Switching the Spin at the B‑Site: How a Low‑to‑High Fe 3+ Spin‑State Transition Selectively Drives CO 2 Reduction in Perovskite Oxides

Solid oxide electrolysis cells (SOECs) co‑electrolyze CO 2 and H 2 O to syngas with high thermodynamic efficiency, but the cathode must do more than catalyze two reductions: it must selectively activate CO 2 in the presence of competing H 2 O. Here we report that B‑site doping of LaCaFeO 3 perovskite with Co, Ni, or Cu resolves this competition by switching Fe 3+ from a low‑spin (t 2g 5 e g 0 ) to a high‑spin (t 2g 3 e g 2 ) configuration. The dopant induces lattice contraction, which elongates Fe─O bonds, reduces the crystal‑field splitting energy, and triggers the spin‑state switch. This switch reconstructs the electronic structure of Fe–O–M (M = Cu, Co, Ni) bridge sites, the genuine active centers for CO 2 activation. The reconstructed bridge asymmetrically polarizes adsorbed CO 2 , selectively weakening one C─O bond and steering the cleavage toward CO production. The consequence is a pronounced shift in selectivity: the CO faradaic efficiency rises from 32% to over 90%, the co‑electrolysis current density increases by up to 228%, and the CO/H 2 ratio becomes tunable from 0.14 to 11.8. By establishing a mechanistic pathway from lattice distortion through spin‑state transition to catalytic selectivity, this study identifies spin‑state engineering as a design dimension that operates beyond the explanatory reach of conventional geometric descriptors.