Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells
Graphical abstract
Introduction
Hydrogen-operated polymer electrolyte membrane (PEM) fuel cells have received a great deal of attention as a green alternative to conventional combustion engines due to their high efficiency and zero emission of pollutants [1], [2], [3]. Despite significant advances, a number of problems, such as the use of platinum, have yet to be overcome. The sluggish electrocatalytic kinetics of the oxygen reduction reaction (ORR) requires Pt as a catalyst, which is an expensive material. The high cost of Pt is a serious limitation to the commercialization of PEM fuel cells. Therefore, it is of great interest to synthesize advanced electrocatalysts for ORR. In an attempt to achieve this goal, various Pt-based alloy catalysts with less expensive 3d-transition metals, such as PtCo [4], [5], [6], PtNi [7], [8], [9], PtCu [10], [11] and PtFe [12], [13], have been extensively developed to improve the activity toward ORR and to reduce the consumption of Pt. However, serious durability issues have been raised in the acidic environment of PEM fuel cells.
Another important route for improving the activity of catalysts is to control the shape of the Pt nanoparticles. In general, the activity of metallic catalysts depends strongly on their morphology, which determines the distinct facets and atomic surface arrangements. Significant research has been devoted to synthesizing shape-controlled Pt nanoparticles and evaluating their electrocatalytic activity [14], [15], [16], [17], [18], [19], [20]. The tetrahexahedral Pt nanoparticles with 24 high-index facets were synthesized and found to exhibit high activity with respect to the electro-oxidation of formic acid and ethanol [21]. The enhanced catalytic activity was reported for one-dimensional structures, such as Pt nanotubes [22], [23] and Pt nanowires [24], [25]. Because of the high surface to volume ratio, a hollow structure such as a nanocage was synthesized via galvanic replacement, and its improved activity toward electrochemical methanol oxidation was reported [26], [27]. Recently, Pt nanodendrites with rich edges and corner atoms were prepared and found to exhibit superior electrocatalytic activity due to the electronic configurations effect [28], [29], [30], [31], [32], [33], [34].
Although there have been several reports regarding the improved electrocatalytic activity of shape-controlled Pt-based catalysts, it is unclear if such high activity could also be observed under actual fuel cell operating conditions. It is important to note that most of these studies evaluated the catalytic properties in an aqueous electrolyte using a conventional three-electrode system. Due to the difference in configuration between the three-electrode system and the fuel cell operating environment, it is necessary to demonstrate that the results drawn from the three-electrode system are applicable to actual fuel cells. In addition, the stability of a shape-controlled catalyst with respect to time has not been thoroughly examined. Therefore, an investigation of the catalytic behavior and durability of the shape-controlled Pt catalysts in PEM fuel cell environments is necessary.
Here, the ORR activity of Pt nanodendrites in a fuel cell station under H2/O2 conditions is evaluated. The durability of the Pt nanodendrites is examined in the fuel cell station using an accelerated durability test (ADT) up to 20,000 cycles, and the performance loss after ADT is examined by measuring the changes in the morphology and the active surface area of the Pt.
Section snippets
Preparation of Pt nanodendrites
Pt nanodendrites were synthesized using a previously published protocol [28]. Potassium tetrachloroplatinate (K2PtCl4, Kojima) and tetradecyltrimethylammonium bromide (TTAB, Aldrich) were dissolved in de-ionized water. The mixture was heated to 70 °C with vigorous stirring until the mixture became clear. Then, ascorbic acid was added to the mixture under stirring. After 6 additional hours, the resulting solution was cooled down to room temperature. The products were separated by centrifugation
Results and discussion
Fig. 1 shows HR-TEM images of the as-prepared Pt nanodendrites and the TKK commercial Pt/C catalysts. The image (Fig. 1(a)) indicates that the synthesized Pt nanodendrites have a well-oriented 3D dendritic structure with branches in various directions. The size of each Pt nanodendrites is in the range of 20–30 nm. Most of the shape-controlled electrocatalysts in the literature were tested in the unsupported form. However, in this study, the Pt nanodendrites are supported by carbon for use in
Conclusion
In this study, the Pt nanodendrites were prepared and their oxygen reduction activity in a fuel cell system was explored. Unlike a three-electrode system, the Pt nanodendrites should be supported by carbon to prevent the isolation of the electrocatalytic active sites on the Pt nanodendrites in the catalyst layer of the fuel cells. The carbon supported Pt nanodendrites exhibited a mass activity of 0.54 A mg−1, which is nearly two times higher than that of the TKK commercial Pt/C catalysts. Based
Acknowledgments
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (2009-0093823) and (NRF-2009-C1AAA001-0092926) funded by the Ministry of Education, Science and Technology.
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