Reverse Electrodialysis Stack Research Articles

Improved Power Production in Reverse Electrodialysis Stacks with Ion‐Permselective Woven Net Spacers

In traditional reverse electrodialysis (RED) stacks, the output power is severely lost due to the shadow effect caused by the nonpermselective spacers. To inhibit the spacer shadow effect, a design on ion‐permselective woven net spacer is proposed. By combining the linear sweep voltammetry and electrochemical impedance spectroscopy methods, the effectiveness of permselective woven net spacer is validated together with the shadow effect and concentration polarization measured quantitatively. Additionally, their influence on stack resistance and power production is investigated under various factors. When contrasted to nonpermselective spacers, the use of permselective woven net spacers reduces the spacer shadow effect by 90% whereas also exacerbating the concentration polarization. This results in a higher power density due to a dramatic reduction in resistance. However, compared with permselective spacers, the permselective woven net spacers increase the power density due to its weaker concentration polarization. The factors including the solution concentration, temperature, and spacer thickness have a considerable influence on the power production of stack. Especially, increasing the concentration of concentrated solution alone is most beneficial to improving the output power while the impact of spacer thickness is the weakest.

Green hydrogen production via reverse electrodialysis and assisted reverse electrodialysis electrolyser: Experimental analysis and preliminary economic assessment

In the climate changing context, hydrogen is leading the energy transition path. The present work focuses on the green hydrogen production exploiting salinity gradients via Reverse Electrodialysis (RED), in short-circuit condition (REDSC), and Assisted RED (ARED), studied for the first time in hydrogen production as an improvement of the low current densities generated by RED. An extensive experimental campaign was carried out by feeding a RED stack with different salinity gradients and testing short-circuit-RED and Assisted RED operative conditions. Hydrogen was produced successfully with ∼100 % Faradic Efficiency (FE) and productivity up to 1.7 mol h−1 m−2. Also, the technology was compared with similar technologies and with the most established state-of-the-art electrolysers to identify advantages and disadvantages of the proposed route. Finally, a preliminary economic analysis was carried out and a minimum Levelized Cost of Hydrogen (LCOH) of 3.2 € kg−1H2 was found, thus leaving room for further studies.

Valorization of abandoned mine wastewater for the production of energy in a reverse electrodialysis cell

The study explores the technical feasibility of using abandoned mining site wastewater, specifically acid mine drainage (AMD), as an electrode solution in a reverse electrodialysis (RED) stack for energy production. Through experiments using a RED cell with varying electrode solutions, it was determined that the system achieved a maximum power density of 0.327 W m−2cell pair (5.01 W m−2electrode) when using simple mixtures of Cu2+ and Fe2+ ions, aligning with previous research on the Fe2+/Fe3+ redox pair. Synthetic solutions mimicking AMD composition also yielded promising results. When operating the system with real AMD effluent from an abandoned mine, a slightly lower maximum power density of 0.137 W m−2cell pair was achieved, only 5.5 % lower than with synthetic AMD. An extended-duration test showed a minor increase in pH in the electrode solution, but importantly, no metal deposition occurred on the electrode surface, as the pH consistently remained below 3.5 throughout the test. These findings indicate the potential of AMD as a viable electrode solution, offering a promising avenue for further research in the development of more sustainable RED systems.

Assessment of Data Capture Conditions Effect on Reverse Electrodialysis Process Using a DC Electronic Load

Reverse electrodialysis (RED), an emerging membrane-based technology, harnesses salinity gradient energy for sustainable power generation. Accurate characterization of electrical parameters in RED stacks is crucial to monitoring its performance and exploring possible applications. In this study, a DC electronic load module (DCELM) is implemented in a constant current condition (CC mode) for characterization of lab scale RED process, using a RED prototype in-house designed and manufactured (RU1), at different data capture setups (DCS), on which the total number of steps for data capture (NS) and the number of measurements per step (ρ) are the parameters that were modified to study their effect on obtained electrical parameters in RED. NS of 10, 50, and 100 and ρ of 10 and 20 were used with this purpose. The accuracy of resulting current and voltage steps can be enhanced by increasing NS and ρ values, and according to obtained results, the higher accuracy of resulting output current and voltage steps, with low uncertainty of the average output steps (AOS) inside the operational region of power curve, was obtained using a DCS of NS = 100 and ρ = 20. The developed DCELM is a low-cost alternative to commercial electronic load devices, and the proposed methodology in this study represents an adaptative and optimizable CC mode characterization of RED process. The results obtained in this study suggest that data capture conditions have a direct influence of RED performance, and the accuracy of electrical parameters can be improved by optimizing the DCS parameters, according to the required specifications and the scale of RED prototypes.

Design strategy of cathodic electrocatalysts to effectively suppress inorganic fouling for long-term stability of reverse electrodialysis

In seawater based-electrochemical devices including reverse electrodialysis (RED) and electrolysis devices, inorganic fouling occurs from the cathodic reaction owing to the precipitation of hydroxides and/or carbonates. Inorganic fouling decreases electrocatalytic activity and disrupts the charge transport, thus reducing power density. In this study, we investigated inorganic fouling over various cathodic electrocatalysts prepared for large-scale RED. Notably, nano-Pt/C which has four times higher scaling control than bulk Pt, exhibited a formation of thin and porous foam-like Mg(OH)2, whereas bulk Pt was coated by a thick and dense film-like precipitate. Surface-modified carbon nanostructures showed a stronger anti-fouling behavior than Pt catalysts at high voltages (≥300-cell-paired RED stacks). In particular, acid–air plasma-treated O-rich functionalized C catalysts produced one-dimensional nanostructures, exhibited excellent electrocatalytic activity and allowed rapid charge transport, resulting in a stable RED performance at a power density of 545 ± 1 W m−2 (current density of 35.17 ± 0.02 A/m−2 over 12-h of operation. This study will promote the upscaling of RED systems toward industrial applications that use natural seawater.

Reverse electrodialysis characteristic of the LiBr-ethanol-water ternary solution

Reverse electrodialysis heat engine (REDHE) has a large potential to convert low-grade heat into electrical energy. Working fluids with low distillation temperature and excellent ion migration properties are essential for the performance improvement of the REDHE. Here, we propose an ethanol-assisted LiBr aqueous to simultaneously decrease the distillation temperature and improve the performance of the REDHE. However, the unclear reverse electrodialysis (RED) characteristics of the ternary working fluid greatly block the application of the REDHE system. To this end, the ternary working fluid of LiBr-ethanol-water is respectively applied to a single-stage RED and multi-stage RED (MS-RED) stacks experiments for the first time, which clearly unveil the RED characteristics of the LiBr-ethanol-water ternary solutions under various working conditions. Both the solution compositions (solvent ratio and concentration) and the working parameters (flow velocity and feed temperature) are systematic studied. The concentration has the most excellent effect on the output performance of the RED stack, followed by temperature and flow rate. The single-stage RED could achieve a power density of 1.17 W m−2. Furthermore, the cumulative energy conversion efficiency of the MS-REDs is increased by 6.73 times from 1.48% to 11.45% as the stage number is stacked from 1 to 22 in series.

Effects of multivalent ions on hydrogen production from the salinity gradient between desalination concentrated brine and river by reverse electrodialysis

In order to simultaneously produce hydrogen and reduce the salinity of concentrated wastewater (brine) from the desalination plant, a lab-scale reverse electrodialysis (RED) hydrogen production system driven by salinity gradient energy is designed and its performance is experimentally studied. The energy conversion of the RED system is found affected by the ionic compositions of the desalination concentrated brine (DCB). The effects of ionic types (multivalent cation, anion, and phosphorous salt) and ionic concentration (from 0.01 to 0.05 M) and pH of DCB (from 5 to 11) on the hydrogen production are investigated. The results show that the existence of multivalent ions in brine significantly degrade the capability of output voltage and hydrogen production of the RED system. Compared to the control group, the additions of 0.05 mol·L−1 NaBr and 0.05 mol·L−1 FeCl3 into the simulated brines lead to the output voltage drops of 29 % and 60 %, respectively, when the current is maintained at 0.2A. Additionally, the experiment yields an MHP of 128 mL·h−1 at the current of 0.3A. Generally, the performances of the RED stack can be improved as increasing the pH of an acidic DCB, but declined as the increasing the pH of an alkaline DCB.

Sustainable power generation from salinity gradients by reverse electrodialysis: Influence of divalent ions

The performance of Reverse Electrodialysis (RED) stacks can be significantly degraded by the presence of divalent ions. To gain a deeper understanding of the influence of divalent ions, a RED model based on the Nernst-Planck framework was developed in this study. Using this model, a simulation study was performed when the feed solutions contain a mixture of NaCl and MgCl2. The impact of the divalent ion Mg2+ on RED performance was examined from the perspective of ion transport. The results indicate that divalent counterions decrease membrane permselectivity and membrane potential, while increasing membrane electrical resistance by affecting ion concentrations within the membrane. The uphill transport of divalent ions occurs only when the molar fraction of divalent ions in the feed solutions is relatively low. Specifically, when the feed solution contains only MgCl2 compared to only NaCl, the open-circuit voltage decreases by 36.2%, and the maximum gross power density decreases by 44.8%.

Techno-economic analysis of conversing the low-grade heat to hydrogen by using reverse electrodialysis – Air gap diffusion distillation coupled method for iron and steel industry

This study devotes to the reverse electrodialysis (RED) – air gap diffusion distillation (AGDD) coupled system of converting the low-grade waste heat recovered from the steel production to hydrogen for steelmaking. This approach not only recovers waste heat but also reduces carbon emissions. To assess the feasibility of the coupled system, a techno-economic model has been developed. The influences on the levelized cost of hydrogen (LCOH) of the ion exchange membranes (cost and lifetime), the operating condition of RED stack (current density and feed solution velocity), the AGDD unit (feed solution flowrate), and the numbers of RED stacks in a multistage series system are investigated. The high price of ion exchange membrane is currently the main factor causing non-commercialization, which is almost 6.63–19.9 times larger than the LCOH of hydrogen production from renewable electricity. The membrane lifetime is the critical factor in increasing the maximum permitted membrane cost. Assuming the membranes cost is 1 RMB/m2, the optimal current density, feed solution velocity (in RED stack), and the solution volume flowrate (in AGDD unit) of are 50 A/m2, 0.00075 m/s, and 0.7 m3/h, respectively. The series systems take a considerable advantage in reducing the required AGDD units compared with one stage.

The effect of flow modes on the capture of the energy between concentrated brine and seawater by reverse electrodialysis

The discharge of concentrated brine from the desalination unit waste amounts of salinity gradient energy, which could be harvested and converted to electric energy by reverse electrodialysis (RED). However, the poor power and restricted conversion efficiency obstacle its development. A suitable flow mode of feed solutions in a RED stack can improve energy conversion efficiency. Here, the influences of co– and counter-current flow modes on the performances of a single-stage RED stack and multi-stage stacks harvesting the energy are studied in the work. Based on this, the models of flow mode affecting the RED performances are built and experimentally verified. Results show that the counter-current in a single-stage stack is better than the co-current, and the advantage is more obvious with the concentration increase of the concentrated solution, the concentration decrease of the diluted solution, and the decrease of flow rate. Besides feed parameters, the stack sizes also influence the performances of the single-stage stack in both modes. The counter-current in the multi-stage stacks is also more excellent than the co-current. However, the difference decreases with the increase of stage number. This work provides some support for utilizing the salinity gradient energy between concentrated brine and seawater.

Optimization Study on Salinity Gradient Energy Capture from Brine and Dilute Brine

The power conversion of salinity gradient energy (SGE) between concentrated brine from seawater desalination and seawater by reverse electrodialysis (RED) benefits energy conservation and also dilutes the discharge concentration to relieve the damage to coastal ecosystems. However, two key performance indexes of the maximum net power density and energy conversion efficiency for a RED stack harvesting the energy usually cannot reach the optimal simultaneously. Here, an optimization study on the two indexes was implemented to improve the performance of RED in harvesting the energy. A RED model for capturing the SGE between concentrated brine and seawater was constructed, and the correlation coefficients in the model were experimentally determined. Based on the model, the effects of a single variable (concentration, flow rate, temperature, thickness of the compartment, length of the electrode) on the performance of a RED stack are analyzed. The multi-objective optimization method based on the genetic algorithm was further introduced to obtain the optimal solution set, which could achieve the larger net power density and energy conversion efficiency with coordination. The ranges of optimal feed parameters and stack size were also obtained. The optimal flow velocity of the dilute solution and the concentration of the dilute solution are approximately 7.3 mm/s and 0.4 mol/kg, respectively.

Recovery of transition metal ions with simultaneous power generation by reverse electrodialysis

Renewable energy processes based on salinity gradient differences possess spreading interests as an alternative to energy production methods based on fossil fuels. The reverse electrodialysis (RED) process is used for energy production by mixing seawater and river water. In the proposed solution, the spent battery effluent is applied as a high-concentrated solution for the first time, generating a potential difference in the RED stack. The high-selective cation exchange membrane for cobalt transportation in the RED system was a demanding barrier controlling the separation behaviour. The initial research confirmed the energy production ability from mixing high concentrated (HC) solution of Co2+, Li+ and Ni2+ nitrates and low concentrated (LC) solution containing chloric acid. The maximum extracted energy was estimated at 0.44 W/m2 with an energy efficiency of 45.5%. The presented method is a promising candidate for dealing with battery waste management and converting it into a valuable product such as cobalt salts with the potential for power harvesting.

Effects of electrode rinse solution on performance of hydrogen and electricity cogeneration system by reverse electrodialysis

A harvestable salinity gradient energy exists between the concentrated and diluted electrolyte solutions, which can be converted to the electromotive force of a reverse electrodialysis (RED) stack to drive the power generation and hydrogen production. The performance of a hydrogen and electricity (H&E) cogeneration system by RED method is significantly influenced by the characteristics of the electrode rinse solution (ERS). The study experimentally investigated the effects of ERS concentration, flow rate, electrolyte composition, and active additive on the system's output voltage, power density, and hydrogen and oxygen production. Besides, a surface-active substance with catalytic activity was applied for the first time to the H&E cogeneration system, aiming to decrease the overpotentials of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The results showed that the output voltage increased with an increase in ERS flow rate, but hydrogen production declined. Increasing the concentration of the ERS was beneficial to promoting the output voltage and power density, but an extremely high pH of the ERS was not conducive to hydrogen production. The electrical output performance of the H&E cogeneration system using NaOH solution as the ERS was generally better than that of the corresponding KOH solution. The addition of trioctylmethylammonium chloride (Aliquat-336) was detrimental to the electrical output performance but promoted evolution of hydrogen and oxygen. At a current of 0.3 A and an Aliquat-336 concentration of 0.4 mM, the system achieved a hydrogen production rate of 0.71 ± 0.002 mol·m−2·h−1.

A Self-Adaptive-Step-Size Incremental-Resistance-MPPT Technique for Reverse-Electrodialysis System

Salinity gradient power (SGP) is a kind of promising renewable energy source with huge potential, which can be derived through reverse electrodialysis (RED). Generally, RED stacks operate at either continuous mode or recycle mode, both of which possess respective advantages in different application scenes. Since SGP technology is still at an infant stage, there is a few research focusing on maximum-power-point-tracking (MPPT) techniques of RED stacks, which are crucial for the effective energy utilization of SGP. In this article, based on the generalized hybrid RED model, a self-adaptive-step-size incremental-resistance MPPT technique is proposed to extract maximum power from RED stacks. Not only is it simple and easy to implement but also it realizes self-adjustment of the step size to strike a balance between fast dynamic responses and small oscillations at a steady state. Moreover, according to the unique intrinsic features of RED stacks, the systematic design principles of the sampling interval and scaling factor are detailed, which help facilitate a fast and stable system. Various tests with the proposed technique under different parameter settings, as well as tests with other conventional MPPT techniques, are conducted on a RED stack prototype to validate the effectiveness of the proposed MPPT.

Energetic Valorisation of Saltworks Bitterns via Reverse Electrodialysis: A Laboratory Experimental Campaign.

Concentrated bitterns discharged from saltworks have extremely high salinity, often up to 300 g/L, thus their direct disposal not only has a harmful effect on the environment, but also generates a depletion of a potential resource of renewable energy. Here, reverse electrodialysis (RED), an emerging electrochemical membrane process, is proposed to capture and convert the salinity gradient power (SGP) intrinsically conveyed by these bitterns also aiming at the reduction of concentrated salty water disposal. A laboratory-scale RED unit has been adopted to study the SGP potential of such brines, testing ion exchange membranes from different suppliers and under different operating conditions. Membranes supplied by Fujifilm, Fumatech, and Suez were tested, and the results were compared. The unit was fed with synthetic hypersaline solution mimicking the concentration of natural bitterns (5 mol/L of NaCl) on one side, and with variable concentration of NaCl dilute solutions (0.01-0.1 mol/L) on the other. The influence of several operating parameters has also been assessed, including solutions flowrate and temperature. Increasing feed solutions' temperature and velocity has been found to lower the stack resistance, which enhances the output performance of the RED stack. The maximum obtained power density (corrected to account for the effect of electrodic compartments, which can be very relevant in five cell pairs laboratory stacks) reached around 10.5 W/m2cellpair, with FUJIFILM Type 10 membranes, temperature of 40 °C, and a fluid velocity of 3 cm s-1 (as empty channel, considering 270 μm thickness). Notably, the present study results confirm the large potential for SGP generation from hypersaline brines, thus providing useful guidance for the harvesting of SGP in seawater saltworks all around the world.

A generalized disjunctive programming model for the optimal design of reverse electrodialysis process for salinity gradient-based power generation

Reverse electrodialysis (RED) is an emerging electro-membrane technology that generates electricity out of salinity differences between two solutions, a renewable source known as salinity gradient energy. Realizing full-scale RED would require more techno-economic and environmental assessments that consider full process design and operational decision space from the RED stack to the entire system. This work presents an optimization model formulated as a Generalized Disjunctive Programming (GDP) problem that incorporates a finite difference RED stack model from our research group to define the cost-optimal process design. The solution to the GDP problem provides the plant topology and the RED units’ working conditions that maximize the net present value of the RED process for given RED stack parameters and site-specific conditions. Our results show that, compared with simulation-based approaches, mathematical programming techniques are efficient and systematic to assist early-stage research and to extract optimal design and operation guidelines for large-scale RED implementation.

A reverse electrodialysis cell-modified photocatalytic fuel cell for efficient electricity and hydrogen generation from the degradation of refractory organic pollutants.

Wastewater treatment is typically energy-intensive. To achieve carbon neutrality, new wastewater treatment technologies that have high efficiency and low energy consumption must be developed. In this study, a reverse electrodialysis (RED) cell-modified photocatalytic fuel cell (PRC) for efficient electricity and hydrogen generation from the degradation of refractory organic pollutants is developed and evaluated. A hydrogen evolution cathode was developed and optimized by doping 1.53wt. % Ni-N-C on CoP/NF. The bias voltage generated from the RED stack accelerated the separation of photoinduced holes and electrons on the photoanode, which enhances ampicillin (AMP) degradation and hydrogen production. The RED stack and electrode reactions respectively contribute 72.3 % and 27.7 % to the electricity production of PRC. The output current and cumulative hydrogen generation reach 2.2-3.0mA and 500 μmol/L respectively with 81.8 % AMP removal. Increasing high concentration (HC), flow rate of NH4HCO3 solutions and AMP concentration could increase the electricity and hydrogen generation. Acidic environment is helpful to improve the reaction rate of hydrogen evolution. We believe this study would provide a promising option for wastewater remediation.

Salinity gradient energy conversion by custom-made interpolymer ion exchange membranes utilized in reverse electrodialysis system

Reverse electrodialysis (RED) is one of methods to extract salinity gradient energy between two aqueous solutions with different salt concentrations. In this work, custom-made interpolymer ion exchange membranes were employed in the RED stack. The effects of divalent (Mg2+, Ca2+ and SO42-) and monovalent (Li+, K+ and Cl-) ions in the feed solutions prepared from NaCl salt as a function of such process parameters as number of membrane pairs, flow rate, and salinity ratio on power generation by the RED method were studied. It was shown that the maximum power density of 0.561 W/m2 was reached by using three membrane pairs using 1:45 of salt ratio with a feed flow rate of 120 mL/min using only NaCl salt in the feed solutions. The maximum power density was 0.398 W/m2 at 120 mL/min of the flow rate of the feed solutions composed of 90 wt% NaCl and 10 wt% KCl by using a salt ratio of 1:30 while the lowest power density of 0.246 W/m2 was obtained with a feed flow rate of 30 mL/min in the presence of SO42- ions with a similar salt ratio. Consequently, it was seen that while the presence of divalent ions in NaCl solutions had negative impact on power generation by RED system, the addition of monovalent ions having smaller hydrated radius than that of the Na+ ions contributed positively to the power generation.

Toward Redox-Free Reverse Electrodialysis with Carbon-Based Slurry Electrodes.

Clean and renewable salinity gradient energy can be harvested using reverse electrodialysis (RED). The electrode system is an essential part to convert ionic current into electrical current. In this study, a typical 0.10 × 0.10 m2 RED stack with a cross-flow configuration was used to test carbon-based slurry electrodes (CSEs) to replace the usual redox solutions, like hexacyanoferrate, to enhance the RED process' sustainability, stability, and economic value. Six different slurry compositions comprising activated carbon, carbon black, and graphite powder were tested. The CSE characteristics were systematically studied by measuring viscosity, electrode compartment pressure drop, maximum current density, stability, and performance of power density and energy efficiency. Using a single membrane configuration, the CSE ran continuously for 17 days with a stable output. The application of CSEs for RED, with artificial seawater and river water, using mixing activated carbon and carbon black at a total concentration of 20 wt %, resulted in the best performance with a net power density of 0.7 W·m-2. Moreover, higher current densities up to 350 A·m-2 were tested for ED and shown to be feasible until 150 A·m-2. CSEs show promising versatility for different application modes.

Methyl orange degradation and hydrogen production simultaneously by a reverse electrodialysis−electrocoagulation coupling system powered by salinity gradient energy

To produce hydrogen and treat dye wastewater simultaneously, a salinity gradient energy (SGE) driven reverse electrodialysis（RED）− electrocoagulation (EC) coupling system was designed and investigated experimentally. Energy conversion was achieved by means of the ionic electromigration (across the membranes) and redox reaction (on the electrodes). The iron anode electrode was electro-dissolved and releases Fe2+ (to be oxidized to Fe3+), and OH– was produced at cathode electrode, which were combined to be the flocculent. Additionally, hydrogen was produced at the cathode. Experiments were conducted to investigate the effects of electrodes (surface, material, and structure) and concentration gradients (from 10:1 to 250:1) on dye methyl orange (MO) degradation and hydrogen production. It results that the use of iron electrode as anode is significantly more efficient than the aluminum electrode for MO degradation, but inversely for hydrogen production; as for cathodes, the largest MO degradation rate and hydrogen production are achieved by means of the titanium mesh electrode and titanium plate electrode respectively. When the working current is constant at 0.25 A, the MO degradation rate and hydrogen production are both decreased with the increase of the concentration gradient. When the experiments are carried under the short-circuit current condition, the MO degradation rate reaches 98.1 % in 70 min at the concentration gradient of 50:1, and the maximum hydrogen production is 294.15 ml·h−1 at the concentration gradient of 25:1. Besides, the evolutions of the following parameters have been observed, including the current of RED stacks, pH of solution, and dissolution of electrodes.