Salinity Gradient Power Generation Research Articles

Highly Efficient Conversion of Salinity Difference to Electricity in Nanofluidic Channels Boosted by Variable Thickness Polyelectrolyte Coating.

The inherent limits of the current produced by imposing salinity gradients along a nanofluidic channel having "hard" boundary walls heavily constrain the resulting energy harvesting efficacy, acting as major hindrances against the practicability of harnessing high power density from the mixing of water having different salinities. In this work, the infusion of variable-thickness polyelectrolyte layer of a conical shape is projected to augment salinity gradient power generation in nanochannels. Such a progressive thickening of a charged interfacial layer on account of axially declining ion concentration facilitates the shedding of enhanced numbers of mobile ions, bearing a net charge of equal and opposite to the surface-bound ions, into the mainstream current flow. We show that the proposed design can convert energy at a higher efficiency as compared to both solid-state and available polyelectrolyte layer (PEL)-covered nanochannels. The same is true for the maximum power density at moderate and high concentration ratios including natural salt gradient conditions for which more than 50% increase is achievable. The maximum values achieved for efficiency and power density read 50.3% and 6.6 kW/m2, respectively. Our results provide fundamental insights on strategizing variable-thickness polyelectrolyte layer grafting on the nanochannel interfaces, toward realizing high-performance osmotic power generators by altering the local ionic clouds alongside the grafted layers and enhancing the ionic mobility by inducing a driving potential gradient concomitantly. These findings open up a new strategy of efficient conversion of the power of the salinity difference of seawater and river water into electricity in a nanofluidic framework, surpassing the previously established limits of blue energy harvesting technologies.

Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures.

Our research investigates the impact of auxiliary structures on ion transport in electrochemical systems such as batteries and microscale desalination units, whose importance for sustainable development has increased dramatically in recent decades. The electrochemical systems typically feature ion-selective surfaces, such as electrodes and ion exchange membranes, where ion depletion can cause performance issues including metal dendrite formation and flow instability. Recent research has shown that auxiliary structures in these electrochemical systems can enhance ion transfer near ion-selective surfaces, thereby resolving the instability problem and improving the energy conversion efficiency of the system. Our study leverages recent advancements in nanoscale electrokinetics to model these auxiliary structures as pillar arrays near an ion exchange membrane in a microchannel. We examine how these structures enhance ion transports relative to the characteristic length scale of microchannel depth and pillars' proximity to the ion-selective surface. Results show that the effect of the pillars varies significantly with their placement. Specifically, in deeper microchannels, where electrokinetic convection is stronger, the closer the auxiliary structure is to the ion-selective membrane, the better the ion transfer. However, in the thinner microchannel, the proximity of the auxiliary structure to the ion selective membrane has a less significant correlation with the ion transfer. Therefore, this finding highlights the importance of spatial arrangement of the auxiliary structures in improving the performance of electrochemical devices. Conclusively, this study can help to better understand energy conversion systems such as fuel cells, salinity gradient power generation systems, and electrochemical desalination systems, where auxiliary structures can be used in the vicinity of ion-selective surfaces. Especially, our fundamental electrokinetic study provides an effective means for designing the efficient electrochemical platforms utilizing micro/nanofluidics.

PET-hydrogel heterogeneous membranes that eliminate concentration polarization for salinity gradient power generation

Salinity gradient (blue) energy has been identified as a promising power source and many technologies have been proposed for its recovery, one of which is nanopore power generators. The single-pore/channel membranes selectively transport ions to achieve efficient salinity gradient power generation. However, porous membranes fail due to concentration polarization. Here, we constructed a heterogeneous membrane by covering ion-selective hydrogel on a polyethylene terephthalate (PET) cylindrical nanochannel porous membrane for the first time to eliminate concentration polarization. The gel layer enhances the ion selectivity of cylindrical nanochannels, making heterogeneous membrane present the ion current rectification effect. The gel layer also avoids the negative impact of concentration polarization on the power generation of porous membranes. The results show that PET porous membrane can hardly generate electricity, but the power density of PET-hydrogel heterogeneous membrane can reach 1.92 W/m2 under 50 times salinity gradient, which is superior to commercial ion exchange membranes. Our work provides a strategy (ion-selective hydrogel covering) to eliminate concentration polarization of porous membranes, and shows the prospect of cylindrical nanochannels for salinity gradient power generation.

Design of a flow channel with vortex generators for performance improvement of flow-electrode capacitive mixing under low pressure drop conditions

Salinity gradient power generation is a novel renewable energy technology that generates electricity by exploiting the disparity in salt concentration between bodies of water with high and low salt content, using techniques such as capacitive mixing with flow electrodes (F-CapMix). In this research, we conducted a comprehensive numerical analysis to examine the fluid dynamics and particle behavior of saline water and activated carbon within a flow electrode system equipped with vortex generators on the inner walls of the flow channels. The maximum and net power densities of the F-CapMix for a flow channel with vortex generators were evaluated experimentally. On average, the introduction of vortex generators into the flow channel resulted in a 35.6 % increase in the maximum power density of the F-CapMix compared to a plain channel. Furthermore, when a vortex generator was incorporated, the net power density, considering the pumping power, showed an average enhancement of 26.3 % at flow rates of 15 ml/min or less. Therefore, we anticipate that the flow channel with integrated vortex generators can be effectively utilized in the F-CapMix electrode system with a negligible impact on the pressure drop.

Salinity gradient power generation in sinusoidal nanochannels

Salinity gradient power generation based on nanochannels has drawn much attention for its green and sustainable advantages. However, previous studies have tended to focus on regularly shaped nanochannels, while rough and irregular surfaces are naturally formed during the preparation of nanochannels (similar to sinusoidal nanochannels). In this paper, a model of salinity gradient energy conversion in sinusoidal nanochannels is established. The impacts of the geometrical parameter dimensions of the sinusoidal channel (including initial phase, channel length, average radius, amplitude, and angular frequency) on the power generation performance under different salinity gradients are systematically investigated. The results show that the sinusoidal nanochannel has better salinity gradient power generation performance than cylindrical nanochannel. It obtains the best power generation performance when the initial phase is zero. Besides, increasing the nanochannel length facilitates ion selectivity and energy conversion efficiency. At high concentration difference, decreasing the average radius or increasing the amplitude help increase the maximum output power and enhance the energy conversion efficiency. The maximum output power of about 1.27 pW and energy conversion efficiency of about 32.8% were obtained by optimizing the sinusoidal channel amplitudes. It is also found that increasing the angular frequency is favorable to improving energy conversion performance at the high concentration difference. These results provide helpful information for designing and optimizing salinity gradient energy conversion systems.

Reducing ion concentration polarization by optimizing reservoir size in membrane-based salinity gradient power generation systems

Salinity gradient power generation has attracted much attention for its green and sustainable advantages. However, previous studies have tended to overlook the effect of reservoir size in porous membrane systems on power generation (the reservoir size from nanometer to micrometer were applied for similar-sized nanochannels). In this work, we investigate the ion transport behavior and working performance in the salinity gradient power generation with different reservoir sizes. The results show that there is a significant dependence of reservoir length and edge height on the effect of salinity gradient power generation. Smaller reservoir length or larger edge height corresponds to weaker ion concentration polarization, larger effective concentration difference, and better power generation performance. The regulation of the energy conversion process by reservoir size is significant. The influence of reservoir size on salinity gradient power generation diminishes as the pore distance increases. Sufficiently large reservoir length and edge height are necessary if a more stable output power is to be obtained. This work illustrates the impact of reservoir size on power generation performance and its significant regulatory effect, providing useful information for future selection of reservoir size in porous membrane systems.

Sandwich-structured covalent organic framework membranes for selective sodium ion transport

Reverse electrodialysis (RED) is a promising renewable energy technology for salinity gradient power generation, while its key component (i.e., ion exchange membranes) faces the challenge of the lower energy density caused by the uphill transport of divalent ions (e.g., Mg2+). In this study, we carried out a series of molecular dynamics (MD) simulations to investigate the permselective transport of Na+ and Mg2+ through a novel sandwich-stacked hybrid covalent organic framework (COF) membrane. The alternate set of TpHz and TpPa-SO3H constructed reduced effective pore size ranging from 5.6 to 7.5 Å, producing an offset-block effect on the ion transport. Besides, the tunable interlayer spacing of the hybrid COF membrane could manipulate the rehydration/dehydration behavior of the confined Na+ ions. A suitable set of the interlayer spacing could achieve an adorable Na+/Mg2+ permselectivity and Na+ permeance by the synergistic effect of aperture sieving and the electrostatic affinity. This work provides useful insights into the Na+/Mg2+ permselective mechanism and further helps to design COF-based membranes for realizing a high energy efficiency for the application of RED.

Engineered cellulose nanofibers membranes with oppositely charge characteristics for high-performance salinity gradient power generation by reverse electrodialysis

Reverse electrodialysis (RED) using nanofluidic ion-selective membrane may convert the salinity difference between seawater and river water into electricity. However, heterogeneous modification reactions of cellulose commonly leads to the inhomogeneous distribution of surface charges, thereby hampering the improvement of cellulose-based nanofluidic membranes for energy conversion. Herein, RED devices based on cellulose nanofibers (CNF) membranes with opposite charge characteristics were developed for the generation of salinity gradient power. Anion-CNF membrane (A-CNF) with varying negative charge densities was synthesized using 2,2,6,6-Tetramethylpiperidine 1-oxy radical (TEMPO) oxidation modification, whereas cation-CNF membrane (C-CNF) was prepared through etherification. By mixing artificial seawater and river water, the output power density of CNF RED device is up to 2.87 W m−2. The output voltage of 30 RED units connected in series may reach up to 3.11 V, which can be used to directly power tiny electronic devices viz. LED lamp, calculator, etc. The results of this work provide a feasible possibility for widespread application of ion exchange membranes for salinity gradient energy harvesting.

High Ionic Flux Sub‐Micro Channels Membrane Model for Salinity Gradient Power Generation

AbstractSalinity gradient cells are widely used in power generation from gradient concentration solutions of one electrolyte. High ionic flux sub‐micro channels membrane can offer a high current density for salinity gradient power generation with a significant power output. Herein, based on numerical simulation model with Poisson‐Nernst‐Planck‐Navier‐Stokes (PNP‐NS) equations, a high ionic flux sub‐micro channel model with assumption is introduced that measured current is contributed by Cl− ionic flux across the membrane. An ion channel system is built for experiment, in which a glass membrane with regular symmetric sub‐micro cylindrical channels and high effective channel area is applied, ion conductance and salinity gradient energy conversion property are tested. This PNP‐NS equations model simulation fits experiment well, its mechanism is also studied by the model. In addition, by mixing artificial sea water (0.5 m NaCl) and river water (0.01 mm NaCl), the output power density of this 50‐fold salinity gradient system can achieve 4.16 W m−2, and the highest output power density will be up to 7.91 W m−2 under a 0.5 m/0.001 m 500‐fold salinity gradient. This high ionic flux membrane shows a good performance for practical application in salinity gradient power generation.

In-depth understanding of boosting salinity gradient power generation by ionic diode

Ionic diodes constructed with asymmetric channel geometry and/or charge layout have shown outstanding performance in ion transport manipulation and reverse electrodialysis (RED) energy collection, but the working mechanism is still indistinct. Herein, we systematically investigated RED energy conversion of straight nanochannel-based bipolar ionic diode by coupling the Poisson-Nernst-Planck and Navier-Strokes equations. The effects of nanochannel structure, charging polarity, and symmetricity as well as properties of working fluids on the output voltage and output power were investigated. The results show that as high-concentration feeding solution is applied, the bipolar ionic diode-based RED system gives higher output voltage and output power compared to the unipolar channel RED system. Under optimal conditions, the voltage output of the bipolar channel is increased by ∼100% and the power output is increased by ∼260%. This work opens a new route for the design and optimization of high-performance salinity energy harvester as well as for water desalination.

A photothermal and conductive composite hydrogel membrane for solar-driven synchronous desalination and salinity power generation

A calcium alginate hydrogel and nickel foam composite membrane (CHN) can achieve desalination and salinity gradient power generation by solar energy. The CHN membrane can be used as a universal solar conversion platform to obtain blue energy.

Analysis on the Utilization of Renewable Energy for Carbon Neutralization in Sewage Treatment Facilities

Domestic wastewater treatment facilities(WTF) in Korea consume a lot of energy and generate greenhouse gases in the process of treating contaminated sewage. In response to climate change and the government's carbon neutrality, measures for energy reduction in WTF is an important issue along with water quality management, and simultaneously, diversification of renewable energy based on treatment facilities is essential. The energy independence and renewable energy utilization by facility capacity were analyzed based on sewage statistics (2019) and operation data of the National Sewage Information System. The amount of energy consumed by WTF was analyzed to be 83.6 million TOE, and the renewable energy produced was 10.8 million TOE. In consequence of analysis, the energy independence of domestic WTF in Korea was 12.9%. Among renewable energy, the contribution of biogas was the highest at 83.6%, but the current status of energy independence is somewhat insufficient to energy independence plan of WTF, requiring support for alternatives and policies to produce and secure additional renewable energy. In order to increase and diversify the use of renewable energy, the integrated digestion biogas of organic waste resources, the use of cooling and heating energy based on sewage heat, and the development of salinity gradient power generation were reviewed. Based on this, it was intended to contribute to the preparation of measures to achieve carbon neutrality in sewage treatment facilities.

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.

Progress and prospects of two-dimensional materials for membrane-based osmotic power generation

The electrical energy that can be harnessed from the salinity difference across the sea water and river water interface can be one of the sustainable and clean energy resources of the future. This energy can be harnessed via the nanofluidic channels that selectively permeate ions. The selective diffusion of cations and anions can produce electricity through reverse electrodialysis. Two-dimensional (2D) materials are a class of nanomaterials that hold great promise in this field. Several breakthrough works have been previously published which demonstrate the high electrical power densities of 2D membranes. The ion transportation can be either through the nano-sized in-plane pores or interlayer spacings of 2D materials. This review article highlights the progress in 2D materials for salinity gradient power generation. Several types of 2D membranes with various nano-architectures are discussed in this review article. These include atom-thick 2D membranes with nanopores, 2D lamellar membranes, 2D lamellar membranes with nanopores, 2D/one-dimensional (1D), and 2D/2D hybrid membranes. The fabrication techniques, physical characteristics, ion transportation properties, and the osmotic power generation of these 2D membranes are elaborated in this review article. Finally, we overview the future research direction in this area. It is envisioned that the research on 2D materials can make practical salinity gradient power generation one step closer to reality.

MXene Composite Membranes with Enhanced Ion Transport and Regulated Ion Selectivity.

Two-dimensional (2D) material-based membranes are promising candidates for various separation applications. However, the further enhancement of membrane ion conductance is difficult, and the regulation of membrane ion selectivity remains a challenge. Here, we demonstrate the facile fabrication of MXene composite membranes by incorporating spacing agents that contain SO3H groups into the MXene interlayers. The synthesized membrane shows enhanced ion conductance and ion selectivity. Subsequently, the membranes are utilized for salinity gradient power (SGP) generation and lithium-ion (Li+) recovery. The membrane containing poly(sodium 4-styrenesulfonate) (PSS) as the spacing agent shows a much higher power density for SGP generation as compared to the pristine MXene membrane. Using artificial seawater and river water, the power density reaches 1.57 W/m2 with a testing area of 0.24 mm2. Also, the same membrane shows Li+/Na+ and Li+/K+ selectivities of 2.5 and 3.2, respectively. The incorporation of PSS increases both the size and charge density of the nanochannels inside the membrane, which is beneficial for ion conduction. In addition, the density functional theory (DFT) calculation shows that the binding energy between Li+ and the SO3H group is lower than other alkali ion metals, and this might be one major reason why the membrane possesses high Li+ selectivity. This study demonstrates that incorporating spacing agents into the 2D material matrix is a viable strategy to enhance the performance of the 2D material-based membranes. The results from this study can inspire new membrane designs for emerging applications including energy harvesting and monovalent ion recovery.

Optimizing pore structure of nanoporous membranes for high-performance salinity gradient power conversion

Nanofluidic salinity gradient power (SGP) harvesting has attracted more attention due to its abundant source and easy-to-implement nature, but its applicability is hampered by the high resistance and undesirable ion selectivity of the commercial ion-exchange membranes. The rational design and optimization of advanced membrane architectures are therefore urgently needed, which require a deep understanding of the relationship between ion transport and porous structure. Herein, a regular network model is defined to mimic the nanoporous membrane, allowing us to study the influence of porous structure on the SGP conversion using the Poisson-Nernst-Planck equations. We find that the slight change of inner porous structure can substantially affect the concentration polarization at the membrane-solution interface, which is dominated by the nanopores along the direct transport direction. The SGP conversion can be effectively improved by matching the local pore structure to electric double layer (EDL) structure. The SGP generation is more sensitive to the surface charge density at high salinity gradients. Our results highlight the significance of the precision regulation of porous structure in the SGP conversion, providing theoretical guidance to the design of nanoporous membranes for highly effective SGP harvesting.

Improving the performance of salinity gradient power generation by a negative pressure difference

Although salinity gradient power is a promising renemable energy source, its performance is limited by ion concentration polarization. We show that this problem can be alliviated effectively by applying a pressure gradient, the direction of which is opposite to the concentration gradient. The mechanisms involved are discussed in detail. BackgroundIon concentration polarization (ICP) can lower appreciably the effective concentration gradient across a nanopore, thereby reducing significantly the power generated in salinity gradient power harvesting. MethodsAttempting to alleviate ICP, we propose applying a negative pressure difference across the nanopore. Significant findingsDue to the enhancement in nanopore selectivity, the power generated can be improved appreciably. For example, at a surface charge density of −60 mC/m2, the maximum power can be raised up to ca. 167% when a pressure difference of −6 bar is applied. We show, for the first time, that applying a negative pressure difference is indeed effective.

MOF-Derived Nanoporous Carbon Incorporated in the Cation Exchange Membrane for Gradient Power Generation.

Ion exchange membranes (IEMs), as a part of the reverse electrodialysis (RED) system, play an important role in salinity gradient power (SGP) generation. Structure optimization of IEMs is critical to increase the power production by RED. In this work, metal organic framework (MOF)-derived nanoporous carbons (hollow zeolitic imidazolate framework (ZIF)-derived nanoporous carbons, HZCs) were incorporated in a sulfonated poly (2, 6-dimethyl-1,4-phenylene oxide) (sPPO) membrane to prepare an organic−inorganic nanocomposite cation exchange membrane (CEM). Physicochemical properties, electrochemical properties, and power generation of the synthesized nanocomposite membranes with different HZCs loading were characterized. The results show that the incorporated HZCs could tailor the microstructure of the membrane matrix, providing a superior performance of the nanocomposite membrane. With a HZCs loading of 1.0 wt.%, the nanocomposite membrane possessed the highest permselectivity of 77.61% and the lowest area resistance of 0.42 Ω·cm2, along with a super gross power density of 0.45 W/m2, which was 87.5% (about 1.87 times) higher than that of the blank sPPO membrane. Therefore, incorporating of an appropriate amount of HZCs in the ion-exchange membrane can improve the performance of the membrane, providing a promising method to increase the power generation of the RED system.

Investigations on the effects of operational parameters in reverse electrodialysis system for salinity gradient power generation using central composite design (CCD)

Reverse electrodialysis (RED) can be utilized for the production of renewable energy from salinity gradients. However, there are many key parameters that could influence the performance of RED. This study investigates the use RSM for development of a predictive power density (PD) and open-circuit voltage (OCV) model for the RED system. A three-factor central composite design (CCD) was used to quantify the effects of flow velocity (X1), salinity ratio (X2), and number of cell pairs (X3) towards PD and OCV. A total of 17 experimental data were fitted and ANOVA was used to validate the accuracy of the models. 3D and surface plots were created to foresee the optimal levels of each variable. It was found that flow velocity and salinity ratio have the most dominant influences on the RED performances as compared to number of cell pairs. The predicted PD and OCV values were found to be reasonably fit with the experimental data, validating the predictability of applied models. Therefore, this study suggests that CCD can be considered an effective tool for evaluating and optimizing the RED system using a minimum number of experiments.

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.