Carbon-incorporated Fe₃O₄ nanoflakes: high-performance faradaic materials for hybrid capacitive deionization and supercapacitors

Abstract

Here, we introduce a new strategy using urea for the synthesis of carbon-incorporated 2D Fe₃O₄ (2D-Fe₃O₄/C) nanoflakes under solvothermal conditions with the following pyrolysis process under an inert atmosphere. Thanks to the structural advantages of 2D-Fe₃O₄/C, including 2D flakes providing a larger accessible surface area and exposing more active sites, as well as carbon incorporation promoting electrical conductivity for faster charge transfer, the 2D-Fe₃O₄/C displays a high specific capacitance of 386 F g⁻¹ at 1 A g⁻¹ in a three-electrode system. More importantly, when further assembled into a hybrid supercapacitor with pre-synthesized NiCo-layered double hydroxides as positive electrodes, the assembled supercapacitor device delivers a high-energy density of 32.5 W h kg⁻¹ at 400 W kg⁻¹ and little capacitance loss with bending angles ranging from 0° to 180°. As another capacitive application in desalination, 2D-Fe₃O₄/C also shows a high desalination capacity of 28.5 mg g⁻¹ over 7.5 min, which suggests a very high mean desalination rate of 3.8 mg g⁻¹ min⁻¹. Our results not only highlight the significance of 2D metal oxide nanosheets/nanoflakes, but also hold great potential for high-performance capacitive applications in supercapacitors and desalination.

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Introduction

Transition metal oxides (TMOs) are one of the most widely studied materials in various fields. Among the reported TMOs, Fe₃O₄ has particularly attracted attention due to its simple structure, environmental friendliness, ease of synthesis, and feasibility of commercial production.^1,2 As a result, a series of Fe₃O₄-based materials have been explored in electrochemical applications, including electrocatalysis, batteries, supercapacitors, capacitive deionization (CDI), etc.^3–7 Generally, a simple chemical co-precipitation reaction of a Fe²⁺/Fe³⁺ mixture in a basic medium can easily achieve the mass production of Fe₃O₄ materials,⁸ but it usually produces bulk Fe₃O₄ with medium performance. Obstacles to the high electrochemical performance of Fe₃O₄ materials must be overcome, including poor electrical conductivity, aggregation tende ncy, and slow ion diffusion characteristics;^9,10 however, in reality, there are still many challenges.

It is advantageous to incorporate carbon into Fe₃O₄ materials to improve their electrochemical performance. Incorporating carbon is expected to effectively improve the electrical conductivity and prevent Fe₃O₄ particles from agglomerating.¹¹ For many years, many efforts have been made to generate carbon/Fe₃O₄ hybrids through various methods, resulting in a variety of species, including graphene/Fe₃O₄,¹² carbon nanotubes/Fe₃O₄,¹³ and Fe₃O₄@carbon.¹⁴ Unfortunately, these strategies usually involve multi-step synthesis and expensive synthetic carbon additives, and the process is too complicated and time consuming to be applied on a large scale. Furthermore, the morphological control of the carbon/Fe₃O₄ hybrid is also very instructive for improving the electrochemical performance. The designed nanostructures, especially two-dimensional (2D) nanosheets/nanoflakes, usually exhibit unusual characteristics beyond ordinary structures, including increased accessible surface area, shortened ion diffusion pathway, and more exposed electrochemically active sites.^15,16 However, the construction of the 2D Fe₃O₄ architecture is only realized in some studies with complex synthesis steps and high production costs,¹⁷ and it is still in its infancy and extremely challenging.

To address the issues, herein, we report a facile chemical approach to preparing carbon-incorporated Fe₃O₄ nanoflakes (denoted as 2D-Fe₃O₄/C) that possess superior potential for hybrid capacitive applications in supercapacitors and desalination. It is well known that urea chemistry, that is, the urea-mediated soft-chemical strategy, has recently been developed to prepare transition metal-based nanostructures on a large scale.^18–20 Under the urea-mediated coordination process, a 2D iron-containing complex nanoflake can be successfully obtained in this work, which subsequently can serve as a precursor for 2D-Fe₃O₄/C via pyrolysis. Consequently, 2D-Fe₃O₄/C exhibits several advantages, such as more exposed electrochemically active sites and a shortened ion diffusion pathway from the engineered 2D nanostructure, as well as improved electrical conductivity from carbon incorporation, which thereby endows 2D-Fe₃O₄/C with a superior faradaic capacitive characteristic. As a proof of concept, the obtained 2D-Fe₃O₄/C is subsequently investigated in hybrid supercapacitors and hybrid CDI, revealing that 2D-Fe₃O₄/C delivers a high specific capacitance of 386 F g⁻¹ at 1 A g⁻¹ (in 2 M KOH), superior desalination performance of 28.5 mg g⁻¹ at 1.2 V, and good cycling stability. What's more, 2D-Fe₃O₄/C also exhibits high energy and power density (32.5 W h kg⁻¹ at 400 W kg⁻¹) when coupled with NiCo-layered double hydroxide (NiCo-LDH) positive electrodes. All of these results show the great potential of 2D-Fe₃O₄/C for hybrid supercapacitors and hybrid CDI.

Results and discussion

The iron-containing complex was first fabricated with ferric chloride and urea under solvothermal conditions in ethylene glycol. The detailed synthetic process is described in the experimental section of the ESI.†Fig. 1a and b show field emission scanning electron microscopy (FESEM) images of the as-prepared iron-containing complex precursor, which clearly displays a flake-like morphology with a smooth surface and size ranging from 100 nm to hundreds of nanometers. After that, the iron-containing complex was carbonized in a nitrogen atmosphere at 400, 450, and 500 °C, producing the 2D-Fe₃O₄/C materials named as 2D-Fe₃O₄/C-400, 2D-Fe₃O₄/C-450, and 2D-Fe₃O₄/C-500, respectively. FESEM images of the prepared 2D-Fe₃O₄/C are shown in Fig. 1c, d and Fig. S1 (ESI†), exhibiting that the flake-like morphology with a rough surface has been well retained after pyrolysis. Such a unique structure is expected to favor electrolyte penetration and electron transfer during the charging process. Moreover, elemental mapping images of the representative sample, 2D-Fe₃O₄/C-450 (Fig. S2, ESI†), indicate that the Fe, O, and C elements are homogeneously distributed in the 2D-Fe₃O₄/C nanoflake, suggesting the uniform distribution of Fe₃O₄ within the carbon.

Fig. 1 FESEM images of (a and b) iron-containing complex precursor and (c and d) 2D-Fe₃O₄/C-450.

The phase and structure information of the as-prepared 2D-Fe₃O₄/C samples was investigated through X-ray diffraction (XRD) and Raman measurements (Fig. 2a and b). The XRD patterns in Fig. 2a indicate that all 2D-Fe₃O₄/C samples exhibit typical diffraction peaks at around 30.1°, 35.5°, 43.1°, 57.2°, and 62.8°, indexed to the (220), (311), (400), (511), and (440) planes of Fe₃O₄ (JCPDS PDF#19–0629), respectively. This suggests the generation of Fe₃O₄ at a low pyrolysis temperature. The Raman spectra presented in Fig. 2b indicate the scattering peaks of Fe₃O₄ at 400–800 cm⁻¹, among which the scattering peaks at around 490 and 670 cm⁻¹ refer to the T_2g and A_1g vibration modes of Fe₃O₄, respectively.^21,22 The two broad peaks referring to D- and G-bands in the carbon materials are located at around 1360 and 1580 cm⁻¹, respectively. Remarkably, the relative intensity ratios of the D- to G-band (I_D/I_G) for 2D-Fe₃O₄/C-400, 2D-Fe₃O₄/C-450, and 2D-Fe₃O₄/C-500 are 0.83, 0.83, and 0.85, respectively, which confirms the generation of tremendous defects and disordered structures within the 2D-Fe₃O₄/C samples.²³

Fig. 2 (a) XRD patterns, (b) Raman spectra, (c) N₂ adsorption/desorption isotherms, and (d) pore size distributions of 2D-Fe₃O₄/C.

N₂ adsorption/desorption isotherms were further used to identify the specific surface area (SSA) and pore size distribution of the 2D-Fe₃O₄/C samples (Fig. 2c and d). The N₂ adsorption/desorption isotherms of all 2D-Fe₃O₄/C samples in Fig. 2c evidently exhibit type IV curves with small hysteresis loops. The pore size distributions (Fig. 2d) reveal the presence of abundant mesopores (2–50 nm) in all 2D-Fe₃O₄/C samples. Moreover, the 2D-Fe₃O₄/C-450 sample possesses the highest SSA and pore volume of 90.8 m² g⁻¹ and 0.34 cm³ g⁻¹, respectively, which will positively provide substantial ion accommodation sites and favor the enhancement of the electrochemical performance.

To further identify structural details of 2D-Fe₃O₄/C, the 2D-Fe₃O₄/C-450 was further chosen as the representative example and then investigated by transmission electron microscopy (TEM). As revealed in Fig. 3a and b, 2D-Fe₃O₄/C nanoflakes are composed of assembled nanoparticles that have a size of 10–20 nm. An additional high-resolution TEM (HRTEM) image in Fig. 3c clearly shows that the amorphous carbon is filled within the gap among Fe₃O₄ nanoparticles, which is expected to prevent the agglomeration of assembled Fe₃O₄ nanoparticles and improve the electrical conductivity of the nanoflakes for efficient electron transfer. The lattice fringes of the nanoparticles show interplanar spacings of 0.254 and 0.298 nm, corresponding to the (311) and (220) planes of Fe₃O₄, respectively.²⁴ The selected area electron diffraction (SAED) pattern image of 2D-Fe₃O₄/C-450 (Fig. 3d) indicates a polycrystalline nature, and the diffraction rings are well indexed to the (220), (311), and (400) planes of Fe₃O₄ (JCPDS PDF#19–0629).

Fig. 3 (a and b) TEM, (c) HRTEM, and (d) SAED pattern images of 2D-Fe₃O₄/C-450.

The elemental composition and valence states of the as-synthesized 2D-Fe₃O₄/C-450 were analyzed by X-ray photoelectron spectroscopy (XPS) measurement. The XPS survey spectrum (Fig. 4a) of 2D-Fe₃O₄/C-450 indicates the presence of coexistent Fe, O, and C elements. The high-resolution XPS spectrum of Fe 2p (Fig. 4b) is fitted with two peaks assigned to electron orbitals of Fe 2p_1/2, Fe 2p_3/2, and shakeup satellites. The spin orbits of Fe 2p_1/2 and Fe 2p_3/2 can be further divided into two peaks, implying the presence of Fe²⁺ and Fe³⁺.²⁵ The high-resolution O 1s spectrum (Fig. 4c) can be fitted into three peaks corresponding to the Fe–O bond at 530.2 eV, the C–O bond at 531.4 eV, and the O–H bond at 533.0 eV.²⁶ The high-resolution C 1s spectrum shown in Fig. 4d can be deconvoluted into peaks at 284.6, 286.2, and 288.6 eV, corresponding to C–C, C–O (C–O–Fe), and C=O, respectively.²⁶ The detailed carbon contents within the 2D-Fe₃O₄/C samples are verified by thermogravimetric analysis (Fig. S3, ESI†), revealing values of approximately 13.5, 10.5, and 6.82 wt% for 2D-Fe₃O₄/C-400, 2D-Fe₃O₄/C-450, and 2D-Fe₃O₄/C-500, respectively.

Fig. 4 XPS spectra for (a) survey, (b) Fe 2p, (c) O 1s, and (d) C 1s of 2D-Fe₃O₄/C-450.

Rapid growth of the commercial electrical device market has evoked high demand for a new energy storage system that could supply higher energy and power than traditional systems.^23,27–29 As compared with traditional dielectric capacitors, supercapacitors that could provide higher energy density while maintaining a high power output have been attracting significant interest in recent years.^30–33 Although supercapacitors usually have extremely long cycling lives, their limited energy-to-power ratio is unfavorable for practical application on an industry scale.^34,35 This is because in common symmetrical supercapacitor devices, carbon materials are commonly employed as active materials for electrodes; however, limited by their working principles based on electrical double layer (EDL) theory, the fabricated supercapacitors usually suffer from comparatively inferior energy density and capacitance, although their cycling stability is very good.^36,37 In contrast, TMOs rely on a pseudocapacitive mechanism through a fast redox process to store more charges and, therefore, can achieve higher specific capacitance, although the cycling stability of TMOs is still very poor. By combining TMOs with carbon materials, the resultant TMO/C composites can achieve both excellent cycling stability and high capacitance, which have been widely studied in hybrid supercapacitors that have higher energy/power ratio as compared to common symmetric supercapacitors.³⁸ Currently, the pursuit of new designs and the exploration of suitable electrode materials with excellent electrochemical activity and unique architecture to promote capacitive performance is highly desirable and of great significance.^39,40 As for our 2D-Fe₃O₄/C nanoflakes, the unique 2D architecture can supply more exposed electrochemically active sites and shorten the ion diffusion pathway, and the filled carbon content within the gap among neighbored Fe₃O₄ nanoparticles prevents the aggregation of Fe₃O₄ nanoparticles and improves the electrical conductivity for faster electron transfer. Therefore, the engineered 2D-Fe₃O₄/C are expected to have a superior faradaic capacitive characteristic as compared to other present materials.

To evaluate the potential of 2D-Fe₃O₄/C for hybrid supercapacitors, cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were first carried out to study the capacitance of 2D-Fe₃O₄/C in a three-electrode configuration with 2 M KOH aqueous solution as the electrolyte. Fig. S4a (ESI†) shows the CV curves of all 2D-Fe₃O₄/C electrodes at 5 mV s⁻¹ from −1.2 to 0 V. It is noted that a pair of redox peaks can be clearly observed in all samples, indicating the pseudocapacitive behavior. Furthermore, the CV curves of all of the 2D-Fe₃O₄/C electrodes at scan rates varying from 2 to 50 mV s⁻¹ in 2 M KOH electrolytes are shown in Fig. S4b–d (ESI†), which displays the redox peaks at any scan rate. Based on further electrochemical kinetic analysis (Fig. S5, ESI†), the total capacitances of all 2D-Fe₃O₄/C electrodes are ascribed to the surface capacitive effects and diffusion-controlled process. The EIS measurements were then carried out, and the resulting Nyquist plots are shown in Fig. S6 (ESI†). All curves consist of a small semicircle at high frequency and a steep straight line at low frequency. Evidently, 2D-Fe₃O₄/C-450 exhibits much lower R_ct (0.20 Ω) than those of 2D-Fe₃O₄/C-400 (0.96 Ω) and 2D-Fe₃O₄/C-500 (0.71 Ω), suggesting that the 2D-Fe₃O₄/C-450 electrode displays the lowest charge transfer resistance, which will promote the electrochemical properties.

The GCD curves of all 2D-Fe₃O₄/C electrodes at 1 A g⁻¹ in Fig. 5a, representing distorted triangular shapes, further suggest the redox behavior of 2D-Fe₃O₄/C materials, and the highest capacitance of 386 F g⁻¹ is achieved by 2D-Fe₃O₄/C-450. Fig. 5b shows the GCD curves of 2D-Fe₃O₄/C-450 at 1, 2, 4, 6, 8, and 10 A g⁻¹, displaying all curves as distorted triangular shapes, due to the redox mechanism of 2D-Fe₃O₄/C-450. The corresponding capacitance values of 2D-Fe₃O₄/C-450 versus the other two 2D-Fe₃O₄/C samples are shown in Fig. 5c. As clearly seen, at any current density, the capacitance of 2D-Fe₃O₄/C-450 is the highest, suggesting the best capacitive performance.

Fig. 5 Electrochemical profiles in a three-electrode system in 2 M KOH: GCD curves for (a) 2D-Fe₃O₄/C samples at 1 A g⁻¹ and (b) 2D-Fe₃O₄/C-450 at 1–10 A g⁻¹; (c) specific capacitances versus current densities of 2D-Fe₃O₄/C samples; (d) CV curves of 2D-Fe₃O₄/C-450 and NiCo-LDHs at 5 mV s⁻¹.

Therefore, 2D-Fe₃O₄/C-450 was selected as the negative electrode for constructing a hybrid supercapacitor, with NiCo-LDHs serving as the positive electrode. The charge balance between 2D-Fe₃O₄/C-450 and NiCo-LDHs electrodes is optimized on the basis of the CV curves shown in Fig. 5d, and the mass loading ratio of 2D-Fe₃O₄/C-450 to NiCo-LDHs within the hybrid supercapacitors is determined to be 1.8 : 1. The subsequent two-electrode electrochemical analysis in 2 M KOH aqueous solution by varying the operation potential window indicates that the potential window for 2D-Fe₃O₄/C-450//NiCo-LDHs can be extended to a high value of 1.6 V with excellent reversibility (Fig. S7 ESI†).

A hybrid supercapacitor composed of 2D-Fe₃O₄/C-450 as the negative electrode, NiCo-LDHs as the positive electrode, and KOH-gel as the electrolyte was then constructed. Fig. 6a displays the CV curves of the hybrid supercapacitor at varying scan rates from 2 to 100 mV s⁻¹ between 0 and 1.6 V. The shape of the CV curves does not change much, even at higher scan rates, indicating good reversibility and high rate capability. Fig. 6b shows the distorted GCD curves of the hybrid supercapacitors at varying current densities from 0.5 to 10 A g⁻¹, suggesting the redox behavior. What's more, the hybrid supercapacitor achieves a capacitance retention of 71.5% over 10 000 charge/discharge cycles (Fig. S8, ESI†), revealing good cycling stability.

Fig. 6 Electrochemical profiles of 2D-Fe₃O₄/C-450//NiCo-LDHs hybrid supercapacitors: (a) CV curves at varying scan rates, (b) GCD curves at varying current densities, (c) CV curves at varying bending angles, and (d) Ragone plots.

The feasibility of the hybrid supercapacitor was further investigated by CV curves with diverse bending angles from 0° to 180° (Fig. 6c). Little capacitance loss could be observed after varying the bending angles, suggesting the good potential of our materials for flexible supercapacitor applications. The Ragone plots of the hybrid supercapacitor are illustrated in Fig. 6d. With an energy density of 32.5 W h kg⁻¹ at a power density of 400 W kg⁻¹, our hybrid supercapacitor exhibits performances comparable to those of previous Fe₃O₄-based hybrid supercapacitors, such as Ni(OH)₂//F2RF-150 (4.1 W h kg⁻¹, 661.5 W kg⁻¹),⁴¹ MnHCF//Fe₃O₄/rGO (27.9 W h kg⁻¹, 2183.5 W kg⁻¹),⁴² CPY//C-G/AFC (18.3 W h kg⁻¹, 351 W kg⁻¹),¹² and Fe₃O₄@Fe₂O₃//Fe₃O₄@MnO₂ (26.6 W h kg⁻¹, 500 W kg⁻¹).⁴³

With the worsening of the energy and environmental issues,^44–47 the exploration of new-class water treatments that save energy and are environmentally friendly has taken on new urgency. In this regard, CDI has been developed based on electrical double layer theory and has received increasing interest over the past decade.^48–55 Porous carbons are the most widely used materials for CDI electrodes, including several subfamilies such as graphene,^56,57 activated/templated carbon,^58–62 and metal–organic framework-derived carbons.^63–66 However, the low desalination capacity of carbon materials largely limits the further application of CDI on an industrial scale.⁶⁷ To address this issue, hybrid CDI that uses a redox electrode to replace one of the two carbon electrodes has been developed.^68–70 Generally, TMOs are good candidates for electrode materials for HCDI,^71–74 and, recently, Fe₃O₄ materials have received increasing interest in the HCDI field due to their high desalination performance and great abundance.⁷⁵ In this section, we plan to evaluate the potential of our 2D-Fe₃O₄/C for HCDI applications.

Before desalination tests, the capacitive performance of the 2D-Fe₃O₄/C-450 electrode was first studied by CV curves in 1 M NaCl solution at 10–100 mV s⁻¹ to evaluate its desalination ability (Fig. 7a). The polarization of the electrode is inapparent, which indicates fast charge transfer kinetics, possibly due to a novel two-dimensional flake structure. In addition, the distorted rectangular curves reveal the redox behavior of 2D-Fe₃O₄/C-450 in NaCl solution. We further conducted electrochemical kinetic analysis, as shown in Fig. S9 (ESI†). The inset of Fig. S9a (ESI†) exhibits the profile of log(i) versus log(v), revealing b values between 0.5 and 1 (Fig. S9a, ESI†), which suggests the combined electrochemical behaviors of the surface-controlled charging and diffusion-controlled Na ion-insertion mechanisms. Therefore, the current response at a fixed potential could be expressed by the following equation:^76–78

i (V) = k₁v + k₂v^1/2, (1)

wherein k₁v and k₂v^1/2 correspond to the current contributions from surface-controlled charging and diffusion-controlled Na ion-insertion processes, respectively. The corresponding contributions of surface-controlled charging and diffusion-controlled Na ion-insertion processes can be displayed in the CV curve as shown in Fig. S9b (ESI†). The calculated separate capacitances at 10–100 mV s⁻¹ are shown in Fig. S9c (ESI†), in which the capacitance contribution from the surface-controlled charging process increases with an increased scan rate, and the separate capacitance for the surface-controlled charging process maintains the values of 68.9–71.9 F g⁻¹. Interestingly, these values are close to the calculated capacitance of 67.5 F g⁻¹ through the plot fitting of the total capacitance (C) versus the square root of the scan rate (v^1/2) shown in Fig. S9d (ESI†), referring to the separate surface-controlled charging contribution, which suggests that the deionization behavior of our material follows the combined surface-controlled charging and diffusion-controlled Na ion-insertion mechanisms.

Fig. 7 Electrochemical profiles of 2D-Fe₃O₄/C-450 in NaCl solution: (a) CV curves at varying scan rates; (b) desalination capacity variations at varying voltages; (c) CDI Ragone plots at varying voltages; (d) desalination capacity and maximum desalination rate at varying voltages; (e) cycling desalination profiles over 20 cycles at 1.2 V.

Furthermore, hybrid CDI systems composed of 2D-Fe₃O₄/C-450 and activated carbon electrodes were constructed. Fig. 7b depicts the profiles of the desalination capacity variations for the 2D-Fe₃O₄/C-450 electrode over 7.5 min in a 500 mg L⁻¹ NaCl solution at varying operation voltages from 0.8 to 1.2 V. It is observed that once the voltage is applied between the opposite electrodes, the desalination capacity rapidly increases, reaching the maximum value at 7.5 min, which suggests desalination equilibrium. The corresponding CDI Ragone plots (Fig. 7c) depict that, with the increase of operation voltage, the plots tend to shift toward the region corresponding to higher desalination capacity and rate. The detailed values of desalination capacity and maximum desalination rate at varying voltages are displayed in Fig. 7d, revealing that the desalination capacity of the 2D-Fe₃O₄/C-450 electrode at 1.2 V reaches 28.5 mg g⁻¹ with a maximum desalination rate of 14.0 mg g⁻¹ min⁻¹. For the total desalination process over 7.5 min, the mean desalination rate is 3.8 mg g⁻¹ min⁻¹. As compared with previous reports of TMOs (Table S1, ESI†), these values are still the state of the art, possibly due to the novel 2D nanoflake structure, which provides a large accessible surface area for ion accommodation and a short ion diffusion pathway for mass transport, as well as the significant improvement of the electrical conductivity of the Fe₃O₄ by the introduced carbons. Finally, the cycling stability of the hybrid CDI cell was studied in NaCl solution with a concentration of 500 mg L⁻¹ (Fig. 7e), which shows that a good desalination capacity retention of 83% remains after 20 cycles, suggesting good cyclability.

Conclusions

In this work, we have successfully synthesized 2D-Fe₃O₄/C by the simple carbonization of an iron-containing complex for hybrid capacitive applications of supercapacitors and CDI. The obtained 2D-Fe₃O₄/C composed of carbon-decorated Fe₃O₄ nanoflakes possesses several advantageous features for high electrochemical performance, including 2D nanoflakes that provide more exposed active sites, larger accessible surface areas, and shortened electron diffusion distances, as well as the decorated carbon significantly improving electrical conductivity and preventing the aggregation of Fe₃O₄ nanoparticles. Consequently, 2D-Fe₃O₄/C achieves a maximum specific capacitance of 386 F g⁻¹ at 1 A g⁻¹ in 2 M KOH, and a high-energy density of 32.5 W h kg⁻¹ at 400 W kg⁻¹ when coupled with NiCo-LDHs for constructing a hybrid supercapacitor. Besides the great potential for hybrid supercapacitors, 2D-Fe₃O₄/C also shows a high desalination capacity of 28.5 mg g⁻¹ with a very high mean desalination rate of 3.8 mg g⁻¹ min⁻¹. Overall, we not only provide a new synthetic approach of 2D Fe₃O₄ nanostructures, but also demonstrate the potential of the obtained 2D Fe₃O₄ nanostructures for applications in hybrid supercapacitors and CDI.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21771064 and 51909066), the Innovative Research Team of Anhui Provincial Education Department (2016SCXPTTD), Primary Research and Development Program of Anhui Province (201904a05020087) and the Key Discipline of Materials Science and Engineering of Suzhou University (2017XJZDXK3) is gratefully acknowledged. This work was partially funded by the Researchers Supporting Project (RSP-2020/267), King Saud University, Riyadh, Saudi Arabia. Dr Xu acknowledges the support from the JSPS Postdoctoral Fellowship for Overseas Researchers (20F20338). This work was partially performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0qm00946f

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