Visualization electrochromic-supercapacitor device based on porous Co doped NiO films
Introduction
With the development of sustainable resources, the devices toward energy storage and conversion such as solar cell, supercapacitor and electrochromic smart windows have attracted increasing attention [1], [2], [3]. Electrochromic smart windows can change the color by reversible ion insertion/extraction under low voltages [4], [5]. Supercapacitors, also known as electrochemical capacitors or ultracapacitors, store energy through reversible adsorption of electrolyte ions or redox reactions [6], [7]. Thus, the electrochromic smart windows and supercapacitors have similar working mechanism and device structure. Since Wei’s group [8] successfully synthesized vanadium pentoxide network as an electrochromic supercapacitor material, the door of electrochromic-supercapacitor (ESC) was opened up due to its integrated characteristics of energy storage and electrochromism, which allowed the visualization of the charge state of the ESCs device through the color saturation degree [9], [10], [11].
Nowadays, studies on ESC are not limited to single electrode materials, but gradually turn to ESCs devices. For instance, Jia’s group [12] constructed asymmetry ESC (AESC) device using NiO/PB and WO3, which exhibited excellent electrochromic and energy storage performance. Wang’s group [13] used poly(indole-6-carboxylicacid) (PICA)/TiO2 as AESCs anode material to realize intelligent energy storage. Feng’s group [14] reported a kind of AESCs composed of viologen and EG/V2O5 hybrid nanopaper with high volumetric capacitance and ultra-fast responsive. However, in order to achieve the color change, numerous reports apply a negative voltage to the ESC devices, which limits the intelligence in the practical application. Moreover, most of the previous studies usually have a small potential window, which leads to the low current density of the ESC device. Therefore, structure design is significant to obtain a smart AESC with high performance. NiO, one of the most promising inorganic ESC material, has attracted tremendous attention due to its high theoretical specific capacitance, high transmittance modulation and low cost [15], [16]. It has been report that porous NiO has significantly improved electrochromic [17] and electrochemical performance [18]. In addition, Co doping can enhance the conductivity of NiO, which is beneficial to improve the energy density [19] and electrochromic [20] performance. While there are few studies on the porous and Co doped NiO for ESC devices.
Based on the considerations mentioned above, porous Co doped NiO nanosheet growing on fluorine-doped tin oxide (FTO) glass was successfully prepared and worked as positive electrode material of the AESC devices. Fe2O3 electrode and KOH soaked white nylon membrane were used as the negative electrode and electrolyte and separator, respectively. The constructed AESC devices possess a wide voltage window (from o to 1.8 V), high areal capacitance (10.8 mF cm−2), high energy density (3.84 × 10−3 mW h cm−2) and excellent cyclic stability (84.5% capacitance retention after 2000 cycles). The AESC device displays dark brown in the charged state and becomes light brown gradually as the charging voltage decreases. After charging, the device can light up a red LED.
Section snippets
Construction of the asymmetrical electrochromic-supercapacitor device
Scheme 1 illustrates the basic steps in prepared the asymmetrical electrochromic-supercapacitor (AESC) device. The Co doped NiO compound film was grown on FTO glass by simple hydrothermal and annealed. Before deposition the FTO was washed with de-ionized water, acetone, ethanol and dried at 60 ℃ for 2 h, then treated by plasma for 5 min under air atmosphere. The sample was prepared as follow: different weight ratio of nickel nitrate and cobalt nitrate (the weight percentage of Co is 0, 3%, 5%
Characterization
The phase compositions of the samples were detected by X-ray diffractometer (XRD, Japan Rigaku). The morphology and structure were characterized by scanning electron microscopy (SEM, Zeiss), transmission electron microscopy (TEM, JEOL 2010), high-resolution transmission electron microscopy (HRTEM, JEOL 2010) and selected area electron diffraction (SAED, JEOL 2010). X-ray photoelectron spectroscopy (XPS Thermo Fisher, E. Grinstead, UK) was recorded using Al Kα irradiation as the excitation
Result and discussion
The crystallographic structure of the powders scratched from the as prepared films was carried out by XRD measurement. The diffraction peaks (Fig. 1) in all the patterns are mainly assigned to NiO (JCPDS No. 47-1049) and no cobalt oxides or other impurities peak was detected, this demonstrates that Co doping cannot alter the phase of original NiO structure.
In order to confirm the optimum doping Co content, galvanostatic charge/discharge (GCD) test (in 1 M KOH solution at different current
Conclusion
In this work, an AESC device based on porous Co doped NiO film as positive electrode was designed and fabricated, which exhibits a large potential window of 1.8 V. The AESC device has high areal specific capacitance, high energy density and excellent cycle stability. For the practical application, the device able to light up LED and monitoring the energy storage state through color change. It can potentially be used for application in smart electronics.
CRediT authorship contribution statement
Junying Xue: Conceptualization, Methodology, Software, Writing - original draft. Zhang Xiang: Grammar check of revised draft. Li Wenjie: Validation, Formal analysis, Data Curation. Ying Song: Writing - review & editing. Zhao Jiupeng: Supervision, Project administration, Writing - review & editing. Li Yao: Funding acquisition, Project administration, Software.
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.
Acknowledgments
We thank, National Key Research & Development Program (2016YFB0303903, 2016YFE0201600), and Foundation of Equipment Development Department (6220914010901).
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