Cu porous granular films by in situ oxidation for electrochemical energy storage

Journal Pre-proof Preparation of Cu2O/Cu porous granular films by in situ oxidation for electrochemical energy storage

Min Song, Jianling Zhao, Hongyi Li, Xiaofei Yu, Xiaojing Yang, Laiqi Zhang, Zekun Yin, Xixin Wang PII:

S1572-6657(19)31023-9

DOI:

https://doi.org/10.1016/j.jelechem.2019.113755

Reference:

JEAC 113755

To appear in:

Journal of Electroanalytical Chemistry

Received date:

8 July 2019

Revised date:

5 December 2019

Accepted date:

10 December 2019

Please cite this article as: M. Song, J. Zhao, H. Li, et al., Preparation of Cu2O/Cu porous granular films by in situ oxidation for electrochemical energy storage, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113755

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© 2019 Published by Elsevier.

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Preparation of Cu2O/Cu porous granular films by in situ oxidation for electrochemical energy storage Min Song1, Jianling Zhao1,*, Hongyi Li2, Xiaofei Yu1, Xiaojing Yang1, Laiqi Zhang1, Zekun Yin1, Xixin Wang1,* 1. School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China

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2. School of Materials Science and Engineering, Beijing University of Technology,

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Beijing 100124, P. R. China

Corresponding

author:

Jianling

Zhao

([email protected]);

([email protected]) Tel: +86-22-60204525; Fax: +86-22-60202660

1

Xixin

Wang

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ABSTRACT

In this paper, Cu2O/Cu porous granular films were prepared by electrodeposition in copper sulfate solution containing triethanolamine and in-situ oxidation strategies. The effects of the amount of triethanolamine, electrodeposition time and annealing temperature on the morphology, structure and electrochemical properties of the

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granular film were investigated, and the related reasons were discussed. Results show

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that the CV curves of the as-prepared sample have a pair of redox peaks with good

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symmetry and reversibility in 1 M KOH solution at a potential window from -1 to -0.2

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V, corresponding to the conversion process between Cu2O and Cu. When the amount of triethanolamine is 0.2 wt%, the deposition time is 3 min, and the annealing

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temperature is 200°C, the capacitance of the sample reaches the maximum value of

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158.8 mF/cm2. Moreover, the sample has a good cycling stability with 85.9% of its original capacitance value after 3000 cycles.

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Keywords: Supercapacitor; Negative electrode; Cu2O/Cu; Electrodeposition; In-situ oxidation 1. Introduction

The electrode of a supercapacitor is mainly composed of the current collector and the active material. Generally, the metal with excellent electrical conductivity is the preferred material for current collector. Increasing the specific surface area of the current collector can increase the contact area between the active material and current collector, enhance the effective utilization of the active material, and reduce the internal resistance of the electrode [1]. Therefore, the fabrication of metal current collector with high specific surface area has received extensive attention. For example, 2

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metal current collectors in different shapes such as copper foam [2], nickel nanocone arrays [3], stainless steel meshes [4], and 3D porous silver nonwoven mats [5] have been prepared. Metal oxides are an important class of electrochemically active materials, which mainly store electrochemical energy in pseudocapacitance style. Commonly used

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metal oxides include RuO2 [6,7], MnO2 [8-10], NiO [11], CuO [12-14], Co3O4 [15], FeOx [16], and the like. Among them, copper oxides have the advantages of good

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performance, easy preparation, low cost, and eco-friendly feature. There have been

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many researches on copper oxide electrodes. For example, Purushottam Kumar Singh

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et al. [17] first prepared CuO nanoparticles by an electrochemical discharge method,

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and then bonded the CuO nanoparticles to graphite with a binder to prepare an

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electrode. Feng Yang et al. [18] fabricated nanoporous bamboo-like CuO by hydrothermal and thermal decomposition method, and then pressed CuO powders

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with acetylene black and polytetrafluoroethylene on Ni-foam to form a CuO electrode. K.V. Gurav et al. [19] obtained a Cu(OH)2 electrode on stainless steel substrates by chemical bath deposition in an aqueous solution containing CuSO4 and NH3. Suocheng Wang et al. [20] first carried out femtosecond laser processing and electrochemical anodization of copper foam, and then obtained a CuO flowers electrode after high temperature treatment. Dong He et al. [21] chemically oxidized the surface of copper foam in a solution containing NaOH and (NH4)2S2O8 to prepare a Cu(OH)2 nanorods electrode. Ying Liu et al. [22] first oxidized copper foam with a solution containing NaOH and (NH4)2S2O8, and then calcined and cyclic voltammetry 3

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oxidized in 6 M KOH solution to fabricate Cu@CuO nanorod arrays electrode. These copper oxide electrodes exhibit promising energy storage properties, however, up to now, the obtained copper oxide electrodes often work at a positive potential window, corresponding to

the

conversion

between

Cu(I)

and

Cu(II)

(Cu2O/CuO,

Cu2O/Cu(OH)2, CuOH/CuO, CuOH/Cu(OH)2) [23-25]. As is well known, compared

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with the relatively mature positive electrode materials, the investigation of negative electrode materials is more important for further improving the performance of

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supercapacitors [26,27]. For example, the working voltage range of Li-doped

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CuO/reduced graphene oxide composite electrodes can reach -1 to 0.8 V [28]. In

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addition, it is worth noting that the standard electrode potential of Cu2O/Cu in alkaline

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electrolyte is about -0.6 V (vs.SCE) [29], thus we have enough reasons to speculate

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that Cu2O/Cu might be an underlying negative electrode material for electrochemical energy storage device. However, reports on electrochemical energy storage through

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conversion between Cu2O and Cu are hard to see. In the preparation of supercapacitor electrodes, there are mainly three methods for combining active materials with current collector: (1) pressing the active powders on the current collector [30]; (2) depositing the active materials onto the current collector by different strategies [31,32]; (3) in situ growth of actives on the surface of the current collector [33]. The advantage of method 3 is that the active materials and the current collector have a higher bonding strength and a smaller contact resistance, thus the electrical conductivity is higher, which can improve the effective utilization of the active materials and enhance the performance of the electrode to a large extent 4

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[34]. Based on the above considerations, in this paper, the porous granular copper film was first prepared by electrodeposition on copper foil, and then the copper atoms on the surface of the granular film were directly transformed into Cu2O by in situ thermal oxidation method to obtain the Cu2O/Cu porous granular film. When the as-prepared

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film was used as a supercapacitor electrode, a reversible redox reaction corresponding to the conversion between Cu2O and Cu can occur at the potential window from -1 to

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-0.2 V (vs. SCE), and in this process, the porous granular film acted as both a current

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collector and a participator in the electrode reaction. At a scanning speed of 5 mV/s,

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the area capacitance reaches 158.8 mF/cm2, and the sample has good cycling stability.

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Using the film sample as an anode of an asymmetric supercapacitor would further

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enlarge the operating voltage window and increase energy density of the supercapacitor. Moreover, since the porous granular film was prepared on top of the

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Cu substrate, it would also be used as a flexible electrode. To the best of our knowledge, this is the first report on the performance of electrochemical energy storage of Cu2O/Cu at the negative potential window. Results show that the Cu2O/Cu granular film prepared in this work is a highly promising material to be used as the anode for asymmetrical supercapacitors.

2. Experimental 2.1 Preparation and characterization of Cu2O/Cu porous granular films A copper sheet (99.9%, 20×40×0.2 mm3) was ultrasonically washed with ethanol 5

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and deionized water for 10 minutes. A certain amount of triethanolamine was added to a solution of 100 g of 0.2 M CuSO4 prepared with deionized water and CuSO4•5H2O, and the mixture was used as an electrodeposition solution. Then the cleaned copper sheet was used as a cathode, a platinum sheet as an anode, and the distance between the anode and cathode was 2.0 cm. The electrodeposition experiment was carried out

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under a direct current condition of 50 mA/cm2 at 25°C for 3 min. After the end of the experiment, the copper sheet was taken out, washed three times with deionized water,

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air-dried, and calcined at 200°C for 2 h to obtain a porous granular film (GF) sample.

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When the amount of triethanolamine was 0, 0.2 g, and 0.4 g, respectively, the

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prepared samples were labeled as GF-0, GF-0.2, and GF-0.4. The reagents used in the

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power source (Dahua, Beijing).

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experiments were all of analytical purity. The power source used was DH1719A-5 DC

The morphology and crystal structure of the GF samples were examined using a

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scanning electron microscope (SEM Quanta 450 FEG, 20 KV), a field emission high-resolution transmission electron microscope (TEM Talos F200S, 200 KV) and an X-ray diffractometer (XRD Bruker D8 Discover, Cu-Kα radiation, λ = 1.542 Å). The compositions of the samples were analyzed using an X-ray photoelectron spectroscope (XPS ESCALAB 250Xi) at room temperature. 2.2 Electrochemical measurements All electrochemical tests were performed in a standard three-electrode system with the GF sample as the working electrode, the platinum plate as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode. The 6

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test electrolyte was 1 M KOH solution. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrical impedance spectroscopy (EIS) properties of the GF samples were tested using a CHI660e electrochemical workstation (Chenhua, Shanghai). Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range from 0.01 Hz to 1000 kHz with a 5 mV AC

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perturbation at open circuit potential. The specific capacitance (Cs) of the sample was calculated from the CV curves according to Eq. 1, or calculated from the GCD curves

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according to Eq. 2. In Eq. 1, S is the integrated area of the CV curve, ν is the scan rate

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(V s-1), and ΔV is the potential window (V). In Eq. 2, I is the charge-discharge current

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density (A cm-2), and Δt is the discharge time (s). S 2ν∆V

(1)

I∆t ∆V

(2)

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Cs =

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Cs =

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3. Results and discussion

3.1 Morphologies and characterizations Fig. 1 shows the SEM images of copper granular film samples when the amount of triethanolamine is different. The surface of the sample GF-0 is rough and the copper particles are tightly packed (Fig. 1a); for the sample GF-0.2, the copper particles deposited on the surface form a grape-bunch structure (Fig. 1b), which are loosely packed and have plenty of corners, and the sediment layer has a large amount of pores. The average particle size is about 330 nm, and the particle film thickness is about 2.2 μm (Fig. 1c); the surface of the sample GF-0.4 also has a grape string 7

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structure, but it is sparse (Fig. 1d), which may be partially caused by the falling off of deposited copper particles. The insets in Figs. 1a, 1b and 1d are SEM images of the samples before annealing. It can be seen from the comparison that the morphology of the samples has not changed significantly before and after annealing, indicating that the porous structure of the samples was formed during the electrodeposition of copper

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particles.

Fig. 1 SEM images of copper granular film samples with different addition amounts of triethanolamine (Insets are the SEM images of unannealed samples): (a) surface image of GF-0; (b, c) surface and side images of GF-0.2; (d) surface image of GF-0.4

The results in Fig. 1 suggest that the addition of triethanolamine to the aqueous solution of copper sulfate can significantly change the morphology of the GF samples. The mechanism is that triethanolamine can coordinate with copper ions to form a complex with a large volume and steric effect [35], which would hinder the close packing of the deposited copper atoms, thus a porous granular film rather than a dense film forms. However, when the amount of triethanolamine is too large, the steric 8

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effect is serious, and the adhesion between the copper particles is weak, thus partial shedding happens (Fig. 1d). According to the experimental results, the optimum

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addition amount of triethanolamine is 0.2% by weight.

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Fig. 2 XRD patterns of copper and copper granular film samples with different addition amounts of triethanolamine (a, b); XPS survey spectrum of Cu2p (c) and O1s (d) of sample GF-0.2

Fig. 2a is the XRD patterns of Cu sheet annealed under 200℃ and copper granular film samples with different addition amounts of triethanolamine, and Fig. 2b is the partially enlarged view of Fig. 2a. As can be seen from the figure, all samples have metallic copper diffraction peaks at 43.3°, 50.5°, and 74.2° (JCPDS no. 04-0836), and (111) crystal plane diffraction peak of Cu2O at 36.3° (JCPDS no. 05-0667). The Cu2O peak intensity of copper sheet is the weakest, which of the GF-0 and GF-0.2 samples increases in turn, however, that of the sample GF-0.4 is weakened relative to GF-0.2 (Fig. 2b). 9

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It can be known from the experimental steps that the Cu2O in the sample is transformed by in-situ oxidation of copper particles or copper atoms on the surface of Cu sheet. Therefore, the peak intensity of Cu2O is closely related to the morphology of the samples. The surface of the Cu sheet is smooth, thus the formed Cu2O is less and the peak is the weakest. GF-0.2 sample has a porous granular structure with a

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large specific surface area. The copper particles of GF-0.2 have a high surface energy and are easily oxidized, thus there is maximum Cu2O and the strongest diffraction

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peak at 36.3°. The copper particles of GF-0 are closely packed, and those of the

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sample GF-0.4 are partially detached, consequently, both GF-0 and GF-0.4 have an

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intermediate amount of Cu2O between Cu sheet and GF-0.2.

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In Figs. 2a and 2b, the Cu2O peak of the sample GF-0.2 is the strongest, so its

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chemical composition was further studied by the XPS spectroscopy. The Cu2p spectrum of GF-0.2 consists of two sets of peaks (Fig. 2c). The peaks at 932.5 eV and

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952.5 eV correspond to Cu+2p3/2 and Cu+2p1/2, and the peaks at 934.7 eV and 954.6 eV correspond to Cu2+2p3/2 and Cu2+2p1/2 [36]. According to the relative intensities of the two sets of peaks, it is known that the main composition of the sample is Cu + and a small amount of Cu2+ is also present in the sample. The O1s peak of the sample is at 530.1 eV (Fig. 2d), which corresponds to the Cu-O-Cu bond. The results in Figs. 2c and 2d show that the GF-0.2 sample is mainly composed of Cu2O and also contains a small amount of CuO. However, there are no diffraction peaks of CuO in Fig. 2a and 2b, implying that the CuO exists as an amorphous structure in the sample. Figs. 3a-3d show the CV curves of Cu and different copper granular film 10

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samples at different scan rates. It can be seen from the figure that the CV curves of each sample have a pair of redox peaks and the symmetry and reversibility of the redox peaks of the sample GF-0.2 is the best (Fig. 3c). Fig. 3e gives the areal capacitance versus scan rate curves of each sample. In the order of Cu, GF-0, GF-0.4, and GF-0.2, the capacitance value increases in turn. When the scan rate is 5 mV/s, the

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areal capacitance value is 15.6 mF/cm2, 51.3 mF/cm2, 86.5 mF/cm2 and 158.8 mF/cm2, respectively. The GF-0.2 sample has a capacitance value which is 10 times that of the

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Cu sheet. Fig. 3f displays the GCD curves of different samples at a current density of

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7 mA/cm2. All curves have obvious charge and discharge platforms. In the order of

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Cu, GF-0, GF-0.4, and GF-0.2, the discharge platform becomes larger in turn, and the

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charge and discharge time and the capacitance value increase sequentially. The

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symmetry of the charge and discharge platform of GF-0.2 sample is much better, while that of Cu, GF-0 and GF-0.4 is relatively poor. Furthermore, the results of Fig.

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3f are completely consistent with the CV test results (Figs. 3a-3e).

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Fig. 3 CV curves (a-d), areal capacitance versus scan rate curves (e) and GCD curves (f) of copper and copper granular film samples with different addition amounts of triethanolamine

In Fig. 3, the redox peak position of each CV curve and the charge and discharge platform of each GCD curve are both symmetric at about -0.58 V (vs. SCE), which is substantially the same as the standard electrode potential of Cu2O/Cu [29], therefore, the redox reaction of each sample may be expressed as: Cu2O + H2O + 2e-

2Cu + 2OH-

(3)

The Cu2O in the sample plays a role of an active material. As can be deduced 12

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from Fig. 2 and Fig. 3, the capacitance of the sample is closely related to the Cu2O content, and the capacitance increases as the amount of Cu2O increases. Sample GF-0.2 has the highest Cu2O content and hence the largest capacitance. 3.3 Effect of deposition time In order to investigate the effect of deposition time on the structure and property

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of the above optimal sample GF-0.2, the electrodeposition time was adjusted to 1.5

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min and 4.5 min and all other conditions remained the same. The SEM images of the

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samples prepared at different deposition times are presented in Fig. 4 (the morphology of the sample at 3.0 min is shown in Figs. 1b and 1c). When the deposition time is 1.5

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min, the granular film is sparse, with the diameter of the copper particles and the film

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thickness being about 230 nm and 1.8 μm (Figs. 4a and 4c); while when the

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deposition time is 4.5 min, the granular film is denser and denser toward the bottom, with the diameter of the copper particles and the film thickness being about 450 nm

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and 2.7 μm (Figs. 4b and 4d). Overall, when the deposition time is 3.0 min, the state of the granular film falls in between 1.5 and 4.5 min, with the diameter of the copper particles and the film thickness being about 330 nm and 2.2 μm, respectively (Figs. 1b and 1c).

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Fig. 4 SEM surface and side images of sample GF-0.2 at different deposition times: (a, c) 1.5

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min; (b, d) 4.5 min

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Figs. 5a and 5b display the CV curves of the sample at different deposition

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times (the CV curves at 3 min is shown in Fig. 3c). When the deposition time is 1.5

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min, the symmetry of CV curves is rather poor (Fig. 5a). However, the symmetry and reversibility of the CV curves are much better when the deposition time is 3.0 min and

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4.5 min. Known from Fig. 5c, as the deposition time increases, the capacitance value of the sample GF-0.2 increases first and then decreases, thus the capacitance value of the sample deposited for 3 min is the largest. Fig. 5d shows the GCD curves of sample GF-0.2 at different deposition times (7 mA/cm2). Each GCD curve has obvious charge and discharge platform. In the order of 1.5 min, 4.5 min and 3 min, the charge and discharge platform of the GCD curve becomes larger in turn, and the charge and discharge time and the capacitance value increase sequentially, which agrees well with the CV test results.

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Fig. 5 CV curves (a, b), areal capacitance versus scan rate curves (c) and GCD curves (d) of sample GF-0.2 at different deposition times

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According to Fig. 4 and Figs. 1b and 1c, the deposition time has two aspect

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effects on the morphology of the sample. As the deposition time is extended, on the one hand, the deposition amount of copper atoms increases, and the thickness of the granular film increases, which is favorable for the improvement of the capacitance value of the sample; On the other hand, along with the increasing deposition time, the copper particles grow and the packing between the particles becomes tight until stick together, which causes a decrease in the specific surface area and thus a lower capacitance value of the granular film. When the deposition time is 3.0 min, the two factors are well balanced, and therefore, the sample exhibits the best capacitive performance. 15

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3.3 Effect of annealing temperatures In order to investigate the effect of annealing temperatures, the sample GF-0.2 was annealed at 100°C, 150°C, 200°C, 250°C and 300°C for 2 h, respectively. Fig. 6 is the XRD patterns of the sample GF-0.2 after annealing at different temperatures. Under different annealing temperatures, all samples have three diffraction peaks of

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metallic copper and a (111) crystal plane diffraction peak of Cu2O at 36.3°.

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Furthermore, the (111) crystal plane diffraction peak intensity of Cu2O increases as

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the annealing temperature increases. When the temperature is higher than 200°C, the (200) and (220) crystal planes diffraction peaks of Cu2O appear at 42.3° and 61.4°,

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and the (111) plane diffraction peak intensity of metallic copper at 43.3° decreases

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significantly. When the annealing temperature is 250°C, a weak (111) crystal plane

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diffraction peak of CuO appears at 38.7° (JCPDS no. 48-1548); and when the annealing temperature is 300°C, two strong peaks at 35.5° and 38.7°can be observed,

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corresponding to (11-1) and (111) crystal plane diffraction peaks of CuO.

Fig. 6 XRD patterns of sample GF-0.2 annealed at different temperatures (Fig. b is the enlarged view of Fig. a)

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At different annealing temperatures, the CV curves of the sample GF-0.2 are shown in Figs. 7a-7d (CV curves of the annealed sample at 200°C are shown in Fig. 3c). The CV curves of each sample have a pair of redox peaks. And when the annealing temperatures are 100°C, 150°C and 200°C, the symmetry and reversibility of the redox peaks are better, which are significantly deteriorated when the annealing

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temperatures are increased to 250°C and 300°C, meanwhile, the overall shape of the curves becomes inclined, and the reaction resistance increases remarkably. Fig. 7e

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shows the areal capacitance versus scan rate curves of sample GF-0.2 at different

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annealing temperatures. As the annealing temperature increases, the capacitance of

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sample GF-0.2 increases first and then decreases. When the annealing temperature is

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200°C, the capacitance value of the sample is maximum. Fig. 7f displays the GCD

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curves of sample GF-0.2 at different annealing temperatures (7 mA/cm2). Each GCD curve has a notable charge and discharge platform. In the order of 300°C, 250°C,

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100°C, 150°C, and 200°C, the charge and discharge platform becomes larger in turn, and the charge and discharge time and the capacitance value increase sequentially, which is completely consistent with the CV test results.

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Fig. 7 CV curves (a-d), areal capacitance versus scan rate curves (e) and GCD curves (f) of sample GF-0.2 at different annealing temperatures

Fig. 8 shows the EIS curves of the annealed samples at different temperatures. The high-frequency semicircle of the Nyquist plot is generated by the conversion between Cu+/Cu, and its diameter corresponds to the resistance of the interface charge transfer (Rct). The straight line in the low frequency region reflects the Warburg impedance (Zw) caused by the diffusion of ions at the electrode/electrolyte interface, 18

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and is expressed by the admittance coefficient Y0 (Y0 = 1/Zw). When the annealing temperature is 100°C, the spectrum of the sample is a semicircle, indicating that the electrode resistance is mainly from the charge transfer resistance, and its Rct value is about 9.6 Ω. When the annealing temperature is higher than 100°C, the impedance spectra of the sample consist of a semicircle in the high frequency region and a

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straight line in the low frequency region, and the analog circuit is of the R(C(RW)) type (Fig. 8a). When the annealing temperatures are 150°C and 200°C, the Rct values

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of the samples are 14.7 Ω and 23.2 Ω, respectively, and the Y0 values are 0.1193

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S•sec0.5 and 0.05994 S•sec0.5, implying that both charge transfer resistance and

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Warburg impedance of the sample increase with the annealing temperature. And when

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the annealing temperatures are 250°C and 300°C, the Rct values of the samples are

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1798.8 Ω and 2878.1 Ω, respectively, and the Y0 values are 0.001356 S•sec0.5 and 0.001329 S•sec0.5. Obviously, the interface charge transfer resistance and Warburg

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impedance of the samples increase dramatically (Fig. 8b).

Fig. 8 Nyquist plots of sample GF-0.2 annealed at different temperatures: (a) 100°C, 150°C and 200°C; (b) 250°C and 300°C

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Upon the results of Fig. 6 and Fig. 8, when the annealing temperature is raised from 100°C to 200°C, although the amount of Cu2O formed on the sample surface is increased, the electrode reaction resistance does not change much, so the capacitance value of the sample increases; while when the temperature is 250°C and 300°C, excessive Cu2O is formed on the sample surface, and CuO is also formed, which

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capacitance value of the sample is greatly reduced.

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causes the electrode reaction resistance to increase remarkably, and thus the

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3.4 Effect of test potential window

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In order to investigate the effect of test potential window on the capacitance performance of the sample, CV tests were performed on the sample GF-0.2 over

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different voltage ranges. The results are shown in Fig. 9. Fig. 9a is the CV curves of

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the sample GF-0.2 at a potential window of -0.9 to -0.3 V at different scan rates, the

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shape of which is changed to some extent compared with that of -1 to -0.2 V (Fig. 3c), but the electrode reaction still corresponds to the conversion between Cu and Cu2O (Eq. 3). When the scan rate is 5 mV/s, the specific capacitance of the sample is 105.9 mF/cm2, significantly lower than that at the potential window of -1 to -0.2 V (158.8 mF/cm2), which may be caused by the decrease of the utilization rate of the actives as the potencial window shrinks. Fig. 9b is the CV curves (50 mV/s) of the GF-0.2 at the potential window of -1 to 0.4 V and -1 to 1 V, both of which show a strong oxidation peak around 0.3 V, corresponding to Cu2O [37,38] or Cu [39] converted to CuO. Besides, the CV curve at the potential window of -1 to 1 V (Fig. 9b) exhibits obvious oxygen evolution peak at 0.6~1 V. However, relative to the oxidation peaks, the 20

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reduction peaks of the two curves are small. The results in Fig. 9b suggest that when the potential window is -1 to 0.4 V and -1 to 1 V, Cu2O in the sample GF-0.2 will be oxidized to CuO, and due to the weak reduction effect, it is difficult to achieve a

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reversible redox reaction.

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Fig. 9 CV curves of sample GF-0.2 under different potential windows: (a) -0.9~-0.3 V; (b)

3.5 Cycling performance

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-1~0.4 V and -1~1 V

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Figs. 10a and 10c present the GCD curves of sample GF-0.2 at different current densities and different potential windows. They all have obvious charge and discharge platforms. And when the current density is 6 mA/cm2, the areal capacitances of the sample are 99.0 mF/cm2 (-1~-0.2 V) and 63.4 mF/cm2 (-0.9~-0.3 V) respectively. Figs. 10b and 10d give the cycling stability of sample GF-0.2 at 9 mA/cm2 at different potential windows. After 3000 cycles, the capacitance retentions of the sample are 85.9% (-1~-0.2 V) and 93.3% (-0.9~-0.3 V) respectively. After narrowing the potential window, the capacitance value of the sample was significantly reduced, but the capacitance retention rate was clearly improved. 21

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Fig. 10 GCD curves and cycling stability of sample GF-0.2 under different potential

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windows: (a, b) -1~-0.2 V; (c, d) -0.9~-0.3 V

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To study the cause of the performance degradation of the sample during cycling, XRD, XPS, and TEM analyses were performed on sample GF-0.2 after the cycling experiment. The results are shown in Fig. 11. Apparently, the XRD patterns of the samples before and after cycling are similar, with strong Cu2O (111) crystal plane diffraction peaks and two diffraction peaks of Cu (Fig. 11a). Fig. 11b shows the XPS Cu2p spectrum of the sample. The relative content of Cu+ and Cu2+ in the sample did not change significantly before and after cycling. Figs. 11c and d are TEM and high-resolution TEM images of the sample after cycling. From the TEM image, it can be seen that the inside and outside structures of the sample are different (Fig. 11c). Fig. 11d gives the high-resolution TEM image of the outside part, and the lattice 22

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spacing is about 0.243 nm, which corresponds to the Cu2O (111) crystal plane. The

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results in Figs. 11c and 11d are consistent with Fig. 11a.

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Fig. 11 XRD patterns (a), XPS Cu2p spectrum (b), TEM image (c) and high-resolution TEM

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image (d) of sample GF-0.2 after 3000 cycles

The results in Fig. 11 show that the crystal phase and composition of GF-0.2 did not change significantly before and after 3000 cycles. Therefore, the main decay mechanisms may be that the volume of the active material and the interface state of the Cu-Cu2O have changed by the rapid redox reaction during repeated charge and discharge processes. Table 1 lists the capacitance performances of related copper oxide electrodes reported previously. The potential window of Cu2O/Cu electrode prepared in this work is located at a larger negative potential. The mass capacitance of the active material (ca. 560.5 F/g) is higher and the electrode has better cycle stability. The 23

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relatively good performances of Cu2O/Cu porous granular film electrode can be mainly ascribed to the following virtues: The porous granular copper film was prepared by electrodeposition, and thus the copper particles have good electrical conductivity. The Cu2O active materials was transformed from copper particles by in-situ oxidation, resulting in a small contact resistance between the active materials

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and the current collector. The porous granular copper film has a large specific surface area, and at the same time, there are many relatively large gap between the copper

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particles, which provides much more shuttle channels for the ions in the electrolyte. In

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addition, this structural feature greatly improves the dispersion uniformity and

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utilization of Cu2O in the sample, and concentrates the electrode reaction in a small

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potential range.

Sample

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Table 1 Capacitive performance of copper oxide electrodes reported in the literatures Potential

Electrolyte

capacitance 571.25 F/g （1 A/g） 251.5 F/g （0.25 A/g） 284 F/g （1 A/g） 272 F/g （1.47 A/g） 459 F/g （5 mV/s） 120 F/g （10 mV/s） 200 F/g （5 mA/cm2） 594.27 F/g (0.71A/g) 212 F/g （0.41 mA/mg） 143.63 F/g (0.5 mA/cm2) 348 F/g （1 A/g） 190 F/g （2 mA/cm2） 158.8 mF/cm2 560.5 F/g （5 mV/s）

range 0~0.8 V

1 M KOH

-0.1~0.4 V

6 M KOH

0~0.5 V

1 M KOH

0~0.33 V

6 M KOH

0~1 V

2 M KOH

-0.5~0 V

1 M NaOH

0~0.5 V

6 M KOH

0~0.5 V

6 M KOH

0~0.4 V

6 M KOH

0~0.6 V

1 M KOH

-0.5~0 V

0.1 M KOH

-0.6~0 V

1 M KOH

-1~-0.2 V

1 M KOH

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CuO nanoparticles /graphite Bamboo leaf-like CuO /Ni foam Uniform CuO:Co microspheres /Ni foam granular CuO films /stainless steel Nanoflower-like CuO/Cu(OH)2 films/stainless steel Cu(OH)2 thin films /stainless steel 3D Cu2O@Cu foams nanocomposite Hierarchical CuO nanorod arrays /Cu foam CuO nanosheets /Cu foam CuO/nanoporous Cu

Specific

3D porous gear-like CuO /Cu foil CuO nanoflakes /Cu foil Cu2O/Cu porous granular films /Cu foil

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capacitance retention rate 92% （1000 cycles） 88.79% （500 cycles） 75.6% （1000 cycles） 85% （3000 cycles） 88% （2000 cycles） _

Ref [16] [17] [23] [40] [41] [18]

87.9% （12000 cycles） 96.45% （4000 cycles） 85% （850 cycles） _

[42]

87.9% （2000 cycles） 67% （2000 cycles） 85.9% （3000 cycles）

[44]

[21] [43] [24]

[45] Our work

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4. Conclusions In this paper, Cu2O/Cu porous granular film was successfully prepared by electrodeposition and in-situ oxidation. The sample exhibits good electrochemical energy storage performance with good symmetry and reversibility of Cu2O/Cu redox peaks at -1 to -0.2 V. The amount of triethanolamine, electrodeposition time and

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annealing temperature have significant effects on the morphology and electrochemical

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properties of the sample. When the amount of triethanolamine is 0.2 wt%, the

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deposition time is 3 min, and the annealing temperature is 200°C, the sample has the best morphology and performance. The areal capacitance of the sample is 158.8

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mF/cm2 when the scan rate is 5 mV/s. When the voltage range is enlarged to a positive

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potential, the reversibility of the electrode reaction becomes worse; when the potential

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window is -0.9 to -0.3 V, the capacitance value of the sample decreases significantly. At a current density of 9 mA/cm2, the sample capacitance retentions are respectively

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85.9% (-1~-0.2 V) and 93.3% (-0.9~-0.3 V) after 3000 cycles. The good capacitive properties of the Cu2O/Cu porous granular film are mainly attributed to its special structure, which endows the film with good electrical conductivity, large specific surface area and low resistance. The negative working potential, large area capacitance value, and good cycling stability of Cu2O/Cu porous granular film make it a highly promising candidate for using as anode materials in electrochemical energy storage devices.

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Acknowledgements This work is supported by the Key Basic Research Programme of Hebei Province of China (17964401D), the National Natural Science Foundation of China (51972095, U1607110) and Beijing Municipal Commission of Education Foundation (KZ201610005002).

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Journal Pre-proof Author Contributions Section Min Song: Conceptualization, Methodology, Investigation, Writing - Original Draft. Jianling Zhao: Data curation, Writing - Review & Editing, Validation. Hongyi Li: Supervision, Validation. Xiaofei Yu: Supervision. Xiaojing Yang: Supervision. Laiqi Zhang: Data curation.

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Zekun Yin: Data curation.

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Xixin Wang: Conceptualization, Methodology, Writing - Review & Editing.

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Journal Pre-proof Declaration of interests

☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical abstract

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Highlights Cu2O/Cu granular film (GF) was prepared by electrodeposition and in-situ oxidation. The CV curve of GF electrode has a pair of Cu2O/Cu redox peaks with good symmetry.

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 The in-situ oxidation method endows the electrode much smaller contact resistance.

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GF electrode has excellent electrochemical performance at negative potential

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window.

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