Effect of Mg2+ ions on energy generation by Reverse Electrodialysis

Energy harvesting from acid mine drainage using a highly proton/ion-selective thin polyamide film

A huge chemical potential difference exists between the acid mine drainage (AMD) and the alkaline neutralization solution, which is wasted in the traditional AMD neutralization process. This study reports, for the first time, the harvest of this chemical potential energy through a controlled neutralization of AMD using H+-conductive films. Polyamide films with controllable thickness achieved much higher H+ conductance than a commercially available cation exchange membrane (CEM). Meanwhile, the optimal polyamide film had an excellent H+/Ca2+ selectivity of 63.7, over two orders of magnitude higher than that of the CEM (0.3). The combined advantages of fast proton transport and high proton/ion selectivity greatly enhanced the power generation of the AMD battery. The power density was 3.1 W m-2, which is over one order of magnitude higher than that of the commercial CEM (0.2 W m-2). Our study provides a new sustainable solution to address the environmental issues of AMD while simultaneously enabling clean energy production.

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

Resistance of Ion Exchange Membranes in Aqueous Mixtures of Monovalent and Divalent Ions and the Effect on Reverse Electrodialysis

Salinity gradient energy has gained attention in recent years as a renewable energy source, especially employing reverse electrodialysis technology (RED), which is based on the role of ion exchange membranes. In this context, many efforts have been developed by researchers from all over the world to advance the knowledge of this green source of energy. However, the influence of divalent ions on the performance of the technology has not been deeply studied. Basically, divalent ions are responsible for an increased membrane resistance and, therefore, for a decrease in voltage. This work focuses on the estimation of the resistance of the RED membrane working with water flows containing divalent ions, both theoretically by combining the one-thread model with the Donnan exclusion theory for the gel phase, as well as the experimental evaluation with Fumatech membranes FAS-50, FKS-50, FAS-PET-75, and FKS-PET-75. Furthermore, simulated results have been compared to data recently reported with different membranes. Besides, the influence of membrane resistance on the overall performance of reverse electrodialysis technology is evaluated to understand the impact of divalent ions in energy generation. Results reflect a minor effect of sulfate on the gross power in comparison to the effect of calcium and magnesium ions. Thus, this work takes a step forward in the knowledge of reverse electrodialysis technology and the extraction of salinity gradient energy by advancing the influence of divalent ions on energy recovery.

Concepts and Misconceptions Concerning the Influence of Divalent Ions on the Performance of Reverse Electrodialysis Using Natural Waters

Divalent ions have a negative effect on the obtained power and efficiency of the reverse electrodialysis (RED) process when using natural waters. These effects can largely be attributed to the interaction between the various ions and the membranes, resulting in a decreased membrane voltage, an increased membrane resistance, and uphill transport of divalent ions. The aim of this study was to investigate the causes of these differences and, if possible, to find underlying causes. The approach mainly followed that in literature articles that specifically focused on the effect of divalent ions on RED. It transpired that seven publications were useful because the methodology was well described and sufficient data was published. I found two widely shared misconceptions. The first concerns the role of the stack voltage in uphill transport of divalent ions; itis often thought that the open circuit voltage (OCV) must be taken into account, but it is plausible that the voltage under working conditions is the critical factor. The second debatable point concerns the methodology used to make a series of solutions to study the effect of divalent ions. Typically, solutions with a constant number of moles of salt are used; however, it is better to make a series with a constant ratio of equivalents of those salts. Moreover, it is plausible that the decreased voltage can be explained by the inherently lower Donnan potential of multi-charged ions and that increased resistance is caused by the fact that divalent ions-with a lower mobility there than the monovalent ions-occupy relatively much of the available space in the gel phase of the membrane. While both resistance and voltage play a decisive role in RED and probably also in other membrane processes like electrodialysis (ED), it is remarkable that there are so few publications that focus on measurements on individual membranes. The implications of these results is that research on the effect of divalent ions in RED, ED and similar processes needs to be more structured in the future. Relatively simple procedures can be developed for the determination of membrane resistance in solutions of mixtures of mono- and divalent salts. The same applies to determining the membrane potential. The challenge is to arrive at a standard method for equipment, methodology, and the composition of the test solutions.

Effect of Co-Existing Ions on Salinity Gradient Power Generation by Reverse Electrodialysis Using Different Ion Exchange Membrane Pairs

This study investigates the influence of co-existing ions on the salinity gradient power generation performance of the reverse electrodialysis (RED) using three different commercial ion exchange membrane pairs. The feed solutions, including the mixture of two different salts, were prepared with 90 wt.% of NaCl and 10 wt.% of LiCl, KCl, CaCl₂, MgCl₂ or Na₂SO₄ by keeping the salt ratio between high concentrate solution and low concentrate solution constant as 1:30 (g/g) at various flow velocities (50, 125 and 200 mL/min). It was observed that the divalent ions exhibited a negative impact on the performance of the RED system due to their high valence and low ionic mobility depending on their high hydrated radius and low diffusion coefficients compared to those of the monovalent ions. On the other hand, the effect of the monovalent ions differed according to the properties of ion exchange membranes used in the RED stack. When the power generation performances of ion exchange membrane pairs employed in the RED stack were compared, it was considered that Neosepta AMX and CMX membranes provided the highest power density due to their low membrane thicknesses, low electrical resistances, and relatively high ion exchange capacities compared to other two commercial ion exchange membrane pairs.

Power Generation Performance of Reverse Electrodialysis (RED) Using Various Ion Exchange Membranes and Power Output Prediction for a Large RED Stack

Reverse electrodialysis (RED) power generation using seawater (SW) and river water is expected to be a promising environmentally friendly power generation system. Experiments with large RED stacks are needed for the practical application of RED power generation, but only a few experimental results exist because of the need for large facilities and a large area of ion-exchange membranes (IEMs). In this study, to predict the power output of a large RED stack, the power generation performances of a lab-scale RED stack (40 membrane pairs and 7040 cm² total effective membrane area) with several IEMs were evaluated. The results were converted to the power output of a pilot-scale RED stack (299 membrane pairs and 179.4 m² total effective membrane area) via the reference IEMs. The use of low-area-resistance IEMs resulted in lower internal resistance and higher power density. The power density was 2.3 times higher than that of the reference IEMs when natural SW was used. The net power output was expected to be approximately 230 W with a pilot-scale RED stack using low-area-resistance IEMs and natural SW. This value is one of the indicators of the output of a large RED stack and is a target to be exceeded with further improvements in the RED system.

Evaluation of ion transport properties characterizing concentration polarization in membrane-solution system under different factors

The ion transport properties across the membrane under conditions of different membrane types, solution concentrations, flow rates and temperatures were investigated in a four-compartment reactor. By combining linear sweep voltammetry and chronopotentiometry, the limiting current density (I lim), the ion transition time (τ) and the difference between ion transport numbers in the membrane and the solution (t m–t s) were determined. And the diffusion boundary layer thickness (δ) of the membrane-solution system at steady-state conditions was measured by electrochemical impedance spectroscopy. The results show that the use of Selemion membrane and the increase of solution concentration, flow rate and temperature, I lim and τ increase, t m–t s and δ decrease. This means the concentration polarization of the system is weaker and complete concentration polarization is more difficult to occur. At the same time, I lim, τ and t m–t s are strongly related to solution concentration and temperature, while the diffusion boundary layer thickness is mainly affected by solution concentration and flow rate. Additionally, I lim of anion exchange membranes is larger than that of cation exchange membranes due to the difference in migration rates of anion and cation.

Response of salinity gradient power generation to inflow mode and temperature difference by reverse electrodialysis

Sustainable utilization has been becoming the core idea of concentrated seawater disposal, which makes the harvest of salinity gradient power based on reverse electrodialysis (RED) become one of the important ways. As the important factors affecting RED performance, different flow orientations along the membrane and solution temperature have been studied in the previous researches. However, there are still some details that need to be clarified. In this study, the inflow mode was further detailed investigated. The results showed that after eliminating the interference of bubbles in the counter-current, the co-current was still better than the counter-current; when the solution of HCC (high concentration compartment) and LCC (low concentration compartment) was circulated for 3 h, the concentration of concentrated seawater discharge liquid was reduced by 6.93%, which was conducive to reducing the negative impact on the marine ecological environment. Meanwhile, the response of salinity gradient power generation to temperature difference was that high temperature had a positive effect on power density, and the order was both the HCC and LCC (0.44 W m^-2) > LCC (0.42 W m^-2) > HCC (0.39 W m^-2). Although the RED performance was more sensitive to the temperature rise of LCC, the positive temperature difference between HCC and LCC is a more practical advantage because the temperature of concentrated seawater in HCC is usually high. These new observations could provide supports for the industrial development of RED in generating electricity economically and reducing the negative environmental impact of concentrated seawater.

Heat to Hydrogen by RED-Reviewing Membranes and Salts for the RED Heat Engine Concept

The Reverse electrodialysis heat engine (REDHE) combines a reverse electrodialysis stack for power generation with a thermal regeneration unit to restore the concentration difference of the salt solutions. Current approaches for converting low-temperature waste heat to electricity with REDHE have not yielded conversion efficiencies and profits that would allow for the industrialization of the technology. This review explores the concept of Heat-to-Hydrogen with REDHEs and maps crucial developments toward industrialization. We discuss current advances in membrane development that are vital for the breakthrough of the RED Heat Engine. In addition, the choice of salt is a crucial factor that has not received enough attention in the field. Based on ion properties relevant for both the transport through IEMs and the feasibility for regeneration, we pinpoint the most promising salts for use in REDHE, which we find to be KNO3, LiNO3, LiBr and LiCl. To further validate these results and compare the system performance with different salts, there is a demand for a comprehensive thermodynamic model of the REDHE that considers all its units. Guided by such a model, experimental studies can be designed to utilize the most favorable process conditions (e.g., salt solutions).

Renewable Power Generation by Reverse Electrodialysis Using an Ion Exchange Membrane

Reverse electrodialysis (RED) is a promising technology to extract sustainable salinity gradient energy. However, the RED technology has not reached its full potential due to membrane efficiency and fouling and the complex interplay between ionic flows and fluidic configurations. We investigate renewable power generation by harnessing salinity gradient energy during reverse electrodialysis using a lab-scaled fluidic cell, consisting of two reservoirs separated by a nanoporous ion exchange membrane, under various flow rates (qf) and salt-concentration difference (Δc). The current-voltage (I-V) characteristics of the single RED unit reveals a linear dependence, similar to an electrochemical cell. The experimental results show that the change of inflow velocity has an insignificant impact on the I-V data for a wide range of flow rates explored (0.01–1 mL/min), corresponding to a low-Peclet number regime. Both the maximum RED power density (Pc,m) and open-circuit voltage (ϕ0) increase with increasing Δc. On the one hand, the RED cell’s internal resistance (Rc) empirically reveals a power-law dependence of Rc∝Δc−α. On the other hand, the open-circuit voltage shows a logarithmic relationship of ϕ0=BlnΔc+β. These experimental results are consistent with those by a nonlinear numerical simulation considering a single charged nanochannel, suggesting that parallelization of charged nano-capillaries might be a good upscaling model for a nanoporous membrane for RED applications.

Principles of reverse electrodialysis and development of integrated-based system for power generation and water treatment: a review

Reverse electrodialysis (RED) is among the evolving membrane-based processes available for energy harvesting by mixing water with different salinities. The chemical potential difference causes the movement of cations and anions in opposite directions that can then be transformed into the electrical current at the electrodes by redox reactions. Although several works have shown the possibilities of achieving high power densities through the RED system, the transformation to the industrial-scale stacks remains a challenge particularly in understanding the correlation between ion-exchange membranes (IEMs) and the operating conditions. This work provides an overview of the RED system including its development and modifications of IEM utilized in the RED system. The effects of modified membranes particularly on the psychochemical properties of the membranes and the effects of numerous operating variables are discussed. The prospects of combining the RED system with other technologies such as reverse osmosis, electrodialysis, membrane distillation, heat engine, microbial fuel cell), and flow battery have been summarized based on open-loop and closed-loop configurations. This review attempts to explain the development and prospect of RED technology for salinity gradient power production and further elucidate the integrated RED system as a promising way to harvest energy while reducing the impact of liquid waste disposal on the environment.

Facile Designed Manganese Oxide/Biochar for Efficient Salinity Gradient Energy Recovery in Concentration Flow Cells and Influences of Mono/Multivalent Ions

Development of effective, environmentally friendly, facile large-scale processing, and low-cost materials is critical for renewable energy production. Here, MnO_x/biochar composites were synthesized by a simple pyrolysis method and showed high performance for salinity gradient (SG) energy harvest in concentration flow cells (CFCs). The peak power density of CFCs with MnO_x/biochar electrodes was up to 5.67 W m^-2 (ave. = 0.91 W m^-2) and stabilized for 500 cycles when using 1 and 30 g L^-1 NaCl, which was attributed to their high specific capacitances and low electrode resistances. This power output was higher than all other reported MnO₂ electrodes for SG energy harvest due to the synergistic effects between MnO_x and biochar. When using a mixture with a molar fraction of 90% NaCl and 10% KCl (or Na₂SO₄, MgCl₂, MgSO₄, and CaCl₂) in both feed solutions, the peak power density decreased by 2.3-40.1% compared to 100% NaCl solution with Ca²⁺ and Mg²⁺ showing the most pronounced negative effects. Our results demonstrated that the facile designed MnO_x/biochar composite can be used for efficient SG energy recovery in CFCs with good stability, low cost, and less environmental impacts. When using natural waters as the feed solutions, pretreatment would be needed.

Power Generation Performance of a Pilot-Scale Reverse Electrodialysis Using Monovalent Selective Ion-Exchange Membranes

Reverse electrodialysis (RED) is a promising process for harvesting energy from the salinity gradient between two solutions without environmental impacts. Seawater (SW) and river water (RW) are considered the main RED feed solutions because of their good availability. In Okinawa Island (Japan), SW desalination via the reverse osmosis (RO) can be integrated with the RED process due to the production of a large amount of RO brine (concentrated SW, containing ~1 mol/dm³ of NaCl), which is usually discharged directly into the sea. In this study, a pilot-scale RED stack, with 299 cell pairs and 179.4 m² of effective membrane area, was installed in the SW desalination plant. For the first time, asymmetric monovalent selective membranes with monovalent selective layer just at the side of the membranes were used as the ion exchange membranes (IEMs) inside the RED stack. Natural and model RO brines, as well as SW, were used as the high-concentrate feed solutions. RW, which was in fact surface water in this study and close to the desalination plant, was utilized as the low-concentrate feed solution. The power generation performance investigated by the current-voltage (I-V) test showed the maximum gross power density of 0.96 and 1.46 W/m² respectively, when the natural and model RO brine/RW were used. These are a 50-60% improvement of the maximum gross power of 0.62 and 0.97 W/m² generated from the natural and model SW, respectively. The approximate 50% more power generated from the model feed solutions can be assigned to the suppression of concentration polarization of the RED stack due to the absence of multivalent ions.

Deployable Membrane-Based Energy Technologies: the Ethiopian Prospect

Membrane-based energy technologies are presently gaining huge interest due to the fundamental engineering and potentially broad range of applications, with economic advantages over some of the competing technologies. Herein, we assess the potential deployability of the existing and emerging membrane-based energy technologies (MEnT) in Ethiopia. First, the status of the current energy technologies is provided along with the active energy and environmental policies to shape the necessary research strategies for technology planning and implementation. Ethiopia is a landlocked country, which limits the effective extraction of energy, for instance, from seawater using alternative, clean technologies such as reverse electrodialysis and pressure retarded osmosis. However, there exists an excess off-grid solar power (up to 5 MW) and wind which can be used to drive water electrolyzers for hydrogen production. Hydrogen is a versatile energy carrier that, for instance, can be used in fuel cells providing zero-emission solutions for transport and mobility. Although Ethiopia is not among the largest CO2 emitters, with more than 90% energy supply obtained from waste and biomass, the economic and industrial growth still calls for alternative CO2 capture and use technologies, which are highlighted in this work. We believe that the present work provides (i) the status and potential for the implementation of MEnT in Ethiopia (ii) and basic guidance for researchers exploring new energy pathways toward sustainable development in developing countries.

Energy Harvesting from Brines by Reverse Electrodialysis Using Nafion Membranes

Ion exchange membranes (IEMs) have consolidated applications in energy conversion and storage systems, like fuel cells and battery separators. Moreover, in the perspective to address the global need for non-carbon-based and renewable energies, salinity-gradient power (SGP) harvesting by reverse electrodialysis (RED) is attracting significant interest in recent years. In particular, brine solutions produced in desalination plants can be used as concentrated streams in a SGP-RED stack, providing a smart solution to the problem of brine disposal. Although Nafion is probably the most prominent commercial cation exchange membrane for electrochemical applications, no study has investigated yet its potential in RED. In this work, Nafion 117 and Nafion 115 membranes were tested for NaCl and NaCl + MgCl2 solutions, in order to measure the gross power density extracted under high salinity gradient and to evaluate the effect of Mg2+ (the most abundant divalent cation in natural feeds) on the efficiency in energy conversion. Moreover, performance of commercial CMX (Neosepta) and Fuji-CEM 80050 (Fujifilm) cation exchange membranes, already widely applied for RED applications, were used as a benchmark for Nafion membranes. In addition, complementary characterization (i.e., electrochemical impedance and membrane potential test) was carried out on the membranes with the aim to evaluate the predominance of electrochemical properties in different aqueous solutions. In all tests, Nafion 117 exhibited superior performance when 0.5/4.0 M NaCl fed through 500 µm-thick compartments at a linear velocity 1.5 cm·s-1. However, the gross power density of 1.38 W·m-2 detected in the case of pure NaCl solutions decreased to 1.08 W·m-2 in the presence of magnesium chloride. In particular, the presence of magnesium resulted in a drastic effect on the electrochemical properties of Fuji-CEM-80050, while the impact on other membranes investigated was less severe.

Design of Monovalent Ion Selective Membranes for Reducing the Impacts of Multivalent Ions in Reverse Electrodialysis

Reverse electrodialysis (RED) represents one of the most promising membrane-based technologies for clean and renewable energy production from mixing water solutions. However, the presence of multivalent ions in natural water drastically reduces system performance, in particular, the open-circuit voltage (OCV) and the output power. This effect is largely described by the “uphill transport” phenomenon, in which multivalent ions are transported against the concentration gradient. In this work, recent advances in the investigation of the impact of multivalent ions on power generation by RED are systematically reviewed along with possible strategies to overcome this challenge. In particular, the use of monovalent ion-selective membranes represents a promising alternative to reduce the negative impact of multivalent ions given the availability of low-cost materials and an easy route of membrane synthesis. A thorough assessment of the materials and methodologies used to prepare monovalent selective ion exchange membranes (both cation and anion exchange membranes) for applications in (reverse) electrodialysis is performed. Moreover, transport mechanisms under conditions of extreme salinity gradient are analyzed and compared for a better understanding of the design criteria. The ultimate goal of the present work is to propose a prospective research direction on the development of new membrane materials for effective implementation of RED under natural feed conditions.

Reverse Electrodialysis Chemical Cell for Energy Harvesting from Controlled Acid-Base Neutralization

We report a novel reverse electrodialysis (RED) chemical cell that integrates RED with acid/base neutralization. This RED neutralization process (REDn) approximately doubled the power density compared to a conventional RED stack (REDc), thanks to the additional salinity gradients established by H⁺ and OH^- ions as a result of the neutralization reaction. Detailed analysis shows that the power performance, i.e., the open circuit voltage and power density, of the REDn cell was greatly limited by concentration polarization and uphill transport of ions. Addressing these issues could potentially lead to an order of magnitude improvement in power density as predicted by the Nernst equation. The current study provides a simple strategy for effectively extracting energy from the neutralization of waste acid and base solutions. Future studies shall further explore the treatment of acid mine drainage and landfill leachate with the RED chemical cell as well as its extension to a wider range of reactions.

Assessing the behavior of the feed-water constituents of a pilot-scale 1000-cell-pair reverse electrodialysis with seawater and municipal wastewater effluent

Reverse electrodialysis (RED) has vast potential as a clean, nonpolluting, and sustainable renewable energy source; however, pilot-scale RED studies employing real waters remain rare. This study reports the largest RED (1000 cell pairs, 250 m²) with municipal wastewater effluent (1.3-5.7 mS/cm) and seawater (52.9-53.8 mS/cm) as feed solutions. The RED stack was operated at a velocity of 1.5 cm/s and the pilot plant produced 95.8 W of power (0.38 W/m²_{total membrane} or 0.76 W/m²_{cell pair}). During operation of the RED, the inlet design of the stack, comprising thin spacers, and the water dissociation reaction at the cathode were revealed as vulnerabilities of the stack. Specifically, pressure drops at the fluid inlet parts had the most detrimental effects on power output due to clogged spacers around the inlet parts. In addition, precipitates resulting in inorganic fouling were inevitable during the water dissociation reaction due to significant potential generated by the stack in the cathode chamber. Na⁺ and Cl^- accounted for the majority of ions transferred from seawater to wastewater effluent through ion exchange membranes (IEMs). Moreover, some divalent cations in seawater, Mg²⁺ and Ca²⁺, were also transferred to the wastewater effluent. Some organics with relatively low molecular weights in the wastewater effluent passed through the IEMs, and their hydrophobic properties elevated the specific UV absorbance (SUVA) level in the seawater.

Long-run operation of a reverse electrodialysis system fed with wastewaters

The performance of a Reverse ElectroDialysis (RED) system fed by unconventional wastewater solutions for long operational periods is analysed for the first time. The experimental campaign was divided in a series of five independent long-runs which combined real wastewater solutions with artificial solutions for at least 10 days. The time evolution of electrical variables, gross power output and net power output, considering also pumping losses, was monitored: power density values obtained during the long-runs are comparable to those found in literature with artificial feed solutions of similar salinity. The increase in pressure drops and the development of membrane fouling were the main detrimental factors of system performance. Pressure drops increase was related to the physical obstruction of the feed channels defined by the spacers, while membrane fouling was related to the adsorption of foulants over the membrane surfaces. In order to manage channels partial clogging and fouling, different kinds of easily implemented in situ backwashings (i.e. neutral, acid, alkaline) were adopted, without the need for an abrupt interruption of the RED unit operation. The application of periodic ElectroDialysis (ED) pulses is also tested as fouling prevention strategy. The results collected suggest that RED can be used to produce electric power by unworthy wastewaters, but additional studies are still needed to characterize better membrane fouling and further improve system performance with these solutions.

Divalent Cation Removal by Donnan Dialysis for Improved Reverse Electrodialysis

Divalent cations in feedwater can cause significant decreases in efficiencies for membrane processes, such as reverse electrodialysis (RED). In RED, power is harvested from the mixing of river and seawater, and the obtainable voltage is reduced and the resistance is increased if divalent cations are present. The power density of the RED process can be improved by removing divalent cations from the fresh water. Here, we study divalent cation removal from fresh water using seawater as draw solution in a Donnan dialysis (DD) process. In this way, a membrane system with neither chemicals nor electrodes but only natural salinity gradients can be used to exchange divalent cations. For DD, the permselectivity of the cation exchange membrane is found to be crucial as it determines the ability to block salt leakage (also referred to as co-ion transport). Operating DD using a membrane stack achieved a 76% reduction in the divalent cation content in natural fresh water with residence times of just a few seconds. DD pretreated fresh water was then used in a RED process, which showed improved gross and net power densities of 9.0 and 6.3%, respectively. This improvement is caused by a lower fresh water resistance (at similar open circuit voltages), due to exchange of divalent for monovalent cations.