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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Design of Three Dimensional Interconnected Hierarchical MicroMesoporous Structure of Graphene as Support Material for Spinel NiCoO Electrocatalyst toward Oxygen Reduction Reaction 2

4

Tingwei Zhang, Zhongfang Li, Zhixu Zhang, Likai Wang, Peng Sun, and Suwen Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08692 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Design of Three Dimensional Interconnected Hierarchical Micro-mesoporous Structure of Graphene as Support Material for Spinel NiCo2O4 Electrocatalyst toward Oxygen Reduction Reaction

Tingwei Zhang, Zhongfang Li*, Zhixu Zhang, Likai Wang, Peng Sun, Suwen Wang

School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255049, P R China

*Corresponding author: Tel. & Fax: +86 533 2786290, E-mail: [email protected] 266# Xincun West Road, Zibo City, Shandong Province, P R China, 255049

1


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Abstract The development of cost-effective electrocatalysts with high performance for oxygen reduction reaction (ORR) is a main problem for fuel cells and metal-air batteries. Herein, a novel interconnected hierarchical three dimensional graphene (3D-HG) is firstly fabricated by the templated method, where coal tar pitch and CaCO3 were used as carbon precursor and template, respectively. The pyrolysis of CaCO3 could produce CaO and CO2 at 800 oC, where the obtained CaO can serve as the template and CO2 can promote the formation of micropores to facilitate the interconnect of the mesopores in 3D-HG. Spinel NiCo2O4 is synthesized on 3D-HG by hydrothermal method, which is denoted as NiCo2O4/3D-HG. The as-prepared NiCo2O4/3D-HG catalysts retain the hierarchical micro-mesoporous structure of graphene. Benefiting from the mass transport convenience of the hierarchical structure, NiCo2O4/3D-HG exhibits high-performance half-wave potential (0.82 V vs. RHE) better than NiCo2O4/rGO (0.79 V), NiCo2O4/CNTs (0.73 V) and 20 wt% Pt/C (0.81 V), and shows higher durability than 20 wt% Pt/C. This work proves that NiCo2O4/3D-HG is a cost-effective electrocatalyst for fuel cells or metal-air batteries.

2



Introduction With the depletion of fossil resources and the resulting environmental pollutions, the development of new technology for energy conversion and storage systems is urgently needed.1-4 Nowadays, metal-air batteries (MABs) and fuel cells have received growing interest owing to high specific energy, high conversion efficiency, as well as low emission.5-7 MABs including zinc–air,8 lithium–air,9 magnesium-air10 and aluminum-air batteries11 have attracted much research interest owing to unlimited and free supply of air as the main battery reactant.12 MABs are not only mass produced, but also accepted as viable energy sources in electric vehicles and hybrid electric vehicles.13 However, owing to sluggish oxygen reduction reaction (ORR) kinetics at the cathode in MABs, it is crucial for the commercial applications to exploit the high-performance ORR electrocatalysts.14-15 It is known that platinum-based materials have been investigated as a promising efficient ORR electrocatalysts,16 while the large-scale commercial applications are hindered by prohibitive cost and scarcity.17-18 It is urgent to develop cost-effective non-precious metal catalysts for ORR.19-20 Recently, a large amount of attention have been paid to numerous non-precious electrocatalysts such as transition metal oxides,18, 21-22 metal chalcogenides,23-24 metal nitrogen-doped carbon materials25-27 and carbon-based catalysts, etc.28-30 Owing to low cost, environmental friendliness and ease for fabrication, the transition metal oxides have attracted extensive concerns, especially mixed valence oxides with spinel structure.31-32 As reported, spinel NiCo2O4 shows high electrocatalytic activity toward 3


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ORR regarding the Ni-Co double metal active sites.33 However, the electrocatalytic activity of spinel NiCo2O4 also sustain some issues including low electronic conductivity, rarely available surface area and low exposure of active sites.34 Therefore, strategies should be provided to enhance their electrochemical performance. The synergistic coupling between spinel NiCo2O4 and carbon-based materials such as graphene or carbon nanotube (CNTs) is a promising approach to increase active sites and electronic conductivity due to the interaction between metal oxides and carbon structure.27,

35

Graphene, a two dimensional one-atom layered

carbon-based material with versatile properties, including high electrical properties, large surface area and excellent electrical conductivity.36 However, the irreversible aggregation owing to π−π interaction between the graphene sheets. Metal oxides nanoparticles (NPs) supported on graphene are easily sandwiched between aggregated graphene layers, which result in loss of the ORR activity.37 Three dimensional (3D) porous carbon materials may provide a solution, which has been demonstrated to have short ion/electron transport pathway and more active sites for ORR.38 To address such a problem, 3D graphene is a state-of-the-art solution, which maintains the superior properties of graphene, and prevents graphene sheets from agglomeration.39 In addition, 3D graphene with porous structure shows high specific surface area and more defects.40 Particularly, 3D hierarchical graphene (3D-HG) with porous structure is more favorable, which not only can enrich the defects served as the reasonable anchor sites for metal oxides NPs but also further promote the diffusion and mass transport of ORR.41 The synergistic interactions between NiCo2O4 and 3D-HG 4



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improve the coordination environment and the interface charge density of active sites.42 3D-HG has been prepared by various methods successfully, including template method, direct deposition, self-assembly.43-46 The morphology and properties of 3D-HG can be controlled by preparation via template method with different 3D templates. Using a different mass ratio of the template and carbon source, the graphene layers in 3D-HG are controlled.47 Herein, the synthesis process of NiCo2O4/3D-HG is illustrated in Scheme 1. A novel interconnected hierarchical 3D-HG was firstly fabricated by the templated method, where coal tar pitch and nano CaCO3 were used as carbon precursor and template,

respectively.

Spinel

NiCo2O4

was

synthesized

on

3D-HG

via

hydrothermal method, which was denoted as NiCo2O4/3D-HG. The fused ring aromatic hydrocarbons are the main constituent of coal tar pitch, which is a help to form the structure of graphene through high-temperature carbonization and dehydrogenative. There are several reasons to choose CaCO3 as the template agent. Firstly, CaCO3 is among the most abundant salt on earth. Secondly, the pyrolysis of CaCO3 could produce CaO and CO2 at 800 oC, where the obtained CaO can serve as the template and CO2 can promote the formation of micropores to facilitate the interconnect of the mesopores in 3D-HG. Finally, the removal of the template can be achieved with ease after carbonization. Furthermore, NiCo2O4/3D-HG with hierarchical porous structure is conducive to boost mass transfer for improving catalytic performance. This study not only offers a potential catalyst for MABs but also a facile approach to design electrocatalysts for commercial applications. 5


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Scheme 1. The synthesis routes of the NiCo2O4/3D-HG catalyst.

Experimental Section Chemicals Nickel acetate (NiC4H6O4·4H2O, 99.5%), cobalt acetate (CoC4H6O4·4H2O, 99%) and potassium hydroxide (KOH, 85%) were purchased from Alfa Aesar. Multi-walled carbon nanotubes (purity: >95%) were purchased from Beijing DK nano technology Co., Ltd. Nafion (5 wt %) was obtained from Du Pont Co., Ltd. Commercial Pt/C was purchased from Energy Chemicals. Preparation of 3D-HG Coal tar pitch (6 g) was dispersed in xylene (60 mL) with stirring for 15 min. KOH (6 g) and 50 nm CaCO3 (12 g) were ground and added to xylene (30 mL) under stirring for 15 min, and then added to the coal tar pitch solution. The mixture was evaporated to a paste at 100 oC under magnetic stirring. The paste was dried in a vacuum oven at 60 °C for 12h. The gained product was sufficiently ground into a powder which was transferred into a porcelain boat and calcined for 2 h under an Ar gas flow at 800 °C with a heating rate of 3 °C min−1. After that, the gained black powder was washed by 1 M HCl solution and deionized (DI) water for several times, respectively. Finally, the obtained product was dried in a vacuum oven at 70 °C for 12 6



h and denoted as 3D-HG (approximately 1.1 g). Preparation of NiCo2O4/3D-HG NiC4H6O4·4H2O (1 mmol) and CoC4H6O4·4H2O (2 mmol) were dissolved in DI water (20 mL). 3D-HG (0.7 g) was dispersed in DI water (20 mL) to form a homogenous solution. These two mixtures were mixed under stirring, and then NH4OH (5 mL) was added to above mixture. After stirring 20h at 80 °C, the mixture was poured into a 50 mL autoclave and kept at 150 °C for 3 h. The reaction mixture centrifugal washed for several times. The obtained powder was dried under a vacuum oven at 70 °C for 12 h and sintered under an Ar atmosphere at 400 °C for 3 h with a heating rate of 2 °C min−1. The product was denoted as NiCo2O4/3D-HG. Graphene oxide (GO) was obtained by a modified Hummers’ method.48 The samples of NiCo2O4/CNTs or NiCo2O4/rGO were fabricated using the same method, by employing carbon nanotube or GO as the carbon precursor, respectively.

Characterization The morphology of the catalysts was investigated by transmission electron microscope (TEM, JEOL, JE-Me2100). XRD pattern was analyzed with a Bruker D8 diffractometer with Cu Kα radiation (λ = 0.1541 nm) from 10° to 80°. XPS spectra were recorded using an ESCALab220i-XL spectrometer. Raman spectra were characterized on LabRAM XploRA micro Raman spectrometer (HORIMA Company). The Barrett-Emmett-Teller (BET) surface areas were characterized by N2 adsorption/desorption on a TriStar II 3020 instrument. 7


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Electrochemical Measurements All electrochemical properties were investigated using the RDE technique on a CHI 660C electrochemical Workstation controlled by a computer. RDE measurements were performed in a three-electrode system. The glassy carbon disk was used as the working electrode. The Hg/HgO was used as a reference electrode and a carbon plate was used as a counter electrode. The ink was prepared as follows: finely ground catalyst (2 mg) was ultrasonically in ethanol (1 mL) and 5 wt % Nafion solution (0.01 mL) for 30 min. Catalyst ink (10 μL) was dripped onto the surface of a glassy carbon disk (0.1075 cm2). Electrochemical measurements were carried out in 0.1 M KOH solution with O2-saturated at 25 °C with sweep rate 5 mV s−1. The durability experiments were tested for NiCo2O4/3D-HG and 20 wt% Pt/C using accelerated durability tests (ADT) and chronoamperometry tests (rotation rate: 1600 rpm). All potentials were all converted to RHE (ERHE = EHg/HgO+0.14 + 0.059 PH) in this work. Results and Discussion Figure 1 showed XRD patterns of 3D-HG and NiCo2O4/3D-HG. The XRD pattern of 3D-HG has two diffraction peaks. A broad peak was observed at approximately 25° can be indexed to the (002) plane and a peak was observed at approximately 43° can be indexed to the (100) plane, which was attributed to the stacking of graphene sheets.49 The XRD pattern of NiCo2O4/3D-HG showed that the diffraction peaks at 2θ= 18.8, 31.1, 36.6, 44.6, 55.4, 59.5 and 64.9° can be indexed to spinel phase of NiCo2O4 (JCPDS no. 20-0781).50 No other diffraction peaks were 8



observed, implying that CaCO3 and CaO template was washed away. Meanwhile, a diffraction peak was observed at 25°, which was indexed to the (002) plane of graphene, implying the amorphous nature of 3D-HG.51

Figure 1 XRD pattern of 3D-HG and NiCo2O4/3D-HG.

Further structural insights were examined by TEM and HRTEM in Figure 2 and Figure S1. As shown in Figure S1 a, the image of 3D-HG possessed evident foam-like hierarchical porous structure, revealing that the 3D-HG had multi-layered graphene structure. The HRTEM image (Figure S1 b) of 3D-HG indicated that it had interconnected mesopore structures. NiCo2O4/CNTs consisted of NiCo2O4 NPs were coated with the surface of CNTs in Figure S1 c, and the HRTEM image (Figure S1 d) of NiCo2O4/CNTs displayed that the measured interplanar spacing of 0.29 nm was in accordance with the (220) plane of NiCo2O4.52 Similarly, the TEM image of NiCo2O4/rGO displayed that NiCo2O4 NPs were dispersed in the graphene sheets with obvious aggregation in Figure S1 e. The HRTEM image (Figure S1 f) of NiCo2O4/rGO displayed that the measured interplanar spacing of 0.15 nm was in accordance with the (511) lattice planes of NiCo2O4.53 The TEM image of 9


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NiCo2O4/3D-HG displayed that the NiCo2O4 were uniformly deposited on 3D-HG in Figure 2a. NiCo2O4/3D-HG comprised evenly and evident multilayer graphene structure. 3D-HG with hierarchical porous structure had more active sites, which was beneficial for evenly distribution and uniformly deposition of NiCo2O4 on 3D-HG surface. The HRTEM image of NiCo2O4/3D-HG displayed that the measured interplanar spacing of 0.25 nm was in accordance with the (311) plane of NiCo2O4 in Figure 2b,42 which was consistent with the XRD results in Figure 1. To further examine the presence of NiCo2O4, the energy-dispersive X-ray spectrum (EDX) mapping measurement was used to demonstrate the elemental distribution of NiCo2O4/3D-HG. As shown in Figure 2d-h, the Ni, Co and O atoms were uniformly distributed throughout the 3D-HG surface. The atomic ratio of Co : Ni was about 2:1 in Figure S2.

Figure 2 (a) TEM image of NiCo2O4/3D-HG; (b) HR-TEM image of NiCo2O4/3D-HG; (c) Typical STEM image; (d-h) elemental mapping images of C, O, Co and Ni.

The Raman spectra (Figure 3) of NiCo2O4/3D-HG and 3D-HG showed a sharp G band at 1590 cm−1and a sharp D band at 1350 cm−1, respectively. The sharp D band 10



corresponded to defective graphitic structures and the G band corresponded to the sp2 hybridized carbon atom.54 The relative intensity of ID/IG corresponded to the extent of defective carbon structures.55 The calculated ID/IG ratios of 3D-HG and NiCo2O4/3D-HG

were

1.02

and

1.18,

respectively.

The

ID/IG

ratios

of

NiCo2O4/3D-HG is higher than 3D-HG owing to increase the defects, demonstrating that the spinel NiCo2O4 NPs anchored on the 3D-HG.41 There was a weak peak 2D band at 2800 cm−1, and the relative intensity of G and 2D (IG/I2D) represented the number of layers in graphene. The ratios of IG/I2D for 3D-HG and NiCo2O4/3D-HG were 5.2 and 5.8, respectively, demonstrating that 3D-HG and NiCo2O4/3D-HG possessed the multilayered structure of graphene.56 Meanwhile, the ratio of IG/I2D for 3D-HG was smaller than NiCo2O4/3D-HG, indicating that the layers of 3D-HG were less than that of NiCo2O4/3D-HG. The above results demonstrated that NiCo2O4 NPs anchored on 3D-HG, which was in accordance with TEM result in Figure 2a.

a: 3D-HG b: NiCo2O4/3D-HG

D-band G-band

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b a 500

ID/IG=1.18

2D-band

ID/IG=1.02

1000

1500

2000

Raman Shift

2500

3000

-1 (cm )

Figure 3 Raman spectra of 3D-HG and NiCo2O4/3D-HG.

The elemental composition and surface electronic states of NiCo2O4/3D-HG 11


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were analyzed by XPS. Figure S3 a clearly indicated that the main components were C, O, Co and Ni elements. The atomic ratios of C, O, Co, and Ni were 87.5, 9.6, 1.7 and 1.2 %, respectively. The Ni/Co atom ratio in the obtained NiCo2O4/3D-HG was approximately 1:2. In addition, there were no other peaks of CaCO3 and CaO revealing that the template was completely removed, which was well agreement with XRD result in Figure 1. In Ni2p spectrum (Figure 4a), the two fitted peaks located at 855.4 and 862.2 eV were assigned to Ni2p3/2 and satellite, respectively, and the other two fitted peaks located at 872.8 and 880.6 eV were assigned to Ni2p1/2 and satellite, respectively, implying valence of the Ni was Ni2+.33 Similarly, in Co2p spectrum (Figure 4b), the two fitted peaks located at 779.9 and 786.8 eV were assigned to Co2p3/2 and satellite, respectively, and the other two fitted peaks located at 794.8 and 804.2 eV were assigned to Co2p1/2 and satellite, respectively, implying valence of the Co was Co3+.34 The result showed that the spinel NiCo2O4 had been prepared, which was well agreement with the expected results. As depicted in Figure S3 b, the Ols showed the strong peak at 529.7 eV owing to M-O-M, and the other two peaks at 531.6 and 532.9 eV was attributed to O=C-O and C=O.53 Figure S3 c displayed the spectra of Cls, and the three fitting peaks at 288.9, 285.4 and 284.9 eV were attributed to C=O, C-O and C-C, respectively.56 The pore width distribution and specific surface area of NiCo2O4/3D-HG were characterized by N2 adsorption-desorption isotherm method. As depicted in Figure 4c, the Nitrogen adsorption and desorption isotherm can be classified as type I and type IV, confirming the presence of the micropore and mesopore structure.57 The BET 12



surface areas of NiCo2O4/3D-HG were 521.9 m2 g-1. Figure 4d showed the pore distribution of NiCo2O4/3D-HG. The mesopore size ranges form 25-50 nm and the particle size of CaCO3 template was 50 nm.The decrease in pore size was ascribed to the deposition of spinel NiCo2O4 on the surface of the 3D-HG, which was in accordance with the result from Raman spectra in Figure 3. The presence of micropore structure was ascribed to generated CO2 and the presence of mesopore structure was ascribed to the template. The interconnected hierarchical porous structure and high surface area contribute to accelerate mass transport and improve the ORR performance.58 (a)

(b)

(c)

(d)

Figure 4 XPS spectra of NiCo2O4/3D-HG catalyst: (a) Ni 2p spectrum; (b) Co 2p spectrum; (c) Nitrogen adsorption-desorption isotherm and (d) Pore size distribution of NiCo2O4/3D-HG.

The catalytic activity of catalysts was investigated by rotating disk electrode 13


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(RDE) measurements in O2-saturated 0.1 M KOH solution. As depicted in Figure S4, NiCo2O4/3D-HG showed no redox peak in the N2-saturated electrolyte. In comparison, a well-define ORR peak appeared in the O2-saturated electrolyte, implying distinct catalytic activity for ORR. Figure 5a displayed the linear sweep voltammogram (LSV) curves of NiCo2O4/3D-HG, NiCo2O4/rGO, NiCo2O4/CNTs and 20 wt% Pt/C. The onset potential (Eonset) of NiCo2O4/3D-HG (0.95 V) was better than those of NiCo2O4/rGO (0.91 V), NiCo2O4/CNTs (0.87 V) and can be comparable with 20 wt% Pt/C (0.95 V). Moreover, the half-wave potential (E1/2) of NiCo2O4/3D-HG (0.82 V vs. RHE) was better than those of NiCo2O4/rGO (0.79 V), NiCo2O4/CNTs (0.73 V) and 20 wt% Pt/C (0.81 V). The above results revealed that NiCo2O4/3D-HG possessed the best ORR activity. The electrocatalytic activity of NiCo2O4/3D-HG was greatly modified by using 3D-HG as the support. NiCo2O4/3D-HG with interconnected hierarchical porous structure contributed to promoting mass transfer for further improvement the electrocatalytic activity. To further gain the ORR kinetics of NiCo2O4/3D-HG, the LSV curves were carried out by RDE with different speeds (625, 900, 1225, 1600, 2025 rpm) in O2-saturated solution. The kinetic parameters were calculated by the Koutecky-Levich (K-L) equation: 1 1 1 1 1     1/2 J JL JK B JK

(1)

B  0.62nFC0 ( D0 ) 2/3 v 1/6

(2)

J K  nFkC0

(3)

14



Where J is the measured current density, JK and JL are the O2 diffusion-limiting current densities and the kinetic, respectively, ω is the rotation speed (ω=2πN, N is the linear rotation speed), n is the number of electrons transferred. F (F=96485 C mol−1) is the Faraday constant, C0 is the bulk concentration of O2, ν is the kinematic viscosity of the electrolyte, and k is the electron-transfer rate constant.33 The electron transfer number (Figure 5b) of NiCo2O4/3D-HG was 3.94 - 3.97 in the potential ranges of +0.40 to +0.60 V, revealing that NiCo2O4/3D-HG catalyze O2 reduction mainly through a four-electron transfer pathway. Furthermore, the numbers of electrons transferred by NiCo2O4/3D-HG, NiCo2O4/rGO and NiCo2O4/CNTs at different potentials were also measured, as depicted in Figure 5c. The electron transfer numbers of NiCo2O4/3D-HG, NiCo2O4/rGO and NiCo2O4/CNTs were 3.81, 3.74 and 3.53 V at 0.7 V, respectively, and the number of electrons transferred was 3.97, 3.87 and 3.77 at 0.5 V, respectively. The results demonstrated that NiCo2O4/3D-HG possessed excellent selectivity toward ORR process. Moreover, all the samples catalyzed O2 reduction predominantly through a four-electron transfer pathway. The Tafel plot was analyzed to assess the ORR kinetics of NiCo2O4/3D-HG. Figure 5d showed Tafel plots of NiCo2O4/3D-HG, NiCo2O4/rGO, NiCo2O4/CNTs and 20 wt% Pt/C. Tafel slopes of 66.7, 75.1, 83.5 and 69.6 mV dec-1 were observed, respectively. The lowest Tafel slope of NiCo2O4/3D-HG further proved that it had the best ORR performance. The electrocatalytic activity of different samples was shown in Table S1. In addition, the catalytic activity of NiCo2O4/3D-HG compared with other NiCo2O4-based materials in literature was presented in table 1. All of the data 15


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were measured in 0.1 M KOH solution and all the potential values were changed to RHE. As shown in table 1, NiCo2O4/3D-HG exceeded many reported values, implying that it was a promising electrocatalyst for ORR. These above results verified the superior activity of NiCo2O4/3D-HG, which was attributed to the interconnected hierarchical 3D-HG Structure. The unique hierarchical porous structure of NiCo2O4/3D-HG combining micropore and mesopore, which is beneficial to ensure efficient mass transfer and evidently facilitate ORR diffusion kinetics.59 (a)

(b)

(c)

(d)

Figure 5 (a) LSV curves of NiCo2O4/3D-HG, NiCo2O4/rGO, NiCo2O4/CNTs and 20 wt% Pt/C. (b) LSV curves of NiCo2O4/3D-HG at different rotation rates from 625 to 2025 rpm (inset displays K-L plots of NiCo2O4/3D-HG between +0.40 and +0.60 V ). (c) Numbers of electron transfer at different potentials. (d) Tafel plots of NiCo2O4/3D-HG, NiCo2O4/rGO, NiCo2O4/CNTs and 20 wt%Pt/C. 16



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Table 1 Electrocatalytic activity overview of typical reported NiCo2O4-based materials catalysts NiCo2O4 catalysts

Eonset V (vs. RHE)

E1/2 V (vs. RHE)

Ref.

NiCo2O4/3D-HG

0.95

0.82

This work

NiCo2O4-NG/C

0.95

0.80

60

NiCo2O4NFs/rGO

0.92

0.79

61

NiO/NiCo2O4

0.92

0.78

62

N-NiCo2O4/C

0.90

0.78

63

NiCo2O4 nanowire array

0.84

0.68

64

NiCo2O4/rGO

0.88

0.78

65

NiCo2O4 hollow

0.78

0.67

66

NiCo2O4/CNTs

0.93

0.81

67

NiCo2O4/PGR

0.84

0.68

68

NiCo2O4/CMK

0.87

0.79

33

The ORR durability experiment of NiCo2O4/3D-HG, NiCo2O4/rGO and 20 wt% Pt/C catalysts were investigated by accelerated durability tests (ADT) and chronoamperometry tests. The E1/2 of NiCo2O4/3D-HG shifted by 16 mV drop after cyclic voltammetry scanning for 5000 cycles in Figure 6a. In comparison, the E1/2 of NiCo2O4/rGO shifted by 25 mV in Figure 6b and the E1/2 of 20 wt% Pt/C showed a higher negative shift of 29 mV in Figure 6c. Furthermore, the current-time (i-t) curves of NiCo2O4/3D-HG, NiCo2O4/rGO and 20 wt% Pt/C were recorded at 0.6 V (vs. RHE) in Figure 6d. It can be seen that the i-t curve of NiCo2O4/3D-HG electrode decreased by about 13.6% after 50000 s of continuous operation. In contrast, 17


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NiCo2O4/rGO and 20 wt% Pt/C electrode decreased by about 25.2% and 29.2%, respectively, under the same conditions, demonstrating that the NiCo2O4/3D-HG had lower attenuation. Obviously, the experiments result demonstrated that the electrochemical durability of NiCo2O4/3D-HG was better than NiCo2O4/rGO and 20 wt% in alkaline media, perhaps thanked hierarchical 3D-HG as support for spinel NiCo2O4 NPs, which enhanced stability against aggregation. (a)

(b)

(c)

(d)

Figure 6 The accelerated durability tests (ADT) of NiCo2O4/3D-HG (a), NiCo2O4/rGO (b) and 20 wt% Pt/C (c); (d) Current-time (i-t) curve of NiCo2O4/3D-HG, NiCo2O4/rGO and 20 wt% Pt/C.

Conclusions NiCo2O4/3D-HG catalyst was prepared via a facile hydrothermal method where spinel NiCo2O4 was supported on hierarchical 3D-HG. NiCo2O4 NPs were well dispersed on 3D-HG and the NiCo2O4/3D-HG retained the interconnected hierarchical 18



porous graphene structure. NiCo2O4/3D-HG exhibited high-performance half-wave potential (0.82 V vs. RHE) better than NiCo2O4/rGO (0.79 V), NiCo2O4/CNTs (0.73 V) and 20 wt% Pt/C (0.81 V), and showed higher durability than 20 wt% Pt/C. NiCo2O4/3D-HG catalyzed O2 reduction mainly through a four-electron process. NiCo2O4/3D-HG with hierarchical porous structure contributed to boosting mass transfer for improving the electrochemical ORR performance. The results demonstrated that NiCo2O4/3D-HG could provide an exciting guideline to synthesize cost-effective ORR catalysts for fuel cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Authors *Z. F. LI. E-mail: [email protected].

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant nos. 21776167, 21805170 and 21276148), the State Key Laboratory of Chemical Engineering (Tianjin University) (Grant no. SKL-ChE-14B01). 19


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