Preparation of Nanocrystalline Nb-Doped SnO<sub>2</Sub> on Mesoporous Carbon for PEFC Electrocatalysts

Sasaki, Kazunari

Pt-based catalysts on carbon support (Pt/C) is widely used as polymer electrolyte fuel cell (PEFC) electrocatalyst. However, under high potential at the cathode, Pt detachment and aggregation due to carbon corrosion can occur. Therefore, conventional Pt/C electrocatalysts have a difficulty in durability[1]. SnO₂ is known to be stable at high potentials in a PEFC cathode environment. Our research group has developed an electrocatalyst that uses carbon as a support framework, with SnO₂ supported on it and Pt catalyst particles on SnO₂ (Pt/SnO₂/Carbon), achieving high durability [2-4]. However, the electronic conductivity of tin oxide is relativity low and the ORR activity with SnO₂-based support tends to be inferior to that of Pt/C, the standard catalyst.

Therefore, the objective of this study is to develop an alternative electrocatalyst that exhibits high ORR activity while maintaining high durability by using mesoporous carbon (MC) with nano-sized mesopores as a support framework and reducing the amount of SnO₂ used by suppressing its grain growth.

Tin oxide nanoparticles were supported on mesoporous carbon (CNovel🄬, TOYO TANSO) using ethoxide reagents. Tin oxide was doped with a small amount of niobium to improve the conductivity of SnO₂ [5]. Electrocatalysts were prepared by loading of Pt particles of a few nm in diameter on the prepared supports using the Pt acetylacetonate method [6].

Pt loading was examined by ICP Atomic Emission Spectroscopy. The crystallite size of SnO₂ and Pt, and possible alloying of Pt and Sn were checked by XRD measurements. The electrochemical surface area (ECSA) of Pt was obtained by cyclic voltammetry (CV). The oxygen reduction reaction (ORR) activity was evaluated by determining the activation-dominant current value in the rotating disk electrode (RDE) measurement. The durability of the catalysts was evaluated by applying 60,000 of potential cycles between 1.0 and 1.5 V_RHE simulating the start-up and shutdown of PEFC systems. The microstructure of electrocatalysts was observed using a field emission electron scanning microscope (SU9000, Hitachi High-Tech Corporation).

Figure 1 shows (a) scanning and (b) transmission electron microscope images of the electrocatalysts before Pt loading (Sn_0.98Nb_0.02O₂/MC). It was confirmed that tin oxide was loaded within the mesopores besides the MC surface. Furthermore, as shown in the transmission image in Fig. 1(b), the particle size of SnO₂ on the surface is about 10 nm, while the particle size of SnO₂ within the mesopores is about 5 nm, indicating that the presence of mesopores suppresses particle growth. Latest results of electrochemical properties of Pt-based electrocatalysts using Nb-doped SnO₂ and MC after Pt catalyst decoration will be reported in this presentation.

References

T. Takahashi, T. Ikeda, K. Murata, O. Hotaka, S. Hasegawa, Y. Tachikawa, M. Nishihara, J. Matsuda, T. Kitahara, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 169, 044523 (2022).
Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018).
S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14), F1164 (2018).
T. Yoshizumi, M. Nagamine, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 92 (8), 479 (2019).
K. Sasaki, F. Takasaki, Z. Noda, S. Hayashi, Y. Shiratori, and K. Ito, ECS Trans., 33 (1), 473 (2010).
A. Hayashi, H. Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochim Acta., 53, 6117 (2008).

Preparation of Nanocrystalline Nb-Doped SnO2 on Mesoporous Carbon for PEFC Electrocatalysts

Preparation of Nanocrystalline Nb-Doped SnO₂ on Mesoporous Carbon for PEFC Electrocatalysts