Robust one-step synthesis of bismuth molybdate nanocomposites: A promising negative electrode for high end ultracapacitors

Highlights

• Aurivillius structured bismuth molybdate systems are successfully prepared by single-pot, rapid microwave combustion method.

• PXRD and Rietveld refinement reveals the formation of orthorhombic koechliniteγ-Bi₂MoO₆ phase for both samples.

• An enhanced C_sp of 662.8 Fg^-1 with 87.4 % stable retention after 2000 cycles for Bi₂MoO₆: La, which is higher than Bi₂MoO₆.

• Energy density and power density upgraded rapidly after the substitution of La ions in the bismuth molybdate.

Abstract

Rare earth metal (La) doped aurivillius structured bismuth molybdate (Bi₂MoO₆: La) has been prepared by a single pot microwave-assisted combustion method. The characteristics of the bare Bi₂MoO₆ and Bi₂MoO₆: La catalysts have been analyzed systematically using different characterization techniques. The crystalline phase purity of the nanostructures has been analyzed by PXRD and Rietveld refinement technique with an average crystallite size approximately 28 nm (Bi₂MoO₆) and 25 nm (Bi₂MoO₆: La). The surface morphologies have been nanoplatelets for both samples. The Bi₂MoO₆: La structure ensures maximum capacitance of 662.8 F g⁻¹ at 5 mVs⁻¹. Also, they performed good cyclic stability of 87.4% over 2000 cycles with a current density of 5 mA cm⁻². The prepared active material serves as a novel negative electrode material for supercapacitors.

Introduction

The combustion synthesis (CS) method is a powerful, rapid and low-cost method for the manufacture of different mechanically functional materials. Nowadays, CS has turned into a well-known methodology for the preparation of nanomaterials and is rehearsed around 65 nations. As of late, various imperative leaps have made forward in this area, outstandingly to the better improvement of new catalysts and nano-carriers through properties superior to those of conventional materials. In recent years, researchers have focused on the CS capacities for materials structural enhancement, energy-saving, ecological assurance and also the significance of industrialization is moreover grasped [[1], [2], [3]]. Solution Combustion Synthesis (SCS), a best type of CS, is an adaptable, simple and easy with less preparation time, permits viable production of an assortment of nanosized substances. This nano synthesis procedure includes a self-supporting reaction in the homogeneous arrangement of various oxidizers and fuels. Contingent upon the kind of precursor together with circumstances utilized for the procedure association, SCS may take place as either volume or layer-by-layer engendering combustion manner. This procedure capitulates nano-size oxides, as well as permits, a consistent homogeneous dissolving of rare-earth ion by a single step in a solitary advance [4]. Amid synthesis of catalysts and electrode materials possess the lion's share of recently published articles. The most recent improvements in the SCS system focused on materials applications [3, [5], [6], [7], [8]]. The nano-phosphors is an intriguing issue in CS. The scope of nano-phosphor based compounds was arranged by SCS [7]. It is apparent that urea keeps on being the favoured fuel to phosphor-based nanomaterial preparations. A substantial number of articles has been published on materials for energy devices, supercapacitors, batteries, dye-synthesized solar cells, solid oxide fuel cells, photocatalysis, direct methanol fuel cell, etc. [3,[7], [8], [9], [10], [11], [12]]. Nanocrystalline powders are synthesized by microwave combustion techniques for widespread applications [3,8]. Recently, catalysts synthesized by SCS (viz: metal-doped ceria and titania) for the environmental application have demonstrated that the doping in those oxides consists of ionic substitution and not metal substitution responses superior catalysis, which cannot be achieved by any chemical synthetic route [13]. This material is utilized as a three-path impetus in autos. These facile and economical procedures for the synthesis of the bolstered catalysts hold an incredible guarantee for environmental remediation.

Recently, bismuth-based semiconductors have pulled in broad consideration because of their photo-response in the visible region. A characteristic aurivillius oxide called Bi₂MO₆(M = Mo/W/V) possess layered Bi oxide were of unique enthusiasm owing to the improvement of their dielectric, catalytic, luminescent, ion-conductive, and synergist properties [14,15]. Current outcomes exposed that Bi₂MoO₆ could execute as a phenomenal dynamic material meant for water splitting and organic degradation in UV light illumination [16]. The reliance on the properties of NPs with the size and shape is a marvel of both essential logical intrigues, with numerous reasonable and innovative appliances [17]. For instance, nano-catalysts were accepted to be a superior performer because of their high surface-to-volume proportion and partition productivity of photo-generated electrons or holes [18]. Bi₂MoO₆ is preferable to be utilized in the visible region of the electromagnetic spectrum owing to their reduced bandgap in the region of 2.5–2.8 eV [19]. Conversely, the rapid recombination of electrons and holes decrease the catalyst efficacy. So, developing epic nanostructures by base up systems might be soundly planned.

Also, researchers have a keen interest in using metal molybdates for energy conversion and storage devices. Molybdate based electrodes have established momentous curiosity in lithium-based batteries and supercapacitors because of steady crystal structure and redox behavior of metals [3,8]. Generally, bismuth molybdate exists in α, β and γ phase with different chemical formulas Bi₂Mo₃O₁₂, Bi₂Mo₂O₉ and Bi₂MoO₆ respectively. These compounds and their combinations are conversant because of their higher catalytic performance in selective oxidation of hydrocarbons to a valuable product [15,16,20]. The γ-Bi₂MoO₆ phase is more attractive than the other two owing to its layered structure and displays awesome catalytic activity [21]. Though bismuth molybdate is well-known for many applications mostly in catalysis, its utilization as electrode materials is limited. Development of supercapacitors that can deliver high energy density and power density with good cyclic retention are used in electric vehicles and versatile electronic gadgets in modern industry to replace batteries to meet the current needs. Though researchers gained sufficient improvement in supercapacitors, their electrode materials are yet restricted mostly because of low capacitance (C based electrodes) [22], highcost (Ru based electrodes) [23], and not eco-friendly (S based electrodes) [24], which has extremely frustrated the advancement of supercapacitors. Metal oxide-based electrode is considered as a potential candidate for elite supercapacitors due to their multiple oxidation states with respectable electrical conductivity [3,25]. Gujar et al. (2006) prepared electrode for supercapacitor using Bi₂O₃ thin film, which performed good electrochemical reversibility but experiences very low C_sp of 98 Fg^-1 at 20 mVs^-1 scan rates [26]. Yuan et al. (2009) suggested a bismuth-based negative electrode on carbon achieved maximum C_sp of 232 Fg^-1 at the sweep rate of 5 mVs⁻¹ [27]. Even though some improvement is adopted in the preparation and electrochemical properties of bismuth based negative electrode materials in supercapacitors, the electrochemical kinetics, and energy stockpiling mechanism even continueto be a great contest.

The synthesis method is one of the key factors which control the particle size, morphology, and electrochemical of metal oxide nanoparticles. Recently, Wang et al. synthesized carbon decorated metal oxide nanocomposites (NiFe₂O₄/N-GN/ZnO and Mn₃O₄/MWCNTs) and PANI/graphene aerogel through one-step hydrothermal method and in-situ polymerization techniques to exterminate electromagnetic pollution problem as a prospective high-performance microwave absorber in practical applications [[28], [29], [30]]. But we have chosen the microwave irradiation method for the present work because it is a simple and novel method and it requires a less time, notably reduces the time of reaction from days and hours to minutes or even seconds for the synthesis of nanoparticles [3]. The synthetic technique posses several advantages over other methods in terms of enhanced reaction kinetics, selective materials coupling, rapid volumetric heating, high reaction rate, increased product yield, most purity product, etc. than many other conventional methods [3]. Moreover while synthesis; it has the capability of restraining the by-products. These desirable properties arising out of microwave synthesis are due to the following reasons: increase of the heating rate of the reaction mixture, uniform heating, change of the association between the species within the reaction mixture, superheating, creation of hot spots and enhancement of the dissolution of the precursor [31].

In this work, Bi₂MoO₆ nanocrystals and Bi₂MoO₆: La nanocrystals were synthesized via a single pot microwave combustion method; a prompt kind of SCS technique and its electrochemical performance were compared. The structural properties of the prepared pure and La-doped bismuth molybdates were investigated by the Rietveld refinement method [32] using JANA2006 [33] software. Though Bi-based oxides haven been reported recently for widespread applications in many fields, limited articles were published for energy storage applications. Recently, Shameem et al. (2019) reported the synthesis of metal molybdate by microwave-assisted combustion method and its electrochemical performance were compared posing superior advantages over other methods [3]. Comparing the pure and La-doped Bi₂MoO₆, Bi₂MoO₆: La nanocomposites expected to have much higher electrochemical activity. The preparation of a negative electrode for supercapacitors is a great challenge for the next-generation energy-storage devices. Herein the present study, Bi₂MoO₆: La composites expected to have a much higher specific capacity with good cyclic stability than pure bismuth molybdate, a novel negative electrode for supercapacitors.