C Catalyst for

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Layered Transition Metal Oxynitride Co3Mo2OxN6x/C Catalyst for Oxygen Reduction Reaction Li An, Zhonghong Xia, Peikai Chen, and Dingguo Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10793 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Layered Transition Metal Oxynitride Co3Mo2OxN6-x/C Catalyst for Oxygen Reduction Reaction Li An, Zhonghong Xia, Peikai Chen, Dingguo Xia*

Beijing Key Lab for Theory and Technology of Advanced Batteries Materials, College of Engineering, Peking University, Beijing 100871, P. R. China *Address correspondence to: [email protected] KEYWORDS. alkaline fuel cell, layered transition metal oxynitride, Co3Mo2OxN6-x /C, oxygen reduction reaction, catalyst ABSTRACT. Transition metal oxynitrides have now garnered growing interest in our quest for highly efficient alternatives to Pt in direct methanol alkaline fuel cells. Herein, carbon supported Co3Mo2OxN6-x was synthesized via a simple two-step approach wherein the reactants undergo refluxing and heat treatment in NH3. For the as-prepared Co3Mo2OxN6-x catalyst, uniformly dispersed on the XC-72, with the particles size averaged at 5 nm, the catalytic activities towards oxygen reduction reaction in alkaline media are related to the commercial Pt/C, such as the comparable onset potential (0.9 V vs RHE), half-wave potential (0.8 V vs RHE) and even higher specific activity (82.7 mA cm-2 at 0.7 V). Significantly, the Co3Mo2OxN6-x catalyst was highly stable in terms of 95% current retention after 12 h chronoamperometry measurement, indicative of favorable prospect for the non-noble cathodic catalyst in alkaline fuel cell. 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

INTRODUCTION Direct methanol alkaline fuel cells (DMAFCs) are regarded as promising energy conversion facilities for their high energy efficiency, high power density and low or zero emissions.1-3 However, the application of DMAFCs on a large scale is still hindered largely ascribe to its high cost, scarce supply and weak durability of Pt, which is currently used as the commercial cathodic catalyst of oxygen reduction reaction (ORR).4-6 To address the above challenges, the goals of intensive research have been achieved to explore the alternative non-precious metal catalysts with remarkable ORR properties and excellent stability to Pt-based noble metal catalysts, such as an extensive range of catalysts based on metal-free N (B/P/S/O)-doped carbon materials,7-10 transition metal chalcogenides,11,12 transition metal oxides,13,14 non-Pt transition-metal intermetallics,15-18 and transition-metal oxynitrides.19-21 Among them, transition metal oxynitrides and nitrides have aroused considerable attention for their potential application in fuel cell, since they possess high electrical and thermal conductivities, tailorable electronic properties and chemical resistance to corrosion in aqueous media.20,21-23 Previously, studies on monometallic nitrides (Ti, Nb, Ta) exhibited high onset potential and superior stability in acidic solution.24,25 Our group also investigated the catalytic activity of MoN/C, CoN/C synthesized by ammonolysis from different precusor in acid and alkaline solution.26,27 Compared to monometallic nitrides, bimetallic nitrides or oxynitrides by the tuning of electronic structure are potentially good candidates for ORR catalysts owning to their catalytic properties. Indeed, the supported bimetallic Co-W-O-N catalysts have substantial onset potential 2 ACS Paragon Plus Environment

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(0.74 V) in acid solution.28 Additionally, Khalifah’ group synthesized CoxMo1-xNz by the temperature programmed reduction that showed excellent ORR activity both in acid and alkaline solution.20 Meanwhile, bimetallic oxynitrides Co-Mo-O-N, Co-Mo nitride synthesized by Oyama’ and Hu’ group also exhibited excellent hydrodenitrogenation, ORR activities, respectively.19,22 Enormous advancements in catalytic performance have been attained; however, it remains a real challenge to search for new catalysts with high activity as well as long-term stability. In our present work, novel carbon supported Co3Mo2OxN6-x nanoparticles are produced by a facile two-step method, and they are of uniform distribution in particles size averaged at approximately 5 nm. Co3Mo2OxN6-x/C catalyst was shown to have high ORR properties and long-term durability. Furthermore, its high methanol tolerance in alkaline solution also proved that Co3Mo2OxN6-x/C is a good candidate catalyst for ORR in DMAFCs application. EXPERIMENTAL SECTION Catalyst synthesis. The synthesis of Co3Mo2OxN6-x/C catalyst was performed following the previously reported method.27,29 In short, 150 mL o-xylene solution contained with 0.2 mmol Co(acac)2, 0.2 mmol Mo(CO)6, and 47 mg Vulcan XC-72 carbon was placed into a 250 mL three-neck flask. Second, the mixture was treated in an ultrasound bath for 3 hours and was heated at 140 ~155°C for 2.5~4.5 hours in an oil bath at. Lastly, the resulting product was filtered, washed several times with absolute ethyl alcohol and dried in a vacuum oven at 80 °C overnight. The as-obtained precursor was put in tube furnace and treated under the flow NH3 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

atmosphere at 450~650 °C for 2.5~4.5 hours. As a control experiment, the Mo5N6/C catalyst was also prepared. For comparison, the different ratio of Co/Mo (1:1, 1:2, 1:3) were also synthesized. Materials Characterization. XRD patterns were performed using the D8 Advance X-ray diffractometer (Bruker, Gemany) with Cu Kα radiation (λ= 1.5406 Å); the radiation sources are 40 kV and 40 mA, respectively. The transmission electron microspectroscopy (TEM) images were taken on a TECNAI-F20 microscope and FEI operating at 200 kV. The Scan transmission electron microspectroscopy (STEM) work was carried using JEM-ARM200F microscope. The information of catalyst composition was obtained with ICP-AES (PROFILE SPEC, Leeman). The analysis of surface status was determined by using XPS (AXIS-Ultra instrument from Kratos Analytical, Al Kα radiation, hν=1486.6 eV). The soft XAS data were performed at beamline 4B7B station of Beijing Synchrotron Radiation Facility (BSRF) and the operating conditions of the storage ring was 2.5 GeV with a maximum current of 300 mA. Electrochemical Measurements. The electrocatalytic activity of the catalysts for ORR were evaluated in a three-electrode configuration connected to an electrochemical workstation (BioLogic SP240). The Hg/HgO (0.1 M KOH) was used as the reference electrodes and the glassy carbon slice was used as the counter electrode. The glassy carbon rotating disk electrode with electrocatalyst was used as the working electrode. The homogeneous catalyst ink was produced by dispersing 5.0 mg catalyst and two drops of 2% Nafion solutions into 1 mL isopropanol under ultrasonic condition for 4 ACS Paragon Plus Environment

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


~30 min. 20 µL catalyst inks was loaded on the glassy carbon disk (geometric area 0.1257 cm-2).

All the potentials were calibrated to the reversible hydrogen electrode (RHE), as described in conversion method (E

(RHE)

= E

(Hg/HgO, 0.1M KOH)

+ 0.933 V).15 The

electrodes of the electrocatalyst were cleaned in the range of 0.2−1.2 V at 50 mV s-1 to get the cycling stability. The onset electrode potential for the ORR is defined as the potential where the additional cathodic current begins to be observed on the voltammogram.30 The kinetic current densities and the electron transfer number of the ORR for Co3Mo2OxN6-x/C and commercial Pt/C catalysts can be determined by the K−L equation.31 1 1 1 1 1 = + = + ݆ ݆௞ ݆௟ ݆௞ ‫ ߱ܤ‬ଵൗଶ ݆௞ = ݊‫ܥ݇ܨ‬଴ B = 0.62nFA‫ܥ‬଴ ߥ ିଵ/଺ (‫ܦ‬଴ )ଶ/ଷ Where jk is kinetic current densities, jl is the diffusion limiting, n is the number of electrons transferred, j is the actual measured current density, ω is the angular frequency of rotation, F is the Faraday’s constant, C0 and D0 are the concentration and the diffusion coefficient of O2 in 0.1 M potassium hydroxide (KOH) media, respectively. ν is the viscosity of the electrolyte, A is the electrode’s area, k is the rate constant of electron transfer. All RRDE measurements were carried out on a RRDE-3A (ALS, Japan) device in 0.1 M KOH (O2 saturated) solution at the speed of 1600 rpm. The disk potential was 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

swept at 5 mV s-1 and the potential of the ring was kept at 1.4 V vs. RHE. Hydrogen peroxide yields [H2O2 (%)], the number of electrons transferred (n) were calculated using the equations below32:

Hଶ Oଶ (%) = 200 ×

n=4×

Iୖ /N Iୖ N + Iୈ

ID I ID + ND

Herein, N is the ring collection efficiency and the value of N is 0.422. IR is the ring currents, ID is the disk currents. RESULTS AND DISCUSSION The XRD pattern in Figure 1a confirms that the precursor can be assigned to the cobalt oxide doped molybdenum, the diffraction peaks of the precursor didn’t change with the reflux temperature and time (Figure S1a, b). After the precursor was heat-treated at 550 °C under NH3 atmosphere, the XRD pattern can be attributed to the Mo5N6–like body-centered cubic crystal structure (JCPDS, No. 51-1326), corresponding to the (004), (110), (114), and (300) planes, respectively (Figure 1b). The peak located at about 25° can be assigned to the (002) crystalline plane of carbon (XC-72). ICP result suggests that the ratio of Co/Mo in the as–obtained sample is 3:2. Therefore, the formula of the as-obtained metal oxynitride can be determined as Co3Mo2OxN6-x. Figure S1c presents the variation of the XRD patterns of the as-obtained products based on the heat treatment temperature under NH3 gas. With the temperature increasing, the intensities of the peaks obviously become higher, 6 ACS Paragon Plus Environment

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicative of the growing crystallinity of the material. However, compared with the Mo5N6 XRD pattern, the intensity of diffraction peak at 44° and 63° become larger, which is relevant with the substitution of Co to Mo atoms. Meanwhile, the diffraction peaks of the annealed product didn’t change with the heat treatment time (Figure S1d). The product with different Co:Mo molar ratio (1:1, 1:2, 1:3) were also synthesized with the same experimental condition. Interestingly, the Co3Mo2OxN6-x composition has no significant change with the various Co: Mo molar ratio and shows good reproducibility (Figure S2), which is obviously different from the reported references.19, 20 The morphologies and particle size of Co3Mo2OxN6-x/C catalyst were observed by the TEM. Figure 2a, b exhibit images of the Co3Mo2OxN6-x/C precursor with amorphous morphology, which is in accordance with the XRD results in absence of obvious diffraction peaks except for the graphite carbon at 25o. Interestingly, the phase transformation from initial disorder to final ordered structure was achieved when annealed at 550 °C under NH3 and Co3Mo2OxN6-x particles are homogeneously distributed on the XC-72 support with an average of around 5 nm (Figure 2d-f). High-resolution TEM (HRTEM) images show the spaces of lattice fringes are 0.249 nm and 0.259 nm, respectively. Considering that the substitution of Co to Mo results in the shift of XRD peaks towards higher angle, the spacing of 0.249 nm and 0.259 nm could be attributed to (110) and (004) plane of the bimetallic oxynitride Co3Mo2OxN6-x/C , respectively (Figure 2c). Additionally, the particle aggregation

7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

occurred partially for the catalyst when the annealing temperature increased up to 650 °C (Figure S3). The XPS spectra for the Co3Mo2OxN6-x/C was performed to investigate the surface chemical states. As shown in Figure 3a, the Co 2p XPS spectra exhibiting two characteristic peaks located at 780.6 and 795.6 eV, corresponding to the Co 2p3/2 and Co 2p1/2 spin-orbit peaks, comes from the dominant Co2+ and minor Co3+ species33 owing to the presence of Co-O or Co-N. The main peaks of Mo 3d5/2 appear at around 229.7 eV and 232.2 eV, arising from Mo (IV) and Mo (VI) species (Figure 3b), respectively. The binding energy located at 235.2 eV is attributed to Mo 3d3/2 of Mo (VI) species, which comes in pairs as the previous.34, 35The integral areas between the d5/2 and d3/2 doublets are 3:2 and the energy gap is 3.15 eV, keeping in accordance with the theoretical value. Given the difference electronegativity of Mo (2.16) versus Co (1.88), 35,

36

an electron transfer from Co to Mo reasonably lead to the lower

binding energy shift of Mo 3d peaks. In other words, the Co sites possess a more suitable electrostatic charge, which can be considered as the electrocatalytic active sites for the ORR. The N 1s data demonstrate two different peaks at 397.3, 399.8 eV, corresponding to pyridinic N, pyrrolic N, 37,38 respectively (Figure 3c), while the peak at 398.4 eV is from the contribution of nitrogen bound to the metal (M−N),13 such as Co(Mo)-N. Additionally, the O 1s spectrum of Co3Mo2OxN6-x/C (Figure S4a) can be divided into three peaks at 530.2, 532.2, and 533.6 eV, corresponding to the C-OH or C-O-C, C=O, and metal-bound oxygen, respectively.9 The result of oxygen K-edge


Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


absorption spectra of Co3Mo2OxN6-x/C also demonstrates the interaction between transition and O as shown in Figure S4b.

Electrochemical measurements of these catalysts were performed to evaluate the electrocatalytic ORR activity in N2- or O2-saturated 0.1 M KOH solution. Both cyclic voltammetry (CV) curves (Figure 4a) of Co3Mo2OxN6-x/C catalyst in N2- and O2saturated electrolyte exhibit one well-defined oxidation peaks at around 1.0 V, which can be ascribed to redox of Co species.27 While the cathodic peak of 0.79 V, which is about 30 mV and 10 mV more positive compared with that of Mo5N6/C and Co3Mo2OxN6-x/C-500 oC catalysts (Figure S5a, Figure S6a), respectively, in O2 saturated 0.1 M KOH can be attributed to electro-catalytic reduction of oxygen on the Co3Mo2OxN6-x/C catalyst, suggestive of high electro-catalytic activity for ORR. Rotating disk electrode (RDE) measurement was further performed to assess the ORR activity and kinetics properties on the Co3Mo2OxN6-x/C compared to Mo5N6/C, carbon (C), and commercial Pt/C catalysts in O2-saturated 0.1 M KOH solution (Figure 4d). The Co3Mo2OxN6-x/C catalyst exhibits the 0.9 V onset potential (Eonset) and 0.8 V vs. RHE half-wave (E1/2) potential, which are similar with commercial Pt/C and higher than that of Mo5N6/C, C catalysts, respectively (Figure 4d). That means the Co3Mo2OxN6-x/C catalyst shows excellent catalytic activity toward ORR compared to commercial Pt/C, Mo5N6/C and C catalysts. Moreover, the Co3Mo2OxN6-x/C catalyst displayed diffusion-limiting current slightly higher than commercial Pt/C catalyst and noticeably higher than that of Co3Mo2OxN6-x/C-500 oC, Mo5N6/C, C catalysts, demonstrating that Co3Mo2OxN6-x/C catalyst shows outstanding catalytic activity 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

toward ORR. Furthermore, the specific kinetic activity is also recognized as an important indicator to evaluate the catalytic performance of catalyst. The Co3Mo2OxN6-x/C catalyst ORR specific kinetic current density at 1600 rpm is about 82.7 mA cm-2 at 0.7 V, which is higher than the commercial Pt/C (50.5 mA cm-2) and 3.8, 9, 25 times that of Co3Mo2OxN6-x/C-500

o

C, Mo5N6/C and C catalysts,

respectively, as illustrated in Figure 4e and Figure S6b. The excellent activity toward the ORR may be contributed to the difference of electropositive between Co and Mo result in the observed mixture of valence states in Co3Mo2OxN6-x/C.35,36 Additionally, the Co3Mo2OxN6-x/C catalyst shows high diffusion-limiting current (~5.5 mA cm-2), which was superior to the Mo5N6/C (~4.0 mA cm-2) and C (~4 mA cm-2) catalysts and even comparable to commercial Pt/C (~5.0 mA cm-2), which may be related to the effect of good conductivity and high specific surface area.31,39 Meanwhile, relative to transition (oxy)nitrides, transition metal oxides or metal-free heteroatom-doped carbon materials ORR catalysts reported by other groups as shown in Table S1, the Co3Mo2OxN6-x/C catalyst demonstrates comparable or even higher ORR activity. The ORR catalytic pathways of the layered transition metal oxynitrides catalysts were further demonstrated by observing the generation of peroxide species (HO2−) during the ORR route via rotating ring-disk electrode (RRDE) measurements. Figure 5a demonstrate the yielded HO2– species for the Co3Mo2OxN6-x/C catalyst is below 10%, indicating the number of electron transfer is 3.9 across the potential range of 0.2-0.9 V, which is well in line with the result determined from the K-L plots on account of the polarization curves (Figure 4b, c), indicating that the ORR favors a dominated 4e10 ACS Paragon Plus Environment

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pathway, that is, the O2 can be reduced to OH- in direct.40 In contrast, the Mo5N6/C catalyst (Figure 5b) produced above 15% HO2− and the number of electron transfer is 3.7 between 0.2 and 0.8 V, also consist with the K-L plots (Figure S5b, c). Interestingly, the decreased HO2− yield validates the enhancement of ORR properties after the partial Mo substituted by Co in the Co3Mo2OxN6-x/C catalyst. This enhancement of activity could be easily due to the role of Co-doping modified the electronic structure of Co3Mo2OxN6-x/C catalyst. The ORR catalytic stability of Co3Mo2OxN6-x/C and Mo5N6/C catalysts were evaluated by chronoamperometric measurements at 0.75 V (Figure 6a). The Co3Mo2OxN6-x/C catalyst exhibits an excellent stability, retaining 95% of the initial current even after 12 h, while Mo5N6/C and commercial Pt/C electro-catalysts only maintain 80% and 60% of their initial current.27 The accelerate durability tests (ADT) were carried out by cycling the potential in the range of 0.6~1.0 V in O2-saturated 0.1 M KOH at 200 mV s-1. After 10, 000 cycles, the onset potential and E1/2 of Co3Mo2OxN6-x/C catalyst show a slight negative shift of about 5 mV and 10 mV as shown in Figure 6b, respectively, which is much lower than Mo5N6/C (5 mV, 15 mV negative shift, Figure S7a), and Pt/C (30 mV, 40 mV negative shift, Figure S7b), verifying the superior stability of Co3Mo2OxN6-x/C catalyst in alkaline media. Moreover, the tolerance to the small organic molecule (methanol, CO) crossover poisoning effect is very important consideration for DMAFCs. To investigate the ability of Co3Mo2OxN6-x/C and Mo5N6/C against methanol tolerance, LSV was measured in 0.1 M KOH (O2-saturated) containing 1 M methanol. After adding 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

methanol to the solution (Figure 6c and d), Co3Mo2OxN6-x/C and Mo5N6/C catalysts show high selectivity to ORR with no response in ORR activity while the commercial Pt/C catalyst shows a declining activity due to the occurrence of methanol oxidation reaction.27 These results unambiguously exhibit excellent tolerance to methanol poisoning effects of Co3Mo2OxN6-x/C and Mo5N6/C catalysts and their better durability as a promising cathode catalyst for alkaline methanol fuel cell.

CONCLUSION In summary, we demonstrate a simple two-step strategy to synthesize the non-noble metal carbon supported Co3Mo2OxN6-x catalyst for highly efficient ORR in alkaline solution. The as-prepared Co3Mo2OxN6-x/C catalyst is uniformly dispersed on support with particle size of ca. 5 nm and shows remarkable catalytic activity, long-term durability and even rivaling that of commercial Pt/C. The tuning of the electronic structure by electronegativity manipulation in the enhancement of ORR performance of Co3Mo2OxN6-x/C provides a versatile strategy for the further design and fabrication of non-noble metal cathodic catalyst in alkaline fuel cell. Conflict of Interest: The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (11179001) and the National High Technology Research and Development Program (No.2012AA052201, No.2012AA110102). The STEM work was carried using JEM-ARM200F microscope (Beijing National Laboratory for Condensed


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Matter Physics, Institute of Physics). The soft XAS data were collected at beamline 4B7B station of Beijing Synchrotron Radiation Facility. Supporting Information Available: Additional experimental and characterization details (e.g., XRD, XPS, XAS and electrochemical characterization) are described in Supporting Information.

REFERENCES 1. Winter, M.; Brodd, R. J. What Are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245−4269. 2. Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J. A Review on Non-Precious Metal Electrocatalysts for PEM Fuel Cells. Energy Environ. Sci. 2011, 4, 3167−3192. 3. Liang, Y.; Li, Y., Wang, H.; Dai, H. Strongly Coupled Inorganic/Nanocarbon Hybrid Materials for Advanced Electrocatalysis. J. Am. Chem. Soc. 2013, 135, 2013−2036. 4. Morozan, A.; Jousselme, B.; Palacin, S. Low-Platinum and Platinum-Free Catalysts for the Oxygen Reduction Reaction at Fuel Cell Cathodes. Energy Environ. Sci. 2011, 4, 1238–1254. 5. Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem., Int. Ed. 2006, 45, 2897–2901. 6. Du, C. Y.; Zhao, T. S.; Yang, W. W. Effect of Methanol Crossover on the 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

Cathode Behavior of a DMFC: A Half-Cell Investigation. Electrochim. Acta 2007, 52, 5266–5271. 7. Chang, Y.; Hong, F.; He, C.; Zhang, Q.; Liu, J. Nitrogen and Sulfur Dual-Doped Non-Noble Catalyst Using Fluidic Acrylonitrile Telomer as Precursor for Efficient Oxygen Reduction. Adv. Mater. 2013, 25, 4794–4799. 8. Choi, C. H.; Chung, M. W.; Kwon, H. W.; Park, S. H.; Woo, S. I. B, N- and P, N-Doped Graphene as Highly Active Catalysts for Oxygen Reduction Reactions in Acidic Media. J. Mater. Chem. A 2013, 1, 3694–3699. 9. Silva, R.; Voiry, D.; Chhowalla, M.; Asefa, T. Efficient Metal-Free Electrocatalysts for Oxygen Reduction: Polyaniline-Derived N- and O‑Doped Mesoporous Carbons. J. Am. Chem. Soc. 2013, 135, 7823−7826. 10. Zhao, S.; Liu, J.; Li, C.; Ji, W.; Yang, M.; Huang, H.; Liu, Y.; Kang, Z. Tunable Ternary (N, P, B)-Doped Porous Nanocarbons and Their Catalytic Properties for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2014, 6, 22297−22304 11. Wang, H.; Liang, Y.; Li, Y.; Dai, H. Co1−xS–Graphene Hybrid: A High-Performance Metal Chalcogenide Electrocatalyst for Oxygen Reduction. Angew. Chem., Int. Ed. 2011, 50, 10969 –10972. 12. Feng, Y.; Alonso-Vante, N. Carbon-Supported Cubic CoSe2 Catalysts for Oxygen Reduction Reaction in Alkaline Medium. Electrochim. Acta 2012, 72, 129–133. 13. Huo, R.; Jiang, W. J.; Xu, S.; Zhang, F.; Hu, J. S. Co/CoO/CoFe2O4/G Nanocomposites Derived from Layered Double Hydroxides towards Mass Production of Efficient Pt-Free Electrocatalysts for Oxygen Reduction Reaction. 14 ACS Paragon Plus Environment

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanoscale 2014, 6, 203–206. 14. Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Rapid Room-Temperature Synthesis of Nanocrystalline Spinels as Oxygen Reduction and Evolution Electrocatalysts. Nat. Chem. 2011, 3, 79−84. 15. Zhang, N.; Chen, X.; Lu, Y.; An, L.; Li, X.; Xia, D.; Zhang, Z.; Li, J. Nano-Intermetallic AuCu3 Catalyst for Oxygen Reduction Reaction: Performance and Mechanism. Small 2014, 10, 2662−2669. 16. Lu, Y.; Zhang, N.; An, L.; Li, X.; Xia, D. Synthesis of High Dispersed Intermetallic Ag4Sn/C and Its Enhanced Oxygen Reduction Reaction Activity. J. Power Sources 2013, 240, 606−611. 17. Zhang, N.; Yan, H.; Chen, X.; An, L.; Xia, Z.; Xia, D. Origins for the Synergetic Effects of AuCu3 in Catalysis for Oxygen Reduction Reaction. J. Phys. Chem. C 2015, 119, 907−912. 18. Holewinski, A.; Idrobo, J. C.; Linic, S. High-Performance Ag–Co Alloy Catalysts for Electrochemical Oxygen Reduction. Nature Chem. 2014, 6, 828−834. 19. Sun, T.; Wu, Q.; Che, R.; Bu, Y.; Jiang, Y.; Li, Y.; Yang, L.; Wang, X.; Hu, Z. Alloyed Co−Mo Nitride as High-Performance Electrocatalyst for Oxygen Reduction in Acidic Medium. ACS Catal. 2015, 5, 1857−1862. 20. Cao, B.; Veith, G. M.; Diaz, R. E.; Liu, J.; Stach, E. A.; Adzic, R. R.; Khalifan, P. G. Cobalt Molybdenum Oxynitrides: Synthesis, Structural Characterization, and Catalytic Activity for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 10753–10757. 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

21. Zhang, K.; Zhang, L.; Chen, X.; He, X.; Wang, X.; Dong, S.; Han, P.; Zhang, C.; Wang, S.; Gu, L.; Cui, G. Mesoporous Cobalt Molybdenum Nitride: A Highly Active Bifunctional Electrocatalyst and Its Application in Lithium–O2 Batteries. J. Phys. Chem. C, 2013, 117, 858−865. 22. Yu, C. C.; Ramanathan, S.; Oyama, S. T. New Catalysts for Hydroprocessing: Bimetallic Oxynitrides MI–MII–O–N (MI, M II= Mo, W, V, Nb, Cr, Mn, and Co): Part I. Synthesis and Characterization. J. Catal. 1998, 173, 1−9. 23. Bem, D. S.; Lampe-Önnerud, C. M.; Olsen, H. P.; Loye, H. C. Synthesis and Structure of Two New Ternary Nitrides: FeWN2 and MnMoN2. Inorg. Chem. 1996, 35, 581−585. 24. Ohnishi R.; Takanabe K.; Katayama M.; Kubota J.; Domen K. Nano-nitride Cathode Catalysts of Ti, Ta, and Nb for Polymer Electrolyte Fuel Cells: Temperature-Programmed Desorption Investigation of Molecularly Adsorbed Oxygen at Low Temperature. J. Phys. Chem. C, 2013, 117, 496–502. 25. Chen, J.; Takanabe, K.; Ohnishi, R.; Lu, D.; Okada, S.; Hatasawa, H.; Morioka, H.; Antonietti, M.; Kubota, J.; Domen, K. Nano-sized TiN on Carbon Black as an Efficient Electrocatalyst for the Oxygen Reduction Reaction Prepared Using an mpg-C3N4 Template. Chem. Commun. 2010, 46, 7492−7494. 26. Xia, D.; Liu, S.; Wang, Z.; Chen, G.; Zhang, L.; Zhang, L.; Hui, S.; Zhang, J. Methanol-tolerant MoN Electrocatalyst Synthesized Through Heat Treatment of Molybdenum

Tetraphenylporphyrin for Four-Electron Oxygen Reduction

Reaction. J. Power Sources, 2008, 177, 296−302. 16 ACS Paragon Plus Environment

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. An, L.; Huang, W.; Zhang, N.; Chen, X.; Xia, D. A Novel CoN Electrocatalyst with High Activity and Stability toward Oxygen Reduction Reaction. J. Mater. Chem. A 2014, 2, 62−65. 28. Ando,

T.;

Izhar,

S.;

Tominaga,

H.;

Nagai,

M.

Ammonia-Treated

Carbon-Supported Cobalt Tungsten as Fuel Cell Cathode Catalyst. Electrochim. Acta 2010, 55 2614–2621. 29. An, L.; Yan, H.; Chen, X.; Li, B.; Xia, Z.; Xia, D. Catalytic Performance and Mechanism of N-CoTi@CoTiO3 Catalysts for Oxygen Reduction Reaction. Nano Energy 2015, 20, 134–143. 30. Takasu Y.; Yoshinaga N.; Sugimoto W. Oxygen reduction behavior of RuO2/Ti, IrO2/Ti and IrM (M: Ru, Mo, W, V) Ox/Ti binary oxide electrodes in a sulfuric acid solution. Electro. Commun. 2008, 10, 668–672 31. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nature Mater. 2011, 10, 780–786. 32. Jiang, W. J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L. J.; Wang, J. Q.; Hu, J. S.; Wei, Z.; Wan, L. J. Understanding the High Activity of Fe−N−C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe−Nx. J. Am. Chem. Soc. 2016, 138, 3570–3578. 33. Wu, Z. S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H. M. Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance. ACS Nano 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

2010, 4, 3187–3194. 34. Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G. Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-Noble Metal Electrocatalysts for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 19186– 19192. 35. Hu, X. K.; Qian, Y. T.; Song, Z. T.; Huang, J. R.; Cao, R.; Xiao, J. Q. Comparative Study on MoO3 and HxMoO3 Nanobelts: Structure and Electric Transport. Chem. Mater. 2008, 20, 1527–1533. 36. Lin, H.; Liu, N.; Shi, Z.; Guo, Y.; Tang, Y.; Gao, Q. Cobalt-Doping in Molybdenum-Carbide Nanowires Toward Efficient Electrocatalytic Hydrogen Evolution. Adv. Funct. Mater. 2016, 26, 5590–5598. 37. He, Q.; Li, Q.; Khene, S.; Ren, X.; López-Suárez, F. E.; Lozano-Castello, D.; Bueno-López, A.; Wu, G. High-Loading Cobalt Oxide Coupled with Nitrogen-Doped Graphene for Oxygen Reduction in Anion-Exchange-Membrane Alkaline Fuel Cells. J. Phys. Chem. C 2013, 117, 8697−8707. 38. Liang, H. W.; Wei, W.; Wu, Z. S.; Feng, X.; Müllen, K. Mesoporous Metal– Nitrogen-Doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction. J. Am. Chem. Soc. 2013, 135, 16002−16005. 39. Lin, L.; Zhu, Q.; Xu, A. W. Noble-Metal-Free Fe−N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136, 11027−11033.


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40. Zhang, G.; Xia, B.; Wang, X.; Lou, X. W. Strongly Coupled NiCo2O4-rGO Hybrid Nanosheets as A Methanol-Tolerant Electrocatalyst for the Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 2408–2412.

Figure 1. Powder XRD patterns a) as-prepared supported Co3Mo2OxN6-x/C precursor, b) The prepared Co3Mo2OxN6-x/C, Mo5N6/C compounds and Mo5N6 (JCPDS NO. 51-1326) annealed at 550 °C under NH3 atmosphere.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

Figure 2. TEM images (a, b) of the Co3Mo2OxN6-x/C precursor at different magnifications; (c) HRTEM images of Co3Mo2OxN6-x/C nanoparticles; (d, e) TEM images of the Co3Mo2OxN6-x/C nanoparticles (scale bar: 20 nm), respectively; (f) size distribution of Co3Mo2OxN6-x/C nanoparticles.

Figure 3. XPS spectra of (a) Co 2p, (b) Mo 3d and (c) N 1s core level in Co3Mo2OxN6-x/C composition.


Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) CV curves of Co3Mo2OxN6-x/C catalyst at a scan rate of 50 mV s-1 in N2 or O2 saturated 0.1 M KOH solutions. (b) RDE voltammetric response for the ORR in O2-saturated 0.1M KOH at a scan rate of 10 mV s-1 at different rotation rates; (c) K-L plots for Co3Mo2OxN6-x/C catalyst at different potentials. (d) ORR polarization curves for Co3Mo2OxN6-x/C, Mo5N6/C, commercial Pt/C and C catalysts in O2-saturated 0.1 M KOH at a scan rate of 10 mV s-1 at 1600 rpm for comparison. e) The specific activity comparison of Co3Mo2OxN6-x/C, Mo5N6/C, commercial Pt/C and C at different potentials.

Figure 5. (a, b) Hydrogen peroxide yield and corresponding electron transfer number of Co3Mo2OxN6-x/C and Mo5N6/C catalysts at various potentials in O2 saturated 0.1 M KOH solution at a scan rate of 5 mV s-1 at 1600 rpm.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

Figure 6. (a) Chronoamperometric responses of Co3Mo2OxN6-x/C, Mo5N6/C catalysts at 0.75 V in O2-saturated 0.1 M KOH, which are normalized to the initial current responses. b) RDE polarization curves of the Co3Mo2OxN6-x/C catalyst before and after 10, 000 cycles in O2-saturated 0.1 M KOH. Potential cycling was carried out between 0.6 and 1.0 V versus RHE at 200 mV s-1. (c, d) RDE voltammetric responses to injection of 1 M methanol into O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s-1 for Co 3Mo2OxN6-x/C and Mo5N6/C catalysts modified electrodes, respectively.

TOC


Page 23 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C Catalyst for

Recommend Documents