Strategies to synthesize carbon-supported elctrocatalysts with reduced Pt content and increased activity for use in low temperature fuel cells

Abstract

[eng] Conventional catalysts for polymer electrolyte fuel cells are based on Pt supported on porous carbons, generally carbon blacks. However, Pt is expensive and scarce and is poisoned by CO, which is present in the hydrogen obtained by reforming natural gas and it is also generated as an intermediate in the oxidation of methanol. In addition, carbon blacks have some drawbacks, such as their microporosity, impurities, and low electrochemical and thermal stability. In this thesis, nanoparticle catalysts with a core- shell structure have been synthesised and characterised, with the Ni or Cu core and the Pt shell supported on advanced carbons, in order to reduce the amount of Pt used and increase its catalytic activity against the reduction of oxygen and the oxidation of methanol and CO, as well as its stability. Different procedures have been used to synthesize the catalysts, with initial deposition of Ni or Cu nanoparticles by chemical reduction on different carbons and subsequent galvanic exchange with Pt. The catalysts have been characterised by transmission electron microscopy, X-ray photoelectron spectroscopy, voltammetry cyclic and linear scanning voltammetry on rotating disk electrode. The results of the structural and electrochemical analyses, supported by computational calculations of model atomic clusters, are consistent with the formation of nanoparticles with a diameter of 2-5 nm, a nucleus enriched with Ni or Cu and an essentially Pt shell. They presented greater tolerance to CO than the commercial Pt/C, due to the electronic effect of the metallic nucleus on the Pt, which enhanced the desorption of CO. The calculation of the CO adsorption energies on different active centres showed that the presence of surface defects could affect the CO tolerance of the catalysts, which could be critical for nanoparticles that are too small, as observed experimentally. In the case of PtNi, the two anodic peaks observed in the oxidation of CO suggest the presence of two distinct structural domains on the catalyst surface, probably Pt in Ni-rich hexagonal domains and in Pt-rich cubic domains, without forming a solid solution between Ni and Pt. Its activity against methanol oxidation is also greater than that of Pt/C due to the ligand effect of Ni on Pt, which also increases with the incorporation of Ru species due to its bifunctional effect. Contrary to the case of PtNi, a solid solution of Cu is formed in Pt, attributable to its compatibility of size and crystallization system, cubic (hexagonal in the case of Ni). Also, in the case of PtCu, better catalytic activity is observed against the oxidation of methanol than for commercial Pt/C, due to the geometric and ligand effects of Cu on Pt. This makes them interesting for the reduction of oxygen in the cathodes of direct methanol fuel cells, since it decreases the negative effect of methanol transport through the membrane. The dispersion of PtCu nanoparticles on commercial carbon nanofibers and nanotubes, as well as commercial and synthetic mesoporous carbons, also leads to catalysts that are more tolerant to CO than commercial Pt/C, due to the electronic effects of Cu on Pt discussed above. Better activities for oxygen reduction than on commercial Pt/C, both mass and specific, are achieved on various supports, while its relative stability is increased with respect to the latter. Mesoporous carbon supports synthesised from chitosan are of special interest, since chitosan is an abundant, non-toxic, nitrogen-rich natural polysaccharide present in crustacean shells. Also taking into account that P20 silicon oxide was used as a template in the synthesis procedure, the mesoporous carbon obtained was low cost and of great added value. Adjusting the microporosity-mesoporosity ratio of the carbons through different synthetic procedures, specific activities against oxygen reduction and methanol oxidation were obtained with mesoporous carbons derived from chitosan, greater than those of Pt/C and that of PtCu catalysts supported on commercial mesoporous carbons.