Long Cyclic Life in Manganese Oxide-Based ... - ACS Publications

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Long Cyclic Life in Manganese Oxide-Based Electrodes Zhaoming Wang,†,‡,§ Qingqing Qin,†,|| Wei Xu,§ Jian Yan,*,‡,⊥ and Yucheng Wu*,‡,⊥ ‡

School of Materials Science and Engineering and ||Institute of Industry & Equipment Technology, Hefei University of Technology, Hefei 230009, China ⊥ Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei 230009, China § Key Lab of Material Physics, Institute of Solid State Physics, Hefei 230031, Anhui People’s Republic of China

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S Supporting Information *

ABSTRACT: Long cyclic life is very important to the practical application of the pseudocapacitors. A systematic study has been carried out to reveal what key factors and how they affecting the cycling behaviors of manganese oxides. The specific capacitance degradation of MnOx is usually attributed to the so-called “dissolution” issue. Our results indicate that “dissoluted MnOx” is in the form of the “flotsam” derived from the detached active materials instead of Mn2+ in the solution, which causes color change of electrolyte and the loss of specific capacitance. During the cycling, the morphology of manganese oxides transformed to flowerlike flakes regardless of the starting structures. After that, it tends to form nanowires especially at elevated temperatures. According to the relative low electrochemical utility of nanowires, specific capacitance might decrease at this stage. These results put forward new questions on charge storage mechanism. Besides, electrochemical oxidation of MnOx leads to an increase in specific capacitance. The cycling behavior of MnOx is mainly determined by these three factors. Excitingly, a very stable cycling performance with no capacitance degradation over 40 000 cycles has been achieved in MnO2 hierarchical sphere-based electrodes. This study provides insightful understanding of the fundamental cycling behavior of MnOx-based electrodes and useful instructions for developing highly stable supercapacitors. KEYWORDS: manganese oxides, cycling stability, dissolution issue, morphology transformation, supercapacitor

1. INTRODUCTION

Generally, the capacitance degradation of MnOx-based electrodes is about several percent after thousands cycles.12−19 Dissolution of MnOx as Mn2+ into electrolyte is commonly accepted to be the major reason causing capacitance degradation.20,21 Recently, several groups have observed morphology and structure evolution which may affect the cycling stability.20,22 It is exciting that there are several groups reporting manganese oxide electrodes with very good cycling stability.23−26 For example, Sun et al. reported that a todorokite-type manganese oxide electrode can remain stable up to more than 20 thousands.23 Zhu et al. reported the birnessite-type MnO2 hierarchical nanoflower exhibiting capacitance retention of 97.5% over 10 000 cycles.25 It suggests that good electrochemical stability could be achieved in different manganese oxides electrodes. However, the key factor(s) dominating the cycling performance is still unknown. The influence of dissolution and morphology evolution on the cycling stability is required to be clarified in detail.

Triggered by environmental issues and the decreasing supply of fossil fuels, researchers have focused on developing new and clean energy resources.1−5 As energy storage devices, supercapacitors could deliver high power and exhibit a long cycling life.1,2,6,7 These featured advantages normally belong to the electrical double layer capacitors (EDLCs, based on carbon materials).2,6 But, EDLCs suffer from low energy density.2,6 Great efforts have been put on to improve the energy density by using redox based materials like metal oxides, nitrides and conducting polymers.8−11 As one of the typical metal oxides, MnO2, with high theoretical specific capacitance, high voltage window in aqueous electrolyte, low cost, environmental benignity, and abundant source, is considered as a very promising material for supercapacitors.8,9 However, MnO2 exhibits low power density and cycling stability. In practical applications, reliable and stable output is very important to the supercapacitors. Long life of supercapacitor could also reduce the cost of maintenance and replacement. Therefore, it is necessary to develop manganese oxide-based electrodes with long and stable cycling capacitive performance. © 2016 American Chemical Society

Received: April 24, 2016 Accepted: June 27, 2016 Published: June 27, 2016 18078

DOI: 10.1021/acsami.6b04883 ACS Appl. Mater. Interfaces 2016, 8, 18078−18088

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ACS Applied Materials & Interfaces

suggests that the nanoparticles are well-crystalline. As shown in the high-resolution transition electron microscopy (HRTEM) image (Figure 2c), the marked interplanar d-spacings of MNs are about 0.31 and 0.49 nm, which corresponds to that of the (112) and (101) lattice planes of tetragonal Mn3O4 (JCPDS 18−0803), respectively. This is in agreement with the XRD characterization. For the sample of MHSs, there are a large amount of spherelike structures with diameter about hundreds nanometers as shown in Figure 2d. The TEM image (Figure 2e) depicts that the MHS is composed with very fine flakes. In the HRTEM image (Figure 2f), the marked interplanar d-spacing of about 0.24 nm could correspond to that of the (211) lattice planes of α-MnO2 (JCPDS, 44−0141). The CMSs were synthesized by carbonized the polydopamine coating layers of MHSs. Figure 2g, h depict the morphology of CMSs, which are still of sphere shape. The energy-dispersive X-ray spectroscopy (EDS) attached to the FE-SEM was employed to detect the chemical composition. The signals from elements of Mn, O, C, and Si are shown in the EDS pattern (Figure S1) clearly. It suggests the successful coating of carbon layer. The Si signal is from the Si substrate. 2.2. Electrochemical Characterization. In the work, MNs, CMSs, and MHSs are employed as active materials. The information on the samples is listed in Table 1. The long-term cycling performance of these samples are tested by cyclic voltammetry (CV, 100 mV s−1) at various conditions shown in Figure 3. Because this work focus on the cycling stability of manganese oxides based electrodes, the specific capacitance is calculated based on the mass of active materials without considering the charge contribution from graphite paper substrate.19 Figure 3a indicates that the three samples exhibit good cycling performance at room temperature (25 °C). The specific capacitance of MNs-#1 increased fast in the first 1000 cycles and reached its highest value. After 12 000 cycles the specific capacitance still remains 96.7% of its highest value. The electrolyte started to show yellowish color, indicating the “dissolution of Mn species”, from about 3000−4000 cycles. However, before the end of the test, the electrolyte became clear again. Brown precipitations have been found on the bottom of the cell. The specific capacitance of CMSs-#1 kept increasing during the test. No color change was observed. Normally, carbon coating layers are employed to prevent dissolution of Mn species into electrolyte, which could enhance the cycling stability. This is in agreement with common expectation. As to the MHSs-#1, the specific capacitance increased about 5% at a low speed after 40 000 CV cycles. The electrolyte kept clear suggesting no dissolution issue. This good cycling stability is even better than previously reported results as mentioned above.23−25 The cycling performance was also evaluated at low and high temperatures as shown in Figure 3b. Similar to our previous report, the cycling stability (MNs-#2) is quite good without capacitance degradation at 5 °C.14 The specific capacitance also exhibits a fast increase initially and keeps increasing at a slow speed subsequently. No dissolution was observed. At high temperature (50 °C), MNs-#3 and MHSs-#2 both show lower cycling stability. The specific capacitance of MNs-#3 increase at the beginning and decrease to about 89.5% of the maximum specific capacitance after 10000 cycles. The MHSs-#2 remains about 85.3% of the specific capacitance. After testing of the two samples, the electrolytes become yellow color indicating serious dissolution. However, the yellow color diminished before the

The aim of this work is to provide a deep understanding on the cycling stability of manganese oxides based electrodes including Mn3O4 nanoparticles (MNs), carbon-coated MnOx spheres (CMSs), and MnO2 hierarchical spheres (MHSs). The “dissolution” of Mn species under different temperatures and pH values are discussed. The Mn species in the solution is in the form of MnOx:Na tiny nanostructures instead of Mn2+. It often occurs in the morphology transformation process. Manganese oxides tend to transform to flowerlike flakes first and nanowires subsequently at room temperature (or low temperature). At higher temperature (50 °C), this process would be accelerated. The loss of active materials could induce degradation of capacitance. The change of electrochemical utility of manganese oxides in different morphologies is also responsible for the variation in specific capacitance. The influence of electrochemical oxidation of MnOx on specific capacitance is also discussed. These findings provide insights on the cycling stability of manganese oxides and put forward new questions on current understanding of the charge storage mechanism.

2. RESULTS AND DISCUSSION 2.1. Material Characterization. In this work, the investigation of cycling stability was carried out using MNs, CMSs and MHSs as active materials. Figure 1 shows the typical

Figure 1. XRD patterns of the as-prepared MN, CMS, and MHS samples.

X-ray diffraction (XRD) patterns of the as-prepared samples. The peaks from MNs could be indexed to the tetragonal Mn3O4 (JCPDS 18−0803). The major peaks from CMSs could march with the tetragonal Mn3O4 (JCPDS 18−0803). Meanwhile, the two peaks marked with “*” might belong to the cubic MnO (JCPDS 07−0230). CMSs are the mixture of the two phases, which are poorly crystalline as indicated by the broad and week peaks. Similar to previous reported nano-MnO2, MHSs could be identified as α-MnO2 (JCPDS, 44−0141).19,27 The morphology of as-synthesized samples characterized by field emission scanning electron microscopy (FE-SEM) is shown in Figure 2. Figure 2a depicts that MNs are composed with a large amount of Mn3O4 nanoparticles with size around tens nanometers. Because carbon nanotubes (CNTs) are used as sacrifice template, MNs exhibit chainlike morphology. The transmission electron microscopy (TEM) image (Figure 2b) indicates that the MNs exhibit no preferred shape. The selected area electron diffraction (SAED, inset in Figure 2b) pattern 18079


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Figure 2. (a) FE-SEM image of the MNs; (b) low-magnification TEM image of the MNs and the inset denoting the SAED pattern; (c) typical HRTEM image of MNs showing that it is composed of well crystallized nanoparticles; (d) FE-SEM image of the MHSs with diameters of about hundreds nanometers; (e) TEM image of the MHSs indicating the small flakes; (f) HRTEM of the area marked with rectangular shown in segment (e); (g, h) low- and high-magnification FE-SEM images of the CMSs.

value of about 6 and 5, the MNs-#5 and MNs-#6 exhibit similar trend of specific capacitance vs cycles with capacitance degradation of about 3.2% after 15 000 cycles and 1.5% after 20 000 cycles, respectively. It suggests that the cycling stability of MNs is a little better in acid condition. Color change was observed during the test of these three samples. Note that the

end of test as brown perceptions found at the bottom of the cell. The effect of pH value on the cycling performance was also evaluated as depicted in Figure 3c. The cycling behavior of MNs-#4 (pH 9) is quite similar to that of MNs-#1. The specific capacitance shows a capacitance degradation of about 3.4% after 17 000 cycles. While tested in the electrolytes with a pH 18080


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ACS Applied Materials & Interfaces Table 1. Information of the Tested Samples mass (mg)

T (°C)

cycles

%

CMSs#1 MHSs#1 MHSs#2 MHSs#3 MHSs#4 MNs-#1

0.64

25

12 000

>100

Pt

clear

0.51

25

40000

>100

Pt

clear

0.5

50

10 000

Pt

0.72

50

20 000

Pt

yellow to clear yellow

0.68

25

10 000

CFP

clear

1.00

25

12 000

96.7

Pt

MNs-#2 MNs-#3

0.37 0.33

5 50

30 000 10 000

>100 89.5

Pt Pt

MNs-#4 MNs-#5

0.32 0.38

25 25

17 000 15 000

96.6 96.8

Pt Pt

9 6

MNs-#6

0.36

25

20 000

98.5

Pt

5

MNs-#7

0.31

50

10 000

89.7

Pt

9

MNs-#8

0.31

50

20 000

92.7

Pt

5

MNs-#9 MNs#10 MNs#11

0.28 0.69

25 50

10 000 10 000

Pt Pt

yellow to clear clear yellow to clear yellow slightly yellow slightly yellow yellow to clear yellow to clear yellow yellow

0.33

25

10 000

CFP

clear

sample

85.3

counter pH

color of electrolyte

electrolyte for the testing of MNs-#6 was clear before about 15 000 cycles. After several tens hours, the electrolytes became clear and brown precipitations were observed. At high temperature (50 °C), the specific capacitance of MNs-#7 tested in the electrolyte with pH 9 degraded much faster than that of MNs-#8 (pH 5) as shown in Figure S2. It is worth to note that the electrolytes for the test of the two samples became clear before the end of cycling test while the specific capacitance kept decreasing. 2.3. Factors Affecting Cycling Stability. 2.3.1. Electrochemical Oxidation for Specific Capacitance Enhancement. MNs and CMSs exhibit a significant enhancement in specific capacitance, which is worth to be discussed first. In previous reports, activation is thought to be the reason inducing initial capacitance increase of manganese oxides electrodes.12 However, in this case, the capacitance enhancement should be mainly due to the electrochemical oxidation.28,29 In recent reports, this phenomenon is well discussed. As depicted in the CV curves at different cycles (see Figure S3), a pair of redox peaks centered at about 0.5 V could be found for MNs-#1 and CMSs-#1. The intensity of the peaks increased initially and decreased subsequently. It suggests the electrochemical oxidation process occurred at the relative early stage of the cycling test. The detailed surface chemical composition of the samples is further characterized by XPS as shown in Figure S4. The Mn (2p1/2) and Mn (2p3/2) peaks of pristine MNs are observed at 653.6 and 642.0 eV, while the peaks of MNs-#5 are shift to 654.1 eV for Mn (2p1/2) and 642.6 eV for Mn (2p3/2). It suggests the enhancement of oxidation state of Mn4+ after cycling test.24,28−31 Meanwhile, XPS data also suggest the presence of Na and Pt, which are introduced during the cycling

Figure 3. (a) Plotted curves exhibiting the specific capacitances of the samples as a function of the cycle number measured at a scan rate of 100 mV s−1; (b) the curves depicting the cycling stability of MNs and MHSs tested at low or high temperatures at 100 mV s−1; (c) plotted curves of the specific capacitances of MNs vs cycle number measured in 1 M Na2SO4 solution with different pH value at 100 mV s−1.

test. The precipitation collected from the electrolyte is mainly MnOx with the presence of Na and Pt. For comparison, the CV curves (Figure S3c) of MHSs-#1 are absent of redox peaks and exhibit almost overlapped box-like shape. The slight specific capacitance enhancement could not be caused by oxidation or activation. There should be other reason, which could be attributed to the morphology transformation as discussed in the following section. Note that the specific capacitances obtained at low scan rates or low current densities are also very stable as shown in Figure S5. 2.3.2. Dissolution Issue and Morphology Transformation. As revealed in the above cycling measurements, when the samples exhibit capacitance degradation, “dissolution” of Mn species into electrolyte could be observed. It seems to be coincident with normal expectation. However, during the 18081


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17.5%. As mentioned above, no Mn was detected in the electrolyte. It indicates that the Mn species in the electrolyte could not be in form of Mn2+. Furthermore, the electrolyte after “dissolution of Mn species” is yellowish or brown while the color of Mn2+ solution is colorless or pink. In a very recent report about the in situ TEM observation of the charge− discharge reactions of liquid Li-oxygen battery cathode, the authors presented that the lost active materials into electrolyte might be the detached lithium peroxides in the form of flotsam caused by the liquid flow agitation.32 It is rational to deduce that the MnOx in the yellow electrolyte is somehow in the form of “flotsam” generated during the morphology transformation. We further checked the morphology of CMSs-#1 after cycling test. It is surprising that flaks were also found as shown in Figure S7. The carbon shell, which often serves as protection layer, could not prevent the “dissolution of Mn species”. The transport of Mn during the formation of flaks might be through solid diffusion process. The factor(s) triggered the generation of “flotsam” of MnOx is not clear considering that the electrolyte normally changed to yellow color at relatively higher temperature during the test of MNs and MHSs. On the other hand, the above results suggest that the specific capacitance does not always decrease with the color change of electrolyte. The specific capacitance could decrease even if the electrolyte remains clear. It suggests that other reason(s) may affect the cycling stability of MnOx. In the following section, the relationship between morphology evolution and cycling stability is discussed. 2.3.3. Morphology Evolution and Electrochemical Utility. To get more insights on the morphology transformation, we characterized morphology of MnOx electrodes tested at high temperature by FE-SEM. Figure 5a, b depicts the morphology of MNs-#3 after cycling test, which is composed of nanowires. The MHSs-#2 also exhibits wirelike morphology with some particles remaining. It suggests that at higher temperature, nanowires are more favored during the cycling test. It might be determined by the thermal stability as revealed in previous reports.19,33 Further study was carried to monitor the morphology evolution by FE-SEM observation of the samples collected at various cycles, which typically are 100th, 500th, 2000th, and 10000th cycle. Since part of the sample was cut for FE-SEM observation, the capacitances are normalized to the value of second cycle of each stage. The cycle stability of MNs-#9 shown in Figure 6a is similar to that of MNs-#1 presented in Figure 3a. The color change was observed after 4000th cycle. The morphologies observation (Figure 6b) indicates that the flakes formed very fast even at 100th cycle. No big change was observed at 500th cycle (Figure 6c). It is interesting that a small amount of MNs directly transformed to nanowires (Figure 6d). After that these flakes tend to grow bigger with cycles up to 2000th. At the end of test, the flakes are bigger in size, and part of them transformed to nanowires (Figure 6e). When tested at 50 °C, MNs-#10 exhibited a fast increase of specific capacitance initially as shown in Figure S8a. From 100th cycle to 2000th cycle, it reached its highest value and started to decrease. During the test, part of the electrode materials had peeled off from the substrate leading to a significant decrease in specific capacitance. Here, this result is shown with the aim that there might be various random or unexpected factors causing capacitance degradation. At the last stage, the specific capacitance decreased at a relative low speed. The electrolyte became yellow after testing. Morphology evolution is shown in

cycling test of MNs-#1, MNs-#3, and MHSs-#3, it was found that the electrolyte became clear while the specific capacitance was still decreasing. It conflicts with normal understanding on the dissolution issue. Furthermore, we have measured the mass of Mn “dissolved” in the electrolyte by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), which is collected after electrolyte became clear. The results are shown in Table 2. No Table 2. Mass of Mn Tested by ICP-MS in the Electrolytes for Different Samples mass of Mn (ng mL−1) pH value temperature (°C)

MNs-#5

MNs-#7

MNs-#8

4.4 6 25

9 50

5 50

Mn was detected in alkaline condition (MNs-#7), which can be predicted. However, it is “abnormal” that no Mn was detected in acid condition (MNs-#8). The mass of Mn (MNs-#5) is only 5 ng mL−1, which is much lower than expected. It discloses that there is no Mn2+ in the electrolyte. The collected brown precipitation was characterized by FE-SEM (Figure 4a and b). It is composed with very fine nanostructures, which induces the color change of electrolyte. It is rational to deduct that the mass loss of active materials would lead to the decrease of specific capacitance. However, our previous work reports that the specific capacitance does not decrease when the electrolyte color became yellowish.19 These “contradictory” results indicate that there might be some changes happening to the active materials. The morphologies of the samples after cycling test were checked by FE-SEM. Interestingly, the nanoparticles (MNs-#1) transformed to thin flakes with a bit like flower as depicted in Figure 4c and d. TEM was also carried out to monitor the detailed morphological and structural change of the electrode materials. Figure 4e is a TEM image of the flake obtained after cycling (MNs-#1). The SAED pattern (inset in Figure 4e) reveals its polycrystalline nature. The HRTEM image indicates that particles are very small, with size around several nanometers, and not well crystallized, as shown in Figure 4f. The FFT (fast flourier transform, inset in Figure 4f) pattern also indicates the polycrystalline nature. Some flakes were also found with rod-like layered structure as reveal in Figure S6a and b, which is somehow similar to that of the morphology of the MHSs-#1 after cycling (Figure 4g and h). The presence of Na was confirmed by EDS (Figure S6c) too. The majority of the MHSs-#1 after cycling is also flakelike structures that are slightly different. These flakes are somehow like fabrics, part of which is transformed to nanowires. A small amount of nanowires were also found. In previous reports, morphologies have been observed by several groups. Li et al. published their findings on the structure change of multiwall carbon nanotube/MnO2 composites electrodes, which could lead to significant capacitance degradation.22 They also found the mass change of electrodes during charge−discharge process and an overall loss of mass. The relationship between the mass loss and capacitance degradation is not reported. Recently, Ivey’s group reported that after 500 CV cycles test, MnO2 changed to petal-like thin nanosheets following a dissolution−redeposition process.20 They proposed that MnO2 dissolved into electrolyte as Mn2+ cations. A mass loss was detected to be about 12.2% of its original value, whereas the capacitance degradation was about 18082


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Figure 4. (a, b) Low- and high-magnification FE-SEM images of the precipitation collected from electrolyte; (c, d) low- and high-magnification FESEM images of the MNs-#1 after cycling test; (e) low-magnification TEM image of the flake of MNs-#1 and the inset denoting the SAED pattern; (f) typical HRTEM image of the flake in e and the inset showing the FFT pattern of the area marked; (g, h) FE-SEM images of the CMSs-#1 after cycling test showing the morphology transformation to flakes with the presence of nanowires.

Figure S8b−e. MNs changed to small flakes, then grew into bigger flakes, and finally transformed mainly to nanowires mixed with flakes. It is interesting that at 50 °C, MHSs-#3 directly transformed to nanowires as depicted in Figure 7a−e. This process occurred slowly. Below 500th cycle, only a small amount of could be found. Even at 20 000th cycle, there are still many MHSs remained. Figure 7f shows that the specific capacitance decreased regularly from 500th cycle. The electrolyte exhibited yellow color at the middle stage of test and became clear before the end of the measurement. As reported previously, wirelike MnO2 exhibits a relative low electro-

chemical utility considering that ultrathin nanowires of MnO2 exhibit high specific capacitance.19,34 Accordingly, the capacitance degradation of MHSs-#3 should be attributed to the morphology transformation. It is worth to note that the specific capacitance increase when the MHSs transform to flakes as which are expected to show higher electrochemical utility due to its thin structure.19 As revealed by XPS characterization, Pt was found in all tested samples. To exclude the influence of Pt on morphology evolution, annealed carbon fiber paper (CFP) were used as counter electrode for the cycling test of MNs-#11 and MHSs18083


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MnOx:Na tiny nanostructures are generated and vanished during the cycling test, which causes the reorganization of the MnOx, namely, the morphology transformation. The MnOx:Na tiny nanostructures might be detached from MnOx and lost into the electrolyte under liquid flow agitation.32 With increase of the MnOx:Na tiny nanostructures in the electrolyte, it starts to show yellowish color or brown color. Note that temperature is an important factor that induces the MnOx:Na tiny nanostructures lost into the electrolyte. However, after a certain period of time, these MnOx:Na tiny nanostructures might aggregate and become brown precipitation finally. The electrolyte becomes clear again. The loss of active materials in the form of MnOx:Na tiny nanostructures (or flotsam) clearly would cause capacitance degradation. Currently, it is still not known that what triggers and controls such morphology evolution. Further detailed study is required. These results also suggest that it is better to be careful with the morphology control and carbon coating based strategies for improving the performance of MnOx-based electrodes as they could not maintain their structures during the cycling test. However, if the detachment of MnOx:Na tiny nanostructures into electrolyte as “flotsam” could be prevented and the morphology could be kept at flakes for a relatively long time, very good stability of MnOx-based electrodes can be achieved as demonstrated by the MHSs with specific capacitance remaining stable for 40 000 cycles.

Figure 5. (a, b) Low- and high-magnification FE-SEM images of the MNs-#3 tested at 50 °C denoting the presence of short nanowires; (c, d) Low- and high-magnification FE-SEM images of the MHSs-#2 after cycling test at 50 °C showing the morphology transformed to long nanowires.

#4. Results are shown in Figures S9 and S10. For MNs-#11, no capacitance degradation was observed (see Figure S9a). The morphology evolution process of the MNs-#11 is similar to that of MNs-#9 (Figure S9b−e). But, there are more nanowires at the end of the test, indicating a relatively fast speed of morphology evolution. It clearly reveals the trend that flakes transform to nanowires during long-term cycling test. MHSs-#4 did not show decrease in specific capacitance either (see Figure S10a). The morphology evolution (Figure S10b−e) also indicates the trend of flakes to nanowires considering that the final structure is a combination of flake and nanowires. Such morphology is a little different from that of MHSs-#1, which might be due to the absence of Pt. As revealed by these results, manganese oxides tend to form flakes first and then transform to nanowires finally during the cycling test. This might be determined by the better thermal stability of MnOx nanowires.33 Note that the morphology transformation is also influenced by heteroatoms like Pt. Since manganese oxides with different morphologies exhibit different electrochemical utilities, morphology transformation could induce change of specific capacitance. However, the specific capacitance of MnOx is not simply determined by its morphology. The size, contact, and other factors of MnOxbased electrodes could also affect the electrochemical utility. Therefore, the specific capacitance of the tested samples does not always decrease immediately when the nanowires show up (see Figures S9a and S10a). But, when nanowires formed directly, the specific capacitance of MHSs-#3 exhibits a clear decrease during the cycling test (See Figure 7) as mentioned above. Normally, it is accepted that the charge−discharge occurs only on the surface or near-surface of MnOx.35 However, our results reveal that the charge−discharge process is more complicated as MnOx could not maintain its structure. It puts forward new questions on current understanding of charge storage mechanism. The structure change of MnOx in discharge−charge process might be similar to that of lithium oxides in Li-oxygen battery as reported previously.32 In brief, during the discharge process, Na ions react with MnOx to form MnOx:Na tiny nanostructures. In the charge process, the majority of Na ions are deintercalated into electrolyte. Such

3. CONCLUSIONS In this paper, a comprehensive study on the cycling performance of MnOx-based electrode using MNs, MHSs and CMSs as active materials is carried out. The key factors dominating the cyclic behaviors of MnOx are disclosed. Dissolution of MnOx into electrolytes is actually caused by “flotsam”, which is detached active materials from electrode and suspended in the electrolyte resulting in color change of electrolyte. Clearly, the loss of active materials would induce a decrease in specific capacitance. On the other hand, if the value of x in the MnOx is less than 2, electrochemical oxidation might occur during the cycling test, which could enhance the specific capacitance. In the charge−discharge process, the MnOx endures a morphology evolution process showing the trend to form flakes first and then transform to nanowires. Different morphology indicates different electrochemical utility. The specific capacitance would change along with the morphology evolution process. These three key factors are often combined and exhibit complicated effect on the cycling stability. More deep studies are required. Nevertheless, the MHS-based electrodes show a very promising stability with specific capacitance remaining stable during 40 000 cycle’s evaluation. It stands a hope to achieve stability comparable to that of carbon-based electric double-layer capacitors. This work provides insights on the cycling stability of MnOx-based electrode and instructions for developing high-performance supercapacitors. 4. EXPERIMENTAL SECTION Synthesis of MHSs, CMSs, and MNs. A very simple method is employed to fabricate MHSs. First, 4.6 mmol of KMnO4 (reagent grade) was added into 50 mL of deionized water (DI water). Then, 50 mL of MnSO4 (6.9 mmol) solution was mixed with KMnO4 solution with stirring. The dark brown precipitation product was obtained immediately. The product was centrifuged and washed with DI-water 18084


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Figure 6. (a) Plotted curve of the capacitance retention of the MNs-#9 as a function of the cycle number of each stage, measured at a scan rate of 100 mV s−1; (b) FE-SEM image of the sample obtained at 100th cycle; (c, d) FE-SEM images at different areas of sample obtained at 500th cycle; (e, f) FE-SEM images of the sample obtained at 2000th and 10000th cycles, respectively. several times. Next, the product was dried at room temperature (25 °C). The CMSs were obtained by carbonizing the polydopamine coating layer of MHSs. Typically, 350 mg of MHS powder was put into 50 mL of dopamine solution (4 mg mL−1) followed by adding 50 mL of trisbuffer solution (20 mM) under stirring for about 8 h. The product was centrifuged and washed with DI water several times. After drying in air, the product was annealed in a tube furnace at 600 °C for 120 min with an Ar flow of 150 standard cubic centimeters per minutes. The MNs are fabricated using a sacrificial template (CNTs) method. First, CNTs (100 mg) were put into 50 mL of dopamine solution (4 mg mL−1) followed by adding 50 mL of tris-buffer solution (20 mM) under stirring. After about 8 h, the products were collected by centrifuge and washed several times. Then, the products were dried in air. The dried powder (∼50 mg) was added into 50 mL of KMnO4 (1 mmol) solution with stirring. The products were centrifuged and

washed with DI water several times. The products were then dried and heated up to 350 °C for 5 min using a hot plate as heater. CNTs were burned during the heating process. Characterization. The phase of the product was identified by XRD (X’Pert PRO MPD), using Cu Kα (λ = 0.15406 nm) radiation at 50 kV and 50 mA in a 2θ range from 20 to 80°. The morphology and structure of the products were characterized by field-emission scanning electron microscopy (SU8020) with an EDX, TEM (JEOL 2010) with EDS and HRTEM. The mass of Mn in the electrolyte is detected by ICP-MS (Thermo fisher Scientific, X Series 2). Electrochemical Measurement. The MNs, MHSs, and CMSs electrodes were prepared by casting a mixture of 80 wt % active material with 15 wt % super-p and 5 wt % polyvinylidenefluoride binder, dissolved in N-methylpyrrolidone on graphite paper substrates followed by drying at 100 °C for 12 h. All substrates were cut into 18085


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Figure 7. (a−e) FE-SEM images of MHSs-#3 obtained at 100th, 500th, 2000th, 10000th, and 20000th cycles, respectively; (f) plotted curve of the capacitance retention of the MHSs-#3 as a function of the cycle number of each stage, measured at 50 °C under a scan rate of 100 mV s−1. pieces with a size of 1.0 cm × 2.0 cm. Active materials were coated on the substrate with area about 1.0 cm × 1.0 cm. Electrochemical measurements were carried out by an electrochemical analyzer (Autolab Potentiostat, PGSTAT101). The threeelectrode cell consisted of Ag/AgCl as the reference electrode, Pt or CFP as the counter electrode and the as-synthesized sample as the working electrode. The CFP was annealed at 400 °C (heating rate 2 °C min−1) for 120 min in air using a tube furnace. Aqueous solutions of 1 M Na2SO4 (50 mL) were served as electrolyte. The pH value was adjusted by adding NaOH or H2SO4 measured by pH meter (METTLER TOLEDO, FE20). CV was done at different scan rates of 5, 10, 20, 50, and 100 mV s−1. Galvanostatic charge/discharge curves were measured at different current densities to evaluate the power density and energy density. The potential window in the range of −0.1−0.9 V was used in all the measurements. The cycling stabilities tested at different temperatures were carried out in temperature controller box with accuracy ±1 °C (Shanghai Shuli,

SPC-70). The detailed information on the tested samples is listed in Table 1. It includes loading mass, temperature, measured cycles, capacitance retention, counter electrode, pH value, and color of electrolyte.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04883. SEM image and EDS spectrum of CMSs (Figure S1); cycling stability of MNs at different pH (Figure S2); CV curves of MNs, CMSs, and MHSs (Figure S3); XPS data of MNs (Figure S4); specific capacitances of MHSs (Figure S5); TEM and EDS of MHs after cycling test (Figure S6); SEM images of CMSs after cycling test 18086


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(Figure S7); cycling performance and SEM images of MNs and MHSs (Figure S8−S10) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (86) 551-6290 2680. *E-mail: [email protected]. Author Contributions †

Z.W. and Q.Q. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Natural Science Foundation of China (Grants 51301162 and 21503065) and the General Financial Grant from the China Postdoctoral Science Foundation (Grant 2015M571924). The authors thank the staff in the Analytical and Testing Center of HFUT for their assistance in the materials characterization.

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