Ni Foam-Supported Carbon-Sheathed NiMoO4 Nanowires as

Research Article pubs.acs.org/journal/ascecg

Ni Foam-Supported Carbon-Sheathed NiMoO4 Nanowires as Integrated Electrode for High-Performance Hybrid Supercapacitors Zhaojie Wang,§,† Guijuan Wei,§,† Kun Du,† Xixia Zhao,† Ming Liu,† Shutao Wang,† Yan Zhou,† Changhua An,*,‡ and Jun Zhang*,† †

College of Science, State Key Laboratory of Heavy Oil Processing and College of Chemical Engineering, China University of Petroleum, No. 66, Changjiang West Road, Huangdao District, Qingdao 266580, P. R. China ‡ Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, College of Chemistry and Chemical Engineering, Tianjin University of Technology, No. 391, Bin Shui West Road, Xi Qing District, Tianjin 300384, P. R. China S Supporting Information *

ABSTRACT: Rational design of hierarchical nanostructure arrays as integrated electrodes with the capability of storing energy has been studied extensively. However, a low electronic/ionic transport rate and structural instability hampered their practical application. In this study, we have fabricated carbon-sheathed NiMoO4 nanowires standing on nickel foam (NF) and employed as a free-standing electrode for supercapacitor. The unique structure revealed remarkable electrochemical behavior including a high areal capacitance, ∼70% capacitance retention at 100 mA cm−2, and an stability during cycling (86% retention after 50,000 cycles). In addition, an NF@NiMoO4@C//activated carbon hybrid supercapacitor presents 201.3 F g−1 of specific capacitance along with an 72.4 W h kg−1 of energy density. The carbon sheath, which prevents the structural pulverization of NiMoO4 and provides another conductive path together with Ni foam, is responsible for the superior electrochemical performances. Our work demonstrates an improved step toward rational design of high-performance integrated electrodes for a supercapacitor with a new vision for theoretical and practical applications. KEYWORDS: NiMoO4, Carbon, Ni foam, Integrated electrode, Supercapacitor

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collection.14,15 More importantly, the resultant composites can act as integrated electrodes or binder-free electrodes for SCs directly.16−18 For example, Cao et al. prepared MnMoO4 nanoplates arrays on Ni foam for advanced electrochemical properties, which presented an areal capacity of 1.15 F cm−2 at 4 mA cm −2 . 19 Xiao and co-workers prepared NiMoO4 nanosheets on conductive Ni foam and provided a specific capacitance of 1694 F g−1 at 1 A g−1.20 Unfortunately, the cycle lifespan and rate capacity behavior are still inferior to the commercial requirements of SCs because of the structural deformation and serious aggregation. Hybridization of conductive carbonaceous materials with active pseudocapacitive electrode materials has been proved be effective to address the issues.21−23 Thus, coating a carbon nanoshell on the nanostructured electrode materials to make a protection of the binder-free electrodes directly grown on the substrate opens an alternative avenue to the extensive advance in electrochemical energy storage devices. In this work, we have rationally designed a class of hierarchical carbon-sheathed NiMoO4 nanowire arrays on a three-dimensional Ni foam skeleton (NF@NiMoO4@C) as an

INTRODUCTION With high concerns about an energy shortage and environmental issues, tremendous attention has been attracted to design and develop novel energy storage technologies, i.e., supercapacitors (SCs).1−3 The fatal disadvantage of SCs is their unsatisfying energy density of E = CV2/2.4−6 Therefore, the parameters to evaluate their electrochemical performances are mainly determined by advanced electrode materials,7 which should meet the requirements of rapid ion/electrons transport kinetics and superior stability. Mixed ternary metal oxides, such as NiCo 2 O 4 , 8 ZnCo 2 O 4 , 9 NiMoO 4 , 10 CoMoO 4 , 11 and MnMoO4,12 have been considered as promising alternatives to conventional binary transition metal oxides. The features in chemical compositions and structural synergy afford their surprising electrochemical performance. Moreover, the existence of different valence states in these mixed metal centers offers considerable electronic conductivity in the charging/ discharging process.13 To achieve a high-performance electrode of single-phase ternary metal oxides, great efforts have been poured into the rational design of hierarchical nanostructure arrays on the surfaces of highly conductive substrates. The well-defined hierarchical arrays present fascinating advantages such as exposing abundant electroactive sites accessible on large surface area, accelerating ion transport, and facilitating electron © 2017 American Chemical Society

Received: March 11, 2017 Revised: May 6, 2017 Published: June 3, 2017 5964

DOI: 10.1021/acssuschemeng.7b00758 ACS Sustainable Chem. Eng. 2017, 5, 5964−5971

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ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of NiMoO4 nanowires on NFs obtained at (a) 6 h, (b) 12 h, and (c) 24 h. (d) Corresponding XRD patterns of NiMoO4 nanowires grown on Ni foams at different reaction times.

Figure 2. (a, b) SEM images of NiMoO4@PDA core−shell nanowires on NFs with different high magnification. (c) SEM and (d) TEM images of NiMoO4@C core−shell nanowires. The thickness of carbon shell was ∼10 nm (inset, panel d).

current collector of Ni foam. A specific capacity up to 6.14 F cm−2 at 5 mA cm−2 has been achieved on NF@NiMoO4@C nanoarchitecture electrode, which is 2 times larger than that of the bare sample. The cycle performance was boosted to 50,000 times with 86.0% of the initial specific remained.

integrated electrode for SCs. Considering the high electrical conductivity of one-dimensional (1D) NiMoO4 nanowires (NWs), the structure directly aligned on NF would offer superior electron transport by robust electrical contact. However, multi-redox reactions in a period of electrochemical cycling make bare NiMoO4 very fragile, easily deforming its structures. Coating carbon as a versatile surface engineering protocol can maintain the nanowire array architecture and protect the structure from deformation. The conductive carbon nanoshells additionally offer other conductive path besides the

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EXPERIMENTAL SECTION

Synthesis of NiMoO4 NWs on Ni Foam. A piece of Ni foam (3.5 cm × 6 cm) was treated in ethanol, acetone, and distilled water under sonication, respectively. Here, 0.72 g of Ni(NO3)2·6H2O and 0.6 g of 5965


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Figure 3. (a) Full XPS spectrum of NiMoO4@C. High-resolution spectra of (b) C 1s, (c) Ni 2p, and (d) Mo 3d.

Figure 4. (a) CV curves of the NiMoO4-6h electrode at various scan rates. (b) Galvanostatic discharge behavior of the NiMoO4-6h electrode. (c) CV curves of the NiMoO4 NW electrodes prepared at different reaction times at the same scan rate of 30 mV s−1. (d) Dependence of specific capacitances of the four bare NiMoO4 NWs vs current density. Na2MoO4·7H2O were mixed in 50 mL of distilled water. The mixture with pretreated NF was transferred into a Teflon-lined stainless steel autoclave before maintaining at 150 °C for several hours. After the autoclave was cooled, the NF depositing with the green sample was washed and dried.

Preparation of NF@NiMoO4@C Nanoarchitecture. Here, 53 mg (0.28 mmol) of dopamine hydrochloride was dispersed into 27 mL Tris-buffer solution (10 mM, pH 8.5). The as-pretreated NF with NiMoO4 NWs was immersed in the dopamine solution and stewed in the dark for 24 h. The resultant NFs were further annealed at 500 °C 5966


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Figure 5. (a) CV curves and (b) galvanostatic discharge curves tested under different scan rates and current densities, respectively. (c) Areal specific capacitance of NiMoO4-6h before and after carbon sheathing changes with current densities. (d) Cycling stability of a NiMoO4@C electrode operated at 200 mA cm−2 for 50,000 cycles. Three insets in panel d, bottom, present the original profiles for the first, middle, and last 10 charge− discharge profiles, respectively.

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in N2 for 3 h. Finally, the mass loading of NiMoO4@C core−shell NWs on Ni foam was ∼2 mg cm−2. Characterization. A Philips X’Pert diffractometer with Cu Kα radiation (λ = 0.15418 nm) was applied to collect the X-ray diffraction (XRD) information. Field-emission scanning electronic microscopy (FESEM, Hitachi S-4800) and transmission electronic microscopy (TEM, JEM-2100UHR) were used to characterize the microscopic features. X-ray photoelectronic spectra (XPS) were determined on a VG Escalab 250 spectrometer equipped with an Al anode (Al Kα = 1846.6 eV). Infrared spectroscopy (IR) was obtained on an IR spectrometer (Thermo Nicole, NEXUS, America). Electrochemical Performance Evaluation. Electrochemical performance evaluations were operated with an electrochemical workstation (CHI 760E, Shanghai Chenhua, China). The active integrated electrode with an immersed area of 1 cm2 was used as the working electrode. A standard saturated calomel electrode (SCE) was used as the reference electrode. A platinum sheet (1 cm × 1 cm) was used as the counter electrode. The specific capacitance was evaluated according to the two equations: C=

C=

I Δt SΔV

I Δt mΔV

RESULTS AND DISCUSSION The morphologies and structure of time-dependent samples at different reaction times were tracked by SEM and XRD, as

(1) Figure 6. Scheme of carbon nanoshells on providing an alternative conductive path together with a conductive NF and helping maintaining the integration of NWs standing on NF.

(2)

where I is the constant current during the charge/discharge process (A), t is discharge time (s), ΔV is the potential window (V), S is the geometrical area (cm2) of the working electrode, and m is the mass of electroactive materials (g). The NF@NiMoO4@C cathode and activated carbon anode were used to assemble a hybrid supercapacitor. All the electrodes were tested at ambient temperature in aqueous KOH of 2 M.

revealed in Figure 1. Interestingly, NiMoO4 nanowires with ∼70 nm in diameter formed arrays after the reaction proceeded for 6 h (Figure 1a). When the reaction increased to 12 h, the diameter of the nanowires increased to ∼120 nm (Figure 1b). Further prolonging the reaction to 24 h, some NiMoO4 nanoflowers composed of a lamellar structure around NWs 5967


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S1). After the carbonization process under N2 atmosphere, the NiMoO4@C core−shell NWs exhibit rough surfaces (Figure 2c, d). The HRTEM image of a single NW (inset, Figure 2d) indicates that the carbon layer was uniformly and continuously formed along the NiMoO4 NWs with a thickness of ∼10 nm. The chemical composition and oxidation state of the NiMoO4@C sample can be identified by XPS. The full survey spectrum in Figure 3a indicates the existence of C, O, Ni, and Mo in our sample. The C 1s spectrum in high resolution (Figure 3b) signifies a number of functional groups besides sp2 and sp3 hybridized carbon including hydroxyl (−C−O) and carboxyl (−COO) groups, which may form an effective interaction with NiMoO4.24 The main binding energy peaks of Ni 2p3/2 (855.6 eV) and Ni 2p1/2 (873.5 eV) are separated by 17.9 eV demonstrating the presence of an Ni2+ oxidation state.25 In addition, two peaks at 232.6 and 235.9 eV are visible in the Mo 3d region (Figure 3d). The peaks and the fission width of 3.1 eV are from the Mo6+ oxidation state.26,27 The obtained architecture of the NF@NiMoO4 and NF@ NiMoO4@C NWs were directly applied as working electrodes without using any insulating binder and conducting additive. First, the bare NF@NiMoO4 NWs obtained from varying reaction times were also measured and compared. Figure 4a gives typical CV curves of NiMoO4-6h NWs with different scan rates. The obvious reduction and oxidation peaks reveal that the capacitance features are governed by reversible Faradaic reactions.28,29 With an increase in scan rate, the peak current increases linearly. It is a doping/dedoping process controlled by the diffusion of electrolyte ions. At the same time, the cathodic and anodic peaks shift to the opposite directions, which derives from a resistance effect.30 The galvanostatic charge−discharge (GCD) behavior was further tested under different current densities from 0 to 0.48 V (vs SCE). Figure 4b provides the

Figure 7. Electrochemical impedance spectra of NF@NiMoO4-6h and NF@NiMoO4@C electrodes.

were observed (Figure 1c), indicating that the assembled NWs might grow into nanoflowers following the ripening process. The XRD pattern (Figure 1d) can be assigned to monoclinic NiMoO4 (PDF Card 86-0361). Typically, the morphology of carbon-sheathed NiMoO4-6h nanowires standing on NFs was identified by SEM and TEM images. It can be seen in Figure 2a that the rough surfaces of the Ni skeleton after dopamine polymerizing outside NiMoO4 NWs and the distinct cross-linked network can be clearly observed. The NiMoO4@PDA core−shell NWs have an average diameter of ∼100 nm (Figure 2b). Moreover, the formation of PDA was also confirmed by FT-IR spectra (Figure

Figure 8. (a) CV curves of the NF@NiMoO4@C//AC hybrid SC with varying voltage windows at 50 mV s−1. (b) CV curves and (c) GCD curves of the NF@NiMoO4@C//AC hybrid SC at different scan rates and current densities, respectively. (d) Cycling behavior of the NF@NiMoO4@C//AC hybrid SC at 20 A g−1. 5968


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with the increment of the cycle number. In detail, its initial areal capacitance was 2.18 F/cm2 and still retains 1.87 F/cm2 after 50,000 cycles by repeating the harsh redox reaction. The insets in Figure 5d were the original profiles for the first 10 GCD cycles, 10 GCD cycles in middle tests, and the last 10 GCD cycles, respectively. The results further prove that the NiMoO4@C electrode exhibits ultralong cycling life at high current density. In general, the carbon sheath buffers the inner volume expansion and prevents the structural pulverization of NiMoO4 even when it experienced harsh cycle redox reactions. The shape analysis of the used NiMoO4@C electrode after 50,000 cycles (Figure S4) shows almost no change occurs, implying its good stability. The enhancement in electrochemical performance can benefit a lot from carbon coating. During a repeated charge− discharge process, the structural deformation caused by the volume expansion would be prevented, and the initial integration of NW arrays would maintain. Particularly, the carbon sheath provides an alternative conductive path together with the NF substrate as a current collector. In other words, the construction of dual conductive paths would additionally improve the electrode conductivity (Figure 6), which is helpful to the integrated electrode. The electrochemical impedance spectroscopy supports this hypothesis in experiments. The corresponding Nyquist plots of the NiMoO4@NF and NiMoO4@C@NF electrodes are shown in Figure 7. Obviously, in the a high frequency area, NF@NiMoO4@C exhibited a smaller semicircle in the impedance plots NF@NiMoO4-6h electrodes, indicating a lower charge−transfer resistance was obtained after carbon coating. At the same time, in a lowfrequency area, the NiMoO4@C integrated electrode changes in a more straight line, which means the electrolyte and proton diffuse more efficiently. We speculate that the surface carbon sheath not only hopefully increases the electrical conductivity but also guarantees the accessible penetration of electrolyte ions.35 In order to explore its practical application, we also assembled the hybrid supercapacitors using an as-synthesized NF@NiMoO4@C composite and activated carbon. Before assembling, CV comparison in each three-electrode systems of both AC electrodes and NF@NiMoO4@C electrodes was measured at the same scan rate of 20 mV s−1 (as shown in Figure S5). According to the charge balance (q+ = q−) between the NF@NiMoO4@C and AC electrodes, the mass balance can be quantified by m+/m− = (C‑*ΔV−)/(C+*ΔV+). Here, C+ and C− are the capacitances of the respective electrode, and ΔV+ and ΔV− are the corresponding voltage windows for charging and discharging. As a result, the loading mass of the negative electrode is ∼3.5 mg cm−2 after balance. Figure 8a presents the CV curves of NF@NiMoO4@C//AC two-electrode hybrid supercapacitor scanned at a rate of 50 mV s−1 as varying voltage windows. The typical redox peaks still existed, and it indicates the battery-like characteristics of the electrode because of the Faradaic reaction of NiMoO4. Its GCD results versus changing current densities are illustrated in Figure 8c. The corresponding specific capacitance is listed in Table S3. As expected, the Coulombic efficiencies are greater than 85% at high current densities. A cycling stability test was performed for the NF@ NiMoO4@C//AC hybrid supercapacitor by a successive GCD test at 20 A g−1 for 4000 cycles. The specific capacitance and retention ratio of the hybrid supercapacitor changing with cycle number increment are shown in Figure 8d. After 4000 cycles,

discharge curves of NiMoO4-6h NWs at 5, 10, 20, 30, and 100 mA cm−2, respectively. A distinct plateau occurred during each discharge process in accordance with the battery-like characteristics from the CV results. As is known, the electrochemical property of an electrode material greatly depends on its microstructure, such as morphology and size. For comparison, CV curves of NiMoO4-6h, NiMoO4-12h, and NiMoO4-24h NWs at 30 mV s−1 are illustrated in Figure 4c. All the CV curves are similar in redox peaks, while each enclosed area is different. Compared to the others, the area of the NiMoO4-6h electrode is the largest, meaning the electrode possesses the highest supercapacitive performance. The electrochemical measurement results of the NiMoO4-12h and NiMoO4-24h electrodes are illustrated in Figures S2 and S3, respectively. According to their galvanostatic discharge profiles at the corresponding current density, the calculated results of areal specific capacitance are plotted and compared in Figure 4d. It is obvious that the NiMoO4-6h electrode owns the highest specific capacitance at each tested current density. As considered from SEM images in Figure 1, with prolonging the reaction, the diameter of the NiMoO4 NWs was increased. In addition, NiMoO4 nanoflowers detached from a conductive Ni substrate can be observed (Figure 1c) after reacting for 24 h, which are different from NiMoO4 NWs depositing on Ni foam directly. The evolution structures from a long reaction time block the electronic/ionic transport, and the capacitances decreased accordingly. The areal specific capacitances of NiMoO4-6h are calculated to be 3.48 F cm−2 at 5 mA cm−2, but only 44.8% remained at 30 mA cm−2. Further enhancement by surface engineering, such as carbon shell protection, is necessary for capacitance and stability improvement. The electrochemical performance of the optimized NF@ NiMoO4@C core−shell NWs was further evaluated and is shown in Figure 5. Remarkably, the redox peaks of NiMoO4-6h (Figure 4a) are nearly encompassed by the NiMoO4@C electrode, indicating that the inside NiMoO4 NWs is not blocked from participating in the Faradaic reaction despite the existence of carbon covers. The specific capacitance of the NiMoO4@C NWs related to the galvanostatic discharge profiles of NiMoO4@C NWs are shown in Figure 5b. As shown in Figure 5c, it reaches 6.14, 5.19, 4.50, 4.38, and 4.17 F cm−2 (3070, 2595, 2250, 2190, and 2085 F g−1) at a current density of 5, 10, 20, 30, and 100 mA cm−2, respectively. Evidently, our NF@NiMoO4@C hybrid electrode provides a much higher areal capacitance than the bare NF@NiMoO4, and almost 70% of the capacitance is retained at 100 mA cm−2. This outstanding rate behavior is attributed to the unique sheathing characteristics of the carbon nanoshell in major. The results indicate that both the vconductive Ni foam substrate and carbon sheath are responsible for the enhanced storage capacity. Moreover, the specific capacitance in this work is much higher than many previously reported NiMoO 4 nanostructures, such as NiMoO4 nanosheet (1221.2 F g−1 at 5 A g−1) and NiMoO4 nanotubes (864 F g−1 at 1 A g−1). It is even higher than those composite electrodes of CoMoO4− NiMoO4·xH2O bundles (988 F g−1 at 5 A g−1) and NiMoO4@ CoMoO4 nanospheres (1300 F g−1 at 5 A g−1).31−34 More comparisons with the previously reported values over NiMoO4based nanoarchitectures are summarized in Tables S1 and S2. For practical application, the cycling lifespan at large charge/ discharge current is very important. For the NiMoO4@C electrode (Figure 5d), the specific capacitance decreases slightly 5969


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ACS Sustainable Chemistry & Engineering only 1.96% of its initial value deteriorated, indicating a distinguished electrochemical stability. The hybrid supercapacitors for practical applications were further evaluated by the power density (P) and energy density (E), which are considered as vital factors in real applications. They can be analyzed by the following equations:36 I∫ E=

t (Umin)

t (Umax )

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P = E/t

*E-mail: [email protected] (C. An). *E-mail: [email protected] (J. Zhang). ORCID

(3)

Changhua An: 0000-0003-2227-5008 Author Contributions

(4)

§

Z. Wang and G. Wei contributed equally to this work.

where I is the current during charge/discharge process, U is the cell voltage, and t is the discharge time in GCD curves. As listed in Table S3, our hybrid supercapacitor can deliver a high specific energy density of 72.4 W h kg−1 at a power density of 851.8 W kg−1 and remains 61.0 W h kg−1 with a high power density of 21.7 kW kg−1. As far as we know, the remarkable electrochemical performance benefits a lot from the unique design with Ni foam-supported carbon-sheathed NiMoO4 nanowires. Compared with those electrodes prepared by traditional powder and slurry, the advantages of a binder-free electrode material are significant. For example, the fabrication process is simpler, and the resistances of interface and contact can be greatly decreased as well. Moreover, the carbon sheath provides an alternative conductive path together with the NF substrate and effectively protects the NiMoO4 nanowires from structural pulverization during harsh cycle redox reactions. This rational design offers the hybrid supercapacitors outstanding energy storage performance in electrochemistry.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51402362 and 21471160), Shandong Natural Science Foundation (ZR2014EMQ012 and ZR2016BM12), Fundamental Research Funds for the Central Universities (16CX05016A, 15CX05045A, and 15CX08010A), and start-up fund from TJUT.

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REFERENCES

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CONCLUSIONS In conclusion, we have constructed an integrated electrode with dual conductive paths by carbon sheathing the binder-free NiMoO4 NWs on Ni Foam. The as-prepared electrode presents remarkable electrochemical energy storage properties with a high specific capacitance of 6.14 F cm−2 at 5 mA cm−2, high rate performance (4.17 F cm−2 at 100 mA cm−2), and outstanding stability (86% of initial specific capacitance remained after 50,000 cycling), which are much higher than those reported values. The NF@NiMoO4@C//AC hybrid supercapacitor was evaluated with high capacitance, high energy density, and longterm lifespan. The carbon sheath can not only act as a sheath to buffer the inner NiMoO4 volume expansion but can also provide another conductive path for electrons transferring quickly together with conductive Ni substrate. The present work provides a facile way to design an advanced integrated electrode for developing novel energy storage devices.

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AUTHOR INFORMATION

Corresponding Authors

U (t )dt

m

power densities of NF@NiMoO4-6h@C//AC hybrid supercapacitor from GCD. (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00758. FT-IR spectra of NiMoO4-6h@PDA core−shell nanowires; electrochemical properties of the NiMoO4 nanowires; SEM image of NF@NiMoO4-6h@C electrode after 50,000 continuous cycles; CV comparison of AC electrode and NF@NiMoO4-6h@C electrode; electrochemical performances of the electrodes based on NiMoO4 nanostructures in previous reports; calculated results of specific capacitance, energy densities, and 5970


Research Article

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Ni Foam-Supported Carbon-Sheathed NiMoO4 Nanowires as

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