Atomically Dispersed Cobalt- and Nitrogen-Codoped Graphene

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Atomically dispersed cobalt- and nitrogen-codoped graphene towards bifunctional catalysis of oxygen reduction and hydrogen evolution reactions Xudong Wen, Lu Bai, Min Li, and Jingqi Guan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00105 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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

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ACS Sustainable Chemistry & Engineering

Atomically

dispersed

cobalt-

and

nitrogen-codoped

graphene

towards

bifunctional catalysis of oxygen reduction and hydrogen evolution reactions

Xudong Wen, Lu Bai, Min Li, and Jingqi Guan*

Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, JieFang Road 2519, Changchun 130021, PR China. *E-mail:

[email protected]

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 high-efficiency and stable single-site catalysts for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is an attractive strategy to substitute precious metal platinum for addressing energy and environmental issues. Here, we adopt a simple annealing strategy to embed atomic cobalt into nitrogen-doped graphene for efficient catalysis of HER and ORR. The as-prepared catalyst shows high activity and durability for HER with low overpotential of 182 V and 172 mV at 10 mA cm-2 in 0.5 M H2SO4 and 0.1 M KOH, respectively. In addition, it also exhibits excellent ORR performance in both alkaline and acid electrolytes, including a positive onset potential (0.97 V versus 0.79 V) and half-wave potential (0.87 V versus 0.69 V), and a low Tafel slope (46.1 mV·dec-1 versus 48.2 mV·dec-1). Meanwhile, it manifests remarkable electrochemical stability, and strong tolerance to methanol crossover. The atomic cobalt coordinated with several nitrogen atoms should be the active centers for HER and ORR. The good stability of electrochemical HER and ORR should be attributed to the stable Co-Nx moieties and the strong interaction between the Co-Nx and graphene support. Our work provides a facile strategy for developing high-efficiency and durable HER and ORR electrocatalysts.

KEYWORDS: Single-site; Nitrogen-doped graphene; Co-Nx; Hydrogen evolution reaction; Oxygen reduction reaction

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INTRODUCTION With rapid consumption of limited fossil resource and incremental greenhouse effect caused by the deteriorating environment, the development of new technologies to generate clean and sustainable energy is a key to address current environmental and energy issues.1-3 Electrochemical oxygen reduction reaction (ORR) plays a vital role in energy storage and conversion,4 while hydrogen evolution reaction (HER) plays a key role in the production of clean and sustainable hydrogen energy.5-6 Development of highly active, resource-rich, and eco-friendly ORR and HER electrocatalysts to replace the noble platinum is the core technology for practical applications. Although platinum based materials exhibited excellent catalytic activity for ORR and HER, which have been commercially applied in both acid and alkaline systems,6 the scarcity, high price and weak stability in ORR immensely hinder their extensive commercial applications.7 Transition metal composites (Fe, Co, and Ni) and heteroatom (e.g. B8, N9, P10-11 and S12-13) codoped materials showed high HER and ORR activity, which have promising potentials to replace the precious metal Pt1,

14-17.

Among them, carbon

materials, especially graphene materials, are of special interest owing to good electrical conductivity, excellent thermal stability, and large specific surface area.18 However, pure graphene does not show high catalytic performance for HER and ORR, which needs doping metal and/or nonmetal heteroatoms to enhance its electrocatalytic activity.18 It is reported than transition metals (Fe and Co) modified N-doped graphene (NG) showed good activity for ORR.4, 3


19

The active metal


coordinated to several nitrogen atoms (M-Nx) would modulate the electronic configuration of the active site and increase catalytic activity for HER and ORR.4, 20-24 The graphitic nitrogen atom can provide electrons to form a conjugated system, increase the nucleophilicity of the adjacent carbon, and effectively improve the kinetics of the ORR.25 Meanwhile, the pyridinic nitrogen determines the onset potential of ORR.18 For HER, the electrocatalytic activity is mainly concentrated on the kinetic energy barrier of hydrogen release and the adsorption energy of hydrogen, which has a strong dependence on the atomic distribution at the interface of the two phases.26 In this work, we reported a high-efficiency single-site Co/N codoped graphene catalyst (Co@NG) prepared through a facile annealing strategy, which showed outstanding electrocatalytic activity for HER and ORR in both acidic and alkaline media, which shows a promising potential to substitute the noble Pt.

EXPERIMENTAL SECTION Materials Synthesis Graphene oxide (GO) was synthesized on the basis of a modified Hummers method, which was washed with 5 vol.% hydrochloric acid for several times.13 The residual manganese in GO is less than 1.0 ppm as determined by ICP-AES. Then, 0.2 g GO and 5.6 mg CoCl2·6H2O (Co/GO = 0.7 wt.%) was dispersed in 50 mL H2O, which was sonicated for 1 h. After water was dislodged by a rotary evaporator, the remaining solid was annealed at 750 °C under a NH3 atmosphere for 2 hours to obtain 4


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Co@NG-750 (yield: ca. 96 mg). For comparison, Co@NG-T (T = 450, 550, 650, and 850 °C) was prepared by a similar synthetic procedure with the above synthesis of Co@NG-750 except the different calcination temperature. Co@G-750 was prepared by annealing the precursor of CoCl2/GO at 750 °C under an Ar atmosphere for 2 hours. G-750 and NG-750 was prepared by annealing GO at 750 °C under an Ar atmosphere and a NH3 atmosphere for 2 hours, respectively.

RESULTS AND DISCUSSION Structural characterizations The morphology of Co@NG-750 electrocatalyst was studied by electron microscope characterization as depicted in Figure 1. From Figure 1a, we can observe that Co@NG-750 shows a folded layered structure, which is typically observed for the graphene materials. No obvious Co/CoOx nanoparticles can be observed from TEM and HAADF-STEM images (Figure S1) in agreement with the XRD results, where no obvious X-ray diffraction peaks due to Co/CoOx nanoparticles indicates the highly dispersion of cobalt species (Figure S2). The atomically dispersed cobalt can be clearly observed from Figure 1b. The composition and element distribution of the Co@NG-750 catalyst were further characterized by the SEM-EDX mapping (C, N, O and Co) as shown in Figs. 1c–d. It can be observed that the Co and N elements are uniformly and closely dispersed onto the N-doped graphene, implying that the possible coordination of Co atoms with N to form Co-Nx site. In addition, 5



Co@NG-750 (440 m2·g−1) shows larger specific surface area than that (220 m2·g−1) of Co@G-750 (Figure S3). The Co content in the Co@NG-750 catalyst is ca. 1.3 wt.% as determined by ICP-AES analysis.

d

500 nm

Figure 1. (a) TEM image, (b) HAADF-STEM image, (c) SEM image used in the EDS mapping test, and (d) the corresponding EDS mapping of Co@NG-750.

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Intensity (a.u.)

Co@NG-750 Co@G-750

O 1s

a

C 1s

b

C-C

N 1s

C 1s

C-N C-O

Co@NG-750

Co@G-750

800 600 400 200 Binding Energy (eV) Co 2p3/2 Sat.

Co 2p1/2

0 290

285 280 Binding Energy (eV)

d

Co 2p

Intensity (a.u.)

c

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


Intensity (a.u.)

Page 7 of 35

Sat.

Co@NG-750

pyridinic-N

Co-Nx pyrrolic-N graphitic-N

Co@G-750

810

800 790 780 Binding Energy (eV)

410

405 400 395 Binding Energy (eV)

Figure 2. (a) XPS survey spectra, and high-resolution XPS spectra of C 1s (b), and Co 2p (c) for Co@G-750 and Co@NG-750; and (d) high-resolution XPS spectrum of N 1s for Co@NG-750.

The surface composition and possible species of Co@NG-750 and Co@G-750 was further analyzed by XPS. From the survey spectra in Figure 2a, it can be seen that C, O and Co elements exist in Co@G-750, while C, O, N and Co are found in Co@NG-750. The surface C, O, and Co content in Co@G-750 is determined by XPS to be 87.55, 12.13, and 0.32 mol.%, respectively; while the surface C, N, O, and Co content in Co@NG-750 is ca. 88.0, 7.92, 3.88, and 0.20 mol.%, respectively (Table 7



S1). The C1s XPS spectrum of Co@NG-750 is decomposed into three peaks, ascribed to C-O (286.5 eV), C-N (285.5 eV), and C-C (284.6 eV) (Figure 2b).27 The high-resolution Co2p displays two satellite peaks, characteristic of CoII (Figure 2c). The Co2p3/2 and Co2p1/2 is located at 781.0 and 797.0 eV, respectively, indicating that CoII ions are mainly presented in both Co@G-750 and Co@NG-750 samples.28-29 The N1s XPS spectrum of Co@NG-750 could be fitted into four peaks, due to pyridinic-N (398.4 eV), Co-Nx (399.6 eV), pyrrolic-N (400.5 eV), and graphitic-N (401.4 eV), respectively (Figure 3d).29-30

ORR performance The ORR properties of the obtained catalysts were first evaluated in 0.1 M KOH electrolyte. The ORR performance of G-750, NG-750, Co@G-750 and Co@NG-750 was tested by cyclic voltammetry (CV) (Figure 3a). Compared with the CV curses collected in N2-saturated electrolyte, a prominent cathodic peak due to oxygen reduction can be seen in O2-saturated solution for these electrocatalysts, suggesting effective ORR activity. However, compared with the Co-free samples, the cathodic peak of Co-doped samples shifts positively and the cathodic peak position of Co-/N-codoped sample is the most positive, highlighting the most effective ORR activity of Co@NG-750. LSV polarization curves were further performed to illustrate ORR performance (Figure 3b). The onset potential at 0.1 mA cm-2 for Pt/C, G-750, NG-750, Co@G-750, and Co@NG-750 is 0.95, 0.80, 0.84, 0.86, and 0.97 V vs RHE, respectively. Moreover, the half-wave potential (E1/2) for Pt/C, G-750, NG-750, 8


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Co@G-750, and Co@NG-750 is 0.82, 0.64, 0.75, 0.78, and 0.87 V vs RHE, respectively. The E1/2 of Co@NG-750 surpasses that of Pt/C and some outstanding cobalt-based ORR electrocatalysts (Table 1). In addition, the diffusion-limiting current density (∼5.4 mA·cm-2) of Co@NG-750 is higher than those of G-750, NG-750, Co@G-750, and Pt/C. The electrocatalytic kinetics of Co@NG-750 is studied (Figure 3c). The current usually increases with raising rotation speed owing to the acceleration of O2 diffusion.31 The Koutecky−Levich (K−L) plots of Co@NG-750 show good linearity and parallelism, implying the first-order ORR kinetics.32 The electron transfer number (n) is determined to be ca. 4.0 at potentials from 0.2 to 0.6 V on Co@NG-750, suggestive of a 4e- reaction route in Figure 3d.33

9



0 b

a

-1

-2

J (mA cm )

-2

J (mA cm )

Co@NG-750

Co@G-750

NG-750

N2

-4

-6 0.2

0.4 0.6 0.8 1.0 Potential (V vs. RHE)

J (mA cm )

2 -1

-2 -4

G-750 NG-750 20%Pt/C Co@G-750 Co@NG-750

0.30 d

c

600 900 1200 1600 2000

-1

-2

-3

-6 0.2

0.2 0.4 0.6 0.8 1.0 1.2 Potential (V vs. RHE)

0

-2

-5

G-750

O2

J (mA cm )

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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0.25 0.20 0.15


n = 4.0 0.6 V 0.5 V 0.4 V 0.3 V 0.2 V

0.08 0.10 0.12 -1/2 (s1/2/rad)

Figure 3. (a) CV curves of G-750, NG-750, Co@G-750, and Co@NG-750 in N2 and O2-saturated 0.1 M KOH. (b) Polarization curves of Pt/C, G-750, NG-750, Co@G-750, and Co@NG-750 at 1600 rpm. (c) Polarization curves of Co@NG-750 at various rotation speeds. (d) K–L plots of Co@NG-750 measured in O2-saturated 0.1 M KOH.

10


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0 b -1

a -2

J (mA cm )

-2

J (mA cm )

Co@NG-850 Co@NG-750

Co@NG-650 Co@NG-550

-2 -3

Co@NG-450 Co@NG-550 Co@NG-650 Co@NG-750 Co@NG-850

-4 -5

N2

Co@NG-450

O2

0.2 0.4 0.6 0.8 1.0 1.2 Potential (V vs. RHE) 0.40 c

-6 0.2

-450 NG Co@

0.35

-550 NG Co@

5 2 .9

dec mV -1

ec Vd

m 5 2 .2 -1

-850 NG Co@

0.30

m

-1

Co@

0.25

5 1 .1

ec Vd

50 650 NG-

7 NGCo@

c V de

.6 m

50 4

-1

c V de

0.4 0.6 0.8 1.0 Potential (V vs. RHE) Co@G-750, 50.92 mFcm-2 Co@NG-450, 37.87 mFcm-2 Co@NG-550, 42.07 mFcm-2 Co@NG-650, 44.24 mFcm-2 Co@NG-750, 94.97 mFcm-2 Co@NG-850, 113.23 mFcm-2

12

-1

j (mA cm-2)

Overpotential (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


10 8

d

6 4 2

6.1 m

-0.9 -0.6 -0.3 0.0 -2 logJ (mA cm )

0

0

20 40 60 80 100 Scan Rate (mV/s)

Figure 4. (a) CV curves of Co@NG-T in N2 and O2-saturated 0.1 M KOH. (b) Polarization curves of Co@NG-T at 1600 rpm. (c) Tafel plots for Co@NG-T. (d) Capacitive j vs scan rate for Co@G-750 and Co@NG-T. The linear slope is equivalent to twice of the double-layer capacitance Cdl.

The influence of annealing temperature on the catalytic activity of Co@NG was studied. As shown in Figure 4a, all the CV curves of Co@NG-T show a strong reduction peak at 0.82-0.88 V for O2-saturated electrolyte but no such response is seen for N2 saturated electrolyte, confirming that all the Co@NG-T can catalyze 11



Page 12 of 35

ORR. Figure 4b displays LSVs of Co@NG-T. The onset potential for Co@NG-450, Co@NG-550, Co@NG-650, Co@NG-750, and Co@NG-850 is approximately 0.90, 0.90, 0.95, 0.97, and 0.95 V vs RHE, respectively, while the diffusion limited current is 3.50, 3.87, 4.56, 5.46, and 4.60 V vs RHE, respectively. The E1/2 (0.87 V vs RHE) of Co@NG-750 is also superior to Co@NG annealed at 450, 550, 650, and 850 °C, indicating that the optimal annealing temperature for Co@NG should be 750 °C.

Table 1. ORR performance on some Co-based catalysts in 0.1 M KOH solution Catalyst

Eonset

Ehalf-wave

Ref.

(V vs RHE)

(V vs RHE)

Co@NG-750

0.97

0.87

This work

L-CCNTs-Co-700

0.86

0.80

34

Co3O4/N-rmGO

0.88

0.83

35

Co@C-NCNTs

0.87

0.80

36

Co@Co3O4@C-CM

0.93

0.81

37

Co-CoO/NrGO

0.88

0.78

38

Co0.50Mo0.50OyNz/C

0.92

0.76

39

ZnCoO4

0.87

0.84

40

Co3O4-doped PCP//NRGO

0.97

0.86

41

The Tafel plots obtained for Co@NG-T are shown in Figure 4c. The slope of 46.1 mV dec-1 is obtained for Co@NG-750, which is smaller than that for Co@NG-450 12


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(52.9 mV·dec-1), Co@NG-550 (52.2 mV·dec-1), Co@NG-650 (51.1 mV·dec-1), and Co@NG-850 (50.6 mV·dec-1). The Tafel slope of Co@NG-750 is close to 40 mV·dec-1, implying that the rate-determining step of ORR should be the protonation of O2- on [email protected],

42-43

The ECSA of Co@NG-T annealed at different

temperature was compared. The Cdl is obtained by plotting the linear slope of the Δj (|jcharge−joff

charge|)

against the scan rates, which represents the relevant ECSA. As

shown in Figure 4d and Figures S4-S9, Co@NG-750 shows an ECSA of 1415 cm2, which is larger than Co@NG-450 (525 cm2), Co@NG-550 (552 cm2), Co@NG-650 (637 cm2), Co@NG-850 (1187 cm2), and Co@NG-450 (472 cm2). The ECSA of Co/N-codoped graphene is larger than that of Co-doped graphene, suggesting that nitrogen doping can increase the ECSA.

13


80 Co@NG-750 Pt/C

60 40 20 0

0

120

40 20

Co@NG-750 Pt/C

80 60 40 20 0

0

-1 -2

Stop CO

b

CH3OH

0

Co@NG-750 Pt/C

CO

Page 14 of 35

100

Time (s)

100 60

120

5000 10000 15000 20000 25000

c

80

Normalized Current (%)

a

100

J (mA cm )

Normalized 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

Normalized current (%)


200 400 600 800 1000 Time (s) NG-750 Co@G-750 Co@NG-750

d

-2 -3 -4 -5

0 0

-6 0.2

200 400 600 800 1000 Time (s)


Figure 5. (a) Chronoamperometric responses of Co@NG-750 and Pt/C at 0.70 V in 0.1 M KOH solution. (b) Chronoamperometric responses of Pt/C and Co@NG-750 at 0.7 V in 0.1 M KOH followed by 3 M methanol. (c) Tolerance to carbon monoxide of Co@NG-750 compared with 20 wt% Pt/C electrocatalyst at 0.7 V. (d) LSV polarization curves for oxygen reduction on NG-750, Co@G-750 and Co@NG-750 catalysts in 0.1 M KOH with (dot line) and without (solid line) 10 mM KSCN.

The ORR stability of Co@NG-750 and the commercial 20 wt% Pt/C was measured through chronoamperometric tests. As exhibited in Figure 5a, Co@NG-750 shows a very slow current attenuation and maintains 93.5% of the initial current 14


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density, much higher than Pt/C (80.5%) after 25,000 s. Unlike the Pt-based catalysts, Co@NG-750 shows better tolerance to methanol. Furthermore, the effect of carbon monoxide on the ORR performance of Co@NG-750 and Pt/C was studied. As demonstrated in Figure 5c, both Co@NG-750 and Pt/C show attenuation in the current density after carbon monoxide is bubbled into the electrolyte, suggesting that carbon monoxide can poison the active sites of Co@NG-750 and Pt/C due to the strong coordination ability of CO with the metal sites. However, compared with Pt/C, Co@NG-750 shows stronger tolerance to carbon monoxide as manifesting by its slower attenuation in the current density. To evaluate the active sites of Co@NG-750 for ORR, KSCN-poisoning experiment was carried out. It is well-known that SCNions have strong complexation ability to metal ions.44 As depicted in Figure 5d, the catalytic activities of NG-750 remains almost constant after introduction of SCN- to the electrolyte since the N/C active centers are inert to SCN- ions. However, the catalytic activity of Co@G-750 and Co@NG-750 decreases significantly after the addition of KSCN, indicating the poisoning of the Co-O and Co-N sites. However, compared with Co@G-750, Co@NG-750 shows more influence by KSCN, indicating that different active sites are presented in the two samples.

15


a

Co@NG-750

J (mA cm-2)

N2 O2

Co@G-750

NG-750

Co@NG-750

O2

Co@NG-650

J (mA cm-2)

J (mA cm-2)

Co@NG-850

N2

0 b -1 -2 -3 -4

-1 -2 -3

Co@NG-550

-4

Co@NG-450

-5

0 G-75

19

-1

0.8

NG-750

V dec

140.3 m

-1

-7 Co@G

50 125.

ec 8 mV d

0.6 2 mV Co@NG-750 48.

0.4

-1

dec -1

Pt/C 68.7

mV dec

-0.9 -0.6 -0.3 0.0 -2 logJ (mA cm )

Normalized current (%)

1.0

-1

c V de 8.0 m

Co@NG-450 Co@NG-550 Co@NG-650 Co@NG-750 Co@NG-850

0.0 0.2 0.4 0.6 0.8 1.0 Potential (V vs. RHE)

0.0 0.4 0.8 1.2 Potential (V vs. RHE) e

G-750 NG-750 Co@G-750 Co@NG-750 Pt/C

-6 0.0 0.2 0.4 0.6 0.8 1.0 Potential (V vs. RHE) 0 d


c

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-5

G-750

Overpotential (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

J (mA cm-2)


120

f

100 80 60 40 20 0

0

1000020000300004000050000

Time (s)

Figure 6. (a) CV curves of G-750, NG-750, Co@G-750, and Co@NG-750 in N2 and O2-saturated 0.5 M H2SO4. (b) Polarization curves of Pt/C, G-750, NG-750, Co@G-750, and Co@NG-750 in 0.5 M H2SO4 at 1600 rpm. (c) CV curves of Co@NG-T in N2 and O2-saturated 0.5 M H2SO4. (d) Polarization curves of Co@NG-T in 0.5 M H2SO4 solution at 1600 rpm. (e) Tafel plots for G-750, NG-750, Co@G-750, Co@NG-750, and Pt/C. (f) Chronoamperometric responses of 16


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Co@NG-750 kept at 0.5 V in O2-saturated 0.5 M H2SO4.

The ORR properties of the prepared catalysts were further studied in 0.5 M H2SO4 solution. From the CV curves of G-750, NG-750, Co@G-750, and Co@NG-750 obtained under the same test conditions (Figure 6a), all the samples show a more defined cathodic ORR peak in O2-saturated electrolyte than in N2-saturated electrolyte. However, compared with G-750, NG-750, and Co@G-750, Co@NG-750 shows a much more positive peak potential, indicating the enhanced ORR activity. LSV curves of these samples were further conducted to assess the catalytic activity. As manifested in Figure 6b, the onset potential for Pt/C, G-750, NG-750, Co@G-750, and Co@NG-750 is ca. 0.89, 0.38, 0.41, 0.53, and 0.79 V vs RHE, respectively; while the E1/2 on these catalysts is 0.76, 0.15, 0.23, 0.33, and 0.69 V vs RHE, respectively. The E1/2 for the Co@NG-750 in 0.5 M H2SO4 solution is only 70 mV more negative than that for Pt/C, but it is positively shifted by 540 mV, 460 mV, and 360 mV, relative to that for the G-750, NG-750, and Co@G-750, respectively. However, the recorded Tafel slope for the Co@NG-750 is 48.2 mV·dec-1, which is smaller than that (68.7 mV·dec-1) for Pt/C, demonstrating different rate-determining steps on the two catalysts (Figure 6c). The rate-determining step on Pt/C is the first electron reduction of oxygen to form adsorbed intermediates, while it on Co@NG-750 should be the protonation of O2- on the active sites.35, 45 Meanwhile, the chronoamperometric measurement was also conducted in 0.5 M H2SO4 to further verify the long-term stability. As depicted in Figure 6d, the retention of ORR current 17



for Co@NG-750 is over 99% at 0.5 V vs. RHE even after continuous operation for over 50000 s, demonstrating its excellent ORR stability in acidic electrolyte.

HER performance The HER properties of the prepared catalysts were first investigated in 0.1 M KOH. Figure 7a exhibits the LSV curves achieved using a typical three-electrode configuration for G-750, NG-750, Co@G-750, Co@NG-750, and Pt/C. As expected, Pt/C shows excellent HER activity with overpotential of only 112 mV to obtain 10 mA cm−2. G-750, NG-750, and Co@G-750 show a poor HER activity with overpotential of 711 mV, 577 mV, and 459 mV at 10 mA cm−2, respectively. However, the Co@NG-750 manifests good HER performance at a low overpotential of 172 mV at 10 mA·cm−2, which is lower than that of most reported values achieved on Co-based HER catalysts in alkaline solutions (Table 2). The HER properties of the Co@NG annealed at different temperatures were evaluated. From the LSV curves in Figure 7b, the overpotential at 10 mA cm−2 is 505 mV, 343 mV, 256 mV, 172 mV, 213 mV for Co@NG-450, Co@NG-550, Co@NG-650, Co@NG-750, and Co@NG-850, respectively.

18


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Table 2. HER performance on some Co-based catalysts Catalyst

Electrolyte

η @ 10

Tafel slope

mA·cm-2

(mV·dec-1)

Ref.

(mV) Co@NG-750

0.5 M H2SO4

182

49.3

This work

Co@NG-750

0.1 M KOH

172

42.5

This work

CoS2 HNSs

1.0 M KOH

193

100

12

Co-NRCNTs

0.5 M H2SO4

260

69

46

CoSe2 MP/CC

0.5 M H2SO4

193

50

47

Co@NC/NCNS-800 1.0 M KOH

219

55.8

48

Co3S4 polyhedra

0.5 M H2SO4

380

85.3

49

Co0.6Mo1.4N2

0.1 M KOH

320

80

50

CoP

0.1 M KOH

210

129

51

CoMoP NPs

0.5 M H2SO4

178

60.5

52

Co@NiCoP

0.5 M H2SO4

276

43

53

The HER properties were further evaluated in 0.5 M H2SO4 solution. Pt/C is included as a reference, which shows superior HER performance with an overpotential of only 35 mV at 10 mA·cm−2 (Figure 7c). Similar to the alkaline media, G-750, NG-750, and Co@G-750 show poor HER activity in acidic electrolyte with an overpotential of 657 mV, 531 mV, and 351 mV at 10 mA·cm−2, respectively. However, the Co@NG-750 requires only 182 mV at 10 mA·cm−2, which is far less 19



than G-750, NG-750, and Co@G-750. The LSV results of Co@NG-T show that the overpotential of Co@NG-750 is 177 mV, 112 mV, 81 mV, and 20 mV less than that of Co@NG-450, Co@NG-550, Co@NG-650, and Co@NG-850, respectively, further suggesting the superior HER performance of Co@NG-750 (Figure 7d). In addition to the excellent HER activity, the Co@NG-750 shows lower Tafel slope (49.3 mV·dec-1) than G-750 (163.7 mV·dec-1), NG-750 (108.4 mV·dec-1), and Co@G-750 (70.0 mV·dec-1) (Figure 7e). Moreover, the high HER activity of Co@NG-750 is durable in alkaline electrolyte. The current density at -0.2 V vs RHE can be remained at J = -7±1 mA·cm−2 for more than 15 h (Figure 7f).

20


Page 20 of 35

Page 21 of 35

0 b

-2 -4 -6

0.1 M KOH G-750 NG-750 Co@G-750 Co@NG-750 Pt/C

-8

-2

J (mA cm-2)

J (mA cm-2)

0 a

-4 -6

0.1 M KOH Co@NG-450 Co@NG-550 Co@NG-650 Co@NG-750 Co@NG-850

-8

2

J (mA cm-2)

J (mA cm-2)

-10 -10 -0.4 -0.2 0.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential (V vs. RHE) Potential (V vs. RHE) 0 d G-750 c NG-750 0 Co@G-750 Co@NG-750 -2 -2 0.5 M HPt/CSO -4 -4 4

-6

-6

0.5 M H2SO4 Co@NG-450 Co@NG-550 Co@NG-650 Co@NG-750 Co@NG-850

-8

-8

-10 -10 -0.8 -0.6 -0.4 -0.2 0.0 -0.4 -0.3 -0.2 -0.1 0.0 Potential (V vs. RHE) Potential (V vs. RHE) 0.6 0 e f 0.5 M H SO dec -1

0 16 G-75

0.4 0.2

c 8.4 mV de

0 Co@G-750 70.

4

0.1 M KOH

-1

NG-750 10

-1

mV dec

Co@NG-750 49.3

0.0

2

V

3.7 m

J (mA cm-2)

Overpotential (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


-1

mV dec

-5 -10

-1

Pt/C 18.4 mV dec

-0.9 -0.6 -0.3 0.0 -2 logJ (mA cm )

-15

0

5 10 Time (h)

15

Figure 7. (a) LSV curves of G-750, NG-750, Co@G-750, Co@NG-750, and Pt/C in 0.1 M KOH. (b) LSV curves of Co@NG-T in 0.1 M KOH. (c) LSV curves of G-750, NG-750, Co@G-750, Co@NG-750, and Pt/C in 0.5 M H2SO4. (d) LSV curves of Co@NG-T in 0.5 M H2SO4. (e) Tafel plots for G-750, NG-750, Co-G-750, Co-NG-750, Pt/C in 0.5 M H2SO4. (f) Plot of current density vs time for the Co@NG-750 at -0.2 V vs RHE in 0.1 M KOH and 0.5 M H2SO4. 21



It has been reported that N-doped carbon materials can catalyze ORR.1 The pyrolysis temperature would influence the N-doping content and type. Although there are still controversies about the ORR active sites of N-doped graphene, some results showed that graphitic-N determines the limit current density, while pyridinic-N is in favor of improving the electrocatalytic ORR activity.54 Additionally, the ORR activity of N-doping graphene can be further improved by incorporating cobalt ions into the system to form Co-Nx sites. Density functional theory (DFT) calculations revealed that single cobalt sites can accelerate the hydrogenation of O2* species,55 and the intrinsic ORR activity of CoN4 is higher than that of [email protected] From Table S1, we can find that the N content of Co@NG-750 is the highest among the series of Co@NG-T, which provides more opportunity to coordinate with the Co sites and form Co-Nx active sites for ORR. If the Co-Nx sites are poisoned by CO or SCN- ions, the ORR activity would decrease significantly (Figure 5), demonstrating their positive role in ORR. The Co-Nx sites are not only active in ORR, but also active in HER. DFT calculations indicated that Co-Nx sites show much lower activation barrier (0.93 eV) than graphitic-N sites (1.71 eV).58 The fact that Co-Nx sites poisoned by KSCN would significantly decrease the HER activity of Co@NG-750 in both acidic and alkaline electrolytes (Figure S10), demonstrating the positive role of Co-Nx sites in HER. In addition, the Co@NG annealed at 750 °C shows higher nitrogen content than those annealed at other temperatures, which results in forming more active Co-Nx sites, and thus possessing better HER performance. 22


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CONCLUSIONS In summary, we demonstrated single Co sites embedded in nitrogen-doped graphene generate active sites for HER and ORR in both acidic and alkaline media. The electrochemical ORR tests indicate that the half-wave potential of the Co@NG-750 is 0.87 V and 0.69 V in 0.1 M KOH and 0.5 M H2SO4, respectively. The electrochemical HER tests indicate that the overpotential of the Co@NG-750 is 172 mV and 182 mV at 10 mA cm-2 in 0.1 M KOH and 0.5 M H2SO4, respectively. The superior electrochemical performance may be ascribed to the existence of active single-site Co-Nx and two-dimensional N-doped graphene matrix. This facile annealing strategy can be readily applied to prepare other high-efficiency single-site graphene-based materials for diverse electrocatalytic applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional HAADF-STEM; XRD; N2 adsorption–desorption isotherms; Double-layer capacitance measurements; Additional LSV curves; Elemental compositions

AUTHOR INFORMATION Corresponding Authors 23



Page 24 of 35

*E-mail: [email protected] (J.G.).

ORCID Jingqi Guan: 0000-0002-8498-1963 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jilin Province (20180101291JC).

REFERENCES (1) Gewirth, A. A.; Varnell, J. A.; DiAscro, A. M. Nonprecious Metal Catalysts for Oxygen Reduction in Heterogeneous Aqueous Systems. Chem. Rev. 2018, 118, 2313-2339, DOI 10.1021/acs.chemrev.7b00335. (2) Cai, S.; Meng, Z.; Tang, H.; Wang, Y.; Tsiakaras, P. 3D Co-N-doped hollow carbon spheres as excellent bifunctional electrocatalysts for oxygen reduction reaction and oxygen evolution reaction. Appl. Catal. B-Environ 2017, 217, 477-484, DOI 10.1016/j.apcatb.2017.06.008. (3) Zhang, G.-R.; Woellner, S. Hollowed structured PtNi bifunctional electrocatalyst with record low total overpotential for oxygen reduction and oxygen evolution reactions.

Appl.

Catal.

B-Environ

2018,

10.1016/j.apcatb.2017.09.066. 24


222,

26-34,

DOI

Page 25 of 35 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


(4) Chen, Y.; Ji, S.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Single-Atom Catalysts: Synthetic Strategies and Electrochemical Applications. Joule 2018, 2, 1242-1264, DOI 10.1016/j.joule.2018.06.019. (5) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086, DOI 10.1039/c4cs00470a. (6) Zheng, Y.; Jiao, Y.; Vasileff, A.; Qiao, S.-Z. The Hydrogen Evolution Reaction in Alkaline Solution: From Theory, Single Crystal Models, to Practical Electrocatalysts. Angew. Chem., Int. Ed. 2018, 57, 7568-7579, DOI 10.1002/anie.201710556. (7) Lin, R.; Cai, X.; Zeng, H.; Yu, Z. Stability of High-Performance Pt-Based Catalysts for Oxygen Reduction Reactions. Adv. Mater. 2018, 30, 1705332, DOI 10.1002/adma.201705332. (8) Tabassum, H.; Guo, W.; Meng, W.; Mahmood, A.; Zhao, R.; Wang, Q.; Zou, R. Metal-Organic Frameworks Derived Cobalt Phosphide Architecture Encapsulated into B/N Co-Doped Graphene Nanotubes for All pH Value Electrochemical Hydrogen Evolution. Adv. Energy Mater. 2017, 7, 1601671, DOI 10.1002/aenm.201601671. (9) Zhang, J.; Dai, L. Nitrogen, Phosphorus, and Fluorine Tri-doped Graphene as a Multifunctional Catalyst for Self-Powered Electrochemical Water Splitting. Angew. Chem., Int. Ed. 2016, 55, 13296-13300, DOI 10.1002/anie.201607405. (10) Wang, Z.; Liu, H.; Ge, R.; Ren, X.; Ren, J.; Yang, D.; Zhang, L.; Sun, X. Phosphorus-Doped Co3O4 Nanowire Array: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting. ACS Catal. 2018, 8, 2236-2241, DOI 25



Page 26 of 35

10.1021/acscatal.7b03594. (11) Zhang, X.; Zhang, X.; Xu, H.; Wu, Z.; Wang, H.; Liang, Y. Iron-Doped Cobalt Monophosphide Nanosheet/ Carbon Nanotube Hybrids as Active and Stable Electrocatalysts for Water Splitting. Adv. Funct. Mater. 2017, 27, 1606635, DOI 10.1002/adfm.201606635. (12) Ma, X.; Zhang, W.; Deng, Y.; Zhong, C.; Hu, W.; Han, X. Phase and composition controlled synthesis of cobalt sulfide hollow nanospheres for electrocatalytic

water

splitting.

Nanoscale

2018,

10,

4816-4824,

DOI

10.1039/c7nr09424h. (13) Zhu, Q.-L.; Xia, W.; Akita, T.; Zou, R.; Xu, Q. Metal-Organic Framework-Derived

Honeycomb-Like

Open

Porous

Nanostructures

as

Precious-Metal-Free Catalysts for Highly Efficient Oxygen Electroreduction. Adv. Mater. 2016, 28, 6391-6398, DOI 10.1002/adma.201600979. (14) Ren, Q.; Wang, H.; Lu, X.-F.; Tong, Y.-X.; Li, G.-R. Recent Progress on MOF-Derived Heteroatom-Doped Carbon-Based Electrocatalysts for Oxygen Reduction Reaction. Adv. Sci. (Weinheim, Ger.) 2018, 5, UNSP 1700515, DOI 10.1002/advs.201700515. (15) Yang, L.; Zeng, X.; Wang, W.; Cao, D. Recent Progress in MOF-Derived, Heteroatom-Doped Porous Carbons as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells. Adv. Funct. Mater. 2018, 28, 1704537, DOI 10.1002/adfm.201704537. (16) Xu, Y.; Kraft, M.; Xu, R. Metal-free carbonaceous electrocatalysts and 26


Page 27 of 35 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


photocatalysts for water splitting. Chem. Soc. Rev. 2016, 45, 3039-3052, DOI 10.1039/c5cs00729a. (17) Mahmood, N.; Yao, Y.; Zhang, J.-W.; Pan, L.; Zhang, X.; Zou, J.-J. Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions. Adv. Sci. (Weinheim, Ger.) 2018, 5, UNSP 1700464, DOI 10.1002/advs.201700464. (18) Hu, C.; Dai, L. Carbon-Based Metal-Free Catalysts for Electrocatalysis beyond the

ORR.

Angew.

Chem.,

Int.

Ed.

2016,

55,

11736-11758,

DOI

10.1002/anie.201509982. (19) Zhang, L.; Wilkinson, D. P.; Liu, Y.; Zhang, J. Progress in nanostructured (Fe or Co)/N/C non-noble metal electrocatalysts for fuel cell oxygen reduction reaction. Electrochim. Acta 2018, 262, 326-336, DOI 10.1016/j.electacta.2018.01.046. (20) Sun, T.; Zhao, S.; Chen, W.; Zhai, D.; Dong, J.; Wang, Y.; Zhang, S.; Han, A.; Gu, L.; Yu, R.; Wen, X.; Ren, H.; Xu, L.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Single-atomic cobalt sites embedded in hierarchically ordered porous nitrogen-doped carbon as a superior bifunctional electrocatalyst. Proc. Natl. Acad. Sci. 2018, 115, 12692-12697, DOI 10.1073/pnas.1813605115. (21) Sun, T.; Zhang, S.; Xu, L.; Wang, D.; Li, Y. An efficient multifunctional hybrid electrocatalyst: Ni2P nanoparticles on MOF-derived Co,N-doped porous carbon polyhedrons for oxygen reduction and water splitting. Chem. Commun. 2018, 54, 12101-12104, DOI 10.1039/c8cc06566g. (22) Sun, T.; Xu, L.; Li, S.; Chai, W.; Huang, Y.; Yan, Y.; Chen, J. 27



Page 28 of 35

Cobalt-nitrogen-doped ordered macro-/mesoporous carbon for highly efficient oxygen reduction

reaction.

Appl.

Catal.

B-Environ

2016,

193,

1-8,

DOI

10.1016/j.apcatb.2016.04.006. (23) Sa, Y. J.; Woo, J.; Joo, S. H. Strategies for Enhancing the Electrocatalytic Activity of M-N/C Catalysts for the Oxygen Reduction Reaction. Top. Catal. 2018, 61, 1077-1100, DOI 10.1007/s11244-018-0935-0. (24) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657, DOI 10.1021/acs.chemrev.5b00462. (25) Yang, H. B.; Miao, J.; Hung, S.-F.; Chen, J.; Tao, H. B.; Wang, X.; Zhang, L.; Chen, R.; Gao, J.; Chen, H. M.; Dai, L.; Liu, B. Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: Development of highly efficient metal-free bifunctional electrocatalyst. Sci. Adv. 2016, 2, UNSP e1501122, DOI 10.1126/sciadv.1501122. (26) Wang, D.-Y.; Gong, M.; Chou, H.-L.; Pan, C.-J.; Chen, H.-A.; Wu, Y.; Lin, M.-C.; Guan, M.; Yang, J.; Chen, C.-W.; Wang, Y.-L.; Hwang, B.-J.; Chen, C.-C.; Dai, H. Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS2 Nanosheets-Carbon Nanotubes for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 1587-1592, DOI 10.1021/ja511572q. (27) Ni, Y.; Chen, Z.; Kong, F.; Qiao, Y.; Kong, A.; Shan, Y. Pony-size Cu nanoparticles confined in N-doped mesoporous carbon by chemical vapor deposition for efficient oxygen electroreduction. Electrochim. Acta 2018, 272, 233-241, DOI 28


Page 29 of 35 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


10.1016/j.electacta.2018.04.002. (28) Guan, J.; Ding, C.; Chen, R.; Huang, B.; Zhang, X.; Fan, F.; Zhang, F.; Li, C. CoOx nanoparticle anchored on sulfonated-graphite as efficient water oxidation catalyst. Chem. Sci. 2017, 8, 6111-6116, DOI 10.1039/c7sc01756a. (29) Chen, B.; He, X.; Yin, F.; Wang, H.; Liu, D.-J.; Shi, R.; Chen, J.; Yin, H. MO-Co@N-Doped Carbon (M = Zn or Co): Vital Roles of Inactive Zn and Highly Efficient Activity toward Oxygen Reduction/Evolution Reactions for Rechargeable Zn-Air

Battery.

Adv.

Funct.

Mater.

2017,

27,

1700795,

DOI

10.1002/adfm.201700795. (30) Zhang, M.; Dai, Q.; Zheng, H.; Chen, M.; Dai, L. Novel MOF-Derived Co@N-C Bifunctional Catalysts for Highly Efficient Zn-Air Batteries and Water Splitting. Adv. Mater. 2018, 30, 1705431, DOI 10.1002/adma.201705431. (31) Wang, Y.; Ding, X.; Wang, F.; Li, J.; Song, S.; Zhang, H. Nanoconfined nitrogen-doped carbon-coated MnO nanoparticles in graphene enabling high performance for lithium-ion batteries and oxygen reduction reaction. Chem. Sci. 2016, 7, 4284-4290, DOI 10.1039/c5sc04668h. (32) Mosa, I. M.; Biswas, S.; El-Sawy, A. M.; Botu, V.; Guild, C.; Song, W.; Ramprasad, R.; Rusling, J. F.; Suib, S. L. Tunable mesoporous manganese oxide for high performance oxygen reduction and evolution reactions. J. Mater. Chem. A 2016, 4, 620-631, DOI 10.1039/c5ta07878d. (33) Yu, L.; Yang, J. F.; Guan, B. Y.; Lu, Y.; Lou, X. W. Hierarchical Hollow Nanoprisms Based on Ultrathin Ni-Fe Layered Double Hydroxide Nanosheets with 29



Page 30 of 35

Enhanced Electrocatalytic Activity towards Oxygen Evolution. Angew. Chem., Int. Ed. 2018, 57, 172-176, DOI 10.1002/anie.201710877. (34) Liang, Z.; Fan, X.; Lei, H.; Qi, J.; Li, Y.; Gao, J.; Huo, M.; Yuan, H.; Zhang, W.; Lin, H.; Zheng, H.; Cao, R. Cobalt-Nitrogen-Doped Helical Carbonaceous Nanotubes as a Class of Efficient Electrocatalysts for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2018, 57, 13187-13191, DOI 10.1002/anie.201807854. (35) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780-786, DOI 10.1038/nmat3087. (36) Li, R.; Wang, X.; Dong, Y.; Pan, X.; Liu, X.; Zhao, Z.; Qiu, J. Nitrogen-doped carbon nanotubes decorated with cobalt nanoparticles derived from zeolitic imidazolate

framework-67

for

highly

efficient

oxygen

reduction

reaction

electrocatalysis. Carbon 2018, 132, 580-588, DOI 10.1016/j.carbon.2018.02.003. (37) Xia, W.; Zou, R.; An, L.; Xia, D.; Guo, S. A metal-organic framework route to in situ encapsulation of Co@Co3O4@C core@bishell nanoparticles into a highly ordered porous carbon matrix for oxygen reduction. Energy Environ. Sci. 2015, 8, 568-576, DOI 10.1039/c4ee02281e. (38) Liu, X.; Liu, W.; Ko, M.; Park, M.; Kim, M. G.; Oh, P.; Chae, S.; Park, S.; Casimir, A.; Wu, G.; Cho, J. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts. Adv. Funct. Mater. 2015, 25, 5799-5808, DOI 10.1002/adfm.201502217. (39) Cao, B.; Veith, G. M.; Diaz, R. E.; Liu, J.; Stach, E. A.; Adzic, R. R.; Khalifah, 30


Page 31 of 35 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


P. G. Cobalt Molybdenum Oxynitrides: Synthesis, Structural Characterization, and Catalytic Activity for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 10753-10757, DOI 10.1002/anie.201303197. (40) Liu, Z.-Q.; Cheng, H.; Li, N.; Ma, T. Y.; Su, Y.-Z. ZnCo2O4 Quantum Dots Anchored

on

Nitrogen-Doped

Carbon

Nanotubes

as

Reversible

Oxygen

Reduction/Evolution Electrocatalysts. Adv. Mater. 2016, 28, 3777-3784, DOI 10.1002/adma.201506197. (41) Hou, Y.; Wen, Z.; Cui, S.; Ci, S.; Mao, S.; Chen, J. An Advanced Nitrogen-Doped Graphene/Cobalt-Embedded Porous Carbon Polyhedron Hybrid for Efficient Catalysis of Oxygen Reduction and Water Splitting. Adv. Funct. Mater. 2015, 25, 872-882, DOI 10.1002/adfm.201403657. (42) De Koninck, M.; Marsan, B. MnxCu1-xCo2O4 used as bifunctional electrocatalyst in alkaline medium. Electrochim. Acta 2008, 53, 7012-7021, DOI 10.1016/j.electacta.2008.02.002. (43) Wen, X.; Yang, X.; Li, M.; Bai, L.; Guan, J. Co/CoOx nanoparticles inlaid onto nitrogen-doped carbon-graphene as a trifunctional electrocatalyst. Electrochim. Acta 2018, 296, 830-841, DOI 10.1016/j.electacta.2018.11.129. (44) Wan, X.; Wang, H.; Yu, H.; Peng, F. Highly uniform and monodisperse carbon nanospheres enriched with cobalt-nitrogen active sites as a potential oxygen reduction electrocatalyst.

J.

Power

Sources

2017,

346,

80-88,

DOI

10.1016/j.jpowsour.2017.02.030. (45) Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, 31



J.-S.; Wei, Z.; Wan, L.-J. Understanding the high activity of Fe–N–C electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of Fe–N x. J. Am. Chem. Soc. 2016, 138, 3570-3578, DOI 10.1021/jacs.6b00757. (46) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmekova, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem., Int. Ed. 2014, 53, 4372-4376, DOI 10.1002/anie.201311111. (47) Liu, Q.; Shi, J.; Hu, J.; Asiri, A. M.; Luo, Y.; Sun, X. CoSe2 Nanowires Array as a 3D Electrode for Highly Efficient Electrochemical Hydrogen Evolution. ACS Appl. Mater. Interf. 2015, 7, 3877-3881, DOI 10.1021/am509185x. (48) Li, M.; Liu, T.; Bo, X.; Zhou, M.; Guo, L. A novel flower-like architecture of FeCo@NC-functionalized ultra-thin carbon nanosheets as a highly efficient 3D bifunctional electrocatalyst for full water splitting. J. Mater. Chem. A 2017, 5, 5413-5425, DOI 10.1039/c6ta09976a. (49) Huang, Z.-F.; Song, J.; Li, K.; Tahir, M.; Wang, Y.-T.; Pan, L.; Wang, L.; Zhang, X.; Zou, J.-J. Hollow Cobalt-Based Bimetallic Sulfide Polyhedra for Efficient All-pH-Value Electrochemical and Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 1359-1365, DOI 10.1021/jacs.5b11986. (50) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550-557, DOI 10.1038/nmat3313. 32


Page 32 of 35

Page 33 of 35 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


(51) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0-14. J. Am. Chem. Soc. 2014, 136, 7587-7590, DOI 10.1021/ja503372r. (52) Ma, Y.-Y.; Wu, C.-X.; Feng, X.-J.; Tan, H.-Q.; Yan, L.-K.; Liu, Y.; Kang, Z.-H.; Wang, E.-B.; Li, Y.-G. Highly efficient hydrogen evolution from seawater by a low-cost and stable CoMoP@C electrocatalyst superior to Pt/C. Energy Environ. Sci. 2017, 10, 788-798, DOI 10.1039/c6ee03768b. (53) Bai, Y.; Zhang, H.; Liu, L.; Xu, H.; Wang, Y. Tunable and Specific Formation of C@NiCoP Peapods with Enhanced HER Activity and Lithium Storage Performance. Chem.-Eur. J. 2016, 22, 1021-1029, DOI 10.1002/chem.201504154. (54) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ. Sci. 2012, 5, 7936-7942, DOI 10.1039/c2ee21802j. (55) Zhang, H.; Zhou, W.; Chen, T.; Guan, B. Y.; Li, Z.; Lou, X. W. A modular strategy for decorating isolated cobalt atoms into multichannel carbon matrix for electrocatalytic oxygen reduction. Energy Environ. Sci. 2018, 11, 1980-1984, DOI 10.1039/c8ee00901e. (56) 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 33



Page 34 of 35

CoN4 site. Nano Energy 2018, 46, 396-403, DOI 10.1016/j.nanoen.2018.02.025. (57) Sun, X.; Li, K.; Yin, C.; Wang, Y.; Jiao, M.; He, F.; Bai, X.; Tang, H.; Wu, Z. Dual-site oxygen reduction reaction mechanism on CoN4 and CoN2 embedded graphene:

Theoretical

insights.

Carbon

2016,

108,

541-550,

DOI

10.1016/j.carbon.2016.07.051. (58) Zhang, L.; Liu, W.; Dou, Y.; Du, Z.; Shao, M. The Role of Transition Metal and Nitrogen in Metal-N-C Composites for Hydrogen Evolution Reaction at Universal pHs. J. Phys. Chem. C 2016, 120, 29047-29053, DOI 10.1021/acs.jpcc.6b11782.

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For Table of Contents Use Only

H2O

H2 Co-N-C

O2

H2O

An atomically dispersed Co- and N-codoped graphene catalyst shows excellent electrochemical HER and ORR performance in both acidic and alkaline media.


Atomically Dispersed Cobalt- and Nitrogen-Codoped Graphene

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