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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Oxygen Evolution Reaction on Pristine and Oxidized TiC (100) Surface in Li-O Battery 2
Yingying Yang, Xiaowan Xue, Yuan Qin, Xudong Wang, Man Yao, Zhenhai Qin, and Hao Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12845 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018
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The Journal of Physical Chemistry
Oxygen Evolution Reaction on Pristine and Oxidized TiC (100) Surface in Li-O2 Battery Yingying Yang, Xiaowan Xue, Yuan Qin, Xudong Wang, Man Yao*, Zhenhai Qin, Hao Huang* School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT Oxidized TiC (100) surface has been constructed to compare the surface stability and catalytic performance with the pure TiC (100) surface in Li-O2 battery by first-principle calculations. The stable oxidized surface was determined by calculating the formation energies of TiC (100) surface under various O coverage rates. The interfacial catalytic models of LixO2 (x=2, 1 and 0) molecules adsorbed on pristine and oxidized TiC (100) surface were used to simulate the OER process. The results of thermodynamics calculation indicate that the oxidized TiC (100) surface has smaller O2 evolution barrier and lower charge voltage. The electron-withdrawing O layer plays an important role in increasing charge transfer from O2x− (x=2, 1 and 0) to substrate, which is helpful for O22− oxidation and Li-O bond activation. The theoretical results have well verified experimental findings of oxidized TiC surface as the potential state of TiC cathode material in Li-O2 battery. The surface-modified strategy of stronger electron-withdrawing layer is proposed to improve OER catalysis in Li-O2 battery, which provides a way for designing more active catalysts. Keywords: Li-O2 battery; OER; TiC (100); oxidized; first-principle calculations
*
Corresponding author: Tel: 86-411 84707347; Fax: 86-411 84709248.
Email:
[email protected] (M. Yao),
[email protected] (H. Huang). 1
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1. INTRODUCTION Rechargeable lithium-oxygen (Li-O2) batteries have received wide attention in recent years because of their high energy density (~1700 Wh/kg), which is 5-10 times higher than that of conventional Li-ion batteries (LIBs) [1-4]. Hence, they are more suitable to electric vehicles and other high-energy application devices [5-7]. In nonaqueous aprotic Li-O2 batteries, the net electrochemical reaction is 2Li++2e-+O2↔Li2O2, with the positive discharging reaction is oxygen reduction reaction (ORR) and the reverse charging reaction is oxygen evolution reaction (OER). However, there are several problems that limit their practical application such as low ORR/OER rate, low round-trip efficiency, short cycle life and electrolyte instability [8-11]. All these issues are related to the sluggish kinetics of OER and ORR, especially the decomposition of discharge product Li2O2 which is an insulator. To solve these issues, reactive catalysts including noble metals [12-13], metal oxides [14-15], bimetallic alloys [16-17], perovskites[18], nitride and carbide of transition metal [19-20] are studied to achieve smaller ORR/OER overpotentials, larger capacity and cycle number. TiC material has been extensively investigated in experiments as the electrochemical catalyst in Li-O2 battery, which has good electrical conductivity (3×107 S/cm), reduces side reactions greatly and displays better reversible decomposition/formation of Li2O2. The TiC-based cathode can reduce side reactions caused by electrolyte and electrode decomposition, and even maintain more than 98% capacity after 100 cycles in a rechargeable aprotic Li-O2 battery [20]. The ordered mesoporous carbon material decorated by TiC as bifunctional catalyst obviously reduces the ORR/OER overpotentials and increases the specific capacity of Li-O2 battery [21]. However, the catalysis of TiC is affected by the 2
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The Journal of Physical Chemistry
TiO2-rich surface layer (along with some TiOC) in Li-O2 battery, because TiC surface is unstable in nanoscale and readily oxidized by interacting with O2 and Li2O2 molecules, which have been proved by the following experimental and calculation results. The XPS and LEED results show that Ti atoms are easily oxidized to form a disordered TiOx (1.5
