English

• With the rapid expansion of the global economy and improved living standards, increasing focus is being placed on green and sustainable energy technologies [1–4]. Hydrogen energy, recognized as a promising alternative to fossil fuels, has been extensively studied for its potential to mitigate urban pollution and reduce carbon emissions [5–10]. Among various hydrogen production methods, the electrolysis of water is particularly advantageous owing to its low energy consumption and zero emissions [11–14]. Noble metal-based catalysts, especially platinum (Pt), are widely used in the hydrogen evolution reaction (HER) for their exceptional activity and selectivity [15]. However, the scarcity and high cost of Pt significantly limit its large-scale practical applications [16]. Despite their excellent electrocatalytic performance, Pt-based catalysts often face structural stability issues, such as leaching and aggregation, due to their high surface energy under the working conditions. Thus, the research on the cost-effective, high-performance, and stable metal-carbon nanomaterials with low Pt content is of great importance for water splitting.

  Non-noblemetal electrocatalysts, particularly those based on earth-abundant transition metals like Fe, Co, and Ni, have been widely studied as promising alternatives for the HER [17–19]. Research has demonstrated that transition metal-based carbon materials are effective HER catalysts, with Ni-based nanomaterials receiving considerable attention because of their Pt-like electronic configuration and strong electrocatalytic activity [20]. For instance, Cheng and co-workers designed a Ni-based metal-organic framework that successfully grown at the local metal catalytic sites during the thermal decomposition and enhance catalytic HER efficiency [21]. Moreover, bimetallic materials often exhibit superior catalytic properties compared to monometallic ones as a result of synergistic effects [22,23]. The combination of Pt and Ni will promote electron transfer, simultaneously optimizing Pt utilization and enhancing catalytic activity [24]. For example, Xu and colleagues synthesized PtNi alloy through a simple solvothermal method, yielding a highly efficient HER catalyst [25]. It remains a great challenge of leveraging the different interactions between Ni-based coordination polymer and Pt-based species, which leads to the formation of various PtNi alloys that further enhance the overall catalytic performance.

  This study presents the facile synthesis of PtNi nanoparticles embedded into N-doped porous carbons using different Pt inclusion methods. Specifically, two types of bimetallic Pt_inNi-ABDC (5-aminoisophthalate) and Pt_exNi-ABDC were obtained via in-situ and ex-situ Pt doping of Ni-ABDC coordination polymers, respectively. After carbonization, the PtNi alloys were integrated in graphitic carbon layers, with Pt_inNi-NC exhibiting exceptional catalytic properties and considerable durability in alkaline media during the HER. The optimal Pt_inNi-NC demonstrated a fast and imposing HER response in the potential range of -0.5∼0.0 V vs. RHE, which just required a small overpotential of 29 mV at 10 mA/cm² in 1.0 mol/L KOH, even surpassing commercial 20 wt% Pt/C. The synergistic effects of the hierarchically porous carbon structures, multi-atom doping, and abundant bimetallic alloys will enhance the activity, stability, and cost-effectiveness of Pt_inNi-NC, effectively reducing the amount of precious Pt required. The current study can be further extended into other transition metal-based coordination polymer-derived metal-carbon nanomaterials, which are potentially utilized in the energy conversion and storage technologies.

  First of all, nickel acetate and 5-aminoisophthalic acid (H₂ABDC) are used to synthesize strip-shaped coordination polymers, labeled as Ni-ABDC (Table S1 in Supporting information). For detailed information on the preparation process of precursors, please see the Experimental section in Supporting information. There are one deprotonated ABDC^2- linker and two terminally coordinated water molecules in the asymmetric unit of Ni-ABDC (Fig. S1 in Supporting information). In the single-crystal structure of Ni-ABDC, each ABDC^2- coordinates with three neighbouring Ni(Ⅱ) centers using its two carboxylates and one NH₂ group, forming a 6-coordinated octahedral geometry (Fig. S2 in Supporting information). In Fig. S3 (Supporting information), it is further extended by bridging ABDC^2- to achieve the 1-dimensional chain along the c-axis. Notably, some intermolecular interactions are clearly observed in this 3-dimensional supermolecule, including pi-pi stacking and hydrogen bonding (Fig. S4 in Supporting information). These intermolecular forces promote dense packing of the nitrogen atoms in NH₂ groups, enhancing their interaction with guest molecules such as K₂PtCl₄ [26]. Through both in-situ and ex-situ inclusion methods, two types of Pt-doped bimetallic precursors were successfully synthesized, denoted as Pt_inNi-ABDC and Pt_exNi-ABDC, respectively (Figs. 1a and b). Fig. 1c displays four sharp and intense diffraction peaks of (011), (101), (101), and (020) in the as-synthesized Ni-ABDC and Pt_exNi-ABDC, indicating their high crystallinity and phase purity. However, Pt_inNi-ABDC exhibits no characteristic peak likely due to structural destruction by in-situ doping of PtCl₄^2-. FT-IR spectra in Fig. 1d contain signals associated with the COO-M²⁺ (1610-1560 cm^-1) to manifest the coordination between carboxylate and Ni(Ⅱ). As for Ni-ABDC, the N-H stretching vibration peak at 3295 cm^-1 is clearly observed, but it decreases dramatically for in-situ Pt-doped sample. In Fig. 1e, thermogravimetric analysis (TGA) shows that Pt_inNi-ABDC has good thermal stability up to around 400 ℃. This composite starts to decompose and stabilize between 400 ℃ and 600 ℃, with complete carbonization occurring at 1000 ℃. As a result, an optimized temperature of 600 ℃ was selected for the preparation of PtNi-based carbon nanomaterials to avoid severe particle agglomeration with maximum of active sites.

  Figure 1

  

  Figure 1.  (a, b) Two Pt inclusion methods to obtain Pt-doped Ni-ABDC. (c) PXRD patterns, (d) FT-IR spectra, (e) TGA curves of Ni-ABDC, Pt_inNi-ABDC and Pt_exNi-ABDC.

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  With two above inclusion methods, Pt(Ⅱ) atoms have been successfully incorporated in Ni-ABDC through competitive coordination, leveraging various intermolecular forces. Notably, the original microporous environment in the initial Ni-ABDC structure can be well retained with the ex-situ doping method. In contrast, the in-situ doping significantly altered the morphology and structure of the coordination polymers, as Pt atoms strongly coordinated with basic amino groups, thus disrupting the effective growth of Ni-ABDC [27,28]. The morphology of all prepared samples were characterized using the scanning electron microscopy (SEM) images. The pre-fabricated Ni-ABDC owns a strip-like shape with a smooth surface, and Pt_exNi-ABDC shows the preserved structure (Figs. 2a and b, Figs. S5 and S6 in Supporting information). On accounts of the influence of different intermolecular forces, the in-situ formed Pt_inNi-ABDC forms an amorphous nanostructure (Fig. 2c and Fig. S7 in Supporting information). Following carbonization, the Ni-NC material displays a lamellar structure with numerous nickel nanoparticles uniformly dispersed on N-doped carbon strips, and the morphology of Pt_exNi-ABDC is largely reserved (Figs. 2d and e, Figs. S8 and S9 in Supporting information). However, the SEM image of Pt_inNi-NC material presents the rough porous structure after carbonization, which is a typical phenomena calcined from amorphous precursor (Fig. 2f and Fig. S10 in Supporting information). In Figs. 2g and h, it clearly shows that the obvious lattice fringe spacings of 0.223 nm are consistent with the (111) plane of the PtNi alloy. The collected energy-dispersive X-ray spectroscopy (EDS) pattern, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, and elemental mappings of Pt_inNi-NC confirm the uniform distribution of C, N, Ni, and Pt (Fig. 2j). This Pt_inNi-NC series is featured with abundant bimetallic particles and a porous nanostructure that will provide favorable pathway for subsequent catalytic reactions. In contrast, the control sample Pt_exNi-NC shows a significantly lower concentration of Pt in the in-situ formed PtNi alloy; for more details please see Fig. S11 (Supporting information).

  Figure 2

  

  Figure 2.  SEM images of (a) Ni-ABDC, (b) Pt_exNi-ABDC, (c) Pt_inNi-ABDC, (d) Ni-NC, (e) Pt_exNi-NC, (f) Pt_inNi-NC. (g, h) HRTEM images, (i) EDS pattern, (j) HAADF-STEM and elemental mapping images of Pt_inNi-NC.

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  In Fig. 3a, all these Ni-ABDC-derived carbon materials exhibit Type-Ⅲ adsorption-desorption curves. It is indicative of that both in-situ and ex-situ Pt doping pose an influence the porous structure of these carbon nanomaterials. The corresponding Brunauer-Emmett-Teller (BET) surface areas for Ni-NC, Pt_inNi-NC and Pt_exNi-NC are calculated to be 125.17, 116.75, 165.67 m²/g, respectively (Table S2 in Supporting information). Meanwhile, the pore size distribution (PSD) curves reveal that Ni-NC, Pt_inNi-NC and Pt_exNi-NC are mesoporous materials, suggesting that the obtained Pt_inNi-NC and Pt_exNi-NC are intrinsically endowed with abundant structural defects, large surface area, and mesopores, which potentially lead to enhanced electrochemical activity [29,30]. Similarly, these Pt-doped carbon nanomaterials show a similar trend to those of their precursors (Fig. S12 in Supporting information). In Fig. 3b, the PXRD pattern reveals two characteristic peaks at 43.9° and 51.2° in Pt_inNi-NC and Pt_exNi-NC samples, corresponding to the (111) and (200) planes of bimetallic PtNi (PDF #97-064-6295), respectively. Within the enlarged 2θ region of 40°-55°, it is noted that the more Pt-doped Pt_inNi-NC presents diffraction peaks close to bimetallic PtNi phase. In contrast, the pure Ni-NC sample showcases three distinct diffraction peaks of 44.5°, 51.8° and 76.3°, assigned to the metallic Ni phase (PDF #04-0850). Fig. 3c presents the Raman spectra with two prominent peaks at approximately 1365 cm^-1 and 1600 cm^-1, which are ascribed to the D peak (caused by defects and disorder in the carbon matrix) and the G peak (associated with ordered graphitic carbon), respectively. The calculated I_D/I_G ratio of 0.97 for Pt_inNi-NC is higher than that of Pt_exNi-NC (0.94) and Ni-NC (0.89), indicating that the in-situ doping of Pt species disrupts coordination environment, reduces graphitization, and creates more defects. In this case, these structural defects in PtNi-carbon nanomaterials will expose additional active sites and promote subsequent electrocatalytic activity.

  Figure 3

  

  Figure 3.  (a) N₂ isotherms and PSD curves. (b) PXRD patterns. (c) Raman spectra of Ni-NC, Pt_exNi-NC and Pt_inNi-NC.

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  X-ray photoelectron spectroscopy (XPS) was performed to examine the electronic structure and coordination environment of Ni-NC, Pt_inNi-NC and Pt_exNi-NC. The full survey spectra confirm the coexistence of Ni, C and N in all samples, while Pt can be detected only in Pt_inNi-NC and Pt_exNi-NC, consistent with the EDS data (Fig. 4a). The high-resolution C 1s spectra clearly give a broad peak which is divided into four components at 284.4, 285.6, 286.8, and 288.9 eV in Fig. 4b, belonging to C-C sp³, C-N/C-O, and C=N/C=O bonds, respectively. In Fig. S13 (Supporting information), the high-resolution N 1s spectra verify the presence of four chemical environments: pyridinic N (398.8 eV), pyrrolic N (400.5 eV), graphitic N (401.6 eV) and oxidized N (402.9 eV). In this case, residual nitrogen is preserved in more stable form after high temperature calcination, further confirming that N heteroatoms are embedded into the carbon matrix. In Ni 2p spectra, the Ni 2p_3/2 peaks at 853.4 eV and the Ni 2p_1/2 peaks at 870.1 eV are ascribed to metallic Ni(0), suggesting a significant presence of Ni(0) in Pt_inNi-NC (Fig. 4c) [31]. While two additional peaks at 855.8 eV and 872.2 eV belong to Ni(Ⅱ) 2p_3/2 and Ni(Ⅱ) 2p_1/2, respectively, along with two satellite peaks at 857.6 eV and 873.8 eV. In the deconvoluted Pt 4f spectra, the metallic state is indicated by peaks at 71.4 eV (Pt 4f_7/2) and 74.6 eV (Pt 4f_5/2), and the oxidized state by peaks at 72.3 eV (Pt 4f_7/2) and 75.5 eV (Pt 4f_5/2) in Pt_inNi-NC (Fig. 4d). However, in Pt_exNi-NC, there are no obvious peaks of Pt 4f likely due to the relatively small amount of Pt doping into precursor. This XPS analysis confirms the successful encapsulation of in-situ formed PtNi nanoparticles within the N-doped carbon derived from Ni-ABDC, especially in the Pt_inNi-NC sample.

  Figure 4

  

  Figure 4.  (a) Full survey XPS spectra, and the deconvoluted spectra of (b) C 1s, (c) Ni 2p, and (d) Pt 4f of Ni-NC, Pt_exNi-NC and Pt_inNi-NC.

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  The electrocatalytic activity of these PtNi-carbon nanomaterials was studied using a three-electrode system in 1.0 mol/L KOH. In order to prevent the platinum contamination, a graphite rod served as the counter electrode [32]. The linear sweep voltammetry (LSV) curves for all electrocatalysts, including commercial Pt/C, were recorded at ambient temperature with a scan rate of 5 mV/s (Fig. 5a and Fig. S14 in Supporting information). The results indicate that the effective Pt incorporation in Pt_exNi-NC and Pt_inNi-NC improves their HER activity compared to Ni-NC. Remarkably, Pt_inNi-NC exhibits a lower overpotential (η₁₀) of 29 mV to achieve a current density of 10 mA/cm², surpassing Ni-NC (278 mV), Pt_exNi-NC (260 mV), and Pt/C (37 mV), highlighting its superior HER performance. The Tafel plots (Fig. 5b) further reinforce these findings, with Pt_inNi-NC showing a much smaller Tafel slope (47.4 mV/dec) compared to Ni-NC (277.2 mV/dec), Pt_exNi-NC (228.7 mV/dec), and Pt/C (47.9 mV/dec). This suggests that the synergistic interaction between Ni and Pt active sites effectively reduces the overpotential of hydrogen evolution [33,34]. The electrochemically active surface area (ECSA), estimated from the double-layer capacitance (C_dl), also points to the superior activity of Pt_inNi-NC (Fig. S15 in Supporting information). As shown in Fig. 5c, the C_dl value of Pt_inNi-NC is 4.47 mF/cm², significantly higher than Pt_exNi-NC (3.68 mF/cm²), Ni-NC (0.9 mF/cm²), and Pt/C (12.16 mF/cm²), indicating that Pt_inNi-NC exposes more active sites. In Fig. 5d and Fig. S16 (Supporting information), Nyquist plots reveal that Pt_inNi-NC has the smallest semicircle diameter at high frequencies (4.1 Ω), indicating a lower charge transfer resistance compared to Ni-NC (9.91 Ω), Pt_exNi-NC (4.62 Ω), and Pt/C (8.2 Ω). Furthermore, the catalytic stability of Pt_inNi-NC was assessed via Amperometric measurements at a constant potential of 1.65 V vs. RHE. It is obviously that Pt_inNi-NC exhibits superior stability compared to Pt/C (Fig. 5e). After 200 h of continuous electrolysis, the optimized Pt_inNi-NC retains a high retention of 91.2% of its initial activity at 10 mA/cm², with only a slight increase in overpotential of 21 mV, indicating excellent long-term durability (Fig. 5f).

  Figure 5

  

  Figure 5.  (a) LSV curves, (b) Tafel slopes, (c) C_dl profiles and (d) EIS curves of Pt/C, Ni-NC, Pt_exNi-NC and Pt_inNi-NC. (e) The i-t stability test, and (f) LSV curves of Pt_inNi-NC before and after 200 h.

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  Ordinarily, the relatively minor decline in the activity of materials could be ascribed to the corrosion of the nanocatalyst particles. These particles, being sustained by the porous carbon framework, are affected during the process of electrolysis in alkaline medium. In order to further confirm the structural robustness of our carbon catalysts, we have carried out several characterizations on the catalysts after prolonged measurements. It can be found that after the HER durability test, TEM images show that the in-situ formed PtNi alloy nanoparticles in the Pt_inNi-NC material were found to be well retained with a partial decomposition (Fig. S17a in Supporting information), which reasonably led to a small decline in electrocatalytic performance. In addition, the weakened characteristic peaks in the obtained PXRD pattern are also observed, indicating that the catalyst retains the initial composition (Fig. S17b in Supporting information). As shown in Figs. S17c-f (Supporting information), the collected XPS spectra showed four elements C, N, Ni and Pt to verify that the element composition of Pt_inNi-NC did not change after the long-term cycle test. It has been found that when the deconvoluted C 1s, N 1s, Ni 2p, and Pt 4f spectra of Pt_inNi-NC are scrutinized before and after the HER cycle test, only minute changes are observable. This observation strongly indicates that the surface electronic states of Pt_inNi-NC maintain a high level of consistency [35,36]. In addition, through a comparative analysis of the experimentally measured hydrogen quantity and the theoretically computed hydrogen amount, the Faradaic efficiency can be obtained, in which cathodic electrolysis is performed at 10 mA/cm² in 1.0 mol/L KOH solution. The experimental results show that the actual hydrogen production rate is consistent with the calculated value, exhibiting a high Faradaic efficiency of 95.1%–98.6% in Fig. S18 and Table S5 (Supporting information).

  In summary, two Pt_inNi-ABDC and Pt_exNi-ABDC coordination polymer precursors were successfully synthesized using both in-situ and ex-situ inclusion methods. The in-situ approach significantly disrupted the effective growth of Ni-ABDC nanostrips for the strong interaction between Pt and ABDC, resulting in an amorphous nanostructure. After pyrolysis, the calcined Pt_inNi-NC was featured with a rough porous material, exhibiting a high concentration of bimetallic PtNi nanoparticles uniformly distributed across N-doped carbon strips. The strong synergistic effect between Pt and Ni atoms enabled Pt_inNi-NC to realize satisfactory HER performance. It showed a low η₁₀ value of 29 mV, a Tafel slope of 47.4 mV/dec, and a robust stability with negligible current drop after 200 h, superior to three control samples. The present work underscores the rational design of Pt-doped transition metal-carbon nanomaterials to achieve competitive electrocatalytic HER activity and durability.

  Declaration of competing interest

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

  CRediT authorship contribution statement

  Yuting Fu: Writing – original draft, Validation, Methodology, Formal analysis, Data curation. Haoran Wang: Validation, Formal analysis. Nan Li: Validation, Formal analysis. Lujiao Mao: Formal analysis. Xusheng Wang: Validation, Formal analysis. Qipeng Li: Supervision, Funding acquisition. Jinjie Qian: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

  Acknowledgments

  This work was financially supported by National Natural Science Foundation of China (No. 21601137), Basic Science and Technology Research Project of Wenzhou, Zhejiang Province (No. G20240038), and the Special Basic Cooperative Research Programs of Yunnan Provincial Undergraduate Universities Association (Nos. 202101BA070001-031, 202101BA070001-042 and 202301BA070001-093), Yunnan Province Young and Middle-aged Academic and Technical Leaders Reserve Talent Project (No. 202105AC160060) and Yunnan Province High-level Talent Training Support Program “Youth Top Talent” Project (2020).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110890.

  
  
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  Figure 1  (a, b) Two Pt inclusion methods to obtain Pt-doped Ni-ABDC. (c) PXRD patterns, (d) FT-IR spectra, (e) TGA curves of Ni-ABDC, Pt_inNi-ABDC and Pt_exNi-ABDC.

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  Figure 2  SEM images of (a) Ni-ABDC, (b) Pt_exNi-ABDC, (c) Pt_inNi-ABDC, (d) Ni-NC, (e) Pt_exNi-NC, (f) Pt_inNi-NC. (g, h) HRTEM images, (i) EDS pattern, (j) HAADF-STEM and elemental mapping images of Pt_inNi-NC.

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  Figure 3  (a) N₂ isotherms and PSD curves. (b) PXRD patterns. (c) Raman spectra of Ni-NC, Pt_exNi-NC and Pt_inNi-NC.

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  Figure 4  (a) Full survey XPS spectra, and the deconvoluted spectra of (b) C 1s, (c) Ni 2p, and (d) Pt 4f of Ni-NC, Pt_exNi-NC and Pt_inNi-NC.

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  Figure 5  (a) LSV curves, (b) Tafel slopes, (c) C_dl profiles and (d) EIS curves of Pt/C, Ni-NC, Pt_exNi-NC and Pt_inNi-NC. (e) The i-t stability test, and (f) LSV curves of Pt_inNi-NC before and after 200 h.

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