**4. Cell architectures and CDI designs**

#### **4.1. Flow between electrodes**

structures and are not property adsorption materials because of the lack of surface area. Others are normally referred to as amorphous carbon. Earlier studies in the last few decades showed that carbon electrodes are promising in CDI cells because of their very high specific surface area, and thereby resulting in better electrosorption and higher salt rejections. Common carbon electrodes, which are utilized for CDI, are activated carbon, activated carbon powder, activated carbon cloth, carbon aerogel, carbon nanotubes, carbon nanofibers,

1. High specific surface area; it allows for high electrical capacity and high ion capacity to hold considerable quantity of ions (high adsorption). However, not the entire surface area

**2.** High electrical conductivity; it offers higher ion capacity [9, 31]; one may increase electrical conductivity by coating the electrode surface with dielectric materials. Myint et al. studied the CDI electrode made of nano/micro-sized zinc oxide/ACC to achieve better performance. Metallic or metal-like (e.g., metal oxide; titania) electronic conductivity guarantees that the whole electrode surface of all particles is charged with low-voltage gradients within the carbon. Low energy dissipation and low heating are achieved by having a high electronic conductivity [29, 31, 32]. Jia et al. reported that titania-modified-ACC increased

**3.** High stability: high physical, chemical and electrochemical electrodes' stability over a wide range of pH values, and the ability to tolerate frequent voltage changes is important

**4.** High and improved hydrophilicity: good wetting behavior, by introducing hydrophilicity,

**5.** Lower spacing between the two electrodes and large spacer electrostatic permittivity (short distances between EDLs) [9]. Laxman et al. added an ion exchange membrane in CDI cells

**6.** Fast ion mobility within the pore network: bottlenecks or very small pores cause diffusional limitations and limit the kinetics. This concerns the porosity within carbon particles as well as the pore structure of the entire CDI electrode, considering, for example, interpar-

**7.** Low costs and scalability: low costs are important for large-scale applications [9, 13].

**8.** High bio-inertness: for long-term operation biofouling needs to be avoided in surface or

**9.** Low contact resistance between the current collector and the porous electrode to avoid a large voltage drop; thus, a low interfacial resistance is required from the electrode to the

**10.** Good processability, moldable into film electrodes based on compacted powders, fibers

calculated from experimental methods may be available to ions [9, 13, 29].

adsorption sites on the electrode surface and showed good reversibility [9].

ensures that the whole pore volume participates in the CDI process [9, 13].

ordered mesoporous carbon and graphene [9, 13].

to ensure longevity and system stability [9, 37].

to achieve better surface energy and stability [31].

ticle distances and electrode thickness [13].

brackish water treatment [13].

current collector [9, 13].

or monoliths [13].

**3.1. Selected parameters for an ideal electrode**

26 Desalination and Water Treatment

In this architecture, CDI contains a pair of porous carbon electrodes parted by a spacer where feed water flows (feed water flows perpendicular to the applied electric field direction (see **Figure 4A**)). Flow between electrodes, which is also known as flow by electrodes, is the oldest and most used CDI architecture and was widely employed in various experiment works, including, but not limited to, removing salt from numerous feed waters, inspecting novel electrode materials performance and performing fundamental studies of salt sorption on porous electrodes. Traditional CDI design has advantages over newer designs due to its simplicity (no membranes or flow electrodes), which can potentially lower the system cost and reduce fouling issues [17, 38–42].

#### **4.2. Flow-through electrodes**

This architecture is defined as a CDI cell, with a pair of porous carbon electrodes parted by a thinner spacer, in which the feed goes directly through the electrodes and parallel to the applied electric field direction (**Figure 4B**). Flow-through electrodes system is used in a threeelectrode cell to study fundamental performance parameters such as charge efficiency. Flowthrough electrodes allow faster cell charging relative to flow-between systems. The primary

cells with lesser cell ionic resistance and faster desalination due to diffusion timescale reduc-

Activated Carbon Cloth for Desalination of Brackish Water Using Capacitive Deionization

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29

This architecture uses ion exchange membranes on the separator side of each electrode (**Figure 4C**). As in electrodialysis cells, the feed water channels in membrane CDI (MCDI) cells are bounded by an anion exchange membrane (AEM) and a cation exchange membrane (CEM). The main benefit of the addition of membranes to the CDI cell is to improve the charge efficiency (which is linked to the efficiency of cell energy). Membranes may be tailored to have selectivity between different ions of the same charge sign which offers an additional level of tunability for complex multi-ion systems. The benefit of charging a CDI cell with constant current rather than constant voltage is that constant current allows for constant cell effluent concentration, which was first demonstrated on an MCDI cell which has the well-established advantage of improving the system's charge efficiency and sorption capacity because of the addition of ion exchange membranes, but this added a significant membrane cost as com-

In this architecture, the flow between electrode CDI cell is modified by using a surface-treated carbon Anode (negative surface charge via a chemical surface treatment), leading to the case of inverted-CDI (I-CDI, **Figure 4D**). Inverted cell demonstrates inverted behavior, whereby cell charging results in desorption of ions and cell discharging results in ion electrosorption [17, 46].

A hybrid cell architecture combines a battery electrode (sodium manganese oxide) and a capacitive electrode (porous carbon) in a single desalination cell (**Figure 4E**). Hybrid systems enable high salt adsorption of ~ 31 mg/g as compared to traditional capacitive CDI cells which

In this architecture, CDI cell utilizes two battery electrodes (**Figure 4F**) for better salt adsorption. The Faradaic reactions in electrodes are tuned to consume a single species (such as chloride or sodium) and may not be able to significantly remove (or affect) other present species [17, 47].

In this design, feed water is pumped through electrode compartments for salinity treatment (**Figure 4G–I**). Carbon flow electrodes (FCDI) have two major benefits: first, feed water flowing through a single cell can be desalinated continuously, as the active carbon particle discharge (formation of brine) can occur as a separate process downstream of the cell, and a second major benefit is that FCDI can effectively increase the capacitance available for better

tion governing the removal of salt from the electrodes [17, 38, 42, 43].

pared to other cell components [17, 13, 41–45].

achieve up to about ~15 mg/g [17, 47–49].

**4.3. Membrane CDI**

**4.4. Inverted-CDI**

**4.5. Hybrid CDI**

**4.6. Desalination battery**

**4.7. Carbon flow electrodes**

**Figure 4.** CDI architectures using static electrodes, including (A) flow between electrodes, (B) flow-through electrode, (C) membrane CDI and (D) inverted CDI. (E) and (F) show architectures which utilize static electrodes that depart from purely capacitive behavior, including (E) hybrid CDI and (F) a desalination battery. (G)–(I) show CDI architectures with flow electrodes, including systems with (G) feed-in electrodes, (H) feed-between electrodes and (I) membrane flow electrode CDI [17].

benefit of this architecture is to remove (or decrease) the separator layer which also serves as the feed flow channel, thus allowing a less separator thickness (from normally 200–500 μm to about 10 μm). The reduction of spacer thickness between electrodes allows for more compact cells with lesser cell ionic resistance and faster desalination due to diffusion timescale reduction governing the removal of salt from the electrodes [17, 38, 42, 43].

#### **4.3. Membrane CDI**

This architecture uses ion exchange membranes on the separator side of each electrode (**Figure 4C**). As in electrodialysis cells, the feed water channels in membrane CDI (MCDI) cells are bounded by an anion exchange membrane (AEM) and a cation exchange membrane (CEM). The main benefit of the addition of membranes to the CDI cell is to improve the charge efficiency (which is linked to the efficiency of cell energy). Membranes may be tailored to have selectivity between different ions of the same charge sign which offers an additional level of tunability for complex multi-ion systems. The benefit of charging a CDI cell with constant current rather than constant voltage is that constant current allows for constant cell effluent concentration, which was first demonstrated on an MCDI cell which has the well-established advantage of improving the system's charge efficiency and sorption capacity because of the addition of ion exchange membranes, but this added a significant membrane cost as compared to other cell components [17, 13, 41–45].

#### **4.4. Inverted-CDI**

In this architecture, the flow between electrode CDI cell is modified by using a surface-treated carbon Anode (negative surface charge via a chemical surface treatment), leading to the case of inverted-CDI (I-CDI, **Figure 4D**). Inverted cell demonstrates inverted behavior, whereby cell charging results in desorption of ions and cell discharging results in ion electrosorption [17, 46].

#### **4.5. Hybrid CDI**

A hybrid cell architecture combines a battery electrode (sodium manganese oxide) and a capacitive electrode (porous carbon) in a single desalination cell (**Figure 4E**). Hybrid systems enable high salt adsorption of ~ 31 mg/g as compared to traditional capacitive CDI cells which achieve up to about ~15 mg/g [17, 47–49].

#### **4.6. Desalination battery**

In this architecture, CDI cell utilizes two battery electrodes (**Figure 4F**) for better salt adsorption. The Faradaic reactions in electrodes are tuned to consume a single species (such as chloride or sodium) and may not be able to significantly remove (or affect) other present species [17, 47].

#### **4.7. Carbon flow electrodes**

**Figure 4.** CDI architectures using static electrodes, including (A) flow between electrodes, (B) flow-through electrode, (C) membrane CDI and (D) inverted CDI. (E) and (F) show architectures which utilize static electrodes that depart from purely capacitive behavior, including (E) hybrid CDI and (F) a desalination battery. (G)–(I) show CDI architectures with flow electrodes, including systems with (G) feed-in electrodes, (H) feed-between electrodes and (I) membrane flow

benefit of this architecture is to remove (or decrease) the separator layer which also serves as the feed flow channel, thus allowing a less separator thickness (from normally 200–500 μm to about 10 μm). The reduction of spacer thickness between electrodes allows for more compact

electrode CDI [17].

28 Desalination and Water Treatment

In this design, feed water is pumped through electrode compartments for salinity treatment (**Figure 4G–I**). Carbon flow electrodes (FCDI) have two major benefits: first, feed water flowing through a single cell can be desalinated continuously, as the active carbon particle discharge (formation of brine) can occur as a separate process downstream of the cell, and a second major benefit is that FCDI can effectively increase the capacitance available for better desalination than that of static electrode CDI systems. Therefore, FCDI can desalinate higher salinity streams than static CDI systems. Subsequently, FCDI has to evolve gradually to be economically suitable for sea water desalination [17, 47, 50].

be difficult to drop a high salinity level by utilizing small electrode mass and dimensions

**Figure 6** shows the effect of applied potential on SAC and charge efficiency at different activated carbon fibers (ACFs) electrodes with various BET surface area (SBET'). It was observed that there will be improvements in both SAC and charge efficiency if higher voltages are applied and higher electrode SBET is achieved. The effect of ACC treatment (with KOH or HNO<sup>3</sup>

different treatment times) on the removal efficiency has been studied and shown that lower treatment (3 h) is favored to achieve a high CDI performance; see **Figure 7**. Conversely, longer CDI desalination times (4 min) are preferred and the desalination cycle may be terminated

> **Applied voltage (V)**

**Figure 7.** Changes in NaCl concentration (initial conductivity of 2000 mS/cm) with time for ACC. (a) ACC treated in

ACC 984 1000 1.6 7 25 5.4 [31]

ACC/ZnO 1300 100 1.2 7 22 8.5 [29]

ACC/titania 1180 5844 0 ~ 1.2 1440 50 8.1 [55]

**Table 2.** Various activated carbon cloth (ACC) electrodes and their CDI treatment performance.

**Operation time (min)**

 100 1.2 12.5 15 — [32] 5844 1 300 18 1.75 [54] 100 1.2 7 15 5.8 [29] 5844 0 ~ 1.2 1440 12 — [55]

637 1000 1.6 7 15 8.1 [31] — 100 1.2 12.5 35 — [32]

1890 5844 1 300 40 4.3 [54] 546 500 1.2 200 45 — [56]

**Salt Rejection (%)**

**Salt adsorption capacity, SAC, (mg/g)**

http://dx.doi.org/10.5772/intechopen.76838

**Ref.**

and 0.6 g) which is the case of any CDI designed for lab-scale analysis [51, 52].

Activated Carbon Cloth for Desalination of Brackish Water Using Capacitive Deionization

for

31

(e.g., 3 × 3 cm2

**Carbon electrode**

KOH and (b) in HNO3

**Specific surface area (m2 /g)**

[53].

**Initial salt concentration (mg/L)**

#### **5. ACC-CDI performance analysis**

Removal efficiency of the CDI cells has been reviewed and analyzed with respect to many parameters (operating conditions). The effect of the feed flow rate and the initial ionic concentration on the removal efficiency is shown in **Figure 5**. It was found that salt rejections decrease at high flow rate; this is because the separation process requires high contact time between the electrode surface and the salt solution. Furthermore, high feed concentrations would result in reduced removal efficiency but would increase electrosorption capacity (SAC) since high amounts of salts will fill up more carbon pores and yield in higher adsorbed salts per electrode mass. Removal efficiency decreases at higher salt concentrations because it would

**Figure 5.** Removal efficiency and electrosorption capacity as a function of (a) flow rate and of (b) initial concentration of NaCl solution [51].

**Figure 6.** Activated carbon fibers (ACFs) with different SBET' (BET surface area): (a) amount of electrosorption and (b) charge efficiency against applied voltage [52].

be difficult to drop a high salinity level by utilizing small electrode mass and dimensions (e.g., 3 × 3 cm2 and 0.6 g) which is the case of any CDI designed for lab-scale analysis [51, 52].

**Figure 6** shows the effect of applied potential on SAC and charge efficiency at different activated carbon fibers (ACFs) electrodes with various BET surface area (SBET'). It was observed that there will be improvements in both SAC and charge efficiency if higher voltages are applied and higher electrode SBET is achieved. The effect of ACC treatment (with KOH or HNO<sup>3</sup> for different treatment times) on the removal efficiency has been studied and shown that lower treatment (3 h) is favored to achieve a high CDI performance; see **Figure 7**. Conversely, longer CDI desalination times (4 min) are preferred and the desalination cycle may be terminated

**Figure 7.** Changes in NaCl concentration (initial conductivity of 2000 mS/cm) with time for ACC. (a) ACC treated in KOH and (b) in HNO3 [53].


**Table 2.** Various activated carbon cloth (ACC) electrodes and their CDI treatment performance.

**Figure 5.** Removal efficiency and electrosorption capacity as a function of (a) flow rate and of (b) initial concentration of

desalination than that of static electrode CDI systems. Therefore, FCDI can desalinate higher salinity streams than static CDI systems. Subsequently, FCDI has to evolve gradually to be

Removal efficiency of the CDI cells has been reviewed and analyzed with respect to many parameters (operating conditions). The effect of the feed flow rate and the initial ionic concentration on the removal efficiency is shown in **Figure 5**. It was found that salt rejections decrease at high flow rate; this is because the separation process requires high contact time between the electrode surface and the salt solution. Furthermore, high feed concentrations would result in reduced removal efficiency but would increase electrosorption capacity (SAC) since high amounts of salts will fill up more carbon pores and yield in higher adsorbed salts per electrode mass. Removal efficiency decreases at higher salt concentrations because it would

economically suitable for sea water desalination [17, 47, 50].

**5. ACC-CDI performance analysis**

30 Desalination and Water Treatment

**Figure 6.** Activated carbon fibers (ACFs) with different SBET' (BET surface area): (a) amount of electrosorption and (b)

NaCl solution [51].

charge efficiency against applied voltage [52].

once we reach a stable product concentration; by doing this, one can estimate the highest achievable rejection for a specific CDI cell; see **Figure 7**.

**Author details**

Hisham A. Maddah1

**References**

2017;**5**(4):551-563

2018. pp. 1-25

**106**(1-3):1-9

\* and Mohammed A. Shihon2

1 Department of Chemical Engineering, King Abdulaziz University, Rabigh, Saudi Arabia 2 Department of Chemical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia

Activated Carbon Cloth for Desalination of Brackish Water Using Capacitive Deionization

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[1] Maddah HA, Alzhrani AS. Quality monitoring of various local and imported brands of bottled drinking water in Saudi Arabia. World Journal of Engineering and Technology.

[2] Maddah HA. Modeling the feasibility of employing solar energy for water distillation. In: Handbook of Environmental Materials Management. Springer International Publishing;

[3] Maddah H, Chogle A. Biofouling in reverse osmosis: Phenomena, monitoring, control-

[4] Matin A, Khan Z, Zaidi SMJ, Boyce MC. Biofouling in reverse osmosis membranes for seawater desalination: Phenomena and prevention. Desalination. 2011;**281**(1):1-16 [5] Kang G, Cao Y. Development of antifouling reverse osmosis membranes for water treat-

[6] Maddah HA, Chogle AM. Applicability of low pressure membranes for wastewater treatment with cost study analyses. Membrane Water Treatment. 2015;**6**(6):477-488 [7] Fane AG. Membranes for water production and wastewater reuse. Desalination. 1996;

[8] Maddah HA et al. Determination of the treatment efficiency of different commercial membrane modules for the treatment of groundwater. Journal of Materials and

[9] Jia B, Zhang W. Preparation and application of electrodes in capacitive deionization

[10] Oren Y. Capacitive deionization (CDI) for desalination and water treatment - past, pres-

[11] Blair John W, Murphy George W. Electrochemical demineralization of water with porous electrodes of large surface area. Saline Water Conversion. 1960;**27**(27):206-223

[12] Murphy GW, Caudle DD. Mathematical theory of electrochemical demineralization in

(CDI): A state-of-art review. Nanoscale Research Letters. 2016;**11**(1):1-25

ent and future (a review). Desalination. 2008;**228**(1-3):10-29

flowing systems. Electrochimica Acta. 1967;**12**(12):1655-1664

ling and remediation. Applied Water Science. 2016;**7**(6):2637-2651

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Environmental Science. 2017;**8**(6):2006-2012

\*Address all correspondence to: hmaddah@kau.edu.sa

ACC-CDI performance was evaluated to check for the system feasibility for water treatment by comparing the literature results for different studied ACC-CDI systems. Important parameters associated with the used electrode (e.g., specific surface area), electrolyte solution (e.g., initial salt concentration) and experiment operating conditions (e.g., applied voltage and operation time) were gathered and reported in **Table 2**. Observed salt rejections and SACs were gathered for non-composite ACC and composites ACC/ZnO and ACC/titania electrodes to be compared separately. The highest achieved rejections were 25, 35 and 50% for ACC, ACC/ZnO and ACC/titania, respectively, and the maximum observed SACs were 5.8, 8.5 and 8.1 for ACC, ACC/ZnO and ACC/titania, respectively.
