Construction of Ni₃S₂-Ni_xP_y/NF@NiFe LDH with heterogeneous interface to accelerate catalytic kinetics of overall water splitting

Abstract

Reasonable design of a kind of non-noble metal catalyst with high catalytic performance, good stability and low cost is of great significance to realize the overall water splitting. Herein, a bottom-up method is proposed to synthesize NiFe layered double hydroxides (NiFe LDHs) coated sulfide/phosphide heterostructure on the nickel foam (Ni₃S₂-Ni_xP_y/NF@NiFe LDH) through hydrothermal method combined with electrodeposition. Low overpotentials of only 248 and 298 mV are required to achieve the current densities of 50 and 100 mA cm⁻² for OER. Meanwhile, the HER performance only needs 137 mV at a current density of 10 mA cm⁻².

GRAPHICAL ABSTRACT

KEYWORDS:

• Electrocatalyst
• heterogeneous interface
• water splitting

1. Introduction

Hydrogen energy has attracted extensive research due to its cleanliness and stability [1,2]. Among them, electrocatalysis, which splits water into hydrogen without carbon emissions, is considered an ideal energy strategy for the future [3]. However, electrocatalytic water splitting faces the challenge of slow kinetic reaction, which has prompted researchers to develop efficient catalysts with low dynamic overpotentials to achieve robust water splitting [4–6]. At present, in the field of electrolysis of water, although precious metals and oxides such as Pt, RuO₂ and IrO₂ have excellent activity as catalysts for hydrogen evolution reaction (HER) or OER, these precious metals have the disadvantages of scarce resources, high prices, and poor durability [7–10]. Some transition metal phosphides [11–15], sulfides [16–19], selenides [20,21], carbides [22,23] and oxides [16,24] have been reported as promising HER or OER electrocatalyst. Most of the catalysts currently studied are only active in the half-reaction of HER or OER, and very few of them are active both HER and OER under the same pH conditions [25,26]. Therefore, it is of great significance to find a dual-function electrocatalyst that is efficient, cheap and rich in earth resources for overall water splitting [27].

Among them, Ni₃S₂ electrocatalyst has been extensively studied due to its high conductivity and unique structural configuration [28,29]. Through the hybridization of Ni₃S₂ with foreign active species, the use of heterogeneous interface engineering to improve the catalytic activity has been extensively studied and has made significant progress [30,31]. For example, Xu et al. successfully constructed a heterogeneous interface between MoS₂ and NiS₂ through a hydrothermal method, which further improved the catalytic water splitting performance of NiS₂ [32]. Meanwhile among various non-noble electrocatalysts, transition metal phosphides (TMPs) have emerged as ideal electrocatalysts for electrocatalytic water splitting [11]. In the HER process, the metal and phosphorus sites of TMPs can form hydrides and capture protons. To date, TMPs including Mo-, Co-, Fe-, and Ni-phosphides have been reported to have reasonably active HER in acidic or basic solutions with considerable amounts of metal phosphorus such as Ni and Co-related Compounds also showed effective OER performance. For example, Jiao et al. successfully prepared Mo-doped Ni2P nanotube-structured catalysts for water splitting [13].

On the other hand, as a kind of two-dimensional material, layered double hydroxides have attracted much attention because of their good redox activity, simple preparation and controllable composition [33–35]. A large number of studies repeatedly demonstrated that LDHs materials, especially NiFe LDHs, have high OER activity [36,37]. The improvement of HER performance by heterojunction and the excellent OER performance of NiFe LDH itself provide us with a new idea. Here, a new type of heterogeneous 3D nano-flowers structure (Ni₃S₂-Ni_xP_y/NF@NiFe LDH) was successfully synthesized on the nickel foam (NF) through the hydrothermal process, phosphating treatment and electrodeposition. The Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst exhibits low OER overpotentials of 248 and 298 mV at 50 and 100 mA cm⁻², also showing low HER overpotential of 137 mV at 10 mA cm⁻²_.

2. Results and discussion

Figure 1 schematically illustrates the synthetic process of Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst. The Ni₃S₂-Ni_xP_y heterostructure was fabricated via a facile vulcanization and phosphating. After phosphating, the bottom-up layered Ni₃S₂-Ni_xP_y/NF nanoflowers with abundant heterogeneous interfaces are generated. Subsequently, the self-supported Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst was synthesized by electroplating. The abundant heterogeneous interfaces between Ni₃S₂ and Ni_xP_y and the formation of NiFe LDH ultra-thin nanosheets might realize fast energy transfer and electron transfer. Hence, we expect that the Ni₃S₂-Ni_xP_y@NiFe LDH/NF electrode can display good bifunctional electrocatalytic activity to HER/OER under alkaline conditions.

Figure 1. Schematic illustration of synthesis procedures of Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst.

We analyzed and confirmed the crystal phase structure of Ni₃S₂-Ni_xP_y/NF@NiFe LDH through XRD. As shown in Figure 2(a), three very strong diffraction peaks at 44.49°, 51.85° and 76.38° belongs to the NF substrate. Meanwhile, we can see that the diffraction peaks of Ni₃S₂-Ni_xP_y/NF@NiFe LDH are the same as those of Ni₃S₂ (JCPDS: 44-1418) [38,39]. Index to other diffraction peaks are Ni₂P (JCPDS: 74-1385) and Ni₁₂P₅ (JCPDS: 74-1381). The formation of Ni_xP_y mixture (Ni₂P and Ni₁₂P₅) is attributed to the polycrystalline phase of nickel-based phosphate in the compound phosphating chemistry [25]. The peaks at 11.79°, 23.77°, 35.89°, 37.60°, 47.02°, 60.28° and 61.66° are the same as those of NiFe LDH (JCPDS: No.49-0188) (003), (006), (009), (104), (018), (110) and (113) planes [40].

Figure 2. (a) XRD patterns of the Ni₃S₂-Ni_xP_y/NF@NiFe LDH and Ni₃S₂-Ni_xP_y/NF. (b) High-resolution of XPS profiles of Ni 2p, (c) S 2p, (d) P 2p, (e) Fe 2p and (f) O 2s.

As shown in Figure S1 and Figure 2(b–f), investigation on the chemical states of Ni₃S₂-Ni_xP_y/NF@NiFe LDH is conducted by XPS. There are two main peaks at 855.8 and 873.4 eV in Figure 2(b), which can be distributed as Ni 2p_3/2 and Ni 2p_1/2 of the spin–orbit doublets of Ni²⁺, respectively. After phosphating, the binding energies of Ni 2p_1/2 and Ni 2p_3/2 display sequentially positive shifts of 0.7 and 0.6 eV, respectively. It shows that phosphating can adjust the electronic structure of Ni₃S₂, produce a large number of Ni₃S₂-Ni_xP_y heterogeneous interfaces, and cause more partial positive/negative charges (δ⁺/δ⁻). Meanwhile, there are two obvious peaks at 855.9 and 873.6 eV as satellite peaks. As shown in Figure 2(c), the S 2p spectrum shows a 2p_1/2 peak at 163.3 eV and a 2p_3/2 peak at 162.2 eV, indicating the presence of S in the catalyst. In Figure 2(d), the P 2p spectrum shows that the binding energies of P 2p_3/2 and P 2p_1/2 are 134.25 and 135.45 eV, respectively, showing the successful introduction of phosphorus after surface phosphating. Figure 2(e) exhibited two peaks at 727.5 and 711.9 eV corresponding to Fe 2p_1/2 and Fe 2p_3/2, suggesting the existence of Fe³⁺. The peak of O 1s at 531.35 eV can be seen in Figure 2(f), which is derived from the OH^- after NiFe LDH electrodeposition. In Table S2, by analyzing the area occupied by the peaks of different elements in XPS, the atomic ratios can be obtained.

Figures S2, S3 and Figure 3(a) are the FESEM images of the Ni₃S₂-Ni_xP_y/NF@NiFe LDH at different stages, and thus the morphological evolution process can be disclosed. Inset Figure 3(a) is the enlarger part of Ni₃S₂-Ni_xP_y/NF@NiFe LDH, showing that the nanosheets are uniformly distributed. From the TEM image of Ni₃S₂-Ni_xP_y/NF@NiFe LDH (Figure 3(b)), we can observe flower cluster structure composed of nanosheets. The staggered arrangement of these nanosheets effectively increases the probability that the active site will provide an efficient electron transport channel.

Figure 3. Ni₃S₂-Ni_xP_y/NF@NiFe LDH: (a) FESEM image, (b) Low-resolution and (c) high-resolution TEM images (d) elemental mapping. (e) Microscopic schematic diagram of interface electron transfer between Ni₃S₂-Ni_xP_y and NiFe LDH.

The fine structure was further revealed by HRTEM shown in Figure 3(c,e). The fringe spacing of 0.286 nm is attributed to the (110) plane of Ni₃S₂. Furthermore, the fringe spacing of 0.234 nm corresponds to the (112) characteristic plane of Ni₁₂P₅, while the lattice spacing of 0.286 nm points to the (110) plane of Ni₂P. Figure 3(d) shows the energy spectrum distribution diagram, and the signals of Ni, Fe, O, P and S elements are distributed uniformly. Besides, the signal intensities of S and P are weaker than those of Ni, Fe and O, which displays that NiFe LDH is completely coated on the surfaces of Ni₃S₂-Ni_xP_y heterostructure.

 We studied the HER performance of the Ni₃S₂-Ni_xP_y/NF@NiFe LDH and the control groups under the same conditions in Figure 4(a,b). The onset overpotential is calculated at current density of 1 mA cm⁻². By contrast, Ni₃S₂-Ni_xP_y/NF@NiFe LDH displays the smallest overpotential of 134 mV at 10 mA cm⁻², which is lower than other samples.

Figure 4. (a) LSV polarization curves, (b) corresponding overpotentials at 10 and 20 mA cm⁻², and (c) tafel plots for OER. (d) Nyquist plots with a frequency range of 0.01–100 kHz. (e) Theoretical models for the H* adsorbed on the surface of Ni₃S₂ and P-Ni₃S₂ and (f) Gibbs free-energy diagram for H* adsorption.

Meanwhile, the tafel slope is used to study the possible reasons for the superior performance of Ni₃S₂-Ni_xP_y/NF@NiFe LDH, as shown in Figure 4(c). The Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst has the lowest tafel slope, only 120.26 mV dec⁻¹. Significantly lower than other samples. Similarly, the impedance values are shown in the Figure 4(d), which can further prove that Ni₃S₂-Ni_xP_y/NF@NiFe LDH has the smallest charge transfer impedance and the largest electrochemical surface area.

To shed light on the effect of the constructed Ni₃S₂-Ni_xP_y heterostructure on the chemisorption of hydrogen and oxygen-containing intermediates, DFT calculations are performed. Considering that the Ni₃S₂-Ni₂P is a monometallic heterostructure, we use the P doped Ni₃S₂ (P-Ni₃S₂) model to represent the Ni₃S₂-Ni₂P interface as shown in Figure S4. The chemisorption free energies of hydrogen (ΔG_H*) are first calculated to evaluate the catalytic performance for HER, and the optimized adsorption structures of H on the surface of the catalysts are depicted in Figure 4(e). According to Figure 4(f), ΔG_H* of H adsorption on the Ni₃S₂ (S site), Ni₃S₂ (Ni site), P-Ni₃S₂ (S site) and P-Ni₃S₂ (Ni site) surfaces are 0.632, 0.261, 0.642 and 0.174 eV, respectively. However, ΔG_H* of the P-Ni₃S₂ (P site) dramatically changes to −0.052 eV, which is relatively close to the ideal ΔG_H* of 0 eV for the catalyst-H* state. Note that, the P site of the P-Ni₃S₂ model can serve as the most active sites for HER, indicating that the construction of a reasonable Ni₃S₂-Ni_xP_y heterostructure can successfully improve the HER performance.

We also studied the OER performance of the Ni₃S₂-Ni_xP_y/NF@NiFe LDH and the control groups in the same alkaline medium (1 mol L⁻¹ of KOH).

As shown inFigure 5(a), after IR compensation of electrolyte resistance, we used linear sweep voltammetry (LSV) to detect its OER activity. The Ni₃S₂-Ni_xP_y/NF@NiFe LDH exhibits a considerable activity with only 248 and 298 mV at 50 and 100 mA cm⁻² of current densities. In order to explore the contribution of NiFe LDH coating to the catalytic performance, we prepared Ni₃S₂-Ni_xP_y/NF catalyst. As shown in Figure 5(b), the catalyst requires 366 mV of overpotential at 50 mA cm⁻², showing that the NiFe LDH ultra-thin nanosheets can effectively improve its OER performance. As shown in Figure 5(c) the calculated Tafel slopes of Ni₃S₂-Ni_xP_y/NF@NiFe LDH are separately 40.57 mV dec⁻¹, which prove that Ni₃S₂-Ni_xP_y/NF@LDH catalyst has the highest reaction rate and fastest kinetics for water oxidation. We also compared the electrochemical impedance spectroscopy (EIS) of the catalyst materials in Figure 5(d). Compared with Ni₃S₂/NF, Ni₃S₂/NF@NiFe LDH and Ni₃S₂-Ni_xP_y/NF, Ni₃S₂-Ni_xP_y/NF@NiFe LDH has the lowest charge transfer resistance, indicating the fastest charge transfer. In order to further obtain the possible reasons for the excellent OER activity of Ni₃S₂-Ni_xP_y/NF@NiFe LDH, the electrochemically active surface area (ECSA) was measured by the double-layer capacitance (C_dl). In Figure S5a-d and Figure S6, through the CV curve of different samples we have calculated their C_dl. Ni₃S₂-Ni_xP_y@NiFe LDH/NF presents the largest C_dl value (45.31 mF cm⁻²) compared to other samples’ C_dl value. The largest C_dl value means that Ni₃S₂-Ni_xP_y@NiFe LDH/NF constructs more effective catalytic sites to enhance electrocatalytic performance and accelerate the catalytic kinetics, indicating that Ni₃S₂-Ni_xP_y/NF@NiFe LDH has the highest intrinsic activity and the most abundant catalytic sites.

Figure 5. (a) LSV polarization curves, (b) corresponding overpotentials at 50 and 100 mA cm⁻² and (c) tafel plots for OER. (d) Nyquist plots with a frequency range of 0.01–100 kHz. (e) Theoretical model of P-Ni₃S₂@NiFe LDH heterostructure. (f) Chemisorption free energies of hydrogen (ΔEad_-H) and hydroxide (ΔEad_-OH).

Similarly, in order to further verify the excellent OER performance of the catalyst, DFT calculation was performed. The results show that the previously constructed P doped Ni₃S₂ (P-Ni₃S₂) model has a significant improvement in the catalytic activity of HER but not that of OER. According to the results, the catalytic activity improvement for OER is not obvious, because there is a small decrease of the chemisorption free energy ΔE_ad-OH (0.307 eV) compared with Ni₃S₂ (0.465 eV). However, through Figure 5(a), we find that the OER performance of Ni₃S₃-Ni_xP_y/NF is significantly better than that of Ni₂S₃/NF, just showing that three-dimensional nano-flower structure has a great effect on the improvement of its catalytic performance.

Similarly, In order to further verify the influence of NiFe LDH on OER performance, DFT calculation was performed. The P-Ni₃S₂@NiFe LDH interface is constructed as shown in Figure 5(e), Figures S10 and Figure S11. We first calculate the interfacial adhesion energy between the NiFe LDH (001) surface and the P-Ni₃S₂ (001) surface. According to the results, the E_adhesion is −3.94 eV, which means there is a strong interaction energy between the NiFe LDH (001) surface and the P-Ni₃S₂ (001) surface. The chemisorption-free energies of hydrogen (ΔE_ad-H) and hydroxide (ΔE_ad-OH) on the (001) surface of Ni₃S₂, P-Ni₃S₂, and the P-Ni₃S₂@NiFe-LDH are shown in Figure 5(f). The ΔE_ad-H of H adsorption on the Ni₃S₂, P-Ni₃S₂ (Ni site), P-Ni₃S₂ (P site) and the P-Ni₃S₂@NiFe-LDH surfaces are 0.270, 0.172, −0.078 and 0.142 eV, respectively. The ΔE_ad-OH of OH adsorption is 0.465, 0.725, 0.307, and −0.539 eV respectively. It is clear that the ΔE_ad-H of P-Ni₃S₂@NiFe-LDH possesses a small increment, while ΔE_ad-OH shows significant decrease. Therefore, the formed interface between P-Ni₃S₂ and NiFe-LDH facilitates the rupture of the O-H bonds of the H₂O molecule to accelerate the OER process, and the effect for HER is negligible simultaneously.

Long-term durability is also a significant parameter for evaluating the electrochemical activity. Figure S13a shows no significant degradation even after 27 h of stability test at the current densities of 50 and 10 mA cm⁻², respectively, exhibiting the stable OER and HER activities. As shown in Figure S13b, after the OER cycle, the weak Ni substrate peak has completely disappeared, indicating that Ni is completely oxidized. At the same time, in Figure S13c, it can be observed that the Fe 2p orbital peak has a significant positive shift and weakens after the OER cycle, indicating that the surface Ni and Fe elements exhibit higher oxidation states [20]. Then we compare and analyze the XPS of the O 2s element before and after the OER cycle in Figure S13d. The high-resolution O 1s spectrum further verified the formation of nickel monoxide after the OER process.

We used FESEM to characterize the Ni₃S₂-Ni_xP_y/NF@NiFe LDH electrode after 27 h OER cycles. In Figure S13e and Figure S13f, we can observe that the surface of Ni₃S₂-Ni_xP_y/NF@NiFe LDH after the OER cycle retains the flower-like structure and the surface becomes rougher. The high-resolution O 1s spectrum (Figure S13d) further verified the formation of NiO after the OER process [25]. In Figure S9, the TEM image of Ni₃S₂-Ni_xP_y/NF@NiFe LDH after OER test is almost the same as before OER test. Meanwhile, as shown in Figure S13g the EDS images show the uniform dispersion of Ni, Fe, S, P and O in the Ni₃S₂-Ni_xP_y/NF@NiFe LDH.

Since Ni₃S₂-Ni_xP_y/NF@NiFe LDH has good catalytic activity for OER and HER in 1 mol L⁻¹ of KOH solution, a two-electrode configuration was assembled by employing Ni₃S₂-Ni_xP_y/NF@NiFe LDH as bifunctional catalyst for overall water splitting. Meanwhile, Ni₃S₂-Ni_xP_y/NF@NiFe LDH || Ni₃S₂-Ni_xP_y/NF@NiFe LDH, Ni₃S₂/NF@NiFe LDH || Ni₃S₂/NF@NiFe LDH, Ni₃S₂-Ni_xP_y/NF || Ni₃S₂-Ni_xP_y/NF and Ni₃S₂/NF || Ni₃S₂/NF were also assembled for comparison. Figure S14a-b. manifests the assembled Ni₃S₂-Ni_xP_y/NF@NiFe LDH || Ni₃S₂-Ni_xP_y/NF@NiFe LDH couple can drive 10, 20 and 50 mA cm⁻² current densities at a cell voltage of only 1.54, 1.62 and 1.82 V, respectively, which are lower than other samples.

In addition, as presented in Figure S14c and Figure S12, the calculated density of the states (DOS) of P-Ni₃S₂ and Ni₃S₂-Ni_xP_y/NF@NiFe-LDH shows that Ni₃S₂ doped with P and the interface between P-Ni₃S₂ and NiFe-LDH enhances the carrier density near the Fermi level, compared with that of pure Ni₃S_2, NiFe-LDH and P-Ni₃S₂. The higher electron density at the Fermi level leads to the higher electrical conductivity, because the elevated electron transfer helps to the enhanced electrochemical performance to accelerate the catalytic kinetics of electrochemical overall water splitting, which is consistent with the results of previous impedance tests in Figure 4(d) and Figure 5(d).

3. Conclusion

We report a novel bottom-up method to successfully construct the high-performance Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst for overall water splitting. Confirmed by experimental and theoretical calculation results, the doping of P and the strong coupling interactions between P-Ni₃S₂ and NiFe-LDH could effectively optimize the electronic structure and chemisorption energy, synergistically boosting the HER/OER activity and stability, and significantly improving the catalytic activity. Furthermore, the assembled cell Ni₃S₂-Ni_xP_y/NF@NiFe LDH || Ni₃S₂-Ni_xP_y/NF@NiFe LDH can drive 10, 20, and 50 mA cm⁻² current densities at ultralow cell voltage of only 1.54, 1.62 and 1.82 V, which are superior to the most reported catalysts. Meanwhile, the durability of the catalyst can be as long as 27 h. The Ni₃S₂-Ni_xP_y/NF@NiFe LDH catalyst may be a potential application candidate for efficient overall water splitting.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by National Natural Science Foundation of China [grant numbers 51871107, 52130101, 1631004].