Interface Coupling of Ni-Co Layered Double Hydroxide Nanowires

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Interface Coupling of Ni-Co Layered Double Hydroxide Nanowires and Cobalt-based Zeolite Organic Frameworks for Efficient Overall Water Splitting Wenxia Chen, Yiwei Zhang, Guangliang Chen, Yuming Zhou, Xin Xiang, and Kostya (Ken) Ostrikov ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06627 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Sustainable Chemistry & Engineering

Interface Coupling of Ni-Co Layered Double Hydroxide Nanowires and Cobalt-based Zeolite Organic Frameworks for Efficient Overall Water Splitting

Wenxia Chena, Yiwei Zhanga*, Guangliang Chenb, Yuming Zhoua*, Xin Xianga, and Kostya (Ken) Ostrikovc

aSchool

of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic

Functional Materials and Engineering Laboratory, Nanjing 211189, P. R. China. bKey

Laboratory of Advanced Textile Materials and Manufacturing Technology and Engineering

Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China. cSchool

of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology,

Brisbane, QLD 4000, Australia.

*Corresponding

authors. E-mail: [email protected]; [email protected]. Tel: +86 25

52090617; Fax: +86 25 52090617.

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Abstract: Hydrogen is a source of sustainable and

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clean energy poised to replace fossil fuels.

Bi-functional electrocatalysts are actively pursued to simultaneously drive the two key hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for hydrogen production by electrolysis water. One of the most promising candidates based on bimetallic layered double hydroxide salts (LHSs) and cobalt-based organic framework (ZIF-67) suffer from poor interface coupling. Herein, we present a new approach based on fusing NiCo LHSs nanowire arrays with ZIF-67, to fabricate three-dimensional flower-like structures on a Ni-Fe foam support. To improve interfacial coupling and catalytic performance, simple oxidation, carbonization, sulfurization and selenization are performed to study the effects of different post-treatments and discover the optimum bi-functional electrocatalysts. The optimized S-doped catalyst reveals the highest electrocatalytic characteristic quantified by the low overpotentials of 170 and 100 mV for OER and HER at 10 mA cm-2 in 1 M KOH, respectively. This outstanding electrocatalytic property is ascribed to strong interfacial coupling between the NiCo-LHSs and ZIF-67 derivatives, as well as the rational electronic structures, dense catalytic active sites and large specific surface area. This work opens new prospects for fabricating efficient and low-cost electrocatalysts for renewable hydrogen energy production.

Keywords:

hydrogen

production,

metal-organic

derivatives,

electrocatalysis, overall water splitting

Introduction 2


hierarchical

structures,

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Water electrolysis has driven to study as a potential technology for sustainable production of renewable hydrogen fuels. In the electrolysis process, cost-effective and high-efficiency electrocatalysts are required to enhance sluggish reaction kinetics1-4. To address this limitation, Pt and Ru/IrO2 are widely used as efficient catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Unfortunately, their scarcities and high cost pose limitations for large-scale application5-7. This is why the development of state-of-the-art catalysts for HER and OER made of earth-abundant materials is a highly topical issue, especially if the same bi-functional electrocatalyst could be used for both reactions. To achieve this goal, considerable contributions have been achieved to investigate high-efficiency and cost-effective electrocatalysts, such as transition-metal sulfides8,9, nitrides10, phosphides11,12 and layered double hydroxides13,14. Among these materials, layered double hydroxide electrocatalysts based on Ni or Co exhibit high performance for HER and OER reactions15-17. Although significant progress has been achieved, major challenges remain for the layered double hydroxides because of the poor conductivity and insufficiently catalytically active surface. These and some other factors limit the electrocatalytic activity and stability in the whole HER and OER process. The in situ growth of NiCo-LHSs on conductive support (nickel foam and carbon nanotubes) has been suggested as a possible way for improving the activity of the catalysts. The catalytic performance of LHSs may also be tuned by hierarchical structuring and elemental composition. Metal-organic frameworks (MOFs) and their derivatives have recently been suggested as efficient precursors or templates to fabricate hollow and porous hybrid materials because of their well-tunable properties, porous structures and good compatibility with electrode materials and 3



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electrolytes. Several studies reported ZIF-derived compounds as promising electrode materials due to their unique structural features. For example, ZIF-67 intertwined with polypyrrole (PPy) nanofibers exhibited superior OER activity and stability in operation18. Furthermore, the ZIF-67-derived Co-based nanoarchitectures boosted the oxygen reduction and evolution reactions19. The NiCo-LHSs and ZIF-derived composites exhibit good OER and HER performance, respectively, although in different electrolytes. However, complex electrode fabrication processes may reduce the number of OER or HER active sites for overall water splitting in the same electrolyte. Grafting Co-MOFs onto NiCo-LHSs surface followed by changing the distribution of electrons and the exposure of the accessible reactive sites, appears promising for facilitating both HER and OER process. However, traditional approaches to merely combine two materials (like NiCo-LHSs and MOFs in this work) that normally catalyze each reaction separately, may be not work because of the unavoidable interaction between the two materials. Therefore, this materials combination should be further modified to achieve significant performance improvements. To obtain the superior electrocatalytic activity, one particularly challenging task is to optimize the interface coupling between the NiCo-LHSs and ZIF-67 materials leading to synergistic effects. In order to fuse these two materials together and improve the interface coupling, post-processing for two catalytic materials is needed, such as oxidation, carbonization, sulphidation and selenation, potentially leading to a discovery of the most effective bi-functional electrocatalysts enabling substantial reduction of the energy cost for overall water splitting. Herein, we produce NiCo-LHSs grafted by Co-based MOFs and fuse them together to improve the interface coupling by simple oxidation, carbonization, sulphidation and selenation, leading to 4


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unique hierarchically structured electrode materials for overall water splitting. The synthetic route (Scheme 1) for these composites involves the following steps: (i) NiCo-LHSs with ultrafine nanowire arrays on NiFe foam support were prepared via hydrothermal method. (ii) The Co-MOF (ZIF-67) polyhedron was grown in situ on the NiCo-LHSs, and then, the two composites reassembled to form a 3D hydrangea-like flower morphology. (iii) The 3D nanostructure was further formed by oxidation (NCO@Co3O4), carbonization (NCO@C), sulfurization (NCO@CoS) and selenation (NCO@CoSe) leading to a discovery of the most effective bi-functional electrocatalyst for OER and HER, respectively (Scheme 1). As the catalyst is grown in situ directly on support material (NiFe foam) in a hydrothermal process, Co reacted preferentially with Ni in the supporting foam. The produced catalysts exhibited extraordinary catalytic property and excellent stability for overall water splitting, surpassing to those most advanced electrocatalysts. Particularly, the NCO@CoS shows the optimal HER and OER performances with small overpotentials of 170 and 100 mV at 10 mA cm-2, respectively, outperforming some noble metal-based and other related catalysts20,21. The good catalytic performance is ascribed to the strong interfacial coupling between the NiCo-LHSs and ZIF-67 derivatives, and the rational design of the electronic structure, the accessibility of dense reactive sites as well as the high specific surface area. This work presents a new approach to develop advanced bi-functional electrocatalysts based on synergistic effects of LHSs and metalorganic derivatives, thus opening new avenues for inexpensive, abundant and non-toxic catalysts for hydrogen economy and other electrochemical applications.

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Scheme 1. A schematic of the preparation of flower-like hybrid material NCO@M (M = Co3O4, C, CoS, and CoSe) for overall water splitting.

Experimental Section Synthesis of NiCo-LHSs/NiFe foam (NFF). To clean the surface dirt, the NFF (1 × 1 cm2) was treated using acetone, HCl and deionized water, respectively. Thereafter, 2910 mg Co(NO3)2·6H2O, 2908 mg Ni(NO3)2·6H2O and 2400 mg urea were mixed together with 40 mL water. Then, the mixture and NFF were shift into a 100 mL autoclave. After reaction, this final sample was rigorously washed until the reaction system cooled down, and the as-prepared sample was then dried at 60 °C. Preparation of NiCo-LHSs@ZIF-67/NFF (marked as NCN@ZIF-67). The ZIF-67 6


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crystallization in situ growth on NFF/NCN surface was analyzed. 2463 mg 2-methylimidazole and 297 mg Co(NO3)2·6H2O were added in 40 mL methanol, and the as-prepared NFF/NCN samples were added into the precursor for 2, 4, 6 and 8 h. The resulting NCN@ZIF-67 was cleaned, and dried in vacuum at 60 °C. Preparation of NCO@M (M = Co3O4, C, CoS and CoSe). The NCO@Co3O4 was developed by the cracking of NCN@ZIF-67. The NCO@M was formed by cracking of NCN@ZIF-67 at 800 °C for 3 h in N2 atmosphere. For the NCO@CoS, firstly, the thioacetamide (1 mmol) was aedded in 20 mL ethanol, and the NCO@Co3O4 was shifted into the above solution for 2 h at 120 °C. The NCO@CoSe was obtained by the selenation of NCO@Co3O4. In brief, selenium powder (1 mmol) and NaBH4 (1 mmol) were mixed with 60 mL ethanol and deionized water, after that, this NCO@Co3O4 was added into the mixture at 180 °C for 16 h. All the obtained NCO@M (M = Co3O4, C, CoS and CoSe) were washed thoroughly with ethanol and deionized water. Preparation of Co3O4 derived from ZIF-67. Co(NO3)2·6H2O (291 mg) samples and 25 mL methanol were mixed to form a homogeneous solution. Following, 328.4 mg 2-methylimidazole were added into 25 mL of methanol. Then, the mixed solution aged at 25 °C for 24 h. Then, the ZIF-67 were collected and washed. Finally, this dried ZIF-67 was calcined at 350 °C to obtain Co3O4 hollow polyhedron. Characterization. X-ray diffractometry (XRD, Rigaku) was applied to investigate the crystalline structures of these products. TEM (Model JEM-2100 JEOL) system equipped with EDX spectrometer can monitor the microstructure of the synthesized materials. The morphology and structure of the nanocomposite catalyst was characterized using SEM (JEOL, Japan). The chemical bonds of these products were discussed by FT-IR spectrometry (Nicolet 6700) and X-ray 7



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photoelectron spectroscopy (XPS, VG, Scientific). The Raman spectra were collected by BX51 (Thermo Fisher) spectrometer. The specific surface area of these products were studied by BET method (ASAP 2020, USA). Electrochemical measurements. An electrochemical workstation featuring a three-electrode configuration (Model CHI750E) was used to test electrochemical property. The NFF, NCN@ZIF-67 and its derivative NCO@M (M = Co3O4, C, CoS and CoSe) materials were used directly as a working electrode, as well as Hg/HgO electrode and carbon rod were the reference and the counter electrodes, respectively. The loading amount of these catalysts (5 mg) for NCO@M (M = Co3O4, C, CoS and CoSe) structures remained the same before and after the reaction. The tests were performed in 1 M KOH, and the linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), chronopotentiometric and chronoamperometric curves were obtained. The electrochemically active surface area (ECSA) was tested according to CV method.

Results and Discussion Characterization of the nanomaterials. SEM and TEM were used to study the morphology of samples. Compared to the optimized ZIF-67@NiCo-LHSs /NFF precursor (Figure S1 and Figure S 2e and 2f), the obtained NCO@Co3O4 with many hollow polyhedral particles change from a 3D hydrangea-like flower structure to a few nanorods though the annealing process (Figure 1a). A possible reason for this behavior may be due to the separation effect of the packed nanosheets during the calcining process. In this case, a synergistic effect occurs in the interior of the ion channels. TEM images reveal that the NCO@Co3O4 sample retains the flower-like structure with a 8


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hollow polyhedral morphology from ZIF-67 derivatives (Figure 1b). Moreover, TEM images further verify the existence of nanorods among the packed nanosheets. After the carbonization treatment, the N-doped porous carbon sample retains the flower-like structure, while the polyhedrons lose the original ZIF-67 morphology (Figure 1c). Additionally, TEM images reveal that a layer of carbon doped with N is coated on the lotus-flower-like hierarchical structure made of NCO and Co3O4, which is verified in the inset in Figure 1d. For NCO@CoS, the derived NCO@CoS catalyst retains the overall morphology of ZIF-67@NiCo-LHSs /NFF sample reasonably well (Figure 1e). Importantly, sulfur particles connect with Co2+, forming loose and porous structure coverage on the surface. From the TEM image, it is clear that the flower-like structure of NCO@CoS was not damaged during the process of sulfuration, whereas the CoS with hollow structure can be seen (Figure 1f, inset). In addition, the TEM and HRTEM images of NCO@CoS have also been obtained at higher magnification. These images reveal the regular flower-like shape and close-packed sulfur particles, thereby suggesting that NCO@CoS samples have the highest chemical bond strength between the Co and S elements (Figure S 4a). HRTEM image of the NCO@CoS reveals two different lattice fringes, and the two d-spacings of the lattice fringes are 0.33 and 0.17 nm, matching with the (220) and (222) planes of CoNiO2 and Co9S8, respectively (Figure S 4b). Figure S 4c shows that the TEM-EDX mapping images display four elements including cobalt, sulfur, nickel and oxygen. The distribution of cobalt, sulfur, nickel and oxygen elements appears uniform across the mapping area of NCO@CoS flower-like structures. After selenation, the derived NCO@CoSe catalysts retains the flower-like morphology (Figure 1g and 1h). The corresponding TEM in the inset of Figure 1g confirms the smooth integration of the 9



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hollow polyhedrons with the nanoflakes. Nevertheless, the size of the derived CoSe (Figure 1g) is larger than that of the CoS particles (Figure 1e), and the increased size may inhibit the electrocatalytic activity of NCO@CoSe.

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Figure 1. The SEM (left) and TEM (right) images of prepared products: (a) and (b) of NCO@Co3O4, (c) and (d) of NCO@C, (e) and (f) of NCO@CoS, (g) and (h) of NCO@CoSe samples. Meanwhile, the crystallinity and chemical structures of these as-obtained derivatives were analyzed by XRD, see Figure 2a. After oxidation, the NCO@Co3O4 mainly shows diffraction peaks at 31.40°, 36.94°, 59.43° and 65.80°, corresponding to the (220), (311), (511) and (440) lattice planes, respectively. The peak at 62.21° is ascribed to the (220) lattice planes, representing the CoNiO2 from the NiCo-LHSs/NFF precursor. The XRD patterns of NCO@C samples originate from Co3O4, cobalt carbide and reduced metallic cobalt. This corresponding peaks at 31.40°, 40.56°, 54.70°, 57.12° and 59.43° match with the (220), (222), (004) (221) and (511) lattice planes, respectively. It is known that the peaks at 31.40° and 59.43° can be defined as the (220) and (511) lattice planes of Co3O4, while the peaks at 40.56°, 54.70° and 57.12° are assigned to (222), (004) and (221), respectively. For comparison, the Co3O4 powder directly derived from ZIF-67 was also characterized by XRD (Figure S6). One can see a minor changes of the diffraction peaks from the spinel Co3O4 phase, which indicates that NCO@C is mainly composed of the Co3O4 and metallic cobalt. For NCO@CoS, these diffraction peaks at 31.40°, 36.94° and 62.21°can be noticed clearly, which are assigned to (222), (311) and (220) lattice planes of the Co9S8 and Co3O4, respectively. After the selenation treatment, diffraction peaks at 34.74°, 36.34° and 51.10° matched the (111), (012) and (643) lattice planes of CoSe2 and CoSe3, respectively. Interestingly, we were not able to resolve the XRD peaks corresponding to CoNiSx, FeSx/FeSex compounds that could have been generated during hydrothermal sulfuration or selenization. This means these compounds do not form, are 11



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likely to be in an amorphous form, or the amount of these compounds is below the XRD detection limit. These CoNiSx, FeSx/FeSex compounds are thus not expected to make any significant contributions to the electrocatalytic process. Figure 2b shows the Raman spectroscopy of the as-prepared composites. In particular, the NCO@C reveals two bands at about 1370 (ID) and 1580 cm-1 (IG), and the value of ID / IG (0.87) manifests that the composite is graphitized, consistently with previous reports22,23, which is beneficial for facilitating ion transfer, increasing the conductivity and leading to higher catalytic activity. Furthermore, charge separation and recombination were examined by photoluminescence PL spectra (Figure 2c), generated using an excitation wavelength of 350 nm. The observed decreasing fluorescence emissions confirms fast electrons transfer between NCO and the above derivatives24. Among them, the NCO@CoS exhibits the lowest recombination rate of carriers, which will be favorable for improving the electronic conductivity of the catalyst. The chemical composition of the derivatives was studied by FT-IR spectroscopy. As shown in Figure 2d, these peaks at 2980-2900, 2350 and 1151 cm-1 match with C-H or C-O stretching vibrations and the peaks around 1625 and 1500-1400 cm-1 are agree with C=C bond, respectively25. While the peak at 1067 cm-1 is consistent with C=O groups, which mainly exists in NCO@Co3O4 and NCO@C composites. It is likely that the C=O groups are formed during the annealing process. However, these absorbance peaks disappear in the sulfurization and selenation processes and are replaced by a new peak at 1151 cm-1 representing characteristic peaks of C-H groups. Besides, the peaks at around 800-550 cm-1 are ascribed to X-O, O-X-O and X-O-X (X = Co and Ni) bonds, confirming the existence of Ni and Co in the material structures26. Apparently, these results are 12


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consistent with the EDX (Figure S5) and XRD (Figure 2a) measurements.

Figure 2. (a) XRD patterns, (b) Raman spectra, (c) photoluminescence spectra and (d) FT-IR spectra of NCO@Co3O4, NCO@C, NCO@CoS, and NCO@CoSe samples, respectively.

In order to further quantify the composition of those composites, the valence states O 1s, C 1s, S 2p and Se 3d from NCO@Co3O4, NCO@C, NCO@CoS and NCO@CoSe samples were analyzed by XPS. The O 1s is fitted with two metal oxygen bonds at 529.3 and 531.0 eV (Figure 3a), and they correspond to Ni-O or Co-O in NCO@Co3O4 and some defect states of oxygen, respectively27,28. The C 1s exhibits three strong peaks at 284.3 and 285.6 eV in Figure 3b, which are ascribed to C=C and C=O groups, respectively29. These peaks form mainly from the pyrolysis of 13



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2-methylglyoxaline. Figure 3c exhibits the S 2p spectrum at 163.32 eV corresponding to S 2p3/230-32. More importantly, we detect other components such as oxidized S, which is centered at 168.89 eV and can be de-convoluted by one main peak and one shakeup satellite peak33,34. The S-doped complex with only one kind of S-bonding configuration is thus produced, and can be used to study physical and chemical properties of S-doped Ni-Co composites. Besides, the Co 2p spectrum of the NCO@CoS reveals the main peaks at 785.3 and 798.4 with two satellite peaks at 788.6 and 803.6, which corresponds to Co 2p3/2 and Co 2p1/2, and the peak at 782.2 is assigned to the Co2+ (Co 2p3/2) species35. This result confirms the coexistence of Co2+ and Co3+ in NCO@CoS composite in Figure S 7a. The peaks at 59.2 and 54.7 eV are corresponding to Se 3d5/2 and Se 3d3/2, respectively, suggesting the -2 charge state for Se36. The Co 2p in NCO@CoSe reveals that the binding energies at 781.5 and 797.6 eV are attributed to the Co 2p3/2 and Co 2p1/2, followed by two satellite peaks located at 786.2 and 802.8 eV37,38, as shown in Figure S 7b. Therefore, the XPS spectra further confirm the successful synthesis of composite flower-like nanostructures and suggests a strong interaction between the NCO and cobalt compound. Through the analysis of the valence states of cobalt elements in NCO@CoS and NCO@CoSe, the Co source participating in sulfurization or selenization should originate from ZIF-67 and NiCo-LHSs compound because the corresponding precursors exist in the form of metal oxides before the sulfurization or selenization.

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Figure 3. XPS spectra: (a) O 1s of NCO@Co3O4, (b) C 1s of NCO@C, (c) S 2p of NCO@CoS and (d) Se 3d of NCO@CoSe.

Electrocatalytic properties. The electrocatalytic water splitting occurs through the two HER and OER half reactions. The OER of NCO@M composites was measured in a three-electrode device. It is found that the NCO@CoS exhibits an earlier OER onset potential (1.34 V vs. RHE) than other composites (Figure S 8a and Figure S 9a). In addition, the corresponding overpotentials measured from the samples in Figure 4a at 10 and 50 mA cm-2 are displayed in Figure 4b. The NCO@CoS needs the lowest overpotential of 170 mV, which is 270, 250, 120, 170 and 50 mV less than those for NFF, ZIF-67@NiCo-LHSs /NFF, NCO@Co3O4, NCO@C and NCO@CoSe catalysts 15



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at 10 mA cm-2, respectively. Moreover, our catalyst shows superior OER activity than its counterparts (Table S1) and many reported Co-based materials (Table S2). The higher current density (50 mA cm-2) was also further comparison. Remarkably, irrespective at 10 or 50 mA cm-2, the overpotential follows the trend: NCO@CoS < NCO@CoSe < NCO@Co3O4 < NCO@C < NiCo-LHSs@ZIF-67/NFF < NFF, suggesting the wonderful OER character of the NCO@CoS catalyst. Possibly, this phenomenon can be caused by their unique 3D heterogeneous structure and facile ion diffusion between NCO and M. Figure 4c displays the Tafel slope of NFF, ZIF-67@NiCo-LHSs/NFF (named as NCN@ZIF-67) and NCO@M. The NCO@CoS exhibits the lowest value of 76 mV dec-1 (Table S1), and it is smaller relative to the NCO@CoSe (87 mV dec-1), NCO@Co3O4 (95 mV dec-1), NCO@C (107 mV dec-1), NCN@ZIF-67 (182 mV dec-1) and NFF (198 mV dec-1) (Figure 4c). This finding demonstrates that the NCO@CoS with a smaller Tafel slope exhibits a higher OER rate. The LSV curve of NCO@CoS is carried out under different scan rates. From Figure 4d, there is almost no change discovered in the LSV curves of NCO@CoS when increases the scan rate. This measurements was performed in a low overpotential region of the mass transport process, which is beneficial for the electrolyte penetration and the rapid release of gas products. Consequently, the active material with an effective electron transport results in the NCO@CoS with smallest Tafel slope.39-41 To further investigate the remarkable OER performances of NCO@M electrodes, the catalytic activity of NCO@M derivative electrodes was quantified by the electrochemical surface areas (ECSA) with CV methods. This CV curves were recorded with different scan rates in the 20-100 mV s-1 range (Figure S 10a, 10c, 10e and 10g). The ECSA of the NCO@CoS are 395 cm2, which is 16


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1.02, 2.0, and 3.29 times higher than of NCO@CoSe, NCO@Co3O4 and NCO@C, based on the calculated values of Cdl (Figure S 10b, 10d, 10f and 10h). Notably, the highest ECSA of the NCO@CoS leads to the highest OER catalytic activity compared to the other derivatives. Further, the above conclusions also suggest that NCO@CoS provides more catalytic active sites and more effective transfer of reactants. The value of the specific surface area is also studied by N2-adsorption/desorption, which is classified as the IV-type isotherms, suggesting the existence of mesopores in our catalysts, as shown in Figure S11. The NCO@CoS has the maximum surface area (232.8 m2 g-1), which is much larger compared to NCO@Co3O4 (137.2 m2 g-1), NCO@C (115.6 m2 g-1) and NCO@CoSe (210.5 m2 g-1), which is favorable for both HER and OER process. Moreover, the 3D hierarchical structure grown in situ on the NFF substrate shows superior OER catalytic performance due to its effective electron transfer and fast ion diffusion compared to other electrocatalytic materials42. The operational stability as another critical parameter to assess practical applications of the catalysts in the OER process is shown in Figure 4e. It displays the stable curves of the NCO@CoS in OER process with 24 h continuous operation. Additionally, a long-term durability of NCO@CoS electrode was also obtained (Figure 4f), and the overpotential had almost no change to continue reaching 10 mA cm-2 after 104 cycles. This conclusion manifests that the NCO@CoS with excellent catalytic towards OER. The unexceptionable OER performances of NCO@CoS are caused by the larger ECSA and porous heterogeneous structure, offering more exposed active sites to enhance the OER.

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Figure 4. (a) LSV curves for OER, (b) Overpotentials diagram, (c) Tafel slopes from different products, (d) LSV curves of NCO@CoS measured under different scan rates, (e) Chronopotentiometric curve of NCO@CoS, (f) LSV curves of NCO@CoS before and after104 CV cycles. Furthermore, the HER feature of NCO@M derivatives is also explored in the same device. 18


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The LSV curves of NCO@M derivatives for HER are displayed in Figure 5a. Surprisingly, the NCO@CoS exists the smallest onset potential, and the corresponding overpotentials at 10 and 50 mA cm-2 are presented (Figure 5b). It can be seen clearly the NCO@CoS with the smallest overpotential of 100 mV, which is 10, 30 and 60 mV lower than those for NCO@CoSe, NCO@Co3O4 and NCO@C catalysts at 10 mA cm-2, respectively. Besides, the NCO@CoS shows excellent HER activity compared to its counterparts (Figure S 8b and Figure S 9b) and several previously reported catalysts (Table S4). As can be observed from the LSV curves of OER and HER, the S-NiCo LHSs/NFF shows better catalytic activity than NiCo LHSs/NFF. It can be found that the electrocatalytic properties of the NCO@CoS have been greatly improved through sulfur doping compared to the NCN@ZIF-67 precursor (Figure 4a and Figure 5a). In other words, the introduction of sulfur regulates the electronic structure of the compound, and the produced metal sulfide promote the OER and HER process. Therefore, the active sites in these catalysts are mainly metal-S, metal-Se, metal-O and metal-C sites. Furthermore, this trend of the overpotential is NCO@CoS < NCO@CoSe < NCO@Co3O4 < NCO@C at 50 mA cm-2, meaning that the NCO@CoS also has excellent HER performance at the same time. Therefore, these NCO@M derivatives possess higher OER and HER characteristic than the NiCo-LHSs/NFF and ZIF-67, manifesting the strong interfacial coupling between the NiCo-LHSs and ZIF-67, enhancing the electrochemical property of the composites. Figure 5c displays the Tafel slopes of NCO@M derivatives, which confirms that the Tafel slope value of NCO@CoS (68 mV dec-1) is smaller than NCO@CoSe (75 mV dec-1), NCO@Co3O4 (92 mV dec-1) and NCO@C (136 mV dec-1). Similar to the above OER measurement results, the NCO@CoS catalyst shows the greatest HER property among the NCO@M derivatives, requiring a 19



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low overpotentials of only 118 mV at 10 mA cm-2 (Table S1). Additionally, Figure 5d exhibits the electrochemical impedance spectra (EIS) of NCO@M derivatives, which is conducted to explore their catalytic activities. As can be seen, the NCO@Co3O4, NCO@C, NCO@CoS and NCO@CoSe have low charge transfer resistances, and the corresponding values are 7.5, 6.0, 4.6 and 5.1 Ω, respectively. It demonstrates the NCO@CoS can promote kinetics process. This operational stability of the NCO@CoS for HER catalyzed is presented in Figure 5e, suggesting the stability of the material with 24 h continuous operation. Furthermore, the LSV curves of NCO@CoS display negligible activity degradation even after the 104 cycles compared with the first CV scanning cycle (Figure 5f), suggesting the excellent HER performance with long-term stability. The superior activity as well as long-term durability of NCO@CoS makes it a promising alternative for high efficiency electrocatalytic material for HER reaction.

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Figure 5. (a) LSV curves for HER, (b) Overpotentials diagram, (c) Tafel slopes of different products (a), (d) EIS spectra of NCO@Co3O4, NCO@C, NCO@CoS and NCO@CoSe, respectively, (e) Chronopotentiometric curve of NCO@CoS, (f) LSV curves of NCO@CoS before and after 104 CV cycles.

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Overall water splitting. To evaluate the practical application for overall water splitting, we also measured HER and OER catalytic performance of our best catalyst (NCO@CoS) in a two-electrode cell (Figure 6a), displayed in Figure 6b. Figure 6c reveals that the cell voltage of NCO@CoS electrode for overall water splitting being is 1.54 V between two electrodes to reach the 10 mA cm-2, which indicates superior bifunctional water splitting feature than the reported catalysts42,43. In contrast, when commercial NFF substrates served as anode and cathode for overall water splitting, a higher voltage of 1.85 V between two electrodes was needed at 10 mA cm-2. In other words, the commercial NFF electrode is inferior to our NCO@CoS. As shown in Figure 6d, the voltage of NCO@CoS approaches the voltage difference (1.52 V) between OER and HER at 10 mA cm-2. The electrocatalyst with high-efficiency and activity makes it a promising candidate for water splitting in practical applications. According to the systematic experimental analysis and the performance tests in water splitting, a mechanism is raised to illustrate the highest catalytic activity of NCO@CoS. It can be ascribed to considerable active sites and the interface coupling between the derivatives of ZIF-67 and LHSs. This unique structural advantage leads to the close contact between water molecules and catalysts through the sufficiently wide porous channels. Particularly, the sulfur particle doping in NCO@CoS helps tune the electronic structure and improve the uniformly of distribution of active sites, which are beneficial for water splitting. Additionally, sulfur dopants not only lead to new multi-atom interactions, they also accelerate electron transfer between different sites, thereby reducing the energy barrier of the reaction intermediates. Moreover, heteroatom doping can tune the electronic structure of the metal active sites, tune its binding energy to the reaction species, and, consequently, increase the reactivity.44 22


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Figure 6. (a) The optical image of NCO@CoS for overall water-splitting, and (b) the corresponding illustration. (c) LSV curve of NCO@CoS and the commercial NNF in a two-electrode configuration, (d) LSV curves of NCO@CoS for HER and OER.

Conclusion We have presented a new and efficient strategy to fuse NiCo-LHSs and ZIF-67 by simple oxidation, carbonization, sulphidation and selenation. A series of 3D hierarchically structured NCO@M (M = Co3O4, C, CoS and CoSe) composites featuring strong interface coupling showed promising performance as bi-functional electrocatalysts for HER and OER reactions. Among these composite materials, the NCO@CoS revealed the best electrocatalytic behavior for overall water 23



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splitting. The NCO@CoS was utilized as the cathode and anode in alkaline condition and achieved a low voltage of 1.54 V at 10 mA cm-2, showing excellent electrocatalytic stability. These results herald superior performance in overall water splitting than Co-based materials. The excellent performance may be ascribed to the exotic hierarchical structure with strong electronic coupling between NiCo-LHSs and MOFs-derivatives phase, as well as rational distribution of electronic states as well as effective accessibility of dense catalytically active sites, high specific surface area and the high conductivity of the support material. The outcomes of this work are promising for the development of next-generation catalytic materials for electrocatalysis and other renewable energy applications.

Supporting Information SEM images, XRD diffraction pattern, TEM images, TEM-EDX spectrum, XPS spectra, LSV curves and N2 adsorption/desorption isotherms of the intermediates and as-obtained products. Comparison table for catalytic activity in this work and the reported catalysts. Table of the ECSA value of the as-prepared samples.

Acknowledgements This work was supported by the financial supports of the National Natural Science Foundation of China (Grant No. 21878047, 21676056 and 51673040), Graduate student scientific research innovation program of Jiangsu Province (KYCX18_0132), Scientific Research Foundation of Graduate School of Southeast University (YBPY1880), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (JNHB-006), Qing Lan Project of Jiangsu Province (1107040167) and A Project 24


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Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (1107047002). K.O. thanks the Australian Research Council for partial support.

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The NCO@CoS catalyst shows optimal overall water splitting performance due to sulfur dopants, leading to multi-atomic synergy, accelerated electron transfer and reduced energy barriers of the reaction intermediates to enhance OER and HER activity. This work provides new prospects for electrolysis of water.

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Interface Coupling of Ni-Co Layered Double Hydroxide Nanowires

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