Engineering the Activity and Stability of Pt-Alloy Cathode Fuel-Cell Electrocatalysts by Tuning the Pt-Pt Distance

One of the main obstacles to the commercialisation of low-temperature fuel cells is the slow kinetics of the oxygen reduction reaction (ORR). In order to decrease the ORR overpotential and reduce the Pt loading we need to develop more active and stable electrocatalysts. A fruitful strategy for enhancing the cathode activity is to alloy Pt with transition metals [1-2]. However, alloys of Pt and late transition metals are typically unstable under fuel-cell conditions. Herein, we present experimental and theoretical studies showing the trends in activity and stability of novel cathode catalysts based on alloys of Pt and lanthanides.

Sputter-cleaned, polycrystalline Pt₅Gd shows a five-fold increase in ORR activity [3], relative to Pt at 0.9 V in 0.1 M HClO₄. The rest of the Pt₅Ln (Ln = lanthanide) tested present at least a 3-fold enhancement in activity [4,5]. In all cases, a Pt overlayer with a thickness of few Pt layers is formed. Accordingly, the effect of alloying Pt is to impose strain onto the Pt overlayer [3,4]. It is likely that this strain would be relaxed by defects [6]. Moreover, the activity of the Pt₅Ln catalysts vs. the Pt-Pt distance shows a volcano relationship (Fig. A) [5]. Pt₅Ln electrocatalysts are highly stable, as shown in Fig. B [4]. We show, for the first time, that the Pt-Pt distance not only controls the activity, but also the stability of these catalysts [5].

[1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B 2005, 56, 9.

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

[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., in preparation, 2014.

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

Fig. (A) ORR kinetic current density at 0.9 V vs. RHE as a function of the lattice parameter and the Pt-Pt distance for Pt₅Ln and Pt. (B) Kinetic current density of Pt₅Ln and Pt before and after a stability test consisting of 10 000 cycles between 0.6 V and 1.0 V vs. RHE in an O₂-saturated 0.1 M HClO₄ electrolyte.