Desalination performance of silver/activated carbon hybrid capacitive deionization under constant-current operation

Jaehan Lee

doi:10.4491/eer.2026.051

Environ Eng Res > Volume 31(5); 2026 > Article

Lee: Desalination performance of silver/activated carbon hybrid capacitive deionization under constant-current operation

Research

Environmental Engineering Research 2026; 31(5): 260051.

Published online: March 25, 2026

DOI: https://doi.org/10.4491/eer.2026.051

Desalination performance of silver/activated carbon hybrid capacitive deionization under constant-current operation

Jaehan Lee^†

Department of Biological and Chemical Engineering, College of Science and Technology, Hongik University, 2639 Sejong-ro, Sejong-si 30016, Republic of Korea

^†Corresponding author: E-mail: ljh0322@hongik.ac.kr, Tel: +82-44-860-2514, Fax: +82-44-866-6940, ORCID: 0000-0002-8191-1396

Received January 26, 2026 Revised March 17, 2026 Accepted March 24, 2026

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Capacitive deionization (CDI) is a promising desalination technology but is often limited by the low ion removal capacity of carbon-based electrodes. In this study, a hybrid capacitive deionization (HCDI) system employing an asymmetric silver/activated carbon (Ag/AC) electrode configuration was investigated under constant-current operation and compared with a conventional membrane capacitive deionization (MCDI) system. Desalination performance was evaluated using CDI Ragone plot analysis, while galvanostatic charge–discharge measurements were used to examine the potential evolution of individual electrodes. The results show that the Faradaic Ag/AgCl reaction stabilizes the cell voltage and expands the usable potential window of the activated carbon electrode. As a result, the HCDI system consistently outperformed MCDI, achieving a maximum salt adsorption capacity of 36.5 mg g⁻¹ in a 10 mM NaCl solution, more than twice that of MCDI (17.0 mg g⁻¹), along with a faster ion removal rate. These findings demonstrate that integrating Faradaic electrodes effectively overcomes the intrinsic capacity limitations of conventional CDI systems.

Keywords: Brackish water desalination, Carbon composite electrode, Constant-current operation, Hybrid capacitive deionization, Membrane capacitive deionization, Silver electrode

Graphical Abstract

Keywords: Brackish water desalination, Carbon composite electrode, Constant-current operation, Hybrid capacitive deionization, Membrane capacitive deionization, Silver electrode

1. Introduction

Capacitive deionization (CDI) is one of the electrochemical desalination technologies that removes ions from saline water through electrochemical adsorption and desorption on electrode surfaces. This method is gaining attention for brackish water desalination due to its high energy efficiency, simple operation, and the fact that it is an eco-friendly process that does not require complex pretreatment steps. The CDI system consists of two electrodes with large surface areas that do not undergo chemical reactions. The feed water to be treated flows between these electrodes. When a direct current or voltage is applied across the electrodes, ions in the feed water form an electrochemical double layer at the electrode surfaces and become adsorbed [1–14]. This phenomenon is similar to the principle behind supercapacitors, which are a type of energy storage device. As ions are adsorbed onto the electrode surfaces, the desalination of the feed water proceeds.

Research on capacitive deionization (CDI) has primarily focused on optimizing cell architectures, developing advanced electrode materials, and enhancing desalination efficiency. However, conventional CDI systems face significant challenges, including limited desalination capacity due to the inherently low accessible surface area of carbon electrodes and the trade-off between surface area and electrical conductivity. To overcome these limitations, membrane capacitive deionization (MCDI) was introduced, incorporating ion-exchange membranes positioned adjacent to each electrode to improve ion selectivity and desalination efficiency [15–17]. Although MCDI has shown enhanced performance compared to conventional CDI, the limited desalination capacity of carbon-based materials remains a critical barrier.

To further address these limitations, a novel concept termed hybrid capacitive deionization (HCDI) has been developed, in which Faradaic electrode materials are integrated into conventional CDI systems to enhance ion storage through reversible redox reactions. Various Faradaic materials have been investigated for this purpose, including transition metal oxides such as sodium manganese dioxide (Na_xMnO₂), Prussian blue analogues, and metal phosphates. In addition, silver/silver chloride (Ag/AgCl) electrodes have been widely studied as effective materials for ion removal [18–33]. In HCDI, ion removal occurs via Faradaic redox reactions at one electrode, while counter ions are simultaneously adsorbed onto an activated carbon electrode through capacitive behavior. During the subsequent discharge phase, ions captured by both electrodes are released back into the solution, completing the operational cycle. This hybrid approach significantly improves desalination capacity relative to conventional CDI systems [34–37]. Despite the promising potential of HCDI, a comprehensive understanding of its performance characteristics is still limited, hindering its further development and widespread adoption as a competitive desalination technology.

In this study, the desalination performance of a hybrid capacitive deionization (HCDI) system employing silver/activated carbon (Ag/AC) electrodes was systematically evaluated under constant-current operating conditions, as illustrated in Fig. 1. Silver electrodes exhibit several advantages among Faradaic electrode materials used in capacitive deionization. Notably, the Ag/AgCl redox couple provides a high theoretical charge storage capacity of approximately 250 mAh g⁻¹, enabling efficient ion capture during operation. Furthermore, the Ag/AgCl conversion reaction occurs at a relatively constant potential, resulting in a stable electrochemical response throughout the reaction process [32]. Due to these characteristics, the HCDI system employing a silver/activated carbon electrode was adopted in this study, as it enables a clear comparison of performance with conventional CDI. Comprehensive comparisons were conducted with a conventional membrane capacitive deionization (MCDI) system, focusing on key performance indicators including desalination capacity and desalination rate. In addition, overall system performance was assessed using CDI Ragone plot analysis [35], complemented by detailed electrochemical characterization to evaluate the competitive advantages of the proposed HCDI system over MCDI.

2. Materials and Methods

2.1. Electrode Fabrication

Activated carbon electrodes were prepared by mixing activated carbon (MSP-20, Kansai Coke and Chemicals, Japan), conductive carbon black (Super P, Timcal Graphite and Carbon, Switzerland), and polytetrafluoroethylene (PTFE, Sigma-Aldrich, USA) as a binder in an ethanol-based solution at a mass ratio of 86:7:7. The resulting slurry was processed into sheet electrodes with a thickness of 200–300 μm using a roll press machine (ROTECH, MP200, Republic of Korea). Silver composite electrodes were fabricated by mixing silver particles (1–2 μm, Henqiu Technology Co., China), carbon black (Super P), and PTFE at a mass ratio of 7:2:1. This mixture was also formed into sheet electrodes with a thickness of 100–120 μm using the same roll press method. All electrodes were subsequently dried in a vacuum oven at 60°C for 12 hours to remove residual solvents. The surface morphology of the fabricated electrodes was examined using a scanning electron microscope (SEM; JSM-IT700HR, JEOL, Japan).

2.2. Electrochemical Tests

The electrochemical properties of the fabricated electrodes were characterized using a potentiostat (PARSTAT 2273, Princeton Applied Research, USA) and a battery cycler (WBCS300, WonA Tech Co., Republic of Korea) via cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements by an electrochemical analysis cell (Fig 2 (a)). CV experiments were conducted in a three-electrode configuration using 1 M NaCl as the electrolyte. The counter electrode was either an activated carbon or a silver/silver chloride (Ag/AgCl) electrode, while a saturated Ag/AgCl (KCl) electrode served as the reference. GCD measurements were carried out using both two-electrode and three-electrode configurations, following previously reported cell assembly methods. The electrochemical cells were assembled with either activated carbon/activated carbon or silver/activated carbon electrode combinations. As separators, glass fiber membranes (Whatman, USA) were employed.

2.3. Deionization Performance Test

Fig. 2(b) illustrates the schematic diagram of the capacitive deionization (CDI) module used in this study, along with the internal cell configuration (electrode diameter: 20 mm). In the membrane capacitive deionization (MCDI) setup, activated carbon electrodes were placed on both sides of the cell, and a cation exchange membrane (CMX, Astom Co., Japan) and an anion exchange membrane (AMX, Astom Co., Japan) were positioned between them. For the hybrid capacitive deionization (HCDI) configuration, a cation exchange membrane was placed on the surface of the activated carbon electrode to enable selective cation adsorption, while a silver electrode was used as the counter electrode. The masses of the silver and activated carbon electrodes were 55.9 ± 1.4 mg and 37.5 ± 0.9 mg, respectively.

To assess system performance under various conditions, parameters such as feedwater concentration and operational settings were systematically varied. Constant current was applied using a battery cycler (WBCS300, WonA Tech Co., Republic of Korea), while the flow rate was maintained at 2 mL/min using a peristaltic pump (Simdos 10, KNF, Switzerland). Desalination performance was evaluated based on changes in effluent concentration, measured using a conductivity meter (3581-10C, HORIBA, Japan). All experiments were conducted in a temperature-controlled chamber maintained at 25°C.

2.4. Calculations and Analytical Methods

Four equations were employed to evaluate and compare the electrochemical and deionization performance of the electrodes: specific capacitance (Eq. 1), specific capacity (Eq. 2), salt adsorption capacity (SAC, Eq. 3), average salt adsorption rate (ASAR, Eq. 4), and energy consumption (Eq. 5).

The specific capacitance of the AC electrodes was calculated using Eq. (1):

(1)

C (F / g) = \frac{1}{V \times M_{e}}

where C is the specific capacitance (F/g_electrode); I is the current (A); V is the potential scan rate (V/s) and M_e is the mass of the electrode (g).

The specific capacity of both the AC and Ag electrodes was determined using Eq. (2):

(2)

Q (\frac{m A h}{c m^{2}}) = t \times I \times \frac{1 m A h}{3.6 C} \times \frac{1}{r^{2}}

where t is the operation time (sec), I is the applied current (A), r is the radius of the electrode (cm²).

The salt adsorption capacity (SAC), representing the total amount of salt removed during operation, was calculated by Eq. (3):

(3)

S A C (m g / g) = \frac{M_{w} \int (C_{i} - C_{e}) φ d t}{M_{e}}

The average salt adsorption rate (ASAR) was evaluated using Eq. (4):

(4)

A S A R (m g / g / s) = \frac{M_{w} \int (C_{i} - C_{e}) φ d t}{M_{e} t}

The specific energy consumption (SEC) for salt removal during the deionization process was determined using Eq. (5):

(5)

Energy consumption (W h / m o l) = \frac{I \int V d t}{\int (C_{i} - C_{e}) φ d t}

where C_i is the concentrations of the influent (mg/L), C_e is the concentrations of the effluent (mg/L), is the flow rate (mL/min), M_w is the molecular weight of NaCl (58.44 g/mol), M_e is the total mass of the electrodes, I is the applied current (A), V is the cell voltage during operation (V), and t is the duration of deionization (sec).

3. Results and Discussion

Fig. 3 shows SEM images and CV curves of the silver (Ag) and activated carbon (AC) electrodes. As observed from the SEM images, both the AC and Ag powders exhibit particle sizes ranging from 10 to 50 μm with irregular, rock-like morphologies. The electrode surfaces appear notably rough due to the aggregation and bonding of active materials (activated carbon or silver), carbon black, and polytetrafluoroethylene (PTFE).

CV analysis was conducted on each electrode using a three-electrode configuration with a 1 M NaCl aqueous electrolyte at a scan rate of 2.0 mV/s. In Fig. 3(c), clear redox peaks corresponding to the reversible electrochemical reactions between chloride ions and silver are observed. These peaks occur within a potential window that prevent water electrolysis at neutral pH, confirming the suitability of the electrode for selective chloride ion removal [35,39,40]. Conversely, Fig. 3(d) shows that the AC electrode exhibits a rectangular CV profile, indicative of ideal capacitive behavior and characteristics of typical electric double-layer capacitors. The specific capacitance of the AC electrode was calculated to be approximately 120 F/g, highlighting its capability for efficient saline water deionization through electrostatic adsorption mechanisms.

Fig. 4(a) and (b) present the conductivity changes, voltage, and current curves observed during constant-current charge-discharge experiments using the silver/activated carbon HCDI system (feed solution: 10 mM NaCl, current density: 1.25 mA/cm²). As indicated by the changes in conductivity, desalination was successfully achieved in the HCDI system, where desalination occurred during the charging process and regeneration occurred during the discharging process. During charging step, chloride ions in the solution reacted with the silver electrode to form silver chloride (AgCl), releasing electrons, while sodium ions were adsorbed onto the negatively charged activated carbon electrode. Thus, the removal of ions from water was achieved by combining the Faradaic reaction at the silver electrode with the non-Faradaic reaction at the activated carbon electrode, enabling repeated usage due to the reversible reaction at the silver electrode.

To compare the performance of the HCDI system with a conventional MCDI system, an MCDI cell utilizing cation exchange and anion exchange membranes was tested under identical conditions (feed solution: 10 mM NaCl, voltage range: −0.6 to 1.2 V, current density: 1.25 mA/cm²). Fig. 4(c) and (d) show conductivity changes and voltage curves over time during charging and discharging processes for both the HCDI and MCDI systems. As evident from the data, the HCDI system exhibited significantly longer charge-discharge times compared to the MCDI system, indicating a higher desalination capacity. Specifically, the salt adsorption capacity of the HCDI system was found to be 36.5 mg g⁻¹, more than twice that of the MCDI system (16.1 mg g⁻¹) at a current density of 1.25 mA/cm². However, due to the resistance of silver chloride and polarization effects from chemical reactions, the HCDI system showed a larger IR drop (voltage decrease) when switching from the charging to discharging process (MCDI: 0.093 V; HCDI: 0.156 V). Based on the calculation using Eq. (5), the specific energy consumption was determined to be 24.2 Wh mol⁻¹ for HCDI and 22.9 Wh mol⁻¹ for MCDI. The slightly higher energy consumption observed in HCDI is attributed to the resistance associated with the Ag/AgCl reaction.

Fig. 5 provides a comparative evaluation of the desalination behavior of MCDI and HCDI systems operated under varying current densities. As shown in Fig. 5(a) and (c), increasing the applied current density led to a reduction in total operation time for both systems, while preserving the characteristic conductivity response associated with constant-current adsorption and desorption processes. To more clearly elucidate the trade-off between desalination capacity and ion removal rate, the salt adsorption capacity (SAC) and average salt adsorption rate (ASAR) were analyzed using CDI Ragone plots, as shown in Fig. 5(b) and (d). The Ragone plots reveal a clear inverse relationship between SAC and ASAR, reflecting the well-known trade-off between adsorption capacity and rate in CDI systems. Importantly, both SAC and ASAR increased with increasing feed concentration, highlighting the beneficial role of higher ionic strength in reducing solution resistance and facilitating ion transport under constant-current operation.

The performance comparison between the two systems shows that HCDI consistently outperformed MCDI across the operation conditions. At a feed concentration of 10 mM NaCl, the HCDI system achieved a maximum SAC of 36.5 mg g⁻¹, more than twice that of the MCDI system (17.0 mg g⁻¹). This substantial enhancement originates from the asymmetric electrode configuration of HCDI, where a Faradaic silver electrode provides additional ion removal capacity beyond the electrostatic adsorption mechanism of activated carbon electrodes used in MCDI. As a result, HCDI overcomes the intrinsic capacity limitations associated with capacitive carbon-based systems. In addition, HCDI also exhibited better ion removal rate, as evidenced by its higher ASAR values across all operating conditions. This kinetic advantage is consistent with previous reports and can be attributed to the rapid and reversible Ag/AgCl conversion reaction occurring on micro-sized silver particles. Such rapid Ag/AgCl conversion reaction is enabling faster ion uptake and release [40]. As a result, HCDI simultaneously exhibits enhanced desalination capacity and sustained adsorption rate. This suggests that the incorporation of Faradaic electrodes redefines the capacity–rate relationship, allowing HCDI to bridge the gap between high salt adsorption capacity and rapid ion removal. This comparative analysis underscores the advantages of hybrid architectures over conventional MCDI and highlights the potential of HCDI as a high-performance desalination system.

To investigate the electrochemical properties of the enhanced desalination performance observed in the silver/activated carbon (Ag/AC) HCDI system, GCD experiments were conducted using two different cell configurations: an activated carbon/activated carbon (AC/AC) electrode pair and a silver/activated carbon (Ag/AC) electrode pair. Fig. 6(a) presents the voltage profiles of each cell measured in a 1 M NaCl solution at a current density of ±1.25 mA/cm². As shown in the result, the cell employing the Ag/AC electrodes exhibits significantly larger charge and discharge capacities than the AC/AC cell, indicating that the Ag/AC system can store a larger amount of charge within the same voltage window.

Fig. 6(b) and 6(c) show the potential variations of the positive and negative electrodes of symmetric AC/AC and asymmetric Ag/AC cells, respectively. During the charge–discharge process, referenced to an Ag/AgCl (saturated KCl) electrode. In the case of the activated carbon electrode, charge storage occurs through the formation of an electrical double layer by ion adsorption on the electrode surface, resulting in a potential that increases proportionally with the stored charge. In contrast, the silver electrode stores charge via a Faradaic phase transformation reaction (Ag + Cl⁻ ↔ AgCl), and therefore its potential remains nearly constant during GCD, as shown in Fig. 6(c).

Owing to this fundamental difference in charge-storage mechanisms, the Ag/AC system operates analogously to a hybrid capacitor, allowing a wider potential window to be utilized on the activated carbon electrode compared to the symmetric AC/AC system, demonstrating that the combination of the high capacity of the Faradaic electrode and the expanded usable potential range of the activated carbon electrode enables the hybrid CDI system to remove a larger amount of ions than conventional capacitive deionization.

4. Conclusions

This study demonstrated that a silver/activated carbon (Ag/AC) hybrid capacitive deionization (HCDI) system operated under constant-current conditions exhibits markedly enhanced desalination performance compared to conventional membrane capacitive deionization. Owing to the asymmetric charge-storage mechanism, in which Faradaic Ag/AgCl conversion at the silver electrode stabilizes cell voltage and expands the usable potential window of the activated carbon electrode, the HCDI system achieved a significantly higher salt adsorption capacity, reaching up to 36.5 mg g⁻¹ at 10 mM NaCl, compared to 17.0 mg g⁻¹ for MCDI under 10 mM NaCl solution. In addition, the HCDI system consistently exhibited faster ion removal performances, as reflected by higher average salt adsorption rates across a wide range of operating conditions. These results highlight the effectiveness of integrating Faradaic electrodes into CDI architectures and provide practical design guidelines for developing high-capacity and rapid electrochemical desalination systems.

Notes

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2025-02223368) and the 2026 Hongik University Research Fund, Republic of Korea.

Author Contributions

J.L. (Associate Professor) conducted all the experiments, wrote the manuscript, and revised the manuscript.

Conflict of Interest

The authors declare that they have no conflict of interest.

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Fig. 1

Schematic diagram of the comparison analysis of HCDI and MCDI systems via constant current operation.

Fig. 2

(a) Schematic diagram of electrochemical analysis cell. (b) Illustration of the CDI module for the desalination tests of HCDI and MCDI (1: current collector, 2: Ag electrode, 3: spacer, 4: cation exchange membrane, 5: AC electrode, 6: anion exchange membrane)

Fig. 3

Scanning electron microscope (SEM) images of (a) silver composite and (b) activated carbon sheet electrodes, showing surface morphology. Cyclic voltammetry (CV) curves measured at a scan rate of 2 mV/s in 1 M NaCl solution for (c) the silver composite electrode and (d) the activated carbon electrode.

Fig. 4

(a) Conductivity variation of the effluent and (b) corresponding current profiles in the HCDI system during operation. (c) Comparison of effluent conductivity and (d) current response between MCDI and HCDI systems under constant current conditions (1.25 mA/cm²) at a flow rate of 5 mL/min in 10 mM NaCl solution.

Fig. 5

Conductivity profiles of (a) MCDI and (c) HCDI measured in 10 mM NaCl solution under various current densities. CDI Ragone plots of (b) MCDI and (d) HCDI systems evaluated at different feed concentrations (10, 20, and 50 mM NaCl) under constant-current operation. The Ragone plots represent the relationship between salt adsorption capacity (SAC) and average salt adsorption rate (ASAR), providing insight into the desalination performance of each system.

Fig. 6

GCD profiles of symmetric AC/AC and asymmetric Ag/AC cells measured in 1 M NaCl at a current density of ±1.25 mA/cm². (a) Full cell voltage as a function of specific capacity of AC/AC and Ag/AC configuration. Potential profiles of each electrode in (b) the AC/AC and (c) the Ag/AC cell referenced to Ag/AgCl (KCl sat’) electrode.