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Towards Lower Overpotential through Improved Electron Transport Property: Hierarchically Porous CoN Nanorods Prepared by Nitridation for Lithium-Oxygen Batteries Shumao Xu, Qian-Cheng Zhu, Michelle Harris, Tong-Heng Chen, Chao Ma, Xiao Wei, Hua-Sheng Xu, Yong-Xian Zhou, Yu-cai Cao, Kai-Xue Wang, and Jie-Sheng Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02805 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016
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Towards Lower Overpotential through Improved Electron Transport Property: Hierarchically Porous CoN Nanorods Prepared by Nitridation for Lithium-Oxygen Batteries
Shu-Mao Xu,1 Qian-Cheng Zhu,1 Michelle Harris,1 Tong-Heng Chen,1 Chao Ma,1 Xiao Wei,1 Hua-Sheng Xu,2 Yong-Xian Zhou,2 Yu-Cai Cao,3 Kai-Xue Wang,1* and Jie-Sheng Chen1*
1
Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China 2
Shanghai Key Laboratory of Catalysis Technology for Polyolefins, Shanghai Research
Institute of Chemical Industry, Shanghai 200062, P. R. China 3
State Key Laboratory of Polyolefins and Catalysis, Shanghai Research Institute of Chemical
Industry, Shanghai 200062, P. R. China
*Corresponding author: KXW, Department of Chemistry and Chemical Engineering, Shanghai, 200240, China. Email:
[email protected] ACS Paragon Plus Environment
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ABSTRACT To lower the overpotential of a lithium-oxygen battery, electron transport at the solid-to-solid interface between the discharge product Li2O2 and the cathode catalyst is of great significance. Here we propose a strategy to enhance electron transport property of the cathode catalyst by the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms to improve electron density at Fermi energy after nitridation. Hierarchically porous CoN nanorods were obtained by thermal treatment of Co3O4 nanorods under ammonia atmosphere at 350 ºC. Compared with that of the pristine Co3O4 precursor before nitridation, the overpotential of the obtained CoN cathode was significantly decreased. Moreover, specific capacity and cycling stability of the CoN nanorods were enhanced. It is assumed that the discharged products with different morphologies for Co3O4 and CoN cathodes might be closely associated with the variation in the electronic density induced by occupancy of nitrogen atoms into interstitial sites of metal lattice after nitridation. The nitridation strategy for improved electron density proposed in this work is proved to be a simple but efficient way to improve the electrochemical performance of metal oxide based cathodes for lithium-oxygen batteries.
KEYWORDS: low overpotential, lithium−oxygen batteries, interfacial contact, nitride, electron density, densely packed
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The aprotic lithium-oxygen batteries (LOBs) have received considerable attention recently due to their high theoretical energy densities (13,200 Wh Kg-1), which is highly desirable for long-range electric vehicles.1-3 Although tremendous progress has been made in the past few years, many issues, particularly the large overpotential,4-6 have to be addressed to turn this alluring technology into reality. The high overpotential of LOB is mainly attributed to the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the oxygen electrode,7 the electron transport barrier of the poorly conducting discharge product Li2O2,6,
8-10
an inferior contact between Li2O2 and the catalyst upon
cycling,11-13 and undesired side reactions such as electrolyte decomposition induced by the existence of Li2O2 and other intermediate products.10,
14
Tremendous efforts have been
devoted to identifying effective electrocatalysts to lower the overpotential of LOBs. Nanostructured noble metals, such as gold,15 platinum16 and palladium14, 17, 18 have been proved to be effective in lowering the overpotential of LOBs. However, it is less competitive for these noble metals as cathode materials in LOBs because of their relatively low specific energy density and high price.6, 15 Porous transition metal oxides, such as Co3O4,6 TiO2,19, 20 MnO2,21 with merits of low cost and accessible pores are regarded as promising catalysts for LOBs. However, due to the relatively poor electronic conductivity, large overpotential also remains a big issue for these metal oxide based LOBs.6, 21, 22 The electron conductivity is usually a limiting factor for OER progress taking place at the Li2O2/catalyst interface.23 During the discharge process of LOBs, O2 is reduced by electrons from the catalyst, forming Li2O2 with Li+. Upon charging, a reversible process with the release of O2 and Li+ takes place. Thus, enhanced electron conductivity of cathode catalysts by nitridation might lower the overpotential of LOBs. The substitution of oxygen in metal oxides with nitrogen would ACS Paragon Plus Environment
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increase the electron density at Fermi energy and consequently promote the electron transport properties of the generated nitride materials. The nitridation of the generally used metal oxide-based catalysts is proposed as a feasible way to generate LOB catalysts with lower overpotential. Moreover, porous structure is beneficial for permeation of oxygen,6 transportation of ions, infiltration of electrolyte within the electrode,6, 24 and the deposition of Li2O2 species during the discharge process with plenty of space.16 However, the application of the nitride-based catalysts with hierarchical pores in LOBs and relative mechanisms remain largely unexplored, compared with corresponding metal oxides and carbides. Herein, hierarchically porous CoN nanorods were prepared through a one-step low-temperature ammonia nitridation approach by use of porous Co3O4 nanorods as precursors and self-templates. Both Co3O4 precursor and hierarchically porous CoN nanorods were employed as cathode catalysts for LOBs and their electrochemical performances were evaluated by galvanostatic charge/discharge. After replacement of the oxygen atoms in Co3O4 precursor with nitrogen through nitridation, the overpotential is significantly decreased, while the specific capacity and the cycling stability are obviously improved. This superior catalytic activity of CoN might be attributed to the increase of electron density after nitridation based on discharge products in Co3O4 and CoN cathodes with different morphologies on designated cycling. The high electron density state on the surface of catalyst would promote the formation of densely packed Li2O2, contributing to the low overpotential. This nitridation strategy might extend to other metal oxides, and this work sheds new light on the development of cathode catalysts with improved electron density for lower-overpotential LOBs. ACS Paragon Plus Environment
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Firstly, porous Co3O4 nanorods were prepared following a procedure reported in the literature.25 Cobalt chloride and urea were hydrothermally reacted at 180 ºC for 16 h and subsequently the hydrothermal product was calcined at 500 ºC. Then, hierarchically porous CoN nanorods were prepared by thermally treating the calcined Co3O4 precursor in ammonia atmosphere at 350 ºC. Figure 1 displays X-ray diffraction (XRD) patterns of the precursor Co3O4 nanorod and the corresponding CoN nanorod after nitridation. The diffraction peaks for Co3O4 can be readily assigned to the face-centered cubic structure with Fd3m space group (JCPDS No. 42-1467). No other diffraction peaks can be observed in the XRD pattern, indicating that the precursor is a pure phase of Co3O4. After nitridation, all diffraction peaks attributed to Co3O4 disappear and new diffraction peaks emerge. The peaks located at 36.2°, 42.2°, 61.3°, 73.3°, and 76.8° (2θ) can be indexed as the (111), (200), (220), (311), and (222) diffractions, respectively, of cubic-structured CoN with space group of F-43m (JCPDS No. 083-0831). The XRD analyses indicate that Co3O4 is successfully transformed to CoN through one-step nitridation under ammonia atmosphere at 350 ºC. Transmission electron microscopy (TEM) observation reveals that the Co3O4 precursor has nanorod morphology with an average diameter of 200 nm and a length of several micrometers (Figure 2a). The nanorods are constructed by interconnected Co3O4 crystallites with particle size in the range of 40–100 nm (Figure 2b). The aggregation of these crystallites leaves plenty of void space among the nanoparticles, generating a porous structure. Distinct lattice fringe with spacing of approximately 2.0 Å is observed in the high-resolution TEM (HRTEM) image, ascribed to the (400) plane of cubic structured Co3O4 (Figure
2c).
The
porous
nanorod
morphology
is
well
preserved
after
temperature-programmed nitridation under ammonia atmosphere (Figure 2d). Both the ACS Paragon Plus Environment
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diameter and length of CoN nanorods are similar to those of as-synthesized Co3O4 nanorods, indicating that Co3O4 nanorods function as not only precursor but also self-sacrificial template. However, wormhole-like mesopores are observed in the CoN nanoparticles (Figure 2e). The formation of these mesopores is attributed to the escape of the gas during the nitration of Co3O4 with NH3. These mesoporous channels within the CoN nanoparticles together with the void space among nanoparticles form a hierarchically porous structure. The hierarchically porous structure can facilitate the permeation of oxygen into the electrode and provide plenty of space for the deposition of discharge products. Lattice fringes with interplanar spacing of approximately 2.5 Å observed in the HRTEM can be assigned to the (111) plane of cubic CoN (Figure 2f). The corresponding Fast Fourier Transformation (FFT) image of the mesoporous CoN nanorod (Inset in Figure 2f) displays individual spots, indicating the relatively high crystallinity of obtained CoN nanorods. The hierarchically porous CoN and porous Co3O4 nanorods were employed as bifunctional catalysts for oxygen electrodes of LOBs (Figure 3). At a current density of 100 mA g-1, an initial discharge capacity of 1,394 mAh g-1 is achieved for the CoN nanorods, much higher than that of Co3O4 (890 mAh g-1) (Figure 3a). The CoN nanorods exhibit a charge capacity of 1,496 mAh g-1, giving a coulombic efficiency of 93.2 %. The charge profile of CoN nanorods exhibits typical three regions characterized by distinct difference in slope (Figure 3a). The first region I at low voltage (ca. 3.3 V) is associated with the oxidation of the highly conducting LiO2-like phase on the rims of toroids and the transformation of toroids to disc-shaped Li2O2 particles.10 The following slope in region II is associated with the oxidation of crystalline Li2O2 shells. The particle size of Li2O2 shrinks in this range of voltage. A nearly flat plateau (III) is associated with the oxidation of the ACS Paragon Plus Environment
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remaining quasi-amorphous Li2O2 cores in parallel to the side reactions (e.g. oxidation of Li2CO3).10 Upon cycling, discharge capacities of 2nd and 5th cycle increase slightly with the same voltage plateau as the 1st cycle. The charge-discharge overpotential of CoN is 1.32 V, lower than that of Co3O4 (1.55 V), demonstrating that the overpotential of original metal oxide-based LOBs is successfully decreased by nitridation (Figure 3b). Both Co3O4 and CoN exhibit a similar discharge plateau potential in 2.58 V, attributable to the formation of Li2O2 products during the discharge process. The cycling performance of Co3O4 and CoN is shown in Figure 3c. For CoN nanorods, an increase in the specific discharge capacity during initial 10 cycles is observed, ascribed to the activation process. CoN cathodes also show relatively good cycling stability, particularly for the initial 40 cycles. For Co3O4 nanorods, decrease in the specific capacity is observed upon cycling. Typical cyclic voltammetry (CV) curves of CoN at a scanning rate of 0.5 mV s-1 are shown in Figure 3d. The cathodic peak at 2.38 V is ascribed to the oxygen reduction. The first anodic peak at 3.25 V during the 2nd cycle is ascribed to formation of off-stoichiometric Li2−xO2 compounds through the topotactical delithiation of the discharge product Li2O2.26 These off-stoichiometric superoxides can be immediately decomposed. Another peak at 4.23 V is attributed to the reaction of Li2O2→O2+2e-+Li+.19 The peak current density and integrated area are nearly unchanged in the subsequent discharge/charge cycles, indicating good cycling stability of the CoN cathodes. To illustrate the obtained superior catalytic performance of metal nitride, morphologies of initial discharge products on the surface of hierarchically porous CoN and porous Co3O4 cathodes were investigated by TEM. After 1st cycle at a current density of 100 mA g-1, nanoparticles and coatings of discharge products can be observed distributed on the outer ACS Paragon Plus Environment
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surface of the full-discharged CoN and Co3O4 nanorods (Figure 4a-c). Distinct lattice fringes with interplanar spacing of approximately 1.6 Å (Figure 4d,e), attributable to the (110) lattice plane of Li2O2 demonstrate the Li2O2 phase of the nanoparticles and coatings on the surface of discharged CoN and Co3O4 nanorods. The nanoparticles on the surface of CoN nanorod have a lying or inbuilt disc-like morphology (Figure 4c). The different morphologies of lying and inbuilt disc-like Li2O2 can be explained by two competitive ORR mechanisms involving an intermediate LiO2* species.9 One mechanism is based on the reduction of the intermediate LiO2* species adsorbing on the active site of catalyst, while the other is related to the chemical disproportion of solvated LiO2* species from the electrolyte. When intermediate LiO2* adsorbs on the active site of catalyst, its further reduction at active sites on the surface of catalyst leads to heterogeneous nucleation of Li2O2. And successive nucleation and growth of Li2O2 at sites on the surface generates inbuilt disc-like Li2O2. However, not all exposed surface of catalyst has appropriate oxygen binding energy and high affinity for LiO2* adsorption. When LiO2* binds weakly to the surface, solvation of the LiO2* species into the electrolyte occurs at a faster rate than the direct surface reduction of LiO2*. The chemical disproportion of the solvated LiO2* species leads to form lying disc-like Li2O2 (2LiO2*→Li2O2+O2).9 The inbuilt and lying disc-like discharged products can be mostly decomposed after subsequent recharging processes (Figure S1). Compared with densely packed Li2O2 on the discharged CoN nanorod (Figure 4c), the toroid in Co3O4 cathode after 1st cycle is composed of many small particles (Figure 4e). For densely packed toroid Li2O2 on the discharged CoN nanorod, its surface half-metallic state27,28 allows electrons to migrate on the surface of Li2O2 particle, leading to relatively even decomposition of Li2O2 (Figure 4f). Although small Li2O2 particles on Co3O4 nanorod can be decomposed more easily than ACS Paragon Plus Environment
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those large crystalline ones on CoN nanorod, electron transport may be difficult for the majority of Li2O2 remote from the surface sites of catalyst and the existence of plenty of particles boundaries (Figure 4g). The decomposition of Li2O2 during the charging process would mainly occur at the Li2O2/catalyst interface, the root of the particles.23 Consequently, the loosely packed Li2O2 particles in Co3O4 cathode are apt to detach from the catalyst surface and sweep away into the electrolyte, leading to large overpotential. Therefore, the overpotential of CoN cathode is smaller than that of Co3O4 cathode in initial cycles. The preferred growth active site of Li2O2 and its relevant particle size is closely related with the electronic density of states and charge density contour plots on the surface of catalyst.2, 9 The formation of relatively unbroken toroid-like discharge product in CoN cathode might be closely associated with the electronic density of states and charge density contour plots on the surface of catalyst. Transition metallic nitrides are formed by occupancy of nitrogen atoms into interstitial sites of metal lattice.29,
30
Volume expansion and
corresponding lattice expansion of metal occurs, narrowing of the metallic d-band gap. Moreover, the mixing of d-orbitals of metal and p-orbitals of nitrogen to form d,p-bonding increases the electron density at Fermi energy (Figure S2). The s-p or s-p-d hybrid-electrons are responsible for long-range order properties because they are the outermost and itinerant electrons. Therefore, the Fermi energy and the electronic density of CoN with face-centered cubic structure are higher than those of Co3O4. Fast electron transport on the surface of CoN strongly favors nucleation and growth of solvated supersaturated LiO2* in electrolyte at sites to form large densely packed toroid Li2O2 (Figure 5a). Slow electron transportation on the surface of Co3O4 will decrease the formation rate of solvated LiO2*. Reaction of a second LiO2* molecule and a Li+ cation with a solvated LiO2* forms an intermediate LiO2 dimers ACS Paragon Plus Environment
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with very short lifetime (Figure 5b).31 The disproportion of LiO2 dimer leads to the formation of Li2O2 with release of O2. It was proposed that Li2O2 could be dissolved in the electrolyte with redox mediators generated by side reactions, such as electrolyte decomposition during the previous cycling.32 Consequently, solvated Li2O2, after reaching a supersaturated state, will heterogeneously nucleate and grow on surface sites to form a large toroid Li2O2 accumulated by plenty of small Li2O2 particles. To further understand the obtained superior catalytic performance of metal nitride, morphology evolution of discharge products on the surface of CoN nanorods at different discharge/charge stages was investigated by SEM. The lying and the inbuilt disc-like Li2O2 can be found on the surface of the full-discharged CoN nanorod after 1st cycle (Figure 6a), consistent with the corresponding TEM observation (Figure 4c). Upon cycling, irreversible decomposition and continuous accumulation of Li2O2 lead to large disc-like Li2O2 in micron size decorated on CoN nanorods (Figure 6c). After 60th cycle, the discharged CoN electrode was almost completely covered with large disc-like Li2O2 (Figure 6d). These micron-sized disc-like Li2O2 cannot be easily decomposed during the subsequent recharging process (Figure 6b), accounting for the inferior performance with capacity decay and enlarged overpotential of CoN electrode for extended cycling. When discharged/charged for 100 cycles, toroid-like Li2O2 in several micrometers can be found on the CoN nanorods (Figure 6e). The reason for these toroid-like Li2O2 in no contact with the catalyst might also be explained by the through-solution mechanism for the nucleation and aggregation of solvated intermediate LiO2* in the electrolyte. In summary, a nitridation strategy through the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms has been proved to be effective in ACS Paragon Plus Environment
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lowering the overpotential of LOBs. Hierarchically porous CoN nanorods were successfully obtained by thermal treatment of Co3O4 nanorods under ammonia atmosphere at 350 ºC. The substitution of oxygen with nitrogen would increase the electron density beneficial for fast electron transport to form densely packed Li2O2 on the surface of catalyst. The surface half-metallic state of densely packed Li2O2 allows electron transport between Li2O2 and catalyst for even decomposition of Li2O2, lowering the overpotential.. However, slow electron transport on the surface of Co3O4 nonorod is inclined to form toroid Li2O2 composed of many particles. The existence of plenty of particles boundaries and the subsequent detachment of these loosely packed Li2O2 particles into electrolyte might result in a large overpotential of metal oxides based LOBs. Moreover, generated hierarchical pores within the CoN nanorod can provide extra channels for mass transportation and void space for the deposition of discharge products, contributing to the improved electrochemical performance. This work sheds a new light to lower overpotential of metal oxide based LOBs by improving electron density at Fermi energy after nitridation.
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■ ASSOCIATED CONTENT * S Supporting Information Material synthesis, cell assembly and characterization, and SEM image of the full-charged CoN electrode after 1st cycle, schematic diagram of the formation of metal-nitrogen linkage. This material is available free of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected] *E-mail:
[email protected] Notes The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program (2014CB932102, 2013CB934102), the National Natural Science Foundation of China (21271128, 21331004, 51472158) and Shanghai Shuguang Project.
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FIGURE LEGENDS Figure 1. XRD patterns of Co3O4 nanorods and corresponding CoN. Figure 2. TEM and HRTEM images of (a-c) Co3O4 nanorods; and (d-f) as-synthesized CoN nanorods, respectively. Inset in f is the corresponding Fast Fourier Transformation (FFT) image. Figure 3. (a) Discharge/charge profiles of the 1st, 2nd, and 5th cycles of the CoN electrode. (b) The initial discharge/charge profiles of Co3O4 and CoN electrode. (c) Cycling performance of Co3O4 and CoN. (d) Cyclic voltammetry curves of CoN electrode at a scanning rate of 0.5 mV s-1. The galvanostatic discharge/charge and cycling test are performed within a voltage window of 2.3 to 4.3 V at a current density of 100 mA g-1. Figure 4. TEM images of the full-discharged (a) CoN electrode and (b) Co3O4 electrode after 1st cycle. (c) The enlarged TEM image of a, and (d) the HRTEM of c, (e) The enlarged TEM image of b. Inset in e is the corresponding HRTEM image. Schematic illustrations of oxygen evolution reaction in (f) CoN and (g) Co3O4 electrodes. Figure 5. Illustration of two discharge reaction mechanisms based on an initial oxygen reduction at an active site followed by solution phase reactions and growth at a surface nucleation site of (a) CoN and (b) Co3O4 catalysts. Figure 6. SEM images of (a) the full-discharged CoN electrode after 1st cycle, (b) the charged CoN electrode after 60th cycle, the full-discharged CoN electrode after (c) 30th, (d) 60th and (e) 100th cycle.
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FIGURES
200
Figure 1
311 222
220 311 20
30
40
2θ /
o
533 622
440
422
400
511
Co3O4
222
220
111 10
CoN
111
Relative Intensity
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60
(Cu-Kα)
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Figure 2
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Figure 3
CoN Voltage / V
b
5 nd
2
th
5
5.0
Co3O4
4.5 4.0
4
Voltage / V
a
3.5
st
1
1.55 V
1.32 V
3.0
3
2.5
III
II
I
2
2.0
CoN
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0
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600
0
800 1000 1200 1400 1600 -1
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Capacity / mAh g
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Capacity / mAh g-1
d
Co3O4
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0.3
nd
2
CoN Current / mA
Capacity / mAh g-1
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0.0
800
rd
-0.3
400
3
0 0
10
20
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Cycle number
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2.5
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3.5
Potential / V
4.0
4.5
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Figure 4
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Figure 5
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Figure 6
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The table of contents entry To lower the overpotential of a lithium-oxygen battery, electron transport at the solid-to-solid interface between the discharge product Li2O2 and the cathode catalyst is of great significance. Here we propose a strategy to enhance electron transport property of the cathode catalyst by the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms to improve electron density at Fermi energy after nitridation. The substitution of oxygen with nitrogen would increase the electron density beneficial for fast electron transport to form densely packed Li2O2 on the surface of catalyst. The surface half-metallic state of densely packed Li2O2 allows electron transport between Li2O2 and catalyst for even decomposition of Li2O2, lowering the overpotential.. However, slow electron transport on the surface of Co3O4 nonorod is inclined to form toroid Li2O2 composed of many particles. The existence of plenty of particles boundaries and the subsequent detachment of these loosely packed Li2O2 particles into electrolyte might result in a large overpotential of metal oxides based LOBs. KEYWORDS: low overpotential, lithium−oxygen batteries, interfacial contact, catalysts, nitride, electron density Shu-Mao Xu, Qian-Cheng Zhu, Michelle Harris, Tong-Heng Chen, Chao Ma, Xiao Wei, Hua-Sheng Xu, Yong-Xian Zhou, Yu-Cai Cao, Kai-Xue Wang, and Jie-Sheng Chen
Towards Lower Overpotential through Improved Electron Transport Property: Hierarchically Porous CoN Nanorods Prepared by Nitridation for Lithium-Oxygen Batteries
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