This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. NRF-2017R1A2A2A05001329). The authors of the manuscript are grateful to the Sogang University, Seoul, Republic of Korea.

1. Experimental requirement
<ol>
	<li>Purchase ion-exchange ionomer polymer, E-550 sulfonated-PEEK polymer fiber to prepare CEM and FAA-3 to prepare AEM. Ensure that all ionomer polymers are stored in a clean, dry, and dust-free environment before use.</li>
	<li>Use high purity (&#62;99%) solvents, including N-Methyl-2- pyrrolidone with molecular weight 99.13 g mol-1 and N, N-Dimethylacetamide with molecular weight 87.12, for preparing homogeneous ionomer solution. Ensure all analytical grade chemicals and solvents are used for membrane preparation as received without any further purification.</li>
	<li>After the membranes&#39; activation process, immediately immerse all membranes in a 0.5 M NaCl solution for better performance. After activation of both membranes, drying is not required. Water with resistivity is 18.2 M&#937; at room temperature was used throughout the synthesis of the membrane.</li>
	<li>Characterize membrane properties using a dry membrane. The detailed description of the characterization techniques and their physicochemical properties such as ion-exchange capacity, ion-conductivity, thickness, thermal analysis, and surface morphology, are as presented in the literatures10,11.</li>
	<li>Use a cutter to shape the membrane for CEM and AEM to the RED stack size with an active area of 49 cm2, as displayed in Figure 2.</li>
	<li>For the RED stack fabrication, make an alternate CEM and AEM arrangement, separated by spacer and gasket; a real picture of the working RED stack is presented in Figure 3a, and its schematic diagram of each layer is illustrated in Figure 3b.
	<ol>
		<li>First, place the PMMA plate facing electrode upside; now, place the rubber gasket and spacer on it, then place the CEM. After that, place the silicone gasket with the spacer on the CEM then place the AEM on it. Similarly, add the silicon gasket and spacer on the top of AEM followed by CEM. Now, place the end PMMA plate, rubber gasket, and spacer followed with tightening using screw and nut bolts.</li>
	</ol>
	</li>
	<li>After assembling the RED stack, check the free flow of the high-concentration (HC), low-concentration (LC), and rinse the solutions one by one. Any crossflow or leakage is required to be eliminated before the measurement.</li>
	<li>Prior to the current and voltage measurement, monitor the flow rate of salt solutions and pressure gauge reading and make sure it gets stabilized. Make sure all the connections are in the exact place before the measurement starts. Avoid touching the RED stack and its connecting tubes while the measurement is running. 
	&#8203;NOTE: HC and LC solution flow from their compartments to discard&#160;compartment through a peristaltic pump, pressure gauge, and RED stack, respectively.</li>
	<li>Use galvanostat method for the measurement of current and voltage, the source meter instrument connected to the RED stack through crocodile clips.</li>
</ol>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/62309/62309fig02.jpg" /> 
Figure 2: Size and shape of the prepared membrane, gasket, and spacer for the fabrication of reverse electrodialysis. (a) outer silicone gasket, (b) outer spacer and inner spacer, (c) inner silicone gasket, (d) cation-exchange membrane, (e) anion-exchange membrane, and (f) gasket and membrane assembly. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/62309/62309fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/62309/62309fig03.jpg" /> 
Figure 3:&#160;Reverse electrodialysis stack. (a) setup of reverse electrodialysis stack with connecting tubes, and (b) schematic illustration of different layers, including PMMA endplates, electrodes, gasket, spacer, CEM, and AEM. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/62309/62309fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Ion-exchange membrane preparation
NOTE: The amount of precursor material was optimized for obtaining a membrane with 18 cm diameter and ~50 &#181;m thickness.
<ol>
	<li>Cation-exchange membrane
	<ol>
		<li>Take 5 wt% of sulfonated-PEEK fibers in a 250 mL round bottom flask and dissolve the fibers in Dimethylacetamide (DMAc) as a solvent having molecular weight 87.12 g mol-1. Shake the flask for 10 min so that all ionomer polymers settle down.</li>
		<li>Place a magnetic bar in the flask and then keep the mixture in the silicon oil bath, followed by vigorously stirring at 500 rpm for 24 h at 80 &#176;C to obtain a homogenous solution.</li>
		<li>Filter the sulfonated-PEEK solution through a 0.45 &#181;m pore size Polytetrafluoroethylene (PTFE) filter.</li>
		<li>After that, pour the filtered solution onto a circular glass dish with a diameter of 18 cm. Ensure that all air bubbles are removed using an air blower before placing the Petri dish in the oven.</li>
		<li>Place the Petri dish inside an oven for drying out the solution at 90 &#176;C for 24 h, resulting in ~50 &#181;m thick free-standing membrane. Do this for extracting free-standing membrane: To peel off the membrane from the Petri dish, fill the Petri dish with warm distill water (~60 &#176;C) and let it stand for 10 min untouched. The free-standing membrane will automatically come out.</li>
		<li>For membrane activation, immerse the prepared free-standing membrane in 1 M sulfuric acid (H2SO4)&#160;aqueous solution, i.e., 98.08 g, in 1 L of distilled water, and incubate for 2 h at 80 &#176;C. 
		NOTE: This step will ensure the removal of foreign particles and other chemicals such as solvents that will reduce the possibility of membrane from fouling.</li>
		<li>Wash the soaked membrane with 1 L of distilled water for 10 min, at least three times at room temperature.</li>
	</ol>
	</li>
	<li>Anion-exchange membrane
	<ol>
		<li>Dissolve FAA-3 ionomer solution 10 wt.% in N-Methyl-2-pyrrolidone (NMP) solvent.</li>
		<li>Keep the solution for stirring at room temperature for 2 h at ~500 rpm.</li>
		<li>After that, filter the solution using the mesh with 100 &#181;m pore size.</li>
		<li>Pour ~30 mL filtered solution into a circular glass Petri dish with a diameter of 18 cm. Ensure that all air bubbles were removed using an air blower before placing the glass Petri dish in the oven. The drying process takes place at 100 &#176;C for 24 h.</li>
		<li>To obtain a free-standing membrane, pour hot distilled water into the glass Petri dish and keep it for at least 10 min. Now peel off the membranes and place in 1 liter of sodium hydroxide (NaOH) solution (concentration 1M and molecular weight 40 g mol-1) for 2 h.</li>
		<li>Then, wash the membrane thoroughly with 1 L of distilled water for 10 min, at least three times in ambient condition. 
		&#8203;NOTE: All prepared membranes were stored in the 0.5 M NaCl solution overnight before using it in the RED stack. So that the membrane conductivity gets enhanced and can achieve stabilized output performance during the measurement of the RED stack. Table 1 describes the membrane properties10,11.</li>
	</ol>
	</li>
</ol>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Specification</td>
			<td>Unit</td>
			<td>CEM</td>
			<td>AEM</td>
		</tr>
		<tr>
			<td>Swelling degree&#160;</td>
			<td>%</td>
			<td>5&#177;1</td>
			<td>1&#177;0.5</td>
		</tr>
		<tr>
			<td>Charge density or Ion exchange capacity&#160;</td>
			<td>meq/g</td>
			<td>1.8</td>
			<td>~1.6</td>
		</tr>
		<tr>
			<td>Mechanical properties 
			(Tensile strength)</td>
			<td>MPa</td>
			<td>&#62;40</td>
			<td>40-50</td>
		</tr>
		<tr>
			<td>Elongation to Break&#160;</td>
			<td>%</td>
			<td>~42</td>
			<td>30-50</td>
		</tr>
		<tr>
			<td>Young Modulus (MPa)</td>
			<td></td>
			<td>1500&#177;100</td>
			<td>1000-1500</td>
		</tr>
		<tr>
			<td>Conductivity at room temperature</td>
			<td>S/cm</td>
			<td>~0.03</td>
			<td>~0.025</td>
		</tr>
		<tr>
			<td>Permselectivity</td>
			<td>%</td>
			<td>98-99</td>
			<td>94-96</td>
		</tr>
		<tr>
			<td>Thickness</td>
			<td>&#956;m</td>
			<td>50&#177;2</td>
			<td>50&#177;3</td>
		</tr>
		<tr>
			<td>Solvent</td>
			<td>-</td>
			<td>Dimethylacetamide (DMAc)</td>
			<td>N-methyl-2-pyrrolidone (NMP)</td>
		</tr>
	</tbody>
</table>
Table 1: Membranes properties. Summary of both cation-exchange and anion-exchange membrane properties.
3. Fabrication of reverse electrodialysis
<ol>
	<li>Assembly of RED stack
	<ol>
		<li>Prepare a model solution using 0.6 M NaCl for high concentration (HC) and 0.01 M NaCl for low concentration (LC) compartments12. 
		NOTE: Here, river water is considered a low concentration salt solution, and seawater is represented as a high concentration salt solution.</li>
		<li>Prepare 5 L of high concentration and low concentration solution in a large container connected with the tubes. Keep the solutions stirring at ambient conditions (room temperature) for at least 2 h before it is used in the RED stack.</li>
		<li>Prepare the mixture of 0.05 M of [Fe (CN)6]-3/ [Fe (CN)6]-4 and 0.3 M NaCl in 500 mL water as a rinse solution for RED.</li>
		<li>Connect all three solution containers with RED stack using rubber tubes through the peristaltic pump and pressure gauges. Use the tube of size L/S 16 for rinse solution, and use the tube of size L/S 25 for HC and LC solution.</li>
		<li>To make a RED stack, take two endplates made-up of polymethyl methacrylate (PMMA). Connect both endplates horizontally face to face with nuts, bolts, and washers using 25 Nm force using a digital wrench driver. The thickness of PMMA endplates 3 cm, and the path of the flow channels was designed in plates for HC, LC, and rinse solution by a driller2.</li>
		<li>Place two mesh electrodes made from metal Titanium (Ti) coated with a mixture of Iridium (Ir) and Ruthenium (Ru) in a 1:1 ratio and place at the end of the PMMA plates. Both end electrodes are connected with the crocodile clip of the source meter. 
		&#8203;NOTE: Both PMMA end plates are equipped with mesh electrodes, both electrodes were layered with a square shape spacer, and the PMMA endplate covered with a rubber gasket facing inside. After that, CEM and AEM are placed alternatively, separated by silicone gasket and spacer, as shown in Figure 3.</li>
		<li>Install silicon gaskets, polymer spacers, and ion-exchange membranes (CEM and AEM) layer by layer, as presented in the schematic diagram Figure 4 and Figure 5. Ensure the active area of electrodes, both membranes, outer and inner spacer, outer and inner gasket is 7 x 7 = 49 cm2.</li>
		<li>Pass high-concentration and low-concentration solutions from respective compartments by peristaltic pumps, as displayed in the schematic diagram in Figure 4.</li>
		<li>Circulate the rinse solution in the outer electrode and membrane compartments in recirculation mode using peristaltic pumps. The flow rate used for the rinse solution is 50 mL min-1.</li>
		<li>Fixed flow rate is used for analyzing the performance of each membrane. In this experiment, we have used 100 mL min-1 through a peristaltic pump.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/62309/62309fig04.jpg" /> 
Figure 4: Schematic representation of the tube connection with reverse electrodialysis stack. Connection of reverse electrodialysis with peristaltic pumps, high-concentration solution container, low-concentration solution container, rinse solution container, and discard solution container. It also shows the spacer&#39;s alignment with both an anion exchange membrane (AEM) and cation exchange membrane (CEM). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/62309/62309fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/62309/62309fig05.jpg" /> 
Figure 5: Schematic diagram of different layers in the reverse electrodialysis setup. (a) Cross-section view of a schematic illustration of reverse electrodialysis shows the flow direction of the high-concentration solution, low-concentration solution, and electrode rinse solution. Other components such as electrodes, outer and inner gaskets, outer and inner spacers, cation-exchange membrane, and anion-exchange membrane. (b) Front view of the stack, which shows the flow direction of a solution. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/62309/62309fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
4. Measurement of reverse electrodialysis
<ol>
	<li>Power calculation
	<ol>
		<li>Let the high concentration, low concentration, and rinse solution, run through the stack at least for 5 min. Measure the RED output performance by a source meter, which is connected to both electrodes of the RED stack13.</li>
		<li>Calculate the RED stack&#39;s current-voltage characteristics in terms of power density using the galvanostat method. 
		NOTE: In the galvanostat method, a constant current is applied across electrodes and measures the resulting current. The resulting current is the current generated due to the electrochemical reaction in the stack. The measurement is carried out under 0.05 V static voltage with a fixed sweep current that is 10 mA.</li>
		<li>The maximum power density for the RED stack is measured with the help of the following equation 1. 
		<img alt="Equation 2" src="/files/ftp_upload/62309/62309eq2.jpg" />&#160;(1) 
		Here, Pmax is the maximum power density of the RED stack (Wm-2), Ustack is the voltage (V) produced by the membrane in the stack, Istack is the recorded current (A), and Amem is the active area of the membranes (m2).</li>
	</ol>
	</li>
</ol>

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E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Salt+concentration+differences+alter+membrane+resistance+in+reverse+electrodialysis+stacks.">Salt concentration differences alter membrane resistance in reverse electrodialysis stacks.</a> Environmental Science &amp; Technology Letters. 1, 36-39 (2014).</li></ol>

The authors declare no conflicts of interest.

The RED&#39;s working principle is mainly dominated by the membrane&#39;s physicochemical properties, which is a crucial part of the RED system, as illustrated in Figure 3. Here, we describe the fundamental characteristics of the membrane for delivering a high-performance RED system. Membrane&#39;s specific ion permeability makes it pass one type of ions through their polymer nanochannel. As the name suggests, CEM can pass cation from one side to another and restricts anion, whereas AEM can pass anion and restricts cation. As shown in Figure 2, all membranes were shaped into a RED stack size containing inlet and outlet passage for flow solution. The amount of ion exchanged through the membrane is directly proportional to the membrane&#39;s conductivity and, therefore, the power output of the stack24. The movement of ions in the ion-exchange membrane works on the Donnan exclusion principle25. The charge group attached with the polymer backbone repels the same charge present in the solution. Thus, higher the charge density greater will be the repulsion, which usually depends on the perm selectivity. Generally, in RED cells, ions&#39; movement takes place through the membrane from higher concentration to lower concentration of the solution. This ion transport from one compartment to another through the membrane gives an open circuit voltage and current values, which is used to calculate the net power output of the cell26.
The RED stack&#39;s performance mainly depends on the ion exchange capacity and swelling density of CEM- and AEM-based membranes27. It is observed that the higher the ion-exchange capacity of the CEM and AEM, the better is the conductivity. However, the higher ion-exchange capacity of the membrane leads to high swelling, easily deteriorating the membrane&#39;s mechanical strength. Thus, it is essential to optimize swelling density and the membranes&#39; conductivity for better and reliable cell performance. On the other hand, it is also crucial to optimize the stack resistance with the function of the feed solution&#39;s flow in both the compartments. As the flow rate increases, the stack resistance decreases, and the output cell performance increases. Theoretically, RED stack resistance is given by equation 8.
<img alt="Equation 9" src="/files/ftp_upload/62309/62309eq9.jpg" />&#160;(8)
N is the number of cell pairs (alternate arrangement of anion- and cation-exchange membranes), A is the effective area of both the membranes (m2), RA is the anion exchange membrane resistance (&#8486; m2), RC is the cation exchange membrane resistance (&#8486; m2), dc is the thickness of the compartment with the concentrated solution (m), kc is its ionic conductivity (S m-1 ), dd is the thickness of the compartment with the diluted solution (m), kd is its ionic conductivity (S m-1), and Re is the electrode resistance (&#8486;). Reducing the stack resistance is an essential factor for enhancing the net output power, but other factors also influence the cell performance28, which also need to be considered. The spacer shadow effect, the flow of feed solution, compartment width, and concentration of feed solution, the schematic illustration of the RED cell are presented in Figure 5.
In RED cells, the membrane acted as a limiting factor and required a stable high conducting membrane. Apart from that, both CEM and AEM are required to have comparable ion-conducting properties so that the cell can produce an efficient and optimized power output. Degradation of ion-exchange capacity and salt accumulation also need to be taken into account for reliable RED performance. Novel membrane material and state-of-the-art device architecture may further improve cell performance in the coming future and will pave a path for future research direction.

Energy harvesting from natural resources is an economical method that is environmentally friendly, thereby making our planet green and clean. Several processes have been proposed until now to extract energy, but reverse electrodialysis (RED) has an enormous potential to overcome the energy crisis issue1. Power production from Reverse electrodialysis is a technological breakthrough for the decarbonization of global energy. As the name suggests, RED is a reverse process, where the alternate cell compartment is filled with the high-concentrated salt solution and low-concentrated salt solution2. The chemical potential generated by the salt concentration difference across the ion-exchange membranes, collected from the electrodes at the compartment end.
Since the year 2000, many research articles have been published, providing insight into the RED theoretically and experimentally3,4. Systematic studies on the operation conditions and reliability studies under stress conditions improved the stack architecture and enhanced the overall cell performance. Several research groups have diverted their attention toward RED&#39;s hybrid application, such as RED with desalination process5, RED with solar power6, RED with reverse osmosis (RO) process5, RED with the microbial fuel cell7, and RED with the radiative cooling process8. As mentioned earlier, there is a lot of scope in implementing RED&#39;s hybrid application to solve the energy and clean water problem.
Several methods have been adopted to enhance the RED cell&#39;s performance and the membrane&#39;s ion-exchange capacity. Tailoring the cation-exchange membranes with different types of ions using sulfonic acid group (-SO3H), phosphonic acid group (-PO3H2), and carboxylic acid group (-COOH) is one of the effective ways to alter the physicochemical properties of the membrane. Anion-exchange membranes are tailored with ammonium groups (<img alt="Equation 1" src="/files/ftp_upload/62309/62209eq1.jpg" />)9. The high ionic conductivity of AEM and CEM without deteriorating the membrane&#39;s mechanical strength is the essential parameter for selecting an appropriate membrane for device application. The robust membrane under stress conditions provides mechanical stability to the membrane and enhances the device&#39;s durability. Here, a unique combination of high-performance free-standing sulfonated poly (ether ether ketone) (sPEEK) as cation-exchange membranes with FAA-3 as anion-exchange membranes are used in the RED application. Figure 1 shows the flow chart of the experimental procedure.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/62309/62309fig01.jpg" /> 
Figure 1: Procedure chart. The flow chart presents the procedure adopted for the preparation of ion-exchange membrane followed by the process for measurement of reverse electrodialysis. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/62309/62309fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>AEM based membrane</td><td>Fumion</td><td>P1810-194</td><td>Ionomer</td></tr><tr><td>CEM based membrane</td><td>Fumion</td><td>E550</td><td>Ionomer</td></tr><tr><td>Digital torque wrench</td><td>Torqueworld</td><td>WP2-030-09000251</td><td>wrench</td></tr><tr><td>Labview software</td><td>Natiaonal Instrument</td><td>-</td><td>Software</td></tr><tr><td>Laptop</td><td>LG</td><td>-</td><td>PC</td></tr><tr><td>Magnetic stirrer</td><td>Lab Companion</td><td>-</td><td>MS-17BB</td></tr><tr><td>N, N-Dimethylacetamide</td><td>Sigma aldrich</td><td>271012</td><td>Chemical</td></tr><tr><td>N-Methyl-2- pyrrolidone</td><td>Daejung</td><td>872-50-4</td><td>Chemical</td></tr><tr><td>Peristaltic pump</td><td>EMS tech Inc</td><td>-</td><td>EMP 2000W</td></tr><tr><td>Potassium hexacyanoferrate(II) trihydrate</td><td>Sigma aldrich</td><td>P3289</td><td>Chemical</td></tr><tr><td>Potassium hexacyanoferrate(III)</td><td>Sigma aldrich</td><td>244023</td><td>Chemical</td></tr><tr><td>Pressure Gauge</td><td>Swagelok</td><td>-</td><td>Guage</td></tr><tr><td>Reverse electrodialysis setup</td><td>fabricated in lab</td><td>-</td><td>Device</td></tr><tr><td>RO system pure water</td><td>KOTITI</td><td>-</td><td>Water</td></tr><tr><td>Rotary evaporator</td><td>Hitachi</td><td>YEFO-KTPM</td><td>Induction motor</td></tr><tr><td>Sodium Chloride</td><td>Sigma aldrich</td><td>S9888</td><td>Chemical</td></tr><tr><td>Sodium Hydroxide</td><td>Merk</td><td>1310-73-2</td><td>Chemical</td></tr><tr><td>Source meter</td><td>Keithley</td><td>-</td><td>2410</td></tr><tr><td>Spacer</td><td>Nitex, SEFAR</td><td>06-250/34</td><td>Spacer</td></tr><tr><td>Sulfuric acid</td><td>Daejung</td><td>7664-93-9</td><td>Chemical</td></tr><tr><td>Tube</td><td>Masterflex tube</td><td>96410-25</td><td>Rubber tube</td></tr></tbody></table>

ion-exchange membranes for the fabrication of reverse electrodialysis device

Reverse electrodialysis (RED) is an effective way to generate power by mixing two different salt concentrations in water using cation-exchange membranes (CEM) and anion-exchange membranes (AEM). The RED stack is composed of an alternating arrangement of the cation-exchange membrane and anion-exchange membrane. The RED device acts as a potential candidate for fulfilling the universal demand for future energy crises. Here, in this article, we demonstrate a procedure to fabricate a reverse electrodialysis device using laboratory-scale CEM and AEM for power production. The active area of the ion-exchange membrane is 49 cm2. In this article, we provide a step-by-step procedure for synthesizing the membrane, followed by the stack&#39;s assembly and power measurement. The measurement conditions and net power output calculation have also been explained. Furthermore, we describe the fundamental parameters that are taken into consideration for obtaining a reliable outcome. We also provide a theoretical parameter that affects the overall cell performance relating to the membrane and the feed solution. In short, this experiment describes how to assemble and measure RED cells on the same platform. It also contains the working principle and calculation used for estimating the net power output of the RED stack using CEM and AEM membranes.

Net power output 
RED cell generally generates electrical energy from the salinity gradient of the salt solution, i.e., ions&#39; movement in the opposite direction through the membrane. To assemble the RED stack correctly, one needs to align all the layers, including electrodes, gaskets, membranes, and spacers in the stack carefully, as demonstrated in the schematic diagram in Figure 4 and Figure 5. If the stack is not perfectly aligned, two problems may arise: (i) HC and LC solution crossflow may occur in the stack and (ii) leakage of the solution in the stack may occur. It is necessary to eliminate both the problems before starting the actual measurement of power output. Other parameters need to be fixed, including the HC and LC solution&#39;s flow rate, pumping pressure, and applied voltage, to obtain efficient power output. To estimate the RED stack&#39;s net power, one needs to deduct the hydrodynamic power loss from obtained net power10. The maximum power output is obtained from the RED stack by multiplying the obtained voltage and current. In contrast, the active area and number of the membrane pairs must be divided to obtain the stack&#39;s actual power density, as given by equation 114,15. The total power obtained from the RED stack is subtracted by a hydrodynamic power loss or pumping power loss generated by the pump and given by the following equation 2.
<img alt="Equation 3" src="/files/ftp_upload/62309/62309eq3.jpg" />(2)
Here, Ploss is a hydrodynamic pumping power loss (W m-2) produced in the RED stack by internal loss. Pmax is the maximum power (W m-2) obtained from the experiment. The highest net power output reported for RED is 1.2 W m-2 using river water and seawater by Vermaas16. Power loss is represented as a difference of pressure at inlet and outlet of HC and LC solution at the stack and given by pressure drop (&#916;P), flow rate (Q), and pump efficiency (&#951;pump)17,18.
<img alt="Equation 4" src="/files/ftp_upload/62309/62309eq4.jpg" />&#160;(3)
Here, QH and QL are the flow rate (mL mim-1) of a high-concentration solution and low concentration solution in mL min-1&#160;and&#160;&#916;PH&#160;and &#916;PL is the pressure drop at the high -concentration side and low concentration compartment in Pa. Here, the measured pressure drop from the pressure gauge for the HC compartment is 11,790 Pa and LC compartment is 11,180 Pa. The calculated pumping power loss (Ploss) is 0.038 W m-2.
Theoretical parameter estimation 
Basically, A RED system is made up of two different types of ion-exchange membranes, gasket, pump, spacers, and electrode. The pressure drop across the RED stack is estimated theoretically using the Darcy-Weisbach equation11,19. In an ideal RED system, a laminar flow of solution in an infinite wide uniform channel is used for calculating the pressure drop.
<img alt="Equation 5" src="/files/ftp_upload/62309/62309eq5.jpg" />&#160;(4)
Here, dh (m) is the channel&#39;s hydraulic diameter, whereas the hydraulic diameter for an infinite wide channel is 2h. Other parameters is the viscosity of water (Pa&#183;s), tres is the residence time (s), L is the length of the membrane (cm). In RED stack, sPEEK as CEM and FAA-3 as AEM is used, and the distance between both membranes is given by the term b, which is directly proportional to the hydraulic diameter&#39;s value in the case of the profiled membrane, and &#34;h&#34; is the intermembrane distance (m), is given by equation 520.
<img alt="Equation 6" src="/files/ftp_upload/62309/62309eq6.jpg" />&#160;(5)
For an infinite wide channel, the value calculated from equation 6 is usually much lower than the finite wide channel&#39;s value. The values obtained are low in magnitude, which is due to the non-uniformity of inlet and outlet of feed solutions. The spacer mesh restricts the flow of aqueous salt solutions due to the spacer shadow effect, resulting in an increment in pumping power. Placing the value obtained from the ratio of surface to volume (Ssp&#160;/ Vsp) of spacer mesh in the formula,&#160;&#949; is the porosity, one can estimate the thickness of spacer-filled channels from equation 621,22.
<img alt="Equation 7" src="/files/ftp_upload/62309/62309eq7.jpg" />&#160;(6)
The spacer thickness and the other parameters, including open ratio, mesh opening, and wire diameters, are kept constant in all the compartments. Both HC and LC compartments used the same solution (NaCl) with different concentrations. Therefore, it is easy to initialize the parameters, and theoretical pumping loss can be given by equation 723.
<img alt="Equation 8" src="/files/ftp_upload/62309/62309eq8.jpg" />&#160;(7)
Where, A is the active membrane area in m2 and Q feed solution flow rate in m3 s-1. Here,&#160;&#956; is the viscosity of water measured in Pa&#183;s, L&#160;is the length of membrane given by cm, and tres is a residence time in second.
The performance of the RED stack 
The RED stack&#39;s output performance was investigated using one cell pair at a fixed flow rate of 100 mL min-1. The feed solution&#39;s concentration was also kept fixed for a higher concentration (0.6 M), and a lower concentration (0.01 M) prepared from NaCl salt. It is observed that the maximum power density is 0.69 W m-2 at 100 mL min-1, and the net power density is 0.66 W m-2 as presented in Figure 6. Higher flow rate and high ion-exchange capacity play a significant role in obtaining better cell performance because ions&#39; transport is more active at a higher flow rate. On the other hand, it decreases the diffusion-boundary-layer resistance at the interface. The difference in the salinity gradient of the salt concentration gives rise to the open-circuit voltage, as illustrated in Figure 6. This voltage depends on the internal resistance of the RED stack and other parameters. It is noted that as the current density increases, the voltage start decreases whereas, initially the power density of the cell increases obtaining maxima at a certain current density value and then drops down. This decrease in the power density is due to an increase in the stack&#39;s internal resistance, as shown in Figure 6.
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/62309/62309fig06.jpg" /> 
Figure 6: Output performance of the reverse electrodialysis device: (a) variation of output voltage with varying current, and (b) net power density with a varying current density of the RED stack. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/62309/62309fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Ion-Exchange Membranes for the Fabrication of Reverse Electrodialysis Device at JoVE.com

Ion-Exchange Membranes for the Fabrication of Reverse Electrodialysis Device

We demonstrate the fabrication of a reverse electrodialysis device using a cation-exchange membrane (CEM) and anion-exchange membrane (AEM) for power generation.

Reverse electrodialysis (RED) is an effective way to generate power by mixing two different salt concentrations in water using cation-exchange membranes ...

ion-exchange-membranes-for-fabrication-reverse-electrodialysis

Nanyang Technological University

Research

JoVE Journal

Engineering

10.3K Views.  Energy Engineering Lab, Sogang University. We demonstrate the fabrication of a reverse electrodialysis device using a cation-exchange membrane (CEM) and anion-exchange membrane (AEM) for power generation.

Video: Ion-Exchange Membranes for the Fabrication of Reverse Electrodialysis Device

Electrochemically and Bioelectrochemically Induced Ammonium Recovery

Streams such as urine and manure can contain high levels of ammonium, which could be recovered for reuse in agriculture or chemistry. The extraction of ammonium from an ammonium-rich stream is demonstrated using an electrochemical and a bioelectrochemical system. Both systems are controlled by a potentiostat to either fix the current (for the electrochemical cell) or fix the potential of the working electrode (for the bioelectrochemical cell). In the bioelectrochemical cell, electroactive bacteria catalyze the anodic reaction, whereas in the electrochemical cell the potentiostat applies a higher voltage to produce a current. The current and consequent restoration of the charge balance across the cell allow the transport of cations, such as ammonium, across a cation exchange membrane from the anolyte to the catholyte. The high pH of the catholyte leads to formation of ammonia, which can be stripped from the medium and captured in an acid solution, thus enabling the recovery of a valuable nutrient. The flux of ammonium across the membrane is characterized at different anolyte ammonium concentrations and currents for both the abiotic and biotic reactor systems. Both systems are compared based on current and removal efficiencies for ammonium, as well as the energy input required to drive ammonium transfer across the cation exchange membrane. Finally, a comparative analysis considering key aspects such as reliability, electrode cost, and rate is made.
This video article and protocol provide the necessary information to conduct electrochemical and bioelectrochemical ammonia recovery experiments. The reactor setup for the two cases is explained, as well as the reactor operation. We elaborate on data analysis for both reactor types and on the advantages and disadvantages of bioelectrochemical and electrochemical systems.

We demonstrate the extraction of ammonium from an ammonium-rich stream using an electrochemical and a bioelectrochemical system. The reactor setup, operation and data analysis are discussed.

Streams such as urine and manure can contain high levels of ammonium, which could be recovered for reuse in agriculture or chemistry. The extraction of ...

Chemistry

Fabrication Process of Silicone-based Dielectric Elastomer Actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tunable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied.

This manuscript shows the fabrication process for the manufacture of dielectric elastomer soft actuators based on silicone membranes. The three key stages of production are presented in detail: blade casting of thin silicone membranes; pad printing of compliant electrodes; and the assembly of all the components.

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an ...

Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed electric body force has to be induced by ionic motions in liquid. On the other hand, positive and negative charges attract each other, and electroneutrality is maintained everywhere in equilibrium conditions. Furthermore, an increase in an applied voltage has to be suppressed to avoid water electrolysis, which causes the solutions to become unstable. Usually, EHD flows can be induced in non-aqueous solutions by applying extremely high voltages, such as tens of kV, to inject electrical charges. In this study, two methods are introduced to generate EHD flows induced by electrical charge separations in aqueous solutions, where two liquid phases are separated by an ion-exchange membrane. Due to a difference in the ionic mobility in the membrane, ion concentration polarization is induced between both sides of the membrane. In this study, we demonstrate two methods. (i) The relaxation of ion concentration gradients occurs via a flow channel that penetrates an ion-exchange membrane, where the transport of the slower species in the membrane selectively becomes dominant in the flow channel. This is a driving force to generate an EHD flow in the liquid. (ii) A long waiting time for the diffusion of ions passing through the ion-exchange membrane enables the generation of an ion-dragged flow by externally applying an electric field. Ions concentrated in a flow channel of a 1 x 1 mm2 cross-section determine the direction of the liquid flow, corresponding to the electrophoretic transport pathways. In both methods, the electric voltage difference required for an EHD flow generation is drastically reduced to near 2 V by rectifying the ion transport pathways.

The rectification of ion transport pathways is an effective method to generate one-directional ion-dragged electrohydrodynamic flows. By setting an ion-exchange membrane in a flow channel, an electrically polarized condition is generated and causes a liquid flow to be driven when an electric field is externally applied.

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed ...

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 &#176;C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and ...

Three-Dimensionally Printed Microfluidic Cross-flow System for Ultrafiltration/Nanofiltration Membrane Performance Testing

Minimization and management of membrane fouling is a formidable challenge in diverse industrial processes and other practices that utilize membrane technology. Understanding the fouling process could lead to optimization and higher efficiency of membrane based filtration. Here we show the design and fabrication of an automated three-dimensionally (3-D) printed microfluidic cross-flow filtration system that can test up to 4 membranes in parallel. The microfluidic cells were printed using multi-material photopolymer 3-D printing technology, which used a transparent hard polymer for the microfluidic cell body and incorporated a thin rubber-like polymer layer, which prevents leakages during operation. The performance of ultrafiltration (UF), and nanofiltration (NF) membranes were tested and membrane fouling could be observed with a model foulant bovine serum albumin (BSA). Feed solutions containing BSA showed flux decline of the membrane. This protocol may be extended to measure fouling or biofouling with many other organic, inorganic or microbial containing solutions. The microfluidic design is especially advantageous for testing materials that are costly or only available in small quantities, for example polysaccharides, proteins, or lipids due to the small surface area of the membrane being tested. This modular system may also be easily expanded for high throughput testing of membranes.&#160;

Design and fabrication of a three-dimensionally (3-D) printed microfluidic cross-flow filtration system is demonstrated. The system is used to test performance and observe fouling of ultrafiltration and nanofiltration (thin film composite) membranes.

Minimization and management of membrane fouling is a formidable challenge in diverse industrial processes and other practices that utilize membrane ...

Bioengineering

Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO2/Si/SiO2 Wafers for Green Desalination

Desalination through direct contact membrane distillation (DCMD) exploits water-repellent membranes to robustly separate counterflowing streams of hot and salty seawater from cold and pure water, thus allowing only pure water vapor to pass through. To achieve this feat, commercial DCMD membranes are derived from or coated with water-repellent perfluorocarbons such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF). However, the use of perfluorocarbons is limiting due to their high cost, non-biodegradability, and vulnerability to harsh operational conditions. Unveiled here is a new class of membranes referred to as gas-entrapping membranes (GEMs) that can robustly entrap air upon immersion in water. GEMs achieve this function by their microstructure rather than their chemical make-up. This work demonstrates a proof-of-concept for GEMs using intrinsically wetting SiO2/Si/SiO2 wafers as the model system; the contact angle of water on SiO2 is &#952;o &#8776; 40&#176;. Silica-GEMs had 300 &#956;m-long cylindrical pores whose diameters at the (2 &#956;m-long) inlet and outlet regions were significantly smaller; this geometrically discontinuous structure, with 90&#176; turns at the inlets and outlets, is known as the &#34;reentrant microtexture&#34;. The microfabrication protocol for silica-GEMs entails designing, photolithography, chrome sputtering, and isotropic and anisotropic etching. Despite the water loving nature of silica, water does not intrude silica-GEMs on submersion. In fact, they robustly entrap air underwater and keep it intact even after six weeks (&#62;106 seconds). On the other hand, silica membranes with simple cylindrical pores spontaneously imbibe water (&#60; 1 s). These findings highlight the potential of the GEMs architecture for separation processes. While the choice of SiO2/Si/SiO2 wafers for GEMs is limited to demonstrating the proof-of-concept, it is expected that the protocols and concepts presented here will advance the rational design of scalable GEMs using inexpensive common materials for desalination and beyond.

Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO2/Si/SiO2 Wafers for Green Desalination

Presented here is a stepwise protocol for realizing gas-entrapping membranes (GEMs) from SiO2/Si wafers using integrated circuit microfabrication technology. When silica-GEMs are immersed in water, the intrusion of water is prevented, despite the water-loving composition of silica.

Desalination through direct contact membrane distillation (DCMD) exploits water-repellent membranes to robustly separate counterflowing streams of hot and ...

Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, compliant and biomimetic nature of this deformation, actuators made of the laminate have received increasing interest in soft robotics and (bio)medical applications. However, methods to easily fabricate the active material in large (even industrial) quantities and with a high batch-to-batch and within-batch repeatability are needed to transfer the knowledge from laboratory to industry. This protocol describes a simple, industrially scalable and reproducible method for the fabrication of ionic carbon-based electromechanically active capacitive laminates and the preparation of actuators made thereof. The inclusion of a passive and chemically inert (insoluble) middle layer (e.g., a textile-reinforced polymer network or microporous Teflon) distinguishes the method from others. The protocol is divided into five steps: membrane preparation, electrode preparation, current collector attachment, cutting and shaping, and actuation. Following the protocol results in an active material that can, for example, compliantly grasp and hold a randomly shaped object as demonstrated in the article.

This article describes a fast and simple manufacturing process of ionic electromechanically active composite materials for actuators in biomedical, biomimetic and soft robotics applications. The key fabrication steps, their importance for the actuators&#39; final properties, and some of the main characterization techniques are described in detail.

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, ...

Merging Ion Concentration Polarization between Juxtaposed Ion Exchange Membranes to Block the Propagation of the Polarization Zone

The ion concentration polarization (ICP) phenomenon is one of the most prevailing methods to preconcentrate low-abundance biological samples. The ICP induces a noninvasive region for charged biomolecules (i.e., the ion depletion zone), and targets can be preconcentrated on this region boundary. Despite the high preconcentration performances with ICP, it is difficult to find the operating conditions of non-propagating ion depletion zones. To overcome this narrow operating window, we recently developed a new platform for spatiotemporally fixed preconcentration. Unlike preceding methods that only use ion depletion, this platform also uses the opposite polarity of the ICP (i.e., ion enrichment) to stop the propagation of the ion depletion zone. By confronting the enrichment zone with the depletion zone, the two zones merge together and stop. In this paper, we describe a detailed experimental protocol to build this spatiotemporally defined ICP platform and characterize the preconcentration dynamics of the new platform by comparing them with those of the conventional device. Qualitative ion concentration profiles and current-time responses successfully capture the different dynamics between the merged ICP and the stand-alone ICP. In contrast to the conventional one that can fix the preconcentration location at only ~5 V, the new platform can produce a target-condensed plug at a specific location in the broad ranges of operating conditions: voltage (0.5-100 V), ionic strength (1-100 mM), and pH (3.7-10.3).

The protocol for a novel ion concentration polarization (ICP) platform that can stop the propagation of the ICP zone, regardless of the operating conditions is described. This unique ability of the platform lies in the use of merging ion depletion and enrichment, which are two polarities of the ICP phenomenon.

The ion concentration polarization (ICP) phenomenon is one of the most prevailing methods to preconcentrate low-abundance biological samples. The ICP ...

Proton Exchange Membrane Fuel Cells

Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University
The United States consumes a large amount of energy &#8211; the current rate is around 97.5 quadrillion BTUs annually. The vast majority (90%) of this energy comes from non-renewable fuel sources. This energy is used for electricity (39%), transportation (28%), industry (22%), and residential/commercial use (11%). As the world has a limited supply of these non-renewable sources, the United States (among others) is expanding the use of renewable energy sources to meet future energy needs. One of these sources is hydrogen.
Hydrogen is considered a potential renewable fuel source, because it meets many important criteria: it&#8217;s available domestically, it has few harmful pollutants, it&#8217;s energy efficient, and it&#8217;s easy to harness. While hydrogen is the most abundant element in the universe, it is only found in compound form on Earth. For example, it is combined with oxygen in water as H2O. To be useful as a fuel, it needs to be in the form of H2 gas. Therefore, if hydrogen is to be used as a fuel for cars or other electronics, H2 needs to be made first. Thusly, hydrogen is often called an &#8220;energy carrier&#8221; rather than a &#8220;fuel.&#8221;
Currently, the most popular way to make H2 gas is from fossil fuels, through steam reforming of hydrocarbons or coal gasification. This does not reduce dependence on fossil fuels and is energy intensive. A less-used method is by electrolysis of water. This also requires an energy source, but it can be a renewable source, like wind or solar power. In electrolysis, water (H2O) is split into its component parts, hydrogen gas (H2) and oxygen gas (O2), through an electrochemical reaction. The hydrogen gas made through the process of electrolysis can then be used in a Proton Exchange Membrane (PEM) fuel cell, generating an electric current. This electric current can be used to power motors, lights, and other electrical devices.

Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University
The United States consumes a large amount of energy &#8211; the current ...

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Dialysis

Dialysis is a diffusion-based purification process that separates analyte molecules from a complex matrix. This is accomplished by allowing molecules in the solution to pass through a semipermeable membrane into a liquid on the other side. The membrane is usually made of cellulose acetate or cellulose nitrate, and the second liquid must be miscible with the solution. Ions (e.g., chloride or sodium) or organic molecules (e.g., glucose) can pass through the membrane pores, which generally have diameters ranging from 1 to 5 nm. Larger molecules (e.g., proteins, hormones, enzymes) with diameters significantly greater than the pore diameter are retained in the original solution.
Dialysis is an inexpensive technique that is commonly used to purify proteins, hormones, and enzymes. During the procedure, a sample is placed in a dialysis bag, and both ends are sealed. The sealed bag is then immersed into a beaker filled with a solution called the dialysate, which has a composition different from the original solution. As dialysis proceeds, smaller molecules freely diffuse across the membrane, migrating from areas of high concentration to those of low concentration, until equilibrium is reached. Larger molecules that cannot enter the pores are left behind. Dialysis is complete once no net movement of the smaller molecules can occur.
Dialysis is a slow process, so it is often performed overnight or over several days. The driving force for diffusion is provided by the concentration gradient created by differences in analyte concentration across the dialysis membrane. The rate of molecular diffusion can be increased by raising the temperature and decreasing the thickness of the membrane. Because dialysis relies on equilibrium, it often needs to be repeated a few times to decrease the concentration of unwanted small molecules to an acceptable level.

Dialysis separates desired analyte ions or molecules from unwanted molecules, or &#39;interferents&#39;, by allowing the sample solution to diffuse ...