Porous Fe2O3 nanospheres anchored on activated carbon cloth for high-performance symmetric supercapacitors

Author’s Accepted Manuscript Porous Fe2O3 Nanospheres Anchored on Activated Carbon Cloth for High-Performance Symmetric Supercapacitors Jien Li, Yanwei Wang, Weina Xu, Yu Wang, Bin Zhang, Shuang Luo, Xiaoyuan Zhou, Cuilin Zhang, Xiao Gu, Chenguo Hu www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(18)30974-1 https://doi.org/10.1016/j.nanoen.2018.12.061 NANOEN3309

To appear in: Nano Energy Received date: 22 November 2018 Revised date: 19 December 2018 Accepted date: 19 December 2018 Cite this article as: Jien Li, Yanwei Wang, Weina Xu, Yu Wang, Bin Zhang, Shuang Luo, Xiaoyuan Zhou, Cuilin Zhang, Xiao Gu and Chenguo Hu, Porous Fe2O3 Nanospheres Anchored on Activated Carbon Cloth for High-Performance Symmetric Supercapacitors, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.12.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Porous Fe2O3 Nanospheres Anchored on Activated Carbon Cloth for High-Performance Symmetric Supercapacitors Jien Li1, Yanwei Wang1, Weina Xu1, Yu Wang1, Bin Zhang2, Shuang Luo1, Xiaoyuan Zhou1,2, Cuilin Zhang1, Xiao Gu1,3,*, Chenguo Hu1,* 1

Department of Applied Physics, State Key Laboratory of Power Transmission

Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, P.R. China 2

Analytical and Testing Center of Chongqing University, Chongqing 400044, P.R.

China 3

Faculty of Science, Ningbo University, Ningbo 315211, P.R. China

* E-mail: [email protected] (C Hu), [email protected] (X Gu)

1

Abstract Metal

oxides

with

nanostructures

are

promising

electrode

materials

for

supercapacitors, however, their cycling stability and rate performance still need to be improved for practical applications. To promote a device performance, it is important to develop an advanced electrode material as well as to design superior electrode architecture. Herein, porous Fe2O3 nanospheres anchored on activated carbon cloth (Fe2O3@ACC) is prepared as an excellent electrode material, which exhibits a large area specific capacitance up to 2775 mF cm-2 in 3M LiNO3 between -0.8 and 0 V versus SCE. The symmetric structured supercapacitor assembled by two pieces of Fe2O3@ACC electrodes achieves 1.8 V operating voltage in 3M LiNO3 aqueous electrolyte and area specific capacitance of 1565 mF cm-2 with excellent cycling stability, with 95% of initial capacitance retained after 4000 cycles, indicating fast ion-involved redox reactions on its surfaces. The Fe2O3@ACC symmetric supercapacitor can deliver an energy density of 9.2 mWh cm-3 at a power density of 12 mW cm-3 and an energy density of 4.5 mWh cm-3 at a power density of 204 mW cm-3. Li ion adsorption and diffusion mechanism on the (100) and (110) facets of Fe2O3 are explained by the calculations of density functional theory. The facile synthesis method and superior performance of the Fe2O3@ACC composite make it promising as an ideal electrode material for high-performance symmetric supercapacitors.

2

Graphical abstract

Porous Fe2O3 nanospheres anchored on activated carbon cloth is prepared and used as high performance electrode for SC. The Fe2O3@ACC electrode have a large specific capacitance, which is much larger than the reported values for the Fe2O3-based electrodes in neutral aqueous electrolyte. We find for the first time that the Fe2O3@ACC has even higher specific capacitance in positive potential window than that in negative potential window at scan rate larger than 30 mV s-1. The low diffusion potential barrier for Li+ ions on Fe2O3 contributes excellent pseudocapacitive performance.

Keywords: porous; nanospheres; Fe2O3; supercapacitor. 1. Introduction Supercapacitors (SC) as an excellent energy storage device, have attracted quite 3

a lot attention for multifarious applications because of their high power density, excellent cycling stability, fast charge and discharge rates and low maintenance cost [1–5]. SC could be more practical when applying to future high performance electronic devices, where higher energy density, higher power density and better cycle life are needed. To increase the specific energy, either the specific capacitance (C) or the output voltage (V) should be increased, according to the energy density equation (E = 1/2CV2) [6,7]. Organic electrolytes and ionic liquids are often used for higher operating voltage due to their sustention of high voltage without dissociation. However, they are also confronted with such challenges as to increase ionic conductivity and cycle life. Those problems impede them in future applications [8]. Therefore, an alternative strategy is to construct supercapacitors using aqueous electrolytes, which are environmentally friendly and could provide fast ion transportation. Transition metal oxides, such as MoO3, V2O5, Co3O4, RuO2, Bi2O3, MnO2 and FeOx as the electrode materials, have been widely explored to achieve large specific capacitance [9-18]. Recently, Fe2O3, as asymmetric supercapacitors anode material, has been reported in several works [19-21]. However, the surface area of the reported Fe2O3 was relatively small, which limits the specific capacitance, and its electronic conductivity was also poor, ending in low power density. To untangle these obstacles, carbon nanotube, graphene and carbon cloth are used as conductive matrices to support Fe2O3 nanoparticles for superior performance [22-24]. Particularly, the 4

activated carbon cloth (ACC), which possesses excellent electronic conductivity and large surface area, has become the most appreciated matrices for loading Fe2O3 and its flexibility is desirable for wearable electronic devices [25-27]. The dispersion of Fe2O3 nanoparticles on ACC matrices is essential, which determines the electrode performance. So far, most Fe2O3/ACC composites have relatively small specific capacitance due to the poor dispersion. Another obstacle is that most of the synthetic routes have very complicated processes for prepare the Fe2O3/ACC composites with excellent properties, limiting it from large scale production. It is desirable to compose porous nanoparticles with conductive matrices, so that the composites can possess both high surface area and good conductivity [6]. In this work, we designed a facile, one-step hydrothermal method to synthesize porous Fe2O3 nanospheres anchored on the activated carbon cloth (Fe2O3@ACC) at a relatively low temperature of 190℃. Porous Fe2O3 nanospheres distributed on the ACC without aggregation by the control of the concentration of a reactant, and a large areal capacitance (Cs) of 2775 mF cm-2 is obtained in 3M LiNO3 between -0.8 and 0 V versus SCE. As far as we know, this materials reach the highest area specific capacitance value compared with all the Fe2O3-based materials using aqueous electrolytes reported previously [18-20,28-35]. Moreover, we assembled a symmetric supercapacitor device by coupling two pieces of Fe2O3@ACC electrodes and 1.8 V high-voltage in aqueous electrolyte is achieved [36]. This symmetric structure design of supercapacitor is new, for Fe2O3 in asymmetric supercapacitor is commonly used as anode material. As expected, this symmetric Fe2O3@ACC supercapacitor exhibits large energy density with outstanding 5

cycling stability and power capability, indicating fast ion-involved redox reactions on its surfaces. Li ion adsorption and diffusion mechanism on the (100) and (110) facets of Fe2O3 are explained by the calculations of density functional theory. 2. Results and Discussion The prepared Fe2O3@ACC electrode and the structure of symmetric supercapacitor are shown in Scheme 1. A simple hydrothermal method was employed to prepare porous Fe2O3 nanospheres directly grown on the activated carbon cloth as an electrode. Due to the existence of oxidation functional groups on the surface of activated carbon cloth, some covalent chemical bonds were formed on the surface of activated carbon cloth (Supporting Information Figure S1) which is why iron oxide nanospheres can anchor firmly on activated carbon cloth [6], by which the transmission distance of electrolyte ions from the electrode material to the current collector is greatly shortened and conductivity of the electrode is greatly enhanced [37]. This method can also be applied to other transition metal-oxides/ACC composites. The crystallographic structure and phase purity of Fe2O3 nanospheres was analyzed by X-ray diffraction (XRD) pattern as shown in Fig. 1a. All the peaks correspond to the rhombohedral Fe2O3 (JCPDs no. 85-0987) without other characteristic peaks from impurities, suggesting its high purity. The crystalline structure of the as-prepared Fe2O3 is shown in Fig. 1b. The rhombohedral crystal structure of Fe2O3 nanospheres has lattice constants a=b=c=5.43 Å and α=β=γ=55.3°. 6

Energy dispersive X-ray spectroscopy (XPS) spectrum is used to analyze the composition and chemical bonding state of the Fe2O3@ACC. The full XPS spectrum of Fe2O3@ACC contains the signals of C, O and Fe as shown in Fig. 1c. Fe2O3 provides two Fe 2p peaks (711.1, 723.9 eV) and the activated carbon as the substrate layer provides C 1s peak at 285.1 eV. The individual high-resolution XPS spectrum are presented in the supplemental materials. As shown in Fig. S1a, the XPS peak at 711 eV and 724.6 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, and the Fe 2p also have two satellite peaks (716.4, 730.7 eV) are consistent with previous reports [28,29,38]. The peaks at 529.9, 531.3, 532.4 and 535.1 eV are denoted to the core levels of O 1s (Fig. S1b). The O2- of Fe2O3 nanospheres contributes the peak at 529.9 eV [28,39]. O=C-OH, C=O and C-OH on the surface of the carbon cloth contribute the other three peaks [40]. The peak of C 1s is broadened, suggesting the existence of other chemical states of activated CC (Fig. S1c) [26,40]. These extended peaks correspond to C-C, C-OH, C=O and C(O)O, respectively. As shown in Fig. 1d, we can further confirm the existence of iron and oxygen elements. This spectrum shows the characteristic O K- and Fe L2,3-edges, where the different edge details can be used as a fingerprint to determine valence for unknown samples [41,42]. Field emission scanning electron microscopy (FESEM) images reveal the rough surface of chemically treated (activated) carbon fibers, quite different from the smooth surface of untreated CC, suggesting the surface of the carbon fibers is modified (Fig. S2a and S2b). Moreover, from the energy dispersive X-ray 7

spectroscopy (EDS) spectra (Fig. S3) we can see that the oxygen element signal of the activated CC relative to that of the untreated CC is enhanced, as expected. The oxygen ratio [defined as NO/(NO+NC), where NO and NC represent the number of oxygen and carbon atoms] slightly increases from 2.6% to 8.4% after activation. The slightly introduction of oxygen on surface of the carbon cloth could positively affect its electrochemical performance. Fig. 2a and 2b show FESEM images of the Fe2O3 nanospheres on the activated carbon cloth with low and high magnifications, respectively. At low magnification, it can be seen that nanospheres are densely covered on the surface of the activated carbon cloth. At high magnification, it is clear that these particles are roughly spherical and compactly stacked on the carbon fibers. Fig. 2c shows the STEM image of the Fe2O3 nanospheres with size of 20-80 nm, and the porous microstructure of iron oxide nanospheres with pore size of about 2 nm (inset Fig. 2c). Fig. 2d shows the high resolution transmission electron microscopy (HRTEM) image of the Fe2O3 nanospheres, where clear lattice fringes of the (110) plane could be found with interplanar spacing of 0.37 nm. This is well consistent with its XRD pattern in Fig. 1a. EDS (Fig. 2f) shows more evidence on the homogeneous distribution of Fe and O element where the ratio of O:Fe is calculated to be 1.81, very close to the atomic ratio of Fe2O3 (Fig. S4). The nitrogen adsorption-desorption isotherm and the pore size distribution of the Fe2O3@ACC composites are shown in Fig. 3. Mesoporous evidence is shown in the 8

type-IV isotherms of the curve of Fe2O3@ACC composite (Fig. 3a). Fig. 3b shows the pore size distributions calculated by Barret-Joyner-Halenda (BJH) method. Fe2O3@ACC composite shows that the diameters of the pores are below 5 nm and mainly concentrated in 3-4 nm, agreeing with the STEM results. This could be mainly contributed by mesopores distributed on Fe2O3 nanospheres and the space between the Fe2O3 nanospheres. The specific surface areas are 99.1 m2 g−1 for the Fe2O3@ACC composite, and significantly, the surface area is greatly increased compared to the untreated CC (5.3 m2 g-1) [26]. The large surface area of the Fe2O3@ACC composite could speed up the Li ion diffusion to reach the surface of the active material, which may lead to improved capacitive performance for the composite electrodes. Large specific surface area and good conductivity mean that the composite electrode will exhibit excellent electrochemical performance for supercapacitors. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) are conducted in a three-electrode system in 3M LiNO3 electrolyte to investigate the electrochemical performance of Fe2O3@ACC electrode. Fig. 4a presents the charge/discharge curves of the Fe2O3@ACC synthesized at different reactant concentrations between -0.8 and 0 V at 8 mA cm-2, and the corresponding sum of Cs to different reactant concentrations is shown in Fig. S5a. The electrochemical performance of the electrodes reach the maximum at the reactant concentration of 1.75 mmol in the range from 0.75 mmol to 3.5 mmol. The 9

CV curves of Fe2O3@ACC electrode with different area (Fig. S5b) indicate that the relative area of the CV diagram hardly changes, but the inclination of the CV diagram increases due to the increase in internal resistance. The FESEM images (Fig. S6) uncover the cause that there is more uniform and denser distribution of Fe2O3 nanospheres on the activated carbon cloth at concentration of 1.75 mmol and there is no serious agglomeration phenomenon. The galvanostatic charge/discharge curves of the Fe2O3@ACC electrode (1.75 mmol) between -0.8 and 0 V at different current densities from 1 to 10 mA cm-2 (Fig. 4b) and 0.5 to 5 A g-1(Fig. S12a) are presented. The near symmetric GCD curves at different current densities during the charge-discharge processes demonstrate the ideal capacitive behavior of the Fe2O3@ACC electrode. The smaller iR drop suggests a rapid I-V response and minimal resistance effect for the Fe2O3@ACC electrode. The areal and specific capacitance of the Fe2O3@ACC electrode versus current density is shown in Fig. 4c. The areal and specific capacitance of the Fe2O3@ACC electrode in -0.8-0 V can reach 2775 mF cm-2 at a current density of 1 mA cm−2 (1500 F g-1 at 0.5 A g-1), which is much larger than previously reported values for the Fe2O3-based electrodes. Previously reported electrochemical performance of the Fe2O3-based electrodes in aqueous electrolytes are listed in Table1 [18-20,28-35]. Compared with those Fe2O3-based electrodes, the Fe2O3@ACC electrode in this work possesses larger specific capacitance and better rate capability due to the synergetic effect of high conductivity of the activated carbon cloth (ACC) and large capacitance contribution from porous Fe2O3 nanosphere. In addition, the Fe2O3@ACC electrode presents 10

excellent rate performance from -0.8 to 0 V and can still achieve a large areal and specific capacitance of 2155 mF cm-2 at a high current density of 10 mA cm−2 (1181 F g-1 at 5 A g-1). At a current density of 1 mA cm−2, the Fe2O3@ACC electrode can deliver an energy density of 0.25 mWh cm−2 between -0.8 and 0 V. When the current density increases to 10 mA cm−2, the Fe2O3@ACC electrode can still have an energy density of 0.19 mWh cm−2. Fig. 4d displays the CV curves of the Fe2O3@ACC, pristine Fe2O3 (P-Fe2O3) and ACC, inpotential of -0.80 V at a scan rate of 10 mV s-1. Obviously, the area of CV curves of Fe2O3@ACC is larger than that of the P-Fe2O3 and ACC, displaying a much larger capacitance. All these CV curves have quasi-rectangular shapes with no obvious redox peaks, indicating that the continuous surface Faradaic reactions play an essential role in the capacitance. The CV curves of Fe2O3@ACC, P-Fe2O3 and ACC at different scan rates (Fig. S8) and the summary plot of areal specific capacitance under different scan rates (Fig. S9a) indicate the far larger specific capacitance of Fe2O3@ACC than that of P-Fe2O3 and ACC, especially in small scan rate. The improvement of Cs is attributed to strong anchor of iron oxide on the carbon cloth, which enhances the electrical conductivity of the electrode. This can also be confirmed by the electrochemical impedance spectra (EIS). Fig. 4e shows the EIS curves of the Fe2O3@ACC, P-Fe2O3, ACC electrodes between 0.01 Hz and 100 kHz. In the high frequency region, the electrolyte contact resistance (Re) and the charge transfer resistance (Rct) are reflected by the intercepts of the Nyquist plots and the radius of the high frequency arc on the real axis, respectively [43]. The Rct of Fe2O3@ACC electrode is as small as that of the carbon cloth and which is much 11

smaller than that of the pristine Fe2O3 electrode, indicating faster charge transition [44]. At low frequency, much more vertical line indicates faster ion diffusion in Fe2O3@ACC [45]. Besides, the anchored iron oxide on the carbon cloth improves the stability (Fig. 4f), by which the Cs retention of Fe2O3@ACC (93%) is much higher than that of pristine Fe2O3 (74%). After 4000 cycles, the capacitance of the Fe2O3@ACC electrode could retain 92% even after 10000 cycles, only 0.0008% per cycle decay, indicating the excellent long-term cycling stability. From the Fig. S9c, we can see that the Fe2O3 electrode material still maintains a good crystal structure after long-term cycling. The diffraction peak intensity of the electrode material is somewhat weakened by the high intensity peak of carbon element. But it can be matched perfectly with rhombohedral Fe2O3 standard cards (JCPDs no. 85-0987). And the FESEM images show that only a few Fe2O3 nanospheres have fallen off from the carbon cloth after long-term cycling, but most of them are firmly anchored on the surface of the carbon cloth and continue to maintain stable nanosphere crystal structure (Fig. S9d,e,f). For better understanding the atomic-scale mechanism of the adsorption and diffusion of Li ions, DFT calculations are conducted. The adsorption energies, E(ads), of

lithium

ions

on

the

surface

are

calculated

as

Εads   ΕFe2O3  Li   ΕFe2O3   ΕLi , where E(Fe2O3) and E(Fe2O3+Li) are the total energy of Fe2O3 and the total energy of Fe2O3 with Li adsorbed on surface. Based on XRD pattern and TEM results, we calculated the surface properties of 12

rhombohedral Fe2O3 110 and 100 planes. The corresponding different views of Lithium ions adsorbed on the 100 and 110 planes of rhombohedral Fe2O3 are shown in the Fig. S10 and the calculation results of adsorption energy are shown in the Fig. S9b, from which we can see that adsorption energy is -2.1 eV on the 110 plane and -1.9 eV on the 100 plane, indicating Lithium ions can easily adsorb on the surface of rhombohedral Fe2O3 and result in the good surface capacitive properties. To study the ion diffusions on the surface of rhombohedral Fe2O3, the barriers in Li ion diffusion paths on 100 and 110 planes are calculated by the NEB method based on the top view in Fig. 5a,b. The results indicate that it is easier for lithium ions to move on the 110 plane as shown in Fig. 5c, which is consistent with the exposed facets of Fe2O3 nanospheres observed by TEM (Fig. 2d). The existence of hydroxyl functional groups also leads to the electronegativity of Fe2O3 surface, which will attract more cations onto the electrode surface and promote the formation of double layer capacitors. So, there are two kinds of fast response surface capacitance due to the oxidation functional groups on the electrode: (I) non-Faradaic contribution as a result of the double-layer capacitance (cations adsorption/desorption); (II) Surface Faradaic contribution originating from the chemisorbed redox groups [46]. The electrochemical properties of the Fe2O3@ACC electrode at the positive potential window is also investigated (Fig. 6a,b). Fig. 6a shows that the CV curve of the electrode is in ideal rectangular shape under the positive and negative potential windows at 10 mV s-1, but the area of CV is smaller in positive potential than that of 13

negative potential, which is the reason why Fe2O3 is commonly more suitable for anode material. However, the difference of areal capacitance in positive and negative potential windows reduces rapidly with the increase in scan rate and reverses at scan rate larger than 30 mV s-1 as shown in Fig. 6b. Therefore, the Fe2O3@ACC could also be used as cathode material at relatively higher scan rate. To demonstrate this, we carried out further research in the positive potential window by cyclic voltammetry (Fig. S11b), galvanostatic charge/discharge (Fig. S11c), and all the test results suggest that the Fe2O3@ACC electrode in positive potential window has excellent electrochemical performance. The areal capacitance of the Fe2O3@ACC electrode in 0-1 V can reach 874 mF cm-2 at 1 mA cm−2 and 595 mF cm-2 at 10 mA cm−2 (Fig. S11d), which is larger than most previously reported values for the Fe2O3-based electrodes

[18-20,28-33].

This

provides

the

possibility

of

constructing

high-performance symmetric supercapacitor based on the Fe2O3@ACC electrode. In addition, the CV curves of Fe2O3@ACC single electrode test retain an idea rectangular shape even at a wide potential window of -0.8-1 V (Fig. S11a), indicating an SC assembled by two piece of Fe2O3@ACC electrode could achieve 1.8 V cell voltage theoretically. Based on the above results, we assembled a symmetric supercapacitor in neutral aqueous electrolyte, which sustained an ultrahigh operating voltage window from 0 to1.8 V (Fig. 6c). 1.8 V is a relatively high voltage that one single symmetric SC could achieve in aqueous electrolyte. Fig. 6d and Fig. S12b show the GCD curves of our Fe2O3@ACC symmetric SC at current of 1-17 mA cm−2 and 0.5-9 A g-1, where linear voltage–time plots indicate good capacitive performance. 14

The cycling response is shown at continuously variable currents in Fig. 6e and the specific capacitances of the Fe2O3@ACC symmetry device versus different current density is shown in Fig. S12c. The SC device has capacitance of ≈1531, 1375, 1286, 1210, 990, 875 and 750 mF cm−2 at 1, 2, 4, 6, 9, 13 and 17 mA cm−2 (747, 700, 667, 613, 582, 544, 471 and 405 F g-1 at 0.5, 1, 2, 3, 4, 5, 7,and 9 A g-1), respectively. The capacitance restores as the current is reset from 17 mA cm−2 back to 1 mA cm−2, showing excellent stability. Fig. 6f shows its long-term cycling stability at 17 mA cm−2. The capacitance retains 95.0% after 4000 cycles. Coulombic efficiency has been maintained ≈99.0% in the whole process. The performance of the device in longer time cycling is presented in Fig. S13a. Fig. S13b compares the volumetric power density and energy density of the Fe2O3@ACC symmetric SC device to the values reported for other supercapacitors. The maximum energy density of the prepared SC is 9.2 mWh cm-3 at a power density of 12 mW cm-3, and it maintains 4.5 mWh cm-3 at a maximum power density of 204 mW cm-3. This value is considerably higher than those of previously reported Fe2O3-based electrode SC devices such as MnO2//Fe2O3 ASC device (0.35 mWh cm-3 at 100 mW cm-3) [18], Fe2O3-P//MnO2 device (0.42 mWh cm-3 at 10.3 mW cm-3) [19], ASV-FO//V-CO device (0.95 mWh cm-3 at 20 mW cm-3) [20], MnO2//Fe2O3/PPy device (0.22 mWh cm-3 at 165.5 mW cm-3 ) [28], α-Fe2O3@PANI//PANI device (0.35 mWh cm-3 at 301.19 mW cm-3) [29], MnO2 NWs//Fe2O3 NTs device (0.32 mWh cm-3 at 139.1 mW cm-3) [30] and Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device (3.8 mWh cm-3 at 1.13 mW cm-3) [32], and is even comparable to the average value of thin-film lithium battery (0.3–10 mWh 15

cm−3). 3. Conclusion In summary, we have presented a facile method to synthesize porous Fe2O3 nanospheres anchored on ACC with large specific surface area. The amount of deposition of iron oxide on ACC is important for the supercapacitive performance of composite electrode. The Fe2O3@ACC composite electrode shows specific capacitance of about 2775 mF cm-2 at 1 mA cm-2 and 4755 mF cm-2 at 2 mV s-1, which is much larger than the reported values for the Fe2O3-based electrodes in aqueous electrolyte. In addition, the Fe2O3@ACC electrode also exhibits excellent rate capability and stability, 92% capacitance retention after 10000 cycles. The excellent performance of Fe2O3@ACC electrode is attributed to the synergetic effect of porous Fe2O3 nanospheres, the highly conductive ACC, relatively lower adsorption and diffusion barrier on the facet of Fe2O3, and electronegativity on the exposed surface. With two pieces of the prepared Fe2O3@ACC electrode, a 1.8 V high-voltage aqueous symmetric SC is developed with high performance, which can reach a maximum area specific capacitance of about 1565 mF cm-2 at 1 mA cm-2 and exhibits good cycling stability with 95% of initial capacitance retained after 4000 cycles. The Fe2O3@ACC symmetric supercapacitor can deliver an energy density of 9.2 mWh cm-3 at a power density of 12 mW cm-3 and an energy density of 4.5 mWh cm-3 at a power density of 204 mW cm-3, which exhibits higher volumetric energy and power densities than previous Fe2O3-based SC. This work demonstrates that the Fe2O3@ACC is an excellent electrode material. The facile material synthesis and superior performance 16

make the Fe2O3@ACC composite promising for practical applications in high performance energy storage systems. 4. Experimental Section Activation of Carbon Cloth: Appropriate amount of CC (Fuel Cell Materials; plain carbon cloth; 115 g m-2) was immersed in a mixed solution (concentrated sulfuric acid and concentrated nitric acid with the volume ratio about 1:1). After about 48 h, the treated CC was then washed with deionized water and alcohol, and ultrasonic treatment about 3 h. Then dry in a drying oven at 60 ℃ about 24 h. The mass density for activated CC is about 100 g m-2. Synthesis of Fe2O3@ACC composite: The Fe2O3 nanospheres on ACC was fabricated by a one-step hydrothermal method. Typically, 1.75 mmol Fe(NO3)3·9H2O were added into the 40 mL deionized water under stirring. A Teflon vessel with stainless steel shell (autoclave) was filled with the resultant solution and a piece of ACC with the area of 1.5×3 cm2 was placed into the solution, and then autoclave was sealed and kept at 190 °C for 48 h. After cooling down to room temperature, the sample of porous Fe2O3 nanospheres anchored on ACC substrate was obtained after cleaning. The mass loading of Fe2O3 nanospheres was calculated by weight difference of Fe2O3@ACC before and after dilute sulfuric acid treatment (about 1.87 mg cm-2). To clarify the role of combination of Fe2O3 nanospheres and ACC, the Fe2O3 powder was synthesized under the same conditions except adding ACC. The typical nanostructure of the Fe2O3 nanospheres powder was shown in Fig. S9 (Supplementary 17

information). Materials Characterization: The crystallographic property and phase purity of the samples were detected by XRD (PANalytical X’Pert Powder with Cu Kα radiation), and the morphology and microstructure were investigated by FESEM (JSM-7800F) and TEM (FEI Talos F200S). The chemical composition was measured by EDS (FEI Talos F200S), electron energy loss spectroscopy (EELS, TECNAI120 Philips) and XPS (ESCA Lab MKII). The specific surface area was evaluated at 77.3 K by the multipoint Brunauere Emmette Teller (BET, Quantachrome NOVA 4200e). The pore size distribution was measured by the Barrett-Joyner-Halenda (BJH) method. Electrochemical Measurements: A CHI electrochemical workstation (Model CHI660D) was used to measure the electrochemical performance of the prepared electrode in a standard three-electrode system, with Pt plate and SCE as counter and reference electrodes in 3 M LiNO3 electrolyte. CV, GCD and EIS tests of the electrodes

were

performed

in

different

neutral

electrolytes.

Symmetric

supercapacitors were assembled by sandwiching a separator (Whatman 8 mm filter paper) between two Fe2O3@ACC electrodes, and electrochemically characterized in a two-electrode configuration cell in 3 M LiNO3 aqueous electrolyte solution.

The capacitance was calculated according to CV curve using C  where

 IdV , ΔV

 IdV

ΔV  ν

,

and ν represent the area of CV curve, potential window and 18

scanning rate, and GCD plot using C 

I  t , where I, Δt and ΔV represent the v

discharging current, time, voltage (excluding IR drop), respectively. The volumetric energy (mWh cm-3) and powder (mW cm-3) densities of the device were calculated based on E  0.5

E CS  3600 , where Cs is the areal (V ) 2 and P  tdischarg e 3600  d

capacitance, d is thickness of the symmetric SC device, ΔV is discharging voltage, tdischarge is the total discharging time. Theoretical calculation method: To explore lithium ion diffusion on the surface of Fe2O3, the first-principles calculations based on density functional theory (DFT) was employed as implemented in Vienna ab initio simulation package (VASP) [47]. For the exchanged-correlation functional, the general gradient approximation (GGA) of the Perdew-Burke-Ernzerh (PBE) was adopted [48]. To describe the core-valence electron interaction, the project-augmented wave (PAW) method was used [49]. The KohnSham orbitals were expanded in a plane wave basis with an energy cut off of 500 eV. For k-space sampling, we used a 2 × 1 × 1 Monkhorst-Pack grid and for the structural optimization, the force threshold was set as 0.01 eV/Å. For ion diffusion, we employed the nudged elastic band (NEB) method to identify the minimum energy paths.

Acknowledgments This work is supported by NSFC (51572040 and 51772036), the Graduate 19

Scientific Research and Innovation Foundation of Chongqing (CYS17042), Fundamental Research Funds for the Central Universities (2018CDJDWL0011, 106112017CDJXY300002, 2018CDQYWL0046, 2018CDPTCG0001/22), the Natural Science Foundation of Chongqing (cstc2017jcyjAX0307), and the Science and Technology Research Project of Chongqing Municipal Education Commission of China (KJ1400607, KJ1401206).

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Jien Li received his bachelor's degree in physics in 2016 from Chongqing University. Now, he is a doctoral candidate in Chongqing University. His research interests are mainly focused on synthesis and applications of functional nanomaterials for energy storage and conversion, including supercapacitor and nanogenerator.

Yanwei Wang received his B.S. in Chemical engineering and technology (2017) from Guizhou Institute of Technology. Now, he is a postgraduate student under the supervision of Professor Yu Wang in Chongqing University. Currently, his research interests are mainly focused on advanced energy materials for electrocatalysis.

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Weina Xu received her master's degree in Physics in 2016 from Chongqing University. Now, she is a doctoral candidate in Chongqing University and a visiting student in University of Wisconsin-Madison. Her research interests are mainly focused on synthesis and applications of functional nanomaterials for energy conversion and storage, including sensors and supercapacitors.

Yu Wang received his Ph.D. from Peking University in 2007. He is presently working as a professor of Chemistry and Chemical Engineering, and State Key Laboratory of Power Transmission Equipment and System Security in Chongqing University. His works have been cited over 4000 times. His research interests include energy storage and catalytic conversion.

Bin Zhang received his Ph.D. in Beijing University of Technology (2016). He worked as an assistant researcher fellow in Chongqing University from 2017. His research interests mainly focus on the relationship between microstructure and properties of functional materials, e.g. thermoelectric and phase change memory materials.

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Shuang Luo received her bachelor's degree in Physics from Chongqing University in 2017. She is currently a master's degree candidate at College of Physics, Chongqing University. Her research mainly focuses on photo-catalytic oxidation and reduction.

Dr. Xiaoyuan Zhou received her Ph.D. degree in Department of Applied Physics, Hong Kong Polytechnic University (2008). She worked as a postdoc research associate in University of Washington (2008–2010), and University of Michigan (2010–2013). In 2013, she joined Chongqing University as a professor. Her current research is focused on thermoelectric materials and their applications.

Cuiling Zhang received her doctor's degree in Physics in 2013 from Chongqing University, China. Now, she is a postdoctor of Chongqing University, China. Her research interests are mainly focused on functional nanomaterials for energy conversion and storage, including supercapacitors and photocatalyst.

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Xiao Gu graduated from Dept. of Physics in Fudan University 2002, and received his Ph.D. degree in 2007 from Fudan University. He was postdoctoral researcher at University of British Columbia, Vancouver, Canada from 2007 to 2009. From 2010 to 2014, he worked as research associate in Dept. of Environmental Sciences and Engineering of Fudan University. Now he is research professor in Dept. of Applied Physics in Chongqing University. His research interests cover advanced materials, such as energy materials (lithium ion battery, supercapacitors, solar cell) and functional materials such as catalysts etc.

Chenguo Hu is a professor of physics in Chongqing University and the director of Key Lab of Materials Physics of Chongqing Municipality. She received her Ph.D. in Materials from Chongqing University in 2003. Her research interests include methodology of synthesizing functional nanomaterials, design and fabrication of electronic devices, such as nanogenerators and sensors.

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Table 1. Comparison of electrochemical performance for the Fe2O3-based electrodes in aqueous electrolytes. Fe2O3-based electrode

Electrolyte

Potential range

Specific capacitance

Rate capability

[V vs. Ag/AgCl or SCE]

[mF cm-2]

[mF cm-2]

Ref.(year)

α-Fe2O3 nanorods

3 M LiCl

-0.8 to 0 V

382.7 at 0.5 mA cm-2

202 at 8 mA cm-2

(2014) [18]

Fe2O3-P nanorods

1M Na2SO4

-0.8 to 0 V

340 at 1 mA cm-2

112 at 20 mA cm-2

(2018) [19]

Fe2O3-δ nanorods

1 M LiOH

-0.9 to 0 V

423 at 0.5 mA cm-2

200 at 20 mA cm-2

(2018) [20]

α-Fe2O3 nanoarrays

1M Na2SO4

-0.8 to 0 V

382.4 at 0.5 mA cm-2

120.8 at 6 mA cm-2

(2017) [28]

α-Fe2O3 nanowires

1M Na2SO4

-0.8 to 0 V

103 at 0.5 mA cm-2

26.5 at 5.7 mA cm-2

(2015) [29]

Fe2O3 nanotubes

5 M LiCl

-0.8 to 0 V

180.4 at 1 mA cm-2

119.9 at 10 mA cm-2

(2014) [30]

α-Fe2O3@NiO composites

1 M LiOH

-0.2 to 0.8 V

557 at 1 mA cm-2

249.6 at mA cm-2

(2014) [31]

Fe2O3 Nanoplates

3 M KOH

-1 to 0 V

694 at 1 mA cm-2

378.2 at 20 mA cm-2

(2017) [32]

Fe2O3 nanoflakes

1M KOH

-1.2 to -0.2 V

169.2 at 1 mA cm-2

61 at 5mA cm-2

(2018) [33]

Fe2O3 nanoneedles

2M KOH

-1.35 to -0.2 V

1000 at 2.8 mA cm-2

510 at 16.8 mA cm-2

(2018) [34]

Fe2O3 nanocrystals

1M Na2SO4

-0.2 to 1 V

1660 at 2 mA cm-2

1160 at 30 mA cm-2

(2016) [35]

Fe2O3@ACC

3M

-0.8 to 0 V

2775 at 1

2155 at 10

This work

29

composites

mA cm-2

LiNO3

mA cm-2

Scheme 1

Scheme 1. Schematic illustration of the Fe2O3@ACC electrode and the structure of symmetric supercapacitor.

Figure 1

30

Figure 1. a) XRD pattern of the Fe2O3 nanospheres. b) the crystal structure of Fe2O3. c) XPS spectrum of Fe2O3@ACC composite and d) Electron energy loss spectroscopy (EELS) spectrum of Fe2O3 nanospheres.

31

Figure 2

Figure 2. a,b) FESEM images of the Fe2O3 nanospheres on activated carbon cloth at different magnification. c) STEM and d) HRTEM images of the Fe2O3 nanospheres; the insets show high magnification STEM/TEM images. f) Elemental mapping images of Fe, O.

Figure 3 32

Figure 3. N2 adsorption–desorption isotherms a) and BJH pore size distribution plots b) of the Fe2O3@ACC composites at 77.3 K.

33

Figure 4

Figure 4. GCD performance of a) Fe2O3@ACC electrode at different reactant concentrations and b) Fe2O3@ACC electrode (1.75mmol) at different current densities. c) The areal and specific capacitances of the Fe2O3@ACC electrode versus current density. d) CV curve of the Fe2O3@ACC, Pure Fe2O3 nanospheres (P-Fe2O3) and ACC at 10 mV s-1. e) Nyquist plots of the Fe2O3@ACC, P-Fe2O3 and ACC; the inset in e) is the enlarged plots at high frequency. f) Cycling performance of Fe2O3@ACC, P-Fe2O3 at 100 mv s-1. 34

Figure 5

Figure 5. The top view of Lithium ions diffusion path on the a)100 and b)110 facets of Fe2O3. c) The corresponding energy barriers in diffusion paths on 100 and 110 facets, calculated by the NEB method.

35

Figure 6

Figure 6. a) Typical CV curves of Fe2O3@ACC electrode in potential windows of -0.8-0 V and 0-1V at 10 mV s-1. b) Areal capacitances as a function of scan rate of Fe2O3@ACC electrode in the positive and negative potential window. Fe2O3@ACC symmetry device performance within 0-1.8 V in aqueous electrolyte: c) CV curves. d) GCD plots. e) The cycling response at continuously variable currents. f) Cycling performance at a constant current density of 17 mA cm-2.

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Research Highlights



Porous Fe2O3 nanospheres anchored on activated carbon cloth is prepared and used as high performance electrode for SC.



The Fe2O3@ACC electrode have a large specific capacitance, which is much larger than the reported values for the Fe2O3-based electrodes in neutral aqueous electrolyte.



We find for the first time that the Fe2O3@ACC has even higher specific capacitance in positive potential window than that in negative potential window at scan rate larger than 30 mV s-1.



The low diffusion potential barrier for Li+ ions on Fe2O3 contributes excellent pseudocapacitive performance.

37