Spin‐State Modulation of Atomic Iron Sites Enables Efficient CO ₂ Electroreduction in Acid Medium

ABSTRACT Electrochemical carbon dioxide (CO 2 ) reduction (CO 2 RR) in acidic conditions not only enables high CO 2 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 CO 2 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 H 3 O + , significantly inhibiting HER, and reducing the *CO desorption energy; thus, CO 2 RR performance in acidic media is greatly improved. The designed Fe−N 4 O 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−N 4 (7.86 mA cm −2 ). Meanwhile, the catalyst reaches a high j CO of 324.55 mA cm −2 , 80.97% CO 2 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 CO 2 RR performance.