Spin-State Modulation of Atomic Iron Sites Enables Efficient CO₂ Electroreduction in Acid Medium

Electrochemical carbon dioxide (CO₂) reduction (CO₂RR) in acidic conditions not only enables high CO₂ utilization but also reduces the formation of interfacial (bi)carbonate. However, the acidic environment tends to favor the competing hydrogen evolution reaction (HER), which lowers the overall energy efficiency of CO₂ reduction. Here, we use axial oxygen coordination to tune the spin state of iron-nitrogen-carbon sites, shifting from the low-spin (LS, t_2g ⁵ e_g ⁰) to the medium-spin (MS, t_2g ⁴ e_g ¹) state. Experimental results and theoretical simulations show that this medium-spin structure results in spin-electron filling of the 𝜎∗ orbital, weakening the interfacial attraction of H₃O⁺, significantly inhibiting HER, and reducing the *CO desorption energy; thus, CO₂RR performance in acidic media is greatly improved. The designed Fe-N₄O structure achieves a mass activity of 76.17 A mg_Fe ^-1 and CO current densities of approximately 335 mA cm^-2 in acidic conditions, far exceeding those of Fe-N₄ (7.86 mA cm^-2). Meanwhile, the catalyst reaches a high j_CO of 324.55 mA cm^-2, 80.97% CO₂ utilization efficiency, and an energy efficiency of 36.89% in a self-designed proton-exchange-membrane porous-solid-electrolyte reactor. This work highlights the spin-manipulation mechanism for enhancing acidic CO₂RR performance.