Solvothermal synthesis of novel pod-like MnCo2O4.5 microstructures as high-performance electrode materials for supercapacitors

https://doi.org/10.1016/j.ijhydene.2019.11.153Get rights and content

Highlights

  • Pod-like MnCo2O4.5 porous microstructures were solvothermally prepared.

  • A highly specific capacitance of 321 F g−1 at 1 A g−1 was obtained.

  • 69.5% rate capability was reached with current density increasing from 1 to 10 A g−1.

  • 87% capacitance retention after 4000 cycles at 3 A g−1.

Abstract

MnCo2O4.5 pod-like microstructures were successfully prepared through an initial solvothermal reaction in a mixed solvent containing water and ethanol, and combined with a subsequent calcinations treatment of the precursors in air. The total synthetic process was accomplished without any surfactant or template participation. The MnCo2O4.5 pods possessed a specific surface area as high as 73.7 m2/g and a mean pore size of 12.3 nm. The electrochemical performances were evaluated in a typical three-electrode system using 2 M of KOH aqueous electrolyte. The results demonstrated that such MnCo2O4.5 pods delivered a specific capacitance of 321 F/g at 1 A/g with a rate capability of 69.5% at 10 A/g. Moreover, the capacitance retention could reach 87% after 4000 cycles at 3 A/g, suggesting the excellent long-term cycling stability. Furthermore, the asymmetric device was fabricated by using MnCo2O4.5 porous pods as anode and active carbon as cathode. It could deliver a specific capacitance of 55.3 F g−1 at 1 A g−1 and an energy density of 19.65 W h kg−1 at a power density of 810.64 W kg−1. Such superior electrochemical behaviors indicate that the MnCo2O4.5 pods may be served as a promising electrode material for the practical applications of high-performance supercapacitors. The current synthesis is simple and cost-effective, and can be extended to the preparation of other binary metal oxides with excellent electrochemical properties.

Graphical abstract

A simple solvothermal route with a post annealing treatment was used to prepare pod-like MnCo2O4.5 microstructures. These MnCo2O4.5 pods delivered a high specific capacitance of 321 F g−1 at 1 A g−1 in 2 M of KOH electrolyte with a cycling stability of 87% capacitance retention after 4000 cycles at 3 A g−1. The assembled MnCo2O4.5//AC asymmetric supercapacitor could deliver a specific capacitance of 55.3 F g−1 at 1 A g−1 and an energy density of 19.65 W h kg−1 at a power density of 810.64 W kg−1, respectively.

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Introduction

Renewable energy resources are doomed to be the most important ones for the future energy applications, and to compensate the energy crisis caused by the depletion of fossil fuels [1,2]. Wind energy, solar energy, and tidal energy are options among natural clean energy resources. They are yet highly depended on the natural environment and affected by seasonal and regional changes [3,4]. Their continuity and stability cannot be guaranteed, so the technologies for efficient energy storage are required in order to store these resources and output them steadily. Supercapacitors (SCs), a type of green energy storage device, have received increasing attention owing to their high specific capacitance, fast charge-discharge capability, high power density, and long-term cycling stability [5]. However, the practical applications of SCs are limited to some extent due to their low energy density. It is well known that the electrochemical performance of SCs is highly depended on electrode material. Hence, designing electrode materials with superior electrochemical performance is of great significance. The electrochemical performance of an electrode material is variable controlled by many factors, which is a comprehensive qualitative expression of multiple parameters. Moreover, well controlled dimensionality, porosity, size, and crystal structure of the electrode materials have also been regarded as critical factors that may bring some novel and unexpected properties. The specific surface area of material is one of the important factors. During the energy storage process, not all surfaces of electrode material are in full contact with the electrolyte, that is to say, not all contact surfaces are considered to be effective surfaces. To describe the electrochemical behavior of an electrode, the specific surface area is defined as the electrochemically active surface area [6]. The pores that are too large or too small are not conducive to improvement of the specific capacitance. Therefore, when referring to the specific capacitance of an electrode material, the number and pore size are not decisive factors, but the distribution of the pore width is. Generally, mesoporous materials are more suitable to be used for electrode materials. Three types of materials including carbon-based materials, conducting polymers, and transition metal oxides are commonly served as electrode materials [7,8].

Based on the mechanism of energy storage, SCs can be classified as electrical double layer capacitors (EDLCs) and pseudocapacitors (PCs). The energy storage process of pseudocapacitive electrode materials with faradaic activity is different from that of EDLCs with charge accumulation [9,10]. The former involves in chemical reactions and is a chemical behavior, while the latter is a pure physical process. As a matter of fact, PCs usually show higher specific capacitance than that of EDLCs owing to the rich and reversible redox reactions [11]. Therefore, many efforts have been paid to the development of low-cost and high-performance electrode materials for further improving the performances of PCs. Transition metal oxides (TMOs) are rich in nature and capable of undergoing rich redox reactions [12,13]. Thus, TMOs are considered to be electrode materials that have been highly promising. As a classical pseudocapacitive electrode material, the electrochemical properties of RuO2 have been investigated since 1970s. The experimental specific capacitances of RuO2 were one order of magnitude lower than the theoretical value [14]. In addition, elemental ruthenium is expensive, which makes it difficult to achieve commercial applications in high yields [15]. Therefore, cheap transition metal oxides such as NiO [16], SnO2 [3,17], MnO2 [18,19], and Co3O4 [[20], [21], [22]] have been studied as alternative materials for RuO2, and some amazing results have been achieved. Among them, Co3O4 is a well-studied electrode material and possesses ultra-high theoretical specific capacitance (3560 F/g) [23]. Nevertheless, it is still a less ideal issue for cycling stability and rate capability, which is difficult to meet the needs of practical applications.

Partially substituting Co atom in Co3O4 crystal structure with another relatively inexpensive metallic atom to obtain Co-based binary metal oxides may show better electrochemical performances than that of pure Co3O4 [[24], [25], [26]]. This is mainly due to the multi-oxidation valence states of the two elements and the synergistic effect between different metals. Manganese possesses richer oxidation state compared with other transition metal elements. Using Mn to partially substitute Co atoms in Co3O4 crystal structure is expected to obtain better electrochemical behaviors. There are many forms for the manganese-cobalt oxides, such as MnxCo3-xO4, (Mn,Co)3O4, MnCo2O4.5, and so forth. So far, a certain number of literatures on porous MnxCo3-xO4 nanostructures have been reported as electrode materials for SCs [13,[27], [28], [29], [30]]. However, compared with other manganese-cobalt oxides, only a few examples about MnCo2O4.5-based materials have been reported as electrode materials until now. Specifically, it was reported that urchin-like MnCo2O4.5 exhibited a specific capacitance of 129.2 F/g at 0.1 A/g and 108 F/g at 5 A/g, respectively [31]. MnCo2O4.5 nanoparticles (NPs) could be obtained with a grinding-calcination process, and these MnCo2O4.5 NPs delivered 149 F/g at 0.2 A/g with a rate capability of 58.3% at 4 A/g [32]. The low specific capacitances of these MnCo2O4.5 electrode materials are probably generated from the low electrical conductivity and small specific surface area. So it is necessary to adjust the pore size distribution and to increase the specific surface area for further improving the specific capacitance.

Besides, the formation of MnCo2O4.5-based composite can also improve the electrochemical performance of MnCo2O4.5 electrode material. For example, MnCo2O4.5/graphene composite was hydrothermally synthesized at 150 °C for 16 h, and it exhibited 252.3 F/g at 0.5 A/g and 92.6% capacitance retention after 1000 cycles at 1 A/g [33]. MnCo2O4.5 nanoneedle/carbon aerogel could deliver 380 F/g at 0.2 A/g with a rate capability of 50.5% at 10 A/g and 86% capacitance retention after 5000 cycles at 2 A/g [34]. However, a tedious preparation process with high temperature and Ar atmosphere was required for the synthesis of such composite. The carbon aerogels were prepared using chitosan solution through a freezing process (−40 °C for 24 h) and pyrolysis process (800 °C for 3 h under Ar atmosphere), after that, the sample was kept at 700 °C for activation. The final MnCo2O4.5 nanoneedle/carbon aerogel was also experienced a 600 °C calcination process in N2 atmosphere. Co3O4/MnCo2O4.5 core-shell polyhedrons prepared by using ZIF-67 templates possessed a specific capacitance of 636 F/g at 1 A/g and a rate capability of 66.8% at 10 A/g, and 86% capacitance retention was obtained after 5000 charge-discharge cycles at 8 A/g [35]. MnCo2O4.5@Ni(OH)2 (MCN) flower-like arrays were reported to deliver 2544.44 F/g at a current density of 3 A/g, however, the synthesis process involved a complicated three-step process (hydrothermal-annealing-hydrothermal). Under the same synthesis condition as that of MCN, MnCo2O4.5 nanoneedles possessed a specific capacitance of 1166.67 F/g and only 34.8% of its initial capacitance was remained after 3000 cycles at 10 A/g [36]. In addition, MnCo2O4.5@NiCo2O4, MnCo2O4.5@δ-MnO2, and MnCo2O4.5@α-MnO2 were also reported as electrode materials with good electrochemical properties [[37], [38], [39]]. To fabricate these MnCo2O4.5-based composites, complicated synthetic steps were required and toxic organic solvents were used sometimes. These disadvantages may hinder their large-scale production and further applications in SCs field.

In this work, a rapid and simple solvothermal method was employed to prepare novel pod-like MnCo2O4.5 mesoporous microstructures. Such MnCo2O4.5 pods possessed a high specific surface area of 73.7 m2/g, and the porous structure was beneficial for rapid Faradaic reactions. The excellent electrochemical properties were proved by the electrochemical tests in a three-electrode system. The MnCo2O4.5 pods exhibited a high specific capacitance as high as 321 F/g at 1 A/g with a good rate capability of 69.5% at 10 A/g, and possessed an outstanding cycling durability with 87% capacitance retention after 4000 cycles at 3 A/g. An asymmetric device was fabricated by using MnCo2O4.5 porous pods as positive electrode and active carbon as negative electrode. It could deliver a specific capacitance of 55.3 F g−1 at 1 A g−1 and an energy density of 19.65 W h kg−1 at a power density of 810.64 W kg−1, respectively. The current synthetic strategy may be extended to the fabrication of other transition metal oxides, which will bring new opportunities for other types of energy saving devices.

Section snippets

Material preparation

All chemical reagents were in analytical grade and were used without further purification. In a typical process, 10 mL of distilled water and 30 mL of absolute ethanol were firstly mixed under magnetic stirring, then 1 mmol of Mn(Ac)2·4H2O and 2 mmol of Co(Ac)2·4H2O were dissolved into above mixed solvent to obtain a homogeneous pink solution. After that, 20 mmol of urea was added with continuous stirring for another 30 min. The solution was transferred into a 50 mL Teflon-lined stainless

Results and discussion

The crystal structure of the sample obtained by solvothermal reaction at 140 °C for 3 h and post-annealing treatment of the precursor at 450 °C for 4 h was analyzed by powder X-ray diffraction measurement (Fig. 1a), and all the diffraction peaks were identified to that of cubic MnCo2O4.5 according to the standard data (JCPDS No. 32–0297, space group: Fd-3m (227), a = b = c = 8.08 Å, α = β = γ = 90°) [41]. No peaks ascribed to the impurities such as MnCo2O4, Co3O4, and other oxides were

Conclusions

In summary, novel pod-like MnCo2O4.5 porous microstructures were solvothermally prepared combined with a post annealing treatment of the precursor in air. These MnCo2O4.5 pods possessed a large specific surface area of 73.7 m2/g and an average pore size of 12.3 nm. The electrochemical performances of the MnCo2O4.5 pods were evaluated in a three-electrode system using 2 M of KOH aqueous electrolyte. The MnCo2O4.5 pods electrode material delivered a specific capacitance of 321 F/g at 1 A/g with a

Acknowledgments

The authors gratefully acknowledge the financial supports from International Cooperation of Science and Technology Projects in Shanxi Province (201703D421040 and 201803D421092), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Shanxi Province.

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