English

• 0.   Introduction

  In recent years, the global economy has experienced rapid growth alongside a population increase, which has intensified the energy demand. However, the extraction of traditional energy resources has become increasingly challenging, resulting in significant environmental pollution. The development of new clean energy sources is considered a crucial strategy for addressing environmental issues^[1-5]. Among alternative energy sources, hydrogen stands out due to its unique properties. Currently, water electrolysis for hydrogen production is regarded as a promising method. The oxygen evolution reaction (OER) is the rate-determining step (RDS) in the water electrolysis process, and its sluggish kinetics lead to low energy efficiency^[6-8]. While noble metals such as ruthenium (Ru) and iridium (Ir) exhibit excellent catalytic performance for OER, their high costs make them unsuitable for large-scale applications. Consequently, researchers are actively pursuing more cost-effective alternative materials^[9-12].

  Transition metal oxides, hydroxides, sulfides, and phosphides are regarded as promising alternatives on account of their relatively low cost and abundant resources^[13]. These materials can be optimized for catalytic performance through modifying their chemical composition and structure. Among these materials, metal-organic frameworks (MOFs), which are self-assembled polymeric ligands formed by metal ions and organic ligands, have witnessed rapid development in recent decades^[14-15]. Owing to their large surface area, porosity, abundant metal sites, and topological diversity, MOFs have been extensively utilized in sensing, medicine, and catalysis^[16-20]. Additionally, electrical conductivity is a crucial factor in evaluating catalyst performance. Electrocatalysts with high electrical conductivity possess higher electron transfer efficiency and display outstanding catalytic performance^[21-28]. Compared with other metal catalysts, MOFs generally have lower electrical conductivity, mainly attributed to their distinctive metal-inorganic structure. The primary strategy for enhancing the electrical conductivity of MOFs at present is high-temperature calcination. However, high temperatures can cause damage to the active metal sites of MOFs. Additionally, it is regrettable that the oxides based on Ni or Co, which are the most active, it is estimated that the TOFs of each metal site are at least one order of magnitude lower than those of the TOFs in the OER complexes in biological systems^[29-33]. Therefore, designing MOF materials with high activity and stability was the current focus of work.

  In this work, we present the synthesis of a novel Co-MOF {[Co₂(BINDI)(DMA)₂]·DMA}_n (H₄BINDI=N,N′-bis(5-isophthalic acid)naphthalenediimide, DMA=N,N-dimethylacetamide), using naphthalene diimide (NDI) as the ligand and Co²⁺ ions as metal nodes. Furthermore, pyrene (Py), an electron donor, was effectively encapsulated within the porous structure of the Co-MOF through host-guest interactions, resulting in the self-assembled Py@Co-MOF composite. A comprehensive investigation of the catalytic performance of both Co-MOF and Py@Co-MOF electrodes revealed that the latter exhibited enhanced catalytic current and durability in the OER, significantly outperforming the Co-MOF without pyrene. These findings indicate that the presence of coordinated DMA within the Co-MOF promotes the formation of open metal sites (OMS), thereby enhancing catalytic activity. Additionally, the strong non-covalent π-π interactions between pyrene and the Co-MOF are believed to facilitate rapid electron transfer from the electrode to the Co-MOF, contributing to the improved stability of the Co-MOF-based electrode, as illustrated in Scheme 1.

  Scheme 1

  

  Scheme 1.  Illustration of the synthesis process of Py@Co-MOF and the three-electrode system for OER

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  1.   Experimental

  Materials and methods are listed in the Supporting information.

  1.1   Synthesis of Co-MOF and Py@Co-MOF

  A mixture of ligand H₄BINDI (3 mg) and Co(NO₃)₂·6H₂O (3.0 mg) was dissolved in 2.0 mL DMA. The resulting mixture was stirred for 0.5 h before being transferred to a 25 mL Teflon-lined autoclave to undergo heating at 120 ℃ for 24 h. Purplish-red bulk crystals of Co-MOF were obtained after the mixture was cooled slowly to room temperature. Anal. Calcd. for C₂₃H₂₃CoN₃O₈ (%): C, 52.28; H, 4.38; N, 7.95. Found (%): C, 52.72; H, 4.20; N, 7.59. IR (KBr, cm^-1): 3 385 (s), 2 891 (m), 1 718 (s), 1 680 (w), 1 448 (m), 1 397 (m), 1 251 (w), 1 113 (m), 1 113 (w), 770 (w), 718 (w), 651 (m).

  The synthesis step of Py@Co-MOF was similar to that of Co-MOF, with a slight modification. A mixture of H₄BINDI (1.5 mg), Co(NO₃)₂·6H₂O (1.5 mg), and pyrene (6.0 mg) was dissolved in 2.0 mL DMA. The resulting mixture was stirred for 0.5 h before being transferred to a 25 mL Teflon-lined autoclave to undergo heating at 115 ℃ for 3 d. Red crystals were obtained after the mixture had cooled slowly to room temperature.

  1.2   Electrochemistry measurements

  All electrochemical measurements were conducted at room temperature using a CHI760E workstation. The performance of the OER was assessed by employing a standard three-electrode electrochemical system in an alkaline electrolyte (1.0 mol·L^-1 KOH). Co-MOF and Py@Co-MOF/NF samples, each with an area of 1 cm×1 cm, were adopted as working electrodes. The counter electrode and reference electrode were a platinum sheet and an Ag/AgCl electrode, respectively. The electrode was fabricated by dispersing the sample, PVDF, and carbon black in an ethanol solution at a mass ratio of 8∶1∶1, followed by ultrasonic oscillation for 30 min to obtain a homogeneous suspension. Subsequently, this suspension was deposited onto nickel foam and then dried and weighed for subsequent OER testing.

  1.3   Crystal structure determination

  Single-crystal X-ray diffraction data were collected on a XtaLAB (Rigaku synergy S) at approximately 300 K with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm). The crystal structures were ascertained by direct methods and then underwent full-matrix least-squares refinement using the Olex2 software suite. In the final refinement cycles, non-hydrogen atoms were dealt with anisotropic displacement parameters. Organic hydrogen atoms were positioned in calculated locations, and their isotropic displacement parameters were set to 1.2U_eq of the attached atom. Comprehensive crystallographic details, including refinement parameters and experimental conditions, have been systematically compiled in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinements for Co-MOF

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  Parameter	Co-MOF		Parameter	Co-MOF
  Empirical formula	C19H14CoN2O7·C4H9NO		μ / mm-1	0.617
  Formula weight	528.37		F(000)	1 180
  Temperature / K	297		θ range for data collection / (°)	4.032-49.97
  Crystal system	Orthorhombic		Index ranges	-43 ≤ h ≤ 18, -17 ≤ k ≤ 10, -12 ≤ l ≤ 7
  Space group	Imma		Reflection collected	8 006
  a / nm	3.695 85(17)		Independent reflection	2 694 (Rint=0.072 6, Rσ=0.056 4)
  b / nm	1.504 41(8)		Data, number of restraints, number of parameters	2 694, 36, 162
  c / nm	1.050 32(5)		Goodness-of-fit on F2	1.062
  Volume / nm3	5.839 9(5)		Final R indexes [I≥2σ(I)]	R1=0.064 5, wR2=0.182 0
  Z	8		Final R indexes (all data)	R1=0.082 5, wR2=0.202 2
  Dc / (g·cm-3)	1.004		Largest diff. peak and hole / (e·nm-3)	620, -800

  2.   Results and discussion

  2.1   Structural description of Co-MOF

  Single-crystal X-ray diffraction studies show that Co-MOF crystallizes in an orthorhombic crystal system with an Imma space group (Table 1). As shown in Fig.1, the asymmetric unit in Co-MOF contains one crystallographically independent Co²⁺ ion and half a BINDI^4- ligand, one coordinated DMA molecule, and half a lattice DMA molecule (Fig.1a). Co1 is coordinated with one O atom from a DMA molecule and four O atoms from the four different BINDI^4- ligands. The two adjacent Co²⁺ ions are linked by four carboxylate groups to form the paddle-wheel secondary building unit (SBU) [Co₂(COO)₄], which further allows for the formation of extended 2D networks through the expansion of BINDI^4- ligands (Fig.1b). This 2D networks along the a-axis by linking with neighboring carboxylate oxygen atoms on BINDI^4- ligands, creating a 3D framework with the potential porosity of 24.4% after removing the free solvents, according to the free software of PLATON. Furthermore, the structural features of Co-MOF, such as relevant bond lengths (nm), bond angles (°), and π-π stacking interactions, have been compiled in Table S1 and S2.

  Figure 1

  

  Figure 1.  (a) Coordination environment of Co-MOF; (b) 2D network of Co-MOF; (c, d) 3D framework of Co-MOF; (e) 3D fsc topology of Co-MOF

  Red stands for the O atom, blue for the N atom, purple for the Co atom, and gray for the C atom; Thermal ellipsoids are drawn at the 50% probability level; Symmetry codes: A: 1-x, 1-y, 1-z; B: x, 1-y, 1-z; C: 1-x, y, z; D: 0.5-x, y, 0.5-z; E: 0.5-x, 1.5-y, 0.5-z; F: x, 1.5-y, z.

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  From a topological perspective, when the paddle-wheel [Co₂(COO)₄] cluster, BINDI^4- ligands could be simplified as a 4-c, the whole framework of Co-MOF can be viewed as a 4-node 3D fsc framework with the point symbols of {(4⁴·6¹⁰·8)(4⁴·6²)} (Fig.1c-1e).

  2.2   Characterization of Co-MOF

  The powder X-ray diffraction (PXRD) patterns manifested the resemblance of the main framework of Co-MOF and Py@Co-MOF (Fig.S1). It should be emphasized that the key peak positions of Co-MOF were completely consistent with the simulation one, which indicates that Co-MOF has been prepared with good phase purity. In addition, a nearly identical trend of thermogravimetric analysis (TGA) curves for Co-MOF and Py@Co-MOF further verified the similar skeleton of the materials. As shown in Fig.S2, the observed 9.00% weight loss below 173 ℃ can be attributed to the desorption of approximately half of the adsorbate DMA molecules initially confined within the porous structure and surface of the material. It is worth noting that, compared with Co-MOF, the weightlessness of Py@Co-MOF decreased when the temperature was higher than 400 ℃. It was confirmed that some DMA solvent molecules were replaced by pyrene guest molecules.

  2.3   Electrocatalytic performance

  The as-prepared samples were evaluated for their OER activity by the linear scan voltammogram (LSV) in a 1.0 mol·L^-1 KOH solution. As shown in Fig.2, the Py@Co-MOF/NF samples exhibited lower overpotentials and smaller Tafel slopes compared to both the Co-MOF/NF and Py/NF electrodes. Notably, Py@Co-MOF/NF demonstrated superior electrocatalytic performance. At a current density of 10 mA·cm^-2, the overpotentials were measured at 246 mV for Py@Co-MOF/NF, 261 mV for Co-MOF/NF, and 279 mV for Py/NF (Fig.2a). The overpotentials of Py@Co-MOF prior most of the MOF-based materials. Similarly, the Tafel slope for Py@Co-MOF/NF was determined to be 54.8 mV·dec^-1, which was lower than those observed for Co-MOF/NF (57.2 mV·dec^-1) and Py/NF (70.4 mV·dec^-1) (Fig.2b). These remarkable outcomes imply that Py@ Co-MOF/NF manifests a notably diminished degree of polarization loss and exhibits more highly efficient kinetics during the process of water oxidation. The as-prepared Py@Co-MOF/NF is also a comparable active catalyst for OER with the overpotential and potential difference at 10 mA·cm^-2 between the as-prepared catalyst (Table 2)^[34-40]. The observed phenomenon suggests that Py@Co-MOF possesses superior attributes and capabilities in the domain associated with OER reaction, which might potentially give rise to significant advancements and enhancements in related applications and research fields.

  Figure 2

  

  Figure 2.  OER LSV curves (a) and Tafel plots (b) of Py@Co-MOF/NF-, Co-MOF/NF-, Py/NF-, and bare NF-based electrode as characterized in 1.0 mol·L^-1 KOH aqueous electrolyte

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  Table 2

  

  Table 2.  Comparison of OER performance for Py@Co-MOF with other OER electrocatalysts

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  Catalyst	Electrolyte	Current density / (mA·cm-2)	Overpotential / mV	Tafel slope / (mV·dec-1)	Ref.
  CoOx	1.0 mol·L-1 KOH	10	420	42	[34]
  FeNi/NiFe2O4@NC-800	1.0 mol·L-1 KOH	10	316	60	[35]
  Co3O4/CoMoO4 hollow nanosphere	1.0 mol·L-1 KOH	10	370	59	[36]
  MOF(Fe1-Co3)550N	1.0 mol·L-1 KOH	10	390	72.9	[37]
  NiFe LDH/carbon nanotubes	1.0 mol·L-1 KOH	10	247	—	[38]
  FeCo alloy/CNF	1.0 mol·L-1 KOH	10	557	—	[39]
  Ni60Co30M10	1.0 mol·L-1 KOH	10	287	41	[40]
  Py@Co-MOF	1.0 mol·L-1 KOH	10	246	54.8	This work

  To acquire a more distinct comprehension of the outstanding OER performance of Py@Co-MOF/NF, the electrochemical double-layer capacitance (C_dl) of the electrode material was ascertained by means of CV curves obtained at diverse scanning rates within the non-Faradaic region (Fig.3a-3c). The experimental outcomes reveal that the electrochemically active surface area (ECSA) of the three electrodes is directly proportional to the sweep rate of the double-layer capacitor. The ECSA values for Py@Co-MOF/NF, Co-MOF/NF, and bare NF were 40.3, 25.0, and 20.1 cm², respectively (Fig.3d). Notably, Py@Co-MOF/NF manifests the highest ECSA value, suggesting that the Py@Co-MOF/NF sample possesses a larger electrochemically active surface area and abundant active sites, which facilitate the enhancement of OER activity^{[12, 41-44]}. The LSV curves normalized by ECSA indicate that Py@Co-MOF/NF exhibits the highest intrinsic catalytic activity. These results further imply that the moderate transformation of Py@Co-MOF/NF can conspicuously improve the intrinsic catalytic activity of the electrocatalysts.

  Figure 3

  

  Figure 3.  CV curves of Co-MOF/NF (a), bare NF (b), and Py@Co-MOF/NF (c) for estimating the ECSA; Corresponding plots of capacitive currents vs scan rates for various OER catalysts (d)

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  Electrochemical impedance spectroscopy (EIS) was also employed to elucidate the electrocatalytic kinetics of the fabricated electrodes. The Nyquist plots reveal that the Py@Co-MOF/NF exhibited the smallest semicircle diameter, indicating the lowest charge transfer resistance (Fig.4a). Whereas it showed a larger semicircle diameter, the charge transfer ability of Py@Co-MOF/NF was significantly enhanced after the introduction of the pyrene molecules. The outcome was highly consistent with the electrocatalytic activity of these catalysts, which further proved that the introduction of pyrene molecules indeed improved the OER properties of the fabricated electrode. The outstanding catalytic activity of Py@Co-MOF/NF can be ascribed to the synergy effect of the π-π interaction between MOF and pyrene molecules in the composite and the active sites which formed by the OMS. Furthermore, the chronoamperometric technique demonstrated that Py@Co-MOF/NF maintained its performance after 24 h at a current density of 10 mA·cm^-2 (Fig.4b). The LSV curve obtained post-stability test exhibited minor deviations at higher current densities and resembled the curves obtained before the test (Fig.4c).

  Figure 4

  

  Figure 4.  (a) EIS of Co-MOF/NF-, Py@Co-MOF/NF-, and bare NF-based electrode; (b) J-t curve of Py@Co-MOF/NF-based electrode; (c) Comparison of OER LSV curve of Py@Co-MOF/NF-based electrode before and after the stability experiment after 24 h

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  2.4   Electrocatalytic mechanism

  To further elucidate the specific impact of active sites in the electrode materials, DFT was employed to calculate the electron structure of the Co-MOF model regarding the adsorption free energy of OH^*, O^*, OOH^*, and the variation of Gibbs free energy in the fundamental steps of OER. Fig.5a depicts the basic steps of the OER process on the Co-MOF. Experimental outcomes reveal that the calculated energy barriers of the three electrochemical steps are 0.09, 0.85, and 0.28 eV, respectively, in the discrete unit. Fig.5b indicates that the formation of O^* demands the highest Gibbs free energy and constitutes the RDS of OER, which is due to the OH^* intermediate has a weak adsorption affinity for the Co site, resulting in a high energy barrier of 0.85 eV for OH^* conversion to O^*. These discoveries suggest that Co is a preferred site for OER electrochemical processes and offer insights into its potential applications for enhancing water cracking efficiency in various catalytic processes. Additionally, the integration of electron-rich pyrene into electron-deficient Co-MOF induces significant charge transfer interactions in the host-guest system, which might be another crucial reason why Py@Co-MOF/NF exhibits outstanding OER properties.

  Figure 5

  

  Figure 5.  Detailed electron transfer process (a) and the plot of the Gibbs free energy of Co-MOF (b) in the OER reaction

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  3.   Conclusions

  In summary, the Py@Co-MOF composite catalyst has been developed to enhance electrical conductivity through the establishment of non-covalent π-π interactions between pyrene and Co-MOF. The OER performance of the Py@Co-MOF composite catalyst was significantly improved, achieving an overpotential of 246 mV at a current density of 10 mA·cm^-2, which was superior to both Co-MOF-based and pyrene-based electrodes, noted for their low cost and good stability. Additionally, DFT calculations indicate that the formation of O^* within the OER process of the Co-MOF catalyst constitutes the RDS, primarily due to the weak adsorption affinity of OH^* intermediates at the Co site. This results in a high energy barrier of 0.85 eV for the conversion of OH^* to O^*. Overall, the insights gained from this study provide a potential pathway for the preparation of composite MOF-based electrochemical catalysts utilizing post-synthetic modification and compositional strategies.

  
  Acknowledgements: This work was supported by the Natural Science Foundation of Shanxi Province (Grant No.202303021212288), Lüliang Key Research and Development Project for the introduction of high-level scientific and technological Talents (Grant No.2024RC29), the introduction of doctoral research start-up fund (Grant No.2210371814), and the Talent Program Foundation of Dezhou University (Grant No.2020xjrc217). Competing interests: The authors declare no competing financial interest.
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Illustration of the synthesis process of Py@Co-MOF and the three-electrode system for OER

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  Figure 1  (a) Coordination environment of Co-MOF; (b) 2D network of Co-MOF; (c, d) 3D framework of Co-MOF; (e) 3D fsc topology of Co-MOF

  Red stands for the O atom, blue for the N atom, purple for the Co atom, and gray for the C atom; Thermal ellipsoids are drawn at the 50% probability level; Symmetry codes: A: 1-x, 1-y, 1-z; B: x, 1-y, 1-z; C: 1-x, y, z; D: 0.5-x, y, 0.5-z; E: 0.5-x, 1.5-y, 0.5-z; F: x, 1.5-y, z.

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  Figure 2  OER LSV curves (a) and Tafel plots (b) of Py@Co-MOF/NF-, Co-MOF/NF-, Py/NF-, and bare NF-based electrode as characterized in 1.0 mol·L^-1 KOH aqueous electrolyte

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  Figure 3  CV curves of Co-MOF/NF (a), bare NF (b), and Py@Co-MOF/NF (c) for estimating the ECSA; Corresponding plots of capacitive currents vs scan rates for various OER catalysts (d)

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  Figure 4  (a) EIS of Co-MOF/NF-, Py@Co-MOF/NF-, and bare NF-based electrode; (b) J-t curve of Py@Co-MOF/NF-based electrode; (c) Comparison of OER LSV curve of Py@Co-MOF/NF-based electrode before and after the stability experiment after 24 h

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  Figure 5  Detailed electron transfer process (a) and the plot of the Gibbs free energy of Co-MOF (b) in the OER reaction

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  Table 1.  Crystal data and structure refinements for Co-MOF

  Parameter	Co-MOF		Parameter	Co-MOF
  Empirical formula	C19H14CoN2O7·C4H9NO		μ / mm-1	0.617
  Formula weight	528.37		F(000)	1 180
  Temperature / K	297		θ range for data collection / (°)	4.032-49.97
  Crystal system	Orthorhombic		Index ranges	-43 ≤ h ≤ 18, -17 ≤ k ≤ 10, -12 ≤ l ≤ 7
  Space group	Imma		Reflection collected	8 006
  a / nm	3.695 85(17)		Independent reflection	2 694 (Rint=0.072 6, Rσ=0.056 4)
  b / nm	1.504 41(8)		Data, number of restraints, number of parameters	2 694, 36, 162
  c / nm	1.050 32(5)		Goodness-of-fit on F2	1.062
  Volume / nm3	5.839 9(5)		Final R indexes [I≥2σ(I)]	R1=0.064 5, wR2=0.182 0
  Z	8		Final R indexes (all data)	R1=0.082 5, wR2=0.202 2
  Dc / (g·cm-3)	1.004		Largest diff. peak and hole / (e·nm-3)	620, -800

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  Table 2.  Comparison of OER performance for Py@Co-MOF with other OER electrocatalysts

  Catalyst	Electrolyte	Current density / (mA·cm-2)	Overpotential / mV	Tafel slope / (mV·dec-1)	Ref.
  CoOx	1.0 mol·L-1 KOH	10	420	42	[34]
  FeNi/NiFe2O4@NC-800	1.0 mol·L-1 KOH	10	316	60	[35]
  Co3O4/CoMoO4 hollow nanosphere	1.0 mol·L-1 KOH	10	370	59	[36]
  MOF(Fe1-Co3)550N	1.0 mol·L-1 KOH	10	390	72.9	[37]
  NiFe LDH/carbon nanotubes	1.0 mol·L-1 KOH	10	247	—	[38]
  FeCo alloy/CNF	1.0 mol·L-1 KOH	10	557	—	[39]
  Ni60Co30M10	1.0 mol·L-1 KOH	10	287	41	[40]
  Py@Co-MOF	1.0 mol·L-1 KOH	10	246	54.8	This work

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