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Cite This: ACS Appl. Energy Mater. 2018, 1, 331−335
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*,†,§ †
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 S Supporting Information *
ABSTRACT: Herein, we first report a binder-free electrode with in situ synthesized MoP nanoflakes on the surface of carbon cloth and its application in nonaqueous 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 toward the formation/ decomposition of Li2O2.
KEYWORDS: Li−O2 battery, oxygen reduction reaction, oxygen evolution reaction, bifunctional catalysts, MoP have been widely commercially utilized.5 As anode materials of a Li-ion battery6 and an efficient catalyst for hydrogen evolution reaction (HER),7 TMPs have also been reported. Recently, TMPs have been undergoing intensive research as potential OER and/or ORR catalysts. Hu first reported that Ni2P helps to improve the performance of OER in an alkaline medium.8 Huang systematically studied ORR catalytic properties of M2P (M = Mn, Co, Ni)@CNTs and found that 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 nonaqueous LOBs. Herein, we first present our recent efforts in developing a binder-free electrode with MoP nanoflakes as efficient bifunctional oxygen catalysts for LOBs, which exhibit considerable energy efficiency as well as enhanced cycling stability.
1. INTRODUCTION Nowadays, the whole world is focused on developing new energy sources 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 a bottleneck hindering the commercial application of LOBs.1 As two important fundamental electrochemical processes in the cathode of LOBs, both the oxygen reduction reaction (ORR) during discharge to generate Li2O2 and the oxygen evolution reaction (OER) during the charging process to decompose Li2O2 have attracted tremendous interest for enhancing their low kinetics. Efficient bifunctional oxygen catalysts are of key importance.1 Pt, Pd, and Ru noble metal 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 non-noble metal bifunctional catalysts to enhance sluggish oxygen kinetics (including carbon materials, transition-metal oxides, and metal carbides).2−4 Transition-metal phosphides (TMPs) are an important class of functional compounds due to the rich bonding type itself. As highly efficient catalysts for hydrodesulfurization reaction, they © 2018 American Chemical Society
2. RESULTS AND DISCUSSION Binder-free electrodes 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 Received: December 16, 2017 Accepted: January 31, 2018 Published: January 31, 2018 331
DOI: 10.1021/acsaem.7b00299 ACS Appl. Energy Mater. 2018, 1, 331−335
Letter
ACS Applied Energy Materials
Figure 1. SEM and TEM images of MoO2 and MoP nanoflakes, respectively. (a, a-1, and a-2) MoO2 nanoflakes; (b, b-1, and b-2) MoP nanoflakes.
Figure 2. XRD patterns of as-prepared MoO3, MoO2, and MoP at different preparing processes, respectively (A). High-resolution XPS spectra: C 1s (B), Mo 3d (C), and P 2p (D).
homogeneous ∼2 nm thick amorphous thin film on the surface of the MoP nanoparticle (Figure 1b-2), which perhaps is a MoOx passivation layer formed during the cooling process.10−12 Furthermore, a 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 μm and a length of over 10 μm, and it is difficult to fully convert this to MoP during the phosphating process (Figure S1). Structures of as-prepared products at different preparing processes were first investigated using XRD (Figure 2A). According to their standard PDF cards, all characteristic peaks
Scheme S1 (Supporting Information). Figure 1 presents 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 (Figure 1a,b); moreover, the surface of a MoO2 nanoflake is smooth (Figure 1a-1), while that of a MoP nanoflake is coarse (Figure 1b-1), which is a benefit as it provides more active sites for ORR and OER. HRTEM images shown in Figure 1a-2,b-2 verify the existence of MoO2 and MoP nanoflakes with prominent lattice fringes identified by lattice analysis. Interestingly, there appears to be a 332
DOI: 10.1021/acsaem.7b00299 ACS Appl. Energy Mater. 2018, 1, 331−335
Letter
ACS Applied Energy Materials
Figure 3. CV curves in LITFSI-TEGDME electrolyte with a scanning rate of 0.2 mV s−1 (A) and full discharge−charge plots at an actual current of 0.1 mA (B) of LOBs based on pure CC, P-doped CC, and MoP@CC electrodes, respectively. Rate capability at different actual current (C) and cycling performance with a cut capacity of 0.25 mAh at a current of 0.1 mA (D) of MoP@CC electrode based LOBs.
Figure 4. SEM images of MoP@CC electrodes at different discharge/charge statuses: (a) fresh; (b, c) after discharge at 2.5 and 2.2 V for 6 h, respectively; (d) after charge at 4.0 V for 6 h. (e) XRD patterns of pristine, discharged, and charged MoP@CC electrodes. (f) XPS spectra of Li 1s after long-term discharged and charged cycles, respectively.
species (0 < δ < 4) in MoP could demonstrate a doublet characteristic.14 Furthermore, two peaks at 233.1 and 236.3 eV come from the Mo−O bond of MoO3, which confirms the presence of a passivation layer; the results agreed with that of TEM (Figure 1b-2).10,15 Figure 2D presents the XPS spectrum of P 2p. Two peaks located at 129.5 and 130.3 eV suggest the formation of a Mo−P bond; moreover, the peak at 133.8 eV is associated with a P−O bond because of surface oxidation.16 Figure 3A shows cyclic voltammetry plots of pure carbon cloth (CC), P-doped CC (with a P-doped content of 0.33 wt % as presented in Figure S2), and MoP@CC electrode based LOBs, respectively. It can be observed that MoP@CC electrode based LOBs exhibit a more positive ORR onset potential (∼2.89 V), compared with that of pure CC electrode based LOBs without catalysts (∼2.76 V), and a higher cathodic
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 (Figure 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 a C−O bond, which is probably due to the absorbed oxygen or carbon dioxide;13 in addition, there appears to be a fitted peak at 285.3 eV, which could be associated with a C−P bond, suggesting that the P atom was simultaneously doped into the carbon fiber during the phosphating process. The deconvolution of the Mo 3d spectrum (Figure 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 the Moδ+ 333
DOI: 10.1021/acsaem.7b00299 ACS Appl. Energy Mater. 2018, 1, 331−335
Letter
ACS Applied Energy Materials
indicating that the MoP nanoflakes contribute to the reversible generation and decomposition of Li2O2 leading to enhanced LOB performance. In order to more clearly characterize the discharge/charge products, XPS measurements were performed after the cycling stability test. Figure 4f presents Li 1s spectra of LOBs after discharge and charge, respectively. The spectrum could be divided into two peaks, in which 55.2 eV corresponds to Li2O2, while the peak located at 54.4 eV could be assigned to Li2CO3, which perhaps comes from decomposition of electrolyte after long-term operations.21 Furthermore, it can be also observed that the peak of Li2O2 after charging is smaller than that after discharging, indicating excellent catalytic activity of the MoP nanoflakes.
current peak. Moreover, MoP@CC electrode based LOBs also show a lower OER onset potential and higher current peak during anodic scans. An overpotential of ∼0.81 V for the MoP@CC electrode has been obtained between the ORR and OER process, while that of a pure CC electrode is ∼0.99 V, indicating better catalytic activities of MoP nanoflakes. The galvanostatic discharge−charge curves of LOBs using MoP@ CC, P-doped CC, and pure CC electrodes, respectively, at an actual current of 0.1 mA, are shown in Figure 3B. In comparison with the other two electrodes, the MoP@CC electrode displays a higher discharge voltage (∼2.75 V) and a lower charge plateau (∼4.28 V). Furthermore, the MoP@CC electrode exhibits a real discharge capacity of 4.15 mAh compared to that of 1.37 mAh for the P-doped CC electrode and 0.39 mAh for the pure CC electrode. Besides, the LOBs based on the MoP@CC electrode show a reasonable round-trip efficiency of 95%, much higher than that of the pure CC electrode (68%) (Figure S3). These phenomena suggest that MoP catalysts could effectively reduce the overpotential of LOBs and enhance the output capacity, and therefore confirm its catalytic effect in Li−O2 batteries. Figure 3C shows the first full discharge/charge properties of the MoP@CC electrode at different actual currents of 0.1, 0.2, and 0.3 mA. With the increase of current from 0.1 to 0.3 mA, the real discharge capacity decreases from 4.15 to 3.30 mAh, with a retention ratio of over 79%, indicating excellent rate capability. Cycling stabilities of the MoP@CC electrode based LOBs were examined at an actual current of 0.1 mA with a fixed capacity of 0.25 mAh. As shown in Figure 3D, although the overpotential increased to a certain extent, the discharge and charge capacities were maintained at 0.25 mAh for 400 cycles with a stable discharging terrace (>2.5 V) and charging voltage (
