Engineering Oxygen Vacancies into LaCoO3 Perovskite for Efficient

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Engineering Oxygen Vacancies into LaCoO3 Perovskite for Efficient Electrocatalytic Oxygen Evolution Yao Lu, Aijing Ma, Yifu Yu, Rui Tan, Chengwei Liu, Peng Zhang, Dan Liu, and Jianzhou Gui ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05717 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Sustainable Chemistry & Engineering

Engineering Oxygen Vacancies into LaCoO3 Perovskite for Efficient Electrocatalytic Oxygen Evolution Yao Lu,† Aijing Ma,†,‡ Yifu Yu,§ Rui Tan,† Chengwei Liu,† Peng Zhang,† Dan Liu*,†, and Jianzhou Gui*,†,‡ †

Tianjin Key Laboratory of Green Chemical Technology and Process Engineering, School of Chemistry & Chemical Engineering, Tianjin Polytechnic University, Binshui West Road 399, Tianjin, 300387, P. R. China ‡ State Key Laboratory of Separation Membranes and Membrane Processes, School of Material Science and Engineering, Tianjin Polytechnic University, Binshui West Road 399, Tianjin, 300387, P. R. China § Department of Chemistry, School of Science, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Yaguan Road 135, Haihe Education Park, Tianjin, 300072, P. R. China *E-mail: [email protected] (LIU); [email protected](GUI)

ABSTRACT: Oxygen vacancies (OVs) can improve the catalytic activities in oxygen evolution reaction (OER). Although considerable effort has been devoted to increasing the OVs concentration in the electrocatalysts, limited OVs have been created by current techniques so far. Here, we, for the first time, engineered (i.e. created) abundant OVs into perovskites by element doping and plasma treatment. The results revealed that more OVs were manufactured by combination Sr doping with Ar plasma treatment, leading to improved OER activity and high stability of LaCoO3 perovskite. The La1-xSrxCoO3-δ (x=0.3) sample with Ar plasma treatment (denoted as Sr-0.3-p) showed high OER activity and stability due to the existence of rich OVs, which provided large amount as well as high intrinsic activity of active sites in OER. The combination of two OVs-creating techniques provides an efficient strategy to develop OVs-rich catalysts for various applications. 1

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KEYWORDS: OER; perovskite; Sr doping; plasma treatment; oxygen vacancies. INTRODUCTION The global energy crisis and environmental issue are driving researchers to develop renewable and clean energy to substitute the traditional fossil fuels.1 Water electrolysis is considered as one of the eco-friendly and sustainable methods to supply high-purity hydrogen energy. However, as a concomitant reaction of hydrogen production in water electrolysis, oxygen evolution reaction (OER) is undergoing the slow kinetics, which requires excess energy consumption and thus results in lower energy efficiency.2-4 The commercial OER catalysts, such as RuO2 and IrO25,6, are suffering from the difficulty of wide application because the noble-metal catalysts are precious and scarce. Therefore, it is urgent to develop inexpensive, efficient and stable OER catalysts as the substitution for noble-metal materials. During the past decades, transition metal-based oxides have been widely adopted as OER catalysts.7-9 Among them, perovskites have attracted more and more attention because of their cheap price, excellent structural stability and adjustable physical-chemical properties.10-18 Unfortunately, the OER performance of perovskites still needs to be further improved. It has been widely reported that oxygen vacancies (OVs) in transition metal-based oxides can efficiently improve their electrocatalytic 2


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performance.19-21 For example, Wang et al. proved that OVs generated in transition metal oxides can improve the electronic conductivity of catalysts and create more active sites, enhancing the electrocatalytic activity for OER.19 Xie group demonstrated that the OVs could efficiently increase the reactivity of active sites and OER activity by decreasing the adsorption energy of H2O on the surface of catalysts.20 To date, various techniques have been developed to create OVs, including calcination treatment under controlled atmosphere,22 doping,19 nonstoichiometric synthesis24, and plasma treatment.14 Although significant progress has been made in the area of preparing OVs-rich electrocatalysts, present works mostly focused on the adoption of solely technique to create limited OVs. It is reasonable to expect that the combination of different OVs-creating techniques can manufacture abundant OVs and thus greatly enhance the OER ability of perovskite-type oxides. Herein, by choosing LaCoO3 perovskite as model OER catalysts, we created OVs in perovskites by adopting two techniques simultaneously. This should be the first time to adopt the combination strategy of creating OVs in perovskites, as far as we know. First, we introduced OVs through substitution lower valence Sr2+ ion for the La3+ ion. Then, the Ar plasma treatment technique was adopted to further manufacture OVs (Figure 1a). The results demonstrated that OVs originated from the combination of two different techniques can improve the OER performance of LaCoO3 3



perovskite. Moreover, the electrocatalysts with abundant OVs showed high stabilities. RESULTS AND DISCUSSION The obtained samples were firstly examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It can be clearly seen that the average size decreased after the partial substitution Sr2+ cations for La3+ cations (Figure S1, Figure 1b). Moreover, the samples of Sr-0.3 and Sr-0.3-p showed similar size and crystalline structure (Figure S2), indicating that plasma treatment had little effect on the morphology of perovskites (Figure 1b,c). The smaller particle size would result in larger special surface areas (Table S1), which may benefit the electrochemical activity. The characterization of X-ray diffraction (XRD), depicted in Figure 1d, confirmed that the as-prepared samples presented a rhombohedral perovskite-type phase. Notably, it was observed that the diffraction peaks gradually shifted toward small diffraction angles with the increasing Sr2+ in perovskites (Figure 1e). This was ascribed to the successive replacement of smaller La3+ cation (rLa3+= 0.136 nm) by bigger Sr2+ cation (rSr2+= 0.144 nm), leading to the expansion of perovskite unit cell.25 Moreover, phase segregation in the form of SrCoOx (JCPDS 49-0692) started to be detected in Sr-0.5 sample, suggesting the limited Sr accommodation in La1-xSrxCoO3-δ perovskite lattice. In the spectra of Fourier Transform infrared spectroscopy (FT-IR), 4


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the vibration peaks at 1465 and 859 cm-1 demonstrated the formation of carbonate species in the Sr-0.5 sample (Figure S3).26,27 Comparing with Sr-0.3, the diffraction peaks of Sr-0.3-p showed little change. It implied that Ar plasma treatment had a negligible effect on the bulk structure of the sample, which was in agreement with the result before.28

Figure 1. (a) Scheme for engineering (i.e. creating) OVs into LaCoO3 perovskite; SEM images of (b) Sr-0.3 and (c) Sr-0.3-p samples; (d) XRD patterns of La1-xSrxCoO3-δ perovskites; (e) Zoom-in of the patterns (d) in the 2θ range of 46.5o~48.5o

X-ray photoelectron spectroscopy (XPS) was adopted to characterize the concentration of OVs. As shown in Figure 2a, four peaks were 5



obtained after the deconvolution of O 1s spectra. The peaks at 528.4, 530.0, 530.9 and 532.0 eV can be assigned to the lattice oxygen species (O1), OVs (O2), the surface-adsorbed oxygen species or the hydroxyl groups (O3) and the surface-adsorbed water (O4),29-33 respectively. Table S2 listed the relative concentration of each oxygen species in perovskites, which was obtained from the area ratio of corresponding peak. It can be seen that both Sr doping and Ar plasma treatment can increase the concentration of OVs. The sample of Sr-0.3-p exhibited higher OVs concentrations than the others, suggesting that it is an effective way to engineer (i.e. create) more OVs by a combination of two OVs-creating techniques. The concentration of OVs in all the samples was further confirmed by oxygen temperature-programmed desorption (O2-TPD) measurements. Three main oxygen desorption peaks were seen in the profiles (Figure 2b), which can be ascribed to physical adsorption oxygen (below 450 oC), chemical adsorption oxygen (450-700 oC) and the lattice oxygen in perovskites (above 700 oC).34-37 In O2-TPD profile, the integral area of desorption peak can reflect the concentration of corresponding specials. Previous studies have proven that chemical adsorption oxygen was closely related to the OVs.38-40 The integral area of chemical adsorbed oxygen in TPD profiles were calculated following the order: Sr-0.5 > Sr-0.3-p > Sr-0.3 > Sr-0.1 > Sr-0 (Table S3). Notably, the highest content of chemical adsorbed oxygen in Sr-0.5 may arise from the 6


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impurities (i.e., SrCoOx and carbonate species). All the aforementioned results prove more OVs can be readily created into LaCoO3 perovskite by the adoption of Sr doping and Ar plasma treatment together. It is predictable that the sample with a higher concentration of OVs will show better OER performance.

Figure 2. (a) XPS survey in O 1s region and (b) O2-TPD profiles of La1-xSrxCoO3-δ perovskites

The electrocatalytic OER performances of La1-xSrxCoO3-δ perovskites were measured in 1.0 M KOH (Figure 3). Potentials of reversible hydrogen electrode (RHE) with I-R compensation were adopted in the liner sweep voltammetry (LSV) polarization curves. As shown in Figure 3a, compared to LaCoO3, the Sr-doped catalysts required lower onset potentials and overpotentials relative to the same current densities. Among them, Sr-0.3-p catalyst exhibited the best OER activity, for which the onset potential and overpotential at 10 mA cm-2 were 120 mV and 326 mV, respectively. The Tafel plots (Figure 3b) and Nyquist impedance plots (Figure 3c) were utilized to reflect OER kinetics of catalysts. 7



Similar to the LSV results, Sr-0.3-p showed a smaller Tafel slop (70.8 mV dec−1) and a lower impendence (39.3 Ω) compared with the others, implying faster reaction kinetics. It should be mentioned that the onset potentials, the overpotentials under the same current densities, the Tafel slopes and the charge-transfer resistance of La1-xSrxCoO3-δ catalysts followed the order: Sr-0.3-p < Sr-0.3 < Sr-0.5 < Sr-0.1 < Sr-0, which is in inverse order to that of the concentration of OVs (Figure 2a). It is suggested that OVs played a dominant role in OER performance. The electrochemical stabilities of Sr-0.3 and Sr-0.3-p catalysts were tested by LSV curves initial and after 1000 cycles as well as chronopotentiometric curves. As shown in Figure 3d and its inset, Sr-0.3-p exhibited excellent stability for OER as well as Sr-0.3. Besides, the morphology of Sr-0.3-p kept unchanged after OER performances (Figure S4).

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Figure 3. (a) Polarization curves, (b) Tafel slopes and (c) EIS spectra of La1-xSrxCoO3-δ perovskites (in the top-right corner: EIS fitting model, where the symbols of Rs, Cd and Rct refer to the resistance of electrolyte, capacitive and charge-transfer, respectively.), (d) LSV curves of Sr-0.3 and Sr-0.3-p catalysts initial and after 1000 cycles (inset: chronopotentiometric curves of Sr-0.3 and Sr-0.3-p catalysts at 10 mA cm-2)

Figure 4. (a) Measurements of electrochemical double-layer capacitance at 0.975 V and (b) Current densities per ECSA vs. potential for Sr-0, Sr-0.3 and Sr-0.3-p samples 9



Electrochemically active surface areas (ECSA) of catalysts, which were calculated from electrochemical double-layer capacitance at 0.975 V, have been measured in order to account for the high activity of Sr-0.3-p catalyst in OER (Figure 4a and Figure S5). It was shown that the ECSA of Sr-0.3-p was 60 cm-2ECSA, which was higher than that of Sr-0.3 (52.5 cm-2ECSA) and Sr-0 (7.5 cm-2ECSA) catalysts. The results indicated that more active sites existed in Sr-0.3-p due to the massive OVs. Figure 4b exhibited the LSV curves with the current density normalized to ECSA. Interestingly, Sr-0.3-p still showed better performance than Sr-0.3 and Sr-0 catalysts, revealing the superior intrinsic activity of active sites in the Sr-0.3-p catalyst. A conclusion can be drawn from the above results that the catalyst treated by Sr doping and the following Ar plasma treatment can not only create more OVs in LaCoO3 perovskite but also increase the intrinsic activity of active sites, resulting in the improvement of OER activity. CONCLUSION In summary, we have successfully created more OVs in LaCoO3 perovskite by Sr doping and Ar plasma treatment. By the combination of two techniques, we achieved the outstanding OER performance due to the existence of abundant OVs, which acted as active sites and showed high intrinsic activity. This finding offers us a simple and effective strategy to create OVs and thus improve the OER activity. Moreover, the proposed 10


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strategy of combining different OVs-creating techniques and the immense potential of transition metal-based oxides will provide new insights into understanding the structure-property relationships of catalysts and explore more widespread application for OVs-rich materials. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/ acssuschemeng.xxxxxxx. Experimental section, SEM images, TEM and HRTEM images, FT-IR spectra, CV curves and Table S1-S3 (PDF). AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] (Liu); [email protected] (Gui) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21706191 and 21576211), China Postdoctoral Science Foundation (Grant No. 2017M610164), Program for Tianjin Innovative Research Team in Universities (Grant No.TD13-5031) and Tianjin 131 Research Team of Innovative Talents.

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Synopsis Abundant oxygen vacancies were created into LaCoO3 perovskites by the combination of Sr doping and Ar plasma treatment. TOC

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Engineering Oxygen Vacancies into LaCoO3 Perovskite for Efficient

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