Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 779−784
pubs.acs.org/JPCL
Synergistic Effects of C/α-MoC and Ag for Efficient Oxygen Reduction Reaction Lujie Cao,†,§ Pengpeng Tao,†,§ Minchan Li,†,§ Fucong Lyu,† Zhenyu Wang,† Sisi Wu,† Wenxi Wang,† Yifeng Huo,† Li Huang,*,‡ and Zhouguang Lu*,† †
Department of Materials Science & Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China Department of Physics, Southern University of Science and Technology, Shenzhen 518055, P. R. China
‡
S Supporting Information *
ABSTRACT: It remains challenging to prepare highly active and stable catalysts from earth-abundant elements for the oxygen reduction reaction (ORR). Herein we report a facile method to synthesize cost-effective heterogeneous C/α-MoC/Ag electrocatalysts. Rotating disc electrode (RDE) experiments revealed that the obtained C/α-MoC/Ag exhibited much superior catalytic performance for ORR than that of C/Ag, C/α-MoC, or even the conventional Pt/C. First-principles calculations indicated that the enhanced activity could be attributed to the efficient synergistic effects between Ag and α-MoC/C by which the energy barrier for O2 dissociation has been substantially reduced. Furthermore, Li−air and Al−air cells were assembled to demonstrate the unprecedented electrochemical performance of C/α-MoC/Ag nanocomposites surpassing the Pt/C. Thus experimental results and theoretical calculations together showed that the heterogeneous C/α-MoC/Ag nanocomposites are a promising alternative to platinum for applications in industrial metal-air batteries.
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carbide has attracted significant recent research attention.37 In general, molybdenum carbide exists in four forms: α-MoC, βMo2C, γ-MoC, and η-MoC.21,33 β-Mo2C, γ-MoC, and η-MoC have very similar hexagonal crystal structures with different stacking sequences. In particular, the β-Mo2C, the most stable phase having an ABAB packing of the metal planes, has been extensively investigated.22,24,27−29,33,37−41 However, the facecenter cubic phase α-MoC with an ABCABC stacking sequence has been rarely touched.32,33,42 Molybdenum carbides alone exhibited very limited activity for ORR,24,26,29 but molybdenum carbides are excellent substrates for loading active component like Pt or Pd to considerably replace the noble metal, demonstrating comparable or even better catalytic activity toward oxygen reduction and particularly improving the stability of catalysts in various electrolytes.23,37,43,44 Metallic Ag has been generally utilized as an electrocatalyst in air electrode for Al−air battery, demonstrating superior activity and stability.45 We demonstrate that α-MoC-promoted Ag can be an intriguing candidate considered as a high efficient catalyst for oxygen reduction. It is well known that as the dimensions of catalysts are reduced, their activity increases markedly. Traditionally, high-temperature solid-state methods have to be used to synthesize α-MoC with good crystallinity, unavoidably yielding microscale aggregated particles having small specific surface area and poor catalytic activities.
n recent years, increasing research efforts have been devoted to metal air batteries due to their exceptionally high theoretical energy density (8135 Wh kg−1 for Al−air and 11 400 Wh kg−1 for Li−air), good reliability, and low cost.1−3 Although Al−air battery is not rechargeable, it has many additional attractive features, such as abundant resources, nontoxicity, zero-emission, easy recycling of Al(OH)3 byproduct, and fast replacement of Al electrode, which matches very well with the requirements of electric vehicles.4 However, it still remains a challenge to implement Al−air or Li−air batteries on a commercial scale. The bottleneck is to develop an effective electrocatalyst to improve the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air electrode.2,5,6 Although noble metals such as Pt or Pd have exhibited good ORR or OER activities, large-scale commercial application has been largely hindered by their high cost, low abundance, and weak durability.3,7−10 Therefore, it is highly attractive and urgent to explore low-cost, feasible, and effective alternatives to noble metals as electrocatalysts for fabricating air electrodes for Li−air and Al−air batteries.7,11,12 Transition-metal carbides including tungsten carbide,13−16 vanadium carbide,17,18 and molybdenum carbide,19−25 possessing special catalytic activities,26 have been extensively studied as catalysts27−29 or catalyst supports15,30,31 in many reactions including hydrogenation,32 water splitting,19,33 and desulfurization.34 In addition to its Pt-like characteristics, transition-metal carbides have high electronic conductivity35 and are highly corrosion-resistant under both alkaline and acid conditions.36 As representative transition metal carbides, molybdenum © 2018 American Chemical Society
Received: December 19, 2017 Accepted: January 30, 2018 Published: January 30, 2018 779
DOI: 10.1021/acs.jpclett.7b03347 J. Phys. Chem. Lett. 2018, 9, 779−784
Letter
The Journal of Physical Chemistry Letters
addition, the ambiguous diffraction peak centered at around 26.2° can be assigned to the graphite (002) plane, revealing graphitization of carbon in these samples. The particle size of αMoC calculated by the Scherrer’s formula is ∼5 nm, which is much smaller than that of the α-MoC particles prepared by other approaches, for instance, high temperature, hydrothermal, or sol−gel. More importantly, the obtained α-MoC ultrafine nanoparticles were single phase without any impurities. Hence, the method is of importance for the mass production of ultrafine metal carbides nanoparticles. Compared with C/Ag, the broader XRD diffraction peaks associated with Ag in C/αMoC/Ag suggest that the particle size of Ag diminishes to some extent, implying that the presence of α-MoC largely inhibits the growth of Ag particles. Meanwhile, Figure 2a,c shows the typical TEM images of the C/α-MoC sample. It is clear that the particles are in spherical
Alternatively, we utilized the ion exchange, followed by lowtemperature calcination method to prepare uniform α-MoC ultrafine nanoparticles with size
