Solvothermal synthesis dependent structural, morphological and electrochemical behaviour of mesoporous nanorods of Sm₂NiMnO₆

Abstract

Polycrystalline nanorods of Sm₂NiMnO₆ with monoclinic P₂1/n symmetry have successfully been synthesized via solvothermal process. The mesoporous nature of nanorods as confirmed by average pore diameter of 10 nm was clearly visible in transmission electron micrographs. These nanorods exhibited high specific surface area of ~48 m²/g with presence of Sm³⁺, Ni²⁺, Mn⁴⁺ ions on the surface of Sm₂NiMnO₆ crystallites. Further, the electrode of mesoporous Sm₂NiMnO₆ nanorods exhibited specific capacitance of 825.6 F/g at scan rate of 2 mV/s and ~86% retention of specific capacitance for ~1500 cycles at constant current density of 4 A/g.

Introduction

The rare earth driven double perovskites with formula R₂NiMnO₆ (R-rare earth cations) owing unique electronic configuration of {Xe} 4f ⁿ⁻¹ 5d⁰⁻¹ 6s² (where n = 1–15) is a widely studied special class of materials with interesting physical properties such as ferromagnetism, magneto-resistance and giant-dielectric constant [1,2]. The Sm₂NiMnO₆, member of this series has been in attraction due to its multiferroic behaviour, and promising technological and device applications [2,3]. The reports on experimental investigations of structural, dielectric and magnetic behaviour of Sm₂NiMnO₆ have been available in literature [[2], [3], [4], [5], [6], [7], [8]]. The Lekshmi et al. have shown the ferromagnetic insulating behaviour of Sm₂NiMnO₆ coupled with magneto-dielectric effect that makes it further more attractive for spintronic applications [5]. The reports on theoretical calculations via density functional theory (DFT) for Sm₂NiMnO₆ on electronic band structure and optical properties are available [2,9]. The DFT calculations have shown that Sm₂NiMnO₆ is metallic in nature with very strong hybridization near the Fermi level [9]. The Sm₂NiMnO₆ exhibited direct optical bandgap of 1.41 eV with high value of absorption coefficient in visible region by DFT calculations which lies close to well-known light absorbing materials frequently used in the fabrication of solar cells [2]. Further, manganese is found to be the most suitable additive and is most commonly used in the synthesis of rare earth based oxide double perovskite structures due to manganese exhibiting multiple valence/oxidation states i.e., Mn²⁺, Mn³⁺, Mn⁴⁺ [8].

Many binary metal oxides investigated show excellent electrochemical behaviour, however, over the last few years; ternary or higher order metal oxides have attracted the researchers' attention in the energy research [10,11]. Ternary/higher order oxides are able to provide additional redox active sites which will result in fast and excellent faradic redox reactions. It will enhance critical electrochemical parameters such as specific capacitance, cyclic stability, energy density, etc. [1,11]. Perovskites materials exhibit characteristics such as unique tunability in composition, high structure stability, electrochemical and thermal stability, multiple flexible oxidation/valence states and good catalytic activity. These properties make them efficient for batteries or electrode materials for supercapacitors [[10], [11], [12], [13]]. In perovskite oxide structures; lanthanides at M site (such as La, Nd, Sm etc.) are usually not taking part in electronic structure near Fermi level, but M-site cations may affect crystal structures, oxidation/valence states of cations at N/N′- sites, active area for electrochemical activity, porosity and oxygen vacancies [1]. Perovskite oxides with such properties along with their structures at nanoscale minimize internal electron transfer resistance, provide enhancement in performance; make them prominent materials in the field of energy research [12,13]. Many rare earth and alkaline earth driven perovskites oxides have experimentally been investigated for fabrication of electrode for electrochemical applications [[13], [14], [15], [16], [17], [18], [19]]. However, the limited reports on the application of double perovskite oxide materials are available [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. The Bavio et al. have investigated electrochemical behaviour of La₂NiMnO₆ double perovskite which belongs to R₂NiMnO₆ series [31]. Further, many cobalt based binary, ternary transition metal oxide have been favourite materials for energy storage applications. But, these are associated with certain complications related to high cost, high thermal expansion coefficient, fast degradation, less chemical stability, excess solubility of cobalt in aqueous solutions for redox reactions [13,24,26]. It is necessary to explore cobalt free compounds in order to overcome these complications. The series R₂NiMnO₆ (R-lanthanide, yttrium) hold a great potential for electrochemical performance such as supercapacitor along with cobalt free nature making them favourable materials for the fabrication of electrode.

The variety of synthesis procedures have been used to synthesize Sm₂NiMnO₆ such as sol-gel, solid state reaction route, Pechini method, sol-gel auto combustion method, etc. [[2], [3], [4], [5],7,8]. To synthesize Sm₂NiMnO₆, solvothermal method has never been used till date. This particular method has its own significance for electrochemical performance due to its ability to produce high aspect ratio, high specific area, porous structure, wide pore size distribution, controlled morphology, and provides excellent charge transport at nanoscale with low agglomeration, etc. [13,32,33]. Further, soft templates and hard templates have been used to synthesize mesoporous materials. These methods involve multistep synthesis, and often a residual template has been left in the final compound which affects the purity [34]. The porosity is also considered to be important characteristics for mass transport of the liquid electrolyte. Subsequently, the solid electrode results in fast and facile faradic redox reaction, providing better discharge rate which, in turn, enhances electrochemical performance [10]. Therefore, template free solvothermal route has been adopted to synthesize mesoporous nanorods of Sm₂NiMnO₆. Further, band gap also plays the important role in electrochemical performance. The lower band gap material results in good value of specific surface area, provides massive active sites with excellent ion conductivity which, in turn, leads to enhancement of specific capacitance with very good cyclic stability as compared to higher band gap materials [[35], [36], [37], [38]]. These characteristics are very important for electrochemical reaction to take place and result in higher specific capacitance, and excellent cyclic stability. The lower band gap [2], multiferroic behaviour [3], and cobalt free nature motivated us to use Sm₂NiMnO₆ as electrode material for electrochemical performance. No experimental study has been reported so far on the solvothermal approach and electrochemical performance of Sm₂NiMnO₆.

The present work gives the new synthesis approach for mesoporous nanorods of Sm₂NiMnO₆. The mesoporous nanorods of Sm₂NiMnO₆ has been fabricated by template free facile solvothermal route; comprehensively investigated for its structural, morphological and electrochemical performance along with calculation of critical parameters such as specific capacitance, energy density and power density.