Synthesis of ordered mesoporous NiCo2O4 via hard template and its application as bifunctional electrocatalyst for Li-O2 batteries

Electrochimica Acta 129 (2014) 14–20

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Synthesis of ordered mesoporous NiCo2 O4 via hard template and its application as bifunctional electrocatalyst for Li-O2 batteries Yuan Li a,b , Liangliang Zou a,b , Jun Li a,b , Kun Guo a,b , Xiaowen Dong a , Xiaowei Li c , Xinzhong Xue a , Haifeng Zhang a,∗ , Hui Yang a,∗ a

Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China University of Chinese Academy of Sciences, Beijing 100039, China c College of Physics, Optoelectronics and Energy of Soochow University, Suzhou 215123, China b

a r t i c l e

i n f o

Article history: Received 15 November 2013 Received in revised form 10 January 2014 Accepted 10 February 2014 Available online 26 February 2014 Keywords: Lithium-oxygen battery Bifunctional electrocatalysts Ordered mesoporous NiCo2 O4 Hard template

a b s t r a c t Ordered mesoporous NiCo2 O4 (OM NiCo2 O4 ) materials have been synthesized via KIT-6 template and used as bifunctional electrocatalyst for rechargeable Li-O2 batteries. Characterization of the catalyst by X-ray diffractometry and transmission electron microscopy confirms the formation of a single-phase, 3-dimensional, ordered mesoporous NiCo2 O4 structure. The as-prepared OM NiCo2 O4 exhibits a specific surface area of 95.5 m2 g−1 with mesoporous peaks between 3 and 5 nm. Linear scanning voltammetric measurements reveal that OM NiCo2 O4 exhibits slightly higher catalytic activity for the oxygen reduction reaction but much higher activity for the oxygen evolution reaction than Ketjen black (KB) carbon. The Li-O2 battery utilizing OM NiCo2 O4 shows a slightly higher discharge voltage plateau and a higher specific capacity of 4120 mAh g−1 than that with pure KB, and the subsequent charge voltage is much lower than that with KB. Moreover, an enhanced cyclability of Li-O2 battery with OM NiCo2 O4 cathode is observed with a capacity retention ratio of 65.4% after 5 cycles. When restricting the discharge capacity at 1000 mAh g−1 , Li-O2 battery with OM NiCo2 O4 cathode shows an improved cyclability and the cut-off voltage is as high as 2.4 V after 20 cycles, suggesting that OM NiCo2 O4 could be as a promising catalyst for Li-O2 batteries. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The Li–O2 battery has recently attracted extensive attentions due to its potential to be used in, for example, future electric vehicles since it can store theoretically 5–10 times higher energy than the state-of-the-art lithium-ion batteries [1]. However, numerous scientific and technical challenges need to be addressed before its practical application, mainly including cyclability, charge/discharge efficiency, rate performance and safety [2]. For a rechargable Li-O2 battery, oxygen reduction/evolution reactions (ORR/OER) are the key barriers during its charge/discharge process. To improve the charge/discharge efficiency and cyclability of the Li-O2 batteries, research efforts have been focused on the development of bifunctional electrocatalysts for both the ORR and OER. These catalysts mainly include metal oxides [3–12], metal nitrides [13–16], and metal nanoparticles [17,18]. Shao-Horn Yang and co-workers first

∗ Corresponding author. Tel.: +86 212 032 1112, fax: +86 212 032 1112. E-mail addresses: [email protected], [email protected] (H. Yang). http://dx.doi.org/10.1016/j.electacta.2014.02.070 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

reported a bifunctional Pt-Au electrocatalyst [17], in which Pt is beneficial for the ORR and Au for the OER. Such a bifunctional Pt-Au electrocatalyst significantly decreases the overvoltage of a Li-O2 battery, thus improving the round-trip efficiency. John B. Goodenough et al. [8] synthesized tetragonal CoMn2 O4 spinel nanoparticles grown on the surface of graphene nanosheets using as a bifunctional electrocatalyst for rechargeable Li-O2 battery with hybrid electrolyte. Recent work by Xinbo Zhang et al. [11] demonstrates that with the aid of porous La0.75 Sr0.25 MnO3 nanotubes, the Li-O2 batteries exhibit rather stable specific capacities above 9000–11000 mAh g−1 for five cycles at 0.025 mA cm−2 . When limiting the capacity at 1000 mAh g−1 , the cycle life could be extended to over 120 cycles, which is much higher than that using carbon. Mesoporous materials might be the promising candidates used for high performance electrodes because of their large specific surface area, interconnected pores, controllable pore sizes, and controllable pore wall compositions [19,20]. Ordered mesoporous materials have the advantages of regular texture, highly structured channel, narrow pore size distribution and adjustable fine structure, and thus resulting in the most promising

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applications. Recently, mesoporous structural materials have received much attention for the development of Li-O2 batteries, including mesoporous carbon [21,22] and mesoporous metal oxides [7,9,12,23,24]. In addition, spinel NiCo2 O4 composite materials have been extensively investigated for electrochemical devices, such as electrochemical capacitors [25–29], fuel cells [30,31], Li-ion batteries [32] and metal-air batteries [9,10,33,34], due to its better electrical conductivity and higher electrochemical activity than the two relevant single-component oxides, nickel oxide and cobalt oxide. Based on above considerations, Guanglei Cui et al. focused on the application of mesoporous NiCo2 O4 nanoflakes in nonaqueous Li-O2 batteries as cathode catalyst, which presents reduced overvoltage and enhanced cyclability. Herein, we report 3-dimentional (3D), ordered mesoporous NiCo2 O4 (OM NiCo2 O4 ) electrocatalyst synthesized via a hard-template method. In contrast with Guanglei Cui’s work (mesoporous NiCo2 O4 nanoflakes, but not ordered structure), our as-prepared NiCo2 O4 has an ordered 3D mesoporous structure with numerous catalytic active sites, which would be more beneficial for the efficient diffusion of both lithium ions and oxygen. The Li-O2 batteries assembled with OM NiCo2 O4 based cathodes exhibit lower overvoltages and improved cyclability along with higher specific capacity and rate capability, which could be attributed to an improvement in Li+ transport with the high specific surface area and ordered 3D mesoporous structure. 2. Experimental 2.1. Sample preparation Highly ordered mesoporous KIT-6 silica templates were synthesized according to previously reported procedures [35–37]. In a typical synthesis process, 0.3 g KIT-6 mesoporous silica template was impregnated with 1 mmol Ni(NO3 )2 ·6H2 O and 2 mmol Co(NO3 )2 ·6H2 O dissolved in 0.3 mL ethanol by stirring at room temperature until ethanol was totally volatilized, and then calcined at 500 ◦ C for 5 h in a muffle stove under air atmosphere. Subsequently, the silica matrix was removed with 2 M NaOH solution at 60 ◦ C for 24 h under magnetic stirring. Finally, the product was filtered and washed with water and ethanol several times, and dried under vacuum at 80 ◦ C for more than 4 h. 2.2. Structural characterizations The structure, morphology and pore distribution of as-prepared OM NiCo2 O4 nanoparticles were examined by X-ray diffraction

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pattern (XRD, Bruker D8 Advance), scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL JEM 200) and BET analyzer (Micromeritics ASAP2020). Before XRD analysis for the cycled electrodes, the electrodes were placed in an argon-filled glove box to avoid exposure into the ambient atmosphere. 2.3. Cell assembly and performance testing In this work, the air electrode was prepared as follows. Ink slurry was prepared by dissolving x wt.% as-prepared OM NiCo2 O4 , (90x) wt.% Ketjen black EC600JD (KB) (where x = 0, 20, 45, 70) and 10 wt.% PTFE as the binder in isopropyl alcohol and dispersed by mechanical stirring and sonication. Then the ink slurry was spread layer-by-layer by hand-painting onto the carbon paper (TGP-H060, E-TEK Inc.) with a carbon loading of 2.0∼2.3 mg cm−2 . The air electrode was then dried under vacuum at 60 ◦ C for 12 h. The glass fibre separator (Whatman) was soaked in 1 M LiCF3 SO3 in TEGDEM electrolyte. Metallic Li foil with a thickness of 0.50 mm was used as the anode. A Swagelok cell with an air hole of 10 mm diameter was used. The cell was assembled in an argon-filled glove box and tested in an oxygen-filled bottle where oxygen pressure was maintained around 1 atm. Galvanostatical discharge-charge performance was evaluated using LAND test system (Wuhan Land Electronic Co. Ltd., China). The capacity is normalized to the total mass of the cathode catalyst including OM NiCo2 O4 and KB carbon. Linear sweep voltammetry (LSV) was conducted in a three-electrode electrochemical cell with a CHI 730B electrochemical workstation. 3. Results and discussion Fig. 1 shows the wide-angle and small-angle XRD patterns of as-prepared NiCo2 O4 . The results are in good agreement to the spinel crystalline structure (JCPDF # 20-0781). The distinct peaks on wide-angle XRD pattern are observed at 2␪ values of 18.91, 31.11, 36.61, 44.61, 59.11 and 64.91, which correspond to (111), (220), (311), (400), (511) and (440) plane reflections of the spinel NiCo2 O4 structure, respectively; confirming the formation of single-phase NiCo2 O4 structure. The small-angle XRD pattern shows a weak 2␪ diffraction peak at 1.1o associated with the ordered mesoporous structure of the as-prepared sample. The morphology and structure of the NiCo2 O4 sample were further characterized by SEM, TEM and high resolution TEM (HR-TEM). As seen from Fig. 2a, the OM NiCo2 O4 particles are microspheres with inerratic heaves on

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80

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Fig. 1. Wide-angle (a) and small-angle (b) XRD patterns of the OM NiCo2 O4 sample.

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Fig. 2. SEM (a), TEM (b) and HRTEM (c, d) images of as-prepared ordered 3D mesoporous NiCo2 O4 .

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NiCo2 O4 sample is of type IV, and exhibits a clear H3 hysteresis loop, which is indicative of the presence of mesopores. The asprepared NiCo2 O4 achieved a BET surface area of 95.0 m2 g−1 . The hierarchical pore size distribution of as-prepared NiCo2 O4 presents mesopores of ca. 3.5-5.0 and 7.5-8.0 nm. Fig. 4 shows LSV curves of the ORR process for OM NiCo2 O4 /KB and KB alone electrodes in O2 -saturated electrolyte of 1 M LiCF3 SO3 in TEGDME and OER process in N2 -saturated electrolyte. For the ORR process (Fig. 4a), the onset potential is higher than the standard potential of the formation of Li2 O2 (2.96 VLi ), suggesting that

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the surface, like Litchi shells. The diameter of the OM NiCo2 O4 particles sizes is mainly about 100-200 nm. Fig. 2b shows a representative TEM image of NiCo2 O4 obtained from the replication of 3D ¯ KIT-6 silica. All of these replicas clearly present extended cubic Ia3d domains of the ordered 3D pore structure. In the high-resolution TEM images (Fig. 2b and 2c), the spacing distances between adjacent fringes are 0.279 and 0.233 nm, which are in conformity with the (220) and (222) lattice spacing distances of spinel NiCo2 O4 , respectively. Fig. 3 shows the N2 adsorption-desorption isotherm and the pore size distribution. The isotherm of the mesostructured

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the reduction current is partly contributed by some side reactions, which might be ascribed to the electrolyte decomposition induced by the catalyst and by the presence of O2 . Nevertheless, the OM NiCo2 O4 /KB electrode exhibits more positive ORR potential and higher current density than KB alone electrode, indicating that OM NiCo2 O4 exhibit higher ORR catalytic activity in 1 M LiCF3 SO3 in TEGDME electrolyte than KB alone. Since the OER refers to the decomposition of lithium oxides, the test of OER activity on OM NiCo2 O4 /KB and KB electrodes are conducted by depositing lithium oxides on the surface through discharge 2500 mAh by normal batteries before the LSV tests. The OER polarization curves in Fig. 4b also indicate a higher OER current density and less positive OER potential on OM NiCo2 O4 than that on KB. These results demonstrate that OM NiCo2 O4 electrocatalyst is also equipped with a good OER catalytic activity, which could improve the charge/discharge efficiency of Li–O2 batteries. Fig. 5a displays the first discharge-charge curves of Li-O2 batteries with different contents of OM NiCo2 O4 at a current density of 0.1 mA cm−2 . As can be seen in Fig. 5a, the specific capacity of Li-O2 battery increases with the addition of small amount of OM NiCo2 O4 catalyst, but further increase in OM NiCo2 O4 content leads to a decrease in specific capacity. In

addition to the higher electrocatalytic activity of OM NiCo2 O4 catalyst, much smaller specific surface area and larger density than KB should be responsible for that. The specific capacities are 3993, 4357, 4120 and 1881 mAh g−1 for the electrodes with 0% (KB alone), 20%, 45% and 70% OM NiCo2 O4 , respectively. Compared with previous reports using mesoporous NiCo2 O4 nanoflakes and 3D NiCo2 O4 nanowire array [9,34] as bifunctional electrocatalysts, the Li-O2 battery using OM NiCo2 O4 presented a significantly enhanced discharging capacity. Besides, the discharge voltage plateau of Li-O2 battery with OM NiCo2 O4 catalyst is about 2.75-2.80 V, which is about 100 mV higher than that with KB alone. In the meanwhile, the charge voltage of Li-O2 battery with OM NiCo2 O4 /KB is found to be much lower than that with KB. The charge plateau decreased from ca. 4.0 V to 3.75 V with a decrease in OM NiCo2 O4 content from 20% to 70%, while no charge plateau is observed with KB electrode. The results are in very good agreement with LSV conclusions. Furthermore, the round-trip capacity efficiency of Li-O2 battery with KB alone is ca. 23.2%, but the efficiencies with OM NiCo2 O4 catalysts are in the range of 78.6-103.0%. The efficiency more than 100% with 45% OM NiCo2 O4 catalyst might be partly due to the electrolyte decomposition during the charging

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Fig. 6. The first discharge-charge curves (a) and capacity retention rate (b) of the Li–O2 batteries with OM NiCo2 O4 /KB and KB at different current densities as well as cycling performance of the batteries with OM NiCo2 O4 /KB (c) and KB (d) with a capacity restriction of 1000 mAh g−1 at 0.1 mA cm−2 .

process. Fig. 5b demonstrates the cycling performance of Li-O2 batteries with OM NiCo2 O4 /KB and KB electrodes between 2.0-4.3 V at a current density of 0.1 mA cm−2 . After 5 cycles, the capacity retention rates of Li-O2 batteries with 20%, 45% and 70% OM NiCo2 O4 content electrodes are 41.9%, 65.3% and 49.8%, respectively, which are much more beyond a retention rate of 11.7% with KB alone electrode. The capacity of the batteries with KB electrode decayed rapidly due to the accumulation of lithium oxides with poor electronic conductivity formed in the discharge process, and scarcely decomposed in the subsequent charge process. Comprehensively considering both specific capacity and cycling performance, the battery with 45% OM NiCo2 O4 content is chosen for the subsequent testing. If without specifying, the OM NiCo2 O4 /KB represents for 45% OM NiCo2 O4 content in the following describe. Fig. 6a shows a performance comparison of the batteries with OM NiCo2 O4 /KB and KB electrodes at various current densities. The first discharge of Li-O2 battery with OM NiCo2 O4 /KB electrode delivers approximately 30% higher capacity than that of KB at the current density of 0.2 and 0.5 mA cm−2 . The discharge capacities decrease with an increase in current density. However, the battery with OM NiCo2 O4 catalyst presents a much better rate capability than that with KB alone, as shown in Fig. 6a and 6b. Further comparison with previous reports on NiCo2 O4 catalysts [9,10] indicates that the battery with OM NiCo2 O4 catalyst shows an improved rate capability. Fig. 6c and 6d show the cycling performance of the batteries by restricting the specific capacity of 1000 mAh g−1 . Although a decrease in the cut-off voltage is observed during the cycling, the battery with OM NiCo2 O4 /KB

electrode exhibits a better capacity retention at the current density of 0.1 mA cm−2 , and after 20 cycles, the cut-off voltage is as high as 2.4 V. After 20 cycles, the voltage drops rapidly, the cut-off voltage drops below 2.0 V after 25 cycles. On the contrary, there is a dramatic decrease in the cut-off voltage of the battery with KB electrode, and after 12 cycles, the cut-off voltage drops below 2.0 V. Fig. 7a shows the XRD patterns of the cathodes obtained before discharge, after discharge and after recharge. From the XRD patterns, Li2 O2 and Li2 CO3 phases formed after discharge, while after recharge, both of them disappeared. According to previous report [38], Li2 CO3 can not be decomposed during the charge process, therefore it is suggested that the presence of weak Li2 CO3 phase might be the reaction between lithium oxides and CO2 in the air. The detailed mechanism is not clear. Fig. 7b depicts the initial morphology of OM NiCo2 O4 /KB electrode. Microspheres are observed on the surface of the electrode due to the aggregation of the catalysts. After discharge, the microspheres are covered by massive blocks as shown in Fig. 7c, implying that the massive blocks are the discharge products, therefore blocking the pathways for oxygen diffusion and electron transfer and leading to the discharge termination. The morphological change of OM NiCo2 O4 /KB after subsequent recharge is shown in Fig. 7d. The massive blocks return back to the microsphere-like structure, but the size of microspheres increases slightly. This manifests that the OM NiCo2 O4 can facilitate the decomposition of discharge products but can not decompose completely, thus leading to an incompletely recovered morphology. This might be due to two aspects: Li2 O2 can not be fully decomposed by OM NiCo2 O4 during charging and/or the

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Fig. 7. XRD patterns of the 45% OM NiCo2 O4 /KB electrode in Li-O2 batteries before discharge (initial), after discharge and after recharge (a), and SEM images of the OM NiCo2 O4 /KB electrodes at different discharge-charge stages: before discharge (b), 1st discharge (c) and 1st recharge (d).

accumulation of irreversible electrolyte decomposition product formed on the surface.

4. Conclusions Ordered 3D mesoporous NiCo2 O4 materials have been synthesized via a hard template as bifunctional electrocatalyst for Li-O2 batteries. The Li-O2 battery using OM NiCo2 O4 as eletrocatalyst exhibits a significantly enhanced rate capability, charge/discharge efficiency and cyclability. Such an improved performance can be ascribed to higher ORR and OER electrocatalytic activity, verified by LSV results. The Li–O2 batteries employing OM NiCo2 O4 as cathode catalyst achieve a high specific capacity of 4120 mAh g−1 , and a better cyclability with a capacity retention rate of 65.4% after 5 cycles. When restricting the capacity at 1000 mAh g−1 , it also shows an obvious improvement in the cyclability, the cut-off voltage is as high as 2.4 V after 20 cycles. That is, the OM NiCo2 O4 could be a promising catalyst for the Li-O2 battery.

Acknowledgements This work was supported by the National Basic Research Program of China (973 Program, 2012CB932800), the National High-Tech. 863 Program of China (2013AA050902), the Natural Science Foundation of China (21103220, 21176119) and Shanghai Science and Technology Committee (11DZ1200400). X. W. Li would like to thank financial support from the Nature Science Foundation of Jiangsu Province (BK2011272).

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