Microwave assisted synthesis of ruthenium electrocatalysts for oxygen

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 0 3 e1 1 0

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Microwave assisted synthesis of ruthenium electrocatalysts for oxygen reduction reaction in the presence and absence of aqueous methanol E. Borja-Arco a, O. Jimeńez Sandoval b, J. Escalante-Garcıá c, A. Sandoval-Gonza´lez a, P.J. Sebastian a,* a

Centro de Investigacioń en Energıá-UNAM, Temixco, Mor. 62580, Me´xico Centro de Investigacioń y de Estudios Avanzados del Instituto Politećnico Nacional (CINVESTAV), Unidad Quere´taro, Apartado Postal 1-798, Quere´taro, Qro. 76001, Me´xico c Centro de Investigaciones Quı´micas-Universidad Autońoma del Estado de Morelos, Av. Universidad 1001, Cuernavaca, Mor. 62210, Me´xico b

article info

abstract

Article history:

In this work we present the synthesis of ruthenium based electrocatalysts for oxygen

Received 15 August 2010

reduction reaction in 0.5 mol L1 H2SO4, using microwave irradiation at different power,

Received in revised form

time and temperature conditions. Ru3(CO)12 and 1,2-dichlorobenzene were used as

10 October 2010

precursor and solvent respectively. The materials obtained were structurally characterized

Accepted 16 October 2010

by FT-IR spectroscopy and X-ray diffraction; their chemical composition was determined

Available online 18 November 2010

by energy-dispersive spectroscopy analysis. The rotating disk electrode technique was used for the electrochemical characterization of the catalysts; the oxygen reduction reac-

Keywords:

tion was performed in the presence and absence of aqueous methanol solutions. The

Microwave irradiation

electrocatalytic activity towards the oxygen reduction reaction is similar to that of ruthe-

Ruthenium catalysts

nium catalysts synthesized using a conventional process reported in the literature.

Oxygen reduction reaction

1.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Introduction

The performance of PEM fuel cells is usually limited by the slow oxygen reduction kinetics and the cross-over effect in direct methanol fuel cells. Another important feature is that the oxygen reduction reaction (ORR) must proceed via the overall four-electron transfer to form water. Platinum and platinum alloys have been the most employed electrocatalysts in fuel cells; however, even with these materials the kinetics of the ORR is slow. Furthermore, there is a great interest in the development of alternative materials for this reaction. Transition metal carbonyl clusters have shown a good electrocatalytic activity towards the ORR, with tolerance to the presence of methanol molecules. However, many of these electrocatalysts have been

synthesized using a conventional heating method of the precursor, i.e., using organic solvents at their refluxing temperature with much longer synthesis times (20 h) [1e11], or by pyrolysis of some metal carbonyl compounds for 5 h [12,13]. This could be a slow and relatively inefficient method for transferring energy into the system because it depends on convection currents and the thermal conductivity of the various materials that must be penetrated, and generally results in the temperature of the vessel being higher than that of the reaction mixture. This is particularly true if reactions are performed under reflux conditions, whereby the temperature of the bath fluid is typically kept at 10e30 C above the boiling point of the reaction mixture in order to ensure an efficient reflux. In addition, a temperature gradient can develop within the sample and

* Corresponding author. Tel.: þ52 55 56229841; fax: þ52 55 56229742. E-mail addresses: sjp@cie.unam.mx, ejboa@cie.unam.mx (P.J. Sebastian). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.051

104


Table 1 e Chemical composition of the ruthenium electrocatalysts determined by EDS experiments (wt %). Element Ru C O

180 /80 W/30 min

180 /100 W/30 min

180 /100 W/60 min

89.05 9.92 0.97

83.12 13.01 4.19

74.19 23.44 2.38

local overheating can lead to product, substrate or reagent decomposition. In contrast, microwave irradiation produces efficient internal heating by direct coupling of microwave energy with the molecules (solvents, reagents, catalysts) that are present in the reaction mixture. Microwave irradiation, therefore, raises the temperature of the whole volume simultaneously whereas in the conventionally heated vessel, the reaction mixture in contact with the vessel wall is heated first. The very efficient internal heat transfer results in minimized wall effects, which may lead to the observation of the so-called specific microwave effects, for example in the context of diminished catalyst deactivation [14]. Recently, it has been reported the synthesis of catalysts (Pt and Pt alloys) for PEMFC applications using microwave irradiation, mainly as anodes in DMFC [15e17]. These results showed an enhancement on their activity towards methanol oxidation, with current densities higher than that of commercial platinum and platinum alloys. On the other hand, Hong Zhu et al. [18] have reported studies on PtAuSn/C nanoparticles for ethanol electrooxidation, as cathode catalyst in DMFC. Parisa Nekooi et al. [19] synthesized CoSe nanoparticles as an alcohol tolerant oxygen reduction catalyst. Ayse Bayrakceken et al. [20] improved the carbon dioxide tolerance of PEMFC electrocatalyst by using microwave irradiation technique, while Glaspell et al. [21] reported the synthesis of Au and Pd nanoparticle catalysts for CO oxidation. The scope of this study is to further analyze the effect of the presence of methanol on the microwave synthesized ruthenium based oxygen reduction catalysts.

The working electrode for the rotating disk electrode (RDE) study was prepared by mixing 1.7 mg of Vulcan XC-72 (Cabot) and 0.3 mg of the catalyst with 10 mL of 5% Nafion solution (ElectroChem) in an ultrasonic bath. 2 mL of the resulting mixture was deposited on a glassy carbon disk electrode and

Fig. 1 e FT-IR spectra for a) Ru3(CO)12 and the ruthenium catalysts synthesized under the following conditions: b) 180 C/30 min/80 W c) 180 C/30 min/100 W d) 180 C/ 60 min/100 W.

Fig. 2 e X-ray diffraction patterns for a) Ru3(CO)12 and the ruthenium catalysts synthesized under the following conditions: b) 180 C/30 min/80 W, c) 180 C/30 min/100 W, d) 180 C/60 min/100 W.

2.

Experimental

2.1. Synthesis and structural characterization of the catalysts The ruthenium monometallic electrocatalysts were synthesized using 0.063 mmol of the precursor triruthenium dodecacarbonyl [Ru3(CO)12, Aldrich], mixed with 5 mL of 1,2dichlorobenzene (b.p. 178e180 C, Aldrich) and treated thermally using microwave irradiation at different power (80, 100 and 200 W) and time (30 and 60 min) conditions (Table 1) and at 180 C. The product obtained was washed with isopropyl alcohol (J. T. Baker) and dried at room temperature. The electrocatalysts synthesized were structurally characterized using reflectance FT-IR spectroscopy, on a Perkin-Elmer-GX3 spectrometer, with the samples dissolved in FT-IR grade KBr. For X-ray diffraction ˚) studies, a Rigaku D/max-2100, with Cu Ka1 irradiation (1.5406 A was used. A Philips XL30ESEM microscope was used to obtain energy-dispersive X-ray spectra (EDS) of the catalysts.

2.2.

Electrochemical experiments

2.2.1.

Electrode preparation


dried at room temperature. The cross-sectional (geometrical) area of the disk electrode was 0.072 cm2.

2.2.2.

Equipment

Measurements were carried out at 25 C in a conventional electrochemical cell with a three-electrode arrangement. A mercury sulfate electrode (Hg/Hg2SO4/0.5 mol L1 H2SO4; abbreviated as MSE) was used as reference (MSE ¼ 0.680 V/NHE), which was connected to the cell through a bridge with a Luggin capillary. The potentials were referred to the normal hydrogen electrode (NHE). The counter electrode was a graphite rod and 0.5 mol L1 H2SO4 was used as electrolyte, which was prepared with 98% sulfuric acid (J. T. Baker) and deionized water (18.2 MUcm). A potentiostat/galvanostat (Solartron 1287) and a PC with CorreWare software were used for the electrochemical measurements. A Radiometer Analytical BM-EDI101 glassy carbon rotating disk electrode (with a CTV101 speed control unit) was used for the voltammetry studies.

2.2.3.

105

scanning (cyclic voltammetry) between 0 and 0.98 V/NHE at 20 mV/s until no variation on the voltammogram was observed (30 cycles). On the other hand, a 30% Pt/Vulcan XC72 electrode was used for comparison, for which a scanning between 0 and 1.58 V/NHE range was performed, at 50 mV s1 rate. The temperature of the system was maintained at 25 C.

2.2.3.2. Linear sweep voltammetry (LSV). The electrolyte was saturated with pure oxygen (Infra; UHP) for 15 min. Polarization 2 curves in the presence of oxygen were obtained in the EO oc to 0 V/ 2 to 0.2 V/NHE NHE range for the new materials and in the EO oc range for the 30% Pt/Vulcan XC-72 electrode, at 5 mV s1 rate. Rotation rates ranged from 100 to 900 rpm. CV and LSV curves were also obtained in the presence of methanol (2.0 mol L1), under the conditions described above and in Section 2.2.3.1.

3.

Results

3.1.

Structural characterization

ORR

2.2.3.1. Cyclic voltammetry (CV). Prior to all measurements, the electrolyte was purged with nitrogen (Infra; UHP) for the activation of the electrode. This activation was done by

Fig. 1 shows the FT-IR spectra of the Ru3(CO)12 precursor as well as those of the ruthenium electrocatalysts prepared by

Fig. 3 e Cyclic voltammograms in the absence and presence of 2 mol LL1 methanol for the ruthenium catalysts synthesized under the following conditions: a) 180 C/30 min/80 W, b) 180 C/30 min/100 W, c) 180 C/60 min/100 W, d) 30%Pt/Vulcan XC-72. The electrolyte was 0.5 mol LL1 H2SO4; the sweep rate was 20 mV/s for the ruthenium electrocatalysts and 50 mV/s for Pt.

106


Fig. 4 e ORR current-potential curves in the absence and presence of 2 mol LL1 methanol for the ruthenium catalysts synthesized under the following conditions: a) 180 C/30 min/80 W, b) 180 C/30 min/100 W, c) 180 C/60 min/100 W, d) 30% Pt/Vulcan XC-72. The electrolyte was 0.5 mol LL1 H2SO4 and the sweep rate was 5 mV/s.

microwave irradiation. The precursor shows the well known strong carbonyl stretching vibration bands around 2040 cm1, as well as a group of bands around 570 cm1, which have been assigned to carbonyl deformation modes, dM-CO [22]. Such carbonyl stretching bands (at ca. 2859 and 2920 cm1) are absent in the infrared spectra of the ruthenium catalysts, thus indicating that the latter virtually lost all the carbonyl groups present in the precursor reagent. This is confirmed by the corresponding XRD patterns (Fig. 2), which show peaks characteristic of metallic ruthenium (2q w40e50 ). These peaks are very broad at the base, indicating the small metal particle size. This was confirmed by calculations using the Scherrer formula [23], which indicated an average ruthenium particle size of 5 nm for b)180 C/30 min/80 W, 4.5 nm for c) 180 C/30 min/ 100 W and 6.5 nm for d) 180 C/60 min/100 W respectively. On the other hand, Table 1 shows the chemical composition of the ruthenium catalysts, where it is possible to observe the presence of carbon and oxygen in their chemical composition.

3.2.

Electrochemical characterization

3.2.1.

Cyclic voltammetry

The cyclic voltammograms of the different ruthenium electrodes in the absence and presence of 2 mol L1 methanol

solution are shown in Fig. 3, along with that for 30% Pt/Vulcan XC-72 for comparison. It can be observed that in agreement with the chemical composition reported in Table 1, the cyclic voltammograms of the catalysts with a higher oxygen content (Fig. 3b and c) show a cathodic peak in the 0.2e0.4 V/NHE range, which is attributed to the presence of ruthenium oxides, RuOx [24,25]. Such oxides form in amounts not large enough to be detected by X-ray diffraction measurements. These materials show the presence of hydrogen and oxygen evolutions peaks, in the cathodic 0e0.1 V/NHE and anodic 0.85 V/NHE regions respectively. Such processes are favored for materials with higher oxygen concentrations. Another important feature is that the ruthenium catalysts do not show methanol oxidation peaks [26,27], in contrast with the platinum electrode, which exhibits very sharp peaks (Fig. 3d) indicating its high activity for the methanol oxidation process.

3.2.2.

Linear sweep voltammetry

Fig. 4 shows the polarization curves of the ruthenium electrocatalysts for the electrochemical reduction of molecular oxygen in 0.5 mol L1 H2SO4, in the absence and presence of 2 mol L1 methanol solution, along with that of 30%Pt/Vulcan XC-72 as reference. These curves show the three distinct regions characteristic of ORR processes taking place on


107

Fig. 5 e Experimental (at 0.5 V/NHE) and theoretical (2 and 4 electrons) KouteckyeLevich plots in the absence and presence of 2 mol LL1 methanol for the ruthenium catalysts synthesized under the following conditions: a) 180 C/30 min/80 W, b) 180 C/30 min/100 W, c) 180 C/60 min/100 W, d) 30%Pt/Vulcan XC-72.

a catalyst’s surface. 1) the kinetic region, where the current, ik, is independent of the rotation velocity; 2) the mixed control region, where the behavior is determined by kinetic as well as diffusion processes; and 3) the mass-transfer region, where the diffusion current, id, is a function of the rotation velocity. It can be observed that the material with the lowest oxygen concentration (Fig. 4a) does not show the peak at 0.2e0.4 V/NHE in the mass-transfer region observed for the materials with a higher oxygen content (Fig. 4b and c). These features are in agreement with the corresponding cyclic voltammograms (Fig. 3) and chemical compositions (Table 1). The most important feature observed is that the polarization curves are practically unchanged by the presence of methanol (Fig. 4aec), thus confirming the tolerance of the ruthenium catalysts to this contaminant during the ORR. In contrast, the Pt electrode exhibits a mixed potential due to the simultaneous methanol oxidation and oxygen reduction reactions on its surface [28], which in turn caused the net cathodic current onset to shift negatively by ca. 0.5 V/NHE (Fig. 4d). The analysis of the catalytic current as a function of the rotation rate was done with the help of the KouteckyeLevich relation:

1 1 B ¼ þ i ik u1=2

(1)

where i is the measured disk current, ik the kinetic current, u is the electrode rotation speed in rpm and B is a constant given by [29]: B¼

!

1 2=3

200nFAv1=6 DO2 CO2

(2)

where n is the number of electrons exchanged per mol of O2, F the Faraday constant, A the catalytic effective surface area, n the kinematic viscosity of the electrolyte, DO2 the oxygen diffusion coefficient and CO2 the bulk oxygen concentration in the electrolyte. The theoretical KouteckyeLevich slopes can be calculated from Eq. (2). The values used in this work were, 0.01 cm2 s1 for the kinematic viscosity, 1.4 105 cm2 s1 for the oxygen diffusion coefficient and 1.1 106 mol cm3 for the bulk oxygen concentration [1]. The KouteckyeLevich plots are usually used to estimate the number of electrons involved during the oxygen reduction. Fig. 5 shows the theoretical (with n ¼ 2 and 4; A, the electrode geometric area ¼ 0.072 cm2) and experimental KouteckyeLevich plots in the absence and presence of 2 mol L1 methanol at a given potential value

108


Fig. 6 e ORR mass-transfer-corrected Tafel plots in the absence and presence of 2 mol LL1 methanol for the ruthenium catalysts synthesized under the following conditions: a) 180 C/30 min/80 W, b) 180 C/30 min/100 W, c) 180 C/60 min/ 100 W, d) 30%Pt/Vulcan XC-72.

(0.4 V/NHE) for the ruthenium electrocatalysts, along with that of the 30%Pt/Vulcan electrode. In general, the experimental plots show that all the electrocatalysts show characteristics closer to the plots obtained for a four-electron process, than those for a two-electron route. Hence, the new electrocatalysts most likely reduce the oxygen molecules directly to water. Another important feature is that the linearity and parallelism observed in these KouteckyeLevich plots (even in

the presence of methanol) can be associated with a first order reaction with respect to the oxygen dissolved in the electrolyte [1,30]. The differences between the experimental and theoretical KouteckyeLevich plots may result from the exposed catalytically active area of the materials, which might be higher than the geometric ones [1]. Fig. 6 shows the mass-transport corrected Tafel plots (log [i,id/(id-i)] vs. E) for the ORR in the absence and presence of

Table 2 e Open circuit potential values and ORR kinetic parameters of the ruthenium electrocatalysts in O2-saturated 0.5 mol LL1 H2SO4, at 25 C. The values for Ruy(CO)n [7] and 30%Pt/Vulcan are included for comparison purposes. Power Watts

Time Minutes

[CH3OH] mol L1

EOC V/NHE

b mV/decade

a

180

80

30

180

100

30

180

100

60

0 2 0 2 0 2 0 2 0 2

0.850 0.800 0.851 0.846 0.854 0.846 0.763 0.763 1.006 0.5416

185.65 183.16 154.56 159.93 163.62 172.25 181.48 181.61 91.46 e

0.3201 0.3231 0.3828 0.3699 0.3617 0.3425 0.3264 0.3259 0.6469 e

Temperature C

Ruy(CO)na 30% Pt/Vulcan

a Reference [7].

j0 mA/cm2 1.24 1.36 2.69 2.99 4.14 4.42 2.07 1.72 2.25

E03 E03 E04 E04 E04 E04 E04 E04 E04 e


methanol solutions for ruthenium and platinum electrocatalysts, obtained from Eq. (3) [29]. ik ¼

i,id id i

(3)

Table 2 summarizes the kinetic parameters (obtained from the Tafel Plots) and open circuit potentials (EOoc2 ) of the ruthenium electrocatalysts reported in this work, compared to those of Ruy(CO)n materials reported in the literature using the conventional thermolysis method [7], and of 30%Pt/Vulcan XC-72. It can be observed that in the absence of methanol all the ruthenium catalysts show EOoc2 values 0.85 V/NHE, and that these potential values are not affected in an important way by the presence of this contaminant. Only the electrocatalyst synthesized at 180 C/80 W/30 min shows w0.05 V/NHE decrease. Although the Pt electrode shows the highest EOoc2 value in the absence of methanol (∼1.0 V/NHE), due to the presence of a mixed potential [28], it shows the lowest value in the presence of this contaminant. The Tafel slope is a parameter related to the reaction mechanism [31]; all the materials show in general Tafel slopes higher than platinum (in the absence of methanol), which suggests that the rate-determining step does not correspond to a single electron-transfer, according to the following scheme [32], O2 þ Hþ þ e /O2 H

(4)

On the other hand, the charge transfer coefficient (a) is a kinetic parameter related to the free energy of the reaction (symmetry of the reaction barrier) [33]. In this sense, the platinum electrode shows the highest a value in the absence of methanol, i.e., the largest decrease of reaction free energy. However, in the presence of this contaminant platinum loses its activity for the ORR. The ruthenium electrocatalysts show similar a values among them, higher than 0.32, and most importantly such values are not significantly modified by the presence of methanol in the electrolyte. One of the most important kinetic parameters is the exchange current density (j0), since it is proportional to the rate constant (k) of the reaction [29]. All the electrocatalysts, including platinum, show similar exchange current densities (∼104 mA cm2). Only the ruthenium catalyst synthesized at 180 /80 W/30 min shows a higher j0 value, as expected from their higher Tafel slopes. According to Table 2, there is not an important effect of time and power synthesis conditions on the kinetic parameters and open circuit potential values of the Ru electrocatalysts. Although these materials showed similar kinetic parameters with respect to Ruy(CO)n catalysts synthesized by a conventional method [7], they showed open circuit potentials ∼100 mV higher than Ruy(CO)n. Furthermore, it has been improved the synthesis method for these kinds of materials using microwave irradiation, because it was possible to avoid long synthesis periods (∼ 20 h), obtaining a similar electroactivity with those materials obtained using a conventional method.

4.

Conclusions

Ruthenium electrocatalysts for the oxygen reduction reaction have been synthesized using microwave irradiation. All the

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materials are constituted mainly by metallic particles, and probably with some kind of residual ruthenium carbide and ruthenium oxide. These electrocatalysts show an activity for the ORR comparable to Ruy(CO)n type reported in the literature. In addition, these ruthenium catalysts show tolerance to the presence of up to 2 mol L1 methanol during the ORR, in contrast to platinum. These properties make them potential candidates as cathodes in PEMFCs and DMFCs, dismissing the negative effects showed by platinum in the presence of methanol. With the microwave method it is possible to decrease reaction times used in the conventional thermolysis processes, from 20 h to only 30 min, with similar electroactivity results for the ORR.

Acknowledgements The authors wish to thank R. A. Mauricio-Sańchez and J. E. Urbina-Alvarez (CINVESTAV-Quere´taro) for valuable technical assistance. A postdoctoral grant (E. Borja-Arco) from UNAM is also acknowledged. This work was done as part of the CONACYT project 100212 and DGAPA project IN103410.

references

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