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Article Cite This: ACS Omega 2017, 2, 9280−9286

Monodispersed Ru Nanoparticles Functionalized Graphene Nanosheets as Efficient Cathode Catalysts for O2‑Assisted Li−CO2 Battery Liangjun Wang,†,‡,○ Wenrui Dai,§,∇,○ Lipo Ma,∥ Lili Gong,‡ Zhiyang Lyu,§ Yin Zhou,§ Jia Liu,⊥ Ming Lin,# Min Lai,† Zhangquan Peng,*,∥ and Wei Chen*,‡,§,∇ †

School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, Jiangsu, China ‡ Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore § Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore ∥ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin 130022, China ⊥ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore # Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, 138634 Singapore ∇ National University of Singapore (Suzhou) Research Institute, Suzhou 215123, China S Supporting Information *

ABSTRACT: In Li−CO2 battery, due to the highly insulating nature of the discharge product of Li2CO3, the battery needs to be charged at a high charge overpotential, leading to severe cathode and electrolyte instability and hence poor battery cycle performance. Developing efficient cathode catalysts to effectively reduce the charge overpotential represents one of key challenges to realize practical Li−CO2 batteries. Here, we report the use of monodispersed Ru nanoparticles functionalized graphene nanosheets as cathode catalysts in Li−CO2 battery to significantly lower the charge overpotential for the electrochemical decomposition of Li2CO3. In our battery, a low charge voltage of 4.02 V, a high Coulomb efficiency of 89.2%, and a good cycle stability (67 cycles at a 500 mA h/g limited capacity) are achieved. It is also found that O2 plays an essential role in the discharge process of the rechargeable Li−CO2 battery. Under the pure CO2 environment, Li−CO2 battery exhibits negligible discharge capacity; however, after introducing 2% O2 (volume ratio) into CO2, the O2-assisted Li−CO2 battery can deliver a high capacity of 4742 mA h/g. Through an in situ quantitative differential electrochemical mass spectrometry investigation, the final discharge product Li2CO3 is proposed to form via the reaction 4Li+ + 2CO2 + O2 + 4e− → 2Li2CO3. Our results validate the essential role of O2 and can help deepen the understanding of the discharge and charge reaction mechanisms of the Li−CO2 battery.

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can promote the battery discharge process.7 The room temperature rechargeable Li−CO2 battery was realized by Liu et al., with commercial Ketjen black cathode, exhibiting a capacity of 1032 mA h/g at a current density of 30 mA/g.6 Owing to the structural advantages, carbon nanotubes and graphene were also introduced into the Li−CO2 batteries as cathode materials, displaying high discharge capacities of 5786 and 6600 mA h/g at the current density of 100 mA/g,

INTRODUCTION Lithium−air (Li−air) battery has been considered as a promising alternative to power electric vehicles using renewable electricity due to its extremely high theoretical energy storage capacity.1−4 One practical issue is to develop the Li−air battery that can work efficiently even in the CO2-contaminated O2 environment. Therefore, intensive research efforts have been devoted to the investigation of the working mechanism of lithium−oxygen (Li−O2) battery in the presence of CO2, as well as the development of the so-called lithium−CO2 (Li− CO2) battery.5−15 Li−CO2 battery was investigated by Archer et al. at various temperatures, and found that high temperature © 2017 American Chemical Society

Received: October 6, 2017 Accepted: December 1, 2017 Published: December 29, 2017 9280

DOI: 10.1021/acsomega.7b01495 ACS Omega 2017, 2, 9280−9286

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Figure 1. (a) FESEM of Ru/GNSs. (b, c) TEM images of Ru/GNSs at different magnifications. Inset in (c) is the corresponding HRTEM image of a single Ru particle, with the white scale bar of 2 nm.

respectively.5,16 Similar to the insulating discharge product of Li2O2 in the Li−O2 battery that usually leads to a high charge overpotential, the development of Li−CO2 battery encounters the same challenge. As a wide band gap insulator, the discharge product of Li2CO3 is difficult to be fully decomposed or usually be decomposed at very high potentials,17−20 thereby resulting in a series of issues such as low energy efficiency, low Coulombic efficiency, electrolyte decomposition, and poor cycle stability.21,22 Hence, it is crucial to rationally design advanced cathode catalysts to effectively decompose Li2CO3 with a low charge overpotential. For the Li−CO2 battery, it was proposed to work via a reversible reaction of 4Li + 3CO2 → 2Li2CO3 + C.10 However, in other studies, the cells delivered negligible discharge capacity in the pure CO2 atmosphere.8,23 Takechi et al. reported that only a very limited capacity of 66 mA h/g was delivered, but a high capacity of ∼4000 mA h/g could be achieved by introducing 10% O2 (volume ratio) into CO2. It was suggested that during the discharge process, CO2 cannot be directly electrochemically reduced, instead, O2 was first reduced to superoxide anion radical (O•− 2 ) and then rapidly consumed by CO2 to achieve a high discharge capacity.8 Therefore, it is still controversial whether CO2 electrochemical reduction exists in the Li−CO2 battery. Therefore, more systematic studies are needed to unveil the working mechanism of Li−CO2 battery. In this work, we develop the graphene nanosheets functionalized with monodispersed Ru nanoparticles with an average size of 2 nm as efficient cathode catalyst to effectively promote the decomposition of Li2CO3 at a low charge potential of 4.02 V, thereby achieving a relatively high Coulombic efficiency and a good cycle stability. We also investigate the discharge and charge reaction mechanism of Li−CO2 and O2-assisted Li− CO2 batteries. Our results confirm the essential role of O2 in Li−CO2 batteries during the discharge process, where O2 is first electrochemically reduced at the cathode and further chemically reacts with CO2 to form the discharge product of Li2CO3 as derived from an in situ quantitative differential electrochemical mass spectrometry (DEMS) analysis of the O2-assisted Li−CO2 battery.

that Ru nanoparticles are homogeneously dispersed on GNSs with an average size of about 2 nm. As shown in the highresolution TEM (HRTEM) image in the inset of Figure 1c, the spacing between adjacent fringes is ca. 0.21 nm, corresponding to the (101) lattice spacing of Ru. This can be further confirmed by the X-ray diffraction (XRD) result in Figure S1, where the broad peak at around 24° can be indexed to the diffraction peak (002) of the reduced graphene oxide and other peaks to the metallic Ru.24,25 The specific surface area of Ru/ GNSs is determined to be 121.05 m2 /g from the N 2 adsorption−desorption measurement (Figure S2a). A bimodal pore size distribution located at 6 is observed (Figure S2a, inset), which can effectively promote efficient electrolyte impregnation and gas/Li+ diffusion.26,27 X-ray photoelectron spectroscopy (XPS) and Raman measurements were further used to characterize the Ru-functionalized GNSs (Figures S3 and S2b). The Ru content in Ru/GNSs hybrid is 54 wt %, as determined by the thermogravimetric analysis (TGA) measurement (Figure S2c). Batteries were measured under three different atmospheres, i.e., pure CO2, CO2 with 2% O2 (Figure 2), and pure O2 (Figure S4). CO2 was purified by Agilent gas clean system to minimize the influence of H2O and O2 (H2O < 0.1 ppm, O2 < 50 ppb). Under the pure CO2 environment, negligible discharge capacity is obtained for both Ru/GNSs and the bare GNSs cathodes. By introducing 2% O2 into CO2, the batteries with Ru/GNSs and bare GNSs cathodes exhibit a high

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RESULTS AND DISCUSSION The as-prepared Ru/GNSs possess a typical porous wrinkle-like structure as revealed by the field emission scanning electron microscopy (FESEM) image in Figure 1a. Transmission electron microscopy (TEM) images (Figure 1b,c) indicate

Figure 2. Full discharge/charge curve of Ru/GNSs and bare GNSs cathodes in CO2 and CO2 with 2% O2 atmospheres at a current density of 0.08 mA/cm2. 9281


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Figure 3. (a) Raman spectra, (b) XRD patterns, and (c) Li 1s XPS spectra of Ru/GNSs cathode cycled in CO2 with 2% O2 atmosphere at different states.

Figure 4. FESEM images of Ru/GNSs cathode in CO2 with 2% O2 atmosphere at (a) pristine, (b) half discharge, (c) full discharge, and (d) charge states, scale bars 1 μm. The inset in (c) is the corresponding enlarged FESEM image, scale bar 100 nm.

by the appearance of a peak at 1086 cm−1 in the Raman profile in Figure 3a, XRD patterns in Figure 3b, and Li 1s peak with a binding energy at 55.7 eV in Figure 3c after discharge.24,28,29 It is also found that the discharge product of Li2CO3 can be almost fully decomposed during the charge process, as revealed by the disappearance of these Li2CO3-related features in Raman/XRD/XPS spectra in Figure 3 after the full charge process. This suggests that Ru/GNSs are effective electrochemical catalysts to decompose Li2CO3. The morphology evolution of Ru/GNSs cathode during the discharge/charge processes was investigated by FESEM (Figure 4). Upon half discharge, the discharge product of Li2CO3 appears as sheet-like aggregations decorated on the cathode (Figure 4b). The discharge product completely covers the cathode surface and evolves into floccule-like morphology after a full discharge process (Figure 4c). This morphology is completely different from the filmlike morphology of Li2O2 for the Ru/GNSs cathode in the pure O2 atmosphere (Li−O2 battery) operated under the same current density (Figure 5).

discharge capacity of 4742 and 5385 mA h/g, respectively. In this context, the battery is termed O2-assisted Li−CO2 battery. As shown in Figure 2, both batteries show a voltage plateau of 2.76 V on discharge, similar to that of Ru/GNSs in the pure O2 (Figure S4). During the recharge process, the O2-assisted Li− CO2 battery with Ru/GNSs exhibits a low charge voltage of 4.02 V. Moreover, 4230 mA h/g capacity (89.2%) can be recharged at a low cutoff voltage of 4.2 V; in contrast, for the battery with bare GNSs, only 1721 mA h/g (31.9%) capacity can be recharged even at a high cutoff voltage of 4.5 V. This comparison indicates that Ru nanoparticles can effectively promote the decomposition of the discharge product in the O2assisted Li−CO2 battery. At a high current density of 0.16 mA/ cm2, the battery with Ru/GNSs can still deliver a discharge capacity of 3865 mA h/g, with an average voltage of 2.67 V, and a charge capacity of 3000 mA h/g, with an average voltage of 4.10 V (Figure S5). The discharge product of the O2-assisted Li−CO2 battery using Ru/GNSs is mainly comprised of Li2CO3, as evidenced 9282


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Figure 5. FESEM images of Ru/GNSs cathode in O2 atmosphere after (a) discharge and (b) charge. White scale bars, 1 μm.

Figure 6. DEMS results of gas consumption and evolution during discharge (a) and charge (b) of the O2-assisted Li−CO2 cells based on Ru/GNSs. The cell was measured with a fixed capacity of 50 mA h/g at a constant current of 0.4 mA.

reaction, and hence the discharge reaction can be described as 4Li+ + 2CO2 + O2 + 4e− → 2Li2CO3, with the overall O2/CO2 consumption ratio of 0.5.18 This coincides well with our DEMS result. As shown in Figure 6b, during the charge process, CO2 is released at a nearly constant rate of about 2 × 10−3 μmol/s, but there is no apparent release of O2 during the whole charge process. By comparing the rate of CO2 consumption and evolution during a full discharge/charge cycle, CO2 recovery efficiency is quantified to be ca. 0.98 (Table 1). Three Li2CO3 decomposition pathways have been proposed in previous studies.10,31 The oxidative reaction (2Li2CO3 → 4Li+ + 4e− + O2 + 2CO2) can be first ruled out because no O2 is detected during the whole charge process. For the second pathway, 2Li2CO3 + C → 4Li+ + 3CO2 + 4e−, with the ratio between CO2 evolution during charge and CO2 consumption during discharge being 1.5, is inconsistent with the CO2 recovery efficiency (0.98) from the DEMS result. As the third possibility, Li2CO3 can be decomposed into CO2 and O•− 2 , and the generated O•− can be further consumed via reactions with the 2 electrolyte, as demonstrated in a previous study.31 In this case, CO2 evolution rate during charge process is equal to the CO2 consumption rate during discharge process, in good agreement with our DEMS result (0.98). We further evaluate the cycle ability of the O2-assisted Li− CO2 batteries. 3178 mA h/g (67%) capacity can be retained at the third cycle (Figure S6a) in the case of Ru/GNSs, with a nearly complete decomposition of Li2CO3 in the cycling potential window. In contrast, a very low capacity of 206 mA h/ g is obtained at the third discharge process (Figure S6b) for the bare GNSs due to the cathode passivation induced by the accumulation of the undecomposed Li2CO3 (Figure S7). The

After the charge process, the cathode restores to its original morphology (Figure 4d), suggesting the almost full decomposition of the discharge product of Li2CO3. To explore the discharge and charge working mechanisms of the O2-assisted Li−CO2 battery with the Ru/GNSs cathode, in situ DEMS was conducted to monitor the O2 and CO2 consumption/evolution rate during a discharge/recharge cycle with a fixed capacity of 50 mA h/g at a constant current of 0.4 mA for both discharge and charge processes, as shown in Figure 6. The simultaneous consumption of O2 and CO2 is observed throughout the whole discharge process (Figure 6a), and the consumption ratio of O2/CO2 is quantified to be about 0.44, close to 0.5 (Table 1). During the discharge process in the Table 1. Gases Consumed and Evolved in a Full Cycle of Li− O2/CO2 Cell Quantified with DEMSa charge passed

CO2 quantity

O2 quantity

cathode

D (a)

R (a)

C (a)

E (a)

C (a)

E (a)

Ru/GNSs

1.866

1.866

0.794

0.780

0.349

∼0

μmol/mg, the quantities of charge and gas have been normalized to the total mass of the respective cathodes. D: discharge; R: recharge; C: consumption of gas during discharge; E: evolution of gas during recharge. a

presence of O2, it is proposed that O2 can be first electrochemically reduced at the cathode to form Li2O2, which further chemically reacts with CO2 forming the thermodynamically more stable Li2CO3 (2Li2O2 + 2CO2 → 2Li2CO3 + O2). In another pathway, CO2 can react with the intermediate O2− and finally form Li2CO3.30 For both proposed mechanisms, O2 is the only oxidant to catalyze the discharge 9283


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Figure 7. Cycling performance of (a) Ru/GNSs and (b) GNSs cathode in CO2 with 2% O2 atmosphere at a limited capacity of 500 mA h/g.

into the O2 electrochemistry in the Li−CO2 batteries with noble metal catalysts.

cycle stability is then evaluated by employing the widely adopted capacity-limiting method.32,33 As shown in Figure 7, when the capacity is limited at 500 mA h/g, the O2-assisted Li− CO2 battery with Ru/GNSs cathode can be operated for 67 cycles at a current density of 0.16 mA/cm2, which is superior to the battery with the bare GNSs cathode (12 cycles). It demonstrates that Ru nanoparticles enhance recyclability via promoting the deposition/decomposition of Li2CO3. It has been reported that Ru is able to efficiently oxidize LiOH at low voltages.34 Therefore, Ru can be considered as a promising catalyst for practical Li−air batteries operated in ambient air, where Li2O2, LiOH, and Li2CO3 coexist as discharge products.35−37 Causes of battery failure after 67 cycles are scrutinized. After disassembling the battery at the 60th cycle, the cathode surface is covered by undecomposed side products (Figure S8). These side products originate from the decomposition of dimethyl sulfoxide (DMSO) electrolyte caused by the superoxide anion radicals O•− 2 during cycling. Although some of them can be decomposed upon recharge, side products increase upon recharge due to the electrolyte decomposition at high potentials, as previously reported.21,38,39 The continuous consumption of electrolyte and the accumulation of side products together result in battery deterioration.21 Therefore, it is crucial to further search for a more stable electrolyte for the Li−O2 and O2-assisted Li−CO2 batteries.

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EXPERIMENTAL SECTION Material Synthesis. Ru/GNSs were prepared by an impregnation method followed by heat treatment in 5% H2/ Ar gas. Typically, 51.5 mg RuCl3·xH2O (Sigma-Aldrich) and 4 mL graphene oxide (GO, 5 g/L from graphene supermarket) were dispersed in deionized water with constant stirring and sonication. The final product was obtained by freeze-drying the well-dispersed solution, followed by annealing in 5% H2/Ar gas at 300 °C for 5 h. Characterization. X-ray diffraction (XRD) measurements were conducted on a Bruker, D8-Advance X-ray diffractometer with Cu radiation (Cu Kα = 0.15406 nm). Thermogravimetric analysis (TGA) was performed by using TA instrument 2960 at a heating rate of 10 °C/min from room temperature to 900 °C under air flow. Brunauer−Emmett−Teller (BET) surface area was measured by nitrogen sorption at 77 K on a surface area and pore size analyzer (Quantachrome Instruments NOVA 2200e). The pore size distribution was obtained by using the Barrett−Joyner−Halenda method. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM, JEOL 2010) and field emission scanning electron microscopy (FESEM, JEOL JSM 6700F) were employed for material characterization. All of the electrodes after discharge and charge were disassembled, rinsed with acetonitrile, and naturally dried in Ar-filled glovebox before further characterization. For the characterization of the O2-assisted Li−CO2 battery electrodes, XPS system was specially configured with the XPS preparation chamber directly connected to a glovebox, which can significantly reduce the air contamination. The peak positions were corrected by referencing the C 1s peak position of carbon (284.6 eV). Raman data were collected with a 532 nm excitation laser, and the equipment was specially configured so that electrodes can be kept in Ar protective atmosphere during the entire measurement. Battery Assembly and Testing. The cathode was prepared by mixing 90% active material (Ru/GNSs or GNSs) with 10% poly(vinylidene difluoride) (PVDF) in N-methyl-2pyrrolidone dispersant. The obtained slurry was then coated onto Toray Carbon Paper (TGP-H-060) and dried at 100 °C overnight before use. A typical mass loading of the cathode catalyst was 0.4−0.5 mg/cm2 and the specific capacity was calculated based on the total mass of the cathode catalyst. The Li−CO2, O2-assisted Li−CO2, and Li−O2 batteries were assembled based on a coin-cell structure in an Ar-filled glovebox (H2O < 0.1 ppm, O2 < 0.4 ppm). The lithium foil

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CONCLUSIONS In summary, monodispersed Ru nanoparticles functionalized graphene nanosheets have been employed as cathode catalysts in the rechargeable O2-assisted Li−CO2 battery. It is found that O2 plays an essential role in the realization of the rechargeable Li−CO2 battery. Only with the existence of O2 (2% in this study) can the Li−CO2 battery deliver an impressive discharge capacity of 4742 mA h/g, with a discharge voltage of 2.76 V at a current density of 0.08 mA/cm2; in contrast, under the pure CO2 environment, the battery can hardly work. Through in situ DEMS measurements, we propose that during the discharge process, O2 is first electrochemically reduced to form the intermediate O•− 2 or Li2O2, which further chemically reacts with CO2 to form the final discharge product of Li2CO3. With the help of Ru nanoparticles, the discharge product of Li2CO3 can be electrochemically decomposed at a relatively low charge potential of 4.02 V (0.08 mA/cm2 current density), and the battery can operate for 67 cycles at a limited capacity of 500 mA h/g (0.16 mA/cm2 current density). Our results provide new understandings of the Li−CO2 batteries and new insights 9284


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(2) Wang, J.; Li, Y.; Sun, X. Challenges and Opportunities of Nanostructured Materials for Aprotic Rechargeable Lithium-Air Batteries. Nano Energy 2013, 2, 443−467. (3) Park, M.; Sun, H.; Lee, H.; Lee, J.; Cho, J. Lithium-Air Batteries: Survey on the Current Status and Perspectives Towards Automotive Applications from a Battery Industry Standpoint. Adv. Energy Mater. 2012, 2, 780−800. (4) Kraytsberg, A.; Ein-Eli, Y. The Impact of Nano-Scaled Materials on Advanced Metal−Air Battery Systems. Nano Energy 2013, 2, 468− 480. (5) Zhang, Z.; Zhang, Q.; Chen, Y.; Bao, J.; Zhou, X.; Xie, Z.; Wei, J.; Zhou, Z. The First Introduction of Graphene to Rechargeable Li-CO2 Batteries. Angew. Chem., Int. Ed. 2015, 54, 6550−6553. (6) Liu, Y.; Wang, R.; Lyu, Y.; Li, H.; Chen, L. Rechargeable Li/CO2O2 (2:1) Battery and Li/CO2 Battery. Energy Environ. Sci. 2014, 7, 677−681. (7) Xu, S.; Das, S. K.; Archer, L. A. The Li−CO2 Battery: A Novel Method for CO2 Capture and Utilization. RSC Adv. 2013, 3, 6656− 6660. (8) Takechi, K.; Shiga, T.; Asaoka, T. A Li−O2/CO2 Battery. Chem. Commun. 2011, 47, 3463−3465. (9) Li, X.; Yang, S.; Feng, N.; He, P.; Zhou, H. Progress in Research on Li-CO2 Batteries: Mechanism, Catalyst and Performance. Chin. J. Catal. 2016, 37, 1016−1024. (10) Yang, S.; Qiao, Y.; He, P.; Liu, Y.; Cheng, Z.; Zhu, J.-j.; Zhou, H. A Reversible Lithium-CO2 Battery with Ru Nanoparticles as a Cathode Catalyst. Energy Environ. Sci. 2017, 10, 972−978. (11) Xie, Z.; Zhang, X.; Zhang, Z.; Zhou, Z. Metal-CO2 Batteries on the Road: CO2 from Contamination Gas to Energy Source. Adv. Mater. 2017, 29, No. 1605891. (12) Wang, X.-G.; Wang, C.; Xie, Z.; Zhang, X.; Chen, Y.; Wu, D.; Zhou, Z. Improving Electrochemical Performances of Rechargeable LiCO2 Batteries with an Electrolyte Redox Mediator. ChemElectroChem 2017, 4, 2145−2149. (13) Qiao, Y.; Yi, J.; Wu, S.; Liu, Y.; Yang, S.; He, P.; Zhou, H. LiCO2 Electrochemistry: A New Strategy for CO2 Fixation and Energy Storage. Joule 2017, 1, 359−370. (14) Zhang, X.; Wang, X.-G.; Xie, Z.; Zhou, Z. Recent progress in rechargeable alkali metal-air batteries. Green Energy Environ. 2016, 1, 4−17. (15) Xu, S.; Lau, S.; Archer, L. A. CO2 and ambient air in metaloxygen batteries: steps towards reality. Inorg. Chem. Front. 2015, 2, 1070−1079. (16) Zhang, X.; Zhang, Q.; Zhang, Z.; Chen, Y.; Xie, Z.; Wei, J.; Zhou, Z. Rechargeable Li-CO2 Batteries with Carbon Nanotubes as Air Cathodes. Chem. Commun. 2015, 51, 14636−14639. (17) Ling, C.; Zhang, R.; Takechi, K.; Mizuno, F. Intrinsic Barrier to Electrochemically Decompose Li2CO3 and LiOH. J. Phys. Chem. C 2014, 118, 26591−26598. (18) Gowda, S. R.; Brunet, A.; Wallraff, G. M.; McCloskey, B. D. Implications of CO2 Contamination in Rechargeable Nonaqueous LiO2 Batteries. J Phys. Chem. Lett. 2013, 4, 276−279. (19) Meini, S.; Tsiouvaras, N.; Schwenke, K. U.; Piana, M.; Beyer, H.; Lange, L.; Gasteiger, H. A. Rechargeability of Li-Air Cathodes PreFilled with Discharge Products Using an Ether-Based Electrolyte Solution: Implications for Cycle-Life of Li-Air Cells. Phys. Chem. Chem. Phys. 2013, 15, 11478−11493. (20) Gallant, B. M.; Mitchell, R. R.; Kwabi, D. G.; Zhou, J.; Zuin, L.; Thompson, C. V.; Shao-Horn, Y. Chemical and Morphological Changes of Li-O2 battery Electrodes Upon Cycling. J. Phys. Chem. C 2012, 116, 20800−20805. (21) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The Carbon Electrode in Nonaqueous Li-O2 Cells. J. Am. Chem. Soc. 2013, 135, 494−500. (22) Lyu, Z. Y.; Zhang, J.; Wang, L. J.; Yuan, K. D.; Luan, Y. P.; Xiao, P.; Chen, W. CoS2 Nanoparticles-Graphene Hybrid as a Cathode Catalyst for Aprotic Li-O2 Batteries. RSC Adv. 2016, 6, 31739−31743. (23) Yin, W.; Grimaud, A.; Lepoivre, F.; Yang, C.; Tarascon, J.-M. Chemical Vs Electrochemical Formation of Li2CO3 as a Discharge

and 0.1 M lithium perchlorate (LiClO4) in DMSO were used as anode and electrolyte, respectively. The obtained coin cells were transferred to home-made pressure-tight glass containers and refilled with purified CO2, CO2 with 2% O2, or O2 for 20 min before test. The battery galvanostatic discharge/charge test was carried out on the LAND multichannel battery testing system at room temperature. DEMS Measurements of the Aprotic O2-Assisted Li− CO2 Cells. A quantitative DEMS was used to study the noble metal−catalyzed O2-assisted Li−CO2 battery cells. A homemade O2-assisted Li−CO2 battery cell with two glued PEEK capillary tubes as purge gas inlet and outlet (Swagelok type) was linked to a commercial magnetic sector mass spectrometer (Thermo Fisher) by a specially designed gas-purging system. The flow rate of purge gas was 5 mL/min. During the discharge process, a mixture of Ar/O 2 /CO 2 (volume ratio 4.99:47.70:47.61) was used as the carrier gas for the purpose of quantifying CO2 and O2 consumption. Ar acted as the internal tracer gas with known constant flux. For charging the O2-assisted Li−CO2 cells, high-purity Ar was used as carrier gas. All of the cells were tested at the current of 0.4 mA with restrained capacity of 50 mA h/g for discharge and recharge.

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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/acsomega.7b01495. Characterization of Ru/GNSs (XRD, BET, Raman, TGA and XPS); full discharge/charge curve of Ru/GNSs in pure O2 atmosphere; full discharge/charge curve of Ru/ GNSs cathode in CO2 with 2% O2 atmosphere (0.16 mA/cm2); full discharge/charge cycling performance of Ru/GNSs and GNSs cathodes and FESEM images of GNSs cathode after three full discharge/charge cycles in CO2 with 2% O2 atmosphere; FESEM image of Ru/ GNSs electrode in CO2 with 2% O2 atmosphere after 60th cycle (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.P.). *E-mail: [email protected] (W.C.). ORCID

Wenrui Dai: 0000-0002-3426-005X Wei Chen: 0000-0002-1131-3585 Author Contributions ○

L.W. and W.D. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors acknowledge the financial support from National Natural Science Foundation of China (91645102), Natural Science Foundation of Jiangsu Province BK20170005 and Singapore MOE grant R143-000-593-112.

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