Tunable strain drives the activity enhancement for oxygen reduction reaction on Pd@Pt core-shell electrocatalysts

Abstract

An effective way to tune the surface reactivity of catalysts in electrocatalysis is by engineering their surface strain. Traditionally, activity enhancement for the oxygen reduction reaction (ORR) can be attributed to both strain and ligand effects for Pt-based catalysts. Herein, we successfully use variable shell thickness to tune surface strain and thus tailor the ORR catalytic activity of core-shell electrocatalysts in acid media. Increasing reaction temperature from 140 °C to 180 °C in a typical one-pot solvothermal method increases the thickness of Pt shells from 3.0 to 14.0 monolayers, by increments of 3 monolayers per 10 °C (3 ML/10 °C). The surface strains of −1.85% to −0.18% are achieved with increasing Pt shell thickness. Relative to a commercial Pt/C catalyst, the optimum mass activity of a Pd@Pt3.4L/C catalyst (0.95 A mgPt−1) is found to be greater by a factor of 5.3. The theoretical study on the strain-activity relation reveals that Pd@Pt3.4L/C possesses 1.85% of surface compression, and the optimum oxygen binding energy (0.15 eV). The Pd@PtnL/C catalysts show desirable stability compared with commercial Pt/C catalyst after 8000 cycles, especially Pd@Pt5.3L/C, with 97.7% of initial mass activity.