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Contents lists available atScienceDirect

Chemical Engineering Journal

journal homepage:www.elsevier.com/locate/cej

Ultrathin Co

3

O

4

nanosheet clusters anchored on nitrogen doped carbon nanotubes/3D graphene as binder-free cathodes for Al-air battery

Yisi Liu

^b,c

, Lishan Yang

^a

, Bo Xie

^b

, Nian Zhao

^b

, Lun Yang

^b

, Faqi Zhan

^d

, Qiyun Pan

^b

, Juanjuan Han

^b

, Xiuzhang Wang

^b

, Junming Liu

^e

, Jie Li

^c

, Yahui Yang

^a,^⁎

aNational & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China

bInstitute of Advanced Materials, Hubei Normal University, Huangshi 415000, China

cSchool of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

dState Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China

eNational Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

H I G H L I G H T S

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^{A novel Co}³^O⁴/NCNTs/3D graphene was prepared by in-situ CVD and thermal processes.

•

The ORR performance of Co3O4/ NCNTs/3D graphene outperforms that of Pt/C catalyst.

•

^Co³^O⁴/NCNTs/3D graphene can be used as binder-free electrode in Al-air battery.

•

Our work offers insights to develop NCNTs/3D graphene based binder- free cathodes.

G R A P H I C A L A B S T R A C T

A R T I C L E I N F O Keywords:

Chemical vapor deposition Oxygen reduction reaction NCNTs/3D graphene Binder-free

Aluminum-air coin cells

A B S T R A C T

Great efforts have been focused on exploring high-efficiency and stable catalysts towards oxygen reduction reaction (ORR) and reducing the use of polymer binders in cathodes for metal-air batteries. In consideration of synergistic effects, unique porous structure and improved properties, a composite of carbon nanotubes and three- dimensional graphene (CNTs-3D graphene) developed via a well-controlled route is urgently required. Herein, an ultrathin Co3O4nanosheet clusters anchored on nitrogen-doped carbon nanotubes/3D graphene (NCNTs/3D graphene) composite was prepared by chemical vapor deposition (CVD) along with thermal treatments over a Ni foam substrate. The cyclic voltammetry (CV) and linear sweep voltammograms (LSVs) results indicate that the Co3O4/NCNTs/3D graphene composite possesses remarkable electrocatalytic performance towards ORR in alkaline condition compared with NCNTs/3D graphene, Co3O4/3D graphene, and 3D graphene catalysts, even outperforming the commercial 20 wt% Pt/C catalyst. Moreover, the Al-air coin cell employing Co3O4/NCNTs/3D graphene as binder-free cathode obtains an open circuit voltage of 1.52 V, a specific capacity of 482.80 mAh g⁻¹ at the discharge current density of 1.0 mA cm⁻², and a maximum power density (Pmax) of 4.88 mW cm⁻², which

https://doi.org/10.1016/j.cej.2019.122681

Received 10 May 2019; Received in revised form 5 August 2019; Accepted 31 August 2019

⁎Corresponding author.

E-mail addresses:yliu88@hbnu.edu.cn(Y. Liu),yangyahui2002@sina.com(Y. Yang).

Available online 03 September 2019

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are superior to the Al-air coin cell using Co3O4/3D graphene, Pt/C, and traditional Co3O4/NCNTs/3D graphene electrodes. This work supplies valuable insights to develop scalable binder-free cathodes using NCNTs/3D graphene based electrocatalysts with excellent catalytic activity towards ORR in Al-air systems.

1. Introduction

Growing demand for energy leads to the rapid consumption of fossil fuel and intensifies the outbreak of environmental problems. The pressure from energy and environment greatly drives people to seek efficient and renewable energy storage and transformation technologies [1,2]. Recently, metal-air batteries have been comprehensively studied, because of their two to ten times higher theoretical energy densities than state-of-the-art Li-ion batteries (LIBs) [3]. Particularly, Al-air batteries stand out from various metal-air batteries and are recognized as “green new energy utilization technologies for the 21th century”, due to their characteristics of higher theoretical voltage and energy density, lower price, use security, and excellent stability [4–7]. In addition, aluminum is a cheap, abundant, recyclable and environmental friendly metal[8]. Oxygen reduction reaction is the most important cathode process of Al-air system. However, its sluggish kinetics is the key factor affecting the performance of air electrode so as to restrict the efficiency and performance of Al-air battery [9,10]. On the other hand, in the traditional design of air electrodes, polymer binders, such as poly- tetrafluoroethylene and polyvinylidene fluoride will bring a series of problems and affect the electrode activity, such as the increase of non- active electrode material, the blockage of pore structure, the decrease of electrical conductivity and the occurrence of side reactions. To mitigate these above mentioned problems, the concept of “binder-free” electrode has been put forward. Integrating catalytic active components with conductive substrates by means of covalent bond or chemical adsorption so as to achieve the dual purpose of transferring electrons and activating the substrates. Therefore, one of the key opportunities to improving the performance of Al-air battery is to design and prepare binder-free air electrodes with high-efficient ORR electrocatalysts. Re- cently, some research teams have designed and prepared binder-free air electrodes, and assembled them into metal-air batteries[10–12]. Noble- metal-based materials are regarded as the most outstanding ORR catalysts[11,12], but the valuableness and terrestrial scarcity seriously obstruct their large-scale applications. Hence, it is highly desirable to explore high-efficiency non-precious metal catalysts to solve the issues.

Nowadays, three-dimensional (3D) carbon nanomaterials for electrocatalysis have gained more attention because of their high specific surface area, good mechanical integrity and facilitated ion diffusion/

charge transfer paths[13–16,56–58]. Thereinto, 3D graphene possesses great advantages, such as high electronic conductivity, excellent chemical stability, etc, which is widely utilized in electrocatalysis [15,17,18,59,60]. For example, Wang et al.[17]proved that few-layer 3D graphene could be regarded as an high-efficiency electrocatalyst towards ORR. Xue et al.[19]reported that 3D B, N co-doped graphene foams prepared by a modified chemical vapor deposition (CVD) method could be used as highly efficient ORR catalysts. In addition, CNTs are also regarded as excellent catalyst supports[20–23]in metal-air systems due to their high tensile modulus, high specific surface area, mesoporous structure, and good electrical properties. Zhao et al.[23]

prepared spinel MnCo2O4nanoparticles partly imbedded in N-doped CNTs and discovered their high ORR and OER catalytic performance, even surpassing the activity of Pt/C, RuO2, and IrO2bifunctional catalysts in metal-air batteries. The enhanced catalytic activity is con- tributed to the synergetic effect from the nitrogen groups in the NCNTs and the spinel Mn-Co oxide particles.

It is worth mentioning that incorporating 1D CNTs into 3D graphene to prepare CNTs-graphene hybrids with interconnected architecture is one promising direction for electrocatalytic research, considering that the 3D structure and synergistic effects can prevent aggregation and

enhance the stability of electrocatalysts. This design allows for larger accessible porous structures and controllable modifications such as the introduction of appropriate dopants (N, P, S, et al) to further improve electrocatalytic activity[24,25]. In numerous candidate dopants, N is considered to be an outstanding element for the chemical doping of carbon materials, because its atomic size is similar to C, and the lone pair electrons can form delocalized conjugated system with carbon skeleton of sp2 hybridization [26], which makes the spin density, charge distribution and electronic state density change, producing more active sites[27]. Ma et al.[24]synthesized a three-dimensional composite catalyst (NCNTs/G) of NCNTs grown on graphene by the pyrolysis of pyridine over a graphene sheet supported Ni catalyst, which showed high electrocatalytic activity and selectivity towards ORR in alkaline condition, indicating its potential application in fuel cells.

Ratso and co-authors[28]reported a nitrogen-doped nanocarbon catalyst (FLG/MWCNT) showed improved catalytic performance towards ORR in alkaline condition comparable with the commercial Pt/C catalyst induced by a higher content of nitrogen doping. The 3D NCNTs- graphene hybrid has been extensively studied as an alternative support towards ORR. Furthermore, CNTs-graphene functionalized with inorganic nanocrystals (transition metal,[29]metal oxides,[30,31]sul- fides,[32,33]nitrides,[2]and carbides[34]) show synergistic effects and significant improvement in electrocatalysis. These inorganic nanocrystals are maily focused on nanoparticles, two-dimensional (2D) nanomaterials modified CNTs-graphene hybrids are still rare. 2D nanomaterials with large specific surface area can combine with 3D NCNTs-graphene to promote the process of electron transfer and mass transport.

In this work, a composite of ultrathin Co3O4 nanosheet clusters anchored on NCNTs/3D graphene was synthesized by optimized CVD and thermal-treatment processes. The Co3O4/NCNTs/3D graphene hybrid catalyst has a robust and well-developed hierarchical porous structure. The combination of the unique architecture with abundant Co-N species and C-N active sites enables the catalyst to exhibit excellent catalytic performance towards ORR in alkaline condition, even outperforming the commercial Pt/C catalyst. An Al-air coin cell was assembled using Co3O4/NCNTs/3D graphene as the binder-free air electrode and exhibited high open-circuit voltage of 1.52 V and specific capacity of 482.80 mAh g⁻¹ at the discharge current density of 1.0 mA cm⁻², demonstrating Co₃O₄/NCNTs/3D graphene electrodes have great potential to be applied in metal-air batteries.

2. Experimental

2.1. Preparation of 3D graphene

3D graphene was synthesized by a single-step chemical vapor deposition (CVD) method. A piece of Ni foam was placed on a clean alumina boat in the center of a quartz tube. Vacuum grease was used to make sure that quartz tube was completely sealed against ambient air.

The quartz tube was purged by Ar and H2gases with 100:10 sccm for 15 min. Then, the furnace temperature was set to 1000 °C for at least 15 min to ensure all potential surface contaminants and oxide layer are reduced and removed from the substrate surface. Next, CH4gas was applied at 15–20 sccm for 90 min, while keeping Ar and H₂running.

Afterwards, the furnace was closed and gradually cooled down to room temperature with Ar and H2running.

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2.2. Preparation of NCNTs/3D graphene

Nitrogen doped carbon nanotubes (NCNTs) were synthesized by floating catalyst chemical vapor deposition (FCCVD). The Al2O3layer was coated on 3D graphene by atomic layer deposition (ALD). To assist with the growth of vertical NCNTs, an Al2O3layer was initially de- posited on the surface of as-prepared graphene/Ni foam using atomic layer deposition (ALD). Before the tube furnace started running, high- purity Ar gas was imported to the tube for 0.5 h to purge out the air of the system. 2.0 g imidazole and 0.2 g ferrocene were separately dis- solved in 10 mL of acetonitrile to be used as the carbon source and catalyst precursor, respectively. When the temperature reached 856 °C, the ferrocence and imidazole solutions were in turn sprayed onto graphene. The synthesis time was 5 min. The Ar gas flow was fixed to 60 sccm during the whole process.

2.3. Preparation of Co3O4/NCNTs/3D graphene composite

For details, 0.29 g Co(NO3)2·6H2O and 0.24 g urea were ultra- sonically dispersed in a mixture of 20 mL of ethanol and 20 mL of deionized water, and transferred into a 50 mL Teflon-lined autoclave, then, a piece of NCNTs/3D graphene was immersed into the solution and hydrothermally treated at 95 °C for 10 h. The product was washed by centrifuging with deionized water and ethanol for several times, and dried at 70 °C overnight. Finally, the as-prepared product was put in a tubular furnace and calcined in Air under 300 °C for 2 h at a heating rate of 5 °C min⁻¹. Co₃O₄/3D graphene was synthesized using the same process without growing NCNTs. In order to make a complete contrast, the mechanical mixture of CMO, NCNTs and 3D graphene was prepared by ball-milling.

2.4. Materials characterization

Detailed measurement requirements of X-ray diffraction spectroscopy, field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM), Raman spectra, X- ray photoelectron spectroscopy, and nitrogen adsorption-desorption isotherm measurements are described in theSupporting Information.

2.5. Electrochemical measurements

The electrochemical characterizations were operated in a standard three-electrode system at room temperature using a rotating disk electrode as work electrode setup with a bipotentiostat and rotation

control (Pine Instruments). Preparation of the catalyst ink and test methods was reported as our previous published paper [35]

(Supporting Information).

2.6. Fabrication of aluminum-air coin cells

The detail procedures of fabricating an Al-air coin cell were described as follows [36]: the negative electrode was 1.0 cm² square metal aluminum foil (purity achieves 99.9%); the binder-free air electrode (positive electrode) was fabricated by cutting Co3O4/NCNTs/3D graphene into pieces with the diameter of 10 mm; the separator was a glass fiber paper with 16 mm in diameter; the electrolyte was 2 mol L⁻¹ KOH solution, respectively. Before assembling, the separator was needed to be soaked in 2 mol L⁻¹KOH solution for 10 min, and 1.0 mL of electrolyte was dropwise added into the negative electrode. The traditional air electrode was fabricated by coating Co₃O₄/NCNTs/3D graphene (after etching in 3 M HCl) ink on Ni foam. The catalyst ink is composed of 90 wt% catalyst powders and 10 wt% binder (polytetra- fluoroethylene) in isopropyl alcohol. Finally, the negative and positive electrodes, and the separator were enclosed into a commercial stainless steel coin cell shell and gasket. The cell discharge tests were carried out by a multichannel battery testing system (LANHE CT2001A). The cells were discharged galvanostatically at room temperature under ambient condition.

3. Results and discussions 3.1. Physical characterizations

The synthesis strategy of Co3O4/NCNTs/3D graphene was schema- tized in Scheme 1. The Co3O4/NCNTs/3D graphene composite was prepared by a three-step synthetic approach, including chemical vapor deposition and thermal treatment. Co3O4/NCNTs/3D graphene is a composite product of 3D, 2D, and 1D materials. The X-ray diffraction (XRD) patterns of 3D graphene, NCNTs/3D graphene, Co3O4/3D graphene and Co3O4/NCNTs/3D graphene are shown in Fig. 1. Three strong diffraction peaks belong to Ni foam, the broad peak around 22°

belongs to 3D graphene, and the diffraction peak at around 26° belongs to NCNTs, respectively. The characteristic diffraction peaks of Co3O4/ 3D graphene and Co3O4/NCNTs/3D graphene at 2θ= 19.00°, 31.34°, 36.38°, 59.35°, 64.94° can be indexed to (1 1 1), (2 2 0), (3 1 1), (5 1 1) and (4 4 0) planes of Co₃O₄structure (JCPDS card No. 43-1003) with a space group Fd-3 m(2 2 7).

As shown in the SEM images of 3D graphene/Ni foam (Fig. S1),

Scheme 1.Schematic illustration for the synthesis of Co3O4/NCNTs/3D graphene.

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continuously wrinkled graphene network was decorated on Ni foam with a macroporous structure. The SEM images of NCNTs/3D graphene (Fig. S2) show that well-defined NCNTs arrays of approximately 50 μm in length and 50 nm in diameter were grown on 3D graphene surface with high density in a stable linkage, constructing a porous 3D interconnected carbon skeleton, facilitating fast reactant diffusion and rapid

electron transfer. As shown inFig. 2a, b, ultrathin Co3O4nanosheet clusters with a thickness of about 20 nm were uniformly anchored on the NCNTs surface with high density. The ultrathin structure can facilitate electrons to reach the surface of NCNTs rapidly and take full advantage of catalytic materials. The energy dispersive X-ray (EDX) mapping images of a selective region (Fig. 2c) reveal the existence and uniform distribution of Co, C, Ni, O and N elements, further demonstrating Co3O4nanosheet clusters wrapped around the NCNTs surface.

The EDX mapping images of a wider range region (Fig. S4) show uniform distribution of Co₃O₄on the surface of nitrogen doped carbon materials.

As shown in TEM images of Co3O4/NCNTs/3D graphene (Fig. 3a and b), bamboo-like NCNTs with about 30 nm in diameter are vertically grown on 3D graphene, and ultrathin Co3O4 nanosheet clusters are wrapped around NCNTs surface. Fig. 3c exhibits the selected area electron diffraction (SAED) patterns of Co₃O₄/NCNTs/3D graphene, demonstrating polycrystalline structure of Co3O4[37]. HRTEM images (Fig. 3d and inset) display good crystalline nature of Co3O4nanosheet clusters. The distinct lattice fringe spacings (d-spacing) are observed to be 0.247 nm, 0.205 nm, and 0.175 nm, which correspond to the (4 0 0), (3 1 1), and (2 2 0) planes of Co3O4, respectively.

The structural composition and defectiveness were further eval- uated by Raman spectroscopy (Fig. 4a). Two representative peaks at 1590 and 2750 cm⁻¹are detected for all samples, corresponding to the G band and 2D band of carbon materials, respectively. The G band corresponds to the doubly degenerateE_2gphonons at the Brillouin zone center[51]. The D band is attributed to the breathing mode ofsp²-rings and requires a defect for its activation by an intervalley double- Fig. 1.X-ray diffraction (XRD) spectra of NCNTs/3D graphene, Co3O4/3D

graphene and Co3O4/NCNTs/3D graphene hybrids, respectively.

Fig. 2.(a, b) Field-emission scanning electron microscopy images of Co3O4/NCNTs/3D graphene at different magnifications; (c) Selected area for elemental mapping;

Elemental mapping images of (d) C, (e) Co, (f) O, (g) Ni, and (h) N.

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resonance (DR) Raman process[38]. The 2D band is due to the same second-order, intervalley DR process, but no defects are required for its activation [38]. No obvious D band was observed for 3D graphene, indicating a well-ordered graphitic structure and low defect degree. The emerging weak D bands for NCNTs/3D graphene and Co3O4/3D graphene demonstrate increased significant defects induced by NCNTs and Co3O4 nanosheets, resulting in more active sites for electrocatalytic activity. Generally, the intensity ratio of the D band to G band,ID/IG, can be utilized for judging the degree of disorder/order of carbon nanomaterials[52]. The higherID/IGfor Co3O4/NCNTs/3D graphene in- dicates its higher defect level, which introduces more active sites resulting in excellent electrocatalytic activity. Additionally, the peaks observed at around 650 cm⁻¹can be attributed to the Co-O functional groups[53].

Fig. 4b exhibits that the XPS spectrum of Co3O4/NCNTs/3D graphene possesses obvious N peak compared with that of Co3O4/3D graphene, demonstrating the doping of nitrogen. In the high-resolution C1s spectrum of Co3O4/NCNTs/3D graphene (Fig. 4c), three obvious peaks belonging to CeC, CeN, and OeC]O groups are observed, suggesting the incorporation of N atoms[39]. The high-resolution Co2p spectrum (Fig. 4d) is composed of two pairs of spin-orbit peaks, demonstrating the coexistence of Co(Ⅱ) and Co(III)[40]. The redox of Co species can be regarded as an assistor in charge transferring[41]. The O1s spectrum (Fig. 4e) reveals two oxygen states: oxygen in Co3O4and the residual oxygen-containing groups of NCNTs/3D graphene [42].

The high-resolution N1s spectrum (Fig. 4f) can be divided into four peaks centered at 398.7 eV, 399.6 eV, 401.2 eV, and 406.0 eV, which are attributed to pyridinic N, pyrrolic N, quaternary N, and pyridinic N oxide, respectively[43]. Pyridinic N refers to N atoms that bond with two carbon atoms at the edges or defects of graphene and contribute a p electron to theπsystem[44]. Pyrrolic N refers to N atoms that bond

into the five-membered ring and contribute twopelectrons to theπ system[54]. Quaternary N refers to N atoms that replace carbon atoms in the hexagonal ring[45]. In addition, pyridinic N oxide refers to N atoms that bond with two carbon atoms and one oxygen atom. It is well known that the enhanced catalytic activity towards ORR can be attributed to pyridinic N and pyrrolic N[45,55]. Comparing with high- resolution C1s, Co2p and N1s spectra of Co3O4/3D graphene (Figs. S5 and S6), there is no obvious change in C1s except the appearance of C-N species for Co3O4/NCNTs/3D graphene, while the binding energies of Co2p and N1s for Co3O4/NCNTs/3D graphene shift towards lower energy direction, probably as a result of the combination between Co and N[46], producing Co–Nx–C active sites. The Co–Nxfunctional groups in carbon matrix are considered as the active sites, which can facilitate the dissociation of O₂dissociation and the desorption of H₂O in the ORR process[47,48].

A high surface area (SBET) and hierarchically porous structure can provide more active sites and efficient mass transport, and bring out a better catalytic activity for ORR. The N2 adsorption–desorption isotherms for NCNTs/3D graphene, Co3O4/3D graphene and Co3O4/ NCNTs/3D graphene are shown inFig. 5. They both present type IV isotherms with H3 hysteresis, suggesting the coexistence of mesopores from slits between Co3O4 nanosheets and macropores from 3D graphene. The calculatedSBETand average pore diameter are 311.8 m²·g⁻¹ and 31.7 nm for NCNTs/3D graphene, and 305.4 m²·g⁻¹and 11.4 nm for Co3O4/3D graphene, and 361.5 m²·g⁻¹ and 11.1 nm for Co3O4/ NCNTs/3D graphene, respectively. The higher specific surface area of Co3O4/NCNTs/3D graphene provides more active sites leading to improved catalytic activity. These mesopores and macropores are helpful to ensure efficient exposure of active sites and mass transport in the ORR process[33].

Fig. 3.(a, b) TEM images of Co3O4/NCNTs/3D graphene at different magnifications; (c) SAED (selected area electron diffraction) pattern; (d and inset) HRTEM image of Co3O4/NCNTs/3D graphene.

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3.2. Electrocatalytic performance of the electrocatalysts for oxygen reduction reaction

Fig. 6a compares the cyclic voltammograms of 3D graphene, Co3O4/ 3D graphene, NCNTs/3D graphene, mechanical mixture, and Co₃O₄/ NCNTs/3D graphene catalysts in O2-saturated 0.1 M NaOH aqueous solution. The voltammetric features of all the catalysts reveal the ty- pical oxygen reduction peaks in the region between 0.54 V and 0.84 V (vs. RHE), indicating their ORR catalytic activity. Co3O4/NCNTs/3D graphene exhibits a more positive ORR peak potential of 0.77 V (vs.

RHE) compared to NCNTs/3D graphene (0.65 V vs. RHE), Co3O4/3D graphene (0.65 Vvs.RHE), mechanical mixture (0.66 Vvs.RHE), and 3D graphene (0.59 Vvs.RHE), suggesting Co3O4/NCNTs/3D graphene has better electrocatalytic activity. Additionally, linear sweep voltammograms at the rotation rate of 1600 rpm in O2-saturated 0.1 M KOH were measured to further study the ORR catalytic kinetics (Fig. 6b). It is observed that Co3O4/NCNTs/3D graphene has an onset potential (E_onset) of 0.89 V (vs.RHE), which is more positive than that of NCNTs/

3D graphene (0.78 Vvs.RHE), Co3O4/3D graphene (0.84 Vvs.RHE), 3D graphene (0.64 Vvs.RHE), mechanical mixture (0.69 Vvs.RHE), and 20 wt% Pt/C (0.87 Vvs.RHE), demonstrating the higher catalytic activity of Co3O4/NCNTs/3D graphene. The half-wave potential (E1/2) of Co3O4/NCNTs/3D graphene (0.80 Vvs.RHE) is more positive than the

comparative catalysts, indicating its faster ORR kinetics resulting in better catalytic performance. Moreover, the current plateau for Co₃O₄/ NCNTs/3D graphene represents a diffusion-controlled process corresponding to the efficient four-electron-dominated ORR pathway[49].

As observed fromFig. 6c, Co₃O₄/NCNTs/3D graphene exhibits a Tafel slope of 66.18 mV dec⁻¹ far less than Co3O4/3D graphene (75.98 mV dec⁻¹), NCNTs/3D graphene (102.10 mV dec⁻¹), and 3D graphene (128.92 mV dec⁻¹), mechanical mixture (96.90 mV dec⁻¹), and even surpassing the 20 wt% Pt/C catalyst (74.79 mV dec⁻¹), suggesting that Co3O4/NCNTs/3D graphene possesses faster reaction kinetics.Fig. 6d displays obvious higher mass activity at −0.199 V (vs.

Ag/AgCl) for Co3O4/NCNTs/3D graphene (0.148 A mg⁻¹) compared with other catalysts, suggesting the best electrocatalytic activity. These results emphasized improved ORR catalytic activity of Co3O4/NCNTs/

3D graphene, which could be ascribed to the synergetic effect from four aspects: (1) Charge redistribution can be caused by introducing N into the graphitic skeleton to promote the adsorption of oxygen; (2) A small number of Co can form Co-N_xactive centers and selectively accelerate the formation of N-C active sites; (3) The electronic interactions between 3D graphene and NCNTs, and between NCNTs and anchored Co₃O₄nanosheets effectively improve the ORR catalytic performance;

(4) The unique porous structure provides more accessible catalytic active sites and electron transfer pathways. The electrocatalytic properties Fig. 4.(a) Raman spectra of the 3D graphene, NCNTs/3D graphene, Co3O4/3D graphene, and Co3O4/CNTs/3D graphene. The laser excitation wavelength is 532 nm.

(b) XPS spectra of Co3O4/CNTs/3D graphene. High-resolution XPS spectra of Co3O4/CNTs/3D graphene: (c) C 1s, (d) Co 2p, (e) O 1s, and (f) N 1s, respectively.

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including onset potentials, half-wave potentials, and Tafel slopes are summarized inTable S1. Furthermore, the outstanding ORR catalytic activity of Co3O4/NCNTs/3D graphene is comparable with or surpasses many recently reported 3D graphene or NCNTs based electrocatalysts in alkaline condition (seeTable S1).

ORR can proceed by two pathways in alkaline medium: direct four- electron reduction pathway and successive two-electron reduction pathway [50]. The electron transfer numbers (n) are calculated by Koutecky-Levich (K-L) equations (Supporting Information). The LSV curves at a serious of rotation rates from 400 rpm to 1600 rpm and corresponding K-L plots at different potentials of Co3O4/NCNTs/3D graphene were exhibited inFig. 6e, f. The calculatednof 3.97 is close to the Pt/C catalyst (4.0,Fig. S7), indicating the high selectivity for 4e reduction path. In addition, the n of Co3O4/NCNTs/3D graphene is higher than that of Co3O4/3D graphene (3.65, Fig. S8), further demonstrating the significant effect of NCNTs on ORR catalytic activity.

The ORR long-term stability of Co3O4/NCNTs/3D graphene was com- paratively investigated by chronoamperometric method (Fig. 7). After 12000 s of continuous operation at constant voltage of −0.3 Vvs.Ag/

AgCl, Co3O4/NCNTs/graphene showed an efficient current density retention rate of 97.8%, far higher than that of the Pt/C catalyst (75.6%

retention), indicating improved ORR stability under alkaline condition.

3.3. The performance of Al-air coin cells using electrocatalysts as binder- free cathodes

To this end, in order to explore the feasibility of using Co3O4/ NCNTs/3D graphene as binder-free air electrodes in Al-air batteries, we fabricated a series of Al-air coin cells with Co3O4/NCNTs/3D graphene and Co₃O₄/3D graphene binder-free air electrodes, and Pt/C and Co3O4/NCNTs/3D graphene traditional air electrodes. Al-air coin cells were assembled as shown inFig. 8a. The traditional Pt/C and Co3O4/ NCNTs/3D graphene air electrodes were fabricated by coating Pt/C catalyst and Co3O4/NCNTs/3D graphene (after etching in 3 M HCl) ink on Ni foam with the diameter of 10 mm, then drying at 60 °C for 2 h to achieve a loading of approximately 5.0 mg cm⁻².Fig. 8b exhibits the discharge behaviors of the coin cells at a constant current density of 1.0 mA cm⁻²under ambient condition. The coin cell assembled with Co3O4/NCNTs/3D graphene as the air electrode exhibits higher open circuit voltage of 1.52 V and potential plateau of 1.42 V than that of Co3O4/3D graphene (1.48 V and 1.23 V), Pt/C (1.51 V and 1.38 V), and traditional Co3O4/NCNTs/3D graphene (1.52 V and 1.40 V) electrodes.

When normalized to the mass of consumed Al, the coin cell with Co3O4/ NCNTs/3D graphene electrode displays longer discharge time and higher specific capacity of 482.80 mAh g⁻¹ than that of Co3O4/3D Fig. 5.N2adsorption-desorption isotherm and BJH absorption pore size distribution of the (a, b) NCNTs/3D graphene, (c, d) Co3O4/3D graphene and (e, f) Co3O4/ NCNTs/3D graphene, respectively.

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graphene electrode (316.59 mAh g⁻¹), Pt/C electrode (361.74 mAh g⁻¹), and traditional Co3O4/NCNTs/3D graphene (372.60 mAh g⁻¹) (Fig. 8c). The discharge results further confirm better electrocatalytic activity and stability of Co3O4/NCNTs/3D graphene binder-free electrode. As listed inTable S2, the Al-air coin cell with Co3O4/NCNTs/3D graphene binder-free electrode is comparable

with many previous reported Al-air batteries.

In order to further investigate the performance of the binder-free electrodes, the cell polarization and power density curves of the coin cells with Co3O4/NCNTs/3D graphene and Co3O4/3D graphene were recorded, as shown inFig. 9. Co3O4/NCNTs/3D graphene produced a higher maximum power density (Pmax) of 4.88 mW cm⁻²than that of Co3O4/3D graphene electrode (4.02 mW cm⁻²), which can be con- tributed to that NCNTs produces more defects and Co-Nxactive sites.

Based on the above discussion, there is no doubt that Co₃O₄/NCNTs/3D graphene can be regarded as binder-free electrodes for practical applications in Al-air batteries and NCNTs/3D graphene based electrodes may drive the tremendous advancement of next-generation flexible batteries.

4. Conclusions

In summary, Co3O4 nanosheet clusters anchored on NCNTs/3D graphene composite were fabricated by CVD and thermal strategies.

The Co3O4/NCNTs/3D graphene composite exhibits superior electrocatalytic performance towards ORR to its counterparts and the commercial 20 wt% Pt/C catalyst, which can be attributed to the 3D interconnected network, the large specific surface area with hierarchical porosity, the exposure of Co-Nxmoieties and C-N active sites, as well as the synergistic effect between Co3O4nanosheets and NCNTs/3D graphene. The discharge performance of coin cells with binder-free electrodes demonstrates that the positive role of Co-N_xactive sites derived from Co3O4 and N species. This work demonstrates the promising Fig. 6.(a) Cyclic voltammograms of 3D graphene, NCNTs/3D graphene, Co3O4/3D graphene, Co3O4/ NCNTs/3D graphene, mechanical mixture, and 20 wt% Pt/C electrocatalysts in O2-saturated 0.1 M KOH aqueous solution; (b) LSV curves of 3D graphene, NCNTs/3D graphene, Co3O4/3D graphene, Co3O4/NCNTs/3D graphene, mechanical mixture, and 20 wt% Pt/C electrocatalysts on a RDE electrode in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm with a scan rate of 5 mV s⁻¹; (c) Tafel slopes from LSV curves at 1600 rpm; (d) Mass activities at −0.199 V for all of the catalysts; (e) LSV curves of Co3O4/NCNTs/3D graphene in O2-saturated 0.1 M KOH solution at various rotation rates with a scan rate of 5 mV s⁻¹; and (f) Corresponding Koutecky-Levich plots at different potentials.

Fig. 7.Current-time chronoamperometric response of Pt/C, Co3O4/NCNTs/3D graphene electrocatalysts at −0.30 V in O2-saturated 0.1 M KOH solution.

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application of the Co3O4/NCNTs/3D graphene composite as binder-free cathode for Al-air batteries and would inspire more breakthroughs in the development of novel NCNTs/3D graphene based electrodes.

Acknowledgements

This study was supported by the National Nature Science Foundation of China (No. 21878340 and 21471054), the Hubei Key Laboratory Analysis & Reuse Technology (Hubei Normal University, No. PA180103), the Research Project of Hubei Provincial Department of Education (No. B2018147).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://

doi.org/10.1016/j.cej.2019.122681.

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