Electrochemical performance of Bi2O3 supercapacitors improved by surface vacancy defects
Introduction
The continuous increase in environmental pollution due to excessive use of traditional fossil fuels (coal, petroleum, and natural gas) renders it imperative to develop ultrahigh-performance and environment-friendly energy storage equipment [[1], [2], [3], [4], [5]]. Supercapacitors with a fast charging/discharging mechanism and a long cyclic life can provide sufficient energy for modern electronic devices and new energy automobiles [6,7]. Therefore, the development of advanced supercapacitors has received widespread attention. Carbon, conductive polymers, and metal oxides are the three main electrode materials for supercapacitors, each with their own advantages and disadvantages in the application of supercapacitors. Although carbon materials have satisfactory electrical conductivity, their lower specific capacitance (SC) is a bottleneck for industrial applications. The SC of polymers is relatively higher; however, their cycling stability is poor. In comparison to the first two types of materials, metal oxides can store energy by fast and reversible redox reactions and generally have a higher theoretical SC. At present, RuO2 [8], MnO2 [9], NiO [10], and Co3O4 [11] exhibiting high theoretical SC are the metal oxide materials mainly used for supercapacitor electrodes.
Bi2O3 with a proper bandgap is a promising material in the field of catalysis [12,13]. There are several studies on Bi2O3 in catalysis. Recent studies show that Bi2O3 has a theoretical capacity of 690 mA h g-1 and a relatively wide potential window (-1–0 V) [[14], [15], [16]], making it potentially useful in supercapacitors. Shinde et al. [17] synthesized a mesoporous micro-sponge-ball-like Bi2O3 electrode with a SC of 559 F g-1 at 0.4 A g-1 and a wide potential window of 1 V. Huang et al. [18] successfully synthesized a rod-shaped Bi2O3 electrode with a high SC of 528 F g-1 at 5 mV s-1. Qiu et al. [19] prepared ultrathin Bi2O3 nanowires with a SC of 691.3 F g-1 at 2 A g-1. Furthermore, Bi2O3 is one of the few metal oxides with a negative potential window and can, therefore, serve as an anode material for asymmetric capacitors (ASCs). Although carbon materials are the most commercially available cathode materials, their lower SC directly limits the greater energy density. If carbon materials are replaced by metal oxides with higher capacitance, the energy density of the ASCs would be greatly improved. For instance, Li et al. [20] prepared a CF@NiCo2O4//ECNF@Bi2O3 ASC with a high SC of 25.1 Wh kg-1 at 786.2 W kg-1. Shinde et al. [21] fabricated a Bi2O3//MnCO3QDs/NiH–Mn–CO3 ASC, with a specific energy of 47 Wh kg-1. However, due to its poor conductivity, Bi2O3 leads to unsatisfactory electrochemical performance. In order to boost the conductivity of Bi2O3, it is generally combined with carbon or conductive polymers materials [[22], [23], [24]]. In addition, creating oxygen vacancy defects in metal oxides is a classic way to improve their electrical conductivity. Moreover, oxygen vacancies can not only improve conductivity but also introduce reactive sites in metal oxides [25].
In the present research, two types of Bi2O3 powders with different morphologies were synthesized for supercapacitor electrodes by calcination (denoted as c-Bi2O3) and two-step hydrothermal methods (denoted as h-Bi2O3). Vacancy clusters of with different concentrations were observed in these two samples by positron annihilation lifetime spectroscopy (PALS). In comparison to the calcined c-Bi2O3 electrode, the h-Bi2O3 one had more surface defects. The studies on the electrochemical performance of Bi2O3 show that the defects act not only as shallow donors to improve the conductivity of Bi2O3 but also as active sites for the insertion/extraction of the K+ ions to increase the pseudo-capacitance of Bi2O3.
Section snippets
Synthesis of Bi2O3
Bi2O3 precursors were prepared through a traditional hydrothermal route. About 2.328 g of Bi(NO3)3·5H2O was dissolved in 48 mL of glycerol and absolute ethanol (volume ratio of 1:1). After the complete dissolution of the solid, the solution was poured into an 80 mL reactor and maintained at 160 °C for 3 h. The resultant solid was washed, dried, and then used as the precursor. Subsequently, the resultant precursors were processed by two different synthetic routes. In the first synthetic route,
Characteristics of h-Bi2O3 and c-Bi2O3
The XRD pattern spectra of the precursor, h-Bi2O3, and c-Bi2O3 powders are displayed in Fig. 1a. The precursor obtained by the one-step hydrothermal method exhibits a typical halo peak of an amorphous material, indicating a basic amorphous structure (green curve). The XRD patterns of h-Bi2O3 and c-Bi2O3 reveal the α-Bi2O3 phase with the lattice constants of a = 5.850 Å, b = 8.165 Å, and c = 7.510 Å, which matched well with JCPDS No.71–0465, indicating that the same crystal phase was achieved
Conclusion
In summary, Bi2O3 with different morphologies were synthesized and used for supercapacitor electrodes. In spite of essentially the same surface areas, pore distribution, and total pore volume, the h-Bi2O3 electrode exhibited much better electrochemical performance than the c-Bi2O3 electrode because of the higher concentration of surface vacancy clusters in the former, as revealed by PALS. In comparison to pure oxygen vacancies, the defects, due to their larger volume, could allow
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We thank the National Key R&D Program of China (2019YFA0210003) and the National Natural Science Foundation of China (12075172, 11875209, 11705029) for financial support.
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