Facile Synthesis of Highly Active and Robust NiâMo Bimetallic

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Letter

A Facile Synthesis of Highly Active and Robust Ni-Mo Bimetallic Electrocatalyst for Hydrocarbon Oxidation in Solid Oxide Fuel Cells Bin Hua, Meng Li, Ya-Qian Zhang, Jian Chen, Yi-Fei Sun, Ning Yan, Jian Li, and Jing-Li Luo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00109 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016

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ACS Energy Letters

A Facile Synthesis of Highly Active and Robust Ni-Mo Bimetallic Electrocatalyst for Hydrocarbon Oxidation in Solid Oxide Fuel Cells Bin Hua a, Meng Li b, Ya-Qian Zhang a, Jian Chen d, Yi-Fei Sun Jian Li b, Jing-Li Luo a, *

a,

*, Ning Yan c, *,

a

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada

b

School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China

c

Van’t Hoff Institute for Molecular Sciences (HIMS), University of Amsterdam, Amsterdam, 1098XH, The Netherlands

d

National Institute for Nanotechnology, Edmonton, Alberta T6G 2M9, Canada

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Abstract In this work, we report a novel Ni-Mo bimetallic alloy decorated with multi-microcrystals as an efficient anode catalyst for hydrocarbon-fueled SOFCs. We show that these Ni-Mo bimetallic alloys are highly active, thermally stable and sulfur/coke tolerant electrocatalysts for hydrocarbon oxidation. When fueled with CH4-50 ppm H2S, the SOFC shows a maximum power density of 0.594 W cm-2 at 800 °C. Most significantly, this bimetallic catalyst also offers sustained and steady power output in the prolonged test, suggesting that it is a promising anode catalyst with the superior sulfur/coke tolerance and thermal stability.

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TOC

Here we demonstrate that the Ni-Mo bimetallic alloys are highly active, thermally stable and sulfur/coke tolerant electrocatalysts for hydrocarbon oxidation in solid oxide fuel cells.

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Despite that hydrogen-powered fuel cell has provided an efficient and eco-friendly pathway for the energy conversion with water and heat as the only byproducts, directly utilizing hydrocarbons in fuel cells is gaining increasing attention 1-5

. In this context, solid oxide fuel cells (SOFCs) have a big advantage over other

types of fuel cells due to their excellent fuel flexibility, i.e., most hydrocarbons fuels, including natural gas, gasified biomass and coal gas, can theoretically be used for power generation

4-6

. However, some serious challenges are always associated with

hydrocarbon-fueled SOFCs for decades. The major obstacle is that the conventional Ni-based anode easily suffers from coking in the absence of decoking agents (e.g., H2O, CO2, catalysts)

4, 7-9

. Another issue in the SOFC implementation is that the

commercially available hydrocarbon fuels, e.g., natural gas, usually contain a small amount of sulfur species, causing sulfur-induced deactivation of the Ni anode 7, 10, 11. Because of the excellent coking resistance and sulfur tolerance, ceramic oxides, particularly perovskite oxides, have received considerable attention to replace the Ni-based cermets as anodes for hydrocarbon-fueled SOFCs. These anodes, derived from SrTiO3 12, LaCrO3

13, 14

and SrMoO3

15, 16

, have been proven to be able to resist

coking and sulfur poisoning. Unfortunately, the use of these alternative anodes usually compromises the cell performance more or less, and brings concerns regarding the physical/chemical compatibility issues with the other cell components. Introducing Ni alloy is another potential solution to prevent coke formation and alleviate sulfur poisoning. The strong interaction between the electrons of Ni 3d and those of C 2p orbitals leads to rapid carbon formation on Ni surface. Alloying Ni effectively adjusts 4


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its atomic orbital and electron cloud, and in turn, enhances the coking resistance. Transition metals, such as Cu, Co or Fe, are typical alloying elements, the formed Ni based alloys have been widely investigated as heterogeneous catalysts used in H2S-containing hydrocarbons

9, 17-19

. Nonetheless, incorporating Cu undermines the

cell performance and durability because of the catalytically inactive nature and the low sintering stability of Cu 20. While Fe and Co containing electrocatalysts display good overall performances in hydrocarbon fuels, apparent degradations of fuel cells are still observed in the long run 20-22. Besides the alloying approach, the particle sizes of catalysts also have a significant impact on the catalytic activity, redox stability and chemical stability of the SOFC electrodes 23, 24. We therefore envision that the development of nano-sized Ni alloy catalysts is a promising direction in combining the features of good coke/sulfur resistance, improved thermal stability and excellent activity of hydrocarbon oxidation. In this work, we report a novel Ni-Mo bimetallic alloy decorated with multi-microcrystals as a sulfur resistant anode catalyst of hydrocarbon-fueled SOFCs. Its morphological characteristics, catalytic activity, thermal stability, coking resistance and sulfur tolerance are discussed. The facile infiltration technique was employed to synthesize the Ni-Mo anode catalyst (see supporting information, SI). Figure 1a demonstrates the X-ray diffraction (XRD) pattern of the Ni-Mo (molar ratio 3:1) loaded Y2O3 stabilized ZrO2 (YSZ) composite. The Ni-Mo alloys consisted of NiMo, Ni3Mo and Ni4Mo intermetallic compounds after 4 h reduction in H2 at 800 °C. Monometallic phases, i.e., Ni and Mo, 5


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were hardly seen in the prepared sample. According to the Ni-Mo phase diagram, three

ordered

intermetallic

phases,

i.e.,

Ni4Mo,

Ni3Mo

and

NiMo,

are

thermodynamically stable between 400 and 850 °C. The dissolution of Mo atoms into the lattice of the Ni and the formation of the intermetallic alloy caused the increase of lattice distortion, reducing the mobility of atoms and dislocation 25, 26. Sequentially, a mixture of Ni4Mo, Ni3Mo and NiMo, rather than pure Ni3Mo intermetallic phase (the stoichiometric ratio of metals in the precursor), was produced after annealing in H2. The X-ray photoelectron spectroscopy (XPS) spectra of Ni-Mo catalyst show that the nano-sized metallic particles were liable to being oxidized after air exposure, which is in line with our previous study 5, 10: In the Ni 2p and Mo 3d envelopes of the as-reduced sample (Figure S1), both Ni-Mo alloys and the oxides (possibly NiO, Ni(OH)2, MoO2, MoO3) were identified. Since these oxides are not likely to present under the SOFC operation conditions in the anode compartment, we used Ar ion sputtering to remove the surface oxides before acquiring the XPS spectra (see Figures 1b and 1c). Compared with those of monometallic phase, the binding energies shifted apparently, suggesting the formation of the alloys. The resulting alternations of the electronic structures might drastically affect the chemical and physical properties of the catalyst, suppressing carbon formation and preventing sulfur poisoning. The transmission electron microscopy (TEM) analysis results supported the conclusion that the bimetallic Ni-Mo alloys formed (see Figs. S2 and 2). The surface oxide layer, if any, was too thin to be detected (see EDX-spot analysis in Figure S2a). Interestingly, the high resolution TEM (HRTEM) images reveal a petal-like surface 6


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structure, comprising multi-microcrystals, of the Ni-Mo nanosphere (Figures 2a, 2b and S2a). The ring-like selected area electron diffraction (SAED) pattern further confirms this crystallographic feature. These microcrystals had irregular shapes with various dimensions ranging from sub-nanometer to several nanometers. Such surface structure has been commonly reported to be more catalytically active than conventional nanoparticles

27-29

. On the contrary, the monometallic Ni nanospheres,

prepared by the identical method, were single crystals with no surface microcrystals being observed (Figure S2b). The performances of the Ni-Mo bimetallic catalyst and Ni monometallic catalyst were then studied in the anode supported SOFCs. Specifically, the single cells were composed of the infiltrated Ni-Mo/YSZ anode or Ni/YSZ anode (~1 mm), dense YSZ electrolyte

(~10

µm),

Gd0.1Ce0.9O1.9

(GDC)

barrier

layer

(~2

µm)

and

La0.6Sr0.4Co0.2Fe0.8O3-δ/GDC cathode (~30 µm) [see details in SI]. In H2, the Ni-Mo/YSZ cell demonstrated lower maximum power density (MPD, 0.94 W cm-2) compared with that of the Ni/YSZ cell (1.06 W cm-2). The attenuation is not significant in this study though Mo is not as active as Ni towards the H2 electrochemical oxidation. Since the polycrystalline shell on the Ni-Mo surface is more active than the bulk material, the performance of Ni-Mo cell was comparable to that of pure Ni (the MPD is only 10% lower). Generally, the infiltrated metallic nanoparticles are susceptible to sintering effects 30. To evaluate the durability of the infiltrated Ni-Mo nanospheres, Ni-Mo/YSZ cell was subjected to isothermal treatment at 800 °C for 50 h, the infiltrated Ni/YSZ 7


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cell was used as the control for comparison. The heat treatment usually imposed a negative effect on the cell performance because the continuous agglomeration of the fine metallic nanoparticles decreased the active area and destroyed their three-dimensional network. Thus, the performance of Ni/YSZ cell degraded dramatically after the isothermal treatment. In contrast, the degradations of Ni-Mo/YSZ cell were inconspicuous. As Figure 3b shows, the MPD of Ni-Mo/YSZ cell (0.87 W cm-2) was much higher than that of Ni/YSZ cell (0.52 W cm-2) after the isothermal treatment. This performance degradation pertains to the evolution of the microstructure. Initially, the as-reduced nanoparticles with the average size of 25 nm were uniformly distributed on the YSZ scaffold for both Ni and Ni-Mo cells (Figures 3c, 3e and S4). After the isothermal treatment, the fine nanoparticles in the Ni/YSZ anode formed much larger particles (Figure 3d, the apparently agglomerated ones were marked by orange arrow), whereas those in Ni-Mo/YSZ anode did not exhibit appreciable difference (Figure 3f). The microstructure changes were also evidenced in the Brunauer-Emmett-Teller (BET) specific surface area analysis inserted in the corresponding images (Figures 3c to 3f). The specific surface area of Ni/YSZ cell decreased from 2.48 cm2g-1 to 1.32 cm2g-1, whereas that of Ni-Mo/YSZ cell merely decreased from 2.51 cm2g-1 to 2.17 cm2g-1. The excellent thermal stability was pertinent to the incorporation of Mo that is a high melting-point element. In addition, the presence of long-range-ordered superlattice structures in the intermetallic compound could effectively reduce the atoms diffusion rate as well as the dislocation 8


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mobility at elevated temperatures, restraining the growth of nano-sized particles while rendering a more sintering-resistant catalyst. Ni-Mo/YSZ cell also showed enhanced coking resistance. The cell performance and durability of Ni/YSZ and Ni-Mo/YSZ cells were studied in humidified CH4 to evaluate their carbon deposition resistances. As illustrated in Figure S5a, both Ni/YSZ and Ni-Mo/YSZ cells showed reasonable open circuit voltages (OCVs) of 1.07-1.08 V, and good MPDs of between 0.77-0.83 W cm-2 in humidified CH4. However, they exhibited entirely different stability behaviors at 800 °C and under 0.7 A cm-2 current load (Figure 4a). Ni/YSZ cell showed a voltage decay of 0.0213 V h-1 within 24 h timespan. Confirmed by the SEM observation in Figure 4b, this prominent degradation was mainly caused by the severe carbon depositions rather than the particle agglomeration. After the stability test, the MPD of Ni/YSZ cell decreased from 0.83 W cm-2 to 0.37 W cm-2 (Figure S5). In contrast, the voltage of the Ni-Mo/YSZ cell degraded slightly in the initial stage and stabilized at 0.73 V after ~12 h (Figure 4a). The J-V/P curves and the SEM images of the Ni-Mo/YSZ cell (Figures 4c and S5) offered supporting evidences of its improved coking resistance. In fact, there was no apparent carbon species, e.g., carbon fibers, carbon beads 8, emerged in Ni-Mo/YSZ anode after a 24h test in humidified CH4 (Figure 4c). The initial performance degradation of Ni-Mo/YSZ might be attributed to the sintering effect rather than the coke formation. Most interestingly, even in CH4-50 ppm H2S, the Ni-Mo/YSZ cell achieved excellent performance and stability. To exclude the performance degradation resulted 9


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from sintering, the cell was pre-isothermally treated at 800 °C in H2 for 12 h. Then the output voltage of Ni-Mo/YSZ cell maintained at ~0.737 V at 800 °C under 0.7 A cm-2 constant current density (Figure 4d). The J-V/P curves of the same cell before and after the stability test in CH4-50 ppm H2S implied that the activity of the Ni-Mo bimetallic catalyst has not been weaken under such harsh operational conditions. The SEM and EDX (Figures 4e and 4f) analysis results of the Ni-Mo/YSZ anode after the test evidently showed that neither carbon depositions nor severe particle agglomeration had ever occurred during the 120 h test, suggesting that Ni-Mo is an extremely coke/sulfur tolerant and durable anode catalyst of SOFC. The Ni-Mo bimetallic catalyst is the mixture of NiMo, Ni3Mo and Ni4Mo intermetallic compounds with a polycrystalline shell. We hypothesized that the Ni-rich intermetallic phases are more catalytically active in this catalyst system, facilitating the electro-oxidation of fuels. The Ni-poor intermetallic phases are more tolerant to and coking and sulfur poisoning (see the schematic in Figure S6). Indeed, the previously discussed electronic effects after Mo incorporation could be a key factor contributing to the improved coke/sulfur resistance (see Figure 1). Besides, carbon deposition is always initiated from the catalytic C-C bond formation on the surface of the catalyst, requiring the presence of at least two carbon-activation sites. the Mo atoms, which are much less active than those of Ni in terms of methane activation, effectively diluted the active site of Ni, minimizing the amount of adjacent Ni atoms that sequentially suppressed the bonding of C-C. In addition, the possibly increased internal methane reforming effect due to the alloying can also contribute to 10


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the enhanced coke resistance. In summary, we have demonstrated that the Ni-Mo bimetallic catalyst is an excellent anode catalyst of SOFC, showing excellent activity and stability in both hydrogen and methane. It is thus a promising anode catalyst replacing the conventional Ni catalyst for H2S-containing hydrocarbon fueled SOFCs.

ACKNOWLEDGEMENT This work was supported by the Natural Sciences and Engineering Research Council of Canada. N. Yan thanks the support from the Research Priority Area of Sustainable Chemistry from the University of Amsterdam.

AUTHOR INFORMATION Corresponding Author Email: [email protected] (J.-L. Luo); [email protected] (N. Yan); [email protected] (Y.-F. Sun)

Notes The authors declare no competing financial interests.

SUPPORTING INFORMATION Supporting Information Available: Experimental details, additional SEM and TEM images, electrochemical test results 11


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and schematic diagram are included.

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Figure 1

(a) X-ray diffraction pattern of Ni-Mo/YSZ composite; (b) and (c) XPS spectra of Ni2p and Mo3d after Ar ion sputtering.

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Figure 2

(a) The STEM image of the YSZ supported Ni-Mo bimetallic nanospheres; (b) the EDX elemental mappings of the Ni-Mo nanosphere; (c) a typical HRTEM image of Ni-Mo nanosphere; (d) the enlarged views showing the edges of the Ni-Mo nanosphere.

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Figure 3

The current density-voltage/-power density plots (in H2 at 850 °C) of Ni and Ni-Mo cells (a) before and (b) after 50 h heat treatment in H2 at 850 °C; the microstructure of (c) as-reduced Ni anode, (d) Ni anode after heat treatment (large particles were marked), (e) as-reduced Ni-Mo anode and (f) Ni-Mo anode after heat treatment.

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Figure 4

(a) The stability test of Ni and Ni-Mo cells in humidified CH4 at 800 °C for 24 h; The microstructure of (b) Ni and (c) Ni-Mo anodes after 24 h test in humidified CH4 at 800 °C for 24 h; (d) the stability test of Ni-Mo cell in CH4-50 ppm H2S at 800 °C and 0.7 A cm-2, and the current density-voltage/power density plots of the same cell before and after stability test; (e) and (f) the microstructure and EDX spectrum of Ni-Mo anode after test in CH4-50 ppm H2S at 800 °C for 120 h.

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Facile Synthesis of Highly Active and Robust NiâMo Bimetallic

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Facile Synthesis of Highly Active and Robust NiâMo Bimetallic