Turning Carbon Atoms into Highly Active Oxygen Reduction Reaction

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Turning Carbon Atoms into Highly Active Oxygen Reduction Reaction Electrocatalytic Sites in Nitrogen-Doped Graphene Coated Co@Ag Mengsi Li, Changlai Wang, Yang Yang, Peng Jiang, Zhiyu Lin, Guoliang Xia, Kang Yang, Pengping Xu, and Qianwang Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02573 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Sustainable Chemistry & Engineering

Turning Carbon Atoms into Highly Active Oxygen Reduction Reaction Electrocatalytic Sites in Nitrogen-Doped Graphene Coated Co@Ag Mengsi Li1†, Changlai Wang1†, Yang Yang, Peng Jiang 1, Zhiyu Lin1, Guoliang Xia1, Kang Yang1, Pengping Xu1, Qianwang Chen1,2,* 1

Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science &

Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, No. 96, JinZhai Road, Hefei 230026, China. 2

High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences,

Hefei 230031, China. *Corresponding author e-mail: [email protected] †

Mengsi Li and Changlai Wang contributed equally.

Abstract In recent years, 3d transition metals or alloys encapsulated by graphene layers (M@NG) have been emerging as prospective electrocatalysts especially for hydrogen evolution reactions (HER) and oxygen evolution reactions (OER). However, this strategy is limited in preparation of high-performance oxygen reduction reaction (ORR) catalysts. Herein we prepared Co@Ag bimetallic nanoparticles embedded in nitrogen-rich graphene layers via pyrolysis metal organic frameworks (MOFs). The catalyst displays extraordinarily high ORR performance with a high onset potential of 0.989V and a half-wave potential of 0.872V in 0.1M KOH, respectively. Moreover, it shows a superb long-term stability performance after 5000 cycles on account of the carbon layers which protect the material from corrosion as well as high methyl alcohol-tolerance under methanol environments. Density functional theory calculations suggest that carbon atoms, which is adjacent to nitrogen dopants in Co@Ag@NC, are active sites for ORR. Especially, Ag “mantle” in Co@Ag@NC contributes a great deal to tune electronic structure of carbon active sites, thereby promoting the activity of carbon atoms and enhancing ORR kinetics. This unique Co core/Ag mantle structure can boost the overall ORR activity of M@NG while keeping cost-effectiveness due to the very low requirement of Ag content, providing a new insight to design other efficient electrocatalysts. 1

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Key words: metal–organic frameworks, oxygen reduction reaction, electron transfer，Co core/Ag mantle structure，tuning electronic structure

Introduction Currently, the increasing demands for renewable energy have motivated more and more intensive research in clean energy conversion technologies. Metal-air batteries are promising to become leading clean energy on account of its high energy density, attractive prices and environmentally friendiness. But the utilization of metal-air batteries in mobile and vehicle power devices is greatly dependent on the rate of ORR process which is consist of reducing oxygen back to water.

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Up to now, Although much progress has been achieved on potential alternatives, the

noble metal Pt still plays an irreplaceable role in delivering adequate rates.5-7 Efficient as it is, the expensive price, scarcity and poor long-term stability largely impede its widespread applications. Beyond that, the toxic effects (e.g. CO or methanol poisoning) on Pt-based materials during the reaction, together with other inevitable problems, will result in the deactivation of catalyst, thus blocking its commercialization. Thus, the search for poisoning-resistant noble metal based electrocatalysts with low cost and good stability is of great significance. To date, carbon materials especially graphene have gained increasing attention on account of their economically low price, superior electrical conductivity as well as strong tolerance to extreme environments , which would be a promising catalyst for replacing Pt.8-9 Even though pristine graphene exhibits inert activity as electrocatalysts, it is possible to tune its electronic structures and activities by introducing heteroatoms especially nitrogen dopants.10-11 Besides, very recently, our group and other researchers proved that the combination of graphene shells with metal core (M@NG) could further enhance the electrochemical activity of graphene, particularly for hydrogen evolution reactions.12-15 However, the ORR activity of most reported M@NG especially with non-precious metal core is still far away from the commercial Pt/C.16-17 The noble metals with the unique 4d electronic track arrangement often display excellent performance, but the high price still hinder their large-scale commercialization. For decades, two main strategies for creating cost-effective catalysts have emerged, including maximizing atom utilization with reduced noble metal loadings and using other cheaper platinum

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group metals.18-19 In most cases, a host of nanostructured architectures have aroused extensive interest, including core-shell, yolk-shell, janus, single-hole hollow etc.20-24 In particular, atomically dispersed hybrids or individual metal monolayers anchored on supports were developed for their diverse characteristics on surface property, chemical composition and functionality with minimal noble metal usage.18 In this respect, Hunt and co-workers demonstrated that Pt or bimetallic PtRu monolayers anchored on Ti0.1W0.9C nanoparticles, which is supported by carbon. The catalyst showed an remarkable activity over the commercial precious metal materials for various electrocatalytic reactions.25 Moreover, our previous studies have proven that a fraction of noble-metal atoms on the outmost layer of alloy core is more conducive to enhance HER performance when compared to the homogeneous counterparts.12, 26-27 The modified chemical properties of the surfaces could be put down to the changed average energy of the surface , which is induced by the ligand effects and strong metal–N interactions.12, 25, 28 Unfortunately, It is still a substantial challenge to achieve precise control over the content of noble-metal on the outside shell and interior core. The main obstacle lies in the phase-segregation of the noble metals, namely, the noble metals should coat the partner surfaces while avoiding alloying with the underlying core. The past researches indicate that the adsorption energies, which correlates with structural features and surface electronic structures of one catalyst, can be modulated by maneuvering the compositions and chemical states.28-29 It turns out that the bimetallic nanohybrids may underwent segregation of metals in response to the reaction environments.

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As a matter of fact, the

immiscibility of the constituents and the diverse surface energies should account for the segregation behavior in the bimetallic alloy.30-32 Interestingly, previous research has suggested that the distinction of work function between two metals would facilitate the accumulation of negative charges, driven by the interfacial polarization.30-31, 33 Among the platinum group materials, Silver (17 $/oz) is more economically advantageous over other noble metals, such as Pt (1020 $/oz), Pd (1092 $/oz), Rh (1725 $/oz) and so on. To our knowledge, silver has aroused extensive attention as OER34 and carbon dioxide reduction reaction (CO2RR) materials35. For Ag-Co binary system, the bimetallic nanoparticles would undergo dramatic segregation of Co and Ag atoms owing to the different surface energies.33, 36 On the other hand, encapsulating alloy into carbon cages will prevent core from corroding under extreme conditions.37-38 Besides, the implanting heteroatom dopants (e.g. N) in the carbon framework can induce the accumulation of charges and 3



consequently raise the density of electrocatalytic active sites, thereby accelerating relevant electrocatalytic process.12-13, 26-27, 37-38 Enlightening by these considerations, Co@Ag nanoparticles embedded in nitrogen-doped graphene layers (Co@Ag@NC) were successfully achieved by a facile strategy using Ag-doped metal organic frameworks. The formed Co@Ag bimetallic nanoalloy core, accompanied by the abundant N dopants in graphitic carbon shells, offers excellent activities in alkaline media. As a catalyst for ORR, our Co@Ag@NC catalyst exhibited a low onset overpotential and a half-wave potential of 872 mV at 2.6mA cm−2, which was even superior to Pt/C catalyst.

Results and Discussion Synthesis and characterization of Co@Ag@NC Scheme 1 illustrated the synthesis route of our prepared Co@Ag@NC catalyst. The Co3[Co(CN)6]2 precursor (Scheme 1a) and Ag-doped Co3[Co(CN)6]2 precursor (Scheme 1b) in this experiment were synthesized at ambient temperature. In this work, we got the different precursor products assigned as S-X-MOF (X=1, 2, 3) after adding into an aqueous solution containing a various amount of AgNO3. The detailed information of experiment is displayed in the Supporting Information. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images (Figure 1a,b,d,e, Figure S2,S3a,b) showed the as-prepared Co3[Co(CN)6]2 doped by Ag retained the cubic morphology. The average size of these particles was approximately 100 nm. According to the thermogravimetric analysis (Figure S1), the decomposition and carbonization of the Ag-doped Co3[Co(CN)6]2 started at approximately 400°C. The resultant products (Scheme 1c) were gained via one-step annealing at 700°C under nitrogen atmosphere, hereinafter recorded as S-X (X= 1,2,3), respectively. From the FESEM results and the TEM images (Figure 1c,f, Figure S2,S3c,d) , we can see that all the samples were collapsed into irregular nanoparticles trapped within carbon layers after annealed in nitrogen atmosphere. As can be seen in Scheme 1d, the final product was composed of a bimetallic Ag-Co nanoparticle core and highly N-doped carbon shells, and the latter would provide more eletrocatalytic active sites as well as avoiding corrosion, which was testified by the following structure characterization. Significantly, during pyrolysis process, most cobalt atoms tend to be surrounded by Ag atoms, which is beneficial to the electrons transfer from alloy core to the coated graphene layers, thus 4


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promoting the ORR activity.

Scheme 1 | The schematic illustration of the synthesis route of S-2.

(a) Co3[Co(CN)6]2 precursor, (b)

Ag-doped Co3[Co(CN)6]2, (c) Co@Ag nanoparticles embedded in carbon layers, (d) the Co@Ag@NC nanoparticle enlarged form（c）and the illustration for oxygen reduction reaction as an electrocatalyst.

Figure 1 | (a, b, c) FESEM images of Co3[Co(CN)6]2, S-2-MOF, S-2, respectively. (d, e, f) TEM images of Co3[Co(CN)6]2, S-2-MOF, S-2 , respectively.

The TEM images and the high-resolution TEM (HRTEM) images of S-2 (Figure 2 a, b, c) clearly showed that the small nanoparticles with Co-rich core/Ag-rich surface were encapsulated within N-doped graphene layers. Particularly, the thin graphene shells consisting of 2–5 layers of carbon atoms were in favour of the electron-transfer process from inner nanoalloy to outside

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graphene layers. In addition, it also revealed the representative lattice fringes identified well with the crystal lattice of (111) plane of the cubic Ag phase with a space of 0.236 nm, and that the lattice fringes with the spacing of 0.221、0.204nm and 0.251 nm were corresponded to Co (110)、 (111) and (220) planes, respectively. Most critically, as observed from Figure 2d–i, the images of both elemental mapping of a random single small particle and EDS line profiles ulteriorly indicated the noble metal Ag atoms are more inclined to be distributed outsider of the nanoalloy core. The emergence of this nanostructure is possibly resulted from the low surface energy of Ag.33, 36 Apart from this, we can gain further information for the elements distribution of the Ag-Co nanoparticle core from the selected area EDS measurements (Figure 2j-l).

Figure 2 | Characterization of Co@Ag@NC hybrid. (a-c) Representative transmission electron microscopy image and HRTEM images of S-2. (d) The TEM image of a randomly selected single nanoparticle of S-2. (e-h) The images of the corresponding elemental mapping of Co, Ag, C, N on the randomly selected single nanoparticle in image 2d, respectively. (i-l) EDS line proﬁles along the yellow line recorded on the single nanoparticle in image 2d and the TEM image and quantitative results (EDX, weight ratio) measured from the exterior of alloy core recorded in image 2d, respectively.

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Figure 3 | (a) XRD patterns of S-1, S-2, S-3. (b) Raman spectra of S-1, S-2 and S-3.

The structural information of as-prepared products was identified by X-ray diffraction (XRD). Similar diffraction features of them were observed in Figure 3a.

Obviously, the larger the

additive amount of AgNO3 （S-1

Turning Carbon Atoms into Highly Active Oxygen Reduction Reaction

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