Dynamically Unveiling Metal-Nitrogen Coordination during Thermal Activation to Design High-Efficient Atomically Dispersed CoN4 Active Sites

We elucidate the structural evolution of CoN4 sites during thermal activation by developing a zeolitic imidazolate framework (ZIF)-8-derived carbon host as an ideal model for Co2+ ion adsorption. Subsequent in situ X-ray absorption spectroscopy analysis can dynamically track the conversion from inactive Co-OH and Co-O species into active CoN4 sites. The critical transition occurs at 700 degrees C and becomes optimal at 900 degrees C, generating the highest intrinsic activity and four-electron selectivity for the oxygen reduction reaction (ORR). DFT calculations elucidate that the ORR is kinetically favored by the thermal-induced compressive strain of Co-N bonds in CoN4 active sites formed at 900 degrees C. Further, we developed a two-step (i.e., Co ion doping and adsorption) Co-N-C catalyst with increased CoN4 site density and optimized porosity for mass transport, and demonstrated its outstanding fuel cell performance and durability.

Automated summary

The structural evolution of CoN4 sites during thermal activation was studied using a ZIF-8-derived carbon host. It was found that the critical transition occurs at 700 degrees C with optimal conversion at 900 degrees C, resulting in the highest intrinsic activity and four-electron selectivity for the ORR.