Facile Chemical Synthesis and Potential Supercapattery Energy Storage Application of Hydrangea-type Bi2MoO6.

Soft chemical synthesis is used to obtain a hydrangea-type bismuth molybdate (Bi2MoO6) supercapattery electrode that demonstrates considerable energy/power density and cycling life. Structure and morphology studies, initially, reveal a phase-pure polycrystalline and hydrangea-type surface appearance for Bi2MoO6, which upon testing in an electrochemical energy storage system displays supercapattery behavior, a combination of a supercapacitor and a battery. From the power law, an applied-potential-dependent charge storage mechanism is established for the Bi2MoO6 electrode material. A Trasatti plot evidences the presence of inner and outer surface charges. The hydrangea-type Bi2MoO6 electrode demonstrates a specific capacitance of 485 F g-1 at 5 A g-1 and a stability of 82% over 5000 cycles. An assembled symmetric supercapattery with a Bi2MoO6//Bi2MoO6 configuration demonstrates energy and power densities of 45.6 W h kg-1 and 989 W kg-1, respectively. A demonstration elucidating the lighting up of three light-emitting diodes, connected in series, by the symmetric supercapattery signifies the practical potentiality of the as-synthesized hydrangea-type Bi2MoO6 electrode in energy storage devices.

Introduction

Energy generation, storage, harvesting, and transportation are highly essential to maintain the cost of living in developed as well as in underdeveloped countries as the development of any country is more or less dependent on the availability of resources. In the upcoming years, the demand for energy will rapidly increase globally and become 10-fold higher than the present requirement; as a result there is a need to acquire energy from modern products and services.[1] Therefore, the need for enhanced-performance, inexpensive, and pollution-free or ecofriendly energy production systems is increasing day by day. Several efforts are being made to upgrade the performance of energy storage devices, including supercapacitors and batteries.[2] Supercapacitors have found a niche in the industrial, academic, and research market owing to their special electrochemical properties, such as high energy/power density, stability, galvanostatic charging and discharging processes, impedance, etc.[3,4] Various materials including carbon,[5−8] transition-metal oxides,[9−11] and conducting polymers[12,13] have been envisaged as electrode materials in supercapacitors. Generally, carbon-based materials demonstrate a high power density and a low energy density but a long cycling life.[14] Transition-metal oxides offer a fast and reversible surface redox reaction with higher specific capacitance (SC) and, therefore, are frequently preferred in commercial energy storage products.[15] But poor conductivity,[16] low stability,[17] limited rate capability, etc., are few of their demerits.[18] Various strategies such as tuning nanostructures at the nanoscale dimension,[19,20] adatom in the host active material matrix to increase electrical conductivity,[21] and mixing two or more active materials with a fast redox reaction[22−24] were applied successfully in the past to solve these issues to some extent. Improving the redox reactions and electrical conductivity via mixing of two or more metal oxides is a modern approach in metal oxides, which was previously adopted in metal alloys; thereby, considerable research activities are underway in this direction.[25−28] Electrode materials of these composites, sometimes called hybrid materials, demonstrate higher electrochemical supercapacitor performance than individual components. In addition, by forming a composite of two metal oxide electrodes of different properties, i.e., a supercapacitor and a battery, a supercapattery can be easily achieved.[29−31] Moreover, in addition to supercapattery performance, both conductivity and surface area of the resultant electrode are optimum. Bismuth molybdate (Bi2MoO6),[31−33] CoMoO4,[34] NiMoO4,[35] etc., are a few of these supercapattery electrode materials. On account of its unique optical and electrical properties, in addition to its application as a supercapacitor, Bi2MoO6 was used as a photocatalyst and a gas sensor in the past.[33,36,37] Aurivillius, since it was described by Aurivillius for the first time, bismuth oxides demonstrate the general structural formula, (Bi2O2)2+(ABO3)2–, consisting of (ABO3)2+ perovskite layers between two (Bi2O2)2+ layers.[38,39] Bi2MoO6 with different nanostructures has been used in energy storage devices, in particular, supercapacitors, i.e., pseudocapacitors, in the literature.[29−33] However, the as-obtained SC performance of Bi2MoO6, compared to known layered double hydride electrode materials, was lower, which could be attributed to different potential windows, various morphologies, and synthesis processes. Thus, many research groups are actively engaged in overcoming these issues for a better electrochemical performance. Senthilkumar et al. reported nanoplate-like Bi2MoO6 by a combustion method that exhibited an SC of 342 F g–1.[29] Ma et al. reported a facile hydrothermal reaction for large-scale growth of Bi2MoO6 nanosheets and hierarchical nanosheet-type nanotubes by a reflux method with an SC of 37.3 and 171.3 F g–1,[32,33] respectively. Samdani et al. successfully obtained Bi2MoO6 nanoplates that demonstrated an SC of 322.85 F g–1.[40] Yu et al.[41] reported flower-like Bi2MoO6 hollow microspheres via a simple hydrothermal method with SC as high as 182 F g–1. Yesuraj et al. documented electrochemical properties of aggregated irregular Bi2MoO6 with an SC of 193 F g–1 via a sonochemical method.[42]

In this work, we have synthesized hydrangea-type Bi2MoO6 by a simple and cost-effective wet chemical method at ambient temperature. Ascribed to its unique and uniform structure, special morphology and large surface area, and easy charge transport in the electrochemical process, a better supercapattery performance was anticipated. A symmetric supercapattery device with a Bi2MoO6//Bi2MoO6 configuration was assembled and electrochemical properties were measured. At the end of this article, with this symmetric supercapattery, three differently colored light-emitting diodes (LEDs), connected in series, were illuminated for nearly ∼18 min with moderate brightness, evidencing the commercial benefits of synthesized hydrangea-type Bi2MoO6.

Results and Discussion

Growth Mechanism

In the wet chemical method, direct formation of hydrangea-type Bi2MoO6 onto Ni-foam, with a three-dimensional (3D) metal architecture, depends on the adsorption and ion-by-ion condensation, which is highly suitable and used in the past for producing nanostructures of mixed metal oxides. We proposed a plausible growth mechanism for Bi2MoO6 by considering adopted synthetic conditions. Bi2MoO6 could be a result of (a) nucleation, (b) aggregation, (c) random arrays, and (d) crystal growth (as illustrated in Scheme ).

Scheme 1

Schematic Presenting the Growth of Hydrangea Flower-Type Bi2MoO6 (Photograph Courtesy of PVS)

Moreover, a plausible reaction mechanism could be as follows: the Bi3+ ions might be attracted toward the triethanolamine (TEA) molecule to form an unstable intermediate complex Bi[N(CH2–CH2–OH)] (eq ). Then, NaOH solution was added into it to increase the pH to ∼10[43] (eq ). MoO42–, obtained from the Na2MoO4·2H2O precursor solution, according to eq , was dissolved in deionized water and slowly added to the Bi[N(CH2–CH2–OH)] solution, so as to obtain a transparent and clear solution.[43] This solution was kept at 353 K for 3 h. Moreover, Bi[N(CH2–CH2–OH)] could lose its stability by activating the bismuth sites, and a π-allyl group could coordinate to a molybdenum ion, which is bridged by an oxygen atom forming a Bi–O–Mo bond through MoO42–.[44] At the same time, the reaction was allowed to proceed in the same container by attaching OH– to Bi3+ to form a stable Bi2MoO4(OH)2 (eq ) compound. Under heating conditions, Bi2MoO4(OH)2 gradually converted into Bi2MoO6 (eq ). Moreover, during the transformation process, an anisotropic growth of Bi2MoO6 could interconnect the molecules with each other. Finally, when Bi2MoO4(OH)2 was completely transformed into Bi2MoO6, a kinetically stable Bi2MoO6 framework was obtained.[45]

Morphology Evolution and Structural Elucidation Studies

The morphology evolution study of Bi2MoO6 is presented in Figure . The field-emission scanning electron microscopy (FE-SEM) images scanned at high/low magnifications are displayed in Figure a–c, where a uniform growth of Bi2MoO6 over Ni-foam is evidenced (Figure S2). The general form of the product is shown in Figure a, suggesting a high yield and surface uniformity. A close-up view of the FE-SEM image shown in Figure b demonstrates a petal-type architecture as in hydrangea with an average diameter of 2 (±02) μm. From Figure c, it is inferred that there are a considerable number of crevices between them as well as among individual petals, suggesting the availability of several paths for easy electrolyte ion penetration followed by percolation and mass transport. The high-magnification image (Figure c) suggests the development of a hydrangea-type architecture composed of dozens of curly two-dimensional (2D) nanopetals of ∼70 (±05) nm thickness. The air gap between the petals was 580 (±20) nm. These open and free interspaced gaps between the petals of the hydrangea-type architecture of Bi2MoO6 would be an excellent morphology for electrochemical energy storage device applications.[43] The surface composition of Bi2MoO6 obtained from FE-energy-dispersive X-ray spectroscopy measurements is shown in Figure d–f. A uniform distribution of Bi, Mo, and O on the hydrangea Bi2MoO6 surface is confirmed. The spectrum showed peaks of Bi and Mo at 2.5 eV and of oxygen at 0.5 eV, suggesting the presence of these elements in the sample product, i.e., Bi2MoO6. The elemental mapping of Bi2MoO6, as shown in Figure S3, clearly corroborates a ratio of 24:13:63 atom % for Bi, Mo, and O, providing the quantitative signature of Bi2MoO6. The high-resolution transmission electron microscopy (HR-TEM) image (Figure g) highlights the group of petals belonging to hydrangea with fine edges and the (Figure g, inset). The obtained lattice interplanar distance of 0.32 nm in the HR-TEM image corresponds to the (134) crystallographic plane of Bi2MoO6 (Figure h). The selected area electron diffraction (SAED) pattern, shown in Figure i, suggests the existence of a polycrystalline crystal structure due to the formation of highly concentrated circular rings, rather than spots and a foggy cloud, for Bi2MoO6. The X-ray diffraction (XRD) pattern was used to confirm the phase purity of Bi2MoO6. In Figure a, the XRD pattern of Bi2MoO6 is presented, where the obtained sharp and well-defined diffraction peaks were in accordance to JCPDS card no. 00-022-0112, revealing the formation of phase-pure orthorhombic Bi2MoO6. The diffraction peaks of Bi2MoO6 appearing at 2θ = 15.90, 27.33, 31.56, 32.63, 45.41, 53.77, and 57.22°, as shown in Figure a, were assigned to the (220), (134), (414), (522), (074), (922), and (816) crystal planes, respectively. The peak positions and respective planes were well indexed, and there were no additional peaks detected from impurities.

Figure 1

(a–c) FE-SEM images at different magnifications showing hydrangea flowers, (d–f) elemental mappings of Bi, Mo, and O, (g) TEM, (h) high-magnification HR-TEM images, and (i) SAED pattern of Bi2MoO6.

Figure 2

(a, b) XRD and full-range X-ray photoelectron spectroscopy (XPS) survey spectra of the Bi2MoO6 electrode. Enlarged XPS spectra of (c) Bi 4f, (d) Mo 3d, and (e) O 1s. (f) Nitrogen adsorption–desorption isotherms of the Bi2MoO6 electrode (inset shows the Barrett-Joyner-Halenda (BJH) pore-size distribution plot).

The XPS survey spectrum (see Figure b) clearly demonstrates the existence of Bi, Mo, and O elements in Bi2MoO6. Figure c–e shows the magnified XPS scans of Bi 4f, Mo 3d, and O 1s. In the Bi 4f spectrum (see Figure c), two XPS peaks at ∼158.4 and 163.8 eV were identified, assigned to Bi3+ 4f7/2 and Bi3+ 4f5/2, respectively.[43] In the Mo 3d pattern (Figure d), the binding energies at 231.5 and 234.7 eV were assigned to Mo 3d5/2 and Mo 3d3/2 of Mo6+, respectively. In Figure e, the peaks at 529.2 and 530.3 eV were due to Bi–O (lattice O) and Mo–O, respectively.[40] To characterize the porous nature of Bi2MoO6, the nitrogen adsorption/desorption isotherm was measured and is shown in Figure f. The as-prepared Bi2MoO6 exhibited a specific surface area of 47.11 m2 g–1. In the isotherms, the hysteresis loops matched those of mesopores, as the average pore diameter was 13.04 nm, which, eventually, would play a significant role in the electrochemical charge/mass transport processes by promoting the penetration of electrolyte ions deep into an electrode material in addition to the near surface with extensive surface redox reactions. Therefore, the as-obtained mesoporous and high surface area hydrangea-type Bi2MoO6 electrode on Ni-foam would demonstrate a better electrochemical performance than the planar and rigid architectures.[47]

Electrochemical Analyses

The electrochemical performance of the as-prepared hydrangea-type Bi2MoO6 electrode was measured in a 1 M KOH solution as the electrolyte. The SC values were measured from the cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) curves. Figure a presents the comparative cyclic voltammetry (CV) curves of blank Ni-foam and Bi2MoO6 electrodes at a fixed scan rate of 100 mV s–1. The CV curves recorded at different scan rates ranging from 5 to 100 mV s–1 and in the voltage window of −1.0–0.0 V (Figure b) show the redox peaks attributed to the Faradaic reactions from Bi(III) to Bi metal of Bi2MoO6.[46] The Faradaic redox reaction of Bi2MoO6 in 1 M KOH electrolyte is similar to that of Bi2O3. The quasi-reversible Faradaic reactions were evidenced by the presence of redox peaks in the CV curve (i.e., Bi3+ to Bi0). For Bi2MoO6, a single reduction peak (R1) with high current intensity could be due to the reduction of Bi3+ to Bi0 (−0.82 V), and, on the other hand, the two anodic peaks (O1 and O2) found at −0.48 and −0.30 V accounted for the oxidation of Bi0 to Bi metal and the oxidation of Bi metal to Bi3+, respectively. The possible oxidation and reduction processes taking place during the redox reaction[40,46,47] could be as followsOxidation peaks at “O1” and “O2” were attributed to the presence of a minor amount of Bi metal situated at the Bi metal/electrolyte interface, and the oxidation peak O2 along with the voltage plateau indicated the oxidation of Bi metal to Bi(III)[48] as followsThe above equations support the occurrence of the quasi-Faradaic signature, where with an increase in scan rate the oxidation peak potential shifts to a more positive direction and the reduction peak potential to the negative direction.[43,49] Well-defined redox peaks in the CV profile are obtained only for Bi2O3, as it was found earlier that the molybdenum (Mo) element in Bi2MoO6 cannot participate in redox reactions.[50] The molybdenum redox species could lead to the improvement of the SC values by increasing the electrical conductivity.[50] In addition to this, the anodic/cathodic peak current position of each CV curve was increased with increasing scan rate from 5 to 100 mV s–1, suggesting the existence of sufficient not only electronic but also ionic transport in the case of the Bi2MoO6 electrode[51,52] (see Figure b). Interestingly, the anodic/cathodic peak shifted in both potential regions (i.e., negative and positive), with increasing scan rate due to the occurrence of a charge diffusion polarization effect.[52] As a result of the quasi-reversibility of the material, the oxidation/reduction peaks were shifted to the positive and negative potential sides with increasing scan rate from 5 to 100 mV s–1, which could be due to (a) high Ohmic resistance, (b) slow electron transfer kinetics, and (c) low ionic diffusivity of the battery-type materials.[53,54] However, the redox peaks were clearly visible even at 100 mV s–1, indicating fast charge transfer, i.e., capacitive-type behavior in Bi2MoO6.[55]

Figure 3

(a) Comparative CVs of Ni-foam and the Bi2MoO6 electrode scanned at 100 mV s–1. (b) CV measurements of the Bi2MoO6 electrode at scan rates of 5–100 mV s–1. (c) log(i) vs log(ν). (d) Calculated capacitive and intercalation current of the Bi2MoO6 electrode at various scan rates. (e) GCD curve and (f) electrochemical impedance spectroscopy (EIS) (inset as a high-frequency region) plots of the Bi2MoO6 electrode. (g) Stability study of the Bi2MoO6 electrode. (h, i) Morphologies of the Bi2MoO6 electrode after 5000 cycles at different magnifications.

SC, one of the important electrochemical parameters, decreased with respect to the scan rate, which could be due to the low movement and delayed interaction time of the electrolyte ions within the electrode material. The maximum SC of 495 F g–1 was obtained at 5 mV s–1. The total SC is the algebraic sum of the SC contributions from the inner and outer surface charges. Therefore, the SC contributions were determined using Trasatti plots, where the Y-intercept of the linear fit between the SC and the square root of the scan rate (υ1/2) at υ = 0 (Figure S4a) gives the total SC, i.e., 523 F g–1 in the present case. At a lower scan rate, electrolyte ions have unlimited access to the electrode surface; therefore, a large amount of charge can be stored on both the inner and outer sides of the electrode. Likewise, the Y-intercept of the linear fit between the SC and υ–1/2 at υ = ∞ (Figure S4b) provides the SC contribution from the outer surface, i.e., 124 F g–1 in the present case. A prolonged time, due to a low scan rate, allows the electrolyte ions to access majority of electrode material for excess redox reactions. Hence, the obtained SC is that contributed by only the inner surface. Furthermore, the SC contribution from the inner surface was 399 F g–1. The Bi2MoO6 electrode stored a large amount of charge on the inner surface because of the voids and crevices, providing good accessibility to the electrolyte ions to penetrate deeply into the electrode material and high surface area, responsible for excessive reactions.[55,56] A power law was used to systematically differentiate the relative contributions from either capacitive or battery-type mechanisms to the total charge stored in the electrodes.[57] The scan rate-dependent peak current can be defined as followswhere I is the current (A), υ is the scan rate (V s–1), and a and b are adjustable parameters. The value of b was derived from the slope of the linear fit of log(υ) against log(i) at a fixed potential (V). If the value of b is smaller than 0.5, the current obeys a diffusion-controlled battery-type mechanism; however, if it is above/close to 1, the material follows a capacitive mechanism. Figure c shows a plot of log(i) and log(υ) at different potentials, i.e., −0.4888, −0.4000, −0.2912, and −0.1815 V. At the initial oxidation potential of −0.4888 V, b was 0.93, suggesting the dominance of a capacitive mechanism in Bi2MoO6. At the peak potential of 0.4000 V, b was 0.58, indicating the existence of a battery-type charge storage mechanism. Subsequently, values of b were 0.65 and 0.71 at the end of the oxidation potential range, i.e., 0.2912 and 0.1815 V, representing the involvement of a mixed charge storage mechanism. Hence, the Bi2MoO6 electrode material could store a charge through both mechanisms. The amount of charge stored by the battery/capacitive mechanism in the redox peak regions was measured through a power law equation as[56,57]It can be changed towhere ip is the peak current, and s1υ and s2υ1/2 are the current contributions from the capacitive and intercalation mechanisms. We obtained a linear plot for the Bi2MoO6 electrode in between ip/υ1/2 vs υ1/2 at 0.2912 V (Figure S4c), whose slope and intercept were s1 and s2, respectively. The capacitive/intercalation current calculated using the above equations is summarized in Figure d, where, at a lower scan rate, the diffusion-controlled current was higher because the electrolyte ions could penetrate more deeply into the Bi2MoO6 electrode material due to the hydrangea-type architecture of Bi2MoO6. At a high scan rate, the capacitive current increased. This measurement supported the conclusion that the Bi2MoO6 electrode could have contributions from both battery/capacitive-controlled intercalation kinetics, suggesting a supercapattery behavior.[58] To avail more information on the capacitance properties of the Bi2MoO6 electrode, galvanostatic charge/discharge measurement was carried out. To evaluate the SC of the Bi2MoO6 electrode, GCD measurements were attempted, where the SC value decreased from 485 to 65 F g–1 (Figure e). It could be due to the easy and rapid penetration of electrolyte ions into the Bi2MoO6 electrode and the accessibility of its whole area by electrolyte ions. The GCD curves showed two regions: a steep voltage drop region, attributed to the internal resistance, and a prolonged plateau of voltage, due to the involvement of a quasi-Faradaic process in the supercapattery Bi2MoO6 electrode, illustrating excellent electrochemical reversibility with fast charge transfer kinetics.[53] The Bi2MoO6 electrode could store charge using the capacitive/battery-type mechanism in the respective potential-dependent/independent regions.[57] The SCs of the Bi2MoO6 electrode at applied current densities are shown in Figure S5. The SC values were decreased with an increase in current density because of the slowed migration of electrolyte ions and charge transfer during the electrochemical reaction. The EIS (Figure f) plot was more vertical in the high-frequency and low-frequency ranges, suggesting the presence of capacitive- and battery-type contributions. In short, from the Nyquist plot, the supercapattery nature of Bi2MoO6[46] is corroborated. The 0.67 Ω semi-circular diameter in the higher frequency region, which is a charge-transfer resistance, was considerably smaller, revealing the existence of fast transfer of electrolyte ions across the electrolyte/electrode interface. The lower value of around 0.52 Ω of series resistance of the Bi2MoO6 electrode confirmed a good ionic response. The electrochemical reaction on the Bi2MoO6 electrode surface, i.e., a hydrangea-like architecture with arbitrary petals of high surface area, could enhance the conductive corridors by providing shorter diffusion routes.[59] Such electrochemical performance of the Bi2MoO6 electrode was facilitated by many interconnecting ultrathin porous petals by affording more active sites for efficient electrolyte ion transport on the active material surfaces. Moreover, the open and free interspaces between these petals and the mesoporous character of the hydrangea-type Bi2MoO6 electrode could serve as ion reservoir channels, responsible for shortening the ion diffusion length from the external electrolyte to the interior surfaces, thus, potentially, improving the intercalation/de-intercalation rate of ions and, thereby, increasing the utilization of active materials.[60,61] The long-term cycle stability of electrode materials is another key factor from the point of view of practical applications. The long-term cycle stability of the as-prepared Bi2MoO6 electrode material was evaluated by repeating charge–discharge tests at 5 A g–1, 5000 times. As shown in Figure g, it can be seen that the SCs of the Bi2MoO6 electrode material dramatically decreased to 82% from 1 to 5000 cycles, revealing its negligible degradation.[62,63]Figure h,i show the FE-SEM images, where no significant change in the surface appearance of the Bi2MoO6 electrode even after 5000 sequential cycles is noted. A few thick nanopetals of the Bi2MoO6 electrode material were destroyed after the cycling. Interestingly, the surface of the hydrangea-type Bi2MoO6 electrode collapsed upon forming a less crystalline or amorphous structure, which eventually could be one of the reasons for the drop in the cycling performance, as the Bi2MoO6 electrode might repeatedly undergo insertion/extraction of OH– ions.[61] The as-synthesized Bi2MoO6 electrode material maintained considerable quantity without disturbing the surface morphology even after 5000 cycling tests, demonstrating minimal dissolution of the active material into the electrolyte solution. A comparative data presenting the electrochemical performance of previously reported Bi2MoO6 electrode materials, obtained using different synthesis methods in different morphologies, given in Table confirm the higher/comparable performance of the present Bi2MoO6 electrode over others.

Table 1

Literature Review of Reported Synthesis Methods and Obtained Electrochemical Performance of Bi2MoO6 as Pseudocapacitive Electrode Materials

working electrode	synthesis method	morphology	potential window/(electrolyte)	specific capacitance (SC)	stability (cycle)	references
Bi2MoO6	combustion	nanoplates	–1.0 to 0.0 V	342 F g–1 (1 mA cm–2)	100% (500)	(29)
			1 M NaOH
Bi2MoO6	hydrothermal	nanosheet array	–0.4 to −0.1 V	37.3 F g–1 (2 A g–1)	89% (1000)	(32)
			1 M KCl
Bi2MoO6	reflux	hierarchical nanotubes	–0.5 to 0.6 V	171.3 F g–1 (0.585 A g–1)	92.4% (1000)	(33)
			6 M KOH
Bi2MoO6	solvothermal	nanopetals	–0.2 to 0.6 V	322.85 F g–1 (1 A g–1)	42% (10 000)	(40)
			6 M KOH
Bi2MoO6	hydrothermal	flower-like	0.0 to 0.6 V	182 F g–1 (1 A g–1)	80% (3000)	(41)
			3 M KOH
Bi2MoO6	sonochemical	aggregated irregular shaped	–1.3 to 0.0	193 F g–1 (2 A g–1)	91% (1000)	(42)
			1 M NaOH
Bi2MoO6	wet chemical	hydrangea flowers	–1.0 to 0.0	485 F g–1 (5 A g–1)	82% (5000)	present work
			1 M KOH

Bi2MoO6//Bi2MoO6 Symmetric Supercapattery Device Performance

Based on the above discussions, the Bi2MoO6 electrode showed a lengthened potential window from 0.0 to 1.5 V, and so a symmetrical supercapattery device was assembled to test its commercial benefits. The overall process of device fabrication is elaborated in Figure . For a symmetric supercapattery cell, Bi2MoO6 electrodes of the same quality (including Ni–F and mass of Bi2MoO6) were used as negative and positive electrodes in the presence of a polypropylene separator in a plastic cylindrical tube (Figure a,b). To design a portable symmetric Bi2MoO6 electrode device, two Bi2MoO6 electrodes were round-folded in the form of a sandwich-type structure using a flexible polypropylene separator. The separator paper was placed between these two electrodes so as to avoid grounded connection of the portable device. Furthermore, these sandwiched Bi2MoO6 electrodes were inserted into a plastic cylindrical tube vertically, as shown in Figure d, into which a 1 M KOH electrolyte was poured and the wire contacts were drawn carefully from each electrode via the seal of the plastic tube (see Figure d,e). Finally, Figure f demonstrates that the Bi2MoO6//Bi2MoO6 cell is ready for electrical operation process. All electrochemical tests (CV, GCD profiling, and cycling stability) of the Bi2MoO6 symmetric supercapattery device were performed and reported. The CV and charge–discharge curves of the Bi2MoO6//Bi2MoO6 symmetric supercapattery device at different scan rates are shown in Figure a,b, respectively.

Figure 4

Digital photograph images of (a) and (b) assembly of the Bi2MoO6 electrode connected to wires, separator, and plastic bottle, (c) folded electrodes with a sandwiching separator, (d) sandwiched round-folded electrodes kept in the plastic bottle, (e) electrolyte added into the plastic bottle device, and (f) display of the actual fabricated device in the laboratory (photograph courtesy of P.V.S.).

Figure 5

(a) CV, (b) GCD, (c) SC curves, and (d) Ragone plot of the Bi2MoO6//Bi2MoO6 symmetric supercapattery device (inset shows the device performance based on reported data), (e) schematic device configuration and mechanism of the Bi2MoO6//Bi2MoO6 symmetric supercapattery device, and (f) scheme presenting cycling stability over 5000 cycles of the Bi2MoO6//Bi2MoO6 symmetric supercapattery device (photograph courtesy of PVS).

The Bi2MoO6//Bi2MoO6 symmetrical supercapattery device showed decrement in the SC values from 40.5 to 25 F g–1 as the current densities increased from 1 to 5 F g–1 (Figure c). The Ragone plot of the symmetric supercapattery device at different current densities is given in Figure d, where the above results were based on the mass of the two active electrodes. It can be understood that the Bi2MoO6//Bi2MoO6 symmetrical supercapattery device exhibited a remarkable energy density/power density (45.6 W h kg–1/989 W kg–1), which is greater than those of symmetrical cells reported previously for CS@Bi2MoO6//CS@Bi2MoO6 (10.8 W h kg–1/410 W kg–1),[40] RuO2/graphene//RuO2/graphene (11 W h kg–1/76 W kg–1),[64] Ni@FeCo2O4@MnO2//Ni@FeCo2O4@MnO2 (22.2 W h kg–1/978.3 kW kg–1),[65] and GR/BiVO4//GR/BiVO4 (45.69 W h kg–1/800 kW kg–1)[66] devices. The energy density/power density value (45.6 W h kg–1/989 W kg–1) obtained in the present work was appealing. Electrochemical charge storage mechanisms of the Bi2MoO6//Bi2MoO6 symmetric supercapattery are displayed briefly as a schematic in Figure e.

In addition, to check the practical feasibility of the as-developed Bi2MoO6//Bi2MoO6 symmetrical supercapattery device, three cells were connected in series and charged through an external power source (at 4.5 V voltage) for 5 min and finally discharged through three differently colored LEDs, where the LEDs with considerable light intensities were illuminated for ∼18 min without any fluctuation, demonstrating the promising future of the Bi2MoO6//Bi2MoO6 symmetrical cell device (Figure a–i).

Figure 6

(a) Assembly of three Bi2MoO6//Bi2MoO6 symmetrical supercapattery devices in series that were used to illuminate a panel of three differently colored LEDs, (b–i) change in the brightness of the three LEDs operated with three series-connected Bi2MoO6 symmetric supercapattery devices with time (photograph courtesy of PVS et al.).

Figure a presents an assembly of the three Bi2MoO6//Bi2MoO6 symmetrical supercapattery devices in series with a panel of three differently colored (green, yellow, and red) LEDs. Figure b highlights an actual image at the initial glowing time of less than 1 min, Figure c–f display images with nearly 5 min time intervals that remained almost the same up to ∼17 min, then the green LED was turned off (Figure g). After a few seconds, the yellow LED went off (see Figure h) and lastly, the red LED stopped glowing after 30 s (see Figure i), suggesting the importance of the Bi2MoO6//Bi2MoO6 symmetrical device with moderate energy/power density in developing supercapattery devices. The stability of the Bi2MoO6//Bi2MoO6 symmetrical supercapattery device was tested using GCD operations, and the results are displayed in Figure f. Over about 5000 cycles at a current density of 1 A g–1, the SC based on the total mass of the two electrodes was about 31.6 F g–1, which corresponds to 78% of its initial capacitance (40.5 F g–1), evidencing the moderate chemical stability and mechanical robustness of the as-fabricated hydrangea-type Bi2MoO6 symmetric supercapattery device before its commercial use.

Conclusions

In summary, hydrangea-type Bi2MoO6 with upright standing narrow petals (separated by a considerable number of crevices) of high surface area was successfully synthesized by a simple and inexpensive wet chemical method. These special properties of Bi2MoO6 would lead to a smaller ion diffusion length and an easy electrolyte ion transfer for better electrode surface utilization and performance. In the electrochemical studies, the hydrangea-type Bi2MoO6 electrode exhibited an SC of 485 F g–1 at 5 A g–1 and supercapattery character. A symmetric supercapattery device assembly of Bi2MoO6 delivered an energy density of 45.6 W h kg–1 at a power density of 989 W kg–1. Finally, the symmetric supercapattery device successfully illuminated three differently colored LEDs with their maximum intensity. With this motivation, future work to synthesize and design other binary metal oxides/sulfides/selenides, etc., in 2D and 3D morphologies for various applications, such as in gas sensors, solar cells, electrocatalysts for water splitting, etc., is underway.

Experimental Section

Chemicals

Bismuth(III) nitrate pentahydrate [Bi(NO3)3·5H2O], sodium molybdate dehydrate (Na2MoO4·2H2O), sodium hydroxide (NaOH), and triethanolamine (TEA) were obtained from Sigma-Aldrich. Concentrated nitric acid (HNO3) was obtained from Junsei Chemical. All chemicals were of analytical grade and were used without any further purification.

Bi2MoO6 Synthesis

The synthesis of Bi2MoO6 via a wet chemical method was considered. Briefly, a solution of 0.1 M Bi(NO3)3·5H2O dissolved in 5 mL of HNO3 and 4 mL of triethanolamine (TEA) was prepared in 50 mL of deionized water. A 0.1 M NaOH solution was mixed with constant stirring by maintaining a speed of 150 rpm so as to form a clear transparent solution (∼10.5 pH).[43] Finally, 0.2 M Na2MoO4·2H2O was dissolved in 50 mL of deionized water. A clear solution was formed and the Na2MoO4 solution was added into the above mixed solution. The whole solution mixture was stirred for 1 h and then well-cleaned pieces of Ni-foam (3 × 3 cm2) were inserted vertically into the solution, and the reaction was allowed to run at 353 K for 3 h. At the end of the reaction time, whitish Bi2MoO6 directly deposited on the Ni-foam, which was washed with deionized water several times, dried, and air-annealed at 427 K for 2 h for removing any residual hydroxide species. The formation of Bi2MoO6 was evidenced as the white product was changed to a biscotti-type product (synthesis process is given in Figure S1 of the Supporting Information). Further, various characterization tools and electrochemical analyses were employed for physical and electrochemical measurements, as discussed earlier.[9,43]