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N-doped graphene hybrid as an efficient catalyst for oxygen reduction reaction
Inorganic Chemistry Communications 106 (2019) 128–134
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Short communication
A thin slice-like Co3O4/N-doped graphene hybrid as an efficient catalyst for oxygen reduction reaction Jun Wu, Jinxing Wang, Yang Lv, Xianbao Wang
T
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Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China
GRAPHICAL ABSTRACT
The thin slice-like Co3O4-N-Gr with high contents of N atoms shows the outstanding electrocatalytic activity, enhanced stability and methanol tolerance for ORR.
ARTICLE INFO
ABSTRACT
Keywords: N-doped graphene Oxygen reduction reaction Slice-like CO3O4 Electrocatalysts
A thin slice-like Co3O4/N-doped graphene (Co3O4-N-Gr) was prepared through a hydrothermal reaction. The asprepared Co3O4-N-Gr displayed an onset potential and half-wave potential of 0.98 and 0.86 V, which were 40 and 80 mV larger than that of Pt/C (0.94 and 0.78 V), while its Tafel slope (31.4 mV dec−1) was much lower than Pt/C (44.7 mV dec−1). Moreover, the current density retained 96.25% after working for 6000 s and had no change after adding methanol. These suggest that the Co3O4-N-Gr is promising as an efficient electrocatalyst for oxygen reduction reaction (ORR) in alkaline medium when compared with Pt/C and other reported non-precious catalysts.
1. Introduction With the rapid depletion of traditional fossil energy, the searches for sustainable green and environment-friendly new energy have become the strategic policy of the world [1–3]. As a new green energy source, fuel cells have been paid more and more attentions. After a long period of development, many applications on fuel cells have been reported, such as in the field of drones [4,5]. At present, there are many basic
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factors affecting fuel cell catalysts, such as catalytic activity, electrochemical active area, anti-toxicity，mass specific activity, stability, etc. [6]. The commercial ORR catalysts usually contain expensive noble metal of Pt, which are not only poor in durability but also easily cause catalyst poisoning after adsorbing CO or methanol [7]. To solve this problem, non-noble metal catalysts have been widely investigated [8]. A large number of studies proved that the introduction of heteroatoms (B, P, N, S, etc.) in carbon materials can significantly enhance the
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.inoche.2019.06.005 Received 7 April 2019; Received in revised form 25 May 2019; Accepted 3 June 2019 Available online 04 June 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Preparation route to Co3O4-N-Gr.
catalytic activity of ORR [9–12]. For example, N-doped carbon nanotubes, N-doped graphene, N-doped porous carbon structures, and two or three elements co-doped carbon materials show excellent catalytic performance in ORR [13–16]. However, although these materials present good stabilities on ORR catalysis, their catalytic behavior is still poorer than Pt/C-based catalysts. Recently, some researchers introduced transition metals (Co, Fe, etc.) in N-doped carbon materials to obtain metal-nitrogen-carbon (M-N-C) composite catalysts [17–20]. These catalysts are more prominent towards ORR, and it was possibly due to the synergistic effect of metal ions and pyridinic-N or graphitic-N [21]. Among them, Co-N-C systems are more conducive to mass transfer due to their abundant porosities and more exposed CoeN active centers, thus possessing better catalytic activities than commercial Pt/C systems [22]. For example, Ma et al. synthesized porous Co3C/Co-N-C/ G modified graphitic carbon [23]. Wang et al. prepared Co nanoparticles embedded in N-doped carbon (Co/N-OMCNS-800) [24]. However, the onset and half-wave potentials of their ORR performances are still not as good as those of Pt/C, which may be attributed to their uneven pore size or morphology, making the mass transfer blocked and the density of active centers decreased. Therefore, the controlling of the homogeneous morphology and pore is still challenging for developing highly efficient Co-N-C catalysts. Herein, we successfully prepared Co3O4 nanoparticles-embedded N-doped graphene (Co3O4-N-Gr), which was made of self-assembled micro-sheets through the coordination of Co2+/Co3+ and the synergistic effect of Co and N-doped graphene. The resulting hybrid showed efficient catalytic activity and high stability for ORR in alkaline solution. Its onset potential and half-wave potential even surpass than that of commercial Pt/C catalyst, which may be attributed to the reduction of the agglomeration and dissolution of nanoparticles between the geometric constraints of nanoparticles and their adjacent carbon atoms and massive exposure of active centers [25], resulting in high mass transfer efficiency, as well as the synergistic interaction of Co3O4 nanoparticles and N-Gr. Therefore, this work provides a new idea for NNM based graphene catalysts with homogeneous morphology distribution.
bought from Alfa Aesar. 5% Nafion was achieved from Dupont. Ethanol, xylosic alcohol, melamine, hexadecyl trimethyl ammonium Bromide (CTAB), N-Methylpyrrolidone (NMP), CoCl2·6H2O, H2SO4 and KOH were acquired from Sinopharm Chemical Reagent Co. Ltd. All chemicals reagents were used directly without further purification and the ultrapure water was synthesized freshly by a Kertone Ultrapure Water System P60-CY (Kertone Water Treatment Co. Ltd., resistivity = 18.25 MΩ cm). 2.2. Co nanoparticles embedded in N-doped porous carbon (Co-N-PC) 2 g xylosic alcohol was first ultrasonically dispersed in 30 mL deionized water for about 30 min. Then, 1 g CTAB was added slowly and the mixture was stirred for 30 min to form dispersion. As follow, 1 g melamine was added in the dispersion and stirred for another 30 min. Subsequently, 2 g CoCl2·6H2O was added and ultrasonically dispersed for the last 30 min. The as-prepared uniform dispersion was shifted to a Teflon-lined reactor and held at 180 °C for 16 h to achieve Co-based precursor and the obtained pink powders were washed by deionized water for several times and dried at 70 °C overnight. Soon afterwards, the products were transferred to quartz tube and annealed at 800 °C for 2 h with a heating rate of 5 °C min−1 before cooled down to room temperature under Ar gas. Finally, we noted the black powders as Co-NPC. For contrast, Co-N-C was synthesized by the same procedure, in addition to the use of CTAB. 2.3. Co/CoO nanoparticles embedded in N-doped porous carbon based on graphene (Co/CoO-N-PC/Gr) To obtain Co/CoO-N-PC/Gr, we just added 30 mg GO in 30 mL deionized water for ultrasonically dispersion of about 2 h. The followed steps were as same as the preparation of Co-N-PC. We noted the black powders used CTAB or not as Co/CoO-N-PC/Gr and Co/CoO-N-C/Gr. 2.4. Co3O4 nanoparticles embedded in N-doped graphene (Co3O4-N-C/Gr)
2. Experimental
Co/CoO-N-PC/Gr was added in 100 mL 0.5 M H2SO4. Then, the products were stirred at 85 °C for 6 h. After activation, the sample was washed by deionized water some times and collected after dry at 70 °C overnight. We noted the black fluffy powders as Co3O4-N-Gr (Scheme 1).
2.1. Chemicals GO powders used for preparing Co3O4-N-C/Gr were purchased from Hunan Chemical Ltd. Hunan, China. Commercial 20% Pt/C catalyst was 129
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2.5. Electrochemical measurements
to the diffractions from (111), (200) and (222) planes of orthorhombic CoO (JCPDS No. 43-1004) [25], respectively. And there are two faint protuberances around 12.9° and 26.4°, corresponding to the GO (001) and rGO (002), respectively, suggesting that GO has a slight reduction accompanied by a weak oxidation of Co. After activation treatment, a large number of Co/CoO were removed, and a small amount Co/CoO were further oxidized into Co3O4 by GO. The major diffraction peaks of Co3O4-N-Gr at 19.0°, 31.3°, 36.8°, 38.5° 44.8°, 59.3° and 65.2° can be assigned to the diffractions from (111), (220), (311), (400), (511) and (440) planes of orthorhombic Co3O4 (JCPDS No. 43-1003) [27], respectively, which is consistent with the TEM results. Meanwhile, the peak at 26.4° becomes more obvious, proving that the GO is further reduced into rGO. Raman spectroscopy is an important approach to characterize the degree of defects in materials. In Fig. 1F, the ID/IG of Co-N-PC is a little higher than that of Co-N-C, suggesting that the addition of CTAB makes the structure produce a little more pores and defects, which are consistent with Fig. S1. Furthermore, the introduction of GO further enhances the defect degree of Co-N-PC. When the rGO is formed, Co3O4-N-Gr shows the highest defect degree [28,29], which is thus favorable for ORR employed to investigate the chemical state and the content of each element in Co3O4-N-Gr. The surveyed spectrum of Co3O4-N-Gr in Fig. 2A shows that the atomic ratio of C:O:N:Co is 87.62:7.85:4:0.52 with the highest N content in the samples (Fig. S3 and Table S1), indicating that the thin slice is easy to dope with N atoms, which is beneficial for ORR. The highresolution N 1s spectrum of Co3O4-N-Gr (Fig. 2B) exhibits three peaks at 398.4 eV (pyridinic N), 399.7 eV (pyrrolic N) and 401.3 eV (graphitic N), respectively [20,24,29]. The high contents of pyridinic N (37.31 at. %) and graphitic N (32.47 at.%) had been proved as active sites for ORR in early reports, which are higher than the other samples (Fig. S3 and Table S2). In Fig. 2C, the O 1s spectrum displays three peaks at 530.9, 532.3 and 533.5 eV, assigned to cobalt oxides, C]O (O-C=O) and CeOeC (C-O-OH), respectively, [30,31] which are different from the others. It may cause the complexation of Co3+ and graphene (Fig. S3). The high resolution of Co 2p spectrum of Co3O4-N-Gr shows six main peaks (Fig. 2D). The peaks at 780.1 (Co3+) and 783.4 eV (Co2+) are assigned to Co 2p3/2, while the peaks at 796.1 (Co3+) and 799.8 eV (Co2+) are assigned to Co 2p1/2, and the peaks at around 787.7 and 803.6 eV are corresponding to weak shakeup satellites, indicative of the formation of Co3O4 [28,32,33], which are really different from those contained Co/CoO (Fig. S3). The high-resolution of Co 2p spectrum of Co3O4-N-Gr is consistent with the lattice fringes of high magnification TEM images and XRD patterns. The pyridinic N can coordinate with the Co element, leading to the formation of the CoeN moieties. Before the activation of the samples, we firstly studied the effects of annealing temperature (from 700 to 900 °C) on the catalytic activity, and obtained the best result at 800 °C (Fig. S4). To further investigate the catalytic performances of Co-N-C catalysts, the electrocatalytic activities of the Co3O4-N-C/Gr was first evaluated by Cyclic Voltammetry (CV) in a 0.1 M KOH electrolyte in O2 or N2(Fig. 3A). A well-defined ORR peak occurs at 0.79 V in O2-saturated electrolyte, while, not obvious peak can be observed under N2. It confirms the remarkable catalytic activity of Co3O4-N-Gr for ORR. The linear sweep voltammetry (LSV) curves in Fig. 3B display that Co3O4-NGr has an onset potential of 0.98 V, which is obviously more positive than 0.94 V of Pt/C. electrolyte in O2 or N2 (Fig. 3A). A well-defined ORR peak occurs at 0.79 V in O2-saturated electrolyte, while, not obvious peak can be observed under N2.It confirms the remarkable catalytic activity of Co3O4-N-Gr for ORR. The linear sweep voltammetry (LSV) curves in Fig. 3B display that Co3O4-N-Gr has an onset potential of 0.98 V, which is obviously more positive than 0.94 V of Pt/C. It suggests that the excellent conductivity and mass-transfer efficiency of graphene. Also, the synergistic effects of Co ions and N atoms make the thin slice-like Co3O4-N-Gr have outstanding electrocatalytic activity for ORR. Furthermore, the half-wave potential of Co3O4-N-Gr (0.86 V) is also 80 mV larger than Pt/C (0.78 V). Compared to the Co3O4-N-C/Gr,
The IM6 electrochemical workstation (Zahner-Electrik, Germany) assumed all the electrochemical tests with a normal three-electrode system at ambient temperature. A nude or modified glassy carbon electrode (GCE) or rotating disk electrode (RDE) as a working electrode; a Pt electrode and a saturated calomel electrode (SCE) were acted as the counter electrode and the reference electrode, respectively. Before the preparation of the catalytic electrode, the GCE (ϕ3 mm) or RDE (ϕ5 mm) were burnished by 0.3 and 0.05 mm alumina powders and flushed with ultrapure water. The electrodes were subsequently washed with absolute ethyl alcohol and ultrapure water by using an ultrasonic unit and natural air dry. To obtain the ink, 1 mg as-prepared product was dispersed in 500 μL of NMP and 480 μL deionized water by ultrasonic for 30 min to get a uniform dispersion. Subsequently, 20 μL of 5% Nafion was dropped to the aforementioned dispersion with ultrasonic concussion for more than 30 min to achieve approximate 1 mg mL−1 dispersion. In the cyclic voltammetry (CV) tests, 3 μL of the as-prepared dispersion was dropped upon the GCE exterior with a pipette gun and parched under infrared lamp. In the linear sweep voltammetry (LSV) tests, 10 μL of the above dispersion was covered upon the 0.196 cm2 disk of RDE exterior, which formed a catalyst filling quantity of 0.05 mg cm−2. For further research the ORR dynamics of the Co-N-C catalysts, the linear sweep voltammetry (LSV) was measured with the rotational speed between 400–2025 rpm in the above O2-saturated 0.1 M KOH solution. Koutecky-Levich (K-L) plots were collected at distinct potentials. The rake ratios of the linearity fit lines were used to compute the electron transfer number on the basic of the K-L equation as follow: [25].
J
1
= (B
1/2 ) 1
+ JK
1
B = 0.2nFCo (Do )2/3 (v )
1/6
where J was the tested current density, JK represented the dynamiclimiting current density, stands for the electrode rotating speed, n was the electron transfer number, F represented the Faraday constant (F = 96,485C mol−1), Co was the volume concentration of O2 (Co = 1.2 × 10−6 mol cm−3), Do represented the diffusivity of O2 in 0.1 M KOH (Do = 1.9 × 10–5 cm2 s−1), and v was the dynamic viscosity (v = 0.01 cm2 s−1). The constant 0.2 was putted to use when the rotation speed was conveyed in rpm. 3. Results and discussion The scanning electron microscopy (SEM) image shows that the Co3O4-N-Gr is composed of homogeneous thin slices, while the inset images reveal that the elements of C, N, O and Co are distributed uniformly in the catalyst (Fig. 1A). This structure is different from other samples mainly because of Co/CoO and porous carbon are dissolved in the activation treatment (Figs. S1, S2 and Table S1). In Fig. 1B, Co3O4 nanoparticles are homogeneously embedded in the N-doped graphene. Moreover, it's clear that lattice fringe spaces of 0.143, 0.156, 0.201, 0.245, 0.285 and 0.467 nm are consistent with the (440), (511), (400), (311), (220) and (111) planes of cubic Co3O4 spinel-phase, respectively (Fig. 1C and D). From Fig. 1A and B, it is found that Co3O4-N-Gr has a homogeneous morphology which is beneficial for mass transfer and active center exposure. To further investigate the crystal structure of Co-N-C catalysts, powder XRD measurement was performed (Fig. E). The major diffraction peaks of Co-N-PC and Co-N-C at 44.2°, 51.5° and 75.8° are corresponding to the diffraction from (111), (200) and (220) planes of orthorhombic metal Co (JCPDS No. 15-0806), [25] respectively, indicating that the carbon materials are amorphous. When the GO was added, the Co was somewhat oxidized and formed Co/CoO complex. Compared with the pure Co, the new weak peaks of Co/CoON-PC/Gr and Co/CoO-N-C/Gr at 36.5°, 42.4° and 61.5° can be assigned 130
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Fig. 1. (A) SEM images and (inset) the corresponding elemental mapping images of Co3O4-N-C/Gr images of carbon, nitrogen, oxygen and cobalt. (B) TEM and (inset) local HRTEM images of Co3O4-N-C/Gr. (C) and (D) High magnification TEM images of Co3O4-N-C/Gr. (E) XRD patterns of different samples. (F) Raman spectra of different samples.
the onset potentials (half-wave potentials) of Co-N-C, Co-N-PC, Co/ CoO-N-C/Gr and Co/CoO-N-PC/Gr are 0.78 V (0.60 V), 0.80 V (0.65 V), 0.81 V (0.66 V) and 0.83 V (0.72 V), respectively, and the corresponding CV curves also show that the onset potential of Co3O4-N-Gr is more positive (Fig. S4). These indicate that the Co3O4 nanoparticles embedded in N-doped graphene structure significantly affects its electrocatalytical behavior for ORR, which is supported by XPS characterization and Raman analysis. It is worth noting that the LSV has a peak for all Co-containing samples, which may be due to the less efficient oxygen diffusion at the beginning, resulting from the slow electrolyte flow in this slice-like structure.[34] Moreover, the Co3O4-N-Gr exhibits a large limited diffusion current density of 5.4 mA cm-2, suggestive of a large amount of exposing active sites towards ORR. As is known to all, the Tafel slope is an important parameter to measure the efficiency of
ORR. Fig. 3C shows the Tafel plots of Co-N-C samples and Pt/C, derived from the corresponding LSV data. The Tafel slope of Co3O4-N-Gr is 31.4 mV dec-1 at low overpotential, which is much lower than 44.7 mV dec1 of Pt/C. And the Tafel slopes of other catalysts are 123.5, 90.9, 64.3 and 59.3 mV dec-1, respectively, which are much more higher than that of Co3O4-N-Gr and Pt/C. Such results are superior to some recent reports, as shown in Fig. S6 and Table S3. In order to gain more information on the ORR dynamics of the Co3O4-N-Gr catalyst, LSV tests were then carried out with different spin velocities at a scanning speed of 5 mV s-1, as displayed in Fig. 3D. Apparently, with the increase of electrolyte diffusion, the diffusion current density increases as the spin velocities rise. The homologous Koutecky-Levich (K-L) plots at various potentials (0.21–0.61 V) meet a good linear relationship (inset in Fig. 3D), and the isoelectron transfer numbers (n) of Co3O4-N-Gr are 131
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Fig. 2. (A) The XPS survey spectra of the Co3O4-N-Gr catalyst. High-resolution XPS spectra of (B) N 1s, (C) O 1s and (D) Co 2p onto the carbon materials, which is considered as the efficient active centers in ORR [26].
calculated to be 3.96 (close to that of Pt/C), which is in accordance with the Koutecky-Levich equation (the calculations were listed in the experimental section in detail) [25,26]. These indicate that a four-electron transfer reaction plays a major role in the ORR process of Co3O4-NGr. Besides the catalytic activity of Co3O4-N-Gr for ORR, the stability and methanol tolerance are two other important indicators for evaluating catalysts. The constancy of the Co3O4-N-Gr and Pt/C is measured at 0.7 V for 6000 s in an O2-saturated 0.1 M KOH electrolyte. Fig. 3E shows that the Co3O4-N-Gr displayed extremely slow damping of the current in the course of the successive chronoamperometric test with a value of 96.25% even after 6000 s. In contrast, the relative current of Pt/C is decreased remarkably to 65.96%, verifying that the Co3O4-N-Gr has good steady catalytic active sites. Moreover, the chronoamperometric responses of Co3O4-N-Gr and Pt/C are recorded after adding methanol (3 M) to electrolyte (Fig. 3F). No changes can be found during the dropping process, while there is a notable decrease of current density for Pt/C. The above results demonstrate that the Co3O4-N-Gr had pretty better selectivities and methanol tolerance for ORR.
16 h); secondly, Co/CoO-N-PC/Gr is obtained by annealing (800 °C, 2 h, Ar); finally, Co3O4-N-Gr is achieved by activation (85 °C, 6 h, 0.5 M H2SO4). The catalyst is composed of homogeneous thin slices with high N contents and high proportions of pyridinic N and graphitic N. The formation of this sheet structure is directly related to the coordination of Co2+/Co3+. Impressively, the as-prepared catalyst Co3O4-N-Gr shows the catalytic activity comparable to the commercial Pt/C catalyst, but better electrochemical stability and methanol poisoning resistance, which due to the synergistic effect of Co3O4 nanoparticles and N-doped graphene. Significantly, our research can provide a new strategy for developing other transition metal and nitrogen-doped carbon catalysts. It is worth noting that our research can provide new strategies for the development of other transition metals and nitrogendoped carbon catalysts. Based on the previously mentioned standards for the preparation of fuel cell catalysts, we have found that transition metals, as well as transition metal oxides, are supported on graphene and have good catalytic properties. We believe that the replacement of precious metal catalysts by non-precious metals is a very viable option if commercialization is to be achieved. The synergistic effect of nonnoble metal oxides and nitrogen-doped graphene can improve the catalytic efficiency and greatly reduce the cost of preparing the catalyst.
4. Conclusions In summary, we have prepared Co3O4 nanoparticles-embedded Ndoped graphene (Co3O4-N-Gr) through three phase reactions: firstly, Co-N-C precursor is synthesized by hydrothermal reaction (180 °C, 132
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Fig. 3. (A) CV curves of the Co3O4-N-Gr catalyst at a scan rate of 50 mV s−1 in 0.1 M KOH solution saturated with N2 and O2. (B) LSV curves for different samples in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm at a scan rate of 5 mV s−1. (C) Tafel plots of different samples derived by the mass-transport correction of corresponding RDE data. (D) LSVs with various rotation rates of Co3O4-N-Gr. The inset indicates the corresponding K-L plots at different potentials. (E) The accelerated durability by i-t curves of Co3O4-N-Gr and 20 wt% Pt/C catalysts at 0.7 V in an O2-saturated 0.1 M KOH electrolyte. (F) Methanol crossover effect test of Co3O4-N-Gr and 20 wt% Pt/C upon the addition of 3 M methanol at around 150 s in an O2-saturated 0.1 M KOH solution at 0.7 V.
Acknowledgements
Appendix A. Supplementary material
This work is financially supported by the Ministry of Science and Technology of the People's Republic of China (Grant 2016YFA0200200) and Wuhan Science and Technology Bureau of China (2018010401011280). Finally, we appreciate the kind help from Prof. Shimin Wang and Prof. Yunbin He for their help in using the electrochemical workstation and XPS, respectively.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.06.005. References [1] X.C. Liu, G.C. Wang, R.P. Liang, L. Shi, J.D. Qiu, Environment-friendly facile
133
Inorganic Chemistry Communications 106 (2019) 128–134
J. Wu, et al.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
[16]
[17]
[18] [19]
synthesis of Pt nanoparticles supported on polydopamine modified carbon materials, J. Mater. Chem. A 1 (2013) 3945. B. Tabah, I.N. Pulidindi, V.R. Chitturi, L.M. Reddy Arava, A. Varvak, E. Foran, A. Gedanken, Solar-energy-driven conversion of biomass to bioethanol: a sustainable approach, J. Mater. Chem. A 5 (2017) 15486–15506. D. Guo, K. Cai, Y. Wang, A distinct mutual phase transition in a new PVDF based lead-free composite film with enhanced dielectric and energy storage performance and low loss, J. Mater. Chem. C 5 (2017) 2531–2541. Z.F. Pan, L. An, C.Y. Wen, Recent advances in fuel cells based propulsion systems for unmanned aerial vehicles, Appl. Energy 240 (2019) 473–485. Z.F. Pan, L. An, T.S. Zhao, Z.K. Tang, Advances and challenges in alkaline anion exchange membrane fuel cells, Prog. Energ. Combust. 66 (2018) 141–175. W. Yang, Y. Zhang, X. Liu, L. Chen, J. Jia, In situ formed Fe–N doped metal organic framework@carbon nanotubes/graphene hybrids for a rechargeable Zn–air battery, Chem. Comm. 53 (2017) 12934–12937. E. Antolini, Palladium in fuel cell catalysis, Energy Environ. Sci. 2 (2009) 915. C.R. Raj, A. Samanta, S.H. Noh, S. Mondal, T. Okajima, T. Ohsaka, Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 4 (2016) 11156–11178. H. Jiang, Y. Wang, J. Hao, Y. Liu, W. Li, J. Li, N and P co-functionalized threedimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction, Carbon 122 (2017) 64–73. S. Agnoli, M. Favaro, Doping graphene with boron: a review of synthesis methods, physicochemical characterization and emerging applications, J. Mater. Chem. A 4 (2016) 5002–5025. Y. Qiu, J. Huo, F. Jia, B.H. Shanks, W. Li, N- and S-doped mesoporous carbon as metal-free cathode catalysts for direct biorenewable alcohol fuel cells, J. Mater. Chem. A 4 (2016) 83–95. Y. Zan, Z. Zhang, H. Liu, M. Dou, F. Wang, Nitrogen and phosphorus co-doped hierarchically porous carbons derived from cattle bones as efficient metal-free electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 5 (2017) 24329–24334. D. Guo, Z. Tian, J. Wang, X. Ke, Y. Zhu, Co2N nanoparticles embedded N-doped mesoporous carbon as efficient electrocatalysts for oxygen reduction reaction, Appl. Surf. Sci. 473 (2019) 555–563. G.A. Ferrero, K. Preuss, A.B. Fuertes, M. Sevilla, M.M. Titirici, The influence of pore size distribution on the oxygen reduction reaction performance in nitrogen doped carbon microspheres, J. Mater. Chem. A 4 (2016) 2581–2589. A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, K. Tammeveski, Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal–nitrogen–carbon catalysts for alkaline membrane fuel cells, J. Mater. Chem. A 6 (2018) 776–804. R. Liu, H. Zhang, S. Liu, X. Zhang, T. Wu, X. Ge, Y. Zang, H. Zhao, G. Wang, Shrimpshell derived carbon nanodots as carbon and nitrogen sources to fabricate threedimensional N-doped porous carbon electrocatalysts for the oxygen reduction reaction, Phys. Chem. Chem. Phys. 18 (2016) 4095–4101. M. Hoque, S. Zhang, M.L. Thomas, Z. Li, S. Suzuki, A. Ando, M. Yanagi, Y. Kobayashi, K. Dokko, M. Watanabe, Simple combination of a protic salt and an iron halide: precursor for a Fe, N and S co-doped catalyst for the oxygen reduction reaction in alkaline and acidic media, J. Mater. Chem. A 6 (2018) 1138–1149. M. Li, Y. Xiong, X. Liu, C. Han, Y. Zhang, X. Bo, L. Guo, Iron and nitrogen co-doped carbon nanotube@hollow carbon fibers derived from plant biomass as efficient catalysts for the oxygen reduction reaction, J. Mater. Chem. A 3 (2015) 9658–9667. X. Qiao, J. Jin, H. Fan, Y. Li, S. Liao, In situ growth of cobalt sulfide hollow
[20] [21] [22] [23]
[24]
[25] [26]
[27] [28]
[29] [30]
[31] [32] [33] [34]
134
nanospheres embedded in nitrogen and sulfur co-doped graphene nanoholes as a highly active electrocatalyst for oxygen reduction and evolution, J. Mater. Chem. A 5 (2017) 12354–12360. F. Tang, H. Lei, S. Wang, H. Wang, Z. Jin, A novel Fe-N-C catalyst for efficient oxygen reduction reaction based on polydopamine nanotubes, Nanoscale 9 (2017) 17364–17370. M. Sun, D. Davenport, H. Liu, J. Qu, M. Elimelech, J. Li, Highly efficient and sustainable non-precious-metal Fe–N–C electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 6 (2018) 2527–2539. Q. Yu, C. Wu, J. Xu, Y. Zhao, J. Zhang, L. Guan, Nest-like assembly of the doped single-walled carbon nanotubes with unique mesopores as ultrastable catalysts for high power density Zn-air battery, Carbon 128 (2018) 46–53. X.-X. Ma, X.-Q. He, T. Asefa, Hierarchically porous Co3C/Co-N-C/G modified graphitic carbon: a trifunctional corrosion-resistant electrode for oxygen reduction, hydrogen evolution and oxygen evolution reactions, Electrochim. Acta 257 (2017) 40–48. H. Wang, W. Wang, M. Asif, Y. Yu, Z. Wang, J. Wang, H. Liu, J. Xiao, Cobalt ioncoordinated self-assembly synthesis of nitrogen-doped ordered mesoporous carbon nanosheets for efficiently catalyzing oxygen reduction, Nanoscale 9 (2017) 15534–15541. C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Synthesis of phosphorus-doped graphene and its multifunctional applications for oxygen reduction reaction and lithium ion batteries, Adv. Mater. 25 (2013) 4932–4937. T. Palaniselvam, V. Kashyap, S.N. Bhange, J.-B. Baek, S. Kurungot, Nanoporous graphene enriched with Fe/Co-N active sites as a promising oxygen reduction electrocatalyst for anion exchange membrane fuel cells, Adv. Funct. Mater. 26 (2016) 2150–2162. Y. Su, Y. Zhu, H. Jiang, J. Shen, X. Yang, W. Zou, J. Chen, C. Li, Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions, Nanoscale 6 (2014) 15080–15089. Y. Chen, J. Li, T. Mei, X.g. Hu, D. Liu, J. Wang, M. Hao, J. Li, J. Wang, X. Wang, Low-temperature and one-pot synthesis of sulfurized graphene nanosheets via in situ doping and their superior electrocatalytic activity for oxygen reduction reaction, J. Mater. Chem. A 2 (2014) 20714–20722. H.-S. Park, S.-B. Han, D.-H. Kwak, J.-H. Han, K.-W. Park, Fe nanoparticles encapsulated in doped graphitic shells as high-performance and stable catalysts for oxygen reduction reaction in an acid medium, J. Catal. 370 (2019) 130–137. Y. Yao, Z. Chen, A. Zhang, J. Zhu, X. Wei, J. Guo, W.D. Wu, X.D. Chen, Z. Wu, Surface-coating synthesis of nitrogen-doped inverse opal carbon materials with ultrathin micro/mesoporous graphene-like walls for oxygen reduction and supercapacitors, J. Mater. Chem. A 5 (2017) 25237–25248. V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis, C. Galiotis, Chemical oxidation of multiwalled carbon nanotubes, Carbon 46 (2008) 833–840. Z. Jiang, Z.-J. Jiang, T. Maiyalagan, A. Manthiram, Cobalt oxide-coated N- and Bdoped graphene hollow spheres as bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions, J. Mater. Chem. A 4 (2016) 5877–5889. R. Nie, J. Shi, W. Du, W. Ning, Z. Hou, F.-S. Xiao, A sandwich N-doped graphene/ Co3O4 hybrid: an efficient catalyst for selective oxidation of olefins and alcohols, J. Mater. Chem. A 1 (2013) 9037. B. Wang, L. Xu, G. Liu, P. Zhang, W. Zhu, J. Xia, H. Li, Biomass willow catkinderived Co3O4/N-doped hollow hierarchical porous carbon microtubes as an effective tri-functional electrocatalyst, J. Mater. Chem. A 5 (2017) 20170–20179.
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A thin slice-like Co3O4/N-doped graphene hybrid as an efficient catalyst for oxygen reduction reaction Jun Wu, Jinxing Wang, Yang Lv, Xianbao Wang
T
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Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China
GRAPHICAL ABSTRACT
The thin slice-like Co3O4-N-Gr with high contents of N atoms shows the outstanding electrocatalytic activity, enhanced stability and methanol tolerance for ORR.
ARTICLE INFO
ABSTRACT
Keywords: N-doped graphene Oxygen reduction reaction Slice-like CO3O4 Electrocatalysts
A thin slice-like Co3O4/N-doped graphene (Co3O4-N-Gr) was prepared through a hydrothermal reaction. The asprepared Co3O4-N-Gr displayed an onset potential and half-wave potential of 0.98 and 0.86 V, which were 40 and 80 mV larger than that of Pt/C (0.94 and 0.78 V), while its Tafel slope (31.4 mV dec−1) was much lower than Pt/C (44.7 mV dec−1). Moreover, the current density retained 96.25% after working for 6000 s and had no change after adding methanol. These suggest that the Co3O4-N-Gr is promising as an efficient electrocatalyst for oxygen reduction reaction (ORR) in alkaline medium when compared with Pt/C and other reported non-precious catalysts.
1. Introduction With the rapid depletion of traditional fossil energy, the searches for sustainable green and environment-friendly new energy have become the strategic policy of the world [1–3]. As a new green energy source, fuel cells have been paid more and more attentions. After a long period of development, many applications on fuel cells have been reported, such as in the field of drones [4,5]. At present, there are many basic
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factors affecting fuel cell catalysts, such as catalytic activity, electrochemical active area, anti-toxicity，mass specific activity, stability, etc. [6]. The commercial ORR catalysts usually contain expensive noble metal of Pt, which are not only poor in durability but also easily cause catalyst poisoning after adsorbing CO or methanol [7]. To solve this problem, non-noble metal catalysts have been widely investigated [8]. A large number of studies proved that the introduction of heteroatoms (B, P, N, S, etc.) in carbon materials can significantly enhance the
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.inoche.2019.06.005 Received 7 April 2019; Received in revised form 25 May 2019; Accepted 3 June 2019 Available online 04 June 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Preparation route to Co3O4-N-Gr.
catalytic activity of ORR [9–12]. For example, N-doped carbon nanotubes, N-doped graphene, N-doped porous carbon structures, and two or three elements co-doped carbon materials show excellent catalytic performance in ORR [13–16]. However, although these materials present good stabilities on ORR catalysis, their catalytic behavior is still poorer than Pt/C-based catalysts. Recently, some researchers introduced transition metals (Co, Fe, etc.) in N-doped carbon materials to obtain metal-nitrogen-carbon (M-N-C) composite catalysts [17–20]. These catalysts are more prominent towards ORR, and it was possibly due to the synergistic effect of metal ions and pyridinic-N or graphitic-N [21]. Among them, Co-N-C systems are more conducive to mass transfer due to their abundant porosities and more exposed CoeN active centers, thus possessing better catalytic activities than commercial Pt/C systems [22]. For example, Ma et al. synthesized porous Co3C/Co-N-C/ G modified graphitic carbon [23]. Wang et al. prepared Co nanoparticles embedded in N-doped carbon (Co/N-OMCNS-800) [24]. However, the onset and half-wave potentials of their ORR performances are still not as good as those of Pt/C, which may be attributed to their uneven pore size or morphology, making the mass transfer blocked and the density of active centers decreased. Therefore, the controlling of the homogeneous morphology and pore is still challenging for developing highly efficient Co-N-C catalysts. Herein, we successfully prepared Co3O4 nanoparticles-embedded N-doped graphene (Co3O4-N-Gr), which was made of self-assembled micro-sheets through the coordination of Co2+/Co3+ and the synergistic effect of Co and N-doped graphene. The resulting hybrid showed efficient catalytic activity and high stability for ORR in alkaline solution. Its onset potential and half-wave potential even surpass than that of commercial Pt/C catalyst, which may be attributed to the reduction of the agglomeration and dissolution of nanoparticles between the geometric constraints of nanoparticles and their adjacent carbon atoms and massive exposure of active centers [25], resulting in high mass transfer efficiency, as well as the synergistic interaction of Co3O4 nanoparticles and N-Gr. Therefore, this work provides a new idea for NNM based graphene catalysts with homogeneous morphology distribution.
bought from Alfa Aesar. 5% Nafion was achieved from Dupont. Ethanol, xylosic alcohol, melamine, hexadecyl trimethyl ammonium Bromide (CTAB), N-Methylpyrrolidone (NMP), CoCl2·6H2O, H2SO4 and KOH were acquired from Sinopharm Chemical Reagent Co. Ltd. All chemicals reagents were used directly without further purification and the ultrapure water was synthesized freshly by a Kertone Ultrapure Water System P60-CY (Kertone Water Treatment Co. Ltd., resistivity = 18.25 MΩ cm). 2.2. Co nanoparticles embedded in N-doped porous carbon (Co-N-PC) 2 g xylosic alcohol was first ultrasonically dispersed in 30 mL deionized water for about 30 min. Then, 1 g CTAB was added slowly and the mixture was stirred for 30 min to form dispersion. As follow, 1 g melamine was added in the dispersion and stirred for another 30 min. Subsequently, 2 g CoCl2·6H2O was added and ultrasonically dispersed for the last 30 min. The as-prepared uniform dispersion was shifted to a Teflon-lined reactor and held at 180 °C for 16 h to achieve Co-based precursor and the obtained pink powders were washed by deionized water for several times and dried at 70 °C overnight. Soon afterwards, the products were transferred to quartz tube and annealed at 800 °C for 2 h with a heating rate of 5 °C min−1 before cooled down to room temperature under Ar gas. Finally, we noted the black powders as Co-NPC. For contrast, Co-N-C was synthesized by the same procedure, in addition to the use of CTAB. 2.3. Co/CoO nanoparticles embedded in N-doped porous carbon based on graphene (Co/CoO-N-PC/Gr) To obtain Co/CoO-N-PC/Gr, we just added 30 mg GO in 30 mL deionized water for ultrasonically dispersion of about 2 h. The followed steps were as same as the preparation of Co-N-PC. We noted the black powders used CTAB or not as Co/CoO-N-PC/Gr and Co/CoO-N-C/Gr. 2.4. Co3O4 nanoparticles embedded in N-doped graphene (Co3O4-N-C/Gr)
2. Experimental
Co/CoO-N-PC/Gr was added in 100 mL 0.5 M H2SO4. Then, the products were stirred at 85 °C for 6 h. After activation, the sample was washed by deionized water some times and collected after dry at 70 °C overnight. We noted the black fluffy powders as Co3O4-N-Gr (Scheme 1).
2.1. Chemicals GO powders used for preparing Co3O4-N-C/Gr were purchased from Hunan Chemical Ltd. Hunan, China. Commercial 20% Pt/C catalyst was 129
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2.5. Electrochemical measurements
to the diffractions from (111), (200) and (222) planes of orthorhombic CoO (JCPDS No. 43-1004) [25], respectively. And there are two faint protuberances around 12.9° and 26.4°, corresponding to the GO (001) and rGO (002), respectively, suggesting that GO has a slight reduction accompanied by a weak oxidation of Co. After activation treatment, a large number of Co/CoO were removed, and a small amount Co/CoO were further oxidized into Co3O4 by GO. The major diffraction peaks of Co3O4-N-Gr at 19.0°, 31.3°, 36.8°, 38.5° 44.8°, 59.3° and 65.2° can be assigned to the diffractions from (111), (220), (311), (400), (511) and (440) planes of orthorhombic Co3O4 (JCPDS No. 43-1003) [27], respectively, which is consistent with the TEM results. Meanwhile, the peak at 26.4° becomes more obvious, proving that the GO is further reduced into rGO. Raman spectroscopy is an important approach to characterize the degree of defects in materials. In Fig. 1F, the ID/IG of Co-N-PC is a little higher than that of Co-N-C, suggesting that the addition of CTAB makes the structure produce a little more pores and defects, which are consistent with Fig. S1. Furthermore, the introduction of GO further enhances the defect degree of Co-N-PC. When the rGO is formed, Co3O4-N-Gr shows the highest defect degree [28,29], which is thus favorable for ORR employed to investigate the chemical state and the content of each element in Co3O4-N-Gr. The surveyed spectrum of Co3O4-N-Gr in Fig. 2A shows that the atomic ratio of C:O:N:Co is 87.62:7.85:4:0.52 with the highest N content in the samples (Fig. S3 and Table S1), indicating that the thin slice is easy to dope with N atoms, which is beneficial for ORR. The highresolution N 1s spectrum of Co3O4-N-Gr (Fig. 2B) exhibits three peaks at 398.4 eV (pyridinic N), 399.7 eV (pyrrolic N) and 401.3 eV (graphitic N), respectively [20,24,29]. The high contents of pyridinic N (37.31 at. %) and graphitic N (32.47 at.%) had been proved as active sites for ORR in early reports, which are higher than the other samples (Fig. S3 and Table S2). In Fig. 2C, the O 1s spectrum displays three peaks at 530.9, 532.3 and 533.5 eV, assigned to cobalt oxides, C]O (O-C=O) and CeOeC (C-O-OH), respectively, [30,31] which are different from the others. It may cause the complexation of Co3+ and graphene (Fig. S3). The high resolution of Co 2p spectrum of Co3O4-N-Gr shows six main peaks (Fig. 2D). The peaks at 780.1 (Co3+) and 783.4 eV (Co2+) are assigned to Co 2p3/2, while the peaks at 796.1 (Co3+) and 799.8 eV (Co2+) are assigned to Co 2p1/2, and the peaks at around 787.7 and 803.6 eV are corresponding to weak shakeup satellites, indicative of the formation of Co3O4 [28,32,33], which are really different from those contained Co/CoO (Fig. S3). The high-resolution of Co 2p spectrum of Co3O4-N-Gr is consistent with the lattice fringes of high magnification TEM images and XRD patterns. The pyridinic N can coordinate with the Co element, leading to the formation of the CoeN moieties. Before the activation of the samples, we firstly studied the effects of annealing temperature (from 700 to 900 °C) on the catalytic activity, and obtained the best result at 800 °C (Fig. S4). To further investigate the catalytic performances of Co-N-C catalysts, the electrocatalytic activities of the Co3O4-N-C/Gr was first evaluated by Cyclic Voltammetry (CV) in a 0.1 M KOH electrolyte in O2 or N2(Fig. 3A). A well-defined ORR peak occurs at 0.79 V in O2-saturated electrolyte, while, not obvious peak can be observed under N2. It confirms the remarkable catalytic activity of Co3O4-N-Gr for ORR. The linear sweep voltammetry (LSV) curves in Fig. 3B display that Co3O4-NGr has an onset potential of 0.98 V, which is obviously more positive than 0.94 V of Pt/C. electrolyte in O2 or N2 (Fig. 3A). A well-defined ORR peak occurs at 0.79 V in O2-saturated electrolyte, while, not obvious peak can be observed under N2.It confirms the remarkable catalytic activity of Co3O4-N-Gr for ORR. The linear sweep voltammetry (LSV) curves in Fig. 3B display that Co3O4-N-Gr has an onset potential of 0.98 V, which is obviously more positive than 0.94 V of Pt/C. It suggests that the excellent conductivity and mass-transfer efficiency of graphene. Also, the synergistic effects of Co ions and N atoms make the thin slice-like Co3O4-N-Gr have outstanding electrocatalytic activity for ORR. Furthermore, the half-wave potential of Co3O4-N-Gr (0.86 V) is also 80 mV larger than Pt/C (0.78 V). Compared to the Co3O4-N-C/Gr,
The IM6 electrochemical workstation (Zahner-Electrik, Germany) assumed all the electrochemical tests with a normal three-electrode system at ambient temperature. A nude or modified glassy carbon electrode (GCE) or rotating disk electrode (RDE) as a working electrode; a Pt electrode and a saturated calomel electrode (SCE) were acted as the counter electrode and the reference electrode, respectively. Before the preparation of the catalytic electrode, the GCE (ϕ3 mm) or RDE (ϕ5 mm) were burnished by 0.3 and 0.05 mm alumina powders and flushed with ultrapure water. The electrodes were subsequently washed with absolute ethyl alcohol and ultrapure water by using an ultrasonic unit and natural air dry. To obtain the ink, 1 mg as-prepared product was dispersed in 500 μL of NMP and 480 μL deionized water by ultrasonic for 30 min to get a uniform dispersion. Subsequently, 20 μL of 5% Nafion was dropped to the aforementioned dispersion with ultrasonic concussion for more than 30 min to achieve approximate 1 mg mL−1 dispersion. In the cyclic voltammetry (CV) tests, 3 μL of the as-prepared dispersion was dropped upon the GCE exterior with a pipette gun and parched under infrared lamp. In the linear sweep voltammetry (LSV) tests, 10 μL of the above dispersion was covered upon the 0.196 cm2 disk of RDE exterior, which formed a catalyst filling quantity of 0.05 mg cm−2. For further research the ORR dynamics of the Co-N-C catalysts, the linear sweep voltammetry (LSV) was measured with the rotational speed between 400–2025 rpm in the above O2-saturated 0.1 M KOH solution. Koutecky-Levich (K-L) plots were collected at distinct potentials. The rake ratios of the linearity fit lines were used to compute the electron transfer number on the basic of the K-L equation as follow: [25].
J
1
= (B
1/2 ) 1
+ JK
1
B = 0.2nFCo (Do )2/3 (v )
1/6
where J was the tested current density, JK represented the dynamiclimiting current density, stands for the electrode rotating speed, n was the electron transfer number, F represented the Faraday constant (F = 96,485C mol−1), Co was the volume concentration of O2 (Co = 1.2 × 10−6 mol cm−3), Do represented the diffusivity of O2 in 0.1 M KOH (Do = 1.9 × 10–5 cm2 s−1), and v was the dynamic viscosity (v = 0.01 cm2 s−1). The constant 0.2 was putted to use when the rotation speed was conveyed in rpm. 3. Results and discussion The scanning electron microscopy (SEM) image shows that the Co3O4-N-Gr is composed of homogeneous thin slices, while the inset images reveal that the elements of C, N, O and Co are distributed uniformly in the catalyst (Fig. 1A). This structure is different from other samples mainly because of Co/CoO and porous carbon are dissolved in the activation treatment (Figs. S1, S2 and Table S1). In Fig. 1B, Co3O4 nanoparticles are homogeneously embedded in the N-doped graphene. Moreover, it's clear that lattice fringe spaces of 0.143, 0.156, 0.201, 0.245, 0.285 and 0.467 nm are consistent with the (440), (511), (400), (311), (220) and (111) planes of cubic Co3O4 spinel-phase, respectively (Fig. 1C and D). From Fig. 1A and B, it is found that Co3O4-N-Gr has a homogeneous morphology which is beneficial for mass transfer and active center exposure. To further investigate the crystal structure of Co-N-C catalysts, powder XRD measurement was performed (Fig. E). The major diffraction peaks of Co-N-PC and Co-N-C at 44.2°, 51.5° and 75.8° are corresponding to the diffraction from (111), (200) and (220) planes of orthorhombic metal Co (JCPDS No. 15-0806), [25] respectively, indicating that the carbon materials are amorphous. When the GO was added, the Co was somewhat oxidized and formed Co/CoO complex. Compared with the pure Co, the new weak peaks of Co/CoON-PC/Gr and Co/CoO-N-C/Gr at 36.5°, 42.4° and 61.5° can be assigned 130
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Fig. 1. (A) SEM images and (inset) the corresponding elemental mapping images of Co3O4-N-C/Gr images of carbon, nitrogen, oxygen and cobalt. (B) TEM and (inset) local HRTEM images of Co3O4-N-C/Gr. (C) and (D) High magnification TEM images of Co3O4-N-C/Gr. (E) XRD patterns of different samples. (F) Raman spectra of different samples.
the onset potentials (half-wave potentials) of Co-N-C, Co-N-PC, Co/ CoO-N-C/Gr and Co/CoO-N-PC/Gr are 0.78 V (0.60 V), 0.80 V (0.65 V), 0.81 V (0.66 V) and 0.83 V (0.72 V), respectively, and the corresponding CV curves also show that the onset potential of Co3O4-N-Gr is more positive (Fig. S4). These indicate that the Co3O4 nanoparticles embedded in N-doped graphene structure significantly affects its electrocatalytical behavior for ORR, which is supported by XPS characterization and Raman analysis. It is worth noting that the LSV has a peak for all Co-containing samples, which may be due to the less efficient oxygen diffusion at the beginning, resulting from the slow electrolyte flow in this slice-like structure.[34] Moreover, the Co3O4-N-Gr exhibits a large limited diffusion current density of 5.4 mA cm-2, suggestive of a large amount of exposing active sites towards ORR. As is known to all, the Tafel slope is an important parameter to measure the efficiency of
ORR. Fig. 3C shows the Tafel plots of Co-N-C samples and Pt/C, derived from the corresponding LSV data. The Tafel slope of Co3O4-N-Gr is 31.4 mV dec-1 at low overpotential, which is much lower than 44.7 mV dec1 of Pt/C. And the Tafel slopes of other catalysts are 123.5, 90.9, 64.3 and 59.3 mV dec-1, respectively, which are much more higher than that of Co3O4-N-Gr and Pt/C. Such results are superior to some recent reports, as shown in Fig. S6 and Table S3. In order to gain more information on the ORR dynamics of the Co3O4-N-Gr catalyst, LSV tests were then carried out with different spin velocities at a scanning speed of 5 mV s-1, as displayed in Fig. 3D. Apparently, with the increase of electrolyte diffusion, the diffusion current density increases as the spin velocities rise. The homologous Koutecky-Levich (K-L) plots at various potentials (0.21–0.61 V) meet a good linear relationship (inset in Fig. 3D), and the isoelectron transfer numbers (n) of Co3O4-N-Gr are 131
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Fig. 2. (A) The XPS survey spectra of the Co3O4-N-Gr catalyst. High-resolution XPS spectra of (B) N 1s, (C) O 1s and (D) Co 2p onto the carbon materials, which is considered as the efficient active centers in ORR [26].
calculated to be 3.96 (close to that of Pt/C), which is in accordance with the Koutecky-Levich equation (the calculations were listed in the experimental section in detail) [25,26]. These indicate that a four-electron transfer reaction plays a major role in the ORR process of Co3O4-NGr. Besides the catalytic activity of Co3O4-N-Gr for ORR, the stability and methanol tolerance are two other important indicators for evaluating catalysts. The constancy of the Co3O4-N-Gr and Pt/C is measured at 0.7 V for 6000 s in an O2-saturated 0.1 M KOH electrolyte. Fig. 3E shows that the Co3O4-N-Gr displayed extremely slow damping of the current in the course of the successive chronoamperometric test with a value of 96.25% even after 6000 s. In contrast, the relative current of Pt/C is decreased remarkably to 65.96%, verifying that the Co3O4-N-Gr has good steady catalytic active sites. Moreover, the chronoamperometric responses of Co3O4-N-Gr and Pt/C are recorded after adding methanol (3 M) to electrolyte (Fig. 3F). No changes can be found during the dropping process, while there is a notable decrease of current density for Pt/C. The above results demonstrate that the Co3O4-N-Gr had pretty better selectivities and methanol tolerance for ORR.
16 h); secondly, Co/CoO-N-PC/Gr is obtained by annealing (800 °C, 2 h, Ar); finally, Co3O4-N-Gr is achieved by activation (85 °C, 6 h, 0.5 M H2SO4). The catalyst is composed of homogeneous thin slices with high N contents and high proportions of pyridinic N and graphitic N. The formation of this sheet structure is directly related to the coordination of Co2+/Co3+. Impressively, the as-prepared catalyst Co3O4-N-Gr shows the catalytic activity comparable to the commercial Pt/C catalyst, but better electrochemical stability and methanol poisoning resistance, which due to the synergistic effect of Co3O4 nanoparticles and N-doped graphene. Significantly, our research can provide a new strategy for developing other transition metal and nitrogen-doped carbon catalysts. It is worth noting that our research can provide new strategies for the development of other transition metals and nitrogendoped carbon catalysts. Based on the previously mentioned standards for the preparation of fuel cell catalysts, we have found that transition metals, as well as transition metal oxides, are supported on graphene and have good catalytic properties. We believe that the replacement of precious metal catalysts by non-precious metals is a very viable option if commercialization is to be achieved. The synergistic effect of nonnoble metal oxides and nitrogen-doped graphene can improve the catalytic efficiency and greatly reduce the cost of preparing the catalyst.
4. Conclusions In summary, we have prepared Co3O4 nanoparticles-embedded Ndoped graphene (Co3O4-N-Gr) through three phase reactions: firstly, Co-N-C precursor is synthesized by hydrothermal reaction (180 °C, 132
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Fig. 3. (A) CV curves of the Co3O4-N-Gr catalyst at a scan rate of 50 mV s−1 in 0.1 M KOH solution saturated with N2 and O2. (B) LSV curves for different samples in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm at a scan rate of 5 mV s−1. (C) Tafel plots of different samples derived by the mass-transport correction of corresponding RDE data. (D) LSVs with various rotation rates of Co3O4-N-Gr. The inset indicates the corresponding K-L plots at different potentials. (E) The accelerated durability by i-t curves of Co3O4-N-Gr and 20 wt% Pt/C catalysts at 0.7 V in an O2-saturated 0.1 M KOH electrolyte. (F) Methanol crossover effect test of Co3O4-N-Gr and 20 wt% Pt/C upon the addition of 3 M methanol at around 150 s in an O2-saturated 0.1 M KOH solution at 0.7 V.
Acknowledgements
Appendix A. Supplementary material
This work is financially supported by the Ministry of Science and Technology of the People's Republic of China (Grant 2016YFA0200200) and Wuhan Science and Technology Bureau of China (2018010401011280). Finally, we appreciate the kind help from Prof. Shimin Wang and Prof. Yunbin He for their help in using the electrochemical workstation and XPS, respectively.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.06.005. References [1] X.C. Liu, G.C. Wang, R.P. Liang, L. Shi, J.D. Qiu, Environment-friendly facile
133
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[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
[16]
[17]
[18] [19]
synthesis of Pt nanoparticles supported on polydopamine modified carbon materials, J. Mater. Chem. A 1 (2013) 3945. B. Tabah, I.N. Pulidindi, V.R. Chitturi, L.M. Reddy Arava, A. Varvak, E. Foran, A. Gedanken, Solar-energy-driven conversion of biomass to bioethanol: a sustainable approach, J. Mater. Chem. A 5 (2017) 15486–15506. D. Guo, K. Cai, Y. Wang, A distinct mutual phase transition in a new PVDF based lead-free composite film with enhanced dielectric and energy storage performance and low loss, J. Mater. Chem. C 5 (2017) 2531–2541. Z.F. Pan, L. An, C.Y. Wen, Recent advances in fuel cells based propulsion systems for unmanned aerial vehicles, Appl. Energy 240 (2019) 473–485. Z.F. Pan, L. An, T.S. Zhao, Z.K. Tang, Advances and challenges in alkaline anion exchange membrane fuel cells, Prog. Energ. Combust. 66 (2018) 141–175. W. Yang, Y. Zhang, X. Liu, L. Chen, J. Jia, In situ formed Fe–N doped metal organic framework@carbon nanotubes/graphene hybrids for a rechargeable Zn–air battery, Chem. Comm. 53 (2017) 12934–12937. E. Antolini, Palladium in fuel cell catalysis, Energy Environ. Sci. 2 (2009) 915. C.R. Raj, A. Samanta, S.H. Noh, S. Mondal, T. Okajima, T. Ohsaka, Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 4 (2016) 11156–11178. H. Jiang, Y. Wang, J. Hao, Y. Liu, W. Li, J. Li, N and P co-functionalized threedimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction, Carbon 122 (2017) 64–73. S. Agnoli, M. Favaro, Doping graphene with boron: a review of synthesis methods, physicochemical characterization and emerging applications, J. Mater. Chem. A 4 (2016) 5002–5025. Y. Qiu, J. Huo, F. Jia, B.H. Shanks, W. Li, N- and S-doped mesoporous carbon as metal-free cathode catalysts for direct biorenewable alcohol fuel cells, J. Mater. Chem. A 4 (2016) 83–95. Y. Zan, Z. Zhang, H. Liu, M. Dou, F. Wang, Nitrogen and phosphorus co-doped hierarchically porous carbons derived from cattle bones as efficient metal-free electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 5 (2017) 24329–24334. D. Guo, Z. Tian, J. Wang, X. Ke, Y. Zhu, Co2N nanoparticles embedded N-doped mesoporous carbon as efficient electrocatalysts for oxygen reduction reaction, Appl. Surf. Sci. 473 (2019) 555–563. G.A. Ferrero, K. Preuss, A.B. Fuertes, M. Sevilla, M.M. Titirici, The influence of pore size distribution on the oxygen reduction reaction performance in nitrogen doped carbon microspheres, J. Mater. Chem. A 4 (2016) 2581–2589. A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, K. Tammeveski, Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal–nitrogen–carbon catalysts for alkaline membrane fuel cells, J. Mater. Chem. A 6 (2018) 776–804. R. Liu, H. Zhang, S. Liu, X. Zhang, T. Wu, X. Ge, Y. Zang, H. Zhao, G. Wang, Shrimpshell derived carbon nanodots as carbon and nitrogen sources to fabricate threedimensional N-doped porous carbon electrocatalysts for the oxygen reduction reaction, Phys. Chem. Chem. Phys. 18 (2016) 4095–4101. M. Hoque, S. Zhang, M.L. Thomas, Z. Li, S. Suzuki, A. Ando, M. Yanagi, Y. Kobayashi, K. Dokko, M. Watanabe, Simple combination of a protic salt and an iron halide: precursor for a Fe, N and S co-doped catalyst for the oxygen reduction reaction in alkaline and acidic media, J. Mater. Chem. A 6 (2018) 1138–1149. M. Li, Y. Xiong, X. Liu, C. Han, Y. Zhang, X. Bo, L. Guo, Iron and nitrogen co-doped carbon nanotube@hollow carbon fibers derived from plant biomass as efficient catalysts for the oxygen reduction reaction, J. Mater. Chem. A 3 (2015) 9658–9667. X. Qiao, J. Jin, H. Fan, Y. Li, S. Liao, In situ growth of cobalt sulfide hollow
[20] [21] [22] [23]
[24]
[25] [26]
[27] [28]
[29] [30]
[31] [32] [33] [34]
134
nanospheres embedded in nitrogen and sulfur co-doped graphene nanoholes as a highly active electrocatalyst for oxygen reduction and evolution, J. Mater. Chem. A 5 (2017) 12354–12360. F. Tang, H. Lei, S. Wang, H. Wang, Z. Jin, A novel Fe-N-C catalyst for efficient oxygen reduction reaction based on polydopamine nanotubes, Nanoscale 9 (2017) 17364–17370. M. Sun, D. Davenport, H. Liu, J. Qu, M. Elimelech, J. Li, Highly efficient and sustainable non-precious-metal Fe–N–C electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 6 (2018) 2527–2539. Q. Yu, C. Wu, J. Xu, Y. Zhao, J. Zhang, L. Guan, Nest-like assembly of the doped single-walled carbon nanotubes with unique mesopores as ultrastable catalysts for high power density Zn-air battery, Carbon 128 (2018) 46–53. X.-X. Ma, X.-Q. He, T. Asefa, Hierarchically porous Co3C/Co-N-C/G modified graphitic carbon: a trifunctional corrosion-resistant electrode for oxygen reduction, hydrogen evolution and oxygen evolution reactions, Electrochim. Acta 257 (2017) 40–48. H. Wang, W. Wang, M. Asif, Y. Yu, Z. Wang, J. Wang, H. Liu, J. Xiao, Cobalt ioncoordinated self-assembly synthesis of nitrogen-doped ordered mesoporous carbon nanosheets for efficiently catalyzing oxygen reduction, Nanoscale 9 (2017) 15534–15541. C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Synthesis of phosphorus-doped graphene and its multifunctional applications for oxygen reduction reaction and lithium ion batteries, Adv. Mater. 25 (2013) 4932–4937. T. Palaniselvam, V. Kashyap, S.N. Bhange, J.-B. Baek, S. Kurungot, Nanoporous graphene enriched with Fe/Co-N active sites as a promising oxygen reduction electrocatalyst for anion exchange membrane fuel cells, Adv. Funct. Mater. 26 (2016) 2150–2162. Y. Su, Y. Zhu, H. Jiang, J. Shen, X. Yang, W. Zou, J. Chen, C. Li, Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions, Nanoscale 6 (2014) 15080–15089. Y. Chen, J. Li, T. Mei, X.g. Hu, D. Liu, J. Wang, M. Hao, J. Li, J. Wang, X. Wang, Low-temperature and one-pot synthesis of sulfurized graphene nanosheets via in situ doping and their superior electrocatalytic activity for oxygen reduction reaction, J. Mater. Chem. A 2 (2014) 20714–20722. H.-S. Park, S.-B. Han, D.-H. Kwak, J.-H. Han, K.-W. Park, Fe nanoparticles encapsulated in doped graphitic shells as high-performance and stable catalysts for oxygen reduction reaction in an acid medium, J. Catal. 370 (2019) 130–137. Y. Yao, Z. Chen, A. Zhang, J. Zhu, X. Wei, J. Guo, W.D. Wu, X.D. Chen, Z. Wu, Surface-coating synthesis of nitrogen-doped inverse opal carbon materials with ultrathin micro/mesoporous graphene-like walls for oxygen reduction and supercapacitors, J. Mater. Chem. A 5 (2017) 25237–25248. V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis, C. Galiotis, Chemical oxidation of multiwalled carbon nanotubes, Carbon 46 (2008) 833–840. Z. Jiang, Z.-J. Jiang, T. Maiyalagan, A. Manthiram, Cobalt oxide-coated N- and Bdoped graphene hollow spheres as bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions, J. Mater. Chem. A 4 (2016) 5877–5889. R. Nie, J. Shi, W. Du, W. Ning, Z. Hou, F.-S. Xiao, A sandwich N-doped graphene/ Co3O4 hybrid: an efficient catalyst for selective oxidation of olefins and alcohols, J. Mater. Chem. A 1 (2013) 9037. B. Wang, L. Xu, G. Liu, P. Zhang, W. Zhu, J. Xia, H. Li, Biomass willow catkinderived Co3O4/N-doped hollow hierarchical porous carbon microtubes as an effective tri-functional electrocatalyst, J. Mater. Chem. A 5 (2017) 20170–20179.