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 (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'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.
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'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's hybrid application to solve the energy and clean water problem.
Several methods have been adopted to enhance the RED cell's performance and the membrane'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 ()9. The high ionic conductivity of AEM and CEM without deteriorating the membrane'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'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.
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. Please click here to view a larger version of this figure.
1. Experimental requirement
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. Please click here to view a larger version of this figure.
Figure 3: 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. Please click here to view a larger version of this figure.
2. Ion-exchange membrane preparation
NOTE: The amount of precursor material was optimized for obtaining a membrane with 18 cm diameter and ~50 µm thickness.
Specification | Unit | CEM | AEM |
Swelling degree | % | 5±1 | 1±0.5 |
Charge density or Ion exchange capacity | meq/g | 1.8 | ~1.6 |
Mechanical properties (Tensile strength) |
MPa | >40 | 40-50 |
Elongation to Break | % | ~42 | 30-50 |
Young Modulus (MPa) | 1500±100 | 1000-1500 | |
Conductivity at room temperature | S/cm | ~0.03 | ~0.025 |
Permselectivity | % | 98-99 | 94-96 |
Thickness | μm | 50±2 | 50±3 |
Solvent | – | Dimethylacetamide (DMAc) | N-methyl-2-pyrrolidone (NMP) |
Table 1: Membranes properties. Summary of both cation-exchange and anion-exchange membrane properties.
3. Fabrication of reverse electrodialysis
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's alignment with both an anion exchange membrane (AEM) and cation exchange membrane (CEM). Please click here to view a larger version of this figure.
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. Please click here to view a larger version of this figure.
4. Measurement of reverse electrodialysis
Net power output
RED cell generally generates electrical energy from the salinity gradient of the salt solution, i.e., ions' 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's flow rate, pumping pressure, and applied voltage, to obtain efficient power output. To estimate the RED stack'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'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.
(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 (ΔP), flow rate (Q), and pump efficiency (ηpump)17,18.
(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 and ΔPH and ΔPL 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.
(4)
Here, dh (m) is the channel's hydraulic diameter, whereas the hydraulic diameter for an infinite wide channel is 2h. Other parameters is the viscosity of water (Pa·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's value in the case of the profiled membrane, and "h" is the intermembrane distance (m), is given by equation 520.
(5)
For an infinite wide channel, the value calculated from equation 6 is usually much lower than the finite wide channel'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 / Vsp) of spacer mesh in the formula, ε is the porosity, one can estimate the thickness of spacer-filled channels from equation 621,22.
(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.
(7)
Where, A is the active membrane area in m2 and Q feed solution flow rate in m3 s-1. Here, μ is the viscosity of water measured in Pa·s, L 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's output performance was investigated using one cell pair at a fixed flow rate of 100 mL min-1. The feed solution'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' 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's internal resistance, as shown in Figure 6.
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. Please click here to view a larger version of this figure.
The RED's working principle is mainly dominated by the membrane'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'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'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' 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'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's mechanical strength. Thus, it is essential to optimize swelling density and the membranes' 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'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.
(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 (Ω m2), RC is the cation exchange membrane resistance (Ω 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 (Ω). 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.
The authors have nothing to disclose.
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.
AEM based membrane | Fumion | P1810-194 | Ionomer |
CEM based membrane | Fumion | E550 | Ionomer |
Digital torque wrench | Torqueworld | WP2-030-09000251 | wrench |
Labview software | Natiaonal Instrument | – | Software |
Laptop | LG | – | PC |
Magnetic stirrer | Lab Companion | – | MS-17BB |
N, N-Dimethylacetamide | Sigma aldrich | 271012 | Chemical |
N-Methyl-2- pyrrolidone | Daejung | 872-50-4 | Chemical |
Peristaltic pump | EMS tech Inc | – | EMP 2000W |
Potassium hexacyanoferrate(II) trihydrate | Sigma aldrich | P3289 | Chemical |
Potassium hexacyanoferrate(III) | Sigma aldrich | 244023 | Chemical |
Pressure Gauge | Swagelok | – | Guage |
Reverse electrodialysis setup | fabricated in lab | – | Device |
RO system pure water | KOTITI | – | Water |
Rotary evaporator | Hitachi | YEFO-KTPM | Induction motor |
Sodium Chloride | Sigma aldrich | S9888 | Chemical |
Sodium Hydroxide | Merk | 1310-73-2 | Chemical |
Source meter | Keithley | – | 2410 |
Spacer | Nitex, SEFAR | 06-250/34 | Spacer |
Sulfuric acid | Daejung | 7664-93-9 | Chemical |
Tube | Masterflex tube | 96410-25 | Rubber tube |