Individual High-Quality N-Doped Carbon Nanotubes Embedded with

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Individual High-Quality N-doped Carbon Nanotubes Embedded with Non-Precious Metal Nanoparticles towards Electrochemical Reaction Shengbo Zhang, Qilong Wu, Lei Tang, Yuge Hu, Mengyun Wang, Jiankang Zhao, Mei Li, Jinyu Han, Xiao Liu, and Hua Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14536 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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ACS Applied Materials & Interfaces

Individual

High-Quality

N-doped

Carbon

Nanotubes Embedded with Non-Precious Metal Nanoparticles towards Electrochemical Reaction Shengbo Zhang,a† Qilong Wu,c† Lei Tang,a† Yuge Hu,b Mengyun Wang,a Jiankang Zhao,a Mei Li,a Jinyu Han,a Xiao Liu, b,a* Hua Wang a* a

S. Zhang, L. Tang, M. Wang, J. Zhao, M. Li, Prof. J. Han, Dr. X. Liu, Dr. H. Wang

Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, P. R. China. b

Y. Hu, Dr. X. Liu

Key Laboratory of Pesticide & Chemical Biology of the Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China. c

Q. Wu

School of Chemical Engineering and Technology, Tianjin University of Technology, Tianjin 300384, P. R. China. †

These authors contributed equally.

KEYWORDS: N-doped carbon nanotubes, carbon-coated metal nanoparticle, ZIF-8, electrocatalytic reactions, interface effect ABSTRACT: Developing highly active and stable non-precious metal catalysts for electrochemical reactions is desirable but remains a great challenge. Herein, we report a novel metal ion adsorption-pyrolysis strategy for the controllable ZIF-8 derived synthesis of 1 ACS Paragon Plus Environment

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individual high-quality N-doped carbon nanotubes embedded with well-dispersed nonprecious metal nanoparticles, which exhibit superior electrocatalytic activity and stability for electrochemical CO2 reduction reaction, oxygen reduction reaction and oxygen evolution reaction. Experimental analysis and density functional theory calculations indicate that the remarkable electrocatalytic activities are mainly attributed to the interface effects for the efficient electron transfer from metal nanoparticles to the N-doped carbon shell, as well as the large specific areas, unique tube structures, appropriate doping, high graphitization degree and robust frameworks. The high reaction stability is attributed to that the multiwalled graphitic carbon shells could efficiently prevent metal nanoparticles from aggregation, corrosion and oxidation. This novel synthetic strategy presents a facile universality for synthesizing Ndoped carbon nanotubes structures and will provide a guideline for developing low-cost, highly active and stable electrocatalytic materials for sustainable energy conversion. 1. Introduction Nitrogen-doped carbon nanomaterials embedded with non-precious metal nanoparticles have been regarded as efficient electrocatalysts for CO2 reduction (CO2RR), oxygen reduction (ORR) and oxygen evolution reactions (OER).1-5 In particular, nitrogen-doped carbon with cylindrical structures has drawn much research attention because of their significant advantages, such as high thermal and chemical stability, large surface areas, channel confinement effect, unique armor protection of metal nanoparticles and efficient electron transfer.6-10 However, the preparation of uniform heteroatom-doped nanotubes are usually complicated with low yields and the dispersion of metal nanoparticles is random. More facile approaches to synthesize high-quality N-doped carbon nanotubes (NCNT) with the fine encapsulation of metal nanoparticles are still in demand. Metal-organic frameworks (MOFs) have been used as novel precursor templates for the preparation of carbon materials since the first example by Xu using MOF-5 as a template.11-18 2 ACS Paragon Plus Environment

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Recently, Lou and Mai independently reported the synthesis of hollow carbon frameworks with N-doped carbon nanotube composites on the outer sphere by using ZIF-67 as precursors.17,18 This method is mainly based on the MOF containing Fe, Co, and Ni metals as a precursor and assisted by a reducing gas. Li and co-workers later designed a novel hybrid nanostructure with CoP nanoparticles embedded in a N-doped carbon nanotube hollow polyhedron derived from core-shell [email protected] As a subclass of MOFs, zeolitic imidazolate frameworks-8 (ZIF-8), easily prepared and low-cost, has been proved to be an ideal precursor for the formation of N-doped carbon materials in view of its abundance of distributed C and N building blocks, and particularly the concurrent evaporation of Zn during carbonization at high temperature. In this way, other desirable metals may be introduced through adsorbing the metal ions in the frameworks with advantage of its microporous structure before carbonization. Worthy of note, the metals, such as Ni, Fe and Co, are efficient catalysts for producing carbon nanotubes.7,16-18 All above inspired us to develop a metal ion adsorption-pyrolysis strategy for the large-scale synthesis of N-doped carbon nanotubes encapsulated with well-dispersed non-precious metal nanoparticles using ZIF-8 as the precursor. Herein, we report a novel adsorption-pyrolysis method through the dispersion of ZIF-8 into Ni(NO3)2, Fe(NO3)2 or Co(NO3)2 methanol solutions. The adsorbed Ni, Fe or Co ions were first in situ reduced into nanoparticles by carbon derived from the thermal decomposition of imidazole, and successively they catalyzed the growth of N-doped carbon nanotubes. As a result, individual high-quality N-doped carbon nanotubes embedded with uniform Ni, Fe or Co nanoparticles were successfully obtained. On account of high graphitization degree, large surface areas, the heteroatom introduction and well-dispersed metal nanoparticles coated with carbon layers, the as-prepared catalysts demonstrated remarkable features for both electrochemical CO2 reduction reaction and oxygen electrocatalysis. In particular, N-doped carbon nanotubes with the imbedding of Ni exhibited 3 ACS Paragon Plus Environment


an outstanding activity and stability in CO2RR with 82.4% of CO Faradaic Efficiency (FE) at -0.70 V vs. reversible hydrogen electrode (RHE), comparable with the carbon-supported metal single atom catalysts. While Fe-based NCNT showed comparable ORR activity to commercial Pt/C with onset potential of 0.97 V and half-wave potential of 0.85 V vs. RHE, and Co-based NCNT gave a markedly higher OER activity over the commercial IrO2. XPS characterization and DFT calculations indicate that higher catalytic performances are mainly attributed to the electron enrichment around the interface between the metal particles and the carbon shell. Additionally, the high stability is owing to that the multiwalled graphitic carbon shells efficiently prevent metal nanoparticles from aggregation, corrosion and oxidation. 2. Experimental Section Synthesis of ZIF-8 cubes: ZIF-8 cubes are prepared by slightly modifying a previously reported method.19 In a typical synthesis, 2.80 g Zn(NO3)2·6H2O was dissolved in 160 mL deionized water. 38.4 g 2-Methylimidazole and 0.068 g CTAB were dissolved in 480 mL deionized water. The 2-Methylimidazole solution was poured into the Zn(NO3)2·6H2O solution and the mixed solution was stirred for 10 min at room temperature. The resulting suspension was transferred to a PTFE hydrothermal reactor at 120 ºC for an additional 6 h. After cooling to room temperature, the solid product were washed with deionized water, followed by methanol, and collected by centrifugation. Finally, the product was dried under vacuum at 60 °C. Synthesis of CN: For synthesis of CN, 100 mg ZIF-8 cubes was placed in a furnace and heated to 950 °C with heating rate of 5 °C / min for 2 h under nitrogen atmosphere. The obtained sample was denoted CN. Synthesis of Ni-NCNT, Fe-NCNT and Co-NCNT: In a typical synthesis, 100 mg ZIF8 cubes was dispersed in 10 mL methanol under ultrasound for 10 min at room temperature. 40 mg Ni(NO3)2·6H2O was dissolved in 10 mL methanol. The ZIF-8 cubes solution was 4 ACS Paragon Plus Environment

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poured into the Ni(NO3)2·6H2O solution and the mixed solution was stirred at 50 °C up to solvent evaporation. Then the sample was placed in a furnace and heated to 950 °C with heating rate of 5 °C / min for 2 h under nitrogen atmosphere. The obtained sample was denoted Ni-NCNT. Fe-NCNT and Co-NCNT were also synthesized by using the similar strategy, except for the replacement of Ni(NO3)2·6H2O with Fe(NO3)2·6H2O and Co(NO3)2·6H2O, respectively. The obtained sample were denoted Fe-NCNT and Co-NCNT, respectively. Synthesis of ZIF-8 rhombic dodecahedra and Ni-NCNT: ZIF-8 rhombic dodecahedra are prepared by slightly modifying a previously reported method.20 In a typical synthesis, 1.116 g Zn(NO3)2·6H2O was dissolved in 30 mL methanol. 1.232 g 2-Methylimidazole was dissolved in 30 mL methanol. The Zn(NO3)2·6H2O solution was poured into the 2Methylimidazole solution under ultrasound for 10 min at room temperature. Then the resulting suspension was grown under static at 35 ºC for an additional 12 h. The solid product were centrifuged and washed with deionized water and methanol. Finally, the product was dried under vacuum at 60 °C for overnight. Ni-NCNT was also synthesized by using the similar strategy. Synthesis of Ni-NCNT by using different precursor: The Ni-NCNT were also synthesized by using Cyanamide, Dicyandiamide, Guanidine hydrochloride, Melamine and Dopamine as precursor, respectively, according to the similar strategy, except for the replacement of 950 °C with 750 °C. Electrochemical CO2 reduction measurements: The experiments were performed in a custom-made two-compartment cell, in which the working electrode was separated from thecounter electrode by Nafion membrane to hinder the re-oxidation of the products on the counter electrode. Each compartment contained 50 ml electrolyte (0.1 M KHCO3 solution) with approximately 50 ml headspace. Pt plate (1×2 cm2) was used as a counter electrode and saturated calomel electrode (Hg/Hg2Cl2) as a reference electrode. All potentials were 5 ACS Paragon Plus Environment


converted to those against a reversible hydrogen electrode (RHE) according to E (RHE) = E (Hg/Hg2Cl2) + 0.24 V + 0.059 × pH. The ink was prepared by dispersing 5 mg catalysts and 60 µL Nafion solution (5 wt%) in 440 µL ethanol solution. The working electrode was prepared by drop casting 10 µL of the homogeneous ink onto a carbon paper. Linear sweep voltammetry (LSV) was performed with a scan rate of 50 mV s-1 in N2-saturated 0.1 M KHCO3 and CO2 saturated 0.1 M KHCO3 electrolyte. The gas phase products were analyzed by GC equipped with a thermal conductivity detector (TCD). Liquid products were analyzed by a 1H nuclear magnetic resonance (1H NMR) spectrometer using dimethyl sulphoxide (DMSO) as an internal standard. Electrochemical oxygen reaction measurements: ORR measurements were performed in a three-electrode cell by a RDE at ambient temperature. An Ag/AgCl (3.5 M KCl) and platinum foil were used as the reference and counter electrodes, respectively. A modified glassy carbon electrode (GCE, d=3 mm) served as a working electrode. Before test, an N2/O2 flow was used through the electrolyte in the cell about 30 min to saturate it with N2/O2. The cyclic voltammetry (CV) profiles are obtained in N2- or O2-saturated 0.1 M KOH solution with a scan rate of 20 mV s-1. RDE/RRDE tests for the ORR are measured in O2-saturated 0.1 M KOH solution at different rotation rates with a scan rate of 10 mV s-1. Linear sweep voltammograms (LSV) for the OER are obtained using a RDE (1600 rpm) in 1.0 M KOH solution at a scan rate of 5 mV s-1. DFT simulations: All calculations were performed using the Vienna Ab Initio Simulation Package (VASP). Projector augmented wave (PAW) potentials were used to represent the effective cores. The Perdew−Burke−Ernzerhof (PBE) functional was used to calculate exchange and correlation energy. The cutoff energy of the plane wave basis set is 400 eV. For geometric optimization, the force convergence was set to be lower than 0.02 eV/Å on atoms during optimization. Brillouin zone sampling was employed using a Monkhorst-Packing grid 3×3×1. A vacuum space of 20 Å has been used in all models. 6 ACS Paragon Plus Environment

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3. Results and discussion 3.1. Synthesis of CN, Ni-NCNT, Fe-NCNT and Co-NCNT Scheme 1 illustrates the synthesis route of the N-doped carbon nanotubes with the encapsulation of metal nanoparticles. First, the ZIF-8 nanocubes prepared according to the typical method19 were dispersed in Ni(NO3)2, Fe(NO3)2 and Co(NO3)2 methanol solution under ultrasound stirring, respectively. In this way, the Ni2+, Fe2+ and Co2+ metal ions were entered into the pores of ZIF-8 frameworks. Subsequently, the resulting mixture was pyrolyzed at 950 °C under the nitrogen atmosphere after the evaporation of solvents. Due to the initial decomposition of imidazole at high temperature, the formed C was supposed to reduce the adsorbed metal ions to produce metal nanoparticles, which afterwards catalyzed the carbon and nitrogen to grow N-doped carbon nanotubes. Meanwhile, the metal nanoparticles were wrapped inside carbon nanotubes. With completely evaporating Zn during pyrolysis, the only metal components Ni, Fe and Co remained. The obtained black materials are denoted as Ni-NCNT, Fe-NCNT and Co-NCNT, respectively. As a comparison, the pure ZIF-8 cubes without any additional metal ions were also pyrolyzed at the same conditions, named as CN.

Scheme 1. Schematic illustration for the preparation of Ni-NCNT, Fe-NCNT and Co-NCNT by using ZIF8 cubes as a precursor.

The as-synthesized ZIF-8 cubic crystals with the size of ~250 nm are composed of C, N and Zn atoms uniformly distributed on nanoparticles (Figure S1). The typical crystal pattern 7 ACS Paragon Plus Environment


and porous structure of ZIF-8 cubes are demonstrated in Figure S2 and S3 respectively. After pyrolysis for 2 h at 950 °C, it can be seen the cubic morphology was retained from Figure 1a1-c1. Some mesoporous structure appeared on the surfaces likely due to the evaporation of Zn species (boiling point of 907 °C).15,20 Furthermore, the EDX mapping analysis show that only C and N are distributed on the CN cubes, indicating the complete removing of Zn (Figure 1d1-g1). These above results demonstrate that only cubic-shaped porous carbon materials were formed in case of pyrolyzing the pure ZIF-8. When nickel, iron and cobalt ions were introduced inside ZIF-8, respectively, black solids Ni-NCNT, Fe-NCNT and Co-NCNT were obtained after the pyrolysis. Figure 1a2-h2 show the scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) and EDX mapping analysis images of Ni-NCNT. Large-scale individual nanotubes have been successfully formed from Figure 1a2. The nanotubes have a uniform diameter of about 40 nm and were embedded with some small nanoparticles ranging from 5 to 20 nm, which were confirmed to be Ni nanoparticles from HRTEM analysis (Figure 1c2). Ni nanoparticles were completely surrounded by the multiwalled graphitic carbon shells with an interplanar spacing of 0.36 nm corresponding to C(002). The carbon shell could not only efficiently prevent Ni nanoparticles from aggregation, corrosion and oxidation but also transfer electrons from metal nanoparticles outside. Furthermore, the EDX mapping analysis of Ni-NCNT (Figure e2-h2) indicates the uniform distribution of C and N in the nanotube frameworks. Besides, Fe-NCNT and Co-NCNT with nanotube structures were also synthesized successfully from Figures 1a3-h3 and 1a4-g4. Most of the nanotubes display unique bamboo-like structures. These above results suggest that the Ni, Fe and Co could turn ZIF-8 cubes into nanotube structures in a large-scale. To the best of our knowledge, such uniform and individual high-quality N-doped carbon nanotubes with the

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encapsulation of non-precious metal nanoparticles using ZIF-8 as precursors have not been reported before.

Figure 1. The SEM, TEM, HAADF-STEM images and the elemental mapping images of samples (a1-g1) for CN; (a2-h2) for Ni-NCNT; (a3-h3) for Fe-NCNT; (a4-g4) for Co-NCNT.

The nitrogen adsorption-desorption isotherms of Ni-NCNT, Fe-NCNT and Co-NCNT show type-IV isotherms with pronounced hysteresis loops, suggesting the existence of mesoporous structures in the N-doped carbon nanotubes (Figure 2a). The BET surface areas were 350, 242 and 216 m2 g-1 for Ni-NCNT, Fe-NCNT and Co-NCNT, respectively, mainly dominated by mesopores (226, 192 and 172 m2 g-1), corresponding to the hollow nanotube 9 ACS Paragon Plus Environment


channels (Table S1). Moreover, these three samples show pore sizes larger than 5 nm and higher pore volume (~ 0.3 cm3 g-1) (Figure S8, Table S1). Raman spectroscopy was implemented to further characterize the structural properties of N-doped carbon nanotubes (Figure 2b). The two dominant peaks at 1350 and 1580 cm-1 illustrate the characteristic D and G bands of carbon, which are assigned to the sp2 graphitic carbon and disordered or defect carbon, respectively. Figure 2b and Table S2 show the ID/IG intensity ratio of 0.69, 0.98 and 0.94 for Ni-NCNT, Fe-NCNT and Co-NCNT, respectively, lower than that of CN (1.05) (Figure S5), indicating a higher graphitization degree after the formation of nanotube structures. The crystalline nature of all materials was investigated by X-ray diffraction (XRD) (Figure 2c). The peak at 26°corresponds to the (002) facets of graphitic carbon for carbon materials. In addition, for Ni-NCNT, the peaks located at 44.5, 51.8 and 76.5°can be indexed to Ni(111), Ni(200) and Ni(220). For Fe-NCNT, the peaks at 43.9, 44.9, 48.7 and 49.3°are attributed to the Fe and Fe3C phase. For Co-NCNT, the peaks at 45.0, 51.8 and 76.5° correspond to the Co(111), Co(200) and Co(220). The XRD results of Ni-NCNT, Fe-NCNT and Co-NCNT are consistent with the HRTEM results. Furthermore, the surface chemical compositions were investigated by X-ray photoelectron spectroscopy (XPS). The highresolution N 1s spectra (Figure 2d) reveal the presence of the pyridinic (398.5 eV), pyrrolic (399.9 eV), graphitic (401.0 eV) and N-Ox (403.5 eV). Based on CHN elemental analysis, the N contents are approximately 3.7, 3.4 and 3.2 wt% for Ni-NCNT, Fe-NCNT and Co-NCNT, respectively. It is worth mentioning that N doping can significantly improve the electronic conductivity and electrochemical reaction activity of graphitic carbon nanotubes, which makes N-doped carbon nanotubes ideal candidates for efficient electrocatalysts.18,21 Additionally, these XPS spectra (Figure S12) and inductively coupled plasma (ICP) (Table S2) analysis of Ni-NCNT, Fe-NCNT and Co-NCNT reveal the presence of Ni, Fe and Co species, respectively, but without Zn species after pyrolysis, which further proved that Zn was 10 ACS Paragon Plus Environment

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completely evaporated under such high temperature. More importantly, the binding energy of high-resolution C 1s peak for Ni-NCNT, Fe-NCNT and Co-NCNT has an obviously downshift compared with that in CN (Figure 2e), which is possibly due to the strong interactions between the confined metal nanoparticles and carbon nanotubes. Generally, densities of states (DOS) by density functional theory (DFT) calculations are applied as an effective parameter for M-N (metal and nitrogen) doped carbon materials to illustrate the doping effects on electronic structures.6,18 Herein, we build simple models containing a fouratom-metal cluster on one layer N doped graphene for describing Ni-NCNT, Fe-NCNT and Co-NCNT, respectively (Figure S13). Obviously, the calculated total DOS of as-prepared MNCNT catalysts show pronounced states from -2 eV to the Fermi level comparing with that of N doped graphene without metal (Figure 2f). The increased total DOS near Fermi level are supposed to derive from the electron transfer between metal nanoparticles and carbon layers, which can enhance the electron densities of NCNT shells. The above results indicate that the confined metal nanoparticles render electrons transfer to activate the outer shell of carbon nanotubes, which provide more active sites for efficient electrocatalytic reactions.



Figure 2. (a) Nitrogen adsorption-desorption isotherms, (b) Raman spectra, (c) The XRD patterns and (d) High-resolution N 1s XPS spectra of Ni-NCNT, Fe-NCNT and Co-NCNT. (e) High-resolution C 1s XPS spectra and (f) the total density of states of CN, Fe-NCNT, Co-NCNT and Ni-NCNT.

A possible mechanism for the formation of metal and nitrogen co-doped carbon nanotubes (taking Ni-NCNT as an example) was illustrated in Figure 3a. First, the Ni2+ metal ions were adsorbed into the pores of ZIF-8 cubes homogeneously with ultrasound assistance. Subsequently, the resulting mixture was pyrolyzed and the organic ligands of ZIF-8 were carbonized. Then, Ni nanoparticles were formed via in situ reduction of carbon derived from the initial decomposition of imidazole in ZIF-8, which was well documented by TGA (Figure 3f). Once the metal Ni was formed, it could instantaneously catalyze the carbon and nitrogen from imidazole to grow N-doped carbon nanotubes. To better understand the formation and 12 ACS Paragon Plus Environment

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evolution process of the Ni-NCNT, the time-dependent experiments were performed and the intermediate products were characterized by SEM (Figure 3b-e). Before pyrolysis, the Ni/ZIF8 cubes remained almost intact with a slight roughness due to the introduction of Ni2+ (Figure 3b). When pyrolyzing at 950 °C time for 0.5 h, the surface became much rougher and some carbon nanotubes grew up from the surfaces of the cubes (Figure 3c). Prolonging the pyrolysis time to 1.0 h, most of the cubic structures were decomposed into irregularly shaped substrates while more nanotubes formed (Figure 3d). When the pyrolysis time was further extended to 2 h, the ZIF-8 cubes were completely transformed into isolated nanotube structures (Figure 3e). The entire evolution process was also monitored by recording the XRD patterns of resulting intermediate products (Figure 3g). It demonstrated that Ni nanoparticles were produced during pyrolysis process, which catalyzed the formation of nanotubes from ZIF-8 cubes. To further testify the effect of nickel on the formation of nanotubes, a series of control experiments with different nickel contents were conducted. As described in the Figure 1a1,b1, only the previous ZIF-8 nanocube structures were obtained in the absence of nickel, instead of nanotubes. When the mass ratio of ZIF-8 to nickel nitrate was 100:5, no nanotubes were obtained (Figure S14a), mainly attributed to the formation of single-atom nickel in the carbon frameworks because of small amount of nickel substituted for Zn node.15,20 When further increasing the mass ratio to 100:20, some nanotubes appeared on some irregularly shaped substrates (Figure S14b). In case of the mass ratio of 100:40, large-scale nanotubes were produced as shown in the Figure 1a2 and Figure 3e. These results indicate that an appropriate amount of nickel ions is a key factor for the formation of large-scale nanotubes. Furthermore, the final pyrolysis temperature is also critical. When the pyrolysis temperature was reduced to 750 °C, only carbon materials with aggregated nanosheets were obtained (Figure S15a,b). Whereas at 850 °C, some carbon nanospheres appeared (Figure S15c,d). Therefore, we



conclude that the pyrolysis temperature, pyrolysis time, proper content of nickel are altogether responsible for the formation of uniform and large-scale Ni-NCNT.

Figure 3. (a) Schematic illustration of the transformation from cubic ZIF-8 to the final Ni-NCNT structure. The SEM images of morphology evolution for Ni-NCNT at different pyrolysis stages: (b) before pyrolysis, (c) 0.5 h, (d) 1 h and (e) 2 h. (f) TGA of ZIF-8, Ni/ZIF-8 and Ni(NO3)2 under nitrogen atmosphere. (g) XRD patterns of Ni-NCNT at different pyrolysis times.

It is known that the ZIF-8 crystals can be tailored with various special morphologies.22-24 Accordingly, we extend our method to the preparation of metal and nitrogen co-doped carbon nanotubes (taking Ni as an example) by using the ZIF-8 rhombic dodecahedral as precursor (Figure 4a). The SEM and TEM images (Figure 4b,c) show that these ZIF-8 possess uniform rhombic dodecahedral shape and smooth surface. Subsequently, the large-scale carbon nanotube structures were obtained in presence of Ni2+ after sufficient pyrolysis as shown in the Figure 4d,e. These Ni and N co-doped carbon nanotubes are well characterized by SEM, TEM, EDX mapping images (Figure 4d-i and Figure S22) and XRD pattern characterization 14 ACS Paragon Plus Environment

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(Figure S23). In order to further prove the universality of the mechanism of the strategy, in addition to ZIF-8, various other chemical precursors containing C and N elements were also investigated. As shown in Figure S26, the Ni-NCNT were synthesized in a high-yield by using melamine, dicyandiamide, cyanamide, dopamine and guanidine hydrochloride as precursors, respectively, according to our strategy, except for the replacement of 950 °C with 750 °C (Scheme S1). These SEM, TEM and EDX mapping images show that the morphology and structure of these nanotubes are similar to those obtained by using ZIF-8 as precursor (Figure S26d1-d3 and Figure S26e1-e3). However, for cyanamide as the precursor, most of the Ni nanoparticles were dispersed outside of the nanotubes and acid leaching experiment also confirmed it (Figure S26c1, Figure S27). These above results indicate that our strategy for synthesis of high-yield metal and nitrogen co-doped carbon nanotubes has a very good universality and shows great potential in the development of different precursors containing C and N elements. It should be pointed out that the nanotube structures obtained by using ZIF-8 as the precursor are more uniform and free-standing. Moreover, the metal nanoparticles are dispersed inside of the nanotubes and can be completely coated by the carbon shell due to the porous nature of ZIF-8.



Figure 4. (a) Schematic illustration for the preparation of Ni-NCNT by using ZIF-8 rhombic dodecahedra as precursor. (b) SEM and (c) TEM images of ZIF-8 rhombic dodecahedra. (d) SEM, (e) TEM, (f) Dark field SEM image and (g-i) The elemental mapping images of Ni-NCNT: (g) C, (h) N and (i) Ni.

3.2. Electrocatalytic CO2RR N-doped carbon materials with the encapsulation of non-precious metal nanoparticles have been proved to be a kind of fascinating catalysts for a series of important reactions, including CO2 hydrogenation reaction,25,26 reductive amination reaction.27,28 However, its application in electrocatalytic CO2 reduction reaction has been rarely developed so far. On the other hand, single-atom metal-N-C materials have been widely applied for electrical CO2RR, mainly due to their simple metal complex structures and well-defined catalytic active sites.2932

Compared to single-atom metal-N-C, nitrogen-doped carbon encapsulated metal

nanoparticles could be easily prepared in a large scale. The interface effects between the metal nanoparticles and the carbon layers could be regulated and promote the electron transfer properties, which inspires us to systematically study the electrocatalytic CO2RR of Ni-NCNT, Fe-NCNT and Co-NCNT. First, the CO2RR performance of the as-prepared samples were evaluated in N2- and CO2-saturated KHCO3 solution (0.1 M) by linear sweep voltammetry (LSV) test within the potentials of 0.0 ~ -0.7 V vs. RHE. As shown in Figure 5a, CN, NiNCNT and Fe-NCNT exhibit higher current densities under CO2 than N2 atmosphere when the potential was more negative, indicating that these three electrodes are active for electrocatalytic CO2 reduction. While for Co-NCNT, the current density was almost the same under CO2 and N2 conditions, indicating it was inactive for CO2RR. At the same applied potential, Ni-NCNT shows the highest net current density (jCO2-jN2), followed by Fe-NCNT. Moreover, the onset potentials for CO2 reduction on Ni-NCNT and Fe-NCNT (-0.35 V vs. RHE) are much more positive than that of CN (-0.48 V), demonstrating that Ni-NCNT and Fe-NCNT can reduce CO2 more efficiently than CN. 16 ACS Paragon Plus Environment

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Furthermore, the constant potential electrolysis was conducted in CO2-saturated 0.1 M KHCO3 solution to evaluate the selectivity of CO2RR at different potentials in the range of 0.5 V to -0.9 V. The detected main products formed by the reduction of CO2 were CO, and the byproduct H2 generated by concurrent proton reduction, while no liquid products were detected by 1H nuclear magnetic resonance spectroscopy (1H NMR) (Figure S29). The Faradaic Efficiency (FE) of a given product is generally applied to represent the product selectivity. Figures 5b illustrates the dependence of FE for CO formation on the applied potential. Clearly, a maximum CO FE of 82.4% for Ni-NCNT was achieved at a smaller potential of -0.7 V, comparable to those of most reported single-atom or precious metal nanoparticles catalysts (Table S3), which was about 2.7 times than that of CN. Fe-NCNT also exhibits a considerable selectivity of CO with 48.2% FE at a potential of -0.6 V. DFT results demonstrated the local electron density of Ni-NCNT was higher than that of Fe-NCNT (Figure 2f) and Raman spectra show higher graphitization degree of Ni-NCNT (Figure 2b), which might lead to the more efficient CO2RR activity of Ni-NCNT than Fe-NCNT. Figure 5c reveals that Co-NCNT produced H2 dominantly over CO with approximate 100% of H2 FE, consistent with the results of LSV in Figure 5a. To understand the kinetic mechanism of CO2RR on CN, Ni-NCNT and Fe-NCNT electrodes, we studied the thermodynamic pathway of CO2 to CO on the basis of Tafel plot, shown in Figure 5d. The value of Tafel slope always indicates a possible rate-determining step. Theoretically, Tafel slope of 118 mV dec-1 means that one-electron reduction of CO2 is the rate-determining step.31-32 Obviously, Ni-NCNT, Fe-NCNT and CN gave the Tafel slopes of 149, 157 and 167 mV dec-1 respectively, indicating that the rate determining step is oneelectron reduction of CO2 to form a surface adsorbed COOH* intermediate.33-35 Subsequently, the adsorbed COOH* intermediate combines with protons and another electron to form adsorbed *CO and H2O (g), and finally the desorption of CO based on free energy pathways calculations results (Figure S30). Additionally, M-NCNT electrocatalysts, especially Ni17 ACS Paragon Plus Environment


NCNT, show smaller value of Tafel slope than CN, demonstrating that Fe- or Ni-doped NCNT have improved CO2RR thermodynamic kinetics, which is in agreement with the LSV analysis. On account of higher electron densities on the outer carbon shells, free energy pathways and higher graphitization (ID/IG=0.69), Ni-NCNT performed better in the activation of CO2 and stabilizing the formed COOH* intermediate, thus accelerating CO production, desorption and suppressing HER as well.6,7,17,18 Furthermore, a long-time stability of 10 h experiments for Ni-NCNT was conducted to test the stability of Ni-NCNT (Figure 5e). No obvious decay in terms of current density and CO FE was observed, which is mainly attributed to that the multiwalled graphitic carbon shells efficiently prevent metal nanoparticles from aggregation, corrosion and oxidation. In order to testify the advantage of metal nanoparticles encapsulated in the nitrogendoped carbon nanotubes, we also tried to synthesize the immobilization of Ni nanoparticles on the outside of nanotubes. Interestingly, when cyanamide was used as the C,N precursor instead of ZIF-8, most of the Ni nanoparticles were located outside the nanotubes from the TEM and SEM images [Figure S26(c1-c3)]. The Ni nanoparticles without the coating of carbon shell was mainly attributed to the lack of porosity in bulk cyanamide to confine the Ni precursor.20 LSV results in Figure 5f indicate that Ni-NCNT formed by cyanamide is less active in CO2RR than Ni-NCNT from ZIF-8 under the same reaction conditions. Combined with the XPS results and DFT simulations, the interface effects between metal nanoparticles and N-doped carbons worked and promoted the catalytic performance, corroborating the advantages of the characteristic structures of Ni metal nanoparticles encapsulated in the channels of nanotubes.


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Figure 5. CO2 reduction reaction activities. Linear sweep voltammetry of (a) CN, Ni-NCNT, Fe-NCNT and Co-NCNT in CO2-saturated and in N2-saturated 0.1 M KHCO3 at a scan rate of 50 mV/s-1. (b) CO Faradaic efficiency, (c) H2 Faradaic efficiency and (d) Tafel plots for CO formation of CN, Ni-NCNT, FeNCNT and Co-NCNT. (e) The stability of Ni-NCNT at a applied potentials of -0.7 V (vs RHE) during 10 h. (f) Linear sweep voltammetry of Ni-NCNT(ZIF-8) and Ni-NCNT (Cyanamide).

3.3. Electrocatalytic ORR and OER Besides electrical CO2RR, the oxygen reduction reaction (ORR) activity of the asprepared catalysts was investigated in 0.1 M KOH solution. LSV results in Figure 6a reveal that Fe-NCNT and Co-NCNT exhibit higher current densities than CN and Ni-NCNT. Particularly, Fe-NCNT shows comparable activity to commercial Pt/C with onset potential of 0.97 V and half-wave potential of 0.85 V vs. RHE beyond that of most reported catalysts (Table S4). The Tafel plots (Figure S33a) also demonstrate the improved thermodynamic kinetics for ORR over Fe-NCNT and Co-NCNT, comparing with those of Ni-NCNT and CN. In addition, Fe-NCNT catalyst presents a preferable four-electron pathway for ORR with

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