One-step microwave synthesis of FeSe2@CNT as high-performance supercapacitor anode material

Abstract In this work, FeSe2@CNT nanocomposite anode materials are fabricated using a convenient and efficient one-step microwave method. The FeSe2@CNT electrode exhibits superior electrochemical performance. The carbon nanotube structure effectively mitigates the severe volume change in the FeSe2@CNT electrode during continuous charge and discharge. Thus, the capacitance value of 573 F g−1 and capacitance retention of 81.9% for 3000 constant current charge and discharge cycles are achieved. In addition, the asymmetric device assembled with FeSe2@CNT as the anode and NiSe2@CNT as the cathode exhibits high Energy density (E can reach 41.32 Wh kg−1 when P is 800 W kg−1), and the device achieves 82.6% capacity retention after 3000 cycles.


Introduction
The gradual depletion of natural resources and the burning of fossil fuels have caused massive emissions of greenhouse gases, such as carbon dioxide. [1,2]9][10][11][12] The performance of supercapacitors depends mainly on their electrode materials. [13][16][17] The mismatch of asymmetric supercapacitors (ASCs) limits their practical application because of the large capacitance difference between the positive and negative electrodes.[20] Therefore, the further development of high-quality ultracapacitor anode materials is an urgent priority.
As anode materials for supercapacitors, Fe-based materials are very promising because of their potential for stable and extensive operation at negative potentials, the multiple valence states of Fe 0 , Fe 2þ , and Fe 3þ of iron make them functionally rich in redox chemical processes and low raw material cost. [21,22]Thus far, Fe 2 O 3 , [23] Fe 3 O 4 , [24] and FeS, [14] have been widely used as supercapacitor anode materials.In particular, selenides have large ionic radii, high electrical conductivity, and low electronegativity making their electrochemical performance superior to that of oxides and sulfides. [25,26]For instance, Liang et al. [27] prepared necklace-shaped FeSe 2 by electrostatic spinning and selenization treatment.Its capacitance decayed rapidly through 100 cycles at 2 A g À1 .Pure compounds are usually subject to rapid capacity decay because of volume changes and structural fragmentation during charge and discharge. [28,29][32] Kang et al. [33] prepared the carbon-doped Fe 2 O 3 composites using a rapid singlestep combustion method.The electrochemistry gives a capacitance of 192.6 F g À1 , and the capacity can be sustained at 82% after 1000 cycles.Park et al. [34] synthesized Fe 3 O 4 nanoparticles via the microwave solvothermal method and grew them uniformly on CNT as supercapacitor electrodes.The synthesized Fe 3 O 4 /CNT composites have a reversible capacitance of 187.1 F g À1 (1 A g À1 ) with a stability of 80.2% after 1000 cycles.Therefore, the combination of CNT and FeSe 2 is expected to solve the long-term cycling durability of iron selenide.
In this study, FeSe 2 @CNT nanocomposite anode materials are rapidly prepared using the microwave method.The electrode obtains excellent transmission dynamics characteristics with specific capacitance up to 573 F g À1 .The voids in the CNT structure can accommodate the accompanying volume changes, allowing the electrode sheet to remain at 81.9% after 3000 cycles.In addition, the ASC device is constructed using NiSe 2 @CNT successfully prepared in the laboratory as the positive electrode and FeSe 2 @CNT prepared in this work as the negative electrode.The constructed ASC device has high energy density (41.32 Wh kg À1 ) and high power density (8000 W kg À1 ).Moreover, the device demonstrates excellent long cycle life (82.6%after 3000 cycles) with good application prospects in practical applications.NiSe 2 @CNT was obtained from the laboratory. [35]2.Synthesis of FeSe 2 @CNT The synthesis of FeSe 2 @CNT was proceeded by a one-step microwave process.First, 40 mg of FeCl 2 ·4H 2 O and 20 mg of Se were ground homogeneously.The powder was transferred to a glass vial mixed with a mixture of C 2 H 8 N 2 (1600 mL) and C 2 H 6 O 2 (800 mL).Then, the CNTs were added and sonicated until well dispersed.The homogeneous mixture was transferred to a crucible (10 mL) with copper oxide and reacted in a Panasonic microwave oven.Finally, the black powder was acquired in microwave irradiation at 800 W 120 s. Figure 1 shows the basic flow of the preparation of FeSe 2 @CNT nanoparticles.The samples were prepared at different content, microwave time and power conditions, as shown in Table 1.

Material characterization
X-ray powder diffraction (XRD; Ultima IV; 2h range of 10 À80 ) and X-ray photoelectron spectroscopy (XPS; Thermo K-Alpha) were utilized to analyze the crystal structure and chemical element categories of the electrode materials.Scanning electron microscopy (SEM; Sigma 300) and energy-dispersive X-ray spectroscopy (EDS) were selected to observe the microscopic morphology of the materials and the compositional analysis of the material microregions.

Electrochemical characterization
First, the mass ratio was 8:1:1, the composite (FeSe 2 @CNT), acetylene black and polyvinylidene fluoride were ground to form a homogeneous slurry.Then, this slurry was coated on 1 Â 1 cm 2 of nickel foam.Finally, the active substance-loaded nickel foam was placed at 100 C for 10 hrs and then extruded (10 MPa) with blank nickel foam on a press.In the case of the three-electrode system, 6 M KOH was used as an electrolyte, and Hg/HgO and Pt sheets were the reference and counter electrodes, correspondingly.The energy storage properties of the prepared materials were further evaluated using a series of measurement techniques on an electrochemical workstation (CHI660E) at Shanghai C&H Instruments Co.The calculated specific capacitance was acquired from the following equation [36] : where Dt (s) and I (A) refer to the time and current of discharge, respectively, and m (mg) denotes the weight of the activated substance.
For two-electrode systems, the anode is FeSe 2 @CNT and the cathode is NiSe 2 @CNT.The loading mass of the electrode is obtained from the formula below [37] : where m þ and m À indicate load weights of positive and negative electrode sheet, DV stands for the voltage window.The calculation of E and P for ASCs is calculated by Equations ( 3) and (4) [38,39] : Figure 1.The basic process for preparing FeSe 2 @CNT.
The essential composition and chemical valence analysis of FeSe 2 @CNT was evaluated using XPS.In Figure 3(a), Fe, Se, O, and C elements are detected in the FeSe 2 @CNT survey spectrum.The O element possibly originates from the oxygen-containing functional group of CNT. [40]As Figure 3(b) shows, the C1s spectrum has two obvious peaks at 284.66 and 285.8 eV, which are ascribed to C-C and C-O, respectively. [41]The peaks that appear at 706.7 and 719.6 eV are labeled as Fe 2p 3/2 and Fe 2p 1/2 , with the characteristic peak one of FeSe 2 , indicated in the spectrum of Figure 3(c)  (Fe 2p). [42]The two primary peaks at 54.5 and 55.6 eV binding energies in Figure 3(d) (Se 3d) correlate to Se 3d 5/2 and Se 3d 3/2 . [43]The XPS characterization results of FeSe 2 @CNT prove the presence of these elements.This result agrees with the XRD detection data.The XRD pattern of Figure S1(a) shows the successful synthesis of FeSe 2 .Furthermore, XPS detection spectra are shown in Figure S1(b-d).
The SEM images of CNT and FeSe 2 are presented in Figure 4(a,b), respectively.Figure 4(c-h) present the SEM images of FeSe 2 @CNT under different conditions.As Figure 4(c) shows, the FeSe 2 particles are uniformly loaded on CNT structural voids.The CNT content affects the microscopic morphology of the material.The introduction of a relatively high amount of CNT is not favorable for the formation of FeSe 2 .However, the FeSe 2 nanoparticles could not be uniformly bound to the CNT structure because of the introduction of a relatively low amount of CNT. Figure 4(f-i) are SEM images under different microwave radiation conditions.At low microwave power (600 W) or short time (90 s), fewer FeSe 2 particles were produced due to insufficient microwave energy.However, the excess energy made the products pile up together and form agglomerates when the microwave power is increased (1000 W) or the time is extended (150 s).The EDS diagram of sample C3 indicates the occurrence of C, Fe and Se elements.As seen in Figure 4(j), the atomic ratio of Fe and Se is 2.7:4.7,which is approximately 1:2, consistent with the ratio of FeSe 2 @CNT.These results further confirm the successful preparation of FeSe 2 @CNT nanocomposites.In addition, the EDS of FeSe 2 (Figure S2(c)) shows an atomic ratio of Fe:Se (30.4:69.6)close to 1:2, suggesting the successful synthesis of the nanosheets.

Effect of differential experimental conditions
The FeSe 2 @CNT samples were synthesized by varying the carbon content and microwave conditions.The specific capacitance values were compared (Figure 5).As it is shown in Figure 5(a), the comparison of the specific capacitance values at different current densities demonstrates that the specific capacitance values of FeSe 2 @CNT nanocomposites are much higher than those of FeSe 2 nanosheets and pure CNTs. Figure 5(b) compares the specific capacitance values for different carbon contents at the same microwave time (120 s) and power (800 W).The capacitance value increases first and decreases later with increasing carbon content, suggesting that the proper incorporation of CNTs can strengthen the electrical conductivity of the composite and reduce the gathering of FeSe 2 particulates.However, the excessive introduction of CNTs (9 mg) leads to a decrease in specific capacitance due to the precursors cannot fully react.Figure 5(c) gives the ratio capacity of the samples at the same carbon content (6 mg) concerning different microwave irradiation.At low microwave reaction power (600 W) or short time (90 s) conditions, insufficient microwave energy prevents the complete reaction of raw materials and low specific capacitance.However, at high microwave power (1000 W) or excessively long time (150 s) conditions, the composite material accumulates clumps because the microwave energy is too high.Therefore, the experimental results show that the best reaction conditions for the prepared FeSe 2 @CNT nanocomposites are CNT mass of 6 mg, microwave power of 800 W, microwave time of 120 s, and their specific capacitance can reach 573 F g À1 .

Electrochemical properties of FeSe 2 @CNT
Under three-electrode electrochemistry measurements, all samples were measured in a 6 M KOH solution.Figure 6(a) illustrates the typical CV curves obtained for pure CNTs, FeSe 2 , and FeSe 2 @CNT at 100 mV s À1 .FeSe 2 @CNT has the greatest confinement area, suggesting that it offers the largest capacitance value.The GCD curves of the three samples at 1 A g À1 are shown in Figure 6(d), with specific capacitances of 40, 237, and 573 F g À1 , respectively.The results are consistent with the CV curves.
The GCD curves for different conditions shown in Figure 6(d-f) also indicate that sample C3 has excellent electrochemical properties.In addition, the area of the CV closure curve of sample C3 remains the largest among all other samples, which implies that the substance loses a significant amount of charge during energy storage.To clarify the sample Cs, the GCD curves in the scope of À1 to 0 V are given in Figure 6(e-f).Sample C3 has the highest capacitance value because of the uniform growth of nanoparticles on the CNT. Figure 6(g) gives the CV curves of sample C3 in the operating voltage window of À1.2 to 0 V at 5-100 mV s ̶ 1 .For the CV  curves, the oxidation peak (from À0.6 to À0.38 V) shifts toward high voltage, whereas the reduction peak (from À1.0 to À1.2 V) shifts toward low voltage, as a result of the internal diffusion resistance of the material, and their shapes do not change significantly with increasing scan rate.Figure 6(h) describes the GCD curves of sample C3 with Cs of 573, 500, 476, 444, and 403 F g À1 .Figure 6(i) is the plot of the multiplicative performance of sample C3 at different current densities.The capacitance value of this sample is still 521 F g À1 at 1 A g À1 after multiple charges/discharges, displaying a favorable electrochemical performance.Table 2 provides data on the electrochemical properties of FeSe 2 @CNT in comparison with other materials.
As shown in Figure 7(a), Nyquist plots from the 1 kHz to 100 kHz range are used to study the kinetic and resistive behavior of charge transport occurring in strong alkaline solutions of electrode materials.The diameter of the semicircle indicates Rct (charge transfer resistance) and the distance to the X-axis represents Rs (internal resistance of the solution).The verticality of the slope expresses the degree of rapidity in the diffusion of electrolyte ions from the electrolyte to the electrode surface.The higher the slope is, the faster the diffusion rate is, and vice versa.Sample C3 (6 mg, 800 W, 120 s) has the highest slope, the lowest resistance (0.86 X), good conductivity and fast charge transfer behavior.The obtained results in Figure 7(b) reveal that the FeSe 2 @CNT electrode has good cycling performance.Under 10 A g À1 through 3000 cycles, the FeSe 2 @CNT electrode material maintains 81.9% of its preliminary capability, while the FeSe 2 capacitance rapidly decreases to 43.9% of its original capacity after 500 cycles.The comparison of Nyquist plots before and after 3000 cycles of the three-electrode cycle (Figure 7(c)) shows that the slope of the curve and the internal resistance decrease after cycling due to structural collapse.
The calculation of the CV fit to sample c to determine the b-value investigated the charge storage kinetics.The b-value was calculated from the following [51] : where b represents a linear change in a straight line.If the value of b is 1, then it symbolizes a bilayer process controlling charge storage; if b is 0.5, then the pseudocapacitance process controls charge storage.Figure 7(d) shows the value of 0.73055 in the case of the anodic oxide peak of the FeSe 2 @CNT, whereas the case of the cathodic reduction peak has a value of 0.71497.This result points out that diffusion control and capacitance control work together.In  addition, the percentage contribution of the charge storage at either sweep speed can be determined by the following equation [52] : where k 1 v denotes the surface control capacitance, and k 2 v 1=2 is the diffusion control capacitance.From Figure 7(e), it can be observed that the surface control capacitance occupies 62.0% of the total capacitance (50 mV s À1 ).At either scan rate (Figure 7(f)), the ion transport shortens as the rate increases.This phenomenon causes a decrease in the percentage of diffusion-controlled capacitance and an increase in the percentage of surface-controlled capacitance.

Conclusion
In summary, FeSe 2 @CNT nanocomposites were synthesized by the microwave method successfully.Compared with pure FeSe 2 , the presence of CNT effectively improves the volume change during FeSe 2 ion exchange and increases the redoxactive site, leading to a significant improvement in capacitance and cycling performance.The prepared FeSe 2 @CNT composites exhibit remarkable electrochemical reversibility and can realize a capacitance value of 573 F g À1 (1 A g À1 ) with a retention rate of 81.9% after 3000 consecutive cycles.The asymmetric NiSe 2 @CNT//FeSe 2 @CNT supercapacitors have a wide operating voltage of 1.6 V and a maximum power density of 41.32 Wh kg À1 and a maximum energy density of 8000 W Kg À1 .The loss is only 17.4% after 3000 GCD cycles.The green and convenient synthesis method and excellent material performance suggest that FeSe 2 @CNT is a promising anode material.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 6 (
Figure 6(b,c) have shown the CV curves of FeSe 2 @CNT samples under different microwave reactions with voltages ranging from À1.2 to 0 V.The possible reaction equations for the CV curves with distinct redox peaks at different sweep rates are as follows:

Figure 7 .
Figure 7. (a) Nyquist plots of FeSe 2 @CNT composites under different conditions; (b) cycling stability of FeSe 2 and FeSe 2 @CNT; (c) Nyquist plots before and after cycling; (d) charge storage kinetic plot of the best sample; (e) plot of the capacitance contribution for FeSe 2 @CNT (6 mg,800 W,120 s) at 50 mV s À1 ; (f) percentage contribution of the control process to the total capacitance at 5-100 mV s À1 .

Advanced
Materials and Chemical Engineering under [Grant 2021SX-AT006]; China-Belarus Joint Laboratory for the Belt and Road Initiative under [Grant ZBKF2022031101]; and Key Research Program of Lvliang City under [Grant 2021GXYF-1-21].

Table 1 .
FeSe 2 @CNT obtained at different CNT content, microwave power and time.
C: different CNT content; T: different microwave reaction time; P: different microwave reaction power.

Table 2 .
The electrochemical properties of FeSe 2 @CNT are compared with other materials.