Spherical mesoporous Fe–N–C catalyst for the air cathode of membrane-less direct formate fuel cells.

Fe–N–C catalysts with excellent performance regarding the oxygen reduction reaction (ORR) have aroused enormous interest in direct-formate fuel cells (DFFCs). However, their limited mass transfer ability, insufficient ORR active sites, and complex fabrication processes remain significant obstacles to the widespread application of Fe–N–C catalysts. Herein, we propose a simple hydrothermal-annealing method with agarose powders to synthesize a uniform spherical Fe–N–C catalyst (∼3 μm) with well-developed mesopores (Fe/rG@C/H-Agar-900). The resultant Fe/rG@C/H-Agar-900 catalyst possesses rich oxygen-containing functional groups and enhanced interconnected pores, which can significantly boost the content of catalytic sites and facilitate mass transport, resulting in a high content of active sites. In the meantime, the mesopore content of Fe/rG@C/H-Agar-900, which can facilitate the formation of the three-phase gas/electrolyte/catalyst interfaces, was optimized by varying the annealing temperature. As a result, the Fe/rG@C/H-Agar-900 demonstrates a half-wave potential of 0.91 V vs. RHE, nearly four-electron pathway selectivity, excellent durability, and excellent formate tolerance for ORR. Furthermore, when used as the air cathode in membrane-less DFFCs, the Fe/rG@C/H-Agar-900-based device exhibits a remarkable peak power density of 24.5 mW cm−2, significantly outperforming the 20 wt% commercial Pt/C. This research facilitates the synthesis of an advanced Fe–N–C catalyst and promotes the practical development of membrane-less DFFCs. [Display omitted] • Spherical mesoporous carbon precursors were prepared by a hydrothermal-annealing method. • Fe–N–C catalyst prepared from agarose powder were used as the air-cathode catalyst for membrane-less DFFCs. • Annealing treatment expands the mesoporous structures of hydrocarbons. • Fe/rG@C/H-Agar-900 exhibits a highly positive E 1/2 (0.91 V vs. RHE) for ORR in alkaline solution. • A P max of 24.5 mW cm−2 is achieved by Fe/rG@C/H-Agar-900, which is 31% higher than that of Pt/C. [ABSTRACT FROM AUTHOR]