Synthesis of Platinum-Rare Earth Metal Alloy Catalysts for Proton Exchange Membrane Fuel Cells

Electrocatalysts for low temperature fuel cells are based on noble metals and can be promoted by development of alloys of platinum with e.g. late transition metals such as Ni and Co.[1,2] It has been reported that Pt alloys with rare earth (RE) metals exhibit remarkable activity and high stability towards the oxygen reduction reaction.[3] Due to the very low standard reduction potentials and high oxophilicity of rare earth metals, preparation platinum alloys in form of nanoparticles meets great challenges for chemical routes of co-reduction. A one-step method is recently developed to prepare Pt−RE nanoalloys from a mixture of solid state precursors consisting of metal halides in their hydrate forms, a suitable nitrogen-rich compound and a carbon support.[4,5] The synthesis involves in situ formation of a carbon-nitrogen network, onto which the two metals are atomically dispersed. The coordination with nitrogen sites stabilizes the metals, which, upon thermal decomposition under a mild reducing atmosphere, leads to the formation of Pt-RE nanoalloys. The carbon support is introduced either as carbon blacks or carbon supported platinum powder. The pre-formed, either in or ex situ, platinum nanoparticles, facilitates the reduction of rare earth metals thermodynamically driven by the negative alloying free energy. A series of pure crystal phases such as Pt5RE (e.g. La, Ce, Nd), and Pt3RE (e.g. Y, Gd, Tb) have been synthesized in form of carbon supported nanoparticles with tunable metal loadings and alloy particle sizes. One of the typical catalysts is the Pt5Ce/C in a metal to carbon ratio of 30 wt% and an average alloy particle size of ca. 5.5 nm which exhibits an area specific activity of 3.7 times and a mass activity of 1.9 times higher than those of the Pt/C analogue. Further understanding and improvement of the process are in progress and the updated results will be presented in this talk. [1]. J. K. Norskov et al., Nature Chemistry 1, 37-46 (2009) [2]. X. Q. Huang et al., Science 348, 1230-1234 (2015) [3]. M. Escudero-Escribano et al., Science 352, 73-76 (2016) [4]. Y. Hu et al., J. Am. Chem. Soc. 142, 953–961 (2020) [5]. Y. Hu et al., Chem. Mater. 33, 2, 535–546 (2021)