Facile synthesis of MOF-derived ultrafine Co nanocrystals embedded in a nitrogen-doped carbon matrix for the hydrogen evolution reaction

Abstract

Non-precious metal-based catalysts with low cost and rich reserves are emerging as promising alternatives for Pt-based catalysts for the hydrogen evolution reaction (HER). Herein, we reported a facile and large-scale strategy for Co nanocrystals embedded in a nitrogen-doped carbon matrix through direct carbonization of Co-based metal–organic frameworks (MOFs) in N₂. After carbonization, the ultrafine Co nanocrystals are well dispersed in the nitrogen-doped carbon matrix. We further studied their electrocatalytic properties toward the HER in acidic media and found that the as-prepared catalyst exhibited enhanced catalytic activity and stability.

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Introduction

Hydrogen production through splitting of water has attracted increasing attention in light of the declining availability of fossil fuels and their negative environmental impacts.^1,2 Electrochemical reduction of water has been widely investigated as a very facile technique to produce hydrogen in several developing clean-energy technologies but it typically requires noble metal catalysts, such as Pt and IrO₂, to facilitate low overpotential and fast kinetics in the hydrogen evolution reaction (HER).³ However, the high cost and scarcity of these noble metals impede these approaches for large-scale applications and commercialization.^4,5 Therefore, it is desirable to design and produce efficient and cost-effective alternatives using non-precious electrocatalysts for the HER.

Recently, non-precious metals electrocatalysts, such as Fe and FeCo, have been developed to replace Pt in HER.^6,7 Nevertheless, there is a general problem that these non-precious metals or alloys are not stable in a narrow range, especially for acidic conditions. Very recently, carbon-encapsulated transition metal nanoparticles and their alloys are emerging as promising alternatives with unique stability for HER catalysts.^8,9 In the catalytic process, the carbon shell with a few layers thickness can prevent direct contact of the inert active species with reactants and electrolyte solutions. In addition, the core–shell structure can effectively avoid the aggregation of the inert active species, resulting in excellent durability of the catalysts. For example, Fei et al. synthesized Co@graphene nanocomposites as highly efficient and durable HER electrocatalysts under acidic condition.¹⁰ Bao' group reported FeCo alloys embedded in carbon nanotubes, which exhibited excellent catalytic activity for HER.⁷ The strategy for the composites with non-precious metal as core and carbon as shell indeed can simultaneously promote the catalytic activity and stability of the catalysts through the synergism and protection from carbon shell. However, the previously synthetic strategies for these composites always contained complex and expensive CVD (chemical vapor deposition) methods or multi-step synthesis process, which hinder large-scale applications and commercialization.^7–9 Metal–organic frameworks (MOFs) with tunable metal centers and organic linkers have attracted particular attention in recent year as templates or precursors for fabricating nanostructured materials through thermolysis.^11–14 Lou's group used prussian blue Fe₄[Fe(CN)₆]₃ as a template and precursor for the direct synthesis of Fe₂O₃ microboxes.¹⁵ Zheng et al. developed this MOF-derived strategy to prepare highly nitrogen-doped carbon materials through direct carbonization of nitrogen-containing MOFs in N₂.¹⁶ Based on previously experimental and theoretical results, nitrogen doping can significantly improve the property of carbon-based materials for HER.^17–19 Therefore, transition metal nanocrystals embedded in nitrogen-doped carbon can be successfully obtained through elaborate design MOFs structure with nitrogen-containing linkers and transition metal centers.

Herein, we developed a facile and scalable in situ strategy to synthesize Co nanocrystals embedded homogeneously in nitrogen-doped carbon matrix (Co@N–C) through direct carbonization of a zeolitic imidazolate framework material (ZIF-67) in N₂. The in situ generated nitrogen-doped carbon matrix can effectively avoid the aggregation of Co nanocrystals during the carbonization process. In addition, the nitrogen-doped carbon matrix can facilitate electron and mass transport during the catalytic process. Combining the advantage of ultrafine Co nanocrystals and nitrogen-doped carbon matrix, the as-prepared hybrid structure exhibited outstanding catalytic performance for HER.

Experiment section

Material preparation

All chemicals are of analytical grade, and were used without any further purification. The typical synthetic experiments were as follows. Solution A: 249.0 mg of cobalt nitrate hexahydrate was dissolved in 25.0 ml methanol under agitated stirring to give a pink solution. Solution B: 328.0 mg of 2-methylimidazole was also dissolved in 25.0 ml methanol under agitated stirring to give a transparent solution. The solution B was directly poured into the solution A under stirring. Stirring was stopped after combining the component solutions. After 24 h, the resulting purple solid was collected by centrifugation, washed with methanol several times, and finally dried in an oven at 60 °C. To obtain Co@N–C-600, Co@N–C-700 and Co@N–C-800, the resulting MOF precursor ZIF-67 was directly carbonized at different targeted temperatures (600, 700 and 800 °C) for 2 h with a heating rate of 10 °C min⁻¹ in N₂.

Material characterization

The powder X-ray diffraction (XRD) patterns of all samples were recorded with a X-ray diffractometer (Japan Rigaku D/MAX-γA) equipped with Cu-Kα radiation (λ = 1.54178 Å) over the 2θ range of 10–70°. Field emission scanning electron microscopy (FE-SEM) images were collected on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a Hitachi H-800 transmission electron microscope using an accelerating voltage of 200 kV, and high-resolution transmission electron microscope (HRTEM) (JEOL-2011) was operated at an acceleration voltage of 200 kV. Thermogravimetric analysis (TGA) was carried out using a Shimadzu-50thermoanalyser under nitrogen flow. X-ray Photoelectron Spectrum (XPS) was performed on an ESCALAB 250 X-ray Photoelectron Spectrometer with Al Kα radiation.

Electrochemical measurement

The electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI 660D). To prepare the working electrodes, 4 mg of the as-prepared samples and 30 μL of Nafion solution (Sigma Aldrich 5 wt%) were dispersed in 1 ml ethanol solution and were sonicated for 1 h to form a homogenous ink. Then 5 μL of the above dispersion (containing 19.42 μg of the as-prepared sample) was drop-cast onto a glassy carbon electrode (3 mm in diameter) to give a mass loading of 27.8 μg cm⁻². Furthermore, a Ag/AgCl electrode and a platinum foil were served as the reference electrode and counter electrode, respectively. All of the potentials were calibrated to a reversible hydrogen electrode (RHE). The potential cycles were conducted in 0.5 M H₂SO₄ solution with continuous N₂ gas in the potential region from 0.05 to 1.05 V at a sweep rate of 50 mV s⁻¹ until the cyclic voltammetry curves (CV) are unchanged. Linear sweep voltammetry (LSV) polarization curves were performed at scan rate of 2 mV s⁻¹ under a flow of N₂ gas. The HER stability of the as-prepared samples in solution was evaluated as described above in a potential orange from −0.3 to 0.1 V.

Results and discussion

ZIF-67 was synthesized by a room-temperature precipitation method and was further used as a precursor for the synthesis of Co@N–C. The size and morphology of the as-prepared ZIF-67 precursor were investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), respectively, as shown in Fig. 1. It can be clearly seen that the precursor is composed of a great deal of polyhedrons with an average size of approximately 1 μm and exhibits solid structure. Fig. 1c shows X-ray powder diffraction (XRD) patterns of the ZIF-67 precursor and their simulated results. The relative intensity and peak positions of the precursor in the XRD patterns are agreement with the simulated one, indicating the formation of pure ZIF-67 crystals.^20,21 To investigate the carbonization process, the thermogravimetric analysis (TGA) curve of the ZIF-67 precursor was measured under a nitrogen flow at a heating rate of 10 °C min⁻¹. Fig. 1d shows that the ZIF-67 precursor underwent a significant weight loss when heating to approximately 570 °C, which can be attributed to the decomposition of the host frameworks. On the basis of TGA results, different temperatures (600, 700 and 800 °C) were chosen to carbonization of the ZIF-67 precursor.

Fig. 1 (a) FESEM image, (b) TEM image, (c) XRD patterns and (d) TG curves of the as-prepared ZIF-67.

In contrast to the sharp peaks of the as-prepared ZIF-67 precursor, as shown in Fig. 2a, Co@N–C particles obtained at different carbonized temperatures showed broadened and less-resolved peaks. The broad peak at approximately 25° in the patterns are assigned to diffractions from the (002) plane of graphite-type carbon sheets. The peaks at 44.2 and 51.5° in the three samples can be assigned to metallic Co. Additionally, a small amount of CoO nanocrystals was readily generated possibly due to existence of solvent molecules in the frameworks of ZIF-67, which provided oxygen species to oxide of Co to CoO during the carbonization process. With increase of carbonization temperature, the peak for CoO is gradually disappeared, while the intense peaks for Co were detected, implying the formation of Co phase with higher crystallization degree. Raman spectrum of Co@N–C-600 particles is shown in Fig. 2b. The characteristic D band and G band are observed at approximately 1350 and 1580 cm⁻¹, respectively. The high I_D/I_G intensity ratio of Co@N–C-600 particles exhibits the formation of large amount of defects, which suggests that large amount of nitrogen atoms were doped in the carbon layers. In addition, a second-order band was observed at approximately 2700 cm⁻¹ and the shape of the D and 2D bands are characteristic features of few-layered graphene.^22,23

Fig. 2 (a) XRD patterns of the as-prepared Co@N–C particles obtained at different carbonized temperatures and (b) Raman spectrum of the as-prepared Co@N–C-600 particles.

The X-ray photoelectron spectroscopy (XPS) spectra of the as-prepared samples were exhibited in Fig. 3. It should be noted that the characteristic peaks of C, N, and Co exhibit in Fig. 3a. The nitrogen content of Co@ N–C-600 particles is the highest among the three samples with nitrogen content of 13.92 atom%. Nevertheless, as the carbonized temperature increased to 700 and 800 °C, the nitrogen content decreased to 9.79 and 6.02 atom%, respectively. In addition, as shown in Fig. 3b, the N 1s spectrum of Co@N–C-600 can be deconvoluted into three individual peaks that are assigned to pyridinic N (N-6, 398.6 eV), pyrrolic N (N-5, 399.6 eV), and quaternary N (N-Q, 401.1 eV), respectively.^24–26 The percentage of pyridinic, pyrrolic and quaternary nitrogen are 38.9, 38.1 and 23.0 atom%, respectively.

Fig. 3 (a) XPS survey spectra f the as-prepared Co@N–C particles obtained at different carbonized temperatures and (b) N 1s of the as-prepared Co@N–C-600 particles.

The conversion of ZIF-67 precursor to Co@N–C-600 particles was investigated by FESEM and TEM. As shown in Fig. 4a, it was clearly seen that Co@N–C-600 retain the pristine shape similar to the precursor, and the surface becomes rough due to the release of gaseous molecules (e.g. CO₂, H₂O) during the carbonization process. Fig. 4b exhibits a typical TEM image of Co@N–C-600 particles, and the sharp contrast over the whole particles demonstrates that ultrafine Co nanocrystals are distributed homogenously in the porous nitrogen-doped carbon matrix. Fig. 4c and d shows high-resolution TEM (HRTEM) images which obtained from the outside edge of the Co@N–C-600 polyhedron, and it further exhibits that Co nanocrystals well-dispersed with a particle size of less than 3 nm in the carbon matrix. In addition, the periodic lattice fringe with an interplanar distance of approximately 2.03 Å comes from the (111) plane of metallic Co, and Co nanocrystals were coated with graphene layers stacked in parallel with an adjacent interplay distance of approximately 0.34 nm. This observation is consistent with the d-spacing of the (002) crystal plane of bulk graphite. Fig. 2e–h show typical TEM energy dispersive spectrometer (EDS) elemental mapping images of a single Co@N–C-600 polyhedron, revealing the extremely homogenous distribution of Co, C and N in the whole Co@N–C-600 polyhedron. Furthermore, as shown in Fig. 5, with increase of carbonization temperature, the resulted Co@N–C-700 and Co@N–C-800 exhibit similar morphologies to Co@N–C-600. Nevertheless, high carbonization temperature results in small Co nanocrystals to adhere to each other to create larger Co nanocrystals.

Fig. 4 (a) FESEM image, (b) TEM image, (c and d) HRTEM images and (e–h) EDS mapping images of Co@N–C-600 particles.

Fig. 5 (a) FESEM and (b) TEM images of Co@N–C-700 particles; (c) FESEM and (d) TEM images of Co@N–C-800 particles.

Nitrogen sorption experiments were performed to obtain information on the specific surface area and pore size of the samples. The adsorption/desorption curves for the samples are shown in Fig. S1.† The Brunauer–Emmett–Teller (BET) specific surface area of the Co@N–C-600 is 14.1 m² g⁻¹. However, when the carbonization temperature was reached to 700 and 800 °C, the BET specific surface area for the Co@N–C-700 and Co@N–C-800 is 44.3 m² g⁻¹ and 97.4 m² g⁻¹, respectively. The pore size for the samples is shown in Table S1.† It can be clearly seen that the pore size decreases with increase of carbonization temperature. The large amounts of smaller Co nanocrystals in the Co@N–C-600 can result in its lower BET specific surface area. But its smaller Co nanocrystals provide more active sites for electrochemical reactions for HER.

As shown in Fig. 6a, compared with Co@N–C-700 particles and Co@N–C-800 particles, the Co@N–C-600 particles exhibits an enhanced electrocatalytic activity with a small onset overpotential of approximately 96 mV and overpotential of 339 mV at 10 mA cm⁻² versus RHE. As a control experiment, the electrocatalytic activity of commercial Pt catalyst (40 wt% Pt/C) was measured. The Co@N–C-600 particles resulted in a Tafel slope of 119 mV per decade compared to Tafel slope of 121 and 123 mV per decade for the Co@N–C-700 particles and Co@N–C-800 particles, respectively. The low Tafel slope for the Co@N–C-600 particles showed that this material catalyzes the HER through a Volmer–Heyrovsky mechanism.^27,28 The enhanced electrocatalytic activity of the Co@N–C-600 particles toward HER can be attributed to its highest nitrogen content among the as-prepared three samples because nitrogen-doping can improve electrical conductivity of carbon matrix and provide large amount of active sites for electrocatalysis.^29,30 A stability test of the Co@N–C-600 catalyst was performed by cycling the potential between −0.3 and 0.1 V in 0.5 M H₂SO₄ at a scan rate of 50 mV s⁻¹. Fig. 7 shows the HER polarization curves for the Co@N–C-600 particles before and after different potential cycles. There was no observable degradation in HER activity after 5000 cycles, indicating that the Co@N–C-600 particles are stable catalysts for HER. As shown in Fig. S2,† we also measured i–t curve of Co@N–C-600 particles at overpotential of 340 mV vs. HRE for 10 h. It can be clearly seen that the current density showed a slight decrease in the first two hours and remained almost unchanged (larger than 10 mV) during the following measurement, which further indicates its long-term stability toward HER.

Fig. 6 (a) The polarization curves of the as-prepared samples versus RHE and (b) the Tafel plots of the as-prepared samples.

Fig. 7 The polarization curves of the Co@N–C-600 particles after different cycles.

Conclusion

In summary, we have reported a facile approach for the synthesis of ultrafine Co nanocrystals embedded in nitrogen-doped carbon matrix through direct carbonization of MOFs. The Co nanocrystals are small in size (<3 nm) and evenly distributed in the nitrogen-doped carbon matrix. This well-designed core–shell structure can effectively avoid the aggregation of the ultrafine Co nanocrystals during the catalytic process. When tested as a HER electrocatalyst, the as-prepared Co@N–C-600 particles exhibit superior stability and HER activity with overpotential of 339 mV at 10 mA cm⁻² in acidic electrolyte. The enhanced electrocatalytic performance of Co@N–C-600 particles can be attributed to the synergistic effects between the ultrafine Co nanocrystals and nitrogen-doped carbon matrix.

Acknowledgements

This work was supported by the National Natural Science Foundation (NSFC, 21371009), Anhui provincial Natural Foundation (No. 1608085QB34) and Science and Technology Commission of Shanghai Municipality (No. 14DZ2261000).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10020a

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