Nickel Nanoparticles Encapsulated in Nitrogen-Doped Carbon

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Nickel Nanoparticles Encapsulated in Nitrogen-Doped Carbon Nanotubes as Excellent Bifunctional Oxygen Electrode for Fuel Cell and Metal−Air Battery Guoyu Zhong,† Simin Li,† Shurui Xu,† Wenbo Liao,† Xiaobo Fu,*,† and Feng Peng*,‡ †

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/24/18. For personal use only.

School of Chemical Engineering and Energy Technology, Key Laboratory of Distributed Energy Systems of Guangdong Province, Dongguan University of Technology, No.1 Daxue Road, Songshan Lake, Dongguan 523808, China ‡ School of Chemistry and Chemical Engineering, Guangzhou University, 230 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou, 510006, China S Supporting Information *

ABSTRACT: Developing low cost, high-performance, and durable bifunctional catalysts for oxygen reduction and oxygen evolution reactions is critical for a commercial application of fuel cells and metal−air batteries. Nitrogen-doped carbon nanotubes encapsulated nickel nanoparticles are prepared through a simple pyrolysis procedure with melamine and nickel chloride hexahydrate as precursors. The catalyst is featured by nickel nanoparticles encapsulated inside nitrogen-doped carbon nanotubes, with abundant surface nitrogen doping. The optimized catalyst exhibits proximate oxygen reduction activity to platinum/carbon catalyst, comparable oxygen evolution activity to ruthenium dioxide catalyst, and better stability to noble metal catalysts in alkaline medium. The oxygen electrode activity parameter (the gap between the potential of oxygen evolution at 10 mA cm−2 and the half-wave potential of oxygen reduction) of the as-prepared catalyst is 0.754 V, which is among the state-ofthe-art bifunctional electrocatalysts reported to date. To explore the active sites, a series of catalysts with different bulk nickel and surface nitrogen contents are synthesized and served as the oxygen reduction and oxygen evolution reactions catalysts. The results reveal that the oxygen reduction activity of this catalyst arises from the doped nitrogen, while the oxygen evolution activity originates from the encapsulated nickel nanoparticles. KEYWORDS: Bifunctional electrocatalysts, Fuel cells, Metal−air batteries, Oxygen reduction reaction, Oxygen evolution reaction, Active sites

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INTRODUCTION

scarce resources of these precious metals impede the application of fuel cells and metal−air batteries.3,4 Furthermore, these precious metal catalysts usually exhibit a nice catalytic performance along only one reaction direction for the oxygen electrode, which is not competent for rechargeable metal−air batteries and fuel cells.6 Hence, many studies have focus on finding the non-noble metals bifunctional catalysts for ORR and OER. At present, transition metal (TM) catalysts are one of the greatest potential bifunctional catalysts owing to

Currently, there is a great demand for sustainable and clean power, because of the increasing environmental problems and the limited reserves of fossil fuels. Among many new energy technologies, fuel cells and metal−air batteries have attracted great attention for their high transformation efficiency and high energy density.1 The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are crucial to the efficiencies of fuel cells and metal−air batteries, are sluggish in reaction kinetics and require catalysts to expedite the reactions.2,3 Pt-based catalysts are the best ORR catalysts,4 while IrO2 and RuO2 are the state-of-the-art OER electrocatalysts.5 Nevertheless, the high price, poor stability, and © XXXX American Chemical Society

Received: July 24, 2018 Revised: September 8, 2018 Published: September 12, 2018 A

DOI: 10.1021/acssuschemeng.8b03582 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) SEM, (b) TEM, (c) HRTEM, and (d) elemental-mapping images of Ni@NCNTs-800.

their high activity and low cost.7−9 Particularly, doping with heteroatoms or loading on conductive materials can substantially enhance the activity of transition metal catalysts, such as transition metal oxide supported on graphene,10 carbonsupported transition metal/nitrogen compounds (MeNx/C),11 metal nitrides/phosphides/sulfides,12−14 and so on. However, the transition metal catalysts display poor stability, especially in acid, as a result of the dissolution, the conglomeration, and the exfoliation. Recently, Bao15 reported a new ORR catalyst of iron encapsulated in carbon nanotubes (CNTs). They suggested that the high ORR performance originates from the encapsulated iron particles which decreased the local work function of the surface carbon. We also prepared nitrogendoped carbon nanotubes (NCNTs) with encapsulated Fe3C nanoparticles as ORR catalyst.16 It was proven that the doped N is the main active site for ORR and the inner Fe3C with surface carbon forms the synergetic effect to enhance ORR activity. This kind of catalyst was also proved to be efficient in OER, such as TMs encapsulated in single layer graphene,17 Co@Co3O4 encapsulated in CNTs,18 and TMs encapsulated in NCNTs.19,20 Those studies indicated transitional metal encapsulated in the N-doped carbon matrix (M@NxC), showing high performance and durability for ORR and OER, is one of the most promising bifunctional catalysts.21 Even so, there are still two indistinct problems for the M@NxC bifunctional catalyst. First, except Fe or Co, other transition metals encapsulated structures are rarely reported in the literature. However, those transition metals are high potential; for example, the higher oxidation state of Ni is very active for OER. Second, the roles of the encapsulated metal and the doped N atom in ORR and OER are unclear. As reported in previous studies, metal nanoparticles encapsulated in the carbon matrix15,19,22 and doped nitrogen16,23,24 both are

identified as the most probable active sites, which are simultaneously responsible for OER and ORR. But as far as we know, nickel is very active to OER and inactive to ORR; the doped N is just the opposite. Therefore, it is significant to synthesize Ni encapsulated in the nitrogen-doped carbon matrix as model catalyst, to severally distinguish the active sites of M@NxC for ORR and OER. Herein, we introduced a simple approach for the preparation of nitrogen-doped carbon nanotube with encapsulated Ni nanoparticles (Ni@NCNTs) via a solid-state thermal reaction. It is demonstrated that catalyst structures, active species composition, and catalytic performances of Ni@NCNTs are primarily engineered by the synthesis step and the thermal treatment temperature. Accordingly, the decent Ni@N-CNTs catalyst after optimization exhibits superior bifunctional catalytic activity and long-term stability toward ORR and OER under alkaline conditions. The roles of the encapsulated metal and the doped N atom in ORR and OER have been discussed.

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EXPERIMENTAL SECTION

Catalysts Preparation. The Ni@NCNTs catalysts were prepared by simply pyrolyzing nickel chloride and melamine. A typical preparation is as follows: 2 g of NiCl2·6H2O, 4 g of melamine, and 30 mL of ethanol solution were stirred for 1 h. The suspension was dried at 70 °C. The dry precursors were heated to an appointed temperature from 650 to 1000 °C with a rate of 6 °C min−1 for 2 h in N2 atmosphere. The obtained black powder was pickled in 6 M HCl for 12 h at 25 °C to remove the unstable phases and then washed in deionized water. The products were named as Ni@NCNTs-X with X being the annealing temperature. For comparison, the products were also pickled in 6 M HCl for 0, 1, 2, 4, 8 h, which were named as Ni@ NCNTs-X-Y (Y indicated the pickling times, 0−8 h). To adjust the doped nitrogen content of Ni@NCNTs, postprocessing is as follows. Typically, 200 mg Ni@NCNTs-800 were B


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ACS Sustainable Chemistry & Engineering heated to 700 °C at 6 °C min−1, maintained at 700 °C for 0.5 h, and then cooled to room temperature in N2 flow. The obtained samples were named as Ni@NCNTs-800-T-t with T being the post-treatment temperature and t being the post-treatment time. CNTs were provided by Tsinghua University. The amorphous carbon and the residual metal catalyst were removed.25 N-doped CNTs (NCNTs) were prepared as described in the literature.26 Structural Characterizations. Scanning electron microscopy (SEM, Merlin, Zeiss Co.) and transmission electron microscopy (TEM, JEOL, JEM2100F) were used to examine the microstructure of the catalysts. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo ESCALAB 250XI using Al Ka radiation (1486.6 eV) with the C 1s peak at 284.8 eV as standard. The energy dispersion spectrum (EDS) was measured by X-MaxN20 (Oxford Co.) affiliated to the SEM. X-ray diffraction spectra (XRD) were measured on an Xray diffractometer (Bruker D8 ADVANCE, Germany) using Cu Ka radiation. The thermogravimetric analysis (TGA) was tested on a Netzsch STA 409 PC analyzer. The Brunauer−Emmett−Teller (BET) specific surface area was detected by a TriStar II 3020 analyzer. Electrochemical Measurements. Electrochemical tests were performed on a CHI 760D electrochemical analyzer assembled with a rotating disk electrode (RDE, the glassy carbon disk diameter of 5 mm, pine Instrument Co.). Graphite rod and Ag/AgCl electrode served as the counter and reference electrodes, respectively. The potentials reported in this work were calibrated to reversible hydrogen electrode (RHE) using the following equation:

E RHE = EAg/AgCl + 0.059 × pH + 0.197

the structures similar to that of Ni@NCNTs-800, as shown in Figure S1 and Figure S2. Specifically, it can be observed that the nanoparticles keep the size and amount constant with the preparation temperature rising from 700 to 900 °C, as shown in Figure S3. However, the samples Ni@NCNTs-650 and Ni@ NCNTs-1000 show shorter carbon nanotubes and more impurities than Ni@NCNTs-800 in Figure S1. The bulk component of Ni@NCNTs catalysts was detected by EDS and TGA. The EDS results (Table S1) reveal that the nitrogen and nickel contents decrease with the preparation temperature increasing. The Ni@NCNTs-800 contains C (72.37 wt %), N (2.01 wt %), O (1.50 wt %), and Ni (24.12 wt %). The as-prepared samples at different temperatures have similar bulk Ni content of 24.42−27.70 wt %, except for the Ni@NCNTs-1000 of 15.89 wt %. The Ni content encased in Ni@NCNTs is higher than that in the reported Ni@NCNTs (8.22 wt %).19 On the other hand, with increasing synthesized temperature the bulk N content first increased, reached the maximum value (3.95 wt %) at 700 °C, and then decreased. TGA test (Figure S4) reveals Ni content of 28.77 wt % in Ni@ NCNTs-800, close to the EDS result. The phase structures of Ni@NCNTs catalysts were investigated by XRD, as shown in Figure 2. At 600 °C, the

(1)

27

where pH = 13 in 0.1 M KOH. The catalyst suspensions were fabricated as follows: 2 mg of catalyst was ultrasonically dispersed in a mixture of 100 μL of acetone, 385 μL of deionized water, and 15 μL of 5% Nafion solution. A 20 μL suspension was pipetted on the glassy carbon electrode surface and air-dried at room temperature. Catalyst loading was 251.2 mg cm−2 for all electrochemical measurements, without additional explanation. The ORR activity was investigated by linear scan voltammogram (LSV) in the oxygen-saturated 0.1 M KOH with a scan rate of 10 mV s−1, from 0 to 1 V vs RHE electrode. The transferred electron number (n) of ORR is calculated based on the Koutecky−Levich equation (Supporting Information). The durability of the catalysts was evaluated by cyclic voltammetry (CV) on a 4 mm glassy carbon disk in oxygen-saturated 0.1 M KOH. The OER activity was investigated by the LSV in 0.1 M KOH solution with a scan rate of 10 mV s−1, from 1 to 1.85 V vs RHE. The OER stability was evaluated based on the accelerated degradation test.17 The LSV of OER was recorded after various numbers CV scanned from 0.5 to 1.5 V (vs. RHE) at 100 mV s−1.

Figure 2. XRD patterns of Ni@NCNTs prepared at different temperatures.

obtained sample only shows a weak diffraction peak of Ni at 2θ = 44.3°,30 indicating that the sample is mainly amorphous carbon containing Ni. The XRD patterns further demonstrate the existence of graphitic carbon and Ni phase in Ni@NCNTs, when the preparation temperature is above 600 °C. As shown in Figure 2, the broad peak at 2θ = 26.5° corresponds to the (002) plane of graphitic carbon. Moreover, the sharp peaks at around 2θ = 44.3°, 51.6°, and 75.8° correspond to the (111), (200), and (220) planes of single nickel.28 No other diffraction peaks were detected, indicating the nanoparticles are mainly nickel. The height of graphite carbon peaks for samples synthesized above 700 °C is almost the same, which suggests a similar crystallinity. The surface composition of Ni@NCNTs was detected by high-resolution XPS, as shown in Figure 3a and Table S2. The samples display four characteristic peaks of carbon (C 1s), nitrogen (N 1s), oxygen (O 1s), and nickel (Ni 2p) at the binding energy of 284, 400, 532, and 860 eV, which is consistent with the EDS result. Contrary to the EDS results, for Ni@NCNTs-800, the XPS results show that the surface nitrogen content is as high as 7.91 atom %, while the surface

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RESULTS AND DISCUSSION Catalyst Characterization. The microstructures of the typical sample Ni@NCNTs-800 were examined via SEM (Figure 1a) and TEM (Figure 1b). The Ni@NCNTs-800 shows typical bamboo-like carbon nanotubes with abundant defects, crinkles, and encapsulated Ni nanoparticles. The highresolution TEM (HRTEM) image in Figure 1c reveals that the Ni particles were coated with several clingy graphitic layers, which stabilizes the Ni particles in solutions. The lattice distance is 0.203 nm, corresponding to the (111) crystal face of the Ni phase.28 The four elements C, Ni, N, and O were detected by the EDS elemental-mappings (Figure 1d). The C and N were evenly dispersed in carbon nanotubes, which suggest the uniform dispersion of the doped N atoms, while Ni and O elements were mostly distributed in the nanoparticles. The presence of the O element can be attributed to the nickel oxides formed on the surface of the nanoparticle by air oxidation.29 The Ni@NCNTs-700 and Ni@NCNTs-900 show C


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Figure 3. (a) XPS survey, (b) high-resolution N 1s spectra and (c) high-resolution Ni 2p spectra of Ni@NCNTs samples.

graphitic layer is only ∼2 nm, even smaller than the XPS detection depth (∼10 nm). Incorporating the EDS and XPS results, it is reasonably concluded that the Ni is mostly wrapped inside NCNTs, while nitrogen species is mostly dispersed on the surface of NCNTs. The increase of the preparation temperature from 650 to 1000 °C affects surface nitrogen content and decreases the bulk Ni content.

nickel content is only 1.51 atom %. The surface nitrogen content increases as synthesized temperature increases, arrives at the maximum at 800 °C, and then decreases as synthesized temperature further increases, while the surface nickel content is low in the range of 1.26−2.14 atom %. These trace Ni, that are detected by XPS, can be attributed to the Ni nanoparticles covered with thin graphitic layers. As shown in Figure 1c, the D


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ACS Sustainable Chemistry & Engineering Table 1. Quantitative XPS Analysis of the Ni@CNTsa Sample

N/(C+N) (%)

NP (%)

NPyr (%)

NQ (%)

NOX (%)

Nads (%)

Ni@NCNTs-650 Ni@NCNTs-700 Ni@NCNTs-800 Ni@NCNTs-900 Ni@NCNTs-1000 [email protected] [email protected] Ni@NCNTs-800-900-1h

7.27 7.81 8.40 7.57 5.65 7.84 7.03 5.80

25.1 27.4 30.5 29.7 19.5 32.1 23.3 24.5

34.0 33.3 28.7 23.0 24.0 26.0 22.5 19.2

23.4 23.2 24.2 29.2 36.6 26.6 35.9 35.7

7.8 10.0 10.2 11.3 10.7 8.9 9.2 12.6

9.7 6.1 6.4 6.8 9.1 6.5 9.1 8.0

a

NP: pyridinic N, NPyr: pyrrolic N, NQ: quaternary N, NOX: N-oxides, Nads: chemisorbed N.

Figure 4. (a) Nitrogen adsorption−desorption curves and (b) pore size distributions of Ni@NCNTs.

The impact of preparation temperature on the nitrogen species was also analyzed by deconvolving the N 1s spectra. Five different N-species can be assigned, representing pyridinic-N (398.6−398.8 eV), pyrrolic-N (400.3 eV), graphitic/quaternary-N (401.5 eV), N-oxides (403 eV), and chemisorbed N (405 eV),26,31 as shown in Figure 3b and Table 1. Pyridinic-N, pyrrolic-N, and quaternary-N are the dominant nitrogen species for Ni@NCNTs. The chemisorbed N was detected in the Ni@NCNTs due to N2 adsorbed or encapsulated in NCNTs. The main N-species changes with preparation temperature from 650 to 1000 °C, the pyrrolic-N decreases from 34.0 to 24.0%; the graphitic-N increases from 34.4 to 36.6%; the pyridinic-N first increases from 25.1 to 30.5% and then decreases to 19.5%. The results indicate that the unstable pyrrolic-N transforms into pyridinic-N and then the pyridinic-N transforms into graphitic-N with the increase of preparation temperature.31 Moreover, Figure 3c shows the Ni 2p high-resolution XPS of Ni@NCNTs. The Ni 2p spectra are composed of two main spin orbitals; there are 2p3/2 and 2p1/2 at 855.4 and 873.0 eV, respectively. In general, the Ni 2p3/2 and 2p1/2 spectra show a complicated shape with satellite peaks at higher binding energy than the main peaks owing to multielectron excitation.32 Deconvolving the Ni 2p spectra, the peak at 852.0 eV can be ascribed to nickel, the strong peaks at 855.4 and 873.0 eV can be attributed to nickelous oxide, and the peaks at 861.0 and 879.0 eV belong to nickel hydroxide. The presence of nickelous oxide and nickel hydroxide is attributed to the surface oxidation of Ni nanoparticles by wet air. 33 Furthermore, the weak peak of nickel is attributed to the shallow detection depth of XPS. It is noteworthy that there is no characteristic peak of nickelous oxide or nickel hydroxide in the XRD spectrum of Ni@NCNTs (Figure 2) due to the

amorphous nickelous oxide and nickel hydroxide. Based on the above, it is easy to understand that the Ni 2p spectrum of Ni@ NCNTs does not change with preparation temperature. The nitrogen adsorption−desorption isotherms and the pore size distributions (PSD) of Ni@NCNTs are displayed in Figure 4. All isotherms exhibit adsorption hysteresis at the large P/P0 range of ca. 0.4−1.0, pointing out the existence of mesopores. The PSD of those samples reveals that their porosities are mainly divided into two mesopore and one macropore systems with sizes concentrated in ∼2.8 nm, 28 nm, and ∼100 nm, respectively. BET surface area, pore volume, and pore size are summarized in Table S3. All the samples exhibit BET surface areas of ∼200 m2 g−1, pore size of 10.92− 15.23 nm, and pore volume of 0.61−0.76 cm3 g−1. These results indicate that the Ni@NCNTs prepared at 650−1000 °C show the same porosity. ORR/OER Activity and Stability. The ORR performances of Ni@NCNTs-800 were investigated and contrasted with those of CNTs, NCNTs, and a 20 wt % Pt/C catalyst by LSV and Koutecky−Levich (K-L) plots in 0.1 M KOH, as shown in Figure 5a and 5b. The Ni@NCNTs-800 exhibits high onset potential of ca. ∼0.970 V, half-wave potential of ca. ∼0.830 V, and large limiting current of ca. ∼4.65 mA cm−2, which is superior to the NCNTs and almost the same as the Pt/C catalyst. The half-wave potential of Ni@CNTs-800 is slightly 15 mV less than that of Pt/C catalyst. This reveals a higher catalytic activity of Ni@NCNTs compared with NCNTs. As shown in Figure S5e, the current density of Ni@NCNTs-800 increases rapidly when the rotation speed increases from 200 to 2500 rpm. The Koutecky−Levich curves (j−1 vs ω−1/2) at various scanned potentials display good linear relation (Figure 5b), and the slopes are similar at the range of 0.3−0.6 V, which suggest similar electron transfer numbers for oxygen reduction. E


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Figure 5. (a) ORR polarization curves. (b) Koutecky−Levic plots and electron transfer number n (inset) of Ni@NCNTs-800, CNTs, NCNTs, and Pt/C. (c) OER polarization curves and (d) tafel plots of Ni@NCNTs-800, CNTs, NCNTs, RuO2, and Pt/C. (e) Bifunctional catalytic activities of various catalysts for both ORR and OER. The polarization curves were obtained at a rotation speed of 1600 rpm in 0.1 M KOH solution at a scan rate of 10 mV s−1.

was investigated according to the ORR peak current densities at different cyclic voltammetries, as shown in Figure S7. After 10,000 cycles, the peak current of Ni@NCNTs-800 remains approximate 83.5% of its original one, while Pt/C catalyst shows an obvious decrease in our prior work.34 The OER activities of Ni@CNTs-800, NCNTs, RuO2, and 20 wt % Pt/C catalysts were investigated by LSV from 1.0 to 1.75 V (Figure 5c). Ni@CNTs-800 catalyst exhibits much lower onset potential and greater current density than that of

The results of NCNTs and Pt/C catalysts are shown in Figure S6 under the same conditions. The electron transfer numbers (n) of all the catalysts at various potentials were calculated based on the Koutecky−Levich equation. The n value of Ni@ NCNTs-800 rises from 3.75 to 3.98 over the range of 0.3−0.6 V, which is slightly higher than ∼3.73 of NCNTs and lower than ∼4.0 of Pt/C, as shown in the inset of Figure 5b. The result indicates a dominated four-electron oxygen reduction process of Ni@NCNTs-800. The stability of Ni@NCNTs-800 F


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Figure 6. (a) ORR and (b) OER polarization curves of Ni@NCNTs-800 with different acid leaching times. (c) ORR and (d) OER polarization curves of Ni@NCNTs prepared at 650−1000 °C. (e) ORR and (f) OER polarization curves of Ni@NCNTs heat-treated at 700 and 900 °C. Those curves were obtained at a rotation speed of 1600 rpm in 0.1 M KOH solution at a scan rate of 10 mV s−1.

as shown in Figure S9. After long CV sweeps, the anodic peak of Ni(II)/Ni(III) increased obviously, because the Ni has been exposed for the oxidation of carbon.37 The XPS results in Table S2 and Figure S10 also demonstrated the exposing of Ni for Ni@NCNTs-800 after OER measurement. After 9,000 potential cycles, the Ni@CNTs-800 retained a good performance to the OER. Moreover, the potential values of 10 mA cm−2 at different cycles were recorded in Figure S9. The potential of Ni@NCNTs-800 exhibits an increase from +1.584 V to +1.632 V. These suggest that Ni@NCNTs-800 is a stable catalyst for the OER. To further evaluate the bifunctional catalytic activities, the common activity parameter ΔE (ΔE = Ej=10 mA,OER − E1/2,ORR)38 was calculated. As shown in Figure 5e, the Ni@ NCNTs exhibited the smallest ΔE (∼0.754 V), which is smaller than Pt/C (0.955 V) and NCNTs (1.055 V). The ΔE of Ni@NCNTs surpasses most of the bifunctional catalysts and equals the best bifunctional catalysts, such as RuO2 (1.511 V),19 N-graphene/CNT (0.96 V),21 Co3O4/NG (0.77 V),10

commercial Pt/C catalysts and NCNTs, which is comparable to that of the state-of-the-art RuO2 catalyst (Sigma-Aldrich). The oxidation peak at ca. 1.36 V in the LSV of the Ni@ NCNTs was attributed to an oxidation of Ni(II) to Ni(III),35 which is also detected by CV in Figure S8. The overpotential desired at the current density of 10 mA cm−2 (Ej=10 mA) is an important performance parameter of OER, which is relevant to solar fuel synthesis.36 Ni@CNTs-800 catalyst exhibits an Ej=10 mA of ∼1.584 V, which is ca. 220 mV less than that of Pt/ C and close to Ej=10 mA = 1.548 V of RuO2. To gain additional insights into the OER process on Ni@CNTs-800, the Tafel plots of the potential vs log (current density) were recorded in Figure 5d. The calculated results of tafel slopes are ∼71, ∼84, ∼111, ∼121, and ∼191 mV dec−1 for RuO2, Ni@NCNTs-800, NCNTs, CNTs, and commercial Pt/C catalysts, respectively. Ni@NCNTs-800 displays a small Tafel slope close to the RuO2 catalyst, revealing the outstanding intrinsic OER kinetics of Ni@NCNTs-800. To evaluate the durability, an accelerated stability test was adopted.17 The LSVs for OER were recorded after long CV scans from 0.5 to 1.5 V (vs RHE) at 100 mV s−1, G


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Figure 7. Dependences of (a) ORR and (b) OER activity on the gross nitrogen content, the amounts of various nitrogen containing functionalities, and bulk Ni content.

Co−N−C (0.74 V),39 Fe@N−C (0.86 V),40 and so on, as shown in Table S5. Discussion of the ORR/OER Active Sites. The Ni@ NCNTs-800 exhibits significantly superior ORR/OER activity, but the origin of its high activity is still not clear. According to the references in Table S5,15,17,41−43 the excellent ORR/OER activity of Ni@NCNTs could be attributed to three potential active sites: (1) the doped N species, (2) the encapsulated Ni with surface carbon, and (3) the trace amount of Ni on the carbon nanotube. It is noteworthy that most references imply that the active sites of such a bifunctional catalyst are simultaneously responsible for both ORR and OER. Nevertheless, to our knowledge, nickel is very active to OER and inactive to ORR; the doped N is just the opposite. Hence, we suspect that Ni@NCNTs could devote active sites separately to ORR and OER. To explore the active site of Ni@NCNTs for the ORR/OER, a variety of experiments were performed and analyzed. To probe the effect of surface Ni on ORR/OER, the Ni@ NCNTs prepared at 800 °C were also acid pickled in 6 M HCl for 0, 1, 2, 4, 8 h. As shown by SEM (Figure S1f) and XRD (Figure S11), the Ni@NCNTs-800−0h contains a lot of Ni particle on the NCNTs. Contrasting the bulk component (Table S1) of Ni@NCNTs-800-0h to that of Ni@NCNTs-800 (acid leaching 12h), it is found that the content of Ni decreases from 39.44 to 24.42% after acid leaching. The Ni@NCNTs800-0h also shows a small surface area of ∼62.64 m2 g−1 for the high Ni content, as shown in Figure 4 and Table S3. As revealed by SEM, XRD, EDS, and BET, the acid pickling decreases the Ni content without destroying the nanotube structure. Figure 6a reveals an increase of ORR activity as acid pickling increases from 0 to 12 h, which indicates that the higher surface Ni content leads to a lower ORR activity. Hence, it can be concluded that the surface Ni is adverse to ORR. But for OER, there is an opposite phenomenon. Figure 6b shows that the Ni@NCNTs-800-0h exhibits higher OER activity than Ni@CNTs-800. This observation indicates that the Ni on the outside surface is facilitated to OER.

According to the characterization, the Ni@NCNTs prepared at 650−1000 °C display approximate morphology, porosity, BET surface area, and graphitization but different surface nitrogen content and bulk Ni content. Therefore, these samples can be used as a model catalyst to probe the effects of the surface nitrogen and the inner Ni on ORR/OER. As shown in Figure 6c, Ni@NCNTs prepared at 650−1000 °C exhibit approximate onset potential, half-wave potential, and LSV shape for ORR. Moreover, the transferred electron numbers of samples show a similar trend and value at various scan potentials, which proves a uniform ORR mechanism, as shown in Figure 5b and Figure S5. However, the ORR current density of Ni@NCNTs is strongly dependent on preparation temperature. The ORR current density increases with the preparation temperature increasing from 650 to 800 °C. When the preparation temperature further increases, the ORR current density decreases. The above results suggest that the active sites of Ni@NCNTs remain the same as that of CNTs. Combining with EDS (Table S1) and XPS (Table S2), the results clearly show that the ORR activities of Ni@NCNTs are related with the surface nitrogen but not affected by bulk Ni content. To exclude the possible effect of nitrogen, the Ni@NCNTs800 was thermal-treated again under 700 and 900 °C. As shown by EDS and XPS in Table S1 and S2, the second thermal treatment of Ni@NCNTs-800 results in a decrease of the surface nitrogen but without affecting its bulk Ni content. The obtained samples exhibit lower ORR activity in terms of reduction currents (Figure 6e), especially for samples with higher thermal-treated temperature and longer thermal-treated time. On the other hand, the second thermal treatment does not obviously reduce the transferred electron numbers of Ni@ NCNTs-800 (Figure S12). The result suggests that the ORR mechanism of thermal-treated samples remains consistent with that of Ni@NCNTs-800. As shown in Figure 6f, the OER activity of [email protected] is almost the same as that of Ni@NCNTs-800. But, the Ni@NCNTs-800-900-1h shows a lower OER activity than that of Ni@NCNTs-800. H


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encapsulated Ni nanoparticle mainly contributes to the OER activity, separately. All the investigations above provide a new insight into development of transitional metal encapsulated in the N-doped carbon matrix and designing the excellent bifunctional catalysts for ORR and OER.

This result clearly indicates that the doped nitrogen plays a key role in the ORR catalysis and mildly facilitates the OER. The Ni@NCNTs with different N and Ni contents provide us an opportunity to explore the structure-performance relationship of Ni@NCNTs as ORR/OER catalysts. First, the kinetic current density Jk at 0.7 V was calculated (Supporting Information) and correlated with the amount of different N species and Ni, as shown in Figure 7a. In consideration of the low graphitized degree and ORR activity, the Ni@NCNTs-650 was not used for the correlation analysis. The Ni@NCNTs-800-0h was not selected for the adverse effect of the surface Ni, as proved above. There is not a monotonic relation between the pyrrolic nitrogen content and the ORR activity or between the quaternary nitrogen content and the ORR activity, while a weak dependence of the pyridinic nitrogen content and the ORR activity is displayed in Figure 7a. The failure to directly correlate activity with the amount of a specific N species indicates that the ORR does not distinguish the exact roles of N species, probably due to the structural complexity of doped nitrogen.26 The ORR activity shows a stronger dependence of the gross nitrogen content than that of pyridinic nitrogen content. This result further demonstrated that various nitrogen species affect together on ORR catalysis. This view helps to explain why pyridinic nitrogen, pyrrolic nitrogen, or quaternary nitrogen is considered a major active site for ORR.44−46 It is noteworthy that there is no relationship between the bulk Ni content and the ORR activity. For the Ni@NCNTs catalysts, it seems that the Ni encapsulated in CNTs hardly dominates the ORR activity, which is different from the Fe3C@NCNTs catalysts.16 It is probably due to the weak synergistic effect between the encapsulated Ni and outer surface graphitic carbon, as discussed by Yihua Zhu.19 Second, the potentials at current density of 10 mA cm−2 were correlated with the amount of nitrogen-containing groups and Ni, in Figure 7b. To exclude the influence of Ni content, the Ni@NCNTs-800-0h and Ni@NCNTs-1000 were not selected for the correlation analysis of nitrogen-containing groups. There is no evident correlation of the gross nitrogen contents or the specific nitrogen contents with the OER potentials. It only shows a tentative tendency that the more gross nitrogen content is, the lower OER overpotential displays. This could be due to the synergistic effect of doped N for OER.47 A good correlation of Ni content with the OER potential is revealed in Figure 7b. It indicated that the Ni content correlates with the activity of Ni@NCNTs well. Therefore, it can be concluded that the Ni is the main active site, together with doped nitrogen serving a synergetic effect to enhance OER activity. The above results indicate that doped N and encapsulated Ni each perform their own function to ORR and OER, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03582. Calculation method of apparent transferred electron number for ORR, supplemental tables S1−S5 and figures S1−S12 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X. Fu). *E-mail: [email protected] (F. Peng). ORCID

Guoyu Zhong: 0000-0002-6797-1990 Feng Peng: 0000-0002-5154-6666 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (No. 21373091), the Guangdong Provincial Science and Technology Project (No. 2017A030313090, 2016A010104013), and Natural Science Foundation of Guangdong Province (No. 2018A030310004).

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REFERENCES

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CONCLUSIONS In conclusion, NCNTs with encapsulated Ni nanoparticles were synthesized by a simple pyrolysis method. The tubular morphology with encapsulated Ni and doped N was observed. The as-prepared Ni@NCNTs catalyst exhibits excellent activity and stability toward ORR and OER in alkaline medium. A series of Ni@NCNTs with different bulk Ni and surface nitrogen content are prepared by varying the synthesized temperature and second thermal treating. The correlations of structures and performances suggest that the doped nitrogen primarily acts as the ORR active site and the I


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Nickel Nanoparticles Encapsulated in Nitrogen-Doped Carbon

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