An integrated bioelectrochemical system coupled CO₂ electroreduction device based on atomically dispersed iron electrocatalysts

Integrating a bioelectrochemical system with CO₂ electroreduction (CO₂ER) can achieve recovery of resources and conversion of value-added chemicals, but it still faces challenge of high overpotential and poor selectivity. Herein, we report a CO₂ER catalyst with iron bonded to nitrogen atoms (Fe-N_x) anchored hierarchical porous carbon (Fe SA-NC) by a molecular-confined pyrolysis strategy. Owing to the high surface area and atomic-level Fe-N₄ sites, Fe SA-NC possessed superior CO₂-to-CO conversion performance with a low overpotential of 90 mV, a small Tafel slope of 92 mV dec⁻¹, and high Faradaic efficiency of 95.9% at −0.5 V, superior to almost all previously reported Fe-N_x based carbon materials for CO₂ER. Experimental results manifested the atomic-level Fe-N₄ sites in carbon frameworks with a single Fe atom coordinating four N atoms. Theoretical calculations revealed Fe-N₄ sites weaken the free energy for the formation of *COOH intermediate, and the short Fe-C bond length in the structure of *COOH absorbed on Fe-N₄ sites accelerated the electron transfer from Fe-N₄ centers to *COOH, thus boosting the reaction kinetics. An integrated device with cathodic Fe SA-NC and bioanode can recover energy and carbon resource from wastewater, delivering maximum current and CO production rate of 1.54 ± 0.05 mA and 33.66 ± 0.58 mmol g⁻¹_cat h⁻¹.