C Supercapacitive Performances through Mixed

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Improve Ni(OH)2/C supercapacitive performances through mixed solvents and thermal treatment of XC-72 Renjie Qu, Shuihua Tang, Jiahao Zhang, and Lingshan Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04011 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Sustainable Chemistry & Engineering

Improve Ni(OH)2/C supercapacitive performances through mixed solvents and thermal treatment of XC-72 Renjie Qu†,‡, Shuihua Tang*,†,‡, Jiahao Zhang†,‡, Lingshan Wu†,‡ †

State Key Lab of Oil and Gas Reservoir Geology & Exploitation, Southwest Petroleum

University, Chengdu, 610500, P R China. ‡

School of Materials Science and Engineering, Southwest Petroleum University, Chengdu,

610500, P R China. E-mail: [email protected]; Tel/Fax: +86-2883032879 KEYWORDS: XC-72, Mixed solvents, Thermal treatment, Solvothermal method, α,βNi(OH)2/XC-72

ABSTRACT: A Ni(OH)2/XC-72 nanocomposite was first synthesized via a solvothermal method in mixed solvents of ethylene glycol and H2O, using smaller amount of ethylene glycol not only reduces the cost, but also enhances the electrochemical performances of the composite. X-ray diffraction patterns indicate that α-Ni(OH)2 and β-Ni(OH)2 are coexistent in the Ni(OH)2/XC-72 composite. The capacitance of α,β-Ni(OH)2/XC-72 increases from 1649.3 F g-1 of α-Ni(OH)2/XC-72 and 1232.5 F g-1 of β-Ni(OH)2/XC-72 to 1803.1 F g-1 at 1 A g-1. To further improve the electrochemical performances of Ni(OH)2/XC-72 composite, XC-72 was calcined in an Ar flow at 600 °C (denoted as T-XC-72) to remove some oxygen-containing groups, thus

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increase its electronic conductivity and also gain more defects as nuclei for Ni(OH)2 particle deposition. The obtained Ni(OH)2/T-XC-72 composite delivers a capacitance of 2252.8 F g-1 at 1 A g-1, which is much higher than that of Ni(OH)2/XC-72. Furthermore, 78.1% of capacitance can be retained at 20 mV s-1 with respect to 74.8% of Ni(OH)2/XC-72. What’s more, it shows a stability of 80.5% over 1000 cycles at 10 A g-1, which is also more superior to 75% of Ni(OH)2/XC-72. Therefore, this work may provide a cost-effective route to synthesize highperformance electrode materials for supercapacitors.

■ INTRODUCTION Severe global warming, limited available fossil fuels and energy safety issues have put forward to unparalleled challenges for rapidly developing renewable resources and sustainable energy conversion and storage systems.[1-4] Among a wide variety of candidates, benefiting from the relatively high power density, fast charge/discharge rate and fantastic cyclic stability, supercapacitor (SC) has emerged as a promising electrochemical energy storage device,[5-8] while the relatively low energy density hindered its wide applications. Therefore researchers have been focusing on how to achieve electrode materials simultaneously with high energy density and high power density. According to the equation E=½CV2, two effective strategies to enhance the energy density (E) can be clarified: one is to enlarge the working voltage of SC devices and the other is to increase the specific capacitance of electrode materials. Using “waterin-salt” or “hydrate-melt” electrolyte has been demonstrated to be a useful strategy to efficiently enlarge the operating voltage, and thus improve the energy density. While the ultrahigh concentration of this kind of electrolyte is so expensive that extremely restricts their further


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widely applications.[9] On the other hand, exploring high-performance electrode materials has been verified to be a very powerful route to achieve high energy density of SC devices.[10] In this regard, a plenty of transition metal compounds have been researched. For instance, Zhao et al. synthesized amorphous FeOOH through one-step electrodeposition, the FeOOH electrode exhibited a relatively high capacitance of 867 F g-1, and still kept 93.3% of its initial capacitance after 3000 cycles at 50 mV s-1, which demonstrated an excellent cycle stability.[11] Wei et al. synthesized MnO2/graphene composite by self-limiting deposition of MnO2 on the graphene sheets under the assistant of microwave irradiation, the MnO2/graphene electrode not only displayed a capacitance of approximately 310 F g-1 at 2 mV s-1 but also maintained nearly 74% of initial capacitance at an extremely high scan rate of 500 mV s-1.[12] Han et al. prepared a hollow core-shell NiO by hydrothermal method and followed by calcination, the received NiO was found to be highly crystalline, meanwhile the specific capacitance values was calculated to be 448 F g-1 at 0.5 A g-1, which was higher than the ever reported values.[13] Although the capacitance of transition metal compounds is much higher than that of carbon materials, while there is still a long way for their commercial applications. Nowadays it is particularly urgent to explore electrode materials with excellent electrochemical performances for SCs. Ni-based hydroxides and related composites have been proven to be one of the most promising pseudocapacitive materials owing to the high theoretical capacitances, high safety, low cost and insignificant environment concerns of Ni(OH)2.[14-17] Extensive researches aiming at Ni(OH)2 with diverse crystal structures have been reported recently. For instance, Wang et al. developed a nanoflower-like α-Ni(OH)2/graphene composite, it exhibited a great specific capacitance of 1632 F g-1 at a current density of 1 A g-1.[18] Zhang et al. reported highly crystallized β-Ni(OH)2 nanoplates uniformly grown on reduced graphene oxide nanofibers through hydrothermal


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method, the composite demonstrated a specific capacitance of 1433 F g-1 at 5 mV s-1 and favorable rate capability of 68.8% at 40 mV s-1.[19] Yuan et al. anchored amorphous Ni(OH)2 on expanded graphite by in-situ electrodeposition, the electrode displayed a fantastic specific capacitance of 1719.5 F g-1 at 1 A g-1 and a good rate capability of 68.6% at 10 A g-1.[20] Above all, previous literature have reported on various crystal structures of Ni(OH)2 as an anode electrode in supercapacitors, while little attention has been paid to the research of α-Ni(OH)2 and β-Ni(OH)2 coexistence. In this work, we propose a highly efficient, easily operated strategy to use mixed solvents to synthesize α,β-Ni(OH)2 composites, not only can reduce the cost by using smaller amount of ethylene glycol, but also expect to enhance their supercapacitive performances through synergetic effects of α-Ni(OH)2 and β-Ni(OH)2. XC-72 was demonstrated to be a promising carbon host to synthesize Ni(OH)2/XC-72 composite on account of its high conductivity, favorable thermal/chemical stability and high percentage of mesoporous structure. Sui et al. synthesized Ni(OH)2/XC-72 composite via a rapid microwave-assisted heating method, the composite delivered a superior specific capacitance of 1560 F g-1 at 1 A g-1, and its retention of specific capacitance was about 71% after 1000 cycles at a high scan rate of 100 mV s-1.[15] To further enhance the supercapacitive performances of Ni(OH)2/XC-72, Qin et al. pre-treated the XC-72 carbon black in a mixed acid of concentrated H2SO4 and HNO3 to obtain a functionalized carbon material (f-XC-72), then the surface oxygencontaining groups acted as anchors for Ni(OH)2 nanoparticles and the as-prepared Ni(OH)2/fXC-72 composite demonstrated a barely same initial specific capacitance of 1596 F g-1 as the Ni(OH)2/XC-72, but the cycling stability greatly increased from 71% to 84.5% after 1000 cycles at 100 mV s-1.[21] The above results indicate that surface properties of XC-72 will dramatically affect the supercapacitive performances of the Ni(OH)2/XC-72 composite. In this work, we also


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want to remove partial surface oxygen containing groups, and then investigate the stability effects of different combinations between C-O-Ni(OH)2 and C-Ni(OH)2. Herein firstly we will prepare a Ni(OH)2/XC-72 composite with α-Ni(OH)2 and β-Ni(OH)2 coexistence by simply using ethylene glycol and H2O as mixed solvents. Then the as-received XC-72 will be modified by thermal treatment (denoted as T-XC-72) to remove some surface oxygen functional groups, thus to increase its electronic conductivity and gain more defects as nuclei for Ni(OH)2 growth. The modified Ni(OH)2/T-XC-72 composites will be characterized, and their electrochemical performances will be investigated. Based on the electrochemical results of the modified Ni(OH)2/T-XC-72 composites, the effects of acid and thermal treatments of carbon materials will be clarified.

■ EXPERIMENTAL Synthesis of Ni(OH)2/XC-72 composite All solvents and chemicals were of analytical grade and directly used as received without further

purification.

Solvothermal

method

was

used

to

synthesize

Ni(OH)2/XC-72

composite.[22] In a typical preparation, 20 mg of Vulcan XC-72 carbon black powder (from Cabot Company) was dispersed into mixed solvent with 15 mL deionized (DI) water and 15 mL ethylene glycol (EG), then 3.2 mL of NiCl2 ethylene glycol solution (0.1 M) was added and stirred for a certain time. Subsequently, 0.5 M NaOH ethylene glycol solution was added drop by drop with continuous stirring till the pH value of 11.5. The suspension was transferred into an airtight Teflon-lined reactor of 50 mL capacity and held at 90 °C for 18 h. After auto-cooling to room temperature, the product was gathered after filtration, rinsing with DI water and ethanol in turns for several times, finally dried in a vacuum oven at 60 °C overnight. To achieve the optimum performance, the volume ratio of DI water to EG was varied from 0:3 to 1:2, 1:1, 2:1


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and 3:0, corresponding to Ni(OH)2/XC-72-EG, Ni(OH)2/XC-72-12, Ni(OH)2/XC-72-11, Ni(OH)2/XC-72-21 and Ni(OH)2/XC-72-W. Furthermore, T-XC-72 was obtained by thermal treatment of the as-received XC-72 at 600 °C for 2 h and then served as a host for the growth of Ni(OH)2, the composite was synthesized in mixed solvents with DI/EG volume ratio of 1:1, which was denoted as Ni(OH)2/T-XC-72. Scheme 1 illustrates the fabrication procedure of Ni(OH)2/XC-72.

Scheme 1. Schematic diagram of the preparation of Ni(OH)2/XC-72. Fabrication of electrodes and measurement of electrochemical performances The electrode was fabricated as follows. Firstly, nickel foam was cleaned by acetone, deionized water and ethanol in sequence to remove surface impurities. Meanwhile, Ni(OH)2/XC72 was mixed with carbon nanotubes and polytetrafluroethylene (PTFE) with a mass ratio of 85:10:5 in ethanol to obtain a homogeneous slurry, then the slurry was distributed onto a dimension of 1cm × 1cm Ni foam and the mass loading of each electrode was controlled at ca. 1 mg cm-2. After dried at 60 °C in vacuum for 6 h, the electrode was finally pressed at 10 MPa and used as the working electrode. The electrochemical measurements were conducted in a three-electrode configuration with 6 M KOH solution as the electrolyte, Hg/HgO electrode as the reference electrode, and Pt coil as the counter electrode. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and


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electrochemical impedance spectroscopy (EIS) were performed on a 302N AutoLab Potentiostat (Metrohm Holland). Characterization Powder X-ray diffraction (XRD Philips X’pert PRO MPD) with Cu Kα radiation (λ=0.15406 nm) was employed to analyze the crystal structure of the composites. IR spectra were examined on a Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer ranged from 4000 to 400 cm1

. Raman spectra of XC-72 and T-XC-72 were conducted on a Renishaw inVia Raman

spectrometer. The scanning electron microscope (SEM, ZEISS EV0 MA15) and transmission electron microscope (TEM, Carl Zeiss LIBRA®200FE) were used to characterize the morphology and structure of the samples. The specific surface area and pore-size distribution were analyzed on a Micromeritics JW-BK100C under 77 K. The real content of Ni(OH)2 in the composite was tested by using thermogravimetric analysis (TGA) (NETZSCH STA 449 F3 Jupiter®) from room temperature to 1000 °C with a heating rate of 10 K min-1 in air atmosphere. X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi) was further employed to analyze the surface chemical species of the composite.

■ RESULTS AND DISCUSSION Structure and morphology of Ni(OH)2/EG


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Figure 1. (a) XRD patterns of the as-prepared composite synthesized by solvothermal method with different volume ratios of H2O to EG; (b) XRD patterns of XC-72, T-XC-72, Ni(OH)2/XC72-11, and Ni(OH)2/T-XC-72. XRD patterns of Ni(OH)2/XC-72 synthesized in mixed solvents have been recorded in Fig. 1(a). We can see that the Ni(OH)2/XC-72-EG composite prepared in pure EG displays relatively weak and broad peaks, but the peaks are totally corresponding to the standard spectrum of PDF#38-0715, which can be ascribed to α-Ni(OH)2. The Ni(OH)2/XC-72-W composite was prepared in pure water, and the as-received Ni(OH)2 can be completely ascribed to β-Ni(OH)2 according to the standard spectrum of PDF#01-1047. While with the volume ratio of EG/H2O increasing, we can see that the peak intensities of β-Ni(OH)2 at 33.1°, 38.5°, 52.1°, 59.1°, and 62.7° become lower and lower, the peak widths become broader and broader. These results indicate that β-Ni(OH)2 particles become smaller, and/or maybe significant amount of αNi(OH)2 appears. Then we further observe the peak at around 19° of β-Ni(OH)2 is strongly influenced by the peaks at 11.3° and 22.7° of α-Ni(OH)2, leading to the emergence of two new broad peaks between 11.3° and 22.7°, indicating that α-Ni(OH)2 and β-Ni(OH)2 in the composite are coexistent at molecular level. The phenomenon indicates α,β-Ni(OH)2 formed rather than physically mixed α-Ni(OH)2 and β-Ni(OH)2. These changes of crystal structures indicate the solvent system has a big influence on the crystal structure of Ni(OH)2.[18, 23, 24] Then XC-72, T-XC-72 and the corresponding Ni(OH)2 composites are also characterized by XRD. As shown in Fig. 1(b), T-XC-72 exhibits more broad and dwarf diffraction peaks at 26° and 43.5°, which can be attributed to a removal of the surface oxygen-containing groups of XC72 and an increase of the structural defects. We also notice that Ni(OH)2/T-XC-72 displays a similar XRD pattern to Ni(OH)2/XC-72-11, this indicates that surface properties of carbon


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materials do not affect the formation of α-Ni(OH)2 and β-Ni(OH)2, only the solvents play a crucial role on Ni(OH)2 phases. According to Scherrer’s formula, the crystalline size of Ni(OH)2 in Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 composites are 14.4 nm and 10.3 nm, the formation of defects on the surface after thermal treatment roughens the surface of XC-72, and finally causing the decreasing particle size of Ni(OH)2.

Figure 2. (a) FTIR spectra of XC-72, T-XC-72, Ni(OH)2/XC-72-11, and Ni(OH)2/T-XC-72 composite; (b) Raman spectra of XC-72 and T-XC-72. FTIR spectra of XC-72, T-XC-72, Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 are recorded and shown in Fig. 2(a). It can be simply find that all peaks of T-XC-72 are weaker than XC-72 under the same analytical conditions, indicating that partly oxygen functional groups disappear after thermal treatment, this is in consistent with the XRD results. Considering the FTIR spectra of Ni(OH)2/XC-72 and Ni(OH)2/T-XC-72, the characteristic band at around 3400 cm-1 is attributed to the stretching vibration of O–H,[25-27] which may be mainly originated from EG and H2O molecule. In addition, the bands at 1630 cm-1 and 1110 cm-1 are corresponding to C=C vibration and C–O–C symmetrical stretching vibration,[21, 28-30] and the band at 1378 cm-1 can be ascribed to the characteristic absorption of carbonate ions.[29, 30] Meanwhile, the bands between 700 cm-1 and 500 cm-1 are owing to stretching vibrations of Ni-OH and Ni-O bending


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mode.[31-33] The resultant surface defects of T-XC-72 are further confirmed by Raman spectra shown in Fig 2(b). More intensive D-band and G-band are observed, which represent more defects on T-XC-72 surface and better electrical conductivity,[34] thus the ID/IG ratio of T-XC72 increases from 0.59 to 0.77 after thermal treatment. This change will be useful to obtain smaller Ni(OH)2 particles and strengthen the interaction between carbon materials and Ni element, therefore enhances the cycling stability of the Ni(OH)2/T-XC-72 composites.

Figure 3. (a) Nitrogen adsorption-desorption isotherms and pore size distribution profiles (inset) of XC-72 and T-XC-72; (b) TGA curves of XC-72, T-XC-72 and Ni(OH)2/T-XC-72 composite. The specific surface area (SSA) and pore size distribution of XC-72 and T-XC-72 are evaluated by the nitrogen adsorption-desorption measurement at 77 K. The isotherm plots of XC72 and T-XC-72 exhibit a typical type IV reversible sorption/desorption profile with a significant hysteresis loop as shown in Fig. 3(a), which demonstrates that large number of pores in carbon hosts are mesoporous.[35-37] The SSA of XC-72 and T-XC-72 are respectively calculated to be 263.1 m2 g-1 and 320.4 m2 g-1. The inset profiles display the pore size distribution, after thermal treatment the average pore size of T-XC-72 increases to 7.1 nm, which is bigger than 6.4 nm of XC-72, the change may come from the carbon corrosion during the process to remove some oxygen groups.


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TGA measurement was conducted at a heating rate of 10 K min-1 from room temperature to 1000 °C in an air flow to evaluate the content of Ni(OH)2, the corresponding TGA curves are shown in Fig. 3 (b). Both of XC-72 and T-XC-72 show a small weight loss before 100 °C, which is ascribed to the evaporation of absorbed water molecules; the second stage of the weight loss mainly comes from the combustion of XC-72 and T-XC-72, while the decomposition temperature of T-XC-72 is slightly lower than that of XC-72, indicating the bulk particles of XC72 are not affected during thermal treatment, only surface oxygen containing groups are removed to produce surface defects, thus leads to T-XC-72 slightly less stable and easier oxidation than the as-received XC-72;[38-40] their residues are almost zero, means whether XC-72 or T-XC-72 are pure without any metal elements. So the real content of Ni(OH)2 in the Ni(OH)2/T-XC-72 composite was estimated as well, three typical stages of weight loss can be observed in the curve. Same as the carbon materials, the first step of nearly 5 wt% weight loss at approximately 150 °C is originated from the evaporation of the absorbed water molecules on T-XC-72 surface and the loss of the Ni(OH)2 crystalline water, the second stage of mass decease from 150 °C to 300 °C is ascribed to the decomposition of Ni(OH)2 to NiO,[41] while about 36.0 wt% of weight loss from 300 °C to 710 °C mainly comes from the combustion of T-XC-72.[19] After calculated, the final content of Ni(OH)2 is 57.1 wt%, which is very close to the nominal content of 60.0 wt%.


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Figure 4. XPS spectrum of Ni(OH)2/T-XC-72 composite (a) survey spectrum; (b) Ni 2p; (c) C1s; (d) O1s. To further explore the element valence states and composition of the Ni(OH)2/T-XC-72 composite, XPS was conducted and the results are shown in Fig. 4(a-d). The survey spectrum in Fig. 4 (a) displays only carbon, oxygen, and nickel element. Fig. 4 (b) shows the spectrum of Ni2p, two separate peaks at 855.8 eV and 873.6 eV belong to Ni2p3/2 and Ni2p1/2, and the spinenergy separation is 17.8 eV, indicating the existence of Ni2+ in the as-prepared composite;[22] with the additional satellite peaks centred at 861.5 eV and 879.8 eV, which are characteristics of Ni(OH)2 phase.[42] The corresponding C1s and O1s spectra are shown in Fig. 4(c-d), the relatively high peak at 284.5 eV is assigned to C-C/C=C,[43-45] while the peak at 285.75 eV comes from C-Ni, which demonstrates the interaction of Ni(OH)2 and T-XC-72. As for oxygen bonding configurations, the weak peak at 532.3 eV is ascribed to C-O and the peaks at 530.83 eV and 532.04 eV are associated with -OH and Ni-O,[46, 47] which also demonstrate the existence of Ni(OH)2 in the composite.


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Figure 5. (a) SEM image; (b and c) TEM image; (d) HAADF-STEM image of Ni(OH)2/T-XC72. The morphologies of the Ni(OH)2/T-XC-72 composite are shown in Fig. 5. From the SEM (Fig. 5 a) and HAADF-STEM images (Fig. 5 d), we can observe that the grey particles are carbon materials and the white ones are Ni(OH)2 particles, Ni(OH)2 particles are uniformly distributed on T-XC-72, and several primary T-XC-72 particles aggregate together to form big secondary ones, the Ni(OH)2 are grown into big particles during hydrothermal process. TEM images (Fig. 5 b) show that Ni(OH)2 nanoparticles are uniformly located at the surface of T-XC-


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72, and it can be found in Fig. 5 (c) that the diameter of Ni(OH)2 nanoparticles are nearly 10 nm, and the size and morphology are similar to those in STEM images. Supercapacitor performances of the Ni(OH)2-based composites

Figure 6. Electrochemical performances of Ni(OH)2/XC-72 composites synthesized with different volume ratios of H2O to EG. (a) CV curves @2 mV s-1; (b) GCD curves @1 A g-1; (c) Rate capabilities calculated by CV; (d) Rate capabilities calculated by GCD. The electrochemical performances of the electrodes are evaluated by CV and GCD tests in a three-electrode configuration with 6 M KOH electrolyte. The CV and GCD curves of the Ni(OH)2/XC-72 composites synthesized with different volume ratios of H2O/EG are shown in Fig. 6(a-b). When we use pure EG as the solvent to prepare Ni(OH)2/XC-72-EG composites, after 50-cycles activation at 50 mV s-1, some of α-Ni(OH)2 is converted to β-Ni(OH)2, thus two


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anodic peaks can be observed at the CV curve of Ni(OH)2/XC-72-EG.[48] While when we adopt pure water as the solvent, only a pair of peaks emerge, indicating only β-Ni(OH)2 exists. As the H2O/EG volume ratio increases, the first anodic peak gradually disappeared, the composite shows a significant phase transformation, from the initial α-Ni(OH)2 phase to the final βNi(OH)2 phase. This phenomenon is also reflected in the discharge platform. This change reveals that the mixed solvents have great effects on the crystal structures of Ni(OH)2. The corresponding specific capacitances and rate capabilities are shown in Fig. 6 (c-d), after calculating, the initial specific capacitances of Ni(OH)2/XC-72-EG, Ni(OH)2/XC-72-12, Ni(OH)2/XC-72-11, Ni(OH)2/XC-72-21 and Ni(OH)2/XC-72-W are 1649.3, 1678.4, 1803.1, 1650.8, 1232.5 F g-1, respectively. The initial performance of Ni(OH)2/XC-72-11 is the best in both steady and transient tests, and the rate capability is also good, because α-Ni(OH)2 has larger interlayer spacing and better conductivity than β-Ni(OH)2, which can favor electrolyte ions diffusions, thus improve the initial capacitance;[49] while β-Ni(OH)2 is more stable than αNi(OH)2 in alkaline media.[50] Therefore, two-phase coexistence of α-Ni(OH)2 and β-Ni(OH)2 will exert synergistic effects and greatly enhance the electrochemical performances.

Figure 7. (a) CV and (b) GCD curves of Ni(OH)2/XC-72-11 composite.


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Fig. 7 (a-b) display the CV and GCD curves of Ni(OH)2/XC-72-11. According to CV curves, we can see that when the scan rate increase, the ratio of ia (anodic current) to ic (cathode current) almost keeps constant and pa (anodic potential) to pc (cathode potential) do not shift significantly, which indicate Ni(OH)2/XC-72-11 composite possesses good reaction activity and fantastic reversibility.[48, 49] The composite shows an excellent specific capacitance of 1504.4 F g-1 at a scan rate of 2 mV s-1 and still possesses 1017.8 F g-1 at 40 mV s-1. Based on GCD curves, the specific capacitances calculated by the discharging time at 1, 2, 5, 10, and 20 A g-1 are respectively 1803.1, 1487.0, 1281.3, 1200.0, and 1130.0 F g-1.


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Figure 8. Comparison of electrochemical performances of Ni(OH)2/XC-72-11 and Ni(OH)2/TXC-72. (a) CV curves at 2 mV s-1; (b) GCD profiles at current density of 1 A g-1; Specific capacitances at different (c) scan rates and (d) current densities; (e) Correlation between the redox current peaks and the square roots of scan rates; (f) Nyquist plots. In order to obtain smaller Ni(OH)2 particles and increase the electronic conductivity of carbon materials, XC-72 was calcined at 600 °C for 2 h to remove some oxygen-containing groups and increase some defects at the surface of XC-72 particles. Fig. 8 (a and b) presents the electrochemical performances of Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 composites. The two composites present identical anode potential and cathode potential in CV and same discharging platform in GCD, while the Ni(OH)2/T-XC-72 demonstrates larger current density and longer discharging time, so the specific capacitance of Ni(OH)2/T-XC-72 is far superior to that of Ni(OH)2/XC-72-11. This can be deduced that Ni(OH)2 particle size is smaller on T-XC72 than that on XC-72, because more defects on T-XC-72 can be nuclei for growth of Ni(OH)2 particles. The specific capacitances of Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 varied with scan rates and current densities are presented in Fig. 8 (c and d). The specific capacitance of Ni(OH)2/T-XC-72 composite can reach 1803.8 F g-1 at 2 mV s-1 and 2252.8 F g-1 at 1 A g-1 while just 1504.0 F g-1 at 2 mV s-1 and 1803.1 F g-1 at 1 A g-1 for Ni(OH)2/XC-72-11, and as the scan


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rate increases from 2 to 20 mV s-1, the rate capabilities of Ni(OH)2/T-XC-72 and Ni(OH)2/XC72-11 remain 78.1% and 74.8%, respectively. In addition, the IR drop of Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 in Fig. 8(b) are 0.0019 V and 0.0014 V, Which demonstrate the Ni(OH)2/TXC-72 composite possesses relatively high conductivity. As compared to the previous work,[21] in which carbon black XC-72 was pre-treated by the mixed concentrated HNO3 and H2SO4 (denoted as f-XC-72), this Ni(OH)2/T-XC-72 composites shows much better performances (2252.8 F g-1) than the Ni(OH)2/f-XC-72 composite (1569 F g-1) under the identical testing condition. The enhanced performances of Ni(OH)2/T-XC-72 may be ascribed to the following merits: (1) The defects of T-XC-72 increase and more sites are provided for the nucleation of nickel hydroxide, and stronger M-C interactions ensure the nanoparticles more tightly anchored on the surface of T-XC-72. (2) The conductivity of T-XC-72 is better than XC-72, Ni(OH)2 nanoparticles directly grown on the surface of T-XC-72 will facilitate the electron transfer and ion migration during the fast Faradaic redox reaction. (3) The synergistic effect of two-phase Ni(OH)2 coexistence can improve the initial specific capacitance and cyclic stability. The performances of Ni(OH)2/T-XC-72 composite are compared with the latest reported Nibased composites, as recorded in Table 1. We can see that its comprehensive performances are better or comparable to the data in below references. Table 1 Comparison of the electrochemical performances of our work and related literature.

Specific capacitance

Cycle stability

(F g-1)

(1000 cycles)

Ni(OH)2/XC-72

1560 @1 A g-1

71% @100 mV s-1

[15]

Ni(OH)2/f-XC-72

1569 @1 A g-1

84.5% @100 mV s-1

[21]

Samples

Ref.


18

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Ni(OH)2/AC

1038@1 A g-1

-

[17]

ZnO/Ni(OH)2

1830 @2 A g-1

80% @10 A g-1

[51]

α-Ni(OH)2/GO

[email protected] A g-1

-

[52]

Ni(OH)2-MnO2/rGO

1985@1 A g-1

75% @8 A g-1

[53]

α-Ni(OH)2

636 @5 A g-1

49.5%(10000cycles @10 A g-1 )

[54]

Ni/Co hydroxides

2654.9@1 A g-1

77%(1500cycles @10 A g-1)

[42]

α-Ni(OH)2

348 mAh g−1@5 A g-1

70% @100 A g-1

[55]

Ni(OH)2/XC-72-11

1803.1@1 A g-1

75% @10 A g-1

This work

Ni(OH)2/T-XC-72

2252.8@1 A g-1

80.5% @10 A g-1

This work

To further explore the energy storage mechanism of Ni(OH)2/T-XC-72 composite, the anodic and cathodic peak current densities of the Ni(OH)2/T-XC-72 (Fig. 8e) are plotted as a function of the square roots of the scan rates (ν1/2). As shown in the graph, the anodic and cathode peak current densities increase linearly with the square roots of the scan rates, abiding a diffusioncontrolled process at the electrode/electrolyte interface.[56, 57] Electrochemical impedance spectroscopy (EIS) was employed to further understand the kinetics of electrode processes.[58] Fig. 8 (f) shows the Nyquist plots of Ni(OH)2/T-XC-72 and Ni(OH)2/XC-72-11, the inset is the enlarged plots at high frequency range. In general, Rs of Ni(OH)2/T-XC-72 is identical to that of Ni(OH)2/XC-72-11, both are 0.34 Ω; while the Rct of Ni(OH)2/T-XC-72 is smaller than that of Ni(OH)2/XC-72-11; and at low frequency range, Ni(OH)2/T-XC-72 displays a nearly vertical line when compared to Ni(OH)2/XC-72-11, indicating T-XC-72 is easier for ion diffusion process and can further improve the capacitive behavior of Ni(OH)2/T-XC-72 composites.


19


Page 20 of 31

Figure 9. (a) Cycling performances of Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 at a current density of 10 A g-1; (b) Nyquist plots of Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 after 1000 cycles, the insets are equivalent circuit (upper) and the enlarged plots at high frequency range (lower). The long-term cycling stabilities of the Ni(OH)2/T-XC-72 and Ni(OH)2/XC-72-11 at a constant current density of 10 A g-1 for 1000 cycles were also measured, as shown in Fig. 9(a). It can be clearly seen that Ni(OH)2/T-XC-72 exhibits more excellent cycling stability of 80.5% after 1000 cycles, while only 76% for Ni(OH)2/XC-72-11 composite. As XC-72 was pre-treated by acid, the stability of Ni(OH)2/f-XC-72 was 84.5% after 1000 cycles @100mV s-1,[21] which seems better than both Ni(OH)2/T-XC-72 and Ni(OH)2/XC-72-11. The above results indicate that surface oxygen-containing groups on XC-72 might be more effective than surface defects to avoid aggregation of Ni(OH)2 nanoparticles. However, it is well-known that CV is a transient measuring technique and GCD is a steady one, so we currently cannot draw such a conclusion. Whatever, the point of whether surface oxygen-containing groups or surface defects holding Ni(OH)2 nanoparticles more tightly than the pristine surface is positive, indicating the surface properties of carbon materials affect the cycle stability of the composites greatly. Fig. 9(b) exhibits the EIS plot of Ni(OH)2/XC-72-11 and Ni(OH)2/T-XC-72 after 1000 cycles, the


20

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equivalent circuit and enlarged plots at high frequency are placed inside. The Rs value of Ni(OH)2/XC-72-11 composite increases slightly from 0.34 Ω to 0.39 Ω, while the Rct increases dramatically and the capacitive behavior weakens, which could be resulted from Ni(OH)2 particles growth during cycling process. For the Ni(OH)2/T-XC-72 composite, the Rct barely changes, it is likely due to the highly active Ni(OH)2/T-XC-72 composite with sufficient access to electron transport after the cycling process.[59] There are barely no changes in diffusion and capacitive behaviors after 1000 cycles, which indicate that Ni(OH)2 particles do not aggregate during cycling, this may be due to the defects at the surface of T-XC-72 can hold Ni(OH)2 tightly through Ni–C interaction just like Ni–O–(C) interaction via the surface oxygen-containing groups, and then avoid Ni(OH)2 movement. We also notice an unusual phenomenon that its Rs decreases from 0.34 Ω to 0.29 Ω, this might be due to the active material exfoliation. Overall, the Ni(OH)2/T-XC-72 composite shows better electrochemical performances including initial performance and cycling stability than the Ni(OH)2/XC-72-11.

■ CONCLUSION Ni(OH)2/XC-72 composite with α-Ni(OH)2 and β-Ni(OH)2 coexistence has been synthesized in mixed solvents, and the composite shows an improved initial capacitance and higher rate capability. By thermal treatment of XC-72, some of the oxygen functional groups on the surface of XC-72 are removed, the obtained Ni(OH)2/T-XC-72 composite exhibits much more excellent specific capacitance of 2252.8 F g-1 and cycle stability of 80.5% over 1000 cycles. This work will provide a helpful route to improve electrochemical performances and can be applied in practical supercapacitor devices.

■ AUTHOR INFORMATION


21


Page 22 of 31

Corresponding Author * Tel/Fax: +86-2883032879. E-mail: [email protected] (Shuihua Tang) Funding Sources The Open Project of Fuel Cells & Hybrid Electric Power Key Lab, Chinese Academy of Sciences (KLFC201702). International Technology Collaboration of Chengdu Science and Technology Division, the Open Project from State Key Lab of Catalysis (N-14-1). The Innovative Research Team of Southwest Petroleum University (2015CXTD04). Notes The authors declare no competing financial and content interests.

■ Acknowledgements This work was supported by the Open Project of Fuel Cells & Hybrid Electric Power Key Lab, Chinese Academy of Sciences (KLFC201702), International Technology Collaboration of Chengdu Science and Technology Division, the Open Project from State Key Lab of Catalysis (N-14-1),

and

the

Innovative Research

Team

of Southwest

Petroleum

University

(2015CXTD04).

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SYNOPSIS


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Sustainable and high-performance Ni(OH)2/XC-72 composite was firstly synthesized in mixed solvents through a simple microwave heating method, and more defects on carbon materials can further improve its specific capacitance and cycle stability.


31

C Supercapacitive Performances through Mixed

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