N/S Co-Doped Mesoporous Carbon Hollow Spheres: Toward Efficient and Durable Oxygen Reduction

Abstract

This study reports the design of N- and S-doped ordered mesoporous carbon hollow spheres (OMCHS) as metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media. Three electrocatalysts were synthesized using molecular precursors: (i) 2-thiophenemethanol (S-OMCHS), (ii) 2-pyridinecarboxaldehyde/2-thiophenemethanol (N1-S-OMCHS), and (iii) pyrrole/2-thiophenemethanol (N2-S-OMCHS). Among them, S-OMCHS exhibited the best activity (Eonset = 0.88 V, E½ = 0.81 V, n ≈ 3.95), surpassing both co-doped analogs. After conducting an accelerated degradation test (ADT), S-OMCHS and N1-S-OMCHS showed improved catalytic behavior and outstanding long-term stability. Surface analysis confirmed that performance evolution correlates with heteroatom reorganization: S-OMCHS retained and regenerated thiophene-S and C=O/quinone species, while N1-S-OMCHS converted N-quaternary into N-pyridinic/pyrrolic, both enhancing O2 adsorption and *OOH reduction through synergistic spin–charge coupling. Conversely, oxidation of N and loss of thiophene-S in N2-S-OMCHS led to partial deactivation. These results establish a direct link between surface chemistry evolution and electrocatalytic durability, demonstrating that controlled heteroatom doping stabilizes active sites and sustains the four-electron ORR pathway. The approach provides a rational design framework for next-generation, metal-free carbon electrocatalysts in alkaline fuel cells and energy conversion technologies.