Non-conductive Spacers Research Articles

Enhanced nitrate removal from groundwater using a conductive spacer in flow-electrode capacitive deionization

Flow-electrode capacitive deionization (FCDI) represents a promising approach for ion separation from aqueous solutions. However, the optimization of spacer, particularly for nitrate-contaminated groundwater systems, has often been overlooked. This research comprehensively investigates the influence of using a conductive (carbon cloth, CC) spacer on nitrate removal performance within FCDI system, comparing it to a non-conductive (nylon net, NN) spacer. In both CC and NN FCDI systems, it is unsurprisingly that nitrate removal efficiency improved notably with the increasing current density and hydraulic retention time (HRT). Interestingly, the specific energy consumption (SEC) for nitrate removal did not show obvious fluctuations when the current density and HRT varied in both systems. Under the auspiciously optimized process parameters, CC-FCDI attained a 20% superior nitrate removal efficiency relative to NN-FCDI, accompanied by a notably diminished SEC for CC-FCDI, registering at a mere 28% of NN-FCDI. This great improvement can be primarily attributed to the decrement in FCDI internal resistance after using conductive spacer, which further confirmed by electrochemical tests such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Upon prolonged continuous nitrate removal at the optimized conditions, the CC-FCDI system achieved a consistent 90% nitrate removal efficiency with a low SEC of 2.7–7.8 kWh/kg NO3-N, underscoring its steady performance. Overall, this study highlights the pivotal importance of careful spacer design and optimization in realizing energy-efficient groundwater treatment via FCDI.

Experimental Study on Ion Transport in Microfluidic Electrodialysis Using Partially Masked Ion Exchange Membranes.

Electrodialysis using anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) has been widely used for water desalination and the management of various ionic species. During commercial electrodialysis, the available area of an ion-exchange membrane is reduced by a non-conductive spacer that is in contact with the AEM/CEM. Although multiple reports have described the advantages or disadvantages of spacers, fewer studies have explored the effects of spacers on the mass transport effect of the reduced membrane area excluding the fluid flow change. In this paper, we present our experimental studies concerning mass transport in microfluidic electrodialysis systems with partially masked ion-exchange membranes. Six different types of masking membranes were prepared by the deposition of non-conductive films on parts of the membranes. The experimental results showed that the overlapped types (in which masking was vertically aligned in the AEM/CEM) exhibited a larger electrical conductance and better current/energy efficiency, compared with the non-overlapped types (in which masking was vertically dislocated in the AEM/CEM). We also observed that a reduction in the unit length of the unmasked ion-exchange membrane enhanced overall mass transport. Our results demonstrate the effects of patterned membranes on electrical resistance and desalination performance; they also identify appropriate arrangements for electromembrane systems.

Influence of spacers on mass transport in electromembrane desalination systems

In electromembrane desalination systems, ion transport near ion exchange membranes induces inevitable concentration polarization, which complicates mass transport. To mitigate the effect of concentration polarization and increase mass transport, spacers which promote mixing and change the flow velocity distribution in the channel. The authors investigated the effect of several spacers (from one to three) on the electromembrane system and varied the size and shape of the spacers, distance to the membranes and flow rate. It has been shown that the presence of spacers actually makes it possible to redistribute the speed and direction of the flow in the system and to increase the current efficiency. The article analyzes the impact of the spacers in the distant exorbitant conditions on mass transport and electroconvective vortices. In the article, we calculated and compared the current-voltage characteristics in the ohmic region, the “plateau” region and overlimiting region for channels with and without spacers. The work investigates fundamental laws depending on the location, size and number of spacers. This paper investigates the effect of non-conductive spacers. It has been shown that, for example, the presence of several spacers (three) near the center of the channel, slightly shifted towards the anion-exchange membrane, increases the mass transport by about 24% -25%. In addition, the influence of spacers on the emergence and development of electroconvective vortices has been studied.

A numerical investigation of charged ion transport in electrodialyzers with spacers

Charged ion transport in electrodialyzers with ion mixing spacers was numerically investigated solving Navier-Stokes and Nernst-Planck equations in a three-dimensional Cartesian coordinate system. An electrically nonconductive spacer placed in the membrane channels in the electrodialyzer enhances three-dimensional hydrodynamic and ionic mechanical dispersion, resulting in a substantial increase in the effective mass transfer coefficient over the membrane surfaces. This mechanical dispersion retards depletion of ions and increases the value of the limiting current density, which is of great importance in practical applications of electrodialyzers. Full detailed three-dimensional numerical computations were conducted, for the first time, to investigate the effect of the ion mixing spacers on the limiting current density, using a voxel method based on Cartesian staggered grids along with no-slip velocity and harmonic mean averaging boundary treatments. The results obtained from the three-dimensional numerical simulation were found in good agreement with the experimental data available in the literature.

Low Energy Membrane Distillation: A Numerical Study on The Role of Conductive Spacers

Thermal conductivity is one of the most influencing DCMD performance parameters. Having larger value in the membrane results in lower thermal efficiency and mass flux. However, increasing the conductivity of the fluid, membrane’s surfaces, or by means of higher conductive gap have reported unique performance improvements in DCMD. In this work, computational fluid dynamics (CFD) has been used to evaluate the role of highly conductive spacers on the performance of a direct contact membrane distillation (DCMD) system. The cylindrical shape spacer is considered in this work which placed directly on the membrane surfaces. The numerical model assumes a 2D steady state, conjugated heat, and turbulent Navier-Stokes flow. Mass flux is estimated by integrating a combined Knudsen and Poiseuille flow model. The effect of adding conductive and non-conductive spacers on temperature polarization coefficient, mass flux and thermal efficiency as well as the total transmembrane heat has been investigated at different incoming flow velocities. Results showed that copper spacers showed better performance than polymer spacers, but a significant enhancement was not observed from the no spacer case. The cylindrical shape spacers although created turbulence zone, it brought with it localized circulating zone that lead to deteriorating each of the mass flux and thermal efficiency when operating the flow at medium to high Reynolds number.

Multi-physical modelling of reverse electrodialysis

Reverse electrodialysis (RED) is an electrochemical membrane process that directly converts the energy associated with the concentration difference between two salt solutions into electrical energy by means of a selective controlled mixing. The physics of RED involves the interaction of several phenomena of different nature and space-time scales. Therefore, mathematical modelling and numerical simulation tools are crucial for performance prediction. In this work, a multi-physical modelling approach for the simulation of RED units was developed. A periodic portion of a single cell pair was simulated in two dimensions. Fluid dynamics was simulated by the Navier-Stokes and continuity equations, and ion transfer by the Nernst–Planck approach along with the local electroneutrality condition. The Donnan exclusion theory was implemented in order to simulate interfacial phenomena. A sensitivity analysis of the process performance was carried out. Different membrane/channel geometrical configurations were investigated, including flat membranes, either with or without non-conductive spacers, and profiled membranes. The influence of feeds concentration/velocity was also evaluated. Results confirmed that, with respect to the ideal case of plane (empty) channels and planar membranes, non-conductive spacers always reduce the power produced, while profiled membranes may or may not perform better, depending on stack features and operating conditions.

Computational fluid dynamics (CFD) assisted analysis of profiled membranes performance in reverse electrodialysis

Implementation of reverse electrodialysis (RED) is economically limited by the relatively high ion-exchange membranes price. Additionally, the shadow effect of non-conductive spacers reduces the membrane area available for counter-ion transport and increases the stack electric resistance. A promising alternative could be utilization of profiled membranes, since the reliefs formed on their surface keeps the membranes separated and provides channels for solutions flow. Herein, we have simulated, through computational fluid dynamics (CFD) tools, fluid behavior in channels formed by various profiled membranes. The highest net power density values were obtained for corrugations shape and arrangement in a form of chevrons due to the increase of the available membrane area and an excellent balance between enhancement of mass transfer and the increase of the pressure drop in the channel. When properly designed, corrugated membranes may offer a better performance even compared to the case of conductive spacers. The proposed membrane corrugation design in not limited to the RED application, and could be also extended to other electromembrane processes, such as electrodialysis and Donnan dialysis, in which high ionic mass transport rates are desirable at as low as possible energy costs.

Enhanced Ionic Conductivity and Power Generation Using Ion-Exchange Resin Beads in a Reverse-Electrodialysis Stack.

Reverse electrodialysis (RED) is a promising technique for harvesting energy by mixing seawater with river water. The energy production is usually limited by ionic conductivity in dilute compartments of a RED system. Novel tests were conducted in this research, which used ion-exchange resin beads (IERB) to replace nonconductive spacer fabrics in RED compartments with dilute NaCl solution in a modified stack containing Fumasep FKS and Fumasep FAS membranes. We compared the conductivity of an IERB packed bed with that of an inert glass-beads-packed bed as a control to confirm IERB's effectiveness. When applied in a RED system, IERB decreased the stack resistance by up to 40%. The maximum gross power density improved by 83% in the RED stack compared to that in a regular RED stack at 1.3 cm/s average linear flow velocity. IERB-filled stack resistance was modeled. The model results fit well with experimental data, thereby confirming the effectiveness of the new approach presented here. The net power density is also estimated based on the measured pressure drop and pumping energy model. Both gross and net power density was improved by over 75% at higher flow rate. A net power density of 0.44 W/m(2) was achieved at a cell thickness of 500 μm. To the best of our knowledge, this research is the first to study the impact of IERB on power generation and establishes a new approach to improving the power performance of a RED system.

Understanding the Enhancement Mechanisms of Surface Plasmon‐Mediated Photoelectrochemical Electrodes: A Case Study on Au Nanoparticle Decorated TiO2 Nanotubes

Surface plasmon resonance (SPR) enhancement in photocatalyst and photovoltaics has been widely studied and different enhancement mechanisms have been established based on different heterostructure interface configurations. This work is intended to unveil the mechanisms behind charge or energy transfer in different plasmonic configurations of metal particle–semiconductor interfaces, especially with a dielectric layer. For this purpose, a series of composite photoelectrodes based on anodic TiO2 nanotube (TONT) backbones coated with Au, Al2O3, or both are designed and characterized systematically. In conjunction with both experimental measurements and numerical simulations, it is revealed that in the TONT‐Al2O3‐Au electrode system (i.e., a thin nonconductive spacer between semiconductor and metal), the enhancement is dominantly governed by SPR‐mediated hot‐electron injection rather than conventional‐thought near‐field electromagnetic enhancement. Among all configurations, the TONT‐Au‐Al2O3 electrode shows the best photoresponse in both UV and visible regions. The superior visible light response of the TONT‐Au‐Al2O3 electrode is ascribed to the Al2O3 intensified local electromagnetic field that enhances the hot‐electron injection through the TiO2‐Au interface, and an effective surface passivation by the Al2O3 coating.

Chemisorption and Electrochemical Activity of Thiophenols at Well-Defined Pd(111) Surfaces: Studies by LEED, AES, HREELS, and Electrochemistry

The chemisorption and electrochemical activity of 2 ,5 -dihydroxythiophenol (DHT) and 2-(8mercaptooctyl)-1,4-benzenediol (DHOT) on well-defined Pd(111) surfaces were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energyloss spectroscopy (HREELS), and electrochemistry (EC). Results confirm that DHT is chemisorbed in two discrete orientations such that at low concentrations, DHT is oxidatively bound to the surface through the diphenol and mercapto groups as quinonoid and S moieties, respectively, whereas at high concentrations, the molecule is coordinated oxidatively through the –SH group in a vertical S- η 1

Patterned ion exchange membranes for improved power production in microbial reverse-electrodialysis cells

Power production in microbial reverse-electrodialysis cells (MRCs) can be limited by the internal resistance of the reverse electrodialysis stack. Typical MRC stacks use non-conductive spacers that block ion transport by the so-called spacer shadow effect. These spacers can be relatively thick compared to the membrane, and thus they increase internal stack resistance due to high solution (ohmic) resistance associated with a thick spacer. New types of patterned anion and cation exchange membranes were developed by casting membranes to create hemispherical protrusions on the membranes, enabling fluid flow between the membranes without the need for a non-conductive spacer. The use of the patterned membrane decreased the MRC stack resistance by ∼22 Ω, resulting in a 38% increase in power density from 2.50 ± 0.04 W m−2 (non-patterned membrane with a non-conductive spacer) to 3.44 ± 0.02 W m−2 (patterned membrane). The COD removal rate, coulombic efficiency, and energy efficiency of the MRC also increased using the patterned membranes compared to the non-patterned membranes. These results demonstrate that these patterned ion exchange membranes can be used to improve performance of an MRC.

Micro-structured membranes for electricity generation by reverse electrodialysis

Reverse electrodialysis (RED) is a technology for extracting salinity gradient power by contacting waters with different salinity, i.e. seawater and river water, through ion exchange membranes. Conventionally, non-conductive spacers are used to separate these ion exchange membranes from each other in RED. The power output is hampered by these non-conductive elements which increase electrical resistance in the RED stack. To eliminate the use of these spacers, structured anion exchange membranes with a structure height of 100µm were prepared by casting a polymer solution on stainless steel molds followed by solvent evaporation. These self-standing membranes with straight-ridge, wave and pillar structures as well as similarly prepared flat membranes were installed on the river water side in a RED stack (where electrical resistance is the highest). 38% higher gross power density and 20% higher net power density were achieved with the pillar-structured membranes when compared to that of flat membranes with spacers. Further optimization of the structure geometry in combination with the possibility to cast membranes of different chemistries offer a huge potential for further development of homogeneous membranes with the desired electrochemical and physical properties, which could provide high power densities in RED.

Colloidal silica nanoparticle-assisted structural control of cellulose nanofiber paper separators for lithium-ion batteries

Porous structure-tuned cellulose nanofiber paper separators (designated as S-CNP separators) are demonstrated as a promising alternative to commercial polyolefin separators for use in lithium-ion batteries. A new architectural strategy based on colloidal silica (SiO2) nanoparticle-assisted structural control is presented to overcome the difficulty in forming controllable porous structure of pure cellulose nanofiber paper separators (designated as CNP separators) from densely-packed cellulose nanofibers (CNFs). The new S-CNP separators proposed herein incorporate SiO2 nanoparticles as a CNF-disassembling agent (i.e., as non-conductive spacer particles). This structural uniqueness allows loose packing of CNFs, thereby facilitating the evolution of more porous structure. The unusual porous structure of S-CNP separators can be fine-tuned by varying SiO2 contents in the CNF suspension. Notably, the S-CNP separator (fabricated with 5 wt.% SiO2 content) exhibits the highest ionic conduction due to the well-balanced combination of nanoporous structure and separator thickness, thus contributing to excellent cell performance. This study underlines that the colloidal SiO2 nanoparticle-directed structural tuning of CNPs offers a promising route for the fabrication of advanced paper separators with optimized attributes and functionality.

Power generation using profiled membranes in reverse electrodialysis

Reverse electrodialysis (RED) is a technology to obtain energy from the salinity difference between salt water and fresh water. Traditionally, ion exchange membranes, separated by non-conductive spacers, are used in this technology. As an alternative for these non-conductive spacers, in this work, heterogeneous ion exchange membranes were hot pressed in the dry state to create a profiled membrane comprising 230–245 μm ridges (in wet state) on one side of the membrane. Stacking such profiled membranes creates channels for the feed water, thus make the use of spacers obsolete. The performance of a RED-stack with such profiled membranes was compared for the first time with a RED-stack with traditional, non-conductive spacers. The ohmic resistance was significantly lower for the stack with profiled membranes compared to that with spacers, whereas the boundary layer resistance was higher. This resulted in slightly higher power densities for the stack with profiled membranes. Despite this small improvement, profiled membranes have a strong future development potential. Experimental data show that the hydraulic friction is much lower for the stack with profiled membranes and hence higher Reynolds numbers are possible than in a stack with spacers. Furthermore, profiling membranes allow much freedom to create new profile geometries where a hydrodynamic flow can be combined with efficient mixing in the boundary layers.

Membrane with integrated spacer

Many membrane processes are severely influenced by concentration polarisation. Turbulence promoting spacers placed in between the membranes can reduce the diffusional resistance of concentration polarisation by inducing additional mixing. Electrodialysis (ED) used for desalination suffers from concentration polarisation in particular. Using spacers there leads to higher cell resistance, and therefore to higher power consumption, because of the induction of the spacer shadow effect: ions do not migrate in areas where the spacer is located. The use of ion-conductive spacers can reduce this spacer shadow effect, however the spacers are still rather thick and the cell resistance stays high. This work tries to overcome the disadvantages of thick spacers while keeping the beneficial effect of turbulence promotion. We present the preparation and characterisation of a novel cation exchange membrane where the spacer is formed directly on the membrane surface and is hence integral part of the membrane. This membrane with integrated spacer is formed by surface patterning of a drying polymer solution in contact with a regular membrane spacer where capillary forces pull the solution towards the spacer strands where they solidify. Peeling of the original spacer leaves the membrane with the spacer topology integrated in its surface. Characterisation of this novel membrane in an ED process improves process hydrodynamics while avoiding the resistance increase corresponding to the shadow effect of a non-conductive spacer. Having a spacer directly on the membrane surface has also the advantage of a much simpler membrane module assembly since the spacer becomes superfluous.

Ion conductive spacers for increased power generation in reverse electrodialysis

Reverse electrodialysis (RED) is a sustainable energy source that converts the free energy of mixing of two solutions with different salinity directly into electrical energy. High power outputs are predicted theoretically, but the practical power output and the efficiency of the process are still relatively low. Next to concentration polarization phenomena, especially the spacer shadow effect, which occurs due to the use of non-conductive spacers, has a high impact on the net power output. Non-conductive spacers block the ionic transport in the stack thus reducing the area effectively available for ionic transport. In this work we developed and applied ion conductive spacers for RED, which eliminates the spacer shadow effect and obviously improves the overall process performance. We combined theoretical calculations with direct current and alternating current experimental stack characterization methods to quantify this effect on stack resistance and power output in RED process under different hydrodynamic conditions. The use of ion conductive spacers led to a decrease in stack resistance and an increase in power density of a factor 3 compared to the use of non-conductive spacers made of PVC with the same open area and shape. This result increases the impact of RED as renewable energy source and brings it closer to economical viability and practical implementation.

Eddy currents and electrical surface charges

AbstractIn eddy current calculations, the displacement current in the non‐conducting space surrounding the eddy current region is usually neglected. This assumption enforces that the electric charge density and the accompanying normal components of the eddy current density on the surface of the eddy current region must vanish.If the field exiting source currents are not accompanied by charges this assumption may yield acceptable results for the eddy current distribution. However, if the field exiting source currents are accompanied by charges, this assumption may lead to totally wrong results for the current distribution in the eddy current region. An example is given which makes plain this point.To obtain correct results it is not necessary to employ the full set of Maxwell's equations capable to describe wave propagation phenomena also outside the eddy current region. It is shown in the paper that by replacing the displacement current density in the field describing equations by a specifically chosen current density function makes it possible to determine eddy currents and surface charges within the quasi‐stationary calculation scheme for arbitrary field exciting source currents which may or may not be accompanied by charges. The solution obtained in this way is shown to be unique. Copyright © 2003 John Wiley & Sons, Ltd.

Transient current distribution in a sphere by feeding a current on the surface from a loop of arbitrary shape

The complete electromagnetic field of an arbitrary transient current distribution in a sphere can be represented by a superposition of a transverse electric and a transverse magnetic field. In this paper, the transient current distribution in a permeable and conducting hollow sphere is calculated for an exciting current of arbitrary time dependence which is fed into the sphere via the surface from an open loop of arbitrary shape. The original configuration is separated into two characteristic configurations whose current distributions are derived from a transverse electric and a transverse magnetic second order potential. The transverse magnetic field is determined by the position of the feeding points on the surface of the sphere. The transverse electric field is influenced only by the contour of the exciting loop in the nonconducting space. If the exciting loop is closed outside the hollow sphere, the transverse magnetic field vanishes and the solution for the transverse electric field can be used to describe the shielding of the transient electromagnetic field by a spherical screen. The general transient solution is proved for the special case of a sinusoidal time dependence and compared with the direct solution of the differential equation for the complex amplitudes of field quantities. An energy balance is introduced which describes the energy flow during the transient process.

Power-law fluid flow of a hydromagnetic free jet

A laminar two-dimensional viscous conductin free jet flow of a power-law non-Newtonian incompressible viscous fluid immersed in nonconducting space is studied. A constant magnetic field transverse to the axis of the jet is applied and the electrical conductivity is taken as an integral power of the axial velocity. The method of Sherbnin (1973) is used to solve the problem. Numerical values of the maximum velocity, the boundary-layer thickness and the discharge rate of the jet are calculated and represneted graphically.

Fiber optic link for digital audio interface

A description is given of a fiber optic link for digital audio interfaces that is able to transmit 3 Mb/s biphase signals up to 10 m with the pulse-width distortion at less than +or-25 ns. The fiber optic transmitting module consists of a GaAlAs light-emitting diode emitting at 660 nm wavelength, and a transmitting IC. These chips are mounted on a lead frame and molded by a clear resin. This molded unit is assembled into a receptacle that has a self-tapping hole for easy attachment to the rear panel of audio equipment. It also has preformed pins for presetting to the printed circuit board before soldering. The fiber-optic receiving module consists of a Si-p-i-n photodiode, and a receiving IC, and is assembled with these chips in the same way as the transmitting module. The conductive receptable electromagnetically shields the receiving module, but a nonconductive spacer attached to the receptacle electrically isolates it from rear panel. The link is intended for use in compact disk (CD) players, digital audio tape (DAT) recorders, video cassette recorders (VCRs), and digital receivers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>