Multiphase interface coupling of Ni-based sulfide composites for high-current-density oxygen evolution electrocatalysis in alkaline freshwater/simulated seawater/seawater†

Jing Liang^a, Zhifeng Zhao*^b, Zhanhua Su^b, Weili Qu^a, Rui Guo^a, Xiaofeng Li*^a and Yongchen Shang*^a
aCollege of Chemistry and Chemical Engineering, Harbin Normal University, Harbin, 150025, China. E-mail: lixiaofeng@hrbnu.edu.cn; yongchenshang@163.com
^bCollege of Chemistry, Guangdong University of Petrochemical Technology, Maoming, 525000, China. E-mail: zhifengzhao1980@163.com

First published on 28th August 2024

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1. Introduction

Electrochemical water splitting is a clean and pollution-free way to generate hydrogen energy.^1–4 The theoretical thermodynamic potential required to drive water splitting is 1.23 V.⁵ However, owing to the complicated four-electron transfer process of the oxygen evolution reaction (OER) in practical applications, a higher overpotential is required to attain considerable current density values.⁶ It is well known that industrial electrochemical water splitting occurs in alkaline freshwater electrolyte. In fact, seawater electrolysis is also inspired because seawater accounts for about 96.5% of the Earth's water reserves.^7–9 Therefore, high-performance electrocatalysts for the OER should be explored to reduce the overpotential at high current densities in alkaline freshwater and seawater.^10–12

At present, precious metal oxides such as RuO₂ and IrO₂ are efficient OER catalysts, but their scarcity, poor stability and exorbitant cost hinder large-scale applications.^13,14 Over the past few years, non-noble electrocatalysts for the OER have garnered extensive attention, such as Fe_x–Ni₂S₃/NF,¹⁵ NiFeS/NF,¹⁶ CoNPs@C,¹⁷ Ru–Ni(Fe)P₂/NF,¹⁸ S-NiFeSe₂ and Ni₃S₂/Co₃S₄,^19,20 and so on.^21–27 However, most of these transition metal-based catalysts have been shown to maintain excellent performance only at small current densities. Liu's team prepared Ni₃S₂@NF with an overpotential of 260 mV at 20 mA cm⁻²,²⁸ while Wang's group assembled Fe MOF-Ni₃S₂, which displayed an overpotential of 243 mV at a current density of 100 mA cm⁻².²⁹ For industrial applications, OER catalysts are usually operated at current densities over 500 mA cm⁻² at low overpotentials. For example, Ha et al. reported V, Li-doped Ni₃S₂ as industrial bifunctional catalysts, in which V-Ni₃S₂, Li-Ni₃S₂ and Ni₃S₂ required excessive energy potentials of 363 mV, 410 mV and 486 mV to deliver 500 mA cm⁻² in 1 M KOH.³⁰ Yu's team fabricated S-(Ni,Fe)OOH/NF, which possesses an overpotential of 355 mV at 1000 mA cm⁻² in alkaline media.³¹ Sun et al. prepared an NiCoHPi@Ni₃N/NF catalyst with an overpotential of 425 mV in 1 M KOH + 0.5 M NaCl solution at 500 mA cm⁻².³² Zou's team synthesized Ni-Fe-OH@Ni₃S₂/NF with an overpotential of 530 mV at 1000 mA cm⁻² in 1 M KOH.³³ Nevertheless, it is still difficult to realize industrialization because of complicated technology, and unsatisfactory catalytic activity and/or unamenable long-term stability at high current densities.

Taking into account the industrial requirements, potential electrocatalysts for the OER should meet the following criteria: (i) excellent electrical conductivity;^34,35 (ii) high activity, long-term durability and high corrosion resistance under oxidation conditions; and (iii) abundant raw materials and simple preparation process. Ni-based sulfides have been considered as one of the most advantageous electrocatalysts owing to their abundance, low cost and superior stability.^36,37 Among them, NiS and Ni₃S₂ have remarkable electrochemical conductivity and intrinsic activity for the OER.^38–40 For NiS, there are generally two phases: NiAs structure (hexagonal, h-NiS) and millerite (rhombohedral, r-NiS).⁴¹ The r-NiS has metallic conductivity and h-NiS has semiconductor properties, conductivity and catalytic activity.⁴² Moreover, we considered whether an NiS heterostructure comprising r-NiS and h-NiS could be constructed to enhance OER activity. Ni₃S₂ exhibits inherently high conductivity and abundant Ni–S and Ni–Ni bonds, which ensure rapid electron conduction and facilitate the production of the OER intermediate (OOH*).⁴³ Furthermore, NiOOH derived from nickel sulphide is generally self-doped with sulfur residues, and thus the multivalent sulfur coordination inside can reasonably be expected to mediate the adsorption of key OER intermediates,⁴⁴ and provide electronic conductivity and chloride repulsion.^27,45 Meanwhile, the multiphase interfaces tend to have more defects and disordered atomic arrangements, which help ionic diffusion and electron transfer to accelerate the catalytic dynamics. Therefore, we utilized multiphase Ni-based sulfide interface engineering to modulate the electronic structure, which meets industrial demands for OER catalysts.

Herein, Ni₃S₂ and two crystalline phases of NiS were grown on NF by a solvothermal method. The synthetic route to NNSN/NF-xh (x = 8, 11 and 14) is shown in Scheme 1. Detailed characterization and electrochemical testing are summarized in the ESI.† The NNSN/NF-11h electrode needs overpotentials of only 377 mV, 451 mV and 476 mV at 1000 mA cm⁻² for the OER in alkaline freshwater, simulated seawater and seawater, respectively. Meanwhile, the NNSN/NF-11h possesses long-term stability at 300 mA cm⁻². This work verifies the significance of multiphase interface coupling in optimizing the performance for high current density electrocatalytic OERs.

2. Results and discussion

2.1. Characterization of NNSN/NF

The phase compositions of the samples were determined via X-ray diffraction (XRD). According to Fig. S1,† the three samples are all the same substances. As shown in Fig. 1a, the peaks at 18.4°, 32.2°, 35.7°, 40.5°, 48.8° and 52.6° can be assigned to the (110), (300), (021), (211), (131) and (401) crystalline planes of r-NiS (PDF# 00-012-0041). The peaks at 21.8°, 31.1°, 37.8°, 50.1° and 55.3° correspond to the (101), (110), (003), (211) and (300) planes of Ni₃S₂ (PDF# 00-044-1418). The diffraction peaks at 30.1°, 34.6°, 45.7° and 53.5° were ascribed to the (100), (101), (102) and (110) crystalline planes of h-NiS (PDF# 03-065-3419). In addition, the diffraction peaks at 44.6°, 51.9° and 76.6° belong to the (111), (200) and (220) crystalline planes of NF (PDF# 01-077-8341).

The morphologies of the samples were observed via scanning electron microscopy (SEM) images. As shown in Fig. 1(b–d), a microstructure of laminated sheets and rough particles is seen to have grown on the surface of NF. The number of particles increased gradually with the extension of reaction time. Transmission electron microscopy (TEM) images reveal fragments and attached particles (Fig. 1e). High-resolution transmission electron microscopy (HRTEM) images of NNSN/NF-11h indicate that the lattice fringes of 0.287 and 0.186 nm correspond to the Ni₃S₂ (110) and r-NiS (131) crystal planes (Fig. 1f), respectively, and the interplanar spacing of 0.171 nm was individually measured in accordance with the (110) planes of h-NiS (Fig. 1g). The energy-dispersive X-ray spectroscopy (EDX) spectrum shows the atomic ratio of Ni and S (Fig. S2†). Mapping confirms that Ni and S elements are uniformly distributed in the sample (Fig. 2a–c). The selected-area electron diffraction (SAED) pattern confirms the polycrystalline nature of NNSN/NF-11h (Fig. 2d).

The oxidation states of NNSN/NF-xh (x = 8, 11 and 14) were investigated through X-ray photoelectron spectroscopy (XPS). The XPS survey spectra manifest the existence of Ni and S atoms in NNSN/NF-xh (x = 8, 11 and 14) (Fig. S3†). The Ni 2p spectrum of NNSN/NF-11h exhibits Ni 2p_1/2 and Ni 2p_3/2 peaks at 875.1 and 857.1 eV (Fig. 2e), which corresponded to Ni³⁺, while the Ni 2p_1/2 and Ni 2p_3/2 peaks at 873.2 and 855.9 eV are ascribed to Ni²⁺, respectively. The peaks at 880.3 and 861.7 eV are the satellite peaks,^46,47 and the peaks located at 870.5 and 853.4 eV originate from NF.⁴⁸ The Ni²⁺ and Ni³⁺ spin-orbital doublets of NNSN/NF-11h were shifted more positively than those of NNSN/NF-8h (874.6/856.9 eV, 873.2/855.8 eV) and NNSN/NF-14h (874.4/856.6 eV, 873.1/855.7 eV). The positive shift of the peaks for Ni ion species leads to boosted empty d orbitals of Ni, which contributes to the improved binding with the OH* intermediate and finally reduces the Tafel value of the OER. The XPS results clearly indicate the distinction of electrons distributed in NNSN/NF-xh (x = 8, 11 and 14). The ratios of Ni²⁺/Ni³⁺ for the NNSN/NF catalysts are 0.9989 (8 h), 1.0049 (11 h) and 0.9962 (14 h), respectively. Besides, the S 2p spectrum of NNSN/NF-11h reveals that the two peaks at 163.2 and 161.8 eV belong to S 2p_1/2 and S 2p_3/2, respectively (Fig. 2f).^16,49 The peak at 168.2 eV is attributed to the S–O bond produced by surface oxidation of the samples.⁵⁰ Compared with the S 2p spectra of other samples, the relative peak intensity of the S–M bond increases with the extension of reaction time. The specific surface areas of the samples were studied by N₂ adsorption–desorption experiments (Fig. S4†). The specific surface area of NNSN/NF-11h is estimated as 1.46 m² g⁻¹, which is larger than those of NNSN/NF-8h (0.75 m² g⁻¹) and NNSN/NF-14h (0.32 m² g⁻¹); thus NNSN/NF-11h exposes more active sites. The UV-vis absorption spectra of NNSN/NF-xh (x = 8, 11 and 14) were recorded at room temperature (Fig. S5†). The absorbance spectra of NNSN/NF-xh (x = 8, 11 and 14) show absorbance peaks at 267 nm and 285 nm. The band gap energy (E_g) values are detected at 4.17 and 4.46 eV for NNSN/NF-11h (Fig. S6†), which are smaller than those of NNSN/NF-8h (4.18 and 4.47 eV) and NNSN/NF-14h (4.22 and 4.48 eV). These results manifest the existence of optical band gaps and semiconductor behavior for NNSN/NF-xh (x = 8, 11 and 14).

2.2. Electrocatalytic OER activity of NNSN/NF

The electrocatalytic OER performances of the NNSN/NF-xh (x = 8, 11 and 14) samples were evaluated in alkaline freshwater/simulated seawater/seawater by linear sweep voltammetry (LSV) (Fig. 3a–c). The OER overpotentials of the NNSN/NF-xh (x = 8, 11 and 14) electrocatalysts at 10, 100 and 1000 mA cm⁻² are shown in Fig. 3d–f. To obtain a current density of 100 mA cm⁻², NNSN/NF-11h only achieves overpotentials of 285, 302 and 298 mV, which are lower than those of NNSN/NF-8h and NNSN/NF-14h in 1 M KOH, 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater solutions, respectively. To further assess catalytic activities, NNSN/NF-11h needs overpotentials of 377, 451 and 476 mV to obtain a current density of 1000 mA cm⁻² in 1 M KOH, 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater solutions, respectively. The overpotentials for seawater electrolysis at 1000 mA cm⁻² are well below the 480 mV threshold for chloride oxidation, which is a crucial prerequisite for commercial applications.

The OER kinetics processes are evaluated using Tafel plots. Fig. 3g–i show Tafel slopes of 45.93, 50.82 and 54.66 mV dec⁻¹ for NNSN/NF-11h in 1 M KOH, 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater solutions, respectively, which are smaller than those of NNSN/NF-8h (55.88, 61.54 and 73.23 mV dec⁻¹) and NNSN/NF-14h (78.24, 75.11 and 103.29 mV dec⁻¹). The overpotentials and Tafel slopes are also comparable with those of reported Ni-based and noble metal electrocatalysts (Tables S1–S4†), which exhibit the superior catalytic activity of NNSN/NF-11h.

All the NNSN materials were loaded on NF with loading amounts of NNSN being 0.400 μg cm⁻² (8 h), 0.767 μg cm⁻² (11 h) and 1.067 μg cm⁻² (14 h). We can see at 285 mV overpotential (1.5 V vs. RHE) that the mass activity of NNSN/NF-11h is 130.37 A g⁻¹; this mass activity is ∼1.4 times higher than that of NNSN/NF-8h (90.28 A g⁻¹) and ∼4.2 times higher than that of NNSN/NF-14h (30.73 A g⁻¹), respectively (Fig. 4a). Even at higher potentials, the mass activity of NNSN/NF-11h reaches an incredible value, far exceeding that of the contrasted catalysts, verifying the expansion of its active sites. A small charge transfer resistance (R_ct) value of 1.118 Ω was obtained for NNSN/NF-11h, which is lower than those of NNSN/NF-8h (1.336 Ω) and NNSN/NF-14h (1.756 Ω) (Fig. 4b). The double-layer capacitance (C_dl) was calculated by measuring cyclic voltammetry (CV) values to assess the electrochemical surface area (Fig. S7–S9†). Fig. 4c shows that the C_dl value of NNSN/NF-11h is 20.09 mF cm⁻², which is higher than those values of NNSN/NF-8h (15.32 mF cm⁻²) and NNSN/NF-14h (13.79 mF cm⁻²). Based on the above results, it has been discovered that an appropriate reaction time can increase electrical conductivity and the number of active sites. More than that, the ECSA was also investigated using the C_dl tests (ECSA = C_dl/C_s) to estimate the catalytic activity, in which C_s is the specific capacitance for a flat surface and taken as 40 μF cm⁻² in alkaline electrolytes.⁵¹ Notably, NNSN/NF-11h provides a larger ECSA (502.25 cm²), which is higher than those of NNSN/NF-8 h (383.00 cm²) and NNSN/NF-14 h (344.75 cm²), revealing the maximum electrochemically active surface area and the highest number of active sites for NNSN/NF-11h among all of the samples obtained.

The OER performance of the electrocatalyst is significantly impacted through its hydrophilicity. The stronger the hydrophilicity, the faster the mass transfer rate, thus facilitating the adsorption of water molecules during the reaction.⁵² Besides, the enhancement of hydrophilicity will result in a more pronounced aerophobic performance, which can accelerate the rapid elimination of gas bubbles. For bare NF, its contact angle is 150.51° (Fig. S10†), indicating its poor hydrophilicity. However, upon establishment of the NNSN/NF-xh (x = 8, 11 and 14) catalysts, the contact angle decreases to 0° (Movies S1–S3†), revealing their excellent hydrophilicity.

Furthermore, the LSV curves of NNSN/NF-11h after 3000 CV cycles are shown in Fig. 4d–f. After 3000 CV cycles, the overpotential only increased by about 15, 30 and 22 mV at 1000 mA cm⁻² in 1 M KOH, 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater solution, respectively. At 300 mA cm⁻², the catalyst activity is almost unchanged throughout 100 h in 1 M KOH solution (inset of Fig. 4d). The current curve displays a gradual attenuation trend, which is close to 16% in 1 M KOH + 0.5 M NaCl solution (inset of Fig. 4e), and about 15% attenuation after 100 h in 1 M KOH + seawater solution (inset of Fig. 4f). Compared with some reported catalysts, NNSN/NF-11h still possesses a certain durability advantage.

To shed further light on the evolution of the NNSN/NF-11h catalyst for OER activity, the composition, morphology and chemical state of the NNSN/NF-11h catalyst after long-term OER tests were studied by XRD, SEM, TEM and XPS. The XRD patterns before and after the 3000 CV cycles are shown in Fig. 5a. The test results show that the diffraction peaks of r-NiS and h-NiS disappear, and there are only diffraction peaks of Ni₃S₂. This is because Ni²⁺ is oxidized to Ni³⁺ during the OER process, while the Ni₃S₂ component is sufficiently inert that its majority in the bulk remains.^53–55 The SEM image of NNSN/NF-11h shown in Fig. 5b manifests tiny variations. Compared with the sample before the OER, the surface of the laminated sheets after the reaction has a small number of fragments, and the rough particles become smooth. The TEM image results match well with the SEM image (Fig. 5c). The EDX elemental mapping shows Ni, S and O elements, but the signal for the S element is greatly weakened, and the signal for O content on the catalyst surface strengthens after OER testing, implying the loss of the S element is due to the enhanced oxidation process during the OER process (Fig. 5d–f). The surface chemical states of NNSN/NF-11h before and after the reaction are further reflected by XPS analysis (Fig. 6a). Compared with Ni 2p before stability, the Ni 2p XPS spectrum after stability shows that Ni⁰ disappears, while the content of Ni³⁺ increases, which reveals that the surface of NNSN/NF-11h is oxidized after the reaction (Fig. 6b). Fig. 6c reflects a similar S 2p signal before and after the OER test, but the relative peak intensity of the S–M bond reduces and the strength of the S–O bond increases as evidence of the oxidation process (Fig. 6c). The O 1s spectrum shows the peaks at 531.1, 531.9 and 533.2 eV are ascribed to Ni–O bonds, the increased hydroxyl bonds on the surface and physically/chemically adsorbed water (Fig. 6d),⁵⁶ confirming the existence of NiOOH species after the OER reaction.

To investigate whether ClO⁻ is produced during the OER process in the 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater electrolytes, we analyzed the products in the 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater electrolytes after 3000 CV cycles. As shown in Fig. S11,† the colour of the electrolyte is unchanged after titration with 0.5 M KI, indicating that NNSN/NF-11h is OER-specific in the 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater solutions.

3. Conclusion

In summary, the NNSN/NF-xh (x = 8, 11 and 14) electrodes were prepared by a one-step solvothermal method. The optimized NNSN/NF-11h shows excellent electrocatalytic activities and supra-low overpotentials at 1000 mA cm⁻² in 1 M KOH, 1 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater solutions, respectively. Besides, interface interactions and phase transfers modulated the electronic structure, exposed more active sites and repelled Cl⁻ from causing corrosion, thereby improving the OER activity. This study offers an effective strategy for developing high performance Ni-based sulfide OER catalysts to meet industrially relevant current densities.

Data availability

All relevant data are contained within the manuscript and its ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors gratefully acknowledge financial support from the Project of Talent Recruitment of Guangdong University of Petrochemical Technology (2019rc052 and 2019rc054) and the Project of Application and Innovation of the Maoming Green Chemical Industry Research Institute (MMGCIRI-2022YFJH-Y-025).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt01673d

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