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Capacitive deionization technology for brackish water treatment has been widely studied but currently still suffers from relatively low efficiency. Therefore, a rational design of promising electrode materials has become an urgent task. In this study, a so-called pore-on-pore strategy was developed by chemical activation of the intrinsic porous renewable precursor to generate a micro–meso hierarchical porous carbon. The synergistic effect of the biocomponent and activation agent is the major mechanism for the high porosity. The synthetic carbon has a very high specific surface area (2147.43 m2 g–1). In addition, the carbon exhibits good performance in terms of electroadsorption, with adsorption capacities of 19.35 and 30.64 mg g–1 in solutions with initial concentrations of 500 and 1500 mg L–1 NaCl under 1.2 V applied voltage, respectively. A capacitance retention of 98.7% and charging efficiency of 98.3% were achieved after 100 uninterrupted charge and discharge cycles, indicating relatively good regeneration performance. This research provides a bright way to develop an excellent performance electrode from renewable resources for environmental applications.

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The world is suffering from chronic water shortage due to the increasing population, water pollution and industrialization. Desalinating saline water offers a rational choice to produce fresh water thus resolving the crisis. Among various kinds of desalination technologies, capacitive deionization (CDI) is of significant potential owing to the facile process, low energy consumption, mild working conditions, easy regeneration, low cost and the absence of secondary pollution. The electrode material is an essential component for desalination performance. The most used electrode material is carbon-based material, which suffers from low desalination capacity (under 15 mg·g−1). However, the desalination of saline water with the CDI method is usually the charging process of a battery or supercapacitor. The electrochemical capacity of battery electrode material is relatively high because of the larger scale of charge transfer due to the redox reaction, thus leading to a larger desalination capacity in the CDI system. A variety of battery materials have been developed due to the urgent demand for energy storage, which increases the choices of CDI electrode materials largely. Sodium-ion battery materials, lithium-ion battery materials, chloride-ion battery materials, conducting polymers, radical polymers, and flow battery electrode materials have appeared in the literature of CDI research, many of which enhanced the deionization performances of CDI, revealing a bright future of integrating battery materials with CDI technology.

Electrochemical deionization (EDI) is hopefully the next generation of water treatment technology. Bismuth (Bi) is a promising anode material for EDI, due to its high capacity and selectivity toward Cl–, but the large volume expansion and severe pulverization aggressively attenuated the EDI cycling performance of Bi electrodes. Herein, carbon-layer-encapsulated nano-Bi composites (Bi@C) were prepared by a simple pyrolysis method using a Bi-based metal–organic framework as a precursor. Bi nanoparticles are uniformly coated within the carbon layer, in which the Bi–O–C bond enhances the interaction between Bi and C. Such a structure effectively relieves the stress caused by volume expansion by the encapsulation effect of the carbon layer. Moreover, the introduction of a carbon skeleton provides a conductive network. As a consequence, the Bi@C composite delivered excellent electrochemical performance with a capacity of 537.6 F g–1 at 1 mV s–1. The Cl– removal capacity was up to 133.5 mg g–1 at 20 mA g–1 in 500 mg L–1 NaCl solution. After 100 cycles, the Bi@C electrode still maintains 71.8% of its initial capacity, which is much higher than the 26.3% of the pure Bi electrode. This study provides a promising strategy for improving EDI electrode materials.

Layered double hydroxides (LDHs) are perceived as a hopeful capacitive deionization (CDI) faradic electrode for Cl– insertion due to its tunable composition, excellent anion exchange capacity, and fast redox activity. Nevertheless, the self-stacking and inferior electrical conductivity of the two-dimensional structure of LDH lead to unsatisfactory CDI performance. Herein, the three-dimensional (3D) hollow nanocage structure of CoNi-layered double hydroxide/carbon composites is well designed as a CDI anode by cation etching of the pre-carbonized ZIF-67 template. C/CoNi-LDH has a unique 3D hollow nanocage structure and abundant pore features, which can effectively suppress the self-stacking of LDH sheets and facilitate the transport of ions. Moreover, the introduced amorphous carbon layer can act as a conductive network. When employed as the CDI anode, C/CoNi-LDH exhibited a high Cl– removal capacity of 60.88 mg g–1 and a fast Cl– removal rate of 18.09 mg g–1 min–1 at 1.4 V in 1000 mg L–1 NaCl solution. The mechanism of the Cl– intercalation pseudo-capacitance reaction of C/CoNi-LDH is revealed by electrochemical kinetic analysis and ex situ characterization. This study provides vital guidance for the design of high-performance electrodes for CDI.

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Developing a high-performance Cl-storage electrode is a crucial issue for capacitive deionization (CDI). Iron-/nitrogen-doped carbon hybrid composites with densely dispersed ultrafine Fe-based nanoparticles are promising candidates for Cl-storage electrodes, yet further improvement of Fe-based nanoparticles prone to agglomeration is strongly desired. Hereby, a hybrid electrode with ultrafine iron carbide nanoparticles encapsulated in graphene/chitosan-derived N-doped carbon (Fe 3 C@GNC) is successfully constructed via facile one-step pyrolysis of aerogel composites. The encapsulation effect of graphene can effectively confine Fe 3 C nanoparticles in the carbon matrix, enabling stable and dispersive ultrafine Fe 3 C nanoparticles, and chitosan also enables N-doping. Also, a satisfactory conductive system with synergistically long-and short-range conductive networks is successfully generated by the graphene/N-doped carbon matrix. The Fe 3 C@GNC electrode exhibits a typical pseudocapacitive behavior, with a specific capacitance of up to 305.33 F g −1 and a dominant capacitive contribution of up to 96%. As a Cl-storage electrode for CDI, it delivers a Cl − adsorption capacity as high as 82.08 mg g −1 with a retention rate of 74.2% for 150 cycles. Furthermore, it is revealed that the Cl − storage mechanism of Fe 3 C@ GNC is a pseudocapacitance effect induced by the reversible Fe 2+ /Fe 3+ redox couple, which can achieve fast reaction kinetics and structural stability in the CDI process.