Activation of the Oxygen Reduction Reaction at an Interface-Regulated Pt/Nb-SnO2 Catalyst
Authors: Shi Guoyu , Tryk Donald A. , Iiyama Akihiro , Kakinuma Katsuyoshi 
Published: December 2020

The sluggish oxygen reduction reaction (ORR) kinetics on carbon-supported Pt catalysts has impeded the large-scale commercialization of polymer electrolyte fuel cells (PEFCs). More active ORR electrocatalysts are highly desired to improve PEFC performance with low Pt cathode loading. The adoption of alternative support material such as doped SnO2 represents a promising way forward for improving the activity of Pt-based catalysts while maintaining high durability.1 However, the correlation between the microstructure of the Pt/doped SnO2 and ORR activity remains ambiguous. In the present work, we, for the first time, show that the ORR kinetics of Pt can be boosted by the PtSn alloy formed at the interface between the loaded Pt nanoparticles and the Nb-SnO2 support.
The Nb-SnO2 support was synthesized by the flame oxide-synthesis method. The Pt nanoparticles were loaded on the Nb-SnO2 by the colloidal method. The Pt loading amount in the Pt/Nb-SnO2 catalyst was determined by plasma-mass spectroscopy (ICP-MS). The microstructure of the Pt/Nb-SnO2 was characterized by scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDX). A commercial catalyst c-Pt/C (TEC10E30E, TKK) was used for comparison. The electrochemical measurements were carried out by use of the rotating disk electrode (RDE) technique from 20 to 80 oC. The experimental procedure was the same as that described earlier.1 All of the electrode potentials in this paper are given versus the reversible hydrogen electrode (RHE).
The Pt loading of Pt/Nb-SnO2 was measured to be 16.3 wt.%. The catalyst was uniformly dispersed on the support with an average Pt particle size of 2.7 ± 0.5 nm. EDX line scan analysis shows that Sn diffused into the Pt particle to form a PtSn alloy near the interface between Pt and Nb-SnO2, as illustrated in Fig. 1a. As shown in Fig. 1b, the Pt/Nb-SnO2 achieved 1.54 times higher specific activity and 1.47 times higher mass activity than c-Pt/C. Fig. 1c shows Arrhenius plots of the apparent rate constant k
app for the ORR. Good linear relationships between the logarithm of k
app and 1/T are observed for both electrodes. The k
app values for c-Pt/C and Pt/Nb-SnO2 at 80 °C were, respectively, about 12 and 7 times higher than those at 20 °C. Thus, elevating the operating temperature significantly enhanced the ORR kinetics, which makes it possible to reduce Pt loadings in PEFCs. The apparent activation energy E
a of c-Pt/C was calculated to be 36 kJ mol-1, which is well consistent with the result of Yano et al.2 Impressively, Pt/Nb-SnO2 exhibited a lower E
a, 27 kJ mol-1, which is responsible for the high k
app. The enhanced ORR kinetics is ascribed to the modified adsorption of oxygen species on the Pt surface by the PtSn alloys at the support-Pt interface, as revealed by DFT calculations.

Acknowledgement

This work was supported by funds for the JSPS KAKENHI Grant Number 20H02839 from the Japan Ministry of Education, Culture, Sports, Science and Technology. We are also grateful to Prof. Minoru Inaba and Prof. Hideo Daimon from Doshisha University for kindly providing the instruction of the RDE system (controlling temperature).

References

[1] K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).
[2] H. Yano, E. Higuchi, H. Uchida, and M. Watanabe, J. Phys. Chem. B, 110, 16544 (2006).

Figure 1