Microwave-Assisted Synthesis of Co-Coordinated Hollow Mesoporous

Jun 15, 2015 - Transition-metal-/metal-oxide-loaded mesoporous carbon materials with hollow structures are thought to have great potential as catalyst...
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Microwave-Assisted Synthesis of Co-Coordinated Hollow Mesoporous Carbon Cubes for Oxygen Reduction Reactions Lisong Chen,† Xiangzhi Cui,† Min Wang,† Yanyan Du,† Xiaohua Zhang,† Gang Wan,† Linlin Zhang,† Fangming Cui,‡ Chenyang Wei,† and Jianlin Shi*,† †

State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China ‡ Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, P. R. China S Supporting Information *

ABSTRACT: Transition-metal-/metal-oxide-loaded mesoporous carbon materials with hollow structures are thought to have great potential as catalysts, especially in the areas of sustainable chemistry and energy conversion. However, it is hard to load transition metals/metal oxides onto carbon materials while keeping the carbon materials unchanged through traditional after-treatment processes, thus making it difficult to determine the true roles of the transition metal/metal oxide and carbon in the reactions. Here, Co-coordinated hollow mesoporous carbon cubes (CoMHMCCs) were prepared by a microwave-assisted approach in the presence of ethylene glycol and hollow mesoporous carbon cubes (HMCCs). The synthesized CoMHMCCs inherited most advantages of the HMCCs, such as large surface area and pore volume, uniform pore size distribution, and hollow mesoporous structure, and the Co species was found to coordinate with the N atoms in the Ndoped hollow mesoporous carbon cubes. The synthesized CoMHMCCs exhibited a much enhanced oxygen electroreduction reaction activity (∼50 mV deviation from Pt/C), a high selectivity (number of electrons transferred = 3.7−3.9), and excellent electrochemical stability (as low as 12 mV negative shift of half-wave potential after 5000 potential cycles) as a result of a synergetic catalytic effect.

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INTRODUCTION Because of the slow kinetics of the oxygen reduction reaction (ORR), Pt/C or Pt-based alloys are almost the only choice for polymer electrolyte fuel cells (PEFCs). However, the high cost and poor durability of Pt-based cathodic catalysts have become key barriers to the commercialization of PEFCs.1,2 Therefore, the design and development of high-activity and long-durability non-precious-metal ORR catalysts is of great importance and is becoming the focus of PEFCs research.3−8 Among recent reports, porous carbons doped with transition-metal (such as Fe,4,9−13 Co,14 Mn,15,16 Ag,17 and Cu18) oxides, chalcogenides, carbides, and macrocyclic complexes and one or more types of heteroatoms19 (such as N5,20−24 and S25−28) have been extensively investigated. Among them, metal-oxide- or -nitride-loaded heteroatom-doped porous carbons (M−N−C) present the most promising alternatives to Pt-based catalysts.6,29 Great efforts have been expended, and many achievements have been made. However, the results are still far from satisfying.30,31 On one hand, the preparation of this type of material mainly relies on the pyrolysis of nitrogen and transition-metalcontaining precursors, which is energy-intensive and results in a composite of carbon and metal/metal-oxide nanoparticles, metal carbide, and metal sulfide.6 Furthermore, the presence of a transition metal will have a significant effect on the © XXXX American Chemical Society

microstructure, electron conductivity, Brunauer−Emmett− Teller (BET) surface area, and so on, which can play an equally or even more important role than the chemical composition in controlling the overall kinetics of the catalytic process. Therefore, it is hard to compare reactant accessibility and reactive-site exposure among different catalysts.32,33 As a result, apart from undesirable catalytic activity, the true catalytic site remains controversial.7,9,34−36 On the other hand, because of the strong covalent bonding between carbon and dopant atoms, heteroatom-doped carbon materials are of good durability, whereas metal-oxide-loaded materials suffer from catalytic performance deterioration as a result of the loss of transition-metal species. Therefore, it is necessary, first, to load transition-metal species on carbon materials without affecting the structure and properties of the carbon support to determine the actual role of the transition metal in the ORR. More importantly, the transition-metal species should be coordinated to the carbon materials for good durability. Microwave-assisted synthesis is a rapid, energy-efficient, and environmentally friendly approach for nanomaterials.37 Scientists have synthesized different types of nanomaterials by this Received: April 6, 2015 Revised: May 25, 2015

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carbon cubes (HMCCs) were first synthesized according to our previously reported method by magnesium thermal reduction followed by acid etching.45 To obtain CoMHMCCs, as illustrated in Figure 1A, 50 mg of HMCCs was added to 30 mL of ethylene glycol, followed by cobalt acetate and sodium hydroxide. After 30 min in an ultrasound bath, the mixture was placed in a microwave oven at 500 W for 90 s. After the resulting material had been centrifuged, washed, and dried several times, CoMHMCCs were obtained. For comparison, microwave-treated hollow mesoporous carbon cubess (MHMCCs) were also prepared by a similar process without the addition of cobalt acetate. Physical Characterization. Field-emission scanning electron microscopy (FE-SEM) images and energy-dispersive X-ray (EDX) spectra were obtained on a Hitachi S-4800 scanning electron microscope, whereas transition electron microscopy (TEM) images were collected on a JEOL-2010F electron microscope operated at 200 kV. Powder X-ray diffraction (XRD) patterns of the as-prepared samples were recorded on a Rigaku D/Max-2550 V X-ray diffractometer with a Cu Kα radiation target (40 kV, 40 mA) at a scanning rate of 4°/min. X-ray photoelectron spectroscopy (XPS) spectra were collected on a VG Micro MK II instrument operated at 150 W using monochromatic Mg Kα X-rays at 1253.6 eV, and spectrum calibration was performed by taking the C 1s electron peak (BE = 285 eV) as an internal reference. Raman spectra were recorded on a DXR Raman microscope with a 532-nm excitation length (Thermo Scientific, Madison, WI). Nitrogen adsorption−desorption measurements were performed using Micromeritics TriStar 3000 analyzer at 77 K, and the specific surface area and pore size distribution were calculated using the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. Electrochemical Characterization. All electrochemical measurements were carried out on a CHI 760E electrochemical workstation (CH Instruments, Austin, TX) with a standard three-electrode cell at room temperature. A platinum wire and Ag/AgCl (3 M KCl) were used as the counter and reference electrodes, respectively. A 0.1 M KOH solution was used as the electrolyte for electrochemical measurements. A rotating ring-disk electrode (RRDE) with a Pt ring (4-mm inner diameter and 6-mm outer diameter) and a glass carbon disk (6-mm diameter) served as the substrate for the working electrode for evaluating the ORR activity and selectivity of various catalysts. Prior to use, the glassy carbon electrode was polished with alumina slurries (1.0, 0.3, and 0.05 μm, in sequence) and ultrasonically cleaned thoroughly with pure water between each polishing step. Catalyst ink was prepared by blending catalyst power (10 mg) with 2 mL of mixed solution (1:1 ethanol/water, volume scale) and 50 μL of Nafion solution (5%) in the ultrasonic bath. Cyclic Voltammetry. For each test, 20 μL of catalyst ink was transferred onto the glassy carbon substrate, yielding a catalyst loading level of 0.35 mg cm−2. The catalysts were characterized by cyclic voltammetry (CV) at room temperature. The CV curves were obtained by a cycling scan after being purged with N2 or O2 for 15 min. Rotating-Disk Electrode (RDE) Measurements. Rotating-disk electrode (RDE) measurements were performed with ALS RRDE3A rotating ring disk electrode apparatus. RDE measurements were carried out in oxygen-saturated 0.1 M KOH at rotating rates varying from 400 to 2025 rpm at a scan rate of 5 mV s−1. Linear sweep voltammetry (LSV) on the RDE was performed on a 3-mm-diameter RDE. Rotating Ring-Disk Electrode (RRDE) Measurements. Catalyst inks and electrodes were prepared by the same method as above. The disk electrode was scanned at a rate of 5 mV s−1, and the ring potential was constant at 0.5 V vs Ag/AgCl. The number of electrons transferred, n, was calculated as

method.38 Compared to heating in a furnace, use of microwaves provides fast and uniform heating of solutions, so that homogeneous nucleation can be obtained within a much shortened crystallization period.39 Moreover, as the reduction of metal precursors and nucleation of metal clusters are accelerated by the rapid microwave heating, small monodisperse nanostructures can be obtained without impacting the metal support.40 Mesoporous carbon materials have large surface areas, large pore volumes, and uniform pore size distributions, which benefit the carrying of oxygen and delivery of generated electrons,41−43 and hollow structures can store electrolyte and shorten the diffusion path of both oxygen and electrons.9,43,44 Therefore, a hollow-structured mesoporous carbon material should be an efficient oxygen reduction catalyst/support. Recently, we reported the synthesis of hollow mesoporous carbon cubes (HMCCs)45 having a large surface area, uniform pore size distribution, cube morphology, and hollow structure, as well as a few-layer graphene structure, making them a promising catalyst support candidate for the ORR. Here, we report the synthesis of Co-coordinated hollow mesoporous carbon cubes (CoMHMCCs) and their properties as an efficient oxygen electroreduction catalyst approaching Pt/ C in alkaline solutions. Co was chosen because its redox potential is close to the potential of oxygen reduction into water.39 The catalyst was prepared by a microwave-assisted method, as shown in Figure 1A. Highly dispersed small cobalt

Figure 1. (A) Schematic illustration of the synthesis process of CoMHMCC. (B) SEM and (C) TEM (inset, HRTEM) images of CoMHMCC.

oxide was successfully coordinated on the HMCCs. The microwave process was demonstrated to have little effect on the surface area, pore size distribution, or uniform hollow mesoporous structure of the carbon support and would thus enable the contribution of the Co species to oxygen reduction. A much enhanced ORR activity of CoMHMCCs was obtained compared to that of HMCCs.

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n=4

EXPERIMENTAL SECTION

Catalyst Synthesis. To synthesize Co-coordinated hollow mesoporous carbon cubes (CoMHMCCs), hollow mesoporous

Id Id + Ir /N

The HO2− percentage was determined as B

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Figure 2. (A) XRD patterns, (B) Raman spectra, (C) nitrogen adsorption−desorption isotherms, and (D) corresponding pore size distributions of the synthesized samples. (E,F) High-resolution (E) N 1s and (F) Co 2p3/2 spectra of CoMHMCC.

HO2− percentage = 200

Ir / N Id + Ir /N

Information). The Co species were distributed homogeneously throughout the sample. According to the semiquantitative results of EDX analysis, about 1.52 at. % Co was loaded on the HMCCs (Figure S3 and Table S1, Supporting Information). Compared with literature reports (Table S2, Supporting Information), the Co content in this study was moderate. To further validate the successful loading of cobalt species, XRD patterns (Figure 2A) of the synthesized materials were also recorded. All three samples showed clear peaks at about 23.9° and 43.5°, corresponding to the (002) and (100) diffraction planes of graphite, respectively. Two more peaks, at about 31.9° and 39.1°, can be seen in the pattern of CoMHMCCs, that belong to cobalt oxide. However, these two peaks are too weak to determine the crystalline structure. From the above information, it is proposed that cobalt oxide nanoparticless are ultrasmall, highly dispersed, and low in content; therefore, it is hard to find the nanoparticles in the SEM and TEM images and also difficult to obtain a strong XRD peak. To investigate the changes in the textural properties of the synthesized samples, such as surface area and pore size distribution, nitrogen adsorption−desorption isotherms were measured; they are shown in Figure 2C, with the corresponding pore size distributions in Figure 2D. The corresponding BET surface areas and pore volume data are summarized in Table S3 (Supporting Information). MHMCCs show a slightly smaller BET surface area and pore volume than HMCCs, probably because of the higher graphitic degree. It should be noted that

where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring. N was determined to be 0.40 from the reduction of K3Fe(CN)6.

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RESULTS AND DISCUSSION

First, two questions stimulated us to investigate the microstructure and element composition, namely, whether and what kind of Co species is coordinated to the HMCCs, and whether the microstructure would be changed by the coordinated Co species. To begin, the synthesized samples were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From the SEM images (Figure 1B; Figure S1A, Supporting Information), both the MHMCCs and CoMHMCCs are cubic in large view. From the damaged cubes, it is easy to see the hollow structure of the carbon cubes. TEM images of both the MHMCCs (Figure S1B, Supporting Information) and CoMHMCCs (Figure 1C) validate the hollow structures of all of the carbon cubes in both samples. All of these characteristics are similar to those of the precursor HMCCs. Therefore, the microwave process had little effect on the morphology and structure of the carbon materials. From high-resolution TEM images (Figure 1C, inset), it is hard to find any nanoparticles other than the carbon cubes; however, element mapping analysis validated the presence of Co species in the synthesized CoMHMCC samples (Figure S2, Supporting C

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Figure 3. (A) CV curves of the synthesized samples in oxygen-saturated 0.1 M KOH at a sweep rate of 50 mV s−1. (B) LSV curves on a rotating-disk electrode of the synthesized samples and Pt/C in oxygen-saturated 0.1 M KOH at a sweep rate of 5 mV s−1 at a rotation speed of 1600 rpm. (C) Number of electrons transferred and (D) H2O2 yield as functions of the electrode potentials of HMCC, MHMCC, and CoMHMCC calculated from the RRDE data at a rotation speed of 1600 rpm in oxygen-saturated 0.1 M KOH. All measurements were performed at room temperature.

the limited difference in pore volumes between MHMCCs and CoMHMCCs indicates that the ultrasmall cobalt oxide nanoparticles are mainly loaded on the surface of the carbon cubes, instead of blocking the porous structures, and this will benefit the enhancement of the ORR activity. However, the BET surface area and pore volume of CoMHMCCs are much lower than those of HMCCs and MHMCCs resulting from the loading of heavier Co species. The pore size distributions of these three samples are all centered at about 3.7 nm, indicating little effect of the microwave process on the mesoporous structure. Similar Raman curves were obtained for the synthesized samples (Figure 2B). For all curves, a G band at about 1570 cm−1 and a D band at 1340 cm−1 are apparent. Notably, the ratios of the intensities of the G and D bands, IG/ID, for MHMCCs and CoMHMCCs are slightly higher than that for HMCCs (the values are 1.09, 1.12, and 1.12 for HMCCs, MHMCCs, and CoMHMCCs, respectively), which support the results of higher graphitization after microwave treatment from the XRD patterns and nitrogen adsorption−desorption isotherms. Generally, the 2D band of Raman spectrum is the scale bar of the layers of graphene-based materials, which can be observed when the number of graphene layers is no more than 10. Here, as can be seen in Figure 2B, the distinct 2D band at about 2674 cm−1 indicates a few-layer graphene structure for all three samples. As discussed above, the synthesized CoMHMCCs inherited all of the merits of HMCCs, such as hollow mesoporous structure, large surface area and pore volume, and few-layer graphene structure, and cobalt oxide was successfully loaded to form CoMHMCCs. Moreover, after the microwave heating treatment, the prepared MHMCCs and CoMHMCCs showed much enhanced graphitization contents compared to the HMCCs, which favors electron transfer during the ORR process.

Surface elemental compositions were measured by X-ray photoelectron spectroscopy (XPS). As can be seen in Figure S3 (Supporting Information), four peaks at about 285.02, 400.12, 532.43, and 781.97 eV corresponding to C 1s, N 1s, O 1s, and Co 2p, respectively, were observed. The elemental atomic contents for C, N, O, and Co were found to be 84.85%, 1.1%, 12.31%, and 1.74%, respectively, which are similar to the EDX results. (Note that the doped N in the CoMHMCCs comes from the HMCCs, as indicated in our previous report.45) The surface elemental contents of N, O, and Co were higher than the bulk contents, indicating higher concentrations on the surface than in the bulk, which is in favor of the ORR as the ORR takes place only on the surface of the catalysts.46 The interlayer of the catalyst plays an important role in electron transfer, and a lower doping amount therein is beneficial. The high-resolution N 1s spectrum was deconvolved into four peaks at 398.2, 400.3, 401.3, and 403.5 eV, corresponding to pyridinic N, pyridonic N, graphitic N, and oxidized N, respectively (Figure 2E). The peak centered at 398.2 eV should also include the contribution of N bonding to the cobalt (Co−N) on account of the small difference between the binding energies of Co−N and pyridinic N.47 According to refs 48 and 49, graphitic N is effective in increasing the limiting current density, whereas pyridinic N and pyridonic N favor improvements in the onset potential, and oxidized N has little effect on the oxygen reduction reaction. Here, little oxidized N was found after deconvolution. The dominating pyridinic/pyridonic N and graphitic N are highly favorable for the oxygen reduction process. Although a significant amount of Co (1.74 at. %) was incorporated in the synthesized CoMHMCC samples as measured by XPS and also confirmed by XRD; however, we could hardly find metal/metal-oxide nanoparticles in the HRTEM images. Therefore, it was deduced that most cobalt atoms are in a coordinated state. The high-resolution Co 2p3/2 spectrum can be deconvoluted into three components (Figure 2F). The peaks at 780.1 and 781.9 eV can be attributed to D

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Figure 4. LSV curves before and after 5000 cycles of (A) CoMHMCC, (B) HMCC, and (C) MHMCC in oxygen-saturated 0.1 M KOH at a sweep rate of 5 mV s−1 and a rotation speed of 1600 rpm.

showed higher diffusion limiting current densities at −0.5 V [4.17 mA cm −2 for HMCCs and MHMCCs and 5.1 mA cm−2 for CoMHMCCs, which are similar to that (5.62 mA cm−2) for Pt/C and the theoretical value of 5.7 mA cm−2]. The high diffusion limiting currents result from the high surface areas of the synthesized samples. Furthermore, rotating ring-disk electrode (RRDE) measurements were performed to determine the ORR pathways. The percentage of peroxide (HO2−) species with respect to the total amount of oxygen reduction products and the number of electrons transferred were calculated from the ring current and disk current (Figure 3C,D). The yield of peroxide species for HMCCs was as high as 60−93% in the potential range from −0.8 to −0.2 V, and the corresponding number of electrons transferred was less than 3.4, especially in the high potential range from −0.4 to −0.2. This indicates a two-electron/fourelectron combined pathway process. On one hand, the twoelectron process will result in a decrease in the energy conversion efficiency, and on the other hand, H2O2 as the main product will corrode both the membrane and the catalyst, thereby decreasing the lifetime of the fuel cell system. The H2O2 yield of MHMCCs was between 40% and 50%, much lower than that of HMCCs. The number of electrons transferred for MHMCCs was around 3.4 throughout the potential range from −0.8 to −0.2 V, indicating the significant effect of the graphitization level on the oxygen reduction pathways. For CoMHMCCs, the yield of peroxide was much lower at less than 20%, and the corresponding number of electrons transferred ranged from 3.7 to 3.9 across the potential range from −0.8 to −0.2 V, indicating a dominating fourelectron pathway of the ORR process. Therefore, as discussed above, the loading of Co species is important not only for the

nitrogen- and oxygen-coordinated cobalt species, respectively,30,48 and the peak at 787.1 eV is a shakeup (satellite) peak. The coordination of Co with oxygen was validated to be in favor of oxygen reduction performance. These nitrogen- or oxygen-coordinated cobalt species should play a significant role in the oxygen reduction reaction and greatly enhance the ORR activity. The ORR properties of the as-prepared samples were first evaluated by cyclic voltammetry (CV) in 0.1 M KOH solution on a glass carbon electrode. As can be seen in Figure S4 (Supporting Information), all three samples showed distinct oxygen reduction peaks in oxygen-saturated 0.1 M KOH, compared to that in nitrogen-saturated solutions. When the spectra are compared (Figure 3A), HMCCs show the lowest oxygen reduction activity, whereas MHMCCs show a much more positive onset potential and half-wave potential and much higher peak current, indicating a much enhanced ORR activity compared to that of HMCCs, most probably resulting from the higher graphitization content of the MHMCCs. Remarkably, CoMHMCCs showed the highest ORR performance among these three samples, with an onset potential of about −0.1 V and a peak potential of about −0.23 V. This suggests that Co species strongly coupled with the HCCMs during the microwave process, which is in favor of the ORR process14,31,50 To gain further insight into the ORR performances of CoMHMCCs, MHMCCs, and HMCCs, rotating-disk electrode (RDE) measurements were performed in oxygen-saturated 0.1 M KOH solutions at a scan speed of 5 mV s−1. For comparison, commercial 20 wt % Pt/C was also examined. As can be seen from Figure 3B, similar trends in ORR performance with CV curves could also be observed. The half-wave potential difference between the CoMHMCCs and Pt/C was about 50 mV at the same mass loading. Additionally, all three catalysts E

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Langmuir enhanced onset potential but also for the inhibition of H2O2 generation. Tolerance to the methanol crossover effect is important for fuel cells catalysts, especially in direct methanol fuel cells. A CoMHMCC sample was measured against the electrooxidation of methanol in 3 M methanol with 0.1 M KOH solution saturated with oxygen. It is known that the oxygen reduction current of commercial Pt/C significantly decreases as a result of rapid methanol chemical oxidation. As can be seen in Figure S7 (Supporting Information), compared to the curve in 0.1 M KOH, the LSV curve in 0.1 M KOH with 3 M methanol shifted toward the negative side by as low as about 30 mV with similar diffusion-limited currents, indicating the excellent methanol tolerance properties of CoMHMCCs. To investigate the durability of the catalysts for the ORR, all three samples were tested by cycling CV curves between 0 and −1.0 V at 50 mV s−1 in oxygen-saturated 0.1 M KOH (Figure 4). After 5000 continuous potential cycles, the half-wave potential shifted toward the negative side by about 24, 22, and 12 mV for HMCCs, MHMCCs, and CoMHMCCs, respectively, demonstrating the excellent durability of CoMHMCCs. The microwave treatment and Co coordination both enhanced the durability of the catalysts. Based on the CV and LSV measurements, it is evident that the ORR performance of CoMHMCCs can be substantially enhanced, which can be confirmed by the more positive onset potential, higher half-wave potential, and larger diffusionlimited current density compared to the other two samples. These differences can be ascribed to the increased graphitization degree, the Co coordination, and the negligible changes in the mesostructures of the synthesized catalysts after Co coordination. Furthermore, the microwave treatment also resulted in enhanced catalytic stability and a more promising four-electron pathway for the ORR process. A possible mechanism for the enhanced ORR performance of CoHMHCCs, in addition to the increased graphitization degree of the mesoporous carbon support, which is favorable for enhancing electron conduction, can be proposed as follows: First, as both nitrogen and Co species can be considered as active sites for the ORR, the coordination of Co species offers more active sites and reduce oxygen molecules into active O2−/O2− through the redox reactions of Co2+/Co3+.52 Second, as measured by XPS, the surface oxygen content increases as a result of microwave treatment, and meanwhile, pyridine species, which is the dominating nitrogen species in CoHMHCCs, can stabilize singlet dioxygen by forming a stable adduct (Figure 5A).46 As the chemisorption of molecular oxygen is the most critical step of the ORR, the formation of a stable adduct between pyridine and molecular oxygen greatly decreases the chemisorption energy and accelerates the ORR process; this is, at least in part, responsible for the enhancement of the ORR performance of CoMHMCCs. In more detail, oxygen adsorption on catalysts can be classified into three types, namely, lateral-group type, end-group type, and bridge type. Generally, lateral-group-type and bridge-type adsorptions are favorable for oxygen chemisorption and reduction. However, lateral-group-type chemisorption usually occurs on the Pt surface, and bridgetype adsorption occurs only when the distance between two active sites is close to the length of an oxygen−oxygen bond, whereas end-group-type adsorption is not favorable for the ORR. For transition macrocyclic metal complex analogues, oxygen can be adsorbed in a manner similar to lateral-group-

Figure 5. Illustrations of (A) the reaction between pyridine and oxygen molecules that leads to the formation of a stable adduct and thus decreases the oxygen chemisorption energy,46 (B) the isolated transition macrocyclic metal−nitrogen complex, and (C) oxygen atoms adsorbed on the transition macrocyclic metal−nitrogen complex.51 In panels B and C, the central red sphere represents a metal Fe or Co atom; blue, yellow, and gray spheres represent oxygen, nitrogen, and carbon atoms, respectively.

type adsorption: One atom lies right on the central transitionmetal atom, and the other is farther from the metal atom, thus aligned on the bisecting line of the two neighboring bonds between the central metal atom and the nearest N atoms (Figure 5B,C).51 Therefore, the formation of Co coordination to N atoms will contribute to the oxygen chemisorption. Considering all of the effects of Co doping, synergetic catalytic effects among nitrogen activation favorable for oxygen adsorption by Co coordination, oxygen activation by the redox of Co2+/Co3+, and promoted carbon graphitization in microwave treatment can be understood in the enhanced oxygen electroreduction performance of CoMHMCCs.53 Therefore, the design and optimization of these synergetic components is of great importance in enhancing the oxygen reduction properties of non-precious-metal catalysts.

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CONCLUSIONS

In summary, Co-coordinated hollow mesoporous carbon cubes have been synthesized through a microwave-assisted approach that is rapid; low-cost; environmentally friendly; and more importantly, retains the mesostructure features of the carbon support unaffected. As a result, the prepared CoMHMCC catalyst featured a large surface area and pore volume and a uniform pore size distribution and exhibited a substantially enhanced oxygen reduction activity and durability in alkaline environments. The oxygen chemisorption on the surface of the catalyst promoted by Co coordination itself and the accompanying N-atom activation by Co coordination, together with the increased graphitization degree in microwave treatment, are proposed to be responsible for the synergetic enhancement of the ORR performance. The microwaveassisted synthesis method is an efficient approach for combining metal-oxide and nanocarbon materials and, therefore, can be used in various areas. F

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables: SEM and TEM images of MHMCC, element mapping of CoMHMCC, EDX spectrum of CoMHMCC, elemental contents of CoMHMCC and comparsion of the Co content with references, BET surface area and pore volume data of synthesized samples, XPS survey spectrum of CoMHMCC, CV and LSV curves of MHMCC and CoMHMCC in 0.1 M KOH, LSV curves of CoMHMCC in oxygen-saturated 0.1 M KOH and 0.1 M KOH with 3 M methanol. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b01256.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: + 86-21-52413122. Tel.: + 86-21-52412712. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of this research by National Key Basic Research Program of China (2013CB933200), National Natural Science Foundation of China (Grants 51202278, 51202288), Natural Science Foundation of Shanghai (12ZR1435200) and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education).

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b01256 Langmuir XXXX, XXX, XXX−XXX