English

• INTRODUCTION

  Molecular hydrogen (H₂) is considered as a green and sustainable feedstock with high energy density for hydrogen economy in the future.^[1-6] Currently, electrochemical water splitting driven by renewable energy is a promising method to generate H₂.^{[7, 8]} Because of the superior activity and stability, platinum (Pt)-based catalysts are the main choice for commercial.^[9-11] However, largescale H₂ production is limited by the scarcity and high price of Pt-group precious metals.^[12-14] To address this issue, efficient and low-cost electrocatalysts are highly imperative to design and develop for hydrogen production.^[15-18]

  Iron is currently one of the most abundant and cheapest transition metals. Thus, considerable efforts have been put into the exploration of Fe-based electrocatalyst, such as sulfides,^{[19, 20]} phosphides,^[21-25] and carbides,^[26-28] for hydrogen production. Among these catalysts, as a promising HER catalyst, the property of pure FeP is unsatisfactory. Interestingly, the fascinating synergistic effects derived from the heterojunction interface of the combination of different types of materials can provide multiple functional sites during hydrogen evolution reaction.^[29] Thus, the heterostructured catalysts are a promising choice to design catalyst with high performance and even new functionality.^[30] For example, Niu et al. have synthesized the hierarchical CoP-FeP branched heterostructures which exhibit remarkable electrocatalytic performance towards HER under acidic conditions and a 30 mV performance improvement over pure FeP.^[31] However, most of the reported heterojunction structures are bimetallic or trimetallic heterojunctions.^[32] The structures of iron-based heterojunctions without the introduction of second-class metals are rarely reported and are full of challenging.

  Herein, we propose an effective method to synthesize Fe₄C/FeP heterostructured catalysts with hollow box structure and carbon coat using Prussian blue (PB) as the precursor, which exhibits excellent hydrogen evolution activity and stability. The in-situ carbonization of organic ligands forms the outside carbon layer as armor, which can prevent them from the destruction of acidic electrolyte. During the hydrogen evolution reaction (HER), Fe₄C/FeP exhibits remarkable catalytic activity superior to pure FeP, with a Tafel slope of 70.7 mV dec^-1 and an overpotential of 180 mV to achieve 10 mA cm^-2. Besides, the target catalyst shows no significant degradation after 5000 cycles of CV testing, indicating excellent durability of the catalyst.

  RESULTS AND DISCUSSION

  As illustrated in Scheme 1, Fe₄C/FeP possesses a cubic framework with cavity and carbon armor, which was synthesized via a two-step method, including the calcination of Prussian Blue (PB) precursor in air and the following phosphating process. In order to reveal the crystalline phase structure, Fe₄C/FeP, Fe₃C and PB were characterized by X-ray diffraction (XRD). As shown in Figure 1a, PB (PDF No. 01-073-0689) is calcined to form Fe₃C (PDF No. 01-077-0255), which is reduced to Fe₄C after phosphating. Besides, the XRD pattern of Fe₄C/FeP suggests the formation of FeP (PDF No. 01-078-1443) and Fe₄C (PDF No. 01-089-4053). Concretely, the diffraction peaks of Fe₄C/FeP approximated at 30.851°, 32.748°, 48.315° and 56.078° belong to (002), (011), (211) and (212) crystallographic planes of FeP. Also, the peaks at 40.247°, 46.815°, 68.363° and 82.416° are well-consistent with (111), (200), (220) and (311) crystal planes of Fe₄C, implying the coexistence of FeP and Fe₄C formed on Fe₄C/FeP.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the preparation of Fe₄C/FeP.

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  Figure 1

  

  Figure 1.  (a) XRD of PB, Fe₃C and Fe₄C/FeP. (b, c) XPS of Fe 2p and P 2p in FeP and Fe₄C/FeP.

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  The X-ray photoelectron spectroscopy (XPS) is employed to investigate the chemical composition and the change in electronic states of Fe₄C/FeP. As depicted in Figure S1, the characteristic peaks of Fe, P, C and O can be observed. As for Fe 2p in Fe₄C/FeP (Figure 1b), the peak has been divided in two couples of Fe 2p_3/2 and Fe 2p_1/2. The binding energies at 707.2 and 720.2 eV are corresponding to the Fe-P species.^{[33, 34]} Besides, the peaks at 710.4 and 724.3 eV belong to Fe-O bonds.^[35] Notably, a negative shift can be observed in the Fe-P of Fe₄C/FeP, indicating that the electron structure of FeP has redistributed. In addition, the P 2p peaks (Figure 1c) from Fe₄C/FeP have two couples of P 2p_3/2 and P 2p_1/2, which indicates the formation of bond between Fe and P. Another peak at around 133.5 eV belongs to phosphate formed by surface oxidation.^{[36, 37]} It also exhibits negative shift in Fe₄C/FeP relative to that of FeP. This phenomenon can be attributed to the improved electron transfer between Fe and P. The peak at 285 eV in C 1s spectrum of Fe₄C/FeP (Figure S2) is attributed to graphitic carbon, C-C or C=C.^{[38, 39]} It is noted that peak intensity of graphitic carbon is strong, which is essential to the improvement of conductivity and stability of Fe₄C/FeP.^[40]

  The morphologic and structural characteristics of Fe₄C/FeP and other contrast samples are investigated by scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). SEM images in Figure 2a and b show that Prussian blue as the precursor has a uniform and evenly distributed nanocube structure. After calcination in air, the surface of Fe₃C (Figure 2c-d) becomes rougher. As shown in Figure 2e-f, the overall morphology of Fe₄C/FeP remains largely unchanged after phosphating, in which the rough and porous surface can provide numerous active sites to enhance its electrocatalytic activity. As a comparison, the surface of FeP obtained by direct phosphating of the precursor Prussian blue (Figure S3) is smoother than that of Fe₃C and Fe₄C/FeP, which may reduce the exposure of active sites.

  Figure 2

  

  Figure 2.  SEM images of (a, b) PB, (c, d) Fe₃C and (e, f) Fe₄C/FeP.

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  The inside hollow structure of Fe₄C/FeP is verified by TEM images. As shown in Figure 3a, PB as a precursor has a solid cubic structure with the size of around 206 nm, which is highly dispersed and uniform in size (Figure S4), while after calcination and phosphating, the Fe₄C/FeP possesses a large cavity inside (Figure 3b). In addition, there is a uniformly dispersed and completely wrapped carbon layer on the outer surface of the catalyst (Figure 3c), and the thickness of the carbon layer is about 6 nm which can be obtained from the insert figure. It is speculated that the carbon layer is obtained by carbonization of cyanide groups uniformly dispersed in the PB structure, which acts as the armor to prevent corrosion and also enhance the conductivity of catalyst. To further explore the detailed structural information of Fe₄C/FeP, HRTEM image is exhibited in Figure 3d, in which the distinct lattice fringes of 0.193 and 0.288 nm can be found corresponding to the (200) crystal plane of Fe₄C and (002) crystal plane of FeP, respectively. Besides, there is a distinct heterojunction interface between Fe₄C and FeP. This result is consistent with the conclusion from XRD and XPS. In addition, there is a distinct heterojunction interface between Fe₄C and FeP, which can induce electron redistribution at the interface. This is beneficial to the surface electron modulation of the heterostructure and enhances the intrinsic catalytic activity.^[32] Figure S5 and S6 are the TEM images of Fe₃C and FeP with partial hollow cavity. It is speculated that cyano group in PB will gradually decompose when the temperature rises,^[41] and the calcination temperature is the key point to control the formation of Fe₄C/FeP box with hollow structure.

  Figure 3

  

  Figure 3.  (a) TEM image of PB. (b, c) TEM images and (d) HRTEM image of Fe₄C/FeP.

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  As illustrated in Figure 4a and b, linear scan voltammetry (LSV) curve of Fe₄C/FeP is measured with a scan rate of 5 mV s^-1 on the reversible hydrogen electrode (RHE) scale. As contrast samples, Pt/C, GCE, PB, FeP and Fe₃C are also characterized for comparison. The tested potentials are converted to reversible hydrogen electrode (RHE), calculated by the Nernst equation. Fe₄C/FeP exhibits excellent electrocatalytic activity, and only needs 180 mV to achieve 10 mA cm^-2, which is much lower than that of other samples. By contrast, FeP needs 340 mV to reach 10 mA cm^-2. This phenomenon may be due to the heterojunction structure formed by Fe₄C and FeP, benefiting from the heterojunction interface with high catalytic activity. Besides, Fe₄C/FeP electrode has lower overpotential than many active HER catalysts under acidic media (Table S1). As illustrated in Figure 4c, Fe₄C/FeP shows a low Tafel slope (70.7 mV dec^-1) which follows the mechanism of Volmer-Heyrovsky, where the adsorbed hydrogen atom prefers to couple with another proton in the electrolyte and a new electron to evolve H₂.^[42] And the Tafel slopes of other contrast samples including Fe₃C (111.8 mV dec^-1), FeP (122.2 mV dec^-1), PB (669.9 mV dec^-1) and Pt/C (37 mV dec^-1) are also demonstrated. In order to further prove the conductivity of Fe₄C/FeP, the analysis of electrochemical impedance spectroscopy is carried out by Nyquist plots in high-frequency semicircle region at the voltage of -0.35 V vs. RHE, as shown in Figure 4d. Fe₄C/FeP (351 Ω) shows the smallest semicircle in the Nyquist plots compared with PB (7.2 GΩ), FeP (1807 Ω) and Fe₃C (9.1 GΩ), suggesting the existence of carbon layer enhanced the conductivity of target catalyst and accelerated the charge transfer. As shown in Figure 4e, the double-layer capacitance (C_dl) of Fe₄C/FeP is estimated to be 0.564 mF cm^-2, which is superior to that of other contrastive samples, suggesting more active sites can be exposed due to the porous structure and heterojunction interfaces between Fe₄C and FeP.

  Figure 4

  

  Figure 4.  HER performance of Fe₄C/FeP and other contrast samples in 0.5 M H₂SO₄. (a) LSV curves, (b) Comparison of overpotentials at 10 mA cm^-2, (c) Tafel plots, (d) Nyquist plots, (e) Double-layer capacitances, (f) LSV curves of Fe₄C/FeP and FeP before and after thousands of CV cycles at a scan rate of 100 mV s^-1.

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  Furthermore, to evaluate the long-term stability of Fe₄C/FeP, 5000 cycles of continuous CV test at a scan rate of 100 mV s^-1 is carried out in acid solution (Figure 4f). Obviously, LSV of Fe₄C/FeP after 5000 cycles CV test shows no significant change. On the contrast, FeP alone shows noticeable decay after only 1000 cycles. The excellent durability of Fe₄C/FeP may be attributed to the optimized electron density of both Fe₄C and FeP, and formed highly active heterojunction interfaces. Besides, carbon layer formed during high temperature calcination plays an important role in both high activity and durability. On one hand, the existence of graphitic carbon coating can enhance the conductivity of target catalyst and accelerates the charge transfer. On the other hand, the armor can also prevent the catalyst from corrosion of electrolyte, which contributed to the excellent stability of Fe₄C/FeP. Meanwhile, chronoamperometry measurement is also carried out (Figure S7). The current density (20 mA cm^-2) of Fe₄C/FeP maintains nearly invariable during 15 h test under a constant voltage of -0.37 V (vs. RHE) for hydrogen evolution reaction.

  Combined with the above experimental conclusions, the superior catalytic activity of Fe₄C/FeP could be attributed to the synergistic effects of porous structure, carbon coating and heterojunction interface function between Fe₄C and FeP (Figure 5). Specifically, the porous structure can promote the contact between the active sites and the electrolyte, which is conducive to the progress of the catalytic reaction. In addition, the formation of carbon layer after carbonization at high temperature can decrease the contact electrical resistance and lead to fast electron transportation, which further improves the hydrogen production rate.^{[43, 44]} Moreover, the heterojunction structure of Fe₄C and FeP can accelerate electron transfer between Fe and P, thus improving their electronic structures to help the catalyst for highly active electrolysis of water for hydrogen production. Finally, carbon layer as armor can enhance the stability of the Fe₄C/FeP in acidic electrolyte.

  Figure 5

  

  Figure 5.  Schematic diagram of the heterojunction interface function between Fe₄C and FeP in Fe₄C/FeP.

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  CONCLUSIONS

  In summary, the hollow Fe₄C/FeP box with heterojunction interface and carbon armor is successfully synthesized via a two-step method, including the calcination in air and the following phosphating. The Fe₄C/FeP which derives from PB (Fe₄[Fe(CN)₆]₃) exhibits excellent hydrogen evolution activity. The enhanced catalytic activity can be ascribed to the porous structure, graphitic carbon armor and heterojunction interface function between Fe₄C and FeP, which optimizes the electronic structure and accelerates electron transfer. The long-term stability can be attributed to the carbon layer as armor to prevent it from corrosion in acidic solution. This work may provide a synthetic approach to produce a series of carbon-coated and heterojunction structure of transition metal phosphides for water electrolysis.

  EXPERIMENTAL

  To prepare PB, 0.01 mol K₄[Fe(CN)₆] was added into 40 mL HCl (0.1 M) within 3 g PVP under stirring. Then the solution was reacted at 80 ℃ for 12 h. After centrifugation and washing, PB was collected.

  Subsequently, 0.1 g PB was calcined under argon atmosphere for 2 h at 700 ℃ with a temperature ramp of 2 ℃ min^-1. The obtained samples were marked as Fe₃C.

  0.01 g Fe₃C and 0.2 g NaH₂PO₂·H₂O were placed in a tube furnace with two separate porcelain boats under argon atmosphere for 2 h at 350 ℃. After the apparatus was cooled to room temperature, products were obtained which are denoted as Fe₄C/FeP. The materials, physical characterization and electrochemical measurements can be found in the supplementary material.

  
  ACKNOWLEDGEMENTS: This work is financially supported by National Natural Science Foundation of China (52174283) and Innovation Fund Project for Graduate Students of China University of Petroleum (East China) (No. CXJJ-2022-23). COMPETING INTERESTS
  The authors declare no competing interests.
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0102
  For submission: https://mc03.manuscriptcentral.com/cjsc
  ADDITIONAL INFORMATION
  
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• 

  Scheme 1  Schematic illustration of the preparation of Fe₄C/FeP.

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  Figure 1  (a) XRD of PB, Fe₃C and Fe₄C/FeP. (b, c) XPS of Fe 2p and P 2p in FeP and Fe₄C/FeP.

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  Figure 2  SEM images of (a, b) PB, (c, d) Fe₃C and (e, f) Fe₄C/FeP.

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  Figure 3  (a) TEM image of PB. (b, c) TEM images and (d) HRTEM image of Fe₄C/FeP.

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  Figure 4  HER performance of Fe₄C/FeP and other contrast samples in 0.5 M H₂SO₄. (a) LSV curves, (b) Comparison of overpotentials at 10 mA cm^-2, (c) Tafel plots, (d) Nyquist plots, (e) Double-layer capacitances, (f) LSV curves of Fe₄C/FeP and FeP before and after thousands of CV cycles at a scan rate of 100 mV s^-1.

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  Figure 5  Schematic diagram of the heterojunction interface function between Fe₄C and FeP in Fe₄C/FeP.

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