For the rational design of high surface area catalysts for PEMFCs, it is crucial to understand the influence of their physical properties on the catalytic performances. To systematically investigate the influence of the physical properties on the performance, each parameter needs to be individually varied while keeping the other parameters constant. To date, many research groups have reported Pt particle size effects on the ORR activity. However, in most cases different commercial Pt/C catalysts are compared; that means that not only the Pt particle size but also other parameters, such as Pt loading and support material, are different from catalyst to catalyst. In addition, several research groups have discussed the Pt loading (on the support) effect on the ORR activity. However, in conventional catalyst synthesis methods, the Pt particle size increases with increasing the Pt loading. In our research group, the colloidal “toolbox” synthesis approach has been applied to prepare Pt/C catalysts, and it has been demonstrated that Pt loading can be varied while keeping the Pt particle size constant [1]. Recently, we have also achieved that the Pt particle size can be reproducibly controlled in the “toolbox” synthesis. Using this synthesis method, in the present study, we systematically investigated for the first time how two important parameters – the particle size and the particle proximity (metal loading) – affect the ORR activity of high surface area catalysts by changing the Pt particle size and the inter-particle distance independently.

The “toolbox” synthesis approach consists of two main steps: a suspension of colloidal Pt nanoparticles (NPs) is prepared via an alkaline ethylene glycol route, and then the NPs are deposited onto carbon support. The Pt particle size can be controlled by changing the molar ratio between Pt precursor (H₂PtCl₆) and NaOH in the reaction solution. Applying this approach, Pt particle size and Pt loading can be independently varied. In this work, 10-70 wt. % Pt/C catalysts (in nominal loading) were prepared by depositing 3 differently sized Pt NPs (1.8, 2.8, and 3.9 nm) on Vulcan XC 72R. The prepared Pt/C catalysts were carefully characterized via the thin film-rotating disk electrode (TF-RDE) method in 0.1 M HClO₄ without incorporation of Nafion ionomer. Especially, pH of the catalyst inks were adjusted to be around 10, because dispersion of the catalyst inks and homogeneity of the catalyst thin films highly depend on the ink pH, as we previously reported [2]. By optimizing the catalyst ink formulation and the catalyst thin film fabrication for each catalyst, uniform catalyst thin films without coffee rings or any pronounced agglomeration were prepared.

In the TF-RDE measurements, it was observed for all the Pt NPs sizes that the ORR specific activity (SA) increases with decreasing the inter-particle distance (increasing particle proximity) and approaches that of bulk (polycrystalline) Pt. Interestingly, it was seen that the proximity effect on the SA became more significant with a decrease in particle size. In addition, the highest ORR mass activity (MA) is achieved by the smallest Pt NPs at high Pt loading. A similar trend was also observed in the Pt reduction peak potential of cyclic voltammograms measured in Ar purged electrolyte. The reduction peak potential increases with decreasing the inter-particle distance, and approaches that of the bulk Pt. The proximity effect on the reduction peak potential is also most significant for the smallest Pt NPs. Furthermore, by comparing the Pt/C catalysts with the lowest Pt loading for each Pt particle size, the “pure” particle size effect independent of the proximity effect can be evaluated. It was observed that both SA and Pt reduction peak potential increase with increasing the particle size and approach those of the bulk Pt. From these results it is confirmed that both the particle size effect and the proximity effect exist. Furthermore, it is indicated that the Pt NPs become more bulk-like either by increasing the particle size or decreasing the inter-particle distance.

X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) measurements were also conducted to further investigate the origin of the two effects. It was found that the Pt atoms in the Pt/C catalysts become less oxophilic with a decrease in the inter-particle distance at constant particle size. The same phenomenon was seen when the Pt particle size was increased. These results indicate that an increase of the SA by the both effect stem from a weakening of the OH_ads intermediates.

References:

[1] Speder et al., Phys. Chem. Chem. Phys., 2013, 15, 3602

[2] Inaba et al., J. Power Sources, 2017, 353, 19