Synthesis of carbon-supported Pd-Sn catalyst by ultrasonic irradiation for oxygen reduction reaction

Abstract

Carbon-supported Pd-Sn (Pd-Sn/C) catalyst was prepared under ultrasonic irradiation, and its electrocatalytic activity for oxygen reduction reaction (ORR) was evaluated in 0.5 M KOH. TEM images showed that the prepared Pd-Sn/C catalyst particles are smaller in average size than carbon-supported Pd (Pd/C) catalyst particles. XRD and XPS results indicated the small particle size and the electronic interaction between Pd and Sn for the Pd-Sn/C catalyst. The Pd-Sn/C catalyst has a higher ORR activity than the Pd/C catalyst in alkaline media. In addition, the Pd-Sn/C catalyst showed a lower Tafel slope and a larger number of electrons transferred for ORR, compared with those of the Pd/C catalyst. These results indicate that Sn influences both the kinetics and the mechanism of ORR. Based on these results, the Pd-Sn/C catalyst prepared by using ultrasonic irradiation can be expected as a promising ORR catalyst in alkaline media.

Introduction

Direct methanol fuel cell (DMFC) has been attracting attention as a promising alternative energy source because of the high theoretical energy density and the advantage of using the liquid fuel [1], [2]. However, currently available DMFC systems suffer from technical problems such as a low electrocatalytic activity and the methanol crossover, which means the permeation of methanol from the anode side to the cathode side of the cell through the electrolyte membrane [3]. Moreover, platinum metal, which is widely used as the electrocatalyst in DMFC, is expensive, and its limited supply poses a serious problem in commercialization of fuel cell technology [4]. Accordingly, the cathode catalyst of DMFC requires a high activity for oxygen reduction, methanol tolerance and a low cost.

Many studies have been carried out to reduce the loading of Pt by increasing the utilization efficiency of Pt and/or to replace it with a less expensive material [5], [6], [7]. Especially, platinum-free catalysts for oxygen reduction reaction (ORR), such as non-platinum metals combinations, metal oxides, chalcogenides, inorganic and organometallic complexes, have been studied [8], [9], [10], [11], [12], [13], [14]. Though they generally exhibit a lower catalytic activity, recent studies on Pd-based electrocatalysts show that they have remarkable activities for oxygen reduction. Bard et al. reported that palladium-based electrocatalysts such as Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%), Pd–Ti (Pd:Ti = 50:50 atom%), and Pd–Co–Mo (Pd:Co:Mo = 70:20:10 atom%) which were synthesized by the conventional borohydride reduction method and the reverse microemulsion method, show a reasonable catalytic activity comparable to that of Pt for ORR in PEMFC at 60 °C [15], [16], [17]. Ota et al. reported that palladium-based alloys such as Pd–Co, Pd–Ni, and Pd–Cr, prepared by rf sputtering, showed a higher ORR electrocatalytic activity than Pd in sulfuric acid solution, although it was lower than Pt. Those Pd alloys had no electrocatalytic activity for methanol oxidation [18].

However, several studies have reported on the dissolution of the transition metals in the alloy catalysts upon exposure to acids, although some of the alloys have been found to exhibit a higher electrocatalytic activity than pure metals [19], [20]. Wieckowski et al. reported that the cobalt metal in Pt₃Co alloy catalyst dissolved in acidic media, whereas it was stable in alkaline media [21]. The alkaline condition has many advantages [22], [23]. The kinetics of both oxygen reduction and methanol oxidation reactions are more favorable in alkaline media than in acidic media. Therefore, a non-Pt catalyst can be used in alkaline media. In addition, it seems possible to suppress the methanol crossover in direct alkaline methanol fuel cell [22], because the movement of hydroxide ions proceeds in the direction opposite to the movement of fuel crossover through the membrane, i.e., the direction of hydroxide ion movement is from cathode to anode. Thus, the management of water would be easy to perform in this system.

We have attempted to synthesize non-Pt catalysts using ultrasonic irradiation, in view of the known fact that the ultrasonic reduction method generates noble metal nanoparticles with a much smaller size, a larger surface area, and a narrower size distribution than those prepared by other methods [24]. This method was expected to simplify the synthesis of bimetal nanoparticles with a specific structure such as the core-shell structure. Because this method does not use a reducing agent, no process for its removal is needed. For these reasons, the ultrasonic reduction method is expected to be a promising method of catalyst preparation for the fuel cell. In our previous studies, Pd-based nanoparticles were prepared under ultrasonic irradiation, and their activities toward the ORR have been evaluated in alkaline solution by using ITO electrodes modified with Pd-based nanoparticles. Pd-Sn nanoparticles prepared under ultrasonic irradiation showed a better ORR activity than Pt and Pd nanoparticles in alkaline media in the absence of methanol. Moreover, the Pd-Sn nanoparticles showed a higher ORR activity than Pt in the presence of 1 M methanol [25].

In this work, we investigated the possibility of utilizing ultrasonic irradiation for the preparation of carbon-supported metal catalyst by incorporating supporting carbon particles in the synthesis solution, and we demonstrated the potentiality of carbon-supported Pd-Sn catalyst as the oxygen reduction catalyst in alkaline media. We established a one-pot method for synthesizing Pd-Sn/C catalyst under ultrasonic irradiation and evaluated its electrocatalytic activity for oxygen reduction in alkaline media.