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

Abstract

• A newly developed hollow structural Fe-N-C catalyst is synthesized by a template-free method. • Fe-N sites are embedded in an ultrasmall, ultrathin, and interconnected porous carbon structure. • The exposure and utilization of Fe-N sites was maximized by twice acid treatment. • The optimal catalyst (HNP-16) exhibited higher E 1/2 and faster ORR kinetics than Pt/C. • HNP-16 delivered a P max of 37.6 mW cm −2 in a membraneless DFFC, 140% higher than Pt/C. 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.