Design of a Three-Dimensional Interconnected Hierarchical Micro

Oct 30, 2018 - The development of cost-effective electrocatalysts with high performance for oxygen reduction reaction is the main problem for fuel cel...
0 downloads 0 Views 1MB Size
Subscriber access provided by UNIV OF LOUISIANA

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 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

The Journal of Physical Chemistry

248x178mm (96 x 96 DPI)

ACS Paragon Plus Environment

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

ACS Paragon Plus Environment

Page 2 of 26

Page 3 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

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

ACS Paragon Plus Environment

Page 4 of 26

Page 5 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

Page 6 of 26

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

ACS Paragon Plus Environment

Page 7 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

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

ACS Paragon Plus Environment

Page 8 of 26

Page 9 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

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

ACS Paragon Plus Environment

Page 10 of 26

Page 11 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

The Journal of Physical Chemistry

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

Page 12 of 26

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

ACS Paragon Plus Environment

Page 13 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

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

ACS Paragon Plus Environment

Page 14 of 26

Page 15 of 26 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

The Journal of Physical Chemistry

(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

ACS Paragon Plus Environment

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

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

ACS Paragon Plus Environment

Page 16 of 26

Page 17 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

Page 18 of 26

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

ACS Paragon Plus Environment

Page 19 of 26 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

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

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

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

ACS Paragon Plus Environment

Page 20 of 26

Page 21 of 26 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

The Journal of Physical Chemistry

References 1.

Hou, H.; Xu, Q.; Pang, Y.; Li, L.; Wang, J.; Zhang, C.; Sun, C., Efficient Storing Energy

Harvested by Triboelectric Nanogenerators Using a Safe and Durable All-Solid-State Sodium-Ion Battery. Adv Sci 2017, 4, 1700072. 2.

Armel, V.; Hindocha, S.; Salles, F.; Bennett, S.; Jones, D.; Jaouen, F., Structural Descriptors of

Zeolitic–Imidazolate Frameworks Are Keys to the Activity of Fe–N–C Catalysts. J Am Chem Soc 2017, 139, 453-464. 3.

Wang, W.; Chen, W.; Miao, P.; Luo, J.; Wei, Z.; Chen, S., Nacl Crystallites as Dual-Functional

and Water-Removable Templates to Synthesize a Three-Dimensional Graphene-Like Macroporous Fe-N-C Catalyst. ACS Catal 2017, 7, 6144-6149. 4.

Chen, X.; Chang, J.; Yan, H.; Xia, D., Boron Nitride Nanocages as High Activity Electrocatalysts

for Oxygen Reduction Reaction: Synergistic Catalysis by Dual Active Sites. J Phys Chem C 2016, 120, 28912-28916. 5.

Tatara, R.; Leverick, G. M.; Feng, S.; Wan, S.; Terada, S.; Dokko, K.; Watanabe, M.; Shao-Horn,

Y., Tuning NaO2 Cube Sizes by Controlling Na+ and Solvent Activity in Na–O2 Batteries. J Phys Chem C 2018, 122, 18316-18328. 6.

Sun, P.; Li, Z.; Wang, S.; Yin, X., Performance Enhancement of Polybenzimidazole Based High

Temperature Proton Exchange Membranes with Multifunctional Crosslinker and Highly Sulfonated Polyaniline. J Membr Sci 2018, 549, 660-669. 7.

Gewirth, A. A.; Varnell, J. A.; DiAscro, A. M., Nonprecious Metal Catalysts for Oxygen

Reduction in Heterogeneous Aqueous Systems. Chem Rev 2018, 118, 2313-2339. 8.

Cano, Z. P.; Park, M. G.; Lee, D. U.; Fu, J.; Liu, H.; Fowler, M.; Chen, Z., New Interpretation of

the Performance of Nickel-Based Air Electrodes for Rechargeable Zinc–Air Batteries. J Phys Chem C 2018. DOI: 10.1021/acs.jpcc.8b06243 9.

Belova, A. I.; Kwabi, D. G.; Yashina, L. V.; Shao-Horn, Y.; Itkis, D. M., Mechanism of Oxygen

Reduction in Aprotic Li–Air Batteries: The Role of Carbon Electrode Surface Structure. J Phys Chem C 2017, 121, 1569-1577. 10. Li, Y.; Zhang, X.; Li, H.-B.; Yoo, H. D.; Chi, X.; An, Q.; Liu, J.; Yu, M.; Wang, W.; Yao, Y., Mixed-Phase Mullite Electrocatalyst for Ph-Neutral Oxygen Reduction in Magnesium-Air Batteries. Nano Energy 2016, 27, 8-16. 11. Xu, Y.; Zhao, Y.; Ren, J.; Zhang, Y.; Peng, H., An All-Solid-State Fiber-Shaped Aluminum–Air Battery with Flexibility, Stretchability, and High Electrochemical Performance. Angew. Chem.-Int. Edit 2016, 55, 7979-7982. 12. Wu, X.; Chen, F.; Jin, Y.; Zhang, N.; Johnston, R. L., Silver–Copper Nanoalloy Catalyst Layer for Bifunctional Air Electrodes in Alkaline Media. ACS Appl Mater Inter 2015, 7, 17782-17791. 13. Shinde, S. S.; Lee, C.-H.; Sami, A.; Kim, D.-H.; Lee, S.-U.; Lee, J.-H., Scalable 3-D Carbon Nitride Sponge as an Efficient Metal-Free Bifunctional Oxygen Electrocatalyst for Rechargeable Zn–Air Batteries. ACS Nano 2017, 11, 347-357. 14. Choi, C. H.; Baldizzone, C.; Polymeros, G.; Pizzutilo, E.; Kasian, O.; Schuppert, A. K.; Ranjbar Sahraie, N.; Sougrati, M.-T.; Mayrhofer, K. J. J.; Jaouen, F., Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium. ACS Catal 2016, 6, 3136-3146. 15. Kulkarni, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K., Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction. Chem Rev 2018, 118, 2302-2312. 16. Yang, X.; Roling, L. T.; Vara, M.; Elnabawy, A. O.; Zhao, M.; Hood, Z. D.; Bao, S.; Mavrikakis, 20

ACS Paragon Plus Environment

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

M.; Xia, Y., Synthesis and Characterization of Pt–Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett 2016, 16, 6644-6649. 17. Bordley, J. A.; El-Sayed, M. A., Enhanced Electrocatalytic Activity toward the Oxygen Reduction Reaction through Alloy Formation: Platinum–Silver Alloy Nanocages. J Phys Chem C 2016, 120, 14643-14651. 18. Grewal, S.; Macedo Andrade, A.; Nelson, A. J.; Thai, K.; Karimaghaloo, A.; Lee, E.; Lee, M. H., Critical Impact of Graphene Functionalization for Transition Metal Oxide/Graphene Hybrids on Oxygen Reduction Reaction. J Phys Chem C 2018, 122, 10017-10026. 19. Han, Y., Wang, Y., Chen, W., Xu, R., Zheng, L., Zhang, J., Luo, J., Shen, R., Zhu, R., Cheong, W., et al., Hollow N-Doped Carbon Spheres with Isolated Cobalt Single Atomic Sites: Superior Electrocatalysts for Oxygen Reduction. J Am Chem Soc 2017, 139, 17269-17272. 20. Wang, W.; Kuai, L.; Cao, W.; Huttula, M.; Ollikkala, S.; Ahopelto, T.; Honkanen, A. P.; Huotari, S.; Yu, M.; Geng, B., Mass‐Production of Mesoporous MnCo2O4 Spinels with Manganese(Iv)‐ and Cobalt(Ii)‐Rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis. Angew. Chem.-Int. Edit 2017, 56, 14977-14981. 21. Liu, X., Liu, W., Minseong, K., Minjoon, P., Min, G., Pilgun, O., Sujong, C., Suhyeon, P., Anix, C., Wu, G., et al., Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts. Adv Funct Mater 2015, 25, 5799-5808. 22. Zhang, Z.; Li, Z.; Sun, C.; Zhang, T.; Wang, S., Preparation and Properties of an Amorphous MnO2/CNTs-OH Catalyst with High Dispersion and Durability for Magnesium-Air Fuel Cells. Catal Today 2017, 298, 241-249. 23. Xia, Z.; An, L.; Chen, P.; Xia, D., Non-Pt Nanostructured Catalysts for Oxygen Reduction Reaction: Synthesis, Catalytic Activity and Its Key Factors. Adv Energy Mater 2016, 6, 3136-3146. 24. Ganesan, P.; Prabu, M.; Sanetuntikul, J.; Shanmugam, S., Cobalt Sulfide Nanoparticles Grown on Nitrogen and Sulfur Codoped Graphene Oxide: An Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions. ACS Catal 2015, 5, 3625-3637. 25. Lin, L.; Yang, Z. K.; Jiang, Y.-F.; Xu, A.-W., Nonprecious Bimetallic (Fe,Mo)–N/C Catalyst for Efficient Oxygen Reduction Reaction. ACS Catal 2016, 6, 4449-4454. 26. Kramm, U. I.; Herrmann-Geppert, I.; Behrends, J.; Lips, K.; Fiechter, S.; Bogdanoff, P., On an Easy Way to Prepare Metal–Nitrogen Doped Carbon with Exclusive Presence of MeN4-Type Sites Active for the Orr. J Am Chem Soc 2016, 138, 635-640. 27. Shen, H., Gracia-Espino, E., Ma, J., Zang, K., Luo, J., Wang, L., Gao, S., Mamat, X., Hu, G., Wagberg, T., et al., Synergistic Effects between Atomically Dispersed Fe−N−C and C−S−C for the Oxygen Reduction Reaction in Acidic Media. Angew. Chem.-Int. Edit 2017, 56, 13800-13804. 28. Qin, L., Yuan, Y., Wei, W., Lv, W., Niu, S., He, Y., Zhai, D., Kang, F., Kim, J., Yang, Q., et al., Graphene-Directed Formation of a Nitrogen-Doped Porous Carbon Sheet with High Catalytic Performance for the Oxygen Reduction Reaction. J Phys Chem C 2018, 122, 13508-13514. 29. Yang, H. B., Miao, J., Hung, S., Chen, J., Tao, H., Wang, X., Zhang, L., Chen, R., Gao, J., Chen, H., et al., Identification of Catalytic Sites for Oxygen Reduction and Oxygen Evolution in N-Doped Graphene Materials: Development of Highly Efficient Metal-Free Bifunctional Electrocatalyst. Sci. Adv 2016, 2, e1501122. 30. Liang, H.-W.; Zhuang, X.; Brüller, S.; Feng, X.; Müllen, K., Hierarchically Porous Carbons with Optimized Nitrogen Doping as Highly Active Electrocatalysts for Oxygen Reduction. Nat Commun 2014, 5, 4973. 21

ACS Paragon Plus Environment

Page 22 of 26

Page 23 of 26 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

The Journal of Physical Chemistry

31. Zhao, Q.; Yan, Z.; Chen, C.; Chen, J., Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond. Chem Rev 2017, 117, 10121-10211. 32. Ge, B.; Li, K.; Fu, Z.; Pu, L.; Zhang, X.; Liu, Z.; Huang, K., The Performance of Nano Urchin-Like NiCo2O4 Modified Activated Carbon as Air Cathode for Microbial Fuel Cell. J Power Sources 2016, 303, 325-332. 33. Bo, X.; Zhang, Y.; Li, M.; Nsabimana, A.; Guo, L., NiCo2O4 Spinel/Ordered Mesoporous Carbons as Noble-Metal Free Electrocatalysts for Oxygen Reduction Reaction and the Influence of Structure of Catalyst Support on the Electrochemical Activity of NiCo2O4. J Power Sources 2015, 288, 1-8. 34. Tong, X.; Chen, S.; Guo, C.; Xia, X.; Guo, X.-Y., Mesoporous NiCo2O4 Nanoplates on Three-Dimensional Graphene Foam as an Efficient Electrocatalyst for the Oxygen Reduction Reaction. ACS Appl Mater Inter 2016, 8, 28274-28282. 35. Zhang, J.; Dai, L., Nitrogen, Phosphorus, and Fluorine Tri-Doped Graphene as a Multifunctional Catalyst for Self-Powered Electrochemical Water Splitting. Angew. Chem.-Int. Edit 2016, 55, 13296-13300. 36. Chen, R.; Yan, J.; Liu, Y.; Li, J., Three-Dimensional Nitrogen-Doped Graphene/Mno Nanoparticle Hybrids as a High-Performance Catalyst for Oxygen Reduction Reaction. J Phys Chem C 2015, 119, 8032-8037. 37. Yu, D.-X.; Wang, A.-J.; He, L.-L.; Yuan, J.; Wu, L.; Chen, J.-R.; Feng, J.-J., Facile Synthesis of Uniform Aupd@Pd Nanocrystals Supported on Three-Dimensional Porous N-Doped Reduced Graphene Oxide Hydrogels as Highly Active Catalyst for Methanol Oxidation Reaction. Electrochim Acta 2016, 213, 565-573. 38. Cao, G.-L.; Yan, Y.-M.; Liu, T.; Rooney, D.; Guo, Y.-F.; Sun, K.-N., Three-Dimensional Porous Carbon Nanofiber Networks Decorated with Cobalt-Based Nanoparticles: A Robust Electrocatalyst for Efficient Water Oxidation. Carbon 2015, 94, 680-686. 39. Li,

C.-S.;

Sun,

Y.;

Lai,

W.-H.;

Wang,

J.-Z.;

Chou,

S.-L.,

Ultrafine

Mn3O4

Nanowires/Three-Dimensional Graphene/Single-Walled Carbon Nanotube Composites: Superior Electrocatalysts for Oxygen Reduction and Enhanced Mg/Air Batteries. ACS Appl Mater Inter 2016, 8, 27710-27719. 40. Zhu, X.; Zhang, P.; Xu, S.; Yan, X.; Xue, Q., Free-Standing Three-Dimensional Graphene/Manganese Oxide Hybrids as Binder-Free Electrode Materials for Energy Storage Applications. ACS Appl Mater Inter 2014, 6, 11665-11674. 41. Zhang, T.; Li, Z.; Wang, L.; Sun, P.; Zhang, Z.; Wang, S., Spinel MnCo2O4 Nanoparticles Supported on Three-Dimensional Graphene with Enhanced Mass Transfer as an Efficient Electrocatalyst for the Oxygen Reduction Reaction. Chemsuschem 2018, 11, 2730-2736. 42. Yuan, H.; Li, J.; Yang, W.; Zhuang, Z.; Zhao, Y.; He, L.; Xu, L.; Liao, X.; Zhu, R.; Mai, L., Oxygen Vacancy-Determined Highly Efficient Oxygen Reduction in NiCo2O4/Hollow Carbon Spheres. ACS Appl Mater Inter 2018, 10, 16410-16417. 43. Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H.-M., Three-Dimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nat Mater 2011, 10, 424-428. 44. Wu, J., Liu, M., Sharma, P., Yadav, R., Ma, Lu., Yang, Y., Zou, X., Zhou, X., Vajtai, R., Yakobson, B., et al., Incorporation of Nitrogen Defects for Efficient Reduction of CO2 Via Two-Electron Pathway on Three-Dimensional Graphene Foam. Nano Lett 2016, 16, 466-470. 22

ACS Paragon Plus Environment

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

45. Xia, X. H.; Chao, D. L.; Zhang, Y. Q.; Shen, Z. X.; Fan, H. J., Three-Dimensional Graphene and Their Integrated Electrodes. Nano Today 2014, 9, 785-807. 46. Xia, D., Yi, K., Zheng, B., Li, M., Qi, G., Cai, Z., Cao, M., Liu, D., Peng, L., Wei, D., et al., Solvent-Free Process to Produce Three Dimensional Graphene Network with High Electrochemical Stability. J Phys Chem C 2017, 121, 3062-3069. 47. Sun, C.; Li, Z.; Zhong, X.; Wang, S.; Yin, X.; Wang, L., Three-Dimensional Graphene-Supported Cobalt Phthalocyanine as Advanced Electrocatalysts for Oxygen Reduction Reaction. J Electrochem Soc 2018, 165, F24-F31. 48. Chen, J.; Li, Y.; Huang, L.; Li, C.; Shi, G., High-Yield Preparation of Graphene Oxide from Small Graphite Flakes Via an Improved Hummers Method with a Simple Purification Process. Carbon 2015, 81, 826-834. 49. Zhao, B.; Huang, S.-Y.; Wang, T.; Zhang, K.; Yuen, M. M. F.; Xu, J.-B.; Fu, X.-Z.; Sun, R.; Wong, C.-P., Hollow SnO2@Co3O4 Core–Shell Spheres Encapsulated in Three-Dimensional Graphene Foams for High Performance Supercapacitors and Lithium-Ion Batteries. J Power Sources 2015, 298, 83-91. 50. Xia, Z.; An, L.; Chen, P.; Xia, D., Non-Pt Nanostructured Catalysts for Oxygen Reduction Reaction: Synthesis, Catalytic Activity and Its Key Factors. Adv Energy Mater 2016, 6, 1600458. 51. Wang, L.; Zhao, X.; Lu, Y.; Xu, M.; Zhang, D.; Ruoff, R. S.; Stevenson, K. J.; Goodenough, J. B., CoMn2O4 Spinel Nanoparticles Grown on Graphene as Bifunctional Catalyst for Lithium-Air Batteries. J Electrochem Soc 2011, 158, A1379-A1382. 52. Mondal, A. K.; Su, D.; Chen, S.; Xie, X.; Wang, G., Highly Porous NiCo2O4 Nanoflakes and Nanobelts as Anode Materials for Lithium-Ion Batteries with Excellent Rate Capability. ACS Appl Mater Inter 2014, 6, 14827-14835. 53. Lee, D. U.; Kim, B. J.; Chen, Z., One-Pot Synthesis of a Mesoporous NiCo2O4 Nanoplatelet and Graphene Hybrid and Its Oxygen Reduction and Evolution Activities as an Efficient Bi-Functional Electrocatalyst. J Mater Chem A 2013, 1, 4754. 54. Wang, N.; Li, T.; Song, Y.; Liu, J.; Wang, F., Metal-Free Nitrogen-Doped Porous Carbons Derived from Pomelo Peel Treated by Hypersaline Environments for Oxygen Reduction Reaction. Carbon 2018, 130, 692-700. 55. Li, Y.; Yang, J.; Huang, J.; Zhou, Y.; Xu, K.; Zhao, N.; Cheng, X., Soft Template-Assisted Method for Synthesis of Nitrogen and Sulfur Co-Doped Three-Dimensional Reduced Graphene Oxide as an Efficient Metal Free Catalyst for Oxygen Reduction Reaction. Carbon 2017, 122, 237-246. 56. Lu, X.; Li, Z.; Yin, X.; Wang, S.; Liu, Y.; Wang, Y., Controllable Synthesis of Three-Dimensional Nitrogen-Doped Graphene as a High Performance Electrocatalyst for Oxygen Reduction Reaction. Int J Hydrogen Energ 2017, 42, 17504-17513. 57. Liu, H.; Cao, Y.; Wang, F.; Huang, Y., Nitrogen-Doped Hierarchical Lamellar Porous Carbon Synthesized from the Fish Scale as Support Material for Platinum Nanoparticle Electrocatalyst toward the Oxygen Reduction Reaction. ACS Appl Mater Inter 2014, 6, 819-25. 58. Jiang, L.; Mi, L.; Wang, K.; Wu, Y.; Li, Y.; Liu, A.; Zhang, Y.; Hu, Z.; Liu, S., Promoting the Electrochemical Performances by Chemical Depositing of Gold Nanoparticles inside Pores of 3d Nitrogen-Doped Carbon Nanocages. ACS Appl Mater Inter 2017, 9, 31968-31976. 59. Yu, H.; Fisher, A.; Cheng, D.; Cao, D., Cu,N-Codoped Hierarchical Porous Carbons as Electrocatalysts for Oxygen Reduction Reaction. ACS Appl Mater Inter 2016, 8, 21431-9. 60. Wan, L.-l.; Zang, G.-l.; Wang, X.; Zhou, L.-a.; Li, T.; Zhou, Q.-x., Tiny Crystalline Grain 23

ACS Paragon Plus Environment

Page 24 of 26

Page 25 of 26 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

The Journal of Physical Chemistry

Nanocrystal NiCo2O4/N-Doped Graphene Composite for Efficient Oxygen Reduction Reaction. J Power Sources 2017, 345, 41-49. 61. Lin, S.; Shi, X.; Yang, H.; Fan, D.; Wang, Y.; Bi, K., Reduced Graphene Oxide-NiCo2O4 Nanoflowers as Efficient Electrocatalysts for the Oxygen Reduction Reaction. J Alloy Compd 2017, 720, 147-155. 62. Cui, Z.; Wang, S.; Zhang, Y.; Cao, M., Engineering Hybrid between Nickel Oxide and Nickel Cobaltate to Achieve Exceptionally High Activity for Oxygen Reduction Reaction. J Power Sources 2014, 272, 808-815. 63. Li, M.; Zhang, H.; Xiao, T.; Zhang, B.; Yan, J.; Chen, D.; Chen, Y., Rose Flower-Like Nitrogen-Doped NiCo2O4/Carbon Used as Cathode Electrocatalyst for Oxygen Reduction in Air Cathode Microbial Fuel Cell. Electrochim Acta 2017, 258, 1219-1227. 64. Jin, C.; Lu, F.; Cao, X.; Yang, Z.; Yang, R., Facile Synthesis and Excellent Electrochemical Properties of NiCo2O4 Spinel Nanowire Arrays as a Bifunctional Catalyst for the Oxygen Reduction and Evolution Reaction. J Mater Chem A 2013, 1, 12170. 65. Zhang, G.; Xia Bao, Y.; Wang, X.; Lou Xiong, W., Strongly Coupled NiCo2O4‐rGO Hybrid Nanosheets as a Methanol‐Tolerant Electrocatalyst for the Oxygen Reduction Reaction. Adv Mater 2013, 26, 2408-2412. 66. Wang, J.; Fu, Y.; Xu, Y.; Wu, J.; Tian, J.-H.; Yang, R., Hierarchical NiCo2O4 Hollow Nanospheres as High Efficient Bi-Functional Catalysts for Oxygen Reduction and Evolution Reactions. Int J Hydrogen Energ 2016, 41, 8847-8854. 67. Ma, C.; Xu, N.; Qiao, J.; Jian, S.; Zhang, J., Facile Synthesis of NiCo2O4 Nanosphere-Carbon Nanotubes Hybrid as an Efficient Bifunctional Electrocatalyst for Rechargeable Zn–Air Batteries. Int J Hydrogen Energ 2016, 41, 9211-9218. 68. Mtukula, A. C.; Shen, J.; Bo, X.; Guo, L., High Utilization Efficiency of NiCo2O4 Supported on Porous Graphene as Noble Metal-Free Catalysts for Oxygen Reduction Reaction. J Alloy Compd 2016, 655, 229-237.

24

ACS Paragon Plus Environment

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

TOC Graphic

25

ACS Paragon Plus Environment

Page 26 of 26