Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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The RED Fouling Monitor : A novel tool for fouling analysis
Bodner, E.J. ; Saakes, M. ; Sleutels, T. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2019
Journal of Membrane Science 570-571 (2019). - ISSN 0376-7388 - p. 294 - 302.
Fouling analysis - Ion-exchange membranes - Organic fouling - RED Fouling Monitor - Salinity gradient

RED is a technology for harvesting energy using the salinity gradient between river (RW) and seawater (SW). Membrane fouling can decrease the net power density. Fouling inhibition might be indispensable. For implementing antifouling strategies more detailed insights upon fouling are required. In RED stacks investigations of single membranes are practically impossible. We introduce the RED Fouling Monitor, in which one side of a single ion-exchange membrane in contact to a foulant-containing feed stream can be studied under OCV and current conditions. Fouling is detectable in four configurations: (1) SW/AEM, (2) RW/AEM, (3) SW/CEM and (4) RW/CEM. Functionality is provided by a novel flow-through salt bridge enabling ionic connection and the incorporation of reference electrodes in close proximity to the membrane surface. The results indicate a stable, reproducible performance under un-fouled conditions. Upon SDBS exposure RW/AEM fouling showed a more pronounced fouling response than SW/AEM fouling. Fouling is partly attributable to the current density and the current field direction. An irreversible, internal fouling of the AEM is indicated when exposed to SDBS in SW. RW/AEM fouling shows to be reversible. With prospect to future systematic investigations this tool can be used to test various configurational, operational designs, different pre-treatment schemes and the fouling potential of feed streams at different seasons. This will result in valuable insights for new constructional sites for future RED plants.

Energy-Efficient Ammonia Recovery in an Up-Scaled Hydrogen Gas Recycling Electrochemical System
Kuntke, Philipp ; Rodrigues, Mariana ; Sleutels, Tom ; Saakes, Michel ; Hamelers, Hubertus V.M. ; Buisman, Cees J.N. - \ 2018
ACS sustainable chemistry & engineering 6 (2018)6. - ISSN 2168-0485 - p. 7638 - 7644.
Ammonia recovery - Electrochemical system - Hydrogen recycling - Up-scaling

Nutrient and energy recovery is becoming more important for a sustainable future. Recently, we developed a hydrogen gas recycling electrochemical system (HRES) which combines a cation exchange membrane (CEM) and a gas-permeable hydrophobic membrane for ammonia recovery. This allowed for energy-efficient ammonia recovery, since hydrogen gas produced at the cathode was oxidized at the anode. Here, we successfully up-scaled and optimized this HRES for ammonia recovery. The electrode surface area was increased to 0.04 m2 to treat up to 11.5 L/day (∼46 gN/day) of synthetic urine. The system was operated stably for 108 days at current densities of 20, 50, and 100 A/m2. Compared to our previous prototype, this new cell design reduced the anode overpotential and ionic losses, while the use of an additional membrane reduced the ion transport losses. Overall, this reduced the required energy input from 56.3 kJ/gN (15.6 kW h/kgN) at 50 A/m2 (prototype) to 23.4 kJ/gN (6.5 kW h/kgN) at 100 A/m2 (this work). At 100 A/m2, an average recovery of 58% and a TAN (total ammonia nitrogen) removal rate of 598 gN/(m2 day) were obtained across the CEM. The TAN recovery was limited by TAN transport from the feed to concentrate compartment.

Concentration Gradient Flow Batteries : salinity gradient energy systems as environmentally benign largescale electricity storage
Egmond, Willem Johannes van - \ 2018
Wageningen University. Promotor(en): Cees Buisman, co-promotor(en): H.V.M. Hamelers. - Wageningen : Wageningen University - ISBN 9789463438421 - 160
Performance of an environmentally benign acid base flow battery at high energy density
Egmond, W.J. van; Saakes, M. ; Noor, I. ; Porada, S. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2018
International Journal of Energy Research 42 (2018)4. - ISSN 0363-907X - p. 1524 - 1535.
acid base flow battery - bipolar membrane - co-ion transport - energy efficiency - ion exchange membranes - renewable energy storage - sustainable materials
An increasing fraction of energy is generated by intermittent sources such as wind and sun. A straightforward solution to keep the electricity grid reliable is the connection of large-scale electricity storage to this grid. Current battery storage technologies, while providing promising energy and power densities, suffer from a large environmental footprint, safety issues, and technological challenges. In this paper, the acid base flow battery is re-established as an environmental friendly means of storing electricity using electrolyte consisting of NaCl salt. To achieve a high specific energy, we have performed charge and discharge cycles over the entire pH range (0–14) at several current densities. We demonstrate stable performance at high energy density (2.9 Wh L−1). Main energy dissipation occurs by unwanted proton and hydroxyl ion transport and leads to low coulombic efficiencies (13%–27%).
(Bio)electrochemical ammonia recovery : progress and perspectives
Kuntke, P. ; Sleutels, T.H.J.A. ; Rodríguez Arredondo, M. ; Georg, S. ; Barbosa, S.G. ; Heijne, A. Ter; Hamelers, Hubertus V.M. ; Buisman, C.J.N. - \ 2018
Applied Microbiology and Biotechnology 102 (2018)9. - ISSN 0175-7598 - p. 3865 - 3878.
Ammonia recovery - Bioelectrochemical systems - Electrochemical systems - Total ammonia nitrogen - Wastewater treatment
In recent years, (bio)electrochemical systems (B)ES have emerged as an energy efficient alternative for the recovery of TAN (total ammonia nitrogen, including ammonia and ammonium) from wastewater. In these systems, TAN is removed or concentrated from the wastewater under the influence of an electrical current and transported to the cathode. Subsequently, it can be removed or recovered through stripping, chemisorption, or forward osmosis. A crucial parameter that determines the energy required to recover TAN is the load ratio: the ratio between TAN loading and applied current. For electrochemical TAN recovery, an energy input is required, while in bioelectrochemical recovery, electric energy can be recovered together with TAN. Bioelectrochemical recovery relies on the microbial oxidation of COD for the production of electrons, which drives TAN transport. Here, the state-of-the-art of (bio)electrochemical TAN recovery is described, the performance of (B)ES for TAN recovery is analyzed, the potential of different wastewaters for BES-based TAN recovery is evaluated, the microorganisms found on bioanodes that treat wastewater high in TAN are reported, and the toxic effect of the typical conditions in such systems (e.g., high pH, TAN, and salt concentrations) are described. For future application, toxicity effects for electrochemically active bacteria need better understanding, and the technologies need to be demonstrated on larger scale.
Tailoring ion exchange membranes to enable low osmotic water transport and energy efficient electrodialysis
Porada, S. ; Egmond, W.J. van; Post, J.W. ; Saakes, M. ; Hamelers, H.V.M. - \ 2018
Journal of Membrane Science 552 (2018). - ISSN 0376-7388 - p. 22 - 30.
Electrical resistance - Electrodialysis - Ion exchange membrane - Osmosis - Water desalination
Ion exchange membranes have been applied for water desalination since the 1950s in a process called electrodialysis, ED. Parallel to the transport of ions across ion exchange membranes, water molecules are transported from diluate to concentrate compartments reducing ED efficiency. In this study tailor made meshed membranes were prepared by embedding polymeric meshes with significantly reduced open area into an ion conductive polymer. These membranes were characterized to assess their transport properties. It is shown that by changing mesh open area, material and surface properties, it is possible to significantly reduce osmotic water transport. Polyamide mesh embedded in a cation exchange polymer showed an eightfold decrease of the water mass transport coefficient. Unexpectedly, osmotic water transport was not affected when the same mesh material was embedded in an anion exchange polymer. A decrease of the osmotic water transport for meshed anion exchange membranes was achieved by using a polyethylene terephthalate mesh. Despite the associated electrical resistance increase, application of meshed membranes increased diluate yield and allowed for more energy efficient operation in case ED is confined to a low current density regime.
Membrane Selectivity Determines Energetic Losses for Ion Transport in Bioelectrochemical Systems
Sleutels, Tom H.J.A. ; Heijne, Annemiek ter; Kuntke, Philipp ; Buisman, Cees J.N. ; Buisman, Cees J.N. ; Hamelers, Hubertus V.M. - \ 2017
ChemistrySelect 2 (2017)12. - ISSN 2365-6549 - p. 3462 - 3470.
BES - ion exchange membrane - MEC - MET - MFC
Ion transport through ion exchange membranes in Bioelectrochemical Systems (BESs) is different from other electrochemical cells as a result of the complex nature of the electrolyte, as the electrolytes in BESs contain many other cations and anions than H + and OH − . Moreover, these other cations and anions are generally present in high concentrations and therefore determine the ion transport through the membrane. In this work, we provide a theoretical framework for understanding ion transport across ion exchange membranes in BESs. We show that the transport of cations and anions other than H + and OH − determines the pH gradient between anode and cathode, and on top of that, also determines the membrane potential. Experimental data for microbial electrolysis cells with cation and anion exchange membranes are used to support the theoretical framework. In case of cation exchange membranes, the total potential loss consists of both the pH gradient and the concentration gradient of other cations, while in case of anion exchange membranes, the total potential loss is lower because part of the pH gradient loss can be recovered at the membrane. The presented work provides a better theoretical understanding of ion transport through ion exchange membranes in general and in BESs specifically.
Hydrogen Gas Recycling for Energy Efficient Ammonia Recovery in Electrochemical Systems
Kuntke, Philipp ; Rodríguez Arredondo, Mariana ; Widyakristi, Laksminarastri ; Heijne, Annemiek ter; Sleutels, Tom H.J.A. ; Hamelers, Hubertus V.M. ; Buisman, Cees J.N. - \ 2017
Environmental Science and Technology 51 (2017)5. - ISSN 0013-936X - p. 3110 - 3116.

Recycling of hydrogen gas (H2) produced at the cathode to the anode in an electrochemical system allows for energy efficient TAN (Total Ammonia Nitrogen) recovery. Using a H2 recycling electrochemical system (HRES) we achieved high TAN transport rates at low energy input. At a current density of 20 A m-2, TAN removal rate from the influent was 151 gN m-2 d-1 at an energy demand of 26.1 kJ gN -1. The maximum TAN transport rate of 335 gN m-2 d-1 was achieved at a current density of 50 A m-2 and an energy demand of 56.3 kJ gN -1. High TAN removal efficiency (73-82%) and recovery (60-73%) were reached in all experiments. Therefore, our HRES is a promising alternative for electrochemical and bioelectrochemical TAN recovery. Advantages are the lower energy input and lower risk of chloride oxidation compared to electrochemical technologies and high rates and independence of organic matter compared to bioelectrochemical systems. (Chemical Equation Presented).

Load ratio determines the ammonia recovery and energy input of an electrochemical system
Rodríguez Arredondo, Mariana ; Kuntke, Philipp ; Heijne, Annemiek Ter; Hamelers, Hubertus V.M. ; Buisman, Cees J.N. - \ 2017
Water Research 111 (2017). - ISSN 0043-1354 - p. 330 - 337.
Complete removal and recovery of total ammonia nitrogen (TAN) from wastewaters in (bio)electrochemical systems has proven to be a challenge. The system performance depends on several factors, such as current density, TAN loading rate and pH. The interdependence among these factors is not well understood yet: insight is needed to achieve maximum ammonium recovery at minimal energy input. The aim of this study was to investigate the influence of current density and TAN loading rate on the recovery efficiency and energy input of an electrochemical cell (EC). We therefore defined the load ratio, which is the ratio between the applied current and the TAN loading rate. The system consisted of an EC coupled to a membrane unit for the recovery of ammonia. Synthetic wastewater, with TAN concentration similar to urine, was used to develop a simple model to predict the system performance based on the load ratio, and urine was later used to evaluate TAN transport in a more complex wastewater. High fluxes (up to 433 gN m−2 d−1) and recovery efficiencies (up to 100%) were obtained. The simple model presented here is also suited to predict the performance of similar systems for TAN recovery, and can be used to optimize their operation.
In-situ carboxylate recovery and simultaneous pH control with tailor-configured bipolar membrane electrodialysis during continuous mixed culture fermentation
Arslan, D. ; Zhang, Y. ; Steinbusch, K.J.J. ; Diels, L. ; Hamelers, Hubertus V.M. ; Buisman, C.J.N. ; Wever, H. de - \ 2017
Separation and Purification Technology 175 (2017). - ISSN 1383-5866 - p. 27 - 35.
Bipolar membrane - Electrodialysis - Fermentation - ISPR - Short chain carboxylates

Anaerobic fermentation of organic waste streams by mixed culture generates a mixture of short chain carboxylic acids. To avoid inhibitory effects of the acids or their consumption in internal conversion reactions in the mixed culture environment, in-situ recovery of acids can be beneficial. In this study, electrodialysis with bipolar membranes (EDBM) was applied to a mixed culture fermentation on organic waste streams using a novel EDBM stack with “direct contact” operation mode. We could demonstrate simultaneous recovery of carboxylates from the fermenter by the EDBM stack while in-situ generation and transport of hydroxyl ions to the fermenter allowed direct pH control. Experiments showed productivity increase after EDBM coupling to the fermenter, and complete elimination of external base consumption. It was also observed that EDBM was able to drive the mixed culture fermentation towards acetate and propionate type of carboxylates.

Energy efficiency of a concentration gradient flow battery at elevated temperatures
Egmond, W.J. van; Starke, U.K. ; Saakes, M. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2017
Journal of Power Sources 340 (2017). - ISSN 0378-7753 - p. 71 - 79.
Aqueous electrolyte - Charge/discharge efficiency - Concentration gradient flow battery - Large scale electricity energy storage - Reverse electrodialysis - Stationary batteries

Fast growth of intermittent renewable energy generation introduces a need for large scale electricity storage. The Concentration Gradient Flow Battery (CGFB) is an emerging technology which combines Electrodialysis with Reverse Electrodialysis into a flow battery which is able to safely store very large amounts of energy in environmental friendly NaCl solutions. In this work, (dis)charge efficiency, energy density and power density are both theoretically and experimentally investigated. Fifteen constant current experiments (−47.5 to +37.5 A m−2) are performed at 40 °C and two experiments (−32.5 and 15 A m−2) at 10 and 25 °C. The magnitudes of the three main energy dissipation sources (internal resistance, water transport and co-ion transport) are measured and mitigation strategies are proposed. The effect of current density, state of charge and temperature on the dissipation sources is analysed. Water transport is shown to cause hysteresis, lower (dis)charge efficiencies and lower energy capacity. At constant current and with increasing temperature, internal resistance is reduced but unwanted water transport is increased. This study reports charge efficiencies up to 58% and discharge efficiencies up to 72%. Full charge or discharge of the battery is shown inefficient. The optimal operating range is therefore introduced and identified (concentration difference Δm > 0.5 and energy efficiency η > 0.4).

Gas-permeable hydrophobic membranes enable transport of CO2 and NH3 to improve performance of bioelectrochemical systems
Sleutels, Tom H.J.A. ; Hoogland, Biense ; Kuntke, P. ; Heijne, A. ter; Buisman, C.J.N. ; Hamelers, Hubertus V.M. - \ 2016
Environmental Science : Water Research & Technology 2 (2016). - ISSN 2053-1400 - p. 743 - 748.
Application of bioelectrochemical systems (BESs), for example for the production of hydrogen from organic waste material, is limited by a high internal resistance, especially when ion exchange membranes are used. This leads to a limited current density and thus to large footprint and capital costs. Ion transport between anode and cathode compartment is one of the factors determining the internal resistance. The aim of this study was to reduce the resistance for ion transport in a microbial electrolysis cell (MEC) through the ion exchange membrane by shuttling of CO2 and NH3 between anode and cathode. The transport of these chemical species was enabled through the use of a hydrophobic TransMembraneChemiSorption module (TMCS) that was placed between anolyte and catholyte circulation outside the cell. The driving force for transport was the pH difference between both solutions. The transport of CO2 and NH3 resulted in an increase in current density from 2.1 to 4.1 A m−2 for a cation exchange membrane (CEM) and from 2.5 to 13.0 A m−2 for an anion exchange membrane (AEM) at 1 V applied voltage. The increase in current density was the result of a lower ion transport resistance through the membrane; this resistance was 60% lower for the CEM, as a result of NH3 recycling from cathode to anode, and 82% for the AEM, as a result of CO2 recycling from anode to cathode with TMCS, compared to experiments without TMCS.
Gas-permeable hydrophobic tubular membranes for ammonia recovery in bio-electrochemical systems
Kuntke, P. ; Zamora, P. ; Saakes, M. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2016
Environmental Science : Water Research & Technology 2 (2016)2. - ISSN 2053-1400 - p. 261 - 265.

The application of a gas-permeable hydrophobic tubular membrane in bio-electrochemical systems enables efficient recovery of ammonia (NH3) from their cathode compartments. Due to a hydrogen evolution reaction at the cathode, no chemical addition was required to increase the pH for continuous NH3 recovery from wastewater.

Revisiting Morrison and Osterle 1965 : The efficiency of membrane-based electrokinetic energy conversion
Catalano, J. ; Hamelers, H.V.M. ; Bentien, A. ; Biesheuvel, Maarten - \ 2016
Journal of Physics-Condensed Matter 28 (2016)32. - ISSN 0953-8984
charged nanopore - desalination - electrokinetic energy conversion

We revisit Morrison and Osterle (1965) who derived a phenomenological expression for the 'figure-of-merit' βEK of the electrokinetic energy conversion (EKEC) of a pressure difference into electric energy (and vice versa) using charged nanotubes, nanopores or ion-exchange membranes. We show the equivalence with Morrison and Osterle of a novel expression of βEK derived by Bentien et al (2013). We analyze two physical models for ionic and solvent flow which directly relate βEK to nanopore characteristics such as pore size and wall charge density. For the uniform potential model, we derive an analytical expression as a function of pore size, viscosity, ion diffusion coefficients and membrane charge density, and compare results with the full space-charge model by Osterle and co-workers as a function of pore size and ion diffusion coefficient. We present a novel expression for βEK for salt solutions with ions with unequal diffusion coefficients (mobilities) and show that to increase βEK the counterion mobility must be low and the coion mobility high.

The concentration gradient flow battery as electricity storage system : Technology potential and energy dissipation
Egmond, W.J. Van; Saakes, M. ; Porada, S. ; Meuwissen, T. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2016
Journal of Power Sources 325 (2016). - ISSN 0378-7753 - p. 129 - 139.
Aqueous based battery - Flow batteries - Ion-exchange membranes - Large scale electricity energy storage - Reverse electrodialysis - Salinity gradient energy

Unlike traditional fossil fuel plants, the wind and the sun provide power only when the renewable resource is available. To accommodate large scale use of renewable energy sources for efficient power production and utilization, energy storage systems are necessary. Here, we introduce a scalable energy storage system which operates by performing cycles during which energy generated from renewable resource is first used to produce highly concentrated brine and diluate, followed up mixing these two solutions in order to generate power. In this work, we present theoretical results of the attainable energy density as function of salt type and concentration. A linearized Nernst-Planck model is used to describe water, salt and charge transport. We validate our model with experiments over wide range of sodium chloride concentrations (0.025-3 m) and current densities (-49 to +33 A m-2). We find that depending on current density, charge and discharge steps have significantly different thermodynamic efficiency. In addition, we show that at optimal current densities, mechanisms of energy dissipation change with salt concentration. We find the highest thermodynamic efficiency at low concentrate concentrations. When using salt concentrations above 1 m, water and co-ion transport contribute to high energy dissipation due to irreversible mixing.

Selective short-chain carboxylates production : A review of control mechanisms to direct mixed culture fermentations
Arslan, D. ; Steinbusch, K.J.J. ; Diels, L. ; Hamelers, H.V.M. ; Strik, D.P.B.T.B. ; Buisman, C.J.N. ; Wever, H. De - \ 2016
Critical Reviews in Environmental Science and Technology 46 (2016)6. - ISSN 1064-3389 - p. 592 - 634.
Biomass conversion - carboxylates - mixed culture fermentation - operational parameters - organic waste - volatile fatty acids

Anaerobic digestion of organic residual streams can be directed to produce carboxylates such as acetate, propionate, and n-butyrate, which can be either directly used in industry or converted into other valuable compounds. This paper reviews the studies working with mixed culture conversion of organic substrates toward carboxylates. It draws connections between standard fermentation parameters and the carboxylate product concentrations and composition. The use of more concentrated carbohydrate-rich substrates, at longer residence times and at neutral pH ranges, stimulates total acid production. When increasing pH to the neutral range, acetate and propionate fractions are elevated. High propionate concentrations and fractions are infrequently reported and mainly appear on high-protein-containing feedstock. High n-butyrate fraction <70% is usually found when pH > 6, at longer retention times or organic loading rates, under CO2 atmosphere or on substrates with high lactate concentrations. The review concludes with prospects for further developments related to the carboxylate platform.

On-line method to study dynamics of ion adsorption from mixtures of salts in capacitive deionization
Dykstra, J.E. ; Dijkstra, J. ; Wal, A. van der; Hamelers, H.V.M. ; Porada, S. - \ 2016
Desalination 390 (2016). - ISSN 0011-9164 - p. 47 - 52.
Capacitive Deionization - Electrosorption - Ionic mixtures - Salt removal - Selective ion removal

Capacitive Deionization (CDI) is a water desalination technology that adsorbs ions into two oppositely polarized porous carbon electrodes, under the action of an applied voltage. Here, we introduce a novel method to analyze the effluent concentration of multiple ionic species in mixtures of salt solutions by directing the outflow of a CDI cell to an inductively coupled plasma optical emission spectroscopy (ICP-OES) instrument. Compared to previous methods based on manual sampling, the on-line use of ICP-OES allows collecting more accurate time-dependent ion adsorption data, and therefore, ion dynamics can be studied even at very short half-cycle times. We use this method to study ion adsorption from a mixed solution containing two monovalent cations with similar radius, namely potassium and sodium. We find that potassium ions are preferentially adsorbed over sodium ions, due to their higher mobility. Furthermore, we compare our experimental findings with a novel multicomponent electromigration model that calculates dynamic adsorption of ions from solutions of multiple salts. Whereas we find good agreement between data and theory at low half cycle times, we observe a considerable discrepancy at higher values.

Chain Elongation with Reactor Microbiomes: Open-Culture Biotechnology To Produce Biochemicals
Angenent, L.T. ; Richter, H. ; Buckel, W. ; Spirito, C.M. ; Steinbusch, K.J.J. ; Plugge, C.M. ; Strik, D.P.B.T.B. ; Grootscholten, T.I.M. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2016
Environmental Science and Technology 50 (2016)6. - ISSN 0013-936X - p. 2796 - 2810.
Chain elongation into medium-chain carboxylates, such as n-caproate and n-caprylate, with ethanol as an electron donor and with open cultures of microbial consortia (i.e., reactor microbiomes) under anaerobic conditions is being developed as a biotechnological production platform. The goal is to use the high thermodynamic efficiency of anaerobic fermentation to convert organic biomass or organic wastes into valuable biochemicals that can be extracted. Several liter-scale studies have been completed and a first pilot-plant study is underway. However, the underlying microbial pathways are not always well understood. In addition, an interdisciplinary approach with knowledge from fields ranging from microbiology and chemical separations to biochemistry and environmental engineering is required. To bring together research from different fields, we reviewed the literature starting with the microbiology and ending with the bioprocess engineering studies that already have been performed. Because understanding the microbial pathways is so important to predict and steer performance, we delved into a stoichiometric and thermodynamic model that sheds light on the effect of substrate ratios and environmental conditions on product formation. Finally, we ended with an outlook
Parallel up-scaling of Capacitive Mixing (CapMix) system enhances the specific performance
Liu, Fei ; Donkers, Tim F.W. ; Wagterveld, R.M. ; Schaetzle, Olivier ; Saakes, Michel ; Buisman, Cees J.N. ; Hamelers, Hubertus V.M. - \ 2016
Electrochimica Acta 187 (2016). - ISSN 0013-4686 - p. 104 - 112.
Capacitive Donnan Potential - Capacitive Mixing - Ion exchange membrane - Salinity gradient energy - Wire-shaped electrode

Given the considerable amount of energy dissipated in the salinity gradient at the point where a river flows into the sea, we investigate the technology of capacitive mixing (CapMix), an attractive technology for generating electrical power from this gradient. We determine the performances of multiple wire-shaped electrode pairs connected either in series or in parallel in a CapMix system. The bundles of pairs were immersed in synthetic river and seawater. Pairs connected in parallel and placed next to each other allowed for 18% more energy extraction than the total energy extracted by the same number of pairs individually. An even higher additional energy gain is possible if contact resistances are further minimized. The improvement is due to the additional flow paths for ions between electrode pairs in parallel connection, reducing the total internal resistance. The highest power density achieved (in terms of the mass of activated carbon material used) was 2.7 mW/g, which was higher than the power densities that have been achieved previously using a flat plate CapMix cell (1.26 mW/g) and a wire electrode cell (0.34 mW/g). The lower ohmic resistance in the parallel system was identified using a current distribution model and experimental measurements.

Theory of Water Desalination by Porous Electrodes with Immobile Chemical Charge
Biesheuvel, P.M. ; Hamelers, H.V.M. ; Suss, M.E. - \ 2015
Colloids and Interface Science Communications 9 (2015). - ISSN 2215-0382 - p. 1 - 5.

In capacitive deionization (CDI), water is desalinated by storing ions in electrical double layers (EDLs) within the micropores of charged porous carbon electrodes. Recent experiments using chemically modified electrodes have shown differing, novel phenomena such as "inverted CDI," "enhanced CDI," and "inversion peaks." We here present an EDL and dynamic model which includes immobile chemical charge in the micropores and show that the models predict these disparate experimental observations. Our model also makes predictions for a previously undiscovered operational regime with higher salt adsorption, which we term extended voltage CDI.

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