Microwave-Assisted Synthesis of a Stainless Steel Mesh-Supported

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 11069-11079

Microwave-Assisted Synthesis of a Stainless Steel Mesh-Supported Co3O4 Microrod Array As a Highly Efficient Catalyst for Electrochemical Water Oxidation Amol R. Jadhav, John Marc C. Puguan, and Hern Kim* Department of Energy Science and Technology, Smart Living Innovation Technology Center, Myongji University, 116 Myongji-ro Cheoin-gu, Yongin, Gyeonggi-do 17058, Republic of Korea

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S Supporting Information *

ABSTRACT: With the rising energy crisis and increasingly serious environmental issues, clean energy sources like fuel cell and lithium−air batteries are attracting the attention of the whole world. The direct fuel cell, especially, is believed to be the quintessential replacement for conventional sources of energy because of its high energy conversion capacity. Electrochemical water splitting has an important role in such sustainable energy technologies. Catalysts play a worthwhile role in water splitting, especially the oxygen evolution reaction (OER). Engineering fine micro/nanostructures with subtle morphologies represents an effective strategy to enhance the activity of the resultant catalyst toward OER through exposing abundant electrochemically active sites. So, here we report a Co3O4@SUS catalyst, the well-defined Co3O4 microrods were successfully anchored onto the stainless steel mesh substrate with the assistance of diethylenetriamine using microwave irradiation, utilizing a commercially available microwave instrument. Co3O4@SUS possesses outstanding catalytic activity toward water oxidation. In water oxidation, the current density of 10 mA cm−2 was achieved at 298 mV overpotential with a low Tafel slope of 105 mV dec−1. In addition to low overpotential, Co3O4@SUS was stable under conditions of continuous O2 evolution for an extended period (24 h). The results show a highly efficient, scalable, and low-cost method for developing highly active and stable OER electrocatalysts in alkaline solution. KEYWORDS: Oxygen evolution reaction, Overpotential, Tafel slope, Electrocatalyst

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INTRODUCTION Our modern life is built on energy; most of this energy comes from burning fossil fuels like coal, oil, etc. Burning fossil fuel releases huge amounts of CO2, NOx, and SOx gases which trap heat near the earth and cause a rise in global temperature. Finding alternatives to fossil fuel is a global necessity, and scientists have made great efforts to develop clean and sustainable sources of energy.1−5 With the rising energy crisis, and increasingly serious environmental issues, clean energy sources like fuel cells and lithium−air batteries have the attention of the whole world. Electrocatalysis provides a green method for conversion and storage of energy. However, the serious challenge in electrolysis is the targeted efficiency because of the limitations in the performance of the oxygen evolution reaction (OER). This reaction involves the four electron pathway in both acidic and alkaline medium6,7 and it is associated with many energy conversion and storage devices such as metal−air batteries, water electrolyzers, solar fuel production, etc. The OER involves a stepwise four-electron transfer process. The O−H bond breaking and the subsequent O−O bond formation have to overcome high energy barriers, which make the OER the rate-determining step.8 The traditional understanding of the OER mechanism on metal oxides involves the concerted four electron−proton transfer © 2017 American Chemical Society

steps on metal-ion centers at their surface, and the generated O2 is derived from water. But recently, Grimaud and coworkers proved that the generated oxygen molecule during OER can come from lattice oxygen using in situ 18O isotope labeling mass spectroscopy. 9 This insight opens new possibilities for developing highly active catalyst using nonconcerted proton−electron transfer steps and lattice oxygen redox processes. Currently, noble metal oxides such as RuO2 and IrO2 are still considered to be the most efficient OER catalysts, but their practical applications are limited by scarcity and high cost.10 The hydroxide and oxide species of transition metals such as Mn, Fe, Co, and Ni have attracted considerable research attention as they have demonstrated comparable water oxidation activity in comparison with their noble metal counterparts under alkaline conditions.11−20 Among all the candidates, cobalt oxide based materials have been broadly explored as active water oxidation catalyst via photochemical and electrochemical approaches because of their nontoxic, earth abundant, superior activity, and stability in alkaline conReceived: August 30, 2017 Revised: September 20, 2017 Published: September 30, 2017 11069

DOI: 10.1021/acssuschemeng.7b03027 ACS Sustainable Chem. Eng. 2017, 5, 11069−11079

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Mechanism for a Controlled Synthesis of Co3O4@SUS

ditions.21 Therefore, cobalt oxide base catalysts can be the most promising candidate to replace precious metal-based catalysts for OER. However, because of the lower electrical conductivity than their metal counterparts, the catalytic activity of these metal oxides is still limited. The electrical conductivity of the catalyst can be enhanced by depositing the material on a highly conductive carbon substrate,22,23 which allows simultaneous relaxation of mass and charge transfer limitations leading to outstandingly high catalytic activity.24 However, the need of drop casting of such materials onto the surface of the current collector by preparing a slurry of the electroactive material, conductivity enhancer, and a binder causes a decrease in contact area between the catalyst and electrolyte, resulting in high resistance and reduced electrocatalytic performance. Moreover, at high current such catalyst peel off from the current collecting substrate, thereby causing degradation of catalytic activity.25 Alternatively, such electroactive material can be directly grown on the surface of a current collector such as nickel foam,26 copper foil,27 carbon cloth or paper,28,29 FTO,30,31 stainless-steel,32 and nickel foils.33 In our current work, stainless steel mesh (SUS) was used as the substrate, since it has a high physical robustness and is chemical resistance in both the acidic and basic environment. Additionally, it has a relatively low electrical resistivity of 74 mΩ·cm.32 These physical properties of SUS suggest it being a good support material for an electrochemical process such as OER. In order to scale up the mass production, it is also important to consider the simplicity and practicability of synthesis methods. Microwave heating techniques have been implanted in the chemistry laboratories during the past years. In the field of organic synthesis, microwave synthesis methods are well researched.34 Nowadays, this method is used for the synthesis of polymers,35 inorganic materials, and nanomaterials.22,36 In nanomaterials synthesis research, the microwave irradiation method is a promising technology for the development of preparation and modification protocols due to the strong interaction of microwave irradiation with carbon and metal chelated complexes.37,38 Herein, we used microwave irradiation methods to synthesize stainless steel mesh selfsupported Co3O4 microrods with a simple postcalcination

process. For the Co3O4 electrocatalyst reported here, we observed that it can generate 10 mA cm−2 current density at a much lower overpotential of 298 mV. This is one of the lowest overpotentials among the known Co3O4 based OER catalysts, even the activity is similar to RuO2 and IrO2 catalysts, which are some of the best OER performing catalysts.10,32,39 It is worth mentioning that the new electrocatalyst can effectively catalyze OER without any additional modification. The main objective of this work is to provide a simple and scalable strategy for the preparation of self-supported spinel oxide material using a proper anchoring agent and substrate. So the prepared electrode materials could be useful as a potential candidate for energy and environmental related applications.

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EXPERIMENTAL SECTION

Preparation of Co3O4@SUS and Co3O4-LNS. All the analytical grade chemicals are commercially available and were used as received. Triply distilled water (DI) was used during all the experimental processes. In a typical experiment, 2 mL diethylenetriamine (DETA) was dissolved in 50 mL DI water at room temperature. To the above solution a piece of cleaned stainless steel mesh (SUS, 316L, 300 mesh) with a size of 1 cm × 2 cm was immersed into the solution. The above solution was stirred for 10 min and then 1 g of Co(NO3)2·6H2O was added in the solution. The reaction mixture was further stirred for 10 min and then placed in the chamber of commercially available microwave workstation. The solution was heated in a microwave oven under a medium high mode power 400 W at 100 °C for 5 min, then 50 mL 1 M KOH was added slowly to the hot solution and again the reaction vessel was placed in microwave oven for 10 min. After 10 min silver colored SUS was converted to pink colored SUS. The above solution was then cooled to room temperature and filtered and washed with DI water several times. The precursors were dried in a vacuum oven at 60 °C overnight and then annealed in air at 400 °C for 3 h with a heating ramp of 3 °C min−1 to obtain product named as Co3O4@SUS. Co3O4 layered nanosheets (Co3O4-LNS) were synthesized following the same procedure without placing the SUS substrate. Materials Characterization. Thermogravimetric analysis (TGA, model scinco TGA N-1000) was used to assess the annealing process of the prepared precursors. The heating of the sample was carried out at a range of 25 to 800 °C with a rate of 10 °C min−1 under a continuous purge of air. The powder X-ray diffraction (XRD) patterns were recorded on analytical X’pert MPD diffractometer, with Cu Kα 11070


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ACS Sustainable Chemistry & Engineering Scheme 2. Proposed Formation Mechanism for Synthesis of Co3O4-LNS

radiation at 40 kV. The scanning regions of the diffraction angle, 2θ, were 5−90°, and radiation was detected with a proportional detector. X-ray photoelectron spectroscopy (XPS) was conducted using a KAlpha (Thermo Electron) with a Mg Kα as a source and the C 1s peak at 284.6 eV as an internal standard. Functional groups in the prepared materials were observed using a Fourier-transform infrared (FTIR) spectrum (Varian 2000), whose resolution was about 8 cm−1 and the scanning number was 32 in the 4000−400 cm−1 range. The morphology of prepared samples was characterized using fieldemission scanning electron microscope (FESEM, Hitachi, S-3500N) and a transmission electron microscope (TEM, JEOL JEM-200CX); elemental detection exploited using energy dispersive X-ray spectrometer (EDX). Electrochemical Measurements. Electrochemical measurements were carried out using ZIVE SP1 electrochemical workstation (Won ATech Co., Ltd., South Korea). All the measurements were performed in a conventional three electrode cell with deposited SUS as a working electrode, platinum wire as a counter electrode and Ag|AgCl (1 M KCl) as a reference electrode. Oxygen evolution reaction performance was investigated using linear sweep voltammetry. Electrochemical impedance spectroscopy (EIS) was recorded in the frequency range of 100 kHz to 100 mHz with a sinusoidal wave amplitude of 10 mV. Chronoamperometry (CA) measurements were performed to understand the activity and stabilization of nanomaterials toward electrocatalytic water oxidation reaction.

ions on SUS surface. Further, the addition of KOH and microwave heating allows the recrystallization of rod shaped Co(OH)2. Successful formation of the Co(OH)2 phase was confirmed by XRD data (Figure S1a in the Supporting Information, reference code 01-074-1057). In the absence of SUS substrate, the morphology of Co3O4 is completely different. The possible reason behind different structural morphology of Co3O4-LNS was demonstrated in Scheme 2. In the absence of SUS substrate, the DETA forms a complex with a Co2+ ion in aqueous solution, then the cage like Cocomplex is able to stand in a strong alkaline solution.42 When the above solution was heated in a microwave oven, thermal relaxation of ligand and metal ion take place facilitating the easy access for OH− ion to interact with Co2+ ions, which led to formation of flower like Co(OH)2 nanomaterial. Time dependent study of the morphology reveals that the hexagonal sheets of Co(OH)2 form initially (5 min MW treatment) and further microwave heating for 5 min convert the hexagonal sheets into flower like layered sheets (Scheme 2). TG measurement was initially performed to assess the follow-up annealing process of the as-prepared precursors. Figure S2 (in the Supporting Information) shows the TGA plot of as prepared precursors, as depicted in Figure S2 (blue curve), the SUS is stable up to 600 °C under air atmosphere,32 a small amount of weight gain observed after 600 °C because of aerial oxidation of SUS surface. Figure S2 (red curve) shows the TGA curve of Co(OH)2 (scratched from Co(OH)2@SUS) sample undergoes a weight loss of 15.54% in the multistep weight loss process involving the decomposition of precursors and dehydration. In first step 1.5% weight loss was observed in the temperature range of 25−211 °C, this is because of the decomposition of water absorbed on the surface of the material.32 The second weight loss from 211−400 °C is associated with the conversion of Co hydroxides to spinel Co3O4.42 The weight loss after 400 °C was construed as the decomposition of spinel Co3O4. So to get spinel phase of Co3O4, 400 °C selected as annealing temperature. X- ray diffraction (XRD) was used to collect the crystallographic information of samples. The XRD pattern of Co3O4 from Co3O4@SUS (Figure 1, blue curve) can be assigned for

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RESULTS AND DISCUSSION Preparation and Characterizations of Co3O4@SUS and Co3O4-LNS. Co3O4 microrods were deposited onto the stainless steel mesh (SUS) using microwave heating method, as described in the experimental part. Scheme 1 demonstrates that in synthesizing specific shaped Co3O4@SUS electrodes, the DETA played an important role. As shown in Scheme 1, the synthesis process of the Co3O4@SUS was described as above. First, the DETA effectively interact with stainless steel using amino group via electrostatic adsorption or physisorption. Subsequently, the inductive effect and specific interaction helps to introduce Co2+ cation to DETA.40 Meanwhile, it is proved that the polar weak capping agent have a tendency to adsorb on (110) plane.41 Thus, polar DETA should adsorb on (110) that contributing to the growth of rod like morphology. DETA acts as an anchoring agent which facilitates the attachment of Co2+ 11071


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The chemical and electronic state of both Co3O4@SUS and Co3O4-LNS were investigated by X-ray photoelectron spectroscopy (XPS) analysis. For Co3O4@SUS catalyst following information was obtained. The best deconvolution of the Co 2p fine spectra was achieved by fitting to six components (Figure 2a), including two pairs of spin−orbit doublets that indicate the coexistence of divalent and trivalent cobalt and their two shakeup satellite peaks. The peak of 780.83 and 796.39 eV binding energy corresponds to the Co2+ and fitting peak of 779.59 and 794.72 eV binding energy should be for Co3+. The peaks at binding energy 788.42 and 803.93 eV are the shakeup satellite peaks.43,44 As shown in Figure 2b, the O 1s XPS spectra decomposed into three components: lattice oxygen O2− (529.84 eV), peroxide ions O− (530.92 eV), and superoxide ions O2− (532.32 eV).45 In this sample C 1s peak is also present, the presence of carbon may arise due to the decomposition of surface adsorbed DETA during annealing. The peak appeared at 284.47, 285.07, and 286.07 eV (Figure S3a, in the Supporting Information) corresponds to C−C and C−O−C bond.42,46 On the other hand, XPS analysis of Co3O4LNS gives the following results. First, the high-resolution Co 2p spectrum shows two different types of Co species (Figure 2c). The absolute peaks near 780.78 and 796.27 eV binding energies were assigned as Co2+, whereas the other distinct peaks having 779.54 and 794.53 eV binding energies belong to Co3+. The gap distance between the Co 2p3/2 and Co 2p1/2 peak is approximately 15.23 eV, which is the identification of the standard Co3O4 spectra and these results are consistent with previously reported Co3O4 materials.47 In the O 1s region, Co3O4-LNS shows same three peaks as like Co3O4@SUS (Figure 2d), but the intensity of all three peaks is different; the peak intensity ratio of lattice oxygen peak (529.8 eV) and other two peaks is lower in Co3O4-LNS than that of Co3O4@SUS, the lattice oxygen percentage is higher in Co3O4@SUS catalyst than Co3O4-LNS catalyst. The relative atomic ratio of O1/(O2 + O3) on the surface of the Co3O4 could be obtained by

Figure 1. X-ray diffraction patterns of the synthesized Co3O4@SUS, SUS, and Co3O4-LNS.

cubic Co3O4 (JCPDS card no. 01-076-1802, S.G. Fd3m/227, a0 = b0 = c0 = 8.0720 Å) confirming the phase purity of annealed sample. Figure 1 (green curve) shows the XRD pattern of Co3O4-LNS prepared without SUS substrate, and it also perfectly matches with spinel Co3O4 (JCPDS card no. 01-0761802). Then, the Scherrer’s equation was used to calculate the average crystallite sizes of Co3O4 from CO3O4@SUS and simple Co3O4-LNS by putting full width half-maximum values of their most intense peak.

D=

kλ β cos θ

Where k stands for Scherrer coefficient which is around 0.89, λ represents X-ray wavelength, β stands for full-width-halfmaximum (fwhm), and θ, for Bragg’s angle. The measured average crystallite sizes for Co3O4@SUS and Co3O4-LNS were 33.41 and 29.33 nm, respectively. A very small difference in crystallite size of Co3O4 was observed in two samples.

Figure 2. Representative XPS spectra, Co3O4@SUS (a) Co 2p, (b) O 1s, and Co3O4-LNS (c) Co 2p, (d) O 1s. 11072


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Figure 3. (a, b) FE-SEM, (c) TEM, and (d) HR-TEM, of Co3O4@SUS. (inset) SAED pattern for corresponding TEM images.

Figure 4. (a, b) FE-SEM, (c) TEM, and (d) HR-TEM of Co3O4-LNS. (inset) SAED pattern for corresponding TEM images.

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Figure 5. (a) OER polarization curves of Co3O4@SUS, Co3O4-LNS on SUS, RuO2 on SUS, and SUS electrode. (b) Corresponding Tafel slope. (c) Electrochemical impedance spectra of Co3O4@SUS, Co3O4-LNS on SUS, and SUS. (inset) Equivalent circuit. (d) Chronoamperometry (i−t) curves of Co3O4@SUS electrode in 1 M KOH solution at fixed potential of 0.500 V (vs Ag|AgCl).

comparing the area that the fitted curve covered. It could be clearly observed that the atomic ratio of O1/(O2 + O3) on the Co3O4@SUS (0.75) is higher than that of free-standing Co3O4LNS (0.52). This indicates that relatively more lattice oxygen sites are present in the Co3O4@SUS than that of Co3O4-LNS Also, noticeably there is a small positive shift toward higher binding energy observed in Co 2p spectra of Co3O4@SUS than that of Co3O4-LNS material (Figure S3c in the Supporting Information), which confirms the strong binding of Co3O4 microrod and SUS surface in [email protected] The deposition of the Co3O4 microrod on the surface of SUS substrate was also confirmed by FT-IR spectroscopy. As we see in Figure S4 (in the Supporting Information), the Co3O4@SUS sample shows two strong peaks at lower frequencies (553.70, 659.71 cm−1) which are the characteristic stretching vibration of Co−O bond in cobalt oxide, and those peaks are absent in bare SUS substrate.49 The physical appearance of the electrodes is shown in Scheme 1 (optical photographs). As we can see, before any treatment, the color of the stainless steel mesh is silvery white, but after deposition of Co(OH)2 the color of the mesh changes to pink which is also uniform in distribution. Lastly, after air annealing the Co(OH)2 converts into Co3O4 which make the SUS mesh color black, which suggests the successful deposition of a uniform Co3O4 layer. The surface morphology of prepared materials was characterized by FE-SEM and TEM analysis. Figure S5 (in the Supporting Information) shows the FE-SEM image of a bare SUS substrate; as we see in Figure S5a (in the Supporting Information) the surface of SUS substrate is plain and the average diameter of a single SUS wire is around 40 μm. Figure S1b, c, and d (see the Supporting Information) shows the FE-SEM images of Co(OH)2@SUS, in which we can clearly see the complete deposition of Co(OH)2 rod on the SUS surface. There was no morphology difference observed in

Co3O4@SUS compared with Co(OH)2@SUS. Figure 3a and b shows the FE-SEM images of Co3O4@SUS. Figure 3b reveal the uniform deposition of Co3O4 microrods of average diameter 400−500 nm on the surface of SUS. During developing the Co(OH)2 microrod crystals on the surface of SUS simultaneously some excess precipitation of Co(OH)2 also take place in to the solution. Thereby in between SUS mesh pores some aggregated rodlike particles were observed (Figure 3a). The Figure 3b inset shows high-resolution FE-SEM image of Co3O4@SUS, in which we can clearly see the morphology of Co3O4 microrod formed by sticking the nanorod plates to each other. Furthermore, the morphology of Co3O4@SUS was examined by TEM analysis. Figure 3d shows the HR-TEM images of Co3O4@SUS. The lattice spacing of 4.68 Å observed in Figure 3d was perfectly matched with the theoretical interplane spacing of spinel Co3O4 (111) plane. The selected area electron diffraction (SAED) pattern (inset of Figure 3d) displays distinct circular lines parallel to the rod axis, which correspond to (111), (311), (222), and (440) planes for Co3O4@SUS, the d spacing and these results are in good agreement with the XRD observations. On the other hand, the SUS substrate free prepared Co3O4 morphology analysis done by FE-SEM and TEM analysis. Figure 4 shows the FE-SEM and TEM image of Co3O4, as shown in Figure 4a, the Co3O4 prepared in absence of SUS substrate having a flowerlike morphology. The high-resolution image (Figure 4b) shows the flowerlike morphology made by many Co3O4 layered circular and hexagonal sheets. TEM image reveals that these sheets are porous in nature (inset of Figure 4c). In the Co3O4-LNS HRTEM, 4.66 Å lattice spacing was observed (Figure 4d) which perfectly matches with the theoretical interplane spacing of the spinel Co3O4(111) plane. The SAED pattern (inset Figure 4d) displays distinct circular lines parallel to the rod axis, which corresponds to (440), (220), (311), and (400) planes for 11074


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Table 1. Comparison of Electrocatalytic Activities of Co3O4@SUS for OER with Literature Reported Catalystsa

a

no.

catalyst

substrate/electrode

electrolyte

overpotential

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Co3O4@SUS Co3O4 RGO−Co3O4 yolk-shell nanocages Co3O4@MWCNT Porous Co3O4 nanosheets Co3O4/rGO hierarchically porous Co3O4/C nanowire arrays Co3O4‑δ-QDs Co3O4@C Co3O4@C-MWCNTs Co/Co3O4-NG Co3O4@C/CP plasma-engraved Co3O4 nanosheets Co@Co3O4/NC-2 Co3O4/NBGHSs Co3O4 Co3O4/N-rmG Co3O4C-NA Co3O4-MTA

SUS GC GC GC GC carbon paper Ni foam carbon paper carbon paper GC GC carbon paper Ti substrate GC GC Ni foam GC Cu foil Ni foam

1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 0.1 M NaOH 0.5 M H2SO4 0.1 M KOH 0.1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 1 M KOH

η10 = 298 mV η10 = 302 mV η10 = 450 mV η10 = 309 mV η10 = 368 mV η10 = 290 mV η30 = 318 mV η30 = 270 mV η10 = 310 mV η10 = 320 mV η10 = 437 mV η10 = 370 mV η10 = 300 mV η10 = 410 mV η10 = 470 mV η10 = 290 mV η10 = 310 mV η10 = 290 mV η150 = 360 mV

this work 42 60 23 61 62 63 64 65 66 67 65 68 69 70 71 72 73 53

Overpotential, η calculated by using the formula, η = ESHE − 1.23 V.

activity of different catalysts.50 Also, the current density 10 mA cm−2 is metric for practical solar fuel synthesis.51 From the Figure 5a curves, it can be observed that Co3O4@SUS exhibits the best electrocatalytic activity with a low onset potential and a low overpotential of 298 mV at a current density of 10 mA cm−2; a remarkably lower overpotential than previously reported cobalt oxide based materials (Table 1). While the other electrodes Co3O4-LNS on SUS, RuO2 on SUS, and SUS show current density of 10 mA cm−2 at an overpotential of 343, 202, and 355 mV respectively. Also, it is noticeable that the Co3O4@SUS electrode reach 50 mA cm−2 current density at a very low overpotential of 391 mV. The OER kinetics of the Co3O4@SUS, Co3O4-LNS on SUS, RuO2 on SUS, and bare SUS mesh electrode have been investigated using Tafel plots. The corresponding Tafel plots are constructed in Figure 5b. According to the Tafel equation η = b log j + a, where, η is the overpotential, b is the Tafel slope, j is the current density, and a is the intercept relative to the exchange current density. The Co3O4@SUS catalyst exhibits a Tafel slope of 105 mV dec−1 in 1 M KOH (Figure 6b) which is lower than the Co3O4-LNS on SUS (164 mV dec−1), RuO2 on SUS (163 mV dec−1), and SUS (122 mV dec−1). These results suggest that the Co3O4@SUS catalyst exhibit the most favorable reaction kinetics due to the rodlike morphology; according to previous studies, rod shaped nanomaterial has a higher surface to volume ratio and thereby considerably accelerates the reaction rates,8,52−55 and such materials demonstrate excellent activity because of the high energy of its (110) crystal plane that reduces the oxidation− reduction potential gap and thus speeds up the reaction rate.56,57 Electrochemical impedance spectroscopy (EIS) analysis was carried out to understand the nature of charge transfer phenomenon at the electrode−electrolyte interface. Figure 5c represents the Nyquist plot of EIS data and the corresponding equivalent circuit. In which Rs indicates the electrolyte resistance, and R1 and Q1 are sheet resistance and the modified double layer capacitance respectively, arising from the gathering of electron charges on the surface of the sheet. R2 indicates the charge transfer resistance at the electrode−

Co3O4, the d spacing of these results also consistent with the XRD observations. Elemental analysis was also performed to study the surface changes occur on the SUS substrate after microwave treatment. As shown in Table S1 (in the Supporting Information), after microwave treatment followed by annealing, the surface of SUS is completely covered by Co3O4. On the surface of bare SUS various metals are present (Table S1 in the Supporting Information), but after deposition of the Co3O4 microrod, the surface composition shows a negligible amount of Fe. Only 2.8% Fe and 1.3% Cr are visible on the surface of Co3O4@SUS, the remaining percentage of metal is only cobalt; which was further confirmed by elemental mapping (Figure S6a and b in the Supporting Information). On the basis of all of the above-discussed results, the prepared electrode material features can effectively promote the active sites on the material surface to expose to the electrolyte, which consequently appears in their excellent electrocatalytic properties toward the water oxidation reaction. Compared to the traditional methods, proposed synthesis method is simple, more environmentally friendly, scalable, and well-suited for large scale production. Electrochemical Tests. The OER performance of the prepared catalyst has been evaluated using a typical three electrode setup in 1 M KOH electrolyte. The performance of the reference materials, including commercially available RuO2 and Co3O4-LNS pasted on SUS (denoted as RuO2 on SUS and Co3O4-LNS on SUS, respectively), and bare SUS was added for comparison. Cyclic voltammetry measurements were carried out to stabilize the catalyst. The samples were first cycled in the potential range of 0 to 0.6 V (vs Ag|AgCl) to get a constant voltammogram. Subsequently, the electrochemical activity of prepared electrode toward OER was investigated by linear sweep voltammetry (LSVs). LSVs were recorded for all samples using scan rate of 5 mV s−1 in potential window 0−0.9 V (vs Ag|AgCl). Figure 5a shows the corresponding polarization data of Co3O4@SUS, Co3O4-LNS on SUS, RuO2 on SUS, and SUS. Onset potential is the most significant factor in evaluating the performance of OER catalyst, but it is difficult to observe exact value. Therefore, the value of the potential at 10 mA cm−2 is considered as the more reliable value to compare the OER 11075


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ACS Sustainable Chemistry & Engineering electrolyte interface the conclusive factor in OER process.58 EIS data fitting with an equivalent circuit consisting of constant phase elements provides the critical information on surface roughness and nature of distribution of the catalytic active sites on the electrode surface.58 W represents Warburg impedance and signifies the diffusion controlled behavior of ions through the film. The charge transfer resistance of Co3O4@SUS, Co3O4LNS on SUS, and SUS electrodes are 1.38, 15.57, and 7.66 Ω, respectively. The lowest charge transfer resistance of Co3O4@ SUS indicates that the self-supported Co3O4 catalyst facilitates the electron transfer more readily at the electrode−electrolyte interface and favors the OER process. To further investigate the surface impact of OER catalyst, the double layer capacitance (Cdl) is measured using CV in a nonfaradaic potential range (−0.10 to 0.10 V vs Ag|AgCl) under different scan rates (Figure S7 in the Supporting Information). A active surface area of the catalyst is normally proportional to its electrocatalytic activity, which is strongly associated with the double layer capacitance at the solid−liquid interface.59 A plot of scan rate against current density exhibits a linear relationship, and its slope is Cdl (Figure S7 b). The Cdl values of SUS, Co3O4@SUS, and Co3O4-LNS on SUS are 6.75, 38.68, and 4.61 mF cm−2, respectively. Co3O4-LNS on SUS electrode was fabricated by preparing a slurry of a binder (Nafion), Co3O4-LNS, and DMF. The binder molecules may block the active reaction sites, which led to the diminished active ECSA of Co3O4-LNS on SUS than that of SUS. It is clear that the double layer capacitance of Co3O4@SUS electrode is much higher than that of other electrodes, which suggests that the Co3O4@SUS electrode has much higher electrocatalytic active surface area than that of other tested electrode materials, thus it might be the possible reason that Co3O4@SUS shows higher catalytic activity toward OER. Grimaud et al. proved that the generated oxygen molecules during the oxygen evolution reaction came from lattice oxygen of metal oxides.9 As previously discussed in XPS analysis, the percentage of lattice oxygen in Co3O4@SUS catalyst is much more than that of a Co3O4-LNS catalyst. More lattice oxygen sites provide the reaction sites for OER, which results the high OER performance of the Co3O4@SUS electrode. The stability of the OER catalyst is also important for water splitting. Stability tests of all the prepared electrodes were carried out by chronoamperometry (CA) using a constant applied potential 0.500 V (vs Ag|AgCl). The current−time (i− t) curve shows a high stability for Co3O4@SUS catalyst (Figure 5d). Co3O4@SUS shows excellent stability toward OER with 78% retention of current even after 24 h. These results show that Co3O4@SUS catalyst displays excellent stability toward OER. We have also conducted the detailed post-mortem analysis for Co3O4@SUS after 24 h CA test. The FE-SEM image shows that the even after 24 h test, the morphology of the Co3O4@SUS remains unchanged (Figure S8c and d, in the Supporting Information). Also, there is no variation in the XRD pattern of Co3O4@SUS, and the phase is similar to Co3O4; these post-mortem analysis results of Co3O4@SUS suggesting that the catalyst is stable under a condition of continuous O2 evolution for an extended period (Figure S8a, in the Supporting Information). Such outstanding performance of the Co3O4@ SUS electrode is due to the following reason: The electrochemical active surface area of Co3O4@SUS catalyst is higher than that of other tested catalysts. The lower charge transfer resistance of Co3O4@SUS suggests a highly facilitated OER

process. The improved lattice oxygen percentage in Co3O4@ SUS catalyst provides more active sites for the OER.

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CONCLUSION In summary, a facile diethylenetriamine (DETA) assisted microwave irradiation and post annealing process were employed to synthesize a self-supported Co3O4 microrod array on stainless steel mesh (Co3O4@SUS). In this synthesis route, DETA acts an anchoring agent, which anchors the Co2+ ion on the surface of stainless steel. The higher percentage of lattice oxygen sites and positive shift of Co 2P binding energy was observed in Co3O4@SUS electrode over that of Co3O4LNS. In the electrochemical oxygen evolution reaction the current density 10 mA cm−2 was achieved at 298 mV overpotential with a low Tafel slope of 105 mV dec−1 for Co3O4@SUS electrode. Electrocatalytic results demonstrate that Co3O4@SUS exhibits advanced catalytic activity and outstanding cycling stability for the oxygen evolution reaction owing to unique morphology, porous nanostructure, and low charge transfer resistance. Importantly the scalable strategy for preparing Co3O4@SUS is a potential method for its practical applications in electrochemical water oxidation reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03027. Loading of catalysts, the thermodynamic potential calculations for OER, elemental composition data of catalysts, XRD spectrum and FE-SEM images of Co(OH)2@SUS, TGA graph of all precursors, C 1s XPS spectra of Co3O4@SUS, and Co3O4-LNS, FTIR spectra, FE-SEM images of bare SUS, elemental mapping, CV for Cdl, XRD and FE-SEM images of Co3O4@SUS after 24 h chronoamperometry test (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 82 31 330 6688. Fax: +82 31 336 6336. E-mail: [email protected] (H.K.). ORCID

Hern Kim: 0000-0002-6137-3201 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP). Grants were funded by the Ministry of Trade, Industry & Energy (MOTIE) (No. 20174010201160) and by the National Research Foundation of Korea (NRF). A grant was funded by the Ministry of Education (No. 2009-0093816), Republic of Korea.

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Microwave-Assisted Synthesis of a Stainless Steel Mesh-Supported

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