Exploring the Lanthanide Contraction to Tune the Activity and Stability of Pt

The high platinum loadings required to compensate for the slow kinetics of the oxygen reduction reaction (ORR) impede the widespread uptake of polymer electrolyte membrane fuel cells. In order to improve the ORR kinetics and reduce the Pt loading, we can tailor the electronic properties of the Pt surface atoms by means of alloying Pt with other metals. Researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co during the last decades. However, these compounds typically degrade under fuel cell reaction conditions, due to dealloying. In contrast, alloys of Pt and lanthanides present very negative enthalpy of formation [1,2], which should increase their resistance to degradation. Herein we present eight novel Pt-lanthanide and Pt-alkaline earth ORR electrocatalysts: Pt5La, Pt5Ce, Pt5Sm, Pt5Gd, Pt5Tb, Pt5Dy, Pt5Tm and Pt5Ca [3]. All the materials are highly active, presenting a 3 to 6-fold activity enhancement over Pt. Pt5Tb is the most active polycrystalline Pt-based catalyst reported in the literature. A Pt overlayer with a thickness of few Pt layers is formed onto the bulk alloys by acid leaching [1-3]. Notably, the experimental ORR activity as a function of the bulk lattice parameter and the Pt-Pt distance follows a volcano relation [3], with Pt5Tb presenting the highest initial activity while Pt5Gd is the most active after 10 000 cycles stability test between 0.6 and 1.0 V versus the reversible hydrogen electrode. We use the lanthanide contraction to control strain effects and tune the electrocatalytic activity, stability and reactivity of Pt [3]. References [1] M. Escudero-Escribano, A. Verdaguer-Casadevall, P. Malacrida, U. Gronbjerg, B.P. Knudsen, A.K. Jepsen, J. Rossmeisl, I.E.L. Stephens, I. Chorkendorff, J. Am. Chem. Soc. 2012, 130, 16476. [2] P. Malacrida, M. Escudero-Escribano, A. Verdaguer-Casadevall, I.E.L. Stephens, I. Chorkendorff, J. Mater. Chem. A 2014, 2, 4234. [3] M. Escudero-Escribano, P. Malacrida, M.H. Hansen, U.G. Vej-Hansen, A. Velazquez-Palenzuela, V. Tripkovic, J. Schiotz, J. Rossmeisl, I.E.L. Stephens, I. Chorkendorff, Science 2016, 352, 73.