FeCo Alloy Nanoparticles Coated by an Ultrathin N-Doped Carbon

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FeCo Alloy Nanoparticles Coated by Ultrathin N-doped Carbon Layer and Encapsulated in Carbon Nanotubes as Highly Efficient Bifunctional Air Electrode for Rechargeable Zn-air Batteries Shasha Li, WEIHENG CHEN, Hongzhou Pan, Yuwei Cao, Zhongqing Jiang, Xiaoning Tian, Xiaogang Hao, Thandavarayan Maiyalagan, and Zhong-Jie Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00307 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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ACS Sustainable Chemistry & Engineering

FeCo Alloy Nanoparticles Coated by Ultrathin N-doped Carbon Layer and Encapsulated in Carbon Nanotubes as Highly Efficient Bifunctional Air Electrode for Rechargeable Zn-air Batteries Shasha Li†,§, Weiheng Chen§, Hongzhou Pan‡, Yuwei Cao‡, Zhongqing Jiang‡,§,*, Xiaoning Tian§, Xiaogang Hao†,*, Thandavarayan Maiyalagan¶, Zhong-Jie Jiang£,* † Department

of Chemical Engineering, Taiyuan University of Technology, 79 Yingze

West Main Street, Taiyuan, 030024, China. ‡

Key Laboratory of Optical Field Manipulation of Zhejiang Province, Department of

Physics, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou, 310018, China. §

Department of Materials and Chemical Engineering, Ningbo University of

Technology, 201 Fenghua Road, Jiangbei District, Ningbo, 315211, China. ¶

Electrochemical Energy Laboratory, Department of Chemistry, SRM Institute of

Science and Technology, SRM Nagar, Kattankulathur, 603203, India. £

Guangdong Engineering and Technology Research Center for Surface Chemistry of

Energy Materials & Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, College of Environment and Energy, South China University of Technology, 382 Outer Ring Road, Guangzhou University City, Panyu District, Guangzhou, 510006, China. *Corresponding authors E-mail: [email protected] or [email protected] (Z. Jiang). E-mail: [email protected] (X. Hao). E-mail: [email protected] (Z.-J. Jiang).

1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

ABSTRACT Development of inexpensive, efficient and stable non-precious-metal-based bifunctional catalysts for oxygen reduction (ORR) and evolution (OER) reactions remains an enormous challenge. This work reports an excellent bifunctional electrocatalyst consisting of ultrathin N-doped carbon (1-3 graphitic carbon layers) coated Fe1.2Co nanoparticles and N-doped carbon nanotubes (Fe1.2Co@NC/NCNTs). The Fe1.2Co@NC/NCNTs has an extremely low Fe/Co content (6.7 wt.%), but with highly efficient and durable bifunctionality for ORR and OER. Specifically, the Fe1.2Co@NC/NCNTs exhibits onset potential (Eonset=0.842 V vs. RHE) and half-wave potential (E1/2= 0.82 V vs. RHE) for ORR and onset potential of 1.43 V vs. RHE and overpotential of 355 mV at 10 mA cm-2 for OER. The potential gap (△E) between E1/2 of ORR and EOER at 10 mA cm-2 (E j = 10) for the Fe1.2Co@NC/NCNTs is 0.765 V, which surpasses the commercial Pt/C and Ir/C catalysts and most state-of-art bifunctional catalysts previously reported. Most notably, when used in the Zn-air battery, the Fe1.2Co@NC/NCNTs exhibits superior efficiency and durability to the Pt-Ir/C catalysts. This strongly suggests that the Fe1.2Co@NC/NCNTs can be used as an efficient bifunctional catalyst with potential applications in the field of clean electrochemical energy storage and conversion technologies. Keywords: N-doped carbon nanotubes, FeCo alloy, Bifunctional electrocatalyst, Zn-air batteries, all-solid-state


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INTRODUCTION With growing environmental awareness and rising cost of using fossil fuels, it is undoubted that the development of the clean, sustainable and renewable new energy technologies is very urgent for the sustainable development of human beings. Direct electrochemical conversion between oxygen and water has attracted great attention, due to its vital role in fuel cells (FCs) and metal-air batteries (MABs).1-3 Especially, oxygen

reduction

(ORR)

and

evolution

(OER)

reactions

are

paramount

electrochemical processes, whose sluggish kinetics and large overpotentials have considered as the main bottlenecks hindering the commercial applications of MABs and FCs in the real-world devices.3-9 Presently, precious-metal based catalysts, such as platinum (Pt) and its alloys, are the state-of-art catalysts for ORR, but they exhibit low OER activities; Iridium (Ir) and ruthenium (Ru) oxide-based catalysts have extraordinary OER activities, but they possess low ORR activities.3,

7, 10

These

catalysts are also confronted with the problems of high cost, scarcity, and poor stability, greatly limiting the comprehensive applications of these precious metal based catalysts in ORR and OER.7, 11-12 To improve the performance of MABs and FCs and promote their widespread uses, it is necessary to develop efficient and inexpensive bifunctional electrocatalysts for both ORR and OER, although it remains a huge challenge since the catalysts efficient for ORR are often unfavorable to OER and vice versa. Recent efforts have been devoted to the search of non-noble-metal based ORR/OER bifunctional electrocatalysts. Earth-abundant three-dimensional (3d) transition metals (TMs), such as Fe, Co, and Ni, have been widely used as the catalysts for ORR and OER in alkaline environments.13-16 Especially, when these metals are integrated to form intermetallic alloys, such as FeCo,16-21 NiCo,14,

22

FeNi,23-25 superb catalytic activities for ORR and OER are expectable because alloying allows for integration of functionalities of individual metals and provides a possibility to produce new functionalities through intermetallic interactions.11, 14, 21, 26 3 ACS Paragon Plus Environment


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However, the alloyed metal nanoparticles are still facing the problems of poor stability and significant self-aggregation, particularly in the harsh electrochemical environments, when they are subjected to ORR and OER. Strategies to tackle these problems are therefore required to promote the uses of the alloyed metal nanoparticles in the catalytic ORR and OER applications. The carbon shell coating is considered as a promising approach to improve the bifunctional electrocatalytic activities and stability of the alloyed metal nanoparticles, because coating can protect the alloyed metal nanoparticles from degradation and aggregation by keeping them from the direct contact with the electrolyte solutions.19-22,

27

Especially when there are

heteroatoms in the carbon shell, the differences in the electronegativity between carbon atom and heteroatom will alter the charge distribution and electronic properties of the carbon shell, providing a stronger interaction between metal core and heteroatom doped carbon shell and thereby higher catalytic activities.5,

11, 28-30

The

work by Bao et al. has proposed that to achieve high catalytic activities the coating with an ultrathin carbon shell (90%). The select area electron diffraction (SAED) inset of Figure 1c shows the rings and scattering dots, which are assignable to the diffraction from graphitic carbon shell and the crystalline nanoparticles, respectively.24, 36-37 Control experiments show that when the same reaction procedure was conducted for the synthesis of the Fe@NC/NCNTs and the Co@NC/NCNTs (Figure S1) using the single metal sources the Fe and Co nanoparticles with larger sizes are obtained. This result strongly suggests the great importance of the fabrication of the Fe1.2Co@NC/NCNTs with small sized Fe1.2Co nanoparticles using the metal source with the co-existence of CoPc and FePc. Most notably, the Fe1.2Co nanoparticles in the Fe1.2Co@NC/NCNTs exhibit a good resistance to the acid etching. No obvious changes in the morphology and size of the Fe1.2Co nanoparticles are observed when the Fe1.2Co@NC/NCNTs is soaked in a 0.5 M H2SO4 solution. The high resistance of the Fe1.2Co nanoparticles towards the acid etching could be attributed to their specifical NC coated structure, which is well isolate them from the direct contact with protons.


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Figure 1. (a) SEM image of Fe1.2Co@NC/NCNTs. The Fe1.2Co@NC nanoparticles and the NCNTs are indicated by the red and blue arrows, respectively. (b, c) TEM and (d) HRTEM images of the Fe1.2Co@NC/NCNTs. The inset in (c) is SAED of the Fe1.2Co@NC/NCNTs. (e) Elemental mapping images of the Fe1.2Co@NC/NCNTs.

Intensity (a. u.)

Fe1.2Co@NC/NCNTs

O 1s O 1s N 1s

Co 2p

O 1s N 1s

Fe 2p

Co 2p Fe 2p O 1s N 1s

1000

800

600

400

200

0

Binding energy (eV)

CNTs O=C-O









Fe1.2Co@NC/NCNTs Co-ionic state

Sat.

(e)

C-N/C-O

sp2 C-C or C-H

Fe@NC/NCNTs C-N/C-O

sp2 C-C or C-H

C=O

292

O=C-O/ C-N/C-O O=C-N  C=O

290

288

286

284

282

pyridinic-N

graphtic-N

FeNx

oxidized-N

Fe1.2Co@NC/NCNTs

736 732 728 724 720 716 712 708 704

Binding energy (eV)

(f)

CNTs

H2O(adsorbed)/C=O O-C-O/N-O

Adsorbed O2

Co@NC/NCNTs OLattice

402

400

398

396

Fe@NC/NCNTs

H2O(adsorbed)/C=O

Adsorbed O2

O-C-O/N-O

OLattice

Fe1.2Co@NC/NCNTs H2O(adsorbed)/C=O

Fe(Co)Nx

Binding energy (eV)

Adsorbed O2

O-C-O/N-O

pyridinic-N

pyrrolic-N graphtic-N

404

Fe0

Fe-Nx

H2O(adsorbed)/C=O

pyrrolic-N

406

Fe-Nx

Fe-ionic Fe-ionic Fe-N x state state Fe0 Sat. Sat.

Co0

Fe@NC/NCNTs

408

Fe2p

Fe0

CoNx

oxidized-N

280

Fe-ionic state Fe Sat. 0

Fe1.2Co@NC/NCNTs

oxidized-N

sp2 C-C or C-H

Binding energy (eV)

Co-Nx

Co@NC/NCNTs

graphtic-N





Co-ionic state Co-Nx Sat. Co0

808 804 800 796 792 788 784 780 776 772

Fe1.2Co@NC/NCNTs 

Co0

Fe 2p3/2

Fe 2p1/2

Fe-ionic Fe-N x state Sat.

pyridinic-N

C=O



Co-Nx

Fe@NC/NCNTs

pyrrolic-N

Co@NC/NCNTs

O=C-O/ O=C-N

Co-Nx Co0

sp2 C-C or C-H

C-O





Co-ionic state

C=O

O=C-O/ O=C-N 

Sat.

Co-ionic state Sat.

Binding energy (eV)

(d) 

Co 2p1/2

(c)

Intensity (a. u.)

Intensity (a. u.)

Fe@NC/NCNTs

Co 2p3/2 Co2p

Co@NC/NCNTs

C 1s

Intensity (a. u.)

(b)

CNTs Co@NC/NCNTs

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


Adsorbed O2

O-C-O/N-O

394


540

537

OLattice

534

531

528

Binding energy (eV)

525


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Figure 2. (a) XPS survey spectra of the samples; (b) High-resolution Co 2p spectra of Co@NC/NCNTs and Fe1.2Co@NC/NCNTs; (c) High-resolution Fe 2p spectra of Fe@NC/NCNTs and Fe1.2Co@NC/NCNTs; (d) C 1s, (e) N 1s and (f) O 1s spectra of the samples. The

elemental

composition

and

chemical

environments

of

the

Fe1.2Co@NC/NCNTs are examined by XPS. Figure 2a shows that the Fe1.2Co@NC/NCNTs comprises C, N, O, Fe and Co. This is well consistent with the EDS mappings of the Fe1.2Co@NC/NCNTs in Figure 1e, which show homogeneous distributions of C and N and scattered distributions of Fe and Co in the Fe1.2Co@NC/NCNTs. The quantitative analysis shows that the contents of Fe and Co in the Fe1.2Co@NC/NCNTs is 1.13 and 0.95 (Table S1), respectively, further confirming that the molar ratio of Fe:Co in the Fe1.2Co@NC/NCNTs was ~1.2:1. The high-resolution XPS spectrum of Co 2p3/2 in Figure 2b exhibits the distinct peaks at 779.8 eV, 781.5 eV,38-40 783.6 eV and 787.6 eV ascribable to zero-valence cobalt, Co-Nx, the oxidation state Co of 2+ and the satellite peak, respectively.41-43 Likewise, the peak corresponding to Co 2p1/2 can be also deconvoluted with four components, corresponding to Co0, Co-Nx, Co2+ and the satellite peak, respectively. The presence of Co-Nx suggests an interaction between Co and nitrogen. This also demonstrates that the carbon shell of the Fe1.2Co nanoparticles have a nitrogen doped structure. Similarly, the deconvolution of the Fe 2p peak in Figure 2c shows the peaks corresponding to metallic Fe (710.2 eV for Fe 2p3/2, and 721.9 eV for Fe 2p1/2) and Fe-Nx (712.4 eV for Fe 2p3/2, and 723.8 eV for Fe 2p1/2),33, 42, 44 demonstrating that the homogeneous distribution of the Fe-Nx active sites on all surface area of the Fe1.2Co@NC/NCNTs catalyst. More interesting is that the binding situation of Fe(Co)-Nx species inside Fe1.2Co@NC/NCNTs shifts to a higher binding energy than those of Fe@NC/NCNTs and Co@NC/NCNTs, implying less charge density on central Fe or Co atoms. This detail is a clear indication of the interaction between the Fe1.2Co alloy species and the N components.21, 29 Figure 2d shows the C 1s spectrum for the Fe1.2Co@NC/NCNTs can be deconvoluted with five components, 8 ACS Paragon Plus Environment

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corresponding to the graphitic C, C–N/C–O, C=O, O–C=O/N–C=O and the π–π* shakeup satellite peak, respectively.10-11, 45 The most prominent peak corresponding to graphitic C in the fitted C 1s spectrum implies that the presence of the graphitic materials in the Fe1.2Co@NC/NCNTs, consistent with the fact that there exist the CNTs in the Fe1.2Co@NC/NCNTs. The deconvolution of the N 1s spectra (Figure 2e) shows the peaks corresponding to the pyridinic N (398.7 eV), Co (Fe)-Nx (399.7 eV), pyrrolic N(400.8 eV), graphitic N(401.7 eV), and oxidized N(403.3 eV), respectively.11,

32, 46-47

Co (Fe)-Nx could be assigned to four nitrogen atoms

coordinated to an iron or cobalt atom, as displayed in the Scheme 1. The presence of the peak ascribable to Co (Fe)-Nx further supports that the Fe1.2Co nanoparticles in the Fe1.2Co@NC/NCNTs are coated with nitrogen doped carbon, which is formed from the pyrolysis reaction of melamine and the FePc/CoPc. The homogenous distribution of N in Fe1.2Co@NC/NCNTs, as indicated in Figure 1e, suggests a uniform doping in the carbon substrate. It is worth noting that the Co (Fe)-Nx species are considered as the dominant electrocatalytic active centers in the ORR reaction. The Fe@NC/NCNTs and Co@NC/NCNTs exhibit the XPS spectra very comparable to those of the Fe1.2Co@NC/NCNTs, except for the absence of the peaks corresponding to Co in the Fe@NC/NCNTs and Fe in the Co@NC/NCNTs. Table S2 summarizes of the deconvoluted results of the C 1s, N 1s, and O1s peaks for the Fe@NC/NCNTs, the Co@NC/NCNTs and the Fe1.2Co@NC/NCNTs. As shown in Figs. 2d-f and Table S2, the deconvoluted C 1s and N 1s spectra for the Fe@NC/NCNTs and the Co@NC/NCNTs

show

the

similar

components

with

those

of

the

Fe1.2Co@NC/NCNTs, indicating that the Fe@NC/NCNTs, the Co@NC/NCNTs and the Fe1.2Co@NC/NCNTs have a very similar structure. It is also confirmable by their FTIR spectra in Figure 3a, which shows a good similarity in their spectra profiles. Worth noting is that the Fe1.2Co@NC/NCNTs contains higher contents of oxygen atoms in comparison to the CNTs, the Fe@NC/NCNTs and Co@NC/NCNTs, as revealed by the XPS survey spectra shown in Figure 2a. This excess oxygen could be attributed to the adsorbed O2 on the surface of the Fe1.2Co@NC/NCNTs (Figure 2f 9 ACS Paragon Plus Environment


and Table S2), suggesting that the introduction of bimetallic of Fe1.2Co would increase the adsorption of oxygen to the surface of the Fe1.2Co@NC in comparison to the monometallic Fe@NC and Co@NC. The high adsorption of oxygen on Fe1.2Co@NC/NCNTs will greatly contribute to its potential application as an electrocatalyst for ORR, because the adsorption of oxygen on the surface of electrocatalyst has been recognized as an important step during the ORR.7, 48-50

Wavenumbers (cm-1)

ID/IG=1.14

80 60

CNTs Co@NC/NCNTs Fe@NC/NCNTs Fe1.2Co@NC/NCNTs

40 20

0

200

400

o

600

Temperature ( C)

800

1000

1500

2000

-2

2500

Raman shift (cm )

Quantity Adsorbed (cm3 g-1 STP)

100

Fe1.2Co@NC/NCNTs

20 15 10 5

CNTs

C (101)

FeCo Alloy JCPDS:65-4131 10

20

30

(f)

CNTs (83.7 m2 g-1) Co@NC/NCNTs (132.8 m2 g-1) Fe@NC/NCNTs (117.1 m2 g-1) Fe1.2Co@NC/NCNTs (181.0 m2 g-1)

25

Fe1.2Co@NC/NCNTs

(200)

Fe@NC/NCNTs ID/IG=0.94

(e)

(d)

0

ID/IG=1.08

500

3500 3000 2500 2000 1500 1000 500

Co@NC/NCNTs

(110)

Fe1.2Co@NC/NCNTs

CNTs

ID/IG=0.84

Intensity (a.u.)

Fe@NC/NCNTs

C (002)

Co@NC/NCNTs

40

50

60

70

2 Theta (o)

80

CNTs Co@NC/NCNTs Fe@NC/NCNTs Fe1.2Co@NC/NCNTs

dV(d)(cm3nm-1g-1)

Intensity (a. u.)

(c)

(b)

CNTs

Intensity (a. u.)

(a)

Mass fracture (%)

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 10 of 33

0 0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/Po)

1.0

0

5

10

15

Pore Width (nm)

20

Figure 3. (a) FT-IR spectra, (b) Raman spectra, (c) XRD patterns, (d) TGA curves, (e) Nitrogen adsorption/desorption isotherms and (f) pore-size distributions of the samples. Figure 3b shows that the Raman spectrum of the Fe1.2Co@NC/NCNTs exhibits two prominent peaks at ∼1338 and ∼1569 cm−1, which are the characteristic vibrations of the D and G band of graphitic structure, respectively.43, 51 However, due to the low loading of Fe1.2Co nanoparticles and the low sensitivity to Raman rays, the corresponding characteristic peaks are not discernable. It is worth noting that the G band of the Fe1.2Co@NC/NCNTs exhibits a shift to the low wavenumber, in comparison with that of the pure CNTs. The electron coupling between Fe1.2Co nanoparticles and the NCNTs leads to the extended delocalization of π electrons, eventually causing spectrum shift.52-53 The intensity ratio of D and G bands (ID/IG) is 10 ACS Paragon Plus Environment

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often used to evaluate the fraction of the defects in the graphitic materials

53-54,

since

the D band at 1336 cm−1 is related to the defects and amorphous carbon atoms, while G band at 1568 cm−1 is related to the tangential oscillation and vibrations in all sp2 hybrids of graphitic carbon.37, 51, 53, 55 The ID/IG ratio of the Fe1.2Co@NC/NCNTs is 1.14, which shows a significant increase in comparison to that of the CNTs (ID/IG=0.84). This might be due to the reason that the growth of the Fe1.2Co nanoparticles and the doping of N bring more structural defects or microstructural rearrangement to the CNTs.45-46,

56

Similar results can be obtained from the

Co@NC/NCNTs and Fe@NC/NCNTs as shown in Figure 3b. The XRD pattern in Figure 3c indicates that the Fe1.2Co@NC/NCNTs exhibits distinct peaks at 25.8° and 42o, corresponding to the (002) and (101) planes of the CNTs,

11, 57

except these two peaks, the Fe1.2Co@NC/NCNTs also exhibit the

relatively small peaks at 2θ of 44.8o and 65.3o, which can be assigned to the (110) and (200) planes of the Fe1.2Co nanoparticles (JCPDS# 65-4131), respectively.16, 31-32 The relatively low intensities of these peaks evidence the low content of metal nanoparticles in the Fe1.2Co@NC/NCNTs. It is well consistent with their TEM images shown above, which show that the Fe1.2Co nanoparticles are coated by ultrathin N-doped carbon layer. Indeed, the low metal content of the Fe1.2Co@NC/NCNTs can be further demonstrated by their TGA analysis. As shown in Figure 3d, the weight percentage of Fe1.2Co nanoparticles in the Fe1.2Co@NC/NCNTs are extremely low (~ 6.7 wt.%). Likewise, the Fe@NC/NCNTs and the Co@NC/NCNTs (XRD patterns are shown in Figure S2) synthesized in a similar way also exhibit low metal contents of ~ 7.4 wt.% and ~ 6.1 wt.%, respectively. The texture property of the Fe1.2Co@NC/NCNTs is analyzed by their N2 adsorption/desorption isotherms. Figure 3e indicates the N2 adsorption/desorption isotherms of the Fe1.2Co@NC/NCNTs display a type IV adsorption–desorption behavior with a H3-type hysteresis loop. The specific surface area (SSA) of the Fe1.2Co@NC/NCNTs estimated using the multi-point Brunauer–Emmett–Teller (BET) method is 181.0 m2 g-1, which is larger than those of the CNTs (83.7 m2 g-1), 11 ACS Paragon Plus Environment


Fe@NC/NCNTs (117.1 m2 g-1) and the Co@NC/NCNTs (132.8 m2 g-1). The higher SSA indicates that the Fe1.2Co@NC/NCNTs can expose more active sites accessible to the catalytic reactions, which is beneficial to improve their bifunctional catalytic performance for ORR and OER.46, 48-49 Figure 3f displays the pore size distribution plot of the Fe1.2Co@/NC/NCNTs, which shows that the Fe1.2Co@NC/NCNTs exhibit a strong and narrow pore distribution peaks centered at 1.4 nm and 3.4 nm. It suggests the co-existence of micro/meso-pores in this sample. These hierarchical pores are very inclined to the adsorption and transportation of O2, improving the ORR and OER activities of the Fe1.2Co@NC/NCNTs.31,

42

Although the Co@NC/NCNTs and the

Fe@NC/NCNTs also exhibit the strong and narrow pore distribution peaks at 1.4 nm and 3.4 nm, their intensities are relatively weaker. This might be the reason that the Fe1.2Co@/NC/NCNTs exhibit higher SSA than those of the Co@NC/NCNTs and the Fe@NC/NCNTs. The higher SSA and the smaller size of the Fe1.2Co nanoparticles are helpful in improving the availability of Fe1.2Co@/NC/NCNTs to the catalytic reactions and thereby boost the ORR and OER activities.

Co@NC/NCNTs Fe@NC/NCNTs Fe1.2Co@C/CNTs Fe1.2Co@NC/NCB Fe1.2Co@NC/NCNTs

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Potential (V vs. RHE)

(b) 0 -1 -2

(c)1.0

CNTs NC/NCNTs Co@NC/NCNTs Fe@NC/NCNTs Fe1.2Co@C/CNTs Fe1.2Co@NC/NCB Fe1.2Co@NC/NCNTs 20 wt.% Pt/C


CNTs NC/NCNTs

Current Density (mA cm-2)

(a) Current Density (mA cm-2)

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 33

0.9

-3

0.8

-4

CNTs (75 mV dec-1) NC/NCNTs (68 mV dec-1) Co@NC/NCNTs (59 mV dec-1) Fe@NC/NCNTs (64 mV dec-1) Fe1.2Co@C/CNTs (62 mV dec-1) Fe1.2Co@NC/NCB (56 mV dec-1) Fe1.2Co@NC/NCNTs (52 mV dec-1) 20 wt.% Pt/C (61 mV dec-1)

0.7

-5 -6 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1



0.6

-0.9

-0.6

-0.3

0.0

0.3

Log |j (mA cm-2)|

0.6

-2

0.25 0.20

0.2

0.15 0.02

0.03



-3

-0.5

0.04

-0.5

(rpm

0.05 )

400rpm 800rpm 1200rpm 1600rpm 2000rpm 2400rpm

-4 -5 -6 -7 0.2

0.4

0.6

0.8



0.00

-4

(h)

1.80

60 40 20 0

(j)


100

1.75

CNTs NC/NCNTs Co@NC/NCNTs Fe@NC/NCNTs Fe1.2Co@C/CNTs Fe1.2Co@NC/NCB Fe1.2Co@NC/NCNTs 20 wt.% Ir/C

1.3

1.4

1.5

1.70 1.65

1.6

1.7

E E1/2

0.8

1.8

1.55

Fe1.2Co@NC/NCB (69 mV dec-1) Fe1.2Co@NC/NCNTs (57 mV dec-1) 20 wt.% Ir/C (72 mV dec-1)

0.0

0.5

1.0

NC/NCNTs

CNTs

100 80 60 40 20 0 4

Fe@NC/NCNTs

Fe1.2Co@C/CNTs

Fe1.2Co@NC/NCNTs

Co@NC/NCNTs Fe1.2Co@NC/NCB

20 wt.% Pt/C

3 2

CNTs Fe@NC/NCNTs

1 0

NC/NCNTs Fe1.2Co@C/CNTs

Fe1.2Co@NC/NCNTs

0.2

Co@NC/NCNTs Fe1.2Co@NC/NCB

20 wt.% Pt/C

0.3 0.4 0.5 0.6 Potential (V vs. RHE)

0.7

(i)20

CNTs (109 mV dec-1) NC/NCNTs (95 mv dec-1) Co@NC/NCNTs (77 mV dec-1) Fe@NC/NCNTs (64 mV dec-1) Fe1.2Co@C/CNTs (66 mV dec-1))

1.60


1.0


-2

(f)

-6 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Potential (V vs. RHE)

1.0

(g)

80

CNTs NC/NCNTs Co@NC/NCNTs Fe@NC/NCNTs Fe1.2Co@C/CNTs Fe1.2Co@N/C/NCB Fe1.2Co@NC/NCNTs 20 wt.% Pt/C

0.4

H2O2 (%)

0.30

n

0 -1

(e)0.6

0.35


1

j-1 (mA-1 cm2)


(d) 2

Potential (V)

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 13 of 33

1.5

Log |j (mA cm-2)|

2.0

15 E=0.765V

10 0

E (H2O/O2)

5 0 -5

-10 0.4

0.6

0.8

1.0

1.2

1.4


1.6

1.8

OER E10 1.4

This Work

1.2

*

1.0

0.6

0.8 0.6

0.4

0.4 0.2

0.2

0.0

/ 2 w 0 PC O 4 Ss O Ts Ts -1 50 ed 00 Ts lo oS Ts -C O 4 PFC Ts N 45 N C H G M 3 -x ) s -7 N -7 ol 2 N N ) M dd N C C -r xC /N C C o/ /h e/ o( G Co T /N C N BG @ /C be N /N /N O4 4 N nO -N N C O 4 (H Ni NiC oF @ / 4 m / / @ H ixC -CN NC O n 3 e G e e /C O o 2 o 4 C M o N D N e2 O C xne iF C o3 soZ C oF eN –N o– o3 N N C oF C H2 @ C C eL he C Fe Fe o@ C M C i 3F N 2Co Fe iF @ ap C N 1. gr Fe nN Fe o,

N

-C

N

F

/3

0.0

D

Figure 4. (a) CV curves in O2 (solid) and N2 (dash) saturated 0.1 M KOH solution at a scan rate of 10 mV s-1. (b) LSV curves in the O2-saturated 0.1 M KOH solution at the rotation rate of 1600 rpm. (c) IR-corrected Tafel plots. (d) LSVs of the Fe1.2Co@NC/NCNTs in the O2 saturated 0.1 M KOH at different rotation rates. The inset in (d) is corresponding K-L plots of the Fe1.2Co@NC/NCNTs. (e) RRDE of LSV curves in O2-saturated 0.1 M KOH at 1600 rpm (f) Peroxide yield (top panel) and electron transfer number (n, bottom panel) obtained based on the corresponding RRDE data in (e). (g) LSVs of the OER in 0.1 M KOH. (h) Tafel plots for the OER. (i) Overall polarization curve of the Fe1.2Co@NC/NCNTs in the entire ORR and OER region. (j) Comparison of the ORR half-potential, OER overpotential, and △E of the Fe1.2Co@NC/NCNTs with those of catalysts reported.



To evaluate the electrocatalytic activities of the Fe1.2Co@NC/NCNTs for ORR, the CV was measured in O2 or N2 saturated 0.1 M KOH. Figure 4a shows that the CV curve of the Fe1.2Co@NC/NCNTs in the N2 saturated KOH solution displays virtually featureless voltammetric currents. Conversely, in the O2 saturated solution, a well-defined cathodic peak located at 0.815 V, corresponding to the reduction of oxygen, could be observed. It indicates that the Fe1.2Co@NC/NCNTs can be used as the catalysts for ORR. To gain insight into the ORR by the Fe1.2Co@NC/NCNTs, RDE measurement was conducted. For comparison, the catalytic performance of the commercial 20 wt.% Pt/C was also studied. Figure 4b shows that the ORR onset (Eonset) and half-wave potentials (E1/2) of the Fe1.2Co@NC/NCNTs are ~ 0.842 and ~ 0.82 V, respectively, which are close to the commercial Pt/C (Eonset and E1/2 of the Pt/C are 0.964 and 0.858 V, respectively, as shown in Figure 4b). In addition, the catalytic current density of Fe1.2Co@NC/NCNTs is even higher than that of the Pt/C at the potential below 0.8 V, suggesting that the Fe1.2Co@NC/NCNTs is more kinetically facile toward ORR than the Pt/C. More importantly, in comparison with the NC/NCNTs and the pure CNTs, the Fe1.2Co@NC/NCNTs displays more pronounced ORR peak, more positive Eonset and E1/2, and higher current density. This suggests that the presence of Fe1.2Co@NC, even at the extremely low content, plays a major contribution on the high catalytic activity of the Fe1.2Co@NC/NCNTs. Most notably, the Fe1.2Co@NC/NCNTs exhibits higher catalytic activities for ORR than the Co@NC/NCNTs and the Fe@NC/NCNTs. Since the FeCo nanoparticles only exhibit a slightly lower particle size than the Fe and Co nanoparticles, we will attribute the alloying of the FeCo nanoparticles the main reason resulting in the higher catalytic performance of the Fe1.2Co@NC/NCNTs. Worthnoting is that the catalytic activity of the Fe1.2Co@NC/NCNTs is also higher than that of the Fe1.2Co@C/CNTs (Figure 4b) without N doping. This strongly suggests that Co-Nx and Fe-Nx sites in the Fe1.2Co@NC/NCNTs are responsible for their higher ORR activity. Meanwhile, it is interesting to note that the Fe1.2Co@NC/NCB showed relatively inferior ORR activities compared with Fe1.2Co@NC/NCNTs, indicating that the better performance 14 ACS Paragon Plus Environment

Page 14 of 33

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also comes from the NCNTs, which provides an interconnected conductivity network and easy transfer of electrons during the ORR.

3, 24

All these observations shown

above strongly indicate that the high catalytic activity of the Fe1.2Co@NC/NCNTs is a consequence of the coexistence of nitrogen doped carbon shell, Fe1.2Co alloy nanoparticles, and interconnected conductivity networks of NCNTs. Worthnoting is that the catalytic activity of the Fe1.2Co@NC/NCNTs is close to that of the CoFe/N−C with relatively higher metal content (~8.0 wt.%), which were synthesized by higher temperature calcination of Fe and Co salts in the presence of Vulcan XC-72 carbon black and urea.25 To obtain information on the oxygen adsorption mechanism and oxygen reduction kinetic of the Fe1.2Co@NC/NCNTs, their diffusion-corrected Tafel curve (η = b log j + a, where η is overpotential, j is current density, and b is Tafel slope) was further plotted. Figure 4c shows that the ORR by the Fe1.2Co@NC/NCNTs exhibits a Tafel slope of 52 mV dec-1 at the low current density region, which is much lower than those of the ORR on the pure CNTs (75 mV dec-1), the NC/NCNTs (68 mV dec-1), the Co@NC/NCNTs (59 mV dec-1), the Fe@NC/NCNTs (64 mV dec-1), the Fe1.2Co@C/CNTs (62 mV dec-1), the Fe1.2Co@NC/NCB (56 mV dec-1), and the Pt/C (61 mV dec-1). The lower Tafel slope implies faster kinetics, indicating that the Fe1.2Co@NC/NCNTs can quickly reach a higher catalytic current density under lower applied potential. This result is in good agreement with the LSV result in Figure 4b, which shows that the Fe1.2Co@NC/NCNTs catalyst has a higher limiting diffusion current. The ORR catalytic activity and reaction kinetics of the Fe1.2Co@NC/NCNTs and the contrast samples are analyzed by the LSVs recorded at rotation speeds from 400 rpm to 2400 rpm (Figure 4d and S3). The Koutecky-Levich (K-L) plots corresponding inset of Figure 4d shows linear relationships between J–1 and ω–0.5, signifying that the ORR catalyzed by the Fe1.2Co@NC/NCNTs catalysts follows first-order reaction kinetics, with regard to the concentration of the dissolved O2. The slopes of the K-L plots in Figure 4d calculated that the electron transfer number is >3.9, indicating that the Fe1.2Co@NC/NCNTs during the ORR proceeds via a 15 ACS Paragon Plus Environment


Page 16 of 33

favorable 4e pathway. The RRDE technique is employed to further analysed the electron transfer number (n) with simultaneous to obtain the HO2- yield. Figure 4e shows that the ORR LSV curves of the Fe1.2Co@NC/NCNTs obtained from the RRDE technique exhibits a profile similar with that of the RDE technique. Based on the ring and disk currents, the electron transfer number (n) is estimated to be larger than 3.95 and the HO2- yield is lower than 4.0% in the potential from 0.2 to 0.7 V (Figure 4f). The both values are close to those of the Pt/C, further demonstrating that the ORR by the Fe1.2Co@NC/NCNTs follows a 4e pathway.

Table 1.

Bifunctional electrocatalytic activities of different catalysts for ORR and

OER.

Catalyst

EORR/V vs. RHE

EOER/V vs.

ΔE/V vs.

Half-wave

RHE

RHE

potential

at 10 mA

(EORR-EOER)

cm-2 CNTs

0.687

1.722

1.035

NC/NCNTs

0.707

1.643

0.936

Co@NC/NCNTs

0.81

1.636

0.826

Fe@NC/NCNTs

0.808

1.623

0.815

Fe1.2Co@C/CNTs

0.797

1.616

0.819

Fe1.2Co@NC/NCB

0.812

1.639

0.827

Fe1.2Co@NC/NCNTs

0.82

1.585

0.765

The OER activity the Fe1.2Co@NC/NCNT is also assessed by LSV curves measured on the RDE at 1600 rpm in the O2-saturated 0.1 M KOH solution (as shown in Figure 4g). For comparison, the pure CNTs, NC/NCNTs, Co@NC/NCNTs, Fe@NC/NCNTs, Fe1.2Co@C/CNTs, Fe1.2Co@NC/NCB and 20 wt.% Ir/C were also 16 ACS Paragon Plus Environment

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studied for the OER. It is worth noting that the onset potential of Fe1.2Co@NC/NCNTs is the lowest, which is 1.43 V, and the current density of 10 mA cm-2 can be reached by only 355 mV overpotential. This value is even ~ 20 mV lower than that of Ir/C (a benchmark catalyst for the OER), indicating that the Fe1.2Co@NC/NCNTs is more catalytically efficient for the OER than the Ir/C. Meanwhile, the high OER performance of the Fe1.2Co@NC/NCNTs is also confirmable by the smaller Tafel slope of 70 mV dec-1 than those of the pure CNTs (109 mV dec-1), NC/NCNTs (95 mV dec-1), Co@NC/NCNTs (77 mV dec-1), Fe@NC/NCNTs (64 mV dec-1), Fe1.2Co@C/CNTs (66 mV dec-1), Fe1.2Co@NC/NCB (69 mV dec-1) and 20 wt.% Ir/C electrodes (72 mV dec-1), suggesting that the OER by the Fe1.2Co@NC/NCNTs is more kinetically favorable (Figure 4h). The results shown above clearly indicates that the Fe1.2Co@NC/NCNTs is an excellent bifunctional catalyst, and its potential difference (i.e., △E =E j = 10 – E1/2, with the OER potential E j = 10

being taken at 10 mA cm −2current density, while the ORR potential E1/2 being

taken at half-wave potential) further confirms this point. The lower is the △E value, the better is the bifunctional activity of the catalyst.

29, 58

Most strikingly, Figure 4i

shows that △E for the Fe1.2Co@NC/NCNTs is 0.765 V, which is much lower than those of most previously reported state-of-art bifunctional catalysts, strongly suggesting that the Fe1.2Co@NC/NCNTs is one of the best bifunctional catalysts among those reported (Table S3 and Figure 4j). Based on the above results, the outstanding electrocatalytic activity of Fe1.2Co@NC/NCNTs is probably attributed to the following reasons: (1) The synergistic interaction between Fe and Co in metal alloy of Fe1.2Co, which promotes the bifunctional catalytic activities of the Fe1.2Co@NC/NCNTs, as verified by the fact that the Fe1.2Co@NC/NCNTs displays much higher electrocatalytic activities than Fe@NC/NCNTs and Co@NC/NCNTs for the ORR and OER as shown in Figure 4b and g. (2) The N doping carbon layer and the presence of the CNTs, which induce the change of charge distribution, allowing for a ready adsorption/desorption of oxygen, and facilitate high electric conductivity, allowing for easy transfer of charges. (3) The 17 ACS Paragon Plus Environment


Page 18 of 33

strong electronic coupling between the Fe1.2Co nanoparticles and the N doped-CNTs and the formation of Fe-Nx and Co-Nx bonds, which are helpful to generate more efficient catalytically active sites for ORR to improve the catalytic activity of the Fe1.2Co@NC/NCNTs. (4) The presence of N doped-CNTs, which favors the formation of the interpenetrated conductivity network structure, improving the electrical conductivity of the Fe1.2Co@NC/NCNTs to promote the fast electron transport and offering rapid reaction kinetics during the catalytic process. As demonstrated in Figure S4, the Fe1.2Co@NC/NCNTs exhibits much higher ORR catalytic

activities

than

Fe1.2Co@NC.

Likewise,

Fe@NC/NCNTs

and

Co@NC/NCNTs also displayed outstanding ORR catalytic activities, compared with that of Fe@NC and Co@NC electrocatalysts. Notably, this could be further certified by the better catalytic activities of Fe1.2Co@NC/NCNTs in comparison to the Fe1.2Co@NC/NCB, since the NCB are prone to aggregation which greatly reduces the active sites for both ORR and OER. (5) The high specific surface area, which facilitates more active sites expose and accessible to the ORR and OER, and increases the interface area of solid/electrolyte during electrochemical reaction. As shown in Figure 3e, the Fe1.2Co@NC/NCNTs exhibits a specific BET surface area of 181.0 m2g-1, which is larger than those of the Fe@NC/NCNTs, the Co@NC/NCNTs, the CNTs and most of the CNTs supported M/N/C nanocomposites reported previously. (6) Encapsulate Fe1.2Co nanoparticles into ultrathin N-doped carbon (1-3 graphitic carbon layers), which can effectively prevent the aggregation and dissolution of FeCo alloy nanoparticles in acidic medium, and maintain the highly electrocatalytic stability. At the same time, the electron permeation from the FeCo alloy wrapped in the inner layer can further promote the electrocatalytic reaction on the surface of carbon layer. The Fe1.2Co@NC/NCNTs is further demonstrated to be stable and tolerant to methanol and CO crossover. As shown in Figure 5a, after 10000s of the ORR, Fe1.2Co@NC/NCNTs still maintained a highly effective oxygen reduction activity with an activity loss of only 5%. However, 70% and 25% decay in the ORR activity were observed for the Fe1.2Co@NC/NCB and NC/NCNTs over 10000 s continuous 18 ACS Paragon Plus Environment

Page 19 of 33

operation, respectively. The electrocatalytic activity of Pt/C catalyst decreased ~30% after 10000 s electrocatalytic reaction, which was mainly due to the aggregation of Pt nanoparticles and the dissociation of Pt nanoparticles from the carbon supports.

39, 59

The much slower decay in the current density of Fe1.2Co@NC/NCNTs is a consequence of the synergistic contribution arising from the highly stable graphitic structure of the NCNTs and the coating of the Fe1.2Co nanoparticles by the NC, which well avoids the structure variation of the Fe1.2Co@NC/NCNTs and suppress the agglomeration/dissolution of the Fe1.2Co nanoparticles during the ORR. Furthermore, with the introduction of methanol and CO, the Fe1.2Co@NC/NCNTs shows a negligible catalytic activity change, indicating its high catalytic selectivity for ORR against methanol oxidation and CO poisoning (Figure 5b and c). This is contrast to the Pt/C, where a dramatic decrease of its activity was observed immediately after the introduction of CO and methanol due to the adsorption of CO or the blockage of active sites on Pt nanoparticles by methanol oxidation products.59 These results clearly suggesting that the Fe1.2Co@NC/NCNTs is more stable for the ORR with higher durability towards the poisoning of methanol and CO than the commercial Pt/C. Likewise, the Fe1.2Co@NC/NCNTs also exhibit good OER stability in the 0.1 M KOH solutions. As shown in Figure 5d, the Fe1.2Co@NC/NCNTs could remain highly active for the OER with a nearly constant operating potential after 3500 s chronopotentiometry test. The above results clearly demonstrate that the Fe1.2Co@NC/NCNTs can be used as superior and durable electrocatalysts for ORR and OER. (b)

(a)

100

100

Relative 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


80

80 Addition of methanol


60 40 20 0

0

2000

4000

60 40


20

6000

Time (s)

8000

0

10000

200


400

600

Time (s)

800

1000


(d)

(c) 100 Addition of CO

80


60 40 20 0

200

400

600

800

Time (s)


1.70


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 20 of 33

1.65 1.60 1.55

CNTs Co@NC/NCNTs Fe1.2Co@C/CNTs Fe1.2Co@NC/NCNTs

1.50 1.45

1000

0

500

NC/NCNTs Fe@NC/NCNTs Fe1.2Co@NC/NCB 20 wt.% Pt/C

1000 1500 2000 2500 3000 3500

Time (s)

Figure 5. (a) Durability evaluation at 0.75 V in the O2-saturated 0.1 M KOH solution. Chronoamperometric response of ORR (b) by introducing 3 M methanol and (c) by introducing additional CO. (d) Chronopotentiometry curves for OER at a constant current density of 10 mA cm-2. Inspired by the outstanding OER and ORR activities of Fe1.2Co@NC/NCNTs, we also assembled rechargeable Zn-air battery with the use of the Fe1.2Co@NC/NCNTs and a Zn foil as the cathode and the anode, respectively, to investigate the potential use of the Fe1.2Co@NC/NCNTs as the air cathode for practical applications. For comparison, the Zn-air battery with using the Pt-Ir/C (the catalyst consisting of the commercial Pt/C catalyst with the highest catalytic activity of ORR and Ir/C catalyst with the highest catalytic activity of OER) as the air cathode were also assembled. Figure 6a shows that the battery containing the Fe1.2Co@NC/NCNTs affords an open circuit voltage of 1.43 V, which is higher than that with the Pt-Ir/C (1.4 V). Especially, as displayed in Figure 6b, the battery with Fe1.2Co@NC/NCNTs can deliver a maximum power density of 194 mW cm-2, which surpasses that of the Pt-Ir/C cathode (160 mW cm−2). The result clearly demonstrates the outperformance of the Fe1.2Co@NC/NCNTs over the Pt-Ir/C. Furthermore, the discharge-charge polarization curves in Figure 6c shows that the Zn-air battery (6 M KOH and 0.2 M zinc acetate) with the Fe1.2Co@NC/NCNTs exhibit a slightly lower voltage gap between charge and discharge in comparison to that with the Pt-Ir/C, indicating the excellent

charge/discharge

performance

of

the

Zn-air

battery

with

the

Fe1.2Co@NC/NCNTs. When the Zn-air battery assembled by Fe1.2Co@NC/NCNTs 20 ACS Paragon Plus Environment

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was cycled at 10 mAcm-2, the initial charge potential and discharge potential were 1.98 V and 1.24 V, respectively, and the potential gap was about 0.74 V (Figure 6d). After 50 h (over 300 cycles), the potential gap only increased by 0.04V, indicating a high durability of the battery with the Fe1.2Co@NC/NCNTs. This is different from the battery with Pt-Ir/C where a much higher increase in the voltage gap (0.20 V) is observed under the same condition. This is due to the fact that Pt-Ir/C suffers from not only the carbon support corrosion, but also the peeling and agglomeration of Pt and Ir nanoparticles on the carbon support during the working process.

49, 58

The TEM

images in Figure 6e show that the morphologies of the Fe1.2Co@NC/NCNTs is barely changed and no aggregation of the Fe1.2Co nanoparticles observed after 300 cycles of the charge and discharge. This well explains why the battery with the Fe1.2Co@NC/NCNTs exhibits a high durability. Figure 6f shows that the Zn-air battery assembled by Fe1.2Co@NC/NCNTs can maintain high efficiency with no voltage drop observed at the constant discharge current of 5 mA cm-2 for 20 h or 50 mA cm-2 for 15 h, while for the battery with Pt-Ir/C, a significant voltage drop could be observed when it is discharged for a long time. The result strongly suggests that the Fe1.2Co@NC/NCNTs is more suitable for practical applications in Zn-air batteries. As the discharge continues, the zinc foil becomes thinner and the soluble zinc salt in electrolyte solution increases. When all zinc is consumed, the battery eventually stops working. Figure 6g shows the specific capacities normalized to the consumed Zn mass are ~ 641.1 mA h gZn-1 and 585.6 mA h gZn-1 (the corresponding energy density is 820.5 Wh kgZn-1 and 644.3 Wh kgZn-1) at current densities of 10 mA cm-2 and 100 mA cm-2,

respectively.

The

above

analysis

clearly

demonstrates

that

the

Fe1.2Co@NC/NCNTs are highly efficient electrocatalysts for both ORR and OER and available for Zn-air batteries with great potential to replace the noble metal/carbon based materials. Figure 6h shows an all-solid-state Zn-air micro-battery with a high OCV of 1.324 V, in which the carbon film was placed next to zinc foil coated by PVA gel and exposed to ambient air. A red LED could be powered by an all-solid-state Zn-air 21 ACS Paragon Plus Environment


micro battery, and there is no obvious performance decay even when the battery was bent. This test clearly demonstrates the promising applications of our material in a variety of flexible and wearable optoelectronics (e.g., LEDs) and many other systems.

(b)

(c)3.0

Power Density (mWcm-2) Voltage (V)

Fe1.2Co@NC/NCNTs

1.4

200

20 wt.% Pt-Ir/C

Voltage (V)

1.2

0.8

100

0.6

50

0

6

0

12

Fe1.2Co@NC/NCNTs 20 wt.% Pt-Ir/C

2.0 1.5 1.0

0.4

(d)

Charging

2.5

150

1.0

100

200

300

400


Cycle number 18

282

288

294

3

47

48

49

Discharging

0.5 0.0

0 500

0

100

200

300

400

-2

Current Density (mA cm )

500

300

Voltage (V)

2.2 2.0 1.8 1.6 1.4 1.2

Fe1.2Co@NC/NCNTs

1.0 0

20 wt.% Pt-Ir/C 1

2

Time (h)

50

(g)

(f)1.5

1.4

1.4

Voltage (V)

j=5 mA cm-1

Voltage (V)

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 33

1.3 j=50 mA cm-1

1.2 1.1 1.0

Fe1.2Co@NC/NCNTs

5

10

Time (h)

1.0

0.8 -1

20 wt.% Pt-Ir/C

0

1.2

15

20

0.6

10 mA cm -1 100 mA cm 0

100

200

300

400

500

600

-1

Specific Capacity (mAh g )

700

Figure 6. (a) Schematic representation of a rechargeable Zn-air battery. (b) Polarization curves (V~i) and corresponding power density plots of the Zn-air batteries with the Fe1.2Co@NC/NCNTs and the Pt/C. (c) Charge and discharge polarization curves of rechargeable Zn-air batteries. (d) Cycling curves of rechargeable Zn-air batteries at the current density of 10 mA cm−2. (e) TEM images of the Fe1.2Co@NC/NCNTs after 300 cycles at the current density of 10 mA cm−2. (f) Long-time discharge curves of the primary batteries with Fe1.2Co@NC/NCNTs and Pt-Ir/C at two different current densities. (g) Discharge curves of the primary Zn-air batteries with Fe1.2Co@NC/NCNTs until complete consumption of Zn at two different current densities. Specific capacity was normalized to the mass of consumed Zn. (h) Schematic illustration and photograph of a red LED (1.4 V) powered by the 22 ACS Paragon Plus Environment

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all-solid-state rechargeable Zn-air micro battery and photograph of the all-solid-state Zn-air micro battery with an open circuit voltage of 1.324 V.

CONCLUSIONS In summary, the Fe1.2Co@NC/NCNTs consisting of ultrathin N-doped carbon (1-3 graphitic carbon layers) coated Fe1.2Co nanoparticles and NCNTs has been successfully synthesized by the calcination of the FePc-CoPc/CNTs in the presence of melamine. The obtained Fe1.2Co@NC/NCNTs has an extremely low metal content, but exhibit super-high catalytic activity for ORR and OER. Specifically, when used for the ORR, the Fe1.2Co@NC/NCNTs can deliver Eonset=0.842 and E1/2= 0.821 V vs. RHE, respectively, which are close to those of the Pt/C. When tested as the catalyst for the OER, the Fe1.2Co@NC/NCNTs only needs an overpotential of 350 mV to achieve a current density of 10 mA cm-2, indicating its OER activity is higher than that of the Ir/C. Most importantly, the Fe1.2Co@NC/NCNTs shows a △E of 0.765 V, which is lower than those of Pt/C and Ir/C catalysts and most of the previously reported non-noble

metal

based

bifunctional

electrocatalysts.

In

addition,

the

Fe1.2Co@NC/NCNTs is also stable for both ORR and OER and show good durability to methanol and CO for the ORR poisoning. These results strongly suggest that the Fe1.2Co@NC/NCNTs could be used as bifunctional electrocatalysts for both ORR and OER with low cost, but higher electrocatalytic activity. With the excellent ORR and OER activity and high durability, the Fe1.2Co@NC/NCNTs is usable as an air-cathode for portable, flexible Zn-air batteries. Remarkably, the battery with the Fe1.2Co@NC/NCNTs displays a lower voltage gap of charge-discharge curves, along with good cycling stability over 50 h. The results show that Fe1.2Co@NC/NCNTs could be used as an efficient electrocatalyst for metal-air batteries and has great potential to replace state-of-art metal/carbon-based materials.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXXXX. This file provides more detailed information regarding certain Chemicals and reagents; Detailed synthesis method of samples; Physical Characterization; Electrocatalytic activity evaluation; Relative percentages of the atoms in the Co@NC/NCNTs, Fe@NC/NCNTs and Fe1.2Co@NC/NCNTs; SEM and TEM images of CNTs, Fe@NC/NCNTs and Co@NC/NCNTs; XRD spectra of Fe@NC/NCNTs and Co@NC/NCNTs; Relative percentages of the C, O, and N containing components in the CNTs, Co@NC/NCNTs, Fe@NC/NCNTs and Fe1.2Co@NC/NCNTs; LSV curves at various different rotation rates for the ORR and corresponding K-L plots; LSV curves of Co@NC, Fe@NC, Fe1.2Co@NC, Co@NC/NCNTs, Fe@NC/NCNTs and Fe1.2Co@NC/NCNTs; Comparison of the catalytic bifunctionality of the Fe1.2Co@NC/NCNTs with those reported.

AUTHOR INFORMATION *Corresponding authors E-mail: [email protected] or [email protected] (Z. Jiang). E-mail: [email protected] (X. Hao). E-mail: [email protected] (Z.-J. Jiang). Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by Science Foundation of Zhejiang Sci-Tech University (No. 18062245-Y), the Guangdong Provincial Natural Science Foundation (No. 2017A030313092), the Guangdong Innovative and Entepreneurial Research Team Program (No. 2014ZT05N200), the Ningbo Natural Science Foundation 24 ACS Paragon Plus Environment

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(No. 2017A610059), and “the Fundamental research funds for the central university” of South China University of Technology (No. 2018ZD25).

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TOC

Synopsis FeCo alloy nanoparticles coated by ultrathin N-doped carbon layer and encapsulated in carbon nanotubes were prepared by a simple, green and sustainable method for application in sustainable energy conversion devices.


FeCo Alloy Nanoparticles Coated by an Ultrathin N-Doped Carbon

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