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Article

Carbon-Encapsulated Ni Nanoparticles Catalysts Derived from Ni-Hexamine Coordination Frameworks for Oxygen Reduction Reaction and Oxygen Evolution Reaction

by
Huoxing Huang
1,
Jiaxing Huang
1,
Guoyu Zhong
1,*,
Shurui Xu
1,
Hongwei Chen
2,*,
Xiaobo Fu
1,
Shimin Kang
1,
Junling Tu
1,
Yongxiao Tuo
3,
Wenbo Liao
1 and
Baizeng Fang
1,*
1
Guangdong Provincial Key Laboratory of Distributed Energy Systems, School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan 523808, China
2
School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, China
3
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(4), 338; https://doi.org/10.3390/catal15040338
Submission received: 6 March 2025 / Revised: 25 March 2025 / Accepted: 28 March 2025 / Published: 31 March 2025
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Versions Notes

Abstract

Developing efficient bifunctional oxygen reduction (ORR) and oxygen evolution (OER) electrocatalysts is critical for renewable energy technologies. Noble metal catalysts face limitations in cost, scarcity, and bifunctional compatibility. Herein, we report the synthesis of nickel nanoparticles encapsulated in nitrogen-doped carbon nanosheets (Ni@NC-T) via a solvothermal polymerization and pyrolysis process using a Ni-hexamine coordination framework (NiHMT) as a precursor. The Ni@NC-900 catalyst exhibits superior ORR and OER activity under alkaline conditions, with an ORR performance (half-wave potential = 0.86 V) comparable to commercial Pt/C and an OER overpotential of only 430 mV at 10 mA cm−2. Structural analysis indicates that the hierarchical porous structure and high specific surface area (409 m2 g−1) of Ni@NC-900 facilitate the exposure of active sites and enhance mass transport. The surface-doped nitrogen species, predominantly in the form of pyridinic N and graphitic N, promote electron transfer during the ORR. Furthermore, its application as a bifunctional cathode in rechargeable zinc-air batteries results in a high power density of 137 mW cm−2, surpassing the performance levels of many existing carbon-based bifunctional catalysts. This work highlights a facile strategy for the fabrication of transition metal-based catalysts encapsulated in MOF-derived carbon matrices, with promising potential for energy storage and conversion devices.

Graphical Abstract

1. Introduction

Renewable and environmentally friendly energy technologies, such as fuel cells and metal–air batteries, are urgently needed to address current environmental issues and energy shortages [1,2]. However, the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as the key cathodic and anodic reactions, respectively, hinder their large-scale application and development [3,4,5]. To date, platinum (Pt)-based materials have been widely studied to accelerate the ORR, while iridium (Ir) and ruthenium (Ru)-based materials are the most effective catalysts for the OER [6,7,8]. However, these noble metal-based catalysts are not ideal for bifunctional applications where both ORR and OER need to be efficiently catalyzed. The limited abundance, high cost, and durability concerns of noble metals present significant obstacles to their large-scale application and long-term viability, particularly in light of the growing demand for renewable energy solutions [9]. Therefore, the development of highly active, cost-effective, and earth-abundant bifunctional electrocatalysts remains a critical and urgent challenge.
Recently, carbon-based catalysts have attracted considerable attention for ORR due to their exceptional electrical conductivity, tunable structure, and cost-effectiveness [10,11]. The ORR activity of these carbon materials can be enhanced by elemental doping (e.g., nitrogen, sulfur, phosphorus, and halogen) or by introducing defects that improve their catalytic properties [12,13,14,15,16]. For example, studies on nitrogen-doped carbon nanotubes and graphene have shown that nitrogen doping alters the electronic structure of carbon materials, generating additional active sites conducive to ORR [17,18]. However, while these materials demonstrate good ORR performance, their OER capabilities remain suboptimal. Transition metal and nitrogen co-doped carbon nanomaterials (M-N-C, where M = Fe, Co, or Ni) are considered to be some of the most promising bifunctional catalysts that address both ORR and OER [19,20]. The inclusion of metal components in these catalysts introduces active sites capable of facilitating both reactions [21]. In particular, Fe-N-C catalysts have been widely studied and have shown high ORR and OER activities that rival or even surpass those of platinum-based catalysts (Pt/C). The primary active sites in these catalysts include atomic-level Fe, Fe nanoparticles, Fe3C, and Fe-Nx complexes [22,23,24]. However, M-N-C catalysts face challenges related to poor stability and durability due to the dissolution, agglomeration, and spalling of exposed metal active components, particularly in acidic environments [25]. Recently, ORR catalysts with transition metals encapsulated in nitrogen-doped carbon matrices (M@NC) have been widely reported, such as Fe3C nanoparticles encapsulated in carbon nanotubes, Fe nanoparticles encapsulated in monolayer graphene, Co nanoparticles encapsulated in graphitic carbon rods, etc. [26,27]. Studies have demonstrated that this structure, characterized by a substantial electronegativity difference, close proximity, and strong interaction between the metal core and carbon shell, is highly conducive to charge transfer between the core and shell [28]. Jung et al. [29] reported novel ORR catalysts of metal particles encapsulated in graphene shells. Theoretical calculations and electrochemical analyses reveal that the high ORR activity results from the carbon shell reducing the work function through a strong interaction with the electron-donating metal particles. Additionally, the M@NC structure effectively prevents the dissolution and aggregation of the metal centers, resulting in excellent bifunctional activity and stability for both ORR and OER. However, M@NC catalysts still face some challenges. First, the most common method for preparing M@NC catalysts is the pyrolysis of nitrogen-containing organic molecules or polymers and metal salt hybrids, which usually require additional nitrogen and carbon sources as soft templates [30]. This complicated multi-step preparation process limits the large-scale synthesis of these catalysts. Second, apart from Fe and Co, transition metal-encapsulated structures with high potential are rarely reported. For instance, Ni exhibits excellent ORR and OER activity in alkaline environments, which can effectively enhance the bifunctional activity of M@NC catalysts. Additionally, compared to noble metals and most other metals, Ni offers superior economic benefits, making it more suitable for large-scale electrocatalytic applications. These limitations highlight the need to develop a more convenient method for the preparation of nickel particles encapsulated in nitrogen-doped graphitic carbon nanosheets as bifunctional electrocatalysts.
Herein, we report a novel solvothermal coordination-pyrolysis strategy for the synthesis of N-doped porous carbon-encapsulated Ni nanoparticles (denoted as Ni@NC-T), which serve as a highly efficient electrocatalyst for ORR. Unlike conventional multi-step approaches, Ni@NC-T is fabricated through the facile one-step pyrolysis of a uniquely designed Ni(II)-hexamine coordination framework (NiHMT). The NiHMT precursor is synthesized via solvothermal polymerization using hexamine (HMT) as both a ligand and nitrogen source, coupled with NiCl2 as the metal center. The resulting Ni@NC-T features a pioneering N-doped carbon matrix that simultaneously (i) encapsulates uniformly dispersed Ni nanoparticles to prevent agglomeration and (ii) incorporates pyridinic/graphitic-N dopants to enhance charge transfer and O2 adsorption. This synergistic metallic Ni core structure, protected by conductive N-rich carbon shells, confers exceptional activity and stability for ORR/OER. Notably, the performance of a Ni@NC-900 air electrode supported on nickel foam surpasses that of noble metal electrodes in rechargeable zinc–air batteries (ZABs).

2. Results and Discussion

2.1. Morphology and Structural Characterizations of the NiHMT

The preparation process of Ni@NC-T is presented in Figure 1a. First, Ni-MOF was synthesized by mixing NiCl2·6H2O with HMT under solvothermal conditions, and the resulting coordination framework is referred to as NiHMT. As shown in the SEM images (Figure 1b,c), the synthesized NiHMT exhibits a microsheet-like structure with a length of approximately 0.8–4.0 μm, a width of 300–800 nm, and a thickness of 80–130 nm. The XRD patterns of NiHMT differ significantly from those of NiCl2 and pure HMT (Figure 1d), indicating that the HMT ligand has been successfully coordinated with Ni2+ to form a novel Ni-MOF crystal. Previous studies have shown that, in the absence of additional bridging ligands, the molecular formula of the polymer configuration generated by the coordination of Ni2+ with a single ligand can be [NiClx(HMT)y(H2O)z]n. To explore the possible coordination structures of NiHMT, elemental analysis was conducted to determine the molecular formula units. Based on the data presented in Table S1, the elemental composition of NiHMT is found to be 18.76 wt% C, 13.03 wt% N, and 4.6 wt% H. The possible structural formulae of NiHMT and their C, N, and H elemental contents are listed in Table S2. Based on the ratios of C, N, and H elements, the molecular formula unit of NiHMT is deduced to be [NiCl2(HMT)(H2O)3]n. In addition, the coordination environment was further examined using Fourier transform infrared (FT-IR) spectroscopy (Figure 1e). In the FT-IR spectrum of pure HMT, two distinct absorption bands at 996 cm−1 and 1235 cm−1 correspond to C-N stretching vibrations [31]. Upon coordination with Ni2+ in the N iHMT complex, these C-N bands split into two separate peaks, confirming that HMT functions as a bidentate bridging ligand in the complex [32]. Furthermore, in the fingerprint region (400–900 cm−1), several sharp peaks from the free HMT become weaker upon coordination, providing additional evidence for the formation of the proposed coordination structure [33]. This analysis suggests that HMT coordinates with NiCl2 through a bidentate mode, stabilizing the structure and influencing the spectral features observed in FT-IR.

2.2. Morphology and Structural Characterizations of the Ni@NC-T

NiHMT was used as a precursor and pyrolyzed at high temperatures in a nitrogen atmosphere. N-doped porous carbon-encapsulated Ni nanoparticle catalysts were obtained after HCl pickling and designated as Ni@NC-T (T represents the pyrolysis temperature). The morphology and structure of a typical sample, Ni@NC-900, were analyzed using SEM (Figure 2a) and TEM (Figure 2b,c). Ni@NC-900 has a coral-like structure consisting of encapsulated nickel nanoparticles cross-stacked with fractured nanosheets of various sizes, many of which protrude outwards, resulting in a rough and uneven surface with numerous defects and pores. SEM images of samples pyrolyzed at different temperatures (Figure S1a–d) reveal similar morphological structures, retaining the lamellar morphology of the NiHMT precursor, especially in Ni@NC-900. This indicates that the NiHMT framework structure did not completely collapse during pyrolysis. Instead, the organic framework decomposed and transformed into smaller carbon nanosheets while the nickel sites aggregated into metallic nanoparticles. TEM and high-resolution TEM (HR-TEM) images (Figure 2b,c) show numerous voids in the carbon matrix left by acid etching of surface nickel particles, with aggregated dark regions indicating nickel nanoparticles encapsulated within the graphitic carbon. The graphitic carbon layer is approximately 8 nm thick and provides stability to the Ni particles in an acidic solution. The lattice spacing of the Ni particles is 0.203 nm, corresponding to the (111) crystallographic plane of the Ni phase, as confirmed by selected area electron diffraction (SAED) results (Figure 2c inset). EDS elemental mappings (Figure 2d) identified the presence of carbon (C), nitrogen (N), oxygen (O), and nickel (Ni). C and N were uniformly distributed throughout the carbon matrix, suggesting homogeneous nitrogen doping throughout the material. Ni was predominantly concentrated in the nanoparticles, while small amounts of Ni and O were uniformly dispersed throughout the carbon matrix, probably due to the surface oxidation of residual Ni after the pickling process [34]. These results confirm the successful encapsulation of Ni nanoparticles in a porous, nitrogen-doped carbon matrix, together with the retention of some MOF-derived structural features during pyrolysis.
XRD analyzed the phase structures of the Ni@NC-T catalysts, as shown in Figure 3a. All samples exhibit distinct diffraction peaks around 2θ = 44.3°, 51.6° and 76.1°, which align with the PDF card data for nickel (PDF#89-7128) and correspond to its (111), (200) and (220) crystal planes, respectively. Additionally, the samples display a broad diffraction peak at 2θ = 21–27°, corresponding to the (002) crystal plane of low graphitization carbon [35]. Notably, the intensity of this graphitic carbon peak decreases as the pyrolysis temperature increases from 700 to 1000 °C, which suggests that a disordering process occurs within the graphitic carbon at higher temperatures, resulting in increased defects in the crystal structure. Furthermore, for samples synthesized at temperatures above 900 °C, a sharp peak appears at ~27° for the (002) plane of the graphitic carbon, indicating that some of the amorphous carbon is transformed into more highly graphitized graphitic carbon at higher temperatures [36,37]. Despite this change in the carbon phase, the intensity of the Ni diffraction peaks remains almost unchanged, indicating that the crystallinity of the nickel nanoparticles remains consistent over the different pyrolysis temperatures. This finding suggests that the variations in calcination temperature primarily affect the carbon phase rather than the crystalline structure of the Ni nanoparticles. Raman spectroscopy was further conducted to analyze the graphitization degree of the synthesized samples, with the results shown in Figure 3b. The D band is associated with disordered structures in carbon materials, while the G band represents the vibrations of sp2-hybridized carbon atoms in graphitic carbon, as indicated by the prominent peaks at 1350 cm−1 and 1580 cm−1, respectively. The intensity ratio (ID/IG) of these two peaks is commonly used to evaluate the degree of disorder or graphitization in carbon materials [38,39]. The Raman spectroscopy results reveal that the ID/IG value of Ni@NC-T increases from 1.15 to 1.32 as the preparation temperature increases, indicating an increase in defects within the carbon matrix. These defects, probably caused by nitrogen doping, can serve as potential active sites to enhance the ORR process. As the temperature increases to 1000 °C, the ID/IG value of Ni@NC-100 decreases, which is attributed to the enhanced graphitization of the sample at high temperatures.
The N2 adsorption–desorption isotherms (Figure 3c) were measured to further investigate the pore structure and surface area of Ni@NC-T. A distinct adsorption hysteresis is observed in the N2 adsorption–desorption isotherms in the high P/P0 range of 0.4–1.0, indicating the presence of abundant mesopores. The corresponding pore size distribution (Figure 3d) shows the coexistence of mesopores and macropores, with pore sizes centered around ~2.8 nm, ~15 nm, and ~51 nm. As summarized in Table S3, the BET surface area and pore volume of the samples vary significantly with different preparation temperatures. In particular, the BET surface area and pore volume of Ni@NC-900 are as high as 409.85 m2 g−1 and 1.09 cm3 g−1, respectively. These results confirm that a hierarchical porous carbon encapsulation structure has been successfully fabricated, which enhances mass transport and active site exposure during the ORR process [40,41].
The surface composition of the Ni@NC-T catalysts was analyzed by XPS, and the results are summarized in Figure 4a and Table S4. The XPS spectra reveal the presence of four characteristic peaks corresponding to carbon (C 1s), nitrogen (N 1s), oxygen (O 1s), and nickel (Ni 2p), which are in agreement with the elemental mappings from EDS. XPS analysis shows that the surface carbon content increases from 82.26% to 92.34% with increasing synthesis temperature, which can be attributed to enhanced graphitization at higher temperatures. Conversely, the surface nitrogen content gradually decreases from 11.69% to 3.93%, reflecting the release of nitrogen species from the NiHMT precursor during pyrolysis. The surface nickel content is relatively low, ranging from 1.2% to 0.42%, which is expected due to the encapsulation of nickel nanoparticles within a thin carbon layer (~8 nm thick), smaller than the XPS detection depth (~10 nm). The reduced nickel content at higher temperatures suggests that increased graphitization limits the exposure of nickel at the surface. However, Ni@NC-900 exhibits a slight increase in nickel content compared to other samples, probably due to optimized encapsulation conditions at this temperature.
The effect of preparation temperature on the nitrogen species in Ni@NC-T catalysts was further examined by deconvoluting the high-resolution N 1s XPS spectrum (Figure 4b and Table S5). For Ni@NC-900, the N 1s spectrum was deconvoluted into several nitrogen species: pyridinic N (398.8 eV), pyrrolic N (400.0–400.3 eV), graphitic N (401.3–401.5 eV) and oxidized N (403.1 eV) [42,43]. As the pyrolysis temperature increases from 700 to 1000 °C, there is a significant decrease in the content of pyrrolic N from 25.06% to 7.61%. In contrast, the content of graphitic N and nitrogen oxides (N-oxides) increases significantly from 20.07% to 43.49% and from 8.14% to 25.59%, respectively. Pyridinic N shows an initial increase to 46.76% at 800 °C before decreasing to 23.31% at 1000 °C. These results indicate that the dominant nitrogen species in Ni@NC-T are pyridinic N and graphitic N. As the preparation temperature increases, the less stable pyrrolic N (NPyr) tends to convert first to pyridinic N (NP), which subsequently converts to the more stable graphitic N (NG) and N-oxides (NOx) [42]. This conversion of nitrogen species results in variations in both the content and proportions of different nitrogen species within the sample. For example, in the typical Ni@NC-900 sample, the combined content of pyridinic and graphitic nitrogen reaches 70.04%. The synergistic effect between NP and NG plays a critical role in enhancing the catalytic activity of Ni@NC-T catalysts for ORR [44]. The evolution of nitrogen species with temperature reflects their central role in influencing catalytic performance by providing active sites for ORR.
Furthermore, the chemical states of Ni have been characterized by the high-resolution Ni 2p spectra of Ni@NC-T (Figure 4c). The main peaks, located at 855.3 eV and 872.5 eV, correspond to the two spin orbitals, 2p3/2 and 2p1/2, respectively. After deconvolution, the weak peak at 852.9 eV is attributed to metallic nickel, indicating that a trace amount of nickel has been detected in the thin carbon layer by XPS, further confirming the formation of metallic Ni. As the treatment temperature increases, the intensity of this peak increases. Higher treatment temperatures may promote the reduction of high-valent nickel species to metallic Ni, thereby increasing the metallic Ni content. At the same time, as the temperature rises, the pyrolysis and volatilization of organic carbon may reduce the thickness of the carbon layer, leading to enhanced exposure to metallic Ni. The strong peaks at 855.3 eV and 872.5 eV are assigned to NiO, while the peaks at 858.1 eV and 875.2 eV correspond to Ni(OH)2, indicating surface oxidation of the Ni nanoparticles. Additionally, the two higher energy peaks (~862.7 eV and ~880.0 eV) are identified as satellite peaks of the Ni 2p spectra [45]. The results suggest that the Ni 2p spectra of the samples are minimally affected by changes in preparation temperature. Based on these results, it can be clearly concluded that Ni nanoparticles encapsulated in N-doped carbon have been successfully synthesized using a one-step Ni-MOF pyrolysis strategy.

2.3. Electrochemical ORR/OER Performance and ZAB Testing

The ORR electrochemical activity of the resulting catalysts was first evaluated in an oxygen-saturated 0.1 M KOH alkaline electrolyte and compared with that of commercial 20 wt% Pt/C catalysts. As shown in the LSV curves in Figure 5a, the onset potential of all catalysts is almost identical to that of Pt/C (0.93–0.96 V). However, the half-wave potential follows this order: Ni@NC-700 < Ni@NC-1000 < Ni@NC-800 < Ni@NC-900. Among them, Ni@NC-900 shows the most favorable ORR catalytic performance, with an onset potential of 0.96 V and a half-wave potential of 0.80 V, values that are comparable to commercial 20 wt% Pt/C (0.95 V and 0.83 V, respectively). This indicates that its ORR catalytic activity is close to that of commercial Pt/C. Additionally, the diffusion-limiting current density of Ni@NC-900 is ~16% higher than that of 20 wt% Pt/C, suggesting that the hierarchical porous N-doped carbon nanosheet structure of Ni@NC-900 is conducive to the diffusion of reactants and products. To further understand the electrochemical kinetics of the ORR, the Tafel slope was calculated from the potential vs. log(current density) data of the LSV curves. As shown in Figure 5b, the Tafel slopes of the Ni@NC-T samples prepared at different temperatures (700–1000 °C) are 118, 83, 79, and 85 mV dec−1, respectively, all higher than 73 mV dec−1 of Pt/C. Among these samples, Ni@NC-900 exhibits the smallest Tafel slope, approaching that of 20 wt% Pt/C, indicating a superior intrinsic catalytic activity for ORR. As shown in Figure S3, the LSV curves of all the samples were recorded at different rotational speeds by adjusting the speed of the rotating disk electrode. The electron transfer numbers (n) of the catalysts at different potentials were then calculated using the Koutecky–Levich equation (Supporting Information). For Ni@NC-900, the current density increases significantly as the rotational speed increases from 200 to 2500 rpm (Figure S3e). The Koutecky–Levich plots (j−1 vs. ω−1/2) at potentials from 0.2 to 0.7 V exhibit excellent linearity, with comparable slopes in the 0.2–0.6 V range (Figure 5c), suggesting similar electron transfer numbers during the ORR process. The calculated electron transfer numbers for all as-prepared samples and the Pt/C catalyst are summarized in the inset of Figure 5c. In the illustration of the inset, the value of the electron transfer number is slightly greater than 4, which may be caused by mismatched empirical constants, background current interference, non-ideal hydrodynamic conditions, and multi-reaction pathway interference [46,47]. The average electron transfer numbers (n) for Ni@NC-T samples are ~3.8, with an average of 3.9 for Ni@NC-900, which is slightly lower than the number of ~4.0 for Pt/C. These results indicate that Ni@NC-900 follows a four-electron oxygen reduction reaction pathway comparable to that of the Pt/C catalyst, which is an efficient ORR mechanism.
Benefiting from the superior ORR performance of Ni@NC-900, its OER performance was further evaluated using LSV in an oxygen-saturated 0.1 M KOH alkaline electrolyte. The results were compared with those of commercial RuO2 and 20 wt% Pt/C catalysts (Figure 5d). The Ni@NC-900 displays a lower onset potential of 1.30 V than those of RuO2 (1.43 V) and Pt/C (1.52 V). In the oxidation zone (1.30–1.85 V), Ni@NC-900 exhibits a low onset potential, with significant oxidation currents appearing after ~1.38 V. Its current density reaches 10 mA cm−2 at 1.66 V. Compared to the standard water-splitting potential (1.23 V), the overpotential of Ni@NC-900 is ~430 mV, close to that of commercial RuO2 (360 mV), while Pt/C requires a higher overpotential of 610 mV to reach 10 mA cm−2. To further analyze the intrinsic OER kinetics of Ni@NC-900, Tafel plots of potential vs. log(current density) were recorded (Figure S4). The calculated Tafel slope for Ni@NC-900 is lower than that of Pt/C, indicating that Ni@NC-900 has excellent intrinsic OER catalytic activity.
In addition to high ORR and OER activity, excellent durability is also critical for the practical application of catalysts. The durability of Ni@NC-900 and commercial Pt/C in an alkaline electrolyte was evaluated using chronoamperometry (i-t) measurements, as shown in Figure 6a. After continuous operation at a constant voltage of −0.7 V for 20,000 s, the current density of Ni@NC-900 decreases by only ~10%, while the Pt/C catalyst exhibits a faster and more significant decrease in ORR current density, indicating that Ni@NC-900 has superior ORR durability compared to commercial Pt/C. Additionally, the methanol tolerance of Ni@NC-900 was characterized using the same i-t method. As shown in Figure 6b, when 3M methanol was added at approximately 300 s, Ni@NC-900 maintained a relatively stable current density, while the Pt/C catalyst showed a significant decrease in current density. This indicates that Ni@NC-900 has better methanol tolerance than commercial Pt/C, highlighting its potential as an ORR catalyst for methanol fuel cell cathodes.
Due to its excellent bifunctional catalytic activity and stability, Ni@NC-900 was used as a bifunctional electrocatalyst in a ZAB to evaluate its practical performance in energy devices (Supporting Information). Using Ni@NC-900 as the air cathode, pure zinc foil as the anode, and a mixture of 6 M KOH and 0.2 M Zn(OAc)2 as the electrolyte, the home-made aqueous zinc–air battery successfully powered an LED bulb (Figure S5). The open-circuit potential of the self-fabricated ZAB was measured to be 1.47 V, which is comparable to that of the Pt/C catalyst (Figure 6c). Furthermore, the polarization curve was recorded, and power densities were calculated during discharge tests of the ZAB to evaluate its output power. As presented in Figure 6d, the Ni@NC-900-based ZAB achieves a power density of up to 137 mW cm−2 at a current density of 226 mA cm−2, surpassing many carbon-based catalysts reported in the literature (Table S7). Therefore, the catalyst shows promising potential for practical applications in energy storage devices such as zinc–air batteries.
Although the catalyst presented in this study demonstrates excellent electrocatalytic activity and stability, we acknowledge that there are still some issues in both the preparation technology and practical applications. For instance, while the synthesis strategy enhances the catalyst’s performance, the graphite carbon layer on the catalyst remains relatively thick (>8 nm), which affects the catalytic activity of the metal center. Moreover, in practical applications, this could lead to high costs and complexities in experimental conditions. Additionally, in real-world environments, the long-term stability and cycling performance of the catalyst still require further validation. The preparation process should be optimized to facilitate large-scale production and application. Therefore, future work should focus on improving the catalyst’s production efficiency and stability and reducing costs.

3. Experimental Section

3.1. Preparation of NiHMT

NiHMT precursors are prepared by solvothermal reaction of nickel chloride with hexaminohexene (HMT). The specific steps are as follows: Dissolve 3.3912 g NiCl2·6H2O and 1.0 g HMT in 30 mL and 50 mL anhydrous ethanol, respectively, and then add the dissolved NiCl2·6H2O solution to the HMT solution in drops, stirring at room temperature for 2 h at a stirring speed of 400 rpm. After the reaction is complete, the solution is dried at 150 °C for 24 h to obtain a powdered NiHMT product. The reaction is carried out at room temperature, and the solvothermal reaction is performed in anhydrous ethanol solvent to ensure the stability of the Ni-HMT complex.

3.2. Preparation of Ni@NC-T

NiHMT precursors are heated to a specified temperature in a nitrogen atmosphere. The heating rate is 5 °C min−1, and the N2 flow rate is maintained at 50 mL min−1. The pyrolysis reaction takes place in the temperature range of 700 °C to 1000 °C with a reaction time of 2 h. After heating, the sample was cooled to room temperature, ground, and acid-soaked in 6 M hydrochloric acid solution for 24 h, then washed with deionized water and dried. After drying, a second grinding process was performed to obtain the final carbon-coated nickel nanoparticles, named Ni@NC-T (T stands for pyrolysis temperature). The acid-leaching process is designed to remove residual metal chloride and ensure the purity of the carbon-coated nickel nanoparticles.
HMT was selected as an organic ligand mainly because HMT, as a bidentate ligand, can coordinate with nickel ions to form a stable Ni-HMT complex. Through this coordination, HMT helps to maintain the good dispersion of nickel, avoiding the problem of metal nickel agglomeration or non-uniform particles during the pyrolysis process, ensuring that the final metal nickel has a uniform distribution and small particle size [48].

3.3. Materials Characterization

The morphology and microstructure of the samples were examined by scanning electron microscopy (SEM, Hitachi Regulus 8100, Hitachi, Chiyoda City, Japan) and transmission electron microscopy (TEM, FEI Talos 200S G2, Thermo Fisher Scientific, Waltham, MA, USA). The Energy Dispersive Spectrum (EDS) was measured by X-MaxN20 affiliated with the SEM (Oxford Instruments, Abingdon, UK). X-ray photoelectron spectroscopy (XPS) was measured on a Thermo Scientific K-Alpha (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al Kα (1486.6 eV) source, and charge shifts were corrected by using the C 1s peak of adventitious carbon at 284.8 eV. X-ray diffraction (XRD) spectra were obtained with an X-ray diffractometer (DX 2800, Haoyuan Instrument Co., Ltd., Dandong, China) using Cu Kα radiation. The organic element (C, N, and H) content of the NiHMT precursor was measured using an elemental analyzer (Thermo Scientific FLASHSMART, Waltham, MA, USA). Fourier transform infrared (FT-IR) spectroscopy was performed on a PerkinElmer infrared spectrometer (Spectrum Two PE, PerkinElmer, Waltham, MA, USA). Raman spectroscopy characterization was performed on a WITec Raman imaging spectrometer (Alpha300 access, Oxford Instruments, Abingdon, UK). The N2 adsorption–desorption isotherms were obtained at 77 K using a mesoporous analyzer (TriStar II 3020, Micromeritics Instrument Co., Norcross, GA, USA), and the specific surface area and pore size distribution were calculated.

3.4. Electrochemical Measurements

The polarization curve (LSV) and Tafel analysis were performed to reveal the catalytic performance of the catalyst at the point. Electrochemical experiments were performed on an Autolab electrochemical workstation (Multi Autolab M204, Metrohm, Herisau, Switzerland) assembly equipped with a glassy carbon (GC) rotating disk electrode (RDE, diameter 5 mm, Pine Instrument Co.). Electrochemical studies were carried out using a standard three-electrode cell with graphite rod and Ag/AgCl electrode as the counter and reference electrodes, respectively, and catalyst-loaded GC as the working electrode. The potentials obtained in this work were converted to reversible hydrogen electrode (RHE) according to the following equation:
E RHE = E Ag / AgCl + 0.197 + 0.059 × pH
ORR and OER measurements were performed in 0.1 M KOH electrolyte (pH = 13.0, measured with a pH meter). The catalyst inks were prepared by mixing 4 mg of catalyst powder with 100 μL acetone, 15 μL Nafion (5 wt%), and 385 μL deionized water and dispersing the mixture by ultrasound for 30 min. A drop of 20 μL of catalyst ink was then coated on the surface of the GC electrode and allowed to dry naturally at room temperature (loading 0.8 mg cm−2). Prior to each measurement, N2 was always blown into 0.1 M KOH solution for 30 min to purge the electrolyte, and the current density value obtained under N2-saturated conditions as a background was then subtracted from that measured in the O2-saturated electrolyte. The prepared catalysts were first subjected to CV tests in the voltage range of 0 to 1.2 V at a scan rate of 0.1 V/s. A series of LSV tests were then performed at a scan rate of 5 mV s−1 and a speed of 200 to 2500 rpm.

4. Conclusions

In summary, we have prepared the NiHMT precursor using a simple solvothermal reaction. The nickel nanoparticles encapsulated in nitrogen-doped carbon nanosheets were synthesized by pyrolysis of this MOF precursor under the N2 atmosphere. The Ni@NC-900 catalyst features nickel nanoparticles encapsulated in nitrogen-doped graphite nanosheets, with pyridinic N and graphitic N as the predominant nitrogen species, accounting for up to 70%. Remarkably, the catalyst retains the sheet-like structure of the MOF precursor, exhibiting a stacked lamellar morphology, and it has a hierarchical mesoporous-macropore structure with a specific surface area of up to 409 m2 g−1. In alkaline electrolytes, Ni@NC-900 exhibits ORR catalytic activity close to that of commercial Pt/C and high OER activity with an overpotential of only 430 mV at 10 mA cm−2, which is lower than that of Pt/C and most other carbon-based catalysts. The rechargeable ZAB based on Ni@NC-900 as a bifunctional cathode provides a high power density of up to 137 mW cm−2. Detailed structural and property studies reveal that the remarkable catalytic performance of Ni@NC-900 is due to synergistic interactions between multiple active sites (including carbon-coated nickel particles and N doping), which provide efficient electron transfer pathways. Moreover, the well-hierarchical pore structure and high specific surface area facilitate the exposure of potential active sites and promote mass transfer. This work presents a simple and efficient synthetic strategy for the preparation of transition metal materials encapsulated in a carbon matrix derived from MOFs intended for energy storage and conversion applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15040338/s1, Figure S1: SEM images of (a) Ni@NC-700, (b) Ni@NC-800, (c) Ni@NC-900, and (d) Ni@NC-1000. Figure S2: FT-IR spectra of Ni@NC-900. Figure S3: LSVs, K-L plots and electron transfer number n (inset) of (a, b) Ni@NC-700, (c, d) Ni@NC-800, (e, f) Ni@NC-900, (g, h) Ni@NC-1000 and (i, j) Pt/C at different rotation rates in oxygen-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1. Figure S4: OER Tafel plots of Ni@NC-900, RuO2, and Pt/C. Figure S5: The ZAB using Ni@NC-900 as the air cathode successfully powered an LED light. Table S1: Elemental analysis test results of the NiHMT precursor (Sample was tested twice). Table S2: The element contents calculated from the possible molecular formulas. Table S3: Specific surface area and pore volume of Ni@NC-T samples. Table S4: Summary of elemental composition of the synthesized catalysts calculated from XPS. Table S5: Quantitative XPS Analysis of the Ni@NC-T. Table S6: Comparison of the ORR electrocatalytic performance of Ni@NC-T and 20% Pt/C in 0.1 M KOH. Table S7. Comparison of the power density for Zn-air batteries. Refs. [49,50,51,52,53,54,55,56] are cited in Supplementary Materials.

Author Contributions

H.H.: Conceptualization, Methodology, Investigation, Writing—Original draft. J.H.: Conceptualization, Data Curation, Methodology, Writing—Original draft. G.Z.: Conceptualization, Data Curation, Methodology, Writing—Original draft, Funding Acquisition, Project Administration. S.X.: Data Curation, Formal Analysis. H.C.: Methodology, Data Curation. X.F.: Validation, Formal analysis. S.K.: Supervision, Resources, Funding Acquisition. J.T.: Supervision, Visualization. Y.T.: Supervision, Visualization. W.L.: Formal Analysis, Validation. B.F.: Formal Analysis, Validation, Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Guangdong Basic and Applied Basic Research Foundation (No. 2022A1515140108; No. 2023A1515140049; No. 2023B1515120062), Guangdong Provincial University Innovation Team Project (No. 2023KCXTD038), Key Projects of Social Science and Technology Development in Dongguan (No. 20231800936352), High-Level Talents Program (No. 2023JC10L014) of the Department of Science and Technology of Guangdong Province.

Data Availability Statement

Data are available upon request from the authors.

Acknowledgments

The authors thank the researchers in the Shiyanjia Lab (www.shiyanjia.com) for their help with XPS. The authors acknowledge the assistance of the Analytical and Testing Center of Dongguan University of Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Schematic illustration of the preparation of NiHMT and Ni@NC-T. (b,c) SEM images of NiHMT. (d) XRD patterns of NiHMT, HMT, and NiCl2·6H2O. (e) FT-IR spectra of HMT and NiHMT precursors.
Figure 1. (a) Schematic illustration of the preparation of NiHMT and Ni@NC-T. (b,c) SEM images of NiHMT. (d) XRD patterns of NiHMT, HMT, and NiCl2·6H2O. (e) FT-IR spectra of HMT and NiHMT precursors.
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Figure 2. (a) SEM and (b) TEM images of the Ni@NC-900 sample. (c) HR-TEM image of a single nickel nanoparticle encapsulated in graphitic carbon (inset: selected area electron diffraction). (d) HAADF-STEM image and EDX mapping images of Ni@NC-900.
Figure 2. (a) SEM and (b) TEM images of the Ni@NC-900 sample. (c) HR-TEM image of a single nickel nanoparticle encapsulated in graphitic carbon (inset: selected area electron diffraction). (d) HAADF-STEM image and EDX mapping images of Ni@NC-900.
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Figure 3. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption–desorption isothermal curves, and (d) pore size distributions of Ni@NC-T prepared at different temperatures.
Figure 3. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption–desorption isothermal curves, and (d) pore size distributions of Ni@NC-T prepared at different temperatures.
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Figure 4. (a) XPS survey, (b) high-resolution N 1s spectra, and (c) high-resolution Ni 2p spectra of the Ni@NC-T samples.
Figure 4. (a) XPS survey, (b) high-resolution N 1s spectra, and (c) high-resolution Ni 2p spectra of the Ni@NC-T samples.
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Figure 5. (a) ORR polarization curves, (b) Tafel plots, (c) Koutecky–Levic plots of Ni@NC-900 and electron transfer number n (inset) of all as-prepared catalysts and 20 wt% Pt/C. (d) OER polarization curves of Ni@NC-900, Pt/C, and RuO2.
Figure 5. (a) ORR polarization curves, (b) Tafel plots, (c) Koutecky–Levic plots of Ni@NC-900 and electron transfer number n (inset) of all as-prepared catalysts and 20 wt% Pt/C. (d) OER polarization curves of Ni@NC-900, Pt/C, and RuO2.
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Figure 6. (a) Chronoamperometric responses for the Ni@NC and Pt/C at 0.7 V (900 rpm), (b) with the addition of 3 M methanol at about 300 s. (c) Open-circuit voltage versus time for zinc–air battery with Ni@NC-900 and 20% Pt/C air cathodes, respectively. (d) Discharge polarization and power density curves of the Ni@NC-900 and commercial Pt/C.
Figure 6. (a) Chronoamperometric responses for the Ni@NC and Pt/C at 0.7 V (900 rpm), (b) with the addition of 3 M methanol at about 300 s. (c) Open-circuit voltage versus time for zinc–air battery with Ni@NC-900 and 20% Pt/C air cathodes, respectively. (d) Discharge polarization and power density curves of the Ni@NC-900 and commercial Pt/C.
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Huang, H.; Huang, J.; Zhong, G.; Xu, S.; Chen, H.; Fu, X.; Kang, S.; Tu, J.; Tuo, Y.; Liao, W.; et al. Carbon-Encapsulated Ni Nanoparticles Catalysts Derived from Ni-Hexamine Coordination Frameworks for Oxygen Reduction Reaction and Oxygen Evolution Reaction. Catalysts 2025, 15, 338. https://doi.org/10.3390/catal15040338

AMA Style

Huang H, Huang J, Zhong G, Xu S, Chen H, Fu X, Kang S, Tu J, Tuo Y, Liao W, et al. Carbon-Encapsulated Ni Nanoparticles Catalysts Derived from Ni-Hexamine Coordination Frameworks for Oxygen Reduction Reaction and Oxygen Evolution Reaction. Catalysts. 2025; 15(4):338. https://doi.org/10.3390/catal15040338

Chicago/Turabian Style

Huang, Huoxing, Jiaxing Huang, Guoyu Zhong, Shurui Xu, Hongwei Chen, Xiaobo Fu, Shimin Kang, Junling Tu, Yongxiao Tuo, Wenbo Liao, and et al. 2025. "Carbon-Encapsulated Ni Nanoparticles Catalysts Derived from Ni-Hexamine Coordination Frameworks for Oxygen Reduction Reaction and Oxygen Evolution Reaction" Catalysts 15, no. 4: 338. https://doi.org/10.3390/catal15040338

APA Style

Huang, H., Huang, J., Zhong, G., Xu, S., Chen, H., Fu, X., Kang, S., Tu, J., Tuo, Y., Liao, W., & Fang, B. (2025). Carbon-Encapsulated Ni Nanoparticles Catalysts Derived from Ni-Hexamine Coordination Frameworks for Oxygen Reduction Reaction and Oxygen Evolution Reaction. Catalysts, 15(4), 338. https://doi.org/10.3390/catal15040338

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