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Composition-Dependent Pseudocapacitive Properties of SelfSupported Nickel-Based Nanobelts Jun Song Chen,*,†,‡ Song Peng Huang,† and Daniel John Blackwood*,† †

Department of Materials Science and Engineering, National University of Singapore, Singapore 117574, Singapore State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of People’s Republic of China, 610054 Chengdu, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: In this work, we have developed a system to prepare selfsupported nickel-based nanobelts of similar morphology but varying chemical compositions. After a simple hydrothermal treatment, phase-pure selfsupported nickel sulfate hydroxide (Ni(SO4)0.3(OH)1.4) nanobelts can be directly prepared on a nickel foam substrate, which can be converted into NiO nanobelts by calcination without any change in the morphology. Alternatively, the pristine Ni(SO4)0.3(OH)1.4 nanobelts can be transformed into Ni3S2 nanobelts via a facile hydrothermal sulfidization. When these nanobelts were applied in supercapacitors they demonstrated contrasting performances, with the Ni3S2 nanobelts showing a high reversible capacitance of about 3.5 F cm−2 at 10 mA cm−2 over 1000 cycles, while the Ni(SO4)0.3(OH)1.4 sample delivered insignificant capacitance. This huge difference in electrochemical activity relates to dramatically different amounts of charge being stored within their bulks, with charge and discharge rates limited by the bulk ionic resistance rather than interfacial charge transfer resistance. Overall it is concluded that chemical composition has a more dominant role than morphology in determining the physicochemical properties of these materials.

1. INTRODUCTION Supercapacitors are an important component in the development of high-performance energy storage devices to tackle the ever increasing global energy demand.1−3 Based on how the charges are stored, supercapacitors can be generally divided into two types.1,4,5 The first type, known as electrochemical doublelayer capacitors (EDLCs), stores charge at the surface of the electrodes by electrostatic interaction. Porous carbons are usually used as electrode materials for EDLCs because of their high surface area allowing for the adsorption of a large amount of charge.5,6 The second type are called pseudocapacitors, where the charges are stored by fast redox reactions in a wide range of metal-based compounds such as oxides or hydroxides.1 Because of the unique mechanism, pseudocapacitors present a greater energy storage capability than EDLCs, as well as hold the promise of closing the energy density gap with batteries.1 Among all the possible electrode materials for pseudocapacitors, nickel-based compounds have attracted the most attention because of their abundance, environmental friendliness, and good thermal and chemical stabilities.7 This is a very large family consisting of many different chemical compositions such as nickel hydroxide,8−15 nickel oxide,16 nickel sulfides,17−40 as well as nickel-containing mixed oxides and sulfides.41−47 Even though these materials have demonstrated promising performance in various applications such as batteries, supercapacitors, and electrochemical catalysis, there appear to be no prior studies comparing the pseudocapacitive properties of nickel-based compounds with different chemical phases, © XXXX American Chemical Society

which could be due to the difficulty of achieving the desired chemical composition while maintaining the same morphology of the materials. In order to better understand the composition-dependent properties of nickel-based compounds, we developed a system to synthesize self-supported nickel-based nanobelts with different chemical compositions. After a simple hydrothermal treatment, self-supported nickel sulfate hydroxide (Ni(SO4)0.3(OH)1.4) nanobelts can be directly grown on a nickel foam substrate. These nanobelts can then be transformed into NiO by a heat treatment without any change in the morphology. Alternatively, the pristine Ni(SO4)0.3(OH)1.4 nanobelts can also be converted into a phase-pure Ni3S2 counterpart in situ, by a simple hydrothermal sulfidization process. When these nickel-based nanobelts are applied for supercapacitor application, they exhibited distinct performances: the Ni3S2 sample demonstrates a high capacitance of about 3.5 F cm−2 at a current rate of 10 mA cm−2 for 1000 cycles, while the NiO showed a much poorer performance, and the pristine Ni(SO4)0.3(OH)1.4 nanobelts delivered insignificant capacitance. Such a set of contrasting electrochemical activities are primarily dependent on the different compositions, giving rise to a dramatically different charge transfer kinetics. Received: January 15, 2017 Revised: March 16, 2017 Published: March 20, 2017 A

DOI: 10.1021/acs.jpcc.7b00452 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION Material Synthesis. The pristine Ni(SO4 ) 0.3 (OH)1.4 sample was synthesized by a simple hydrothermal method. The substrate Ni foam was first cleaned with ethanol and DI water by ultrasonication for 10 min. Briefly, 0.237 g of NiSO4· 6H2O precursor was added into 30 mL of aqueous solution. After full dissolution, the reaction medium was transferred into a polytetrafluoroethylene-lined stainless steel autoclave, and a piece of cleaned Ni foam with a size of 2 cm × 2 cm was placed into the solution. The solution was then kept at 150 °C for 20 h, after which it was naturally cooled to room temperature. The sample was then flushed with a copious amount of DI water and ethanol and dried at 60 °C overnight. NiO nanobelts were obtained by calcining as-prepared Ni(SO4)0.3(OH)1.4 sample at 400 °C in air for 2 h with a ramping rate of 2 °C·min−1. Ni3S2 nanobelts were synthesized by putting the Ni(SO4)0.3(OH)1.4 sample into 30 mL of aqueous solution containing 0.1 g of thioacetamide and under a hydrothermal system at 180 °C for 20 h. Material Characterization. The morphology of the samples was analyzed using a Zeiss Field Emission Scanning Electron Microscope (FESEM) equipped with an energydispersive spectroscope and a JEM2010F transmission electron microscope (TEM). The crystallographic information was obtained using a Bruker D8 Advancer (Cu Kα, λ = 1.54 Å). The chemical state of different elements was studied using highresolution X-ray photoelectron spectroscopy with a Kratos Axis UltraDLD X-ray photoelectron spectrometer equipped with an Al Kα X-ray source (1486.6 eV). All peaks were referenced to C 1s at 284.5 eV. Raman spectroscopy was performed using a LabRam HR Evolution Raman microscope with excitation via an Ar ion laser at 514 nm. Electrochemical Measurements. The electrochemical tests were conducted on a Bio-Logic SP-200 electrochemical workstation. The electrolyte was 1 M NaOH aqueous solution, and the counter and reference electrode are Pt and standard calomel electrode (SCE), respectively. The cyclic voltammetry was conducted with a voltage window of 0−0.8 V (vs SCE) at different scan rates, while the galvanostatic charge−discharge tests were performed under various current rates. Electrochemical impedance spectroscopy (EIS) was carried out after charging at open-circuit potential with a superimposed 5 mV sinusoidal (root-mean-square) perturbation over the frequency range from 100 kHz to 0.01 Hz.

Figure 1. Scanning electron microscopy (SEM; A, B), transmission electron microscopy (TEM; C) and high-resolution (HR) TEM (D) images of the Ni(SO4)0.3(OH)1.4 nanobelts. (E) shows the element mapping of the sample.

elements, Ni, O, and S, can be observed, suggesting a uniform chemical composition of the sample. Crystallographic information on the sample was gathered by X-ray diffraction (XRD) (Figure S1A), and all the nonsubstrate peaks can be attributed to monoclinic Ni(SO4)0.3(OH)1.4 phase (JCPDS card No.: 41-1424, Space Group: P, a = 7.89 Å, b = 2.96 Å, c = 13.63 Å). In thermogravimetric analysis (TGA) (Figure S1B), some weight loss occurred before 200 °C due to the evaporation of moisture content,51 while the major weight loss between 200 and 400 °C of about 1.17 wt % can be associated with the decomposition of the Ni(SO4)0.3(OH)1.4.51,52 From the TGA data, we understand that the pristine Ni(SO4)0.3(OH)1.4 nanobelts can be completely decomposed by 400 °C, and we thus calcined the sample at this temperature in order to get NiO nanobelts. From the SEM images in Figure 2A it can be seen that the morphology of the nanobelts is well retained after the heat treatment, and they remain in good contact with the Ni foam substrate. Under a higher magnification (Figure 2B), the 1D structure is still intact without any rupture along the longitudinal axis. Some broadening of the nanobelts on the transversal plane can be observed, leading to the increased width of 400 nm to several micrometers. Under TEM observation, the calcined nanobelts are shown to have a highly

3. RESULTS AND DISCUSSION Self-supported Ni(SO4)0.3(OH)1.4 nanobelts were first synthesized on a Ni foam substrate by a simple hydrothermal method, with the morphology displayed in Figure 1. Consistent with previous reports,48−50 the sample demonstrates a structure that consists of belt-like ribbons tens of micrometers in length that form densely packed bundles on the substrate. Under a higher magnification (Figure 1B), these nanobelts are shown to have a width of about 100−200 nm and a thickness of a few tens of nanometers. Such an isotropic one-dimensional (1D) structure is confirmed under transmission electron microscopy (TEM; Figure 1C), with the high-resolution (HR) TEM image (Figure 1D) depicting some visible lattice fringes with an interplanar distance of about 0.5 nm, which corresponds to the (102) crystal plane of the Ni(SO4)0.3(OH)1.4 phase. From localized element mapping (Figure 1E), even distributions of the major B


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Figure 2. SEM (A,B) and TEM (C) images, XRD pattern (D), X-ray photoelectron spectrum of Ni 2p (E), and Raman spectrum (F) of NiO nanobelts. The inset in C shows a magnified image of this sample. The asterisks in D mark the peaks due to the Ni foam substrate, and the two blue circles indicate the peaks due to NiO. The standard pattern of NiO is shown at the bottom of panel D.

Figure 3. SEM (A−C) images and XRD pattern (D) of Ni3S2 nanobelts. The asterisk in D marks the peak due to the Ni foam substrate.

that are widely reported for NiO-based materials.54,55 The “shoulder” peaks at the higher binding energies are known as “nonlocal satellites”,56 but their exact origin is still controversial.56 These could be due to the formation of Ni2O3 on the surface of the sample54,55 or the presence of structure defects.56 The Raman spectrum of the calcined nanobelts is depicted in Figure 2F. Consistent with a previous report of Luo et al.,57 two absorption peaks are present at 538 and 1088 cm−1, due to the Ni−O oscillation.57 The above data confirm that the pristine nanobelts have been transformed into NiO after the heat treatment. Alternatively, the as-prepared Ni(SO4)0.3(OH)1.4 nanobelts can be facilely converted into Ni3S2 by a simple hydrothermal

porous structure (Figure 2C, inset), the pores possibly being generated by the removal of the OH− and SO4− groups. No distinct peaks due to the NiO phase could be observed by XRD (Figure 2D). Such a phenomenon was also reported previously for Ni foam-supported NiO obtained by calcining the nickel oxalate precursor by Yang et al.,53 being attributed to the low crystallinity of the as-obtained nanobelts after the heat treatment. The lack of XRD data meant that other characterizations were required to confirm the NiO phase. X-ray photoelectron spectroscopy (XPS) was used to evaluate the valence state of Ni 2p (Figure 2E). It is apparent that the two main peaks at about 855 and 873 eV can be deconvoluted into the two smaller peaks C


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Figure 4. Electrochemical analysis: cyclic voltammetry (CV) of the Ni3S2 nanobelts (A) and the corresponding areal capacitance calculated from these CVs (B). Discharge curves of the Ni3S2 nanobelts at different current densities (C), and the corresponding areal capacitance calculated from these curves (D). The initial ten charge−discharge cycles of the Ni3S2 nanobelts at a current density of 10 mA cm−2 (E) and the long-term cyclic performance at 10 mA cm−2 of the same sample (F). In B, D, and F, the performances of the three samples, Ni(SO4)0.3(OH)1.4 (I), NiO (II), and Ni3S2 (III) nanobelts, are compared.

sulfidization process33 without causing significant changes to their structure. As shown in Figure 3A, the array structure is well retained after the chemical transformation and remains firmly attached to the surface of the substrate. With a closer look (Figure 3B), the 1D nanobelts are still intact, and no apparent ruptures can be observed. At an even higher magnification (Figure 3C), some wrinkle-like structures appear on the surface of the nanobelts. A similar morphology was also observed during the formation of nickel sulfide on other substrates.22 The XRD pattern of the sample is shown in Figure 3D, and all the diffraction peaks can be assigned to rhombohedral Ni3S2 (JCPDS No.: 44-1418, S. G.: R32, a0 = b0 = 5.7454 Å, c0 = 7.1350 Å). No peaks due to other phases can be identified, suggesting high phase purity for the sample after the sulfidization treatment. The energy-dispersive spec-

troscopy (EDX) results of the major elements from these three samples on Ni foam substrates are listed in Table S1, confirming that the calcination removed the sulfate to produce nickel oxide and the hydrothermal sulfidization treatment successfully resulted in a nickel sulfide. The electrochemical properties of the three samples were then evaluated in a three-electrode system. Cyclic voltammograms (CVs) of the Ni3S2 nanobelts at various sweep rates in 1 M NaOH electrolyte are presented in Figure 4A, whereas those of the pristine Ni(SO4)0.3(OH)1.4 and NiO nanobelts are included in Figure S2. As shown in these figures, all three samples have very similar CV profiles, with a pair of peaks associated with a reversible redox couple located at 0.25 V vs SCE (negative scan) and 0.45 V vs SCE (positive scan), despite the fact that the three samples have very different current D


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quite stable for the rest of the test. As expected, the pristine Ni(SO4)0.3(OH)1.4 sample exhibits a very low capacitance for the entire test. In order to understand the reason for the dramatically different supercapacitor performances of the three samples despite their similar morphologies, electrochemical impedance analysis was carried out in 1 M NaOH electrolyte at the opencircuit potential in the charged state, with the profiles displayed in Figure S4. It can be observed that these samples exhibit similar impedance spectra, with a depressed semicircle in the high frequency range and a steep up-sloping line in the low frequency domain. These spectra can be modeled using the same equivalent circuits shown as insets in Figure S4A, with all the fitting data for each circuit element listed in Table S3. It can be discovered that the Ni3S2 sample exhibits the lowest leakage resistance RL and charge transfer resistance RCT, implying the highest ionic conductivity and most efficient redox reaction among the three samples. This leads to its highest CPEDL (i.e., a constant phase element representing the double-layer capacitance arising from the storage of charges on the surface of the Ni3S2 film) value of 1.64 F cm−2, while the NiO and Ni(SO4)0.3(OH)1.4 samples show higher RL and also lower CPEL (denotes the capacitance originated from the Faradaic charge transfer processes)8 of 0.96 and 0.14 F cm−2, respectively. Such a trend is consistent with the discharge data presented in Figure 4F. It is worth noting that both CPEL and RL are more than 2 orders of magnitude larger than CPEDL and RCT, suggesting that the majority of the charge is stored in the bulk rather than the surface of the electrode and that the charge and discharge rates are limited by the bulk ionic resistance rather than the interfacial charge transfer resistance. Considering that all three samples have very similar belt-like 1D nanostructure morphologies, the reason for the contrasting pseudocapacitive performance is because of their distinct chemical compositions, giving rise to very different charge transfer kinetics. This ultimately leads to the best performance of Ni3S2 with the most efficient redox reactions and the insignificant electrochemical activity of the pristine Ni(SO4)0.3(OH)1.4 with a charge transfer resistance almost an order of magnitude higher.

densities. The direct use of the pristine Ni(SO4)0.3(OH)1.4 phase for electrochemical application is rare,58 and the mechanism of the Faradaic reactions displayed in Figure S2A has not been explicitly discussed. However, for NiO59 and Ni3S2,22 the redox couple involves the oxidation of Ni(II) to Ni(III) and can be attributed to the following reactions, respectively NiO + OH− ↔ NiOOH + e−

(1)

Ni3S2 + 3OH− ↔ Ni3S2 (OH)3 + 3e−

(2)

This feature suggests the pseudocapacitive properties of the three samples. The areal capacitance of the three samples can be obtained from the corresponding CV curves using the equation60,61 C=

1 Av(Va − Vc)

∫V

Vc

I(V )dV

(3)

a

where A is the area of the electrode; v is the scan rate; and (Va − Vc) is the voltage window of the scan. Figure 4B shows the areal capacitance values for the three samples, from which it is very clear that the Ni3S2 sample possesses the highest capacitance at all scan rates. Even at the highest scan rate of 50 mV s−1, it also shows a high capacitance of about 1 F cm−2, while the NiO nanobelts exhibit a lower capacitance of 0.6 F cm−2. The pristine sample demonstrates almost no electrochemical activity at all scan rates. The CV curves of the bare nickel foam were presented in Figure S3A, and the corresponding capacitance was calculated in Figure S3B, showing that the substrate produced negligible capacitance at these scan rates. This suggests that the high capacitance of the as-prepared sample is originated from the Ni3S2 active material. Figure 4C depicts the discharge curves of Ni3S2 nanobelts at different current rates, with the equivalent curves of the other two samples shown in Figure S2. The areal capacitance can again be calculated using the following equation22 (4)

C = I × Δt ÷ ΔV −2

where I is the current density in mA cm ; Δt is the discharge time in s; and ΔV is the voltage window in V. As displayed in Figure 4D, both the Ni3S2 and NiO nanobelts show very similar capacitances at these current rates, with the former being typically 20% higher, while the pristine Ni(SO4)0.3(OH)1.4 has a charge storage capability that is only about 10% of the other two compounds. All the detailed capacitance values of the three samples are listed in Table S2 for an easy comparison. Figure 4E illustrates the charge−discharge voltage profiles of a Ni3S2 sample for a few initial cycles 10 mA cm−2, and a high Coulumbic efficiency of more than 80% can be calculated by dividing the discharge capacitance by the charge capacitance.44 The long-term charge−discharge cycling performances of the three samples are presented in Figure 4F. It is apparent that the Ni3S2 nanobelts have the highest capacitance among the three samples. It starts with a capacitance of about 2.5 F cm−2, which gradually increases to more than 3 F cm−2 in the course of the first 500 cycles and continues to increase and reaches about 3.5 F cm−2 at the end of 1600 cycles. The initial increase in capacitance is commonly observed in supercapacitor analysis,22 possibly being due to an activation process where more active sites become available because of the structural change induced by the charge−discharge processes. For the NiO nanobelts, its initial capacitance of about 1.8 F cm−2 drops dramatically to below 1 F cm−2 during the first 400 cycles, after which it stays

4. CONCLUSION In summary, we have developed a hydrothermal method to synthesize self-supported Ni(SO4)0.3(OH)1.4 nanobelts on Ni foam substrates. With additional chemical modifications, the pristine sample can be facilely converted into NiO and Ni3S2 with negligible change in the structure. When these 1D materials are applied as electrode materials for supercapacitors they demonstrated contrasting pseudocapacitive performance: at a high current rate of 10 mA cm−2, the Ni3S2 nanobelts exhibit a very high reversible capacitance of about 3.5 F cm−2 for 1000 charge−discharge cycles, while the pristine Ni(SO4)0.3(OH)1.4 sample shows insignificant electrochemical activities with a capacitance lower than 0.5 F cm−2. Such different properties can be attributed to their distinct chemical phases, which give rise to diverse bulk charge storage abilities, leading to their vastly different supercapacitor performance. Based on these data, it can be concluded that the chemical composition has a more dominant role than morphology in determining the physicochemical properties of the materials. E


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00452. XRD pattern, TGA data, EDX results, CV curves and capacitances, charge−discharge profiles, EIS spectra, and the fitted data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun Song Chen: 0000-0003-0116-8481 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP Award No. NRFCRP 10-2012-6).

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REFERENCES

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