Facile synthesis of 3D flower-like porous NiO architectures with an excellent capacitance performance

Abstract

3D flower-like porous NiO architectures were synthesized via a facile and green method. When applied as an electrode material for supercapacitors, the as-prepared 3D flower-like porous NiO exhibited an outstanding electrochemical performance, such as a high specific capacitance (1609 F g⁻¹ at 2 A g⁻¹), and a long-term cycling stability (4.3% loss after 3000 cycles). The excellent electrochemical performance is mainly attributed to the morphology of the porous ultrathin nanosheets self-assembling into flower-like architectures, which facilitate fast and efficient diffusion of the electrolyte ions. Thus, it is expected that the 3D flower-like porous NiO can be a perspective electrode material for electrochemical energy storage applications.

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1 Introduction

Supercapacitors have gained great attention in energy storage applications due to high power density, long lifespan, excellent safety, simple charging circuit and no memory effect.^1–3 Over the past decade, three major classes of materials have been studied as electrode materials for supercapacitors, such as carbon materials, transition metal oxide/hydroxides and conductive polymers.^3,4 Particularly, transition metal oxides can afford higher energy density for supercapacitors than carbon materials and better electrochemical stability than conductive polymers,³ so transition metal oxides including RuO₂,⁵ NiO,⁶ MoO₃,⁷ MnO₂,⁸ and Co₃O₄,⁹ are suggested to be promising candidates for next-generation high-capacitance energy storage devices. Among them, NiO is one of the most attractive electrode materials for supercapacitors due to its high theoretical capacitance of 2584 F g⁻¹, highly electroactive nature, robust chemical/thermal stability, environmental benignity, low cost and abundance in the earth.^3,6,10 Unfortunately, the real specific capacitance of NiO-based electrodes is still far below the theoretical value. For example, previous studies on the supercapacitor performance of different NiO based nanostructures, such as nanoparticles,¹¹ nanowires,¹² nanotubes,¹³ nanocolumns,¹⁴ nanoslices,¹⁴ nanoplates,¹⁴ nanosheets,¹⁵ thin flim,¹⁶ nanospheres,^17–19 nanoflowers^20,21 etc., have a specific capacitance within ∼182 F g⁻¹ to ∼1297 F g⁻¹ (a detailed list in Table S1†). To solve this problem, electrode materials with superior nanostructured NiO should be fabricated to improve electrochemical performance due to the fact that the electrochemical behavior mainly depends on the morphology, porosity, surface area, and wetting characteristics of nanostructured electrodes.^6,22,23 The well-designed three dimensional (3D) microstructures such as ultraporous structure and high surface structure are expected to provide solutions to the practical needs of energy-storage devices, which will show significant performance advantages over current state-of-the-art technologies.²⁴ Previous studies also demonstrated that porous and 3D nanoarchitectures exhibited significant performance advantages at electrochemical performance.^10,24–27 Therefore, in order to utilize NiO as practical energy-storage devices in the future, it is of great importance to develop a facile and green method to prepare 3D flower-like porous NiO architectures with high capacitance and good stability for supercapacitors.

Herein, we demonstrate the design and synthesis of 3D hierarchical porous NiO flower-like architectures via a facile and green hydrothermal route for supercapacitor applications. These 3D hierarchical flower-like architectures are self-assembled by ultrathin and porous NiO nanosheets, which facilitate fast and efficient diffusion of the electrolyte ions.²⁶ As an electrode material for supercapacitors, the 3D flower-like porous NiO exhibited outstanding electrochemical performance due to its unique structure, such as high specific capacitance (1609 F g⁻¹ at 2 A g⁻¹), and a long-term cycling stability (4.3% loss after 3000 cycles). The specific capacitance of the 3D flower-like porous NiO are superior to previously reported Ni-based supercapacitor materials.^11–21,28 More importantly, these hierarchical flower-like porous NiO with outstanding electrochemical performance can be synthesized by facile and green method, so that it can be an excellent candidate for the supercapacitor materials.

2 Experimental section

2.1 Materials preparation

All of the reagents in the experiments were analytical grade and purchased from Sinopharm. They were used without further purification. In a typical synthesis, 5 mmol of nickel acetate tetrahydrate (C₄H₆NiO₄·4H₂O) was dissolved in deionized water (36 mL) and ethylene glycol ((CH₂OH)₂, 10 mL) under stirring. Subsequently, glucose (0.005 g, 0.028 mmol) was added into above solution and the mixture was stirred for another 10 min. The whole mixture was then transferred into a 60 mL polytetrafluoroethylene (PTFE) (Teflon)-lined autoclave and maintained at 180 °C for 16 h in an electric oven. After being cooled to room temperature naturally, the precipitates were collected by centrifugation, and washed with deionized water and ethanol for several times, then dried at 60 °C overnight to get blue powders. Afterwards, the samples were calcined in a muffle furnace at 450 °C for 2 h in air. After being cooled to room temperature naturally, the NiO powders were obtained and collected for characterization and application.

2.2 Materials characterization

As-prepared products were characterized with D/max-2550 PC X-ray diffractometer (XRD, Rigaku, Cu-Kα radiation), a scanning electron microscopy (SEM, S-4800), and a transmission electron microscopy (TEM, JEM-2100F). The specific surface area and pore volume of the products were measured by Brunauer–Emmett–Teller (BET) and Barett–Joyner–Halenda (BJH) methods (Micromeritics, ASAP2020).

2.3 Electrochemical measurement

Electrochemical performances of the as-obtained products were performed on Autolab (PGSTAT302N potentiostat) using a three electrode mode in 6 M KOH solution. Working electrodes were prepared by mixing the as-synthesized NiO products (80 wt%) with acetylene black (15 wt%), and polyvinylidene fluoride (PVDF, 5 wt%) in N-methylpyrrolidinone (NMP). The mixture was first coated onto the surface of a piece of nickel foam sheet, and then dried at 120 °C for 4 h to remove solvent. Then the sheets with mixture were finally pressed under 10 MPa to form the working electrode. The mass loading of flower-like porous NiO on nickle foam was calculated to be 0.616 mg. The reference electrode and counter electrode were Ag/AgCl electrode and platinum foil (1.5 cm × 1.5 cm), respectively. The specific capacitance of the electrodes is calculated from the cyclic voltammetry (CV) curves by the following equation:^25,29

C = Q/mΔV (1)

where Q (C) is the average charge during the charging and discharging processes, m (g) is the mass of the active materials in the electrodes, and ΔV (V) is the potential window.

The specific capacitance of the electrodes is calculated by discharge curves according to the following formula:^25,30

C = IΔt/mΔV (2)

where I (A g⁻¹) is the constant discharge current, Δt (s) is the discharge time, ΔV (V) is voltage interval of the discharge.

3 Results and discussion

The 3D hierarchical Ni(OH)₂ flower-like architectures (Fig. S1†) were prepared by a simple and green (using water and ethylene glycol as solvent) hydrothermal route, then annealed at 450 °C for 2 h in air to form final NiO products. The XRD pattern and SEM images of the final NiO products are showed in Fig. 1. The XRD pattern of as-prepared NiO sample matches well with the standard cubic phase NiO (JCPDS card no. 47-1049) (Fig. 1a). There are no diffraction peaks from any other impurities, which indicates the high purity of the bunsenite products. The low-magnification SEM image of the as-prepared NiO sample (Fig. 1b) shows that the final products successfully inherited the flower-like architectures of the Ni(OH)₂. The medium SEM images (Fig. 1c and d) of NiO samples reveal that NiO have a flower-like architectures with diameters ranging from 5–10 μm, and the architectures are self-assembled with dense nanosheets. The “petals” of NiO “flower” (Fig. 1e and f) was composed of many porous and ultrathin sheets (∼5 to 9 nm in thickness). Compared with the Ni(OH)₂ precursor, the surface of NiO sheets turned to more rough after annealing process. The ultrathin nanosheets of the NiO “flower” used in supercapacitor have the advantages of shortening the diffusion distance, increasing the utilization of active material, accommodating the volume changes during faradaic reaction.^27,31

Fig. 1 (a) XRD patterns of the as-synthesized 3D hierarchical NiO flower-like architectures, (b, c and d) low-, medium- and high magnification SEM images of as-synthesized 3D hierarchical NiO flower-like architectures. (e and f) Low- and high-magnification SEM images of NiO nanosheets.

The structures of the as-synthesized 3D hierarchical NiO flower-like architectures were further characterized by TEM. The TEM image of 3D hierarchical NiO flower-like architectures (Fig. 2a) shows the edge of 3D hierarchical structure were ultrathin nanosheets, indicating the 3D hierarchical structure were assembled with 2D ultrathin nanosheets. The TEM image of these nanosheets is given in Fig. 2b, which remarkably demonstrates that each NiO sheet is full of numerous fairly uniform mesoporous (such as marked with blue circles), which is due to the fact that the solvent, and gases are released and lost during the intermediates' decomposition/oxidation through thermal annealing. The crystal structure of the nanosheets was further characterized by HRTEM observations (Fig. 2c). The interplanar spacing of the nanosheets is ∼0.24 nm corresponding to the d-spacing for the (110) crystal planes of standard cubic phase of NiO, which further confirmed high purity of the bunsenite products.

Fig. 2 (a) TEM images of 3D hierarchical NiO flower-like architectures. (b and c) TEM and HRTEM images of NiO nanosheets. (d) Typical nitrogen-adsorption–desorption isotherm and pore-size distribution curve (inset) of the 3D hierarchical NiO flower-like architectures.

The porous structure of the 3D hierarchical NiO flower-like architectures was further evaluated by Brunauer–Emmett–Teller (BET) N₂-adsorption–desorption analysis. As shown in Fig. 2d, the 3D hierarchical NiO flower-like architectures have a Brunauer–Emmett–Teller (BET) surface area as high as 89.7 m² g⁻¹ and exhibit the type IV isotherm, indicating mesoporous structure. The pore size distribution exhibits a sharp peak centered at a mean value of 3.6 nm, illustrating fairly uniform mesoporous distribute on the NiO nanosheets (Fig. 2d, insert). The high-quality void volume (large surface area) of 3D architectures, most particularly a fully interconnected 3D mesoporous structure, can facilitate the solvent infiltration and penetration of electrolyte, and promote reaction species to quickly access the electrode.^24,27 Thus, a large surface area and uniform pores distribution have great potential for large specific capacitances of 3D hierarchical NiO flower-like architectures.

In order to understand the formation of the 3D hierarchical Ni(OH)₂ flower-like architectures, the effect of the reaction time on its morphology was studied. The structural evolution of the Ni(OH)₂ precursor is showed in Fig. 3. It can be clearly observed that micro-sphere with rough surface (Fig. 3a) was synthesized after reacting for 0.5 h. When the reaction time was prolonged to 2 h, the nanoplates grew on micro-sphere surface (Fig. 3b). The nanoplates on the micro-sphere surface became denser after increasing the reaction time to 6 h (Fig. 3c). Accompanying the prolonged reaction time, the solid micro-sphere slowly disappeared, and the nanosheets became thinner and denser. As shown in Fig. 3d, the structure of the precursor finally evolved into 3D hierarchical Ni(OH)₂ flower-like architectures self-assembled with ultrathin nanosheets after reacting for 16 h. Based on the above results, we elucidate a plausible mechanism responsible for the formation of the 3D hierarchical Ni(OH)₂ flower-like architectures (Fig. 3e). In the synthetic process, the ethylene glycol and nickel acetate formed Ni(OH)₂ nucleis in hydrothermal reaction condition. As the reaction proceeded, the Ni(OH)₂ nucleis further grow into the micro-sphere with rugged surfaces. The reaction was developed into Ostwald ripening process as reaction time increasing.³² The crystal particles in the surface of Ni(OH)₂ micro-sphere dissolved, and further precipitated on the surface of Ni(OH)₂ micro-sphere to form the sheet structure due to the effect of ethylene glycol and glucose. With increase of reaction time, the solid Ni(OH)₂ micro-sphere became smaller and smaller, and the nanosheets on the surface became thinner and denser. Finally, the solid Ni(OH)₂ micro-sphere completely dissolved, and formed into 3D hierarchical Ni(OH)₂ flower-like architectures self-assembled with ultrathin nanosheets. We also investigated the effect of glucose the to form 3D hierarchical Ni(OH)₂ flower-like architectures. The Ni(OH)₂ products were fabricated by littery thick flakes and didn't form 3D hierarchical flower-like architectures without adding glucose (Fig. S2a†). While, the morphology of the Ni(OH)₂ products changed into large nubby structures constructed by nanoparticles after adding an excessive amount of glucose (50 mg) into the reaction (Fig. S2b†). So glucose can help to form 3D hierarchical Ni(OH)₂ flower-like architectures.

Fig. 3 SEM images of 3D hierarchical Ni(OH)₂ flower-like architectures precursor synthesized at 180 °C for 0.5 h (a), 2 h (b), 6 h (c) and 16 h (d). (e) Scheme illustrating the growth mechanism of 3D hierarchical Ni(OH)₂ flower-like architectures precursor.

To evaluate the capacitive performance of the as-prepared 3D hierarchical NiO flower-like architectures, it is measured in 6 M KOH solution as electrolyte. The specific capacitance was all examined by CV and galvanostatic charge–discharge (CD) measurements. As shown in Fig. 4a, the CV curves were recorded at different scan rates ranging from 5 to 80 mV s⁻¹, and at the potential window of 0 to 0.5 V. A couple of redox peaks were appeared within the potential windows due to redox reactions related to Ni–O/Ni–O–OH.^6,28 Moreover, with the scan rates increasing, the redox current increased. Based on their corresponding CV curves, the specific capacitances of the as-prepared NiO materials were calculated to be 1574, 1134, 912 and 815 F g⁻¹ at scan rate of 5, 20, 50 and 80 mV s⁻¹, respectively (Fig. S3†). The excellent electrochemical performance of the 3D hierarchical NiO flower-like architectures was further confirmed by CD tests performed at different current densities, as shown in Fig. 4b. According to the previous articles,^25,30 the specific capacitance of the material is calculated to be 1609, 1427, 1268, 1203, 1120, and 1061 F g⁻¹ at current densities of 2, 4, 8, 10, 15, and 20 A g⁻¹ respectively, as shown in Fig. 4c. The value of specific capacitance measured here is much higher than that of the previous reported NiO pseudocapacitance (a detailed list in Table S1†).^11–21 The high specific capacitances can be attributed to the unique 3D porous architectures. The 3D hierarchical NiO flower-like architectures were self-assembled by many interconnecting ultrathin porous sheets, which can afford more active sites for efficient electrolyte ions transportation on both the active materials surface and throughout the bulk. Moreover, the open and free interspaces between the nanosheets and the mesoporous inside the nanosheets can serve as an “ion reservoir” which can shorten ion diffusion length from the external electrolyte to the interior surfaces, significantly improve the intercalation/deintercalation of ions and increase the utilization of active materials.^25,33

Fig. 4 (a) CV curves at different scan rates of 3D hierarchical NiO flower-like architectures electrode. (b) CD curves of 3D hierarchical NiO flower-like architectures electrode at different current densities. (c) Specific capacitance of 3D hierarchical NiO flower-like architectures electrode at different current densities. (d) Cycling performance of 3D flower-like porous NiO electrode during 2000 cycles at a current density of 8 A g⁻¹.

The long-term cycle stability of the electrode materials is another key factor for practical applications. A long-term cycle stability of the as-prepared NiO product as an electrode material was evaluated by repeating charge–discharge test at 8 A g⁻¹ for 3000 cycles. As shown in Fig. 4d, it can be clearly seen that the capacitances of as-synthesized NiO electrode material gradually increased to 110% from 0 to 400 cycles, demonstrating that there is an activation process of the electrode at the beginning period of the CD cycling test. Subsequently, the capacitances of NiO electrode material kept almost constant from 400 to 1050 cycles. Then the capacitances of NiO electrode material began to gradually decrease from 1100 cycles, which can be ascribed to a minor damage of the electrode active materials after experiencing harsh and frequent phase variations during the redox reactions and destruction/reconstruction of the structures.^34,35 After 3000 cycles the specific capacitance of as-synthesized NiO electrode material could retain 95.7% (87.0% of the maximum capacitance), suggesting a steady behavior during these CD cycling tests. This phenomenon mainly attributed to the fact that the 3D flower-like porous NiO architectures can accommodate with the volume change during the repeated insertion/extraction of OH⁻ ions. Notably, the electrolytes of as-synthesized NiO materials maintained limpidity even after 3000 cycling tests, demonstrating a minimal dissolution of active material into the solution, which can be attributed to good cycle stability as active electrode materials.

Electrochemical impedance spectroscopy (EIS) of NiO electrode was measured before and after 3000 cycles, as shown in Fig. S4.† The intercept on the real axis in the high frequency range exhibits the equivalent series resistance (R_s), containing inherent resistances of the electro active material, electrolyte resistance, and contact resistance at the interface/electrolyte and electrode.³⁶ It can be seen that both R_s before and after 3000 cycling tests are very low (0.098 Ω and 0.10 Ω, respectively). Besides, at lower frequencies, the diffusive resistance (Warburg impedance) of the NiO electrode slightly increased after 3000 cycles and the angle between the straight line and real axis in the low frequency region is still larger than 45°, showing that the electrolyte penetration and ion diffusion in the host materials have not reduced notably. Impedance spectra further show the NiO electrode possesses stable electrochemistry ability.

4 Conclusion

In summary, we developed a facile and green method to prepare NiO electrode materials. The NiO preserves the 3D hierarchical flower-like architectures self-assembled by porous ultrathin nanosheets. The 3D flower-like porous NiO were used as supercapacitor electrode materials exhibiting high specific capacitance and good electrochemical stability. The excellent electrochemical performances of the NiO materials could be attributed to their unique 3D hierarchical nanostructure, which could shorten ion diffusion paths and facilitate the rapid migration of electrolyte ions. Thus, excellent electrochemical properties of the NiO material, as well as simple and green synthetic method, could make the present 3D hierarchical flower-like porous NiO as an excellent candidate for the supercapacitor material.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 21171035, 51472049, 51302035, 21176102, 21176215 and 21476136), the Key Grant Project of Chinese Ministry of Education (Grant no. 313015), the PhD Programs Foundation of the Ministry of Education of China (Grant no. 20110075110008 and 20130075120001), the National 863 Program of China (Grant no. 2013AA031903), the Science and Technology Commission of Shanghai Municipality (Grant no. 13ZR1451200), the Fundamental Research Funds for the Central Universities, the Program Innovative Research Team in University (IRT1221), the Shanghai Leading Academic Discipline Project (Grant no. B603) and the Program of Introducing Talents of Discipline to Universities (no. 111-2-04), the Natural Science Foundation of Jiangsu Province (no. BK20131100), the Connotation Construction Project of SUES (no. Nhky-2015-05), and the Sino-German Center for Research Promotion (no. GZ935), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Innovation Program of 5 Shanghai Municipal Education Commission (Grant no. 14ZZ160) and the Open Fund of State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (Grant no. LK1209).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05231a

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