UC Irvine

With the demand for reducing greenhouse gas emissions and developing diverse energy sources, more and more efforts have been made in the advent of hydrogen fuel energy technologies. Transition metal-nitrogen-carbon materials (M-N-C catalysts) are promising electrocatalysts in the hydrogen fuel energy technologies like polymer electrolyte fuel cells (PEFCs) and electrolyzer applications. High-temperature treatment in the inert atmosphere (pyrolysis) is the most common method for the synthesis of M-N-C catalysts and is critical to achieving high electrocatalytic activity and electronic conductivity. To this day, despite many uses and successful implementations in materials manufacturing, pyrolysis has been an entirely empirical technology, with process control and optimization relying exclusively on the “Edisonian” approach. The knowledge gap in the mechanism about how the precursor is being transformed into catalysts hinders further development of the M-N-C catalysts regardless of the precursor class and processing protocols. Herein, this dissertation focused on probing the morphological evolution and chemical transformation of the precursors to M-N-C catalysts via a combination of in situ and ex situ synchrotron and laboratory-based diagnostic techniques. The following processes have been observed and analyzed:i) The pyrolysis process of a mixture of nitrogen-containing charge-transfer organic salt, transition metal (iron) salt, and amorphous silica powder (precursors) leading to M-N-C materials. ii) The re-pyrolysis process of the above-mentioned M-N-C materials leading to state-of-the-art M-N-C catalysis. iii) The pyrolysis process of two commercial highly porous metal-organic frameworks (MOFs) leading to the M-N-C catalysts. It was found that the pyrolyzing temperature might bring an adverse effect on the final product, causing the active sites to decrease and the carbon to be highly graphitized. The carbon graphitization event catalyzed by the metallic particles forming occurs during the heat treatment. The direct observation and thorough fundamental study of the pyrolytic synthesis are expected to benefit in improving the rational design of the M-N-C catalysts.