Highly Conductive Bimetallic NiâFe Metal Organic Framework as

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Highly Conductive Bimetallic Ni-Fe Metal Organic Framework as Novel Electrocatalyst for Water Oxidation Fuqin Zheng, Dong Xiang, Ping Li, Ziwei Zhang, Cheng Du, Zhihua Zhuang, Xiaokun Li, and Wei Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01131 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019

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ACS Sustainable Chemistry & Engineering

Highly Conductive Bimetallic Ni−Fe Metal Organic Framework as Novel Electrocatalyst for Water Oxidation Fuqin Zheng, †, ‡ Dong Xiang, †, ∆ Ping Li, †,§ Ziwei Zhang, †,§ Cheng Du, †,§ Zhihua Zhuang, †,§ Xiaokun Li † and Wei Chen*,†, § †State

Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, Jilin, China ‡University

∆ Faculty

of Chinese Academy of Sciences, No. 19, Yuquan Road, Beijing, 100049, China

of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, 130024,

China §

School of Applied Chemistry and Engineering, University of Science and Technology of

China, No.96, JinZhai Road, Hefei 230026, China

*Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

In recent years, metal-organic frameworks (MOFs) have been extensively investigated for diverse heterogeneous catalysis due to their diversity of structures and outstanding physical and chemical properties. Currently, most related work focuses on employing MOFs as porous substrate materials to fabricate confined nanoparticle or heteroatom-doped electrocatalysts which have to be annealed at high temperature before application. However, the annealing process would destroy the structure completely and lose the intrinsic active sites in MOFs framework. Herein, a simple solvothermal process is used to synthesize a series of Fe/Ni bimetallic MOFs. The as-prepared MOFs are applied directly as highly efficient oxygen evolution reaction (OER) electrocatalysts with no post-annealing treatment. The bimetallic FeNi-MOFs show higher OER activity than single metal MOFs and commercial precious RuO2 catalyst. With the optimized FeNi-MOF as catalyst, the OER current densities of 50 and 100 mA/cm2 can be achieved at the overpotentials of only 270 and 287 mV, respectively. Meanwhile, a small Tafel slope of 49 mV/dec was obtained. Moreover, this catalyst shows high electrochemical stability in strong basic solution. This work demonstrates that through structural optimization, bi-metallic and multi-metallic MOFs may have promising potentials as advanced catalysts for electrochemical energy conversion.

KEYWORDS: Electrocatalysis, catalyst, water splitting, oxygen evolution reaction, metalorganic framework, iron, electrical conductivity


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INTRODUCTION Rapid depletion of conventional fossil fuels, environmental issues and energy crisis have forced people to exploit new clean and recyclable energy sources, such as metal-air batteries, fuel cells and hydrogen energy.1-4 Hydrogen has been widely accepted as a type of clean energy carrier for the next-generation energy devices and it can be efficiently produced through electrolysis of water.2 Unfortunately, because of the proton and multi-electron involved process, electrochemical evolution of oxygen from water has sluggish kinetics and therefore remains a key challenge in modern energy research.1, 5 Among the developed catalysts, iridium oxide and ruthenium oxide are the two prominent oxygen evolution reaction (OER) electrocatalysts, but the low terrestrial abundance, prohibitive cost, easy oxidation to higher valence and dissolution during OER etc. impede their widespread application.6-7 Consequently, searching for efficient transition-metal

based

OER

electrocatalysts,3,

8

such

as

oxides,9-10

hydroxides,11-12

chalcogenides,13-15 nitrides,16 phosphides17 and carbon-based materials,18-19 has attracted tremendous research interests during the past decade. Metal-organic frameworks (MOFs) have aroused tremendous interests in a myriad of research fields because of the tunable surface chemical groups, highly porous structure, large surface area and abundant intrinsic molecular metal sites.20-22 For electrocatalysis, MOFs have been considered as promising templates for the synthesis of metals, metal compounds and carbon-based porous materials by post-calcination treatment, and the prepared electrocatalysts showed high catalytic performances for OER.23-25 However, in the reported strategies, the intrinsic active sites in MOFs may be lost during the calcination treatments at high temperatures and the ordered structure may be also destroyed completely. Till now, only a few of MOFs have been directly used as electrocatalysts for the high electric resistance, instability in highly acidic


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or alkaline media and small pore sizes (MOFs usually have abundant micropores but with only few or no mesopores or macropores).26-27 Thus, to realize the direct application of MOFs as electrocatalysts, the structural parameters have to be carefully optimized, which is very important but is still difficult and challenging. Currently, hybridizing with secondary conductive supports, like polyaniline,28-29 graphene30-31 and so on, is a common method to increase the conductivity of MOFs. However, with this method, the additional components may block the intrinsic micropores of MOFs, which usually impedes the effective mass transport during electrocatalysis. On the other hand, recent studies showed that Fe dopant can effectively improve the catalytic performances of OER electrocatalysts on the basis of Ni metal.32 Although the improvement has still not yet completely understood, increasing the electrical conductivity of these materials has been confirmed to be crucial. For example, Boettcher et al. reported that, when the Fe content reached 25%, the conductivity of Ni1-xFexOOH increased from 0.2 to 6.5 mS/cm, and the overpotential for OER significantly dropped by a value of 200 mV.33 In addition, such phenomenon has also been observed in MOFs. In a recent report, the NiFe-MOF arrays synthesized by Prof. Zhao et al. showed a high electrical conductivity (1 ± 0.2×10-3 S/m) and exhibited a low OER overpotential for 10 mA/cm2 (240 mV), 56 mV-lower than that from NiMOF.34 In another work, Lang et al. synthesized Fe/Ni2.4-MIL-53 composite which showed superior OER catalytic activity with an low overpotential of 244 mV for 10 mA/cm2.35 These studies clearly show that FeNi bimetallic structures may have promising applications in electrocatalysts. However, it is still challenging to design functional MOFs containing both Fe and Ni sites which are directly used as catalytically active centers for OER. MOF-74 is a class of isostructures formed by a variety of transition metal centers (Zn, Ni, Co, Fe, Mn, Mg) with 2,5-dihydroxyterephthalic acid (H4DOBDC) as ligand. Among the reported


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MOF materials, MOF-74 possesses many advantages. Especially, MOF-74 contains coordinately unsaturated metal sites, which provide abundant metal sites in structure. In addition, the fully coordinated oxygen in H4DOBDC makes MOF-74 very stable.36 Due to these structural advantages, MOF-74 could serve as a type of novel OER catalyst after structure optimization. Herein, a simple solvothermal method is used to prepare different FeNi-MOFs, which serve directly as OER electrocatalysts with no post-annealing process. The electrochemical measurements showed that the as-prepared bimetallic FeNi-MOFs have enhanced OER catalytic performance as compared with the initial monometallic Ni-MOF. The present study presents an effective strategy for synthesizing bimetallic MOFs as advanced electrocatalysts without destroying the original structures.

FeNi-MOF

+ Fe

OER

Ni

Scheme 1 Synthesis process of bimetallic FeNi-MOF.

RESULTS AND DISCUSSION The bimetallic FeNi-MOFs were prepared following a previously reported procedure,37 as displayed in Scheme 1. Here, to examine the effect of Fe-precursor on the catalytic properties of the final FeNi-MOF product, in addition to FeSO4, FeCl2 and FeCl3 were also used separately in the synthesis. Meanwhile, four bimetallic FeNi-MOF samples are prepared with the Fe/Ni molar


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ratios of 9:1, 5:1, 3:1, 1:1 using the same organic ligand of H4DOBDC, and the synthesized samples are named as FeNi-DOBDC-1, FeNi-DOBDC-2 FeNi-DOBDC-3 FeNi-DOBDC-4, respectively (Experimental section). XRD was firstly used to characterize the crystal structures of the synthesized MOFs. One can see from Figure 1a that for the monometallic Ni-MOF with NiCl6·6H2O as Ni metal precursor, the XRD pattern agrees well with the simulated XRD spectrum of MOF-74, indicating the successful formation of Ni-MOF-74. However, monometallic Fe-MOF-74 cannot be obtained using FeSO4·7H2O as Fe metal source. As shown in Figure 1a, the diffraction spectrum of the obtained product is much different from that of NiMOF. Meanwhile, the obtained sample shows a midnight blue color, which is very different from that of pure FeSO4(H2O), but similar to that of the complex solution of FeSO4·7H2O and H4DOBDC (Figure S1). Moreover, the FT-IR spectrum of the product shows the existence of DOBDC (1650, 1500, 1420 cm-1, Figure S2). Therefore, the obtained product is possibly the complex of FeSO4(H2O) and H4DOBDC (denoted as Fe-complex). When FeSO4·7H2O was added into the Ni-MOF system, the X-ray photoelectron spectrum (XPS) indicates that NiII can be remained (Figure 1b). However, compared with Ni-MOF, the XRD pattern of the as-prepared FeNi-MOF shows a change (Figure 1a). The major diffraction peak of Ni-MOF at 6.9° shifts to 9.3°, and the diffraction intensity also shows obvious decrease. In terms of other major diffraction peaks, the XRD pattern of FeNi-MOF has also some changes compared with NiMOF. We surmised that a new structure MOF may be formed when Fe is doped into the NiMOF. In addition, no matter how many ferric salts were added, the obtained FeNi-MOFs show similar XRD patterns (Figure S3). Such result indicates that the FeNi-MOF has no change in crystal structure.


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Figure 1 a) XRD spectra of simulated MOF-74, and the prepared Ni-MOF, FeNi-DOBDC-3 and Fe-complex; XPS of b) Ni 2p, c) Fe 2p d) C 1s, e) O 1s from the Ni-MOF, FeNi-DOBDC-3 and Fe-complex, respectively; f) EDS of the FeNi-DOBDC-3.


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As shown in Figure S2, the FT-IR measurements reveal that the prepared Ni-MOF, FeNiMOF and Fe-complex have no absorptions from the protonated H4DOBDC (1680-1715 cm-1), suggesting the full deprotonation of H4DOBDC.38 Moreover, the prepared FeNi-MOF and NiMOF have similar FT-IR spectra, demonstrating that they have a similar coordination between metal centers and ligands. Unfortunately, the crystal structure is still under research and it will be reported in due course. From the Fe 2p XPS (Figure 1c), we can find that Fe in the Fe-complex is +2 (710.0 eV), in accord with the XRD result. Instead, the binding energy of Fe 2p in the bimetallic FeNi-MOFs is 712.7 eV, corresponding to Fe3+. The different valence states of Fe in Fe-complex and FeNi-MOFs may be due to that the coordinated iron ions in MOFs are easier to be oxidized. From the deconvoluted XPS of C 1s (Figure 1d), three peaks located at 284.6, 286.3 and 288.4 eV are assigned to adventitious hydrocarbon, C-O and carbonate species (i.e. -CO3 and C=O bonds), respectively. The O 1s XPS of the prepared samples were fitted into three peaks at 530.5, 531.5 and 532.8 eV (Figure 1e), corresponding to metal–OH (or metal–O), C-O (containing C–OH and C=O) and chemisorbed and dissociated oxygen species, respectively. Meanwhile, all the prepared samples have almost the same C 1s and O 1s binding energies, indicating that they are in the same chemical states (Figure 1d and 1e). For comparison, bimetallic FeNi-MOFs have also been prepared by using FeCl2 or FeCl3 as Fe precursor and interestingly, the XRD spectra of the products suggest that a mixture of MOF-74 and the new structured MOF can be obtained, as shown in Figure S4a. Such results indicate the importance of the usage of FeSO4·7H2O as Fe precursor for the formation of FeNi-MOFs.


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a

b

c 200 nm

2 µm

d

e

50 nm

1 µm

1 μm

f C

O

Ni

S

300 nm

Fe

Figure 2 SEM of a) Ni-MOF, b) FeNi-DOBDC-3 and c) Fe-complex; d) TEM image of FeNiDOBDC-3; e) HAADF-STEM image and f) STEM-EDS mapping images of FeNi-DOBDC-3.

The morphologies of the obtained materials were then examined by SEM. As shown in Figure 2a, the Ni-MOFs exhibit polyhedron shape and with the addition of Fe in Ni-MOFs, the morphology changes to nanosheets assemblies with the nanosheet thickness increasing from 10 to 70 nm (Figure 2b and Figure S5). Under the Fe: Ni ratio of 0.43, nanosheets with mean thickness of 18.3 nm and small irregular particles were formed in the product of FeNi-DOBDC-1 (Figure S5a). When the Fe: Ni ratio increases to 0.8 (determined by ICP, Table S1), regular nanosheet assemblies with thickness around 19 nm can be formed in the FeNi-DOBDC-2 and FeNi-DOBDC-3 (Figure 2b and Figure S5b). However, if the Fe/Ni ratio is too high, much thicker nanosheets with thickness of 33.4 nm containing irregular particles were formed (e.g. FeNi-DOBDC-4, Figure S5d). Therefore, to obtain regular nanosheets, appropriate Fe/Ni ratio is necessary. In addition, the energy-dispersive X-ray spectroscopy (EDS) (Figure 1f) and XPS


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(Figure S6) measurements indicate that the prepared FeNi-MOF contains Fe, Ni metals and S element with the presence of SO42- (Figure S6b). STEM mappings demonstrate that the Fe, Ni, C, O, S elements have highly uniform distributions in the FeNi-MOF (Figure 2e, f). From ICP measurements (Table S1), the ratio of Fe: Ni in the produced FeNi-MOFs is higher than that of the precursors. Such result indicates that Fe element is easier than Ni to coordinate with H4DOBDC.

Figure 3 Nitrogen adsorption and desorption isothermals of a) Ni-MOF, FeNi-DOBDC-3 and Fe-complex b) Different bimetallic FeNi-MOF samples.

The Brunauer–Emmett–Teller (BET) measurements showed that the prepared Ni-MOF has large surface area of 512.6 m2/g due to the large numbers of micropores (Figure 3a). However, when Fe was added, the surface area of FeNi-MOFs dropped rapidly to below 100 m2/g and the structure is predominated by mesopores with the disappearance of most micropores. In addition, The BET measurements indicate that for the bimetallic FeNi-MOFs, the samples with regular


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nanosheets assemblies have higher surface areas than those with irregular morphologies (Figure 3b and Table S1). Different from most of the studies on MOF-based electrocatalysts, here, the intrinsic oxygen evolution reaction (OER) catalytic activities of the obtained bimetallic FeNi-MOFs were studied without the post-calcination process. In the experiments, the FeNi-MOF nanosheets were first electrochemically treated by ~20 cyclic voltammetry scans (at 100 mV/s) from 1.0 to 1.8 V vs. RHE to reach a stable electrochemical state. Then, the catalytic performances of the samples for OER were studied by linear sweep voltammetry (LSV) with the potential scan rate of 10 mV/s in 1.0 M O2-saturated KOH solution. The OER activity of RuO2 was also measured for comparison and a 95% iR correction was applied to all measurements. As shown in Figure 4a, the OER catalytic performances of all the FeNi-MOF catalysts are much higher than the prepared single Ni-MOF or Fe-complex. Meanwhile, among the FeNi-MOFs, FeNi-DOBDC-2 and FeNiDOBDC-3 composed of regular nanosheets exhibit enhanced OER catalytic activities than other irregular FeNi-MOFs. This result demonstrates that the improvement of crystal regularity with nanosheet morphology is beneficial to enhance the catalytic activity of the bimetallic FeNi-MOF towards OER in alkaline solution. Such morphology is better to make most metal centers available to reactants. Meanwhile, these two samples own Fe content of 12-17% (Table S1), which is in accordance with the Alexis T. Bell’s report that the maximum OER activity was achieved with 12-17% Fe incorporation into Ni(OH)2/NiOOH in alkaline electrolyte.39 Notably, for the as-obtained FeNi-DOBDC-2 catalyst, low overpotentials of 274 and 292 mV are needed to obtain the OER current densities of 50 and 100 mA/cm2, respectively. For the FeNi-DOBDC3, the corresponding overpotentials are 270 and 287 mV at 50 and 100 mA/cm2, respectively. The overpotentials obtained from the present FeNi-MOFs are much lower than those from the


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Figure 4 a) Polarization curves and b) the corresponding overpotentials at 50 mA/cm2 and the current densities under an overpotential of 300 mV for OER. c) The corresponding Tafel plots of OER on the Ni-MOF, FeNi-MOFs, Fe-complex and RuO2 in 1.0 M KOH with 95% iR correction with the potential scan rate of 10 mV/s; d) Nyquist EIS spectra of FeNi-DOBDC-3 in 1.0 M KOH at various potentials, insets are the enlarged Nyquist EIS plots and the


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corresponding equivalent circuit model; e) Nyquist EIS plots of OER on Ni-MOF, FeNi-MOFs, Fe-complex at 1.48 V; f) Polarization curves of OER on FeNi-DOBDC-3 before and after 10,000 potential cycles in 1.0 M KOH.

commercial RuO2 (389 and 454 mV at 50 and 100 mA/cm2, respectively), indicating the much higher OER catalytic activities of the bimetallic FeNi-MOF nanosheet assemblies. Moreover, high current density of 155.5 mA/cm2 can be achieved on FeNi-DOBDC-3 at the overpotential of 300 mV (Figure 4b). It should be pointed out that the electrocatalytic properties of these FeNiDOBDC MOFs are comparable to those of the previously reported materials in basic electrolytes (Table S2). The outstanding catalytic property of bimetal FeNi-MOF electrocatalyst mainly attributes to the synergistic effect of the two metals, as other reported bimetallic LDHs and MOFs1, 34-35, 40. By comparing FeNi-DOBDC-3 with MOF(FeCl2) and MOF(FeCl3) (Figure S4b), we can find that the FeNi-DOBDC-3 shows much higher OER catalytic performance, demonstrating that using FeSO4·7H2O precursor as metal source is beneficial for obtaining highly active FeNi-MOF OER catalyst. To get a deeper insight into the OER catalytic properties of these FeNi-MOFs, the Tafel plots were also collected and studied (Figure 4c). As expected, the Tafel slope from FeNi-DOBDC-3 is only 49 mV/dec, much smaller than those of other studied catalysts (FeNi-DOBDC-2, 52 mV/dec; RuO2, 62 mV/dec), and even lower than those of most catalysts shown in Tables S2, suggesting its faster OER catalytic kinetics. The electronic conductivities of the prepared FeNi-MOF catalysts were also studied by electrochemical impedance spectroscopy (EIS). In this study, the EIS plots of all the MOF


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samples are fitted by an equivalent circuit (Figure 4d inset), which consists of solution resistance (Rs), constant phase element (Cd) and electron transfer resistance (Rct) (Figure S7). It can be observed that the Rct of the FeNi-DOBDC-3 catalyst shows rapid decrease with increasing potential from 1.40 to 1.52 V (Figure 4d). Therefore, the Rct is dependent on the applied potential and a larger overpotential can enhance the oxygen evolution reaction.41 Besides, at the same potential (1.48 V), the FeNi-DOBDC-3 has the lowest Rct value of 6 Ω among the studied MOFs (Figure 4e), indicating the importance of conductivity of MOF materials for their electrocatalytic properties. Moreover, the prepared bimetallic MOFs catalysts exhibit lower Rct values than the single metal MOFs at the same potential of 1.48 V (Figure 4e). Therefore, Fe dopant in Ni-MOF offers a faster catalytic kinetics, and the bimetallic FeNi-MOF nanosheets have intrinsically high electrical conductivities, which are beneficial for the oxygen evolution process in basic electrolytes. High durability of an electrocatalyst is also very important for its real applications. Here, accelerated durability test (ADT) was used to evaluate the catalytic durability of FeNi-DOBDC-3 with continuous CV scanning, as shown in Figure 4f. Meanwhile, a chronopotentiometric curve was measured with potential scanning at η = 278 mV for 12 h without iR compensation (Figure 5a). As shown in Figure 4f, the FeNi-DOBDC-3 shows almost no catalytic performance degradation after the ADT test, demonstrating that the FeNi-DOBDC-3 catalyst possesses an excellent catalytic stability in strong basic media. Meanwhile, the chronopotentiometric curve (Figure 5a) of the FeNi-DOBDC-3 shows a negligible loss in the static current and meanwhile, the subsequent LSV curves (Figure 5b) keep almost no change without degradation up to 12 h. Moreover, the XPS spectra and HRTEM images of the FeNi-DOBDC-3 before and after the electrochemical testing (after OER catalysis at 15 mA/cm2 for 1 h) are compared in Figure S8.


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We can see that the Ni 2p and Fe 2p XPS spectra show no much difference after the OER catalysis with only partial oxidation of Fe and Ni. Such oxidation could be induced by the OER process. Also from the HRTEM images before and after the electrochemical testing, the crystal structure has almost no change. These results further confirm that the FeNi-DOBDC-3 has excellent OER catalytic durability.

Figure 5 a) Chronopotentiometric curve (without iR compensation) of FeNi-DOBDC-3 at overpotential of 278 mV; b) LSVs for FeNi-DOBDC-3 before and after the chronoamperometry analysis (without iR compensation).

CONCLUSIONS In summary, a series of bimetallic FeNi-MOF structures with different Fe/Ni ratios are facilely fabricated. Different from most of the reported application of MOFs in electrocatalysts after high-temperature carbonization, the prepared FeNi-MOFs in this study are directly applied as OER electrocatalysts with satisfactory catalytic activity and durability. OER current densities of 50 and 100 mA/cm2 can be achieved from the optimized FeNi-DOBDC-3 at overpotentials of only 270 and 287 mV, respectively. Meanwhile, the FeNi-DOBDC-3 shows a low OER Tafel


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slope of 49 mV/dec. Such outstanding OER catalytic activity could originate from the highly porous structure, large surface area, high conductivity and the synergy intereaction between Fe and Ni in the MOFs. This work demonstrates the promising application of highly conductive MOFs in energy electrocatalysis with optimized structure and composition.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.XXXXXXX. Experimental section, more FTIR, XRD, SEM, XPS and electrocatalytic data. (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21575134, 21633008, 21605006, 21773224), the National Key Research and Development Plan (2016YFA0203200) and K. C. Wong Education Foundation. REFERENCES


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TOC

FeNi-MOF

+ Fe

OER

Ni

Highly conductive FeNi bimetallic MOFs are prepared and used directly as high-performance OER electrocatalyst without the post-calcination treatment.


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Highly Conductive Bimetallic NiâFe Metal Organic Framework as

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