Template-directed synthesis of M-N₄ type catalysts for electrochemical conversion reactions

Nitrogen doped carbons with atomically dispersed metal active sites (M-NCs) are a promising and sustainable catalyst substitute to precious metals for electrochemical CO2 conversion and oxygen reduction reactions. This thesis compares an established Zn-imprinting route to prepared M-NCs with an optimized Mg-imprinting strategy implemented in a water-free NaCl-KCl-MgCl₂ eutectic, yielding N-doped carbons with well-defined M-N₄ motifs. Post-synthetic metalation/transmetalation to Ni-, Fe-, and Co-NCs produces active site structures that predominantly adopt porphyrin like coordination for Fe-N4 and Co-N4 sites, while Ni-NCs favor corrole-type Ni-N4 motifs, a new structural finding supported by XAS together with DFT. Mg-based imprinting approach is resource-efficient and scalable, enabling high carbon yields, controllable metal contents, which are crucial for intrinsic activity comparisons, and well-defined active sites. Screening of electrochemical activity for the carbon dioxide reduction reaction (CO₂RR) began with Mg-imprinted Ni-NCs in a half-cell (rotating disk electrode) configuration. The best-performing material reached CO Faradaic efficiencies up to 83% in a single-cell setup. Furthermore, integrating a Zn-imprinted Co/Zn-NC catalyst into an in-house developed bioelectrocatalytic system (BES) provided proof-of-concept operation under biotic conditions, enabling microbial acetate formation, and demonstrated a bridge between electrocatalysis and microbial C₂+ production. Exploiting the imprinting strategies to produce M-NCs with similar macroscopic properties, a controlled electrochemical study was conducted to compare the ORR performance of Fe-NCs and Co-NCs in alkaline. The study revealed intrinsically inferior performance of the Co-NCs which can be attributed to enhanced peroxide oxidation, consistent with a sequential 2 + 2 e⁻ mechanism in which outer-sphere peroxide formation is followed by inner-sphere reduction at metal sites. Mixed potentials and peroxide reduction and oxidation (PRR/POR) limiting currents emerged as key descriptors, underscoring peroxide management e.g., via secondary catalytic sites, as a rational design lever for next-generation M-NC ORR catalysts.