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In this thesis, nanostructured materials were rationally designed to overcome some of the most critical instability issues of standard catalysts for low temperature fuel cell applications, which can briefly be summarized as: the loss of platinum surface area over time and corrosion of carbon supports, with the first one being a consequence of the dissolution, agglomeration, and detachment of platinum nanoparticles. The first targeted nanostructure consisted of Pt or Pt‐M alloy nanoparticles encapsulated in a porous carbon shell, typically known as yolk‐shell materials. Au, @C, Pt, @C and AuPt, @C yolk‐shell materials were prepared by bottom‐up approaches to produce materials as monodisperse as possible. The first step of the synthesis was the production of the metal or bimetallic colloid by solution methods. The next step was the covering of the colloidal particles with a non‐porous/mesoporous double silica shell. The external mesoporous silica layer was used as exotemplate for the nanocasting of the carbon shell. After the nanocasting process, the double silica shell was selectively leached out, and the final yolk‐shell structure was obtained. A comprehensive study was carried out by varying synthesis parameters, in order to obtain the yolk‐shell material with the highest Pt content and the most robust structural characteristics. Hypothetically, with the use of yolk‐shell materials electrochemical degradation processes such as agglomeration and detachment could be reduced. A first estimation of the electrochemical stability of an optimized AuPt, @C yolk‐shell type catalyst was done by ex‐situ electrochemical half cell measurements. The electrochemical stability was studied simulating start‐up / shut‐down processes in a fuel cell under operation, and the changes of the materials upon degradation were visualized by identical‐location transmission electron microscopy (IL‐TEM) measurements. The electrochemical studies revealed that the accessibility of the bimetallic core was only achieved after a long activation procedure, upon which the structural characteristics of the carbon shell were partially affected. After the activation process, the material presented a rather high stability. Synthesis characterization and electrochemical stability investigations of AuPt, @C yolk‐shell material will be presented in detail in chapter three.
The second concept explored in this thesis was the stabilization of Pt nanoparticles by pore confinement in nanostructured carbon supports. Mesoporous graphitic carbon materials with tailored pore structures were synthesized by nanocasting, and the graphitization process was assisted by transition metal salts. Optimized mesoporous carbon materials were investigated as catalysts supports for the stabilization of Pt nanoparticles under high temperature as well as under electrochemical conditions. Particularly, Hollow Graphitic Spheres (HGS) were studied for the efficient encapsulation and thus stabilization of metal nanoparticles. Sintering stability of Pt nanoparticles was proven by thermal treatment under protective atmosphere up to 900°C. The Pt nanoparticles underwent a controlled and homogeneous particle growth from < 2 nm to 3‐4 nm. This thermal treatment was established as an essential step for the successful confinement of the Pt nanoparticles in the mesopores of HGS and other mesostructured supports. Further functionalization of the HGS support material was intended by nitrogen doping processes. Therefore, nitrogen‐doped Hollow Carbon Spheres (NHCS) were also synthesized with a tailored pore structure for the hosting of Pt nanoparticles. The synthesis and characterization of HGS, NHCS and other mesostructured carbon materials will be presented in chapter four, as well as a comprehensive study of the nanoparticle pore confinement concept.
The particle pore confinement was considered to be a potential structural characteristic to reduce agglomeration and detachment processes of Pt nanoparticles under electrochemical conditions. Additionally, the graphitization of the carbon was aimed to reduce carbon corrosion processes. Therefore, the relations between the Pt nanoparticle confinement in nanostructured carbon supports and their electrochemical performance are presented in chapter five. Electrochemical activity and stability was studied by ex‐situ half cell electrochemical measurements as well as by in‐situ single cell measurements. A first estimation of the electrochemical properties of Pt nanoparticles confined in HGS, obtained ex‐situ by RDE measurements, suggested a superior electrochemical stability than standard Pt/Vulcan fuel cell catalysts. These results were successfully verified under real fuel cell conditions by in‐situ single cell measurements. A deeper understanding of the degradation processes was obtained by identical location scanning and scanning transmission electron microscopy (IL‐SEM/STEM) measurements. IL‐SEM/STEM conclusively proved that during electrochemical cycling the pore confinement significantly suppresses detachment and agglomeration of Pt nanoparticles, two of the most important degradation mechanisms in fuel cell catalysts.
The influence of the morphology of the HGS support was explored by the study of a comparison material, bulk mesoporous graphitic carbon (MGC), which has similar pore structure and graphitization degree as the HGS material, but it does not have the spherical shape and the hollow core. Also, the effect of nitrogen functionalities on catalyst activity and stability was studied using NHCS as catalyst support. Under ex‐situ RDE measurement conditions, the electrochemical properties of Pt nanoparticles confined in MGC and NHCS supports were found to be comparable to the HGS‐based materials. However, under in‐situ single cell measurement conditions the superiority of the HGS‐based materials was recognized. Thus, beyond providing an improved electrocatalyst, this thesis describes the blueprint for targeted improvement of fuel cell catalysts by design of the carbon support, as well as a comprehensive study of the relations between structure and performance of nanostructured catalyst materials for low temperature fuel cell applications.
