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An inverse spinel cobalt-iron oxide and N-doped graphene composite as an efficient and durable bifuctional catalyst for Li-O2 batteries Yudong Gong, Wei Ding, Zhipeng Li, Rui Su, Xiuling Zhang, Jian Wang, Jigang Zhou, Zhiwei Wang, Yihua Gao, Shaoqing Li, Pengfei Guan, Zidong Wei, and Chunwen Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04401 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018
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ACS Catalysis
An inverse spinel cobalt-iron oxide and N-doped graphene composite as an efficient and durable bifuctional catalyst for Li-O2 batteries Yudong Gong,1,2‡ Wei Ding,3‡ Zhipeng Li,1,2‡ Rui Su,4,5‡ Xiuling Zhang,1,2 Jian Wang,6 Jigang Zhou,6 Zhiwei Wang,1,2,8 Yihua Gao,7 Shaoqing Li,1,2 Pengfei Guan,4* Zidong Wei,3* and Chunwen Sun1,2,8*
1
CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China 2 School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China 3 The State Key Laboratory of Power Transmission Equipment & System Security and New Technology；Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China 4 Beijing Computational Science Research Center, Beijing 100193, China 5 Innovative Center for Advanced Materials, Hangzhou Dianzi University, Hangzhou 310018, China 6 Canadian Light Source Inc., University of Saskatchewan, Saskatoon, SK S7N 2V3, Canada 7 Center for Nanoscale Characterization and Devices, Wuhan National Laboratory for Optoelectronics–School of Physics–School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, PR China 8 Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
*
Corresponding authors.
Email:
[email protected], (C. Sun),
[email protected] (P. Guan),
[email protected], (Z. Wei). ‡
These authors contributed equally to this work.
ABSTRACT: Rational design of efficient bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts are critical for rechargeable Li-O2 batteries. Here, we report inverse spinel Co[Co,Fe]O4/nitrogen-doped graphene (NG) composite used as a promising catalyst for rechargeable Li-O2 batteries. The cells with Co[Co,Fe]O4/NG catalyst exhibit high initial capacity, remarkable cyclability, and good rate capability. Moreover, the 1
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overpotential of the Li-O2 batteries is reduced significantly. The improved ORR/OER performances are attributed to the good property of Co[Co,Fe]O4 with an inverse spinel structure toward ORR and the improved electronic conductivity of N-doped graphene. The density functional theory (DFT) shows the rate limitation step for ORR is the growth of the Li2O2 cluster for the inverse spinel surface while the rate limitation step for the OER pathway is the oxidation of Li2O2. The inverse spinel surface in Co[Co,Fe]O4/NG is more active than that of the normal spinel phases for the Li-O2 battery reactions. This work not only provides a promising bifunctional catalyst for practical metal air batteries but also offer a general strategy to rationally design catalysts for various applications.
KEYWORDS: Inverse spinel; Co[Co,Fe]O4/N-doped Graphene; oxygen reduction reaction; oxygen evolution reaction; Li-O2 batteries.
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INTRODUCTION Lithium-air batteries are promising compared to the present lithium-ion batteries (LIBs) due to their superior high energy density (~11140 W h kg-1).1-3 The prerequisite for operation of a rechargeable nonaqueous Li-O2 battery is the reversible formation and decomposition of insoluble Li2O2 on porous cathode during discharge and charge process, respectively.4-8 In order to make the Li-O2 battery with a nonaqueous electrolyte viable, it is required to address the issues including sluggish reaction kinetics, high overpotential, and the formation of insoluble discharge products, etc. The overpotential of discharge process corresponding to oxygen reduction reaction (ORR) commonly is ~0.3V whereas that of the charge process related to the oxygen evolution reduction (OER) gives a value of overpotential about 1.2-1.5 V.9 To decrease polarization gaps, various cathode catalysts have been developed, such as activated carbon materials or graphene based materials,10-12 noble-metals and their alloys,13,14 non-precious metals and their compounds.15-19 Particularly, graphene or heteroatoms-doped graphene has been used as supports of catalysts for Li-O2 batteries due to their 2D structure,20 excellent electrical conductivity21 and large specific surface area.22 However, it is reported that nitrogen doped graphene shows excellent catalytic performances in oxygen catalytic process but it does not show good cyclability.23-25 In contrast, spinel oxide materials, such as Co3O4,26,27 MnCo2O4,28 CuCo2O4,29 have been widely studied as cathode materials for Li-O2 batteries, showing better catalytic performance. To commercialize Li-air batteries, it is highly desired to develop catalysts for Li-O2 batteries with high performance and low cost. 30-32
Herein, we reported a nitrogen-doped graphene supported inverse spinel structure material CoⅡFeⅢCoⅢO4 (denoted as Co[Co,Fe]O4/NG) by partial substitution of Fe atoms for Co atoms at octahedral sites used as a cathode material for Li-O2 batteries. The performance of Li-O2 batteries with Co[Co,Fe]O4/NG catalyst outperforms that of the commercial Pt/C catalyst in terms of low polarization and excellent cycling performance. X-ray absorption near-edge structure (XANES) spectroscopy and scanning transmission X-ray microscopy (STXM) analysis indicates that Co2+ and 3
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Co3+ coexist in Co[Co,Fe]O4/NG. The density functional theory (DFT) shows the rate limitation step for ORR is the growth of the Li2O2 cluster for the inverse spinel surface and the reduction of LiO2 adsorbate for the spinel phase while the rate limitation step for the OER pathway, is the oxidation of Li2O2 and LiO2 for inverse and normal spinel phases, respectively. The inverse spinel surface Co[Co,Fe]O4/NG is more active than that of the normal spinel phases for the Li-O2 battery reactions due to the difference of Co, Fe atoms distribution on the surfaces of the two differnt oxides. All those evidences demonstrate that the Co[Co,Fe]O4/NG is a promising bifunctional catalyst for rechargeable Li-O2 batteries.
EXPERIMENTAL METHODS Synthesis of Co[Co,Fe]O4/NG Synthesis of nitrogen-doped graphene. 0.3 g montmorillonite were dispersed in the 15 mL mixture solution of deionized water and anhydrous alcohol (1:1, volume ratio) by ultrasonic treatment for 30 min. The function of montmorillonite is as pore forming material. Then, 300 mg aniline monomer was added to the suspension with stirring for 48 hours. After that, with added 7.5 mL of 0.5 M H2SO4 solution, the solution was kept stirring for more 1h. (NH4)2S2O8 (dissolved in 2.5 mL of 0.5 M H2SO4 solution) was added drop-wise in the continuously stirring solution with a molar ratio of 1:1 to aniline. The final solution was kept for 1 day in ice water mixture before filter and wash to get the precursor. The solid product was heated under N2 flow at 120 oC and 900 oC for 1 h and 3 h, respectively. The obtained powder was treated by 40% HF to remove montmorillonite. The solid products were washed with deionized water, centrifuged and dried to get N-doped graphene.
Synthesis of Co[Co,Fe]O4/NG. The Co[Co,Fe]O4/NG composite was prepared according to the same method as a our previous work.33 For preparing Co[Co,Fe]O4 nanoparticles, Co(NO3)2·6H2O and FeCl3·6H2O were dissolved in 25 mL deionized water with a molar ratio of 2:1 to form a solution. The brownish solution was heated at 80 oC and about 12 mL of NH3·H2O was added into the solution to adjust the pH 4
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ACS Catalysis
value in the range of 11-12. After stirring for 60min, the obtained solution was transferred to a 100 mL Teflon lined autoclave and kept at 150 oC for 24 h. The product was centrifuged and washed several times with deionized water and dried at 60 oC overnight. The obtained Co[Co,Fe]O4 were mixed with N-doped graphene with a weight ratio of 1:1 by grinding for 1h and then dispersed ultrasonically in absolute alcohol for 1h. Then the suspension was dried by vigorously stirring and the Co[Co,Fe]O4/NG were obtained.
Synthesis of Co[Fe2]O4/NG. The Co[Fe2]O4/NG composite was prepared according to the same method as the literature.33
Characterization of the Materials. XRD diffraction (XRD) test were carried out in a Bruker-AXS D8 diffractometer with Cu Kα radiation in the 2θ range of 10-70o. High resolution transmission electron microscopy (TEM) images, high angle annular dark field scanning TEM (HAADF-STEM) images, and chemical elements mapping using energy dispersive X-ray spectroscopy (EDX) were taken by FEI Tecnai G2 F20 S-Twin TEM with accelerating voltage of 200 kV. The electron energy loss spectrums (EELS) were acquired on FEI Titan G2 60-300 S/TEM equipped with Gatan GIF 963 EELS detector using accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) was performed on a spectrometer with Mg Kα radiation (ESCALAB 250, Thermofisher Co.). The data were fitted by using XPSPEAK software.34 For STXM measurements, powder samples of the Co[Co,Fe]O4/NG and CoFe2O4/NG samples were dispersed in ethanol by brief sonication, and then deposited on a holey carbon film coated TEM grid and dried in the air. STXM was performed at the SM beamline of the Canadian Light Source (CLS), a 2.9 GeV third generation synchrotron facility. The detailed process is similar to those reported in the previous work.34 Raman spectra were conducted with a Confocal Raman Spectrometer (Horiba, France) with an excitation length of 538 nm.
DFT calculations. Spin resolved density functional theory (DFT) calculations are 5
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package35
(VASP).
The
exchange-correlation effect is described by the BEFF-vdW functional36 which includes non-local vdW correction for the adsorption energy. The electron-ion interaction is described by the projector augmented wave potentials. The 1s orbital of the lithium atom is treated as valence in the calculations. The wave functions are expanded by planewave basis with a kinetic energy cutoff of 400 eV. The K-point mesh is set to 5×5×5 and 2×2×1 for crystal and slab calculations, respectively. All structures are relaxed until the per-atom force is less than 0.05 eV/Å. The localized 3d orbital of Co and Fe atoms are treated by the Hubbard U model with effective U values set to 2.0 and 3.0 eV,37 respectively.
Preparation of air catalytic electrodes and cell assembly. The as-prepared Co[Co,Fe]O4/NG was mixed with Super P and polyvinylidene fluoride (PVDF) with a weight ratio of 2:7:1. Then the powder mixture was ground for 30 min before dispersed in N-methyl-2-pyrrolidone (NMP) by ultrasonic treatment for 10 min. Ni foam pieces with an area of about 0.6 cm2 were used as the current collectors. Each current collector was treated in 0.1 M HNO3 for 10 min to remove surface oxidants, then washed by deionized water and alcohol for several time before dried at 60 oC in an oven. The Ni foam pieces were cast by the above slurry and then dried at 120 oC for 12 h in a vacuum oven. The catalyst loading is about 1.6 mg cm-2. The cells were assembled with a standard CR2032 coin cells with holes on the cathode side in an argon-filled glove box with H2O and O2
