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One-pot synthesis of ruthenium nanoparticles embedded nitrogen-doped carbon framework for electrocatalytic hydrogen evolution reaction
Inorganic Chemistry Communications 116 (2020) 107914
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One-pot synthesis of ruthenium nanoparticles embedded nitrogen-doped carbon framework for electrocatalytic hydrogen evolution reaction
T
Ziqi Zhanga,1, Tiejun Wangb,1, Kaida Yaoa, Linchuan Conga, Zhuochen Yua, Lina Qua, ⁎ Miaomiao Qiana, Weimin Huanga,c, a
College of Chemistry, Jilin University, Changchun 130012, China Division of Orthopaedic Traumatology, The First Hospital of Jilin University, Changchun, China c Key Laboratory of Physics and Technology for Advanced Batteries of Ministry of Education, Jilin University, Changchun 130012, China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: ZIF-8 Low-noble catalyst One-pot Hydrogen evolution reaction Nitrogen-doped carbon framework
Hydrogen is one of the most desirable alternatives to fossil fuel due to its cleanliness, renewability and large energy density. Efficient and durable electrocatalyst toward hydrogen evolution reaction (HER) is an urgent need for the sustainable development and long-term environmental conservation. The noble metal(Pt/Ru/Ir) has a favorable performance for HER but the high price and the low reserve hinder its further application. Meanwhile, the non-noble metal electrocatalysts can hardly achieve a satisfactory stability and efficiency. Therefore, this paper reports a Ru embedded N-doped carbon framework material as low-noble metal catalyst which has low loading of Ru (0.39 wt%) and exhibits the desirable overpotential of 181 mV at 10 mA cm−2 in alkaline and 298 mV at 10 mA cm−2 in acidic solution. Besides, it retains 94. 1% current density after durability potentiostatic tests for 30,000 s while the commercial 20 wt% Pt/C can only retain 84. 2%, which suggests a superior stability.
1. Introduction Hydrogen is a clean and renewable energy carrier having a large
energy density [1]. With the deterioration of environmental issues and depletion of traditional fossil fuels, electrocatalytic hydrogen evolution reaction (HER) is drawing more and more attention since it can produce
Corresponding author at: College of Chemistry, Jilin University, Changchun 130012, China. E-mail address: [email protected] (W. Huang). 1 Ziqi Zhang and Tiejun Wang made the equal contribution to the article. ⁎
https://doi.org/10.1016/j.inoche.2020.107914 Received 3 March 2020; Received in revised form 25 March 2020; Accepted 28 March 2020 Available online 02 April 2020 1387-7003/ © 2020 Elsevier B.V. All rights reserved.
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H2 powered by renewable energy, e.g., solar or wind [2,3]. To drive HER with low power consumption and at low potential in practical applications, efficient, durable and affordable electrocatalysts are highly desirable [4]. Pt-based or Pd-based catalysts have been regarded as the most efficient electrocatalysts for HER. However, the high cost and scarcity of them hinder their commercialization [5]. Currently, to decrease the cost, various noble-metal-free electrocatalysts such as transition metal sulfides [6], phosphides [7], carbides [8], nitrides [9], and oxides [10] materials have been used for HER. Unfortunately, these catalysts based on transition metal performed dissatisfactory efficiency, stability or durability [11]. Especially the noble metal has an extremely low overpotential of HER. In order to combine the advantages of noble metal catalyst and non-noble metal catalyst, more and more attentions have been paid to study low-noble metal catalysts. Ruthenium has been a preferred material because of its cheaper price than that of platinum and its superior performance for HER [12]. It has the overpotential of 55 mV in 0.1 M KOH [13]. Therefore, ruthenium is a desirable substitute for Pt. Metal–organic frameworks (MOFs) are composed of organic ligands and tunable metal ion centers featuring large surface areas and diverse structural topologies, which offer inherent advantages and accessible active sites when being used as catalyst [14,15,16]. Zeolitic imidazolate framework (ZIF) series is a kind of MOFs that can be easily synthesized, which is suitable to serve as the base to loading metal atoms in order to enhance electrocatalytic efficiency and increase noble metal atomic utilization rate. More importantly, direct carbonization of ZIF can form nanoporous carbon materials that possess adjustable N and metal atoms contents and species, which provides large surface areas containing considerable active sites and regulate the electronic structure to from a conductive network [14]. In this work, we demonstrated a new low-noble metal electrocatalyst for HER, which was obtained by direct carbonizing Ru doped ZIF-8-based nitrogen carbon mixture (Ru@NC). The Ru@NC was synthesized by one-pot method and was calcined in high temperature and in nitrogen atmosphere to acquire Ru embedded N-doped carbon framework (Ru@NCF). The carbonized Ru@NC provides high surface area and satisfying stability thus it is durable and effective during the process of electrocatalysis. Calcining Ru@NC at high temperature can remove Zn atoms so that Ru atoms can have access to the Zn vacancy. It allows the Ru atoms embedded equably and regularly throughout the N-doped carbon framework and eventually improve the rate of atomic utilization. As a result, the catalyst can reach a low HER overpotential at 181 mV in 1 M KOH. And the loading of Ru is only 0.39 wt% detected by inductively coupled plasma (ICP).
photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250Xi with an excitation source of Al Kα radiation. Xray diffraction (XRD) patterns were carried out using a Empyrean (PANalytical B.V.) with a Cu Kα radiation source (λ1 = 1.5406 Å) operating at 40.0 kV and 40.0 mA, and the diffraction data were recorded in the 2θ range of 5–80° with a scan rate of 4 degrees per min. 2.3. Preparation of Ru@NCF In a typical procedure, 1. 2 g of Zn(OAC)2·2H2O and 2. 0 g of dicyandiamide (DCD) were dissolved in 25 ml of methanol to form a clear solution, which was subsequently injected into 25 ml of methanol containing 1. 8 g of 2-methylimidazole (MeIM). Then, 1 ml, 2 ml or 3 ml of 10−4 M RuCl3 aqueous solution was added into the mixed solution which was subsequently stirred for 24 h at room temperature. The mixed solution was centrifuged and washed with methanol several times and dried in vacuum at 60 °C for overnight to get Ru doped ZIF-8based nitrogen carbon mixture (Rux@NC, x represents the volume of the Ru solution added). ZIF-8-based nitrogen carbon mixture (NC) was synthesized the same without Ru as blank control group. The prepared Rux@NC was heated at 800 °C for 4 h at a rate of 3 °C·min−1 in a tube furnace and in N2 atmosphere to get Ru embedded N-doped carbon framework (Rux@NCF, x represents the volume of the Ru solution added in the last synthetic procedure). The NC was treated the same to get N-doped carbon framework (NCF) as blank control group, too. 2.4. Cathode preparation 5 mg of sample (NC, NCF, Rux@NCF and Pt/C(20 wt%)) was dispersed in 240 μL of isopropyl alcohol, 250 μL of ultrapure water and 20 μL of 5 wt% Nafion mixed solution and ultrasonic for 30 min. 5 μL of liquid was pipetted onto the surface of glass carbon electrode (GCE, ∅ = 3 mm) (loading ~0.7077 mg·cm−2) with natural drying and then served as working electrode. 2.5. Electrochemical measurements Electrochemical measurements were performed by a CHI 660E electrochemical workstation with a three-electrode system, including a working electrode (glassy carbon electrode, GCE, diameter 3 mm, superficial area 0.07065 cm2), a counter electrode (Pt foil, 1 × 1 cm2) and a reference electrode (Hg/HgO(1 M KOH electrolyte) electrode in 1 M KOH solution or Hg/Hg2SO4 (saturated K2SO4 electrolyte) electrode in 0.5 M H2SO4 solution)). The experimental potential values were calibrated by using the following equation: E vs. RHE = E vs. Hg/ HgO + 0.098 + 0.059 pH; E vs. RHE = E vs. Hg/ Hg2SO4 + 0.616 + 0.059 pH. Before and during the test, a N2 flow (20 ml·min−1) was continuously fed to the cathode through the electrolyte in the cell to eliminate polarization. And the linear sweep voltammetry (LSV) experiments were performed in the scan rate of 5 mV s−1 from 0.2 to −0.7 V (vs. RHE) in N2-saturated 0.5 M H2SO4 or 1 M KOH solution, respectively. Electrochemical impedance spectra (EIS) measurement was performed at overpotential of 300 mV (vs. RHE) with frequency from 0.1 to 100,000 Hz and an amplitude of 5 mV. Durability potentiostatic tests were conducted in N2-saturated 1 M KOH solution at −0.4 V (vs. RHE) for Ru3@NCF and −0.1 V (vs. RHE) for Pt/C for 30,000 s by current–time(I-t) measurement(polarization quiet time = 3000 s). The electrochemically active surface area (ECSA) was estimated by CVs, which was tested from −0.80 to −0.70 V (vs. Hg/ HgO) with scan rate from 10 to 100 mV s−1 in 1 M KOH.
2. Experimental section 2.1. Reagents Zincacetate dihydrate (Beijing Chemical Works, C4H6O4Zn·2H2O, analytical reagent), 2-methylimidazole(C4H6N2, aladdin), ruthenium chloride hydrate (35. 0–42. 0% Ru basis, aladdin), 2-methylimidazole (C4H6N2, aladdin), dicyandiamide (C2H4N4, aladdin), methanol (Beijing Chemical Works, CH4O, ≥99. 8%), Pt/C (20 wt%, JM), Nafion (5. 0 wt%, Dupont), Nafion 117 membrane (Dupont), ethyl alcohol (Beijing Chemical Works, C2H5OH, ≥99. 8%), sulfuric acid (Beijing Chemical Works, H2SO4, 98%), potassium hydroxide (Beijing Chemical Works, KOH, 96%) and N2 gas (99. 99%) were employed. All chemical reagents were used as received without further purification. All aqueous solutions were prepared with ultrapure water (resistivity of 18. 25 MΩ cm). 2.2. Physical characterization Transmission electron microscopy (TEM) and elemental mapping were performed on an FEI Tecnai G20/JEM2010 microscope. X-ray 2
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Fig. 1. TEM images (a–c), HAADF-STEM image (d), corresponding mapping images (e–g), high-resolution TEM image (h) and XRD pattern (i) of Ru3@NCF.
3. Result and discussion
which are corresponding to the Ru (1 0 0), Ru (0 0 2) and Ru (1 0 1) planes in XRD pattern, respectively, (Fig. 1i) according to the Bragg’s Law(2dsinθ = nλ). Besides, the Bragg reflections at 37. 39°, 42. 15°, 44. 01°, 58. 32°, 69. 40°, 78. 39°, 82. 22°, 84. 70° and 85. 96° correspond to the indexed planes of hexagonal closest packed (HCP) crystals of Ru (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1), respectively (JCPDS No: 6-0663) [20]. In order to investigate the information of compound surface including chemical composition and element bonding configurations, Xray photoelectron spectroscopy was carried out for Ru3@NCF. As shown in Fig. 1a, it confirms the existence of C, N, O, Ru and Zn. In the C 1s and Ru 3d spectra (Fig. 2b), the peaks were observed at 280.2, 284. 7, 285. 6 and 286. 7 eV, which can be ascribed to the Ru 3d, CeC, CeN, and CeC]O, respectively [21]. Fig. 2c shows the high-resolution spectrum of N 1s, which presents 3 types of N coordination environments at 398. 7 eV for pyridinic-N, 401. 0 eV for graphitic-N, 404. 0 eV for oxidized-N [22]. Meanwhile, the N content is determined to be 9. 98 at.%. N doping in the porous carbon polyhedrons is beneficial to stabilizing metallic Ru, as well as regulating the electronic structure and surface permeability of Ru@NCF catalyst [4]. The Ru 3p peaks (460.9 eV and 482. 7 eV) and Zn auger peak (473. 5 eV) are shown in Fig. 2d. The presences of O 1s and Zn 3p signal at about 532. 1 eV and 1023. 8 eV in the XPS survey are unavoidable, due to the surface oxidation and the incomplete gasification of Zn atoms.
3.1. Physical characterization The precursor was a mixture of Zn2+, Ru3+, MeIM and DCD in methanol solution. Zn2+ and MeIM was able to generate ZIF-8 analogue [17] while Ru3+ can cling to the ZIF-8 analogue by intensely stirring. It is hoped that Ru3+ was reduced to Ru and combined with DCD to form crosslinking nanotubes network at high temperatures [18] while the ZIF-8 analogue can be directly carbonized and serve as the fundamental base for Ru nanoparticles to be fixed. As shown in Fig. 1a, the nanotubes grow at the edge of the material while the internal nanotubes interweave with each other and become a two-dimensional N-doping carbon framework. It can enhance the electron conductivity and strengthen the stability of Ru nanoparticles fixed on it. Meanwhile, the Ru nanoparticles distributed among the framework equably (Fig. 1b) which can significantly boost the atomic utilization thus reducing the cost of electrocatalyst. In addition, partial Ru nanoparticles are wrapped in the nanotubes framework (Fig. 1c) which can increase their endurance [19] and corrosion resistance so that it can be employed in an acidic and alkaline environments. From HAADF-STEM image (Fig. 1d) and its corresponding EDS mapping (Fig. 1e–g), it can be verified that Ru nanoparticles have been dispersed among the N-doping carbon framework. Furthermore, in high-resolution TEM image in Fig. 1h, the Ru lattice distance are 0.234 nm, 0.214 nm and 0.206 nm, 3
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Fig. 2. XPS survey spectrum of Ru3@NCF (a) and the corresponding high-resolution spectra of C 1s and Ru 3d (b), N 1s (c) and Ru 3p (d).
3.2. HER catalytic activities in acidic solution
3.3. HER catalytic activities in alkaline solution
The HER catalytic performance of Rux@NCF was firstly tested in 0.5 M H2SO4. As shown in Fig. 3a, NC and NCF show negligible electrocatalytic activity while Rux@NCF exhibits increasing HER activity as the content of Ru increases. And the Rux@NCF displays more outstanding HER activity than NCF, which suggests the Ru atom may be the active sites of HER. The overpotential at 10 mA cm−2 of Rux@NCF (x = 1, 2, 3) and Pt/C is shown in Fig. 3b, Ru3@NCF exhibits the lowest overpotential of 298 mV compared to 42 mV of 20 wt% Pt/C. To get insight into the kinetics and mechanism for HER, the Tafel plots derived from the corresponding polarization curves are presented in Fig. 3c. The Ru3@NCF displays a Tafel slope of −137. 4 mV dec−1 compared to Pt/ C catalyst (−50.6 mV dec−1). The Tafel slope is the intrinsic properties of the electrocatalytic HER activity, which is related to the rate-determining step of the HER. The lower Tafel slope shows better HER kinetics [23]. In the aqueous electrolyte, H+ obtain electrons to form H atoms and adsorbed on the surface of the working electrode (Volmer reaction, H3O+ + e− → Hads) [11], followed by either an electrochemical desorption step (Heyrovsky reaction, Hads + H3O+ + e− → H2) or a chemical desorption step (Hads + Hads → H2) to form hydrogen molecules [4]. Theoretical Tafel slope of Volmer, Heyrovsky and Tafel reaction are −120, −40 and −30 mV dec−1, respectively. The Tafel slope of the Ru3@NCF was −137.4 mV dec−1, which indicates the Volmer step is the rate determining step [24]. Fig. 3d is the electrochemical impedance spectra (EIS) of Rux@NCF, NCF and Pt/C at −0.3 V (vs. RHE), which indicates that the transfer resistances decrease as the content of Ru increases. The Ru3@NCF has the minimum charge transfer resistance closed to the commercial Pt/C, which promotes the kinetics of the HER.
The HER catalytic performance of Ru was also tested in 1 M KOH. As shown in Fig. 4a, NC and NCF all display negligible electrocatalytic activity. Rux@NCF exhibits better HER activity than it in acidic solution. As shown in Fig. 4b and c, Ru3@NCF exhibits the smaller Tafel slope and the overpotential (72. 5 mV dec−1, 181 mV) compared with Pt/C catalyst (36. 1 mV dec−1, 44 mV), which suggests that Ru3@NCF has a low HER overpotential closed to Pt/C and the HER kinetics of Ru3@NCF is so favorable that the current density of Ru3@NCF will increase faster than Ru1@NCF and Ru2@NCF as the potential goes more negative, corresponding to the result of LSV curves in Fig. 4a. Besides, EIS spectra is also shown in Fig. 4d, illustrating that Rux@NCF has smaller intrinsic resistance and charge transfer resistance than NCF, which facilitates the kinetics towards HER, too. 4. Durability and activities of electrocatalyst In order to investigate the durability and activity of Rux@NCF, the current–time measurement and CVs were carried out in 1 M KOH solution. In order to compare the durability of two catalysts, the potentiostatic tests were conducted in N2-saturated 1 M KOH solution at −0.4 V (vs. RHE) for Ru3@NCF and −0.1 V (vs. RHE) for Pt/C for 30,000 s to maintain a similar current density. As shown in Fig. 5a the LSV curves verify that Ru3@NCF has a smaller current density loss after electrolysis. And in Fig. 5b, Ru3@NCF can retain 94. 1% current density compared to initial current density after continuous electrolysis for 30,000 s while the Pt/C can only retain 84. 2%, which means Rux@NCF is more durable than Pt/C. The ECSA is estimated by CVs (Fig. 5c and d) with the equation ECSA = Cdl/Cs (Cdl is double layer capacitance, Cs is capacitive 4
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Fig. 3. iR-corrected LSV curves of Rux@NCF, NC, NCF and 20 wt% Pt/C (a); Overpotentials of Rux@NCF and Pt/C at 10 mA cm−2 (b); Tafel plots of Ru3@NCF and Pt/C (c) and EIS spectra for HER at −0.3 V (vs RHE) (d). The electrolyte solution: 0.5 M H2SO4.
Fig. 4. iR-corrected LSV curves of Rux@NCF, NC, NCF and 20 wt%Pt/C (a); Overpotentials of Rux@NCF and Pt/C at 10 mA cm−2 (b); Tafel plots of Ru3@NCF and Pt/ C (c) and EIS spectra for HER at −0.3 V (vs RHE) (d). The electrolyte solution: 1 M KOH.
behavior). The larger ECSA means more active sites [25]. Since the Cs of Rux@NCF is unknown, we can estimate the ECSA by comparing the value of Cdl. Cdl can be calculated by the slope of the ΔJ-scan rate line
[5]. The CV was tested from −0.80 to −0.70 V (vs. Hg/HgO) with scan rate from 10 to 100 mV s−1 in 1 M KOH (Fig. 5d). And the Cdl is calculated by Fig. 5d with the ΔJ at −0.75 V (vs. Hg/HgO). The value of 5
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Fig. 5. iR-corrected LSV curves of Ru3@NCF and Pt/C before and after electrolysis (a); I-t curves of Ru3@NCF at −0.4 V (vs. RHE) and Pt/C at −0.1 V(vs RHE) for 30,000 s (b); Ru3@NCF tested from −0.80 to −0.70 V (vs. Hg/HgO) with scan rate from 10 to 100 mV s−1 (c) and its Cdls calculated with the ΔJ at −0.75 V (vs. Hg/ HgO) (d). The electrolyte solution: 1 M KOH.
Cdl(μF cm−2) is half of the slope [26], which is 16.5 μF cm−2 for Ru3@ NCF. It can be assumed that the catalytic effect can be improved with the increasing quantity of catalytically active sites. In some reports [21,27], the density functional theory simulation calculation shows that the introduction of noble metal (Ru/Pt/Ir) into the electrocatalyst can reduce the value of Gibbs free energy (ΔGH*), and thus facilitates HER. This conclusion is consistent with the result of this work.
Declaration of Competing Interest 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. Acknowledgements This work was supported by National Key R&D Program of China (No. 2016YFC1102802).
5. Conclusion
Data availability statement
In summary, we synthesized a new Ru embedded N-doped carbon framework to electrocatalyse HER. This low-noble metal catalyst can reach superior electrocatalytic performance as well as outstanding durability. And the loading of Ru is only 0.39 wt% which represents extraordinary atomic utilization. More importantly, we find a cheap and feasible way to synthesize the low-noble metal embedded N-doped carbon framework by one-pot method and direct carbonization, which provides a promising way to decrease the cost of noble metal catalyst for HER. It is a innovate approach to reduce the price of HER catalyst and enhance the catalytic efficiency and increase its durability at the meantime.
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CRediT authorship contribution statement Ziqi Zhang: Conceptualization, Methodology, Writing - original draft. Tiejun Wang: Validation. Kaida Yao: Formal analysis. Linchuan Cong: Resources. Zhuochen Yu: Software. Lina Qu: Data curation. Miaomiao Qian: Investigation. Weimin Huang: Writing - review & editing, Supervision, Project administration. 6
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electronic structures of nonprecious ternary alloys encapsulated in graphene layers for optimizing overall water splitting activity, ACS Catal. 7 (1) (2017) 469–479, https://doi.org/10.1021/acscatal.6b02573. M. Yurderi, A. Bulut, M. Zahmakiran, M. Gülcan, S. Özkar, Ruthenium(0) nanoparticles stabilized by metal-organic framework (ZIF-8): Highly efficient catalyst for the dehydrogenation of dimethylamine-borane and transfer hydrogenation of unsaturated hydrocarbons using dimethylamine-borane as hydrogen source, Appl. Catal. B 160–161 (2014) 534–541, https://doi.org/10.1016/j.apcatb.2014.06.009. P. Lu, Y. Yang, J. Yao, M. Wang, S. Dipazir, M. Yuan, J. Zhang, X. Wang, Z. Xie, G. Zhang, Facile synthesis of single-nickel-atomic dispersed N-doped carbon framework for efficient electrochemical CO2 reduction, Appl. Catal. B 241 (2019) 113–119, https://doi.org/10.1016/j.apcatb.2018.09.025. X. Zhao, P. Pachfule, S. Li, J.R.J. Simke, J. Schmidt, A. Thomas, Bifunctional electrocatalysts for overall water splitting from an iron/nickel-based bimetallic metal-organic framework/dicyandiamide composite, Angew. Chem. Int. Ed. 57 (29) (2018) 8921–8926, https://doi.org/10.1002/anie.201803136. M. Dabiri, R. Nikbakht, S.K. Movahed, Structuring Ru nanoparticles on magnetic nitrogen doped carbon induces excellent photocatalytic activity for oxidation of alcohols under visible light, J. Photochem. Photobiol., A 379 (2019) 159–170, https://doi.org/10.1016/j.jphotochem.2019.04.044. J. Su, Y. Yang, G. Xia, J. Chen, P. Jiang, Q. Chen, Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped graphene as active electrocatalysts for producing hydrogen in alkaline media, Nat. Commun. 8 (1) (2017) 14969, https://doi.org/10. 1038/ncomms14969. D. Li, Z. Zong, Z. Tang, Z. Liu, S. Chen, Y. Tian, X. Wang, Total water splitting catalyzed by Co@Ir core-shell nanoparticles encapsulated in nitrogen-doped porous carbon derived from metal-organic frameworks, ACS Sustainable Chem. Eng. 6 (4) (2018) 5105–5114, https://doi.org/10.1021/acssuschemeng.7b04777. J. Wang, F. Xu, H. Jin, Y. Chen, Y. Wang, Non-noble metal-based carbon composites in hydrogen evolution reaction: fundamentals to applications, Adv. Mater. 29 (14) (2017) 1605838, https://doi.org/10.1002/adma.201605838. F. Li, X. Zhao, J. Mahmood, M.S. Okyay, S.-M. Jung, I. Ahmad, S.-J. Kim, G.-F. Han, N. Park, J.-B. Baek, Macroporous inverse opal-like MoxC with Incorporated Mo vacancies for significantly enhanced hydrogen evolution, ACS Nano 11 (7) (2017) 7527–7533, https://doi.org/10.1021/acsnano.7b04205. Q. Zhang, H. Zhong, F. Meng, D. Bao, X. Zhang, X. Wei, Three-dimensional interconnected Ni(Fe)OxHy nanosheets on stainless steel mesh as a robust integrated oxygen evolution electrode, Nano Res. 11 (3) (2018) 1294–1300, https://doi.org/ 10.1007/s12274-017-1743-8. M.A. Lukowski, A.S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin, Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets, JACS 135 (28) (2013) 10274–10277, https://doi.org/10.1021/ja404523s. P. Jiang, J. Chen, C. Wang, K. Yang, S. Gong, S. Liu, Z. Lin, M. Li, G. Xia, Y. Yang, J. Su, Q. Chen, Tuning the activity of carbon for electrocatalytic hydrogen evolution via an iridium-cobalt alloy core encapsulated in nitrogen-doped carbon cages, Adv. Mater. 30 (9) (2018) 1705324, https://doi.org/10.1002/adma.201705324.
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
One-pot synthesis of ruthenium nanoparticles embedded nitrogen-doped carbon framework for electrocatalytic hydrogen evolution reaction
T
Ziqi Zhanga,1, Tiejun Wangb,1, Kaida Yaoa, Linchuan Conga, Zhuochen Yua, Lina Qua, ⁎ Miaomiao Qiana, Weimin Huanga,c, a
College of Chemistry, Jilin University, Changchun 130012, China Division of Orthopaedic Traumatology, The First Hospital of Jilin University, Changchun, China c Key Laboratory of Physics and Technology for Advanced Batteries of Ministry of Education, Jilin University, Changchun 130012, China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: ZIF-8 Low-noble catalyst One-pot Hydrogen evolution reaction Nitrogen-doped carbon framework
Hydrogen is one of the most desirable alternatives to fossil fuel due to its cleanliness, renewability and large energy density. Efficient and durable electrocatalyst toward hydrogen evolution reaction (HER) is an urgent need for the sustainable development and long-term environmental conservation. The noble metal(Pt/Ru/Ir) has a favorable performance for HER but the high price and the low reserve hinder its further application. Meanwhile, the non-noble metal electrocatalysts can hardly achieve a satisfactory stability and efficiency. Therefore, this paper reports a Ru embedded N-doped carbon framework material as low-noble metal catalyst which has low loading of Ru (0.39 wt%) and exhibits the desirable overpotential of 181 mV at 10 mA cm−2 in alkaline and 298 mV at 10 mA cm−2 in acidic solution. Besides, it retains 94. 1% current density after durability potentiostatic tests for 30,000 s while the commercial 20 wt% Pt/C can only retain 84. 2%, which suggests a superior stability.
1. Introduction Hydrogen is a clean and renewable energy carrier having a large
energy density [1]. With the deterioration of environmental issues and depletion of traditional fossil fuels, electrocatalytic hydrogen evolution reaction (HER) is drawing more and more attention since it can produce
Corresponding author at: College of Chemistry, Jilin University, Changchun 130012, China. E-mail address: [email protected] (W. Huang). 1 Ziqi Zhang and Tiejun Wang made the equal contribution to the article. ⁎
https://doi.org/10.1016/j.inoche.2020.107914 Received 3 March 2020; Received in revised form 25 March 2020; Accepted 28 March 2020 Available online 02 April 2020 1387-7003/ © 2020 Elsevier B.V. All rights reserved.
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H2 powered by renewable energy, e.g., solar or wind [2,3]. To drive HER with low power consumption and at low potential in practical applications, efficient, durable and affordable electrocatalysts are highly desirable [4]. Pt-based or Pd-based catalysts have been regarded as the most efficient electrocatalysts for HER. However, the high cost and scarcity of them hinder their commercialization [5]. Currently, to decrease the cost, various noble-metal-free electrocatalysts such as transition metal sulfides [6], phosphides [7], carbides [8], nitrides [9], and oxides [10] materials have been used for HER. Unfortunately, these catalysts based on transition metal performed dissatisfactory efficiency, stability or durability [11]. Especially the noble metal has an extremely low overpotential of HER. In order to combine the advantages of noble metal catalyst and non-noble metal catalyst, more and more attentions have been paid to study low-noble metal catalysts. Ruthenium has been a preferred material because of its cheaper price than that of platinum and its superior performance for HER [12]. It has the overpotential of 55 mV in 0.1 M KOH [13]. Therefore, ruthenium is a desirable substitute for Pt. Metal–organic frameworks (MOFs) are composed of organic ligands and tunable metal ion centers featuring large surface areas and diverse structural topologies, which offer inherent advantages and accessible active sites when being used as catalyst [14,15,16]. Zeolitic imidazolate framework (ZIF) series is a kind of MOFs that can be easily synthesized, which is suitable to serve as the base to loading metal atoms in order to enhance electrocatalytic efficiency and increase noble metal atomic utilization rate. More importantly, direct carbonization of ZIF can form nanoporous carbon materials that possess adjustable N and metal atoms contents and species, which provides large surface areas containing considerable active sites and regulate the electronic structure to from a conductive network [14]. In this work, we demonstrated a new low-noble metal electrocatalyst for HER, which was obtained by direct carbonizing Ru doped ZIF-8-based nitrogen carbon mixture (Ru@NC). The Ru@NC was synthesized by one-pot method and was calcined in high temperature and in nitrogen atmosphere to acquire Ru embedded N-doped carbon framework (Ru@NCF). The carbonized Ru@NC provides high surface area and satisfying stability thus it is durable and effective during the process of electrocatalysis. Calcining Ru@NC at high temperature can remove Zn atoms so that Ru atoms can have access to the Zn vacancy. It allows the Ru atoms embedded equably and regularly throughout the N-doped carbon framework and eventually improve the rate of atomic utilization. As a result, the catalyst can reach a low HER overpotential at 181 mV in 1 M KOH. And the loading of Ru is only 0.39 wt% detected by inductively coupled plasma (ICP).
photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250Xi with an excitation source of Al Kα radiation. Xray diffraction (XRD) patterns were carried out using a Empyrean (PANalytical B.V.) with a Cu Kα radiation source (λ1 = 1.5406 Å) operating at 40.0 kV and 40.0 mA, and the diffraction data were recorded in the 2θ range of 5–80° with a scan rate of 4 degrees per min. 2.3. Preparation of Ru@NCF In a typical procedure, 1. 2 g of Zn(OAC)2·2H2O and 2. 0 g of dicyandiamide (DCD) were dissolved in 25 ml of methanol to form a clear solution, which was subsequently injected into 25 ml of methanol containing 1. 8 g of 2-methylimidazole (MeIM). Then, 1 ml, 2 ml or 3 ml of 10−4 M RuCl3 aqueous solution was added into the mixed solution which was subsequently stirred for 24 h at room temperature. The mixed solution was centrifuged and washed with methanol several times and dried in vacuum at 60 °C for overnight to get Ru doped ZIF-8based nitrogen carbon mixture (Rux@NC, x represents the volume of the Ru solution added). ZIF-8-based nitrogen carbon mixture (NC) was synthesized the same without Ru as blank control group. The prepared Rux@NC was heated at 800 °C for 4 h at a rate of 3 °C·min−1 in a tube furnace and in N2 atmosphere to get Ru embedded N-doped carbon framework (Rux@NCF, x represents the volume of the Ru solution added in the last synthetic procedure). The NC was treated the same to get N-doped carbon framework (NCF) as blank control group, too. 2.4. Cathode preparation 5 mg of sample (NC, NCF, Rux@NCF and Pt/C(20 wt%)) was dispersed in 240 μL of isopropyl alcohol, 250 μL of ultrapure water and 20 μL of 5 wt% Nafion mixed solution and ultrasonic for 30 min. 5 μL of liquid was pipetted onto the surface of glass carbon electrode (GCE, ∅ = 3 mm) (loading ~0.7077 mg·cm−2) with natural drying and then served as working electrode. 2.5. Electrochemical measurements Electrochemical measurements were performed by a CHI 660E electrochemical workstation with a three-electrode system, including a working electrode (glassy carbon electrode, GCE, diameter 3 mm, superficial area 0.07065 cm2), a counter electrode (Pt foil, 1 × 1 cm2) and a reference electrode (Hg/HgO(1 M KOH electrolyte) electrode in 1 M KOH solution or Hg/Hg2SO4 (saturated K2SO4 electrolyte) electrode in 0.5 M H2SO4 solution)). The experimental potential values were calibrated by using the following equation: E vs. RHE = E vs. Hg/ HgO + 0.098 + 0.059 pH; E vs. RHE = E vs. Hg/ Hg2SO4 + 0.616 + 0.059 pH. Before and during the test, a N2 flow (20 ml·min−1) was continuously fed to the cathode through the electrolyte in the cell to eliminate polarization. And the linear sweep voltammetry (LSV) experiments were performed in the scan rate of 5 mV s−1 from 0.2 to −0.7 V (vs. RHE) in N2-saturated 0.5 M H2SO4 or 1 M KOH solution, respectively. Electrochemical impedance spectra (EIS) measurement was performed at overpotential of 300 mV (vs. RHE) with frequency from 0.1 to 100,000 Hz and an amplitude of 5 mV. Durability potentiostatic tests were conducted in N2-saturated 1 M KOH solution at −0.4 V (vs. RHE) for Ru3@NCF and −0.1 V (vs. RHE) for Pt/C for 30,000 s by current–time(I-t) measurement(polarization quiet time = 3000 s). The electrochemically active surface area (ECSA) was estimated by CVs, which was tested from −0.80 to −0.70 V (vs. Hg/ HgO) with scan rate from 10 to 100 mV s−1 in 1 M KOH.
2. Experimental section 2.1. Reagents Zincacetate dihydrate (Beijing Chemical Works, C4H6O4Zn·2H2O, analytical reagent), 2-methylimidazole(C4H6N2, aladdin), ruthenium chloride hydrate (35. 0–42. 0% Ru basis, aladdin), 2-methylimidazole (C4H6N2, aladdin), dicyandiamide (C2H4N4, aladdin), methanol (Beijing Chemical Works, CH4O, ≥99. 8%), Pt/C (20 wt%, JM), Nafion (5. 0 wt%, Dupont), Nafion 117 membrane (Dupont), ethyl alcohol (Beijing Chemical Works, C2H5OH, ≥99. 8%), sulfuric acid (Beijing Chemical Works, H2SO4, 98%), potassium hydroxide (Beijing Chemical Works, KOH, 96%) and N2 gas (99. 99%) were employed. All chemical reagents were used as received without further purification. All aqueous solutions were prepared with ultrapure water (resistivity of 18. 25 MΩ cm). 2.2. Physical characterization Transmission electron microscopy (TEM) and elemental mapping were performed on an FEI Tecnai G20/JEM2010 microscope. X-ray 2
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Fig. 1. TEM images (a–c), HAADF-STEM image (d), corresponding mapping images (e–g), high-resolution TEM image (h) and XRD pattern (i) of Ru3@NCF.
3. Result and discussion
which are corresponding to the Ru (1 0 0), Ru (0 0 2) and Ru (1 0 1) planes in XRD pattern, respectively, (Fig. 1i) according to the Bragg’s Law(2dsinθ = nλ). Besides, the Bragg reflections at 37. 39°, 42. 15°, 44. 01°, 58. 32°, 69. 40°, 78. 39°, 82. 22°, 84. 70° and 85. 96° correspond to the indexed planes of hexagonal closest packed (HCP) crystals of Ru (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1), respectively (JCPDS No: 6-0663) [20]. In order to investigate the information of compound surface including chemical composition and element bonding configurations, Xray photoelectron spectroscopy was carried out for Ru3@NCF. As shown in Fig. 1a, it confirms the existence of C, N, O, Ru and Zn. In the C 1s and Ru 3d spectra (Fig. 2b), the peaks were observed at 280.2, 284. 7, 285. 6 and 286. 7 eV, which can be ascribed to the Ru 3d, CeC, CeN, and CeC]O, respectively [21]. Fig. 2c shows the high-resolution spectrum of N 1s, which presents 3 types of N coordination environments at 398. 7 eV for pyridinic-N, 401. 0 eV for graphitic-N, 404. 0 eV for oxidized-N [22]. Meanwhile, the N content is determined to be 9. 98 at.%. N doping in the porous carbon polyhedrons is beneficial to stabilizing metallic Ru, as well as regulating the electronic structure and surface permeability of Ru@NCF catalyst [4]. The Ru 3p peaks (460.9 eV and 482. 7 eV) and Zn auger peak (473. 5 eV) are shown in Fig. 2d. The presences of O 1s and Zn 3p signal at about 532. 1 eV and 1023. 8 eV in the XPS survey are unavoidable, due to the surface oxidation and the incomplete gasification of Zn atoms.
3.1. Physical characterization The precursor was a mixture of Zn2+, Ru3+, MeIM and DCD in methanol solution. Zn2+ and MeIM was able to generate ZIF-8 analogue [17] while Ru3+ can cling to the ZIF-8 analogue by intensely stirring. It is hoped that Ru3+ was reduced to Ru and combined with DCD to form crosslinking nanotubes network at high temperatures [18] while the ZIF-8 analogue can be directly carbonized and serve as the fundamental base for Ru nanoparticles to be fixed. As shown in Fig. 1a, the nanotubes grow at the edge of the material while the internal nanotubes interweave with each other and become a two-dimensional N-doping carbon framework. It can enhance the electron conductivity and strengthen the stability of Ru nanoparticles fixed on it. Meanwhile, the Ru nanoparticles distributed among the framework equably (Fig. 1b) which can significantly boost the atomic utilization thus reducing the cost of electrocatalyst. In addition, partial Ru nanoparticles are wrapped in the nanotubes framework (Fig. 1c) which can increase their endurance [19] and corrosion resistance so that it can be employed in an acidic and alkaline environments. From HAADF-STEM image (Fig. 1d) and its corresponding EDS mapping (Fig. 1e–g), it can be verified that Ru nanoparticles have been dispersed among the N-doping carbon framework. Furthermore, in high-resolution TEM image in Fig. 1h, the Ru lattice distance are 0.234 nm, 0.214 nm and 0.206 nm, 3
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Fig. 2. XPS survey spectrum of Ru3@NCF (a) and the corresponding high-resolution spectra of C 1s and Ru 3d (b), N 1s (c) and Ru 3p (d).
3.2. HER catalytic activities in acidic solution
3.3. HER catalytic activities in alkaline solution
The HER catalytic performance of Rux@NCF was firstly tested in 0.5 M H2SO4. As shown in Fig. 3a, NC and NCF show negligible electrocatalytic activity while Rux@NCF exhibits increasing HER activity as the content of Ru increases. And the Rux@NCF displays more outstanding HER activity than NCF, which suggests the Ru atom may be the active sites of HER. The overpotential at 10 mA cm−2 of Rux@NCF (x = 1, 2, 3) and Pt/C is shown in Fig. 3b, Ru3@NCF exhibits the lowest overpotential of 298 mV compared to 42 mV of 20 wt% Pt/C. To get insight into the kinetics and mechanism for HER, the Tafel plots derived from the corresponding polarization curves are presented in Fig. 3c. The Ru3@NCF displays a Tafel slope of −137. 4 mV dec−1 compared to Pt/ C catalyst (−50.6 mV dec−1). The Tafel slope is the intrinsic properties of the electrocatalytic HER activity, which is related to the rate-determining step of the HER. The lower Tafel slope shows better HER kinetics [23]. In the aqueous electrolyte, H+ obtain electrons to form H atoms and adsorbed on the surface of the working electrode (Volmer reaction, H3O+ + e− → Hads) [11], followed by either an electrochemical desorption step (Heyrovsky reaction, Hads + H3O+ + e− → H2) or a chemical desorption step (Hads + Hads → H2) to form hydrogen molecules [4]. Theoretical Tafel slope of Volmer, Heyrovsky and Tafel reaction are −120, −40 and −30 mV dec−1, respectively. The Tafel slope of the Ru3@NCF was −137.4 mV dec−1, which indicates the Volmer step is the rate determining step [24]. Fig. 3d is the electrochemical impedance spectra (EIS) of Rux@NCF, NCF and Pt/C at −0.3 V (vs. RHE), which indicates that the transfer resistances decrease as the content of Ru increases. The Ru3@NCF has the minimum charge transfer resistance closed to the commercial Pt/C, which promotes the kinetics of the HER.
The HER catalytic performance of Ru was also tested in 1 M KOH. As shown in Fig. 4a, NC and NCF all display negligible electrocatalytic activity. Rux@NCF exhibits better HER activity than it in acidic solution. As shown in Fig. 4b and c, Ru3@NCF exhibits the smaller Tafel slope and the overpotential (72. 5 mV dec−1, 181 mV) compared with Pt/C catalyst (36. 1 mV dec−1, 44 mV), which suggests that Ru3@NCF has a low HER overpotential closed to Pt/C and the HER kinetics of Ru3@NCF is so favorable that the current density of Ru3@NCF will increase faster than Ru1@NCF and Ru2@NCF as the potential goes more negative, corresponding to the result of LSV curves in Fig. 4a. Besides, EIS spectra is also shown in Fig. 4d, illustrating that Rux@NCF has smaller intrinsic resistance and charge transfer resistance than NCF, which facilitates the kinetics towards HER, too. 4. Durability and activities of electrocatalyst In order to investigate the durability and activity of Rux@NCF, the current–time measurement and CVs were carried out in 1 M KOH solution. In order to compare the durability of two catalysts, the potentiostatic tests were conducted in N2-saturated 1 M KOH solution at −0.4 V (vs. RHE) for Ru3@NCF and −0.1 V (vs. RHE) for Pt/C for 30,000 s to maintain a similar current density. As shown in Fig. 5a the LSV curves verify that Ru3@NCF has a smaller current density loss after electrolysis. And in Fig. 5b, Ru3@NCF can retain 94. 1% current density compared to initial current density after continuous electrolysis for 30,000 s while the Pt/C can only retain 84. 2%, which means Rux@NCF is more durable than Pt/C. The ECSA is estimated by CVs (Fig. 5c and d) with the equation ECSA = Cdl/Cs (Cdl is double layer capacitance, Cs is capacitive 4
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Fig. 3. iR-corrected LSV curves of Rux@NCF, NC, NCF and 20 wt% Pt/C (a); Overpotentials of Rux@NCF and Pt/C at 10 mA cm−2 (b); Tafel plots of Ru3@NCF and Pt/C (c) and EIS spectra for HER at −0.3 V (vs RHE) (d). The electrolyte solution: 0.5 M H2SO4.
Fig. 4. iR-corrected LSV curves of Rux@NCF, NC, NCF and 20 wt%Pt/C (a); Overpotentials of Rux@NCF and Pt/C at 10 mA cm−2 (b); Tafel plots of Ru3@NCF and Pt/ C (c) and EIS spectra for HER at −0.3 V (vs RHE) (d). The electrolyte solution: 1 M KOH.
behavior). The larger ECSA means more active sites [25]. Since the Cs of Rux@NCF is unknown, we can estimate the ECSA by comparing the value of Cdl. Cdl can be calculated by the slope of the ΔJ-scan rate line
[5]. The CV was tested from −0.80 to −0.70 V (vs. Hg/HgO) with scan rate from 10 to 100 mV s−1 in 1 M KOH (Fig. 5d). And the Cdl is calculated by Fig. 5d with the ΔJ at −0.75 V (vs. Hg/HgO). The value of 5
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Fig. 5. iR-corrected LSV curves of Ru3@NCF and Pt/C before and after electrolysis (a); I-t curves of Ru3@NCF at −0.4 V (vs. RHE) and Pt/C at −0.1 V(vs RHE) for 30,000 s (b); Ru3@NCF tested from −0.80 to −0.70 V (vs. Hg/HgO) with scan rate from 10 to 100 mV s−1 (c) and its Cdls calculated with the ΔJ at −0.75 V (vs. Hg/ HgO) (d). The electrolyte solution: 1 M KOH.
Cdl(μF cm−2) is half of the slope [26], which is 16.5 μF cm−2 for Ru3@ NCF. It can be assumed that the catalytic effect can be improved with the increasing quantity of catalytically active sites. In some reports [21,27], the density functional theory simulation calculation shows that the introduction of noble metal (Ru/Pt/Ir) into the electrocatalyst can reduce the value of Gibbs free energy (ΔGH*), and thus facilitates HER. This conclusion is consistent with the result of this work.
Declaration of Competing Interest 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. Acknowledgements This work was supported by National Key R&D Program of China (No. 2016YFC1102802).
5. Conclusion
Data availability statement
In summary, we synthesized a new Ru embedded N-doped carbon framework to electrocatalyse HER. This low-noble metal catalyst can reach superior electrocatalytic performance as well as outstanding durability. And the loading of Ru is only 0.39 wt% which represents extraordinary atomic utilization. More importantly, we find a cheap and feasible way to synthesize the low-noble metal embedded N-doped carbon framework by one-pot method and direct carbonization, which provides a promising way to decrease the cost of noble metal catalyst for HER. It is a innovate approach to reduce the price of HER catalyst and enhance the catalytic efficiency and increase its durability at the meantime.
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CRediT authorship contribution statement Ziqi Zhang: Conceptualization, Methodology, Writing - original draft. Tiejun Wang: Validation. Kaida Yao: Formal analysis. Linchuan Cong: Resources. Zhuochen Yu: Software. Lina Qu: Data curation. Miaomiao Qian: Investigation. Weimin Huang: Writing - review & editing, Supervision, Project administration. 6
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