Enabling Pyrochlore-Type Oxides as Highly Efficient Electrocatalysts

Oct 11, 2017 - Furthermore, the obtained improved electrochemical performances of Na–O2 batteries were found to be strongly related to metallic char...
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Research Article Cite This: ACS Catal. 2017, 7, 7688-7694

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Enabling Pyrochlore-Type Oxides as Highly Efficient Electrocatalysts for High-Capacity and Stable Na−O2 Batteries: The Synergy of Electronic Structure and Morphology Na Li,†,‡,§ Yanbin Yin,†,‡,§ Fanlu Meng,†,‡ Qi Zhang,‡ Junmin Yan,*,† and Qing Jiang† †

Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, Department of Materials Science and Engineering, Jilin University, Changchun 130022, People’s Republic of China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: Sodium−oxygen (Na−O2) batteries are proposed to be one of the most promising energy storage technologies due to their high energy density and abundance of sodium resources, while their practical applications still suffer from great challenges, including low capacity and poor cycle stability due to the absence of highly efficient electrocatalysts. In response, herein, as a proof of concept experiment, pyrochlore-type oxides, namely La 2 Sn 2 O 7 (La2−xSrxSn2O7 and La2Sn2−xCoxO7), possessing strongly correlated electronic systems, were employed as model and coupled systems to identify favorable electrocatalysts for Na− O2 batteries. Importantly, we first discovered the relationship between the enhancement of catalytic activities and the insulator to metal transition of pyrochlore-type oxides via systematically tuning the La/Sr or Sn/Co ratio. The obtained La2Co2O7 catalyst simultaneously endows Na−O2 batteries with excellent electrochemical performances, inducing high capacity (up to 20184.2 mAh g−1) and good cycle stability (up to 167 cycles). Furthermore, the obtained improved electrochemical performances of Na−O2 batteries were found to be strongly related to metallic character and the increased specific surface area of pyrochlore-type oxides, which should provide a design guideline to construct highly efficient catalysts for other metal−air batteries in addition to Na−O2 batteries. KEYWORDS: Na−O2 battery, catalytic activity, electronic structure, high capacity, stable

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INTRODUCTION The ever-increasing consumption of electrical energy and the pressing demand of renewable energy have forced us to develop next-generation battery-based energy storage systems with high energy densities that far exceed those of lithium-ion batteries.1−7 Recently, metal−air batteries such as Na−O2 batteries have attracted a great deal of attention as potential electrochemical energy storage devices due to their extremely high energy density,8−13 low cost (30 times cheaper than Li), and natural abundance (2.3−2.8% in the earth’s crust).14−16 Although some progress has been made, Na−O2 batteries are still in their infancy and suffer from daunting challenges such as low capacity and short cycle life, which are closely relevant to the sluggish reaction kinetics of the oxygen cathode, where the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) happen during charge and discharge processes, respectively.17−25 In this context, semiconducting or even insulating earth-abundant transition-metal oxides have been proposed to improve the performance of Na−O2 batteries,26−32 while the formation of Schottky barriers at catalyst−support © XXXX American Chemical Society

and/or catalyst−electrolyte interfaces leads to poor cyclability and stability. Therefore, developing an efficient electrocatalyst to accelerate OER and OER processes for Na−O2 batteries is thus of great importance while still being very challenging. Pyrochlore (A2B2O6O′, A = alkaline-earth or lanthanide cations; B = transition-metal cations) type compounds as one of the important oxide materials have versatile applications in the fields of data storage, superconductors, and catalysts.33−37 As the 3D network of corner-sharing BO6 octahedra generates a cagelike B2O6 framework to provide a conduction path for the electrons, pyrochlore oxides thus have considerable flexibility for compositional variations entailing structural distortions, with A and O′ atoms occupying interstitial sites to form A− O′−A linkages.38−40 Theoretically, introduction of heteroatoms on the A or B sites can lead to valence state changes and structural adjustments (such as B−O or A−O′ bond length, B− Received: June 24, 2017 Revised: September 15, 2017

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DOI: 10.1021/acscatal.7b02074 ACS Catal. 2017, 7, 7688−7694

Research Article

ACS Catalysis O−B bond angle size) and thus modify the electronic properties through influencing the interactions between the 3d electrons of the A site atoms or B site atoms (Scheme 1), which might provide new possibilities for constructing highly efficient bifunctional OER/OER for Na−O2 batteries.41,42 Scheme 1. Tuning the Structure and Electronic Properties of La2−xSrxSn2O7 and La2Sn2−xCoxO7 via Adjusting the La/Sr or Sn/Co Ratioa To Improve the Performance of Na−O2 Batteries

Figure 1. (a) XRD patterns, (b) FTIR spectra, and XPS (c) O 1s and (d) Sn 3d spectra of La2−xSrxSn2O7.

the SnO6 octahedron. The peak suffers from red shift, which should be induced by the changing lengths of Sn−O bonds or a decrease in the O−Sn−O bond angle (Table S1 in the Supporting Information). These changes in Sn−O bond length may influence oxygen mobility of the catalysts, which is expected to result in the differences in the catalyst activity (vide infra). In order to get an in-depth understanding of the electronic structure, we further studied the XPS spectra of all the pyrochlore oxides La2−xSrxSn2O7. As shown in Figure 1c, with an increase in the Sr content, the O 1s spectra change noticeably, which is represented by the surface affinity to oxygen species. In detail, the two primary peaks of O 1s spectra are consistent with lattice oxygen (∼528.9 eV) and surfaceabsorbed oxygen species (∼531.3 eV), respectively. Obviously, the two peaks shift noticeably. When x ≥ 1, namely La0.5Sr1.5Sn2O7 and Sr2Sn2O7, they exhibit slight peak shifts, lattice oxygen peaks being ca. 0.2 eV lower and surface oxygen peaks being ca. 0.4 eV higher. The abundance of surface oxygen species on transforming to metal corresponds to the theoretically predicted stabilizing effect of metallic band structures on surface oxygen species.30 We also examined the XPS valence band spectra of Sn 3d and Sr 3d. As shown in Figure 1d and Figure S4 in the Supporting Information, the two peaks corresponding to Sn 3d and Sr 3d both slightly shift to higher positions, which can be attributed to the change in the Sn−O bond. Once Sr replaces La, the bond length and angle between Sn and O will change, which results in a higher binding energy of Sn−O. In order to verify the Sr atom introduction of the pyrochlore oxides La2Sn2O7, we tested the EDX (Figure S5 in the Supporting Information) and ICP-MS (Table S2 in the Supporting Information) of La2−xSrxSn2O7 samples, which indicate that the Sr content is consistent with the ratio of raw materials. As demonstrated above, the incorporation of Sr has changed the bond length and angle, thus modifying the structural and electronic properties. These changes lead to the emergence of metallic properties with strong reflectivity toward infrared light, as proven by density of states calculations and electrical conductibility shown in Figure S1 and S2 in the Supporting Information, which is expected to enhance the catalyst activity (vide infra).

a

Structural tuning in Table S1 of the Supporting Information and electrical conductivity in Figures S1 and S2 in the Supporting Information.

Herein, as a proof of concept experiment, we first propose and explore the flexible electronic properties of strongly correlated electron systems, namely La2Sn2O7 pyrochlore-type oxides (La2−xSrxCo2O7 and La2Sn2−xCoxO7), for accelerating the OER and ORR processes of Na−O2 batteries. When they are first employed as new cathode catalysts for Na−O2 batteries, in comparison to commercial carbon materials, pyrochlore-type oxides exhibit superior electrocatalytic behavior, which is related to the La/Sr ratio on the A site or Sn/Co ratio on the B site. In addition, La2Sn2−xCoxO7 (x = 2) exhibits better electrochemical performance in comparison to the aforementioned pyrochlore-type oxides. The enhanced catalytic activity is evidently attributed to the synergy of emerging metallic properties (Figures S1 and S2 in the Supporting Information) and changing morphology (or specific surface area increasing).

2. RESULTS AND DISCUSSION Substituent A Site in La2Sn2O7: La2−xSrxSn2O7. We first studied the pyrochlore-type oxides La2−xSrxSn2O7 as the oxides can satisfy the Sr content changing from x = 0 to x = 2. The Xray diffraction patterns (XRD) of La2−xSrxSn2O7 (x = 0, 0.2, 0.5, 1.0, 1.5, 2.0) are shown in Figure 1a. Obviously, all the diffraction patterns are consistent with the standard pattern of La2Sn2O7 (JCPDS No. 130082, Fd3̅m). However, increasing Sr content leads to the peaks shifting gradually to larger angles (Figure 1a inset), which may be due to the lower ionic radius of Sr. In order to understand the structural details, we refined all the XRD patterns of La2−xSrxSn2O7 (Figure S3), and the Rietveld refinement results indicate that Sr successfully replaced the La atom site. Note that the space group Fd3̅m corresponds to the random distribution of A-site ions. FTIR was then performed; as shown in Figure 1b, the peaks around 608 cm−1 should originate from vibration and bending of Sn−O bonds in 7689

DOI: 10.1021/acscatal.7b02074 ACS Catal. 2017, 7, 7688−7694

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which is reported to be relatively stable toward superoxide in comparison with carbonate-based electrolyte.43,44 As shown in Figure 3a, during the discharge process, the cathodes with

The morphologies of the pyrochlore oxide (La2−xSrxSn2O7) samples are shown in Figure 2. With an increase in the Sr

Figure 3. (a) First charge−discharge curves of Na−O2 batteries with or without pyrochlore oxides La2−xSrxSn2O7 as catalysts at a current density of 100 mA g−1 (mass 0.5 mg). (b) Cyclic voltammetry curves and (c) rate capability of Na−O2 batteries with or without pyrochlore oxides as catalysts. (d) Coulombic efficiency of Na−O2 batteries with or without pyrochlore oxides as catalysts.

La2−xSrxSn2O7 catalysts exhibit higher specific capacity in comparison to SP cathode, which indicates the excellent catalytic activities of La2−xSrxSn2O7 for the ORR. In detail, the discharge specific capacity regularly increases along with an increase in x. Moreover, in contrast with the SP cathode, the cathodes with La2−xSrxSn2O7 catalysts show lower overpotential during the charge process, which indicates superior OER performance. In order to demonstrate the superior catalytic activity, we measured the cyclic voltammetry curves of the La2−xSrxSn2O7 catalysts to reflect the ORR and OER processes. In comparison with the SP cathode, the cathodes with La2−xSrxSn2O7 catalysts have higher onset potential and anodic peak voltage, which is consistent with the higher ORR catalytic activity. According to the anodic scan, the cathodic peaks are higher than that of the pure SP cathode, corresponding to superior activity during the OER process. After introduction of Sr atoms, the catalytic activity is improved regularly with an increase in the Sr content, which should result from the change in electronic structure and the enhancement of specific surface area. We then investigated the rate performance of La2−xSrxSn2O7 catalyst. As shown in Figure 3c, the discharge voltage plateau of the La2−xSrxSn2O7/SP cathodes are higher than that of the SP cathode at each current density. The discharge voltage plateau and Coulombic efficiency increase along with an increase in the Sr atom content (Figure 3d). In addition, the Na−O2 battery with Sr2Sn2O7 exhibits much better cyclic performance (vide infra) in comparison to that with other La2−xSrxSn2O7 (x < 2). All of these significant improvements could be attributed to the high catalytic activity derived from higher conductivity (increasing metallicity), porous structure, and higher specific surface area of La2−xSrxSn2O7. In conclusion, the cathode with Sr2Sn2O7 catalyst exhibits the best performance in Na−O2 batteries.

Figure 2. Scanning electron microscopy (SEM) images of pyrochlore oxides La2−xSrxSn2O7: (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 1.0; (e) x = 1.5; (f) x = 2.0 (inset: high-magnification SEM image).

content, the morphology changes noticeably, especially for x ≥ 1.5. The morphology of La2−xSrxSn2O7 (x < 1) changes with a decrease in the size of nanoparticles. However, La0.5Sr1.5Sn2O7 and Sr2Sn2O7 exhibit rodlike and tubelike morphologies, respectively. In detail, the rodlike La0.5Sr1.5Sn2O7 is composed of nanoparticles, which may enhance catalytic activity due to the abundance of pores on the surface. The tubelike Sr2Sn2O7 may possess superior catalytic activity, due to the advantage of morphology, providing more paths and sites for the reactants. We also observed the transmission electron microscopy (TEM) images (Figure S6) of La2−xSrxSn2O7, which are consistent with the SEM images. The high-resolution TEM (HRTEM) images collected from pyrochlore oxides surface exhibit well-resolved lattice fringes with a d spacing of 0.308 nm, corresponding to the (222) plane of La2−xSrxSn2O7 (Figure S7 in the Supporting Information). Introduction of Sr causes morphological changes which may lead to the different catalytic activities of pyrochlore oxides (vide infra). The specific surface area of La2−xSrxSn2O7 was estimated by N2 adsorption−desorption isotherms (Figure S8 in the Supporting Information), which is representative of mesoporous materials with no or few micropores. The specific surface area presents an increasing trend with an increase in x. La2−xSrxSn2O7 (x ≥ 1.5) shows larger surface area and pores, in comparison to those of x < 1. The increasing specific surface area benefits from the decreased nanoparticle size and the novel morphology, which results in the improvement in catalytic activity. The catalytic activities of La2−xSrxSn2O7 catalysts were examined in Na−O2 batteries in comparison with commercial Super P (SP), using 0.5 M NaCF3SO3-TEGDME as electrolyte, 7690

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ACS Catalysis Substituent B Site in La2Sn2O7: La2Sn2−xCoxO7. Inspired by the promising results obtained above, the A site plays an important role in regulating the catalytic properties. To obtain an optimal catalyst, exploring the influence of B site would be absolutely necessary. Therefore, for comparison, other pyrochlore-type oxides (La2Sn2−xCoxO7, x = 0, 0.2, 0.5, 1.0, 1.5, 2.0) were for study. The XRD patterns of La2Sn2−xCoxO7 are consistent with the standard pattern of La2Sn2O7 (Figure 4a), except for a slight shift to larger angle due to the lower

atoms increases. We also analyzed the XPS valence band spectra of Co 2p. As shown in Figure 4d, the Co valence in La2Co2O7 is a mixture of valences +2 and +3, which is different from the case for La2Sn2−xCoxO7 (x < 2). Moreover, even some of the Co atoms in La2Co2O7 are partially oxidized from Co3+ to Co4+, which goes hand in hand with hole doping in the Co 3d bands. When the Co content is increased, the valence of Co changes between +2 and +3. In order to form the stable crystalline structure, the length and angle of Co−O bonds and La−O bonds change noticeably, which is consistent with the theoretical result (Scheme 1 and Figure S1 in the Supporting Information). Then we investigated the morphology of the pyrochlore oxides La2Sn2−xCoxO7 samples as shown in Figure 5. With an

Figure 4. (a) XRD patterns, (b) FTIR spectra, and XPS (c) O 1s and (d) Co 2p spectra of La2Sn2−xCoxO7.

ionic radius of Co2+ or Co3+. The refinement XRD patterns of La2Sn2−xCoxO7 further indicated that the Co atom successfully replaces Sn (Figure S9 in the Supporting Information). As shown in Figure 4b, FTIR peaks in the range 600−700 cm−1 should originate from vibration and bending of Sn−O bonds or Co−O bonds in the SnO6 or CoO6 octahedron. However, with an increase in Co content, the peaks of Co−O bonds should increase and those of Sn−O bonds gradually disappear. Therefore, there is an insulator to metal transition, which shows a trend similar to that of La2−xSrxSn2O7 with respect to both electronic structure evolution and catalytic activity. In order to verify the introduction of Co atoms into the pyrochlore-type oxides La2Sn2O7, we carried out energy dispersive X-ray (EDS) (Figure S11 in the Supporting Information) and ICP-MS tests (Table S3 in the Supporting Information) of La2Sn2−xCoxO7 samples, which indicate that the ratio of Co content is consistent with the ratio of raw materials. To understand the electronic structure information, we tested pyrochlore oxides La2Sn2−xCoxO7 using XPS. As shown in Figure 4c, the O 1s spectra changes noticeably with an increase in the Co content, which represents the surface affinity for oxygen species. Obviously, lattice oxygen peaks of La2Sn0.5Co1.5O7 and La2Co2O7 move to lower binding energy and surface oxygen peaks of La2Sn0.5Co1.5O7 and La2Co2O7 move to higher binding energy. The enrichment of the surface oxygen species on transforming to metal corresponds to the theoretically predicted stabilizing effect of metallic band structures on surface oxygen species. The incorporation of Co atoms in the pyrochlore oxide crystal structure leads to the formation of oxygen vacancies and causes local structure changes, where the coordinative unsaturation of specific Sn

Figure 5. SEM images of pyrochlore oxides La2Sn2−xCoxO7: (a) x = 0; (b) x = 0.2; (c) x = 0.5; (d) x = 1.0; (e) x = 1.5; (f) x = 2.0.

increase in Co content, the morphology remains unchanged, but the nanoparticle sizes decrease greatly, which is very different from the case for La2−xSrxSn2O7. We also measured the TEM images (Figure S12 in the Supporting Information) of La2Sn2−xCoxO7, which generally follow the SEM images. The HRTEM image collected from the pyrochlore oxide La2Co2O7 surface exhibits well-resolved lattice fringes with a d spacing of 0.308 nm, corresponding to the (222) planes of La2Sn2O7 (Figure S13 in the Supporting Information). Introduction of Co results in a decrease in the nanoparticle size, which contributes to the enhancement in catalytic activity. The BET surface area of La2Sn2−xCoxO7 was then studied using N2 adsorption−desorption isotherms (Figure S14 in the Supporting Information). The La2Co2O7 sample exhibits a high surface area of 129.4 m2 g−1 and large pore volume of 1.057 cm3 g−1. The BET surface area exhibits an increasing trend along with the increased Co content, which is related to the smaller crystallite sizes. The variation in surface area results in an improvement in catalytic activity (vide infra). 7691

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Co content (Figure 6d). The gradual enhancement of rate performance and Coulombic efficiency from x = 0 to x = 2 benefits from increasing catalytic activity, which is related to conductivity and nanoparticle size. On the basis of theoretical calculations, we can conclude that the conductivity increases gradually from x = 0 to x = 2, which is consistent with the trend of the catalytic activity enhancement. Moreover, the batteries with La2Sn2−xCoxO7/SP cathodes showed better cyclic performance (Figure 7b); even the La2Co2O7/SP cathode could undergo 167 cycles, which is higher than those of a La2Sn2O7/SP cathode (64 cycles) and an SP cathode (52 cycles). All of these significant improvements could be attributed to the synergistic effect of high electronic conductivity and large surface area of La2Sn2−xCoxO7. In conclusion, the cathode with La2Co2O7 as catalyst exhibits the best performance of Na−O2 batteries, which is attributed to the metallic properties (high conductivity) and smaller nanoparticles (superior surface area and large holes). Electrochemical Performance. These above enhanced electrochemical performances demonstrate that replacing not only the A site atom but also the B site atom in La2Sn2O7 would lead to different catalytic activities. It is worth exploring which plays an even greater role; therefore, we compared the battery performances of La2Sn2−xCoxO7 with La2−xSrxSn2O7 in Figure 7. Obviously, Na−O2 batteries with La2Sn2−xCoxO7/SP cathodes exhibited the best performance, which is expectantly attributed to the different metal band structures. In detail, we tested the electrochemical impedance spectra (EIS) of Na−O2 batteries after several cycles (Figure S15 in the Supporting Information). The EIS of La2Co2O7/SP cathodes are all lower than those of Sr2Sn2O7/SP cathodes, which should promise better cycle performance. Surprisingly, the capacity retention of the La2Co2O7/SP cathode is slightly higher than that of the Sr2Sn2O7/SP cathode, which is similar to the variation in specific capacity (Figure 7c). Better rate performance was also achieved with the La2Co2O7/SP cathode (Figure S17 in the Supporting Information). On the basis of the above results, La2Co2O7 exhibits the best electrochemical performances over other pyrochlore-type oxides mentioned above. This can be explained by the fact that La2Sn2−xCoxO7 can offer more surface oxygen vacancies and active sites in comparison to La2−xSrxSn2O7, which is likely to stabilize intermediate oxygen species during ORR and OER processes. In order to understand the composition of the discharge products with different catalysts, we used Raman spectra, XPS, and XRD for analysis. We found that the discharge products were composed of NaO2 and Na2O2.45−49 In addition, the pyrochlore catalysts have no effect on the composition of discharge products (Figures S18−S20 in the Supporting Information).

We further investigated the electrochemical performance of pyrochlore oxides La2Sn2−xCoxO7 in Na−O2 batteries in comparison with pristine SP cathode, using 0.5 M NaCF3SO3-TEGDME as electrolyte. As shown in Figure 6a,

Figure 6. (a) First charge−discharge curves of Na−O2 batteries with or without pyrochlore oxides La2Sn2−xCoxO7 as catalysts at a current density of 100 mA g−1 (mass: 0.5 mg). (b) Cyclic voltammetry curves and (c) rate capability of Na−O2 batteries with or without pyrochlore oxides La2Sn2−xCoxO7 as catalysts. (d) Coulombic efficiency of Na− O2 batteries with or without pyrochlore oxides La2Sn2−xCoxO7 as catalysts.

in contrast to pure SP cathode, the Na−O2 batteries with a La2Sn2−xCoxO7/SP cathode exhibits superior specific capacity during the discharge process, indicating the superior ORR catalytic activity of La2Sn2−xCoxO7. In detail, the discharge specific capacity of La2Co2O7 can reach 19040 mAh g−1, which is much higher than those of La2Sn2O7/SP cathode (6455.2 mAh g−1) and of SP cathode (5012 mAh g−1). Furthermore, the specific capacity increased regularly along with x from 0 to 2. In addition, during the charge process, the cathode with La2Sn2−xCoxO7 as catalyst shows lower overpotential, which indicates superior OER performance. With an increase in Co atoms, the catalytic activity improved regularly, which is attributed to the changes in the electronic structure (tendency to be metallic) and the gradual decrease in nanoparticle size (increased BET surface area). To confirm the superior catalytic activity, we measured the cyclic voltammetry curves of the Na− O2 batteries with La2Sn2−xCoxO7 as catalyst to reflect the ORR and OER processes (Figure 6b). The cathode with La2Sn2−xCoxO7 as catalyst has a gradually increased onset potential and anodic peak voltage, which is consistent with the high ORR catalytic activity. From the anodic scan, the cathodic peaks are both higher than that of the pure SP cathode, corresponding to the high catalytic activity in the OER process. The obtained and much improved specific capacity of Na−O2 batteries could be attributed to synergistic effects of the high ORR and OER catalytic activity and the unique porous hollow structure of La2Sn2−xCoxO7. We also investigated the rate performance of La2Sn2−xCoxO7 catalyst, which showed that the discharge voltage plateau of La2Sn2−xCoxO7/SP cathodes is higher than that of SP cathode at each current density (Figure 6c). The discharge voltage plateau increases along with an increase in Co atom content. Moreover, the Coulombic efficiency of La2Sn2−xCoxO7 catalysts gradually improved, which is accompanied by the increase in

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CONCLUSION In summary, the electronic properties of pyrochlore-type oxides, namely La2−xSrxSn2O7 and La2Sn2−xCoxO7 (x = 0, 0.2, 0.5, 1.0, 1.5, 2.0), were explored as ORR and OER catalysts in Na−O2 batteries. Gradually substituting atoms at the A or B sites result in the properties tending to be metallic, which is tightly related to significant improvement in catalytic activity for the Na−O2 battery. In addition, La2Sn2−xCoxO7 (x = 2.0) exhibited optimal electrochemical performance, including high capacity (up to 20184.2 mAh g−1) and good cycle stability (up to 167 cycles). This research indicates that the metallic property is an important parameter for oxide catalysts of metal−air batteries, which can improve the catalytic activity, 7692

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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/acscatal.7b02074. Experimental details, detailed analysis data, TEM images, EDS patterns, N2 adsorption−desorption isotherm graphs of pyrochlore oxides, XPS valence band spectra and Rietveld refinements of La 2−x Sr x Sn 2 O 7 and La2Sn2−xCoxO7, and corresponding electrochemical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.Y.: [email protected]. ORCID

Junmin Yan: 0000-0001-8511-3810 Qing Jiang: 0000-0003-0660-596X Author Contributions §

N.L. and Y.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (51522101, 51631004, 51471075, and 51401084) and the Program for JLU Science and Technology Innovative Research Team (2017TD-09).

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REFERENCES

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Figure 7. Cycle performance of Na−O2 batteries at a current density of 200 mA g−1 (mass 0.5 mg) with a limited capacity of 2000 mAh g−1: (a) La2−xSrxSn2O7/SP cathodes and (b) La2Sn2−xCoxO7/SP cathodes. (c) Capacity retention of Na−O2 batteries with the four kinds of electrodes at different current densities.

reduce electronic transmission resistance, and lower the overpotentials during discharging/charging cycles. The electronic structures can be selectively tuned toward targeted interactions with surface adsorbed species by replacing atoms in different sites. Furthermore, the electronic structure−activity and surface−activity relations provide a promising guideline for constructing efficient catalysts in heterogeneous catalysis. 7693

DOI: 10.1021/acscatal.7b02074 ACS Catal. 2017, 7, 7688−7694

Research Article

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DOI: 10.1021/acscatal.7b02074 ACS Catal. 2017, 7, 7688−7694