intercalation in Nickel Hydroxychloride Microspheres: A

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Anion De-/intercalation in Nickel Hydroxychloride Microspheres: A Mechanistic Study of Structural Impact on Energy Storage Performance of Multi-anion-containing Layered Materials Sixian Fu, Liping Li, Yuelan Zhang, Shaoqing Chen, Shaofan Fang, Yuancheng Jing, and Guangshe Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00324 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Energy Materials

Anion De-/intercalation in Nickel Hydroxychloride Microspheres: A Mechanistic Study of Structural Impact on Energy Storage Performance of Multi-anion-containing Layered Materials Sixian Fu,† Liping Li,† Yuelan Zhang,† Shaoqing Chen,‡ Shaofan Fang,‡ Yuancheng Jing,† and Guangshe Li*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, P.R. China ‡

Fujian Institute of Research in Structure of Matter, Chinese Academy of Sciences,

Fuzhou 350002, P.R. China

ABSTRACT: Electrochemical cation de-/intercalation has long been investigated for energy relevant applications, while anion de-/intercalation is comparatively highly challenging, although promising for materials performance promotion. Herein, layered nickel hydroxychloride was selected as a model multi-anion-containing inorganic functional material to study. Hierarchical flower-like microspheres self-assembled from nanosheets were synthesized via a solvothermal method. The as-prepared nickel hydroxychloride was built up from neutral layers of [Ni(OH)3/3Cl3/3] octahedra, showing an expanded interlayer spacing of 0.57 nm. With 1

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this unique microstructure, Cl- deintercalation and OH- intercalation were accomplished through an effective non-electrochemical process. The nickel hydroxychloride Ni(OH)0.99Cl1.01 with a maximum Cl- ion content was found to possess the largest interlayer spacing, which when firstly employed as electrode materials for supercapacitor, delivered an ultrahigh specific capacitance of 3831 F/g at a current density of 1 A/g. For the latter case, Ni(OH)2.18(H3O)0.18 with a maximum OH- content showed a specific capacitance of 1489 F/g at 1 A/g. Expanded interlayer spacing associated with the anion de-/intercalation is the key that enhances ion diffusion kinetics between layers. The methodology of anion de-/intercalation reported in this work would provide hints of exploring novel multi-anion-containing materials with anion de-/intercalation necessary for high-performance energy applications.

KEYWORDS: anion de-/intercalation, nickel hydroxychloride microspheres, interlayer spacing, multi-anion-containing layered materials, energy storage

1. INTRODUCTION Anion de-/intercalation is a promising way to achieve optimum performance of multi-anion-containing inorganic materials for many functional applications, such as advanced catalysis, optoelectronics, photoluminescence, and energy storage.1-4 For example, fast anion-exchange in CsPbX3 nanocrystals (NCs) has been achieved to fine tune the spectrally narrow and bright photoluminescence over the entire visible spectral region.3 Anion intercalation derived three dimensional graphitic foams have 2


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been applied to ultrafast aluminum-ion batteries.5 As anion-intercalation-type electrodes for supercapacitors, perovskite structure has been found a cation leaching effect for capacity change.6 Since most of the functional materials contain more than one kind of anions, crystal structure, electronic structure, defect structure, and the spin structure of the materials may be changed with anion de-/intercalation, resulting in diversified performance and that facilitate technological applications. Comparing to cations, however, anions are relatively larger in ionic radius and have stronger interactions

with

the

central

framework

metal

ions.

In

particular

for

multi-anion-containing inorganic functional materials, distinct bonds would bring abundant chemical properties. Despite of many efforts,7-8 it is still very hard to achieve efficient de-/intercalation of anions from multi-anion-containing inorganic functional material systems when comparing to that of cations at small ionic sizes.

Till now, electrochemical methods, popularly valid for achieving cation de-/intercalation in several material systems,9-11 are not applicable for anion de-/intercalation, particularly for the case of multi-anion-containing inorganic functional material system. The primary reason is due to the existence of over-potential, since many kinds of anions (e.g., S2-, OH-, Cl- and NO3-) would be electrolyzed accompanying with a redox process. Motivated by this, we proposed to adopt a non-electrochemical method by selecting a double-anion layered nickel hydroxychloride as a target to study based on the following considerations: Firstly, nickel hydroxychloride is a typical multi-anion-containing layered compound. Layered materials offer fascinating opportunities for both fundamental studies and 3



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functional applications due to their unique structural and electronic properties.12-16 Relevant results will have a widespread impact. Secondly, based on the law of charge conservation and anion coordination characteristics in layered structures,10, 17 anion de-/intercalation may be realized by a non-electrochemical H2O stirring method. Thirdly, for many inorganic materials, anion de-/intercalation always bring about lattice change, in particular for layered compounds.15, 18, 19 Anion de-/intercalation will change the interlayer spacing between adjacent two layers, so as to achieve the purpose of optimizing the charge storage ability.

In this work, we initially prepared flower-like nickel hydroxychloride microspheres by a template-free solvothermal method. Then, we designed a non-electrochemical H2O stirring method that accomplished an effective anion de-/intercalation of Cl-/OHin nickel hydroxychloride. The electrochemical properties changes induced by anion de-/intercalation are investigated in details. Combining our kinetic analyses and the literature data ever reported,20 we conclude that expanded interlayer spacing associated with the anion de-/intercalation is the key that enhances ion diffusion kinetics. The non-electrochemical methodology of achieving effective anion de-/intercalation

reported

in

this

work

may

be

extended

to

other

multi-anion-containing material systems, which would advance more syntheses of novel high-performance inorganic functional materials for important energy storage applications.

2. EXPERIMENTAL SECTION

4


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2.1. Synthesis of nickel hydroxychloride microspheres

All reagents are analytical grade without further treatment. In a typical procedure, 10 mmol NiCl2·6H2O was firstly dissolved into 60 mL ethyl alcohol to form a homogenous solution under magnetic stirring at room temperature. Subsequently, the solution was transferred into a Teflon-lined autoclave (100 mL), sealed and heated at 180 oC for 12 h. The system was then cooled down to room temperature naturally. The product was collected by centrifugation, washed with deionized water and ethanol for several times and eventually dried in air at 70 oC for 12 h. The obtained yellow powders named as-prepared sample.

2.2. Anion de-/intercalation for nickel hydroxychloride

The as-prepared sample was divided into eight parts averagely. Seven of them were stirred with H2O for 3, 6, 12, 24, 48, 96, and 192 h, respectively, at a speed of 700 r/min. The products were collected by centrifugation, washed several times with deionized water and ethanol, followed by drying at 70 oC for 6 h. Final samples were thus obtained.

2.3. Synthesis of the contrast sample Ni(OH)2

10 mmol NiCl2·6H2O was firstly dissolved into 50 mL H2O under stirring to form a homogenous solution at room temperature. Then, 10 mL NaOH (2M) was added into the solution at a speed of 20 drops/min under continuous stirring. Subsequently, the solution was transferred into a Teflon-lined autoclave (100 mL), sealed and heated at 5



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180 oC for 12 h. After cooling down to the room temperature, the obtained sample was washed with deionized water and ethanol, and then dried at 70 oC for 6 h in air.

2.4. Material characterization

Powder X-ray diffraction (XRD) patterns of the samples were recorded on a diffractometer (Rigaku miniflex600) with Cu Kα radiation (λ = 0.1518 nm). Step scanning in a 2θ range of 10−80° with intervals of 0.02° was used to collect diffraction data. The morphologies and microstructures of the samples were observed by field-emission scanning electron microscopy (SEM) (HITACHI SU8020) and high-resolution transmission electron microscopy (HRTEM) (Tecnai G2 F30). Elemental mapping was carried out using energy dispersive spectroscopy (EDS) to determine the molar ratios of Cl to Ni in the sample prior to or after anion de-/intercalation. The chemical compositions and oxidation states for the samples were determined using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250). Patterns were calibrated by a primary C 1s at 284.8 eV. Fourier transform infrared spectra (FT-IR) of the samples were conducted on an apparatus IFS-66V/S. Thermo-gravimetric analysis (TGA) was performed on a Netzsch Model STA449 F5 thermal analyzer connected with a mass spectroscopy QMS403D. The data were recorded at a heating rate of 10 °C min-1 from room temperature to 1000 °C. Nitrogen adsorption-desorption isotherm was determined using Brunauer–Emmett–Teller (BET) equation by a surface area analyzer ASAP 2020 (Micromeritics). X-ray absorption fine structure (XAFS) data were collected at 1W1B station in Beijing Synchrotron

6


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Radiation Facility (BSRF).

2.5. Electrochemical measurements

Electrochemical measurements were carried out on an electrochemical workstation (CHI 760E, Chenhua Instruments, China).The test electrodes were prepared using the following steps: 80 wt % of the active material (1.6 mg), 10 wt % of the acetylene black (0.2 mg) and 10 wt % of the poly-tetrauoroethylene (PTFE) were mixed with deionized water and ethanol in mortar, and then the obtained paste was uniformly coated on nickel foams with a coating area of 1 cm2 to make the working electrodes. The pasted working electrodes were dried at 60 °C for 4 h and then pressed at 8 MPa to obtain final electrodes. Electrochemical measurements of the working electrodes were carried out using a three-electrode cell in a 6 M KOH electrolyte. The Pt foil was used as the counter electrode, and a standard calomel electrode (SCE) was taken as the reference electrode. The electrochemical impedance spectra (EIS) were obtained using an AC voltage of 5 mV in the frequency range from 0.01 Hz to 100 kHz. The gravimetric specific capacitances (Csc) were calculated from GCD curves according to the following equation:

Csc = (I∆t)/(m∆V)

(1)

where I (A) is the discharge current, ∆t (s) is the time period during discharge, m (g) is the mass of the active component, and ∆V (V) is the potential window.

3. RESULTS AND DISCUSSION 7



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Figure 1. Schematic diagram that illustrates the anionic de-/intercalation processes of nickel hydroxychloride by non-electrochemical H2O stirring.

Anionic de-/intercalation processes of layered nickel hydroxychloride under constant H2O stirring treatment is schematically illustrated in Figure 1. The parent nickel hydroxychloride is built up from [Ni(OH)3/3Cl3/3] octahedra, delivering a maximum interlayer spacing of ~0.57 nm.21 It is well established that Cl atoms and O atoms within the monolayers occupy the six vertexes of the octahedral sites having strong coordination with central framework Ni atoms, while the interlayer interactions are weak van der Waals force. Such ordered arrangement brings about some remarkable anion-exchange properties with the changes of physical-chemical conditions.22-23 According to Coulomb's law, Cl- ionic radius (181 pm) is larger than OH- (137 pm), so that the interionic attraction between Ni2+ and Cl- is weaker than that between Ni2+ and OH-. Therefore, deintercalation of Cl- ions is easier than OH-, so as to be realized by H2O stirring treatment. With the break of ionic bond between Ni2+ and Cl-, the charge balance is broken, and the nickel hydroxychloride is positively charged. In order to maintain the electrical neutrality, OH- in H2O is more likely to bond with central Ni2+ ions, thus [Ni(OH)3/3(OH)3/3] octahedra are formed. The process of Cl- deintercalation and OH- intercalation can be described by the 8


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following reaction,

NiOHCl(s) + H2O → Ni(OH)2(s) + HCl(aq)

(2)

There are a number of hydrogen bonds formed between the two atomic layers in this step because of the intercalation of OH-, leading to a reduced interlayer spacing. Particularly, the layered structure is not destroyed, and the sample maintains its inherent morphology during the whole process. Previous studies reported that two-dimensional layered materials have a discontinuous structural expansion along c-axis direction as H2O molecules intercalate, which results from the weak interaction between adjacent two layers.24 Continuous H2O stirring progress would makes water molecules intercalate into the interlayer space, which may form water bilayers to bring some changes in properties. All these assumptions were evidenced by the following experimental results.

Figure 2. XRD patterns of (a) as-prepared sample and those after water stirring for a 9



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given period of time: (b) 6 h, (c) 12 h, (d) 24 h, (e) 48 h, (f) 96 h, and (g) 192 h. The peaks denoted by “♦” are assigned to the internal standard of Cu. Straight lines at the bottom are redrawn from the standard diffraction database for Ni(OH)2 (JCPDS card No. 14-0117).

The maintained layered structure of the samples during the whole stirring process is confirmed by XRD measurements. As seen in Figure 2a, the primary diffraction peaks for the as-prepared sample are similar to those of nickel hydroxychloride nanocrystals previously reported by Hu et al.25 Therefore, the as-prepared sample is determined to be nickel hydroxychloride with a brucite-like structure.7 In the structure, nickel hydroxychloride was built up from neutral layers of [Ni(OH)3/3Cl3/3] octahedra, similar to the layered structure of Cd(OH)Cl.21 As the stirring time increases, the diffraction at two theta of 56.2° disappeared, while four other diffractions remarkably arose at two theta of 38.5°, 52.1°, 59.1°, and 62.7°, respectively. When the stirring time reached 192 h, nickel hydroxychloride almost transformed to Ni(OH)2 (JCPDS card No. 14-0117). The structural evolution is further manifested by the enlarged XRD pattern, where the position of the (001) peak shifted towards a higher diffraction angle as the stirring progresses, indicating a reducing tendency of the interlayer spacing along c-axis direction. According to Bragg equation, the d-spacing of original 5.70 Å changed to final 4.55 Å experienced Cl- deintercalation, OH- intercalation and further H2O intercalation. A shoulder peak around two theta of 16o beside the (001) peak is observable when the stirring time reached 24 h, 48 h, 96 h, and 192 h. It is attributed to the formation of water bilayers as reported elsewhere.10 As shown in 10


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Figure 1, Ni(OH)2 layers forms gradually with the intercalation of OH-. OH- species would combine a number of water molecules by hydrogen bonds. The water bilayers appeared when each Ni(OH)2 layer with adsorbed water monolayer are closely stacked. And they give further support to the basic integrity of the Ni(OH)2 layers.26

Figure 3. FE-SEM images of (a, b) the as-prepared sample and those after stirring in 11



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H2O for a period of time: (c, d) 6 h and (e, f) 192 h at different magnifications.

Morphologies of the samples were examined by FE-SEM. The as-prepared sample showed a flower-like spherical shape which is composed of crumpled nanosheets with random orientation (Figure 3a, b). The thickness of the nanosheet is 10 ~ 30 nm (Figure S1). No matter how long the stirring time, the morphologies show no apparent change (Figure 3c-f) (Figure S2). This phenomenon for anion de-/intercalation is totally different from that of electrochemical cation de-/intercalation. For instance, in lithium-ion battery cathode systems, Li+ ion electrochemical insertion/extraction leads to volume change and structural collapse followed by a lattice shrink/expansion.27

Figure 4.

(a) SEM image and the corresponding EDS elemental mapping images for

(b) Ni, (c) O, and (d) Cl in the as-prepared sample. X-ray mapping images for Cl in the samples after stirring in water for (e) 12 h, (f) 24 h, and (g) 48 h. (h) Cl to Ni atomic ratio histogram for the as-prepared samples and those after stirring at different period of time.

To monitor the variation of Cl elemental distribution in the samples during the 12


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stirring in water, EDS mapping was measured. For the as-prepared sample, EDS mappings of Ni, O, and Cl elements are homogenously distributed through the microsphere (Figure 4b-d). We put a spotlight on Cl elemental distribution, and found that Cl signals are enriched within the microsphere. The blue signals in Figure 4e-g become more and more dispersed with stirring time increasing, suggesting a decrease in Cl- content. When stirring for 48 h, Cl- content reduces to quite a low level, since Cl- enriched area is hard to distinguish (Figure 4g). Corresponding Cl to Ni atomic ratio histogram for the samples with different stirring time is depicted in Figure 4h. The ratio values were calculated from EDS data, as listed in Table S1. For the as-prepared sample, the atomic ratio of Cl to Ni is 0.9766. With increasing the periods of stirring time, the atomic ratio continuously decreased to 0.021, far from unity, indicating a non-uniform deintercalation process of Cl- ions. All these evidence that Cl- ions are successfully deintercalated from the parent lattice during the stirring in H2O.

13



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Figure 5. TEM and HR-TEM images of (a, b, c) as-prepared sample and those after stirring in water for a period of time: (d, e, f) 48 h, and (g, h, i) 192 h.

Detailed morphologies and structural information of the samples were also examined by TEM. The thickness of the nanosheets is about 15 nm as labeled by yellow arrows in Figure 5b. And the nanosheets were assembled by numerous nanoparticles with a dimension about 3 nm, similar to the previous reports.21 There appeared clear lattice spacings of 0.202 and 0.207 nm (Figure 5c). When stirring in H2O for 48 h and 192 h, the lattice spacings of 0.232 nm and 0.145 nm were observed, 14


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respectively (Figure 5f and i), which correspond to the planes (011) and (111) of hexagonal Ni(OH)2. The nanoparticles for nanosheets in Figure 5b disappeared as stirring time increases (Figure 5e and h) because the nickel hydroxychloride was gradually converted into hexagonal Ni(OH)2.

Figure 6. FT-IR spectra of the as-prepared sample and those after stirring in water for given periods of time.

Fourier transform infrared spectroscopy (FT-IR) is an effective method to detect H2O and OH- structure in compounds. To identify the intercalated species in the interlayer of samples, FT-IR spectra of the samples when stirring for different periods of time were comparatively studied, as seen in Figure 6. For the as-prepared sample, there are five distinguishable absorption bands. The strongest one at 3561 cm-1 is

15



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assigned to ν(O-H) stretching vibration in nickel hydroxychloride. Similar vibration is reported in references.21, 25 The second intense absorption at 754 cm- is associated withδ(Ni-O-H) vibration of nickel hydroxychloride.21 In addition to both strong vibrations, a broad peak centered at 3448 cm-1 came from ν(O-H) stretching vibration of absorbed H2O molecules, and a relatively weak peak at 1640 cm-1 is assigned to the bending vibration of these molecules.28 The weakest shoulder peak at 462 cm-1 is attributed to ν(Ni-O) lattice vibration.28, 29 Distinctly, the bands at 3561 and 754 cm-1 became weaker and weaker with prolonging stirring in H2O. Meanwhile, a sharp absorption band at 3642 cm-1 appeared, which is related to ν(O-H) stretching vibration in Ni(OH)2,28, 30 as assigned for brucite-like structure.29 Moreover, in low wave number region, a strong band at 518 cm-1 associated with δ(O-H) lattice vibration becomes more and more intense.28 The above results verify that OH- species gradually intercalates into the lattice, which is followed by a transformation from nickel hydroxychloride to Ni(OH)2. It should be noted that the peak at 3448 cm-1 gradually shifted towards higher wave numbers, such as 3475 cm-1 for a stirring period of time 192 h, which is followed by a broadening in Cl- deintercalated samples. These changes are related to the appearance of ν(O-H) stretching vibration from the absorbed H2O molecules, but also to the contribution of the interlayer H2O molecules and the hydrogen-bonded OH groups in the interlayer, as reported elsewhere.28, 31 These FT-IR results give a strong proof that Cl- deintercalation happens as followed by OH- intercalation into the lattice, and that H2O molecules could intercalate into the adjacent bilayers with stirring. 16


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Figure 7. (a) TGA and DSC curves measured in given atmospheres for the as-prepared sample and that after water stirring for 192 h. (b) MS spectra of H2O and HCl detected for the exhausted gas during the thermal decomposition of the as-prepared sample. As stated above by FT-IR measurements, some OH- species and H2O molecules exist in the layered structure of samples. The amount of intercalated OH- was further characterized using thermo-gravimetric analysis (TGA) and simultaneous thermal analysis–mass spectrometry (STA–MS). After a slowly continuous decrease of mass (~1.09 wt %) up to 335oC, two obvious mass losses can be clearly seen in TG curve of the as-prepared sample (Figure 7a), which gave a total mass loss of 35.69 wt %. The initial mass loss of ~1.09 wt % is ascribed to the removal of physically adsorbed water molecules.32 The subsequent mass loss of ~8.72 wt % around 415 oC is derived from the dehydroxylation of brucite-like octahedral layers in nickel hydroxychloride. The final mass loss with a large stage of ~25.88 wt % around 738 oC is due to the elimination of Cl- in the form of HCl.33 We confirmed the production of H2O and elimination of Cl- by connected mass spectra in Figure 7b. In MS spectrum of H2O 17



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(m/Z=18), most of H2O was detected at the temperature corresponding to the dehydroxylation process. The MS spectrum of HCl (m/Z=36) has a peak at the temperature related to the final mass loss. When stirring in H2O for 192 h, two similar obvious stepwise weight losses combining with a slowly continuous decrease of mass were observed, as shown in Figure 7a, while the temperatures for two step weight losses decreased significantly when comparing to that prior to the stirring. The mass loss below about 150 oC is attributed to the evaporation of the intercalated and physically adsorbed water molecules.34 The abrupt mass loss of ~17.87 wt % around 296 oC associated with the sharp endothermic peak of DSC curve is due to the decomposition of layered structure to form NiO.35 Another endothermic peak with a small weight loss of 5 wt % around 400 oC is probably caused by stepwise H2O production of dehydroxylation process.33, 35 Based on the mass losses in TG curves, the compositions for the as-prepared sample and that after stirring in H2O for 192 h were estimated to be Ni(OH)0.99Cl1.01 and Ni(OH)2.18(H3O)0.18, respectively, suggesting that 1.19 mol OH- and 0.18 mol H2O were intercalated into the structure when the as-prepared sample underwent a stirring for a given period of time 192 h.

18


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Figure 8. XPS spectra of the as-prepared sample and that after stirring for 192 h: (a) survey, (b) Cl 2p, (c) Ni 2p, and (d) O 1s.

To confirm the variations of chemical composition and oxidation state during the water stirring process, we selected the as-prepared sample and that after stirring in water for 192 h to compare their XPS spectra. For the as-prepared sample, the survey spectrum presents Ni, Cl, C and O elements (Figure 8a), which coincides well with the elemental mapping result. Apparently, Cl 2p peak is too weak to distinguish when stirring for 192 h, implying traces of Cl residues in the material after stirring for long time. Detailed information of Cl 2p orbit shown in Figure 8b reveals that Cl 2p spectrum has two split signals at 198.6 eV and 200.1 eV, which correspond to Cl 2p3/2 and Cl 2p1/2, respectively. The deconvolution for Cl 2p demonstrated that there is one 19



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chemical state of Cl in the origin Ni(OH)0.99Cl1.01. Ni 2p XPS spectra of both samples showed two major signals centered at 856.2 eV and 874.2 eV with an energy separation of 17.7 eV (Figure 8c) which can be assigned to Ni 2p3/2 and Ni 2p1/2, respectively, in good agreement with the reported values previously for Ni(OH)2 phase.36, 37 The presence of two remarkable satellite peaks at 862.2 eV and 879.8 eV in Ni 2p region confirms that Ni ions in both samples are divalent. More structural information can be obtained by O 1s spectra in Figure 8d. The as-prepared sample shows only one photoelectron peak at a binding energy of 531.6 eV, corresponding to the lattice oxygen in Ni(OH)0.99Cl1.01.38 When stirring for 192 h, O 1s region is deconvoluted into two peaks: The first one at 530.9 eV corresponds to the oxygen species bonding with nickel and hydrogen, i.e. Ni-O-H in Ni(OH)2.18(H3O)0.18 lattice.38,

39

The second peak at approximately 532.3 eV is assigned to water

molecules which contain two highly overlapped parts. The absorbed water on the surface of materials has O 1s signal at a relatively high binding energy around 532.3 eV. Previous XPS study about Ni-Fe electrocatalyst proposed that intercalated water within the layered structure shows a photoelectron peak at a binding energy in the range between 531.6 and 533.0 eV.40 Accordingly the peak at 532.3 eV is contributed by intercalated water in the interlayer and by the absorbed water on the surface of Ni(OH)2.18(H3O)0.18.

20


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Figure 9. (a) Ni K-edge XANES and (b) corresponding Fourier-transformed EXAFS spectra of the as-prepared sample and Ni(OH)2.

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were used to examine the coordination environment about the as-prepared sample Ni(OH)0.99Cl1.01. Comparing to Ni(OH)2, the Ni absorption edge of the as-prepared sample (Figure 9a) slightly shifted towards lower energies, which can be explained by larger ionic radius and lower electronegativity of Cl atom relative to O atom that result in a change in the average Ni valence to a lower value.41 Fourier-transformed EXAFS curve of Ni(OH)2 is characterized by two main peaks at 1.93 Å and 3.04 Å (Figure 9b), corresponding to the Ni-O and neighboring Ni-Ni coordinations, respectively.42 In comparison with Ni(OH)2, an obvious peak at 2.30 Å for the as-prepared sample emerged, corresponding to Ni-Cl coordination based on the assignment in literature.43, 44 In addition, both Ni-O bond length (1.84 Å) and Ni-Ni bond length (3.19 Å) of the as-prepared sample showed deviations. All these results can be explained by the introduction of Cl- into the lattice. Because Cl atom has a larger radius and lower electronegativity with a lower ability of attracting 21



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electron than O atom, Ni-Cl hybridization degree is less than Ni-O, which shortens the Ni-O bond length, meanwhile lengthens the Ni-Cl bond length. Moreover, the occupation of Cl atom with larger radius leads to octahedral distortion, which increases the disordering of the lattice. As a result, the length of neighboring Ni-Ni bond increases and the peak intensity of the Ni-Ni decreases. Strikingly, a decrease in peak intensity of the first shell Ni-O and an increase in the second shell Ni-Cl for the as-prepared sample imply a relatively higher amount of Ni-Cl bonds and a smaller amount of Ni-O bonds relative to Ni(OH)2.45 These results are consistent with the theoretical model we proposed above, revealing that direct coordination of Cl- ion to Ni2+ can really bring tailoring of layered structures.

The porous structure and specific surface area of materials have a significant influence on the capacitive performance. From N2 adsorption-desorption isotherms (Figure S3), the profile for the as-prepared sample showed a hysteresis loop, suggesting a mesoporous nature.46 The as-prepared sample exhibited a larger Brunauer–Emmett–Teller (BET) surface area than the contrast sample Ni(OH)2, meaning that the hierarchical flower-like microspheres exploded more accessible active sites to contact with the electrolyte.47

22


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Figure 10. (a) Comparative CV curves of the as-prepared sample and the contrast sample Ni(OH)2 electrodes at a scan rate of 10 mV/s; (b) CV curves of the as-prepared sample recorded at various scan rates; (c) GCD curves of the as-prepared sample recorded at various current densities; (d) Specific capacitance of the as-prepared sample calculated from charge/discharge curves at various current densities; (e) Nyquist plots recorded in a frequency range from 0.01 Hz to 100 kHz for the as-prepared sample and that after stirring for 192 h. The inset shows the impedance in the high-frequency region; (f) Cycling performance of the as-prepared 23



Page 24 of 43

sample at a current density of 10 A/g.

The samples were tested as the active materials for electrodes of supercapacitors, and the relevant electrochemical capacitive properties were evaluated in a three-electrode cell in a 6 M KOH aqueous electrolyte by taking Ni(OH)2 as a reference. In such an alkaline solution, nickel foams may undergo a corrosion to grow Ni(OH)2, which however contributes no capacitance (Figure S5). Cyclic voltammetry (CV) curves of the as-prepared sample and contrast sample Ni(OH)2 at a scan rate of 10 mV/s are depicted in Figure 10a. CV curve for the as-prepared sample electrode did not show a well-defined rectangular redox peaks, similar to the previous reports for Ni-based materials, but gave a pair of typical redox peaks at about 0.02 and 0.35 V, implying an ideal pseudocapacitive behavior that involves the faradaic redox reactions of Ni–OH/Ni–O–OH.48, 49 Comparatively, two anodic peaks were observed in contrast sample Ni(OH)2, owing to the oxidation of β-Ni(OH)2 to β-NiOOH and a phase transition from α-Ni(OH)2 to β-Ni(OH)2, respectively.38, 50 Evidently, the as-prepared sample electrode exhibited a significantly higher specific capacitance than Ni(OH)2 electrode, due to its larger enclosed CV curve area and peak current. Figure 10b shows the CV curves of the as-prepared sample at various scan rates from 5 to 100 mV/s. Redox peaks appeared between -0.1 and 0.5 V with increasing the scan rate, confirming the pseudocapacitive behavior and good electrochemical reversibility of the interface redox reaction.51 Moreover, incomplete redox peaks were observed at high scan rate of 100 mV/s and the peak separation between anodic and cathodic peaks increased with the scan rates, which certifies that the limiting step of the 24


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pseudocapacitive reaction is diffusion-controlled process, as reported elsewhere.52

Figure 10c shows the galvanostatic charge–discharge (GCD) curves of the as-prepared sample at different current densities from 1 to 20 A/g in the potential range of -0.1 to 0.35V. The electrochemical polarization of the electrode material may lead to a potential range deviation between GCD curves and CV curves.48 All charge– discharge curves are nonlinear and asymmetric with a distinct discharge plateau. This phenomenon further supports the pseudocapacitive characteristics that involve a faradaic behavior of the electrode. Based on the discharge curves in Figure 10c, the specific capacitances were calculated via Eqn (1), as illustrated in Figure 10d. For the as-prepared sample, a remarkable specific capacitance of 3831 F/g was obtained at a current density of 1 A/g. The outstanding capacitive performance is superior to most well-designed Ni-hydroxylation materials listed in Table S3. All these indicate the promising applications of the as-prepared sample electrode for energy storage. Even at a high current density of 20 A/g, a capacitance around 1767 F/g can be retained. The capacitance decay with the increase of current densities is mainly due to the incremental voltage drop and insufficient active material involved in the redox reactions at high current densities.52

The electronic and ionic conductivities of the as-prepared sample were characterized by electrochemical impedance spectroscopy (EIS) measurements in frequency range from 100 kHz to 0.01 Hz at open circuit potential with amplitude of 5 mV. As shown in Figure 10e, the Nyquist plots of the as-prepared sample electrode

25



Page 26 of 43

and contrast sample Ni(OH)2 electrode are composed of a linear component in the low-frequency region and a semicircle (inset, Figure 10e) in the high-frequency region. The overall impedance is described as follow,

= +

(3)

where Z is the coverall impedance (Ω), j is the imaginary unit, ω is the angular frequency (Hz), Rs is the solution resistance (Ω), Cdl is the double-layer capacitance (F), and Rct is the charge-transfer resistance (Ω).53 From the diameter of the semicircle in the high-frequency region, the charge transfer resistance (Rct) of the electrode could be determined.51, 54 Comparing to the contrast sample Ni(OH)2 in the insert of Figure 10e, the as-prepared sample showed a smaller semicircle diameter that corresponds to a smaller resistance, suggesting a good ionic response and superior electronic conductivity.55 In the low-frequency region, the straight line presents the Warburg impedance (Zw), corresponding to the diffusive impendence of the electrolyte ions into the electrode surface.51, 54 The straight line for the as-prepared sample in the low-frequency region showed an evidently larger slope than the contrast sample Ni(OH)2 electrode, meaning a lower diffusion resistance and better capacitive behavior. EIS results reveal that intercalated Cl- ions could enhance the electrical conductivity of the layered structure to some extent. Based on the previous theories,56, 57

we propose that the coordinated Cl- ions function as pillars, which stably play a

supporting role between the adjacent two layers so as to provide a large interlayer spacing and wide galleries to promote the diffusion and migration of electrolyte ions

26


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during the charge/discharge process, bringing about excellent capacitive performance.

Then, the electrochemical cycling performance of the as-prepared sample was investigated by galvanostatic charge–discharge measurements for 500 cycles at a high current density of 10 A/g, as shown in Figure 10f. Severe attenuation can be found at high current densities, which gave a continuous decrease with increasing the cycle number. 58.1% specific capacitance retention is presented after 500 cycles. Concerning this, we performed XRD (Figure S6) and EDS (Figure S7) measurements for the as-prepared sample after cycles to explore what causes the capacitance degradation. After long time electrochemical cycles in an strong alkali electrolyte, Clwas deintercalated from nickel hydrochloride followed by OH- intercalation, and then Ni(OH)2 phase gradually produced. In the charge-discharge process, Cl- just went through a deintercalation but did not participate in the redox reaction. The electrochemical reaction can be described as follows: Ni(OH)Cl + OH- ⇌ NiOOH + HCl + e-

(4)

Moreover, the Coulombic efficiency of the as-prepared sample is relatively high, with an average value of 97.7%.

27



Page 28 of 43

Figure 11. (a) Enclosed CV curve area and peak current of the as-prepared sample and those obtained after stirring for given periods of time from comparative CV curves at a scan rate of 5 mV/s; (b) Comparative GCD curves of the electrodes at a current density of 2 A/g for the given samples; (c) Specific capacitance of the electrodes as a function of current density; (d) Nyquist plots of the electrodes recorded in a frequency range from 0.01 Hz to 100 kHz. Inset shows the equivalent electrical circuit. To understand the role of Cl- deintercalation on electrochemical performance, a series of samples with different Cl contents were made into electrodes of supercapacitors and measured using the same three-electrode cell configuration in 6 M KOH aqueous solution. Comparative cyclic voltammetry (CV) curves are shown in Figure S8 measured at a scan rate of 5 mV/s. All samples show a good 28


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pseudocapacitive behavior, totally different from those of electric double-layer capacitors (EDLC).58 According to the comparative CV curves (Figure S5), a histogram was given to clearly exhibit the graphing key indicators in Figure 11a. Among all electrodes, the as-prepared sample with the highest Cl content and the largest interlayer distance showing the largest enclosed CV curve area and the highest peak current, and therefore the greatest capacitive ability. As the histogram clearly shows, both CV curve area and peak current similarly decreased along with the stirring time and reached a minimum for a period of stirring time. After that, a slight increment can be found when the stirring time increase to 96 and 192 h, indicating a little rebound of charge-storage capacity. This abnormal observation is attributed to H2O molecules that have been intercalated into the interlayers, which results in the formation of water bilayers, as supported by XRD and FT-IR analysis.

The galvanostatic charge–discharge (GCD) profiles of this series of electrodes with different Cl contents were performed at a current density of 2 A/g in the potential range of -0.1 to 0.35 V (Figure 11b). All charge curves are asymmetric to their discharge counterparts, showing that the reversible faradaic behavior was not altered by the change of Cl content.37 As expected, the as-prepared sample exhibited the most prolonged discharge time and significantly enhanced specific capacitance when compared to other electrodes. Accordingly, the corresponding specific capacitances derived from the discharge curves at different current densities were shown in Figure 11c. The specific capacitances of the electrodes made by as-prepared sample and those after stirring in water for 3, 6, 12, 24, 48, 96, and 192 h at 1 A/g were 3831, 29



Page 30 of 43

2831, 1737, 1491, 1377, 1194, 1428, and 1489 F/g, respectively. When the current density increased from 1 to 10 A/g, 56.4, 65.2, 81.2, 84.5, 78.2, 74.3, 88.8 and 62.1 % capacitances were retained respectively. The electrode made by the sample after stirring for 96 h has the largest capacitance retention among all the electrodes, despite its relatively low initial specific capacitance, indicating an excellent rate capability. Obviously, the specific capacitance decreases and then increases for the samples with the whole water stirring process. We further compared the specific capacitance for as-prepared sample with that for Ni(OH)2, as shown in Table S2. The interlayer spacing is confirmed to be the dominant factor in influencing the capacitive performance.

EIS data recorded for the samples with different Cl contents are studied, as shown in Figure 11d. All Nyquist plots were analyzed based on the equivalent circuit (inset, Figure 11d). Several samples were selected to investigate the variation of resistance. In the high-frequency region, the intercept of the plots at the real axis represents the equivalent series resistance (Rs), consisting of the ionic resistance of the electrolyte, intrinsic resistance of active materials and the contact resistance between the active materials and current collector.51,

59

Careful data analysis showed that all these

samples have almost the same Rs around 0.45Ω. Similar EIS curves in the higher-frequency region suggest little difference of charge transfer resistances (Rct) among these electrodes. However, the inclined straight lines in the low-frequency region displayed diverse slopes, implying quite different Warburg resistances (Zw). It is worth noting that larger slope corresponds to lower ion diffusion resistance and 30


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faster ion absorption onto the electrode surface, suggesting an ideal capacitive behavior.55,

60

The low-frequency plot slope declines from the maximum for the

as-prepared sample and then has a rise when the stirring time kept increasing. That is to say, the as-prepared sample has the lowest Zw, corresponding to the swiftest ionic diffusion and absorption. In other words, the supporting effect of Cl- can greatly accelerate the activation process of the electrodes.61 With the Cl- deintercalation and OH- intercalation, pillars supporting effect gradually became weak, causing interlayer spacing shrinkage so as to hinder transport of ions. Generally, the charge-discharge behavior involves the migration of protons into and out of interlayer galleries. The electrolyte OH- ions dominate the transport process. But the Cl- deintercalated samples have less space for OH-, which will weaken the transportation of protons. In contrast, the as-prepared sample with largest interlayer galleries are well hydrated, leading to easy access of OH- and, consequently, yield excellent electrochemical activities.62 Moreover, the ion diffusion resistance has a reduction trend, similar to those of of CV and GCD. This could be related to H2O intercalation into the interlayer that forms the water bilayers to facilitate ionic diffusion and further enhance ion diffusion kinetics.

4. CONCLUSION

A series of hierarchical flower-like nickel hydroxychloride microspheres were prepared, and anion de-/intercalation was achieved by a non-electrochemical method to give different content of coordinated Cl- ions. The layered structure maintained

31



Page 32 of 43

during the anion de-/intercalation by H2O stirring process. The interlayer spacing decreases with the deintercalation of Cl- ions. Cl- deintercalation is accompanied by OH- intercalation into the lattice and H2O molecules intercalation into the adjacent bilayers. When firstly employed as electrode materials for supercapacitor, the as-prepared nickel hydroxychloride Ni(OH)0.99Cl1.01 with a maximum Cl- ion content showed an ultrahigh specific capacitance of 3831 F/g at a current density of 1 A/g with a vast potential to be applied for advanced supercapacitors. Electrochemical capacities of these materials decay with the deintercalation of Cl- ions. This finding was explained in terms of the fact that the role of Cl- is defined as pillars and could support the adjacent layers in nickel hydroxychloride, which may provide wide tunnels for fast ion diffusion and facilitate efficient charge transport. This non-electrochemical anions de-/intercalation method could be universal to other anions (NO3-, SO42-, Ac-, etc), bringing diverse properties associated with their different chemical natures. The concept described in this work not only allows one to profoundly comprehend the effect of anion de-/intercalation on charge storage in layered materials, but provides hints in fabricating other multi-anion-containing layered materials for important energy storage applications.

■ ASSOCIATED CONTENT Supporting Information Available: High-magnification SEM images, BET and pore-size distribution, XRD patterns before and after cycles, comparative CV curves, EDS spectrum after cycles, table of atomic ratio, and table of capacitive performance comparisons.

■ AUTHOR INFORMATION 32


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Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We acknowledge the technical assistance of Beijing Synchrotron Radiation Facility (BSRF). We thank Dr. Xiyang Wang for XANES spectra analysis and Dr. Shuaikai Xu for electrochemical support. This work is financially supported by National Natural Science Foundation of China (NSFC) (Grants 21571176, 21611530688, 21771171, 21671077, and 21025104).

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intercalation in Nickel Hydroxychloride Microspheres: A

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