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Concentration polarization
Concentration polarization
Concentration polarization
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Concentration polarization is a term used in the scientific fields of electrochemistry and membrane science.

In electrochemistry

[edit]

In electrochemistry, concentration polarization denotes the part of the polarization of an electrolytic cell resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface.[1] Here polarization is understood as the shift of the electrochemical potential difference across the cell from its equilibrium value. When the term is used in this sense, it is equivalent to “concentration overpotential”.[2][3] the changes in concentration (emergence of concentration gradients in the solution adjacent to the electrode surface) is the difference in the rate of electrochemical reaction at the electrode and the rate of ion migration in the solution from/to the surface. When a chemical species participating in an electrochemical electrode reaction is in short supply, the concentration of this species at the surface decreases causing diffusion, which is added to the migration transport towards the surface in order to maintain the balance of consumption and delivery of that species.[vague]

Fig. 1. Fluxes and concentration profiles in a membrane and the surrounding solutions. In Fig. a, a driving force is applied to a system initially at equilibrium: the flux of a selectively permeating species in the membrane, , is higher than its flux in solution, . Higher flux in the membrane causes decreasing concentration at the upstream membrane/solution interface, and increasing concentration at the downstream interface (b). Concentration gradients gives rise to diffusion transport, which increases the total flux in solution and decreases the flux in the membrane. In steady state, .

In membrane science and technology

[edit]

In membrane science and technology, concentration polarization refers to the emergence of concentration gradients at a membrane/solution interface resulted from selective transfer of some species through the membrane under the effect of transmembrane driving forces.[4] Generally, the cause of concentration polarization is the ability of a membrane to transport some species more readily than the other(s) (which is the membrane permselectivity): the retained species are concentrated at the upstream membrane surface while the concentration of transported species decreases. Thus, concentration polarization phenomenon is inherent to all types of membrane separation processes. In the cases of gas separations, pervaporation, membrane distillation, reverse osmosis, nanofiltration, ultrafiltration, and microfiltration separations, the concentration profile has a higher level of solute nearest to the upstream membrane surface compared with the more or less well mixed bulk fluid far from the membrane surface. In the case of dialysis and electrodialysis, the concentrations of selectively transported dissolved species are reduced at the upstream membrane surface compared to the bulk solution. The emergence of concentration gradients is illustrated in Figs. 1a and 1b. Fig. 1a shows the concentration profile near and within a membrane when an external driving force is just applied to an initially equilibrium system. Concentration gradients have not yet formed. If the membrane is selectively permeable to species 1, its flux () within the membrane is higher than that in the solution (). Higher flux in the membrane causes a decrease in the concentration at the upstream membrane surface () and an increase at the downstream surface (), Fig. 1b. Thus, the upstream solution becomes depleted and the downstream solution becomes enriched in regard to species 1. The concentration gradients cause additional diffusion fluxes, which contribute to an increase of the total flux in the solutions and to a decrease of the flux in the membrane. As a result, the system reaches a steady state where . The greater the external force applied, the lower . In electrodialysis, when becomes much lower than the bulk concentration, the resistance of the depleted solution becomes quite elevated. The current density related to this state is known as the limiting current density.[5]

Concentration polarization strongly affects the performance of the separation process. First, concentration changes in the solution reduce the driving force within the membrane, hence, the useful flux/rate of separation. In the case of pressure driven processes, this phenomenon causes an increase of the osmotic pressure gradient in the membrane, which reduces the net driving pressure gradient. In the case of dialysis, the driving concentration gradient in the membrane is reduced.[6] In the case of electromembrane processes, the potential drop in the diffusion boundary layers reduces the gradient of electric potential in the membrane. Lower rate of separation under the same external driving force means increased power consumption.

Moreover, concentration polarization leads to:

  • Increased salt leakage through the membrane
  • Increased probability of scale/fouling development

Thus, the selectivity of separation and the membrane lifetime are deteriorated.

Generally, to reduce the concentration polarization, increased flow rates of the solutions between the membranes as well as spacers promoting turbulence are applied [5, 6]. This technique results in better mixing of the solution and in reducing the thickness of the diffusion boundary layer, which is defined as the region in the vicinity of an electrode or a membrane where the concentrations are different from their value in the bulk solution.[7] In electrodialysis, additional mixing of the solution may be obtained by applying an elevated voltage where current-induced convection occurs as gravitational convection or electroconvection. Electroconvection is defined [8] as current-induced volume transport when an electric field is imposed through the charged solution. Several mechanisms of electroconvection are discussed.[9][10][11][12] In dilute solutions, electroconvection allows increasing current density several times higher than the limiting current density.[11] Electroconvection refers to electrokinetic phenomena, which are important in microfluidic devices. Thus, there is a bridge between membrane science and micro/nanofluidics.[13] Fruitful ideas are transferred from microfluidics: novel conceptions of electro-membrane devices for water desalination in overlimiting current range have been proposed.[14][15]

References

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Concentration polarization

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from Grokipedia
Concentration polarization is a fundamental transport phenomenon in electrochemical and membrane-based systems, characterized by the development of steep concentration gradients of ionic species near ion-selective surfaces, such as electrodes or perm-selective membranes, under an applied . This occurs when ions are selectively transported—typically counter-ions passing through the surface while co-ions are rejected—leading to ion depletion zones on the anodic side and enrichment zones on the cathodic side, which alters the local conductivity and increases system resistance. In electrolytic cells and processes like , concentration polarization arises from the interplay of , , and , often resulting in a limiting where further increase in voltage yields minimal additional ion flux due to near-zero salt concentration at the selective interface. This effect is exacerbated by differences in ion diffusivities, with slower-diffusing s (e.g., cations in many electrolytes) dictating the ambipolar transport rate, and can manifest as non-Ohmic behavior in current-voltage characteristics, resembling diode-like rectification. Over-limiting currents may occur through secondary mechanisms such as electro-osmotic instabilities or surface reactions, but polarization generally hinders efficiency by promoting and scaling on membrane surfaces. The phenomenon is particularly pronounced in micro- and nanofluidic systems, where amplified (up to 30-fold) within depletion zones enhance electrokinetic effects like fluid pumping and particle , enabling applications in biomolecular preconcentration, , and devices such as fuel cells. Mitigation strategies, including pulsatile flows or membrane modifications, are essential to minimize polarization's adverse impacts on processes like and battery performance, underscoring its critical role in advancing sustainable electrochemical technologies.

Fundamentals

Definition and Basic Principles

Concentration polarization is a transport-limited phenomenon characterized by the establishment of concentration gradients of species near a selective surface, such as an electrode or membrane, due to differences in the transport rates of those species compared to the bulk fluid. This results in local concentrations at the surface that deviate significantly from the uniform bulk concentrations, often leading to enhanced or depleted levels of solutes or ions adjacent to the interface. At the core of this process is the diffusion boundary layer (DBL), a thin adjacent to the surface where diffusive dominates over , creating steep concentration gradients. The DBL distinguishes itself from the well-mixed bulk solution, where concentrations remain relatively constant, and its thickness typically ranges from micrometers to millimeters depending on flow conditions and species diffusivities. Concentration polarization arises under general prerequisites such as selective across the interface—where certain are preferentially permitted or rejected—and imbalances between convective and diffusive fluxes that prevent rapid replenishment of depleted or accumulated . The concept was first systematically described in early 20th-century through Walther Nernst's diffusion layer model, which introduced the idea of a stagnant layer near governing mass transport limitations. In electrochemical contexts, concentration polarization manifests as an that increases the required voltage to sustain current flow by altering reactant availability at the . In membrane processes, it reduces the effective driving force for by elevating solute concentrations on the feed side or depleting them on the permeate side, thereby diminishing flux efficiency.

Physical Mechanisms

Concentration polarization originates from an imbalance between the rate of species consumption or production at a reactive surface and the rate of their replenishment or removal through transport processes in the surrounding fluid phase. The key transport mechanisms involved are , driven by random molecular motion; migration, which occurs under the influence of in charged systems; and , arising from bulk fluid motion. When these mechanisms fail to match the surface reaction rate, a concentration develops near the surface, leading to polarization. Diffusive transport, the dominant mechanism in stagnant or low-flow conditions, follows Fick's first law, which states that the diffusive is proportional to the negative of concentration, J=−D∂c∂xJ = -D \frac{\partial c}{\partial x}, where DD is the diffusion coefficient and cc is the concentration./Kinetics/09:_Diffusion) This forms within the diffusion boundary layer adjacent to the surface. Several factors influence the extent of concentration polarization. Surface selectivity, such as charge-based repulsion or size-based exclusion, determines which species are preferentially rejected or transported, exacerbating imbalances for specific solutes. Flow conditions play a : in , limited mixing allows thicker boundary layers and more severe polarization, whereas turbulent flow enhances convective replenishment, thinning the layer and mitigating . The is quantitatively related to the (ShSh), a dimensionless parameter that correlates the with system hydrodynamics, typically expressed as Sh=f([Re](/page/Reynoldsnumber),[Sc](/page/Schmidtnumber))Sh = f([Re](/page/Reynolds_number), [Sc](/page/Schmidt_number)), where [Re](/page/Reynoldsnumber)[Re](/page/Reynolds_number) is the and [Sc](/page/Schmidtnumber)[Sc](/page/Schmidt_number) is the ; higher ShSh values indicate improved and reduced polarization. Concentration polarization manifests in two general forms: depletion, characterized by a lower concentration of reactant near the surface compared to the bulk fluid, which limits supply; and accumulation, where rejected or product species build up to higher concentrations at the surface, potentially driving back-diffusion or secondary effects. These can occur under steady-state conditions, where the concentration profile stabilizes with balanced fluxes, or transiently, during initial operation when gradients evolve over time until equilibrium. Experimental techniques enable direct observation of these gradients. Interferometry visualizes concentration variations by detecting changes in refractive index within the boundary layer, providing non-invasive profiles of the polarization zone. Microelectrodes, positioned near the surface, measure local concentrations with high spatial resolution, allowing quantification of depletion or accumulation in real time.

In Electrochemistry

Phenomenon Description

In electrochemical systems, concentration polarization manifests as the development of ion concentration gradients at the electrode-electrolyte interface, arising when the electrochemical reaction rate surpasses the mass transport rate of reactants via diffusion, migration, or convection. This imbalance depletes reactant species or accumulates products near the electrode surface, altering the local concentrations compared to the bulk electrolyte. As described in foundational electrochemical theory, this phenomenon directly contributes to concentration overpotential, which is the additional voltage required to sustain the reaction under non-equilibrium conditions. The primary effects of concentration polarization include diminished reaction rates due to limited reactant supply at the interface, elevated cell voltage to overcome the gradient, and a shift in the that reduces overall system efficiency. In cathodic processes, such as metal ion reduction, the depletion of cations like metal ions near the lowers the surface concentration, impeding further . Conversely, in anodic processes, the buildup of oxidized products or anions creates a barrier to additional oxidation reactions. These effects integrate with reaction kinetics by coupling mass transport limitations to the Butler-Volmer rate expression, where surface concentrations influence the exponential current-potential dependence. Notable examples occur in practical electrochemical devices. In lithium-ion batteries, concentration polarization at high discharge currents restricts Li⁺ ion availability at the , leading to capacity fade and reduced power output. In fuel cells, it exacerbates voltage losses at elevated current densities by limiting oxygen transport to the , thereby decreasing . In chlor-alkali , the process is observed through ion depletion at the and accumulation at the , though industrial designs minimize its impact via optimized spacing. Concentration polarization is distinct from ohmic polarization, which stems from electrolyte resistance, and activation polarization, which involves charge transfer kinetics at the electrode; it predominantly governs performance limitations at high current densities where mass transport becomes the rate-controlling step.

Quantitative Analysis

The quantitative analysis of concentration polarization in electrochemistry relies on models that describe mass transport limitations near the electrode surface, primarily through diffusion within a boundary layer. The Nernst diffusion layer model provides a foundational approximation by assuming a stagnant layer of thickness δ adjacent to the electrode, where convective transport is negligible and the concentration profile is linear. This thickness is expressed as δ = D / k, with D representing the diffusion coefficient of the reacting species and k the mass transfer coefficient, which depends on hydrodynamic conditions such as flow rate or rotation speed. This model leads to the concept of limiting , the maximum current achievable before the surface concentration of the reactant drops to zero. For a reduction process, the limiting is given by iL=nFDCbδi_L = \frac{n F D C_b}{\delta} where n is the number of electrons transferred, F is Faraday's constant, and C_b is the bulk concentration of the reactant. This equation highlights how polarization intensifies as current approaches i_L, with thinner boundary layers (higher k) mitigating the effect by enhancing . The concentration overpotential arising from this depletion can be quantified using the Nernst equation applied to the surface concentration. For cathodic processes, it is ηconc=RTnFln⁡(1−iiL)\eta_{\text{conc}} = \frac{RT}{nF} \ln \left(1 - \frac{i}{i_L}\right) where R is the and T is ; this logarithmic term reflects the exponential relationship between potential and concentration ratio at the surface. At currents much less than i_L, η_conc is small, but it diverges as i nears i_L, severely limiting cell performance. In steady-state conditions, the flux of the reacting species at the surface (J_1^s) equals the flux through the or interface (J_1^m), ensuring : J_1^s = J_1^m. This equality underpins the analysis of polarization in systems like batteries or electrolyzers, where deviations indicate transport bottlenecks. For more detailed investigations beyond the simplified Nernst layer, numerical simulations solving the Nernst-Planck equations are essential. These coupled partial differential equations describe ion fluxes due to , migration, and : Ji=−Di∇ci−ziDiFRTci∇ϕ+civJ_i = -D_i \nabla c_i - \frac{z_i D_i F}{RT} c_i \nabla \phi + c_i \mathbf{v} where J_i is the flux of species i, c_i its concentration, z_i its charge, ϕ the electric potential, and v the velocity field; continuity requires ∇ · J_i = 0 in steady state. Combined with Poisson's equation for charge balance, these simulations capture nonlinear effects like space charge regions near limiting currents, enabling predictions of polarization in complex geometries.

In Membrane Science and Technology

Occurrence in Membrane Processes

Concentration polarization arises in membrane processes due to the selective of components, where faster-permeating (such as or specific ions) pass through the more readily than others, creating concentration gradients across a at the membrane surface. This results in elevated solute concentrations on the feed side near the wall, particularly for rejected solutes, as described by the film theory model where the acts as a diffusive resistance. In pressure-driven membrane processes like (RO), (UF), and nanofiltration (NF), concentration polarization manifests as solute accumulation at the membrane-feed interface, driven by convective transport toward the surface and diffusive back-transport. In RO, this buildup increases the local , counteracting the applied transmembrane pressure and thereby reducing permeate ; for instance, measurements in NF systems show wall concentrations up to 1.8 times the bulk for sulfate solutions at low velocities. UF experiences similar effects with macromolecular solutes concentrating near the surface, while NF demonstrates salt rejection (e.g., 45.9% for NaCl with NF 270 membranes), exacerbating polarization under higher pressures. Electrically driven processes, such as , exhibit concentration polarization through ion depletion on the diluate side and accumulation on the concentrate side of ion-exchange membranes, forming thin diffusion boundary layers that increase electrical resistance. This ion asymmetry causes voltage drops across the cell, with initial resistance rises up to dramatic levels in the first few minutes of operation, reducing overall process efficiency by as much as 60%. The primary impacts of concentration polarization include flux decline, modeled by the solution-diffusion equation J=A(ΔP−Δπ)J = A (\Delta P - \Delta \pi), where JJ is the permeate flux, AA is the membrane permeability, ΔP\Delta P is the transmembrane pressure difference, and Δπ\Delta \pi is the difference that rises due to polarization-enhanced solute concentrations at the wall. Additionally, it promotes gel layer formation from highly concentrated solutes, initiating that further impairs performance, particularly at low cross-flow velocities in UF/NF. In gas separation , especially high-performance inorganic types like zeolite-based systems, polarization depletes the more permeable gas (e.g., CO₂ in CH₄/CO₂ mixtures) at the retentate interface, reducing separation by up to 10% if unaccounted for in models. A key quantitative metric is the polarization modulus β=CwallCbulk>1\beta = \frac{C_{wall}}{C_{bulk}} > 1, which quantifies accumulation by the ratio of wall to bulk solute concentration; values range from 1.5 to 1.8 in NF/RO under typical conditions, increasing with and decreasing with .

Mitigation Techniques

Mitigation techniques for in membrane processes aim to enhance coefficients at the membrane surface, thereby thinning the solute-depleted and improving in systems such as (RO), ultrafiltration (), and (). These established approaches, developed prior to 2023, encompass hydrodynamic enhancements, optimized module configurations, and adjustable operating conditions to counteract the accumulation of rejected solutes. By promoting or secondary flows, such methods reduce the thickness of the concentration , directly tying into the fundamental physical mechanisms of polarization. Flow enhancement techniques primarily involve increasing cross-flow velocity to generate turbulence that disrupts the and minimizes solute buildup. In RO and ED modules, turbulence promoters and spacers are widely used; for example, net-type spacers oriented at 45° to the feed flow direction promote mixing, elevating the and boosting permeate flux by 2–5 times relative to empty channels, though at the cost of higher pressure drops (up to 160 times greater). Ladder-type spacers, suited for low-salinity applications, further reduce concentration polarization while lowering energy demands compared to conventional designs. These elements increase by 1.7–10 times in ED, as demonstrated in early studies on mesh-type promoters. Module design innovations focus on inducing secondary flows to sustain high shear rates near the , effectively sweeping away polarized solutes. Spacer-filled channels in spiral-wound RO modules, such as those with zigzag configurations, enhance hydrodynamics and while mitigating losses in specific feed conditions. Vortex promoters generate localized to destabilize the , and rotating systems—employing disk or cylindrical rotations—achieve shear rates up to 3×1053 \times 10^5 s−1^{-1}, yielding permeate es 3–5 times higher than cross-flow setups in UF and RO, with up to 70% energy savings at moderate speeds. Thicker spacers (e.g., 0.86 mm) in these designs also lower propensity linked to polarization. Operational strategies offer practical adjustments to alleviate concentration polarization without module redesign. Pulsed flow creates intermittent disruptions that induce secondary circulations, reducing the polarized layer's thickness and enhancing in UF systems for applications like clarification. In UF hollow fiber modules, air sparging introduces bubbles that promote meandering flows, with optimal superficial velocities around 0.3 m/s effectively controlling polarization and by increasing instability in the feed stream. pH adjustments in ED processes modify charge interactions at ion-exchange surfaces, inhibiting scale precipitation exacerbated by polarization and enabling operation at higher currents. Electrochemical aids, particularly in ion-exchange membrane ED, leverage applied electric fields to drive electroconvection, which actively mixes the depleted diffusion layer and extends mass transfer beyond limiting currents. Equilibrium electroconvection, powered by surface charge-induced electroosmosis, scales with zeta potential and counters polarization in underlimiting regimes, while non-equilibrium forms at overlimiting currents involve extended space charge regions that amplify convection, especially on hydrophobic membranes. These techniques' performance is quantified via the kk, expressed as k=[Sh](/page/Sherwoodnumber) Ddh,k = \frac{[Sh](/page/Sherwood_number) \, D}{d_h}, where [Sh](/page/Sherwoodnumber)[Sh](/page/Sherwood_number) is the Sherwood number (convective-to-diffusive transfer ratio), DD is the , and dhd_h is the ; higher kk values indicate reduced polarization, guiding module optimization in laminar or turbulent flows. Empirical Sherwood correlations, often tied to Reynolds and Schmidt numbers, enable prediction of flux limitations across RO and ED configurations.

Recent Advances

Innovations in Electrochemistry

Recent advancements in have focused on mitigating concentration polarization through nanoscale , particularly in nanoporous membranes where electrokinetic mechanisms enhance transport. In confined environments, such as carbon nanotubes or 2D MXene supports, spatial restrictions alter oxidation processes by stabilizing intermediates and optimizing pathways, leading to reduced limitations. For instance, a 2025 study on space-confined electrocatalysis for demonstrated that nanoconfinement in 3D frameworks increases local reactant concentrations and improves mass transport, achieving overpotentials as low as ~30 mV at 10 mA cm⁻² for (HER) due to enhanced charge transfer along 1D anisotropic pathways. These mechanisms counteract polarization by promoting efficient selectivity and minimizing buildup in high-rate electrochemical devices. Interface manipulation techniques in membrane electrolysis have emerged as key strategies for efficiency gains, primarily through surface modifications that lower and alleviate concentration gradients. Nanofiber-modified porous electrodes (PTEs), such as vertically aligned carbon nanofibers coated with , enable low loading (0.1 mg cm⁻²) while enhancing catalyst adhesion and conductivity, resulting in superior polarization curves and reduced mass losses during operation. Similarly, post-2023 modifications to porous layers (PTLs) via mechanical abrasion and chemical increase surface and remove oxides, decreasing voltage losses by up to 60 mV at 2 A cm⁻², thereby suppressing concentration polarization in proton exchange membrane water electrolyzers (PEMWE). These approaches, detailed in recent patents and papers, facilitate better access and electron pathways, boosting overall device performance without excessive material use. Advanced materials, particularly ion-exchange membranes engineered for electroconvection enhancement, have shown promise in reducing polarization effects. As of 2025, heterogeneous cation- and anion-exchange membranes leverage electroconvection vortices to sustain overlimiting currents, where gravitational convection intensities reach 150-350 μm s⁻¹ depending on salt concentration (0.01-0.1 M KCl), effectively dispersing depleted layers and extending operational current densities beyond classical limits. Polarizing currents control this convection symmetrically on diluate and concentrate sides, suppressing unwanted electroconvection at higher bulk concentrations and improving ion transfer efficiency in electrodialysis systems. Such enhancements prioritize material innovations like profiled surfaces to amplify vortex formation, providing a scalable solution for polarization mitigation in electrochemical separations. Device-specific innovations in fuel cells and batteries emphasize 3D electrode architectures to minimize the diffusion boundary layer (DBL), directly impacting limiting currents in high-rate applications. In 2025 operando studies, 3D flow-through electrodes, modeled after mm-sized particles, reduce DBL thickness to 100-600 μm under flow velocities of 0.1-2.1 mm s⁻¹, enabling sustained Faradaic reactions and moderating gradients that exacerbate polarization. For lithium batteries, rapid diffusion pathways in high-loading cathodes diminish concentration polarization, supporting power densities suitable for fast-charging scenarios with minimal . In hybrid Ni-Fe systems, 3D nickel electrodes maintain current densities comparable to standalone electrolyzers while curbing DBL effects, achieving stable performance over extended cycles. These designs collectively enhance limiting current densities by 20-50% in demanding conditions, underscoring their role in scalable . Additionally, emerging 2025 breakthroughs incorporate AI-optimized electrode designs for predicting and mitigating concentration polarization. models integrated with operando data reduce CP effects by up to 30% in lithium- batteries by dynamically adjusting microstructures for improved . Emerging metrics for polarization assessment leverage spatially resolved in-situ to provide real-time insights into electrochemical interfaces. A 2023 utilized in-situ to map concentration polarization at zinc-based electrodes, revealing how hydrated modulation suppresses interfacial gradients and improves reversibility. (FLIM) further enables operando visualization of local pH and dynamics in 3D electrodes at 9 μm resolution, quantifying plume formation and enhancements under varying currents (up to 143 mA g⁻¹). These techniques, integrated with theoretical modeling, offer unprecedented spatiotemporal data for optimizing device efficiency beyond classical analyses.

Advances in Membrane Technology

Recent advances in membrane technology have focused on developing sophisticated models and strategies to quantify and mitigate concentration polarization (CP) in forward osmosis (FO) processes, particularly through innovative evaluation methods that account for feed solution effects. A 2025 study introduced a new quantitative approach using the water transmission coefficient (η_WT), defined as the ratio of experimental water flux to theoretical flux, to assess CP severity in FO membranes. This method revealed that internal CP dominates the osmotic pressure drop, contributing 60% to 80% under varying NaCl concentrations (0.5–1.5 mol/L), while external CP plays a lesser role. The internal CP coefficient (β) is calculated via equations linking water flux (J), mass transfer coefficient (k), and membrane structural parameter (S), showing that higher organic feed concentrations (e.g., 500 mg/L humic acid) exacerbate β by up to 0.29%, reducing η_WT and overall efficiency; draw solutions like CaCl₂ further intensify this compared to NaCl. In inorganic and gas separation membranes, post-2023 research has emphasized rigorous impact assessments and targeted reductions in CP for emerging high-performance materials. A 2024 analysis applied (CFD) and systemic modeling to evaluate CP in hollow-fiber inorganic membranes for upgrading, finding that CP significantly diminishes purification rates—particularly when CO₂ exceeds 1000 GPU—while moderately affecting recovery. The effects intensify with larger fiber diameters and higher operating pressures, potentially limiting the of these membranes despite their high selectivity. Reduction strategies highlighted include optimizing geometric parameters and operating conditions to minimize buildup, though no universal mitigation was identified without trade-offs in permeance. Enhancements in (ED) systems have leveraged advanced spacer configurations and adjustments to curb CP, as detailed in a 2023 meta-synthesis . Optimized spacers, such as those promoting turbulent mixing at higher Reynolds numbers, effectively disrupt boundary layers and reduce CP by enhancing , though they introduce flow resistance and a "shadow effect" that can limit active area. Increasing solution similarly boosts and minimizes CP but elevates energy demands due to higher pumping power. The concludes that no single optimization dominates; instead, hybrid approaches balancing spacer geometry (e.g., diagonal net vs. ladder-type) with control yield the best CP mitigation, tailored to specific ED configurations. Hybrid membrane systems integrating catalytic nanofiltration (NF) have addressed CP challenges in pollutant degradation, exemplified by a 2023 investigation into naproxen removal. In this setup, one-side-coated NF membranes with catalytic supports activated peroxymonosulfate (PMS) for radical generation, achieving 97% naproxen removal on the permeate side and 12% on the feed side under optimal PMS-to-naproxen ratios. orientation (support layer facing feed vs. permeate) modulated CP, with feed-side catalysis showing heightened CP effects that concentrated reactants but risked decline; permeate-side operation mitigated this while maintaining high degradation efficiency. This hybrid approach demonstrates how CP can be leveraged for enhanced local concentrations in , provided orientation and flow conditions are controlled. Broader applications of these advances extend to microfluidic and bio-inspired membranes, where CP management is critical for scalability and efficiency, as outlined in recent reviews. A 2025 review on microfluidic electrochemical systems highlights concentration polarization (ICP) in H-shaped microchannels with cation-exchange membranes, achieving 95-98% salt rejection and 40-45% recovery at energy costs of 4–8 kWh/m³, with low risk due to depletion zone formation; parallelization of channels addresses throughput limitations (e.g., from 20 μL/min to higher scales via microporous enhancements). In bio-mimetic contexts, aquaporin-embedded membranes (e.g., AQP1 or AQPZ in bilayers or matrices) reduce CP-related declines by enabling high permeance (up to 80% recovery in hybrids) through molecular sieving, though challenges persist in large-scale salt rejection. A 2024 roadmap on further underscores post-2023 innovations like biomimetic artificial channels in thin-film composites and photothermal nanomaterials in , which counteract CP and temperature polarization to approach theoretical energy minima (1.1 kWh/m³ for RO) while boosting recovery beyond 50%. Additionally, 2025 developments in graphene-oxide hybrid membranes have achieved over 55% recovery rates with energy consumption below 2 kWh/m³ through self-cleaning surfaces that minimize CP accumulation.

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