Nitrogen-Doped Carbon Boosting Fe 2 O 3 Anode Performance for Long-Life Supercapacitors

Here, we report Fe 2 O 3 /N-doped carbon (Fe 2 O 3 /CN) composites via one-step facile calcination process by using FeOOH and PANI as precursor. The results show that N-doped carbon is helpful to enhance the electrochemical properties of Fe 2 O 3 . N-doped carbon not only enhances the conductivity of Fe 2 O 3 electrode, but also alleviates the volume expansion of Fe 2 O 3 in the process of repeated charge and discharge. In addition, the synergistic effect of Fe 2 O 3 and N-doped porous carbon makes the composites show higher capacitive properties (538.7 mF/cm 2 at 5 mA/cm 2 ) and cycle life (100% retention after 2000 cycles). In addition, its superior electrochemical performance is also proved in symmetrical supercapacitor. After 4900 cycles with current density of 10 mA/cm 2 , its capacity retention rate is 100%.


Introduction
Compared with other energy storage devices (such as Lithium-ion battery, Sodium ion battery, Zinc ion battery e.g.), chemical capacitors, also known as supercapacitors, have the advantages of high-power density, fast charge and discharge speed and long cycle life [1,2,3,4]. However, the relatively low energy density of supercapacitors still hinders their large-scale practical application [5,6,7]. It is very necessary to develop advanced supercapacitors with the comprehensive advantages of high energy density [8, 9,10]. According to the energy density formula the energy density is proportional to the speci c capacitance and voltage of the supercapacitor. The speci c capacitance of supercapacitor is closely related to the performance of electrode material [11,12,13]. Compared with the reported positive materials, the speci c capacitance of negative materials is often unsatisfactory. In order to construct high performance supercapacitors, it is urgent to develop negative materials with high capacity and high cycle stability. [14].
Among the negative electrode materials, Fe 2 O 3 has been widely studied because of its low price, high theoretical capacity, natural abundance and non-toxicity [15,16,17,18,19,20,21]. Nevertheless, there are still some problems in the process of constant current charge and discharge, such as poor conductivity and large volume change, resulting in serious electrode pulverization, particle cracking and capacity loss [22,23]. In order to overcome these problems, various strategies have been developed, such as surface coating with conductive materials and constructing nanostructured materials with different morphologies [24,25,26,27,28]. Among these methods, the design of Fe 2 O 3 /carbon composites with nano structure shows great application prospects in commercial applications because of their economical preparation methods [29]. Speci cally, as a unique protective coating, carbon has been widely used in electrode materials because of its excellent electronic conductivity, chemical stability and large surface area [30,31]. The research shows that Fe 2 O 3 /carbon composites not only improve the conductivity of Fe 2 O 3 , but also enhance the electrochemical properties of Fe 2 O 3 . For example, carbon coated Fe 2 O 3 nanorods on carbon ber have recently been reported and exhibit a speci c capacity of 384.3 mAh/g at a current density of 2 A/g [32]. In order to further improve the high-rate performance and long cycle stability of Fe 2 O 3 /carbon composites, the modi cation of carbon materials is also a feasible and effective strategy.
Doping of some heteroatoms (nitrogen (N), sulfur (S), phosphorus (P) and boron (B) can introduce more defects, disordered structures and more active sites, so as to improve the electrochemical performance [33,34]. Although some studies on the application of Fe 2 O 3 /heteroatom doped carbon composites in supercapacitors have been reported, the electrochemical properties are not satisfactory.
Here, Fe 2 O 3 /N-doped carbon composites (Fe 2 O 3 /CN) are successfully prepared by one-step calcination process with FeOOH and polyaniline (PANI) as precursors. N-doped carbon improves the conductivity of the whole composite, so as to improve the cycle stability. The N-doped carbon layer provides a protective shell to adapt to the volume change of Fe 2 O 3 during charge and discharge, which leads to the improvement of the stability of the whole electrode. The results show that Fe 2 O 3 /CN has better electrochemical properties than pure Fe 2 O 3 . Fe 2 O 3 /CN has excellent rate performance (e.g., 538.75 mF/cm 2 at 5 mA/cm 2 and 360 mF/cm 2 at 20 mA/cm 2 ) and excellent cycle performance (capacity retention rate of 100% after 2000 cycles at 25 mA/cm 2 ). In addition, we also construct a symmetrical supercapacitor with a capacity retention rate of 100% after 4900 cycles at 10 mA/cm 2 . These promising results show that Fe 2 O 3 /CN can be used as ideal electrode materials for high magni cation and long-life supercapacitor.

Preparation of Fe 2 O 3 /N-doped carbon (Fe 2 O 3 /CN).
The Fe 2 O 3 /CN electrode was manufactured by the following steps [35,36]. Firstly, FeOOH was grown directly on carbon cloth substrate by hydrothermal method, and polyaniline was grown on FeOOH in situ.
Speci cally, 0.54 g of ferric chloride hexahydrate (FeCl 3 · 6H 2 O) and 0.284 g of sodium sulfate (Na 2 SO 4 ) were dissolved in 40 ml of deionized water under magnetic stirring. The prepared aqueous solution was transferred to a 100 mL hydrothermal autoclave, in which four hydrophilic treated carbon cloths (1cm× 2cm) were xed. Then, the autoclave was heated to 120°C and maintained for 7 h. After cooling to room temperature, the obtained carbon clothes were washed with deionized water to remove impurities.
Secondly, soak the dried carbon cloth in 0.01 mol/L aniline solution (0.1 mol/L phytic acid), and then add 0.6 mol/L ammonium persulfate solution to initiate polymerization. Subsequently, transferring the carbon cloth to a refrigerator at 3 ℃ for 8 h. After the reaction, the obtained product was washed three times with ethanol and deionized water and dried overnight at 60°C. Finally, the obtained carbon cloth was annealed in nitrogen at 400°C for 2 h.

Characterization
The morphology and structure of the products were examined by scanning electron microscopy (SEM, Hitachi s-4800, accelerating voltage of 5 kV, Japan). The phase composition of the sample was determined by X-ray diffraction (XRD) at 2 θ = Identi cation shall be carried out in the range of 10-70°.
The electronic states of the samples were studied by X-ray photoelectron spectroscopy (XPS, Phi 5000 versa probe). Its electrochemical properties were studied on ChI 660E electrochemical workstation (Chenhua ChI 660E). Fe 2 O 3 and Fe 2 O 3 /CN are directly used as working electrodes on carbon ber cloth.

Electrochemical measurements
The electrochemical test was evaluated by a standard three electrode system in 2.0 mol/L KOH aqueous solution. Platinum foil electrode is used as counter electrode. The Hg/HgO electrode was used as the reference electrode. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry (GCD) measurements were performed on the ChI660E electrochemical workstation to determine the electrochemical performance. The EIS is performed in the frequency range of 0.01 Hz and 100 kHz. The speci c capacitance (C, mF/cm 2 ), energy density (E, mWh/cm 2 ) and power density (P, mW/cm 2 ) are calculated by the following equation, Where I is the constant current during the GCD discharge process, S is the area of active 3. Results And Discussion Fe 2 O 3 /CN was successfully synthesized on carbon cloth by hydrothermal, in-situ growth and calcination, and the synthesis process is shown in Scheme 1. In short, macro woven carbon cloth with good exibility and conductivity is used as the support of composite structure. Precursors are formed on the surface of carbon cloth by hydrothermal and in-situ synthesis. Fe 2 O 3 /CN composites are synthesized by one-step calcination, including loaded Fe 2 O 3 nanorod arrays and N-doped carbon coatings to ensure effective electrolyte penetration and further inhibit the pulverization of active substances.
The morphologies of Fe 2 O 3 and Fe 2 O 3 /CN are rstly examined by SEM. As shown in Fig. 1a and c, Fe 2 O 3 nanorods are uniformly and densely covered with carbon cloth, with a diameter of 100 nm and clear twodimensional characteristics (Fig. 1c). SEM images (Fig. 1b, d) show that the N-doped carbon layer has many pores, and the N-doped carbon is attached to Fe 2 O 3 nanorods. The SEM image of magni cation shows that N-doped carbon is stacked by many nano rods with a diameter of 50-100 nm, which is the reason for the porosity of N-doped carbon layer (Fig. 1d). In addition, the presence of N-doped carbon alleviates the mechanical stress caused by the volume expansion of Fe 2 O 3 , and reduces the powder damage during Fe 2 O 3 charge and discharge.
The crystal structure of Fe 2 O 3 and its composites are con rmed by XRD, and XRD pattern of carbon cloth,  [37]. And no impurity peak is observed in the whole pattern, indicating the high purity of the nal product. It is worth noting that the peak of Fe 2 O 3 /CN becomes stronger and sharper, indicating a slight increase in crystallinity. Fig. 3b shows Raman spectra to characterize the structure and surface chemistry of Fe-O, respectively [41]. The illustration in Fig. 3c shows the high-resolution spectrum of the Fe 2p region, indicating that Fe 2p is composed of Fe 2p 3/2 and Fe 2p 1/2 , corresponding to the binding energies of 711.2 and 724.8 eV, respectively [42]. Between the peaks of Fe 2p 3/2 and Fe 2p 1/2 , the satellite peak of 718.8 eV can explain the pure trivalent property. The result corresponds to the crystal structure of Fe 2 O 3 in XRD. In the C 1s spectrum, the three peaks can be attributed to C-N (288.8 eV), C-O (285.9 eV) and C-C (284.7 eV), respectively, which proves that N atoms are successfully doped in carbon materials (Fig. 3d) [43]. In addition, the N 1s spectrum can be tted into two peaks (Fig. S1). The rst peak at 398.5 eV corresponds to pyridine-N [44]. The second peak at 400.3 eV belongs to graphite-N. Pyridine-N provides redox active sites for additional pseudo capacitance, while graphite nitrogen reduces the charge transfer resistance due to the enhancement of conductivity. Therefore, Fe 2 O 3 /CN is successfully prepared, which is consistent with the results of Raman spectrum XRD and SEM.
The prepared electrodes used a three-electrode system to evaluate their electrochemical properties in 2 mol/L KOH electrolyte. The electrochemical properties of Fe 2 O 3 and Fe 2 O 3 /CN electrodes are studied by cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS). Fig. 4a [30,31,32]. This phenomenon is due to the increase of capacitance caused by the enhancement of the conductivity of the whole electrode material by N-doped carbon. Fig. 3b shows the GCD curves of Fe 2 O 3 and Fe 2 O 3 /CN electrodes at 15 mA/cm 2 current density. The longer the discharge time of Fe 2 O 3 /CN electrode, it shows that it has higher capacitance than Fe 2 O 3 electrode, which is consistent with CV results. This is attributed to the fact that the presence of N-doped carbon promotes the improvement of the capacitance of iron oxide [45]. The conductivity enhancement of  Figure S2 shows the cyclic stability of Fe 2 O 3 /CN and Fe 2 O 3 . In the initial cycle, the charge transfer resistance (R ct ) decreases due to the activation of the electrode, so the electrode capacity increases. It is worth noting that after 2000 cycles at a high current density of 25 mA/cm 2 , the capacitance retention is 100% ( Figure S2), which is higher than that of bare metal Fe 2 O 3 electrode. These results con rm the effectiveness of our strategy. The performance of Fe 2 O 3 electrode is greatly improved by surface Ndoping carbon.
In order to further evaluate the possibility of Fe 2 O 3 /CN electrode material in practical energy storage application, a symmetrical supercapacitor is assembled with Fe 2 O 3 /CN as cathode and anode, and tested in 2 mol/L KOH electrolyte. Fig. 5a shows the CV curves recorded by the Fe 2 O 3 /CN//Fe 2 O 3 /CN symmetrical supercapacitor device at different scanning rates of 10 to 200 mV/s in the 0-1.0 V voltage window. All CV curves are quasi rectangular and highly symmetrical, re ecting good reversibility and typical capacitance behavior. As shown in Fig. 5b, the symmetrical GCD curve with excellent coulomb e ciency and triangular shape further proves that the symmetrical supercapacitor device we assembled has excellent energy storage performance. The Fe 2 O 3 /CN//Fe 2 O 3 /CN symmetric supercapacitor devices show good cyclic performance. Fig. S4 shows that after 4900 cycles at a high current density of 10 mA/cm 2 , the capacitance retention is 100%. In addition, we further calculate the energy density and power density of the equipment, and the results are shown in Fig. 5d.
It is impressive that Fe 2 O 3 /CN//Fe 2 O 3 /CN symmetric supercapacitor devices can achieve an energy density of 0.011 mWh/cm 2 at a power density of 0.4 mW/cm 2 , and can still maintain an energy density of 4 mWh/cm 2 at a power density of 0.0062 mW/cm 2 . Although the device performance may be different due to electrode material preparation methods and electrochemical test conditions, under similar test conditions, our device performance is better than most reported supercapacitor devices.
Based on these results, Fe 2 O 3 /CN//Fe 2 O 3 /CN symmetric supercapacitor device shows excellent electrochemical behavior, which is attributed to the synergy between the components. As a powerful protective shell, N-doped carbon plays an extremely important role in the volume change of Fe 2 O 3 , which can prevent the corrosion and stress deformation of active materials. N-doped carbon can improve the conductivity of Fe 2 O 3 /CN electrode, effectively improve the stability of active material in long-term cycle and improve the capacitance of active material. In addition, the N-doped carbon loaded on Fe 2 O 3 array has a unique pore structure and retains the channel for electrolyte ions to quickly approach the electrode surface. Therefore, Fe 2 O 3 /CN//Fe 2 O 3 /CN symmetric supercapacitor devices have excellent capacitance characteristics and is an attractive candidate material in commercial energy storage equipment.

Conclusion
In short, Fe 2 O 3 /CN electrodes are fabricated using a simple one-step calcination route. Compared with a high current density of 25 mA/cm 2 ). In addition, the assembled Fe 2 O 3 /CN//Fe 2 O 3 /CN symmetric supercapacitor device provides a maximum energy density of 0.011 mWh/cm 2 at a power density of 0.4 mW/cm 2 , and can still maintain an energy density of 4 mWh/cm 2 at a power density of 0.0062 mW/cm 2 . In addition, it also has an excellent capacity retention of 100% of the original capacitance after 4900 cycles at a high current density of 10 mA/cm 2 . This study provides a feasible method for assembling

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