NiO synthesized by electrochemical dealloying as efficient electrode materials for supercapacitors

Journal Pre-proofs Nanospherical Cu2O/NiO synthesized by electrochemical dealloying as efficient electrode materials for supercapacitors Yidong Miao, Xuping Zhang, Yanwei Sui, Enyuan Hu, Jiqiu Qi, Fuxiang Wei, Qingkun Meng, Yezeng He, Zhi Sun, Yaojian Ren, Zhenzhen Zhan PII: DOI: Reference:

S0167-577X(20)30005-7 https://doi.org/10.1016/j.matlet.2020.127300 MLBLUE 127300

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Materials Letters

Received Date: Revised Date: Accepted Date:

24 November 2019 25 December 2019 2 January 2020

Please cite this article as: Y. Miao, X. Zhang, Y. Sui, E. Hu, J. Qi, F. Wei, Q. Meng, Y. He, Z. Sun, Y. Ren, Z. Zhan, Nanospherical Cu2O/NiO synthesized by electrochemical dealloying as efficient electrode materials for supercapacitors, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127300

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Nanospherical Cu2O/NiO synthesized by electrochemical dealloying as efficient electrode materials for supercapacitors

Yidong Miao1, Xuping Zhang 2, *, Yanwei Sui2, Enyuan Hu2, Jiqiu Qi2, Fuxiang Wei2, Qingkun Meng2, Yezeng He2, Zhi Sun2, Yaojian Ren 2, Zhenzhen Zhan2 1. China University of Mining and Technology, Xuzhou 221116, People’s Republic of China

2. School of Materials Science and Physics, China University of Mining and Technology, Xuzhou 221116, People’s Republic of China

*E-mail addresses: [email protected]

Abstract In this study, Cu2O/NiO with nanosphere structure has been successfully synthesized by dealloying a copper foil coated with Zn-Cu-Ni alloy films. First, Zn-Ni films were electrodeposited on copper foil. Next, the Zn-Cu-Ni films were produced by annealing and the Cu2O/NiO was obtained after Zn-dealloying and oxidation. Due to the advantage of the tunable nanosphere structure and dealloying synthesis method, Cu2O/NiO has excellent supercapacitor properties, such as excellent cycling stability (94.5% after 5000 cycles) and high specific capacitance (2255.5 mF/cm2 in 1mA/cm2). These data suggested that this lowcost oxide Cu2O/NiO with nanosphere morphology synthesized by dealloying method could be a promising electrode material for supercapacitors. Keywords: Nanosphere, Cu2O/NiO, Supercapacitors, Electronic materials, Microstructure

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1. Introduction In the last two decades, as a result of the extensive growth of the electronics devices and hybrid electrical vehicles, energy demand has been increasing [1]. Supercapacitors, one of the dominant energystorage devices for portable and grid-level applications, which have attracted considerable attention owning to higher power density, shorter charge transfer time and low-cost [2]. In general, the performance of supercapacitor usually depends on the electrode material [1, 3]. As a result, a lot of new electrode materials have been designed to improve the performance of supercapacitor [4-6]. Transition metal oxides (TMOs) are important materials for supercapacitor and photoelectrical devices [3, 7-9], which exhibit multiple advantages, such as low cost, high theoretical capacity, and environmental friendliness [4]. However, its low rate and fast capacity fading during charging-discharging process hinders large-scale practical application [5]. To overcome these limitations, a tremendous amount of effort has been made in structural design and synthesis methods to improve structural stability and conductivity, such as tremella-like NiO [5], Cu2O with various micromorphology [2, 7-9]. Among these strategies, dealloying is an approach to obtain nanoporous metal with well-interconnected structures, which is helpful to release the stress during the electrochemical cycling, thus providing high performance and long cycle lifespan to the device [10, 11]. There are some studies report the fabrication of NPM (films or wires) by dealloying into current collector for Li-ion battery anodes [10], catalysis [12] and functional nanometer components [13], but only few reports on supercapacitor by dealloying [14]. Herein, we have successfully synthesized the Cu2O/NiO with nanosphere structure by dealloying, and copper foil coated with Zn-Ni-Cu alloy film by electrodepositing and annealing was used as a precursor. The Cu2O/NiO with nanosphere structure shows high specific capacitance (2255.5 mF/cm2) and excellent

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cycling stability (94.5% after 5000 cycles). 2. Experimental Section 2.1. Synthesis of Cu2O/NiO/copper foil (CF) All materials and reagents employed were of analytical grade and used without further purification. 0.74 g Zn (NO3)2∙6H2O, 7.27 g of Ni (NO3)2∙6H2O and 1.55 g H3BO3 were dissolved in 50 mL of deionized water, and the copper foil (20mm x 10 mm x 0.1mm, purity 99.99%) was washed by HCl, NaOH, deionised water and ethanol respectively. Electrodeposition was carried out on a CHI660E electrochemical workstation by using a three-electrode system containing the above electrolyte, copper foil, Hg/HgO electrode and platinum slice were used as the working, reference and counter electrodes, respectively. The deposited precursors were washed by deionized water and placed in a tube furnace and annealed at 150℃ for 60 min under an Ar atmosphere. Electrochemical dealloying of the precursors were conducted in 3.5 w.t% KCl, the samples were oxidized at 200 ℃ for 120 min. The gravimetric capacitance Cm (mF/cm2) was calculated according to the following formula: 𝐶𝑚 =

𝐼 × Δ𝑡 𝑆 × Δ𝑉

where I, ∆t, S and ∆V are the discharge current (mA), discharge time (s), area of active material (cm2) in the working electrode and voltage range (V), respectively. 2.2. Characterization The structures of the samples were characterized by X-ray diffraction (XRD, Bruker D8). The microstructures, morphologies images of the samples were investigated by scanning electron microscopy (SEM, FEI QuantaTM250), the TEM, HRTEM and SAED images were obtained from JEM 2100F. X-ray photoelectron spectroscopy (XPS) was conducted on ESCALAB 250Xi X-ray photoelectron spectroscope.

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The electrochemical measurements were carried out in a three-electrode system (CHI660E) with 2.0 M KOH solution as electrolyte. 3. Results and Discussion

Fig. 1. (a) Schematic for the formation of Cu2O/NiO/CF; (b) XRD patterns of three samples; (c) survey spectrum, (d) Ni 2p, (e) Cu 2p. The stepwise preparation of the Cu2O/NiO/CF electrode is presented in Fig. 1a. Fig. 1b shows the XRD patterns of the Cu2O/NiO/CF electrode sample, apart from the diffraction peaks of Cu substrate (JCPDS no.04-0836), the XRD pattern of the sample exhibits diffraction peaks at 2θ angles of 29.6, 36.4, 42.3, 61.3 and 73.5°, corresponding to the (110), (111), (200), (220), (311) crystalline planes of Cu 2O (JCPDS no.050667), and the peaks at 2θ angles of 37.2, 43.2, 62.8,75.4°, corresponding to the (111), (200), (220), (311) crystalline planes of NiO (JCPDS no. 47-1049), respectively. Fig. 2i displays the HRTEM image of Cu2O/NiO sample, clearly reveals sets of lattice fringes with interplanar spacing of 0.213 nm and 0.246 nm, corresponding to (200) and (111) plane of Cu2O (JCPDS no.05-0667), while the interplanar spacing of 0.209 nm and 0.241 nm corresponding to (200) and (111) plane of NiO (JCPDS no.47-1049). The results 4

above indicating that the Cu2O/NiO/CF has been successfully synthesized. X-ray photoelectron spectra (XPS) were carried out to explore the chemical valence states of Cu2O/NiO/CF. The full spectra of the Cu2O/NiO/CF sample were shown in Figure 1c, which demonstrate the presence of Ni, Cu, O and C elements in Cu2O/NiO/CF. The Ni 2p peaks of Cu2O/NiO/CF could be decomposed into Ni2+ signals at 855.3 and 872.9 eV [5], as well as Ni3+ signals at 857.2 and 874.4 eV [15]. As for Cu 2p spectra (Fig. 1e), the Cu 2p peaks at 934.1 (Cu 2p3/2) and 954.1eV (Cu 2p1/2), indicative of the presence of Cu2+ [16], and the two peaks at 932.5 and 952.3eV correspond to the Cu 2p3/2 and Cu 2p1/2 peaks of Cu+ species [2].

Fig. 2. SEM images of (a) after deposition; (b) after annealing; (c) after dealloying and oxidation; (d-h) 15-Cu2O/NiO/CF; (i)TEM, HRTEM and SAED image of 2-Cu2O/NiO/CF. The morphologies of Zn/Ni /CF precursor and Cu2O/NiO/CF are observed by scanning electron microscopy (SEM). Fig. 2(a-c) shows the evolution of the morphology during the preparation process, Fig. 2a shows the micrograph of the Zn-Ni alloy film on the copper foil by electrodeposition, after 150 ℃ annealing, tiny particles were formed on the surface of the Zn-Ni alloy film, which formed nanospheres after dealloying and oxidation, showing superior morphology inheritance. In order to investigate the effect of deposition time and dealloying time on the morphology of the samples, a series of samples were synthesized, details

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were shown in Table 1. As shown in Fig. 2d-f, after 300s of dealloying, the best nanosphere structure were obtained on 2-Cu2O/NiO/CF, and no nanosphere was obtained on 3-Cu2O/NiO/CF due to excessive zinc removal. Because of too short dealloying time, the ball structure was not fully formed on 4-Cu2O/NiO/CF, while the structure became rough and collapsed on 5-Cu2O/NiO/CF by reason of too long dealloying time. To evaluate Cu2O/NiO/CF as a potential electrode material for supercapacitor, CV and GCD tests are performed to measure electrochemical performance. The charge-discharge mechanism of Cu2O/NiO can be expressed as follows: [2, 5] 𝑁𝑖𝑂 + 𝑂𝐻 − ↔ 𝑁𝑖𝑂𝑂𝐻 + 𝑒 − 1 1 𝐶𝑢2 𝑂 + 𝐻2 𝑂 + 𝑂𝐻 − ↔ 𝐶𝑢(𝑂𝐻)2 + 𝑒 − 2 2 Fig. 3a showed the CV curves of the Cu2O/NiO/CFs with different electrodeposition time at a scan rate of 100 mV/s. The curves present obvious redox peaks, the enveloping area of CV curve becomes larger with the increase of scanning speed. Remarkably, the enclosed CV curve area of 2-Cu2O/NiO/CF was larger than those of samples, which can be further intuitive observed through GCD curves. As shown in Fig. 3b, 2Cu2O/NiO/CF (E300D300) electrode displayed a much longer discharge time in comparison with the other electrodes at 1mA/cm2, which coincided with the trend of CV plots. Similar results could also be obtained from the Fig.3c-d, which proved that performance can be improved by nanosphere morphology. Fig.3e shows the cyclic voltammetry curves of 2-Cu2O/NiO/CF at different scanning speeds. With the increase of sweep speed, the peak position of redox peak shifts gradually due to polarization, but the shape of the curve remains basically unchanged, which indicates that it has good electrochemical reversibility. The specific capacitances of 2-Cu2O/NiO/CF electrode are 2255.5, 2164, 1833, 1520,1168 mF/cm2 at current densities of 1, 2, 5, 8 and 10 mA/cm2 respectively, demonstrating good rate characteristics of 51.8% (Fig. 3f). Fig.

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3g showed cycling life of the 2-Cu2O/NiO/CF electrode at 30 A/g for 5000 cycles. After 5000 cycles, the specific capacitances still maintain 94.5% of its initial value, revealing excellent cyclic stability. Fig. 3h present the Nyquist plots before and after 5000 cycles. It can be clearly seen that intercept at high frequency region became smaller after cycles while the larger slope of the straight line of 2-Cu2O/NiO/CF at lowfrequency area demonstrated a faster ion diffusion that was favorable for enhanced redox reaction activity.

Fig. 3. (a-d) CV and GCD curves of samples with different E-time and D-time at 50 mV/s and 1 mA/cm2; (e) CV and (f) GCD curves of 2-Cu2O/NiO/CF at different scan rates; (g) Cycling performance of 2Cu2O/NiO/CF (h) Nyquist plots of 2-Cu2O/NiO/CF before and after cycling. 4. Conclusions In summary, we have developed a dealloying method to synthesize Cu2O/NiO on the copper foil, the resultant materials possessed nanosphere structure. The Cu2O/NiO with nanosphere structure shows high specific capacitance (2255.5 mF/cm2) and excellent cycling stability (94.5% after 5000 cycles). This work may shed some light on the viable synthesis of low-cost oxides with scalable morphology for supercapacitors.

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Conflict of interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 51671214 and 51871238); Xuzhou Science and Technology Project (Grant No. KC18075). Reference [1] X.Y. Lang, A. Hirata, T. Fujita, M.W. Chen, Nature Nanotechnology 6 (2011) 232-236. [2] C.C. Wan, Y. Jiao, J. Li, Journal of Materials Chemistry A 5 (2017) 17267-17278. [3] V. Augustyn, P. Simon, B. Dunn, Energy & Environmental Science 7 (2014) 1597-1614. [4] L.N. Xu, J. Li, H.B. Sun, X. Guo, J.K. Xu, H. Zhang, X.J. Zhang, Frontiers in Chemistry 7 (2019). [5] K. Han, Y. Liu, H. Huang, Q. Gong, Z. Zhang, G. Zhou, Rsc Advances 9 (2019) 21608-21615. [6] J.Y. Seok, J. Lee, M. Yang, Acs Applied Materials & Interfaces 10 (2018) 17223-17231. [7] J. Wei, Z.G. Zang, Y.B. Zhang, M. Wang, J.H. Du, X.S. Tang, Optics Letters 42 (2017) 911-914. [8] T.W. Zhou, Z.G. Zang, J. Wei, J.F. Zheng, J.Y. Hao, F.L. Ling, X.S. Tang, L. Fang, M. Zhou, Nano Energy 50 (2018) 118-125. [9] Z.G. Zang, Applied Physics Letters 112 (2018). [10] Q.B. Yun, Y.B. He, W. Lv, Y. Zhao, B.H. Li, F.Y. Kang, Q.H. Yang, Advanced Materials 28 (2016) 6932-+. [11] R. Wang, Y.W. Sui, S.F. Huang, Y.G. Pu, P. Cao, Chemical Engineering Journal 331 (2018) 527-535. [12] X.F. Xing, D.Q. Han, Y.F. Wu, Y. Guan, N. Bao, X.H. Xu, Materials Letters 71 (2012) 108-110. [13] N.Y. Xing, L.X. Lian, Y. Liu, Y. Shi, Materials Letters 254 (2019) 125-128. [14] N. Wang, B.L. Sun, P. Zhao, M.Q. Yao, W.C. Hu, S. Komarneni, Chemical Engineering Journal 345 (2018) 31-38. [15] J. Zhao, Z.J. Li, X.C. Yuan, Z. Yang, M. Zhang, A. Meng, Q.D. Li, Advanced Energy Materials 8 (2018) 14. [16] X.T. Zhang, J.Y. Zhou, W. Dou, J.Y. Wang, X.M. Mu, Y. Zhang, A. Abas, Q. Su, W. Lan, E.Q. Xie, C.F. Zhang, Journal of Power Sources 383 (2018) 124-132. Table 1 sample preparation parameters Sample (#)

Electrodeposition

Dealloying

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Area Specific Capacitance

Current/Time (mA/s)

Current/Time (mA/s)

(mF/cm2 at 1mA/cm2)

1

20/150

20/300

487.7

2

20/300

20/300

2255.5

3

20/450

20/300

1675

4 5

20/300 20/300

20/150 20/450

907.5 1167

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Highlights: 1.Cu2O/NiO/copper foil has been successfully synthesized by dealloying a copper foil coated with Zn-Cu-Ni alloy films. 2.The resultant Cu2O/NiO possesses scalable nanosphere structure. 3. The Cu2O/NiO with nanosphere structure shows high specific capacitance (2255.5 mF/cm2) and excellent cycling stability (94.5% after 5000 cycles).

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Yidong Miao: Conceptualization, Methodology, Writing- Original draft preparation, Investigation Xuping Zhang: Writing - Review & Editing, Resources Yanwei Sui: Supervision, Writing - Review & Editing Enyuan Hu: Data Curation Jiqiu Qi: Investigation Fuxiang Wei: Conceptualization Qingkun Meng: Visualization Yezeng He: Visualization Zhi Sun: Project administration Yaojian Ren: Visualization Zhenzhen Zhan: Visualization

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