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Effect of simultaneous electrochemical deposition of manganese hydroxide and polypyrrole on structure and capacitive behavior
Journal Pre-proof Effect of simultaneous electrochemical deposition of manganese hydroxide and polypyrrole on structure and capacitive behavior
Tamene Tamiru Debelo, Masaki Ujihara PII:
S1572-6657(20)30008-4
DOI:
https://doi.org/10.1016/j.jelechem.2020.113825
Reference:
JEAC 113825
To appear in:
Journal of Electroanalytical Chemistry
Received date:
15 October 2019
Revised date:
10 December 2019
Accepted date:
4 January 2020
Please cite this article as: T.T. Debelo and M. Ujihara, Effect of simultaneous electrochemical deposition of manganese hydroxide and polypyrrole on structure and capacitive behavior, Journal of Electroanalytical Chemistry(2020), https://doi.org/ 10.1016/j.jelechem.2020.113825
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© 2020 Published by Elsevier.
Journal Pre-proof Effect of simultaneous electrochemical deposition of manganese hydroxide and polypyrrole on structure and capacitive behavior
Tamene Tamiru Debelo and Masaki Ujihara* e-mail: [email protected] Graduate Institute of Applied Science and Technology, National Taiwan University of Science
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and Technology, 43 Keelung Road, Taipei 10607, Taiwan, Republic of China
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Keywords: polypyrrole, manganese oxide, electrodeposition, pseudocapacitor, nanocomposite
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Abstract
Nanocomposite electrodes were prepared by single-phase galvanostatic electrodeposition on a
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stainless-steel mesh in an aqueous solution of manganese nitrate and pyrrole. On the anode, the deposited film (anodic film) was composed of spherical particle aggregates of almost pure
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polypyrrole, which strongly adhered to the stainless steel. In contrast, highly porous nanosheets of Mn(OH)2 effectively formed on the cathode and became Mn oxides with a trace of carbon
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material after calcination at 200 °C (calcined cathodic film). Both the obtained anode and calcined cathodic films on the stainless-steel meshes were characterized as capacitive materials. The anodic film showed capacitive behavior, while the calcined cathodic film exhibited a more pseudocapacitive nature. The specific capacitance of the anodic and calcined cathodic films was analyzed from the CV curves (510 and 270 F/g at 5 mV/s) and galvanostatic charge-discharge curves (430 and 220 F/g at current densities of 0.27 A/g and 0.23 A/g, respectively). During the cycle test, the calcined cathodic film gradually increased the capacitance due to ion diffusion into the MnO2 nanostructures over time, while the polypyrrole in the anodic film repeated adsorption/desorption of dopant ions during the redox processes, which led to degradation of the capacitive properties.
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1. Introduction Presently, in this field of study, considerable focus is paid to improving energy-storage devices to create portable, flexible, wearable, lightweight devices with reduced cost, enhanced efficiency, less pollution, and a reliable power supply [1]. Among them, supercapacitors have the advantages of high power density, less charge-discharge time compared to batteries, and superior energy density relative to conventional capacitors. The electrodes of supercapacitors have a high
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surface area, which allows rapid and substantial ion exchange as an electrical double layer capacitor (EDLC). Since supercapacitors store charge on the surfaces of the electrode materials,
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less electrode degradation happens relative to batteries, resulting in better cycle stability.
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However, the reaction on the surface limits the capacity. The energy density of a supercapacitor is several orders lower than that of a battery. To solve this drawback of supercapacitor
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electrodes, various energy storage mechanisms have been suggested, such as formation of an electrochemical double layer, which stores energy at the interface of the electrode/electrolyte;
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pseudocapacitor electrodes, which store energy on the electrode surface by a Faradic process; and battery-type electrodes in which the specific capacitance is potential window dependent and
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the electrode is assembled using hybrid and electrochemical double layer electrodes [2-4]. The most commonly suggested material for pseudocapacitors is conductive polymers such as
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polyaniline and polypyrrole (PPy), which possess advantages such as high conductivity,
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flexibility, simple production, and low price. However, they are not yet marketed because of their low cycle stability. Among conductive polymers, PPy films produced by electrochemical deposition techniques from aqueous solutions with TsO-, SO4- and NO3- have been studied [5, 6]. Galvanostatic and potentiostatic electropolymerization of PPy films on stainless steel in a nitratecontaining solution has been reported to result in better conductivity and adherence [7, 8]. In addition, the galvanostatic electrochemical polymerization of Ppy at different current densities can lead to different conductivities in anionic surfactants and result in superior conductivity in alkaline aqueous solutions. To facilitate Faradic and non-Faradic reactions on an electrode, structural control of PPy films is also being developed. Porous nanobelt, nanobrick, and nanosheet films of PPy have been electrodeposited in neutral NO3- media using voltammetry experiments [8, 9] [10, 11]. However, MnO2 and Mn(OH)2 are also promising materials for
Journal Pre-proof pseudocapacitors. Different from other metal oxides, the redox reactions of Mn oxides occur over a wide range of potentials and allow the pseudocapacitive charge-discharge property [12]. The disadvantage of these Mn compounds is their low conductivity [13]. Therefore, Mn compounds are hybridized with other flexible and highly conductive materials [14]. Nanocomposites of MnO2 with PPy are obtained by chemical methods [15]. The nanostructure of hybrid materials is crucial for achieving rapid electrochemical reactions on an electrode; however, most of the materials consist of spherical particles of Mn oxides [15, 16]. Nanorods and nanosheets of MnO2 have also been designed, but they require complicated processes for
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hybridization with other components [17, 18]. As a convenient synthesis method, nanostructured
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MnO2 and Mn(OH)2 can be electrochemically deposited at once on the anode and cathode in a single experiment [19].
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Furthermore, the effect of current density should be considered to avoid the dissolution of
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metallic electrodes. The macroscopic structure of electrodes is also important for device stability. Recent progress on supercapacitors has included the use of three-dimensional (3D) printing
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technology for electrode fabrication. This technology is promising for extending the surface area and high mass loading of active materials on flexibly designed electrodes. [20, 21] In this study,
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we examine a simple method of electrochemical deposition of active materials on a stainlesssteel mesh to achieve 3D structure.
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Thus, in this study, we propose a simple method to prepare flexible and binder-free electrodes
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for pseudocapacitors. PPy and Mn oxides are simultaneously deposited on stainless-steel mesh as electrodes in neutral media. The materials deposited on two electrodes (anode and cathode) were characterized, and their electrochemical properties as the active materials of pseudocapacitors were examined. The results will be helpful for designing nanocomposite electrodes for supercapacitors. 2. Experimental Section 2.1 Materials and Characterization: Pyrrole (99 %), isopropanol (99.5 %), Mn(NO3)2, HNO3 (69 %), KNO3, and acetone were received from Acros Organics UK. Both working and counter electrodes were prepared from stainless-steel mesh (SS: SUS316L and 500 mesh). Ultra-pure water with a resistivity of 18.2
Journal Pre-proof MΩcm was freshly prepared with an apparatus (Yamato, Japan) and used throughout our experiments. Each chemical was used without further purification. The surface morphology of the materials was observed by scanning electron microscopy (SEM), and elemental analysis was also carried out by energy dispersive X-ray spectroscopy (EDS) using field emission scanning electron microscopy (FE-SEM; JEOL JSM-6500F). Crystal structure was investigated by X-ray diffraction (XRD; Bruker, D8) in the range of 2θ = 10° to 70°, employing Cu kα radiation with a wavelength of 0.15406 nm with 40 kV and 200 mA.
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Functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR; Nicolet Thermo Scientific 6700). The sample was mixed with KBr, pressed into a pellet and analyzed in
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transmittance mode.
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2.2 Synthesis of the PPy film and Mn(OH)2 electrodes
The active materials were simultaneously synthesized under ambient temperature in alkaline
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media following a similar procedure as previously reported with some modification [9, 19]. First,
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two SS meshes (1 cm x 4 cm) were mechanically polished and washed with water, HNO3 (10 %), acetone, and isopropanol, sequentially, for 10 minutes each and dried in vacuum overnight.
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Second, the mass of each bare SS was recorded before electrode deposition. Third, an aqueous solution (60 ml) of KNO3 (0.1 M), MnNO3 (0.1 M), and pyrrole monomer (0.1 M) was prepared.
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Then, 20 ml of the mixture was transferred to a glass container with the three electrode system; two SS meshes for both the working electrode and counter electrode along with a reference
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electrode (Ag/AgCl in 3 M KCl), and 1 cm2 of each SS electrode (1 cm x 1 cm) was immersed in the electrolyte. The remaining SS on the electrolyte was connected to the external circuit. Then, galvanostatic electrodeposition was performed at a current density of 5 mA/cm2 for 15 minutes using a galvanostat/potentiostat (Hokuto-Denko, model HA-151B) under moderate stirring. After deposition, the working electrode (WE) and the counter electrode (CE) were rinsed with water and dried in vacuum overnight. The CE was further calcined for 1 hour at 200 °C in open air. Finally, the mass of each electrode was recorded to obtain the masses of active material deposited on each electrode. 2.3 Electrochemical measurements
Journal Pre-proof The electrochemical performance of the obtained electrodes was checked by a conventional three-electrode system in an aqueous solution of KNO3 (2 M) for the film on the WE and Na2SO4 (1 M) for the film on the CE, where Ag/AgCl (3 M KCl) and platinum foil were used as the reference electrode and CE, respectively. The cyclic voltammetry (CV) curves were obtained in the range of 0 to 0.8 V at different scan rates for each electrode. The galvanostatic chargedischarge (GCD) measurements were also performed at different current densities within the same potential range as the CV measurements vs the Ag/AgCl reference electrode. The electrochemical impedance spectroscopy (EIS) results were performed at an amplitude of 10 mV
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conducted at
ambient
temperature
using
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Zahner CIMPS-X
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measurements
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in the frequency range from 0.053 Hz to 100 kHz and described in terms of a Nyquist plot. All
photoelectrochemical work station together with the Thales software package (Xpot-26366).
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3.1 Electrodeposition on the WE and CE
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3. Results and discussion
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In a mixture of pyrrole and manganese salt, PPy, Mn oxides, and Mn hydroxides could be electrochemically deposited on the electrodes. The WE was set as the anode (the potential varied
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to satisfy the current). In this study, pyrrole could be electrochemically polymerized by the following mechanism: In the first phase, the pyrrole monomer is oxidized to form a radical
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cation, which is followed by the formation of pyrrole dimers. In the second phase, the large conjugation in the dimer makes oxidation easier, and oxidation is followed by the formation of
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trimers, oligomers, and PPy as black films [9, 22]. Mn2+ could also be oxidized to Mn3+ and Mn4+, which would precipitate as MnOOH, Mn2O3, Mn3O4, MnO(OH)2, and MnO2 at a neutral pH [23-25]. In contrast, the potential of the CE was reductive. Therefore, NO3- could be reduced into nitrite and OH-, which resulted in the precipitation of Mn(OH)2 on the CE [19]. During the reaction, the electrolyte changed from pale pink to yellow and finally dark brown. This change suggests that the reaction products, such as the pyrrole oligomer and Mn hydroxides/oxides, were partially soluble in water and homogeneously mixed in the reaction system. After deposition, a black film was observed on the WE, and a brown film was formed on the CE. Notably, the film on the CE did not stably form on the SS when the electrochemical deposition was carried out without pyrrole. After annealing the CE at 200 °C, the film was black. The average mass of the active material deposited on the WE and CE was approximately 2.6 mg and
Journal Pre-proof 1.3 mg, respectively. Hereafter, the film formed on the WE is called the anodic film, and the annealed film developed on the CE is calcined cathodic film. 3.2 Morphological, functional group, structural, and elemental characterization SEM images of the anodic film are shown in Figure 1(a, b). Aggregates of spherical PPy nanoparticles were observed on the surface of the SS electrodes. The morphology of the PPy film resembled that of the PPy films chemically deposited on the SS in NO3- and MoO42- electrolyte media [7]. The cross-sectional analyses revealed the formation of strongly adhered PPy films on
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the SS with an average thickness of ~2 μm. On the other hand, flower-like nanosheets were observed on the CE after the heat treatment, as reported (Figure 1c) [19]. The cross-sectional
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SEM indicated that the nanosheets developed vertically with an average thickness of ~2 µm on
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the CE (Figure 1d) as well. These structures were not substantially different from the noncalcined film (see Figure S1 in supporting information). EDS analysis was performed on both
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electrodes to analyze the elemental composition (Figure 1e and f). In addition to the metallic elements from SS (Fe, Cr, and Ni), other elements were confirmed: the anodic film exhibited
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atomic percentages of C (67.11 %), N (18.59 %), and O (14.31 %), while the calcined cathodic film contained C (22.38 %), O (52.79 %), and Mn (24.83 %). The cathodic film before
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calcination also consisted of C, O, and Mn (Figure S1e and f in supporting information): C (46.14 %), O (41.71 %) and Mn (12.15 %). The lower Mn ratio before calcination suggests the
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decomposition of organic compounds and dehydration of Mn compounds during the calcination. Each spectrum indicated that the film that formed on the WE was almost pure PPy with NO3-
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ions as a dopant [26], and the calcined cathodic film consisted mainly of Mn oxides (Mn:O ≈ 1:2) and carbon materials (hereafter, MnO2@carbon). The unassigned peaks corresponded to substrate constituent alloys such as iron, chromium, zirconium and platinum. The carbon materials could be byproducts of the PPy synthesis, such as PPy oligomers. The film on the CE did not stably form on the SS without pyrrole in the synthesis system, and the carbon materials acted as the binder.
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Figure 1 SEM images of the (a) anodic film from the top view and (b) cross-sectional view; (c)
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anodic and (f) calcined cathodic films.
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calcined cathodic film from the top view and (d) cross-sectional view; EDX spectra of the (e)
The functional groups in the films were characterized by FTIR (Figure 2). The anodic film
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exhibited a weak band at 790 cm-1, which indicated the NH2 vibration of pyrrole, while the band at 930 cm-1 could be assigned to the in-plane bending vibration of C=C (Figure 2a). The band at
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approximately 1045 cm-1 could be assigned to the in-plane deformation of C-H and N-H. In addition to these characteristic bands, the bands at approximately 1192 cm-1, 1384 cm-1, and
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1552 cm-1 and a broad band at 3400-3700 cm-1 could be assigned to C-N stretching, C-N stretching or interlayer vibration modes of doped NO3- [9, 27], pyrrole ring stretching (C=C and C-C), and –OH groups, respectively, confirming PPy formation [28-30]. From Figure 2b, the two high bands at approximately 505 and 623 cm-1 are the typical absorption bands of the Mn-O bond: the vibration of octahedral [MnO6] in MnO2 [31]. Although the band at 623 cm-1 was also observed for the anodic film, no significant absorption was observed at 505 cm-1. The bands at 1095 and 1630 cm-1 corresponded to Mn-O-H vibration [32]. The bands at 1384 cm-1 and 34003700 cm-1 could be assigned to nitrate, as mentioned above. The band at 2933 cm-1 belongs to CH stretching, which corresponds to the carbon materials and is consistent with the EDX spectrum [28]. Before calcination, the absorption bands of the materials resembled those of calcined film (Figure 2c), except the widening of the band around 1130 cm-1 and the intensification of band at
Journal Pre-proof 1630 cm-1 of the Mn-O-H peak against the bands at 505 and 623 cm-1. These changes suggest that more hydroxyl groups existed in the cathodic film before calcination as expected from the
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EDX results described above (Figure S1f in the supporting information).
Figure 2 FT-IR spectra of materials scraped from the (a) anodic film, (b) calcined cathodic film
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and (c) cathodic film before calcination.
The crystal structure of the film grown on both SS meshes was confirmed by XRD (Figure 3).
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Figure 3a shows the XRD pattern of the anodic film, and the broad peak centered at approximately 25° is the typical peak of amorphous PPy [22]; the remaining two sharp and
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intense peaks at 44° and 50° correspond to the SS substrate [33]. The XRD pattern of the
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calcined cathodic film has broad peaks, which suggest small crystal grains (Figure 3b). The highly intense peaks at approximately 44° and 50° are from the SS mesh, as explained above, while the peaks at 19°, 29°, 34°, 35.5°, 59°, and 65° correspond to the α-MnO2 planes of (200), (310), (450), (211), (521), and (002), respectively [34]. Throughout these characterizations, Mn compounds were not observed in the anodic film, which could be explained by two reasons: (1) PPy is electropolymerized by initiation with a cation radical that is positively charged and repels Mn2+ via electrostatic repulsive forces [35]. (2) Even if Mn4+ formed on the anode, Mn4+ could oxidize pyrrole and be reduced to water-soluble Mn2+ [29].
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Figure 3 XRD patterns of the (a) anodic film and (b) calcined cathodic film.
3.3 Electrochemical performance
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The CV measurement was performed with the anodic film at different scan rates in an aqueous
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electrolyte (2 M KNO3) from 0 to 0.8 V (Figure 4a). The CV curve was nearly rectangular at a low scan rate. The rise in the scan rate caused a minor distortion in the rectangular shape due to
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the ohmic polarization of PPy [36]. The diffusion rate of ions could also limit the specific capacitance [37]. The rapid increase in the oxidative wave from 0.7 V indicates oxidation of PPy
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[7]. Figure 4b shows the CV curve for the calcined cathodic film. The CV curve deviated from
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the rectangular shape, which indicated Faradic reactions at the electrode.
Figure 1 CV curves: (a) for the anodic film and (b) calcined cathodic film at different scan rates. The electrochemical behavior of the films was also measured by the GCD method (Figure 5). The GCD curves for the anodic film were nearly symmetrical triangles, which indicated the capacitive behavior of the electrode (Figure 5a). A slightly curved arc was observed at 0.7 V vs Ag/AgCl due to the PPy oxidation as the CV curves illustrate above [7]. The GCD curve at high
Journal Pre-proof current density was more symmetrical than at low current density due to reduced degradation of PPy in a short time. In contrast, the GCD curve of MnO2@carbon at different current densities ranging from 0.23 to 0.77 A/g exhibited a deviation from the symmetrical triangular shapes, which suggested the
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pseudocapacitive nature of this material at lower current densities (Figure 5b).
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Figure 5 GCD curves of the (a) anodic film at different current densities from 0.27 to 3.08 A/g
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and the (b) calcined-cathodic film at current densities from 0.23 to 0.77 A/g.
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The specific capacitance for each electrode was determined from the CV and GCD
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∫
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measurements using equations 1 and 2, respectively [38]. equation 1 equation 2
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where Cg is the gravimetric specific capacitance (F/g), I is the discharge current (A), t is the discharge time (s), m is the mass of the active or loaded electrode (g), v is the scan rate (mV/s), and V is the potential window (V). The specific capacitances obtained from the CV curves are 510 and 270 F/g at 5 mV/s, while those from the GCD curve are 430 and 220 F/g at a current density of 0.27 A/g and 0.23 A/g for the anodic and calcined cathodic electrodes, respectively (Figure 4, 5). The stability of each electrode was also evaluated by the capacitance retention of the materials (Figure 6). For the anodic film, a GCD cycle was performed in the range from 0 to 0.8 V at 11 A/g, and the capacitance retention decreased to 62 % after 2,000 cycles (Fig 6a). The 38 % loss in the cycling test could be attributed to 3 reasons: (1) deformation and collapse of the PPy film
Journal Pre-proof caused by mechanical stress in the dedoping process of NO3- [39], (2) damage to the polymer chain, such as over-oxidation of pyrrole due to monomer incorporation in the polymer matrix chain [40] or low pH effects under over-oxidative conditions [41] and reactions with oxygen, and (3) dissolution of the SS-based electrode [5]. However, no detachment of the active material was observed after repeatedly drying and bending the electrode before the electrochemical performance test, which indicated that the anodic film was sufficiently adherent and tough, as confirmed by the cross-sectional SEM image (Figure 1). As mentioned above, a potential above 0.7 V would cause the oxidation of PPy and result in low capacitance retention. The GCD cycles
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at lower range (from -0.2 to 0.6 V) could decrease the oxidation of PPy, and the capacitance
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retention remained at 76.5 % after 1,600 cycles (see Figure S2 in the supporting information). Figure 6b compares the CV curves at a scan rate of 5 mV/s of an anodic film before and after
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GCD for 2,000 cycles at 11 A/g in a potential window of 0 to 0.8 V, resulting in 69 % capacitance retention, consistent with the GCD result. The CV curve after the GCD cycles was
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reduced relative to that for a fresh electrode; however, the shape was still rectangular even after
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2,000 cycles, indicating that the characteristics of the material were not drastically changed.
Figure 6 (a) Capacitance retention vs cycle number for an anodic film where the insets are the GCD curves for the first and last 6 cycles in a potential window of 0 to 0.8 V, (b) CV curves of an anodic film at a 5 mV/s scan rate before and after 2,000 GCD cycles, (c) GCD curve of an anodic film at 0.4 A/g before and after the 1,000 cycle stability test, and (d) specific capacitance vs. cycle number for the calcined cathodic film where the insets are the first (left) and last (right) cycle GCD curves. A comparison of the GCD curves for the same electrode obtained before and after GCD cycling at 0.4 A/g is shown in Figure 6c. The curves revealed that the discharge time for the electrode
Journal Pre-proof before the cycle test was 656 seconds, while it was 608 seconds for the same electrode after the cycle test, which demonstrates a capacitance retention of 93 %. This higher capacitance retention (93 % at 0.4 A/g and 77 % at 11 A/g after 1,000 cycles) indicates that the high current density results in apparent degradation of the electrode, which can be avoided at low current density. This mechanism may be due to the dedoping process of counter ions, as mentioned above. On the other hand, the specific capacitance of the calcined cathodic film changed differently. In the very early stage (cycle number < 16), the specific capacitance decreased, which could be attributed to a partial corruption of the film by swelling-shrinking behavior during the charge-
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discharge process. Then, the film adapted, and the specific capacitance gradually increased with
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the cycle number. The highest specific capacitance was obtained after approximately 1,000 cycles (Figure 6d). This improvement was due to the complete activation of the highly porous
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structure of the electrode (see Figure 1); ion intercalation into the inner part of the active material
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needed sufficient time [42, 43]. The GCD curves for the calcined cathodic film showed that greater than 90 % of the maximum capacitance was maintained in the range of 800-1,551 cycles
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at 0.78 A/g. After 1,200 cycles, a gradual decay of the capacitance retention was observed, which could be attributed to the degradation of the active materials. A thick film with nanostructures
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can be unstable under charge-discharge cycles. Although greater mass loading on the electrode will result in higher capacitance, a thinner film facilitates ion diffusion into the inner part of the
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active electrode and increases the specific capacitance. [44,45] Therefore, the film structure should be considered to balance the properties. Throughout the cycle tests, the charging-
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discharging times for both electrodes were close to each other (insets in Figure 6a and d), indicating the high columbic efficiency and good reversibility of the electrodes [46]. The columbic efficiency was determined from the ratio of the discharge time to the charging time [47]. The anodic film had a columbic efficiency of ~97 % in the first 2,000 cycles, while that of the calcined cathodic film was 95.4 % in the first 1,551 cycles. The lower columbic efficiency of MnO2@carbon could be due to the Faradic reaction of MnO2. To analyze the charge-transfer mechanism, EIS was also measured and plotted as a Nyquist plot (Figure 7).
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Figure 7 EIS curves of the (a) anodic film and (b) calcined cathodic film before and after chargedischarge cycles. The model for analyses is inserted in b. The EIS curve of the anodic film before the cycle test contained an offset region with an xintercept (real axis-intercept), which included the electrolyte resistance (Rs), electrode resistance,
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contact resistance between the electrode and current collector, and current collector resistance.
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The semicircular arc in the high-frequency region corresponds to the charge transfer resistance (Rct) between the electrode/electrolyte interface, [48] and the slope of the Warburg resistance
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region connects to a vertical line in the low-frequency region [49]. Using a circuit model, as inserted in Figure 7b, the analysis of this material gave the resistance components of Rs = 1.35 Ω
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and Rct = 0.6 Ω for the electrode before the cycle test, and Rs = 4.9 Ω and Rct = 1.54 Ω for the
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electrode after 2,000 GCD cycles in potential window of 0 to 0.8 V. The low charge-transfer resistance reveals the high capacitive nature of the material, while the increase in Rct after the
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cycle test indicates lower capacitive performance after the test, which coincide with the CV and GCD curves. This degradation is likely due to the electrochemical stability of PPy and
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expansion/contraction during ion intercalation/deintercalation [50]. In contrast, the Nyquist plot of MnO2@carbon in the 1 M Na2SO4 electrolyte showed a larger
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semicircular arc before the cycle test, which indicated a higher Rct between the electrode and the electrolyte (Figure 7b). The resistance of this electrode was respectively analyzed as Rs = 0.9 Ω and Rct = 16 Ω before the cycle test and Rs = 2.11 Ω and Rct = 28.92 Ω after the cycle test. Although degradation was observed in the MnO2@carbon, its ratio was lower than that of PPy. The high Rct contributed to the lower capacitive behavior and more Faradic nature of this electrode, as confirmed by the CV and GCD curves. The linear line with a low slope in the lowfrequency region suggested the complex structures of the electrode, which limited the ionic diffusion. The slope became steeper after the cycle test, suggesting a change in electrode morphology. 4. Conclusion
Journal Pre-proof The in situ hybridization of PPy and MnO2 on SS meshes was examined by electrochemical deposition of the active materials in an aqueous mixture of pyrrole and Mn(NO3)2. Aggregates of spherical PPy nanoparticles were formed on the anode during single-phase galvanostatic electrodeposition at a current density of 5 mA/cm2, while flower-like nanosheets of Mn(OH)2 simultaneously developed on the cathode and changed to MnO2@carbon after calcination at 200 °C. For the deposition of Mn(OH)2 on the cathode, the carbon materials, which could be byproducts of the PPy synthesis, worked as the binder to stabilize the nanosheets on the SS base. The electrochemical characterization (CV, GCD, and EIS) of the PPy film indicated typical
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capacitive behavior, while the less conductive Mn(OH)2@carbon exhibited a less capacitive
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nature. The specific capacitances of these two electrodes (PPy and MnO2@carbon) were calculated to be 430 F/g and 220 F/g at a current density of 0.27 A/g and 0.23 A/g, respectively,
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from the GCD curves, and the CV measurements were 510 F/g and 270 F/g at 5 mV/s, respectively. The relatively high specific capacitance of PPy could be due to its high
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conductivity. The nanosheet formation of MnO2@carbon as a stable film on the electrode was
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determined to be an advantage in preparing the MnO2 nanostructure. The synthesis method used in this study is simple and can also be applied to other nanocomposites of conducting polymers
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and metal oxides, such as polyaniline and Ni/Co oxides.
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Funding: This work was supported by the Ministry of Science and Technology, R.O.C. [MOST
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108-2221-E-011-093], and the Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology. References 1. 2. 3. 4. 5.
Mecklenburg, M., et al., Aerographite: ultra lightweight, flexible nanowall, carbon microtube material with outstanding mechanical performance. 2012. 24(26): p. 3486-3490. Augustyn, V., et al., Pseudocapacitive oxide materials for high-rate electrochemical energy storage. 2014. 7(5): p. 1597-1614. Dubal, D.P., et al., Hybrid energy storage: the merging of battery and supercapacitor chemistries. 2015. 44(7): p. 1777-1790. Jiang, W., et al., Investigation on electrochemical behaviors of NiCo 2 O 4 battery-type supercapacitor electrodes: the role of an aqueous electrolyte. 2017. 4(10): p. 1642-1648. Ferreira, C., et al., Electrosynthesis of strongly adherent polypyrrole coatings on iron and mild steel in aqueous media. 1996. 41(11-12): p. 1801-1809.
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Mei, B.-A., et al., Physical interpretations of Nyquist plots for EDLC electrodes and devices. 2017. 122(1): p. 194-206. Huang, Y., et al., Nanostructured polypyrrole as a flexible electrode material of supercapacitor. 2016. 22: p. 422-438.
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Journal Pre-proof Tamene Tamiru Debelo: Data curation, Investigation, Formal analysis, and Writing- Original draft preparation.
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Masaki Ujihara: Conceptualization, Methodology, Resources, Supervision, Writing- Reviewing and Editing, Visualization, Project administration, Funding acquisition
Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphical abstract
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Highlights:
From a mixture of pyrrole and Mn2+, active materials were simultaneously deposited.
Polypyrrole aggregates on anode and Mn(OH)2 nanosheets were respectively formed.
The polypyrrole exhibited the specific capacitance of 510 F/g at 5 mV/s.
Calcination converted the Mn(OH)2 nanosheet to Mn oxide with carbonaceous material.
Charge-discharge cycles improved the capacitance of Mn oxide by the ion diffusion.
Jo
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Tamene Tamiru Debelo, Masaki Ujihara PII:
S1572-6657(20)30008-4
DOI:
https://doi.org/10.1016/j.jelechem.2020.113825
Reference:
JEAC 113825
To appear in:
Journal of Electroanalytical Chemistry
Received date:
15 October 2019
Revised date:
10 December 2019
Accepted date:
4 January 2020
Please cite this article as: T.T. Debelo and M. Ujihara, Effect of simultaneous electrochemical deposition of manganese hydroxide and polypyrrole on structure and capacitive behavior, Journal of Electroanalytical Chemistry(2020), https://doi.org/ 10.1016/j.jelechem.2020.113825
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© 2020 Published by Elsevier.
Journal Pre-proof Effect of simultaneous electrochemical deposition of manganese hydroxide and polypyrrole on structure and capacitive behavior
Tamene Tamiru Debelo and Masaki Ujihara* e-mail: [email protected] Graduate Institute of Applied Science and Technology, National Taiwan University of Science
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Keywords: polypyrrole, manganese oxide, electrodeposition, pseudocapacitor, nanocomposite
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Abstract
Nanocomposite electrodes were prepared by single-phase galvanostatic electrodeposition on a
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stainless-steel mesh in an aqueous solution of manganese nitrate and pyrrole. On the anode, the deposited film (anodic film) was composed of spherical particle aggregates of almost pure
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polypyrrole, which strongly adhered to the stainless steel. In contrast, highly porous nanosheets of Mn(OH)2 effectively formed on the cathode and became Mn oxides with a trace of carbon
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material after calcination at 200 °C (calcined cathodic film). Both the obtained anode and calcined cathodic films on the stainless-steel meshes were characterized as capacitive materials. The anodic film showed capacitive behavior, while the calcined cathodic film exhibited a more pseudocapacitive nature. The specific capacitance of the anodic and calcined cathodic films was analyzed from the CV curves (510 and 270 F/g at 5 mV/s) and galvanostatic charge-discharge curves (430 and 220 F/g at current densities of 0.27 A/g and 0.23 A/g, respectively). During the cycle test, the calcined cathodic film gradually increased the capacitance due to ion diffusion into the MnO2 nanostructures over time, while the polypyrrole in the anodic film repeated adsorption/desorption of dopant ions during the redox processes, which led to degradation of the capacitive properties.
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1. Introduction Presently, in this field of study, considerable focus is paid to improving energy-storage devices to create portable, flexible, wearable, lightweight devices with reduced cost, enhanced efficiency, less pollution, and a reliable power supply [1]. Among them, supercapacitors have the advantages of high power density, less charge-discharge time compared to batteries, and superior energy density relative to conventional capacitors. The electrodes of supercapacitors have a high
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surface area, which allows rapid and substantial ion exchange as an electrical double layer capacitor (EDLC). Since supercapacitors store charge on the surfaces of the electrode materials,
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less electrode degradation happens relative to batteries, resulting in better cycle stability.
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However, the reaction on the surface limits the capacity. The energy density of a supercapacitor is several orders lower than that of a battery. To solve this drawback of supercapacitor
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electrodes, various energy storage mechanisms have been suggested, such as formation of an electrochemical double layer, which stores energy at the interface of the electrode/electrolyte;
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pseudocapacitor electrodes, which store energy on the electrode surface by a Faradic process; and battery-type electrodes in which the specific capacitance is potential window dependent and
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the electrode is assembled using hybrid and electrochemical double layer electrodes [2-4]. The most commonly suggested material for pseudocapacitors is conductive polymers such as
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polyaniline and polypyrrole (PPy), which possess advantages such as high conductivity,
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flexibility, simple production, and low price. However, they are not yet marketed because of their low cycle stability. Among conductive polymers, PPy films produced by electrochemical deposition techniques from aqueous solutions with TsO-, SO4- and NO3- have been studied [5, 6]. Galvanostatic and potentiostatic electropolymerization of PPy films on stainless steel in a nitratecontaining solution has been reported to result in better conductivity and adherence [7, 8]. In addition, the galvanostatic electrochemical polymerization of Ppy at different current densities can lead to different conductivities in anionic surfactants and result in superior conductivity in alkaline aqueous solutions. To facilitate Faradic and non-Faradic reactions on an electrode, structural control of PPy films is also being developed. Porous nanobelt, nanobrick, and nanosheet films of PPy have been electrodeposited in neutral NO3- media using voltammetry experiments [8, 9] [10, 11]. However, MnO2 and Mn(OH)2 are also promising materials for
Journal Pre-proof pseudocapacitors. Different from other metal oxides, the redox reactions of Mn oxides occur over a wide range of potentials and allow the pseudocapacitive charge-discharge property [12]. The disadvantage of these Mn compounds is their low conductivity [13]. Therefore, Mn compounds are hybridized with other flexible and highly conductive materials [14]. Nanocomposites of MnO2 with PPy are obtained by chemical methods [15]. The nanostructure of hybrid materials is crucial for achieving rapid electrochemical reactions on an electrode; however, most of the materials consist of spherical particles of Mn oxides [15, 16]. Nanorods and nanosheets of MnO2 have also been designed, but they require complicated processes for
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hybridization with other components [17, 18]. As a convenient synthesis method, nanostructured
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MnO2 and Mn(OH)2 can be electrochemically deposited at once on the anode and cathode in a single experiment [19].
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Furthermore, the effect of current density should be considered to avoid the dissolution of
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metallic electrodes. The macroscopic structure of electrodes is also important for device stability. Recent progress on supercapacitors has included the use of three-dimensional (3D) printing
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technology for electrode fabrication. This technology is promising for extending the surface area and high mass loading of active materials on flexibly designed electrodes. [20, 21] In this study,
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we examine a simple method of electrochemical deposition of active materials on a stainlesssteel mesh to achieve 3D structure.
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Thus, in this study, we propose a simple method to prepare flexible and binder-free electrodes
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for pseudocapacitors. PPy and Mn oxides are simultaneously deposited on stainless-steel mesh as electrodes in neutral media. The materials deposited on two electrodes (anode and cathode) were characterized, and their electrochemical properties as the active materials of pseudocapacitors were examined. The results will be helpful for designing nanocomposite electrodes for supercapacitors. 2. Experimental Section 2.1 Materials and Characterization: Pyrrole (99 %), isopropanol (99.5 %), Mn(NO3)2, HNO3 (69 %), KNO3, and acetone were received from Acros Organics UK. Both working and counter electrodes were prepared from stainless-steel mesh (SS: SUS316L and 500 mesh). Ultra-pure water with a resistivity of 18.2
Journal Pre-proof MΩcm was freshly prepared with an apparatus (Yamato, Japan) and used throughout our experiments. Each chemical was used without further purification. The surface morphology of the materials was observed by scanning electron microscopy (SEM), and elemental analysis was also carried out by energy dispersive X-ray spectroscopy (EDS) using field emission scanning electron microscopy (FE-SEM; JEOL JSM-6500F). Crystal structure was investigated by X-ray diffraction (XRD; Bruker, D8) in the range of 2θ = 10° to 70°, employing Cu kα radiation with a wavelength of 0.15406 nm with 40 kV and 200 mA.
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Functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR; Nicolet Thermo Scientific 6700). The sample was mixed with KBr, pressed into a pellet and analyzed in
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transmittance mode.
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2.2 Synthesis of the PPy film and Mn(OH)2 electrodes
The active materials were simultaneously synthesized under ambient temperature in alkaline
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media following a similar procedure as previously reported with some modification [9, 19]. First,
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two SS meshes (1 cm x 4 cm) were mechanically polished and washed with water, HNO3 (10 %), acetone, and isopropanol, sequentially, for 10 minutes each and dried in vacuum overnight.
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Second, the mass of each bare SS was recorded before electrode deposition. Third, an aqueous solution (60 ml) of KNO3 (0.1 M), MnNO3 (0.1 M), and pyrrole monomer (0.1 M) was prepared.
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Then, 20 ml of the mixture was transferred to a glass container with the three electrode system; two SS meshes for both the working electrode and counter electrode along with a reference
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electrode (Ag/AgCl in 3 M KCl), and 1 cm2 of each SS electrode (1 cm x 1 cm) was immersed in the electrolyte. The remaining SS on the electrolyte was connected to the external circuit. Then, galvanostatic electrodeposition was performed at a current density of 5 mA/cm2 for 15 minutes using a galvanostat/potentiostat (Hokuto-Denko, model HA-151B) under moderate stirring. After deposition, the working electrode (WE) and the counter electrode (CE) were rinsed with water and dried in vacuum overnight. The CE was further calcined for 1 hour at 200 °C in open air. Finally, the mass of each electrode was recorded to obtain the masses of active material deposited on each electrode. 2.3 Electrochemical measurements
Journal Pre-proof The electrochemical performance of the obtained electrodes was checked by a conventional three-electrode system in an aqueous solution of KNO3 (2 M) for the film on the WE and Na2SO4 (1 M) for the film on the CE, where Ag/AgCl (3 M KCl) and platinum foil were used as the reference electrode and CE, respectively. The cyclic voltammetry (CV) curves were obtained in the range of 0 to 0.8 V at different scan rates for each electrode. The galvanostatic chargedischarge (GCD) measurements were also performed at different current densities within the same potential range as the CV measurements vs the Ag/AgCl reference electrode. The electrochemical impedance spectroscopy (EIS) results were performed at an amplitude of 10 mV
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conducted at
ambient
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using
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Zahner CIMPS-X
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measurements
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in the frequency range from 0.053 Hz to 100 kHz and described in terms of a Nyquist plot. All
photoelectrochemical work station together with the Thales software package (Xpot-26366).
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3.1 Electrodeposition on the WE and CE
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3. Results and discussion
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In a mixture of pyrrole and manganese salt, PPy, Mn oxides, and Mn hydroxides could be electrochemically deposited on the electrodes. The WE was set as the anode (the potential varied
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to satisfy the current). In this study, pyrrole could be electrochemically polymerized by the following mechanism: In the first phase, the pyrrole monomer is oxidized to form a radical
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cation, which is followed by the formation of pyrrole dimers. In the second phase, the large conjugation in the dimer makes oxidation easier, and oxidation is followed by the formation of
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trimers, oligomers, and PPy as black films [9, 22]. Mn2+ could also be oxidized to Mn3+ and Mn4+, which would precipitate as MnOOH, Mn2O3, Mn3O4, MnO(OH)2, and MnO2 at a neutral pH [23-25]. In contrast, the potential of the CE was reductive. Therefore, NO3- could be reduced into nitrite and OH-, which resulted in the precipitation of Mn(OH)2 on the CE [19]. During the reaction, the electrolyte changed from pale pink to yellow and finally dark brown. This change suggests that the reaction products, such as the pyrrole oligomer and Mn hydroxides/oxides, were partially soluble in water and homogeneously mixed in the reaction system. After deposition, a black film was observed on the WE, and a brown film was formed on the CE. Notably, the film on the CE did not stably form on the SS when the electrochemical deposition was carried out without pyrrole. After annealing the CE at 200 °C, the film was black. The average mass of the active material deposited on the WE and CE was approximately 2.6 mg and
Journal Pre-proof 1.3 mg, respectively. Hereafter, the film formed on the WE is called the anodic film, and the annealed film developed on the CE is calcined cathodic film. 3.2 Morphological, functional group, structural, and elemental characterization SEM images of the anodic film are shown in Figure 1(a, b). Aggregates of spherical PPy nanoparticles were observed on the surface of the SS electrodes. The morphology of the PPy film resembled that of the PPy films chemically deposited on the SS in NO3- and MoO42- electrolyte media [7]. The cross-sectional analyses revealed the formation of strongly adhered PPy films on
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the SS with an average thickness of ~2 μm. On the other hand, flower-like nanosheets were observed on the CE after the heat treatment, as reported (Figure 1c) [19]. The cross-sectional
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SEM indicated that the nanosheets developed vertically with an average thickness of ~2 µm on
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the CE (Figure 1d) as well. These structures were not substantially different from the noncalcined film (see Figure S1 in supporting information). EDS analysis was performed on both
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electrodes to analyze the elemental composition (Figure 1e and f). In addition to the metallic elements from SS (Fe, Cr, and Ni), other elements were confirmed: the anodic film exhibited
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atomic percentages of C (67.11 %), N (18.59 %), and O (14.31 %), while the calcined cathodic film contained C (22.38 %), O (52.79 %), and Mn (24.83 %). The cathodic film before
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calcination also consisted of C, O, and Mn (Figure S1e and f in supporting information): C (46.14 %), O (41.71 %) and Mn (12.15 %). The lower Mn ratio before calcination suggests the
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decomposition of organic compounds and dehydration of Mn compounds during the calcination. Each spectrum indicated that the film that formed on the WE was almost pure PPy with NO3-
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ions as a dopant [26], and the calcined cathodic film consisted mainly of Mn oxides (Mn:O ≈ 1:2) and carbon materials (hereafter, MnO2@carbon). The unassigned peaks corresponded to substrate constituent alloys such as iron, chromium, zirconium and platinum. The carbon materials could be byproducts of the PPy synthesis, such as PPy oligomers. The film on the CE did not stably form on the SS without pyrrole in the synthesis system, and the carbon materials acted as the binder.
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Figure 1 SEM images of the (a) anodic film from the top view and (b) cross-sectional view; (c)
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anodic and (f) calcined cathodic films.
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calcined cathodic film from the top view and (d) cross-sectional view; EDX spectra of the (e)
The functional groups in the films were characterized by FTIR (Figure 2). The anodic film
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exhibited a weak band at 790 cm-1, which indicated the NH2 vibration of pyrrole, while the band at 930 cm-1 could be assigned to the in-plane bending vibration of C=C (Figure 2a). The band at
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approximately 1045 cm-1 could be assigned to the in-plane deformation of C-H and N-H. In addition to these characteristic bands, the bands at approximately 1192 cm-1, 1384 cm-1, and
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1552 cm-1 and a broad band at 3400-3700 cm-1 could be assigned to C-N stretching, C-N stretching or interlayer vibration modes of doped NO3- [9, 27], pyrrole ring stretching (C=C and C-C), and –OH groups, respectively, confirming PPy formation [28-30]. From Figure 2b, the two high bands at approximately 505 and 623 cm-1 are the typical absorption bands of the Mn-O bond: the vibration of octahedral [MnO6] in MnO2 [31]. Although the band at 623 cm-1 was also observed for the anodic film, no significant absorption was observed at 505 cm-1. The bands at 1095 and 1630 cm-1 corresponded to Mn-O-H vibration [32]. The bands at 1384 cm-1 and 34003700 cm-1 could be assigned to nitrate, as mentioned above. The band at 2933 cm-1 belongs to CH stretching, which corresponds to the carbon materials and is consistent with the EDX spectrum [28]. Before calcination, the absorption bands of the materials resembled those of calcined film (Figure 2c), except the widening of the band around 1130 cm-1 and the intensification of band at
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EDX results described above (Figure S1f in the supporting information).
Figure 2 FT-IR spectra of materials scraped from the (a) anodic film, (b) calcined cathodic film
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and (c) cathodic film before calcination.
The crystal structure of the film grown on both SS meshes was confirmed by XRD (Figure 3).
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Figure 3a shows the XRD pattern of the anodic film, and the broad peak centered at approximately 25° is the typical peak of amorphous PPy [22]; the remaining two sharp and
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intense peaks at 44° and 50° correspond to the SS substrate [33]. The XRD pattern of the
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calcined cathodic film has broad peaks, which suggest small crystal grains (Figure 3b). The highly intense peaks at approximately 44° and 50° are from the SS mesh, as explained above, while the peaks at 19°, 29°, 34°, 35.5°, 59°, and 65° correspond to the α-MnO2 planes of (200), (310), (450), (211), (521), and (002), respectively [34]. Throughout these characterizations, Mn compounds were not observed in the anodic film, which could be explained by two reasons: (1) PPy is electropolymerized by initiation with a cation radical that is positively charged and repels Mn2+ via electrostatic repulsive forces [35]. (2) Even if Mn4+ formed on the anode, Mn4+ could oxidize pyrrole and be reduced to water-soluble Mn2+ [29].
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Figure 3 XRD patterns of the (a) anodic film and (b) calcined cathodic film.
3.3 Electrochemical performance
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The CV measurement was performed with the anodic film at different scan rates in an aqueous
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electrolyte (2 M KNO3) from 0 to 0.8 V (Figure 4a). The CV curve was nearly rectangular at a low scan rate. The rise in the scan rate caused a minor distortion in the rectangular shape due to
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the ohmic polarization of PPy [36]. The diffusion rate of ions could also limit the specific capacitance [37]. The rapid increase in the oxidative wave from 0.7 V indicates oxidation of PPy
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[7]. Figure 4b shows the CV curve for the calcined cathodic film. The CV curve deviated from
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the rectangular shape, which indicated Faradic reactions at the electrode.
Figure 1 CV curves: (a) for the anodic film and (b) calcined cathodic film at different scan rates. The electrochemical behavior of the films was also measured by the GCD method (Figure 5). The GCD curves for the anodic film were nearly symmetrical triangles, which indicated the capacitive behavior of the electrode (Figure 5a). A slightly curved arc was observed at 0.7 V vs Ag/AgCl due to the PPy oxidation as the CV curves illustrate above [7]. The GCD curve at high
Journal Pre-proof current density was more symmetrical than at low current density due to reduced degradation of PPy in a short time. In contrast, the GCD curve of MnO2@carbon at different current densities ranging from 0.23 to 0.77 A/g exhibited a deviation from the symmetrical triangular shapes, which suggested the
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pseudocapacitive nature of this material at lower current densities (Figure 5b).
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Figure 5 GCD curves of the (a) anodic film at different current densities from 0.27 to 3.08 A/g
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and the (b) calcined-cathodic film at current densities from 0.23 to 0.77 A/g.
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The specific capacitance for each electrode was determined from the CV and GCD
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∫
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measurements using equations 1 and 2, respectively [38]. equation 1 equation 2
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where Cg is the gravimetric specific capacitance (F/g), I is the discharge current (A), t is the discharge time (s), m is the mass of the active or loaded electrode (g), v is the scan rate (mV/s), and V is the potential window (V). The specific capacitances obtained from the CV curves are 510 and 270 F/g at 5 mV/s, while those from the GCD curve are 430 and 220 F/g at a current density of 0.27 A/g and 0.23 A/g for the anodic and calcined cathodic electrodes, respectively (Figure 4, 5). The stability of each electrode was also evaluated by the capacitance retention of the materials (Figure 6). For the anodic film, a GCD cycle was performed in the range from 0 to 0.8 V at 11 A/g, and the capacitance retention decreased to 62 % after 2,000 cycles (Fig 6a). The 38 % loss in the cycling test could be attributed to 3 reasons: (1) deformation and collapse of the PPy film
Journal Pre-proof caused by mechanical stress in the dedoping process of NO3- [39], (2) damage to the polymer chain, such as over-oxidation of pyrrole due to monomer incorporation in the polymer matrix chain [40] or low pH effects under over-oxidative conditions [41] and reactions with oxygen, and (3) dissolution of the SS-based electrode [5]. However, no detachment of the active material was observed after repeatedly drying and bending the electrode before the electrochemical performance test, which indicated that the anodic film was sufficiently adherent and tough, as confirmed by the cross-sectional SEM image (Figure 1). As mentioned above, a potential above 0.7 V would cause the oxidation of PPy and result in low capacitance retention. The GCD cycles
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at lower range (from -0.2 to 0.6 V) could decrease the oxidation of PPy, and the capacitance
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retention remained at 76.5 % after 1,600 cycles (see Figure S2 in the supporting information). Figure 6b compares the CV curves at a scan rate of 5 mV/s of an anodic film before and after
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GCD for 2,000 cycles at 11 A/g in a potential window of 0 to 0.8 V, resulting in 69 % capacitance retention, consistent with the GCD result. The CV curve after the GCD cycles was
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reduced relative to that for a fresh electrode; however, the shape was still rectangular even after
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2,000 cycles, indicating that the characteristics of the material were not drastically changed.
Figure 6 (a) Capacitance retention vs cycle number for an anodic film where the insets are the GCD curves for the first and last 6 cycles in a potential window of 0 to 0.8 V, (b) CV curves of an anodic film at a 5 mV/s scan rate before and after 2,000 GCD cycles, (c) GCD curve of an anodic film at 0.4 A/g before and after the 1,000 cycle stability test, and (d) specific capacitance vs. cycle number for the calcined cathodic film where the insets are the first (left) and last (right) cycle GCD curves. A comparison of the GCD curves for the same electrode obtained before and after GCD cycling at 0.4 A/g is shown in Figure 6c. The curves revealed that the discharge time for the electrode
Journal Pre-proof before the cycle test was 656 seconds, while it was 608 seconds for the same electrode after the cycle test, which demonstrates a capacitance retention of 93 %. This higher capacitance retention (93 % at 0.4 A/g and 77 % at 11 A/g after 1,000 cycles) indicates that the high current density results in apparent degradation of the electrode, which can be avoided at low current density. This mechanism may be due to the dedoping process of counter ions, as mentioned above. On the other hand, the specific capacitance of the calcined cathodic film changed differently. In the very early stage (cycle number < 16), the specific capacitance decreased, which could be attributed to a partial corruption of the film by swelling-shrinking behavior during the charge-
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discharge process. Then, the film adapted, and the specific capacitance gradually increased with
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the cycle number. The highest specific capacitance was obtained after approximately 1,000 cycles (Figure 6d). This improvement was due to the complete activation of the highly porous
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structure of the electrode (see Figure 1); ion intercalation into the inner part of the active material
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needed sufficient time [42, 43]. The GCD curves for the calcined cathodic film showed that greater than 90 % of the maximum capacitance was maintained in the range of 800-1,551 cycles
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at 0.78 A/g. After 1,200 cycles, a gradual decay of the capacitance retention was observed, which could be attributed to the degradation of the active materials. A thick film with nanostructures
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can be unstable under charge-discharge cycles. Although greater mass loading on the electrode will result in higher capacitance, a thinner film facilitates ion diffusion into the inner part of the
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active electrode and increases the specific capacitance. [44,45] Therefore, the film structure should be considered to balance the properties. Throughout the cycle tests, the charging-
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discharging times for both electrodes were close to each other (insets in Figure 6a and d), indicating the high columbic efficiency and good reversibility of the electrodes [46]. The columbic efficiency was determined from the ratio of the discharge time to the charging time [47]. The anodic film had a columbic efficiency of ~97 % in the first 2,000 cycles, while that of the calcined cathodic film was 95.4 % in the first 1,551 cycles. The lower columbic efficiency of MnO2@carbon could be due to the Faradic reaction of MnO2. To analyze the charge-transfer mechanism, EIS was also measured and plotted as a Nyquist plot (Figure 7).
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Figure 7 EIS curves of the (a) anodic film and (b) calcined cathodic film before and after chargedischarge cycles. The model for analyses is inserted in b. The EIS curve of the anodic film before the cycle test contained an offset region with an xintercept (real axis-intercept), which included the electrolyte resistance (Rs), electrode resistance,
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contact resistance between the electrode and current collector, and current collector resistance.
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The semicircular arc in the high-frequency region corresponds to the charge transfer resistance (Rct) between the electrode/electrolyte interface, [48] and the slope of the Warburg resistance
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region connects to a vertical line in the low-frequency region [49]. Using a circuit model, as inserted in Figure 7b, the analysis of this material gave the resistance components of Rs = 1.35 Ω
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and Rct = 0.6 Ω for the electrode before the cycle test, and Rs = 4.9 Ω and Rct = 1.54 Ω for the
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electrode after 2,000 GCD cycles in potential window of 0 to 0.8 V. The low charge-transfer resistance reveals the high capacitive nature of the material, while the increase in Rct after the
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cycle test indicates lower capacitive performance after the test, which coincide with the CV and GCD curves. This degradation is likely due to the electrochemical stability of PPy and
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expansion/contraction during ion intercalation/deintercalation [50]. In contrast, the Nyquist plot of MnO2@carbon in the 1 M Na2SO4 electrolyte showed a larger
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semicircular arc before the cycle test, which indicated a higher Rct between the electrode and the electrolyte (Figure 7b). The resistance of this electrode was respectively analyzed as Rs = 0.9 Ω and Rct = 16 Ω before the cycle test and Rs = 2.11 Ω and Rct = 28.92 Ω after the cycle test. Although degradation was observed in the MnO2@carbon, its ratio was lower than that of PPy. The high Rct contributed to the lower capacitive behavior and more Faradic nature of this electrode, as confirmed by the CV and GCD curves. The linear line with a low slope in the lowfrequency region suggested the complex structures of the electrode, which limited the ionic diffusion. The slope became steeper after the cycle test, suggesting a change in electrode morphology. 4. Conclusion
Journal Pre-proof The in situ hybridization of PPy and MnO2 on SS meshes was examined by electrochemical deposition of the active materials in an aqueous mixture of pyrrole and Mn(NO3)2. Aggregates of spherical PPy nanoparticles were formed on the anode during single-phase galvanostatic electrodeposition at a current density of 5 mA/cm2, while flower-like nanosheets of Mn(OH)2 simultaneously developed on the cathode and changed to MnO2@carbon after calcination at 200 °C. For the deposition of Mn(OH)2 on the cathode, the carbon materials, which could be byproducts of the PPy synthesis, worked as the binder to stabilize the nanosheets on the SS base. The electrochemical characterization (CV, GCD, and EIS) of the PPy film indicated typical
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capacitive behavior, while the less conductive Mn(OH)2@carbon exhibited a less capacitive
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nature. The specific capacitances of these two electrodes (PPy and MnO2@carbon) were calculated to be 430 F/g and 220 F/g at a current density of 0.27 A/g and 0.23 A/g, respectively,
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from the GCD curves, and the CV measurements were 510 F/g and 270 F/g at 5 mV/s, respectively. The relatively high specific capacitance of PPy could be due to its high
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conductivity. The nanosheet formation of MnO2@carbon as a stable film on the electrode was
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determined to be an advantage in preparing the MnO2 nanostructure. The synthesis method used in this study is simple and can also be applied to other nanocomposites of conducting polymers
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and metal oxides, such as polyaniline and Ni/Co oxides.
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Funding: This work was supported by the Ministry of Science and Technology, R.O.C. [MOST
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108-2221-E-011-093], and the Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology. References 1. 2. 3. 4. 5.
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Journal Pre-proof Tamene Tamiru Debelo: Data curation, Investigation, Formal analysis, and Writing- Original draft preparation.
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Masaki Ujihara: Conceptualization, Methodology, Resources, Supervision, Writing- Reviewing and Editing, Visualization, Project administration, Funding acquisition
Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphical abstract
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Highlights:
From a mixture of pyrrole and Mn2+, active materials were simultaneously deposited.
Polypyrrole aggregates on anode and Mn(OH)2 nanosheets were respectively formed.
The polypyrrole exhibited the specific capacitance of 510 F/g at 5 mV/s.
Calcination converted the Mn(OH)2 nanosheet to Mn oxide with carbonaceous material.
Charge-discharge cycles improved the capacitance of Mn oxide by the ion diffusion.
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