High-Loading Nickel Cobaltate Nanoparticles Anchored on Three

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High-Loading Nickel Cobaltate Nanoparticles Anchored on ThreeDimensional N‑Doped Graphene as an Efficient Bifunctional Catalyst for Lithium−Oxygen Batteries Hao Gong, Hairong Xue, Tao Wang, Hu Guo, Xiaoli Fan, Li Song, Wei Xia, and Jianping He* College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China S Supporting Information *

ABSTRACT: The lithium−oxygen batteries have been considered as the progressive energy storage equipment for their expected specific energy. To improve the electrochemical catalytic performance in the lithium−oxygen batteries, the NiCo2O4 nanoparticles (NCONPs) are firmly anchored onto the surface of the N-doped reduced graphene oxide (N-rGO) by the hydrothermal method followed by low-temperature calcination. Compared with the pure metallic oxide, the introduction of the rGO can create the high surface area, which give a good performance for ORR (oxygen reduction reaction), and improve the electrical conductivity between the NCONPs. The high-loading NCONPs also ensure the material to have great catalytic activity for OER (oxygen evolution reaction), and the rGO can be protected by the nanoparticles coating against the side reaction with the Li2O2. The as-synthesized NCO@N-rGO composites deliver a specific surface area (about 242.5 m2 g−1), exhibiting three-dimensional (3D) porous structure, which provides a large passageway for the diffusion of the oxygen and benefits the infiltration of electrolyte and the storage of the discharge products. Owing to these special architectures features and intrinsic materials, the NCO@N-rGO cathode delivers a high specific capacity (6716 mAh g−1), great rate performance, and excellent cycling stability with cutoff capacity of 1000 mAh g−1 (112 cycles) in the lithium−oxygen batteries. The improved electrochemical catalytic activity and the special 3D porous structure make the NCO@N-rGO composites be a promising candidate for Li−O2 batteries. KEYWORDS: NiCo2O4 nanoparticles, 3D N-doped rGO, efficient bifunctional catalyst, long life, lithium−oxygen batteries

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INTRODUCTION In the past few years, the rechargeable lithium−oxygen (Li− O2) batteries have been regarded as the most potential candidate available to meet the demands of the highperformance batteries for a long device operating time.1−3 The theoretical energy density is calculated to be as high as 3500 Wh kg−1, which is nearly 10 times higher than the commercial lithium-ion batteries.2−4 However, there are still plenty of challenges to overcome, including poor cycling stability, low charge/discharge electrical efficiency, and low rate capability.5−7 In a typical Li−O2 batteries, the lithium metal anode is oxidized in the pure O2, and the discharge product is formed in the aprotic electrolyte during discharging (ORR). In the charge process, the Li2O2 electrochemically decomposed to the oxygen (OER).2−4 The discharge products Li2O2 are insoluble in the nonaqueous electrolyte and generate on the surface of the catalyst, which can block the oxygen diffusion and its contact with catalyst. In addition, the formation of Li2O2 can also cause the volumetric change and can damage the cathode structure, resulting in the poor cyclability of the battery.2,8 At present, the porous carbon and carbon-based material is widely adopted as the cathode due to the great electrical © 2016 American Chemical Society

conductivity as well as large surface area and pore volume, which can provide the electrochemical active site for ORR and the storage place for discharge product.5,8,12−15 For instance, Yamauchi and his group11 prepared the functional nanoporous carbon materials with notable electrochemical performance for ORR. Even the Super P can deliver a capacity of ∼3399 mAh gc−1 at a low current density of 50 mA gc−1. What is more, the ordered mesoporous carbon can also deliver a capacity of ∼1812 mAh gc−1 at low current density of 50 mA g−1.16 Compared with the conventional carbon materials, researchers have paid attention to graphene and applied it as a potential cathode material for the Li−O2 batteries for its high specific surface area and electrochemical stability. Recently, graphene, particularly the N-doped graphene, has been explored for the cathode of the Li−O2 batteries, but the charging overpotential of the Li2O2 is still too high, leading to the electrolyte decomposition. To improve the OER performance of graphene, many methods have been proposed.8,17−21 Chen reported that Received: April 22, 2016 Accepted: June 29, 2016 Published: June 29, 2016 18060

DOI: 10.1021/acsami.6b04810 ACS Appl. Mater. Interfaces 2016, 8, 18060−18068

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ACS Applied Materials & Interfaces

Figure 1. Synthesis of schematic diagram of the 3D porous NCO@N-rGO composite prepared by hydrothermal methods.

delivered for 112 cycles, with a narrow potential polarization (∼0.66 V).

RuO2 loaded on graphene can greatly reduce the potential gaps and increase the cycling ability.4 Wang also reported that Ru/ graphene can reduce the potential to 0.335 V at the current rate of 200 mA g−1. Palladium nanoparticle functionalized graphene also shows enhanced discharge capacity of 7690 mA h g−1 and gives a narrow potential polarization.21 The RuO2, Ru, Au, or Pd anchored on the surface of graphene composites deliver the remarkable performance in the lithium oxygen batteries, but an inconvenient truth is the price of the noble metal. Consequently, different kinds of low-price electrocatalysts have been researched for the cathodes for the Li−O2 batteries, like MnCo2O4,22,23 CoO,24,25 MnO2,26,27 and so on. As reported, the NiCo2O4 is cubic spinel phase and the Ni element is embedded in the center of the octahedron, leaving many catalytic active sites for both the ORR and OER.28−31 To further enhance its electrocatalytic performance, the NiCo2O4 is assumed to anchor on the rGO, and the envisaged NiCo2O4/ graphene composites benefit from the following aspect:32−35 (1) The meatal oxides (except for RuO2) are blamed for their poor electrical conductivity, which can be overcome by coating on the graphene. (2) The insoluble discharge products cause the volume change and destroy the electrode structure. The NiCo2O4 is fixed on the surface of the graphene, which can efficiently prevent their aggregation. (3) The graphene has numerous sites for electrochemical reaction due to the large surface area and can also provide efficient diffusion passageway for O2, resulting in the promotion of rate performance and specific capacity. (4) The nitrogen can be doped into the graphene, which can further improve the ORR performance in the Li−O2 batteries. (5) The NiCo2O4 anchored on graphene can restrain the reunion of graphene during the hydrothermal methods, forming the 3D porous structure. Herein, we in situ fabricated the NCONPs dispersed on Ndoped graphene (denoted as NCO@N-rGO) and applied for the cathode material in the Li−O2 batteries. The 3D porous NCO@N-rGO composites were synthesized by a facile coprecipitation of Ni2+ and Co2+ with urea to adjust the pH value and following a calcination process. This is the first time that the NCO@N-rGO hybrids have been applied as the cathode in the Li−O2 batteries with high electrocatalytic activities and great charge−discharge cycles. The NCO@NrGO cathode delivers a high initial capacity of 6716 mAh g−1 and great rate performance. A capacity of 1000 mAh g−1 can be

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RESULTS AND DISCUSSION Figure 1 illuminates the fabrication process of the NCO@NrGO. First, the GO was dispersed in deionized water. The Ni2+ and Co2+ were added into the GO dispersion, and then the metal ion anchored on the oxygen functional groups due to the electrostatic interaction between positively charged metal ions and negatively charged functional groups.39,40 During the hydrothermal process, the urea hydrolyzes to adjust the pH value, and the Ni−Co-carbonate hydroxide precursors could nucleate on the surface of the GO. Moreover, the GO is reduced under the alkaline condition, and the nitrogen, from the urea, can be doped into the rGO. After the annealing in the air, the 3D porous NCO@N-rGO composites were successfully synthesized. The XRD patterns of the as-prepared NCO@N-rGO hybrid is given in Figure 2a. All diffraction peaks can be well fitted to cubic spinel phase of NiCo2O4 (PDF#73-1702). The size of nanoparticles of the NiCo2O4 can be calculated by using the Scherrer equation through the (311) planes: D = 0.89λ/(B cos θ). D represents the particle size, λ is the wavelength of the Xray (λ = 0.154 nm), B is the line broadening of the diffraction peaks, and the θ is the value of the corresponding Bragg angle. The average particle size of the NCO@N-rGO is about 7.3 nm. It is noted that small NCONPS can deliver high specific surface area; thus, the contact between the catalyst and the electrolyte can be further enhanced. Moreover, an additional small and weak broad peak is observed at 24° (2θ), which is indexed to the existence of the reduced graphene oxide and is in accord with the result obtained by the XRD of the rGO in Figure S1. However, the content of NiCo2O4 is about 72.4% (Figure S2) with well crystallization, and the surface of the rGO is fixed with plenty of NCONPs, leading to the weak diffraction peaks of the rGO. The FTIR spectrum of GO, rGO, and NCO@N-rGO is managed to confirm the reduction of the GO as is shown in Figure 2b, and the inset is the magnified graph below the 800 cm−1. In the spectrum of GO, the broad peaks around 3500 cm−1 is attributed to the stretching bonds of −OH, while other labile oxygen functional groups are found at 1731 cm−1 (C O), 1223 cm−1 (C−OH), and 1056 cm−1 (C−O).41−44 After 18061


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Figure 2. (a) Typical XRD pattern of the as-synthesized NCO@N-rGO. (b) FTIR curves of spectra of different samples. (c−f) XPS spectra of survey spectrum (c) Ni 2p, (d) Co 2p, (e) C 1s, and (f) N 1s for NCO@N-rGO composite.

NiCo2O4 shows spin−orbit doublets, characteristic of Co2+ and Co3+ and two shakeup satellites. The fitting peaks of the Co3+ are at 780.8 and 795.5 eV, while the peaks of Co2+ are at 779.8 and 795 eV.30,44 Analogously, there were two spin−orbit doublets and two shakeup satellites in the Ni 2p emission spectrum. Therein the peaks of Ni2+ are found at 853.7 and 871.3 eV, and the fitting peaks of Ni3+ are at 855.3 and 873.5 eV.30,44 Based on the XPS result, the spinel NiCo2O4 is successfully prepared, where the ratio of two atoms (Co:Ni) in the obtained NCO@N-rGO is confirmed to be 2:1. The C 1s spectrum of the NCO@N-rGO hybrids can be decomposed into four dominant peaks at 284.6 eV to CC, 285.7 eV to CN, 286.5 eV to C−O, and 288.6 eV to CO, respectively.48,52,53 The peaks of CN certify the existence of the nitrogen, but the amount of N is only 2.61 at. % from the urea. The N 1s spectrum is consist of few graphitic N (401.1 eV) and most pyridinic N (398.5 eV), which is reported to

the hydrothermal method, the most oxygen functional groups are decreased in the intensity values, indicating the reduction of the GO. Moreover, the decrement of the oxygen functional groups also imply that they can serve as anchoring sites for the effectual decoration of the NCONPs. Compared with the rGO, there are two characteristic bands in the spectrum of NCO@NrGO at around 542 and 659 cm−1,44,45 corresponding to metal−oxygen vibrations. In addition, the sharp peak at 1540 cm−1 can be observed and assigned to the CN stretching.43 Thus, the result conducted from the FTIR can confirm the reduction of the GO and the successful synthesis of the NCO@ N-rGO. The XPS measurement is performed to analyze elemental composition and the surface electronic states of the NCO@NrGO, and the result is shown in the Figure 2c−f. Figure S3 clearly indicates that the main components are Ni, Co, O, N, and C. As shown in the Figure 2d, the Co 2p spectra of 18062


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Figure 3. (a) SEM image of the NCO@N-rGO composite. (b) TEM image of the as-prepared NCO@N-rGO composite. (c, d) HRTEM images of the NCONPs on the N-rGO. (e) TEM image of the needlelike NiCo2O4 without the existence of the GO. (f) HRTEM images of the needlelike NiCo2O4 without the existence of the GO. (g) EDX mapping of the NCO@N-rGO composite.

deliver a high catalytic performance.46−49 Moreover, the intensity of the C−O and CO is quite low, indicating a lower content of the oxygenous groups and the graphene oxide is well reduced. The detailed structural and morphological properties of the NCO@N-rGO composites were examined by scanning electron microscopy (SEM), transmission electron microcopy (TEM), and high-resolution TEM (HRTEM). In Figure 3a, the typical SEM images show that the surface of the N-rGO is completely covered by the NiCo2O4. It is worth noting that the graphene-free nanoparticles are hardly noticed in the SEM images, indicating the good attachment between the NCONPs and the N-rGO. The strong adhesion can also effectively prevent the agglomeration of the NCONPs during the charge/ discharge cycles in the Li−O2 batteries. Moreover, the existence of the NiCo2O4 nanoparticles prevents the agglomeration of the rGO and then constitutes the 3D porous structure, which is beneficial for diffusion of the electrolyte and oxygen and the storage of the Li2O2. The 3D porous structure is confirmed by the Brunauer−Emmett−Teller (BET) method in Figure S4, and the specific surface area is as high as 242.5 m2 g−1. Form the inset graph in Figure S4, the size of mesopore is distributed

around 5 nm, which is in good accordance with the HRTEM result. These mesopores provide the 3D path for the oxygen diffusion and electrolyte infiltration. For further investigation on the detailed microstructure and morphology of the 3D porous NCO@N-rGO, the HRTEM is performed as shown in Figure 3c. From the HRTEM image (Figure 3c), plenty of NCONPs are homogeneously are anchored on the surface of the N-rGO, and the particle size is about 7−8 nm, in good coincidence with the XRD results. It is noted that the deep dark areas in Figure 3b are attributed to the folded graphene formed in the high heat temperature, the same as those reported in the literature.44 Moreover, the NCONPs possess a typical crystalline texture, with a lattice spacing of 0.467 nm in Figure 3d, consistent with the interplanar spacing of the (111) planes for spinel phase NiCo2O4. Moreover, the energy dispersive Xray (EDX) spectrometry mapping of the NCO@N-rGO further confirms the NCONPs homogeneously anchored on the surface of the N-rGO. In comparison, the NiCo2O4 were prepared by the same methods only without using the GO. The morphology of the as-synthesized graphene-free NiCo2O4 is reversely changed, as shown in Figure 3e. The NCONPs form together to be the nanorods, and the lattice fringes of the (222) 18063


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Figure 4. (a) Initial discharge−charge plots of the NCO@N-rGO and NCO/rGO cathodes at a current density of 200 mA g−1. (b) Electrochemical impedance spectra of the NCO@N-rGO and NCO/rGO cathodes before first discharge. (c, d) Discharge/charge curves of the NCO@N-rGO and NCO/rGO cathode from 100 to 1000 mA g−1, respectively. (e, f) Cycling performance of the NCO@N-rGO and NCO/rGO cathodes at the current density of 200 mA g−1. (g) Cycling stability and the variation of the terminal charge and discharge voltages of the NCO@N-rGO cathodes at 200 mA g−1. The galvanostatic discharge and charge curves (c−g) are obtained with a capacity of 1000 mAh g−1.

planes of the spinel NiCo2O4, corresponding to d-spacings of 0.233 nm, are shown in the HRTEM (Figure 3f). The distinction of the morphologies between the NiCo2 O 4 nanorods and the NiCo2O4@N-rGO is schematically illustrated in Figure 1. As reported,39,44 the GO sheets have many oxygencontaining groups, confirmed in the FTIR result, and the metal ion (Ni2+ and Co2+) can be favorably anchored on the groups via electrostatic interactions. During the hydrothermal reaction, the NiCo precursor nuclei bind on the GO surface, and the electrostatic interactions restrain the precursors to spontaneously grow into the NiCo2O4 nanorods. The influence that the GO has on the morphology of NiCo2O4 indicates that the binding force between the N-rGO and the NCONPs are strong

enough to resist the agglomeration during the cycling in the Li−O2 batteries. The NCO@N-rGO and NCO/rGO samples were assembled into 2032-type cells to test the battery performance for Li−O2 batteries, and the first discharge−charge curves of the two batteries are shown in Figure 4a. All the batteries were operated at 200 mA g−1 within 2.3−4.3 V in the dry O2. The initial discharge capacity of the NCO@N-rGO cathode is about 6716 mAh g−1, while the NCO/rGO cathode delivers a lower capacity of about 5422 mAh g−1. The result is further confirmed by the cyclic voltammetry (CV) at a scan rate of 0.1 mV s−1 in Figure S5. The average discharge potential of the NCO@NrGO cathode is slightly higher than the NCO/rGO cathode 18064


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ACS Applied Materials & Interfaces (∼0.1 V), which gives the NCO@N-rGO cathode a narrow charge/discharge potential gap of ∼0.95 V and a high Coulombic efficiency of ∼100%. In comparison, the NCO/ rGO cathode delivers a much higher charge/discharge potential gap of 1.47 V, creating a lower Coulombic efficiency close to 84%. To find out the reason for the narrow gap of the NCO@ N-rGO cathode, the electrochemical impedance spectra (EIS) for two samples were obtained under open circuit and are shown in Figure 4b. The corresponding equivalent circuit is given in the inset image of Figure 4b, where Rct represents the charge-transfer resistance, constant phase element (CPE) results from the charge transfer process at the electrolyte/ electrode interface, and the Rs means liquid phase ohmic resistance.50,51 The lower Rct value of NCO@N-rGO suggests that the electric charge transport has been improved by the successful introduction of the rGO and thus leads to the narrower charge/discharge potential gap of the NCO@N-rGO based battery. The rate performance was obtained with a restrictive capacity of 1000 mAh g−1, and the current density is increased from 100 to 1000 mA g−1. As is shown in Figure 4c,d the NCO@N-rGO cathode exhibits a better rate performance than the NCO/rGO mechanical mixtures. The discharge potential of NCO@N-rGO cathode is about 2.78 V at 100 mA g−1 and even maintains at 2.59 V at 1000 mA g−1 while the charge potential increases only from 3.55 to 3.96 V. However, the NCO/rGO cathode with poor electric charge transport delivers inferior rate performance for that the discharge potential has decreased to 2.45 V and the charge potential reached 4.09 V. The cycling performance of the NCO@N-rGO cathode was further examined at 200 mA g−1 with a limited capacity of 1000 mAh g−1, while the NCO/rGO mechanical mixtures were also performed under the same conditions. As shown in Figure 4e, the NCO@N-rGO cathode shows a discharge platform at 2.79 V and the charge platform is about 3.4 V, indicating a great ORR/OER performance. No severe deterioration can be observed along with the increased cycling numbers. Even increased to 72 cycle in Figure 4g, the discharge potential is still over 2.70 V and the charge potential is lower than 4.40 V, implying excellent cycling stability. The NCO@N-rGO cathode exerts a great cycling performance for almost 112 cycles, and finally the cells cannot be charged to 1000 mA g−1 anymore. For comparison, the NCO/rGO cathode delivers a poor cycling performance, where the charge potential is nearly 4.00 V, and the discharge potential is only 2.69 V in the first cycle. After charging and discharging for 20 cycles, the discharge voltage plateau shows a great dropping, and the charging voltage plateau arrives at the cutoff voltage 4.5 V. The improvement of the rate and cycling performance is mainly contributed to the following reasons. First of all, compared with the NiO and Co3O4, the NiCo2O4 is cubic spinel phase, and the Ni element is embedded in the center of the octahedron, which can slightly improve the electrical conductivity of the transition metal oxides. To further increase the electrical conductivity between the nanoparticles, the rGO is introduced as the conductive substrate. The mechanical mixing cannot guarantee the NCONPs to be well dispersed in contrast with the in situ hydrothermal methods, leading to distant contacts and poor electron transport between the rGO and NCONPs. As a result, when the discharge current is up to 1000 mA g−1, the NCO/rGO cathodes suffer from the poor electron transfer, and the rate performance is inferior to the NCO@N-rGO cathodes. As described in Figure 5, the

Figure 5. Schematic drawing of the NCO@N-rGO electrode.

nanosized NiCo2O4 delivers a diameter of 7−8 nm, and uniformly anchored on the rGO, which gives the catalyst a higher specific surface area. The small size of the NiCo2O4 nanoparticle benefits the sufficient contact between the catalytic active sites and oxygen, which can ensure the electrode materials to catalyze in the both ORR and OER process. What is more, when the NiCo2O4 nanoparticles are fixed on the surface of the graphene, it can not only have higher specific surface area for more catalytic active sites, but also the aggregation of the NiCo2O4 nanoparticles can be prevented for the existence of binding force with the N-rGO. For comparison, the mechanically mixed NCO/rGO cathode material is blamed for the aggregation of catalysts and poor binding force between rGO and NCO nanoparticles. Thus, the volume change during the charge−discharge process can destroy the structure of NCO/rGO cathode and result in the aggregation of the catalysts, which can lead to the decreased catalytical active sites, depressed electrolyte infiltration, and oxygen diffusion throughout the electrode. As illustrated in Figure 5, the 3D porous NCO@N-rGO makes the cathode materials sufficiently contact with electrolyte and oxygen, which can increase the participation of electrode material during the charge/discharge process of the Li−O2 batteries. More importantly, the 3D porous structure provides void volume for the discharge product Li2O2. The discharge product on the NCO@N-rGO cathode was analyzed by the XRD and directly observed by SEM. As shown in Figure 6a, the Li−O2 batteries were discharged to 2.3 V and charged to 4.3 V. Compared with the pristine electrode, two diffraction peaks at around 32.8° and 34.8° are apparently observed in the inset graph, which can be assigned to the (100) and (101) diffraction of Li2O2 (JCPDF card no. 73-1640). All the Li2O2 peaks were rarely observed after recharging back to 4.3 V, indicating that the Li2O2 is decomposed in the charging process. Further investigation on the NCO@N-rGO cathode is performed by SEM, as is shown in Figure 6b−d. The discharge products (Li2O2) with typically toroid-shaped structure are homogeneously deposited on the cathode.13−15 The size of the toroid-shaped Li2O2 particles is ∼300 nm, which is associated with the ORR performance of the cathodes. As reported,23 the catalyst in the nonaqueous media exhibits a similar trend of ORR performance as in aqueous media. Thus, the excellent ORR performance of the NCO@N-rGO is confirmed by the rotating disk electrode (RDE) in the 0.1 M KOH solution under saturated oxygen as shown in Figure S6. After recharging to 4.3 V, the deposited Li2O2 is reversibly decomposed. The 18065


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Figure 6. (a) XRD pattern of pristine, the discharged, and recharged NCO@N-rGO cathodes. SEM images of the (b) pristine, (c) discharged, and (d) charged NCO@N-rGO electrodes. Bruker D8 X-ray diffraction meter at 40 kV voltage and a 200 mA current with monochromatic Cu K radiation (λ = 0.154 056 nm). The PerkinElmer TGA 7 instrument was used to obtain the NiCo2O4 load content from ambient to 900 °C under flowing oxygen. The morphologies were characterized on a Hitachi H-800 transmission electron microscope (TEM). Surface morphologies of the calcined products were analyzed by field emission scanning electron microscope (SEM, FEI, Quanta 400), and the powder samples were pressed on conducting carbon tape before mounting on the microscope sample holder for analysis. The ESCALab220i-XL spectrometer was used to obtain X-ray photoelectron spectroscopy (XPS). Li−O2 Cell Measurements. The electrochemical performance of NCO@N-rGO in Li−O2 batteries was explored by assembling the R2032-type coin cells (MTI Corporation). The O2 electrodes (typically 0.2 mg) were prepared by mixing catalysts:super P:polytetrafluoroethylene (PTFE) binders (5:4:1). The samples were rolled for a long time and then get the slices. The obtained slices were cut into square pieces of 0.5 cm × 0.5 cm and then pasted on carbon paper current collector under a pressure of 5 MPa. The as-prepared electrodes were vacuum-dried at 60 °C for 24 h to evaporate any solvent. The Li−O2 cells were assembled inside the glovebox under an argon atmosphere. The batteries consist of a 10 mm Li disk as the anode, a glass fiber as separator, and 1 M LiTFSI in tetraethylene glycol dimethyl ether (TEGDME) as electrolyte. After being assembled, the cells were transferred into a glass container full of dry oxygen. All the charge/discharge curves are acquired through the CT2001A LAND battery test system (Wuhan, China), and the current density is calculated by the amount of catalysts. The electrochemical workstation (CHI660C) was used to perform the cyclic voltammetry. A Solartron 1260 frequency response analyzer coupled to a Solartron 1287 potentiostat is used to measure the electrochemical ac impedance spectroscopy (EIS) under open-circuit voltage in the frequency range of 10−2−106 Hz.

great catalytic performance for both ORR and OER contributes to the abundant active sites, provided by both NiCo2O4 and the N-rGO.

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CONCLUSION In summary, the 3D porous NCO@N-rGO composite is successfully prepared by the in situ hydrothermal method followed by low-temperature calcination. The NCONPs are uniformly steadily fixed on the surface of the rGO. Besides, the pyridinic N is doped into the reduced graphene oxide, which can further improve the catalytic performance of the NCO@NrGO composite. The electrochemical performance of the 3D porous NCO@N-rGO composite for nonaqueous Li−O2 batteries is explored and shows a high specific capacity (6716 mAh g−1 at 200 mA g−1) and great rate performance. The NCO@N-rGO cathode can charge and discharge for 112 cycles with the limited capacity of 1000 mAh g−1 at 200 mA g−1, indicating a great cycling stability.

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EXPERIMENTAL METHODS

Material Preparation. In a typical synthesis process, graphite oxide (GO) was prepared from natural graphite through a modified Hummers method.36−38 To prepare NCO@N-rGO composites, 50 mg GO was first dispersed in 80 mL of mixed solvent (H2O:ethylene glycol = 1:2) by ultrasonication for an hour. Then 1.5 mmol of Ni(NO3)2·6H2O and 3 mmol of Co(NO3)2·6H2O were dissolved into the above solution along with 54 mmol of urea. After stirring and ultrasonication for several minutes to form a homogeneous aqueous dispersion, the solution was transferred into 80 mL Teflon-lined autoclave and kept at 110 °C for 16 h. The precipitates were centrifugal washing for three times to remove any impurities, followed by freeze-drying overnight. The final products were obtained by annealing at 300 °C for 3 h in air with a heating rate of 2 °C min−1. For a comparison, pure NiCo2O4 and bare rGO were also prepared in the same way in the absence of graphene oxide or metal ion. Then, the NiCo2O4 and the rGO were mechanically mixed with the mass ratio of 3:1 (NCO: rGO) and named NCO/rGO. Structural and Morphological Characterization. The X-ray powder diffraction patterns of the samples were carried out by a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04810. Additional details, including the TEM image, XRD patterns, TG curves, XPS spectra, N2 adsorption− 18066


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desorption isotherm and pore-size distribution curve, cyclic voltammetry (CV) curves of the batteries, and the cyclic voltammetry (CV) and RDE polarization curves performed in the KOH solution (PDF)

AUTHOR INFORMATION

Corresponding Author

*(J.H.) Tel +86 25 52112906; Fax +86 25 52112626, e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support from the National Natural Science Foundation of China (51372115, 11575084) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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High-Loading Nickel Cobaltate Nanoparticles Anchored on Three

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