Maximizing Fe-N exposure by tuning surface composition via twice acid treatment based on an ultrathin hollow nanocarbon structure for highly efficient oxygen reduction reaction

Fe-N-C oxygen reduction reaction (ORR) catalysts are still limited in poor ORR activity due to low Fe-N exposure and utilization. Here, we report a newly developed Fe-N-C catalyst with Fe-N sites embedded in a micro/mesopore-interconnected, ultrasmall (5–45 nm), ultrathin (1.3 nm), and hollow nanocarbon structure, which is synthesized by a facile, scalable, and template-free method by using cheap and safe reagents. Moreover, the exposure of Fe-N is maximized through twice acid treatment to tune the surface composition of the catalysts. The mass sites density of the optimal catalyst HNP-16 reached 45.0 μmol g−1, much higher than that of untreated HNP-0 (7.4 μmol g−1) and common treated HNP-1 (12.2 μmol g−1). Benefiting from a collective contribution of abundant surface Fe-N active sites and structural advantages, HNP-16 exhibits higher half-wave potential and faster ORR kinetics than commercial Pt/C. Detailed electrochemical analysis revealed the relationships between coordination and ORR activity. The higher content of Fe-N results in faster ORR kinetics while O-related species lead to an undesired 2-electron ORR pathway. When HNP-16 was employed as the cathode catalyst in a membraneless direct formate fuel cell, it delivered an accelerated ORR current response and achieved an unprecedented maximum power density of 37.6 mW cm−2, which was 1.4 times higher than that of Pt/C. The superior performance of HNP-16 is attributed not only to the highly exposed Fe-N sites but also to the rapid mass transfer provided by the favorable hollow structure.