Î±-MoC and Ag for Efficient Oxygen Reduction

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Letter

Synergistic Effects of C/#-MoC and Ag for Efficient Oxygen Reduction Reaction Lujie Cao, Pengpeng Tao, Minchan Li, Fucong Lyu, Zhenyu Wang, Sisi Wu, Wenxi Wang, Yifeng Huo, Li Huang, and Zhouguang Lu J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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The Journal of Physical Chemistry Letters

Synergistic Effects of C/α-MoC and Ag for Efficient Oxygen Reduction Reaction Lujie Cao†,§, Pengpeng Tao†,§, Minchan Li†,§, Fucong Lyu†, Zhenyu Wang†, Sisi Wu†, Wenxi Wang†, Yifeng Huo†, Li Huang‡,*, and Zhouguang Lu†,* †Department of Materials Science & Engineering, Southern University of Science and Technology, Shenzhen 518055, P.R. China ‡ Department of Physics, Southern University of Science and Technology, Shenzhen 518055, P.R. China § These authors contributed equally to this work.

AUTHOR INFORMATION Corresponding Author [email protected] (ZG Lu); [email protected] (L Huang)

ACS Paragon Plus Environment

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ABSTRACT. It remains challenging to prepare highly active and stable catalysts from earth-abundant elements for the oxygen reduction reaction (ORR). Herein we report a facile method to synthesize cost-effective heterogeneous C/α-MoC/Ag electrocatalysts. Rotating disc electrode (RDE) experiments revealed that the obtained C/α-MoC/Ag exhibited much superior catalytic performance for ORR than that of C/Ag, C/α-MoC, or even the conventional Pt/C. Firstprinciples calculations indicated that the enhanced activity could be attributed to the efficient synergistic effects between Ag and α-MoC/C by which the energy barrier for O2 dissociation has been substantially reduced. Furthermore, Li-air and Al-air cells were assembled to demonstrate the unprecedented electrochemical performance of C/α-MoC/Ag nanocomposites surpassing the Pt/C. Thus, experimental results and theoretical calculations together showed that the heterogeneous C/α-MoC/Ag nanocomposites are promising alternative to platinum for applications in industrial metal-air batteries.

TOC GRAPHICS

Experimental and theoretical calculations demonstrated that exceptionally high and stable activities toward oxygen reduction reactions can be achieved on the ternary C/α-MoC/Ag nanocomposites arising from the efficient synergistic effects between Ag and α-MoC/C by which the energy barrier for O2 dissociation can be considerably reduced.

KEYWORDS Non-Platinum electro-catalyst, Nano-composite, Oxygen reduction reaction, Activation barrier, and Metal-air batteries.


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In recent years, increasing research efforts have been devoted to metal air batteries due to their exceptionally high theoretical energy density (8135 Wh kg-1 for Al-air, and 11400 Wh kg-1 for Li-air), good reliability, and low cost.1-3 Though Al-air battery is not rechargeable but it has many additional attractive features, such as abundant resources, nontoxicity, zero-emission, easy recycling of Al(OH)3 byproduct, and fast replacement of Al electrode, which matches very well with the requirements of electric vehicles.4 However, it still remains a challenge to implement Al-air or Li-air batteries on a commercial scale. The bottleneck is to develop an effective electrocatalyst to improve the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air electrode.2, 5-6 Although noble metals such as Pt or Pd have exhibited good ORR or OER activities, large-scale commercial application has been largely hindered by their high cost, low abundance and weak durability.3,

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Therefore, it is highly

attractive and urgent to explore low cost, feasible, and effective alternatives to noble metals as electrocatalysts for fabricating air electrodes for Li-air and Al-air batteries7, 11-12. Transition metal carbides including tungsten carbide13-16, vanadium carbide17-18, and molybdenum carbide19-25, possessing special catalytic activities26, have been extensively studied as catalysts27-29 or catalyst supports15, 30-31 in many reactions including hydrogenation32, water splitting19,

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and desulfurization34. In addition to its Pt-like characteristics, transition metal

carbides have high electronic conductivity35 and are highly corrosion resistant in both alkaline and acid conditions36. As representative transition metal carbides, molybdenum carbide has attracted significant recent research attention37. Generally, molybdenum carbide exists in four forms: α-MoC, β-Mo2C, γ-MoC, and η-MoC.21, 33 β-Mo2C, γ-MoC, and η-MoC have very similar hexagonal crystal structures with different stacking sequences. Particularly the β-Mo2C, the most stable phase having an ABAB packing of the metal planes, has been extensively investigated.22, 24,


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However, the face-center cubic phase α-MoC with an ABCABC stacking sequence

has been rarely touched.32-33, 42 Molybdenum carbides alone exhibited very limited activity for ORR.24, 26, 29 But molybdenum carbides are excellent substrates for loading active component like Pt or Pd to considerably replace the noble metal demonstrating comparable or even better catalytic activity toward oxygen reduction and particularly improving the stability of catalysts in various electrolytes.23, 37, 43-44 Metallic Ag has been generally utilized as electrocatalyst in air electrode for Al-air battery demonstrating superior activity and stability.45 Herein, we demonstrate that α-MoC promoted Ag can be an intriguing candidate considered as high efficient catalysts for oxygen reduction. It is well known that as the dimensions of catalysts are reduced, their activity increases markedly. Traditionally, high temperature solidstate methods have to be used to synthesize α-MoC with good crystallinity, unavoidably yielding microscale aggregated particles having small specific surface area and poor catalytic activities. Alternatively, we utilized the ion-exchange followed by low temperature calcination method to prepare uniform α-MoC ultrafine nanoparticles with size less than 10 nm that are homogeneously and stably distributed throughout porous and conductive carbon matrix. And then ultrafine Ag nanoparticles were loaded onto the C/α-MoC nanoparticles to form extremely uniform and stable heterogeneous C/α-MoC/Ag nanocomposites. Furthermore, the size and the distribution of the α-MoC nanoparticles can be finely tuned by simply changing the ion concentration and the heat treatment conditions. The ultrafine α-MoC nanoparticles were extremely stable due to the favorable confinement effects of the nanoporous carbon. More importantly, this ternary composite catalyst can substantially enhance the holistic electrocatalytic activity owing to the efficient synergistic effect arising from the high electronic conductivity of carbon matrix, the ultrafine particle size, and homogeneous distribution of Ag


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and α-MoC nanoparticles, and the promotion effects from the intact interaction between Ag and α-MoC46-48. Additionally the produced porous carbon matrix possessed high graphitic degree due to the catalytic effect of Mo species during the carbonization process endowing good electronic conductivity. Thanks to the aforementioned comprehensive merits, the synthesized C/α-MoC/Ag heterogeneous nanocomposites exhibited excellent electrocatalytic activities toward oxygen reduction and evolution reactions. Deeper insights into the mechanism and process of ORR over the novel electrocatalyst were also explored by theoretical calculation using first-principles calculations within density functional theory (DFT). Finally prototype Li-air and Al-air batteries were assembled by using the C/α-MoC/Ag nanocomposites as electrocatalysts in the air cathodes, both demonstrating very promising electrochemical performance.

Fig. 1. XRD patterns of as-prepared samples: (a) C/Ag, (b) C/α-MoC/Ag, (c) C/α-MoC.

Firstly, we confirmed the as prepared composite by XRD shown in Fig. 1 of the C/Ag, C/αMoC/Ag, and C/α-MoC, respectively. The peaks at 36.4o, 42.3o, 61.3o, 73.4o and 77.3o can be clearly observed, corresponding to the (111), (200), (220), (311), and (222) facets of the α-MoC crystal, respectively. The distinct diffractions at 38.1o, 44.3o, 64.6o and 77.4o are detected and could be attributed to the (111), (200), (220) and (311) planes of metallic Ag. As expected, the XRD pattern of C/α-MoC/Ag as shown in Fig. 1(b) includes all the characteristic diffractions of


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α-MoC and Ag, indicating the co-existence and good crystallinity of the two components. In addition, the ambiguous diffraction peak centered at around 26.2o can be assigned to the graphite (002) plane, revealing graphitization of carbon in these samples. The particle size of α-MoC calculated by the Scherrer’s formula is approximate 5 nm, which is much smaller than that of the α-MoC particles prepared by other approaches for instance high temperature, hydrothermal, or sol-gel. More importantly, the obtained α-MoC ultrafine nanoparticles were single phase without any impurities. Hence, the method is of importance for the mass production of ultrafine metal carbides nanoparticles. Compared with C/Ag, the broader XRD diffraction peaks associated with Ag in C/α-MoC/Ag suggest that the particle size of Ag diminishes to some extent, implying the presence of α-MoC largely inhibits the growth of Ag particles.

Fig. 2. TEM and HRTEM images of as-prepared C/α-MoC (a,c,e), and C/α-MoC/Ag (b,d,f); The inset of (d) displays the corresponding EDS pattern.


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Meanwhile, Fig.2 (a) and (c) show the typical TEM images of the C/α-MoC sample. It is clear that the particles are in spherical shape and uniformly well-dispersed throughout the carbon matrix with extremely fine size. From the randomly selected 100 particles, the average particle sizes were calculated to be about 5 nm. The HRTEM image in Fig. 2(e) explicitly proclaims the different crystal faces of α-MoC owing to good crystallinity. The small size and narrow size distribution of the particles exactly demonstrates the advantages of our method in synthesizing high quality phase-pure α-MoC nanoparticles. After the loading of AgNO3, it is clearly observed from Fig. 2(b) and (d) that Ag particles with average size of less than 5 nm are well-distributed without obvious aggregation around the α-MoC. The EDS pattern in inset of Fig. 2(d) clearly confirms the co-existence of Mo and Ag. HRTEM image in Fig. 2(f) reveals distinct crystal lattices of α-MoC (111), (200) and Ag (111), (200), which further proves the co-existence and good crystallinity of α-MoC and Ag. It was found that the Ag nanoparticles were closely anchored onto the α-MoC nanoparticles.

Fig. 3. HRTEM images of the ternary C/α-MoC/Ag catalysts. (a) demonstrates both the location of Ag nanoparticles on top and in the neighbor of α-MoC nanoparticles and (b) shows the adjacent of Ag and α-MoC nanoparticles.


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To further evaluate the change of Ag and the interface between Ag and α-MoC, HRTEM images of the C/α-MoC/Ag were studied in more detail in Fig. 3. For the comparison, the TEM and HRTEM images of the normal C/Ag composite are shown in Fig. S2. We can clearly see that Ag in the normal C/Ag composite is coated on the glossy surface of carbon, and the diameter of an individual Ag nanoparticle is larger than 15 nm (Fig. S2a). In good contrast, Ag nanoparticles in the C/α-MoC/Ag nanocomposites were either surrounded tightly by a number of MoC nanoparticles (Fig. 3b) or located on top of the MoC nanoparticles (Fig. 3a), and the average grain sizes of Ag are reduced to less than 5 nm. So we speculate that the existence of MoC largely hinders the growth of the Ag particle. What is more, the good contact between α-MoC and Ag particles is verified by the XPS (Fig. S2 e and f) result that the binding energy of both 3d5/2 and 3d3/2 peak of Ag are increased comparing with the C/Ag sample. This is beneficial for the enhancement of catalytic activity and will be illustrated in detail later.

Fig. 4. Linear potential scan curves of oxygen reduction on different catalysts in O2 saturated 0.1mol L-1 KOH solution at 25 oC; 2 mV s-1, 2500 rpm. Curves: (a) C/α-MoC, (b) C/Ag, (c) C/α-MoC/Ag and (d) Pt/C.

Next, the electrocatalytic activities over various catalysts for ORR were studied by RDE voltammograms in 0.1 M KOH. The polarization curves of the air electrodes using C/Pt in the same condition are also included for comparison. Fig. 4 shows typical ORR polarization curves of C/α-MoC, C/Ag, C/α-MoC/Ag, and the commercial Pt/C catalysts. The half-wave potential is


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indicative of the catalytic activity for the ORR, the higher potential, the better ORR activity.49 As shown in the Fig. 4, C/α-MoC can catalyze the ORR but the half-wave potential around -0.37V is quite negative which is not preferable. As a common cathode catalyst for metal-air battery, C/Ag shows relatively better performance with a relatively higher half-wave potential of about 0.21V. Noticeably, the half-wave potential of the C/α-MoC/Ag sample distinctly shifted to a more positive position of approximate -0.145V, which suggests the significant improvement of the ORR performance caused by a significant synergistic effect between the C/α-MoC and Ag. Although the ORR at Pt/C electrode occurred at an even more positive half-wave potential (0.125 V), it is obvious that the mass transfer limited current density at -0.8V over the C/αMoC/Ag sample increases more sharply than that of all other samples including the 20% Pt/C catalysts, indicating that the C/α-MoC/Ag exhibits significantly enhanced catalytic activity for electrochemical oxygen reduction. More importantly, the Ag content in C/α-MoC/Ag only accounts for 6.7 wt%, which is considerably lower than that of 20% Pt/C. Unambiguously, the nanocomposite electrocatalyst displays an efficient synergetic effect towards oxygen reduction leading to exceptional catalytic performance even surpassing the commercial Pt/C. Furthermore, we display the linear potential scan curves of oxygen reduction over the (a) C/α-MoC, (c) C/Ag, and (e) C/α-MoC/Ag catalysts on RDE under different rotating rates in Fig. S3. The corresponding Koutecky–Levich plots at distinctive potentials for various electrocatalysts are described in Fig. S3(b), (d), and (f). The electron transfer numbers per oxygen molecule involved in the reduction reaction were calculated according to the Koutecky– Levich equation 50 and the results are listed in Table S1. It is clear that C/α-MoC catalyzes ORR in a two-electron process similar to the traditional carbon in alkaline solutions. The ORR occurs mainly through an approximate three-electron mechanism in C/Ag that is similar to the previous


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works.7 Noticeably, the C/α-MoC/Ag follows a favorable four-electron mechanism, which is similar to C/Pt under identical testing conditions. The results clearly indicate that the C/αMoC/Ag ternary nanocomposite catalyst performs excellent electrocatalytic activities toward oxygen reduction reaction resulting from the effectively synergistic effect between the C/α-MoC matrix and the well dispersed Ag active centers. Similarly, the enhanced hydrogen evolution reaction performance caused by the synergistic effect and strong interaction between the MoCx nanocrytallites and the continuous incorporated carbon matrix was reported by Wu et al51. Also, Pd/MoC shows excellent activity on ethanol oxidation due to synergistic effect which was published by Yan52. Actually, the metal d-band vacancy which is related to the adsorption strength of the reaction intermediates, is generally considered the most important factor determining the kinetics for ORR. The perfect activity of Pt with a d-band vacancy equal to 0.6 per atom, was proved to have the most appropriate adsorption for oxygen.53 Meanwhile, many reports demonstrated that the formation of α-MoC largely altered the nature of the d band of Mo, which in turn led to catalytic properties that were similar to those of group VIII noble metals, rather than those of the parent metals Mo.54 So the addition of α-MoC can exactly modify the d-band structure of Ag which displays imperfect active centers owing to its less unpaired electrons and give rise to strong interaction with oxygen as demonstrated in the aforementioned DFT calculations.47, 55-56 It was reported that Ag/Pt core-shell nanoparticles formed via the growth of Pt on the core of metallic Ag nanoparticles exhibited excellent ORR performance.57 Based our TEM results, the Ag nanoparticles are surrounded by plenty of α-MoC nanoparticles or confined on top of the α-MoC nanoparticles just like an open Ag/Pt core-shell structure. As a result, the α-MoC may help to bind the initial oxygen, whereas the Ag may facilitate the facile adsorption and desorption of


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oxygen. In this way, the whole four-electron process can be achieved by combining the rapid kinetics of α-MoC for the first two-electron reduction with the fast disproportionation on Ag.58 As a consequence, the heterogeneous nanocomposite catalyst C/α-MoC/Ag demonstrates superior ORR activity as compared to C/Ag and C/α-MoC or even Pt/C. To further explore the ORR performance of as prepared ternary heterogeneous catalyst C/αMoC/Ag, we have fabricated a home-made Li-air and Al-air batteries (refer to Fig. S5-6 and discussions therein). The two applications both show that the ORR performance are highly improved due to the novel and special characteristics of the C/α-MoC/Ag nanocomposite including favorable synergetic effects between the Ag and C/α-MoC components, the homogeneous distribution of the ultrafine Ag/α-MoC nanoparticles throughout the porous carbon matrix, and excellent electronic conductivity of the highly graphitic carbon matrix.

Fig. 5 (Color online) (a) Top view of oxygen molecule adsorbed on Ag4 cluster decorated a-MoC (001) surface. The optimized bond length of O-O is 1.46 Å; (b) top and (c) side view of dissociated oxygen molecule adsorbed on the Ag4 decorated MoC(001) surface. In (a)-(c), the violet, brown, silver and red balls represent Mo, C, Ag, O atoms, respectively. (d) The energy profile along the dissociation of O2 reaction coordinate, with


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the zero of the energy scale taken for the energy of the optimized molecular adsorbed configurations. The red and blue line corresponds to the reaction barrier with and without Ag clusters, respectively.

To better understand the superior ORR activity caused by heterogeneous nanocomposite catalyst C/α-MoC/Ag, we have conducted the first-principles DFT computation as shown in Fig. 5. We first adsorb molecular oxygen on clean α-MoC(001) surface. All possible adsorption sites and O-O bond orientations have been considered. The most stable adsorption configuration is the one where oxygen molecule bridging two Mo atoms, with an adsorption energy of 1.69 eV. In this stable molecularly adsorbed state, the distance between the two O atoms is remarkably elongated, 1.41 Å compared to 1.23 Å for the gas phase isolated molecule. Bader analysis of the charge density indicates that charge transfer (about 0.87 e) into the antibonding 2

orbital of O2

molecule from the surface occurs upon adsorption, leading to activation of the O-O bond and no spin polarization (singlet state). Fig. 5(d) shows the energy profile for O2 dissociation on clean α-MoC(001). The calculated activation barrier is 0.15 eV. Next we consider a Ag tetramer on αMoC(001) as a representative system to investigate how the presence of Ag nanoparticles affects the catalytic performance of α-MoC for ORR. Different starting geometries of Ag4 cluster adsorbed on α-MoC(001) are optimized, the optimal structure of the adsorbed Ag4 is found to be a tetrahedron with the three Ag atoms in direct contact with the substrate binding at the MMC sites (the three-fold hollow site having two metal and one carbon atom neighbors) of MoC(001) (Fig. 5a-c). Various O2 adsorption sites are explored. It is found that O2 molecule prefers to adsorb at the interfacial periphery of Ag nanoparticle with MoC(001) surface with its O-O bond parallel to the surface (Fig. 5a) as in the clean MoC(001), but the O-O distance is much more elongated (1.46 Å), indicating that the Ag tetramer are active toward O2 dissociation. As shown in Fig. 5(d), the dissociation barrier is only 0.07 eV, much lower than that on the clean MoC(001) surface. Furthermore, we tentatively calculate the dissociation barriers when the O2


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adsorbs at the sites apart from the Ag particles on the α-MoC surface, it is found that those barriers are also lower than that on the clean MoC(001) due to the charge polarization occurred upon the contact of Ag particles with α-MoC. These first-principles calculations corroborate that the Ag nanoparticles work in a favorable cooperative way with the α-MoC to actively promote the dissociation of O2 molecule for ORR.

CONCLUSIONS In summary, a cost-effective ternary heterogeneous catalyst C/α-MoC/Ag was synthesized and intensively evaluated as oxygen reduction reaction catalyst in cathodes considered for both Li-air and Al/air batteries for the first time. The results demonstrated that the C/α-MoC/Ag exhibited attractive activity for the ORR with comparable positive half-wave potential and enhanced current density as compared with Pt/C, owing to a special synergistic effect deriving from the high electronic conductivity of carbon matrix, the ultrafine particle size, and homogeneous distribution of Ag and α-MoC nanoparticles, as well as from the promotion effects from the intact interaction between Ag and α-MoC. In addition, the performance of the Li-air and Al-air batteries with a C/α-MoC/Ag cathode were also tested and found to be much better than that of the cells with 20% Pt/C cathode. Density functional theory calculations provided fundamental insights into the material structure and attributed the high electrocatalyst activities to the facile oxygen adsorption and considerably reduced activation barrier of oxygen after the anchoring of even very small amount of Ag (6 wt%) onto the C/α-MoC surface. What deserves to be highlighted is that the replacement of Pt is significant in terms of the cost and the resource limitation. Consequently, this study opens a new perspective for the development of highly active and stable electrocatalysts for the next-generation high energy metal air batteries.


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ASSOCIATED CONTENT Supplementary Information Detailed experimental procedures for the fabrication of air electrode, assemble of L-air and Alair batteries, the DTA analysis of thermal decomposition of Mo-containing resins and the linear potential scan curves and corresponding Koutecky–Levich plots on different catalysts and the calculation and results of electron transfer as well as the galvanostatic charge/discharge voltage profiles of the Li-air cells based on different electrocatalysts. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: [email protected] and [email protected] The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21671096), the Natural Science Foundation of Guangdong Province (No. 2016A030310376), and the Natural Science Foundation of Shenzhen (No. JCYJ20170412153139454 and No. JCYJ20150331101823677).


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Î±-MoC and Ag for Efficient Oxygen Reduction

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