A Hybrid Electrocatalyst with Coordinatively Unsaturated MOF Shell

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A Hybrid Electrocatalyst with Coordinatively Unsaturated MOF Shell and Hollow Ni3S2/NiS Core for OER Application Jingjing Wang, and Hua Chun Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04479 • Publication Date (Web): 04 Jun 2019 Downloaded from http://pubs.acs.org on June 4, 2019

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ACS Applied Materials & Interfaces

A Hybrid Electrocatalyst with Coordinatively Unsaturated MOF Shell and Hollow Ni3S2/NiS Core for OER Application Jingjing Wang and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 *E-mail: [email protected] KEYWORDS: nickel sulfide, metal organic framework, hybrid electrocatalyst, in situ, oxygen evolution reaction ABSTRACT: Metal-organic frameworks (MOFs) have emerged as a promising class of materials. However, their insulating nature has limited their application as electrocatalysts. Herein, we report a heterogeneous nanostructure of Ni-based MOF modified Ni3S2/NiS hollow nanoparticle. The Ni3S2/NiS hollow core is prepared by a sulfuration process from colloidal nickel nanoparticle using dodecanethiol, followed by a low temperature heat treatment in air to remove the adsorbed organic ligands. The thin shell of Ni-based MOF (Ni-BDC) is synthesized using an in situ method, in which the nickel sulfides supply the metal source and the additional terephthalic acid serves as the linker. Serving as an OER catalyst, this hybrid nanocomposite shows superior electrocatalytic performance with a low overpotential of 298 mV at 10 mA∙cm–2 without carbon addition, and a long-time endurability with no detectable activity deterioration, which can be attributed to the synergistic effect of the advantageous heterogeneous structure, combining the hydrophilicity and coordinatively unsaturation of the Ni-BDC shell and the high conductivity and porosity of the Ni3S2/NiS core, as well as the strongly coupled interface between them.

generally quite poor. To improve their catalytic performance, nickel sulfide-based heterogeneous nanostructures, such as MoS2/Ni3S2,16 NiFe-LDH/Ni3S2,17 and PtNi/NiS18 have been developed. These heterogeneous nanostructures, showing the synergistically promoted kinetics on varied active sites and electron-reconfigured interfaces, are indeed superior to their single-component counterparts for OER. To ensure their OER synergy, rational engineering of hetero-structures is necessary, in which multiphasic interfaces of each component should be fully manifested, and the final products should guarantee fast mass/charge transportations and hydrophilic surface properties. In this regard, heterogeneous nanostructures of metal chalcogenides as well as metal-organic frameworks (MOFs) which contain aquo- and hydroxo-rich nodes could be good candidates to work as OER electrocatalysts.19-21 First, MOFs have abundant intrinsic molecular metal sites and accessible structural pores, which could further increase the active sites. Second, the aquo- and hydroxo-rich nodes of MOFs could improve hydrophilic properties, and also facilitate local proton delivery and/or long-range proton transport.22-23 Moreover, via in situ methods, metal species in metal chalcogenides can be conveniently converted to corresponding MOFs with additional organic linkers, which can ensure a coherent hetero-interface. Nevertheless, such heterogeneous nanostructures have been rarely reported.22, 24-27 Herein, we reported a heterogeneous nanostructure of Ni-based MOF modified Ni3S2/NiS, in which the hollow biphasic Ni3S2/NiS was prepared by a sulfuration process from colloidal nickel nanoparticles using dodecanethiol, followed by a low temperature heat treatment in air to remove

1. Introduction In recent years, substantial research effort is being devoted to the development of efficient electrocatalytic hydrogen evolution technologies, for example, water splitting.1 In this relation, it has been found that the oxidation of H2O to O2 is the efficiency-limiting half reaction for the splitting of water to generate H2. So far, noble metals and their corresponding oxides are demonstrated as the state-of-the-art electrocatalysts for oxygen evolution reaction (OER).2-3 However, realization of large-scale hydrogen production requires the development of alternative low-cost electrocatalysts containing only highly abundant elements. In the last decades, a variety of candidate materials based on first-row transition metal (Co, Mo, Fe and Ni), including their metal oxides,4 hydroxides, phosphides,5 phosphates,6-7 and layered double hydroxides,8-9 have shown promising OER activity and stability in alkaline solutions. In addition, transition metal-based chalcogenides (especially sulfides) have also attracted more and more attention serving as electrocatalysts for oxygen evolution because of their low cost, easy fabrication process and good electronic conductivity.10-12 Take nickel sulfides as an example, which have four relatively stable forms, including heazlewoodite (Ni3S2), millerite (NiS), polydymite (Ni3S4) and pyrite (NiS2), a variety of these sulfide compounds have been prepared and investigated as OER catalysts though methods such as electrodeposition,13 liquid phase,14 or gas phase sulfuration15 from nickel-based precursors. However, it is noted that catalytic activity of single-phase nickel sulfide nanostructures toward the OER process is

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the adsorbed organic ligands. The Ni-based MOF was synthesized using an in situ method, where the nickel sulfide supplies the metal source and the additional terephthalic acid served as the linker. The OER catalytic activity of the heterogeneous catalyst shows a significant enhancement over its single-component counterparts, which can be at-

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tributed to the synergistic effect of the advantageous heterogeneous nanostructure, including the hydrophilia and coordinatively unsaturation of Ni-BDC shell, the high conductivity and porosity of Ni3S2/NiS core, as well as the strongly coupled interface between them.

Scheme 1. Schematic illustration of the synthesis process of Ni-BDC modified Ni3S2/NiS (NiSO-BDC) hollow nanoparticles.

noted as NiSO in this work. The effect of different temperatures from 150°C to 250°C was also investigated, including the phase change and OER activities of the resultant samples. 2.5 Removal of organic ligands of Ni3S2 hollow nanoparticles by heat-treatment in a reduction atmosphere. This method was commonly used to remove organic ligands for colloidal nanoparticles in this work and here it served as a control experiment. According to reported processes,28-29 our dried sample of the Ni3S2 hollow nanoparticles (from Section 2.3) was annealed at 400 °C under 5% H2/Ar atmosphere for 2 h, and then cooled naturally. The obtained product was denoted as NiSH. 2.6 Synthesis of Ni-BDC modified NiSO hollow nanoparticles (denoted as NiSO-BDC). To prepare Ni-BDC modified NiSO hollow nanoparticles, 15 mg of NiSO hollow nanoparticles was dispersed in 18 mL of DMF by sonication for 20 min, then 31 mg of terephthalic acid and 50 mg of PVP were added respectively. Finally, the solution was transferred to a 30 mL Teflon lined stainless steel autoclave for solvothermal reaction at 120°C for 24 h. The final product was washed and dried using the same method as descried in Section 2.3. 2.7 Synthesis of pure Ni-BDC MOF. 45 mg of NiCl2 was dissolved in 18 mL of DMF by sonication for 5 min, then 31 mg of terephthalic acid and 50 mg of PVP were added respectively. Finally, the solution was transferred to a 30 mL Teflon lined stainless steel autoclave for solvothermal reaction at 120°C for 24 h. The final product was washed and dried using the same method as descried in Section 2.3. 2.8 Transformation of Ni-BDC to Ni3(OH)2(BDC)2(H2O)2. By a re-solvothermal treatment, the above Ni-BDC could be converted to a reported MOF, Ni3(OH)2(BDC)2(H2O)2; thus we inferred the Ni-BDC to be a precursor of Ni3(OH)2(BDC)2 (H2O)2. Briefly, in this transformative synthesis, 15 mg of Ni-

2. Experimental Section 2.1 Chemicals. The following chemicals were used as received without any further purification: nickel formate dihydrate (Alfa-Aesar), oleylamine (80-90%, Acros-Organics), 1-dodecanethiol (+98%, Aldrich), toluene (AR, VWR), N,N-dimethylformamide (DMF, 99.9%, VWR), nickel chloride hexahydrate (98%, Merck), terephthalic acid (98%, Aldrich), polyvinylpyrrolidone (PVP, 40000, Aldrich), KOH (85%, Merck), ethanol (analytical reagent grade, VWR), ultrapure water (Merck), perfluorosulfonic acid-PTFE copolymer (Nafion solution, 5% w/w in water, Alfa-Aesar). 2.2 Synthesis of colloidal metallic nickel nanoparticles. 50 mL of 0.10 M nickel-formate oleylamine solution and 50 mL of oleylamine were sealed in a glass bottle with a capacity of 250 mL and then kept inside an oil bath at 190oC for 30 min. After the reaction, black suspension (i.e., aminecapped Ni nanoparticles) was obtained, and it was cooled naturally. 2.3 Synthesis of Ni3S2 hollow nanoparticles. In this process, briefly, 4.0 mL of the above obtained suspension of Ni nanoparticles, 20 mL of toluene, and 1.0 mL of 1-dodecanethiol were mixed in a Teflon-lined stainless steel autoclave with a capacity of 30 mL and then kept inside an electric oven at 180oC for 4 h. After the reaction, it was then cooled naturally. The product was collected by washing and centrifuging with anhydrous ethanol for three times, and finally air-dried at 60oC. 2.4 Removal of organic ligands of Ni3S2 hollow nanoparticles by heat-treatment in air. Typically, 30 mg of the dried sample of Ni3S2 hollow nanoparticles was placed in the middle of a tube-furnace, and heated from room temperature to 180°C for 4 h with a ramping rate of 3°C∙ min−1 in air. After heating, the sample was cooled naturally and is de-

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BDC nanoparticles was dispersed in 18 mL of a cosolvent of DMF, ethanol and deionized water (DMF/ethanol/water = 16:1:1), and then the suspension was transferred to a 30 mL Teflon lined stainless steel autoclave for the second solvothermal treatment at 120°C for 24 h, noting that the NiBDC precursor here was prepared from the first solvothermal reaction (Section 2.7). Finally, the product was collected by centrifuging and washed with anhydrous ethanol for several times, and finally air-dried at 60oC. 2.9 Preparation of electrodes for electrochemical measurements. 3.0 mg of each of the above sample powder (i.e., catalyst) was first dispersed in 0.9 mL of a mixed solution of water and ethanol (2:1, v/v), and then 45 μL of Nafion solution (5 wt%) was added. The catalyst suspension was ultrasonically treated for 30 min in order to obtain a homogeneous ink. To prepare the working electrode, 4.2 μL of the mixture ink was dripped onto a polished glassy carbon electrode (GCE) with a diameter of 3 mm (catalyst loading 0.2 mg∙cm−2), then dried at room temperature for 12 h. 2.10 Electrochemical measurements. The electrochemical measurements were conducted on a CH Instruments model 760E electrochemical workstation using a three electrode system: platinum gauze as a counter electrode, Ag/AgCl electrode with 3.0 M KCl as a reference electrode and 1.0 M KOH (pH = 14) as electrolyte. During the measurements, the working electrode was constantly rotated at 1600 rpm to remove generated O2. Before measuring polarization curves, the prepared catalysts were first cycled for about 40 times by cyclic voltammetry (CV) to obtain a stable CV curve. Linear sweep voltammetry was carried out at 5 mV∙s−1 and corrected with 95% iR-compensation. The measured potentials versus the reference electrode Ag/AgCl were converted to a reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE = EAg/AgCl + 0.059 pH + EoAg/AgCl, where EoAg/AgCl = 0.21 V). The relative electrochemical active surface areas of the electrocatalysts were estimated by determining their electrochemical double-layer capacitance (Cdl), which is linearly proportional to the effective surface area. Accordingly, the values of Cdl (mF∙cm−2) were determined from the CV curves measured in a 0.10 V potential range with no Faradaic process at different scan rates referring to the following equation: Jc = 2Cdl∙ν, where Jc and ν are the capacitive current density (mA∙cm−2) and scanning rate (V∙s−1) respectively. By plotting the average capacitive current density at the open circuit potential against the scan rate, half of the measured slope of the plot can be regarded as the Cdl. The ac impedance spectra were performed at frequencies ranging from 100 kHz to 100 mHz at 0.5 V (vs Ag/AgCl). Rotating ring-disk electrode (RRDE) voltammograms were measured on an RRDE configuration (Pine Research Instrumentation). In order to detect the HO2− formation, the ring potential was held constantly at 1.51 V vs. RHE for oxidizing HO2− intermediate in O2-saturated 0.1 M KOH; to prove the oxidation current is from oxygen evolution, a small disk current and a ring potential of 0.40 V vs. RHE were kept and the ring current was collected. The Faradaic efficiency was calculated according to the equation: Faradaic efficiency = Ir/IdN, where Ir and Id are the measured ring and disk currents, respectively, and N is the collection efficiency of RRDE, set at 0.37 in this work.

2.11 Characterization methods. The morphological features of the samples were characterized by transmission electron microscopy (TEM, JEOL JEM- 2010, 200 kV), highresolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, 200 kV) attached with energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, model 7426) and scanning electron microscopy (SEM, JEOL-6700F). The crystal structures were analyzed by XRD, Bruker D8 Advance system with Cu Kα radiation source. The surface compositions and oxidation states were investigated using Xray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) analysis. All binding energies were referenced to the C 1s peak arising from C–C bonds, which was set at 284.5 eV. Chemical bonding information of the studied samples was also gathered with Fourier transformed infrared spectroscopy (FTIR, Bio-Rad FTS-3500ARX). Thermogravimetric analysis (TGA) study was carried out on a thermobalance (TGA-2050, TA Instruments) with a gas flow (air) rate of 50 mL∙min−1 and a heating rate of 5 °C∙min−1. The contact angles were measured using a contact angle meter (Kyowa Interfacial Science Japan, CA-D). Raman spectroscopy was conducted using a Renishaw Raman microscope with the 633 nm line of an Ar ion laser as the excitation source. Carbon, hydrogen, nitrogen and sulfur (CHNS) analysis of the Ni-BDC sample was conducted using an Elementar Vario Micro Cube instrument to confirm its composition. The products of combustion in the CHNS analysis were carried through the system by He carrier. Adjustments for blank, calibration and weights were applied to the final integrated signal and the results were displayed as weight percentages of carbon, hydrogen and nitrogen. N2 adsorption−desorption experiments were performed at 77.3 K after degassing in ﬂowing N2 overnight at 150°C (Quantachrome Instruments NOVA 4200e surface area and pore size analyzer).

3. Results and Discussion Morphological characterization of the prepared samples is reported in Figure 1. Figure 1a, b and Figure S1a-c display the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of metallic nickel nanoparticles prepared from the reaction of nickel formate with oleylamine (OA), where the oleylamine serves as both a reducing agent and a stabilizer. The size of nickel nanoparticles is in the range of 20–50 nm, averaged at about 35.6 nm. After a simple sulfidation process using dodecanethiol as a sulfur source, the nickel nanoparticles can be converted to hazelwoodite phase (Ni3S2). As shown in Figure 1c, d and Figure S1d-f, the Ni3S2 has a hollow morphology as a result of a well-known nanoscale Kirkendall effect; some of them also has additional mesopores in the wall due to the irregular shapes and the size is slightly increased when compared with its nickel precursor. In addition, the surface of the hollow Ni3S2 particles is encapsulated by ligands which are identified to be mainly thiol molecules according to the FTIR spectrum in Figure S1h. Because these organic ligands can impede catalytic activity by blocking catalyst active sites and alter the electronic structure of the material, post treatment is required to get rid of them. The method we adopt here is a low temperature (180 oC) heat treatment in the air environment. Compared to the



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Comparing surface states of original Ni3S2 and NiSO in Figure 2, we can also deduce that the surface of NiSO is slightly oxidized, based on the evidence of the rise in the amount of lattice O (Figure 2b), valence state of Ni (Figure 2c, the decrease in Ni+ species and the increase in Ni2+ species), and amount of oxidized sulfur (Figure 2d, the component at 168.0 eV). Actually, when the heating temperature is raised to 250oC, the NiSO becomes a mixture of nickel oxide and nickel sulfides (NiO, NiS and Ni3S4), according to our investigation on effects of heat-treatment temperature (150°C to 250°C) on the phase stability and OER activities (Figure S5 and Figure S6). However, if the heat treatment is held at high-temperature (350 to 450oC) in a reduction environment, the phase and surface states of Ni3S2 can be maintained (Figure S7d and Figure S8), but their hollow interiors reduce gradually and diminish eventually (Figure S7a-c), accompanied by an inter-particle aggregation, owing to the secondary recrystallization. It is believed that this solidstate ripping process will smooth out surface fine structures, reduce active sites significantly, and result in their poorer electrochemical performance (Figure S7e).

high-temperature (400oC) heat treatment in a reduction environment which is more commonly reported, our method has a better effect of organic ligands removal and avoid aggregation in the meantime (Figure S2). From the SEM and TEM images (Figure 1e, f) we can see that, after heat treatment in air, the surface of NiSO (formed from Ni3S2) hollow nanoparticles become rough and clear, but they still maintain good dispersity. In addition, it can also be found that more mesopores appear in the walls (Figure 1g). Furthermore, upon this heat treatment, there is a partial phase transformation from the original Ni3S2 phase to NiS phase according to the evolution of XRD patterns (Figure 2a). As shown in the high-resolution TEM images and FFT patterns (Figure 1h, i) of two different regions of a NiSO hollow nanoparticle, the interplanar spacings are 0.254 nm and 0.408 nm, which correspond to d021 of NiS and d101 of Ni3S2 respectively. By quantitative analysis, the weight percentages of NiS and Ni3S2 are calculated to be 23.8% and 76.2% respectively (Figure S3). Due to the structural difference between heazlewoodite Ni3S2 (rhombohedral structure) and millerite NiS (hexagonal structure), abundant boundaries and defects are created which account for the surface roughness and addition of mesopores (Figure S4).


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Figure 1. SEM images and TEM images of (a-b) colloidal Ni nanoparticles, (c-d) organic ligands encapsulated Ni3S2 hollow nanoparticles, (e-g) clear NiSO hollow nanoparticles. Colored histogram inset in (b) shows the corresponding size distribution of pristine nickel nanoparticles. (h, i) high-resolution TEM images and their corresponding FFT patterns of two different regions in one NiSO hollow nanoparticle, and (j-m) NiSO-BDC hollow nanoparticles.



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Figure 2. (a) XRD patterns, and (b-d) XPS spectra of Ni3S2 and NiSO hollow nanoparticles.

Finally, by a solvothermal reaction of NiSO hollow nanoparticles with terephthalic acid, the coordination polymer (Ni-BDC) modified NiSO can be achieved. In this process, the NiSO serves as the metal source, terephthalic acid serves as the organic linker and PVP helps to keep the dispersion of the freestanding product. From the SEM image (Figure 1j) we can see that the size of prepared NiSO-BDC nanoparticles is slightly increased when compared with the pristine NiSO (Figure 1e). TEM images (Figure 1k, l) show that the NiSO-BDC exhibits a core-shell structure where the Ni-BDC forms a wrapping shell with a thickness of only 2–5 nm (Figure 1m) on the external surface of the NiSO core. In order to find out the exact composition of the Ni-BDC coating, we used a similar method by replacing the NiSO with NiCl2 to synthesize pure Ni-BDC. The morphological characterization of this Ni-BDC product is given in Figure S9. As SEM (Figure S9a, b) and TEM (Figure S9c, d) images have shown, the Ni-BDC exhibits nanoflake morphology with irregular product shapes. The chemical mappings of the flakelike product show the elementals (including C, O, and Ni) are uniformly distributed, which indicates a homogenous composition across the product. In Figure S10, the C 1s spectrum and O 1s spectrum of the Ni-BDC sample both reveal the characteristic peaks of C=O, which are located at 288.2 eV and 532.7 eV respectively. The Ni 2p spectrum exhibits the onefold +2 oxidation state of Ni, consistent with other Ni based coordination polymers.30

Based on the air-TGA result of Ni-BDC (Figure S11), a weight loss of 5.7 wt% up to 152 °C due to removal of solvent ethanol molecules adsorbed at the surface. Then continuous weight loss of 24 wt% from room temperature to 320 oC corresponds to the loss of coordinated DMF molecules. The main massive losses of 47 wt% occurring between 350 and 390 oC are attributed to the decomposition of organic ligands. Finally, the residue is identified to be NiO after completely decomposition of linker molecules. Combing the results of TGA and CHNS analysis (Table S1), the stoichiometric molecular formula of Ni-BDC can be deduced to be Ni(BDC)(DMF), which will be further elaborated below. In accordance with above results, we can infer the structure of cluster that function as building units for Ni-BDC. As depicted in Figure 3a, on average, each nickel ion is coordinated by four oxygen atoms from four different carboxylate groups and one solvent molecule (e.g., DMF) in a square-pyramidal geometry.31-32 Solvent molecules on each of the two ions can be removed to give open metal sites, which results in unsaturated metal clusters. The existence of these defects necessitates the hydroxo or aquo to complete the coordination of Ni (Figure S12). It has been well reported that these missing-linker induced metal–OH groups are in fact very reactive.33


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Figure 3. (a) Illustration of the structure and calculated formula of Ni-BDC shell. (b) C 1s spectrum of NiSO-BDC. (c) XRD patterns of NiSO-BDC, pure Ni-BDC and simulated Ni3(OH)2(BDC)2(H2O)2. (d) FTIR spectra and (e) Raman spectra of NiSO-BDC and pure Ni-BDC samples. (f) XRD patterns of pure Ni-BDC sample after re-solvothermal treatment in a cosolvent (DMF: water: ethanol = 16:1:1) and simulated Ni3(OH)2(BDC)2(H2O)2.

Figure 3b-e gives the structural and compositional characterizations of NiSO-BDC prepared through a solvothermal treatment at 120oC for 24 h (Section 2.6). First, the C 1s spectrum in Figure 3b shows the characteristic peak of C=O located at 287.9 eV indicating the successful coordination with BDC at the surface of NiSO nanoparticles. Second, from the XRD patterns of NiSO-BDC and pure Ni-BDC given in Figure 3c, we can see that the diffraction peaks (7.7o, 8.7o and13.5o) of NiSO-BDC are in accordance with that of Ni-

BDC, indicating the same chemical substance of shell coating in the NiSO-BDC as that of Ni-BDC. What is more, due to the extraction of Ni ions by BDC molecules, the Ni content in the NiSO core is reduced, which results in the conversion of Ni3S2 to NiS. As shown in the XRD pattern, the intensities of diffraction peaks of Ni3S2 are reduced while those of NiS are increased. The high-resolution TEM image of NiSO-BDC also indicates the main phase of NiSO turns into NiS (Figure S13). Finally, Figure 3d and e gives the FTIR and Raman spectra



of NiSO-BDC and pure Ni-BDC respectively. As shown in Figure 3d, the characteristic vibrations of –COO– (1552.6, 1599.5 cm–1, asymmetric stretching vibrations of –COO–; 1373.0 cm–1, symmetric stretching vibration of –COO–) and ring-out-of-plane vibration of the 1,4-substituted benzene core of the linkers (746.2 cm–1) are both observed in the NiSO-BDC and pure Ni-BDC samples, indicating the successful coordination between nickel ions and BDC2– ligands. In addition, the vibrational frequencies observed at 1647.6, 1109.9 and 1017.2 cm–1 belong to DMF, confirming the presence of Ni coordinating to DMF molecules, as described in Figure 3a.34 Typical Raman spectra of BDC based MOFs were also obtained for the NiSO-BDC and Ni-BDC samples (Figure 3e), which is identical to those of reported data in literature.35 These results indicate the successful synthesis of the coordination polymer-based coating, specifically having the same composition and structure as the Ni-BDC reference. Actually, through a re-solvothermal treatment in a cosolvent (DMF : water : ethanol = 16:1:1), the Ni-BDC phase in Figure 3a can be converted into a well-studied MOF, Ni3(OH)2(BDC)2(H2O)2 (Figure 3f),20 together with a morphological transformation from its irregular shapes to nanosheets (Figure S14). In this light, we can also regard the Ni-BDC as a precursor of Ni3(OH)2(BDC)2(H2O)2; the characterization of the latter MOF synthesized directly using the above cosolvent is shown in Figure S15. However, if the same cosolvent is used at the beginning to synthesize the NiSO-BDC, the faster coordination speed will lead to the generation of bulk MOF. As displayed in Figure S16, the MOF product has a flake-like morphology with the size above 3 μm, while the original NiSO hollow nanoparticles are decorated on the surface of MOF flakes. Due to losing the merit of NiSO-BDC core-shell configuration, this NiSO/MOF product shows a very low OER activity, similar to that of Ni3(OH)2 (BDC)2(H2O)2 (Figure S16f).

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During the synthesis of the NiSO-BDC, different reaction parameters such as reaction time and temperature have been examined. From the TEM images in Figure 4, we have found that the thickness of Ni-BDC shell is highly dependent on the reaction time. According to Figure 1m, the thickness of Ni-BDC coating is at the range of 2–5 nm after 24 h of reaction, while the thickness is below 2 nm for 12 h of reaction and 5–10 nm for 36 h of reaction. A similar trend has been found with reaction temperatures from 100oC to 140oC (Figure S17); an increase in process temperature also gives rise to the increase of thickness of Ni-BDC coating.

Figure 5. (a) FTIR spectra and (b) XRD patterns of Ni-BDC modified NiSO hollow nanoparticles with reaction time of 12 h, 24 h and 36 h respectively. Refer to Figure 3c for the XRD peak assignment for the Ni3S2 and NiS phases. Color codes are the same in (a) and (b).

Figure 5 gives the FTIR spectra and XRD patterns of NiBDC modified NiSO hollow nanoparticles with different reaction times (12, 24 and 36 h) respectively, from which we can get the information on the compositional evolution and phase change during this process. First, upon the increase of reaction times, the intensity of characteristic peaks of car-

Figure 4. TEM images of Ni-BDC modified NiSO hollow nanoparticles prepared with different reaction times (a, b) 12 h, and (c, d) 36 h.


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ACS Applied Materials & Interfaces Summary of the overpotentials at the current density of 10 mA∙cm−2 and Tafel slopes of prepared catalysts. Color codes are the same in (a) and (b).

boxylate group –COO– shows an increasing trend which corresponds to the thickening of Ni-BDC shell (Figure 5a); refer to Figure 3d for the assignment of other IR peaks. Moreover, from the XRD patterns we can also find that the NiSO core go through a phase transformation during the formation of Ni-BDC shell. As elucidated in Figure 5b, the diffraction peaks of Ni3S2 is gradually weakened and the peaks of NiS is getting more and more stronger from 12 h to 36 h, which is consist with previous finding in Figure 3c.

The electrocatalytic activity of Ni-BDC modified NiSO (namely, NiSO-BDC) prepared by different times and temperatures were first investigated to find out the optimized reaction conditions. These tests were carried out in 1.0 M KOH electrolyte without adding carbon, using a standard three-electrode configuration (Section 2.10). Figure 6a and b show representative iR-corrected LSV polarization curves and Tafel slopes of these samples. As a benchmark comparison, the overpotential (η) at the current density of 10 mA∙cm–2 and Tafel slopes in each case were extracted and listed in Figure 6c. Among them, the NiSO-BDC (120oC, 24 h) sample displays the highest OER electrocatalytic activity with the lowest overpotential of 298 mV and the smallest Tafel slope of 58.6 mV∙dec-1, respectively. Moreover, the onset potentials of the peaks prior to the water oxidation around 1.35 to 1.40 V (Figure 6a), which are assigned to the precatalytic oxidation of Ni2+ to Ni3+ or Ni4+, also indicate the highest electrocatalytic activity of NiSO-BDC. The difference in electrocatalytic activity of these samples are mainly determined by the thickness of the Ni-BDC shell. When the reaction time of 12 h or the reaction temperature of 100oC is adopted, the Ni-BDC shell is too thin to compose a homogenous film and part of NiSO core is exposed (Figure 4a, b and Figure S17a, b). On the contrary, when the reaction time of 36 h or the reaction temperature of 140oC is adopted, the thickness of Ni-BDC shell is among 5–10 nm (Figure 4c, d and Figure S17c, d). This will result in the Ni-BDC to be the exclusive active component during OER process, which have been proved to be less active (Figure S16f) without the contribution from the underneath NiSO core. Next, the electrocatalytic activities of the original NiSO, NiSO-BDC, Ni-BDC samples and the reference sample, RuO2, were compared using the same electrolyte and system (the overpotentials and Tafel slopes of all samples tested in this work are summarized in Table 1). As shown in Figure 7a, the core-shell NiSO-BDC sample exhibits enhanced activity when compared to its singe components, NiSO and Ni-BDC due to a synergistic effect, which is also superior to the reference RuO2. Furthermore, the OER kinetics of catalysts were also estimated by Tafel plots in Figure 7b. The slope of NiSO-BDC that reaches 58.6 mV∙dec−1 is also smaller than those of NiSO (72.4 mV∙dec−1), Ni-BDC (165.3 mV∙dec−1) and RuO2 (99.3 mV∙dec−1), indicating a much better kinetic process. Besides, the RRDE tests of NiSO-BDC sample given in Figure S18 suggest almost no formation of hydrogen peroxide and the faradaic efficiency is as high as 96.6 %, comparable with the previous reported electrocatalysts.36-37 Finally, the durability study of the NiSO and NiSO-BDC was conducted at a constant current density of 10 mA∙cm–2. As shown in Figure 7d, the chronopotentiometry curves of NiSO and NiSO-BDC both show a slight potential drop at the beginning which can be ascribed to the activation of NiSO. Finally, the NiSO-BDC shows a good stability after 15 h with no obvious increase of overpotential. After the stability test, the spent NiSO-BDC electrocatalyst was further characterized with TEM technique. As

Figure 6. (a) LSVs and (b) the relevant Tafel plots of prepared electrocatalysts tested in 1.0 M KOH without carbon added. (c)



shown in Figure 8a, the NiSO-BDC could maintain the original hollow morphology. Additionally, XPS spectra were also obtained to identify the surface oxidation of NiSO-BDC after the stability tests. As reported in Figure 8b, the peak positions of Ni 2p shifted to a higher binding energy when compared with the initial sample, where the Ni+ component (also see Figure 2c) disappeared while the Ni3+ emerged, indicating that the surface Ni ions were oxidized after continuous electrochemical tests. In this agreement, the peak of adsorbed O species at 532.9 eV (O 1s spectrum, Figure 8c) after the stability test becomes dominantly strong, whereas

Page 10 of 15

the peak of the lattice O species at 530.8 eV almost diminishes, which indicates the generation of a significant amount of surface phases of hydrated oxide or oxyhydroxide. In Figure 8d, furthermore, the peaks of S 2p after the OER stability tests become much weaker, and the main peak at 168.5 eV is assigned to the oxidized sulfur species in the form of SO3H or SO42−, confirming the oxidation of catalyst surface during the OER process.38 These results are consistent with the XRD pattern of the used NiSO-BDC sample (Figure S19), showing the components mainly converted to Ni(SO4)0.3(OH)1.4 after OER.

Figure 7. (a) LSVs and (b) the relevant Tafel plots of NiSO-BDC and reference samples tested in 1.0 M KOH without carbon added. (c) Summary of the overpotentials at the current density of 10 mA∙cm−2 and Tafel slopes of prepared catalysts. (d) Chronopotentiometry curves for 15 h at a constant current density of 10 mA∙cm–2 for NiSO and NiSO-BDC samples. Color codes are the same in all the above figure panels.


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Figure 8. (a) TEM image of NiSO-BDC after OER test. (b-d) XPS spectra of NiSO-BDC before and after OER test.



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Figure 9. Cyclic voltammetry curves in a non-faradaic potential region under different scan rates of (a) NiSO hollow nanoparticles and (b) NiSO-BDC. (c) Different current density versus scan rate to assess the Cdl of the ECSA. (d) Electrochemical impedance spectra measured at 0.5 V (vs Ag/AgCl). (e-g) contact angles of water droplets on Ni3S2 hollow nanoparticles, NiSO hollow nanoparticles and Ni-BDC modified NiSO hollow nanoparticles respectively.

In order to elucidate the strongly enhanced OER activities of NiSO-BDC, electrochemical double-layer capacitances (Cdl) were measured on the studied samples using the cyclic voltammetry (CV) method. From this, their electrochemically active surface areas (ECSA) can be quantified, since the ECSA is expected to be linearly proportional to the Cdl of electrocatalyst. Figure 9a and b show the cyclic voltammetry curves of NiSO and NiSO-BDC in a non-Faradaic region under different scan rates (10, 20, 30, 40 and 50 mV∙s−1) respectively. By calculating the slopes from the linear relationship of current density difference Δj against the scan rate (see Section 2.10), the Cdl (half of the slopes) of the NiSO-BDC sample was determined to be 126 mF∙cm−2, much bigger than 70 mF∙cm−2 of the NiSO (Figure 9c). This comparison is well consistent with the BET results (Figure S20) showing a slight increase of the surface area and mesopores

of NiSO-BDC compared to that of NiSO, which clearly affirms higher active surface areas and better exposure of the catalytic active sites of the NiSO-BDC. We ascribe this to the additional working metal sites of the Ni-BDC shell that the original NiSO sample does not have and the synergistic effect between the Ni-BDC shell and its NiSO core, noting that herein the thickness of shell has been optimized (Figure 6). Furthermore, to understand the reaction kinetics of the prepared electrodes, the electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 0.1–100 kHz. Basically, the high-frequency semicircle is mainly associated with charge transfer resistance (Rct) of the redox reaction, and the smaller the diameter of semicircle, the smaller the Rct.39-40 As we can see in Figure 9d, the diameter of semicircle of the NiSO-BDC electrode is much smaller than that of the NiSO electrode.


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By further fitting using the circuit model inset in Figure 9d, the impedance parameters were obtained (listed in Table S2). The charge transfer resistance for NiSO-BDC was calculated to be 5.0 Ω, smaller than that of NiSO at 6.4 Ω, indicating a faster charge transfer and favorable reaction kinetics on the heterostructure. This can be mainly ascribed to the improvement of proton transfer under the condition of a slight decrease of NiSO crystallinity (Figure 5b) after the formation of Ni-BTC shell, although it may be unfavorable for electron transfer. In addition, the surface properties also have an obvious improvement. As shown in Figure 9e-g, due to the heat treatment and Ni-BDC modification, the contact angles of water droplets on the Ni3S2, NiSO and NiSO-BDC samples are 156o, 120o, 56o respectively, indicating that the surface hydrophilicity is improved in the sample series. The improvement of the NiSO sample is attributed to the remove of adsorbed organic ligands inherited from the synthesis of Ni3S2 and the significant improvement of the NiSOBDC can be ascribed to the introduction of hydroxo-rich NiBDC shell. Table 1. Summary of overpotentials and Tafel slopes of all electrocatalysts tested in this work. Catalyst (without adding carbon)

Electrolyte

Ni3S2 NiSH

(mV)@10

presented in this work suggest a promising and efficient approach to explore highly active and durable noble-metalfree electrocatalysts based on the state-of-the-art MOFs (e.g., Ni-BDC herein) and more traditional functional materials (e.g., Ni3S2/NiS) with additional configuration engineering (e.g., forming a hollow core-shell structure) for the OER as well as other renewable energy applications.

ASSOCIATED CONTENT Supporting Information Additional SEM images, TEM images, EDX spectrum, element mapping, XRD patterns, FTIR and XPS spectra and electrocatalytic test of prepared samples. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] ACKNOWLEDGMENT

mAcm−2

Tafel slope (mVdec−1)

1.0 M KOH 1.0 M KOH

452 369

225.4 90.5

NiSO-150 oC

1.0 M KOH

351

96.3

NiSO-210 oC

1.0 M KOH

334

61.2

NiSO-250 oC

1.0 M KOH

344

94.6

NiSO

1.0 M KOH

327

72.4

NiSO-BDC

1.0 M KOH

298

58.6

NiSO-BDC 12 h NiSO-BDC 36 h

1.0 M KOH 1.0 M KOH

307 312

68.7 75.6

NiSO-BDC 100oC

1.0 M KOH

315

68.1

REFERENCES

NiSO-BDC 140oC

1.0 M KOH

320

87.3

RuO2

1.0 M KOH

366

99.3

Ni-BDC

1.0 M KOH

404

165.3

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4. Conclusion In summary, Ni-based MOF modified Ni3S2/NiS (i.e., NiSOBDC) hollow nanoparticles have been prepared for OER application, via an in situ method where the nickel sulfides serve as a metal source and the additional terephthalic acid serves as an organic linker. Due to the synergistic effect of the advantageous heterogeneous structure, the NiSO-BDC sample shows enhanced electrocatalytic performance with a low overpotential of 298 mV at 10 mA∙cm–2 and a small Tafel slope of 58.6 mVdec−1 without carbon additive, as well as a long-time endurability of 15 h with no any detectable activity decay. This electrocatalytic performance is superior to those of its singe components, NiSO and Ni-BDC, even that of the reference sample RuO2. In addition, the observed promoting effect is further found to result from the higher hydrophilicity and coordinatively unsaturation of the Ni-BDC shell, high conductivity and porosity of Ni3S2/NiS core, as well as the strongly coupled interface between them. In a wider sense, we believe that the findings

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J.W. would like to thank National University of Singapore for providing her postgraduate scholarship. The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, and National University of Singapore. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.



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TOC Entry Figure

Ni3S2/NiS

15


A Hybrid Electrocatalyst with Coordinatively Unsaturated MOF Shell

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