Hierarchical integration of Fe single-atom sites and Fe2O3 nanoclusters on carbon nanofibers for electrocatalytic nitrate-to-ammonia conversion

Electrocatalytic nitrate-to-ammonia reduction (e-NO 3 RR) provides a sustainable alternative to fossil-fuel-dependent Haber-Bosch processes. However, its practical application is hindered by limited ammonia yield and unsatisfactory faradaic efficiency (FE). In this work, hierarchical porous carbon nanofibers decorated with Fe 2 O 3 nanoparticles and Fe–N–C sites (Fe 2 O 3 /Fe SAC/HCNF) are synthesized through electrospinning-assisted templated pyrolysis. Structural characterization reveals that spatial confinement effect of the polymer suppresses the agglomeration of Fe 2 O 3 during the synthesis process and the core-shell structure enhances the stability Fe species under prolonged operational conditions, while the hierarchical porous hollow fibers serve as an integrated substrate, optimizing both the active sites and mass transfer pathways. The Fe SAC sites preferentially adsorb and activate NO 3 − , while the ultrafine Fe 2 O 3 site provide abundant active hydrogen for the subsequent reduction process. Accordingly, Fe 2 O 3 /Fe SAC/HCNF achieves an FE maximum of 87 % ± 4 % at −0.50 V, with a corresponding areal productivity of 1.5 ± 0.02 mmol h −1  cm −2 and excellent long-term stability. When incorporated into a Zn-NO 3 − battery as the cathode, the Fe 2 O 3 /Fe SAC/HCNF system exhibits outstanding electrochemical performance with high power density of 4.67 mW cm −2 and FE of 85 % ± 2 %. This study establishes a systematic strategy for synergistically enhance the performance of NO 3 RR to ammonia.