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Top-down preparation of active cobalt oxide catalyst Yue Zhou, Cunku Dong, Li-Li Han, Jing Yang, and Xi-Wen Du ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02416 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016
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Top-down preparation of active cobalt oxide catalyst Yue Zhou†,‡, Cun-Ku Dong†, Li-Li Han†, Jing Yang†, Xi-Wen Du*,† †
Institute of New-Energy Materials, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China ‡
Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin University, Tianjin, 300072, China Supporting Information
ABSTRACT: Cobalt oxide is a cheap catalyst for oxygen evolution reaction; however, the low activity limits its practical application. Herein we report the preparation of highly active Co3O4 catalyst via a top-down process, laser fragmentation. The fierce laser irradiation generates fine and clean nanoparticles with abundant oxygen vacancies which simultaneously improve the adsorption energy and electrical conduction. Resultantly, the catalytic performance of product reaches the top level of cobalt oxide, even outperforming the noble-metal catalyst, RuO2.
Water splitting is expected as a promising way to supply a clean and sustainable hydrogen source, which is of crucial importance and great challenge.1-5 The whole water-splitting process involves hydrogen evolution reaction and oxygen evolution reaction (OER), and OER is kinetically unfavorable owing to multiple steps of proton-coupled electron transfer, thus an effective catalyst is indispensable for OER.5-9 To date, noble metal oxides, such as RuO2 and IrO2, have shown high performance on the catalysis of OER.10, 11 However, the prohibitive price and scarcity of these noble metals severely hinder their practical applications. Therefore, it is imminent to explore new catalysts with low price and superior properties. Over the past decade, cobalt oxides emerged as promising non-noble catalysts for OER.12, 13 Nevertheless, most of the catalysts suffer from poor performance which was attributed to the weak adsorption of H2O molecules and their poor conductivity.14 A possible solution is to introduce oxygen vacancies in the catalysts.15-17 On one hand, oxygen vacancies can adsorb H2O molecules efficiently,15, 16 on the other hand, they can delocalize electrons which are easy to be excited to the conduction band, thus increasing the conductivity.17 Traditionally, oxygen vacancies are generated by annealing in reductive atmosphere such as hydrogen16, 18, or reacting with active metals at high temperature.19 However, these high temperature approaches are not energy efficient and usually out of control. Recently, a low temperature technique was developed to form oxygen vacancies by immersing the sample in NaBH4 solution, which largely enhances the performance of the catalyst.20 However, the high overpotential (about 400 mV) and of reduced Co3O4 sample cannot meet the requirements of practical application. Recently, our group exploited a top-down strategy based on laser fragmentation for directly conversing bulk materials into colloidal nanoparticles (e. g. monodisperse quantum dots and metallic nanocrystals).21 Different from bottom-up routes based on chemical reactions, laser fragmentation is intrinsically a physical process in which chemical precursors or surfac-
tants can be excluded completely, thus the products usually expose clean surface with high reactivity.14, 22, 23 Meanwhile, this approach is featured with high temperature, high pressure, rapid heating, and quick cooling, which certainly facilitate the formation of defects (such as oxygen vacancy) in the asprepared nanoparticles.24, 25 Considering the above two aspects, we infer that the top-down approach is advantageous over the conventional chemical routes on preparing active catalyst, although no related work has been reported. In the test of the above hypothesis, we prepare Co3O4 nanoparticles by laser fragmentation of commercial powder, and compare with the nanoparticles by bottom-up synthesis on their performance. The laser-generated catalysts are found to contain a large number of oxygen vacancies, which simultaneously improve the adsorption energy and electrical conductivity. Resultantly, they achieve the best performance ever reported for cobalt oxides, the current density and catalytic kinetics even outperform commercial noble-metal catalyst, RuO2, demonstrating the power of laser fragmentation on producing catalysts.
Figure 1. Synthesis and characterizations of Co3O4 nanoparticles. (a) Schematic illustration on the synthesis of nanoparticles via laser fragmentation. (b) XRD patterns of L-Co3O4, R-Co3O4 and H-Co3O4. (c) TEM image of nanoparticles in L-Co3O4, (d) TEM image of nanoparticles in H-Co3O4.
As schematically shown in Figure 1a, commercial Co3O4 powders (R-Co3O4) were adopted as the raw material for preparing Co3O4 nanoparticles by laser fragmentation (L-Co3O4). In addition, a conventional hydrothermal route was employed to synthesize Co3O4 nanoparticles (H-Co3O4) as the control sample. X-ray diffraction (XRD) patterns of L-Co3O4, RCo3O4 and H-Co3O4 shown in Figure 1b are well indexed to
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cubic Co3O4 (JCPDS 43-1003). This result demonstrates that laser beam merely breaks up the large R-Co3O4 particles without modification of their crystal structure or loss of elements, which is confirmed by Raman spectra and energy dispersive spectrometer (EDS) profiles (see Figure S1). The FTIR results in Figure S2 illustrate that hydroxyl groups exist on the surface of L-Co3O4 nanoparticle, while both acetate and hydroxyl groups are found on the surface of H-Co3O4 nanoparticles, indicating that laser fragmentation can generate cleaner nanoparticles. Transmission electron microscope (TEM) observation indicates that R-Co3O4 is composed of irregular and uneven particles with size of several hundred nanometers (Figure S3). After laser irradiation, the produced L-Co3O4 NPs are uniform and well dispersed with average size of 5.8±1.1nm (Figures 1c, S4a and S4b). Noticeably, the nanoparticles in HCo3O4 show similar uniformity and size distribution (Figures 1d, S4c and S4d)
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sion and storage systems. As shown in Figure 2d, L-Co3O4 demonstrates high stability of the chronopotentiometric response at a constant current density of 10 mA/cm2. After testing the durability for 75 hours, the current remains 92% of its initial value, and the LSV plot is very close to the pristine one. Obviously, the properties of L-Co3O4 are superior to those of H-Co3O4, R-Co3O4, and even RuO2. In fact, the overpotential, current density and catalytic kinetics of L-Co3O4 represent the top level ever reported for cobalt oxide under the same testing conditions (see Table S1). 17, 26-28
Figure 3. (a) The relationship between the current density and the scan rate. (b) EIS spectra of different catalysts recorded at 1.59 V. (c) and (d) XPS spectra of O1s and Co 2p, respectively.
Figure 2. The electrochemical properties of Co3O4 catalysts. (a) CV curves, (b) LSV curves, and (c) Tafel plots of three Co3O4 catalysts. (d) Stability of L-Co3O4, the Insert is polarization curves before and after testing for 75 hours.
Electrocatalytic oxygen-evolution activity was assessed in 1 M aqueous KOH by loading the catalysts on carbon fibers with catalyst weight of 0.3 mg/cm2. Cyclic voltammetry (CV) measurements were firstly performed over a potential range of 1.0 to 1.55 V (vs reversible hydrogen electrode, RHE), and the results were shown in Figure 2a. Compared with R-Co3O4 and H-Co3O4, L-Co3O4 displays a remarkable increase in current density over the entire potential range, demonstrating higher electrochemically active surface area of L-Co3O4.17 The polarization curves measured in 1 M KOH solution were shown in Figure 2b, with RuO2 as the reference. The overpotentials at 10 mA/cm2 of RuO2, L-Co3O4, H-Co3O4 and R-Co3O4 are 298, 294, 336 and 430 mV, respectively. In addition, the catalytic kinetics for OER reaction was evaluated by Tafel plots (Figure 2c). The slope of L-Co3O4 (74 mV/decade) is lower than those of RuO2 (78 mV/decade), H-Co3O4 (88mV/decade) and R-Co3O4 (92mV/decade). Long term durability of catalyst is of great significance for energy conver-
To discover the origin of the excellent performance of LCo3O4, we firstly measured the electrochemical double-layer capacitance (Cdl) and surface roughness factor (Rf) of three Co3O4 samples. As shown in Figures 3a and S5, the current density shows a linear relationship against the scan rate, from which Cdl value is determined by the slope of the line, while Rf can be calculated by dividing the Cdl of catalysts with the Cdl of bulk cobalt oxide (60 µF/cm2). 6 Among the three samples, L-Co3O4 shows the largest Cdl and Rf values (35.4 mF/cm2 and 590, respectively), being more than one and a half times of those for H-Co3O4 (20.7 mF/cm2 and 345) and four times of these for R-Co3O4 (7.35 mF/cm2 and 122.5) (see supporting information Figure S5). Since the nanoparticles in LCo3O4 show a size distribution (and then specific surface area) similar with those in H-Co3O4, the higher Cdl and Rf values of L-Co3O4 should arise from increased active sites created by laser ablation. We also measured the electrical conductivity of three samples by means of electrical impendence spectroscopy (EIS). Figure 3b shows the Nyquist plots tested at 1.52 V vs RHE where current density of L-Co3O4 is 10 mA/cm2, the semicircular diameter of L-Co3O4 is the smallest among the three samples. The corresponding equivalent circuit model is shown in Figure S6, where Rs represents the resistance of the electrolyte, CPE the capacitance phase element and Rct the charge transfer resistance between the catalysts and the electrolyte.
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By fitting the Nyquist plots with the given model, the Rct values of R-Co3O4, H-Co3O4, and L-Co3O4 can be determined as 381.3, 239.6 and 87.1 Ω, respectively, indicating that electrons can transfer more rapidly in L-Co3O4. To find out the intrinsic reason behind the increased active sites and electrical conductivity of L-Co3O4, we adopted X-ray photoelectron spectroscopy (XPS) to distinguish oxygen valence state in the three samples (Figure S7). As shown in Figure 3c, O 1s spectra can be fitted with three major oxygen contributions with the corresponding peaks centered at 529.7, 531.2, and 532.7 eV in sequence. These bands can be attributed to oxygen atoms bound to metal Co atoms (OL), oxygen atoms in low coordinated Co-O bonds (OV) and oxygen atoms in water molecule absorbed on Co3O4 surface (OW), respectively.15, 29, 30 It is obvious that the spectrum of L-Co3O4 exhibits the largest OV peak area (36.22%) among three samples (see Table S2), indicating the highest amount of oxygen vacancies which was recognized as the active sites for OER electrocatalysis process.15 Besides, Co 2p 3/2 and Co 2p 1/2 peaks in Figure 3d can be deconvoluted into Co2+ and Co3+.The peaks located at 781 and 795 eV belong to Co2+ , while those at 797 and 779 eV to Co3+, as documented in literature.31, 32 Obviously, the Co2+ peak of L-Co3O4 is higher than these of other samples, which confirms the presence of more oxygen vacancies in L-Co3O4.
Figure 4. Structural modes of Co3O4 (110) surface for DFT calculation. Perfect Co3O4 (110) surface (a) before and (b) after the adsorption of H2O molecule. Co3O4 (110) surface with an oxygen vacancy (c) before and (d) after the adsorption of H2O molecule. (e) Temperature rise of R-Co3O4 nanoparticles as a function of laser fluence in a single pulse.
The rate-determining step in four-electron OER process is known as the adsorption of H2O onto the surface of catalysts.33 Hence, it is essential to obtain the catalyst with high affinity for water molecules. To investigate the effect of oxygen vacancies on the adsorption of water, we made density functional theory (DFT) calculation on the Co3O4 (110) surface without and with oxygen vacancies (see details in Supporting Information section 4 and Figure S8). After a water molecule adsorbs onto perfect Co3O4 (110) surface (see Figures 4a and 4b), it dissociates immediately into a hydroxyl group and a hydrogen atom. The hydroxyl group prefers to bind with two adjacent cobalt atoms in a bridge-adoption manner, while the hydrogen atom binds with the neighbor oxygen atom (Figure 4b). In contrast, the oxygen vacancy on the Co3O4 (110) surface gives rise to the rearrangement of surficial atoms (Figure
4c), and the dissociative hydroxyl group preferably occupies the oxygen vacancy to saturate the coordination of cobalt atoms (Figure 4d). Our calculation indicates that the oxygenvacant Co3O4 (110) surface possesses larger adsorption energy (2.81 eV) for water molecule than the perfect surface (1.09 eV), thus accelerating the rate-determining step in OER process. To elucidate the formation of oxygen vacancies in L-Co3O4, we calculate the temperature rise and phase transformation of R-Co3O4 under laser irradiation with different beam intensities (see details in Supporting Information section 5), the result was shown in Figure 4e. As the beam intensity exceeds 0.41 J/cm2, the particles in R-Co3O4 gets boiling by laser heating. In our experiment, a laser fluence of 0.6 J/cm2 was adopted for laser experiment, hence, R-Co3O4 was vaporized partially and broken into nanoparticles via trival fragmentation.34 After the laser pulse, the hot nanoparticles were quickly cooled down to ambient temperature by the water medium. Such a heatingquenching process causes the loss of oxygen atoms of RCo3O4 and introduces oxygen vacancies into nanoparticles of L-Co3O4 product.6, 34 Hence, the non-equilibrium laser fragmentation is favorable for generating active sites for catalyzing OER. In view of above, the ultrahigh OER activity of L-Co3O4 can be attributed to three reasons. First, oxygen vacancies on the surface of L-Co3O4 facilitate the adsorption of H2O molecules which, in turn, accelerates the whole OER process. Second, the conductivity of L-Co3O4 is also improved by oxygen vacancies, thus ensuring good electron transport. Last but not the least, the ultrafine nanoparticles in L-Co3O4 possesses clean surface and large specific surface area, thus allowing the adequate exposure of oxygen vacancies. In conclusion, we employed laser fragmentation to introduce oxygen vacancies into Co3O4 nanoparticles and identify their role on electrocatalysis from both theoretical and experimental sides. The laser induced oxygen vacancies can adsorb water molecules efficiently and improve the conductivity; resultantly, the Co3O4 nanoparticles produced by laser ablation shows OER activity outperforming RuO2, a commonly used noble-metal oxide. This study reveals, for the first time, how intensive laser generates oxygen vacancies and how oxygen vacancies improves OER performance, and our work illuminates the green and rapid top-down approach is powerful on producing cheap and active catalysts.
ASSOCIATED CONTENT Supporting Information. Materials, detailed synthetic procedures, characterization methods, calculation and additional experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * E-mail for X.W.D.:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT
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This work was supported by National Key Basic Research Program of China (2014CB931703) and National Natural Science Foundation of China (Nos. 51471115 and 51171127).
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SYNOPSIS TOC
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