A Universal Method to Engineer Metal OxideâMetalâCarbon Interface

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A Universal Method to Engineer Metal Oxide-MetalCarbon Interface for Highly Efficient Oxygen Reduction Lv Lin, Dace Zha, Yunjun Ruan, Zhishan Li, Xiang Ao, Jie Zheng, Jianjun Jiang, Hao Ming Chen, Wei-Hung Chiang, Jun Chen, and Chundong Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01056 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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A Universal Method to Engineer Metal Oxide-Metal-Carbon Interface for Highly Efficient Oxygen Reduction Lin Lv,† Dace Zha,† Yunjun Ruan,† Zhishan Li,† Xiang Ao,† Jie Zheng,§ Jianjun Jiang,† Hao Ming ∥

⊥

Chen, Wei-Hung Chiang, Jun Chen,*, ‡ and Chundong Wang*, †

†

School of Optical and Electronic Information, Huazhong University of Science and Technology,

Wuhan 430074, China. ‡

Department of Materials Science and Engineering, Stanford University, Stanford, California

94305, USA. §

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

∥

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.

⊥

Department of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei 10607, Taiwan. *Correspondence to: [email protected] (C.D.W.); [email protected], [email protected] (J.C.)

ABSTRACT: Oxygen is the most abundant element in the Earth’s crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report an N-doped carbon encapsulated CeO2/Co interfacial hollow structures (CeO2-Co-NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. Firstly, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO2. Secondly, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO2 and Co in CeO2/Co interface are unveiled theoretically by density functional theory (DFT) calculations to justify the increased oxygen absorption for ORR activities improvement. The 1

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reported CeO2-Co-NC hollow nanospheres not only exhibit decent ORR performance with a high onset potential (922 mV vs. RHE), half-wave potential (797 mV vs. RHE) and small Tafel slope (60 mV dec-1) comparable to those of the state-of-the-art Pt/C catalysts, but also possess long-term stability with a negative shift of only 7 mV of the half-wave potential after 2000 cycles and strong tolerance against methanol. This work represents a solid step toward high-efficient oxygen reduction.

KEYWORDS: oxygen reduction, interface, cerium oxide, cobalt, electron interactions

Nowadays, with the threatening of climate change and energy crisis, the increasing demand for clean and sustainable energy sources has become a strong driving force in continuing economic development.1-5 The rapid development of stable, environment-friendly and highly efficient energy conversion/storage devices provide a superior solution to these issues, especially the fuel cells and metal-air batteries, where oxygen reduction reaction (ORR) plays a critical role.6-9 Exploring high-efficient, durable, yet cost-effective catalysts remains highly desired and a challenge.10-13 Traditional active noble metal catalysis, by using such as Pt and Pd, is rapidly developing recently owning to its dominated 4e-ORR process and excellent ORR performance.14-16 However, its further development might be shadowed by high-cost, poor long-term durability, weak tolerance against chemical corrosion such as methanol poisoning.17-19 What is more, the cost of noble metals largely hinders the future large-scale application.20 Cerium oxide (CeO2), benefiting from its high oxygen-storage capacity, remarkable redox competence (Ce3+ ↔ Ce4+) and intrinsically decent electronic/ionic conductivity, which are favorable for ORR catalytic activities and long-term stability.21,22 Compared with CeO2 (110) and (100) surfaces, CeO2 (111) surface exhibits lower surface energy and thus is more stable but could 2


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adsorb O2 in virtue of the coexistence of Ce3+ ion to easily form superoxide on CeO2 (111) surface and Ce3+ ion exhibits increased oxygen adsorption energy due to the synergy, which makes it superior as ORR catalyst.23-25 To engineer the CeO2-based catalysts for ORR, considerable efforts have been devoted into the community.26-30 However, the CeO2-based catalytic efficiency is still insufficient for industrial production and the catalytic mechanism remains unclear. Herein, we report a generalized and facile approach to prepare the hierarchical structure of N-doped carbon-shell protected CeO2/Co (CeO2-Co-NC) hollow spheres with greatly improved oxygen reduction performance. Compared to the commercial Pt/C catalyst, CeO2-Co-NC exhibits high ORR catalytic activity with a high onset potential of 922 mV (vs. RHE), a positive half wave-potential of 797 mV (vs. RHE), diffusion-limited current density of 5.37 mA cm-2, small Tafel slope of 60 mV dec-1 and delivers a decent durability (the half-wave potential exhibits a negative shift of only 7 mV after 2000 cycles). The enhanced ORR performance was attributed to the interfacial effect, boosting the first-order reaction kinetics in virtue of charge redistribution between Co and CeO2. In addition, density functional theory (DFT) calculations were employed to verify the fact that stronger oxygen adsorption was occurred compared with the pristine CeO2 case due to the improved electrical conductivity and intrinsic charge redistribution at the engineered interface of CeO2/Co. With a collection of compelling features, including high-efficiency, low-cost and decent stability, this work is a great advancement for CeO2-based oxygen reduction.

RESULTS AND DISCUSSION The synthesis route of CeO2-Co-NC is illustrated in Figure 1. The CeO2 hollow nanospheres (CeO2-HSs) were firstly synthesized via a facile hydrothermal approach with SiO2 as sacrificial 3


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templates and a subsequent chemical etching process. A detailed synthesis process was presented in the Methods. Subsequently, a different dose (1.4, 2.8 and 5.6 mmol) of Co(NO3)2·6H2O and 250 mg dopamine hydrochloride (DA) was added into the alkaline solution, respectively. The mixed solution was then vigorously stirred for 8 h at room temperature to achieve uniform adsorption of Co2+ ions onto the CeO2-HSs surface. The structural rearrangement in alkaline media will result in the formation of polydopamine (PDA) at the external via a self-polymerization process. With that, the precursor CeO2-Co2+-PDA was produced. Finally, a carbonization treatment was implemented in Ar atmosphere to obtain the carbon encapsulated CeO2/Co heterostructures. To verify it, control experiments were carried out (see Methods for details). To clarify the crystalline structures of the as-prepared samples, powder XRD patterns were collected. As shown in Figure 2a, on one hand, typical peaks located at 44.2°, 51.5° and 75.9° were observed in sample CeO2-Co-NC, indexing to the (111), (200) and (220) plane of face-centered cubic (fcc) metallic cobalt (JCPDS 15-0806). On the other hand, the strong peaks at 28.6°, 33.1°, 47.5° and 56.3°can be respectively indexed to the (111), (200), (220) and (311) plane of fcc-CeO2 (JCPDS 81-0792). While another identified broad peak around 24° was associated to graphitic carbon.31-33 For sample CeO2-Ni-NC, three peaks at 44.4°, 51.6° and 76.1° are consistent with the metallic nickel (JCPDS 01-1258), which affirms that the XRD pattern of CeO2-Ni-NC is quite similar to that of CeO2-Co-NC. However, no carbon features can be identified in XRD pattern of CeO2-Ni-NC, the reason of which may be assigned to the different crystallines of metals and/or the different amount of adsorbed metal ions during the stirring process for the sample preparation. This observation suggests that this simple and straightforward route is a universal method to fabricate metal oxide-metal-carbon (MO-M-C) hierarchical structures. To prove, more 4


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samples, such as CeO2-Co-NC, CeO2-NC, Co-NC, CeO2-Cu-NC, CeO2-Ag-NC, CeO2-Co2+-PDA, CeO2-Co3O4 were prepared and characterized (Figure S1-S2). Raman spectra were also performed for the samples of CeO2, CeO2-Ni-NC and CeO2-Co-NC. One strong peak at around 462 cm-1 was observed in all the three samples (Figure 2b), which is ascribed to the host material, CeO2. The identified D and G peaks at 1348 cm-1 and 1594 cm-1 in sample CeO2-Ni-NC, and the D and G peaks at 1356 cm-1 and 1596 cm-1 in sample CeO2-Co-NC justify the existence of the carbon layer in CeO2-Ni-NC and CeO2-Co-NC.34 The high ratios of D peak to G peak (ID/IG) (1.05 for CeO2-Ni-NC and 1.04 for CeO2-Co-NC) confirm the presence of structural defects in carbon layer assigned to the nitrogen bonded sp2-C and sp3-C species.35,36 In addition, Raman spectra of CeO2-Cu-NC, CeO2-Ag-NC and single NC were shown in Figure S3

for comparison. N2

adsorption measurements were further carried out to disclose the porous structure. A distinct hysteresis loop at relative pressures (P/P0) from 0.5 to 1.0 in the isotherms of CeO2-Co-NC (Figure 2c) and CeO2-NC (Figure S4a), indicating the presence of mesoporous structure in them. While the mesoporous structure was not found in sample Co-NC (Figure S4b), which manifests that the porous structure is originated from hollow CeO2 matrix. The pore size distribution of CeO2-Co-NC (inset of Figure 2c) is centered around 31.8 nm, which is more positive than that of CeO2-NC (around 4.2 nm) (inset of Figure S4a). The Brunauer-Emmett-Teller (BET) surface area and the total pore volume of CeO2-Co-NC were assessed to be 14.4 m2 g-1 and 0.056 cm3 g-1, which are respectively much smaller than those of CeO2-NC (134.3 m2 g-1 and 0.085 cm3 g-1). Although the incorporation of metallic Co nanocrystals into CeO2-NC sharply decreases the BET surface area and the total volume, nonetheless, the overall ORR performance of CeO2-Co-NC is effectively improved. The possible reason could be attributed to the synergistic effect between 5


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CeO2 and Co, which overcomes the negative effect of reduced BET surface area as well as the total volume.37,38 Furthermore, to investigate the substantial variation of the precursor CeO2-Co2+-PDA and CeO2-Co-NC during the pyrolysis process, thermogravimetric analysis (TGA) was performed (Figure S5). The black curve shows that PDA starts to collapse at 210 oC with a continuous weight loss of ~28% until 290 oC. The reduction of cobalt ions and the formation of graphitic carbon almost accomplish at above 300 oC. In addition, the weight variation (vs. temperature) of CeO2-Co-NC upon pyrolyzation in air was also studied. The weight loss caused by the decomposition of carbon is less than the weight increase resulted from the oxidation of Co. This indicates a low content of carbon in CeO2-Co-NC. X-ray photoelectron spectroscopy (XPS) was further performed to understand the chemical states of the as-prepared CeO2-Co-NC. As shown in Figure S6a, Ce, Co, C, and O were well discerned in the survey spectrum. The core level spectra of Ce 3d for CeO2-Co-NC and CeO2-NC is shown in Figure 2d, where eight peaks, locating at 917.3 eV, 908.1 eV, 901.7 eV, 900.0 eV, 898.7 eV, 889.1 eV, 884.8 eV and 883.0 eV (labelled as u1, u2, u3, u4, v1, v2, v3 and v4, respectively) are resolved. Among them, the peaks labelled as v3 and u3 are ascribed to be the state of Ce3+, while other six peaks are associated to the state of Ce4+.39,40 These results validate that Ce3+ and Ce4+ are coexisted in CeO2-Co-NC. It is universally acknowledged that the presence of Ce3+ ions in CeO2 could lead to charge imbalance, and create oxygen vacancies, of which it depends on the concentration of Ce3+ ions.41 The concentrations of Ce3+ ions ρ(Ce3+) in CeO2-NC and CeO2-Co-NC were estimated from the ratio of integrated Ce3+ peaks A(Ce3+) to the sum of Ce3+ and Ce4+ peaks A(Ce4+), as follows: ρ(Ce3+) = [A(Ce3+)]/[ A(Ce3+)+ A(Ce4+)]

(1) 6


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ρ(Ce3+) in CeO2-NC and CeO2-Co-NC were ~0.1528 and ~0.2561, respectively. Given that CeO2-Co-NC holds much more Ce3+ ions than that in CeO2-NC, it evidentially proves that the charge redistribution occurred between CeO2 and Co at the interface to induce abundant oxygen vacancies. In the core level spectrum of Co 2p (Figure 2e), two major peaks at 781.1 eV and 796.9 eV were identified, accompanying with two satellites peaks respectively at 786.1 eV and 803.6 eV, which corresponds to the state of metallic Co.42 In comparison to Co-NC, the Co 2p3/2 and Co 2p1/2 peaks in the XPS spectrum of CeO2-Co-NC positively shift ~0.57 eV, which unequivocally corroborates the strong electron interactions between CeO2 and Co in the as-prepared CeO2-Co-NC. In Figure 2f, it shows that the C1s spectrum of CeO2-Co-NC was deconvoluted into four types of carbon species respectively located at 284.8 eV, 285.6 eV, 286.4 eV and 289.1 eV, which respectively corresponds to C=C (C(i)), C=N&C-O (C(ii)), C-O-C&C-N (C(iii)) and – O-C=O (C(iv)).43,44 The deconvolution results of C1s indicates the formation of heteroatomic bond between oxygen and nitrogen. Besides, the O 1s and N 1s core levels of CeO2-Co-NC are also collected and shown in Figure S6b-c. Particularly, N1s spectrum was deconvoluted into three peaks at 398.9 eV, 400.4 eV and 401.3 eV, respectively, which are ascribed to pyridinic-N, pyrrolic-N and graphitic-N.45,46 Elemental composition of the as-prepared CeO2-Co-NC was analyzed by XPS (Table S1, Supporting Information). Further insight into the synergy between CeO2 and Co was obtained by probing the oxidation state of CeO2 and Co with X-ray adsorption near-edge structure (XANES) spectroscopy. When Co nanoparticles were introduced into the CeO2-NC HSs, the Ce-L3 edge shifted to lower energy (Figure 2g), indicating more Ce3+ was formed; on the other hand, this shift behavior also informs the fact that charges were transferred from Co nanoparticles to CeO2.47,48 In Figure 2h, it also shows that the Co k-edge XANES bands 7


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of catalyst Co-NC was negatively shifted compared to the standard Co foil, suggesting less oxidation states of Co species in Co-NC, the reason of which could be assigned to the protection of the outer carbon layer from being oxidized.49,50 It is worth nothing that some obvious positive shifts of Co k-edge bands (highlighted with dash line) were observed (CeO2-Co-NC vs. Co-NC), verifying the increased oxidation states of Co species after the combination with CeO2 matrix. The Fourier transformed extended X-ray adsorption fine structure (FT-EXAFS) patterns were further collected and shown in Figure 2i, where it shows that the intensity of Co-Co bonding was decreased in sample CeO2-Co-NC compared with the cases of standard Co foil and Co-NC. This observation again informs the formation of Co/CeO2 interface.51 The aforementioned XANES analysis is consistent with the XPS result, which evidences the existence of CeO2/Co interface in our as-prepared CeO2-Co-NC product. The structures and morphology of the as-prepared samples were unveiled by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Sacrificial templates, i.e. SiO2 nanospheres, were synthesized via a modified Stöber method, the diameters of which are about 300 nm (Figure S7). CeO2 hollow nanospheres (HSs) were prepared by firstly depositing CeO2 onto the SiO2 nanospheres surface and then removing SiO2 nanospheres by chemical etching. As shown in Figure 3a, the as-prepared CeO2-HSs are uniform nanosphere with diameters of about 400 nm. The broken ones clearly demonstrate the hollow feature with a wall thickness of ~ 45 nm (Figure S8a). The high-resolution SEM image of broken CeO2-HSs (Figure 3b) indicates that the sphere walls are composed of stacked octahedrons (see the model in the bottom-left inset of Figure 3b). Actually, the CeO2 favors to form octahedron structure in our set growth condition, of which it was confirmed by the 8


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TEM (Figure S8b-c) and SEM images (Figure S9) of the new synthesized CeO2 with the quite similar procedure for preparation of CeO2-HSs (only without SiO2 templates). The formation of the octahedrons is directly induced by intrinsic growth mechanism under the settled condition.52 Structurally, the Co2+ ions in the CeO2-Co2+-PDA precursor were homogeneously adsorbed on the CeO2-HSs surface and then PDA was aggregated at the outer surface of the hybrid CeO2-Co2+ by a wet-chemical approach. To synthesize CeO2-Co-NC, a simple annealing treatment was performed in Argon atmosphere at 800 oC and Co2+ ions on the surface of CeO2-HSs were directly reduced to metallic Co because of the reductive nature of the formed graphitic carbon at the outer surface during the carbonization process, thus forming hierarchical structured CeO2-Co-NC. Clearly, the CeO2-Co-NC inherits the sphere structures of the parent CeO2-HSs and the gear-like corner angles (Figure 3c). An enlarged SEM image of CeO2-Co-NC sphere was demonstrated in Figure 3d, where diverse hillside-like bulges were observed. The external diameters of the hollow nanospheres were increased from 400 nm to 425 nm upon both decorating metallic Co layer and carbon layer coating. In contrast, carbon encapsulated cobalt nanocrystals (termed as Co-NC) were synthesized following the same procedure without adding CeO2-HSs (Figure S10). CeO2-HSs exhibited uniform hollow nanospheres sized of 400 nm with hackly wall as evidenced from the TEM images (Figure 3e). Additionally, the white suspension shown in the inset of Figure 3e verifies that CeO2-HSs are of good hydrophilous. As the TEM image of CeO2-Co-NC shown in Figure 3f, similar to the CeO2-HSs, CeO2-Co-NC inherits the hollow structure nanospheres with quite uniformity, and the photograph (inset of Figure 3f) of the CeO2-Co-NC suspensions indicates its good dispersity for water. The gear-like nanostructures can be identified at the edges of CeO2-Co-NC HSs (Figure 3g-h), which further confirms our conclusion that the CeO2-Co-NC 9


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HSs exactly inherit the shape features of CeO2-HSs. These morphology characterizations could prove that the Co nanoparticles are ultrafine on a very thin layer of carbon. A high-resolution TEM (HRTEM) image was recorded at one edge of CeO2-Co-NC HSs (Figure 3h). The measured interplanar spacing of 0.20 nm and 0.31 nm are respectively assigned to Co and CeO2 , which are indexed well to Co (111) and CeO2 (111) planes (Figure S11). The interfacial structure of CeO2 nanooctahedrons and Co nanoparticles can be further verified by TEM images in other selected regions (Figure S12). Remarkably, an ultrathin graphitic carbon layer was clearly observed at the outmost, serving as the effective physicochemical protection for Co from oxidation and dissociation to electrolytes but this protection would not impede the activation of O2.50,53 In addition, the lattice parameter of the Co nanocrystal is further confirmed by fast Fourier transform (FFT) (Figure 3i) and the plane distance is measured to be 0.20 nm, which is in well consistent with the data of Co (111). Furthermore, as the selected area electron diffraction (SAED) patterns shown in Figure 3j, the cyclically arranged diffraction spots with blurry rings suggest the monocrystal structure nature of Co and the polycrystalline structure feature of CeO2. This is further evidenced by the SAED patterns of CeO2-HSs (Figure S8d). The element composition and distribution of CeO2-Co-NC was validated by TEM energy-dispersive X-ray spectroscopy (TEM-EDS). Composition line-scan profiles across the CeO2-Co-NC HSs were captured (Figure 3k), where we determined that the CeO2/Co interface is surrounded by thin carbon layer since strong carbon peaks were identified in the edges parts compared with the central ones. High angle annular dark field (HAADF) image and the corresponding element mapping were collected (Figure 3l and Figure S13a). It shows that Ce, Co, O, C and N elements were evenly distributed in the entire hollow sphere, indicating that CeO2-Co is actually encapsulated by an N-doped thin 10


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carbon layer, which was in line with energy dispersive X-ray spectroscopy (EDS) (Figure S13b). To investigate the electrocatalytic ORR activity, cyclic voltammetry (CV) tests, rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements were systematically carried out in 0.1 M KOH aqueous solution. In Figure 4a and Figure S14, the cathodic peaks for all the samples are discriminately enhanced in O2-saturated electrolyte in comparison with that in Ar-saturated solution, implying their different catalytic activities. Of all the CV curves of the as-prepared catalysts, the position of cathodic peaks of CeO2-Co-NC are the closest one to the commercial Pt/C catalyst. Linear Sweep Voltammetry (LSV) curves were also examined to evaluate the ORR performance of the CeO2-Co-NC. As shown in Figure 4b and Figure S15, the performance of CeO2-Co-NC is superior to all the other prepared catalysts such as CeO2, CeO2-NC, CeO2-Ni-NC, CeO2-Cu-NC, CeO2-Ag-NC, NC, Co-NC and CeO2-Co3O4. Then the CeO2-Co-NC hollow nanospheres deliver a more positive onset potential of 922 mV (vs. RHE) and half-wave potential of 797 mV (vs. RHE) as well as larger diffusion-limited current density of 5.37 mA cm-2. More detailed results were presented in Table S2. To investigate the effect of Co content on the ORR performance of CeO2-Co-NC, RDE measurements of CeO2-Co-NC with halved Co content (denoted as CeO2-Co0.5-NC) and CeO2-Co-NC with double Co content (termed as CeO2-Co2-NC) were conducted. As shown in Figure S16, it shows that the ORR performance is sharply decreased when Co content is doubled, while it is slightly decreased when Co content was reduced by half. This observation informs that the improved electrocatalytic performance is ascribed to the synergy between CeO2 and cobalt. At a scan rate of 5 mV s-1, the LSV curves of the catalysts at various rotating rates in a three-electrode system in O2-saturated alkaline solution were demonstrated in Figure 4c and Figure S17. Here, the Koutecky-Levich 11


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(K-L) equation is utilized to estimate the ORR kinetics. The K-L plots derived from RDE curves are presented in Figure 4d. The linearity and parallelism characteristics of the K-L plots indicate the first-order reaction kinetics with respect to the electron transfer numbers n at the potential of 0.5 V (vs. RHE).54 The electron transfer numbers n of CeO2, CeO2-NC, Co-NC, CeO2-Ni-NC, CeO2-Co-NC and 20 wt % Pt/C at 0.5 V (vs. RHE) were calculated to be 2.70, 3.62, 3.83, 2.91, 3.98 and 4.09, respectively. Interestingly, the electron transfer numbers of CeO2-Co-NC (n = 3.98) is significantly increased compared with those of the pristine CeO2 (2.70) and CeO2-NC (3.62), suggesting the effectivity of CeO2-Co interface in CeO2-Co-NC and a superior ORR activity of CeO2-Co-NC. On one hand, the enhanced electron transfer numbers of CeO2-Co-NC was attributed to the interface between CeO2 and Co, which allows electron to transfer and leads to more oxygen deficiency in CeO2. On the other hand, the high electron transfer numbers of CeO2-NC (3.62) compared with that of CeO2 (2.70) and CeO2-Ni-NC (2.91) signify the strong interactions between CeO2 and N-doped carbon, which indicates that the excellent electrocatalytic performance of CeO2-Co-NC could show positive impact on the interactions between CeO2 and carbon. To further confirm the ORR reaction pathway of the above catalysts, the RRDE technique is implemented to monitor the generation of peroxide (HO2-) and examine the electron numbers at various potentials (Figure S18 and S19a). As shown in Figure 4e and Figure S19b, the HO2- yield of CeO2-Co-NC ranges from 10.9% to 19.8% and the corresponding electron transfer number n is in the range of from 3.61 to 3.78, which are obviously superior to those of other samples for contrast. These parameters of CeO2-Co-NC are comparable to those of the commercial Pt/C (HO2yield: 8.7% - 16.6%; n: 3.66 - 3.82), which is consistent with the results of K-L plots based on the 12


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RDE measurements. More detailed experimental results for control groups are presented in Table S3. These results indicate that CeO2-Co-NC are endowed with a four-electron transport throughout the ORR process, which is the same with the commercial 20% Pt/C. To compare the ORR kinetics, the Tafel plots were converted from LSV curves recorded at 1600 rpm (Figure S20). The Tafel slope of CeO2-Co-NC was calculated to be ca. 60 mV dec-1, smaller than that of CeO2-NC (94 mV dec-1) and Co-NC (153 mV dec-1), indicating a faster ORR kinetic process of CeO2-Co-NC owing to the CeO2-Co heterostructures. The cycling experiment was carried out by cyclic voltammetry (CV) to assess the stability of CeO2-Co-NC. As shown in Figure 4f, after 2000 continuous cycles the half-wave potential potential exhibited a small negative shift of 7 mV. Chronoamperometric test was further conducted to evaluate the tolerance to methanol (Figure S21). No obvious current decay is observed when 2 M methanol is added into the 0.1 M O2-saturated KOH electrolyte at 2000s, whereas Pt/C suffers from a sharp current decrease (only 50% retention), further manifesting that CeO2-Co-NC not only possesses long-term stability but also have strong tolerance against the methanol poisoning. To demonstrate the advantage of our prepared CeO2-Co-NC, the ORR performances of some very recent CeO2-based catalysts are listed in Table S4. Since oxygen adsorption at the initial step is known to be the rate-determining step (RDS) towards ORR, particularly for oxide catalysts,55 thus, the overpotential for ORR is closely related to the proton and electron transfer to the adsorbed oxygen or hydroxide being bonded to the catalyst surface.56 To theoretically clarify the working mechanism and the role of the interfacial structure of CeO2-Co on the enhanced ORR activity, a DFT-D3 method was employed to evaluate the van der Waals effect on the adsorption of O atoms on both the CeO2 (111) and CeO2 (111)/Co 13


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(111) surfaces as well as the corresponding charge density distributions. A detailed introduction of the computational method was presented in the Methods. The optimized structures of oxygen species on CeO2 (111) and CeO2 (111)/Co (111) heterostructures and other structures for DFT calculations are shown in Figure 5a, Figure 5b and Figure S22. For comparison, the adsorption energy of oxygen species on the (111) surface of pure metallic Co were calculated to be ∆Ead = -0.8 eV, suggesting a very strong adsorption of oxygen species, being with negative impact on the consequent ORR process. After decorating Co nanoparticles on CeO2 (111) surface, the CeO2 (111)/Co (111) heterostructures exhibit a moderate adsorption of oxygen species (∆Ead was decreased from 2.67 eV to 1.39 eV), which facilitates the electrocatalytic ORR process. More values of ∆Ead for other structures were presented in Figure S22. This observation proves the importance of the engineered interface and the exposed sites toward high-performance electrocatalytic activities. Charge densities at CeO2 (111)/Co (111) interface were calculated to substantiate the elusive electronic interactions between CeO2 and Co. As shown in Figure 5c, it is exactly the charge redistribution between CeO2 and Co that gives rise to an electron-rich region on CeO2 and a hole-rich region on Co, corresponding to the newborn Ce3+ peak in PDOS around Fermi level, i.e. the formation of a reduced CeO2 (111). The newborn Ce3+ plays a key role in the ORR performance enhancement, which is well consistent with experimental results.57 As shown in Figure 5d, it is observed that the total density of states (TDOS) for CeO2 (111)/Co (111) heterostructures is nonzero at the Fermi level, indicating it is a more conductive structure compared with CeO2 (111) slab. Moreover, a newborn pronounced Ce3+ peak was observed in the partial DOS (PDOS) of CeO2 (111)/Co (111) around Fermi level (Figure S23, Supporting Information), which indicates that more preferred adsorption sites for oxygen species were created. 14


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This is well consistent with the aforementioned discussion.23,25

CONCLUSIONS In summary, we report a universal method to engineer metal-oxide metal-carbon interface for high-efficiency and stable oxygen reduction with tunable morphology and well-distributed active sites. By tuning the content of Co nanocrystals dispersed on the CeO2 surface, the appropriate charge redistribution could occur at the CeO2/Co interface. The CeO2/Co interface encapsulated by N-doped carbon (CeO2-Co-NC) exhibits superior ORR performance, even comparable to the commercial Pt/C catalyst. With a strong tolerance for methanol poisoning, the CeO2-Co-NC also delivered a decent durability (the half-wave potential exhibited a negative shift of 7 mV after 2000 cycles). As a systematical investigation, the working mechanism of enhanced ORR activity was clarified by DFT calculations. In addition, the synthesis strategy in this work can be extensively applied to prepare various carbon coated metal oxide-metal interface structures. We expect that the reported method in this work will be widely adopted toward low-cost and high-efficiency electrocatalysts.

METHODS Synthesis of cerium oxides hollow nanospheres. Silica (SiO2) nanospheres were used as templates to synthesize CeO2 hollow nanospheres (HSs). Firstly, SiO2 nanospheres were synthesized with a modified Stöber method.58 CeO2 HSs was prepared according to a previous method.59 Typically, 150 mg SiO2 was dispersed in the mixed solution of absolute ethanol (30 mL) and deionized (DI) water (30 mL) and ultrasonicated for half an hour to form a uniform 15


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suspension. Then 8 mmol Ce(NO3)3·6H2O and 0.5 g urea were further added to the above suspension, and stirred for 20 mins. After completely dissolved, they were transferred to Teflon-lined stainless autoclave, sealed and maintained in an oven at 160 oC for 8 h. The collected precipitate was washed with DI water and ethanol absolute several times, respectively, and dried at 60 oC overnight. Finally, SiO2 templates were removed in 2 M sodium hydroxide solution at 50 oC for 8 h to obtain CeO2 hollow nanospheres, and the product was washed and dried. Synthesis of CeO2 HSs-Co2+-PDA. In a typical procedure, 150 mg CeO2 was dispersed in absolute ethanol /DI water (30 mL/30 mL) mixed solution and ultrasonicated for half an hour to form a uniform suspension, 1 mL 25 % NH3·H2O was injected in the suspension. And then 2.8 mmol Co(NO3)2 and 250 mg dopamine hydrochloride was added to the above suspension, and kept stirring for 8 h at room temperature. Finally, the precipitate was separated and washed by vaccum filtration with ethanol absolute and DI water and dried at 60 oC overnight. Co(NO3)2 was replaced with Ni(NO3)2 to synthesize CeO2-Ni2+-PDA in contrast. Synthesis of CeO2-Co-NC. CeO2-Co2+-PDA hybrid materials were annealed for carbonization at 800 oC for 3 h in Argon atmosphere at a ramp rate of 5 oC min-1 and a flow of 40 sccm. The obtained products were washed by the same way above. To further investigate the effect of Co content, different molar quantity of Co(NO3)2 (1.4 mmol, 2.8 mmol and 5.6 mmol) was added into the reaction in the aforementioned procedure for comparison with other conditions unchanged. CeO2-NC, Co-NC, CeO2-Ni-NC, CeO2-Cu-NC and CeO2-Ag-NC was synthesized via the same method. In addition, the CeO2-Co2+-PDA was annealed in air to synthesize CeO2-Co3O4 with other conditions unchanged in contrast. N-doped carbon (NC) layer was prepared by a similar procedure without adding CeO2 and any other metal sources. 16


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Materials characterization. The structure of the as-prepared samples was collected with X-ray Diffraction (XRD, Philips X’Pert Pro; Cu Kα, λ=1.542 Å). Morphology was unveiled using field emission scanning electron microscopy (FESEM, JEOLJSM-7100F) and transmission electron microscopy (TEM, Titan G2 60-300 with image corrector). Raman scattering spectra were recorded on a LabRAM HR800 Horiba JobinYvon. The X-ray photoelectron spectroscopy (XPS) was collected on a Kratos AXIS Ultra DLD-600W XPS system with a monochromatic Al Kα (1486.6 eV) X ray-source. X-ray adsorption near-edge structure (XANES) spectra were measured at SP8 (Japan) 12B2 Taiwan beamline (NSRRC) and analyzed using the Ifeffit Athena software. Thermos gravimetric analysis (TGA) were conducted on a PerkinElmer Diamond TG under air atmosphere at a ramp rate of 5

o

C min-1 from room temperature to 800

o

C.

Brunauer-Emmett-Teller (BET) surface area and pore size distribution data were measured on a Micromeritics ASAP 2020 sorptometer using nitrogen adsorption at 77 K. Electrochemical characterization. All the electrochemical measurements were conducted in a three-electrode configuration under 0.1 M KOH electrolyte at room temperature, in which the samples were directly used as working electrodes, the Ag/AgCl filled with saturated KCl solution serves as reference electrode and a platinum plate as counter electrode, respectively. The potentials vs. Ag/AgCl could be converted to vs. RHE according to the Nernst equation: ERHE = EAg/AgCl + 0.059 pH + 0.197 V. The preparation of working electrodes, CV and LSV measurements, conversion of Kouteckey-Levich plots and calculations of peroxide yield (H2O2 %) and electron transfer number (n) are described in detail (Supporting Note S1). Computational methods. First principle calculations were carried out using the Vienna Ab Initio Simulation Package (VASP) with a plane-wave basis set and a projector-augmented wave 17


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(PAW) method.60-62 The electron-core interactions were represented by generalized gradient approximation (GGA) in the parametrization of Perdew-Burke-Ernzerhof (PBE) pseudopotential. All the calculations are spin polarized. The energy cutoff is 400 eV. A Hubbard U term was also included to solve the underestimate of the Ce (4f) and Co (3d) states. The effective U values of 5.0 eV and 3.5 eV was applied for Ce and Co, respectively.25,62,63 The DFT-D3 method of Grimme was employed to evaluate the van der Waals effect on the adsorption of O atom on CeO2 (111) and CeO2 (111)/Co (111) surface.64 The examined lattice constant for bulk CeO2 is 5.41 Å, well consistent with previous work.25,65,66 The stability of the intermediates O* can be calculated both on a CeO2 (111) surface and CeO2/Co surface (* denotes a site on the surface). The calculated binding energies was defined as the reaction energies of the reaction H O + ∗ → O∗ + H.56

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX. Additional experimental data, including XRD, BET, TGA, XPS, SEM, TEM, Tafel plots, DFT calculated structures and Tables (PDF)

AUTHOR INFORMATION Corresponding authors *Email: [email protected] (C.D. W.) *Email: [email protected], [email protected] (J.C.)

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51502099 and 51571096), Natural Science Foundation of Hubei Province (No. 2016CFB129), and

18


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“the Fundamental Research Funds for the Central Universities”, HUST: 2016YXMS211. C.D.W. particularly acknowledges the Hubei “Chu-Tian Young Scholar” program. The authors appreciate the technical support from the Analytical and Testing Center of Huazhong University of Science and Technology.

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Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62–69. (59) Zhang, J.; Gong, M.; Tian, C.; Wang, C.-A. Facile Synthesis of Well–Defined CeO2 Hollow Spheres with a Tunable Pore Structure. Ceram. Int. 2016, 42, 6088–6093. (60) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total–Energy Calculations Using a Plane–Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (61) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (62) Dudarev, S.; Botton, G.; Savrasov, S.; Humphreys, C.; Sutton, A. Electron–Energy–Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505. (63) Keating, P. R.; Scanlon, D. O.; Morgan, B. J.; Galea, N. M.; Watson, G. W. Analysis of Intrinsic Defects in CeO2 Using a Koopmans–Like GGA+U Approach. J. Phys. Chem. C 2012, 116, 2443–2452. (64) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT–D) for the 94 Elements H–Pu. J. Chem. Phys. 2010, 132, 154104. (65) Wu, X.-P.; Gong, X.-Q.; Lu, G. Role of Oxygen Vacancies in the Surface Evolution of H at CeO2 (111): A Charge Modification Effect. Phys. Chem. Chem. Phys. 2015, 17, 3544–3549. (66) Wang, W.; Thevuthasan, S.; Wang, W.; Yang, P. Theoretical Study of Trimethylacetic Acid Adsorption on CeO2 (111) Surface. J. Phys. Chem. C 2016, 120, 2655–2666.

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Figures and Captions

Figure 1. Schematic illustration of the processing of CeO2-Co-NC and CeO2-NC hollow spheres synthesis.

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Figure 2. Interfacial materials characterization. (a) XRD patterns and (b) Raman spectra of CeO2, CeO2-Ni-NC and CeO2-Co-NC. (c) Nitrogen adsorption/desorption isotherm of CeO2-Co-NC (Inset is the pore distribution). XPS spectra of (d) Ce 3d, (e) Co 2p and (f) C1s. (g) XANES spectra at Ce L3-edge of CeO2-Co-NC, CeO2-NC and CeO2. (h) XANES spectra at Co K-edge of standard Co foil, CeO2-Co-NC and Co-NC. (i) FT-EXAFS spectra of standard Co foil, CeO2-Co-NC and Co-NC.

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Figure 3. Microstructural characterization of CeO2 and CeO2-Co-NC. (a) FESEM image of CeO2 hollow spheres. (b) An enlarged view of (a). Inset of (b) is an individual octahedron. (c, d) Morphologies of CeO2-Co-NC hollow spheres. (e) TEM images of CeO2-HSs. Inset is a photograph of CeO2-HSs suspensions in DI water. (f, g) TEM images of CeO2-Co-NC. Inset of (f) is a photograph of CeO2-Co-NC suspensions in DI water. (h) HRTEM image of CeO2-Co-NC. (i) Fast Fourier Transforms (FFT) images of the selected area of Co. (j) SAED pattern of the CeO2-Co-NC. (k) EDS line-scanning profile across the individual hollow spheres of the inset. (l) HAADF-TEM image of CeO2-Co-NC and the corresponding elemental mapping images.

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Figure 4. Oxygen reduction performance. (a) CV curves of CeO2, CeO2-NC, CeO2-Ni-NC, CeO2-Co-NC and 20% Pt/C in Ar- and O2-saturated 0.1 M KOH aqueous solution. (b) LSV curves of the catalysts in O2-saturated 0.1 M KOH aqueous solution at 1600 rpm. (c) LSV curves of CeO2-Co-NC in O2-saturated 0.1 M KOH aqueous solution at various rotation rates. (d) Koutecky-Levich plots of the samples at 0.5 V (vs. RHE). (e) Peroxide yield (H2O2%) of the catalysts during ORR process (solid line) and the calculated electron transfer number of the samples (dash line). (f) LSV curves of CeO2-Co-NC before and after 2000 cycles at 1600 rpm.

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Figure 5. Atomic-level interfacial electronic structure calculations. Chemisorption models of oxygen atom on (a) CeO2 (111) surface and (b) CeO2 (111)/Co (111) surface. (c) Isosurfaces of local charge density of CeO2 (111)/Co (111). Both yellow and red represent 0.02 e per Å3 isosurfaces, corresponding to the charge accumulation and depletion regions, respectively. (d) TDOS curves of CeO2 (111) and CeO2 (111)/Co (111).

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A Universal Method to Engineer Metal OxideâMetalâCarbon Interface

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