Preparation of Y-doped ZrO₂ coatings on MnO₂ electrodes and their effect on electrochemical performance for MnO₂ electrochemical supercapacitors

Abstract

To enhance the cycling stability and conductivity of manganese dioxide (MnO₂) electrodes for supercapacitors, yttrium (Y) doped zirconia (ZrO₂) (denoted as Y/ZrO₂) is coated on MnO₂ supercapacitor electrodes (Y/ZrO₂@MnO₂ electrodes), so protecting the MnO₂ electrodes in the electrolytes and enhancing the electrochemical performance of MnO₂ electrodes in sodium sulfate electrolytes. The Y/ZrO₂@MnO₂ electrodes are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and XRD analysis. The electrochemical properties of the electrodes are tested and analyzed by galvanostatic charge/discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The Y/ZrO₂@MnO₂ electrodes achieve a specific capacity of 282.1 F g⁻¹ with a specific capacity loss of only 6.3% after 100 cycles at the current density of 50 mA g⁻¹. The results show that MnO₂ particles are successfully deposited by Y-doped ZrO₂ while the Y/ZrO₂@MnO₂ electrodes display better cycling stability and capacity performance. Therefore, this Y-doped ZrO₂ coating is a potential choice to improve the cycling stability and conductivity of MnO₂ electrodes.

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1. Introduction

Since entering the 21st century, with the increasing consumption of fossil fuels for the development of the economy, environment pollution and the energy crisis have become the most important problems that we must deal with. Therefore, there is a great need to look for renewable and sustainable energy sources and develop technologically advanced devices for energy conversion and storage.^1–4 In recent years, electrochemical supercapacitors (ESs), as a new sort of efficient device for energy conversion and storage, have caused extensive concern because of their high power density, fast charge and discharge rate, better cycling stability, and longer life-cycle.^1,2 These properties and characteristics make ESs applicable to many domains, such as electric vehicles, computers, electrochemical power sources, military applications, power distribution systems, etc.^5–7 Therefore, supercapacitors are expected to become a novel energy resource to solve the problems concerning energy storage and environmental pollution.

The electrochemical properties of supercapacitors, such as specific capacity and cycling stability, intimately depend on the electrode materials.² Thus, improving the electrochemical performance of electrode materials is an effective strategy to enhance the electrochemical properties of supercapacitors. Generally, various materials for electrodes of supercapacitors such as carbonaceous materials, conducting polymers and transition-metal oxides have been researched. Among these materials, MnO₂, as a sort of transition-metal oxides, is considered as a promising alternative class of materials for electrochemical supercapacitors, for its superior characteristics such as large theoretical specific capacity (1370 F g⁻¹), environmental safety, low toxicity and natural abundance, as well as low cost.^8–10 In spite of all of the superiorities, commercial applications of supercapacitors based on MnO₂ electrode have not been implemented mostly due to some existing drawbacks about MnO₂ as follows. Firstly, partial dissolution of MnO₂ electrode in the electrolyte happens spontaneously during cycling, leading to the fading capacity.^2,11 Secondly, poor electronic conductivity and ionic conductivity result in large resistance of MnO₂ electrode in supercapacitors.¹² Hence, it is a key task for researchers to solve these disadvantages of MnO₂ material mentioned above.

Among these drawbacks of MnO₂ electrode material mentioned above, the partial dissolution in the electrolyte is the most prominent issue and draws great attention of researchers. Pang and Anderson^13,14 utilized sol–gel method to prepare thin-film MnO₂ electrode, the result of test indicating that more than 10% capacity was lost after 1500 cycles, in 0.1 mol L⁻¹ Na₂SO₄ electrolyte, at a scanning rate of 50 mV s⁻¹. Reddy et al.¹⁵ prepared MnO₂ electrode, at a sweep rate of 5 mV s⁻¹, suffering more than 50% capacity loss after 800 cycles. Recently, Hsieh et al.¹¹ also reported the capacity fading of electrochemical capacitors based on the MnO₂·nH₂O electrodes, ranging from 5% to 30% in 1000 cycles, which was close to current rate and binder content. Moreover, it has been reported that the protective coatings on the electrodes surface are an effective strategy to improve the cycling stability of the electrode and enhance its electrochemical properties.¹⁶ Walz et al.¹⁷ deposited nanoporous silica coating on solid barium ferrate particles and obviously enhanced its stability, comparing to the uncoated materials. Licht et al. have used ceramic material of zirconia (ZrO₂) as protective coatings on the K₂FeO₄ electrodes, and their investigation results indicate that the ZrO₂ coating effectively enhances the stability of these electrodes, simultaneously improves the energy storage capacity of super-iron batteries.^18,19 Nevertheless, as we all know, ZrO₂ can form cubic, tetragonal, and monoclinic phases or orthorhombic phases. Among these phases, the cubic form has caused the extensive concern attributed to its high conductivity, excellent thermal stability, mechanical properties, chemical resistance, and oxygen conductivity. In spite of all of the superiorities, the stability of the cubic form at room temperature is not satisfactory. Therefore, the stabilization of cubic form at room temperature is very necessary.^20,21 It is reported that doping Y to ZrO₂ can stabilize cubic of ZrO₂ at room temperature; meanwhile the conductivity of cubic ZrO₂ at room temperature can be improved as well.²² In addition, Zhang et al.^23,24 successfully investigated and developed Y-doped zirconia coatings on K₂FeO₄ particles, significantly enhancing the stability and conductivity of the coated K₂FeO₄ electrodes and provided a good strategy for our further research.

Based on the previous work and investigation above, it is necessary to research and develop a novel sort of electrode materials for supercapacitors. Therefore, in order to enhance the cycling stability and conductivity of manganese dioxide (MnO₂) electrodes for supercapacitors in this paper, yttrium (Y) doped zirconia (ZrO₂) (denoted as Y/ZrO₂) are coated on MnO₂ electrodes as supercapacitors electrodes (Y/ZrO₂@MnO₂ electrodes), so protecting the MnO₂ electrodes and enhancing the electrochemical performance of MnO₂ electrodes in sodium sulfate electrolytes. And then their structure and electrochemical performance were observed and researched through SEM, TEM, EDX, galvanostatic charge/discharge test, CV and EIS.

2. Experimental

2.1 Materials

Zirconium oxychloride octahydrate (ZrOCl₂·8H₂O) (AR grade, ≥99.0%), ammonia water (NH₃·H₂O) (AR grade, 25–28%), and cetyltrimethyl ammonium bromide (CTAB) (AR grade, 99.0%) were purchased from Tianjin GuangFu fine chemical research institute. Manganous sulfate (MnSO₄·H₂O) (AR grade, ≥99.0%), ethanol absolute (C₂H₅OH) (AR grade, ≥99.7%), and sodium sulfate (Na₂SO₄) (AR grade, ≥99.0%) were supplied by Tianjin Guangfu Fine Chemicals Co., Ltd. Potassium permanganate (KMnO₄) (AR grade, ≥99.0%) and yttrium nitrate (Y(NO₃)₃·6H₂O) (AR grade, ≥99.0%) were obtained from Tianjin Jiangtian chemical Co., Ltd. Acetylene black (AR grade, ≥99.0%, Tianjin Jinqiushi Chemical Co., Ltd.), poly vinylidene difluoride (PVdF) (AR grade, ≥99.0%, Chengdu Chenguang Research Institute of Chemical Industry), nickel foam, and N-methyl pyrrolidone (NMP) (AR grade, ≥99.0%, Tianjin Bo di Chemical Co., Ltd.) were obtained and used as-received.

2.2 Preparation of Y/ZrO₂@MnO₂ electrodes

2.2.1 Preparation of MnO₂ particles. A certain amount of CTAB was dissolved in 67 mL deionized water under magnetic stirring. After CTAB powders were completely dissolved, 0.6320 g KMnO₄ granules were added into the CTAB solution, which was kept stirring for 30 minutes. Then the MnSO₄ aqueous solution (1.014 g MnSO₄·H₂O powders dissolved in 100 mL deionized water) was dropped into KMnO₄ solution by a dropping funnel at a speed of 2 drops per second under powerful magnetic stirring. The whole process was kept at constant temperature of 30 °C. The products were collected by centrifugation, washed by ethyl alcohol and deionized water for several times and dried at 60 °C in the air for 6 hours.

2.2.2 Preparation of Y/ZrO₂@MnO₂ electrodes. Firstly, a certain amount of ZrOCl₂·8H₂O and Y(NO₃)₃·6H₂O was added into 100 mL deionized water in a 250 mL conical flask, kept stirring for 10 minutes after the powders were completely dissolved. Then a certain amount of CTAB was added into the solution. After complete dissolution, the solution was kept stirring for 30 minutes. Finally, 1 g prepared MnO₂ particles were added and then handled with ultrasonic treatment for 30 minutes. After that, the dilute ammonia solution was dropwise added into the prepared solution above slowly by a dropping funnel. The solution pH value was adjusted from 8 to 9 and then the solution was kept stirring for several hours. The whole process is under constant 30 °C. Finally, the products were collected by centrifugation, washed by ethyl alcohol and deionized water for several times and dried for 6 hours at 60 °C in the air. After above reactions, the dried particles were calcined in the air in pipe furnace at 450 °C for 3 h and the Y/ZrO₂@MnO₂ powders were obtained.

2.3 Fabrication of electrodes

The electrodes for evaluating the electrochemical properties were fabricated by mixing Y/ZrO₂@MnO₂ (80 wt%) sample with acetylene black (10 wt%) and PVdF (10 wt%) in a beaker. Then moderate amount of NMP was added into the electrode material to form slurry. After that, the slurry was coated (coating area: 1 × 1 cm²) on a nickel foam (1.8 mm, washed by ethyl alcohol under sonication for 1 h). Next, the electrodes were dried at 60 °C for 3 h under vacuum (vacuum degree: ≥0.08 MPa), and then pressed at 10 MPa for 10 seconds. Finally, the electrodes were dried at 60 °C for 12 h in the air.

2.4 Characterization

2.4.1 SEM analysis. The surface morphologies and structures of the as-prepared samples were observed using a scanning electron microscope (SEM, Hitachi S-4800, Japan).

2.4.2 TEM analysis. The surface morphologies and structures of the as-prepared samples were observed using a transmission electron microscopy (TEM, JEOL JEM-2100F, Japan).

2.4.3 EDX analysis. The content of elements in the surface of the as-prepared samples were observed using a energy dispersive X-ray spectrometer (EDX, Tecnai G2F20 FEI, Netherlands).

2.4.4 XRD analysis. The XRD patterns were recorded using a RINT2000 diffractometer (Rigaku Corporation, Japan) equipped with a CuKα X-ray source. Data were collected in the 2θ range 10–80°.

2.5 Electrochemical tests

Electrochemical tests of the as-prepared electrodes, including the galvanostatic charge/discharge test, the cyclic voltammetry (CV) test, and the electrochemical impedance spectroscopes (EIS) test. All the electrochemical tests were performed using 0.5 mol L⁻¹ aqueous Na₂SO₄ solution as the electrolyte at 25 °C in this study.

2.5.1 Galvanostatic charge/discharge tests. The galvanostatic charge/discharge tests of electrodes were studied via Land Battery Test System (CT2001A, Wuhan Jinnuo Electronic Ltd., China) in the potential range between 0 and 0.9 A at constant current and in the potential range between 0 and 0.8 V. In this paper, the specific capacity is calculated on the basis of the galvanostatic charge/discharge test curve for testing a single electrode. There are the same two electrodes having identical quality in the process, with a current density of 100 mA g⁻¹.

2.5.2 CV tests. CV tests were performed using a CHI650C electrochemical workstation in a three-electrode configuration, with Y/ZrO₂@MnO₂ particles coated Ni foam, Pt foil (1 × 1 cm²) and saturated calomel electrode (SCE) as working, counter and reference electrodes, respectively. The potential voltage range of CV was from −0.5 V to 0.5 V.

2.5.3 EIS tests. EIS tests were also performed using a CHI650C electrochemical workstation in a three-electrode configuration as the same as that in the CV tests. And the electrochemical impedance spectra were measured by imposing a sinusoidal alternating voltage frequency of 10⁻² to 10⁵ Hz.

3. Results and discussion

3.1 Determination of preparation conditions of Y/ZrO₂@MnO₂ particles

The electrochemical performances of Y/ZrO₂@MnO₂ electrodes are affected by many factors during depositing Y/ZrO₂ coatings on the surface of MnO₂ electrodes. Among those factors, the adding amount of ZrOCl₂·8H₂O, the doping amount of Y(NO₃)₃·6H₂O (the mole ratio of Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O) (mol : mol), the concentration of CTAB, the reaction time, and the calcination temperature were observed and researched. To determine the fitting preparation conditions of Y/ZrO₂@MnO₂ particles, a single factor experiment was used for research and five different levels of every factor were chosen in experiments. Finally, electrodes were prepared using Y/ZrO₂@MnO₂ particles to further investigate.

In all the single factor experiments, the specific capacity of produced electrodes mentioned above were evaluated by using the method of galvanostatic charge/discharge test.

3.1.1 Determination of the adding amount of ZrOCl₂·8H₂O. Fig. 1 shows that the effect of the adding amount of ZrOCl₂·8H₂O on the specific capacity of the electrodes. At the initial stage, the specific capacity of electrodes increases with adding amount of ZrOCl₂·8H₂O being increased. When the adding amount of ZrOCl₂·8H₂O is 0.6171 g, the specific capacity of electrodes reaches the highest value of 207.3 F g⁻¹. When the adding amount of ZrOCl₂·8H₂O is increased continually, the specific capacity of the electrodes is decreased sharply. The reason for the result suggests that: when the adding amount of ZrOCl₂·8H₂O is less than 0.6171 g, the electrodes can not be deposited completely, so the coatings are not enough inhibit the fading of specific capacity. And the other hand, the adding amount of ZrOCl₂·8H₂O is more than 0.6171 g, the coatings are so thick that they make electrolyte transfer difficultly and even prevent active materials from contacting with electrolyte, leading to the sharp decrease of the specific capacity. In conclusion, 0.6171 g is chosen as the fitting adding amount of ZrOCl₂·8H₂O.

Fig. 1 The effect of adding amount of zirconium oxychloride on the specific capacity of electrodes.

3.1.2 Determination of the doping amount of Y(NO₃)₃·6H₂O. The curve in Fig. 2 shows the effect of yttrium (Y) doping amount on the specific capacity of electrodes. At the initial stage, the specific capacity of the electrodes increases with the doping amount of Y(NO₃)₃·6H₂O being added. The specific capacity of the electrodes reaches its maximum value of 233.6 F g⁻¹ when the mole ratio between Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O (mol : mol) is 3 : 100. Then the specific capacity of the electrodes starts to decrease with the doping amount of yttrium being increased unceasingly. Especially, the mole ratio between Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O (mol : mol) is more than 5 : 100, the specific capacity of the electrodes decreases sharply. The reason suggests that: the oxygen vacancies are few in the coating when the doping amount of yttrium (Y) is less, so the resistance of charge transfer is strong, which makes the specific capacity low. With increasing the doping amount of Y(NO₃)₃·6H₂O, the coatings of the electrodes contain more yttrium (Y), which increases the oxygen vacancies and decreases the resistance of charge transfer. But when the doping amount of Y(NO₃)₃·6H₂O is too much, the coatings turn into mixtures of zirconium (Zr) and yttrium (Y), which makes the conductivity in the coatings of the electrodes decrease seriously. As the result, the resistance of charge transfer grows highly and the specific capacity decreases much. Thereby, the fitting mole ratio between Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O (mol : mol) is 3 : 100.

Fig. 2 The effect of yttrium doping amount on the specific capacity of electrodes.

3.1.3 Determination of the concentration of CTAB. Fig. 3 shows that the specific capacity of the electrodes increases with the increasing of the concentration of CTAB at the initial stage. The electrodes get the highest specific capacity of 225.3 F g⁻¹ while the concentration of CTAB is 7 mmol L⁻¹. However, when the concentration of CTAB is more than 7 mmol L⁻¹, the specific capacity begins to decline. This result suggests that: CTAB is added to insure that channels are generated in the inorganic coating shells of the electrodes, which makes active materials easily contact with electrolyte. Channels for electrolyte transfer can not be produced enough if the amount of CTAB is little. So, with adding the concentration of CTAB, the quantity of channels increases continuously, as a result the specific capacity of the electrodes augments. However, the formation of inorganic coating shells might be prohibited by excess adding amount of CTAB. Therefore, the fitting adding amount of CTAB is 7 mmol L⁻¹.

Fig. 3 The effect of the concentration of CTAB on the specific capacity of electrodes.

3.1.4 Determination of the reaction time. It is shown in Fig. 4 that the specific capacity increases with the coating reaction time extends. The specific capacity of the electrodes changes slightly when coating reaction time exceeds 2 h. The specific capacity of the electrodes reaches its maximum value of 217.3 F g⁻¹ when coating reaction time is 3 h. Then reaction time reaches 4 h, it has a weak influence on the specific capacity, the value of the specific capacity is 203.5 F g⁻¹. The reason suggests that: short reaction time can not enough impel the growth of the inorganic coating shell, in contrast, overlong reaction time will add unnecessary costs. Therefore, the fitting reaction time should be 3 h.

Fig. 4 The effect of reaction time on the specific capacity of electrodes.

3.1.5 Determination of the calcination temperature. As is shown in Fig. 5, the specific capacity of the electrodes increases with rising calcination temperature at the initial stage and reaches the maximum value (227.5 F g⁻¹) at 450 °C, whereas the specific capacity decreases sharply while calcination temperature exceeds 450 °C.

Fig. 5 The effect of different calcination temperatures on the specific capacity of electrodes.

Actually, different crystal structures of MnO₂ are formed under different calcination temperatures. When the calcinations temperature is below 300 °C, MnO₂ is still in amorphous state. However, α-MnO₂ appears when MnO₂ is heated between 400 and 500 °C and transforms to α-Mn₂O₃ above 500 °C.³ It is reported that if MnO₂ was in amorphous state, it is not stable during cycling as the electrode material, which is adverse to the diffusion of electrolyte. The specific capacity of electrodes decreases sharply when calcination temperature exceeds 450 °C, because the specific capacity of α-Mn₂O₃ is very little. Therefore, 450 °C is adopted as the fitting calcination temperature.

The fitting preparation conditions of Y/ZrO₂@MnO₂ particles are: the mass of ZrOCl₂·8H₂O is 0.6171 g, the mole ratio between Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O (mol : mol) 3 : 100, the adding amount of CTAB 7 mmol L⁻¹, the reaction time 3 h, the calcination temperature 450 °C. Characterizations and electrochemical tests of electrodes prepared under fitting preparation conditions were analyzed and researched as below.

3.2 Characterizations

3.2.1 SEM analysis of materials. The Y/ZrO₂@MnO₂ materials for SEM are prepared under the fitting preparation conditions.

The morphology of pure MnO₂ particles and Y/ZrO₂@MnO₂ particles are shown in Fig. 6. SEM images in Fig. 6(A) reveals that the pure MnO₂ particles perform sphere morphology with smooth surface; meanwhile they have a particle size about 450 nm. Compared to the SEM image of the pure MnO₂ particles, the surface of Y/ZrO₂@MnO₂ particles becomes relatively rough in Fig. 6(B). These results indicate that MnO₂ materials have been indeed coated by Y/ZrO₂ coatings. Besides, by comparing the images in Fig. 6(B) and (C), it is proved that the morphology of Y/ZrO₂@MnO₂ has no obvious changes after 5000 cycles and the inorganic coating shell increases the cycle stability of the electrode.

Fig. 6 SEM images: (A) pure MnO₂ particles (B) Y/ZrO₂@MnO₂ particles (C) Y/ZrO₂@MnO₂ particles after 5000 cycles.

3.2.2 TEM and EDX analysis of materials. The Y/ZrO₂@MnO₂ materials for TEM and EDX are prepared under the fitting preparation conditions.

TEM images in Fig. 7(A)–(C) show that MnO₂ particles are successfully by Y/ZrO₂ coatings. In these images, deep color regions stand for MnO₂ particles and gray sections represent Y/ZrO₂ coatings. The thickness of the coating shells is between 20 nm and 50 nm in Fig. 7(A)–(C). This result is congruent with the result of SEM analysis. Moreover, EDX spectrum indicates that the coating shell includes Y and Zr element, which proves that yttrium is successfully doped into the ZrO₂ coating shells.

Fig. 7 TEM images and EDX spectrum of Y/ZrO₂@MnO₂ particles.

3.2.3 XRD analysis of materials. The P-MnO₂ and Y/ZrO₂@MnO₂ materials for XRD are prepared under the fitting preparation conditions: the mass of ZrOCl₂·8H₂O is 0.6171 g, the mole ratio between Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O (mol : mol) 3 : 100, the adding amount of CTAB 7 mmol L⁻¹; the reaction time 3 h; the calcination temperature 450 °C.

The XRD patterns for P-MnO₂ and Y/ZrO₂@MnO₂ samples are she own in Fig. 8. In the pattern of the two samples, all diffraction peaks can be indexed to the α-MnO₂ according to the JCPDS 44-0141 standard file.²⁵ It can be seen from the Fig. 8 that there are some new diffraction peaks appearing and slight deviation of peak position through comparing the patterns of P-MnO₂ and Y/ZrO₂@MnO₂ samples. The new peaks correspond to the ZrO₂ according to the JCPDS 07-0343 standard file.²⁶ The result indicates the MnO₂ succeed to be coated by ZrO₂. Meanwhile, no extra peaks pertaining to Y are observed, indicating that no significant changes of the structure occurred during the doping process.

Fig. 8 X-Ray diffraction patterns of different electrode materials.

3.3 Electrochemical tests

As indicated by SEM and TEM results, Y/ZrO₂@MnO₂ materials is the potential choice application in energy field. To explore the potential applications in electrochemical energy storage, the Y/ZrO₂@MnO₂ materials are used to make supercapacitor electrodes, and characterized by galvanostatic charge/discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

3.3.1 Galvanostatic charge/discharge tests of electrodes. Cycle life is a key factor to be considered for supercapacitor. Thus, in order to validate the effect of Y/ZrO₂ coatings on the electrodes, the cycle stability tests of pure MnO₂ electrodes and Y/ZrO₂@MnO₂ electrodes are carried out under the same current densities of 50 mA g⁻¹. As seen in Fig. 9(A) and (B), the specific capacity of pure MnO₂ electrodes reaches 257.3 F g⁻¹ at first cycle, and it decreases to 167.9 F g⁻¹ after 100 cycles and 153.0 F g⁻¹ after 5000 cycles, with an overall specific capacity loss of 35% after 100 cycles and 40.5% after 5000 cycles. Under the same current density, the Y/ZrO₂@MnO₂ electrodes get a specific capacity of 282.1 F g⁻¹ at the initial stage, and it decreases to 264.3 F g⁻¹ after 100 cycles and 243.6 F g⁻¹ after 5000 cycles, with a specific capacity loss of only 6.3% after 100 cycles and 13.6% after 5000 cycles. The two figures indicate that Y/ZrO₂ inorganic coating shells increases the specific capacity and cycle stability of the pure MnO₂ electrodes, because Y/ZrO₂@MnO₂ electrodes shows a larger initial specific capacity and a less attenuation of specific capacity.

Fig. 9 The effect of cycle number on the specific capacity of electrodes: (A) 100 cycles (B) 5000 cycles.

The reason suggests that: the protection of Y-doped ZrO₂ coating, which prevents direct contact between MnO₂ electrode and electrolyte, reduces partial dissolution of electrode materials in the electrolyte during cycling test. Simultaneously, the expansion and contraction of MnO₂ crystal lattice is limited by Y-doped ZrO₂ coating, thus the cycle stability of the MnO₂ electrode is improved. Alternatively, the conductivity of ZrO₂ coating is improved by doping yttrium (Y), which decreases surface resistance of ZrO₂ coating on the surface of MnO₂ electrode and charge-transfer resistance of the whole electrode.

3.3.2 CV analysis of electrodes. The effect of scanning rate on the specific capacity of pure MnO₂ electrodes and Y/ZrO₂@MnO₂ electrodes is shown in Fig. 10. As shown in Fig. 10, when the scanning rate is 5 mV s⁻¹, the specific capacity of pure MnO₂ electrode and Y/ZrO₂@MnO₂ electrode are 272.4 F g⁻¹ and 297.1 F g⁻¹, respectively. With increasing scanning rate, the specific capacity of electrodes decreases sharply. As a result, when the scanning rate is 200 mV s⁻¹, the specific capacity of pure MnO₂ electrode and Y/ZrO₂@MnO₂ electrode decline to 77.5 F g⁻¹ and 145.3 F g⁻¹, and the specific capacity retention ratio are around 28% and 49%, respectively. The result indicates that the Y/ZrO₂ inorganic coating shell increases the specific capacity of pure MnO₂ electrode, and makes the decreasing rate of the specific capacity to be less. The reason suggests that: with the scanning rate increasing, the reaction processes of electrode become faster. Consequently, the immigration and emigration of the electrolyte ion from the electrode can not occur timely. And then, the active materials in the electrode can not have fully reaction, leading to the declination of the specific capacity. Therefore, the Y/ZrO₂ inorganic coating shell increases the overall conductivity of the electrode, indeed.

Fig. 10 The effect of scanning rate on the specific capacity of electrodes.

The electrochemical properties of the two electrodes are evaluated using CV tests in 0.5 mol L⁻¹ Na₂SO₄ solution at the scan rate of 50 mV s⁻¹. As shown in Fig. 11, the test result of the first cycle is compared with that of 100 cycles under same conditions. For pure MnO₂ electrode evaluated in Fig. 11(A), although the shape of the CV curves maintain rectangle, the response current decreases obviously after 100 cycles, which implies that the specific capacity of electrode decreases obviously. Then, to compare with the pure MnO₂ electrode, the Y/ZrO₂@MnO₂ electrode is shown in Fig. 11(B), and the CV curves of Y/ZrO₂@MnO₂ electrode change little after 100 cycles, which indicates that the response current almost stays the same. Therefore, by comparing the curves of the two electrodes, it is proved that the inorganic coating shell increases the cycle stability of the electrode. This result is congruent with the conclusion of former analysis.

Fig. 11 Effect of cycle number on the cyclic voltammetry property of electrodes: (A) pure MnO₂ electrode (B) Y/ZrO₂@MnO₂ electrode.

3.3.3 EIS analysis of electrodes. The Nyquist plots of pure MnO₂ electrodes and Y/ZrO₂@MnO₂ electrodes before cycle tests are shown in Fig. 12. In the Nyquist plots, the curve is a depressed arc at high frequency range, corresponding to the charge transfer process. Arc radius stands for the resistance of the charge transfer process and greater radius stands for bigger resistance. The straight line appeared at low frequencies range corresponds to the electrolyte diffusion process. Normally, a higher slope for the impedance line presents a lower resistance of the electrolyte diffusion process. The two curves display that the charge transfer resistance of pure MnO₂ electrodes is seemingly almost the same to that of Y/ZrO₂@MnO₂ electrodes before cycle tests. However, the difference of charge transfer resistance between pure MnO₂ and Y/ZrO₂@MnO₂ electrodes materials will be more clearly found when the depressed arc part is enlarged in Fig. 12. It can be concluded that the Y/ZrO₂ coatings indeed decreases the charge transfer resistance of pure MnO₂ electrodes. Then during the electrolyte diffusion process, the slopes of the two lines are approach still, which implies the inorganic coating shell hinders the diffusion of the electrolyte inside the electrode slightly. Therefore, the two curves in Fig. 12 show that the EIS property of the two electrodes is similar to each other before cycle tests.

Fig. 12 Nyquist plots of pure MnO₂ electrodes and Y/ZrO₂@MnO₂ electrodes.

For further investigation, electrodes are described by an equivalent circuit. As seen in Fig. 13, the equivalent circuit is consisted of the electrolyte resistance (R_s), the charge transfer resistance (R_Ct), the Warburg impedance related to the diffusion in electrolyte (W) and the electrochemical capacitance (C). Based on this equivalent circuit, the values of R_s, R_Ct and W can be obtained via the software of ZsimpWin, and the corresponding results are listed in Table 1. It can be seen from Table 1, the R_s values of two electrodes are very similar because of the consistent electrolyte. The R_Ct values of Y-ZrO₂@MnO₂ electrode are significantly less than the P-MnO₂ electrode and the W values of two electrodes are basically the same. These results indicate that the Y-ZrO₂ coating reduces the charge-transfer resistance of MnO₂ and hardly hinders the diffusion of the electrolyte inside the electrode, which conforms to the Nyquist plots in Fig. 13. The results further demonstrate that the conductivity and the electrochemical properties of the whole MnO₂ electrode can be enhanced by Y-doped ZrO₂ coating significantly.

Fig. 13 EIS equivalent circuit of electrodes.

Table 1 The results of R_s, R_Ct and W for different electrodes

Types of electrode	Rs/Ω cm^2	RCt/Ω cm^2	W/Ω cm^2
P-MnO2	0.874	1.853	0.241
Y-ZrO2@MnO2	0.842	0.921	0.256

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The EIS of pure MnO₂ electrodes during electrochemical cycling is shown in Fig. 14(A). Nyquist plot shows that the radius of depressed arc at high frequencies after 100 cycles is much larger than that after 1 cycle, which implies the resistance of charge transfer process increases with increasing cycle number. The increasing resistance of charge transfer process might be induced by the swells and contracts of MnO₂ lattice during charge–discharge,²⁷ which makes the micro-structure collapse and enlarges internal resistance. Finally, the collapse of the microstructure makes the specific capacity decrease sharply. For the low frequencies in Nyquist plot, the slope of the line after 100 cycles is lower than that after 1 cycle, which implies that the diffusion resistance of electrolyte inside electrodes greatly increases after 100 cycles. This is because the collapse of microstructure hinders the diffusion of electrolyte. The curve in Fig. 14(B) shows the admittance plot of pure MnO₂ electrodes. The knee frequency for pure MnO₂ electrodes is reduced from 8.1 Hz (1 cycle) to 3.7 Hz (100 cycles), implying degraded electrochemical response. According to phase angle–frequency plot in Fig. 14(C), the capacitor response frequency is reduced from 0.12 Hz (1 cycle) to 0.026 Hz (100 cycles). It decreases by 80%, which makes the capacitor response time increases from 8 s to 40 s. As shown in the curves in Fig. 14, the EIS of pure MnO₂ electrodes varies greatly after 100 cycles. The resistance of both charge transfer process and electrolyte diffusion process increases, which reduces the electrochemical response ability and increases capacitor response time.

Fig. 14 Effects of cycles number on EIS of pure MnO₂ electrodes: (A) Nyquist plot (B) admittance plot (C) phase angle–frequency plot.

The effect of cycle numbers on the EIS of Y/ZrO₂@MnO₂ electrodes is shown in Fig. 15. Nyquist plot in Fig. 15(A) shows that the radius of depressed arc at high frequencies after 100 cycles is similar to that after 1 cycle, which implies the resistance of charge transfer process during electrodes process mainly remains the same with increasing cycle numbers. Although MnO₂ lattice continually swells and contracts during charge–discharge process, inorganic coating shells effectively protect the microstructure of electrodes, which enhances the cycle stability of the electrodes. For the straight line at the low frequency range, its slope changes little after 100 cycles, indicating the electrolyte diffusion resistance inside the electrodes hardly changes. Fig. 15(B) shows the admittance plot of Y/ZrO₂@MnO₂ electrodes. The knee frequency is 2.1 Hz (1 cycle) and 1.9 Hz (100 cycles), which implies the capacitor response ability changes little. According to Fig. 13(C), the capacitor response frequency is 0.12 Hz (1 cycle) and 0.098 Hz (100 cycles), which only decreases by 19%. At the same time, capacitor response time increases from 8 s (1 cycle) to 10 s (100 cycles). The curves in Fig. 13 show that the EIS of Y/ZrO₂@MnO₂ electrodes changes little after 100 cycles. The resistance of charge transfer process and electrolyte diffusion process remains largely unchanged, which makes the electrochemical response ability keep the same and the capacitor response time increase little. Compared to the EIS of pure MnO₂ electrodes, the results prove that the inorganic coating shell increases the cycle stability of the electrode indeed.

Fig. 15 Effects of cycles number on EIS of Y/ZrO₂@MnO₂ electrodes: (A) Nyquist plot (B) admittance plot (C) phase angle–frequency plot.

4. Conclusions

In this paper, a cost-effective and simple strategy is presented to design and fabricate novel Y/ZrO₂@MnO₂ material, to improve the cycle stability and conductivity property of electrode for supercapacitor. The fitting preparation conditions of Y/ZrO₂@MnO₂ particles are: the mass of ZrOCl₂·8H₂O is 0.6171 g, the mole ratio between Y(NO₃)₃·6H₂O and ZrOCl₂·8H₂O (mol : mol) 3 : 100, the adding amount of CTAB 7 mmol L⁻¹; the reaction time 3 h; the calcination temperature 450 °C. The results of SEM, TEM, EDX and XRD about electrodes prepared under the fitting preparation conditions show that MnO₂ materials are indeed coated by Y-doped ZrO₂ coatings and Y/ZrO₂@MnO₂ particles possess a core–shell structure. The results of electrochemical tests show that the Y/ZrO₂@MnO₂ electrodes display better cycling stability and capacity performance. Therefore, Y-doped ZrO₂ coating is the potential choice to improve the cycling stability and conductivity of MnO₂ electrode.

Acknowledgements

This project is supported by National Natural Science Foundation of China (No. 21076143), by the key technologies R & D program of Tianjin (15ZCZDSF00160), by the Basic Research of Tianjin Municipal Science and Technology Commission (13JCYBJC20100), by Tianjin Municipal Science and Technology Xinghai Program (No. KJXH2014-05), by China Scholarship Council (No. 201406255018).

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