Interconnected CuS nanowalls with rough surfaces grown on nickel foam as high-performance electrodes for supercapacitors

Abstract

In this study, we have developed a facile in situ growth process to prepare three-dimensional (3D) interconnected CuS nanowalls on Ni foam, which have a large number of nanosheets on both sides of the nanowalls. The fabrication route contains the replacement process of Cu submicrometer-sized cubes on Ni foam and subsequent in situ growth process of interconnected rough CuS nanowalls (RCuS) on Ni foam via a hydrothermal method. The RCuS electrode displays excellent performance, demonstrating a specific capacitance of 1124 F g⁻¹ at a current density of 15 mA cm⁻² and 912.5 F g⁻¹ at 30 mA cm⁻² with a good cycling ability (∼90.7% of the initial specific capacitance remained after 2000 cycles), which is much higher than that of the CuS electrode through a simple hydrothermal (HCuS), taking copper dichloride as the copper source. The excellent electrochemical performance may be attributed to the high conductivity, large specific surface, and strong bonding between nanowalls and the Ni foam. The demonstrated high specific capacity and remarkable rate performance of the CuS nanowalls, together with the flexibility of the nickel foam substrate, make the 3D nanostructured electrode ideally suited for low-cost, high-performance supercapacitors.

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Introduction

The development of new energy storage techniques is a vital link in the application of renewable energy sources.^1,2 Among the current protocols, supercapacitors are considered as one of the most ideal candidates for energy storage due to their fast charging and discharging capability, high power density, long lifespan and operation safety.^3,4 Based on their charge-storage mechanism, supercapacitors are generally classified into two categories: electrical double-layer capacitors (EDLCs), that use carbon-active materials; and pseudocapacitors, that use redox-active materials. Compared with the electrical double-layer capacitors (EDLCs), pseudocapacitors can achieve much higher specific capacitance and energy density as they can provide a variety of oxidation states for efficient redox reactions.^2,4–6 Transition metal oxides, and hydroxides have been widely investigated as electrode materials for pseudocapacitors because of their high theoretical specific capacitance, environmental friendliness and low cost.^3,7–9 However, the poor conductivity of most transition metal oxides and hydroxides impedes the electron transport, resulting in the gradual loss of capacitance and hindering their practical applications in supercapacitors.^10–12 Therefore, it is desirable to design and fabricate electrode materials with high capacitive performance as well as good electrical conductivity.

Recently, extensive research efforts have been devoted to the development of transition metal sulfides.^13,14 Among metal sulfide compounds, CuS is an important transition-metal chalcogenide semiconductor, and it is an inexpensive and abundant material with widespread applications.^15,16 It has been applied as the electrode material in lithium-ion batteries and supercapacitors for its metal-like electronic conductivity (10⁻³ S cm⁻¹) and high theoretical capacity.^17,18 However, it is not favorable for applications in electrode materials for supercapacitors because CuS usually suffers from poor electronic conductivity and large volume expansion which leads to severe capacity fading and low rate performance arising from poor kinetics.¹⁹ Recent studies revealed that poor kinetics are mainly due to the pseudocapacitive effect which is limited only to a region near the surface (or poor utilization of active materials).^20,21 To ensure high capacitances and facile ionic transportation, it is important to prepare highly porous CuS-based materials with a high specific surface area on a suitable current collector. Accordingly, CuS nanostructures, and CuS-carbon composites involving conductive materials of high surface area have been extensively studied.^18,22–24 The nanostructured electrode materials are typically mixed with ancillary materials such as binder or carbon black and then coated on a current collector, which could greatly decrease the conductivity and the overall specific capacitance. Since electroactive nanostructures grown on three-dimensional (3D) conductive substrates provide more efficient electrical contact, they can be used directly as binder-free electrodes, potentially offering a simpler electrode structure with higher energy density. Nickel foam has been used extensively as a supporting substrate for active materials in supercapacitors and batteries due to its high porosity, large surface area, good electric conductivity, and excellent chemical stability in a wide variety of liquid electrolytes.^1,5,6,25 Several methods to grow electroactive nanostructures on Ni foam have been demonstrated, such as chemical bath deposition, hydrothermal treatment and electrodeposition.^20,26–28 All the above methods just use directly the copper metal salt as copper source of metal sulfides, which may not ensure good contact between the nanostructures and the nickel foam leading to the decreased electron transfer.²⁹

Herein, we prepared 3D interconnected CuS nanowalls with rough surfaces directly on Ni foam by a two-step method containing a simple replacement reaction and subsequent hydrothermal process. The interconnected rough CuS nanowalls offer an excellent charge transfer path and give rise to a large specific area. Due to the unique structure and good contact with substrate, the RCuS electrode has a specific capacitance of 1124 F g⁻¹ when tested as electrodes (without adding any binder and conductive agent) and maintain 90.7% of the initial capacity after 2000 cycles at a current density of 15 mA cm⁻², indicating a high-current capacitive behavior and good cycle performance. These results show that RCuS grown on nickel foams could be a promising electrode for high-performance electrochemical capacitors.

Experimental

Reagents and materials

CuCl₂·2H₂O, sulfur powder, Cu powder, HCl (37%) and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used throughout all the experiments. All of the chemicals were of analytical grade and used without further purification.

Synthesis of the RCuS electrode

The CuS nanowalls were grown on nickel foam through a facile hydrothermal process taking Cu particles as precursors. Prior to the synthesis, Ni foam (10 mm × 10 mm × 1 mm) was first treated by acetone to clean the surface. Then, the Ni foam was immersed into a 3 mol L⁻¹ HCl solution for 20 min to remove the surface oxide layer. Finally, the Ni foam was washed thoroughly with deionized water and absolute ethanol. Then, the Ni foam was immersed in a 0.5 mol L⁻¹ CuCl₂ solution under an atmospheric environment without any special treatment. After 30 min immersion the product was cleaned several times by deionized water and the intermediate product Ni foam coated with Cu particles (Cu–Ni) was dried in vacuum at 80 °C for 3 h.

Through an in situ crystallization process, the interconnected CuS nanowalls grew directly on the Ni foam. In a typical procedure, 0.03 g sulfur powder and 70 mL absolute ethanol were added separately into the autoclave, and a piece of Cu–Ni foam was immersed in the reaction solution. The autoclave was sealed and maintained at 80 °C for 12 h. After it cooled down to room temperature naturally, the Ni foam with the product was taken out of the autoclave and washed with deionized water several times. Finally the RCuS was dried in vacuum at 60 °C for 3 h. For comparison purposes, CuS was synthesized on fresh Ni foam (HCuS) with the additional of 0.03 g sulfur powder and 0.5 mmol CuCl₂. The CuS powder (CuS–P) was prepared by using 0.5 mmol Cu powder and 0.03 g sulfur powder. Both HCuS and CuS–P were obtained in a similar hydrothermal process to RCuS. Moreover, the experimental procedure of fresh Ni foam and 0.03 g sulfur powder in a similar hydrothermal process to RCuS was also conducted.

Materials characterization

X-Ray diffraction (XRD) patterns were obtained with a Philip X’pert pro diffractometer using Cu Kα radiation (λ = 0.154056 nm). The morphologies of the samples were characterized using a scanning electron microscope (SEM, JEOL-JSM-6700F, Japan Electronics). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) measurements were performed using a JEM-2100 transmission electron microscope with a field emission gun operating at 200 kV. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method (NOVA 4200e Surface Area and Pore Size Analyzer). The mass changes of the samples were evaluated by a microbalance with a readability of 0.1 mg (Mettler, AL204).

Electrochemical measurements

The electrochemical measurements of the RCuS, HCuS and CuS–P samples were conducted in a three-electrode electrochemical cell containing 2 mol L⁻¹ KOH aqueous solution as the electrolyte. The interconnected CuS nanowalls, CuS nanosheets on Ni foam, were directly used as the working electrodes and the mass loading of RCuS and HCuS is around 2.4 and 2.6 mg cm⁻², respectively (the mass loading of the CuS on Ni foam was obtained by weighing the residue after dissolving CuS in dilute hydrochloric acid). The area of the working electrode immersed into the electrolyte was cut to about 1 cm². The as-obtained CuS–P (80 wt%), acetylene black (15 wt%) and polytetrafluoroethylene (PTFE, 5 wt%) were mixed and then dispersed in ethanol to produce a homogeneous paste. After coating the slurry onto a nickel foam substrate (10 mm × 10 mm × 1.0 mm), the fabricated electrode was dried at 60 °C for 12 h in a vacuum oven. Cyclic voltammetry (CV) measurements were carried out on an electrochemical workstation (CS2350, Wuhan) at a scanning rate at 5 mV s⁻¹ between −0.3 V and 0.6 V at 25 °C with the sample as the working electrode, saturated calomel electrode as the reference electrode, and Pt foil as the counter-electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out on this apparatus in the frequency range of 0.01 Hz to 1000 Hz. Galvanostatic charging and discharging were also performed at the same workstation in a potential range from −0.3 V to 0.5 V at different current densities. The areal capacitance (C_a, F cm⁻²) and specific capacitance (C_s, F g⁻¹) were calculated by the following eqn (1) and (2):

C_a = IΔt/SΔV (1)

C_s = IΔt/mΔV (2)

where I represented the fixed current (A), Δt is the discharging time (s), S was the area of the active working electrode (cm²), m was the mass loading of electrode material on Ni foam (g), and ΔV was obtained from voltage change in one charging or discharging process (V).

Results and discussion

The fabrication processes of the interconnected rough CuS nanowalls grown on Ni foam are illustrated schematically in Fig. 1A. First, based on the reaction (1), a Cu particle layer closely covered on the Ni foam was obtained from the replacement process, which can be taken as the copper source of RCuS. Then through a hydrothermal method, the Cu–Ni was completely converted to RCuS in the presence of sulfur, which resulted in formation of the interconnected CuS nanowalls with a large number of nanosheets grown on both sides. The equations of these chemical reactions can be described as follows:

Ni + Cu²⁺ → Cu + Ni²⁺ (3)

Cu + S → CuS (4)

Fig. 1 Schematic illustrations of (A) the formation process of roughly crossed CuS nanowalls on Ni foam and (B) the basis for the excellent electrochemical performance of the RCuS electrode.

The expected good electrochemical performance of RCuS could be proved from the great advantages of the interconnected rough CuS nanowalls (shown in Fig. 1B); the according reasons are shown as follows. Firstly, these interconnected nanowalls with high conductivity are beneficial for fast electron transport during the electrochemical process. The mesoporous nanostructure could provide efficient ion and electron transport, giving rise to faster kinetics and resulting in high charge/discharge capacities even at high current densities. Secondly, the hierarchical and mesoporous characteristics of the nanowalls result in a large surface area, providing more electroactive sites for faradaic energy storage. In addition, the ultrathin CuS nanosheets grown on both sides of nanowalls could take full advantage of the space between the nanosheets, allowing the electrolyte ions easier access to the surface of the active materials. Finally, the nanowalls directly grown on the nickel foam can ensure good mechanical adhesion and electrical connection to the current collector, avoiding the use of polymer binders and conducting additives, which generally increase the series resistance and the deterioration of the capacitance during the redox reactions.

To illustrate the morphology and microstructure of interconnected rough CuS nanowall arrays, SEM and TEM analysis are performed. Clearly, the precursor Cu submicrometer-sized cubes are fully grown on the surface of Ni foam (Fig. 2A), indicating that the present replacement process is favorable for preparing Cu submicrometer-sized cubes on conductive substrates. The obtained Cu submicrometer-sized cubes are randomly attached on the Ni foam. Close observation of the cubes shows that an individual Cu submicrometer-sized cube has an average diameter of 200–300 nm. These Cu submicrometer-sized cubes can be easily converted to CuS with the addition of sulfur. Through the hydrothermal process it is apparent that the overall nanowall structures are well preserved, from the insert in Fig. 2B. The top-view SEM images show that the interconnected CuS nanowalls are uniformly distributed and adhere firmly to the surface of the Ni foam with an average diameter of around 800 nm (Fig. 2B). The magnified SEM image in Fig. 2C obviously shows that the interconnected CuS nanowalls have rough surfaces with many ultrathin nanosheets grown on both sides. A magnified image (inset in Fig. 2C) shows that the ultrathin nanosheets on the interconnected nanowalls form abundant open spaces; an enlarged view is shown in Fig. S1.† The as-formed unique interconnected nanowall with abundant open spaces can provide more electroactive surface sites, which could resulted in effective penetration of the electrolyte and enhancement of mass/charge transfer at the electrode/electrolyte interface. Next, the unique architecture of RCuS with rough surfaces can be further confirmed through TEM analyses. The TEM images in Fig. 2D and E demonstrate the existence of a sheet-like structure with ultrathin thickness on the nanowall, which is in good agreement with SEM observations (Fig. S1†). The typically magnified image in Fig. 2F clearly presents a large number of mesopores throughout the whole surface of the nanowalls, which may be due to the overlapping nanosheets on their surface. Meanwhile, the specific surface area and average pore diameter of RCuS, tested by Brunauer–Emmett–Teller measurements, are 42.55 m² g⁻¹ and 3.8 nm, respectively. Moreover, the HRTEM image (inset in Fig. 2F) shows lattice fringes with an interplanar distance of around 0.304 nm, matching well with the spacing of the (102) planes of CuS. For the sake of comparison, HCuS was obtained through hydrothermal treatment taking CuCl₂ directly as the copper source. The SEM images of HCuS displayed in Fig. S2† show that irregular CuS nanosheets grow on Ni foam under similar hydrothermal conditions to RCuS. CuS–P employing Cu powder as the copper source was prepared by a similar hydrothermal process to RCuS in the absence of Ni foam. Fig. S3(A–C)† shows the low and high magnification images CuS–P. The image shows that the CuS powder tends to agglomerate without the Ni substrate (Fig. S3A†), which consists of densely packed irregular nanoplatelet structures with a thickness of about 200–300 nm (Fig. S3B and C†).

Fig. 2 FESEM images of (A) Cu–Ni bulk structure (the inset shows a large-area view) and (B and C) interconnected CuS nanowalls with rough surfaces. The insets in (B) and (C) show a large-area view and a high magnification view, respectively. (D–F) TEM images of interconnected rough CuS nanowalls. The inset in (F) shows the corresponding HRTEM image of CuS nanowalls scratched from Ni foam.

The phase structure of the as-prepared products on Ni foam was studied by the XRD technique. Fig. 3 exhibits the typical XRD patterns of the Cu crystallites and interconnected CuS nanowalls supported on Ni foam. The three strong peaks in the XRD patterns are typical peaks coming from the Ni foam. The Cu–Ni curve represents the XRD patterns of the Ni foam after being immersed in the CuCl₂ solution. The diffraction peaks of Cu–Ni can be well indexed to Cu (JCPDS card no. 04-0836) and Ni (JCPDS no. 04-0850), indicating that the Ni foam has been partially replaced by Cu by immersion in a 0.5 mol L⁻¹ CuCl₂ solution for 0.5 h under ambient temperature. In the XRD patterns of the RCuS electrode, except for the peaks originating from the Ni foam substrate, all the diffraction peaks can correspond to the hexagonal CuS (JCPDS card no. 06-0464) and no characteristic peaks from impurities can be detected. According to the characteristics of the XRD patterns, the successful formation of CuS on Ni foam is undoubtable. Moreover, the XRD patterns of CuS–P in the Fig. S3D† shows that all the diffraction peaks can be indexed to the hexagonal phase of CuS. Then the XRD patterns of HCuS electrode also confirm that the hexagonal CuS has been synthesized directly using CuCl₂ as the copper source (Fig. S4†). Fig. S5† is the XRD pattern of the product obtained from a similar hydrothermal process to RCuS with the addition of bare Ni foam and 0.03 g sulfur powder. It shows that there are almost no extra peaks except for Ni. This indicates that the nickel crystals on the Ni foam cannot react with sulfur powder under this experimental process and thus there is no Ni_xS_y existing in the as-prepared CuS electrode. This demonstrates that as-obtained products are pure CuS. All the XRD patterns have a substrate peak around at 23° (the product is placed on a glass slide during the XRD test).

Fig. 3 XRD patterns of (A) Cu–Ni obtained through the replacement reaction, and (B) RCuS electrode prepared after the hydrothermal process.

With the aim of exploring the advantages of this unique architecture for practical applications, we evaluated the electrochemical performance of the as-obtained CuS electroactive materials for supercapacitors. We applied directly the prepared CuS nanowall arrays on Ni foam as an integrated electrode to highlight the merits of the unique architecture in a three-electrode configuration using 2 mol L⁻¹ KOH as the electrolyte. Fig. 4A shows the CV plots range from −0.3 to 0.6 V at a scan rate of 5 mV s⁻¹. A pair of redox peaks around at 0.44 V and 0.31 V (vs. SCE) can be observed for RCuS, which is due to the transition of the redox couple Cu²⁺/Cu in an KOH electrolyte.^17,22,30 In addition, the integrated areas of the CV plots of the RCuS electrode are apparently larger than that of the HCuS electrode, indicating that RCuS electrode materials prepared with the assistance of the replacement method have significantly larger specific capacitances than those of the HCuS obtained directly from hydrothermal processes. This could be ascribed to the unique interconnected CuS nanowalls of the RCuS electrode that can shorten efficiently the electron charge transfer paths; and the ultrathin nanosheets grown on the nanowalls also produce more spaces between the ultrathin CuS nanosheets which can increase the surface area accessibility for electrolyte ion transport. This phenomenon was further confirmed by chronopotentiometry (CP) measurements. As shown in Fig. 4B, the discharge curves of the both samples at a current density of 15 mA cm⁻² show significant deviation from a straight and flat line, indicating that the capacitance mainly comes from the faradaic redox reactions, which is consistent with the CV curves (Fig. 4A). Compared to the HCuS, the discharge time of RCuS nanowalls was subsequently increased from 72 s to 144 s at a current density of 15 mA cm⁻² (within a voltage window of −0.3 to 0.5 V), indicating that the RCuS electrode exhibits higher specific capacitance values than the HCuS electrode, which is in accord with the CV results. Furthermore, the CV plots at different scan rates for RCuS are shown in Fig. S6A.† The redox peaks can be observed in every CV curve even at the higher scan rates, proving the RCuS electrode has good rate performance. Fig. S6B† displays the charge–discharge curves of the RCuS electrode at different current densities. With increasing of current density, without an obvious IR (internal resistance) drop, the RCuS electrode exhibits outstanding electrochemical reversibility. Based on the eqn (2) mentioned above, the specific capacitance of the RCuS nanowalls can be calculated according to the CD curves (Fig. S6B†), and typical data are presented in Fig. 4C. Impressively, the Ni foam supported RCuS nanowall array electrode delivers high specific capacitances of 1124, 1053, 912.5, 864.3, 815.5 and 689.4 F g⁻¹ at current densities of 15, 20, 30, 40, 50 and 80 mA cm⁻², respectively. The HCuS and CuS–P (Fig. S7†), however, only show a specific capacitance of 500.1 and 126.5 F g⁻¹ at the current density of 15 mA cm⁻², respectively. When the current density is increased from 15 to 80 mA cm⁻², 61.3% of the specific capacitance has been retained. To the best of our knowledge, the good electrochemical performance of the RCuS electrode are considerably better than the previously reported values of the CuS nanostructures; for instance, CuS nanowire arrays on Ni foil (305 F g⁻¹ at 0.6 mA cm⁻²),³¹ CuS nanoparticles (101 F g⁻¹ at 1.5 mA cm⁻²),¹⁶ CuS nanocrystals (346.57 F g⁻¹ at 5 A g⁻¹).³² All the results are summarized in Table S1.† Compared with these CuS based materials, our HCuS and CuS–P show a higher or comparable capacitance, which further demonstrates that the as-synthesized RCuS can be an excellent electrode material for use as a high-performance supercapacitor electrode. The reason for the better electrochemical behavior of RCuS in comparison with HCuS may be that the replacement process during the synthetic route could guarantee favorable adhesion and electrical connection between the CuS nanowalls and Ni foam. Meanwhile, the electrochemical performance of CuS–P can prove sufficiently that the binder-free electrode is beneficial for the electrochemical process, and the substrate can effectively prevent the aggregation of CuS.

Fig. 4 (A) A comparison of CV curves of the HCuS electrode and RCuS electrode at a scan rate of 5 mV s⁻¹; (B) a comparison of the CD curves of as-synthesized electrode materials at a current density of 15 mA cm⁻²; (C) specific capacitances of as-obtained RCuS and HCuS electrodes at different current densities; (D) cycling performance of the as-synthesized electrode materials at a current density of 30 mA cm⁻².

The enhanced supercapacitor performance of RCuS electrode with a unique structure can also be manifested by the cycle capability/cycle life, which is an important parameter to evaluate supercapacitors for practical applications. Fig. 4D shows the cycling performance of RCuS and HCuS determined by a galvanostatic charge–discharge technique at a current density of 12.5 A g⁻¹ (30 mA cm⁻²) in the potential window of −0.3 to 0.5 V. For RCuS and HCuS electrodes, the specific capacitances show a slight increase after the first 100 cycles, which is possibly due to the activation process, which means that unused electrochemically active Ni sites of the active material inside the nickel foam electrode are fully exposed to the electrolyte during the cycling process.^29,33 In Fig. 4D, it is shown that the CuS nanowall structure only exhibits a ∼9.3% loss of specific capacitance, whereas the corresponding HCuS electrode under the same hydrothermal method has around 80.5% retention after 2000 repetitive charge–discharge cycles (from 302.1 to 243.3 F g⁻¹). Meanwhile, the factors responsible for the capacitance decrease may involve a change in the shape, the loss of active surface area and an increase of the resistance during the charge–discharge process. On the basis of the above results, we can see that the RCuS has an obviously higher electrochemical stability and capacitance performance than the HCuS produced only by hydrothermal method. This can be explained as follows: firstly, these interconnected nanowalls with high conductivity are advantageous for fast electron transport. The thin and mesporous nanostructure could provide efficient ion and electron transport, giving rise to faster kinetics and resulting in high charge/discharge capacities even at high current densities. Secondly, the thin and mesoporous characteristics of the nanowalls result in a large surface area, providing more electroactive sites for faradaic energy storage. Thirdly, the ultrathin RCuS nanosheets are grown on the nanowalls, and the space between the RCuS nanosheets can be efficiently utilized, allowing the electrolyte ions easier access to the surface of the active materials. Finally, the directly grown nanowalls can ensure good mechanical adhesion and electrical connection to the current collector, avoiding the use of polymer binders and conducting additives, which generally increase the series resistance and the deterioration of capacitance during the redox reactions.

To understand further the better electronic conductivity of the RCuS electrode, we resorted to electrochemical impedance spectroscopy (EIS) carried out at an open circuit potential with an perturbation of 5 mV in the frequency range of 1000 kHz to 0.01 Hz. The Nyquist plots of the EIS spectra of RCuS, HCuS and CuS–P electrodes are shown in Fig. 5 and S7D.† The impedance spectra have a straight line and a semicircular arc. The measured impedance spectra were analyzed using the complex nonlinear least squares fitting method on the basis of the equivalent circuit, which is given in the inset of Fig. 5. The x-intercept of the Nyquist plot corresponds to the equivalent series resistance (R_s), which contains the contributions of the ionic and electronic resistance. The intrinsic resistance of the material is connected to the electronic resistance, while the interfacial resistance is an integration of the resistance among the current collector and particles and the resistance among the interparticles. The incorporation of a constant phase element (CPE) in the equivalent circuit corresponds to the porosity of the electrode and the semi-infinite diffusion of the cations. The diffusion resistance of the electrolyte resistance in the pores and ionic resistance through the pores are coupled with the diffusion resistance. The high-frequency region presenting a semicircular arc is a result of the charge transfer resistance (R_ct), which can be divided into the double layer capacitance at the electrode/electrolyte interface and the faradaic reactions.^34–37 From the diameter of the semicircle arc the R_ct can be calculated. The measured values of R_s for RCuS, HCuS and CuS–P electrodes is 0.22 Ω, 0.29 Ω and 0.79 Ω, respectively, while the charge-transfer resistance R_ct is 0.02 Ω, 0.17 Ω and 0.23 Ω in the same order. This clearly demonstrates that the RCuS electrode has a much better charge-transfer property against the HCuS and CuS powder electrodes. The charge-transfer resistance R_ct, also called the Faraday resistance, is a limiting factor for the specific power of a supercapacitor, hence the low Faraday resistance of our RCuS electrode really means that a high specific power is achievable. The smaller values of R_s and R_ct for RCuS are due to the porous nanostructure of the electrode facilitating the effective access of electrolyte ions to the active electrode material and shortening the ion diffusion path. Based on the above electrochemical performance, it is apparent that the obtained roughly crossed RCuS electrodes in this work are superior to many of the previously reported CuS based materials, as summarized in Table S1.† These results suggest the great promise of this unique hybrid structure for high-performance pseudocapacitive electrodes characterized in terms of their high specific capacitance, excellent rate capability and long cycle life.

Fig. 5 Nyquist plots of the RCuS and HCuS electrodes.

Conclusions

In summary, CuS with an interesting structure has been successfully fabricated via a replacement process, followed by a hydrothermal method. The as-obtained CuS nanowall electrode exhibits an excellent supercapacitor performance of 1124 F g⁻¹ at 15 mA cm⁻² and an excellent rate capability and good cycle life. The outstanding electrochemical performance of the RCuS electrode can be attributed to the interconnected CuS nanowalls directly grown on the Ni foam and the ultrathin CuS nanosheets grown on both sides of the nanowalls, providing efficient transport pathways for electron transfer during the charge–discharge process. Eventually, this leads to superior current capacitive behavior and better cycling performance compared to HCuS and CuS–P. This work shows the cost-effective synthesis and excellent electrochemical performance of CuS nanowalls, which provide great potential as an active electrode for electrochemical supercapacitors.

Acknowledgements

The present work is financially supported by the Startup Research Fund of Zhengzhou University (51090104) and the Key Scientific and Technological Project of Henan Province, China (No. 082101510007).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The XRD patterns and SEM images of HCuS and CuS–P; the electrochemical characterizations of CuS–P electrode and RCuS electrode; a comparison of the electrochemical performance of CuS-based electrode materials reported in previous literature in a table. See DOI: 10.1039/c6ra10327h

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