Metallic Ni3N Quantum Dots as Synergistic Promoter for NiO

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C: Energy Conversion and Storage; Energy and Charge Transport 3

Metallic NiN Quantum Dots as Synergistic Promoter for NiO Nanosheet towards Efficient Oxygen Reduction Electrocatalysis Hui Zhang, Meihuan Liu, Weiren Cheng, Yuanli Li, Wanlin Zhou, Hui Su, Xu Zhao, Peng Yao, and Qinghua Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00235 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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The Journal of Physical Chemistry

Metallic Ni3N Quantum Dots as Synergistic Promoter for NiO Nanosheet towards Efficient Oxygen Reduction Electrocatalysis Hui Zhang,1 Meihuan Liu,1 Weiren Cheng,1,* Yuanli Li,1 Wanlin Zhou,1 Hui Su,1 Xu Zhao,1 Peng Yao,2,* and Qinghua Liu1,* 1National

Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China 2Department

of Electronic Science and Technology, University of Science and Technology of China, Hefei 230027, Anhui, P. R. China

*E-mail: [email protected]; [email protected]; [email protected] Abstract The development of efficient and stable transition-metal based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells is highly desirable, yet great challenge remains. Here, we report a novel Ni3N quantum dots (QDs)/NiO heterostructure material, fabricated by immobilization of metallic Ni3N QDs onto the surface of NiO nanosheet, as a highly-active and durable electrocatalyst for efficient oxygen reduction performance. The electrochemical characterizations and theoretical calculations reveal that a strong interface coupling effect in Ni3N QDs/NiO heterostructure is realized by the interfacial hetero Ni species, synergistically accelerating the dissociation of adsorbed water molecule and reductive kinetics of adsorbed oxygen molecule during the ORR process. Hence, the developed Ni3N QDs/NiO heterostructure yields prominent oxygen reduction performance with a small half-wave potential (E1/2) of 0.76 V and high kinetic current density (Jk) of 15.4 mA cm-2 at 0.7 V, much superior to NiO nanosheet (0.65 V; 0.81 mA cm-2) and comparable to commercial Pt/C catalyst (0.80 V; 13 mA cm-2). In addition, the catalyst shows a long-term catalytic stability with robust methanol tolerance, serving as a promising noble-metal-free ORR catalyst for fuel cells.

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1. Introduction Fuel cells, converting chemical energy of fuels into electric energy, have been actively developed as promising renewable energy devices owing to their simplicity, high energy efficiency, low working temperature, and pro-environment.1 The crucial reaction process of fuel cells includes the four-electron reduction of O2 to form water at the cathode and the oxidation of H2 at the anode.2-4 Generally, the reaction kinetics of the oxygen reduction reaction (ORR) at the cathode is kinetically sluggish, which prevents fuel cells from achieving excellent performance.5-7 Noble-metal based catalysts, such as platinum (Pt)/C, are known as the currently state-of-the-art catalysts for the ORR. However, the scarcity and high-cost of Pt element as well as the low durability of Pt-based catalysts under operating conditions have greatly limited their large-scale application in fuel cells.8-13 Therefore, the discovery and design of cheap and abundant noble-metal-free catalysts, such as transition-metal alternatives, have been paid continuous attention around the world. It is well known that many transition-metal catalysts have long been studied as oxygen-involved electrocatalysts, mainly due to their earth-abundance, high catalytic activity and robust chemical stability.14,15 Among these transition-metal materials, nickel-based catalysts have exhibited high oxygen catalytic activity towards a potential ORR electrocatalysts.16 However, the ORR activity of Ni-based catalyst nowadays is still inferior due to its insufficient electrical conductivity and the lack of oxygen adsorption sites. Numerous efforts such as heterostructure construction and heteroatom doping have been performed to solve these intrinsic drawbacks for Ni-based catalysts. For example, the loading of NiO onto carbon nanotube (NiO/CNT) was found to effectively enhance the ORR performance due to the improved conductivity.17 Furthermore, N-doping was reported to significantly modify the local 2


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electronic structure of Ni sites within nickel oxynitrides towards efficient oxygen-involved electrocatalysis.18 Despite of these progresses, the dissociation of O-O bonds and the release of OH products during ORR process are still the bottleneck problems for Ni-based catalysts due to its strong binding ability with oxo-group. It is well known that the interfacial electron coupling between hetero-valence metal atoms at heterostructure interface is significantly beneficial for adjusting its oxo-group affinity towards high oxygen-relative catalytic activity.19 Consideration of different conductivity and metal-valence-state for nickel oxides and nitrides, it is expected to effectively tune the oxo-group affinity of Ni sites within Ni-based catalyst by construction of heterostructure with metallic nickel nitrides and semi-conductive nickel oxide. Under this way, the thermodynamically favorable adsorption of O2 on nickel nitride and H2O on nickel oxides at heterostructure interface would synergistically promote the dissociation of O-O bond for efficient ORR.20-22 Herein, we have designed a novel nickel nitride/oxide heterostructure of Ni3N quantum dots (QDs)/NiO, with metallic Ni3N QDs anchored on the surface of NiO nanosheet, to realize significant promotion in ORR activity for nickel-based catalysts. Thanks to the well-distribution of nanoscale metallic Ni3N QDs,23-26 the as-prepared Ni3N QDs/NiO heterostructure could exhibit outstanding ORR activity with a low half-wave potential (E1/2) of 0.76 V and high kinetic current density (Jk) of 15.4 mA cm-2 at 0.7 V. Furthermore, it also shows an excellent long-term ORR durability with strong methanol tolerance, as a promising noble-metal-free electrocatalyst for efficient fuel cells. According to the electronic structure characterizations, the intrinsically high ORR catalytic activity of Ni3N QDs/NiO could be attributed to the synergistic effect between Ni3N QDs and NiO nanosheet at the heterostructure interface, which not only thermodynamically facilitates the dissociation of *OO 3



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intermediates but also effectively promotes the mass transfer to accelerate the ORR kinetics. These findings open up a new way to fabricate highly active transition-metal based heterostructure as an efficient oxygen-involved electrocatalyst, which has a wide range of applications in fuel cells.

2. Experimental section 2.1. Synthesis of Ni3N/NiO heterostructure Typically, 1 mmol Ni(NO3)2·6H2O and 0.25 g cetyltrimethylammonium bromide (CTAB) were dissolved in 18 ml mixed solution of distilled water and methanol (v/v = 1:5) under vigorous stirring to form a transparent solution. Subsequently, this mixed solution was transferred into a 25 ml Teflon-lined autoclave, and then was sealed and heated at 150° C for 20 h. When the Teflon-lined autoclave naturally cooling down, the sample was collected by centrifuging the mixture, washed with distilled water and ethanol for several times and then dried at 60 °C overnight. After that, the dry sample was placed in the muffle furnace by a simple calcination reaction at 300 °C at a heating ratio of 3 °C min-1 and kept for 2 h to obtain the NiO power. Finally, the Ni3N QDs/NiO catalysts were prepared by nitrogenization of NiO power under a second heating of 300 °C in ammonia atmosphere for 1h. The Ni3N sample was obtained by nitriding NiO powder at 400 °C in ammonia atmosphere for 1 h in the tube furnace. 2.2. Morphological and structural characterization Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed by using a JEM-2100F microscope with an acceleration voltage of 200 kV. The X-ray diffraction (XRD) patterns were obtained in a Philips

L

$ Pro Super

X-ray diffractometer with Cu MN radiation. The field emission scanning electron 4


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microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) were taken on a Gemini SEM 500 scanning electron microscope. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII instrument with Mg MN 0 O = 1253.6 eV) as the excitation source. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.5 eV. The Ni L-edge X-ray absorption near-edge spectra (XANES) were measured at BL12B-a beamline of NSRL in the total electron yield (TEY) mode by collecting the sample drain current under a vacuum better than 5×10-8 Pa. The beam from the bending magnet was monochromatized utilizing a varied line spacing plane grating and refocused by a toroidal mirror. The energy range is 100-1000 eV with an energy resolution of ca. 0.1 eV. 2.3. Electrochemical performance measurements Electrochemical measurements were performed using an electrochemical workstation (Model CHI760D, CH instruments, Inc., Austin, TX) with a standard three-electrode electrochemical cell, where the prepared electrodes, a carbon rod and Ag/AgCl (saturated KCl) act as the working, auxiliary, and reference electrode, respectively. Before testing, the electrolyte was purged with high-purity N2 or O2 gas for at least 30 min. Linear Sweep Voltammetry (LSV) curves were carried out at a rate of 10 mV s-1 without IR correction after dozens of cyclic voltammetric scans until stable. Electrochemical impedance spectroscopy (EIS) was recorded in the frequency range of 0.1–100000 kHz at a potential of 0.6 V. To avoid the faradaic region, the electrochemical double-layer capacitance (Cdl) was calculated by cyclic voltammetry curves in the region of 0.86–0.96 V at various scan rates ranging from 20 to 200 mV s-1 with 20 mV increments. Tafel plots were derived from the LSV curves of samples. 5



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All the final potentials were calibrated with respect to a reversible hydrogen electrode (RHE). For electrode preparation, 5 mg of catalyst powder and 2 mg of acetylene black was dispersed into a mixture solution of 5 R Nafion (5%, Sigma Aldrich), 250 R ethanol, and 750 R distilled water by ultrasonic to form homogeneous catalyst ink. Then, 3 R of the resulting catalyst ink was evenly deposited onto the clean glassy carbon electrode (3 mm diameter with an electrocatalyst loading of

0.21 mg cm-2).

For comparison, Pt/C electrode was prepared by the same approach as the preparation of Ni-based catalyst, except that the Ni-based catalyst was replaced with commercial 20 wt% Pt/C.

3. Results and discussion The novel Ni3N quantum dots (QDs)/NiO heterostructure catalyst consisting of metallic Ni3N QDs and NiO nanosheet was synthesized through the solvothermal method followed by low-temperature ammonia treatment. The morphology, crystal structure

and

chemical

compositions

of

the

as-prepared

Ni3N

QDs/NiO

heterostructure were demonstrated by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) mapping images and the X-ray diffraction (XRD) patterns. Seen from Fig. 1a, NiO nanosheet with lateral size of about 18 R

was obtained. Furthermore, homogenous

Ni3N QDs with size of ~20 nm are well distributed on the surface of NiO nanosheet as shown in Fig. 1b. Moreover, Fig. 1c shows a large number of Ni3N QDs with high crystalline firmly anchored on the NiO nanosheet surfaces, where the lattice fringes spacing of 0.21 nm can be well indexed to the (002) facets of hexagonal Ni3N as shown in the inset of Fig. 1c.27 In addition, the EDS mapping reveals that the elements of Ni, O and N are homogenously distributed over the Ni3N QDs/NiO heterostructure 6


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-2

O2-saturated 0.1M KOH 1600 rpm Ni3N/NiO

-3 -4

Ni3N -5 0.4

0.6

0.8

86 mV dec

0.8

110 mV dec-1

0.7 0.6 -2.0

1.0

-1.5

-0.5

0.0

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0.09 -1/2

0.12

0.2

400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm

-3 -4 0.4

0.6

0.8

Potential (V vs RHE)

Fig. 2.

Ni3N

1.0

n 3.5

Hydrogen peroxide content

20 10 0 0.3

0.4

0.5

0.6

Pt/C

Ni3N/NiO

CH3OH addition

-2

-1

0.15

(S. rad

H2O2(%)

0.06

NiO

0

(f) -6

Electron transfer number

-1

0.3

n=3.7

-2

-1.0

log[Jk (mA cm-2)]

O2-saturated 0.1M KOH

2

-1

0.4

4

153mV dec-1

4.0

J (cm mA )

0.55V 0.60V 0.65V 0.70V

8

0.3

(e)

0

12

0.6


(d)

16 0.9

Current density (mA cm )

-6

NiO Pt/C

-1

0.9

(c)

Ni3N/NiO Ni3N NiO Pt/C

96 mV dec-1

E1/2 (V vs RHE)

1.0

JK (mA cm-2)

(b)

0 -1


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

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0.7

0.8


Ni3N/NiO Pt/C

-4 E=0.6V O2-saturated

-2

0.1M KOH

0

0

1500

3000

4500

6000

7500

Time (s)

(a) ORR polarization curves of Ni3N QDs/NiO, NiO, Ni3N and Pt/C under 1600 rpm in

0.1 M KOH solution, (b) The corresponding Tafel plots derived from the ORR polarization curves, (c) kinetic current density (Jk) at 0.70 V and half-wave potential (E1/2) for different catalysts, (d) ORR polarization curves of Ni3N QDs/NiO at different rotation rates (inset: K-L plots and electron transfer numbers), (e) Electron transfer number (n; top) and H2O2 yield (bottom) for Ni3N QDs/NiO, and (f) Methanol tolerance tests for Pt/C and Ni3N QDs/NiO.

To evaluate the catalytic activity of Ni3N QDs/NiO heterostructure, the oxygen reduction performance of the catalysts was tested with a rotating disk-electrode (RDE) in O2-saturated 0.1 M KOH solution. As shown in Fig. 2a, the Ni3N QDs/NiO heterostructure catalyst shows a low half-wave potential (E1/2) of 0.76 V with a high diffusion limited current density (JL) of ~4.7mA cm-2, which is very close to those of the benchmarking Pt/C (E1/2: 0.80 V, JL: 5.1 mA cm-2).30 In contrast, the individual Ni3N and NiO catalyst exhibits relatively low catalytic activity with E1/2 of 0.69 and 0.65 V and JL of 3.1 and 2.5 mA cm-2 for Ni3N and NiO, respectively. This indicates that the outstanding ORR catalytic activity of Ni3N QDs/NiO is mainly originated 8


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from its robust interface synergistic effect between the semi-conductive NiO and metallic Ni3N QDs. Moreover, Ni3N QDs/NiO heterostructure shows a much smaller Tafel slope of 86 mV dec-1 relative to that of NiO (153 mV dec-1), Ni3N (110 mV dec-1) and Pt/C (96 mV dec-1) (see Fig. 2b), indicating an improved ORR kinetics at the Ni3N QDs/NiO interface. More importantly, Fig. 2c displays that the kinetic current density (Jk) of Ni3N QDs/NiO is high up to 15.4 mA cm-2 at 0.7 V, evidently larger than that of Pt/C catalyst (13 mA cm-2). In order to deeply understand the ORR kinetics, the RDE test results were recorded under various rotating rates in the O2-saturated 0.1 M KOH for the Ni3N QDs/NiO heterostructure as displayed in Fig. 2d. The inset of Fig. 2d shows the Koutecky-Levich (K-L) plots with prefect linearity and similar slopes. Based on the K-L equation, the electron transfer number (n) was calculated to be approximately 3.7 at potentials ranging from 0.50 to 0.70 V, indicating that the Ni3N QDs/NiO heterostructure catalyst undergoes an efficient four-electron oxygen reduction process. Furthermore, the rotating ring-disk electron (RRDE) measurements reveal that the electron transfer number for Ni3N QDs/NiO heterostructure during ORR process is about 3.78 (Fig. 2d), consistent well with the calculated results derived from K-L plots. Meantime, the peroxide (H2O2) yield of Ni3N QDs/NiO heterostructure is less than 10% at the scanning potential range of 0.3–0.8 V as shown in the inset of Fig. 3d, demonstrating a favorable 4e- ORR for Ni3N QDs/NiO. Moreover, seen from Fig. 2f, the chronoamperometric curves (i–t) results show that the Ni3N QDs/NiO heterostructure could maintain 90% of its catalytic activity after 20 h continuous ORR 9



operation even in the presence of methanol, while the commercial Pt/C is with ~50% current density loss after adding of methanol. Furthermore, the metallic Ni3N QDs are uniformly anchored on the surface of NiO nanosheets by the SEM images and XRD patterns after ORR tests, suggesting a robust long-term stability for Ni3N/NiO heterostructure (Figure S3). These results confirm that the Ni3N QDs/NiO heterostructure is a promising ORR electrocatalyst with high catalytic activity, excellent long-term durability and robust methanol tolerance. (b)

Ni 2p

Ni3N/NiO

2p b 2/3

2p

Sat.

1/2

Sat.

Intensity (a.u.)

Ni L3

a

850

860

870

854.5 eV Ni L2

850

880

855

860

865

870

875

Energy (eV)

Binding Energy (eV)

(c)

(d) NiO Ni3N/NiO

0.3

NiO Ni3N/NiO

2.5 3.64 mF cm-2

2.0

-Z" (kohm)

-2

NiO NiO/NiN

NiO

Intensity (a.u.)

(a)

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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0.2

1.5 1.0

1.78 mF cm-2

0.1

0.5 0.0

20

40

60

80

100

0.0

0

1

Scan rate (mV s-1)

2

3

4

Z' (kohm)

Fig. 3. (a) Ni 2p XPS spectra, (b) Ni L3,2-edge XANES spectra for Ni3N QDs/NiO and NiO, (c) double-layer capacitances for Ni3N QDs/NiO and NiO towards ORR active surface area, (d) Impedance spectrum of the Ni3N QDs/NiO and NiO at 0.6 V vs RHE (inset shows the equivalent electrical circuit model).

10


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To obtain the atomic and electronic structure of Ni3N QDs/NiO heterostructure, X-ray photoelectron spectroscopy (XPS) was performed. As the Ni 2p spectra of NiO shown in Fig. 3a, the two characteristic peaks located at 853.9 and 855.6 eV can be assigned to the Ni 2p3/2 peaks of NiO, suggesting the appearance of Ni2+ and vacancy-induced Ni3+ in octahedral NiO.31 In comparison, a novel peak denoted as a is emerged at 852.6 eV for Ni3N QDs/NiO heterostructure relative to NiO, indicating the presence of Ni1+ within Ni3N QDs/NiO due to the existence of Ni3N QDs.32 Moreover, the main Ni 2p3/2 peak of Ni3N QDs/NiO heterostructure is obviously shifted to high energy side by 0.48 eV compared with that of NiO, indicating a robust interfacial electron transfer between NiO and Ni3N QDs. To further support this conjecture, the Ni L-edge X-ray absorption near-edge spectroscopy (XANES) was conducted. Seen from Fig. 3b, the Ni L3,2-edge of Ni3N QDs/NiO heterostructure shows a rising shoulder peak appeared at ~854.5 eV. This peak is attributed to the emergence of high-spin Ni2+ species, which is caused by the strong electron coupling at Ni3N QDs/NiO interface.33 The presence of high spin Ni2+ species means that there are much more unpaired or mobile electrons in Ni3N QDs/NiO heterostructure, which could endow the Ni3N QDs/NiO with high oxygen reduction catalytic activity. Indeed, the double-layer capacitance (Cdl), closely associated with the electrochemically active surface area (ECSA) of catalyst,34 is calculated to be 3.64 mF cm2 for Ni3N QDs/NiO heterostructure as shown in Fig. 3c, twice that of NiO (1.78 mF cm2). This result indicates that more surface active sites are produced within Ni3N QDs/NiO heterostructure. To clearly clarify the electrochemical behavior of Ni3N QDs/NiO heterostructure, the electrochemical impedance spectroscopy (EIS) was performed under 0.6 V vs 11



RHE in O2-saturated 0.1 M KOH. To gain quantitative analysis, the reasonable equivalent circuit model was built based on the EIS data. As shown in the inset of Fig. 3d, the Rs, Rct, and Rbulk correspond to mass-transfer resistance, charge transfer resistance, internal electrode resistance, respectively, and the constant phase element CPE1 and CPE2 are ascribed to the limited pseudo-faradic reaction and double layer capacitance, respectively.35 It can be seen that the values of Rbulk and Rct are estimated as 0.4 and 0.25 M[ for Ni3N QDs/NiO heterostructure electrode, respectively, much lower than those of NiO (1.5 and 1.2 M[3& This result suggests a fast electron transfer in Ni3N QDs/NiO heterostructure. Furthermore, relative to NiO, the CPE1value of Ni3N QDs/NiO heterostructure is evidently lower (1.8 vs 2.3 mF cm2), while that of CPE2 is much higher (3.8 vs 1.6 mF cm2). This result clearly indicates a larger active surface area and accelerated ORR kinetics for Ni3N QDs/NiO due to the synergistic effect at the Ni3N QDs/NiO interface.

0 Ef

-25 -50 80

DOS (a.u.)

(b)

Total Ni 3d O 2p

25

Formation energy (eV)

DOS (a.u.)

(a) 50

Total Ni 3d O 2p N 2p

40 0

M-*OO for ORR

2

1

Ef

-40 -80

-4

-2

0 Energy (eV)

2

0

4

(c) 0.6

(d) U = 1.23 V

0.4

Free energy (eV)

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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NiO

Ni3N

Ni3N/NiO

ORR process under operation H2O Had

O2 OH

0.2

N

H

Ni

O

O2

0.0

OHad

Ni3N Had NiO

-0.2

Ni3N/NiO -0.4

Pt

O2

*OO

NiO

*OOH

*O

Ni3N/NiO interface

H2O

Reaction coordinate

12


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Fig. 4. (a) DOSs for Ni3N QDs/NiO, (b) Theoretical binding energy calculations of *OO Ni3N QDs/NiO towards ORR, (c) Free energy change \D for ORR mechanism under equilibrium potential of 1.23 V for Ni3N QDs/NiO and NiO, (d) Schematic illustration of the interfacial synergy between NiO and Ni3N for an enhanced ORR under alkaline conditions.

To deeply understand the origin of the high catalytic activity, the first-principles theoretical calculations were carried out to reveal the four-electron ORR mechanism of Ni3N QDs/NiO heterostructure. The electron densities of states (DOSs) for the Ni3N QDs/NiO shown in Fig. 4a exhibit an evidently newly-formed energy level across the Fermi level with a remarkably narrowed bandgap relative to pure NiO.36 These results indicate a high carrier concentration and superior electrical conductivity for Ni3N QDs/NiO heterostructure, which would greatly low the adsorption barrier of surface oxo-contained precursor and simultaneously promote the oxygen-relative catalytic kinetics. Indeed, according to Fig. 4b, the theoretical formation energy of *OO for Ni3N QDs/NiO heterostructure was only 0.32 eV, close to the theoretical values of the commercial Pt catalysts (0.38 eV) and much smaller than that of NiO (1.7 eV) and Ni3N (1.52 eV).37 In addition, the theoretical Gibbs free energy change of reactive intermediate formation under equilibrium potential of 1.23 V during ORR process for Ni3N QDs/NiO heterostructure was further conducted as shown in Fig. 4c. The conversion of O2 into *OO is an endothermic reaction for NiO and requires the highest energy barrier of 0.41 eV compared to the other steps, indicating the rate-limiting step of *OO formation for NiO during ORR process.38 In contrast, the calculated free energy of *OO formation for Ni3N QDs/NiO heterostructure has been significantly reduced by 0.38 eV relative to that of NiO, inferring the overcome of energy barrier of *OOH formation towards accelerated ORR kinetics. Furthermore, the change of the free energy for *OH formation and release becomes smaller relative 13



to NiO, suggesting an accelerated ORR kinetics due to the synergistic effect between metallic Ni3N and semiconducting NiO. More importantly, the adsorbed H2O on Ni sites of NiO could be easily dissociated into H+ and OH-, whereas these yielded H+ would prefer to couple with the *OOH intermediate that was adsorbed on Ni sites of Ni3N. This leads to the cleavage of O-O to form key *OH intermediate and then effectively promotes the ORR kinetics at the Ni3N QDs/NiO interface as shown in Fig. 4d.20 Therefore, the synergetic effect between Ni3N and NiO will endow the Ni3N QDs/NiO heterostructure with efficient ORR activity.

4. Conclusions In summary, we have developed a new type of Ni3N quantum dots (QDs)/NiO heterostructure as a promising ORR electrocatalyst by a hydrothermal method combined with ammonia treatments. The as-synthesized Ni3N QDs/NiO catalyst with metallic Ni3N quantum dots anchored on NiO nanosheet can exhibit superior ORR electrocatalytic activity and good durability in an alkaline medium with a half-wave potential (E1/2) of 0.76 V (vs. RHE) and high kinetic current density (Jk) of 15.4 mA cm-2 at 0.7 V.

According to electronic structure characterizations and theoretical

calculations, the intrinsic ORR catalytic activity is attributed to the synergistic effect between the metallic Ni3N QDs and the semi-conductive NiO nanosheet at the Ni3N QDs/NiO heterostructure interface. The unique electronic structure of Ni3N QDs/NiO heterostructure around the Fermi level not only increases effective oxygen-relative active sites and promotes the electron transfer, but also significantly facilitates the formation of key *OOH and *OH intermediates, contributing to an efficient ORR performance. This finding provides a new path for the preparation of advanced electrocatalyst based on transition-metal towards efficient fuel cells in the future. 14


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Supporting Information The detailed methods of preparation on the Ni3N QDs/NiO catalysts, the SEM, XRD spectra, the electrochemical measurements, and DFT calculation details.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Grants No. 21603207, 11875257 and U1532265), and the Fundamental Research Funds for the Central Universities (WK2310000070).

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TOC Graphic

ORR process under operation H2O Had

O2 OH

N

H

Ni

O

O2 OHad

Ni3N Had NiO

Ni3N/NiO interface

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Metallic Ni3N Quantum Dots as Synergistic Promoter for NiO

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