Abstract
The aim of this PhD thesis project was the development of self-standing Fe-N-C cathodes prepared by electrospinning, in order to achieve hierarchical microporous and macroporous electrodes. Such a structure is desirable to improve accessibility by O2 of the Fe-based active sites for oxygen reduction reaction (ORR), and therefore to improve the performance of Fe-N-C cathodes in proton exchange membrane fuel cell (PEMFC). Due to the lower activity for ORR of Fe-N-C catalysts compared to platinum on carbon, Fe-N-C active layers of ca 100 um thickness are today the state-of-art, about 5 to 10 times thicker than Pt/C layers. For this reason, the optimization of O2 diffusion in Fe-N-C electrodes is important to allow the replacement of Pt by less expensive catalysts.Different approaches were investigated to prepare self-standing Fe-N-C electrodes. In a first approach, a 3D web of polyacrylonitrile (PAN) nanofibers comprising also a Fe precursor and additional porogens was first prepared by electrospinning, and then thermally treated in argon and NH3. This resulted in self-standing electrodes based on microporous carbon nanofibers with Fe-based active sites. In a second approach, iron-doped metal-organic frameworks (MOFs) were grown on 3D web of polymer nanofibers (polyacrylonitrile or polybenzimidazole, PBI), and then thermally treated in argon. For this approach, it was first necessary to establish a reproducible method for the preparation of iron-doped MOFs, which was achieved with ferrocene encapsulation in two different MOFs (SIM-1 and ZIF-8). The ferrocene content and synthetic conditions were separately optimized to achieve the highest ORR activity. The same synthesis was then applied to grow Fe-doped MOFs on 3D webs of either PAN or PBI fibers. The growth of these MOFs was studied either on the polymer fibers, or on such fibers pre-coated with zinc.The Fe-N-C electrodes and materials prepared by these different approaches were characterized for their morphology, structure and Fe speciation by scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy and Fe K-edge x-ray absorption spectroscopy. The self-standing electrodes and/or grinded FeNC powders were electrochemically investigated with rotating disk electrode and single-cell PEMFC.ORR-active materials were obtained with the different approaches, but the most promising one is identified to be the crystalline growth of ferrocene-doped ZIF-8 on a web of cross-linked PBI fibers, followed by pyrolysis in argon. The electrodes were investigated in PEMFC, either after grinding the self-standing FeNC cathodes into a powder or as a self-standing structure. In the latter case, they were functionalized by Nafion before electrochemical measurement in PEMFC. Grinded FeNC cathodes are shown to retain a fibrous structure derived from the electrospinning process, leading to increased macroporosity in the electrodes and good performance in fuel cell. Due to their thickness of only ca 20 µm, several self-standing FeNC layers (functionalized with Nafion) were superimposed in order to reach sufficient overall ORR activity. The concept of self-standing Fe-N-C cathode was validated for the first time, but further optimization of their functionalization by Nafion ionomer is needed to take full advantage of this approach.Future research is therefore needed to functionalize such novel electrode structures by proton-conducting ionomers to optimize the electrode activity and proton conductivity. Such electrode structure can also find application in other electrochemical energy conversion devices such as anion exchange membrane fuel cells, electrochemical supercapacitors and electrochemical CO2 reduction.