Dynamically Unveiling Metal-Nitrogen Coordination during Thermal Activation to Design High-Efficient Atomically...

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Zeolitic imidazolate frameworks (ZIFs) serve as precursors for advanced carbon materials.
• Understanding the formation of active sites in metal-nitrogen-carbon (M-N-C) catalysts is crucial for energy applications.
• Cobalt-nitrogen-oxygen (Co-N-O) species are common intermediates during the synthesis of M-N-C catalysts.

Purpose of the Study:

• To elucidate the structural evolution of cobalt-nitrogen-carbon (CoN4) active sites during thermal activation.
• To develop a model system using ZIF-8 derived carbon for studying Co2+ adsorption and active site formation.
• To optimize the synthesis of Co-N-C catalysts for enhanced oxygen reduction reaction (ORR) activity and fuel cell performance.

Main Methods:

• Synthesis of a ZIF-8 derived carbon host for Co2+ adsorption.
• In situ X-ray absorption spectroscopy (XAS) to dynamically track the conversion of cobalt species.
• Density Functional Theory (DFT) calculations to understand reaction mechanisms and structural effects.

Main Results:

• The conversion of inactive Co-OH and Co-O species to active CoN4 sites occurs between 700°C and 900°C.
• Optimal active site formation and highest ORR intrinsic activity were achieved at 900°C.
• DFT calculations indicated that compressive strain in Co-N bonds at 900°C favors the ORR kinetics.
• A two-step synthesis approach increased CoN4 site density and improved catalyst porosity for mass transport.

Conclusions:

• Thermal activation at 900°C is optimal for generating highly active CoN4 sites in ZIF-8 derived catalysts.
• The developed Co-N-C catalyst exhibits superior fuel cell performance and durability due to optimized active sites and porosity.
• This work provides a mechanistic understanding of active site formation in Co-N-C catalysts for electrochemical applications.