Controlled synthesis and characterization of carbon-supported Pd₄Co nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

Abstract

Carbon-supported Pd₄Co nanoalloy samples (denoted as Pd₄Co/C) have been synthesized by a modified polyol reduction process, involving a mixture of ethylene glycol (a polyol) and a small amount of sodium borohydride as reducing agents, and evaluated as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. The synthesis process offers Pd₄Co nanoparticles with an average particle size of 4.6 nm and a narrow size distribution, which increases to 7.3 and 11.0 nm on annealing, respectively, at 350 and 500 °C in 10% H₂–90% Ar. Although the as-prepared Pd₄Co/C sample exhibits instability during ORR, annealing at 350 and 500 °C enhances the stability significantly due to an increasing degree of alloying between Pd and Co and increasing particle size. While both Pd/C and Pt/C exhibit a drastic decline in catalytic activity on annealing at 350 and 500 °C due to a significant increase in particle size, the Pd₄Co/C sample maintains high catalytic activity even after annealing at 350 °C due to an increase in the degree of alloying. The Pd₄Co/C catalyst offers an added advantage of excellent tolerance to methanol poisoning and lower cost compared to the conventional Pt/C catalyst.

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Broader context

Rapid depletion of fossil fuels and growing environmental concerns pose serious scientific and technological challenges to address the increasing global demand for energy. Fuel cells are appealing in this regard, but the high cost of the currently used platinum electrocatalysts is an impediment to the commercialization of the fuel cell technology. Palladium-based alloy catalysts have recently been found to exhibit high catalytic activity for the oxygen reduction reaction in fuel cells, but with the multiple metals involved, synthesis and processing play a significant role on the degree of alloying, chemical homogeneity, particle size and distribution, and catalytic activity. To address this issue, we present here a novel modified polyol reduction method to obtain carbon-supported Pd₄Co nanocrystals with a high degree of alloying and dispersion on the carbon support while keeping the particle size small. The Pd₄Co catalysts thus obtained exhibit high catalytic activity for the oxygen reduction reaction with excellent tolerance to methanol in direct methanol fuel cells.

1. Introduction

Platinum supported on carbon black is widely used as an electrocatalyst for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC).¹ However, the limited world's supply of platinum catalyst cannot support a widespread commercialization of the technologies. To address this critical challenge, alloying of Pt with other less expensive metals like Co or Ni,^2–4reduction in the catalyst loading,^5–10 and development of alternative electrocatalysts such as metal carbides,¹¹ metal oxides,¹² metal chalcogenides,^13,14 enzymes,^15,16 and platinum-free metal combinations^17–19 have been widely pursed over the years for ORR.

Recently, we reported that palladium-based alloy electrocatalysts such as Pd–Co–Au, Pd–Ti, and Pd–Co–Mo^20–22 show catalytic activity comparable to that of Pt for ORR. However, for structure and surface sensitive reactions such as the ORR, particle size as well as dispersion and compositional homogeneity of the alloy clusters on the carbon support are important factors to obtain good catalytic activity in practical applications. Generally, the Pd-based alloys supported on carbon have been prepared by a conventional reduction of the metal ions with alkali metal borohydrides or by a reduction of the metal ions within microemulsions with borohydrides, followed by heat treatment at 500–900 °C in a reducing atmosphere or by simple thermal treatment of the constituents with H₂ to promote alloy formation.^20–23 Unfortunately, the faster reduction with the strongly reducing borohydrides and the high heating temperatures lead to undesired particle growth, which results in a decrease in surface area and catalytic activity. Development of novel, controlled synthesis approaches that can provide enhanced alloy formation at the lowest temperatures possible while keeping the particle size small at the nanoscale is critical to realize the full potential of the Pd-based alloy electrocatalysts.

Reduction of metal ions by polyols has been pursued by several groups for the synthesis of metal nanoparticles.^24–30 The polyol process, which exploits high-boiling polyalcohol solvents as mild reducing agents at moderate temperatures, offers a facile route for obtaining multi-metallic alloy nanoparticles without requiring significant solid state diffusion over large atomic distances. Size and shape controlled nanocrystals with particle size as small as 2 nm has been obtained by the polyol process.²⁸ However, the conventional polyol process often employs large amounts of stabilizers such as the poly(vinylpyrrolidone) (PVP) polymer to produce mono-dispersed, small particles, resulting in polymer-protected colloidal dispersions. It is difficult to remove the polymer completely from such colloidal dispersions, and the presence of polymer is undesirable for practical applications such as electrocatalysts in fuel cells. In addition, the large differences in the standard reduction potentials of ions like Pd²⁺ and Co²⁺ and the mild reducing power of polyols lead to the formation of hollow nanospheres of the easily reducible components like Pd with no Co.³¹ In other words, it is difficult to reduce ions like Co²⁺ with polyol due to their low reduction potential compared to that of Pd²⁺.

We present in this paper a modified polyol reduction process, involving a mixture of a polyol (ethylene glycol) and a small amount of sodium borohydride as reducing agents as well as much lower amounts of the protective polymer than that used in conventional polyol process. While the use of borohydride helps to reduce the Co²⁺ ions easily, the lower amount of polymer helps to obtain electrocatalysts nearly free from the protective polymer contamination. The reduction products formed are then annealed at a moderate temperature of 350 °C to obtain carbon-supported Pd₄Co nanocrystals with a high degree of alloying and dispersion on the carbon support while keeping the particle size small. Characterization of the Pd₄Co/C electrocatalysts by X-ray diffraction and transmission electron microscopy, an investigation of their electocatalytic activity for ORR in single cell PEMFC and DMFC and by rotating disk electrode (RDE) measurements, and an assessment of their stability in a three-electrode test cell in the presence and absence of methanol are presented.

2. Experimental

2.1 Synthesis

Carbon-supported Pd₄Co electrocatalysts were prepared by a modified polyol reduction process. Required amounts of (NH₄)₂PdCl₄ and CoCl₂·6H₂O were first dissolved in a mixture of ethylene glycol (EG, a polyol) and water, followed by dissolving poly(vinylpyrrolidone) (PVP) in it and adding an aqueous solution of NaBH₄ into it. The mixture was then refluxed, cooled to room temperature, stirred with an appropriate amount of carbon (Vulcan XC 72R), filtered, washed, and dried overnight. The as-prepared powders thus obtained are hereafter designated as Pd₄Co/C-Ap, and were examined by a Perkin Elmer FT-IR System (Spectrum BX) to detect if any PVP is present. The samples were then heated at 350 and 500 °C in a reducing atmosphere, which are denoted hereafter, respectively, as Pd₄Co/C-350 and Pd₄Co/C-500. For a comparison, Pd/C and Pt/C electrocatalysts were also prepared by a similar process and denoted as Pd/C-Ap, Pd/C-350, Pd/C-500, Pt/C-Ap, Pt/C-350 and Pt/C-500. Further details of these synthesis procedures are described in ESI.†

2.2 Structural and microstructural characterizations

The samples synthesized were characterized with a Phllips X-ray diffractometer (XRD) using Cu Kα radiation and a FEI Tecnai G² F20 X-Twin transmission electron microscope (TEM) equipped with energy dispersive spectroscopic (EDS) analysis. The average crystallite sizes of all the samples synthesized were assessed by analyzing the XRD data with the Jade 7.0 program. In addition, the average sizes of selected Pd₄Co samples were also calculated based on a random selection of 70 particles in TEM.

2.3 Electrochemical characterization

2.3.1 Cyclic voltammetry . The stability of the electrocatalyst samples was assessed by cyclic voltammetry (CV) using a glassy carbon (GC) micro-electrode containing the electrocatalyst (working electrode) in a classical three-electrode system with H₂SO₄ or H₂SO₄ + methanol electrolyte solution. A Pt net and a saturated calomel electrode (SCE) were used, respectively, as the counter and reference electrodes. The CV curves were obtained by multiple-cycle scans with a sweep rate of 20 mV s⁻¹. The details of the CV studies are described in ESI.†

2.3.2 Single cell evaluation. The electrocatalytic activity for ORR was assessed by single cell measurements in PEMFC. For the PEMFC measurements, the anode catalyst was a commercial 20 wt% Pt/C (E-TEK) and the cathode catalysts were the synthesized Pt/C, Pd/C and Pd₄Co/C samples. The metal or alloy loadings in both the anode and cathode were 0.4 mg cm⁻². The membrane-electrode assemblies (MEAs) were fabricated by hot-pressing the anode and cathode onto a Nafion 112 membrane (DuPont). The electrochemical performances in PEMFC of the MEAs were evaluated with a single-cell fixture having an active area of 5 cm² and by feeding humidified hydrogen into the anode and humidified oxygen into the cathode. The temperatures of the humidified hydrogen and oxygen were same as that of the cell temperature (60 °C).

The electrocatalytic activity for ORR was also assessed in single cell DMFC. In this case, the anode catalyst was a commercial 40 wt% PtRu/C (E-TEK) and the cathode catalysts were Pd/C-350, Pt/C-350, Pd₄Co/C-350, and a commercial 20 wt% Pt/C. The electrodes were prepared by the same procedure described above for PEMFC. The metal or alloy loadings in the anode and cathode were, respectively, 0.8 and 0.25 mg cm⁻². The electrochemical performances in DMFC of the MEAs fabricated with Nafion 112 membrane (DuPont) were evaluated by feeding a preheated 1 M methanol solution into the anode and humidified oxygen into the cathode. The temperatures of the preheated methanol solution and humidified oxygen were same as that of the cell temperature (65 °C). Further details of the PEMFC and DMFC single cell evaluations are described in ESI.†

2.3.3 Linear polarization measurements. To assess the electrocatalytic activity for ORR further and for extracting kinetic parameters, linear polarization (LP) measurements using a rotating disk electrode (RDE) were carried out with the same three-electrode system as that for the CV tests with H₂SO₄ electrolyte solution saturated with oxygen. A 5 mm diameter glassy carbon rotating disk micro-electrode was taken as the working electrode and the micro-electrode was prepared by the same procedure described above for the CV tests. All the electrode potential values attained in the LP tests were treated with iR correction. The details of the linear polarization studies are described in ESI.†

3. Results and Discussion

3.1 Structural and microstructural analysis

Fig. 1 compares the X-ray diffraction (XRD) patterns of the Pd₄Co/C and Pd/C samples prepared by the modified polyol process before and after heat treatment at various temperatures in the reducing atmosphere. The data indicate the formation of single phase products with reflections characteristic of a face-centered cubic lattice. The reflections of Pd₄Co/C are shifted to higher angles, particularly after annealing at 350 and 500 °C, compared to the reflections of Pd/C, confirming the substitution of smaller Co atoms for the larger Pd atoms and the formation of Pd–Co alloy. The movement of the reflections to higher angles on going from the as-prepared to the 350 and 500 °C Pd₄Co/C samples also suggests an increasing degree of alloying on annealing. Table 1 also compares the crystallite size values obtained from the XRD data using Scherrer formula for Pd₄Co/C, Pd/C, and Pt/C after annealing at various temperatures. The particle size increases with increasing annealing temperature, although the increase is much greater for Pt/C compared to that for both Pd₄Co/C and Pd/C.

Table 1 Particle size and compositional analysis data of the Pd₄Co/C, Pd/C, and Pt/C electrocatalysts

Sample	Average crystallite size from XRD^a (nm)	Average particle size from TEM^b (nm)	Pd : Co atom ratio from TEM-EDS analysis
a Obtained by using the (111) reflections; the numbers in parentheses give the standard deviation. b The numbers in parentheses give the standard deviation.
Pd4Co/C-Ap	4.4 (0.2)	4.6 (2.1)	83 : 17
Pd4Co/C-350	8.5 (0.3)	7.3 (4.9)	82 : 18
Pd4Co/C-500	10.8 (0.2)	11.0 (4.2)	81 : 19
Pd/C-Ap	4.8 (0.2)	—	—
Pd/C-350	8.7 (0.3)	—	—
Pd/C-500	11.5 (0.3)	—	—
Pt/C-Ap	4.5 (0.2)	—	—
Pt/C-350	12.1 (0.3)	—	—
Pt/C-500	18.3 (0.2)	—	—

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Fig. 1 XRD patterns of the Pd₄Co/C and Pd/C catalysts before and after annealing in 10% H₂–90% Ar atmosphere at various temperatures. The dashed vertical line indicates the standard 2θ value corresponding to the (111) reflection of Pd metal.

Fig. 2 and 3 compare the TEM photographs and the particle size distributions of the Pd₄Co/C samples before and after annealing at various temperatures. As seen, the nanosize particles have predominantly spherical morphology with a uniform distribution on the carbon support. The histograms and a Gaussian fit of the TEM data in Fig. 3 indicate a narrow particle size distribution, although the 350 °C sample has a slightly broader distribution than the as-prepared and 500 °C samples. The data indicate an average particle size value of 4.6, 7.3, and 11.0 nm, respectively, for the Pd₄Co/C-Ap, Pd₄Co/C-350, and Pd₄Co/C-500 samples, which agree closely with the crystallite size values obtained from the XRD data (Table 1).

Fig. 2 TEM micrographs of (a) Pd₄Co/C-Ap, (b) Pd₄Co/C-350, and (c) Pd₄Co/C-500.

Fig. 3 Particle size distribution histograms of (a) Pd₄Co/C-Ap, (b) Pd₄Co/C-350, and (c) Pd₄Co/C-500.

Fig. 4 shows the high resolution TEM (HRTEM) photographs of the Pd₄Co/C-Ap, Pd₄Co/C-350, and Pd₄Co/C-500 samples. The data indicate the presence of predominantly twinned or polycrystalline nanocrystals. For each sample, the spacing between the (111) planes are also indicated in Fig. 4. The decrease in the (111) spacing from 0.2259 to 0.2222 nm with increasing annealing temperature confirm the substitution of smaller Co atoms for larger Pd atoms and an increase in the degree of alloying, which is consistent with the XRD data in Fig. 1. Moreover, EDS elemental analysis of the nanocrystals indicate a Pd : Co atom ratio of around 82 : 18 (Table 1), which is close to the nominal composition of 80 : 20.

Fig. 4 High resolution TEM micrographs of the Pd₄Co/C samples: (a) Pd₄Co/C-Ap, (b) Pd₄Co/C-350, and (c) Pd₄Co/C-500.

The TEM and XRD data thus indicate that the addition of a small amount of sodium borohydride to the polyol reduction process helps to reduce nearly all the Co²⁺ ions into Co, which is difficult to achieve by the conventional polyol reduction process employing polyol alone as a reducing agent. On the other hand, the controlled, slow reduction within the polymer-protected colloidal dispersions with the predominantly present mild reducing agent, polyol, offers small particle size with a narrow size distribution. The short diffusion distance within the small (∼5 nm), as-prepared nanoparticles helps to achieve a high degree of alloying at low annealing temperatures of 350–500 °C. The modified polyol method employing a mixture of a polyol and borohydride as reducing agents offers an effective approach to obtain Pd–Co alloys with a high degree of alloying while keeping the particle size small. In addition, FT-IR measurements revealed the absence of PVP in the samples.

3.2 Electrochemical studies

3.2.1 Cyclic voltammetry analysis. Fig. 5 compares the cyclic voltammetry (CV) plots of the Pd₄Co/C-Ap, Pd₄Co/C-350 and Pd₄Co/C-500 samples recorded in 0.5 M H₂SO₄ electrolyte at room temperature. For a comparison, the CV plots of Pd/C-Ap and Pd/C-350 are also shown in Fig. 5(d) and (e). Since all the micro-electrodes for the CV tests contained the same mass of the Pd–Co alloy, the electrochemical surface area of the studied catalysts for ORR can be directly compared from the specific mass current density and peak area values³² of the surface oxide reduction region in Fig. 5. The specific mass cathodic peak area values normalized to the sweep rate (i.e. charge transferred) is used in this study, and the CV scans were carried out for multiple cycles for all the samples in Fig. 5 to assess their stability (durability). Comparing the second cycle scans of the Pd₄Co/C samples, the as-prepared Pd₄Co/C sample shows higher electrochemical surface area (Fig. 5(a)) than the samples annealed at 350 and 500 °C (Fig. 5(b) and (c) ). However, the as-prepared Pd₄Co/C sample shows a decline in electrochemical surface area on cycling as indicated by a decrease in the surface oxygen reduction peak area, suggesting instability.

Fig. 5 Cyclic voltammograms (CV) of the Pd₄Co/C and Pd/C samples in 0.5 M H₂SO₄ solution at a sweep rate of 20 mV s⁻¹ at room temperature: cycling data of (a) Pd₄Co/C-Ap, (b) Pd₄Co/C-350, (c) Pd₄Co/C-500, (d) Pd/C-Ap, and (e) Pd/C-350 and (f) the 15th cycle data of Pd₄Co/C-Ap, Pd₄Co/C-350 and Pd₄Co/C-500. The inset in (f) compares the 15th cycle electrochemical surface area of the three samples.

The decline in electrochemical surface area with cycling becomes less pronounced with increasing annealing temperature, and the Pd₄Co/C-500 sample exhibits little decline in electrochemical surface area on going from 2nd to 15th cycle. The faster decline in the electrochemical surface area of the Pd₄Co/C-Ap sample on cycling in the acidic environment could be due to both the leaching out of the poorly alloyed Co and the smaller particle size (high particle surface area). The former is consistent with the increased degree of alloying between Pd and Co and improved homogeneity on annealing at 350 and 500 °C as indicated earlier by the XRD and TEM data as well as the recent theoretical calculations³³ indicating that Co in Pd–Co alloy is likely to be stripped at lower potentials in acidic solutions. The latter is consistent with the faster decline in the electrochemical surface area of the Pd/C-Ap sample (with no Co) on cycling (Fig. 5(d)) due to smaller particle size while the annealed Pd/C-350 sample exhibits improved stability on cycling (Fig. 5(e)) due to larger particle size. However, the decline in electrochemical surface area on cycling is smaller for the Pd/C-Ap sample compared to that for the Pd₄Co/C-Ap sample as both the leaching out of Co and the smaller particle size contribute to the instability. The decline in electrochemical surface area with smaller particle size (high particle surface area) could be due to either sintering of the smaller particles on potential cycling or easier dissolution in acid. Although one could differentiate these possibilities by a TEM evaluation of the catalyst after CV cycling, the use of Nafion in the microelectrode preparation makes it difficult to establish this unambiguously at the present time. Fig. 5(f) compares the CV scans recorded at the 15th cycle for Pd₄Co/C-Ap, Pd₄Co/C-350 and Pd₄Co/C-500. The inset in Fig. 5(f) compares the peak area for the surface oxide reduction peak at the 15th cycle for the three samples. Although the onset potential does not vary significantly, the electrochemical surface area decreases with increasing annealing temperature due to an increase in particle size and a decrease in particle surface area.

3.2.2 Correlation of the catalytic activity to annealing temperature and particle surface area. Fig. 6 shows the variations with annealing temperature of the catalytic activity for ORR and the particle surface area. The catalytic activities were extracted from the single cell PEMFC data (given in Fig. 7) as the current density in mA cm⁻² at a given cell potential. The particle surface area S in m² g⁻¹ was calculated using the equation S = 6000/dρ for spherical particles,²² where d is the crystallite size (diameter) in nm obtained from the TEM data (Fig. 2) and ρ is the density of the Pd₄Co alloy (11.04 g cm⁻³). This equation is obtained by dividing the surface area of a spherical particle (4πr²) by the mass of the spherical particle (mass = (density) (volume) = (ρ) (4πr³/3)), where r is the radius of the spherical particle (d/2) and ρ is the density of the alloy, and making appropriate changes to obtain the surface area value in m² g⁻¹ when the density and crystallite size values are substituted, respectively, in g cm⁻³ and nm. The inset in Fig. 6 shows the variation of the catalytic activity for ORR with surface area. The data in Fig. 6 demonstrate clearly a decrease in catalytic activity for ORR with increasing annealing temperature due to a decreasing surface area. The main reason for the larger surface area at lower annealing temperatures is the smaller particle size. However, a higher number of catalytically active defects at low annealing temperatures as well as changes in the Pd : Co ratio at the surface and in the near-surface region could also influence the catalytic activity in addition to the particle size. Considering the cyclic voltammetry and single cell data, the Pd₄Co/C-350 sample exhibits a combination of high catalytic activity for ORR and good stability.

Fig. 6 Variations with annealing temperature of the catalytic activity for ORR and the surface area of the Pd₄Co/C sample. The inset shows the relationship between the electrochemical activity and surface area.

Fig. 7 Comparison of the catalytic activities for ORR of the Pd₄Co/C, Pd/C, and Pt/C samples before and after annealing at various temperatures: (a) comparison of the polarization curves recorded in single cell PEMFC of Pt/C-350, Pd/C-350 and Pd₄Co/C-350 with the inset showing a comparison of the catalytic activity of the three samples at 0.73 V; (b) comparison of the linear polarization curves recorded with RDE of Pt/C-350, Pd/C-350 and Pd₄Co/C-350 with the inset showing a comparison of the catalytic activity of the three samples at 0.73 V; (c) comparison of the polarization curves recorded in single cell PEMFC of Pt/C-Ap, Pd/C-Ap, and Pd₄Co/C-Ap with the inset showing a comparison of the catalytic activities extracted from the single cell tests of the three samples at 0.73 V; and (d) comparison of the polarization curves recorded in single cell PEMFC of Pt/C-500, Pd/C-500, and Pd₄Co/C-500 with the inset showing a comparison of the catalytic activities extracted from the single cell tests of the three samples at 0.73 V.

3.2.3 Comparison of catalytic activities. Fig. 7 compares the catalytic activity for ORR of the Pd₄Co/C samples with those of the Pd/C and Pt/C samples prepared under similar conditions. Fig. 7 (a) compares the polarization curves of Pt/C-350, Pd/C-350 and Pd₄Co/C-350 in single cell PEMFC. A comparison of the current density values for the catalysts at 0.73 V, which is in the activation overpotential controlled region of ORR, is shown in the inset in Fig. 7(a). In order to confirm the single cell PEMFC data, linear polarization measurements using a rotating disk electrode (RDE) were conducted on the three catalyst samples annealed at 350 °C with the same three-electrode system as that for the CV test with 0.5 M H₂SO₄ electrolyte solution saturated with oxygen. Fig. 7(b) displays the linear polarization curves of RDE and the inset compares the electrochemical activity of Pt/C-350, Pd/C-350 and Pd₄Co/C-350 by extracting the current density values from the LP-RDE curves at 0.73 V. The trend in the RDE data is consistent with the single cell data in Fig. 7(a).

As seen in Fig. 7(c), the as-prepared Pd/C-Ap and Pd₄Co/C-Ap have lower catalytic activity for ORR than the as-prepared Pt/C-Ap although all of them have a similar particle size of ∼4.5 nm (Table 1). However, although all the three samples exhibit a decrease in catalytic activity on annealing at 350 °C (inset in Fig. 7(a)) and 500 °C (inset in Fig. 7(d)) due to an increase in crystallite size (Table 1), both the Pd/C and Pt/C samples exhibit a much faster decrease in catalytic activity compared to the Pd₄Co/C sample. For example, despite a similar crystallite size of ∼8 nm after annealing at 350 °C (Table 1), the Pd₄Co/C-350 sample exhibits higher catalytic activity than the Pd/C-350 sample and about 60% of the activity of Pt/C-350, indicating that the alloying of Pd with Co enhances the catalytic activity for ORR. This demonstrates that combining a metal like Co that cleaves the O–O bond easily due to the high negative free energy change for oxide formation with a metal like Pd that reduces the adsorbed oxygen easily due to the high positive standard oxidation potential offers an effective strategy to design low cost electrocatalysts.^20,34 In addition, formation a core-shell structure for Pd-based alloys on annealing has been reported by Lima et al.³⁵ due to the segregation of the noble metal Pd to the surface, which could also contribute to the variation in catalytic activity with annealing temperature. Besides, shifts in the Pd:4d band center and thus modifications in the Pd–O bond energy to a balanced level on alloying with Co so that the Pd–O bond is not too strong or too weak, as indicated by experimental and density functional theory (DFT) calculations,³⁶ may make the ORR more facile on alloying. Thus, the slow decrease in the catalytic activity of the Pd₄Co/C sample with annealing temperature compared to the Pd/C and Pt/C samples is due to a partial offset of the decrease in catalytic activity caused by the increasing particle size by the increasing degree of alloying between Pd and Co. Finally, the drastic, much faster decrease in the catalytic activity of Pt/C with annealing temperature is due to a rapid increase in crystallite size with annealing temperature (Table 1).

3.2.4 Methonal tolerence of Pd₄Co electrocatalysts. In DMFC, the tolerance of the cathode catalyst to the methanol that permeates from the anode to the cathode through the polymeric membrane is a critical issue. Accordingly, in order to evaluate the tolerance of the Pd₄Co/C electrocatalyst towards methanol, the CV scans were recorded in 0.5 M H₂SO₄ + 1 M methanol solution. Fig. 8(a)–(d) compare the CV plots (15th cycle) of Pd₄Co/C-Ap, Pd₄Co/C-350, Pd₄Co/C-500, and Pt/C-Ap recorded in the presence and absence of methanol. As seen, Pd₄Co/C catalyst exhibits similar CV curves and electrochemical surface areas in the presence and absence of methanol, indicating the high tolerance of Pd₄Co to methanol. On the other hand, the voltammetric reduction peak disappears on adding methanol to sulfuric acid in the case of Pt/C catalyst (Fig. 8(d)), indicating the poisoning of Pt by methanol. Additionally, while Pt/C exhibits a large methanol oxidation peak during the anodic sweep, the Pd₄Co/C samples do not, illustrating the tolerance of the latter to methanol.

Fig. 8 Comparisons of the cyclic voltammogram of (a) Pd₄Co/C-Ap, (b) Pd₄Co/C-350, (c) Pd₄Co/C-500, (d) Pt/C-Ap, (e) Pd/C-Ap, and (f) Pd/C-350 in 0.5 M H₂SO₄ and in 0.5 M H₂SO₄ + 1 M methanol solution at a sweep rate of 20 mV s⁻¹ at room temperature. The insets compare the electrochemical surface area in the presence and absence of methanol.

To understand the origin of the methanol tolerance of the Pd₄Co catalyst, we also recorded the CV plots of Pd/C-Ap and Pd/C-350 in the presence and absence of methanol, and Fig. 8(e) and (f) display those CV plots at 15th cycle. The Pd catalyst shows similar behavior in the presence and absence of methanol, indicating that Pd possesses high tolerance to methanol. Thus, while Pt is highly active for the methanol oxidation reaction (MOR), the inactivity of Pd for MOR avoids the adsorption of intermediates that could form during MOR on the active sites of Pd–Co alloys, resulting in a superior tolerance of Pd₄Co to methanol during ORR.

Fig. 9 compares the stability of the Pd₄Co/C-350 sample in presence of methanol on subjecting to 15 cycles in the CV scan. The sample exhibits excellent stability in presence of methanol similar to that observed in the absence of methanol in Fig. 5.

Fig. 9 Cyclic voltammograms of the Pd₄Co/C-350 sample recorded in a 0.5 M H₂SO₄ + 1 M methanol solution at a sweep rate of 20 mV s⁻¹ at room temperature, illustrating the stability on cycling.

Fig. 10 compares the polarization curves recorded in single cell DMFC of Pd/C-350, Pt/C-350, Pd₄Co/C-350, and a commercial 20 wt% Pt in carbon electrocatalysts for ORR. As seen, Pd₄Co-350 shows higher open-circuit voltage (OCV) and better catalytic activity with lower polarization loss in the activation polarization region than both the Pd/C-350 and Pt/C-350 electrocatalysts prepared by the same modified polyol process. The performance of Pd₄Co-350 is comparable to that of a state-of-the-art commercial Pt/C catalyst. We believe the better performance of Pd₄Co-350 compared to the Pt/C sample prepared by us by the same modified polyol method is due to both a higher tolerance of the former to methanol poisoning in addition to smaller particle size (Table 1). The combination of high catalytic activity and excellent tolerance to methanol together with a lower cost makes Pd₄Co/C an attractive cathode catalyst for DMFC.

Fig. 10 Comparison of the polarization curves recorded in single cell DMFC of Pd/C-350, Pt/C-350, Pd₄Co/C-350, and a commercial 20 wt% Pt in carbon electrocatalysts for ORR at 65 °C. The methanol concentration was 1 M.

3.2.5 ORR kinetics of Pd₄Co electrocatalysts. In order to obtain the intrinsic electrochemical kinetic parameters, linear polarization data for ORR were collected with the Pd₄Co/C electrocatalysts as well. Fig. 11 compares the Tafel plots of the Pd₄Co/C-Ap, Pd₄Co/C-350 and Pd₄Co/C-500 samples. The Pd₄Co/C-Ap sample exhibits a Tafel slope of 62 mV/dec, which is close to the value of 60 mV/dec typically observed for Pt/C in acid or alkaline environment,³⁷ suggesting a direct four electron process for ORR. For a comparison, the Pd/C sample exhibits a Tafel slope of 71 mV/dec, indicating an increase in efficiency for ORR on alloying Pd with Co. However, the Tafel slope of the Pd₄Co/C-350 sample increases slightly to 69 mV/dec and that of the Pd₄Co/C-500 sample increases significantly to 98 mV/dec. This suggests that a significant increase in particle size may cause a deviation from the four-electron process for ORR, implying an increase in peroxide formation. Further work to study the formation of such intermediates and their effects by extending the RDE setup to a rotating ring-disc electrode (RRDE) is currently in progress in our laboratory.

Fig. 11 ORR Tafel plots of Pd₄Co/C-Ap, Pd₄Co/C-350, and Pd₄Co/C-500 recorded in 0.5 M H₂SO₄ at a slow sweep rate of 5 mV s⁻¹ at room temperature.

4. Conclusions

Carbon-supported, nanostructured Pd₄Co alloys with a controlled particle size and distribution have been synthesized by a modified polyol reduction process and evaluated as electrocatalysts for ORR. Annealing at 350 and 500 °C is found to enhance the stability (durability) significantly due to an increasing degree of alloying between Pd and Co as well as an increasing particle size. Compared to the single element (Pd and Pt) electrocatalysts, the increase in alloying also counteracts partly the decrease in catalytic activity caused by an increase in particle size during annealing at 350 and 500 °C. The sample annealed at 350 °C exhibits a combination of high catalytic activity for ORR and good stability. Linear polarization measurements indicate the ORR with the Pd₄Co catalyst to involve a close to direct four electron process. In addition, the Pd₄Co alloy catalysts exhibit superior tolerance to methanol poisoning compared to the conventional Pt catalysts, resulting in high catalytic efficiency for ORR in single cell DMFC. The high catalytic activity together with the low cost and high tolerance for methanol may make the Pd–Co alloys attractive as cathode electrocatalyst for DMFC.

Acknowledgements

Financial support by the National Science Foundation grant CBET-0651929 and the Welch Foundation grant F-1254 is gratefully acknowledged. The authors thank Mr. Wen Li for his help in evaluating the catalysts in single cell DMFC.

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Footnote

† Electronic Supplementary Information (ESI) available: Experimental details describing synthesis and electrochemical characterization of the electrocatalysts. See DOI: 10.1039/b814708f

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