Co7Se8 Nanostructures as Catalysts for Oxygen Reduction Reaction

Co7Se8 Nanostructures as Catalysts for Oxygen Reduction Reaction with High Methanol Tolerance Jahangir Masud and Manashi Nath* Department of Chemistry, Missouri University of Science & Technology, Rolla, Missouri 65409, United States S Supporting Information *

ABSTRACT: Co7Se8 nanostructures electrodeposited on glassy carbon (GC) electrodes show high efficiency for oxygen reduction reaction (ORR) with high methanol tolerance as compared to Pt electrocatalysts. In the presence of methanol, the onset potential for the ORR at Pt/GC is shifted from 0.931 V (vs reversible hydrogen electrode (RHE)) to 0.801 V (vs RHE), whereas it remains the same (0.811 V vs RHE) at Co7Se8/GC in the presence and absence of methanol in 0.5 M H2SO4 solution. The Co7Se8/GC electrodes also showed high cyclability in the presence of methanol, with no degradation of catalytic performance. It is also noteworthy that the Co7Se8/GC exhibited exclusively a four-electron reduction pathway for ORR and very low H2O2 yield in acidic electrolyte. The admirable performance of Co7Se8/GC catalyst along with its cost-effective nature holds great potential for application in direct methanol fuel cells.

E

relatively high ORR catalytic activity in acidic medium. However, the ORR activities of these materials are still far from practical DMFC application. In this Letter, we report for the first time Co 7 Se 8 nanostructures prepared by simple electrodeposition on glassy carbon (GC) as nonprecious metal-based cathode catalyst active for methanol-tolerant ORR. Co7Se8 nanostructures exhibit superior ORR activity in acid medium via exclusively four-electron reduction pathways in the presence of small organic contaminants such as methanol and exhibits excellent stability and cyclability in 0.5 M H2SO4 solution containing 0.5 M methanol. The Co7Se8 films were prepared by electrodeposition on GC as well as Au-coated glass electrodes as described in the Supporting Information. Electrodeposition grows the catalytic films directly on the electrodes, thereby simplifying the process of cell fabrication by reducing the need for using additives to coat the electrode with the catalyst film. For comparison of ORR activity, Pt was also electrodeposited on GC electrode (details are provided in the Supporting Information). The loadings of the catalysts were calculated to be 0.17 and 0.06 mg cm−2 for Pt and Co7Se8, respectively (see the Supporting Information and Table 1). The crystallinity of the as-prepared electrodeposited film on Au-glass was investigated through powder X-ray diffraction (PXRD) technique. As can be seen from Figure 1a, the PXRD pattern obtained from the film was

lectrochemical oxygen reduction is a crucial and major challenge for several renewable energy technologies, such as fuel cells and metal−air batteries, because of the sluggish oxygen reduction reaction (ORR) activity at the cathode.1 Although Pt-based materials have been proven to be excellent ORR catalysts, the prohibitive cost, scarcity of resources, poor durability, and methanol crossover significantly prohibits their large-scale application.2−5 Particularly, a trace amount of methanol can easily cross over from the anode to the cathode side through the polymer membranes of direct methanol fuel cells (DMFCs), which can react directly with the cathode catalyst and O2 to decrease the cathode potential, resulting in reduced fuel efficiency. Moreover, commercial viability of electrochemical devices for energy conversion and storage can be achieved only through the use of low-cost and earth-abundant raw materials. Therefore, extensive efforts have been devoted toward the development of methanol-tolerant, durable, and low-cost cathode catalysts for the ORR. Recently, low-cost alternative catalysts such as some transition-metal oxides6,7 and metal chalcogenides8,9 have been reported as excellent, durable electrocatalysts with high methanol tolerance. In this regard, selenides of Ni and Co are attracting enormous interest for water electrolysis as well as ORR electrocatalysts with performance surpassing that of the oxides.10−12 In particular, a carbon-supported CoSe2 catalyst synthesized by Alonso-Vante and co-workers has been reported for its methanol-tolerant ORR activity.13 Nekooi and co-workers14 have prepared a CoSe/CB by a microwave-assisted polyol method and measured its methanol-tolerant ORR activity. In another study, Pt-modified CoSe2 electrocatalysts displayed © XXXX American Chemical Society

Received: March 8, 2016 Accepted: April 12, 2016

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DOI: 10.1021/acsenergylett.6b00006 ACS Energy Lett. 2016, 1, 27−31

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters Table 1. Electrochemical and Kinetics Parameters of Co7Se8 Electrocatalyst Compared with Pt I, A g−1 at 0.65 V (mass activity) −2

2 −1

catalyst

loading (μg)

ECSA (cm )

specific ECSA (m g )

0.0 M methanol

Pt/GC Co7Se8 /GC

34.5 10.7

1.0 3.7

2.9 34.5

160.4 172.1

Tafel slopes (mV dec−1)

0.5 M methanol % of loss of I 0.0 M methanol 113.2 170.0

29.4 1.2

59.2, 107.8 60.3, 120.8

0.5 M methanol 68.3, 145.2 60.8, 121.1

Figure 1. (a) PXRD of as-deposited Co7Se8 film. (b) SEM image of Co7Se8/GC, (c) TEM image of Co7Se8/GC, and (d) HRTEM image of the catalyst showing lattice fringes corresponding to ⟨101⟩ planes. Inset shows typical SAED pattern with spots corresponding to ⟨102⟩ planes.

Figure 2. (a) Hydrodynamic voltammograms measured in O2-saturated 0.5 M H2SO4 with and without 0.5 M methanol at a scan rate of 10 mV s−1 in a RRDE setup (rotation rate of 2000, 1600, 1200, 800, and 400 rpm). The Pt ring electrode was maintained at 1.4 V vs RHE. Solid and dotted lines indicate the disk and ring current, respectively. (b) Comparison of ORR activity for Pt/GC and Co7Se8/GC electrodes in O2saturated 0.5 M H2SO4 solution in presence and absence of methanol at 1600 rpm.

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ACS Energy Letters

Figure 3. (a) K−L plots at different potentials of Pt/GC and Co7Se8/GC. (b) Estimation of number of electrons during the ORR activity in O2-saturated 0.5 M H2SO4 solution in presence and absence of methanol, and (c) Tafel plots of the Co7Se8 (green) catalysts compared with Pt (red) in the presence and absence of methanol.

obvious satellite at the higher binding energies of the Co 2p signal can be due to the antibonding orbital between the Co atom and Se atom.16 In the Se 3d spectra, the peak at 54.6 eV is consistent with Se 3d of cobalt selenide, and the peak in the range of 57.0−63.0 eV could be assigned to Co 3p of cobalt selenide.17 The morphology of Co7Se8 was further investigated through TEM (Figure 1c), which confirmed the formation of these extremely thin nanoflakes, with thickness in the order of 10−15 nm. This kind of nanostructured morphology is very ideal for catalytic activities because it has a high degree of exposure of the catalytically active sites both on the surface of the thin flakes as well as the edges. The high-resolution TEM (HRTEM) image of the Co7Se8 (Figure 1d) shows lattice fringes at 2.68 Å corresponding to the ⟨101⟩ interplanar spacing of Co7Se8. The crystallinity of the film was further confirmed by the selected area electron diffraction (SAED) pattern shown as the inset of Figure 1d in which the diffraction spots could be indexed to ⟨102⟩ lattice plane. The electrochemical characterization of the Co7Se8/GC and Pt/GC electrodes was performed by measuring the cyclic voltammograms (CVs) in N2-saturated 0.5 M H2SO4, as shown in Figure S4. A characteristic CV for Pt was obtained with welldefined hydrogen adsorption desorption, double-layer charging, and oxide formation−reduction region at the Pt/GC electrode.18,19 No peak corresponding to hydrogen adsorption−desorption was observed for the Co7Se8/GC electrode.

weakly crystalline and the diffraction pattern could be matched well with that of Co7Se8 phase (JCPDS 04-003-3440). The average particle size was estimated to be around 10 nm by using Scherrer equation (see Supporting Information). Co7Se8 is a selenium-rich cobalt selenide hexagonal phase with both Co and Se in an octahedral coordination (Figure S1 in Supporting Information) and is frequently represented as the Co0.85Se phase. It must be noted that this is the first example of Co7Se8 showing ORR catalytic activity. Morphology of electrodeposited films was studied through scanning electron micrscopy (SEM) and transmission electron microscopy (TEM) imaging. Flakelike nanostructured morphology of Co7Se8 film was observed in the SEM image, as shown in Figure 1b, with random distribution of the thin nanoflakes. These randomly oriented nanoflakes led to a very rough film with porous architecture, which may be one of the key factors for enhanced ORR activity because it increases the active surface area of catalyst. EDS spectra of Co7Se8 as shown in Figure S2 confirmed the presence of Co and Se in the film with a relative atomic Co:Se ratio close to 0.88:1. The electronic state of Co and Se of the electrodeposited film on the GC surface was comprehensively studied by X-ray photoelectron spectroscopy (XPS). The high-resolution XPS spectra shown in panels a and b of Figure S3 show the binding energies of Co 2p and Se 3d, respectively. The first doublet at 781.2 and 796.3 eV indicated the presence of Co2+ as seen in Co0.85Se.15 An 29

DOI: 10.1021/acsenergylett.6b00006 ACS Energy Lett. 2016, 1, 27−31

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ACS Energy Letters Interestingly, Co7Se8/GC exhibited higher double-layer charging current (Figure S5) which is one of the characteristics of a high-efficiency catalyst. The electrochemical active surface areas of Pt and Co7Se8 were estimated as 1.0 and 3.7 cm−2, respectively (Table 1 and Supporting Information). The rotating ring−disk electrode (RRDE) voltammograms were carried out (Figure 2a) to investigate the ORR catalytic activity of Co7Se8/GC and Pt/GC catalysts in the presence and absence of 0.5 M methanol containing 0.5 M H2SO4 solution. Figure 2b shows the comparison of ORR activities at Co7Se8/ GC and Pt/GC electrodes in O2-saturated 0.5 M H2SO4 solution containing 0.0 and 0.5 M of methanol, respectively. In the absence of methanol, Pt/GC shows an onset potential at 0.931 V vs reversible hydrogen electrode (RHE) compared to that of Co7Se8/GC at 0.811 V vs RHE. However, the ORR activity is greatly affected in the presence of methanol at Pt/ GC. The onset potential of Pt/GC in the presence of 0.5 M methanol shifted in the negative direction to a much lower value (onset potential ca. 0.801 V vs RHE), reflecting the wellknown poisoning of the Pt surface in the presence of methanol. However, the activity for ORR on Co7Se8/GC electrode was hardly affected by the presence of 0.5 M of methanol (onset potential remained unchanged, ca. 0.811 V vs RHE), implying that the Co7Se8/GC catalyst was fully tolerant to the presence of methanol. The number of transferred electrons per oxygen molecule involved in the ORR at these electrodes (Pt/GC and Co7Se8/GC) were determined by the Koutecky−Levich equation (see Supporting Information). As shown in the Figure 3a, the Koutecky−Levich plots of jl−1 versus ω−1/2 at various potentials for the voltammograms gave straight lines, the slopes of which were similar to that expected for fourelectron ORR, and the value of n was calculated to be 4.0 for the Pt/GC electrocatalyst while for Co7Se8/GC it was 3.93. Hence, it can be concluded that similar to Pt/GC, Co7Se8/GC exclusively provides four-electron oxygen reduction to water. Additionally, RRDE experiments were performed to estimate the percentage of H2O2 produced during ORR by measuring the ring and disc currents, which also confirmed the number of electrons transferred. As shown in Figure S6, the percentage of H2O2 produced by the catalysts was less than 5% at the potentials ranging from 0.8 to 0.2 V, in a four-electron (4e−) reaction process like Pt/GC (n ≈ 4.0, Figure 3b). As given in Figure 3c, the Pt/GC and Co7Se8/GC electrocatalysts exhibit typical Tafel plots with two slopes, at low and high current densities, respectively. The values of the Tafel slope at Pt/GC in the presence of methanol were higher (68.3 and 145.2 mV/ decade) than in a methanol-free electrolyte (59.2 and 107.8 mV/decade), indicating suppression of ORR activity in the presence of methanol. In contrast, Tafel slopes at the Co7Se8/ GC electrode were not affected by the presence of methanol, also indicating high methanol tolerance of the catalyst. The gravimetric current density (A g−1) of catalysts at 0.65 V (vs RHE) was calculated from the ORR polarization curves at 2000 rpm and is summarized in Table 1. Co7Se8 (172 A g−1) showed better mass activity than the Pt (160 A g−1) and was not affected by the presence of methanol (170 A g−1). However, the activity of the Pt catalyst was decreased by about 30% in the presence of methanol compared to methanol-free solution. The stability of the Co7Se8/GC catalyst was estimated by carrying out ORR experiments for 1000 cycles in O2-saturated 0.5 M H2SO4 solution in the presence of 0.5 M methanol at 1200 rpm (Figure 4). The linear sweep voltammetry (LSV) measurements before and after 1000 cycles of ORR showed

Figure 4. Stability of Co7Se8/GC catalyst by comparing the LSV measurements of the initial sample and after 1000 consecutive cycles in 0.5 M methanol containing 0.5 M H2SO4 solution at 1200 rpm with a scan rate of 10 mV s−1.

similar behavior, indicating high reproducibility and cyclability of the Co7Se8/GC in 0.5 M methanol containing 0.5 M H2SO4 solution for extended period of time. Hence, it can be concluded that unlike Pt/GC, Co7Se8/GC exhibits efficient methanol tolerance with retention of full catalytic activity. The enhanced ORR at Co7Se8 can be related to the presence of Se in the lattice which directly modifies the electronic structure of the active catalyst site (Co) as well as the relative positioning of the conduction and valence bands with respect to water oxidation bands. Typically it has been observed that changing from transition-metal oxide to transition-metal selenide leads to an increase of the conduction and valence band edges, making them closer to the water oxidation−reduction levels.20−22 The relative closeness of the band positions will expectedly lead to better charge transfer at the catalyst−electrolyte interface, thus enhancing the catalytic efficiency. Similar effect has been also reported by Alonso-Vante in a comprehensive report comparing the ORR activity of transition-metal chalcogenide catalysts where they have shown the favorable effect of selenization in increasing the ORR catalytic activity for both Pt-group metals as well other transition metals, especially Co.23 Similar effect has also been reported for Co9Se8 through theoretical as well as experimental study.24 Additionally, these nonstoichiometric compositions such as Co0.85Se (i.e., Co7Se8), being less Se rich and containing the metal in a mixed valent state, might be more beneficial for this kind of catalytic reactions because it can easily support variation of the metal oxidation state, thereby facilitating the reaction pathway. In fact, our recent work with electrodeposited Co7Se8 suggests that this material is an efficient electrocatalyst for oxygen and hydrogen evolution reactions (OER and HER, respectively) in an alkaline medium (Figure S7, unpublished results), thereby validating the above explanation. In summary, we have successfully electrodeposited nanostructured Co7Se8 films on a GC substrate and reported for the first time methanol-tolerant ORR catalytic activity of the Co7Se8/GC electrode. The thin Co7Se8 nanoflakes on a GC electrode exhibit excellent catalytic performance with superior durability and electrocatalytic selectivity toward ORR compared to Pt/GC catalysts in 0.5 M H2SO4 solution containing 0.5 M methanol, which makes it promising as a nonprecious metalbased cathodic catalyst for direct methanol fuel cell applications. Our preliminary studies reveal that Co7Se8 is also active for OER and HER processes, and the potential for 30

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(12) Swesi, A.; Masud, J.; Nath, M. Nickel selenide as high-efficiency catalyst for oxygen evolution reaction. Energy Environ. Sci. 2016, DOI: 10.1039/C5EE02463C. (13) Feng, Y. J.; He, T.; Alonso-Vante, N. Carbon-supported CoSe2 nanoparticles for oxygen reduction reaction in acid medium. Fuel Cells 2010, 10, 77−83. (14) Nekooi, P.; Akbari, M.; Amini, M. K. CoSe nanoparticles prepared by the microwave-assisted polyol method as an alcohol and formic acid tolerant oxygen reduction catalyst. Int. J. Hydrogen Energy 2010, 35, 6392−6398. (15) Song, J. M.; Zhang, S. S.; Yu, S. H. Multifunctional Co0.85SeFe3O4 nanocomposites: controlled synthesis and their enhanced performances for efficient hydrogenation of p -nitrophenol and adsorbents. Small 2014, 10, 717−724. (16) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, W.; Wei, S.; et al. Low overpotential in vacancy-rich ultrathin CoSe2 nanosheets for water oxidation. J. Am. Chem. Soc. 2014, 136, 15670−15675. (17) Jiang, Q.; Hu, G. Co0.85Se hollow nanoparticles as Pt-free counter electrode materials for dye-sensitized solar cells. Mater. Lett. 2015, 153, 114−117. (18) Awaludin, Z.; Suzuki, M.; Masud, J.; Okajima, T.; Ohsaka, T. Enhanced electrocatalysis of oxygen reduction on Pt/TaOx/GC. J. Phys. Chem. C 2011, 115, 25557−25567. (19) Zhan, D.; Velmurugan, J.; Mirkin, M. V. Adsorption/desorption of hydrogen on Pt nanoelectrodes: evidence of surface diffusion and spillover. J. Am. Chem. Soc. 2009, 131, 14756−14760. (20) Xu, Y.; Schoonen, M. A. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am. Mineral. 2000, 85, 543−556. (21) Rasmussen, F. A.; Thygesen, K. S. Computational 2D materials database: electronic structure of transition-metal dichalcogenides and oxides. J. Phys. Chem. C 2015, 119, 13169−13183. (22) Lauer, S.; Trautwein, A. X.; Harris, F. E. Electronic-structure calculations, photoelectron spectra, optical spectra, and Mössbauer parameters for the pyrites MS2 (M = Fe, Co, Ni, Cu, Zn). Phys. Rev. B: Condens. Matter Mater. Phys. 1984, 29, 6774−6783. (23) Alonso-Vante, N. Transition metal Chalcogenides for Oxygen reduction. In Electrocatalysis in Fuel Cells; Shao, M., Ed.; Lecture Notes in Energy; Springer-Verlag: London, 2013; Vol. 9, pp 417−436. DOI: 10.1007/978-1-4471-4911-8_14. (24) Vayner, E.; Sidik, R. A.; Anderson, A. B.; Popov, B. N. Experimental and theoretical study of cobalt selenide as a catalyst for O2 electroreduction. J. Phys. Chem. C 2007, 111, 10508−10513.

practical applications of such multifunctional precious-metalfree electrocatalysts will be very high because their use will lead to significant reduction in cost and complexity of fuel cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00006. Electrodeposition of Co7Se8 and Pt, characterization techniques, calculation of ECSA, description of ORR kinetics, crystal structure of Co7Se8, EDS spectra of Co7Se8, XPS Spectra of Co 2p and Se 3d, and CVs of Pt and Co7Se8, % of H2O2 generated (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The authors acknowledge American Chemical Society Petroleum Research Fund (54793-ND10) for financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Wipula P. R. Liyanage for help with crystal structure illustrations and Materials Research Center for equipment usage.

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REFERENCES

(1) Stoerzinger, K. A.; Risch, M.; Han, B.; Shao-Horn, Y. Recent insights into manganese oxides in catalyzing oxygen reduction kinetics. ACS Catal. 2015, 5, 6021−6031. (2) Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486, 43−51. (3) Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.; Markovic, N. M. The effect of the particle size on the kinetics of CO electrooxidation on high surface area Pt catalysts. J. Am. Chem. Soc. 2005, 127, 6819−6829. (4) Zhu, Y.; Zhou, W.; Chen, Y.; Yu, J.; Xu, X.; Su, C.; Tade, M. O.; Shao, Z. Boosting Oxygen reduction reaction activity of palladium by stabilizing its unusual oxidation states in perovskite. Chem. Mater. 2015, 27, 3048−3054. (5) Sun, M.; Dong, Y.; Zhang, G.; Qu, J.; Li, J. α-Fe2O3 spherical nanocrystals supported on CNTs as efficient non-noble electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2014, 2, 13635−13640. (6) 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. (7) Cao, R.; Lee, J.; Liu, M.; Cho, J. Recent progress in non-precious catalysts for metal-air batteries. Adv. Energy Mater. 2012, 2, 816−829. (8) Gao, M. R.; Jiang, J.; Yu, S. H. Solution-based synthesis and design of late transition metal chalcogenide materials for oxygen reduction reaction (ORR). Small 2012, 8, 13−27. (9) Feng, Y.; Gago, A.; Timperman, L.; Alonso-Vante, N. Chalcogenide metal centers for oxygen reduction reaction: activity and tolerance. Electrochim. Acta 2011, 56, 1009−1022. (10) Feng, Y. J.; He, T.; Alonso-Vante, N. In situ free-surfactant synthesis and ORR- electrochemistry of carbon-supported Co3S4 and CoSe2 nanoparticles. Chem. Mater. 2008, 20, 26−28. (11) Liu, T.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X. Amorphous CoSe film behaves as an active and stable full water splitting electrocatalyst under strongly alkaline conditions. Chem. Commun. 2015, 51, 16683− 16686. 31

DOI: 10.1021/acsenergylett.6b00006 ACS Energy Lett. 2016, 1, 27−31