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Reverse osmosis (RO) is the most important membrane technology for the desalination of water. Measured water and salt fluxes are traditionally analyzed in the context of the solution-diffusion (SD) model which leads to a water permeability, A, and a salt permeability, B. However, this parametrization of the salt flux is not correct for water desalination by RO membranes, because these membranes show markedly different retentions for different feed salt concentrations, a classical observation in the literature, and this effect is not captured by the SD model. Thus, the traditional salt permeability B is not an intrinsic property of these membranes. We present a new analysis for desalination of a 1:1 salt, which follows from a transport theory that is based on the assumption that coions are strongly excluded from the membrane, and we demonstrate that it accurately describes a large dataset of salt retention by an RO membrane as function of pressure and feed salt concentration. This analysis leads to unique values of the water and salt permeabilities, A and B″, not dependent on salt concentration or permeate water flux. Because we now have an improved parametrization, we can more accurately compare different membranes or study in more detail how membrane performance depends on conditions such as salt type and temperature. The new equation can provide guidance for the design of high-performance desalination membranes and for process modeling of desalination systems.

Ion selectivity is regarded as the ability of a system to remove specific ionic species from multi-ionic mixtures. In this PhD thesis, two electrochemical systems, namely capacitive deionization (CDI) and electrodialysis (ED), were studied for their potential to achieve selective ion separations relevant to a wide variety of applications. One such application is the removal of the most common chemical contaminant found in groundwater: nitrate. This PhD thesis combines the study of the selective separation of monovalent ions in CDI and ED. The research comprises a systematic evaluation of variables that influence ion selectivity (ion concentration, applied electrical potential, presence of competing ions). Additionally, theoretical frameworks were developed to describe ion adsorption in porous carbon electrodes and ion transport in ion-exchange membranes (IEMs).Overall, results obtained in this PhD thesis indicate that selectivity results from the interplay between the properties of the ions, the properties of the main constituents of the electrochemical system, i.e., carbon electrodes in CDI and IEMs in ED, and operational parameters.This thesis shows that during electrosorption, ion selectivity is time-dependent, which indicates that the electrical double layers (EDLs) constantly rearrange their structure. Overall, at equilibrium (when there is no transport of ions through the carbon pores), NO3 selectivity does not depend on the initial ion concentration in the bulk solution, but it does depend on the applied cell voltage. It is assumed that selectivity is not the result of rearrangements of the hydration shell, e.g., partial loss of water molecules, of the ion with lower hydration energy, i.e., NO3. The reasoning behind this assumption is that the mean size of the carbon pores is larger than the hydration shells of the adsorbed ions.  Ion selectivity was also studied in newly-designed heterogeneous anion-exchange membranes (AEMs). The membranes were fabricat

Selective transport of specific ions across ion-exchange membranes can be enhanced by controlling membrane properties such as hydrophobicity. Previous studies have shown that hydrophobic membranes enhance transport of ions with low hydration energy, although such membranes often have increased electrical resistance. In the present work, we study the separation of monovalent ions, specifically nitrate and chloride, using newly-designed heterogeneous anion-exchange membranes. These membranes show high selectivity for nitrate over chloride and have low electrical resistance. We use a functionalized polymeric binder (ionomer) and three ion-exchange resins with different hydrophobic groups, i.e., resins with quaternary ammonium groups and methyl, ethyl, and propyl substituents, respectively. We find that in electrolyte solutions with nitrate and chloride, nitrate over chloride selectivity in our membranes increases with increasing length of the alkyl groups. The membrane with propyl groups, i.e., which has the highest selectivity for nitrate, was further tested in electrolyte solutions containing nitrate, chloride, sulfate, and nitrate, chloride, iodate. The transport of sulfate and iodate ions across the membrane with propyl groups was 6% and 2% of the total counterions transport, respectively. For monovalent ions with similar hydrated size it is possible to report selectivity trends based on the ion hydration energy. We find that the chemical structure of the membrane can either promote or hinder the transport of ionic species.

In the field of Capacitive Deionization (CDI), it has become a common notion that constant current (CC) operation consumes significantly less energy than constant voltage operation (CV). Arguments in support of this claim are that in CC operation the endpoint voltage is reached only at the end of the charging step, and thus the average cell voltage during charging is lower than the endpoint voltage, and that in CC operation we can recover part of the invested energy during discharge. Though these arguments are correct, in the present work based on experiments and theory, we conclude that in operation of a well-defined CDI cycle, this does not lead, for the case we analyze, to the general conclusion that CC operation is more energy efficient. Instead, we find that without energy recovery there is no difference in energy consumption between CC and CV operation. Including 50% energy recovery, we find that indeed CC is more energy efficient, but also in CV much energy can be recovered. Important in the analysis is to precisely define the desalination objective function, such as that per unit total operational time –including both the charge and discharge steps– a certain desalination quantity and water recovery must be achieved. Another point is that also in CV operation energy recovery is possible by discharge at a non-zero cell voltage. To aid the analysis we present a new method of data representation where energy consumption is plotted against desalination. In addition, we propose that one must analyze the full range of combinations of cycle times, voltages and currents, and only compare the best cycles, to be able to conclude which operational mode is optimal for a given desalination objective. We discuss three methods to make this analysis in a rigorous way, two experimental and one combining experiments and theory. We use the last method and present results of this analysis.

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%).

Over the past decade, capacitive deionization (CDI) has realized a surge inattention in the field of water desalination and can now be considered as animportant technology class, along with reverse osmosis and electrodialysis.While many of the recently developed technologies no longer use a mechanismthat follows the strict definition of the term "capacitive", these methodsnevertheless share many common elements that encourage treating them withsimilar metrics and analyses. Specifically, they all involve electricallydriven removal of ions from a feed stream, storage in an electrode (i.e., ionelectrosorption) and release, in charge/discharge cycles. Grouping all thesemethods in the technology class of CDI makes it possible to treat evolving newtechnologies in standardized terms and compare them to other technologies inthe same class.

Capacitive deionization (CDI) is an emerging technology for the facile removal of charged ionic species from aqueous solutions, and is currently being widely explored for water desalination applications. The technology is based on ion electrosorption at the surface of a pair of electrically charged electrodes, commonly composed of highly porous carbon materials. The CDI community has grown exponentially over the past decade, driving tremendous advances via new cell architectures and system designs, the implementation of ion exchange membranes, and alternative concepts such as flowable carbon electrodes and hybrid systems employing a Faradaic (battery) electrode. Also, vast improvements have been made towards unraveling the complex processes inherent to interfacial electrochemistry, including the modelling of kinetic and equilibrium aspects of the desalination process. In our perspective, we critically review and evaluate the current state-of-the-art of CDI technology and provide definitions and performance metric nomenclature in an effort to unify the fast-growing CDI community. We also provide an outlook on the emerging trends in CDI and propose future research and development directions.

The recently developed modified Donnan (mD) model provides a simple and useful description of the electrical double layer in microporous carbon electrodes, suitable for incorporation in porous electrode theory. By postulating an attractive excess chemical potential for each ion in the micropores that is inversely proportional to the total ion concentration, we show that experimental data for capacitive deionization (CDI) can be accurately predicted over a wide range of applied voltages and salt concentrations. Since the ion spacing and Bjerrum length are each comparable to the micropore size (few nanometers), we postulate that the attraction results from fluctuating bare Coulomb interactions between individual ions and the metallic pore surfaces (image forces) that are not captured by mean-field theories, such as the Poisson-Boltzmann-Stern model or its mathematical limit for overlapping double layers, the Donnan model. Using reasonable estimates of the micropore permittivity and mean size (and no other fitting parameters), we propose a simple theory that predicts the attractive chemical potential inferred from experiments. As additional evidence for attractive forces, we present data for salt adsorption in uncharged microporous carbons, also predicted by the theory., Denmark. Ministry of Infrastructure and the Environment (Wetsus), Denmark. Ministry for Economic Affairs (Wetsus), European Union (Regional Development Fund), Denmark. Province of Fryslân, Denmark (Northern Netherlands Provinces)

Capacitive technologies, such as capacitive deionization and energy harvesting based on mixing energy (“capmix” and “CO2 energy”), are characterized by intermittent operation: phases of ion electrosorption from the water are followed by system regeneration. From a system application point of view, continuous operation has many advantages, to optimize performance, to simplify system operation, and ultimately to lower costs. In our study, we investigate as a step towards second generation capacitive technologies the potential of continuous operation of capacitive deionization and energy harvesting devices, enabled by carbon flow electrodes using a suspension based on conventional activated carbon powders. We show how the water residence time and mass loading of carbon in the suspension influence system performance. The efficiency and kinetics of the continuous salt removal process can be improved by optimizing device operation, without using less common or highly elaborate novel materials. We demonstrate, for the first time, continuous energy generation via capacitive mixing technology using differences in water salinity, and differences in gas phase CO2 concentration. Using a novel design of cylindrical ion exchange membranes serving as flow channels, we continuously extract energy from available concentration differences that otherwise would remain unused. These results may contribute to establishing a sustainable energy strategy when implementing energy extraction for sources such as CO2-emissions from power plants based on fossil fuels.

In a recent study, Wimalasiri and Zou [1] have reported the use and performance of composite electrodes of carbon nanotubes (CNT) and graphene for application as porous electrodes in capacitive deionization (CDI). While CDI is emerging as an attractive technology for water desalination, and novel electrode materials and composites are important contributions to the advancement of the field, there are several issues in this study that we must comment on. We first address the capacitive deionization (CDI) performance reported by Wimalasiri and Zou [1], namely an adsorption of NaCl in the composite electrodes of 26.42 mg/g at a cell voltage of Vcell = 2.0 V. This value is approximately one order of magnitude higher than what has been reported for this kind of composite material in 2012 by Zhang et al. [2], namely 1.4 mg/g at Vcell = 2.0 V, and very recently by Li et al. [3], namely 0.9 mg/g at Vcell = 1.6 V. The difference in performance is in contrast to the very similar synthesis route of all three studies (i.e., reduction of graphene oxide/CNT composite) and the comparable pore characteristics of the composite electrode (Zhang et al.: 480 m2/g; Li. et al.: up to 450 m2/g; Wimalasiri and Zou: 391 m2/g). As these two examples show, in previous literature, values for salt adsorption in electrodes composed of graphene, CNTs, or composites thereof, are all very close and in the low range 1–3 mg/g, defined per g of both electrodes combined, see Table 1. Such moderate performance is in line with expectations, because the specific surface area of these materials is moderate (i.e., a few hundred m2/g). In contrast, the much higher salt adsorption capacity in CDI known for microporous carbons occurs in conjunction with high specific surface areas in excess of 1000 m2/g [4] and [5]. Thus, it remains unclear how the material reported by Wimalasiri and Zou with a specific surface area below 400 m2/g and pores in the range of 3–4 nm for graphene and 7–8 nm for CNT/graphene compos

In capacitive deionization (CDI) water is desalinated by applying an electrical field between two porous electrodes placed on either side of a spacer channel that transports the aqueous solution. In this work we investigate the equilibrium salt adsorption and the dynamic development of the effluent salt concentration in time, both as function of spacer and electrode thicknesses. The electrode thickness will be varied in a symmetric manner (doubling both electrodes) and in an asymmetric manner, by doubling and tripling one electrode but not the other. To describe the structure of the electrostatic double layer (EDL) which determines the salt adsorption in the micropores of activated carbons, a modified Donnan-model is set up which successfully describes the data, also for situations of very significant electrode thickness ratios. We develop a generalized CDI transport model accounting for thickness variations, which compares favorably with experimental data for the change of the effluent salt concentration in time. These experiments are aimed at further testing our equilibrium and transport models, specifically the assumption therein that in first approximation, for electrodes made of chemically unmodified activated carbon particles, the EDL structure is independent of the sign of the electronic charge. To investigate the relevance of chemical surface charge we also varied pH of the salt solution flowing into the cell.