Inducing Microstrain in Electrodeposited Pt through Polymer Addition for Highly Active Oxygen Reduction Catalysis
Qi Hua, Xinyi Chen, Junfeng Chen, Nawal M. Alghoraibi, Yoon Hyeok Lee, Toby J. Woods, Richard T. Haasch, Steven C. Zimmerman, Andrew A. Gewirth
Abstract
We investigate an approach to tune the d-band center and enhance the oxygen reduction reaction (ORR) activity of Pt material without relying on foreign metals or the process of alloying/dealloying. It is known that Pt exhibits suboptimal ORR catalytic activity due to its strong binding to oxygen, therefore requiring a downshift in the d-band center by approximately 0.2 eV to weaken the Pt-O binding energy and boost ORR kinetics. We found that the d-band center can be tuned by inducing microstrain in the Pt electrodeposit, simply achieved by introducing polymer into the electrodeposition bath. Pt electrodes (Pt-P1 and Pt-PLA) prepared with the addition of poly- N -(6-aminohexyl)acrylamide (P1) or poly- l -arginine (PLA) exhibit improved ORR activity compared to Pt electrodeposited without polymer addition (Pt-alone) in both acidic and basic environments, with the order of activity being Pt-P1 > Pt-PLA > Pt-alone. Pt-P1 exhibits a positive shift of E 1/2 by 90 mV vs Pt-alone in basic solution, comparable to other reported high-activity ORR catalysts. Scanning electron microscopy shows the presence of agglomerates with diameters between 5 and 20 μm and tip-splitting growth structure due to diffusion-limited aggregation on Pt-P1 and Pt-PLA. Characterization using X-ray photoemission spectroscopy and X-ray diffraction, combined with Rietveld refinement analysis reveal a trend of downshifted d-band center, increased microstrain, and slightly increased compressive strain as the ORR activity increased among the three catalysts. The presence of more defective sites on Pt-P1 and Pt-PLA is the cause of the increased microstrain, which further leads to the downshift of the Pt d-band center and enhancement of ORR activity.