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An ultralong cycle-life Li-O2 battery enabled by a MOF-derived rutheniumcarbon composite catalyst with a durable regenerative surface Xiangkun Meng, Kaiming Liao, Jie Dai, Xiaohong Zou, Sixuan She, Wei Zhou, Fei Ye, and Zongping Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05235 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019
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ACS Applied Materials & Interfaces
An ultralong cycle-life Li-O2 battery enabled by a MOF-derived ruthenium-carbon composite catalyst with a durable regenerative surface Xiangkun Meng,†,§ Kaiming Liao,*,†,§ Jie Dai,† Xiaohong Zou,† Sixuan She,† Wei Zhou,† Fei Ye,† Zongping Shao,*,†,‡ † State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China ‡ WA School of Mines: Minerals, Energy and Chemical Engineering (WASM-MECE), Curtin University, Perth, WA 6845, Australia
KEYWORDS Li-O2 battery, electrocatalyst, cycling stability, surface regeneration, Ru-MOF.
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ABSTRACT
The cycling performance of Li-O2 batteries (LOBs), which is an important parameter determining the practical use of this advanced energy technology with ultrahigh energy density, is strongly affected by the nature of oxygen electrocatalyst. As a good oxygen electrode, it should possess good activity for both oxygen evolution reaction and oxygen reduction reaction and superior stability under operating conditions. During the past, oxygen electrodes for LOBs were generally fabricated by loading noble metal nanoparticles on the surface of a porous carbon support. However, the nanoparticles could easily lose contact with the carbon support during the reversible liquid-gas-solid reactions that involve lithium ions, oxygen gas, and Li2O2. Herein, we reported a novel Ru-MOF-derived carbon composite, characterized by stereoscopic Ru nanoparticles distribution within the carbon matrix, as an alternative oxygen catalyst of LOBs, enabling superior operational stability and favorable activity. More specifically, the battery demonstrated stable charge-discharge cycling for up to 800 times (~107 days) at a current density of 500 mA g-1 with low discharge/charge overpotentials (~0.2/0.7 V vs Li). A mechanism of regenerative surface was further proposed to explain the excellent cycling stability of the LOBs through the using of RuMOF-C catalyst. These encouraging results imply an accessible solution to address issues related to the oxygen catalyst for the realization of practical LOBs. 1. INTRODUCTION Lithium-oxygen batteries (LOBs), integrating the “Holy Grail” anode of Li metal with the inexhaustible cathode of O2 gas, have attracted great attentions due to their potential applications in the smart grid and electric vehicles, which can deliver an ultrahigh theoretical energy density of 3500 Wh kg-1 with a thermodynamic potential Uo = 2.96 V based on the reversible reaction of 2Li
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ACS Applied Materials & Interfaces
+ O2 = Li2O2.
1-8
The oxygen electrode performs as an oxygen reduction electrocatalyst under
discharge condition and an oxygen evolution electrocatalyst under charge condition. The practical energy density and cycling stability is closely related to the performance of the oxygen electrode. However, the inherent sluggish kinetics of cathodic oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), insufficient catalyst duration, and vexing side reactions related to the decomposition of battery components (e.g., electrolytes and carbon electrodes), that result in poor rate capability and short span life of LOBs, are the main challenges that hinder the practical realization of the now-established system.9-14 In response to the challenges mentioned above, tremendous efforts have been devoted to developing highly efficient bifunctional electrocatalysts for LOBs, including the use of noble metals (Au, 1 Ir, 2 Ru, 15 Pt, 16 Pd, 17 etc.), noble metal alloys (Pt-Au, 18 Pt-Ir, 19 Au-Ag, 20 etc.), transition metal oxides (RuO2, 14 MnO2, 21 Co3O4, 22 Ti4O7, 23 etc.), nitrides (TiN 24) and carbides (TiC
25)
alone or deposited on carbon supports.
26-29
Among the various materials, carbon-
supported Ru catalysts 30-33 have been most extensively explored for LOBs due to the facts that: (1) Ru has high catalytic activity towards both ORR and OER; 31 (2) the ORR can be significantly promoted by depositing the Ru catalysts on carbon supports;
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(3) carbon materials have high
electronic conductivity, light weight and a large mass-specific surface area;
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(4) Ru is cost-
competitive compared with other noble metals such as Pt, Au, Pd, and Ir; (5) Ru exhibits good catalytic activity for the decomposition of Li2CO3 36-37 and thus avoids the accumulation of the carbonate byproduct upon cycling. In the synthesis of carbon-supported Ru catalysts, two main approaches are highlighted: (1) “physical” approach, in which the Ru nanoparticles are pre-prepared and then the obtained nanoparticles are loaded on the surface of carbon supports through physical adsorption;
30
(2)
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“chemical” approach, in which carbon materials are pre-dispersed in Ru3+-contained solution and followed by heat-treating with reductive agent. 32 However, the surface-loaded Ru nanoparticles are usually not strong enough to endow long-term reversible liquid-gas-solid reaction including lithium ions, oxygen gas, and Li2O2. Once the Ru nanoparticles lose contact with the support because of undesirable carbon decomposition (oxidation) during the OER process,
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sharp
increase in OER overpotential will be experienced. The increase in OER overpotential further accelerates the oxidative decomposition of carbon and the exfoliation of more Ru nanoparticles from the carbon support, leading to accelerating cell performance degradation. Therefore, for the Ru/carbon composite electrode, research efforts should be made to address the stability issue by developing new Ru/carbon architecture that can improve the surface stability. In recent years, carbon-based composite materials derived from metal-organic frameworks (MOFs) have aroused great interest for the next-generation electrochemical energy storage systems such as LOBs and Li-S batteries due to their advantages of controllable structures, excellent electronic conductivity, high porosity, and gravimetric surface areas.
39-40
Considering unavoidable carbon
decomposition/oxidation under hurdle operating condition of LOBs, instead of trying in improving the surface stability, the building of a regenerative electrode surface is more attractive. Herein, we reported a new Ru-carbon composite electrocatalyst for LOBs with stereoscopic Ru nanoparticles distribution in carbon matrix. Fresh Ru nanoparticles will be exposed to perform as new active sites once the surface Ru is lost due to detachment from the carbon oxidation during operation, such regenerative surface then ensures a high cycling stability. The proposed Ru/carbon electrocatalyst architecture can be facilely prepared by using Ru-MOF crystals as precursor. The introduction of Ru into MOF ensures the atomic mixing of Ru and carbon in the precursor, thus maximizing the intrinsic activity of Ru for oxygen electrocatalysis, while the carbon in the MOF
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performed as source of carbon matrix during the subsequent pyrolysis. A LOB with the as-obtained Ru-MOF-C as oxygen electrode demonstrated ultra-stable cycling performance (over 800 cycles, 107 days) at room temperature. This study demonstrates a new way for the development of superior oxygen electrode of LOBs. 2. EXPERIMENTAL SECTION 2.1. Synthesis of Ru-MOF-C catalyst For the synthesis of Ru-MOF-C as an electrocatalyst of Li-O2 battery, Ru-MOF crystal was firstly prepared based on a similar procedure for the synthesis of X3(BTC)2 (X = Mo, Ru) as described in previous papers.
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In detail, RuCl3·nH2O (~1.2 mmol) and 1,3,5-
Benzenetricarboxylic acid (H3BTC, ~0.8 mmol) were dissolved in 8 mL mixed solution (VCH3COOH:VH2O = 1:5.5) to form a mixed solution, which was then poured into 10 mL Teflon liner. After capped tightly in a steel autoclave and kept in an oven at 160 °C for 72 hours, a self-assembly reaction was appeared, leading to the formation of dark green-colored Ru-MOF. After centrifugally collected and washed with deionized water and followed by freeze-dried for 12 h, the as-obtained Ru-MOF crystal was obtained, which was used as the precursor for calcination (700 oC
for 10 min under Ar atmosphere) to obtain the final Ru-MOF-C catalyst.
2.2. Assembly of coin-type LOBs The electrolyte solution (1 mol L-1) used for LOBs was prepared in an Ar-filled glove box (O2
