Multifunctional hollow sandwich structure with many active sites for electronic transfer modulation and its application in energy storage and conversion

Abstract

Revealing the fundamental mechanism of effective, low-cost multifunctional electrocatalysts based on a hollow sandwich structure is desirable for energy storage and conversion. Transition metal sulfides (TMS) and carbon-based heteroatom-doped materials are designed to achieve hollow structures with large specific surface areas. These materials require the well-organized integration of dual carbon layers with different morphologies and active Ni₃S₂ nanosheets to form a multifunctional electrocatalyst. The interactions among the three components also require elucidation. In this work, we synthesized N-doped hollow carbon spheres (NHC) to obtain a large specific surface area and many active sites. Then, hierarchical Ni₃S₂ nanosheets were adhered to the NHC surfaces to form an NHC/Ni₃S₂ nanocomposite. The nanocomposite was encapsulated in reduced graphene oxide (RGO) to form an NHC/Ni₃S₂/RGO sandwich structure. The NHC/Ni₃S₂/RGO nanocomposite showed outstanding performance as a multifunctional catalyst in dye-sensitized solar cells (DSSCs), in the acidic hydrogen evolution reaction (HER), and in the energy storage systems of supercapacitors (pseudocapacitance). A DSSC containing the nanocomposite as a counter electrode achieved a photoelectric conversion efficiency (PCE) of up to 9.03%. The specific capacitance of the supercapacitor containing the nanocomposite was 990.6 F g⁻¹. When the composite was used as a catalytic electrode for the HER, a current density of 10 mA cm⁻² was achieved with an overpotential of only −142 mV. In addition, the Tafel slope (98.1 mV dec⁻¹) in 0.5 M H₂SO₄ solution was small. This work contributes a strategy for the development of multifunctional catalytic materials for energy storage and conversion.

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New concepts

Catalytic materials in many catalytic fields have single characteristics, so they cannot be used in other fields. To develop and use materials more economically, we have studied common material performance requirements across many catalytic fields, and rationally designed multifunctional materials that can be used in many catalytic fields. In this work, we report a hollow sandwich composite consisting of nitrogen-doped hollow carbon spheres, nickel disulfide flakes, and a conductive reduced graphene oxide protecting layer (NHC/Ni₃S₂/RGO). The large specific surface area contributes to the rapid ion transport in the electrochemical reaction, and the hollow structure provides space for ion storage. The composite provides support for catalysis and energy storage and conversion, and achieves the goal of developing one material for multiple purposes. A dye-sensitized solar cell based on a NHC/Ni₃S₂/RGO counter electrode achieved a power conversion efficiency of 9.03%, a supercapacitor containing the nanocomposite had a specific capacitance of 990.6 mF g⁻¹, and as a catalyst for the hydrogen evolution reaction the nanocomposite achieved a current density of 10 mA cm⁻² with an overpotential of only −142 mV and a small Tafel slope (98.1 mV dec⁻¹) in 0.5 M H₂SO₄ solution.

1. Introduction

The development of human society is closely linked with the development and use of energy. The demand for energy is increasing rapidly with economic development, which has depleted reserves of non-renewable energy resources.^1–5 In addition, the large-scale use of fossil fuel presents severe problems with pollution.^6–8 Currently, in response to global initiatives to protect energy resources and the environment, research is focusing on developing energy sources, devices, and products.^9–11 Compared with fossil fuel, new energy sources have larger reserves and produce less pollution. Solar energy is both a primary energy and renewable energy source.^12–14 It is a rich resource that is free to use and does not require transportation. Most importantly, it does not pollute the environment and has the potential to improve people's lives. The non-renewable resources we are currently using were created by the photosynthesis of ancient plankton and other organisms. Dye-sensitized solar cells (DSSCs) are photoelectric devices that simulate photosynthesis to produce energy.^15–17 In addition, hydrogen is a clean and renewable energy source.^18–20 At present, due to their low-cost, sustainability, excellent electrochemical properties, electrochemical clean energy systems consisting of DSSCs and the hydrogen evolution reaction (HER) are an important area of research in energy conversion. These systems require the development of effective electrocatalysts and the improvement of their electrochemical performance. In addition, supercapacitors are an important method of energy storage.^21–24

Typically, research into electrocatalysts has focused on developing synthesis strategies with precise structure control and design; however, assembling highly efficient multifunctional catalyst electrodes remains a major challenge. To improve the electrochemical performance of an advanced catalytic material for energy storage and conversion, multifunctional catalysts with the following properties are required. (1) The catalyst should form homogeneous nanostructures and exhibit good electron transport. (2) The catalyst should have excellent electrocatalytic activity. (3) The catalyst should have high surface area and absorb the maximum number of electrons in the electrolyte. (4) The catalyst should have high stability in electrolytes. (5) The catalyst should be cheap.

Materials with hollow structures are frequently used in energy storage and conversion,^25–27 owing to their large specific surface area for reactions and high capacity for storing electrons, large number of defects, high electrical conductivity, and large number of active sites. Hollow carbon materials have been used in many fields, such as solar cells,²⁸ supercapacitors,²⁹ the oxygen reduction reaction,³⁰ the oxygen evolution reaction,³¹ and the HER,³² and their electrochemical properties are excellent. Transition metal sulfides (TMS) have also attracted attention owing to their excellent properties and extensive applications in electrochemical energy conversion and storage. Nevertheless, there are few examples of the efficient integration of the carbon/metal–sulfide/carbon constituents into a multifunctional catalyst material.

In this work, we designed N-doped hollow carbon spheres (NHC)/Ni₃S₂/reduced graphene oxide (RGO) nanocomposites with unique ternary interfacial structures. First, Ni₃S₂ nanosheets were embedded on the surface of NHCs, forming interconnected networks to ensure the smooth routes for electrolyte ion diffusion in different directions. Introducing RGO had three advantages. First, the electrons diffusion rate was increased by the two-dimensional layered structure of RGO. Second, RGO introduced more surface defects, which took up more electrons and provided a constant supply of electrons for electrochemical reactions. Third, RGO is a flexible carbon material that protects NHC/Ni₃S₂ from electrolyte corrosion. The NHC/Ni₃S₂/RGO nanocomposites were used as a counter electrode (CE) for DSSCs, and photoelectric conversion efficiency (PCE) of 9.03% was achieved, which was higher than that of Pt (7.75%). The NHC/Ni₃S₂/RGO nanocomposite was also as a catalyst in the HER and showed a respectable electrocatalytic performance; it required an overpotential of only −142 mV in 0.5 M H₂SO₄ solution to provide a current density of 10 mA cm⁻². In addition, our multifunctional material could also be used as an active material for supercapacitors and had a good electrochemical performance with a specific capacity of 990.6 F g⁻¹ at a current density of 1 A g⁻¹.

2. Results and discussion

2.1 Morphology and composition

The synthesis of the NHC/Ni₃S₂/RGO composite with a hollow sandwich structure is shown in Fig. 1. First, SiO₂ nanospheres were coated with nitrogen-doped carbon to form SiO₂/N-carbon, and then nickel silicate sheets were anchored to the surface of the N-carbon spheres to form N-carbon/NiSi/SiO₂. The SiO₂ and N-carbon spheres were used as sacrificial templates. The SiO₂ was etched away, leaving N-doped hollow carbon spheres (NHC), which were vulcanized to form NHC coated with Ni₃S₂ nanosheets (NHC/Ni₃S₂) and finally sandwiched with RGO to form a multilayer composite structure (NHC/Ni₃S₂/RGO).

Fig. 1 Synthesis of NHC/Ni₃S₂/RGO composites with a hollow sandwich structure.

SEM and TEM were used to characterize the morphology of the nanocomposites. Fig. 2 shows SEM images at different magnification of NHC/Ni₃S₂ (Fig. 2a and b) and NHC/Ni₃S₂/RGO (Fig. 2c and d). In Fig. 2a, many uniform nanospheres are visible (magnification of 30 000×). Fig. 2b shows that the nanospheres are about 400 nm in diameter (high magnification of 120 000×). Criss-crossing nanosheets are attached to the surface of the nanospheres, and the gaps between the nanosheets are clearly visible. These loose sheets increase the specific surface area of the material, and thus increase the contact area between the electrolyte and the material to minimize dead volume. Fig. 2c and d show images of NHC/Ni₃S₂/RGO, and it is clear that the RGO encloses the complex. The introduction of RGO increases the electronic transport network and protects the material from corrosion and collapse in the acid–base electrolyte. The TEM images (Fig. 2e–h) show that the hollow structure of the Ni₃S₂ nanosheet shell combined with NHC provides abundant electrochemical active sites and a large contact area for the electrolytic electrode for electrochemical reactions.³³ The NHC promotes electron transfer and improves the speed of electrochemical reactions.³⁴ Element mapping by TEM-EDS of the NHC/Ni₃S₂/RGO nanocomposite is shown in Fig. 2i–m. C, N, S, and Ni are uniformly distributed over the hollow sphere shell structure without other impurities.

Fig. 2 (a and b) SEM images of NHC/Ni₃S₂. (c and d) SEM images of NHC/Ni₃S₂/RGO. (e and f) TEM images of NHC/Ni₃S₂. (g and h) TEM images of NHC/Ni₃S₂/RGO. (i) STEM image of NHC/Ni₃S₂ (TEM-EDS technology). (j–m) Corresponding elemental mapping images for (j) C, (k) N, (l) Ni, and (m) S.

2.2 Crystal structure and composition

The crystal structures of NHC/NiSi, NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO were characterized by XRD (Fig. 3a). The characteristic peaks at 21.7°, 31.1°, 38.3°, 44.3°, 55.1° and 77.6° corresponded to the (101), (110), (021), (202), (122) and (223) crystal planes of Ni₃S₂ (JCPSD 44-1418) in the NHC/Ni₃S₂/RGO composite, respectively.³⁵ The XRD pattern of NHC/Ni₃S₂ was similar to that of NHC/Ni₃S₂/RGO, except that the peak strengths at 21.7° or 31.1° were decreased by the surface coverage of RGO, which confirmed the introduction of RGO.

Fig. 3 (a) XRD patterns of NHC, NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO. (b) Wide XPS spectra of all samples. (c) Ni 2p, (d) S 2p, (e) N 1s, and (f) C 1s spectra. (g) N₂ adsorption–desorption isotherms of NHC, NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO. (h) Corresponding pore size distributions of all samples. (i) Specific surface areas and pore size distributions of all samples.

The surface chemical states of the samples were characterized by XPS (Fig. 3b). Only the S 2p (153.9 eV), C 1s (285.3 eV), N 1s (399.6 eV), O 1s (533.5 eV), and Ni 2p (854.7 eV) peaks were observed. There were no impurities, which was in good agreement with the element mapping results. The Ni 2p region (Fig. 3c) contained two peaks at 854.5 and 872.6 eV for Ni 2p_2/3 and Ni 2p_1/2, respectively. The peaks at 874.7 and 856.2 eV corresponded to Ni³⁺, and the peaks at 872.1 and 854.3 eV corresponded to Ni²⁺. The S 2p (Fig. 3d) region contained two peaks at 163.1 and 164.1 eV for S 2p_2/3 and S 2p_1/2, respectively, and the peak at 168.4 eV corresponded to SO_x⁻. The N 1s region is shown in Fig. 3e. The N 1s peak was fitted to the following three components according to the binding energies: pyridinic N (401.1 eV), graphitic N (399.2 eV), and pyrrolic N (398.2 eV).³⁶ The C 1s spectra of NHC/Ni₃S₂/RGO showed typical peaks for C=C (284.6 eV), C–N (285.3 eV), C–O (286.5 eV) and C=O (288.4 eV).³⁷ These results indicated N was incorporated into the hollow carbon sphere.

2.3 Brunauer–Emmett–Teller analysis

The specific surface area and porosity of NHC, NHC/Ni₃S₂, and NHC/Ni₃S₂/RGO were determined by nitrogen adsorption and desorption isotherms (Fig. 3g). NHC had a large specific surface area (750.27 m² g⁻¹), owing to the hollow spherical frame. Adding Ni₃S₂ sheets to the surface of NHC to form NHC/Ni₃S₂ decreased the specific surface area (316.74 m² g⁻¹). However, the introduction of RGO increased the specific surface area to 365.84 m² g⁻¹ for the NHC/Ni₃S₂/RGO composite. This increase was attributed to the large internal space, rough surface structure and the introduction of RGO. The large specific surface area increased the contact area, avoided excessive dead volume, and introduced more active sites, which improved the electrochemical properties. The pore size distribution was evaluated using the Barrett–Joyner–Halenda method (Fig. 3h). The pore size distributions of NHC, NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO were evaluated as 3.89, 6.85, and 5.40 nm, respectively.

2.4 Performance of DSSCs

To demonstrate the application of the as-prepared materials as the CE in DSSCs and to measure the photovoltaic performance, the photocurrent density–voltage (J–V) curves of NHC, NHC/Ni₃S₂, and NHC/Ni₃S₂/RGO (Fig. 4a) were tested under AM 1.5 irradiation (100 mW cm⁻²),³⁸ and the results are shown in Fig. 4b. The PCE of NHC, NHC/Ni₃S₂, NHC/Ni₃S₂/RGO, and Pt as DSSC CEs were 5.52%, 8.53%, 9.03% and 7.75%, respectively. NHC/Ni₃S₂/RGO had the best PCE (9.03%), which was almost twice that of NHC (5.52%), and higher than that of Pt (7.75%). All parameters are listed in Table 1. The photoelectric transformation performance results show that in the optimized structure, the hollow carbon spheres provide huge ion storage, which rapidly provides electrolyte ions for the electrochemical reaction, and the conductivity of the carbon shell also provides a better channel for electron transfer. However, the relatively smooth surface structure of the carbon shell only provides a limited number of active sites for taking up electrons.^8,39–41 In contrast, the increased surface roughness NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO increases the number of active sites, which allows greater take up of electrons, and thus increases the speed of the transformation of the iodine ions.⁴²

Fig. 4 (a) J–V curves of DSSCs with different CEs. (b) PCEs for all samples, including a DSSC with a Pt CE for comparison. (c) IPCE curves of DSSCs with different CEs. (d) Light response for all samples, including a DSSC with a Pt CE for comparison.

Table 1 EIS, Tafel curves, and photovoltaic parameters for DSSCs based on NHC, NHC/Ni₃S₂, NHC/Ni₃S₂/RGO and Pt CEs in the same environment

CEs	R s (Ω cm^2)	R ct (Ω cm^2)	lg J0 (mA cm^−2)	lg Jlim (mA cm^−2)	J sc (mA cm^−2)	V oc (V)	FF (%)	PCE (%)
NHC	19.1 ± 0.01	1.88 ± 0.01	0.44 ± 0.01	1.51 ± 0.01	14.1	0.773	50.57	5.52
NHC/Ni3S2	13.8 ± 0.01	0.25 ± 0.01	0.73 ± 0.01	1.67 ± 0.01	16.3	0.768	67.97	8.53
NHC/Ni3S2/RGO	13.7 ± 0.01	0.14 ± 0.01	0.87 ± 0.01	1.69 ± 0.01	17.7	0.773	66.44	9.03
Pt	13.9 ± 0.01	0.27 ± 0.01	0.56 ± 0.01	1.54 ± 0.01	14.1	0.759	64.14	7.75

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Fig. 4c shows the incident photoelectron conversion efficiency (IPCE) curve for DSSCs containing each material as a photoanode with a sandwich structure. The IPCE spectra show a similar photoelectric response in the wavelength range of 450 to 750 nm. The results show that the photoelectric response values of traditional Pt, NHC/Ni₃S₂/RGO, NHC/Ni₃S₂, and NHC were 61.05%, 71.54%, 50.20% and 44.71%, respectively (Fig. 4d). Therefore, NHC/Ni₃S₂/RGO can be used as a promising alternative electrode for platinum-free DSSCs.

Based on the photoelectric conversion properties, the electrocatalytic properties were examined by cyclic voltammetry (CV) curves in a three-electrode system with a scanning step of 25 mV s⁻¹. Fig. S1a (ESI†) indicates that the NHC/Ni₃S₂/RGO nanocomposite had better catalytic activity than the other as-prepared samples and conventional Pt for I₃⁻ in the electrolyte. The two pairs of redox peaks are labeled Ox1 and Red1, and Ox2 and Red2. Eqn (1) and (2) describe the cycling of I₃⁻ catalyzed by the CE in a DSSC system.⁴³

I₃⁻ + 2e⁻ = 3I⁻, (1)

3I₂ + 2e⁻ = 2I₃⁻. (2)

The strength of the peak represents the redox ability for I₃⁻ of the material, and a strong peak indicates a high current density, which mainly reflects the adsorption and transfer of ions on the surface of the CE material and is also an important factor for the evaluation of iodide current density. The order of peak strength of the materials is NHC/Ni₃S₂/RGO > NHC/Ni₃S₂ > Pt > NHC, showing that NHC/Ni₃S₂/RGO has a huge specific surface area, which not only takes up more electrons and increases the current density, but also achieves super catalytic reduction of I₃⁻. E_pp is another important parameter for CE materials to assess the catalytic activity for I₃⁻ reduction.⁴⁴ In the redox reaction of iodide ions, a narrower E_pp indicates higher catalytic velocity. NHC/Ni₃S₂/RGO had the smallest E_pp value of the CEs, which was 23% lower than the E_pp value of conventional Pt. The values are shown in Fig. S1b (ESI†). The corrosive nature of the electrolyte destroys TMS, which hampers the continuous generation of surface active sites and affects the lifespan of the material. Thus, flexible RGO was used to encapsulate the NHC/Ni₃S₂ nanocomposites and provide a layer of protection. We also performed electrochemical stability analysis on the materials in the electrolyte by increasing the number of CV scans to 30 (Fig. S2, ESI†). Only the CV trace for NHC/Ni₃S₂/RGO remained virtually unchanged, revealing that NHC/Ni₃S₂/RGO had stable electrochemical properties and good corrosion resistance. Incorporating RGO has other advantages, such as creating an RGO network structure that provides more electron transmission paths and increasing the specific surface area, which provides more active sites. Thus, RGO allows excellent conductivity and remarkable catalytic activity to be achieved.⁴⁵

The electrocatalytic properties were also assessed via electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements. An EIS test was performed to evaluate the electron transfer process and exchange current density data at the interface between the electrolyte and the CE (Fig. S3, ESI†). In the figure, the equivalent circuit model, containing series resistance (R_s) and charge transfer resistance (R_ct), was used to fit the recorded data. The results contained two semicircles. The high-frequency region is the semicircle on the left in the Nyquist curve, which is usually related to the electrocatalytic performance of the CE.⁴⁶ The intercept between the left semicircle and the x-axis in the high-frequency region represents R_s for the entire cell. The left semicircle in the high-frequency region represents R_ct, and is the sum of resistances generated during the redox process at the interface between the CE and the electrolyte.⁴⁷ The right semicircle is the low-frequency region, indicating the Nernst diffusion impedance (Z_N) of redox-coupled transport in the electrolyte.⁴⁸ The values are shown in Table 1. The order of R_ct and R_s values for the four samples is NHC/Ni₃S₂/RGO < NHC/Ni₃S₂ < Pt < NHC because NHC/Ni₃S₂/RGO has multiple channels formed by the crossing nanosheets. The results show that NHC/Ni₃S₂/RGO has excellent electrocatalytic properties. The addition of RGO greatly increases the contact area of ions, prevents dead volume, increases the number of electron transmission channels and greatly improves the conductivity. The smaller impedance allows the electrons to flow more smoothly between the electrode material and the electrolyte.

Tafel polarization measurements were carried out in a symmetrical battery (Fig. S4, ESI†). Theoretically, Tafel curves can be divided into the polarization region, Tafel region and diffusion region. The Tafel and diffusion zones are used to elucidate the electrocatalytic activity of the CE in the reduction of I₃⁻.⁴⁹ Exchange current density (J₀) and diffusion-limited current density (J_lim) are extracted from the Tafel region and diffusion region, respectively. The values of J₀ and J_lim are shown in Table 1. Both J₀ and J_lim of NHC/Ni₃S₂/RGO are the largest values among the materials, including Pt, indicating that as a CE, this material has excellent electrocatalytic activity for I₃⁻ reduction, which is also consistent with the electrochemical test results. The excellent catalytic activity of the material was attributed to the hollow spherical shell that has a large specific surface area, the high electrical conductivity of Ni₃S₂ nanosheets, and creation of electronic transmission channels by RGO. Thus, the current density remained high throughout the electrochemical reaction.

2.5 Electrochemical properties of supercapacitors (pseudocapacitance)

To evaluate the electrochemical pseudocapacitance of the electrode materials, we first determined their CV curves with a constant-current charge–discharge technique in a three-electrode system, with KOH solution (6 M) as the electrolyte. The CV curves for NHC, NHC/Ni₃S₂, and NHC/Ni₃S₂/RGO (Fig. 5a) were typical, with a potential window of 0–0.6 V (vs. Ag/AgCl) at a scan rate of 5 mV s⁻¹, indicating typical pseudocapacitive behaviors.⁵⁰ The capacitance is mainly affected by the Faraday redox reaction.⁵¹ All the CV curves consisted of a pair of strong redox peaks corresponding to the conversion reaction of the materials in KOH solution. At a potential of 0.37 V, the CV curves of NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO both had a clear anode oxidation peak, but the peak for NHC/Ni₃S₂/RGO was much higher, demonstrating an excellent oxidation reaction. The cathode peak was observed near a potential of 0.185 V, and corresponded to the reduction process in the Faraday reaction. The anode and cathode peaks were symmetric, showing that the NHC/Ni₃S₂/RGO active material electrode had excellent reversibility. In addition, when the scan rate was increased from 5 to 100 mV s⁻¹, the shape of the CV curves did not change substantially (Fig. S5, ESI†), which indicated that the mass transfer rates were increased, the electron conduction was increased, and the equivalent series resistance was decreased. In addition, the anode peaks moved to the positive pole and the cathode peaks moved to the negative pole because of the internal resistance of the electrode materials.⁵¹

Fig. 5 Electrochemical characterization of NHC, NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO. (a) CV curves at a scan rate of 5 mV s⁻¹. (b) GCD curves at a current density of 1 A g⁻¹. (c) Long-term stability of NHC, NHC/Ni₃S₂ and NHC/Ni₃S₂/RGO at a GCD current density of 1 A g⁻¹. (d) Schematic of eNHC/Ni₃S₂/RGO as an active material for supercapacitors in the electrochemical reaction.

Specific capacitance is a key factor in determining whether an active material can be used as the electrode of a supercapacitor, which is represented by C (F g⁻¹). The specific capacitance is calculated by eqn (3).

Here, Δt is the discharge time, I is the discharge current, the m is the mass of the active material in the electrode system, and ΔV is the charge potential at discharge. The constant-current charge–discharge test has a better response to the ratio capacitance. For a potential window of 0–0.6 V, the constant-current galvanostatic charge/discharge (GCD) curves maintain good symmetry (Fig. 5b). The NHC/Ni₃S₂/RGO electrode active material has the longest discharge time and the largest specific capacitance of the electrode active materials. The specific capacitances of NHC/Ni₃S₂/RGO, NHC/Ni₃S₂ and NHC are 990.6, 733.2, and 540.5 F g⁻¹, respectively, at a current density of 1 A g⁻¹. We also tested the charging and discharging properties of the NHC/Ni₃S₂/RGO electrode active material with current densities from 1 to 10 A g⁻¹ and a potential window of 0–0.6 V (Fig. S6, ESI†). With the increase of current density, the specific capacitance decreased gradually, and the specific capacitance values of 1, 2, 5, 8, and 10 A g⁻¹ were 990.6, 965.3, 958.3, 950.3, and 935.5 F g⁻¹, respectively. Fig. S7 (ESI†) shows the specific capacitances of the materials at different current densities.

The mechanism of the excellent capacitance of the electrodes consisting of the active materials was studied further by electrochemical EIS (Fig. S8, ESI†). Fig. S9 (ESI†) is an enlargement of the high-frequency region. The high-frequency region of the NHC/Ni₃S₂/RGO electrode active material is a pseudo-semicircle, whereas those of the other two materials are not, suggesting the NHC/Ni₃S₂/RGO electrode active material has smaller charge transfer resistance.⁵² In the low-frequency region, the slope of the NHC/Ni₃S₂/RGO electrode active material is higher than those of the other two materials, and indicating that the NHC/Ni₃S₂/RGO electrode has lower diffusion resistance and faster ion velocity.

To evaluate the durability and the performance during long-term cycling of each active material at a current density of 1 A g⁻¹, the GCD curves were measured for each active material for 6000 cycles. After 6000 cycles, the specific capacitance of the NHC/Ni₃S₂/RGO electrode remained at 91.71% of the original value, whereas those of NHC/Ni₃S₂ and NHC were only 70.01% and 44.38%, respectively (Fig. 5c). These results also demonstrate that the NHC/Ni₃S₂/RGO electrode material had more stable electrochemical and cyclic properties that the other electrode materials. Fig. 5d shows the electrochemical mechanism of the materials. NHC/Ni₃S₂/RGO had good specific capacitance because the hollow carbon spheres function as a huge electronic vault, the electrical conductivity of the Ni₃S₂ nanosheets is large, and RGO forms electronic transmission channels. The electrolyte can easily diffuse into NHC/Ni₃S₂/RGO through the open spaces between the crisscross nanosheets. The hollow sandwich structure ensures that NHC/Ni₃S₂/RGO is in close contact with the electrolyte, and the electrons and electrolyte ions can easily diffuse.

2.6 Electrocatalytic acidic HER

Hollow carbon materials and TMS have been widely used for the electrocatalytic HER. However, composite materials consisting of TMS grown on the surface of hollow carbon materials, particularly the hollow sandwich structure formed by combining carbon materials with different morphologies and TMS, have seldom been used in this field. For the acidic electrocatalytic HER, a NHC/Ni₃S₂/RGO catalyst with large pores, high specific surface area and non-noble metals was prepared. The excellent performance of the NHC/Ni₃S₂/RGO nanocomposite in DSSCs and supercapacitors suggested that it may have catalytic activity in the electrocatalytic HER.

To test whether our assumption was correct, we measured the electrochemical HER performance of the NHC/Ni₃S₂/RGO catalyst by linear scanning voltammetry (LSV).⁵³ The HER catalytic performances of NHC/Ni₃S₂/RGO, NHC/Ni₃S₂, NHC and Pt/C (20 wt%) were evaluated with a three-electrode standard test system with a scan range of 0.1 to −0.6 V (vs. RHE), and the electrolyte was H₂SO₄ (0.5 M) (Fig. 6a). The as-prepared catalyst materials were placed on a rotating disk electrode and the scan rate was 5 mV s⁻¹ at 2000 rpm. Owing to the subjectivity of the determination of the onset potential, the overpotential corresponding to the current density of 0.05–5 mA cm⁻² is usually determined as the onset overpotential.⁵⁴ In this paper, the overpotential corresponding to the current density of 0.25 mA cm⁻² was determined as the onset overpotential. The LSV polarization curves show that the onset overpotential of the NHC/Ni₃S₂/RGO catalyst was closer to that of the Pt/C catalyst (20 wt%) in Fig. 6b. The preliminary results showed that the NHC/Ni₃S₂/RGO had good electrocatalytic activity. When the current density was increased to 10 mA cm⁻², the overpotentials of NHC/Ni₃S₂ and NHC were −186.27 and −205.46 mV, respectively, whereas it was only −142 mV for NHC/Ni₃S₂/RGO because of the quicker discharge reaction of H⁺ (2H⁺ → H₂) in acid solution (Fig. S10, ESI†).⁵⁵ For a current density of 15 mA cm⁻², the overpotential of NHC/Ni₃S₂/RGO was −188.13 mV, which was only −45.84 mV higher than that of 10 mA cm⁻², whereas the overpotential of NHC/Ni₃S₂ was −235.05 mV. In addition, the current density increases rapidly with the increase of overpotential and the current density is also an important standard factor for indicating the catalytic activity of catalysts.^53,54 In further comparisons, when the overpotential reached −150 mV, the cathode current density of NHC/Ni₃S₂/RGO was 10.89 mA cm⁻², which was larger than that of NHC/Ni₃S₂ (6.63 mA cm⁻²) and 2.1 times higher than that of NHC (5.11 mA cm⁻²). When the overpotential was further increased to 200 mV, the current density of NHC/Ni₃S₂/RGO reached 16.32 mA cm⁻², which was 1.68 times that of NHC (9.69 mA cm⁻²). The current density of NHC/Ni₃S₂/RGO was much higher than those of NHC/Ni₃S₂ and NHC when the overpotential was the same.

Fig. 6 (a) Standard acidic electrolyte three-electrode system. (b) LSV curves and (c) corresponding Tafel plots for different catalysts in 0.5 M H₂SO₄ solution. (d) Initial and 2000th polarization curves of NHC/Ni₃S₂/RGO.

To explore the electrocatalytic HER activity of NHC/Ni₃S₂/RGO further, the Tafel curves (Fig. 6c) of the four samples were measured with the potential negatively scanned from −0.5 to 0.1 V (vs. RHE) at a scan rate of 5 mV s⁻¹. The Tafel curves were plotted as the overpotential change to the logarithm of the current density. The intrinsic characteristics of electrocatalytic hydrogen evolution materials are determined from the Tafel slopes, which are obtained by fitting the linear part of the Tafel curves.⁵⁵ Generally, in accordance with the values of the Tafel slope, there are three classical reactions in the catalytic mechanism of HER: the Volmer reaction, the Heyrovsky reaction, and the Tafel reaction, which have slopes of 118.2, 39.4, and 29.6 mV dec⁻¹, respectively.^56,57 The Tafel slope of as-prepared NHC/Ni₃S₂/RGO was 98.1 mV dec^−l; thus, the catalytic mechanism of HER matched the Volmer–Heyrovsky reaction. A smaller Tafel slope shows better scaling of the kinetics with voltage and faster kinetics of hydrogen evolution.⁵⁷ The Tafel slope of NHC/Ni₃S₂/RGO was closest to that of 20 wt% Pt/C (35.3 mV dec^−l) (Fig. 6c). The results show that the NHC/Ni₃S₂/RGO has excellent performance for electrocatalytic hydrogen evolution. Nitrogen atom doping changes the crystal structure of the material and rearranges the surface electron distribution, resulting in more surface defects.⁵⁸ The rough surface of the hollow structure greatly increased the specific surface area to produce more hydrogen evolution active sites.⁵⁹ RGO increased the electron transport rate and the current density.

To assess the durability of NHC/Ni₃S₂/RGO as a catalytic material, long-term CV between 0.1 and 0.50 V (vs. RHE) was performed. When the current density was 10 mA cm⁻², the polarization curve period of the catalytic material after 2000 cycles almost overlapped with the initial period, indicating the excellent durability of the material for electrocatalysis (Fig. 6d).^60,61

3. Conclusions

We demonstrated the use of NHC as substrate for preparing NHC/Ni₃S₂/RGO nanocomposites with a multifunctional sandwich structure for use in DSSCs, supercapacitors, and the acidic HER. Electrochemical tests proved that the NHC/Ni₃S₂/RGO nanocomposite exhibited better catalytic activity than NHC, NHC/Ni₃S₂ and Pt, demonstrating the essential role of the sandwich structure and heteroatom doping affecting the electronic modulation from the charge redistribution. The electrolyte easily diffused into the NHC/Ni₃S₂/RGO nanocomposite through the openings between the crisscross nanosheets. The hollow sandwich structure ensured that the NHC/Ni₃S₂/RGO nanocomposite was in close contact with the electrolyte, and that the electrons and electrolyte ions easily diffused in between. The nanocomposite is a promising candidate for improving the performance of various energy technologies and devices.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by National Key R&D Program of China (2017YFA0403503), National Natural Science Foundation of China (11674001), Open Fund for Discipline Construction, Institute of Physical Science and Information Technology (S01003103, Anhui University), Anhui Provincial Natural Science Foundation (1708085MA07, 1708085QE116), the Key Natural Science Research Program of Anhui Educational Committee (KJ2018ZD001), and the doctoral research start-up funds projects of Anhui University (J01003201). We are grateful for the experimental materials provided by Nanjing XianFeng Nanomaterials Technology Co., Ltd. We acknowledge the experimental instruments and technical guidance provided by PINE Company and Hong Kong PHYCHEMi Company.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nh00133f

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