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Low temperature preparation of carbon-supported PdCo alloy electrocatalysts for methanol-tolerant oxygen reduction reaction
Electrochimica Acta 53 (2008) 6662–6667
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Low temperature preparation of carbon-supported Pd Co alloy electrocatalysts for methanol-tolerant oxygen reduction reaction Xiaowei Li, Qinghong Huang, Zhiqing Zou, Baojia Xia, Hui Yang ∗ Energy Science and Technology Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China
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Article history: Received 20 February 2008 Received in revised form 8 April 2008 Accepted 14 April 2008 Available online 20 April 2008 Keywords: Pd Co alloy Nanoparticle Oxygen reduction reaction Methanol tolerance
a b s t r a c t Carbon-supported Pd Co alloy electrocatalysts of different Pd/Co atomic ratios were simply prepared in an aqueous solution at room temperature with NH4 F as a complexing agent and H3 BO3 as a buffer, followed by NaBH4 reduction. As-prepared Pd Co bimetallic nanoparticles show a single-phase face-centered-cubic (fcc) disordered structure, and the mean particle size is found to decrease with increase in Co content. TEM images demonstrated that the as-prepared Pd Co alloy nanoparticles are well dispersed on the surface of the carbon support with a small particle size and a relatively narrow particle size distribution. For example, the average particle size of a Pd2 Co1 /C catalyst is ca. 3.0 nm, which is much smaller than that of the Pd Co/C bimetallic nanoparticles reported by others. An activity evaluation of the oxygen reduction reaction (ORR) on as-prepared Pd Co/C catalysts with a rotating disk electrode (RDE) technique indicated that the maximum ORR mass activity was observed for a Pd:Co atomic ratio of 4:1, but the highest specific activity was found on a Pd:Co atomic ratio of 2:1. Kinetic analysis reveals that the ORR on Pd Co/C catalysts follows a four-electron process leading to water. Moreover, the Pd Co/C catalyst exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst, assessing that it may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell (DMFC). © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Proton-exchange membrane fuel cells (PEMFCs) including direct methanol fuel cell (DMFC) are expected soon to become a major source of clean energy [1–3]. However, there are still several obstacles for their widespread applications, one of which is the high costs of both anode and cathode Pt-based electrocatalysts [4,5]. Pt is widely used as the electrocatalyst for the oxygen reduction reaction (ORR) due to its high catalytic activity and excellent chemical stability in the fuel cell environment. However, Pt is expensive and the limited world’s supply of platinum, which possess serious problems for a widespread commercialization of the fuel cell technology. Thus, research efforts in the development of cathode electrocatalysts have been placed on decreasing the Pt content or replacing it with less expensive materials while maintaining the high ORR activity [6]. Pd-based alloy nanoparticles have been the focus of considerable attention, not only due to lower costs and more abundance but also to the lower reactivity of Pd-based alloys for the adsorption and oxidation of methanol, which is of great importance for the improved methanol tolerance in a DMFC [7,8].
∗ Corresponding author. Tel.: +86 21 32200534; fax: +86 21 32200534. E-mail address: [email protected] (H. Yang). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.04.032
Recent reports have shown that the ORR activity on the Pd alloys is comparable to, or slightly better than that on the Pt/C [9–18]. For example, Bard and co-workers reported that the ORR mass activity on the Pd Co/C catalysts with a Co atomic content of 10–30% is found to be very close to that on the Pt/C, using a scanning electrochemical microscopy method [10]. They also prepared the Pd Co Au, Pd Ti and Pd Co Mo electrocatalysts heat-treated at evaluated temperatures, which showed essentially equal or slightly better performance than Pt for the ORR [11,12]. Adzic group has synthesized a new class of electrocatalysts consisting of Pd Fe alloys prepared by thermal treatment, whose activity can surpass that of the state-of-the-art Pt/C electrocatalysts [17]. More recently, the effect of heat treatment on particle size and the ORR activity of Pd65 Co35 /C alloys has been investigated [19]. It was reported that Pd65 Co35 /C heat-treated at 300 ◦ C showed the highest ORR activity, which was attributed to a small particle size of 8.9 nm and to an alloy formation between Pd and Co. As reported [10–13,17] the particle size of Pd-based catalysts is large, thus there proves to be significant room for improvement in the ORR mass activity. Challenges to be met for the synthesis of improved Pd alloy catalysts include the need for synthesis procedures resulting in catalysts with desirable composition, small particle size and a narrow size distribution. It is known that Pd2+ ion is readily reduced to metallic Pd, which would result in the
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formation of Pd nanoparticles with large particle size of ca. 15 nm [20]. To synthesize the Pd or Pt nanoparticles with small particle size and uniform dispersion on the carbon support, surfactants and polymers have been extensively employed as stabilizing agents [21–26]. Aiming to decrease the particle size of Pd-based catalysts and to increase their ORR activity, here we report a novel strategy for the synthesis of Pd Co bimetallic electrocatalysts for methanoltolerant ORR. The synthesis procedures used here involved (1) the complexing of Pd2+ with NH4 F to decrease the reduction potential of Pd species and (2) the reduction of complex by NaBH4 at room temperature. ORR activities on as-prepared Pd Co/C catalysts were evaluated in an acid medium in the absence and presence of methanol. Also, the ORR kinetics was analyzed using a rotating disk and ring-disk electrode (RDE/RRDE).
2. Experimental 2.1. Catalyst preparation Carbon-supported Pd Co alloy electrocatalysts were prepared in an aqueous solution at room temperature with NH4 F as a complexing agent and H3 BO3 as a buffer, followed by NaBH4 reduction. In brief, 25.0 mg of NH4 F, 125.0 mg of H3 BO3 and 5.6 mL of 0.025 M PdCl2 were added to 10 mL of water under constant stirring for half an hour, followed by adding CoCl2 ·6H2 O into the mixture. Fig. 1 shows the UV–vis absorption spectra (PerkinElmer Moderl Lambda 17 spectrophotometer) of the PdCl2 solution before and after the addition of both NH4 F and H3 BO3 reagents. An absorption peak appears at ca. 295 nm after adding the ligand, indicative of the formation of the complex of Pd2+ with NH4 F, which would decrease the reduction potential of Pd species, probably leading to the formation of Pd-based nanoparticles with a relatively small particle size. Then, XC-72 carbon was added to the mixture under constant stirring. The Co content was calculated according to a Pd:Co atomic ratio of the desired product and the total metal loading within the catalysts was controlled at a weight percent of 20 wt%. Concentrated ammonia was used to adjust the pH value of the mixture to 8–9. Subsequently, NaBH4 solution was added into the above mixture and sonicated for 5 h. Unless stated otherwise, the reaction temperature was kept at room temperature. The resulting suspension was filtered, washed and dried in a vacuum oven at 75 ◦ C overnight. For a comparison, the Pd/C catalyst was prepared with a similar procedure.
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2.2. Physical characterization The analysis of the composition of catalysts was performed with an IRIS Advantage inductively coupled plasma atomic emission spectroscopy (ICP-AES) system (Thermo, America). X-ray diffraction (XRD) measurements utilized a Rigaku D/MAX-2000 ˚ The tube voltdiffractometer with a Cu K␣ radiation (1.54056 A). age was maintained at 40 kV and tube current at 100 mA. Diffraction patterns were collected from 10◦ to 90◦ at a scanning rate of 2◦ /min, and with a step size of 0.02◦ . The mean particle size was calculated from the (2 2 0) diffraction plane using the Scheerer equation. The lattice parameter and Pd Pd interatomic distance were calculated from the (1 1 1) diffraction plane using Bragg equation [17]. The particle size and morphology of the Pd/C and Pd Co/C were analyzed by transmission electron microscopy (TEM). For TEM characterization, the powdered carbon-supported catalysts were ultrasonically dispersed in ethanol solution to obtain uniform catalyst ink. Then, the catalyst ink was mounted onto copper grids covered with holey carbon film and dried in the air. A Technai G2 20s-Twin Microscope (FEP Inc., America), operating at 200 kV was used for TEM observation. 2.3. Electrochemical characterization Porous electrodes were prepared as described previously [27]. Ten milligrams of Pd/C and Pd Co/C catalysts, 0.5 mL of Nafion solution (5 wt%, Aldrich) and 2.5 mL of ultrapure water were mixed ultrasonically. A measured volume (ca. 3 L) of this ink was transferred via a syringe onto a freshly polished glassy carbon disk (GC, 3 mm diameter). After the solvents were evaporated overnight at room temperature, the electrode was used as the working electrode. Each electrode contained ca. 28 g cm−2 of metal. All chemicals were of analytical grade. All solutions were prepared with ultrapure water (Milli-Q, Millipore). Electrochemical measurements were performed using a CHI 730 A Potentiostat and a conventional three-electrode electrochemical cell. The catalytic activity for the ORR was measured with a rotating disk electrode and a rotating ring-disk electrode mounted with a BM-ED 1101 electrode (Radiometer, France). The counter electrode was a glassy carbon plate, and a saturated calomel electrode (SCE) was used as the reference electrode. All potentials, however, are referenced with respect to the reversible hydrogen electrode (RHE). The electrolyte used was 0.1 M HClO4 or 0.1 M HClO4 + 0.5 M CH3 OH. Prior to any electrochemical measurements, the porous electrodes were cycled between 0.05 V and 1.00 V at a scan rate of 50 mV s−1 under N2 until reproducible cyclic voltammograms (CVs) were obtained, in order to remove any contaminants from the electrode. The upper potential was set to ca.1.00 V/RHE so that any change in particle size could be avoided. High-purity nitrogen or oxygen was used for deaeration of the solutions. During the measurements, a gentle gas flow was kept above the electrolyte solution. In the electrochemical measurements, the current densities are normalized to apparent surface area of the GC electrode. All experiments were carried out at a temperature of 25 ± 1 ◦ C. 3. Results and discussion 3.1. Physical characterization of Pd Co/C bimetallic catalysts
Fig. 1. UV–vis absorption spectra of 10−4 M PdCl2 solution: (a) before and (b) after the addition of NH4 F + H3 BO3
The practical compositions of the Pd Co/C catalysts were evaluated by an ICP-AES analysis. The obtained results are very close to the original stoichiometric values, indicating that Pd and Co could be co-reduced and that Co may be alloyed with Pd with such a synthesis method.
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Table 1 Structural and electrochemical characteristics of the Pd/C and Pd Co/C catalysts Catalyst
2 (1 1 1) (◦ )
Crystalline size (nm)
Lattice constant (nm)
Pd Pd bond distance
Electrochemical area (m2 g−1 ) metal
Jk at 0.75 V (mA/cm2 )
Pd1 Co1 /C Pd2 Co1 /C Pd4 Co1 /C Pd11 Co1 /C Pd/C
39.33 39.99 39.82 39.23 39.10
2.6 2.9 3.4 3.8 3.9
0.3964 0.3901 0.3917 0.3974 0.3989
0.2803 0.2758 0.2769 0.2810 0.2820
5.67 10.97 32.62 21.77 16.34
0.250 0.439 0.196 0.127 0.120
Fig. 2 shows the XRD patterns of the Pd Co/C bimetallic catalysts with different Pd/Co atomic ratios. For the sake of comparison, the XRD pattern of a Pd/C catalyst prepared with a similar procedure is also given in the figure. The first peak at ca. 24.7◦ in all the XRD patterns is attributable to the carbon support. The other four peaks are characteristics of face-centered-cubic (fcc) crystalline Pd, corresponding to the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), indicating that all the catalysts mainly resemble the single-phase disordered structure (solid solution). These four diffraction peaks for the Pd Co/C bimetallic catalysts are shifted slightly to higher 2 values with respect to the corresponding peaks in the Pd/C catalyst. Such angle shifts reveal an alloy formation between Pd and Co, and indicate a lattice contraction, which are caused by the incorporation of Co into the Pd fcc structure. The changes in 2 and lattice parameter of different Pd Co/C catalysts are listed in Table 1. Although no reflections of pure Co and its oxides are found, their presence may not be completely ruled out due to their smaller concentration levels and possibly poor crystallinity. The mean particle sizes calculated from XRD patterns for all catalysts are also shown in Table 1. As shown in table, the mean particle size decreases with increase in Co content. The ratio of the peak intensities of Pd (1 1 1) to carbon is the relative crystallinity of the Pd and Pd Co particles. The degrees of relative crystallinity for the Pd/C and Pd4 Co1 /C are 1.14 and 1.26, respectively, which are smaller than the reported value for the Pd/C [20], indicative of a high content of amorphous structure within our samples. The lattice parameters obtained for Pd Co/C bimetallic catalysts are smaller than that of Pd/C, indicative of a lattice contraction upon alloying. The Pd Pd interatomic distances of the Pd Co/C catalysts are also provided. The minimum Pd Pd interatomic distance of ca. 0.2758 nm is found for a Pd2 Co1 /C catalyst. Fig. 3 shows TEM images of the Pd/C, Pd4 Co1 /C and Pd2 Co1 /C catalysts and their corresponding particle size distribution histograms based on the observation of more than 200 nanoparticles. It can be seen that all the nanoparticles of monometallic and alloy catalysts are well dispersed on the surface of the support with a relatively narrow particle size distribution. The mean particle size decreases
Fig. 2. X-ray diffraction patterns of the Pd/C catalyst and the Pd Co/C bimetallic catalysts with different atomic ratios.
with increase in Co content. The average particle diameters are ca. 4.0 nm, 3.5 nm and 3.0 nm for the Pd/C, Pd4 Co1 /C and Pd2 Co1 /C catalysts, respectively, which are in fairly good agreement with the XRD results. The ascertained mean particle size of the Pd Co/C catalysts is much smaller than those reported by other groups for Pd-based catalysts [17,19,31,32], which may be beneficial for an increase in the ORR mass activity. It is worth mentioned that all nanosized Pd alloy catalysts in the literature were prepared at heat treatment temperatures higher than 300 ◦ C. However, in our work, we use NH4 F as a complexing agent with Pd2+ ions to carry out the co-reduction of Pd2+ and Co2+ ions in an aqueous solution, resulting in the better alloying of Pd and Co even at room temperature, thus much smaller nanoparticles of the Pd alloy catalysts could be obtained and the aggregation of nanoparticles could be avoided when annealing. Hence, the preparation procedure used here looks a good method to obtain Pd alloy electrocatalysts with a small particle size and a good dispersion on a support.
3.2. Electrochemical characterization of the Pd Co/C bimetallic catalysts Fig. 4 presents the CVs of different Pd-based catalysts in 0.1 M HClO4 . From the CVs of the Pd-based catalysts, the hydrogen adsorption/desorption peaks and preoxidation/reduction peaks are clearly seen. However, no well-defined hydrogen adsorption/desorption peaks were observed for all the catalysts, suggesting that a high dispersion of the catalysts with disordered surface structure was obtained. Interestingly, the area of the hydrogen adsorption/desorption or oxide formation/reduction region decreases in the order Pd4 Co1 /C > Pd11 Co1 /C > Pd/C > Pd2 Co1 /C > Pd1 Co1 /C. When Co content is more than 25% within the alloy catalyst, the area of hydrogen adsorption/desorption peaks is smaller than that of the Pd/C catalyst, probably due to the surface composition effect. The addition of Co within the catalysts might restrain the dissolution of hydrogen into bulk Pd Co/C as compared to pure Pd catalyst. Such a result is in good agreement with that in the literature [18,19]. For the Pd4 Co1 /C and Pd11 Co1 /C catalysts, their electrochemically active surface area (EASA) is higher than that of the Pd/C, probably due to the smaller particle size and to the formation of relatively rough surfaces during alloying, which is very similar to that on the Pd Fe/C catalyst [17]. The EASA of the catalysts is estimated according to a previously reported procedure [28] and listed in Table 1. In addition, all the catalysts exhibited a similar double-layer behavior, suggesting that all the catalysts have the same resistances to transfer charges. Furthermore, it can be found that the onset potential of the oxide reduction on the Pd2 Co1 /C and Pd4 Co1 /C catalysts shifts to more positive potential, suggesting that the alloying of Pd with appropriate Co content inhibits the chemisorption of OH on the Pd sites at high potentials. This may have a beneficial effect on the oxygen adsorption at low overpotential, and thus leading to an enhancement in the ORR kinetics [33]. Fig. 5 is an activity comparison of the ORR on the Pd/C and Pd Co/C catalysts under similar experimental conditions. For a
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Fig. 3. TEM images and the corresponding particle size distribution histograms of (a) Pd/C, (b) Pd4 Co1 /C and (c) Pd2 Co1 /C catalysts.
comparison, the LSV of the ORR on commercial Pt/C (from E-Tek) with a metal loading of 20% is also given in the figure. From the figure, the ORR on all the catalysts is diffusion controlled when the potential is less than 0.65 V and is under mixed control in the potential region between 0.65 V and 0.85 V. When the potential is higher than 0.85 V, the ORR is under kinetics control in the Tafel region. In the Tafel region and the mixed potential region, it is found that most of the Pd Co/C catalysts have much higher ORR mass activities than a Pd/C, except for the Pd1 Co1 /C catalyst, probably due to the fact that the high Co content within a Pd1 Co1 /C sample results in a Co surface enrichment which prohibits oxygen molecules from adsorption on the Pd atom. The onset potential of the ORR on Pd2 Co1 /C and Pd4 Co1 /C catalysts is found to be the most positive among the Pd Co/C catalysts, which is in good agreement with our CV results. The maximum mass activity for the ORR among all the Pd-based catalysts is found with a Pd:Co atomic ratio of 4:1. Additional RRDE data shown in Fig. 5 also illustrates the ORR pathway [4e− (water formation) or 2e− (hydrogen peroxide formation)] on a Pd4 Co1 /C
catalyst. The ring current is found to be negligible compared to the disk current for potentials above 0.65 V, indicating that the ORR proceeds without peroxide production. The ring current increases when the potential is lower than 0.65 V. The fraction of peroxide, XH2 O2 , at a typical fuel cell operating potential of 0.70 V/RHE, was evaluated from disk current (ID ), ring current (IR ) and collection factor (N = 0.20) according to the equation: XH2 O2 = 2IR /N/(ID + IR /N) [27,29]. The calculated fraction of XH2 O2 for the Pd4 Co1 /C catalyst is 5.0%, indicative of a negligible peroxide production on the Pd4 Co1 /C alloy catalyst. Thus, the ORR follows a four-electron process leading to water formation. Furthermore, the Koutecky–Levich plots (data not shown) for the ORR on the different catalytic electrodes showed that all plots are straight lines with almost the same slope as that of the theoretically predicted line with four-electron reduction of oxygen, again assessing that the oxygen reduction on all Pd-based catalysts follows a four-electron process leading to water. It is worth mentioning that the open circuit potential of the Pd Co alloy catalysts in oxygen-saturated solution is slightly higher than that of the
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Fig. 6. Kinetic current density and Pd Pd bond distance calculated from XRD data against the Co content in Pd Co/C catalysts. Fig. 4. Cyclic voltammograms of the Pd/C and Pd Co/C catalysts in N2 saturated 0.1 M HClO4 solution at a scan rate of 50 mV s−1 .
Pd/C catalyst, suggesting that the oxygen adsorption on the alloy surface is more favored than that on pure Pd surface. It is well known that, for an electrochemical reaction, the structure and composition of the catalysts are two key points in determining the adsorption/catalytic properties. Generally, the activity enhancement in the ORR on Pd-based alloy catalysts could be correlated to the changes in structural parameters [13,27]. The relationship between kinetic current density jk at 0.75 V and the Pd Pd interatomic distance as a function of the atomic percent of Co% in the catalysts is plotted in Fig. 6 to understand the enhanced activity of the alloys and possible role of their structure. The kinetic currents are calculated based on the equation: 1/j = 1/jk + 1/jd , where j and jd are the measured current and limited current, respectively. A “volcano” type relationship was gained. The highest ORR specific activity is obtained for a Pd2 Co1 /C catalyst, corresponding to a Pd Pd bond distance of 0.2758 nm. For some Pt M alloys, the peak in specific activity was observed for an optimal Pt Pt bond distance [30]. Such a conclusion could be extended to the Pd-based electrocatalysts for the ORR. For example, Zhuang and co-workers [31] reported the Pd Co/C catalyst with a Pd Pd bond distance ca. 0.273 nm shows higher activity for the ORR than other ratios of Pd Co/C catalysts in their lab, but still less active
Fig. 5. Linear scan voltammograms of the Pt/C and Pd Co/C catalysts in 0.1 M HClO4 saturated with pure oxygen at a scan rate of 5 mV s−1 and with a rotation speed of 1600 rpm. Current density is normalized to the geometrical surface area of the electrode. The ring current, RRDE data, for hydrogen peroxide production is compared.
than a commercial Pt/C. Zhang et al. synthesized Pd65 Co35 /C catalysts using different reducing agents and found the catalyst with a Pd Pd bond distance of 0.2726 nm had the highest activity for the ORR [32]. The favorable Pd Pd bond distance of 0.2758 nm obtained in our experiment is slightly larger than the reported results. The difference between our results and others may be ascribed to the difference in synthesis methods and to the particle size effects. 3.3. The ORR on the Pd-based catalysts in methanol-containing electrolyte It is well known that the crossover of methanol from the anode to the cathode can lead to a further reduction of the cell voltage by ca. 200–300 mV. Fig. 7 shows the ORR activity on the commercial Pt/C and as-prepared Pd4 Co1 /C alloy catalysts in the presence of 0.5 M CH3 OH. As compared to the ORR in pure HClO4 solution, both catalysts exhibited an increase in overpotential under the same current density in the presence of methanol. For the ORR on pure Pt catalyst in methanol-containing solution [19], the overpotential increases by ca. 200 mV, while only a small negative shift of ca. 15 mV was observed on the Pd4 Co1 /C catalyst. Results clearly indicate that the Pd4 Co1 /C electrocatalyst is very active for the ORR even at a high concentration of methanol. The overpotential of 15 mV on the Pd4 Co1 /C catalyst might reflect the blocking of some active sites for oxygen reduction by the adsorbed methanol or its intermediates from oxidation. All the Pd Co/C catalysts were found to exhibit the
Fig. 7. LSVs of the Pt/C and Pd4 Co1 /C catalysts in 0.1 M HClO4 + 0.5 M CH3 OH saturated with pure oxygen. Other conditions are identical to those of Fig. 5.
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similar characteristics as the Pd4 Co1 /C alloy for the adsorption and oxidation of methanol, thus the Pd Co/C bimetallic alloy might be a good candidate for the ORR in a DMFC. 4. Conclusions Carbon-supported Pd Co bimetallic nanoparticles with relatively small particle size and single-phase fcc disordered structure have been synthesized via a simple procedure at room temperature. The Pd4 Co1 /C catalyst exhibits an observed maximum ORR mass activity, which is close to the commercial Pt/C, while the highest specific activity is obtained on a Pd2 Co1 /C catalyst with a Pd Pd interatomic distance of ca. 0.2758 nm. The enhanced ORR activity of the Pd2 Co1 /C may be ascribed to the compressed Pd lattice via alloy with Co and the downshift of the d-band center of the Pd metal. The excellent tolerance to methanol of the Pd Co/C alloy catalysts implies that Pd-based catalyst may be an economical candidate to replace Pt as a cathode catalyst for a DMFC. Acknowledgments We would like to thank the National Natural Science Foundation of China (20673136, 20706056), the National “863” High-Tech. Research Programs of China (2006AA05Z136, 2006AA04Z342, 2007AA05Z141), the 100 People Plan Program of the Chinese Academy of Sciences and the Pujiang Program of Shanghai City (No. 06PJ14110) for support of this work. References [1] M.S. Dresselhaus, I.L. Thomas, Nature 414 (2001) 332. [2] M.Z. Jacobson, W.G. Colella, D.M. Golden, Science 308 (2005) 1901. [3] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of Fuel Cells Fundamentals, Technology and Applications, vol. 1–4, John Wiley & Sons, Chichester, 2003.
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Low temperature preparation of carbon-supported Pd Co alloy electrocatalysts for methanol-tolerant oxygen reduction reaction Xiaowei Li, Qinghong Huang, Zhiqing Zou, Baojia Xia, Hui Yang ∗ Energy Science and Technology Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 20 February 2008 Received in revised form 8 April 2008 Accepted 14 April 2008 Available online 20 April 2008 Keywords: Pd Co alloy Nanoparticle Oxygen reduction reaction Methanol tolerance
a b s t r a c t Carbon-supported Pd Co alloy electrocatalysts of different Pd/Co atomic ratios were simply prepared in an aqueous solution at room temperature with NH4 F as a complexing agent and H3 BO3 as a buffer, followed by NaBH4 reduction. As-prepared Pd Co bimetallic nanoparticles show a single-phase face-centered-cubic (fcc) disordered structure, and the mean particle size is found to decrease with increase in Co content. TEM images demonstrated that the as-prepared Pd Co alloy nanoparticles are well dispersed on the surface of the carbon support with a small particle size and a relatively narrow particle size distribution. For example, the average particle size of a Pd2 Co1 /C catalyst is ca. 3.0 nm, which is much smaller than that of the Pd Co/C bimetallic nanoparticles reported by others. An activity evaluation of the oxygen reduction reaction (ORR) on as-prepared Pd Co/C catalysts with a rotating disk electrode (RDE) technique indicated that the maximum ORR mass activity was observed for a Pd:Co atomic ratio of 4:1, but the highest specific activity was found on a Pd:Co atomic ratio of 2:1. Kinetic analysis reveals that the ORR on Pd Co/C catalysts follows a four-electron process leading to water. Moreover, the Pd Co/C catalyst exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst, assessing that it may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell (DMFC). © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Proton-exchange membrane fuel cells (PEMFCs) including direct methanol fuel cell (DMFC) are expected soon to become a major source of clean energy [1–3]. However, there are still several obstacles for their widespread applications, one of which is the high costs of both anode and cathode Pt-based electrocatalysts [4,5]. Pt is widely used as the electrocatalyst for the oxygen reduction reaction (ORR) due to its high catalytic activity and excellent chemical stability in the fuel cell environment. However, Pt is expensive and the limited world’s supply of platinum, which possess serious problems for a widespread commercialization of the fuel cell technology. Thus, research efforts in the development of cathode electrocatalysts have been placed on decreasing the Pt content or replacing it with less expensive materials while maintaining the high ORR activity [6]. Pd-based alloy nanoparticles have been the focus of considerable attention, not only due to lower costs and more abundance but also to the lower reactivity of Pd-based alloys for the adsorption and oxidation of methanol, which is of great importance for the improved methanol tolerance in a DMFC [7,8].
∗ Corresponding author. Tel.: +86 21 32200534; fax: +86 21 32200534. E-mail address: [email protected] (H. Yang). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.04.032
Recent reports have shown that the ORR activity on the Pd alloys is comparable to, or slightly better than that on the Pt/C [9–18]. For example, Bard and co-workers reported that the ORR mass activity on the Pd Co/C catalysts with a Co atomic content of 10–30% is found to be very close to that on the Pt/C, using a scanning electrochemical microscopy method [10]. They also prepared the Pd Co Au, Pd Ti and Pd Co Mo electrocatalysts heat-treated at evaluated temperatures, which showed essentially equal or slightly better performance than Pt for the ORR [11,12]. Adzic group has synthesized a new class of electrocatalysts consisting of Pd Fe alloys prepared by thermal treatment, whose activity can surpass that of the state-of-the-art Pt/C electrocatalysts [17]. More recently, the effect of heat treatment on particle size and the ORR activity of Pd65 Co35 /C alloys has been investigated [19]. It was reported that Pd65 Co35 /C heat-treated at 300 ◦ C showed the highest ORR activity, which was attributed to a small particle size of 8.9 nm and to an alloy formation between Pd and Co. As reported [10–13,17] the particle size of Pd-based catalysts is large, thus there proves to be significant room for improvement in the ORR mass activity. Challenges to be met for the synthesis of improved Pd alloy catalysts include the need for synthesis procedures resulting in catalysts with desirable composition, small particle size and a narrow size distribution. It is known that Pd2+ ion is readily reduced to metallic Pd, which would result in the
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formation of Pd nanoparticles with large particle size of ca. 15 nm [20]. To synthesize the Pd or Pt nanoparticles with small particle size and uniform dispersion on the carbon support, surfactants and polymers have been extensively employed as stabilizing agents [21–26]. Aiming to decrease the particle size of Pd-based catalysts and to increase their ORR activity, here we report a novel strategy for the synthesis of Pd Co bimetallic electrocatalysts for methanoltolerant ORR. The synthesis procedures used here involved (1) the complexing of Pd2+ with NH4 F to decrease the reduction potential of Pd species and (2) the reduction of complex by NaBH4 at room temperature. ORR activities on as-prepared Pd Co/C catalysts were evaluated in an acid medium in the absence and presence of methanol. Also, the ORR kinetics was analyzed using a rotating disk and ring-disk electrode (RDE/RRDE).
2. Experimental 2.1. Catalyst preparation Carbon-supported Pd Co alloy electrocatalysts were prepared in an aqueous solution at room temperature with NH4 F as a complexing agent and H3 BO3 as a buffer, followed by NaBH4 reduction. In brief, 25.0 mg of NH4 F, 125.0 mg of H3 BO3 and 5.6 mL of 0.025 M PdCl2 were added to 10 mL of water under constant stirring for half an hour, followed by adding CoCl2 ·6H2 O into the mixture. Fig. 1 shows the UV–vis absorption spectra (PerkinElmer Moderl Lambda 17 spectrophotometer) of the PdCl2 solution before and after the addition of both NH4 F and H3 BO3 reagents. An absorption peak appears at ca. 295 nm after adding the ligand, indicative of the formation of the complex of Pd2+ with NH4 F, which would decrease the reduction potential of Pd species, probably leading to the formation of Pd-based nanoparticles with a relatively small particle size. Then, XC-72 carbon was added to the mixture under constant stirring. The Co content was calculated according to a Pd:Co atomic ratio of the desired product and the total metal loading within the catalysts was controlled at a weight percent of 20 wt%. Concentrated ammonia was used to adjust the pH value of the mixture to 8–9. Subsequently, NaBH4 solution was added into the above mixture and sonicated for 5 h. Unless stated otherwise, the reaction temperature was kept at room temperature. The resulting suspension was filtered, washed and dried in a vacuum oven at 75 ◦ C overnight. For a comparison, the Pd/C catalyst was prepared with a similar procedure.
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2.2. Physical characterization The analysis of the composition of catalysts was performed with an IRIS Advantage inductively coupled plasma atomic emission spectroscopy (ICP-AES) system (Thermo, America). X-ray diffraction (XRD) measurements utilized a Rigaku D/MAX-2000 ˚ The tube voltdiffractometer with a Cu K␣ radiation (1.54056 A). age was maintained at 40 kV and tube current at 100 mA. Diffraction patterns were collected from 10◦ to 90◦ at a scanning rate of 2◦ /min, and with a step size of 0.02◦ . The mean particle size was calculated from the (2 2 0) diffraction plane using the Scheerer equation. The lattice parameter and Pd Pd interatomic distance were calculated from the (1 1 1) diffraction plane using Bragg equation [17]. The particle size and morphology of the Pd/C and Pd Co/C were analyzed by transmission electron microscopy (TEM). For TEM characterization, the powdered carbon-supported catalysts were ultrasonically dispersed in ethanol solution to obtain uniform catalyst ink. Then, the catalyst ink was mounted onto copper grids covered with holey carbon film and dried in the air. A Technai G2 20s-Twin Microscope (FEP Inc., America), operating at 200 kV was used for TEM observation. 2.3. Electrochemical characterization Porous electrodes were prepared as described previously [27]. Ten milligrams of Pd/C and Pd Co/C catalysts, 0.5 mL of Nafion solution (5 wt%, Aldrich) and 2.5 mL of ultrapure water were mixed ultrasonically. A measured volume (ca. 3 L) of this ink was transferred via a syringe onto a freshly polished glassy carbon disk (GC, 3 mm diameter). After the solvents were evaporated overnight at room temperature, the electrode was used as the working electrode. Each electrode contained ca. 28 g cm−2 of metal. All chemicals were of analytical grade. All solutions were prepared with ultrapure water (Milli-Q, Millipore). Electrochemical measurements were performed using a CHI 730 A Potentiostat and a conventional three-electrode electrochemical cell. The catalytic activity for the ORR was measured with a rotating disk electrode and a rotating ring-disk electrode mounted with a BM-ED 1101 electrode (Radiometer, France). The counter electrode was a glassy carbon plate, and a saturated calomel electrode (SCE) was used as the reference electrode. All potentials, however, are referenced with respect to the reversible hydrogen electrode (RHE). The electrolyte used was 0.1 M HClO4 or 0.1 M HClO4 + 0.5 M CH3 OH. Prior to any electrochemical measurements, the porous electrodes were cycled between 0.05 V and 1.00 V at a scan rate of 50 mV s−1 under N2 until reproducible cyclic voltammograms (CVs) were obtained, in order to remove any contaminants from the electrode. The upper potential was set to ca.1.00 V/RHE so that any change in particle size could be avoided. High-purity nitrogen or oxygen was used for deaeration of the solutions. During the measurements, a gentle gas flow was kept above the electrolyte solution. In the electrochemical measurements, the current densities are normalized to apparent surface area of the GC electrode. All experiments were carried out at a temperature of 25 ± 1 ◦ C. 3. Results and discussion 3.1. Physical characterization of Pd Co/C bimetallic catalysts
Fig. 1. UV–vis absorption spectra of 10−4 M PdCl2 solution: (a) before and (b) after the addition of NH4 F + H3 BO3
The practical compositions of the Pd Co/C catalysts were evaluated by an ICP-AES analysis. The obtained results are very close to the original stoichiometric values, indicating that Pd and Co could be co-reduced and that Co may be alloyed with Pd with such a synthesis method.
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Table 1 Structural and electrochemical characteristics of the Pd/C and Pd Co/C catalysts Catalyst
2 (1 1 1) (◦ )
Crystalline size (nm)
Lattice constant (nm)
Pd Pd bond distance
Electrochemical area (m2 g−1 ) metal
Jk at 0.75 V (mA/cm2 )
Pd1 Co1 /C Pd2 Co1 /C Pd4 Co1 /C Pd11 Co1 /C Pd/C
39.33 39.99 39.82 39.23 39.10
2.6 2.9 3.4 3.8 3.9
0.3964 0.3901 0.3917 0.3974 0.3989
0.2803 0.2758 0.2769 0.2810 0.2820
5.67 10.97 32.62 21.77 16.34
0.250 0.439 0.196 0.127 0.120
Fig. 2 shows the XRD patterns of the Pd Co/C bimetallic catalysts with different Pd/Co atomic ratios. For the sake of comparison, the XRD pattern of a Pd/C catalyst prepared with a similar procedure is also given in the figure. The first peak at ca. 24.7◦ in all the XRD patterns is attributable to the carbon support. The other four peaks are characteristics of face-centered-cubic (fcc) crystalline Pd, corresponding to the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), indicating that all the catalysts mainly resemble the single-phase disordered structure (solid solution). These four diffraction peaks for the Pd Co/C bimetallic catalysts are shifted slightly to higher 2 values with respect to the corresponding peaks in the Pd/C catalyst. Such angle shifts reveal an alloy formation between Pd and Co, and indicate a lattice contraction, which are caused by the incorporation of Co into the Pd fcc structure. The changes in 2 and lattice parameter of different Pd Co/C catalysts are listed in Table 1. Although no reflections of pure Co and its oxides are found, their presence may not be completely ruled out due to their smaller concentration levels and possibly poor crystallinity. The mean particle sizes calculated from XRD patterns for all catalysts are also shown in Table 1. As shown in table, the mean particle size decreases with increase in Co content. The ratio of the peak intensities of Pd (1 1 1) to carbon is the relative crystallinity of the Pd and Pd Co particles. The degrees of relative crystallinity for the Pd/C and Pd4 Co1 /C are 1.14 and 1.26, respectively, which are smaller than the reported value for the Pd/C [20], indicative of a high content of amorphous structure within our samples. The lattice parameters obtained for Pd Co/C bimetallic catalysts are smaller than that of Pd/C, indicative of a lattice contraction upon alloying. The Pd Pd interatomic distances of the Pd Co/C catalysts are also provided. The minimum Pd Pd interatomic distance of ca. 0.2758 nm is found for a Pd2 Co1 /C catalyst. Fig. 3 shows TEM images of the Pd/C, Pd4 Co1 /C and Pd2 Co1 /C catalysts and their corresponding particle size distribution histograms based on the observation of more than 200 nanoparticles. It can be seen that all the nanoparticles of monometallic and alloy catalysts are well dispersed on the surface of the support with a relatively narrow particle size distribution. The mean particle size decreases
Fig. 2. X-ray diffraction patterns of the Pd/C catalyst and the Pd Co/C bimetallic catalysts with different atomic ratios.
with increase in Co content. The average particle diameters are ca. 4.0 nm, 3.5 nm and 3.0 nm for the Pd/C, Pd4 Co1 /C and Pd2 Co1 /C catalysts, respectively, which are in fairly good agreement with the XRD results. The ascertained mean particle size of the Pd Co/C catalysts is much smaller than those reported by other groups for Pd-based catalysts [17,19,31,32], which may be beneficial for an increase in the ORR mass activity. It is worth mentioned that all nanosized Pd alloy catalysts in the literature were prepared at heat treatment temperatures higher than 300 ◦ C. However, in our work, we use NH4 F as a complexing agent with Pd2+ ions to carry out the co-reduction of Pd2+ and Co2+ ions in an aqueous solution, resulting in the better alloying of Pd and Co even at room temperature, thus much smaller nanoparticles of the Pd alloy catalysts could be obtained and the aggregation of nanoparticles could be avoided when annealing. Hence, the preparation procedure used here looks a good method to obtain Pd alloy electrocatalysts with a small particle size and a good dispersion on a support.
3.2. Electrochemical characterization of the Pd Co/C bimetallic catalysts Fig. 4 presents the CVs of different Pd-based catalysts in 0.1 M HClO4 . From the CVs of the Pd-based catalysts, the hydrogen adsorption/desorption peaks and preoxidation/reduction peaks are clearly seen. However, no well-defined hydrogen adsorption/desorption peaks were observed for all the catalysts, suggesting that a high dispersion of the catalysts with disordered surface structure was obtained. Interestingly, the area of the hydrogen adsorption/desorption or oxide formation/reduction region decreases in the order Pd4 Co1 /C > Pd11 Co1 /C > Pd/C > Pd2 Co1 /C > Pd1 Co1 /C. When Co content is more than 25% within the alloy catalyst, the area of hydrogen adsorption/desorption peaks is smaller than that of the Pd/C catalyst, probably due to the surface composition effect. The addition of Co within the catalysts might restrain the dissolution of hydrogen into bulk Pd Co/C as compared to pure Pd catalyst. Such a result is in good agreement with that in the literature [18,19]. For the Pd4 Co1 /C and Pd11 Co1 /C catalysts, their electrochemically active surface area (EASA) is higher than that of the Pd/C, probably due to the smaller particle size and to the formation of relatively rough surfaces during alloying, which is very similar to that on the Pd Fe/C catalyst [17]. The EASA of the catalysts is estimated according to a previously reported procedure [28] and listed in Table 1. In addition, all the catalysts exhibited a similar double-layer behavior, suggesting that all the catalysts have the same resistances to transfer charges. Furthermore, it can be found that the onset potential of the oxide reduction on the Pd2 Co1 /C and Pd4 Co1 /C catalysts shifts to more positive potential, suggesting that the alloying of Pd with appropriate Co content inhibits the chemisorption of OH on the Pd sites at high potentials. This may have a beneficial effect on the oxygen adsorption at low overpotential, and thus leading to an enhancement in the ORR kinetics [33]. Fig. 5 is an activity comparison of the ORR on the Pd/C and Pd Co/C catalysts under similar experimental conditions. For a
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Fig. 3. TEM images and the corresponding particle size distribution histograms of (a) Pd/C, (b) Pd4 Co1 /C and (c) Pd2 Co1 /C catalysts.
comparison, the LSV of the ORR on commercial Pt/C (from E-Tek) with a metal loading of 20% is also given in the figure. From the figure, the ORR on all the catalysts is diffusion controlled when the potential is less than 0.65 V and is under mixed control in the potential region between 0.65 V and 0.85 V. When the potential is higher than 0.85 V, the ORR is under kinetics control in the Tafel region. In the Tafel region and the mixed potential region, it is found that most of the Pd Co/C catalysts have much higher ORR mass activities than a Pd/C, except for the Pd1 Co1 /C catalyst, probably due to the fact that the high Co content within a Pd1 Co1 /C sample results in a Co surface enrichment which prohibits oxygen molecules from adsorption on the Pd atom. The onset potential of the ORR on Pd2 Co1 /C and Pd4 Co1 /C catalysts is found to be the most positive among the Pd Co/C catalysts, which is in good agreement with our CV results. The maximum mass activity for the ORR among all the Pd-based catalysts is found with a Pd:Co atomic ratio of 4:1. Additional RRDE data shown in Fig. 5 also illustrates the ORR pathway [4e− (water formation) or 2e− (hydrogen peroxide formation)] on a Pd4 Co1 /C
catalyst. The ring current is found to be negligible compared to the disk current for potentials above 0.65 V, indicating that the ORR proceeds without peroxide production. The ring current increases when the potential is lower than 0.65 V. The fraction of peroxide, XH2 O2 , at a typical fuel cell operating potential of 0.70 V/RHE, was evaluated from disk current (ID ), ring current (IR ) and collection factor (N = 0.20) according to the equation: XH2 O2 = 2IR /N/(ID + IR /N) [27,29]. The calculated fraction of XH2 O2 for the Pd4 Co1 /C catalyst is 5.0%, indicative of a negligible peroxide production on the Pd4 Co1 /C alloy catalyst. Thus, the ORR follows a four-electron process leading to water formation. Furthermore, the Koutecky–Levich plots (data not shown) for the ORR on the different catalytic electrodes showed that all plots are straight lines with almost the same slope as that of the theoretically predicted line with four-electron reduction of oxygen, again assessing that the oxygen reduction on all Pd-based catalysts follows a four-electron process leading to water. It is worth mentioning that the open circuit potential of the Pd Co alloy catalysts in oxygen-saturated solution is slightly higher than that of the
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Fig. 6. Kinetic current density and Pd Pd bond distance calculated from XRD data against the Co content in Pd Co/C catalysts. Fig. 4. Cyclic voltammograms of the Pd/C and Pd Co/C catalysts in N2 saturated 0.1 M HClO4 solution at a scan rate of 50 mV s−1 .
Pd/C catalyst, suggesting that the oxygen adsorption on the alloy surface is more favored than that on pure Pd surface. It is well known that, for an electrochemical reaction, the structure and composition of the catalysts are two key points in determining the adsorption/catalytic properties. Generally, the activity enhancement in the ORR on Pd-based alloy catalysts could be correlated to the changes in structural parameters [13,27]. The relationship between kinetic current density jk at 0.75 V and the Pd Pd interatomic distance as a function of the atomic percent of Co% in the catalysts is plotted in Fig. 6 to understand the enhanced activity of the alloys and possible role of their structure. The kinetic currents are calculated based on the equation: 1/j = 1/jk + 1/jd , where j and jd are the measured current and limited current, respectively. A “volcano” type relationship was gained. The highest ORR specific activity is obtained for a Pd2 Co1 /C catalyst, corresponding to a Pd Pd bond distance of 0.2758 nm. For some Pt M alloys, the peak in specific activity was observed for an optimal Pt Pt bond distance [30]. Such a conclusion could be extended to the Pd-based electrocatalysts for the ORR. For example, Zhuang and co-workers [31] reported the Pd Co/C catalyst with a Pd Pd bond distance ca. 0.273 nm shows higher activity for the ORR than other ratios of Pd Co/C catalysts in their lab, but still less active
Fig. 5. Linear scan voltammograms of the Pt/C and Pd Co/C catalysts in 0.1 M HClO4 saturated with pure oxygen at a scan rate of 5 mV s−1 and with a rotation speed of 1600 rpm. Current density is normalized to the geometrical surface area of the electrode. The ring current, RRDE data, for hydrogen peroxide production is compared.
than a commercial Pt/C. Zhang et al. synthesized Pd65 Co35 /C catalysts using different reducing agents and found the catalyst with a Pd Pd bond distance of 0.2726 nm had the highest activity for the ORR [32]. The favorable Pd Pd bond distance of 0.2758 nm obtained in our experiment is slightly larger than the reported results. The difference between our results and others may be ascribed to the difference in synthesis methods and to the particle size effects. 3.3. The ORR on the Pd-based catalysts in methanol-containing electrolyte It is well known that the crossover of methanol from the anode to the cathode can lead to a further reduction of the cell voltage by ca. 200–300 mV. Fig. 7 shows the ORR activity on the commercial Pt/C and as-prepared Pd4 Co1 /C alloy catalysts in the presence of 0.5 M CH3 OH. As compared to the ORR in pure HClO4 solution, both catalysts exhibited an increase in overpotential under the same current density in the presence of methanol. For the ORR on pure Pt catalyst in methanol-containing solution [19], the overpotential increases by ca. 200 mV, while only a small negative shift of ca. 15 mV was observed on the Pd4 Co1 /C catalyst. Results clearly indicate that the Pd4 Co1 /C electrocatalyst is very active for the ORR even at a high concentration of methanol. The overpotential of 15 mV on the Pd4 Co1 /C catalyst might reflect the blocking of some active sites for oxygen reduction by the adsorbed methanol or its intermediates from oxidation. All the Pd Co/C catalysts were found to exhibit the
Fig. 7. LSVs of the Pt/C and Pd4 Co1 /C catalysts in 0.1 M HClO4 + 0.5 M CH3 OH saturated with pure oxygen. Other conditions are identical to those of Fig. 5.
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similar characteristics as the Pd4 Co1 /C alloy for the adsorption and oxidation of methanol, thus the Pd Co/C bimetallic alloy might be a good candidate for the ORR in a DMFC. 4. Conclusions Carbon-supported Pd Co bimetallic nanoparticles with relatively small particle size and single-phase fcc disordered structure have been synthesized via a simple procedure at room temperature. The Pd4 Co1 /C catalyst exhibits an observed maximum ORR mass activity, which is close to the commercial Pt/C, while the highest specific activity is obtained on a Pd2 Co1 /C catalyst with a Pd Pd interatomic distance of ca. 0.2758 nm. The enhanced ORR activity of the Pd2 Co1 /C may be ascribed to the compressed Pd lattice via alloy with Co and the downshift of the d-band center of the Pd metal. The excellent tolerance to methanol of the Pd Co/C alloy catalysts implies that Pd-based catalyst may be an economical candidate to replace Pt as a cathode catalyst for a DMFC. Acknowledgments We would like to thank the National Natural Science Foundation of China (20673136, 20706056), the National “863” High-Tech. Research Programs of China (2006AA05Z136, 2006AA04Z342, 2007AA05Z141), the 100 People Plan Program of the Chinese Academy of Sciences and the Pujiang Program of Shanghai City (No. 06PJ14110) for support of this work. References [1] M.S. Dresselhaus, I.L. Thomas, Nature 414 (2001) 332. [2] M.Z. Jacobson, W.G. Colella, D.M. Golden, Science 308 (2005) 1901. [3] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of Fuel Cells Fundamentals, Technology and Applications, vol. 1–4, John Wiley & Sons, Chichester, 2003.
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