Datensatz

Zusammenfassung

Increasing energy demand and depletion of fossil resources urge the search for alternative and sustainable energy systems. One promising alternative technology for mobile applications is the polymer electrolyte membrane fuel cell (PEMFC). A major restriction in successful commercialization and implementation of the technology is the availability and high over potential of commonly employed platinum catalysts as well as their lack in long-term stability. In particular, latter is the result of an interplay between different degradation mechanisms. The essential high stability, demanded for real applications, requires the synthesis of advanced electrocatalysts that withstand the harsh operation conditions. In the first part of this work, the synthesis of oxygen reduction electrocatalysts consisting of Pt-Co (i.e. Pt₅Co, Pt₃Co, and, PtCo) alloyed nanoparticles encapsulated in the mesoporous shell of hollow graphitic spheres (HGS) is reported. The mass activities of the activated catalysts depend on the initial alloy composition and an activity increase on the order of 2 to 3 fold, compared to pure Pt@HGS, was achieved. The key point of the investigation is the degradation of PtCo@HGS (showing the highest activity). The impact of dissolution/dealloying and carbon corrosion can be studied without the interplay of other degradation mechanisms that would induce a substantial change in the particle size distribution. Therefore, impact of the upper potential limit (UPL) and the scan rates on the dealloying and ECSA evolution are examined in detail. Besides the incorporation of Pt-Co catalysts, also shape-controlled Pt-Ni catalysts were finely dispersed within the mesoporous shell of the HGS. Pre-impregnated Pt seeds, acting as anchor points for the shell growth, are demonstrated to be a prerequisite for the successful precious metal catalyst dispersion, as omitting the Pt seeds led to severe agglomeration and inhomogeneous particle distribution. A strong increase in specific activity could be obtained, whereas stability enhancement, originated from the encapsulation, was superimposed by agglomerated particles on the graphitic domains of the HGS. Alternative synthesis strategies are reported, enabling large scale synthesis of hollow mesoporous carbon structures. Chemical vapor deposition (CVD) of ferrocene is employed for the synthesis of hollow graphitic spheres (HGS_cvd). Thanks to the precursor, iron particles are embedded into the mesoporous template during the CVD, facilitating the synthesis of highly ordered graphite structures. The effect of the subsequent annealing temperatures on the graphitization degree and on textural properties are highlighted. Ultimately, the HGS_cvd is employed as a support material, significantly improving the stability of finely dispersed nanoparticles in comparison to commercial high surface area carbon materials. Alternatively, the synthesis of hollow mesoporous carbon, following a soft templating method, is reported in a single step. The textural properties can be fine-tuned by subsequent hydrothermal treatments, allowing the control over particle porosity in the range of 3–12nm.