Low-crystalline FeOx@PPy hybridized with (Ni0.25Mn0.75)3O4@PPy to constructed high-voltage aqueous hybrid capacitor with 2.4 V

Journal of Electroanalytical Chemistry 859 (2020) 113828

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Low-crystalline FeOx@PPy hybridized with (Ni0.25Mn0.75)3O4@PPy to constructed high-voltage aqueous hybrid capacitor with 2.4 V Xiaohui Li 1, Xing Zhou 1, Dejian Chen, Long Li, Danyang Zhao, Xintang Huang

⁎

Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan 430079, PR China

A R T I C L E

I N F O

Article history: Received 23 August 2019 Received in revised form 28 November 2019 Accepted 5 January 2020 Available online 08 January 2020 Keywords: Electrochemical Li+ pre-insertion Low-crystalline High-voltage aqueous hybrid capacitor Iron oxide nanorod array Manganese oxide nanoprism array

A B S T R A C T

The operating voltage of aqueous hybrid capacitors are generally limited to 2 V due to the decomposition of water, which significantly impede the progress of energy density. Herein, the porous low-crystalline FeOx nanorod array on carbon cloth is prepared by the novel electrochemical Li+ pre-insertion method, and a 2.4 V high-voltage aqueous hybrid capacitor device is successfully obtained after matching with the nickel doped (Ni0.25Mn0.75)3O4@PPy nanoprisms array. The low-crystalline structure of FeOx preserved during the first Li+ insertion and space created via the elimination of low-crystalline Li2O dramatically provides sufficient electronic and ionic transfer channels. In addition, surface polypyrrole (PPy) stabilization is employed to further enhance electron conductivity and electrode stabilization. Benefitting from increasing active sites, fast ion diffusion and electron transfer the obtained lowcrystalline FeOx@PPy electrode exhibits improved electrochemical performance, especially for capacitance and stability. Moreover, the aqueous hybrid capacitors (Ni0.25Mn0.75)3O4@PPy//FeOx@PPy device delivers a high energy density of 72.4 Wh kg−1 with the ultra-high voltage, and admirable cycling stability (94.7% retention after 4000 cycles). Our work highlights the novel electrochemical Li+ pre-insertion method to achieve superior low-crystalline electrodes materials and designs the high-voltage aqueous hybrid energy storage devices. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The awful aggravation of environment pollution and energy crisis have urgently triggered the development of energy storage system forward for its standing at the forefront of seeking renewable energy alternatives [1,2]. As a promising alternative for advanced energy storage device, supercapacitors (SCs) also called electrochemical capacitors have witnessed extensive expansion owing to their attractive superiorities including ultrahigh power density (P), long lifespan, superior safety and low maintenance cost [3–5]. SCs can be generally classified into two categories according to their intrinsic charge storage mechanism, one is electric double-layer capacitors (EDLCs) which depends on the electrochemical reversible adsorption/desorption of cations and anions at the electrode/electrolyte interfaces; another is pseudocapacitor which relates to the reversible surface Faradic redox reactions [6–9]. In reality, pseudocapacitors generally possess much higher capacity than EDLCs originated from fast and reversible redox reactions. Nevertheless, prior to widespread application, its relatively low energy density (E) compared with commercial lithium ion batteries (LIBs) remains ⁎ Corresponding author. E-mail address: [email protected]. (X. Huang). 1 The authors contributed equally to this work.

http://dx.doi.org/10.1016/j.jelechem.2020.113828 1572-6657/© 2018 Elsevier B.V. All rights reserved.

to be enhanced without sacrificing power density [10,11]. Generally, two predominant strategies have been proposed to address the above issue: 1. boosting the capacitance of electrodes. Considerable efforts have been focused on the rational design of electrodes to provide more redox active sites, facilitate electron transfer and ion diffusion rates, such as constructing hierarchical nanostructure [8,9,12], doping exotic element [13,14], engineering intrinsic defects [15,16], functionalizing surface [17]. 2. broadening the working voltage of devices. According to the formula E = 1/ 2CV2, in which C, V represents specific capacitance and operating potential respectively, thus, it is more effective by enlarging cell operating potential instead of maximizing specific capacitance to promote energy density [18,19]. On one hand, the cell operating voltage is greatly dominated by the decomposition of electrolyte and interaction between electrolyte and electrodes. Generally, organic electrolyte possesses larger operating voltage than aqueous electrolyte owing to the theoretical decomposition of water at 1.23 V in aqueous electrolyte, normally, the lower concentration of H+ or OH– in neutral electrolyte endowing higher overpotential than acid or alkaline ones, which leads to the larger operating voltage. However, the flammability, high-cost, slumped ion diffusion rates deeply impede its practical application. On the other hand, constructing hybrid capacitor configuration can also achieve larger operating voltage ascribed to the electrodes possess different nature for positive and negative polarization [20].

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2.2. Synthesis of porous low-crystalline FeOx@PPy nanorod array on CC

Iron oxide was regarded as a promising anode candidate used for aqueous electrochemical energy storage device owing to its features of remarkable high theoretical capacity, negative operating potential, co-friendliness and natural abundance [21,22]. Despite the notable achievements on the electrochemical performance, it still suffers from low specific capacitance, poor cycling durability and structural instability ascribed to its lower electron conductivity and structural instability [23]. On the other hands, Fe2O3 also attracted widespread concern as promising anode materials in secondary lithium ion batteries [24], the reaction mechanism has been demonstrated to follow “conversion-type”: when Li+ inserted, Fe2O3 is reduced into metallic Fe nanoparticles (<2 nm) which dispersed in the amorphous Li 2O matrix, in the followed desertion process, the Li2O composed and metallic Fe nanoparticles were oxidized to FeO, fortunately, the lowcrystalline structure formed during Li+ insertion preserved in the following charge-discharged process. Recently, amorphous structure or low-crystalline structure have been demonstrated beneficial the electrochemical performance of supercapacitors [25–27], the loose arrangement of atom can accommodate the volume expansion during the redox reactions, which enhance the cycling durability; on the other hand, long-range disorder can dramatically increase redox sites due to facilitating the electrolyte diffusion, achieving higher specific capacity. Based on the above considerations, herein, an electrochemical Li+ pre-insertion method was proposed to transform well-crystalline Fe2O3 nanorod array directly grown on carbon cloth (CC) into porous metallic low-crystalline Fe nanorods array, according to the conversion-type lithiation mechanism, the iron oxide was transformed into pseduamorphous Fe0/Li2O compound. Afterward, ethanol and deionized water were used to get rid of residential Li2O and naturally oxidized to low-crystalline FeOx . To enhance the electrode structural stability and conductivity, polypyrrole (PPy) nanocoating was introduced on FeOx to obtain porous low-crystalline FeOx@PPy electrodes [28,29]. Electrochemical measurement reveals this elaborate material engineering approach can tremendously optimize the electrochemical performance of electrodes, enhanced specific capacitance of 592.8 F g −1 at 1 A g−1 , as well as improved cycling durability, is achieved. To assemble aqueous hybrid capacitor device with high energy density, Ni doped manganese oxide nanoprism array stabilized by PPy with high operating potential from 0 to 1.3 V (vs SCE) was used as a cathode. The introduction of Ni atom and PPy stabilization can dramatically suppress water decomposition and increase electron conductivity [30,31]. As the fabricated aqueous device can operate stably at high-voltage range of 0–2.4 V and exhibits ultrahigh energy density of 72.4 Wh kg−1 at power density of 0.676 KW kg−1 as well as outstanding cycling durability life of 4000 cycles.

Porous low-crystalline FeOx nanorod array was obtained by the electrochemical Li+ insertion method followed by the elimination of Li2O and natural oxidization. Typically, Fe2O3 nanorod array on CC was cut into pieces of 1*1 cm2 and used as working electrode of coin cell battery. The coin cell battery was assembled in Ar filled glovebox with 1 M Li LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DEC, 1:1 by volume) as electrolyte and Li foil as reference and counter electrode. Li+ insertion was conducted on a battery tester by galvanostatic discharge at 50 mA g−1 in the potential range of 0.01–3 V (vs. Li+/Li). After presupposed 0, 3, 6 cycles, the asprepared working electrodes were taken out and washed with thoroughly ethanol and deioned water, respectively. The low-crystalline FeOx was obtained after drying in the air. To realize PPy stabilization, 96.7 mg pyrrole monomer was added into 30 mL ethanol solution containing 0.416 g Ptoluene sulfonic acid (p-TSA), 20 mL aqueous solution containing 0.12 g ammonium persulfate was used as oxidizing agent. CC was firstly immersed in the ethanol solution for 10 min. Afterward, several drops of oxidizing agent solution were dropped on the CC. After drying in the oven at 60 °C overnight, FeOx@PPy was obtained. The PPy directly coated on the CC for comparison. The mass of FeOx and FeOx@PPy samples were weighted to be 1.2 and 1.4 mg cm−2, respectively. 2.3. Synthesis of (Ni0.25Mn0.75)3O4@PPy nanoprism array cathode The (Ni0.25Mn0.75)3O4 nanoprism array on CC was prepared through facile hydrothermal followed by post heat treatment. In detail, 50 mL light green aqueous solution containing 0.912 g Mn(CH3COO)2·4H2O, 0.311 g Ni(CH3COO)2·4H2O, 0.37 g NH4F and 1.5 g urea were transferred into a 100 mL autoclave with a clean CC of 1 × 4 cm2 inside. After heat at 135 °C for 6 h, the (Ni0.25Mn0.75)CO3 precursor was washed by ethanol and deionized water. Then the (Ni0.25Mn0.75)CO3 was annealed in Ar atmosphere at 425 °C for 2 h with a ramp rate of 2 °C min−1. Finally, the (Ni0.25Mn0.75)3O4@PPy was obtained by the above method. Similarly, the Mn3O4 film was prepared in the same procedure without the addition of Ni(CH3COO)2·4H2O. The mass of (Ni0.25Mn0.75)3O4 and (Ni0.25Mn0.75) −2 , respectively. 3O4@PPy nanoarray was weighted to be 1.8 and 2.2 mg cm 2.4. Materials characterization The crystal structure information of these samples was obtained by Xray powder diffraction (XRD, PANalytical X' PertPRO) and Raman spectrometer (LabRAM HR JY-Evolution, 532 nm). The morphology characterization was investigated by scanning electron microscopy (SEM, JEOLJSM6700F) with an accelerating voltage of 20 kV, combing with energy dispersive X-ray spectroscopy (EDX) for element identification. The analysis of elements and valence state was by X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi). The microstructural characterization was researched by transmission electron microscopy (TEM) images were recorded with Philips Tecnai 20 and JEOL JEM-2010 high-resolution transmission electron microscopes.

2. Experimental 2.1. Synthesis of Fe2O3 nanorod array on CC CC was cleaned by ultrasonic washing with ethanol, acetone and deionized water for 15 min, respectively. Fe2O3 nanorod array was grown on CC by a facile hydrothermal treatment followed by annealing in the air. Typically, 0.946 g FeCl3·H2O and 0.497 g Na2SO4 were added into a 70 mL aqueous solution and stirred vigorously to obtained a transparent orange solution. After that, the obtained solution was transferred into a 100 mL autoclave with a CC of 1*4 cm2 in it followed by sealed and heated in an oven at 160 °C for 6 h. After hydrothermal treatment, the black CC substrate was covered with yellow film and washed with ethanol and deionized water for several times, hence FeOOH nanorod array precursor was obtained. After drying at 70 °C overnight, it was annealing in the Ar atmosphere at 450 °C for 2 h, the yellow film turned dark red and Fe2O3 was well prepared. The mass of Fe2O3 nanoarray loaded on CC was weighed to be 1.2 mg cm2.

2.5. Electrochemical measurements For the single electrode test, those electrochemical performances were obtained on a CorrTest Electrochemical workstation (CHI660E) in a three-electrode system using a 1 M Li2SO4 solution as electrolyte at a room temperature. The obtained samples served as working electrode, the Pt foil and calomel electrode served as the counter and reference electrode, respectively. The distance between the working electrode and counter electrode was 2 cm. Cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were carried out. Finally, the electrochemical properties of the self-assembled hybrid capacitor devices were investigated with employing a two-electrode system. 2

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washed thoroughly with ethanol and deionized water to get rid of Li2O due to its solubility and organic residents on the electrode surface. The elimination of Li2O was believed to vacate more spaces inside Fe nanorods, which may facilitate the ion diffusion and provide more redox sites. Then the Fe0 nanorod array was naturally oxidized to low-crystalline FeOx nanorod array for the instability of ultra‑tiny metallic particles when exposed to air. However, loose space inside low-crystalline structure and poor grain electric contacts may reduce the electron conductivity of active materials. To restrain this drawback, one of prevailing strategy is to coat organic conductive polymer. Herein, surface PPy coating was employed to enhance the conductivity of electrodes, besides, PPy can also stabilize electrode structure hence enhance cycling durability.

The specific capacitance (C) was calculated according to the discharged curve in the light of following equation: C¼

I  Δt m  ΔV

ð1Þ

where I (A) represents the discharge current; Δt(s) designates the discharge time; m is total weight of the electrode active material; ΔV (V) corresponds to the potential window. The energy density and power density of the flexible hybrid capacitor devices were calculated based on the following equations: C¼

I  Δt m  ΔV

ð2Þ

E¼

C  ΔV 2 2

ð3Þ

E P¼ Δt

3.1. Characterization and electrochemical measurements of FeOx@PPy anode Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were then employed to investigate the morphological and structural evolution. It is found that the cycle number of Li+ pre-insertion process imposes a significant influence on the crystalline and morphology (Fig. S3): with increasing cycle number, the degree of Li+ pre-insertion is deepening and crystalline is lower. After 6 cycles, the Fe2O3 nanorods suffered fearful structural destruction, all the nanorods contact with each other and pristine morphology is no longer retained, revealing severe pulverization and agglomeration. Thus, we selected the FeOx NPs after 3 cycles which basically preserves its original structure as the experimental condition for the following samples. For the Fe2O3 NRs precursor, the overall SEM image in Fig. 2a reveals vertical and high density nanorods arrays on the surface of carbon cloth, and the TEM images (Fig. 2b and c) present clear lattice fringes and high-crystalline structure. In contrast, there is no obvious change on the morphology for the FeOx (Fig. 2d), but the TEM images (Fig. 2e and f) demonstrates the porous low-crystalline structure after Li+ insertion and elimination. In the case of FeOx@PPy, as shown in Fig. 2g, the morphology was inherited intact after coated PPy layer, and the TEM images (Fig. 2h and i) indicate that the PPy layer (thickness ~5 nm) is entirely stabilized on the surface of nanorod is conformably coated on the surface of porous low-crystalline FeOx NRs arrays through chemical oxidative polymerization [35] and the interior still remain the low-crystalline structure (the insert in Fig. 2i). As descript above, all sample present the similar morphology of nanorods with the diameter of about 80 nm and length of about 500 nm. Additionally, the Energy dispersive X-ray spectroscopy (EDX) of the Fe2O3 NRs and FeOx NRs provides the chemical composition (Fig. S4). Fe and O is the main element in both samples, and there is on obvious Li peak indicating the LiO2 is completely eliminated. Meanwhile, the Brunauer-Emmett-Teller (BET) results also suggest that pores of 2–6 nm of the FeOx NRs have remarkably increased as compared to those of Fe2O3 (Fig. S5), which determine the creation and maintain of more holes during the Li2O process. Thus, it can be confirmed that the porous low-crystal of

ð4Þ

where C is the volumetric capacitance; I (A) refers to the discharge current; Δt (s) stands for the discharge time; ΔV (V) corresponds to the potential; m is the total volume of the hybrid capacitor device. 3. Results and discussion The synthesis procedure of porous low-crystalline FeOx@PPy is schematically illustrated in Fig. 1. Firstly, FeOOH nanorod arrays (NRs) grown vertically on CC was prepared by a facile hydrothermal treatment, afterward, well-crystalline Fe2O3 NRs were obtained by annealing FeOOH NRs in the air at 450 °C for 2 h. To convert well-crystalline Fe2O3 into lowcrystalline structure, Li+ pre-insertion process was conducted by galvanostatic discharge-charge (GCD) measurements on battery tester with coin cell assembled with Fe2O3 NRs as working electrode and lithium foil as both counter and reference (Fig. S1). Galvanostatic discharge profile is presented in Fig. S2, a low discharged current density of 50 mA h g−1 was utilized to prevent high-current-induced structural degradation. The discharge curves possess sloped region in the range of 1.5–0.9 V (vs Li+/ Li) and long plateau at about 0.8 V (vs Li/Li+), which exhibits typical Fe2O3 discharged characteristic, corresponding the Li+ insertion process [32]. The Fe2O3 NRs electrode presents ultrahigh initial discharged specific capacity of 1236.3 mA h g−1, which partially risen from the irreversible formation of solid electrolyte interface (SEI) and decomposition organic electrolyte [33]. As a promising “conversion-type” anode material for LIBs, after initially discharged to cut-off voltage (0.01 V vs Li/Li+) accompanied with the insertion of Li+, the Fe2O3 NRs/CC was transformed into metallic Fe/Li2O nanorods compound, in which ultra‑tiny iron nanocrystal dispersed in Li2O matrix [34]. Afterward, the electrode was taken out and

Fig. 1. Schematically illustration of synthesis process of porous low-crystalline FeOx@PPy nanorod array. 3

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Fig. 2. SEM, TEM and HR-TEM images of Fe2O3 (a–c), FeOx (d–f) and (d) FeOx@PPy (g–i).

configuration with Pt foil as counter electrode and calomel electrode as reference in 1 M Li2SO4 aqueous electrolyte. To investigate the charge storage behavior of Fe2O3, FeOx and FeOx@PPy, cyclic voltammetry (CV) techniques were conducted from −1-0 V (vs SCE) at the scanning rate of 20 mV s−1 (shown in Fig. 4a), all CV curves exhibit similar rectangle shape, revealing the charge storage properties can be attributed to coeffects of EDLC by surface absorption/desorption and Faradic pseudocapacitance arisen from redox couple of Fe2+/Fe3+ [41]. The integral areas which are in proportion to specific capacitance increase successively in accordance with the Fe2O3, FeOx and FeOx@PPy. The remarkable improved specific capacitance after Li+ pre-insertion process may originate from followings: firstly, the elimination of low-crystalline Li2O create numerous pores insides NRs, which can significantly increase redox sites; secondly, low-crystalline structure derived from Li+ preinsertion tremendously facilitate ion diffusion inside NRs and provide more redox sites [42]; finally, PPy stabilization endow nanoarray with fast electron transfer capacity [43]. Fig. 4b shows the CV profiles of FeOx@PPy at different scan rates, with the increasing of scanning rates, the shape CV curves possess unchanged shapes and enlarger integrate area, indicates the excellent electron/ion transport and high redox reversibility. Galvanostatic charge-discharge (GCD) techniques were carried out to test the specific capacitance. Calculated specific capacitance of Fe2O3, FeOx and FeOx@PPy electrodes are 73.2, 361.4, 592.8 F g−1 (Fig. 4c), respectively, revealing that FeOx@PPy achieve specific capacitance of 8.1 times enlarger than Fe2O3 NRs due to the effective Li + pre-insertion process and improved conductivity by PPy. It is worth noting that the mass of PPy is only 14.3% of the FeOx@PPy weight, and the separated PPy electrode is almost no contribution for capacity in this study (Fig. S7). All GCD curves at different current densities possess symmetric linear shapes (Fig. 4d), which are in agreement with the CV curves. The specific capacitance at different current densities was plotted in Fig. 4e, even at high current density of 10 A g−1, the retention of 47.5% was achieved, exhibiting good

the FeOx@PPy NRs sample was obtained after Li + pre-insertion process and PPy coating. X-ray diffraction (XRD) was carried out to investigate the crystal structure and phase composition. As shown in Fig. 3a, all diffraction peaks from FeOOH and Fe2O3 are well indexed to goethite (JCPDS No. 1-401) and α-Fe2O3 (JCPDS No. 1-1053), respectively. No impurity signals are detected except peaks from CC and the intense diffraction peaks, indicating their well-crystalline character and high purity. No obvious signal emerges for FeOx NRs arrays primarily testify the low-crystalline structure, which is in agreement with “conversion-type” reaction mechanism [36]. After PPy coating, the FeOx@PPy possess similar but more weak XRD patterns comparing with FeOx, which may be due to PPy layer coated on the surface. In order to investigate its chemical composition and materials evolution, X-ray photoelectron spectroscopy (XPS) and was performed. Fig. S6 shows the XPS complete survey of the Fe2O3 NRs and the FeOx NRs without the Li2O elimination, which clearly confirms the Li+ insertion in the Fe2O3 during the electrochemical Li+ pre-insertion process. Fig. 3b indicates that the FeOx@PPy is composed of C, N, O and Fe elements, the existence of N reveals the effectiveness of PPy stabilization [37]. High resolution XPS spectrum of Fe 2p and N 1s provide more information of the state of elements(Fig. 3c and d). Core-level XPS spectrum of Fe 2p presents two dominant peaks located at 711.1 and 724.7 eV corresponding Fe 2p1/2 and Fe 2p3/2 with two satellite located at 718.9 and 733.1 eV, respectively. Peaks of Fe 2p1/2 can be deconvoluted into two peaks located at 726.8 and 724.6 eV while two peaks located at 712.7 and 710.8 eV for Fe 2p3/2. Fitting peaks located at 711.1 and 724.6 eV can be ascribed to Fe2+ while peaks located at 712.7 and 726.8 eV can be ascribed to Fe3+ [38]. The existence of Fe2+ and Fe3+ may originate from the Fe0 oxidization when exposed to air, besides, the Li + insertion into Fe2O3 can also create partial Fe2+ [34,39]. Broad peaks from N 1s can be fitted into three peaks located at 400.2, 400.9 and 402.5 eV, which are assigned to -NH−, -NH+and =N+- cations from PPy layer, respectively [40]. The electrochemical measurements of porous low-crystalline FeOx@ PPy nanorod array electrode were carried out in the three-electrode 4

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Fig. 3. (a) XRD patterns of FeOOH, Fe2O3, FeOx and FeOx@PPy. XPS (b) complete survey, (c) core-lever Fe 2p, (d) N 1s of FeOx@PPy sample.

conformal coating also benefits the cycling performance owing to effectively releasing the volume expansion/contraction strength during faradic reactions.

capacities capacity. It is deserved to be mentioned that precursory Fe2O3 NRs shows competitive rate capacity with FeOx@PPy, which because the elimination of Li2O and lowing crystalline may impede the electron transfer. Fortunately, the PPy stabilization makes up the drawback. To further investigate the kinetics of electrode process, the electrochemical impedance spectrum (EIS) was measured on three-type electrodes with alternating current (AC) perturbation amplitude of 5 mV in the frequency range of 0.01–100 kHz. As shown in Fig. 4f, the obtained Nyquist plots exhibits similar shape with a semicircle in the high-frequency region and a slash in the low-frequency area, which are in related with the charge transfer process and diffusion-limited process, respectively [44]. It is instinct that FeOx and FeOx@PPy electrodes possess larger ion transfer coefficient than Fe2O3 electrode according to the slop in low-frequency range which can be ascribed to the porous low-crystalline structure derived from Li+ pre-insertion. The diameter of semicircle related to electron transfer resistance (Rct) for FeOx@PPy is smaller than FeOx indicates the effectiveness of PPy stabilization to increase electrode conductivity. Cycling durability is also a significant factor in terms of electrochemical performance. But the ohmic impedance (Rs) become larger after Li+ pre-insertion, which may be due to the residual SEI (solid electrolyte interface) layer forming in the Li+ pre-insertion process. GCD techniques were performed at 5 A g−1 to estimate the cycling stability of different electrodes. Fig. 4g clearly demonstrates that FeOx@PPy electrode exhibits most enduring cycling stability, retention of 90.5% after 4000 GCD cycles are obtained compared with Fe2O3 electrode (61.3% retained after 700 cycles) and FeOx electrodes (73.5% retained after 700 cycles). The low-crystalline structure preserved during the Li + pre-insertion and pores resulted from the elimination of Li2O produce abundance space, which can release the strength caused by volume expansion thus improving cycling performance. Meanwhile, PPy

3.2. Synthesis, characterization and electrochemical measurements of cathode materials To achieve high energy density hybrid capacitor device, Mn-based oxides have recently attracted considerable attention for its relatively wide operating potential and high theoretical capacity in positive polarity potential [31,45–47]. Herein, (Ni0.25Mn0.75)3O4 nanoprisms was fabricated through hydrothermally growing (Ni0.25Mn0.75)CO3 nanoprisms precursor on CC followed by post-annealing in Ar atmosphere by a modified reported method [31]. In order to further enhance the conductivity and stability of electrodes, PPy was coated on (Ni0.25Mn0.75)3O4 nanoprisms to obtain (Ni0.25Mn0.75)3O4@PPy nanoprisms. Fig. 5a and b depict the morphology of as-prepared (Ni0.25Mn0.75)3O4 nanoprisms array, each fiber is entirely uniformly covered by arrays. Mn3O4 film (Fig. S8) presents irregular rocklike surface morphology of about 2 μm while uniform nanoprisms of around 400 nm in diameter vertically anchored on CC fibers were obtained for (Ni0.25Mn0.75)3O4 after Ni atom doping. It's inspiring that arrays suffer favorably morphological transformation ascribed to Ni atom incorporation, compared with densest irregular rock-like arrays, the nanoprisms array make up more space inside films and achieve higher specific surface area which can significantly benefit the electrolyte ion diffusion and increase redox active sites. Meanwhile, the (Ni0.25Mn0.75)3O4 Despite the key challenge to increasing the conductivity of arrays, the same material engineer strategy was used to coat PPy nanofilm on (Ni0.25Mn0.75)3O4 nanoprisms array. 5

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Fig. 4. (a) CV curves of Fe2O3, FeOx and FeOx@PPy at 20 mV s1. (b) CV curves of FeOx@PPy at various scan rates. (c) GCD curves of Fe2O3, FeOx and FeOx@PPy at a current density of 1 mA g−1. (d) GCD curves of FeOx@PPy at various current densities. (e) Current density dependence of the areal capacitance, (f) Nyquist plots of EIS and (g) cycling performance of the pristine Fe2O3, FeOx and FeOx@PPy. 6

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Fig. 5. (a), (b) FE-SEM images of (Ni0.25Mn0.75)3O4@PPy; XRD patterns in the range of (c) 10–80° and (d) 30–34°; (e) XPS complete survey of (Ni0.25Mn0.75)3O4@PPy.

element substitution and PPy stabilization. With the increase of scanning rates from 5 mV s−1 to 50 mV s−1, the shapes (Fig. S11b) remain unchanged for (Ni0.25Mn0.75)3O4@PPy electrodes illustrating the excellent rate capability. As shown in Fig. S11c, GCD curves of Mn3O4, (Ni0.25Mn0.75)3O4 and (Ni0.25Mn0.75)3O4@PPy at the same current density of 0.75 A g−1 further manifest that the pronounced enhancement of pseudocapacitance for (Ni0.25Mn0.75)3O4@PPy electrodes. (Ni0.25Mn0.75) −1 , which is 3O4@PPy exhibits excellent specific capacitance of 271.2 F g −1 much dominant to (Ni0.25Mn0.75)3O4 (196.2 F g ) and pristine Mn3O4 (105.9 F g−1). To investigate the rate capacity of electrodes, GCD measurements for (Ni0.25Mn0.75)3O4@PPy electrode at different current densities in the range of 0.75–12 A g−1 were plotted to study the rate performance (Fig. S11d and e), a 48.1% retention is achieved when the current density increase to 12 A g−1 exhibiting comparable rate capability with FeOx@ PPy. EIS was employed to investigate the electrical conductivity of the electrodes. According to the Fig. S11f, (Ni0.25Mn0.75)3O4@PPy electrode displays a smaller partial semicircle at high frequency and more vertical line in the low-frequency region, indicating the superior conductivity. The Cycling durability was assessed with chronopotentiometry (CP) technique at 10 A g−1 (Fig. S11g), presented GCD cycling profiles reveal that 94.7% initial specific capacitance is retained after 4000 cycles, the outstanding cycling stability can be ascribed to the Ni element substitution and surface PPy stabilization.

XRD and XPS measurements were performed to confirm the crystal structure and elements chemical states. XRD patterns indicate both MnCO3 and (Ni0.25Mn0.75)CO3 precursor can be well indexed to hexagonal rhodochrosite (JCPDS No. 01-981), meanwhile, peaks from (Ni0.25Mn0.75) CO3 precursor shifts slightly towards higher degree indicating the effective substitution of Ni elements (Fig. S9). After heat treatment in Ar atmosphere and PPy stabilization, all peaks arose from Mn3O4, (Ni0.25Mn0.75)3O4 and (Ni0.25Mn0.75)3O4@PPy can be well indexed to tetragonal hausmannite (JCPDS NO: 01-1127) and manifest the crystal structure after annealing (Fig. 5c). All peaks from (Ni0.25Mn0.75)3O4 and (Ni0.25Mn0.75)3O4@PPy shift towards higher degree compared with Mn3O4 indicate the effective substitution of Ni elements (Fig. 5d) [48]. XPS analysis was performed on (Ni0.25Mn0.75)3O4@PPy to confirm the elemental composition and exact Ni/Mn ratio. The complete survey (Fig. 5e) reveals the existence of Ni, Mn, O, N, C elements, the appearance of N signals indicates the success of PPy stabilization. Ni/Mn ratio was further confirmed to be 14.2/42.0 by calculating the integral area of high-resolution Ni/Mn signals, which approaches 1:3. High-resolution spectrums of Mn2p, Ni2p were fitted to investigate the chemical states (Fig. S10). The electrochemical performance was exhibited in Fig. S11 under threeelectrode configuration with Pt foil as counter and SCE as reference in 1 M Li2SO4 aqueous neutral solution. The CV scans were conducted at 5 mV s−1 in the range of 0–1.4 V (vs SCE) to investigate the charge storage properties of Mn3O4, (Ni0.25Mn0.75)3O4 and (Ni0.25Mn0.75)3O4@PPy electrodes. All the CV (Fig. S11a) curves present similar quasi-rectangular shapes with two pairs of peaks (~0.58 V and 0.89 V vs SCE) corresponding to the reversible redox reactions of Mn2+/Mn3+ and Mn3+/Mn4+ accompanied with the Li+ insertion/desertion. According to the previous report, the electrochemical phase can promotes the formation of a low-crystalline phase, and its reversible redox activity dominantly utilizes transferred electrons, which can exactly suppresses the oxygen evolution at the high potential window [31,49]. The integrate area increasing successively from Mn3O4, (Ni0.25Mn0.75)3O4 and (Ni0.25Mn0.75)3O4@PPy electrodes definitely demonstrates the effectiveness in improving the charge storage capacity of Ni

3.3. Assembly and electrochemical measurements of high-voltage aqueous supercapacitor device Aimed at achieving hybrid capacitor device with high energy density, a device constructed with (Ni0.25Mn0.75)3O4@PPy as cathode and FeOx@PPy as anode was assembled (Fig. 6a). It is essential for cathode and anode to possess separate potential windows and matchable charge storage properties to pursue aqueous hybrid capacitor device of high efficiency. Fig. 6b presents the CV curves at 5 mV s−1 in separate potential window of −17

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Fig. 6. (a) Structural model of as-assembled full cell; (b) CV scans of cathode and anode at separate potential ranges; CV scans of ASCs (c) at different operating voltage; (d) different scan rates in 0–2.4 V; (e) GCD curves at different current densities in 0–2.4 V; (f) rate capacity of the full cell; (g) Ragone plots of as-assembled full cell and those other reported; (h) Cycling stability of as-assembled full cell. 8

X. Li et al.

Journal of Electroanalytical Chemistry 859 (2020) 113828

Declaration of competing interest

0 V for FeOx@PPy electrodes and 0–1.4 V for (Ni0.25Mn0.75)3O4@PPy electrodes which make it feasible to construct high-voltage aqueous hybrid capacitor of 2.4 V. In the exploration of applicable operating voltage range (Fig. 6c), electrodes suffer slight current cell leap when the cut-off voltage was increased to 2.6 V indicating the water decomposition occurred. Finally, the device exhibits a high operating voltage to 2.4 V, which profits from the oxygen evolution suppression of the (Ni0.25Mn0.75)3O4@PPy cathode. The quasi-rectangle shape is well preserved when increasing scanning rates from 5 to 50 mV s−1 (Fig. 6d), demonstrating excellent capacitive behavior and fast electron/ion transport rate. The GCD curves are displayed in Fig. 6e, it is clear that all curves exhibit quasi-triangular shape with linear voltage–time plots, indicating good capacitive behavior. Additionally, the DC curves were determined to exhibit the potential distributions for the full cell (Fig. S12). Positive and negative DC values denote the charge and discharge direction, respectively. The two peaks indicating the main charge/discharge potential regions are at about 2 V, which is consistent with the GCD curves. Furthermore, the open circuit voltages and leakage current of the full cell were monitored in the Fig. S13, from which the open circuit voltage remained above 1 V for >10 h and the leakage current decreased drastically to only 15 μA after 2 h. Moreover, the Fig. 6f shows the rate performance and corresponding energy efficiency curves. The high specific capacitance of 89.2, 75.5, 65.8, 50.8 and 42.8 F g−1 is obtained at the current densities of 0.5, 1, 2, 4 and 8 A g−1 in the potential window of 0–2.4 V respectively, and the energy efficiencies are all above 90%, indicating the superior energy coefficient of utilization. Notably, the calculated specific capacitances are based on the total mass of anode and cathode. To assess the power density and energy density of hybrid capacitor devices, Ragone plots are drawn according to the specific capacitance at different current densities in comparison with other reported ASCs (Fig. 6g), as-fabricated (Ni0.25Mn0.75)3O4@PPy// FeOx@PPy device exhibits remarkable energy density of 72.4 Wh kg−1 at power density of 0.676 kW kg−1, which greatly superior most reported ASCs, such as Fe2O3@NiNTAs//MnO2@NiNTAs [50], Fe2O3 nanoflakes// PPy nanoleaves [51], NiCo2S4//rGO@Fe2O3 [52], MnO2 network//AC [53], Fe3O4@CNTs//PAN film [54]. Besides, high energy density of 24.5 Wh kg−1 is retained even at high power density of 9.9 kW kg−1, which exhibits extraordinary rate capacities. As a key factor when estimating the electrochemical performance of hybrid capacitor devices, long-term stability is valued at 4 A g−1 and shown in Fig. 6h, 92.7% of initial specific capacitance is retained after 4000 cycles.

The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51172085) and “863 Program” national project of China (No. 2013AA031903). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2020.113828.

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4. Conclusion

[25]

In summary, a porous low-crystalline FeOx@PPy nanorod arrays anode was successfully synthesized via electrochemical Li+ insertion and PPy coating. Taking advantage of the low-crystalline, interior ion diffusion channels and improved electrical conductivity, the obtained FeOx@PPy electrode exhibits a high enhanced specific capacitance of 592.8 F g−1, which is almost one order of magnitude larger than pristine Fe2O3, and excellent cycling stability (90.5% retention after 4000 cycles). Additionally, an High-voltage aqueous hybrid capacitor (Ni0.25Mn0.75)3O4@PPy// FeOx@PPy device was fabricated by employing the optimized Fex@PPy nanorod arrays electrode as anode and (Ni0.25Mn0.75)3O4@PPy electrode as cathode, which delivered an ultra-high voltage of 2.4 V and extraordinary energy density of 72.4 Wh kg−1 with a high power density of 9.9 kW kg−1, as well as impressive cycling stability of 4000 cycles, comparable to or better than many of the state-of-the-art devices reported recently. The present work not only provides a promising strategy of multimetal nanocomposites for new type aueous hybrid capacitors, but also is available for practical application.

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Xiaohui Li and Xing Zhou contributed equally to this work. 9

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