Assembly of Soft Electrodes and Ion Exchange Membranes for Capacitive Deionization

The responsible use of water, as well as its reuse and purification, has been a major problem for decades now. In this work, we study a method for adsorbing ions from aqueous solutions on charged interfaces using highly porous electrodes. This water purification process is based on the electric double layer concept, using the method known as capacitive deionization (CDI): If we pump salty solutions through the volume comprised between two porous electrodes while applying a potential difference to them, ions present in the solution are partially removed and trapped on the electrode surfaces. It has been well established that the use of carbon electrodes in combination with ion exchange membranes (membrane-CDI) improves the efficiency of the method above that achieved with bare activated carbon. Another approach that has been tested is based on coating the carbon with polyelectrolyte layers, converting them into “soft electrodes” (SEs). Here we investigate the improvement found when combining SEs with membranes, and it is shown that the amount of ions adsorbed and the ratio between ions removed and electrons transported reach superior values, also associated with a faster kinetics of the process. The method is applied to the partial desalination of up to 100 mM NaCl solutions, something hardly achievable with bare or membrane-covered electrodes. A theoretical model is presented for the ion transport in the presence of both the membrane and the polyelectrolyte coating.This research was funded by MINECO, Spain, grant number PGC2018-098770-B-I00 and RYC-2014-1690

Stacking of capacitive cells for electrical energy production by salinity exchange

Publisher's link: http://www.sciencedirect.com/science/article/pii/S0378775316303676?np=yHa estado embargado durante 1 año, hasta julio 2017In this paper we explore methods for stacking individual cells in order to increase the amount of energy that can be extracted from salinity gradient cycles (capmix methods). Each of the cells consists of a pair of parallel electrodes made of activated carbon particles, either bare or coated by a layer of polyelectrolyte. In these methods, energy is produced based on the modifications in the electrical double layer (EDL) structure in the pores of the carbon particles upon exchange of the salinity of the surrounding medium. In the case of the bare carbon particles, the electrodes are externally charged in presence of high-salt concentration, and discharged after exchanging the solution for a dilute one (sea- vs. river-water cycles). In a first stacking approach, we simply connect the electrodes side-by-side and only the outermost ones can be connected to the external source. Using a stack where potential and current between cells can be measured, it is shown that only the external electrodes seem to work, and it is suggested that this is a consequence of internal short-circuit currents between oppositely charged electrodes of different cells through the electrolyte. In contrast, the side-by-side configuration is operational if coated electrodes are used.MICINN, Spain (Project FIS2013-47666-C3-1R)RYC-2014-16901 (MINECO)FEDER Funds E

Modification of the surface of activated carbon electrodes for capacitive mixing energy extraction from salinity differences

This is an unedited version of this article. The publisher's edited version cab reached in this URL: http://www.sciencedirect.com/science/article/pii/S0021979714006274#The reference for this article is: Marino et al., Journal of Colloid and Interface Science 436(2014) 146-153.The “capacitive mixing” (CAPMIX) is one of the techniques aimed at the extraction of energy from the salinity difference between sea and rivers. It is based on the rise of the voltage between two electrodes, taking place when the salt concentration of the solution in which they are dipped is changed. We study the rise of the potential of activated carbon electrodes in NaCl solutions, as a function of their charging state. We evaluate the effect of the modification of the materials obtained by adsorption of charged molecules. We observe a displacement of the potential at which the potential rise vanishes, as predicted by the electric double layer theories. Moreover, we observe a saturation of the potential rise at high charging states, to a value that is nearly independent of the analyzed material. This saturation represents the most relevant element that determines the performances of the CAPMIX cell under study; we attribute it to a kinetic effect.Departamento de Física Aplicad

Polylectrolyte- versus membrane-coated electrodes for energy production by Capmix salinity exchange methods

La versión final publicada se puede encontrar en: Journal of Power Sources, 302(20): 387-393 (2016). http://dx.doi.org/10.1016/j.jpowsour.2015.10.076In this paper we analyze the energy and power achievable by means of a re- cently proposed salinity gradient technique for energy production. The method, denominated soft electrode or SE, is based on the potential di erence that can be generated between two porous electrodes coated with cationic and anionic polyelectrolytes. It is related to the Capacitive Donnan Potential (CDP) tech- nique, where the electrical potential variations are mostly related to the Donnan potential, of ion-selective membranes in the case of CDP, and of the polyelec- trolyte coating in SE. It is found that although SE is comparable to CDP in terms of energy production, it presents slower rates of voltage change, and lower achieved power. The separate analysis of the response of positively and neg- atively coated electrodes shows that the latter produces most of the voltage rise and also of the response delay. These results, together with electrokinetic techniques, give an idea on how the two types of polyelectrolytes adsorb on the carbon surface and a ect di erently the di usion layer. It is possible to suggest that the SE technique is a promising one, and it may overcome the drawbacks associated to the use of membranes in CDP.MINECO FIS2013-47666-C3-1-RJunta de Andalucía, PE2012-FQM0694European Union 7th Framework Programme (FP7/2007–2013) under agreement No. 25686

Polyelectrolyte-coated carbons used in the generation of blue energy from salinity differences

This is the unedited version of this manuscript. The publisher's edited version can be reached at this URL: http://pubs.rsc.org/en/content/articlelanding/2014/cp/c4cp03527e#!divAbstractIn this work we present a method for the production of clean, renewable electrical energy from the exchange of solutions with different salinities. Activated carbon films are coated with negatively or positively charged polyelectrolytes by well-established adsorption methods. When two oppositely charged coated films are placed in contact with an ionic solution, the potential difference between them will be equal to the difference between their Donnan potentials, and hence, energy can be extracted by building an electrochemical cell with such electrodes. A model is elaborated on the operation of the cell, based on the electrokinetic theory of soft particles. All the features of the model are experimentally reproduced, although a small quantitative difference concerning the maximum opencircuit voltage is found, suggesting that the coating is the key point to improve the efficiency. In the used experimental conditions, we obtain a power of 12.1 mW/m2. Overall, the method proves to be a fruitful and simple approach to salinity-gradient energy production. Obtaining energy from salinity differences asDepartamento de Física AplicadaThe research leading to these results received funding from the European Union 7th Framework Programme (FP7/2007-2013) under agreement No. 256868. Further financial sup- ports from Junta de Andalucía, project PE2012-FQM694, and Ministerio de Economía y Competitividad (Spain), project FIS2013-47666-C3-1-R

Multi-ionic effects on energy production based on double layer expansion by salinity exchange

The edited version of the manuscript can be obtained at: Delgado et al., J. Colloid Interface Science (2014); DOI http://dx.doi.org/10.1016/j.jcis.2014.08.009It has been recently shown that the free energy change upon salinity mixing in river mouths can be harvested taking advantage of the fact that the capacitance of charged solid/liquid interfaces (electrical double layers, EDLs) depends strongly on the ionic composition of the liquid medium. This has led to a new generation of techniques called Capmix technologies, one of them (CDLE or Capacitive energy extraction based on DL Expansion) based precisely on such dependence. Despite the solution composition playing a crucial role on the whole process, most of the research carried out so far has mainly focused on pure sodium chloride solutions. However, the effect of other species usually present in river and sea waters should be considered both theoretically and experimentally in order to succeed in optimizing a future device. In this paper, we analyse solutions of a more realistic composition from two points of view. Firstly, we find both experimentally and theoretically that the presence of ions other than sodium and chloride, even at low concentrations, may lead to a lower energy extraction in the process. Secondly, we experimentally consider the possible effects of other materials usually dispersed in natural water (mineral particles, microbes, shells, pollutants) by checking their accumulation in the carbon films used, after being exposed for a long period to natural sea water during CDLE cycles.Universidad de Granada. Departamento de Física AplicadaThe research leading to these results received funding from the European Union 7th Framework Programme (FP7/2007-2013) under agreement No. 256868. Financial support from Junta de Andalucia (Project PE2012-FQM 694) and MINECO (Project FIS2013-4766-C3-1-R) is also acknowledged. One of us, M.M.F., is grateful to the University of Granada for her FPU grant

Propiedades electrocinéticas de suspensiones coloidales concentradas

Contiene un amplio resumen en inglésTesis Univ. Granada. Departamento de Física Aplicada. Leída el 14 de abril de 200

Electrokinetic detection of the salt-free condition in colloids. Application to polystyrene latexes

Financial support from Ministerio de Ciencia, Innovación y Universidades (Spain) ( GC2018-098770-B-I00 ), and Junta de Andalucía (Spain) and European Funds for Regional Development ( BF-FQM-141-UGR18 , PI20-00233 ) is gratefully acknowledged.Financial support from Ministerio de Ciencia, Innovación y Universidades (Spain) (GC2018-098770-B-I00), and Junta de Andalucía (Spain) and European Funds for Regional Development (BF-FQM-141-UGR18, PI20-00233) is gratefully acknowledged.Because of their singular phenomenology, the so-called salt-free colloids constitute a special family of dispersed systems. Their main characteristic is that the dispersion medium ideally contains only the solvent and the ions compensating exactly the surface charge of the particles. These ions (often called released counterions) come into the solution when the surface groups responsible for the particles charge get ionized. An increasing effort is nowadays dedicated to rigorously compare theoretical model predictions for ideal salt-free suspensions, where only the released counterions are supposed to be present in solution, with appropriately devised experiments dealing with colloids as close as possible to the ideal salt-free ones. Of course, if the supporting solution is aqueous, the presence of atmospheric contamination and any other charged species different from the released counterions in the solution must be avoided. Because this is not an easy task, the presence of dissolved atmospheric CO2 and of H+ and OH− from water dissociation cannot be fully discarded in aqueous salt-free solutions (often denominated realistic in such case). Ultimately, at some point, the role of the released counterions will be comparable or even larger in highly charged concentrated colloids than that of added salts. These topics are covered in the present contribution. The model results are compared with experimental data on the dynamic mobility and dielectric dispersion of polystyrene spheres of various charges and sizes. As a rule, it is found that the model correctly predicts the significance of alpha and Maxwell-Wagner-O'Konski relaxations. Positions and amplitudes of such relaxations are well predicted, although it is necessary to assume that the released counterions are potassium or sodium instead of protons, otherwise the frequency spectra of experimental mobility and permittivity differ very significantly from those theoretically calculated. The proposed electrokinetic evaluation is an ideal tool for detecting in situ the possible contamination (or incomplete ion exchange of the latexes). A satisfactory agreement is found when potassium counterions are assumed to be in solution, mostly if one considers that the comparison is carried out without using any adjustable parameters.Ministerio de CienciaMinisterio de Ciencia, Innovación y Universidades GC2018-098770-B-I00European Regional Development Fund BF-FQM-141-UGR18, PI20-00233Junta de Andalucí

Polymer-induced orientation of nanowires under electric fields

The controlled orientation of metallic wires inside a polymeric medium can enhance desired properties of the composites, such as the electrical conductivity or the optical transmittance. In this work, we study silver nanowire orientation in semidilute suspensions of DNA and find an intriguing effect: under the application of low-frequency AC electric fields with moderate amplitude, the DNA coils can provoke the orientation of the wires in solution. The phenomenon is entirely induced by the polymer, when it is deformed by the application of an electric field. This effect is explained using computer simulations based on excluded volume interactions. Moreover, we experimentally show that such a behaviour is not exclusive of silver nanowire-DNA suspensions, but rather occurs for other particle-polymer systems. This phenomenon can be taken advantage of to achieve strong orientation of particles otherwise insensitive to electric fields

Effect of coating nanostructure on the electrokinetics of polyelectrolyte-coated particles. Grafted vs adsorbed polymer

In this work, the electrokinetic response of nanoparticles suspensions under the action of alternating electric fields is analyzed for the case that the particles are coated with a shell of polyelectrolyte. This is an important field of application of colloidal systems, as the coating is necessary or even unavoidable for control of the stability or functionalization of the particles for a specific use. Characterization of the coating in situ is not an easy task, and electrokinetics can help in answering this question, if one goes beyond the simple routine evaluation of the electrophoretic mobility of the particles, which will provide information on the sign (and perhaps a sort of effective amount) of the surface charge and potential, but little more. The richness and rigour of the information is much more significant if AC fields are used. This is the case for the techniques evaluated in the present investigation, namely, AC electrophoresis and dielectric dispersion. They jointly sweep several decades in frequency, and are sensitive in different manners to the size of the particles, the charge of the core and the shell, the thickness and rigidity of the latter, etc. In addition, an important distinction is made between coatings with a soft or adsorbed layer structure and those with a grafted, ideally radially arranged (brush-like) polyelectrolyte. A model is first described for the AC electrokinetics in both cases, and it is demonstrated that the brush structure magnifies the Maxwell-Wagner (or double layer) polarization, leading to an elevation in the AC mobility for frequencies around the MHz, and it very much raises the amplitude of the alpha- or concentration-polarization relaxation detected in the dielectric dispersion. The distribution of the charge in a thicker region for the brush structure explains these results. Experimental investigations are carried out with silica spheres coated by PDADMAC (+) and PSS (-) polyelectrolytes in the soft-layer case, and by (vinylbenzyl)trimethylammo nium chloride (+) and sodium 4-vinylbenzenesulfonate (-) monomers that were polymerized (grafted) on the particles. The results show that the brush coating produces the expected MW elevation of the mobility, mostly in the case of the cationic polymer, apparently better attached to the particles by electrostatic attraction to the negative charge of the core silica particles. In contrast, a rather monotonous decrease of the mobility in absolute value is measured for the soft coatings, indicating that the inertia of the particles sets out at lower frequencies because of aggregation. Dielectric spectra confirm the better stability in the presence of the grafted polymer, although a low-frequency elevation in the logarithmic derivative of the permittivity is a proof of the existence of aggregates, less abundant in any case than for soft coatings. Dielectric data also confirm the different amounts of charge, larger for grafted cationic layers than for anionic ones. Finally, the model elaborated can fit the experimental results yielding quantitative values of the main parameters of the coated particles, namely, effective size, overall charge of the coating, and thickness and ionic permeability of the latter.FEDER/Junta de Andalucia-Consejeria de Transformacion Econmica, Industria, Conocimiento y Universidades PI20-00233Proyectos I + D + i, Programa Operativo FEDER AndaluciaERDF/Ministry of Science and Innovation -State Research AgencyUniversidad de Granada/CBUA TED2021-131855B-I00 A.FQM.492.UGR2