In-Situ-Grown Mg(OH)2-Derived Hybrid Î±-Ni(OH)2 for Highly Stable

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Letter

In situ Grown Mg(OH)2-Derived Hybrid #Ni(OH)2 for Highly Stable Supercapacitor Mingjiang Xie, Shuyi Duan, Yu Shen, Kai Fang, Yongzheng Wang, Ming Lin, and Xuefeng Guo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00258 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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In situ Grown Mg(OH)2-Derived Hybrid α-Ni(OH)2 for Highly Stable Supercapacitor Mingjiang Xie a, Shuyi Duan a, b, Yu Shen a, Kai Fang a, Yongzheng Wang a, Ming Lin c, Xuefeng Guo∗ a a

Key Lab of Mesoscopic Chemistry MOE, School of Chemistry & Chemical Engineering,

Nanjing University, Nanjing 210093, China. b

Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National

Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China. c

Institute of Materials Research and Engineering (IMRE), 2 Fusionopolis Way, Innovis, #08-03,

Singapore 138634, Singapore. Corresponding Author: Xuefeng Guo, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

As one of the most promising candidates for supercapacitor electrodes, transition metal hydroxides usually suffer from the quick decay in capacity during cycling, mainly caused by the decrease of electroactive surface area resulting from the instability of microstructure/morphology upon fast and repeated charging/discharging. Herein, we fabricated a structure-stable Ni(OH)2 grown on Ni foam via in situ ion-exchange reaction with Mg(OH)2 as sacrificial substrate and effective dopant. The obtained hybrid Ni(OH)2 possesses nanosheet morphology, large surface area (220 m2/g) and achieves an unprecedent cycling stability with a 95 % retention after 10000 cycles. The asymmetric supercapacitors with the hybrid Ni(OH)2 exhibit superior supercapacitive performances with large capacity of 167 F/g and maximum energy density of 57.9 Wh/kg at power density of 1.58 kW/kg. Even at standard power density of 4.0 kW/kg, a high energy density of 49.6 Wh/kg was achieved, making the hybrid Ni(OH)2 a promising candidate for practical supercapacitor devices.

TOC GRAPHICS


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Electrochemical capacitors (ECs)1, one of the most promising energy storage devices, have attracted increasing attention because of their higher power density and longer cycling life than secondary batteries. Pseudocapacitor2-6, one type of ECs, utilizes near-surface redox reactions of transition metal oxides/hydroxides or conducting polymer materials providing very high energy storage capacity while maintaining high power density. Among the pseudocapacitive electrode materials, Ni(OH)2 has been considered as a promising candidate for supercapacitors due to its high theoretical capacity, excellent redox behavior, ease of synthesis, abundant sources, low cost, environmental benignity, etc. 7-11 Despite these attractive features, the actual cycling life reported for various Ni(OH)2 materials usually remains poor, mainly caused by the quick decay of electroactive surface area resulting from the instability of microstructure/morphology upon fast and repeated charging/discharging.12 Therefore, great efforts have been devoted to fabricating various Ni(OH)2 nanostructures to enhance the cycling stability. However, most of them failed to achieve a long cycling life. Recently, there are several nanostructures reported that can largely enhance the cycling stability (80~90 % retention after 10000 cycles). 13-15 However, their special fabrication methods involving the use of toxic NH4F or electrochemical deposition technique limited their further large-scale preparation and applications. In the ceramic research, it’s well known that an oxide structure can be stabilized by certain dopants, such as Y2O3-stabilized ZrO2. Recently, doping electroactive electrode materials with non-electroactive components as dopants in order to enhance ECs performances including the cycle life, has attracted increasing attention.16, 17 For example, transition metal layered double hydroxides (LDHs) with electroactive Ni-OH or Co-OH and non-electroactive Al-OH or Zn-OH have been fabricated and exhibited potential for supercapacitor application.18-21 Although the improvement in ECs performances is encouraging, the cycling stability of the reported various


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types of LDHs remains disappointing (usually less than 90 % retention after 5000 cycles), possibly because the dopants (e.g. Al, Zn) are not very stable in the strongly alkaline electrolyte, leading to instable structure/morphology and inferior cycling stability.22 Up to date, it still remains a challenge to construct a hybrid nickel hydroxide with stable structure/morphology and excellent pseudocapacitive stability by an easily accessed approach.

Figure 1. (a) Schematic illustration for the growth of Ni(OH)2 on Ni foam by ion-exchange reaction method. (b-c) SEM images of the bare Ni foam and Mg-Ni(OH)2. (d-e) TEM and HRTEM images of Mg-Ni(OH)2. (f) STEM and corresponding element mapping of MgNi(OH)2. Herein, as depicted in Figure 1a, we report a facile method to fabricate a hybrid Ni(OH)2 grown on Ni foam via in situ ion-exchange reaction (IER), i.e.


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Ni2+ (aq.) + Mg(OH)2 (s) → Ni(OH)2 (s) + Mg2+ (aq.), in which the prepared Mg(OH)2 with a nanosheet morphology (shown in Figure S1 in supporting information) was employed as sacrificial substrate and effective dopants. Unlike previous synthesis methods 23-25 of Ni(OH)2 and LDHs, it needs neither any extra alkali sources nor heat treatment (hydrothermal or microwave irradiation) for producing OH− ions because the described IER is a spontaneous reaction driven by the difference between the solubility product (Ksp) of Mg(OH)2 and that of Ni(OH)2. Through IER, magnesium (the residual component of magnesium hydroxide in IER) could be simultaneously doped into the final nickel hydroxide (denoted as Mg-Ni(OH)2). To elucidate the key role of the doped magnesium, a dopant-free Ni(OH)2 with the same crystalline phase as Mg-Ni(OH)2, fabricated by hydrothermal method with hexamethylenetetramine as precipitation reagent, is employed as comparison. In our strategy, the direct growth of electrode material on conductive substrate (Ni foam) aims to favor the electrolyte wettability and electron transportation,26-33 while the residual magnesium hydroxide in the hybrid nickel hydroxide plays an important role in stabilizing the structure/morphology since magnesium hydroxide is stable in the strong alkaline electrolyte. Therefore, the decay of electroactive surface area can be effectively controlled since the electroactive surface area is closely related to the morphology.34 The obtained hybrid MgNi(OH)2 achieves an outstanding cycling stability with a 95 % retention after 10000 cycles ( vs 51 % retention only after 3000 cycles for dopant-free one), and the Mg-Ni(OH)2-based asymmetric supercapacitors exhibit a large capacity of 167 F/g and high energy density of 57.9 Wh/kg (at 1.58 kW/kg), showing great potential as supercapacitor. The scanning electron microscope (SEM) images in Figure 1b-1c show that after IER, MgNi(OH)2 with nanosheet morphology and thickness of ∼17 nm was homogenously grown on the


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Ni foam. The nanosheet morphology was further confirmed by the transmission electron microscope (TEM) image in Figure 1d. The high-resolution TEM image (Figure 1e) displays some visible lattice fringes with an equal interplanar distance of 0.232 nm, corresponding to (015) planes of α-Ni(OH)2, indicating that the presented IER method succeeded in preparing crystalline nickel hydroxide. Figure 1f shows STEM image and corresponding element mapping of Mg-Ni(OH)2. The maps of Mg and Ni both show profiles analogous to the original STEM image, indicative of a homogeneous element distribution in the parent material. The elemental components of Mg-Ni(OH)2 were further determined by inductively coupled plasma atomic emission spectroscopy (ICP), in which the molar ratio of Ni/Mg is 13:1.

Figure 2. (a) XRD patterns; (b) N2 sorption isotherms and pore size distributions (PSD) curves (b-inset); (c) Raman patterns and (d) Ni2p XPS spectra of Mg-Ni(OH)2 and dopant-free Ni(OH)2.

The obtained Mg-Ni(OH)2 and dopant-free Ni(OH)2 were further investigated by X-ray diffraction (XRD), nitrogen sorption, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD patterns (Figure 2a) of the two nickel hydroxides both exhibit single crystalline phase of α-Ni(OH)2 (JCPDS: 38-0715), which is consistent with the HRTEM result. Nitrogen


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sorption isotherms (Figure 2b) of the two nickel hydroxides both show type IV curves with a hysteresis loop, indicative of the existence of a porous structure. The pore size distribution (PSD) curves depicted in the inset of Figure 2b, show that the obtained Mg-Ni(OH)2 has bimodal pore size distribution centered at 3.0 and 6.5 nm, while dopant-free Ni(OH)2 has single pore size distribution centered at 3.0 nm. The calculated BET surface area and pore volume listed in Table S1 are 220 m2/g and 0.56 cm3/g for Mg-Ni(OH)2, 42 m2/g and 0.11 cm3/g for dopant-free Ni(OH)2, respectively, indicating that Mg-Ni(OH)2 has richer porosity than Ni(OH)2. Raman spectra (Figure 2c) of the two nickel hydroxides both show a peak at 450 cm-1, ascribed to the NiO stretching vibration of Ni(OH)2.35 The Ni2p X-ray photoelectron spectra (XPS, Figure 2d) display four peaks around 855, 860, 873 and 878 eV that can be indexed to Ni2p3/2, Ni2p3/2 satellite, Ni2p1/2 and Ni2p1/2 satellite signals of Ni(OH)2, respectively. The above measurements indicate that the developed ion-exchange method realized the preparation of hybrid Mg-Ni(OH)2, which possesses nanosheet morphology, rich porosity and large surface area. The electrochemical capacitor (EC) performances of Mg-Ni(OH)2 and dopant-free Ni(OH)2 were firstly investigated in a three-electrode configuration. Figure 3a and Figure S2a show the cyclic voltammograms (CVs) of the two nickel hydroxides at various scan rates, they both display typical pseudocapacitive behaviors with two redox peaks around 0.5 and 0.3 V, ascribed to the Faradaic reaction Ni(OH)2 + OH-

NiOOH + H2O + e-.

Galvanostatic charge/discharge curves (GDC, Figure 3b and Figure S2b) of Mg-Ni(OH)2 and dopant-free Ni(OH)2 at different current densities from 0.5 to 20 A/g show deviation from the typical triangular shape of non-Faradaic electric double-layer capacitor (EDLCs), further evidencing the Faradaic characteristics of the charge storage. Figure 3c shows specific


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capacitances of Mg-Ni(OH)2 and dopant-free Ni(OH)2 at different current densities. For MgNi(OH)2, the capacitances (from 0.5 to 20 A/g) calculated from GDC curves are 1931, 1905 1837, 1565, 1521 and 1496 F/g, respectively, the values are higher than those of dopant-free

Figure 3. Electrochemical performances tested by a three electrode cell in 6.0 M KOH electrolyte. (a) Cyclic voltammograms (CVs) of Mg-Ni(OH)2 at various scan rates, (b) Charge/discharge curves of Mg-Ni(OH)2 at different current densities, (c) Specific capacitances vs current densities, (d) Capacitance retention for the two nickel hydroxides after 10000 and 3000 cycles at 10 A/g respectively, inset are CVs of Mg-Ni(OH)2 at the 1st , 5000th and 10000th cycle.

Ni(OH)2 (maximum capacitance of 1389 F/g) at every current density. The superior capacitance of Mg-Ni(OH)2 as compared to dopant-free Ni(OH)2 may be attributed to its advantage of surface area (220 m2/g vs 42 m2/g). Figure 3d shows the cycling stability test for the two nickel hydroxides obtained by galvanostatic charge/discharge at a constant current density of 10 A/g. Notably, the capacitance retention for Mg-Ni(OH)2 is as high as 95 % after 10000 cycles. In


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sharp contrast, the capacitance retention for dopant-free Ni(OH)2 is as low as 51 % only after 3000 cycles. The CVs (Figure 3d-inset) of Mg-Ni(OH)2 at the 1st , 5000th and 10000th cycle shows a slight change, which further verified the superior cycling stability. As shown in Figure 4, the SEM image of Mg-Ni(OH)2 after 10000 cycles test (Figure 4b) shows almost no change in the nanosheet structure/morphology

Figure 4. SEM images of the two nickel hydroxides before and after long-cycling test at current density of 10.0 A/g. SEM image of Mg-Ni(OH)2 before (a) and after (b) 10000 cycles test; SEM images of dopant-free Ni(OH)2 before (c) and after (d) 3000 cycles test.

compared to the pristine one (Figure 4a), accounting for the excellent cycling stability. For dopant-free Ni(OH)2, repeated charging/discharging of 3000 cycles results in obvious collapse of nanosheet structure as Figure 4d shows, leading to a poor cycle life. The SEM record on structure/morphology variation before and after long-cycling test disclosed the relationship between the capacitance decay and the changes in the microstructure/morphology of the two nickel hydroxides, which is consistent with the finding of Tour et al .14 The stable morphology of


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Mg-Ni(OH)2 can be ascribed to the introduction of magnesium in the nickel hydroxide skeleton. Since magnesium hydroxide is a highly stable component in alkaline electrolyte, it can be expected that the residual Mg(OH)2 can serve as a cement stabilizing the microstructure, providing a stable morphology to resist the long-time charging/discharging. Other dopants reported before, e.g. Al and Zn, are not stable in the alkaline electrolyte. The doped component could be easily etched by the spontaneous reaction, i.e. Al(OH)3 (s) + OH- (aq.) → AlO2-1 (aq.) +H2O or Zn(OH)2 (s) + 2OH- (aq.) → Zn(OH)4-2 (aq.), leading to an instable morphology. Based on the results, it may be concluded that the excellent cycling performance of Mg-Ni(OH)2 mainly derives from the introduction of a stable component of Mg(OH)2 into the skeleton.

Figure 5. Electrochemical performance of asymmetric supercapacitor based on Mg-Ni(OH)2 and dopant-free Ni(OH)2 electrodes in 6.0 M KOH. (a) Cyclic voltammograms (CVs) of MgNi(OH)2 at various scan rates, (b) Charge/discharge curves of Mg-Ni(OH)2 at different current


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densities, (c) Specific capacitances vs current densities, (d) Ragone plots of the two asymmetric cells. To evaluate the practical EC performance of the prepared Mg-Ni(OH)2 and dopant-free Ni(OH)2, asymmetric two-electrode cells were assembled using a commercial activated carbon (AC, TF-B520 surface area of 1991 m2/g) as the negative electrode (see Experimental Section, Figure S3 and S4). The EC performance of the two nickel hydroxides was evaluated by CVs and GDCs within a potential window of 0-1.6 V. Both CVs of the two nickel hydroxides (Figure 5a and Figure S5a) at different scan rates from 5.0 to 200 mV/s show an anodic peak at 1.0 V and a cathodic peak at 0.7 V assigned to the redox reaction of Ni(OH)2 + OH-

NiOOH + H2O + e-.

The GDC curves (Figure 5b and Figure S5b) indicate the redox reactions, which is consistent with the CVs. The specific capacitance of the entire device including both positive and negative electrodes are calculated from GDC curves and shown in Figure 5c. Obviously, the calculated capacitances for the cell based on Mg-Ni(OH)2 are all higher than that of the cell based on dopant-free Ni(OH)2 from current density of 1.0 to 20 A/g. For the cell based on Mg-Ni(OH)2, the maximum capacitance is as high as 167 F/g, the value is much higher than those of dopantfree Ni(OH)2 (maximum capacitance of 98 F/g). The EC performance of the fabricated devices that use the Mg-Ni(OH)2 and dopant-free Ni(OH)2 as electrodes are also reflected by their energy and power densities, as shown in the Ragone plot (Figure 5d). The maximum energy density (Emax) and power density (Pmax) of the Mg-Ni(OH)2 were calculated to be 57.9 Wh/kg and 26 kW/kg, respectively; the values are much higher than those of dopant-free Ni(OH)2 ( Emax of 33.9 Wh/kg, Pmax of 24 kW/kg) and lie at a high level among the recently reported supercapacitors based on nanostructured Ni(OH)2 ( as the listed results shown in Table S2). Obviously, the Mg-


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Ni(OH)2 exhibits higher energy densities than those of Ni(OH)2 at every power density. For a standard power density of 4.0 kW/kg, the Mg-Ni(OH)2 shows a much higher energy density of 49.6 Wh/kg than that of Ni(OH)2 (26.9 Wh/kg). Further, the fabricated device based on the two nickel hydroxides were tested at 10 A/g for cycling stability by repeated discharge/charge. For Mg-Ni(OH)2, up to 91% retention was obtained (Figure S6) after 10000 cycles whereas, the asymmetric capacitor based on dopant-free Ni(OH)2 shows a low capacitance retention of 48 % only after 3000 cycles. The magnesium ion hybridization, open-access nanostructure and large surface

area

of

Mg-Ni(OH)2

nanosheet

grown

on

Ni

foam

ensure

a

stable

microstructure/morphology and provide more electrochemically active surface for redox reactions, contributing to the superior supercapacitor performances and superlong cycling life. In conclusion, a simple approach based on ion-exchange reaction with magnesium hydroxide as counter material and dopant was developed and applied to the fabrication of hybrid nickel hydroxide nanosheets. The hybrid nickel hydroxide nanosheets grown on Ni foam can be directly used as additive-free electrode, it delivers a superior capacitance (of 1931 F/g) when compared to the hybrid-free Ni(OH)2 (of 1389 F/g). More importantly, the obtained Mg-Ni(OH)2 achieves an outstanding cycling stability with 95 % retention after 10000 cycles vs 51 % retention after 3000 cycles for dopant-free one. For asymmetric supercapacitor, the entire device based on hybrid Mg-Ni(OH)2 exhibits a capacitance of 167 F/g with maximum energy density of 57.9 Wh/kg at power density of 1.58 kW/kg and an excellent cycling stability, thus being a promising candidate for supercapacitors. Furthermore, the presented methodology based on ion-exchange reaction is simple and broadly applicable, providing new opportunities for design of many other transition metal hydroxides/oxides electrode materials.


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EXPERIMENTAL METHODS Details are provided in the Supporting Information. ASSOCIATED CONTENT Supporting Information Experimental methods; TEM image of Mg(OH)2; Structural parameters of Mg-Ni(OH)2, Ni(OH)2 and AC; Electrochemical capacitive performances of Ni(OH)2 and AC (TF-B520), Cycling stability test of the asymmetric supercapacitor based on Mg-Ni(OH)2 and Ni(OH)2. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (2009CB623504), the National Science Foundation of China (20773062, 20773063, 21173119, and 21273109), the Fundamental Research Funds for the Central Universities and the Postdoctoral Foundation of Jiangsu (0205003455).


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(24) Ma, X. W.; Li, Y.; Wen, Z. W.; Gao, F. X.; Liang, C. Y.; Che, R. C. Ultrathin beta-Ni(OH)2 Nanoplates Vertically Grown on Nickel-Coated Carbon Nanotubes as High-Performance Pseudocapacitor Electrode Materials. ACS Appl. Mater. Inter. 2015, 7, 974-979. (25) Ji, J.; Zhang, L. L.; Ji, H.; Li, Y.; Zhao, X.; Bai, X.; Fan, X.; Zhang, F.; Ruoff, R. S. Nanoporous Ni(OH)2 Thin Film on 3D Ultrathin-Graphite Foam for Asymmetric Supercapacitor. Acs Nano 2013, 7, 6237-6243. (26) Li, J.; Xiao, J.; Wang, Z.; Wei, Z.; Qiu, Y.; Yang, S. Construction of bicontinuously porous Ni architecture as a deposition scaffold for high performance electrochemical supercapacitors. Nano Energy 2014, 10, 329-336. (27) Li, L.; Xu, J.; Lei, J.; Zhang, J.; McLarnon, F.; Wei, Z.; Li, N.; Pan, F. A one-step, costeffective green method to in situ fabricate Ni(OH)2 hexagonal platelets on Ni foam as binder-free supercapacitor electrode materials. J. Mater. Chem. A 2015, 3, 1953-1960. (28) Min, S.; Zhao, C.; Zhang, Z.; Chen, G.; Qian, X.; Guo, Z. Synthesis of Ni(OH)2/RGO pseudocomposite on nickel foam for supercapacitors with superior performance. J. Mater. Chem. A 2015, 3, 3641-3650. (29) Peng, S.; Li, L.; Wu, H. B.; Madhavi, S.; Lou, X. W. Controlled Growth of NiMoO4 Nanosheet and Nanorod Arrays on Various Conductive Substrates as Advanced Electrodes for Asymmetric Supercapacitors. Adv. Energy Mater. 2015, 5(2), 1401172. (30) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132, 74727477.


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ACS Energy Letters

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(31) Xiong, X.; Ding, D.; Chen, D.; Waller, G.; Bu, Y.; Wang, Z.; Liu, M. Three-dimensional ultrathin Ni(OH)2 nanosheets grown on nickel foam for high-performance supercapacitors. Nano Energy 2015, 11, 154-161. (32) Yan, J.; Fan, Z.; Sun, W.; Ning, G.; Wei, T.; Zhang, Q.; Zhang, R.; Zhi, L.; Wei, F. Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density. Adv. Funct. Mater. 2012, 22, 2632-2641. (33) Zhang, G.; Lou, X. W. General Solution Growth of Mesoporous NiCo2O4 Nanosheets on Various Conductive Substrates as High-Performance Electrodes for Supercapacitors. Adv. Mater. 2013, 25, 976-979. (34) Feng, L.; Zhu, Y.; Ding, H.; Ni, C. Recent progress in nickel based materials for high performance pseudocapacitor electrodes. J. Power Sources 2014, 267, 430-444. (35) Li, W.; Xin, L.; Xu, X.; Liu, Q.; Zhang, M.; Ding, S.; Zhao, M.; Lou, X. Facile synthesis of three-dimensional structured carbon fiber-NiCo2O4-Ni(OH)2 high-performance electrode for pseudocapacitors. Sci. Rep. 2015, 5, 9277.


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In-Situ-Grown Mg(OH)2-Derived Hybrid Î±-Ni(OH)2 for Highly Stable

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