Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction

Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction Minghua Huang, Xianbiao Hou, Tengjia Ni, Zhaozheng Zhang, Jian Zhou, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5541146/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Spin configuration and coordination environment changes have emerged as promising strategies to boost the oxygen evolution reaction (OER) activity. However, achieving the precise and gradual regulation of both spin states and coordination environment to elucidate the structure-activity relationship remains a key priority and is rarely reported. In this work, we successfully induce the gradual transition of spin states of Fe sites from low spin state to a medium spin state and ultimately to high spin state by meticulously adjusting coordination environment within NiFe-MOF, while the Ni sites still keep a low spin state. Experimental and theoretical calculations confirm the precise regulation of spin polarization and electrons migration from the Fe-t 2g to the Fe-e g orbitals with a reduced coordination saturation, which optimizes the spin orbital exchange interactions between Fe and Ni ions and facilitates adsorption of reaction intermediates. The NiFe-MOF-D 3 with unique NiO 6 -FeO 4 geometric structure exhibits low overpotential of 328 mV@1 A cm -2 and 365 [email protected] A cm -2 in alkaline medium. Furthermore, the assembled alkaline electrolyzer also presents remarkable activity (1.77 V@500 mA cm -2 ) and lower cost ($ 0.94) than the target of U.S. Department of Energy ($ 2.00). Physical sciences/Chemistry/Catalysis/Electrocatalysis Physical sciences/Chemistry/Catalysis/Catalytic mechanisms Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The electrochemical water splitting is recognized as one of the prospective technologies to achieve carbon neutrality in the coming decades via producing the high-pure and eco-friendly hydrogen energy source driven by renewable energy.[ 1 – 3 ] The oxygen evolution reaction (OER) at the anode, a fundamental and essential half-reaction of water splitting, necessitates a relatively high thermodynamic potential (over 1.23 V vs. RHE) to overcome the sluggish kinetics because of the multistep four-electron transfer processes, which in turn seriously limits the water splitting efficiency.[ 4 , 5 ] Although noble metal-based materials such as IrO 2 and RuO 2 are first-rate catalysts for improving OER performance, their scarcity and low stability pose significant obstacles to actual commercialization.[ 6 , 7 ] In this regard, transition metal (TM) based catalysts with earth-abundant resources and cost-effectiveness have demonstrated as ideal alternatives to catalyze OER. Currently, theoretical investigations into the TM based OER catalysts have predominantly centered on the adsorption/desorption behavior of oxygen-containing intermediate to diminish the overpotential.[ 8 – 10 ] However, the energy barrier of these intermediates is closely associated with the transition of electron spin state, which is typically forbidden in quantum mechanics without spin-related electron transfer between the diamagnetic singlet state of OH − /H 2 O and the paramagnetic triplet state of O 2 molecule (↑O = O↑).[ 11 , 12 ] Consequently, spin state regulation of OER catalysts plays a pivotal impact on the reaction kinetics and adsorption/desorption behavior of oxygen intermediates. Of note, the TM based catalysts with magnetic characteristics possess the ability to create an appropriate spin selective channel for facilitating the transfer/extraction of required spin-polarized electrons [ 13 ] In this case, the multiple possible spin state of metal cations can be divided into low spin state (LS), medium spin state (MS), and high spin state (HS) based on the electronic occupation on e g orbital.[ 14 ] Tian et al. [ 15 ] reported the transition of Fe spin states from low to high via modulating the surface electronic structure of pentlandite, in which the Fe ions with HS state could accelerate the accumulation of OH − intermediates and endow the catalyst surface with fast electron transfer and reconstruction reaction kinetics. In principle, the alteration in the electron spin states induced by electron transfer/extraction within the 3d orbitals could significantly impact the OER activity, closely associated with the valence state and the surrounding chemical environment of the metal sites. Metal-organic frameworks (MOFs), composed of metal ions (such as Ni, Co, Fe, and Mn ions) and organic carboxylate ligands bound together by strong coordination bonds, may emerge as the ideal platforms and model catalysts for explicitly researching the electron spin characteristics of active sites owing to the flexible structure tunability and isolated active sites.[ 16 , 17 ] It should be noted that the coordination configuration of a metal with surrounding oxygen atoms is commonly regarded as a crucial factor influencing the adsorption energy and catalytic activity by modulating the charge distribution at the active sites.[ 18 ] In addition, according to quantum spin-orbital exchange interactions, the extent of orbital hybridization between the p orbitals of oxygen ligand and the metal d orbitals, driven by the varying charge distribution of active sites, could directly impact the spin-related electron transfer kinetics and adsorption/desorption capabilities[ 13 , 19 , 20 ]. In order to increase the electrocatalytic performance, significant efforts have been made to adjust the spin polarization by altering e g filling of active sites. For instance, Ye’s group fabricated the NiAl- and NiFe-based bimetallic MOFs to investigate the changes in spin states of active sites induced by the introduction of a second metal (Al or Ni), in which the spin state with a shallow hole trap could endow the catalyst with enhanced electron transfer and improved OER kinetics.[ 21 ] Wei and co-workers demonstrated that various organic ligands and functional groups within 2D Co-MOFs could effectively adjust the spin related electronic structure of Co active centers, thus optimizing the d-band centers of Co sites and boosting electrocatalytic performance.[ 22 ] Similarly, Liu et al. reported that the 2,5-Dihydroxy-1,4-benzoquinone organic ligand with redox-active could effectively reduce the d-orbital crystal field splitting energy of Fe ions and form a high spin state, thereby significantly optimizing the energy barrier of OER rate-determination step.[ 23 ] Although the substitution of metal nodes and selection of appropriate ligands are efficient strategies to manipulate the electron spin state for desired catalytic activities, achieving the gradual and precise regulation from LS to MS and then to HS states is difficult and easily overlooked. The most serious challenge lies in the absence of effective methods to accurately alter the ligand field around the metal and induce the extraction or migration of 3d orbital electrons. Therefore, it is imperative to develop a controllable strategy to precisely regulate the spin states and coordination environment, while gaining deeper insight into the structure-activity relationship between the spin configuration and OER activity. Unfortunately, no systematic research progress has been made thus far. Herein, we successfully prepare a series of NiFe-MOF-Dx catalysts with well-defined oxygen coordination environments via a straightforward solvothermal followed by chemical reduction treatment to accurately regulate electron spin states and establish the correlation between electronic structure and OER activity of catalysts. Experimental characterization results demonstrate the formation of NiO 6 -FeO 6 , NiO 6 -FeO 5 , and NiO 6 -FeO 4 configurations within NiFe-MOF-Dx, highlighting the relationship between the coordination configuration and electron spin states of active sites. It could be found that the d-orbital electrons of Fe sites achieve a precise transfer from the Fe-t 2g to the Fe-e g orbitals as the coordination number decreases, leading to increased spin magnetic moment and gradual transform of Fe spin polarization from a LS to a MS and ultimately to HS state, while the Ni sites remain in a LS state and NiO 6 configurations without any changes. In addition, density functional theory (DFT) calculations demonstrate that the spin state change induced by ligand field modulation could increase spin hole in the bridged oxygen ligand and optimize d-band center, thereby enhancing the adsorption capacity of oxygen-containing intermediates and significantly boosting the catalytic activity of OER. Impressively, the resulting NiFe-MOF-D 3 with NiO 6 -FeO 4 configuration, characterized by a high spin state of Fe, displays exceptional high catalytic OER performance with a low overpotential of 365 mV at the ampere-level current density of 1.5 A cm − 2 and small Tafel slope of 44.3 mV dec − 1 in alkaline medium, surpassing the benchmark RuO 2 catalyst. This study unveils the correlation between coordination number and electron spin configuration and delves into the origin of the electron spin-dependent activity behavior, presenting an innovative blueprint for the design and development of MOF-based catalysts. Results and Discussion To intuitively reveal the origin of OER activity of NiFe-MOF catalysts from the electronic level, a strategy to precisely regulate the spin state of active sites is proposed. As schematically illustrated in Fig. 1 a, a series of bimetallic NiFe-MOF-Dx catalysts with well-defined coordination defects are prepared via a straightforward solvothermal strategy followed by chemical reduction treatment. Firstly, the exogenously introduced Ni(NO 3 ) 2 and Fe(NO 3 ) 3 could coordinate with 2-aminoterephthalic acid (H 2 BDC-NH 2 ) ligand to obtain the pristine NiFe-MOF through the solvothermal process. Subsequently, the above pristine NiFe-MOF is dispersed in the ultrapure water to obtain the suspension, then various dosages of reductant agent NaBH 4 (0, 1, 3, 5 mg) are dissolved in the above suspension via the continuous stirring to prepare a series of bimetallic NiFe-MOF-Dx. The unsaturated coordination within the NiFe-MOF-Dx catalysts is formed though the NaBH 4 chemical reduction process. The high oxidation state Fe(III) of Fe-O cluster could be preferentially reduce to Fe(II) by the reductant agent NaBH 4 , while the Fe(II) is unstable and easily further oxidized into Fe(III). The interconversion between Fe(II) and Fe(III) could induce the variation of coordination environment around Fe atoms during the chemical reduction process, thus lead to the possible formation of coordination defects around Fe sites, which may induce the "escape" of electrons within 3d orbitals of Fe, resulting in the controllable change in spin configuration. These as-synthesized catalysts are denoted as NiFe-MOF-D 0 , NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 , respectively. The X-ray diffraction (XRD) technique is firstly carried out to monitor the crystal structural evolution of these catalysts along with variation of added NaBH 4 content. As presented in Fig. 1 b, all these catalysts exhibit the main diffraction peaks at ≈ 9.2, 10.7, 16.7, and 21.3°, which are identical to that of Fe-MIL-88B phase,[ 24 ] indicating their framework structures still maintain well after adding NaBH 4 . However, with the increase of NaBH 4 content, the peaks intensity of these catalysts gradually weakens, indicating the reduced crystallinity and probable formation of the unsaturated coordination defects. More interestingly, with the continuous introduction of NaBH 4 to 5mg, the prominent diffraction peaks have become a little “hump” for the NiFe-MOF-D 3 , displaying the low-crystalline nature. Such crystallinity changes could be attributed to the formation of local defects triggered by the introduction of NaBH 4 .[ 25 ] Raman spectrum is recorded to reveal the functional groups information for these as-synthesized catalysts, as displayed in Fig. 1 c. The main characterization peaks centered at around 1602 and 1412 cm − 1 are attributed to the in- and out-of-phase stretching modes of the carboxylate group, while the peaks captured at 1254 and 810 cm − 1 could be assigned to the H 2 BDC-NH 2 linker stretching mode and C-H bonding vibrations of the benzene ring, respectively.[ 26 ] The results imply the metal ions is successfully coordinated with organic H 2 BDC-NH 2 linkers. It could be found that the peaks intensity gradually reduced as the increase of NaBH 4 content, indicating the raised unsaturated coordination defects. More sufficient evidence can also be obtained via analyzing the electron paramagnetic resonance (EPR) spectra. As presented in Fig. 1 d, it can be seen that these catalysts present the gradual stronger EPR signals located at g = 2.003 with the increase of NaBH 4 content, indicating the introduction of NaBH 4 with different content could significantly modulate the defect concentration within NiFe-MOF.[ 27 ] The morphological structures of NiFe-MOF-D 0 , NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 are investigated via the scanning electron microscopy (SEM) in Fig. 1 e-h, the nanorods morphology can be observed for the NiFe-MOF-D 0 , which are highly oriented and arranged in parallel with obvious gaps between nanorods. After introducing the NaBH 4 , the as-obtained NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 still present the similar nanorods morphology to NiFe-MOF-D 0 , indicating NiFe-MOF is well preserved and without structural collapse after introducing reductant agent NaBH 4 . Interestingly, the regular nanorods morphology usually possess high conductivity and strong capillary force to promote the absorption between electrolyte and nanorod surface, which could endow the catalyst surface with abundant accessible active sites for boosting the OER performance.[ 26 ] As presented in Fig. 1 i and j, the transmission electron microscopy (TEM) images of the NiFe-MOF-D 3 display the nanorods morphology with smooth surface and regular sharp angles at the apexes of the nanorods. For further analysis of the detailed crystal structure of NiFe-MOF-D 3 , the High-resolution TEM (HRTEM) is adopted (Fig. 1 k). It can be found that the lattice fringes of 0.95 nm can be observed, which matches well with the lattice spacing (002) planes. The halo ring further implies the low-crystalline state of NiFe-MOF-D 3 in the selected area electron diffraction (SAED) pattern (Fig. 1 l).[ 28 ] The high angle annular dark-field scanning TEM (HAADF-STEM) and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images reveal uniformly distributed on the nanorods (Figure S1 ). To reveal the effect of coordination environment on the chemical components and valence state, the X-ray photoelectron spectroscopy (XPS) for NiFe-MOF-D 0 and NiFe-MOF-D 3 are performed (Figure S2). As depicted in Fig. 2 a, the high-resolution Fe 2p spectrum of NiFe-MOF-D 0 present two peaks of Fe 2p 3/2 at 712.2 eV and Fe 2p 1/2 at 725.6 eV, accompanied by two satellite peaks, revealing the characteristic features of Fe 3+ .[ 29 ] In the high-resolution Ni 2p spectrum (Fig. 2 b), two discernible peaks at 856.1 and 873.6 eV can be attributed to Ni 2p 3/2 and Ni 2p 1/2 electronic configurations, which offer an indication of Ni 2+ .[ 16 ] In comparison to the XPS spectrum of NiFe-MOF-D 0 , the Fe 2p spectra of NiFe-MOF-D 3 display a subtle negative shift of approximately 0.3 eV, whereas no notable shift is discernible for the Ni 2p spectra. This observation suggests that unsaturated coordination defects may be predominantly localized around the Fe center, thereby inducing alterations in the electronic structure and valence states from Fe 3+ to Fe 2+ . As shown in Fig. 2 c, the high-resolution O 2p spectra of NiFe-MOF-D 3 can be deconvoluted into lattice oxygen (530.5 eV), oxygen vacancies (531.8 eV), and oxygen in carboxylate group (532.4 eV).[ 28 ] Notably, there is a significant reduction in the intensity of the lattice oxygen peak, while a sharper peak can be observed for the oxygen vacancies compared to that of NiFe-MOF-D 0 , further indicating the formation of unsaturated coordination defects within NiFe-MOF-D 3 . The XPS valence band spectra are measured to further investigate the electronic properties of the NiFe-MOF-D 3 , as shown in Figure S3. The valence band maximum energy of NiFe-MOF-D 3 has undergone a negative shift towards the Fermi level, approximately 0.65 eV lower than those of NiFe-MOF-D 2 (0.78 eV), NiFe-MOF-D 1 (1.13 eV), and NiFe-MOF-D 0 (1.47 eV), suggesting the introduction of unsaturated coordination defects can effectively modulate the electronic structure and increase the conductivity. [ 30 ] X-ray absorption structure (XAS) analysis is further conducted to elucidate the local coordination environment and the electronic configuration of NiFe-MOF-D 3 , together with the Fe foil, Ni foil, FeO, Fe 2 O 3 , NiO, and NiFe-MOF-D 0 as the comparison. As shown in Fig. 2 d, Fe K-edge X-ray near-edge structures (XANES) spectrum show that the spectral profile of NiFe-MOF-D 0 is close to that of Fe 2 O 3 reference, implying an average valence state of Fe is + 3. After introducing unsaturated coordination, the Fe K-edge XANES spectrum displays that the adsorption threshold position of the as-formed NiFe-MOF-D 3 has a slightly negative shift, indicating that the valence of Fe in NiFe-MOF-D 3 is between + 2 and + 3. As for Ni K-edge XANES spectra (Fig. 2 e), the profiles of NiFe-MOF-D 3 and NiFe-MOF-D 0 are similar to that of NiO reference, indicating the presence of Ni 2+ . The Fourier transform (FT) k 3 -weighted extended X-ray absorption fine structure (FT-EXAFS) spectra are further analyzed. As shown in Fig. 2 f, the Fe K-edge FT-EXAFS spectra of NiFe-MOF-D 3 and NiFe-MOF-D 0 exhibit a prominent peak located at ~ 1.47 Å, which is mainly attributed to the Fe-O bond of the first coordination shell.[ 15 ] Similar to the analysis of the Fe K-edge, a major peak at 1.56 Å is also observed for the Ni K-edge, which is assigned to the Ni-O bond of the first coordination shell (Fig. 2 g).[ 15 ] The other detectable small peaks related to Fe/Ni-O coordination confirms the formation of NiFe-MOF structure. Wavelet transform (WT) of Fe/Ni K-edge EXAFS spectra is applied to investigate the metal distribution at atomic resolution. As displayed in Fig. 2 h, S7, and S7, the WT contour plots of NiFe-MOF-D 0 and NiFe-MOF-D 3 reveals a single intensity maximum approximately 4.7 Å −1 in k space, attributed to Fe/Ni-O.[ 31 ] The above observation indicates that the Ni/Fe atoms are predominantly dispersed on the NiFe-MOF structure in the form of single atoms. The coordination configuration of the Fe/Ni atom in NiFe-MOF-D 3 and NiFe-MOF-D 0 is further examined by the quantitative EXAFS fitting analysis (Table S1 and S2). The first shell coordination numbers (CNs) for Fe-O and Ni-O in the NiFe-MOF-D 0 are about 5.8 and 6.3, respectively, revealing coordination configuration of FeO 6 -NiO 6 within NiFe-MOF-D 0 . In contrast, after introducing unsaturated coordination defects, the CNs values of Fe-O and Ni-O are 4.4 and 6.1 for the NiFe-MOF-D 3 , respectively. Such indicates that the Fe centers are coordinated with four O atoms, while the Ni centers are still coordinated with six oxygen atoms (NiO 6 -FeO 4 ) in NiFe-MOF-D 3 . The above results suggest that the introduction NaBH 4 could induce the variation of coordination environment and electronic distribution only at around Fe atoms, with a negligible alteration observed at the Ni sites. The Fe L-edge XANES spectra of NiFe-MOF-D 3 , NiFe-MOF-D 0 , and Fe 2 O 3 are thus recorded to further examine the impact of unsaturated coordination defects on Fe 2p orbital electronic configuration. As shown in Fig. 2 i, the Fe L-edge XANES spectra of NiFe-MOF-D 0 and NiFe-MOF-D 3 exhibits a negative shift compared to that of Fe 2 O 3 , further suggesting an Fe valence state between + 2 and + 3. The two peaks located at 708.9 and 712.2 eV could be observed in the Fe L 3 -edge region, which are attributed to Fe 3d t 2g and Fe 3d e g sub-bands, respectively. Notably, compared with the NiFe-MOF-D 0 , there is a noticeable decrease in the peak intensity of Fe 3d t 2g and e g for the target NiFe-MOF-D 3 , suggesting an enhanced Fe 3d unpaired electron occupation. To clarify the possible charge transfer between the t 2g and e g orbitals of NiFe-MOF-D 3 , the O K-edge XANES spectra are further investigated. As shown in Fig. 2 j, the two characteristic peaks captured at 529.2 and 531.6 eV, emphasized by yellow and gray patterns, could be attributed to the hybridization between unoccupied O 2p orbitals and Fe 3d orbitals. Compared with the NiFe-MOF-D 0 , the characteristic peak located at ∼529.2 eV of NiFe-MOF-D 3 became stronger, while the peak at ∼531.6 eV of NiFe-MOF-D 3 became weaker, which could be ascribed to the electron transition from t 2g to e g orbitals.[ 10 ] Based on the above results, we propose that ligand field regulation is an effective strategy to flexibly manipulate the migration of 3d orbital electrons, which could induce the precise modulation of electron spin states of NiFe-MOF catalysts. Moreover, the energy band structures of these NiFe-MOF are determined (Fig. 2 k), and the dz 2 energy level of Fe atoms could be well decreased with spin state transition. The ferromagnetic hysteresis loops and EPR spectra of the catalysts are recorded to probe the unpaired electrons and spin state on Fe ions. As presented in Fig. 3 a, the saturation magnetization gradually enhances with the increase of NaBH 4 content. When the NaBH 4 content is up to 5 mg, the strongest saturation magnetization of NiFe-MOF-D 3 could be observed, which may be attributed to the presence of more unpaired electrons in the NiFe-MOF-D 3 .[ 32 ] The inset of Fig. 3 a is the enlarged view of the ferromagnetic hysteresis loops around H = 0. It can be seen that the NiFe-MOF-D 3 displays a stronger magnetic field (Hc) of 649.81 Oe and higher residual magnetization (Mr) of 0.023 emu g − 1 compared with other catalyst references, indicating the enhanced spin polarization within NiFe-MOF-D 3 . The EPR spectra presents that NiFe-MOF-D 3 have a higher peak intensity at g = 2.001 compared to NiFe-MOF-D 3 , indicating the enhanced unpaired electrons number and spin state (Fig. 3 b).[ 33 ] To further investigate the electron spin state of Fe 3d orbitals for the NiFe-MOF-D 0 , NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 , the zero-field cooling (ZFC) temperature-dependent magnetic susceptibility (M-T) measurements are performed (Fig. 3 c).[ 34 ] The effective magnetic moment (µ eff ) can be determined through the application of the Langevin theory and the Curie-Weiss law. The µ eff value of NiFe-MOF-D 0 , NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 is calculated to be 1.9, 2.3, 3.4, 5.3 µ eff , respectively. The unpaired d electron numbers (n) of Fe 3d orbitals are further obtained via the equation: 2.828 \(\:\sqrt{{{\chi\:}}_{\text{m}}\text{T}}\) = 𝜇 eff = \(\:\sqrt{\text{n}(\text{n}+2)}\) . The average number of the unpaired d electron is calculated to be 4.4 in Fe 3d orbitals for the NiFe-MOF-D 3 , which is higher than that of NiFe-MOF-D 0 (1.2), NiFe-MOF-D 1 (1.5), and NiFe-MOF-D 2 (2.6). Combined with these test results, it can be found that the electrons may undergo an incremental transfer from the Fe-t 2g orbitals to the Fe-e g orbitals with a reduced coordination saturation, leading to a precise change in the spin polarization of Fe sites from LS to MS and then to HS state mixed with Fe 2+ and Fe 3+ . The change of electronic structures corresponded to Fe 3d orbitals of these catalysts could be presented in Figure S9. As is known to all, the optimal filling of the e g orbitals plays a pivotal role in influencing orbital hybridization. [ 15 ] The presence of an unpaired electron in the d z 2 orbital facilitates its ready penetration into the bonding σ-orbital of oxygen.[ 10 ] Consequently, for the Fe 2+/3+ with a HS state, the optimal electrons occupancy in the d z 2 orbital is conductive to the hybridization between the Fe 3d and O 2p orbital, facilitating the generation of oxygen-containing intermediates and spin-dependent charge transfer. To offer a comprehensive perspective on the interpretation of orbital hybridization in NiFe-MOF catalysts, we examine the spin configurations of metal iron and nickel. Based on the aforementioned results, the possible stable local spin configurations of iron and nickel cations are shown in Fig. 3 d. The d z 2 orbitals of Ni 2+ with LS state is empty without electrons, while the d x 2 −y 2 and 𝜋-symmetry d orbitals (d xy , d xz , and d yz ) of Ni 2+ is fully occupied. The schematic representation of the electronic coupling between Ni and Fe provides a clear illustration of the detailed electron transfer pathway. As shown in Fig. 3 e, the electron-electron repulsion is regarded as the dominant interaction between Ni 2+ and bridging O 2− . Except for Fe 2+ with LS state, all Fe cations possess unpaired electrons in 𝜋-symmetry d orbitals, enabling them to interact with bridging O 2− ions through 𝜋-donation. Once the Fe 2+/3+ and Ni 2+ are coupled, electron-electron repulsion within the Ni-O coordination strengthens the 𝜋-donation of the Fe-O bond though the push-pull electronic effect, significantly enhancing the charge transport pathway [ 35 ]. According to the molecular orbital bonding principle, the d z 2 orbitals of metal sites are inclined to interact with 2p z /π 2p * of oxygen-containing intermediates to form strong σ-type bonds, while the weak π-type bonds could be formed by the hybridization of d xz/yz and 2p y/z orbitals [ 36 ]. The interactions between d xy and d x 2 −y 2 orbitals of metal sites and the 2p x/y/z orbitals of the oxygen intermediates are negligible because of the relative inertness of d xy and d x 2 −y 2 compared with d z 2 and d xz/yz orbitals. Therefore, neither the d x 2 −y 2 nor d xy participate in bond formation in the diagram. The bond orders (BOs) of OH* and O* intermediates on Fe sites (Fig. 3 f) are calculated to quantify the adsorption strength (bond order = (number of bonding electrons – number of antibonding electrons)/2). As is known to all, a higher value of BO, a stronger spin-orbital interaction between the metal sites and the intermediates.[ 37 ] It can be seen that the BOs values of OH* are determined to be 1.5, 1.5, 2, and 1.5 on the Fe 2+/3+ (LS), Fe 3+ (MS), Fe 3+ (HS), and Fe 2+ (HS), respectively. The higher BOs value indicate that the OH* intermediates is preferentially adsorbed on HS Fe 3+ , facilitating the initiation of the OER process. Additionally, the higher BOs value of O* on HS Fe 2+/3+ compared to LS Fe 2+/3+ reveal the OER thermodynamic advantage for HS Fe ions due to the strengthened Fe-O bond [ 38 ]. Apparently, Fe ions with high spin polarization exhibit optimal bond order values with reaction intermediates, facilitating the rapid adsorption of OH* species on metal sites and the dissociation of metal-OH bond to ensure the effective recycling of O* and OH* species, achieving a significantly accelerated OER kinetic process. These findings underscore the critical role of spin regulation in enhancing oxygen reactivity. The density functional theory (DFT) calculations are performed to intuitively investigate the change of d-orbital electronic structure with different coordination environments using spin states as descriptors. The spin density patterns of Fe-MOF and NiFe-MOF are shown in Fig. 4 a. In the pure Fe-MOF, where all available sites are occupied by Fe, the next-to-nearest neighbor Fe cations respectively exhibit spin-up and spin-down character, which could lead to band gap opening associated with pronounced Jahn-Teller distortions [ 39 ]. In addition, the oxygen ligand connecting to two Fe ions exhibits a low spin density (0.017 e), indicating the long-range spin-related charge transfer within the Fe-MOF is sluggish. Upon introducing Ni ions to Fe-MOF, the spin density of Fe aligns with that of Ni, and the oxygen ligand between Ni and Fe ions presents a high spin density. This could lead to ferromagnetic quantum spin orbital exchange interactions between Fe and Ni ions, facilitating the formation of a spin channel in the oxygen atoms for fast charge transport. Consequently, it could be considered that the catalytic activity of NiFe-MOF primarily originates from the Fe sites, while the presence of both Ni and Fe in the MOF with Fe as dominant sites is necessary for a fast charge transfer and high OER activity.[ 15 , 40 ] The spin density patterns of NiFe-MOF with different “Fe-O” coordination structures (the structural models are shown in Figure S10-S12, including NiO 6 -FeO 4 , NiO 6 -FeO 5 , and NiO 6 -FeO 6 models) are further calculated to investigate the change of spin polarization of metal and spin-related charge transport dynamics. As depicted in Fig. 4 a, it is apparent that spin density of Fe sites within NiFe-MOF increased significantly with reduced saturated coordination, while there is not notable change in the low spin density of Ni sites, indicating the alteration in the coordination environment could solely modulate the spin polarization of Fe site, with a negligible change observed at the Ni sites. Additionally, the spin density of the oxygen ligand connecting Fe and Ni ions also increases as coordination becomes more unsaturated, suggesting the increased spin hole in the bridged oxygen ligand, which could facilitate accelerated charge transport in the catalysts. The charge density differences are calculated to intuitively understand the change of electronic structure. As shown in Fig. 4 b, it is evident that the Fe sites and bridged oxygen atoms exhibit an electron-deficient and electron-rich state for the three models, respectively, indicating the possible formation of electron transfer channels.[ 15 ] The calculated bader charge of Fe atoms gradually increase from 6.384 e (NiO 6 -FeO 6 model) to 6.417 e (NiO 6 -FeO 5 model) and 6.530 e (NiO 6 -FeO 4 model), which further demonstrates fast charge transfer and redistribution of electrons triggered by unsaturated coordination of Fe sites. The spin-resolved density of states (DOS) on NiFe-MOF with different “Fe-O” structures are determined to explore the change of electronic structure within Fe 3d orbital. As shown in the DOS diagram of Fig. 4 c, the unoccupied e g orbital of Fe gradually approaches the occupied t 2g orbital with the increase of unsaturated coordination. Meanwhile, the bonding state near the Fermi level of NiO 6 -FeO 5 and NiO 6 -FeO 4 models are stronger than that of NiO 6 -FeO 6 model, indicating the electronic migration from t 2g to e g orbital inside Fe sites. [ 10 ] Therefore, we can infer that the Fe-3d orbitals of NiFe-MOF with different unsaturated coordination models exhibit the precise transition from low to medium and then to high spin polarization. Besides, the spin magnetic moment of NiO 6 -FeO 4 model is determined to be 3.724 µ B , larger than that of NiO 6 -FeO 5 (1.794 µ B ) and NiO 6 -FeO 6 (1.449 µ B ) models (Fig. 4 d), further revealing the gradual increased spin polarization with greater unsaturation in coordination. As displayed in Fig. 4 e, the d-band center of Fe sites gradually become close to the Fermi level with the increase of spin polarization, demonstrating the decreased occupation of the anti-bonding state, thus optimizing the adsorption energy between metal Fe site and oxygen intermediates. The adsorption capacity of Fe sites towards OH* and OOH* is investigated from the perspective of bonding by calculating the crystal orbital Hamilton population (COHP). The up/down curves are on behalf of the bonding and antibonding contribution, respectively. For the OH* adsorption, the NiO 6 -FeO 4 model with high spin states exhibits decreased occupation of antibonding state, accompanied by the lower integrated COHP (ICOHP) value (1.864) than that of NiO 6 -FeO 5 (1.802) and NiO 6 -FeO 6 model (1.793), as presented in Figure S13a-c. Meanwhile, a more negative ICOHP in the NiO 6 -FeO 4 model can be observed for the OOH* adsorption compared with the NiO 6 -FeO 5 and NiO 6 -FeO 6 model (Figure S13d-f), indicating the precise modulation of spin states can flexibly adjust the adsorption and desorption behavior of reaction intermediates. The Gibbs free energies (∆G) of each OER step are calculated to further identify the intrinsic catalytic activity of NiFe-MOF model. Figure S14 exhibits the OER pathway at metal Ni and Fe sites of NiO 6 -FeO 6 model, including the following stages: * → OH* → O* → OOH* → O 2 .[ 28 ] For the Ni sites, the transformation of OH* to O* at U = 0 V is deemed the rate-determining step (RDS) due to the largest value of free energy among all steps (Figure S15a). Conversely, the RDS at Fe sites is found to be the third step (O* → OOH*), in which the free energy values of Fe sites (2.04 eV) is lower than that of the Ni sites (2.09 eV). The calculated OER overpotential of Fe sites (0.80 V) is also smaller than that of Ni sites (0.86 V) at U = 1.23 V (Figure S15b). These results indicate that the Fe sites could possess faster reaction dynamics and better activity than that of Ni center, which may serve as the primary active center involved in the OER. For the NiFe-MOF models with different “Fe-O” coordination, the ΔG value of typical OER pathways with U = 0 was calculated to determine the OER activity at the central Fe sites (Fig. 4 g). It is evident that all the elementary reactions within those NiFe-MOF models exhibit uphill trends in the OER process, while the third step involving O* → OOH* demonstrates the highest free energy change for each NiFe-MOF model, indicating that the transition from O* to OOH* intermediate on the Fe sites is regarded as the RDS during OER process. The ΔG value of RDS for NiO 6 -FeO 6 model is concluded to be 2.04 eV, higher than that of NiO 6 -FeO 5 model (1.91 eV) and NiO 6 -FeO 4 model (1.75 eV). The calculated theoretical overpotential on the Fe sites effectively reduces from 0.8 V (NiO 6 -FeO 6 model) to 0.68 V (NiO 6 -FeO 5 model) and 0.52 V (NiO 6 -FeO 4 model), revealing that the NiO 6 -FeO 4 model with Fe high spin state could accelerate the formation of OOH* intermediate and decrease the overpotential (Fig. 4 f). The corresponding structure changes of NiO 6 -FeO 4 model (Fe sites) during OER process is shown in Fig. 4 h. Additionally, the ΔG values of OH* adsorption for NiO 6 -FeO 6 , NiO 6 -FeO 5 and NiO 6 -FeO 4 models are determined to be 0.91, 0.11, and 0.04 eV, respectively. The low OH* adsorption energy indicates the NiO 6 -FeO 4 model with Fe HS state is conducive to adsorbing OH* intermediates for initiating the OER process.[ 41 ] These results theoretically validate the flexible modulation of ligand fields can evidently induce the 3d orbital electron migration to realize precise changes in electron spin states, which could optimize the d-band center and enhance orbital hybridization of Fe 3d and O 2p, thereby accelerating the reaction kinetics and improving the OER performance. The catalytic OER activity of NiFe-MOF-D 0 , NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 with different electron spin configuration of Fe sites are evaluated in a standard three-electrode system containing the 1M KOH electrolyte. All the linear sweep voltammetry (LSV) polarization curves were calibrated with iR-correction to appraise the overpotentials unless noted. As shown in Fig. 5 a and b, compared to NiFe-MOF-D 0 (569 and 670 mV), NiFe-MOF-D 1 (508 and 606 mV), NiFe-MOF-D 2 (424 and 470 mV), the NiFe-MOF-D 3 only require the smaller overpotentials of 328 and 365 mV to drive the ampere-level current density of 1 A cm − 2 and 1.5 A cm − 2 , respectively. Such increased OER activity highlights the significant roles of the modulated electronic structures of metal Fe sites induced by introducing abundant unsaturated coordination defects. Notably, the overpotential of the target NiFe-MOF-D 3 is comparable, or even better than other previously reported MOF-based catalysts, highlighting the outstanding catalytic OER activity (Table S3). Likewise, as displayed in Fig. 5 c, the NiFe-MOF-D 3 also displays the lowest Tafel slope value of 44.3 mV dec − 1 than those of NiFe-MOF (58.7 mV dec − 1 ), NiFe-MOF-D 1 (51.7 mV dec − 1 ), NiFe-MOF-D 2 (47.1 mV dec − 1 ) and commercial RuO 2 (87.2 mV dec − 1 ), indicating its accelerated reaction kinetics, which can be further verified by the electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig. 5 d, it can be found that the charge transfer resistance (R ct ) values of those catalysts develop in general yields the trend: NiFe-MOF-D 3 < NiFe-MOF-D 2 < NiFe-MOF-D 1 < NiFe-MOF-D 0 . The double-layer capacitance (C dl ) value, that is obtained from the cyclic voltammetry curves in a non-Faradaic region (Figure S16), is calculated to evaluate the electrochemical active surface area (ECSA). The calculated C dl value of target NiFe-MOF-D 3 is gained as high as 1.07 mF·cm − 2 , with the NiFe-MOF-D 1 reference (0.97 mF·cm − 2 ) coming next, followed by the NiFe-MOF reference (0.83 mF·cm − 2 ), and the NiFe-MOF-D 2 reference (0.75 mF·cm − 2 ) the last (Fig. 5 e). The highest C dl value implies more roughest surface created in the NiFe-MOF-D 3 , thereby provide more efficient active sites for boosting the OER activity.[ 41 ] The comparing polarization curves normalized by ECSA of the target NiFe-MOF-D 3 and other catalyst references further demonstrate the highest intrinsic OER activity (Figure S17). As observed in Fig. 5 f, the mass activity (MA) calculated by the inverse proportion relation between the current density and mass load are arranged in this order of NiFe-MOF-D 3 > NiFe-MOF-D 2 > NiFe-MOF-D 1 > NiFe-MOF-D 0 . It shows that the NiFe-MOF-D 3 has the highest mass activity value, further indicating the significant intrinsic OER activity among all the above catalysts.[ 16 ] The trends observed in turnover frequency (TOF) values align with those of the MA, with the high spin state yielding superior TOF values (Fig. 5 g and S18). The cyclic voltammetry (CV) and chronopotentiometric (v-t) tests are conducted to assess the stability of NiFe-MOF-D 3 . As depicted in in Fig. 5 h, there is no obvious positive shift observed in polarization curve after successive CV scanning for 20000 cycles. Similarly, the v-t curve recorded at a permanent current density of 1 A cm − 2 displays minimal voltage increases even after a continuous operation for 170h (Fig. 5 i), which significantly surpasses the stability of other reported catalysts (Table S3), suggesting the superior stability and durability of NiFe-MOF-D 3 towards OER in alkaline medium. To simulate industrial conditions of water electrolysis, an alkaline anion exchange membrane water electrolysis (AEMWE) full-cell is assembled to assess catalytic performance, as shown in Fig. 5 j. The as-prepared NiFe-MOF-D 3 is used as the anode, while the Pt/C, known for its high hydrogen evolution reaction activity, was utilized as the cathode (NiFe-MOF-D 3 // Pt/C). As shown in Fig. 5 k, the NiFe-MOF-D 3 // Pt/C two-electrode system only require the low cell voltages of 1.77 V to reach current densities of 500 mA cm − 2 , which is comparable, or even better than the other previously reported catalysts in alkaline electrolyte (Table S4) and reaches the United States Department of Energy (DOE) target. After conducting continuous electrolysis for 1 hour with an AEMWE full cell, the calculated Faraday efficiency was determined to be close to 100% by measuring the volume of generated O 2 using the water drainage method, as shown in the Fig. 5 l. In addition, the homemade electrocatalytic mold could maintain a stable cell voltage at 500 mA cm − 2 for 55 h, highlighting its significant potential for practical application (Fig. 5 m).[ 42 ] To verify potential industrial applications, the cell efficiency and economic efficiency of the electrolyzer is evaluated. The calculated cell efficiency is 70.8%, higher than that of commercial RuO 2 // Pt/C (57.1%) The estimated cost for generating per gasoline-gallon equivalent (GGE) H 2 is as low as $ 0.94 according to the formula listed in supporting information, which is significantly lower than the target of $ 2.00 by 2026 from the U.S. DOE. To elucidate the underlying reasons for highly efficient catalytic activity toward OER by spin state modulation, we carried out a series of control experiments. The adsorption energies of the activated OH* intermediate on the catalyst surface is measured via the methanol oxidation reaction (MOR) method. As depicted in Fig. 6 a and S18, the onset potential of MOR for NiFe-MOF-D 3 displays a negative shift of approximately 90 mV, surpassing those of NiFe-MOF-D 0 (67 mV), NiFe-MOF-D 1 (75 mV), and NiFe-MOF-D 2 (79 mV). This outcome implies that the NiFe-MOF-D 3 possesses relatively high capacity for OH* adsorption, thereby facilitating the first step of the OER process.[ 10 ] In-situ EIS measurements at various potentials are conducted to further explore the reaction kinetics (Figure S20 a and c). It is observed that the semicircles corresponding to NiFe-MOF-D 3 diminished more quickly than that of NiFe-MOF-D 0 as the potential increased, indicating the NiFe-MOF-D 3 is more susceptible to polarization for rapidly adsorbing OH* intermediate at low potentials. Meanwhile, the phase peak values in the Bode diagram also display a similar trend to the semicircles, as shown in Fig. 6 b and S20b. In comparison to the NiFe-MOF-D 0 , the NiFe-MOF-D 3 consistently possess lower phase peak and R ct values at the same potential (Fig. 6 c and S21), further indicating the advantageous OER kinetics to trigger the OER process.[ 43 ] The deuterium kinetic isotope effects (KIEs) are employed to provide evidence for proton transfer kinetics (Figure S22).[ 44 ] Fig. 6 d and e present that the KIE values of NiFe-MOF-D 3 are concluded to be 1.61 in the overpotential regions, lower than that of NiFe-MOF-D 0 (1.78) at the same potentials regions, demonstrating that the proton transfer kinetics of NiFe-MOF-D 3 could be significantly enhanced after enhancing the spin state of Fe sites.[ 10 , 44 ] In-situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra is recorded at different potentials to ascertain the RDS of the OER process. As presented in Fig. 6 f and g, two broad peaks centered at about 1220 and 1050 cm − 1 are attributed to the Si-O-Si stretching band and surface OOH* intermediate in the NiFe-MOF-D 3 , respectively.[ 16 , 45 ] Notably, the characteristic peak of surface OOH* intermediate appeared at an applied voltage of 1.20 V for the NiFe-MOF-D 3 , which is easily increased in intensity as the potential moves positively. By contrast, the OOH* peak for NiFe-MOF-D 0 emerge at about 1.25 V. Consequently, the aforementioned findings robustly affirm that the Fe active sites with a high spin state could significantly accelerate the RDS of O* → OOH* during the OER process for promoting the generation of OOH* intermediate on the NiFe-MOF-D 3 .[ 16 ] Based on these results, flexible control of coordinative environment and spin state is conductive to optimizing the adsorption of OH* intermediates and the reaction barriers of OOH* intermediates, which could dramatically accelerate the electron transfer kinetics (H 2 O→OH* and O*→OOH*), thereby resulting in the outstanding OER performance. Conclusion In summary, this work can not only accurately alter the coordination number around active metal center, but also achieve flexible control of spin state, thereby enhancing the OER activity at the ampere-level current density. Experimental characterizations together with theoretical analysis elucidate that the electrons of Fe 3d orbital undergo a controlled transfer from the t 2g to e g orbitals as coordination number reduced, leading to the increased spin polarization of Fe. However, the Ni sites within NiFe-MOF still maintain the NiO 6 geometric configuration with low spin states. It could be found that the ferromagnetic quantum spins orbital exchange interactions between HS state Fe and LS state Ni ions are more conductive to creating spin channel of bridged oxygen ligand and penetrating the bonding σ-orbitals of oxygen compared to those in MS and LS state Fe, which optimizes the d-band center of metal sites and the adsorption of oxygen intermediates. Therefore, the obtained NiFe-MOF-D 3 catalyst with unique NiO 6 -FeO 4 configuration and high spin state of Fe displays a low overpotential (328 mV @ 1 A cm − 2 and 365 mV @ 1.5 A cm − 2 ) and long-term stability over 170 h in alkaline medium. In addition, the assembled AEMWE flow cell with target NiFe-MOF-D 3 catalyst and Pt/C also presents remarkable activity and stability in 1M KOH. The present work offers theoretical foundation for efficiently improving OER activity through deliberately manipulating the electron spin configurations and ligand field, which can also provide a precise guidance for the design and development of other advanced non-precious metal-based OER catalysts. Methods Materials preparation A mixture consisting of Fe(NO 3 ) 3 ·9H 2 O (0.66 mmol), Ni(NO 3 ) 2 ·6H 2 O (0.33 mmol), and H 2 BDC-NH 2 (1 mmol) was dissolved in a solvent mixture of 30 mL DMF, 2 mL deionized water, and 1mL ethanol. This solution was stirred for 30 minutes to ensure uniformity. Subsequently, the well-mixed solution was transferred to a 50 mL Teflon-lined autoclave and heated in a temperature-controlled oven at 120°C for 15 hours. Upon natural cooling to room temperature, the resultant was thoroughly washed with DMF and ethanol and then dried at 50°C under vacuum. After that, 200 mg of the NiFe-MOF was mixed with varying amounts of NaBH 4 in 5 mL deionized water under vigorous stirring. Then the products were collected by centrifugation with three times washing using deionized water instantly. The final products were dried in a vacuum oven at 50°C for 12 hours. The quantities of NaBH 4 used were controlled as 0 mg, 1 mg, 3 mg, and 5 mg, corresponding to the samples named NiFe-MOF-D 0 , NiFe-MOF-D 1 , NiFe-MOF-D 2 , and NiFe-MOF-D 3 , respectively. Material characterizations The crystal structures and phase compositions of as-prepared catalysts were identified by a Bruker D8 Advanced Diffractometer with Cu Kα radiation (λ = 1.54056 Å). The morphology characterizations were determined by field emission scanning electron microscopy (FESEM, SU 8000) The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) were carried out on the FEI Tecnai G2 F20. The contact angles of those catalysts were tested on JC 2000 DM. The X-ray photoelectron spectroscopy (XPS) was conducted on the ESCALAB 250Xi, equipped with an X-ray source (Al Kα) at hν = 30 eV. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements of Fe and Ni K-edge were performed in fluorescence mode at the beamline BL14W1. DFT Computation We carried out all the DFT calculations in the Vienna ab initio simulation (VASP 6.3.0) code. The exchange-correlation is simulated with PBE functional and the ion-electron interactions were described by the PAW method. The vdWs interaction was included by using empirical DFT-D3 method. The electronic structure of the pure Fe-MOF and the NiFe-MOF were calculated to describe the effect of O vacancy. The NiFe-MOF exposing Fe and Ni site was used to simulate the electrochemical OER. The simplified Cu 3 based MOF was placed in a 15×15×25 Å 3 cubic box, and only the Γ-point was used to sample the Brillouin zone. All atoms are allowed to move freely during the geometry optimization. The cutoff energy is set as 450 eV for all periodic structure. The convergence criteria are set as 0.02 eV A − 1 and 10 − 5 eV in force and energy, respectively. The OER process were analyzed by calculating the reaction free energies of all elementary steps along the reaction coordinates, including (i) the hydroxide ion dissociates into OH groups adsorbed on the catalyst surface (*OH); (ii) *OH further reacts with hydroxide ions to dissociate into O groups (*O), and generates water molecules; (iii) *O reacts with a hydroxide ion and produces an OOH group (*OOH); (iv) eventually O 2 is produced and then released from catalyst. The free energy calculation of species adsorption ( \(\Delta G\) ) is based on following model. Herein Δ E , Δ E ZPE , and Δ S respectively represent the changes of electronic energy, zeropoint energy, and entropy that caused by adsorption of intermediate. The Δ H 0→ T refers to the change in enthalpy when heating from 0K to T K (298K in this work). The entropy of H + +e − pair is approximately regarded as half of H 2 entropy in standard condition. The G U is the applied potential, here is set for 0 V and 1.23 V. Electrochemical characterizations The electrochemical measurements were conducted on an electrochemical workstation based on the three-electrode system, in which the as-prepared catalysts were used as the working electrode, a graphite rod as the auxiliary electrode, and Hg/HgO electrode as the reference electrode in 1M KOH (pH = 13.8) electrolytes, respectively. The measured potential was calibrated to the reversible hydrogen electrode (RHE) according to the equation of E RHE = E + 0.098 V + 0.059 pH. The linear sweep voltammetry (LSV) was performed with iR compensation at a scan rate of 5 mV s − 1 . The prolonged stability test was implemented by using the chronopotentiometric curve without iR compensation. The corresponding Tafel plots were obtained by fitting the polarization curves between the potential and log current density (log j) by the equation: η = b log(j) + a, where b is the Tafel slope. Electrochemical impedance spectroscopy (EIS) spectra were recorded at a selected potential in the frequency range from 100 kHz to 0.01 Hz. The values of TOF were calculated according to the following equation: \(\:\text{T}\text{O}\text{F}=\frac{jA}{4Fn}\) Here, A (cm 2 ) stands for the geometric area of the Ni foam electrode. The number 4 means the four electrons transfer in OER and F equals to the constant of 96485.3 C mol − 1 . n represents the number of active sites. The loading mass (m, mg cm − 2 ) of catalysts and the measured current density j (mA cm − 2 ) was used to calculate the mass activity. $$\:\text{M}\text{a}\text{s}\text{s}\:\text{a}\text{c}\text{t}\text{i}\text{v}\text{i}\text{t}\text{y}=\frac{j}{m}$$ The electrochemical active surface area (ECSA) of catalysts was calculated from the double-layer capacitance (C dl ) according to the equation: $$\:\text{E}\text{C}\text{S}\text{A}\:=\:\frac{{C}_{dl}}{{C}_{s}}$$ Here, the C dl was obtained though the CV measurements in a non-Faradaic region with the scan rates of 20, 40, 60, 80, 100, and 120 mV s − 1 , respectively. The C s is the specific capacitance of an atomically smooth planar surface per unit area under the same electrolyte condition, which is normally between 0.02–0.06 mF cm − 2 . According to our catalytic surface, C s is usually estimated to be 0.04 mF cm − 2 . Declarations Data availability The data that support the findings of this study are available from the corresponding authors upon reasonable request. Source data are provided as a Source data file. Source data are provided with this paper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (52261145700, 22279124), the Natural Science Foundation of Shandong Province (ZR2022ZD30), Qingdao New Energy Shandong Laboratory Open Project (QNESL OP202307), the Fundamental Research Funds for the Central Universities (202262010). the Research Grant Council of Hong Kong (15304023; N_PolyU502/21; CRS_PolyU504/22), the funding for Projects of Strategic Importance of The Hong Kong Polytechnic University (Project Code: 1-ZE2V) and Natural Science Foundation of Guangdong Province (2023A1515012219). Author contributions M.H. and B.H. conceived the idea and supervised the project. 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A general method to probe oxygen evolution intermediates at operating conditions. Joule . 3 , 1498 (2019). Wang, X. et al. Proton capture strategy for enhancing electrochemical CO 2 reduction on atomically dispersed metal-nitrogen active sites. Angew. Chem. Int. Ed. 60 , 11959 (2021). Chen, J. et al. Promoting CO 2 electroreduction kinetics on atomically dispersed monovalent Zn I sites by rationally engineering proton-feeding centers. Angew. Chem. Int. Ed. 61 , 2111683 (2021). Additional Declarations There is NO Competing Interest. Supplementary Files Supportinginformation.docx Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5541146","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":387022269,"identity":"6dd57983-24d4-4a15-91b0-8b04aed50dc0","order_by":0,"name":"Minghua Huang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8ElEQVRIiWNgGAWjYBACAwglAUYMH1AEidHCOIMELRBdzDzEaDGXSH728GuORZ58dI/hZ5uKw/IM7M3bJBhq7uDUYjkjzdxYdptEseGdM8bSOWcOGzbwHCuTYDj2DLfDbiSYSUtuk0jcOCPHQDq37XACg0SOmQRjw2E8WtK/wbQY/7YEaZF/Q0hLjpnkR6CW+UDDpRnBtvAQ0HLmTZk0I1DLBom0MsueM+mGbTxpxRYJx/BoOZ6+TfLntrrE+TOSN9/4UWEtz89+eOONDzW4tYAAODoMDkB5bCAiAa8GYKT/ABLyDQRUjYJRMApGwcgFAGgcUkfAnJkrAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-9622-3131","institution":"Ocean University of China","correspondingAuthor":true,"prefix":"","firstName":"Minghua","middleName":"","lastName":"Huang","suffix":""},{"id":387022270,"identity":"e5267388-3d64-4ff8-9f9f-994a906ce132","order_by":1,"name":"Xianbiao Hou","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Xianbiao","middleName":"","lastName":"Hou","suffix":""},{"id":387022271,"identity":"01d2d864-b9de-4397-9750-6b645bd26667","order_by":2,"name":"Tengjia Ni","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Tengjia","middleName":"","lastName":"Ni","suffix":""},{"id":387022272,"identity":"b4f40e10-b5cd-4ddb-b99a-23599d982898","order_by":3,"name":"Zhaozheng Zhang","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Zhaozheng","middleName":"","lastName":"Zhang","suffix":""},{"id":387022273,"identity":"110f87e8-3b65-47e1-b8d2-22c725fc0157","order_by":4,"name":"Jian Zhou","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Jian","middleName":"","lastName":"Zhou","suffix":""},{"id":387022274,"identity":"b7a933f8-5e05-4f7b-a25d-0aca6b0791f8","order_by":5,"name":"Shucong Zhang","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Shucong","middleName":"","lastName":"Zhang","suffix":""},{"id":387022275,"identity":"233d1d8a-6b7c-4298-b3fe-ca0cb8d3badc","order_by":6,"name":"Shuixing Dai","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Shuixing","middleName":"","lastName":"Dai","suffix":""},{"id":387022276,"identity":"b1616ea4-09aa-450c-af95-30dc5e692a59","order_by":7,"name":"Lei Chu","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Lei","middleName":"","lastName":"Chu","suffix":""},{"id":387022277,"identity":"700a4aa9-5454-4580-9f64-7bfa54e0b1c8","order_by":8,"name":"Huanlei Wang","email":"","orcid":"","institution":"Ocean University of China","correspondingAuthor":false,"prefix":"","firstName":"Huanlei","middleName":"","lastName":"Wang","suffix":""},{"id":387022278,"identity":"324ca31e-f4ad-4af3-b049-a7dff36efd10","order_by":9,"name":"Bolong Huang","email":"","orcid":"https://orcid.org/0000-0002-2526-2002","institution":"The Hong Kong Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Bolong","middleName":"","lastName":"Huang","suffix":""}],"badges":[],"createdAt":"2024-11-28 09:20:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5541146/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5541146/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":70906498,"identity":"68461d22-994d-4338-bb32-2447ea952f1b","added_by":"auto","created_at":"2024-12-09 06:40:09","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1047016,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic illustration for the synthesis of NiFe-MOF-Dx. (b) XRD pattern, (c) Raman spectra, and (d) EPR spectra for those catalysts. The SEM images of (e) NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, (f) NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, (g) NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and (h) NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (i-k) TEM images with different resolutions, and (l) SAED image of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/b473191c21ecb05537cd1312.png"},{"id":70906476,"identity":"d3afae48-1547-42bb-8916-a5592a536989","added_by":"auto","created_at":"2024-12-09 06:40:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":662404,"visible":true,"origin":"","legend":"\u003cp\u003eThe high resolution XPS spectra of (a) Fe 2p, (b) Ni 2p, (c) O 1s for NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (d) Fe K-edge XANES spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, FeO, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and Fe foil. (e) Ni K-edge XANES spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, NiO, and Ni foil. \u0026nbsp;(f) Fe K-edge FT-EXAFS spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, FeO, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and Fe foil. (g) Ni K-edge FT-EXAFS spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, NiO, and Ni foil. (h) Wavelet transform of Fe K-edge EXAFS spectra for NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (i) Fe L-edge XANES spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. (j) O K-edge XANES spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (k) Schematic of the energy level exchange for the DD-Ni-NDA nanosheets after spin state regulation.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/9737fe91b0fc838dfb39fc96.png"},{"id":70906821,"identity":"f778e680-744a-4d25-aaaa-470b36497c84","added_by":"auto","created_at":"2024-12-09 06:48:09","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":480003,"visible":true,"origin":"","legend":"\u003cp\u003eElectronic structure characterization of these catalysts. (a) Magnetic hysteresis loops for these catalysts, (b) EPR spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (c) M-T plots of these catalysts. (d) d-electron configurations of Ni\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e/\u003csup\u003e3+\u003c/sup\u003e cations. (LS: low spin state, MS: medium spin state, HS: high spin state.) (e) Schematic diagram of electronic coupling between Ni and Fe in NiFe-MOF. (f) Orbital interactions between iron cations over NiFe-MOF and oxygen-containing intermediates.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/172d1a3522d22669d7a96d49.png"},{"id":70906487,"identity":"b3b70323-6118-4b0b-8da0-5365f2bf2f9e","added_by":"auto","created_at":"2024-12-09 06:40:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":756706,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The spin density pattern of the different NiFe-MOFs (FeO\u003csub\u003e6\u003c/sub\u003e, NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e, NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e, and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e models). (b) Charge density difference of NiFe-MOFs with different models. (c) Calculated spin-resolved DOS, (d) PDOS for Fe-3d orbitals, and (e) the d-band centers for the NiFe-MOFs with different coordinate models. Free energy diagrams of the NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e, NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e, and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e models within different NiFe-MOF (f) at U = 1.23 V and (g) U = 0 V. (h) The corresponding OER pathway for the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e at Fe sites\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/f8bed7000873256833543ba9.png"},{"id":70906477,"identity":"f271f0a0-eb16-4904-9036-28f65886feed","added_by":"auto","created_at":"2024-12-09 06:40:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":490712,"visible":true,"origin":"","legend":"\u003cp\u003eThe electrochemical measurements in 1M KOH. (a) LSV curves, (b) overpotential at 0.5, 1, and 1.5 A cm\u003csup\u003e-2\u003c/sup\u003e, and (c) corresponding Tafel slopes for NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, and RuO\u003csub\u003e2\u003c/sub\u003e. (d) The EIS, (e) the C\u003csub\u003edl\u003c/sub\u003e values, (f) the mass activity at the overpotential of 240 mV, and (g) TOF values of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (h) LSV curves before and after 20000 cycles, and (i) the time-dependent potential curve at 1 A cm\u003csup\u003e-2\u003c/sup\u003e for NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. (j) The Schematic diagram of electrolytic water reaction cell. (k) LSV polarization curves of the NiFe-MOF-D\u003csub\u003e3 \u003c/sub\u003e// Pt/C and Pt/C/NF//RuO\u003csub\u003e2\u003c/sub\u003e/NF electrolyzers. (l) The theoretically calculated and experimentally measured gas amount vs. time in assembled NiFe-MOF-D\u003csub\u003e3 \u003c/sub\u003e// Pt/C water electrolysis cell. (i) The time-dependent potential curve of NiFe-MOF-D\u003csub\u003e3 \u003c/sub\u003e// Pt/C under 500 mA cm\u003csup\u003e-2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/5eefbde7e8966bd16fa0c652.png"},{"id":70906491,"identity":"8d58a17a-e9b8-4ff0-a049-9ecb9bb0ba6d","added_by":"auto","created_at":"2024-12-09 06:40:08","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":331270,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The difference of onset potential between MOR and OER for these catalysts. (b) Bode plots of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e at different applied potentials in 1.0 M KOH solution. (c) Phase peak values at specific applied potentials in 1.0 M KOH solution of NiFe-MOF-D\u003csub\u003e0 \u003c/sub\u003eand NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. KIE values against potential for pristine (d) NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and (e) NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. In situ ATR-FTIR spectra for (f) NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e and (g) NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/d1368a7bc658ec53f4e78f98.png"},{"id":73905722,"identity":"dd2f1892-14b5-4cd9-aa3a-077956c4c697","added_by":"auto","created_at":"2025-01-15 19:23:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4811439,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/666f0d4a-1ad9-491b-9f90-449b9bbc2c7d.pdf"},{"id":70906481,"identity":"ad6c0aaf-ff2b-47c8-8572-b2d9dfddf85d","added_by":"auto","created_at":"2024-12-09 06:40:07","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4478264,"visible":true,"origin":"","legend":"Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction","description":"","filename":"Supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5541146/v1/eeea52c246bee00fc259e8ba.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe electrochemical water splitting is recognized as one of the prospective technologies to achieve carbon neutrality in the coming decades via producing the high-pure and eco-friendly hydrogen energy source driven by renewable energy.[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] The oxygen evolution reaction (OER) at the anode, a fundamental and essential half-reaction of water splitting, necessitates a relatively high thermodynamic potential (over 1.23 V vs. RHE) to overcome the sluggish kinetics because of the multistep four-electron transfer processes, which in turn seriously limits the water splitting efficiency.[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] Although noble metal-based materials such as IrO\u003csub\u003e2\u003c/sub\u003e and RuO\u003csub\u003e2\u003c/sub\u003e are first-rate catalysts for improving OER performance, their scarcity and low stability pose significant obstacles to actual commercialization.[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] In this regard, transition metal (TM) based catalysts with earth-abundant resources and cost-effectiveness have demonstrated as ideal alternatives to catalyze OER. Currently, theoretical investigations into the TM based OER catalysts have predominantly centered on the adsorption/desorption behavior of oxygen-containing intermediate to diminish the overpotential.[\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] However, the energy barrier of these intermediates is closely associated with the transition of electron spin state, which is typically forbidden in quantum mechanics without spin-related electron transfer between the diamagnetic singlet state of OH\u003csup\u003e\u0026minus;\u003c/sup\u003e/H\u003csub\u003e2\u003c/sub\u003eO and the paramagnetic triplet state of O\u003csub\u003e2\u003c/sub\u003e molecule (\u0026uarr;O\u0026thinsp;=\u0026thinsp;O\u0026uarr;).[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] Consequently, spin state regulation of OER catalysts plays a pivotal impact on the reaction kinetics and adsorption/desorption behavior of oxygen intermediates.\u003c/p\u003e \u003cp\u003eOf note, the TM based catalysts with magnetic characteristics possess the ability to create an appropriate spin selective channel for facilitating the transfer/extraction of required spin-polarized electrons [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] In this case, the multiple possible spin state of metal cations can be divided into low spin state (LS), medium spin state (MS), and high spin state (HS) based on the electronic occupation on e\u003csub\u003eg\u003c/sub\u003e orbital.[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] Tian et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] reported the transition of Fe spin states from low to high via modulating the surface electronic structure of pentlandite, in which the Fe ions with HS state could accelerate the accumulation of OH\u003csup\u003e\u0026minus;\u003c/sup\u003e intermediates and endow the catalyst surface with fast electron transfer and reconstruction reaction kinetics. In principle, the alteration in the electron spin states induced by electron transfer/extraction within the 3d orbitals could significantly impact the OER activity, closely associated with the valence state and the surrounding chemical environment of the metal sites. Metal-organic frameworks (MOFs), composed of metal ions (such as Ni, Co, Fe, and Mn ions) and organic carboxylate ligands bound together by strong coordination bonds, may emerge as the ideal platforms and model catalysts for explicitly researching the electron spin characteristics of active sites owing to the flexible structure tunability and isolated active sites.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] It should be noted that the coordination configuration of a metal with surrounding oxygen atoms is commonly regarded as a crucial factor influencing the adsorption energy and catalytic activity by modulating the charge distribution at the active sites.[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] In addition, according to quantum spin-orbital exchange interactions, the extent of orbital hybridization between the p orbitals of oxygen ligand and the metal d orbitals, driven by the varying charge distribution of active sites, could directly impact the spin-related electron transfer kinetics and adsorption/desorption capabilities[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In order to increase the electrocatalytic performance, significant efforts have been made to adjust the spin polarization by altering e\u003csub\u003eg\u003c/sub\u003e filling of active sites. For instance, Ye\u0026rsquo;s group fabricated the NiAl- and NiFe-based bimetallic MOFs to investigate the changes in spin states of active sites induced by the introduction of a second metal (Al or Ni), in which the spin state with a shallow hole trap could endow the catalyst with enhanced electron transfer and improved OER kinetics.[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] Wei and co-workers demonstrated that various organic ligands and functional groups within 2D Co-MOFs could effectively adjust the spin related electronic structure of Co active centers, thus optimizing the d-band centers of Co sites and boosting electrocatalytic performance.[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] Similarly, Liu et al. reported that the 2,5-Dihydroxy-1,4-benzoquinone organic ligand with redox-active could effectively reduce the d-orbital crystal field splitting energy of Fe ions and form a high spin state, thereby significantly optimizing the energy barrier of OER rate-determination step.[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] Although the substitution of metal nodes and selection of appropriate ligands are efficient strategies to manipulate the electron spin state for desired catalytic activities, achieving the gradual and precise regulation from LS to MS and then to HS states is difficult and easily overlooked. The most serious challenge lies in the absence of effective methods to accurately alter the ligand field around the metal and induce the extraction or migration of 3d orbital electrons. Therefore, it is imperative to develop a controllable strategy to precisely regulate the spin states and coordination environment, while gaining deeper insight into the structure-activity relationship between the spin configuration and OER activity. Unfortunately, no systematic research progress has been made thus far.\u003c/p\u003e \u003cp\u003eHerein, we successfully prepare a series of NiFe-MOF-Dx catalysts with well-defined oxygen coordination environments via a straightforward solvothermal followed by chemical reduction treatment to accurately regulate electron spin states and establish the correlation between electronic structure and OER activity of catalysts. Experimental characterization results demonstrate the formation of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e, NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e, and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e configurations within NiFe-MOF-Dx, highlighting the relationship between the coordination configuration and electron spin states of active sites. It could be found that the d-orbital electrons of Fe sites achieve a precise transfer from the Fe-t\u003csub\u003e2g\u003c/sub\u003e to the Fe-e\u003csub\u003eg\u003c/sub\u003e orbitals as the coordination number decreases, leading to increased spin magnetic moment and gradual transform of Fe spin polarization from a LS to a MS and ultimately to HS state, while the Ni sites remain in a LS state and NiO\u003csub\u003e6\u003c/sub\u003e configurations without any changes. In addition, density functional theory (DFT) calculations demonstrate that the spin state change induced by ligand field modulation could increase spin hole in the bridged oxygen ligand and optimize d-band center, thereby enhancing the adsorption capacity of oxygen-containing intermediates and significantly boosting the catalytic activity of OER. Impressively, the resulting NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e with NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e configuration, characterized by a high spin state of Fe, displays exceptional high catalytic OER performance with a low overpotential of 365 mV at the ampere-level current density of 1.5 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and small Tafel slope of 44.3 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in alkaline medium, surpassing the benchmark RuO\u003csub\u003e2\u003c/sub\u003e catalyst. This study unveils the correlation between coordination number and electron spin configuration and delves into the origin of the electron spin-dependent activity behavior, presenting an innovative blueprint for the design and development of MOF-based catalysts.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003eTo intuitively reveal the origin of OER activity of NiFe-MOF catalysts from the electronic level, a strategy to precisely regulate the spin state of active sites is proposed. As schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, a series of bimetallic NiFe-MOF-Dx catalysts with well-defined coordination defects are prepared via a straightforward solvothermal strategy followed by chemical reduction treatment. Firstly, the exogenously introduced Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e could coordinate with 2-aminoterephthalic acid (H\u003csub\u003e2\u003c/sub\u003eBDC-NH\u003csub\u003e2\u003c/sub\u003e) ligand to obtain the pristine NiFe-MOF through the solvothermal process. Subsequently, the above pristine NiFe-MOF is dispersed in the ultrapure water to obtain the suspension, then various dosages of reductant agent NaBH\u003csub\u003e4\u003c/sub\u003e (0, 1, 3, 5 mg) are dissolved in the above suspension via the continuous stirring to prepare a series of bimetallic NiFe-MOF-Dx. The unsaturated coordination within the NiFe-MOF-Dx catalysts is formed though the NaBH\u003csub\u003e4\u003c/sub\u003e chemical reduction process. The high oxidation state Fe(III) of Fe-O cluster could be preferentially reduce to Fe(II) by the reductant agent NaBH\u003csub\u003e4\u003c/sub\u003e, while the Fe(II) is unstable and easily further oxidized into Fe(III). The interconversion between Fe(II) and Fe(III) could induce the variation of coordination environment around Fe atoms during the chemical reduction process, thus lead to the possible formation of coordination defects around Fe sites, which may induce the \"escape\" of electrons within 3d orbitals of Fe, resulting in the controllable change in spin configuration. These as-synthesized catalysts are denoted as NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, respectively. The X-ray diffraction (XRD) technique is firstly carried out to monitor the crystal structural evolution of these catalysts along with variation of added NaBH\u003csub\u003e4\u003c/sub\u003e content. As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, all these catalysts exhibit the main diffraction peaks at ≈ 9.2, 10.7, 16.7, and 21.3°, which are identical to that of Fe-MIL-88B phase,[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] indicating their framework structures still maintain well after adding NaBH\u003csub\u003e4\u003c/sub\u003e. However, with the increase of NaBH\u003csub\u003e4\u003c/sub\u003e content, the peaks intensity of these catalysts gradually weakens, indicating the reduced crystallinity and probable formation of the unsaturated coordination defects. More interestingly, with the continuous introduction of NaBH\u003csub\u003e4\u003c/sub\u003e to 5mg, the prominent diffraction peaks have become a little “hump” for the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, displaying the low-crystalline nature. Such crystallinity changes could be attributed to the formation of local defects triggered by the introduction of NaBH\u003csub\u003e4\u003c/sub\u003e.[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] Raman spectrum is recorded to reveal the functional groups information for these as-synthesized catalysts, as displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. The main characterization peaks centered at around 1602 and 1412 cm\u003csup\u003e− 1\u003c/sup\u003e are attributed to the in- and out-of-phase stretching modes of the carboxylate group, while the peaks captured at 1254 and 810 cm\u003csup\u003e− 1\u003c/sup\u003e could be assigned to the H\u003csub\u003e2\u003c/sub\u003eBDC-NH\u003csub\u003e2\u003c/sub\u003e linker stretching mode and C-H bonding vibrations of the benzene ring, respectively.[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] The results imply the metal ions is successfully coordinated with organic H\u003csub\u003e2\u003c/sub\u003eBDC-NH\u003csub\u003e2\u003c/sub\u003e linkers. It could be found that the peaks intensity gradually reduced as the increase of NaBH\u003csub\u003e4\u003c/sub\u003e content, indicating the raised unsaturated coordination defects. More sufficient evidence can also be obtained via analyzing the electron paramagnetic resonance (EPR) spectra. As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, it can be seen that these catalysts present the gradual stronger EPR signals located at g = 2.003 with the increase of NaBH\u003csub\u003e4\u003c/sub\u003e content, indicating the introduction of NaBH\u003csub\u003e4\u003c/sub\u003e with different content could significantly modulate the defect concentration within NiFe-MOF.[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eThe morphological structures of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e are investigated via the scanning electron microscopy (SEM) in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee-h, the nanorods morphology can be observed for the NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, which are highly oriented and arranged in parallel with obvious gaps between nanorods. After introducing the NaBH\u003csub\u003e4\u003c/sub\u003e, the as-obtained NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e still present the similar nanorods morphology to NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, indicating NiFe-MOF is well preserved and without structural collapse after introducing reductant agent NaBH\u003csub\u003e4\u003c/sub\u003e. Interestingly, the regular nanorods morphology usually possess high conductivity and strong capillary force to promote the absorption between electrolyte and nanorod surface, which could endow the catalyst surface with abundant accessible active sites for boosting the OER performance.[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ei and j, the transmission electron microscopy (TEM) images of the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e display the nanorods morphology with smooth surface and regular sharp angles at the apexes of the nanorods. For further analysis of the detailed crystal structure of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, the High-resolution TEM (HRTEM) is adopted (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ek). It can be found that the lattice fringes of 0.95 nm can be observed, which matches well with the lattice spacing (002) planes. The halo ring further implies the low-crystalline state of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e in the selected area electron diffraction (SAED) pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003el).[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] The high angle annular dark-field scanning TEM (HAADF-STEM) and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images reveal uniformly distributed on the nanorods (Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo reveal the effect of coordination environment on the chemical components and valence state, the X-ray photoelectron spectroscopy (XPS) for NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e are performed (Figure S2). As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, the high-resolution Fe 2p spectrum of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e present two peaks of Fe 2p\u003csub\u003e3/2\u003c/sub\u003e at 712.2 eV and Fe 2p\u003csub\u003e1/2\u003c/sub\u003e at 725.6 eV, accompanied by two satellite peaks, revealing the characteristic features of Fe\u003csup\u003e3+\u003c/sup\u003e.[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] In the high-resolution Ni 2p spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), two discernible peaks at 856.1 and 873.6 eV can be attributed to Ni 2p\u003csub\u003e3/2\u003c/sub\u003e and Ni 2p\u003csub\u003e1/2\u003c/sub\u003e electronic configurations, which offer an indication of Ni\u003csup\u003e2+\u003c/sup\u003e.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] In comparison to the XPS spectrum of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, the Fe 2p spectra of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e display a subtle negative shift of approximately 0.3 eV, whereas no notable shift is discernible for the Ni 2p spectra. This observation suggests that unsaturated coordination defects may be predominantly localized around the Fe center, thereby inducing alterations in the electronic structure and valence states from Fe\u003csup\u003e3+\u003c/sup\u003e to Fe\u003csup\u003e2+\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, the high-resolution O 2p spectra of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e can be deconvoluted into lattice oxygen (530.5 eV), oxygen vacancies (531.8 eV), and oxygen in carboxylate group (532.4 eV).[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] Notably, there is a significant reduction in the intensity of the lattice oxygen peak, while a sharper peak can be observed for the oxygen vacancies compared to that of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, further indicating the formation of unsaturated coordination defects within NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. The XPS valence band spectra are measured to further investigate the electronic properties of the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, as shown in Figure S3. The valence band maximum energy of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e has undergone a negative shift towards the Fermi level, approximately 0.65 eV lower than those of NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e (0.78 eV), NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e (1.13 eV), and NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e (1.47 eV), suggesting the introduction of unsaturated coordination defects can effectively modulate the electronic structure and increase the conductivity. [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eX-ray absorption structure (XAS) analysis is further conducted to elucidate the local coordination environment and the electronic configuration of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, together with the Fe foil, Ni foil, FeO, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, NiO, and NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e as the comparison. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, Fe K-edge X-ray near-edge structures (XANES) spectrum show that the spectral profile of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e is close to that of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e reference, implying an average valence state of Fe is + 3. After introducing unsaturated coordination, the Fe K-edge XANES spectrum displays that the adsorption threshold position of the as-formed NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e has a slightly negative shift, indicating that the valence of Fe in NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e is between + 2 and + 3. As for Ni K-edge XANES spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee), the profiles of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e are similar to that of NiO reference, indicating the presence of Ni\u003csup\u003e2+\u003c/sup\u003e. The Fourier transform (FT) k\u003csup\u003e3\u003c/sup\u003e-weighted extended X-ray absorption fine structure (FT-EXAFS) spectra are further analyzed. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef, the Fe K-edge FT-EXAFS spectra of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e exhibit a prominent peak located at ~ 1.47 Å, which is mainly attributed to the Fe-O bond of the first coordination shell.[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] Similar to the analysis of the Fe K-edge, a major peak at 1.56 Å is also observed for the Ni K-edge, which is assigned to the Ni-O bond of the first coordination shell (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg).[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] The other detectable small peaks related to Fe/Ni-O coordination confirms the formation of NiFe-MOF structure. Wavelet transform (WT) of Fe/Ni K-edge EXAFS spectra is applied to investigate the metal distribution at atomic resolution. As displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh, S7, and S7, the WT contour plots of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e reveals a single intensity maximum approximately 4.7 Å\u003csup\u003e−1\u003c/sup\u003e in k space, attributed to Fe/Ni-O.[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] The above observation indicates that the Ni/Fe atoms are predominantly dispersed on the NiFe-MOF structure in the form of single atoms.\u003c/p\u003e \u003cp\u003eThe coordination configuration of the Fe/Ni atom in NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e is further examined by the quantitative EXAFS fitting analysis (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and S2). The first shell coordination numbers (CNs) for Fe-O and Ni-O in the NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e are about 5.8 and 6.3, respectively, revealing coordination configuration of FeO\u003csub\u003e6\u003c/sub\u003e-NiO\u003csub\u003e6\u003c/sub\u003e within NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e. In contrast, after introducing unsaturated coordination defects, the CNs values of Fe-O and Ni-O are 4.4 and 6.1 for the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, respectively. Such indicates that the Fe centers are coordinated with four O atoms, while the Ni centers are still coordinated with six oxygen atoms (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e) in NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. The above results suggest that the introduction NaBH\u003csub\u003e4\u003c/sub\u003e could induce the variation of coordination environment and electronic distribution only at around Fe atoms, with a negligible alteration observed at the Ni sites. The Fe L-edge XANES spectra of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e are thus recorded to further examine the impact of unsaturated coordination defects on Fe 2p orbital electronic configuration. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei, the Fe L-edge XANES spectra of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e exhibits a negative shift compared to that of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, further suggesting an Fe valence state between + 2 and + 3. The two peaks located at 708.9 and 712.2 eV could be observed in the Fe L\u003csub\u003e3\u003c/sub\u003e-edge region, which are attributed to Fe 3d t\u003csub\u003e2g\u003c/sub\u003e and Fe 3d e\u003csub\u003eg\u003c/sub\u003e sub-bands, respectively. Notably, compared with the NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, there is a noticeable decrease in the peak intensity of Fe 3d t\u003csub\u003e2g\u003c/sub\u003e and e\u003csub\u003eg\u003c/sub\u003e for the target NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, suggesting an enhanced Fe 3d unpaired electron occupation. To clarify the possible charge transfer between the t\u003csub\u003e2g\u003c/sub\u003e and e\u003csub\u003eg\u003c/sub\u003e orbitals of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, the O K-edge XANES spectra are further investigated. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej, the two characteristic peaks captured at 529.2 and 531.6 eV, emphasized by yellow and gray patterns, could be attributed to the hybridization between unoccupied O 2p orbitals and Fe 3d orbitals. Compared with the NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, the characteristic peak located at ∼529.2 eV of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e became stronger, while the peak at ∼531.6 eV of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e became weaker, which could be ascribed to the electron transition from t\u003csub\u003e2g\u003c/sub\u003e to e\u003csub\u003eg\u003c/sub\u003e orbitals.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] Based on the above results, we propose that ligand field regulation is an effective strategy to flexibly manipulate the migration of 3d orbital electrons, which could induce the precise modulation of electron spin states of NiFe-MOF catalysts. Moreover, the energy band structures of these NiFe-MOF are determined (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ek), and the dz\u003csup\u003e2\u003c/sup\u003e energy level of Fe atoms could be well decreased with spin state transition.\u003c/p\u003e \u003cp\u003eThe ferromagnetic hysteresis loops and EPR spectra of the catalysts are recorded to probe the unpaired electrons and spin state on Fe ions. As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, the saturation magnetization gradually enhances with the increase of NaBH\u003csub\u003e4\u003c/sub\u003e content. When the NaBH\u003csub\u003e4\u003c/sub\u003e content is up to 5 mg, the strongest saturation magnetization of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e could be observed, which may be attributed to the presence of more unpaired electrons in the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e.[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] The inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea is the enlarged view of the ferromagnetic hysteresis loops around H = 0. It can be seen that the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e displays a stronger magnetic field (Hc) of 649.81 Oe and higher residual magnetization (Mr) of 0.023 emu g\u003csup\u003e− 1\u003c/sup\u003e compared with other catalyst references, indicating the enhanced spin polarization within NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. The EPR spectra presents that NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e have a higher peak intensity at g = 2.001 compared to NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, indicating the enhanced unpaired electrons number and spin state (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb).[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e] To further investigate the electron spin state of Fe 3d orbitals for the NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, the zero-field cooling (ZFC) temperature-dependent magnetic susceptibility (M-T) measurements are performed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] The effective magnetic moment (µ\u003csub\u003eeff\u003c/sub\u003e) can be determined through the application of the Langevin theory and the Curie-Weiss law. The µ\u003csub\u003eeff\u003c/sub\u003e value of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e is calculated to be 1.9, 2.3, 3.4, 5.3 µ\u003csub\u003eeff\u003c/sub\u003e, respectively. The unpaired d electron numbers (n) of Fe 3d orbitals are further obtained via the equation: 2.828\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{{{\\chi\\:}}_{\\text{m}}\\text{T}}\\)\u003c/span\u003e\u003c/span\u003e = 𝜇\u003csub\u003eeff\u003c/sub\u003e = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sqrt{\\text{n}(\\text{n}+2)}\\)\u003c/span\u003e\u003c/span\u003e. The average number of the unpaired d electron is calculated to be 4.4 in Fe 3d orbitals for the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, which is higher than that of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e (1.2), NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e (1.5), and NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e (2.6). Combined with these test results, it can be found that the electrons may undergo an incremental transfer from the Fe-t\u003csub\u003e2g\u003c/sub\u003e orbitals to the Fe-e\u003csub\u003eg\u003c/sub\u003e orbitals with a reduced coordination saturation, leading to a precise change in the spin polarization of Fe sites from LS to MS and then to HS state mixed with Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e. The change of electronic structures corresponded to Fe 3d orbitals of these catalysts could be presented in Figure S9. As is known to all, the optimal filling of the e\u003csub\u003eg\u003c/sub\u003e orbitals plays a pivotal role in influencing orbital hybridization. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] The presence of an unpaired electron in the d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbital facilitates its ready penetration into the bonding σ-orbital of oxygen.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] Consequently, for the Fe\u003csup\u003e2+/3+\u003c/sup\u003e with a HS state, the optimal electrons occupancy in the d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbital is conductive to the hybridization between the Fe 3d and O 2p orbital, facilitating the generation of oxygen-containing intermediates and spin-dependent charge transfer.\u003c/p\u003e \u003cp\u003eTo offer a comprehensive perspective on the interpretation of orbital hybridization in NiFe-MOF catalysts, we examine the spin configurations of metal iron and nickel. Based on the aforementioned results, the possible stable local spin configurations of iron and nickel cations are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed. The d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals of Ni\u003csup\u003e2+\u003c/sup\u003e with LS state is empty without electrons, while the d\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003csub\u003e−y\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e and 𝜋-symmetry d orbitals (d\u003csub\u003exy\u003c/sub\u003e, d\u003csub\u003exz\u003c/sub\u003e, and d\u003csub\u003eyz\u003c/sub\u003e) of Ni\u003csup\u003e2+\u003c/sup\u003e is fully occupied. The schematic representation of the electronic coupling between Ni and Fe provides a clear illustration of the detailed electron transfer pathway. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee, the electron-electron repulsion is regarded as the dominant interaction between Ni\u003csup\u003e2+\u003c/sup\u003e and bridging O\u003csup\u003e2−\u003c/sup\u003e. Except for Fe\u003csup\u003e2+\u003c/sup\u003e with LS state, all Fe cations possess unpaired electrons in 𝜋-symmetry d orbitals, enabling them to interact with bridging O\u003csup\u003e2−\u003c/sup\u003e ions through 𝜋-donation. Once the Fe\u003csup\u003e2+/3+\u003c/sup\u003e and Ni\u003csup\u003e2+\u003c/sup\u003e are coupled, electron-electron repulsion within the Ni-O coordination strengthens the 𝜋-donation of the Fe-O bond though the push-pull electronic effect, significantly enhancing the charge transport pathway [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. According to the molecular orbital bonding principle, the d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals of metal sites are inclined to interact with 2p\u003csub\u003ez\u003c/sub\u003e/π\u003csub\u003e2p\u003c/sub\u003e* of oxygen-containing intermediates to form strong σ-type bonds, while the weak π-type bonds could be formed by the hybridization of d\u003csub\u003exz/yz\u003c/sub\u003e and 2p\u003csub\u003ey/z\u003c/sub\u003e orbitals [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The interactions between d\u003csub\u003exy\u003c/sub\u003e and d\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003csub\u003e−y\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals of metal sites and the 2p\u003csub\u003ex/y/z\u003c/sub\u003e orbitals of the oxygen intermediates are negligible because of the relative inertness of d\u003csub\u003exy\u003c/sub\u003e and d\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003csub\u003e−y\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e compared with d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e and d\u003csub\u003exz/yz\u003c/sub\u003e orbitals. Therefore, neither the d\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003csub\u003e−y\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e nor d\u003csub\u003exy\u003c/sub\u003e participate in bond formation in the diagram. The bond orders (BOs) of OH* and O* intermediates on Fe sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef) are calculated to quantify the adsorption strength (bond order = (number of bonding electrons – number of antibonding electrons)/2). As is known to all, a higher value of BO, a stronger spin-orbital interaction between the metal sites and the intermediates.[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] It can be seen that the BOs values of OH* are determined to be 1.5, 1.5, 2, and 1.5 on the Fe\u003csup\u003e2+/3+\u003c/sup\u003e(LS), Fe\u003csup\u003e3+\u003c/sup\u003e(MS), Fe\u003csup\u003e3+\u003c/sup\u003e(HS), and Fe\u003csup\u003e2+\u003c/sup\u003e(HS), respectively. The higher BOs value indicate that the OH* intermediates is preferentially adsorbed on HS Fe\u003csup\u003e3+\u003c/sup\u003e, facilitating the initiation of the OER process. Additionally, the higher BOs value of O* on HS Fe\u003csup\u003e2+/3+\u003c/sup\u003e compared to LS Fe\u003csup\u003e2+/3+\u003c/sup\u003e reveal the OER thermodynamic advantage for HS Fe ions due to the strengthened Fe-O bond [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Apparently, Fe ions with high spin polarization exhibit optimal bond order values with reaction intermediates, facilitating the rapid adsorption of OH* species on metal sites and the dissociation of metal-OH bond to ensure the effective recycling of O* and OH* species, achieving a significantly accelerated OER kinetic process. These findings underscore the critical role of spin regulation in enhancing oxygen reactivity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe density functional theory (DFT) calculations are performed to intuitively investigate the change of d-orbital electronic structure with different coordination environments using spin states as descriptors. The spin density patterns of Fe-MOF and NiFe-MOF are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. In the pure Fe-MOF, where all available sites are occupied by Fe, the next-to-nearest neighbor Fe cations respectively exhibit spin-up and spin-down character, which could lead to band gap opening associated with pronounced Jahn-Teller distortions [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. In addition, the oxygen ligand connecting to two Fe ions exhibits a low spin density (0.017 e), indicating the long-range spin-related charge transfer within the Fe-MOF is sluggish. Upon introducing Ni ions to Fe-MOF, the spin density of Fe aligns with that of Ni, and the oxygen ligand between Ni and Fe ions presents a high spin density. This could lead to ferromagnetic quantum spin orbital exchange interactions between Fe and Ni ions, facilitating the formation of a spin channel in the oxygen atoms for fast charge transport. Consequently, it could be considered that the catalytic activity of NiFe-MOF primarily originates from the Fe sites, while the presence of both Ni and Fe in the MOF with Fe as dominant sites is necessary for a fast charge transfer and high OER activity.[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e] The spin density patterns of NiFe-MOF with different “Fe-O” coordination structures (the structural models are shown in Figure S10-S12, including NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e, NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e, and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e models) are further calculated to investigate the change of spin polarization of metal and spin-related charge transport dynamics. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, it is apparent that spin density of Fe sites within NiFe-MOF increased significantly with reduced saturated coordination, while there is not notable change in the low spin density of Ni sites, indicating the alteration in the coordination environment could solely modulate the spin polarization of Fe site, with a negligible change observed at the Ni sites. Additionally, the spin density of the oxygen ligand connecting Fe and Ni ions also increases as coordination becomes more unsaturated, suggesting the increased spin hole in the bridged oxygen ligand, which could facilitate accelerated charge transport in the catalysts. The charge density differences are calculated to intuitively understand the change of electronic structure. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, it is evident that the Fe sites and bridged oxygen atoms exhibit an electron-deficient and electron-rich state for the three models, respectively, indicating the possible formation of electron transfer channels.[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] The calculated bader charge of Fe atoms gradually increase from 6.384 e (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model) to 6.417 e (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e model) and 6.530 e (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model), which further demonstrates fast charge transfer and redistribution of electrons triggered by unsaturated coordination of Fe sites.\u003c/p\u003e \u003cp\u003eThe spin-resolved density of states (DOS) on NiFe-MOF with different “Fe-O” structures are determined to explore the change of electronic structure within Fe 3d orbital. As shown in the DOS diagram of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, the unoccupied e\u003csub\u003eg\u003c/sub\u003e orbital of Fe gradually approaches the occupied t\u003csub\u003e2g\u003c/sub\u003e orbital with the increase of unsaturated coordination. Meanwhile, the bonding state near the Fermi level of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e models are stronger than that of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model, indicating the electronic migration from t\u003csub\u003e2g\u003c/sub\u003e to e\u003csub\u003eg\u003c/sub\u003e orbital inside Fe sites. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] Therefore, we can infer that the Fe-3d orbitals of NiFe-MOF with different unsaturated coordination models exhibit the precise transition from low to medium and then to high spin polarization. Besides, the spin magnetic moment of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model is determined to be 3.724 µ\u003csub\u003eB\u003c/sub\u003e, larger than that of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e (1.794 µ\u003csub\u003eB\u003c/sub\u003e) and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e (1.449 µ\u003csub\u003eB\u003c/sub\u003e) models (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), further revealing the gradual increased spin polarization with greater unsaturation in coordination. As displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, the d-band center of Fe sites gradually become close to the Fermi level with the increase of spin polarization, demonstrating the decreased occupation of the anti-bonding state, thus optimizing the adsorption energy between metal Fe site and oxygen intermediates. The adsorption capacity of Fe sites towards OH* and OOH* is investigated from the perspective of bonding by calculating the crystal orbital Hamilton population (COHP). The up/down curves are on behalf of the bonding and antibonding contribution, respectively. For the OH* adsorption, the NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model with high spin states exhibits decreased occupation of antibonding state, accompanied by the lower integrated COHP (ICOHP) value (1.864) than that of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e (1.802) and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model (1.793), as presented in Figure S13a-c. Meanwhile, a more negative ICOHP in the NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model can be observed for the OOH* adsorption compared with the NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model (Figure S13d-f), indicating the precise modulation of spin states can flexibly adjust the adsorption and desorption behavior of reaction intermediates.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Gibbs free energies (∆G) of each OER step are calculated to further identify the intrinsic catalytic activity of NiFe-MOF model. Figure S14 exhibits the OER pathway at metal Ni and Fe sites of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model, including the following stages: * → OH* → O* → OOH* → O\u003csub\u003e2\u003c/sub\u003e.[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] For the Ni sites, the transformation of OH* to O* at U = 0 V is deemed the rate-determining step (RDS) due to the largest value of free energy among all steps (Figure S15a). Conversely, the RDS at Fe sites is found to be the third step (O* → OOH*), in which the free energy values of Fe sites (2.04 eV) is lower than that of the Ni sites (2.09 eV). The calculated OER overpotential of Fe sites (0.80 V) is also smaller than that of Ni sites (0.86 V) at U = 1.23 V (Figure S15b). These results indicate that the Fe sites could possess faster reaction dynamics and better activity than that of Ni center, which may serve as the primary active center involved in the OER. For the NiFe-MOF models with different “Fe-O” coordination, the ΔG value of typical OER pathways with U = 0 was calculated to determine the OER activity at the central Fe sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg). It is evident that all the elementary reactions within those NiFe-MOF models exhibit uphill trends in the OER process, while the third step involving O* → OOH* demonstrates the highest free energy change for each NiFe-MOF model, indicating that the transition from O* to OOH* intermediate on the Fe sites is regarded as the RDS during OER process. The ΔG value of RDS for NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model is concluded to be 2.04 eV, higher than that of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e model (1.91 eV) and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model (1.75 eV). The calculated theoretical overpotential on the Fe sites effectively reduces from 0.8 V (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e model) to 0.68 V (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e model) and 0.52 V (NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model), revealing that the NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model with Fe high spin state could accelerate the formation of OOH* intermediate and decrease the overpotential (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). The corresponding structure changes of NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model (Fe sites) during OER process is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh. Additionally, the ΔG values of OH* adsorption for NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e6\u003c/sub\u003e, NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e5\u003c/sub\u003e and NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e models are determined to be 0.91, 0.11, and 0.04 eV, respectively. The low OH* adsorption energy indicates the NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e model with Fe HS state is conducive to adsorbing OH* intermediates for initiating the OER process.[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e] These results theoretically validate the flexible modulation of ligand fields can evidently induce the 3d orbital electron migration to realize precise changes in electron spin states, which could optimize the d-band center and enhance orbital hybridization of Fe 3d and O 2p, thereby accelerating the reaction kinetics and improving the OER performance.\u003c/p\u003e \u003cp\u003eThe catalytic OER activity of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e with different electron spin configuration of Fe sites are evaluated in a standard three-electrode system containing the 1M KOH electrolyte. All the linear sweep voltammetry (LSV) polarization curves were calibrated with iR-correction to appraise the overpotentials unless noted. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and b, compared to NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e (569 and 670 mV), NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e (508 and 606 mV), NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e (424 and 470 mV), the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e only require the smaller overpotentials of 328 and 365 mV to drive the ampere-level current density of 1 A cm\u003csup\u003e− 2\u003c/sup\u003e and 1.5 A cm\u003csup\u003e− 2\u003c/sup\u003e, respectively. Such increased OER activity highlights the significant roles of the modulated electronic structures of metal Fe sites induced by introducing abundant unsaturated coordination defects. Notably, the overpotential of the target NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e is comparable, or even better than other previously reported MOF-based catalysts, highlighting the outstanding catalytic OER activity (Table S3). Likewise, as displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e also displays the lowest Tafel slope value of 44.3 mV dec\u003csup\u003e− 1\u003c/sup\u003e than those of NiFe-MOF (58.7 mV dec\u003csup\u003e− 1\u003c/sup\u003e), NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e (51.7 mV dec\u003csup\u003e− 1\u003c/sup\u003e), NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e (47.1 mV dec\u003csup\u003e− 1\u003c/sup\u003e) and commercial RuO\u003csub\u003e2\u003c/sub\u003e (87.2 mV dec\u003csup\u003e− 1\u003c/sup\u003e), indicating its accelerated reaction kinetics, which can be further verified by the electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, it can be found that the charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e) values of those catalysts develop in general yields the trend: NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e \u0026lt; NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e \u0026lt; NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e \u0026lt; NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e. The double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) value, that is obtained from the cyclic voltammetry curves in a non-Faradaic region (Figure S16), is calculated to evaluate the electrochemical active surface area (ECSA). The calculated C\u003csub\u003edl\u003c/sub\u003e value of target NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e is gained as high as 1.07 mF·cm\u003csup\u003e− 2\u003c/sup\u003e, with the NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e reference (0.97 mF·cm\u003csup\u003e− 2\u003c/sup\u003e) coming next, followed by the NiFe-MOF reference (0.83 mF·cm\u003csup\u003e− 2\u003c/sup\u003e), and the NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e reference (0.75 mF·cm\u003csup\u003e− 2\u003c/sup\u003e) the last (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). The highest C\u003csub\u003edl\u003c/sub\u003e value implies more roughest surface created in the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, thereby provide more efficient active sites for boosting the OER activity.[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e] The comparing polarization curves normalized by ECSA of the target NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e and other catalyst references further demonstrate the highest intrinsic OER activity (Figure S17). As observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef, the mass activity (MA) calculated by the inverse proportion relation between the current density and mass load are arranged in this order of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e \u0026gt; NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e \u0026gt; NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e \u0026gt; NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e. It shows that the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e has the highest mass activity value, further indicating the significant intrinsic OER activity among all the above catalysts.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] The trends observed in turnover frequency (TOF) values align with those of the MA, with the high spin state yielding superior TOF values (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg and S18). The cyclic voltammetry (CV) and chronopotentiometric (v-t) tests are conducted to assess the stability of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e. As depicted in in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh, there is no obvious positive shift observed in polarization curve after successive CV scanning for 20000 cycles. Similarly, the v-t curve recorded at a permanent current density of 1 A cm\u003csup\u003e− 2\u003c/sup\u003e displays minimal voltage increases even after a continuous operation for 170h (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ei), which significantly surpasses the stability of other reported catalysts (Table S3), suggesting the superior stability and durability of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e towards OER in alkaline medium.\u003c/p\u003e \u003cp\u003eTo simulate industrial conditions of water electrolysis, an alkaline anion exchange membrane water electrolysis (AEMWE) full-cell is assembled to assess catalytic performance, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ej. The as-prepared NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e is used as the anode, while the Pt/C, known for its high hydrogen evolution reaction activity, was utilized as the cathode (NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e // Pt/C). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ek, the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e // Pt/C two-electrode system only require the low cell voltages of 1.77 V to reach current densities of 500 mA cm\u003csup\u003e− 2\u003c/sup\u003e, which is comparable, or even better than the other previously reported catalysts in alkaline electrolyte (Table S4) and reaches the United States Department of Energy (DOE) target. After conducting continuous electrolysis for 1 hour with an AEMWE full cell, the calculated Faraday efficiency was determined to be close to 100% by measuring the volume of generated O\u003csub\u003e2\u003c/sub\u003e using the water drainage method, as shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003el. In addition, the homemade electrocatalytic mold could maintain a stable cell voltage at 500 mA cm\u003csup\u003e− 2\u003c/sup\u003e for 55 h, highlighting its significant potential for practical application (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003em).[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e] To verify potential industrial applications, the cell efficiency and economic efficiency of the electrolyzer is evaluated. The calculated cell efficiency is 70.8%, higher than that of commercial RuO\u003csub\u003e2\u003c/sub\u003e // Pt/C (57.1%) The estimated cost for generating per gasoline-gallon equivalent (GGE) H\u003csub\u003e2\u003c/sub\u003e is as low as \u003cspan\u003e$\u003c/span\u003e 0.94 according to the formula listed in supporting information, which is significantly lower than the target of \u003cspan\u003e$\u003c/span\u003e 2.00 by 2026 from the U.S. DOE.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the underlying reasons for highly efficient catalytic activity toward OER by spin state modulation, we carried out a series of control experiments. The adsorption energies of the activated OH* intermediate on the catalyst surface is measured via the methanol oxidation reaction (MOR) method. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and S18, the onset potential of MOR for NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e displays a negative shift of approximately 90 mV, surpassing those of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e (67 mV), NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e (75 mV), and NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e (79 mV). This outcome implies that the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e possesses relatively high capacity for OH* adsorption, thereby facilitating the first step of the OER process.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] In-situ EIS measurements at various potentials are conducted to further explore the reaction kinetics (Figure S20 a and c). It is observed that the semicircles corresponding to NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e diminished more quickly than that of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e as the potential increased, indicating the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e is more susceptible to polarization for rapidly adsorbing OH* intermediate at low potentials. Meanwhile, the phase peak values in the Bode diagram also display a similar trend to the semicircles, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb and S20b. In comparison to the NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e consistently possess lower phase peak and R\u003csub\u003ect\u003c/sub\u003e values at the same potential (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and S21), further indicating the advantageous OER kinetics to trigger the OER process.[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e] The deuterium kinetic isotope effects (KIEs) are employed to provide evidence for proton transfer kinetics (Figure S22).[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e] Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed and e present that the KIE values of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e are concluded to be 1.61 in the overpotential regions, lower than that of NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e (1.78) at the same potentials regions, demonstrating that the proton transfer kinetics of NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e could be significantly enhanced after enhancing the spin state of Fe sites.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eIn-situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra is recorded at different potentials to ascertain the RDS of the OER process. As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef and g, two broad peaks centered at about 1220 and 1050 cm\u003csup\u003e− 1\u003c/sup\u003e are attributed to the Si-O-Si stretching band and surface OOH* intermediate in the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, respectively.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] Notably, the characteristic peak of surface OOH* intermediate appeared at an applied voltage of 1.20 V for the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, which is easily increased in intensity as the potential moves positively. By contrast, the OOH* peak for NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e emerge at about 1.25 V. Consequently, the aforementioned findings robustly affirm that the Fe active sites with a high spin state could significantly accelerate the RDS of O* → OOH* during the OER process for promoting the generation of OOH* intermediate on the NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] Based on these results, flexible control of coordinative environment and spin state is conductive to optimizing the adsorption of OH* intermediates and the reaction barriers of OOH* intermediates, which could dramatically accelerate the electron transfer kinetics (H\u003csub\u003e2\u003c/sub\u003eO→OH* and O*→OOH*), thereby resulting in the outstanding OER performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e "},{"header":"Conclusion","content":"\u003cp\u003eIn summary, this work can not only accurately alter the coordination number around active metal center, but also achieve flexible control of spin state, thereby enhancing the OER activity at the ampere-level current density. Experimental characterizations together with theoretical analysis elucidate that the electrons of Fe 3d orbital undergo a controlled transfer from the t\u003csub\u003e2g\u003c/sub\u003e to e\u003csub\u003eg\u003c/sub\u003e orbitals as coordination number reduced, leading to the increased spin polarization of Fe. However, the Ni sites within NiFe-MOF still maintain the NiO\u003csub\u003e6\u003c/sub\u003e geometric configuration with low spin states. It could be found that the ferromagnetic quantum spins orbital exchange interactions between HS state Fe and LS state Ni ions are more conductive to creating spin channel of bridged oxygen ligand and penetrating the bonding σ-orbitals of oxygen compared to those in MS and LS state Fe, which optimizes the d-band center of metal sites and the adsorption of oxygen intermediates. Therefore, the obtained NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e catalyst with unique NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4\u003c/sub\u003e configuration and high spin state of Fe displays a low overpotential (328 mV @ 1 A cm\u003csup\u003e− 2\u003c/sup\u003e and 365 mV @ 1.5 A cm\u003csup\u003e− 2\u003c/sup\u003e) and long-term stability over 170 h in alkaline medium. In addition, the assembled AEMWE flow cell with target NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e catalyst and Pt/C also presents remarkable activity and stability in 1M KOH. The present work offers theoretical foundation for efficiently improving OER activity through deliberately manipulating the electron spin configurations and ligand field, which can also provide a precise guidance for the design and development of other advanced non-precious metal-based OER catalysts.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003eMaterials preparation\u003c/h2\u003e\n \u003cp\u003eA mixture consisting of Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO (0.66 mmol), Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO (0.33 mmol), and H\u003csub\u003e2\u003c/sub\u003eBDC-NH\u003csub\u003e2\u003c/sub\u003e (1 mmol) was dissolved in a solvent mixture of 30 mL DMF, 2 mL deionized water, and 1mL ethanol. This solution was stirred for 30 minutes to ensure uniformity. Subsequently, the well-mixed solution was transferred to a 50 mL Teflon-lined autoclave and heated in a temperature-controlled oven at 120\u0026deg;C for 15 hours. Upon natural cooling to room temperature, the resultant was thoroughly washed with DMF and ethanol and then dried at 50\u0026deg;C under vacuum. After that, 200 mg of the NiFe-MOF was mixed with varying amounts of NaBH\u003csub\u003e4\u003c/sub\u003e in 5 mL deionized water under vigorous stirring. Then the products were collected by centrifugation with three times washing using deionized water instantly. The final products were dried in a vacuum oven at 50\u0026deg;C for 12 hours. The quantities of NaBH\u003csub\u003e4\u003c/sub\u003e used were controlled as 0 mg, 1 mg, 3 mg, and 5 mg, corresponding to the samples named NiFe-MOF-D\u003csub\u003e0\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e1\u003c/sub\u003e, NiFe-MOF-D\u003csub\u003e2\u003c/sub\u003e, and NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e, respectively.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eMaterial characterizations\u003c/h3\u003e\n\u003cp\u003eThe crystal structures and phase compositions of as-prepared catalysts were identified by a Bruker D8 Advanced Diffractometer with Cu K\u0026alpha; radiation (\u0026lambda;\u0026thinsp;=\u0026thinsp;1.54056 \u0026Aring;). The morphology characterizations were determined by field emission scanning electron microscopy (FESEM, SU 8000) The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) were carried out on the FEI Tecnai G2 F20. The contact angles of those catalysts were tested on JC 2000 DM. The X-ray photoelectron spectroscopy (XPS) was conducted on the ESCALAB 250Xi, equipped with an X-ray source (Al K\u0026alpha;) at h\u0026nu;\u0026thinsp;=\u0026thinsp;30 eV. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements of Fe and Ni K-edge were performed in fluorescence mode at the beamline BL14W1.\u003c/p\u003e\n\u003ch3\u003eDFT Computation\u003c/h3\u003e\n\u003cp\u003eWe carried out all the DFT calculations in the Vienna \u003cem\u003eab initio\u003c/em\u003e simulation (VASP 6.3.0) code. The exchange-correlation is simulated with PBE functional and the ion-electron interactions were described by the PAW method. The vdWs interaction was included by using empirical DFT-D3 method. The electronic structure of the pure Fe-MOF and the NiFe-MOF were calculated to describe the effect of O vacancy. The NiFe-MOF exposing Fe and Ni site was used to simulate the electrochemical OER. The simplified Cu\u003csub\u003e3\u003c/sub\u003e based MOF was placed in a 15\u0026times;15\u0026times;25 \u0026Aring;\u003csup\u003e3\u003c/sup\u003e cubic box, and only the \u0026Gamma;-point was used to sample the Brillouin zone. All atoms are allowed to move freely during the geometry optimization. The cutoff energy is set as 450 eV for all periodic structure. The convergence criteria are set as 0.02 eV A\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV in force and energy, respectively.\u003c/p\u003e\n\u003cp\u003eThe OER process were analyzed by calculating the reaction free energies of all elementary steps along the reaction coordinates, including (i) the hydroxide ion dissociates into OH groups adsorbed on the catalyst surface (*OH); (ii) *OH further reacts with hydroxide ions to dissociate into O groups (*O), and generates water molecules; (iii) *O reacts with a hydroxide ion and produces an OOH group (*OOH); (iv) eventually O\u003csub\u003e2\u003c/sub\u003e is produced and then released from catalyst.\u003c/p\u003e\n\u003cp\u003eThe free energy calculation of species adsorption (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\Delta G\\)\u003c/span\u003e\u003c/span\u003e) is based on following model.\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eHerein \u0026Delta;\u003cem\u003eE\u003c/em\u003e, \u0026Delta;\u003cem\u003eE\u003c/em\u003e\u003csub\u003eZPE\u003c/sub\u003e, and \u0026Delta;\u003cem\u003eS\u003c/em\u003e respectively represent the changes of electronic energy, zeropoint energy, and entropy that caused by adsorption of intermediate. The \u0026Delta;\u003cem\u003eH\u003c/em\u003e\u003csub\u003e0\u0026rarr;\u003cem\u003eT\u003c/em\u003e\u003c/sub\u003e refers to the change in enthalpy when heating from 0K to T K (298K in this work). The entropy of H\u003csup\u003e+\u003c/sup\u003e+e\u003csup\u003e\u0026minus;\u003c/sup\u003e pair is approximately regarded as half of H\u003csub\u003e2\u003c/sub\u003e entropy in standard condition. The \u003cem\u003eG\u003c/em\u003e\u003csub\u003eU\u003c/sub\u003e is the applied potential, here is set for 0 V and 1.23 V.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eElectrochemical characterizations\u003c/h2\u003e\n \u003cp\u003eThe electrochemical measurements were conducted on an electrochemical workstation based on the three-electrode system, in which the as-prepared catalysts were used as the working electrode, a graphite rod as the auxiliary electrode, and Hg/HgO electrode as the reference electrode in 1M KOH (pH\u0026thinsp;=\u0026thinsp;13.8) electrolytes, respectively. The measured potential was calibrated to the reversible hydrogen electrode (RHE) according to the equation of E\u003csub\u003eRHE\u003c/sub\u003e = E + 0.098 V\u0026thinsp;+\u0026thinsp;0.059 pH. The linear sweep voltammetry (LSV) was performed with iR compensation at a scan rate of 5 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The prolonged stability test was implemented by using the chronopotentiometric curve without iR compensation. The corresponding Tafel plots were obtained by fitting the polarization curves between the potential and log current density (log j) by the equation: \u0026eta;\u0026thinsp;=\u0026thinsp;b log(j)\u0026thinsp;+\u0026thinsp;a, where b is the Tafel slope. Electrochemical impedance spectroscopy (EIS) spectra were recorded at a selected potential in the frequency range from 100 kHz to 0.01 Hz. The values of TOF were calculated according to the following equation:\u003cbr\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{T}\\text{O}\\text{F}=\\frac{jA}{4Fn}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003cp\u003eHere, A (cm\u003csup\u003e2\u003c/sup\u003e) stands for the geometric area of the Ni foam electrode. The number 4 means the four electrons transfer in OER and F equals to the constant of 96485.3 C mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. n represents the number of active sites.\u003c/p\u003e\n \u003cp\u003eThe loading mass (m, mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) of catalysts and the measured current density \u003cem\u003ej\u003c/em\u003e (mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) was used to calculate the mass activity.\u003c/p\u003e\n \u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$$\\:\\text{M}\\text{a}\\text{s}\\text{s}\\:\\text{a}\\text{c}\\text{t}\\text{i}\\text{v}\\text{i}\\text{t}\\text{y}=\\frac{j}{m}$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eThe electrochemical active surface area (ECSA) of catalysts was calculated from the double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) according to the equation:\u003c/p\u003e\n \u003cdiv id=\"Equb\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e$$\\:\\text{E}\\text{C}\\text{S}\\text{A}\\:=\\:\\frac{{C}_{dl}}{{C}_{s}}$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eHere, the C\u003csub\u003edl\u003c/sub\u003e was obtained though the CV measurements in a non-Faradaic region with the scan rates of 20, 40, 60, 80, 100, and 120 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The C\u003csub\u003es\u003c/sub\u003e is the specific capacitance of an atomically smooth planar surface per unit area under the same electrolyte condition, which is normally between 0.02\u0026ndash;0.06 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. According to our catalytic surface, C\u003csub\u003es\u003c/sub\u003e is usually estimated to be 0.04 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding authors upon reasonable request. Source data are provided as a Source data file. Source data are provided with this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is financially supported by the National Natural Science Foundation of China (52261145700, 22279124), the Natural Science Foundation of Shandong Province (ZR2022ZD30), Qingdao New Energy Shandong Laboratory Open Project (QNESL OP202307), the Fundamental Research Funds for the Central Universities (202262010). the Research Grant Council of Hong Kong (15304023; N_PolyU502/21; CRS_PolyU504/22), the funding for Projects of Strategic Importance of The Hong Kong Polytechnic University (Project Code: 1-ZE2V) and Natural Science Foundation of Guangdong Province (2023A1515012219).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM.H. and B.H. conceived the idea and supervised the project. X.H., Z.Z. and S.Z. performed the experiments and electrolysis measurements. T.N. and J.Z. carried out the DFT calculations. S.D. and L.C. assisted in the material characterization and discussion. H.W. supervised the electrolysis measurements. X.H. and T.N. wrote the manuscript. M.H. and B.H. reviewed and corrected the manuscript. All authors discussed the results and commented on the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information the online version contains supplementary material available at.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflicts of interest.\u003c/p\u003e\n\u003cp\u003eCorrespondence and requests for materials should be addressed to Bolong Huang or Minghua Huang.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eRoger, I., Shipman, M. A. \u0026amp; Symes, M. D. 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Promoting CO\u003csub\u003e2\u003c/sub\u003e electroreduction kinetics on atomically dispersed monovalent Zn\u003csup\u003eI\u003c/sup\u003e sites by rationally engineering proton-feeding centers. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e\u003cstrong\u003e61\u003c/strong\u003e, 2111683 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":false,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5541146/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5541146/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSpin configuration and coordination environment changes have emerged as promising strategies to boost the oxygen evolution reaction (OER) activity. However, achieving the precise and gradual regulation of both spin states and coordination environment to elucidate the structure-activity relationship remains a key priority and is rarely reported. In this work, we successfully induce the gradual transition of spin states of Fe sites from low spin state to a medium spin state and ultimately to high spin state by meticulously adjusting coordination environment within NiFe-MOF, while the Ni sites still keep a low spin state. Experimental and theoretical calculations confirm the precise regulation of spin polarization and electrons migration from the Fe-t\u003csub\u003e2g\u003c/sub\u003e to the Fe-e\u003csub\u003eg\u003c/sub\u003e orbitals with a reduced coordination saturation, which optimizes the spin orbital exchange interactions between Fe and Ni ions and facilitates adsorption of reaction intermediates. The NiFe-MOF-D\u003csub\u003e3\u003c/sub\u003e with unique NiO\u003csub\u003e6\u003c/sub\u003e-FeO\u003csub\u003e4 \u003c/sub\u003egeometric structure exhibits low overpotential of 328 mV@1 A cm\u003csup\u003e-2\u003c/sup\u003e and 365 [email protected] A cm\u003csup\u003e-2\u003c/sup\u003e in alkaline medium. Furthermore, the assembled alkaline electrolyzer also presents remarkable activity (1.77 V@500 mA cm\u003csup\u003e-2\u003c/sup\u003e) and lower cost ($ 0.94) than the target of U.S. Department of Energy ($ 2.00).\u003c/p\u003e","manuscriptTitle":"Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-09 06:40:00","doi":"10.21203/rs.3.rs-5541146/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b2e7cf94-0a3f-4b87-b7e3-fbe0700a3bef","owner":[],"postedDate":"December 9th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":41242480,"name":"Physical sciences/Chemistry/Catalysis/Electrocatalysis"},{"id":41242481,"name":"Physical sciences/Chemistry/Catalysis/Catalytic mechanisms"}],"tags":[],"updatedAt":"2025-09-30T17:00:56+00:00","versionOfRecord":[],"versionCreatedAt":"2024-12-09 06:40:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5541146","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5541146","identity":"rs-5541146","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}