Activated carbon, Potassium ferricyanide (K₃Fe(CN)₆, ≥98%), Ethanol, Ferric chloride hexahydrate (FeCl₃·6H₂O, ≥98%), Carboxyl Multi-wall Carbon Nanotube (C-CNT), Nickel Chloride (NiCl₂·6H₂O, ≥98%), sodium chloride (NaCl), Polyvinylidene fluoride (PVDF), Graphite powder and N-methyl-2-pyrrolidone (NMP) were obtained from Tianjin Damao Chemical Reagent Partnership. All chemical reagents mentioned in this study did not require subsequent purification.

The NF-PBA was synthesized via a co-precipitation process. Solution A was prepared by dissolving precise molar ratios of NiCl₂·6H₂O and FeCl₃·6H₂O (8:2, 5:5, 2:8) in 50 mL deionized (DI) water, while solution B was prepared by dissolving K₃Fe(CN)₆ in an equal volume DI water, maintaining a total mass ratio of 2:1 between the combined NiCl₂·6H₂O and FeCl₃·6H₂O to K₃Fe(CN)₆. Under intense stirring, solutions A and B were dropwise added to 50 mL DI water, and the mixture was aged for 12 h. The precipitate was washed with DI water and ethanol, then vacuum-dried at 60°C overnight. Ni-PBA, synthesized without FeCl₃·6H₂O, was labeled separately, while NF-PBA-1, NF-PBA-2, and NF-PBA-3 used the 8:2, 5:5, and 2:8 ratios, respectively.

The synthesis of the NF-PBA/CNT composite utilized an in-situ polymerization technique [36]. Solution A, a 0.05 M mixture of NiCl₂·6H₂O and FeCl₃·6H₂O at an 8:2 molar ratio, was dissolved in 50 mL DI water. Solution B, containing 0.0025 M K₃Fe(CN)₆, was prepared in an equal volume DI water. A 0.01 g CNT suspension was sonicated for 0.5 h, and then solution A was added, followed by the dropwise addition of solution B under stirring. The mixture reacted for 3 h at room temperature. The precipitate was washed with DI water and ethanol, then vacuum-dried at 60°C to acquire NF-PBA/CNT-1. The process was repeated with 0.02 g, 0.03 g, and 0.04 g of CNT to yield NF-PBA/CNT-2, NF-PBA/CNT-3, and NF-PBA/CNT-4, respectively. The fabrication procedure is illustrated in Scheme 1.

The morphological characterization of the samples was conducted through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) was used to determine the crystal phases. Fourier transform infrared spectroscopy (FTIR) helped to investigate the materials’ vibrational modes. The specific surface area (SSA) and porosity were determined through the N₂ adsorption/desorption method. X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface chemistry.

The electrochemical properties of Ni-PBA, NF-PBA, and NF-PBA/ CNT were evaluated using an electrochemical workstation (CHI 760E, Chenhua) with a three-electrode setup: the prepared electrodes, a saturated calomel electrode (SCE) as the reference electrode, and a Pt sheet (1×1 cm²) as the counter electrode. To fabricate the working electrodes, a mixture of active components, conductive carbon black, and PVDF was ground in a ratio of 8:1:1. This mixture was then stirred with NMP to form a slurry, which was subsequently coated onto graphite paper. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) were employed to detect electrochemical performances in a 1 M NaCl solution. The specific capacitance (SC, F g⁻¹) in CV and GCD tests is calculated using Eqs. (1), (2).

where Δ V (V) is the potential window, I (A) is the response current, m (g) is the mass of the active component, v (mV s⁻¹) is the sweep rate, and Δ t (s) is the time.

A batch-mode CDI system was employed to evaluate the desalination capability of asymmetric cells. The setup encompassed a CDI cell (Fig. S1), a computer for data acquisition, a conductivity meter, a multi-meter, a peristaltic pump for solution circulation, and a direct-current power supply. The CDI cell was configured by combining a PBA sheet (3×3 cm) as the anode with an activated carbon (AC) sheet (3×3 cm) as the cathode and nickel foam was used as the current collector. An insulating mesh was placed between them to ensure parallel assembly, thus constructing an asymmetric CDI system by integrating distinct synthetic PBA electrode materials with AC. A DDS-307F conductivity meter was monitored online the conductivity changes as 100 mL saline water (100–800 mg L⁻¹) circulated through the CDI module at 20 mL min⁻¹. Moreover, the changes in current were monitored online by a multi-meter (VC8265). The relationship between NaCl concentration and conductivity is depicted in Fig. S2. To determine the performance metrics, specific salt adsorption capacity (SAC, mg g⁻¹), salt adsorption rate (SAR, mg g⁻¹ min⁻¹), and charge efficiency (Λ) were calculated using Eqs. (3), (4), and (5), respectively.

where C₀ and C_e represent initial and equilibrium concentrations, m (g) is the total weight of the active component, t (s) is the deionization time, and V (mL) is the circulated solution volume. I (A), U (V), M (58.5 g mol⁻¹), and F (96485 C mol⁻¹) denote the response current, direct voltage, molecular weight of NaCl, and Faraday’s constant, respectively.

SEM characterization was performed to scrutinize the morphology of Ni-PBA, NF-PBA-2, and NF-PBA/CNT-2. Fig. 1a presents the image of Ni-PBA, revealing a nanoparticle morphology with dimensions of approximately 300–400 nm [41]. Comparatively, Fig. 1b shows the nanoparticle structure of NF-PBA, which closely resembles that of Ni-PBA. Notably, NF-PBA displays a significantly reduced and more uniform particle size distribution, typically within the range of 200–300 nm. This observation indicates that the increase in Fe content did not fundamentally alter the morphological properties of PBA but selectively regulated particle size. Despite this, NF-PBA still exhibits a pronounced tendency towards crystallite aggregation. Fig. 1c depicts the composites of NF-PBA/CNT-2, which are considerably smaller, with sizes spanning from 20–30 nm. The CNT can regulate the PBA crystallization process and optimize the crystallization of NF-PBA-2. Fig. 1d, which displays the TEM image of NF-PBA/CNT-2, reveals that CNT interconnects with NF-PBA-2 to create a 3D network structure [42]. This unique electron transfer channel and 3D conductive framework in NF-PBA/CNT-2 significantly reduce diffusion resistance, both between individual nanoparticles of NF-PBA-2 and among neighboring nanoparticles. In addition, the elemental maps in Fig. 1e show the presence of N, Fe, and Ni, which is consistent with the results of the XPS analyses, and show that Ni and Fe are uniformly dispersed in the composite. The improved structural characteristics of NF-PBA/CNT-2, particularly its smaller particle size and 3D network structure, are expected to contribute favorably to its entire performance.

The XRD patterns of three samples are depicted in Fig. 2a. The distinctive diffraction peaks of Ni-PBA are located at 14.9°, 17.3°, 24.5°, 35.1°, 39.3°, 43.4°, 50.5°, 53.5°, and 57.0° corresponding to the (111), (020), (022), (040), (042), (242), (044), (153), and (062) crystal facets, respectively, which all match the standard-crystal form of Ni-PBA (PDF# 96-101-0374) [43]. Interestingly, the XRD patterns of NF-PBA/CNT-2 and NF-PBA-2 exhibit a similar pattern to that of Ni-PBA, indicating that the alterations in Fe content and the addition of CNT have no impact on the crystal structure. Fig. 2b illustrates the FTIR spectra. The broad absorption bands at 2166 and 2098 cm⁻¹ are attributed to the −C≡N stretching vibrations within the [Fe(CN)₆] ligands, while the low-frequency band at 597 cm⁻¹ corresponds to the Fe-C≡N bond stretching vibrations. The distinct peaks within the range of 2098–2166 cm⁻¹ suggest the presence of divalent and trivalent iron species covalently bonded to carbon. Additionally, the peak at 3380 cm⁻¹ is attributed to the O-H stretching vibrations and the peak at 1607 cm⁻¹ corresponds to the H-O-H bending vibrations of water molecules [44]. These findings reveal crucial insights into the structural features and chemical compositions of the materials under investigation.

The N₂ adsorption-desorption isotherms are shown in Fig. 2c. These materials display Type IV isotherms, indicating their mesoporous nature [43]. The SSA calculated for these materials reveals values of 43.7 m² g⁻¹ for Ni-PBA, 98.4 m² g⁻¹ for NF-PBA-2, and 128.9 m² g⁻¹ for NF-PBA/CNT-2. Notably, the incorporation of CNT into NF-PBA leads to an augmentation in SSA, which is ascribed to the inherently high SSA of CNT and its capability to hinder the aggregation of NF-PBA particles. [36]. This transformation provides more active sites for ion accommodation, improving the material’s performance. Furthermore, the pore size distribution, pivotal for material functionality, significantly influences ion diffusion. The BJH method indicates that NF-PBA/CNT-2 composite primarily exhibits pore sizes ranging from 3–15 nm, which creates favorable conditions for ion diffusion, facilitating the diffusion process within the materials and ultimately improving their overall performance.

Analysis of the survey spectra reveals the presence of Ni, Fe, C, O, and N in all three composites, as depicted in Fig. 2d. Further examination of the Fe 2p spectra shows distinct peaks corresponding to Fe²⁺ and Fe³⁺, indicating different oxidation states at 706.3, 719.2, 708.7, and 721.5 eV (Fig. 2e). Similarly, the Ni 2p spectra displays two pairs of spin-orbit doublet peaks representing Ni²⁺ and Ni³⁺ at 854.0, 877.0, 857.0, and 875.0 eV [45], as shown in Fig. 2f. The satellite peaks at 860.1 and 879.3 eV further confirm the existence of Ni²⁺ in the composite [37].

As illustrated in Fig. 3a, the CV curves of different NF-PBA-x electrodes suggest a complex ion storage mechanism involving both an electrical double-layer and redox capacitive behavior [46]. Notably, when the Ni/Fe ratio reaches 2:8, the electrodes exhibit two pairs of redox peaks, signifying the reversible transitions of Fe^IIN₆/Fe^IIIN₆ and Fe^IIC₆/Fe^IIIC₆ at different potentials [47]. As the content of Ni doping increases, the HS Fe is substituted by Ni. Given that Ni^IIN₆ does not undergo redox reactions during the insertion/extraction of Na⁺, the redox reaction primarily takes place at Fe^IIC₆/Fe^IIIC₆ [48]. Meanwhile, as the content of Ni doping rises, the vacancies and crystalline water within the crystal structure are reduced, providing more adsorption sites for Na⁺ and promoting the redox reaction of Fe²⁺/Fe³⁺. As a result, the electrochemical activity of the NF-PBA-2 electrode can be significantly enhanced in Fig. 3b. However, since Ni does not take part in the reaction within the crystal structure, excessive doping of Ni in NF-PBA-3 will decrease the redox sites of Fe²⁺/Fe³⁺, thereby reducing the electrochemical activity of the material [37]. Fig. 3c displays the GCD curves of NF-PBA-x electrodes at 1 A g⁻¹. The potentials of its charge/discharge plateaus are similar to those of the redox peaks. Combined with the specific capacitance calculations in Fig. S3, NF-PBA-2 maintains a high specific capacitance in the GCD tests.

In Fig. 3d and Fig. 3e, the NF-PBA/CNT-2 has the largest integrated area and the longest discharge time. Moreover, the specific capacitance variation trend of NF-PBA/CNT-2 calculated according to the GCD curves (Fig. S4a) is similar to the result (Fig. S4b) calculated from the CV tests. The specific capacitance trend of the NF-PBA/CNT-x electrodes followed a pattern of initial increase and subsequent decrease, attributed to the change in CNT content. Initially, the addition of CNT improves electron transfer within the composite. However, the composite of excess CNT envelops PBA, which increases the double electric layer effect, prevents Na⁺ from penetrating PBA, and reduces the specific capacitance [42]. The electron transfer and material transfer processes were analyzed by EIS tests as shown in Fig. 3(f). Obviously, the NF-PBA/CNT-2 electrode presents a smaller semicircle than other ratios, which indicates that the NF-PBA/CNT-2 electrode has a stronger electron transfer capability [49].

The CV curves of Ni-PBA, NF-PBA-2, and NF-PBA/CNT-2 shown in Fig. 4a maintain a consistent shape with well-defined symmetrical oxidation/reduction peaks, indicating a reversible intercalation/ deintercalation process of Na⁺ within the crystalline structure of the materials. This process is facilitated by the alteration of the Fe²⁺/Fe³⁺ redox couple, primarily occurring at Fe^IIC₆/Fe^IIIC₆ during the insertion/extraction of Na⁺ [36]. The substitution of high-spin Fe by Ni results in NiIIN6 not undergoing redox reactions. Furthermore, the addition of CNT effectively improves the electrical conductivity of the composite, enhancing electron transfer. Consequently, NF-PBA/CNT-2 exhibits the largest integrated area and retains the highest specific capacitance throughout the scan range, reaching a peak specific capacitance of 286.76 F g⁻¹ at 5 mV s⁻¹ (Fig. 4b). The robust charge/discharge plateau and extended charge/discharge duration of NF-PBA/CNT-2 (Fig. 4c) signify effective redox reactions and Faradaic electrode functionality. The integration of the doped element with CNT leads to a significant enhancement in specific capacity (Fig. 4d), aligning with the values obtained from the CV curves. Besides, the superiority of NF-PBA/CNT-2 in specific capacitance is attributed to its lower [Fe(CN)₆] vacancy and crystal water content [50]. The comprehensive enhancement in performance demonstrated by NF-PBA/CNT-2 showcases its potential for advanced desalination applications.

The efficiencies of charge transfer and ion diffusion of Ni-PBA, NF-PBA-2, and NF-PBA/CNT-2 electrodes were examined through the EIS test. As shown in Fig. 4e, compared with other electrodes, NF-PBA/CNT-2 exhibits the smallest semicircle in the high-frequency region and presents a shorter and steeper straight line in the low-frequency region, which demonstrates its superior electron transfer ability and electrical conductivity [51]. From the charging resistance diagrams (Fig. 4f), the charging resistances of NF-PBA/CNT-2 are significantly lower. This is attributed to ion doping and increased electrical conductivity of CNT, which facilitates ion transport through electrostatic attraction on their surfaces [40]. By studying the dynamics of charge transfer on the electrodes, as shown in Fig. 4g, we find that the linear fit yielded b-values of 0.75 and 0.74, respectively. This result emphasizes the interplay between capacitance- and diffusion-controlled processes in governing the charge transfer efficiency [52]. Moreover, the capacitance behaviors of NF-PBA/CNT-2 at 50 mV s⁻¹ are shown in Fig. 4h. Through integration, the capacitive contribution of NF-PBA/CNT-2 is calculated to be 86.24%. The capacitance contribution rises from 50.64% to 86.24% as the sweep rate alters from 5–50 mV s⁻¹ (Fig. 4i and Fig. S5a–d). A high percentage implies sufficient exposed active sites, favoring rapid ion storage kinetics. In summary, the NF-PBA/CNT-2 electrode demonstrates superior charge transfer capability, enhanced electrical conductivity, and improved ion diffusion properties compared to Ni-PBA and NF-PBA-2 electrodes. These findings underscore the significance of composition regulation and structural design in optimizing the ion storage performance of the composite.

Fig. S6a displays the CV curves of AC at scan rates ranging from 5 mV s⁻¹ to 100 mV s⁻¹. All curves exhibit a quasi-rectangular shape without distinct redox peaks, consistent with typical EDL behavior. As the scan rate increases from 5 mV s⁻¹ to 100 mV s⁻¹, the specific capacitance decreases from 116.01 F g⁻¹ to 55.75 F g⁻¹ (Fig. S6b), attributed to the reduced ion migration time from solution to pores and increased electrode internal resistance at higher scan rates [53]. The GCD curves in Fig. S6c show symmetrical triangular profiles, corroborating the CV results and further demonstrating the excellent electrochemical reversibility of AC. Fig. S6d consists of small semicircular curves and steep lines in the high and low-frequency regions, and similar figures have been reported by other researchers [54]. The equivalent series resistance (Rs) of AC is 2.84Ω.

The desalination capabilities of different modules, Ni-PBA//AC, NF-PBA-2//AC, and NF-PBA/CNT-2//AC, were analyzed under various operating voltages and initial concentrations. In Fig. 5a, the salt adsorption capacity of the NF-PBA/CNT-2//AC cell was found to be 25.61 mg g⁻¹, the highest among the tested modules. It reaches adsorption equilibrium with the maximum adsorption amount, which is 1.24 times that of AC//NF-PBA-2 (20.63 mg g⁻¹) and 1.38 times that of Ni-PBA//AC (18.53 mg g⁻¹). The Ragone plot reveals that NF-PBA/CNT-2//AC has the maximum desalination capacity and the fax test desalination rate at 10.88 mg g⁻¹ min⁻¹. The desalination capacities are calculated at specific voltages ranging from 0.8 V-1.4 V (Fig. 5b), showing an increase in capacity with rising voltage for all modules. The NF-PBA/CNT-2//AC cell demonstrates superior performance across all voltage levels. Upon applying voltage, the redox reactions are swiftly activated, allowing Na⁺ ions to occupy the ionic adsorption sites until equilibrium is reached. This rapid response is attributed to the material’s fast charge-transfer capability and high electrical conductivity, consistent with electrochemical test results [31].

Desalination dynamics in a three-module system were examined and analyzed through extensive experiments. The data obtained was fitted to both pseudo-first-order and pseudo-second-order kinetic models, as shown in Fig. S7 and Fig. 5c, respectively. Subsequent analysis of the correlation coefficients (Table S1 and Table S2) indicates a strong relationship between the experimental results and the pseudo-second-order kinetic model. This revelation suggests that the Na⁺ embedding is primarily influenced by the thermodynamically favorable electrosorption phenomenon [23]. Of the three modules analyzed, the NF-PBA/CNT-2//AC cell exhibits the most robust desalination kinetics. It boasts an impressive correlation coefficient R² of 0.9971 and an exceptional maximum rate constant k of 3.97×10⁻⁴, making it the standout performer among its counterparts. A comparative evaluation of the desalination efficacy of various CDI modules reveals that as the initial concentration increases from 100–800 mg L⁻¹ under 1.4 V, the ion storage capacity of all three modules is significantly improved. Interestingly, the cell NF-PBA/CNT-2 consistently outperforms Ni-PBA//AC and NF-PBA-2//AC cells across various NaCl concentrations, highlighting its superior adaptability and efficiency in diverse settings. The visually represented data in Fig. 5d further underscores the remarkable desalination capabilities of the NF-PBA/CNT-2//AC cell and its excellent performance within different salinity levels.

The effectiveness of desalination in AC//NF-PBA/CNT-2 cell was examined through a series of experiments involving various operating voltages and initial NaCl concentrations. As shown in Fig. 5e, the desalination capacities were measured at 0.8, 1.0, 1.2, and 1.4 V, resulting in escalating values of 9.01, 14.21, 17.45, and 25.6 mg g⁻¹, respectively. These values demonstrated comparable desalination efficiency with the NCP-2 electrode but exhibited marginally lower performance compared to the NCP/CNT composite [36]. The observed difference may primarily originate from the insufficient CNT loading, which limits ionic transport pathways and surface charge density, thereby compromising the overall desalination capacity. Fig. S8 and Fig. S9 systematically characterize the current distribution and charging efficiency of the three modules under varying direct voltages. In Fig. S9, the charging efficiency of the NF-PBA/CNT-2//AC cell increases progressively with voltage elevation, peaking at 1.4 V. This behavior indicates that at 1.4 V charge utilization efficiency reaches its maximum. Notably, the absence of efficiency degradation at 1.4 V directly evidences the effective suppression of side reactions under this operational voltage [36]. By analyzing the salt adsorption rate of the cell under different voltages depicted in the Ragone plots (Fig. 5f), it becomes evident that an increase in voltage leads to a substantial improvement in salt adsorption capacity and ion removal rate. Despite the potential risks associated with high voltages, such as water electrolysis and parasitic reactions [33], the modules exhibit a notable enhancement in intercalation pseudo-capacitance rate, enabling more efficient ion insertion into the electrode material and consequently enhancing desalination performance. Furthermore, the investigation extends to the evaluation of salt adsorption in CDI modules under varying initial NaCl concentrations, showing a salt removal capacity increase from 11.91–32.15 mg g⁻¹ as the concentration elevates from 100–800 mg L⁻¹ (Fig. 5g). The corresponding Ragone plots (Fig. S10) confirm the superior ion removal capability of NF-PBA/CNT-2//AC cell upon exposure to higher NaCl concentrations, positioning the curves for 800 mg L⁻¹ NaCl at the top corner, signifying its exceptional desalination efficiency. These findings showcase the promising desalination potential of NF-PBA/CNT-2//AC cell and underscore the significance of optimizing operating conditions for enhanced salt removal performance.

In the comparison shown in Fig. 5h and Table S3, the desalination capacity and the desalination rate of NF-PBA/CNT-2 significantly surpasses that of other electrode materials such as Co-PBA, NiHCF/rGO, NaFeHCF@CNT, and ZIF-67/CNT. Fig. 5i shows the desalination and regeneration performance of NF-PBA/CNT-2//AC, with the capacity retention hovering around 89% after seven cycles. Notably, this desalination capacity outperforms that of the FeHCF@HGT composite previously reported in the literature [37], highlighting the enhanced capabilities of the NF-PBA/CNT-2//AC system in terms of both desalination efficiency and stability during the cycling process. During this process, NF-PBA suffers from volume expansion, which leads to active site loss and reduced conductivity [34], while the composite structure of CNT-bridged NF-PBA internally enhances the conductivity but does not fully alleviate the lattice stresses caused by crystal expansion. Despite these issues, NF-PBA/CNT-2 demonstrated excellent desalination capabilities, providing a valuable reference for subsequent studies.

The XRD patterns of the NF-PBA/CNT-2 electrode were analyzed before and after the deionization process (Fig. 6a), revealing that the crystal structure remains largely unchanged after cycling. This indicates that the electrode material maintains its excellent morphological and structural integrity throughout the desalination process. In Fig. 6b, a comparison of the XPS spectra of NF-PBA/CNT-2 before and after desalination shows that the peak of Na 1s appears at 1070.6 eV post-desalination (Fig. S11). Previous studies [40] have suggested that PBAs with open frameworks can facilitate Na⁺ diffusion and storage, and the increase of Na⁺ content in NF-PBA/CNT-2 confirms that Na⁺ inserts into the crystal structure successfully during desalination. Further analysis of the XPS spectra reveals changes in the Fe peaks (Fig. 6c), with a decrease in the abundance of Fe³⁺ and an increase in Fe²⁺ after desalination. This suggests a gradual reduction of Fe³⁺ to Fe²⁺ within the crystal structure, possibly due to the embedding of Na⁺. The high-resolution spectrum of Ni 2p after desalination (Fig. 6d) displays similar features to the original NF-PBA/CNT-2 electrode spectra. However, the intensities of the satellite peaks associated with nickel are increased after desalination, indicating potential changes in the crystal structure which is caused by Na⁺ intercalation during the desalination process. Overall, the analysis of the XRD and XPS spectra highlights the structural and chemical changes that occur in NF-PBA/CNT-2 after recycling. The maintenance of structural integrity and the observed changes in metal peaks point to the dynamic nature of electrode materials during desalination.

The mechanism of NF-PBA/CNT-2 in CDI is elucidated as follows (Fig. 7). Initially, the Fe²⁺/Fe³⁺ ion pairs on the PBA electrodes undergo an oxidation reaction, contributing to a pseudo-capacitance that facilitates electron transport. Secondly, Ni doping enhances the electrical conductivity of the composite and optimizes the electronic structure of the PBA. Meanwhile, it reinforces the stability of the crystal framework without participating in redox reactions. Further, the CNT in NF-PBA/CNT-2 forms a network structure with abundant transport channels, providing not only pores and a large SSA for the composite but also enabling efficient electron transfer to the PBA to participate in redox reactions. The CNT also fosters electron conduction pathways between neighboring particles, enhancing the efficiency of the redox cycle. In summary, the dual-element doping in NF-PBA/CNT-2 and the incorporation of CNT reduce vacancies and water of crystallization, significantly enhancing the specific capacity. The adsorption behavior of CNT within the electric double layer, combined with the redox reaction of NF-PBA, synergistically contributes to the efficient removal of ions during desalination.