Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis

international journal of hydrogen energy xxx (xxxx) xxx

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Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis Sanjiang Pan a,b,c,d, Xiangbin Kong a,b,c,d, Qixing Zhang a,b,c,d, Qiaojing Xu a,b,c,d, Manjing Wang a,b,c,d, Changchun Wei a,b,c,d, Ying Zhao a,b,c,d, Xiaodan Zhang a,b,c,d,* a

Institute of Photoelectronic Thin Film Devices and Technology of Nankai University, Tianjin, 300350, PR China Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Tianjin, 300350, PR China c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, China d Renewable Energy Conversion and Storage Center of Nankai University, Tianjin, 300072, China b

highlights

graphical abstract


Changing the electronegativity of substitutes is employed to modify the performance of OER catalysts.
Optimal ratio of low electronegative group (-NH2) shows OER performance with overpotential of 197 mV at 10 mA cm2.
Amorphous

structure

induces

synergistic effects contribute more active sites.

article info

abstract

Article history:

Oxygen evolution reaction (OER) is a limiting factor for water splitting due to multi-step

Received 23 October 2019

transfer of four electrons and sluggish reaction kinetics. At present, metal-organic

Received in revised form

frameworks (MOFs) as OER catalyst still has some shortcomings, such as poor conductiv-

23 January 2020

ity and high-temperature synthesis, which seriously limits its application in electro-

Accepted 29 January 2020

catalysis. Here, we synthesize amorphous metal-organic frameworks (aMOFs) by a simple

Available online xxx

chemical method at room temperature. The X-ray photoelectron spectroscopy (XPS) results indicated that the different electronegativity of substituents could dramatically influence

Keywords:

the metal center in aMOFs. These aMOFs exhibit different OER performances in basic

Oxygen evolution reaction

aqueous solutions, in which the A0.25BeNiFe showed the lowest overpotential of 237 mV at

Electronegativity

10 mA cm2 current density and Tafel slope of 60 mV dec1. These heuristic results

* Corresponding author. Institute of Photoelectronic Thin Film Devices and Technology of Nankai University, Tianjin, 300350, PR China. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.ijhydene.2020.01.229 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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Amorphous metal-organic

demonstrate that the electronegativity of substituents has a profound influence on the

frameworks

metal active sites of the catalysts. This will provide a new strategy for accelerating the

Room temperature

application of MOFs in electrocatalysis and energy conversion. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Electrochemical water splitting is a feasible and environmentally friendly energy usage [1e6]. Photo-voltaic driven electrolysis also can convert solar energy to stored fuel is an ideal way to prevent pollution [7e9]. For both of them, the role of electrocatalyst is very crucial. Due to the sluggish reaction kinetics of oxygen evolution reaction (OER), which is involved with multi-step transfer of four electrons [10e12], restricting the development of the water splitting [13e15]. Classical IrOx and RuO2 catalysts are typical OER catalysts with low overpotential and good stability [16e18]. However, their scarcity and high cost limit their application in catalysis. Transitionmetal oxides and hydroxides/(oxy)hydroxides are the ideal low-cost candidate for alkaline OER catalyst. Nevertheless, their high charge-transfer resistance, poor conductivity and low exposure of surface active sites that severely limit their OER performance [19]. Researchers put low-cost metal center nanoclusters supported on carbon or nitrogenated carbon is a very effective strategy [20e22]. But the substrates usually need high temperature to synthesis. It is crucial to explore new and valid strategy to improve the performance of the low-cost catalysts. When researchers design the OER electrocatalyst, the binding energy of metal to *OH will be the primary consideration. The binding energy between transition metal (Ni, Fe et.) and *OH is relatively high and plays an important role in OER [19]. Therefore, bimetallic FeNi-based catalysts have aroused widespread attention due to their excellent catalytic activity. Nowadays, owing to their unique porous structure and earth-abundant feature, transition metal-based metalorganic frameworks (MOFs) have attracted extensive attention in the fields of gas storage [23e25], separation [26,27] and biosensors [28,29]. In addition, the MOFs is also widely applied in catalysis [30,31]. However, there exists deficiency in conductivity and mass permeability due to organic ligands. Modification of MOF activity by specific methods can improve its catalytic performance. The commonly used improvement strategies of MOFs are shown as below: 1) Changing the dimensions (1D, 2D, 3D) [32,33]; 2) Replacing different active center metals (Ni, Co, Fe and so on) [34e36]; 3) High temperature carbonization treatment [37e39]. However, for these strategies, it is difficult to control and implement. Finding an easier and more effective strategy to improve OER performance is extremely essential and has the practical significance. Modulating electronegativity of substituents on organic ligands may be a superior way to enhance the conductivity of the MOFs. In fact, on most occasions, the electronegativity of atoms could deeply influence the metal active centers of the

materials, which has been confirmed many times. For example, NiOOH [40] and Ni(OH)2/Co(OH)2 [41] supported on Au substrates (relatively strong electronegativity) demonstrate very strong OER activity. Heteroatom doping is another way to modulate the electronegativity of materials. Su’s group confirmed that the high electronegativity of N atom, which led to partial electron migration from Co to pyridinic and/or pyrrolic N [42]. Luo’s group found that the more electronegative P atoms in the Co2P lattice can grab electrons from metal atoms and then significantly enhance its HER performance [43]. However, the MOFs containing organic ligands are completely different from other materials. It seems invalid that direct doping heteroatom in the MOFs. However, some studies have shown that the hydrogen on benzene ring can be replaced by different electronegative groups [44e46]. Therefore, we synthesized a series of aMOFs by changing the electronegativity of substituents. Different electronegativity has significant effect on the structure and performance of aMOFs. The 2-aminoterephthalic acid (B) as a strong electron-donating group (eNH2) and 2-fluoroterephthalic acid (C) as a strong electron-withdrawing group (eF) were employed to modify the structure of MOFs. Through changing the ratio of A, B and C, a series of NiFe-based aMOFs were obtained. From XPS and EIS data, it can be seen that different electronegativity demonstrated deeply influence on the electron transport ability and metal active centers. The excellent electrocatalyst can be obtained through rational modulating electronegativity of substituents in aMOFs. Heterogeneous bimetallic aMOFs (A0.25BeNiFe) exhibits unexpected low overpotential of 237 mV at 10 mA cm2 in 1.0 M KOH electrolyte and Tafel slope of 60 mV dec1. When A0.25BeNiFe is loaded on Ni foam, the lower overpotential 197 mV at 10 mA cm2 in 1.0 M KOH electrolyte will obtain. Besides, these aMOFs can be synthesized at low temperature, which is beneficial for the application in catalysis.

Experimental section Materials NiCl2$6H2O (99.99%, AR, grade) was purchased from SigmaAladrich, FeCl3$6H2O (99.5% AR, grade) from Heowns, Benzenedicarboxylic acid (A) from Tokyo Chemical Industry, 2fluoroterephthalic acid (C) from Ark Pharm, 2aminoterephthalic acid (B), Nafion solution (5 wt%), KOH (99.98%) from Alfa Aesar. Triethylamine (TEA) and N, Ndimethylformamide (DMF) were bought from Aladdin Reagent. All chemicals were used directly without further purification.

Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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Synthesis of A0.25BeNiFe First, DMF (32 ml), ethanol (2 ml) and water (2 ml) were mixed in a 50 ml polytetrafluoroethylene (PE) tube. Then, 0.15 mmol A and 0.60 mmol B were dissolved into above mixed solution, and then treated with ultrasonic. Subsequently, 0.375 mmol NiCl2$6H2O and 0.375 mmol FeCl3$6H2O were added. After dissolving Ni2þ and Fe3þ salts, 0.8 ml TEA was quickly injected into the solution. Then, the solution was stirred for 5 min to obtain a uniform colloidal suspension. Subsequently, the colloidal solution was continuously ultrasonicated for 4 h (40 kHz) under closed conditions. Finally, the products were obtained by centrifugation, washed 3 times with ethanol, and freeze-dried. Other bimetallic aMOFs, AxB-NiFe, BeNiFe and A-NiFe were prepared by the similar procedures with different ligand ratios at the beginning.

Synthesis of A3CeNiFe The preparation process of A3CeNiFe is slightly different from the above methods. DMF (32 ml), ethanol (2 ml) and water (2 ml) were mixed in a 50 ml polytetrafluoroethylene (PE) tube. Next, 0.6 mmol A, 0.15 mmol C and 0.375 mmol NiCl2$6H2O were added into the mixed solution under ultrasonication. After the clarified solutions were obtained, 0.375 mmol FeCl3$6H2O were added. After that, 0.8 ml TEA was quickly injected into the solution. Then, the solution was stirred for 5 min to obtain a uniform colloidal suspension. Subsequently, the colloidal solution was continuously ultrasonicated for 4 h (40 kHz) under closed conditions. Finally, the products were obtained by centrifugation, washed 3 times with ethanol, and freeze-dried. Other bimetallic AyC-NiFe and CeNiFe were prepared according to the similar procedures with different ligand ratios at the beginning.

Synthesis of NiFeeOH To fabricate the NiFeeOH samples, a simple solvothermal method was used. The mole ratios of Ni/Fe (1:1) solution was obtained by dissolving Ni(NO3).26H2O and Fe(NO3).39H2O in 17.5 ml anhydrous ethanol to form a clear solution, and the sum of moles of metal cations were kept 1 mmol. Urea (1.2 mmol) were after added into the solution. Thereafter, the solution were transferred into a Teflon-lined stainless-steel autoclave and heated at 120  C for 12 h. Finally, the products were obtained by centrifugation, washed 3 times with ethanol, and freeze-dried.

Synthesis of A0.25BeNiFe crystal First, DMF (32 ml), ethanol (2 ml) and water (2 ml) were mixed in a 50 ml polytetrafluoroethylene (PE) tube. Then, 0.15 mmol A and 0.60 mmol B were dissolved into above mixed solution, and then treated with ultrasonic. Subsequently, 0.375 mmol NiCl2$6H2O and 0.375 mmol FeCl3$6H2O were added. After dissolving Ni2þ and Fe3þ salts, the mixed solution was transferred into a 45 ml Teflon vessel at 140  C for 48 h under airtight conditions. Finally, the products were

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obtained by centrifugation, washed 3 times with ethanol, and freeze-dried.

Characterization Powder X-ray diffraction (PXRD) data were conducted on a PAN alytical X’Pert Pro Diffractometer operating at a voltage of 40 kV and a current of 40 mA with Cu Ka radiation (l ¼ 1.5418 A) from 10 to 80 (2q). Field emission scanning electron microscopy (FESEM) observations were gathered on a Hitachi S4800 microscope performing at an accelerating voltage of 20.0 kV. Transmission electron microscopy (TEM) images were collected with a JEOLJEM-2010 instrument operating at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were acquired with ESCALab220i-XL electron spectrometer. FEI SEM operated at 1.5 kV was used to characterize the surfaces of the samples. Raman spectroscopy (RTS-HiR-AM) at 532 nm excitation wavelength were used. Fourier transform infrared spectroscopy spectra (FTIR) were collected on a FT-IR spectrometer (Bruker Tensor 27) with a range of 400e4000 cm1 at room temperature.

Electrochemical measurements Electrochemical measurements were performed using a three-electrode system connected to an electrochemical workstation (Autolab PGSTAT302 N) with a built-in EIS analyzer. The working electrode was a GCE (diameter: 3 mm, area: 0.072 cm2) from CH Instruments. Ag/AgCl (with saturated KCl as the filling solution) and graphite electrode were used as the reference and counter electrodes, respectively. Typically, 4 mg of catalyst powder was dispersed in a mixture solution of water (1 mL) and ethanol (0.25 mL), and then a Nafion solution (80 mL, 5 wt% in water) was added. At last, a homogeneous ink was prepared by immersing the suspension in an ultrasonic bath for 30 min. The working electrode was prepared by immersing the GCE into the catalyst ink (catalyst loading 0.21 mg cm2). To load the catalyst on Ni foam (thickness: 1.6 mm, Sigma), 20 mg of catalyst was dispersed in a mixture of water (2 ml) and ethanol (2 ml), and 100 mL of Nafion solution was added. Then, the suspension was sonicated for 30 min to produce a homogeneous ink. Finally, 20 mL catalyst ink was drop-casted on Ni foam with a fixed area of 0.5  0.5 cm2 that was coated with water-resistant silicone glue. The polarization curves were measured at a scan rate of 5 mV s1. EIS measurements were conducted in a static solution at 1.50 V (vs. RHE) on a GCE. The amplitude of the sinusoidal wave was 10 mV, and the frequency scan range was from 100 kHz to 1 Hz. The Cdl of electrocatalysts was used to evaluate the ECSA, which was measured by using cyclic voltammograms in a no Faradaic reaction potential window (1.12e1.22 V vs. RHE) with the scan rates of 10, 20, 40, 80, and 100 mV s1, respectively. The plot of the current density differences (△J ¼ (J1 e J2)/2 at 1.17 V vs. RHE) against the different scan rates has a linear relationship and its slope is Cdl. Unless otherwise stated, all experiments were performed at ambient temperature (25 ± 2  C), and the electrode potential was converted to the RHE scale using equation (1): EðRHEÞ ¼ EðAg = AgClÞ þ 0:197V þ 0:059  PH

(1)

Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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The turnover frequency (TOF) was evaluated by the following standard equation (2) [53]: TOF ¼ ðJ  AÞ = 4  F  m

(2) 2

Here, J is the current density (A cm ) at an overpotential of 0.3 V. A and m represent the area of the electrode and the number of moles of the active materials that were deposited onto the electrode, respectively. F is the Faraday constant (96485 C mol1).

Results and discussion As an example, the fabrication procedure of A0.25BeNiFe was selected as shown in Fig. 1a. The synthetic route of these aMOFs is explained as follows. The environment of N,Ndimethylformamide (DMF), ethanol and water solutions is suitable for these reactions. NiCl2$ 6H2O and FeCl3$ 6H2O were added into the mixture organic solution of A and B. The

Fig. 1 e (a) Schematic illustration of the fabrication procedure of A0.25BeNiFe. Morphology and physical characterization of A0.25BeNiFe: (b) HAADF image, (c) TEM image, (d) HRTEM image, (e) EDS mapping images and SAED image of A0.25BeNiFe. Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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triethylamine (TEA) as an alkaline organic initiator could make eCOOH to eCOOe, and then improve the reaction rate. When TEA was added into the final solution, the suspension was obtained immediately. These aMOFs were synthesized by ultrasonication at room temperature. To gain insight into the difference structures of these electrocatalysts, A0.25BeNiFe (electron-donating modifying), A-NiFe (origin) and A3CeNiFe (electron-withdrawing modifying) were selected. The morphology of A0.25BeNiFe was characterized by transmission electron microscopy (TEM) (Fig. 1b and c). From the High-resolution TEM (HRTEM) image of A0.25BeNiFe, it is clear that there is no crystal lattice in this material (Fig. 1d). Selected area electron diffraction (SAED) image of A0.25BeNiFe with special diffraction halo is the typical feature of amorphous materials (Fig. 1e). X-ray

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diffraction (XRD, Fig. S1) analysis also indicated that the prepared catalyst possessed weak crystalline phase and A0.25BeNiFe Crystal possessed high crystalline phase. Energy dispersive spectroscopy mapping (EDAS mapping) indicated that elements are uniformly distributed on the whole A0.25BeNiFe surface (Fig. 1e). Energy dispersive spectroscopy (EDS) shows that A0.25BeNiFe is composed of C, O, N, Ni, and Fe without any other impurity elements (Fig. S2). Detailed EDS analysis shows that the element ratio of Fe and Ni in A0.25BeNiFe is close to 1:1. In addition, the morphology and elemental analysis of A-NiFe and A3CeNiFe were also tested. The morphologies of A-NiFe and A3CeNiFe are similar to those of A0.25BeNiFe and are displayed by TEM images (Fig. S3b and Fig. S4b). Interestingly, from HRTEM and SAED images, we can clearly see that there are some differences in crystal

Fig. 2 e (a) Raman spectra of A3CeNiFe (black), A-NiFe (blue) and A0.25BeNiFe (red). (b) High resolution XPS Ni 2p spectra of A0.25BeNiFe, A-NiFe and A3CeNiFe. (c) High resolution XPS Fe 2p spectra of A0.25BeNiFe, A-NiFe and A3CeNiFe. (d) High resolution XPS O 1s spectra of A0.25BeNiFe. (e) High resolution XPS O 1s spectra of A-NiFe. (f) High-resolution XPS O 1s spectra of A3CeNiFe. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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morphologies of A0.25BeNiFe, A-NiFe and A3CeNiFe (Figs. S3c and d and Fig. S4c and d). That means the electronegativity of ligands can change the structure of the MOFs. Besides, the ratio of iron to nickel of these two elements is the same, approaching 1:1, and there are no other impurity elements (Fig. S3e and Fig. S4e). To further understand the physical properties of these aMOFs samples, Raman, and X-ray photoelectron spectroscopic (XPS) measurements were employed. The corresponding Raman spectra (Fig. 2a) shows that A0.25BeNiFe has the widest peaks and thus demonstrated the weakest crystalline property [47]. At the same time, the relatively broadened peak also implies the inhomogeneous and defective structure of A0.25BeNiFe [48]. When Raman spectra are amplified, the NieO peaks of A0.25BeNiFe (806.3 cm1) are converted to lower wavenumbers than those of A-NiFe (863.0 cm1) (Fig. S5). However, the NieO peak of A3CeNiFe (861.2 cm1) has no significant difference. This result demonstrated that the B ligand could affect NieO distance and change the chemical activity of the central Ni elements. On the contrary, the effect of C ligand is weaker than that [49]. When different electronegativity of substituents are added to aMOFs, the intrinsic activity of metal sites may change. Therefore, X-ray photoelectron spectroscopy (XPS) measurement is further used for analysis. Compared with A-NiFe (856.43 eV) and A3CeNiFe (856.62 eV), the binding energy of Ni 2p 3/2 in A0.25BeNiFe (856.05 eV) shifted to low binding energy (Fig. 2b). Similarly, the same down-shift of Fe 2p 3/2 peak was also obtained (Fig. 2c). We can see that the binding energy of

Fe 2p 3/2 peak in A3CeNiFe (712.63 eV), A-NiFe (712.40 eV) and A0.25BeNiFe (712.02 eV). These shifts of binding energy suggest that the active metal centers in A0.25BeNiFe achieved more electrons, leading to easier charge transfer from oxygen to Ni and Fe. The electron density of metal active sites was increased and then improved the performance of the catalysts. Therefore, the lowest binding energy of A0.25BeNiFe indicates the reason why the lowest value of electrochemical impedance spectroscopy (EIS) of A0.25BeNiFe is shown below. To deeply understand these materials, the O 1s spectrum is analyzed, and the yellow peak of O1s is attributed to the oxygen vacancy (Fig. 2d) [50]. The blue peak can be attributed to hydroxyl and carbon-oxygen species [50]. Due to the oxygen atoms bound to the Ni, Fe metals, the purple peak appears [51]. Compared with the yellow peak of A0.25BeNiFe (12328.78), ANiFe (7613.60) and A3CeNiFe (4741.64), the yellow peak area of A0.25BeNiFe sample is the largest (Fig. 2e and f). This reveals that A0.25BeNiFe sample has more oxygen vacancies than other counterparts. The synergistic effect of charge transfer and oxygen vacancies influences the OH adsorption on the surface of the catalysts under alkali condition, thus enhancing the performance of the materials. The working electrode for OER performance evaluation was prepared by uniformly depositing 0.21 mg cm2 of catalysts onto a glassy-carbon electrode (GCE). The Linear sweep voltammetry was carried out in a traditional three-electrode cell containing 1 M KOH solution at a slow scan rate of 5 mV s1 without iR-corrected. Thermodynamic OER potential (E0H2O/O2 ¼ 1.229 V) was used as a reference [32]. In order to

Fig. 3 e Evaluation of OER electrochemical activity: (a) LSV curves of A0.25BeNiFe, A0.25BeNiFe Crystal, A-NiFe, A3CeNiFe and NiFeeOH in O2-saturated 1 M KOH solution. (without iR compensation) (b) Overpotential required for j ¼ 10 and 40 mA cm¡2. (c) Tafel plots of A0.25BeNiFe, A0.25BeNiFe Crystal, A-NiFe, A3CeNiFe and NiFeeOH. (d) Electrochemical impedance spectra at 1.50 V of A0.25BeNiFe, A0.25BeNiFe Crystal, A-NiFe, A3CeNiFe and NiFeeOH. Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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compare the electrochemical activity, A0.25BeNiFe Crystal, ANiFe, A3CeNiFe and NiFeeOH catalysts were also studied. Compared with other Ni-Based electrocatalysts, there is no obvious oxidation peak from Ni2þ to Ni3þ(Fig. 3a). A3CeNiFe shows the highest 351 mV overpotential at 10 mA cm2. ANiFe and A0.25BeNiFe Crystal possesses middle overpotential of 310 mV and 307 mV at 10 mA cm2. Delightfully, A0.25BeNiFe has a very low overpotential of 237 mV at 10 mA cm2, smaller than that of NiFeeOH (297 mV at 10 mA cm2) (Fig. 3b). We assume that the A0.25BeNiFe due to amorphous metalorganic frameworks possess more active metal center on the face than crystalline MOFs. So that the A0.25BeNiFe has lower overpotential than A0.25BeNiFe Crystal. In addition, these AxBNiFe, BeNiFe, A-NiFe, and AyC-NiFe also have different OER performances, which also exist the similar performance changes (Fig. S6). At the same time, we also use platinum counter electrodes test OER performance. The electronegativity of substituents in aMOFs has a profound impact on the OER activity. Due to the weak electrical conductivity and electron transfer ability of GCE, when A0.25BeNiFe was deposited onto conductive nickel foam, resulting in a further enhanced OER performance, with Eonset at about 1.396 V and an overpotential of 197 mV at 10 mA cm2 (Fig. S7). For A0.25BeNiFe, the overpotential of 315 mV is needed to reach a current density of 40 mA cm2. However, to achieve the same current density, 38, 51, 54 and 113 mV higher overpotentials were required for NiFeeOH, A-NiFe, A0.25BeNiFe Crystal and A3CeNiFe, respectively, (Fig. 3b). This means that the performance of OER can be further optimized by finding a suitable ligand ratio.

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The Tafel plots obtained from Koutecky-Levich curves were used to evaluate the catalytic kinetics in 1 M KOH oxygen saturated solution. It can be obviously seen that the catalytic kinetics of A0.25BeNiFe is enhanced. Compared with other catalysts, such as A-NiFe (74 mV dec1), A3CeNiFe (76 mV dec1) and NiFeeOH (73 mV dec1), A0.25BeNiFe Crystal (72 mV dec1), A0.25BeNiFe shows the lowest Tafel slope of 60 mV dec1 (Fig. 3c). It is noteworthy that no obvious changes of Tafel slope of the series of AxB-NiFe (within 10 mV dec1), but compared to A-NiFe has an obvious changes (10 mV dec1) (Fig. S8). This indicated that B group added into the A-NiFe can also improve the reaction dynamics. And then, in order to further investigate charge transfer kinetics and Nyquist plots, the EIS was tested. The EIS data show that all the prepared materials have the similar series resistances (Rs) (8.5 ± 0.5 U). The great changes of charge-transfer resistances (Rct) are observed after changing B (electron donating group) into the A-NiFe. Compared with NiFeeOH (30.9 U), A0.25BeNiFe Crystal (33.0 U), A-NiFe (39.0 U) and A3CeNiFe (278.9 U), the A0.25BeNiFe exhibits the smallest Rct value (27.4 U) (Fig. 3d). This result shows that suitable electron donor group B is beneficial to charge transfer and increase reaction rate. On the contrary, the C (electron-withdrawing group) has significant negative effect on mass transfer resistance and reaction rate in OER. Indicating that this difference of electron donating and electron-withdrawing has a profound impact on the OER performance of aMOFs. Moreover, the activities of the A0.25B-NiFe were directly compared with various reported Fe-, Co- and Ni-based electrocatalysts at

Fig. 4 e (a) Capacitive currents as a function of the scan rate to give the double-layer capacitance (Cdl) for A0.25BeNiFe, A-NiFe and A3CeNiFe (on Glassy Carbon Electrode). (b) TOF Comparison of A0.25BeNiFe, A-NiFe, A3CeNiFe at 300 mV overpotential. (c) Chronopotentiometry curves (On Nickel Foam) of A0.25BeNiFe for 36 000 s at 10 mA cm¡2 in 1 M KOH. (d) FT-IR spectra of A0.25BeNiFe and A0.25BeNiFe after 10 h OER. Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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alkaline conditions (Table S1, S2). The A0.25B-NiFe show superior OER performance than most of them. The electrochemically active surface area (ECSA) would be a reliable metric for finding the real surface area. We therefore carried out a simple cyclic voltammetry (CV) measurement to determine the doublelayer capacitance (Cdl) (Figs. S9eS11). These Cdl results of A0.25BeNiFe (0.149 mF cm2), A-NiFe (0.071 mF cm2) and A3CeNiFe (0.061 mF cm2) indicated that the active sites in the A0.25BeNiFe were more exposed (Fig. 4a). The polarization curves of different samples were normalized by the ECSA and mass loading of catalysts. The A0.25BeNiFe still possess a better OER catalytic performance than that of ANiFe and A3CeNiFe, indicating that the A0.25BeNiFe is intrinsically more active than A-NiFe and A3CeNiFe individuals (Fig. S12). We further calculated the turnover frequency (TOF) at an overpotential of 300 mV [52]. The A0.25BeNiFe reached a TOF of 0.105 s1. This value is much higher than that of A-NiFe (0.059 s1) and A3CeNiFe (0.044 s1), supporting the high intrinsic activity of the A0.25BeNiFe as well (Fig. 4b). The robustness and durability are also very important performance parameters for OER [53]. The chronopotentiometric measurements at 10 mA cm2 were applied to investigate the long-term operation stability. The A0.25BeNiFe exhibits superior stability at 10 mA cm2, and no significant potential increase was observed after 10 h (Fig. 4c). The morphology of A0.25BeNiFe has no obviously change in the SEM images before and after 10 h electrocatalytic test at 10 mA cm2 (Fig. S13). Besides, the chemical composition of catalyst and chemical environment of metal center were also tested by XPS (Fig. S14). We could see these obvious peaks of Ni 2p, Fe 2p, O 1s, N 1s and C 1s before and after OER. The Fourier transform infrared spectroscopy (FT-IR) also can give us some information about the catalysts (Fig. 4d). The absorption band at 3464 cm1 and 3332 cm1 indicated the double stretching vibrations of NeH. The appearance of absorption band at 1382 cm1 showed the stretching vibrations of CeN. These absorption bands after 10 h OER are still evident. This means that the eNH2 group is firmly connected to the benzene ring.

Conclusions In summary, we have successfully developed aMOFs with excellent OER performance, which was synthesized by simple ultrasonic method at room temperature. Significantly, we found that the electronegativity of the substitutes in the aMOFs affected the OER performance. The electron-donating substitutes with low electronegativity would increase oxygen vacancies and enhance the charge transfer, and then improve the OER activity. After purposefully adjusting the ratio of substitutes, the A0.25BeNiFe catalyst exhibited excellent electrocatalytic activity, even better than the NiFeeOH catalyst. The catalytic stability of the catalyst was confirmed. Here, we highlight the importance of the suitable electronegativity of substitutes in the aMOFs design. In particular, the eNH2 group could be easily added into the ANiFe, which obviously improved the OER performance. We believe that the strategy of changing the electronegativity of the substitutes will open up a new way to create active heterogeneous OER electrocatalysts.

Acknowledgements The authors gratefully acknowledge the supports from International Cooperation Project of the Ministry of Science and Technology (2014DFE60170), the National Natural Science Foundation of China (Grant No. 61674084), the Overseas Expertise Introduction Project for Discipline Innovation of Higher Education of China (Grant No. B16027), Tianjin Science and Technology Project (Grant No. 18ZXJMTG00220), and the Fundamental Research Funds for the Central Universities, Nankai University (Grant Nos. 63191736, ZB19500204).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.229.

references

[1] Luo J, Im JH, Mayer MT, Schreier M, Nazeeruddin MK, Park NG, Tilley SD, Fan HJ, Graetzel M. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earthabundant catalysts. Science 2014;345:1593e6. [2] Jacobsson TJ, Fjallstrom V, Sahlberg M, Edoff M, Edvinsson T. A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solarto-hydrogen efficiency. Energy Environ Sci 2013;6:3676e83. [3] Zhang GW, Liu G, Liu Y, Qu H, Li J. Highly active and stable catalysts of phytic acid-derivative transition metal phosphides for full water splitting. J Am Chem Soc 2016;138:14686e93. [4] Martin-Sabi M, Soriano-Lopez, Winter RS, Chen JJ, VilaNadal L, Long DL, Galan-Mascaros JR, Cronin L. Redox tuning the Weakly-type polyoxometalate archetype for the oxygen evolution reaction. Nat. Catal. 2018;1:208e13. [5] Seitz LC, Dickens CF, Nishi K, Hikita Y, Montoya J, Doyle A, Kirk C, Vojvodic A, Hwang HY, Norskov JK, Jaramillo TF. A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction. Science 2016;353:1011e4. [6] Wang N, Cao Z, Kong X, Liang J, Zhang Q, Zheng L, Wei C, Chen X, Zhao Y, Cavallo L, Zhang B, Zhang X. Activity enhancement via borate incorporation into a NiFe(oxy) hydroxide catalyst for electrocatalytic oxygen evolution. J Mater Chem A 2018;6:16959e64. [7] Yun S, Vlachopoulos N, Qurashi A, Ahmad S, Hagfeldt A. Dye sensitized photoelectrolysis cells. Chem Soc Rev 2019;48:3705e22. [8] Liu X, Wu B, Chen X, Yan L, Guo H, Li K, Xu L, Lin J. A novel hierarchical Bi2MoO6/Mn0. 2Cd0. 8S Heterostructured Nanocomposite for Efficient Visible-light hydrogen production. Int J Hydrogen energy 2019;45:2884e95. [9] Luo L, Wang Y, Huo S, Lv P, Fang J, Yang Y, Fei B. Cu-MOF assisted synthesis of CuS/CdS (H)/CdS (C): enhanced photocatalytic hydrogen production under visible light. Int J Hydrogen energy 2019;44:30965e73. [10] Seh ZW, Kibsgaard J, Dickens CF, Chorkendorff IB, Norskov JK, Jaramillo TF. Combining theory and experiment in electrocatalysis: insights into materials design. Science 2017;355:4998. [11] Roger I, Shipman MA, Symes MD. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 2017;1:0003.

Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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[12] Hu C, Dai L. Multifunctional carbon-based metal-free electrocatalysts for simultaneous oxygen reduction, oxygen evolution, and hydrogen evolution. Adv Mater 2017;29:1604942. [13] Niu S, Jiang WJ, Wei Z, Tang T, Ma J, Hu JS, Wan LJ. Se-doping activates FeOOH for cost-effective and efficient electrochemical water oxidation. J Am Chem Soc 2019;17:7005e13. [14] Guan J, Duan Z, Zhang F, Kelly SD, Si R, Dupuis M, Huang Q, Chen JQ, Tang C, Li C. Water oxidation on a mononuclear manganese heterogeneous catalyst. Nat. Catal. 2018;1:870e7. [15] Cheng W, Zhao X, Su H, Tang F, Che W, Zhang H, Liu Q. Lattice-strained metaleorganic-framework arrays for bifunctional oxygen electrocatalysis. Nat. Energy 2019;4:115e22. [16] Slavcheva E, Radev Bliznakov S, Topalov G, Andreev P, Budevski E. Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis. Electrochim Acta 2007;52:3889e94. [17] Lee Y, Suntivich J, May KJ, Perry EE, Shao-Horn Y. Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions. J Phys Chem Lett 2012;3:399e404. [18] Jiao Y, Zheng Y, Jaroniec M, Qiao SZ. Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem Soc Rev 2015;44:2060e86. [19] Hunter BM, Gray HB, Muller AM. Earth-abundant heterogeneous water oxidation catalysts. Chem Rev 2016;116:14120e36. [20] Munir A, Haq TU, Hussain I, Qurashi A, Ullah U, Iqbal MJ, Hussain I. Ultrasmall Co@Co(OH)2 nanoclusters embedded in N-enriched mesoporous carbon networks as efficient electrocatalysts for water oxidation. ChemSusChem 2019;12:5117e25. [21] Huang H, Zhou S, Yu C, Huang H, Zhao J, Dai L, et al. Rapid and energy-efficient microwave pyrolysis for high-yield production of highly-active bifunctional electrocatalysts for water splitting. Energy Environ Sci 2020. In press. [22] Munir A, Haq TU, Qurashi A, Rehman HU, Ul-Hamid A, Hussain I. Ultrasmall Ni/NiO nanoclusters on thiolfunctionalized and-exfoliated graphene oxide nanosheets for durable oxygen evolution reaction. ACS Appl Energy Mater 2018;2:363e71. [23] Eddaoudi M, Kim J, Rosi N, Vodak D, Wachter J, O’Keeffe M, Yaghi OM. Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science 2002;295:469e72. [24] Chae HK, Siberio-Perez DY, Kim J, Go Y, Eddaoudi M, Matzger AJ, O’Keeffe M, Yaghi OM. A route to high surface area, porosity and inclusion of large molecules in crystals. Nature 2004;427:523e7. [25] Dybtsev DN, Chun H, Yoon SH, Kim D, Kim K. Microporous manganese formate: a simple metal-organic porous material with high framework stability and highly selective gas sorption properties. J Am Chem Soc 2004;126:32e3. [26] Zhao X, Wang Y, Li D-S, Bu X, Feng P. Metal-organic frameworks for separation. Adv Mater 2018;30:1705189. [27] Meng X, Zhong R-L, Song X-Z, Song S-Y, Hao Z-M, Zhu M, Zhao S-N, Zhang H-J. A new type of double-chain based 3D lanthanide(III) metal-organic framework demonstrating proton conduction and tunable emission. Chem Commun 2014;50:6406e8. [28] Li LM, Jiao XL, Chen DR, Lotsch BV, Li C. Facile fabrication of ultrathin metal-organic framework-coated monolayer colloidal crystals for highly efficient vapor sensing. Chem Mater 2015;27:7601e9. [29] Douvali A, Tsipis AC, Eliseeva SV, Petoud S, Papaefstathiou GS, Malliakas CD, Papadas I, Armatas GS,

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

9

Margiolaki I, Kanatzidis MG, Lazarides T, Manos MJ. Turn-on luminescence sensing and real-time detection of traces of water in organic solvents by a flexible metal-organic framework. Angew Chem Int Ed 2015;54:1651e6. Zhang WN, Lu G, Cui CL, Liu YY, Li SZ, Yan WJ, Xing C, Chi YR, Yang YH, Huo FW. A family of metal-organic frameworks exhibiting size-selective catalysis with encapsulated noble-metal nanoparticles. Adv Mater 2014;26:4056e60. Bonnefoy J, Legrand A, Quadrelli EA, Canivet J, Farrusseng D. Enantiopure peptide-functionalized metal-organic frameworks. J Am Chem Soc 2015;137:9409e16. Zhao SL, Wang Y, Dong JC, He CT, Yin HJ, An PF, Zhao K, Zhang XF, Gao C, Zhang LJ, Lv JW, Wang JX, Zhang JQ, Khattak M, Khan NA, Wei X, Zhang J, Liu SQ, Zhao HJ, Tang ZY. Ultrathin metal-organic framework nanosheets for electrocatalytic oxygen evolution. Nat. Energy 2016;1:1e10. Ding Y, Chen Y-P, Zhang X, Chen L, Dong Z, Jiang H-L, Xu H, Zhou H-C. Controlled intercalation and chemical exfoliation of layered metal-organic frameworks using a chemically labile intercalating agent. J Am Chem Soc 2017;139:9136e9. Diaz Morales OA, Ferrus-Suspedra D, Koper MTM. The importance of nickel oxyhydroxide deprotonation on its activity towards electrochemical water oxidation. Chem Sci 2016;7:2639e45. Rui K, Zhao G, Chen Y, Lin Y, Zhou Q, Chen J, Zhu J, Sun W, Huang W, Dou SX. Hybrid 2D dual-metal-organic frameworks for enhanced water oxidation catalysis. Adv Funct Mater 2018;28:1801554. Jia X, Wang M, Liu G, Wang Y, Yang J, Li J. Mixed-metal MOFderived Co-doped Ni3C/Ni NPs embedded in carbon matrix as an efficient electrocatalyst for oxygen evolution reaction. Int J Hydrogen energy 2019;44:24572e9. Zhao R, Lian ZB, Gao S, Yang C, Zhu BJ, Zhao JL, Qu C, Zou RQ, Xu Q. Puffing up energetic metal-organic frameworks to large carbon networks with hierarchical porosity and atomically dispersed metal sites. Angew Chem Int Ed 2019;58:1975e9. Fang XZ, Jiao L, Zhang R, Jiang HL. Porphyrinic metal-organic framework-templated Fe-Ni-P/reduced graphene oxide for efficient electrocatalytic oxygen evolution. ACS Appl Mater Interfaces 2017;9:23852e8. Zhao A, Zhang L, Xu G, Zhang X, Zhang S, Xiao Y. Hollow ZnxCo1-xSe2 microcubes derived from Metal-Organic framework as efficient bifunctional electrocatalysts for hydrogen evolution and oxygen evolution reactions. Int J Hydrogen energy 2020;45:2607e16. Chakthranont P, Kibsgaard J, Gallo A, Park J, Mitani M, Sokaras D, Kroll T, Sinclair R, Mogensen MB, Jaramillo TF. Effects of gold substrates on the intrinsic and extrinsic activity of high-loading nickel-based oxyhydroxide oxygen evolution catalysts. ACS Catal 2017;7:5399e409. Zhou Y, Zeng HC. Metalehydroxide and gold-nanocluster interfaces: enhancing catalyst activity and stability for oxygen evolution reaction. J Phys Chem C 2016;120:29348e57. Liu ZQ, Cheng H, Li N, Ma TY, Su YZ. ZnCo2O4 quantum dots anchored on nitrogen-doped carbon nanotubes as reversible oxygen reduction/evolution electrocatalysts. Adv Mater 2016;28:3777e84. Zhuang MH, Ou XW, Dou YB, Zhang LL, Zhang QC, Wu RZ, Ding Y, Shao MH, Luo ZT. Polymer-Embedded fabrication of Co2P nanoparticles encapsulated in N,P-doped graphene for hydrogen generation. Nano Lett 2016;16:4691e8. Laali KK, Borodkin GI. First application of ionic liquids in electrophilic fluorination of arenes; Selectfluor™(F-TEDA-BF 4) for “green” fluorination. J Chem Soc Perkin Trans 2002;2:953e7. Lee Y-R, Chung Y-M, Ann W-S. A new site-isolated acidebase bifunctional metaleorganic framework for one-pot tandem reaction. RSC Adv 2014;4:23064e7.

Please cite this article as: Pan S et al., Rational modulating electronegativity of substituents in amorphous metal-organic frameworks for water oxidation catalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.229

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international journal of hydrogen energy xxx (xxxx) xxx

[46] Sonawane SH, Anniyappan M, Athar J, Banerjee S, Sikder AK. Synthesis of bis (propargyl) aromatic esters and ethers: a potential replacement for isocyanate based curators. RSC Adv 2016;6:8495e502. [47] Zhang W, Wu YZ, Qi J, Chen MX, Cao RA. Thin NiFe hydroxide film formed by stepwise electrodeposition strategy with significantly improved catalytic water oxidation efficiency. Adv Energy Mater 2017;7:1602547. [48] Michel D, Jorba MPY, Collongues R. Study by Raman spectroscopy of order-disorder phenomena occurring in some binary oxides with fluorite-related structures. J Raman Spectrosc 2005;5:163e80. [49] Xu L, Jiang Q, Xiao Z, Li X, Huo J, Wang S, Dai L. Plasmaengraved Co3O4 nanosheets with oxygen vacancies and high surface area for the oxygen evolution reaction. Angew Chem Int Ed 2016;55:5277e81. [50] Chang JK, Wu CM, Sun IW. Nano-architectured Co(OH)2 electrodes constructed using an easily-manipulated

electrochemical protocol for high-performance energy storage applications. J Mater Chem 2010;20:3729e35. [51] Hai G, Jia X, Zhang K, Liu X, Wu Z, Wang G. Highperformance oxygen evolution catalyst using twodimensional ultrathin metal-organic frameworks nanosheets. Nanomater Energy 2018;44:345e52. [52] Feng JX, Xu H, Dong YT, Ye SH, Tong YX, Li GR. FeOOH/Co/ FeOOH hybrid nanotube Arrays as high-performance electrocatalysts for the oxygen evolution reaction. Angew Chem Int Ed 2016;55:3694e8. [53] Zhang B, Zheng X, Voznyy O, Comin R, Bajdich M, GarciaMelchor M, Han L, Xu J, Liu M, Zheng L, de Arquer FPG, Dinh CT, Fan F, Yuan M, Yassitepe E, Chen N, Regier T, Liu P, Li Y, De Luna P, Janmohamed A, Xin HL, Yang H, Vojvodic A, Sargent EH. Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016;352:333e7.

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