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  Electrochemical water splitting is one of the most significant routes to generate the high purity H₂ without harmful side products, which can reconcile the environmental deterioration and excessive consumption of fossil fuels.^{[1, 2]} Unfortunately, this strategy still suffers from great challenges because the highly scarce and expensive noble metal-based materials are regarded as the best electrocatalysts to decrease the overpotentials and elevate the sluggish dynamic of the two half-reactions: the hydrogen evolution reaction (HER) at cathode and the oxygen evolution reaction (OER) at anode.^[3-6] Therefore, it is urgent to design and develop inexpensive catalysts extracted from earth-abundant non-noble metal elements as promising alternatives to maximize the water splitting efficiency.^[3-5] Among various alternative catalysts, metal-organic frameworks (MOF) built from metal ions/clusters and organic linkers are of particular attention as they have the advantages including tailorable pore structure, high surface area and fascinating structural diversity.^{[6, 7]} For instance, Li et al. synthesized a series of trimetallic Mn_xFe_yNi-MOF-74 films in-situ grown on nickel foam (NF), which exhibited excellent OER and HER performance in alkaline media.^[8] Wang et al. reported the NiFe-bimetal two-dimensional ultrathin MOFs nanosheets (NiFe-UMNs) with a uniform thickness of ~10 nm as OER electrocatalysts, delivering excellent catalytic OER activity along with outstanding durability.^[9] Despite the considerable progress that has been made, the MOF electrocatalysts still suffer from unsatisfactory electrical conductivity and sluggish mass transfer.^[10]

  Recently, MOF has been successfully utilized as a precursor or sacrificial template to fabricate metal derivatives (oxides, hydroxides, phosphides, sulfides, et al.) for improving electrical conductivity and catalytic activity through the pyrolysis strategy.^[11-13] The high temperature thermal treatments, however, would possibly damage the desirable porous MOF structure, which could result in the aggregation of metal derivatives and deficiency of corresponding active sites, thus decreasing electrocatalytic performance towards water splitting.^{[12, 14]} To resolve these problems, direct coupling of metal derivatives (e.g. metal oxides, hydroxides, and phosphides) with porous MOF materials is regarded as a brilliant way to not only improve the electrical conductivity but also take full advantage of the unique features of MOF.^[15-17] The possible coupling between MOF and metal derivatives could induce synergistic effect to facilitate the adsorption/desorption process of reaction intermediates for promoting water-splitting efficiency. However, there is still much room to boost the catalytic activity. It is well known that metal sulfides with plentiful bridged bonds of metal and sulfur possess a high electrical conductivity and great corrosion resistance, providing a big chance to accelerate the charge transfer and improve catalytic activity.^{[7, 18-20]} To the best of our knowledge, there is rare report on the combination of metal sulfides and MOFs for achieving excellent catalytic performance.

  Herein, a facile one-step solvothermal strategy is applied to fabricate an advanced bifunctional HER/OER electrocatalyst via assembling Fe doped Ni₃S₂ clusters with hierarchical Ni/Fe-based MOF nanosheets array onto Ni foam (denoted as Fe-Ni₃S₂@NiFe-MOF/NF). The marriage of clusters and ordered nanosheet arrays could not only endow the elaborate Fe-Ni₃S₂@NiFe-MOF/NF with large specific surface area, but also facilitate the diffusion of electrolyte ions and the release of produced bubbles, thus boosting the catalytic HER/OER activity compared to the individual catalysts alone. More importantly, the strong electronic interaction between the Fe-Ni₃S₂ clusters and NiFe-MOF nanosheet arrays could accelerate the charge transfer and optimize the binding strength of the HER/OER intermediates on the catalyst surface. Impressively, the optimized Fe-Ni₃S₂@NiFe-MOF/NF not only exhibits excellent catalytic OER activity with a low overpotential of 226 mV at the current density of 100 mA cm^-2, but also displays outstanding catalytic activity towards HER with the requirement of only 170 mV to drive 50 mA cm^-2, which are superior to the Fe-Ni₃S₂/NF and NiFe-MOF/NF in alkaline electrolyte. In addition, the assembled overall water-splitting system with Fe-Ni₃S₂@NiFe-MOF/NF serving as both cathode and anode just requires 1.60 V cell voltage to realize the current density of 10 mA cm^-2.

  As illustrated in Scheme 1, the Fe-Ni₃S₂@NiFe-MOF/NF is in-situ grown on 3D macroporous NF substrate via a simple solvothermal strategy. During the solvothermal process, exogenously introduced Ni and Fe ions would coordinate with the 2-aminoterephthalic acid (H₂BDC-NH₂) ligand to form NiFe-MOF. Meanwhile, thioacetamide (TAA) as a sulfur source decomposes to release S^2- ions, which could directly react with the introduced Ni and Fe to form Fe-Ni₃S₂ on the surface of NiFe-MOF. With the process of reaction, the color of the NF surface completely transforms from silver-gray to brownish black without any speckle, indicating the as-obtained Fe-Ni₃S₂@NiFe-MOF/NF is uniformly covered on the NF substrate (Figure S1). For comparison, the pure NiFe-MOF/NF and Fe-Ni₃S₂/NF are also synthesized using the same procedure as Fe-Ni₃S₂@NiFe-MOF/NF, except for no adding TAA and H₂BDC-NH₂ linkers, respectively.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the fabrication processes of Fe-Ni₃S₂@NiFe-MOF/NF as an effective electrocatalyst for HER/OER.

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  The X-ray diffraction (XRD) technique is conducted to investigate the crystal structure of the as-prepared Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF. As shown in Figure 1a, it is evident that all three samples in XRD pattern display similarly prominent diffraction peaks at 44.5, 51.8, and 76.4°, corresponding to the NF substrate (JCPDS No. 65-2865).^[21] The pure Fe-Ni₃S₂/NF exhibits the characteristic diffraction peaks at 21.9, 31.3, 38.0, 50.1, and 55.4°, which can be indexed to the (101), (110), (003), (113) and (122) of Ni₃S₂ hexagonal planes.^{[17, 22, 23]} It can be seen that the diffraction peaks of Fe-Ni₃S₂/NF slightly shift to the high-angle direction compared to those of the Ni₃S₂ hexagonal structure previously reported,^[23] indicating the possible doping of Fe into Ni₃S₂ framework. For pure NiFe-MOF/NF, the characteristic peaks recorded at 9.2, 10.5, 12.5, 16.7, and 20.7° agree well with what have previously reported for MIL-88B type MOF structure.^[24] As observed, all the above mentioned diffraction peaks of NiFe-MOF/NF and Fe-Ni₃S₂/NF could be clearly detected in the XRD pattern of Fe-Ni₃S₂@NiFe-MOF/NF, demonstrating the successful formation of NiFe-MOF and Fe-Ni₃S₂ in as-fabricated Fe-Ni₃S₂@NiFe-MOF/NF. The Raman spectra are recorded to reveal functional groups of the as-prepared Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF. As shown in Figure S2, the appearance of peaks at ~194, 344 and 677 cm^-1 clearly validate the formation of Ni-S species for pure Fe-Ni₃S₂/NF.^[23] A shoulder peak at 310 cm^-1 can also be observed, which is assigned to the Fe-S species.^[23] The characteristic peaks associated with the vibration mode of the H₂BDC-NH₂ linkers are observed in the spectra of pure NiFe-MOF/NF (Figure 1b). The peaks at ~629, 809, and 866 cm^-1 are related to the stretching mode of C-H from benzene ring,^{[25, 26]} while a new doublet at ~1440 and 1609 cm^-1 is matched well with the in- and out-of-phase stretching modes of the carboxylate group, indicating the successful coordination between Ni/Fe ions and H₂BDC-NH₂ linkers in NiFe-MOF/NF.^{[25, 27]} It is worth noting that the Fe-Ni₃S₂@NiFe-MOF/NF possesses all the Raman signals nearly identical to those of NiFe-MOF/NF and Fe-Ni₃S₂/NF, verifying the coexistence of Fe-Ni₃S₂ and NiFe-MOF, which agree well with the XRD results.

  Figure 1

  

  Figure 1.  (a) XRD patterns of Fe-Ni₃S₂@NiFe-MOF/NF, NiFe-MOF/NF and Fe-Ni₃S₂/NF. (b) Raman spectra of Fe-Ni₃S₂@NiFe-MOF/NF and NiFe-MOF/NF. High-resolution XPS spectra of (c) C 1 s, (d) Fe 2p, (e) Ni 2p, and (f) S 2p for the electrocatalysts.

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  The chemical components and valence state of the as-prepared Fe-Ni₃S₂@NiFe-MOF/NF are probed through X-ray photoelectron spectroscopy (XPS). As shown in Figure S3, the whole XPS survey spectrum demonstrates the presence of Ni, Fe, S and O elements on the surface of Fe-Ni₃S₂@NiFe-MOF/NF. As shown in O 1s spectrum of Fe-Ni₃S₂@NiFe-MOF/NF (Figure 1c), the characteristic peak at 533.7 eV could be attributed to the -OH group originated from the adsorbed H₂O molecules, while the peaks at 531.6 and 532.7 eV are ascribed to the O-C=O and M-O bonds, respectively.^{[28, 29]} Figure 1d presents that the Fe 2p spectrum of Fe-Ni₃S₂@NiFe-MOF/NF could be divided into a pair of peaks at 711.1 and 725.1 eV assigned to Fe 2p_3/2 and Fe 2p_1/2, indicating the existence of Fe³⁺. The characteristic peaks at 856.3 and 874.6 eV for the Ni 2p spectrum can be indexed to the Ni 2p_3/2 and Ni 2p_1/2 (Figure 2e), confirming the +2 oxidation state of Ni in Fe-Ni₃S₂@NiFe-MOF/NF.^{[22, 29, 30]} In the S 2p spectrum of Fe-Ni₃S₂@NiFe-MOF/NF (Figure 2f), the two characteristic peaks centered at 162.9 and 164.6 eV are representative of the S 2p_3/2 and S 2p_1/2, respectively.^[30] Compared to NiFe-MOF/NF, the Ni 2p and Fe 2p peaks are positively shifted by ~0.4 and 1.2 eV for the Fe-Ni₃S₂@NiFe-MOF/NF, respectively. The positive shift of Fe 2p peaks is mainly attributed to the formation of Fe-S bridging units,^[10] which is consistent with the above XRD and Raman results. The S 2p peaks can be observed to be negatively shifted by ~0.3 eV compared to that of the Fe-Ni₃S₂/NF, suggesting the probable existence of electron coupling interaction between NiFe-MOF and Fe-Ni₃S₂, which is conducive to promoting the charge transfer and reaction kinetics of Fe-Ni₃S₂@NiFe-MOF/NF.^[17]

  Figure 2

  

  Figure 2.  (a) SEM images and (b) TEM images of Fe-Ni₃S₂/NF. (c) SEM images and (d) TEM images of NiFe-MOF/NF. (e) SEM images, (f) TEM images, (g) HR-TEM images, (h) SAED pattern, (i) HAADF-STEM image and corresponding elemental mapping images of Fe-Ni₃S₂@NiFe-MOF/NF.

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  The morphology structures of as-prepared Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF are characterized by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). For pure Fe-Ni₃S₂/NF, the 3D honeycomb-like morphology composed of densely interlaced nanosheets can be observed in Figure 2a and 2b. The pure NiFe-MOF/NF displays aggregated nanosheet array structure (Figure 2c and 2d), while the as-fabricated Fe-Ni₃S₂@NiFe-MOF/NF shows the coupled well-dispersed nanosheet arrays with the Fe-Ni₃S₂ nanoclusters. Such nanoclusters could effectively weaken the interaction between MOF nanosheets, e.g. van der Waals forces and p-p stacking, preventing the aggregation of NiFe-MOF nanosheets. TEM images of Fe-Ni₃S₂@NiFe-MOF/NF further unveil that the nanoclusters with the size of 100-200 nm are coated on the surface of 2D nanosheet (Figure 2f). High-resolution TEM (HR-TEM) is performed to further obtain the information of local morphology and microstructure of Fe-Ni₃S₂@NiFe-MOF/NF. As shown in Figure 2g, the Fe-Ni₃S₂@NiFe-MOF/NF possesses a well-defined phase interface on the surface, in which the visible lattice fringes separated by the interplanar spacing of 0.28 and 0.18 nm can be coincided with the d-spacing of the (110) and (113) crystalline planes of the Fe-Ni₂S₃, respectively.^{[17, 22, 30]} Some bright diffraction spots could be clearly observed in the selected area electron diffraction (SAED) pattern (Figure 2h), which correspond to the (003), (211), and (300) planes of Fe-Ni₃S₂, respectively.^[22] It is not visible for the lattice fringes and diffraction pattern that are indexed to the MIL-88B, which may be caused by the damage of NiFe-MOF structure under the high-energy electron flow.^[31] The elemental mapping is conducted to analyze the distribution of different elements in Fe-Ni₃S₂@NiFe-MOF/NF. It can be seen in Figure 2i that the Ni, Fe, O, and C are uniformly distributed over the entire Fe-Ni₃S₂@NiFe-MOF/NF. As observed, the elemental mapping of the collected S and Ni is strong in the clusters portion of Fe-Ni₃S₂@NiFe-MOF/NF, which indicates the formation of Fe-Ni₃S₂ clusters on the NiFe-MOF surface. Such unique structure is expected to accelerate the electron/mass transfer, electrolyte permeation as well as the release of generated gases, which can be effective to promote the catalytic activity.

  The electrochemical OER performances of the resultant Fe-Ni₃S₂@NiFe-MOF/NF, together with NiFe-MOF/NF, Fe-Ni₃S₂/NF, RuO₂ and blank NF for comparison, are evaluated in 1.0 M KOH solution using a standard three-electrode cell configuration. As shown in Figure 3a, the blank NF exhibits negligible anodic currents before 1.65 V vs. RHE. For Fe-Ni₃S₂@NiFe-MOF/NF, NiFe-MOF/NF, and Fe-Ni₃S₂/NF, the appearance of the peaks located at ca.1.37 V vs. RHE implies the surface oxidization of the partial Ni²⁺ to form Ni³⁺.^{[6, 32]} The overpotential to deliver the current density of 100 mA cm^-2 is as low as 226 mV for Fe-Ni₃S₂@NiFe-MOF/NF, which is much smaller than those of NiFe-MOF/NF (292 mV), Fe-Ni₃S₂/NF (272 mV) and commercial RuO₂ (362 mV). The corresponding Tafel slope is applied to evaluate the catalytic OER kinetics.^[33] As shown in Figure 3b, the calculated Tafel slope value of Fe-Ni₃S₂@NiFe-MOF/NF (67.5 mV dec^-1) is lower than those of NiFe-MOF/NF (105.7 mV dec^-1), Fe-Ni₃S₂/NF (90.1 mV dec^-1) and commercial RuO₂ (68.8 mV dec^-1), manifesting its remarkable OER dynamic behavior. As expected, when the potential is fixed at 1.50 V vs. RHE, the Fe-Ni₃S₂@NiFe-MOF/NF can get a high current density of 170 mA cm^-2 (Figure 3c), 3-fold, 2-fold, 23-fold, and 33-fold higher than NiFe-MOF/NF (60.5 mA cm^-2), Fe-Ni₃S₂/NF (92.3 mA cm^-2), RuO₂/NF (7.3 mA cm^-2) and blank Ni foam (5.2 mA cm^-2), respectively. Electrochemical impedance spectroscopy (EIS) is carried out to assess the charge transfer resistance (R_ct) at the electrode/electrolyte interface.^[34] As presented in Figure 3d, Nyquist plots reveal that Fe-Ni₃S₂@NiFe-MOF/NF has the smallest semicircle among all the investigated samples, indicating its lowest R_ct.^[34] With increasing the potential gradually, the semicircle diameter reduced the most rapidly in Fe-Ni₃S₂@NiFe-MOF/NF compared with other counterparts (Figure 3e), indicating the Fe-Ni₃S₂@NiFe-MOF/NF is easier to be polarized during OER process,^[35] which could facilitate the charge transfer linked to the adsorption/desorption of OER reaction intermediates. The electrochemical active surface area (ECSA) is also investigated to evaluate the OER activities by calculating the double-layer capacitance (C_dl), which is obtained via recording the cyclic voltammetry (CV) curves at different scan rates in a non-faradic potential range. The C_dl value of Fe-Ni₃S₂@NiFe-MOF/NF is calculated to be 7.03 mF cm^-2 (Figure 3f), higher than that of NiFe-MOF/NF (6.80 mF cm^-2) and Fe-Ni₃S₂/NF (1.71 mF cm^-2), suggesting that Fe-Ni₃S₂@NiFe-MOF/NF with unique arrays structure could provide more accessible active sites to boost the catalytic OER activity. The LSV curves normalized by ECSA display that the Fe-Ni₃S₂@NiFe-MOF/NF affords higher specific activity than other counterparts (Figure S4), further disclosing the marriage of Fe-Ni₃S₂ clusters and NiFe-MOF nanosheet arrays endows the catalysts with high specific activity.

  Figure 3

  

  Figure 3.  (a) LSV curves at a scan rate of 5 mV·s^-1, (b) the corresponding Tafel plots, (c) the current density at 1.50 V versus RHE and the overpotential at 10 mA·cm^-2 for the Fe-Ni₃S₂/NF, NiFe-MOF/NF, Fe-Ni₃S₂@NiFe-MOF/NF, RuO₂, and the blank NF. (d) EIS spectra, (e) Nyquist plots at various potentials, and (f) the C_dl values for the as-prepared Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF. (g) Chronopotentiometry curve at 50 mA·cm^-2, and (h) LSV curves before and after 1000 CV cycles for the Fe-Ni₃S₂@NiFe-MOF/NF.

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  Apart from the catalytic activity, the prolonged durability is considered as another important parameter to assess the OER performance.^[36] The 1000 cycles are handled for Fe-Ni₃S₂@NiFe-MOF/NF using a successive CV scanning between 1.2 and 1.5 V vs. RHE. As displayed in Figure 3g, the negligible potential loss can be observed from the polarization curve of Fe-Ni₃S₂@NiFe-MOF/NF after 1000 CV cycles compared with the initial one, especially at the high potential region, which suggests that the Fe-Ni₃S₂@NiFe-MOF/NF possesses the exceptional cycling durability during the OER process. In addition, the chronopotentiometric (v-t) measurement is carried out to further evaluate the long-term stability of Fe-Ni₃S₂@NiFe-MOF/NF. It should be noted that the studied Fe-Ni₃S₂@NiFe-MOF/NF exhibits negligible decay of potential after a 36h continuous test with a permanent current density of 50 mA cm^-2 (Figure 3h), further demonstrating the outstanding OER stability. The significantly enhanced OER performance could be ascribed to the hierarchical array structure and the synergistic effect between NiFe-MOF and Fe-Ni₃S₂, which is conducive to facilitating the charge/mass transfer and the adsorption/desorption of OER reaction intermediates.

  The XRD is employed to investigate the structure evolution of Fe-Ni₃S₂@NiFe-MOF/NF after the OER test. As shown in Figure S5, the characteristic peaks of NiFe-MOF completely disappear, while the peaks of Fe-Ni₃S₂ have no obvious change. The results suggest that the NiFe-MOF may undergo structural change from crystalline MOF to amorphous hydroxide,^{[37, 38]} whereas the structure of Fe-Ni₃S₂ can be well kept during the OER process. The disordered hydroxide generated from the conversion of NiFe-MOF may prevent the Fe-Ni₃S₂ clusters from being reconstructed. The SEM images show that the morphology of Fe-Ni₃S₂@NiFe-MOF/NF still maintains the array structure after the OER test, whereas the nanosheets become wrinkled and the thickness obviously increases (Figure S6), suggesting the possible generation of hydroxides during OER process. The XPS technique is applied to further investigate the structural change of Fe-Ni₃S₂@NiFe-MOF/NF during the OER process, as shown in Figure S7. In the O 1s spectrum, the percentage of M-O bond obviously increases and that of O-C=O bond decreases after OER durability test, further suggesting the conversion of NiFe-MOF to hydroxides during OER process.^{[11, 17, 22]} Notably, the binding energy of Ni 2p and Fe 2p peaks present the obviously positive shift for Fe-Ni₃S₂@NiFe-MOF/NF after the OER test, revealing that the metal species in Fe-Ni₃S₂@NiFe-MOF/NF could be converted to be electron-rich state during OER process, which is convenient to the formation of hydroxide.^[37] Therefore, it could be concluded that the formed hydroxide may serve as the real active sites in the OER process.

  Besides the promising applications for OER, the HER activity of Fe-Ni₃S₂@NiFe-MOF/NF is also investigated in 1M KOH solution. It is found that the HER activity of Fe-Ni₃S₂@NiFe-MOF/NF is better than that of NiFe-MOF/NF and Fe-Ni₃S₂/NF, although it is inferior to that of commercial Pt/C (Figure 4a and 4b). The needed HER overpotential of Fe-Ni₃S₂@NiFe-MOF/NF (170 mV) is lower than those of NiFe-MOF/NF (274 mV) and Fe-Ni₃S₂/NF (200 mV) to deliver the current density of 50 mA cm^-2. The Tafel slope of Fe-Ni₃S₂@NiFe-MOF/NF is 96.3 mV dec^-1, smaller than those of pure NiFe-MOF/NF (102.3 mV dec^-1) and Fe-Ni₃S₂/NF (100.1 mV dec^-1), demonstrating the fastest HER kinetics (Figure 4c). Such Tafel slope value suggests that the HER pathway of Fe-Ni₃S₂@ NiFe-MOF/NF corresponds to the Volmer-Heyrovsky mechanism and the desorption of hydrogen is the key rate-determining step.^[39] As can be seen in Figure 4d, the Fe-Ni₃S₂@NiFe-MOF/NF displays the smallest semicircle in the Nyquist plots compared with other counterparts, indicating that the Fe-Ni₃S₂@NiFe-MOF/NF has the fastest electron transfer at the electrode/electrolyte interface. After HER process, the diffraction peaks of NiFe-MOF completely disappear while those of Fe-Ni₃S₂ survive, as shown in the XRD pattern in Figure S8. This may be because the metal sites of NiFe-MOF could be partially reduced and the OH^- could replace the organic ligands of NiFe-MOF during HER process, leading to the structural transformation from NiFe-MOF to metal hydroxide.^{[8, 40]} Therefore, the main HER active sites may be the metal hydroxide formed at the Fe-Ni₃S₂@NiFe-MOF/NF surface.

  Figure 4

  

  Figure 4.  (a) LSV curves, (b) the corresponding overpotentials at current density of 50 mA cm^-2, and (c) Tafel plots of Fe-Ni₃S₂/NF, NiFe-MOF/NF, Fe-Ni₃S₂@NiFe-MOF/NF, Pt/C, and the blank NF. (d) Nyquist plots of the as-fabricated Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF. (e) Schematic illustration of the electrolyzer using Fe-Ni₃S₂@NiFe-MOF/NF as both anode and cathode. (f) LSV curves for the couple of Fe-Ni₃S₂@NiFe-MOF/NF//Fe-Ni₃S₂@NiFe-MOF/NF and RuO₂//Pt/C for water electrolysis (without iR compensation). (g) LSV curves before and after 1000 cycles and (h) chronopotentiometrric curve at 10 mA cm^-2 for the integrated system.

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  Inspired by the outstanding performances of OER and HER, an overall alkaline electrolysis cell is further established by taking Fe-Ni₃S₂@NiFe-MOF/NF as both anode and cathode (Figure 4e). As revealed in Figure 4f, the assembled Fe-Ni₃S₂@NiFe-MOF/NF//Fe-Ni₃S₂@NiFe-MOF/NF system could readily drive water electrolysis with a low cell voltage of 1.60 V to realize the current density of 10 mA cm^-2, which is comparable to the state-of-the-art coupled RuO₂//Pt/C under the corresponding current density. After continuous electrolysis for 1 h with an H-type electrolytic cell, the calculated Faraday efficiency is close to 100% by measuring the volume of generated O₂ and H₂ with water drainage method, as shown in Figure S9. The long-term stability of integrated system could be deemed as one of the central interests for practical applications.^[35] We perform a successive CV scanning between 1.2-1.7 V vs. RHE to investigate the stability for the assembled Fe-Ni₃S₂@NiFe-MOF/NF//Fe-Ni₃S₂@NiFe-MOF/NF system. The obtained polarization curve after 1000 CV cycles exhibits just a little bit shift compared with the initial one, indicating the excellent stability of the assembled system in 1 M KOH solution (Figure 4g). In addition, a chronopotentiometric test is used to evaluate long-term durability at a constant current density of 50 mA cm^-2 (Figure 4h). As observed, the potential of Fe-Ni₃S₂@NiFe-MOF/NF//Fe-Ni₃S₂@NiFe-MOF/NF system exhibits only 5% increase at the current density of 50 mA cm^-2 over a period of 50 h, which hence makes the Fe-Ni₃S₂@NiFe-MOF/NF a great promising candidate for efficient and durable H₂ generation by water splitting.

  CONCLUSION

  In summary, we have successfully designed hierarchical NiFe-MOF nanosheet arrays coupled with Fe-Ni₃S₂ clusters on the surface of Ni foam substrate via a facile one-step solvothermal reaction. The hierarchical array structure and the synergistic effect be-tween NiFe-MOF and attractive Fe-Ni₃S₂ could endow the Fe-Ni₃S₂@NiFe-MOF/NF with abundant active sites, rapid mass/charge transport, and strong bubble-releasing ability during the electrochemical reaction process. Remarkably, the Fe-Ni₃S₂@NiFe-MOF/NF exhibits excellent catalytic activity and stability towards both OER (η = 226 mV at 100 mA cm^-2) and HER (η = 170 mV at 50 mA cm^-2). More significantly, the assembled Fe-Ni₃S₂@NiFe-MOF/NF//Fe-Ni₃S₂@NiFe-MOF/NF system could readily drive the current density of 10 mA cm^-2 only at a low cell voltage of 1.60 V. The work provides useful guidelines for the preparation of high-performance bifunctional electrocatalysts and sheds light on the way for rationally designing the hybrid materials based on the MOF and metal sulfide.

  EXPERIMENTAL

  Materials and Reagents. Ni(NO₃)₂·6H₂O, FeCl₃·6H₂O, 2-aminoterephthalic acid (H₂BDC-NH₂), and thioacetamide (TAA) were purchased from Aladdin Co., Ltd. (Shanghai, China). Potassium hydroxide, N, N-dimethylformamide (DMF), isopropanol, and ethanol were bought from Sinopharm Chemical Reagent Co. The commercial Pt/C catalyst (20 wt%) and RuO₂ powder (99.9%) were obtained from Johnson Matthey Company (Shanghai, China). Nafion solution (5%) was provided by Sigma-Aldrich (MO, USA). All chemicals and reagents were used directly without further purification.

  Synthesis of Fe-Ni₃S₂@NiFe-MOF/NF. The Fe-Ni₃S₂@NiFe-MOF was in situ grown on Ni foam (NF) through a solvothermal method, as illustrated in Scheme 1. Prior to the solvothermal reaction, the commercial NF was ultrasonically treated for 15 min in sequence in 5 M HCl solution, ethanol, and deionized water to clean the surface oxide layers and impurities, followed by drying in vacuum at 50 ℃. In the preparation of Fe-Ni₃S₂@NiFe-MOF/NF, a homogeneous mixture containing 54 mg FeCl₃·6H₂O, 291 mg Ni(NO₃)₂·6H₂O, 181 mg 2-aminoterephthalic acid (H₂BDC-NH₂), 75 mg TAA, 30 mL DMF and 8 mL ethanol was poured into the Teflon-lined stainless-steel autoclave and the pretreated Ni foam was immersed in the above solution. Then, the autoclave wassealed for reaction at 150 ℃ for 15 h. After cooling down to room temperature, the resulting NF was taken out and thoroughly rinsed with ethanol and deionized water for three times before drying at 50 ℃ in vacuum. The loading amount of the Fe-Ni₃S₂@NiFe-MOF on the NF was around 1.2 mg cm^-2.

  Synthesis of NiFe-MOF/NF. The same procedures described above were applied for the preparation of NiFe-MOF/NF except for no TAA involved in the mixed solution.

  Synthesis of Fe-Ni₃S₂/NF. The same procedures described above were applied for the preparation of Fe-Ni₃S₂/NF except that no H₂BDC-NH₂ ligands were added into the solution.

  Preparation of Pt/C and RuO₂. 5 mg Pt/C (20 wt%) or RuO₂ powder (99.9%) was dispersed in the mixture solution of deionized water/isopropanol/5% Nafion (v:v:v = 4:1:0.1). After ultrasonication for 1 h, the as-prepared catalyst ink was uniformly loaded on the surface of NF and dried under infrared light. The mass loading of the Pt/C or RuO₂ catalyst powder on Ni film was ~1.2 mg cm^-2.

  Material Characterizations. The powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced Diffractometer (Cu Ka radiation, λ = 1.5418 Å). Raman measurements were carried out on the Thermo Fisher spectrometer equipped by using a 532 nm laser as the excitation source. The surface morphology, composition and microstructure of the as-prepared samples were characterized via scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM 2100). X-ray photoelectron spectroscopy (XPS) measurements were carried out on the Thermo Scientific ESCALAB 250Xi spectrometer.

  Electrochemical Measurements. The electrocatalytic activities towards HER and OER were tested by the electrochemical workstation (CHI 760E, Shanghai) with a three-electrode system or two-electrode electrochemical cell. The OER and HER performances were investigated in 1.0 M KOH using the NF-based electrodes as the working electrode, the graphite rod and Hg/HgO electrode as counter and reference electrodes, respectively. All potentials for OER and HER in this work were converted to the reversible hydrogen electrode (RHE) potential according to the Nernst equation: E_RHE = E_Hg/HgO + 0.059pH + 0.197. The performances of overall water splitting were measured with a two-electrode configuration cell, in which the Fe-Ni₃S₂@NiFe-MOF/NF was applied as both anode and cathode. The linear sweep voltammetry (LSV) curves were collected at a scan rate of 5 mV s^-1 with automatic iR compensation on the electrochemical workstation. To evaluate the electrochemical active surface area (ECSA), the electrochemical double layer capacitances (C_dl) were obtained by recording the CV curves in a non-Faradaic potential region from 0.401 to 0.501 V vs. RHE at different scan rates of 20, 40, 60, 80, 100, and 120 mV s^-1.

  
  ACKNOWLEDGEMENTS: This work was financially supported by the Natural Science Foundation of Shandong Province (ZR2020ZD10), and the National Natural Science Foundation of China (21775142). The authors declare no competing interests.
  COMPETING INTERESTS
  For submission: https://mc03.manuscriptcentral.com/cjsc
  ADDITIONAL INFORMATION
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0145
  
• 1. [1]

     Li, H.; Di, S.; Niu, P.; Wang, S.; Wang, J.; Li, L. A durable half-metallic diatomic catalyst for efficient oxygen reduction. Energy Environ. Sci. 2022, 15, 1601-1610. doi: 10.1039/D1EE03194E

  2. [2]

     Wang, H.; Liu, X.; Niu, P.; Wang, S.; Shi, J.; Li, L. Porous two-dimensional materials for photocatalytic and electrocatalytic applications. Matter 2020, 2, 1377-1413. doi: 10.1016/j.matt.2020.04.002

  3. [3]

     Wu, Y.; Wang, H.; Ji, S.; Pollet, B. G.; Wang, X.; Wang, R. Engineered porous Ni₂P-nanoparticle/Ni₂P-nanosheet arrays via the Kirkendall effect and Ostwald ripening towards efficient overall water splitting. Nano Res. 2020, 13, 2098-2105. doi: 10.1007/s12274-020-2816-7

  4. [4]

     Ding, W.-L.; Cao, Y.-H.; Liu, H.; Wang, A.-X.; Zhang, C.-J.; Zheng, X.-R. In situ growth of NiSe@Co_0.85Se heterointerface structure with electronic modulation on nickel foam for overall water splitting. Rare Met. 2020, 40, 1373-1382.

  5. [5]

     Wang, C.-Y.; Yang, C.-H.; Zhang, Z.-C. Unraveling molecular-level mechanisms of reactive facet of carbon nitride single crystals photocatalyzing overall water splitting. Rare Met. 2020, 39, 1353-1355. doi: 10.1007/s12598-020-01568-1

  6. [6]

     Zhou, J.; Han, Z.; Wang, X.; Gai, H.; Chen, Z.; Guo, T.; Hou, X.; Xu, L.; Hu, X.; Huang, M.; Levchenko, S. V.; Jiang, H. Discovery of quantitative electronic structure‐OER activity relationship in metal‐organic framework electrocatalysts using an integrated theoretical‐experimental approach. Adv. Funct. Mater. 2021, 31, 2102066. doi: 10.1002/adfm.202102066

  7. [7]

     He, P.; Xie, Y.; Dou, Y.; Zhou, J.; Zhou, A.; Wei, X.; Li, J. R. Partial sulfurization of a 2D MOF array for highly efficient oxygen evolution reaction. ACS Appl. Mater. Interfaces 2019, 11, 41595-41601. doi: 10.1021/acsami.9b16224

  8. [8]

     Zhou, W.; Xue, Z.; Liu, Q.; Li, Y.; Hu, J.; Li, G. Trimetallic MOF-74 films grown on Ni foam as bifunctional electrocatalysts for overall water splitting. ChemSusChem 2020, 13, 5647-5653. doi: 10.1002/cssc.202001230

  9. [9]

     Hai, G.; Jia, X.; Zhang, K.; Liu, X.; Wu, Z.; Wang, G. High-performance oxygen evolution catalyst using two-dimensional ultrathin metal-organic frameworks nanosheets. Nano Energy 2018, 44, 345-352. doi: 10.1016/j.nanoen.2017.11.071

  10. [10]

      Ma, X.; Chang, C.; Zhang, Y.; Niu, P.; Liu, X.; Wang, S.; Li, L. Synthesis of Co-based Prussian blue analogues/dual-doped hollow carbon microsphere hybrids as high-performance bifunctional electrocatalysts for oxygen evolution and overall water splitting. ACS Sustain. Chem. Eng. 2020, 8, 8318-8326. doi: 10.1021/acssuschemeng.0c01974

  11. [11]

      Sun, H.; Lian, Y.; Yang, C.; Xiong, L.; Qi, P.; Mu, Q.; Zhao, X.; Guo, J.; Deng, Z.; Peng, Y. A hierarchical nickel-carbon structure templated by metal-organic frameworks for efficient overall water splitting. Energy & Environ. Sci. 2018, 11, 2363-2371.

  12. [12]

      Jiao, L.; Wang, Y.; Jiang, H. L.; Xu, Q. Metal-organic frameworks as platforms for catalytic applications. Adv. Mater. 2018, 30, e1703663. doi: 10.1002/adma.201703663

  13. [13]

      Lyu, F.; Bai, Y.; Li, Z.; Xu, W.; Wang, Q.; Mao, J.; Wang, L.; Zhang, X.; Yin, Y. Self-templated fabrication of CoO-MoO₂ nanocages for enhanced oxygen evolution. Adv. Funct. Mater. 2017, 27, 1702324. doi: 10.1002/adfm.201702324

  14. [14]

      Duan, J.; Chen, S.; Zhao, C. Ultrathin metal-organic framework array for efficient electrocatalytic water splitting. Nat. Commun. 2017, 8, 15341. doi: 10.1038/ncomms15341

  15. [15]

      Wang, W.; Wang, Z.; Hu, Y.; Liu, Y.; Chen, S. A potential-driven switch of activity promotion mode for the oxygen evolution reaction at Co₃O₄/NiO_xH_y interface. eScience 2022, https://doi.org/10.1016/j.esci.2022.04.004. doi: 10.1016/j.esci.2022.04.004

  16. [16]

      Cho, K.; Han, S. H.; Suh, M. P. Copper-organic framework fabricated with CuS nanoparticles: synthesis, electrical conductivity, and electrocatalytic activities for oxygen reduction reaction. Angew. Chem. Int. Ed. 2016, 55, 15301-15305. doi: 10.1002/anie.201607271

  17. [17]

      Yuan, B.; Li, C.; Guan, L.; Li, K.; Lin, Y. Prussian blue analog nanocubes tuning synthesis of coral-like Ni₃S₂@MIL-53(NiFeCo) core-shell nanowires array and boosting oxygen evolution reaction. J. Power Sources 2020, 451, 227295. doi: 10.1016/j.jpowsour.2019.227295

  18. [18]

      Wu, Z.; Guo, J.; Wang, J.; Liu, R.; Xiao, W.; Xuan, C.; Xia, K.; Wang, D. Hierarchically porous electrocatalyst with vertically aligned defect-rich CoMoS nanosheets for the hydrogen evolution reaction in an alkaline medium. ACS Appl. Mater. Interfaces 2017, 9, 5288-5294. doi: 10.1021/acsami.6b15244

  19. [19]

      Wang, Q.; Tian, Z.-Y.; Cui, W.-J.; Hu, N.; Zhang, S.-M.; Ma, Y.-Y.; Han, Z.-G. Hierarchical flower-like CoS₂-MoS₂ heterostructure spheres as efficient bifunctional electrocatalyst for overall water splitting. Int. J. Hydrog. Energy 2022, 47, 12629-12641. doi: 10.1016/j.ijhydene.2022.02.024

  20. [20]

      Liu, T.; Li, P.; Yao, N.; Kong, T.; Cheng, G.; Chen, S.; Luo, W. Self-sacrificial template-directed vapor-phase growth of MOF assemblies and surface vulcanization for efficient water splitting. Adv. Mater. 2019, 31, e1806672. doi: 10.1002/adma.201806672

  21. [21]

      Xu, X.; Du, P.; Guo, T.; Zhao, B.; Wang, H.; Huang, M. In situ grown Ni phosphate@Ni₁₂P₅ nanorod arrays as a unique core-shell architecture: competitive bifunctional electrocatalysts for urea electrolysis at large current densities. ACS Sustain. Chem. Eng. 2020, 8, 7463-7471. doi: 10.1021/acssuschemeng.0c01814

  22. [22]

      Zhao, M.; Li, W.; Li, J.; Hu, W.; Li, C. M. Strong electronic interaction enhanced electrocatalysis of metal sulfide clusters embedded metal-organic framework ultrathin nanosheets toward highly efficient overall water splitting. Adv. Sci. 2020, 7, 2001965. doi: 10.1002/advs.202001965

  23. [23]

      Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-index faceted Ni₃S₂ nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting. J. Am. Chem. Soc. 2015, 137, 14023-6. doi: 10.1021/jacs.5b08186

  24. [24]

      Senthil Raja, D.; Lin, H.-W.; Lu, S.-Y. Synergistically well-mixed MOFs grown on nickel foam as highly efficient durable bifunctional electrocatalysts for overall water splitting at high current densities. Nano Energy 2019, 57, 1-13. doi: 10.1016/j.nanoen.2018.12.018

  25. [25]

      Hou, X.; Han, Z.; Xu, X.; Sarker, D.; Zhou, J.; Wu, M.; Liu, Z.; Huang, M.; Jiang, H. Controllable amorphization engineering on bimetallic metal-organic frameworks for ultrafast oxygen evolution reaction. Chem. Eng. J. 2021, 418, 129330. doi: 10.1016/j.cej.2021.129330

  26. [26]

      Sun, F.; Wang, G.; Ding, Y.; Wang, C.; Yuan, B.; Lin, Y. NiFe-based metal-organic framework nanosheets directly supported on nickel foam acting as robust electrodes for electrochemical oxygen evolution reaction. Adv. Energy Mater. 2018, 8, 1800584. doi: 10.1002/aenm.201800584

  27. [27]

      Hou, X.; Zhou, J.; Xu, X.; Wang, X.; Zhang, S.; Wang, H.; Huang, M. Morphological modulation of CoFe-based metal organic frameworks for oxygen evolution reaction. Catal. Commun. 2022, 165, 106445. doi: 10.1016/j.catcom.2022.106445

  28. [28]

      Zhu, X.; Dai, J.; Li, L.; Zhao, D.; Wu, Z.; Tang, Z.; Ma, L.-J.; Chen, S. Hierarchical carbon microflowers supported defect-rich Co₃S₄ nanoparticles: an efficient electrocatalyst for water splitting. Carbon 2020, 160, 133-144. doi: 10.1016/j.carbon.2019.12.072

  29. [29]

      Chen, Y.; Zhang, X.; Qin, J.; Liu, R. Transition metal atom doped Ni₃S₂ as efficient bifunctional electrocatalysts for overall water splitting: design strategy from DFT studies. Mol. Catal. 2021, 516, 111955. doi: 10.1016/j.mcat.2021.111955

  30. [30]

      Zhang, W.; Jia, Q.; Liang, H.; Cui, L.; Wei, D.; Liu, J. Iron doped Ni₃S₂ nanorods directly grown on FeNi₃ foam as an efficient bifunctional catalyst for overall water splitting. Chem. Eng. J. 2020, 396, 125315. doi: 10.1016/j.cej.2020.125315

  31. [31]

      Xu, Y.; Li, B.; Zheng, S.; Wu, P.; Zhan, J.; Xue, H.; Xu, Q.; Pang, H. Ultrathin two-dimensional cobalt-organic framework nanosheets for high-performance electrocatalytic oxygen evolution. J. Mater. Chem. A 2018, 6, 22070-22076. doi: 10.1039/C8TA03128B

  32. [32]

      Zhao, Z.; Shao, Q.; Xue, J.; Huang, B.; Niu, Z.; Gu, H.; Huang, X.; Lang, J. Multiple structural defects in ultrathin NiFe-LDH nanosheets synergistically and remarkably boost water oxidation reaction. Nano Res. 2021, 15, 310-316.

  33. [33]

      Zhou, C.; Chen, X.; Liu, S.; Han, Y.; Meng, H.; Jiang, Q.; Zhao, S.; Wei, F.; Sun, J.; Tan, T.; Zhang, R. Superdurable bifunctional oxygen electrocatalyst for high-performance zinc-air batteries. J. Am. Chem. Soc. 2022, 144, 2694-2704. doi: 10.1021/jacs.1c11675

  34. [34]

      Wu, J.; Yu, Z.; Zhang, Y.; Niu, S.; Zhao, J.; Li, S.; Xu, P. Understanding the effect of second metal on CoM (M = Ni, Cu, Zn) metal-organic frameworks for electrocatalytic oxygen evolution reaction. Small 2021, 17, e2105150. doi: 10.1002/smll.202105150

  35. [35]

      Wen, Q.; Yang, K.; Huang, D.; Cheng, G.; Ai, X.; Liu, Y.; Fang, J.; Li, H.; Yu, L.; Zhai, T. Schottky heterojunction nanosheet array achieving high‐current‐density oxygen evolution for industrial water splitting electrolyzers. Adv. Energy Mater. 2021, 11, 2102353. doi: 10.1002/aenm.202102353

  36. [36]

      Xu, H.; Fei, B.; Cai, G.; Ha, Y.; Liu, J.; Jia, H.; Zhang, J.; Liu, M.; Wu, R. Boronization-induced ultrathin 2D nanosheets with abundant crystalline-amorphous phase boundary supported on nickel foam toward efficient water splitting. Adv. Energy Mater. 2019, 10, 1902714.

  37. [37]

      Hu, N.; Du, J.; Ma, Y.-Y.; Cui, W.-J.; Yu, B.-R.; Han, Z.-G.; Li, Y.-G. Unravelling the role of polyoxovanadates in electrocatalytic water oxidation reaction: active species or precursors. Appl. Surf. Sci. 2021, 540, 148306. doi: 10.1016/j.apsusc.2020.148306

  38. [38]

      Cheng, C.-C.; Cheng, P.-Y.; Huang, C.-L.; Raja, D.-S.; Wu, Y.-J.; Lu, S.-Y. Gold nanocrystal decorated trimetallic metal organic frameworks as high performance electrocatalysts for oxygen evolution reaction. Appl. Catal. B: Environ. 2021, 286, 119916. doi: 10.1016/j.apcatb.2021.119916

  39. [39]

      Kumar, A.; Bui, V. Q.; Lee, J.; Jadhav, A. R.; Hwang, Y.; Kim, M. G.; Kawazoe, Y.; Lee, H. Modulating interfacial charge density of NiP₂-FeP₂ via coupling with metallic Cu for accelerating alkaline hydrogen evolution. ACS Energy Lett. 2021, 6, 354-363. doi: 10.1021/acsenergylett.0c02498

  40. [40]

      Qi, L.; Su, Y.-Q.; Xu, Z.; Zhang, G.; Liu, K.; Liu, M.; Hensen, E. J. M.; Lin, R. Y.-Y. Hierarchical 2D yarn-ball like metal-organic framework NiFe(dobpdc) as bifunctional electrocatalyst for efficient overall electro-catalytic water splitting. J. Mater. Chem. A 2020, 8, 22974-22982. doi: 10.1039/D0TA08094B

• 

  Scheme 1  Schematic illustration of the fabrication processes of Fe-Ni₃S₂@NiFe-MOF/NF as an effective electrocatalyst for HER/OER.

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  Figure 1  (a) XRD patterns of Fe-Ni₃S₂@NiFe-MOF/NF, NiFe-MOF/NF and Fe-Ni₃S₂/NF. (b) Raman spectra of Fe-Ni₃S₂@NiFe-MOF/NF and NiFe-MOF/NF. High-resolution XPS spectra of (c) C 1 s, (d) Fe 2p, (e) Ni 2p, and (f) S 2p for the electrocatalysts.

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  Figure 2  (a) SEM images and (b) TEM images of Fe-Ni₃S₂/NF. (c) SEM images and (d) TEM images of NiFe-MOF/NF. (e) SEM images, (f) TEM images, (g) HR-TEM images, (h) SAED pattern, (i) HAADF-STEM image and corresponding elemental mapping images of Fe-Ni₃S₂@NiFe-MOF/NF.

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  Figure 3  (a) LSV curves at a scan rate of 5 mV·s^-1, (b) the corresponding Tafel plots, (c) the current density at 1.50 V versus RHE and the overpotential at 10 mA·cm^-2 for the Fe-Ni₃S₂/NF, NiFe-MOF/NF, Fe-Ni₃S₂@NiFe-MOF/NF, RuO₂, and the blank NF. (d) EIS spectra, (e) Nyquist plots at various potentials, and (f) the C_dl values for the as-prepared Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF. (g) Chronopotentiometry curve at 50 mA·cm^-2, and (h) LSV curves before and after 1000 CV cycles for the Fe-Ni₃S₂@NiFe-MOF/NF.

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  Figure 4  (a) LSV curves, (b) the corresponding overpotentials at current density of 50 mA cm^-2, and (c) Tafel plots of Fe-Ni₃S₂/NF, NiFe-MOF/NF, Fe-Ni₃S₂@NiFe-MOF/NF, Pt/C, and the blank NF. (d) Nyquist plots of the as-fabricated Fe-Ni₃S₂/NF, NiFe-MOF/NF, and Fe-Ni₃S₂@NiFe-MOF/NF. (e) Schematic illustration of the electrolyzer using Fe-Ni₃S₂@NiFe-MOF/NF as both anode and cathode. (f) LSV curves for the couple of Fe-Ni₃S₂@NiFe-MOF/NF//Fe-Ni₃S₂@NiFe-MOF/NF and RuO₂//Pt/C for water electrolysis (without iR compensation). (g) LSV curves before and after 1000 cycles and (h) chronopotentiometrric curve at 10 mA cm^-2 for the integrated system.

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