A New Dual Site Catalyst Design Concept for Promoting Reaction Kinetics of the Oxygen Reduction Reaction
Authors: Shen Xiaochen , Nagai Tomoyuki , Yang Feipeng , Zhou Li Qin , Pan Yanbo , Yao Libo , Wu Dezhen , Liu Yi-Sheng , Feng Jun , Guo Jinghua , Jia Hongfei , Peng Zhenmeng 
Published: December 2020

Introduction

Oxygen reduction reaction (ORR) accounts for a substantial portion of the energy loss of the proton exchange membrane fuel cell system used for vehicle application. Current state of art ORR catalysts are typically Pt-alloy nanoparticles supported on carbon, of which surface Pt is solely the active site and their activity improvement is fundamentally limited by a “scaling relationship” [1]. In this study, we report the preparation of a SnOx/Pt−Cu−Ni heterojunctioned nanostructure as one potential strategy to break the scaling relationship in ORR. Experimental results confirm a significant activity enhancement compared with pristine Pt−Cu−Ni. Theoretical study suggests a dual-site cascade mechanism wherein the first two steps occur on SnOx sites, followed by transfer of the intermediate to adjacent Pt sites for the subsequent steps. This new catalyst design offers a plausible new approach to achieve high ORR activity on Pt alloys by introducing a second active site.

Materials and Methods

Pt−Cu−Ni alloy nanoparticles were synthesized by a solid-state chemistry method involving electrochemical treatment to generate a clean Pt surface prior to use in this study [2]. The as prepared Pt-Cu-Ni catalysts were first de-alloyed and then followed by immersion in SnCl2 solution for various time to coat with SnOx [3]. The resulting heterojunctioned catalysts were characterized by HR-TEM, XPS, XANES, as well as in electrochemical measurement to determine the activity and durability. The density functional theory (DFT) simulations were performed with the Quantum ESPRESSO package to calculate the energy states of reaction intermediates on different surface sites.

Results and Discussion

The conceptual design of the dual site catalyst, as shown in Figure 1(a). involves a possible migration of reaction intermediates during the steps of ORR. Such a migration should be driven by the difference in free energy of the sites. For proof-of-concept, we synthesized Pt−Cu−Ni nanoparticles and then deposited SnOx on the surface to form a heterojunctioned nanostructure (Figure 1(b)). SEM -EDX and XPS analyses confirmed that the SnOx content increased with deposition time, while the Pt:Cu:Ni ratio remained nearly unchanged before and after the deposition. XANES and high resolution XPS characterization suggest the oxidation state of the SnOx is primarily 4+. A decrease of electrochemical active surface area (ECSA) was observed as the deposition time (td) increases (Figure 1 (c)). The mass activity and specific activity (SA) both exhibit a volcano trend with respect to td with the highest values at td=5 min. The apparent SA increased by 40% as compared to the pristine Pt-Cu-Ni, and if only considering the interfacial sites, the estimated specific activity enhancement may be up to 10 folds (Figure 1(d)). While these results provide unambiguous experimental evidence of SnOx’s promoting effect, DFT simulation of the reaction pathways was performed to obtain insight into the possible mechanism (Figure 1(e)). The results suggest the SnOx and the neighboring Pt sites may have cooperated to find the most kinetically favored pathway (Figure 1(f)), wherein the O2 first protonated on SnOx sites to form *OOH and *O intermediates, and then the *O transfers to Pt sites to complete the remaining steps.

Conclusion

This study demonstrates a strategy to bypass the energy barrier bottleneck of ORR, providing one more dimension of design flexibility towards developing highly active electrode catalysts for fuel cell applications.

References

Kulkarni, A. et al., Chem. Rev. 2018, 118 (5), 2302−2312.

Zhang, C. et al., J.
Am. Chem. Soc. 2014, 136 (22), 7805−7808.

Shen, X. et al., J.
Am. Chem. Soc. 2019, 141(24), 9463-9467.

Figure 1