Efficiency of capacitive deionization using carbon materials based electrodes for water desalination

Journal Pre-proof Efficiency of capacitive deionization using carbon materials based electrodes for water desalination

Milene Adriane Luciano, Hélio Ribeiro, Gisele Eva Bruch, Glaura Goulart Silva PII:

S1572-6657(20)30023-0

DOI:

https://doi.org/10.1016/j.jelechem.2020.113840

Reference:

JEAC 113840

To appear in:

Journal of Electroanalytical Chemistry

Received date:

2 March 2019

Revised date:

6 January 2020

Accepted date:

9 January 2020

Please cite this article as: M.A. Luciano, H. Ribeiro, G.E. Bruch, et al., Efficiency of capacitive deionization using carbon materials based electrodes for water desalination, Journal of Electroanalytical Chemistry(2018), https://doi.org/10.1016/ j.jelechem.2020.113840

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© 2018 Published by Elsevier.

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Efficiency of Capacitive Deionization Using Carbon Materials Based Electrodes for Water Desalination

Milene Adriane Luciano1, Hélio Ribeiro1, Gisele Eva Bruch2, Glaura Goulart Silva1* 1

Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de

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Minas Gerais, Belo Horizonte, MG, Brazil. Departamento de Fisiologia e Biofísica, Instituto de Ciências Biológicas,

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Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil.

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Corresponding author: Glaura G. Silva, Departamento de Química, Instituto de

Ciências Exatas, Universidade Federal de Minas Gerais, Avenida Presidente Antônio

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Carlos, 6627, Belo Horizonte, MG, 31270-901, Brazil.

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e-mail: [email protected]

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Abstract

Capacitive deionization (CDI) based on electrosorption has been considered an important methodology for desalination process due to its lower energy consumption, easier electrode maintenance and regeneration, when compared with conventional desalination techniques based on membrane separation or thermal distillation. New

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electrodes for CDI desalination have been prepared in the last years with several types of carbon nanomaterials and inorganic nanoparticles, resulting in a large

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number of articles and patents. Thus, an impressive number of works have recently

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arisen, and new revisions with different point of views appeared. In this short review, it is shown the last advances and contributions in electrodes for CDI, obtained by

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different types and combinations of carbon materials, and their efficiency in the

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desalination process. The new approach of this work is the comparison of the removal efficiency for different salts (not only NaCl), both at articles and patents, to

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produce a quantitative comparison of the electrode materials used in this field. The

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removal efficiency was classified in three levels for the case of NaCl electrosorption (high, moderate and low) and specified also for salts containing Fe3+, Mn2+, Cr3+, Cd2+, NO3-, SO42- for instance. Therefore, this work can be useful as guideline of materials for electrodes for specific applications into this relevant technological and scientific field and its challenges in relation to the increasing demand by fresh water in the world.

Graphical Abstract

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Journal Pre-proof Insert graphical abstract Scheme of different capacitive deionization electrodes based on carbon materials

Keywords Capacitive deionization, CDI carbon electrodes, desalination, specific capacitance, electrosorption capacity, carbon materials. Abbreviations SC specific capacitance, EC electrosorption capacity, AC activated carbon, IC ACC activated carbon cloth, ACF active carbon fiber, ACF900

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inactive carbon,

activated carbon nanofiber at 900ºC, ASAR average salt adsorption rate, BET Brunauer Emmett, Teller method, CB carbon black, CC constant current, MOHC

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mesoporous ordered hierarchical carbon, CDI capacitive deionisation, CFCN carbon nanofilm, CFE carbon felt, CFO carbon foam, CNF activated carbon nanofiber, carbon

nanomaterials,

CNT

carbon

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CNM

nanotubes,

CNT/G

carbon

layer,

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nanotubes/graphene, CV constant voltage, CVE carbon veil, DEL double electric DWCNT double-walled carbon nanotubes,

nanoparticles grown on graphene sheets, graphene/mesoporous

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GE/MC

E-Gr-Fe3O4 etching Fe3O4

G graphene, GH graphene hydrogel, carbon,

GHMCS

graphene-

coated hollow mesoporous carbon spheres, GO graphene oxide, OMC

ordered

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mesoporous carbon, Gr-Fe3O4 Fe3O4 nanoparticles grown on graphene sheets, glycidyl

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GOPF@PS graphene oxide phenolic polymer coated polystyrene spheres, GTMA trimethylammonium,

HCF

hollow

carbon

nanofibers,

HMCS

hollow mesoporous carbon spheres, HOMCs hierarchically ordered mesoporous carbons, HOMC-H hierarchically ordered mesoporous carbons acid activated, HOMC-C hierarchically ordered mesoporous carbons activated

by CO2, HPC

hierarchical porous carbon, IEM ion exchange membranes, IER anion and cation exchanger resins,

MC mesoporous carbon, MCDI MnO2-nanorods

membrane capacitive

deionization,

MnO2-NRs@graphene

graphene,

MnO2-

NPs@graphene

MnO2-nanoparticles graphene, Na@C carbon nanowalls, NC

nanoporous carbon, NCPCs nitrogen-doped cluster-like porous carbons, N-HPC Ndoped hierarchical porous carbon, NPCSs Nitrogen-doped porous carbon spheres, OMC ordered mesoporous carbon, P-CNFA P-doped carbon nanofiber aerogels, 3

Journal Pre-proof PCSs porous carbon spheres, PANI polyaniline, PES polyethersulfone, PF@PS phenolic polymer coated polystyrene spheres, PG30 graphene bonded carbon nanofiber aerogels, PTFE polytetrafluoroethene, PS phenolic polymer spheres, PVA polyvinyl alcohol, RGO reduced graphene oxide, SAC adsorption capacity, SCBFA sugar cane bagasse fly ash, SWCNTs single-walled carbon nanotubes, TiO2-NTs TiO2 nanotubes, TNT Titania, WWAP United Nations World Water Assessment Programme, S-PES sulfonated polyethersulfone, A-PES aminized polyethersulfone, GTMA glycidyl trimethylammonium, PVA polyvinyl alcohol, PGW powder activated carbon, AT anti -Terra, TMA trimethylamine aminophosphorous, TMHDA N,N,N´, N´-

polyacrylonitrile polymer,

GO-PCNF

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tetramethyl hexamethylenediamine, ACNF activated carbon nanofiber, PAN porous carbon nanofiber networks,

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RGO/ACF RGO/ AFC incorporation by spraying using an ultrasonic atomizer, Zn-

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AECNF carbon nanofiber activated by ZnCl2.

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Journal Pre-proof Contents 1. Introduction .......................................................................................................... 6 2. Capacitive Deionization........................................................................................ 9 2.1. CDI system geometries: flow-through and flow-by ......................................... 11 2.2. Important parameters for evaluating the efficiency of the CDI process .......... 15 3. Extraction of ions Na+ and Cl- by using carbon materials ................................... 16 3.1. Efficiency of removal of ions by using carbon nanotubes and graphene in different CDI electrode structures ......................................................................... 19 3.2. Carbon nanomaterials modified with metal oxide nanoparticles .................... 25 3.3. Electrospun carbon nanofibers electrodes ..................................................... 29

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4. Study of extraction of different ions (Na+, K+, Ca2+, Cu2+, As3+, NO3-, SO42-among others) with carbon and metal oxide electrodes ....................................................... 34 4.1. Efficiency of different ions removal in solutions, using electrodes based on activated carbon.................................................................................................... 35

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4.2. Efficiency of different ions removal in solutions during the CDI process using electrodes based on different systems based on carbon materials....................... 36

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4.3. Efficiency of removal of ions in solutions, obtained by several authors, using different electrodes based on different carbon structures during the CDI process 37

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4.4. Performance of CDI electrodes available in patents ...................................... 38 5. Conclusion ......................................................................................................... 41 6. Acknowledgments .............................................................................................. 42

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7. References............................................................ Error! Bookmark not defined.

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1. Introduction

The availability of water bodies for the maintenance of life on the planet is a relevant topic discussed worldwide. The population growth coupled with industrial demand urges the growing global need for fresh water. Around the world, industry consumes about 20% of the water collected, with the remaining ~ 69% for

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agriculture, 8% for domestic consumption, and 3% related to different losses [1]. The United Nations World Water Assessment Program (WWAP) report (2017) predicted that until 2030 the planet will face a water deficit of 40%, unless the

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management of this resource is dramatically improved. This report also discussed

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the environmental neglect of the underdeveloped countries, as they release most of

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their waste water directly into the environment without adequate treatment, favoring a major social and environmental problem. Thus, the availability of water resources is

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not only related to the scarcity of water itself, but mainly due to the inefficiency of the

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management of these resources. Poor water quality is associated with the presence of any substance or agent in quantity that renders the water unacceptable or

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potentially hazardous to the consumer. As water is a powerful solvent, it becomes a vehicle with a large amount of chemical and biological contaminants such as pesticides and herbicides, bacteria, viruses and parasites, heavy metals, sulfides, cyanide, dioxins, organic matter [2], among others. Disasters such as occurred recently in Brazil, the largest Brazilian environmental accident in the Rio Doce basin, with the rupture of the Samarco mining tailings dam in November of 2015, greatly reinforce the concerns about water contamination by chemicals, including heavy metals. Reports showed high concentrations of several metals in the Rio Doce basin rivers, such as: aluminum, 6

Journal Pre-proof total arsenic, barium, total cadmium, calcium, lead, cobalt, copper, chromium, tin, iron, magnesium, manganese, total mercury, nickel, potassium and sodium [2]. Thus, the health of people who depend on the water supply of the Rio Doce basin for direct use (irrigation, fishing and recreation) was directly affected. Considering this, an effective water treatment is necessary. Some heavy metals are not fully extracted from water when using traditional treatment methods [3–5]. The urgent global demand for fresh water, both in general or in dramatical

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cases as the Rio Doce basin and Paraopeba river (Brumadinho) in Brazil, motivates the urge for technologies that are environmentally safe, simple, efficient and capable

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of large-scale application for removal of metals and salts solubilized in water [2]. In

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this context, the present work intends to update the review on the different types of carbon based materials used in the manufacture of electrodes and their efficiency in

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the removal of metallic ions in solutions, applying the technology of capacitive

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deionization, which is based on the principles of electrode electrosorption. Two revisions on CDI were published in the years 2013 [6] and 2014 [7], and

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much advance regarding the CDI materials and technology from these date. In 2015

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also two revisions were reported: Suss et al., 2015, [8] proposed a performance metric nomenclature to help in the rapid growing field; and Liu et al., 2015, [9] compared composite electrodes with pristine carbon electrodes with respect to NaCl removal. Huang et al., 2017, [10] explored the relationship between different carbon materials’ features and electrosorption capacity, mainly for NaCl. Oladunni et al., 2018, [11] presented an interesting discussion about the major shortcomings of carbon electrodes and future prospects for CDI. In this last work, the authors showed a table with the percent of NaCl removed varying between 11% and 98%.

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Journal Pre-proof Other recent reviews should be considered to clarify the relevance of the present work, for instance: Teow et al., 2019, [12] showed the recent use of carbon nanotubes, graphene and zeolites as adsorbents, membrane, supercapacitor, and electrodes for desalination purpose. They discussed about the advances in nanomaterials well as its potential impact on human health and ecosystem. Ahmed et al., 2018, [13] assessed the overall state of the CDI technology about saturation and regeneration of electrodes. They also revisited the recent progress produced in

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carbon surface modifications by metals or metal oxides, membranes, polymers, that resulted in ion removal increase from the water by CDI process. Zhang et al., 2018,

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[14] overviewed about the different types and mechanisms of Faradaic reactions in

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CDI carbon electrodes (such as anodic oxidation and cathodic reduction of oxygen), and their negative and positive effects on water salt removal. They also discussed

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about the main CDI configurations or operational modes and different strategies that

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could avoid the by-products undesired formation, with consequent electrodes deterioration. Tang et al., 2018, [15] reviewed the recent advances in several CDI

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cell architectures, particularly the flow-by and membranes (MCDI), its activities,

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application, operational mode, cell design, Faradaic reactions and theoretical models. Choi et al., 2019, [16] reported a critical review about the recent advances in relation to technical requirements of various applicable areas, with an emphasis on CDI hybrid systems. They discussed about different CDI technologies, its applications in brackish water desalination, water softening, selective ion removal (e.g., heavy metals or phosphate/nitrate), energy efficiency, among others. Differently of these published reviews, our contribution is focused exclusively in the ions’ removal efficiency, which was studied for several carbon electrode architectures applied in CDI.

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Journal Pre-proof An impressive number of works have appeared in the last years in this field, and several revisions addressed these works. However, in most cases, the previous review articles did not provide details about the complete correlation between the type of CDI electrode material and its removal efficiency [6–10,15,16]. This minireview presents a complementary contribution to the field because the main CDI electrochemical parameters, as well as the electrodes removal efficiency reported in articles and patents, were categorized in different levels of ions’ removal efficiency in

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CDI. The experimental data obtained by several authors for the removal of different ions (not only NaCl) by CDI technique were summarized in several tables to provide

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2. Capacitive Deionization

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a guideline of electrodes for specific applications.

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The capacitive deionization has been studied since the 60’s, aiming at water desalination. At that time, one of the obstacles to this method evolution was the

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absence of technology to obtain efficient electrodes, i.e., electrodes with greater

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capacity of ion adsorption [17]. With the development of new materials, showing a potentially greater efficiency, this method has returned to the agenda. The evolution of the number of publications and patents available on the SciFinder site using the theme "capacitive deionization" has demonstrated the evolution of research and development associated with this technology, especially from 2010. Until the last October (2018), there were more than 1,500 articles published and more than 350 patents granted. The CDI method is based on the application of low energy to remove ions from an aqueous solution. When compared to other desalination techniques, CDI has lower energy consumption per volume of treated water. Since it

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Journal Pre-proof is a capacitive process, part of the energy used in the CDI process can be recovered during the regeneration process of the system [18,19]. The only residue obtained is a concentrated salt solution from the regeneration process (desorption), which minimizes environmental impacts [17]. Posey and Morozumi [20] described the modes of energy application (galvanostatic or potentiostatic) for the desalination of water through capacitive deionization. The galvanostatic method has the current circulating in the system

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controlled by means of a resistance. According to the authors, the use of membranes is essential for the application of the galvanostatic mode, since this would be the only

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way to make the concentration of the ions at the exit of the cell remain constant over

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time, due to the effect of co-ions. The other method is potentiostatic, with the electric potential as control variable. Energy consumption is an important parameter when

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comparing CDI in different modes applied for desalination: either in constant voltage

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(CV) or in constant current (CC) [21].

Qu et al. [22] showed by theoretical and experimental studies that the CC mode

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consumes energy at 33.8 kJ.mol-1 per mole of ions (Na+, Cl-) removed, which is only

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28% of CV mode energy consumption (120.6 kJ.mol-1). This energy saving is possible with similar level of salt removals, which is due to less resistive dissipation with CC mode. The models and experiments supported that CC is more energy efficient than CV for equal charge and charging duration for flow-through CDI cell geometry. Most of the published works use the CV based methods, which has the desalination step initially decreasing the effluent salinity and then increasing it again, when this potential is removed. However, in real devices, this can be a problem since it is necessary to produce fresh water with a constant composition over time.

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Journal Pre-proof Then, from this perspective, the galvanostatic operation (CC) would be better indicated, since the salt concentration of the effluent remains at a constant low value during the adsorption and a constant high value during the desorption [23]. The conventional CDI cell uses a pair or set of pairs of carbon electrodes with a thickness ranging from 100 to 300 μm, separated by a spacer (~1 mm) to avoid short circuit and to provide a channel to feed the water to be treated [6,7].

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2.1. CDI system geometries: flow-through and flow-by

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Among the main CDI cells, the most usual architectures are the flow-by model

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(Figure 1a), in which a feed stream flows parallel to charged porous electrodes, and the flow-through model (Figure 1b), where the saline solution is pumped

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perpendicularly to the electrode. The flow-through geometry electrodes have larger

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pores so that the desalinated water flows and the ions are retained at the same time [24]. The flow-through has the advantage of a faster response than the flow-by

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because the latter is limited by the diffusive time scale (based on the time it takes for

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the ions to spread from the spacer channel to the electrodes) [6]. In both cases, when the potential difference between two electrodes is applied, the cations in solution migrate towards the cathode, being retained on its surface due to the formation of the electric double layer, while the anions migrate towards the anode, also being retained by the same phenomenon (adsorption process). If the cell is short-circuited, the retained ions will be released, (desorption process) favoring the cleaning of the electrodes to start a new cycle [22,25]. Remillard et al. [24] demonstrated that the adsorption capacity, average salt adsorption rate and charge efficiency can be directly affected by the difference between these cells geometry.

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Journal Pre-proof For instance, flow-by tends to have higher adsorption capacity and better charge efficiency than flow-through, while flow-through CDI shows a higher average salt adsorption rate, particularly for shorter half-cycle times. Therefore, in both geometry cases, their performance will be proportional to the kinetics of adsorption/desorption of ions from the solution, since the amount of treated water in relation to the feed water will be as large as possible [26].

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Insert Figure 01

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Figure 1. Scheme of different water desalination cell geometries: CDI flow-by (a), and CDI

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flow-through (b).

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Another classification that can be done in CDI cells is related to the pairs of

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electrodes. The pairs are symmetrical when the cathode and anode are identical in chemical composition, considering manufacture and amount of mass. The pairs are

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asymmetrical when any of these factors is changed, i.e., when the anode is different

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from the cathode. Regardless of the geometry of the electrode pairs and the cell architecture, there will always be two steps (electrosorption/desorption) during the CDI method. The determinant characteristics for the efficiency of an electrode to be used in a CDI cell come from its surface properties, specific surface area, the pore structure and the operating life of the electrodes [27]. It has been stated that the CDI method still presents limits of cost, efficiency and scalability when the desalination of water with a salt concentration higher than 10 g.L-1 is the goal. Therefore, it is of great importance to optimize the process to become financially competitive with reverse osmosis and distillation, which are currently the most widely used techniques in large scale desalination systems [28]. 12

Journal Pre-proof Yang et al. [29] demonstrated that the removal of ions from a solution is only efficient when its concentration does not exceed the electrode surface saturation capacity. In addition, the authors verified through the mathematical model that the thickness of the electric double layer undergoes a reduction with the increase of the concentration of the ions in solution, implying directly in reduction of its removal capacity [7]. Different advanced theories about capacitive energy extraction have been developed in relation to the electrical double layers capacitance, where the finite ion size effect

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on the energy output is considered. The porous carbon electrodes use to extract electric energy or to desalinate a solution has been investigated, aiming their

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efficiency to applied in different CDI techniques. These theories in general take

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account two important aspects: a high degree of ions confinement and moderate surface potentials. In these conditions, steric repulsion between ions and the DEL

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overlap should be considered [30]. For instance, Jiménez et al. [31], showed different

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interpretation about the electrical double layers change about the capacitance of in relation to the electrode/solution interface. In their model, they consider that the

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porous electrodes provide huge amounts of surface area, however it given the

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typically small pore size. In this case, different pore radii and particle sizes and possible DEL overlap is considered. The interface and DEL overlap curvature affected directly the capacitive efficiency. They also observed that, the double layer capacitance is maximum to certain values of potential in different electrolyte solution concentration. The consequence about it is that the limited ionic concentration at the particle-solution interface is imposed by the finite size of ions, leads to the interference of different potential ranges. For instance, to low electric potentials the capacitance increases with the ionic strength, while for large potentials it was find the opposite trend [31]. Otherwise, the Donnan (mD) model has been studied by several

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Journal Pre-proof authors. This model considers the ion interactions inside (in the intraparticle pore space, or micropores) and outside of particles (interparticle pore space, or macropores) that depend on the concentration and can be used in DEL in microporous carbons structures. In general, in the mD model, the micropore ion concentration relates to that outside the pores according to the Boltzmann equation in the equilibrium. In this condition there is no transport across the electrode, and the macropore concentration is equal to that of the external solution outside the porous

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electrode [32]. Biesheuvel et al. [32], also demonstrated that in order to describe charge and salt adsorption in CDI porous carbon electrodes, the predictive Donnan

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model can be modified assuming that the ion attractive energy is no longer a fixed

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constant, but is inversely related to the total ion concentration in the pores. According to these authors the important role of electrostatic forces in porous

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electrodes, cannot be described by classical mean-field theories such as the

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Poisson-Boltzmann-Stern model or its mathematical limit for overlapping double layers, the Donnan model [32]. Thus, the ion volume exclusion effect may be high

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enough that instead of an excess adsorption, it has less salt in the pores than in the

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outside solution. The modified Donnan model not only has relevance for modeling the DEL structure in porous electrodes for CDI, but also for membrane CDI, salinity gradient energy, and for energy harvesting from treating CO2 containing power plant flue gas, among others [32]. Jiménez et al. [30], also presented the different DEL theory for the charged electrodes behavior in presence of multiionic solutions. In their model is taken into account the Stern layer existence, DEL overlap, and both rigid spheres concepts of the Carnahan-Starling [33] and Bikerman [30]. In this case, both DEL concepts provide a more realistic description and hence, it is used in order to get predictions of the extracted energy in cycles and the salt adsorbed in the CDI

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Journal Pre-proof technique when multiionic solutions are used. Furthermore, it was observed that the presence of divalent ions diminishes energy adsorbed by cycles in CDI process. Therefore, in face of these great challenges in application of the CDI process, this review intends to be a to its development and efficient deployment.

2.2. Important parameters for evaluating the efficiency of the CDI process

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The capacitive deionization occurs through the phenomenon of electrosorption on the electrode surface, therefore, the specific surface area plays an important role

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on the electrode behavior. However, only this aspect by itself does not define

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electrode performance, because not all pores can be accessible for the electrosorption process. For instance, Landon et al. [34] observed, for electrodes with

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high surface area (400-1100 m2.g-1), that they were incapable to adsorb ions with

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the size of the hydrated ray of the ion. This happened because the hydrated ions were much larger than the pores, impairing their performances. The pore size

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distribution also plays an essential role in the electrical double layer formed on the

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surface of the electrodes, preventing the overlapping of co-ions in order to improve the property of electrosorption in aqueous solution [35]. In this regard, the specific surface area available on the electrode, associated with the pore size distribution on its surface, is the key parameter that determines the efficient CDI performance in the electrosorption process. Mesoporous materials are tailored to be applied to CDI electrodes, because they present specific surface area with ideal pore size distribution, allowing greater accessibility of the ions. The specific capacitance (SC) is another important parameter of the CDI electrodes that directly depends on the materials and their morphology. The SC is

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Journal Pre-proof defined as the amount of charge stored with respect to the electrode mass, when a certain value of electric potential is applied to the system [36]. The tendency to perform well to remove ions in a CDI cell is directly proportional to the specific capacitance [37]. Other important properties expected for materials used in CDI electrodes are the high chemical and thermal stability, mechanical resistance, high electrical conductivity and long lifetime [7]. In this short review, we focused on the comparison of the specific capacitance

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(F.g-1) and electrosorption capacity (mg.g-1) to produce a quantitative comparison among the carbon materials used in this field and look for insights as how to further

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advance the design of materials for the CDI application. Therefore, the main goal of

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this review is to outline which CDI set (cell/electrode material) has been most efficient in the ion removal process. The results obtained by several authors for the removal

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of NaCl in solution using this technique will be summarized in Tables 1, 2 and 3. The

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three tables will separate the results showing high (>78%), moderate and low (<30%) efficiency of removal. For comparison, the best result of efficiency will be showed

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together (in the same table) with lower efficiencies obtained by the same authors. It

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will be shown the different results from the use of several materials, methodologies and experimental conditions. Moreover, the most important features of the works listed in Tables 1, 2 and 3 will be discussed. This work will also address the removal of other types of ions, such as K+, Ca2+, Pb2+, Cu2+, As3+, NO3-, SO42- (among others) along with their CDI cells’ characteristics and performances presented in Tables 4, 5 and 6, with also their main information about water treatment.

3. Extraction of ions Na+ and Cl- by using carbon materials

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As previously discussed, the electrode efficiency is directly linked to the material used in its manufacture. One of the most fundamental characteristics, but not the only one, is its ion adsorption capacity [17,27]. Carbon materials have been extensively used for the manufacture of electrodes due to their high adsorption capacity, high specific surface area, porous structure and high surface reactivity [35,38]. Among the types of carbon materials that are relevant in CDI, we can carbon aerogel [29,39], carbon foam [6],

activated carbon (AC) [40],

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mention:

activated carbon fabric [41], activated carbon cloth [42,43], activated carbon

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nanofiber (CNF) [44], carbon nanospheres [45], carbon black (CB) [46], graphene

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[47], carbon nanotubes (CNT) [48,49] among others. Carbon-based nanomaterials (CNM) have unlocked an array of applications in

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different fields, generating tremendous interest among researchers in academy and

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industry, and have been highly requested in water treatment. Among these materials, CNT and graphene have superior physical properties such as very high aspect ratio

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and surface area, high thermal and electrical conductivity, extraordinary mechanical

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and optical properties [50,51]. CNM can be incorporated as single component form (or hybrid) in other materials (such as polymers, ceramics, etc.), for application in selective membranes [52], electrodes and in other 3D structural composites [53]. Since the CNT and graphene were discovered, their potential use in water treatment processes has been widely investigated. For instance, mixture of these carbon nanomaterials combined with activated carbon resulted in an ion adsorbent CDI nanocomposite with high specific surface area, porosity and efficient active sites for adsorption [50]. These interesting properties make them promising materials for the removal of salty ions from brackish water in CDI cells [54]. These CNM also have a

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Journal Pre-proof wide range of applications such as transistors, solar cells, flexible and transparent electrodes, lithium ion batteries, electrochemical supercapacitors, or as thermal and mechanical reinforcement in composite materials [50,55–57]. Other types of amorphous or semi-amorphous CNM, with high surface area, are also considered suitable as materials to be applied in CDI electrodes. For instance, carbon black (CB) is a very suitable material used as nanofiller in electric conductor pigments, plastics and rubbers. CB has been also used in polymer matrices as reinforcing material due

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to its appreciable mechanical and thermal properties, which can be as well tailored in different systems to generate suitable large range of porosity in composite

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nanomaterials [46].

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Activated carbon (AC) is also appealing as adsorbent material with potential use in the water desalination process, due to its high surface area, large pore

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volume, electrochemical stability and low cost. In general, AC is derived from natural

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sources, such as coconut shells, tea leaves, wood, coal, or by carbonization of synthetic polymers, and can be used as absorbent material in electrodes for

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successfully removing ions from brackish water [58]. On the other hand, carbon

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nanofibers (CNF) also have been proposed for use in CDI for removal of different salty ions, and several studies have showed that CNF has a great effect on the high ion adsorption capacity performance [59,60]. Some other works have reported the success of the mixture of functionalized graphene (or CNT) with CNF aiming the ion adsorption, thus improving the CDI electrodes’ efficiency [54]. Figure 2 shows the morphology of some of these previously mentioned carbon materials.

Insert figure 02

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Journal Pre-proof Figure 2. Morphologies of different carbon materials used in CDI electrodes. (a) activated carbon cloth (ACC) produced with CNF, adapted with permission from [42], (b) carbon spheres, adapted with permission from [45], (c) activated carbon, (d) carbon black, (e) CNT and (f) graphene.

3.1. Efficiency of removal of ions by using carbon nanotubes and graphene in different CDI electrode structures

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The use of different CNM allowed the development of a low energy consumption CDI process without secondary waste, and it has emerged as a new

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desalination technology [61]. The most promising materials for the delivery of pure water are those that incorporate nanoscale characteristics and custom chemical

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properties [62][58]. As mentioned before, AC is currently the most used material for

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CDI, because it is highly porous, favoring the removal of ions and contaminating species from water. However, some studies have shown that the graphene can

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reach a theoretical surface area (2630 m2.g-1) much larger than carbon black

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(900 m2.g-1), or isolated carbon nanotubes (1315 m2.g-1) [63,64]. In addition,

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graphene also showed high electrosorption capacity, which demonstrates its potential as a material for CDI electrode manufacturing [35]. As well as graphene, CNT is also considered as strong candidate for this type of application, due to its high aspect ratio and surface area, and exceptional thermal, mechanical and electrical properties [65]. Table 1 shows results of the use of carbon nanotubes, reduced graphene oxide (RGO), and ordered mesoporous carbon (OMC) with Na+ and Cl- ions electrosorption capacity above 78%, with several results around 90% of removal efficiency. The electrodes developed by Li et al. [35], Wimalasiri et al. [66], Peng et al. [67], El-Deen et al. [68] and Yasin et al. [69] showed mostly a mesoporous structure. The KOH 19

Journal Pre-proof electrode OMC/CNT activation was responsible not only for producing high surface areas and large pore volume, but also for producing a highly mesoporous structure with improved electrosorption capacity, in this case 91.8% [67]. Insert Table 01 Table 1. Efficiency of Na+ and Cl- ions removal in solutions using different carbon-based electrodes during the CDI process. Results of high removal efficiency (>78% for the best

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result in the work).

Table 1 lists the work of Li et al. [35], whose results can be seen in Figure 3a-b.

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This work reports the comparison of different electrosorptive behavior among singlewalled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and

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graphene as materials for CDI electrodes. These CNM were used as conductive

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material in polytetrafluoroethylene (PTFE), to fabricate CDI electrodes in different

na

wt%. In particular, the graphite oxide used was prepared by modified Hummers’ method [75], followed by reduction hydrazine. The DWCNT, SWCNT and graphene

ur

electrodes showed specific surface area of 415, 453 and 77 m2.g-1, respectively.

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However, the kinetic electrosorption studies demonstrated that the CNT samples are more suitable when compared with graphene for use in CDI electrodes (Figure 3a). The electrosorption performance of these electrodes was simulated and the free energy of adsorption (ΔGo) was calculated by Langmuir parameters [35]. The experimental data were also used to compare with the thermodynamic adsorption behaviors of CNT and graphene electrodes by Langmuir and Freundlich isotherms (Figure 3b). They observed that ΔGo decreases from DWCNT (-16.73 KJ.mol-1), SWCNT (-17.82 KJ.mol-1) to graphene (-17.93 KJ.mol-1), (as more negative ΔGo values imply a greater driving force of electrosorption, resulting in an increased electrosorptive capacity). These results suggest that the electrosorption process 20

Journal Pre-proof could be more favorable for graphene electrode. However, according to the authors, other important factors also interfere on the CDI electrode efficiency, such as its specific surface area, pore structure and hydrophilicity.

In this case, due to the

graphene low hydrophilicity, small specific surface in relation to the others CNM electrodes, the adsorption capacity of CDI electrodes based on single-walled or double-walled carbon nanotubes were larger (Figure 3a-b).

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Insert figure 03

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Figure 3. (a) Variation of NaCl concentration with time for CDI based on CNT and graphene with initial concentration around 400 μmol.L-1. The applied electric voltage was 2.0 V.

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Experimental and modelled data, using Langmuir and Freundlich isotherms for CNTs and graphene based on graphite respectively. Solid lines - adjustment curve by Langmuir model;

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lines of points - adjustment curve by the Freundlich model (b). Adapted with permission from

na

[35].

Besides the use of different carbon materials, such as graphene in an isolated

ur

form, it is also possible their combination in CDI hybrid electrodes. However,

Jo

graphene nanosheets can restack due to their strong π-π interactions between their basal planes. Thus, this type of aggregation largely reduces the electrode surface area and the electrochemically active sites, resulting in lower CDI electrosorption capacity. The most common methodology to solve this issue has been adding ―spacers‖ between the graphene nanosheets, such as with metal oxides, conductive polymers, CNT, activated carbon or other different macro, meso and microporous carbons [73]. A series of ions removal efficiency results by CDI applying different types of hybrid carbon electrodes will be discussed herein. For instance, Wimalasiri et al. [66] (Table 1) reported a CDI cell with great efficiency in the removal of Na+ and Cl- ions, by using the mixture of CNT and graphene (CNT/G) as hybrid electrodes 21

Journal Pre-proof (Figure 4a-d). The CNT intercalation between graphene nanosheets increased the graphite plane distances, producing a highly mesoporous architecture (Figure 4c-d). The specific capacitance of CDI cell (obtained in 1M NaCl solution) with the electrodes based on pure graphene was of 140 F.g-1 and increased to 220 F.g-1 when the hybrid CNT/G structures were used, resulting in an expressive 98% of NaCl removal (Figure 4e) (Table 1). Besides its high electrical conductivity, CNT also assists in the formation of interconnections or networks between graphene

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nanosheets, providing channels for faster ion conduction. Moreover, the hybrid CNT/G electrode showed considerably faster salt adsorption/desorption by cycle

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(within an average of 62 min) in relation to the graphene-based electrodes, which

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consumed 112 min, on average (Figure 4f). The electrosorption capacity of the CNT/G composite was 26.42 mg.g-1, which was comparatively higher than graphene

na

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at 22.27 mg.g-1 [66].

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Insert figure 04

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Figure 4. SEM images of (a) CNT, (b) graphene (c) and (d) CNT/G hybrid composites. CV curves of graphene and CNTs/G composite at 5 and 10 mV scan rates (e). CDI performance of graphene and the CNTs/G composite using a 1M NaCl solution. Reproduced with permission from [66].

Mesoporous carbon (MC) and hybrid graphene/mesoporous carbon electrodes (GE/MC) were also produced as CDI electrodes, and comparatively studied by Zhang et al. [61] (Table 1). This hybrid electrode was prepared by direct triblockcopolymer-templating method, with different wt% of graphene (GE) (Figure 5a-c). The GE/MC hybrid composites exhibited similar pore size distributions behavior in

22

Journal Pre-proof relation to the pristine MC, (studied by N2 sorption), demonstrating that the mesoporous structure was well preserved after the incorporation of GE. The CDI process was carried out in an NaCl aqueous solution and supplied to the cell using a pump with a flow rate of 25 mL.min-1. The area increased from 567.7 m2.g-1 for pristine MC to a maximum value of 685.2 m2.g-1 for GE-5%/MC (studied by BET), and decreased with the GE addition, after 5wt%, due to slight agglomeration of the nanosheets. The graphene and GE/MC electrodes SC are shown in Figure 5c. As

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described, the pristine MC electrode exhibited a relatively low SC (35.76 F.g-1). However, when GE was added, the values of SC increased until a maximum of 52.12

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F.g-1 for the hybrid GE-5%/MC electrode. The conductivity curves are shown in

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Figure 5d. The specific capacitance reached by hybrid GE-5%/MC electrode was 731 μg.g-1, much larger than the single MC electrode (590 μg.g-1). Thus, the GE/MC

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specific capacitance improvement was attributed to the better electric conductivity,

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high specific surface area, uniform pore size distribution and no aggregation of graphene nanosheets. These important factors were responsible for its best

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electrosorptive activity, due to efficient double electric layer formation in the

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electrodes and the conductive graphene nanosheets presence (Figure 5d). The electrosorption capacity of the GE/MC composites and mesoporous carbon electrodes was up to 0.6 mg.g-1 in an NaCl aqueous solution with an initial conductivity of 89.5 μmS.cm-1 at 2.0 V. In this way, the GE/MC composite electrodes showed to be promising electrodes for CDI.

23

Journal Pre-proof Insert figure 05

Figure 5. TEM images of (a) MC and (b) GE/MC with 5 wt% of graphene, (c) Specific capacitances of the GE/MC electrodes with various GE wt%. Comparison of CDI curves of the AC, MC and GE/MC 5wt% electrodes in an NaCl aqueous solution (d). Reproduced with permission from [61].

Another strategy to produce CDI electrodes has been the 3D hybrid composites

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fabrication by hierarchical graphene-coated hollow carbon spheres (GHMCS). For instance, the use of GHMCS has been pointed out as the efficient electrode material

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for capacitive deionization. Wang et al. [73] (Table 1) fabricated graphene modified

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materials by simple template-directed method using phenolic polymer coated polystyrene spheres (PF@PS), resulting in graphene-based composites with

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mesoporous hollow nanostructure (Figure 6a). The PF@PS templates were mixed in

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the graphene oxide (GO), after that, the resulted composite (GOPF@PS) was reduced thermally in an inert atmosphere (Figure 6a-e). The GHMCS nanostructures

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showed to have tailored adsorption sites for the formation of electric double layer in

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the CDI electrodes. The CDI performance was carried out in the batch mode apparatus, the conductivity of NaCl aqueous solution after 120 minutes was reduced from of 68.0 μs.cm-1 to 15.8 μS.cm-1, the electrosorption capacity of GHMCS electrode was 2.3 mg.g-1, larger than the hierarchically mesoporous carbon spheres (HMCS) (2.0 mg.g-1) and twice that of single graphene (GR) electrodes (1.0 mg.g-1) (Figure 6f). These structural features induced synergistic effects of graphene and HMCS, leading to remarkable electrochemical properties and excellent CDI performance [73].

This type of 3D carbon architecture is expected to be the

24

Journal Pre-proof foundation for the design and fabrication of high-performance CDI electrodes with high electrochemistry performance.

Insert figure 06

Figure 6. (a) Schematic illustration of graphene coated hollow mesoporous carbon spheres (GHMCS). (b-e) SEM and TEM images of GHMCS, (f) CDI profiles of the GR, HMCS and

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GHMCS conductivity electrodes performance. Reproduced with permission from [73].

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3.2. Carbon nanomaterials modified with metal oxide nanoparticles

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Different modified CDI electrode architectures have also been prepared using

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carbon nanomaterials with several types of metal oxide nanoparticles, as can be seen in Table 1. The incorporation of metallic oxide nanoparticles such as ZrO2, TiO2,

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MnO2 and ZnO can inhibit the tendency of nanocarbon aggregation, notably

ur

improving the electrodes specific capacitance. Especially, ZrO2 is considered an ideal candidate due to its distinct characteristics, such as eco-friendly, good chemical

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stability and good ability to destroy some microorganisms [69]. In addition, ZrO2 also presents photocatalytic degradation characteristic of organic pollutants, and can improve the ion capacity of adsorption in the CDI electrodes [66]. For instance, Yasin et al. [69] prepared RGO with 10% m/m of ZrO2 nanoparticles. Especially, the RGO presented some different hydrophilic functional groups that facilitated the interaction between the graphene nanosheets and the metal oxide nanoparticles. The RGO/10% ZrO2 electrode showed a significant increase in the specific capacitance (452.06 F.g1

). Moreover, this electrode exhibited great cycling stability, excellent salt removal

efficiency (93.03%), and distinct electrosorptive capacity (4.55 mg.g-1) (Table 1) [69]. 25

Journal Pre-proof El-Deen et al. [68] also used graphene-intercalated with MnO2 nanostructures produced by a one-pot reaction, low-time consuming by eco-environmentally method. The scheme and structure morphology of graphene, MnO2-nanorods (MnO2-NRs@ graphene), and MnO2-nanoparticles (MnO2-NPs@ graphene) can be seen in the figure (Figure 7a-b). It was observed that the MnO2 nanoparticles intercalation during the exfoliation process had a great impact to separate the graphene sheets (Figure 7b). The synthesized MnO2-nanorods@graphene electrode revealed excellent

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specific capacitance (292 F.g-1), electrosorptive capacity (5.01 mg.g-1), impressive salt removal efficiency (~ 93%) and good recyclability, in an NaCl solution at 1.2 V,

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as can be observed in Figure 7c, and Table 1. Therefore, the MnO2-NRs@graphene

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has also been considered as an efficient electrode material for CDI applications.

(a) Schematic illustration for the one pot synthesis procedure of MnO2-

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Figure 7.

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Insert figure 07

nanostructures with graphene nanosheets as sandwich architectures. (b) TEM image of

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MnO2-NRs@ graphene. (c) CDI performance of the synthesized materials and AC for

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electrodes in the NaCl solution at 1.2 V. Adapted with permission from [68].

Asymmetric CDI electrodes containing carbon materials, metal, metal oxide, or conductive polymers can not only increase their surface area and wettability, but also enhance the ions electrosorption capacity by faradaic reactions [14]. Silver nanoparticles (anode) and mesoporous ordered hierarchical carbon (MOHC) (cathode) were prepared by Tsai and Doong [74] as asymmetric electrodes in a CDI cell. This arrangement allowed a removal efficiency of 82.6% of NaCl against 62.7% in a symmetric activated carbon CDI cell (Table 1). Porada et al. [6] summarized six possible reaction mechanisms to support CDI performance, including capacitive ion 26

Journal Pre-proof storage, ionic kinetics, chemical surface charge, redox reactions, water chemistry, and carbon oxidation for CDI processes [76]. The faradaic reaction on the surface of the asymmetric electrode results in the occurrence of redox and/or water chemical reaction, leading to the changes of the electrosorption behavior and consequent removal of ions [77]. In addition, the use of metallic silver in the asymmetric anode produces Ag+ ions that react with chloride ions in saline water to form AgCl, contributing to the removal efficiency [74].

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The effects of electrical conductivity, absorption capacity, hydrophilicity, and salt removal rates in CNT/silver electrode incorporated with inactive carbon (IC) were

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investigated by Alencherry et al. [78]. They observed that the silver/CNT

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impregnation changed the electrodes surface area, and considerably increased their electrical conductivity. The silver/CNT impregnation in IC reduced the electrode bulk

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resistivity, and increased charge accumulation, providing a better electrical potential

na

at the electrode interface. The observed electrode improvements were in the electrosorption capacity (from 42% to 60%), resulting in a moderate desalination

ur

capacity (67%) (Table 2).

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With moderate efficiency (between 30% - 78%) in desalination processes of solutions, the electrodes presented in Table 2 show high specific surface area, but not always high specific capacitance, which can contribute to a low efficiency. For instance, Wang et al. [79] showed for the first time the CNT sponge electrodes efficiency by simply compressing them as CDI electrodes without additives or binders. These structures showed moderate removal efficiency. The experimental result of continuous desalination NaCl solutions by CDI flow cell was compared with the Langmuir model, and reported a maximum desalination capacity of 40 mg.g-1 for the same electrode, (this value is higher than observed for other types of carbon

27

Journal Pre-proof materials). This value can be attributed to the higher electrical conductivity, larger effective surface area, and good pore size distribution observed for this type of CNT sponges.

Moreover, this material showed to be very flexible and presented a

continuous three-dimensional mesoporosity suitable to CDI electrodes. The ion exchange resins incorporation to capacity activated carbon was evaluated by Li et al. [80] (Table 2). The use of these resins is very interesting due to their block co-ions (ions with the same charge as the local electrode), which increase

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the salt storage in the electrodes macropores, improving their loading efficiency. At lower concentration of Na+ and Cl- ions in solution, the removal efficiency increased

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24.1% with the use of the resins. However, in more concentrated solutions with Na+

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and Cl-, the removal efficiency decreased 12.5% due to corrosive processes observed in the current collector (Table 2).

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There is also, different type of modification case, for instance Ahualli et al. [81],

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proposed an approach about the electrokinetics in soft particles: a layer of polyelectrolyte (cationic on one electrode, anionic on the opposite one) coats the

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carbon electrodes, converting them in a sort of soft electrode pair. They also

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demonstrated that the co-ion concentration is strongly reduced, and the concentration of the counterion is raised, in the region occupied by the soft layer, as compared to the respective bulk concentrations in the macropore. Its means that the soft layer affects the ion transport from macropores to micropores, enhancing the flow of counterions and blocking that of co-ions, thus justifying a value of charge efficiency close to one.

28

Journal Pre-proof

3.3. Electrospun carbon nanofibers electrodes

Another important strategy to prepare CDI electrodes is the production of nanoparticle networks embedded in different polymer systems by electrospinning method, leading to multi-scale textures. These electrodes are considered monolithic and show a high electrosorption capacity, which is partly caused by the continuous

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structure with improved electrical conductivity [82]. The activated carbon nanofiber web (ACNF) is produced from a polymeric precursor followed by an additional

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treatment to create high porosity that can cause some loss of carbon yield or

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impaired network flexibility. However, when using ACNF as electrode, it is not necessary to combine the use of a polymeric binder and an electrical conductor,

lP

which can increase its internal resistance and block some of pores in carbon

na

materials resulting in lower adsorption capacity. Obviously, the design and manufacture of binder-free monolithic structure materials for CDI will be highly

ur

demanded [44].

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Generally, polyacrylonitrile polymer (PAN) is used as a precursor of electrophilization to obtain carbon nanofiber due to its good spinnability in solution and its high carbon yield. However, during carbonization it is necessary to activate the carbon surface to improve the specific surface area. Activation to improve material porosity may be physical, using CO2, or chemical when is performed by chemical agents such as ZnCl2, KOH e H3PO4 [83]. The fact graphene oxide has several oxygenated groups on its surface favors its strong interaction with the precursor electrospun polymer, that increases the stable and uniform nanofibers formation. Therefore, by adding GO to the polymeric substrate combined with

29

Journal Pre-proof electrospinning it is expected to obtain composite nanofibers with modified porous structure and surface chemical properties [82,84,85]. The work published by Haider et al [86] discusses how ACNFs are produced and how their characteristics are modified as their production parameters vary, such as: applied electric field, distance between needle and collector, flow rate, solution (solvent, polymer concentration, viscosity and solution conductivity) as well as environmental influences such as temperature and humidity. The electrospinning

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technique for building ACNF begins when electric charges move into the polymer solution through the metal needle. This causes instability in the polymer solution as a

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result of induction of charges on the polymer drop. At the same time, reciprocal

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charge repulsion produces a force that opposes surface tension and, finally, the polymer solution flows toward the electric field. An additional increase in the electric

lP

field causes the spherical drop to deform into a conical shape. At this stage, ultrafine

na

nanofibers emerge from the conical polymer drop (Taylor's cone), which are collected in the metal collector kept at an optimal distance. A stable charge jet can be formed

ur

only when the polymer solution has sufficient cohesive force. During the process,

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internal and external loading forces cause the liquid jet to whip towards the manifold. This whip motion allows the solution's polymer chains to extend and slide together, resulting in the creation of fibers small enough to be called nanofibers. G. Wang et al. [44] prepared activated carbon fiber (ACF) webs with a high specific surface area by electrospinning in PAN (Table 2). The ACF webs were stabilized, carbonized and activated in CO2 at 750-900

o

C, prior to use. This

methodology has the advantage of not to need to incorporate binders that would increase the internal electrical resistance, resulting in a lower capacity for electrosorption. SEM image of the ACF web activated at 800 oC is shown in Figure

30

Journal Pre-proof 8a. This ACF showed a regular and flexuous fibrous morphology with an average diameter of 365 nm. The capacitance (at a voltage scan rate of 2 mV.s-1) was between 172-228 F.g-1 as the activation temperature increases (Figure 8b-c). When the activation temperature increased, the ACF pore web became more connected, which allowed a greater accessibility of the ions in solution, thus reflecting in the electrosorption capacity. The salt removal showed a moderate electrosorption capacity of 4.6 mg.g-1 and removal of 36.5 % of Na+ and Cl- ions for the ACF web

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electrode activated at 900 °C (Figure 8d) (Table 2).

By incorporating GO and PAN in the 0.05:1 ratio, Bai et al. [84], obtained

and steam activation. Both GO-PCNF and PCNF

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by carbonization at 800 °C

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porous carbon nanofiber networks (GO-PCNF) prepared by electrospinning, followed

demonstrated higher electrosorption capacities (13.2 mg.g-1 and 9.4 mg.g-1

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respectively) in relation to commercial activated carbon fiber due to their smaller

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diameter and shallower pores. GO-PCNF showed higher mesopore ratio (37% for GO-PCNF and 19% PCNF) and higher electrical conductivity due to GO soaking,

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which could lead to reduction of ion transport obstacle and enhanced dual electrical

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capacitance, when compared to pure PCNF. Dong et al. [85] has opted to enable embedded PAN nanofiber networks with RGO (1- 15 wt%) in CO2. Initially the electrodes were heat treated to 280 °C in air and then to 800 °C in inert atmosphere and then activated with CO2. The electrode containing 10 wt% of RGO (RGO/ACF10) presented the best electrosorption result (7.2 mg.g− 1) due to its high surface area (621 m².g-1) with larger total pore and mesoporous volume than pure nanofiber. Similarly to Dong et al. in 2014 [85], Wang et al. [82] compared the direct incorporation of RGO into nanofiber network by the spraying process using an ultrasonic atomizer. During this step, RGO nanoparticles disperse in the electrophilic

31

Journal Pre-proof structures are fixed in the individual nanofiber surface, creating GO fiber/polymer system. After heat treatment, the ACFs acted as bridges to connect the GO nanosheets. Compared to direct electrospinning RGO/ACF composites, the USRGO/ACF presented a rougher structure where conductive graphene only involved the nanofiber. The pore size distribution showed that the mesopore volume of this composite increased compared to ACF without graphene, and it is great indicative that mesopore structures were formed caused by RGO presence. The RGO

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incorporation by spraying using an ultrasonic atomizer increased electrode sorption capacity up to 9.2 mg.g−1.

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Some work with chemical activation has also been reported. Liu et al. [83] used

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the electrospinning technique to develop a carbon nanofiber with pre-oxidation, followed by carbonization and activation in ZnCl2 (Zn-AECNF). The carbon nanofiber

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obtained by ZnCl2 showed good flexibility, high hydrophilicity and better specific

na

surface area (~430 m².g-1) improving desalination performance (10.52 mg.g-1). Good water wettability is very beneficial for the diffusion and adsorption of ions in the

ur

nanofiber solution, resulting in better surface utilization. The electrode stability was

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investigated by recycling the CDI cell and showed that after three days of charge and discharge recycling, the capacitor can still maintain its good desalination properties. All of these removal efficiency results are shown in Table 3. Despite having a considerable value in the electrosorption capacity, the percentage removal efficiency was very small in all these works.

Insert figure 08

Figure 8. (a) TEM image of activated carbon fiber (ACF800), (b) CV curves for ACF900 at different scan rates of 2–50 mV/s, (c) specific capacitance (SC) as a function of the voltage

32

Journal Pre-proof scan rates and (d) electrosorption behavior of the ACF web electrodes in CDI at 1.6 V. Adapted with permission from [44]. INSERT TABLE 02 Table 2. Efficiency of Na+ and Cl- ions removal in solutions using different carbon electrodes. Results associated with moderate removal efficiency.

It can be seen in Table 3 the electrodes with less efficiency in the desalination

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process when compared to the electrodes of Table 1 and Table 2. Although they are mesoporous carbon nanomaterials doped with metal oxides or nitrogen, the

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efficiency of these electrodes showed to be very small [89,90]. One of the factors that justify their low efficiency, proposed by Gu et al. [89], is associated with high

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concentration of the saline solution for some cases. The hierarchical porous carbons

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(HPC) employed by Li et al. [90] presented some deficiencies such as high internal resistance, poor wettability and trade-off between specific surface area and pore

na

structure that make difficult to use them as electrodes with CDI application. For the

ur

electrode’s electrochemical performance improvement, Li et al. [90] provide HPC

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electrodes doped by the incorporation of nitrogen atoms into the HPC matrix to improve their electrical conductivity and surface wettability. The nitrogen doping induces a large number of defects in the carbon nanostructures, generating accessibility through the pores for the ions, in turn, improving their hydrophilicity and electrosorption capacity. However, their results showed an improvement of only 31.8% in the desalination efficiency of NaCl solution (100 mg.L-1) when compared to the same electrode without doping (Table 3). The low efficiency of CDI electrodes was also studied by Lado et al. [91] by using materials produced from the by-product of fly ash burning. Their results were related to the low volume of mesopores (0.048 cm³.g-1) in relation to the volume of 33

Journal Pre-proof micropores (0.138 cm³.g-1) after the activation at 200 °C (Table 3). Moreover, higher temperatures also were tested, but with even smaller micropores/mesopores volumetric ratio. Liu et al. [92] and Zornitta et al. [1] opted, respectively, for nanocarbon and carbon ball electrodes due to their high specific surface areas (> 1300 m².g-1), however, without success (Table 3). Unfortunately, the cells still showed a low desalination efficiency for these cases, which demonstrates the challenge that should

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be faced in this area. Therefore, in our opinion, to consider all results, the most successful and the less successful, as organized in tables 1 to 3 is essential to

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critically analyze the status of the field at this moment.

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INSERT TABLE 03

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Table 3. Efficiency of Na+ and Cl- ions removal in solutions using different carbon-based

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electrodes. Results associated with low removal efficiency.

4. Study of extraction of different ions (Na+, K+, Ca2+, Cu2+, As3+, NO3-,

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SO42-among others) with carbon and metal oxide electrodes

Although research on capacitive deionization focuses frequently on the solutions of NaCl for desalination optimization, it is also important in several fields to extract other types of contaminating ions in solution, by using selective electrodes. Tables 4, 5 and 6 show the results obtained for removal of different salts in solution using CDI cells with carbon based materials.

34

Journal Pre-proof 4.1. Efficiency of different ions removal in solutions, using electrodes based on activated carbon

Two works published by Gaikwad and Balomajumder [101,102] brought us rich observations, that the solutions diluted ten times are twice as efficient in the removal of fluoride and chromium (VI) ions. The residual biomass of Indian tea was used by them to produce activated carbon with similar to AC commercial efficiency,

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increasing the sustainability of the process. This fact was also observed in the recent work reported by Chong et al. [103], when they produced AC from palm waste. In all

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these cases, the removal efficiency order continued to be linked to the ionic charge of

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the ions in solution, where higher removal efficiency is observed in multi-element solutions as well (Table 4). Although it was not possible to calculate the removal

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efficiency of arsenic ions in solution in the work of Fan et al. [104] (Table 4), it was

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noticed that the values of specific ions removal (mass of ions removed per gram of electrode) are higher than the other values obtained in the works described

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previously. This fact leads us to the certainty of an efficient removal of arsenic ions in

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solution. After that, the efficiency of arsenic ions removal from groundwater, by a single-pass-mode CDI reactor in the presence of multiple ions, was showed by the same authors. Their results indicated that this methodology achieved an effluent arsenic concentration of 0.03 mg.L-1, below the rate of arsenic concentration standard for drinking water and irrigation sources in Taiwan [105]. Presence of other ions had an important influence on the arsenic ion removal from groundwater. The electrosorption results showed the selectivity removal preference ordered as follows: NO3- > SO42- > F- > Cl- (Table 4). The electrosorption selectivity for cations was ordered as follows: Ca2+ > As3+ > Mg2+ > Na+ ~ K+. Moreover, monovalent cations

35

Journal Pre-proof were replaced by divalent cations at the electrode surface in the later period of the electrosorption stage. INSERT TABLE 04

Table 4. Efficiency of different ions removal in solutions, using electrodes based on activated carbon

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4.2. Efficiency of different ions removal in solutions during the CDI process using electrodes based on different systems based on carbon

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materials

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Table 5 is a compilation of carbon materials (CF, CFCN, CNTs, among others),

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with several additives, such as zeolites, metal, metal oxide, HCF, which showed

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different percentage of ions removal. Leonard et al. [109] obtained higher percentage of ions removal of up to 99 wt%. In this work, the addition of metal oxide

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nanoparticles in carbon fibers favored a high ions removal efficiency, even with

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electrodes with low specific surface area (<90 m².g-1). Divalent and trivalent ions were more adsorbed than monovalent ions due to strong interaction between their charges and the double electric layer formed on the surface of the cathode, as was also observed by Porada et al. [6]. As previously discussed, AC electrodes have been preferentially used as electrodes in CDI application, not only because they have high specific surface areas, but also because they allow high specific capacitance values. As shown in Table 4, the values of the surface areas of the activated carbon electrodes were about one hundred times greater than the metal oxide doped carbon fiber electrodes tested by Leonard et al. [109]. However, the removal efficiency of the 36

Journal Pre-proof ions in the solutions was significantly lower (in Table 4) with respect to Table 5 results. The pore size distribution may have influenced these results, by not allowing ions to be efficiently adsorbed. The higher efficiency ratio of ions removal in solutions maintains the direct proportion in relation to the increase of their nuclear charges (Table 5). INSERT TABLE 05 Table 5. Efficiency of different ions removal in solutions during the CDI process using

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electrodes based on different systems of carbon materials.

4.3. Efficiency of removal of ions in solutions, obtained by several

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authors, using different electrodes based on different carbon

lP

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structures during the CDI process

Table 6 lists several results of CDI cells with different carbon structures. For

na

instance, Tsai and Doong [113] and Zafra et al. [39] reported the activation of mesoporous materials with carbon dioxide (CO2). Their results were not very

ur

promising, as was to be expected. Although this process elevates the specific

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surface area of the electrodes, the volume of the original mesopores decreases in favor of increasing the volume of micropores. However, in other Tsai and Doong [109] work, using nitric acid to activate the carbon from sugarcane bagasse, they observed an increase of the specific electrode capacitance by 70%, and the efficiency of removal of calcium (II) ions, in water hardness, by 41.7%. Despite the high surface area and good electrical conductivity characteristics of graphene/PTFE in individual use in CDI electrodes, it presented low values of specific removal as showed in the work of Li et al. [114] (Table 6). These authors worked in a low concentration multielement solution, but graphene/PTFE might 37

Journal Pre-proof probably become clustered by inefficient dispersion. Nevertheless, it was possible to simultaneously remove lower amounts of Fe3+, Ca2+, Mg2+ and Na+ ions. Other promising methodology used to produce CDI cells, with improvement of performance in salt removal, is the inclusion of ion exchange membranes (IEM) on the surface of the electrodes (known as membrane capacitive deionization (MCDI)) [77,115]. The presence of IEM in the electrodes has been considered to prevent co-ions from leaving the electrode region, thereby increasing the ion removal performance of

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MCDI compared to CDI [52,77,116]. In addition, the observable effect of MCDI in electrodes improved their interfacial selectivity, and this subject also needs to be

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better explored.

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INSERT TABLE 06

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Table 6. Efficiency of removal of ions in solutions, obtained by several authors, using

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different electrodes based on different carbon structures during the CDI process.

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4.4. Performance of CDI electrodes available in patents

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We intend here to analyze the carbon electrodes CDI efficiency patented, to get a general view of this field, because the information available in patents is especially important for the technological advance. In this case, the important experimental details about the manufacture of the electrodes are shown in Table 7. Although several CDI patents have been deposited in recent years, not all of them deal exclusively with the manufacture of carbon electrodes. It is recurrent, among the several patents deposited, the non-declaration of the efficiency of these electrodes. Therefore, we will only address patents that presented ion removal efficiency results, as shown in Table 7. 38

Journal Pre-proof Korean patent KR2009/0073808 [120], for instance, describes the use of polyether sulfone (PES) (engineering plastic) as polymer to produce the capacitive electrode for removal of ions in solution without crosslinking reaction. For the cathode manufacture, the PES was previously dried, and then solubilized in a solution of methylsulfuric acid with sulfuric acid. The sulfonated PES (S-PES) was precipitated, filtered and washed with distilled water. S-PES was mixed with dimethylacetamide, which was deposited on a graphite sheet. For the anode, a solid amine polyether

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sulfone (A-PES) was prepared and deposited on a thick graphite sheet.

A salt

removal efficiency of 86 wt% was obtained by applying a potential of 1.5 V, in a 100

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mg.L-1 solution of NaCl.

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The authors of patent US2012/0132519A1 [121] produced a variation of Korean patent KR2009/0073808 [120], by adding active carbon powder to the electrodes

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(Table 7). The potential of 1.4 V was applied in the CDI cell, in a 250 mg.L-1 solution

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of NaCl, and the removal efficiency was improved with the addition of carbon black in the electrodes, attaining 98.2%.

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Patent US2012/199486A1 [122] reports the use of an ion exchange resin in

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both layers of the CDI electrodes. The cathode was produced by a polymer solution obtained from polystyrene (containing the cation exchange group), prepared by the sulfonation reaction (S-PES). For manufacture of the anode, an amination reaction was applied. Both electrodes then had two coating layers with ion exchange resin. As shown in Table 7, in solutions of low NaCl concentration, there were no significant changes in the ion removal efficiency when the electrodes were used with double layer ion exchange resin. In contrast, increasing the solution concentration 10 times, the removal efficiency of the electrodes without ion exchange resin decreased approximately 20%, when compared to the electrode containing two layers.

39

Journal Pre-proof The electrodes presented in patent WO2013/183973A1 [123] were fabricated by coating the active layer with a solution of an ion-selective polymer matrix and cross-linked by photopolymerization and/or thermal polymerization, aiming to form a selective layer of ions with greater durability and adsorption removal efficiency, by introducing a series of ion exchange groups. The crosslinked layer was supposed to prevent accumulation of scale on the electrode when used for a long period of time. The results showed that photopolymerization crosslinking provides slightly higher ion

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removal efficiency than thermopolymerization crosslinking. In both cases, the results were higher than the ones for a non-crosslinked electrode, when used with 1.5 V

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potential in a 250 mg.L-1 solution (Table 7).

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The approach presented in patent US2015/0175449A1 [124] was focused on the use of a wetting additive and Anti-Terra (AT) 250 flocculation controller in the

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manufacture of the cathodes for CDI cells. The anode used was the same for all

na

cells. By analyzing the results presented in Table 7, it can be seen that the efficiency values of NaCl removal are high (above 90 %), even when using the cathode without

ur

the addition of Anti-Terra 250 (PGW-PVA27).

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Another interesting work was the patent under number US2017/0001188A1 [125]. Different proportions of three monomers (2-acrylamide-2-methyl-1-propane sulfonic acid, methylmethacrylate, 2-hydroxyethylmethacrylate) mixed with PVA and sulfosalicylic acid (ion exchange functional group) were used for the production of anodes. The cathode was produced with the PVA blend, GTMA, carbon black, activated carbon and glutaric acid. The best performance for the removal of NaCl ions (93.6%) from a solution with a concentration of 250 mg.L-1, when applied the potential of 1.5 V, was obtained by the cathode manufactured with the mixture of the monomers mentioned above in the proportion of 5:4:1.5. Anodes without the

40

Journal Pre-proof presence of sulfosalicylic acid presented NaCl removal efficiency lower than 90% (Table 7). INSERT TABLE 07

Table 7. Efficiency of CDI cells with carbon electrodes used in patented methods.

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5. Conclusion

The desalination technique used in the CDI method is a very broad scientific

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and technological subject, which should be periodically revised due to the enormous

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volume of work produced annually. The diversity of researches about CDI cells since the 1960s has shown, unquestionably, that this subject remains a challenging field

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that may be considered a technologically emerging area. It is consensual that there

na

are several possibilities of material combinations to use, from carbon nanomaterials, oxides, metallic nanoparticles, by-products derived from the combustion, among

ur

other alternative composite materials. All of them should be better explored in CDI

Jo

electrodes manufacture in order to increment performance. Many issues about the CDI process are still not clear and need to be better elucidated, for instance, the correlation between the microstructure and the best electrodes for each specific CDI application and their impact on the desalination technology for real challenges. Therefore, this work contributes to bring to the forefront of the discussion the removal efficiency of the cells produced from different combinations of materials, including carbon structures, as a new point of view to the field because of the categorization provided for the NaCl level of efficiency as well this data for several other salts. Moreover, this work reported an analysis of the performance in the patents of CDI devices. 41

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6. Acknowledgments

The authors thank to the Brazilian agency CNPq for financial support, and Centro de Microscopia da Universidade Federal de Minas Gerais, Brazil by the cordially provided microscopy images. We also thank Diego N. Vilela for the cordially provided CDI cell scheme.

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The authors declare no conflict or competing financial interest.

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[107] S.-Y. Huang, C.-S. Fan, C.-H. Hou, Electro-enhanced removal of copper ions from aqueous solutions by capacitive deionization, J. Hazard. Mater. 278 (2014) 8–15. [108] M. Dai, L. Xia, S. Song, C. Peng, J.R. Rangel-Mendez, R. Cruz-Gaona, Electrosorption of As (III) in aqueous solutions with activated carbon as the electrode, Appl. Surf. Sci. 434 (2018) 816–821. [109] K.C. Leonard, J.R. Genthe, J.L. Sanfilippo, W.A. Zeltner, M.A. Anderson, Synthesis and characterization of asymmetric electrochemical capacitive deionization materials using nanoporous silicon dioxide and magnesium doped aluminum oxide, Electrochimica Acta. 54 (2009) 5286–5291. [110] Y. Liu, W. Ma, Z. Cheng, J. Xu, R. Wang, X. Gang, Preparing CNTs/CaSelective zeolite composite electrode to remove calcium ions by capacitive deionization, Desalination. 326 (2013) 109–114. [111] K. Wei, Y. Wang, W. Han, J. Li, X. Sun, J. Shen, L. Wang, Fabrication and characterization of TiO2-NTs based hollow carbon fibers/carbon film composite electrode with NiOx decorated for capacitive application, J. Power Sources. 318 (2016) 57–65. [112] J.J. Lado, R.E. Pérez-Roa, J.J. Wouters, M.I. Tejedor-Tejedor, C. Federspill, J.M. Ortiz, M.A. Anderson, Removal of nitrate by asymmetric capacitive deionization, Sep. Purif. Technol. 183 (2017) 145–152. [113] Y.-C. Tsai, R. Doong, Activation of hierarchically ordered mesoporous carbons for enhanced capacitive deionization application, Synth. Met. 205 (2015) 48– 57. [114] H. Li, L. Zou, L. Pan, Z. Sun, Using graphene nano-flakes as electrodes to remove ferric ions by capacitive deionization, Sep. Purif. Technol. 75 (2010) 8– 14. [115] A. Hassanvand, G.Q. Chen, P.A. Webley, S.E. Kentish, A comparison of multicomponent electrosorption in capacitive deionization and membrane capacitive deionization, Water Res. 131 (2018) 100–109. [116] J.E. Dykstra, K.J. Keesman, P.M. Biesheuvel, A. Van der Wal, Theory of pH changes in water desalination by capacitive deionization, Water Res. 119 (2017) 178–186. [117] E. Avraham, M. Noked, A. Soffer, D. Aurbach, The feasibility of boron removal from water by capacitive deionization, Electrochimica Acta. 56 (2011) 6312– 6317. [118] Z. Huang, L. Lu, Z. Cai, Z.J. Ren, Individual and competitive removal of heavy metals using capacitive deionization, J. Hazard. Mater. 302 (2016) 323–331. [119] Q. Ji, C. Hu, H. Liu, J. Qu, Development of Nitrogen-Doped Carbon for Selective Metal Ion Capture, Chem. Eng. J. (2018). [120] G.-S. Gang, T.-I. Kim, W.-G. Son, Capacitive Deionization Electrode using ionexchangeable engineering plastic and Its Manufacturing Method Thereof, KR2009/0073808, 2009. [121] K.S. Kang, J.H. Choi, Capacitive electrode for deionization, and electrolytic cell using same, US2012/0132519A1, 2012. [122] K.S. Kang, W.K. Son, J.H. Choi, N.S. Park, T.I. Kim, Ion-selective capacitive deionization composite electrode, and method for manufacturing a module, US2012/199486A1, 2012. [123] K.S. Kang, W.K. Son, T.I. Kim, M.-Y. Kim, Method of manufacturing capacitive deionization electrode having ion selectivity and CDI electrode module including the same, WO2013/183973 A1, 2013. 49

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São Paulo, November 2019

JELECHEM-D-19-00401

Dear Editor, The authors declare no conflicts of interest or financial intention.

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Regards,

Jo

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na

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re

-p

The authors

51

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Figures and captions

Efficiency of Capacitive Deionization Using Carbon Materials Based Electrodes for Water Desalination

Milene Adriane Luciano1, Hélio Ribeiro1, Gisele Eva Bruch2, Glaura Goulart Silva1*

1

ro of

Milene Adriane Luciano1, Hélio Ribeiro1, Gisele Eva Bruch2, Glaura Goulart Silva1* Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de

Departamento de Fisiologia e Biofísica, Instituto de Ciências Biológicas,

re

2

-p

Minas Gerais, Belo Horizonte, MG, Brazil.

Jo

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na

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Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil.

FIGURE 01

52

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ro of

Figure 1. Scheme of different water desalination cell geometries: CDI flow-by (a), and CDI

-p

flow-through (b).

Jo

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na

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re

FIGURE 02

Figure 2. Morphologies of different carbon materials used in CDI electrodes. (a) activated carbon cloth (ACC) produced with CNF, adapted with permission from [29], (b) carbon spheres, adapted with permission from [32], (c) activated carbon, (d) carbon black, (e) CNT and (f) graphene.

FIGURE 03 53

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Figure 3. (a) Variation of NaCl concentration with time for CDI based on CNT and graphene

re

with initial concentration around 400 μmol.L-1. The applied electric voltage was 2.0 V. Experimental and modelled data, using Langmuir and Freundlich isotherms for CNTs and

lP

graphene based on graphite respectively. Solid lines - adjustment curve by Langmuir model; lines of points - adjustment curve by the Freundlich model (b). Adapted with permission from

Jo

ur

na

[22].

FIGURE 04 54

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Figure 4. SEM images of (a) CNT, (b) graphene (c) and (d) CNT/G hybrid composites. CV curves of graphene and CNTs/G composite at 5 and 10 mV scan rates (e). CDI

lP

performance of graphene and the CNTs/G composite using a 1M NaCl solution.

Jo

ur

na

Reproduced with permission from [53].

FIGURE 05 55

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Figure 5. TEM images of (a) MC and (b) GE/MC with 5 wt% of graphene, (c) Specific capacitances of the GE/MC electrodes with various GE wt%. Comparison of CDI curves of the AC, MC and GE/MC 5wt% electrodes in an NaCl aqueous solution (d).

Jo

ur

na

Reproduced with permission from [48].

56

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FIGURE 06

Figure 6. (a) Schematic illustration of graphene coated hollow mesoporous carbon

lP

spheres (GHMCS). (b-e) SEM and TEM images of GHMCS, (f) CDI profiles of the GR, HMCS and GHMCS conductivity electrodes performance. Reproduced with permission

Jo

ur

na

from [60].

57

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Figure 7.

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FIGURE 07

(a) Schematic illustration for the one pot synthesis procedure of MnO2-

nanostructures with graphene nanosheets as sandwich architectures. (b) TEM image of

na

MnO2-NRs@ graphene. (c) CDI performance of the synthesized materials and AC for

Jo

ur

electrodes in the NaCl solution at 1.2 V. Adapted with permission from [55].

58

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na

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FIGURE 08

Figure 8. (a) TEM image of activated carbon fiber (ACF800), (b) CV curves for ACF900 at

ur

different scan rates of 2–50 mV/s, (c) specific capacitance (SC) as a function of the voltage scan rates and (d) electrosorption behavior of the ACF web electrodes in CDI at 1.6 V.

Jo

Adapted with permission from [31].

59

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Efficiency of Capacitive Deionization Using Carbon Materials Based Electrodes for Water Desalination

Milene Adriane Luciano1, Hélio Ribeiro1, Gisele Eva Bruch2, Glaura Goulart Silva1*

ro of

Tables Table 1. Efficiency of Na+ and Cl- ions removal in solutions using different carbon-based electrodes during the CDI process. Results of high removal efficiency (>78% for the best

Carbon based material

C0

E

-1

NC/MnO2 AC/MnO2 AC SWCNT/PTFE DWCNT/PTFE

22.8

ur

[35]

25.0

na

[70]

(V)

lP

(mg.L )

1.2

2.0

Jo MC

40.0

40.0

2.0

(mg.g-1)

(%)

558.0

128 .0

0.99

81.5

686.0

204.7

0.95

79.3

721.0

98.6

0.32

26.1

0.55

97.6

0.75

81.5

0.46

48.8

415.0

n.a.

78.4 n.a.

n.a.

780.0

567.7

52.10

0.59

72.1

685.2

35.76

0.73

88.8

990.0

88.9

n.a.

91.8

362.0

140.0

22.27

81.0

391.0

220.0

26.42

98.0

370.9

136.6

2.0

SWCNT/graphene CNT/S 32.0

70.0

1.2

graphene

[72]

(F.g-1)

370.7

OMC/CNT: KOH 1:2 800°C [66]

(m².g-1)

330.1

GE/MC [67]

Removal

1.2

SWCNT

[61]

Electrosorption capacity

77.0

0.4SWCNT/PANI

70.0

Specific capacitance

453.0

Graphene/PTFE [71]

Specific surface area

re

Ref.

Experimental conditions

-p

result in the work).

1.2

CNT/N

88.8 n.a.

207.8

119.0

86.9

[68]

graphene/MnO2

70.0

1.2

n.a.

292.0

5.01

93.0

[73]

GHMCS

68.0

1.6

n.a.

43.2

2.3

76.8

60

Journal Pre-proof GR

23.3

silver nanoparticles (anode) [74]

2.4

HOMC biomass sugar cane (cathode)

58.5

1.2

AC RGO/10% ZrO2 [69]

50.0

1.0

29.4

14.7

82.6

9.7

508.0

104.2

629.0

36.6

14.7

62.7

241.2

452.1

6.30

90.0

210.6

48.9

0.77

n.a.

1.2

ro of

GO

Table 2. Efficiency of Na+ and Cl- ions removal in solutions using different carbon electrodes.

Experimental conditions C0

E

CNT sponge

60

[44]

ACF900

95

AC AC/0.2% Ag

600

ur

[78]

na

[79]

Jo

Removal

(mg.g-1)

(%)

1.2

60-80

n.a.

4.3

53.3

1.6

712.0

228.0

4.60

69.2

793.8

6.3

n.a.

42.0

811.1

8.6

n.a.

60.0

489.0

10.0

n.a.

67.0

9.50

58.0

1539.4

91.0 20.8

32.0

12.0

72.0

18.3

28.0

1.2

500

AC

2000 500

Electrosorption capacity

(F.g-1)

AC/0.2% Ag/ CNT

[80]

Specific capacitance

(m².g-1)

(V)

lP

(mg L-1)

Specific surface area

re

Ref.

Carbon based nanomaterial

-p

Results associated with moderate removal efficiency.

1.2

Anodo 1296.7

IER-AC

47.0 Catodo 1297.2

2000 [87]

TNT-5

50

2.0

238.0

23.0

13.2

60.7

[88]

AC/TiZr

50

1.2

n.a.

251.3

3.00

53.1

61

Journal Pre-proof Table 3. Efficiency of Na+ and Cl- ions removal in solutions using different carbon-based electrodes. Results associated with low removal efficiency. Experimental conditions Ref.

Specific surface area

Specific capacitance

Electrosorption capacity

Removal

(m².g-1)

(F.g-1)

(mg.g-1)

(%)

406.4

135.7

2.14

40.0

137.4

112.0

1.51

26.3

AC

836.0

n.a.

0.61

10.5

E-Gr-Fe3O4 (mesoporous)

362.0

128.0

10.30

10.0

Carbon based material

C0

E

(mg.L-1)

(V)

RGO functionalized

Gr-Fe3O4 (mesoporous)

58500

2.0

ro of

[89]

40

RGO

1.6

307.0

75.0

8.60

8.3

120.0

54.0

6.50

6.0

-p

[93]

1321.0

243.0

5.81

20.0

240.0

33.0

2.30

8.7

353.0

55.0

4.30

17.0

730.0

182.6

8.65

29.0

609.0

66.7

6.50

22.0

8.0

<5.0

1664.0

5.0

870.0

40.0

262.4

25.7

39.5

17.5

308.4

36.4

48.7

23.9

756.0

306.4

8.8

17.8

n.a..

235.2

4.1

n.a.

712.0

n.a.

9.4

18.6

740.0

n.a.

10.8

22.0

RGO PCSs-1000 [92]

500

1.6

T200 (SCBFA actived at 200 °C) N-HPC

100

HPC

[1]

ur

carbon felt CFE

na

[90]

600

carbon veil CVE

1.2

lP

[91]

90% fly ash; 10% PVDF (SCBFA - sugar cane bagasse fly ash)

re

(carbon nanospheres)

300

1.2

1.4

Jo

AC (coconut shell) MWCNT/RGO [94]

300

Na@C 100 AC 500

9.0 25.0

1.5

AC [96]

n.a.

2.0

SWCNT/RGO [95]

6.0

1.2

OMC [97]

graphene 3D

572

2.0

124.0

219.6

29.6

6.8

[98]

NCPC-900 nitrogendoped cluster-like porous carbons

100

1.6

1359.0

199.0

12.0

20.3

62

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[99]

phosphorus (P)-doped carbon nanofiber aerogels (P-CNFA)

1000

1.2

728.2

335.6

16.2

28.1

[60]

Graphene bonded carbon nanofiber aerogels (PG30)

500

1.2

542.8

220.0

16.1

25.5

[100]

G/CNT

500

1.2

n.a.

62.9

5.2

7.1

583

105.9

9.4

n.a.

474

151.6

13.2

~16

621

193

7.2

~21

PCNF [84]

450

1.2

GO-PCNF RGO/ACF-10

95

1.2

ro of

[85]

RGO/ACF-10 [82]

300

1.2

500

1.2

n.a.

649

111

9.2

~5

39.0

10.5

~12

~430

ur

na

lP

re

Zn-AECNF (Zn-60)

7.2

Jo

[83]

100

-p

S-RGO/ACF

621

63

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Table 4. Efficiency of different ions removal in solutions, using electrodes based on activated carbon

Experimental conditions Ref.

Carbon based material

C0

E

(mg.L-1)

(V)

Specific surface area

Specific capacitance

Electrosorption capacity

Removal

(m².g-1)

(F.g-1)

(mg.g-1)

(%)

53.4

3.04

59.8

0.13

43.2

1.36

Electrolyte Na+

243.0

+

K AC

[107]

AC

[104]

AC

10.1 1.2

Ca2+

58.0

Mg2+

58.6

Cu2+

50.0

0.8

100.0

1.2

964.0

35.0

SO42-

7.3 1.2

Cl-

6.4

F

AC

964.0

n.a.

24.57

964.0

103.3

70.0

0.03

40.0

0.16

38.0

0.48

79.4

0.34

65.3

82.0

2.56

43.6

7.5

0.23

42.5

0.1

0.002

76.9

0.85

97.1

0.82

94.2

3.67

42.0

3.22

36.8

0.77

88.5

0.74

85.2

2.83

32.4

2.49

28.5

964.0

103.3

ur

Jo

10.0

F-

1.2

n.a.

n.a.

Cr6+ F

-

100.0

Cr6+ F

-

10.0 1.2

Cr6+ F+

[103]

0.32

7.3

1.2

Cr6+

AC palm/rGO

85.1

Mg2+

As3+

AC biomass Tea waste

1.89

8.5

K

[102]

n.a.

Ca

+

AC/graphit e powder

5.03

2+

Na+

[101]

69.0

7.08

na

[105]

lP

-

1.51

re

NO3-

36.4

-p

As3+ As5+

n.a.

ro of

[106]

n.a.

n.a.

100.0

Na

1190.0

5.9

K+

35.3

7.9

2+

Ca

126.0

1.2

1145.0

54.6

n.a.

32.0

Mg2+

34.3

27.0

Ni2+

0.168

68.0

64

Journal Pre-proof Cu2+

0.055

51.0

Pb2+

0.025

52.0

Na

1190.0

5.9

K+

35.3

9.3

Ca2+

126.0

35.0

+

AC/rGO

2+

Mg

34.3

Ni2+

0.168

54.0

2+

0.055

65.0

Pb2+

0.025

44.0

As3+

75.0

1.0

1331.0

53.0

4.7

38.7

-p re lP na ur

AC

32.0

Jo

[108]

33.7

ro of

Cu

838.0

65

Journal Pre-proof Table 5. Efficiency of different ions removal in solutions during the CDI process using electrodes based on different systems of carbon materials.

C0

E

Specific surface area

(mg.L-1)

(V)

(m².g-1)

Experimental conditions Electrolyte +

58.0

96.0

Fe2+

65.0 40.0

Ca2+

38.0

2+

Mg

17.0

Fe

80.0 1.5

3+

750.0

[111]

TiO2NTs/HCF/CFCN-3 (9,35%)

Na2SO4

50.0

Na+

2.0

lP

-p

Ca2+

NO3-

9.0//28.0

n.a. 88.0

selective zeolite CNT/Ca

1.6

90.0 99.0

87.8

127.8

20.8

n.a.

244.9

63.3 56.0 50.0

Ca2+

-

na

NO3 +

136

66.0 1.5

n.a.

n.a. 56.0

Na

5.0

NO3-

12.0

Jo

ur

CF

(%)

Mn2+

[110]

[112]

(F.g-1)

82.0

CF/Al2O3/Mg//CF/Si O2

CF coated with SiO2 (catodo) Al2O3 (anodo)

Removal

30.0

re

[109]

Specific capacitance

Na

ro of

Ref.

Carbon based material

66

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Table 6. Efficiency of removal of ions in solutions, obtained by several authors, using different electrodes based on different carbon structures during the CDI process.

Experimental conditions

Ref.

Carbon based material

C0

E

(mg.L-1)

(V)

Specific surface area

Specific capacitance

Electrosorption capacity

Removal

(m².g-1)

(F.g-1)

(mg.g-1)

(%)

Electrolyte

Fe3+ Ca [114]

Graphene

25.0

Mg2+

2.0

Aerogel activated CO2

Cl-

1290.0

B(OH)3

480.0

Cl-

3550.0 -

NO3

PO43-

HOMC-H activated acid HOMC-C actived CO2

Ca2+

ur

[113]

Cd2+

ACC

2+

Jo

[118]

6200.0

Pb

Cr3+

1440.0

1.2

40.0

1.2

0.55

134.0

n.a.

n.a. 0.52 0.45 27.7 n.a.

6.74

100.5

11.80

86.9

14.25

508.0

51.8

3.26

487.0

88.4

4.62

1517.0

8.7

3.28

1069.0

56.2 103.6

1.2

n.a.

n.a.

n.a.

26.0

Mg2+

70.0

1.2

307.9

103.0

n.a.

Fe ACC CDI

Na+ F-

7.4

1.2

1101

graphene pyrrolic-N

3.0

43.5

3.6

51.0

n.a.

0.083

94.2 268.0

Pb2+

[119]

28.0 76.0

ACC MCDI graphene pyrimidic-N

42.6

18.0

3+

[77]

n.a.

52.5

Na

RGO-ACF20

n.a.

31.9

+

[54]

24.4 31.7

120.3

9500.0

na

HOMC

1.0

re

[39]

ACF

820.0

lP

[117]

Na+

254.0

-p

Na+

ro of

0.62

2+

239.7

102.1

93.0

57.9 1.2

0.083

95.8 198.0

93.0

195.2

259.5 87.4

67

Journal Pre-proof

Table 7. Efficiency of CDI cells with carbon electrodes used in patented methods.

CDI cell Patent

WO2013/183973A1 [123]

1.5

100

86.0

S-PES

A-PES: AC

S-PES: AC

96.3

A-PES: AC:carbon black

S-PES: AC:carbon black

98.2

PVDF: dimethylacetaldehyde: AC

PVDF: dimethylacetaldehyde: AC

A-PES: AC covered with A-PES

A-PES: AC covered with S-PES

A-PES: AC covered with PVDF 4-vinylbenzenesulfonic acid

Dip-coating

1.4

250 72.2

100

91.0

1000

73.0

100

90.0

1000

59.0

1.5

S-PES: AC covered with S-PES

photopolymerization

90.4

Styrene

Polyethersulfone

thermopolymerization

1.5

250

without crosslinking

PGW-PVA27

91.8

PGW-PVA27-AT30

96.8

na

Jo

Hydrophilic polymer (4:5:1,5); PVA Hydrophilic polymer (9:0:1,5); PVA

88.9 81.5

PGW-PVA20-AT10

n.a.

1.5

400

ur

PVA:GTMAc:PGW15

ro of

n.a.

Hydrophilic polymer (4:5:1,5); PVA sulfosalicylic acid US2017/001188A1 [121]

n.a.

A-PES

polystyrene

US2015/175449A1 [124]

Efficiency (%)

-p

US2012/199486A1 [122]

Co NaCl (mg.L-1)

Cathode

re

US2012/132519A1 [121]

E (V)

Anode

lP

KR2009/0073808 [120]

Observation

94.6

AT10-PGW-PVA10

9.2

AT10-PGW-PVA20

95.6 93.6

PVA; GTMA; carbon black; AC; glutaric acid

n.a

1.5

250

86.3 86.3

68

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8