Peilin Huang,Ming Meng,Gang Zhou,Peifang Wang,Wenxian Wei,Hao Li,Rong Huang,Fuchi Liu,Lizhe Liu
Dynamic orbital hybridization triggered spin-disorder renormalization via super-exchange interaction for oxygen evolution reaction
spin-disorder renormalization,dynamic orbital hybridization,super-exchange interaction,oxygen evolution reaction
Significance We propose an intriguing spin renormalized strategy in Fe0.5Co0.5-MOF to accelerate the spin-dependent reaction kinetics in OER, in which the extraordinary super-exchange interaction can be used to reconfigure the domain direction of spin nets through temporarily bonding with dynamic magnetic ions in electrolytes under alternating electromagnetic field stimulation. This spin renormalization treatment from disordering low-spin state to high-spin state facilitates rapid water dissociation and optimal carrier migration, leading to a spin-dependent reaction activity.
Dynamic orbital hybridization triggered spin-disorder renormalization via super-exchange interaction for oxygen evolution reaction

We propose an intriguing spin renormalized strategy in Fe0.5Co0.5-MOF to accelerate the spin-dependent reaction kinetics in OER, in which the extraordinary super-exchange interaction can be used to reconfigure the domain direction of spin nets through temporarily bonding with dynamic magnetic ions in electrolytes under alternating electromagnetic field stimulation. This spin renormalization treatment from disordering low-spin state to high-spin state facilitates rapid water dissociation and optimal carrier migration, leading to a spin-dependent reaction activity.

Water oxidation or oxygen evolution reaction (OER) plays a critical role in exploiting clean and renewable hydrogen (1, 2), which could help reduce the dependence on fossil fuels to achieve carbon neutrality. Essentially, the OER process accompanies with four proton-coupled electrochemical reaction, generally leading to a sluggish reaction kinetics and poor electrochemical performance (3, 4). In the past decades, much attention has been devoted to develop OER electrocatalysts to optimize these four electron-transfer reaction pathways, such as transition metal oxides (5, 6), layered structure materials (7, 8), and perovskites (9, 10). Meanwhile, the electronic structures of catalysts can be regulated by various microstructural strategies (11–14), namely reducing nanostructure sizes, implanting vacancies, doping or alloying, and crystal-phase engineering. These methods can partially regulate the bonding strength between active sites and reactants at adsorption and desorption process, but the overall performance stills unsatisfactory because the spin-dependent charge transfer and orbital interaction are unconsciously underestimated. From the viewpoint of reaction kinetics, the number of unpaired electrons at four-electron OER process is changeable (15, 16), which makes it strongly depend on the domain direction of spin nets. Recent reports also demonstrate that the magnetic potentials acting as selective gates can directly affect the activation energy at rate-limiting step and carrier transfer ability via quantum spin exchange interactions (QSEI) or spin–orbit coupling effect (17–19). These findings inspire us to develop a feasible strategy to reorder the domain direction of spin nets, which could help to achieve a further breakthrough in OER application. However, it is universally known that the domain direction of spin nets has been customized with catalyst formation (20, 21), which is mainly determined by the spatial distribution of spin nets and their spin exchange interaction strength (22, 23). Therefore, the spin configuration cannot be easily regulated by traditional methods if we do not destroy the crystal symmetry or atomic occupation. Consequently, it becomes extremely difficult to understand the intrinsic correlation between spin-dependent catalytic activity and corresponding transformation in spin configuration. According to these considerations, we propose an intriguing dynamic orbital hybridization approach through a super-exchange interaction between magnetic ions in electrolytes and pristine reactive sites to renormalize the catalysts’ spin configuration under the synergetic stimulation in electromagnetic fields. In this concept, the magnetic ions in electrolytes can be driven by electrostatic potentials to generate a temporary orbital hybridization with the reactive sites; meanwhile, this particular super-exchange interaction can renormalize the domain direction of spin nets under an alternating current (AC) magnetic field stimulation. This strategy can be used to control the spin state transformation, because which can successfully break through the spatial limitation for inter-site spin exchange interaction. To confirm our concept, metal–organic frameworks (MOFs) formed by organic ligands coordinated with magnetic elements are designed as typical porous configurations (24–26). The reasons are as follows: 1) the inherent porous structures with a large diameter can be used as magnetic cation channels to conduct a dynamic orbital hybridization more effectively; 2) the magnetic metal sites in MOFs are linked by large-size organic molecules that make their spin exchange interaction becomes very weak to generate a disordering spin configuration, which has an obvious advantage in exhibiting the spin renormalization process; 3) skin effect induced by alternating electromagnetic field in the porous configuration has a significant influence onto forming eddy current owing to the dielectric difference between molecular linkers and magnetic metal sites, where the asymmetrical potential distribution can enforce the coulomb attraction between metal sites and magnetic cations in electrolytes, facilitating spin-related super-exchange interaction. In this work, we propose a new-type bimetallic Fe0.5Co0.5-MOF as the spin-dependent catalyst, in which the directions of spin nets at reactive sites can be renormalized by an intriguing super-exchange interaction from dynamic magnetic ions accompanied with electromagnetic coupling synergistic stimulation, leading to a spin state transformation from disordering low-spin (LS) configuration to high-spin (HS) configuration. The spin-renormalized Fe0.5Co0.5-MOF displays a mass activity of 2,095.1 A gmetal−1 at an overpotential of 0.33 V (see the calculations about mass activity in SI Appendix, Fig. S1 and Table S1), which is about 5.9 times than the pristine sample. More importantly, the spin-renormalized Fe0.5Co0.5-MOF can demonstrate ~99% of its initial performance after 200 h OER application at high current density. This is because that the antibonding or bonding interaction between spin-related catalysts and intermediates play a critical role in regulating the rate-limiting activity barrier and carrier migration.

Based on the crystal theory as shown in Fig. 1A, the octahedral ligands could lead to a series of bonding levels (a1g, eg, t2g) and antibonding levels (a1g*, eg*, t2g*), which are originated from the orbital hybridization between metal center-3d orbitals and ligand-2p orbitals (π/σ). The energy difference between bonding levels and antibonding levels is proportional to the charge transfer from metal centers to ligands, which can directly affect the spin polarization and their spin-exchange interaction. Generally speaking (17–19), the intra-atomic exchange interactions benefit for the high-spin configuration with more unpaired 3d electrons, which makes the d manifolds split into three lower t2g orbitals and two higher eg orbitals. When the oriented metals and ligands are bonded together depending on the crystal symmetry, the bonding interaction and charge transfer could decrease the radial extension of the 3d orbital that increases crystal field effect and reduces the intra-atomic exchange interaction. In this case, the contribution of intra-atomic exchange interaction and crystal filed becomes comparable strength, in which a stronger spin-exchange interaction leads to a high-spin feature but a weaker one gives rise a low-spin behavior. Some slight atomic distortions will be caused to regulate their electronic occupation at 3d orbitals. For a weaker spin-exchange strength in transition metal oxides (marked by M-O), the strong d-d Coulomb interaction makes the antibonding bands split into one empty upper band (UB) and filled lower band (LB) (Left panel in Fig. 1B). The 3d orbtials of metal centers will be protected by M-O bonding level, which cannot provide effective valance electrons to support following catalytic reaction because the energy level of LB is lower than M-O bands. On the contrary, a stronger spin-exchange interaction makes the electronic structure give rise a spin polarization at Fermi level (Right panel in Fig. 1B), which not only reduces the band gap to facilitate coherent electron-spin transport but also can donate half-filled d electrons or empty orbitals to optimize the activation energy at the rate-limiting step (17–19). To better understand this point, the intrinsic catalytic principle for OER is displayed in Fig. 1C, which is strongly depended on the combination of electronic occupation at t2g and eg orbitals. For the reactant chemisorption, the half-filled metal sites require to provide an empty t2g orbital to hybridize with σ orbital of *OH, corresponding to an acceptance of valance electron. Additionally, to decrease the activation energy at *OOH formation, the metal sites must simultaneously possess some separate d electrons that can donate into the antibonding orbitals to weak the orbital interaction with intermediates (27). It is generally accepted that the “acceptance-donation” of electrons between active sites and reactants is the essential interaction, which is strongly related with the 3d-orbtial occupation and their spin exchange interaction. It is important to note that no extra interatomic QSEI can be triggered in MOFs due to the non-nearest-neighbor distribution of magnetic metal centers, which makes the interatomic Coulomb repulsion become comparatively intact. In this case as shown in Fig. 1D, Jahn–Teller distortions emerge to help reduce the electronic repulsions, which might appear along the direction of the highest occupied antibonding orbitals, finally leading to a disorder magnetization direction. Generally speaking, the OER reactive activity in LS configuration will be reduced by the stabilization and additional localization of the electron pairs, Fermi heaps between metal centers and the relative destabilization at the lowest unoccupied orbitals. When one magnetic ion is adsorbed onto the metal site to generate a direct orbital hybridization as shown in Fig. 1E, this additional stronger interatomic QSEI will reduce the pristine radial extension of the d orbitals and their electronic repulsions. As a result, the electronic delocalization will enforce the interatomic QSEI among metal sites to generate a HS configuration (the bottom panel in Fig. 1D), which makes the pristine magnetic moments finally turn to a dominant consistent direction, especially at electromagnetic coupling synergistic stimulation. The adsorption and desorption of magnetic ions onto the catalyst surface can be confirmed by the in-situ Raman observation (SI Appendix, Fig. S2). After removing the magnetic ions, this HS configuration also can be reserved, which can be used to renormalize the magnetic structure by using this dynamic super-exchange interaction. This process can be confirmed by the differences in spin-related magnetization before and after adding some magnetic ions (SI Appendix, Fig. S3). To understand the electromagnetic coupling contribution, the spatial charge redistribution regulated by external AC magnetic field is theoretically demonstrated combining the density function theory with finite element numerical simulation, as shown in Fig. 1F. The charge density at metal sites is obviously stronger than linking organic molecule due to the difference in dielectric response, in which the eddy currents and skin effect induced by external AC magnetic field can lead to an asymmetric electrostatic potential and localized magnetocaloric effect. In this case, the dynamic orbital hybridization between magnetic ions and metal sites and the spin renormalization in this porous MOFs will become easier. Subsequently, the organic ligands (C6H8O6) with lower dielectric response characteristic are suggested to coordinate with magnetic Fe and Co elements to fabricate a bimetallic MOF (Fe0.5Co0.5-MOF), which can be realized using hydrothermal procedure (SI Appendix, Fig. S4). The ultrathin MOF nanosheet arrays are vertically aligned with three-dimensional porous texture to act as a functional electrode, which can be clearly observed in Fig. 2A from scanning electron microscopy image (SEM). The transmission electron microscopy (TEM) in Fig. 2B and the SEM observation (SI Appendix, Fig. S5) disclose that the average thickness of each nanosheet is ~20 nm and the size is about ~300 nm. The X-ray diffraction (XRD) patterns for the Fe0.5Co0.5-MOFs with and without spin-renormalized treatments are compared in Fig. 2C and SI Appendix, Fig. S6, which are completely consistent with the theoretical simulation (28, 29). The slight difference between experiments and simulation can be understood by the fact that simulated result is calculated from the perfect bulk material; however, the realistic nanostructures are complex which could lead to some changes in geometrical structure factor. The elemental mapping in Fig. 2D and energy-dispersive X-ray spectrum (SI Appendix, Fig. S7) demonstrate that the Fe and Co elements are uniformly distributed with an approximate ratio of 1:1 in this structure. It can be observed that the main typical organic chemical bonds are almost unchanged before and after spin-renormalized treatments (SI Appendix, Fig. S8), indicating no obvious crystal destruction occurs. To better confirm this point, the X-ray absorption spectroscopies of Fe0.5Co0.5-MOF with and without spin-renormalized treatments are conducted to disclose the spin reconfiguration effects toward to the atomic structure changes. The X-ray absorption near edge structure (XANES) results of Fe and Co K-edge at Fe0.5Co0.5-MOF in Fig. 2 E and F demonstrate that they possess similar valence state and coordination environment. After spin-renormalized treatments, the Fe K-edge XANES spectra show a slight shift toward higher energy comparing to the pristine sample but the opposite shift occurs at Co K-edge, which can be observed more clearly in the corresponding insets. This result implies that the Fe and Co sites have distinguishable spin-renormalized response due to their different spin polarization feature. Subsequently, Fourier transform analysis about the Fe and Co K-edge extended X-ray absorption fine structure (EXAFS) are adopted to investigate their local atomic configuration (30). The Fourier transform spectra of Fe K-edge in Fig. 2G disclose that the typical peaks located at 1.96 Å corresponds to the nearest Fe-O coordination (29, 30). After spin-renormalized treatments, the Fe-O peak intensity is diluted and this peak position is shifted to a larger bond length. More importantly, the intensity of secondary peak at 3.02 Å originated from Fe-Co coordination become obviously stronger than the pristine sample. These distinguishable changes in XANES results undoubtedly demonstrate that the Janh–Teller distortion induced by interatomic electronic repulsion can be partially suppressed by our suggested spin-renormalized approaches (SI Appendix, Table S2). Furthermore, the spin-related atomic reconfiguration also can be confirmed by the XANES results of Co K-edge in Fig. 2H, in which the bonding interaction and orbital hybridized strength is also changed as spin state transformation. To visually display the spin-renormalized effect, the magnetizations as a function of magnetic field (M-H) for Fe0.5Co0.5-MOF with and without spin-renormalized treatments are compared in Fig. 3A by superconducting quantum interference device (SQUID). Compared with the pristine sample, the long-time (~200 s) treated sample demonstrates an obvious nonlinear hysteresis loop curve and nonzero residual magnetization, showing a typical ferromagnetic characteristic at room temperature. More importantly, the saturation magnetization intensity and coercivity can be enhanced simultaneously with the treatment time, which only can be attributed to the magnetic structure transformation from LS state to HS state. The Mössbauer spectra of Fe0.5Co0.5-MOF in Fig. 3B also disclose that the isomer shift (IS = −0.11) and quadrupole splitting (QS = 0.58) with spin renormalization are obviously distinguishable from pristine sample (IS = −0.23 and QS = 1.09). This is due to the fact that the atomic distortion induced by the interatomic electronic repulsion can be reduced by a dynamic orbital hybridization from magnetic ions, which makes the domain direction of spin nets toward to an unanimity, leading to a change in Zeeman splitting. To disclose this physical mechanism, the energy difference for spin flip between ferromagnetic (FM) and antiferromagnetic (AFM) configuration (acting as HS and LS state) are calculated by the Heisenberg model: ΔEAFM-FM= EAFM − EFM = (J1 + J2 + J3) Si·Sj. Here, Si and Sj are the spin nets, and J1, J2, and J3 correspond to the first-, second-, and third-nearest spin coupling strength, respectively. The spin-flip energy as a function of charge concentration is calculated in Fig. 3C, in which spin flip energy difference for Fe0.5Co0.5-MOF with LS state and HS state is obviously increased as charge concentration, benefiting for a spin flip. This can be used to explain why the eddy currents triggered by alternating electromagnetic field can help to drive a spin state transformation. Interestingly, this phenomenon becomes more sensitive when one magnetic ion hybridizes with the metal site (SI Appendix, Fig. S9), because this dynamic bonding interaction is not stronger than bonding interaction between the organic linkers and metal sites. Therefore, the spin renormalization can be easily achieved by this dynamic super-exchange interaction stimulated by electromagnetic field. To understand this spin renormalization effect, the spin-resolved projected density of state (PDOS) for Fe0.5Co0.5-MOFs with LS and HS state are displayed in Fig. 3 D and E, in which the spin splitting for HS state is obviously stronger than LS state. A larger spin splitting makes the unfilled eg orbitals at conduction band is far away from the half-filled t2g orbitals at valence bands. In this case, the virtual electronic hopping from t2g to eg orbitals could need an external energy to support their spin exchange interaction. If this transformation from LS state to HS state is achieved by dynamic orbital hybridization with AC magnetic field stimulation, this spin-renormalized structure will be stable because the spin exchange interaction at HS state is stronger than the thermal energy corresponding to 300 K. To experimentally confirm this process, the electron energy loss spectroscopy (EELS) measurements of Co K-edge are displayed in Fig. 3F to demonstrate the spin-related electronic structure transformation (31–33). It is universally accepted that the change in Co K-edge is sensitive to the spin-related electronic structure, in which the higher (lower) energy corresponds to the electronic occupation at eg (t2g) orbitals. Compared to pristine sample, the peaks from t2g orbital in spin-renormalized sample is downshifted meanwhile the signals originated from eg orbitals is obviously enhanced, which can be attributed to their stronger spin exchange interaction at HS state. Based on Goodenough–Kanamori rule (34), the spin exchange interaction among half-occupied t2g orbitals generally exhibits an LS characteristic. When the electrons at t2g orbital hop onto eg orbitals, the stable HS feature will be observed due to the formation of electronic reoccupation at eg orbitals. This spin-related electronic reconfiguration clearly demonstrates a magnetic structure transformation triggered by our suggested spin-renormalized strategy. It is important to note that this spin-renormalized process is completely independent of the dynamic dissolution/redeposition of metal sites at the catalysts, which can be confirmed by the comprehensive analysis about SEM and TEM images, XPS spectra, Raman spectra, and in situ structural tracking technology (SI Appendix, Fig. S10-S12). To disclose the relationship between spin renormalization and OER performance, the linear sweep voltammetry curves of Fe0.5Co0.5-MOF electrodes with different treated times (0 to 600 s) are displayed in Fig. 4A. It is important to note that the anodic peaks in the broad OER polarization peaks at 1.3 to 1.5 V versus RHE can be ascribed to the oxidation of metal sites. More interestingly, the anodic peaks of the Fe0.5Co0.5-MOFs are obviously shifted to the lower potentials as spin-renormalized time, meanwhile which are obviously superior than the commercial RuO2 electrode especially at high current density. The detailed relationship between overpotential shifts and spin-renormalized time in (SI Appendix, Fig. S13) confirms that the overpotential at 150 mA cm−2 quickly reduces as spin renormalization for initial 50 s and then slowly incline to a stable state with 1.53 V potential. The optimal spin-renormalized time is about 200 s, because the spin reconfiguration can be triggered and completed at this operating time window. After finishing the spin state transformation, the OER performance begins to keep an unchanged performance. Therefore, the enhancement of OER only can be attributed to the spin renormalization, because some 3d electrons prefer to hop onto eg orbital from t2g orbital at HS state, leading to a spin electronic reoccupation. In this case, the bonding interaction and orbital hybridization between reactive sites and reactants can be optimized to reduce the rate-limiting potential barrier. To make a fair comparison, the mass normalized linear sweep voltammetry (LSV) curves of Fe0.5Co0.5-MOF catalysts on carbon paper with different spin-renormalized treatments are shown in SI Appendix, Fig. S14. To confirm this point, Tafel plots are shown in Fig. 4B to compare the spin-renormalized reaction activity. Compared with the pristine sample, the Tafel slopes are reduced to 68.49, 64.55, 60.77 and 58.63 mV dec−1 with increasing spin-renormalized time, demonstrating a change in spin-renormalized catalytic activity. The relative electrochemical impedance spectra (EIS) are provided in Fig. 4C to exhibit the correlation between conductivities and spin state reconfiguration. The rapid reduction in charge transfer resistance (Rct) and the enhancement of electrochemically active surface area (ECSA) in SI Appendix, Fig. S15 further demonstrate that the spin renormalization not only can accelerate the carrier migration but also can benefit for providing a higher reaction activity. In addition, the MOFs with spin-renormalized treatment also demonstrate an excellent electrochemical stability with only ~1% decay of the initial current density after undergoing 200 h electrochemical test, as shown in Fig. 4D. The original morphology, metal composition and organic linkers keep unchanged after this long-time OER test (SI Appendix, Fig. S16). Meanwhile, some blank experiments are provided in SI Appendix, Figs. S17–S19 to better display the contribution of the spin-renormalized treatments. The relationship between Co/Fe ratio in Fe1-xCox-MOFs and spin-renormalized OER performance in Fig. 4E demonstrate that the overpotentials firstly decrease and reach a minimum value for Fe:Co = 1:1, and then they begin to increase as increasing Co concentration. This is because the pure Fe1Co0-MOF with larger magnetic moment (Fe:3.612 μB) is more sensitive to the spin-renormalized treatment than the Fe0Co1-MOF with a smaller magnetic moment (Co:2.471 μB). On the contrary, the intrinsic reaction activity in Fe0Co1-MOF is higher than pure Fe1Co0-MOF. After considering the comprehensive effects, the Fe0.5Co0.5-MOF sample displays the excellent OER performance, which can be used to explain why we choose this sample as a model. To disclose the contribution of dynamic orbital hybridization with magnetic ions, the catalytic activity of various Fe1-xCox-MOFs treated in different magnetic ion solution are compared in Fig. 4F (relative experimental results are provided in SI Appendix, Fig. S20. The more sensitive spin-renormalized-response happens that the sample and magnetic ions have a similar concentration. Because, the binding energy for Co-Fe bonds is about 2.13 eV, which is higher than formation of Co-Co bond (2.05 eV) or Fe-Fe bonds (1.61 eV). In addition, the relative energies induced by the dynamic orbital interaction between metal sites and magnetic ions are provided in SI Appendix, Fig. S21. The orbital hybridizations between metal sites and magnetic ions prefer to occurs at homogeneous elements, which requires stoichiometric element distribution to support this super-exchange interaction (SI Appendix, Fig. S22). To disclose the spin-renormalized catalytic mechanism, the reaction pathway of reactive sites with forming various intermediates are displayed in Fig. 5A. The free energy differences between two isomeric intermediates are calculated to evaluate the rate-limiting step and activation energy. To simulate the spin renormalization contribution onto catalytic activity, the critical difference for reactive sites with LS and HS state can be considered as spin reconfiguration and then to react with reactants. As a general feature, the Gibbs free energy for spin-renormalized reactive sites with forming reactants can be effectively reduced comparing with pristine sample. Take Fe site for example, the OER prefers to choose the spin-renormalized pathway. Because, the formation of *OOH with HS state acting as rate-limiting barrier merely needs 1.37 eV to be realized, which is obviously lower than that of LS intermediates (1.86 eV). It is interesting to note that the similar selectivity of reaction pathway also occurs at the Co site, in which the determining activation energies for forming Co-*OOH is reduced to 1.03 eV (HS) from 1.30 eV (LS). Based on the lowest principle of energy, the Co sites with HS state play a key factor in determining OER performance, which is strongly related with the spin state reconfiguration. To understand the activation energy differences between Co and Fe site, the charge density difference of adsorbed *OOH at the rate-limiting steps is calculated in Fig. 5B. Herein, the green isosurfaces represent the electron increase and the pink regions correspond to the electron depletion. When the determining intermediates of *OOH is formed onto Co site with HS state, the unpaired O-2p orbital will hybridize with Co-3d orbital. Compared with the LS configuration, the electron redistribution becomes more delocalized at Co-*OOH bonds, which make the charge transfer increase to 0.186 e (HS) from 0.125 e (LS). To understand the difference in relative stability at rate-limiting intermediate, the crystal orbital Hamilton population (COHP) for Co site with HS and LS state is compared in Fig. 5C to demonstrate its bonding behavior with reactants. The relative bonding interaction between intermediates and Fe sites are also shown in SI Appendix, Fig. S23. Different from the weaker Co-*OOH antibonding interaction at LS state, the antibonding strength at Fermi level is obviously reduced for HS configuration, implying that the valence electrons of Co-3d orbitals prefer to partially occupy the half-filled O-2p orbitals. Therefore, the corresponding bonding interaction for Co-*OOH at HS state is stronger than that of LS configuration, thus resulting in a lower activation energy. This spin-related reaction process also can be reflected by the changes in spin density in Fig. 5D, because their orbital interaction can reconfigure the electronic occupation at Co-3d orbitals. It can be observed that the spin density is obviously decreased after adsorbing *OOH, which make the magnetic moment increase to 2.799 μB from 2.672 μB. This is because that the energy levels of Co site at HS-d orbitals and *OOH-π* orbitals are well matched as shown in Fig. 5E, giving rise to partial occupation at the formed d-π* orbitals (35). On the contrary, the weak spin polarization at LS state cannot provide enough exchange stabilization energy to generate the d-π* interactions. Therefore, the large spin polarization at Co-HS state can be partially occupied, and thus forcing *OOH to be of radical nature that can be furthermore activated. When the *OOH group is dehydrogenated, one electron transfers from Co sites to the π* orbitals of *OOH and the orbital hybridization can further weaken *O-OH bond, which is responsible for the lower dissociation barrier at HS configuration. In summary, we have proposed that the dynamic orbital hybridization induced by electromagnetic coupling super-exchange interaction can lead to a spin renormalization, which provides a feasibility to regulate a spin-related charge transfer and orbital interaction between active sites and intermediates. As a result, the spin-renormalized sample demonstrate a super-low overpotential of ~0.29 V at 100 mA cm−2. Our findings open a new door into boosting OER performance in the field of spin-dependent catalysts.

The Fe0.5Co0.5 MOF were synthesized on nickel foam (NF) through a solvothermal method. First, 0.5 mmol iron nitrate nonahydrate and 0.5 mmol cobalt nitrate hexahydrate were simultaneously added into 10 mL deionized water to form a homogeneous solution by vigorous stirring for 45 min under room temperature (denoted as solution A). Then, 0.16 g of organic ligand 2,5-dihydroxyterephthalic acid was dissolved in 5 mL of ethanol and 5 mL of DMF by ultrasonication to ensure adequate dispersion (denoted as solution B). After sufficient dissolution, the solution B was added dropwise to the solution A at a rate of 0.1 mL/s, and then the mixture was stirred under the high-purity argon current protection for 1 h to form a homogeneous solution. Meanwhile, the NF was soaked in 1 M HCl for 20 min, ultrasonicated sequentially in deionized water and ethanol for 0.5 h each to thoroughly remove impurities, and dried at room temperature. The pre-cleaned NF substrate was vertically aligned in the above mixed solution under ultrasonication for 2 h. Thereafter, the homogeneous solution with NF was transferred to a 25-mL Teflon-lined stainless-steel autoclave and then placed in the oven for solvothermal reaction at 125 °C for 12 h. After that, the NF loaded with the final products was taken out, sonicated for 10 min in an ice bath and then alternately rinsed several times with ethanol and deionized water, and then dried in an oven at 60 °C for 10 h under the high-purity argon atmosphere. The value of iron nitrate decreased by 0.1 mmol each sample from 1 to 0 mmol, while the value of cobalt nitrate increased by 0.1 mmol each sample from 0 to 1 mmol. The prepared MOF was denoted as Fe1-xCox-MOF (x = 0 to 1), where x referred to the percent of Fe and Co source mass. The preparation route of these samples with different source masses are the similar as above. The super-exchange interaction was realized in the solution contained by different mass of iron nitrate and cobalt nitrate under the external high-frequency AC magnetic field and direct current (DC) electric field. Briefly, the as-prepared MOFs were further placed into an external high-frequency AC magnetic field using commercial induction set-up operated at 150 kHz as the source of magnetic stimulation (Honghu, HD-25) and DC electric field using DC stabilized power operated at 100 mV as the electrical stimulation source (RIGOL, DP832). Different Fe1-xCox-MOFs samples were treated by electromagnetic treatments, which can be simply named as Fe1-xCox-t (t = 0 − 600 s). The quantity of MOF deposited on NF can be calculated similarly by our previous literature (SI Appendix, Fig. S24).

The electrochemical measurements were tested in an electrochemical workstation (CIMPS, Zahner, Germany) at room temperature using O2-saturated 1 M KOH solution. The NF covered with different samples were used as the working electrode. The commercial Ag/AgCl (saturated KCl) and graphite rod (Alfa Aesar, 99.9995%) were used as the reference and counter electrodes, respectively. The electrolyte was bubbled with high-purity oxygen for at least 20 min to ensure the H2O/O2 equilibrium before the OER kinetics measurements. All reported potentials were calibrated with 100% iR compensation manually via following formulas: Ecorrected =Emeasured – iR (R is the EIS-determined series resistance) and converted with respect to reversible hydrogen electrode (RHE) by using the Nernst function: , where is the standard electrode potential of Ag/AgCl reference electrode (36–39). The linear sweep voltammogram (LSV) was measured in the potential range from 1.2 to 1.6 V versus RHE at a scan rate of 5 mV s−1. The Tafel slopes (b) were estimated by Tafel plots from the linear portion fitting overpotential (η) against log |j| (j, current density) by the Tafel Equation: η=blog(j)+a. To evaluate the catalyst stability, the chronopotentiometry measurements were tested by maintaining the current density of 10 mA cm−2 or 100 mA cm−2 for 200 h. The electrochemical impedance spectra (EIS) were recorded in the open circuit with a sinusoidal voltage of 5 mV amplitude in the range from 100 kHz to 0.1 Hz. The cyclic voltammetry (CV) measurements were performed with various scan rates (1 to 20 mV s−1) at the potential window from 1.00 V to 1.10 V versus RHE with double-layer charging currents (j) to estimate the double-layer capacitances (Cdl) (40). Importantly, the measured results are only slightly depended on the changes in reference electrode and scan frequency, SI Appendix, Fig. S25.

The theoretical calculations are based on the Vienna ab initio simulation package (VASP), with Perdew–Burke–Ernzerhof (PBE) and generalized gradient approximation (GGA) (41, 42). The Monkhorst–Pack k-point grids were adopted as 9 × 9 × 1 and all the atomic forces were converged to be 0.01 eV/Å. In the calculations, the cutoff energy is considered as 500 eV to expand the Kohn–Sham wave functions. In addition, the vacuum space of 20 Å is used into surface slabs, in which the interaction between periodical images can be excluded. The strong d-d Coulomb interactions can be considered by the Hubbard method. In our calculations, the effective Hubbard U = 2 eV for Fe and U = 1 eV for Co are adopted to describe the Fe-3d and Co-3d orbitals. The Gibbs free energy differences for the intermediates including zero-point energy (ZPE) and entropy corrections (ΔS) can be described as: ΔG = ΔE + ΔZPE – TΔS + ΔGpH. Herein, ΔE is the adsorption energy of the intermediates, and ΔGpH = −kBTln (10) × pH is pH-related energy correction. The transition states can be acquired by the climbing image nudged elastic band method.

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