Preparation of NiO nanoflakes under different calcination temperatures and their supercapacitive and optical properties

Accepted Manuscript Title: Preparation of NiO nanoflakes under different calcination temperatures and their supercapacitive and optical properties Author: Jiangshan Zhao Hua Liu Qiang Zhang PII: DOI: Reference:

S0169-4332(16)32008-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.09.128 APSUSC 34057

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APSUSC

Received date: Revised date: Accepted date:

21-5-2016 1-9-2016 25-9-2016

Please cite this article as: Jiangshan Zhao, Hua Liu, Qiang Zhang, Preparation of NiO nanoflakes under different calcination temperatures and their supercapacitive and optical properties, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.09.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation of NiO nanoflakes under different calcination temperatures and their supercapacitive and optical properties Jiangshan Zhao, Hua Liu, Qiang Zhang* School of chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China

ABSTRACT: The NiO nanocrystals were successfully prepared by calcinating Ni(OH)2 precursor synthesized via a facile ion diffusion controlled by ion exchange membrane without adding any solvent or template. X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) isotherm, X-ray photoelectron spectroscopy (XPS) and Ultraviolet–visible (UV-Vis) analysis were used to investigate the crystallinity, morphology, surface and porosity characteristics, chemical composition and optical properties in more detail. The pseudocapacitive behavior of the NiO samples was investigated by cyclic voltammograms (CV) and galvanostatic charge-discharge tests in 2 M KOH. The results analysis reveals that both specific capacitance and surface area decrease with the increase of calcination temperatures. Among the NiO samples, the NiO-400 nanoflakes calcinated at 400 ℃ possess the highest specific capacitance of 381 F g-1 at a current density of 2 A g-1, but much lower than the Ni(OH)2 sample. In addition, the UV-Vis analysis shows that there is a red shift of absorption peak for the three NiO samples with the increasing temperature and the NiO-400 has a broad band gap of 3.3 eV, which renders the material highly interesting for application in photocatalyst.

Keywords: Ion Diffusion; Ion Exchange Membrane; β-Ni(OH)2 Nanoflake; NiO Nanoflakes; Supercapacitors

*Corresponding author: e-mail [email protected], Phone: +8601062131468, Fax: +8601062131468, Address: School of Chemistry, Beijing Institute of Technology, 5 Zhongguancun Street, Haidian District, Beijing 100081, P. R. China

Highlights 1. The NiO nanocrystals were prepared by calcinating Ni(OH)2 nanoflakes synthesized via a facile ion diffusion controlled by ion exchange membrane. 2. The NiO sample calcinated at 400 ℃ exhibits the highest specific capacitance of 381 F g-1 and highest specific surface area of 188.4 m2 g-1. 3. The NiO samples with the lower binding energy are harder to capture OH- than Ni(OH)2 in the process of electrode reaction, which was disadvantageous to charge storage on the electrode. 4. The UV-Vis absorption peak of NiO samples have a red shift with increasing the calcination temperature from 400 to 600 ℃, which is mainly attributed to the defect passivation and increase in crystallinity.

1. Introduction The rapid depletion of fossil fuels, global warming and the deterioration of the environmental pollution are ringing alarm bells to human society in the 21st century. In these circumstances, energy storage and conversion for alternative energy sources have emerged as a key technological challenge with the increasing demands for energy and growing concerns about ecologically sustainable development [1-3]. In the past few years, many efforts have been devoted to develop high performance renewable energy storage and conversion devices, such as lithium-ion batteries (LIBs), electrochemical capacitors (ECs), solar cells, and fuel cells [4-9]. Among the above energy storage/conversion devices, ECs have gained considerable attention owing to their high power and energy density, long and stable cycle life, short charge time, low maintenance cost, minimal safety concerns and environmental benignity [10, 11]. ECs (also called supercapacitors or ultracapacitors) are classified as electric double-layer capacitors (EDLCs) and pseudocapacitors according to different energy-storage mechanisms. EDLCs store charges using reversible adsorption of ions at the electrode/electrolyte interface. Their research has been mainly focused on carbon materials due to low cost, good conductivity and outstanding cycling stability, whereas the specific capacitance of EDLCs is limited by the electrostatic surface ion-adsorption charging mechanism [12, 13]. In contrast, pseudocapacitors store charges by Faradic redox reactions taking place on the surface of the electrode materials that possess various valence/oxidation states and exhibit much higher energy density and capacitance than EDLCs [14]. Transition

metal compounds including oxides/hydroxides, sulfides, and binary metal oxides/hydroxides as well as conducting polymers are widely used as electrode materials for pseudocpacitors [15-19]. Among the materials studied, nickel hydroxides, a typical layered double-hydroxide-type material with two well-identified polymorphs (α and β), represents a promising choice, owing to its high specific capacity, low cost and toxicity, easy preparation, and environmental friendliness [20, 21]. As an excellent electrode material, Ni(OH)2 has been widely investigated because of its higher specific capacitance [22-25], whereas the poor chemical and thermal stability impede its application in industrial production. NiO is a p-type semiconductor with a wide band gap of 3.2-3.8 eV between the valence and conduction bands [26]. It is one of the most promising metal oxides used in photocatalysis, electrochemistry and electrochromics [27]. As the thermal decomposition product of nickel hydroxide, nickel oxide shows much higher thermal stability and also has most the merit of nickel hydroxide, and thus draws more attention of scientists than nickel hydroxide. Usually, the electrochemical performance of NiO materials is greatly dependant on their morphology, porosity, specific surface area, electrical conductivity, ionic transport, etc. [28]. In this context, structures of zero-dimensional (0D) to three-dimensional (3D) NiO mesoporous nanomaterials with different morphologies such as nanoparticles (0D) [29, 30], nanofibers [31], nanorods [32], nanotubes (1D) [33], nanoflakes [34], nanosheets [35] and nanofilms (2D) [36, 37], nanoflowers [38], nanospheres [39, 40], nanosheets arrays grown on substrates [41] and hierarchical mesoporous roselike nanosheets (3D)

[42] have been prepared via various methods including electrodeposition, hydrothermal, electrospun, vapor deposition, sacrificial template, chemical bath deposition, sol-gel, microwave-assisted synthesis, layer-by-layer self-assembly method and solvothermal method. The typical specific capacitances of the electrodes made by these nanostructured NiO materials range from 91.43 to 1860 F g-1, which are still far from theoretical specific capacitance of 2573 F g-1, suggesting the low electrochemical utilization of nickel oxide materials. There are many works in the synthesis of Ni(OH)2 and NiO nanomaterials. However, most of the above synthetic methods usually require high temperature and pressure, expensive equipment, complex operations or templating agents. In other cases, the products show poor performance, which restricts their practical applications. In this study, we synthesized Ni(OH)2 nanoflakes via a fast, facile, low cost and environmental friendly ion diffusion method controlled by ion exchange membrane. The as-synthesized Ni(OH)2 nanoflakes then were calcined at different temperatures to obtain NiO samples. According to the report in the literature, the NiO nanoflakes can enhance the diffusion of electrolyte and provide more paths for diffusion of ions leading to improvement in the electrochemical performance of the material. The NiO nanoflakes also have many other applications such as catalysts, battery electrodes, gas sensors, electrochemical films and photoelectronic devices [43, 44].

2. Experimental 2.1. Materials and Methods Nickel nitrate (Beijing Tong Guang Fine Chemical Company, 98％) and sodium

hydroxide (Beijing Chemical Works, 96％) were all analytical reagent and used as received without any further purification. Deionized water and absolute ethyl alcohol were used as the solvent in all experiments. The Ni(OH)2 nanoflakes were prepared with the method reported in our previous work [45]. In a typical synthesis, 50 mL of 0.5 M nickel nitrate and 50 mL of 1.0 M sodium hydroxide were prepared respectively, and the ion diffusion reaction carried out under the water bath condition and was kept at 70 ℃ for 12 h. The obtained product was repeatedly washed with deionized water and ethanol, and then was dried at 70 ℃ overnight. The NiO samples were obtained by annealing the Ni(OH)2 precursor at 400, 500, 600 ℃ for 2 h with a rising rate of 10 ℃ min-1 in an air atmosphere and denoted as NiO-400, NiO-500, and NiO-600 respectively. For example, NiO-400 denoted the product calcined at 400 ℃ for 2 h. 2.2. Characterization The crystal structure was analyzed with an X-ray powder diffraction (XRD, ULTIMAIV RIGAKU) in the range of 10-90° with a scan rate of 20 °/min. Thermal degradation research was investigated by thermal gravimetric analysis (TG-DTA, Shimadzu DTG-60). The morphology was observed by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2100). The specific surface area and distribution of pore diameter of samples were performed on N2 adsorption-desorption test with MICROMERITICS INSTRUMENT CORP ASAP 2460 specific surface area and pore size distribution instrument. The chemical state of elements on materials surface was

characterized by X-ray photoelectron spectroscopy (XPS) which was recorded on a XPS system (PHI QUANTERA- Ⅱ , ULVAC-PHI, INC., Japan) using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). 2.3. Electrode Preparation and Electrochemical Measurement The nickel foams (0.5 mm thick) were used as the current collector for electrodes. The working electrode was prepared by mixing 80 wt % active materials (Ni(OH)2 or NiO), 15 wt % acetylene black and 5 wt % polytetrafluoroethylene (PTFE), in ethanol and stirring to form slurry. This slurry was coated onto nickel foam substrate with an area of 1 cm-2 and the weight of the loading electrode active materials was about 5 mg. Finally, the electrodes were dried at 80 ℃ for 12 h and then pressed at 10 MPa. Cyclic voltammetry (CV) and chronopotentiometry were carried out using a CHI 760 E electrochemical workstation and the electrolyte used was a 2 M KOH aqueous solution. The electrochemical cell included a three electrode system where platinum foil served as a counter electrode and saturated calomel electrode (SCE) as a reference electrode.

3. Results and discussion 3.1. Structure and morphology

100 95

-20

DTA

90 TG (%)

-10

-30 TG

85

-40

DTA (μV)

endorthermic

0

80 -50 75 0

200

400

600

-60 800

T (℃)

Fig. 1. TG-DTA curves of uncalcined sample. The thermal behavior of β-Ni(OH)2 nanoflakes was investigated by TG-DTA analysis in air atmosphere from ambient temperature to 800 ℃. As shown in Fig. 1, a three-step weight loss, room temperature (RT)-210, 210-285 and 285-500 ℃ was observed from the TG curve. The first minor weight loss occurred between RT and 210 ℃ corresponds to the evaporation of absorbed water. The major weight loss of ~15% occurred between 210 and 285 ℃ is an indication of decomposition of crystalline β-Ni(OH)2 to NiO [46]. The DTA curve shows an endothermic peak with a maximum located at 285 ℃. The temperature range of the endothermic peak in the DTA curve fits well with that of major weight loss in the TG curve, corresponding to endothermic behavior during the decomposition of crystalline β-Ni(OH)2 to NiO. The third weight loss of about 4.6% occurred between 285 and 500 ℃ can be assigned to a continuous thermal depletion of the amorphous Ni(OH)2. The total weight loss between 210 and 500 ℃ is measured to be 19.6%, which corresponds well with the

expected weight loss of 19.4% for the dehydroxylation of Ni(OH)2 to NiO, so 400 ℃ is chosen as initial calcination temperature to ensure the complete decomposition of

(311) (222)

(220)

(200)

(111)

β-Ni(OH)2.

d

30

50

60

70

(202)

(102)

40

(200) (103) (201)

20

(110) (111)

10

(101)

(001)

(100)

c b

80

a 90

2θ (degree)

Fig. 2. XRD patterns of the as-synthesized Ni(OH)2 sample (a) and samples calcined at different temperatures (b NiO-400, c NiO-500 and d NiO-600). The XRD pattern of as-prepared Ni(OH)2 sample is shown in Fig. 2a. The diffraction peaks at 19.2°, 33.0°, 38.0°, 52.1°, 59.5°, 62.6°, 69.0°, 70.2°, 72.7° and 83.0° correspond to (001), (100), (101), (102), (110), (111), (200), (103), (201) and (202) planes of hexagonal β-Ni(OH)2 (JCPDS 14-0117). No other impurity peaks confirms that pure Ni(OH)2 is obtained. The XRD patterns of samples NiO-400, NiO-500, and NiO-600 are shown in Fig. 2b, c, d. All the diffraction peaks are indexed to the cubic phase of NiO. The peaks at 37.0°, 43.2°, 62.8°, 75.4°, and 79.6° correspond to the (111), (200), (220), (311), and (222) planes of NiO (JCPDS

65-5745). No peaks related to β-Ni(OH)2 are observed, showing that β-Ni(OH)2 is completely converted to NiO. The diffraction peaks of NiO crystals become stronger and narrower obviously with increasing calcination temperature, indicating that the crystallinity of NiO becomes higher.

Fig. 3. The FESEM images of Ni(OH)2 nanoflake and NiO nanoflakes obtained at different calcination temperatures (a Ni(OH)2, b NiO-400, c NiO-500, d NiO-600). It is acknowledged that β-Ni(OH)2 is isostructural with brucite and consists of closely stacked 2D Ni(OH)2 principle layers [47]. Fig. 3a shows that the accumulated platelets with a thickness of 15 nm and length of about 100-300 nm. The NiO-400 and NiO-500 samples exhibit the same morphology as the Ni(OH)2 sample. However, the flake structure of NiO-600 sample becomes incomplete and conglomerating, which indicates that serious aggregation phenomenon occurs during the higher calcination

temperature, resulting in the decrease of surface area. The larger holes are generated at 600 ℃, which can be attributed to the crystallization and crystal growth of NiO.

Fig. 4. TEM images of NiO samples at different magnifications (a, b) NiO-400, (c, d)

NiO-500, (e, f) NiO-600. Insets of (b, d, f): the corresponding SAED patterns, high-resolution TEM (HRTEM) micrographs of the sample NiO-400 (g). The detailed structural and morphological features of the three NiO samples are confirmed by TEM measurements. Fig. 4a shows the low-magnification image of sample NiO-400 constituting of stacked nanoflakes. The magnified image clearly reveals that the length of nanoflakes is around 200 nm and the diameter of little mesopores marked by the black arrow is about 2 nm. No obvious morphologic changes for sample NiO-500 are observed from Fig. 4c, and Fig. 4d shows the existence of mesopores in the accumulated nanoflakes. Fig. 4e presents the agglomeration of sample NiO-600 with a lack of lamellar structure. Fig. 4f exhibits that the diameter of irregular pores of sample NiO-600 are much larger than those in samples NiO-400 and NiO-500. As shown in Fig. 4f, it is also found that some larger holes (as directed by the arrow) are formed in the sample NiO-600, owing to the re-crystallization under high temperature. The selected-area electron diffraction patterns reveal that the diffraction rings of sample NiO-400 change to bright spots of sample NiO-600, indicating the improvement of crystallinity with the increase of calcination temperature. In Fig. 4g, the HRTEM image of NiO-400 clearly demonstrates polycrystalline structure of the sample, and the obtained lattice fringes with the spacing about 0.24 nm, 0.21 and 0.15 nm correspond to the (111), (200) and (220) planes of cubic crystalline NiO, respectively. All the TEM results are consistent with aforementioned XRD and SEM results.

0.04 0.03 0.02 0.01 0.00

100

0 20 40 60 80 100 120 140 pore diameter (nm)

50

Adsorption Desorption

250 200 150

0.4 0.6 0.8 Relative Pressure (P/P0)

0.01 0 20 40 60 80 100 120 140 pore diameter (nm)

Adsorption Desorption

50

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

140

0.02

Quantity Absorbed (cm3 g-1 STP)

c

300 dV/dD (cm3 /g·nm)

Quantity Adsorbed (cm3 g-1 STP)

0.02

0.00

1.0

350

150

0.03

0 0.2

200

0.04

100

0

250

b

0.05 dV/dD (cm3/g·nm)

0.05

300

0.01

0.00

100

0 20 40 60 80 100 120 140 pore diameter(nm)

50

Adsorption Desorption

0 0.2

0.4

0.6

0.8

120 100 80 60 40

d

0.012 dV/dD (cm3 / g·nm)

150

Quantity Adsorbed (cm3 g-1 STP)

200

dV/dD (cm3 / g·nm)

Quantity Adsorbed(cm3 g-1 STP)

0.06

250

350

a

300

0.010 0.008 0.006 0.004 0.002 0.000

0

20 40 60 80 100 120 pore diameter (nm)

20

Adsorption Desorption

0

1.0

0.2

Relative Pressure (P/P0)

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Fig. 5. Nitrogen adsorption-desorption isotherms of Ni(OH)2 (a), NiO-400 (b), NiO-500 (c), NiO-600 (d). Insets: the corresponding BJH pore size distribution curves calculated from desorption branch. Table 1 Physical property of four samples. Surface area, SBET (m2 g-1)

Samples

Average pore diameter, AP (nm)

Total pore volume, Vtot (cm3 g-1)

Ni(OH)2

117.5

15.9

0.47

NiO-400

188.4

11.3

0.53

NiO-500

110.5

17.0

0.47

NiO-600

46.3

17.3

0.20

To

characterize

the

specific

surface

area

and

porosity,

nitrogen

adsorption-desorption

isotherm

experiments

were

conducted.

The

absorption-desorption curves and the corresponding Barrette-Joynere-Halenda (BJH) pore size distribution are shown in Fig. 5. All four isotherm profiles can be categorized to type IV with a hysteresis loop observed in the relative pressure range of 0.5-0.85. With increasing calcination temperature, the hysteresis loop shifts to higher relative pressure, indicating larger mesopores. The BJH pore size distribution curves indicate the existence of abundant mesopores in these NiO samples. Table 1 exhibits the results of the measured BET specific surface area and the BJH pore size distribution of the four samples. As shown in Table 1, Ni(OH)2 has a large specific surface area of 117.5 m2 g-1, NiO-400 has the largest specific surface area of 188.4 m2 g-1, while NiO-600 has the smallest specific surface area of 46.3 m2 g-1, due to their structural difference. For NiO-400, the largest surface area comes from the assembly of loosely stacked nanoflakes. The higher specific surface area is favorable for enhancing the electrolyte-material contact area and providing large active reaction area, which is beneficial for electrochemical properties. The increase in pore size and decrease in pore volume with calcination temperature increasing from 400 to 600 ℃ are consistent with SEM and TEM results.

Ni2p

O1s

Intensity (a.u.)

d c b a

850

860

870

880

Binding Energy (eV)

d

Intensity (a.u.)

Ni2p1/2

Ni2p3/2

c b a 526

528

530

532

534

Binding Energy (eV)

Fig. 6. XPS of Ni2p and O1s for Ni(OH)2 nanoflakes prepared (a) and NiO-400 (b), NiO-500 (c) and NiO-600 (d), respectively. XPS is an effective method of surface chemical analysis, which is used to probe more detailed elemental species and chemical states of the as-prepared samples, and the spectra are illustrated in Fig. 6. The Ni2p signal could be deconvoluted into five peaks in the range of 850 to 880 eV. As shown in Fig. 6a, the peaks centered at 853.5, 855.0, and 860.1 eV are attributed to Ni2p3/2, and the peaks located at 872.2 and 878.1 eV are attributed to Ni2p1/2. The peaks of the NiO samples obviously shift to the lower binding energy compared with that of Ni(OH)2 sample, which is due to the oxygen vacancy existing on the surface after calcinations [48]. It is observed that Ni2p peak becomes sharper and the intensity of Ni2p peak increases with elevating the annealing temperature. The deconvoluted O1s core level spectrum shows three peaks at binding energies of 529.1, 530.5 and 531.9 eV (Fig. 6a). The peaks at lower binding energies of 529.1 and 530.5 eV can be ascribed to O2- in octahedral symmetry (bulk O) associated with NiO species and OH- related with Ni(OH)2 respectively [49]. Whereas, the higher binding energy peak (531.9 eV) is ascribed to the H-O-H bond of

residual water [50]. However, the peak at 530.5 eV disappears when Ni(OH)2 transformed into NiO samples, and the new peaks at 527.6 eV can be assigned to bridging O in NiO crystals [51]. The ratio of the area of the peak at 529.1 eV versus the peak at 527.6 eV is 1.38 for NiO-400 and is only 0.72 for NiO-600, indicating that the bridging O increases and the bulk O decreases with the increase of calcination temperature, which will reduce the active sites and specific surface area. There is a little shift to the higher binding energy for both Ni2p and O1s peaks of sample NiO-600 compared with that of samples NiO-400 and NiO-500, which may be attributed to the formation of trace Ni2O3 species. 3.2. Optical properties of NiO samples

0.25

1.0

0.15

a

b c

(Ahν)2/ (eV)2

Absorbance (a.u.)

a 0.20

b 0.5

c 0.0

0.10

2

3 hν /(eV)

4

500 λ /(nm)

600

700

0.05 0.00 200

300

400

800

Fig. 7. Optical absorption spectrums and (Ahν)2–hν curve (inset) for NiO nanoflakes (a-NiO-400, b-NiO-500, c-NiO-600). The band gap is a significant parameter in determining the characteristics of semiconductor and nanomaterials employed in solar industries, which is often

determined from the UV absorption spectrum. Fig. 7 shows the optical absorption spectrum of the three NiO samples dispersed in deionized water. The spectrum shows an absorption peak at 294 nm in the UV region for sample NiO-400, which is assigned to the band gap absorption. There is a red shift and decrease in absorption peak intensity with increasing calcination temperature from 400 to 600 ℃, since higher calcination temperature allows the structure of NiO to reorganize, passivates the crystal defects and increases the crystallinity of NiO samples [Fig. 2], leading to a progressive decrease in band gap [52, 53]. The optical band gap (Eg) can be calculated based on the optical absorption spectrum using the following equation

(Ahν)n= B(hν-Eg)

(1)

Where A is the absorbance, hν is the photo energy, B is a constant relative to the material and n is 2 or 1/2 for direct and indirect transitions, respectively [54]. For NiO, n = 2, since it is a direct band gap semiconductor [55]. The inset of Fig. 7 shows the (Ahν)2 versus hν curve of the direct transitions for the samples. The optical band gap of the absorption peak is obtained by the intersection of the extrapolated linear portion. Eg is found to be 3.30, 3.23 and 3.09 eV for samples NiO-400, NiO-500 and NiO-600, respectively, confirming their potential employment as a photocatalyst. 3.3. Supercapacitive properties of Ni(OH)2 and NiO nanoflakes

40

30 1 mV/s 2 mV/s 4 mV/s 8 mV/s 16 mV/s

20 10 0 -10 -20

10 0 -10

a

-30

2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s

20

Current density (A g-1)

Current density (A g-1)

30

-20

b

-30 0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

Potential (V)

0.4

0.5

15

20

2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s

10

Current density (A g-1)

Current density (A g-1)

0.3

Potential (V)

0

-10

c

-20 0.0

0.1

0.2

0.3

0.4

0.5

2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s

10 5 0 -5

d

-10 0.0

0.1

Potential (V)

0.2

0.3

0.4

0.5

Potential (V)

Fig. 8. CV curves of Ni(OH)2 and NiO samples at various scan rates (a, Ni(OH)2, b, NiO-400, c, NiO-500, d, NiO-600). Apparently, well-defined redox reaction peaks within 0-0.5 V (vs. SCE) are visible in all CV curves, which indicates that the electrochemical capacitance of the Ni(OH)2 or NiO electrode is distinct from that of a pure electric double-layer capacitor having an ideal rectangular shape of CV curve. The appearance of these redox peaks is involved in the following Faradaic redox reactions [56, 57]: Ni(OH)2 + OH- ↔ NiOOH + H2O + e- (a) NiO + zOH- ↔ zNiOOH + (1- z)NiO + ze- (b) With the increase of scan rate, the potentials of the oxidation and reduction peaks

move toward more positive and negative directions respectively, which can be attributed to the strengthened concentration polarization as the scan rate scales up [58]. The potential difference between oxidation peaks (EO) and reduction peaks (ER) from CV curves can be used as a measure of reversibility of the electrode reaction. The smaller the ΔE (EO- ER) value, the more reversible the electrode reaction will be and vice versa [59]. As shown in Fig. 8, ΔE value is 0.23 and 0.12 V for Ni(OH)2 and NiO respectively. The lower ΔE value of NiO electrodes compared with Ni(OH)2 electrode shows better electrochemical reversibility. Gravimetric specific capacitance of Ni(OH)2 and NiO electrodes can be obtained from CV following the equation

CS 

 idu

2vmu

(2)

Where Cs is the specific capacitance (F g-1), i denotes the instantaneous current response in cyclic voltammogram (A), u is the instantaneous potential, v is the scan rate (V s-1), ∆u is the potential range, and m is the deposited weight of the Ni(OH)2 or NiO material on the electrode for the unit area (1 cm-2) dipped in electrolyte. The specific capacitance values of Ni(OH)2 was calculated to be 1740 F g-1 at a scan rate of 1 mV s-1, and was reduced to 816 F g-1 at a scan rate of 8 mV s-1. The specific capacitance values of NiO-400, NiO-500 and NiO-600 electrodes were 365, 230 and 105 F g-1 at a scan rate of 2 mV s-1, and were reduced to 204, 165 and 84 F g-1 at a scan rate of 50 mV s-1 respectively. The capacitance of both Ni(OH)2 and NiO samples decreases with the increase of scan rates，because the growing number of OH-

is not able to diffuse timely to the inner space of electrode materials to react with the inner active sites.

Potential (V vs.SCE)

0.4

4 A g-1 8 A g-1

0.3

12 A g-1 16 A g-1 20 A g-1

0.2 0.1

a

0.0 0

100

200

300

Potential (V vs.SCE)

0.5

0.5

0.4

2 A g-1 4 A g-1

0.3

6 A g-1 8 A g-1 10 A g-1

0.2 0.1

b

0.0 0

400

50

100

Time (s)

200

0.5

0.4

2 A g-1 4 A g-1

0.3

6 A g-1 8 A g-1 10 A g-1

0.2 0.1

c

0.0 0

20

40

60

80

100

Potential (V vs.SCE)

0.5

Potential (V vs.SCE)

150

120

0.4

2 A g-1 4 A g-1

0.3

6 A g-1 8 A g-1 10 A g-1

0.2 0.1

d

0.0 0

10

20

Time (s)

30

40

50

60

Time (s)

e

100 Capacitance retention (%)

250

Time (s)

80 60

Ni(OH)2@4 A g-1

NiO-400@2 A g-1

40 20 0 0

200

400 600 Cycle number

800

1000

Fig. 9. Galvanostatic charge–discharge curves of (a) Ni(OH)2, (b) NiO-400, (c) NiO-500 and (d) NiO-600; (e) Cycle performance of the Ni(OH)2 and NiO-400

electrode at a current density of 4 and 2 A g-1 respectively. To get more information about their potential application in supercapacitors, galvanostatic charge-discharge measurements were carried out in 2 M KOH solution between 0 and 0.5 V (vs. SCE) at various current densities ranging from 2 to 20 A g-1. As shown in Fig. 9, the typical CP plots exhibit good platforms, which further illustrates their pseudocapacitance nature and are quite consistent with the result of CV curves in Fig. 8. The specific capacitance of the electrode materials can be also calculated from the charge-discharge curves according to the following equation:

Cm =

I∗∆t m∗∆V

(3)

where Cm (F g-1) is the specific capacitance, I (A) is the charge-discharge current, Δt (s) is the discharge time of a cycle, m (g) is the mass of the active material, ΔV (V) is the potential window. The specific capacitance values obtained from charge-discharge studies are 1724 F g-1 for Ni(OH)2 electrode at a current density of 4 A g-1 and 381, 210, 111 F g-1 for NiO-400, NiO-500, and NiO-600 electrodes at a current density of 2 A g-1 respectively. The specific capacitance values of NiO-400 and NiO-500 samples are higher than 167 F g-1 reported in the previous paper [60]. The NiO-400 exhibits the highest capacitance among the three NiO samples, which can be attributed to its highest specific surface area. The specific capacitance as well as specific surface area decrease with the increase of calcination temperature. It can be seen that the Ni(OH)2 sample exhibits much higher specific capacitance and current density than NiO samples, which can be attributed to different reaction principles as shown in reactions (a) and (b). The cycling performance of both Ni(OH)2 and NiO-400 is evaluated (see

Fig. 9e). When cycled at 4 A g-1, Ni(OH)2 electrode retains 79.0% of the initial SC after 1000 cycles, and when cycled at 2 A g-1, the NiO-400 retains 89.2% of the initial SC after 1000 cycles, confirming their excellent cyclic stability. This higher capacitance retention implies that the as-synthesized Ni(OH)2 and NiO nanoflakes are suitable material for supercapacitor application. The stoichiometric number of “Z” in reaction (b) represents the electrochemical utilization of NiO electrodes. The reaction (b) suggests that only a fraction of the active nickel sites are involved in the redox reaction (i.e., when Z = 1; the entire electroactive material is involved in the redox process). The value of Z for NiO samples can be calculated from the following equation [61]

Z=

𝐶𝑀∆𝑉 𝐹

(4)

Where C is the specific capacitance value (F g−1) at 2 A g-1, M is the molar mass (74.7 g mol-1), ΔV is the potential window, and F is the faradic constant (96,500 C mol-1). The calculated “Z” values of NiO-400, NiO-500 and NiO-600 are 0.1475, 0.0813, and 0.0430, respectively, which means that about 14.8% of active nickel sites are involved in the redox reaction for NiO-400, and only 8.1% and 4.3% do for NiO-500 and NiO-600. It is well known that the higher valence state corresponds to the higher binding energy, so the binding energy will shift to the higher values when one element is oxidized. According to the above XPS results, the lack of highly electronegative oxygen atoms would lower the binding energy of Ni. It is harder to capture OH- for NiO with the lower binding energy compared with Ni(OH)2 in the process of

energy storage, so most of the NiO species are not involved in the redox reaction, which reduces the effective collision and decreases the utilization of NiO. For this reason, optimization of preparation methods and conditions of NiO is still being carried out, and NiO-based composite materials are also being researched to improve the reaction activity and conductivity of NiO for further increasing its capacitance.

4. Conclusions In summary, three NiO samples have been prepared through calcination of the as-synthesized β-Ni(OH)2 at 400-600 ℃ for 2 h in air. The electrochemical test demonstrates that the as-prepared mesoporous NiO-400 nanoflakes has an excellent specific capacitance of 381 F g-1 owing to its highest BET surface area, but the capacitance of NiO samples is much lower than Ni(OH)2 sample. It is concluded that Ni2+ species with the lower binding energy in NiO samples are harder to capture OHcompared with that in Ni(OH)2, which is disadvantageous to the charge storage, resulting in the lower electrochemical utilization of the active component, thus leading to the lower specific capacitance of NiO. In addition, the UV-vis spectrum of NiO samples shows that there is a red shift with the increase of calcination temperature owing to the defect passivation and increase in crystallinity, and NiO-400 has a wide energy gap of 3.3 eV.

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