Three-Dimensional Hybrid Nanostructures of Fe3O4 Nanoparticles/Vertically-Aligned Carbon Nanotubes for High-Performance Supercapacitors

Abstract

A three-dimensional (3D) hybrid nanostructure of Fe₃O₄ nanoparticles uniformly anchored on vertically-aligned carbon nanotubes (VACNTs) was fabricated by a facile two-step method. Assisted by supercritical carbon dioxide (SCCO₂), the Fe precursor was firstly absorbed on CNT surface and then transformed into Fe₃O₄ nanoparticles by vacuum thermal annealing. Owing to the synergetic effects of well-distributed Fe₃O₄ nanoparticles (~7 nm) and highly conductive VACNTs, the hybrid electrode exhibits a high specific capacitance of 364.2 F g⁻¹ at 0.5 A g⁻¹ within the potential range from −0.9 to +0.1 V in Na₂SO₃ electrolyte and an excellent cycling stability of 84.8% capacitance retention after 2000 cycles at a current density of 4 A/g. This 3D hybrid architecture consisting of aligned CNTs and pseudocapacitive metal oxide may be a promising electrode for high-performance supercapacitors.

1. Introduction

In order to replace non-renewable fossil fuels and solve the air pollution/climate change problems, great efforts have been made to stimulate extensive research on energy storage and conversion systems for the sake of exploring alternative energy sources over recent years [1,2,3,4,5]. Nowadays, supercapacitors (also referred to as electrochemical capacitors) have attracted much attention due to their high power densities, environmental benignity, fast charging–discharging rates, long cycle life and so forth, compared with rechargeable batteries and conventional dielectric capacitors [6,7,8]. According to the charge storage mechanism, supercapacitors may often be classified into electrochemical double-layer capacitors (EDLCs) and pseudocapacitors [9,10]. Carbon-based materials are generally used as electrode materials in EDLCs, such as activated carbon, graphene, and carbon nanotubes, due to their high specific surface area, good stability and high electrical conductivity [11]. However, the limited specific capacitance of EDLCs is far from the ever-growing need of practical energy storage application [12]. On the contrary, the pseudocapacitors (using transition metal oxides/hydroxides and conductive polymers as electrode materials) afford much higher specific capacitance and energy density than those of EDLCs due to the reversible Faradaic redox reaction in charge/discharge process [8,12,13,14,15]. These pseudocapacitive materials, especially the transition metal oxides, often have poor electric conductivity, which results in poor rate performance and limited cycling ability [16,17]. Thus, hybridization of the pseudocapacitive metal oxides with carbonaceous materials represents a promising strategy for developing novel electrode materials with high performance [18].

Up to now, extensive efforts have been devoted to exploring transition metal oxides (TMOs) as electrode materials for pseudocapacitors, such as V₂O₅, VN, TiN, Co₃O₄, MnO₂, MoO_3−x, and Fe₂O₃ [16,19,20,21,22,23,24,25]. Among these materials, Fe₃O₄ is considered a promising pseudocapacitive material due to its fast reversible redox reactions, natural abundance, low cost, and eco-friendly nature [26,27,28]. Nevertheless, as with most transition metal oxides, the low specific surface area and undesirable conductivity of Fe₃O₄ can cause inadequate redox reaction with electrolyte ions, which is unsatisfactory for practical application in supercapacitor [29,30,31]. Thus, in order to overcome these limitations and fully exploit the potential electrochemical benefits of Fe₃O₄, the design of nanostructured Fe₃O₄/carbon composite for supercapacitor electrode has attracted great attention recently [32].

Vertically-aligned carbon nanotubes (VACNTs), with unique 3D hierarchical nanostructures for supporting active materials, are regarded as ideal architectures for electrode materials owing to their large specific surface area, excellent electrical conductivity, and robust chemical stability [32,33,34]. In particular, the well-oriented CNTs within VACNTs may form sufficient 1D “electron highway” to ensure fast charge transport. Moreover, the suitable inter-tube spacing facilitates efficient diffusion of electrolytic ions [35,36]. However, due to the nano-sized inter-tube spacing and hydrophobic nature of CNTs, it is challenging to uniformly grow TMOs on CNT surface by conventional wet chemistry. Moreover, VACNTs are easy to collapse and agglomerate upon contacting with liquid reagents due to capillary forces [33,34]. Therefore, despite the various methods that had been explored, such as electrochemical deposition [37], nebulized ethanol assisted infiltration [35], and dip-casting method [38], it is still a big challenge to fabricate 3D TMOs/VACNTs hybrid composites with satisfactory electrochemical performances.

In this work, Fe₃O₄ nanoparticles-decorated VACNTs hybrids were designed and fabricated by a supercritical carbon dioxide (SCCO₂) assisted process followed by low-temperature annealing. Owing to the unique features of SCCO₂ (gas-like diffusivity and liquid-like dissolving capability), SCCO₂ is beneficial for mass transfer of precursor molecules within the nano-sized space of VACNTs [39]. With an effective approach, Fe₃O₄ nanoparticles with diameters of about 5–10 nm can be successfully decorated on CNTs homogeneously, which are then used as binder-free electrodes for supercapacitors. Profiting from the advantageous architecture with well-dispersed Fe₃O₄ nanoparticles in-situ grown on aligned CNTs, the as-obtained Fe₃O₄/VACNTs composite electrode exhibit a specific capacitance of 364.2 F g⁻¹ at 0.5 A g⁻¹ in the potential window of −0.9 to +0.1 V, excellent rate performance, and a satisfactory cycle stability. Furthermore, the neutral Na₂SO₃ electrolyte used in the current work have advantages including higher safety and better environmental benignity than the harsh acid or alkaline electrolyte. Such an electrochemical performance of the Fe₃O₄/VACNTs composites holds promise for application in advanced supercapacitors.

2. Experimental Section

2.1. Synthesis of VACNTs

Vertically-aligned carbon nanotubes with highly dense and millimeter-long thickness were synthesized by the water-assisted chemical vapor deposition (CVD) at 840 °C, using high-purity ethylene (99.99%) as a carbon source. An Fe film (1.5 nm) with an Al₂O₃ buffer layer (30 nm) was sputtered onto an Si (100) wafer, which was used as the catalyst for CNT growth. High-purity Ar (99.999%) with H₂ (99.999%) gas were used as carrier gas at 1 atm, and the total flow rate was 600 sccm during the growth. A small and controlled amount of water vapor was introduced to increase catalyst lifetime and was supplied by passing a portion of the Ar carrier gas through a water bubbler. In a typical CVD growth, 100 sccm ethylene was introduced with water concentration of 100–200 ppm at 840 °C for 10 min at ambient pressure.

2.2. Fabrication of Fe₃O₄/VACNTs Composites

In a typical process, VACNTs and 30 mg precursor (Iron (III) acetylacetonate) dissolved in 0.75 mL benzene solution were sealed in a high-pressure vessel of 50 mL. Secondly, the vessel was connected to a gas pipeline. Then, the reactor was pre-heated to 50 °C, and CO₂ was introduced to reach the targeted pressure by a syringe pump slowly. After that, the vessel was heated to 100 °C and kept for 6 h, allowing the metallorganic precursor to be dissolved into SCCO₂ fluids and be adsorbed into the VACNTs. Subsequently, the reactor was depressurized and cooled to room temperature slowly. Finally, the precursor-impregnated VACNTs were transferred to a tube furnace and annealed at different temperatures (500, 550, and 600 °C) in a vacuum for 1 h to convert the precursor to iron oxide.

2.3. Characterization

The crystal structures of the products were analyzed by XRD patterns which were performed by an X-ray diffraction (XRD, Advance D8) spectrometer with CuKa radiation (λ = 0.1542 nm) at a scanning rate of 10 °/min. Raman spectra were taken on a Raman Station 400 F with an excitation length of 633 nm. The morphologies of different samples and energy-dispersive X-ray spectroscopy elemental mapping images were viewed using a field emission scanning electron microscopy (FE-SEM, Quanta FEG450). The microstructures of obtained samples were observed by a transmission electron microscopy (TEM, Tecnai G2 F30).

2.4. Electrochemical Measurement

Electrochemical measurements were performed on CHI 750D electrochemical workstation (Shanghai Chenhua, Shanghai, China) at ambient temperature using a three-electrode configuration in 1 M Na₂SO₃ aqueous solution. The as-obtained VACNTs or hybrid samples were pressed onto nickel foam to form a freestanding electrode without any binders, which was then used as the working electrode. A Pt plate and SCE were used as the counter electrode and reference electrode, respectively.

3. Results and Discussions

Scheme 1 illustrates the detailed preparation process of Fe₃O₄/VACNTs hybrid materials. VACNTs act as the 3D porous scaffold for absorption of Fe(acac)₃ precursor and the growth of Fe₃O₄ nanoparticles. Specifically, VACNTs were grown by water-assisted CVD and then Fe(acac)₃ was absorbed on CNT surface with the aid of SCCO₂. Thanks to the unique physicochemical properties of SCCO₂, the aligned structure of VACNTs will be well retained. Subsequently, the Fe(acac)₃-impreganated VACNTs were annealed in vacuum to transform the Fe precursor to Fe₃O₄ nanoparticles.

First of all, the effect of annealing temperature on Fe₃O₄/VACNTs hybrids is investigated. The Fe(acac)₃/VACNTs samples containing 45.2 wt% precursors were annealed at different temperatures (500, 550, and 600 °C) for 1 h in vacuum, and thus the Fe₃O₄/VACNTs hybrids obtained are denoted as Fe₃O₄/VACNTs-500, Fe₃O₄/VACNTs-550 and Fe₃O₄/VACNTs-600, respectively. Figure 1a shows the X-ray diffraction (XRD) patterns of the pristine VACNTs and different Fe₃O₄/VACNTs composites. The relative peak intensities of Fe₃O₄ nanoparticles are slightly weak due to the small amount of nanoparticles available for analysis. Compared with the pure VACNTs, the XRD pattern of Fe₃O₄/VACNTs composites shows some additional peaks at 2θ values of 30.2°, 35.6°, 43.2°, 53.7°, 57.2°, and 62.7°, corresponding to the (220), (311), (400), (422), (511), and (440) planes of Fe₃O₄ (JCPDS file NO.75-0033), respectively. No obvious peaks from other phases were observed, which indicates that the product is a mixture of only two phases: cubic Fe₃O₄ and VACNTs. Figure 1b shows the Raman spectra of the pure VACNTs and Fe₃O₄/VACNTs composites annealed at different temperatures. The D-band located at 1340 cm⁻¹ and the G-band at 1579 cm⁻¹ are characteristic peaks of carbon nanotubes [40,41]. The three Raman peaks observed between 200 and 700 cm⁻¹ are associated with E_g, T_2g and A_1g vibration modes of Fe₃O₄, respectively [42]. Accordingly, the Raman results also confirm the successful synthesis of Fe₃O₄/VACNTs composite without any trace of the impurity phase. Besides, with the increase of annealing temperature, the characteristic diffraction peaks and Raman shifts of Fe₃O₄ have become more obvious, which can possibly be attributed to the aggregation of Fe₃O₄ nanoparticles at a higher temperature.

The morphology and microstructure of the Fe₃O₄/VACNTs composite were examined by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). From the TEM images (Figure 2a,b), homogeneous distribution of Fe₃O₄ nanoparticles on the CNT surface is directly observed. The mean particle size of the Fe₃O₄ is about 7 nm. The HRTEM examination (Figure 2c) shows clear lattice fringes with an interplanar distance of 0.297 nm, which can be attributed to the (220) plane of Fe₃O₄. The morphology of the pure VACNTs and Fe₃O₄/VACNTs hybrids annealed at different temperatures was investigated by FESEM. Figure S1a,b shows the vertically-aligned and dense structures of as-grown VACNTs. Compared with the pristine VACNTs, the nanotubes appear much rougher in Fe₃O₄/VACNTs composites, indicating the possible decoration of the nanoparticles on the CNT surface (Figure 2d–f). Noteworthy, the Fe₃O₄/VACNTs composites annealed at 500 and 550 °C show the well-retained aligned structure. However, when the annealing temperature rises to 600 °C, the CNTs become more distorted and entangled. Therefore, it is inferred that excessive annealing temperature during the decomposition process of the Fe precursor may impair the alignment of VACNTs somehow. Figure S2 presents cross-sectional SEM image of the Fe₃O₄/VACNTs-500 and the corresponding EDS spectrum. Presence of C, O and Fe elements suggests the successful synthesis of the composite without any impurities. The EDS elemental mapping further demonstrates uniform distribution of Fe and O elements on the cross-section (Figure 2g), validating homogeneous dispersion of Fe₃O₄ nanoparticles in the VACNTs. This can be credited to the advantages of SCCO₂. Due to the gas-like diffusivity and zero surface tension of SCCO₂, the Fe precursors could be efficiently delivered and uniformly attached on the CNT surface in the millimeter-long VACNTs [39].

To explore potential application for high-performance supercapacitors, the Fe₃O₄/VACNTs composites pressed onto nickel foam are used as binder-free electrodes. All electrochemical tests were performed in 1 M Na₂SO₃ with SCE as the reference electrode and a Pt mesh as the counter electrode. The cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were conducted. Figure 3a shows the CV curves of the Fe₃O₄/VACNTs-500, Fe₃O₄/VACNTs-550 and Fe₃O₄/VACNTs-600 samples at a scan rate of 20 mV s⁻¹. Here, the CV scans were performed in the potential window between −0.9 V and +0.1 V to prevent degradation of the Fe₃O₄/VACNTs composite electrodes and electrolyte at higher potentials [43]. All of the samples have the roughly rectangular and symmetric CV curves with small redox peaks, which show the typical behaviors of combination of electric double layer capacitance from VACNTs and pseudocapacitance from the redox reaction of Fe₃O₄. As is well-known, the specific capacitance is proportional to the average areas of CV curves. As can be seen from the figure, the CV curves of the Fe₃O₄/VACNTs-550 sample is much wider than those of other samples at the same scan rate. Therefore, these results indicate that annealing at 550 °C may be more appropriate for electrochemical performance. In Figure S3a–c, typical CV curves of the Fe₃O₄/VACNTs composites annealed at various temperature in 1 M Na₂SO₃ electrolyte at different scan rates are shown. A pair of symmetrical redox peaks can be observed at the scan rates ranging from 10 to 200 mV s⁻¹, resulting from the pseudo-capacitive contribution of the redox reaction between the active material and electrolyte ions. In the experiment, the redox reaction occurs by the following mechanism [43]:

(1)$\mathrm{FeO}+{\mathrm{SO}}_3^{2-}\leftrightarrow {\mathrm{FeSO}}_4+2{\mathrm{e}}^{-}$

(2)$2{\mathrm{Fe}}^{\mathrm{II}}\mathrm{O}+{\mathrm{SO}}_3^{2-}\leftrightarrow {\left({\mathrm{Fe}}^{\mathrm{III}}\mathrm{O}\right)}^{+}+{\mathrm{SO}}_3^{2-}\left({\mathrm{Fe}}^{\mathrm{III}}\mathrm{O}\right)+2{\mathrm{e}}^{-}$

However, at a high scan rate, a distinct redox peak was not observed, since solution and electrode resistance can distort the current response at the switching potential, and this distortion is dependent on the scan rate, leading to the incomplete pseudocapacitance response of the electrode material and less evident redox peaks.

Figure 3b and Figure S3d–f show the GCD curves of the Fe₃O₄/VACNTs hybrid electrodes at various current densities (0.5 to 4 A g⁻¹). As shown in Figure S3d–f, the charge-discharge plateau in the charging potential window (from −0.4 to −0.3 V) and the discharging potential window (−0.8 to −0.9 V) suggest typical redox reactions of Fe₃O₄ nanoparticles in the Na₂SO₃ electrolyte, consistent with the redox peaks in the CV curves. Obviously, the discharge time of the Fe₃O₄/VACNTs-550 electrode is longer than that of the other samples (Figure 3b), indicating the higher specific capacitance. The specific capacitance, C_s, was calculated from the galvanostatic charge/discharge curves using the following equation:

(3)$C_S=\frac{I\times \Delta t}{m\times \Delta V}$

where I, △t and m represent discharge current, discharge time, and mass of the active material, respectively, and △V is the voltage drop during discharging [44]. The calculated specific capacitance values were 212.9, 285.3 and 184.7 F g⁻¹ at current density of 0.5 A g⁻¹ for the Fe₃O₄/VACNTs-500, Fe₃O₄/VACNTs-550 and Fe₃O₄/VACNTs-600, respectively. As displayed in Figure 3c, the Fe₃O₄/VACNTs-550 exhibits higher specific capacitance than the other counterparts at all current densities, revealing the superior capacity for energy storage of the Fe₃O₄/VACNTs-550 electrode. When the current density increases eight times, the C_s of Fe₃O₄/VACNTs-550 still hold 93.2% of the initial value, showing an excellent rate performance. However, upon annealed at 600 °C, the composite electrode shows a significant drop in the specific capacitance. It is possible that annealing at the excessively high temperature induces agglomeration of Fe₃O₄ nanoparticles, which results in the lower utilization efficiency of the active materials. Meanwhile, the entanglement of CNTs (shown by SEM observation) may hinder electrolyte ion transportation during electrochemical process.

EIS measurements were also carried out to examine the electrochemical performance of the Fe₃O₄/VACNTs hybrids at different annealing temperature. Typically, Nyquist plots often consist of a semicircle arc in the high-frequency region, and a straight line (in the low-frequency area) indicating diffusion of the electrolyte ions [45]. As shown in Figure 3d, all of these profiles display the semicircle with a small diameter (<3 ohm) and steep line in the low-frequency area, suggesting the small charge-transfer resistance and good capacitive property. Notably, the Fe₃O₄/VACNTs-600 composite exhibits a larger semicircle and smaller slope than that of other samples, suggesting that the Fe₃O₄/VACNTs-600 has higher charge-transfer resistance at the electrode/electrolyte interface and slower ion diffusion rate than the counterparts. This can be ascribed to the agglomeration of Fe₃O₄ nanoparticles and the degradation of CNT alignment [21]. Thus, it is important to keep aligned CNT structure and homogeneous distribution of the Fe₃O₄ nanoparticles for fabrication process of the Fe₃O₄/VACNTs composites. Based on the above electrochemical performance, 550 °C is the appropriate annealing temperature to fabricate a hybrid electrode with optimal performance.

To further evaluate the capacitive performances of the as-prepared Fe₃O₄/VACNTs hybrids, two different weight percentages of Fe₃O₄ after 1 h annealing at 550 °C have been investigated. The weight percentages of Fe₃O₄ is, respectively, 17.2% and 41.9% (noted as Fe₃O₄/VACNTs-17.2% and Fe₃O₄/VACNTs-41.9%, respectively), which is calculated according to the equation:

(4)$wt{\%}_{\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)}=\frac{\left(m_t-m_{VACNTs}\right)}{m_t}\times 100\%$

where m_t is the total mass of Fe₃O₄/VACNTs composites after annealing, m_VACNTs is the mass of pure VACNTs without SCCO₂ treatment. It is noted that two different weight percentages of Fe₃O₄ for composites (17.2 and 41.9 wt%) with the same fabricated process except for using different concentration of precursor. Figure S4a shows the X-ray diffraction (XRD) patterns of the pristine VACNTs and Fe₃O₄/VACNTs composites with different loading mass. Compared with the pure VACNTs, the Fe₃O₄/VACNTs composites XRD pattern shows six extra diffraction peaks located, which can be attributed to the characteristic crystal face reflections of Fe₃O₄. It is noteworthy that the characteristic peaks of Fe₃O₄/VACNTs-41.9% sample are stronger than those of Fe₃O₄/VACNTs-17.2%, which is attributed to the increase of Fe₃O₄ contents in Fe₃O₄/VACNTs-41.9%. As shown in Figure S4b, the Raman spectra of the pristine VACNTs and Fe₃O₄/VACNTs composites with different mass of Fe₃O₄ also proved Fe₃O₄ successfully decorated in VACNTs. Besides, with increase of Fe₃O₄ in VACNTs contents in composites, the characteristic peaks of Fe₃O₄ become stronger and those of CNTs become weaker, but the I_G/I_D ratio of Fe₃O₄/VACNTs-17.2 and −41.9% samples do not have significant change, indicating that the defects increase of carbon nanotubes in composites is mainly due to the excessive annealing temperature rather than high loading mass of Fe₃O₄ in synthetic process.

Figure 4a–d presents SEM images showing the cross sections of Fe₃O₄/VACNTs hybrids with two different weight percentages of Fe₃O₄ nanoparticles at different magnifications. As shown in Figure 4a,b, uniformly distributed nanoparticles can be clearly observed in the SEM images of the cross sections of Fe₃O₄/VACNTs-17.2% sample, especially in Figure 4b. Figure 4c,d shows the SEM images of Fe₃O₄/VACNTs-49.1% sample, in which the nanoparticles can also be clearly seen on the walls of CNTs. Both images show that nanoparticles are homogeneously distributed in the VACNT arrays and the CNTs of the composites still hold parallel to each other without obvious distortion and winding.

Figure 5 shows the electrochemical capacitive performance of pure VACNTs and Fe₃O₄/VACNTs composites with different loading mass of Fe₃O₄. Based on the CV curves at a scan rate of 20 mV/s (Figure 5a), the bare VACNTs has a rather rectangular curve, typically for electric double-layer capacitance, while the CV curves of Fe₃O₄/VACNTs-17.2% and Fe₃O₄/VACNTs-41.9% electrodes show apparent redox peaks (pseudocapacitive behavior). Moreover, for the Fe₃O₄/VACNTs composites, the enclosed area of CV curves is much larger than that of pure VACNTs, suggesting that the specific capacitance is significantly enhanced after the Fe₃O₄ hybridization. Besides, the Fe₃O₄/VACNTs-41.9% possess larger enclosed area and much stronger redox peaks than the Fe₃O₄/VACNTs-17.2%, indicating superior electrochemical properties. The GCD curves shown in Figure 5b also depict the similar dependence of specific capacitance on the Fe₃O₄ loading content. The Fe₃O₄/VACNTs-41.9% shows the much longer discharge time than that of the composite with less Fe₃O₄, demonstrating a higher specific capacitance. Besides, the obvious charge-discharge plateau can be observed for all of the composite electrodes, which is different from the as-grown VACNTs. The specific capacitances derived from the discharge curves at different current densities are summarized in Figure 5c. The specific capacitance of the Fe₃O₄/VACNTs-41.9% sample is 364.2, 330.1, 319.1, 310.8, 308.5 F g⁻¹ at 0.5, 1, 2, 3, 4 A g⁻¹, respectively, which is notably higher than that of the Fe₃O₄/VACNTs-17.2%. In addition, compared with pure VACNTs, the capacitance is significantly increased after the Fe₃O₄ loading. The maximum C_s (364.2 F g⁻¹ at 0.5 A g⁻¹) is notably higher than that of the previously-reported Fe₃O₄-based hybrid electrodes, as shown in Table 1. Noteworthily, when the charge-discharge current density changes from 0.5 A g⁻¹ to 4 A g⁻¹, the Fe₃O₄/VACNTs-41.9% electrode has a capacitance retention of 84.7%, also showing the good rate capability at higher Fe₃O₄ content. EIS measurement shows a similar trend. As illustrated by Nyquist plots in Figure 5d, in the high-frequency region, the semicircle and the intersect with real axis gradually increase with raising the Fe₃O₄ mass, indicating larger charge-transfer resistance and equivalent series resistance (ESR) at higher Fe₃O₄ content. These results can be attributed to the larger amount of Fe₃O₄ nanoparticles with poor electrical conductivity in the composite, which may agglomerate to induce uneven distribution of the Fe₃O₄ on CNTs. In the low frequency region, the VACNTs shows the most ideal straight lines along the imaginary axis, while the slope decreases with the hybridization of Fe₃O₄, suggesting the slightly lower diffusion rate of electrolyte ions in the composite electrodes [46].

As an important parameter for practical application, the cycling stability of Fe₃O₄-decorated VACNTs is further investigated and the results are shown in Figure 6a,b. After 2000 cycles at the current density of 4 A g⁻¹, the Fe₃O₄/VACNTs-41.9% can maintain 84.8% of the initial capacitance and the Fe₃O₄/VACNTs-17.2% may keep 91.6% of the initial capacitance, which both deliver excellent cyclic stability with extraordinary capacity retention. The structural advantages of Fe₃O₄/VACNTs may contribute to the excellent cycling stability. Firstly, the in-situ grown Fe₃O₄ nanoparticles have an intimate contact with CNTs, facilitating fast electron transfer. Secondly, the tiny Fe₃O₄ nanoparticles uniformly dispersed in VACNTs expose large active surface area and ensure efficient utilization of the pseudocapacitive oxide. Thirdly, the free space among Fe₃O₄ nanoparticles alleviates volume expansion and mechanical strain during fast and long-term Faradaic reaction [16]. To sum up, the Fe₃O₄/VACNTs hybrids have a unique nanostructure and excellent electrochemical properties as the electrode materials of supercapacitors.

4. Conclusions

A novel 3D hybrid electrode material with Fe₃O₄ nanoparticles uniformly distributed on VACNTs is fabricated via a facile SCCO₂ assisted method and subsequent vacuum annealing. It is found that the annealing temperature after SCCO₂ treatment has significant effect on the nanostructure and electrochemical properties of the Fe₃O₄/VACNTs composites. The Fe₃O₄/VACNTs composite electrode annealed at 550 °C shows a superior specific capacitance and rate performance compare to the other counterparts. The Fe₃O₄ loading percentage in composites also plays an important role in electrochemical performance of the composite electrodes. The Fe₃O₄/VACNTs with 41.9% of Fe₃O₄ exhibits a higher specific capacitance of about 364.2 F g⁻¹ at 0.5 A g⁻¹ than the electrode with lower Fe₃O₄ content. Furthermore, the Fe₃O₄/VACNTs-41.9% also shows outstanding rate capability and satisfactory cycling stability with an 84.8% capacitance retention. The excellent electrochemical performance makes the 3D Fe₃O₄/VACNTs composite electrodes promising for application in advanced supercapacitors.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figures, Scheme and Table

Scheme 1. Schematic illustration for synthesis of Fe3O4/VACNTs composite.

Figure 1. XRD patterns (a) and Raman spectra (b) of the VACNTs and Fe3O4/VACNTs composites annealed at different temperatures.

Figure 2. TEM (a,b) and HRTEM (c) images of the Fe3O4/VACNTs-550 composites. (d–f) Cross-sectional FESEM images of Fe3O4/VACNTs composites at different annealing temperature (500, 550 and 600 °C, respectively). (g) Cross-sectional SEM image of the VACNTs-550 and corresponding EDS elemental mapping.

Figure 3. Electrochemical performance of the Fe3O4/VACNTs composites annealed at different temperatures in 1M Na2SO3: (a) CV curves at 20 mV/s. (b) GCD curves at 0.5 A/g. (c) Specific capacitance at various current densities. (d) EIS spectra.

Figure 4. Cross-sectional FESEM images of Fe3O4/VACNTs composites at different magnifications. (a,b) Fe3O4/VACNTs-17.2%; (c,d) Fe3O4/VACNTs-41.9%.

Figure 5. Electrochemical performance of the pure VACNTs and Fe3O4/VACNTs composites with different Fe3O4 loading: (a) CV curve at 20 mV/s; (b) GCD curves at 0.5 A/g; (c) Specific capacitances at various current densities; (d) EIS spectra.

Figure 6. Cycling performance of the Fe3O4/VACNTs composites with different Fe3O4 loading at a current density of 4 A g−1: (a) Fe3O4/VACNTs-17.2%; (b) Fe3O4/VACNTs-41.9%.

Table 1

Comparison of the electrochemical performance with other nanocomposites based on Fe₃O₄.

Composites	Special Capacitance(F/g)	Current Density(A/g)	Reference
RGO-Fe3O4	236	1	[31]
Fe3O4/activated biocarbon	342	1	[47]
DE/Fe3O4/PANI	242.9	0.5	[48]
Nanoporous Fe3O4-carbon nanosheets	163.4	1	[49]
Fe3O4/graphene	300	0.4	[50]
Fe3O4/VACNTs	364.2	0.5	This work

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/electrochem3030035/s1, Figure S1: FESEM images of cross sections of pure VACNTs at different magnifications; Figure S2: EDS spectrum of the Fe₃O₄/VACNTs-550 sample; Figure S3: CVs and GCD curves of the Fe₃O₄/VACNTs hybrids annealed at different temperature: (a,d) Fe₃O₄/VACNTs-500; (b,e) Fe₃O₄/VACNTs-500; (c,f) Fe₃O₄/VACNTs-600; Figure S4: XRD patterns (a) and Raman spectra (b) of the pure VACNTs and Fe₃O₄ /VACNTs composites with different loading mass percentage.

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