Novel heterostructure Cu₂S/Ni₃S₂ coral-like nanoarrays on Ni foam to enhance hydrogen evolution reaction in alkaline media

Abstract

Exploring efficient alternatives to precious noble metal catalysts is a challenge. Here, a new type of non-noble metal Cu₂S/Ni₃S₂ heterostructure nanosheet array is fabricated on 3D Ni foam. This electrocatalyst has excellent activity and durability to Hydrogen Evolution Reaction (HER) under alkaline conditions. The synergistic catalysis produced by the {2̄10} and (034) crystal planes and the increase in charge transfer and the number of active sites caused by lattice defects greatly improve the electrocatalytic activity of Ni₃S₂. In the HER process, the Cu₂S/Ni₃S₂ interface increases the formation of S–H bonds, and Cu₂S promotes the transformation during the HER process into S-doped CuO, optimizing the adsorption capacity of S-doped sites for H. Among electrocatalysts made with different feed ratios, Cu₂S/Ni₃S₂/NF-3, for HER, only needs an overpotential of 50 mV to deliver a current density of 10 mA cm⁻². This work provides a promising non-noble metal electrocatalyst for water splitting under alkaline conditions.

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1. Introduction

H₂, with its 142 MJ kg⁻¹ energy density and nonpolluting reproducibility, is a sustainable energy source worthy of research.¹ The application of hydrogen energy urgently requires exploring cleaner, cheaper, and more efficient hydrogen production technology.² A viable clean hydrogen production technology is water splitting, but the overpotential requirements of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) limit the wide application. The slow HER kinetic process enormously limits the overall efficiency.³ Platinum-based electrocatalysts are recognized as the top electrochemical HER catalysts. However, noble metal catalysts cannot be widely used due to practical factors. Commercially available traditional alkaline electrocatalysts are affordable but not sufficiently active,⁴ such as stainless steel,⁵ RANEY® nickel,⁶ and nickel alloys.⁷ Therefore, exploring catalysts that are effective for HER in abundant non-noble metal elements is urgent.

Owing to these practical problems, research has been carried out for decades to find substitutes for noble metals. Among them, high-performance nickel-based materials with excellent electrochemical performance and intrinsic electrocatalytic activity are expected to become the best candidates.^8–11 Among various nickel sulfides, Ni₃S₂ with inherent metallic advantages can be a suitable candidate for HER and OER catalysts.¹² As an effective catalyst active ingredient, Ni₃S₂ is widely used in electrode materials for supercapacitors and lithium–sulfur batteries.¹³ Miaomiao Tong¹⁴ built special 3D Ni₃S₂ nanorods@nanosheets, improved their adhesion properties, and exposed more active sites. However, Ni₃S₂ can still be modified in terms of increasing conductivity, improving catalytic activity, and exposing active sites.^15,16 Owing to the inevitable sulfur vacancies in Ni₃S₂, the regulation of its surface electronic structure has become a way to improve catalytic activity. Metallic element doping is a feasible solution. It can adjust the adsorption/desorption energy, expand the effective surface area, regulate the electronic structure, and exert a synergistic role between ions.^17–19 Relevant literature has reported that Fe,²⁰ Co,²¹ Zn,²² and Mo²³ doping in Ni₃S₂ improves HER or OER performance. However, Cu-doped high-activity Ni₃S₂ catalyst with a great HER performance has rarely been reported. Cu, as a doping element, does have a good effect. Several researchers doped Cu as catalytic materials. Zhang et al.²⁴ reported that the charge transfer ability and surface area of Cu-doped Fe–Co oxide have been greatly improved. Co/Cu-modified NiO designed by Guo, ZG et al.²⁵ proved that Cu doping can activate Ni sites at a low overpotential, thereby increasing conductivity and accelerating charge transfer. Until now, studies have focused on the use of doped modified substrate materials. For example, the recent report of Du's group about Cu doped Ni₃S₂ has an overpotential of 91 mV.²⁶ However, the intrinsic catalytic activity of each substance is different, and it is a novel idea to improve the catalytic activity by constructing a heterostructure. There are few reports on the heterostructure of Cu₂S and Ni₃S₂, which is a direction worth studying. We used the activity of Cu₂S to construct a heterostructure Cu₂S/Ni₃S₂ greatly improved the catalytic activity of Ni₃S₂ for hydrogen evolution.

Based on the above viewpoints, a two-step hydrothermal method was designed to successfully grow Cu₂S/Ni₃S₂ with a stable and regular morphology heterostructure on nickel foam. In the first step, the precursor (Cu–Ni layered double hydroxide [LDH]) of the composite structure of nanosheets and nanowires is prepared by changing the feed ratio and reaction conditions. In the second step, a sulfide ion exchange is used to obtain the target sample (Cu₂S/Ni₃S₂/NF). The synergistic effect between Cu₂S and Ni₃S₂ of heterostructure makes the material obtain excellent HER performance. Cu₂S/Ni₃S₂/NF-3 to achieve a current density of 10 mA cm⁻² only needs an overpotential of 50 mV, which is much lower than the reported Cu doped Ni₃S₂ and Ni–S–Cu systems.²⁶ The number of active sites calculated by the turnover frequency (TOF) is 3.568 × 10⁻⁴ mol. Moreover, the actual surface area of Cu₂S/Ni₃S₂/NF-3 is about 2.2 times that of Ni₃S₂/NF. The high electrocatalytic performance of Cu₂S/Ni₃S₂/NF is mainly manifested in the following aspects: 1. After vulcanization, the surface of the layer structure is “coral-like,” exposing abundant active sites and allowing the active ingredients to be in close contact with the electrolyte. 2. The combination of Cu₂S and Ni₃S₂, Cu₂S is converted into S-doped CuO during the HER process, where CuO introduces a defect level near the Fermi level, accelerates charge transfer and improves intrinsic conductivity.²⁷ 3. The high conductivity of Ni₃S₂ provides a fast charge transfer and promotes the electrocatalytic reaction of the catalyst. 4. High-index {2̄10} of Ni₃S₂ and exposed (034) crystal plane of Cu₂S improve electrocatalytic performance.

2. Experimental section

2.1 Experimental materials preparation

In this experiment, chemicals include ionized water (>18.25 mΩ cm⁻¹, Millipore), and CuCl₂·2H₂O, NiCl₂·6H₂O, CH₄N₂O, NH₄F, Na₂S, all purchased from Sinopharm Chemical Reagent Co., Ltd. The nickel foam (NF) used in all experiment were obtained from Kunshan Desco Electronics Co., Ltd. (Suzhou, China) and the density of NF was 350 g m⁻².

2.1.1 Fabricating Ni–Cu LDH nanosheet array on NF. First, CuCl₂·2H₂O and NiCl₂·6H₂O were dissolved in deionized water(70 mL) at different molar ratios (the specific feed ratio and the corresponding serial number are shown in Table 1), 10 mmol CH₄N₂O and 4 mmol NH₄F were add in above solution. Then the precursor solution, stirred for 10 minutes, was transferred into a 100 mL Teflon-line stainless autoclave. The prepared clean NF (2 × 5 cm) was immersed in it, then reacted at 120 °C for 6 h. After cooling, the obtained Ni–Cu LDH/NF was washed and then placed at 60 °C to dry overnight in a vacuum oven. Fig. 1 shows the schematic diagram of preparation of Cu₂S/Ni₃S₂/NF.

Table 1 Different feed ratios and corresponding numbers

CuCl2·2H2O	NiCl2·6H2O	Corresponding Cu–Ni LDH
2 mmol	2 mmol	Cu–Ni LDH/NF-0
2 mmol	4 mmol	Cu–Ni LDH/NF-1
2 mmol	8 mmol	Cu–Ni LDH/NF-2
2 mmol	18 mmol	Cu–Ni LDH/NF-3
2 mmol	24 mmol	Cu–Ni LDH/NF-4

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Fig. 1 Schematic diagram of preparation of Cu₂S/Ni₃S₂/NF.

2.1.2 Ni–Cu LDH sulfide to Cu₂S/Ni₃S₂. Prepared 0.2mol L⁻¹ Na₂S solution and dried NiCu-LDH were put into Teflon-line stainless autoclave. After reacting at 100 °C for 8 h, the black Cu₂S/Ni₃S₂/NF was obtained. After the Cu₂S/Ni₃S₂/NF is washed and dried, the subsequent characterization and performance can be performed.

2.2 Preparation for material characterizations and electrochemical measurements

The X-ray diffraction (XRD) of Ni–Cu LDH and Cu₂S/Ni₃S₂ was measured using a PANalytical Empyren equipment at 45 kV and 40 mA with a Cu target. The scanning electron microscopy (SEM) images of Ni–Cu LDH and Cu₂S/Ni₃S₂ were from Zeiss Sigma 500. Energy dispersive spectroscopy (EDS) analyses were recorded by FEI Talos 200s. The valence states of the elements on the electrode surface were obtained by an ESCALAB 250Xi X-ray photon spectrometer (XPS). Transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution TEM (HRTEM), and scanning TEM were all obtained by using a JEM2100F microscope.

All electrochemical tests use CHI660B electrochemical workstation. A three-electrode system consisting of graphite, saturated calomel electrodes (SCE, KCl saturated) and the sample to be tested was made up in 1 M KOH at 25 °C. The potentials measured in the experiment has been calibrated, and the potential (vs. SCE) was converted into a reversible hydrogen electrode (RHE) through the Nernst equation:²⁸ E_vs.RHE = E _vs.SCE + 0.242 V + 0.059 pH. The polarization curves were corrected by the equation:²⁹ E_corrected = E_vs.RHE − iR linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and continuous CV cycles were used to evaluate the electrocatalytic performance of the catalyst.

3. Experimental results

3.1 Cu₂S/Ni₃S₂ structure analysis

NF with 3D conductive network structure and macropores was used as a base material for electrocatalyst growth. After the two-step hydrothermal reaction mentioned above, the silver white NF changed to brick red (the NiCu LDH precursor) and then black (Cu₂S/Ni₃S₂/NF). The SEM images in Fig. 2(a)–(f) can visually present the microscopic morphology of the sample. Fig. 2(a)–(c) show the SEM images of NiCu LDH. NiCu LDH is uniformly anchored on the NF substrate in a sheet-like manner. After further increasing the magnification, “nano fluff” was evenly distributed on the nanosheets. Compared with NiCu LDH, the SEM images of Cu₂S/Ni₃S₂/NF did not change significantly, as shown Fig. 3(d)–(f), indicating that the required LDH layered structure can exist stably. After vulcanization, the “nano fluff” is transformed into a “coral” with a rough surface, exposing more active sites. The SEM picture of Cu₂S/Ni₃S₂/NF after 58 h chronopotentiometry test is placed in the support information. As shown in Fig. S1,† some cracks appeared on Cu₂S/Ni₃S₂ layer after the test. And the original thin nanosheets are transformed into a stacked coral layer. This transition from “nano fluff” to “rough coral” increases the specific surface area and helps improve electrocatalytic activity.

Fig. 2 (a–c) SEM image of NiCu LDH-3, (d–f) SEM image of Cu₂S/Ni₃S₂/NF-3, (g) XRD pattern of Ni₃S₂/NF, Cu₂S/Ni₃S₂/NF and NF, (h) EDS image of Cu₂S/Ni₃S₂/NF.

Fig. 3 (a) XPS survey spectrum for Cu₂S/Ni₃S₂. XPS spectra of Cu₂S/Ni₃S₂ in the (b) Ni 2p, (c) Cu 2p and (d) S 2p regions.

Furthermore, Fig. 2(g) shows the XRD pattern of Ni₃S₂/NF and Cu₂S/Ni₃S₂/NF. Both XRD spectra have the same three strong peaks, based foam nickel (PDF #70-1849). The diffraction peaks of both at 2θ = 21.75°, 31.10°, 37.78°, 49.73°, and 55.16° correspond to Ni₃S₂ (PDF #44-1418) (101), (110), (003), (113), and (122) crystal planes, respectively. Obviously, the characteristic peak of Cu₂S (PDF #33-0490) appeared in the XRD spectrum (indicated by the orange line) after Cu was added. The XRD spectrum qualitatively showed the existence of Cu₂S/Ni₃S₂. EDS confirms that Ni, Cu, and S are present in Cu₂S/Ni₃S₂/NF sample, and their atomic ratio is close to the feed ratio. The Cu₂S/Ni₃S₂/NF after 58 h chronopotentiometry test was characterized by XRD. As shown in the ESI Fig. S2,† after the 58 h stability test, the XRD spectrum of Cu₂S/Ni₃S₂/NF showed a CuO peak (PDF #89-5899) that had not appeared before the test. The diffraction peaks at 2θ = 35.6°, 38.8°, and 48.7° correspond to CuO (−111), (111), and (−202) crystal planes, respectively. The intensity of the Cu₂S peak weakened, indicating that Cu₂S was partially converted to CuO.

In addition, the element composition of Cu₂S/Ni₃S₂/NF-3 can be obtained by XPS. The overall XPS spectrum of Cu₂S/Ni₃S₂/NF-3 is shown in Fig. 3(a), where Ni, Cu, and S are present. Fig. 3(b) shows the XPS spectrum of the Ni 2p region, where the intensities of the Ni 2p_1/2 and Ni 2p_3/2 peaks are 855.48 eV and 873.23 eV, respectively, indicating that Ni exists in the form of Ni₃S₂.³⁰ The peaks of 879.2 and 860.8 eV are the concomitant satellites.^31,32 The small peak on the far right (852.8 eV) is a typical metal nickel sulfide or metal nickel peak.³³ In the XPS spectrum of Cu 2p region shown in Fig. 3(c), 932.03 eV is the peak of Cu 2p_3/2 in Cu₂S, and 951.93 eV is the peak of Cu 2p_1/2.³⁴ The last picture in Fig. 3(d) is the XPS spectrum of S 2p, where 162.48 and 161.48 eV are attributed to S 2p_1/2 and 2p_3/2 in Ni₃S₂, respectively.³⁵ And 162.25 eV is attributed to S 2p_3/2 in Cu₂S. All XPS spectra fully prove that the synthesized sample is Cu₂S/Ni₃S₂. After 58 h chronopotentiometry test, the XPS spectra of the electrode is placed in the ESI Fig. S3.† In Fig. S3(b)† shows that Ni 2p_3/2 and Ni 2p_1/2 of Ni₃S₂ are the main strong peaks, which can match the conclusion of XRD (Fig. S2†). Moreover, in Fig. S3(b).† The Cu 2p spectra (Fig. S3(c)†) shows Cu⁺ and Cu²⁺ peaks. Among them, the Cu 2p_3/2 and Cu 2p_1/2 match with the Cu₂S and the strong Cu²⁺ satellite match with the CuO detected by XRD (Fig. S2†). Compared with the original XPS spectra, the intensity of S 2p (Fig. S3(d)†) is reduced after the chronopotentiometry test. We suspected that the heterostructure Cu₂S/Ni₃S₂ catalyst is converted to sulfur-doped CuO in this process. It is reported that the ΔG_H* of sulfur-doped CuO is lower than that of pure CuO.³⁶ The heterostructure between Cu₂S/Ni₃S₂ promotes interface electron transfer. The Ni sites at the Cu₂S/Ni₃S₂ interface interact with O in water molecules to adsorb water molecules on the surface. After that, the water molecules adsorbed on the Ni site interact with hydrogen bonds or form S–H bonds, which accelerate the adsorption and dissociation of water and increase the speed of the Volmer step.^37–39 It is reported that S optimizes the free energy of H* (ΔEH*) adsorption of CuO, and the O site of CuO near the S-doped site increased the H adsorption capacity.⁴⁰

Fig. 4(a) clearly shows the TEM image of Cu₂S/Ni₃S₂/NF layer structure stacked on top of one another. The SAED pattern of Cu₂S/Ni₃S₂/NF in Fig. 4(b) shows typical polycrystalline diffraction rings. These calibrated diffraction rings can correspond well to the peak of the XRD spectrum. Comparing with the PDF card confirmed it to be Ni₃S₂. The diffraction ring of the (034) crystal planes of Cu₂S can also be observed, which corresponds to the strong peak appearing after Cu doping in the XRD spectrum. The exposed (034) crystal planes of Cu₂S is beneficial for the improvement of the HER performance of the catalyst. The HRTEM lattice fringe image of Cu₂S/Ni₃S₂ is in Fig. 4(c). The crystal planes with interplanar spacings of 0.287, 0.234, and 0.237 nm correspond to the (110), (021), and (003) crystal planes of Ni₃S₂ (PDF#44-1418), respectively. The exposed crystal surface of the catalyst has an important influence on its catalytic performance. The angle between the two (110) is 60°, indicating that the thermodynamically stable {001} crystal planes is exposed. The angle between (003) and (021) is about 70.5°, then high-index {2̄10} is exposed.⁴¹ Research by Liang-Liang Feng et al.⁴¹ showed that the synergistic catalysis produced by the nanosheet array and the exposed {2̄10} high-index facets help improve electrocatalytic performance. Numerous disordered defects and the exposed (034) crystal planes of Cu₂S can also be observed in the HRTEM image, indicating that the exposed active sites and electrical conductivity can be increased.⁴² Moreover, the mapping of Cu₂S/Ni₃S₂ nanosheet in Fig. 4(d) reflects that Ni, S, and Cu are evenly distributed on the nanosheets, which is conducive to the uniform dispersion of active sites.

Fig. 4 (a) TEM image of Cu₂S/Ni₃S₂ layer structure, (b) SAED pattern and (c) HRTEM image of Cu₂S/Ni₃S₂, (d) mapping of Ni, Cu, S and overlap images of Cu₂S/Ni₃S₂ nanosheet.

3.2 Catalyst electrochemical performance analysis

3.2.1 Catalyst HER performance. The LSV of Pt/C, Ni₃S₂/NF, and Cu₂S/Ni₃S₂/NF-x (x represents various feed ratios) was evaluated in 1 M KOH solution. Comparing the polarization curves clearly shows that Pt/C has the best HER performance (η₁₀ = 31 mV), whereas Ni₃S₂/NF requires 180 mV. The Cu₂S/Ni₃S₂/NF-3 curve, which is closest to Pt/C curve, requires an overpotential of 50 mV at 10 mA cm⁻². The overpotentials of five samples are shown in Table 2. Compared with Ni₃S₂/NF, the HER performance of Cu₂S/Ni₃S₂/NF is indeed improved much. However, experiments have shown that the amount of Cu cannot be too much, which is consistent with Byung Keun Kim's report⁴³ that excessive Cu will increase the internal stress and cause the coating to fall off, thereby reducing catalytic performance. Cu₂S/Ni₃S₂/NF was also compared with the results of other scientific researchers. Detailed data can be found in Table 3.

Table 2 Comparison of overpotential of different samples

Catalysts	Current density (j mA cm^−2)	Overpotential (η/mV)
Pt/C	10	31
Ni3S2/NF	10	180
Cu2S/Ni3S2/NF-0	10	182
Cu2S/Ni3S2/NF-1	10	159
Cu2S/Ni3S2/NF-2	10	122
Cu2S/Ni3S2/NF-3	10	50
Cu2S/Ni3S2/NF-4	10	134

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Table 3 Comparison of the electrocatalysts performance of Cu₂S/Ni₃S₂/NF and other Ni₃S₂

Catalysts	Electrolyte Solution	Overpotential (η10)
Cu2S/Ni3S2/NF-3	1 M KOH	50 mV
S-v-Ni3S2−xPx−4 (ref. 44)	1 M KOH	89 mV
Mo-doped Ni3S2 (ref. 45)	1 M KOH	90 mV
V-doped Ni3S2/NiS^46	1 M KOH	85 mV
CoNi2S4/Ni3S2@NF^47	1 M KOH	171 mV

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In alkaline solution, HER follows Volmer–Tafel or Volmer–Heyrovsky mechanism.⁴⁸ They all consist of three steps: 1. Discharge (H₂O + e⁻ → H* + OH⁻, Volmer reaction) 2. Electrochemical desorption (H₂O + H* + e⁻ → H₂ + OH⁻, Heyrovsky reaction) and 3. Recombination (2H* → H₂, Tafel reaction). The calculated Tafel slope of Cu₂S/Ni₃S₂/NF-3 is 107 mV dec⁻¹ (lower than 120 mV dec⁻¹), indicating that hydrogen evolution is mainly limited by the Volmer reaction and follows the Volmer–Heyrovsky mechanism. The Tafel slope of Cu₂S/Ni₃S₂/NF-3 is clearly much smaller than that of Ni₃S₂/NF, indicating that Cu₂S/Ni₃S₂/NF-3 catalytic reaction kinetics is faster in an alkaline medium, and the catalytic activity of HER is better.⁴⁹

Turnover frequency (TOF) can be used to reflect intrinsic activity.⁵⁰ First, CV curves were measured in a ph = 7 phosphate buffer saline solution at a scan rate of 50 mV s⁻¹ and a voltage range of −0.2–0.6 V. Then, the number of active sites was calculated from the method reported by Merki.⁵¹ In Table 4, the active sites loaded on the Cu₂S/Ni₃S₂/NF-3 surface is 3.568 × 10⁻⁴ mol, which is more than Ni₃S₂/NF (1.625 × 10⁻⁴ mol). This result may be related to the coral-like surface on the nanosheet shown in the SEM image. This specific morphology can expose more active sites. The calculated TOFs are shown in Fig. 5(d). The overpotentials of Ni₃S₂/NF, Cu₂S/Ni₃S₂/NF-2∼4 at TOF of 0.2 S⁻¹ are 182, 168, 72, and 119 mV, respectively. In Fig. 6(a), TOFs denote that Cu₂S/Ni₃S₂/NF-3 has a higher catalytic activity.

Table 4 Number of active sites of Cu₂S/Ni₃S₂/NF-x and Ni₃S₂/NF

Catalysts	Number of active sites (×10^−4 mol)
Ni3S2/NF	1.625
Cu2S/Ni3S2/NF-2	2.639
Cu2S/Ni3S2/NF-3	3.568
Cu2S/Ni3S2/NF-4	3.533

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Fig. 5 (a) iR-Corrected linear sweep voltammetry curves of Ni₃S₂/NF and Cu₂S/Ni₃S₂/NF-x (x represents various feed ratios) for HER in 1 M KOH at 5 mV s⁻¹, (b) Tafel plots of Ni₃S₂/NF, Pt/C and Cu₂S/Ni₃S₂/NF-3, (c) CVs of Ni₃S₂/NF and Cu₂S/Ni₃S₂/NF-x at pH = 7 at the scan rate of 50 mV s⁻¹, (d) TOF curves of Ni₃S₂/NF and Cu₂S/Ni₃S₂/NF-x, (e) polarization curves of Ni₃S₂/NF and Cu₂S/Ni₃S₂/NF-x normalized by the ECSA, (f) measured capacitive currents plotted as a function of scan rate.

Fig. 6 (a) The chronopotentiometry curve of Cu₂S/Ni₃S₂/NF at constant current density of 10 mA cm⁻². (b) The polarization curves of Cu₂S/Ni₃S₂/NF-3 before and after 2000 CV cycles. (c) Stability test of Cu₂S/Ni₃S₂/NF-3 carried out at multiple currents. (d) Impedance Nyquist plots of Cu₂S/Ni₃S₂/NF-2, Cu₂S/Ni₃S₂/NF-3 and Cu₂S/Ni₃S₂/NF-4.

Electrochemical Active Surface Area (ECSA) test was carried out. First, the CV curves were measured at various scan rate. Then, Half of the values of the positive and negative current density differences (Δj) at the median value of the scanning range are plotted versus the CV scanning rates in Fig. 5(f). The electrochemical double layer charge (C_dl) value can be obtained from the fit slope. Cu₂S/Ni₃S₂/NF-3 and Ni₃S₂/NF are both cut into 1 cm × 1 cm rectangles, with the same geometric surface area. The C_dl of Cu₂S/Ni₃S₂/NF-3 (17.95 mF cm⁻²) is approximately 2.2 times that of Ni₃S₂/NF (8.23 mF cm⁻²), indicating that the actual surface area of Cu₂S/Ni₃S₂/NF-3 is larger. From Fig. S4,† the surface loading of Cu₂S/Ni₃S₂ is 2.8 mg cm⁻².

The chronopotentiometry curve was measured to evaluate the mechanical strength and excellent mass transfer performance of Cu₂S/Ni₃S₂/NF-3. In Fig. 6(a), Cu₂S/Ni₃S₂/NF can maintain 95.1% activity stably for at least 58 hours. The fluctuation of the curve in the first 30 minutes is due to the gradual increase in the potential during the activation phase caused by the removal of the hydroxide/oxide on the electrode surface.⁵² A CV was performed 2000 times continuously from 100 mV to −300 mV. In Fig. 6(b), the HER performance of Cu₂S/Ni₃S₂/NF-3 decreased slightly after the CV and need 70 mV to 10 mA cm⁻². Multistep chronopotentiometry (from 10 mA to 100 mA with 10 mA interval) was used to evaluate the mass transfer and stability of Cu₂S/Ni₃S₂/NF-3. The electrode reaction kinetics can be studied by EIS. The Nyquist diagrams of Cu₂S/Ni₃S₂/NF-2,3,4 were obtained at 200 mV. The equivalent circuit in Fig. 6(d): a constant-phase element (CPE) connected in parallel with a charge transfer resistance (R_ct), then an electrolyte resistance (R_s) is connected in series. The fitting line represented by the red line is semicircular, which means that the charge transfer controls the entire HER. R_ct can be determined in the semicircular low-frequency region to reflect the electron transport efficiency.⁵³ The R_ct values of Cu₂S/Ni₃S₂/NF-2,3,4 are 2.44 Ω, 0.79 Ω, and 1.98 Ω, respectively. Cu₂S/Ni₃S₂/NF-3 has the smallest R_ct, which means fast electron transfer. The excellent HER performance of Cu₂S/Ni₃S₂/NF-3 also echoes this result. By contrast, a small R_s value indicates that the bonding between the catalyst and the current collector is good.⁵⁴ The impedance results of Cu₂S/Ni₃S₂/NF-3 are consistent with the previous HER results, which can prove that it can be a candidate with excellent HER kinetics and outstanding electron transport performance.

4. Conclusion

Overall, the heterostructure Cu₂S/Ni₃S₂ were successfully synthesized by two-step hydrothermal method and exhibited excellent HER activity. Compared with the recently reported system, Cu₂S/Ni₃S₂/NF-3 has a lower overpotential (η₁₀ = 50 mV). After vulcanization, the coral-like rough surface exposes more active sites. The heterostructure of Cu₂S/Ni₃S₂ exposes specific {2̄10} crystal planes as well as a large number of defects and (034) crystal planes, which helps expose more active sites. The structural characterization is consistent with the electrochemical test results. In HER process, Cu₂S/Ni₃S₂ interface increase the formation of S–H bonds, optimizing the adsorption capacity of S-doped sites for H. And Cu₂S promotes the transformation of the HER process into S-doped CuO. Therefore, the combination of Cu₂S and Ni₃S₂ further increases the catalytic activity.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Authors very thanks for the help from the friends and the authoritative opinion and suggestions from the reviewers.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra07514d

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