Shape Control, Crystalline Conversion and Pseudocapacitance Properties of Mn3O4: Effects of Yb3+ Doping①

YOU Jun-Hu GUO Yo-Zu ZHAO Yo NI Zhi-Yun GUO Rui,

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Shape Control, Crystalline Conversion and Pseudocapacitance Properties of Mn3O4: Effects of Yb3+Doping①

YOU Jun-HuaaGUO Yao-ZuaZHAO YaoaNI Zhi-YuanbGUO Ruib,c②

a(110870)b(110819)c(066004)

We report a facile method for the synthesis of manganese oxide (Mn3O4) nanorodsvia the direct reaction of MnCl2and H2O2by doping Yb3+ions at room temperature and air atmosphere. The Mn3O4:Yb3+samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CVs), electrochemical impedance spectroscopy (EIS), and charging-discharging test (CD). The results show thattrace Yb3+doping (6 at%) could effectively induce crystalline transformation of Mn3O4from cubic system (space group-3) to tetragonal system (space group41/) and incite the morphology changing from irregular particles to uniform nanorods. When Yb3+doping amount is 3%, the capacitance of Mn3O4reaches the maximum, 246 F/g, which is related to the morphology change and the corresponding decrease of impedance.

Mn3O4, Yb3+, shape control, crystalline conversion, pseudocapacitor;

1 INTRODUCTION

Manganese oxides (such as MnO, Mn2O3, Mn3O4, and MnO2) store electrochemical energy by simul- taneous injection of electrons and charge-compensa- ting cations, as other electroactive transition metal oxides[1, 2]. Among them, Mn3O4-based materials have attracted increased interest because of its superior electrochemical performance, environmen- tally friendly nature, and low cost[4-6]. In addition, most of the reports have focused on improving the specific surface area of Mn3O4nanomaterials but the effects of morphology and crystal phase on electro- chemical performance should not be ignored. For example, Shaik et al. reported tetragonal phase Mn3O4thin film grown on stainless steel substrates exhibited a specific capacitance of 568 Fg-1at a current density of 1 Ag-1in 1 M Na2SO4aqueous electrolyte with excellent capacitance retention of 93% even after 5000 cycles[3]. Different morpho- logies of Mn3O4prepared by various methods have also been reported, such as amorphous Mn3O4prepared by successive ionic layer adsorption and reaction (SILAR) method and galvanostatic anodic deposition method[4, 7], Mn3O4nanosheets prepared in xylene at 90 °C and in air atmosphere[3], Mn3O4composite loaded on CMK-3[8], Mn3O4with ant-cave structure[9], Mn3O4nanosheets prepared by a solvo- thermal method with a high-performance pseudoca- pacitors[10], Mn3O4nanoparticles via a one-pot method[11], well encapsulated Mn3O4octahedra in graphene nanosheets by a dealloying method[12], and so on. Among them, Mn3O4nanowires or nanorods were usually synthesized by hydrothermal method[13], whose excellent electrochemical properties caught our attention. Considering the large-scale application and low cost, nanomaterials should be prepared by a facile method. Then our group has devoted many efforts to improving the morphology of Mn3O4nanorods. In addition, there are few studies on improving the morphology and properties of Mn3O4by ion doping.

In this work, we propose a cost-effective, large- scale synthesis method for Mn3O4nanorods. The product prepared by the direct reaction of MnC12with H2O2in alkaline conditions is a mixture of amorphous cubic Mn3O4and trace nanorod-like tetragonal Mn3O4. When Yb3+ions were added during the reaction, it was interesting to find that the amorphous Mn3O4is converted to uniform nanorods under the incitement of Yb3+ion.

To the best of our knowledge, this facile prepara- tion method of Mn3O4nanorods in aqueous solution at room temperature and air pressure have not been reported. Excitingly, Yb3+ion doping further impro- ved the pseudocapacitive properties from the 161 F/g of pure Mn3O4to the 246 F/g of Mn3O4:3%Yb3+.

2 EXPERIMENTAL

All the reagents used for synthesis were of analytical grade. Typically, Mn3O4or Mn3O4:Yb3+was synthesized as follows[14-17].Stoichiometric MnCl2, H2O2, and NaOH were dissolved in deioni- zed water in air atmosphere under vigorous stirring for 4 h. Then the resulting solution was kept at room temperature for about 12 h. Purple precipitant was collected by filtration, washed with ethyl ether and dried in air. The chemical reaction between H2O2, NaOH and Mn2+ions leads to the deposition of Mn3O4, which could be described as Eq. 1.

Here the NaOH is base and in basic medium Mn2+is an unstable state, hence Mn2+is partly oxidized to Mn3+[4].

The XRD patterns were recorded on a D/Max-RB X-ray diffractometer (Rigaku) using Cuirradia- tion from 10° to 90°.The powder morphologies were characterized using SEM (Zeiss Supra 55).

The mixture of Mn3O4or Mn3O4:Yb3+, acetylene black, andPolyvinylidene Fluoride (PVDF) with the weight ratio of 7:2:1was used to prepare the working electrode. The typical mass loading of the active material in each electrode is about 10.0 mg. The mixture was coated onto 1 cm2of nickel foam (known mass). The prepared electrodes were dried at 80 ℃ for about 12 h[12, 14, 18]. All electrochemical measurements were performed in a three-electrode system, such as the prepared nickel foam electrode (working electrode), a platinum electrode (counter electrode), and a saturated calomel electrode (SCE, reference electrode). For all electrochemical measurements, 1 M Na2SO4was used as the electro- lyte and the experiments were done at ambient temperature, which was typically 25 ℃. The specific capacitance was calculated by integrating the area under the CV curve to obtain the charge () and then divided by the mass of electroactive material (), scan rate (), and potential window (Δa?c) according to Eq. 2.

In addition, the specific capacitance can be calcu- lated from the galvanostatic charging-discharging function according to Eq. 3.

Δis the discharging time, Δis the potential window, andis mass of the electroactive material.

Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10 mHz～100 kHz in the amplitude of 5 mV[12, 19]. Constant current charge-discharge tests were performed in the voltage range of 0～0.8 V (SCE).

3 RESULTS AND DISCUSSION

XRD was first employed to investigate the phase structure ofpure Mn3O4and Mn3O4:Yb3+. As shown in Fig. 1a,themain diffraction peaks of pure Mn3O4and Mn3O4: 3% Yb3+are well indexed to cubic Mn3O4phase (corresponding to that of JCPDS 13-0162). However, it is clear that theintensity of the peak at 18.2o corresponding tothe (111) plane of cubic Mn3O4structure disappeared after Yb3+doping beyond 6% and the peaks at 18.0o and 28.9o are clearly observed, indicating that Mn3O4transfers from a cubic system to a tetragonal system (corresponding to that of JCPDS 24-0734) under the incitement of Yb3+ions. The change in the micro- structure of pure Mn3O4and Mn3O4:Yb3+is shown in Fig. 1b-1f. Fig. 1b shows the SEM image of pure Mn3O4, which exhibits a mixture of amorphous particles and trace short rod-like particles. However, all morphologies of Mn3O4:Yb3+are short rod- shaped and the surface is very smooth. With the increase of Yb3+ion doping, the short rod-like particles tend to agglomerate slightly and the length/diameter ratio of Mn3O4nanorods also gradually increases (Fig. 1b-e). The morphological changes of Mn3O4observed in SEM are consistent with the results of XRD, which further verify the induction effects of Yb3+doping, that is, Yb3+ion doping first causes the morphological changes of Mn3O4but does not induce the crystal lattice conversion. When the Yb3+doping amount exceeds 6%, the crystal lattices of all the particles are converted to tetragonal phase. Taking into account that the synthesis conditions are unchanged, the morphological change and crystalline lattice change of Mn3O4particles are directly related to the doping of Yb3+ions.

Fig. 1. XRD patterns of pure Mn3O4:Yb (a) and SEM images of the pure Mn3O4(b) and Mn3O4: 3%Yb3+(c), 6%Yb3+(d), 9%Yb3+(e), 12%Yb3+(f)

The pure Mn3O4and Mn3O4:Yb electrodes were used in the supercapacitor and their performances were tested using cyclic voltammograms (CV) technique. The CVs were measured at different voltage scan rates and shown in Fig. 2.The specific capacitance is proportional to the area under the CV curve. Importantly, the specific capacitance was measured at a high mass loading of 10.0 mg/cm2, which is higher than that investigated in other recent reports on Mn3O4[13]. It could be seen that the CV behavior of pure Mn3O4is similar to those of Mn3O4:Yb3+. The CVs curves of all samples remain rectangular at low scan speeds, indicating that the electrode process is reversible in this condition. When the scan speed increases, the CVs are asymmetric, indicating an irreversible redox process. When the scan speed is 5 mV/s, the specific capa- citance is the largest for each sample, which is shown in Fig. 2f. It is found the specific capacitance of pure Mn3O4is about 161 F/g. When 3% Yb3+is doped, although the cubic phase still dominates, the specific capacitance increases to 246 F/g[13], a 53% increase compared to pure Mn3O4. After the crystal lattices convert from cubic phase to tetragonal phase completely, the specific capacitance of Mn3O4: 6%Yb3+drops to 135 F/g, a 16% decrease compared to pure Mn3O4. Subsequently, as the Yb3+doping amount increases, the capacitance decreases gra- dually. The above results show that the change in morphology by Yb3+doping increases the specific capacitance of the cubic phase Mn3O4and the specific capacitance of the tetragonal Mn3O4is smaller than the cubic phase Mn3O4.

Fig. 2. Cycling performance of pure Mn3O4(a) and Mn3O4doped by Yb3+(3%, b; 6%, c; 9%, d; 12%, e) at different scan speeds, and the plots of the specific capacitancescan speed (f)

Fig. 3. Corresponding Nyquist plots (a) and Galvanostatic charging-discharging curves for pure Mn3O4and Mn3O4: Yb3+at 2 A/g current density (b)

Electrochemical impedance spectroscopy (EIS), a powerful technique for the investigation of capacitive behavior, has been also used to explore the effects of Yb3+doping on electrochemical performances of Mn3O4. The EIS measurements for pure Mn3O4and Mn3O4:Yb3+electrodes were conducted at open circuit voltage state using fresh cells. As observed in Fig. 3a, all Nyquist plots show a sloping line from in the high-frequency region to the low-frequency region. By comparison, the biggest difference for these Nyquist plots lies in the absolute values of the impedance in the low-frequency region due to Yb3+doping. When the Yb3+doping amount is 3%, the decrease in the imaginary part of the impedance means that the properties of the interface between the electrode and the active material is improved, which also indicates that the adsorption/desorption activation energy of the redox species on the surface of Mn3O4is more moderate at this time. Taking account of a high mass loading, a smaller impedance is helpful for the electron injection into active substance, yielding a large specific capacitance of Mn3O4:3%Yb3+. When the Yb3+doping amount is beyond 3%, the real part of the impedance increases gradually as the crystal lattice transforms into a tetragonal phase, implying the internal resistance of the tetragonal phase is higher. Fig. 3b shows galvanostatic charging-discharging curves of pure Mn3O4and Mn3O4:Yb3+electrodes at 2 A/g current density. For current densities at 2 A/g, thedrops of pure Mn3O4, Mn3O4:9%Yb3+and Mn3O4:12%Yb are larger, which is greatly in agreement with the results of EIS. The charging times of Mn3O4:3%Yb3+and Mn3O4:6%Yb3+are similar. But the charging current of Mn3O4:3%Yb3+rises faster and the discharging time of Mn3O4:3%Yb3+is longer, indicating a greater specific capacity, which is also consistent with the EIS.

4 CONCLUSION

In this investigation, for the first time, we success- fully prepared rod-shaped Mn3O4using a facile method at room temperature and air pressure by doping Yb3+ions. The Yb3+doping not only changed the morphology of Mn3O4but also converted the crystal lattice of Mn3O4from cubic phase into tetragonal phase. When the Yb3+doping amount is 3%, the specific capacitance reaches the maximum, 246 F/g. As the doping amount of Yb3+ions con- tinues to increase, the specific capacitance of the tetragonal Mn3O4begins to decrease due to the increase of impedance. The specific capacitances of Yb3+-doped tetragonal Mn3O4are always smaller than that of cubic Mn3O4, indicating that cubic Mn3O4is a more promising electrode material for electrochemical capacitor.

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11 May 2018;

16 July 2018

①Supported by the National Natural Science Funds Youth Project of China (No. 51704064), the Fundamental Research Funds for the Central Universities (No. N162302001), Hebei Province Higher Education Science and Technology Research Project(No.ZD2017309), the Scientific and Technological Research and Development Plan of Qinhuangdao City (201701B063), the further support fund of Key Laboratory of Nanomaterials and Photoelectrocatalysis in Qinhuangdao City (201705B021), and the Northeastern University at Qinhuangdao Campus Research Fund (XNK201602)

. Born in 1979. Tel: 15076015448, E-mail: guorui@mail.neuq.edu.cn

10.14102/j.cnki.0254-5861.2011-2067