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Ordered mesoporous spinel CoFe2O4 as efficient electrocatalyst for the oxygen evolution reaction
Journal of Electroanalytical Chemistry 840 (2019) 409–414
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Short communication
Ordered mesoporous spinel CoFe2O4 as efficient electrocatalyst for the oxygen evolution reaction
T
Yarong Huang, Weiwei Yang , Yongsheng Yu , Sue Hao ⁎
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MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China
ARTICLE INFO
ABSTRACT
Keywords: Cobalt ferrite Hard template Mesoporous structure Oxygen evolution
Oxygen evolution reaction (OER) from water using transition metal-based catalyst is of significance for the wide spread of future renewable energy materials. Herein, we propose a novel strategy to synthesize cobalt iron oxides (CoFe2O4) with different ordered mesoporous structure by using a facial hard template method. Benefiting from the ordered mesoporous structure, the optimized electrocatalyst achieved a small overpotential of 342 mV to reach 10 mA cm−2 current density, a low Tafel slope of 57.1 mV dec−1 in a 1 M KOH aqueous medium, and excellent stability for 15 h, which OER performance is the best after the comparison of lasted reported CoFe2O4based electrocatalysts. This work not only provides an approach for synthesizing efficient OER catalysts but also highlights a novel tactic to optimize the OER electrocatalysts according to regulate the porous structure.
1. Introduction The growing requirement for energy resources and the increasing concerns toward environment pollution from conventional mineral fuels, which stimulated research interest about energy storage and conversion from alternative clean energy [1]. Hydrogen (H2), as a promising alternative sustainable energy has received much attention in the effective conversion of conventional fossil fuels. Water splitting consists of two typical process, HER (hydrogen evolution reaction) and OER (oxygen evolution reaction), is considered as one of the cleanest and nonpolluting methods to produce H2. However, the half-reaction of OER is the key factor of sluggish kinetic reactions because of the high barrier over the multiple steps. Therefore, effective electrocatalysts are applied to reduce the barrier and accelerate the reaction, thus the efficiency of energy conversion can be improved. Noble metal oxides, commercial RuO2 and IrO2 are the most effective OER electrocatalysts [2,3]. Thus, it is indispensable to construct new OER electrocatalysts with high efficiency, low cost, and good durability for the practical applications. Up to date, numerous efforts upon 3d transition metal oxides electrocatalysts have been made, NiO [4], Co3O4 [5], MnO2 [6] and Fe2O3 [7] owing to their high catalytic activities, relatively low cost, and environmental friendliness. Compare to commercial RuO2 or IrO2, these materials usually show high catalytic activity toward OER process. Due to synergetic effects in mixed metal oxides, introducing one or more
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foreign metals can improve the catalytic activity that compared to the single oxides, Therefore, spinel structured metal oxides, such as CuFe2O4 [8], NiFe2O4 [9] and CoFe2O4 [10], have attracted great interest to enhance the catalytic activities of the iron-based oxides [11]. Particularly, CoFe2O4 is considered a promising catalyst because replacement of Fe2+ by Co2+ in the spinel lattice of Fe3O4 could significantly improve the OER activities [12]. Spinel compounds prepared by template free methods usually show irregular shape, large particle size and low surface area [13]. Precise control of the morphology is deemed an effective route to enhance the electrocatalytic performance of nanocatalysts, especially nanomaterials with ordered mesoporous have their architectural advantages of large specific surface area, interconnected pores, regular texture, and thus could be the most promising catalyst candidates for the electrocatalytic applications [14–16]. The synthesis of mesoporous spinel structured FeNi oxide of OER electrocatalysts with highly efficient catalytic performance has been reported [17–20]. Landon and his co-worker showed that mixed Fe-Ni oxide electrocatalysts using hard template exhibit the excellent OER performance [21]. Nickel cobalt oxide materials with ordered mesoporous prepared by a nanocasting method have also been reported as efficient OER catalysts [22]. Numerous active sites accelerate the electron transfer and mass transport that derived from the large surface area and high porosity of porous structure, and thus enhance the catalytical performance [23–25]. Therefore, preparing ordered mesoporous spinel compounds could be considered as an efficient
Corresponding authors. E-mail addresses: [email protected] (W. Yang), [email protected] (Y. Yu).
https://doi.org/10.1016/j.jelechem.2019.04.010 Received 16 February 2019; Received in revised form 29 March 2019; Accepted 1 April 2019 Available online 09 April 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 840 (2019) 409–414
Y. Huang, et al.
18 mW, spectral range: 200–1000 cm−1). The X-ray diffraction (XRD) patterns were recorded using a PANanalytical X'Pert powder with Cu Kα radiation (λ = 1.5418 Å). The specific surface areas and pore diameter distribution of the catalysts were calculated on a Beishide 3H-2000PS1 Gas Sorption. X-ray photoelectron spectroscopic (XPS) measurements were conducted on a Perkin-Elmer PHI 5000C ESCA using twin anode Mg Kα (1253.6 eV) radiation.
route to improve the electrocatalytic performance for the OER. In this work, cobalt iron oxides with ordered mesoporous were prepared and their catalytic activity for OER was studied systematically. The CoFe2O4 with different ordered mesoporous structure was prepared by using a facial hard template method. And the effects of the microstructure, BET surface area and porosity of hard-templated oxides on the OER performance of CoFe2O4 were investigated. The results show that ordered mesoporous CoFe2O4 fabricated via SBA-15 template not only exhibits the best OER catalytic activities with a small overpotential of 342 mV at a current density of 10 mA cm−2 in a 1 M KOH (low Tafel slope of 57.1 mV dec−1), but also exhibit excellent tolerance toward OER process in an alkaline medium.
2.4. Electrochemical characterization of catalytic activities Electrochemical activities of the as-prepared materials were measured in a three-electrode system (CHI660). Commercial glassy carbon electrode (GCE; diameter: 3 mm) as the working electrode, Pt foil as counter electrode and Ag/AgCl as reference electrode in a 1 M KOH electrolyte. Electrode was prepared as details: 2 mg as-prepared materials or commercial RuO2 were dispersed in a mixture solution contain 1 mL distilled water and 20 μL Nafion solution (5 wt%, Alfa Aesar). After sonication for 30 min, GCE electrode surface was modified with 5 μL catalyst ink and dried in the air (Catalysts loading: 0.142 mg cm−2). Linear sweep voltammetry (LSV) with 80% iR compensation was performed at a scan rate of 10 mV s−1 to get polarization curves. The Nernst equation (ERΗΕ = EAg/AgCl + 0.197 + 0.059 pH, pH = 13.69) was used to convert potentials to the reversible hydrogen electrode (RHE) scale. Electrochemical impedance spectroscopy (EIS) was recorded without iR compensation (1.6 V vs. RHE). The frequency range is from 100 KHz to 0.01 Hz (amplitude: 5 mV). The electrochemical active surface areas (ECSAs) were measured at different scan rates of 5, 10, 20, 30, 40 and 50 mV s−1.
2. Experiment section 2.1. Chemicals P123 (Mw = 5800) was purchased from sigma-Aldrich. Hydrochloric acid (HCl), buty alcohol, Tetrathylorthosilicate (TEOS), ferric nitrate (Fe(NO3)3·9H2O), cobalt nitrate (Co(NO3)3·9H2O), sodium hydroxide (NaOH) and ethanol were purchased from Aladdin. 2.2. Synthesis of catalysts Ordered mesoporous SiO2 hard templates (SBA-15, KIT-6 and MCM41) were prepared according to the literature [26–28]. The synthesis process was shown in Supporting information. 1 g of KIT-6-100 (SBA-15-120, MCM-41) impregnated in 4 mL ethanol solution containing 0.8 M metal precursor (Co:Fe = 1:2), vigorous stirring was applied to ensure that the precursor solution and template can be well mixed. After the solvent completely evaporated at room temperature, the vessel was placed on a hitting plate at 50 °C overnight. The powder was sintered at 500 °C in the air for 5 h (heating rate: 1 °C/min). After cooling down to the room temperature, the powder was transferred into 50 mL of 2 M NaOH solution to leach the SiO2 template for 24 h. The product was washed with distilled water and collected by centrifugation for several times. Finally, ordered mesoporous CoFe2O4 was obtained by dried the products at 60 °C. CoFe2O4 nanocasting electrocatalysts prepared from KIT-6-100, SBA-15-120 and MCM-41 were denoted as KIT-6-100-CoFe2O4, SBA-15-120-CoFe2O4 and MCM-41-CoFe2O4. The process for preparation of SBA-15-120CoFe2O4 was illustrated in Scheme 1.
3. Results and discussion 3.1. Characterization of catalyst The structure of nanocast materials is determined by hard template. To obtain the morphology information of SiO2 template, SEM images of three ordered mesoporous SiO2 hard templates are measured in Fig. S1. From SEM images, it can be seen that SBA-15 has undulating-ribbon morphology (Fig. S1a), KIT-6 displays an irregular appearance (Fig. S1b) and MCM-41 (Fig. S1c) shows polyhedral morphology. The detailed structure of SBA-15, KIT-6 and MCM-41 are further investigated by TEM, which shows that all the templates prepared in this work have good ordered mesoporous structure (Fig. S2). To examine the details of texture parameters, N2 adsorption was applied in the experiments. Fig. S3 shows the surface area of as-prepared SiO2 template. The N2 adsorption isotherms indicate that the isotherm plots (Type IV) and the hysteresis loop shows that all the templates are ordered mesoporous materials. Considering the potential applications of the hard templates (SBA15, KIT-6 and MCM-41) for preparing ordered mesoporous nanomaterials, spinel CoFe2O4 was prepared by the nanocasting route. The TEM images (Fig. 1) reveal that the ordered mesoporous CoFe2O4 is well preserved after SiO2 template leaching. In Fig. 1a, the uniform nanowire structure with ordered mesopores among nanowires can be
2.3. Catalyst characterization Scanning electron microscopy (SEM) analysis was carried out on a JEOL JSM-6360 SEM unit. Transmission electron microscopy (TEM) was conducted on a JEOL JEM-1400. High resolution transmission electron microscopy (HRTEM) images and corresponding selected area electron diffraction (SAED) pattern were obtained using a Talos F200X instrument. Raman spectroscopy measurements of the powder sample were performed using a Renishaw Invia Raman microscope at room temperature (Renishaw U.K., He-Ne laser: 633 nm, laser source:
Scheme 1. Schematic illustration of the preparation process of SBA-15-120-CoFe2O4 catalyst. 410
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Fig. 1. TEM images of as-prepared CoFe2O4 (a. SBA-15-120-CoFe2O4 c. KIT-6-100-CoFe2O4 d. MCM-41-CoFe2O4), HR-TEM image along with SAED pattern of SBA15-120-CoFe2O4 (b).
the weak peak at 334 cm−1 was matched with Eg [30]. The results coincide with the XRD studies. The parameter of surface area and distribution of pore size about asprepared ordered mesoporous CoFe2O4 were characterized at 77 K by N2 adsorption-desorption isotherm measurements (Fig. 3). The isotherms (type IV) with obvious hysteresis loops, indicate that CoFe2O4 prepared by hard-templating method possessed mesoporous nature. CoFe2O4 prepared by SBA-15-120, KIT-6-100 and MCM-41 exhibit similar surface areas of 163.3, 151.6 and 165.5 m2 g−1, respectively. SBA-15-120-CoFe2O4 with a pore volume of 0.79 cm3 g−1 is higher than KIT-6-100-CoFe2O4 (0.59 cm3 g−1) and MCM-41-CoFe2O4 (0.17 cm3 g−1). The higher pore volume exposed more active sites which make SBA-15-120-CoFe2O4 shows excellent electrocatalytic oxygen evolution performance. The detailed parameters are summarized in Table 1. In order to further confirm the existence of Co, Fe, O, XPS measurements were conducted. Fig. 4a and Fig. 4b show the spectra of Co 2p and Fe 2p in ordered mesoporous CoFe2O4. The peaks around 779.9 and 781.7 eV in Fig. 4a are two main doublets of Co 2p3/2, the peak located at 795.9 eV is associated with Co 2p1/2. Two Peaks around at 786.6 and 804 eV are satellite peaks. All these peaks are characteristic features of Co2+. The peak position of Fe 2p3/2 is shown in Fig. 4b. The binding energies of Fe 2p3/2 and Fe 2p1/2 are located at 710.6, 712.1 and 724.6 eV, respectively. The satellite peak is clearly distinguishable at 718.6 eV. These results are consistent with the Co 2p and Fe 2p binding energy for CoFe2O4 [31–33].
observed from TEM images. In this case, the silica walls of hard template behave as barriers between mesoporous to prevent the further growth of the nanocrystals and makes the nanomaterials inside SBA-15120 pores maintain nanowire structure. Fig. 1b shows HRTEM image recorded on SBA-15-120-CoFe2O4 exhibiting clear lattice fringes (dspacing: 0.25 nm), which is well matched with the interplanar spacing of CoFe2O4 phase (311) plane [29]. The SAED pattern shown in the inset of Fig. 1b reveals that the as-synthesized mesoporous CoFe2O4 possess a well-defined polycrystalline structure. Fig. 1c shows that KIT6-100-CoFe2O4 samples also have periodic mesoporous structures and the well-ordered mesoporous structure is fit with the template, it indicates that after leaching process the different pore channels are replicated. The narrower pore size and thinner pore wall of MCM-41 than SBA-15-120, the TEM image of MCM-41-CoFe2O4 (Fig. 1d) shows that some agglomerations and the ordering of CoFe2O4 replica is not obviously because of the low interconnection between pore channels, but the mesoporous structure still exists, the pore size distribution curves of MCM-41-CoFe2O4 illustrate this result (Fig. 3b). The crystal structure of prepared CoFe2O4 was also measured by XRD. The results show the diffraction peaks of all three samples in Fig. 2a, the peaks located at 30.02°, 35.36°, 42.97°, 53.31°, 56.82° and 62.39° are well matched with (220), (311), (400), (422), (511) and (440) the lattice planes of the CoFe2O4 (JCPDS no. 22-1086). No other phases or impurities are observed, suggesting that the samples prepared by different hard templates are spinel structure of CoFe2O4. Furthermore, Raman spectroscopy was applied to analyze the cationic distribution and the strain of as-prepared samples. As shown in Fig. 2b, obvious peaks around at 470 and 680 cm−1 were well matched with T2g and A1g (1) which belong to the phonon modes of spinel compounds, 411
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Fig. 2. (a) XRD patterns of as-prepared CoFe2O4 and (b) Raman spectra of as-prepared CoFe2O4.
catalysts at a sweep rate of 10 mV s−1. Obviously, SBA-15-120-CoFe2O4 exhibits the most outstanding OER activity with the smallest onset overpotential of ~1.572 V. SBA-15-120-CoFe2O4 required a relatively low overpotential of 342 mV to achieve the 10 mA cm−2 current density, which is much lower than KIT-6-100-CoFe2O4 (360 mV) and MCM41-CoFe2O4 (383 mV), and close to RuO2 (248 mV). In order to understand the catalytic OER kinetics, Tafel plots of the as-prepared CoFe2O4 samples was obtained. Fig. 5b shows the Tafel slope of the SBA-15-120-CoFe2O4 (57.1 mV dev−1), which is lower than that of KIT-6-100-CoFe2O4 (58.4 mV dec−1) and MCM-41-CoFe2O4 (85.2 mV dec−1), suggesting that the SBA-15-120-CoFe2O4 catalyst has the fastest OER kinetics. EIS analysis is employed to assess the conductance of the electrocatalysts under 1.6 V (Fig. 5c), and the measured impedance data were fitted using a series R(CR) equivalent circuit. As expected, the Rct of ordered mesoporous SBA-15-120-CoFe2O4 (16.26 Ω) is much lower than that of KIT-6-100-CoFe2O4 (38.12 Ω) and MCM-41-CoFe2O4 (48.21 Ω), indicating that SBA-15-120-CoFe2O4 holds the faster charge transfer kinetic. These results are in accordance with the OER results. In order to unveil the mechanism of enhanced OER performance, we tested electrochemical active surface area (ECSA) based on their electrochemical double-layer capacitance (Cdl). Cdl is conducted in the non-Faradic region with cyclic voltammetry at different scan rates (Fig. S4). As shown in Fig. 5d, it is clearly revealed that the Cdl of the SBA-15-120-CoFe2O4 (4.1 mF cm−2) is larger than that of (2.2 mF cm−2) and MCM-41-CoFe2O4 KIT-6-100-CoFe2O4 −2 (1.6 mF cm ). The large Cdl indicates that SBA-15-120-CoFe2O4 exposes the highest active sites, which supposed to be the possible reasons that improve OER performance. To assess long-term electrochemical OER stability of the SBA-15120-CoFe2O4, we probe the durability of catalyst under continuous potential scanning conditions. As seen in Fig. 6a, our obtained samples disclose in alkaline medium at a scan rate of 10 mV s−1. As observed, there is almost no attenuation after 1000 cycles. Furthermore, we also observed a long-term durability at voltage of 1.57 V (Fig. 6b). The current density is no obvious vibration during the continuous operation
Fig. 3. N2 adsorption-desorption isotherm curves of as-prepared CoFe2O4. Table 1 N2 adsorption desorption isotherm results of as-prepared materials. Sample SBA-15-120 KIT-6-100 MCM-41 SBA-15-120-CoFe2O4 KIT-6-100-CoFe2O4 MCM-41-CoFe2O4
Surface area (m2 g−1)
Pore volume (cm3 g−1)
905.3 1090.2 1385.9 163.3 151.6 165.5
1.57 1.56 1.04 0.79 0.59 0.17
3.2. Oxygen evolution catalytic activity The electrocatalytic OER properties of ordered mesoporous CoFe2O4 prepared by different hard templates were examined by a three-electrode configuration, and the commercial RuO2 was measured for direct comparison. Fig. 5a shows the OER polarization curves of different
Fig. 4. XPS spectra of the Co 2p and Fe 2p regions of as-prepared CoFe2O4. 412
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Fig. 5. (a) Polarization curves, (b) Tafel plots, (c) electrochemical impedance spectra at potential of 1.6 V vs. RHE, and (d) charging current density difference plotted against scan rate of as-prepared CoFe2O4.
for 15 h. These results suggest that the superior durability of SBA-15120-CoFe2O4. We also further investigated the morphology of SBA-15120-CoFe2O4 catalyst after stability test using TEM. There is no obvious change on morphology and SBA-15-120-CoFe2O4 catalyst is still the ordered mesoporous nanowire structure, as shown in Fig. S5. The comparison of recent CoFe2O4 electrocatalysts for OER in alkaline electrolytes is shown in Table S1, suggesting that SBA-15-120-CoFe2O4 catalyst with ordered mesoporous structure has the higher catalytic activity, favorable kinetics, and good durability for OER catalysis.
catalyst prepared via a facial hard template method. The effects of the microstructure, BET surface area and porosity of hard-templated oxides on the OER performance of CoFe2O4 were investigated. The SBA-15120-CoFe2O4 catalyst exhibits the best OER catalytic activities with smallest overpotential of 342 mV at 10 mA cm−2 current density, lowest Tafel slope of 57.1 mV dec−1 and longtime tolerance, compared with other reported CoFe2O4-based OER electrocatalysts. The lowest transfer resistance and the highest active sites in SBA-15-120-CoFe2O4 catalyst should be the possible reasons for enhanced OER performance. This research offers a novel route for the development of ordered mesoporous electrocatalysts with high-performance, low cost and environmental friendliness for the oxygen evolution reaction.
4. Conclusion In summary, we have reported an ordered mesoporous CoFe2O4
Fig. 6. (a) Polarization curves of SBA-15-120-CoFe2O4 after 1000 cycles stability test at 10 mV s−1 in 1 M KOH, (b) chronoamperometry curves at a constant potential of 1.57 V vs. RHE of SBA-15-120-CoFe2O4.
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Acknowledgements
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Short communication
Ordered mesoporous spinel CoFe2O4 as efficient electrocatalyst for the oxygen evolution reaction
T
Yarong Huang, Weiwei Yang , Yongsheng Yu , Sue Hao ⁎
⁎
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China
ARTICLE INFO
ABSTRACT
Keywords: Cobalt ferrite Hard template Mesoporous structure Oxygen evolution
Oxygen evolution reaction (OER) from water using transition metal-based catalyst is of significance for the wide spread of future renewable energy materials. Herein, we propose a novel strategy to synthesize cobalt iron oxides (CoFe2O4) with different ordered mesoporous structure by using a facial hard template method. Benefiting from the ordered mesoporous structure, the optimized electrocatalyst achieved a small overpotential of 342 mV to reach 10 mA cm−2 current density, a low Tafel slope of 57.1 mV dec−1 in a 1 M KOH aqueous medium, and excellent stability for 15 h, which OER performance is the best after the comparison of lasted reported CoFe2O4based electrocatalysts. This work not only provides an approach for synthesizing efficient OER catalysts but also highlights a novel tactic to optimize the OER electrocatalysts according to regulate the porous structure.
1. Introduction The growing requirement for energy resources and the increasing concerns toward environment pollution from conventional mineral fuels, which stimulated research interest about energy storage and conversion from alternative clean energy [1]. Hydrogen (H2), as a promising alternative sustainable energy has received much attention in the effective conversion of conventional fossil fuels. Water splitting consists of two typical process, HER (hydrogen evolution reaction) and OER (oxygen evolution reaction), is considered as one of the cleanest and nonpolluting methods to produce H2. However, the half-reaction of OER is the key factor of sluggish kinetic reactions because of the high barrier over the multiple steps. Therefore, effective electrocatalysts are applied to reduce the barrier and accelerate the reaction, thus the efficiency of energy conversion can be improved. Noble metal oxides, commercial RuO2 and IrO2 are the most effective OER electrocatalysts [2,3]. Thus, it is indispensable to construct new OER electrocatalysts with high efficiency, low cost, and good durability for the practical applications. Up to date, numerous efforts upon 3d transition metal oxides electrocatalysts have been made, NiO [4], Co3O4 [5], MnO2 [6] and Fe2O3 [7] owing to their high catalytic activities, relatively low cost, and environmental friendliness. Compare to commercial RuO2 or IrO2, these materials usually show high catalytic activity toward OER process. Due to synergetic effects in mixed metal oxides, introducing one or more
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foreign metals can improve the catalytic activity that compared to the single oxides, Therefore, spinel structured metal oxides, such as CuFe2O4 [8], NiFe2O4 [9] and CoFe2O4 [10], have attracted great interest to enhance the catalytic activities of the iron-based oxides [11]. Particularly, CoFe2O4 is considered a promising catalyst because replacement of Fe2+ by Co2+ in the spinel lattice of Fe3O4 could significantly improve the OER activities [12]. Spinel compounds prepared by template free methods usually show irregular shape, large particle size and low surface area [13]. Precise control of the morphology is deemed an effective route to enhance the electrocatalytic performance of nanocatalysts, especially nanomaterials with ordered mesoporous have their architectural advantages of large specific surface area, interconnected pores, regular texture, and thus could be the most promising catalyst candidates for the electrocatalytic applications [14–16]. The synthesis of mesoporous spinel structured FeNi oxide of OER electrocatalysts with highly efficient catalytic performance has been reported [17–20]. Landon and his co-worker showed that mixed Fe-Ni oxide electrocatalysts using hard template exhibit the excellent OER performance [21]. Nickel cobalt oxide materials with ordered mesoporous prepared by a nanocasting method have also been reported as efficient OER catalysts [22]. Numerous active sites accelerate the electron transfer and mass transport that derived from the large surface area and high porosity of porous structure, and thus enhance the catalytical performance [23–25]. Therefore, preparing ordered mesoporous spinel compounds could be considered as an efficient
Corresponding authors. E-mail addresses: [email protected] (W. Yang), [email protected] (Y. Yu).
https://doi.org/10.1016/j.jelechem.2019.04.010 Received 16 February 2019; Received in revised form 29 March 2019; Accepted 1 April 2019 Available online 09 April 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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18 mW, spectral range: 200–1000 cm−1). The X-ray diffraction (XRD) patterns were recorded using a PANanalytical X'Pert powder with Cu Kα radiation (λ = 1.5418 Å). The specific surface areas and pore diameter distribution of the catalysts were calculated on a Beishide 3H-2000PS1 Gas Sorption. X-ray photoelectron spectroscopic (XPS) measurements were conducted on a Perkin-Elmer PHI 5000C ESCA using twin anode Mg Kα (1253.6 eV) radiation.
route to improve the electrocatalytic performance for the OER. In this work, cobalt iron oxides with ordered mesoporous were prepared and their catalytic activity for OER was studied systematically. The CoFe2O4 with different ordered mesoporous structure was prepared by using a facial hard template method. And the effects of the microstructure, BET surface area and porosity of hard-templated oxides on the OER performance of CoFe2O4 were investigated. The results show that ordered mesoporous CoFe2O4 fabricated via SBA-15 template not only exhibits the best OER catalytic activities with a small overpotential of 342 mV at a current density of 10 mA cm−2 in a 1 M KOH (low Tafel slope of 57.1 mV dec−1), but also exhibit excellent tolerance toward OER process in an alkaline medium.
2.4. Electrochemical characterization of catalytic activities Electrochemical activities of the as-prepared materials were measured in a three-electrode system (CHI660). Commercial glassy carbon electrode (GCE; diameter: 3 mm) as the working electrode, Pt foil as counter electrode and Ag/AgCl as reference electrode in a 1 M KOH electrolyte. Electrode was prepared as details: 2 mg as-prepared materials or commercial RuO2 were dispersed in a mixture solution contain 1 mL distilled water and 20 μL Nafion solution (5 wt%, Alfa Aesar). After sonication for 30 min, GCE electrode surface was modified with 5 μL catalyst ink and dried in the air (Catalysts loading: 0.142 mg cm−2). Linear sweep voltammetry (LSV) with 80% iR compensation was performed at a scan rate of 10 mV s−1 to get polarization curves. The Nernst equation (ERΗΕ = EAg/AgCl + 0.197 + 0.059 pH, pH = 13.69) was used to convert potentials to the reversible hydrogen electrode (RHE) scale. Electrochemical impedance spectroscopy (EIS) was recorded without iR compensation (1.6 V vs. RHE). The frequency range is from 100 KHz to 0.01 Hz (amplitude: 5 mV). The electrochemical active surface areas (ECSAs) were measured at different scan rates of 5, 10, 20, 30, 40 and 50 mV s−1.
2. Experiment section 2.1. Chemicals P123 (Mw = 5800) was purchased from sigma-Aldrich. Hydrochloric acid (HCl), buty alcohol, Tetrathylorthosilicate (TEOS), ferric nitrate (Fe(NO3)3·9H2O), cobalt nitrate (Co(NO3)3·9H2O), sodium hydroxide (NaOH) and ethanol were purchased from Aladdin. 2.2. Synthesis of catalysts Ordered mesoporous SiO2 hard templates (SBA-15, KIT-6 and MCM41) were prepared according to the literature [26–28]. The synthesis process was shown in Supporting information. 1 g of KIT-6-100 (SBA-15-120, MCM-41) impregnated in 4 mL ethanol solution containing 0.8 M metal precursor (Co:Fe = 1:2), vigorous stirring was applied to ensure that the precursor solution and template can be well mixed. After the solvent completely evaporated at room temperature, the vessel was placed on a hitting plate at 50 °C overnight. The powder was sintered at 500 °C in the air for 5 h (heating rate: 1 °C/min). After cooling down to the room temperature, the powder was transferred into 50 mL of 2 M NaOH solution to leach the SiO2 template for 24 h. The product was washed with distilled water and collected by centrifugation for several times. Finally, ordered mesoporous CoFe2O4 was obtained by dried the products at 60 °C. CoFe2O4 nanocasting electrocatalysts prepared from KIT-6-100, SBA-15-120 and MCM-41 were denoted as KIT-6-100-CoFe2O4, SBA-15-120-CoFe2O4 and MCM-41-CoFe2O4. The process for preparation of SBA-15-120CoFe2O4 was illustrated in Scheme 1.
3. Results and discussion 3.1. Characterization of catalyst The structure of nanocast materials is determined by hard template. To obtain the morphology information of SiO2 template, SEM images of three ordered mesoporous SiO2 hard templates are measured in Fig. S1. From SEM images, it can be seen that SBA-15 has undulating-ribbon morphology (Fig. S1a), KIT-6 displays an irregular appearance (Fig. S1b) and MCM-41 (Fig. S1c) shows polyhedral morphology. The detailed structure of SBA-15, KIT-6 and MCM-41 are further investigated by TEM, which shows that all the templates prepared in this work have good ordered mesoporous structure (Fig. S2). To examine the details of texture parameters, N2 adsorption was applied in the experiments. Fig. S3 shows the surface area of as-prepared SiO2 template. The N2 adsorption isotherms indicate that the isotherm plots (Type IV) and the hysteresis loop shows that all the templates are ordered mesoporous materials. Considering the potential applications of the hard templates (SBA15, KIT-6 and MCM-41) for preparing ordered mesoporous nanomaterials, spinel CoFe2O4 was prepared by the nanocasting route. The TEM images (Fig. 1) reveal that the ordered mesoporous CoFe2O4 is well preserved after SiO2 template leaching. In Fig. 1a, the uniform nanowire structure with ordered mesopores among nanowires can be
2.3. Catalyst characterization Scanning electron microscopy (SEM) analysis was carried out on a JEOL JSM-6360 SEM unit. Transmission electron microscopy (TEM) was conducted on a JEOL JEM-1400. High resolution transmission electron microscopy (HRTEM) images and corresponding selected area electron diffraction (SAED) pattern were obtained using a Talos F200X instrument. Raman spectroscopy measurements of the powder sample were performed using a Renishaw Invia Raman microscope at room temperature (Renishaw U.K., He-Ne laser: 633 nm, laser source:
Scheme 1. Schematic illustration of the preparation process of SBA-15-120-CoFe2O4 catalyst. 410
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Fig. 1. TEM images of as-prepared CoFe2O4 (a. SBA-15-120-CoFe2O4 c. KIT-6-100-CoFe2O4 d. MCM-41-CoFe2O4), HR-TEM image along with SAED pattern of SBA15-120-CoFe2O4 (b).
the weak peak at 334 cm−1 was matched with Eg [30]. The results coincide with the XRD studies. The parameter of surface area and distribution of pore size about asprepared ordered mesoporous CoFe2O4 were characterized at 77 K by N2 adsorption-desorption isotherm measurements (Fig. 3). The isotherms (type IV) with obvious hysteresis loops, indicate that CoFe2O4 prepared by hard-templating method possessed mesoporous nature. CoFe2O4 prepared by SBA-15-120, KIT-6-100 and MCM-41 exhibit similar surface areas of 163.3, 151.6 and 165.5 m2 g−1, respectively. SBA-15-120-CoFe2O4 with a pore volume of 0.79 cm3 g−1 is higher than KIT-6-100-CoFe2O4 (0.59 cm3 g−1) and MCM-41-CoFe2O4 (0.17 cm3 g−1). The higher pore volume exposed more active sites which make SBA-15-120-CoFe2O4 shows excellent electrocatalytic oxygen evolution performance. The detailed parameters are summarized in Table 1. In order to further confirm the existence of Co, Fe, O, XPS measurements were conducted. Fig. 4a and Fig. 4b show the spectra of Co 2p and Fe 2p in ordered mesoporous CoFe2O4. The peaks around 779.9 and 781.7 eV in Fig. 4a are two main doublets of Co 2p3/2, the peak located at 795.9 eV is associated with Co 2p1/2. Two Peaks around at 786.6 and 804 eV are satellite peaks. All these peaks are characteristic features of Co2+. The peak position of Fe 2p3/2 is shown in Fig. 4b. The binding energies of Fe 2p3/2 and Fe 2p1/2 are located at 710.6, 712.1 and 724.6 eV, respectively. The satellite peak is clearly distinguishable at 718.6 eV. These results are consistent with the Co 2p and Fe 2p binding energy for CoFe2O4 [31–33].
observed from TEM images. In this case, the silica walls of hard template behave as barriers between mesoporous to prevent the further growth of the nanocrystals and makes the nanomaterials inside SBA-15120 pores maintain nanowire structure. Fig. 1b shows HRTEM image recorded on SBA-15-120-CoFe2O4 exhibiting clear lattice fringes (dspacing: 0.25 nm), which is well matched with the interplanar spacing of CoFe2O4 phase (311) plane [29]. The SAED pattern shown in the inset of Fig. 1b reveals that the as-synthesized mesoporous CoFe2O4 possess a well-defined polycrystalline structure. Fig. 1c shows that KIT6-100-CoFe2O4 samples also have periodic mesoporous structures and the well-ordered mesoporous structure is fit with the template, it indicates that after leaching process the different pore channels are replicated. The narrower pore size and thinner pore wall of MCM-41 than SBA-15-120, the TEM image of MCM-41-CoFe2O4 (Fig. 1d) shows that some agglomerations and the ordering of CoFe2O4 replica is not obviously because of the low interconnection between pore channels, but the mesoporous structure still exists, the pore size distribution curves of MCM-41-CoFe2O4 illustrate this result (Fig. 3b). The crystal structure of prepared CoFe2O4 was also measured by XRD. The results show the diffraction peaks of all three samples in Fig. 2a, the peaks located at 30.02°, 35.36°, 42.97°, 53.31°, 56.82° and 62.39° are well matched with (220), (311), (400), (422), (511) and (440) the lattice planes of the CoFe2O4 (JCPDS no. 22-1086). No other phases or impurities are observed, suggesting that the samples prepared by different hard templates are spinel structure of CoFe2O4. Furthermore, Raman spectroscopy was applied to analyze the cationic distribution and the strain of as-prepared samples. As shown in Fig. 2b, obvious peaks around at 470 and 680 cm−1 were well matched with T2g and A1g (1) which belong to the phonon modes of spinel compounds, 411
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Fig. 2. (a) XRD patterns of as-prepared CoFe2O4 and (b) Raman spectra of as-prepared CoFe2O4.
catalysts at a sweep rate of 10 mV s−1. Obviously, SBA-15-120-CoFe2O4 exhibits the most outstanding OER activity with the smallest onset overpotential of ~1.572 V. SBA-15-120-CoFe2O4 required a relatively low overpotential of 342 mV to achieve the 10 mA cm−2 current density, which is much lower than KIT-6-100-CoFe2O4 (360 mV) and MCM41-CoFe2O4 (383 mV), and close to RuO2 (248 mV). In order to understand the catalytic OER kinetics, Tafel plots of the as-prepared CoFe2O4 samples was obtained. Fig. 5b shows the Tafel slope of the SBA-15-120-CoFe2O4 (57.1 mV dev−1), which is lower than that of KIT-6-100-CoFe2O4 (58.4 mV dec−1) and MCM-41-CoFe2O4 (85.2 mV dec−1), suggesting that the SBA-15-120-CoFe2O4 catalyst has the fastest OER kinetics. EIS analysis is employed to assess the conductance of the electrocatalysts under 1.6 V (Fig. 5c), and the measured impedance data were fitted using a series R(CR) equivalent circuit. As expected, the Rct of ordered mesoporous SBA-15-120-CoFe2O4 (16.26 Ω) is much lower than that of KIT-6-100-CoFe2O4 (38.12 Ω) and MCM-41-CoFe2O4 (48.21 Ω), indicating that SBA-15-120-CoFe2O4 holds the faster charge transfer kinetic. These results are in accordance with the OER results. In order to unveil the mechanism of enhanced OER performance, we tested electrochemical active surface area (ECSA) based on their electrochemical double-layer capacitance (Cdl). Cdl is conducted in the non-Faradic region with cyclic voltammetry at different scan rates (Fig. S4). As shown in Fig. 5d, it is clearly revealed that the Cdl of the SBA-15-120-CoFe2O4 (4.1 mF cm−2) is larger than that of (2.2 mF cm−2) and MCM-41-CoFe2O4 KIT-6-100-CoFe2O4 −2 (1.6 mF cm ). The large Cdl indicates that SBA-15-120-CoFe2O4 exposes the highest active sites, which supposed to be the possible reasons that improve OER performance. To assess long-term electrochemical OER stability of the SBA-15120-CoFe2O4, we probe the durability of catalyst under continuous potential scanning conditions. As seen in Fig. 6a, our obtained samples disclose in alkaline medium at a scan rate of 10 mV s−1. As observed, there is almost no attenuation after 1000 cycles. Furthermore, we also observed a long-term durability at voltage of 1.57 V (Fig. 6b). The current density is no obvious vibration during the continuous operation
Fig. 3. N2 adsorption-desorption isotherm curves of as-prepared CoFe2O4. Table 1 N2 adsorption desorption isotherm results of as-prepared materials. Sample SBA-15-120 KIT-6-100 MCM-41 SBA-15-120-CoFe2O4 KIT-6-100-CoFe2O4 MCM-41-CoFe2O4
Surface area (m2 g−1)
Pore volume (cm3 g−1)
905.3 1090.2 1385.9 163.3 151.6 165.5
1.57 1.56 1.04 0.79 0.59 0.17
3.2. Oxygen evolution catalytic activity The electrocatalytic OER properties of ordered mesoporous CoFe2O4 prepared by different hard templates were examined by a three-electrode configuration, and the commercial RuO2 was measured for direct comparison. Fig. 5a shows the OER polarization curves of different
Fig. 4. XPS spectra of the Co 2p and Fe 2p regions of as-prepared CoFe2O4. 412
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Fig. 5. (a) Polarization curves, (b) Tafel plots, (c) electrochemical impedance spectra at potential of 1.6 V vs. RHE, and (d) charging current density difference plotted against scan rate of as-prepared CoFe2O4.
for 15 h. These results suggest that the superior durability of SBA-15120-CoFe2O4. We also further investigated the morphology of SBA-15120-CoFe2O4 catalyst after stability test using TEM. There is no obvious change on morphology and SBA-15-120-CoFe2O4 catalyst is still the ordered mesoporous nanowire structure, as shown in Fig. S5. The comparison of recent CoFe2O4 electrocatalysts for OER in alkaline electrolytes is shown in Table S1, suggesting that SBA-15-120-CoFe2O4 catalyst with ordered mesoporous structure has the higher catalytic activity, favorable kinetics, and good durability for OER catalysis.
catalyst prepared via a facial hard template method. The effects of the microstructure, BET surface area and porosity of hard-templated oxides on the OER performance of CoFe2O4 were investigated. The SBA-15120-CoFe2O4 catalyst exhibits the best OER catalytic activities with smallest overpotential of 342 mV at 10 mA cm−2 current density, lowest Tafel slope of 57.1 mV dec−1 and longtime tolerance, compared with other reported CoFe2O4-based OER electrocatalysts. The lowest transfer resistance and the highest active sites in SBA-15-120-CoFe2O4 catalyst should be the possible reasons for enhanced OER performance. This research offers a novel route for the development of ordered mesoporous electrocatalysts with high-performance, low cost and environmental friendliness for the oxygen evolution reaction.
4. Conclusion In summary, we have reported an ordered mesoporous CoFe2O4
Fig. 6. (a) Polarization curves of SBA-15-120-CoFe2O4 after 1000 cycles stability test at 10 mV s−1 in 1 M KOH, (b) chronoamperometry curves at a constant potential of 1.57 V vs. RHE of SBA-15-120-CoFe2O4.
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Acknowledgements
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