(745b) Engineering the Active Site for Oxygen Electroreduction in Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are very attractive as potentially zero-emission sources of power. However, the high platinum loadings required to compensate for the slow kinetics of the oxygen reduction reaction (ORR) impede the widespread uptake of PEMFCs. In order to reduce the Pt loading, researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co. However, these compounds typically degrade under fuel cell conditions, due to dealloying. In contrast, alloys of Pt and lanthanides present exceptionally negative alloying energy [1-2], which should increase their resistance to degradation.

Polycrystalline Pt₅Gd shows a 5-fold increase in ORR activity [2], relative to Pt. The active phase consists of a compressed Pt overlayer formed by acid leaching. Mass-selected Pt_xGd nanoparticles present a significant ORR activity enhancement as compared to pure Pt nanoparticles, Pt_xGd 8 nm showing 3.6 A (mg Pt)^-1 mass activity [3,4]. The activity of Pt_xGd nanoparticles correlates strongly with compressive strain [3]. In order to study the trends in ORR activity for different levels of compression, we studied eight novel polycrystalline Pt-lanthanide and Pt-alkaline earth electrocatalysts [5]. Pt₅Tb is the most active polycrystalline Pt-based catalyst ever reported. All the materials present a 3 to 6-fold activity enhancement over Pt. Notably, the experimental ORR activity (kinetic current density measured at 0.9 V vs. the reversible hydrogen electrode, j_k (0.9 V vs. RHE)) versus the bulk lattice parameter determined by X-ray diffraction follows a â??volcanoâ? relation. We use the lanthanide contraction to control strain effects and tune the electrocatalytic activity, stability and reactivity.

References

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

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

[3] A. Velazquez-Palenzuela, et al. J. Catal. 2015, 328, 297.

[4] C.M. Pedersen, M. Escudero-Escribano, A. Velazquez-Palenzuela, L.H. Christensen, I. Chorkendorff, I.E.L. Stephens, Electrochim. Acta 2015, 179, 647.

[5] M. Escudero-Escribano, et al., Science 2016, 352, 73.