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In this thesis the synthesis and the electrochemical characterization of advanced nanostructured catalysts for the application in Proton Exchange Membrane Fuel Cells (PEM-FCs) are discussed. The catalysts consist of platinum and platinum alloy nanoparticles embedded in spherical mesoporous carbon supports. Several aspects related to the use of the embedded catalysts for the oxygen reduction reaction (ORR) are explored.
In the first part of the thesis (chapter 2), the new concept of confined-space alloying of Pt-Ni nanoparticles is described. The textural features of a unique mesoporous carbon nanostructure, made of hollow graphitic spheres (HGS) were exploited to prepare highly dispersed Pt-Ni bimetallic catalyst that showed remarkable mass activities coupled with an excellent stability over an extended accelerated degradation protocol. The carbon support was found to have a dual function: (i) it acts as physical confinement during the alloying treatment, preventing the excessive sintering of the nanoparticles; at the same time (ii) it plays a major role in suppressing some of the catalyst degradation mechanisms (i.e., particles detachment and agglomeration). The embedded Pt-Ni@HGS catalysts yield mass activities that are 2-3.5 folds higher than pure Pt (depending on the alloy composition). Most importantly, the enhanced activity is completely maintained over an extended degradation protocol within the normal operation range of an ORR catalyst in PEM-FCs. It was found that the confined-space alloying synthetic method not only represents an interesting strategy to prepare efficient ORR catalysts, but it is also a valuable tool to improve our current understanding of the degradation of Pt-alloys nanoparticles, as discussed in chapter 2.8. Indeed, since the sintering (agglomeration) of the nanoparticles was suppressed thanks to their confinement, it was possible to elucidate the impact of de-alloying degradation mechanisms (as a function of alloy composition and type of degradation protocols) separately from other parallel degradation processes, providing an unprecedented in-depth insight into this elusive degradation mechanism.
The electrochemical activation of the encapsulated catalysts was found to be a decisive step for the formation of stable catalysts. However, it is not possible to carry out such procedure in a PEM-FC, as the dissolved metal cations would damage the membrane and the catalyst layer. In order to overcome this issue, the possibility to substitute the electrochemical activation protocol with an equivalent chemical step was explored (chapter 3). On this account, a new ozone-based procedure was developed. Under optimized conditions, the new scalable post-synthesis treatment, which allows for the control of the chemical potential without utilizing any electrochemical equipment, assures the complete activation and the simultaneous surface de-alloying of PtNi@HGS, without compromising the stability of the catalyst.
The use of nanostructured mesoporous carbon materials in electrocatalysis is a relatively unexplored field. Consequently, there are still many aspects to uncover, especially with respect to the actual suitability of this class of materials in PEM-FC electrodes, and in in relation to real benefits of the mesoporous framework at the catalyst layer (CL) level. The third part of the thesis (chapter 4) tackles these questions through the synthesis and electrochemical investigation of two comparable carbon structures (hollow and full spheres) that share the same features at the nano-scale level, but have different morphological properties. The performance of these model systems was investigated in comparison to the behavior of a conventional catalyst. The thorough in situ study proposed in chapter 4.4 corroborated some of the observations (i.e., pore confinement effect) made via the ex situ analysis. At the same time, this analysis evidenced some important factors (location of the ionomers within the catalyst layer, relevance of the operating conditions) that could not be detected via the ex situ investigation.
The final part of the thesis (chapter 5) is dedicated to the development of new strategies to synthesize hollow mesoporous carbon spheres, in replacement of the currently employed hard-templating methods. As a matter of fact, synthesis scalability issues, especially for the HGS system, are the main bottleneck for the extensive use of these nanostructured materials in catalysis. Consequently, there is a strong motivation to develop a simple and scalable method to prepare Hollow Mesoporous Carbon Spheres (HMCS), so to substitute the current procedure to prepare HGS. The new synthetic procedure is based on the deposition of a homogeneous hybrid polymer/silica composite shell on the outer surface of silica spheres through the surfactant-assisted simultaneous polycondensation of silica and polymer precursors in a colloidal suspension. Such composite materials can be further processed to give HMCS. The flexibility of this method allows for an independent control of the morphological (i.e. core diameter and shell thickness) and textural features of the carbon spheres. In particular, it is demonstrated that the size of the pores within the mesoporous shell can be precisely tailored over an extended range (2-20 nm) by simply adjusting the reaction conditions. In a similar fashion, also the specific carbon surface area as well as the total shell porosity can be tuned. The possibility to directly modify the shell textural properties by varying the synthetic parameters in a scalable process represents a distinct asset over the multi-step hard-templating
(nanocasting) routes. Thus, the new synthetic procedure could lead to the expansion, perhaps even beyond the lab-scale, of advanced carbon nanostructured supports for applications in catalysis.
