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Facile Electrochemical Fabrication of Porous Fe2O3 Nanosheets for Flexible Asymmetric Supercapacitor Ting Li, Hang Yu, Lei Zhi, Wenliang Zhang, Liqin Dang, Zong-Huai Liu, and Zhibin Lei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04330 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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The Journal of Physical Chemistry

Facile Electrochemical Fabrication of Porous Fe2O3 Nanosheets for Flexible Asymmetric Supercapacitor

Ting Li, Hang Yu, Lei Zhi, Wenliang Zhang, Liqin Dang, Zonghuai Liu, Zhibin Lei*

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, and School of Materials Science and Engineering, Shaanxi Normal University, 620 West Chang’an Street, Xi'an, Shaanxi, 710119, China, Fax: 86-29-81530702; Tel: 86-29-81530810; Email: [email protected]

*Corresponding Authors: Prof. Zhibin Lei, School of Materials Science and Engineering, Shaanxi Normal University, 620 West Chang’an Street, Xi'an, Shaanxi, 710119, China. Tel: 86-29-81530810; Fax: 86-29-81530702; Email: [email protected]

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ABSTRACT

Fe2O3 is one of promising negative electrodes for asymmetric supercapacitor. But the rather low conductivity and poor cycling stability hinder its practical application. Herein, we report a facile electrochemical deposition technique to prepare porous Fe2O3 nanosheets on carbon fabric (CF-Fe2O3). The obtained Fe2O3 displays highly porous nanosheet structure with reduced ion diffusion length and significantly enhanced conductivity. As a flexible supercapacitor electrode, the CF-Fe2O3 hybrid electrode delivers a large areal capacitance of 1.56 F cm−2, good rate capability and excellent cycling stability without capacitance decay after 5000 cycles. A packaged solid-state supercapacitor device based on the flexible CF-Fe2O3 electrode exhibits an areal capacitance of 842 mF cm−2, a volumetric energy density of 6.75 mWh cm−3, together with superior cycling performance which remains 93% capacitance after 4000 cycles. Moreover, the device performance is well retained under various bending and twisting states.

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1. INTRODUCTION The ever-increasing growth of flexible and portable electronics has triggered the rapid development of flexible energy storage devices that have high-energy density, fast charge-discharge rate and excellent cycling stability.1-4 Battery and supercapacitor have become two kinds of the most important energy storage devices that have received wide attentions over the past several years. In particular, supercapacitor hold great potential as backup power source in view of its fast energy delivery, quick charge-discharge rate and extraordinarily long cycling life. However, as compared with batteries, the low energy density of supercapacitor arising from double-layer charge storage largely restricts its wide practical applications.5-7 In general, energy density of a supercapacitor can be boosted by increasing the specific capacitance of electrode materials,8-10 or by configuring a supercapacitor cell with a wide-voltage-window. For example, the energy density of a supercapacitor could be significantly boosted when using the ionic liquid as the electrolytes.11,12 But ionic liquid has relatively low ion conductivity and slow ion kinetics due to its large ion size and the high viscosity, which largely limits its applications as high-rate supercapacitors. Alternatively, building an aqueous-based asymmetric supercapacitor (ASC) represents another efficient route to boosting the energy density.13-15 To this end, a good matching between positive and negative electrode is the key to maximize the performance of the device in aqueous electrolyte. Recently, although MnO2, double hydroxides16-20 and cobalt oxides21 have been widely explored as positive 3



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electrodes, most of current ASCs utilizes the low-capacity porous carbons as negative electrodes.22-24 This configuration leads to a low gravimetric and/or volumetric energy density of the device because more carbon negative electrode is required to balance the charges stored in the positive electrode. Iron oxides have emerging as one of promising negative electrodes for ASC with respect to its large theoretical capacity (3625 F g−1),25 nature abundance, stable and more negative working potential in alkaline electrolyte.26-32 As Fe2O3 electrodes store energy through reversible Faradaic redox reactions occurring at their outmost surface layers, great efforts have been devoted to integrating nanostructred Fe2O3 with various conductive substrates for getting a better performance. However, the performance of current Fe2O3 electrodes is still far from their practical applications due to their poor electrical conductivity (~10−14 S cm−1),33,34 low specific capacitance (120−300 F g−1),26,35 and limited electrochemical stability. Moreover, the mass loading of Fe2O3 on conductive substrates is rather low (~1.0 mg cm−2),36,37 and the interfacial interaction between the substrates and active Fe2O3 is not strong enough. These limitations largely hinder Fe2O3 from serving as negative electrode for high-performance ASCs. Herein, we report the highly porous Fe2O3 nanosheets vertically grown on carbon fabric (CF) by a facile electrochemical deposition technique in combination with a low-temperature post-annealing. The Fe2O3 on the conductive CF displays sheet-like and highly porous network which has significantly improved electrochemically active surface area and potentially reduced ion pathway length. Depending on the 4


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electrodeposition time, mass loading of Fe2O3 nanosheets on CF (denoted as CF-Fe2O3-x hybrid, with x representing the deposition time in hour) increases from 1.24 to 4.53 mg cm−2, while the sheet thickness of Fe2O3 varies from 17 to 41 nm and the conductivity of the hybrids is in the range of 351−628 S m−1. In particular, the CF-Fe2O3-4h deliver a maximal areal capacitance of 1.56 F cm−2 with 102% capacitance retention after 5000 cycles in 2.0 M KOH aqueous electrolyte. More importantly, a packaged flexible solid-state ASC device based on the CF-Fe2O3 hybrid can deliver a maximum specific energy density of 6.75 mWh cm−3, while retaining the normal electrochemical performance under various bending or twisting states. These capacitive performances are ascribed to the unique CF-Fe2O3 hybrid which is composed of highly porous Fe2O3 nanosheets strongly coupled on the interconnected and conductive CF substrates. 2. EXPERIMENTAL SECTION 2.1. Preparation of CF-Fe2O3 Negative Electrode The commercially available CF was first refluxed at 100 °C in a mixed solution containing 20 mL concentrated HNO3 and 60 mL concentrated H2SO4 for 5 h to get a hydrophilic surface nature. Growth of FeOOH nanosheets on CF was performed by a modified galvanostatic technique as reported previously.38 The CF with dimensional size of 2.5 × 1.0 cm2 was used as the working electrode, while Pt foil and Ag/AgCl were used as the counter and reference electrode, respectively. A mixed solution containing 0.2 M CH3COONa, 0.1 M Na2SO4 and 0.1 M Fe(NH4)2(SO4)2·6H2O was used as electrolyte. After electrodeposition at constant current density of 0.125 mA 5



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cm–2 for different durations, samples were ultrasonicated and dried at 40 °C. In the following step, samples were thermally annealed at 400 °C for 2.5 h (with a heating rate of 1 °C min−1) in flowing N2 to convert FeOOH into α-Fe2O3. The mass loading of α-Fe2O3 on CF before and after electrodeposition was determined using a microbalance with an accuracy of 0.01 mg. The composite electrodes were denoted as CF-Fe2O3-xh with x representing the deposition time in hours. 2.2. Preparation of CF-Co3O4 Positive Electrode For the deposition of Co3O4, 0.05 M Co(NO3)2·6H2O was used as aqueous electrolyte, while the potentiostatic technique at working potential of −1.0 V (vs Ag/AgCl) was applied.39 After electrochemical deposition for different time, the products were thermally annealed at 350 °C in air atmosphere for 150 min to convert cobalt oxide into Co3O4. By controlling the potentiostatic deposition time of 10, 20, 30 and 40 min, Co3O4 mass loading of 1.12, 2.02, 2.96 and 3.87 mg cm−2 on CF could be obtained, respectively, which were denoted as CF-Co3O4-xm with x representing the deposition time in minutes. 2.3. Characterization Methods The morphologies and structure of the samples were examined by field-emission scanning electron microscopy (FESEM) on SU8020 and field-emission transmission electron microscope (FETEM, Tecnai G2 F20 S) with an acceleration voltage of 200 kV. The Brunauer–Emmett–Teller (BET) surface area of samples were determined using the BET method with the adsorption data in the relative pressure (P/P0) range of 0.05–0.2. Pore size distribution (PSD) curves were derived from adsorption branch 6


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using the nonlocal density functional theory (NLDFT) model assuming the slit pore geometry. The phase structure of samples was characterized on a DX-2700 X-ray diffractometer with Cu Kα radiation of wavelength (λ = 0.154 nm). X-ray photoelectron spectroscopy (XPS) spectra were collected on an AXIS ULTRA spectrometer (Kratos Analytical) using a monochromatized Al Ka X-ray source (1486.71 eV). The surface wettability was evaluated by performing the static water contact angle measurement on the Dataphysics OCA 20 contact angel system at room temperature, with 2.0 µL water droplets as the indicator. The conductivity of all samples were measured by a standard four-probe technique. Conductivity for each sample was measured for five times to get an average value. The error bars represent the standard errors of the conductivity deviated from the average value. 2.4. Electrochemical Measurements The electrochemical performances of the electrode materials were evaluated in 2.0 M KOH aqueous electrolyte by a three-electrode system on a Gamry Reference 3000 electrochemical

workstation.

Cyclic

voltammetry

(CV),

galvanostatic

charge-discharge and electrochemical impedance spectroscopy (EIS) were measured. In a three-electrode cell, the obtained CF-Fe2O3 and CF-Co3O4 hybrids were directly used as working electrode, while Pt plate and Ag/AgCl electrode were used as the counter electrode and the reference electrodes, respectively. The areal specific capacitance, C (F cm–2) of the electrode material was calculated from the galvanostatic discharge curves according to the following equation: C = I × ∆t/(∆V × S), where I is the discharge current (A), ∆t is the discharge time (s), ∆V is the voltage 7



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change (V) excluding voltage drop (IR drop) in the discharge process, and S is the geometric area of the active material contacting with the electrolyte.40,41 2.5. Solid-State ASC Fabrication The flexible solid-state ASC was fabricated with CF-Co3O4-20m as positive electrode, CF-Fe2O3-4h as negative electrode and PVA-KOH as solid-state electrolyte. The size of each electrode is typical of 1.0 × 1.5 cm2. Gel electrolyte of PVA-KOH was prepared by adding 3 g PVA in 30 mL water, followed by heating at 85 °C under continually stirring to get a clear solution. Afterward, 30 mL of 2 M KOH solution was dropped and the mixture was stirred for another 1 min. Finally, the mixture was cooling down until a clear PVA-KOH sol was obtained. The CF-Co3O4-20m and CF-Fe2O3-4h electrode were immersed into the KOH-PVA sol for 3 min, followed by solidification at room temperature. This process was repeated for three times to ensure sufficient PVA-KOH electrolyte in the ASC cell. The cellulose film (TF4030 NKK) was used as separator to sandwich the two electrodes. After solidification of PVA-KOH electrolyte, a mechanical robust full cell could be packed and tested. The areal specific capacitance Ca (F cm–2) of the ASC was calculated according to the equation: Careal = I × ∆t/(∆V × S),20 where I, ∆t, ∆V and S are discharge current (A), discharge time (s), voltage change (V) excluding the IR drop, and the face-to-face area (cm2) of the positive and negative electrodes, respectively. Ragone plot correlating the volumetric energy (mWh cm–3) and power density (mW cm–3) was derived from E = CV × V 2/(2 × 3.6) and P = 3600 × E/∆t, where CV (F cm–3) is the volumetric capacitance of the full cell including the separator, and V (V) is the cell 8


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voltage excluding IR drop.16 3. RESULTS AND DISCUSSION 3.1 Preparation and Characterization of CF-Fe2O3 Hybrids For uniform deposition of Fe2O3 nanosheets on CF, the CF was treated in a mixed acids containing 20 mL concentrated HNO3 and 60 mL concentrated H2SO4 for 5 h. This treatment does not dramatically decrease the conductivity of CF (1373 vs 1210 S m−1) but can significantly improve the wetting ability as indicated by water contact angle measurement. As shown in Figure 1a, the contact angles of water on pristine CF shows a large angle of 130.2 ± 0.2°, which remains at 129.8 ± 0.2° after dropping water for 30 s, suggesting a highly hydrophobic surface nature of pristine CF. In contrast, water can rapidly spread over the acid-treated CF substrate once it contact with the substrate, implying an improved wetting ability after acid treatment (Figure 1a). Figure S1a compares the XPS spectra of CF before and after this treatment. The increase of oxygen atomic concentration from 5.8 to 17.1 at% confirms the present of numerous oxygen functional groups. These oxygen functional groups exist in the form of C–O at 286.2 eV, C=O at 287.1 eV, and –COOH at 289.0 eV (Figure S1b).42,43 In the following electrodeposition step, a galvanostatic technique was applied at a current density of 0.125 mA cm–2. The Fe3+ generated by electrochemical oxidation of Fe2+ reacts with OH− in the alkaline electrolyte to precipitate FeOOH on CF as confirmed by XRD in Figure S2a. Thereby, the whole electrodeposition process involves an electrochemical reaction (Fe2+ ↔ Fe3+ + e−), a precipitation reaction (Fe3+ + 3OH− ↔ Fe(OH)3) and a decomposition reaction (Fe(OH)3 ↔ FeOOH + H2O).38 In the second step, the CF-FeOOH was converted into CF-Fe2O3 by thermal annealing in 9



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flowing Ar,33,44 while the acid treated CF exhibited a negligible weight loss of ~2.5wt% at temperature

Facile Electrochemical Fabrication of Porous ... - ACS Publications

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