C core–shell nanocatalysts for the oxygen reduction reaction

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 x x x ( 2 0 1 3 ) 1 e6

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RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction J.L. Reyes-Rodrı´guez, F. Godı´nez-Salomo´n, M.A. Leyva, O. Solorza-Feria* Departamento de Quı´mica, Centro de Investigacio´n y Estudios Avanzados del IPN, Av. IPN 2508, Col. San Pedro Zacatenco, A. Postal 14-740, 07360 Me´xico D.F., Mexico

article info

abstract

Article history:

Kinetics study of the oxygen reduction reaction (ORR) taking place on carbon supported

Received 24 August 2012

Co@Pt/C coreeshell nanocatalysts, were characterized by using the rotating ring-disc

Received in revised form

electrode (RRDE) technique and results compared to that of Pt/C (Etek). The supported cat-

4 December 2012

alyst was successfully synthesized through a simple procedure through sequential colloidal

Accepted 5 December 2012

chemical reduction of Co and a galvanic replacement by Pt. X-ray diffraction results of the

Available online xxx

Co@Pt/C nanoparticles showed important structural changes which can be responsible for the observed enhanced catalytic activity. XRD Rietveld refinement confirmed the presence

Platinum

of metallic Pt and Co in the relative weight fraction of 12 wt% and 88 wt% respectively, as  and Co lattice expansion (i.e. 3.6170  well as a Pt lattice contraction (i.e. 3.8741 A) A). Elec-

Oxygen reduction reaction

trochemical results support the formation of a coreeshell structure with double enhanced

Coreeshell nanocatalyst synthesis

ORR activity and similar peroxide generation of the Co@Pt/C catalyst with respect to Pt/C

Hydrogen peroxide

(Etek).

Proton exchange membrane fuel cell

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Keywords:

reserved.

1.

Introduction

Nowadays Pt catalyst continues being the principal metallicbased element used for the oxygen reduction reaction (ORR) which is carried out on the cathodic side of a proton exchange membrane fuel cell (PEMFC). Nonetheless, in order to meet large-scale PEMFC production, several technical and economic issues such as electrocatalytic activity, stability, durability, etc, should be previously resolved [1]. According to the New Energy and Industrial Technology Development Organization (NEDO), the target of Pt loading, which has a great impact on the cost in the mass-production stage, is to get less than 0.1 g/kW energy production [2]. One strategy for reducing the cost is to decrease the amount of Pt loading by the efficient use of this metal [3]. In this sense, the development of a new based Pt catalysts, with higher electrocatalytic activity and improved active surface area utilization results critical for achieving the loading target.

In terms of the catalytic activity and high Pt utilization, coreeshell catalysts have been in the spotlight recently [4e6]. Because only the surface atoms during the electrocatalytic reactions are involved, modulating the particle configuration to promote the coreeshell shape, increases the electrochemically available Pt. Besides, the mismatch atoms radii between the Pt on top and the other elements constituting the core (i.e. Ni, Co, Fe, Ag etc), promotes changes in the interatomic distance which in turn modify the electronic interaction promoting, in most cases, a higher activity [7]. In a fundamental point of view, it is widely known that ORR on Pt proceeds mainly via a four-electron transfer process with a two-electron side reaction [8]. The last has enormous impact mainly during PEMFC operation, since formation of H2O2 as a byproduct tends to attack the polymer membrane and accelerate the Pt dissolution leading to a deterioration of the membraneelectrode-assembly, MEA, durability [9]. From a fundamental

* Corresponding author. Tel.: þ52 55 5747 3715; fax: þ52 55 5747 3389. E-mail address: [email protected] (O. Solorza-Feria). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.12.031

Please cite this article in press as: Reyes-Rodrı´guez JL, et al., RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2012.12.031

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point of view, the desired route of the oxygen reduction is the 4e pathway leading to H2O formation; however, a partial 2e reduction to H2O2 is also possible. Peroxide derived species boost a chain of oxidation reactions by attacking the carboxylic end groups of the Nafion polymer used in the PEMFC [10], decreasing considerably its useful lifetime. Thus, decreasing the H2O2 release, as well as increasing the catalyst intrinsic activity is of paramount importance in the design of an ORR electrocatalyst. In this work, the catalyst preparation, structural and electrochemical characterization were used as a basis for the determination of the intrinsic properties of Co@Pt coreeshell electrocatalyst and to compare this, in terms of the ORR kinetics, with that obtained on Pt/C (E-tek). Carbon supported coreeshell catalyst, was tested under the same experimental conditions as Pt/C (E-tek) electrocatalyst, and results led to Co@Pt/C coreeshell electrocatalyst to be considered as a cathode electrode in PEMFCs.

2.

Experimental

2.1.

Catalyst synthesis

Cobalt nanoparticles were synthesized by the borohydride reduction method in ethanol following a procedure reported previously [11]. Shortly, 1.5 g of CoCl2$6H2O (6.31 mM) and 2.04 g of tetrabutyl ammonium bromide (TBAB, 6.32 mM) as capping agent, were dissolved in 700 mL of ethanol and magnetically stirred during 1 h in a N2 atmosphere. Then, 0.975 g of NaBH4 (25.3 mM) dissolved in 100 mL of ethanol were added drop by drop to promote cobalt reduction. The Co was physically supported, through ultrasound stirring, on Vulcan carbon XC-72 estimating a 40% metal loading expressed as a weight percent. The Co@Pt/C coreeshell catalyst was prepared by galvanic replacement mixing 212.5 mg of Co/C catalyst with 45.6 mg of H2PtCl6$6H2O, which corresponds to a nominal weight Co:Pt ratio of 80:20. Procedure details have been reported elsewhere [4].

2.2.

The working electrode was prepared according to Schmidt et al. [13]. Nominal Pt loading around 19 mg cm2geo and 15 mg cm2geo were used for both Pt/C-Etek and Co@Pt/C respectively. Additionally, the CO-stripping charge was used to determine the electrochemical surface area (ECSA) [14]. For CO stripping measurements, the catalyst surface was previously saturated with CO by bubbling CO through the electrolyte solution polarizing the electrode at 0.1 V/RHE for 5 min. Then the remaining CO was purged by flowing N2 for 10 min before measurements which were performed at scan rate of 20 mV s1.

2.3.

X-ray diffraction

Crystal structure identification was performed through low temperature (173 K) two-dimensional X-ray diffraction technique (XRD), using a BruckereNonius (model Kappa-CCD) diffractometer with monochromatic Mo-Ka radiation (l ¼ 0.7107  A) in a 2q range from 15 to 62.5 [15]. The crystallite size determination and the quantitative phase analysis were analyzed by Rietveld refinement [16], using the Topas academic software. The mean crystallite size was calculated from the (111) Pt diffraction plane.

2.4.

Transmission electron microscopy

Particle size, morphology and distribution were evaluated through transmission electron microscopy (TEM) using a TECNAI instrument operated at an accelerated voltage of 300 kV. Nanoparticles size distribution was calculated based on a random counting of particles using the Digital Micrograph software. The carbon-supported catalysts were ultrasonically dispersed in isopropanol to obtain a uniform catalyst ink, then, a drop of the slurry was deposited onto a carboncovered copper grid.

3.

Results and discussion

3.1.

Crystal structure and morphology characterization

3.1.1.

Transmission electron microscopy (TEM)

Electrochemical measurement

The electrochemical measurements were conducted in a three-compartment electrochemical cell using a ring-disk electrode setup with an Autolab PGSTAT302 bi-potentiostat and rotation control (Pine Instrument). The Pt ring electrode was set at 1.4 V/RHE. Pt mesh and saturated calomel electrode (SCE) were used as counter and reference electrode respectively, however all potentials are referred to the reversible hydrogen electrode (RHE). In order to avoid chloride contamination, SCE was placed outside the cell and electrochemically connected by a porcelain Luggin capillary. The collection efficiencies were determined for both catalysts in a typical 0.5 Na2SO4 þ 10 mM K3Fe(CN)6 deaerated solution using the thin film rotating ring disk configuration. The disk potential was negatively swept from 0.7 to 0.0 V/NHE at 20 mV s1, maintaining a constant ring potential of 1.4 V. After different experiments, the collection efficiency remain unchanged at N ¼ 0.20  0.02 and N ¼ 0.22  0.04 for Co@Pt/C and Pt/C-Etek, respectively. This procedure is in agreement with data reported in the literature [12].

Fig. 1 shows representative low-magnification micrographs as well as histograms of Pt/C-Etek and Co@Pt/C catalysts. The TEM image of the trademarked catalyst (Pt/C-Etek) showed Pt nanoparticles with sphere like shape, well distributed and rather uniform on carbon. On the other side, Co@Pt/C catalysts exhibited spherical morphology but a significant amount of agglomerated particles were also detected. The centered insets show the size distribution as determined by measuring nanoparticle sizes on a series of TEM micrographs. The fitting of a lognormal distribution to the size distribution results in an average particle size of 2.6 nm for Pt/C-Etek and 3.8 nm for Co@Pt/C. It is important to state that agglomerated particles were not considered for a Co@Pt/C sample, which may explain the little discrepancy with XRD results (Table 1).

3.1.2.

X-ray diffraction (XRD)

The quantitative phase analysis and crystallites size were determined by using X-ray diffraction after full-profile Rietveld

Please cite this article in press as: Reyes-Rodrı´guez JL, et al., RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2012.12.031

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Fig. 1 e Low resolution TEM micrographs for Pt/C-Etek (A) and Pt@Co/C (B) samples. The platinum distributions considering several regions are shown in the histograms.

refinement, the data are summarized in Table 1. According to previous reports [17], the determination of lattice parameters using Rietveld refinement is not common; using standard methods like this may lead to incorrect conclusions about the structural data. In this sense, we simply determined parameters which were corroborated by other characterization techniques (i.e. TEM). Fig. 2A and B, shows XRD patterns for Pt/CEtek and Co@Pt/C catalysts respectively, after removing the background. The blue line corresponds to the fitted data and the small graphics on the top correspond to differences between fitted and experimental values. The catalysts phase diagram agrees with the reflection of a Face Centered Cubic (FCC) crystal structure (Fm3m) for both catalysts. On Pt/C-Etek (Fig. 2A) only the metallic Pt phase was detected, and the catalyst showed a crystallite size of 2.4 nm. The Co@Pt/C

Table 1 e Values of particle size and lattice constants deduced from analyzed samples. Catalyst

Particle size (nm) TEM

Pt/C-Etek Pt@Co/C

2.6  0.56 3.8  1.7

CO stripping

1.9  0.1 7.6  0.9

XRD Pt

Co

2.4 3.2

e 1.4

(Fig. 2B) showed qualitatively, a diffraction pattern like Pt/CEtek sample, however subtle differences were detected: i) the diffraction peaks center were shifted toward higher angles and ii) a more prominent tail at higher angles can also be observed, where the latter reflect the presence of a second phase different to Pt. Thus, the Rietveld analysis allows the detection of two phases which were assigned to Co and Pt, and these are denoted in the figure by the pink and gray lines respectively. The positive displacement of the diffraction angles associated to Pt phase on Co@Pt/C with respect to those from Pt/C-Etek, was qualitatively rationalized with a tension process caused by the compression of the lattice crystal of Pt because it attempts to adopt the Co structure during the galvanic replacement. The quantitative phase analysis reveals the presence of Pt and Co in the relative weight fraction of 12 and 88 wt%, respectively with an experimental error of 0.57%. The metal ratio is moreeless consistent with the nominal ratio. The estimation of the crystallite size considering the reflection from Pt on the shell was 3.2 nm and 1.4 nm from the reflection corresponding to Co in the core; this agrees with results found through TEM.

3.2.

Electrochemical characterization

3.2.1.

Cyclic voltammetry

The catalysts were electrochemically characterized by cyclic voltammetry (CV). As a systematic procedure, the potential

Please cite this article in press as: Reyes-Rodrı´guez JL, et al., RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2012.12.031

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Counts (a.u.)

600

450

A)

(111)

Pt/C-Etek

(200)

(220)

300

(311)

(222) (004) (311)

(042)

150 15

20

25

30

35

40

45

50

Diff.

2 Tetha 10 5 0 -5 -10

B) Counts (a.u.)

Co@Pt/C

240

--- 12 wt% Pt --- 88 wt% Co

200

160 15

20

25

30

35

40

45

50

2 Theta Fig. 2 e XRD two theta scan after background subtractions for samples Pt/C-Etek (A) and Pt@Co/C (B). Figure on top corresponds to differences between experimental and fitted parameters. (Blue line) fitted parameters through Rietveld refinement.

immersion of the working electrode was controlled at 0.2 V to avoid possible Co oxidation (i.e. Co@Pt/C sample) during the sample immersion, and then cycled several times between 0.05 V and 1.2 V. For the Co@Pt/C sample, no processes assigned to the Co oxidation was observed during the first

6

A)

Pt/C-Etek 40% Co@Pt/C

scan (i.e. w0.3 V), this indicates that Co-atoms are not exposed to the electrolyte since the Pt shell completely encapsulate the Co core. Fig. 3A shows CV’s normalized with respect to the geometrical surface area for Co@Pt/C and Pt/CEtek catalysts in N2 purged 0.5 M H2SO4. Note that qualitatively the CV’s show similar features, and are very similar to that of Pt/C electrode reported in H2SO4 [18] although some differences can be observed between them. The hydrogen region (0.05e0.4 V) for the Pt/C-Etek shows two desorption peaks around 0.14 V and 0.205 V which correspond mainly to the presence of (110) terrace domain. On the other side, the Co@Pt/C show peaks at 0.15 V, 0.21 V and 0.27 V. The two first have been ascribed to the (110) terrace domain and the third one to the adsorption state on (100) sites [19]. This result confirms little differences on crystalline surface domain on both catalysts. In the oxide formation/reduction region (E > 0.7 V), the onset potential related to OHad formation is apparently lower on Pt/C-Etek than on Co@Pt/C. At the same time, the oxide reduction potential during the back scan direction is more positive for Co@Pt/C. These features were related with both, smaller particle size from Pt/C-Etek catalyst and an intrinsic higher activity toward oxygen reduction from Pt@Co/C sample, respectively. Different studies have shown that the onset of OH adsorption shifts to a lower potential as grain size decrease, namely, the oxophilicity of the catalyst increase [20]. On the other side, it has been shown that modifying the electronic properties of Pt, by alloying with transition metals, shifts the onset of OH adsorption to higher potentials and leads to an enhancement in the ORR rate [21], which can be associated to positive shifting during a reverse sweep. The CO stripping technique (Fig. 3B) was used to determine the Pt electrochemical surface area of the Pt/C-Etek and Co@Pt/C catalysts. The curves were recorded in CO-free solution after completely surface saturation by adsorbing CO at 0.1 V. The CO stripping peak area was obtained by subtraction of the second sweep. The Pt/C-Etek show higher current density per mass unity than Co@Pt/C due to a smaller particle size; considering a spherical shape for both catalysts, the particle size determined by CO stripping charge was 1.9  0.1 nm and 7.6  0.9 nm, respectively

B) @ 20 mV s

4

-2

5

0.727 V

2

J (mA cmgeo)

6

0.822 V -1

4

0

3

-2

2

x7

-4 -6

1 -1

@ 100 mV s

-2

40 30 20 10 0 -10

J (mA cmgeo)

Diff.

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0

-8 -1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 E (Volts/RHE)

0.4

0.6 0.8 1.0 E (Volts/RHE)

1.2

Fig. 3 e (A) Cyclic base voltammograms and (B) N2-purged COad stripping voltammograms on Pt/C-Etek (Pt loading L2 19 mg cmL2 geo ) and Co@Pt/C (Pt loading 15 mg cmPt ). CV and CO-stripping were carried out in 0.5 M H2SO4 at room temperature. Please cite this article in press as: Reyes-Rodrı´guez JL, et al., RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2012.12.031

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(B) Ring

3

-1

1600 rpm, 20 mV s

4.5

1

4.0 3.5

H2O2 / %

0 -1 -2 JD (mA cmgeo)

ER = 1.4 V

2

3.0 2.0

(A) Disk

1.5 1.0

-2

(C)

2.5

0.5 0.0 0.0

Pt/C-Etek Co@Pt/C

Pt/C-Etek Co@Pt/C 0.2

0.4

0.6

0.8

1.0

E (Volts/RHE)

-3 -4 0.0

0.2

0.4

0.6

0.8

1.0

E (Volts/RHE)

Fig. 4 e (A) Steady-state polarization curves for ORR on Pt/ L2 C-Etek ð20 mg cmL2 Pt Þ and Co@Pt/C ð15 mg cmPt Þ in O2 saturated 0.5 M H2SO4 at 1600 rpm. (B) Simultaneously recorded ring current density (JR) for a ring potential of ER [ 1.4 V. (C) Percentage of H2O2 formation during O2 reduction.

(Table 1). The inconsistency between the particle size determined by CO-stripping and the other techniques (i.e. XRD and TEM) may be due to the presence of particles agglomeration and/or incomplete stripping of the capping agent during the washing step. An incomplete stripping of surfactant molecules may blocks a surface area, avoiding CO adsorption on those sites and decreasing the number of active surface atoms which are consider during electrochemical surface area determination.

3.2.2.

O2 reduction activity and H2O2 monitoring

After CV characterization and determination of the collection efficiencies, the catalysts were evaluated using the RRDEtechnique for the evaluation of the oxygen reduction kinetic parameters as well as for monitoring the hydrogen peroxide production. The measurements were carried out in a cathodic

0.6

-1 -1 2 JL (mA cmgeo)

0.95

(B)

(A) 0.5

sweep at 20 mV s1 at various rotation rates, although we only show those at 1600 rpm. The potentiodynamic curves for both catalysts (Fig. 4A), show that the oxygen reduction is under mixed kinetic-diffusion control at potentials higher than 0.7 V, followed by a well-defined limiting-diffusion control at lower potentials. On the ring (Fig. 4B), the current remains practically zero at potentials higher than w0.85 V and increase as potentials decrease. According to previous reports [8], above 0.8 V the oxygen reduction process proceeds practically by a four-electron pathway and this number decrease at lower potentials due to the formation and subsequent chemical dissociation of H2O2. Fig. 4C shows that the hydrogen peroxide yield and the production of H2O2 on both catalysts is essentially the same. To evaluate the kinetic parameters of these catalysts, their Tafel region is plotted in Fig. 5A. The Tafel slopes were fitted in the same potential region, i.e. 0.9e0.8 V. It is important to mention that every catalyst was evaluated at least three times even with different electrode Pt loadings to corroborate reproducibility. The Tafel slope values for Co@Pt/ C and Pt/C-Etek were very similar, 89 mV/dec and 84 mV/dec, respectively and in agreement with the literature [4]. Also, these values agree with theoretical 2.3 RT/F at lower overpotential (E > 0.8 V), as expected for the first electron transfer rate determining step. The positive shifting from Co@Pt/C sample with respect to Pt/C-Etek represents a kinetic gain of about two times in the specific current density at a given potential, i.e. 0.85 V. This enhancement of specific activity may be related with a positive shifting of the adsorption potential of the OH species, which could be influenced by both the Pt lattice contraction, as observed by X-ray, and electronic change as result of PteCo orbital’s interaction as proposed in the literature [7]. Fig. 5B shows the inverse of the overall limiting current density ðJL 1 Þ, as a function of the inverse square root of the rotation rate (u1/2), known as KouteckyeLevich plot. The plot shows a linear relation between J1 vs u1/2, indicating a first order kinetic and in agreement with the theoretical slope value, i.e. Bth ¼ 12.63  102 mA cm2 rpm1/2, calculated considering a four-electron transfer process leading to water formation, i.e., O2 þ 4Hþ þ 4e / 2H2O [22].

Pt/C-Etek Co@Pt/C

-2 -1/2 11.1 ± 0.89 mA cm rpm

0.90 -1 89 ± 6 mV dec

0.4

0.85

0.3 -2 -1/2 10.2 ± 0.64 mA cm rpm

0.2 0.1

-1 84 ± 10 mV dec

E (Volts/RHE)

JR (µA)

4

0.80

Pt/C-Etek Co@Pt/C

0.75 0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 -2.4 -2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4

1/2 1/2 w (rpm )

-2 Log Jk (mA cmPt)

Fig. 5 e KouteckyeLevich plots (A) and mass-transfer corrected Tafel plots (B) in O2 saturated 0.5 M H2SO4. Please cite this article in press as: Reyes-Rodrı´guez JL, et al., RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2012.12.031

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Conclusions [7]

Well defined carbon supported Co@Pt coreeshell nanocatalysts with around 3.8 nm in size were synthesized through sequential colloidal chemical reduction and galvanic replacement. X-ray diffraction for the Co@Pt/C sample showed important structural changes. The Rietveld refinement confirmed the presence of two phases assigned to metallic Pt and Co in the relative weight fraction of 12 wt% and 88 wt%, respectively. The change in the reflection profile suggests a strong interaction between Pt and Co which resulted in a Pt lattice contraction (i.e. 3.8741  A) and a Co lattice expansion (i.e. 3.6170  A). The CV in the O2 free electrolyte resembled that of pure Pt and confirms a complete Pt shell covering Co-core. The RRDE measurement shows a catalytic activity enhancement of around twice for the O2 reduction reaction and a similar peroxide production on the Co@Pt/C catalyst, compared to Pt/CEtek under the same measurement conditions. High catalytic activity, low peroxide production and ultra-low Pt loading, promotes the Co@Pt coreeshell as an outstanding candidate for proton exchange membrane fuel cell applications.

[8]

[9]

[10] [11]

[12]

[13]

[14]

Acknowledgments We gratefully acknowledge the support of the National Council of Science and Technology, CONACYT (grant FOINS 2250-6) and Science and Technology Institute, ICYTDF (grants PICCO 10-3; PICSO 11-24). FGS thanks ICYTDF for the postdoctoral fellowship.

[15]

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Please cite this article in press as: Reyes-Rodrı´guez JL, et al., RRDE study on Co@Pt/C coreeshell nanocatalysts for the oxygen reduction reaction, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2012.12.031