membrane. electrodialysis, the anisotropic membrane is much superior to the isotropic membrane.

(a)

(b)

Figure 8. Cont. Energies 2020, 13, 904 9 of 15 Energies 2020, 13, x FOR PEER REVIEW 9 of 15

(c)

FigureFigure 8. 8.A A comparison comparison of: of: (a) power(a) power densities, densities, (b) di(bff)usion diffusion potentials, potentials, and ( cand) area (c specific) area specific resistances ofresistances the anisotropic of the anisotropic nanoporous nanoporo anodicus aluminum anodic aluminum oxide membrane oxide membrane (Da = 2 ( nm,Da = L2 anm,= 2 Lµa m)= 2 andμm) the isotropicand the nanoporousisotropic nanoporous anodic aluminum anodic aluminum oxide membrane oxide formembrane various combinationsfor various combinations of concentrations. of concentrations. Why is there a very large difference in power densities between two membranes? As shown in Why is there a very large difference in power densities between two membranes? As shown in Figure8b, at cH of 0.5 M and cL of 0.01 M, the diffusion potentials of the anisotropic membrane and theFigure isotropic 8b, at membrane cH of 0.5 M are and 53 cL mV of 0.01 and M, 18 the mV, diffusion respectively. potentials The presence of the anisotropic of the active membrane layer increases and thethe di isotropicffusion potentialmembrane by are factor 53 mV of and 2.9. 18 On mV, the respec othertively. hand, The as presence shown inof Figurethe active8c, layer the area increases specific the diffusion potential by factor of 2.9. On the other hand, as shown in Figure 8c, the area specific resistances of the anisotropic membrane and the isotropic membrane are 0.047 Ωm2 and 0.039 Ωm2, resistances of the anisotropic membrane and the isotropic membrane are 0.047 Ωm2 and 0.039 Ωm2, respectively. The presence of the active layer increases the area specific resistance by a factor of 1.2. respectively. The presence of the active layer increases the area specific resistance by a factor of 1.2. As shown in Equation (5), the power density is proportional to the square of the diffusion potential As shown in Equation (5), the power density is proportional to the square of the diffusion potential and is inversely proportional to the area specific resistance. As a result, the positive effect of the active and is inversely proportional to the area specific resistance. As a result, the positive effect of the active layerlayer on on the the power power densitydensity caused by by an an increase increase in in the the diffusion diffusion potential potential is much is much greater greater than than the the negativenegative e effectffect caused caused byby increaseincrease in the area area spec specificific resistance. resistance. Therefore, Therefore, the the power power density density of the of the anisotropicanisotropic membrane membrane isis muchmuch greater than that that of of the the isotropic isotropic membrane. membrane. AccordingAccording toto EquationEquation (3), the transference transference number number for for the the anions anions t cant can be becalculated calculated from from the the didiffusionffusion potential, potential, the the brine brine solution solution concentration, concentration, and and the the dilute dilute solution solutionconcentrations. concentrations. At cH of 0.5 M0.5 and Mc andL of 0.01cL of M,0.01 transference M, transference numbers numbers for the for anions the anions of the of anisotropic the anisotropic membrane membrane and the and isotropic the membraneisotropic membrane are 0.79 and are 0.60,0.79 and respectively. 0.60, respectively. Therefore, Therefore, the ion selectivitythe ion selectivity is increased is increased by the by presence the ofpresence the active of the layer active [6], andlayer the [6], transference and the transferen numberce number increases increases as the influenceas the influence of the of surface the surface charges ofcharges the nanopore of the nanopore on the ion on transport the ion transport phenomena phen growsomena largergrows comparedlarger compared to the to influence the influence of the of bulk ionthe concentration bulk ion concentration in the nanopore. in the Asnanopore. a result, As the a transferresult, the number transfer increases number with increases the increase with the of the Dukhinincrease number, of the Dukhin which is number, used to characterizewhich is used the to ratio characterize of bulk tothe surface ratio of charge bulk carriers.to surface The charge Dukhin numbercarriers.Du Theof Dukhin the nanopore number is Du given of the as nanopore [2] is given as [2] 4σ Du = 4σ (6) Du =DFc()+ c (6) DF(cH + cLL)

Here,Here,σ σand andD Dare are thethe surfacesurface charge density density and and the the pore pore diameter, diameter, respectively. respectively. As Asshown shown in in Equation (6), when cH and cL are fixed, the Dukhin number increases with a decrease in the pore Equation (6), when cH and cL are fixed, the Dukhin number increases with a decrease in the pore diameter.diameter. In In thethe presentpresent study, the pore diameter diameter of of the the active active layer layer is ismuch much smaller smaller than than that that of the of the support layer. As a result, Dukhin number for the active layer is much larger than that of the support support layer. As a result, Dukhin number for the active layer is much larger than that of the support layer, and the transference number of the active layer is larger than that of the support layer. layer, and the transference number of the active layer is larger than that of the support layer. Therefore, Therefore, the ion selectivity is increased by the presence of the active layer. the ion selectivity is increased by the presence of the active layer. It is worth noting that the very thin active layer with the pore length of 2 μm causes the dramatic It is worth noting that the very thin active layer with the pore length of 2 µm causes the dramatic change in the transference number from 0.60 to 0.79 at cH of 0.5 M and cL of 0.01 M [15,17], and the ion change in the transference number from 0.60 to 0.79 at c of 0.5 M and c of 0.01 M [15,17], and the ion H L Energies 2020, 13, 904 10 of 15 selectivity increases rapidly when the pore length is changed from 0 to several micrometers. On the other hand, the ion selectivity increases very slowly when the pore length is changed from several micrometers to several tens of micrometers [17]; it is because the ion selectivity strongly depends of the pore length for the short nanopores where the Goldman approximation is not satisfied. On the other hand, the ion selectivity is almost independent to the pore length for the long nanopores where the Goldman approximation is satisfied. Therefore, the active layer with the pore length of several micrometers can effectively increase the ion selectivity of the membrane. As a result, the diffusion potential and the power density of the anisotropic membrane are much greater than those of the isotropic membrane, respectively. It is also worth noting that there is only a small difference in the area specific resistances between the anisotropic membrane and the isotropic membrane at cH of 0.5 M and cL of 0.01 M. Generally, the electrical resistance of the nanopore is proportional to the pore length and is inversely proportional to the square of the pore diameter. Therefore, if the active layer length of the anisotropic membrane is equal to the support layer length, the area specific resistance of the anisotropic membrane should be much greater than that of the isotropic layer due to the presence of the active layer. In this case, the power density of the anisotropic membrane is much smaller than that of the isotropic membrane. However, in the present study, the active layer length is smaller than one-twentieth of the support layer length. In this case, the presence of the active layer does not significantly increase the area specific resistance of the anisotropic membrane. As a result, the power density of the anisotropic membrane can be much greater than that of the isotropic membrane. In addition, Figure8 also indicates the following characteristics of the di ffusion potential, area specific resistance, and power density for the anisotropic membrane.

(a) As shown in Figure8b, the di ffusion potential increases with an increase in the brine solution concentration cH and a decrease in the dilute solution concentration cL. This is explained by the fact that the diffusion potential value is proportional to the logarithm of the ratio of the brine and dilute solution concentrations (Equation (3)). (b) In Figure8b, the di ffusion potentials for the same concentration ratio of 10 are marked by the red circles. In general, as the concentration increases, the ion selectivity decreases, because Dukhin number shown in Equation (6) decreases as the concentration increases. Therefore, the diffusion potential decreases with an increase in the concentration under the fixed concentration ratio (Equation (3)). (c) As shown in Figure8c, the area specific resistance decreases with an increase in the brine and dilute solution concentrations. It is because the number densities of the cations and anions in the nanopores increases proportionately to the concentration. Since the active layer, which has higher resistance compared to the resistance of the support layer, is in contact with the dilute solution, the resistance is more affected by changes in the dilute solution concentration than in the brine solution concentration. (d) As depicted in Figure8a, the power density under the fixed dilute solution concentration increases monotonically as the brine concentration increases. This is because the power density increases with an increase in the diffusion potential and a decrease in the area specific resistance (Equation (5)). On the other hand, under the fixed brine concentration, there is an optimal dilute solution concentration that maximizes the power density. If the dilute solution concentration is excessively high, the power density is low since the diffusion potential is negligible. If the dilute solution concentration is excessively low, the power density is low too because the area specific resistance is excessive.

The characteristics of the diﬀusion potential, area speciﬁc resistance, and power density for the isotropic membrane shown in Figure8 are similar to those for the anisotropic membranes, as mentioned previously. Energies 2020, 13, x FOR PEER REVIEW 11 of 15

worth comparing power densities of the anisotropic membrane and the isotropic membrane for various combinations of the concentrations. To clearly compare the power densities, the ratio of the power density of the anisotropic membrane to that of the isotropic membrane was calculated for various combinations of the concentrations from the results shown in Figure 8a and is presented in Figure 9. As shown in the figure, the power density of the anisotropic membrane is higher than that of the isotropic membrane for all combinations of concentrations. This is mainly because the diffusion potential of the anisotropic membrane is much higher than that of the isotropic membrane regardless of the concentrations. Nonetheless, an increase in resistance due to the presence of the active layer is not significant. So far, it was successfully shown that the anisotropic membrane is better than the isotropic membrane for reverse electrodialysis. However, to make better power generators with the anisotropic membranes, it is important to investigate the effects of engineering parameters on the power generation performance of the anisotropic membrane. Therefore, the influence of some engineering parameters on the power generation performance was tested by comparing the anisotropic membranes with two different active layer lengths and three different active layer pore diameters. Figure 10 shows a comparison between the power densities, diffusion potentials, and area specific resistances for the anisotropic membranes with two different active layer lengths and three Energiesdifferent2020, 13, 904 active layer pore diameters. As shown in Figure 10a, the change in the power density caused11 of 15 by the change in the length of the active layer is more than the change in the power density caused by the change in the nanopore diameter of the active layer. Therefore, the length of the active layer is Ina Figuremore 8importanta, it was foundengineering that the parameter highest powerthan the density nanopore of the diameter anisotropic of the membrane active layer. is 7.2 The times higherfollowing than that facts of were the isotropic also recognized membrane from Figure at cH 10:of 0.5 M and cL of 0.01 M. However, this does not mean that the anisotropic membrane has greater power density than the isotropic membrane (a) The effects of the dilute and brine solution concentrations on the power density, diffusion regardlesspotential, of the brine and solutionarea specific concentration resistance of the and anisotropic the dilute membrane solution are concentration. similar regardless Therefore, of the it is worth comparinglength of the power active densitieslayer and the of thepore anisotropic diameter the membrane active layer. and the isotropic membrane for various(b) combinations As shown in of Figure the concentrations. 10b, the longer Tothe clearly length compareof the active the powerlayer, the densities, higher the the diffusion ratio of the power densitypotential. of the It is anisotropic because the membraneion selectivity to of that the ofactive the layer isotropic increases membrane as the active was layer calculated length for various combinationsincreases, as explained of the concentrations earlier. On the from other the hand, results the diffusion shown inpotential Figure tends8a and to increase is presented with in Figure9. Asa showndecrease in in the the figure, nanopore the diameter. power density It is because of the the anisotropic ion selectivity membrane of the active is higher layer thanis greater that of the isotropicwhen membrane the nanopore for alldiameter combinations of the active of concentrations.layer is smaller, due This to ishigher mainly Dukhin because number the shown diffusion in Equation (6). potential of the anisotropic membrane is much higher than that of the isotropic membrane regardless (c) As depicted in Figure 10c, the longer the length of the active layer with high electrical resistance, of the concentrations. Nonetheless, an increase in resistance due to the presence of the active layer is the higher the electrical resistance. On the other hand, the larger the nanopore diameter of the not significant.active layer, the larger the area specific resistance.

FigureFigure 9. Ratios 9. Ratios of the powerof the power density density of the anisotropicof the anisotro membranepic membrane to the to power the power density density of the of isotropic the membraneisotropic for variousmembrane combinations for various combinations of concentrations. of concentrations.

So far, it was successfully shown that the anisotropic membrane is better than the isotropic membrane for reverse electrodialysis. However, to make better power generators with the anisotropic membranes, it is important to investigate the effects of engineering parameters on the power generation performance of the anisotropic membrane. Therefore, the influence of some engineering parameters on the power generation performance was tested by comparing the anisotropic membranes with two different active layer lengths and three different active layer pore diameters. Figure 10 shows a comparison between the power densities, diffusion potentials, and area specific resistances for the anisotropic membranes with two different active layer lengths and three different active layer pore diameters. As shown in Figure 10a, the change in the power density caused by the change in the length of the active layer is more than the change in the power density caused by the change in the nanopore diameter of the active layer. Therefore, the length of the active layer is a more important engineering parameter than the nanopore diameter of the active layer. The following facts were also recognized from Figure 10:

(a) The effects of the dilute and brine solution concentrations on the power density, diffusion potential, and area specific resistance of the anisotropic membrane are similar regardless of the length of the active layer and the pore diameter the active layer. (b) As shown in Figure 10b, the longer the length of the active layer, the higher the diffusion potential. It is because the ion selectivity of the active layer increases as the active layer length increases, as explained earlier. On the other hand, the diffusion potential tends to increase with a decrease Energies 2020, 13, 904 12 of 15

in the nanopore diameter. It is because the ion selectivity of the active layer is greater when the nanopore diameter of the active layer is smaller, due to higher Dukhin number shown in Equation (6). (c) As depicted in Figure 10c, the longer the length of the active layer with high electrical resistance, the higher the electrical resistance. On the other hand, the larger the nanopore diameter of the active layer, the larger the area specific resistance. Energies 2020, 13, x FOR PEER REVIEW 12 of 15 In the results so far, as mentioned in Section2 and shown in Figures1 and2b, a low concentration In the results so far, as mentioned in Section 2 and shown in Figures 1 and 2b, a low concentration solution filled the reservoir on the active layer side of the membrane. However, with the anisotropic solution filled the reservoir on the active layer side of the membrane. However, with the anisotropic membranemembrane not not being being symmetrical, symmetrical, the the results depended depended on onthe theinstallation installation direction. direction. In Figure In 11, Figure the 11, the resultsresults for for thethe firstfirst installation installation direction direction with with a lo aw low concentration concentration solution solution filling fillingthe reservoir the reservoir on the on the activeactive layer layer sideside areare comparedcompared with with the the results results for for the the second second installation installation direction direction with witha high a high concentrationconcentration solution solution filling filling the the reservoir reservoir on on thethe activeactive layer layer side. side. As As shown shown in Figure in Figure 11, the11, first the first installationinstallation direction direction has ahas larger a larger diffusion diffusion potential, potentia similarl, similar area area specific specific resistance, resistance, and and a largera larger power densitypower compared density tocompared the second to the configuration. second configuration. This is mainly This is mainly because because the concentration the concentration of the of solutionthe in thesolution active layerin the is active lower layer when is lower a low when concentration a low concentration solution filled solution the filled reservoir the reservoir on the active on the layer side ofactive the layer membrane. side of the As membrane. a result, the As Dukhina result, the number Dukhin is number smaller, is andsmaller, the and ion the selectivity ion selectivity is greater is greater for this case. Therefore, whenever an anisotropic membrane is used for reverse for this case. Therefore, whenever an anisotropic membrane is used for reverse electrodialysis, a low electrodialysis, a low concentration solution should be brought into contact with the active layer side concentrationof the membrane. solution should be brought into contact with the active layer side of the membrane.

(a)

(b)

Figure 10. Cont.

Energies 2020, 13, 904 13 of 15 Energies 2020, 13, x FOR PEER REVIEW 13 of 15 Energies 2020, 13, x FOR PEER REVIEW 13 of 15

(c) (c) Figure 10. A comparison of: (a) power densities, (b) diffusion potentials, and (c) area specific FigureFigure 10. A 10. comparison A comparison of: (a )of: power (a) power densities, densities, (b) di ff(usionb) diffusion potentials, potentials, and (c )and area (c specific) area specific resistances resistances of the anisotropic nanoporous anodic aluminum oxide membranes with two different of theresistances anisotropic of nanoporous the anisotropic anodic nanoporous aluminum anodic oxide aluminum membranes oxide with membranes two different with lengthstwo different and three lengths and three different pore diameters of the active layer for various combinations of differentlengths pore and diameters three different of the active porelayer diameters for various of the combinations active layer offor concentrations. various combinations of concentrations. concentrations.

(a) (a)

(b) (b)

Figure 11. Cont. Energies 2020, 13, 904 14 of 15 Energies 2020, 13, x FOR PEER REVIEW 14 of 15

(c)

FigureFigure 11. 11.A comparisonA comparison of: of: ( a()a) power power densities,densities, ( (b)) diffusion diffusion potentials, potentials, (c) ( carea) area specific specific resistances resistances of the anisotropicof the anisotropic nanoporous nanoporous anodic anodic aluminum aluminum oxide oxide membranes membranes with with two ditwofferent different install install directions for variousdirections combinations for various combinations of concentrations. of concentrations.

4. Conclusions4. Conclusions In thisIn study,this study, power power generation generation from from concentration concentration gradient gradient by reverse by reverse electrodialysis electrodialysis in anisotropic in anisotropic nanoporous anodic aluminum oxide membranes was experimentally investigated for the nanoporous anodic aluminum oxide membranes was experimentally investigated for the first time. first time. The experiments were carried out for various combinations of concentrations. The Theexperimental experiments results were carried showed out that for the various power combinationsdensity of the ofanisotropic concentrations. membrane The was experimental higher than results showedthat of that the theisotropic power membrane density for of all the combinations anisotropic of membrane concentrations. was The higher highest than power that density of the that isotropic membranewas measured for all from combinations the anisotropic of concentrations. membrane was The15.0 highestmW/m2, powerand it was density 7.2 times that higher was measured than that from the anisotropicfrom the isotropic membrane membrane. was It 15.0 is because mW/m the2, andpositive it was effect 7.2 of times the active higher layer than on the that power from density, the isotropic membrane.caused by It isincrease because in thethe positivediffusion e potential,ffect of the is activemuch layergreater on than the powerthe negative density, effect caused caused by by increase in theincrease diffusion in the potential, area specific is much resistance. greater The than ion the selectivity negative of e theffect active caused layer by increases increase rapidly in the area when specific resistance.the pore Thelength ion is selectivitychanged from of 0 the to several active micrometers layer increases because rapidly the ion when selectivity the pore strongly length depends is changed fromon 0 the to severalpore length micrometers for the short because nanopores the ionwhere selectivity the Goldman strongly approximation depends on is thenot satisfied. pore length As a for the result, the diffusion potential of the anisotropic membrane is much greater than that of the isotropic short nanopores where the Goldman approximation is not satisfied. As a result, the diffusion potential membrane. On the other hand, there is only a small difference in the area specific resistances between of thethe anisotropic anisotropic membranemembrane and is much the isotropic greater thanmembrane, that of because the isotropic the active membrane. layer length On is the smaller other hand, therethan is only one-twentieth a small di offference the support in the layer area length. specific As resistances a result, the between power thedensity anisotropic of the anisotropic membrane and the isotropicmembrane membrane, is much greater because than that the activeof the isot layerropic length membrane. is smaller The experiments than one-twentieth on the anisotropic of the support layermembranes length. As with a result, various the active power layer density lengths of and the pore anisotropic diameters membrane were conducted. is much It was greater shown than that that of the isotropicthe length membrane.of the active layer The is experiments a more importan on thet engineering anisotropic parameter membranes than the with nanopore various diameter active layer lengthsof the and active pore layer. diameters Additionally, were conducted. it was shown It wasthat a shown low concentration that the length solution of the should active be layer brought is a more importantinto contact engineering with the parameteractive layer thanside of the the nanopore membrane diameter whenever of an the anisotropic active layer. membrane Additionally, is used it was for reverse electrodialysis. shown that a low concentration solution should be brought into contact with the active layer side of the membraneAuthor Contributions: whenever Investigation, an anisotropic Y.L., membraneH.J.K., and D.-K.K. is used wr foriting—original reverse electrodialysis. draft preparation, D.-K.K., writing—review and editing, D.-K.K. Author Contributions: Investigation, Y.L., H.J.K., and D.-K.K. writing—original draft preparation, D.-K.K., writing—reviewFunding: This and research editing, was D.-K.K. supported All authorsby the Nano-Material have read and Technology agreed to theDevelopment published Program version through of the manuscript. the National Research Foundation of Korea (NRF) and funded by the Ministry of Science and ICT (NRF-2011- Funding:0030285).This research was supported by the Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Science and ICT (NRF-2011-0030285). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publishpublish the results.the results.

Energies 2020, 13, 904 15 of 15

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