Single Cobalt Atom and N Codoped Carbon Nanofibers as Highly

Publication Date (Web): August 29, 2017 ... breakthrough of the highly durable TM-N/C ORR catalyst could open an avenue for affordable and durable fue...
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Single Cobalt Atom and N Codoped Carbon Nanofibers as Highly Durable Electrocatalyst for Oxygen Reduction Reaction Qingqing Cheng,†,‡ Lijun Yang,§ Liangliang Zou,† Zhiqing Zou,† Chi Chen,† Zheng Hu,§ and Hui Yang*,† †

Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China University of the Chinese Academy of Sciences, Beijing 100039, China § Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡

S Supporting Information *

ABSTRACT: Transition-metal and nitrogen-codoped carbon-based (TMN/C) catalysts are promising candidates for catalyzing the oxygen reduction reaction (ORR). However, TM-N/C catalysts suffer from insufficient ORR activity, unclear active site structure, and poor durability, particularly in acidic solution. Herein, we report single Co atom and N codoped carbon nanofibers (Co−N/CNFs) catalyst with high durability and desirable ORR activity in both acidic and alkaline solutions. The half-wave potential of the ORR shows a negligible decrease after a 10 000-cycle accelerated durability test. The high ORR durability is originated from the structural stability of the atomically dispersed Co-based active site, as revealed by probing analysis and density functional theory calculations. A passive direct methanol fuel cell with the Co−N/CNFs cathode delivers a maximal power density of 16 mW cm−2 and a remarkable stability during a 200 h test, demonstrating the application potential of Co−N/CNFs. The breakthrough of the highly durable TM-N/C ORR catalyst could open an avenue for affordable and durable fuel cells. KEYWORDS: cobalt single atoms, durability, transition-metal/nitrogen-doped carbon nanofibers, oxygen reduction reaction

1. INTRODUCTION The oxygen reduction reaction (ORR) limits the energyconversion efficiency in fuel cells due to its sluggish kinetics. Currently, the commercialization of fuel cells containing noblemetal catalysts is hindered by their prohibitive cost and low stability. Hence, extensive efforts have been devoted to developing highly efficient and durable nonprecious metal ORR catalysts,1−3 such as transition-metal-coordinated macrocyclic compounds,4 transition-metal carbides or nitrides,5,6 metal-free heteroatom-doped or defective carbon,7−12 and transition metal (such as Fe and Co) and nitrogen codoped carbon (TM-N/C) catalysts.13−16 TM-N/C catalysts are currently considered as the most promising candidates to replace Pt-based catalysts. Various strategies have been used to improve the ORR activity of the TM-N/C catalysts, e.g., by creating micro/ mesoporous-carbon structure,17−19 metal carbide encapsulated in N-doped carbon,20−22 and single-atom TM-N/C catalysts.23,24 However, the elusive active sites plague TM-N/C catalysts due to their unpredictable synthesis and complex structures. The role of transition metals as active sites is hitherto unknown.25−27 Meanwhile, their instability, particularly in acidic solution, greatly limits the practical application. Wang et al.28 developed a metal−organic-framework (MOF)© 2017 American Chemical Society

derived Fe−N/C catalyst with a desirable stability that showed a decrease of the half-wave potential (E1/2) by 40 mV after a 10 000-cycle accelerated durability test (ADT). Xu et al.29 reported a self-supported Fe−N/C-800 catalyst that displayed a greatly improved durability with a ∼29 mV negative shift in E1/2 after an ADT. We have developed a manganese oxide-induced strategy to high-performance Fe−N/C catalyst with highly exposed active sites and an even smaller E1/2 of 11 mV after a 10 000-cycle ADT.19 Regardless, these Fe-based TM-N/C catalysts still suffer from ORR degradation. In addition, as a part of the Fenton reagent, Fe ions can promote the decomposition of H2O2 into highly reactive free radicals,30 which will attack the membrane,31 active site, and carbon support.32 In contrast, Co is not a component of the Fenton reagent, and thus, such an oxidative attack will not occur.33,34 Furthermore, a single Co atom anchored in a carbon skeleton would be difficult to demetallate, probably preventing the loss of the active sites, which would improve the ORR stability. Therefore, a single Co atom and N codoped Co−N/C catalyst Received: July 14, 2017 Revised: August 23, 2017 Published: August 29, 2017 6864

DOI: 10.1021/acscatal.7b02326 ACS Catal. 2017, 7, 6864−6871

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ACS Catalysis could be a promising candidate as a durable ORR catalyst in acidic solution. Herein, we developed a new type of single Co atom and Ncodoped carbon nanofibers (Co−N/CNFs) as an efficient ORR catalyst, exhibiting desirable electrocatalytic activity and excellent durability. The E1/2 of the Co−N/CNFs showed a negligible decrease after a 10 000-cycle ADT in both acidic and basic electrolytes, which was rarely reported to date. A combination of electrochemical characterizations and density functional theory (DFT) calculations demonstrated the effectiveness of the single Co atoms on the high durability and structural stability of Co-based active site. A passive direct methanol fuel cell (DMFC) with Pt-free Co−N/CNFs as the cathode catalyst exhibited a maximal power density of 16 mW cm−2 and high stability.

beamline of Shanghai Synchrotron Radiation Facility (SSRF) and analyzed with Ifeffit Athena software. 2.4. Electrochemical Measurements. Prior to use, the glass carbon rotating disk electrode (RDE, 5 mm in diameter) and rotating ring disk electrode (RRDE, SD: 0.2475 cm2) were polished with 0.3 and 0.05 μm Al2O3 slurry, and washed in ultrapure-water and ethanol. For the electrochemical tests in 0.1 M HClO4 solution, the TM-N/C catalyst inks were prepared by mixing 12 mg of the catalyst, 0.1 mL of 5% Nafion, 0.3 mL of isopropanol (IPA) and 0.6 mL of ultrapure-water. After ultrasonic dispersion, 10 μL of the suspension was carefully dropped onto the freshly polished RDE or RRDE, resulting in a catalyst loading of 0.6 mg cm−2. For the tests in 0.1 M KOH solution, the catalyst inks (include 20 wt % Pt/C catalyst) were composed of 10 mg of catalyst, 0.2 mL of 5% Nafion solution, 0.6 mL of IPA and 1.2 mL of ultrapure-water. Four microliters of the solution was dropped onto RDE to obtain a catalyst loading of 0.1 mg cm−2. Electrochemical measurements were performed using a CHI 730E workstation. The glass carbon and Hg/Hg2SO4 (0.1 M HClO4) or Ag/AgCl (0.1 M KOH) were used as the counter and reference electrodes, respectively. The RDE and RRDE tests were conducted in O2-saturated 0.1 M HClO4 and 0.1 M KOH electrolytes at a rotating speed of 1600 rpm and with a scan rate of 10 mV s−1 unless stated otherwise. The accelerated durability tests (ADTs) were carried out by the potential-cycling between 0.6 and 1.0 V (vs RHE) at a scan rate of 50 mV s−1 in O2-saturated electrolytes, according to U.S. Department of Energy protocol. The Koutecky−Levich (K−L) plots can be obtained at various rotating speeds. The electron transfer number (n) can be calculated from the K−L equation:

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Co−N/CNFs, CoP-N/CNFs, and Fe−N/CNFs Catalysts. Initially, 1.0 g of polyacrylonitrile (PAN) powder was dissolved into 12.5 g of dimethylformamide (DMF) under stirring at 60 °C for 2 h to form a yellow solution with a PAN content of 8 wt %. Then, 2.0 g of 4-dimethylaminopyridine (DMAP) powder and 150 mg of cobalt acetate (Co(Ac)2) or ferrous acetate (Fe(Ac)2) were added into the above solution and stirred at room temperature for 2 h. The prepared electrospun solution was transferred into a 5 mL plastic syringe to conduct a typical electrospinning at 35 °C with a relative humidity of ca. 45%. The applied voltage was 16−18 kV, and the working distance between the collector and syringe end was ∼13 cm. The supply rate for the solution was 0.12 mm min−1. The fibrous film prepared above was dried under vacuum at 60 °C for 12 h and then stabilized by annealing in a muffle at 250 °C under air atmosphere for 2 h with a heating rate of 2 °C min−1. Thereafter, the film was placed on a porcelain boat and conducted to heat-treatment at 900 °C for 2 h under N2 atmosphere with a rate of 3 °C min−1 for Co−N/CNFs, and 950 °C for 2 h with a rate of 10 °C min−1 for CoP-N/CNFs and Fe−N/CNFs. The prepared samples were immersed in 5 M H2SO4 solution and stirred at 80 °C for 24 h. The final products were washed by ultrapurewater and dried under vacuum at 60 °C. 2.2. Synthesis of Nitrogen-Doped CNFs and CNFs Catalysts. The nitrogen-doped CNFs (N/CNFs) and CNFs catalysts were synthesized with the same method as above. In brief, 1.0 g of PAN or poly(ethylene oxide) (PEO) powder was dissolved into 12.5 g of DMF at 60 °C for 2 h to form a clear solution with the polymer content of 8 wt %, respectively. After the electrospinning, high-temperature pyrolysis, and posttreatment process, the catalysts were obtained and denoted as N/CNFs or CNFs. 2.3. Physical Characterization. The transmission electron microscopy (TEM) was performed on FEI Tecnai 30F. The high-angle annular dark field scanning TEM (HAADF-STEM) images were obtained by the Titan Cubed Themis G2 300. The Bruker AXS D8 ADVANCE powder X-ray diffractometer with a Cu Kα (λ = 1.5418 Å) radiation source was used to collect the X-ray diffraction (XRD) patterns of the samples. X-ray photoelectron spectroscopy (XPS) was carried out on a Kratos AXIS UltraDLD with Al Kα radiation, referenced to C 1s binding energy (BE) of 284.45 eV. The X-ray absorption nearedge structure (XANES) and extended X-ray adsorption fine structure (EXAFS) spectra were conducted on BL14W1

1 1 1 1 1 = + = + J JL JK JK Bω1/2 B = 0.62nFC0D2/3V −1/6

where J is the apparent current density, JK and JL are the kinetic and limiting current densities, ω is the angular velocity of the disk, n is the electron transfer number, F is the 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 electrolytes (1.9 × 10−5 cm2 s−1), and V is the kinematic viscosity of the 0.1 M KOH (0.011 cm2 s−1) and 0.1 M HClO4 (0.0089 cm2 s−1). Hydrogen peroxide yields and the electron transfer number (n) can be calculated through the following equations: H 2O2 % = 200 ×

n=4×

Ir N

Id +

Ir N

Id Id +

Ir Id

where Id and Ir are the disk and ring currents, respectively. The current collection efficiency for the Pt ring is N = 0.37. The ring potential is kept at a potential of 1.35 V/RHE. 2.5. Preparation of Membrane Electrode Assembly and Performance Evaluation of the DMFC. For the passive DMFCs, Nafion 115 was selected as the proton exchange membrane, which was pretreated in 5% H2O and 0.5 M H2SO4 at 80 °C for 2 h. The membrane electrode assembly (MEA) 6865

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Figure 1. (a) TEM image and (b) enlarged TEM image of Co−N/CNFs (inset: SAED image). (c) HAADF-STEM image and (d) EDX mapping of Co−N/CNFs. (e) Aberration-corrected STEM image and (f) enlarged image of Co−N/CNFs (single Co atoms are highlighted by orange circles).

Figure 2. (a) C K-edge XANES spectra of Co−N/CNFs and N/CNFs. (b) Co K-edge XANES and (c) Fourier-transform EXAFS spectra of Co− N/CNFs and reference samples. (d) XPS survey of Co−N/CNFs (e) C 1s and (f) N 1s high-resolution XPS spectra of Co−N/CNFs.

with a 4 cm2 active area was fabricated by blade-coated method. The catalyst inks were prepared by dispersing PtRu/C (anode) and Pt/C or Co−N/CNFs (cathode) with 20 wt % Nafion for Pt/C and 33 wt % Nafion for Co−N/CNFs in IPA/H2O solution under ultrasonic for 3 h. The anode catalyst loading was 4 mg cm−2, and the cathode catalyst loading was 1 mg cm−2 for Pt/C or 4 mg cm−2 for Co−N/CNFs. Thereafter, the prepared electrodes were pressed at 135 °C with 15 kg cm−2 for 180 s. The prepared MEAs were activated in water for 12 h and 2 M methanol for 24 h, respectively. The performance of the MEA was measured on an Arbin Fuel Cell Testing System (Arbin Instrument Inc., U.S.A.). The anode was fed with 4 M methanol. The cathode was operated

under air-breathing mode. All the tests were conducted at 40 °C with a relative humidity of 30−40%. The lifetime test was performed at a constant current density of 20 mA cm−2 by continually feeding with 4 M methanol solution. 2.6. DFT Calculation. The stability of MOxN4G (x = 0, 1, 2) was investigated by calculating the free energy of ion exchange (Table S1) between metal (or oxides) ions and protons in a thermodynamic scheme: MOx N4G + 2H+ = H 2N4G + MOx 2 +

H2N4G is protonated N4G. The free energy of the reaction is ΔG = μH N G + μMO2+ − μMO N G − 2μH+ 2 4

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x

x 4

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Figure 3. (a) LSVs of ORR on the Co−N/CNFs and Pt/C (b) K−L plots derived from LSVs of ORR on Co−N/CNFs at various rotating speeds. (c) Durability test of Co−N/CNFs for 10000 cycles in O2-saturated 0.1 M HClO4. (d) LSVs of ORR on the Co−N/CNFs and Pt/C (e) K−L plots derived from LSVs of ORR on Co−N/CNFs at various rotating speeds. (f) Durability test of Co−N/CNFs for a 10000-cycle in O2-saturated 0.1 M KOH. For all tests, catalyst loading for Co−N/CNFs is 0.6 mg cm−2 (0.1 M HClO4) or 0.2 mgcm−2 (0.1 M KOH) and Pt/C is 0.1 mg cm−2, respectively. (g) STEM image of single nanofiber. (h) Aberration-corrected STEM image. (i) EDX-mapping for Co−N/CNFs after a 10000-cycle ADT test.

are uniformly distributed over the CNFs, as indicated by the energy dispersive X-ray (EDX) mapping (Figure 1d). The superposition of these heteroatoms on nanoscale indicates that Co atoms might be bonded with N or C and anchored into the carbon skeleton. Additionally, aberration-corrected HAADFSTEM images presented in Figure 1e and 1f show bright dots on the carbon matrix, clearly indicating the existence of single Co atoms. Additional HAADF-STEM images (Figure S2) of the Co−N/CNFs at different regions further prove the existence of single Co atoms within the Co−N/CNFs. XANES and EXAFS were conducted to investigate the local chemical state and coordination environment of C, N, and Co atoms in the catalysts. XANES spectroscopy at the normalized C K-edge (Figure 2a) for the Co−N/CNFs displays two main peaks at ∼284.3 eV (aromatic carbon species35,36) and ∼291.2 eV (sp3-hybridized σ* band), similar to the case for the N/ CNFs. The high peak intensity of the σ* band for the Co−N/ CNFs is attributed to the high degree of graphitization after Co doping. The N K-edge spectrum (Figure S3a) displays two peaks corresponding to pyridinic N and graphitic N, confirming

The chemical potential of a proton in solution can be calculated as μ H+ =

μH 2

2

+ RT ln c H+

3. RESULTS AND DISCUSSION The prepared Co−N/CNFs with uniform diameters of ∼150 nm are visible by TEM (Figure 1a), and no Co particles are observed. Amorphous carbon consisting of randomly orientated graphitic domains is observed in the enlarged TEM image (Figure 1b). The ring-like selected area electron diffraction (SAED, inset in Figure 1b) pattern indicates the poor crystallinity of Co−N/CNFs catalyst. XRD pattern (Figure S1) shows only two broad peaks at 26.2° and 43.3° corresponding to partially graphitized carbon, indicating that the metal aggregations can be completely removed after acidleaching. Furthermore, no obvious bright areas are observed in the HAADF-STEM image (Figure 1c), further confirming the absence of Co-containing nanoparticles. Co, N, and C elements 6867

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Figure 4. LSVs before and after a 10 000-cycle ADT of ORR on (a) CoP-N/CNFs and (b) Fe−N/CNFs in O2 saturated 0.1 M HClO4 solution at rotating rate of 1600 rpm with a scan rate of 10 mV s−1. Effect of CN− ions on the ORR activity for (c) Co−N/CNFs, (d) CoP-N/CNFs, (e) Fe− N/CNFs, (f) N/CNFs in 0.1 M HClO4.

the codoping of N element in the carbon skeleton. Furthermore, the Co K-edge XANES spectrum (Figure 2b) shows a white-line peak for the Co−N/CNFs at a position between those of Co foil and Co(Ac)2, indicating the electronic state of the Co species is between Co(0) and Co(II) with a porphyrinic planar structure for the single Co site.25 Fouriertransform EXAFS spectroscopy of the Co−N/CNFs in Figure 2c presents a primary peak located at ∼1.42 Å, corresponding to the Co−N scattering path. Compared with the spectra for Co foil and cobalt oxide, no Co−Co scattering path at ∼2.17 Å is observed, and the Co−O path is relatively weak, indicating that the single Co atom possesses a Co−N local structure in the carbon matrix, again confirming the existence of single Co

atoms. The XPS survey (Figure 2d) shows four peaks corresponding to C 1s, N 1s, O 1s, and Co 2p with weight contents of 83.86%, 2.30%, 13.20%, and 0.66%, respectively. The low peak intensity of Co element is mainly due to the low doping content. The C 1s spectrum (Figure 2e) can be deconvoluted into four peaks, corresponding to CC (284.6 eV), CN (285.7 eV), C−N (286.5 eV) and O−CO (287.8 eV), respectively. The Co 2p3/2 signal (Figure S3b) centering at 779.7 eV is situated between Co (II) and Co (0) with a binding energy of Co 2p3/2 at 780.6 and 778.5 eV, respectively,23 which is in good agreement with the XANES results. The N 1s spectrum (Figure 2f) is deconvoluted into pyridinic-N, pyrrolicN, graphitic-N, and oxidized-N37 with the relative percentage of 6868

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Figure 5. (a) Relative ion-exchange energy profiles of FeN4-G and CoN4-G with protons after the adsorption of molecular and atomic oxygen. (b) Relative energy profiles for the ORR processes on FeN4-G and CoN4-G. (c) Steady-state polarization curves of passive DMFCs with Pt/C (1.0 mg cm−2) and Co−N/CNFs (4 mg cm−2) as cathode catalysts fed with 4 M methanol at 40 °C. (d) The 200-h stability test of a passive DMFC with the cathode catalyst prepared by Co−N/CNFs at a current density of 20 mAcm−2.

with a negligible negative shift of E1/2 after a 10 000-cycle ADT, which is much superior to Pt/C with a ∼40 mV shift of E1/2 even after a 3000-cycle ADT (Figure S6a). In basic electrolyte, the Co−N/CNFs simultaneously display efficient ORR activity with an onset potential of 0.92 V and E1/2 of 0.82 V (Figure 3d), a nearly 4-electron process (inset in Figure 3e) and superior durability with nearly a 1 mV negative shift of E1/2 (Figure 3f) when comparing the Pt/C catalyst (Figure S6b). The morphology and element composition of Co−N/CNFs catalyst after the durability test are investigated by STEM, EDX-mapping, and XPS analysis. The results show that no metal aggregation but single Co atoms are identified along the Co−N/CNFs with the diameter of ∼150 nm after 10 000 potential cycling, as indicated in Figure 3g,h. The ICP analysis indicates that nearly no Co species was detected in the solution after ADT test, verifying the structural stability of single Co atoms anchored in carbon skeleton. Besides, the Co, N atoms are still homogeneously distributed over the fiber as shown in Figure 3i. The element contents derived from EDX-mapping (Table S1) and XPS survey (Figure S7a) remain unchangeable in comparison with the pristine Co−N/CNFs. The N 1s XPS (Figure S7b) spectrum also displays similar N-functionalities and ratios, revealing that the nitrogen species are intact. These results mentioned above strongly assess the excellent structural stability of the Co−N/CNFs catalyst, which would facilitate the outstanding ORR durability.

31.56%, 12.21%, 48.72% and 7.50%, respectively. Clearly, pyridinic N and graphitic N are the dominant peaks in the N 1s XPS spectrum, which not only are favorable for ORR activity38−40 but also serve as anchoring sites for single Co atoms.23 The ORR characteristics of the Co−N/CNFs and Pt/C catalysts are evaluated by linear sweep voltammetry (LSV) in O2-saturated 0.1 M HClO4 solution, as illustrated in Figure 3a. As expected, the Co−N/CNFs show a desirable ORR activity, with an onset potential and E1/2 of 0.82 and 0.70 V/RHE, respectively. The ORR activity is rather good but still lower than that of Pt/C with an onset potential and E1/2 of 0.94 and 0.83 V, respectively. For a better comparison, the CNFs and N/ CNFs were prepared to evaluate the catalytic activity of N and Co dopants in the carbon skeleton. As shown in Figure S4a, the ORR activity in acidic solution follows the order of CNFs < N/ CNFs < Co−N/CNFs. The CNFs without N and Co shows nearly inactive toward ORR, definitely indicating that the desirable ORR activity of Co−N/CNFs is mainly attributed to the codoping of N and single Co atoms. From K−L plots, derived from the LSVs at different rotating speeds in Figure 3b, the calculated electron-transfer number (inset in Figure 3b) for the ORR on the Co−N/CNFs is ca. 3.4. The RRDE test (Figure S4b) shows a H2O2 yield of ∼30%. Although the H2O2 yield is high, the Co−N/CNFs exhibit excellent H2O2 tolerance in comparison with the Pt/C catalyst (Figure S5). Strikingly, the Co−N/CNFs exhibit outstanding durability (Figure 3c), 6869

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better ORR activity than the CoN4-G model, which is in fairly good agreement with our experimental results. On the basis of its good ORR activity, excellent durability, and methanol resistance (Figure S12), the Co−N/CNFs was employed as the cathode catalyst in the passive DMFCs. The open-circuit voltage and the maximum power density of a passive DMFC at 40 °C (Figure 5c) with Pt-free Co−N/CNFs cathode are 0.91 V and 16 mW cm −2 , respectively; demonstrating a comparable performance to the case with Pt/C catalyst (0.75 V and 25 mW cm−2). The lifetime test (Figure 5d) of the passive DMFC shows a nearly constant cell voltage of ∼0.30 V after discharging for 200 h, indicating the excellent stability of the cathode. Therefore, the Co−N/CNFs could be a promising ORR candidate to replace the Pt/C in DMFCs.

To further clarify the unique effect of the single Co atoms on the ORR durability in acidic electrolyte, Co nanoparticles and N codoped CNFs (CoP-N/CNFs) was prepared. The existence of Co nanoparticles can be confirmed by XRD and EXAFS (Figure S8a, S8b). Clearly, the ORR on CoP-N/CNFs shows a ca. 34 mV negative shift of E1/2 after a 10 000-cycle ADT (Figure 4a), further supporting the stability of single Co atoms anchored in the carbon matrix. For comparison, Fe and N codoped CNFs (Fe−N/CNFs) catalyst was also prepared to examine the difference between Co- and Fe-based TM-N/C catalysts. Obviously, The Fe−N/CNFs catalyst displays more positive ORR onset potential (0.87 V/RHE), higher electrontransfer number calculated by K−L equation (∼3.8, Figure S9a), and lower H2O2 yield (∼11%) monitored by RRDE test as shown in Figure S9b than Co−N/CNFs. However, a ∼59 mV negative shift of E1/2 on the Fe−N/CNFs (Figure 4b) is observed after 10 000-cycle ADT, indicating inferior stability. Therefore, the comparison of CoP-N/CNFs and Fe−N/CNFs directly validates the effectiveness of single atoms and Co-based CNFs for improved durability in acidic solution. To explore the origin of the high durability, we probed the possible role of transition metal within the active sites of the TM-N/C catalysts in acidic solution. As expected, the Co−N/ CNFs catalyst shows ORR activity degradation after the addition of 5 mM CN− (Figure 4c), SCN−, or EDTA− (Figure S10) ions into 0.1 M HClO4 solution. Similar decays are observed for CoP-N/CNFs (Figure 4d, Figure S11a) and Fe− N/CNFs (Figure 4e, Figure S11b), thus indicating that the transition metals are at least part of the ORR active sites. Interestingly, the ORR activity on the Co−N/CNFs and CoPN/CNFs can gradually recover to their original levels upon using fresh electrolyte, indicative of good structural stability of the Co-containing active site. In contrast, the ORR activity on the Fe−N/CNFs is unrecoverable. Furthermore, N/CNFs (Figure 4f) exhibits poor ORR activity, but no obvious activity decay is observed after the addition of the poisoning ions, again assessing the crucial role of transition metal during the ORR. However, the roles of N and C species during the ORR still remain unclear.41,42 To further identify the origins of the structural stability of the Co−N/CNFs, we conducted DFT calculations on TM-N4graphene (G) models. Figure 5a shows the ion-exchange energy landscapes for FeN4-G and CoN4-G after the adsorption of molecular and atomic oxygen under acidic environment. After geometry optimization, O2 adsorbs in parallel and end-on modes for FeN4-G and CoN4-G, respectively (insets in Figure 5a), although the initial configurations of these adsorptions are the same. The parallel O2 adsorption causes an obvious increase of the ion-exchange energy (∼0.7 eV) for FeN4-G, whereas the end-on O2 adsorption for CoN4-G slightly lowers this energy (Table S2). In other words, the Fe-containing center becomes less stable in the N4-G structure after O2 adsorption. By contrast, the structural stability of CoN4-G slightly increases after the adsorption of O2 and O, which could explain the excellent ORR durability of the Co−N/CNFs catalyst in acidic electrolyte. Furthermore, the free-energy diagrams of the ORR as indicated in Figure 5b, were calculated to compare the catalytic activity between the Co- and Fe-based TM-N/C catalysts. The green line represents the theoretical ideal freeenergy diagram of the ORR, with an equivalent free-energy drop for each step43 upon electron transfer. Obviously, FeN4-G possesses the closest profile to the ideal diagram, indicating a

4. CONCLUSIONS In summary, we have successfully synthesized single Co atom and N codoped carbon nanofibers with a high durability and desirable ORR activity in both acidic and alkaline electrolytes. The formation of single Co atoms within the catalysts has been confirmed by aberration-corrected HAADF-STEM and EXAFS. The ORR E1/2 value on Co−N/CNFs remains nearly unchanged after a 10 000-cycle ADT, which is the most stable performance for TM-N/C catalysts so far. The unprecedented durability of the Co−N/CNFs is well understood with the intrinsic structural stability of the Co-containing active site in acidic solution by the comparative DFT calculations on FeN4-G and CoN4-G models. A passive DMFC with the Co−N/CNFs cathode exhibits the excellent stability as expected and the comparable performance to the case with Pt/C catalyst. From an important aspect of the structure−activity-durability relationship for Co−N/CNFs catalysts, it is anticipated that single-atom Co rather than Fe could be an efficient active component for the development of durable TM-N/C catalyst for a wide range of important energy conversion reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02326. Catalysts preparation details; physical and electrochemical measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zheng Hu: 0000-0002-4847-899X Hui Yang: 0000-0001-5013-0469 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Research programs (SQ2017YFJC020259), Natural Science Foundation of China 216673275) and the BL14W1 beamline at Synchrotron Radiation Facility (SSRF). 6870

National Key the National (21533005, the Shanghai

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ACS Catalysis

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DOI: 10.1021/acscatal.7b02326 ACS Catal. 2017, 7, 6864−6871