Timeline on the application of intercalation materials in Capacitive Deionization
Singh, K. ; Porada, S. ; Gier, H.D. de; Biesheuvel, P.M. ; Smet, L.C.P.M. de - \ 2019
Desalination 455 (2019). - ISSN 0011-9164 - p. 115 - 134.
Capacitive deionization is a water desalination technology in which ions are stored in electrodes in an electrochemical cell construction, connected to an external circuit, to remove ions present in water from various sources. Conventionally, carbon has been the choice of material for the electrodes due to its low cost, low contact resistance and high specific surface area, electronic conductivity, and ion mobility within pores. The ions in the water are stored at the pore walls of these electrodes in an electrical double layer. However, alternative electrode materials, with a different mechanism for ion and charge storage, referred to as ion intercalation, have been fabricated and studied as well. The salt adsorption performance exhibited by these materials is in most cases higher than that of carbon electrodes. This work traces the evolution of the study of redox activity in these intercalation materials and provides a chronological description of major developments in the field of Capacitive Deionization (CDI) with intercalation electrodes. In addition, some insights into the cell architecture and operation parameters are provided and we present our outlook of future developments in the field of intercalation materials for CDI.
Energy consumption in capacitive deionization – Constant current versus constant voltage operation
Dykstra, J.E. ; Porada, S. ; Wal, A. van der; Biesheuvel, P.M. - \ 2018
Water Research 143 (2018). - ISSN 0043-1354 - p. 367 - 375.
Capacitive deionization - Constant current and constant voltage operation - Minimizing energy consumption - Optimizing salt adsorption
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.
Capacitive deionization with wire-shaped electrodes
Mubita, T.M. ; Porada, S. ; Biesheuvel, P.M. ; Wal, A. van der; Dykstra, J.E. - \ 2018
Electrochimica Acta 270 (2018). - ISSN 0013-4686 - p. 165 - 173.
Amphoteric donnan model - Capacitive deionization - Dynamic ion adsorption theory - Wire-shaped electrodes
Capacitive deionization is a desalination technology to remove ions from aqueous solution in a cyclic manner by applying a voltage between pairs of porous electrodes. We describe the dynamics of this process by including a possible rate limitation in the transport of ions from the interparticle pore space in the electrode into intraparticle pores, where electrical double layers are formed. The theory includes the effect of chemical surface charge located in the intraparticle pores, which is present in the form of acidic and basic groups. We present dynamic data of salt adsorption for electrodes with and without coated ion-exchange membranes. Experiments were conducted in a CDI cell geometry based on wire-shaped electrodes placed together. The electrodes consisted of graphite rods coated with a layer of porous carbon. To fabricate this layer, we examined two procedures that involve the use of different solvents: acetone and N-methyl-2-pyrrolidone (NMP). We found that electrodes prepared with acetone had a lower salt adsorption compared to electrodes prepared with NMP. At equilibrium, the theory is in agreement with data, and this agreement underpins the effect of chemical surface groups on electrode performance. Under dynamic conditions, our theory describes reasonably well desalination cycles.
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%).
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.
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.
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.
Complementary surface charge for enhanced capacitive deionization
Gao, X. ; Porada, S. ; Omosebi, A. ; Liu, K.L. ; Biesheuvel, P.M. ; Landon, J. - \ 2016
Water Research 92 (2016). - ISSN 0043-1354 - p. 275 - 282.
Amphoteric Donnan model - Capacitive deionization - Enhanced salt removal - Extended working voltage window - electrodes - carbon - desalination - water treatment - ionization - elektrodes - koolstof - ontzilting - waterzuivering - ionisatie
Commercially available activated carbon cloth electrodes are treated using nitric acid and ethylenediamine solutions, resulting in chemical surface charge enhanced carbon electrodes for capacitive deionization (CDI) applications. Surface charge enhanced electrodes are then configured in a CDI cell to examine their salt removal at a fixed charging voltage and both reduced and opposite polarity discharge voltages, and subsequently compared to the salt removal of untreated electrodes. Substantially improved salt removal due to chemical surface charge and the use of a discharge voltage of opposite sign to the charging voltage is clearly demonstrated in these CDI cycling tests, an observation which for the first time validates both enhanced CDI and extended-voltage CDI effects predicted by the Donnan model [Biesheuvel et al., Colloids Interf. Sci. Comm., 10.1016/j.colcom.2015.12.001 (2016)]. Our experimental and theoretical results demonstrate that the use of carbon electrodes with optimized chemical surface charge can extend the CDI working voltage window through discharge voltages of opposite sign to the charging voltage, which can significantly enhance the salt adsorption capacity of CDI electrodes. Thus, in addition to carbon pore size distribution, chemical surface charge in carbon micropores is considered foundational for salt removal in CDI cells.
Water desalination via capacitive deionization : What is it and what can we expect from it?
Suss, M.E. ; Porada, S. ; Sun, X. ; Biesheuvel, P.M. ; Yoon, J. ; Presser, V. - \ 2015
Energy & Environmental Science 8 (2015)8. - ISSN 1754-5692 - p. 2296 - 2319.
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.
Enhanced charge efficiency and reduced energy use in capacitive deionization by increasing the discharge voltage
Kim, T. ; Dykstra, J.E. ; Porada, S. ; Wal, A. van der; Yoon, J. ; Biesheuvel, P.M. - \ 2015
Journal of Colloid and Interface Science 446 (2015). - ISSN 0021-9797 - p. 317 - 326.
activated carbon electrodes - desalination performance - water desalination - porous-electrodes - constant-current - adsorption rate - oxide - ions - electrosorption - removal
Capacitive deionization (CDI) is an electrochemical method for water desalination using porous carbon electrodes. A key parameter in CDI is the charge efficiency, ¿, which is the ratio of salt adsorption over charge in a CDI-cycle. Values for ¿ in CDI are typically around 0.5-0.8, significantly less than the theoretical maximum of unity, due to the fact that not only counterions are adsorbed into the pores of the carbon electrodes, but at the same time coions are released. To enhance ¿, ion-exchange membranes (IEMs) can be implemented. With membranes, ¿ can be close to unity because the membranes only allow passage for the counterions. Enhancing the value of ¿ is advantageous as this implies a lower electrical current and (at a fixed charging voltage) a reduced energy use. We demonstrate how, without the need to include IEMs, the charge efficiency can be increased to values close to the theoretical maximum of unity, by increasing the cell voltage during discharge, with only a small loss of salt adsorption capacity per cycle. In separate constant-current CDI experiments, where after some time the effluent salt concentration reaches a stable value, this value is reached earlier with increased discharge voltage. We compare the experimental results with predictions of porous electrode theory which includes an equilibrium Donnan electrical double layer model for salt adsorption in carbon micropores. Our results highlight the potential of modified operational schemes in CDI to increase charge efficiency and reduce energy use of water desalination
Extraction of Energy from Small Thermal Differences near Room Temperature Using Capacitive Membrane Technology
Sales, B.B. ; Burheim, O.S. ; Porada, S. ; Presser, V. ; Buisman, C.J.N. ; Hamelers, H.V.M. - \ 2014
Environmental Science & Technology Letters 1 (2014)9. - ISSN 2328-8930 - p. 356 - 360.
charged membranes - performance - electrodes - systems
Extracting electric energy from small temperature differences is an emerging field in response to the transition toward sustainable energy generation. We introduce a novel concept for producing electricity from small temperature differences by the use of an assembly combining ion exchange membranes and porous carbon electrodes immersed in aqueous electrolytes. Via the temperature differences, we generate a thermal membrane potential that acts as a driving force for ion adsorption/desorption cycles within an electrostatic double layer, thus converting heat into electric work. We report for a temperature difference of 30 degrees C a maximal energy harvest of similar to 2 mJ/m(2), normalized to the surface area of all the membranes.
Carbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation
Porada, S. ; Hamelers, H.V.M. ; Bryjak, M. ; Presser, V. ; Biesheuvel, P.M. ; Weingarth, D. - \ 2014
Journal of Materials Chemistry. A, Materials for energy and sustainability 2 (2014)24. - ISSN 2050-7488 - p. 9313 - 9321.
graphite powder suspensions - activated carbon - electrochemical polarization - salinity differences - porous-electrodes - constant-current - co2 capture - desalination - ions - performance
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.
Carbon Nanomaterials for Water Desalination by Capacitive Deionization
Biesheuvel, P.M. ; Porada, S. ; Wal, A. van der; Presser, V. - \ 2014
In: Carbon Nanomaterials, Second Edition / Gogotsi, Y., Presser, V., CRC Press - ISBN 9781439897812 - p. 419 - 460.
Attractive forces in microporous carbon electrodes for capacitive deionization
Biesheuvel, P.M. ; Porada, S. ; Levi, M. ; Bazant, M.Z. - \ 2014
Journal of Solid State Electrochemistry 18 (2014)5. - ISSN 1432-8488 - p. 1365 - 1376.
electrical double-layer - porous-electrodes - activated carbon - water desalination - energy-consumption - poisson-boltzmann - mesoporous carbon - aqueous-solutions - constant-current - ions
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.
Wireless desalination using inductively powered porous carbon electrodes
Kuipers, J. ; Porada, S. - \ 2013
Separation and Purification Technology 120 (2013). - ISSN 1383-5866 - p. 6 - 11.
membrane capacitive deionization - water desalination - nanofiber webs - brackish-water - energy - recovery - optimization - graphene - battery - sensors
Water desalination by capacitive deionization (CDI) uses electrochemical cell pairs formed of porous carbon electrodes, which are brought in contact with the water that must be desalinated. Upon applying a cell voltage or current between the electrodes, ions are electrosorbed and water is produced of a reduced salinity. Such cells are directly connected to the electrical circuit via current collectors and wires. In this work we demonstrate for the first time wireless desalination by porous carbon electrode cells. Here, the cells are charged, at constant current, by wireless energy transfer via the mechanism of resonant inductive coupling (RIC) by the use of an external transmitting coil that induces a magnetic field which is picked up by an energy receiving circuit which charges the electrodes one relative to the other. We present data for wireless power transfer, for charge transfer between one electrode and the other, and desalination degree, at various levels of the maximum cell voltage in cycles of a typical duration of a few minutes. In the present work, one wireless desalination cell is placed within the transmitting coil, with the two porous electrodes placed parallel. A future design may use optimized spherical desalination capsules, placed in a packed bed or continuous fluidized bed water desalination reactor.
Direct prediction of the desalination performance of porous carbon electrodes for capacitive deionization
Porada, S. ; Borchardt, D. ; Oschatz, M. ; Bryjak, M. ; Atchison, J.S. ; Keesman, K.J. ; Kaskel, S. ; Biesheuvel, P.M. ; Presser, V. - \ 2013
Energy & Environmental Science 6 (2013). - ISSN 1754-5692 - p. 3700 - 3712.
carbide-derived carbon - reduced graphene oxide - pore-size - activated carbon - water desalination - composite electrodes - salinity difference - charge efficiency - selective removal - energy-storage
Desalination by capacitive deionization (CDI) is an emerging technology for the energy- and cost-efficient removal of ions from water by electrosorption in charged porous carbon electrodes. A variety of carbon materials, including activated carbons, templated carbons, carbon aerogels, and carbon nanotubes, have been studied as electrode materials for CDI. Using carbide-derived carbons (CDCs) with precisely tailored pore size distributions (PSD) of micro- and mesopores, we studied experimentally and theoretically the effect of pore architecture on salt electrosorption capacity and salt removal rate. Of the reported CDC-materials, ordered mesoporous silicon carbide-derived carbon (OM SiC-CDC), with a bimodal distribution of pore sizes at 1 and 4 nm, shows the highest salt electrosorption capacity per unit mass, namely 15.0 mg of NaCl per 1 g of porous carbon in both electrodes at a cell voltage of 1.2 V (12.8 mg per 1 g of total electrode mass). We present a method to quantify the influence of each pore size increment on desalination performance in CDI by correlating the PSD with desalination performance. We obtain a high correlation when assuming the ion adsorption capacity to increase sharply for pore sizes below one nanometer, in line with previous observations for CDI and for electrical double layer capacitors, but in contrast to the commonly held view about CDI that mesopores are required to avoid electrical double layer overlap. To quantify the dynamics of CDI, we develop a two-dimensional porous electrode modified Donnan model. For two of the tested materials, both containing a fair degree of mesopores (while the total electrode porosity is 95 vol%), the model describes data for the accumulation rate of charge (current) and salt accumulation very well, and also accurately reproduces the effect of an increase in electrode thickness. However, for TiC-CDC with hardly any mesopores, and with a lower total porosity, the current is underestimated. Calculation results show that a material with higher electrode porosity is not necessarily responding faster, as more porosity also implies longer transport pathways across the electrode. Our work highlights that a direct prediction of CDI performance both for equilibrium and dynamics can be achieved based on the PSD and knowledge of the geometrical structure of the electrodes
Energy consumption in membrane capacitive deionization for different water recoveries and flow rates, and comparison with reverse osmosis
Zhao, R. ; Porada, S. ; Biesheuvel, P.M. ; Wal, A. van der - \ 2013
Desalination 330 (2013)2. - ISSN 0011-9164 - p. 35 - 41.
ion-exchange membranes - brackish-water - porous-electrodes - desalination - carbon - plant - optimization - performance - electrochemistry - experience
Membrane capacitive deionization (MCDI) is a non-faradaic, capacitive technique for desalinating brackish water by adsorbing ions in charged porous electrodes. To compete with reverse osmosis, the specific energy consumption of MCDI needs to be reduced to less than 1 kWh per m3 of freshwater produced. In order to investigate the energy consumption of MCDI, we present here the energy consumption, and the fraction of energy that can be recovered during the ion desorption step of MCDI, as a function of influent concentration, water flow rate and water recovery. Furthermore, the energy consumption of MCDI based on experimental data of our lab-scale system is compared with literature data of reverse osmosis. Comparing with literature data for energy consumption in reverse osmosis, we find that for feed water with salinity lower than 60 mM, to obtain freshwater of ~ 1 g TDS/L, MCDI can be more energy efficient.
Review on the science and technology of water desalination by capacitive deionization
Porada, S. ; Zhao, R. ; Wal, A. van der; Presser, V. ; Biesheuvel, P.M. - \ 2013
Progress in Materials Science 58 (2013)8. - ISSN 0079-6425 - p. 1388 - 1442.
activated carbon cloth - electrical double-layer - controlled ion exchange - carbide-derived carbon - porous-electrodes - charge efficiency - brackish-water - composite electrodes - aerogel electrodes - aqueous-solutions
Porous carbon electrodes have significant potential for energy-efficient water desalination using a promising technology called Capacitive Deionization (CDI). In CDI, salt ions are removed from brackish water upon applying an electrical voltage difference between two porous electrodes, in which the ions will be temporarily immobilized. These electrodes are made of porous carbons optimized for salt storage capacity and ion and electron transport. We review the science and technology of CDI and describe the range of possible electrode materials and the various approaches to the testing of materials and devices. We summarize the range of options for CDI-designs and possible operational modes, and describe the various theoretical–conceptual approaches to understand the phenomenon of CDI
Comment on “Carbon nanotube/graphene composite for enhanced capacitive deionization performance” by Y. Wimalasiri and L. Zou
Biesheuvel, P.M. ; Porada, S. ; Presser, V. - \ 2013
Carbon 63 (2013). - ISSN 0008-6223 - p. 574 - 575.
electrodes - nanotubes
In a recent study, Wimalasiri and Zou  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 , 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. , namely 1.4 mg/g at Vcell = 2.0 V, and very recently by Li et al. , 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  and . 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 composites would provide such a high salt adsorption capacity and high capacitance (220 F/g = 0.56 F/m2). We like to note that the stated salt adsorption capacity of 26 mg/g is per gram of CNT/graphene, and not per gram of total electrode, which would reduce the number to 19 mg/g because 28 mass% of the total electrode is composed of binder and graphite.
Water Desalination Using Capacitive Deionization with Microporous Carbon Electrodes
Porada, S. ; Weinstein, L. ; Dash, R. ; Wal, A.F. van der; Bryjak, M. ; Gogotsi, Y. ; Biesheuvel, P.M. - \ 2012
ACS Applied Materials and Interfaces 4 (2012)3. - ISSN 1944-8244 - p. 1194 - 1199.
carbide-derived carbon - activated carbon - brackish-water - seawater desalination - aerogel electrodes - charge efficiency - aqueous-solutions - electrosorption - technology - adsorption
Capacitive deionization (CDI) is a water desalination technology in which salt ions are removed from brackish water by flowing through a spacer channel with porous electrodes on each side. Upon applying a voltage difference between the two electrodes, cations move to and are accumulated in electrostatic double layers inside the negatively charged cathode and the anions are removed by the positively charged anode. One of the key parameters for commercial realization of CDI is the salt adsorption capacity of the electrodes. State-of-the-art electrode materials are based on porous activated carbon particles or carbon aerogels. Here we report the use for CDI of carbide-derived carbon (CDC), a porous material with well-defined and tunable pore sizes in the sub-nanometer range. When comparing electrodes made with CDC with electrodes based on activated carbon, we find a significantly higher salt adsorption capacity in the relevant cell voltage window of 1.2–1.4 V. The measured adsorption capacity for four materials tested negatively correlates with known metrics for pore structure of the carbon powders such as total pore volume and BET-area, but is positively correlated with the volume of pores of sizes