Modification of Transition Metal Catalysts and Their Application in the Oxygen Reduction Reaction

Abstract

Fuel cells, as a novel energy source, have garnered widespread attention due to their high efficiency, environmental friendliness, and exceptional reliability. The oxygen reduction reaction (ORR) serves as the critical cathode reaction in fuel cells, whose sluggish kinetics necessitate the use of highly efficient catalysts. To date, platinum (Pt)-based materials exhibit the highest ORR performance, establishing themselves as commercial catalysts for fuel cells. However, Pt’s high cost, limited resources, and poor thermal stability have driven research into efficient non-precious metal catalysts. Transition metal-nitrogen-carbon (M-N-C) catalysts have emerged as promising alternatives due to their stability, high surface area, excellent conductivity, and low manufacturing costs. Among these, Fe-N-C catalysts have garnered significant attention for exhibiting Pt-like activity in ORR. However, issues such as low loading and utilization of Fe-N 4 active sites and inherently low intrinsic activity in Fe-N-C catalysts have prevented their catalytic performance from reaching an ideal state. To enhance the ORR performance of Fe-N-C catalysts, it is essential to select an appropriate carbon support to anchor a large number of iron (Fe) single atoms and to regulate the electronic structure of Fe-N 4 sites through targeted defect engineering, thereby improving the intrinsic activity of Fe-N 4 active sites. This work employs three-dimensional ZIF-8 as a precursor, iron nitrate as the Fe source, and utilizes urea etching to synthesize the single-atom Fe-N-C-2 catalyst in a one-step process. Numerous Fe atoms are uniformly distributed on the porous carbon framework with a dodecahedral structure, significantly increasing the loading and utilization of Fe-N 4 active sites. Simultaneously, urea etching selectively breaks C-N bonds adjacent to Fe-N 4 active centers, creating carbon vacancies and disrupting the symmetry of the Fe-N 4 local electronic structure, thereby enhancing its intrinsic activity. Consequently, the Fe-N-C-2 catalyst exhibits outstanding ORR activity in acidic electrolytes, achieving an E 1/2 value of 0.934 V in acidic media, along with excellent environmental durability and cycling stability.