Nano-Fe 3 O 4 /Carbon Nanotubes Composites by One-Pot Microwave Solvothermal Method for Supercapacitor Applications

: Carbon nanotubes (CNTs) are being increasingly studied as electrode materials for supercapacitors (SCs) due to their high electronic conductivity and chemical and mechanical stability. However, their energy density and speciﬁc capacitance have not reached the commercial stage due to their electrostatic charge storage system via a non-faradic mechanism. Moreover, magnetite (Fe 3 O 4 ) exhibits higher speciﬁc capacitance originating from its pseudocapacitive behaviour, while it has irreversible volume expansion during cycling. Therefore, a very interesting and facile strategy to arrive at better performance and stability is to integrate CNTs and Fe 3 O 4 . In this study, we demon-strate the microwave-solvothermal process for the synthesis of Fe 3 O 4 nanoparticles uniformly grown on a CNT composite as an electrode for SCs. The synthesized Fe 3 O 4 /CNT composite delivers a reversible capacitance of 187.1 F/g at 1 A/g, superior rate capability by maintaining 61.6% of 10 A/g (vs. 1 A/g), and cycling stability of 80.2% after 1000 cycles at 1 A/g.

Carbon nanotubes (CNTs) have attracted much attention as an electrode for SCs, where the charge can be electrostatically stored with non-faradic reaction, because of their good chemical stability, high mechanical strength, and excellent electrical conductivity [9][10][11][12][13]. However, the use of CNTs materials as commercial electrodes is limited due to their low specific capacitance originating from the non-faradic process, which requires further enhancement in the specific capacitance [9][10][11][12][13].
However, the continuous charging and discharging process of Fe 3 O 4 can lead to irreversible volume change, resulting in severe electrochemical performance decay. Therefore, the modification of the Fe 3 O 4 -based electrode is important to satisfy the high capacitance, stable cycling performance, and superior rate capability required for SCs.
A desirable approach to resolve these problems is the incorporation of redox-active Fe 3 O 4 nanoparticles into the electrically conductive CNT matrix. This approach can offer enhanced specific capacitance while preserving cycling stability and rate capability. The fabrication of Fe 3 O 4 /CNT hybrid structure with uniform nanoparticles is challenging as it requires a complicated process including multi-and time-consuming steps and the use of dangerous chemical reagents. Thus, developing a simple and eco-friendly synthesis strategy to enhance the electrochemical performance of electrode materials is crucial.
Herein, we report the synthesis of a well-designed Fe 3 O 4 /CNT structure using a straightforward strategy relying on a simple microwave-solvothermal method, where CNT serves as an effective heating template for facilitating nucleation of Fe 3 O 4 nanoparticles. Uniform Fe 3 O 4 nanoparticles with high purity were deposited onto the surface of CNTs using the microwave-solvothermal process. The synthesized Fe 3 O 4 /CNT architecture shows high specific capacitance, rate capability, and good cycling performances.

Materials and Methods
The nano-Fe 3 O 4 /CNTs composite was synthesized following the microwave solvothermal method. CNTs (0.1 g) were sonicated in diethylene glycol (DEG, >99%, Fluka, 70 mL). Subsequently, FeCl 2 4H 2 O (0.207 g, Aldrich), FeCl 3 6H 2 O (0.507 g, Junsei), ammonium hydroxide (15 mL, 30 wt %, Junsei), and sodium acetate (1.32 g, Sigma Aldrich) were included in the solution, which was then loaded into 100 mL Teflon vessels sealed and placed in a microwave system (MARS-5, CEM Corporation). The reaction mixture was heated to 180 • C where it was maintained at that temperature for 10 min. Then, the mixture was naturally cooled down to room temperature.
A field-emission scanning electron microscopy (FE-SEM; JEOL 6340F) and a transmission electron microscopy (TEM; 200 kV; CM200, Philips) were used to analysed the surface morphology and structure of the nano-Fe 3 O 4 /CNTs composites, respectively. The structural properties were studied by performing powder X-ray diffraction (XRD) measurements (Rigaku, Japan). Moreover, an X-ray photoelectron spectroscopy (XPS; 15 kV, 150 W; ESCALAB 250, Thermo Electron Corporation) was used to evaluate the surface chemistry of nano-Fe 3 O 4 /CNTs, and a thermogravimetric analyser (TGA, STA 409 PC, Netzsch) allowed to conduct thermo-gravimetric studies using a ramp rate of 10 • C/min in air atmosphere from room temperature to 800 • C.
For the electrochemical test, the slurry was made with 80 wt.% active materials (nano-Fe 3 O 4 -CNT), 10 wt.% conductive additives (Super-P), and 10 wt.% binder (Polyvinylidene fluoride) using 1-methyl pyrrolidone and cast with active mass loading of 1-1.3 mg/cm 2 (Platinum substrate). Electrochemical tests, including cyclic voltammogram (CV) and galvanostatic charge-discharge (GCD) curves, were conducted using a potentiostat (Biologics VMP3) and a 1 M Na 2 SO 3 solution in a −1.0-0 V potential window vs. SCE. The mass of the nano-Fe 3 O 4 /CNT composite was used to normalize the current response in the CV and GCD curves. The respective specific capacitances (Cs) were determined by using half integrated area of the GCD curve to obtain the charge (Q) and subsequently dividing the charge by the mass of the electrode (m) and the width of the potential window (∆V) following equation: Cs = Q/m ∆V.

Results and Discussion
In the nano-Fe 3 O 4 /CNT composite development, the CNTs functioned as a template for the selective heterogeneous nucleation and growth of Fe 3 O 4 nanoparticles. In particular, the carbonaceous materials absorb the microwave energy, converting it into heat to provide a high temperature on the surface of CNTs relative to the solution. In general, because the nucleation of metal ions occurs on hot spots, the Fe 3

O 4 precursor in the solution is
Energies 2021, 14, 2908 3 of 8 considered to be selectively nucleated and uniformly grown on the surface of the CNTs, as shown schematically in Figure 1. Additionally, the microwave-solvothermal method could favour the process owing to its distinct advantages over conventional hydrothermal or solvothermal methods, such as lower synthesis temperature, higher reaction rate, shorter reaction time, and smaller resulting particles [24,25].

Results and Discussion
In the nano-Fe3O4/CNT composite development, the CNTs functioned as a template for the selective heterogeneous nucleation and growth of Fe3O4 nanoparticles. In particular, the carbonaceous materials absorb the microwave energy, converting it into heat to provide a high temperature on the surface of CNTs relative to the solution. In general, because the nucleation of metal ions occurs on hot spots, the Fe3O4 precursor in the solution is considered to be selectively nucleated and uniformly grown on the surface of the CNTs, as shown schematically in Figure 1. Additionally, the microwave-solvothermal method could favour the process owing to its distinct advantages over conventional hydrothermal or solvothermal methods, such as lower synthesis temperature, higher reaction rate, shorter reaction time, and smaller resulting particles [24,25].  Figure S2a,b). The selected area electron diffraction (SAED) pattern of Fe3O4/CNT exhibits distinct diffraction rings corresponding to the (440), (400), (311), and (220) crystalline planes of Fe3O4, identifying that the highly crystalline Fe3O4 nanoparticles grew on the CNT (Figure 2d) [19,26,27]. As shown in Figure 2e, the TEM image displays the size of iron oxide nanoparticles grown uniformly on CNT ranges from 2 to 4 nm. Such a uniform distribution of iron oxide particles of nano-Fe3O4/CNT originated from dissolution and precipitation of the iron oxide precursor on the surface of CNTs during the microwave-solvothermal process. In addition, a high magnification TEM image of Fe3O4/CNT composite shows the formation of a crystalline pattern with a d-spacing of 0.251 nm, which corresponds to the (311) plane of Fe3O4.  [19,26,27]. The XRD pattern for Fe 3 O 4 /CNT also shows the presence of CNT, which was clearly confirmed from the broad peak of (200) located at 25.4 • . As mentioned previously, the major peak corresponding to the (311) plane shown in the XRD pattern matches very well with the observed lattice fringe spacing of 0.251 nm for the Fe 3 O 4 crystalline phase (Figures 2f and 3a).   [19,26,27]. The XRD pattern for Fe3O4/CNT also shows the presence of CNT, which was clearly confirmed from the broad peak of (200) located at 25.4°. As mentioned previously, the major peak corresponding to the (311) plane shown in the XRD pattern matches very well with the observed lattice fringe spacing of 0.251 nm for the Fe3O4 crystalline phase (Figures 2f and  3a). The chemical properties such as composition and bonding nature of Fe3O4/CNT composite are characterized by XPS analysis (Figure 3b,c). The full-scale XPS spectra for Fe3O4/CNT exhibit C1s and O1s at 285.4 eV and 529.1 eV, as shown in Figure 3b. In addition, the peaks centred at 710 eV and 725 eV assigned to the Fe 2p3/2 and Fe 2p1/2 states (Figure 3c) corresponded to the Fe3O4 of nano-Fe3O4/CNTs [24]. Note that the peaks of Fe The chemical properties such as composition and bonding nature of Fe 3 O 4 /CNT composite are characterized by XPS analysis (Figure 3b,c). The full-scale XPS spectra for Fe 3 O 4 /CNT exhibit C1s and O1s at 285.4 eV and 529.1 eV, as shown in Figure 3b. In addition, the peaks centred at 710 eV and 725 eV assigned to the Fe 2p 3/2 and Fe 2p 1/2 states (Figure 3c) corresponded to the Fe 3 O 4 of nano-Fe 3 O 4 /CNTs [24]. Note that the peaks of Fe 2p 1/2 are detected at approximately 724.5 eV and 727.0 eV, respectively, indicating the formation of mixed oxides of Fe (II) and Fe (III). Moreover, the peak of Fe 2p 3/2 comprises two peaks at approximately 711.1 eV and 713.9 eV, respectively, which further confirm the formation of Fe 3 O 4 [28]. The results clearly indicate the successful formation of  Figure S3 implies that no capacitance associated with the used Pt-substrate is present. CV curves of Fe 3 O 4 /CNT composite electrodes present a rectangular and symmetrical shape with redox peaks, which is indicative of the combined contribution of the double layer capacitive and pseudocapacitive behaviours from CNT and Fe 3 O 4 , respectively. The pseudocapacitive reaction mechanism of Fe 3 O 4 in 1 M Na 2 SO 3 electrolyte is based on the reversible redox reactions between Fe 2+ and Fe 3+ accompanied with the intercalation of SO 3 2− ions from electrolyte [23,29,30]: Energies 2021, 14, x FOR PEER REVIEW 6 of 8 of Fe3O4 in 1 M Na2SO3 electrolyte is based on the reversible redox reactions between Fe 2+ and Fe 3+ accompanied with the intercalation of SO3 2-ions from electrolyte [23,29,30]: The b values were derived from the cathodic and anodic peaks for CV curves of the Fe3O4/CNT composite using log(i) vs. log(v) plots ( Figure S4a).  The b values were derived from the cathodic and anodic peaks for CV curves of the Fe 3 O 4 /CNT composite using log(i) vs. log(v) plots ( Figure S4a).
where a and b are constants determined by the intercept and slope of the log(i)-log(v) plots, respectively. The b-value of 0.5 indicates a diffusion-controlled behaviour, whereas the value of 1 indicates a surface pseudocapacitance-dominated behaviour. The b-values of the cathodic peak, anodic peak 1, and anodic peak 2 for Fe 3 O 4 /CNT composite are 0.977, 0.975, and 0.833, respectively, implying a pseudocapacitive-controlled behaviour ( Figure S4b). Fe 3 O 4 /CNT composite curves retain a similar shape with distinct and reversible redox peaks as the scan rate increased from 5 to 300 mV/s. The specific capacitance of Fe 3 O 4 /CNT composite was also calculated from the GCD curves (Figure 4c) as 187.1, 163.3, 145.9, 132.6, and 115.4 F/g at current densities of 1, 2, 3, 5, and 10 A/g, respectively. The calculated specific capacitances of Fe 3 O 4 /CNT composite are higher than that of pristine CNTs (~50 F/g) [30]. Our synthesized Fe 3 O 4 /CNT composite has the superior specific capacitance compared to those measured in a sodium-based electrolyte such as Na 2 SO 3 and Na 2 SO 4 reported in previous studies [24,27,30,31]. This result was due to the uniform Fe 3 O 4 nanoparticles grown on CNTs that effectively improved the capacitive behaviour of CNT by adding a pseudocapacitive reaction. The GCD curves of Fe 3 O 4 /CNT still remained symmetric and exhibited high specific capacitance even at the high current density of 10 A/g, suggesting good rate capability with superior reversibility. These results are consistent with those from CV curves. High rate capability results from the highly electronic conductive CNT matrix, as it provides a practical pathway for fast ion/electron transport.
Besides the rate performance, the cycle stabilities of the Fe 3 O 4 /CNT composite electrode are evaluated at a current density of 1 A/g (Figure 4d). The specific capacitance of Fe 3 O 4 /CNT composite was measured as 150 F/g after 1000 cycles, showing high capacitance retention of 80.2%. The loss of specific capacitance that was approximately 20% might be due to the inevitable pulverization arising from the volume expansion of Fe 3 O 4 particles. However, this implies that the conductive CNT matrix and the strong connection between CNT and Fe 3 O 4 nanoparticles could reduce the drastic volume expansion during the charge and discharging process, which is beneficial for good cycle stability. The excellent capacitor performance of the Fe 3 O 4 /CNT composite can be due to the synergistic effect between the high pseudocapacitive Fe 3 O 4 and the superior electronic conductive carbon matrix.

Conclusions
In summary, we synthesized a nano-Fe 3 O 4 /CNT structured electrode consisting of uniform Fe 3 O 4 particles' growth on the CNT matrix to enhance the specific capacitance, rate capability, and cycle stability of Fe 3 O 4 for SCs. Superior electrochemical performances of Fe 3 O 4 /CNT composite are attributed to the combined contribution from Fe 3 O 4 nanoparticle and CNT matrix: (i) Fe 3 O 4 nanoparticles showed a pseudocapacitive behaviour originating from the redox reactions with SO 3 2− ions and iron oxides. (ii) Excellent electroconductive CNT provided fast ion/electron transport and robust electrode structure with the double layer capacitive behaviour.