Rationally Reconstructed Metal–Organic Frameworks as Robust Oxygen Evolution Electrocatalysts

Reconstructing metal–organic framework (MOFs) toward a designed framework structure provides breakthrough opportunities to achieve unprecedented oxygen evolution reaction (OER) electrocatalytic performance, but has rarely, if ever, been proposed and investigated yet. Here, the first successful fabrication of a robust OER electrocatalyst by precision reconstruction of an MOF structure is reported, viz., from MOF‐74‐Fe to MIL‐53(Fe)‐2OH with different coordination environments at the active sites. Due to the radically reduced eg–t2g crystal‐field splitting in Fe‐3d and the much suppressed electron‐hopping barriers through the synergistic effects of the O species the efficient OER of in MIL‐53(Fe)‐2OH is guaranteed. Benefiting from this desired electronic structure, the designed MIL‐53(Fe)‐2OH catalyst exhibits high intrinsic OER activity, including a low overpotential of 215 mV at 10 mA cm−2, low Tafel slope of 45.4 mV dec−1 and high turnover frequency (TOF) of 1.44 s−1 at 300 mV overpotential, over 80 times that of the commercial IrO2 catalyst (0.0177 s−1).Consistent with the density functional theory (DFT) calculations, the real‐time kinetic simulation reveals that the conversion from O* to OOH* is the rate‐determining step on the active sites of MIL‐53(Fe)‐2OH.


Introduction
Metal-organic frameworks (MOFs) are a type of crystalline porous materials, which have become competitive oxygen evolution reaction (OER) catalysts largely due to their inherent high porosity and surface area as well as their designability. [1] Understanding the structure-property relationship is crucial for enhancing the activity and durability of the catalysts for different electrochemical reactions in different environments. [2] Although the significance of the electrocatalytic Herein, we report the first successful molecular reconstruction of the interior frameworks of MOFs by a simple in situ solvothermal modulation procedure, and the demonstration of the underlying mechanism for their high OER performance. MOF-74-Fe was used as the prototype with hexagonal channels and coordinated phenolic hydroxyl groups, and its modulated form MIL-53(Fe)-2OH was rationally designed and fabricated with rhombic channels and uncoordinated phenolic hydroxyl groups (Figure 1a). H 4 DOBDC can be selectively deprotonated at only the two carboxylic groups, and consequently form the bidentate anion [H 2 DOBDC] 2− , which enables the formation of MIL-53(Fe)-2OH leaving the phenolic hydroxyl groups uncoordinated. The characterizations of XRD, XAS, SEM, and TEM are in good consistence with each other, and proved that the pure MIL-53(Fe)-2OH and MOF-74-Fe structures have been successfully synthesized by controlling the solvent ratio and concentration. Density functional theory (DFT) calculations have revealed the highly reduced e g -t 2g crystal field splitting in Fe-3d of MIL-53(Fe)-2OH, where the fast electron transfer pathways are mediated by the omnipresent molecularly assembled O sites. Under the leverage of the desirable electronic structure, the modulated MIL-53(Fe)-2OH with uncoordinated phenolic hydroxyls exhibits high intrinsic OER activity with a turnover frequency per metal site of 1.44 s −1 at 1.53 V versus RHE, which is 81 times higher than that of the commercial IrO 2 catalyst.
Moreover, the overpotentials of the MIL-53(Fe)-2OH catalyst are 215 and 314 mV at 10 and 500 mA cm −2 , respectively, which are lower than those of the best MOF catalysts reported to date. Capitalizing on the MOF platform with molecular-level precision, this work is able not only to offer insightful understanding of the structure-property relationship but also to provide an innovative design strategy for systematically optimizing MOF-based electrocatalysts for OER and other important but challenging reactions.

Results and Discussion
Inspired by the promise of the rational design presented above, MIL-53(Fe)-2OH and MOF-74-Fe catalysts were synthesized by using 2,5-dihydroxyterephthalic acid (H 4 DOBDC) as ligand and iron as the metal component. Synthesis of MOFs involves solid-to-solid rearrangement and intermediate formation. [13] As shown in Figure 1a and Figure S1 (Supporting Information), H 4 DOBDC have two carboxylic groups rigidly located at an angle of 180° (1,4-benzenedicarboxylic acid functionalities) and two phenolic functional groups. [14] Deprotonation of H 4 DOBDC can form two kinds of ligand anions: 2,5-dihydroxyterephthalato anion ([H 2 DOBDC] 2− ) with reported the pKa value of 7.31 and 2,5-dioxidoterephthalato tetra-anion ([DOBDC] 4− ) with the www.advmat.de www.advancedsciencenews.com pKa value of 26.67. [15] Alkaline environment can be formed spontaneously through the solvothermal decomposition, even if base is absent from the starting reagents. [13a] The quadridentate ligand [DOBDC] 4− can be easily formed, which tends to be incorporated into MOF-74-Fe ( Figure S2, Supporting Information). [13a] By controlling the solvent ratio and concentration, H 4 DOBDC can be selectively deprotonated at only the two carboxylic groups, and consequently form the bidentate anion [H 2 DOBDC] 2− , which leads to the formation of MIL-53(Fe)-2OH ( Figure S3, Supporting Information). [16] Meanwhile, DFT calculations have also revealed the distinctly different electronic structures of these two MOFs. From the electronic distributions near the Fermi level (E F ), it is evident that the bonding orbitals in MOF-74-Fe are dominated by the carbon chains with only limited contributions from the Fe sites, leading to the relatively weak electroactivity of Fe (Figure 1b). In MIL-53(Fe)-2OH, however, Fe sites display the highly electron-rich feature, indicating high electroactivity (Figure 1c). Meanwhile, O sites of the phenolic groups and the connection points also facilitate the electron transfer within MIL-53(Fe)-2OH for improved OER performance. The highly distinct electronic structures are responsible for the different OER performances of these two MOFs. The projected partial density of states (PDOSs) in Figure 1d further reveals the distinct electronic structures. Notably, Fe-3d orbitals show a large e g -t 2g splitting of 3.43 eV, leading to the large barriers for electron transfer in MOF-74-Fe. OH-s,p orbitals locate in the deep position, which mainly serves as the electron reservoir, and the resulting limited overlap between Fe-3d and OH-s,p orbitals increases the difficulties of site-to-site electron transfer. In contrast, the e g -t 2g splitting of Fe-3d orbitals has been significantly alleviated to 1.19 eV in MIL-53(Fe)-2OH, which supports much more efficient electron transfer from the MOF to the intermediates (Figure 1e). C-s,p orbitals cover a broad range in both MOFs to facilitate electron depletion. More importantly, two different types of O sites have synergistically improved the OER performance. The O-s,p orbitals from Fe-O-Fe chains have shown a close position toward the E F with good overlap with both Fe-3d and OH-s,p orbitals, indicating that the phenolic groups in MIL-53(Fe)-2OH are able to ensure the fast site-to-site electron transfer.
The crystal structures of the MIL-53(Fe)-2OH and MOF-74-Fe catalysts were investigated by X-ray powder diffraction (XRD). The experimental XRD pattern of the MIL-53(Fe)-2OH sample matches well with the MIL-53(Fe) structure (CCDC No. 734218) in the Imma space group with the unite cell parameters of a = 17.84 Å, b = 6.87 Å, c = 11.84 Å, and good agreement factors of weight-profile R-factor R wp = 7.3% and underweighted R-factor R p = 5.6% (Figure 2a) [17] The Raman spectra reveal the skeleton phonon modes of the carboxylate, phenolate, and benzene rings, and FeO in the obtained MOFs ( Figure S4, Supporting Information). [18] The four vibrational peaks appeared at ≈1606, 1397, 1535, and 1302 cm −1 for the MIL-53(Fe)-2OH are ascribed to the in-and out-of-phase stretching region of the carboxylate group, ν(COO−) vibration, and v(C−O) vibration, respectively. [18,19] The peaks at 600 and 404 cm −1 are assigned to CH stretching region of the benzene ring and  FeO bonds vibration, respectively. [18,19b] By Raman spectroscopy analysis, although MIL-53(Fe)-2OH and MOF-74-Fe have similar coordinated structures, the vibration of hydroxyl group, CO stretching vibration of the hydroxyl groups, carboxyl groups, and FeO bond vibration are obviously different, indicating that the coordination environments are distinct in these two MOFs. Furthermore, Fourier transform infrared (FT-IR) spectroscopy was also employed to confirm the formation of MOFs ( Figure S5, Supporting Information). The 1211 cm −1 peak of MIL-53(Fe)-2OH is assigned to the C−O stretching vibration of the hydroxyl groups. [20] The coordinated hydroxyl groups in MOF-74-Fe are difficult to form hydrogen bonds, resulting in the absorption peak moving to lower frequency. [21] Therefore, the C−O stretching vibration peak of MOF-74-Fe shifts to 1192 cm −1 . Importantly, the absorption peak at 3404 cm −1 for MIL-53(Fe)-2OH, which is absent for MOF-74-Fe, is related to the variation of phenolic hydroxyl group (νOH), suggesting the successful fabrication of the two target MOF structures. [22] The XPS survey spectrum for MIL-53(Fe)-2OH reveals the presence of C (62.4 at%), O (34.3 at%), and Fe (3.3 at%) ( Figure S6 and Table S1, Supporting Information). High-resolution C 1s XPS spectra confirm the composition of the CC/C−C in aromatic rings (284.7 eV), the C−O (286.4 eV), and the carboxylate group (OC−O, 288.8 eV) for both the MIL-53(Fe)-2OH and MOF-74-Fe samples ( Figure S7, Supporting Information). [6,23] The O 1s XPS spectrum of MOF-74-Fe can be deconvoluted into three energy peaks at 530.8, 531.9, and 533.7 eV, which are ascribed to the Fe−O bonds, the carboxylate of the organic ligands, and absorbed water, respectively ( Figure S8, Supporting Information). [23] The O 1s XPS peak at 532.8 eV for MIL-53(Fe)-2OH is assigned to C−OH bonds. The fitting data showed that the content ratio of the Fe−O bond in MOF-74-Fe was slightly higher than that of MIL-53(Fe)-2OH, presumably related to the different coordination environments of the Fe ions with phenolic hydroxyl and carboxyl groups. [24] The highresolution XPS spectrum of MIL-53(Fe)-2OH at Fe 2p can be deconvoluted into four characteristic peaks shown in Figure S9 (Supporting Information), which are assigned to Fe 3+ (712.4 and 725.6 eV) and associated shakeup satellites (716.6 and 730.3 eV), respectively. [25] X-ray absorption spectroscopy (XAS) was performed to further elucidate the fine structure of the synthesized MOFs. As shown in the X-ray absorption near edge spectra (XANES) (Figure 2c (Table S3, Supporting Information) exhibit a higher FeO coordination number of MIL-53(Fe)-2OH (6.67) than that of MOF-74-Fe (6.28) in the first shell, which is perhaps related to the higher oxidation state of Fe in MIL-53(Fe)-2OH. The lowest intensity of the pre-edge peak at ≈7114 eV appears in MIL-53(Fe)-2OH, indicating the excellent symmetric octahedra [FeO 6 ] structure in MIL-53(Fe)-2OH, which is in good agreement with the simulated MIL-53(Fe) structure (CCDC: 734 218) shown in Figure 2a and Figure S3 (Supporting Information). The pre-edge peak intensity of MOF-74-Fe is slightly higher, indicating the slightly lowered symmetry of the coordination sites. The coordination environment of Fe in MIL-53(Fe)-2OH is actually quite different from that of MOF-74-Fe. [26] This effectively suggests that the different oxygenic coordination environment alters the local electron density of the metal sites and thus impacts the catalytic activity. [27] The Fourier transform of the extended XAFS (EXAFS) spectra at the Fe K-edge and the corresponding fitting results are summarized in Figure 2d, Figures S11 and S12 (Supporting Information). The main peak of Fourier transforms (FTs) curves for MIL-53(Fe)-2OH (1.59 Å) and MOF-74-Fe (1.64 Å) correspond to the nearest Fe−O coordination, with no indication of the formation of FeFe bond. [28] The signal of wavelet transforms (WT) for the k 3 -weighted Fe K-edge EXAFS curves related to Fe−Fe bonds has been detected in the Fe foil but not in MIL-53(Fe)-2OH and MOF-74-Fe, giving a strong indication that there are only atomically dispersed Fe sites in the MOF samples (Figure 2e). The maximum intensity signal at ≈5.1 Å −1 for MIL-53(Fe)-2OH is related to the FeO bond, which exhibits a positive shift in contrast to that of MOF-74-Fe (≈4.75 Å −1 ), indicating the different electronic structure between them. The Fe K-edge EXAFS data reveal a FeO bond length of 1.976 ± 0.04 Å and the FeOC bond length of 3.179 ± 0.008 Å for MIL-53(Fe)-2OH, which show great consistency with the simulated structure based on the XRD analysis (Table S3 and Figure S13, Supporting Information). Of note, the FeO bond length of MOF-74-Fe is slightly longer (1.996 ± 0.045 Å) than that of MIL-53(Fe)-2OH, and tallies with its lower oxidation state, which is also very much consonant with the XRD analysis ( Figure S14, Supporting Information). The different electronic structures of MIL-53(Fe)-2OH from MOF-74-Fe optimize the adsorption/desorption ability of oxygenic intermediates (OH*, OOH*, and O*) and thus improves the catalytic OER activity. [18,29] N 2 adsorption-desorption isotherms reveal that the Brunauer-Emmett-Teller (BET) surface of MIL-53(Fe)-2OH is 7.3 m 2 g −1 , which is lower than that of the MOF-74-Fe (10.4 m 2 g −1 ) (Figure S15, Supporting Information). These two Fe-based MOFs catalysts exhibit an obvious adsorption hysteresis loop typical of the type-III isotherm, manifesting the microporous character. The measured pore sizes of MIL-53(Fe)-2OH and MOF-74-Fe are mainly concentrated at 1.48 and 1.59 nm, respectively, which are in good agreement with their crystal structure (Figure 3a,b). The morphologies of MIL-53(Fe)-2OH and MOF-74-Fe samples were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 3c, Figures S16 and S17  Table S2, Supporting Information).
The electrocatalytic OER performances of MIL-53(Fe)-2OH and MOF-74-Fe samples were studied in a three-electrode system in the O 2 -saturated 1.0 m KOH solution at room temperature. The polarization curves were obtained at a scan rate of 1 mV s −1 in O 2 -saturated electrolyte. According to the polarization curves shown in Figure 4a, the MIL-53(Fe)-2OH catalyst exhibits the lowest overpotential of 215 mV at a current density of 10 mA cm −2 , compared to that of the MOF-74-Fe (242 mV), and IrO 2 (335 mV) catalysts. The Tafel slope of the MIL-53(Fe)-2OH catalyst (45.4 mV dec −1 ) is also lower than that of MOF-74-Fe (49.5 mV dec −1 ), IrO 2 (99.7 mV dec −1 ), and NF (108.2 mV dec −1 ) (Figure 4b; Figure S20 and Table S4, Supporting Information). The electrochemical active surface area (ECSAs) of the electrocatalyst is proportional to the electrochemical double-layer capacitance (C dl ), which can be determined by measuring the scanning rate-dependent cyclic voltammetry (CV) of the non-Faraday region ( Figure S21, Supporting Information). The measured C dl value of MIL-53(Fe)-2OH is 4.2 mF cm −2 , which is close to that of the MOF-74-Fe (4.1 mF cm −2 ) and larger than that of IrO 2 (3.0 mF cm −2 ) ( Figure S22, Supporting Information). As shown in the CV curve of MIL-53(Fe)-2OH ( Figure S23, Supporting Information), it can be observed that the redox peaks occur between 1.2 and 1.5 V, which are attributed to the redox of Ni 2+ (Fe 2+ ), Ni 3+ (Fe 3+ ), and Ni 4+ (Fe 4+ ). [30] As calculated from geometric current densities and ECSAs, the highest specific current density (j ECSA ) is achieved over MIL-53(Fe)-2OH (3.24 mA cm −2 ), which is much higher than that of MOF-74-Fe (1.42 mA cm −2 ) and IrO 2 (0.08 mA cm −2 ) catalysts, indicating its superior intrinsic activity for OER ( Figure S24, Supporting Information). To further illustrate the electrode reaction kinetics during the catalytic OER process, electrochemical impedance spectroscopy (EIS) was performed. As shown in Figure 4c, the Nyquist plots reveal that MIL-53(Fe)-2OH has an ultralow charge transfer resistance (R ct ) of ≈0.65 Ω at the overpotential of 300 mV (at 1.53 V vs RHE) in alkaline condition, which is lower than that of the MOF-74-Fe (0.80 Ω) and IrO 2 (18.32 Ω) catalysts, indicating a much faster charge transfer on the MIL-53(Fe)-2OH surface in the electrochemical reaction process. These results firmly demonstrated the excellent activity of MIL-53(Fe)-2OH for OER. In addition, the turnover frequency (TOF) and the mass activity (MA) of the catalysts were also determined to illustrate the intrinsic activity at the constant overpotential of 300 mV (Figure 4d). [18,31] More details of the kinetic model have been presented in section 1.4 and 1.5 of the Supporting Information. The high TOF of 1.44 s −1 is obtained for the MIL-53(Fe)-2OH catalyst, prominently exceeding that of MOF-74-Fe (0.59 s −1 ) and IrO 2 (0.0177 s −1 ) ( Figure S25, Supporting Information). The MA value of MIL-53(Fe)-2OH could reach as high as 357.90 A g −1 , significantly larger than those of MOF-74-Fe (153.60 A g −1 ) and IrO 2 (62.34 A g −1 ) ( Figure S26 and  www.advmat.de www.advancedsciencenews.com catalytic OER activities in terms of the lower Tafel slope and lower overpotential, which exceeds most of the recently reported MOFs OER catalysts, as shown in Figure 4e, Tables S6 and S7 (Supporting Information). [2b,11b,19a,25b,32] A double-electrode electrolyzer of Pt/C || MIL-53(Fe)-2OH was assembled to assess the catalyst activity for overall water splitting, and its high catalytic activity was demonstrated with a cell voltage of 1.59 V easily driving a current density of 100 mA cm −2 ( Figure S27, Supporting Information). Furthermore, the Faradaic efficiency (FE) of the MIL-53(Fe)-2OH catalyst was as high as 96.4%, representing the nearly full utilization of all charges for OER ( Figures S28 and S29, Supporting Information).
The OER performances of MOF-74-Fe and MIL-53(Fe)-2OH have further been elucidated by DFT calculations from both electronic structures and reaction energy. In MOF-74-Fe, the size of e g -t 2g splitting increases with the coordination numbers (Figure 5a). The existence of OH vacancy also cannot evidently lower the e g -t 2g splitting, demonstrating that electron transfer   Figure S30, Supporting Information). For MIL-53(Fe)-2OH, the carbon sites in the benzene ring are more electroactive than that in OCO and the formation of OH vacancy further improves the electron transfer (Figure 5b). This indicates that the existence of the electroactive phenolic groups is able to not only facilitate the electron transfer but also improve the overall electroactivity. Since the electronic structures are highly correlated with the OER performances, the PDOSs of key intermediates on both MOFs are demonstrated (Figure 5c,d). For MOF-74-Fe, it is noted that the upshifting trend of the σ orbitals from O-species has shown an evident deviation at OOH*, which potentially increases the barriers for the conversion from O* to OOH*. Meanwhile, such a deviation is absent in MIL-53(Fe)-2OH, which achieves the efficient conversions of intermediates with low-energy barriers. These results further confirm that the superior OER performances of MIL-53(Fe)-2OH originate from the optimal electronic structures modulated by the phenolic groups. The loss of OH groups in both MOFs shows high-energy costs of 3.34 and 2.65 eV in MOF-74-Fe and MIL-53(Fe)-2OH, respectively ( Figure S31, Supporting Information). This illustrates that the involvement of OH groups from MOF to promote the OER is challenging. More importantly, this further reveals that the more electroactive Fe and O sites in MIL-53(Fe)-2OH are the key factors for the highly efficient OER process. The reaction energy change of OER has been compared between the two MOFs. With U = 0 V, both MOFs display the largest energy cost at the conversion from O* to OOH* as the rate-determining step ( Figure S32  www.advmat.de www.advancedsciencenews.com in MIL-53(Fe)-2OH is smaller than that of the MOF-74-Fe, which is consistent with the results of electronic structures. After introducing the equilibrium potential, we notice that the improved OER performances of MIL-53(Fe)-2OH are attributed to suitable binding of OH*, which results in a reduced energy barrier of 0.23 eV than that of the 0.32 eV in MOF-74-Fe (Figure 5e; Figure S33, Supporting Information). On the other side, the over-binding of OH* leads to a continuous uphill trend in MOF-74-Fe, which has affected the efficiency of the OER process. The reaction mechanism of OER has been demonstrated through the structural configurations ( Figures S34  and S35, Supporting Information). Notably, the frameworks of MOF are able to remain relatively stable during the adsorption of the intermediates. Moreover, the selectivity between 2e − and 4e − OER has been compared based on the conversion reaction of O*. For both MOF-74-Fe and MIL-53(Fe)-2OH, the 4e − OER is more preferred due to the much smaller energy barrier of O* → OOH* than O* → ½ O 2 , indicating the high selectivity toward 4e − OER. In recent years, there have been many discussions on the OER mechanisms including the associated evolution mechanism (AEM) and lattice oxygen mechanisms (LOM). [33] Due to the unique coordination environments of the Fe sites in both MOF-74-Fe and MIL-53(Fe)-2OH, the formation of neighboring vacancies is highly unlikely. In particular, our calculations have proved that the loss of neighboring OH groups requires high-energy costs ( Figure S31, Supporting Information), indicating that LOM is not feasible for our proposed MOF structures.
The chronopotentiometry responses (E-t) curve in Figure 6a demonstrates that the as-synthesized MIL-53(Fe)-2OH and MOF-74-Fe catalysts possess good durability after the OER test for >100 h at 100 mA cm −2 . The polarization curves of the MIL-53(Fe)-2OH and MOF-74-Fe catalysts after the stability test have no obvious changes of the overpotential before and after the chronopotentiometry test at the high-current density of 100 mA cm −2 ( Figure S36, Supporting Information). The overpotential of MIL-53(Fe)-2OH after 100 h of the stability test increases only 2 mV at the current density of 100 mA cm −2 , which suggests that the MIL-53(Fe)-2OH exhibits excellent long-term stability (Figure 6b). The overpotential after the stability test increases 10 mV at the current density of 100 mA cm −2 for MOF-74-Fe catalyst ( Figures S37 and S38, Supporting Information). C dl values of the MIL-53(Fe)-2OH (3.8 mF cm −2 ), MOF-74-Fe (3.9 mF cm −2 ), and IrO 2 (2.9 mF cm −2 ) catalysts exhibit no obvious changes comparing to their initial ones ( Figures S39-S41, Supporting Information). The overpotential at the current density of 100 mA cm −2 for MIL-53(Fe)-2OH after 100 h of stability test (266.8 mV) only increases 0.6 mV compared to the initial value (266.2 mV), which is much lower than that of MOF-74-Fe (5.9 mV) and IrO 2 (32.4 mV). Compared with MOF-74-Fe and IrO 2 , the R ct value of MIL-53(Fe)-2OH also shows the minimum increase after the stability test (0.26 Ω) compared to MOF-74-Fe (0.80 Ω) and IrO 2 (21.1 Ω) at the overpotential of 300 mV, confirming the superior electrochemical durability of the MIL-53(Fe)-2OH catalyst (Figure 6c; Figures S42-S44, Supporting Information). The OER LSV curves recorded before and after 10 000 CV cycles are almost unchanged, further proving the excellent durability of the MIL-53(Fe)-2OH catalysts ( Figure S45, Supporting Information). Furthermore, XRD, XPS, TEM, and SEM were performed to probe the essential difference in the catalysts' structure during the OER process. As shown in Figures S46 and S47 (Supporting Information), the XRD patterns of MIL-53(Fe)-2OH and MOF-74-Fe after the OER stability test at 100 mA cm −2 for 100 h agree well with that of FeO(OH) (ICSD No. 94 874), which demonstrates the formation of iron hydroxides. [34] The structure and electronic properties of MIL-53(Fe)-2OH after stability test was further analyzed by XPS. The XPS analysis discloses the presence of Fe (4.3 at%), C (36.5 at%), and O (59.2 at%) elements in MIL-53(Fe)-2OH catalysts after OER for 100 h, in which O content was significantly higher than that of the initial MIL-53(Fe)-2OH (Table S1, Supporting Information). As shown in Figure S48a (Supporting Information), the high-resolution XPS spectrum of Fe 2p shows deconvoluted peaks for Fe 3+ (711.9 and 725.8 eV). [20a,35] The O 1s spectrum can be split into three peaks at 530.7, 532.8, and 534.1 eV, which are ascribed to the FeO, COH, and absorbed water, respectively ( Figure S48b, Supporting Information). [36] These results suggest a structural transformation of the MIL-53(Fe)-2OH catalyst. [19b,20a,37] The morphology of MIL-53(Fe)-2OH and MOF-74-Fe after stability test was observed by SEM and TEM. As shown in Figure S49 (Supporting Information), the MIL-53(Fe)-2OH is transformed from the original regular octahedral structure to the morphology of bulk stacking. For MOF-74-Fe, the polyhedral prism morphology is transformed into sheets ( Figure S50 The structure-property dependency of the MIL-53(Fe)-2OH catalyst during the OER process was further investigated by the real-time kinetic simulation. Following the procedure of our previous work, [38] the standard activation free energies for the five elementary reaction steps ( OA 0 G ∆ * for the oxidative adsorption step,  also in good agreement with the DFT calculations. Figure 6e shows the adsorption isotherms of the reaction intermediates. The fractional coverages of the intermediates, θ OH , θ O , and θ OOH , are closely correlated to their standard free energy. The low O 0 G ∆ accelerates the formation and adsorption of O*, leading to the high coverage of O* on the MIL-53(Fe)-2OH surface. In the higher overpotential range, the consumption rate of O* to form OOH* increases, resulting in the decrease of θ O and the increase of the fractional coverage of OOH*. The strong adsorption of the intermediates on the MIL-53(Fe)-2OH surface promotes the generation of the FeO(OH) phase during the OER process. It is worth noting from Figure 6f that the activation energies for the rate-determining step are very close on the initial and post-tested MIL-53(Fe)-2OH catalyst, confirming the robustness of the catalyst with an invariant OER activity even after the long-term OER operational testing.
Although MIL-53(Fe)-2OH catalyst exhibits excellent OER stability within the testing time interval, the chemical structure of the MIL-53(Fe)-2OH catalyst has changed after the OER stability testing, so its kinetic behavior and intermediate coverages may also change after the OER stability test. Indeed, the OER kinetics of the post-tested MIL-53(Fe)-2OH catalyst is dependent on both the formation of OOH* and the adsorption of OH* with an increased energy barrier of 280 meV compared to the initial one (171 meV) (Figure 6g)   than those of the initial counterpart, confirming a changed kinetic behavior of the MIL-53(Fe)-2OH catalyst during the OER process ( Figure S56, Supporting Information). Thus, the real-time kinetic simulation opens a new direction in exploring the OER mechanism and can help to develop robust OER electrocatalysts with competitive activity and stability.

Conclusion
This study has proposed and demonstrated a feasible synthesis strategy to rationally reconstruct the MOF structure from the phenolic hydroxyl coordinated MOF-74-Fe with hexagonal channels to uncoordinated MIL-53(Fe)-2OH with rhombic channels. XAS analyses and DFT calculations have revealed that the different electronic structures of Fe sites in MOF-74-Fe and MIL-53(Fe)-2OH due to the precisely tuned ligand fields, which further lead to the varied electroactivity toward the OER process. It is found that the much-reduced e g -t 2g splitting of Fe-3d orbitals with electroactive O sites in the MIL-53(Fe)-2OH guarantees the efficient OER with lowered overpotential at the rate-determining step. Leveraging on the desirable electronic structure, the purpose-built MIL-53(Fe)-2OH catalyst delivers remarkably fast OER kinetics with low overpotential of 215 mV at 10 mA cm −2 , small Tafel slope of 45.4 mV dec −1 , competitive TOF value of 1.44 s −1 and mass activity of 153.60 A g −1 at the overpotential of 300 mV. Furthermore, the real-time kinetic simulation elucidates a pronouncedly changed kinetic behavior of MIL-53(Fe)-2OH during the OER process. By combining the advantages of homogeneous and heterogeneous catalysts, this work offers a novel design strategy based on the molecular MOF reconstruction with delicate modulations of coordination environments and electronic structures of active sites, which will open a new direction for the future development of efficient OER electrocatalysts.

Experimental Section
Synthesis of Hydroxyl Functionalized Metal-Organic Framework MIL-53(Fe)-2OH: First, H 4 DOBDC (0.20 g) and FeCl 3 ·6H 2 O (0.27 g) were dissolved in DMF (12 mL). Ultrasonicated for 5 min and then the uniform mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated for 24 h at 110 °C. Cooling it to room temperature, it was filtered by a microporous filter membrane and washed with EtOH and DI water three-time. Finally, the MIL-53(Fe)-2OH was dried in a drying oven at 50 °C for 12 h.

Synthesis of Fe-Metal-Organic Frameworks-74 (MOF-74-Fe):
Fe-Metalorganic Frameworks-74 (MOF-74-Fe) was prepared according to literature reports. [25b] Typically, FeCl 2 ·4H 2 O (0.10 g) and H4DOBDC (0.22 g) were dissolved in 7.5 mL DMF-EtOH-water mixture (1:1:1 (v/v/v)) under magnetic stirring at room temperature to form a mixture solution. The obtained mixture solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave to heat at 120 °C for 24 h. The powder was collected by washing/filtered with EtOH and DMF several times to remove organic residues and dried at 100 °C overnight.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.