Huang, Chong: Synthesis of Metal Carbodiimide Electrocatalysts for Oxygen Evolution/Reduction and Nitrate Reduction. , Dissertation, Universität Zürich, 2025
10.5167/uzh-281215
Open Access Version
Abstract:
The electrochemical conversion of abundant environmental feedstocks into high-value small fuel molecules powered by renewable energy is emerging as a promising strategy for storing excess renewable power and enabling a fossil-free energy supply. Electrocatalysis plays a central role in this process, facilitating the efficient synthesis and utilization of these fuel molecules. Electrocatalysts are crucial for accelerating the relevant electrochemical reactions and ensuring the selective formation of desired products. Therefore, the development of cost-effective electrocatalysts with high activity, selectivity, and durability is essential for the practical implementation of these energy conversion technologies. Metal carbodiimides, as a distinct and less explored class of functional materials, feature hybrid structures based on the linear [NCN]2− ligands and show unique physicochemical properties for a variety of applications. In this thesis, metal carbodiimides were investigated as electrocatalysts for three key reactions: the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and the nitrate reduction reaction (NO3−RR). Operando/in situ spectroscopic techniques were employed to uncover the true catalytically active sites and phases of metal carbodiimides during these electrochemical processes. The four-electron ORR is a pivotal cathodic process in fuel cells. Transition metal-nitrogen-carbon (M–N–C) based catalysts have emerged as promising alternatives to platinum-group metals for catalyzing ORR. However, the presence of diverse and complex coordination environments around the active sites poses significant challenges to accurately understanding the structure-function relationships in M–N–C catalysts. Herein, we explored cobalt carbodiimide (CoNCN) with unambiguously characterized Co–N–C moieties and large Co–Co interatomic distances as promising ORR electrocatalysts. Crystalline CoNCN flakes with preferential exposure of (100) facets were supported on oxidized carbon nanotubes (O-CNTs), which assisted charge redistribution around Co centers. The obtained CoNCN/O-CNT electrocatalyst displayed a catalytic activity comparable to that of commercial Pt/C catalysts and good long-term stability with the attenuation of only 6 mV in half-wave potential after 20,000 cyclic voltammetry cycles. Quasi in situ X-ray emission spectroscopy and X-ray absorption spectroscopy (XAS) experiments confirmed the structural stability of the Co–N–C motifs during electrocatalysis. These results offer an ideal material system for understanding the structure-function correlation between well-defined M–N–C moieties and ORR activity. OER is a bottleneck for the conversion of water and clean energy into chemical fuels through electrocatalysis. In-depth insights into the surface structural evolution of real active species on catalysts during the OER process are of great significance for knowledge-driven catalyst design. Herein, we developed iron-doped cobalt carbodiimide (CoxFe1−xNCN) nanoparticles as efficient OER pre-catalysts with stable overpotential for at least 290 hours. Advanced structural characterizations disclosed the presence of abundant structural defects in low-crystalline (LC) CoxFe1−xNCN. Operando XAS and X-ray diffraction studies revealed that these intrinsic structural defects could accelerate the irreversible surface reconstruction in LC-CoxFe1−xNCN. This promoted the generation of high-valence metal oxyhydroxides as real active OER phases, resulting in a lower overpotential compared to highly crystalline CoxFe1−xNCN. The present study highlights the introduction of structural defects as an effective approach for the rational design of efficient OER electrocatalysts.