Synthesis of Ordered Mesoporous Carbon as Support ...

ECS Transactions, 61 (29) 39-47 (2014) 10.1149/06129.0039ecst ©The Electrochemical Society

Synthesis of Ordered Mesoporous Carbon as Support for Pt-Co Alloys: Evaluation as an Alcohol-Tolerant ORR Catalyst for Direct Oxidation Fuel Cells D. Morales-Acostaa, N. M. Sanchez-Padillab, F. J. Rodríguez-Varelaa,b a

Programa de Nanociencias y Nanotecnología, CINVESTAV-IPN, Unidad Saltillo, Av. Industrial Metalurgica 1062, Ramos Arizpe, Coahuila 25900, México. b Programa de Sustentabilidad de los Recursos Naturales y Energía, CINVESTAV-IPN, Unidad Saltillo, Av. Industrial Metalurgica 1062, Ramos Arizpe, Coahuila 25900, México.

In this work the Pt-Co/OMC as oxygen reduction reaction (ORR) cathode was evaluated under fuel-free and fuel-propanol in acid electrolyte conditions. By RDE measurements the ORR on the alloy seems to proceed via 4 e- transfer mechanism in acid media. In presence of 2-Propanol the alloy shows a high degree of tolerance. The results obtained from the Pt-Co/OMC alloy were compared to those of a Pt/OMC cathode.

Introduction Direct 2-Propanol acid fuel cells (DPFC) are attractive as a power energy density of 7080 Wh L-1, almost 1.5 times the energy density of methanol (4820 Wh L-1), lower crossover of the propanol fuel through the polymer membrane, safe storage and transportation compared to direct methanol fuel cells (DMFC)(1-3). The complete electro-oxidation for 2-Propanol is and 18 e- process (4): CH3CHOHCH3 + 5H2O 3CO2 + 18H+ + 18e- (5). The challenges associated with developing practical DMFC or DPFC include poisoning of anode electrocatalysts by intermediate species obtained from fuel oxidation, crossover of fuel from the anode to the cathode and cathode poisoning by fuel (2, 5). However, 2-Propanol have been showed a smaller crossover rate than methanol through Nafion membranes (2). In order to overcome these drawbacks, efforts are mainly focused in the development of new cathode electrocatalysts based on Pt alloys that do not adsorb fuel while maintaining an excellent activity for the oxygen reduction reaction. The improvement in the ORR electrocatalysis has been ascribed to the change in the Pt-Pt interatomic distance and to the increased Pt d-band vacancy. The metal nanoparticles are normally supported on carbon materials in order to maximize their surface area and decrease the total amount of metal employed (6). It had been reported that the nature of the catalyst and the presence of a support influences the catalyst activity and the reaction selectivity. Pt-Co/MWCNT has shown excellent properties as an alcohol-tolerant ORR catalyst for direct oxidation fuel cells (7). Ordered Mesoporous Carbon (OMC) has received great attention because of its potential use as electrocatalysts support for fuel cell electrodes due to its relatively large pores (compared to Carbon black, Vulcan) that facilitate mass transfer, and very high surface area, which allows a high concentration of active sites per mass of catalyst (8-13). Platinum electrocatalysts supported on OMC have been prepared as alternative catalysts

39

Downloaded on 2015-01-07 to IP 148.247.22.254 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

ECS Transactions, 61 (29) 39-47 (2014)

for PEM fuel cells (14, 15). Their performance has been compared with that of a commercial Pt-carbon black on carbon cloth electrode (E-TEK) for fuel cells applications. Recently, Pt-Co bifunctional nanoparticles supported on OMCs were found to have superior electrocatalytic activity and the tolerance to methanol crossover during ORR as compared to commercial electrocatalysts (12). In such report, Pt-Co/OMC as ORR cathode was evaluated under fuel-free and propanol-containing acid electrolyte conditions. In this work, we report the synthesis and performance evaluation of Pt-Co/OMC electrocatalyst for the ORR under DPFC conditions. Ordered mesoporous carbon support was synthetized by a simple method via self-assembly in aqueous phase. The electrochemical behavior of the alloy was studied by RDE technique. The results were compared to those measured for a Pt/OMC under the same conditions.

Experimental Reagents and physical characterization

Ultrapure 18.3 MΩ water and high-purity N2, as well as analytical grade 2-Propanol and H2SO4 (Aldrich) solutions were used during the experiments. Physicochemical characterization

The synthesized catalysts were characterized by X-ray diffraction (XRD). The diffraction patterns were acquired over 20-100 degrees with 0.05 steps in a X-pert MPD Phillips diffractometer using Cu Kαradiation operating at 43 kV and 35 mA. A Philips XL30 E-SEM scanning electron microscope was used for SEM image characterization while energy dispersive X-ray (EDX) was employed for elemental analysis of the catalysts. The morphology was investigated with a JEOL transmission electron microscope Model JEM-ARM200 operated at 200 kV. Support and catalysts synthesis, electrode preparation and electrochemical set-up

The OMC powders were synthesized by organic-organic self-assembly in aqueous phase according to a procedure described elsewhere (16) with some modifications. Briefly, resorcinol and formaldehyde (Sigma-Aldrich) were pre-polimerized and Pluronic® F-127 (Sigma) triblock copolymer was used as the template to allow the formation of the mesostructure in acidic conditions. Finally, the removal of template and carbonization was carried out by means of calcination at 900 °C under N2 atmosphere. Previous to the catalysts deposition, the OMC support was chemically modified. Firstly, the OMC was put in HNO3/H2SO4 (1:4 V/V) under refluxing and stirred conditions (17). Later, the wet powder was washed with deionized water until the pH of the filtrated solution was 7. Finally, the obtained powder was dried. The Pt-Co catalyst with a nominal Pt:Co atomic ratio of 40:60 was prepared by an impregnation method on OMC and reduced with NaBH4. Briefly, 150 mg of carbon support was firstly dispersed in 100 mL of deionized water in an ultrasonic bath for 30

40



min. After that, H8Cl6N2Pt (Aldrich, 99.99%) and CoCl2.6H2O (Sigma-Aldrich, 98%) solutions were added to the aqueous slury containing OMC under continuous magnetic stirring for 90 min. After that NaBH4 (Sigma-Aldrich, 98.5%) was slowly dropped into this mixture and vigorously stirred for 1 h. The molar ratio between the metal and the reducing agent was 1:5. The resulting solution was filtered, washed and dried. Overall, the same methodology was followed for the preparation of Pt/OMC. The electrochemical behavior of the catalysts in acid media was evaluated with a Voltalab PGZ30 Potentiostat/Galvanostat. A platinum mesh was used as counter electrode and Silver/Silver Chloride (Ag/AgCl=0.22 V/NHE) as the reference electrode, although all potentials in the text are referred to NHE (Normal Hydrogen Electrode). Glassy carbon disk with a cross-sectional area of 0.196 cm2 was used as a support for the thin films and used as an ink-type working electrode. The catalytic ink for working electrodes was prepared with 10 mg of the corresponding catalyst (20% wt. Pt-Co on OMC), 1 mL of de isopropylic alcohol and 20 µL of Nafion (5 % wt, DuPont), which were mixed by ultrasound in order to form a colloidal catalytic ink (10 mgcatal mL-1). An aliquot of 10 µL of the solution was dispersed onto a glassy carbon disc, and let to dry. CVs were carried out to activate the electrocatalysts surface in N2-satured electrolyte (0.5 M H2SO4) in the 0.05 to 1.2 V at 20 mV s-1 (not showed). The ORR measurements were performed by the linear sweep voltammetry (LSV) technique in oxygen-saturated electrolyte in the potential range of 1 to 0.1 V in a Pine equipment (AFCBP1) under rotating conditions (400, 800, 1200, 2000 rpm). All ORR experiments were performed at the scan rate 2 mV s-1 at 25 °C. For tolerance tests, the ORR measurements in presence of 0.125, 0.250 and 0.5 M C3H8O (in 0.5 M H2SO4) were performed in the same potential range.

Results and discussion Figure 1 shows the XRD patterns of Pt-Co/OMC and Pt/OMC. The broad diffraction peak at ca. 24◦ is attributed to C (0 0 2) (11). The diffractograms show the characteristics of Pt crystalline face-centered-cubic (fcc) structure, with peaks corresponding to the planes (111), (200), (220), (222) and (311) at around 39.7, 46.1, 67.5 and 81.4. As can be seen, all platinum diffraction peaks in the Pt-Co catalyst were shifted to higher 2θ values with respect to the peaks in Pt/OMC. The shift to higher angles reveals the alloy formation between Pt and Co, which is caused by the incorporation of Co atoms in the fcc structure of Pt. This is corroborated by the values of lattice parameters obtained for both materials (0.391 and 0.389 nm for Pt/OMC and Pt-Co/OMC, respectively). These results correlate well with those reported previously (7). The average particle size calculated from the (220) diffraction peaks are 2.1 and 2 nm for Pt/OMC and PtCo/OMC, respectively. The physicochemical characteristics of both catalysts are shown in table 1.

41


Pt [3 1 1]

Pt [2 2 0]

Intensity / a. u.

C [0 0 2]

Pt [2 0 0]

Pt [1 1 1]


PtCo/OMC

Pt/OMC 20

30

40

50

60

70

80

90

100

2θ / Degrees

Figure 1. XRD patterns of Pt-Co/OMC and Pt/OMC catalysts.

Figure 2 shows STEM and HR-STEM images of Pt/OMC. It is possible to observe the formation of agglomerates on OMC (fig. 2a). The electrocatalyst displays crystalline features as indicated in the electron diffraction patterns inserted in figure 2b. Fourier analysis carried out in the image of figure 2b, allowed to determine a distance between adjacent planes of d¼ = 0.222 nm, ascribed to Pt (111). Finally, the particle size observed is similar to that obtained from XRD (see table 1).

(a)

(b)

d= 0.222 nm Pt [1 1 1]

50 nm

5 nm

Figure 2. STEM anf HR-STEM images of Pt/OMC.

42



Table 1. Chemical composition and physicochemical parameters of catalysts. Catalyst

Atomic ratio Pt:Co

2θ

Lattice Parameter (nm)

Average crystallite (nm)

Pt/ OMC

1:0

68.2

0.391

2.1

PtCo/ OMC

0.4:0.6

67.5

0.389

2

The electrocatalytic activity and kinetics of ORR on Pt-Co/OMC and Pt/OMC have been evaluated through the polarization curves on a rotating disk electrode (RDE), performed in 0.5 M H2SO4 electrolyte solution saturated with O2. Figure 3 shows the ORR polarization curves at Pt-Co/OMC under several rotation rates. The Pt/OMC polarization curve at 2000 rpm is also included. All the polarization curves reveal three distinguishable potential regions: kinetic (E > 970 mV), mixed (0.510 mV < E < 860 V) and diffusion controlled (E < 550 mV). The curves in Figure 3 show that the onset potential of the ORR is similar at the two cathodes, although some differences arise in the mixed controlled region. At 50 µA/cm2, the ORR potential (at 2000 rpm) on Pt-Co/OMC is 980 mV, slightly higher than the 960 mV measured at Pt/OMC. This behavior demonstrates a facile kinetics at the bimetallic catalyst for the ORR. The Pt-Co/OMC cathode in this work shows enhanced onset potential for the ORR compared to similar catalysts reported in a previous work (7).

0

Current density (mA/cm²)

PtCo/OMC -1

-2

400 -3

800 -4

1200 Pt/OMC@2000

1600 -5

2000 0

200

400

600

800

1000

Potential (mV vs SHE)

Figure 3. Polarization curves of the ORR at the Pt-Co/OMC and Pt/OMC catalysts in 0.5 M H2SO4 solution. Scan rate: 2 mV s-1. Mass activity plots of both catalysts are shown in Figure 4. Over the complete potential interval, the mass current density of Pt-Co/OMC is significantly higher in comparison with that of Pt/OMC.

43



1.00 0.95

PtCo/OMC

0.90

E (V vs SHE)

0.85

Pt/OMC

0.80 0.75 0.70 0.65 0.60 1E-7

1E-6

1E-5

1E-4

1E-3 -1

im (A mg

Pt

0.01

0.1

)

Figure 4. Mass activity curves of Pt-Co/OMC and Pt/OMC. ω=2000 rpm. Scan rate: 2 mV s-1.

0.6

PtCo/OMC

j-1 / mA-1 cm-2

0.5

0.4

0.3

V vs SHE 0.65 0.6 0.55 0.5 4e-

0.2

0.1

0.0 0.00

0.01

0.02

0.03 -1/2

ω

/ rpm

0.04

0.05

-1./2

Figure 5. Levich-Koutecky plots corresponding to the experimental data of Pt-Co/OMC (from Figure 3). The theoretical slope for a 4 e- reaction mechanism is also shown. According to the Levich-Koutecky equation, 1/j = 1/jk+1/jd = 1/jk+1/Bω1/2, a relationship between the disc current density (jd) and the kinetic current density (jk) can be established (7). Figure 5 shows the plots considering the experimental data of Pt-

44



Co/OMC at different potential values. The theoretical slope for a 4 e- transfer reaction is also included. As expected, the plots are all straight lines, parallel to the theoretically calculated slope. The slopes of Pt-Co/OMC agree with a diffusion controlled reduction process indicating first order kinetics with respect to O2. This result suggests that the ORR on Pt-Co/OMC alloy may proceed via an overall mechanism that leads preferentially to the formation of H2O. The crossover of 2-Propanol from the anode to the Pt cathode, is one of the major problems of DPFCs which can decrease the cell potential. A competitive reaction therefore occurs between ORR and 2-Propanol oxidation, needing electrocatalysts tolerant to the fuel. With the aim to evaluate the ORR activity in a propanol containing electrolyte, ORR experiments in O2 saturated with several fuel concentrations have been performed. Fig. 6 depicts polarization curves of Pt-Co/OMC for ORR in propanolcontaining electrolyte (0.125, 0.250 and 0.5 M). As a comparison, the response of Pt/OMC with 0.125 M propanol concentration is included.

PtCo/OMC ORR @ 1600 rpm 2-Prop / M 0.125 0.250 0.5

Current density (mA/cm²)

4

2

0

-2

-4

Pt/OMC [0.125 M] -6 0

200

400

600

800

1000

Potential (mV vs SHE)

Figure 6. ORR of Pt-Co/OMC in the presence of different concentrations of C3H8O at 1600 rpm. Supporting electrolyte: 0.5 M H2SO4. Scan rate: 2 mV s-1. As can be seen, the ORR curve of Pt/OMC is significantly affected be the presence of the fuel, as compared to the ORR curves in fig. 5. It is clear that on Pt/OMC, both propanol oxidation and ORR can take place simultaneously as deduced from the positive current observed at potentials at ca. 500 mV. Also, there is an important potential shift towards more negative potentials in the presence of propanol. Under these conditions, some active sites at Pt/OMC may have been be poisoned by adsorbed species arising from the propanol oxidation reaction. On the other hand, the Pt-Co/OMC cathode shows a higher tolerance to the organic molecule than Pt/OMC. There is a shift in onset potential, but smaller than that of Ptalone. Moreover, a small decrease in the reduction current density is recorded with 0.125 M propanol, although the intensity of the associated peak is considerably lower compared

45



to the peak at Pt/OMC. At higher fuel concentrations (0.25 and 0.5 M) the ORR overpotential increases and the current density is diminished but no oxidation currents are observed. Even if the chemical composition of the alloy may not be the optimized one, these results demonstrate a high catalytic activity of Pt-Co/OMC for the ORR, with high selectivity towards the ORR and tolerance to propanol. The high performance of PtCo/OMC can be ascribed to several factors: 1) the unique structure of Pt-Co and the nanoparticles size effect that promote a higher active sites availability (7), 2) the synergetic effect between Co and Pt atoms in which Co can act as an electron donor, modifying the Pt-Pt interplanar distances (18), and 3) the role of OMC, facilitating electron transfer which promotes the ORR (19). These results lead to the conclusion that Pt-Co/OMC electrodes are suitable for use as propanol-tolerant catalysts for the ORR in acidic media.

Conclusions The catalytic activity of Pt-Co/OMC for the ORR in the absence and presence of different 2-Propanol concentrations was evaluated. The RDE results indicated that the ORR at Pt-Co/OMC follows a nearly 4 e- transfer mechanism. The mass activity plot showed a higher performance of Pt-Co/OMC compared to Pt/OMC over a 0.95-0.7 potential interval. Even if the chemical composition of the Pt-Co/OMC alloy has not been optimized, its high catalytic activity for the ORR and good selectivity characteristics have been shown in this work. The results indicated that Pt-Co/OMC electrodes are suitable for use as propanol-tolerant cathode catalysts in acidic media.

Acknowledgments This work has been partially supported by the Mexican Council of Science and Technology (CONACyT). DMA and NMSP thank CONACyT for Posdoctoral fellowship and MSc scholarship, respectively.

References 1. D. Cao and S. H. Bergens, J. Power Sources, 124, 12 (2003). 2. Z. Qi, M. Hollett, A. Attia and A. Kaufman Electrochem. Solid-State Lett., 5, A129 (2002). 3. S. Sen Gupta and J. Datta, Journal of Chemical Sciences, 117, 337 (2005). 4. S. L. Blair and W. L. Law, in Electrocatalysis of Direct Methanol Fuel Cells, p. 527, Wiley-VCH Verlag GmbH & Co. KGaA (2009). 5. A. S. Aricò, S. Srinivasan and V. Antonucci, Fuel Cells, 1, 133 (2001). 6. E. Antolini, Applied Catalysis B: Environmental, 88, 1 (2009). 7. D. Morales-Acosta, D. López de la Fuente, L. G. Arriaga, G. Vargas Gutiérrez and F. J. Rodríguez Varela, International Journal of Electrochemical Science, 6, 1835 (2011). 8. S. H. Joo, K. Kwon, D. J. You, C. Pak, H. Chang and J. M. Kim, Electrochim. Acta, 54, 5746 (2009).

46



9. N.-I. Kim, J. Y. Cheon, J. H. Kim, J. Seong, J.-Y. Park, S. H. Joo and K. Kwon, Carbon, 72, 354 (2014). 10. S.-H. Liu, C.-C. Chiang, M.-T. Wu and S.-B. Liu, Int. J. Hydrogen Energy, 35, 8149 (2010). 11. S.-H. Liu and J.-R. Wu, Int. J. Hydrogen Energy, 37, 16994 (2012). 12. S.-H. Liu, F.-S. Zheng and J.-R. Wu, Applied Catalysis B: Environmental, 108– 109, 81 (2011). 13. S. Shrestha, S. Asheghi, J. Timbro and W. E. Mustain, Carbon, 60, 28 (2013). 14. J. B. Xu and T. S. Zhao, RSC Advances, 3, 16 (2013). 15. F. A. Viva, M. M. Bruno, E. A. Franceschini, Y. R. J. Thomas, G. Ramos Sanchez, O. Solorza-Feria and H. R. Corti, Int. J. Hydrogen Energy, 39, 8821 (2014). 16. J. Xu, A. Wang and T. Zhang, Carbon, 50, 1807 (2012). 17. D. Morales-Acosta, M. D. Morales-Acosta, L. A. Godinez, L. Álvarez-Contreras, S. M. Duron-Torres, J. Ledesma-García and L. G. Arriaga, J. Power Sources (2011). 18. D. Morales-Acosta, J. Ledesma-Garcia, L. A. Godinez, H. G. Rodríguez, L. Álvarez-Contreras and L. G. Arriaga, J. Power Sources, 195, 461 (2010). 19. S.-H. Liu, S.-C. Chen and W.-H. Sie, Int. J. Hydrogen Energy, 36, 15060 (2011).

47
