Hydrothermal vs electrodeposition: How does deposition method affect the electrochemical capacitor performance of manganese dioxide?

Author’s Accepted Manuscript Hydrothermal vselectrodeposition: howdoes deposition method affect the electrochemical capacitor performance of manganese dioxide? Xiao-bo Li, Guang-ri Xu www.elsevier.com/locate/ceri

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S0272-8842(17)30641-7 http://dx.doi.org/10.1016/j.ceramint.2017.04.036 CERI15016

To appear in: Ceramics International Received date: 28 March 2017 Revised date: 6 April 2017 Accepted date: 6 April 2017 Cite this article as: Xiao-bo Li and Guang-ri Xu, Hydrothermal vselectrodeposition: howdoes deposition method affect the electrochemical capacitor performance of manganese dioxide?, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrothermal vselectrodeposition: howdoes deposition method affect the electrochemical capacitor performance of manganese dioxide?

Xiao-bo Li*, Guang-ri Xu Department of Chemistry and chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China

*

E-mail: [email protected] (Dr. X. B. Li)

ABSTRACT: Hydrothermal and electrodeposition are widely utilized methods in the preparation of MnO2 nanomaterials for electrochemical capacitor application. In this paper, these two approaches were applied to synthesize nanoscale MnO2, and their electrochemical properties were fully investigated in different electrolyte environments. Because of different mechanisms, the hydrothermal method gave an ultrathin layer of MnO2 nanosheets with a large accessible electrochemical surface area, while the electrodeposition method resulted in the formation of MnO2 nanorods with a rather limited opening area. The electrochemical performance of hydrothermally prepared MnO2 was much better in specific capacitance and overall resistance, except the cycling durability, as a result of its unique structure. Keywords: Hydrothermal, electrodeposition, MnO2, microstructure, electrochemical capacitor

1. Introduction

Recently, electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, have received extensive interest because of their high power density, fast charge/discharge rate capability, high cycling durability, fast charge propagation dynamics, as well as simple principles and low maintenance cost.[1] Based on the energy storage mechanism, ECs are generally classified into electric double layer capacitors (EDLCs), which store energy at the double layer, and pseudocapacitors where energy is stored via the fast and reversible Faradaic reaction.[2] So far, the most widely utilized EC electrode materials are porous carbons,[3] metal oxides[4–8] and conducting polymers.[9] Among them, the best specific capacitance was obtained from RuO2 with a value higher than 1300 F·g-1.[10] However, high cost and rareness are the main constraints of RuO2 for further application in ECs. Therefore, it necessitates researchers to explore inexpensive alternative metal oxide materials with comparable electrochemical performance. MnO2 is believed to be one of the most attractive candidates due to its high theoretical capacitance (1370 F·g-1), low cost and existence as an abundant resource.[7] In order to prepare feasible ECs with MnO2 electrodes, many efforts have investigated the synthesis of MnO2 materials with various morphologies, including nanowires,[11, 12] nanotubes,[13–15] nanoflakes,[16] spheres,[17–20] nanorods,[21–24] nanoflowers,[25– 27] nanosheets[28, 29] and nanoparticles.[30] Nevertheless, MnO2 has a rather poor electrical conductivity (10-5~10-6 S·cm-1), which greatly limits its potential application in energy storage systems.[31, 32] It was reported that the composites of MnO2 with highly conductive materials such as metal,[14, 28] carbon foams and nanoparticles,[15, 23]

carbon nanotubes [16, 19, 26] and graphene [18, 27, 30] are significantly effective for solving this problem. On the other hand, many techniques such as hydrothermal,[11, 13, 16, 17, 24, 33, 34] electrodeposition,[14, 15, 23, 26, 27, 35–37] self-limiting redox,[25, 30, 38–41] chemical vapor deposition,[19] precipitation[21] and ultraviolet irradiation and ultrasonic assisted methods[42, 43] have been developed to prepare MnO2 materials with desired properties. Hydrothermal and electrodeposition approaches are thetwo most investigated methods for the synthesis of MnO2. To the best of our knowledge, the direct impact of these two techniques on the structure and electrochemical performance of collected MnO2 has not been reported in detail until now. Here, MnO2 was synthesized by both hydrothermal and electrodeposition approaches. The differences in morphologies and structures of the MnO2 prepared accordingly are investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersivespectrometry (EDS). The deviations of electrochemical properties of the synthesized MnO2 from different procedures are characterized using cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) in both neutral and alkaline electrolytes. It is anticipated that our findings will provide an insight into the growth mechanism of MnO2 utilizing these two techniques, and the corresponding analysis will be potentially helpful for the science in this field.

2. Experimental

2.1. Materials and instruments

KMnO4, Mn(CH3COO)2, Na2SO4 and NaOH were used as purchased without further purification. All the chemical reagents used in this study were of analytical grade. A CHI660E electrochemical workstation was used with a standard three-electrode test pool, where a platinum sheet and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. All potentials in this study were referred to this reference electrode unless stated otherwise. The morphologies and microstructures were established by focused-ion-beam SEM (ZEISS AURIGA) and TEM (ZEISS LIBRA 200). The chemical composition and crystallographic information were obtained by XRD (RigakuD/max 2500, Cu Kα), using a Krotos Axis Ultra X-ray photoelectron spectroscope (Al Kα source).

2.2. The experimental setup for the synthesis of MnO2

The experimental setups for the synthesis of MnO2 materials on nickel foam by hydrothermal and electrodeposition routes (namely MN-1 and MN-2, respectively) are illustrated in Figure 1. In the hydrothermal preparation (Fig. 1a), the MnO2 is formed through the decomposition of KMnO4 based on the following equation:[16]

4MnO4 + 2H2O → 4MnO2 + 4OH- + 3O2 (1)

During the reaction, the former MnO2 nanoparticles serve as the nucleation sites where the latter MnO2 deposits on, as there is no introduction of any other structuring agents.[16] As time progresses, the layer structure, which is constructed through the electrostatic interaction between negatively charged [MnO6] and K+ ions in the layer space, grows to a larger size.[34] The insertion of crystal water into the interlayer space leads to the expansion of the interstitial distance between the layers, forming a loose structure of low crystallinity but increasing the accessible and exposed surface area for electrolyte ions; this would be potentially helpful for the electrochemical performance of MnO2. In the electrodeposition synthesis (Fig. 1b), MnO2 forms through the electrochemical oxidation of Mn2+. Newly formed MnO2 nanoparticles is anchored at the place where the electrons can transfer with the lowest resistance, resulting in a compact structure of the collected MnO2 with low accessible surface area.

2.3. Synthesis of MN-1 and MN-2

First, nickel foam (1×1.5 cm) was carefully cleaned with 3 M HCl solution, ethanol and deionized water in sequence to remove the surface NiO layer. MN-1 was synthesized in a typical procedure as follows. Nickel foam was placed against the wall of a Teflon-lined stainless steel autoclave containing a homogeneous solution (63.2 mg KMnO4in 40 mL deionized water). Afterwards, the autoclave was sealed and maintained at 160°C for 4 h. Finally, the Ni foam was taken out and cleaned with distilled water several times to

remove the remaining reactants and then dried at 60°C for 6 h in an oven. The mass loading was approximately 0.5 mg. MN-2 was electrochemically deposited onto Ni foamat room temperature in electrolyte (pH 7.0) composed of Mn(CH3COO)2 and Na2SO4 (0.1 M of each), where a piece of cleaned Ni foam was used as the working electrode. The cyclic voltammetric technique was conducted by sweeping the potential between +0.10 and +0.70 V at a scan rate of 20 mV·s−1 for 5 min. The Ni foam was then thoroughly rinsed with deionized water and absolute ethanol several times, and finally dried at 60°C in air. The mass loading was approximately 0.7 mg.

2.4. Electrochemical measurements

The electrochemical measurements were performed in either 1.0 M Na2SO4 or 2.0 M KOH electrolyte with the above-mentioned three-electrode system, where the MnO2loaded nickel foam was directly used as the working electrode. The specific capacitances of the products can be calculated from GCD curves according to the following equation:

C=

I ´ Dt mDV

(2)

where I (A) is the charge-discharge current, DV (V) is the potential window during the charge or discharge process, m (g) is the mass of the electroactive materials in the

electrodes and Dt (s) is the discharge time.

3. Results and discussion

The EC performances of MnO2are strongly dependent on both surface properties and crystalline structure of the synthesized MnO2 based on the following mechanisms:[44]

(MnO2)surface + M+ + e-↔ (MnO2M)surface

(3)

MnO2 + xM+ + xe-↔MxMnO2 (4)

where M+ is the cation (H+, Li+, Na+ or K+). Reaction (3) is based on the surface adsorption of electrolyte ions, while reaction (4) assumes that cations can intercalate/deintercalate in the MnO2 lattice during the electrochemical process. Accordingly, the valence of Mn in MnO2 will alter between Mn(IV) and Mn(III) along with the above electrochemical reactions. Considering the mechanism governing the process, the larger accessible surface area and sufficient gap in the MnO2 lattice which let the ions transfer are two important factors for the better electrochemical capacitor performances of MnO2.[44] To understand the morphologies of the products, SEM images of MN-1 (Fig. 2a and 2b)

and MN-2 (Fig. 2c and 2d) are presented in Figure 2. As can be seen, MN-1 is mainly composed of MnO2 nanosheets with a larger opening area which can facilitate the internal ion transfer. The MnO2 nanosheets array vertically on the nickel substrate which serves as both mechanical support for the MnO2 and current collector in the following electrochemical tests. The nickel surface is fully utilized by the homogeneous covering of MnO2 without any visible voids. However, in MN-2, MnO2 arrays into isolated spherical assemblies mainly composed of MnO2 nanorods on the substrate surface with a rather limited opening area. Moreover, many apparent voids are clearly observed. The elemental distributions of both MN-1 and MN-2are characterized by EDS mapping (Fig. S1, see the supplementary information). The microstructures of MN-1 and MN-2 are further characterized by TEM (Fig. 3). The thin films of MnO2 in MN-1 are clearly observed (Fig. 3a and 3b) with a large area of micron size and low thickness of several nanometres. This microstructure of MnO2 is more beneficial for the transfer of ions between the electrolyte and MnO 2 surface. The polycrystalline structure of MnO2 in MN-1 is clearly shown in the SAED pattern (inset in Fig. 3b). The amorphous MnO2 nanorods in MN-2 are displayed in Figures 3c and 3d, and show diameters of 10–20 nm and length up to several hundred nanometres. All observations for MnO2 from both SEM and TEM images confirm the proposed growth mechanism discussed above. The poor crystallinity of MnO2 in both MN-1 and MN-2 is furthermore verified by XRD (Fig. 4); almost all the peaks have disappeared except that one small peak is observed in MN-1 (inset in Fig. 4a). To investigate the corresponding EC performance of the products, Figure 5a displays the CV curves at 50 mV·s-1,and Figure 5b presents the GCD curves at 1 A·g-1of MN-1 and

MN-2 in 1.0M Na2SO4 electrolyte. Typical rectangular shapes without obvious redox peaks of the CV curves are observed for the two samples, indicating ideal capacitor behavior and fast charge/discharge characteristics.[11, 19, 25, 28, 30] This good electrochemical capacitor property and rapid reversible redox reaction are also confirmed by the symmetric GCD curves.[11] Derived from the unique thin layer structure, MN-1 has a much larger accessible surface area and more channels for the ion transport from the electrolyte to the surface of the MnO2. Furthermore, the loose property of the MnO2 lattice greatly enhances the charge transfer of Na+ in the MnO2 bulk. Herein, the specific capacitance (Fig. 5d) of MN-1 is about four times that of MN-2. The scan rate and current density dependent performances in 1.0M Na2SO4electrolyteof MN-1 and MN-2 are illustrated in Figure S2. The deviations of electrochemical capacitor performances between MN-1 and MN-2 are similar in tendency through the whole scanning and current densityrange; MN-1 performs much better than MN-2 in the whole ECs tests. EIS technique is also applied to investigate the impedance in 1.0M Na2SO4electrolyteof MN1 and MN-2 in detail (Fig. 5c). Normally, the EIS results for ECs application can be classified into three regimes: (i) at the high frequency area, the intercept at the real part corresponds to the combination of contact resistance between MnO2 and current collector, intrinsic resistance of the MnO2 host and the ionic resistance of electrolyte;(ii) at the middle frequency region, the surface property associated with Faradic charge transfer resistance dominates; (iii) at the low frequency zone, the straight line with a slope whether vertical to the real part (ideal capacitor behavior without any diffusion resistance) or a finite value (for real capacitor with diffusion resistance).[16, 17, 34, 36, 39] The intercept in Figure 5c is similar for MN-1 and MN-2, which implies that thesetwo

samples have a similar combination resistance. The slope of the MN-1 curve is higher than that of the MN-2 curve, indicating a lower diffusion resistance of MN-1 which is caused by the wide open thin layer structure. However, no typical semicircle in the middle frequency region is observed in these EIS tests. The electrochemical characterizations of MN-1 and MN-2 in 2.0M KOH electrolyteare given in Figure 6, and the corresponding rate-dependent testing results are provided in Figure S3. In the CV curves at 50 mV·s-1 (Fig. 6a), apparent redox peaks are observed for both MN-1 and MN-2, which can be assigned to the intercalation/deintercalation of K+ in the MnO2 lattice, as a result of its smaller solvated size compared with that of Na+. This electrochemical reaction process is also confirmed by the plateau in the GCD curves at 1 A·g-1 (Fig. 6b). The gap between the specific capacitances (Fig. 6d) of MN-1 and MN-2 is larger than that obtained in 1.0M Na2SO4 electrolyte. This is because of the enhanced pseudocapacitances from the intercalation/deintercalation of K+(rather than Na+) in the MnO2 lattice. The EIS curve of MN-1 in Figure 6c is similar to that in Figure 5c, and there is no challenge to calculate the corresponding identical resistances. Nevertheless, the EIS curve of MN-2 in Figure 6c is quite strange, and the relevant analysis is difficult and needs further examination. The durability tests of MN-1 and MN-2 were performed by GCD at 5 A·g-1 in both 1.0M Na2SO4 and 2.0M KOH electrolyte; the results are illustrated in Figure 7. After 1000 cycling tests, MN-1 maintains the capacitance retention of 65.3% and 103.2% in Na2SO4 and KOH, respectively. Meanwhile, MN-2 performs much better because of the limited exposed and accessible surface area and stronger corrosion resistance of nanorod structures compared with that of ultrathin nanosheets, and holds the retention of 91.7%

and 200% in Na2SO4 and KOH, respectively. The increasing capacitance in KOH after cycling is derived from the creation of defects from the charge-discharge process into the MnO2 matrix.

4. Conclusion

In conclusion, the MnO2 nanomaterials were synthesized by both hydrothermal and electrodeposition approaches. As a result of the different growth mechanisms of MnO2, the MnO2 prepared by thehydrothermal technique is composed of a thin layer of MnO2 nanosheets with a much larger opening area and loose microstructure while the MnO2 synthesized by electrodeposition is composed of nanorods and has a limited electrochemical accessible surface area. Because of the different structures and intrinsic poor electrical and ionic conductivity of MnO2, the MnO2 prepared by the hydrothermal method performs much better as electrochemical capacitor electrodes in both specific capacitance and overall resistance than those synthesized by electrodeposition. However, the electrodeposited MnO2 gives higher durability after 1000 cycling tests in both Na2SO4 and KOH electrolytes, benefiting from the low exposed surface area and stronger corrosion resistance of the nanorod structures.

Acknowledgements

The authors sincerely acknowledge the financial supports provided by The Education Department of Henan Province (Grant no. 17A150010).

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Figures and captions Fig.1 Illustrative diagram of different MnO2 architecture decorated nickel foam using (a) hydrothermal or (b) electrodeposition method.

Fig.2 FIB/SEM images of (a, b) MN-1, and (c, d) of MN-2. Fig.3 TEM image of (a, b) MnO2 in the MN-1 structure and SAED pattern (inset), and (c, d) MnO2 in the MN-2 structure and SAED pattern (inset). Fig.4 XRD patterns of (a) MN-1, the inset displays the peak of MnO2 and (b) MN-2. Fig.5 Electrochemical properties of electrode materials measured using a three-electrode system in 1.0 M Na2SO4 aqueous electrolyte: (a) CVs of MN-1 and MN-2 at a scan rate of 50 mV·s-1 and (b) GCD curves of MN-1 and MN-2 at a current density of 1.0 A·g-1. (c) Nyquist plot of MN-1 and MN-2. (d) Calculated specific capacitance of each electrode for various current densities. Fig.6 Electrochemical properties of electrode materials measured using a three-electrode system in 2.0 M KOH aqueous electrolyte: (a) CVs of MN-1 and MN-2 at a scan rate of 50 mV·s-1 and (b) GCD curves of MN-1 and MN-2 at a current density of 1.0 A·g-1. (c) Nyquist plot of MN-1 and MN-2. (d) Calculated specific capacitance of each electrode for various current densities. Fig.7 Long-term cycling performance of electrodes with the increase of charge-discharge numbers at a current density of 5 A·g-1 in (a) 1.0 M Na2SO4 aqueous electrolyte and (b) 2.0 M KOH aqueous electrolyte.

Fig.1

(a)

(b)

Fig.2

Fig. 3

10

20

30

40

50

2q (degree)

60

70

80

(220)

24

2q (degree)

(200)

(220) 22

(b) Ni

Ni (200)

20

MN-2

Intensity (a.u.)

(101)

(a)

MnO2

Intensity (a.u.)

Intensity (a.u.)

MN-1

(111)

(111)

Fig. 4 (XRD)

10

20

30

40

50

2q (degree)

60

70

80

Fig.5 10

(a)

6

(b)

0.8

MN-1 MN-2

0.6

Potential (V)

Current density (A g-1)

8

4 2 0

MN-1 MN-2

0.4 0.2 0.0

-2 -4

50 mV s-1

1.0 A g-1

-0.2

-6 -0.2

0.0

0.2

0.4

0.6

0

0.8

100

200

300

400

Time (s)

Potential (V) 400

1500

(d)

25

900

20

-Z" (Ohm)

-Z" (Ohm)

-1

MN-1 MN-2

1200

Specific capacitance (F g )

(c)

600

MN-1 MN-2

15 10 5

300

0 0

5

10 Z' (Ohm)

15

20

0

MN-1 MN-2

300

200

100

0 0

500

1000

Z' (Ohm)

1500

0

2

4

6

8 -1

Current density (A g )

10

Fig. 6

MN-1 MN-2

MN-1 MN-2

0.3

10 0 -10 -20

(b)

0.4

Potential (V)

20

0.2 0.1 0.0 -0.1

1.0 A g-1

-0.2

50 mV s-1

-0.3

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0

100

200

Potential (V)

(c) -1

2

1

0

0.8

1.2

1.6

2.0

2.4

400

500

500

Specific capacitance (F g )

MN-1 MN-2

3

300

2.8

3.2

(d)

400

300

MN-1 MN-2

200

100

0

2

4

6

8 -1

Z' (Ohm)

600

Time (s)

4

-Z" (Ohm)

Current density (A g-1)

0.5

(a)

30

Current density (A g )

10

60

retention 65.3%

45 30 15

retention 91.7%

0

5Ag 0

-1 200

400

400

(a)

MN-1 MN-2

75

600

Cycle number

800

-1 Specific capacitance (F g )

-1 Specific capacitance (F g )

Fig. 7

1000

(b)

MN-1 MN-2 300

retention 103.2% 200

100

retention 200.0% 5Ag 0

200

400

600

Cycle number

800

-1 1000