What Is the Optimum Strain for Pt Alloys for Oxygen Electroreduction?
Sputter-cleaned, polycrystalline Pt5Gd shows a 5-fold increase in ORR activity , relative to Pt. All the Pt-lanthanide alloys are at least 3 times more active than Pt for the ORR [3-5]. A compressed Pt overlayer is formed onto the bulk alloy. Accordingly, the effect of alloying Pt is to impose strain onto the Pt overlayer [3-5]. It is likely that this strain would be relaxed by defects . The activity of the Pt-based electrocatalysts versus the lattice parameter in the bulk shows a volcano relationship (Fig. 1A). The lattice parameter is presented as a new descriptor that controls both the activity and stability of these materials . The best performance (activity-stability) is achieved by Pt5Gd. Furthermore, mass-selected PtxGd nanoparticles synthesised by the gas aggregation technique present a significant ORR activity enhancement as compared to pure Pt nanoparticles, PtxGd 8 nm showing 3.6 A (mg Pt)-1 mass activity (Fig. 1B) , surpassing the highest activity reached with PtxY nanoparticles . The activity of PtxGd nanoparticles also correlates strongly with compressive strain. Our results demonstrate that we can engineer both the activity and stability by tuning the Pt-Pt distance.
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Fig 1. (A) ORR kinetic current density as a function of the lattice parameter and the Pt-Pt distance for Pt5Ln and Pt. (B) Mass activity of PtxGd, PtxY and Pt nanoparticles. All activity values were taken at 0.9 V vs. RHE, from cyclic voltammetry recorded at 50 mV s-1 and 1600 rpm in O2-saturated 0.1M HClO4.