Iron-nitrogen containing carbon catalysts for oxygen reduction in fuel cells

Doktorsavhandling, 2018

New solutions to efficiently convert energy are needed to mitigate global climate changes and sustain the needs of the growing population. An energy device with high potential is the proton exchange membrane (PEM) fuel cell. PEM fuel cells can convert chemical energy to electrical energy with extraordinary high conversion efficiency. However, a drawback with PEM fuel cells is their high price, mainly caused by extensive use of the expensive noble metal platinum as catalyst in the fuel cell cathode. A reduced catalyst price (ideally by replacing platinum) would make PEM fuel cells’ cost-competitive in comparison to other energy conversion devices. Consequently, the development of inexpensive noble metal-free catalysts, active for the oxygen reduction reaction (ORR) has become a hot research topic.

In this thesis, we designed and synthesized iron-chelating nitrogen-functionalized ordered mesoporous carbon (Fe-OMC) catalysts, active for ORR in PEM fuel cells. One focus of the work was to gain a deeper understanding about the operational mechanism of the active sites in the catalysts. Another focus was to evaluate the influence of some synthesis variables and gain new insight into the formation mechanism of the materials to thereby achieve more active catalysts. Several steps are covered in the thesis; synthesis of Fe-OMCs, physical characterization of the catalysts, structural investigation of the active sites, and electrochemical evaluation of the catalysts in a single cell PEM fuel cell. Characterization methods, such as N2-sorption, Raman, X-ray diffraction, and Small Angle X-ray scattering, were used to investigate the physical properties of the catalysts. Whereas, electron paramagnetic resonance(EPR) and nuclear magnetic resonance (NMR) spectroscopies were used to study the active sites and iron-nitrogen interactions in both the precursor mixtures and the final catalysts. In this thesis EPR spectroscopy was (for the first time) shown to be a useful method to study Fe-OMC (and possibly otherFe‑N/C) catalysts. It was shown that EPR spectroscopy can contribute with substantial information regarding the iron species, oxygen radicals, and delocalized electrons in the Fe-OMCs. Essential information about the iron species such as type, oxidation state, geometry and interaction with oxygen was obtained. Of more general importance, a number of crucial pretreatment steps of the EPR samples were identified and found necessary to employ prior to the measurements to obtain high quality EPR data.

Furthermore, additional information about the Fe-Nx chelate structures acting as catalytically active sites was successfully obtained by comparing catalysts prepared from precursors with different functional groups. The results confirmed that nitrogen is involved in the formation of active sites in the Fe-OMC catalysts. Iron-nitrogen interactions could be observed in the precursor mixing step of the catalyst preparation and were correlated to catalytic activities in the final catalysts. Synthesis parameters such as hydration state of the iron salt, precursor aging time, iron to N/C-precursor ratio, iron salt anion, were shown to influence the performance of the prepared catalysts. Finally, by modifying the precursor composition and employing an alternative template etch method, improvements of the mesoporous carbon properties were achieved.

With the results obtained, we made progress in the understanding and tuning of Fe-OMC catalysts. Even though the catalytic performance of these new noble metal-free catalysts is still inferior the commercial platinum-based catalysts, they seem to have potential to compete with them and hopefully, eventually replace them in applications for PEM fuel cells.