Carbon-supported Pd–Fe electrocatalysts for oxygen reduction reaction (ORR) and their methanol tolerance

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Abstract

Carbon-supported palladium-iron bimetallic electrocatalysts of different Pd to Fe ratios (1:1, 2:1, 3:1) were prepared by a low temperature single step co-reduction method in alkaline media without any stabilizing agents. The physical characterizations of catalysts were done using XRD and TEM. The electrochemical characterizations were done using underpotential deposition (upd) of hydrogen and copper. Electrocatalytic activities for oxygen reduction reaction (ORR) were investigated and compared to that of standard Pt/C catalyst. The carbon-supported Pd–Fe bimetallic catalysts showed higher surface area than that of pure Pd/C. The half-wave potential of ORR on as-prepared palladium-iron bimetallic catalysts was shifted positively by ∼75–100 mV from that of Pd/C in oxygen saturated 0.1 M HClO4 solution. The highest catalytic activity was obtained with Pd3Fe/C catalyst and it was close to that of standard Pt/C. XRD analysis did not show any noticeable shift in peak position due to alloying. In presence of methanol, carbon-supported Pd and Pd–Fe bimetallic catalysts showed superior ORR selectivity and activity unlike Pt/C. The peroxide generation on Pd3Fe/C – the best of Pd based electrocatalyst – was comparable to that on Pt/C. These catalysts, prepared at low temperature and without any further heat-treatment, gave activities that were free from effects of crystallite size, segregation, and enrichment of precious metal content that might happen at high temperature.

Highlights

► Pd–Fe/C catalysts prepared at low temperature did not show any lattice contraction. ► These catalysts show ∼75–100 mV positive shift in half-wave potential of ORR relative to Pd/C. ► Peroxide formation on Pd–Fe/C bimetallic catalysts is higher than that on Pt/C. ► Oxide formation on Pd–Fe/C is shifted to higher potential by ∼50 mV from that of Pd/C. ► Thermodynamic effects seem to operate on Pd–Fe/C catalysts.

Introduction

Platinum based materials – the most commonly used oxygen reduction catalysts in low temperature fuel cells – have several limitations. High overpotential, peroxide formation due to incomplete reduction of oxygen to water, high cost, and low availability of platinum limits the widespread commercialization of both polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) [1], [2], [3], [4]. Extensive research reported in the literature is focused on improving the existing activity of Pt and on reducing the precious metal catalyst loading. In DMFCs – when Pt is used as the cathode catalyst – the problems are exacerbated by the methanol crossover from the anode that results in mixed potential at the cathode. Therefore, a non-Pt catalyst with high activity and selectivity towards ORR is urgently required. The strategies adopted to achieve these objectives include development of catalysts based on chalcogenides, porphyrins, and carbides [5], [6], [7]. However, their intrinsic activity and long term stability are far inferior to Pt.

Recent reports show that catalytic activity of Pd can be improved by alloying with transition metals like Co, Fe, Ni etc. [8], [9], [10]. Moreover, Pd is less expensive, more abundant, and catalysts based on Pd were claimed to be methanol tolerant, and more stable than non-precious metal catalysts in acidic media. Thus, alternative materials, which are less expensive and at the same time having comparable or better activity than that of Pt, continues to attract considerable research interest.

Conventionally, carbon-supported alloy catalysts for fuel cell applications were prepared by a multi-step process [10], [11]. In this procedure, first carbon-supported precious metal catalyst (typically 20 wt.%) were produced, and subsequently subjected to high temperature-treatment of ca. 500–900 °C with transition metal precursors. The effect of various reducing agents and heat-treatment to different temperatures were well documented in the literature [12], [13], [14], [15]. In all cases heat-treatment was a deciding step in the preparation of catalysts. The high temperature-treatment, besides being energy intensive, increases the crystallite size [10] and degree of alloying [16]. Often the activity improvement was explained on the basis of surface segregation of precious metal, high degree of alloying, lattice contraction, and optimal crystallite size due to high temperature-treatment [10], [11], [16], [17], [18]. Therefore, it is difficult to identify the contribution of each of these factors to activity improvement.

There are some reports on synthesis of catalysts at low temperature (ca. 200 °C) in the literature; often, these methods involve complex stabilizing agents, inert atmosphere, and organic solvents [12], [16], [19], [20], [21]. Moreover, various synthetic strategies – polyol synthesis, sol–gel method, and micro-emulsion method – were reported to improve the activity of alloy catalysts [20], [21], [22]. Even then, the ORR activities reported are much lower than that of the alloy catalysts prepared at high temperature [10]. In this paper, we report the preparation of Pd and Pd–Fe bimetallic catalysts by a low temperature single step co-reduction method. They were characterized using both Hupd and Cuupd. Their ORR activity, methanol tolerance, and peroxide formation were investigated without heat-treatment of the catalysts.

Heat-treatment results in sintering, and limits the preparation only to supported catalysts. The low temperature synthesis allows the activity of Pd/C to be compared with that of carbon-supported Pd–Fe catalysts with similar crystallite size, and without any surface segregation that might happen because of high temperature-treatment.

Section snippets

Catalyst synthesis

Palladium nitrate (Pd(NO3)2.2H2O) (40% Pd), iron(II) chloride tetrahydrate (FeCl2.4H2O), sodium hydroxide (NaOH) and sodium borohydride (NaBH4), all procured from Merck, were used in the catalysts preparation.

Nano-clusters of palladium with iron were prepared by a single-step borohydride reduction at 80 °C. In a typical synthesis, 100 mg of Pd(NO3)2.2H2O was added to ca. 400 ml of water in a 500 ml beaker and the pH of the solution was adjusted to 10 by adding (5 wt.%) NaOH solution. Required amount

Morphology and surface electrochemistry

Fig. 1 shows XRD patterns of carbon-supported Pd and Pd–Fe bimetallic catalysts. The peak at 24.5° in all XRD patterns was attributed to the Vulcan XC-72 carbon support. All catalysts showed main characteristic peaks of face centered cubic (fcc) crystalline Pd demonstrating multiphase disordered structure [16]. The lattice parameter obtained from full profile fitting and the corresponding Pd–Pd bond distance for all as-prepared catalysts is shown in Table 1. On calculating the lattice parameter

Conclusion

Carbon-supported Pd–Fe nanoparticle catalysts were synthesized by a low temperature single step co-reduction method. The XRD patterns of the as-prepared catalysts did not reveal any noticeable shift in peak positions. The particle size obtained from TEM and XRD matches each other, and it was in 3–5 nm range. The surface areas of all Pd–Fe bimetallic catalysts were higher than that of Pd/C and the surface areas calculated from Hdes and Cuupd were comparable to each other. Repeated cycling in 0.1 M

Acknowledgements

We would like to thank Department of Science and Technology (DST), Government of India for funding the project (Grant No. SR/S1/PC – 27/2008). We express our gratitude to Indian Institute of Technology Bombay (IITB) for supporting and rendering facilities for our project. Sophisticated analytical instrument facility (SAIF) and Metallurgical Engineering and Material Science, IITB is also acknowledged for characterization of our samples.

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