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Article

Biomass-Derived Catalysts with Dual Functions for Electrochemical Water Splitting

by
Wangchuang Zhu
,
Xinghua Zhang
,
Qi Zhang
,
Lungang Chen
,
Xiuzheng Zhuang
* and
Longlong Ma
*
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(14), 3592; https://doi.org/10.3390/en18143592
Submission received: 2 June 2025 / Revised: 3 July 2025 / Accepted: 6 July 2025 / Published: 8 July 2025
(This article belongs to the Section A4: Bio-Energy)
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Versions Notes

Abstract

With the continuous consumption of fossil energy and the related environmental problems, clean energy, especially the hydrogen energy-derived water electrolysis, has attracted wide attention. However, as a result of the high energy consumption of water electrolysis and the limitations of single-function catalysts, there is an urgent need for cheap and simple-to-make bifunctional catalysts. In this work, based on the NiFe-LDH that is usually used for OER (Oxygen Evolution Reaction), doping of heteroatoms was carried out and a bifunctional catalyst could be then prepared using biomass as the carbon source. The preparation of catalyst precursors and in situ reduction were performed through the coupling process of hydrothermal and pyrolysis to enhance the electrolytic activity of the catalyst. Results showed that the overpotentials required to reach a current density of 10 mA·cm−2 for the HER and OER processes were 305.2 mV and 310.4 mV, respectively, which are superior to the commercial catalysts. In the subsequent characterization, the structural characteristics of the catalyst support and their structure–activity correlation with active metals were systematically investigated by TEM, XRD, and XPS analysis, providing mechanistic insights into the catalytic behavior. The basic catalytic mechanisms of HER and OER were also obtained: the HER process was due to the formation of a Ni3Fe alloy structure during catalyst preparation, which changed the electronic structure of the catalyst, while the OER process was induced by the formation of a NiOOH intermediate. The research results are expected to provide new ideas and data support for the preparation of bifunctional catalysts.

1. Introduction

Given the ongoing consumption of fossil energy and the escalating severity of environmental issues, people have gradually turned their attention to the development of sustainable energy [1,2]. Among sustainable energy sources, hydrogen energy, with its advantages such as high energy density, environmental friendliness, and zero carbon emissions, has become one of the focuses of extensive attention [1,3,4]. Hydrogen production through electrolysis of water is more environmentally friendly than technologies such as methane reforming, and it only requires the use of sustainable energy sources (such as wind and solar energy), thus attracting widespread attention [5,6]. The development of cost-effective, high-efficiency catalysts represents a critical pathway toward reducing the energy requirements of electrolysis processes, a fundamental prerequisite for their large-scale industrial implementation. While conventional industrial-scale electrolysis has predominantly relied on precious metal-based catalysts, growing concerns regarding their scarcity and prohibitive costs have prompted a significant research focus on transition metal alternatives.
Considerable progress has been achieved in the design and optimization of transition metal-based electrocatalysts, with several systems now demonstrating performance metrics approaching those of their noble metal counterparts. Zhang et al. [7] prepared self-supported NiCoPx nanoarrays on felt using a wet chemical–hydrothermal method combined with an in situ phosphidation reaction. This catalyst has a wide pH range of applicability and exhibits excellent HER catalytic performance in alkaline, neutral, and acidic media, and can maintain high stability. Guo et al. [8] reconstructed the surface structure of the catalyst via a unique electrochemical cathodic reduction process and finally obtained an OV@Fe-NiTe catalyst rich in oxygen vacancies. This catalyst only requires an overpotential of 245 mV to reach a current density of 100 mA·cm−2 in 1 M KOH solution, showing excellent OER catalytic performance. Liu et al. [9] prepared a new type of NiFe-LDH@CS using a two-step hydrothermal method. This catalyst uses carbon as a carrier, which not only does not destroy the crystal structure of NiFe-LDH, but also improves its OER performance. The overpotential required to reach a current density of 100 mA·cm−2 in 1 M KOH solution is 372 mV.
Although the utilization cost of transition metal catalysts is lower than that of noble metals, the conductivity of transition metals is relatively poor [2,10]. Therefore, in addition to the requirements of high surface area, high stability, unique structural features, and easy accessibility for the carrier, the carrier also needs to compensate for the poor conductivity of transition metals. Carbon materials can meet the requirements of catalysts for carriers well. Currently, the main carbon materials used in catalyst preparation include carbon nanotubes [11,12,13], graphene [14,15,16], and spherical carbon [17,18,19]. Most of the carbon materials used have unique nanostructures, which can provide a large surface area, have high structural and performance stability, and have suitable active sites, which can load transition metal catalysts well. Among them, biomass-derived carbon materials are the most promising in terms of likelihood to become the carriers of carbon-based catalysts due to their abundant and easily accessible raw materials, rich surface-active functional groups, and unique structural features.
Wang et al. [20] prepared Fe/FeP hydrogen evolution reaction (HER) catalysts using Ginkgo biloba leaves via phosphating pyrolysis. This catalyst only requires an overpotential of 92 mV to achieve a current density of 10 mA·cm−2. Liu et al. [21] used Chinese fir wood to prepare Fe, N-induced NiCo spinel catalysts by reconstructing the metal structure on the catalyst surface. This catalyst has a high OER catalytic effect, with an overpotential of 280 mV at a current density of 10 mA·cm−2. These studies demonstrate the application prospects of biomass-derived carbon materials in catalyst preparation. However, these catalyst studies mainly focus on single catalytic performance and do not consider the matching of dual electrodes [22]. Different catalyst electrodes will bring additional production and maintenance costs, and the optimal pH range for different catalysts is also difficult to match. Therefore, research on bifunctional catalysts is particularly important [23]. Hoang Van Chinh et al. [24] prepared Ni/NiO and N-doped activated carbon catalysts using cauliflower leaves. The results showed that this catalyst had a good water electrolysis effect, with initial overpotentials of 180 mV and 346 mV for HER and OER reactions in 0.1 M KOH solution, respectively, and a current density of 10 mA·cm−2 was achieved at a full water-splitting voltage of 1.688 VRHE. Liu et al. [25] synthesized Pd/NiFe-LDH catalysts using a hydrothermal method and ultrasonic assistance. The results showed that NiFe-LDH itself has excellent OER catalytic performance, and the introduction of Pd also provides active sites for the HER reaction, enhancing the bifunctional catalytic performance of the catalyst.
These studies have all focused on mechanism research regarding metal oxides and the coexistence of metals, but it is difficult to control the content of different valence states of elements in the preparation of catalysts. Therefore, this work will be based on the preparation of NiFe-LDH catalysts and prepare NiFe alloy bifunctional catalysts through the hydrothermal pyrolysis coupling method, and explore the mechanism of the catalytic process through a series of characterization tests.

2. Experimental Section

2.1. Materials

C6H12O6 (AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Ni(NO3)2·6H2O (99.9%), Fe(NO3)3·9H2O (99.9%), Na2B4O7 (99%), and CO(NH2)2 (99.5%) were purchased from Aladdin (Bay City, MI, USA). NaH2PO4 (99%) and CH4N2S (99%) were purchased from Thermo Scientific (Waltham, MA, USA). KOH (99.9%) was purchased from Ron Reagent (Irvine, CA, USA). Nafion (5% wt) reagent was purchased from Dupont (Wilmington, DE, USA). The water used in the experiment was purified by a water purification system (with a pure water resistance of 18.2 MΩ·cm−1). None of the reagents were further purified and all were used directly.

2.2. Preparation of NiFe-Based Carbonaceous Precursor

Briefly, 3 g of glucose, and amounts of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O (molar ratio = 5:1) were dissolved in a mixture of 15 mL deionized water and 15 mL ethylene glycol. Subsequently, 0.07 mol of Na2B4O7 (or 0.07 mol of CO(NH2)2, 0.07 mol of Na2HPO4, and 0.07 mol of CH4N2S) were added, followed by thorough stirring until complete dissolution had occurred. The resultant solution was transferred into a Teflon-lined stainless-steel autoclave and maintained at 180 °C for 16 h in an oven. Afterward, the sample was retrieved and subjected to suction filtration. The solid product was first rinsed with deionized water, followed by washing with anhydrous ethanol to remove organic residues on the surface. Finally, the material was re-rinsed with deionized water, then dried under vacuum at 60 °C for 24 h, yielding carbon-based materials denoted as NiFe@BCS, NiFe@NCS, NiFe@PCS, and NiFe@SCS. These catalysts were placed in a boat and put into a tube furnace. Under the condition of N2 flow, the temperature was raised to 600 °C at a rate of 5 °C/min and maintained for 2 h, then cooled to room temperature. The samples obtained were sealed with nitrogen and stored in a drying oven at 50 °C for future use. The catalysts were respectively named NiFe@BCS-600, NiFe@NCS-600, NiFe@PCS-600, and NiFe@SCS-600. In addition, catalysts with different doping sequences were prepared, and the main difference lies in the doping of heteroatoms by impregnation after hydrothermal treatment. The catalysts were respectively named NiFe@TBCS-600, NiFe@TNCS-600, NiFe@TPCS-600, and NiFe@TSCS-600.

2.3. Heat Treatment of the Precursor Under Different Pyrolysis Conditions

The precursor catalyst, after hydrothermal treatment, was placed in a boat and put into a tube furnace. It was heated at a rate of 5 °C/min to 500, 600, 700, and 800 °C, respectively, held at each temperature for 2 h, and then cooled to room temperature. The precursor, after hydrothermal treatment, was placed in a boat and put into a tube furnace. It was heated at rates of 2.5 °C/min and 10 °C/min, respectively, to 700 °C, held at this temperature for 2 h, then cooled to room temperature, and the samples were sealed with nitrogen and stored in a drying oven at 50 °C for future use. The precursor catalyst, after hydrothermal treatment, was placed in a boat and put into a tube furnace. It was heated at a rate of 5 °C/min to 700 °C, held at this temperature for 4 h, then cooled to room temperature, and the sample was sealed with nitrogen and stored in a drying oven at 50 °C for future use.

2.4. Electrochemical Testing

Catalyst electrode preparation: The catalyst ink was prepared by ultrasonically dispersing a precisely weighed amount of the catalyst sample in a 3:1 (v/v) deionized water/ethanol solvent mixture containing 5 wt% Nafion binder for 60 min at 25 ± 2 °C, followed by drop-casting 5 μL of the homogeneous suspension onto a pre-polished glassy carbon electrode (3 mm diameter) and drying at 60 °C for 30 min to obtain the working electrode.
Electrochemical testing: The experiments were conducted using a CHI760E electrochemical workstation (Shanghai Chenhua Co., Ltd. (Shanghai, China). All electrochemical measurements were performed in 1 M potassium hydroxide (KOH) aqueous solution. First, the electrolytic cell (three-electrode system) was assembled. CV (Cyclic Voltammetry) activation was performed on the catalyst. For OER testing, the CV voltage range was 0.2 to 0.9 VHg/HgO, and, for HER testing, the CV voltage range was −0.9 to −1.5 VHg/HgO. The scan rate was 10 mV/s, and the cycle number was 10 times. After activation, LSV (linear sweep voltammetry) performance testing was performed on the catalyst. The OER test voltage range was 0.2 to 0.9 VHg/HgO, and the HER test voltage range was −0.9 to −1.5 VHg/HgO. The scan rate was 10 mV/s. After completion, OCP (Open Circuit Potential) testing of the open-circuit voltage was performed, with a test duration of 400 s. Then, CV curve testing was performed within ±0.05 V of the open-circuit voltage as the standard, with 10 cycles at scan rates of 5 mV/s, 10 mV/s, 20 mV/s, 40 mV/s, and 80 mV/s. After testing, Electrochemical Impedance Spectroscopy (EIS) measurements were performed with an AC amplitude of 10 mV over a frequency range of 106 Hz to 0.1 Hz.

2.5. Characterization and Analytical Methods

Conventionally, the performance of a catalyst is mainly reflected in two aspects: the structure of the catalyst carrier and the metal nanoparticles loaded on the catalyst. To obtain more comprehensive information about the catalyst, the following two major characterization methods were used to further understand the catalyst.
Firstly, in terms of the morphology of the catalyst carrier, the surface morphology and structure of the catalyst were studied by scanning electron microscopy (scanning electron microscope, SEM; S-4800, HITACHI, Tokyo, Japan) and transmission electron microscopy (Transmission Electron Microscope, TEM; F200, JEOL, Tokyo, Japan). Meanwhile, the distribution of metal atoms was measured by energy dispersive X-ray spectroscopy (EDS; Thermo Scientific, Waltham, MA, USA). Additionally, the specific surface area of the catalyst was analyzed by the American-Micromeritics-ASAP2460 gas adsorption analyzer using the Brunner−Emmet−Teller (BET) method. The adsorption/desorption processes were carried out for 8 h at a temperature of −186 °C with a relative pressure (P/P0) ranging from 0.01 to 0.99 using N2. Finally, the functional groups on the surface of the catalyst were detected by Fourier transform infrared spectroscopy (Fourier transform infrared spectroscopy, FTIR; Bruker Vertex 70 spectrophotometer, Karlsruhe, Germany).
Secondly, in terms of the metal nanoparticles loaded on the catalyst, the surface chemical properties of the catalyst were studied by X-ray photoelectron spectroscopy (X-ray Photoelectron Spectroscopy, XPS), particularly the valence states of metallic elements and surface species on the support material, within a depth range of 0.1–1 nm. The XPS spectra were obtained on the American-Thermo Scientific-NEXSA spectrometer, which was equipped with an Al(Ka) X-ray radiation source (hν = 1486.6 eV), with an energy of 20 eV, an energy step of 0.1 eV, and a dwell time of 0.1 s. Additionally, X-ray diffraction (X-ray Diffraction, XRD; PANalytical, X’Pert PRO, Almelo, The Netherlands) was performed on the catalyst. The test was conducted using Cu Ka radiation (λ = 0.15406 nm), and the scanning conditions were adjusted from 5° to 80° at a step size of 0.0167 in the 2θ range. Finally, Raman spectroscopy (Raman spectra; LabRAM HR800-LS55, HORIBA Scientific, Lyon, France) was used to analyze the carbon components of the catalyst. The light source was provided by a Nd-YAG laser at 532 nm, and the scanning range selected was between 1800 cm−1 and 1000 cm−1.

3. Results and Discussion

3.1. Selection of Bifunctional Catalysts and Determination of Pyrolysis Conditions

3.1.1. HER Performance of Bifunctional Catalysts

First, the types and methods of element doping were screened. Figure S1a,b shows the HER performance curves of the catalysts doped with B, N, P, and S elements by the one-pot method and two-step (hydrothermal + impregnation) method, respectively. It can be seen from the figure that the HER catalytic performance of the catalysts doped with B and S elements (NiFe@BCS-600, NiFe@TBCS-600, NiFe@SCS-600, and NiFe@TSCS-600) by both the one-pot method and two-step method was not ideal. However, the catalytic performance of the NiFe@NCS-600 catalyst by the one-pot method was excellent, with an overpotential of 301.04 mV at a current density of 10 mA·cm−2 (Table S1). However, the performance of NiFe@TNCS-600 was very poor. The possible reason is that the two-step doping method failed to form a suitable composite structure between the N element and Ni, Fe metal ions, resulting in a poorer catalytic performance than NiFe@NCS-600. Therefore, the process conditions of the NiFe@NCS-600 catalyst were optimized.
Figure 1 shows the electrochemical performance at different pyrolysis temperatures. It can clearly be seen from Figure 1a that the HER performance of the catalyst increases with the increase of pyrolysis temperature. The higher the pyrolysis temperature, the more obvious the change in overpotential at a larger current density. Among the samples, the NiFe@NCS-800 catalyst had the best HER catalytic effect, with an overpotential of 284.3 mV at a current density of 10 mA·cm−2 and 406.2 mV at a current density of 100 mA·cm−2. The possible reasons are that, as the temperature increases, the carbonization degree of the carbon material carrier of the catalyst increases, which improves the overall conductivity of the catalyst and reduces the resistance of electron transfer; the increase in temperature causes more changes in the valence state of metals, and the NiFe alloy structure formed between different metals becomes more stable, making it easier for hydrogen ions (H*) to adsorb and desorb; the increase in temperature causes the organic matter in the catalyst after hydrothermal treatment to volatilize, thereby increasing the reaction contact area of the catalyst [26]. Figure 1b shows the Electrical Double-Layer Capacitor (Cdl) at different temperatures. It can be seen that, as the temperature increased, the overall double-layer capacitance continuously decreased. Theoretically, a larger Cdl indicates that the catalyst has a larger specific surface area and may provide more active sites. As the temperature increased, the decrease in Cdl indicated a reduction in the active sites of the catalyst. However, the catalytic performance of the catalyst continued to improve, and the catalytic efficiency of the active sites of the catalyst might have been enhanced due to other factors [27]. Possible factors include the following: the increase in temperature continuously improves the carbonization of the catalyst, thereby enhancing the conductivity of the catalyst and increasing the electron transfer rate and catalytic efficiency; the formation of an alloy phase (Ni3Fe) between metals provides more active sites and reduces the activation energy of the reaction, enhancing the catalytic efficiency. Therefore, the overall catalytic efficiency continued to increase.
Subsequently, Figure 1c shows the HER electrochemical impedance spectra. As can be observed, as the pyrolysis temperature increased, the interfacial charge transfer resistance (Rct) in the impedance spectra gradually decreased (Table S2), and at high temperatures, the interfacial charge transfer resistance (Rct) gradually stabilized. A smaller interfacial transfer resistance indicated that the charge transfer process was easier and had better reaction kinetics [28]. Similarly, in Figure 1d, the Tafel slope of the catalyst decreased with the increase in temperature and remained stable at higher pyrolysis temperatures. This indicates that high temperatures can effectively improve the reaction kinetics of the catalyst and enhance its catalytic performance. Additionally, different Tafel slopes correspond to different rate-limiting steps. For the HER catalytic process, when the Tafel slope is 120 mV·dec−1, the corresponding rate-limiting step is the Volmer step [29], that is, the proton adsorption process becomes the rate-limiting step of the HER process.

3.1.2. OER Performance of Bifunctional Catalysts

In addition, OER performance tests were also conducted for the catalysts doped with B, N, P, and S. The OER performance curves of different heteroatom-doped catalysts prepared by the one-pot and two-step methods are shown in Figure S2a,b, respectively. It can be seen that, although the OER catalytic performance of NiFe@PCS-600 and NiFe@TPCS-600 was excellent (Table S1), with overpotentials of 260.3 mV and 284.7 mV at a current density of 10 mA·cm−2, respectively, their HER performance was relatively weak, with overpotentials of 536.8 mV and 489.1 mV at 10 mA·cm−2. However, the HER performance of NiFe@NCS-600 was excellent (Tables S3 and S4), and its OER performance was also superior to that of commercial catalysts. Therefore, NiFe@NCS-600 was ultimately selected as the initial bifunctional catalyst for the pyrolysis condition tests.
Figure 2 shows the electrochemical performance of NiFe@NCS catalysts obtained at different temperatures. Figure 2a presents the LSV polarization curves of the catalysts. It can be observed that the initial overpotential of the catalysts decreased with increasing temperature, with values of 397.48 mV (at 500 °C) > 318.6 mV (at 600 °C) > 310.44 mV (at 700 °C) > 299.8 mV (at 800 °C). Moreover, the OER catalytic performance of the catalysts at high current densities was significantly improved with increasing temperature and gradually stabilized at 700 °C and 800 °C. The overpotentials at a current density of 50 mA·cm−2 were 387.8 mV and 371.5 mV, respectively, and at 100 mA·cm−2 were 462.7 mV and 443.4 mV, respectively. The possible reasons are as follows: (1) elevated temperature promotes carbonization of the carbon support material, leading to a higher graphitization degree that significantly improves electrical conductivity and facilitates electron transfer kinetics; (2) thermal treatment induces dynamic changes in metal valence states and stabilizes the NiFe alloy structure, thereby optimizing hydrogen adsorption/desorption energetics (ΔGH*) for more efficient proton reduction; (3) temperature-driven volatilization of residual organic species from hydrothermal synthesis effectively increases the catalyst’s accessible surface area by removing pore-blocking contaminants and exposing additional active sites. Figure 2b shows the double-layer capacitance (Cdl) of the catalysts at different temperatures. It can be seen that the double-layer capacitance of the catalysts gradually increased with increasing temperature, with a value of 8.33 mF·cm−2 at 800 °C. This indicates that the increase in temperature led to an increase in the active sites and reaction active area of the catalyst, which supports the previous conjecture [30]. Figure 2c shows the OER electrochemical impedance spectra. The interfacial charge transfer resistance (Rct) of the catalysts gradually decreased with increasing temperature (Table S2). Although Rct slightly increased from 700 °C to 800 °C, overall, Rct decreased with increasing temperature, indicating that the increase in temperature may change the interface structure of the catalyst, thereby facilitating the charge transfer in the catalytic process [31]. The Tafel slope in Figure 2d gradually decreased with increasing temperature, indicating that the catalyst has higher reaction kinetics at higher temperatures. Furthermore, the Tafel slope remained stable under high-temperature conditions, which may be due to the fact that the influence of temperature on improving the chemical reaction performance of the catalyst had reached its upper limit, and thus that further increasing the temperature cannot enhance the reaction kinetics of the catalyst.
Naturally, in addition to the effect of pyrolysis temperature, the heating rate and holding time were also tested. Figure S3 shows the electrochemical LSV polarization curves at different heating rates and holding times. It can be seen that the initial overpotential of the catalyst remained basically unchanged at different heating rates, and that the overpotential at higher current densities slightly decreased. This indicates that the heating rate has a minor impact on the catalytic performance of the catalyst and is a secondary factor. Similarly, the initial overpotential of the catalyst remained basically unchanged at different holding times. However, under larger current densities, the performance of the catalyst under the 4 h holding condition deteriorated. This may be because the longer holding time leads to excessive carbonization of the catalyst, resulting in a decrease in conductivity. Under large current densities, the charge transfer rate slows down, and thus a higher overpotential is required to complete the OER catalytic reaction.

3.2. Analysis of Structural Morphology and Metallic States

3.2.1. Morphology Analysis of Bifunctional Catalyst

To further understand the catalytic mechanism of the catalyst, further characterization of the catalyst was carried out. Figure 3 shows the SEM images of the catalysts at different pyrolysis temperatures. It can be seen from the figure that, as the temperature increases, the initially spherical catalyst gradually condensed and its volume gradually decreased. The overall carbon–oxygen ratio increased, as seen from the surface element content detected by EDS, indicating that the catalyst was gradually aromatizing with the increase in pyrolysis temperature. Additionally, as the temperature rose, defects gradually appeared on the surface of the catalyst, which may have been due to the volatilization of organic matter inside the catalyst at high temperatures, gradually breaking through the surface structure, which also increases the surface area of the catalyst. At 700 °C, small spherical attachments gradually appeared on the surface of the catalyst, and at 800 °C, the spherical attachments gradually increased in size. EDS detection showed that the content of NiFe on the surface of the catalyst gradually increased with the increase in temperature, indicating that metals were loaded on the surface of the catalyst to form NiFe metal complexes at higher temperatures, and that these complexes gradually grew with the increase in temperature. In addition, EDS analysis revealed that the C:O ratio on the catalyst surface changed to 9.5 (at 500 °C), 11.32 (at 600 °C), 9.92 (at 700 °C), and 9.22 (at 800 °C) with increasing pyrolysis temperature. This indicates that during pyrolysis below 700 °C, oxygen (O) was removed from the catalyst more efficiently than carbon (C), while above 700 °C, carbon removal became more pronounced, leading to a decrease in the relative C:O ratio, which then gradually stabilized.
Figure 4 shows the TEM test images of the NiFe@NCS-700 catalyst. From Figure 4a, it can be seen that the metal particles on the surface of the catalyst were small, with an average particle size of 12.2 nm, and the distribution of metal particles was relatively uniform, with no obvious agglomeration. Figure 4b shows the lattice image of the metal particles on the surface of the catalyst, from which the lattice stripes with widths of 0.177 nm, 0.205 nm, and 0.126 nm can be clearly seen, corresponding to the (200), (111), and (220) crystal planes of the alloy Ni3Fe, which also indicated the alloy structure on the surface of the catalyst. Figure 4c–f shows the element distribution maps on the surface of the catalyst, showing that the metals Ni and Fe and the non-metal N were uniformly distributed on the carbon carrier, further proving the successful loading and uniformity of the elements on the carbon carrier.
Figure 5 shows the BET and desorption pore size distribution of the catalysts at different pyrolysis temperatures. The BET adsorption–desorption images of the catalysts at temperatures from 500 to 800 °C all belong to type IV isotherms, indicating that the catalysts have mesoporous structures. As the temperature increased, the BET curves of the catalysts showed more obvious hysteresis loops, which are more consistent with type H3, indicating that the pore type of the catalysts is slit-shaped pores. From the pore size distribution diagram, it can be seen that the pore size distribution of the four catalysts was concentrated below 50 Å, which again proves the mesoporous structure of the catalysts. Of course, there was a distribution of micropores in the pore size distribution of NiFe@NCS-600, but with the increase in temperature, the existence of micropores gradually disappeared. In addition, the intensity of the single peak in the figure gradually increased with the increase in temperature, indicating that the material had more regular mesoporous channels with the increase in temperature.
Table 1 shows the specific surface area and pore size of the catalysts based on desorbed N2. It can be seen that the specific surface area of the catalysts gradually increased with the increase in temperature, and remained stable after reaching 700 °C. A larger specific surface area means a larger reaction active area and more reaction active sites, which can further improve the catalytic performance of the catalysts. The pore volume of the catalysts showed an overall upward trend with the increase in temperature, which was beneficial to the mass transfer process of catalysis. The pore size gradually decreased with the increase in temperature and remained stable after 700 °C. An increase in pore volume and decrease in pore size can improve the adsorption capacity of a material, promote the adsorption of reactants and intermediate products, and thereby improve the catalytic performance.
Figure 6 shows the surface characterization of the catalysts at different pyrolysis temperatures. Figure 6a presents the FTIR image of the catalyst. The synthetic peak between 3200 and 3680 cm−1 was mainly attributed to the -OH bond, and multiple peaks could be observed within this broad peak, which may be due to the variety of -OH bonds within the catalyst, resulting in peak overlap. The peak between 1500 and 1600 cm−1 was mainly attributed to the C=C bond, which mainly constituted the aromatic structure. Three accumulated peaks existed in this range, corresponding to the vibrations of the three C=C bonds in the aromatic ring. Additionally, as the temperature gradually increased, the peak of the C=C bond gradually rose and slightly decreased at 800 °C, indicating that the catalyst gradually became more aromatic with increasing temperature. The slight decrease at 800 °C may be due to the enhanced carbonization of the carbon-based catalyst at higher temperatures, where independent C=C bonds may be disrupted, which is consistent with the results shown by SEM. Certainly, due to the incorporation of N elements in the catalyst, there may also be peaks caused by the vibration of C=N (pyridine nitrogen) bonds in this range, leading to the superposition of two peak positions. The peaks between 1300 and 1430 cm−1 and at 777 cm−1 were likely attributable to the C-H bond, caused by the vibration of the bonds on the aromatic ring and the bonds of incompletely graphitized carbon materials. Among them, the intensity of the bond at 1317 cm−1 decreased with increasing temperature, indicating that the degree of graphitization of the material increased with temperature. Meanwhile, the peak at 1387 cm−1 also became smaller, possibly due to the increase in the conjugated system with rising temperature. The peak at 1127 cm−1 was caused by the vibration of the C-O bond. The peak at 614 cm−1 was due to the metal bonds M-O (where M represents Ni or Fe) formed by the doped metals.
Figure 6b shows the X-ray diffraction (XRD) pattern. It can be seen from the figure that the catalyst mainly presented peaks at 2θ of 44.1°, 51.4°, and 75.6°, which correspond to the (111), (200), and (220) crystal planes of Ni3Fe alloy, respectively [32]. This alloy possesses a face-centered cubic structure, and its XRD diffraction peaks overlap with those of elemental Ni, suggesting that elemental Ni is also present in the catalyst. Moreover, the peak intensities at these three positions gradually increased with the rise in temperature, indicating that high temperature can promote the formation of metal alloys and elemental metals. The reason why the catalyst can act as a bifunctional catalyst is most likely due to the formation of the Ni3Fe alloy structure. Additionally, as the temperature gradually increased, a small peak appeared at 26.2°, corresponding to the (200) crystal plane of C. The appearance of this peak indicated that the catalyst gradually graphitized with the increase in temperature, which is consistent with the results of SEM and FTIR.

3.2.2. Metallic States Analysis of Bifunctional Catalysts

In addition to structural characterization of the catalyst, elemental characterization was also conducted. Figure 7a shows the 2p orbital spectra of Ni in the catalyst, where 851.7 eV and 869.2 eV correspond to the peaks of elemental Ni, 854 eV and 871.6 eV correspond to Ni2+, 855.7 eV and 873.7 eV correspond to Ni3+, and 860.6 eV and 879.3 eV are two satellite peaks [33,34]. The presence of elemental Ni may be due to the reducing property of the organic matter in the catalyst during the pyrolysis process, which reduces the metal. Figure 7d shows the distribution diagram of Ni content in the catalyst at different pyrolysis temperatures. It can be seen that, with the increase in temperature, the peak area ratio of elemental Ni generally followed an upward trend, and the performance of the catalyst gradually improved. Thus, it can be seen that the formation of elemental Ni on the surface of the catalyst was beneficial to the overall catalytic performance of the catalyst. Furthermore, the increase in temperature gradually enhanced the peak area of Ni3+ on the surface of the catalyst, and the peak area ratio of Ni2+ and Ni3+ gradually decreased, that is, the relative content of Ni3+ gradually decreased, indicating that the catalytic effect of the catalyst itself was mainly improved by the existence of elemental Ni and Ni2+. Combined with the XRD pattern, it can be seen that the overall structure of the catalyst is a Ni3Fe alloy structure. It is speculated that, in addition to the alloy structure formed on the surface of the catalyst, there is also a small amount of metal oxide coverage, which leads to the detection of strong valence peaks on the surface of the catalyst. Figure 7b is the 2p orbital spectra of Fe. It can be seen that the surface of the catalyst mainly consisted of three valence states of Fe, where the peak positions of elemental Fe are 706.3 eV and 719.1 eV, the peak positions of Fe2+ are 709.9 eV and 722.4 eV, and the peak positions of Fe3+ are 712.1 eV and 724.9 eV [34]. Figure 7e is the distribution diagram of Fe content in the catalyst at different temperatures. It can be seen that, with the increase in temperature, although the peak intensity of Fe3+ gradually exceeded that of Fe2+, the overall content of Fe3+ gradually decreased, while the relative content of elemental Fe gradually increased, indicating that the reduction ratio of Fe gradually increased at higher temperatures. Thus, it can be seen that the Ni3Fe alloy on the surface of the metal plays an important role in the overall catalytic performance of the catalyst, which is consistent with the results shown by XRD.
Certainly, in Figure 7c, the N on the surface of the catalyst was mainly divided into four peaks, where 397.9 eV corresponds to pyridine N, 399.5 eV corresponds to pyrrole N, 401.2 eV corresponds to graphitic N, and the peak greater than 402 eV corresponds to oxidized N. It can be seen from the figure that, with the increase in temperature, the peak intensity of pyrrole N gradually increased, and the peak area ratios were 25%, 34.3%, 36.3%, and 31.9%, respectively, indicating that the increase in temperature increased the content of pyrrole N, but excessively high temperatures would cause the decomposition of pyrrole N [34,35]. Correspondingly, the peak area of graphitic N gradually increased with the increase in temperature, reaching 5.9%, 9.1%, 12.9%, and 16.1%, respectively. It can be seen that the increase in temperature, to a certain extent, enhanced the graphitization degree of the catalyst. The peak position of oxidized N shifted with the change in temperature, which may be due to the inconsistent forms of oxidized N in the catalyst, and the peak position shifted to a lower binding energy position at high temperatures. Figure 7f shows the distribution diagram of N content in the catalyst at different temperatures. The figure shows that, with the increase in temperature, the N content in the catalyst gradually decreased, mainly due to the decrease of pyridine N and pyrrole N, but at higher temperatures, the N content tended to be stable, indicating that the structure of N in the catalyst gradually stabilizes, and the influence of temperature decreases.
As shown in Figure 8b, the inorganic oxygen content initially increased and then remained relatively stable as the temperature rose. The ratio of the peak areas of C-O and C=O bonds first increased and then remained relatively stable, indicating that at higher temperatures, the trends of C-O and C=O bonds are basically the same. From the EDS elemental content spectrum, it can be seen that the overall oxygen content of the catalyst decreased, suggesting that the degree of aromatization (or graphitization) of the catalyst gradually increased, and the contents of C-O and C=O bonds decreased overall. This can be confirmed from Figure 8a, where with the increase in temperature, the peak area of C=O bonds gradually decreases from 12% to 8%, and the peak area of C=C bonds increases from 23% to 28%. The aromatization of the catalyst improved [34,35]. Figure 8c shows the D and G peaks in the Raman spectrum. As the temperature rose, the ID/IG ratios were 1.70, 1.67, 1.60, and 0.99 respectively. It can be seen that, as the temperature increased, the ID/IG ratio of the catalyst gradually decreased, meaning the proportion of amorphous carbon in the catalyst gradually decreased, and the proportion of the graphitic structure increased. This indicates that the degree of graphitization of the catalyst increased with the rise in pyrolysis temperature, which is consistent with the structures shown by SEM, XRD, and FTIR.

3.3. Mechanism Analysis of Bifunctional Catalysts

Based on the above experimental results, it can be concluded that the catalytic function of the bifunctional catalyst is mainly due to the presence of Ni3Fe alloy, which provides active sites for catalysis. Therefore, the catalytic process mainly occurs on the alloy phase.
In an alkaline environment, the HER catalytic mechanism is shown in Figure 9b. Initially, water in the electrolyte adsorbs onto the surface of the Ni3Fe alloy and reacts to release electrons, forming adsorbed hydrogen ions (H*) and generating OH−. Then, the adsorbed hydrogen ions (H*) combine with water molecules in the electrolyte and release electrons to form adsorbed hydrogen gas on the catalyst surface and generate OH−. The process shown in the figure represents the Heyrovsky reaction. When there are more adsorbed hydrogen ions on the catalyst surface, the Tafel reaction may also occur, where the adsorbed hydrogen ions couple to form hydrogen gas on the catalyst surface. Finally, the adsorbed hydrogen gas desorbs from the catalyst surface to release hydrogen gas. During this process, the catalyst surface has a strong adsorption and desorption capacity for hydrogen ions. Additionally, the catalyst support has good electrical conductivity and mesoporous characteristics, which facilitate electron and mass transfer, overall promoting the HER reaction.
In an alkaline condition, the OER catalytic mechanism is shown in Figure 9a. Firstly, OH− in the electrolyte adsorbs onto the catalyst surface and combines with Ni atoms to form NiOH and lose electrons. Then, it combines with OH− in water and loses electrons to generate water and form NiO on the catalyst surface. Afterward, it combines with OH− in the electrolyte and loses electrons to form NiOOH. Finally, it combines with OH− in the electrolyte and loses electrons to generate water and form NiOO on the catalyst surface. Ultimately, oxygen desorbs from the catalyst surface to release oxygen gas. During this process, Ni on the catalyst surface participates in the reaction, and under the synergistic effect of Fe atoms, the electronic structure around Ni is altered, making the intermediates NiO and NiOOH more easily formed and promoting the oxygen evolution reaction (OER). In the expressions in the figure, M represents Ni (or Fe) atoms.

4. Conclusions

This study develops high-performance bifunctional electrocatalysts for water splitting by engineering heteroatom-doped NiFe-LDH through an integrated hydrothermal-pyrolysis strategy. We systematically compared one-pot and two-step (hydrothermal-impregnation) methods for incorporating B, N, P, and S dopants, with comprehensive electrochemical evaluation revealing that N-doped catalysts synthesized via the one-pot approach exhibited optimal HER/OER activity. Detailed investigation of pyrolysis parameters demonstrated that, while increasing temperature enhanced catalytic performance, prolonged holding time induced excessive carbonization of the support material, leading to deteriorated conductivity, whereas heating rate showed minimal influence. Advanced characterization (SEM/TEM/XPS/XRD/FTIR) uncovered three key mechanistic insights: (1) the formation of Ni3Fe alloy phases on catalyst surfaces significantly promoted bifunctional activity, (2) N-doping modified the carbon support structure with pyrrolic-N identified as the primary active species, and (3) OER catalysis proceeded through high-valence intermediate formation. These findings provide fundamental guidance for designing efficient transition metal-based electrocatalysts, offering both theoretical understanding and practical synthesis protocols for advancing water electrolysis technologies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en18143592/s1, Figure S1. HER activity of doped catalysts; Figure S2. Heteroatom-doped catalysts for OER; Figure S3. Effect of heating rate and holding time on electrochemical properties; Figure S4. EDS analysis of catalyst at different pyrolysis temperatures; Figure S5. Stability test curve graph of bifunctional catalysts; Figure S6. LSV performance curves of catalysts with different NiFe ratios; Figure S7. Electrochemical performance curves of catalysts with different NiFe ratios; Figure S8. SEM images of catalysts with different NiFe ratios; Figure S9. BET images of catalysts with different NiFe ratios; Table S1. The overpotential of different catalysts at a current density of 10 mA/cm2; Table S2. Fitting parameters of electrochemical impedance spectroscopy (EIS) for bifunctional catalysts; Table S3. A comparison of the performance of transition metal catalysts in OER tests; Table S4. A comparison of the performance of transition metal catalysts in HER tests. References [26,36,37,38,39,40,41,42,43] are cited in Supplementary Materials.

Author Contributions

X.Z. (Xiuzheng Zhuang) supervised and designed the research. W.Z. summarized the relevant information, wrote the original paper, and assisted with the analysis of data. Q.Z., X.Z. (Xinghua Zhang), L.C. and L.M. reviewed and corrected the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported financially by National Natural Science Foundation of China (52306232, 52236010), National Science Foundation for Post-doctoral Scientists of China (2023M740597), Jiangsu Funding Program for Excellent Postdoctoral Talent (2023ZB175), the Postdoctoral Fellowship Program of CPSF (GZC20230425), and Fundamental Research Funds for the Central Universities (2242022R10058).

Data Availability Statement

Data supporting the findings of this study are available from the corresponding authors upon reasonable request. Correspondence and requests for materials should be addressed to X.Z.

Acknowledgments

We thank eceshi (www.eceshi.com) for the BET, TEM and XPS analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Electrochemical characterization of catalysts at different pyrolysis temperatures. (a) LSV curves of HER at different temperatures, (b) double-layer capacitance of HER at different temperatures, (c) Nyquist curves of HER at different temperatures, (d) Tafel slope graphs of HER at different temperatures.
Figure 1. Electrochemical characterization of catalysts at different pyrolysis temperatures. (a) LSV curves of HER at different temperatures, (b) double-layer capacitance of HER at different temperatures, (c) Nyquist curves of HER at different temperatures, (d) Tafel slope graphs of HER at different temperatures.
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Figure 2. Electrochemical performance of NiFe@NCS catalysts at different temperatures. (a) LSV curves of OER at different temperatures, (b) double-layer capacitors of OER at different temperatures, (c) Nyquist curves of OER at different temperatures, (d) Tafel slope graphs of OER at different temperatures.
Figure 2. Electrochemical performance of NiFe@NCS catalysts at different temperatures. (a) LSV curves of OER at different temperatures, (b) double-layer capacitors of OER at different temperatures, (c) Nyquist curves of OER at different temperatures, (d) Tafel slope graphs of OER at different temperatures.
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Figure 3. SEM images of the catalyst at different pyrolysis temperatures. (a) NiFe@NCS-500, (b) NiFe@NCS-600, (c) NiFe@NCS-700, (d) NiFe@NCS-800.
Figure 3. SEM images of the catalyst at different pyrolysis temperatures. (a) NiFe@NCS-500, (b) NiFe@NCS-600, (c) NiFe@NCS-700, (d) NiFe@NCS-800.
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Figure 4. TEM image of NiFe@NCS-700 catalyst. (a) TEM image and particle size distribution, (b) crystal lattice and diffraction pattern, (c–f) mapping images of different elements.
Figure 4. TEM image of NiFe@NCS-700 catalyst. (a) TEM image and particle size distribution, (b) crystal lattice and diffraction pattern, (c–f) mapping images of different elements.
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Figure 5. BET images of catalysts at different temperatures. (a) NiFe@NCS-500, (b) NiFe@NCS-600, (c) NiFe@NCS-700, (d) NiFe@NCS-800.
Figure 5. BET images of catalysts at different temperatures. (a) NiFe@NCS-500, (b) NiFe@NCS-600, (c) NiFe@NCS-700, (d) NiFe@NCS-800.
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Figure 6. Surface characterization of the catalysts at different pyrolysis temperatures. (a) FTIR curves of the catalyst at different pyrolysis temperatures, (b) XRD images of the catalyst at different heat engine temperatures.
Figure 6. Surface characterization of the catalysts at different pyrolysis temperatures. (a) FTIR curves of the catalyst at different pyrolysis temperatures, (b) XRD images of the catalyst at different heat engine temperatures.
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Figure 7. XPS spectra of catalysts with different pyrolysis temperatures. (a) Ni 2P orbital spectra of NiFe@NCS-700, (b) Fe 2P orbital spectra of NiFe@NCS-700, (c) N 1S orbital spectra of NiFe@NCS-700, (d) Ni content distribution map, (e) Fe content distribution map, (f) N content distribution map.
Figure 7. XPS spectra of catalysts with different pyrolysis temperatures. (a) Ni 2P orbital spectra of NiFe@NCS-700, (b) Fe 2P orbital spectra of NiFe@NCS-700, (c) N 1S orbital spectra of NiFe@NCS-700, (d) Ni content distribution map, (e) Fe content distribution map, (f) N content distribution map.
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Figure 8. XPS characterization spectra and Raman spectra of the catalysts. (a) XPS C 1S orbital spectra, (b) XPS O 1S orbital spectra, (c) catalyst Raman spectra. Note: All gray curves in the figure correspond to the actual data.
Figure 8. XPS characterization spectra and Raman spectra of the catalysts. (a) XPS C 1S orbital spectra, (b) XPS O 1S orbital spectra, (c) catalyst Raman spectra. Note: All gray curves in the figure correspond to the actual data.
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Figure 9. Mechanism diagram of catalyst electrolysis of water. (a) HER catalytic process mechanism diagram, (b) OER catalytic process diagram.
Figure 9. Mechanism diagram of catalyst electrolysis of water. (a) HER catalytic process mechanism diagram, (b) OER catalytic process diagram.
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Table 1. Specific surface area and pore size of N2 desorbed catalysts at different pyrolysis temperatures.
Table 1. Specific surface area and pore size of N2 desorbed catalysts at different pyrolysis temperatures.
Thermophysical PropertiesNiFe@NCS-500NiFe@NCS-600NiFe@NCS-700NiFe@NCS-800
BET Surface Area
(m2·g−1)		55.89131.36191.26190.94
Pore volume
(cm3·g−1)		0.1450.2020.1430.178
Pore Size (Å)115.4181.2758.2658.87
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Zhu, W.; Zhang, X.; Zhang, Q.; Chen, L.; Zhuang, X.; Ma, L. Biomass-Derived Catalysts with Dual Functions for Electrochemical Water Splitting. Energies 2025, 18, 3592. https://doi.org/10.3390/en18143592

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Zhu W, Zhang X, Zhang Q, Chen L, Zhuang X, Ma L. Biomass-Derived Catalysts with Dual Functions for Electrochemical Water Splitting. Energies. 2025; 18(14):3592. https://doi.org/10.3390/en18143592

Chicago/Turabian Style

Zhu, Wangchuang, Xinghua Zhang, Qi Zhang, Lungang Chen, Xiuzheng Zhuang, and Longlong Ma. 2025. "Biomass-Derived Catalysts with Dual Functions for Electrochemical Water Splitting" Energies 18, no. 14: 3592. https://doi.org/10.3390/en18143592

APA Style

Zhu, W., Zhang, X., Zhang, Q., Chen, L., Zhuang, X., & Ma, L. (2025). Biomass-Derived Catalysts with Dual Functions for Electrochemical Water Splitting. Energies, 18(14), 3592. https://doi.org/10.3390/en18143592

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