What Is the Optimum Strain for Pt Alloys for Oxygen Electroreduction?

In order to make low-temperature fuel cells commercially viable, it is crucial to develop oxygen reduction catalysts based on more active, stable and abundant materials. A fruitful strategy for enhancing the oxygen reduction reaction (ORR) activity is to alloy Pt with transition metals [1]. However, commercial alloys of Pt and late transition metals such as Ni, Co or Fe are typically unstable under fuel-cell conditions [2]. The very negative enthalpy of formation of alloys of Pt and lanthanides could provide them with greater long term stability than Pt and late transition metals. Herein, we show the trends in activity and stability novel Pt-lanthanide (Pt-Ln) alloys as efficient ORR catalysts.

Sputter-cleaned, polycrystalline Pt5Gd shows a 5-fold increase in ORR activity [3], 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 [6]. 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 [5]. 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) [7], surpassing the highest activity reached with PtxY nanoparticles [8]. 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.

References

[1] I.E.L. Stephens, A.S. Bondarenko, U. Grønbjerg, J. Rossmeisl, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 6744.

[2] S. Chen, H.A. Gasteiger, K. Hayakawa, T. Tada, Y. Shao-Horn, J. Electrochem. Soc. 2010, 1571, A82.

[3] M. Escudero-Escribano, et al., J. Am. Chem. Soc. 2012, 130, 16476.

[4] P. Malacrida, M. Escudero-Escribano, A. Verdaguer-Casadevall, I.E.L. Stephens, I. Chorkendorff, J. Mater. Chem. A 2014, 2, 4234.

[5] M. Escudero-Escribano, et al., to be submitted, 2014.

[6] P. Strasser, et al., Nature Chem. 2010, 2, 454.

[7] A. Velázquez-Palenzuela, et al., J. Catal., accepted, 2014.

[8] P. Hernández-Fernández, et al., Nature Chem. 2014, 6, 732.

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 Pt_xGd, Pt_xY 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.