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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.2015.10.076 ; In 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-R ; Junta de Andalucía, PE2012-FQM0694 ; European Union 7th Framework Programme (FP7/2007–2013) under agreement No. 256868

The following article has been submitted to/accepted by Journal of Renewable and Sustainable Energy. After it is published, it will be found at http://dx.doi.org/10.1063/1.4976326. ; Blue energy or salinity difference energy takes advantage of the free energy released in the mixture of two solutions with different salinity concentrations as it happens continuously in river mouths. Among the large number of available techniques that aim to harness blue energy, capmix (or capacitive mixing) methods allow to directly extract electrical energy without the need of any electromechanical converter such as turbines or heat engines. The main goal of this article is to analyze the potential of blue energy by capmix methods in Central America. So far, blue energy studies have been principally carried out in countries from the global North. Therefore, we describe experimental results with real sea and river waters from the Gulf of Fonseca, an area of special interest due to its hydrographic richness, which is situated among Honduras, El Salvador and Nicaragua. An electrochemical cell, which consists of a pair of activated carbon electrodes coated with cationic and anionic polyelectrolyte layers respectively, is used in the experiments. The cell voltage in open circuit (OCV) is used as a measure of the performance of the capmix process. It is found that the OCV is larger when natural river water is used instead of low salinity NaCl solutions. The rainy season in which the experiments were performed reduced the ionic content of the river, increasing the salinity difference with ocean waters. The feasibility of capmix as a means of clean energy production is discussed. ; Eureka SD Project (agreement number 2013-2591), Erasmus Mundus programme of the European Union. ; Ministerio de Economía y Competitividad of Spain (Project FIS2013-47666-C3- 1-R) ; Junta de Andalucía (Spain) project PE-2012- FQM0694 ; EU FEDER FUNDS

This is an unedited version of this paper. The publisher version can be reached in this URL: http://pubs.acs.org/doi/abs/10.1021/es500634f ; In recent years, the capacitance of the interface between charged electrodes and ionic solutions (the electric double layer) has been investigated as a source of clean energy. Charge is placed on the electrodes either by means of ion-exchange membranes or of an external power source. In the latter method, net energy is produced by simple solution exchange in open circuit, due to the associated decrease in the capacitance of the electric double layer. In this work, we consider the change in capacitance associated with temperature variations: the former decreases when temperature is raised, and, hence, a cycle is possible in which some charge is put on the electrode at a certain potential and returned at a higher one. We demonstrate experimentally that it is thus viable to obtain energy from electric double layers if these are successively contacted with water at different temperatures. In addition, we show theoretically and experimentally that temperature and salinity variations can be conveniently combined to maximize the electrode potential increase. The resulting available energy is also estimated. ; Departamento de Física Aplicada ; The research leading to these results received funding from the European Union 7th Frame-work Programme (FP7/2007-2013) under agreement No. 256868. Further Financial supports from Junta de Andalucía, project FQM 694, and Ministerio de Economía y Competitividad (Spain), project FIS2013-47666-C3-1-R.

Capacitive energy extraction based on double layer expansion (CDLE) is the name of a new method devised for extracting energy from the exchange of fresh and salty water in porous electrodes. It is based on the change of the capacitance of electrical double layers (EDLs) at the electrode/solution interface when the concentration of the bulk electrolyte solution is modified. The use of porous electrodes provides huge amounts of surface area, but given the typically small pore size, the curvature of the interface and EDL overlap should affect the final result. This is the first aspect dealt with in this contribution: we envisage the electrode as a swarm of spherical particles, and from the knowledge of their EDL structure, we evaluate the stored charge, the differential capacitance and the extracted energy per CDLE cycle. In all cases, different pore radii and particle sizes and possible EDL overlap are taken into account. The second aspect is the consideration of finite ion size instead of the usual point-like ion model: given the size of the pores and the relatively high potentials that can be applied to the electrode, excluded volume effects can have a significant role. We find an extremely strong effect: the double layer capacitance is maximum for a certain value of the surface potential. This is a consequence of the limited ionic concentration at the particle- solution interface imposed by the finite size of ions, and leads to the presence of two potential ranges: for low electric potentials the capacitance increases with the ionic strength, while for large potentials we find the opposite trend. The consequences of these facts on the possibility of net energy extraction from porous electrodes, upon changing the solution in contact with them, are evaluated. ; Departamento de Física Aplicada, Grupo de Física de Interfases y Sistemas Coloidales,Facultad de Ciencias, Universidad de Granada. ; The research leading to these results received funding from the European Union 7th Framework Programme (FP7/2007-2013) under Agreement No. 256868. Further financial support to S.A. from Junta de Andalucía (Spain) Project PE-2008-FQM3993.

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.2014.08.009 ; It 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 Aplicada ; The 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.

NOTICE: this is the author's version of a work that was accepted for publication in Journal of Power Sources. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Power Sources (2014) DOI:http://dx.doi.org/10.1016/j.jpowsour.2013.12.125 ; The capacitive mixing procedure for energy extraction based on Double Layer Expansion (CDLE) belongs to the group of so-called CAPMIX techniques, which aim at obtaining energy from the salinity difference between fresh and sea waters. Specifically, the CDLE technique takes advantage of the voltage rise that occurs when sea water is exchanged for river water in a pair of porous electrodes which jointly behave as an electrical double layer supercapacitor. In this article, we deal with some experimental aspects that are key for optimizing the extracted energy, and have not been analyzed yet with sufficient detail. This investigation will help in evaluating those parameters which we need to be fixed in a future CDLE device. These include the charging potential, the durations of the different cycle steps, the load resistance used, and the porosity and hydrophilicity of the carbon. ; Departamento de Física Aplicada, Universidad de Granada ; The research leading to these results received funding from the European Union 7th Framework Programme (FP7/2007-2013) under agreement No. 256868. Further financial support from Junta de Andalucía (Spain), project PE-2008-FQM3993 is also gratefully acknowledged.