Layered Bimetallic Iron-Nickel Alkoxide Microspheres as High

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Layered Bimetallic Iron-Nickel Alkoxide Microspheres as High-performance Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Mei Wang, Jing Jiang, and Lunhong Ai ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Sustainable Chemistry & Engineering

Layered Bimetallic Iron-Nickel Alkoxide Microspheres as High-performance Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Mei Wang, Jing Jiang, and Lunhong Ai* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, No. 1 Shida Road, Nanchong 637002, P.R. China

*Corresponding Author E-mail: [email protected] (L. Ai) Tel/Fax: +86-817-2568081

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ABSTRACT x-glycerolates

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Herein, we report a simple solvothermal process to synthesize a series of FexNi1(FeNiGly, x=0, 0.25, 0.5, 0.75 and 1) and explore them as high-performance

electrocatalysts for oxygen evolution reaction (OER) in alkaline media. These FeNiGly samples bear unique structures of hierarchical microspheres assembled by the interlaced ultrathin nanosheets or loosely aggregated nanoparticles, which ensure the electrocatalytic systems more efficient and accessible for the OER process. The FeNiGly exhibits an excellent OER activity with quite low overpotential of ~320 and ~380 mV to achieve the current density of 10 and 50 mA cm-2 in 1.0 M KOH solutions, respectively. Moreover, the FeNiGly also presents a good durability in alkaline electrolytes. The superior OER performance would be associated with the unique structures and strong electronic interaction between Fe and Ni in the FeNiGly.

KEYWORDS

Oxygen evolution; Electrocatalysis; Alkoxides; Microspheres

INTRODUCTION

The oxygen evolution reaction (OER) is an elementary half reaction and play a critical role in energy storage and conversion technologies, including rechargeable metal-air batteries, regenerative fuel cells, and water electrolysis cells for the hydrogen generation.1-3 A major challenge to efficiently accomplish these conversion process is the intrinsically sluggish kinetics of OER arising from the complex multistep proton-coupled electron transfer process, which brings about high overpotential to proceed reaction. At present, the iridium (IrO2) and ruthenium (RuO2) oxides have been regarded as the most famous and active electrocatalysts to improve the OER efficiency.4,5 However, their large-scale applications are largely limited due to their scarcity and high price. In this context, considerable research effort has been carried out toward exploring nonprecious metals and their derivatives as OER electrocatalysts.


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Currently, various alternatives based on earth-abundant first-row transition metals including oxides, hydroxides, sulfides, selenides, nitrides, and phosphides have been explored as efficient and potential OER electrocatalysts for replacement of precious catalysts.6-12 Of special note, all of these catalysts easily converted into corresponding oxyhydroxides under OER conditions, which are proposed as the key catalytically active metal species for the OER. Such oxyhydroxide species in situ formed on the surface of parent materials show more active than the corresponding ones produced through direct synthesis.13 Among these first-row transition metal materials, oxides/(oxy)hydroxides containing both Fe and Ni have emerged as promising class of electrocatalysts for boosting OER under alkaline conditions, which bear substantially higher activity than ones containing only Ni or Fe. Indeed, Bell and his coworker employed in situ Raman spectra to unravel the OER nature of electrodeposited Ni-Fe oxide films and confirmed that the presence of Fe altered the redox properties and local environment of Ni to enhance OER activity.14 Similarly, several intensive research works also observed the enhanced OER activity in FeNi-oxyhydroxides, because Fe as a synergy component not only induces the partial-charge transfer to activate Ni surrounded centers, but also endows catalytic systems with the increased charge-carrier density.15-17 Despite these exciting advances, we note that the current research mainly focuses on electrocatalytic OER systems of Fe,Ni-based oxides/(oxy)hydroxides to realize the synergistically enhanced OER activities. It is therefore essential to extend this rule for the design and search other FeNi-containing materials with the structural similarity to the Fe,Nioxides/(oxy)hydroxides. Metal glycerolate, as a typical metal alkoxide, is a very interesting system to achieve this purpose, because they provide a layer structure of stacked metal-oxygen sheets separated by bonded glycerolate anions, similar to the anion-intercalated hydroxides,18,19 which also has been


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used to design electroactive materials for different applications.20-24 Motivated by above considerations, we herein report a simple solvothermal process to synthesize a series of FexNi1-xglycerolates (FeNiGly, x=0, 0.25, 0.5, 0.75 and 1) that is composed of hierarchical nanosheetbuilt microspheres. It is unraveled that strong electronic coupling between Fe and Ni in the FeNiGly and partial in situ formed Fe(Ni)OOH active phase is favorable for OER catalysis. As a result, the developed FeNiGly exhibits superior OER activity with quite low overpotential of 320 and 380 mV at current density of 10 and 50 mA cm-2, respectively and long-term stability in alkaline electrolytes. To our best of knowledge, this is a first example on catalytically OERactive metal glycerolates. EXPERIMENTAL SECTIONS Materials Fe(NO3)3·9H2O, Ni(NO3)3·6H2O, KOH, glycerol and isopropanol were purchased from Aladin Ltd (Shanghai, China). All chemicals used in this study were of commercially available analytical grade and used without further purification. Synthesis of hierarchical iron nickel-glycerolate microspheres Hierarchical FeNi-glycerolate microspheres were synthesized by a facile one-step solvothermal method. Typically, 0.2 mmol of Fe(NO3)3·9H2O and 0.2 mmol of Ni(NO3)3·6H2O were dissolved in glycerol (8 mL) and isopropanol (40 mL) with continuously stirring for 30 min to obtain a clear solution. The mixture solution was then transferred to a Teflon-lined stainless steel autoclave and kept at 180 °C for 6 h. The product (abbreviated as Fe0.5Ni0.5Gly) was separated by centrifugation, washed with ethanol three times and dried at 80 °C for 24 h. The other FeNi-glycerolates were also prepared as control samples by similar procedure as for


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Fe0.5Ni0.5Gly by changing the Fe/Ni feeding ratios. Specifically, the feeding ratio of Fe to Ni is 1:0 for the preparation of FeGly, 3:1 for Fe0.75Ni0.25Gly, 1:3 for Fe0.25Ni0.75Gly, and 0:1 for NiGly, respectively. The total amount of metal precursors (Fe + Ni) was kept constant as 0.4 mmol. Characterization The powder X-ray diffraction (XRD) measurements on the synthesized samples were recorded on a Rigaku Dmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). The molecular structures of the synthesized samples were tested using a Fourier transform infrared spectrometer (FTIR, Nicolet 6700). The morphologies of the synthesized samples were observed with a Hitachi S4800 field scanning electron microscope (SEM) combined with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) measurements on the synthesized samples were recorded on a Perkin-Elmer PHI 5000C spectrometer using monochromatized Al Kα excitation. All binding energies were calibrated by using the contaminant carbon (C1S = 284.6 eV) as a reference. Electrochemical measurements The electrochemical measurements were performed on an electrochemical workstation (CHI 660E, Shanghai Chenhua) with a standard three-electrode system in an electrolyte solution of 1.0 M KOH, using a glassy carbon electrode (GCE) modified with various catalysts as the working electrode, platinum wire as a counter electrode and Ag/AgCl as a reference electrode. For preparing the working electrode, 5 mg of catalyst powders and 10 µL of a 5 wt% Nafion solution were ultrasonically dispersed in 1 mL of a mixed water-alcohol (3:1 v/v) solution to form a homogeneous ink. 5 µL aliquot of catalyst ink was drop-casted onto the glassy carbon electrode


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with the diameter of 3 mm for the electrochemical measurements. The potential was converted from Ag/AgCl to RHE by Nernst equation: ERHE = EAg/AgCl+ 0.197 + 0.059 pH

(1)

Linear sweep voltammetry (LSV) with an ohmic drop (iR) correction (ηcorrection = η - iR) were scanned from 0 to 1.0 V versus Ag/AgCl with a sweep rate of 5 mV s -1 to obtain the polarization curves and 1 mV s-1 for Tafel plots. The electrical double layer capacitor (Cdl) data was evaluated from double-layer charging curves using cyclic voltammograms (CVs) in a potential range of 1.12-1.22 V versus RHE at different scan rates (5-50 mV s-1). AC impedance measurements were evaluated at a potential of 1.623 V (versus the RHE) from 10-2 to 106 Hz with an AC amplitude of 5 mV. RESULTS AND DISCUSSION Bimetallic iron nickel glycerolate microspheres were prepared by solvothermal treatment of metal nitrate solution in glycerol and isopropanol. The phase structure of the FeNi-glycerolates (FexNi1-xGly) was determined by X-ray diffraction (XRD). As shown in Figure 1a, all the FeNiGly samples show a similar XRD profile, where the typical diffraction peaks are clearly observed at 10.8º, 19.7º, and 35.6º, corresponding to interlayer spacing lamellar structure with the characteristic of stacked metal-oxygen sheets separated by bonded glycerolate anions.18,19 These diffraction patterns are ascribed to the metal-glycerolate phase, which are in good agreement with those of the metal-alkoxides typically produced in glycerol.25-27 Estimated from low angle (10.8º), the interlayer spacing of FeNiGly is about 0.82 nm. In order to further reveal the molecular structure of the FeNiGly, the Fourier transform infrared (FTIR) spectroscopy were recorded. As shown in Figure 1b, all the samples almost exhibit identical spectra due to their


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inherently existing glycerol moiety, which are similar to those of reported metal-glycerolate.27-29 The absorption bands centered at 3400 and 2858 cm-1 are attributed to the hydrogen-bonded O−H groups stretching vibrations and the characteristic C−H stretching vibrations due to the presence of the organic alkoxide in the FeNiGly. The band at 1650 cm-1 arises from the H−O−H bending vibration, suggesting the existence of adsorbed water in the samples. The band at 1423 cm-1 corresponds to O–H bending vibrations, while the intense band at 1124 cm-1 is assigned to the C–O stretching vibrations. In addition, the band at 622 cm-1 is associated with metal-oxygen stretching vibrations. The thermal behavior of the FeNiGly was also investigated by using thermogravimetric analysis (TGA). A large weight loss of around 44.7% in the range of 25300 °C from the TGA curve suggests a number of organic components in the FeNiGly (Figure S1, Supporting Information). These results support the formation of iron nickel glycerolates.

Figure 1. (a) XRD patterns and (b) FTIR spectra of the FeNiGly.


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The morphologies of the synthesized FeNiGly samples were examined by scanning electron microscopy (SEM). Figure 2 and Figure S2 give the typical SEM images of the FeNiGly samples with different magnifications. As shown in Figure S2a, the FeGly sample is composed of a large quantity of high-quality and uniform microspheres with good dispersion. These spheres display a rough surface and a relatively narrow size distribution. The average size of FeGly spheres is estimated to be about 800 nm. Figure 2a and 2b reveal the detailed microstructure features of the FeGly sample. The FeGly spheres exhibit unique three-dimensional flower-like hierarchical architectures, which actually consist of numerous two-dimensional nanosheets nearly perpendicular to the spherical surface. The nanosheets of about 10 nm in thickness interweave together and interlaced like petals to ultimately form the unique open architectures with abundant voids and channels, which facilitates the electrolyte access during the electrocatalysis process. The Fe0.25Ni0.75Gly sample (Figure 2c and 2d) also consists of a larger number of nanosheet-built microspheres, similar to those of FeGly sample. When further increasing Ni amounts, the resulting Fe0.5Ni0.5Gly (Figure 2e and 2f) and Fe0.25Ni0.75Gly (Figure 2g and 2h) samples appear to be two kinds of microspheres, that is, close-packed nanoparticle-assembled microspheres and loosely packed nanosheet-assembled microspheres. Interestingly, the NiGly sample (Figure 2i and Figure S2f) bears different morphology from that of Fe-containing samples, which shows rather coarse surface and looks like a nanoparticle-aggregated coral microstructure.


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Figure 2. SEM images of FeNiGly samples: (a,b) FeGly, (c,d) Fe0.75Ni0.25Gly, (e,f) Fe0.5Ni0.5Gly, (g,h) Fe0.25Ni0.75Gly and (i) NiGly. Figure 3a is a general transmission electron microscopy (TEM) image of Fe0.5Ni0.5Gly, which reveals the sample consists of two types of monodispersed submicrospheres with solid structure, consistent with SEM observation. The nanosheet-assembled microspheres with rough surface can be clearly seen from enlarged image shown in Figure 3b. Meanwhile, the high-angle annular dark-field scanning TEM (HAADF-STEM) image in shown Figure 3c evidences the solid nature of spheres. Figure 3d is representative energy dispersive X-ray spectroscopy (EDS) analysis for the Fe0.5Ni0.5Gly sample. The signals from constituted elements including iron, nickel, oxygen, and carbon can be clearly detected. The HAADF-STEM elemental mapping images (Figure 3e)


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suggests the existed elements of C, O, Fe and Ni seem to be evenly distributed through the microspheres. Additionally, a typical high-resolution TEM (HRTEM) image shown in Figure 3f reveals that no obvious lattice fringes are observed for the Fe0.5Ni0.5Gly sample, confirming its amorphous structure.

Figure 3. TEM images (a, b), HAADF-STEM image (c), EDS spectrum (d), elemental mapping images (e) and HRETM images of Fe0.5Ni0.5Gly. The elemental composition and electronic structure of the Fe0.5Ni0.5Gly were determined by the X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum (Figure S3, Supporting Information) illustrates that the Fe0.5Ni0.5Gly is composed of Fe, Ni, C and O elements. Figure 4a shows the high-resolution XPS spectrum of the C 1s, which can be deconvoluted into two components at 284.6 and 286.2 eV, attributing to the C–C and C–O bonds from glycerol moiety in Fe0.5Ni0.5Gly, respectively. The high-resolution XPS spectrum of the O 1s (Figure 4b) can be fitted by two components at 529.8 and 532.2 eV, which are assigned to the C–O bonds from


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glycerol moiety and the metal–O bonds in Fe0.5Ni0.5Gly, respectively. Figure 4c shows the highresolution XPS spectra of Fe 2p. The binding energies of 710.7 and 724.9 eV with a satellite signal at 719.0 eV suggest the oxidation state of iron is primarily Fe3+ in Fe0.5Ni0.5Gly.30 Figure 4d is the high-resolution XPS spectra of Ni 2p, which shows the two strong peaks at 854.6 and 872.5 eV with their corresponding satellites, indicating the existence of Ni2+ in Fe0.5Ni0.5Gly.31,32 Of note, Ni 2p XPS spectrum of Fe0.5Ni0.5Gly shows a positive shift of ∼0.6 eV compared with NiGly (Figure S4, Supporting Information), indicating the electronic interaction between Ni and Fe in the Fe0.5Ni0.5Gly.33,34

Figure 4. XPS spectra of Fe0.5Ni0.5Gly: (a) C 1s, (b) O 1s, (c) Fe 2p, and (d) Ni 2p.


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To examine performances of the FeNiGly as OER electrocatalysts, the samples were drop-cast onto a glassy carbon electrode. The electrochemical tests were carried out in a standard threeelectrode system in 1.0 M KOH electrolyte solutions. Linear sweep voltammetry (LSV) measurements were performed to evaluate the OER activities of FeNiGly samples. Figure 5a shows the polarization curves of the FeNiGly in 1.0 M KOH solution at a scan rate of 5 mV·s-1 in comparison with commercial RuO2. As expected, the RuO2 reference exhibits the superior OER activity. Both the monometallic FeGly and NiGly present poor OER activity. It is noteworthy that the bimetallic FeNiGly samples exhibit greatly enhanced OER activity with much earlier onset potential and significantly lower overpotential for OER. As for FexNi1-xGly (x=0, 0.25, 0.5, 0.75 and 1) samples, the variation of OER activity along with the Fe content is not monotonous. The overpotential at the same catalytic current density decreases with increasing Fe content to 50% (Fe0.5Ni0.5Gly) and then increases upon further increase in the Fe content beyond 50%. This may be explained that the proper introduction of Fe species can induces the partial-charge transfer to render the Ni sites more OER-active, owing to a synergistically electronic interaction between Ni and Fe in the FeNiGly. However, in this case, as the FeGly exhibits the poor activity for the OER, excessive Fe (>50%) leads to a great increase in the Fe amount in the FeNiGly, which becomes a main contributor to OER and results in an opposing effect on the OER activity. This phenomenon is similar to recent observation in the other reported FeNi-based OER electrocatalytic systems.35-38 More specifically, to deliver a current density of 10 mA cm-2 (a critical metric in solar fuel production), the operating overpotential for Fe0.5Ni0.5Gly is ~320 mV, which is 150, 57, 26, and 85 mV smaller than that for FeGly, Fe0.75Ni0.25Gly, Fe0.25Ni0.75Gly, and NiGly, respectively (Figure 5b). More importantly, the Fe0.5Ni0.5Gly only demands overpotentials of ~380 and ~420 mV to accomplish a current


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density of 50 and 100 mA cm-2, respectively. This performance compares favourably to most of the reported Fe,Ni-based electrocatalysts, such as Ni0.9Fe0.1Ox film (~336 mV),39 Ni-Fe oxide/Ni foam (~377 mV),40 NiFe2O4 nanorods (~342 mV),41 Ni-Fe oxide (~328 mV),37 Ni-Fe hydroxides/N-doped graphene (~337 mV, 0.1 M KOH),42 exfoliated Ni-Fe hydroxide/defective graphene (~210 mV),43 NiFe layered double hydroxide (~302 mV),44 Ni@C (~370 mV),45 and Ni0.9Fe0.1@NC (~330 mV).38 Additionally, the Faradaic efficiency (FE) of generated oxygen was also measured in a gas-tight H-type electrochemical cell and result is shown in Figure S5. The FE is almost over 94% during 70 min of electrolysis. Meanwhile, the chronopotentiometric curve (Figure S6, Supporting Information) of the Fe0.5Ni0.5Gly exhibits a slight change after nearly 25 h of continuous measurement, suggesting the high durability of the Fe0.5Ni0.5Gly. To investigate the kinetics of the FeNiGly towards the electrocatalytic OER, the Tafel slope was determined by fitting the polarization curves into the Tafel equation: η = b log j + a

(2)

where η is the overpotential, j is the current density, and b is the Tafel slope. To avoid possible polarization effects and the capacitive current, we reduce the scan rate of the polarization curves to 1 mV s-1. The corresponding curves in Figure 5c shows that the Fe0.5Ni0.5Gly bear a smaller value of Tafel slope of 50 mV dec-1 in comparison to FeGly (262 mV dec-1), Fe0.75Ni0.25Gly (82 mV dec-1), Fe0.25Ni0.75Gly (71 mV dec-1), and NiGly (148 mV dec-1), which is slightly larger than the RuO2 reference (47 mV dec-1), demonstrating its faster OER kinetics than that of the others.


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Figure 5. (a) Polarization curves of the FeNiGly samples and RuO2 reference at a scan rate of 5 mV s-1 in 1.0 M KOH solution. (b) Comparison of overpotentials at a current density of 10 mA cm-2 afforded by the FeNiGly samples. (c) Tafel plots of the FeNiGly samples and RuO2 reference for OER electrocatalysis at a scan rate of 1 mV s-1 in 1.0 M KOH solution. (d) Nyquist plots of the FeNiGly samples measured at a potential of 1.623 V (versus RHE) in 1.0 M KOH solution. Electrochemical impedance spectroscopy (EIS) analysis on the FeNiGly was carried out to understand the difference on OER performances. The Nyquist plots measured at a potential of 1.623 V versus RHE are given in Figure 5d, which are well fitted to a two-time constant equivalent circuit model. It is obvious that the order of charge-transfer resistance for FeNiGly


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samples (FeGly: 85.8 Ω, Fe0.75Ni0.25Gly (7.5 Ω), Fe0.5Ni0.5Gly (3.1 Ω), Fe0.25Ni0.75Gly (4.4 Ω), and NiGly: 20.3 Ω) coincides with the order of OER activity, where Fe0.5Ni0.5Gly yields the

Figure 6. CVs of the FeGly (a), NiGly (b) and Fe0.5Ni0.5Gly (c) measured at different scan rates from 5 to 50 mV s-1. (d) Capacitive currents at 1.18 V (versus RHE) as a function of scan rate for estimating ECSA. smallest charge-transfer resistance during this process, thus achieving the most efficient charge transport for OER electrocatalysis. To further gain insight into the intrinsic activity of the FeNiGly samples, we estimate the electrochemical surface area (ECSA) through cyclic voltammetry (CV) measurements of the electrochemical double-layer capacitance (Cdl), which can be calculated from the linear relationship of the capacitive current against the scan rate. The CVs from 5 to 50 mV s-1 were recorded in a potential range of 1.12-1.22 V versus RHE for all


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the Fe0.5Ni0.5Gly samples (Figure 6a-c and Figure S7, Supporting Information). As clearly illustrated in Figure 6d, the Cdl values of FeGly, Fe0.75Ni0.25Gly, Fe0.5Ni0.5Gly, Fe0.25Ni0.75Gly, and NiGly are 0.43, 0.49, 0.63, 0.51, and 0.48 µF cm-2, respectively. Due to high Cdl value representing the large ECSA, the Fe0.5Ni0.5Gly thus has a largest ECSA among FeNiGly samples , agreeing with its OER performance. In order to shed light on the electrocatalytic OER process of the FeNiGly samples, a series of characterizations were conducted. We first analyze the CV cycles of the FeNiGly samples (Figure S8, Supporting Information). Clearly, the FeGly did not show any redox features, while the NiGly presents the distinct Ni2+/Ni3+ redox peaks with the oxidation of Ni2+ to Ni3+ at 1.45 V vs RHE. The introduction of Fe into the NiGly induces the positive shift of Ni2+/Ni3+ redox and the decrease in peak area, consistent with previous reports on Fe-Ni based electrocatalysts.14,36,46 This observation reveals a synergism between Ni and Fe centers, resulting in a strong electronic interaction between Ni and Fe to modify the electronic structure of the catalyst.33 This can be further verified by the fact that yellow colored Fe0.5Ni0.5Gly after OER electrocatalysis becomes darkened (Figure 7a). Inspired by these results, we speculate that the structure and chemical state of the catalyst surface may change during the reaction. As observed from SEM images (Figure S9, Supporting Information), the post-OER Fe0.5Ni0.5Gly basically retains its original microspherical morphology after OER catalysis, but XRD analysis on post-OER Fe0.5Ni0.5Gly (Figure 7b) illustrates that the Fe0.5Ni0.5Gly undergo the partial phase structure change, in which some new prominent peaks at about 34º and 60º associated with Fe(Ni)OOH can be clearly seen along with the dominant FeNiGly phase. Similarly, the peak of Ni 2p3/2 in the XPS spectrum (Figure 7c) of the post-OER Fe0.5Ni0.5Gly shifts to higher binding energies of ~1.1 eV and presents a slight broadening, implying the transformation of the Ni(II)O structure to Ni(II)(OH)2


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that is subsequent a partial oxidative conversion from Ni(II) to Ni(III) as a consequence of the formation of the active Ni(III)OOH phase under catalytic conditions.47 Consistent with previous observations,15,35,36 the Fe 2p XPS spectrum (Figure 7d) did not undergo an obvious shift, but showed a slight broadening under OER condition, confirming that Fe is consistently in oxidation state +3 and assigned to Fe species in FeOOH.48,49

Figure 7. (a) Optical photographs, (b) XRD patterns, (c) Ni 2p and (d) Fe 2p XPS spectra of the Fe0.5Ni0.5Gly before and after OER electrocatalysis. Based on the above analysis of the facts, we propose that the excellent OER performances of the FeNiGly are ascribed to the following aspects. First, the FeNiGly has an optimal structural characteristic for OER electrocatalysis. Metal glycerolates hold lamellar structure with


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considerable interlayer spacing, which gain enough space for accommodation of reactants. Also, the bonded glycerolate anions can flip their coordination mode during the redox switching process of metal cations and facilitate the OER process.50,51 In viewpoint of microstructure, ultrathin-nanosheet assembled microspheres endows them with unique open structure bearing abundant voids and channels, which ensures the effective mass transport of electrolyte and gaseous products and exposes more accessible active sites during the electrocatalysis process. Second, it is generally accepted that water adsorption on the active site of the catalyst surface is the elementary step for water oxidation. The FeNiGly is a typical alkoxide, which render it very hydrophilic with a small water contact angle of 52.3° (Figure S10, Supporting Information) caused by oxygen-containing units including glycerolate anions and metal-oxygen sheets, thus facilitating the easy adsorption of water onto catalyst surface. Third, strong electronic coupling between Fe and Ni in the FeNiGly evidenced by CVs and XPS analysis, and partial in situ formed Fe(Ni)OOH active phase is favorable for OER catalysis. Incorporation of Fe-sites into NiOOH structure have more optimal OER intermediate energetics, thus reducing the required overpotential for OER.15,52 CONCLUSIONS In summary, we have synthesized a series of FexNi1-x-glycerolates (FeNiGly, x=0, 0.25, 0.5, 0.75 and 1) nanosheet-assembled microspheres by a facile solvothermal process. The structural feature makes the systems efficient and accessible for the eletrocatalytic OER process. The FeNiGly shows pronounced activity towards OER, as evidenced by the observable low overpotential to achieve the large current density and a long-term durability in alkaline electrolytes. The superior OER performance would be ascribed to the unique structures and strong electronic interaction between Fe and Ni in the FeNiGly.


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ASSOCIATED CONTENT Supporting Information: TGA curves, SEM images, survey and Ni 2p XPS spectrum, Faradic efficiency measurement, chronopotentiometric curve, CVs and water contact angle measurement of FeNiGly. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21771147, 51572227), Sichuan Youth Science and Technology Foundation (2013JQ0012), Major Cultivating Foundation of Education Department of Sichuan Province (Grant 17CZ0036), Meritocracy Research Funds of CWNU (17YC007, 17YC017) and Innovative Research Team of CWNU (CXTD2017-1). REFERENCES (1)

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Synergetic hierarchical architectures of FeNi-glycerolate microspheres have been explored as robust electrocatalysts for oxygen evolution reaction


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Layered Bimetallic Iron-Nickel Alkoxide Microspheres as High

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