English

• The overuse of fossil fuels has caused an energy crisis and a series of environmental problems. Hence, developing new cleaner energy sources (solar, wind, tidal, and hydrogen) which are ample and eco-friendly, is crucial [1-4]. Hydrogen is a promising option due to its high energy density and zero carbon emissions [5,6]. Electrocatalytic water splitting has erupted as a pivotal and sustainable method for hydrogen extraction, but the sluggish conversion kinetics of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) limited the efficiency [7-12]. The OER plays a key role in many energy conversion processes such as water splitting and rechargeable metal-air batteries [13]. In particular, the OER has a high thermodynamic barrier due to the four consecutive proton-coupled electron transfers [14]. Based on that, higher catalytic activity noble metals have been chosen as electrocatalytic materials in water splitting, like Pt/C for HER and RuO₂ or IrO₂ for OER [15-19]. Yet, the high cost and insufficiency of noble metals cannot meet the requirements of large-scale actual production and hinder the commercialization of water electrolysers. Accordingly, it is of great interest to explore economical, efficient, and stable non-noble metal-based electrocatalytic materials for OER towards water splitting [20-22].

  FeNi-based bimetallic materials have been regarded as promising OER electrocatalysts because of their low cost, abundant reserves, and inherent activity [23,24]. Additionally, FeNi-based materials feature a unique three-dimensional (3D) electronic structure that binds OER intermediates in an alkaline medium, like FeNi layered hydroxides [25], metal-organic frameworks [26], FeNi-based (oxygen) hydroxides [27-29], FeNi-based oxides [30-32], FeNi alloys [33,34] and FeNi-based non-oxides [35-38]. The volcano diagram depicts that the binding energy of nickel-based materials is slightly lower than that of noble metal-based catalysts, indicating that the theoretical eta of OER is small. However, affected by the complex OER process and characterization methods, determining the precise active center remains a challenge, and the accurate active site for OER remains controversial. The previous literature proved that the introduction of Ni and Fe atoms is crucial for regulating the electronic structure around the active site and improving OER dynamics, thus exhibiting better performance than that of pure Ni or Fe-based alloys, oxides, or hydroxides [39-42].

  Besides, by manipulating anions with different electronegativities, the electronic structure and redox processes of transition metal ions also can be modulated. Notably, anions of transition metal compounds (MeX, X = P, Se, S, and O) catalysts tend to capture water molecules during the OER process [43-55]. Meanwhile, comparing the effects of various anions on performance within the same conditions is challenging because of unpredictable factors such as interactions and anion manipulation, which influence activities and morphology, respectively. In addition, there is still a lack of a fundamental understanding of the anion substitution that would reveal the relationship between the anion species and catalytic performance. Hoffmann cyano-bridged coordination polymer (HCP) can be converted into metal phosphides, selenides, sulfides, and oxides with the original structure, and the morphology after conversion can remain unchanged [56-59].

  Herein, the FeNi HCP was selected as a precursor to synthesize a series of FeNi-based materials (FeX/NiX, X = P, Se, S, and O) [60-65]. The fabrication routine of the superstructure and its derivations is schematically shown in Fig. 1. Polyvinylpyrrolidone (PVP) was used as a surfactant to synthesize Fe[Ni(CN)₄] nanowire self-assembled superstructure (Fe-Ni HCPS), then anion-modified catalysts were obtained through phosphating, selenizing, sulfurizing, and oxidizing treatments. The electronic structure, redox process, and OER catalytic mechanism show that substitution with less electronegative anions leads to an increase in electron density around the metal atoms, causing a transfer of binding energy. Based on DFT calculations, we found that anions with lower electronegativity improve the metallicity of the catalyst to increase conductivity and optimize the adsorption behavior of oxygen intermediates, thereby improving OER electrocatalytic activity. As an OER electrocatalyst, the FeNi selenide exhibits enhanced activity and stability compared with commercial RuO₂ at a high current density.

  Figure 1

  

  Figure 1.  Schematic illustration of the synthesis and anion modification of the Fe[Ni(CN)₄] nanowire self-assembled superstructure.

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  The phase composition of the Fe[Ni(CN)₄] nanowire self-assembled superstructure (FeNi HCPS) was confirmed by X-ray diffraction (XRD) analysis (Fig. S1a in Supporting information). All diffraction peaks are well-matched with the reported Fe(H₂O)₂[Ni(CN)₄]·H₂O bulk phase. From Fourier transform-infrared (FT-IR), it can see the existence of −C≡N (2156.1 cm⁻¹), Ni−C≡N (581.4 cm⁻¹), and Fe-N (621.3 cm⁻¹) signal peaks (Fig. S1b in Supporting information). The above characterization proves that the preparation of the samples is indeed FeNi HCPs. Energy dispersive spectrometer (EDS) can be seen that the sample contains C, N, O, Fe, and Ni elements, and no other impure elements were detected (Fig. S1c in Supporting information). FT-IR and EDS spectra further prove the composition of the product is FeNi HCPS. The morphology of the as-prepared FeNi HCP is characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). Figs. 2a and b show high uniformity of the FeNi HCPS, which is composed of nanowires, with a length of ~20 µm. From Fig. 2c, it shows that the shape is composed of wires. The forming process is to first superimpose new wires irregularly on the wire. As the wires become longer, more and more wires are superimposed, making the FeNi HCPS continue to grow longer and wider until it is stable (Fig. S2 in Supporting information). FeNi HCPS can be observed in the transmission electron microscope (TEM) images (Figs. 2d and e), and it can be seen that the middle is thicker than both sides, proving the superimposed growth of FeNi HCP nanowires. The high-resolution TEM (HRTEM) image of the clear lattice fringes (Fig. 2f). The interplanar spacings of 3.5 and 4.4 Å correspond to the (200) and (121) planes, respectively (Fig. 2g). The scanning transmission electron microscopy (STEM) images and corresponding elemental mapping confirm the homogeneous distribution of C, N, O, Ni, and Fe elements within FeNi HCPS (Fig. 2h). Further, the XPS spectra results verify the presence of elements in FeNi HCPS, according to Fe 2p XPS spectra, in which part of the Fe²⁺ is oxidized to Fe³⁺, C 1s XPS spectra and N 1s XPS spectra can be seen that FeNi HCPS has C≡N, (Fig. S3 in Supporting information). We conducted a preliminary exploration of the formation reasons of FeNi HCPS. The effect of NH₄⁺ on the superstructure is excluded (Fig. S4 in Supporting information). Two additional experiments were performed to verify that oxidation of Fe²⁺ to Fe³⁺ was the reason for the superstructure formation: The first one was synthesized with (NH₄)₂Fe(SO₄)₂ and Fe₂(SO₄)₃, and the superstructure still existed (Fig. S5 in Supporting information); while the other one was synthesized with only Fe₂(SO₄)₃ without superstructure appearing (Fig. S6 in Supporting information). Such results confirm that the presence of Fe²⁺ and Fe³⁺ is crucial for uniform superstructure formation. Moreover, citrate was regarded as an additive in tuning the morphology of the superstructure: The size becomes thicker and shorter when lowering the dosage of citrate (Fig. S7 in Supporting information). In contrast, the size would be longer and thinner because an overdose of citrate triggers the formation of Fe²⁺-citrate complex which leads to a decrease in the nucleation rate.

  Figure 2

  

  Figure 2.  (a–c) FESEM images and (d, e) TEM images of the Fe[Ni(CN)₄]. (f, g) HRTEM image of the Fe[Ni(CN)₄] in (e). (h) STEM images and the corresponding elemental mapping images of a Fe[Ni(CN)₄].

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  FeNi composites were derived from FeNi HCPS as the precursor and O₂, P, Se, and S powder as the anion sources. XRD spectra results confirm that the phosphating sample prepared under these conditions is a composite phase of Fe₂P and Ni₂P. FTIR spectra reveal that the sample has infrared characteristic peaks of Ni−P (1635.2 cm⁻¹, 745.7 cm⁻¹) and Fe−P (536.9 cm⁻¹), further proving that the synthesized sample is Fe₂P/Ni₂P. EDS spectra demonstrate the sample contains elements P, Fe, and Ni (Fig. S8 in Supporting information). XRD spectra results confirm that the selenizing sample prepared under these conditions is a composite phase of Fe₃Se₄ and Ni₃Se₄. FTIR spectra reveal that the sample has infrared characteristic peaks of Ni−Se (1616.1 cm⁻¹) and Fe−Se (621.0 cm⁻¹, 481.1 cm⁻¹) further proving that the synthesized sample is Fe₃Se₄/Ni₃Se₄. EDS spectra demonstrate the sample contains elements Se, Fe, and Ni (Fig. S9 in Supporting information). XRD spectra results confirm that the sulfide sample prepared under these conditions is a composite phase of Fe₇S₈ and NiS. FTIR spectra reveal that the sample has infrared characteristic peaks of Ni−S (1636.7 cm⁻¹) and Fe−S (1095.9 cm⁻¹, 629.4 cm⁻¹) further proving that the synthesized sample is Fe₇S₈/NiS. EDS spectra demonstrate the sample contains elements S, Fe, and Ni (Fig. S10 in Supporting information). XRD spectra results confirm that the oxidizing sample prepared under these conditions is a composite phase of Fe₃O₄ and NiO. FTIR spectra reveal that the sample has infrared characteristic peaks of Ni−O (1636.7 cm⁻¹) and Fe−O (579.7 cm⁻¹) further proving that the synthesized sample is Fe₃O₄/NiO. EDS spectra demonstrate the sample contains elements O, Fe, and Ni (Fig. S11 in Supporting information).

  The FESEM images show that four FeNi bimetallic derivations possess the same morphology, size, and thickness as that of the FeNi HCPS (Figs. 3a and b, Fig. S12 in Supporting information). Among of above, the selected area electron diffraction (SAED) pattern shows that the Fe₃Se₄/Ni₃Se₄ is polycrystalline (Fig. 3c). The scanning TEM (STEM) and corresponding elemental mapping images suggest that Se, Ni, and Fe elements are distributed homogeneously within the Fe₃Se₄/Ni₃Se₄ (Fig. 3d).

  Figure 3

  

  Figure 3.  (a, b) FESEM and (c) TEM image (inset: the corresponding SAED image), (d) STEM image and the corresponding elemental mapping images, (e) HRTEM image (inset: the corresponding FFT) and (f) enlarged image from the dot line rectangle in (e) of the Fe₃Se₄/Ni₃Se₄. (g) Distribution of the charge density difference at Fe₃Se₄/Ni₃Se₄ heterointerfaces, in which yellow and blue represent positive and negative electronic clouds, respectively.

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  To decipher the relationship between Fe₃Se₄ and Ni₃Se₄, high-resolution TEM (HRTEM), fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) were conducted. Figs. 3e and f show that the (111) plane of Fe₃Se₄ and the (002) plane of Ni₃Se₄ exhibit a lattice match, resulting in a heterostructure. The differential charge density based on the density functional theory (DFT) calculation reveals noticeable electron transfer (from Fe₃Se₄ to Ni₃Se₄) at both phases, indicating the formation rationality of the Fe₃Se₄/Ni₃Se₄ (Fig. 3g), and Fe₂P/Ni₂P, Fe₇S₈/NiS, and Fe₃O₄/NiO (Fig. S13 in Supporting information).

  The electrocatalytic OER performance of the Fe₃Se₄/Ni₃Se₄, Fe₂P/Ni₂P, Fe₇S₈/NiS, and Fe₃O₄/NiO was investigated in a 1 mol/L KOH solution using a standard three-electrode configuration. As shown in Fig. 4a, Fe₃Se₄/Ni₃Se₄ affords more negative onset potential and higher current density than other catalysts. The Fe₃Se₄/Ni₃Se₄ catalyst exhibits a current density of 10 mA/cm² at a potential of 1.54 V versus RHE (316 mV), lower than that of RuO₂ (335 mV), Fe₂P/Ni₂P (357 mV), Fe₇S₈/NiS (379 mV), and Fe₃O₄/NiO (464 mV). Moreover, the current density rises as potential increases, faster than that of other catalysts, indicating catalysts modified with selenium ions have better catalytic activity.

  Figure 4

  

  Figure 4.  (a) LSV curves, (b) Tafel plots, (c) Mass activities, (d) TOF, (e) EIS curves, (f) DOS, and (g) ECSA curves of the Fe₃Se₄/Ni₃Se₄ and other catalysts. (h) CP test of the Fe₃Se₄/Ni₃Se₄ catalyst at the current density of 10 mA/cm². (i) LSV curves of the Fe₃Se₄/Ni₃Se₄ after CP test. (j) Schematic illustration of the proposed OER pathways on the Fe₃Se₄/Ni₃Se₄ catalyst. (k) Calculated energetic reaction pathway for the OER process on the surface of the Fe₃Se₄/Ni₃Se₄ and other catalysts.

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  The electrocatalytic kinetics for OER was further examined by Tafel plots (Fig. 4b). The Fe₃Se₄/Ni₃Se₄ catalyst features a lower Tafel slope (70.1 mV/dec) than that of Fe₇S₈/NiS (70.4 mV/dec), RuO₂ (73.9 mV/dec) and Fe₃O₄/NiO (95.2 mV/dec), indicative of enhanced kinetics for OER. A smaller Tafel slope suggests effective electrocatalysis, as it verifies that the rate-determining step occurs towards the end of the multi-electron transfer reaction. Since the Tafel slope of Fe₂P/Ni₂P is lower than that of Fe₃Se₄/Ni₃Se₄, the current density of the P-modified catalyst increases significantly faster after about 1.65 V. More than that, the mass activity of Fe₃Se₄/Ni₃Se₄ at an overpotential of 300 mV is also the highest among counterparts with a value of 70.0 A/g (Fig. 4c). Besides, Fe₃Se₄/Ni₃Se₄ showed a high turnover frequency (TOF) of 19.8 s⁻¹, superior than that of Fe₂P/Ni₂P (5.1 s⁻¹), Fe₇S₈/NiS (2.7 s⁻¹), Fe₃O₄/NiO (0.8 s⁻¹), and RuO₂ (4.9 s⁻¹) (Fig. 4d).

  The results of the electrochemical impedance spectroscopy (EIS) demonstrate that the charge-transfer resistance is as follows: Fe₃Se₄/Ni₃Se₄ < Fe₂P/Ni₂P < Fe₇S₈/NiS < Fe₃O₄/NiO (Fig. 4e). The local electron densities of the above-mentioned catalyst models at the atomic level were determined by density functional theory (DFT). The density of states (DOS) of Fe₃Se₄/Ni₃Se₄ across the Fermi level is higher than that of other catalyst materials (Fig. 4f and Fig. S14 in Supporting information), following EIS data, suggesting metallic properties in terms of carrier concentration and electrical conductivity of Fe₃Se₄/Ni₃Se₄. Therefore, anions with low electronegativity can enhance the conductivity, thus improving its catalytic activity. In addition, the OER electrocatalytic performance of other catalysts with different Se mass ratios was also evaluated. Fig. S15 (Supporting information) depicts that the Fe₃Se₄/Ni₃Se₄ with the Fe[Ni(CN)₄]: Se mass ratio of 1:1 exhibits the best electrocatalytic performance. The Fe₃Se₄/Ni₃Se₄ offers the lowest overpotential, Tafel slope and impedance, coupled with the highest mass activity, electrochemical active area and conversion frequency.

  The results show that the Cdl value of Fe₃Se₄/Ni₃Se₄ exceeds that of other catalysts, suggesting a greater electrochemically active area and more OER active sites. (Fig. 4g and Fig. S16 in Supporting information), suggesting that the considerably reduced charge-transfer resistance of Fe₃Se₄/Ni₃Se₄ may result from the addition of the Se ions. To exclude the factor of the varied number of active sites on different catalysts, the LSV curves are replotted using ECSA-normalized current density (Fig. S17 in Supporting information). The result reveals that Fe₃Se₄/Ni₃Se₄ still has the highest OER activity, indicating that the intrinsic activity of the catalytically active sites on the Fe₃Se₄/Ni₃Se₄ catalyst is higher than those of the other three catalysts. Besides the activity, the high stability of the electrocatalysts toward OER is also critical for energy conversion systems. The long-term durability of the Fe₃Se₄/Ni₃Se₄ was tested by chronopotentiometry (CP) measurements (Fig. 4h), and the polarization curves obtained are similar to those at the beginning (Fig. 4i). Impressively, the catalyst could maintain for at least 100 h under the current density of 10 mA/cm². After the OER test, both XRD and FTIR measurements could detect the Fe₃Se₄/Ni₃Se₄ signals also can be detected by XRD and FT-IR measurements, and the superstructure-like morphology kept well, supporting the high stability of the catalytic performance of the selenide. (Fig. S18 in Supporting information). Nevertheless, some parts of the catalyst transform from crystalline cyanide/selenide hybrid to amorphous oxide/hydroxide-related species after the durability test, as revealed by the XRD, FTIR, EDS data (Fig. S19 in Supporting information).

  The OER processes occurring on these four catalysts were also simulated, in which the adsorption modes of the corresponding intermediates (such as *OH, *O, and *OOH) on the active sites were calculated based on DFT (Fig. 4j and Fig. S20 in Supporting information). Then the energetic reaction pathway can be obtained accordingly (Fig. 4k). It is found that the rate-determining steps are not all the same for these catalysts, i.e., the second step (from *OH to *O) for the Fe₃Se₄/Ni₃Se₄ and Fe₃O₄/NiO, but the third step (from *O to *OOH) for the Fe₂P/Ni₂P and the fourth step (from *OOH to O₂) for Fe₇S₈/NiS. The energy barrier of the rate-determining step is calculated as 0.4, 1.28, 1.76, and 1.82 eV for the Fe₃Se₄/Ni₃Se₄, Fe₂P/Ni₂P, Fe₇S₈/NiS and Fe₃O₄/NiO, respectively. This indicates the catalytic activity of the activity sites follows the sequence of Fe₃Se₄/Ni₃Se₄ > Fe₂P/Ni₂P > Fe₇S₈/NiS > Fe₃O₄/NiO, consistent with the above experimental analysis. The Ni 2p_3/2 and Fe 2p_3/2 binding energies of the catalysts with those anions that have relatively low electronegativity display a positive shift (Fig. S21 in Supporting information). The positive shift suggesting that the electron density of metal atoms is reduced, which leads to the fast desorption of O-containing intermediates and thus enhances the intrinsic activity for OER, in accordance with adsorption free energy data [66,67]. In addition, the concentrations of Ni³⁺ and Fe²⁺ of Fe₃Se₄/Ni₃Se₄ are higher than other catalysts (Tables S2 and S3 in Supporting information), which is probably improving catalytic activity in electrocatalysis [68]. The complete substitution of relatively low electronegative elements could affect electron intensity localization around metal atoms, also being in good agreement with DOS [69]. Low electronegativity improves the metallicity of the catalyst, thereby improving the conductivity of the catalyst. Since both selenide and phosphide have metallic properties, they both have good conductivity. However, the energy required for the rate-determining step of adsorption free energy for selenide is lower than that for phosphide.

  In conclusion, we have successfully synthesized The FeNi HCPS self-assembled superstructure for four anionic modifications. The FeNi HCPS was formed into an irregular superposition of wires and converted into Fe₂P/Ni₂P, Fe₃Se₄/Ni₃Se_4, Fe₇S₈/NiS, and Fe₃O₄/NiO. Among them, the Fe₃Se₄/Ni₃Se₄ catalyst delivers enhanced electrocatalytic OER performance, with a low overpotential of 316 mV at 10 mA/cm², surpassing those of the other Fe/Ni-based catalysts in this work. Moreover, the Fe₃Se₄/Ni₃Se₄ catalyst possesses high conductivity, enhances the adsorption ability of intermediate products, accelerates the rate-determining step, and consequently results in improved electrocatalytic performance. Accordingly, this work provides a facial yet efficient synthetic approach for designing and synthesizing high-performance bimetal-based heterostructure electrocatalysts with a unique structure for OER electrocatalysis.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Weibin Shen: Writing – original draft, Investigation, Data curation, Conceptualization. Jie Liu: Validation, Software, Data curation. Gongyu Wen: Writing – review & editing, Validation. Shuai Li: Data curation. Binhui Yu: Data curation. Shuangyu Song: Data curation. Bojie Gong: Data curation. Rongyang Zhang: Data curation. Shibao Liu: Data curation. Hongpeng Wang: Data curation. Yao Wang: Validation, Software. Yujing Liu: Validation. Huadong Yuan: Validation. Jianming Luo: Validation. Shihui Zou: Validation. Xinyong Tao: Writing – review & editing, Supervision, Funding acquisition. Jianwei Nai: Writing – review & editing, Validation, Supervision, Funding acquisition, Conceptualization.

  Acknowledgments

  This work is supported by the National Natural Science Foundation of China (Nos. 52222317, 21902144, 52225208), the ''Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang'' (No. 2020R01002), the Natural Science Foundation of Zhejiang Province (No. LZ23E020002), and the Fundamental Research Funds for the Provincial Universities of Zhejiang (No. RF-C2023002).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110184.

  
  
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  Figure 1  Schematic illustration of the synthesis and anion modification of the Fe[Ni(CN)₄] nanowire self-assembled superstructure.

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  Figure 2  (a–c) FESEM images and (d, e) TEM images of the Fe[Ni(CN)₄]. (f, g) HRTEM image of the Fe[Ni(CN)₄] in (e). (h) STEM images and the corresponding elemental mapping images of a Fe[Ni(CN)₄].

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  Figure 3  (a, b) FESEM and (c) TEM image (inset: the corresponding SAED image), (d) STEM image and the corresponding elemental mapping images, (e) HRTEM image (inset: the corresponding FFT) and (f) enlarged image from the dot line rectangle in (e) of the Fe₃Se₄/Ni₃Se₄. (g) Distribution of the charge density difference at Fe₃Se₄/Ni₃Se₄ heterointerfaces, in which yellow and blue represent positive and negative electronic clouds, respectively.

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  Figure 4  (a) LSV curves, (b) Tafel plots, (c) Mass activities, (d) TOF, (e) EIS curves, (f) DOS, and (g) ECSA curves of the Fe₃Se₄/Ni₃Se₄ and other catalysts. (h) CP test of the Fe₃Se₄/Ni₃Se₄ catalyst at the current density of 10 mA/cm². (i) LSV curves of the Fe₃Se₄/Ni₃Se₄ after CP test. (j) Schematic illustration of the proposed OER pathways on the Fe₃Se₄/Ni₃Se₄ catalyst. (k) Calculated energetic reaction pathway for the OER process on the surface of the Fe₃Se₄/Ni₃Se₄ and other catalysts.

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