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MoP nanoflakes as efficient electrocatalysts for rechargeable Li-O2 batteries Minghui Wei, Yong Luo, chao jin, Jing Sui, Zhangjun Wang, Cong Li, and Ruizhi Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00299 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018
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ACS Applied Energy Materials
MoP Nanoflakes As Efficient Electrocatalysts For Rechargeable Li-O2 Batteries Minghui Wei†, §#, Yong Luo†, §#, Chao Jin†, §*, Jing Sui‡, Zhangjun Wang†, §, Cong Li †, § , Ruizhi Yang†, §* † Soochow Institute for Energy and Materials InnovationS, College of Physics, Optoelectronics and Energy & Collaborative Innovation Centre of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China. ‡ College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China § Jiangsu Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies，Soochow University, Suzhou 215006, China.
ABSTRACT: Herein, we firstly report a binder-free electrode with in-situ synthesized MoP nanoflakes on the surface of carbon cloth and its application in non-aqueous Li-O2 batteries (LOBs). The assembled LOBs exhibit improved discharge/charge capability (achieving actual 4.15 mAh at a current of 0.1 mA and cycle stability (400 cycles without capacity fading), which should be attributed to the superior electrocatalytic activity of MoP nanoflakes towards the formation/decomposition of Li2O2.
KEYWORDS: Li-O2 battery; Oxygen reduction reaction; Oxygen evolution reaction; Bifunctional catalysts; MoP
1. INTRODUCTION Nowadays, whole world are focused on developing new energy because of worsening energy and environmental issues. Rechargeable Li-O2 batteries (LOBs), in which the theoretical energy density is 11,140 Wh kg-1 and close to that of gasoline, have been considered as the most promising power devices for future electrical vehicles and/or hybrid electric vehicles. However, some challenges exist, such as sluggish kinetics of electrode reactions, large overpotential gap, instability of electrolyte, low rate capability and short cycle life, which form bottleneck hindering the commercial application of LOBs.1 As two importantly fundamental electrochemical processes in cathode of LOBs, both oxygen reduction reaction (ORR) during discharge to generate Li2O2 and oxygen evolution reaction (OER) during charging process to decompose Li2O2 have attracted tremendous interest to enhance their low kinetics. Efficient bifunctional oxygen catalysts are of key importance 1. Pt, Pd and Ru noble metal
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catalysts show the best overall catalytic activity toward the ORR/OER, but, high cost and scarcity make them not feasible for large scale applications. Considerable efforts are desired to explore alternative none-noble metal bifunctional catalysts to enhance sluggish oxygen kinetics (including carbon materials, transition metal oxides, metal carbides).2-4 Transition-metal phosphides (TMPs) are an important class of functional compounds due to rich bonding type itself. As highly efficient catalysts for hydrodesulfurization reaction, they have been widely commercially utilized.5 As anode materials of Li-ion battery,6 and efficient catalyst for hydrogen evolution reaction (HER)7, TMPs have also been reported. Recently, TMPs have been undergoing intensively research as potential OER and/or ORR catalyst. Hu firstly reported Ni2P helps to improve the performance of OER in alkaline medium.8 Huang systematically studied ORR catalytic properties of M2P (M=Mn, Co, Ni)@CNTs, and found Co2P and Mn2P exhibit better ORR activities, while Ni2P shows inferior performance.9 Although TMPs have demonstrated excellent catalytic activities in aqueous solution for HER, ORR and OER, to the best of our knowledge, they have not yet been investigated in rechargeable non-aqueous LOBs. Herein, we firstly present our recent efforts in developing binder-free electrode with MoP nanoflakes as efficient bifunctional oxygen catalysts for LOBs, which exhibit considerable energy efficiency as well as enhanced cycling stability.
2. RESRULTS AND DISCUSSION
Fig. 1, SEM and TEM images of MoO2 and MoP nanoflakes, respectively. (a), (a-1) and (a-2) for MoO2 nanoflakes; (b), (b-1) and (b-2) for MoP nanoflakes. Binder-free electrode with MoP nanoflakes on the surface of carbon cloth (noted as MoP@CC) were in-situ prepared through a facile hydrothermal reaction and then a phosphating process. The detailed preparation procedure was displayed in Scheme S1 (Electronic Supplementary Information, ESI†). Fig. 1 presents
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ACS Applied Energy Materials
scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images to characterize morphologies of typical MoO2@CC and MoP@CC samples, respectively. It can be clearly observed that formed MoO2 and MoP are nanoflakes with a thickness of ~100 nm (Fig. 1a,b), moreover, the surface of MoO2 nanoflake is smooth (Fig. 1a-1), while that of MoP nanoflake is coarse (Fig. 1b-1), which benefits providing more active sites for ORR and OER. HRTEM images shown in (Fig. 1a-2 and b-2) verify the existence of MoO2 and MoP nanoflakes with prominent lattice fringes identified by lattice analysis. Interestingly, there appears a homogenous ~2 nm thick amorphous thin film on the surface of MoP nanoparticle (Fig. 1b-2), which perhaps is a MoOx passivation layer formed during the cooling process.10-12 Furthermore, MoO3@CC sample was also prepared, and then phosphatization was directly performed. However, big hexagonal prisms were formed with a diameter of over 2.5 um and a length of over 10 um, and it is difficult to fully convert to MoP during phosphating process (Fig. S1, ESI†).
Fig. 2, XRD patterns of as-prepared MoO3, MoO2 and MoP at different preparing processes, respectively (A). High-resolution XPS spectrums, C 1s (B), Mo 3d (C) and P 2p (D). Structures of as-prepared products at different preparing processes were firstly investigated using XRD (Fig. 2A). According to their standard PDF cards, all
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characteristic peaks of MoO3, MoO2 and MoP could be well indexed without other impurities, respectively. Chemical state and composition of MoP@CC were characterized by high-resolution XPS. In regard to the C 1s spectrum (Fig. 2B), it could be divided into three peaks, in which 284.6 eV corresponds to graphenic carbon atoms, while the peak located at 286.6 eV could be assigned to C-O bond, which is probably due to the absorbed oxygen or carbon dioxide,13 in addition, there appears a fitted peak at 285.3 eV, which could be associated with C-P bond, suggested P atom was simultaneously doped into carbon fiber during phosphating process. The deconvolution of the Mo 3d spectrum (Fig. 2C) includes three doublets. Two stronger peaks located at 228.3 and 231.5 eV are associated with MoP, and a spin energy separation of 3.2 eV between them indicates that Moδ+ species (0
