Controllable synthesis of 3D hierarchical bismuth compounds with good electrochemical performance for advanced energy storage devices

Abstract

Three dimensional (3D) hierarchical structures have attracted rapidly increasing attention in the energy storage field because they can facilitate electron and ion transport and electrolyte/electrode contact which results in full utilization of active materials. Here, a series of 3D hierarchical bismuth (Bi)-based compounds with different sizes and morphologies have been controllably synthesized by adjusting the Bi³⁺/urea molar ratio. Electrochemical characterizations indicated that the prepared Bi-based compounds exhibit high specific capacitance and superior rate capability in aqueous alkaline electrolyte (832 F g⁻¹ at 1 A g⁻¹, and still maintained 90% of the initial level at 20 A g⁻¹), which can be attributed to their novel hierarchical structure.

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1. Introduction

Supercapacitors have gained increasing interest in recent times due to their high power density (1–10 kW kg⁻¹), long cycle life and bridging function for the power/energy gap between traditional capacitors and batteries.^1,2 To a great extent, the performance of the supercapacitor is mainly governed by the properties of the electrode materials.³ Therefore, most researchers have been focused on the exploration of various means to improve the electrochemical properties of existing materials or the synthesis of new electrode materials with good electrochemical performance. In addition, the morphology and microstructure of the materials also have significant influence on their physical and chemical properties.⁴ Thus, fabrication of hierarchical microstructure with desired shape, which is very important in material science and industry, has been a research hotspot.^3,5 Considerable nano-materials with different and controlled shapes such as nanoflakes,⁶ nanocubes,⁷ nanowires,⁸ urchin or flowerlike structures,⁹ and hollow structures¹¹ have been developed for the possible applications as electrode materials for supercapacitors. Among these, three dimensional (3D) hierarchical structures, organized from 1D or 2D nano-building units, have attracted rapidly increasing attention due to their favoring of electrolyte/electrode contact which results in full utilization of active materials.^12,13

Bismuth (Bi)-based compounds, due to their low cost and toxicity, high oxygen conducting, good photocatalytic and dielectric permitting behaviors, have already been widely used as catalysts, optical materials, gas sensors and supercapacitors.^14–17 In the field of supercapacitors, Bi-compounds have exhibited high capacitance and superior rate performance.^17,18 For examples, Bi₂O₃ thin film, synthesized via electrodeposition, was first investigated for its usage in supercapacitor by Gujar and co-workers,¹⁹ and a high specific capacitance of 98 F g⁻¹ was achieved in aqueous NaOH solution. In recent years, researches on Bi-based materials with better composite structures and the corresponding charge storage mechanism have been performed.^20–23 Tong et al. prepared rippled Bi₂O₃ nanobelts through electrodeposition route for supercapacitor applications.²⁰ Nanosized Bi₂O₃ has been deposited on highly ordered mesoporous carbon through a simple deposition method.²¹ A specific capacitance of the loaded Bi₂O₃ is achieved as high as 1305 F g⁻¹ at 1 mV s⁻¹; however, this value sharply decreases with the increase of the sweep rate, which is a fatal weakness as electrode material for supercapacitor. A composite structure fabricated by electrodepositing Bi₂O₃ onto titania nanotube arrays, exhibited a capacitance of 430 mF cm⁻².¹⁷ Bi₂O₃ flowers grown on carbon nanofiber paper (CNF) were synthesized by Hu and co-workers and used as negative electrode to combine with MnO₂/CNF paper to form a flexible asymmetric supercapacitor (ASC).²⁴ This ASC delivers a high capacitance of 97 mF cm⁻² (25.2 F g⁻¹) and energy density of 43.4 μW h cm⁻² (11.3 W h kg⁻¹). Despite this, Bi-based compounds have not been studied in depth for their capacitance behavior, and it is still highly desirable and challenging to develop Bi-based electrode with good performance. 3D hierarchical Bi-based microspheres are rarely reported for supercapacitor application, although they are highly desirable for achieving enhanced electron transport, improved ionic diffusion, and excellent rate capability.^4,25 Therefore, it is imperative to develop systematic and rational techniques to construct well-defined hierarchical structures for supercapacitor.

In this study, 3D hierarchical Bi-based microspheres with different morphologies have been synthesized via a simple solvothermal method. The size and microstructure of the samples can be controlled by simply adjusting the molar ratio of Bi³⁺/urea. With deliberately adjusting the amount of urea, average size of the Bi-based samples decreased gradually from 40 to 6 μm with microstructure evoluted greatly. Electrochemical characterization indicates that the Bi-based compounds exhibit high specific capacitance owing to their novel microstructures.

2. Experimental section

2.1 Sample preparation

All the chemicals were of analytical grade and used as received without further purification. In a typical synthesis of the Bi-based microspheres, 1 mmol Bi(NO₃)₃·5H₂O (0.485 g) was added into 30 mL ethylene glycol (EG) to form a clear solution. Afterwards, a given amount of urea was added into the above solution under stirring to ensure that all the regents were dispersed homogeneously. The resulting solution was transferred into a 50 mL Teflon autoclave and kept at 160 °C for 12 h. After being cooled down to room temperature, the product was collected and washed with deionized water and absolute alcohol, and dried at 40 °C. The synthesized samples were denoted as Bi-1, Bi-2, Bi-3, Bi-4 and Bi-5 based on the dosage of urea (2, 5, 10, 15, and 30 mmol, respectively).

2.2 Characterization

The morphology and microstructure of the as prepared samples were characterized by field-emission scanning electron microscope (FESEM, JSM-6701F, JEOL, Japan) and transmission electron microscope (TEM, FEI Tecnai G2 F20). The crystallographic structure of the samples was analyzed by X-ray diffraction (XRD, Rigaku D/Max-2400 diffractometer using Cu-Kα radiation and graphite monochromator, λ = 1.54056 Å). Nitrogen adsorption and desorption apparatus (Micromeritics ASAP 2020, America) was employed to investigate the surface area and the pore size distribution of the synthesized materials.

2.3 Electrochemical measurements

Electrochemical measurement was carried out using an electrochemical working station (CHI660C, Shanghai, China) in a three-electrode configuration at room temperature. A platinum (Pt) sheet and an Hg/HgO electrode were served as the counter electrode and the reference electrode, respectively. The working electrode was prepared by mixing 85 wt% electroactive material, 10 wt% carbon black, 5 wt% polytetrafluoroethylene (PTFE) with a few drops of ethanol to form a homogeneous slurry. The slurry was pasted onto the nickel foam and dried at 60 °C. After solvent was evaporated, the resulting paste was pressed at 10 MPa to a nickel gauze and dried at 70 °C for 12 h in air. The loading mass of the active materials on each electrode was about 5 mg. CV scans were recorded from −0.1 V to −1.1 V (vs. Hg/HgO) at different scan rates, and the galvanostatic charge–discharge tests were carried out in the potential range of −0.1 V to −1.1 V (vs. Hg/HgO) in 6 M KOH aqueous solution. Cycle life test was taken by Land Battery Test System (Wuhan Kingnuo Electronic Company, China).

3. Results and discussion

3.1 Structure and morphology characterization

The phase and composition of samples were examined by X-ray diffraction (XRD) as presented in Fig. 1. When 2 mmol urea was added, the XRD pattern of Bi-1 indicates that a transition characteristic structure of [Bi₆O₄](OH)₄(NO₃)₆·H₂O was obtained.²⁶ With increasing the amount of urea, characteristic peak at 11.5° gradually weakened and finally disappeared, indicating the complete disappearance of transition state and the generation of β phase Bi₂O₃ (JCPDs no. 76-0147) with typical parameters of a = 7.738, b = 7.738 and c = 5.731. The diffraction peaks at 28.34°, 32.84°, 47.16° and 55.84° correspond to the (201), (220), (222) and (421) diffraction planes of Bi₂O₃, respectively.

Fig. 1 XRD patterns of the as prepared samples.

SEM images were applied to directly observe the hierarchical structure of the prepared samples, as shown in Fig. 2. Evidently, not only the average size ranged from 40 to 6 μm (Fig. S1 in ESI†), but also the structure and morphology of the prepared samples evoluted greatly with increasing the amount of urea from 2 to 30 mmol. Thus, the amount of urea plays a crucial role in the formation of hierarchical structure. With 2 mmol urea, flower-like [Bi₆O₄](OH)₄(NO₃)₆·H₂O microstructure built from 2D thick plates with diameters of dozens of micrometers was obtained (Fig. 2a). Increasing urea to 5 mmol, it is observed that the prepared microspheres are also composed of thick plates, but with much smaller size and nano-whiskers growing on the plates (Fig. 2b). With further increasing urea to 10 and 15 mmol, the average size of the synthesized microspheres continued to decrease and a unique 3D hierarchical structure formed with nanowires being curled and connected between each plates (Fig. 2c and d). The well-defined microspheres disposed separately from each other, representing a unique monodispersed structure in dimension and shape. In the case of 30 mmol urea, it is worth noting that microspheres with diameter of 6 μm are formed with nanoparticles rather than nanowires, as shown in Fig. 2e. As stated above, our results suggest that the morphological characteristics of the synthesized Bi-based compounds can be simply controlled by adjusting the Bi³⁺/urea molar ratio. The detailed microstructures of the samples were further proved by TEM observation, as shown in the inset images of Fig. 2. It can be clearly seen that the microstructure changes gradually with increasing the amount of urea, which is consistent with the SEM results.

Fig. 2 SEM and TEM images of the synthesized samples: Bi-1 (a₁–a₄), Bi-2 (b₁–b₄), Bi-3 (c₁–c₄), Bi-4 (d₁–d₄), Bi-5 (e₁–e₄).

In order to understand the formation process of the Bi₂O₃ microstructure, samples with the reaction time increasing from 1 to 6, 12, 18, and 24 h were prepared with 5 mmol urea (Fig. S2†). It can be seen clearly that microspheres composed of nanoplates were formed quickly within 1 h with the assistance of EG and urea molecules. In this fabrication, EG can not only coordinate with Bi³⁺ but also form dimer or trimmer at high temperature to act as template for Bi³⁺ to form lamellar structures.^27,28 After reacting 12 h, nano-whisker structures appeared on the surface of the nanoplates. This can be explained by the rolling mechanism of layered structures, which reports that materials with natural or artificial lamellar structures can roll into nanotubes/wires under appropriate experimental conditions.^29–32 When large amounts of urea were present or prolong the reaction time, nanowires grew longer and longer on the plates and 3D structure formed with nanowires being curled and connected between each plates. In this process, urea molecules could absorb onto the layers and link the different layers through intermolecular hydrogen-boding interactions.^33,34 These links provide the possibility of these layers exfoliating and rolling up to form the nanowire structure during the solvo-thermal process. When a mixed solvent of 15 mL EG and 15 mL H₂O was used instead of pure EG, nanoparticles of Bi₂O₂CO₃ (JCPDs no. 84-1752) was obtained (Fig. S3†). This may explained by the presence of water, which can reduce the coordination ability of the EG molecule with the Bi³⁺ ions and accelerate the decomposition of urea to CO₃²⁺ ions.^28,35 Thus, Bi₂O₂CO₃ product was obtained. Our understanding of the formation mechanism is still limited and deeper investigation is in progress.

The pore structure of the Bi-based microspheres was further investigated by N₂ absorption and desorption measurement, and the results are provided in Fig. 3. The isotherms, according to the International Union of Pure and Applied Chemistry (IUPAC) classification, can be identified as type IV.^10,36 The obvious hysteresis loop observed in the range of ca. 0.6–1.0 P/P₀ is close to H₃ type hysteresis loop, revealing the presence of meso/macro-porous structure.^12,37 From Bi-1 to Bi-5, specific surface area and pore volume exhibited an increase tendency from 6.23 to 67.89 m² g⁻¹ and 0.0227 to 0.3187 cm³ g⁻¹ (Table. S1†), respectively, corresponding to the morphology evolution from thick plates based microflowers to smaller 3D hierarchical spheres. Such unique 3D hierarchical and porous structure may provide more efficient transport pathways for electrolyte ions which is critical for electrochemical and other applications.

Fig. 3 N₂ adsorption/desorption isotherms and BJH pore size distribution curves of Bi-samples.

3.2 Electrochemical properties of the Bi-based samples

The electrochemical characteristics of the Bi-based compounds were investigated in 6 M KOH electrolyte. Fig. 4a shows the CV curves of Bi-based compounds from −0.1 V to −1.1 V at a scan rate of 1.5 mV s⁻¹. Obviously, for all the electrodes, a pair of redox peaks is detected, indicating that the reversible capacity is mainly based on the faradaic redox mechanism.³⁸ XRD pattern of the fully discharged electrode is also shown in Fig. S4.† After fully discharged, diffraction peaks at 2θ = 27.1°, 37.8°, 39.6°, 48.5°, 55.6°, 62.1° and 64.4° appear and can be indexed as the (012), (104), (110), (202), (024), (116) and (122) planes of Bi metal (JCPDs, 44-1246), indicating the generation of Bi metal, while peaks located at 44.5°, 51.9° and 76.5° are the characteristic diffraction peaks of Ni foam (JCPDs, 04-0850). Therefore, the redox reaction of the electrode can be described by the following equation:

Bi₂O₃ + 3H₂O + 6e⁻ ↔ 2Bi + 6OH⁻

Fig. 4 Electrochemical performance of the Bi-based compounds measured in 6 M KOH electrolyte. (a) CV curves at 1.5 mV s⁻¹, (b) charge–discharge profiles at a current density of 1 A g⁻¹, (c) the plots of the corresponding specific capacitance as a function of current density, (d) cycling life of the electrodes at a current density of 5 A g⁻¹ (500 cycles).

Moreover, the CV curves are almost symmetric, revealing good reversibility of the oxidation and reduction processes.³⁹ On the other hand, as the scan rates increase from 1.5 to 10 mV s⁻¹, the current response increases accordingly and the shapes of these curves show no significant change (Fig. S5†).

The galvanostatic charge–discharge curves of Bi-based compounds are shown in Fig. 4b. Apparently, a distinct plateau region can be observed, exhibiting the typical characteristic of pseudo-capacitance, which is consistent with the CV results. The specific capacitance can be calculated from the discharge curves according to the equation of C = IΔt/(mΔV), where C (F g⁻¹) is the specific capacitance, I (mA) is charge–discharge current, Δt (s) is the discharge time, m (mg) is the mass of the active material, and ΔV (V) represents the potential drop during discharge. At a current density of 1 A g⁻¹, the values of the Bi-compound samples were calculated to be 614, 698, 821, 832, and 830 F g⁻¹, which correspond to the specific capacitances of the Bi-1, Bi-2, Bi-3, Bi-4 and Bi-5 samples, respectively. As expected, the specific capacitance of the samples presents an increase tendency with improving the Bi³⁺/urea molar ratio. This result should be associated with the variations of structure and specific surface area of the presented electrodes. Specific surface areas of 6.23, 29.25, 32.67, 45.31 and 67.89 m² g⁻¹ were obtained corresponding to Bi-1, Bi-2, Bi-3, Bi-4 and Bi-5 (Table S1†). The higher surface area will lead to higher efficiency for Bi₂O₃ utilization.²⁰ The 3D hierarchical structure, in which the nanoplates separated from each other on one side and are interconnected by nanowires with each other on the other hand, is beneficial for the contact of electrolyte and material surface and shorten the path ways of ions diffusion,^40,41 thereby exhibiting better electrochemical performance.

For practical application, it is critical for supercapacitors to maintain large capacitance even at high current density. Fig. S6† displays galvanostatic charge–discharge curves of the Bi-based compounds conducted at different current densities from 1 A g⁻¹ to 20 A g⁻¹. The corresponding specific capacitance for different Bi-electrodes as a function of current density is shown in Fig. 4c. Remarkably, all the samples manifest superior rate capability. Taking Bi-4 for example, high specific capacitance values of 833 and 750 F g⁻¹ have been obtained at current densities of 1 and 20 A g⁻¹, respectively. Namely, there is still around 90% of its initial capacitance retained as the current density increasing from 1 to 20 A g⁻¹.

Moreover, electrochemical properties of the Bi-4 electrode in different electrolyte have also been studied in depth. CV curves and typical charge and discharge curves of Bi-4 electrode in different electrolyte were shown in Fig. S7,† and the corresponding specific capacitances of the electrode calculated from CV curves according to the equation of C = (∫IdV)/(νmV) were displayed in Fig. S8a.† In addition, specific capacitances of 832 F g⁻¹, 788 F g⁻¹, 631 F g⁻¹, 539 F g⁻¹ and 15 F g⁻¹ could be obtained from the discharge curves at 1 A g⁻¹, corresponding to the electrolytes of 6 M KOH, 4 M KOH, 2 M KOH, 2 M NaOH and 1 M Na₂SO₄, respectively (Fig. S8b†). A highest capacitance was obtained in 6 M KOH, which happens since the plenty of ions participating in the redox reaction.^22,42 The enhanced specific capacitance in KOH electrolyte as compared to NaOH may be due to the smaller hydrated radius and higher conductivity of K⁺ ions (3.31 Å, 73 cm² Ωmol⁻¹) than Na⁺ ions (3.58 Å, 50 cm² Ωmol⁻¹),^43,44 which will favor the ionic mobility and interaction with the electrode material.

The cycle stability of the Bi-based compounds has been further tested by performing continuous charge–discharge cycles at a constant discharge current density of 5 A g⁻¹ for 500 cycles (Fig. 4d). It can be seen that the capacitance value of Bi-4 maintains 56% level of its original capacitance after 500 times of cycling. The decrease of the capacitance with cycling might be accounted to the peeling off and microstructure variation of the active materials during the Faradic reaction.^23,45 In addition, high Coulombic efficiency of approximately 100% is achieved during entire cycles (Fig. S9†), demonstrating that no significant gas evolution occurs in the cycling voltage region.⁴⁶

Asymmetric supercapacitor using Bi₂O₃ as negative electrode and active carbon (AC) as positive electrode has been fabricated. The mass ratio of Bi₂O₃/AC was chosen to balance the charge of the two electrodes based on the equation of q = C × ΔE × m, and the specific capacitance values were calculated based on the total mass of electroactive materials loading on both electrodes. As shown in Fig. S10,† a specific capacitance of 84 F g⁻¹ was calculated according to the discharge curve at 0.2 A g⁻¹ and could still maintain at 44 F g⁻¹ at 4 A g⁻¹.

4. Conclusions

In summary, 3D hierarchical Bi-based microspheres are constructed through a simple and controllable solvothermal approach. The results demonstrate that the amount of urea, EG and reaction time play crucial roles in the formation of hierarchical structure. With increasing the amount of urea from 2 to 30 mmol, average size of the samples decreased gradually from 40 μm to 6 μm with obvious microstructure evolution. When using 15 mmol urea, 3D Bi₂O₃ hierarchical structure composed of nanoplates with nanowires curled and connected between each other was obtained and exhibited excellent electrochemical performance in 6 M KOH electrolyte, i.e., 832 F g⁻¹ at 1 A g⁻¹, and can still maintain 90% of level at 20 A g⁻¹. It is anticipated that such a designed hierarchical architecture will be helpful to the exploration of electrode materials and development of energy storage devices.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09760f

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