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FeNi-based bimetallic MIL-101 directly applicable as an efficient electrocatalyst for oxygen evolution reaction
Microporous and Mesoporous Materials 286 (2019) 92–97
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
FeNi-based bimetallic MIL-101 directly applicable as an efficient electrocatalyst for oxygen evolution reaction
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Qiang Wanga,b, Congcong Weia,b, Dandan Lia,b, Wenjun Guoa,b, Dazhong Zhonga,b, Qiang Zhaoa,b,∗ a b
Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, PR China Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan, 030024, Shanxi, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal–organic frameworks Electrocatalyst Oxygen evolution reaction Synergistic effect Water oxidation
Metal–organic frameworks (MOFs) have been widely used in the field of electrocatalysis in recent years. In most reports, an MOF is used as a sacrificial template to prepare a highly efficient electrocatalyst by calcination. Here, a series of FeNi bimetallic MIL-101 materials has been synthesized by a solvothermal method and directly applied for the oxygen evolution reaction (OER). These MOFs contain a large number of catalytically active MO6 octahedra and exposed hydrophilic carboxyl groups, which are believed to enhance OER activity. In addition, after the introduction of Ni, the catalytic activity is significantly improved by synergy between the metals. Among the studied compositions, Fe2Ni-MIL-101 showed the best OER performance, requiring a low overpotential of just 237 mV to drive a current density of 20 mA cm−2 in 1 M KOH solution, corresponding to a low Tafel slope of 44.2 mV dec−1. Moreover, the electrochemical properties remained virtually unchanged during a 40-h long-term stability test. These findings enrich the application of MOFs materials in the field of electrocatalysis.
1. Introduction The energy crisis and environmental issues are becoming more prominent due to excessive use of fossil fuels [1], and hence the quest for clean and sustainable alternatives to fill the growing energy gap is becoming extremely urgent [2,3]. Hydrogen is an ideal candidate to replace traditional fossil fuels. Electrochemical water decomposition provides a clean and effective way to produce pure hydrogen on a large scale [4,5]. The oxygen evolution reaction (OER), which plays an important role in various energy storage and conversion processes, is considered to be the main bottleneck preventing effective water decomposition for hydrogen generation [6–8]. Moreover, the anodic OER is a four-electron transfer (4OH− → 2H2O + O2 + 4e−). Due to the sluggish kinetics of the multi-step proton-coupled electron transfer, the OER has always limited the efficiency of electrochemical systems [9]. Current research indicates that the precious metal Pt and noble metal oxides (RuO2, IrO2) exhibit the best HER and OER activities, but they are not suitable for large-scale applications [10]. Therefore, great efforts have been directed towards developing efficient and stable OER catalysts, such as metal oxides, hydroxides, chalcogenides, nitrides, and phosphides, as well as carbon-based materials [11–18]. Furthermore,
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many reports have shown that transition metal layered double hydroxides (LDHs) can serve as excellent and stable OER catalysts [19–22]. Transition metal materials exhibit superior OER performance. In recent years, MOFs have attracted extensive attention in the field of electrocatalysis, benefiting from their intrinsic large specific surface areas, adjustable pores, variable metal ions, and organic ligand diversity [23]. MOFs have many intrinsic metal sites, but they are rarely used for electrocatalysis because of their poor conductivity and small pore size [24,25] For the application of MOFs in water electrolysis, the preparation of porous nanomaterials by post-treatment of MOFs has attracted wide interest [26]. They are usually calcined to afford metal oxide and porous carbon materials to improve their performance [27]. However, this method may destroy the structure and lead to the loss of organic ligands [28]. The combination of a conductive material with an MOF can enhance electrical conductivity, which has attracted wide attention, but this method leads to a decrease in the number of active sites [29,30]. Therefore, direct application of MOFs as catalysts for water electrolysis remains a significant challenge. Among the many known transition metal MOFs, Fe-based MOFs have high hydrothermal and chemical stabilities, endowing them with great potential as anodic OER catalysts. Fe-MIL-101 has many
Corresponding author. Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, PR China. E-mail address: [email protected] (Q. Zhao).
https://doi.org/10.1016/j.micromeso.2019.05.040 Received 22 February 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 18 May 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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Scheme 1. Schematic illustration of the synthesis of FeXNiY-MIL-101, and its direct use as an anodic oxygen evolution catalyst.
to cool to room temperature. The precipitate obtained was washed four times each with DMF and ethanol and dried at 60 °C for 8 h Fe3Ni-MIL101 ((0.5 mmol Ni(NO3)2·6H2O, 145.3 mg):(1.5 mmol FeCl3·6H2O, 405 mg)), FeNi-MIL-101 ((1 mmol Ni(NO3)2·6H2O, 290.7 mg):(1 mmol FeCl3·6H2O, 270 mg)) and FeNi2-MIL-101 ((1.32 mmol Ni(NO3)2·6H2O, 383.8 mg):(0.66 mmol FeCl3·6H2O, 178.2 mg)) were prepared by the same method, varying the Fe:Ni molar ratio as appropriate.
advantages as an anodic OER catalyst, and the raw materials for its synthesis are inexpensive and readily available [31]. Meanwhile, FeMIL-101 has suitable mesoporosity and a rigid zeolite-type crystal structure composed of quasi-spherical cages in two modes (2.9 and 3.4 nm) accessible through windows of ca. 1.2 and 1.6 nm [32–34], which is conductive to full contact with water molecules. Moreover, coordinatively unsaturated metal sites (CUS) can be easily produced in the structure of MIL-101 by heating the material in a vacuum [35]. This is beneficial to expose more active sites. Moreover, with Fe3+ as the central metal ion, its high potential can significantly improve the stability of Fe-MIL-101 [36]. During the one-pot solvothermal process for the synthesis of bimetallic MOFs, the central metal ions are partially replaced to form a stable structure [37]. Compared with single-metal MOF catalysts, bimetallic-based MOF catalysts show enhanced activity due to a synergistic effect between the metals [38,39]. Herein, we describe a simple strategy for the synthesis of a series of bimetallic Fe–Ni MOFs (Scheme 1). Bimetallic MIL-101 samples with different Fe–Ni ratios have been synthesized by a solvothermal method. This is a convenient way to prepare bimetallic MOFs with different metal ratios and sizes by adjusting the molar ratio and molar amounts of the reactants. Bimetallic MIL-101, as a direct electrocatalytic water oxidation catalyst, provides abundant metal sites as catalytic centers. A synergistic effect between the metals after introducing Ni into Fe-MIL101 enhances the activity. The increase in electron density of Ni ions in bimetallic MIL-101 improves its electron-transport ability, which is considered to be a further reason for the improved catalytic performance in the OER.
2.2.2. Preparation of Fe-MIL-101 Pure Fe-MIL-101 was synthesized according to a previously reported method [40]. FeCl3·6H2O (540 mg, 2 mmol) and H2bdc (166 mg, 1 mmol) were dissolved in DMF (10 mL) and the mixture was left to stand at 120 °C for 20 h. The product was collected by centrifugation, washed three times each with ethanol and DMF, and dried at 60 °C for 8 h.
2.2.3. Preparation of Fe-MIL-100 Pure Fe-MIL-100 was synthesized according to Yoon et al. [41]. Typically, Fe powder (277.5 mg), 1,3,5-Benzenetricarboxylic acid (687.5 mg), hydrofluoric acid (30%, 200 μL), and nitric acid (65%, 190 μL) were well dispersed in ultrapure water (20 mL) in a Teflonlined steel autoclave. The autoclave was then placed in an oven at 150 °C for 12 h. After cooling to room temperature, the light-orange solid product was collected by filtration and washed with ultrapure water. The as-synthesized Fe-MIL-100 was further purified by placing it in water at 80 °C for 3 h to remove residual unreacted ions, and then in ethanol at 60 °C for 3 h until no more colored impurities diffused into the medium. The highly purified MIL-100(Fe) was then collected by centrifugation at 10000 rpm for 5 min. The solid was finally dried at 80 °C under vacuum overnight.
2. Experimental 2.1. Materials FeCl3·6H2O, Ni (NO3)2·6H2O, N, N-dimethylformamide (DMF), and ethanol were purchased from Sinopharm. KOH, RuO2, 1,3,5Benzenetricarboxylic acid (H3btc) and terephthalic acid (H2bdc) were purchased from Aladdin Ltd. (Shanghai, China). All chemicals were used as received without further purification. Nickel foam (NF; thickness: 1.5 mm) was purchased from The Source of Power Battery Material Co., Ltd. (Shanxi, China).
2.2.4. Preparation of nano Fe2O3 Nano Fe2O3 samples were prepared by calcining the above Fe-MIL101 in air in a muffle furnace at 350 °C, 450 °C, and 550 °C for 2 h, attained at a heating rate of 5 °C/min.
2.2.5. Preparation of nano Fe2O3/NiO in 2:1 ratio Firstly, bulk Ni-MOF is synthesized in the following typical way. 0.166 g of PTA and 0.096 g of Ni (NO3)2·6H2O were dissolved in 20 mL DMF with stirring at room temperature. Then, 2 mL of NaOH aqueous solution (0.4 M) was slowly added to the above solution drop by drop. After that, the mixture was transformed into a Teflon-lined stainlesssteel autoclave with a capacity of 40 mL. The autoclave was maintained at 100 °C for 8 h, and then naturally cooled to room temperature. The resulting precipitate was thoroughly washed several times with DMF and alcohol, respectively. Nano NiO were prepared by calcining the above Ni-MOF in air in a muffle furnace at 350 °C for 2 h, attained at a heating rate of 5 °C/min.
2.2. Preparation of the samples 2.2.1. Preparation of Fe3Ni-MIL-101, Fe2Ni-MIL-101, FeNi-MIL-101, and FeNi2-MIL-101 In a typical synthesis of Fe2Ni-MIL-101, 1,4-BDC (1 mmol, 16.6 mg), Ni (NO3)2·6H2O (0.66 mmol, 191.9 mg), and FeCl3·6H2O (1.32 mmol, 356.4 mg) were added to DMF (10 mL) in a Teflon vessel (20 mL). The mixture was stirred for 30 min to form a dark-yellow solution. The vessel was then transferred to a stainless-steel autoclave, which was placed upright in an oven, heated at 110 °C for 20 h, and then allowed 93
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in which characteristic peaks at 856.6 and 874.3 eV can be assigned to Ni 2p2/3 and Ni 2p1/2 electronic configurations, respectively [42]. The Fe 2p spectrum (Fig. 2b) features two main peaks at 711.7 and 725.5 eV, which can be ascribed to Fe 2p3/2 and Fe 2p1/2, together with a satellite peak at 718.5 eV, implying Fe3+ species [43]. With the introduction of Ni, the electronic structure of oxygen is enriched, and the binding energy in the O 1s spectrum is positively shifted. The O 1s spectrum of FeNi-MIL-101 (Fig. 2c) can be deconvoluted into three peaks at 530.9, 531.9, and 532.6 eV, corresponding to O in Fe (Ni)–O, Fe (Ni)–O–H, and O–C]O, respectively [44,45]. Furthermore, the C 1s spectra reveal the presence of the same functional groups in Fe-MIL-101 and Fe2Ni-MIL-101 (Fig. 2d), namely C]C (284.5 eV), C]O (285.9 eV), and O–C]O (288.5 eV) [46,47]. These synthetic MOFs could be directly used in electrocatalytic OER without annealing or doping with conductive materials, which is attractive from a practical perspective. In order to optimize the electrochemical performances of these MOFs, we varied a number of parameters, such as the catalyst loading, the activation time, and the activation temperature. The OER activities of the resultant MOFs were investigated in a standard three-electrode electrochemical cell with 1.0 M KOH as the electrolyte. All of the catalysts were applied to pretreated NF to afford the working electrodes. As shown in Fig. S3-S4†, when 3 mg cm−2 was applied, and activation was conducted for 3 h at 150 °C, the best OER performance was obtained. For comparison, FeMIL-100 was also synthesized, and its identity was confirmed by its XRD pattern and SEM images (Fig. S5-S6†). We also prepared nanoparticles, nanospheres, and nanorods of Fe2O3 by calcination of Fe-MIL101 at different temperatures (Fig. S13-16†). and then Nano Fe2O3: NiO in 2:1 ratio was also prepared (Fig. S17†). Fig. 3a and b show the steady-state polarization curves and Tafel slopes of blank NF, RuO2/NF, nano Fe2O3, Fe-MIL-100/NF, Fe-MIL101/NF, and Fe2Ni-MIL-101/NF. As shown in Fig. 3a, the LSV curves feature an oxidation peak at around 1.35 V, which can be attributed to the formation of Ni3+ in the OER process. In addition, this phenomenon is also considered to be corresponding to a larger oxidation current [48]. In Fig. 3b, it can be seen that Fe-MIL-101/NF (41.4 mV dec−1) and Fe-MIL-100/NF (41.5 mV dec−1) show lower Tafel slopes than blank NF (90.6 mV dec−1), RuO2/NF (138.6 mV dec−1), and nano Fe2O3/NF (45.9 mV dec−1). Fe2Ni-MIL-101/NF also exhibits favorable OER kinetics, with a Tafel slope of only 44.2 mV dec−1. In Fig. 3c, the blank NF curve shows negligible OER performance, requiring an overpotential of 375 mV at a current density of 20 mA cm−2. The as-prepared RuO2/NF gave a current density of 20 mA cm−2 at an overpotential of 323 mV, whereas Fe-MIL-101 and Fe-MIL-100 supported on NF showed greatly improved performances, requiring much lower overpotentials of 251 and 253 mV at the same current density. As shown in Fig. S6c-d† and Fig. S9a-b†, Fe-MIL-101 has a bigger specific surface area and a pore size than Fe-MIL-100. Therefore, Fe-MIL-101 exhibits better OER performance. Nano Fe2O3 also showed a good OER performance, requiring an overpotential of 268 mV at a current density of 20 mA cm−2. Bimetallic Fe2Ni-MIL-101/NF exhibited the best performance, requiring an overpotential of only 237 mV at a current density of 20 mA cm−2, much lower than those of previously reported highly active OER catalysts (Table S1†). Moreover, Fe2Ni-MIL-101/NF apparently exhibit superior OER performance than the simple mixture of nano Fe2O3 and NiO (Fig. S18), suggesting crucial interaction between Ni and Fe species. Fig. 3d shows that Fe2Ni-MIL-101/NF still performed efficiently, incurring only a small loss of current density, after 500 CV cycles at a scan rate of 50 mV/s. The obtained polarization curve is similar to the initial curve. In addition, the chronopotentiometric curve of Fe2Ni-MIL-101/NF at a current density of 20 mA cm−2 was also obtained (Fig. S12c†), which showed no discernible decline. SEM images acquired after long-term stability tests also indicated the good structural stability of the catalyst (Fig. S12a-b†). In investigating the reasons for the excellent oxygen evolution properties of these MOFs, some possible influencing factors were
2.2.6. Preparation of RuO2/NF electrodes Nickel foam (NF, 1 cm × 3 cm) was first activated with dilute hydrochloric acid (2.0 M HCl) and then repeatedly washed with deionized water and ethanol. RuO2 (2 mg) and a suspension of Nafion® (10 μL, 5 wt%) were then dispersed in a mixture of water (400 μL) and ethanol (50 μL) with sonication for 30 min. The catalytic ink thus obtained was dropped onto Ni foam (1 cm × 1 cm) and dried at 60 °C for 8 h. 2.3. Characterization of the materials X-ray diffraction (XRD) patterns were measured on a Bruker D8 Advance X-ray diffractometer employing Cu-Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) were performed with a Hitachi SU8010 instrument (Hitachi, Tokyo, Japan) equipped with a Horiba Xmax 50 EDX system. Brunauer–Emmett–Teller (BET) surface areas and pore size distributions of the catalysts were measured with a Micromeritics TriStar II 3020 instrument based on the adsorption of N2 at 77 K. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F instrument (Tokyo, Japan) operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed with a WSCAL-ab 220i-XL spectrometer (VG Scientific, Sussex, UK) employing Al-Kα radiation (hν = 1486.6 eV, 150 W). 2.4. Electrochemical measurements Electrochemical measurements were made with a Princeton electrochemical workstation (PARSTAT MC, Princeton, USA) in a standard three-electrode system. Catalysts loaded on NF were used as working electrodes (1 cm × 1 cm), with a mercury/mercury (II) oxide electrode (Hg/HgO) as the reference, and a Pt rod as the counter electrode. Each catalyst suspension was prepared by dispersing MOF (3 mg) in 0.265 mL of a solution comprising ultra-pure water (200 μL), ethanol (50 μL), and 0.5 wt% Nafion® suspension (10 μL) followed by ultrasonication for 30 min. Each suspension was then dropped onto pretreated NF and dried at room temperature. Linear sweep voltammetry (LSV) was carried out in 1.0 M KOH at a scan rate of 2 mV/s. All the potentials were calibrated as reversible hydrogen electrode (RHE) based on the formula: ERHE = EHg/HgO + 0.098 V + 0.059 pH. The OER overpotential (η) was obtained according to the following formula: η = ERHE-1.23 V. Long-term durability tests were performed by chronopotentiometric measurements. 3. Results and discussion Bimetallic MIL-101 was successfully synthesized, as verified by the following characterization data. A SEM image of bimetallic MIL-101 is shown in Fig. 1a and Fig. S10†, which reveals the same uniform octahedral particles as seen for Fe-MIL-101 (Fig. S1†). The PXRD patterns show that bimetallic MOFs with different molar ratios exhibit the same diffraction peaks as Fe-MIL-101 (Fig. 1c). Energy-dispersive X-ray spectroscopy (EDX) mappings and X-ray photoelectron spectra (XPS) confirmed that these bimetallic MOFs were made up of the same elements (Figs. 1d and 2). The EDX mappings reveal that the various elements were uniformly distributed, and ICP-AES results of the elemental contents were consistent with the molar ratio of the raw materials (Fig. S11†). TEM images revealed the crystal structure (Fig. 1b). With increasing Ni content, the diffraction peak and the specific surface area decreased (Fig. 1c and Fig. S9†), and the crystallinity of bimetallic electrochemical measurements supported the decreased surface area (Fig. S8†). X-ray photoelectron spectroscopy (XPS) was applied to analyze the surface compositions and surface electronic states of Fe-MIL-101 and Fe2Ni-MIL-101. Compared with Fe-MIL-101, it was clearly apparent that Ni was additionally present in Fe2Ni-MIL-101. The presence of Ni2+ was demonstrated by the high-resolution Ni 2p spectrum (Fig. 2a), 94
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Fig. 1. (a) SEM and (b) TEM images of Fe2Ni-MIL-101; (c) PXRD patterns of simulated Fe-MIL-101, Fe-MIL-101, Fe3Ni-MIL-101, Fe2Ni-MIL-101, FeNi-MIL-101, and FeNi2-MIL-101; (d–g) EDX elemental mappings of Fe2Ni-MIL-101.
Fig. 2. High-resolution XPS spectra of (a) Ni 2p, (b) Fe 2p, (c) C 1s, and (d) O 1s of Fe-MIL-101/NF and Fe2Ni-MIL-101/NF. 95
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Fig. 3. (a) Steady-state polarization curves and (b) Tafel plots of blank NF, RuO2/NF, nano Fe2O3, Fe-MIL-100/NF, Fe-MIL-101/NF, and Fe2Ni-MIL-101/NF. (c) The required overpotentials obtained from OER polarization curves at a current density of 20 mA cm−2. (d) LSV curves of Fe2Ni-MIL-101/NF before and after 500 CV cycles. All experiments were carried out in 1.0 M KOH.
Fig. 4. CV curves at different scan rates in the range of 0.32 and 0.40 V vs. RHE for (a) Fe-MIL-101/NF and (b) Fe2Ni-MIL-101/NF, plots of the charging current density against scan rates of (c) Fe-MIL-101/NF and (d) Fe2Ni-MIL-101/NF.
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References
explored. We used a CV method to calculate the double-layer capacitance (Cdl) [49], which is related to the electrode surface area (ECSA). The slope of a linear plot of non-faradaic capacitance current as a function of scan rate is equal to Cdl (Fig. 4a and b). The results showed Fe2Ni-MIL-101 to have the largest ECSA, with more active sites available at the solid–liquid interface. The Cdl evaluated for Fe2Ni-MIL-101/ NF is 10.36 mF cm−2, 1.97 mF cm−2 higher than that for Fe-MIL-101/ NF (8.39 mF cm−2) (Fig. 4c and d). The similarity of the ECSA values for these catalysts implies that they have similar numbers of active sites. Compared with the nano Fe2O3, Fe-MIL-100, and Fe-MIL-101, Fe2NiMIL-101 has a critical synergistic effect between Fe and Ni. Furthermore, Fe2Ni-MIL-101 contain Fe predominantly as Fe3+, under OER conditions, in principle partial-charge transfer between Fe and Ni2+/ 3+ , which could enhance the activity. The excellent OER performance of the Fe2Ni-MIL-101/NF catalyst is clearly influenced by the presence of Ni. In Fe/Ni-based OER catalysts, Ni is generally considered to be the reactive center for oxygen evolution [50], and the presence of Fe can facilitate the elevation of Ni to a higher valence state [51]. Moreover, during the process of OER, FeO6 and NiO6 octahedra within the MOF can be partially oxidized to Fe2O3 and NiOOH as active centers, which can promote the conversion of OH− into oxygen molecules [52]. The synergy between the uniformly distributed Fe–Ni active centers in the catalyst also effectively improves the OER performance. The superior performance of Fe2Ni-MIL-101 is also due to its intrinsic porous structure, and the large specific surface area exposes more active sites. In particular, during the OER process, some organic ligand structures are destroyed to form evenly distributed mesopores of about 10 nm (Fig. S19b†), which are conducive to the mass-transfer process, promote the desorption of gas generated in the process of water electrolysis, and enhance contact between water molecules and the active sites. At the same time, the carbon-based skeleton is abnormally stable, and it can effectively inhibit agglomeration of active sites, thereby reducing overpotential and improving stability.
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4. Conclusions In summary, we have successfully synthesized a series of bimetallic MIL-101 materials in one step by a solvothermal method. The prepared MOFs have been directly used as OER catalysts. By virtue of the intrinsic porous structure and synergy between the metals, bimetallic MIL-101 materials show excellent electrochemical performances. Among them, Fe2Ni-MIL-101 showed the best OER performance, requiring a potential of only 237 mV at 20 mA cm−2, with a small Tafel slope of 44.2 mV dec−1. Moreover, it also exhibited excellent stability upon chronoamperometric cycling for up to 40 h. Our research enriches the direct application of MOF materials as catalysts for water electrolysis. In future work, some other transition metals may be incorporated to further enhance the oxygen evolution performance through synergy between the metals. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21878202, 21476153), the Research Project Supported by Shanxi Scholarship Council of China (No. 2017-041), the Natural Science Foundation of Shanxi Province (No. 201801D121052), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.05.040. 97
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
FeNi-based bimetallic MIL-101 directly applicable as an efficient electrocatalyst for oxygen evolution reaction
T
Qiang Wanga,b, Congcong Weia,b, Dandan Lia,b, Wenjun Guoa,b, Dazhong Zhonga,b, Qiang Zhaoa,b,∗ a b
Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, PR China Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan, 030024, Shanxi, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal–organic frameworks Electrocatalyst Oxygen evolution reaction Synergistic effect Water oxidation
Metal–organic frameworks (MOFs) have been widely used in the field of electrocatalysis in recent years. In most reports, an MOF is used as a sacrificial template to prepare a highly efficient electrocatalyst by calcination. Here, a series of FeNi bimetallic MIL-101 materials has been synthesized by a solvothermal method and directly applied for the oxygen evolution reaction (OER). These MOFs contain a large number of catalytically active MO6 octahedra and exposed hydrophilic carboxyl groups, which are believed to enhance OER activity. In addition, after the introduction of Ni, the catalytic activity is significantly improved by synergy between the metals. Among the studied compositions, Fe2Ni-MIL-101 showed the best OER performance, requiring a low overpotential of just 237 mV to drive a current density of 20 mA cm−2 in 1 M KOH solution, corresponding to a low Tafel slope of 44.2 mV dec−1. Moreover, the electrochemical properties remained virtually unchanged during a 40-h long-term stability test. These findings enrich the application of MOFs materials in the field of electrocatalysis.
1. Introduction The energy crisis and environmental issues are becoming more prominent due to excessive use of fossil fuels [1], and hence the quest for clean and sustainable alternatives to fill the growing energy gap is becoming extremely urgent [2,3]. Hydrogen is an ideal candidate to replace traditional fossil fuels. Electrochemical water decomposition provides a clean and effective way to produce pure hydrogen on a large scale [4,5]. The oxygen evolution reaction (OER), which plays an important role in various energy storage and conversion processes, is considered to be the main bottleneck preventing effective water decomposition for hydrogen generation [6–8]. Moreover, the anodic OER is a four-electron transfer (4OH− → 2H2O + O2 + 4e−). Due to the sluggish kinetics of the multi-step proton-coupled electron transfer, the OER has always limited the efficiency of electrochemical systems [9]. Current research indicates that the precious metal Pt and noble metal oxides (RuO2, IrO2) exhibit the best HER and OER activities, but they are not suitable for large-scale applications [10]. Therefore, great efforts have been directed towards developing efficient and stable OER catalysts, such as metal oxides, hydroxides, chalcogenides, nitrides, and phosphides, as well as carbon-based materials [11–18]. Furthermore,
∗
many reports have shown that transition metal layered double hydroxides (LDHs) can serve as excellent and stable OER catalysts [19–22]. Transition metal materials exhibit superior OER performance. In recent years, MOFs have attracted extensive attention in the field of electrocatalysis, benefiting from their intrinsic large specific surface areas, adjustable pores, variable metal ions, and organic ligand diversity [23]. MOFs have many intrinsic metal sites, but they are rarely used for electrocatalysis because of their poor conductivity and small pore size [24,25] For the application of MOFs in water electrolysis, the preparation of porous nanomaterials by post-treatment of MOFs has attracted wide interest [26]. They are usually calcined to afford metal oxide and porous carbon materials to improve their performance [27]. However, this method may destroy the structure and lead to the loss of organic ligands [28]. The combination of a conductive material with an MOF can enhance electrical conductivity, which has attracted wide attention, but this method leads to a decrease in the number of active sites [29,30]. Therefore, direct application of MOFs as catalysts for water electrolysis remains a significant challenge. Among the many known transition metal MOFs, Fe-based MOFs have high hydrothermal and chemical stabilities, endowing them with great potential as anodic OER catalysts. Fe-MIL-101 has many
Corresponding author. Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, PR China. E-mail address: [email protected] (Q. Zhao).
https://doi.org/10.1016/j.micromeso.2019.05.040 Received 22 February 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 18 May 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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Scheme 1. Schematic illustration of the synthesis of FeXNiY-MIL-101, and its direct use as an anodic oxygen evolution catalyst.
to cool to room temperature. The precipitate obtained was washed four times each with DMF and ethanol and dried at 60 °C for 8 h Fe3Ni-MIL101 ((0.5 mmol Ni(NO3)2·6H2O, 145.3 mg):(1.5 mmol FeCl3·6H2O, 405 mg)), FeNi-MIL-101 ((1 mmol Ni(NO3)2·6H2O, 290.7 mg):(1 mmol FeCl3·6H2O, 270 mg)) and FeNi2-MIL-101 ((1.32 mmol Ni(NO3)2·6H2O, 383.8 mg):(0.66 mmol FeCl3·6H2O, 178.2 mg)) were prepared by the same method, varying the Fe:Ni molar ratio as appropriate.
advantages as an anodic OER catalyst, and the raw materials for its synthesis are inexpensive and readily available [31]. Meanwhile, FeMIL-101 has suitable mesoporosity and a rigid zeolite-type crystal structure composed of quasi-spherical cages in two modes (2.9 and 3.4 nm) accessible through windows of ca. 1.2 and 1.6 nm [32–34], which is conductive to full contact with water molecules. Moreover, coordinatively unsaturated metal sites (CUS) can be easily produced in the structure of MIL-101 by heating the material in a vacuum [35]. This is beneficial to expose more active sites. Moreover, with Fe3+ as the central metal ion, its high potential can significantly improve the stability of Fe-MIL-101 [36]. During the one-pot solvothermal process for the synthesis of bimetallic MOFs, the central metal ions are partially replaced to form a stable structure [37]. Compared with single-metal MOF catalysts, bimetallic-based MOF catalysts show enhanced activity due to a synergistic effect between the metals [38,39]. Herein, we describe a simple strategy for the synthesis of a series of bimetallic Fe–Ni MOFs (Scheme 1). Bimetallic MIL-101 samples with different Fe–Ni ratios have been synthesized by a solvothermal method. This is a convenient way to prepare bimetallic MOFs with different metal ratios and sizes by adjusting the molar ratio and molar amounts of the reactants. Bimetallic MIL-101, as a direct electrocatalytic water oxidation catalyst, provides abundant metal sites as catalytic centers. A synergistic effect between the metals after introducing Ni into Fe-MIL101 enhances the activity. The increase in electron density of Ni ions in bimetallic MIL-101 improves its electron-transport ability, which is considered to be a further reason for the improved catalytic performance in the OER.
2.2.2. Preparation of Fe-MIL-101 Pure Fe-MIL-101 was synthesized according to a previously reported method [40]. FeCl3·6H2O (540 mg, 2 mmol) and H2bdc (166 mg, 1 mmol) were dissolved in DMF (10 mL) and the mixture was left to stand at 120 °C for 20 h. The product was collected by centrifugation, washed three times each with ethanol and DMF, and dried at 60 °C for 8 h.
2.2.3. Preparation of Fe-MIL-100 Pure Fe-MIL-100 was synthesized according to Yoon et al. [41]. Typically, Fe powder (277.5 mg), 1,3,5-Benzenetricarboxylic acid (687.5 mg), hydrofluoric acid (30%, 200 μL), and nitric acid (65%, 190 μL) were well dispersed in ultrapure water (20 mL) in a Teflonlined steel autoclave. The autoclave was then placed in an oven at 150 °C for 12 h. After cooling to room temperature, the light-orange solid product was collected by filtration and washed with ultrapure water. The as-synthesized Fe-MIL-100 was further purified by placing it in water at 80 °C for 3 h to remove residual unreacted ions, and then in ethanol at 60 °C for 3 h until no more colored impurities diffused into the medium. The highly purified MIL-100(Fe) was then collected by centrifugation at 10000 rpm for 5 min. The solid was finally dried at 80 °C under vacuum overnight.
2. Experimental 2.1. Materials FeCl3·6H2O, Ni (NO3)2·6H2O, N, N-dimethylformamide (DMF), and ethanol were purchased from Sinopharm. KOH, RuO2, 1,3,5Benzenetricarboxylic acid (H3btc) and terephthalic acid (H2bdc) were purchased from Aladdin Ltd. (Shanghai, China). All chemicals were used as received without further purification. Nickel foam (NF; thickness: 1.5 mm) was purchased from The Source of Power Battery Material Co., Ltd. (Shanxi, China).
2.2.4. Preparation of nano Fe2O3 Nano Fe2O3 samples were prepared by calcining the above Fe-MIL101 in air in a muffle furnace at 350 °C, 450 °C, and 550 °C for 2 h, attained at a heating rate of 5 °C/min.
2.2.5. Preparation of nano Fe2O3/NiO in 2:1 ratio Firstly, bulk Ni-MOF is synthesized in the following typical way. 0.166 g of PTA and 0.096 g of Ni (NO3)2·6H2O were dissolved in 20 mL DMF with stirring at room temperature. Then, 2 mL of NaOH aqueous solution (0.4 M) was slowly added to the above solution drop by drop. After that, the mixture was transformed into a Teflon-lined stainlesssteel autoclave with a capacity of 40 mL. The autoclave was maintained at 100 °C for 8 h, and then naturally cooled to room temperature. The resulting precipitate was thoroughly washed several times with DMF and alcohol, respectively. Nano NiO were prepared by calcining the above Ni-MOF in air in a muffle furnace at 350 °C for 2 h, attained at a heating rate of 5 °C/min.
2.2. Preparation of the samples 2.2.1. Preparation of Fe3Ni-MIL-101, Fe2Ni-MIL-101, FeNi-MIL-101, and FeNi2-MIL-101 In a typical synthesis of Fe2Ni-MIL-101, 1,4-BDC (1 mmol, 16.6 mg), Ni (NO3)2·6H2O (0.66 mmol, 191.9 mg), and FeCl3·6H2O (1.32 mmol, 356.4 mg) were added to DMF (10 mL) in a Teflon vessel (20 mL). The mixture was stirred for 30 min to form a dark-yellow solution. The vessel was then transferred to a stainless-steel autoclave, which was placed upright in an oven, heated at 110 °C for 20 h, and then allowed 93
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in which characteristic peaks at 856.6 and 874.3 eV can be assigned to Ni 2p2/3 and Ni 2p1/2 electronic configurations, respectively [42]. The Fe 2p spectrum (Fig. 2b) features two main peaks at 711.7 and 725.5 eV, which can be ascribed to Fe 2p3/2 and Fe 2p1/2, together with a satellite peak at 718.5 eV, implying Fe3+ species [43]. With the introduction of Ni, the electronic structure of oxygen is enriched, and the binding energy in the O 1s spectrum is positively shifted. The O 1s spectrum of FeNi-MIL-101 (Fig. 2c) can be deconvoluted into three peaks at 530.9, 531.9, and 532.6 eV, corresponding to O in Fe (Ni)–O, Fe (Ni)–O–H, and O–C]O, respectively [44,45]. Furthermore, the C 1s spectra reveal the presence of the same functional groups in Fe-MIL-101 and Fe2Ni-MIL-101 (Fig. 2d), namely C]C (284.5 eV), C]O (285.9 eV), and O–C]O (288.5 eV) [46,47]. These synthetic MOFs could be directly used in electrocatalytic OER without annealing or doping with conductive materials, which is attractive from a practical perspective. In order to optimize the electrochemical performances of these MOFs, we varied a number of parameters, such as the catalyst loading, the activation time, and the activation temperature. The OER activities of the resultant MOFs were investigated in a standard three-electrode electrochemical cell with 1.0 M KOH as the electrolyte. All of the catalysts were applied to pretreated NF to afford the working electrodes. As shown in Fig. S3-S4†, when 3 mg cm−2 was applied, and activation was conducted for 3 h at 150 °C, the best OER performance was obtained. For comparison, FeMIL-100 was also synthesized, and its identity was confirmed by its XRD pattern and SEM images (Fig. S5-S6†). We also prepared nanoparticles, nanospheres, and nanorods of Fe2O3 by calcination of Fe-MIL101 at different temperatures (Fig. S13-16†). and then Nano Fe2O3: NiO in 2:1 ratio was also prepared (Fig. S17†). Fig. 3a and b show the steady-state polarization curves and Tafel slopes of blank NF, RuO2/NF, nano Fe2O3, Fe-MIL-100/NF, Fe-MIL101/NF, and Fe2Ni-MIL-101/NF. As shown in Fig. 3a, the LSV curves feature an oxidation peak at around 1.35 V, which can be attributed to the formation of Ni3+ in the OER process. In addition, this phenomenon is also considered to be corresponding to a larger oxidation current [48]. In Fig. 3b, it can be seen that Fe-MIL-101/NF (41.4 mV dec−1) and Fe-MIL-100/NF (41.5 mV dec−1) show lower Tafel slopes than blank NF (90.6 mV dec−1), RuO2/NF (138.6 mV dec−1), and nano Fe2O3/NF (45.9 mV dec−1). Fe2Ni-MIL-101/NF also exhibits favorable OER kinetics, with a Tafel slope of only 44.2 mV dec−1. In Fig. 3c, the blank NF curve shows negligible OER performance, requiring an overpotential of 375 mV at a current density of 20 mA cm−2. The as-prepared RuO2/NF gave a current density of 20 mA cm−2 at an overpotential of 323 mV, whereas Fe-MIL-101 and Fe-MIL-100 supported on NF showed greatly improved performances, requiring much lower overpotentials of 251 and 253 mV at the same current density. As shown in Fig. S6c-d† and Fig. S9a-b†, Fe-MIL-101 has a bigger specific surface area and a pore size than Fe-MIL-100. Therefore, Fe-MIL-101 exhibits better OER performance. Nano Fe2O3 also showed a good OER performance, requiring an overpotential of 268 mV at a current density of 20 mA cm−2. Bimetallic Fe2Ni-MIL-101/NF exhibited the best performance, requiring an overpotential of only 237 mV at a current density of 20 mA cm−2, much lower than those of previously reported highly active OER catalysts (Table S1†). Moreover, Fe2Ni-MIL-101/NF apparently exhibit superior OER performance than the simple mixture of nano Fe2O3 and NiO (Fig. S18), suggesting crucial interaction between Ni and Fe species. Fig. 3d shows that Fe2Ni-MIL-101/NF still performed efficiently, incurring only a small loss of current density, after 500 CV cycles at a scan rate of 50 mV/s. The obtained polarization curve is similar to the initial curve. In addition, the chronopotentiometric curve of Fe2Ni-MIL-101/NF at a current density of 20 mA cm−2 was also obtained (Fig. S12c†), which showed no discernible decline. SEM images acquired after long-term stability tests also indicated the good structural stability of the catalyst (Fig. S12a-b†). In investigating the reasons for the excellent oxygen evolution properties of these MOFs, some possible influencing factors were
2.2.6. Preparation of RuO2/NF electrodes Nickel foam (NF, 1 cm × 3 cm) was first activated with dilute hydrochloric acid (2.0 M HCl) and then repeatedly washed with deionized water and ethanol. RuO2 (2 mg) and a suspension of Nafion® (10 μL, 5 wt%) were then dispersed in a mixture of water (400 μL) and ethanol (50 μL) with sonication for 30 min. The catalytic ink thus obtained was dropped onto Ni foam (1 cm × 1 cm) and dried at 60 °C for 8 h. 2.3. Characterization of the materials X-ray diffraction (XRD) patterns were measured on a Bruker D8 Advance X-ray diffractometer employing Cu-Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) were performed with a Hitachi SU8010 instrument (Hitachi, Tokyo, Japan) equipped with a Horiba Xmax 50 EDX system. Brunauer–Emmett–Teller (BET) surface areas and pore size distributions of the catalysts were measured with a Micromeritics TriStar II 3020 instrument based on the adsorption of N2 at 77 K. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F instrument (Tokyo, Japan) operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed with a WSCAL-ab 220i-XL spectrometer (VG Scientific, Sussex, UK) employing Al-Kα radiation (hν = 1486.6 eV, 150 W). 2.4. Electrochemical measurements Electrochemical measurements were made with a Princeton electrochemical workstation (PARSTAT MC, Princeton, USA) in a standard three-electrode system. Catalysts loaded on NF were used as working electrodes (1 cm × 1 cm), with a mercury/mercury (II) oxide electrode (Hg/HgO) as the reference, and a Pt rod as the counter electrode. Each catalyst suspension was prepared by dispersing MOF (3 mg) in 0.265 mL of a solution comprising ultra-pure water (200 μL), ethanol (50 μL), and 0.5 wt% Nafion® suspension (10 μL) followed by ultrasonication for 30 min. Each suspension was then dropped onto pretreated NF and dried at room temperature. Linear sweep voltammetry (LSV) was carried out in 1.0 M KOH at a scan rate of 2 mV/s. All the potentials were calibrated as reversible hydrogen electrode (RHE) based on the formula: ERHE = EHg/HgO + 0.098 V + 0.059 pH. The OER overpotential (η) was obtained according to the following formula: η = ERHE-1.23 V. Long-term durability tests were performed by chronopotentiometric measurements. 3. Results and discussion Bimetallic MIL-101 was successfully synthesized, as verified by the following characterization data. A SEM image of bimetallic MIL-101 is shown in Fig. 1a and Fig. S10†, which reveals the same uniform octahedral particles as seen for Fe-MIL-101 (Fig. S1†). The PXRD patterns show that bimetallic MOFs with different molar ratios exhibit the same diffraction peaks as Fe-MIL-101 (Fig. 1c). Energy-dispersive X-ray spectroscopy (EDX) mappings and X-ray photoelectron spectra (XPS) confirmed that these bimetallic MOFs were made up of the same elements (Figs. 1d and 2). The EDX mappings reveal that the various elements were uniformly distributed, and ICP-AES results of the elemental contents were consistent with the molar ratio of the raw materials (Fig. S11†). TEM images revealed the crystal structure (Fig. 1b). With increasing Ni content, the diffraction peak and the specific surface area decreased (Fig. 1c and Fig. S9†), and the crystallinity of bimetallic electrochemical measurements supported the decreased surface area (Fig. S8†). X-ray photoelectron spectroscopy (XPS) was applied to analyze the surface compositions and surface electronic states of Fe-MIL-101 and Fe2Ni-MIL-101. Compared with Fe-MIL-101, it was clearly apparent that Ni was additionally present in Fe2Ni-MIL-101. The presence of Ni2+ was demonstrated by the high-resolution Ni 2p spectrum (Fig. 2a), 94
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Fig. 1. (a) SEM and (b) TEM images of Fe2Ni-MIL-101; (c) PXRD patterns of simulated Fe-MIL-101, Fe-MIL-101, Fe3Ni-MIL-101, Fe2Ni-MIL-101, FeNi-MIL-101, and FeNi2-MIL-101; (d–g) EDX elemental mappings of Fe2Ni-MIL-101.
Fig. 2. High-resolution XPS spectra of (a) Ni 2p, (b) Fe 2p, (c) C 1s, and (d) O 1s of Fe-MIL-101/NF and Fe2Ni-MIL-101/NF. 95
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Fig. 3. (a) Steady-state polarization curves and (b) Tafel plots of blank NF, RuO2/NF, nano Fe2O3, Fe-MIL-100/NF, Fe-MIL-101/NF, and Fe2Ni-MIL-101/NF. (c) The required overpotentials obtained from OER polarization curves at a current density of 20 mA cm−2. (d) LSV curves of Fe2Ni-MIL-101/NF before and after 500 CV cycles. All experiments were carried out in 1.0 M KOH.
Fig. 4. CV curves at different scan rates in the range of 0.32 and 0.40 V vs. RHE for (a) Fe-MIL-101/NF and (b) Fe2Ni-MIL-101/NF, plots of the charging current density against scan rates of (c) Fe-MIL-101/NF and (d) Fe2Ni-MIL-101/NF.
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References
explored. We used a CV method to calculate the double-layer capacitance (Cdl) [49], which is related to the electrode surface area (ECSA). The slope of a linear plot of non-faradaic capacitance current as a function of scan rate is equal to Cdl (Fig. 4a and b). The results showed Fe2Ni-MIL-101 to have the largest ECSA, with more active sites available at the solid–liquid interface. The Cdl evaluated for Fe2Ni-MIL-101/ NF is 10.36 mF cm−2, 1.97 mF cm−2 higher than that for Fe-MIL-101/ NF (8.39 mF cm−2) (Fig. 4c and d). The similarity of the ECSA values for these catalysts implies that they have similar numbers of active sites. Compared with the nano Fe2O3, Fe-MIL-100, and Fe-MIL-101, Fe2NiMIL-101 has a critical synergistic effect between Fe and Ni. Furthermore, Fe2Ni-MIL-101 contain Fe predominantly as Fe3+, under OER conditions, in principle partial-charge transfer between Fe and Ni2+/ 3+ , which could enhance the activity. The excellent OER performance of the Fe2Ni-MIL-101/NF catalyst is clearly influenced by the presence of Ni. In Fe/Ni-based OER catalysts, Ni is generally considered to be the reactive center for oxygen evolution [50], and the presence of Fe can facilitate the elevation of Ni to a higher valence state [51]. Moreover, during the process of OER, FeO6 and NiO6 octahedra within the MOF can be partially oxidized to Fe2O3 and NiOOH as active centers, which can promote the conversion of OH− into oxygen molecules [52]. The synergy between the uniformly distributed Fe–Ni active centers in the catalyst also effectively improves the OER performance. The superior performance of Fe2Ni-MIL-101 is also due to its intrinsic porous structure, and the large specific surface area exposes more active sites. In particular, during the OER process, some organic ligand structures are destroyed to form evenly distributed mesopores of about 10 nm (Fig. S19b†), which are conducive to the mass-transfer process, promote the desorption of gas generated in the process of water electrolysis, and enhance contact between water molecules and the active sites. At the same time, the carbon-based skeleton is abnormally stable, and it can effectively inhibit agglomeration of active sites, thereby reducing overpotential and improving stability.
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4. Conclusions In summary, we have successfully synthesized a series of bimetallic MIL-101 materials in one step by a solvothermal method. The prepared MOFs have been directly used as OER catalysts. By virtue of the intrinsic porous structure and synergy between the metals, bimetallic MIL-101 materials show excellent electrochemical performances. Among them, Fe2Ni-MIL-101 showed the best OER performance, requiring a potential of only 237 mV at 20 mA cm−2, with a small Tafel slope of 44.2 mV dec−1. Moreover, it also exhibited excellent stability upon chronoamperometric cycling for up to 40 h. Our research enriches the direct application of MOF materials as catalysts for water electrolysis. In future work, some other transition metals may be incorporated to further enhance the oxygen evolution performance through synergy between the metals. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21878202, 21476153), the Research Project Supported by Shanxi Scholarship Council of China (No. 2017-041), the Natural Science Foundation of Shanxi Province (No. 201801D121052), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.05.040. 97