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Hollow N-Doped Carbon Spheres with Isolated Cobalt Single Atomic

Nov 6, 2017 - Unexpectedly, ISAS-Co/HNCS exhibited the highest ORR activity, E1/2 = 0.773 V vs RHE, which approaches that of commercial Pt/C (0.790 V,...
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Hollow N-doped Carbon Spheres with Isolated Cobalt Single Atomic Sites: Superior Electrocatalysts for Oxygen Reduction Han Yunhu, Yang-Gang Wang, Wenxing Chen, Ruirui Xu, Li Rong Zheng, Jian Zhang, Jun Luo, RongAn Shen, Youqi Zhu, Weng-Chon Cheong, Chen Chen, Qing Peng, Dingsheng Wang, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Hollow N-doped Carbon Spheres with Isolated Cobalt Single Atomic Sites: Superior Electrocatalysts for Oxygen Reduction Yunhu Han,†, Δ Yang-Gang Wang,†, Δ Wenxing Chen,†, Δ Ruirui Xu, § Lirong Zheng, ‡ Jian Zhang, † Jun ‖ † † † † † Luo, Rong-An Shen, Youqi Zhu, Weng-Chon Cheong, Chen Chen, Qing Peng, Dingsheng Wang*, † and Yadong Li*, † †

Department of Chemistry, Tsinghua University, Beijing 100084, China

‡

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ǁ Center for Electron Microscopy, Tianjin University of Technology, Tianjin 300384, China § School of Chemistry & Chemical Engineering, Shanxi Normal University Supporting Information Placeholder ABSTRACT: To search a low-cost, ultrastable and highly efficient non-precious metal catalyst substituting Pt for oxygen reduction is extremely urgent, especially in acidic media. Herein, we developed a template-assistant-pyrolysis (TAP) method to obtain a unique Co catalyst with isolated single atomic sites anchored on hollow N-doped carbon sphere (ISAS-Co/HNCS). Both the single sites and the hollow substrate endow the catalyst with excellent ORR performance. The half-wave potential in acidic media approaches that of Pt/C. Experiments and density functional theory (DFT) have verified that isolated Co sites are the source for the high ORR activity by significantly increasing the hydrogenation of OH* species. This TAP method is also demonstrated to be effective in preparing a series of ISAS-M/HNCS, which provides opportunities for discovering new catalysts.

For commercial application of proton-exchange membrane fuel cells, the foundational barrier is the high cost of Pt-based catalysts for oxygen reduction reaction (ORR).1, 2 Therefore, the exploration of non-precious metal catalysts that can substitute Pt has received high attention. Currently, alkaline ORR is more favorable to the platinum substitution because of the fast kinetics and some ternary M-N-C materials have taken the superior or comparable catalytic performance than Pt/C.3-8 Although tremendous effort has been paid for exploring non-precious metal catalysts in acidic media, it is still a huge challenge and significantly sluggish than in the alkaline environment.9-11 To achieve the substitution of Pt-based catalysts, two key aspects need to be considered. One is increasing the exposed active sites. The coming isolated single atomic site catalysts afford an opportunity for obtaining the maximum atom efficiency and exposing the most active sites.12-18 Compared to non-precious nanomaterials, isolated single atomic site catalysts are more stable, especially in acidic media.8, 19 Another significant point is substrate, which is required to both anchor well and expose extremely the active sites and promote the transported properties of ORRrelevant species in materials.20-22

Here, we developed a template-assistant-pyrolysis (TAP) method to prepare a hollow N-doped carbon sphere with isolated single Co atomic sites (ISAS-Co/HNCS). It possesses an atomically dispersed cobalt sites for increasing the exposed active sites and hollow substrate for supporting the transporting of ORR-relevant species. Both them endow it an outstanding ORR activity (E1/2 of 0.773 V) in O2-saturated 0.5 M H2SO4 solution. The half-wave potential approaches that of the commercial Pt/C, and exceeds the cobalt-free hollow N-doped carbon sphere (HNCS, E1/2 of 0.571 V) and the solid N-doped carbon with atomically dispersed Co sites (ISAS-Co/SNCS, E1/2 of 0.708 V). Furthermore, ISASCo/HNCS shows superior anti-methanol poison and stability, in which scarcely obvious current change in the presence of 1.0 M MeOH and little ORR polarization curve shift after 10000 cycles. Experiments and DFT calculations have demonstrated that the outstanding ORR activity is ascribed mainly to the atomically dispersed cobalt sites by significantly increasing the hydrogenation of OH* species. According to the TAP method, a series of ISAS-M/HNCS (M = Cu, Fe, etc.) can be fabricated, which demonstrated the universality of this method. To synthesize ISAS-Co/HNCS (Figure 1a), the SiO2 template was first prepared. The SiO2 was dispersed in Co-TIPP/TIPP solution as template before added in another monomer. The obtained mixed solution performed quaterisation (Scheme S1). The collected powder (Figure S1, S2) was thermally treated under flowing hydrogen/argon and then etched with sodium hydroxide to remove SiO2 template. Transmission electron microscopy (TEM, Figure 1b-c) and scanning electron microscopy (SEM, Figure S3) were measured to examine the morphology of ISAS-Co/HNCS. From Figure 1b, we can see that ISAS-Co/HNCS remains the SiO2 shape and its wall thickness is roughly 5 nm (Figure 1c). To elucidate the existing form of cobalt atoms, we performed aberration corrected high-angle annular dark-field scanning transmission electron microscope (AC HAADF-STEM) measurements (Figure 1d) with sub-angstrom resolution. In Figure 1e, the representative Co, N, and C mappings of ISAS-Co/HNCS by electron energyloss spectroscopy (EELS) demonstrate the N, Co heteroatoms are evenly distributed on the hollow substrate. X-ray diffraction pat-

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tern of ISAS-Co/HNCS displays no peaks of cobalt nano-particles can be found (Figure S4). Co content is about 2.2 wt% measured by inductively coupled optical emission spectrometer (ICP-OES) analysis. The binding states of nitrogen were investigated by Xray photoelectron spectroscopy (XPS). The graphitic (401.6 eV), pyrrolic (400.6 eV) and pyridinic (398.6 eV) nitrogen species for N1s are coexistance (Figure S5).

Figure 1. (a) Schematic illustration of the synthesis of ISASCo/HNCS. (b, c) TEM images of ISAS-Co/HNCS. (d) AC HAADF-STEM image of ISAS-Co/HNCS, isolated bright dots marked with light-green cycles are cobalt atoms. (e) HAADFSTEM image and corresponding EDX element mapping of ISASCo/HNCS, C (violet), N (red) and Co (green). To further investigate the structure of Co species in atomic level, X-ray absorption fine structure (XAFS) measurements were carried out. From Figure 2a, we can see Co K-edge X-ray absorption near edge structure (XANES) spectra of ISAS-Co/HNCS with Co Foil, CoO and Co3O4 references. According to the absorption edge of ISAS-Co/HNCS in Figure 2a, Co atom valence situates between that of Co0 and Co2+. Fourier-transformed (FT) k3-weighted extended X-ray absorption fine structure (EXAFS) spectra of ISAS-Co/HNCS in Figure 2b just shows one main peak at 1.32 Å, which is thought to correspond to the Co-N/C first coordination shell, and no obvious Co-Co peak (2.17 Å) or other high-shell peaks are observed. Wavelet transform (WT) was used to analyze Co K-edge EXAFS oscillations. In Figure 2f, the WT maximum at 4 Å-1 for ISAS-Co/HNCS could be considered as the Co-N(C) bonding and no intensity maximum corresponded to CoCo is detected, compared with the WT plots of Co foil, CoO and Co3O4 in Figure 2c-e. Therefore, we can believed that Co atoms are atomically dispersed through AC HAADF-STEM and XAFS results. EXAFS fitting was performed to extract the structure

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parameters and obtain the quantitative chemical configuration of Co atoms (Figure 2h). The obtained coordination number of center atom Co is about 4 and the mean bond length of ISASCo/HNCS is 1.98 Å (Table S1). The fitting curves are exhibited in Figure 2g, h. The data indicate that in the ISAS-Co/HNCS the Co atom is coordinated by 4 N atoms (Figure 2h: inset). In contrast, the best-fit results of Co foil are displayed in Figure S6.

Figure 2. (a) XANES spectra at the Co K-edge of ISASCo/HNCS, CoO, Co3O4 sample and Co foil. (b) Fourier transform (FT) at the Co K-edge of ISAS-Co/HNCS, CoO, Co3O4 sample and Co foil. (c-f) Wavelet transform (WT) of Co foil, CoO, Co3O4 and ISAS-Co/HNCS, respectively. (g) Corresponding EXAFS fitting curves of ISAS-Co/HNCS at k space. (h) Corresponding EXAFS fitting curves of ISAS-Co/HNCS at R space; inset: the schematic model of ISAS-Co/HNCS, Co (pink), N (blue), and C (gray). The ORR activity of ISAS-Co/HNCS was investigated by steady-state linear sweep voltammetry (LSV) using rotating disk electrode (RDE) technique in O2-saturated 0.5 M H2SO4 at room temperature. The CVs of ISAS-Co/HNCS were measured in O2saturated and N2-saturated 0.5 M H2SO4, a clear reduction peak appeared on the CVs profile for the O2-saturated case in comparison to the N2-saturated case (Figure S7). ISAS-Co/HNCS showed an outstanding ORR activity (Figure 3a). For comparison, HNCS and ISAS-Co/SNCS were measured to evaluate the importance of metal-center and hollow structure (Figure S8-S10). Unexpectedly, ISAS-Co/HNCS exhibited the highest ORR activity E1/2 of 0.773 V vs. RHE, which is approached that of the commercial Pt/C (0.790 V, Figure 3a) and is one of the best values of non-precious metal catalysts (Table S2, Figure 3a). HNCS (0.571 V) and ISASCo/SNCS (0.708 V) exhibited a poor ORR activity. These results verified metal-N-C sites is the source for ORR rather than N-C sites and the importance of hollow substrate for ORR. LSV curves were measured at different rotation rate (Figure 3b). Figure 3b inset represented corresponding Koutecky-Levich (K-L) plots. The value of electron transfer number (n) was about

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3.87 calculated by the K-L equation. The selectivities of the oxygen four-electron reduction for ISAS-Co/HNCS and Pt/C were studied using the RRDE technique. The H2O2 yield of ISASCo/HNCS remained below 12.2% at all potentials and dropped to 0.76% at 0.8 V, corresponding to a high electron-transfer number of 3.76-3.96 (Figure S11). ISAS-Co/HNCS showed a lowest tafel slope of 34 mV dec-1 than those of HNCS (171 mV dec-1), ISASCo/SNCS (59 mV dec-1) and Pt/C (53 mV dec-1) suggesting its superior kinetics for ORR (Figure 3c).

(Figure S14). It suggested that ISAS-Co/HNCS had outstanding tolerance to methanol. Besides of the hollow structure, we believed that single dispersed cobalt sites are also responsible for excellent ORR activity. To show this, we prepared Co-NPs/HNCS as contrast. As shown in Figure S15, some inhomogenous Co nanoparticles were observed on the hollow substrate. Co-NPs/HNCS was evaluated the ORR activity in O2-saturated 0.5 M H2SO4 media. After 1000 cycles, the ORR property of Co-NPs/HNCS became much better with the dissolution of cobalt nanoparticles in acidic media but the retention of single Co sites (Figure S16-S18). Compared with fresh Co-NPs/HNCS, the E1/2 of acid-leached Co-NPs/HNCS increased significantly by 32 mV (Figure S19), which suggesting that single dispersed cobalt sites are responsible for the improvement of activity. To explore the origin of the high ORR reactivity of ISASCo/HNCS, DFT calculations were carried out. The calculated free energy pathways of the four-electron ORR reaction processes are shown in Figure 3f and the computational details are presented in support information (Figure S20, Table S5, S6). The general ORR under acid condition are composed of five elementary steps: (1) O2(g) + *  O2*; (2) O2* + H+ + e− OOH*; (3) OOH* + H+ + e− O* + H2O; (4) O* + H+ + e− OH*;

Figure 3. (a) ORR polarization curves. (b) ORR polarization curves at different rotating rates (inset: K-L plots). (c) Corresponding tafel plots obtained from the RDE polarization curves. (d) ORR polarization curves before and after 5000 cycles and 10000 cycles. (e) ORR polarization curves of ISAS-Co/HNCS in O2-saturated 0.5 M H2SO4 without and with 1.0 M CH3OH, 10 mV/s and 1600 rpm. (f) Free energy paths of ORR on ISASCo/HNCS and Co particles at PH=0 and T=298 K. U denotes the applied electric voltage at the cathode. The stability was vital for the electrochemical catalyst. The accelerated durability test (ADT) was performed by cycling the catalyst between 0.4 and 0.85 V at a scan rate of 50 mV/s under O2-saturated atmosphere. After 5000 cycles and 10000 cycles, the half-wave potential of ISAS-Co/HNCS exhibited a little negative shift approximate 3 mV and 7 mV(Figure 3d), which is lowest than the reported values for non-precious metal catalysts (Table S2), suggesting the outstanding stability in acidic media (Figure 3d). However, the half-wave potential for Pt/C showed an obviously negative shift about 36 mV caused by bisulfate adsorption (Figure S12). AC HAADF-STEM (Figure S13) showed the single dispersed metal active sites were still homogeneously distributed after ADT. ISAS-Co/HNCS and Pt/C were measured in both O2saturated 0.5 M H2SO4 and O2-saturated 0.5 M H2SO4 + 1.0 M methanol solutions. From Figure 3e, we can see there was no obvious change in the current density on ISAS-Co/HNCS after injecting 1.0 M methanol into the electrolyzing solution. However, for the Pt/C, the cathodic oxygen reduction peak vanished and a typical inverse peak of the methanol oxidation appeared in CV

(5) OH* + H+ + e−H2O, where the later four steps correspond to the four-electron reduction processes. We note that the second hydrogenation step could also proceed as OOH* + H+ + e−(2OH)*, which is, however, energetically much less favorable than the step OOH* + H+ + e− O* + H2O and is not considered in the present study. In Figure 3f, it is clearly demonstrated that on ISAS-Co/HNCS catalyst all the electron transfer steps are exothermic at U=0 V and thus the free energy pathway is downhill. With the potential increased above to 0.54 V, the first reduction step (i.e. O2* + H+ + e− OOH*) and the last electron transfer step (i.e. OH* + H+ + e−H2O) will become endothermic in succession. In contrast, it is shown that even at U=0 V on Co particle both the first and last reduction steps are endothermic by 0.17 eV and 0.63 eV, respectively. This suggests that Co particles should exhibit much less reactivity than ISAS-Co/HNCS. According to the free energetics, the rate determining step (RDS) on Co particle is the last reduction step. Therefore, the high ORR reactivity of ISAS-Co/HNCS is attributed to the significant improvement of the last elementary step at the single Co site, which facilitates the easier proton and charge transfer to the adsorbed *OH species. This is also confirmed by the calculated binding energies in Table S6 where it shows that the OH species binds to the single Co site weaker than the Co particle by 1.13 eV and should be much easier to be removed. To verify the generality of the TAP method, we extended this method to fabricate hollow spheres with single metal sites by using other metalloporphyrin (M-TIPP, M = Fe, Cu, etc.) as monomers. As shown in Figure 4, by AC HAADF-STEM, we can observe many individual bright dots in ISAS-Fe/HNCS and ISASCu/HNCS. The results of HAADF-STEM (Figure S21, S22), XRD (Figure S23) and XAFS (Figure 4a-b, Figure S24-S27 and Table S3, S4) also demonstrated iron and copper atoms were atomically dispersed on hollow N-doped carbon spheres.

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This work was supported by China Ministry of Science and Technology under Contract of 2016YFA (0202801), the National Natural Science Foundation of China (21521091, 21390393, U1463202, 21471089, 21671117), and the National Postdoctoral Program for Innovative Talents (BX201600084). We thank the 1W1B station for XAFS measurement in Beijing Synchrotron Radiation Facility (BSRF).

REFERENCES

Figure 4. (a) Fourier transform at the Cu K-edge of ISASCu/HNCS, CuO sample and Cu foil. (b) Fourier transform at the Fe K-edge of ISAS-Fe/HNCS, FeO, Fe2O3 sample and Fe foil. (c, d) TEM images of ISAS-Cu/HNCS and ISAS-Fe/HNCS; inset: AC HAADF-STEM image of ISAS-Cu/HNCS and ISASFe/HNCS. Single atoms are highlighted by the green circles. In conclusion, we reported a TAP method to prepare a serials of materials with single metal sites embedded in hollow nitrogendoped carbon substrate (ISAS-M/HNCS, M = Co, Cu, Fe, etc.). Interestingly, the ISAS-Co/HNCS exhibits an excellent activity for ORR under acidic media because of the feature of the single sites and the hollow substrate. It shows also a good methanol tolerance and great stability. Experiments and DFT have verified single dispersed cobalt sites are responsible mainly for the outstanding ORR activity by significantly increasing the hydrogenation of OH* species. This work provides not only an important method to preparation of new ISAS catalysts, but also affords a potential non-precious electrocatalyst for substituting Pt in acidic media.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures; characterization methods, and additional tables and figures (PDF).

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

Author Contributions Δ

Y.H., Y.W. and W.C. contributed equally.

Notes

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The authors declare no competing financial interests.

ACKNOWLEDGMENT

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