MS-14-P-1988 HRTEM and STEM-EELS study of thin films Pt-CeOx nanocatalysts for on-chip fuel cell technology

In recent years, cerium oxide based materials such as Pt-cerium oxide have received much attention because of their excellent catalytic properties for a variety of reactions. Among others, fuel cells offer an interesting application field of these materials. It was shown that sputtered thin cerium oxide films containing Pt, which had been deposited on the anode side of a fuel cell, exhibited a higher specific power compared to a conventional Pt−Ru catalyst [1-2]. Besides the large scale fuel cells, there is also an increasing interest in miniature fuel cells fabricated on silicon, which could be used as an on-chip power supply for portable electronic devices.

In this study, nanometric Pt-ceria thin films were characterized by TEM after elaboration by physical vapor deposition on various substrates (silicon, carbon foils, carbon nanotubes …). The deposited layers exhibited different morphologies linked to the different substrates [3-4]. More particularly, deposition carried out directly on silicon substrate is linked to flat surface layer whereas elaboration on carbon substrates (nanotubes, carbon foils, intermediate carbon layer grown on silicon substrate) is linked to the presence of porous surfaces. In addition to the substrate type, many effects as the formation of carbides or silicates at the interface, an interaction of ceria with platinum and the presence of the porosity influenced also the structure and the chemistry of the deposited layers.

In all samples, crystallites corresponding mainly to CeO₂ and to a less extent to CeC₂ crystallographic structures were observed (Fig. 1). STEM-EELS measurements have been carried out on layers grown on silicon with and without intermediate carbon layer. Data analysis of the M_4,5 white lines of cerium have pointed out a variation of cerium oxidation state from Ce⁴⁺ to a mixture of Ce³⁺ and Ce⁴⁺ depending of the localization of the measurement (Fig. 2).

[1] C. Xu, R. Zeng et al, Electrochimica Acta 51 (2005) p.206

[2] N.V. Skorodumova, S.I. Simak et al, Physical review letters 89 (2002) p.166601

[3] S. Bruyere, A. Cacucci et al, Surface and coating technology. 227, (2013) p.15

[4] M. Dubau, J. Lavkova et al, ACS Applied materials interfaces 6 (2014) p.1213