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Effects of Temperature on Electrochemical Properties of Bismuth Oxide/Manganese Oxide Pseudocapacitor Chi-Huey Ng, Hong-Ngee Lim, Shuzi Hayase, Zulkarnain Zainal, Suhaidi Shafie, and Nay-Ming Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04980 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Industrial & Engineering Chemistry Research

Effects of Temperature on Electrochemical Properties of Bismuth Oxide/Manganese Oxide Pseudocapacitor Chi Huey Ng1,3, Hong Ngee Lim1,2*, Shuzi Hayase3, Zulkarnain Zainal1, Suhaidi Shafie4, Nay Ming Huang5 1

Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia. 2

Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia. 3

Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan.

4

Department of Electrical and Electronic Engineering, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia.

5

New Energy Science & Engineering Programme, University of Xiamen Malaysia, Jalan SunSuria, Bandar SunSuria, 43900 Sepang, Selangor Darul Ehsan, Malaysia. Hong-Ngee Lim (Corresponding author*) Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia. Tel: +60 3 8946 7494 E-mail address: [email protected]

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ABSTRACT In this study, a temperature investigation is conducted on a bismuth oxide/manganese oxide (Bi2O3/MnO2) supercapacitor to determine how temperature affects the performances of the supercapacitor. Energy and power densities of 9.5 Whkg-1 and 102.6 Wkg-1 are obtained at 60 °C, respectively, which are approximately twice the values for supercapacitors at 0 °C and 1.37-fold higher than 30 °C. Additionally, the supercapacitors achieve energy densities of 4.9 Whkg-1 and 6.9 Whkg-1, and power densities of 53.8 Wkg-1 and 74.8 Wkg-1 at 0 °C and 30 °C, respectively. Interestingly, the hybrid Bi2O3/MnO2 active materials exhibit superior stability and reversibility, retaining 95% of the original capacitance at 30 °C and >75% at the high temperature of 60 °C. Although the cooler supercapacitor exhibits a slightly higher resistive performance, its excellent capacitance retention upon continuous charging/discharging measurement at 0 °C shows its potential for use as an all-weather compatible supercapacitor in the automotive sector. KEYWORDS: temperature, bismuth oxide, manganese oxide, supercapacitor, superior stability


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INTRODUCTION A green energy device does not solely rely on high electrochemical performance, but must also possess temperature durability to meet and fulfill the various application needs. A supercapacitor should be able to operate under mild to severe weather conditions in electric vehicles. It is common for car batteries to malfunction in winter (below -1.1 °C). Hence, a device that is weather resistant, in addition to having a high capacitive performance, is preferable, in order to replace the conventional battery application. A supercapacitor offers high power density and fast charging properties,1 which can increase the acceleration rate and decrease the charging time (in just a few minutes) of a vehicle relative to a standard electric car battery. Only a few temperature investigations have been reported, with most supercapacitor studies focusing on applications at room temperature. A single-wall carbon nanotube coin cell supercapacitor employing tetraethylammonium tetrafluoroboratepolypropylene carbonate as the electrolyte was measured at 25–100 °C in 2009. This supercapacitor exhibited a high power density of approximately 55 kWkg-1 at a current density of 100 Ag-1. The insignificant cell damage to the supercapacitor upon operation at a current density of 100 mAg-1 to 100 Ag-1 at temperature of 25–100 °C showed the superiority of its electrodes.2 In 2013, a three-electrode manganese oxide-electrospun carbon nanotubeembedded carbon nanofiber electrode was reported with a high capacitance of 546 Fg-1 at 75 °C, which showed a 120 Fg-1 increment compared to that at 25 °C.3 Pseudocapacitive materials, which store chargers via reversible redox reactions, generally render higher capacitance than electric double layer (EDL) materials.4 Importantly, the pseudocapacitive materials provide excellent redox reaction on or near the surface for high capacitance surge, coupled with fast charge/discharge capability.5 Among the transition metal oxides, MnO2 has drawn considerable attention owing to its low cost, great abundance, and environmentally friendly properties, which show its promise as an electrode material for supercapacitor applications.6 Generally, a MnO2 electrode theoretically exhibits a specific capacitance of 1370 Fg-1 for an electron per manganese atom redox process. The performance of MnO2 could be limited by poor intrinsic electrical conductivity and cyclic instability. In addition, the dense morphologies and aggregation of MnO2 nanosheets could impede the penetration of electrolyte ions within the nanocomposite matrix.7 In 2011, an α-MnO2 nanorod supercapacitor electrode was developed, which delivered a capacitance of 166.2 Fg18

. A low capacitance requires the incorporation or doping of another pseudocapacitive

material to serve as a scaffold to anchor the MnO2, and consequently improve the


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conductivity of the electrode materials. Additionally, the synergism between the active materials enhanced the capacitance performance. A Co3O4@MnO2 nanocomposite was reported with a specific capacitance of 480 Fg-1 (0.7 Fcm-2) at 2.67 (Ag-1), along with a cyclic retention of 97.3% after 5000 cycles.9 The application of bismuth-based active material such as (BiO)2CO3 in photocatalytic reaction and energy storage system is substantially increasing owing to its large contact area, excellent conductivity, and responsiveness to UV and visible light irradiation.10,11 Likewise, bismuth oxide (Bi2O3) has drawn considerable attention because of its large energy band gap (2.8 eV),12 good oxygen-ion conductivity, high dielectric permittivity, non-toxicity, and biocompatible. In addition, Bi2O3 has a long durability cyclic performance, depending on its polymorphic form. Bi2O3 can exist in five polymorphs, where low-temperature α–Bi2O3 (monoclinic) and high-temperature δ–Bi2O3 (cubic) are highly stable.13 Because of its direct band gap and photoluminescence properties, Bi2O3 has been widely applied in gas sensors and solid oxide fuel cells, and has been used as a photocatalyst for water splitting and pollutant decomposing. Additionally, the layered structure of Bi2O3 promotes the employment of Bi2O3 as an interesting host for energy storage. The layered property of Bi2O3 can be described to the alternating layers of bismuth atoms, which are parallel to the (100) plane of the cell and oxide ions in the c-axis direction.14,15 The addition of Bi2O3 has successfully skyrocketed the capacitance of an activated carbon supercapacitor from 106.5 Fg-1 to 332.6 Fg-1.16 A two-electrode-configured Bi2O3/MnO2 device was fabricated as a symmetric supercapacitor via a one-step facile hydrothermal process. Nickel foam was employed as the deposition platform and current collector because of its high porosity and bendable features. This work focused on the performances of this supercapacitor, specifically the shelf-life of the supercapacitor at various temperatures of 0 °C, 30 °C, and 60 °C. A supercapacitor with a prolonged lifespan and weather sustainability are the key performances requirements for multi-application viabilities to replace the battery in the automotive sector. The temperaturedependent Bi2O3/MnO2 supercapacitor exhibited excellent electrochemical performances without compensating for its stability performances. Interestingly, the Bi2O3/MnO2 supercapacitor possessed superior stability (>75%) after 1000 continuous charge/discharge cycles from 0 °C to 60 °C.


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EXPERIMENTAL SECTION Materials. Nickel foam was obtained from Goodfellow Cambridge Ltd. Nylon filter membrane was purchased from Membrane Solutions, LLC, US. KMnO4 (>99–100.5%) was obtained from R&M Chemicals, Malaysia, while bismuth oxide (Bi2O3) with a purity of 99.999% was purchased from Sigma-Aldrich, Malaysia. Fabrication of Bi2O3/MnO2 electrode. First, 1 mM of Bi2O3 and 10 mM of KMnO4 powder were dispersed in 40 ml of deionized water. A 2 × 3 cm2 piece of nickel foam was immersed in the prepared solution and subjected to hydrothermal treatment at 140 °C for 24 h. The deposited nickel foam was then dried at 60 °C for 6 h and calcined at 350 °C for 2 h. Fabrication of symmetric supercapacitor. A two-electrode-configuration symmetric supercapacitor was prepared by stacking the Bi2O3/MnO2 electrodes separated by a piece of 1 M Na2SO4 soaked filter paper. The supercapacitor was fitted into a Swagelok for electrochemical measurements. For temperature tests, the supercapacitor was either cooled at 0 °C or heated at 60 °C for 1 h prior to the electrochemical measurements and was constantly cooled in an ice box or heated on a hot plate throughout the test. For measurement at 30 °C, the supercapacitor was left in an ambient condition. Material characterizations. The chemical states and compositions of the products were analyzed using X-ray photoelectron spectroscopy (XPS) by a PHI Quantera II with Spherical Capacitor Analyzer (SCA), whereas the crystalline structures of the samples were analyzed using a D8 Advance, Bruker AXS Germany (Cu, Kα one radiation) X-ray diffraction (XRD) technique. The surface morphologies of the samples were investigated using a Quanta 400F FESEM equipped with an EDX feature. Fourier transform infrared (FTIR) spectroscopy was performed using the attenuated total reflectance of a Perkin Elmer 1650, FTIR spectrophotometer. RAMAN spectroscopy was conducted using a WITec Raman Microscope. Transmission electron microscopy (TEM) was conducted using a Hitachi, HT-7700. Electrochemical measurements. The as-fabricated symmetric supercapacitor was subjected to cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) measurements. The applied working potential for the CV measurements was in the range of 0–1 V, measured from scan rates of 100 to 2 mVs-1. The specific capacitance of

the symmetric

supercapacitor was calculated from


the

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charge/discharge profile using Equation (1), where C is the specific capacitance in farads per gram (Fg-1), I is the charge/discharge current, ∆t is the discharging time, ∆V is the potential window, and m is the mass of the electrode. While k =2 is the average mass of electrode is used and k =4 when total mass of both electrodes is used.17 C=

∆ ∆

Equation (1)

The energy density and power density of the symmetric supercapacitor were calculated using Equations (2) and (3), respectively.18 = 12 Equation (2) =

Equation (3)

where Ecell is the energy density in Whkg-1, Ccell is the specific capacitance of an electrode obtained from CV, V is the potential window obtained from the discharge curve, Pcell is the power density in Wkg-1, and t refers to the discharge time. RESULTS AND DISCUSSION Structural properties of supercapacitor electrode. FESEM images demonstrated that the spherical Bi2O3 particles (Figure 1a) were enveloped by 2D sheet-like MnO2 (Figure 1b) after 24 h of the hydrothermal process, as depicted in Figure 1c, which is in great agreement with the TEM image. Bi2O3 was featureless before undergoing the hydrothermal process (Figure S1). The elemental mapping confirmed the existence of Bi2O3 and MnO2 over the nickel foam (Figure S2). The TEM image (Figure 1d) indicated the contrast between Bi2O3 and MnO2 in the hybrid. The light color in the TEM image represents the ultrathin MnO2 nanosheet, while the darker region indicates the Bi2O3 nanosphere (white arrows). The formation of MnO2 sheets and stacked MnO2 sheets is depicted in the TEM image. The HRTEM (Figure 1e) image reveals visible lattice fringes with equal interplanar distances of 0.37 and 0.27 nm, which correspond to MnO2,19 indicating the wrapping of MnO2 over the Bi2O3 material. In addition, the diffraction rings displayed in the SAED pattern indicate that Bi2O3/MnO2 co-exist in the polycrystalline structure. The XRD profile (Figure 2a) shows the good embedment of the highly crystalline Bi2O3/MnO2 composite over the entire surface of the blank Ni foam, which agrees with the


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FESEM and TEM images. The good adhesion of the Bi2O3/MnO2 over the nickel foam scaffold was confirmed through the tremendous decrease in the intensity peak of the nickel foam upon the introduction of active materials. The diffraction peaks detected at 43°, 52°, and 77° correspond to the lattice planes of (111), (200), and (220), which indicate the crystallographic planes of nickel.20-23 The major detection point for the nickel foam is ascribed to the low concentration of precursors used. In addition, the XRD profile shows the absence of additional crystalline peaks, which is evidence that an impurity free nanocomposite was successfully synthesized. Figure 2b has further evidence for the deposition of the Bi2O3/MnO2 hybrid composite on the nickel foam substrate. The FTIR peaks were slightly blue shifted and the intensity of the peaks diminished upon the deposition of Bi2O3 and MnO2 pseudocapacitive materials. The presence of Mn-O bending vibrations for the [MnO6] octahedron was found at the peak of ~452 cm-1, suggesting that MnO2 was present within the Bi2O3/MnO2 hybrid composite.24 The blue shift in wavelength could be due to the strong bond energy between the Bi2O3 and MnO2 in the composite.25 The existence of Bi2O3/MnO2 is further reflected by the Raman spectroscopies (Figure 2c). The prominent peak at 634 cm-1 corresponds to the symmetric stretching vibration of the Mn-O lattice, which is perpendicular to the MnO6 octahedron double chains of MnO2.24,26 In addition, the peak at 560 cm-1 represents the Mn-O stretching vibration in the basal plane of the MnO6 sheet, indicating the formation of δ-MnO2.27 Meanwhile, the Bi2O3 nanostructure exists in the α– Bi2O3 (monoclinic) phase.28 The XPS analysis was conducted to study the chemical composition and surface electronic state of the Bi2O3/MnO2 electrode. The wide scan of Bi2O3/MnO2 explicitly revealed the presence of the Bi, Mn, and O elements, as reflected in Figure 3a. The narrow scan of the Bi4f spectrum showed two deconvoluted peaks (164 eV and 158 eV) of Bi4f5/2 and Bi4f7/2, respectively (Figure 3b). The peak separation of 5.3 eV thus clarified the existence of the Bi element in an oxidation state of +3.29,30 In addition, the Mn2p spectrum shows two deconvoluted peaks at 654 eV and 642.2 eV, which correspond to the Mn2p1/2 and Mn2p3/2 spin-orbit peaks, respectively (Figure 3c).31 It thus shows the domination of Mn4+ within the binary system.32 Figure S3 additionally proves the formation of MnO2 with a peak separation of 4.6 eV, as expected from the Mn3s spectrum.33 The Mn-O-Mn and Mn-O-H peaks are positioned at 529 eV and 532 eV, respectively, as depicted in Figure 3d.32 The deconvoluted peak at 530 eV represents the oxide group of Bi2O3. The obvious peak at 531 eV is linked to


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the Ni2p3 spin-orbit, which is attributed to the nickel foam current collector, and the peak situated at 532.9 eV represents the C=O functional group. Formation mechanism of Bi2O3/MnO2 materials. The formation of 2D MnO2 occurred through the decomposition of KMnO4, as depicted in Equation (4), as the Bi3+/Bi couple theoretically recorded a lower redox potential (+0.308 V vs. SHE) than the MnO4-/MnO2 couple (+1.679 V vs. SHE), which promotes the spontaneous reduction process of MnO4- to MnO2. The MnO4- ions oxidized water with concurrent oxygen evolution, which resulted in the precipitation of MnO2:15, 34-39 4 + 2 → 4 + 4 + 3

Equation (4)

The transformation of MnO2 and Bi2O3 into MnOOH and BiOOH, respectively, is hypothesized to have occurred during the hydrothermal process (a) in an enclosed selfgenerated pressurized autoclave system (Equations (5) and (6)). The formation of BiOOH occurred through the destruction of Bi2O3 particles upon contact with water.39 4 + 2 → 4 +

Equation (5)

2 ! + 2 → 4

Equation (6)

A schematic diagram that represents the plausible growth mechanism for the Bi2O3/MnO2 precursors is presented in Scheme 1. The decomposed MnO4- nanoparticles resulted in the formation of ultrathin MnOOH nanosheets,36 which enveloped the BiOOH, as evidenced through the FESEM and TEM images. The immediately formed intermediate species MnOOH and BiOOH were subsequently transformed into a pure oxide lattice structure, which was more compact, and an impurity free Bi2O3/MnO2 electrode upon calcination (b). Electrochemical performances of pseudocapacitor under influence of temperature. The performances of the fabricated pseudocapacitor at various temperatures were investigated with the goal of determining the kinetics and ion diffusion mechanism of the supercapacitor, which will govern the rate capability and specific capacitance of the supercapacitor in future commercial applications. The voltammogram areas (Figure 4a) are precisely parallel to the capacitive performances of the supercapacitor. The large cyclic voltammogram area of the supercapacitor measured at 60 °C compared to those at 30 °C and 0 °C corresponded to a high specific capacitance of 150.2 Fg-1 at a scan rate of 2 mVs-1. The specific capacitances of the supercapacitor obtained at temperature of 0 °C, 30 °C, and 60 °C are depicted in Table 1. The increase in capacitance was caused by enhanced ion mobility at a high temperature, which promoted greater charge transfer and storage capability.


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Additionally, the evaporation of electrolyte at a high temperature could induce the physisorption of electrolyte ions and give rise to Faradaic currents, which enhanced the capacitance performance.2 However, the current response deviated slightly from the equilibrium state in the high potential region at a higher temperature, which implied the occurrence of electrochemical polarization.3 The sequential increase in specific capacitance from the galvanostatic charge/discharge, as shown in Figure 4b, gave evidence that the application of temperature promoted electrochemical activation, which consequently resulted in a high capacitance performance compared to that at 0 °C. The good linear voltage-time profiles at 60 °C compared to those at 30 °C and 0 °C from the prolonged discharge time implied a good capacitive behavior and better charge storage capability at a higher temperature. This phenomenon suggested that the application of heat provided extra energy to trigger the chemical reaction. The Nyquist plot (Figure 4c) shows that the supercapacitor had a slightly greater resistance performance at 0 °C than at 30 °C and 60 °C. The supercapacitor was highly resistive at 0 °C, giving an ESR of 2.76 Ω and Rct of 5.65 Ω, as a result of the crystallization of the Na2SO4 electrolyte under a cold influence. Meanwhile, the low ESR of 0.42 Ω and Rct of 0.6 Ω found for the supercapacitor at 60 °C could be ascribed to the expansion of the nanocomposite matrix, which facilitated the diffusion of electrolyte ions. In addition, the more vertical line of the 60 °C supercapacitor at the lower frequency region was indicative of the ideal capacitive behavior of the electrodes and lower diffusion resistance at a high temperature.3 The cyclic retentive performance of the electrode under the influence of the various temperatures was also investigated, as depicted in Figure 4d. The 60 °C supercapacitor could still retain 75% of its original capacitance after extensive expansion and contraction processes, even under prolonged heating. The degradation and deterioration of the nanocomposite matrix upon continuous expansion and contraction processes, especially under prolonged high-temperature measurements, stimulated the dissolution of the MnO2.3,40 The influence of heating is hypothesized to exaggerate the mechanical and electrical faults between the MnO2 and Bi2O3 and the framework destruction, thereby leading to a disabled charge storage process. Moreover, the morphological change and phase transition inevitably hastened the material’s degradation. Comparably, the supercapacitor measured at 30 °C showed a 5% capacitive fade, with negligible capacitance loss for the 0 °C supercapacitor, proving the good electrochemical reversibility of the Bi2O3/MnO2 electrodes. In addition, the excellent capacitance retention was ascribed to the milder resistive performance, which facilitated the


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movement of the electrolyte ions into the inner pores of the nanomaterial. Interestingly, the increase in the capacitance from 0 to 30 °C could be the result of the defrosting of the Na2SO4 electrolyte, which hastened the diffusivity of the electrolyte ions. In contrast, the abrupt decrease in the capacitance from 79.4 Fg-1 to 39.4 Fg-1 at 1 Ag-1 (60 °C to 30 °C) could be attributed to the partial degradation of the active materials upon heating. Nevertheless, it still possessed high cyclic stability (more than 100% capacitance retention) after 1000 charging/discharging cycles. Overall, the electrochemical performances of the supercapacitor at 60 °C were better than those of the supercapacitors tested at 0 and 30 °C, which is consistent with the results of other reported works (Table 2). Table 2 shows that Bi2O3/MnO2 supercapacitor (our work) measured through a two-electrode system exhibited good cycling stability where 95% of its capacitance retained after 1000 charge/discharge cycles (90% capacitance retained after 2000 charge/discharge cycles), as opposed to NiO//AC supercapacitor which retained 86% of its capacitance after 800 cycles. It thus shows that the synergistic effect between Bi2O3 and MnO2 has contributed to high capacitance and cyclic stability.

The Ragone plot illustrates the energy storing and power delivery capability of the supercapacitor at various temperatures. The milder resistance performance of the Bi2O3/MnO2 supercapacitor at 60 °C enhanced the energy storage ability and power delivery efficiency, with energy and power densities of 4.7 Whkg-1 and 1691.5 Wkg-1 at 100 mVs-1, respectively. Meanwhile, at a scan rate of 3 mVs-1, the supercapacitor could still achieved an energy density of 9.5 Whkg-1 and power density of 102.6 Wkg-1. The energy density achieved at lower scan rate is higher owing to highly efficient infusion and de-fusion of electrolyte ions within the nanocomposite matrix of the supercapacitor. Comparatively, the supercapacitor exhibited twofold decreases in its energy (4.9 Whkg-1) and power densities (53.8 Wkg-1) at 3 mVs-1 when it was tested at 0 °C, which could be attributed to compromised diffusive ability of the electrolyte due to partial freezing electrolyte (Figure 5a and Table 1). Figure 5b and 5c delineate the indistinguishable resistive performances of the supercapacitor for various defrosting times. The high capacitance retention ability implies an efficient charge diffusion, transportation, and excellent storage capability of the supercapacitor even under cold conditions. As observed, the surface scaffold of the 0 °C Bi2O3/MnO2 electrode (Figure 6a) was coarser than those at 30 °C (Figure 6b) and 60 °C (Figure 6c) after being exposed to retention measurement under cold conditions. The rough surface could have resulted from the crystallization of the electrolyte and active materials, ACS Paragon Plus Environment

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which would impede the facilitation of electrolyte ions, and consequently lead to a lower capacitance performance. Cracks were observed on the 60 °C coated surface and a smoother surface (30 °C) was presented upon contact with the electrolyte during the continuous cycling measurement. All in all, the ultra-high durability, stability, and reversibility of the Bi2O3/MnO2 supercapacitor, regardless of the temperature, are accredited to the inclusion of Bi2O3 within the MnO2 matrix. Figures S4a and S4b present the electrochemical performances of Bi2O3, MnO2, and Bi2O3/MnO2 supercapacitors. The synergistic effects in the Bi2O3/MnO2 composite not only resulted in 1.3-fold and 1.5-fold enhancement of specific capacitance, compared to Bi2O3 and MnO2 supercapacitors, respectively (Figure S4a), but also alleviated its resistive performance. As observed through the magnified EIS spectra, MnO2 nanomaterial exhibited greater equivalent series resistance (ESR) than Bi2O3 and Bi2O3/MnO2 composite. The poorer electrochemical performances of MnO2 could be due to the agglomeration of dense MnO2 nanosheets (Figure 1), consequently impedes the facilitation of electrolyte ions. With the inclusion of Bi2O3, the resistivity of Bi2O3/MnO2 was alleviated, as shown in Figure S4b. The cyclic retentive profile (Figure S4c) shows a highly stable and durable Bi2O3 electrode material, which could retain 84% of its original capacitance after 2000 galvanostatic cycles, compared to the MnO2 electrode material (61%). The addition of Bi2O3 in the MnO2 matrix made a major contribution to the sustainability and reversibility of the hybrid supercapacitor. Evidently, the stability and durability performances were dramatically improved from 61% (solely MnO2) to 90% upon the insertion of Bi2O3 upon 2000 cyclic test (95% of its capacitance retained after 1000 galvanostatic cycles). The Nyquist plots (Figure S4d) before the cyclic tests and after 100, 500, and 1000 cyclic tests show no obvious distinctions, which suggest a negligible or minor degradation of the electrode, demonstrating the properties of a highly sustainable and reversible energy storage device, which is highly suitable for automotive applications.41 Charging and discharging mechanism of Bi2O3/MnO2 electrode. During the charging and discharging processes, two plausible reversible redox reactions of the 2D MnO2 have been proposed based on its intercalation and de-intercalation process of Na+, as shown in Equation (7):44,45 + "#$ + % & ↔ (& "#$ )


Equation (7)

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In addition, the adsorption of cations on the electrode surface from the electrolyte is proposed in Equation (8): ( )*+,-. + "#$ ↔ (& "#$ )*+,-.

Equation (8)

The charge storage of the Bi2O3 material in the aqueous electrolyte undergoes a quasiconversion reaction, instead of the conventional intercalation mechanism, which indicates a change in the valence state of bismuth (

!$

↔ / ) during the repeatable charge/discharge

process (Equation (9)). ! + 3 + 6% & ↔ 2

/

+ 6&

Equation (9)

CONCLUSION In summary, temperature investigation tests unveiled the excellent reversibility and sustainability of the Bi2O3/MnO2 supercapacitor. The symbiosis effect between the Bi2O3 and MnO2 materials enhanced the electrochemical performances of the binary composite supercapacitor, and also dramatically improved the cyclic reversibility, sustainability, and stability of the Bi2O3/MnO2 supercapacitor, regardless of the temperature influence. Excellent electrochemical performances were found for the Bi2O3/MnO2 supercapacitor, typically at a high temperature (60 °C) compared to those measured at 0 and 30 °C. This showed that the supercapacitor with a longer defrosting period possessed better electrolyte ion diffusivity, which consequently contributed to enhanced capacitive performances. The superior stability and reversibility performances of the Bi2O3/MnO2 supercapacitor under any conditions (up to >75%) show its suitable for use in seasonal countries and the automotive sector. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. FESEM image of Bi2O3 particle before hydrothermal process, elemental maps of Ni, Mn, O, and Bi elements for Bi2O3/MnO2, XPS analysis: determination of oxidation state of MnO2 from Mn3s plot, cyclic retention comparison between Bi2O3/MnO2, MnO2, and Bi2O3 supercapacitors, and Nyquist plots of Bi2O3/MnO2 supercapacitor after life cycle performances at room temperature. AUTHOR INFORMATION Corresponding Author


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*E-mail address: [email protected]

Notes The authors declare no conflict of interest. ACKNOWLEDGEMENT All the authors contributed equally to this work. This work was supported by the Fundamental Research Grant Scheme (01-01-16-1872FR) from the Ministry of Higher Education of Malaysia. REFERENCES (1) Deng, B.; Lei, T.; Zhu, W.; Xiao, L.; Liu, J. In-Plane Assembled Orthorhombic Nb2O5 Nanorod Films with High-Rate Li+ Intercalation for High-Performance Flexible Li-Ion Capacitors. Adv. Funct. Mater. 2018, 28, 1704330-n/a. (2) Masarapu, C.; Zeng, H. F.; Hung, K. H.; Wei, B. Effect of Temperature on The Capacitance of Carbon Nanotube Supercapacitors. ACS Nano 2009, 3, 2199-2206. (3) Wang, J. G.; Yang, Y.; Huang, Z. H.; Kang, F. Effect of Temperature on The PseudoCapacitive Behavior of Freestanding MnO2@Carbon Nanofibers Composites Electrodes in Mild Electrolyte. J. Power Sources 2013, 224, 86-92. (4) Jiang, Y.; Zhou, C.; Liu, J. A Non-Polarity Flexible Asymmetric Supercapacitor with Nickel Nanoparticle@ Carbon Nanotube Three-Dimensional Network Electrodes. Energy Storage Materials 2018, 11, 75-82. (5) Zuo, W.; Li, R.; Zhou, C.; Li, Y.; Xia, J.; Liu, J. Battery-Supercapacitor Hybrid Devices: Recent Progress and Future Prospects. Adv. Sci. 2017, 4, 1600539-n/a. (6) Zuo, W.; Xie, C.; Xu, P.; Li, Y.; Liu, J. A Novel Phase-Transformation Activation Process toward Ni–Mn–O Nanoprism Arrays for 2.4 V Ultrahigh-Voltage Aqueous Supercapacitors. Adv. Mater. 2017, 29, 1703463-n/a. (7) Unnikrishnan, B.; Wu, C. W.; Chen, I. W. P.; Chang, H. T.; Lin, C. H,; Huang, C. C. Carbon Dot-Mediated Synthesis of Manganese Oxide Decorated Graphene Nanosheets For Supercapacitor Application. ACS Sustainable Chem. Eng. 2016, 4, 3008-3016. (8) Li, Y.; Xie, H.; Wang, .; Chen, L. Preparation and Electrochemical Performances of αMnO2 Nanorod for Supercapacitor. Mater Lett. 2011, 65, 403-405. (9) Wang, J. G.; Kang, F.; Wei, B. Engineering of MnO2-Based Nanocomposites for HighPerformance Supercapacitors. Prog Mater Sci. 2015, 74, 51-124. (10) Li, Q.; Hao, X.; Guo, X.; Dong, F.; Zhang, Y. Controlled Deposition of Au on (BiO)2CO3 Microspheres: The Size and Content of Au Nanoparticles Matter. Dalton Trans. 2015, 44, 8805-8811. (11) Ma, J.; Zhu, S.; Shan, Q.; Liu, S.; Zhang, Y.; Dong, F.; Liu, H. Facile Synthesis of Flower-Like (BiO)2CO3@MnO2 and Bi2O3@MnO2 Nanocomposites for Supercapacitors. Electrochim. Acta 2015, 168, 97-103. (12) Ma, M. G.; Zhu, J. F.; Sun, R. C.; Zhu, Y. J. Microwave-Assisted Synthesis of Hierarchical Bi2O3 Spheres Assembled from Nanosheets with Pore Structure. Mater Lett. 2010, 64, 1524-1527.


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Figures and Figure Captions

Figure 1. FESEM images of (a) Bi2O3 after hydrothermal process, (b) MnO2, and (c) Bi2O3/MnO2 nanocomposite, along with (d) TEM image, (e) HRTEM, and (f) SAED of Bi2O3/MnO2 nanocomposite. Inset of Figure 1d shows the encapsulation of 2D MnO2 ultrathin sheet over Bi2O3 particles.


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Figure 2. (a) XRD profile and (b) FTIR spectrum of blank Ni foam, Bi2O3, MnO2, and Bi2O3/MnO2, and (c) Raman spectroscopies of Bi2O3, MnO2, and Bi2O3/MnO2.


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Figure 3. XPS analysis of (a) wide scan of Bi2O3, MnO2, and Bi2O3/MnO2, and narrow scan of (b) Bi4f, (c) Mn2p, and (d) O1s. The MnO2 is postulated to be Mn4+ because it has a different binding energy of 4.6 eV, which is the closest to the theoretical value of 4.8 eV, as determined from the Mn3s plot, as shown in Figure S3.


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Scheme 1. Plausible growth of Bi2O3/MnO2 on nickel foam substrate during hydrothermal process. It shows that the MnO2 nanosheet is compactly covering the Bi2O3 sphere after calcination.


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Figure 4. (a) Cyclic voltammogram, (b) galvanostatic charge/discharge, (c) Nyquist plot, and (d) life cycle of Bi2O3/MnO2 supercapacitors at different temperatures. Inset shows the magnified EIS spectra of Bi2O3/MnO2 supercapacitors at different temperatures.


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Figure 5. (a) Ragone plot of Bi2O3/MnO2 supercapacitor at temperatures of 0, 30, and 60 °C. (b) EIS profile and (c) life cycle of cooled (0 °C) Bi2O3/MnO2 supercapacitor at various defrosting periods (30, 60, and 120 min). The Bi2O3/MnO2 supercapacitor was cooled at 0 °C for 1 hour, subsequently defrosted at various defrosting periods (30, 60, and 120 min).


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Figure 6. FESEM images of Bi2O3/MnO2 (a) before life cycle at 30 °C and (b)–(d) after life cycle measurements at 0 °C, 60 °C, and 30 °C, respectively.


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Tables and Table Captions

Table 1 Tabulated energy and power density, and specific capacitance of Bi2O3/MnO2 supercapacitors. The specific capacitances are extracted from cyclic voltammetry (CV) at 2 mVs-1.

Specific capacitance (Fg-1) Energy density (Whkg-1) Power density (Wkg-1)

0 °C

30 °C

60 °C

76.4

136.4

150.2

4.98

6.93

9.50

53.78

74.84

102.59


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Table 2 Electrochemical performances of present and reported supercapacitor studies under temperature influences or at room temperature. Active

Electrolyte

Capacitance

Comments

Ref.

materials MnO2@CNF

0.5M

546 Fg-1 at 1 Ag-1

Exhibited

Na2SO4

(3-electrode system)

stability (95.3%) at 25°C;

good

diminishing

cycling

at

3

75°C th

(82.4%) after 1000 cycles. T-NT/Bi2O3

Bi2O3/Cu

Bi2O3@MnO2

NiO//AC

340 mF at 3 mAcm-2

75% at room temperature


after 500th cycles.

22 mFcm-2 at 20 mVs-1

Measured


temperature.

1 molL-1

93.1 Fg-1 at 1 Ag-1

Measured

Na2SO4


temperature.

PVA-5M

61.5 Fg-1 at 0.1 Ag-1

Life

KOH-H2O


attenuated 14% after 800th

1M NaOH

1M NaOH

cycle

30

at

room

43

at

room

11

profile

42

cycles at 25°C. Bi2O3/MnO2

1M Na2SO4

150.2 Fg-1 at 2 mVs-1 -1

-2

(79.4 Fg or 158 mFcm -1

~

95

%

capacitance

This

retained at 30°C, opposed

work

at 1 Ag )

to 75% at 60°C upon


1000th cycles.

~1.5-fold

increment

in

capacitance upon heated.


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