Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Deep Eutectic Solvent-Mediated Hierarchically Structured Fe-Based Organic−Inorganic Hybrid Catalyst for Oxygen Evolution Reaction Chenyun Zhang, Baohua Zhang, Zhonghao Li,* and Jingcheng Hao Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, No. 27, Shanda Nanlu, Jinan 250100, P.R. China
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S Supporting Information *
ABSTRACT: The genuine active sites toward oxygen evolution reaction (OER) are usually generated via the in situ transformation of initial electrocatalysts during anodic oxidation. In principle, all the catalysts possessing such a transition might be active toward OER. However, current OER electrocatalysts are still mainly limited in metal oxides, (oxy)hydroxides, phosphides, and chalcogenides, etc. Little attention is paid to the rational design of electrocatalysts based on the transformation-induced strategy. Herein, we demonstrated a Fe-based organic−inorganic hybrid strategy for the construction of OER electrocatalysts based on the in situ transformation. Hierarchically three-dimensional (3D) iron alkoxides of glycerol were synthesized in choline chloride (ChCl)/glycerol deep eutectic solvent (DES). During the OER, the as-prepared Fe-based organic− inorganic hybrid can transform into FeOOH, an active species for electrocatalysis, exhibiting sustainable electrocatalytic performance. It only requires 280 mV to offer a current density of 10 mA cm−2 and has a small Tafel slope of 47 mV decade−1 in 1 M KOH solution, outperforming most reported Fe-based catalysts. This work provides the concept of DES-mediated organic−inorganic hybrid materials as transformation-induced catalysts toward OER, thus casting new light on the rational design of advanced electrocatalysts for water splitting. KEYWORDS: deep eutectic solvent, iron alkoxide, organic−inorganic hybrid, oxygen evolution reaction, electrocatalyst
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INTRODUCTION Iron(III) alkoxides are one kind of well-known Fe-based organic−inorganic hybrid, in which iron alkoxides of ethylene glycol and glycerol are widely reported.1−3 Though iron alkoxide hybrids have been studied since the mid-20th century,4 the synthesis approaches and applications have not drawn enough attention for many years. Until now, iron alkoxides still follow the traditional synthesis method which uses ferric ion to react with alcohols and often need surfactants or additives to control their structures.5,6 Their application fields are relatively narrow, which are mainly limited in acting as precursors of iron oxide or iron (oxy)hydroxide6,7 and catalysts for some organic reactions.8,9 The dramatic concern over clean energy and the environment has given rise to widespread study of electrochemical water splitting, which is an important way to obtain hydrogen.10,11 The water splitting to hydrogen must be accompanied by the oxygen evolution reaction (OER), a kinetically sluggish process. In the ongoing process of exploration for efficient and cheap catalysts, earth-abundant iron group elements have been generally acknowledged to be promising alternatives to noble metal electrocatalysts for OER.12−14 Among others, Fe-based nanomaterials are extremely appealing catalysts. The reasons include the following: (1) Fe is the most abundant metallic element on the earth. So, it is cheaper than the other iron group elements; © XXXX American Chemical Society
(2) Fe is less toxic than Co and Ni and has inherent sustainability and interesting physicochemical properties; and (3) Fe possesses rich redox properties for O2 activation.15 Nowadays, the efforts on Fe-based electrocatalysts for water splitting focus mainly on iron oxide, iron (oxy)hydroxide, iron phosphide, and so on.16−21 In fact, the genuine active sites toward OER are usually generated via the in situ transformation of the initial electrocatalysts during anodic oxidation.22−26 Although plenty of work concentrate on the mechanism of true active sites of the catalysts during OER,27,28 little attention is paid to the rational design of electrocatalysts based on the transformation-induced strategy. As a family of materials, organic−inorganic hybrids are rich in structure and components.29,30 Specifically, some organic− inorganic hybrids can transform into corresponding (oxy)hydroxides during OER, which is well-known active site during the electrolysis. However, the rational design of conventional organic−inorganic hybrids containing Fe elements as catalysts which can effectively generate active sites toward OER is rarely reported. Therefore, it is very urgent to develop advanced OER electrocatalysts based on these kinds of hybrids. Iron alkoxide, an ordinary but unnoticed organic−inorganic hybrid, might be Received: January 28, 2019 Accepted: April 8, 2019
A
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 1. XRD pattern (a), FTIR spectra (b), XPS spectra (c), and TG analysis (d) of iron alkoxide of glycerol organic−inorganic hybrid (Fe_DES).
for the rational engineering of advanced catalysts in water splitting.
a promising catalyst for water splitting. However, it has never been explored for electrocatalytic performance for OER until now. Deep eutectic solvents (DESs) are systems obtained by mixing two or even three high melting-point materials, driving to liquid by hydrogen bonding interactions between them.31−33 As a new type of ionic liquid analog, DESs have unique features, such as low cost, nontoxicity, and biodegradability.34 DESs are found to be suitable for synthesizing functional micro/nanomaterials as benign solvents. It is interesting that they can modulate nucleation and crystallization of micro/nanomaterials as template providers, offering advanced structures and properties.35,36 Taking the synthesis of iron oxide nanostructures as examples, Fe3O4 and Fe2O3 have been fabricated in ChCl/urea DES.37−39 Their phases, size, and morphologies are well-controlled by DESs. Moreover, the DESs can also participate in the synthesis of nanomaterials as reactants. Mu and co-workers synthesized urchin-like NiCo2S4 nanostructures40 and crumpled Fe3S4 nanosheets41 via a onestep solvothermal synthesis method in polyethylene glycol 200/thiourea. Therefore, DESs play multiple roles for the controlled synthesis of advanced micro/nanomaterials. In this work, we demonstrated a Fe-based organic−inorganic hybrid strategy for the construction of OER electrocatalysts based on the transformation-induced design via DES-mediated approaches. Using ChCl/glycerol DES as a reaction medium without any surfactants or additives, a hierarchically threedimensional (3D) iron alkoxide of glycerol hybrid nanostructure was successfully obtained. This hybrid presents excellent electrocatalytic efficiency and high stability for OER in alkaline medium along with the transformation. This DES-mediated synthesis strategy together with the concept of organic− inorganic hybrid as an OER catalyst might open up the avenue
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EXPERIMENTAL SECTION
Materials. Choline chloride (purity, 99.5%), glycerol (C3H8O3) (purity, 99.5%), hydrated iron nitrate (Fe(NO3)3·9H2O) (purity, 98.5%), and iridium oxide (IrO2) (purity, 99.9%) were bought from Aladdin industrial Corp. Ni foam (denoted as NF, and its bulk density was 0.23 g/cm3 with thickness of 0.5 mm) was purchased from Changsha Lyrun Material Co., Ltd. Synthesizing Iron Alkoxide (Denoted as Fe_DES) in DES. A ChCl/glycerol (molar ratio of 1:2) DES was synthesized according to the literature.42,43 The typical synthesis procedure of iron alkoxide is as follows. 121.2 mg (0.3 mmol) Fe(NO3)3·9H2O was dissolved in 3 mL pre-prepared DES by ultrasonic treatment for 10 min and then was transferred into a Teflon-lined stainless autoclave, holding at 150 °C for 11 h. After reaction, the autoclave was naturally cooled to ambient temperature. The prepared iron alkoxide was washed with ethanol repeatedly to remove organic byproducts. Finally, the product was put into a vacuum drying oven under room temperature overnight. For comparison, the synthesis of Fe-based catalyst (Fe_Gly) in pure glycerol was performed with a similar procedure as mentioned above, except that 3 mL glycerol was used. Material Characterization. X-ray diffraction (XRD) characterization was conducted on Bruker D8A A25 X-ray diffractometer with Cu Ka radiation at 40 kV and 40 mA. Transmission electron microscopy (TEM) characterization was examined on a JEM 1400 TEM. Scanning electron microscopy (SEM) was conducted on a ZEISS MERLIN Compact. Energy-dispersive X-ray analysis (EDX) and EDX mapping were conducted on OXFORD-instruments XMaxN. X-ray photoelectron spectroscopy (XPS) was measured using a photoelectron spectrometer ESCALAB 250 XI. Fourier transform infrared spectroscopy (FTIR) was characterized using Thermo Nicolet iS50 FT-IR. Thermogravimetric analysis (TGA) was performed on SDT Q600 thermal analyzer under oxygen heating from room temperature to 800 °C with 5 °C min−1. The static contact angles (SCAs) were performed using OCA 15EC (Dataphysics, B
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 2. SEM images of low and high magnification (a−c) and EDX mapping (d) of the as-synthesized Fe_DES. Germany) by the sessile drop method using a deionized water droplet (2.5 μL) to place on the material surfaces. Electrochemical Measurements. The electrochemical test was measured by an electrochemical workstation (CHI model 760E). In our experiment, Ni foam modified with the as-obtained Fe-based catalyst acted as the working electrode, a saturated calomel electrode (SCE) was employed as reference electrode while a graphite rod was used as counter electrode. Preparing the Working Electrode. First, the Fe-based electrocatalyst (5.0 mg) and 5% Nafion solution (40 μL) were dispersed in 1 mL ethanol, and the mixture was ultrasonicated for 20 min. Second, the pretreatment of Ni foam was as follows. Ni foam (1 cm × 0.5 cm) was cleaned in 1 M HCl for 10 min by ultrasonic treatment with subsequent washing using ethanol and deionized water for several times. Third, the above catalyst dispersion was modified on pretreated NF according to the mass loading of 0.6 mg cm−2. Electrochemical Characterization. Linear sweep voltammetry (LSV) was implemented at 5 mV s−1 in different concentrations of KOH solution. Stability measurements were performed by cyclic voltammogram (CV), and chronoamperometry (CA) measurement in 1 M KOH solution. The CV was obtained at 100 mV s−1 for 12 000 cycles and recorded polarization curves every 2000 cycles. Chronoamperometry studies were measured at 1.53 and 1.56 V for 48 h, respectively. The electrochemical impedance spectroscopy (EIS) measurements was obtained with the frequency range from 0.01 Hz to 100 kHz, and their alternating voltage is 5 mV. Electrochemical active surface areas (ECSA) of OER electrocatalysts were tested through measuring the double-layer capacitance by CV treatment between 1.37 and 1.47 V (vs RHE) with a scanning rate from 10 to 100 mV s−1. All the data was with respect to the reversible hydrogen electrode (RHE). The turnover frequency (TOF) was obtained according to the formula based on the literature.44
is chosen as a benign reaction medium, in which glycerol is not only a hydrogen bonding donor but also a reactant. After Fe(NO3)3 is added into DES, the hydrogen bonding is formed between nitrate anion and glycerol, while Fe3+ is strongly chelated by glycerol. Therefore, Fe(NO3)3 is readily integrated into the DES matrix by hydrogen bonding and coordination bonding. The ability that DES as a medium effectively brings the reactants together with a prestructuring effect has been demonstrated by some reports.32,36 Then the yellow−green iron glycerate hybrid (Fe_DES) is fabricated at 150 °C for 11 h through a solvothermal method. The as-obtained iron alkoxide was characterized using X-ray diffraction (XRD) pattern. In Figure 1a, diffraction peaks at 2θ = 10.8°, 20.8°, and 62.7° are consistent with metal alkoxide reported previously, in which the strong diffraction peak at 10.8° is indexed to the characteristic (001) crystal face of iron alkoxide.1,4,6,45 Fourier transformed infrared (FTIR) spectra further reveal the compositions of product (Figure 1b). The broad band at 3407.2 cm−1 can be attributed to the OHstretching vibration, which comes from Fe_DES, residual glycerol, or adsorbed water molecules. The absorption band lying in the 2858.9 cm−1 domain is characteristic of the C−H stretching mode. The strong peaks at 1049.6 and 832.1 cm−1 correspond to CH2−, CH2−OH, or C−C. All of these peaks suggest the existence of the organic constituents in the asprepared product. The peaks at 707.7 and 596.7 cm−1 are attributed to Fe−O vibration of iron alkoxide, confirming the formation of iron alkoxide. As illustrated in Figure 1c, the X-ray photoelectron spectra (XPS) agree well with the surface peak for Fe(III) 2p. The 710.34 eV peak is consistent with Fe(III) 2p3/2, accompanied by a satellite peak centered at 716.99 eV. The 724.37 eV peak with a satellite peak centered at 730.90 eV is assigned to Fe(III) 2p1/2. This result confirms the existence of Fe(III). In Figure S1 of the Supporting Information (SI),
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RESULTS AND DISCUSSION The iron alkoxide of glycerol nanospheres were fabricated using a DES-mediated strategy. The ChCl/glycerol (1:2) DES C
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 3. Polarization curves (a), Tafel plots (b), polarization curves before and after cyclic voltammogram treatment (c), and chronoamperometry study under current densities of 20 as well as 100 mA cm−2, respectively, (d) of the Fe_DES catalyst in 1 M KOH solution.
(Figure S5 of the SI) displays that the mole ratio of C:O:Fe fits well with the iron glycerate hybrid, further confirming the formation of iron alkoxide. In the literature,7 an iron alkoxide of ethylene glycol with flower structure was synthesized via ethylene glycol mixed with oleylamine. However, the morphology and composition is different from ours since our product is the iron alkoxide of glycerol with hierarchical structure. To explore the template role of DES, a control experiment using pure glycerol as medium was tested under the same reaction conditions. Unlike in DES, the product from glycerol (Fe_Gly) is revealed to be amorphous by the XRD pattern (Figure S6a of the SI). The morphology is irregular as observed from both SEM (Figure S6b, c of the SI) and TEM images (Figure S6d−f of the SI). The difference in structure between Fe_DES and Fe_Gly suggests that DES has the ability to modulate the growth process and structure; whereas in glycerol, a common neutral organic medium, it is very difficult to induce controllable growth. It is known that hydrogen bonding, existing in both ionic and molecular environments, constructs the unique structure of DES.36 It is this kind of special feature of DES that provides the prestructuring effect and induces the product to grow along the preferred crystallographic directions. This phenomenon has been demonstrated by many studies.6,31−33,35,37 Therefore, DES plays a strategic role on the shape of iron alkoxide, causing formation of the hierarchically seedlike 3D structure. The template effect of DES is further confirmed via systematically studying reaction intermediates at various stages. In the initial 1 h, iron ion first coordinates with glycerol involved in DES, driving the nucleation of primary particles. From SEM images (Figure S7a, b of the SI), we can see that 3D nanospheres are obtained with nanobulges uniformly on the surface, and their size is 158 ± 11 nm in diameter (Figure S7c of the SI). The XRD pattern reveals that the nanospheres
the O 1s peak is located at 530.6 eV, which agrees with the metal−oxygen band.46 The thermal behavior was investigated by thermogravimetric analysis (TGA), as illustrated in Figure 1d. From room temperature to 800 °C, the weight of asprepared product has sharp decline from about 200 to 400 °C along with the corresponding mass loss of near 40%, which can be ascribed to the decomposition of organic constituents in iron alkoxide of glycerol.47,48 This value basically matches the theoretical mass loss (44%) of Fe-DES converted to Fe2O3, which further confirms the successful formation of iron alkoxide of glycerol. The brownish red residual after TG measurement is demonstrated to be hematite (α-Fe2O3) (JCPDS card 33-0664) via XRD pattern (Figure S2 of the SI). These results indicate that the iron alkoxide organic− inorganic hybrid is successfully fabricated in the ChCl/glycerol DES. Figure 2a−c show the typical scanning electron microscopy (SEM) images of Fe_DES. Obviously, as-synthesized Fe_DES exhibits 3D nanospheres with some nanobulges on the surface, just like the seed of sycamore. The diameter of nanospheres is 566 ± 20 nm (Figures 2a, b, and S3 of the SI). A broken nanosphere is observed in the high-magnification SEM image (Figure 2c), which provides the detailed morphology. We can see that it possesses hollow structure with a void space. The wall thickness observed is about 280 nm, and its cavity is approximately 140 nm in diameter. The SEM images further reveal that the outer layer is constructed by a large number of nanowedges. They crossedly self-assemble toward the center of the sphere, forming a 3D nanostructure. The structure of Fe_DES can be further confirmed by transmission electron microscope (TEM) (Figure S4 of the SI), whose rough surface of the nanosphere corresponds to nanowedges in the SEM images. Energy-dispersive X-ray (EDX) elemental mapping reveals that three elements (C, O and Fe) are homogeneously distributed in as-synthesized iron alkoxide (Figure 2d). EDX D
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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performance for water splitting. The better OER performance of Fe_DES in comparison with Fe_Gly could be attributed to the 3D structure of Fe_DES, which assures adequate contact between electrolyte and electrode and provides enough accessible channels for O2 escape and electrolyte diffusion. The turnover frequency (TOF) was calculated at 1.58 V, and it was found that Fe_DES displayed a TOF of 0.114 s−1. Furthermore, the double-layer capacitance (Cdl) was measured to assess the electrochemically active surface area (ECSA). The Cdl of Fe_DES is found to be 64 mF cm−2 (Figure S12 of the SI), better than Ni 2 P(O)/Fe 2 P(O), 54 NiFe-LDH, 55 (NixFe1−x)2P,56 etc. Therefore, this unambiguously demonstrates that Fe_DES has effective accessibility of active surface areas depending on its special hierarchical nanostructure and, then, can offer superior OER rates. To understand interfacial properties and the electrode kinetics of the catalysts, we measured the electrochemical impedance spectra (EIS). As shown in Figure S13 of the SI, the charge transfer resistance (Rct) of Fe_DES is 4.1 Ω, illustrating that 3D nanospheres own the high charge-transfer kinetics and low charge transfer resistance. In order to further study the electrocatalytic properties of Fe_DES, we replace the Ni electrode with glassy carbon electrode (GCE) using the same loading mass of catalyst (Figure S14 of the SI). The experiment result manifests that Fe_DES/GCE requires higher overpotential (η10 = 380 mV) (Figure S14a and Table S2 of the SI), and exhibits higher Tafel slope (72 mV dec−1) than Fe_DES/NF (Figure S14b and Table S2 of the SI). It can be inferred that the decrease in catalytic activity is caused by poor conductivity of Fe_DES catalyst, just like many other Fe-based compounds reported suffered from poor conductivity. 57,58 However, mixing conductive carbon black (CB, Vulcan XC-72) into iron alkoxide (the mass ratio of Fe_DES: CB of 2:1) modified on GCE (denoted as Fe_DES+CB/GCE) can lower the overpotential to η10 = 320 mV with Tafel slope to 54 mV dec−1 (Figure S14 and Table S2 of the SI). These comparative experiments prove that Fe_DES have the inherently high catalytic efficiency for OER. The electrocatalytic durability has been considered to be a very important aspect in catalyst evaluation since stability is a basic element for industrial application. The durability of Fe_DES during OER was explored in 1 M KOH solution by cyclic voltammogram (CV) and chronoamperometry (CA) measurement, respectively (Figure 3c and d). The CV treatment was tested at 100 mV s−1 for 12 000 cycles. During this process, the polarization curve is measured every 2000 cycles. Note that the initial in Figure 3c means the curve after 40 cycles of linear sweep voltammetry. As shown in Figure 3c, the polarization curves remain almost coincident. Compared with the electrochemical durability measured at the number of scanning cycles (1000−3000 cycles) in the literature,59,60 Fe_DES displays superior electrocatalytic stability in an alkaline environment. The chronoamperometry study also demonstrates its high stability. We conduct the chronoamperometry study under two current densities, 20 and 100 mA cm−2, respectively. Figure 3d suggests that Fe_DES catalyst can maintain these two types of constant current densities for 48 h, in which almost no obvious decay is observed. So Fe_DES can be used as excellent electrocatalysts with extraordinarily electrochemical durability for OER. The excellent OER electrocatalytic activity and durability of Fe-based organic−inorganic hybrid catalysts urge us to
are iron alkoxide of glycerol hybrid (Figure S8 of the SI), that is, Fe_DES nanomaterials have already formed, growing in the initial 1 h, indicating that the formation process is very rapid. With extension of time to 8 h, the nanospheres become larger than the initial 1-h product, and the size increases to 316 ± 15 nm (Figure S7d−f of the SI). Moreover, the surface of the nanospheres is rougher and evolves to the seedlike structure. Meanwhile, it starts to grow into hollow, as observed from a broken nanosphere in high-magnification SEM image (Figure S7e of the SI). Growing to 11 h, the typically 3D nanospheres discussed above are obtained (Figure 2a−c). The OER electrocatalytic activities of the DES-mediated iron glycerate hybrid were investigated modified on Ni foam in 1 M KOH solution. For comparison, control experiments to measure the catalytic behavior of bare Ni foam and Fe_Gly were conducted. Polarization curves were obtained by employing linear sweep voltammetry (LSV) as presented in Figure 3a. It is found that the bare Ni foam as electrode displays a large overpotential (η10 = 380 mV) to reach current densities of 10 mA cm−2. Figure 3a presents the polarization curve of Fe_Gly after activation with 40th LSV. We found that it has an overpotential of 330 mV to offer 10 mA cm−2, which is better than many reported Fe-based catalysts, such as Fe2O3/ carbon nanotube,49 Fe1.9F4.75·0.95 H2O,50 Co-doped/Fe3O4,51 and so on. The Fe_Gly does not show the oxidation peak between 1.4 and 1.5 V. However, the oxidation peak between 1.4 and 1.5 V appears in the polarization curve of Fe_Gly after activation with the 70th LSV (Figure S9 of the SI). As expected, a significant improvement in the oxygen evolution activity has been observed when Fe_DES is used as the electrocatalyst. In Figure S10 of the SI, the polarization curves indicate an activation process from the 1st to 40th polarization curves for Fe_DES due to its changes in chemical composition and morphology in the alkaline electrolyte, which will be discussed in the following. After 40 cycles of polarization curves, a stable polarization curve is reached, and the overpotential only required 280 and 300 mV to achieve 10 and 20 mA cm−2, respectively. It is worth mentioning that offering a larger current density of 200 mA cm−2 only needs 350 mV. The electrocatalytic performance is more perfect than that of Fe-based catalysts reported previously (Table S1 of the SI). Moreover, Fe-DES is superior to IrO2 (η10 = 327 mV) on Ni foam as an OER electrocatalyst (Figure 3a), further indicating its efficient electrocatalytic activity. The Tafel plots were further explored (Figure 3b). The bare Ni foam as an electrode gives a high Tafel slope of 132 mV dec−1 under our experimental conditions. After modification with Fe_Gly on Ni foam, the Tafel slope can lower to 53 mV dec−1. Excitingly, Fe_DES can exhibit the lower Tafel slope of 47 mV dec−1, which is smaller than that of Fe2O3/CNT (62 mV dec−1),52 Fe(TCNQ)2/Fe (110 mV dec−1),26 Fe@C-NG/NCNTs (163 mV dec−1),53 and IrO2 (62 mV dec−1). It is well-known that a smaller Tafel slope is more beneficial for practical applications as it remarkably increases the OER rate. It can be inferred from the above results that the catalytic activity of Fe_DES is superior to that of Fe_Gly. In addition, Fe_DES can also offer the high catalytic performance in low pH environments. In 0.1 M KOH electrolyte (pH = 13), overpotential η10 is 350 mV, and the Tafel slope is 102 mV dec−1 (Figure S11 of the SI), which is comparable to the values of other reported Fe-based OER electrocatalysts (Table S1 of the SI). This is the first time we have used a pure iron alkoxide of glycerol to catalyze OER, and fortunately, we find it possesses promising electrocatalytic E
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 4. FTIR pattern (a), XRD pattern (b), and XPS spectra (Fe 2p (c), O 1s (d)) for conversion product of Fe_DES catalyst after catalysis at 20 mA cm−2 under 1.53 V for 48 h.
Fe_DES could transform into Fe2O3 instead of FeOOH (Figure S15 of the SI). The generation of new species generally accompanies structural evolution. As shown in Figure 5, the SEM images
systematically probe the transformation of the catalyst in composition and morphology during the electrocatalytic process. To detect the transforming species of catalyst after the catalytic reaction with potential sweep, the electrode modified with Fe_DES was ultrasonic treated after catalyzing 48 h at 20 mA cm−2 under 1.53 V (Figure 3d) and residue powder of the Fe_DES catalyst was obtained. FTIR spectra of the transforming species show that there is no characteristic absorption band of C−H stretching mode in the 2850−2960 cm−1 domain (Figure 4a), indicating iron alkoxide disappears during the OER. The transforming species was characterized via XRD pattern. As shown in Figure 4b, diffraction peaks at 2θ = 44.39, 51.85, and 76.49 are indexed to the (111), (200), and (220) planes of Ni (JCPDS card 87-0712),61 which should peel off from Ni foam during the ultrasonic treatment. The other diffraction peak positions in Figure 4b clearly reveal that the transforming species after catalysis is lepidocrocite (γFeOOH) (JCPDS card 44-1415).62 Further evidence for the formation of FeOOH can be obtained from XPS spectra.62,63 XPS peaks located at 711.08 eV with satellite 718.87 eV as well as 724.70 eV with satellite 732.33 eV refer Fe 2p3/2 and Fe 2p1/2 of FeOOH, respectively (Figure 4c). Peaks of O 1s are located at 530.34 and 531.85 eV, respectively, which is consistent with O 1s of FeOOH (Figure 4d). The above results show that as-prepared iron alkoxide is transformed in situ into FeOOH under the electrocatalytic process. In 2018, Lee and co-workers have reported the transformation from iron alkoxide to FeOOH treated in KOH solution.7 In our work, Fe_DES reacts with the alkaline electrolyte via adsorbing OH−, forming iron hydroxide, and in turn forms FeOOH through dehydration. For comparison, we soaked Fe_DES into 1 M KOH solution for different time. It was found that
Figure 5. SEM images of low (a) and high (b) magnification for conversion product of Fe_DES catalyst after catalysis at 20 mA cm−2 under 1.53 V for 48 h.
reveal that transformed species after OER basically maintain the original morphology and size, of which the nanosphere is about 538 ± 30 nm in diameter (Figure S16 of the SI). However, the catalyst undergoes reconstruction of surface morphology during electrocatalytic treatment, and the seedlike structure evolves to the wrinkled surface. The conversion of composition and morphology plays an important role in realizing the excellent activity and stability of Fe_DES. It can be attributed to several reasons.23,64 In composition terms, iron (oxy)hydroxide, a new species generated in situ during OER, has been demonstrated to be an active OER electrocatalysts by many studies.7,65,66 Therefore, FeOOH species originated from iron alkoxide will activate and continuously catalyze the OER. In morphology terms, the wrinkled morphology which appeared through the F
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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self-adjustment of structure has been proven to be conducive to catalytic behavior.66 This structure can ensure adequate contact between electrolyte and electrode, while providing enough accessible channels for O2 escape and electrolyte diffusion. In addition, it is reported that suitable wettability of the catalyst is favorable for catalytic reaction.67 After the chronoamperometry study, the electrode covered with the residue of Fe_DES catalysts presents static contact angles (SCAs) of 50.5° (Figure S17 of the SI), indicating its excellent hydrophilic property. The highly hydrophilic surface will further facilitate the catalytic activity due to the easy access of electrolyte to the catalyst surface. Obviously, the change in composition and morphology of Fe_DES results in the combination of higher amounts of available active sites, better charge transport, and stronger interaction with the electrolyte, availing electrocatalytic performance toward OER.
CONCLUSION In summary, based on the in situ transformation-induced strategy, we developed an iron alkoxide organic−inorganic hybrid via the DES-mediated synthesis process as an OER electrocatalyst. Hierarchical 3D iron alkoxide with a special seedlike structure is created in ChCl/glycerol DES, which acts as not only a benign medium but also a reagent as well as a template. For the first time, iron alkoxide is employed as an electrocatalyst toward OER. DES-mediated iron alkoxide can in situ transfer into active species FeOOH during the electrocatalytic process. The transforming species can sustainably catalyze OER, driving excellent catalytic performance along with the high durability. The present DES-mediated synthesis strategy together with the concept of organic− inorganic hybrid materials as OER catalysts not only provides iron alkoxide as a new kind of OER electrocatalyst but also offers a guideline for the rational design of advanced transformation-based electrocatalysts for water splitting. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00197.
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REFERENCES
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Article
Additional data for XPS spectra, XRD pattern for residue after TG analysis, particle size distribution, TEM images, EDX image, SEM images, polarization curve, the relationship of number of polarization curves and catalytic activity, comparison of OER performance, cyclic voltammograms, electrochemical impedance spectra, and water contact angle (PDF)
AUTHOR INFORMATION
Corresponding Author
*Fax: (+86) 531-88564750. Tel.: (+86) 531-88363821. Email:
[email protected]. ORCID
Zhonghao Li: 0000-0003-0699-300X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant No. 21673128). G
DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsaem.9b00197 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
