High performance flexible hybrid supercapacitors based on nickel hydroxide deposited on copper oxide supported by copper foam for a sunlight-powered rechargeable energy storage system

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Abstract

Herein, an integrated system combining solar cells with a hybrid supercapacitor for operating a homemade windmill device was assembled, achieving energy conversion, storage and utilization. As a candidate for positive electrode of hybrid supercapacitor devices, battery-like Ni(OH)2@CuO@Cu binder-free electrode was fabricated by a two-step process at ambient temperature. CuO@Cu was prepared by chemical oxidation method to act as the supporting electrode for electrochemical deposition of Ni(OH)2. Various deposition times (30, 50, 90, 150 and 200 s) were investigated to optimize the energy storage characteristics of the resulting Ni(OH)2@CuO@Cu electrode materials. Among all the samples, Ni(OH)2@CuO@Cu-150 exhibited the largest areal capacity of 7063 mC cm−2 at 20 mA cm−2, and was therefore chosen as the positive electrode in a hybrid supercapacitor device. Using N-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh cm−2 at a power density of 1.6 mW cm−2. Two hybrid supercapacitor devices were connected in series to successfully lighten up a red LED for 12 min 39 s, while three devices assembled in series were able to successfully power a three-digit digital display for 1 min 28 s. Interestingly, the hybrid supercapacitor device, charged by solar cells, further operated a homemade windmill device for 59 s, achieving sunlight-powered integration system. All of the findings suggested the practical application potential of the hybrid supercapacitor based on Ni(OH)2@CuO@Cu composite as energy storage device.

Introduction

In the last few years, the problems associated with environment pollution, fossil fuels decline, and global warming have drawn increasing concern [1], [2]. To pursue a sustainable life, people have concentrated more on clean and low cost energy sources like solar energy, wind energy, rain, tides, waves, geothermal heat and hydrogen energy [3], [4], [5]. Furthermore, the development of electric vehicles and new wearable electronics promotes the exploration of flexible energy storage devices with high energy density [6], [7], [8]. As energy storage devices, supercapacitors become more and more favored, because of their obvious benefits like low cost, high power density, fast charge–discharge speed, and so on [9], [10]. However, most of supercapacitors encounter the problem of low energy density (E) compared with batteries, hindering their widespread application in our daily life [11], [12]. According to the correlation of energy density with capacitance and potential (E=12CV2), high energy density can be reached through enhancing the capacitance (C) and/or the operating potential window (V). Hence, constructing a hybrid supercapacitor device represents an attractive route to broaden the operating potential window, further enhancing the energy density [13], [14], [15].

Usually, a hybrid supercapacitor device consists of a positive battery-like electrode with high capacity, and a negative carbon-based electrode with a wide working potential window [16], [17]. For example, Wang et al. constructed a hybrid supercapacitor device by employing metal silicates as an effective positive electrode and activated carbon as negative electrode material in PVA-KOH gel electrolyte, achieving high energy density of 4.6 mWh cm−3 [18]. Dong et al. assembled NiSi/GO composite//activated carbon (AC) hybrid supercapacitor device, exhibiting an energy density of 0.37 Wh m−2 [19]. Zhang et al. fabricated a hybrid supercapacitor using NiO/C/rGO and a hierarchical porous carbon derived from sodium citrate, delivering a high energy density of 35.9 Wh kg−1 [20]. Among all the battery-like electrode materials, Ni(OH)2 with large theoretical specific capacitance (2082 F g−1) has drawn plenty of interest. Wang et al. prepared nanostructured nickel silicate-nickel hydroxide composite (NiSi-Ni(OH)2) with a relatively high charge storage property of 476.4 F g−1 at 2 A g−1 [21]. Jiang et al. fabricated 3D porous Ni(OH)2/Ni electrode and achieved a specific capacity of 414 mC cm−2 at 10 mA cm−2 [22]. Xiong et al. reported the formation of ultrathin Ni(OH)2 nanosheets on nickel foam (NF), demonstrating an areal capacity of 2160 mC cm−2 at 2 mA cm−2 [23]. Zou et al. synthesized Ni(OH)2 nanosheets on electrochemically activated carbon cloth, exhibiting a specific capacity of 918 mC cm−2 at 2 mA cm−2 [24]. Shi et al. prepared Ni(OH)2-Cu electrode, which delivered 3464 mC cm−2 at 1 mA cm−2 [25]. Even though the electrode materials achieved a good performance, the recorded capacity was far below the theoretical value. Therefore, there is enough room for alternative methods to improve the properties of Ni-based electrodes.

In this study, we synthesized Ni(OH)2 through electrochemical deposition on Cu foam current collector, owing to its three-dimensional (3D) structure and good conductivity. Before Ni(OH)2 electrochemical deposition, CuO was formed on Cu foam (CuO@Cu) by chemical oxidation, followed by thermal annealing at 190 °C for 3 h. The electrodeposition of Ni(OH)2 was investigated for various time spans (30, 50, 90, 150 and 200 s) to gain some insights on the effect of deposition time on the electrochemical properties of the resulting Ni(OH)2@CuO@Cu electrode materials. Among them, Ni(OH)2@CuO@Cu-150 composite exhibited the largest active surface area of 512.6 cm2 and areal capacity of ~ 7063 mC cm−2 at 20 mA cm−2. However, in absence of CuO nanostructured layer, Ni(OH)2@Cu-150 achieved an areal capacity of 3667.2 mC cm−2 at 20 mV s−1, which is much lower than that of Ni(OH)2@CuO@Cu-150 (7742 mC cm−2 at 20 mV s−1), indicating the importance of CuO nanostructured layer to facilitate electron and ion transfer to further enhance the electrochemical performance of the electrodes. Hence, Ni(OH)2@CuO@Cu-150 was investigated as a positive electrode to assemble a hybrid supercapacitor device, while N-doped reduced graphene oxide coated on nickel foam (N-rGO/NF) was applied as a negative electrode. The device displayed a large areal energy density of 130.4 μWh cm−2 at a power density of 1.6 mW cm−2. For practical applications, two hybrid supercapacitor devices were assembled in series to successfully lighten up a red LED for 12 min 39 s. In addition, a three-digit digital display was powered for 1 min 28 s using three devices in series. Finally, a sunlight-powered integration system based on the hybrid supercapacitor device, solar cells, and a homemade windmill device was constructed. The hybrid supercapacitor was charged by the solar cells, which further powered the windmill device and ensured its continuous operation for 59 s, indicating the successful practical application of the hybrid supercapacitors based on Ni(OH)2@CuO@Cu composites as energy storage devices.

Section snippets

Materials and methods

All the reagents were of analytical grade and used without any further purification. Nickel(II) nitrate tetrahydrate [Ni(NO3)2·4H2O], sodium hydroxide (NaOH), ammonium persulphate [(NH4)2S2O8], hydrochloric acid (HCl) and acetone were obtained from Sigma-Aldrich (France).

Copper foam (Cu foam) was purchased from Kunshan Guangjiayuan new materials Company (China). Graphene oxide (GO) was purchased from Graphitene (UK). Nickel foam (NF) was obtained from Jiayisheng Company (China).

The water used

Results and discussion

The main target of the present work was to prepare a highly performant positive electrode for hybrid supercapacitors and further use it for the construction of a sunlight-powered rechargeable energy storage system. Ni-based electrodes display high theoretical capacitance values. Therefore, we investigated Ni(OH)2 modified CuO@Cu [Ni(OH)2@CuO@Cu] as a binder-free electrode for supercapacitors. As illustrated in Fig. 1, Cu foam was partially oxidized at ambient temperature to produce Cu(OH)2@Cu

Conclusion

In conclusion, Ni(OH)2@CuO@Cu composites were prepared by electrochemical deposition for different times (30, 50, 90, 150 and 200 s) of Ni(OH)2 on CuO@Cu. Among all the prepared electrodes, Ni(OH)2@CuO@Cu-150 achieved the largest areal capacity of 7063.2 mC cm−2 at 20 mA cm−2. Therefore, a hybrid supercapacitor consisting of Ni(OH)2@CuO@Cu-150 positive and N-rGO/NF negative electrodes was assembled. The energy storage device attained a high areal energy density of 130.4 μWh cm−2 at a power

CRediT authorship contribution statement

Min Li: Investigation, Formal analysis, Writing - original draft. Ahmed Addad: Investigation, Formal analysis. Pascal Roussel: Investigation, Formal analysis. Sabine Szunerits: Investigation, Formal analysis. Rabah Boukherroub: Conceptualization, Validation, Writing - review & editing, Supervision.

Declaration of Competing Interest

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.

Acknowledgements

The authors gratefully acknowledge financial support from the Centre National de la Recherche Scientifique (CNRS), the University of Lille, and the Hauts-de-France region. Min Li thanks Chinese government for the China Scholarship Council fellowship.

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