Beneficial synergy of adsorption–intercalation–conversion mechanisms in Nb₂O₅@nitrogen-doped carbon frameworks for promoted removal of metal ions via hybrid capacitive deionization

Abstract

Capacitive deionization (CDI) is an emerging water purification technology, but the ion adsorption capacity of traditional carbon-based CDI electrodes is still unsatisfactory. Herein, a novel faradaic electrode by anchoring Nb₂O₅ nanoparticles on nitrogen-doped carbon frameworks as anodes and activated carbon (AC) as cathodes in a hybrid capacitive deionization (HCDI) system was originally developed to capture Na⁺ ions via adsorption–intercalation–conversion mechanisms. The synergistic effects of the nanostructure and carbon coating were beneficial to enhancing electrical conductivity and offering fast Na⁺ ion diffusion pathways. Impressively, the HCDI system demonstrated an excellent ion adsorption capacity of 35.4 mg g⁻¹ in a 500 mg L⁻¹ NaCl solution at 1.2 V as well as stable regeneration ability. In situ Raman and ex situ XPS measurements unraveled that the mechanism of ion removal from water was the reversible redox reaction of Nb₂O₅. The new overall understanding of the synergistic effects opens opportunities for the design of HCDI systems for efficient removal of metal ions from saline water.

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Environmental significance

The shortage of clean water has become a global issue as a consequence of population growth and climate change. Less than 3% of all water on earth is fresh water, and only 1.2% of all fresh water is surface water, which can be directly utilized. With this limited amount of usable fresh water, desalination of saline water and brackish water offers a promising solution to the supply of clean water. Capacitive deionization (CDI) has gained increasing attention due to its unique advantages of low energy consumption, low cost, rapid regeneration, and environment friendliness. Unfortunately, the ion adsorption capacity of traditional carbon-based CDI electrode materials is still unsatisfactory. Therefore, a novel faradaic electrode by anchoring Nb₂O₅ nanoparticles on the nitrogen-doped carbon frameworks (Nb₂O₅@N–C-1) in a hybrid capacitive deionization (HCDI) system was originally developed to capture Na⁺ ions via adsorption–intercalation–conversion mechanisms. Benefitting from the synergistic effects of structure optimization (nanostructure) and surface engineering (nitrogen-doped carbon coating), the Nb₂O₅@N–C-1 electrode demonstrated superior CDI performance in terms of ion adsorption capacity and ion adsorption rates. Furthermore, in situ Raman and ex situ XPS analyses were conducted to verify the adsorption–intercalation–conversion mechanism of ion removal from water during the CDI process. This work presents a new strategy to design highly efficient HCDI systems for efficient removal of metal ions from saline water.

1 Introduction

In the recent decades, the shortage of clean water has become a global issue as a consequence of population growth and climate change.^1–3 Less than 3% of all water on earth is fresh water, and only 1.2% of all fresh water is surface water, which can be directly utilized. With this limited amount of usable fresh water, desalination of saline water and brackish water offers a promising solution to the supply of clean water.^4,5 Traditional desalination methods have been examined to remove ions from seawater efficiently. Unfortunately, some of the methods suffer from various drawbacks including massive energy consumption, high cost and significant environmental impacts.^6,7 Therefore, it is imperative to develop cost-effective and eco-friendly desalination technologies as promising alternatives. Capacitive deionization (CDI) has gained increasing attention due to its unique advantages of low energy consumption, low cost, rapid regeneration, and environment friendliness.^8,9 When applying a low potential on the two electrodes, the ions in solution can be quickly adsorbed and harvested on the oppositely-charged electrodes. Thus, clean water is obtained.^10,11

The ion removal capacity of CDI is closely dependent on the electrode materials. Carbon-based materials, such as carbon aerogels, carbon nanotubes, activated carbon, mesoporous carbon, and reduced graphene, have been extensively explored as CDI electrode materials due to the advantages of high surface area, porous structure, and electrochemical stability.^12–15 Unfortunately, one of the major limitations of carbon-based materials is the unsatisfactory ion adsorption capacity.^16,17 In carbon-based electrodes, the ions removed from a solution are reserved on the carbon surface based on the electric double layer (EDL) theory, in which the removal capacity is mainly determined by the pores and surface area. In addition, the co-ion effect would restrain more ions from gathering and cause adsorbed ions to easily return to the solution.^18,19 To overcome the drawback, hybrid capacitive deionization (HCDI) has been developed. HCDI systems consist of one carbon electrode and one faradaic electrode.^20,21 In general, anions (such as Cl⁻ ions) are electrostatically adsorbed on the carbon-based materials. But cations (such as Na⁺ ions) are not only removed by surface adsorption, but also captured through a charge transfer reaction.^22,23 Compared with those EDL-based electrodes, HCDI systems deliver a higher ion removal capacity and faster ion removal rates.^24,25 The selection of faradic materials for HCDI systems usually favored those with substantially improved energy storage domains.^26,27 Back in 2012, Pasta²⁸et al. selected Na_2−xMn₅O₁₀ and Ag as the electrodes and obtained good desalination performance. After that, a series of faradic electrode materials, such as MnO₂, TiO₂, SnS₂, MoS₂, Na₄Ti₉O₂₀, NaTi₂(PO₄)₃, and Prussian blue, have been developed and proved to gain excellent ion removal capacities.^29–32 Wang³³et al. used the hollow carbon@MnO₂ to capture Na⁺ ions through a redox reaction and obtained a high removal capacity of 30.7 mg g⁻¹. Our former work³⁴ showed that synthesized MoS₂–graphene materials reserve/convert Na⁺ ions via a faradaic reaction. The obtained electrodes demonstrated a high volumetric adsorption capacity of 14.3 mg cm⁻³ in a 500 mg L⁻¹ NaCl solution at 1.2 V.

Orthorhombic Nb₂O₅ (T-Nb₂O₅), as a representative Na⁺ ion capturing material with an intercalation–conversion type, is a promising candidate for an HCDI electrode due to the intrinsic structural advantage.^35,36 The (001) planes of Nb₂O₅ have a larger interplanar spacing (3.9 Å) than the size of Na⁺ ions (2.04 Å), which may be suitable for Na⁺ ion diffusion.³⁷ Unfortunately, the low electrical conductivity (≈3 × 10⁻⁶ S cm⁻¹) and the sluggish diffusion of Na⁺ ions from aggregation tendency may hinder the further application of Nb₂O₅. Two effective methods can be adopted to enhance the electrical conductivity and decrease the diffusion length for electrons/ions: (1) nanostructure engineering can make the materials have low dimensions, which offers shorter pathways and faster Na⁺ ion diffusion;^38,39 (2) a composite with carbon can stabilize the materials and form an interconnected network for fast transport of electrons and Na⁺ ions.^40,41 Throughout all sorts of carbon-based supports, graphene gains much attention due to its large surface area, good chemical/physical stability, and superb electrical conductivity.⁴² Moreover, the heteroatoms (such as nitrogen) doped in the carbon skeleton could boost electrical conductivity, introduce more defects, and improve hydrophilicity.^43,44 Graphitic carbon nitride (g-C₃N₄), as a suitable nitrogen precursor, has a relatively high nitrogen content of 57% and an sp² hybridized carbon structure. The high level of pyridine type nitrogen can offer abundant lone electron pairs to capture Nb atoms as benign ligands.⁴⁵ 2-Methylimidazole, as another benign N ligand, can coordinate with Nb and prohibit the large aggregation of Nb₂O₅ nanocrystals during the heating process at elevated temperature.⁴⁶

Herein, we originally designed Nb₂O₅ anchored on nitrogen-doped carbon frameworks (denoted Nb₂O₅@N–C-1) via simple assembly and pyrolysis as anodes and activated carbon (AC) as cathodes. The obtained Nb₂O₅@N–C-1 has uniform Nb₂O₅ nanoparticles tightly anchored on the graphene networks. The N-doping improved the electrical conductivity and produced localized highly-reactive regions. Benefitting from the synergistic effects of structure optimization (nanostructure) and surface engineering (nitrogen-doped carbon coating), the Nb₂O₅@N–C-1 electrode demonstrated superior CDI performance in terms of ion adsorption capacity and ion adsorption rates. Furthermore, in situ Raman and ex situ XPS analyses were conducted to verify the adsorption–intercalation–conversion mechanism of ion removal from water during the CDI process.

2 Materials and methods

First of all, graphene oxide (GO) was prepared by modified Hummer's method,³⁹ and g-C₃N₄ was obtained by a thermal decomposition/polymerization process.⁴⁶ Specifically, 0.02 g of GO and 0.1 g of g-C₃N₄ were mixed in 50 mL of methanol after ultrasonic treatment for 1 h. Next, 0.11 g of NbCl₅ was added into the mixture after ultrasonic treatment for 0.5 h to form solution alpha. Meanwhile, 0.17 g of 2-methylimidazole (2-MeIM) was dispersed in 30 mL of methanol after ultrasonic treatment for 0.5 h to form solution beta. Then, solution beta was added dropwise into solution alpha with stirring, and the mixed solution was left at room temperature for 12 h. The Nb/2MeIM/g-C₃N₄/GO precursors were gained by centrifugation with methanol and drying at 60 °C for 12 h. After that, the dried material was calcined at 800 °C (3 °C min⁻¹) for 2 h under an N₂ atmosphere to obtain the Nb₂O₅@N–C-1 (NbCl₅ and 2-MeIM with a mole ratio of 1 : 1) composite. Simultaneously, Nb₂O₅@N–C-2 (0.11 g of NbCl₅ and 0.085 g of 2-MeIM with a mole ratio of 2 : 1), Nb₂O₅@N–C-0.5 (0.11 g of NbCl₅ and 0.34 g of 2-MeIM with a mole ratio of 1 : 2), Nb₂O₅ (only utilization of NbCl₅) and N–C (without utilization of NbCl₅) were synthesized through the same procedure.

3 Results and discussion

3.1 Characteristic analysis

These Nb₂O₅@N–C composites were synthesized via an assembly approach followed by a pyrolysis method at 800 °C under a N₂ atmosphere, as shown in Fig. 1. In the assembly process, Nb⁵⁺ ions were readily adsorbed on the surface of the negatively charged GO/g-C₃N₄ complex and further coordinated with 2-methylimidazole (2-MeIM) to form the Nb/2MeIM/g-C₃N₄/GO precursors (Fig. S1a and b†). After the pyrolysis treatment, the Nb₂O₅@N–C composites were successfully prepared. The morphology and microstructure of the obtained samples were observed and verified by SEM and TEM (Fig. S2†). These electron microscopy images revealed that the Nb₂O₅ nanoparticles with an average size of around 50 nm (Fig. S2c, f, and i†) were tightly anchored on the nitrogen-doped graphene sheets. Besides, the density of the Nb₂O₅ nanocrystals increased with the improvement of Nb⁵⁺ utilization, from Nb₂O₅@N–C-0.5 (Fig. S2a and b†) and Nb₂O₅@N–C-1 (Fig. S2d and e†) to Nb₂O₅@N–C-2 (Fig. S2g and h†). The contact between the Nb₂O₅ nanoparticles and graphene network (Fig. 2a) could supply numerous pathways for ion diffusion and electron transfer, which would improve the electrochemical performance. The HRTEM image of Nb₂O₅@N–C-1 (Fig. 2b) displays lattice fringes with a spacing of 0.393 nm, which are well attached to the (001) planes of the orthorhombic Nb₂O₅ phase. In Fig. S3,† prominent diffraction rings were observed in the selected area electron diffraction (SAED) patterns, which confirmed the existence of the (001), (180), and (200) planes of the orthorhombic Nb₂O₅ phase. Furthermore, the relevant elemental mapping images of Nb₂O₅@N–C-1 demonstrated the uniform distributions of C, N, O, and Nb elements over the selected area, which proved that the Nb₂O₅ nanoparticles were homogeneously anchored on the nitrogen-doped graphene networks (Fig. S4†).

Fig. 1 Schematic illustration of Nb₂O₅@N–C composites as anodes for hybrid capacitive deionization.

Fig. 2 (a) TEM image, (b) HRTEM image, (c) XRD pattern of Nb₂O₅@N–C-1, and (d) Raman spectra of Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2.

The formation and crystalline structure of the Nb₂O₅ nanocrystals in these Nb₂O₅@N–C composites were verified by XRD analysis, as shown in Fig. 2c and S5.† These characteristic diffraction peaks located at 22.6°, 25.7°, 28.4°, 28.9°, 36.6°, 37.0°, 46.2°, 50.9°, and 54.9°, could be appropriately indexed to the (001), (041), (180), (200), (181), (201), (002), (331), and (371) planes of the orthorhombic Nb₂O₅ phase (PDF#30-0873). To figure out the composition and structural properties of the correlative carbon supports, Raman analysis of the obtained samples was conducted (Fig. 2d). Two typical carbon-based characteristic peaks of disordered carbon (D band) and sp² bonded ordered graphitic carbon (G band) were observed at around 1350 and 1599 cm⁻¹. The value of I_D/I_G was normally related to the degree of structural disorder and the number of defects. The I_D/I_G values of Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2 were 1.19, 1.16, and 1.11, revealing the abundant vacancies and defects. Moreover, the degree of disorder decreased with the increase of metal contents. Besides, the three small peaks at 121, 248, and 660 cm⁻¹ were the characteristic bands of Nb₂O₅, which correspond to the vibrations of octahedra, the bending vibration of the Nb–O bond, and the stretching vibration of the Nb–O bond,^47,48 respectively. The pore structure characteristics of the Nb₂O₅@N–C composites were obtained by N₂ adsorption/desorption measurements. Both the BET specific surface area and pore volume decreased as the amount of Nb₂O₅ in the composite increased (Table S1†). Nb₂O₅@N–C-0.5 exhibited the largest BET specific surface area of 78.6 m² g⁻¹, in comparison to Nb₂O₅@N–C-1 (34.1 m² g⁻¹) and Nb₂O₅@N–C-2 (29.5 m² g⁻¹) (Fig. S6a†). Moreover, the pore size distribution plots indicated the mesoporous structures of the three samples, which could supply abundant diffusion channels for ion transportation. Specifically, Nb₂O₅@N–C-1 showed a similar pore size and volume distribution compared to Nb₂O₅@N–C-0.5, which are much higher and richer than those of Nb₂O₅@N–C-2 in the range of 2–10 nm (Fig. S6b†). To further investigate the surface chemical elements and bonding states of the Nb₂O₅@N–C composites, XPS analysis was conducted, as shown in Fig. S7.† The Nb 3d, C 1s, N 1s, and O 1s signals were obviously seen in the full-scan spectra of Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2, proving the successful integration of Nb₂O₅ in the structure of the carbon frameworks. Moreover, the formation of C–N bonds (Fig. S8†) confirmed the insertion of N atoms into the carbon plane. The N 1s peak consisted of four types of N, namely pyridinic N, pyrrolic N, graphitic N, and oxide N (Fig. S9†). The Nb–O bond (Fig. S10†) verified the presence of Nb₂O₅. The atomic percentage of the Nb element (Table S2†) increased from Nb₂O₅@N–C-0.5 to Nb₂O₅@N–C-2, which was consistent with the Nb precursor utilization in the synthesis process. Since the electrochemical properties and CDI performance were tested in NaCl aqueous solution, the hydrophilicity of the electrode materials was also a vital indicator. We evaluated the hydrophilicity by dynamic water contact angle test (Fig. S11†). The results showed that Nb₂O₅@N–C-1 has a smaller contact angle (74.29°) than Nb₂O₅@N–C-0.5 (75.42°) and Nb₂O₅@N–C-2 (84.14°). The good hydrophilicity of Nb₂O₅@N–C-1 was due to the porous structure and abundant N content within the carbon frameworks.

3.2 Electrochemical performance

The electrochemical performance of the Nb₂O₅@N–C composites was firstly evaluated by cyclic voltammetry (CV). The CV plots were obtained at scan rates ranging from 1 to 100 mV s⁻¹ (Fig. S12a–c†). For every composite, the integrated area of the CV curve was gradually enlarged with the increase of scan rates. At the same scan rate, the Nb₂O₅@N–C-1 curve has the largest integrated area, followed by the Nb₂O₅@N–C-0.5 and Nb₂O₅@N–C-2 curves. As a result, Nb₂O₅@N–C-1 possessed the highest specific capacitance among these composites at any scan rate (Fig. S12d†). To gain a deeper insight into the charge transfer mechanisms in Nb₂O₅@N–C-1, we quantitatively divided the contributions of the capacitive and diffusion-controlled effects from the CV plots by using a slower scan rate of 0.1 mV s⁻¹. In Fig. 3a, a redox peak appeared at around −0.13 V, demonstrating a faradic reaction of Na⁺ ions inserting into the Nb₂O₅@N–C-1 electrode. Therefore, the Na⁺ ions are removed by the synergistic effects of electro-adsorption of the nitrogen-doped carbon frameworks and the faradaic reaction of Nb₂O₅ (Fig. S13†). The surface capacitive (shaded areas) and diffusion-controlled contributions of Nb₂O₅@N–C-1 at 0.1 mV s⁻¹ were 64.3% and 35.7%, respectively.^37,49 The results confirmed that Nb₂O₅ played a leading role in capturing Na⁺ ions. Fig. S14a–c† show the galvanostatic charge–discharge (GCD) plots of Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2 at a current density of 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 A g⁻¹. The GCD plots demonstrated analogous symmetric triangular shapes, exhibiting benign electrochemical reversibility. Besides, when the current density was raised from 0.2 to 1.2 A g⁻¹, the Nb₂O₅@N–C-1 electrode showed the longest discharge time, which manifested the largest capacitance in accordance with the CV results. Additionally, Nb₂O₅@N–C-1 showed a lower potential drop (iR) than other materials at the initial discharge process (Fig. S14d†), suggesting a better electrical resistance. Cycling stability was another vital parameter for electrode materials. In Fig. 3b, the GCD plots maintained the original shape after 10 000 cycles at 10 A g⁻¹, manifesting an outstanding cycle-to-cycle durability. The superior electrochemical property of Nb₂O₅@N–C-1 was further demonstrated by electrochemical impedance spectroscopy (EIS), as shown in Fig. 3c. The Nyquist plots consisted of a semicircle in the medium frequency region corresponding to charge-transfer resistance (R_ct), and a straight segment in the low frequency region corresponding to Warburg diffusion impedance (W). An equivalent circuit (inset of Fig. 3c) was employed to simulate these EIS curves, in which R_s and CPE_ct represent the solution resistance and constant phase part, respectively.⁵⁰ Nb₂O₅@N–C-1 demonstrated the smallest semicircle among the three electrodes, indicating the lowest charge-transfer impedance. According to the fitting results (Table S3†), the R_ct values were 14.66, 18.52, and 23.94 Ω for Nb₂O₅@N–C-1, Nb₂O₅@N–C-0.5, and Nb₂O₅@N–C-2, respectively. Furthermore, the diffusion coefficient (D_Na⁺) of Nb₂O₅@N–C-1 was 4.18 × 10⁻¹⁵ cm² s⁻¹, much larger than those of Nb₂O₅@N–C-0.5 (2.49 × 10⁻¹⁵ cm² s⁻¹) and Nb₂O₅@N–C-2 (2.27 × 10⁻¹⁵ cm² s⁻¹), indicating enhanced Na⁺ ion diffusion and adsorption (Fig. 3d). Therefore, the excellent electrochemical properties may endow Nb₂O₅@N–C-1 with good CDI performance.

Fig. 3 (a) Total current (green line) and capacitive current (shaded regions) of Nb₂O₅@N–C-1 at 0.1 mV s⁻¹, (b) 10 000 GCD cycle curves of Nb₂O₅@N–C-1 in the first 10 cycles and the last 10 cycles at 10 A g⁻¹, (c) EIS curves of Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2, inset: the equivalent circuit to simulate EIS curves, and (d) the corresponding plots of real part impedance (Z′) vs. square root of the angular frequency (ϖ^−1/2) in the Warburg region.

3.3 Capacitive deionization of water

The batch-mode CDI experiments were conducted to evaluate the removal capacity of the electrode pairs. In Fig. 4a, the deionization behaviors of Nb₂O₅//AC, Nb₂O₅@N–C-0.5//AC, Nb₂O₅@N–C-1//AC and Nb₂O₅@N–C-2//AC were analyzed in a 500 mg L⁻¹ NaCl solution at 1.2 V. It was observed that the ion adsorption capacity (IAC) increased quickly at first and then the trend gradually slowed down within the required time. The Na⁺ ions were removed through electro-adsorption and electrochemical reactions with Nb₂O₅@N–C, while the negatively charged Cl⁻ ions were electrically-adsorbed by the AC electrode. After 120 min, the IAC of Nb₂O₅@N–C-1//AC reached 35.4 mg g⁻¹, much higher than those of Nb₂O₅@N–C-0.5//AC, Nb₂O₅@N–C-2//AC, and Nb₂O₅//AC which were 29.2 mg g⁻¹, 19.4 mg g⁻¹, and 10.4 mg g⁻¹, respectively. The result confirmed that the combinations of the Nb₂O₅ composite with the nitrogen-doped carbon framework can enhance the removal capacity of pure metal oxides. In addition to IAC, the ion adsorption rate (IAR) is another key indicator of the CDI performance. Fig. 4b shows the Ragone plots of IAR vs. IAC of the four electrodes. IAR was located at a high level firstly and moved downwards with the increase of IAC. The Ragone plot of Nb₂O₅@N–C-1//AC was located at the top right corner, suggesting that Nb₂O₅@N–C-1//AC has the fastest ion adsorption rate and the highest removal capacity among the four electrode materials. Additionally, to demonstrate the advantages of Nb₂O₅@N–C-1//AC over simple carbon substrates and the advantages of the nitrogen-doped carbon framework over commercial AC, the desalination behaviors of Nb₂O₅@N–C-1//AC along with N–C//AC and AC//AC were tested in a 500 mg L⁻¹ NaCl solution at 1.2 V. Unsurprisingly, the Nb₂O₅@N–C-1//AC composites exhibited the largest IAC and the quickest IAR (Fig. S15†). The outstanding performance of Nb₂O₅@N–C-1//AC was further confirmed in Fig. S16† where Nb₂O₅@N–C-1//AC exhibited the greatest concentration reduction (CR) (0.88 mM) and the fastest average ion adsorption rate (AIAR) (0.50 mg g⁻¹ min⁻¹) among all the electrode materials. The charge utilization and energy loss in the CDI process could be measured by charge efficiency. The charge efficiencies of Nb₂O₅@N–C-0.5//AC, Nb₂O₅@N–C-1//AC, Nb₂O₅@N–C-2//AC, and Nb₂O₅//AC were calculated using the current transient curves in a 500 mg L⁻¹ NaCl solution at 1.2 V. As shown in Fig. 4c, Nb₂O₅@N–C-1//AC has a higher charge efficiency (0.70) than Nb₂O₅@N–C-0.5//AC (0.58), Nb₂O₅@N–C-2//AC (0.46) and Nb₂O₅//AC (0.31). A series of trade-off curves between the inverse of specific energy consumption (SEC⁻¹) and IAR were also recorded in a 500 mg L⁻¹ NaCl solution at 1.2 V. In Fig. 4d, the trade-off curve of Nb₂O₅@N–C-1//AC was always above the curves of Nb₂O₅@N–C-0.5//AC, Nb₂O₅@N–C-2//AC, and Nb₂O₅//AC. This means that Nb₂O₅@N–C-1//AC reached the fastest IAR in the same SEC⁻¹ compared with other electrodes, demonstrating the most efficient energy utilization. The synergistic effects of the electro-adsorption of the nitrogen-doped carbon framework and the faradaic reaction of Nb₂O₅ facilitated ion diffusion and ion adsorption. The deionization property of Nb₂O₅@N–C-1//AC was further analyzed at various initial concentrations of NaCl solution and different applied voltages. In Fig. 4e, it was clear that the IAC of Nb₂O₅@N–C-1//AC increased from 18.1 and 27.4 to 35.4 mg g⁻¹ as the NaCl concentrations increased from 100 and 300 to 500 mg L⁻¹ at 1.2 V. In general, a higher NaCl concentration reduced the ionic resistance and facilitated fast ion adsorption. Compared with other reported metal-based electrode materials (Table S4†), Nb₂O₅@N–C-1 exhibited an excellent deionization behavior with a high ion adsorption capacity. Moreover, the Ragone plot moved to the top right with the increase of NaCl concentration, verifying a faster IAR and a higher IAC in a 500 mg L⁻¹ solution (Fig. S17a†). Fig. 4f presents the IAC of Nb₂O₅@N–C-1//AC at distinct applied voltages of 0.8, 1.0, and 1.2 V. The change of voltages was consistent with the change of NaCl concentrations. Notably, IAC increased from 4.9 and 19.1 to 35.4 mg g⁻¹ when the voltage was raised from 0.8 and 1.0 to 1.2 V. The Ragone plots (Fig. S17b†) also showed a faster IAR and a higher IAC at a higher voltage. The enhanced deionization performance was due to the stronger electrostatic force at a higher applied voltage. The comparison between the CR and AIAR in different combinations of NaCl concentration and voltage also showed that the best performance was achieved when the initial NaCl concentration was 500 mg L⁻¹ and the applied voltage was 1.2 V (Fig. S18†). The regeneration ability was crucial to justifying a deionization material for further applications. The continuous cyclic adsorption–desorption experiments of Nb₂O₅@N–C-1//AC were conducted in a 100 mg L⁻¹ NaCl solution (Fig. S19†). The adsorption process was started by applying a 1.2 V voltage, and the subsequent desorption process was realized by short circuiting at 0 V. In every cycle, the electrical conductivity dropped to a low level during the adsorption process and recovered to the initial level during the desorption process. The water recovery (WR) was 33.3%, the energy consumption was 100 J g⁻¹ and energy efficiency was 19.58% under the same measurement conditions in one cycle. The deionization rate demonstrated general stability over 20 consecutive adsorption–desorption cycles. After the regeneration test, the Nb₂O₅ nanoparticles were still tightly anchored on the nitrogen-doped graphene sheets (Fig. S20a and b†). But the size of the Nb₂O₅ nanoparticles slightly decreased, which could shorten the ion transport pathways and provide rapid diffusion channels in a long-term operation (Fig. S20c†). The above results proved that Nb₂O₅@N–C-1 possessed outstanding regeneration ability.

Fig. 4 (a) IAC plots of the Nb₂O₅, Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2 electrodes in a 500 mg L⁻¹ NaCl solution with a working voltage of 1.2 V and a flow rate of 40 mL min⁻¹, (b) the corresponding Ragone plots of IAR vs. IAC of the three electrodes, (c) current transient curves and charge efficiency (inset figure) of the Nb₂O₅, Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2 electrodes in a 500 mg L⁻¹ NaCl solution with a working voltage of 1.2 V and a flow rate of 40 mL min⁻¹, (d) plots of the trade-off between SEC⁻¹ and IAR of the Nb₂O₅, Nb₂O₅@N–C-0.5, Nb₂O₅@N–C-1, and Nb₂O₅@N–C-2 electrode materials, (e) IAC plots of Nb₂O₅@N–C-1 at different initial concentrations of NaCl solution with a working voltage of 1.2 V and a flow rate of 40 mL min⁻¹, and (f) IAC plots of Nb₂O₅@N–C-1 in a 500 mg L⁻¹ NaCl solution with different working voltages and a flow rate of 40 mL min⁻¹.

3.4 Mechanisms of ion removal

The mechanisms of ion removal from water were investigated by in situ Raman characterization. Fig. 5a shows the in situ Raman spectra when the CV plots (a scan rate of 0.1 mV s⁻¹) reached different stages of ion adsorption and desorption (Fig. S21†). During the ion adsorption process from 0.5 to −0.5 V, the evolution of each band could be summarized as follows. (1) The V_Hi band kept nearly constant from 0.5 to 0.1 V. When the potential decreased to −0.1 V, the V_Hi band demonstrated a red shift and the relative intensity increased, which was consistent with the CV curves in which a prominent cathodic peak appeared at this range. (2) The V_Mid band presented a similar trend with the V_Hi band. No noticeable change was observed from 0.5 to 0.1 V. After that, the V_Mid band has split into two small bands from −0.1 to −0.5 V. (3) The V_Lo band, after being static at the range of 0.5 to 0.1 V, demonstrated an increased relative intensity from −0.1 V. In contrast, the evolution of the three bands in the Na⁺ ion deintercalation process was the exact opposite of the above-mentioned Na⁺ ion intercalation. Specifically, during the ion desorption process from −0.5 to 0.5 V, the V_Hi band was blue-shifted with a reduced intensity; the V_Mid bands merged into one band, and the V_Lo band also decreased in intensity. The opposite evolutions of these Raman band groups of Nb₂O₅ verified the reversible structural conversion via Na⁺ ion intercalation/deintercalation.^51,52 Moreover, ex situ XPS analysis was used to observe the valence changes in the Nb element during the ion adsorption and ion desorption process. As shown in Fig. S22,† there was a clear blue shift from the pristine state to the adsorption state and the subsequent red shift from the adsorption state to the desorption state. The reversible shift of Nb 3d_5/2 and Nb 3d_3/2 revealed that the valence changes of Nb had a strong correlation with the Na⁺ ion conversion process. The Nb 3d XPS spectra of the three states were divided into individual peaks to provide more information about the removal mechanisms (Fig. 5b). At the pristine state, Nb⁵⁺ played a significant role while Nb⁴⁺ played a minor role. After that, most of the Nb⁵⁺ ions were reduced into Nb⁴⁺ when the electrode adsorbed sufficient Na⁺ ions. Finally, most of the Nb⁴⁺ ions were oxidized to Nb⁵⁺ after the applied voltage was removed. The above XPS results indicated that the Na⁺ ion conversion process arose from the reversible redox reaction of the Nb⁵⁺/Nb⁴⁺ pair in the Nb₂O₅@N–C-1 electrode.^53–55 The Na⁺ ion insertion/extraction reaction could be expressed as Nb₂O₅ + x Na⁺ + x e⁻ ⇔ Na_xNb₂O₅.^38,53 Meanwhile, Cl⁻ ions were adsorbed on the surface of the positively charged AC during the ion adsorption process, and were released back to the salt solution during the ion desorption process. Therefore, the removal mechanisms of Na⁺ ions and Cl⁻ ions could be distinctly illustrated as shown in Fig. 5c.

Fig. 5 (a) In situ Raman tests of Na⁺ ion adsorption/desorption behaviors in Nb₂O₅@N–C-1, (b) the corresponding valence change of Nb in the pristine state and during the adsorption and desorption processes, and (c) schematic illustration of the ion removal mechanisms in Nb₂O₅@N–C-1.

4 Conclusions

In summary, the exceptional CDI performance of Nb₂O₅@N–C-1 could be explained by the synergistic effects of the electro-adsorption of the nitrogen-doped carbon framework and the faradaic reaction of Nb₂O₅. GO, g-C₃N₄ and 2-MeIM derived nitrogen-doped carbon (N–C) as flexible supports showed superb cycling stability. Besides, the introduction of N–C enhanced the electrical conductivity of Nb₂O₅ and realized better Na⁺ ion diffusion. Nb₂O₅in situ growth on the N–C framework increased the ion diffusion channels and facilitated the fast transport of Na⁺ ions. Owing to the combination of the electro-adsorption/desorption mechanism of N–C and the insertion/extraction process of Nb₂O₅, the Nb₂O₅@N–C-1 electrode displayed an excellent ion removal capacity. Inspiringly, the novel HCDI system equipped with Nb₂O₅@N–C-1//AC exhibited a superb IAC (35.4 mg g⁻¹ in a 500 mg L⁻¹ NaCl solution at 1.2 V), fast IAR, high charge efficiency, low energy loss, and good regeneration ability. In situ Raman and ex situ XPS analyses verified that the mechanisms of ion removal were reversible intercalation and the faradaic reaction of Nb₂O₅. This work presents a new strategy to design highly efficient HCDI systems for water purification.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the support from the National Natural Science Foundation of China (21906101; 21722704) and the Science and Technology Commission of Shanghai Municipality (19DZ2293100; 18DZ2281400).

Notes and references

1. T. Oki and S. Kanae, Global Hydrological Cycles and World Water Resources, Science, 2006, 313, 1068.
2. C. J. Vörösmarty, P. B. McIntyre, M. O. Gessner, D. Dudgeon, A. Prusevich, P. Green, S. Glidden, S. E. Bunn, C. A. Sullivan, C. R. Liermann and P. M. Davies, Global threats to human water security and river biodiversity, Nature, 2010, 467, 555–561.
3. G. Wang, T. Yan, J. Zhang, L. Shi and D. Zhang, Trace-Fe-Enhanced Capacitive Deionization of Saline Water by Boosting Electron Transfer of Electro-Adsorption Sites, Environ. Sci. Technol., 2020, 54, 8411–8419.
4. J.-F. Pekel, A. Cottam, N. Gorelick and A. S. Belward, High-resolution mapping of global surface water and its long-term changes, Nature, 2016, 540, 418–422.
5. M. Elimelech and W. A. Phillip, The Future of Seawater Desalination: Energy, Technology, and the Environment, Science, 2011, 333, 712.
6. J. E. Dykstra, R. Zhao, P. M. Biesheuvel and A. van der Wal, Resistance identification and rational process design in Capacitive Deionization, Water Res., 2016, 88, 358–370.
7. C. Zhang, D. He, J. Ma, W. Tang and T. D. Waite, Faradaic reactions in capacitive deionization (CDI)-problems and possibilities: A review, Water Res., 2018, 128, 314–330.
8. S. P. Surwade, S. N. Smirnov, I. V. Vlassiouk, R. R. Unocic, G. M. Veith, S. Dai and S. M. Mahurin, Water desalination using nanoporous single-layer graphene, Nat. Nanotechnol., 2015, 10, 459–464.
9. C. He, J. Ma, C. Zhang, J. Song and T. D. Waite, Short-Circuited Closed-Cycle Operation of Flow-Electrode CDI for Brackish Water Softening, Environ. Sci. Technol., 2018, 52, 9350–9360.
10. P. Liu, T. Yan, L. Shi, H. S. Park, X. Chen, Z. Zhao and D. Zhang, Graphene-based materials for capacitive deionization, J. Mater. Chem. A, 2017, 5, 13907–13943.
11. T. Wang, C. Zhang, L. Bai, B. Xie, Z. Gan, J. Xing, G. Li and H. Liang, Scaling behavior of iron in capacitive deionization (CDI) system, Water Res., 2020, 171, 115370.
12. J. Zhang, T. Yan, J. Fang, J. Shen, L. Shi and D. Zhang, Enhanced capacitive deionization of saline water using N-doped rod-like porous carbon derived from dual-ligand metal-organic frameworks, Environ. Sci.: Nano, 2020, 7, 926–937.
13. Z. U. Khan, T. Yan, J. Han, L. Shi and D. Zhang, Capacitive deionization of saline water using graphene nanosphere decorated N-doped layered mesoporous carbon frameworks, Environ. Sci.: Nano, 2019, 6, 3442–3453.
14. W. Tang, J. Liang, D. He, J. Gong, L. Tang, Z. Liu, D. Wang and G. Zeng, Various cell architectures of capacitive deionization: Recent advances and future trends, Water Res., 2019, 150, 225–251.
15. F. Ji, L. Wang, J. Yang, X. Wu, M. Li, S. Jiang, S. Lin and Z. Chen, Highly compact, free-standing porous electrodes from polymer-derived nanoporous carbons for efficient electrochemical capacitive deionization, J. Mater. Chem. A, 2019, 7, 1768–1778.
16. Y. Chen, J. Chen, C. Lin and C. Hou, Integrating a supercapacitor with capacitive deionization for direct energy recovery from the desalination of brackish water, Appl. Energy, 2019, 252, 113417.
17. F. Yu, L. Wang, Y. Wang, X. Shen, Y. Cheng and J. Ma, Faradaic reactions in capacitive deionization for desalination and ion separation, J. Mater. Chem. A, 2019, 7, 15999–16027.
18. J. Cao, Y. Wang, L. Wang, F. Yu and J. Ma, Na₃V₂(PO₄)₃@C as Faradaic Electrodes in Capacitive Deionization for High-Performance Desalination, Nano Lett., 2019, 19, 823–828.
19. L. Wang, J. E. Dykstra and S. Lin, Energy Efficiency of Capacitive Deionization, Environ. Sci. Technol., 2019, 53, 3366–3378.
20. M. Li and H. G. Park, Pseudocapacitive Coating for Effective Capacitive Deionization, ACS Appl. Mater. Interfaces, 2018, 10, 2442–2450.
21. F. Zhou, T. Gao, M. Luo and H. Li, Heterostructured graphene@Na₄Ti₉O₂₀ nanotubes for asymmetrical capacitive deionization with ultrahigh desalination capacity, Chem. Eng. J., 2018, 343, 8–15.
22. F. Xing, T. Li, J. Li, H. Zhu, N. Wang and X. Cao, Chemically exfoliated MoS₂ for capacitive deionization of saline water, Nano Energy, 2017, 31, 590–595.
23. Y. Zhao, B. Liang, X. Wei, K. Li, C. Lv and Y. Zhao, A core-shell heterostructured CuFe@NiFe Prussian blue analogue as a novel electrode material for high-capacity and stable capacitive deionization, J. Mater. Chem. A, 2019, 7, 10464–10474.
24. K. Wang, Y. Liu, Z. Ding, Y. Li, T. Lu and L. Pan, Metal-organic-frameworks-derived NaTi₂(PO₄)₃/carbon composites for efficient hybrid capacitive deionization, J. Mater. Chem. A, 2019, 7, 12126–12133.
25. Z. Yue, T. Gao and H. Li, Robust synthesis of carbon@Na₄Ti₉O₂₀ core-shell nanotubes for hybrid capacitive deionization with enhanced performance, Desalination, 2019, 449, 69–77.
26. B. W. Byles, D. A. Cullen, K. L. More and E. Pomerantseva, Tunnel structured manganese oxide nanowires as redox active electrodes for hybrid capacitive deionization, Nano Energy, 2018, 44, 476–488.
27. W. Peng, W. Wang, G. Han, Y. Huang and Y. Zhang, Fabrication of 3D flower-like MoS₂/graphene composite as high-performance electrode for capacitive deionization, Desalination, 2020, 473, 114191.
28. M. Pasta, C. D. Wessells, Y. Cui and F. La Mantia, A Desalination Battery, Nano Lett., 2012, 12, 839–843.
29. Z. Y. Leong and H. Y. Yang, A Study of MnO₂ with Different Crystalline Forms for Pseudocapacitive Desalination, ACS Appl. Mater. Interfaces, 2019, 11, 13176–13184.
30. X. Wen, M. Zhao, M. Zhang, X. Fan and D. Zhang, Efficient Capacitive Deionization of Saline Water by an Integrated Tin disulfide Nanosheet@Graphite Paper Electrode via an in Situ Growth Strategy, ACS Sustainable Chem. Eng., 2019, 8, 1268–1275.
31. M. Ding, S. Fan, S. Huang, M. E. Pam, L. Guo, Y. Shi and H. Y. Yang, Tunable Pseudocapacitive Behavior in Metal-Organic-Framework-Derived TiO₂@Porous Carbon Enabling High-Performance Membrane Capacitive Deionization, ACS Appl. Energy Mater., 2019, 2, 1812–1822.
32. B. Min, J. Choi and K. Jung, Improved capacitive deionization of sulfonated carbon/titania hybrid electrode, Electrochim. Acta, 2018, 270, 543–551.
33. S. Wang, G. Wang, T. Wu, C. Li, Y. Wang, X. Pan, F. Zhan, Y. Zhang, S. Wang and J. Qiu, Membrane-Free Hybrid Capacitive Deionization System Based on Redox Reaction for High-Efficiency NaCl Removal, Environ. Sci. Technol., 2019, 53, 6292–6301.
34. J. Han, T. Yan, J. Shen, L. Shi, J. Zhang and D. Zhang, Capacitive Deionization of Saline Water by Using MoS₂–Graphene Hybrid Electrodes with High Volumetric Adsorption Capacity, Environ. Sci. Technol., 2019, 53, 12668–12676.
35. B. Deng, T. Lei, W. Zhu, L. Xiao and J. Liu, In-Plane Assembled Orthorhombic Nb₂O₅ Nanorod Films with High-Rate Li⁺ Intercalation for High-Performance Flexible Li-Ion Capacitors, Adv. Funct. Mater., 2018, 28, 1704330.
36. P. Guo, K. Sun, X. Shang, D. Liu, Y. Wang, Q. Liu, Y. Fu and D. He, Nb₂O₅/RGO Nanocomposite Modified Separators with Robust Polysulfide Traps and Catalytic Centers for Boosting Performance of Lithium-Sulfur Batteries, Small, 2019, 15, e1902363.
37. X. Wang, Q. Li, L. Zhang, Z. Hu, L. Yu, T. Jiang, C. Lu, C. Yan, J. Sun and Z. Liu, Caging Nb₂O₅ Nanowires in PECVD-Derived Graphene Capsules toward Bendable Sodium-Ion Hybrid Supercapacitors, Adv. Mater., 2018, 30, e1800963.
38. D. Cao, Z. Yao, J. Liu, J. Zhang and C. Li, H-Nb₂O₅ wired by tetragonal tungsten bronze related domains as high-rate anode for Li-ion batteries, Energy Storage Mater., 2018, 11, 152–160.
39. G. Wang, J. Feng, L. Dong, X. Li and D. Li, Antimony (IV) Oxide Nanorods/Reduced Graphene Oxide as the Anode Material of Sodium-ion Batteries with Excellent Electrochemical Performance, Electrochim. Acta, 2017, 240, 203–214.
40. K. Tang, T. Z. X. Hong, L. You and K. Zhou, Carbon-metal compound composite electrodes for capacitive deionization: synthesis, development and applications, J. Mater. Chem. A, 2019, 7, 26693–26743.
41. B. W. Byles, B. Hayes-Oberst and E. Pomerantseva, Ion Removal Performance, Structural/Compositional Dynamics, and Electrochemical Stability of Layered Manganese Oxide Electrodes in Hybrid Capacitive Deionization, ACS Appl. Mater. Interfaces, 2018, 10, 32313–32322.
42. J. Han, L. Shi, T. Yan, J. Zhang and D. Zhang, Removal of ions from saline water using N, P co-doped 3D hierarchical carbon architectures via capacitive deionization, Environ. Sci.: Nano, 2018, 5, 2337–2345.
43. J. Zhang, J. Fang, J. Han, T. Yan, L. Shi and D. Zhang, N, P, S co-doped hollow carbon polyhedra derived from MOF-based core-shell nanocomposites for capacitive deionization, J. Mater. Chem. A, 2018, 6, 15245–15252.
44. P. Liu, H. Wang, T. Yan, J. Zhang, L. Shi and D. Zhang, Grafting sulfonic and amine functional groups on 3D graphene for improved capacitive deionization, J. Mater. Chem. A, 2016, 4, 5303–5313.
45. R. Wang, T. Yan, L. Han, G. Chen, H. Li, J. Zhang, L. Shi and D. Zhang, Tuning the dimensions and structures of nitrogen-doped carbon nanomaterials derived from sacrificial g-C₃N₄/metal-organic frameworks for enhanced electrocatalytic oxygen reduction, J. Mater. Chem. A, 2018, 6, 5752–5761.
46. G. Wang, J. Deng, T. Yan, J. Zhang, L. Shi and D. Zhang, Turning on electrocatalytic oxygen reduction by creating robust Fe-N_x species in hollow carbon frameworks via in situ growth of Fe doped ZIFs on g-C₃N₄, Nanoscale, 2020, 12, 5601–5611.
47. Q. Deng, F. Chen, S. Liu, A. Bayaguud, Y. Feng, Z. Zhang, Y. Fu, Y. Yu and C. Zhu, Advantageous Functional Integration of Adsorption-Intercalation-Conversion Hybrid Mechanisms in 3D Flexible Nb₂O₅@Hard Carbon@MoS₂@Soft Carbon Fiber Paper Anodes for Ultrafast and Super-Stable Sodium Storage, Adv. Funct. Mater., 2020, 30, 1908665.
48. X. Jiao, Q. Hao, X. Xia, Z. Wu and W. Lei, Metal organic framework derived Nb₂O₅@C nanoparticles grown on reduced graphene oxide for high-energy lithium ion capacitors, Chem. Commun., 2019, 55, 2692–2695.
49. H. Kim, E. Lim, C. Jo, G. Yoon, J. Hwang, S. Jeong, J. Lee and K. Kang, Ordered-mesoporous Nb₂O₅/carbon composite as a sodium insertion material, Nano Energy, 2015, 16, 62–70.
50. Q. Ji, X. Gao, Q. Zhang, L. Jin, D. Wang, Y. Xia, S. Yin, S. Xia, N. Hohn, X. Zuo, X. Wang, S. Xie, Z. Xu, L. Ma, L. Chen, G. Z. Chen, J. Zhu, B. Hu, P. Müller-Buschbaum, P. G. Bruce and Y. J. Cheng, Dental Resin Monomer Enables Unique NbO₂/Carbon Lithium-Ion Battery Negative Electrode with Exceptional Performance, Adv. Funct. Mater., 2019, 29, 1904961.
51. D. Chen, J. H. Wang, T. F. Chou, B. Zhao, M. A. El-Sayed and M. Liu, Unraveling the Nature of Anomalously Fast Energy Storage in T-Nb₂O₅, J. Am. Chem. Soc., 2017, 139, 7071–7081.
52. J. Meng, Q. He, L. Xu, X. Zhang, F. Liu, X. Wang, Q. Li, X. Xu, G. Zhang, C. Niu, Z. Xiao, Z. Liu, Z. Zhu, Y. Zhao and L. Mai, Identification of Phase Control of Carbon-Confined Nb₂O₅ Nanoparticles toward High-Performance Lithium Storage, Adv. Energy Mater., 2019, 9, 1802695.
53. Y. Li, H. Wang, L. Wang, Z. Mao, R. Wang, B. He, Y. Gong and X. Hu, Mesopore-Induced Ultrafast Na⁺-Storage in T-Nb₂O₅/Carbon Nanofiber Films toward Flexible High-Power Na-Ion Capacitors, Small, 2019, 15, e1804539.
54. F. Liu, X. Cheng, R. Xu, Y. Wu, Y. Jiang and Y. Yu, Binding Sulfur-Doped Nb₂O₅ Hollow Nanospheres on Sulfur-Doped Graphene Networks for Highly Reversible Sodium Storage, Adv. Funct. Mater., 2018, 28, 1800394.
55. Y. Lian, D. Wang, S. Hou, C. Ban, J. Zhao and H. Zhang, Construction of T-Nb₂O₅ nanoparticles on/in N-doped carbon hollow tubes for Li-ion hybrid supercapacitors, Electrochim. Acta, 2020, 330, 135204.

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Footnote

† Electronic supplementary information (ESI) available: Materials, structural characterization, electrochemical characterization, CDI test, SEM images, TEM images, particle size distribution, SAED pattern, EDS mappings, XRD patterns, N₂ adsorption–desorption isotherms, BJH pore size distribution plots, XPS spectra, dynamic water contact angle analysis, CV curves, specific capacitance, GCD curves, IR drops, plots of IAC vs. deionization time, Ragone plots, comparison between AIAR and CR, regeneration test, elemental quantification, fitted parameters in EIS, and comparison of IAC with other reported materials. See DOI: 10.1039/d0en01003k

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