A bionic carbon framework activates the oxygen lattice sites of NiFe₂O₄ to enhance electrochemical water splitting

Abstract

A well-designed carbon-supported material enables the promotion of efficient oxygen evolution. Herein, a NiFe₂O₄ catalyst supported by a bionic carbon framework (BCF) was synthesized, and the elements doped into the BCF facilitate electron transfer between oxygen and active metal centers. The as-prepared catalyst exhibits a low overpotential and Tafel slope, and the design approach presented here represents a promising method to design electrocatalysts for water splitting.

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With the development of industry and the requirement for green energy, electrocatalytic water splitting for hydrogen production and carbon emissions reduction has become the best choice.^1–3 However, the whole electrocatalytic system is restricted by the oxygen evolution reaction (OER), which is controlled by a four-electron transfer process.^4–7 In recent years, NiFe₂O₄ has emerged as one of the most promising non-precious metal-based electrocatalysts for the OER. Despite the potential of NiFe₂O₄, it still has poor electrical conductivity and a limited number of active sites, and thus a lot of strategies, such as morphology control, modulation of the material composition and defect engineering, have been utilized to solve these issues with the aim of achieving a low cost and highly active electrocatalyst for water splitting.^8–10

A favorable carrier not only disperses the catalyst better but also modulates the electronic structure and enhances the electrocatalytic performance. Carbon supports are frequently employed as conductive frameworks for catalysts. However, existing studies typically focus on doping with a single element, which at the same time has a limited effect on altering the reaction mechanism by which NiFe₂O₄ operates.^11–14 This constraint significantly hinders the efficiency of most carbon-composite NiFe₂O₄ catalysts in the oxygen evolution reaction (OER). Additionally, the utilization of these carbon materials tends to be relatively insufficient.^15–19 Constructing a bionic structure from diatomite is an effective method to address the issue above.^20–22

There are two potential mechanisms for NiFe₂O₄ in the OER in alkaline electrolytes, and the lattice oxygen mechanism (LOM) is considered to be the most promising pathway.^23–26 Compounding and doping are great solutions for improving the catalytic performance of NiFe₂O₄ by forcing the material to operate mainly under LOM control rather than a switchable OER route.^27,28 However, these methods often require rare-earth or precious metals and involve complex synthesis methods, which limits their scalability for large-scale applications. Moreover, the ability to control the shift in catalytic mechanisms by doping the complex rather than NiFe₂O₄ itself remains an area that is yet to be fully explored.

Herein, diatomite (De) was used as a bionic hard template to construct a sulfur and nitrogen co-doped bionic carbon framework (SNC) on which the NiFe₂O₄ catalyst was loaded. Characterized by high conductivity and more active sites, in alkaline media, the NiFe₂O₄/SNC electrode delivered a small overpotential of 242 mV at 10 mA cm⁻² and exhibited a lower Tafel slope of 91.42 mV dec⁻¹ (RuO₂ : 140.2 mV dec⁻¹). Additionally, its excellent OER performance was confirmed through chemical probe studies and pH-dependent analysis, which revealed the activation of the LOM in NiFe₂O₄. The electrode also demonstrated outstanding durability, maintaining more than 98% of its initial performance at 10 mA cm⁻² over 48 h of continuous electrocatalysis, significantly outperforming its NiFe₂O₄ counterpart.

The NiFe₂O₄/SNC material was synthesized in four steps (details provided in the ESI†), with the corresponding schematic of the process illustrated in Fig. 1a. As shown in Fig. S1 (ESI†), the sulfur and nitrogen-doped bionic carbon framework was synthesized via the polymerization of metanilic acid (MA) and p-toluenesulfonic acid (PA) on the surface of the diatomite. Subsequently, NiFe layered double hydroxides (LDH) were synthesized in situ on the polymer framework, and the composites were reduced to NiFe₂O₄/SNC@De by heating at 900 °C in nitrogen (Fig. S2, ESI†). Finally, the NiFe₂O₄/SNC was obtained after removing the diatomite hard templates using KOH (Fig. S3, ESI†). The multi-hollow, cake-like morphology of the diatomite template is clearly visible in scanning electron microscopy (SEM) image shown in Fig. 1b, confirming the successful synthesis of the bionic structure through this method. The sheet-like NiFe₂O₄ structure is uniformly distributed and retains the pores from the diatomite template, as observed in the higher magnification image in Fig. 1c. The pore size distribution and specific surface area (SSA) of NiFe₂O₄/SNC were measured using the Brunauer–Emmett–Teller (BET) method (Fig. S4, ESI†), which indicated that the pores on the surface are predominantly mesoporous (average pore size = 6.18 nm, SSA = 19.97 m² g⁻¹). The transmission electron microscopy (TEM) images in Fig. 1d reveal the sulfur and nitrogen-doped carbon structure beneath the NiFe₂O₄ nanosheets. The high-resolution TEM (HRTEM) image in Fig. 1e shows clear lattice fringes corresponding to the (220) crystal planes of NiFe₂O₄/SNC, demonstrating the high crystallinity of the material. In contrast, the disordered carbon structure of SNC does not exhibit lattice fringes due to its low crystallinity. The uniformity of the NiFe₂O₄/SNC synthesis and the successful doping of sulfur and nitrogen into the carbon structure were further confirmed by the Ni, Fe, O, S, and N elemental mapping results shown in Fig. 1f.

Fig. 1 (a) Synthesis of NiFe₂O₄/SNC; (b) and (c) SEM images of the as-prepared NiFe₂O₄/SNC; (d) TEM image of NiFe₂O₄/SNC and (e) HRTEM image and (f) elemental mapping of Ni, Fe, O, S and N from NiFe₂O₄/SNC.

The X-ray diffraction (XRD) pattern of NiFe₂O₄/SNC exhibited a set of peaks corresponding to NiFe₂O₄ (JCPDF no. 74-2081), as shown in Fig. 2a. The specific peaks associated with diatomite (JCPDF no. 76-0935) were absent, indicating the complete etching of the diatomite structures from NiFe₂O₄/SNC, as well as in the other samples. The Fourier-transform infrared (FTIR) spectrum of NiFe₂O₄/SNC is presented in Fig. S5 (ESI†). Characteristic peaks for the O–H band, Fe–O stretching, and Ni–O stretching of NiFe₂O₄ were observed at approximately 3436.5 cm⁻¹, 600.9 cm⁻¹, and 436.8 cm⁻¹, respectively.²⁹ The peaks associated with SNC appeared at 1653.3 cm⁻¹, 1408.4 cm⁻¹, and 1006.6 cm⁻¹, which were attributed to C–N, N–H, and C=S stretching vibrations, respectively.³⁰

Fig. 2 (a) XRD pattern of NiFe₂O₄/SNC; (b) N 2p XPS spectra and (c) S 2p XPS spectra of SNC, NC and C; (d) Ni 2p XPS spectra, (e) Fe 2p XPS spectra and (f) O 1s XPS spectra of NiFe₂O₄/SNC, NiFe₂O₄/NC and NiFe₂O₄/C.

To confirm the success of the N and S doping into the carbon framework and to assess the electronic changes induced by doping, X-ray photoelectron spectroscopy (XPS) measurements were conducted on C@De, NC@De, and SNC@De for N and S, and on NiFe₂O₄/C, NiFe₂O₄/NC, and NiFe₂O₄/SNC for O, Fe, and Ni. The corresponding XPS spectra of N 1s (Fig. 2b) displayed two distinct peaks, confirming the successful doping of nitrogen into the lattice of NC@De and SNC@De. Moreover, the blue peak exhibited a positive shift (≈1.22 eV) from NC to SNC, demonstrating that the addition of sulfur enhances electron transfer between C and N. The XPS spectra of S 2p (Fig. 2c) confirm the successful doping of sulfur into the SNC@De lattice. A peak at 163.98 eV corresponds to S 2p_3/2, indicating a strong interaction between sulfur and the carbon framework. In the XPS spectrum of O 1s (Fig. 2d), peaks at 530.04 eV and 530.75 eV correspond to oxygen–metal bonds and hydroxyl groups, respectively, consistent with previous reports for NiFe₂O₄.³¹ The XPS spectra of Fe 2p for NiFe₂O₄/SNC (Fig. 2e) displayed two main peaks at 711.44 eV and 724.80 eV, corresponding to Fe²⁺ 2p_3/2 and Fe²⁺ 2p_1/2, respectively, and two peaks at 713.08 eV and 726.34 eV, corresponding to Fe³⁺ 2p_3/2 and Fe³⁺ 2p_1/2. Compared to NiFe₂O₄/NC and NiFe₂O₄/C, the peak positions in the O 1s spectrum were shifted positively, while those in the Fe 2p spectrum were shifted negatively, indicating enhanced electron transfer from O to Fe after doping. Similarly, doping increased the valence of the Ni species (Fig. 2f), suggesting enhanced electron transfer from O to Ni. Moreover, comparing the N-doped (≈0.26 eV for Fe, ≈0.92 eV for Ni) and S, N co-doped (≈0.40 eV for Fe, ≈1.29 eV for Ni) samples with the undoped sample, it was found that co-doping more effectively enhanced electron transfer, improving the binding energy. These results demonstrate that co-doping significantly promotes electron transfer between lattice oxygen and the catalytically active centers, thereby enhancing oxygen evolution performance.

Using a typical three-electrode system, the OER activity of NiFe₂O₄/SNC was evaluated in 1 M KOH solution without IR compensation, using a platinum mesh as a counter electrode and an Hg/HgO electrode as the reference electrode. For comparison, NiFe₂O₄/NC, NiFe₂O₄/C, RuO₂ and Ni foam were also tested as comparison working electrodes. The linear sweep voltammetry (LSV) curves in Fig. 3a show that NiFe₂O₄/SNC can drive the OER with a lower overpotential (η₁₀) of 242 mV at 10 mA cm⁻² than NiFe₂O₄/NC (337 mV), NiFe₂O₄/C (347 mV) and RuO₂ (502 mV). In addition, the NiFe₂O₄/SNC electrode has a lower Tafel slope which indicates faster OER kinetics, as shown in Fig. 3b. The Tafel slope of 91.42 mV dec⁻¹ is lower than that of 132.5 mV dec⁻¹ for NiFe₂O₄/NC, 124.6 mV dec⁻¹ for NiFe₂O₄/C and 140.2 mV dec⁻¹ for RuO₂. The results reveal that S doping of the carbon can increase the catalytic activity of the electrode and enhance the reconstruction process of the pre-catalyst to the real catalyst.^10,27 Furthermore, the capabilities of NiFe₂O₄/NC are superior to those of NiFe₂O₄/C, indicating that the presence of N leads to some positive effects on the catalytic properties. Electrochemical impedance spectroscopy (EIS) plots were obtained for a better understanding of the charge transfer efficiency of the electrodes. Fig. S6 (ESI†) shows the EIS plots and the equivalent circuit diagram of the NiFe₂O₄/SNC, NiFe₂O₄/NC and NiFe₂O₄/C electrodes, which confirms that the N,S-doped carbon layer reduces the charge transfer resistance and effectively promotes electron transfer, leading to enhanced OER activity.

Fig. 3 (a) LSV curves of the as-prepared catalysts in KOH electrolyte (1.0 mol L⁻¹) without internal resistance (IR) compensation, scan rate: 5 mV s⁻¹; (b) corresponding Tafel plots at low potentials; (c) current density (ΔJ = J_a − J_c) versus scanning rate and the corresponding linear slopes for NiFe₂O₄/SNC, NiFe₂O₄/NC, NiFe₂O₄/C and RuO₂; (d) comparison of the performance of NiFe₂O₄/SNC in alkaline electrolyte with other reported OER catalysts.

To further investigate the factors affecting the OER activity, the double layer capacitance (C_dl) was estimated by cyclic voltammetry tests at different sweep rates in the non-Faraday region. As the conversion between the electrochemical active surface area (ECSA) and C_dl is positively correlated, a higher C_dl can justify a larger ECSA. As shown in Fig. 3c and Fig. S7 (ESI†), the C_dl values are detected to be 2.22 mF cm⁻², 1.93 mF cm⁻², 1.90 mF cm⁻² and 1.16 mF cm⁻² for NiFe₂O₄/SNC, NiFe₂O₄/NC, NiFe₂O₄/C and RuO₂, respectively. A larger ECSA means more active sites. Compared with NiFe₂O₄/NC and NiFe₂O₄/C, the N and S co-doping of the carbon layers also effects the ECSA, giving NiFe₂O₄/SNC the highest value. In comparison with other OER catalysts from the last two years, NiFe₂O₄/SNC exhibits extraordinary performance, as shown in Fig. 3d and Table S1 (ESI†). Long-term stability is a key parameter that we used to evaluate the stability of NiFe₂O₄/SNC by continuously running the OER operation for 48 h at a current density of 10 mA cm⁻². The small voltage decay (≈1.8%) that we observed indicated that NiFe₂O₄/SNC has excellent stability for the OER (Fig. S8, ESI†).

To gain deeper insights into the influence that the S and N doping in the carbon layer has on OER activity, the OER mechanisms of the samples were investigated by using a chemical probe to identify the oxidized oxygen intermediate. The LOM route involved peroxo-like (O₂²⁻) and superoxol-like (O²⁻) negative species, and the O₂²⁻ species can serve as an indicator of whether the OER proceeds via the LOM route.²⁸ Owing to its strong interaction with the O₂²⁻ species, the tetramethylammonium cation (TMA⁺) was added to the electrolyte to probe the presence of O₂²⁻ species.^23,24 As shown in Fig. 4a, The OER activity of NiFe₂O₄/SNC decreased remarkably when the electrolyte was changed to TMAOH. In contrast, the OER activity of NiFe₂O₄/NC and NiFe₂O₄/C only decreased slightly (Fig. 4b), reflecting that the OER process on the NiFe₂O₄/SNC electrode proceeded via the LOM route. Moreover, the pH dependence of the OER activity for NiFe₂O₄/SNC, NiFe₂O₄/NC and NiFe₂O₄/C is shown in Fig. 4c and Fig. S9 (ESI†). The OER activity of NiFe₂O₄/SNC was highly dependent on the pH of the electrolyte, while the pH dependence was considerably weaker for NiFe₂O₄/NC and NiFe₂O₄/C. Furthermore, the current densities of NiFe₂O₄/SNC, NiFe₂O₄/NC and NiFe₂O₄/C at 1.6 V as a function of the pH were plotted, as shown in Fig. 4d. These results confirmed that the OER activity of NiFe₂O₄/SNC had a much higher pH dependence than that of NiFe₂O₄/NC and NiFe₂O₄/C. The higher pH dependence of NiFe₂O₄/SNC implied that S doping may trigger the LOM pathway with a non-concerted proton–electron transfer process.^26,31 In other words, S,N doping of the carbon layer trigged the transition of the OER mechanism of NiFe₂O₄ from the AEM to the LOM.²³

Fig. 4 (a) and (b) Polarization curves for NiFe₂O₄/SNC, NiFe₂O₄/NC and NiFe₂O₄/C in 1.0 M KOH or 1.0 M TMAOH; (c) LSV tests for NiFe₂O₄/SNC in different concentration KOH solution; (d) current densities of NiFe₂O₄/SNC, NiFe₂O₄/NC and NiFe₂O₄/C at 1.6 V vs. RHE as a function of pH value.

In summary, a series of highly efficient hybrid bionic NiFe₂O₄-based electrodes were developed using a multi-step sacrificial template method. NiFe₂O₄/SNC exhibits exceptionally high catalytic activity and remarkable durability. The outstanding OER performance of NiFe₂O₄/SNC can be attributed to the following factors: (i) the catalysts, synthesized via the described method, have the natural three-dimensional porous structure of diatoms, which provides a greater number of active sites; (ii) the carbon layer with defects (doping), formed through the polymerization of organic compounds followed by high-temperature calcination, enhances the electrical conductivity of the electrodes; and (iii) this is the first instance where sulfur and nitrogen co-doping in the carbon layer has effectively shifted the OER mechanism of NiFe₂O₄ from the AEM to the LOM pathway, significantly improving the catalytic performance of the catalyst. The well-designed NiFe₂O₄/SNC material provides novel insights into the transition of the OER mechanism in NiFe₂O₄-based catalysts induced by elemental doping in the supporting complexes rather than by directly modifying the catalysts themselves.

This work was supported by the financial support of the National Natural Science Foundation of China (grant no. 22405052 and 52378217), and the China Postdoctoral Science Foundation (grant no. 2024M750491). The project was supported (No. 2024 cdjqyjcyj−001) by the fundamental research funds for the central universities, and the Taishan Industry Leading Talents Project that provided valuable data and analyzed the prospect of water splitting electrocatalysts.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc01015b

‡ These authors contribute equally to this work.

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