Isolated FeN3 sites anchored hierarchical porous carbon nanoboxes for hydrazine-assisted rechargeable Zn-CO2 batteries with ultralow charge voltage

Abstract

Zn-CO₂ batteries (ZCBs) are promising for CO₂ conversion and electric energy release. However, the ZCBs couple the electrochemical CO₂ reduction (ECO₂R) with the oxygen evolution reaction and competitive hydrogen evolution reaction, which normally causes ultrahigh charge voltage and CO₂ conversion efficiency attenuation, thereby resulting in ~90% total power consumption. Herein, isolated FeN₃ sites encapsulated in hierarchical porous carbon nanoboxes (Fe-HPCN, derived from the thermal activation process of ferrocene and polydopamine-coated cubic ZIF-8) were proposed for hydrazine-assisted rechargeable ZCBs based on ECO₂R (discharging process: CO₂ + 2H⁺ → CO + H₂O) and hydrazine oxidation reaction (HzOR, charging process: N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻). The isolated FeN₃ endows the HzOR with a lower overpotential and boosts the ECO₂R with a 96% CO Faraday efficiency (FE_CO). Benefitting from the bifunctional ECO₂R and HzOR catalytic activities, the homemade hydrazine-assisted rechargeable ZCBs assembled with the Fe-HPCN air cathode exhibited an ultralow charge voltage (decreasing by ~1.84 V), excellent CO selectivity (FE_CO close to 100%), and high 89% energy efficiency. In situ infrared spectroscopy confirmed that Fe-HPCN can generate rate-determining *N₂ and *CO intermediates during HzOR and ECO₂R. This paper proposes FeN₃ centers for bifunctional ECO₂R/HzOR performance and further presents the pioneering achievements of ECO₂R and HzOR for hydrazine-assisted rechargeable ZCBs.