Pt nano-particles dispersed on carbon black (Pt/C) are widely used as electrocatalysts for polymer electrolyte fuel cells (PEFCs). For the implementation of low-cost PEFCs, reducing the amount of expensive Pt is the important issue. Since the oxygen reduction reaction (ORR) occurs on the surface of the Pt nano-particles, core-shell catalysts in which Pt atomic-layer shell is coated on metallic nano-particle core are desired in reducing the Pt usage. Although such core-shell catalysts exhibit very high mass activity compared with the commercial Pt/C, their durability still remains as a challenge (1). If expensive metals such as Au and Pd are used as the core, the cost reduction effect is also limited. According to previous studies, the durability of the PEFC electrocatalyst can be improved by using conductive oxides including SnO₂ as an electrocatalyst support which has thermochemical stability under the PEFC cathode conditions (2-4). In principle, it is possible to develop electrocatalysts with both higher durability and ORR mass activity by employing such stable oxide as the core, instead of noble metal. Here, the objective of this study is to directly deposit Pt on SnO₂ on an atomic level by applying a photochemical procedure towards the oxide-core Pt-shell electrocatalyst preparation.

Experimental

Vapor grown carbon fiber (VGCF-H), carbon nanotube (CNT), and graphitized carbon black (GCB) were applied as the conductive backbone (fillers) of the electrocatalysts. Nb-doped SnO­­₂ or SnO₂ was prepared on each carbon-based material via the ammonia co-precipitation, microwave homogeneous precipitation, and SnO₂ sol impregnation. Pt deposition on SnO₂ was made using photochemical procedure for on an atomic-level. Since this photochemical reaction occurs only on semiconductors, the selective Pt deposition on SnO₂ may be possible. Support materials powders, H₂PtClO₆•6H₂O, and HCOOH were added to ultrapure water, followed by stirring and irradiation of UV (and VIS) light in order to prepare Pt/SnO₂/carbon-filler electrocatalysts.

Nanostructure of the electrocatalysts obtained was observed by FE-SEM and STEM. ICP analysis was also made to quantify Pt loading. Electrochemical characterization of these electrocatalysts was carried out by half-cell measurements. Electrochemical surface area (ESCA) was measured by cyclic voltammetry (CV), and oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (i_k) after the rotating disk electrode (RDE) measurement.

Results and discussion

Agglomeration of Pt nanoparticles could be suppressed by shortening UV irradiation time and reducing formic acid concentration, resulted in improved ECSA and mass activity. High-resolution STEM observation suggested that a part of Pt was deposited near an atomic level on the SnO₂ surface which could be a Pt-shell of the electrocatalysts. SnO₂ particle size was about 20 nm after the ammonia co-precipitation. We succeeded to prepare SnO₂ core with a diameter of about 3 nm by applying the microwave homogeneous precipitation and the SnO₂ sol impregnation (Figs. 1 and 2). In addition, it was revealed that dispersibility of SnO₂ was improved by an activation treatment with HNO₃ for carbon fillers.

References

[1] M. Inaba, and H. Daimon, ECS trans., 50 (2) 65 (2012).

[2] A. Masao, S. Noda, F. Takasaki, K. Ito, and K. Sasaki, Electrochem. Solid-State Lett., 12 (9), (2009).

[3] F. Takasaki, S.Matsuie, Y. Takabatake, Z. Noda, A. Hayashi, Y. Shiratori, K. Ito, and K. Sasaki, J. Electrochem. Soc., 158 (10), B1270 (2011).

[4] K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).