Electrochemical supercapacitor based on multiferroic BiMn2O5
Graphical abstract
Introduction
Oxides of Bi and Mn are of special interest for application in ES due to the multiple valence states of Bi and Mn. Many investigations were focused on the analysis of capacitive behavior of MnO2 for application in positive electrodes of aqueous ES [1], [2], [3], [4]. The charge storage mechanism of MnO2 is pseudocapacitive, it can be described by the following reaction [5]:MnO2 + A+ + e− ↔ A(MnO2)where A+ = Li+, Na+, K+, H+. Equation (1) indicates that high electronic and ionic conductivities are necessary [6], [7], [8] in order to utilize capacitive properties of MnO2 in ES electrodes. The theoretical specific capacitance [5] of MnO2 is about 1370 F g−1. However, the increase in active mass loading resulted in significantly lower capacitances [9] due to low conductivity of MnO2. This problem was addressed by the fabrication of MnO2-carbon nanotube (CNT) composites [9], [10], [11], [12]. The electrodes with mass loading above 10 mg cm−2 usually show capacitances [9] below 150–200 F g−1 at low scan rates and poor capacitance retention at high scan rates. Recently, it was recognized [13] that reporting only gravimetric capacitance as well as other mass normalized characteristics, such as power and energy densities, may not give a realistic picture of the ES performance due to strong dependence of the gravimetric characteristics on the electrode mass. The area normalized capacitance of 2.8–5.9 F cm−2 was achieved [14], [15], [16] for MnO2–CNT and MnO2–graphene electrodes with high mass loadings at low scan rates, however the capacitance decreased with increasing scan rate.
The capacitive behavior of Bi2O3 and composites was investigated for application in negative electrodes [17], [18], [19], [20], [21], [22] of aqueous ES. The cyclic voltammogram of Bi2O3 electrodes deviated significantly from ideal box shape and showed redox peaks [17], [18], [21]. Thin Bi2O3 films, prepared by electrodeposition [17], showed a capacitance of 98 F g−1 (0.022 F cm−2) in NaOH electrolyte. The specific capacitance of 255 F g−1 was reported for Bi2O3–graphene composite [18]. The charging mechanism [23], [24] of Bi2O3 was described by the reaction:Bi2O3 + A+ + e− ↔ A(Bi2O3)where A+ = Li+, Na+. Recently asymmetric ES was prepared [24], containing MnO2 positive electrode and Bi2O3 negative electrode, which showed a specific capacitance of 97 mF cm−2. The literature data on capacitive properties of Mn and Bi oxides have generated our interest in the investigation of complex oxides, such as BiMn2O5. The investigation of complex oxides is now an important avenue [25], [26], [27] for the development of advanced electrode materials for ES.
BiMn2O5 belongs to the group of multiferroic materials, which exhibit ferroelectric and magnetic properties, magnetoelectric effect and other functional properties [28], [29]. BiMn2O5 has an orthorhombic structure with ordered Mn4+ and Mn3+ ions. The Mn4+ ions are located in [Mn4+O6] octahedra [30], which share ages and form a linear chain along the c-axis. The octahedra are interconnected by [Mn3+O5] square pyramids, containing Mn3+ ions [30]. The 6s2 electrons of the highly polarizable Bi3+ cations contribute to the large distortion of the [BiO8] polyhedrons [30] and favor the formation of ferroelectric ordering [31] in BiMn2O5 in the direction of b-axis below the antiferromagnetic Neel temperature TN = 40 K. Moreover, BiMn2O5 exhibits interesting properties at higher temperatures, which are not well understood. In the previous investigation [32] we discovered a pyroelectric effect in the range of 77–500 K in the a-axis direction of BiMn2O5 monocrystals. The pyroelectric properties were also reported [33], [34] for polycrystalline BiMn2O5 in a wide temperature range. The pyroelectric current changed its sign with switching the direction of the poling field of the polycrystalline material [34]. The investigations of dielectric properties of BiMn2O5 revealed high dielectric constant [35] in the range of 105–106 at room temperature, which is promising for applications in capacitors. The studies of frequency dependence of the dielectric constant revealed [35] a low temperature dielectric relaxation, attributed to the hopping of charge between Mn3+ and Mn4+, and high temperature dielectric relaxation, related to oxygen defects. The analysis of the dielectric constant at different conditions [33] suggested a relaxor type ferroelectric behavior at high temperatures. However, no direct evidence of ferroelectric hysteresis behavior was observed at room temperature [30]. Investigations were focused on the analysis of multiple valence states [35], [36] of Mn and Bi under the influence of different factors and their influence on electrical properties of BiMn2O5.
The goal of our investigation was the fabrication and testing of BiMn2O5 based electrodes and devices. The approach was based on the use of hydrothermal synthesis for the fabrication of submicrometre BiMn2O5 particles. An important finding was the possibility of the fabrication of BiMn2O5–MWCNT composite electrodes using Celestine blue dye as a co-dispersant for BiMn2O5 and MWCNT. The results presented below showed excellent capacitive performance of BiMn2O5–MWCNT electrodes, which exhibited high capacitance at low scan rates and outstanding capacitance retention at high scan rates and high active mass loadings. These results paved the way to the fabrication of ES devices with high power-energy characteristics, based on the BiMn2O5–MWCNT electrodes. Mowing toward this goal we fabricated and tested asymmetric ES, containing BiMn2O5–MWCNT positive electrodes and AC–CB negative electrodes, which showed excellent performance in a voltage window of 1.8 V in an aqueous electrolyte.
Section snippets
Materials
Celestine blue, Bi(NO3)3, KMnO4, MnCl2∙4H2O, KOH, Na2SO4, and poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVB, average MW = 50 000–80 000) were purchased from Aldrich company. MWCNT were purchased from Bayer Company. Ni foams with 95% porosity were provided by Vale Company. For synthesis of BiMn2O5 particles, 0.6 mmol of Bi(NO3)3 was dissolved in 40 ml of deionized water, and then 0.8 mmol of MnCl2∙4H2O and 0.4 mmol of KMnO4 were added. Stirring of the solution was performed for
Experimental results
XRD studies of as-prepared material confirmed the formation of BiMn2O5. The diffraction pattern presented in the Fig. 1 is in agreement with the JCPDS file 027-0048 of BiMn2O5. The SEM images of the BiMn2O5 powder at different magnifications are presented in Fig. 2(A,B). The SEM image at low magnification (Fig. 2A) indicated low agglomeration of the powder. The analysis of the image (Fig. 2B) at higher magnification indicated the formation of submicrometre particles. The diameter of the
Conclusions
Submicrometre particles of multiferroic BiMn2O5 were prepared by a hydrothermal method. It was demonstrated for the first time that BiMn2O5–MWCNT composite can be used as a new active material for positive electrodes of ES. Celestine blue dye can be used as a co-dispersant for BiMn2O5 and MWCNT for the formation of BiMn2O5–MWCNT composites from colloidal suspensions. The composite BiMn2O5–MWCNT electrodes with high mass loading and high active material to current collector mass ratio showed a
Acknowledgments
The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada for the financial support and Vale Canada for the donation of Ni foam.
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