Facile synthesis of Bi₂O₃@MnO₂ nanocomposite material: A promising electrode for high performance supercapacitors

Highlights

• Room-temperature synthesizing Bi₂O₃, MnO₂ and Bi₂O₃@MnO₂ electrode materials over graphite rod.

• The X-ray confirm the formation of the Bi₂O₃@MnO₂ composite matrix.

• Bi₂O₃@MnO₂ composite demonstrated 350 F g⁻¹ of specific capacitance @10 A g⁻¹ in 6 M KOH electrolyte.

• The 28 Wh kg⁻¹ energy density at 1395 W kg⁻¹ power density of Bi₂O₃@MnO₂ composite electrode.

• Enables to light a “CNED” panel designed with 42 LEDs in full-brightness for 45 s.

Abstract

Room-temperature successive ionic layer adsorption and reaction (SILAR) electroless chemical method has been proposed for synthesizing Bi₂O₃, MnO₂ and Bi₂O₃@MnO₂ electrode materials over graphite rod. The flake-type Bi₂O₃ on MnO₂ granules increases active sites on preventing the agglomeration of MnO₂ to easy and fast electrolyte ions percolation for higher energy storage performance. The X-ray photo-spectroscopy investigation provides the evidence for the formation of the Bi₂O₃@MnO₂ composite matrix. The as-prepared compose electrode material tested for its electrochemical characterizations endows 350 F g⁻¹ of specific capacitance (SC) @10 A g⁻¹ which is better than that of an individual counterpart. Furthermore, the 28 Wh kg⁻¹ energy density at 1395 W kg⁻¹ power density of its symmetric electrochemical supercapatter i.e. summation of battery and supercapacitor performance of the Bi₂O₃@MnO₂//Bi₂O₃@MnO₂ is again superior than that of individuals and also those reported previously for Bi₂O₃, MnO₂ and Bi₂O₃@MnO₂-based symmetric electrochemical storage devices. This enables to light a “CNED” panel designed with 42 LEDs in full-brightness for 45 s, suggesting a commercial potential of the as-obtained electroless Bi₂O₃@MnO₂ composite electrode material in energy storage devices.

Introduction

Technology is budding with fast speed, but familiar power devices are inadequate to cope up with required necessities [1,2]. For example, an increasing demand of durable and high power batteries is struggling for durable power and power consumption, every year, in automobile industries. Due to inadequacy of batteries the popularity of electric drive vehicles is held back. The spectacular rise of smart phones, tablets, laptops, drones and electronic good items has signified an importance of electrochemical supercapacitors (ESs) for higher power density [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. Though, ESs avail higher power density (>10 kW kg⁻¹) [12] and moderate cycling life then other devices in energy storage applications [13] their inadequate energy density mitigate direct involvement in a smart energy storage devices where both high power and energy are highly essential [[14], [15], [16], [17]]. Therefore, energy storage devices need to be developed with either novel electrode materials or architectures for higher power and energy density and a superb cycling stability with chemical and thermal stability and mechanical robustness [18,19]. Till now, various electrode materials including carbonaceous, metal oxides, chalcogenides, nitrides, hydroxides, layered double hydroxides, and conducting polymers etc., have effectively been envisaged as ESs electrode materials [[20], [21], [22]]. Several debates are active to identify whether metal oxides/chalcogenides are either battery type or supercapacitor type. Though carbonaceous electrodes explore a high surface area their wide range of applications are majorly constrained by their toxic character and higher charge transfer resistance [23]. Due to presence of several reversible faradaic redox reactions the use of transition metal oxides/hydroxides/layered double hydroxides/chalcogenides is essential for higher energy density and specific capacitance (SC). However, their low specific surface area and poor surface/electrolyte interface wettability mitigate the overall electrochemical performance followed applications [[24], [25], [26], [27]]. Therefore, for increasing all three parameters together, with an extended potential window feature, it is essential to construct and design higher performance and durable composite electrode materials by combining battery and ES properties. Graphene oxide (GO)-based electrode materials offer a large surface area, excellent mechanical strength, electrical conductivity as well as moderate electrochemical properties [[28], [29], [30]]. Lin et al. coated manganese oxide (MnO₂) over graphite by an immersion method that demonstrated 556 mF cm⁻² SC in 0.5 M LiCl electrolyte solution [31]. George et al. prepared nanoparticles of MnO₂ on graphite by a redox deposition process as ES [32] and the one developed by Deepi et al. decorating the Bi₂O₃ over graphene exhibited 136.76 F g⁻¹SC and 95% retention even after 1000 cycles [33]. Ruthenium oxide has been one of the mostly preferred electrode candidates in ES devices but its expensiveness and toxic character are serious hurdles before using it into commercial products [[34], [35], [36]]. Alternatively, MnO₂ is envisaged for several times due to its different oxidation states and positive operating window with theoretical specific capacitance of 1370 F g⁻¹ [[37], [38], [39], [40]]. Poor cycling stability and electric conductivity, from the practical point of view, hinders in obtaining a high theoretical capacitance [41]. On the other hand, bismuth oxide (Bi₂O₃) due to its quasi faradaic redox reactions and negative potential window, stable crystal structure, various morphologies, and mesoporous character is a potential alternative candidate to carbonaceous materials. In short, both electrodes are finding electrochemical properties in different operating windows therefore, by assuming an extended potential window and higher energy storage performance, in the present work; it was our aim to obtain a composite electrode with these two materials [[42], [43], [44], [45]]. In the literature, a very few reports are available on synthesis of the Bi₂O₃–MnO₂ composite electrode. Li et al. prepared Bi₂O₃-coated amorphous MnO₂ for ES applications [46]. To check the performance of pseudocapacitor affected by the temperature, Ng et al. synthesized Bi₂O₃/MnO₂ electrode, but that exhibited only 9.5 WhKg⁻¹ of energy density [47]. Furthermore, Ma et al. reported a flower-like morphology of bismuth-subcarbonate@mangnese oxide and Bi₂O₃@MnO₂ by a hydrothermal method [48]. Most of the reported research works employed either expensive synthesis techniques with limited product electrode materials quantity and area or prolonged synthesis time. Secondly, it is known that for producing these electrode materials the use of different methods is mandatory as bismuth nitrate is stable in nitric acid and manganese sulphate is sable in both i.e. acidic as well as in basic media. Thereby, the present work deals with a room-temperature and cost-effective electroless synthesis of the Bi₂O₃@MnO₂ composite electrode using successive ionic layer adsorption and reaction (SILAR) chemical method over graphite-rod (GR) as this method offers a perfect control on the deposition process and expected quantity (both structure and phase) of electrode materials, as instead of atoms, ions are building blocks [[49], [50], [51]]. The Bi₂O₃@MnO₂ composite electrode has demonstrated higher; (i) electrochemical ES performance, (ii) cycling stability, and (iii) electrochemical accessibility and ionic conductivity with the possibility of a dual charge storage mechanism i.e. double layer for GR and faradaic for Bi₂O₃@MnO₂. The growth mechanism, structural elucidation, elemental mapping, surface morphological evolution etc, of Bi₂O₃, MnO₂ and Bi₂O₃@MnO₂ electrode materials were measured and reported. The assembled and tested Bi₂O₃@MnO₂//Bi₂O₃@MnO₂ symmetric device has endowed 144 F g⁻¹ SC and 29 Wh kg⁻¹ energy density at 1395 W kg⁻¹ of power density. Using two symmetric devices in series a panel “CNED” (Centre for Nanomaterials & Energy Devices) of 42 LEDs was lighted with its full-bright intensity for nearly 45 s suggesting potentiality of synthesis method and commercial prospectus of developed electrode materials.