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

Sunday, 5 October 2014: 14:40
Sunrise, 2nd Floor, Galactic Ballroom 7 (Moon Palace Resort)
M. Escudero-Escribano, P. Malacrida, U. G. Vej-Hansen, V. Tripkovic, J. Rossmeisl, I. E. L. Stephens, and I. Chorkendorff (Technical University of Denmark)
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.