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MOF-derived carbon materials mounted with highly dispersed Ru and MoO3 for rechargeable Li-O2 cathode yield enhanced cyclability Jing Li, Yijie Deng, Limin Leng, Kailing Sun, Lulu Huang, Huiyu Song, and Shijun Liao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05005 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019
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ACS Sustainable Chemistry & Engineering
MOF-derived carbon materials mounted with highly dispersed Ru and MoO3 for rechargeable Li-O2 cathode yield enhanced cyclability Jing Li, Yijie Deng, Limin Leng, Kailing Sun, Lulu Huang, Huiyu Song, Shijun Liao* The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory of New Energy Technology of Guangdong Universities, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wu Shan Road, Tianhe District, Guangzhou, Guangdong, 510641, China *Corresponding authors, E-mail addresses:
[email protected], fax +86 20 87113586.
Abstract: A composite cathode catalyst for Li-O2 batteries using ruthenium and molybdenum trioxide loaded on MOF-derived carbon materials (MDC) is designed and prepared by an impregnation and hydrogen reduction process. The catalyst exhibits high capacity and excellent cycling stability, and the Ru nanoparticles (NPs) and MoO3 significantly enhance the catalyst’s performance. For our optimal catalyst, Ru-MoO3/MDC, the first discharge capacity and charge capacity reach up to 5343 and 5950 mAh g-1 at a discharge rate of 100 mA g–1, with a discharge voltage plateau of 2.6 V. It also exhibits excellent cycling stability over 160 cycles at a limited capacity of 600 mA h g–1. The addition of Ru NPs greatly improves the cathode’s capacity and effectively reduces the battery’s charge overpotential. The further addition of MoO3 significantly improves the material’s cycling stability, although it seems to have no influence on the cathode’s capacity. The morphology of the discharge products changes from donut to yo-yo shaped with further MoO3 loading. We ascribe the high performance of our composite catalyst to: the high surface area of the MDC’s rich porous structure, which is ideal for the discharge products; the addition of Ru and MoO3, which enhance the catalyst’s activity and stability; and synergetic effects between carbon, Ru, and MoO3. Key words: Li-O2 batteries, MOFs derived carbon, Oxygen reduction and oxygen 1
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evolution, Ruthenium, Molybdenum trioxide.
Introduction Owing to their extremely high theoretical energy density1-2, nonaqueous lithium–air (oxygen) batteries are considered a promising next generation of energy storage systems for electric vehicles. However, the sluggish kinetics of the oxygen reduction reaction (ORR) is a crucial problem, resulting in large overpotential and poor cyclability and thereby seriously restricting the further development of these batteries. In the last decade, tremendous effort has been devoted to improving the performance of Li-O2 cells by exploring efficient cathode catalysts.3-6 Among the numerous ones developed, including carbon materials7-8, metal alloys9, metal oxides9-10, and perovskite11-12, carbon-based materials have attracted significant attention due to their high specific surface area, good conductivity, diverse structures, and good ORR performance. However, this type of catalyst has some drawbacks. Generally, doped carbon catalysts present good ORR activity but less than desirable OER activity, resulting in higher overpotential during charging, random overgrowth of products on the material’s surface, less than complete decomposition during the charging process, and poor battery cycling performance. Recently, MOF-derived carbon catalysts have attracted increasing research interest in the field of electrochemical energy storage, including fuel cells13-16, lithium-ion batteries17-19, supercapacitors20-21, Li-S batteries,22-23 CO2 reduction24 and metal–air batteries25-28, due to their high surface area and regular meso/macroporous structure, derived from their MOF precursors. However, previous catalysts usually have exhibited poor cycling performance due to their poor OER activity. To improve the OER performance and cycling stability of MOF-derived catalysts, some researchers have proposed composites of MOF-derived carbon and metals or other compounds. For example, Wu et al.29 prepared a transition metal-based nitrogen-doped nanocarbon catalyst by pyrolyzing a mixture of cage-containing MOFs, iron acetate, and dicyandiamide; the resulting N-Fe-MOF catalyst delivered a 2
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discharge capacity of 5300 mA h gcat–1 in a nonaqueous Li-O2 battery. Zhang et al.30 prepared a MIL-100(Fe)-derived -Fe2O3/carbon composite catalyst. The first discharge capacity of a Li-O2 cell with their composite catalyst as the cathode reached 5970 mA h g–1. Ruthenium has been widely investigated as an effective promoter for carbon-based cathode catalysts in Li-O2 batteries. Previously, we reported a Co/Fe co-doped rGO catalyst mounted with Ru NPs, which achieved a capacity of 23,000 mA h g–1; in that study, we intensively investigated the improvement achieved with Ru.31 Inspired by others’ research and building upon our previous work, herein we describe the design and preparation of a MOF-derived carbon catalyst mounted with Ru NPs and molybdenum trioxide (MoO3) as promoters. We find that the addition of Ru is effective in reducing the charge overpotential, the addition of MoO3 can significantly enhance the cyclic stability of Li-O2 batteries, and the high surface area and porous structure of MOF-derived carbon ensure the discharge products are highly dispersed and easily decomposed. As a result, the first discharge capacity and charge capacity of a battery with Ru-MoO3/MDC as the cathode reach up to 5343 and 5950 mA h g-1 at 100 mA g–1, and the first average discharge/charge voltages are 2.6 V/4.1 V, respectively. Importantly, a battery with this Ru-MoO3/MDC cathode exhibits ultrahigh cyclability of over 160 cycles at 100 mA g−1 with a limited capacity of 600 mA h g–1.
Experimental Synthesis of MOF (iron-doped ZIF-8)-derived carbon (MDC) MOF (iron-doped ZIF-8)-derived carbon material (MDC) was prepared following a previously developed method.32-33 1.75 g 2-methylimidazole was dissolved in 12.5 ml methanol (solution A); 0.25 g Zn(NO3)2·6H2O and 0.25 g iron(III) acetylacetonate were dissolved in 6.25 ml methanol (solution B). Solution B was then added to solution A, and the mixture was stirred vigorously for 24 h at room temperature to form the MOF precursor. The precursor was collected by 3
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centrifugation, washed with methanol, and dried in a vacuum oven, then put in a tubular furnace for pyrolysis at 350°C for 1 h and at 900°C for 3 h at a rate of 5°C·min–1 in Ar flow. Finally, the pyrolyzed material was leached with 0.5 mol·L–1 H2SO4 solution at 80°C for 12 h, followed by washing, drying, and further heating at 900°C for 1 h to achieve further graphitization.
Preparation of Ru-MoO3/MDC Ru-MoO3/MDC composite catalysts were synthesized by an incipient-wetness impregnation and H2-reduction method. The MDC was impregnated in a RuCl3·3H2O and MoCl5 ethanol solution, mixed ultrasonically for 2 h and then allowed to dry naturally. Afterwards, the sample was reduced in a H2 tubular furnace at 200°C for 2 h. For our optimized catalyst, the recipe Ru loading is 20 wt%, and the molar ratio of Ru/Mo is 3:1. (The actual molar ratio of Ru/Mo is 2.5:1, the contents of ruthenium and molybdenum trioxide are 17.3% and 8.4%, respectively, by the ICP analysis). Ru-MoO3/MDC represents our optimized catalyst in this paper, except specified specially. Further, for comparison purpose, single Ru- or MoO3-supported catalysts were prepared via the same procedures but omitting impregnation in either RuCl3·3H2O or MoCl5 ethanol solution.
Characterization X-ray diffraction (XRD) patterns were collected using a TD-3500 powder diffractometer (Tongda, China) with Cu-K radiation sources. Scanning electron microscopy (SEM) was conducted on a SU8220 field emission scanning electron microscope (HITACHI, Japan). Transmission electron microscopy (TEM) was performed with a JEM-2100HR transmission electron microscope (JEOL, Japan). Specific surface areas were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption–desorption on a Tristar II 3020 gas adsorption analyzer (Micromeritics, USA). XPS was performed on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo-VG Scientific, USA). 4
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Assembly and measurement of Li/O2 batteries Electrochemical measurements were carried out using 2032 coin-type cells with holes for oxygen access at the cathode side, which were assembled in an argon-filled glovebox (MIKROUNA, H2O
