Identifying the intrinsic active site in bimetallic Co₃S₄/Ni₃S₂ feathers on MXene nanosheets as a heterostructure for efficient oxygen evolution reaction

Abstract

Exploring economical, efficient and stable electrocatalysts toward the oxygen evolution reaction (OER) is crucial for the advancement of water splitting to optimize sustainable energy conversion technologies. Herein, Co₃S₄/Ni₃S₂ feathers are fixed on MXene nanosheets supported by nickel foam as a NF@MXene@Co₃S₄/Ni₃S₂ heterogeneous electrocatalyst through hydrothermal reaction. The catalyst exhibits excellent OER performance in alkaline medium (1M KOH) with a low overpotential of 186 mV at the current density of 10 mA cm⁻² and a small Tafel slope of 97 mV dec⁻¹, showing long-term durability over 24 h and good stability after 1000 cyclic voltammetry (CV) cycles. Theoretical calculation further identified that the key intrinsic active catalytic site in this bimetallic catalyst is the nickel atom of Ni₃S₂, which enables adsorption of the intermediates, due to a lower Gibbs free energy change of the rate-determining step of *O to *OOH. The effective identification of the intrinsic catalytically active site in the heterostructure contributes to the regulation of composites and the development of electrocatalysis applications in the OER.

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Introduction

Due to the increasingly severe issues of environmental pollution and energy shortages caused by the massive consumption of fossil fuels, numerous investigations have been devoted to developing clean, sustainable and renewable energy sources, among which electrochemical water splitting for hydrogen production is an ecologically friendly and economically viable solution.^1–5 However, the oxygen evolution reaction (OER) suffers from sluggish kinetics involving four-electron-proton coupled transfer, compared with the two-electron-transfer hydrogen evolution reaction (HER), and it is regarded as the rate-determining step for water splitting.^6–10 Accordingly, the design and synthesis of high-efficiency electrocatalysts for OER is essential to improve the conversion efficiency of water splitting.

Currently, precious metal catalysts such as RuO₂ and IrO₂ exhibit high catalytic activity for OER under alkaline conditions, but their exorbitant price and scarce abundance restrict their large-scale practical applications.^11–15 Thus, developing an earth-abundant water oxidation catalyst with the properties of low-cost, effectiveness, high energy efficiency, and long-term durability is of great importance for improvement of the sluggish kinetics of OER.^16,17 Hence, researchers have concentrated on the design of alternative OER catalysts based on 3d transition metals, for instance, forming mixed metal oxides,^18,19 sulfides,^20–22 selenides,^23–25 nitrides,^26–28 phosphides,^29,30 their hybrids^31–33 and so on. Among the above alternatives, transition metal sulfides (TMSs) with semiconductor properties and better conductivity can facilitate the charge transfer process,³⁴ and are regarded as the most promising candidate to replace precious metals. Co₃S₄ and Ni₃S₂ often respectively emerge as ideal OER catalysts.^35,36 However, when these two active TMSs are combined in bimetallic heterojunction materials, the real active site of Co₃S₄/Ni₃S₂ for OER has not been probed. Moreover, since TMSs suffer from the tendency of common agglomeration, resulting in poor durability and a reduced chemically active area,^37,38 suitable support materials are required. The two-dimensional (2D) material MXene shows excellent electrocatalytic properties due to its high conductivity, elemental diversity and rich surface terminations, which can be introduced into a heterostructure employed as the substrate.^39–41 Hao et al., grew CoFe-LDH (layered dihydroxide) vertically on the MXene substrate, effectively inhibiting the aggregation of CoFe-LDH, which exposed more active sites of the heterostructure and enhanced the efficiency of OER.⁴² In addition, Yu et al., verified that the interfacial interaction between FeNi-LDH and MXene gives rise to electronic coupling with the formation of prominent charge transfer.⁴³ Inspired by these works, integrating TMSs with MXene to form a heterostructure is anticipated to produce enhanced performance OER electrocatalysts.

In this work, we vertically grew bimetallic Co₃S₄/Ni₃S₂ feathers on MXene nanosheets supported by nickel foam, forming a multidimensional feather structure, NF@MXene@Co₃S₄/Ni₃S₂, through a simple hydrothermal reaction. With the two precursors NF@MXene and NF@MXene@CoNi-LDH, and the control samples NF@MXene@Co₃S₄, NF@MXene@Ni₃S₂, NF@Co₃S₄/Ni₃S₂ for comparison, these heterostructures were investigated for their OER performance under alkaline conditions. The results demonstrate that the NF@MXene@Co₃S₄/Ni₃S₂ electrode can achieve an impressive overpotential of 186 mV at the current density of 10 mA cm⁻² and a Tafel slope of 97 mV dec⁻¹ in contrast to NF@MXene (260 mV and 147 mV dec⁻¹) and NF@MXene@CoNi-LDH (236 mV and 122 mV dec⁻¹). MXene prevents the Co₃S₄/Ni₃S₂ feathers from agglomeration and improves the conductivity of the catalyst, providing more numerous rich reactive sites compared to the control samples. Density functional theory (DFT) further revealed that the real intrinsic active sites of the bimetallic Co₃S₄/Ni₃S₂ feathers are mainly concentrated on the nickel atom of Ni₃S₂. This study contributes insight to the fabrication of heterostructures as more effective OER electrocatalysts.

Experimental

Preparation of Ti₃C₂T_x MXene

The delaminated Ti₃C₂T_x MXene was synthesized by etching Ti₃AlC₂ powder with HCl/LiF as previously reported.^44–46 2 g of LiF was added into 50 mL of a 9 M HCl solution and stirred for 30 min at 300 rpm. Then, 2 g of Ti₃AlC₂ was slowly added into the solution and stirred for 30 h at 35 °C with a stirring rate of 100 rpm. After that, the mixture was washed repeatedly with deionized water, and centrifuged (5000 rpm, 5 min) until the pH of the supernatant was greater than 6. The precipitate was then re-dispersed in 100 mL of deionized water and sonicated with an ultrasound water bath (kept below 4 °C) at 500 W for 1 h under N₂ flow, followed by centrifugation at 3500 rpm for 1 h to obtain the Ti₃C₂T_x colloidal solution. After drying, 1 mL of the Ti₃C₂T_x colloid was weighed to calculate its concentration, yielding 1.2 g, then configured into a 1 mg mL⁻¹ solution.

Preparation of NF@MXene

To prepare NF@MXene, the nickel foam (NF, 1 × 2 cm²) was ultrasonically cleaned with hydrochloric acid (6 M), acetone, ethanol and deionized water, and dried in a vacuum for 6 h at 60 °C. The NF was added into 2 mg mL⁻¹ CTAB for 30 min and immersed into 1 mg  mL⁻¹ MXene for 30 min, then dried in a vacuum for 6 h at 60 °C.

Preparation of NF@MXene@CoNi-LDH

In a typical procedure, 1.20 mmol of Co(NO₃)₂·6H₂O and 0.40 mmol of Ni(NO₃)₂·6H₂O were added into 20 mL of deionized water and sonicated for 10 min. Then, NF@MXene was immersed into the solution and aged for 1 h and transferred into a 50 mL Teflon autoclave. 3.33 mmol of urea and 20 mL of deionized water were added into the mixture and it was kept at 120 °C for 4 h to obtain the NF@MXene@CoNi-LDH precursor. The precursor was washed with deionized water and dried in a vacuum at 60 °C.

Preparation of NF@MXene@Co₃S₄/Ni₃S₂

The NF@MXene@CoNi-LDH precursor was hydrothermally vulcanized to prepare the product. In detail, the precursor with 0.62 mmol of Na₂S·9H₂O was added to 40 mL of deionized water and transferred into a 50 mL Teflon autoclave, which was kept at 160 °C for 12 h, washed with deionized water and dried in a vacuum at 60 °C to gain the NF@MXene@Co₃S₄/Ni₃S₂ product.

Results and discussion

The heterostructure NF@MXene@Co₃S₄/Ni₃S₂ was obtained via a simple hydrothermal reaction (Fig. 1). Scanning electron microscopy (SEM) was initially used to observe the morphology of the as-synthesized NF@MXene@Co₃S₄/Ni₃S₂ and its two precursors. As shown in Fig. 2a, MXene is uniformly dispersed over the surface of the NF, then, fuzz-like CoNi-LDH grows on the MXene after the hydrothermal process (Fig. 2b). During the final vulcanization reaction under hydrothermal conditions, TMS-Co₃S₄/Ni₃S₂ converts the fuzz to feathers, leading to the much rougher surface shown in Fig. 2c. Without MXene, the material NF@Co₃S₄/Ni₃S₂ agglomerates and forms irregular clusters (Fig. S1†). Transmission electron microscopy (TEM) imaging shows the great transparency of the Co₃S₄/Ni₃S₂ feather, reflecting its ultrathin feature (Fig. 2d). The energy dispersive spectroscopy (EDS) spectra confirmed the existence of C, O, S, Ti, Co and Ni elements, and the mapping images suggest a uniform dispersion of the six elements in NF@MXene@Co₃S₄/Ni₃S₂ (Fig. 2e). The high-resolution TEM (HRTEM) image reveals the layered property of this heterostructure with MXene-supported Co₃S₄/Ni₃S₂ (Fig. 2f), and the lattice fringes indicated the good crystalline character of Co₃S₄, Ni₃S₂ and MXene with lattice spacings of 0.550 nm, 0.287 nm and 0.256 nm, indexed to the (111), (110) and (110) crystal planes of Co₃S₄, Ni₃S₂ and MXene, respectively (Fig. 2g–i).

Fig. 1 Schematic illustration of the preparation strategy of NF@MXene@Co₃S₄/Ni₃S₂.

Fig. 2 SEM images of (a) NF@MXene, (b) NF@MXene@CoNi-LDH and (c) NF@MXene@Co₃S₄/Ni₃S₂. (d) TEM image, (e) HAADF image and corresponding EDS elemental mapping of C, O, S, Ti, Ni, and Co and their overlay distribution of NF@MXene@Co₃S₄/Ni₃S₂. (f–i) HRTEM images of NF@MXene@Co₃S₄/Ni₃S₂.

The structure of NF@MXene@Co₃S₄/Ni₃S₂ was confirmed using powder X-ray diffraction (PXRD) with bare NF, MXene and NF@MXene as references. The weak peaks of MXene are covered by the strong signals of the NF pattern in the PXRD pattern of NF@MXene. New peaks were detected after the hydrothermal treatment of NF@MXene, located at 31.4°, 50.2°, and 55.1°, and 31.1°, 50.2°, 54.6°, and 55.2°, which respectively correspond to the Co₃S₄ (JCPDS No. 42-1448) and Ni₃S₂ (JCPDS No. 44-1418), evidencing the successful synthesis of the heterostructure NF@MXene@Co₃S₄/Ni₃S₂ (Fig. 3a). Moreover, the surface elemental composition and chemical state of the elements of NF@MXene@Co₃S₄/Ni₃S₂ were observed using X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum indicates that NF@MXene@Co₃S₄/Ni₃S₂ consists of C, O, S, Ti, Co, and Ni elements (Fig. S2†), among which the elements C, O and Ti are derived from Ti₃C₂T_x MXene.⁴⁷ In the Ti 2p XPS spectrum, four peaks centred at 464.0, 461.6, 458.3 and 455.6 eV can be observed, wherein the peaks at 455.6 and 461.6 eV are ascribed to the Ti–C bonds, and those at 458.3 and 464.0 eV can be assigned to the Ti–O bonds.(Fig. S3†)^48–50 The XPS spectrum of S 2p contains spin–orbit doublet peaks at 162.1 eV (S 2p_3/2) and 163.3 eV (S 2p_1/2), and two shake-up satellite peaks at approximately 168.0 and 169.1 eV ascribed to the S_n²⁻ and SO_x species (Fig. 3b).^44,51 The XPS spectrum of Co 2p consists of two pairs of spin–orbit doublets. The first doublet at 781.1 eV (Co 2p_3/2) and 796.6 eV (Co 2p_1/2) and the second doublet at 784.1 eV (Co 2p_3/2) and 797.8 eV (Co 2p_1/2) are attributed to Co²⁺ and Co³⁺ of Co₃S₄, respectively, while their accompanying satellite peaks are located at 791.6 and 802.6 eV (Fig. 3c).^52–54 In the Ni 2p spectrum, two spin–orbit peaks locate at 856.1 eV (Ni²⁺ 2p_3/2) and 873.6 eV (Ni²⁺ 2p_1/2) along with two typical satellite peaks of Ni²⁺ at 861.4 and 879.4 eV, and the minor peak at 852.4 eV belongs to the metallic Ni of the NF substrate (Fig. 3d).^37,55,56 The above analysis indicates that the composite NF@MXene@Co₃S₄/Ni₃S₂ was successfully manufactured.

Fig. 3 (a) PXRD patterns of NF, MXene, NF@MXene and NF@MXene@Co₃S₄/Ni₃S₂. High-resolution XPS spectra of (b) S 2p, (c) Co 2p and (d) Ni 2p regions of NF@MXene@Co₃S₄/Ni₃S₂.

In order to evaluate the effectiveness of the conductivity of the MXene nanosheets and the activity of the TMS-Co₃S₄/Ni₃S₂ feathers, the OER electrocatalytic performance of this heterostructure was appraised using a standard three-electrode cell in the potential range of 0.85–2.00 V versus the reversible hydrogen electrode (RHE) at a scan rate of 10 mV s⁻¹ in 1.0 M KOH solution. In order to eliminate the influence of the nickel oxidation peak, the OER LSV polarization curves were obtained using the backward sweep, which can effectively evaluate the OER performance of as-synthesized catalysts (Fig. S4†).⁵⁷ For comparison, a commercial RuO₂ benchmark loaded on NF (denoted as NF@RuO₂), the two precursors NF@MXene and NF@MXene@CoNi-LDH, and bare NF were also studied as control catalysts. Evidently, the featherlike NF@MXene@Co₃S₄/Ni₃S₂ catalyst exhibited superior electrocatalytic performance compared to the other electrodes. According to the linear sweep voltammetry (LSV) curves of the evaluated catalysts, NF@MXene@Co₃S₄/Ni₃S₂ can reach a current density of 10 mA cm⁻² at a low overpotential (denoted as η₁₀) of 186 mV (1.416 V vs. RHE), smaller than those of NF (429 mV), NF@MXene (260 mV), NF@MXene@CoNi-LDH (236 mV) and NF@RuO₂ (399 mV) (Fig. 4a). Moreover, the samples exhibit the same trend for η₁₀₀ as that of the LSV curves (Fig. 4b). Along with the MXene preventing Co₃S₄/Ni₃S₂ from agglomerating, the overpotential at a current density of 10 mA cm⁻² of NF@MXene@Co₃S₄ (259 mV) with more active sites is smaller than that of NF@Co₃S₄/Ni₃S₂ (306 mV), but larger than that of NF@MXene@Ni₃S₂ (221 mV) with Ni₃S₂ instead of Co₃S₄ in NF@MXene@Co₃S₄ (Fig. S5†). The reaction kinetics of the electrocatalysts were evaluated using Tafel slopes with fitted linear regions. The Tafel slope of NF@MXene@Co₃S₄/Ni₃S₂ (97 mV dec⁻¹) is smaller than those of NF (286 mV dec⁻¹), NF@MXene (147 mV dec⁻¹), NF@MXene@CoNi-LDH (122 mV dec⁻¹) and NF@RuO₂ (259 mV dec⁻¹) (Fig. 4c), implying the fastest kinetics and the highest OER catalytic activity of NF@MXene@Co₃S₄/Ni₃S₂. The electrochemical active surface area (ECSA) of a catalyst is linearly proportional to the electrochemical double-layer capacitance (C_dl), which is estimated by measuring the non-faradaic potential range from the cyclic voltammetry (CV) curves at different scan rates (Fig. 4d and S6†). The calculated C_dl value of NF@MXene@Co₃S₄/Ni₃S₂ is 8.95 mF cm⁻², much larger than those of NF (1.34 mF cm⁻²), NF@MXene (2.15 mF cm⁻²), NF@MXene@CoNi-LDH (3.30 mF cm⁻²) and NF@RuO₂ (2.31 mF cm⁻²) (Fig. 4e), suggesting that the feather-like Co₃S₄/Ni₃S₂ fixed on the MXene nanosheets supported by nickel foam as a catalyst has a greater amount of catalytically active sites, contributing to better OER performance. Electrochemical impedance spectroscopy (EIS) tests were performed in a frequency range of 100 kHz-0.01 Hz under the open-circuit potential condition with an alternate current perturbation of 10 mV. A simplified equivalent circuit was used to fit the EIS data and to extract the series resistance (R_s), charge-transfer resistance (R_ct) and constant phase elements (CPE), where R_s, R_ct and CPE represent the series resistance, correspondingly accounting for the transport resistance, charge transfer resistance and electrode/electrolyte interface capacitance. The Nyquist plots further evidence the fastest electron transfer rate and accelerated faradaic process on the NF@MXene@Co₃S₄/Ni₃S₂ electrode, as shown in Fig. 4f.

Fig. 4 The OER performances of different catalysts in 1M KOH electrolyte. (a) LSV curves at 10 mV s⁻¹, (b) overpotentials at 10 mA cm⁻² and 100 mA cm⁻², (c) Tafel slopes, (d) CV curves at non-faradaic potential, (e) C_dl values and (f) Nyquist plots.

Due to the Co₃S₄/Ni₃S₂ feathers fixed on the MXene nanosheets with varied active sites and good electrical conductivity, NF@MXene@Co₃S₄/Ni₃S₂ exhibits superior performance with a lower overpotential at 10 mA cm⁻² compared with previously developed electrocatalysts (Fig. 5a, Table S1†). In addition, long-term stability is a vital parameter when investigating the electrochemical performance of an electrocatalyst. The long-term durability of NF@MXene@Co₃S₄/Ni₃S₂ was evaluated by continuous CV scanning and chronoamperometric tests. The OER curve almost completely overlaps the initial one after 1000 CV cycles (Fig. 5b) and the straight i-t curve proves that the current density remains stable with minimal decay for 24 h at a constant potential of 1.62 V, corroborating the electrocatalytic durability of this heterojunction structure (Fig. 5c). Further, the SEM image shows that NF@MXene@Co₃S₄/Ni₃S₂ still maintains its initial feather-like morphology, and the EDS elemental mapping confirmed the uniform distribution of C, O, S, Ti, Co and Ni after the OER stability test, the same as their initial one (Fig. 5d and e). In addition, the structure verified by PXRD and Brunauer–Emmett–Teller (BET) surface area of NF@MXene@Co₃S₄/Ni₃S₂ showed no obvious alteration after the stability test (Fig. S7 and S8, Table S2†), which again confirms the catalytic stability of this heterostructure for OER.

Fig. 5 (a) Comparison of overpotentials at 10 mA cm⁻² of different electrocatalysts in the literature. (b) The LSV curves of NF@MXene@Co₃S₄/Ni₃S₂ before and after 1000 CV cycles. (c) Chronoamperometry curve of the durability test of NF@MXene@Co₃S₄/Ni₃S₂. (d) SEM image and (e) EDS elemental mapping of NF@MXene@Co₃S₄/Ni₃S₂ after the stability test.

Density Functional Theory (DFT) calculation was performed to understand the mechanism of the OER activity of Co₃S₄ and Ni₃S₂ in the NF@MXene@Co₃S₄/Ni₃S₂ electrocatalyst to ascertain the actual active site in the heterostructure. The reaction coordinates during OER of Co₃S₄ and Ni₃S₂ are schematically depicted in Fig. 6a and b, respectively. The details of the optimized structures with adsorptive active intermediates are shown in Fig. S9–S12,† respectively. It was found that Co and Ni atoms behave as the main active sites for the adsorption of intermediates. According to the Gibbs free energy change of homologous active sites, we found that the conversion *O + OH⁻ → *OOH + e⁻ is the rate-determining step for both Co₃S₄ and Ni₃S₂ based catalysts synthesized in this work. The calculated free energy changes ΔG₁, ΔG₂, ΔG₃ and ΔG₄ for Co₃S₄ and Ni₃S₂ are shown in Table S3;† for the * + OH⁻ → *OH + e⁻ step, the Co₃S₄ group is more negative than the Ni₃S₂ group, which means that Co₃S₄ has a higher binding energy for OH. However, in the *OH + OH⁻ → *O + H₂O + e⁻ step, Co₃S₄ needs an additional energy input of 0.16 eV (1.39–1.23 = 0.16 eV), while the value for Ni₃S₂ is 0 eV (1.23–1.23 = 0 eV), exactly equal to a standard hydrogen potential that does not require additional energy input. The next step, *O + OH⁻ → *OOH + e⁻, further intensifies the additional energy input of Co₃S₄ (Fig. 6c and d). The highest energy input required for the overpotential of Co₃S₄ is 0.41 V, compared with 0.37 V for Ni₃S₂, which certifies that the nickel atom plays a pivotal role as the active site in this heterogeneous structure.

Fig. 6 The optimized absorptive structures of (a) Co₃S₄ and (b) Ni₃S₂ at the four reaction coordinates during OER. Free energy diagram at 1.23 V for OER of (c) Co₃S₄ and (d) Ni₃S₂.

Conclusions

To sum up, a heterostructure, NF@MXene@Co₃S₄/Ni₃S₂, based on the homogeneous distribution of feathers supported by 2D nanosheets was successfully produced using a hydrothermal process. It shows excellent catalytic performance toward OER in an alkaline medium (1M KOH), manifesting a low overpotential of 186 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 97 mV dec⁻¹ with good structural stability. Based on theoretical calculation, the nickel atom in NF@MXene@Co₃S₄/Ni₃S₂ performs as the key active site, giving a lower Gibbs free energy change from *O to *OOH. Consequently, the NF@MXene@Co₃S₄/Ni₃S₂ catalyst is a promising catalyst for OER and provides a new strategy for energy conversion.

Author contributions

Yang Li: conceptualization, writing – original draft, visualization, and data curation. Qixuan Du: writing – original draft and visualization. Jian Cui: data curation and visualization. Xiao Chen: conceptualization, data curation, and visualization. Hongwei Yang: validation, supervision, and resources. Hua Qian: supervision, writing – review & editing, and resources.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (No.22075143) and Hongwei Yang thanks the Qing Lan Project for the grant.

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Footnote

† Electronic supplementary information (ESI) available: Additional electrochemistry, characterization and computation data, Fig. S1–S12 and Tables S1–S3. See DOI: https://doi.org/10.1039/d2qi01830f

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