Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for

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Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn-Air Batteries Yingying Guo, Pengfei Yuan, Jianan Zhang, Yongfeng Hu, Ibrahim Saana Amiinu, Xin Wang, Jigang Zhou, Huicong Xia, Zhibo Song, Qun Xu, and Shichun Mu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08721 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn-Air Batteries Yingying Guo,a Pengfei Yuan,c Jianan Zhang,*,a Yongfeng Hu,d Ibrahim Saana Amiinu,b Xin Wang,a Jigang Zhou,d Huicong Xia,a Zhibo Song,a Qun Xu,*,a and Shichun Mu*,b a

College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P. R.

China. Emails: [email protected] (J. N. Zhang) and [email protected] (Q. Xu) b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Wuhan 430070, P. R. China. Email: [email protected] c

International Joint Research Laboratory for Quantum Functional Materials of Henan Province,

and School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China d

Canadian Light Source 44 Innovation Boulevard Saskatoon, SK, S7N 2V3, Canada

KEYWORDS: atomic boron-doped Co-N-C species, oxygen evolution reaction, oxygen reduction reaction, Zn-air batteries, electrocatalyst

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ABSTRACT: Structural and compositional engineering atomic-scaled metal-N-C catalysts is important yet challenging in boosting their performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here, a boron (B)-doped Co-N-C active sites confined in hierarchical porous carbon sheets (denoted as Co-N,B-CSs) were obtained by soft template selfassembly pyrolysis method. Significantly, the introduced B element gives an electron deficient site, that can active the electron transfer around the Co-N-C sites, strength the interaction with oxygenated species, and thus accelerate reaction kinetics in the 4e- processed ORR and OER. As a result, the catalyst showed Pt-like ORR performance with a half-wave potential (E1/2) of 0.83 V versus (vs.) RHE, the limiting current density is about 5.66 mA cm-2, and high durability (almost no decay after 5000 cycles) than Pt/C catalyst. Moreover, a rechargeable Zn–air battery device comprising this Co-N,B-CSs catalyst shows superior performance, open-circuit potential of ～ 1.4 V, a peak power density of ～100.4 mW cm-2, as well as excellent durability (128 cycles for 14h of operation). DFT calculations further demonstrated that the coupling of Co-Nx active sites with B atoms prefers to adsorb O2 molecule in side-on mode and accelerates ORR kinetics.

Metal-air batteries with promising sustainable energy conversion are highly dependent on the activity and stability of the cathodic oxygen reduction reaction (ORR) and anodic oxygen evolution reaction (OER).1-6 Platinum (Pt)-based materials have been identified as the most boosted ORR catalysts, whereas Ru- and Ir-based catalysts are effective in OER process.7-10 However, these noble-metal-containing catalysts are plagued by scarcity, high price and limited stability, which inspire researchers to seek effective, highly active and durable bifunctional electrocatalysts for ORR and OER.11-13


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As reported, a wide range of nonprecious metal electrocatalysts show promising activity in ORR/OER, including earth-abundant transition metal, metal-free heteroatom-doped carbon, and metal-nitrogen-doped carbons (M-N-C).14 Among them, the M-N-C active sites containing materials are considered to be the most promising alternatives to metal catalysts for ORR. This is because density functional theory (DFT) calculations suggest that the possible metal-N4 sites could be as active as Pt for O2 adsorption and subsequent O=O bond breaking during the ORR.15,16 In particular, atomically dispersed Co-Nx-C active sites with the critical structure of nitrogen coordinated with a Co atom located in 3D nano-architecture (graphene aerogels,17 carbon nanotube,18 porous carbon,19,20 and g-C3N4,21 so on), have been considerable attractive, since they can afford maximum atom utilization efficiency and expose most active sites.22-24 However, to achieve a breakthrough in promoting the efficiency, intensively studied atomically dispersed Co-Nx-C catalysts are frequently hindered by low content of active sites and low surface area of carbon matrix. To this end, the key to promote the catalytic activity of M-N-C catalysts is properly enhancing the per unit area reactive efficiency of M-N-C active sites for oxygen reactions. It is widely accepted that the ORR activity is strongly correlated with the adsorption of oxygen and formation of superoxide through a one-electron reduction (O2 + e- [O2(ads)]-). This generally is viewed as initial ORR steps, with O2 adsorption proposed by Morcos and Yeager to be the rate-determining step.25 Attractively, the electron-deficient B-doped sites in carbon are favorable for O2 adsorption because they can break the electroneutrality of matrix to create charged sites.2629

Therefore, doping B within Co-N-C catalysts is expected to be a promising strategy to enhance

Co-based catalysts in both ORR and OER. Here, we put forward Co-N-C active sites confined in N,B-co-doped carbon nanosheets (Co-N,B-CSs) via the self-assembly pyrolysis method using


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soft template. Thanks to the preferred O2 adsorption on electron-deficient B atoms doped Co-NC sites, it lowers the oxygen reaction activation barriers on Co-N-C. Therefore, although possessing relatively low surface area, Co-N,B-CSs exhibit superior electrocatalytic activity towards both ORR and OER under alkaline conditions. Furthermore, a rechargeable Zn-air battery device fabricated with Co-N,B-CSs shows a small charge-discharge voltage gap and long-term stability. Undoubtedly, this work will promote development of inexpensive non-noble metal hybrid materials for applications in ORR/OER electrocatalysis. RESULTS AND DISCUSSION As illustrated in Scheme 1, the preparation of Co-N,B-CSs was performed via a soft template self-assembled pyrolysis method. A dry gel precursor was first made by mixing boric acid, urea, polyethylene glycol (Mw=2000, PEG-2000), Co(NO3)2⋅6H2O (as Co source) and water and then drying at 120 °C for 10 h, followed by carbonization at 900 °C (see details in the supporting informantion). Figure S1 shows the transmission electron microscope (TEM) images of the asobtained sample. Interestingly, the high temperature pyrolysis can guarantee the formation of two-dimensional (2D) carbon sheets with existence of Co nanoparticles (NPs) in the pores. The sample is denoted as Co/Co-N,B-CSs. By contrast, the TEM image (Figure S2) of the asprepared sample without the addition of PEG-2000 shows aggregated particle morphology with bulk Co, demonstrating the structure-induced effect of the surfactant PEG-2000 on forming 2D sheet-like structure for Co/Co-N,B-CSs during the carbonization. Urea, as is well-known, is the precursor of graphitic carbon nitride (g-C3N4) nanosheets which can be utilized as sacrificial template to generate the 2D nanosheet morphology.30 To confirm this, we have shown the TEM images of a sample obtained by pyrolysis at 550 oC in Ar atmosphere for 4h under 2 °C min-1 followed by removing the Co containing species, and it giving a nanosheet structure (Figure S3).


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Accordingly, after removing Co NPs by acid treatment, Co-N,B-CSs (carbonized at 900 oC) reveals 2D sheets with crumples, representing internal porous structure (Figure 1a). The corresponding large-magnified TEM images (Figure 1b&S4) demonstrate that an individual sheet consists of numerous macropores with diameter of 30-50 nm created by the etching of Co NPs in Co/Co-N,B-CSs. Notably, the pores size of Co-N,B-CSs is larger than that of the Co NPs in Co/Co-N,B-CSs, probably caused by fusion and shrinkage of the metallic cobalt. Coinciding with these results, the pore size distribution calculated from the N2 sorption data focuses in 16-20 nm, confirming the coexistence of mesopores and macropores (Figure S5a, Table S2). The hierarchical porous structure and the sheet-like morphology deeply play an important role in providing more exposure of active sites and facilitating the mass transport during the catalytic process. Moreover, the structure of Co-N,B-CSs was further analyzed by the X-ray diffraction (XRD) and Raman spectroscopy. As shown in Figure S5b, the XRD pattern shows the very broad characteristic 2θ peaks at 26° and 44°, indicating the disordered structure of the carbon sheet.31 Meanwhile, the Raman spectrum of Co-N,B-CSs displays the intensity ratio of two main bands located at 1354 and 1591 cm-1 (ID/IG) is 1.03, further confirming the defective structure of carbon nanosheets (Figure S5c). Based on our previous work, the disordered structure is expected to play a vital role in increasing content of the atomically dispersed Co-Nx species. The high-angel annular dark field scanning transmission electron microscopy (HAADFSTEM), X-ray photoelectron spectroscopy (XPS), and the Co k-edge X-ray adsorption near-edge structure (XANES) and extended X-ray adsorption fine structure (EXAFS) spectroscopy Co kspace of Co-N,B-CSs shows different oscillation curves were conducted to confirm the atomically dipersed B-doped Co-Nx species in Co-N,B-CSs. As shown in Figure 1c, a great number of atomic-scaled Co can be identified by the isolated bright dots and marked with arrows.


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As a further evidence, the HAADF-STEM image and STEM mapping displays C, Co, N, and B elements in the entire architecture (Figure 1e-h), which is agreement with the XPS analysis (Figure S5d). Significantly, Co-N,B-CSs give a lower Co content of 0.52 at.% detected by X-ray photoelectron spectroscopy (XPS) measurement than most of the reported metal-Nx catalysts. As shown in Figure 1i, the high-resolution N1s XPS spectrum for Co-N,B-CSs shows five features (pyridinic N, N-6; Co-Nx; pyrrolic N, N-5; graphitic N, N-Q; oxidized N, N-O). The Co-Nx bond with the binding energy of 398.32 eV, associated with the cobalt-nitrogen functional moieties, indicates that there are abundant Co-Nx active sites in Co-N,B-CSs.31 Additionally, the highresolution B1s XPS spectrum (Figure S5e) shows three valence state of boron element, including the B-C bond (190.27 eV), B-N bond (191.37 eV), and B-O bond (193.2 eV), implying replacement of C atoms by B atoms as adjacent atoms with N atoms.26,31 Due to that N atoms can induce positive polarization of C atoms, while low electronegativity B atoms can be positively polarized, the positively polarized N-C and C-B bonds favor attraction of O2 molecules during the O2 reaction process, further enhancing the ORR an OER performance of Co-N,B-CSs. Therefore, both the HAADF-STEM and XPS results suggested that the coupled hybrid materials, atomicscaled Co-N-C species uniformly disperse on B,N-co-decorated carbon sheets, was successfully synthesized.

Additionally, XANES and EXAFS spectra were performed to further study the coordination environment of cobalt, as X-ray absorption spectroscopy is sensitive to the electronic structure and coordination properties of both crystalline and amorphors forms of species. As shown in Figure 1j, Co K-edge XANES spectra of Co-N,B-CSs and Co-N-Carbon show different spectral features (in terms of both edge postion and peak area), with the formal Co oxidation state between that of the Co in Co foil and CoO references.32,33 Co in Co-N,B-CSs is more reduced


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since its edge jump is at a lower energy compared to Co-N-Carbon. More importantly, the peak area (area after the edge jump) in Co-N-Carbon is much higher. The peak area (after normalization, as shown) is directly proportional to the number of vacancies in the Co np and nd orbitals.34 Because of the electron donor property of boron, there is less density of unoccupied state in Co-N,B-CSs, compared to Co-N-Carbon. On the other hand, EXAFS analysis (R-space results shown in Figure 1k) clearly indicates that there is no Co-Co interaction observed in both Co-N,B-CSs and Co-N-Carbon, suggesting that Co is well atomically dispersed in carbon network. The lack of Co-O peak in the R-space result of these two samples, as observed at 1.64 Å in CoO, also confirmed these two samples are not cobalt oxide like.34 The multiple peaks in the low R region (0.95, and 1.5 Å) could be attributed to interactions between Co and N and C in these samples, including the Co-B for the Co-N,B-CSs sample. Taken together, the data indicate that the Co is atomically dispersed in the N and B doped carbon matrix, and it is in the ionic state with nitrogen and boron atoms in the configuration. With the advance of the configuration, the B doping coupled with the atomic-scaled Co-N-C active sites could modify the electronic properties, create more active sites and improve the electronic conductivity, thus allowing the enhancement of the electrocatalytic activity for ORR and OER (Figure S6).34

The electrocatalytic performances towards ORR for the as-prepared materials were first evaluated by rotating disk electrode (RDE) techniques in O2-saturated 0.1 M KOH solution, respectively. As the control experiments a catalyst synthesized without B doping (as Co-N-carbon) was also tested to better understand the important role of doped B and atomic scale Co-N-C active sites on enhancing the ORR for Co-N,B-CSs. As shown in the SEM and TEM images in Figure S7, Co-N-carbon exhibit similar porous nanosheet structure to Co-N,B-CSs. Figure 2a, S8,9 shows the comparison of linear scan voltammogram (LSV) and cyclic voltammogram (CV)


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curves, as well as the kinetic current densities for Co-N,B-CSs, Co/Co-N,B-CSs, Co-N-carbon, and Pt/C catalyst (details are in the Table S3). The Co-N,B-CSs exhibits a pronounced Pt/C-like ORR activity with a half-wave potential (E1/2) of 0.83 V and limiting current density (JL) of 5.66 mA cm-2, which overperforms that of Co/Co-N,B-CSs (0.803 V, 4.7 mA cm-2) and Co-N-carbon (0.640 V, 3.13 mA cm-2) catalysts. To further gain information on the ORR mechanism, the Tafel slopes were determined by mass transport correction from the mixed kinetic-diffusion controlled region. As shown in Figure 2b, the fast ORR kinetics for Co-N,B-CSs is further confirmed by its Tafel slope (64 mV dec-1), close to that of Pt/C catalyst (68 mV dec-1). To quantitatively investigate the ORR activity of Co-N,B-CSs, a series of LSV curves are presented in Figure 2c at different rotating speeds (400-2050 rpm). Likewise, calculated from the Koutechy-Levich plots (K-L, j−1 vs. ω−1/2), the electron transfer number per oxygen molecule (n) for Co-N,B-CSs is determined to be ~4.0. In particular, calculated from the rotating ring-disk electrode (RRDE) data, the H2O2 yield measured for Co-N,B-CSs (highest yield is 2.2%) is much less than that for Pt/C (the highest yield is 5.9%) at all potentials, implying the value of n is between 3.98-4.00 at 0.200.50 V, well consistent with the results obtained from the K-L plots (Figure S10a). Taken together, it suggests an efficient oxygen reduction activity via a dominant four-electron pathway.

An excellent catalytic selectivity for cathode reactions against fuel poisoning is important for an efficient ORR electrocatalyst, especially displays the superior tolerance to methanol crossover of Co-N,B-CSs over Pt/C and Co-N-Carbon catalysts. More significantly, Co-N,BCSs exhibit a slight current decrease retention over 2000s during continuous operation, whereas Pt/C exhibits a dramatic current loss with only 64% retention. Moreover, the Co-N,B-CSs catalyst also reveals excellent long-term stability after 5000 cycles at 0.1 to -0.8 V with a scan


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rate of 500 mV s-1 in O2-saturated 0.1 M KOH solution, more stable than the commercial Pt/C (Figure 2e&S11).

Figure 2f shows excellent ORR and OER bifunctional electrocatalytic activities for Co-N,BCSs, which can be judged by the potential difference between OER and ORR (△E = E

j=10

–

E1/2, the OER potential is taken at a current density of 10 mA cm−2, while the ORR potential is taken at half-wave). As a reference, we also used a state-of-the-art OER electrode with RuO2 nanoparticles, and found that Co-N,B-CSs possesses a lower onset potential than the RuO2 and Pt/C, and △E is minimal of 0.83 V. This is probably because not only the N atoms (such as pyrrole-like, graphitic nitrogen atoms) can positively polarize the adjacent C atom but also the electronegativity B atom can activate the C atoms and Co-N-C active sites, providing carbon chemistry between carbon involving active sites (Co-N-C, B-C, and N-C) and oxygen molecular.35

The superior ORR and OER bifunctional electrocatalytic activities of Co-N,B-CSs inspired us to construct a rechargeable Zn-air battery with the same configuration as that of the primary counterpart but using 6 M KOH with 0.2 M zinc acetate as the electrolyte (Figure 3a).36,37 Figure 3b shows the maximum power density of the Zn-air battery using the Co-N,B-CSs catalyst were determined to be 100.4 mW cm−2, much higher is higher than that of Pt/C catalysts (66.3 mW cm−2). The battery with the Co-N,B-CSs air-cathode shows voltage plateaus of ~1.20 V at a current density of 10 mA cm−2 (Figure 3c). The as-formed Zn-air battery has an opencircuit voltage of 1.43 V with Co- N,B-CSs as the air cathode (Figure S12). Figure 3d shows the discharge and charge polarization curves for Zn-air batteries with Co-N,B-CSs as the ORR and OER bifunctional catalyst.


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Motivated by this initial performance, the robustness of Co-N,B-CSs cathode was further demonstrated by cycling at a current density of 5 mA cm−2 (Figure 3e). The Zn-air battery produces an initial charge potential of 2.25 V and discharge potential of 0.9 V, with a small voltage gap of 1.35 V and a high round-trip efficiency of 58%. After 128 cycles test (about 14 h), the Co-N,B-CSs air cathode shows a slight performance loss with a small increase in the voltage gap by 0.2 V. A lower charge-discharge voltage gap is observed for the Zn-air battery with the Co-N,B-CSs air-cathode, indicating the robust rechargeability. The observed performance decay for Co-N,B-CSs is most probably due to the inevitable exposure to positive potential during OER that causes the carbon oxidization and the loss of active sites, porous offers fast mass diffusion pathways and high efficient single Co atoms dispersion, while the inner sheet can supply good electrically conduct.

A potential challenge to rational design of electrochemical energy devices is the realization of lab-scale performance. To demonstrate this, we have further show the applicability and promise of Co-N,B-CSs as a Zn-air battery cathode electrocatalyst for portable electronic devices. In this regard, a scalable all-solid state Zn-air battery consisting of a free-standing Co-N,B-CSs aircathode, zinc foil anode, alkaline poly(vinyl alcohol) (PVA) gel electrolyte, and pressed (for strengthen conductivity) nickel foam current collector (the catalyst loading on the air electrode was 0.5 mg cm-2) was fabricated via the schematics shown in Figure S13. We also constructed a discharge Zn-air batteries with the same configuration as that of the counterpart. Interestingly, a high OCV of 1.345 V is obtained in Figure 3f, where it only needs three batteries in series to power blue light-emitting diodes (LEDs, 8 mm, ~3.4 V) with an open circuit voltage of 4.058 V (Figure 3g). Galvanostatic cycling profiles of Co-N,B-CSs based all-solid state battery at the current density of 2 mA cm-2 (Figure 3h).


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We propose that the outstanding activity and high stability of Co-N,B-CSs catalyst depend on the following two aspects: 1) the high density of isolated atomic-scaled Co-N-C boosts electron transfer ability and provides high active sites for ORR; 2) the introduction of B atoms into the carbon matrix can raise the un-balanced charge distribution and thus positively polarize C atom and Co-N-C sites, which favors the adsorption of oxygen species.38 To demonstrate this, we performed first-principles calculations using DFT methods to determine how the addition of B reduces the H2O2 production and favors the four electrons ORR pathway (Figure 4). As shown in Table S5, Co-N3B and Co-N4 moieties at the edge of an armchair graphene nanoribbon exhibit the lowest free energies, indicating their most structural stability. In this way, we used the Co-N3B and Co-N4 doped graphene nanoribbon as model reference to represent the difference of Co-N,B-CSs and Co-N-Carbon. Considering the four electrons paths of ORR and OER on CoN4-CSs and Co-N3B-CSs in alkaline solution, free energy for each elementary step was calculated by combining the enthalpy and the harmonic entropy, which are listed in Table S5-S7. Figure 4a&b show the highest occupied molecular orbital plot of the corresponding O2 adsorption configuration of Co-N,B-CSs. Due to the higher electronegativity of oxygen, the adsorbed functional groups attract electrons from surrounding carbon atoms, resulting in charge redistribution on those carbon atoms. The electron-deficient carbon B-doped sites in carbon can break the electroneutrality of matrix to create charged sites, favorable for O2 adsorption. To this end, we suppose that most of Co-N-C active sites are located at the edge. As shown in Figure 4c, the free energy changes of Co-N3B-CSs catalyst during the ORR process decrease slightly from 0 to 0.073 eV in the first step, which is significantly lower than that for Co-N4-CSs (-0.014 eV). Given an applied potential of 0.24 V, Co-N,B-CSs in each step during the ORR process are


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energetically downhill, significantly lower than that for Co-N4-CSs (0.48 V), which supplies a theoretical evidence for highly efficient ORR catalytic activity of Co-N,B-CSs.

CONCLUSIONS In summary, the M-N-C active site type of atomically dispersed B-doped Co-N-C active sites, evidenced by HAADF-STEM, XPS, XANES, and EXAFS characterizations, have been achieved and boosted bifunctional ORR and OER activities approaching that of Pt/C and RuO2 along with significantly enhanced stability. Furthermore, the catalyst exhibits good performances when equipped in liquid and all-solid rechargeable Zn-air batteries than Pt/C. Significantly, the control experiments and DFT calculations confirmed that the introduction of B atoms can strength the positively polarization of the Co-N-C, N-C, and C-B moieties, which favors the capture of oxygen species, and thus accelerate reaction 4e- processed kinetics in ORR and OER. Besides, the porous layered structure can supply a great number of exposed edges sites. This work highlights that the atomic reactive efficiency of M-N-C active sites can be enhanced by decoration the sites with hetero-atoms (B,P,S) for oxygen electrocatalysis. The outstanding characteristics of our cost-effective Co-N,B-CSs catalyst make it a promising candidate for energy conversion and storage devices.

EXPERIMENTAL METHODS Chemicals. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was obtained from Beijing Chemical Reagent Company (Beijing, China). Boric acid (H3BO2) and urea were bought from Xilong Chemical Technology Co., Ltd. Polyethylene glycol (PEG) (Mw=2000) was purchased from Beijing Tianjin Kermel Chemical Reagent Co., Ltd. sulfuric acid (H2SO4) were obtained from the Tianjin Chemical Factory. Nafion (5.0 wt%) were purchased from Sigma-Aldrich. Pt catalyst


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(20% Pt supported on Vulcan XC-72 carbon) was obtained from Johnson Matthey. All chemicals were used as received without any further purification Deionized water was used in all experiments. Synthesis of Co-N,B-CSs. Co-N,B-CSs were synthesis via a hydrothermal method. Typically, 5 g Urea (24 mmol), 0.051 g Co(NO3)2⋅6H2O (0.68 mmol), 0.15 g boric acid and 0.5 g PEG-2000 were dissolved in 50 mL water under stirring and dried in an oven at 120 °C for 10 h. Then, the temperature was further raised to 900 °C at a ramp of 10 °C min-1 and kept at 900 °C for 6 h under Ar atmosphere. For the removal of Co NPs, the as-obtained material was immersed in 0.5 M H2SO4 for 24 h. After washing with deionized water for several times, Co-N,B-CSs were obtained. As the control-experiments, the synthesis of Co-N carbon and Co-N,B-CSs carbon bulk was similar with the above method but Co/Co-N,B-CSs, respectively.

ASSOCIATED CONTENT Supporting Information. Additional experimental details, figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected]


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ORCID Jianan Zhang: 0000-0002-7559-1090

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21571157, U1604123, and 51173170), Outstanding Young Talent Research Fund of Zhengzhou University (No. 1521320001), The Young Outstanding Teachers of Univeristy in Henan Province (2016-130), and the Open Project Foundation of Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) (2017-29), Nankai University, and Open Project Foundation of State Key Laboratory of Inorganic Synthesis and Preparation of Jilin University. The XAS measurement is conducted at the Canadian Light Source (CLS). CLS is supported by the NSERC, NRC, CIHR of Canada, and the University of Saskatchewan. REFERENCES (1) Bruce. D.; Haresh, K.; Jean-Marie, T. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (2) Wang, X.; Lu, X.; Liu, B.; Chen, D.; Tong, Y.; Shen, G. Flexible Energy-Storage Devices: Design Consideration and Recent Progress. Adv. Mater. 2014, 26, 4763-4782. (3) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y. Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chem. Int. Ed. 2017, 56, 610-614. (4) Kraytsberg, A.; Ein-Eli, Y. The Impact of Nano-Scaled Materials on Advanced Metal–Air Battery Systems. Nano Energy 2013, 2, 468-480.


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Figures

Scheme 1. Synthetic procedure of the Co/N-B doped Carbon Nanosheet (Co-N,B-CSs).


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Figure 1. (a) SEM, (b) TEM, (c-d) HAADF-STEM images, (e-h) EDS mapping element mapping of C, Co, N, and B for Co-N,B-CSs (scale bar is 300 nm). (i) The high-resolution XPS spectrum of N 1s for Co-N,B-CSs. (j) XANES spectra and (k) Fourier transformed EXAFS spectra of the Co foil and Co-N,B-CSs and Co-N-Carbon. R (Å), distance in angstroms. k, wave number.


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Figure 2. (a) LSV curves of Co-N,B-CSs, Co/Co-N,B-CSs, Co-N-Carbon and Pt/C catalysts in O2-saturated 0.1 M KOH solution. (b) Tafel slopes of Co-N,B-CSs and Pt/C. (c) LSV curves at different rotation rates in rpm. Inset is the corresponding K–L plot with a sweep rate of 5 mV s-1. (d) Chronoamperometric response for Co-N,B-CSs, Co-N-Carbon and Pt/C electrode at 0.75 V (vs. RHE) after the introduction of 9.7 mL of CH3OH into 230.3 mL of 0.1 M KOH solution. (e) ORR polarization LSV and CV curves of Co-N,B-CSs measurement before and after 5000 potential cycles at the scan rate of 50 mV s-1 with the rotation speed of 1600 rpm. (f) LSV curves of different catalysts for both ORR and OER in 0.1 M KOH at 1600 rpm a sweep rate of 5mV s1

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Co-N,B-CSs Pt/C

10 mA cm-2

Figure 3. (a) Schematic representation of the rechargeable Zn-air battery. (b) Polarization and power density curves of the Zn-air batteries of the Co-N,B-CSs and Pt/C catalysts. (c) Discharge curves of the Zn-air battery with Co-N,B-CSs as catalyst at 10 mA cm-2 current densities. (d) Charge and discharge polarization curves. The catalyst loading on the air electrode was 0.5 mg cm-2. (e) Performance of the battery cycling at the current densities of 5 mA cm-2. (f) Photograph of a Co-N,B-CSs based all-solid-state Zn-air micro battery with an open circuit voltage of 1.345V and (g) a lighted green LED (8 mm, ~3.4 V) powered by three all-solid-state Zn-air batteries interconnected in series. (h) Galvanostatic cycling profiles of Co-N,B-CSs based allsolid state battery at the current density of 2 mA cm-2.


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Figure 4. (a,b) Opetimized geometry of the corresponding O2 adsorption configuration on the Co-N,B-CSs system. (c) Free-energy paths of ORR on Co-N3B-CSs and Co-N4-CSs systems during the ORR in alkaline solution at the equilibrium potential of U=0 V and 0.24 V for CoN3B-CSs and U=0 V, 0.48 V (Co-N4-CSs).


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Table of Content

Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn-Air Batteries Yingying Guo,a Pengfei Yuan,c Jianan Zhang,*,a Yongfeng Hu,d Ibrahim Saana Amiinu,b Xin Wang,a Jigang Zhou,d Huicong Xia,a Zhibo Song,a Qun Xu,*,a and Shichun Mu*,b


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Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for

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