Ni_0.9Co_1.92Se₄ nanostructures: binder-free electrode of coral-like bimetallic selenide for supercapacitors

Abstract

Coral-like Ni_0.9Co_1.92Se₄ nanostructured materials have been prepared through a simple and controlled two-step solvothermal method, which present a handsome electrochemical performance. The specific capacitance reaches 1021.1 F g⁻¹ under the current density of 2 mA cm⁻² over a 0.5 V electrode potential window with an areal capacitance of 6.43 F cm⁻². A superior rate capability of 77% is achieved with discharge rates increasing from 2 to 50 mA cm⁻², as well as a good cycling stability of 88.39% after 5000 cycles. Furthermore, a good energy density of 26.29 W h kg⁻¹ is achieved under the power density of 265 W kg⁻¹ when assembled into a Ni_0.9Co_1.92Se₄//AC asymmetric supercapacitor with the operating potential window extended to 1.5 V. The high performance can be attributed to the coral-like architectures with rich redox reactions, high conductivity and transport rate for both electrons and electrolyte ions. Our results suggest that the coral-like Ni_0.9Co_1.92Se₄ nanostructured electrode materials may be a good choice for supercapacitors.

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Introduction

In recent decades, supercapacitors have received much attention due to their high electrochemical performance, good cycling stability, fast charge–discharge capability and low maintenance cost.^1–4 These significant advantages provide supercapacitors with great potential for applications in future energy storage devices, especially in high power electric devices, mobile communications, electric vehicles and battery electronic equipment.^5,6 Among these applications, we know that mobile communications and battery electronic equipment are developing toward smaller, portable versions, which result in higher requirements for their power supplies. Consequently, electrode materials with high specific capacitance and energy density for supercapacitors show advantages for these power supply applications. Although significant advances have been achieved in supercapacitors, high-performance energy density without losing power density remains insufficient. It is commonly found that high specific capacitance is poorly maintained in most electrode materials, especially under high current density and power operation. Because the specific capacitance retention is largely determined by the morphology, size, high surface-to-volume ratio of the electrode materials, it is necessary to identify good electrode materials with good electronic conductivity, high specific capacitance retention and high energy density.^6,7

Many types of metal oxides,^8–12 hydroxide¹³ and sulphides^14,15 have been studied for supercapacitors. Nickel or cobalt based electrode materials usually be used because of their fast faradaic reactions in electroactive materials surface, offering promotion of high energy density without power density loss in the meantime. Among the various metals, bimetallic nickel and cobalt oxides^16–24 and sulphides^25–29 possess nice electrochemical performance due to their particular structures and morphologies than the monometallic materials.⁷ We have reported multiple hierarchical NiCo₂O₄ structures with superior electrochemical performance.²⁰ Recently, significant research effort has been expended on sulphides capacitor electrodes because they have a lower band gap and higher conductivity than oxides and provide better rate capability.²⁷ The obvious reason for the better performance of metal sulphides is the lower electronegativity of sulphur relative to oxygen in the studied materials. It is well known that lower electronegativity facilitates redox reactions, providing accordingly improved electrode material performance. Similar to the interest in metalsulphides, there have also been a few reports on metal selenides. Additionally, metal selenides have excellent electrical conductivity for applications to multiple research areas. Specifically, three-dimensional hierarchical GeSe₂ nanostructures³⁰ and hollow Co_0.85Se nanowire arrays on carbon fibre paper³¹ have been reported. However, the specific capacitance and energy density of these monometallic materials are much lower than other electrode materials. The reason for the low performance of three-dimensional hierarchical GeSe₂ nanostructures may be that germanium is not a transition metal. It is well known that transition metals have multiple oxidation states, therefore, they have greater electrochemical activity than other metals. Hollow Co_0.85Se nanowire arrays also perform more weakly than many other bimetallic nickel and cobalt based electrode materials, because the mutual synergy of nickel and cobalt in bimetallic electrode materials provide additional advantages. To our knowledge, bimetallic nickel and cobalt selenide for supercapacitors have rarely been reported, it is worth while to synthesize bimetallic selenides with novel nanostructures for high performance supercapacitors.

Based on the above considerations, we prepared a coral-like Ni_0.9Co_1.92Se₄ nanostructured material on Ni foam for supercapacitor applications. The Ni_0.9Co_1.92Se₄ nanostructures perform well with an absolute advantage in capacitance retention relative and power density to many similar materials. What's more, our work further extends the research on the unique structure, in which the coral-like building blocks of the Ni_0.9Co_1.92Se₄ nanostructures are composed of nanoparticles with sizes of a few tens of nanometres. Comparison with monometallic selenide in additional experiments shows that the unique nanostructures of Ni_0.9Co_1.92Se₄ nanostructures exhibit effectively reduced electrolytes diffusion resistance and shorted electron and ion transport pathways, which contribute to the optimization of the high electrochemical performance.

Results and discussion

Initially, the EDX pattern was recorded to check the elemental purity of the as-prepared electrode materials. The EDX data demonstrates that the samples are mainly composed of nickel, copper and selenium, except a little oxygen signal which may be from moisture and oxygen adsorbed on the surface of sample,^31,37 as shown in Fig. S1.† This result demonstrates that the as-prepared samples are Ni–Co selenide materials. The Ni/Co/Se atomic ratio was further analysed and confirmed by Inductively Coupled Plasma (ICP), and the quantitative analyses to determine the atom percentage were tabulated as shown in Fig. 1a. Through a simple physical calculation, the detected atomic ratio of Ni/Co/Se is approximately 0.9 : 1.92 : 4. According to both analyses, it can be safely concluded that the as-prepared electrode materials prepared in this work have a chemical composition of Ni_0.9Co_1.92Se₄.

Fig. 1 (a) ICP spectra and high-resolution (b) Ni 2p, (c) Co 2p, (d) Se 3d XPS spectra of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam.

To determine the chemical state of the elemental composition, the as-prepared Ni_0.9Co_1.92Se₄ samples were discussed by XPS measurements shown in Fig. 1b–d. The high-resolution Ni 2p and Co 2p spectra are fitted with two spin–orbit doublets and two shakeup satellites using a Gaussian fitting method, and the intensity of Ni 2p spectra is stronger. The Ni 2p binding energy peaks at approximately 853.20 and 870.28 eV are assigned to Ni²⁺, and those at 855.99 and 873.45 eV are assigned to Ni³⁺ in the Ni 2p spectra. Likewise, the peaks at 778.58 and 793.45 eV indicate Co³⁺, and those at 780.91 and 797.05 eV indicate Co²⁺ in the Co 2p spectra.³¹ These results essentially match the reported Co 2p and Ni 2p spectrum data for bimetallic nickel and cobalt oxides²⁴ and sulphides.^34–36 In Fig. 1d, the peaks at 54.10 and 54.98 eV represent the Se 3d binding energy, and the broad peak near 58.9 eV indicates Se²⁻ in the Se 3d spectra, and the peak at 59.25 eV can be assigned to oxidized Se.^32,33,38

To check the phase purity of the as-prepared Ni_0.9Co_1.92Se₄ samples, XRD patterns were recorded, and we scratched out the Ni foam to prevent the very strong Ni signal from influencing the XRD results. The XRD power analysis suggests the successful preparation of the Ni–Co precursors as shown in Fig. S2.†The XRD pattern of the Ni–Co precursors is accord with those of Co(CO₃)_0.5(OH)·0.11H₂O (JCPDS no. 48-0083) and Ni₃(CO₃)(OH)₄·4H₂O (JCPDS no. 16-0164), which suggests that the Ni–Co precursors can be considered a cobalt carbonate hydroxide hydrate with a fraction of cobalt substituted by nickel. And we searched the JCPDS card database and found that the corresponding peaks of the as-prepared Ni_0.9Co_1.92Se₄ samples are accord with the peak positions of Ni₃Se₄ (JCPDS no. 18-0890) and Co₃Se₄ (JCPDS no. 89-2001), as shown in Fig. 2, which also suggests that the as-prepared Ni_0.9Co_1.92Se₄ samples also can be considered cobalt partially substituted by nickel with the metals/selenium ratio of ∼3/4. Furthermore, the result is consistent with the EDX, XPS and ICP measurements.

Fig. 2 XRD spectra of the Ni_0.9Co_1.92Se₄ nanostructures powder.

The morphology of the as-prepared Ni_0.9Co_1.92Se₄ samples and the Ni–Co precursors were examined by FESEM. Fig. 3a shows the SEM image of Ni foam, the large spaces among the interconnected nickel atoms provide faster ion transport to the surface of the electroactive, which effectively reduces the diffusion resistance for supercapacitor applications. The different magnifications in Fig. 3b and c show that the Ni–Co precursor nanowires are uniformly grown on the Ni foam. The Ni_0.9Co_1.92Se₄ nanostructures present a coral-like morphology as shown in Fig. 3d–f, each subunit is separate and individually connected to the Ni foam, which may facilitate electronic transport and ion transport to the whole active area. Also, Fig. 4a shows the FESEM image of Ni_0.9Co_1.92Se₄ nanostructures at a high magnification. Compared with the Ni–Co precursor nanowires, the coral-like building blocks of the Ni_0.9Co_1.92Se₄ nanostructures are composed of nanoparticles with sizes of a few tens of nanometers. In general, the structure and morphology are well preserved after selenization process, including the particularity and novelty. Meanwhile, compared to the monometallic selenide of Co_0.85Se in Fig. S6,† the unique nanostructures exhibit effectively reduce diffusion electrolyte resistance, which contribute to the optimization of high electrochemical performance.

Fig. 3 FESEM images of (a) Ni foam; (b and c) Ni–Co precursors on Ni foam; (d–f) Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at different magnifications.

Fig. 4 (a) FESEM, (b, d and e) TEM and (f and g) HRTEM images of Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at different magnifications and (c) corresponding SAED pattern.

The detailed morphology and structure characteristics of coral-like Ni_0.9Co_1.92Se₄ nanostructures were further investigated by TEM and HRTEM characterizations. As shown in Fig. 4b, d and e, the Ni_0.9Co_1.92Se₄ nanostructures are made of nanoparticles, and the surface is densely wrinkled. The diameter of the nanoparticles is approximately 50 nm, which is in good agreement with the results shown in the FESEM images. The lattice fringes shown in Fig. 4f and g can be indexed to the crystal plane of the cubic phase, which further confirms the formation of Ni_0.9Co_1.92Se₄ nanostructures. Additionally, the corresponding SAED pattern as shown in Fig. 4c reflects the polycrystalline features of Ni_0.9Co_1.92Se₄ nanostructures and the diffused rings is in agreement with the XRD analysis.

In view of the appealing electrochemical performance of the coral-like Ni_0.9Co_1.92Se₄ nanostructures in supercapacitors, we further examined their electrochemical performance in three-electrode systems. Fig. 5a exhibits the CV curves of Ni_0.9Co_1.92Se₄ nanostructures on Ni foam with scan rates changing from 1 to 20 mV s⁻¹. With increasing scan rate, the anodic and cathodic peaks shift toward positive and negative potential, respectively, and the redox current increases obviously because of the diffusion of ions. Fig. 5b shows the GCD curves of Ni_0.9Co_1.92Se₄ nanostructures with current densities changing from 2 to 50 mA cm⁻² with a potential window from 0 to 0.5 V. The obvious plateau regions of the GCD curves clearly implies the pseudocapacitive characteristics and the symmetric shape demonstrates the splendid reversibility of the redox reactions, which is accord with the CV curves. In order to further comparison, the CV curves at 5 mV s⁻¹ and the GCD curves at 3 mA cm⁻² of the Ni foam, Co_0.85Se, Ni–Co precursor and Ni_0.9Co_1.92Se₄ nanostructures on Ni foam are presented in Fig. 5c and S8.† The CV integral area for the Ni_0.9Co_1.92Se₄ nanostructures is larger than those observed for the three other substances. The discharge time of the Ni_0.9Co_1.92Se₄ nanostructures is more than twice those of Co_0.85Se, and much larger than that of the Ni–Co precursor at the same current density. As for the conductive Ni foam substrate, it can be neglected for the Ni_0.9Co_1.92Se₄ nanostructures. Fig. 5d shows the specific capacitance and coulombic efficiency of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam. It is worthing noting that the specific capacitance reaches 1021.1 F g⁻¹ under the current density of 2 mA cm⁻² with an areal capacitance of 6.43 F cm⁻², and the specific capacitance remains 785.7 F g⁻¹ at a considerable current density, and 77% retention of the capacitance is kept, implying a good rate capability.

Fig. 5 Electrochemical performances: (a) the CV curves of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at the scan rates ranging from 1 to 20 mV s⁻¹; (b) the GCD curves of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at the current densities ranging from 2 to 50 mA cm⁻²; (c) comparison of the Ni foam, Co_0.85Se, Ni–Co precursor and Ni_0.9Co_1.92Se₄ nanostructures on Ni foam of CV curves at a scan rate of 5 mV s⁻¹; (d) the specific capacitance and coulombic efficiency of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at various current densities.

Cycling stability is an important requirement for the supercapacitors application. Fig. 6a shows the cycling performance of Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at 20 mA cm⁻². After 5000 cycles, we are pleasantly surprised to find that the cycling stability is relatively stable, over 88.39% of the specific capacitance retention can be retained all over the 5000 cycles. The electrical equivalent circuit of the impedance spectra is shown inside Fig. 6b and c, which includes bulk solution resistance (R_s), charge-transfer (R_ct), constant phase angle element (Q), specific capacitance (C_sp) from the Faraday redox reaction, and Warburg impedance (W) to determine the ions diffusion to the electroactive surface.²⁰ Fig. 6b shows the electrochemical impedance spectra (EIS) and EIS over the higher frequency region of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam in the initial cycle and after 5000 cycles in the frequency range from 10 mHz to 100 kHz at an AC amplitude of 5 mV. Compared with the equivalent series resistance of initial cycle and after 5000 cycles (0.0273 Ω and 0.1946 Ω, respectively), the little change clearly shows the lower resistive nature of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam after cycling, as well as the better cycling stability. The EIS comparison of Co_0.85Se and Ni_0.9Co_1.92Se₄ nanostructures is shown in Fig. 6c. The Warburg line of Ni_0.9Co_1.92Se₄ nanostructures is much longer than that of Co_0.85Se, which indicates better electrolyte diffusion into the electrode material. The low frequency line of the Ni_0.9Co_1.92Se₄ nanostructures is perpendicular to the X axis, which present the favourable ion diffusion characteristics relative to the Co_0.85Se. And the high frequency loop of the Ni_0.9Co_1.92Se₄ nanostructures shows much more inconspicuous from the inside part of Fig. 6c, which indicates the less charge-transfer resistance between the electroactive material and electrolyte interface. Additionally, the longer Warburg line and the lower equivalent series resistance fully embody that the Ni_0.9Co_1.92Se₄ nanostructures are advantageous as an active material in contact with the electrolyte.

Fig. 6 (a) The cycling performance of Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at a current density of 20 mA cm⁻². (b) EIS of the Ni_0.9Co_1.92Se₄ nanostructures on Ni foam at initial cycle and after 5000 cycles with the electrical equivalent circuit inside used for fitting impedance spectra; (c) EIS comparison of Co_0.85Se and Ni_0.9Co_1.92Se₄ nanostructures on Ni foam with the imaginary part (Y axis) vs. the real part (X axis).

To further demonstrate the potential for practical application of the Ni_0.9Co_1.92Se₄ nanostructures, an asymmetric supercapacitor is assembled with Ni_0.9Co_1.92Se₄ nanostructures used as the positive electrode and activated carbon as the negative electrode. The electrochemical performance of the activated carbon (AC) is shown in Fig. S10.† The specific capacitance of the AC is 166 F g⁻¹ under the current density of 2 mA cm⁻². To balance the charge storage between positive and negative electrodes, two electrodes are matched by the mass balance method.³⁹

where m⁺ and m⁻ are the mass, C_sp⁺ and C_sp⁻ are the specific capacitance, ΔV⁺ and ΔV⁻ are the potential windows of positive and negative electrodes, respectively.

The two electrodes possess different voltage window of Ni_0.9Co_1.92Se₄ (0–0.6 V) and AC (−1 to 0 V) shown in Fig. S11a,† so the voltage of Ni_0.9Co_1.92Se₄//AC asymmetric supercapacitor can be reached to 1.5 V, as shown in Fig. S11b.† From the GCD curves at different current densities in Fig. S11c,† the specific capacitance of the Ni_0.9Co_1.92Se₄//AC asymmetric supercapacitor can be achieved shown in Fig. S11d† according to the eqn (2). As we all know, asymmetric supercapacitor can increase the energy density by increasing the operating potential window.³¹ And a high energy density under the premise of high power density is a significant parameter for a good supercapacitor. Fig. 7 shows the Ragone plot of the calculated energy and power density at various current densities according to the eqn (4) and (5). The energy density changes from 26.29 to 8.08 W h kg⁻¹, when the power density ranges from 265 to 1058 W kg⁻¹. Evidently, in this work, the electrochemical characteristics of Ni_0.9Co_1.92Se₄ nanostructures is significantly higher than that of monometallic selenide and Ni–Co precursor, and this performance is also remarkable compared with previously reported values for metal selenide materials for supercapacitors as shown in Table S1.†

Fig. 7 Ragone plot of the estimated energy density and power density of the Ni_0.9Co_1.92Se₄//AC asymmetric supercapacitor at various charge–discharge rates.

Experimental

Preparation of electrode materials

The chemicals and solvents in this work are analytical grade and used without any purification treatment. Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O and urea are purchased from Aladdin. Hydrazine hydrate, NH₄F and Na₂SeO₃·5H₂O are purchased from SCR.

Firstly, Ni foams (1 × 1 cm²) were disposed by consecutive sonication with acetone, 6 M HCl, DIW (distilled water), absolute alcohol, and vacuum dried for 6 hours at 80 °C. Then 2 mmol Ni(NO₃)₂·6H₂O and 4 mmol Co(NO₃)₂·6H₂O were dissolved in a 100 mL Teflon-lined stainless steel autoclave with 80 mL DIW, and added 24 mmol urea and 12 mmol NH₄F to form a pink solution. After stirring to fully dissolved, the pretreated Ni foam was transferred into the autoclave, which was sealed and heated for 6 hours at 120 °C to prepare Ni–Co precursors. After the autoclave cooled down below 30 °C, the samples were cleaned by DIW and absolute alcohol, and vacuum dried for 6 hours at 80 °C.

Then, the Ni–Co precursors on Ni foam were treated for the selenization process. 200 mg Na₂SeO₃·5H₂O and 10 mL hydrazine hydrate were added into 70 mL absolute ethanol solution with the Ni–Co precursors on Ni foam in a 100 mL Teflon-lined stainless steel autoclave for 12 hours at 140 °C. After the autoclave cooled down below 30 °C, the samples were cleaned by DIW and absolute alcohol, and vacuum dried for 6 hours at 80 °C. These steps resulted in Ni_0.9Co_1.92Se₄ nanostructures on Ni foam.

Electrochemical measurements

In this work, we used a three-electrode system to obtain electrochemical data, and Ni_0.9Co_1.92Se₄ nanostructures on Ni foam pressed for 60 s at 10 MPa were used as the working electrode, a platinum network as the counter electrode, Hg/HgO as the reference electrode, 3 M KOH solution as the electrolyte. Galvanostatic charge–discharge (GCD) data were gained by LANDMon (LANHE, CT2001A). Cyclic voltammetry (CV) data were gained by electrochemical analyser (Chenhua, CHI660C). Electrochemical impedance spectroscopy (EIS) data were gained by potentiostat/galvanostat (Ametek, PAR 2273).

Calculations

Specific capacitance (C_sp F g⁻¹), areal capacitance (C_arl F cm⁻²), energy density (E W h kg⁻¹), power density (P kW kg⁻¹) values were calculated from chronoamperometry measurements by the following equations:³¹where I is the current density, Δt is the discharge time, ΔV is the potential window (excluding the IR drop), S is the geometrical area of the electrode, Δm is the mass of the electrode.

Characterizations of electrode materials

The phase purity of the electrode materials was examined by XRD (Bruker, D8-Advance). The structures and morphologies were examined by FESEM (Hitachi, SU8020) and TEM (FEI, Tecnai G2 F20). Elemental composition analyses of the electrode materials were performed using XPS (Thermo, ESCALAB 250Xi) and ICP (Agilent, 7700ce).

Conclusions

In summary, we have successfully grown a coral-like Ni_0.9Co_1.92Se₄ nanostructured material on Ni foam by a simple and controlled two-step solvothermal method for supercapacitors. Furthermore, cobalt partially substituted by nickel in this work leads to synergy between the two materials, and confers on the Ni_0.9Co_1.92Se₄ nanostructures not only a brand-new appearance but also high electrochemical performance relative to monometallic selenide. We have found that Ni_0.9Co_1.92Se₄ nanostructures on Ni foam shows a high specific capacitance of 1021.1 and 785.7 F g⁻¹ and a superior areal capacitance of 6.43 and 4.95 F cm⁻² at current densities of 2 and 50 mA cm⁻², as well as a ascendant rate capability of 77%. What's more, the electrode still shows a superior cycling stability after 5000 cycles with the capacitance retention of 88.39% at a considerable current density of 20 mA cm⁻². When assembled into Ni_0.9Co_1.92Se₄//AC asymmetric supercapacitor based on the Ni_0.9Co_1.92Se₄ nanostructures positive electrode and activated carbon negative electrode, the operating potential window increased to 1.5 V, and a high energy density of 26.29 W h kg⁻¹ is achieved at the power density of 265 W kg⁻¹. More importantly, the unique coral-like Ni_0.9Co_1.92Se₄ nanostructures composed of nanoparticles with sizes of a few tens of nanometers can availably reduce the diffusion resistance of the electrolytes, and shorten the transport pathways of electrons and ions. Our results suggest that the Ni_0.9Co_1.92Se₄ nanostructured materials may supply a good choice for supercapacitors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21266018); the Natural Science Foundation of Inner Mongolia, P. R. China (No. 2010MS0218); Science and technology projects of Science and Technology Department of Inner Mongolia Autonomous Region, P. R. China (No. 20110401 and No. 20130409); the Ministry of Science and Technology China-South Africa Joint Research Program (No. CS08-L15); the National Research Foundation (South Africa, CHN14033166025); the Program for Young Talents of Science and Technology in the Universities of Inner Mongolia Autonomous Region (No. NJYT-15-A04).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17825a

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