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Electrocatalytically Active Graphene supported MMo Carbides (MNi, Co) for Oxygen Reduction Reaction
Accepted Manuscript Title: Electrocatalytically Active Graphene supported MMo Carbides (M
S0013-4686(16)31910-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.09.023 EA 27953
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
26-4-2016 3-9-2016 6-9-2016
Please cite this article as: Huanping Yang, Jilei Liu, Jin Wang, Chee Kok Poh, Weijiang Zhou, Jianyi Lin, Zexiang Shen, Electrocatalytically Active Graphene supported MMo Carbides (MNi, Co) for Oxygen Reduction Reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.09.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrocatalytically
Active
Graphene
supported MMo
Carbides (M=Ni, Co) for Oxygen Reduction Reaction
Huanping Yang a, Jilei Liu,*b,
c, d
Jin Wang,b Chee Kok Poh,c Weijiang Zhou,*e
Jianyi Lind and Zexiang Shen* b,d
a
b
Zhejiang University of Science & Technology, Liuhe road 318, Hangzhou, China Division of Physics and Applied Physics, Nanyang Technological University, Singapore
637371 c
Heterogeneous Catalysis, Institute of Chemical Engineering and Sciences, A*star, 1 Pesek
Road, Jurong Island, Singapore 627833 d
e
Energy Research Institute @ NTU, Nanyang Technological University, Singapore 639798 School of Mechanical and Aerospace Engineering, Nanyang Technological University,
Singapore 637553 Corresponding authors : J. L. Liu ([email protected] / [email protected]); W. J. Zhou ([email protected]); Z. S. Shen ([email protected]).
1
Abstract Graphene-supported NiMo and CoMo carbides with monodisperse feature were prepared and their electrocatalytic activity toward for the ORR was investigated. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide species (i.e., MoC and Mo2C, respectively) and in carbide particle size distribution. The ORR tests revealed that CoMo carbide/G exhibit better electrocatalytic performance than NiMo carbide/G in terms of on-set potential, limiting current density and durability. Our present results may open new way for the synthesis of molybdenum carbides, and also suggested that strategies for tuning the molybdenum carbide species within bi-metallic carbides may lead to further enhancements in ORR activity.
Keywords: electrocatalytic, Graphene, Molybdenum carbides, oxygen reduction reaction
1. Introduction Owing to the high energy yield and low environment impact, polymer electrolyte fuel cells such as direct methanol fuel cells (DMFC) and proton exchange membrane fuel cells (PEMFC) represent one of the most promising energy conversion devices available today.[1-3] Cathodic reduction of oxygen and anodic oxidization of hydrogen or alcohol take places at the cathode and anode in these devices. The major technological and economic bottlenecks for the industrial viability of polymer electrolyte fuel cells are the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode.[4, 5] Platinum supported on carbon is the most widely used cathode electrocatalyst for ORR to date. However, the prohibitive cost and weak durability of
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platinum based catalysts limit their large-scaled applications. The development of noble-metal free materials as alternative cathode catalysts is therefore highly desirable.[6] Various noblemetal free catalysts, such as transition metal chalcogenides,[7, 8] transition metal oxides[9-11]/ /nitride[4, 12, 13]/carbide,[14] macrocycles,[5], metal-organic framework,[15] transition metal oxynitrides,[16-18] and metal-free heteroatom-doped carbonaceous materials[19-22], have been investigated. Transition metal carbides are of particular interest as electrocatalysts for ORR due to their high electrical conductivity, good corrosion and poisoning resistance, and“platinum” like electronic and catalytic properties[23, 24]. Numbers of metal carbides, including tungsten carbides,[25] vanadium carbides[26], iron carbides[27], and molybdenum carbides[28, 29] were found to exhibit good ORR activity due to the synergistic effects arising from charge transfer between the metal carbide and Pt. Unfortunately, the synthesis of nano-structured carbides with high surface areas and free of aggregations remain a great challenge although approaches such as metal complexes pyrolysis,[30] carbon thermal reaction[31] and ultrasound irradiation of metal carbonyl compounds,[32] have been proposed. Herein we report the facile synthesis of nanostructured M (Ni/Co)-MoxC (x=1 or 2) nanocatalysts supported on graphene using a simple pyrolysis approach. These catalysts show wellcontrolled structures of nanoparticles encased by uniform graphene layers. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide components (i.e. MoC and Mo2C, respectively) and in carbide particle size distribution. This feature provides a unique model materials for probing the ORR active sites of the catalysts. 2. Experimental Section 2.1 Material synthesis Graphene was prepared via electrochemical exfoliation methods[33, 34].
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Synthesis of NiMo carbide/graphene (NiMo carbide/G). Typically, a mixture of 0.1 mM Ni(NO3)2 • H2O and 0.1 mM (NH4)6Mo7O24• H2O in 35 ml DI water was added to 50 ml of 1 mg/ml graphene dispersed aqueous solution. The mixture was sonicated at room temperature for 20 min until a homogeneous solution was achieved. The mixture solution was further stirred overnight at 80C. The resulting samples was washed with DI water, collected by centrifugation and freeze-dried before annealed at 800 C for 2h in the flow of 400 sccm Ar. CoMo carbide/graphene (CoMo carbide/G). The synthesis procedure of CoMo carbide/G was similar to that of NiMo carbide/G except replacing nickel nitrite with cobalt nitrite. 2.2 Material Characterization Transmission electron microscopy (TEM, JEM-2010, 200 kV), field-emission scanning electron microscopy (FESEM, JEM-7600F, 10.0 kV), X-ray diffraction (XRD, Bruker D-8 Avance), and X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 250 spectrometer (Thermo Electron, Altrincham, U.K) (Al KX-ray source (1486 eV)) were used to investigate morphology features and element information of samples. Thermogravimetric analysis (TGA) was conducted on a TGA Q500 (Thermal Analysis Instruments, Burlington, MA) in air, with a flow rate of 60 mL min-1 and a temperature ramp rate of 10 C min-1 from room temperature to 800 C. 2.3 Electrochemical evaluation Cyclic voltammerty (CV) measurements were performed in a standard three-electrode cell (Reference electrode: saturated Ag/AgCl; Counter electrode: platinum foil) using an Autolab PGSTAT302 electrochemical test system (Eco Chemie, Netherland). The working electrode was prepared with a similar method to that reported previously[33]. Briefly, 10 mg of active material was ultrasonically dispersed into 1 ml of 2-propanl containing Nafion solution (5 wt%, dupont)
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to form a catalyst ink. 10 l of the catalyst ink was coated on the glassy carbon disk (5 mm in diameter) and dried at 80 C for 10 min. Pure GO, Pt-loaded carbon catalyst (Pt/C 20% on Vulcan XC-72R) and 50% NiMo carbide/graphene (NiMo carbide/G) + 50% CoMo carbide/graphene (CoMo carbide/G) with the same amount were also studied for comparison by using the same electrode configuration. Oxygen reduction reaction (ORR) measurement was conducted in a 0.1 M KOH aqueous electrolyte saturated with oxygen at a scan rate of 2 mV/s on the rotating-disk electrode system (Eco Chemie, Netherland). The bare glassy carbon electrode was tested without catalyst casting as comparison. All measurements were carried out at room temperature. The Koutecky-Levich equation was used to calculate the number of electron transferred (n)[33]. 3. Results and discussion The bi-metallic (Ni/Co)-MoxC (x=1 or 2) nano-catalysts were successfully synthesized on graphene supports by a one-step pyrolysis approach. X-ray diffraction (XRD) patterns of the resultant products (Fig. 1) show that NiMo carbide is a mixture of MoC (JCPDS #65-6664) and Mo2C (JCPDS #65-8766) with MoC acting as the dominate specie. Besides these two phases, the trace of MoNi (JCPDS 65-6903) is also detected in the NiMo carbide/G. On the other hand, the Mo2C (JCPDS #65-8766) dominates the CoMo carbide/G, in addition to some traces of metallic cobalt and Co3Mo(JCPDS 65-3519). These results indicate that Ni and Co precursor effect the carbide components of the final products although underlying mechanism remains unclear until now. The difference in components for NiMo carbide/G and CoMo carbide/G provides a unique model materials for probing the ORR active sites of the catalysts. The representative SEM images show that monodisperse (Ni/Co)-MoxC (x=1 or 2) nanoparticles with diameter ranging from 5 - 100 nm distributed uniformly over graphene layers
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(Figs. 2a & 2d). These features were further confirmed by TEM images in Figs. 2b and 2e. The CoMo carbide exhibit relative smaller particle size compared to that of NiMo carbide. HRTEM analysis (Figs. 2c & 2f ) verify the high crystalline properties of NiMo / CoMo carbides and the intact contact between them and graphene. This is favour for effective charge transfer, giving rise to enhanced electrocatalytic performance. The fast Fourier transformation (FFT) electron diffraction pattern (inset in Fig.2c) reveals the (100), (101) and (103) planes of Mo2C, agreeing well with XRD results. Similarly, the (100) and (001) planes of MoC were identified for NiMo carbide (inset in Fig. 2f). In addition, benefiting from the much smaller particle size of CoMo carbide, the BET results (Figs. 2g &2h) given the specific surface areas of 86.4 m2/g for CoMo carbide/G, which is larger than that of NiMo carbide/G (68.8 m2/g).This porous structure could facilitate the electrolyte ion diffusion and thus maximize charge transfer process. The surface composition is of particular interest and the chemical state of various elements in catalysts were investigated by X-ray photoelectron spectroscopy (XPS). The deconvolution of C1s spectra presents the peaks at 284.5, 285.5 and 286.7 eV, which were assigned to graphitic carbon (C-C), C-O and O-C=O, respectively (Figs 3a & 3b). The oxygen function group contents were estimated to be 60.2% for NiMo carbide/G and 57.6% for CoMo carbide/G, suggesting the higher metal oxidization states in NiMo carbide/G (Tab.S1). The Co 2p3/2 data exhibits a dominant peak at 781.8 eV corresponding to Co2+/3+, in addition to the small peak at 779.0 eV owing to Co metal (Fig. 3c).[35] Similar phenomenon was also observed for Ni 2p spectrum (Fig. 3d). The high-resolution Mo 3d XPS (Fig. 3e) revealed that the peak at 228.5 eV was attributed to Mo2+, stemming from Mo2C.[36] The peak at 231.6 was assigned to MoO2,[37] and the peaks at 232.3 and 235.5 eV were attributed to MoO3, as a consequence of surface oxidization.[18, 35] The Mo 3d in NiMo carbide/G (Fig. 3f) was similar to those of CoMo carbide/G, except for the
6
absence of the peak that corresponds to Mo2+, indicating that MoC is the dominant specie. These results consistent well with XRD results. A certain amount of N was also introduced due to the use of (NH4)6Mo7O24• H2O as precursor. The N content is calculated to be 3.8% for NiMo carbide/G and 4.5% for CoMo carbide/G, respectively. Diverse-distribution of N-doping types were also identified and quantified via the deconvolution of N1s spectra (Tab. S1 and Fig. S1). The decent amount of N content and the diverse distribution of N-doping types ensure the effective electrocatalytic activity of both NiMo carbide/G and CoMo carbide/G.[38] The ORR activity of NiMo carbide/G and CoMo carbide/G was evaluated by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements (Fig. 4). The results were compared with that of a commercial Pt/C catalyst, pure GO and 50% NiMo/G+50% CoMo/G. Different from the featureless CV curves in N2-saturated KOH solution, significant ORR peaks were observed for all catalysts in the O2-saturated KOH solution, corroborating the ORR catalytic activity of NiMo/CoMo carbide materials (Fig. 4a). The reduction peaks locate at -0.29 V (vs. Ag/AgCl) for NiMo carbide/G and at -0.21 V (vs. Ag/AgCl) for CoMo carbide/G, respectively. The stronger and more positive cathodic peak in CoMo carbide/G indicate that O2 can be reduced more easily on it than on NiMo carbide/G. This is further confirmed ORR polarization curves in Fig. 4b. A more positive on-set potential (-0.07 V) accompanying with a much larger current response was detected for CoMo carbide/G than that of NiMo carbide/G (0.13 V), 50% NiMo/G+50% CoMo/G (-0.09 V), and pure GO (-0.17 V). The current density for CoMo carbide/G is 3.3 mA /cm2 at -0.4 V and 4.0 mA/cm2 at -0.8 V, which is higher than that of NiMo carbide/G (2.0 mA/cm2 at -0.4 and 2.7 mA/cm2 at -0.8 V) (Fig. 4c), 50% NiMo/G+50% CoMo/G (2.9 mA/cm2 at -0.4 and 3.4 mA/cm2 at -0.8 V), pure GO (1.1 mA/cm2 at -0.4 and 2.0 mA/cm2 at -0.8 V) (Fig. S2). These results corroborate that i) it is the bi-metallic carbide that 7
contribute more to the oxygen reduction activity, and ii) the CoMo carbide exhibits much better electrocatalytic performance than NiMo carbide. The mass activities (at -0.8 V) were estimated to be 5.3 A/g for NiMo carbide/G and 7.8 A/g for CoMo carbide/G, respectively. Nevertheless both NiMo carbide/G and CoMo carbide/G are still inferior to commercial Pt/C catalyst and more work are still needed to optimize their performance further. RDE files (Figs. 5a &5d) illustrate that the limiting current density increases with increasing rotation rate, indicating kinetic-limiting behaviour of ORR.[39, 40] The linear Koutecky-Levich (K-L) plots (Figs. 5b & 5e) were derived at different potentials, suggesting the first-order reaction kinetics toward the concentration of O2 on both catalysts from -0.3 to -0.9 V. The transferred electron number (n) was calculated to be 2.0-3.8 depending on different potentials and catalyst types (Fig. 4d). At potentials -0.5 V, 2.5 transferred electrons was determined for NiMo carbide/G, indicating that a two-electron pathway is the dominant process, consisting well with CV and LSV results. While a nearly four electron process was indentified for CoMo carbide/G with a much larger n value (3.6 at -0.5 V). These features corroborate the better electrocatalytic properties of CoMo carbide/G catalyst. To reveal the underlying reasons for the enhanced electrocatalytic activity of CoMo carbide/G over NiMo carbide/G, the electrochemical impedance spectroscopy (EIS) tests at different potentials were conducted. Nyquist plots for NiMo carbide/G and CoMo carbide/G (Figs. 5c&5f) clearly depict two time constants or two arcs, one appearing as a small depressed semicircle in the high-frequency region and the other appearing as a large semicircle in the following low frequency region. The high-frequency arc is related to the electronic transport and exhibit no change in diameter. Whereas the diameter of the low-frequency arc (Figs. 5c&5f) is found to be potential dependent. This clearly illustrates that the relaxation corresponding to this arc is due to
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the oxygen-reduction charge-transfer process.[41, 42] Fitting results (Tab. S2) reveal that the CoMo carbide/G displays much smaller charge-transfer resistance than NiMo carbide/G, although similar electron transport impedances were detected for both catalysts. The enhanced charge-transfer is attributed to i) more active sites associated with the much smaller CoMo carbide particle sizes (Fig.2); ii) the larger specific surface area that facilitate the electrolyte ion diffusion and thus maximize charge transfer process; iii) low oxygen function group contents (Table S1); iv) the high N content and more diverse distribution of N-doping types (Tab. S1 and Fig. S1); v) the presence of a small amount of Co metal (Fig. 1) that ensures good electric conductivity; and vi) the intrinsic high electrocalatytic properties of Mo2C over MoC.[43-45] Durability is another critical factor for evaluating a ORR catalysts. The polarization curves of NiMo carbide/G and CoMo carbide/G were collected continuously in O2-saturated 0.1 M KOH. After 5,000 cycles. The catalysts exhibit similar curves with a slightly decay of cathodic currents (with limited current density retention of 94% for CoMo carbide/G and 91% for NiMo carbide/G, respectively), and nearly the same onset potential (Fig. 6), corroborating good durability and practical feasibility. 4. Conclusions Graphene-supported NiMo and CoMo carbides with monodisperse feature were prepared via a facile one-step pyrolysis approach. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide species (i.e., MoC and Mo2C, respectively) and in carbide particle size distribution. This feature provides a unique model materials for probing the ORR active sites of the catalysts. The ORR tests revealed that CoMo carbide/G exhibit better electrocatalytic performance than NiMo carbide/G in terms of on-set potential, limiting current density and durability. The function improvement is due to i) more active sites associated with the much
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smaller CoMo carbide particle sizes (Fig.2); ii) the larger specific surface area that facilitate the electrolyte ion diffusion and thus maximize charge transfer process; and iii) low oxygen function group contents (Table S1); iv) the presence of a small amount of Co metal (Figure 1) that ensures good electric conductivity; and v) the intrinsic high electrocalatytic properties of Mo2C over MoC. Our present results open new way for the synthesis of molybdenum carbides (i.e. MoC or Mo2C), and also suggested that strategies for tuning the molybdenum carbide components within bi-metallic carbides may lead to further enhancements in ORR activity.
Conflict of interest The authors declares that there is no conflict of interest. Acknowledgments The authors acknowledge financial support from the Energy Research Institute, NTU, Singapore. We also acknowledge the financial support from Ministry of Education, Tier 1(Grant No. M4011424.110) and Tier 2 (Grant No. M4020284.110)
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Fig.1. Materials Characterization. Comparative XRD patterns of NiMo carbide/G and CoMo carbide/G electrocatalysts.
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Fig. 2. Materials Characterization. CoMo carbide/G (a, b, c, g). NiMo carbide/G. (d, e, f, h). Typical SEM images(a, d) (inset is the magnified SEM image), TEM images (b, e) and HRTEM images (c, f) (inset is the fast Fourier transformation (FFT) pattern of the rectangle area). Comparative N2 adsorption/desorption isotherms of CoMo carbide/G (g) and NiMo carbide/G (h).
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Fig. 3. XPS Characterization. Left column: CoMo carbide/G. (a) C 1s, (c) Ni 2p and (e) Mo 3d. Right column: NiMo carbide/G. (b) C 1s, (d) Co 2p and (f) Mo 3d.
15
Fig. 4. ORR activity evaluation. Cyclic voltammetry (CV) curves (a), RDE polarization curves (b), and the specific activity and mass activity of NiMo carbide/G, CoMo carbide/G and 20% Pt/C at -0.8 V (c) (NiMo carbide/G, blue area; CoMo carbide/G, pink area; 20% Pt/C, grey area). (d) Calculated transferred electron number.
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Fig. 5. ORR activity evaluation. RDE polarization curves (a, d), Koutechy-Levich plots at a potential range of -0.30 to -0.90 V (vs. Ag/AgCl) (b, e), and EIS plots (c, f). Top part: CoMo carbide/G; bottom part: NiMo carbide/G.
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Fig. 6. ORR activity evaluation. Polarization curves for the ORR on NiMo carbide/G (a) and CoMo carbide/G, before and after 5,000 potential cycles. Sweep rate, 2 mV/s; rotating rate, 1600 rpm.
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S0013-4686(16)31910-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.09.023 EA 27953
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
26-4-2016 3-9-2016 6-9-2016
Please cite this article as: Huanping Yang, Jilei Liu, Jin Wang, Chee Kok Poh, Weijiang Zhou, Jianyi Lin, Zexiang Shen, Electrocatalytically Active Graphene supported MMo Carbides (MNi, Co) for Oxygen Reduction Reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.09.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrocatalytically
Active
Graphene
supported MMo
Carbides (M=Ni, Co) for Oxygen Reduction Reaction
Huanping Yang a, Jilei Liu,*b,
c, d
Jin Wang,b Chee Kok Poh,c Weijiang Zhou,*e
Jianyi Lind and Zexiang Shen* b,d
a
b
Zhejiang University of Science & Technology, Liuhe road 318, Hangzhou, China Division of Physics and Applied Physics, Nanyang Technological University, Singapore
637371 c
Heterogeneous Catalysis, Institute of Chemical Engineering and Sciences, A*star, 1 Pesek
Road, Jurong Island, Singapore 627833 d
e
Energy Research Institute @ NTU, Nanyang Technological University, Singapore 639798 School of Mechanical and Aerospace Engineering, Nanyang Technological University,
Singapore 637553 Corresponding authors : J. L. Liu ([email protected] / [email protected]); W. J. Zhou ([email protected]); Z. S. Shen ([email protected]).
1
Abstract Graphene-supported NiMo and CoMo carbides with monodisperse feature were prepared and their electrocatalytic activity toward for the ORR was investigated. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide species (i.e., MoC and Mo2C, respectively) and in carbide particle size distribution. The ORR tests revealed that CoMo carbide/G exhibit better electrocatalytic performance than NiMo carbide/G in terms of on-set potential, limiting current density and durability. Our present results may open new way for the synthesis of molybdenum carbides, and also suggested that strategies for tuning the molybdenum carbide species within bi-metallic carbides may lead to further enhancements in ORR activity.
Keywords: electrocatalytic, Graphene, Molybdenum carbides, oxygen reduction reaction
1. Introduction Owing to the high energy yield and low environment impact, polymer electrolyte fuel cells such as direct methanol fuel cells (DMFC) and proton exchange membrane fuel cells (PEMFC) represent one of the most promising energy conversion devices available today.[1-3] Cathodic reduction of oxygen and anodic oxidization of hydrogen or alcohol take places at the cathode and anode in these devices. The major technological and economic bottlenecks for the industrial viability of polymer electrolyte fuel cells are the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode.[4, 5] Platinum supported on carbon is the most widely used cathode electrocatalyst for ORR to date. However, the prohibitive cost and weak durability of
2
platinum based catalysts limit their large-scaled applications. The development of noble-metal free materials as alternative cathode catalysts is therefore highly desirable.[6] Various noblemetal free catalysts, such as transition metal chalcogenides,[7, 8] transition metal oxides[9-11]/ /nitride[4, 12, 13]/carbide,[14] macrocycles,[5], metal-organic framework,[15] transition metal oxynitrides,[16-18] and metal-free heteroatom-doped carbonaceous materials[19-22], have been investigated. Transition metal carbides are of particular interest as electrocatalysts for ORR due to their high electrical conductivity, good corrosion and poisoning resistance, and“platinum” like electronic and catalytic properties[23, 24]. Numbers of metal carbides, including tungsten carbides,[25] vanadium carbides[26], iron carbides[27], and molybdenum carbides[28, 29] were found to exhibit good ORR activity due to the synergistic effects arising from charge transfer between the metal carbide and Pt. Unfortunately, the synthesis of nano-structured carbides with high surface areas and free of aggregations remain a great challenge although approaches such as metal complexes pyrolysis,[30] carbon thermal reaction[31] and ultrasound irradiation of metal carbonyl compounds,[32] have been proposed. Herein we report the facile synthesis of nanostructured M (Ni/Co)-MoxC (x=1 or 2) nanocatalysts supported on graphene using a simple pyrolysis approach. These catalysts show wellcontrolled structures of nanoparticles encased by uniform graphene layers. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide components (i.e. MoC and Mo2C, respectively) and in carbide particle size distribution. This feature provides a unique model materials for probing the ORR active sites of the catalysts. 2. Experimental Section 2.1 Material synthesis Graphene was prepared via electrochemical exfoliation methods[33, 34].
3
Synthesis of NiMo carbide/graphene (NiMo carbide/G). Typically, a mixture of 0.1 mM Ni(NO3)2 • H2O and 0.1 mM (NH4)6Mo7O24• H2O in 35 ml DI water was added to 50 ml of 1 mg/ml graphene dispersed aqueous solution. The mixture was sonicated at room temperature for 20 min until a homogeneous solution was achieved. The mixture solution was further stirred overnight at 80C. The resulting samples was washed with DI water, collected by centrifugation and freeze-dried before annealed at 800 C for 2h in the flow of 400 sccm Ar. CoMo carbide/graphene (CoMo carbide/G). The synthesis procedure of CoMo carbide/G was similar to that of NiMo carbide/G except replacing nickel nitrite with cobalt nitrite. 2.2 Material Characterization Transmission electron microscopy (TEM, JEM-2010, 200 kV), field-emission scanning electron microscopy (FESEM, JEM-7600F, 10.0 kV), X-ray diffraction (XRD, Bruker D-8 Avance), and X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 250 spectrometer (Thermo Electron, Altrincham, U.K) (Al KX-ray source (1486 eV)) were used to investigate morphology features and element information of samples. Thermogravimetric analysis (TGA) was conducted on a TGA Q500 (Thermal Analysis Instruments, Burlington, MA) in air, with a flow rate of 60 mL min-1 and a temperature ramp rate of 10 C min-1 from room temperature to 800 C. 2.3 Electrochemical evaluation Cyclic voltammerty (CV) measurements were performed in a standard three-electrode cell (Reference electrode: saturated Ag/AgCl; Counter electrode: platinum foil) using an Autolab PGSTAT302 electrochemical test system (Eco Chemie, Netherland). The working electrode was prepared with a similar method to that reported previously[33]. Briefly, 10 mg of active material was ultrasonically dispersed into 1 ml of 2-propanl containing Nafion solution (5 wt%, dupont)
4
to form a catalyst ink. 10 l of the catalyst ink was coated on the glassy carbon disk (5 mm in diameter) and dried at 80 C for 10 min. Pure GO, Pt-loaded carbon catalyst (Pt/C 20% on Vulcan XC-72R) and 50% NiMo carbide/graphene (NiMo carbide/G) + 50% CoMo carbide/graphene (CoMo carbide/G) with the same amount were also studied for comparison by using the same electrode configuration. Oxygen reduction reaction (ORR) measurement was conducted in a 0.1 M KOH aqueous electrolyte saturated with oxygen at a scan rate of 2 mV/s on the rotating-disk electrode system (Eco Chemie, Netherland). The bare glassy carbon electrode was tested without catalyst casting as comparison. All measurements were carried out at room temperature. The Koutecky-Levich equation was used to calculate the number of electron transferred (n)[33]. 3. Results and discussion The bi-metallic (Ni/Co)-MoxC (x=1 or 2) nano-catalysts were successfully synthesized on graphene supports by a one-step pyrolysis approach. X-ray diffraction (XRD) patterns of the resultant products (Fig. 1) show that NiMo carbide is a mixture of MoC (JCPDS #65-6664) and Mo2C (JCPDS #65-8766) with MoC acting as the dominate specie. Besides these two phases, the trace of MoNi (JCPDS 65-6903) is also detected in the NiMo carbide/G. On the other hand, the Mo2C (JCPDS #65-8766) dominates the CoMo carbide/G, in addition to some traces of metallic cobalt and Co3Mo(JCPDS 65-3519). These results indicate that Ni and Co precursor effect the carbide components of the final products although underlying mechanism remains unclear until now. The difference in components for NiMo carbide/G and CoMo carbide/G provides a unique model materials for probing the ORR active sites of the catalysts. The representative SEM images show that monodisperse (Ni/Co)-MoxC (x=1 or 2) nanoparticles with diameter ranging from 5 - 100 nm distributed uniformly over graphene layers
5
(Figs. 2a & 2d). These features were further confirmed by TEM images in Figs. 2b and 2e. The CoMo carbide exhibit relative smaller particle size compared to that of NiMo carbide. HRTEM analysis (Figs. 2c & 2f ) verify the high crystalline properties of NiMo / CoMo carbides and the intact contact between them and graphene. This is favour for effective charge transfer, giving rise to enhanced electrocatalytic performance. The fast Fourier transformation (FFT) electron diffraction pattern (inset in Fig.2c) reveals the (100), (101) and (103) planes of Mo2C, agreeing well with XRD results. Similarly, the (100) and (001) planes of MoC were identified for NiMo carbide (inset in Fig. 2f). In addition, benefiting from the much smaller particle size of CoMo carbide, the BET results (Figs. 2g &2h) given the specific surface areas of 86.4 m2/g for CoMo carbide/G, which is larger than that of NiMo carbide/G (68.8 m2/g).This porous structure could facilitate the electrolyte ion diffusion and thus maximize charge transfer process. The surface composition is of particular interest and the chemical state of various elements in catalysts were investigated by X-ray photoelectron spectroscopy (XPS). The deconvolution of C1s spectra presents the peaks at 284.5, 285.5 and 286.7 eV, which were assigned to graphitic carbon (C-C), C-O and O-C=O, respectively (Figs 3a & 3b). The oxygen function group contents were estimated to be 60.2% for NiMo carbide/G and 57.6% for CoMo carbide/G, suggesting the higher metal oxidization states in NiMo carbide/G (Tab.S1). The Co 2p3/2 data exhibits a dominant peak at 781.8 eV corresponding to Co2+/3+, in addition to the small peak at 779.0 eV owing to Co metal (Fig. 3c).[35] Similar phenomenon was also observed for Ni 2p spectrum (Fig. 3d). The high-resolution Mo 3d XPS (Fig. 3e) revealed that the peak at 228.5 eV was attributed to Mo2+, stemming from Mo2C.[36] The peak at 231.6 was assigned to MoO2,[37] and the peaks at 232.3 and 235.5 eV were attributed to MoO3, as a consequence of surface oxidization.[18, 35] The Mo 3d in NiMo carbide/G (Fig. 3f) was similar to those of CoMo carbide/G, except for the
6
absence of the peak that corresponds to Mo2+, indicating that MoC is the dominant specie. These results consistent well with XRD results. A certain amount of N was also introduced due to the use of (NH4)6Mo7O24• H2O as precursor. The N content is calculated to be 3.8% for NiMo carbide/G and 4.5% for CoMo carbide/G, respectively. Diverse-distribution of N-doping types were also identified and quantified via the deconvolution of N1s spectra (Tab. S1 and Fig. S1). The decent amount of N content and the diverse distribution of N-doping types ensure the effective electrocatalytic activity of both NiMo carbide/G and CoMo carbide/G.[38] The ORR activity of NiMo carbide/G and CoMo carbide/G was evaluated by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements (Fig. 4). The results were compared with that of a commercial Pt/C catalyst, pure GO and 50% NiMo/G+50% CoMo/G. Different from the featureless CV curves in N2-saturated KOH solution, significant ORR peaks were observed for all catalysts in the O2-saturated KOH solution, corroborating the ORR catalytic activity of NiMo/CoMo carbide materials (Fig. 4a). The reduction peaks locate at -0.29 V (vs. Ag/AgCl) for NiMo carbide/G and at -0.21 V (vs. Ag/AgCl) for CoMo carbide/G, respectively. The stronger and more positive cathodic peak in CoMo carbide/G indicate that O2 can be reduced more easily on it than on NiMo carbide/G. This is further confirmed ORR polarization curves in Fig. 4b. A more positive on-set potential (-0.07 V) accompanying with a much larger current response was detected for CoMo carbide/G than that of NiMo carbide/G (0.13 V), 50% NiMo/G+50% CoMo/G (-0.09 V), and pure GO (-0.17 V). The current density for CoMo carbide/G is 3.3 mA /cm2 at -0.4 V and 4.0 mA/cm2 at -0.8 V, which is higher than that of NiMo carbide/G (2.0 mA/cm2 at -0.4 and 2.7 mA/cm2 at -0.8 V) (Fig. 4c), 50% NiMo/G+50% CoMo/G (2.9 mA/cm2 at -0.4 and 3.4 mA/cm2 at -0.8 V), pure GO (1.1 mA/cm2 at -0.4 and 2.0 mA/cm2 at -0.8 V) (Fig. S2). These results corroborate that i) it is the bi-metallic carbide that 7
contribute more to the oxygen reduction activity, and ii) the CoMo carbide exhibits much better electrocatalytic performance than NiMo carbide. The mass activities (at -0.8 V) were estimated to be 5.3 A/g for NiMo carbide/G and 7.8 A/g for CoMo carbide/G, respectively. Nevertheless both NiMo carbide/G and CoMo carbide/G are still inferior to commercial Pt/C catalyst and more work are still needed to optimize their performance further. RDE files (Figs. 5a &5d) illustrate that the limiting current density increases with increasing rotation rate, indicating kinetic-limiting behaviour of ORR.[39, 40] The linear Koutecky-Levich (K-L) plots (Figs. 5b & 5e) were derived at different potentials, suggesting the first-order reaction kinetics toward the concentration of O2 on both catalysts from -0.3 to -0.9 V. The transferred electron number (n) was calculated to be 2.0-3.8 depending on different potentials and catalyst types (Fig. 4d). At potentials -0.5 V, 2.5 transferred electrons was determined for NiMo carbide/G, indicating that a two-electron pathway is the dominant process, consisting well with CV and LSV results. While a nearly four electron process was indentified for CoMo carbide/G with a much larger n value (3.6 at -0.5 V). These features corroborate the better electrocatalytic properties of CoMo carbide/G catalyst. To reveal the underlying reasons for the enhanced electrocatalytic activity of CoMo carbide/G over NiMo carbide/G, the electrochemical impedance spectroscopy (EIS) tests at different potentials were conducted. Nyquist plots for NiMo carbide/G and CoMo carbide/G (Figs. 5c&5f) clearly depict two time constants or two arcs, one appearing as a small depressed semicircle in the high-frequency region and the other appearing as a large semicircle in the following low frequency region. The high-frequency arc is related to the electronic transport and exhibit no change in diameter. Whereas the diameter of the low-frequency arc (Figs. 5c&5f) is found to be potential dependent. This clearly illustrates that the relaxation corresponding to this arc is due to
8
the oxygen-reduction charge-transfer process.[41, 42] Fitting results (Tab. S2) reveal that the CoMo carbide/G displays much smaller charge-transfer resistance than NiMo carbide/G, although similar electron transport impedances were detected for both catalysts. The enhanced charge-transfer is attributed to i) more active sites associated with the much smaller CoMo carbide particle sizes (Fig.2); ii) the larger specific surface area that facilitate the electrolyte ion diffusion and thus maximize charge transfer process; iii) low oxygen function group contents (Table S1); iv) the high N content and more diverse distribution of N-doping types (Tab. S1 and Fig. S1); v) the presence of a small amount of Co metal (Fig. 1) that ensures good electric conductivity; and vi) the intrinsic high electrocalatytic properties of Mo2C over MoC.[43-45] Durability is another critical factor for evaluating a ORR catalysts. The polarization curves of NiMo carbide/G and CoMo carbide/G were collected continuously in O2-saturated 0.1 M KOH. After 5,000 cycles. The catalysts exhibit similar curves with a slightly decay of cathodic currents (with limited current density retention of 94% for CoMo carbide/G and 91% for NiMo carbide/G, respectively), and nearly the same onset potential (Fig. 6), corroborating good durability and practical feasibility. 4. Conclusions Graphene-supported NiMo and CoMo carbides with monodisperse feature were prepared via a facile one-step pyrolysis approach. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide species (i.e., MoC and Mo2C, respectively) and in carbide particle size distribution. This feature provides a unique model materials for probing the ORR active sites of the catalysts. The ORR tests revealed that CoMo carbide/G exhibit better electrocatalytic performance than NiMo carbide/G in terms of on-set potential, limiting current density and durability. The function improvement is due to i) more active sites associated with the much
9
smaller CoMo carbide particle sizes (Fig.2); ii) the larger specific surface area that facilitate the electrolyte ion diffusion and thus maximize charge transfer process; and iii) low oxygen function group contents (Table S1); iv) the presence of a small amount of Co metal (Figure 1) that ensures good electric conductivity; and v) the intrinsic high electrocalatytic properties of Mo2C over MoC. Our present results open new way for the synthesis of molybdenum carbides (i.e. MoC or Mo2C), and also suggested that strategies for tuning the molybdenum carbide components within bi-metallic carbides may lead to further enhancements in ORR activity.
Conflict of interest The authors declares that there is no conflict of interest. Acknowledgments The authors acknowledge financial support from the Energy Research Institute, NTU, Singapore. We also acknowledge the financial support from Ministry of Education, Tier 1(Grant No. M4011424.110) and Tier 2 (Grant No. M4020284.110)
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Fig.1. Materials Characterization. Comparative XRD patterns of NiMo carbide/G and CoMo carbide/G electrocatalysts.
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Fig. 2. Materials Characterization. CoMo carbide/G (a, b, c, g). NiMo carbide/G. (d, e, f, h). Typical SEM images(a, d) (inset is the magnified SEM image), TEM images (b, e) and HRTEM images (c, f) (inset is the fast Fourier transformation (FFT) pattern of the rectangle area). Comparative N2 adsorption/desorption isotherms of CoMo carbide/G (g) and NiMo carbide/G (h).
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Fig. 3. XPS Characterization. Left column: CoMo carbide/G. (a) C 1s, (c) Ni 2p and (e) Mo 3d. Right column: NiMo carbide/G. (b) C 1s, (d) Co 2p and (f) Mo 3d.
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Fig. 4. ORR activity evaluation. Cyclic voltammetry (CV) curves (a), RDE polarization curves (b), and the specific activity and mass activity of NiMo carbide/G, CoMo carbide/G and 20% Pt/C at -0.8 V (c) (NiMo carbide/G, blue area; CoMo carbide/G, pink area; 20% Pt/C, grey area). (d) Calculated transferred electron number.
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Fig. 5. ORR activity evaluation. RDE polarization curves (a, d), Koutechy-Levich plots at a potential range of -0.30 to -0.90 V (vs. Ag/AgCl) (b, e), and EIS plots (c, f). Top part: CoMo carbide/G; bottom part: NiMo carbide/G.
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Fig. 6. ORR activity evaluation. Polarization curves for the ORR on NiMo carbide/G (a) and CoMo carbide/G, before and after 5,000 potential cycles. Sweep rate, 2 mV/s; rotating rate, 1600 rpm.
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