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Liquid-to-gas transition derived cobalt-based nitrogen-doped carbon nanosheets with hierarchically porous for oxygen reduction reaction
Journal Pre-proofs Full Length Article Liquid-to-Gas Transition Derived Cobalt-based Nitrogen-Doped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction Zhenlu Zhao, Qiqi Sha, Kongshuo Ma, Yizhong Lu PII: DOI: Reference:
S0169-4332(20)30121-5 https://doi.org/10.1016/j.apsusc.2020.145365 APSUSC 145365
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Applied Surface Science
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15 October 2019 12 December 2019 10 January 2020
Please cite this article as: Z. Zhao, Q. Sha, K. Ma, Y. Lu, Liquid-to-Gas Transition Derived Cobalt-based Nitrogen-Doped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145365
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Liquid-to-Gas Transition Derived Cobalt-based NitrogenDoped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction
Zhenlu Zhao,*a, b Qiqi Sha,a Kongshuo Mac and Yizhong Lua
a
School of Material Science and Engineering, University of Jinan, Jinan 250022, Shandong, China.
b
Department of Bionano Engineering, Hanyang University, Ansan 426-791, South Korea.
c
State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, Jilin, China.
*E-mail:
[email protected]
Keywords: Carbon nanosheets, Hybrid nanostructure, Synergistic effect, Oxygen reduction reaction
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Abstract Here, Co@CoOx-based three-dimensional nitrogen-doped graphene-like carbon nanosheets (denoted as 3D HPN-GCNS/Co@CoOx) are fabricated by controllable grading-pyrolysis strategy, in which they show 3D fluffy and hierarchically porous nanostructure in and out-of-plane; large numbers of Co@CoOx-based nanoparticles uniformly embedded on graphene-like C nanosheets and mesoporous active edge introduce more ORR active sites. When applied for ORR, hybrids exhibit a half-wave potential (E1/2) of 0.87 V and an onset potential (Eonset) of 0.92 V in 0.1 M KOH, comparable to that of commercial 20% Pt/C. The hybrids display better methanol tolerance and stability, with almost no change of ORR performance before and after 5000 cycles. H2O2 yield in ORR process is less than 5% and number of electron transfer is higher than 3.9. The remarkable features are mainly ascribed to synergistic enhancement effect, in which 3D fluffy and hierarchically porous nanostructure in and out-of-plane supplies large accessible specific activity area, allows species to efficiently participate in ORR and facilitates fast mass transfer; graphene-like C nanosheets with N-rich dopant promote O2 adsorption and assure fast electron transport; large numbers of Co@CoOx-based nanoparticles introduce more ORR active sites. All the advantages synergistically enhance the ORR efficiency and make it a highly efficient ORR catalyst.
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1. Introduction The gradual depletion of fossil energy and the aggravation of environmental pollution have forced human society to put more attention and research into the development of clean and sustainable new energy [1-3]. The development of fuel cells and large-scale commercial promotion are an important solution to the current problem. Although Pt-based ORR catalysts have extremely high catalytic efficiency and have been widely used, they have high cost and rare reserves. Disadvantage such as easy poisoning limits its large-scale commercial application. In response, a variety of nonPt-based catalysts have been developed, including transition metal nitrides [4-7], transition metal oxides [8-14], conductive polymers [15-17], doped carbon-based catalysts [18-26], and monoatomic catalysts [27-31], etc. In contrast, transition metalbased catalysts, especially cobalt (Co), are widely considered to be ideal catalysts for non-Pt-based ORR catalysts due to their high catalytic activity, high stability and low cost [32]. Recently, a large amount of work has been devoted to the development of Co-based catalysts on different carbon materials to improve the stability and catalytic activity of the ORR catalyst in process, and achieved significant results [33-43]. However, it is still a challenge to simplify the preparation steps while improving the catalytic activity and stability. At present, many investigations have placed nitrogen-doped carbon materials containing cobalt elements, and significant progress has been made. Bimetallic alloy catalysts containing cobalt and metal core-shell nanomaterial catalysts were developed. However, the preparation process is complicated, and the catalytic activity, methanol 3
tolerance and stability need to be further improved. In order to further improve the ORR performance and simplify the preparation of the Co-based ORR catalyst, we demonstrate an in-situ controllable grading pyrolysis strategy for the synthesis of Co@CoOx-nanoparticles-based
three-dimensional
nitrogen-doped
graphene-like
carbon nanosheets with fluffy and hierarchically porous nanostructure (denoted as 3D HPN-GCNS/Co@CoOx). We have selected a precursor obtained by mixing 2methylimidazole and Co(NO3)2·6H2O. As confirmed in Fig. S1-S2, the precursor is liquefied at 180 °C (‘liquid phase step’ in the liquid-to-gas transition process), and then nitrate decomposition releases a large amount of gas (‘gas phase step’ in the liquid-togas transition process), which is called “liquid-to-gas” transition process. That is, we use the gas generated by the decomposition reaction of nitrate to rapidly expand the structure of the precursor in the liquefied state, making its structure become fluffy. The precursor in Fig. S1a is powder before 180 ° C heat treatment and the sample in Fig. S1b become honeycomb fluffy after 180 ° C heat treatment. Further pyrolysis forms a graphitized three-dimensional layered porous nanostructured material inlaid with Co@CoOx-nanoparticles. The remarkable features of high conductivity of the carbon material, well-developed in and out-of-plane pores, and large amount of Co@CoOxnanoparticles have greatly promoted the electron and mass transport and assured excellent and synergistically enhanced ORR catalytic efficiency. The half-wave potential and onset potential of the hybrid electrocatalysts reach 0.87 V and 0.92 V in 0.1 M KOH, respectively, much higher ORR activity than that of commercial 20% Pt/C. More importantly, they display better methanol tolerance and higher stability, with 4
almost no change of the ORR performance before and after 5000 cycles. And the H2O2 yield of the 3D HPN-GCNS/Co@CoOx in the ORR catalysis process has been less than 5%, the number of electron transfer (n) has been consistently higher than 3.9, which is close to the theoretical value of 4.00 for Pt/C, indicating a complete 4e ORR pathway. Specifically, this simple yet versatile strategy implies highly promising for commercial application of electrochemical storage/conversion systems.
2. Experimental Section 2.1 Synthesis of 3D HPN-GCNS/Co@CoOx precursor Co(NO3)2·6H2O (5.82 g, 0.02 mol) was dissolved in 100 ml of methanol solvent under vigorous stirring to obtain solution I; The 2-methylimidazole (3.08 g, 0.0375 mol) was dissolved in 100 ml of methanol solvent to obtain a solution II; then the solution II was slowly poured into the solution I under vigorous stirring to obtain a purple mixed solution III. The mixed solution III was allowed to stand at room temperature, and after drying completely, the purple precursor was obtained. 2.2 Synthesis of 3D HPN-GCNS/Co@CoOx The purple precursor was scraped off from the inner wall or bottom of the beaker, and 0.2 g of purple precursor was placed in the interior of the porcelain boat, heated to 180 °C (3°C/min) and kept for half an hour. Then, the temperature was further raised to 920°C (3°C/min) and held for 3.5 hours to obtain 3D HPN-GCNS/Co@CoOx. 2.3 Material characterizations
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Scanning electron microscopy (SEM, FEI QUANTA FEG 250 field emission scanning electron microscope produced by FEI, USA, secondary electron image resolution of 1.04 nm) and transmission electron microscope (TEM, LM1-125 produced by FEI, USA) were performed. The crystal information of the sample was crystal information by X-ray diffraction (XRD, D8 Advance X-ray diffractometer manufactured by Bruker, Germany). The valence and content of the elements in the sample were obtained by X-ray photoelectron spectroscopy (XPS, PX13 0031 X-ray photoelectron spectroscopy analyzer, Bruker, Germany). The pore size and distribution of the samples and the Brunauer-Emmett-Teller (BET) specific surface area data were obtained using a porosimeter (BET, Pore Master-60 Fully Automatic Porosity Meter from Quantachrome Instruments). The thermogravimetric data obtained by a synchronous thermal analyzer (TAG, TGA/DSC1/1600HT manufactured by METTLER) characterizes whether the precursor has liquefied upon decomposition of the released gas. Raman spectral analysis (HR Evolution, manufactured by HORIBA Scientific) was obtained using a 633 nm laser as a light source. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM, LM2-031, Bruker, Germany) and energy spectrometer (EDS, LM2-031, Bruker, Germany) were used. 2.4 Electrochemical characterizations All electrochemical data were tested at room temperature by a CHI760E. A rotating ring disk electrode (RRDE, manufactured by Japan ALS, the diameter of the GC disk is 4 mm, and the inner and outer diameter of the Pt ring is 5 mm and 7 mm, respectively.) and a rotating disk electrode (RDE, the diameter of the GC disk is 5 mm) are using as 6
the working electrode, and the reference electrode is a saturated calomel electrode (SCE). Pt wire is the counter electrode. All potentials involved in this study were referenced to a reversible hydrogen electrode (RHE) (ERHE = ESCE + pH × 0.059 V + 0.242 V). The potential of the ESCE reference electrode was 0.242 V. The loading of the catalyst on the electrode was 0.99 mg/cm2. Cyclic voltammetry (CV) measurements were tested after purging N2 or O2 gas for 30 min in the potential range from -0.8 to 0.2V versus SCE. RDE/RRDE tests for the ORR were measured in O2-saturated 0.1 M KOH solution at rotation speeds from 100 to 2500 rpm in the potential range from -0.8 to 0.2V versus SCE. The transferred electron number (n) was calculated by the K-L equation. Further, the percentage of peroxide species (HO2−) with respect to the total generated oxygen reduction products and the electron reduction number (n) were calculated.
3 Results and discussion The synthesis process of 3D HPN-GCNS/Co@CoOx is illustrated in scheme 1. The synthetic raw materials, 2-methylimidazole and Co(NO3)2·6H2O, were firstly mixed to obtain the 3D HPN-GCNS/Co@CoOx precursor. As shown in Fig. S3, the purple precursor is a three-dimensional nanosheet structure. Subsequently, the 3D HPNGCNS/Co@CoOx hybrids electrocatalysts were fabricated by controllable grading pyrolysis strategy. The morphology and crystal structure of the 3D HPNGCNS/Co@CoOx were examined by electron microscopy techniques and XRD. As shown in Fig. 1a-b, nanosheets assembly with a 3D fluffy structure were synthesized 7
as expected. They are loosely packed and exhibit large numbers of pores out of plane and interconnected tunnels throughout the entire assembly, facilitating the mass and electron transport. Moreover, large numbers of Co@CoOx-based nanoparticles are uniformly embedded on the surface of the thin nanosheets. It can be further confirmed consistent by the typical TEM (Fig. 1c). From the high angle annular dark fieldscanning transmission electron microscope (HAADF-STEM) images (Fig. 1d) and corresponding element mapping images, the nanoparticles embedded in nanosheets is about 20 nm in diameter, and distribution of cobalt element and oxygen element on the nanosheets is consistent with the distribution of nanoparticles. The N element is evenly distributed on the ultrathin nanosheets, preliminarily indicating that N heteroatoms can be uniformly doped. To further infer the composition of the nanoparticles on the nanosheets, we took a high-resolution TEM (HRTEM) image of the 3D HPNGCNS/Co@CoOx (Fig. 1f-g) and performed an XRD (Fig. 1e) pattern analysis. The lattice spacing of 0.2 nm in Fig. 1f corresponds to the (111) crystal plane of Co, located at 44.3° peak in the XRD pattern; the lattice spacing of 0.17 nm and 0.125 nm is attributed to the (200) and (220) crystal plane of Co, respectively; and in the XRD pattern, they correspond to the peaks at 51.5° and 75.9°, respectively. While the lattice spacing of 0.2 nm in Fig. 1g represents the (111) crystal plane of C, fixed at the 44.3° peak in the XRD pattern. All the results can be determined that the nanoparticles on the nanosheets contain Co0 and cobalt oxide. Additionally, the composition and oxidation states of the hybrids were further examined by XPS. The high-resolution Co 2p spectrum in Fig. 2a shows the peaks of 8
Co 2p3/2 and Co 2p1/2 and satellite peaks; specifically, the peaks at 778.4 eV, 779.2 eV and 781 eV in Co 2p3/2 and peaks at 794 eV, 795 eV and 797.5 eV in Co 2p1/2 are attributed to the Co0, Co3+ and Co2+, further confirming that the nanoparticles embedded on the surface of the hybrids are coexistence of Co0 and cobalt oxide-based nanoparticles (denoted as Co@CoOx). More excitedly, in the high-resolution N 1s spectrum (Fig. 2b), the peaks at 398.8 eV, 400.4 eV, and 401.2 eV are referred to pyridinic N, pyrrolic N and graphitic N, respectively [44]. The atomic percentage of N dopant reached 8.4% (Fig. S4). The rich N provides plenty of ORR catalytic active sites for 3D HPN-GCNS/Co@CoOx [21, 22, 32, 45]. Combined with the high resolution O 1s and C 1s XPS (Fig. S5-S6), Raman spectrum (Fig. 2c) shows the graphene-like D band (1327cm-1) and G band (1587cm-1) of the 3D HPN-GCNS/Co@CoOx (ID/IG = 0.97), indicating carbon nanosheets show graphitization [45, 46]. To further investigate the nanostructure of the 3D HPN-GCNS/Co@CoOx, we performed a BET test (Fig. 2d) on 3D HPN-GCNS/Co@CoOx. Excitingly, the 3D HPN-GCNS/Co@CoOx exhibits a specific surface area of 191 m2 g-1, and through the BJH pore size distribution map at the upper left, the pore size distribution on 3D HPNGCNS/Co@CoOx is concentrated to 2.5 nm. This matches the pore size in the Fig. 2d (insert) and Fig. S7, implying that the 3D HPN-GCNS/Co@CoOx is mesoporous in plane of C nanosheets, which provides favorable conditions for the mass transfer process of 3D HPN-GCNS/Co@CoOx during the ORR reaction [47-49]. All of the experiments in the electrochemical test section were performed in 0.1M KOH, and 3D HPN-GCNS/Co@CoOx was compared with the commercial 20 wt% Pt/C 9
catalyst. The cyclic voltammetry (CV) (Fig. S8) was first tested under nitrogen and oxygen saturation, respectively. Compared to that under nitrogen saturation, a significant oxygen reduction peak appeared in 3D HPN-GCNS/Co@CoOx under oxygen saturation conditions, demonstrating that 3D HPN-GCNS/Co@CoOx has ORR catalytic activity. As shown in the linear sweep voltammetric curve (LSV) (Fig. 3a), the Eonset and E1/2 of 3D HPN-GCNS/Co@CoOx is 0.92 V and 0.87 V at 1600 rpm/min, while the 20 wt% Pt/C shows an Eonset of 0.93V and an E1/2 of 0.83V (Fig. S9). The kinetic parameter (Fig. 3b) is analyzed by Koutecky-Levich. The electron-transfer number (n) is greater than 3.9, further proving that 3D HPN-GCNS/Co@CoOx has high ORR catalytic activity; in order to further explore the catalytic activity of 3D HPNGCNS/Co@CoOx, we verified the number of electron transfer at 1600 rpm/min and tested the yield of hydrogen peroxide (H2O2%) of the 3D HPN-GCNS/Co@CoOx (Fig.3c). The results show that the n of 3D HPN-GCNS/Co@CoOx is always higher than 3.9, and H2O2% is always lower than 5%. Combined with the current density of the ring and disk on the RRDE (Fig. S10), the disk current density is much larger than the ring current density; in short, the 3D HPN-GCNS/Co@CoOx has extremely high ORR catalytic efficiency, close to the complete 4e process. As shown in Fig. 3d, the slope of 20 wt% Pt/C is 85 mV/dec, and the slope of 3D HPN-GCNS/Co@CoOx is 80 mV/dec, indicating that 3D HPN-GCNS/Co@CoOx has a good kinetic process for ORR. Subsequently, methanol tolerance was performed (Fig. 3e).The LSV curve of 3D HPNGCNS/Co@CoOx displays almost no change after adding 1 M methanol into the 0.1 M KOH; by monitoring n and H2O2% yield before and after adding methanol (Fig. 3f), the 10
difference of the 3D HPN-GCNS/Co@CoOx is very small and the peak current of CV is the same (Fig.S11), concluding that 3D HPN-GCNS/Co@CoOx has excellent methanol resistance. Furthermore, stability is estimated. After 5000 cycles, the half-wave potential shifted to the left by 7 mV (Fig. 4a). After catalysing the oxygen reduction reaction for 30,000 s at 0.5 V (vs RHE), the catalytic activity remained 87.66% (Fig. S12); the overall structure still maintains a complete 3D fluffy and hierarchically porous nanosheets structure, although thickness of the nanosheets has increased slightly after continuous five hours reaction (Fig. S13). Additionally, the hybrids after 10 h acid treatment displays a weaker but still considerable ORR activity (Fig. S14). By monitoring the n and H2O2% (Fig. 4b) before and after the cycle, n is increased by 0.04 and H2O2% decreased by 2% after the cycle, indicating that the 3D HPN-GCNS/Co@CoOx maintains excellent ORR catalytic activity. Remarkably, the good ORR activity also makes it very attractive among the recent reported Co-based electrocatalysts (Table S1). The remarkable features of high activity, favourable kinetics, and strong durability are mainly ascribed to the synergistic effect of structure and composition of the 3D HPNGCNS/Co@CoOx: (ⅰ) the hybrids show 3D fluffy and hierarchically porous nanostructure in and out-of-plane, resulting in large accessible specific activity area, allowing the species to efficiently participate in ORR and facilitating the fast mass transfer; (ⅱ) the graphene-like C nanosheets with N-rich dopant promote O2 adsorption and assure fast electron transport; (ⅲ) large numbers of Co@CoOx-based nanoparticles uniformly embedded on the C nanosheets introduce more ORR active sites. The 11
features synergistically enhance the ORR performance and make it a promising lowcost, highly efficient ORR catalyst.
4. Conclusion In summary, on the basis of endogenous derivatization, we developed a controllable grading-pyrolysis strategy for the synthesis of 3D HPN-GCNS/Co@CoOx and exploited it as highly effective electrocatalyst for ORR. The hybrids exhibit a comparable ORR onset potential and higher ORR activity than the 20 wt% Pt/C. Additionally, they have good methanol tolerance and stability, making it a promising non-precious-metal catalyst. It is expected that it will promote ORR commercialization.
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Acknowledgments We acknowledge the financial support from the National Natural Science Foundation (Grant No. 21605057), Natural Science Foundation of Shandong Province (No. ZR2016BQ07), Open Founds of State Key Laboratory of Electroanaytical Chemistry (SKLEAC201907), and Study Abroad Fund.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/xxx.
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Scheme 1. Synthesis schematic of 3D HPN-GCNS/Co@CoOx.
23
Fig. 1. Typical (a, b) SEM, (c) TEM, (d) STEM images
and the corresponding
element mapping of Co, O, and N of the 3D HPN-GCNS/Co@CoOx. (e) XRD patterns of 3D HPN-GCNS/Co@CoOx. (f, g) HRTEM images of the 3D HPNGCNS/Co@CoOx.
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Fig. 2. Deconvoluted XPS (a) Co 2p and (b) N 1s of the 3D HPN-GCNS/Co@CoOx. (c) Raman spectra and (d) N2 adsorption-desorption isotherms of 3D HPNGCNS/Co@CoOx (Inset: mesoporous size distribution determined by the BJH method and high-resolution TEM of the sample).
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Fig. 3. (a) LSV curves of the 3D HPN-GCNS/Co@CoOx at different speeds in 0.1 M KOH. (b) Corresponding K-L curves ( j-1 vs ω-1/2) from the LSV plots shown in (a). (c) Electron transfer number and H2O2 yield curve and (d) Tafel plot of 3D HPNGCNS/Co@CoOx at 1600rpm/min in 0.1 M KOH. (e) Methanol tolerance and
(f)
electron transfer number and H2O2 yield curve of 3D HPN-GCNS/Co@CoOx at 1600rpm/min in 0.1 M KOH with and without 1 M methanol.
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Fig. 4. (a) LSV curves of before and after 5000 cycles, (b) the electron transfer number and H2O2 yield curve of 3D HPN-GCNS/Co@CoOx at 1600rpm/min in 0.1M KOH.
27
Highlights
The hybrids were successfully fabricated by a controllable grading pyrolysis strategy.
Graphene-like C nanosheets with N-rich dopant show in and out-of-plane nanostructure.
Co-based nanoparticles uniformly embedded on fluffy and hierarchically C nanosheets.
The remarkable features synergistically enhance the ORR catalytic efficiency.
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Liquid-to-Gas Transition Derived Cobalt-based NitrogenDoped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction Zhenlu Zhao,*a, b Qiqi Sha,a Kongshuo Mac and Yizhong Lua
3D HPN-GCNS/Co@CoOx as catalysts for ORR: The hybrids were fabricated by a controllable grading-pyrolysis strategy. The hybrids show 3D fluffy and hierarchically porous nanostructure in and out-of-plane, resulting in large accessible specific activity area, allowing the species to efficiently participate in ORR and facilitating the fast mass transfer; the graphene-like C nanosheets with N-rich dopant promote O2 adsorption and assure fast electron transport; large numbers of Co@CoOx-based nanoparticles uniformly embedded on the C nanosheets and mesoporous active edge introduce more ORR active sites. The remarkable features synergistically enhance the ORR catalytic efficiency, making it a promising low-cost, highly efficient ORR catalyst.
Keywords: Carbon nanosheets, Hybrid nanostructure, Synergistic effect, Oxygen reduction reaction
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
30
Zhenlu Zhao conceived and designed the project. QiQi Sha and Kongsuo Ma prepared the nanomaterials, conducted the structural and characterizations, and performed electrical measurements. Zhenlu Zhao, QiQi Sha, Kongsuo Ma and Yizhong Lu analyzed and discussed the data. Zhenlu Zhao and QiQi Sha wrote the paper.
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S0169-4332(20)30121-5 https://doi.org/10.1016/j.apsusc.2020.145365 APSUSC 145365
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
15 October 2019 12 December 2019 10 January 2020
Please cite this article as: Z. Zhao, Q. Sha, K. Ma, Y. Lu, Liquid-to-Gas Transition Derived Cobalt-based Nitrogen-Doped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145365
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© 2020 Published by Elsevier B.V.
Liquid-to-Gas Transition Derived Cobalt-based NitrogenDoped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction
Zhenlu Zhao,*a, b Qiqi Sha,a Kongshuo Mac and Yizhong Lua
a
School of Material Science and Engineering, University of Jinan, Jinan 250022, Shandong, China.
b
Department of Bionano Engineering, Hanyang University, Ansan 426-791, South Korea.
c
State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, Jilin, China.
*E-mail:
[email protected]
Keywords: Carbon nanosheets, Hybrid nanostructure, Synergistic effect, Oxygen reduction reaction
1
Abstract Here, Co@CoOx-based three-dimensional nitrogen-doped graphene-like carbon nanosheets (denoted as 3D HPN-GCNS/Co@CoOx) are fabricated by controllable grading-pyrolysis strategy, in which they show 3D fluffy and hierarchically porous nanostructure in and out-of-plane; large numbers of Co@CoOx-based nanoparticles uniformly embedded on graphene-like C nanosheets and mesoporous active edge introduce more ORR active sites. When applied for ORR, hybrids exhibit a half-wave potential (E1/2) of 0.87 V and an onset potential (Eonset) of 0.92 V in 0.1 M KOH, comparable to that of commercial 20% Pt/C. The hybrids display better methanol tolerance and stability, with almost no change of ORR performance before and after 5000 cycles. H2O2 yield in ORR process is less than 5% and number of electron transfer is higher than 3.9. The remarkable features are mainly ascribed to synergistic enhancement effect, in which 3D fluffy and hierarchically porous nanostructure in and out-of-plane supplies large accessible specific activity area, allows species to efficiently participate in ORR and facilitates fast mass transfer; graphene-like C nanosheets with N-rich dopant promote O2 adsorption and assure fast electron transport; large numbers of Co@CoOx-based nanoparticles introduce more ORR active sites. All the advantages synergistically enhance the ORR efficiency and make it a highly efficient ORR catalyst.
2
1. Introduction The gradual depletion of fossil energy and the aggravation of environmental pollution have forced human society to put more attention and research into the development of clean and sustainable new energy [1-3]. The development of fuel cells and large-scale commercial promotion are an important solution to the current problem. Although Pt-based ORR catalysts have extremely high catalytic efficiency and have been widely used, they have high cost and rare reserves. Disadvantage such as easy poisoning limits its large-scale commercial application. In response, a variety of nonPt-based catalysts have been developed, including transition metal nitrides [4-7], transition metal oxides [8-14], conductive polymers [15-17], doped carbon-based catalysts [18-26], and monoatomic catalysts [27-31], etc. In contrast, transition metalbased catalysts, especially cobalt (Co), are widely considered to be ideal catalysts for non-Pt-based ORR catalysts due to their high catalytic activity, high stability and low cost [32]. Recently, a large amount of work has been devoted to the development of Co-based catalysts on different carbon materials to improve the stability and catalytic activity of the ORR catalyst in process, and achieved significant results [33-43]. However, it is still a challenge to simplify the preparation steps while improving the catalytic activity and stability. At present, many investigations have placed nitrogen-doped carbon materials containing cobalt elements, and significant progress has been made. Bimetallic alloy catalysts containing cobalt and metal core-shell nanomaterial catalysts were developed. However, the preparation process is complicated, and the catalytic activity, methanol 3
tolerance and stability need to be further improved. In order to further improve the ORR performance and simplify the preparation of the Co-based ORR catalyst, we demonstrate an in-situ controllable grading pyrolysis strategy for the synthesis of Co@CoOx-nanoparticles-based
three-dimensional
nitrogen-doped
graphene-like
carbon nanosheets with fluffy and hierarchically porous nanostructure (denoted as 3D HPN-GCNS/Co@CoOx). We have selected a precursor obtained by mixing 2methylimidazole and Co(NO3)2·6H2O. As confirmed in Fig. S1-S2, the precursor is liquefied at 180 °C (‘liquid phase step’ in the liquid-to-gas transition process), and then nitrate decomposition releases a large amount of gas (‘gas phase step’ in the liquid-togas transition process), which is called “liquid-to-gas” transition process. That is, we use the gas generated by the decomposition reaction of nitrate to rapidly expand the structure of the precursor in the liquefied state, making its structure become fluffy. The precursor in Fig. S1a is powder before 180 ° C heat treatment and the sample in Fig. S1b become honeycomb fluffy after 180 ° C heat treatment. Further pyrolysis forms a graphitized three-dimensional layered porous nanostructured material inlaid with Co@CoOx-nanoparticles. The remarkable features of high conductivity of the carbon material, well-developed in and out-of-plane pores, and large amount of Co@CoOxnanoparticles have greatly promoted the electron and mass transport and assured excellent and synergistically enhanced ORR catalytic efficiency. The half-wave potential and onset potential of the hybrid electrocatalysts reach 0.87 V and 0.92 V in 0.1 M KOH, respectively, much higher ORR activity than that of commercial 20% Pt/C. More importantly, they display better methanol tolerance and higher stability, with 4
almost no change of the ORR performance before and after 5000 cycles. And the H2O2 yield of the 3D HPN-GCNS/Co@CoOx in the ORR catalysis process has been less than 5%, the number of electron transfer (n) has been consistently higher than 3.9, which is close to the theoretical value of 4.00 for Pt/C, indicating a complete 4e ORR pathway. Specifically, this simple yet versatile strategy implies highly promising for commercial application of electrochemical storage/conversion systems.
2. Experimental Section 2.1 Synthesis of 3D HPN-GCNS/Co@CoOx precursor Co(NO3)2·6H2O (5.82 g, 0.02 mol) was dissolved in 100 ml of methanol solvent under vigorous stirring to obtain solution I; The 2-methylimidazole (3.08 g, 0.0375 mol) was dissolved in 100 ml of methanol solvent to obtain a solution II; then the solution II was slowly poured into the solution I under vigorous stirring to obtain a purple mixed solution III. The mixed solution III was allowed to stand at room temperature, and after drying completely, the purple precursor was obtained. 2.2 Synthesis of 3D HPN-GCNS/Co@CoOx The purple precursor was scraped off from the inner wall or bottom of the beaker, and 0.2 g of purple precursor was placed in the interior of the porcelain boat, heated to 180 °C (3°C/min) and kept for half an hour. Then, the temperature was further raised to 920°C (3°C/min) and held for 3.5 hours to obtain 3D HPN-GCNS/Co@CoOx. 2.3 Material characterizations
5
Scanning electron microscopy (SEM, FEI QUANTA FEG 250 field emission scanning electron microscope produced by FEI, USA, secondary electron image resolution of 1.04 nm) and transmission electron microscope (TEM, LM1-125 produced by FEI, USA) were performed. The crystal information of the sample was crystal information by X-ray diffraction (XRD, D8 Advance X-ray diffractometer manufactured by Bruker, Germany). The valence and content of the elements in the sample were obtained by X-ray photoelectron spectroscopy (XPS, PX13 0031 X-ray photoelectron spectroscopy analyzer, Bruker, Germany). The pore size and distribution of the samples and the Brunauer-Emmett-Teller (BET) specific surface area data were obtained using a porosimeter (BET, Pore Master-60 Fully Automatic Porosity Meter from Quantachrome Instruments). The thermogravimetric data obtained by a synchronous thermal analyzer (TAG, TGA/DSC1/1600HT manufactured by METTLER) characterizes whether the precursor has liquefied upon decomposition of the released gas. Raman spectral analysis (HR Evolution, manufactured by HORIBA Scientific) was obtained using a 633 nm laser as a light source. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM, LM2-031, Bruker, Germany) and energy spectrometer (EDS, LM2-031, Bruker, Germany) were used. 2.4 Electrochemical characterizations All electrochemical data were tested at room temperature by a CHI760E. A rotating ring disk electrode (RRDE, manufactured by Japan ALS, the diameter of the GC disk is 4 mm, and the inner and outer diameter of the Pt ring is 5 mm and 7 mm, respectively.) and a rotating disk electrode (RDE, the diameter of the GC disk is 5 mm) are using as 6
the working electrode, and the reference electrode is a saturated calomel electrode (SCE). Pt wire is the counter electrode. All potentials involved in this study were referenced to a reversible hydrogen electrode (RHE) (ERHE = ESCE + pH × 0.059 V + 0.242 V). The potential of the ESCE reference electrode was 0.242 V. The loading of the catalyst on the electrode was 0.99 mg/cm2. Cyclic voltammetry (CV) measurements were tested after purging N2 or O2 gas for 30 min in the potential range from -0.8 to 0.2V versus SCE. RDE/RRDE tests for the ORR were measured in O2-saturated 0.1 M KOH solution at rotation speeds from 100 to 2500 rpm in the potential range from -0.8 to 0.2V versus SCE. The transferred electron number (n) was calculated by the K-L equation. Further, the percentage of peroxide species (HO2−) with respect to the total generated oxygen reduction products and the electron reduction number (n) were calculated.
3 Results and discussion The synthesis process of 3D HPN-GCNS/Co@CoOx is illustrated in scheme 1. The synthetic raw materials, 2-methylimidazole and Co(NO3)2·6H2O, were firstly mixed to obtain the 3D HPN-GCNS/Co@CoOx precursor. As shown in Fig. S3, the purple precursor is a three-dimensional nanosheet structure. Subsequently, the 3D HPNGCNS/Co@CoOx hybrids electrocatalysts were fabricated by controllable grading pyrolysis strategy. The morphology and crystal structure of the 3D HPNGCNS/Co@CoOx were examined by electron microscopy techniques and XRD. As shown in Fig. 1a-b, nanosheets assembly with a 3D fluffy structure were synthesized 7
as expected. They are loosely packed and exhibit large numbers of pores out of plane and interconnected tunnels throughout the entire assembly, facilitating the mass and electron transport. Moreover, large numbers of Co@CoOx-based nanoparticles are uniformly embedded on the surface of the thin nanosheets. It can be further confirmed consistent by the typical TEM (Fig. 1c). From the high angle annular dark fieldscanning transmission electron microscope (HAADF-STEM) images (Fig. 1d) and corresponding element mapping images, the nanoparticles embedded in nanosheets is about 20 nm in diameter, and distribution of cobalt element and oxygen element on the nanosheets is consistent with the distribution of nanoparticles. The N element is evenly distributed on the ultrathin nanosheets, preliminarily indicating that N heteroatoms can be uniformly doped. To further infer the composition of the nanoparticles on the nanosheets, we took a high-resolution TEM (HRTEM) image of the 3D HPNGCNS/Co@CoOx (Fig. 1f-g) and performed an XRD (Fig. 1e) pattern analysis. The lattice spacing of 0.2 nm in Fig. 1f corresponds to the (111) crystal plane of Co, located at 44.3° peak in the XRD pattern; the lattice spacing of 0.17 nm and 0.125 nm is attributed to the (200) and (220) crystal plane of Co, respectively; and in the XRD pattern, they correspond to the peaks at 51.5° and 75.9°, respectively. While the lattice spacing of 0.2 nm in Fig. 1g represents the (111) crystal plane of C, fixed at the 44.3° peak in the XRD pattern. All the results can be determined that the nanoparticles on the nanosheets contain Co0 and cobalt oxide. Additionally, the composition and oxidation states of the hybrids were further examined by XPS. The high-resolution Co 2p spectrum in Fig. 2a shows the peaks of 8
Co 2p3/2 and Co 2p1/2 and satellite peaks; specifically, the peaks at 778.4 eV, 779.2 eV and 781 eV in Co 2p3/2 and peaks at 794 eV, 795 eV and 797.5 eV in Co 2p1/2 are attributed to the Co0, Co3+ and Co2+, further confirming that the nanoparticles embedded on the surface of the hybrids are coexistence of Co0 and cobalt oxide-based nanoparticles (denoted as Co@CoOx). More excitedly, in the high-resolution N 1s spectrum (Fig. 2b), the peaks at 398.8 eV, 400.4 eV, and 401.2 eV are referred to pyridinic N, pyrrolic N and graphitic N, respectively [44]. The atomic percentage of N dopant reached 8.4% (Fig. S4). The rich N provides plenty of ORR catalytic active sites for 3D HPN-GCNS/Co@CoOx [21, 22, 32, 45]. Combined with the high resolution O 1s and C 1s XPS (Fig. S5-S6), Raman spectrum (Fig. 2c) shows the graphene-like D band (1327cm-1) and G band (1587cm-1) of the 3D HPN-GCNS/Co@CoOx (ID/IG = 0.97), indicating carbon nanosheets show graphitization [45, 46]. To further investigate the nanostructure of the 3D HPN-GCNS/Co@CoOx, we performed a BET test (Fig. 2d) on 3D HPN-GCNS/Co@CoOx. Excitingly, the 3D HPN-GCNS/Co@CoOx exhibits a specific surface area of 191 m2 g-1, and through the BJH pore size distribution map at the upper left, the pore size distribution on 3D HPNGCNS/Co@CoOx is concentrated to 2.5 nm. This matches the pore size in the Fig. 2d (insert) and Fig. S7, implying that the 3D HPN-GCNS/Co@CoOx is mesoporous in plane of C nanosheets, which provides favorable conditions for the mass transfer process of 3D HPN-GCNS/Co@CoOx during the ORR reaction [47-49]. All of the experiments in the electrochemical test section were performed in 0.1M KOH, and 3D HPN-GCNS/Co@CoOx was compared with the commercial 20 wt% Pt/C 9
catalyst. The cyclic voltammetry (CV) (Fig. S8) was first tested under nitrogen and oxygen saturation, respectively. Compared to that under nitrogen saturation, a significant oxygen reduction peak appeared in 3D HPN-GCNS/Co@CoOx under oxygen saturation conditions, demonstrating that 3D HPN-GCNS/Co@CoOx has ORR catalytic activity. As shown in the linear sweep voltammetric curve (LSV) (Fig. 3a), the Eonset and E1/2 of 3D HPN-GCNS/Co@CoOx is 0.92 V and 0.87 V at 1600 rpm/min, while the 20 wt% Pt/C shows an Eonset of 0.93V and an E1/2 of 0.83V (Fig. S9). The kinetic parameter (Fig. 3b) is analyzed by Koutecky-Levich. The electron-transfer number (n) is greater than 3.9, further proving that 3D HPN-GCNS/Co@CoOx has high ORR catalytic activity; in order to further explore the catalytic activity of 3D HPNGCNS/Co@CoOx, we verified the number of electron transfer at 1600 rpm/min and tested the yield of hydrogen peroxide (H2O2%) of the 3D HPN-GCNS/Co@CoOx (Fig.3c). The results show that the n of 3D HPN-GCNS/Co@CoOx is always higher than 3.9, and H2O2% is always lower than 5%. Combined with the current density of the ring and disk on the RRDE (Fig. S10), the disk current density is much larger than the ring current density; in short, the 3D HPN-GCNS/Co@CoOx has extremely high ORR catalytic efficiency, close to the complete 4e process. As shown in Fig. 3d, the slope of 20 wt% Pt/C is 85 mV/dec, and the slope of 3D HPN-GCNS/Co@CoOx is 80 mV/dec, indicating that 3D HPN-GCNS/Co@CoOx has a good kinetic process for ORR. Subsequently, methanol tolerance was performed (Fig. 3e).The LSV curve of 3D HPNGCNS/Co@CoOx displays almost no change after adding 1 M methanol into the 0.1 M KOH; by monitoring n and H2O2% yield before and after adding methanol (Fig. 3f), the 10
difference of the 3D HPN-GCNS/Co@CoOx is very small and the peak current of CV is the same (Fig.S11), concluding that 3D HPN-GCNS/Co@CoOx has excellent methanol resistance. Furthermore, stability is estimated. After 5000 cycles, the half-wave potential shifted to the left by 7 mV (Fig. 4a). After catalysing the oxygen reduction reaction for 30,000 s at 0.5 V (vs RHE), the catalytic activity remained 87.66% (Fig. S12); the overall structure still maintains a complete 3D fluffy and hierarchically porous nanosheets structure, although thickness of the nanosheets has increased slightly after continuous five hours reaction (Fig. S13). Additionally, the hybrids after 10 h acid treatment displays a weaker but still considerable ORR activity (Fig. S14). By monitoring the n and H2O2% (Fig. 4b) before and after the cycle, n is increased by 0.04 and H2O2% decreased by 2% after the cycle, indicating that the 3D HPN-GCNS/Co@CoOx maintains excellent ORR catalytic activity. Remarkably, the good ORR activity also makes it very attractive among the recent reported Co-based electrocatalysts (Table S1). The remarkable features of high activity, favourable kinetics, and strong durability are mainly ascribed to the synergistic effect of structure and composition of the 3D HPNGCNS/Co@CoOx: (ⅰ) the hybrids show 3D fluffy and hierarchically porous nanostructure in and out-of-plane, resulting in large accessible specific activity area, allowing the species to efficiently participate in ORR and facilitating the fast mass transfer; (ⅱ) the graphene-like C nanosheets with N-rich dopant promote O2 adsorption and assure fast electron transport; (ⅲ) large numbers of Co@CoOx-based nanoparticles uniformly embedded on the C nanosheets introduce more ORR active sites. The 11
features synergistically enhance the ORR performance and make it a promising lowcost, highly efficient ORR catalyst.
4. Conclusion In summary, on the basis of endogenous derivatization, we developed a controllable grading-pyrolysis strategy for the synthesis of 3D HPN-GCNS/Co@CoOx and exploited it as highly effective electrocatalyst for ORR. The hybrids exhibit a comparable ORR onset potential and higher ORR activity than the 20 wt% Pt/C. Additionally, they have good methanol tolerance and stability, making it a promising non-precious-metal catalyst. It is expected that it will promote ORR commercialization.
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Acknowledgments We acknowledge the financial support from the National Natural Science Foundation (Grant No. 21605057), Natural Science Foundation of Shandong Province (No. ZR2016BQ07), Open Founds of State Key Laboratory of Electroanaytical Chemistry (SKLEAC201907), and Study Abroad Fund.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/xxx.
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Scheme 1. Synthesis schematic of 3D HPN-GCNS/Co@CoOx.
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Fig. 1. Typical (a, b) SEM, (c) TEM, (d) STEM images
and the corresponding
element mapping of Co, O, and N of the 3D HPN-GCNS/Co@CoOx. (e) XRD patterns of 3D HPN-GCNS/Co@CoOx. (f, g) HRTEM images of the 3D HPNGCNS/Co@CoOx.
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Fig. 2. Deconvoluted XPS (a) Co 2p and (b) N 1s of the 3D HPN-GCNS/Co@CoOx. (c) Raman spectra and (d) N2 adsorption-desorption isotherms of 3D HPNGCNS/Co@CoOx (Inset: mesoporous size distribution determined by the BJH method and high-resolution TEM of the sample).
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Fig. 3. (a) LSV curves of the 3D HPN-GCNS/Co@CoOx at different speeds in 0.1 M KOH. (b) Corresponding K-L curves ( j-1 vs ω-1/2) from the LSV plots shown in (a). (c) Electron transfer number and H2O2 yield curve and (d) Tafel plot of 3D HPNGCNS/Co@CoOx at 1600rpm/min in 0.1 M KOH. (e) Methanol tolerance and
(f)
electron transfer number and H2O2 yield curve of 3D HPN-GCNS/Co@CoOx at 1600rpm/min in 0.1 M KOH with and without 1 M methanol.
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Fig. 4. (a) LSV curves of before and after 5000 cycles, (b) the electron transfer number and H2O2 yield curve of 3D HPN-GCNS/Co@CoOx at 1600rpm/min in 0.1M KOH.
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Highlights
The hybrids were successfully fabricated by a controllable grading pyrolysis strategy.
Graphene-like C nanosheets with N-rich dopant show in and out-of-plane nanostructure.
Co-based nanoparticles uniformly embedded on fluffy and hierarchically C nanosheets.
The remarkable features synergistically enhance the ORR catalytic efficiency.
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Liquid-to-Gas Transition Derived Cobalt-based NitrogenDoped Carbon Nanosheets with Hierarchically Porous for Oxygen Reduction Reaction Zhenlu Zhao,*a, b Qiqi Sha,a Kongshuo Mac and Yizhong Lua
3D HPN-GCNS/Co@CoOx as catalysts for ORR: The hybrids were fabricated by a controllable grading-pyrolysis strategy. The hybrids show 3D fluffy and hierarchically porous nanostructure in and out-of-plane, resulting in large accessible specific activity area, allowing the species to efficiently participate in ORR and facilitating the fast mass transfer; the graphene-like C nanosheets with N-rich dopant promote O2 adsorption and assure fast electron transport; large numbers of Co@CoOx-based nanoparticles uniformly embedded on the C nanosheets and mesoporous active edge introduce more ORR active sites. The remarkable features synergistically enhance the ORR catalytic efficiency, making it a promising low-cost, highly efficient ORR catalyst.
Keywords: Carbon nanosheets, Hybrid nanostructure, Synergistic effect, Oxygen reduction reaction
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Zhenlu Zhao conceived and designed the project. QiQi Sha and Kongsuo Ma prepared the nanomaterials, conducted the structural and characterizations, and performed electrical measurements. Zhenlu Zhao, QiQi Sha, Kongsuo Ma and Yizhong Lu analyzed and discussed the data. Zhenlu Zhao and QiQi Sha wrote the paper.
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