Nitrogen-Doped Carbon NanotubeâGraphene Frameworks with

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N-doped Carbon Nanotube-Graphene Frameworks with Encapsulated Fe/Fe3N Nanoparticles as Catalysts for Oxygen Reduction Yan Zheng, Fei He, Jiaming Wu, Delong Ma, Huailin Fan, Shufei Zhu, Xiang Li, Yizhong Lu, Qing Liu, and Xun Hu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00506 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Nano Materials

N-doped Carbon Nanotube-Graphene Frameworks with Encapsulated Fe/Fe3N Nanoparticles as Catalysts for Oxygen Reduction Yan Zheng†, Fei He†, Jiaming Wu†, Delong Ma†, Huailin Fan†, Shufei Zhu†, Xiang Li†, Yizhong Lu†, Qing Liu‡ and Xun Hu*† †Country school of Material Science and Engineering, University of Jinan, Jinan, 250022, P. R. China. ‡Country Key Laboratory of Low Carbon Energy and Chemical Engineering, College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, P. R. China.

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ABSTRACT: Iron and nitrogen co-doped carbon materials hold broad application prospects in the oxygen reduction reaction (ORR) due to their abundant reserves, low cost and excellent catalytic activity. In this study, a N-doped carbon nanotube-graphene framework with encapsulated Fe/Fe3N nanoparticles (Fe-N-CNT@RGO) is designed and synthesized by annealing a mixture of iron acetylacetonate, dicyandiamide and graphene oxide via a one-step calcination strategy. Fe-N-CNT@RGO has a better ORR catalytic activity than reduced graphene oxide, nitrogen-doped graphene and N-doped carbon nanotubes with encapsulated Fe/Fe3N nanoparticles with respect to the onset potential, limiting-current density and kinetic current density. Fe-N-CNT@RGO also has high stability and a high discharging cell voltage, which approaches to those of Pt/C in Zn-air batteries. The relationship between the structure and activity of Fe-N-CNT@RGO demonstrates that the high density of Fe-N and pyridinic-N sites, moderate wettability and positive zeta potential promote the exposure of the active sites, accelerate the transmission of hydrated oxygen and enhance the adsorption of HO2- for the 4eORR.

KEYWORDS:

Carbon

nanotube-graphene

composites,

nitrogen-doped,

Fe/Fe3N

nanoparticles, wettability, zeta potential, oxygen reduction reaction, Zn-air batteries


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1. Introduction Batteries are one of the most promising ways to achieve energy conversion by electrochemical methods.1-3 Metal-air batteries are attracting increasing attention due to their safety, high energy density and low environmental burden.4,5 The efficiency of the oxygen reduction reaction (ORR) plays a key role in metal-air batteries.6-8 Therefore, it is meaningful to develop an electrocatalyst with a high ORR activity. At present, the most efficient electrocatalysts for the ORR are Pt-based catalysts.9-11 However, the high cost and poor stability of Pt-based catalysts limit their large-scale application. Therefore, it is of great significance to develop a catalyst with low cost and excellent stability for the ORR. In the 1960s, cobalt phthalocyanine was found to exhibit ORR activity under alkaline conditions, which opened up the application of non-precious metals as catalysts for the ORR.12 To date, extensive efforts have been made to design catalysts with a high ORR catalytic activity and stability. This design is mainly based on the consideration of the following two aspects: ⅰ) Sufficient and well-designed active sites should be embedded in the substrate to ensure a high ORR activity;13-15 ⅱ) the supporting materials are expected to provide an accessible catalytic environment for the ORR.16-18 Active sites and supporting materials are two key factors in affecting the ORR activity of catalysts. With the respect to active sites, carbon materials functionalized with a nonprecious metal of Fe or Co have been found to exhibit an outstanding ORR performance.19-22 For example, Tang et al. adopted a simple method to synthesize Fe-N-C materials by assembling hollow carbon nanospheres and graphene as a


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substrate, and the samples exhibited a superior ORR performance.23 Li et al. developed a new strategy for the pyrolysis of a bimetallic zinc/cobalt-organic framework to prepare Co single atoms on nitrogen-doped porous carbon, which presented a superior ORR performance and outstanding stability.24 Hou et al. fabricated N-doped carbon nanosheet networks with Co nanoparticles by combining hydrolysis and pyrolysis methods, and the results confirmed that the abundant exposure of active sites enabled the catalyst to show excellent ORR and OER activities.25 The supporting material influences not only the dispersion of active sites but also the coordination between the substrate and active sites, which impact the performance of ORR catalysts.26-28 Therefore, the preparation of suitable supporting materials is critical to obtain electrocatalysts with a high ORR activity. Among the various supporting materials, sp2-hybridizated 1D carbon nanotubes (CNTs) and 2D graphene can meet the above-mentioned requirements as a substrate, owing to their excellent stability, good conductivity and facile functionalization.29 Therefore, the combination of 1D CNTs and 2D graphene is expected to form a 3D carbon network for use as a supporting material. In addition, the doping of nitrogen into the carbon nanostructure can promote oxygen adsorption by forming multiple possible configurations, such as pyrrolic nitrogen, graphitic nitrogen and pyridinic nitrogen.30 N-doped CNTs or/and graphene coupled with transition-metal-based electrocatalysts, especially Fe-based catalysts combined with a 3D carbon network, have been regarded as attractive ORR catalysts.31-35 In principal, the 3D carbon network is beneficial to the effective exposure of more ORR active sites. Furthermore, such a 3D structure also contributes by accelerating the diffusion of ORR-related species, such


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as O2 and H2O, leading to an enhanced ORR activity. Nevertheless, how the combination of the N-doped CNTs/graphene and transition metals affects the performances of a catalyst is not yet been fully understood. Hence, to explore the relationship between structure and activity for the N-doped CNTs and graphene composites with transition metals is highly desirable. Based on the above consideration, a N-doped CNTs and graphene framework with encapsulated Fe/Fe3N nanoparticles (Fe-N-CNT@RGO) was designed and synthesized in this study by annealing a mixture of iron acetylacetonate, dicyandiamide (DCDA) and graphene oxide (GO). Fe-N-CNT@RGO was optimized by designing a series of contrastive samples for the ORR activity test. Additionally, to verify the ORR activity in practical applications, Fe-NCNT@RGO was also assembled into Zn-air batteries. More importantly, the relationship between the structure and activity was also established by correlating the surficial properties of the catalyst with the ORR activity. Although the pyrolysis of Fe, N sources and carbon to obtain Fe-N/C catalysts has been widely used,36-38 in this wok, the relationship between the structure of a catalyst and its ORR activities is revealed by using different carbon supports such as N-doped CNTs or/and graphene, which will shed some light on the design of efficient Febased catalysts combined with a 3D carbon network for use in the ORR. 2. Experimental Section 2.1. Materials. Graphite (Aladdin Reagent Co. Ltd. 99.95%), sulfuric acid (H2SO4, Yuandong Fine Chemical Co., Ltd., 95~98%), phosphoric acid (H3PO4, Tianjin Fuyu Fine Chemical Co., Ltd., ≥85%), potassium permanganate (KMnO4, Sinopharm Chemical Reagent Co., Ltd, ≥99.5%), hydrogen peroxide (H2O2, Sinopharm Chemical Reagent Co., Ltd., 30%), iron(III) ACS Paragon Plus Environment

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acetylacetonate (C15H21FeO6, Shanghai Macklin Biochemical Co. Ltd., 98%), dicyandiamide (C2H4N4, Shanghai Macklin Biochemical Co. Ltd.,), hydrochloric acid (HCl, Laiyang Fine Chemical Co., Ltd., 36~38%), ethanol (CH3CH2OH, Tianjin Fuyu Fine Chemical Co., Ltd., ≥99.7%), Nafion® perfluorinated resin solution (C9HF17O5S, DuPont, 5 wt%), potassium hydroxide (KOH, Sinopharm Chemical Reagent Co., Ltd., ≥85%) and Pt/C (HiSPECTM 3000, Alfa Aesar, 20 wt%) were used in this study without further purification. 2.2. Preparation of graphene oxide. The preparation procedure for graphene oxide (GO) was followed an improved Hummers’ method.39 Typically, 144 mL of H2SO4 and 16 mL of H3PO4, with a volumetric ratio of 9, were poured into a 500 mL round bottom flask. Then, 1.2 g of graphite powder and 7.2 g of KMnO4 were gradually added with stirring for 30 min at 0 °C and then for 12 h raised to 55 °C. Afterwards, 5 mL of H2O2 (30%) was added dropwise until no bubbles formed. The fluffy graphite oxide powder was finally obtained by centrifuging, washing, ultrasonicating and freeze-drying. 2.3. Preparation of Fe-N-CNT@RGO. First, 50 mg of GO, 10 g of dicyandiamide (DCDA) and different amounts of iron (III) acetylacetonate (Fe(acac)3) (0.0158, 0.0315, 0.0631, 0.0946 and 0.1261 g) were mixed by grinding for 2 h. The different mass ratios of Fe/GO precursors (5%, 10%, 20%, 30% and 40%) corresponded to the products of 5% Fe-N-CNT@RGO, 10% Fe-N-CNT@RGO, 20% Fe-N-CNT@RGO, 30% Fe-N-CNT@RGO and 40% Fe-N-CNT@RGO, respectively. The produced mixture was calcined at 700 °C with a ramping rate of 10 °C/min and was kept for 2 h under argon to obtain the Fe-N-CNT@RGO composite. The as-prepared Fe-N-CNT@RGO composite was further treated with a 1 M HCl solution to remove the


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agglomerated and unprotected iron particles generated during the pyrolysis process. To compare with 30% Fe-N-CNT@RGO, reduced graphene oxide (RGO), nitrogen-doped graphene (N-RGO) and Fe-N-CNT samples were prepared according to the same procedure by using GO, GO/DCDA and iron (III) acetylacetonate/DCDA as the corresponding precursors, respectively. The mixture of Fe-N-CNT and RGO was also prepared via ball milling. Samples with different mass ratio of DCDA and GO were prepared by changing the amount of GO to 25 mg or 100 mg respectively, the addition of DCDA and (Fe(acac)3) was kept unchanged. Based on the mass ratio of Fe and GO, Fe-N-CNT@RGO used the amount of GO for 25 mg and 100 mg were denoted as 60% Fe-N-CNT@RGO and 15% Fe-N-CNT@RGO respectively. 2.4. Electrochemical measurements. The electrochemical measurements were conducted with an electrochemical workstation (CHI 760E, CH Instruments, China) by using a saturated calomel electrode (SCE) and platinum (Pt) foil. The working electrode was a rotating glassy carbon electrode (5 mm in diameter) with 500 μg cm-2 catalyst. To exclude possible pollution of the ORR activity originating from the Pt counter electrode, a control experiment using a graphite rod as the counter electrode was performed. The results in Figure S1 showed that both LSV curves were almost coincident, indicating the efficient ORR activity of Fe-N-CNT@RGO was not from the possible Pt pollution.

2.4.1. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements. The CV and LSV curves were recorded in the 0.1 M KOH electrolyte saturated with O2 and N2 from 0.2 to 1.0 V vs. RHE (E (RHE) = E (SCE) + 0.245 + 0.059×pH). The scanning rates for CV and LSV were 50 and 10 mVs-1, respectively. The onset potential (Eonset) and half-wave ACS Paragon Plus Environment

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potential (E1/2) corresponded to the potential at 5% and 50% of diffusion-limited current density in the scanning cathode, respectively.40

2.4.2. Stability and methanol poisoning test. The stability of samples was scanned at 0.8 V vs. RHE at 1600 rpm in O2-saturated 0.1 M KOH electrolyte for 7200 s. The methanol poisoning test was scanned under the above same conditions except for 1 M methanol.

2.4.3. The kinetic current test. The kinetic current (Jk) of the ORR was calculated based on the Koutecky-Levich equation: J-1 = Jk-1 + JL-1 = Jk-1 + (Bω1/2)-1

(1)

B = 0.62nFC0(D0)2/3υ-1/6

(2)

Jk = nFkC0

(3)

in which J, Jk and JL represent the measured, kinetic-limited and diffusion-limited current densities, respectively, ω is the angular velocity of the rotation electrode, n is the number of exchange electron in the ORR, F is the Faraday constant, which equals 96485 C mol-1, C0 and D0 represent the concentration and diffusion coefficient of O2, which equal 1.2×10-3 M, 1.9×105

cm2 s-1, respectively in 0.1 M KOH, υ is the kinematic viscosity of the electrolyte, which

equals to 0.01 cm2s-1, and k is the electron transfer rate constant.

2.4.4. RRDE test. The electron transfer number (n) and the hydrogen peroxide yield (H2O2%) can be determined by RRDE tests and calculated by the following equations: H2O2(%) = 200 ∗

𝐼𝑟 𝑁

(4)

𝐼𝑟

𝐼𝑑 + 𝑁


8

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𝑛=4∗

𝐼𝑑

(5)

𝐼𝑟

𝐼𝑑 + 𝑁

where Id, and Ir represent the disk current and the ring current respectively, and N is the current collection efficiency (N = 0.29).

2.4.5. Zn-air battery assembly: A Zn-air battery was installed to test the traditional stacktype battery performance. First, the 1 mg cm-2 air electrode was prepared by casting the catalyst ink on carbon paper (SLT-10D, Kunshan). A polished Zn foil with a thickness of 0.5 mm was used as the anode and 6.0 M KOH with 0.2 M ZnCl2 was used as the electrolyte. 2.5. Characterizations. The internal phases of the samples were measured by using an X-ray diffractometer (XRD, Ultima IV X–ray, Japan) with a Cu target (Kα–radiation source, λ = 1.5406 Å). The morphologies and elemental mapping distributions of the samples were observed by high- resolution transmission electron microscopy (HRTEM) (FEI Talos, Czech). The Raman measurements were performed on a LabRAM HR Evolution (France) with a laser wavelength of 532 nm. The contact angles were determined by the contact angle measuring instrument (OCA15Pro, Dataphysics, Germany). The specific surface area and pore size distribution of the pretreated samples were estimated from N2 adsorption-desorption isotherms (Pioneer, SSA–6000, China). The X-ray photoelectron spectroscopy (XPS) spectra were obtained by using a Thermo Fisher ESCALAAB 250Xi (Czech). The zeta potentials of the samples were measured by using a zeta potential analyzer (Malvern, Zetasizer Nano, ZS90). 3. Results and discussion 3.1. Structural characterization analysis


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Scheme. 1 shows the synthetic routes for the different catalysts. RGO, N-RGO, Fe-N-CNT and Fe-N-CNT@RGO were synthesized by using GO as the supporting material, DCDA as C and N sources and Fe(acac)3 as the iron source. It should be noted that the mentioned Fe-NCNT@RGO and Fe-N-CNT in the following text represent the samples prepared via pyrolysis of the precursors with a mass ratio of 30% for Fe/GO by optimizing the ORR performance. Scheme 1. Routes for synthesis of the different catalysts.

3.1.1. XRD, Raman spectra and N2 adsorption-desorption isotherm analyses The crystalline structures of the different catalysts are shown in Figure 1a. The characteristic peaks for graphitic carbon, Fe3N (PDF#49-1664) and Fe (PDF#65-4899) were observed in FeN-CNT@RGO and Fe-N-CNT. The diffraction peak of RGO was only found at 2θ = 26.6°, which was indexed to the (002) crystalline plane of graphitic carbon. After doping with nitrogen, N-RGO exhibited diffraction peaks at 2θ = 43.0° and 2θ = 26.6° for the (100) and (002)


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crystalline planes of graphitic carbon, respectively. The XRD patterns of Fe-N-CNT@RGO with different contents of the Fe element are presented in Figure S2a. The results showed that the samples were all composed of Fe, Fe3N and C. The intensity of the Fe and Fe3N peaks increased with the content of Fe. The Raman spectrum of Fe-N-CNT@RGO is displayed in Figure 1b. The peaks located at ∼1356 and ∼1583 cm-1 corresponded to the D-band and G-band of this sample, respectively. The peak intensity ratio of ID/IG for Fe-N-CNT@RGO was 1.07. By contrast, the peak intensity ratios of ID/IG for RGO (Figure 1b), N-RGO (Figure S2b) and Fe-N-CNT (Figure S2b) were 0.88, 1.02 and 1.00, respectively. The higher ratio of ID/IG for Fe-N-CNT@RGO implied that the introduction of nitrogen and Fe produced more defects and disorder, contributing to the increment in active sites and the catalytic activity. 41,42 The specific surface areas and the pore size distribution of Fe-N-CNT@RGO were further analyzed by N2 adsorption-desorption characterizations which are shown in Figure 1c and d. Fe-N-CNT@RGO exhibited a typical type Ⅳ isotherm. The specific surface area of Fe-NCNT@RGO was 64 m2 g-1 with a meso-microporous feature. By contrast, the specific surface areas of RGO, N-RGO and Fe-N-CNT were 336, 9 and 60 m2 g-1, respectively (Figure S3a-f). The decreased specific surface areas of N-RGO was possibly caused by the aggregation and overlap of graphene oxide sheets during the pyrolysis process of dicyandiamide and graphene oxide.43,44 More defects in Fe-N-CNT and Fe-N-CNT@RGO led to the lower specific surface areas than that of RGO.41 The relationship between the BET specific surface areas of samples and the ORR catalytic performance will be discussed later.


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

 Fe3N

  

Intensity (a.u.)





D

Fe-N-CNT@RGO 

G



 

(b)

 Fe

Intensity (a.u.)

(a)

Fe-N-CNT





N-RGO

Fe-N-CNT@RGO

ID/IG = 1.07

RGO

ID/IG = 0.88

RGO

10

20

30

40 50 60 2Theta (degree)

70

80

90

(c)

dV/dP Pore Volume (cm3 g-1 nm-1)

250 200

Volume (cm3 g-1)

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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150 Adsorption Desorption

100 50 0

0.0

0.2

0.4

0.6

Relative pressure (P/P0)

0.8

1.0

1000 0.05

1500

2000

Raman shift (cm-1)

(d)

0.04 0.03 0.02 0.01 0.00 0

10

20

30

40

50

60

70

80

Pore diameter (nm)

Figure 1. (a) XRD pattern of different catalysts. (b) Raman spectra of RGO and Fe-NCNT@RGO. (c) N2 adsorption-desorption isotherm of Fe-N-CNT@RGO. (d) Pore size distribution of Fe-N-CNT@RGO.

3.1.2. Morphology analyses TEM and HRTEM characterizations were further carried out to probe the morphological structures of the different catalysts. As shown in Figure 2, N-RGO exhibited a sheet-like morphology (Figure 2a). For Fe-N-CNT, many metal nanoparticles were encapsulated in the carbon nanotubes which were intertwined with each other (Figure 2b). The average size of


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the nanoparticles was ca. 34 nm (Figure S4a), while the distribution of the particles was uneven, and they were partially agglomerated (Figure 2b). It should be noted that Fe-NCNT@RGO was composed of layered graphene sheets and intertwined CNTs, in which Fe and Fe3N nanoparticles were encapsulated by graphitic carbon layer and CNTs (Figure 2c). The introduction of graphene into Fe-N-CNT@RGO resulted in a more uniform distribution of metal nanoparticles, which possessed a smaller size (25 nm) than those of Fe-N-CNT (34 nm) (Figure S4b). This result was attributed to the oxygen-containing functional groups of GO in the mixture of GO, DCDA and Fe(acac)3 for Fe-N-CNT@RGO that were able to promote a uniform distribution of the Fe ions by coordination.45 After the subsequent pyrolysis, small and well-distributed metal nanoparticles (Figure S4b) were observed for Fe-N-CNT@RGO compared with those of Fe-N-CNT, similar to another reported result.46 The HRTEM image of Fe-N-CNT@RGO in Figure 2d further shows that the diameter of the carbon nanotubes was approximately 20 nm. The Fe and Fe3N nanoparticles appeared on the top or inside of the CNTs and were distributed randomly on the graphene. As shown in Figure 2d, the inner Fe/Fe3N nanoparticles were wrapped by carbon layers with a 4 nm wall thickness. This geometric limitation of Fe/Fe3N not only inhibited the dissolution and agglomeration of internal Fe/Fe3N nanoparticles under severe conditions but also enriched the electron density on the surface of carbon, which could promote the surface reaction.47 In addition, Figure 2e shows that the high-resolution lattice stripe with the widths of 0.202 nm and 0.208 nm corresponded to the (110) and (111) crystal planes of Fe and Fe3N, respectively. The EDS mapping of a selected region of Fe-N-CNT@RGO (Figure 2f) indicated that C and N were


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uniformly distributed on the surface of the CNTs and RGO, while the metallic iron-related species mainly were concentrated inside of the CNTs. All of the above results confirmed the formation of Fe-N-CNT@RGO.

Figure 2. TEM images of (a) N-RGO, (b) Fe-N-CNT and (c)Fe-N-CNT@RGO. (d, e) HRTEM images and (f) EDS mapping images of the Fe-N-CNT@RGO.

3.1.3. XPS analysis To further explore the electronic structure and elemental composition on the surface of the samples, the XPS characterization was conducted. As shown in Figure 3a, Fe-N-CNT@RGO contained Fe, N, O and C. The N 1s spectra in Figure 3b for Fe-N-CNT@RGO was deconvolved into five peaks located at 397.7, 398.7, 399.2, 400.4 and 401.8 eV, corresponding to pyridinic-


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N, Fe-N, pyrrolic-N, graphitic-N and oxidized-N, respectively.41,48 Clearly, N-RGO was only fitted as pyridinic-N, pyrrolic-N, graphitic-N and oxidized-N. This result suggested the introduction of Fe could promote the formation of an Fe-N configuration,48 which might be conducive to improving the ORR activity and will be investigated latter. The Fe 2p spectrum of Fe-N-CNT@RGO exhibited four peaks (Figure 3c). Among them, the peaks located at 706.7 and 720.3 eV corresponded to the zero-valence metallic Fe species, while the peaks at 711.3 and 722.1 eV corresponded to the Fe species in Fe-N configuration. 48,49 (b)

Survey

O 1s

Fe 2p

N 1s

Intensity (a.u.)

C 1s

Fe-NPyridinic N Pyrrolic N Graphitic N Oxidized N

Fe-N-CNT@RGO

Fe-N-CNT

(c)

N 1s

Intensity (a.u.)

(a) Intensity (a.u.)

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


Fe0 2p3/2 Fe 2p Satellite peak Fe3+ 2p1/2 Fe2+ 2p3/2 Fe3+ 2p3/2

Fe2+ 2p1/2

N-RGO

800

600

400

Binding energy (eV)

200

408

404

400

396

Binding energy (eV)

392 740

730

720

710

700

Binding energy (eV)

Figure 3. XPS spectra of Fe-N-CNT@RGO:(a) survey spectrum, (b) N 1s spectra and (c) Fe 2p spectrum. 3.2. Electrochemical analysis The Fe-N-CNT@RGO prepared by the pyrolysis of precursors with a mass ratio of 30% for Fe/GO exhibited the best ORR activity (Figure S5a). Figure 4a presents the electrocatalytic activities of the optimized Fe-N-CNT@RGO catalysts toward the ORR in 0.1 M KOH, which were firstly evaluated by the CV method. Compared to the N2 saturated solutions result, an evident peak at 0.81 V vs. RHE appeared in the O2-saturated electrolyte, indicating that Fe-N-


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CNT@RGO could catalyze the reduction of O2. The electrocatalytic activities of the prepared catalysts and commercial Pt/C were further evaluated by LSV by using RDE at 1600 rpm (Figure 4b). Among a series of comparative samples, the trend for the limiting current density was as follows: RGO < N-RGO < Fe-N-CNT < Fe-N-CNT@RGO. Particularly, among the prepared samples, Fe-N-CNT@RGO presented the highest limiting current density of approximately 4.43 mA cm-2 (at 0.5 V vs. RHE). The onset potential (Eonset) and half-wave potential (E1/2) of Fe-N-CNT@RGO were 0.93 V and 0.79 V vs. RHE, respectively, close to those of Pt/C (Eonset = 0.92 V, and E1/2 = 0.82 V), and this result indicated an efficient ORR activity. The Eonset and E1/2 of Fe-N-CNT@RGO were also close to those of most previously reported Fe/N/C ORR catalysts (Table S1). Fe-N-CNT, N-RGO and RGO showed poorer ORR activity than did Fe-N-CNT@RGO (Figure 4b), as confirmed by the lower Eonset and E1/2 of these samples. Furthermore, the mixtures of Fe-N-CNT and RGO prepared via ball milling, 60% Fe-N-CNT@RGO and 15% Fe-N-CNT@RGO were all exhibited lower ORR activities than that of the optimized Fe-N-CNT@RGO (Figure S5b). The above electrochemical results indicated that Fe, N and the substrates (CNT and graphene) might have contributed to a synergistic effect that improved the electrocatalytic ORR activity. The further investigation into the structure-activity relationships of the catalysts will be discussed below. By increasing the rotations speeds, Fe-N-CNT@RGO exhibited a typical increasing current, indicating that the Fe-N-CNT@RGO-catalyzed ORR was a diffusion-controlled process (Figure S5c and d). Moreover, according to the ORR polarization curves and the corresponding Koutecky-Levech (K-L) plots, Fe-N-CNT@RGO exhibited the largest kinetic current (1.9 mA


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cm-2) among those of RGO (0.1 mA cm-2), N-RGO (0.1 mA cm-2) and Fe-N-CNT (1.5 mA cm2)

(Figure 4c). Furthermore, the Tafel slope of Fe-N-CNT@RGO was 83 mV/dec, approaching

that of Pt/C (87 mV/dec) (Figure 4d). The H2O2 yield during the ORR was further evaluated by the rotating ring-disk electrode (RRDE) measurements. As shown in Figure 4e, the H2O2 yield over the Fe-N-CNT@RGO catalysts was 0 ~ 13% at 0.4 ~ 0.8 V vs. RHE, which was lower than that of Fe-N-CNT, N-RGO and RGO. Based on the RRDE results and equation (5), the electron transfer number (n) of FeN-CNT@RGO was calculated to approach 4, indicating a near four-electron ORR pathway (Figure 4e). The durability test (Figure 4f) revealed that the relative current of Fe-NCNT@RGO remained at 94% after 7200 s. Although the activity decreased, the stability of the Fe-N-CNT@RGO was superior to that of the commercial Pt/C (71%). Moreover, the durability of Fe-N-CNT@RGO before and after catalyzing 10000 cycles of the ORR was further tested by LSV in an O2-saturated alkaline solution (Figure S6). The half-wave potential Fe-NCNT@RGO exhibited only a 12 mV negative shift after 10000 cycles, clearly less than that of Pt/C (50 mV). In addition, adding 1 M methanol resulted in a slight loss in the ORR activity of Fe-N-CNT@RGO compared with that of Pt/C during the i-t test (Figure 4g). These results indicated that the high stability and reduced crossover effect of Fe-N-CNT@RGO were superior to those of Pt/C. Although Fe-N-CNT@RGO showed excellent activity towards to the ORR compared with that of RGO, N-RGO and Fe-N-CNT for ORR, the detailed reason has not yet been understood. Owing to a strong chelating interaction with iron ions,50 SCN− was used to poison the Fe sites


17


of Fe-N-CNT@RGO in acidic and alkaline electrolytes. As shown in Figure S7a, after adding 10 mM of SCN- to the acidic electrolyte, the Eonset and E1/2 of Fe-N-CNT@RGO were apparently negatively shifted, demonstrating clearly that the catalytic active sites of Fe-NCNT@RGO should include Fe cations. In the alkaline electrolyte, the Eonset and E1/2 of Fe-NCNT@RGO were barely changed after adding 10 mM SCN− (Figure S7b), which was similar to the reported results.51 It should be noted that the ORR activity of Fe-N-CNT@RGO did not reduce completely after the SCN- poisoning in the acidic electrolyte, which was probably attributed to the contribution of the pyridinic-N active site.52

0.0 -0.5

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2 1

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6000

(g)

50 0 100

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200

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elative current (%) elative current (%)

Figure 4. (a) CV curves of Fe-N-CNT@RGO in a N2 or O2-saturated 0.1 M KOH with a scan

elative current (%)

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 38

rate of 50 mV s-1. (b) LSV curves of different catalysts and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1. (c) Kinetic current density (at 0.85 V) of RGO, NRGO, Fe-N-CNT and Fe-N-CNT@RGO. (d) Tafel slope of Fe-N-CNT@RGO and Pt/C. (e) H2O2


18

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yield (black) of Fe-N-CNT@RGO, Fe-N-CNT, N-RGO and RGO and the electron transfer number (n) (red) of Fe-N-CNT@RGO (dotted line). (f) Current-time curves and (g) chronoamperometric responses to the addition of 1 M methanol for Fe-N-CNT@RGO and Pt/C. 3.3. Structure-activity relationships During the oxygen reduction process, the type and content of doped N could greatly affect the ORR performance by changing the electron density of the materials. In this case, the detailed contents of different N species and Fe in RGO, N-RGO, Fe-N-CNT and Fe-NCNT@RGO were analyzed and are listed in Table S2, while the type and content of N for the samples are shown in Figure 5a. It should be noted that although the N element contents of different catalysts are diverse and also affect the ORR performance, not all N acts as an ORR active site. According to the reported results, the pyridinic-N and Fe-N are usually considered to be the ORR active sites.38,52 Therefore, the content of pyridinic-N and Fe-N with respect to Ntotal ((pyridinic-N + Fe-N)/Ntotal) was analysed carefully to associate it with the ORR performance. Compared with that N-RGO and Fe-N-CNT, the total content of pyridinic-N and Fe-N ((pyridinic-N + Fe-N)/Ntotal) in Fe-N-CNT@RGO accounted for the highest proportion of all the nitrogen species. Accordingly, Fe-N-CNT@RGO showed the highest Eonset and Jk, which were the two important parameters for the evaluation of the ORR performance. When the ratio of (pyridinic-N + Fe-N)/Ntotal increased to a certain amount, the impact on Eonset was no longer evident (Figure 5b). Figure 5c shows the relationship between Jk and


19


(pyridinic-N + Fe-N)/Ntotal. The increase in the ratio of (pyridinic-N + Fe-N)/Ntotal when going from RGO to N-RGO was not apparently connected to their kinetic current values. The kinetic current of Fe-N-CNT increased distinctly and was higher than that of N-RGO. Further increases in the ratio of (pyridinic-N + Fe-N)/Ntotal revealed that the kinetic current of the samples also increased and that Fe-N-CNT@RGO exhibited the highest Jk among all the samples. However, it was noted that there was no clear relationship among the BET specific surface areas, onset potential and kinetic current density (Figure S8), which was consistent with many reported results.53,54 0.95

100

80 60 40

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Pyridinic N Fe-N Pyrrolic N Graphitic N Oxidized N

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Content of N (%)

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(a)

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(

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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(c)

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40

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N + Fe-N) / Ntotal (%)

0

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(Pyridinic

40

N + Fe-N) / Ntotal (%)

Figure 5. (a) Type and content of N for samples. (b) Relationship between the Eonset and (pyridinic-N + Fe-N)/Ntotal. (c) Relationship between the Jk and (pyridinic-N + Fe-N)/Ntotal. During the ORR process, O2 enters the active sites in the form of a hydrated O2 molecule, and the good surface wettability of the catalysts facilitates the reaction.55 However, an excessively hydrophilic surface probably makes the active sites flooded by water, resulting in a reduced ORR activity.56 Therefore, a mild surface wettability of a catalyst is theoretically more beneficial for enhancing the ORR activity. To quantitatively analyze the surface differences of the catalysts, the contact angle experiment was further performed. As shown in


20

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Figure 6(a), the contact angles of N-RGO, Fe-N-CNT and Fe-N-CNT@RGO were 95°, 120° and 105°, respectively. The mild surface wettability of Fe-N-CNT@RGO was attributed to the probable synergistic effect between the CNTs and graphene modulating the N distribution of Fe-N-CNT@RGO (Figure 3b and Table S2).57 A similar result in that the simultaneous introduction of CNTs and graphene could improve the surface wettability of graphene-CNT hybrids was also reported by Chen et al.58 In addition, the surface charge of the catalysts was monitored by zeta potential measurements, because this property could modulate the ORR activity by affecting the adsorption and desorption of ORR-related intermediates.59 Figure 6(b) presents the zeta potentials of RGO, N-RGO, Fe-N-CNT and Fe-N-CNT@RGO as 6.17, -4.10, -11.65 and 15.23 mV, respectively. Considering the lowest HO2- yield was over Fe-N-CNT@RGO (Figure 4e), the high positively zeta potential of Fe-N-CNT@RGO likely contributed to enhance the adsorption of the HO2- intermediates to further reduction through the electrostatic interactions present under alkaline conditions. In contrast, the negatively charged surfaces of Fe-N-CNT and N-RGO and the poor positively charged surface of RGO led to the facile desorption of HO2-, resulting in a high HO2- yield and poor 4e- ORR activity in comparison with those of Fe-N-CNT@RGO (Figure 4e). In this regard, the CNTs and graphene in Fe-NCNT@RGO could synergistically improve the surface charge of the catalyst toward enhancing the adsorption of HO2- for the 4e- ORR. It should be noted that efficient ORR catalysts with superior-to-Pt/C activity are expected to be realized upon the further optimization of the structures, densities of active sites and the catalytic surface properties of catalysts.


21


3.4. Zn-air battery analysis The efficient ORR activity of Fe-N-CNT@RGO was further stimulated to evaluate its utility in a rechargeable liquid Zn-air battery. Figure 7a shows that the open circuit voltage of Fe-NCNT@RGO was 1.44 V, which was as high as that of Pt/C. Figure 7b reveals the polarization curves and corresponding power density curves of the battery, which suggested the maximum power density of Fe-N-CNT@RGO was superior to that of Pt/C. The long-term galvanostatic discharge measurements at 5 mA cm2 indicated that voltage of Fe-N-CNT@RGO was 1.25 V, close to that of commercial Pt/C as the air cathode (Figure 7c). It can be seen from the inset of Figure 7c that two Zn-air batteries were assembled in series to power four light-emitting diodes (LEDs). The above results demonstrated that Fe-N-CNT@RGO is expected to be an alternative to Pt/C. 20

(a)

95° N-RGO 120° Fe-N-CNT 105° Fe-N-C@RGO

Zeta potential (mV)

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 22 of 38

(b)

10

0

N-RGO RGO


-10

Figure 6. (a) Contact angles of water on the surface of N-RGO, Fe-N-CNT and Fe-NCNT@RGO. (b) Zeta potential of different catalysts.


22

Page 23 of 38

1.2

Potential (V vs. Zn)

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0.8 Pt/C Fe-N-CNT@RGO

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300

600

900

1200

1500

1800

Time (min)

Figure 7. (a) Open circuit voltage curves of Fe-N-CNT@RGO and Pt/C. (b) Polarization curves and corresponding power density plots of the battery with Fe-N-CNT@RGO and Pt/C. (c) Long term galvanostatic discharge of the fabricated Zn-air batteries (d) Photograph of a series of LED powered by two liquid Zn-air batteries in series. 4. Conclusion In summary, an efficient Fe-N-CNT@RGO electrocatalyst was fabricated via a one-step calcination method using iron acetylacetonate, dicyandiamide and graphene oxide as precursors. The as-obtained Fe-N-CNT@RGO possessed a carbon matrix of CNTs and graphene, which was embedded with the active sites of iron atoms, doped N and Fe-N.


23


Page 24 of 38

Compared with that of RGO, N-RGO and Fe-N-CNT, Fe-N-CNT@RGO displayed the highest ORR activity, a superior methanol tolerance and a long-term stability in an alkaline medium. Additionally, Fe-N-CNT@RGO also presented a high charge-discharge performance and cycle durability when it was integrated into a rechargeable zinc-air battery. The high ratio of (pyridine-N + Fe-N)/Ntotal, mild surface wettability and appropriate positive zeta potential endowed Fe-N-CNT@RGO with an efficient ORR activity. These unique structural characteristics could promote the exposure of the accessible active sites, improve the surface charges and accelerate the transmission of hydrated O2, which finally enhanced the ORR activity of Fe-N-CNT@RGO. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website at https://doi.org/XXX LSV curves of Fe-N-CNT@RGO by using different counter electrode (Figure S1); XRD pattern of catalysts of Fe-N-CNT@RGO with different content of Fe and the Raman spectrum of NRGO and Fe-N-CNT (Figure S2); N2 adsorption-desorption isotherm and sizes distribution of nanoparticles for RGO, N-RGO and Fe-N-CNT (Figure S3); Sizes distribution of nanoparticles for Fe-N-CNT and Fe-N-CNT@RGO (Figure S4); Comparison of the ORR electrocatalytic performance for different Fe-N/C catalysts and Pt/C in 0.1M KOH which reported in literatures (Table S1); LSV curves of Fe-N-CNT@RGO with different content of Fe, LSV curves


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of the mixture of Fe-N-CNT and RGO prepared via ball milling and samples prepared with different mass ratio between DCDA and GO, LSV curves of Fe-N-CNT@RGO with different rotating speed and Corresponding Koutecky-Levich plots at different potential (Figure S5); ORR polarization plots of Fe-N-CNT@RGO and Pt/C before and after 10,000 potential cycles (Figure S6); Linear sweep voltammograms of Fe-N-CNT@RGO on a rotating disk electrode (1600 rpm) in 0.5 M H2SO4 (with and without NaSCN) at a scan rate of 5 mV s-1 and linear sweep voltammograms of Fe-N-CNT@RGO on a rotating disk electrode (1600 rpm) in 0.1 M KOH (with and without NaSCN) at a scan rate of 5 mV s-1 (Figure S7);The relationship among the BET specific surface areas, onset potential (Eonset) and kinetic current density (Jk) (Figure S8) (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51876080), the Strategic International Scientific and Technological Innovation Cooperation Special Funds of National Key R&D Program of China (No. 2016YFE0204000), the Program


25


Page 26 of 38

for Taishan Scholars of Shandong Province Government, the Recruitment Program of Global Young Experts (Thousand Youth Talents Plan), the Natural Science Fund of Shandong Province (ZR2017BB002), the Key R&D Program of Shandong Province (2018GSF116014) and the National Natural Science Fund (Youth Fund, 21503008).


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REFERENCES (1) Mei, H.; Yang, M.; Shen, Y.; He, F.; Zhou, Z.; Chen, X.; Yang, Y.; Liu, S.; Zhang, Y. Noncovalent Pre-organization of Molecular Precursors: A Facile Approach for Engineering Structures and Activities of Pyrolyzed Co-N-C Electrocatalysts. Carbon 2019, 144, 312320. (2) He, F.; Mi, L.; Shen, Y.; Chen, X.; Yang, Y.; Mei, H.; Liu, S.; Mori, T.; Zhang, Y. Driving Electrochemical Oxygen Reduction and Hydrazine Oxidation Reaction by Enzymeinspired Polymeric Cu(3,3'-diaminobenzidine) Catalyst. J. Mater. Chem. A 2017, 5, 1741317420. (3) Zhao, Z.; Yuan, Z.; Fang, Z.; Jian, J.; Li, J.; Yang, M.; Mo, C.; Zhang, Y.; Hu, X.; Li, P.; Wang, S.; Hong, W.; Zheng, Z.; Ouyang, G.; Chen, X.; Yu, D. In Situ Activating Strategy to Significantly Boost Oxygen Electrocatalysis of Commercial Carbon Cloth for Flexible and Rechargeable Zn-Air Batteries. Adv. Sci. 2018, 5, 1800760-1800764. (4) Lin, H.; Chen, D.; Lu, C.; Zhang, C.; Qiu, F.; Han, S.; Zhuang, X. Rational Synthesis of N/SDoped Porous Carbons as High Efficient Electrocatalysts for Oxygen Reduction Reaction and Zn-Air Batteries. Electrochim. Acta 2018, 266, 17-26. (5) Zhang, C.; Zhang, G.; Li, H.; Chang, Y.; Chang, Z.; Liu, J.; Sun, X. Interfacial Dehalogenation-enabled Hollow N-doped Carbon Network as Bifunctional Catalysts for Rechargeable Zn-air Battery. Electrochim. Acta 2017, 247, 1044-1051.


27


Page 28 of 38

(6) Quílez-Bermejo, J.; González-Gaitán, C.; Morallón, E.; Cazorla-Amorós, D. Effect of Carbonization Conditions of Polyaniline on Its Catalytic Activity towards ORR. Some Insights about the Nature of the Active Sites. Carbon 2017, 119, 62-71. (7) Zhang, X.; Yang, Z.; Lu, Z.; Wang, W. Bifunctional CoNx Embedded Graphene Electrocatalysts for OER and ORR: A Theoretical Evaluation. Carbon 2018, 130, 112-119. (8) Xiang, Q., Liu, Y., Zou, X., Hu, B., Qiang, Y., Yu, D., Yin, W., Chen. C. Hydrothermal Synthesis of a New Kind of N-doped Graphene Gel-like Hybrid as an Enhanced ORR Electrocatalyst. ACS Appl. Mater. Interfaces, 2018, 10, 10842-10850. (9) Mamtani, K.; Jain, D.; Dogu, D.; Gustin, V.; Gunduz, S.; Co, A. C.; Ozkan, U. S. Insights into Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) Active Sites for Nitrogen-doped Carbon Nanostructures (CNx) in Acidic Media. Appl. Catal. B-

Environ. 2018, 220, 88-97. (10)Khan, I. A., Qian, Y., Badshah, A., Nadeem, M. A., Zhao, D. Highly Porous Carbon Derived from MOF-5 as a Support of ORR Electrocatalysts for Fuel Cells. ACS Appl. Mater.

Interfaces 2016 8, 17268-17275. (11)Thomas, M., Illathvalappil, R., Kurungot, S., Nair, B. N., Mohamed, A. A. P., Anilkumar, G. M., T. Hareesh, U. S. Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered Precursors of Nitrogen-doped Porous Carbon for Efficient Oxygen Reduction Reaction (ORR) Electrocatalysis. ACS Appl. Mater. Interfaces 2016, 8, 29373-29382.


28

Page 29 of 38 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


(12)Yang, M., Liu, Y., Chen, H., Yang, D., Li, H. Porous N-doped Carbon Prepared from Triazine-based Polypyrrole Network: a Highly Efficient Metal-free Catalyst for Oxygen Reduction Reaction in Alkaline Electrolytes. ACS Appl. Mater. Interfaces 2016, 8, 2861528623. (13)Xiao, M.; Zhu, J.; Ma, L.; Jin, Z.; Ge, J.; Deng, X.; Hou, Y.; He, Q.; Li, J.; Jia, Q.; Mukerjee, S.; Yang, R.; Jiang, Z.; Su, D.; Liu, C.; Xing, W. Microporous Framework Induced Synthesis of Single-atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N4 Active Site Identification Revealed by X-ray Absorption Spectroscopy. ACS Catal. 2018,

8, 2824-2832. (14)Zhang, C.; Zhang, W.; Zheng, W. Transition Metal-Nitrogen-Carbon Active Site for Oxygen Reduction Electrocatalysis: Beyond the Fascinations of TM-N4. ChemCatChem 2018, 8, 2824-2826. (15)Xiao, M.; Zhang, H.; Chen, Y.; Zhu, J.; Gao, L.; Jin, Z.; Ge, J.; Jiang, Z.; Chen, S.; Liu, C.; Xing, W. Identification of Binuclear Co2N5 Active Sites for Oxygen Reduction Reaction with More than One Magnitude Higher Activity than Single Atom CoN4 Site. Nano

Energy 2018, 46, 396-403. (16)Lehmann, K.; Yurchenko, O.; Melke, J.; Fischer, A.; Urban, G. High Electrocatalytic Activity of Metal-free and Non-doped Hierarchical Carbon Nanoballs towards Oxygen Reduction Reaction. Electrochim. Acta 2018, 269, 657-667.


29


Page 30 of 38

(17)Zhang, L.; Wang, X.; Wang, R.; Hong, M. Structural Evolution from Metal–Organic Framework to Hybrids of Nitrogen-doped Porous Carbon and Carbon Nanotubes for Enhanced Oxygen Reduction Activity. Chem. Mater. 2015, 27, 7610-7618. (18)Fan, Z.; Yan, J.; Zhi, L.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M.; Qian, W.; Wei, F. A ThreeDimensional Carbon Nanotube/Graphene Sandwich and Its Application as Electrode in Supercapacitors. Adv. Mater. 2010, 22, 3723-3728. (19)Hossen, M. M.; Artyushkova, K.; Atanassov, P.; Serov, A. Synthesis and Characterization of High Performing Fe-N-C Catalyst for Oxygen Reduction Reaction (ORR) in Alkaline Exchange Membrane Fuel Cells. J. Power Sources 2018, 375, 214-221. (20)Mahmood, J.; Li, F.; Kim, C.; Choi, H.-J.; Gwon, O.; Jung, S.-M.; Seo, J.-M.; Cho, S.-J.; Ju, Y.-W.; Jeong, H. Y.; Kim, G.; Baek, J.-B. Fe@C2N: A Highly-Efficient Indirect-Contact Oxygen Reduction Catalyst. Nano Energy 2018, 44, 304-310. (21)Zhu, C.; Shi, Q.; Xu, B. Z.; Fu, S.; Wan, G.; Yang, C.; Yao, S.; Song, J.; Zhou, H.; Du, D.; Beckman, S. P.; Su, D.; Lin, Y. Hierarchically Porous M-N-C (M = Co and Fe) Single-Atom Electrocatalysts with Robust M-Nx Active Moieties Enable Enhanced ORR Performance.

Adv. Energy Mater. 2018, 8, 1801956-1801960. (22)Xu, J.; Wu, C.; Yu, Q.; Zhao, Y.; Li, X.; Guan, L. Ammonia Defective Etching and NitrogenDoping of Porous Carbon toward High Exposure of Heme-derived Fe–Nx Site for Efficient Oxygen Reduction. ACS Sustain. Chem. Eng. 2017, 6, 551-560.


30

Page 31 of 38 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


(23)Tan, H.; Tang, J.; Henzie, J.; Li, Y.; Xu, X.; Chen, T.; Wang, Z.; Wang, J.; Ide, Y.; Bando, Y.; Yamauchi, Y. Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets and Creation of Iron-Nitrogen-Doped Porous Carbon for Oxygen Reduction. ACS Nano 2018,

12, 5674-5683. (24)Yin, P.; Yao, T.; Wu, Y.; Zheng, L.; Lin, Y.; Liu, W.; Ju, H.; Zhu, J.; Hong, X.; Deng, Z.; Zhou, G.; Wei, S.; Li, Y. Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts. Angew. Chem. Int. Ed. 2016, 55, 10800-10805. (25)Huang, X.; Zhang, Y.; Shen, H.; Li, W.; Shen, T.; Ali, Z.; Tang, T.; Guo, S.; Sun, Q.; Hou, Y. N-Doped Carbon Nanosheet Networks with Favorable Active Sites Triggered by Metal Nanoparticles as Bifunctional Oxygen Electrocatalysts. ACS Energy Lett. 2018, 3, 29142920. (26)Demidova, Y. S.; Suslov, E. V.; Simakova, I. L.; Mozhajcev, E. S.; Korchagina, D. V.; Volcho, K. P.; Salakhutdinov, N. F.; Simakov, A.; Murzin, D. Y. One-pot Monoterpene Alcohol Amination over Au/ZrO2 Catalyst: Effect of the Substrate Structure. J. Catal. 2018,

360, 127-134. (27)Briega-Martos, V.; Herrero, E.; Feliu, J. M. Effect of pH and Water Structure on the Oxygen Reduction Reaction on Platinum Electrodes. Electrochim. Acta 2017, 241, 497509.


31


Page 32 of 38

(28)Cazetta, A. L.; Spessato, L.; Bedin, K. C.; Souza, I. P. F. A.; Araújo, R. A.; Martins, A. F.; Silva, T. L.; Silva, R.; Almeida, V. C. Metal-Free Ovalbumin-Derived N-S-Co-Doped Nanoporous Carbon Materials as Efficient Electrocatalysts for Oxygen Reduction Reaction. Appl. Surf. Sci. 2019, 467-468, 75-83. (29)Dai, L. Carbon-Based Catalysts for Metal-Free Electrocatalysis. Curr. Opin. Electrochem. 2017, 4, 18-25. (30)Dacheng, W. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9, 1752-1758. (31)He, Q.; Li, Q.; Khene, S.; Ren, X.; López-Suárez, F. E.; Lozano-Castelló, D.; Bueno-López, A.; Wu, G. High-Loading Cobalt Oxide Coupled with Nitrogen-Doped Graphene for Oxygen Reduction in Anion-Exchange-Membrane Alkaline Fuel Cells. J. Phys. Chem. C 2013, 117, 8697-8707. (32)Okamoto, Y. First-Principles Molecular Dynamics Simulation of O2 Reduction on Nitrogen-Doped Carbon. Appl. Surf. Sci. 2009, 256, 335-341. (33)Higgins, D. C.; Hoque, M. A.; Hassan, F.; Choi, J.-Y.; Kim, B.; Chen, Z. Oxygen Reduction on Graphene–Carbon Nanotube Composites Doped Sequentially with Nitrogen and Sulfur. ACS Catal. 2014, 4, 2734-2740.


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(34)Liu, C. D.; Li, G. L.; Cheng, G. C.; Hao, C.; Chen, S. M.; Xie, Y. Y. Enhancing Oxygen Reduction Reaction Durability via Coating Graphene Layers on the Iron-Nitrogen Supported Carbon Nanotubes. RSC Adv. 2016, 6, 73581-73588. (35)Wang, X.; Li, Q.; Pan, H.; Lin, Y.; Ke, Y.; Sheng, H.; Swihart, M. T.; Wu, G. Size-Controlled Large-Diameter and Few-Walled Carbon Nanotube Catalysts for Oxygen Reduction.

Nanoscale 2015, 7, 20290-20298. (36)Yin, H.; Zhang, C.; Liu, F.; Hou, Y. Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction. Adv. Funct. Mater. 2014,

24, 2930-2937. (37)Wang, S.; Yu, D.; He, X. A Facile Strategy to Fabricate Nitrogen-Doped Graphene AerogelSupported Fe3N Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction. New J. Chem. 2017, 41, 1755-1764. (38)Li, J.; Yu, F.; Wang, M.; Lai, Y.; Wang, H.; Lei, X.; Fang, J. Highly Dispersed Iron Nitride Nanoparticles Embedded in N Doped Carbon as a High Performance Electrocatalyst for Oxygen Reduction Reaction. Int J Hydrogen Energy 2017, 42, 2996-3005. (39)Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Alexander, S.; Zhengzong, S.; Alexander, S.; Alemany, L. B.; Wei, L.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806-4810.


33


Page 34 of 38

(40)He, F.; Chen, X.; Shen, Y.; Li, Y.; Liu, A.; Liu, S.; Mori, T.; Zhang, Y. Ionic Liquid-Derived Fe–N/C Catalysts for Highly Efficient Oxygen Reduction Reaction without any Supports, Templates, or Multi-step Pyrolysis. J. Mater. Chem. A 2016, 4, 6630-6638. (41)He, L.; Wang, G.; Wu, X.; Wen, Z.; Zhang, W. N-Doped Graphene Decorated with Fe/Fe3N/Fe4N Nanoparticles as a Highly Efficient Cathode Catalyst for Rechargeable Li−O2 Batteries. ChemElectroChem 2018, 5, 2435-2441. (42)Wang, B.; Qin, Y.; Tan, W.; Tao, Y.; Kong, Y., Smartly Designed 3D N-doped Mesoporous Graphene for High-Performance Supercapacitor Electrodes. Electrochim. Acta 2017, 241, 1-9. (43)Guo, H.-L.; Su, P.; Kang, X.; Ning, S.-K. Synthesis and Characterization of Nitrogen-Doped Graphene Hydrogels by Hydrothermal Route with Urea as Reducing-Doping Agents. J.

Mater. Chem. A 2013, 1, 2248-2255. (44)Wang, T.; Wang, L.; Wu, D.; Xia, W.; Zhao, H.; Jia, D. Hydrothermal Synthesis of Nitrogen-Doped Graphene Hydrogels Using Amino Acids with Different Acidities as Doping Agents. J. Mater. Chem. A 2014, 2, 8352-8361. (45)Dong, Y.; Li, J.; Shi, L.; Xu, J.; Wang, X.; Guo, Z.; Liu, W. Graphene Oxide–Iron Complex: Synthesis, Characterization and Visible-Light-Driven Photocatalysis. J. Mater. Chem. A 2013, 1, 644-650.


34

Page 35 of 38 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


(46)Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Mullen, K. 3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 9082-9085. (47)Wei, D.; Zidong, W.; Siguo, C.; Xueqiang, Q.; Tao, Y.; Jinsong, H.; Dong, W.; Li-Jun, W.; Shahnaz Fatima, A.; Li, L. Space-Confinement-Induced Synthesis of Pyridinic- and Pyrrolic-Nitrogen-Doped Graphene for the Catalysis of Oxygen Reduction. Angew.

Chem. Int. Ed. 2013, 52, 11755-11759. (48)Wang, M.; Yang, Y.; Liu, X.; Pu, Z.; Kou, Z.; Zhu, P.; Mu, S. The Role of Iron Nitrides in the Fe-N-C Catalysis System towards the Oxygen Reduction Reaction. Nanoscale 2017, 9, 7641-7649. (49)Wang, S.; Yan, X.; Wu, K.-H.; Chen, X.; Feng, J.-M.; Lu, P.; Cheng, H.-M.; Cheng, H.-M.; Liang, J.; Dou, S. X. A Hierarchical Porous Fe-N Impregnated Carbon-Graphene Hybrid for High-Performance Oxygen Reduction Reaction. Carbon 2019, 144, 798-804. (50)Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You, Y.; Liu, J. G.; Wu, X. L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S. G. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem.

Soc. 2014, 136, 10882-10885.


35


Page 36 of 38

(51)Zhou, X.; Bai, Z.; Wu, M.; Qiao, J.; Chen, Z. 3-Dimensional Porous N-doped Graphene Foam as a Non-Precious Catalyst for the Oxygen Reduction Reaction. J. Mater. Chem. A 2015, 3, 3343-3350. (52)Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of NitrogenDoped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts.

Science 2016, 351, 361-365. (53)Wei, Q.; Zhang, G.; Yang, X.; Chenitz, R.; Banham, D.; Yang, L.; Ye, S.; Knights, S.; Sun, S. 3D Porous Fe/N/C Spherical Nanostructures as High-Performance Electrocatalysts for Oxygen Reduction in both Alkaline and Acidic Media. ACS Appl. Mater. Interfaces 2017,

9, 36944-36954. (54)Shen, H.; Gracia-Espino, E.; Ma, J.; Zang, K.; Luo, J.; Wang, L.; Gao, S.; Mamat, X.; Hu, G.; Wagberg, T.; Guo, S. Synergistic Effects between Atomically Dispersed Fe-N-C and C-SC for the Oxygen Reduction Reaction in Acidic Media. Angew. Chem. Int. Ed. 2017, 56, 13800-13804. (55)Hao, G. P.; Sahraie, N. R.; Zhang, Q.; Krause, S.; Oschatz, M.; Bachmatiuk, A.; Strasser, P.; Kaskel, S. Hydrophilic Non-Precious Metal Nitrogen-Doped Carbon Electrocatalysts for Enhanced Efficiency in Oxygen Reduction Reaction. Chem. Commun. 2015, 51, 1728517288.


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Page 37 of 38 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


(56)Wang, Y. C.; Huang, L.; Zhang, P.; Qiu, Y. T.; Sheng, T.; Zhou, Z. Y.; Wang, G.; Liu, J. G.; Rauf, M.; Gu, Z. Q.; Wu, W. T.; Sun, S. G. Constructing a Triple-Phase Interface in Micropores to Boost Performance of Fe/N/C Catalysts for Direct Methanol Fuel Cells. ACS

Energy Letters 2017, 2, 645-650. (57)Lee, W. J.; Maiti, U. N.; Lee, J. M.; Lim, J.; Han, T. H.; Kim, S. O. Nitrogen-Doped Carbon Nanotubes and Graphene Composite Structures for Energy and Catalytic Applications.

Chem. Commun. 2014, 50, 6818-6830. (58)Dong, X.; Chen, J.; Ma, Y.; Wang, J.; Chan-Park, M. B.; Liu, X.; Wang, L.; Huang, W.; Chen, P. Superhydrophobic and Superoleophilic Hybrid Foam of Graphene and Carbon Nanotube for Selective Removal of Oils or Organic Solvents from the Surface of Water.

Chem. Commun. 2012, 48, 10660-10662. (59)Schmies, H.; Hornberger, E.; Anke, B.; Jurzinsky, T.; Nong, H. N.; Dionigi, F.; Kühl, S.; Drnec, J.; Lerch, M.; Cremers, C.; Strasser, P. Impact of Carbon Support Functionalization on the Electrochemical Stability of Pt Fuel Cell Catalysts. Chem. Mater. 2018, 30, 72877295.


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For Table of Contents Only

High ratio of (pyridine-N + Fe-N)/Ntotal, mild surface wettability and appropriate positive zeta potential endowed Fe-N-CNT@RGO with an efficient ORR activity.


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