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Co Nanoparticles Encapsulated in N-Doped Carbon Nanosheets: Enhancing Oxygen Reduction Catalysis without Metal-Nitrogen Bonding Xinlei Zhang, JingJing Lin, Shuangming Chen, Jia Yang, Li Song, Xiaojun Wu, and Hangxun Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11120 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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ACS Applied Materials & Interfaces

Co Nanoparticles Encapsulated in N-Doped Carbon Nanosheets: Enhancing Oxygen Reduction Catalysis without Metal-Nitrogen Bonding Xinlei Zhang,† Jingjing Lin,‡ Shuangming Chen,§ Jia Yang,† Li Song,§ Xiaojun Wu,‡,* and Hangxun Xu†,* †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and

Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China. ‡

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and

Engineering, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China §

National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience,

University of Science and Technology of China, Hefei, Anhui 230029, China Keywords: Cobalt Nanoparticles, Carbon Nanosheets, Electrocatalysis, Oxygen Reduction Reaction, N-Doping

ABSTRACT: It is known that introducing metal nanoparticles (e.g., Fe and Co) into N-doped carbons can enhance the activity of N-doped carbons toward oxygen reduction reaction (ORR). However, introducing metals into N-doped carbons inevitably causes the formation of multiple active sites. Thus, it is challenging to identify the active sites and unravel mechanisms

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responsible for enhanced ORR activity. Herein, by developing a new N-heterocyclic carbene (NHC)-Co complex as the nitrogen- and metal-containing precursor, we report the synthesis of N-doped carbon nanosheets embedded with Co nanoparticles as highly active ORR catalysts without direct metal-nitrogen bonding. Electrochemical measurements and X-ray absorption spectroscopy indicate that the carbon-nitrogen sites surrounding Co nanoparticles are responsible for the observed ORR activity and stability. DFT calculations further reveal that Co nanoparticles could facilitate the protonation of O2 and thus promote the ORR activity. These results provide new prospects in rational design and synthesis of heteroatom-doped carbon materials as nonprecious metal catalysts for various electrochemical reactions.

INTRODUCTION Oxygen reduction reaction (ORR) at the cathode is critically important for metal-air batteries and fuel cells.1-5 Currently, Pt-based precious metal catalysts are the most used materials for ORR in small-scale prototype devices. However, the prohibitive cost and low abundance of Pt have impeded the large-scale commercialization of above clean energy technologies.5,6 This long-term challenge has stimulated extensive efforts to develop low-cost alternatives for ORR. Since the first report of using macrocyclic Co-phthalocyanine as an ORR electrocatalyst in alkaline media in 1964,7 nonprecious metal electrocatalysts containing transition metal-nitrogen coordination sites (M-N, M=Co, Fe, etc) have attracted enormous interests.8-22 They can be synthesized via pyrolysis of suitable carbon precursors under inert or reducing atmospheres (e.g., NH3 or H2) with the presence of metal salts.23 The direct contact of N source and metal species in the precursors, however, usually leads to the formation of carbon catalysts with various potentially active sites including M-N species, N-doped carbons, and metal


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nanoparticles/oxides/carbides/sulfides.24-42 Therefore, the nature of the active sites responsible for ORR remains elusive due to the heterogeneity occurred during the high-temperature synthesis. Consequently, unraveling the active sites and other important parameters that can enhance ORR activity is highly desirable for rational design and synthesis of high-performance ORR catalysts. Recently, intensive investigations on the structure and composition of catalysts comprised of Fe/Fe3C nanoparticles and Fe-N species have revealed that the coexistence of nanoparticles and Fe-N species was crucial to achieve the prominent ORR performance.24-26 The Fe/Fe3C nanoparticles could boost the ORR activity of Fe-N sites whereas N-doped carbons still contributed to the observed ORR activity. Meanwhile, Mukerjee et al. and Gewirth et al. synthesized novel core-shell structured catalysts that were devoid of any Fe-N species and ambiguously proved that the embedded Fe nanoparticles were not directly involved in the oxygen reduction pathway. In contrast, the N-doped carbon shells were responsible for the observed activity and stability.43,44 Although these studies have expanded our general understanding of the importance of Fe and Fe3C nanoparticles in enhancing ORR activity of Ndoped carbons,24-26,43,44 much less attention has been paid to elucidate the role of Co nanoparticles and Co-N species in influencing the oxygen reduction pathways.45-48 In this work, starting from precursor design, we demonstrate that N-heterocyclic carbene (NHC)-Co complex without direct Co-N bonding can be used as a single-component precursor to prepare N-doped carbon nanosheets embedded with Co nanoparticles (hereafter referred to as the Co-NHC catalyst) as highly active ORR catalysts. In this approach, planar tridentate NHC ligands are used to ‘anchor’ Co atoms to form stable Co-C bonds without Co-N coordination, which is expected to avoid the formation of Co-N species during pyrolysis. By combining


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electrochemical measurements and X-ray absorption studies, we show that the obtained Co-NHC catalyst exhibits prominent ORR activity without existence of Co-N species. Our DFT calculations further indicate that the enhanced ORR activity could be attributed to the electron transfer from Co nanoparticles to the N-doped carbon sheets, leading to a charge redistribution and decrease of work function. RESULTS AND DISCUSSION

Figure 1. (a) Schematic illustration of the synthetic procedures for the NHC-Co coordination polymer network. (b) High resolution Co 2p XPS spectrum of the NHC-Co coordination polymer network. (c) Schematic representation of the Co-NHC catalyst along with the potential ORR process. The chemical structure of the tridentate-NHC ligand and the formation of NHC-Co complex are schematically shown in Figure 1a. The tris-benzimidazolium salts with a conformationally rigid core structure were employed as precursors for the corresponding tridentate-NHC ligands. This is to ensure that geometrically well-constrained tridentate-NHC ligands with three carbene sites could be in situ generated by deprotonation upon treating with a


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strong base.49-51 Thus, after coordination with CoCl2, an extended 2D coordination polymer network was obtained (Figure 1a). Meanwhile, the steric effect of tert-butyl substituent could prevent the aggregation of network and avoid the close contact between Co and N. The formation of stable Co-C bond was confirmed by X-ray photoemission spectroscopy (XPS) with an obvious Co-C peak at ~780.5 eV (Figure 1b). X-ray absorption near-edge spectroscopy (XANES, Figure S1) also indicated the formation of Co-C bond (Table S1).28,52,53 Fourier transform infrared spectroscopy (FTIR) also confirmed the reaction between the NHC ligands and CoCl2 as the peak at 523 cm−1 is the characteristic stretching vibration of Co−C bond (Figure S2).54 The as-prepared NHC-Co coordination polymer network was then subjected to pyrolysis at 900 oC under Ar atmosphere for 3 h to obtain carbon nanosheets. During pyrolysis, Co atoms were reduced and sintered into Co nanoparticles (Figure S3). To remove unstable and electrochemically inactive species, the pyrolyzed product was further treated with hot HCl solution overnight. Subsequently, the leached sample was annealed under the same pyrolysis temperature to afford the Co-NHC-900 catalyst for ORR (Figure 1c). After this pyrolysis and acid washing process, the Co contents decreased from 11.01 wt% to 6.94 wt% as determined from inductively coupled plasma-atomic emission spectrometry (ICP-AES).


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Figure 2. (a) TEM image of the Co-NHC-900 catalyst. (b) High-resolution TEM image of a single Co nanoparticle encapsulated in a graphitic carbon shell with corresponding SAED pattern displayed in the inset. (c) HAADF-STEM image of the Co-NHC-900 catalyst and the corresponding elemental mapping images of Co, C, and N. The detailed microstructure was first examined using transmission electron microscope (TEM) which revealed that the Co-NHC-900 catalyst exhibited a sheet-like structure embedded with Co nanoparticles (Figure 2a). This sheet-like struture was also confirmed using scanning electron microscopy (SEM) (Figure S4). High-resolution TEM image (Figure 2b) further showed that Co nanoparticles were encapsulated by a crystalline carbon shell which can be easily identified as the (002) plane of graphitic carbon with an interlayer spacing of ~0.34 nm. Selected area electron diffraction (SAED) pattern is shown in the inset of Figure 2b. The brighter and bigger spots are attributed to the diffraction from Co nanoparticles with (111) plane of the β-Co phase. Meanwhile, the ring-like diffraction pattern with dispersed bright spots could be ascribed to the (002) plane of graphitic carbon.55 Both crystalline Co and carbon were identified in powder X-ray diffraction (PXRD) pattern (Figure S5). The formation of the graphitic carbon shell is believed to be critical in achieving stable and active ORR by enhancing the electronic conductivity and resistance to corrosion.56,57 Moreover, as shown in Figure 2c, elemental mapping reveals that both carbon and nitrogen are homogeneously distributed in the carbon skeleton. In contrast, the strong Co signals only intensify in the area of nanoparticles, indicating the existence of Co nanoparticles. No Co signals could be found in the carbon skeleton besides Co nanoparticles (Figure S6), implying that Co mainly exists in the catalyst as nanoparticles without obvious Co-N species.24


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N2 sorption isotherms of the Co-NHC-900 catalyst can be identified as type-IV isotherms (Figure S7). The steep increase in N2 uptake at the low pressure region and the small hysteresis loop indicates the coexistence of both micropores and mesopores. The measured BrunauerEmmett-Teller (BET) surface area is 1204 m2/g. XPS was conducted to investigate the surface chemical composition of the Co-NHC-900 catalyst (Figure S8). Deconvolution of the highresolution N 1s peak yielded three peaks at 398.6, 400.1, and 401.3 eV corresponding to pyridinic, pyrrolic, and graphitic nitrogens, respectively.37,58,59 The high content of pyridinic N (61%) should be beneficial for the ORR.60,61 No obvious Co-N bonding could be identified from N 1s peak.28,53,62 Meanwhile, the Co content measured from XPS (1.25 wt%) is much lower than that obtained from ICP-AES (6.94 wt%), suggesting that the Co species are embedded within the carbon shells.

Figure 3. (a) LSV curves of the Co-NHC-900 catalyst with other control samples and Pt/C in O2-saturated 0.1 M KOH. Electrode rotation speed, 1600 rpm; scan rate, 10 mV/s. (b) Peroxide yield

and

calculated

electron

transfer

number

of

the

Co-NHC-900

catalyst.

(c)

Chronoamperometric responses of Co-NHC-900 and Pt/C catalysts at 0.75 V vs. RHE. The ORR activity of Co-NHC-900 in O2-saturated 0.1 M KOH was investigated using rotating disk electrode (RDE) measurements (Figure 3a). The Co-NHC-900 catalyst exhibited a prominent half-wave potential (E1/2) of ~0.85 V vs. RHE, which is comparable to that of the commercial Pt/C catalyst (0.84 V). The Co-NHC-900 catalyst exhibited a Tafel slope of 63


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mV/decade (Figure S9), indicating that the transfer of the first electron is probably the ratedetermining step.63 The electron transfer number (n=4, Figure S10) calculated from the Koutechy-Levich (K-L) equation is consistent with that derived from the rotating ring-disk electrode (RRDE) measurements over the entire potential region (n=3.96, Figure 3b and S11), suggesting a four-electron reduction pathway for ORR. RRDE results also showed that the peroxide yield remained below 5% at all potentials (Figure 3b), indicating the Co-NHC-900 catalyst has extremely high ORR catalytic efficiency.64,65 The Co-NHC-900 catalyst also shows high stability for ORR as confirmed by the chronoamperometric tests (Figure 3c). Approximately 95% of the original current density was retained for the Co-NHC-900 catalyst, whereas the Pt/C catalyst displayed a much higher current loss of ~40%. Further examination of the Co-NHC-900 catalyst after durability test revealed that the sheet-like structure and Co nanoparticles were still retained (Figure S12). The tolerance to methanol crossover was also evaluated and the Co-NHC900 catalysts clearly exhibited superior performance to the Pt/C catalyst (Figure S13). Obviously, the ORR performance of the Co-NHC-900 catalyst is among the best reported Co-based electrocatalysts (Table S2). In order to reveal the role of Co nanoparticles in influencing the ORR activity, several control experiments were carried out (Figure 3a). The LSV results indicated that the NHC-900 catalyst (N-doped carbons synthesized by direct pyrolysis of the NHC ligands without Co) showed the much inferior ORR activity to the Co-NHC-900 catalyst. The variation in diffusionlimited current density could be ascribed to the improved graphitization and increased surface area in the presence of Co during pyrolysis (Figure S14). Meanwhile, the PXRD pattern of NHC900 only showed one broad peak around 23°~26° (Figure S15) which was in striking contrast with the diffraction peak at 26.5° observed in Co-NHC-900. This observation further confirmed


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that Co could facilitate graphitization during pyrolysis.27 CN- ions could strongly coordinate with transition metals and poison the M-N sites for ORR in alkaline electrolytes.23,64 In our study, the presence of CN- ions would not influence the ORR activity of the Co-NHC-900 catalyst, suggesting that the outer surface of Co-NHC-900 is free of Co-N species. However, these physically isolated Co nanoparticles within carbon shells are still critical for the observed ORR activity as the Co-NHC-900 catalyst exhibited diminished ORR activity after ball milling and removal of the majority of Co nanoparticles. Therefore, the encapsulated Co nanoparticles must interact with the N-doped graphitic carbon shell and affect the electronic properties of the outer active sites where O2 is activated and reduced, as schematically shown in Figure 1c.37 The importance of Co nanoparticles in enhancing the ORR activity was also confirmed in 0.1 M HClO4 electrolyte (Figure S16). Similarly, SCN- poisoning experiment further supported that the Co-N species were not present in the Co-NHC-900 catalyst (Figure S16).23,24,64

Figure 4. (a) Co K-edge XANES of Co-NHC-900. (b) Fourier-transformed Co K-edge EXAFS spectra. Co-TPP and Co foil were used for reference. Electrochemical studies have unambiguously confirmed that the Co-N species which are commonly observed in the Co- and N-containing carbon materials were not present in the Co-


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NHC-900 catalyst. To further understand the nature of ORR active sites, Co K-edge XANES and EXAFS spectra were used to determine the local bonding environments of Co in Co-NHC-900. Figure 4a shows the normalized XANES spectra for the Co-NHC-900 catalyst along with commercial cobalt(II) tetraphenylporphyrin (Co-TPP) and Co foil. Obviously, the Co K-edge spectrum in Co-NHC-900 is very similar to that of Co foil, implying metallic nature of Co in the Co-NHC-900 catalyst. This observation also reveals the absence of Co-N species in the characterized Co-NHC-900 catalyst.52,66 The corresponding Fourier transformation of Co K-edge from EXAFS spectra are shown in Figure 4b. The quantitative information on the local structures was obtained by fitting EXAFS data using the IFEFFIT package (Table S1). Obviously, the Co K-edge in Co-NHC-900 is dominated by a peak at ~2.49 Å, which can be fitted with Co-Co scattering referring to the Co foil.66 This observation suggests that the Co atoms in the precursor are predominantly converted to metallic Co. In contrast, Co-TPP exhibits a dominant peak at 1.94 Å assigned to Co-N distance due to the coordination of Co and N in porphyrin. More importantly, this peak is absent in the Co-NHC-900 catalyst, further confirming that there are no detectable Co-N moieties. Thus, we successfully demonstrate a novel approach to synthesize Ndoped carbon nanosheets encapsulated with Co nanoparticles without direct Co-N bonding.


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Figure 5. (a) The partial DOS projected on C and N’s p orbitals in freestanding N-doped graphene layer (top) is compared with those in N-doped graphene supported by Co substrate. The Fermi level is aligned at 0 eV. (b) Top view (top) and side view (bottom) of the difference charge density plot of N-doped graphene supported on Co substrate. (c) The computed freeenergy diagrams for ORR in alkaline media (pH=13 and T= 298K) at 0 V on N-doped graphene with and without the Co substrate, respectively. The optimized structures of a four-step, fourelectron ORR pathway on N-doped graphene are inserted. The white, red, blue, and grey balls represent hydrogen, oxygen, nitrogen, and carbon atoms, respectively.


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Density functional theory (DFT) calculations were performed to reveal the interactions between Co nanoparticles and the outer N-doped graphitic carbon shells (see details in supporting information). To simplify the calculation, a model with a layer of N-doped graphene on the surface of Co (111) was used (Figure S17). Figure 5a shows the calculated density of states (DOS) projected on p orbitals of C and N atoms. Although pristine graphene is a zerobandgap semiconductor, N-doped graphene is metallic with increased DOS at the Fermi level. For N-doped graphene on Co substrate, d orbitals of Co atoms have significant contribution to the DOS at the Fermi energy level, implying strong coupling between d orbitals of Co atoms and p orbitals of C and N atoms in N-doped graphene. In addition, electrons are transferred from Co substrate to N-doped graphene. Bader charge analysis indicates that about 3 electrons per super cell are transferred from Co substrate to N-doped graphene, which slightly enhances the Fermi energy level of N-doped graphene of ~0.5 eV. The deformation charge density clearly indicates the charge redistribution on N-doped graphene due to the charge effect of Co substrate (Figure 5b). Consequently, the charge transfer and shift of work function (Figure S18) are expected to enhance the electrocatalytic activity of the N-doped graphene.37,60 To further understand the possible effects of Co substrate on ORR pathways, the potential energy profiles for ORR were investigated (Figure 5c). Since the ORR catalyzed by Co-NHC900 is a four-electron process, four elementary steps of ORR reaction including the adsorption of O2 (*O2), OOH (*OOH), O(*O), and OH (*OH) species on N-doped graphene were considered here. As shown in Figure 5c, the protonation step of O2 could be considered as the ratedetermining step in the entire ORR process. Thus, the ORR activity could be determined by direct comparison of the free energy for the protonation of O2 on N-doped graphene layer before and after introducing Co substrate. Our calculations clearly indicate that the free energy change


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for the protonation of O2 on pristine N-doped graphene layer is 1.901 eV, which is much higher than that of the N-doped graphene supported with Co substrate (0.154 eV). Therefore, DFT calculations unambiguously prove that Co nanoparticles encapsulated in N-doped carbon nanosheets could lead to enhanced ORR activity.37,67,68 CONCLUSIONS In summary, we developed a novel NHC-Co coordination polymer network as the unique precursor for successful encapsulating Co nanoparticles into N-doped carbon nanosheets without direct Co-N bonding. The as-prepared Co-NHC-900 catalyst exhibits prominent ORR performance with excellent durability. The encapsulated Co nanoparticles could facilitate the protonation of O2 and enable the full four-electron reduction pathway on the metal-free active sites. The results from XPS, XANES, EXAFS, and electrochemical measurements confirm the absence of Co-N species in the Co-NHC-900 catalyst, which in turn allows us to conclusively attribute the ORR activity to the N-doped carbons. DFT calculations confirm that the enhanced ORR activity arises from the electron transfer from Co nanoparticles to the outer N-doped carbon sites, leading to a decreased local work function on the carbon surface. Although further investigations are required to elucidate the mechanism responsible for the formation of such a unique structure in Co-NHC-900 during pyrolysis, our study identifies the role of metallic Co nanoparticles in heteroatom-doped carbon catalysts in enhancing ORR performance, thus providing new prospects in design and synthesis of nonprecious metal electrocatalysts in the near future. EXPERIMENTAL SECTION


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Materials.

2,3,6,7,10,11-hexabromotriphenylene

and

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2,3,6,7,10,11-hexatertbutylamino

triphenylene were synthesized according to the reported procedures.51,69 All syntheses were performed using standard Schlenk technique under nitrogen atmosphere unless otherwise stated. Triethyl orthoformate and potassium tert-butoxide were purchased from Energy Chemical (Shanghai). KH was purchased from Sigma-Aldrich and anhydrous CoCl2 was purchased from Alfa Aesar. Anhydrous solvents were purchased from Sinopharm Chemical Reagent Co. Ltd, which were purified with sodium and benzophenone and degassed by three freeze-pump-thaw cycles. All other reagents were used as received from commercial suppliers. Synthesis of Triphenylene-based Tris(azolium) Salt. A mixture of 2,3,6,7,10,11hexatertbutylaminotriphenylene (920 mg, 1.4 mmol), HCl (0.39 mL, 4.62 mmol), and trimethyl orthoformate (40 mL) was refluxed under aerobic conditions for 16 h. Subsequently, Et2O was added to the cooled reaction mixture and the precipitated solid was collected by filtration. 1H NMR (300 MHz, DMSO-d6): 9.20 (s, 6H, CH arom), 9.17 (s, 3H, NCHN), 2.04 (s, 54 H,C(CH3)3). Synthesis of NHC-Co Coordination Polymer Network. The overall synthetic scheme is shown in Scheme S1 in the supporting information. Potassium hydride (42 mg, 5 eq.) and potassium tert-butoxide (1 mg) were added to the solution of the obtained solid (200 mg) in THF (15 mL). The solution was allowed to stir at R.T. for 2 hours. Subsequently, anhydrous CoCl2 (28 mg, 0.75 eq) was added to the mixture. After 12 h, the product was obtained by centrifugation and further washed with dry THF for 3 more times. The as-obtained sample was then dried under vacuum at 60 ℃ overnight. Synthesis of Co-NHC-900 Catalyst. The obtained NHC-Co coordination polymer network was pyrolyzed at 900 °C for 6 hours in argon atmosphere (The sample was heated from room


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temperature to 900 °C at a rate of 5 °C/min and then maintained at 900 °C for additional 3 hours.). The pyrolyzed product was immersed in 1 M HCl at 90 ℃ for 24 hours followed by thorough washing with water for 3 times. Then the products were subjected to another pyrolysis under the same temperature to afford the final Co-NHC-900 catalyst. Characterization. Nuclear magnetic resonance (NMR) spectra were recorded using a Bruker 400 MHz spectrometer. PXRD patterns were collected from a Japan Rigaku DMax-γA rotation anode X-ray diffractometer (λ = 1.54178 Å). Fourier transform infrared (FTIR) spectra as shown in the supporting information were obtained from a Bruker VECTOR-22 323 FTIR spectrometer. ICP-AES results were measured by using the Atomscan Advantage spectrometer. The JEM2100F field-emission transmission electron microscope was used to acquire TEM and HRTEM images used in this work. Meanwhile, elemental mapping images were recorded using the Gatan GIF Quantum 965 instrument. XPS results were qcuired from the Thermo ESCALAB 250 X-ray photoelectron spectrometer. The N2 sorption isotherms were measured using a Micromeritics Tristar 3020 instrument. Co K-edge X-ray absorption spectroscopy (XAS) measurements were carried out at the beamline 1W1B in Beijing Synchrotron Radiation Facility (BSRF). Detailed data acquisition and processing methods can be found in our previous work.70 Electrochemical Measurements. The experimental setup used in this work for electrochemical measurements was the same to our previous reports.62,70 The working electrode is a 5.0 mm glassy carbon rotating disk electrode (PINE Research Instrumentation). A graphite rod and saturated calomel electrode were used as the counter and reference electrodes, respectively. To acquire accurate results, the reference electrode should be calibrated with respect to RHE before each experiments. The procedures to deposit catalyst inks onto the glassy carbon electrode can


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be found in previous reports62,70 and the final catalyst loading amount was 0.3 mg/cm2. The cyclic voltammetry experiments were performed in 0.1 M KOH electrolyte saturated with Ar/O2. The scan rate was 10 mV s-1. Meanwhile, RDE tests were conducted in 0.1 M KOH saturated with O2 (scan rate of 10 mV s-1). The electron transfer number for ORR was calculated by Koutecky-Levich equation: 1 1 1 1 1 = + = 1/2 + J JL JK Bω JK B=0.62nFC0D02/3v-1/6 where J is the measured current density, JK and JL are the kinetic and diffusion-limiting current densities, respectively, ω is the angular velocity, n is transferred electron number, F is Faraday constant (96485 C mol-1), C0 is the bulk concentration of O2 (1.2×10-6 mol cm-3), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm2 s-1), and v is the kinetic viscosity of the electrolyte (0.01 cm2 s-1). In this work, rotating ring-disk electrode (RRDE) measurements were also performed to assess the four-electron selectivity of the catalyst. The scan rate of the disk electrode was 10 mV s-1 and the ring electrode potential was set to 1.2 V vs. RHE. Accordingly, the electron transfer number (n) and the hydrogen peroxide yield (%H2O2) were calculated by the following equations:

%H2O2=200 n=4

ir/N id + ir/N

id id + ir/N

where id and ir are the disk and ring currents, respectively. N is the ring current collection efficiency which was determined to be ~37 %.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional TEM images, XPS spectra, PXRD patterns, electrochemical measurements, and computational details (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H. X.) *E-mail: [email protected] (X. W.) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT H. X. and X. W. acknowledges the funding support from the MOST (2015CB351903, 2017YFA0207301, and 2016YFA0200602), National Natural Science Foundation of China (51402282, 21474095, and 21573204), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), and the Fundamental Research Funds for the Central Universities. Partial results were obtained from the Catalysis and Surface Science Endstation in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The calculations are supported by USTCSCC, SCCAS, Tianjin, and Shanghai Supercomputer Centers. REFERENCES


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