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Molybdenum Nitride/N-Doped Carbon Nanospheres for Lithium‑O2 Battery Cathode Electrocatalyst Kejun Zhang,†,‡,§ Lixue Zhang,†,§ Xiao Chen,† Xiang He,# Xiaogang Wang,† Shanmu Dong,† Lin Gu,# Zhihong Liu,† Changshui Huang,† and Guanglei Cui*,† †
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China University of Chinese Academy of Sciences, Beijing, 100039, China # Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ‡
S Supporting Information *
ABSTRACT: Molybdenum nitride/N-doped carbon nanospheres (MoN/N−C) are synthesized by hydrothermal method followed by ammonia annealing. The as-prepared MoN/N−C nanospheres manifest considerable electrocatalytic activity toward oxygen reduction reaction in nonaqueous electrolytes because of its nanostructure and the synergetic effect between MoN and N−C. Furthermore, the MoN/N−C nanospheres are explored as cathode catalyst for Li−O2 batteries with tetra-(ethylene glycol) dimethyl ether as the electrolyte. The assembled batteries deliver alleviated overpotentials and improved battery lifespan, and their excellent performances should be attributed to the unique hierarchical structure and high fraction of surface active sites of cathode catalyst. KEYWORDS: MoN/N−C nanospheres, synergetic effect, electrocatalytic activity, oxygen reduction reaction, Li−O2 batteries
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potential applications in fuel cell and Li−O2 battery.27−29 Bulk MoN usually exhibits limited ORR activity probably because of their large particle size and low specific surface area.27 Some efforts have been made to enhance the electrocatalytic activity of MoN.27−29 For instance, carbon-supported MoN nanoparticles have been developed to maximize the electroactive surface area of catalyst and improve their catalytic activity.27 As well-known, highly efficient catalyst needs high surface areas or high active sites as well as fast mass transfer of the reactants and products to and from the catalytic sites. Therefore, it is of great significance to further explore the catalytic activity by rational design and facile synthesis of nanostructured catalysts materials. Herein, the molybdenum nitride/N-doped carbon nanospheres (MoN/N−C) with much active sites and mesopores were designed to improve the ORR catalytic activity of MoN material. The MoN nanospheres are prepared by reduction of MoO2 nanospheres in ammonia atmosphere with cyanamide as the structure confinement agent. Because of the unique structure and composition, the MoN/ N−C nanosphere catalyst displays excellent electrocatalytic activity in nonaqueous electrolytes. It is demonstrated that an enhanced performance of the rechargeable Li−O2 battery is obtained when MoN/N−C nanospheres are used as the cathode catalyst. Such a transition metal nitride could alleviate
INTRODUCTION The oxygen reduction reaction (ORR) is an important electrochemical reaction in a variety of electrochemical energy storage and conversion devices such as fuel cells and metal-air batteries.1−7 Recently, ORR in nonaqueous electrolyte has received considerable attention in rechargeable Li−O2 battery, which represents an emerging energy storage system for electric vehicle applications due to its high theoretical energy storage capacity.8−13 An ideal electrocatalyst should facilitate a complete reversibility of oxygen reduction/evolution reactions (ORR/OER) for (2Li + O2 ←→ Li2O2) with little negative impact to the electrolyte. Recently, several non-noble metal materials such as carbon materials,14,15 metal oxides,16,17 perovskite,18,19 and pyrochlore20,21 have been widely explored as efficient catalysts or promoters for potential application in Li−O2 battery, which enhances battery efficiency and improves battery lifespan by reducing or eliminating electrolyte decomposition. However, despite continuous efforts, the role of the catalyst in promoting ORR (discharge) or OER (charge) in Li−O2 batteries still remains controversial. Due to their unique characteristics, transition metal nitrides have been shown to have excellent catalytic activities in a variety of reactions.22−25 It is known that the catalytic and electronic properties of transition metal nitrides are governed by their bulk and surface structure and stoichiometry.26 Among numerous transition metallic nitrides, MoN, which exhibits high Pt-like electrocatalytic activities, electronic conductivity and chemical stability, has recently attracted extensive attention for © 2013 American Chemical Society
Received: January 17, 2013 Accepted: April 1, 2013 Published: April 1, 2013 3677
dx.doi.org/10.1021/am400209u | ACS Appl. Mater. Interfaces 2013, 5, 3677−3682
ACS Applied Materials & Interfaces
Research Article
Figure 1. (a) Typical SEM image of MoN/N−C nanospheres. (b, c) Corresponding TEM image of MoN/N−C nanospheres, inset in c is HRTEM image of MoN/N−C nanospheres. (d) Nitrogen adsorption−desorption isotherms of MoN/N−C nanospheres and the pore-size distributions. were obtained on a JASCO FT/IR-6200 instrument from 2000 to 400 cm−1 with a resolution of 2 cm−1. Electrochemical Characterization. Li−O2 cells consisted of a lithium metal anode and an O2 electrode. The O2 electrodes (typically 1.0 mg) were prepared by mixing 40 wt % catalysts with 40 wt % super P and 20 wt % polytetrafluoroethylene (PTFE) binders, or 70 wt % super P with 30 wt % PTFE. The samples were rolled into slices and cut into square pieces of 0.5 cm × 0.5 cm, and then pasted on a stainless steel current-collector under a pressure of 5 MPa. Electrochemical experiments were carried out by using a swagelok cell with a hole drilled only on the cathode of current collector to enable oxygen flow in. The Li−O2 cells were assembled inside the glovebox under argon atmosphere (
