Single-atom iron-decorated hierarchically porous carbon nanofibers with carbon vacancy-enhanced performance for zinc-air batteries and supercapacitors

The high-efficiency utilization of metal single-atom sites plays a pivotal role in improving the performance of energy conversion and storage systems. However, it remains a challenge to construct efficient single-atom sites while controlling the structural characteristics of the carbon support to achieve dual functionality in both applications. Herein, hierarchically porous carbon nanofibers immobilized with single-atom-dispersed FeN₄ sites (denoted as Fe_SA/N-HPCNF) are synthesized. The hierarchically porous structure and optimized electronic structure of the FeN₄ sites induced by adjacent carbon vacancies endow Fe_SA/N-HPCNF with significantly boosted performance in both oxygen reduction reaction (ORR) and supercapacitors (SCs). Impressively, it achieves one of the best performances among reported bifunctional materials for ORR and SCs, including a high alkaline ORR half-wave potential of 0.895 V and a large specific capacitance of 331.8 F g⁻¹. Theoretical calculation reveals that the carbon vacancy induces an asymmetric charge distribution in the adjacent FeN₄ site, which facilitates the adsorption/desorption behavior of oxygen-containing intermediates, consequently accelerating the ORR kinetics. Also, the carbon vacancy facilitates electron transfer from Fe to the coordinated N, leading to more accumulation of negative charge on the N sites, which contributes to more H⁺ adsorption, consequently enhancing the charge storage capability. Both zinc-air batteries and coin cell SCs employing Fe_SA/N-HPCNF exhibit outstanding electrochemical energy conversion and storage performance as well as exceptional cycle stability, demonstrating its great potential in bifunctional applications.