3D Carbon Coated Tree-Like Ni3S2 Superstructures on Nickel Foam

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Energy, Environmental, and Catalysis Applications

3D Carbon Coated Tree-Like Ni3S2 Superstructures on Nickel Foam as Binder-Free Bifunctional Electrodes Xiong Nie, Xiangzhong Kong, Dinesh Selvakumaran, Linzhen Lou, Junrong Shi, Ting Zhu, Shuquan Liang, Guozhong Cao, and Anqiang Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13813 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Applied Materials & Interfaces

3D Carbon Coated Tree-Like Ni3S2 Superstructures on Nickel Foam as Binder-Free Bifunctional Electrodes Xiong Nie,† Xiangzhong Kong,† Dinesh Selvakumaran,† Linzhen Lou,† Junrong Shi,† Ting Zhu,†, * Shuquan Liang,† Guozhong Cao,†, ‡ Anqiang Pan†,* †

School of Materials Science and Engineering, Central South University, Hunan,

410083, China ‡

Department of Materials Science and Engineering, University of Washington, Seattle,

98195, USA * Corresponding authors: [email protected] (A.Q. Pan); [email protected] (T. Zhu)

1

ACS Paragon Plus Environment

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ABSTRACT Three-dimensional (3D) nanostructures are commonly endowed with numerous active sites, large specific surface area and good mechanical strength, which makes them as an efficient candidate for energy storage and conversion. Herein, by considering the advantages of 3D nanostructures we successfully fabricated carbon coated nickel sulfide on nickel foam (C@NS@NF) with a unique 3D tree-like superstructures via a two-step hydrothermal process. In virtue of its hierarchical superstructures, 3D tree-like architecture and carbon shell encapsulation, the as-fabricated carbon coated Ni3S2 can be directly served as binder-free bifunctional electrodes for supercapacitor and hydrogen evolution reaction (HER), where high specific areal capacitances (6.086 F cm-2 at 10 mA cm-2) for supercapacitors and low overpotential (92 mV at 10 mA cm-2) for electrocatalyst have been demonstrated. These inspiring results of this material makes it as a potential candidate for energy storage and conversion.

KEYWORDS: Ni3S2, Tree-like superstructures, Binder-free, Supercapacitors, Hydrogen evolution reaction

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1. INTRODUCTION With the ever-growing energy demand and impending energy crisis, renewable energy utilization has drawn tremendous attentions over the last decade.1 Storage and conversion of intermittent renewable energy have been regarded as a promising way to alleviate the energy crisis.2 Among various strategies for energy storage and conversion, supercapacitor is treated as one of the most promising devices because of their great advantages including low cost, long cycle lifetime and high power density.3-4 Another appealing strategy is the hydrogen evolution reaction (HER), and the most interesting aspect of HER is unlike other chemical reactions, it does not harm the atmosphere by releasing greenhouse gases.5-6 Owing to the above mentioned advantages, a growing scale of efforts have been devoted to exploring low-cost, environmentally friendly and high-performance electrode materials for both supercapacitors and HER.7-11 3D nanomaterials have gained increasing interests in energy storage and conversion in recent years, because they not only possess large specific surface areas, but also maintain good mechanical stability.12-14 In addition, with spatial configuration of numerous subunits, the 3D structures can further enhance its physicochemical and catalytic properties by providing more active sites and charge transfer channels.15-16 Meanwhile, transition metal based sulfides (TMSs) such as nickel sulfides (NiSx) have attracted extensive research interests owing to their low cost, earth abundance, environmental compatibility and good electroactivity.17-18 Till now, various TMSs with 3D structural features and chemical compositions have been prepared to achieve better electrochemical/catalytic properties.19-21 For instance, nanocages

materials22-23,

and

hollow

boxes

nanomaterials24

3


exhibit

better


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supercapacitive performance compared to their bulk counterparts. Wang et al. synthesized highly durable nickel sulfide microspheres on NF, with a low overpotential for HER.25 Besides, our group previously reported a 3D multi-shell cobalt

sulfides/oxides

that

exhibited

good

electrochemical

performance.26

Nevertheless, the synthesis of 3D nanostructures still remains an enormous challenge due to the tedious and complicated manufacturing process.27 Additionally, the 3D nanostructures also suffer from poor mechanical stability and low conductivity, which hampers their future applications. Building the carbon coated 3D hierarchical nanostructures on conductive backbones could be an effective strategy to address the above issues, since these conductive backbones not only offer advantageous structural stability but provide large surface area.28-30 In addition, the carbon coated active materials with 3D hierarchical structures can serve as conductive additive-free and binder-free electrodes for electrochemical reactions.31 Herein, we designed a facile hydrothermal approach to fabricate a 3D tree-like carbon coated Ni3S2 supported on NF as binder-free electrodes for supercapacitor and HER applications. The 3D tree-like Ni3S2 nanostructures were first grown on NF by a facile solution strategy. Subsequently, the carbon coated 3D Ni3S2 nanostructures were obtained through a second hydrothermal reaction with glucose as the carbon source. With the unique 3D hierarchical structure and enhanced electrical conductivity after carbon coating, the as-fabricated electrode exhibited high specific areal capacitance (6.1 F cm-2 at 10 mA cm-2), remarkable rate capability, outstanding cycling stability, and great energy density (52.9 µWh cm-2). Moreover, it also revealed a low overpotential (92 mV at a current density of 10 mA cm-2) for HER with a long-term durability (32 h).

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2. EXPERIMENTAL SECTION Synthesis of Tree-Like NS@NF. Tree-like Ni3S2 nanostructures on NF were synthesized by a hydrothermal strategy. In a typical synthesis, a piece of NF (1 × 3 cm2) was submerged in a Teflon-lined stainless steel autoclave containing a 30 mL of aqueous solution and a specific amount of thioacetamide (TAA, C2H5NS). Subsequently, the autoclave was sealed and heated at 120 oC for 12 h. After cooling down to room temperature (RT), the resultant sample was rinsed with deionized water and ethanol before drying in a vacuum oven at 50 oC for 12 h. Synthesis of C@NS@NF. A piece of the as-prepared NS@NF was dipped fully into 30 mL of glucose aqueous solution and the solution was heated at 180 oC for 6 h, followed by a similar rinsing process and drying process as mentioned above to accomplish the carbon coating process. The samples obtained from the glucose dosages of 5, 10, 15 g/L are marked as C@NS@NF-5, C@NS@NF-10 and C@NS@NF-15, respectively. Besides, their mass loading records during the synthesis process were shown in Table S1. Materials Characterizations. X-ray diffraction (XRD, Rigaku D/max 2500, Cu Kα radiation, λ = 0.1518 nm) was employed to characterize the crystalline structures. Raman tests were conducted by a Hariba LabRAM HR spectrometer. A scanning electron microscope (SEM, Quanta FEG 250) equipped with energy dispersive spectrometer

(EDS)

and

transmission

electron

microscope

(TEM,

Nova

NanoSEM230) were used to detect the morphologies and compositions. In order to evaluate the carbon content, thermogravimetric analysis (TGA, NETZSCH STA 449C) was conducted. X-ray photoelectron spectroscopy (XPS, ThermoFisher-VG Scientific, Britain) was performed to study the crystallographic structures. 5



Electrochemical

Measurements

for

Supercapacitors.

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Electrochemical

measurements for supercapacitors were carried out in a three-electrode system in a 2 M KOH aqueous solution using a CHI 760D electrochemical workstation. The as-prepared materials (1 × 1 cm2 nominal planar area) were directly served as working electrodes, saturated calomel electrode (SCE) and platinum foil (1 × 1 cm2) were used as the reference and counter electrode, respectively. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were conducted. The specific areal capacitance of the electrode was calculated from the GCD curve according to the equation11:

Ca =

I∆t A∆V

(1)

, where Ca (F cm-2) is the specific areal capacitance, I (mA) is the current density, ∆t (s) is the discharge time, A (cm2) is the electrode area and ∆V (v) is the discharge potential window. The as-obtained samples and activated carbon (AC) were assembled to the asymmetric supercapacitor (ASC) devices in a fresh 2 M KOH aqueous solution. The mass loading of AC electrode was adjusted based on the charge balance (q+ = q-) between two electrodes. Thus, the mass ratio would follow the equation below: m+ / m- = (C- × ∆V-) / (C+ × ∆V+ )

(2)

, where m+, m- represent the mass loading of active materials of cathode and anode, C+, C- are the specific capacitances of cathode and anode, and ∆V+, ∆V- refer to the operating voltage windows for cathode and anode. The GCD and CV profiles were obtained in a potential range of 0~1.5 V. Besides, the C@NS@NF-10 electrode and AC electrode were cut into desired size and fabricated into a R2032 coin cell using 6


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porous glassy fiber paper as the separator.

Electrochemical Measurements for Hydrogen Evolution Reaction. The electrocatalytic measurements including linear sweep voltammetry (LSV), EIS and chronoamperometry (j-t) were performed in a 1 M KOH aqueous solution by a three-electrode system, using SCE and carbon rod as the reference and counter electrode, respectively. The mass loading of the commercial Pt/C coating on NF was ~0.4 mg. The as-synthesized NS@NF, C@NS@NF-5, 10 and 15 were directly served as the working electrodes. LSV was measured at the scan rate of 5 mV s-1, and the Tafel slope was obtained on the basis of the LSV curve. EIS for HER was conducted at the potential of 150 mV vs. RHE (reversible hydrogen electrode) with a frequency range from 0.01 Hz to 100 kHz. The cycle durability of the C@NS@NF-10 electrode was tested by the chronoamperometry at an overpotential of 150 mV for 32 hours.

3. RESULTS AND DISCUSSION Figure 1 shows the schematic illustration of the preparation of 3D C@NS@NF. First, the Ni cations released by the inducement of TAA decomposition reacted with S2- anions to form nickel sulfide species at the interface of NF and solvent. With the reaction time going on, the 3D tree-like superstructures were finally formed by the force of recrystallization process. Inspired by a previous report about the growth mechanism of tree-like Ni3S232 and Ni3Se2,33 we assume that the growth mechanism of tree-like architectures may involve three periods. Firstly, NF acted as both nickel source and substrate, while TAA served as both sulfide source and ligand. Next, Ni ion released from NF would be absorbed by TAA to form the corresponding Ni-TAA 7



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complex.32 Subsequently, due to the weak interaction between TAA and Ni during the heating process, Ni ion would react with the S ion released from TAA to generate Ni3S2 nuclei. Finally, the Ni3S2 nuclei would grow along with the favored direction leading to the main trunks formation of the Ni3S2 trees.32 In addition, with the reaction time increasing, some Ni3S2 particles would grow along with other inferior preferred directions. Thus, the branches would be generated along with the trunks and the 3D tree-like nanostructures were finally formed.33 The tree-like Ni3S2 were further coated by a thin carbon layer by a subsequent hydrothermal process. The carbon content can be controlled by different dosages of glucose.

Figure 1. Schematic illustration of the preparation of 3D C@NS@NF.

Figure

S1

displays

the

XRD

patterns

of

NS@NF,

C@NS@NF-5,

C@NS@NF-10 and C@NS@NF-15. The well-defined peaks at 21.7°, 31.1°, 37.8°, 49.7° and 55.2° can be indexed to (101), (110), (003), (113) and (122) crystal planes of Ni3S2 (JCPDS no. 44-1418),34 while the typical peaks at 44.5°, 51.8° and 76.4° can be ascribed to (111), (200) and (220) crystal planes of nickel (JCPDS no. 04-0850).35 By comparing the XRD pattern of the NS@NF with the patterns of other samples, no obvious peaks of carbon were detected on the samples before and after carbon coating, 8


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which may due to the amorphous as well as the low content of carbon. In addition, there are no other peaks, implying the high purity of the products. For further studying the crystal structures of as-prepared samples, the Raman spectra of NS@NF, C@NS@NF-5, C@NS@NF-10 and C@NS@NF-15 were obtained, as illustrated in Figure S2. All the Raman spectra show two evident peaks at 301 cm-1 and 347 cm-1, which can be indexed to the typical vibration of Ni3S2.36 Additionally, the spectra of the samples after carbon coating process, presents another two peaks at 1330 cm-1 and 1590 cm-1, which can be indexed to the D and G bands of coated carbon. Through the above Raman spectra comparison between NS@NF and other samples, we can conclude that carbon was successfully coated on the NS@NF after carbon coating process. The structures and morphology of as-synthesized composites were investigated by SEM, TEM and HRTEM. As displayed in Figure 2a, the NF was densely covered by tree-like Ni3S2 after hydrothermal process. A higher magnification image (Figure 2b) demonstrates the details of the as-prepared NS@NF-10, where the Ni3S2 tree-like superstructures were evidently observed to be vertically supported on the NF. Figure 2c shows the SEM image of the tree-like superstructures after carbon coating process. No obvious structural destructions were observed, indicating the robustness of the as-formed 3D hierarchical structures. A typical TEM image (Figure 2d) clearly shows the branches of the tree-like structures. Figure 2e further reveals that the branches of the trees are composed of the ultrathin nanoflakes. A HRTEM image of the sample is displayed in Figure 2f, where the typical lattice spacing distance of 0.18318 nm, 0.23 nm can be found and indexed to the (113), (003) crystal planes of Ni3S2, respectively. In order to see the carbon shell more clearly, TEM test was conducted for the C@NS@NF-10 with an image shown as Figure S3. As

9



revealed by the TEM result, the Ni3S2 branch was clearly encapsulated by the carbon shell whose thickness is around 6 nm.

Figure 2. SEM (a-c), TEM (d and e) and HR-TEM (f) images of NS@NF (a and b) and C@NS@NF-10 (c-f).

Additional SEM images of C@NS@NF-5, 10 and 15 are displayed in Figure 3 with corresponding EDS results. Apparently, the tree-like structures of all the three samples are well retained after the carbon coating. It should be noted that the tree-like superstructures of C@NS@NF-15 are shortened and composed of small building blocks after carbon coating, which could be caused by a thickest carbon coating among all the three samples. Moreover, the carbon contents on their surfaces for C@NS@NF-5, 10 and 15 are 8.42%, 15.36% and 21.34%, respectively. To evaluate the weight percent of carbon in the as-prepared samples, TGA was conducted with the results displayed in Figure S4. Evidently, the TGA curve of the NS@NF shows a flat curve, because Ni3S2 is quite stable in air when temperature is lower than 600 oC.37 While all the other carbon coated samples exhibited a weight loss before 350 oC which may be caused by organic molecules and adsorbed water. Subsequently, they 10


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also experienced a sharp weight loss around 400 oC, which could be assigned to the oxidation of the carbon.38 Thus, the carbon contents of the C@NS@NF-5, 10 and 15 calculated from the curves are 1.03 %, 2.07 % and 2.92 %, respectively. These results show that the carbon contents of as-synthesized samples could be controlled by the concentrations of added glucose.

Figure 3. SEM images (a, b, d, e, g and h) and EDX patterns (c, f and i) of the C@NS@NF-5 (a-c), C@NS@NF-10 (d-f) and C@NS@NF-15 (g-i).

XPS was conducted to study the surface element compositions of as-synthesized C@NS@NF-10. The survey spectrum is illustrated in Figure 4a, apart from the peaks of Ni, S, C and O, no other elemental peaks were observed. As shown in the Ni 2p spectrum (Figure 4b), the XPS spectra of Ni 2p are well fitted with two spin-orbit doublet peaks as well as two shakeup satellite peaks (Sat.). The two peaks observed at 11



binding energies of 855.3 eV and 872.9 eV are typical peaks of Ni2+, and the other two peaks located at around 857 eV and 873.9 eV are particularly assigned to Ni3+ ions. Besides, at the high binding energy sides of Ni 2p1/2 and Ni 2p3/2, there are two satellite peaks of nickel.35, 39

Figure 4. XPS spectra of the C@NS@NF-10: Survey spectrum (a), Ni 2p spectrum (b), C 1s spectrum (c) and S 2p spectrum (d).

As displayed in the C 1s spectrum (Figure 4c), it consists of four parts. Two main peaks at binding energies of 284.5 eV and 285.2 eV are typically characteristic of sp2 and sp3 of carbon. The other two weak peaks centered at 286.8 eV and 288.3 eV are ascribed to carbon atoms in different oxygen functional group: C-O and C=O. The reduced level of oxygen functional groups indicates a considerable de-oxygenation of glucose during the hydrothermal reaction.39 Moreover, the existence of C 1s spectrum again proves that the carbon was successfully coated on the surface of 3D tree-like Ni3S2 superstructures. Figure 4d displays the core region spectrum of S 2p region, 12


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with peaks centered at 162.3 eV and 163.6 eV ascribed to S 2p3/2 and S 2p1/2, respectively. Another S 2p peak at around 168.3 eV is the satellite peak of sulfur, indicating the surface of sulfur species at particular higher oxidation states.11

Figure 5. CV curves obtained at 20 mV s-1 (a), linear fitting of Ip vs. v1/2 curves for the redox peaks (b), areal and specific capacitances at various current densities (c), Ragone plots (d) and cycling performances at 80 mA cm-2 (e) of electrodes fabricated from the as-prepared samples.

To investigate the electrochemical properties of these materials, they were assembled into a three-electrode configuration for measurements. First, in order to evaluate the electrochemically active surface area of these samples, double layer capacitances (Cdl) were tested in a non-faradaic region. The Cdl values of NS, C@NS@NF-5, C@NS@NF-10 and C@NS@NF-15 calculated from their respective CV curves (Figure S5) are 25.7, 19.2, 24.1 and 15.8 mF cm-2, respectively (carbon contribution excluded40-41). Based on the mass loadings of active materials and reported calculation method,42 the ECSA values of these samples were estimated to be 10.7, 8.0, 10.0 and 6.6 m2 g-1, respectively. Figure 5a demonstrates the CV curves of these samples at the same scan rate of 20 mV s-1 in a potential window of -0.1~0.7 V 13



Page 14 of 34

(vs. SCE). These CV curves display redox peaks during the cathodic and anodic sweeps, owing to the reversible faradic redox reaction of Ni2+/ Ni3+ with the help of the alkaline electrolyte. The possible reaction is shown below43-44: Ni3 S2 +3OH- ↔ Ni3 S2 (OH)3 +3e-

(3)

The enclosed area of the NS@NF is the smallest among these samples, implying its inferior electroactivity. This may result from the poor electron conductivity which weakens the capacity of Ni3S2. On the contrary, the current densities of other samples are much higher, indicating the advantages of the introduced carbon species. Inferred from the CV curves of NS, C@NS@NF-5 and C@NS@NF-10, with the increase of added glucose, the related enclosed area was enlarged. This may be caused by the enhancement of electron and ion transfer rate, generated by in-situ coated carbon shell. Nevertheless, superfluous carbon coating would enlarge the distance of electron transfer (recall the structural change in Figure 3g and h), therefore, the capacitance of C@NS@NF-15 is lower than that of C@NS@NF-10. The CV curves are illustrated in Figure S6, from which the redox peaks can maintain their shapes at high scan rates, indicating its rapid charge/discharge response. This phenomenon can be attributed to the uniform tree-like morphology, which greatly enlarges the specific area of the samples, and the substrate, NF, offering a channel for electron transfer. As shown in Figure 5b, the linear correlation between peak current densities and the corresponding square root of scan rates, reveals the great reversibility of diffusion-controlled reactions. In addition, the fitting line of the C@NS@NF-10 owns the biggest slope, suggesting its best reversibility among the obtained samples. Besides, Figure S7 demonstrates the Nyquist plots of the NS@NF, C@NS@NF-5, C@NS@NF-10 and C@NS@NF-15 electrodes. Derived from the points that cross with the horizontal axis, their equivalent series resistance (ESR) values are 0.51, 0.45, 0.41 and 0.43 Ω, 14


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respectively45. In addition, the charge transfer resistance (Rct) is quite small, because no evident semicircles could be observed46. Furthermore, at low-frequency range, the linear part related to the diffusion limitation shows a typical capacitive behavior.47 The C@NS@NF-10 shows the biggest slopes in the low frequency regions and the lowest ESR, indicating that the C@NS@NF-10 has the best electrical conductivity among all the samples. The discharge curves of the obtained samples at various current densities ranging from 10 mA cm-2 to 80 mA cm-2 are illustrated in Figure S8. The corresponding specific areal capacitances are illustrated in Figure 5c. The capacitance of the NS@NF (1.868 F cm-2 at 10 mA cm-2) is much smaller than that of other carbon coated samples owing to its poor conductivity. The C@NS@NF-10 delivers a higher capacitance (6.086 F cm-2) than C@NS@NF-5 (4.534 F cm-2) and C@NS@NF-15 (4.540 F cm-2) at 10 mA cm-2, which is consistent with the analysis of CV curves. Better capacitance retentions can be achieved by introducing carbon shell, which can be concluded from Figure S9. For the C@NS@NF-10, high specific areal capacitance of 6.086, 4.878, 4.268, 3.600, 3.100 and 2.432 F cm-2 can be achieved at the current densities of 10, 15, 20, 30, 50, 80 mA cm-2, respectively. Particularly, the capacitance of C@NS@NF-10 (2.432 F cm-2) is much higher than that of NS (0.496 F cm-2) at 80 mA cm-2, which can be ascribed to the carbon shell. Figure 5d compares the Ragone plots of all the samples. The C@NS@NF-10 sample delivers a high energy density of 52.8 µW h cm-2 at a power density of 625 µW cm-2, higher than that of NS (16.2 µW h cm-2), C@NS@NF-5 (39.4 µW h cm-2) and C@NS@NF-15 (39.4 µW h cm-2), respectively, demonstrating the superiority of the C@NS@NF-10 electrode. The long-term cycling stability at 80 mA cm-2 of all the electrodes are displayed in Figure 5e. After 5000 cycles, the capacitance of C@NS@NF-10 still remains 2.109 F cm-2 15



Page 16 of 34

with a capacity retention of 87.4%, indicating a good cycling stability. Other samples demonstrate good cycling durability as well, but deliver lower capacitances, which is consistent with the above discharge curves. Besides, on the cycle curve of the C@NS@NF-15, there was a slight capacitance fluctuation around 2500 cycles which may result from the mass loss of active material during the durability test.48 Table 1 summarizes the electrochemical performance of some previously reported nickel-based materials. The C@NS@NF-10 sample in our research owns outstanding electrochemical behavior as binder-free supercapacitor, indicating its potential of practical applications in energy storage. Table 1. Comparison of specific areal capacitance of the reported nickel-based materials. Electrolyte

I (mA cm-2)

C (mF cm-2)

Reference

3D C@NS@NF

2M KOH

10

6086

This work

CC@NiCo2S4

2M KOH

2.5

633

[49]

Co3O4@NiCo2O4

2M KOH

1

4350

[50]

Ni3S2/CoNi2S4/NF

6M KOH

1.6

1948

[51]

Ni3S2/oxidized NF

3M KOH

10

2370

[17]

ECO-Ni3S2/NF

1M KOH

8

2035

[18]

NiCo2S4/CFP

2M KOH

1

1190

[52]

CoMoO4@CoNiO2

2M KOH

5

5310

[53]

Ni@NiO

6M KOH

8

2000

[54]

Ni3S2@NF

6M KOH

4

2741

[43]

CoxNi1-x(OH)2/NiCo2S4

1M KOH

4

2860

[55]

Materials

16


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To further study the potential application of C@NS@NF-10 electrode, we fabricated an ASC device using AC as anode and C@NS@NF-10 as cathode. Figure S10 illustrates the supercapacitor performance of the AC electrode. The shape of the CV curve is almost a rectangular, revealing the typical capacitive behavior of AC.56 According to the GCD results of AC and C@NS@NF-10, the loading mass of AC is 8.1 mg while the loading mass of active materials is 6.0 mg. Additionally, the specific capacitance is calculated on the basis of the entire mass of as-prepared materials and AC. Figure 6a illustrates the energy storage mechanism of the C@NS@NF-10//AC supercapacitor. During the charging process, the C@NS@NF-10 cathode can store energy through the oxidation reaction of Ni3S2, while the AC anode can accumulate electrons via cation adsorption. During the discharging process, the electrodes can release energy by the reduction reaction and ion desorption. Besides, EIS for all the samples fitting for the asymmetric device configuration was tested to further ensure that the C@NS@NF-10 electrode is the best candidate among all the as-prepared samples for ASC and the corresponding Nyquist plots were demonstrated in Figure S11. An equivalent circuit (inset of Figure S11) fitting for the EIS plots consists of Rs, Rct, Cdl and Warburg impedance (W). From the Nyquist plots, the X-intercept at high frequency corresponds to the Rs related to the electrochemical process while the semicircle diameter represents the Rct corresponding to the mass transfer process.57-58 Noticeably, the C@NS@NF-10//AC delivers a low Rs value (1.67 Ω) which is smaller than

the

NS@NF//AC

(2.41

Ω),

C@NS@NF-5//AC

(2.31

Ω),

and

C@NS@NF-15//AC (1.68 Ω). Meanwhile, the C@NS@NF-10//AC exhibits the lowest Rct value (0.17 Ω). Therefore, the C@NS@NF-10//AC is the best sample among all the as-fabricated device for ASC due to the low resistance of C@NS@NF-10 and the smallest contact resistance between electrodes and substrate. 17



Figure 6. Depiction of charge storage mechanism (a), CV curves at various scan rates (b), GCD curves at various current densities (c), specific capacitances calculated from different current densities (d) and cycling performance at a current density of 100 mA cm-2 of the C@NS@NF-10//AC (e). The inset in (e) shows an optical picture of the C@NS@NF-10//AC system.

To find the suitable voltage window for the device, CV curves at different voltage windows were tested as shown in Figure S12. There is an obvious polarization observed in the CV curve when voltage higher than 1.5 V, while there is no distinct increment of enclosed CV area then the voltage window enlarged to 1.6 18


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V.24 Thus, the optimum voltage window for the as-prepared ASC device was determined to be 0-1.5 V. Figure 6b displays the CV curves of the device at various scan rates with a potential window from 0 to 1.5 V. These curves exhibit the typical features of both pseudocapacitance and double-layer capacitance. Besides, Figure 6c shows the GCD curves of the ASC device, which demonstrates almost linear variation in the whole voltage window. This phenomenon indicates fast charge/discharge behavior and ideal capacitive feature.59 On the basis of the total mass of AC and C@NS@NF-10, the calculated capacitances at various current densities further demonstrate its rate capability. As displayed in Figure 6d, the device can deliver 89, 77, 71, 63, 58 and 54 F g-1 at the current densities of 10, 15, 20, 30, 40 and 50 mA cm-2, respectively, which indicates its superior rate capability. As another key factor, the Ragone plots were usually used to evaluate the performance of asymmetric supercapacitors.60-61 The power densities and energy densities were calculated based on the following equations: E = 1/2 × Cm ×∆V2

(4)

P = E / ∆t

(5)

, where E represents the energy density, Cm is the specific capacitance calculated by the total mass loading of two electrodes, ∆V refers to the operating window voltage, P is the power density, and ∆t refers to the discharge time. As shown in Figure S13, the C@NS@NF-10//AC device exhibits a high energy density of 27.5 Wh kg-1 at a power density of 532 W kg-1, and it can still remain 16.7 Wh kg-1 at a power density of 2660 W kg-1. In addition, the superior energy densities obtained by the as-fabricated device was compared to other nickel based ASC devices at Table S2. Figure 6e shows long-term cycle durability of the C@NS@NF-10//AC supercapacitor at a current 19



density of 100 mA cm-2. After 5000 charge/discharge cycles, the ASC device delivers a high capacitance retention of 94.72%. Such an excellent performance in rate and cycling stability can be ascribed to the following reasons: (1) Growing electro-active material on NF enhances the structural robustness and electrical conductivity. (2) Homogeneous tree-like morphology can offer more active area between the electrode and electrolyte. (3) The carbon shell can facilitate charge transport, which can excavate the potential of as-prepared NS in energy storage and conversion. For further investigation of its potential for real-life application, a coin cell asymmetric supercapacitor was assembled by employing C@NS@NF-10 as cathode and AC as anode with a 2M KOH as electrolyte. According to Figure S14a, the potential window 0-1.5 V is a reasonable working voltage window. As displayed in Figure S14b, the CV curves obtained at various scan rates show the features of both double-layer capacitance and pseudocapacitance. Based on the charge/discharge curves in Figure S14c, the capacitances of the coin cell ASC were calculated and displayed in Figure S14d. The device exhibits 77, 69, 60, 54, and 48 F g-1 at the current densities of 15, 20, 30, 40 and 50 mA cm-2, respectively. Moreover, long cycles GCD was tested to characterize its long-term durability as displayed in Figure S14e. After 5000 charge/discharge cycles, the coin cell still delivers a high capacitance retention of 93.42%. In addition, EIS analysis of the coin cell asymmetric supercapacitor was also performed before and after the cycling test in the frequency range from 0.01 Hz to 100 kHz with an amplitude of 5 mV. The corresponding Nyquist plots are illustrated in Figure S14f and the inset graph is the equivalent circuit fitting for the plots. Only a minor rise of the Rs value and Rct can be detected after 5000 GCD cycles indicating a great cycling durability, which may ascribe to the advantageous structure of the C@NS@NF-10 electrode. 20


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To explore its electrocatalytic performance, the as-prepared electrodes and commercial Pt/C electrode were tested in a typical three-electrode configuration. As illustrated in Figure 7a, the C@NS@NF-10 shows a low overpotential (η) of 96 mV at the current density of 10 mA cm-2 normalized by geometric surface area (GSA), while the NS@NF requires a overpotential of 166 mV, indicating that the amorphous carbon shell has greatly enhanced the HER performance of the 3D tree-like superstructures and activated their surfaces. Besides, the C@NS@NF-5 and the C@NS@NF-15 require overpotentials of 130 mV and 110 mV, respectively, to achieve 10 mA cm-2 normalized by GSA. These results reveals the best HER activity of the C@NS@NF-10 among all the samples, but the performance is still worse than the commercial Pt/C that only requires an overpotential of 39 mV at 10 mA cm-2 (Shown in Figure 7a). Figure 7b shows the Tafel curves of the as-obtained electrodes and commercial Pt/C electrode. Owing to the fact that the lowest Tafel slope indicates the most efficient HER kinetics62, the C@NS@NF-10 electrode displays a great HER kinetics performance because of its small Tafel slope (97 mV dev-1). To gain further insights into the reason of the superior catalytic performance of C@NS@NF-10, we adopted EIS to test the electrode kinetics. As presented in the obtained Nyquist plots (Figure S15) fitted by the equivalent circuit shown in the inset, the carbon coated samples all deliver smaller semicircular diameters than that of the NS@NF, proving that the coated carbon can enhance the electrical conductivity. Moreover, different amount of carbon coating would result in different electrical conductivity improvement, and the C@NS@NF-10 electrode exhibits the smallest semicircular diameter (related to the charge transfer resistance Rct), which indicates its best charge transfer ability among all the samples. To understand the original differences and intrinsic activity of HER performances of these samples40, we normalized the LSV 21



curves by ECSA and the corresponding curves are shown in Figure 7c. Obviously, their HER performance exhibits the following sequence: Pt/C > C@NS@NF-15 > C@NS@NF-10 > C@NS@NF-5 > NS@NF. It indicates that the porous carbon shell can boost HER performance. In addition, the HER performance were improved gradually with the growing amount of coated carbon.

Figure 7. Electrocatalytic performances: LSV curves (a) normalized by GSA, Tafel slopes (b) normalized by GSA, and LSV curves (c) normalized by ECSA of the NS@NF, C@NS@NF-5, C@NS@NF-10 C@NS@NF-15 and commercial Pt/C electrodes, respectively; an optical picture showing a working electrode (d) and stability performance for 32 h at an overpotential of 150 mV (e) of the C@NS@NF-10 normalized by GSA.

Besides, to identify who plays a major role in HER, we prepared a sample of C@NF-10 by a similar method for C@NS@NF-10 and tested its LSV curve. The LSV curve was compared with that of C@NS@NF-10 in Figure S16. The C@NF-10 requires an overpotential of 243 mV to achieve a geometric current density of 10 mA cm-2, which is much larger than that of C@NS@NF, while the corresponding overpotential of NS@NF is 166 mV (Figure 7a). Based on the results, we could conclude that nickel sulfides played a major role in HER and the carbon layers have 22


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improved the catalytic performances.

Stability is another important factor to judge the performance of an electrocatalyst. Hence, the long-term stability of the C@NS@NF-10 was tested by chronoamperometry measurement (j-t) with a constant overpotential (150 mV). Figure 7d is a digital photo taken during the LSV measurements, which demonstrates a stable evolution of hydrogen bubbles on the surface of the C@NS@NF-10 electrode. The stability test of C@NS@NF-10 was also carried out with an overpotential of 150 mV and the result has been shown as Figure 7e. A decrease of the current density during the first 8 h can be observed and it may be ascribed to the mass loss of the catalytic material. Furthermore, a steady current density of 17 mA cm-2 can be maintained for another 24 h without any noticeable current decay, indicating its excellent electrocatalytic durability.

4. CONCLUSION In summary, we have successfully synthesized the carbon coated 3D tree-like Ni3S2 on NF using a two-step facile hydrothermal route. The 3D superstructures on NF can facilitate the electrolyte penetration, reduce ions and electrons transport resistance and offer numerous active sites for electrochemical reactions. Moreover, the carbon layer can further enhance the electric conductivity of the electrodes while the underlying substrate NF ensure the high-performance and binder-free electrodes for direct use in charge storage and conversion. When employed as electrodes for supercapacitors and HER, the obtained composites exhibited good pseudocapacitive performance as well as superior electrocatalytic HER performance. 23



ASSOCIATED CONTENT Supporting Information

The mass loading records during the synthesis process; The comparison of XRD patterns, Raman spectra, TGA curves and electrochemical data of the obtained samples; TEM image of the C@NS@NF-10 dispersed after ultrasonic process.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (A.Q. Pan); [email protected] (T. Zhu). ORCID Ting Zhu: 0000-0001-5567-3809; Guozhong Cao: 0000-0003-1498-4517; Anqiang Pan: 0000-0002-7605-1192.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Program for New Century Excellent Talents in University (NCET-13-0594), the National Natural Science Foundation of China (No. 51302323, 51374255), the Innovation Project of Central South University (2017CX001).

REFERENCES 24


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3D Carbon Coated Tree-Like Ni3S2 Superstructures on Nickel Foam

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